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Celcon POM Processing and Troubleshooting Guide

1. Ensure molding machine has free flowing check ring Move mold to machine where shot size is 50 75 of capacity Nozzle drool Lower nozzle temperature Lower material temperature Minimize cushion Delay sprue break time Decrease mold open time Use nozzle with smaller diameter orifice Use decompression Poor Weld Lines Increase injection pressure Increase screw forward time Raise mold temperature Vent cavity at area of weld line Raise melt temperature Increase injection speed Relocate gate to alter flow pattern Provide overflow well at weld area Check for leaking check ring 32 Celcon acetal copolymer Table 4 8 Troubleshooting Guide for Injection Molding Celcon Acetal Copolymer continued SYMPTOMS POSSIBLE SOLUTIONS Ticona Short Shots Pit Marks and Surface Ripples Increase feed Increase injection pressure Increase mold temperature Decrease cushion Increase injection speed Increase injection time Add vents Use larger machine Check for leaking check ring Sink marks Increase injection pressure Increase screw forward time Maintain proper cushion Reduce tool temperature Increase injection speed Increase feed Enlarge gate or runner Splay Marks Delamination Decrease injection speed Reduce injection rate Enlarge gate size Reduce cylinder temperature Increase mold temperature Dry material S
2. 5 2 3 Screen Pack A screen pack of assorted screen sizes e g 20 60 80 20 mesh should be placed immediately in front of the screw to filter out any unmelted particulates which could lead to defective products or abrasion of the inner head and die surfaces The screen pack also serves to maintain back pressure which prevents uneven parison flow due to surging The screen pack should be replaced at regular intervals 5 2 4 Breaker Plate A breaker plate should be used to hold the screen pack firmly in place while interfering as little as possible with a smooth flow of the melt stream A breaker plate is a steel plate usually about 6 4 mm 1 4 in thick and perforated with closely spaced 6 4 1 4 in holes The holes in the breaker plate are frequently chamfered on the side facing the screw to improve melt flow 5 2 5 Die Head A die head and adapter is generally placed between the breaker plate and the parison die to guide the melt stream to the die entrance M any assemblies require that the melt stream turn 90 as it approaches the die To encourage smooth flow and avoid undesirable 35 weld lines flow pin spiders with helical deflectors machined to a knife edge at the point of convergence are often used The melt stream can flow freely around such a die pin without dead spots where material could hang up A flow divider of this type is shown in Figure 5 1 Fig 5 1 Flow Pin Divider to Promote Smooth Flow and A
3. Increase stock pressure Increase parison wall thickness Poor Pinch Off Weld Reduce blowing pressures Use wider pinch off blades Walls too Thick Decrease screw speed Change die dimensions as required Walls too Thin Increase screw speed Change die dimensions as required Fig 5 7 Landed Pinch off For Improved Impact Strength MOLD CAVITY 0 60 100 Land A 39 Fig 5 8 Blow Molded Fluidic Capacitor i Celcon acetal copolymer 6 Extrusion H igher molecular weight grades of C elcon acetal copolymers higher viscosity than for injection molding such as C elcon M 25 are recommended for extrusion Sheets up to about 6 4 mm 0 25 in thick and tubes with wall thicknesses 0 38 to 1 0 mm 0 015 to 0 040 in can be extruded Call Product Information Services at 1 800 833 4822 for recommendations on specific grades of C elcon acetal for each type of job requirement The information on equipment given below applies generally to all types of extrusion Additional specific details for high speed tubing film and sheet and profile extrusion are also covered in this C hapter 6 1 Equipment 6 1 1 Materials of Construction At extrusion temperatures C elcon acetal copolymer is not affected by contact with copper zinc iron nickel brass or bronze The designer therefore has freedom in selecting the most cost effective material for a specific application for di
4. Conical 150 included angle Tip shape Die Steel is commonly used for die construction Chrome plating of all internal surfaces is recommended for long production runs Brass and beryllium copper are sometimes used for short runs because they are easy to machine and are good heat conductors Both straight through and cross head dies may be used with straight through dies preferred in most cases Because of the melt elasticity and strength of C elcon acetal the die should be 15 20 oversize in width and 10 in thickness It is recommended that land length to thickness ratio should be 8 1 15 1 to provide the correct level of exit velocity back pressure and mixing to give a smooth lustrous surface to the extruded part Sizing and cooling Air from an air ring or profiled copper tubing should be blown on the surface of the part Water is generally not recommended for cooling Celcon profiles 44 Celcon acetal copolymer For sizing of simple thin shapes the extrudate should be passed through a set of brass or aluminum sizing plates with the first plate 5 8 oversize and the others progressively smaller Complex shapes can be sized by passing through a number of adjustable brass fingers which are appropriately positioned to produce the desired cross section 6 4 2 Processing Profile extrusion processing conditions can vary significantly depending on the geometry of the part thickness equipment resin grad
5. Deposits in M old Troubleshooting Blow Molding Blow Molding Methods Extrusion Blow M olding Injection Blow Molding Equipment Extruder Screws Screen Pack Breaker Plate Die H ead Ticona 5 2 6 5 2 7 5 2 8 5 3 5 3 1 5 3 2 5 3 3 5 4 5 4 1 5 4 2 5 4 3 6 1 6 1 1 6 1 2 6 1 3 6 1 4 6 1 5 6 1 6 6 2 1 6 2 2 6 3 1 6 3 2 6 3 3 6 4 6 4 1 6 4 2 7 1 7 2 7 2 1 7 3 1 7 3 2 7 3 3 7 4 Celcon acetal copolymer Die H opper M olds Processing Parameters Barrel Temperature M old Temperature Blowing Pressure Effects of Process Variables on Part Dimensional Stability and Part Q uality M old Shrinkage Surface A ppearance Impact Strength Extrusion Equipment Materials of C onstruction Extruder Barrel Screw Design Screen Pack H ead and Die Design H opper H igh Speed Tubing Extrusion Equipment Processing Parameters Profile Extrusion Equipment Processing Parameters Troubleshooting Profile Extrusion Equipment Processing Rotational Casting Equipment M olds Particle Size Processing Parameters Resin Drying Conditions Part H eating O ven Parameters Part Cooling Rates Troubleshooting Ticona Overview General Guidelines Dimensional Stability Injection Molding Blow Molding Extrusion Rotational Casting Celcon acetal copolymer List of Tables Table 1 1 Regulatory Listings Table 2 1 Typical Start U p Conditions Table
6. 0 050 in Shrinkage in in 60 90 120 Blow Pressure psi Ticona 5 4 Effects of Process Variables on Part Dimension and Quality 5 4 1 Mold Shrinkage M old shrinkage for blow molded C elcon acetal generally ranges from 2 5 M old shrinkage is dependent on such factors as mold temperature cooling time blow pressure and wall thickness The effect of these variables on shrinkage are shown in Figures 5 3 through 5 6 Average shrinkage values obtained in around 3 4 ounce container with a nominal 1 27mm 0 050 in wall thickness are shown in Figures 5 3 5 4 and 5 5 while the effect of wall thickness on shrinkage in a relatively hot 127 C 260 F mold is seen in Figure 5 6 Fig 5 4 Effect of Cooling Time on Shrinkage a ee Mold Temperature 127 C 260 F Melt Temperature 188 C 370 F Blow Pressure 85 psi Wall Thickness 1 27 mm 0 050 in Shrinkage in in o S S le 10 20 30 Cooling Time Seconds Fig 5 6 Effect of Wall Thickness on Shrinkage Mold Temperature 127 C 260 F Melt Temperature 188 C 370 F Cooling Time 20 Seconds Blow Pressure 85 psi Shrinkage in in 0 020 0 040 0 060 0 080 0 100 Wall Thickness in 38 Celcon acetal copolymer In general a blown container will shrink slightly more in its length than in its diameter and slightly more in the neck area than in other sections 5 4 2 Surface Appearance The surface appearance of blown Celc
7. considered This will automatically adjust molding conditions to provide consistently satisfactory parts O pen loop systems which are less costly can be set up to stop the molding cycle or sound an alarm to indicate that molding conditions to provide satisfactory parts have changed and should be readjusted to prevent rejects Both types of systems are suitable for molding C elcon acetal 4 4 Processing Before placing C elcon acetal in a molding machine it is highly recommended that you refer to Chapter 2 of this publication G eneral Guidelines for safe handling and processing information The M aterial Safety D ata Sheets for specific grades of C elcon acetal will also provide additional information related to safety handling and use and may be obtained by calling 1 800 526 4960 4 4 1 Typical Molding Conditions Typical processing conditions for injection molding C elcon acetal are shown in Table 4 4 The recommendations should be used as an initial guide and fine tuned as necessary for each specific application 4 4 2 Melt Temperature M ost moldings are made using a melt temperature in the range 182 199 C 360 390 F to facilitate processing and provide good quality parts at minimum cycle M elt temperatures substantially above 199 C 390 F should be accompanied by a corresponding decrease in residence time to avoid overheating and possible degradation of the resin This can be achieved by using a smaller c
8. using a faster fill rate may marginally increase packing pressure and eliminate the problem Be careful not to exceed the critical melt shear rate which may lead to resin flow lines splay and pit marks Refer to the discussion on excessive melt shear during runnerless molding page 4 5 for further comments 4 2 5 Sprue Bushings Standard sprue bushings with a taper of 2 1 2 per side perform satisfactorily with C elcon acetal The sprue diameter should be larger than the mating end of the molding machine nozzle to prevent an undercut and facilitate ejection of the sprue The end of the sprue bushing which mates with the runner should be larger than the diameter of the runner and be radiused at the junction O pposite the junction of the sprue bushing and the runner provision should be made for acold slug well and a standard Z or other design sprue puller The sprue puller pin should be kept below the runner system to prevent interference with resin flow Secondary sprues used for gating in 3 plate molds should havea taper of 2 3 included angle and should also be radiused where they join the runner The sprue size must be larger than the maximum wall thickness of the molded part 4 2 6 Runners In designing arunner system it is preferable to restrict the length and diameter to minimize the amount of material that has to be recycled Runners should be as short as possible and adequate in cross sectional diamet
9. 2 2 Effect of Remolding Regrind on the Properties of U nreinforced Celcon A cetal Table 2 3 Effect of Remolding Regrind on the Properties of Glass C oupled Celcon A cetal Table 3 1 Effect of Processing Conditions on Part Shrinkage Table 3 2 Shrinkage Before and A fter Annealing Table 3 3 Recommended Annealing Procedure Table 4 1 Typical Injection M olding Screw for Plasticizing C elcon Acetal Copolymer Table 4 2 Typical Runner Size Recommendations for Celcon A cetal Copolymer Table 4 3 Recommended G ate Dimensions for Rectangular E dge G ates Table 4 4 Typical Start U p Conditions Table 4 5 A pproximate C ycle Times as a F unction of Wall Thickness U nreinforced G rades Table 4 6 Effect of M olding Conditions on M echanical Properties U nreinforced G rades Table 4 7 Typical M olding C onditions for Shrinkage Range U nreinforced G rades Table 4 8 Troubleshooting Guide Injection M olding C elcon Acetal C opolymer Table 5 1 Comparison of Injection and Extrusion Blow M olding Processes Table 5 2 Typical Screw C haracteristics to Plasticize Celcon Acetal Copolymer Blow M olding Table 5 3 Typical Blow M olding Conditions Table 5 4 Troubleshooting Guide Blow molding Table 6 1 Recommended M etering Screw Dimensions for Extruding C elcon A cetal Table 6 2 Typical C onditions for Tubing Extrusion Table 6 3 H igh Speed Tubing Extrusion Troubleshooting Guide Table 6 4 Typical C onditions for Film and Sheet Extrusion Table 6 5 Film a
10. CPM Technology CPM technology has been eminently successful in making it possible to produce high quality parts shot after shot for months and even years In the basic process pressure sensing instruments called transducers are placed in the mold usually in the cavity or at the gate to track pressures during every cavity filling incident Since the cavity pressure is normally proportional to the amount of molten polymer present in the cavity before solidification you thus have a convenient means to determine shot to shot reproducibility of the process Should one cavity filling incident not agree with the norm the parts that are produced in that specific cycle are usually segregated into a reject parts bin or the like These systems use a computer to analyze the data and make it rather easy to do diagnostic studies on a molding problem Then you can make any corrections that may be required to optimize the whole molding process The net result is that it allows one to produce parts that fall within very tight tolerances on weight dimensions etc Since out of spec parts are not 28 Celcon acetal copolymer produced or are caught when they are made the amount of post molding QC can be reduced Because process fine tuning can usually be done through the judicious use of the data generated at the time of operation it is often possible to reduce the variability of molded part product qua
11. If a mold cavity is not properly vented trapped air can hold the plastic material away from the cavity surface interfere with heat transfer in the cavity and result in a poor finish on the molded part Vents 0 051 0 102 mm 0 002 0 004 in deep should be provided at the mold parting line in areas where air entrapment is likely M olds can be vapor honed or sand blasted to provide a matte surface which prevents sticking and facilitates part ejection 36 Celcon acetal copolymer 5 3 Processing The extruder should be started up by the procedures outlined in Chapter 6 of this publication As the extrudate issues from the die the cold parison will appear rough and translucent R aising the temperature produces a well converted clear transparent parison If parison temperature is too high bubbles and slight discoloration will appear Each different blow molding job will require some variation in operating conditions to optimize the production process U sually the lowest material melt temperature should be used consistent with obtaining a fully plasticized melt no unmelted pellets to provide a satisfactory parison with maximum melt strength Item Ticona 5 3 1 Barrel Temperature Initially barrel temperature profiles should be relatively flat but after operation is well underway some benefit will be derived from raising the temperature in the feed section and lowering the temperature of the succeeding zones to estab
12. Several factors affect surface finish including condition of the mold surface itself mold temperature cavity pressure part configuration wall thickness resin melt viscosity and flow pattern A check list of the key parameters is shown below For mold surface condition and surface temperature Check mold surfaces for nicks blemishes etc M Check for worn surfaces from glass reinforced resins M Make sure the melt temperature is not on the low side this can lead to abrasion from reinforced and filled resin grades Mold surface temperatures should be high enough to prolong freezing of the melt in the cavity and gate allowing better pressure trans mission to the part extremities Surface pit marks and visible flow lines are indications of low mold surface temperature A minimum mold surface temperature of 82 C 180 F is recommended for thin walled parts lt 1 5 mm or 0 06 in or less Lower surface temperatures may be satisfactory for thicker walled parts but precautions should be taken against increased post molding shrinkage Celcon acetal copolymer For cavity pressure M Packing pressure must be adequate to force the melt against the mold surface and keep it there until a cooled surface film has formed to insure adequate reproduction of the surface If the pres sure drop from the gate to the furthest point of fill is too high the frozen skin may pull away from the mold surface as the resin s
13. Ticona A business of Celanese AG 2000 Ticona Printed in USA 00 303 3M 0700 8 a c O O O D S T 7 Q O O ne A
14. about 10 micro inches R M S Although surface roughness decreases with increasing die temperature adie temperature of 230 C 446 F usually yields the best balance of minimum surface roughness and good appearance Typical processing conditions are shown in Table 6 2 Sizing Techniques An external sizing sleeve submerged in a water bath will provide good results A series of plates or rollers should follow the sleeve to keep the tube below the water level in line with the external sleeve This will prevent chatter marks on the tube In conjunction with the air pressure injected through the mandrel tube diameter is controlled by the film of water trapped between the tube and the external sizing sleeve Tubing with diameter 2 5 5 8 mm 0 100 0 230 in can be made using a 6 2mm 0 245 in diameter sleeve Increasing the inside diameter of the tube is accomplished using air pressure of 0 07 0 35 kg cm 1 5 psig transmitted into the tube via the die mandrel Vacuum sizing is generally conducted at higher line speeds and may also be used for tubing extrusion of Celcon acetal A draw down ratio of 1 5 to 2 1 is recommended The sizing plates should gradually decrease in diameter in the direction of extrusion The last plate should be 1 to 5 oversize depending on the line speed to allow recrystallization and shrink down to the desired final diameter Orientation When producing tubing biaxial orientation of the extru
15. as can partial systems such as hot sprues A wide variety of drop designs are acceptable 23 Greater than 101 6 Up to 4 7 92 0 312 including hot tip hot edge angle gate torpedo angle tip multi tip and E type nozzles M achine system suppliers can provide extensive design services to determine the best drops for a specific application A variety of gate configurations can be used for processing C elcon acetal in hot runners including systems which provide thermal freeze off Valve gates especially hydraulic designs work well with parts requiring zero vestiges G enerally gates should be relatively unrestricted and should not subject the melt to shear rates higher than 1 500 2 000 sec at polymer melt temperatures Excessive shear may result in melt fracture G ate design and location influence mold filling patterns and affect mechanical properties dimensions and surface finish T he gate land should be a minimum 1mm 0 040 in Tips should be hardened to reduce wear especially with reinforced or filled systems and should be designed to be easily replaced when excessively worn Temperatures need to be accurately controlled in all melt channels Thermocouple placement is critical It is recommended that control systems based on proportional integral derivative PID algorithms be used These systems anticipate temperature fluctuations and account for thermal inertia when regulating heaters The result is much fine
16. conditions can have a significant effect on the mechanical properties of fiberglass reinforced grades due to glass breakage Increased glass breakage can be expected to occur mostly with increasing screw rotational speed and to a lesser extent with increasing injection speed and back pressure Tensile strength and modulus will decrease with increasing screw rotational speed due to breakage of glass fibers M old shrinkage will increase with increasing screw rotational speed for the same reason W hen molding fiberglass reinforced C elcon acetal screw speed and back pressure should be kept to aminimum 20 30 rpm and 0 psi respectively Injection speed should be medium consistent with adequate fill of the mold cavities with flash Celcon Ticona acetal copolymer Table 4 7 Typical Molding Conditions for Maximum Shrinkage of Unreinforced Celcon Acetal Copolymer Melt temperature C F 180 210 360 410 Mold temperature C F 121 250 Injection pressure MPa psi 35 5 000 Injection speed Minimum Screw rotational speed has little effect on mold shrinkage of unreinforced Celcon grades 4 8 Molding Problems 4 8 1 Deposits on Molds In those rare instances when deposits may build up on mold cavity and runner surfaces the most likely causes are inadequate venting mold surfaces that are colder than recommended or excessive shear heating by injecting at high rates through small diameter
17. copolymer C onventional 2 plate 3 plate and runnerless molds may all be used 4 2 2 Mold Bases M old bases should be fabricated in a suitable steel grade and be made sturdy enough with pillars to adequately support the cavities and the cores without buckling of the retainer plates during injection molding They should also be large enough to accommodate water cooling channels to provide uniform mold temperature This operation is essential to produce acceptable parts 4 2 3 Mold Cavities and Cores The selection of steels for the mold can be critical to its successful performance J ust as resins are formulated to satisfy processing and performance requirements steels are alloyed to meet the specific needs for mold fabrication processing and its intended use There are many different parts to the mold eg cavity gates vents pins cores slides etc and these may have different requirements For example some applications may require a mold steel with high hardness to resist wear and abrasion at the parting line while another application may require toughness to resist mechanical fatigue U sually steels with higher hardness and wear resistance properties tend to be more brittle and steels with higher toughness will show less wear resistance The selection process of the tool steels should include input from the tool steel supplier the mold designer and mold fabricator in addition to the resin supplier Post treatment of the m
18. ensure minimum flow restriction The mating surfaces between the screw tip A and the check ring seal E and the check ring seal and the screw F should be cylindrical and machined flush to ensure no projections into the flow A II surfaces contacting the flow should have a surface finish better than 16 r m s and all flow channels should be free from sharp turns Although standard unopened containers of C elcon acetal usually do not have to be dried a vented barrel machine with a two stage extraction screw may be used if the level of moisture encountered during molding is high enough to warrant removal W hen improved melting is required to reduce the cycle time a barrier flight may be introduced in the first stage The barrier flight clearance should be 1 02 1 52 mm 0 040 0 060 in 4 1 2 Nozzles Conventional free flow and reverse taper nylon type nozzles fitted with a heater band for temperature Fig 4 2 Recommended Check Valve bic Check Ring Screw Tip D Check Ring Seal 3 2 40 30 30 control of the nozzle are recommended for C elcon acetal copolymer Caution Nozzle designs with positive shut off devices are not recommended for safety reasons although they have been successfully used Formaldehyde gas may be released from Celcon acetal in the molding process particularly if left at elevated temperature in the heated barrel for an extended period This gas must be free to escape through
19. following manuals C elcon acetal copolymer Short Term Properties C E 4 Designing with Plastic The Fundamentals TD M 1 and Designing with C econ acetal copolymer C E 10 They are available by contacting your local Ticona sales representative by calling our Technical Information H otline at 1 800 833 4882 or on our web site www ticona com Comments and suggestions for improving this and other Ticona literature are always welcome and may be sent to us at the above phone number by writing to us at the address shown on the back cover or by e mailing the webmaster on our internet site Celcon acetal copolymer Table of Contents Introduction 1 Overview 11 Chemistry of A cetal Copolymers 1 2 General C haracteristics 1 3 Product Types 1 4 Regulatory C odes and A gency Listings 1 5 Product Support 2 General Guidelines 2 1 Storage and H andling 2 2 Safety and H ealth Information 2 3 Flammability 2 4 Drying 2 5 Processing Start U p 2 6 Changing from A nother R esin 2 7 Changing from a D ifferent G rade of C elcon Acetal C opolymer 2 8 Processing Shutdown 2 9 Use of Regrind 2 10 Secondary O perations 2 10 1 Finishing 2 10 2 Surface Treatment 3 Dimensional Stability 3 1 Shrinkage C aused by Processing Injection M olding 3 2 Part Warpage 3 3 Post M olding Shrinkage 3 4 Annealing 3 5 Tolerances 3 6 M oisture A bsorption 4 Injection Molding 4 1 Equipment 4 1 1 Barrel and Screw
20. make brake cable tubing with O D 5 8mm 0 230 in and wall thickness 0 64mm 0 025 in at a rate of 91 m min 300 ft min amachine must deliver 82 kg hr 180 Ib hr of molten resin Since this is the upper limit for a2 1 2 in or 60mm extruder a 3 1 2 in or 90 mm extruder is necessary Table 6 1 Recommended Metering Screw Dimensions for Extrusion Screw Diameter mm in Channel Depth Metering mm in 1 78 0 070 2 79 0 110 3 18 0 125 3 56 0 140 Zone Length as of screw length Feed mm in Feed Transition Metering 7 11 0 28 35 30 35 11 18 0 44 35 30 35 12 70 0 50 35 30 35 14 72 0 56 35 30 35 38 1 1 5 63 5 2 5 88 9 3 5 114 3 4 5 42 Celcon acetal copolymer Water Bath A three axis positioning water bath is needed to properly align the bath to the die and extrusion path This alignment is critical in high speed extrusion for good surface finish Bath length may vary from 1 8 3 7 meters 6 12 ft with temperature maintained typically between 16 C 60 F and 38 C 100 F 6 2 2 Processing Die and Head Temperature The die and head temperatures should be in the range of 190 230 C 374 446 F for C elcon acetal copolymer A temperature profile with a die temperature of 10 38 C 50 100 F greater than the head temperature gives the best results In practice for every 6 C 10 F increase in melt temperature surface roughness is reduced by
21. of Remolding on the Properties of Fiberglass Reinforced Celcon Acetal Ist 3rd 5th Property Molding Molding Molding Tensile yield strength MPa Value 110 92 5 85 6 percent retention 81 7 75 6 Tensile modulus MPa Value 8 280 7 660 6 970 percent retention 92 6 84 2 Flexural modulus MPa Value 7 250 6 830 6 350 percent retention 94 1 87 6 2 10 Secondary Operations 2 10 1 Finishing Celcon acetal can be readily machined drilled punched buffed sawed sanded and routered by methods commonly used on soft metals such as brass and aluminum It is a good idea to direct a jet of cool compressed air on the machined area to prevent overheating and sticking of the shavings to the molded part H igh tool speed and slow feed is recommended Avoid excessive speeds and pressures Standard metal working tools are satisfactory for machining C elcon acetal 2 10 2 Surface Treatment Celcon parts can be surface treated by laser marking printing labeling and hot stamping Black laser markable grades are available which can also possess good ultraviolet light stability if required such as for many automotive applications These products produce extremely robust white markings for applications such as bar codes graphic or alphanumeric characters and 2 D symbology Ticona Printing by conventional silk screen dry offset direct techniques etc require special inks for satisfactory adhesion C onventional surface
22. runners and tiny gates To prevent deposit build up assure adequate venting of the cavities and runners use mold surface temperatures of 82 C 180 F or higher make sure runners have thicker cross sections than the maximum wall thickness of the molded part and gates larger than 1 mm 0 040 in To remove the deposits we recommend heating in a hot 82 C 180 F detergent solution for several hours followed by rigorous scrubbing An alternate approach for small cavity blocks would be to immerse inverted in a hot 82 C 180 F detergent solution within an ultrasonic bath for several hours A fter deposit removal the cavities should be dried and treated with a rust preventative 4 8 2 Troubleshooting Injection Molding Table 4 8 lists common injection molding problems with possible corrective actions in the preferred order of implementation Table 4 8 Troubleshooting Guide for Injection Molding Celcon Acetal Copolymer SYMPTOMS POSSIBLE SOLUTIONS Brittleness Lower material temperature Check for contamination Decrease regrind level Burn marks Decrease injection speed Decrease injection pressure Improve venting Increase gate size Discoloration Purge machine Lower cylinder temperature Decrease back pressure Lower screw speed Decrease nozzle temperature Reduce injection speed Eliminate contamination from feed Examine nozzle and cylinder for hold up points
23. significant advantages compared to other molding techniques for the following reasons E Low equipment and mold costs M Little or no scrap M Easy adaptability for short production runs E M ultiple products and multiple colors can be molded simultaneously Celcon acetal copolymer has been rotomolded into parts up to 20 feet long and weighing hundreds of pounds For information on specific grades and processing parameters please call Product Information Services at 1 800 833 4882 O ne disadvantage of rotational casting is the potential for weak points in the rotomolded part since no pressure is applied to promote complete melding of all part sections For applications where parts will be exposed to one or more impact rigorous testing is recommended to ensure adequate strength over the total part surface 7 1 Equipment Celcon acetal can be readily rotomolded using any type of commercially available machine including a carousel clamshell The shuttle and rock and roll type machines are used most often to produce longer and heavier parts 7 2 Molds For small and medium sized parts cast aluminum is commonly used because of its good heat transfer characteristics and cost effectiveness O ne major drawback of using cast aluminum is that its surface is easily damaged Sheet metal molds are normally used for prototyping and for extremely large parts Electro or vapor formed nickel molds give e
24. 10 in mandrel o d 10 2mm 5 84mm ratio 1 74 drawdown 0 400 in 0 230 in i d 4 57mm 2 79mm 0 180 in 0 110 in 1 6 blow up ratio The tubing produced in this manner will be less brittle than that extruded without biaxial orientation Celcon acetal copolymer Troubleshooting Table 6 3 lists some typical problems for high speed tubing extrusion and suggested solutions 6 3 Film and Sheet Extrusion 6 3 1 Equipment For highest efficiency along barrel extruder L D 24 1 is recommended This should have a metering screw with at least a five turn metering section to ensure melt homogeneity Standard center feed dies may be used The die manifold can bein a straight line or bent to forma Y but the latter is preferred because it provides a more uniform flow from the die An adjustable choker bar which acts as a valve can be used to regulate the thickness across the sheet The choker bar combines with the die lip to give enough back pressure to force the plastic out to the ends of the manifold Thick or wide sheets require longer lands and more choking to ensure die fill out Die lands should be 38 51 mm 1 5 2 0 long depending on the thickness of the die opening Either a two or three rolled stack system may be used The three roll system is preferred because it provides greater precision and control and provides a glossy finish on both sides of the product Rolls may also be textured to pr
25. 330 210 410 182 360 171 340 204 400 166 330 171 340 204 400 Adapter 188 370 166 330 204 400 Die Zone 1 188 370 Zone 2 Zone 3 166 330 204 400 204 400 166 330 210 410 210 410 191 375 Melt 210 410 196 385 199 390 Screw rpm 30 45 Current Amps 100 95 Back pressure MPa psi 8 3 1 200 11 0 1 600 Blow pressure MPa psi 37 0 83 120 0 83 120 0 45 65 Celcon acetal copolymer 5 3 2 Mold Temperature A mold temperature of as high as 138 C 280 F is often used to achieve the optimum in part quality A mold temperature below 93 C 200 F is not recommended 5 3 3 Blowing Pressure Typical blowing pressure for C elcon acetal is in the range of 0 69 0 90 MPa 100 130 psi Pressures below 0 55 M Pa 80 psi can be used but are not recommended Blow pressures for C elcon acetal are higher than required for polyethylene and consequently require higher clamping forces on the mold Fig 5 3 Effect of Mold Temperature on Shrinkage Melt Temperature 188 C 370 F Cooling Time 20 Seconds Blow Pressure 85 psi Wall Thickness 1 27 mm 0 050 in Shrinkage in in 0 80 120 160 200 240 280 Mold Temperature F Fig 5 5 Effect of Blow Pressure on Shrinkage Mold Temperature 127 C 260 F Melt Temperature 188 C 370 F Cooling Time 20 Seconds Wall Thickness 1 27 mm
26. 4 1 2 Nozzles 4 1 3 Plasticizing Capacity 4 1 4 Clamping Force 4 2 M old Design 4 2 1 General Criteria 4 2 2 Mold Bases 4 2 3 Mold Cavities and Cores 4 2 4 Mold Surface Finish 15 17 17 18 18 Ticona Overview General Guidelines Dimensional Stability Injection Molding Blow Molding Extrusion Rotational Casting Celcon acetal copolymer 4 2 5 4 2 6 4 2 7 4 2 8 4 2 9 4 2 10 4 2 11 4 2 12 4 2 13 4 2 14 4 3 4 3 1 4 3 2 4 4 4 4 1 4 4 2 4 4 3 4 4 4 4 4 5 4 5 4 5 1 4 5 2 4 5 3 4 5 4 4 5 5 4 5 6 4 6 4 6 1 4 6 2 4 7 4 7 1 4 7 2 4 8 4 8 1 4 8 2 5 1 5 1 1 5 1 2 5 2 5 2 1 5 2 2 5 2 3 5 2 4 5 2 5 Sprue Bushings Runners Runnerless M olding M olded In Inserts O utsert M olding Gating Vents Cooling Channels Draft Parting Line Auxiliary Equipment M old Temperature C ontrol U nits Process C ontrol Processing Typical M olding Conditions M elt Temperature M old Surface Temperature Injection Pressure Cavity Pressure M easurement CPM Technology Cushion Injection Speed Solidification Time Decompression Settings Screw Speed CycleTime Process O ptimization C onducting a D esign of Experiments D O E Quality Control of M olded Parts Part Weight Part Dimensions Effect of M olding Conditions on M echanical Properties U nreinforced C elcon A cetal Grades G lass mineral C oupled C elcon Acetal Grades M olding Problems
27. 5 0 165 1 02 0 04 24 Celcon Ticona acetal copolymer Fig 4 5 Some Basic Gate Designs Suitable for Celcon Acetal Side or Edge Secondary Pius Restricted or Pin 3 plate mold Diaphragm Internal Ring External Ring Cut runner as close as possible to part x Parting Line 0 040 0 060 in diameter 1 00 1 50 mm Submarine 25 Celcon acetal copolymer Gate Size Gate size should be selected so that the molten plastic in the gate freezes before the second stage pressure is released thereby preventing backflow of the plastic Recommended gate sizes for rectangular edge gates are given in Table 4 3 for various ranges of thickness The smaller gate dimension should be one half to two thirds of the maximum part wall thickness The minimum diameter recommended for around gate is 1 0 mm 0 040 in preferably greater than 1 5 mm 0 060 in Although parts have been successfully produced with gates as small as 0 5 mm 0 020 in These gate sizes should be restricted to very small parts weighing less than 1 gram wall thicknesses of less than 0 5 mm 0 020 in Gate Location G ating in areas of the molded parts which will be subjected to high stress bending or impact during use should be avoided G ates should generally be located in the thickest cross section of the part and bein a position so that the initial flow of plastic into the mold impinges on a wall This wil
28. 61 Items including plumbing components beverage dispensers etc for contact with potable water Underwriters Laboratories UL Various UL ratings for flammability electrical and thermal service use International Association of Food Industry Suppliers IAFIS Sanitary Standards 3A United States Department of Agriculture USDA Approved for direct contact use with meat and poultry products ASTM D 4181 Supersedes ASTM D2133 Military Specification LP 392 A Mil P 6137A MR General Material Specification SAE J2274 Society of Automotive Engineers Global Specifications ISO 9988 1 2 Designation system processing conditions and testing protocols for comparative properties Celcon Ticona acetal copolymer Power Seat Belt Transmission G ear Set Ceicon M90 Acetal C opolymer Provides L ubricity Low Coefficient of Friction Resistance to Lubricants M echanical Strength F atigue Endurance Surface H ardness Dimensional Stability Ease of Molding Celcon acetal copolymer 2 General Guidelines 2 1 Storage and Handling C elcon acetal copolymer should be stored in its original container on pallets in adry place O pen containers should be carefully resealed before returning to storage In the winter containers of resin should be brought into the warm processing area at least 24 hours prior to use and allowed to come to room temperature before opening If this i
29. DOE Process optimization can be handled in anumber of ways but the technique that is recognized as the preferred one by injection molding experts is called the DOE an abbreviation for Design of Experiments Before the DOE made its entry into the field of process optimization the most widely used technique was to conduct along series of experiments where one process variable was changed at atime e g the temperature or the pressure or the speed and the effect noted So for example mold temperature might be raised from an initial value of 80 F to 200 F in steps while keeping all the other molding parameters fixed Samples would be collected at each step for testing Then after setting mold temperature at the best value the next variable e g injection pressure would then be studied in a similar way Table 4 5 Approximate Cycle Times as a Function of Wall Thickness Unreinforced Grades Wall thickness mm in Approximate total cycle time secs 1 6 1 16 20 3 2 1 8 30 6 4 1 4 50 12 7 1 2 75 Celcon acetal copolymer The steps repeated until all variables were studied Thus the process optimization would be studied one variable at a time O bviously the whole process was a long and involved one and in most cases its use never really led to the best conditions The big shortcoming was that the interactions among injection molding parameters would not be studied directly In co
30. Processing Celcon 1cona Version CE 6 1Page 40 Removed for Presentation Clarity Celcon Ticona acetal copolymer Foreword The Celcon A cetal C opolymer Processing and Troubleshooting guide is written for plastics processors who require specific information on start up processing techniques and troubleshooting using this versatile group of products M aterial handling techniques resin drying conditions and health and safety issues are also included Chapters 1 and 2 cover an introduction to Celcon acetal copolymer grades and physical characteristics regulatory and flammability listings start up and shutdown procedures and the safety and health aspects pertaining to handling C elcon acetal copolymer This information is pertinent to all processing methods These two chapters should be read before attempting to process any grade of C elcon acetal copolymer Chapter 3 is devoted to the important topic of molded part dimensional stability including part shrinkage annealing dimensional tolerances and the effect of moisture absorption on part dimensions Each of the final four chapters of the manual describes a specific processing technique injection molding extrusion blow molding and rotational casting and includes a troubleshooting section Information on machine settings mold design and where appropriate screw design is also included For moreinformation on material characteristics and part and mold design consult the
31. The discontinuous method is suitable only for processing resins which are not heat sensitive and is not recommended for C elcon acetal 5 1 2 Injection Blow Molding Injection blow molding is a two stage process for manufacturing completely finished thermoplastic containers Ticona A dvantages of injection over extrusion blow molding are the ability to mold a finished neck on a container with good dimensional control and better dimensional control of wall thickness This results in better product quality less material usage and a minimum of waste material to be reworked The major advantages and disadvantages of each method are summarized in Table 5 1 5 2 Equipment 5 2 1 Extruder A ny conventional commercially available extruder can be used to plasticize melt C elcon acetal with little difficulty for usein blow molding It is highly recommended that the selected machine have a screw with an L D length to diameter ratio of at least 16 1 and preferably higher 20 1 or 24 1 The use of a machine with a higher L D ratio allows more uniform mixing of the molten resin eliminates resin memory and provides a more uniform melt temperature In selecting an extruder it is recommended that particular attention be given to the quality of the temperature controls on the machine Parison temperature control is critical for trouble free blow molding 5 2 2 Screws Screws for the extrusion of Celcon acetal parisons should be of
32. adherent inks are available in addition to special ones which penetrate the material and offer outstanding abrasion resistance Printing inks usually require a high temperature bake for best adhesion Transfer labels exhibit moderate to good adhesion to C elcon acetal As many as four colors can be applied using this method D ecoration with transfer labels is usually less costly than silk screening for multiple color decoration and when a very large number millions of parts are involved Paper labels the lowest cost decorating method are available in many standard types including some with heat or pressure sensitive adhesives which give unusually good bonding to C elcon acetal surfaces H ot stamping can be used as long as recommended procedures and foil laminates are used to obtain satisfactory adhesion For information on machining consult Chapter 12 Machining and Surface O perations of Designing with C elcon A cetal C opolymer C E 10 or call Product Information Services at 1 800 833 4882 Celcon acetal copolymer 3 Dimensional Stability When manufacturing parts from C elcon acetal it is important to understand the factors which may cause dimensional changes T he dimensional effects of shrinkage both in mold and post molding annealing and moisture absorption are discussed in this chapter 3 1 Shrinkage Caused by Processing Injection Molding M any factors influence mold shrinkage They includ
33. apacity machine relative to the shot size decreasing the overall cycle time or if practical increase the number of cavities in the mold Celcon acetal should never be processed above 238 C 460 F If overheating is observed or suspected lower the cylinder temperature and purge Celcon acetal copolymer Ticona Table 4 4 Typical Start Up Conditions Cylinder temperature C F 182 360 Rear 188 370 Center 193 380 Front 198 390 Nozzle Melt temperature C F 182 199 360 390 Mold surface temperature C F 82 121 180 250 Ist stage injection pressure MPa 10 psi 76 138 11 0 20 0 2nd Hold injection pressure MPa 10 psi 76 138 11 0 15 0 Ist stage injection fill time sec 22 8 Ist stage injection speed moderate Back pressure MPa psi O 0 3 0 50 Use higher end of range if running color concentrates Screw rotational speed rpm 20 40 Overall cycle secs 15 60 Depending on wall thickness Drying the overheated material dropping the purgings into water Stay away from the nozzle and the machine hopper as much as possible to avoid inhaling any fumes Provide local exhaust to remove off gases 4 4 3 Mold Surface Temperature M old surface temperature required for C elcon acetal ranges from 38 127 C 100 260 F with 93 C 200 F being the most common Typically a mold tem
34. at it can be done almost contemporaneous with the part s molding i e the weight can be determined just seconds after the part is ejected from the mold Thus the molder can obtain almost immediate feedback thus allowing very timely corrective actions to be taken The procedure that is normally followed is to determine the average weight of known good parts along with the range of deviations from the average e g 10 6 0 03 grams Then a systematic Ticona weight check would be made on all production parts as they are produced Any part over or under the range 10 57 to 10 63g would be set aside while those within the range would be kept for the usual subsequent QC Causes could be identified and corrective actions taken to improve production efficiencies 4 6 2 Part Dimensions In the same way that part weight was determined directly after molding part dimensions can be used as a QC check just as soon as the part is ejected from the mold While we know that dimensions change with time after molding this could be factored into the analysis What one would do is to measure the particular dimension s as a function of time after molding e g 0 50 1 4 8 12 18 and 24 hours N ext plot these data and extrapolate the plotted line back to zero time This would then give you a dimension for parts right off the mold and indicate what the dimension would be 24 hours later 4 7 Effect of Moldin
35. ave been successfully designed with no draft and have exhibited little problem with part ejection H owever we suggest a minimum draft of 1 2 1 per side for best results 4 2 14 Parting Line Parting lines should be located away from aesthetically important areas but should not complicate mold construction W here appearance is important the parting line should be placed in an area where the line will be concealed such as an inconspicuous edge of the part an area of changing geometry or on a shoulder 26 Celcon acetal copolymer 4 3 Auxiliary Equipment 4 3 1 Mold Temperature Control Units Three types of mold temperature control units are commercially available and suitable for molding parts of C elcon acetal 1 N on pressurized water circulating units in which the reservoirs are open to the atmosphere 2 Pressurized water circulating units 3 Pressurized oil circulating units To maintain a mold surface temperature of 93 C 200 F the mold temperature control unit must usually be operated in the range of 104 110 C 220 230 F to compensate for heat loss in water lines platens etc In anon pressurized unit using water only these temperatures cannot be attained because the water will boil off If the heaters gaskets etc in such a unit are operable at these high temperatures the boiling point of the water may be safely raised by the addition of ethylene glycol A solution of 60 ethylene glycol
36. cally modified to decrease static build up for applications such as conveyer belt links and audio and video cassette hubs and rollers Electrically Conductive These electrically conductive EC grades are used for applications requiring low electrical resistance and or rapid dissipation of static build up Some electrically conductive grades contain carbon fibers and exhibit high strength and stiffness Impact Modified The impact modified TX products are formulated to provide moderate to high levels of improvement in impact strength and greater flexibility compared to the standard product Ticona Laser Markable The laser markable LM grades of C elcon acetal copolymer have been developed with enhanced capability for laser printing T hese black products produce extremely robust white markings for applications such as bar codes graphic or alphanumeric characters and 2 D symbology O ne of the grades has good ultraviolet light stability Both grades have excellent toughness and dimensional stability for many applications including automotive parts Colors M ost grades are available in natural black and custom colors There are standard color concentrates available for use in all natural grades Custom color concentrates can also be made 1 4 Regulatory Codes and Agency Listings M any grades of C elcon acetal are in compliance with or approved under a variety of agency specifications and regulatory standards as shown
37. con acetal copolymer in the cylinder the nozzle must not be blocked This is one of the main reasons a nozzle heater band is recommended See Table 2 1 for the typical start up conditions The procedure for starting a machine with C elcon acetal copolymer already in the cylinder is as follows Table 2 1 Typical Start Up Conditions Cylinder Temperature Rear 360 F 182 C Center 370 F 188 C Front 380 F 193 C Nozzle 390 F 198 C Melt Temperature 360 390 F 182 199 C measured by air shot Mold Temperature 180 250 F 82 121 C 1st Stage Injection Pressure 11 000 20 000 psi 75 138 MPa 2nd Stage Hold Pressure 11 000 20 000 psi 75 138 MPa Ist Stage Injection Fill Time 2 5 seconds Ist Stage Injection Speed moderate Screw Speed 20 40 rpm Back Pressure O 50 psi Higher end if using concentrates Cushion 1 8 1 4 in Drying 180 F 82 C for 3 hrs Usually not necessary Ticona Set the nozzle temperature at 400 420 F 204 216 C and cylinder temperatures to 250 275 F 121 135 C Asthe nozzle and cylinder come up to temperature the nozzle orifice should be watched for signs of drooling When drooling occurs indicating that the material in the nozzle is fully molten cylinder temperatures may be raised to 370 380 F 188 193 C A few purge shots should then be taken at reduced injection pressure and spee
38. crew for Extrusion Metering Transition pn yp nr 41 Celcon Ticona acetal copolymer M Thechannel depth ratio i e the ratio of the channel depth in the feed zone to that in the metering zone h h should be between 3 and 4 5 A channel depth ratio of 4 is recommended for optimum results M The feed section should occupy about 35 of the screw length the transition zone about 30 and the metering zone about 35 Recommended dimensions for extrusion screws are given in Table 6 1 6 1 4 Screen Pack A 20 60 100 60 20 mesh screen pack is generally recommended preceding the breaker plate when extruding unfilled resins This will remove most unmelted contaminants It is especially important to use a screen pack when regrind is used T he screen pack also helps to increase back pressure and minimize surging while improving mixing If the resin contains a filler or reinforcement a screen pack is not used 6 1 5 Head and Die Design Straight through crosshead or offset dies may be used to extrude C elcon acetal copolymer All inside surfaces should be highly polished and streamlined If there are any areas of stagnation or hold up resin in such areas could degrade and result in discolored streaks in the extrudate Low resistance dies may not provide sufficient back pressure In such cases relatively long lands are recommended As arule of thumb land length for circular cross sections should be at l
39. d in this publication Please consult the nearest Ticona Sales O ffice or call the numbers listed above for additional technical information Call C ustomer Services for the appropriate M aterials Safety D ata Sheets M SD S before attempting to process these products Celcon acetal copolymer is not intended for use in medical or dental implants 50 Ticona Products offered by Ticona Celcon and Hostaform acetal copolymer POM GUR ultra high molecular weight polyethylene UHMWPE Celanex thermoplastic polyester Impet thermoplastic polyester Vandar thermoplastic polyester alloy Riteflex thermoplastic polyester elastomer Vectra liquid crystal polymer LCP Celstran long fiber reinforced thermoplastics Fortron polyphenylene sulfide PPS Celanese Nylon 6 6 PA 6 6 Topas cyclic olefin copolymer COC Encore recycled thermoplastic molding resins North America Europe Asia Pacific Ticona Ticona GmbH Polyplastics Co Ltd 90 M orris Avenue Industriepark H ochst K asumigaseki Bldg 6th FI Summit N J 07901 Building C 657 2 5 K asumigaseki 3 chome 65926 Frankfurt Chiyoda ku Tokyo 100 Technical Information Japan 1 800 833 4882 Technical Information 49 0 69 305 4653 Telephone N umber TE 81 3 3593 2411 1 800 526 4960 Customer Service 49 0 69 305 31949 Fax Number Visit our Internet Site at http www ticona com for further information 81 3 3593 2455 9 Celanese
40. d with no booster If there is no blockage cylinder and nozzle temperatures as well as other conditions may be adjusted as desired the heating cylinder closed to the mold and molding started on cycle W hen C elcon acetal copolymer is started in an empty cylinder nozzle temperature should be set at 400 420 F 204 216 C and cylinder temperature at 370 390 F 188 199 C Using low pressure approximately 5 000 psi and slow injector speed pack Celcon resin into the cylinder with several short deliberate strokes of the screw plunger A fter a few minutes commence another series of strokes and repeat this procedure As the material melts successive packing strokes will work the material farther into the cylinder until it is full A few air shots should then be taken to clear the cylinder of any air bubbles which may have been entrapped during the packing process Cylinder and nozzle temperatures cycle time and other conditions may then be adjusted as required and molding may proceed In all cases once C elcon acetal copolymer is introduced into the cylinder it should be kept moving to prevent overheating If adelay of more than 15 minutes is anticipated the cylinder should be backed away from the mold and the machine purged on cycle or manually every few minutes If a longer delay is expected it is recommended that the machine be shut down entirely following the procedure outlined under Shutting Down aM achine w
41. date is the best way to reduce brittleness H eavy walled tubing can be biaxially 43 Ticona Table 6 2 Typical Conditions for Tubing Extrusion Machine 3 1 2 in L D Ratio 24 1 75 Hp 56 kW Amp Rating 192 Size of Motor Screw type Metering Compression ratio 3 1 Metering zone depth 3 6 mm 0 14 in Screw speed 33 RPM Screen pack 20 40 80 40 20 mesh Sizing Four plates 6 4 mm 2 1 2 in apart 1 7 1 mm 0 281 in diameter 2 6 2 mm 0 242 in diameter 3 5 6 mm 0 220 in diameter 4 5 2 mm 0 204 in diameter Die dimensions Mandrel Bushing Die temperature 4 3 mm 0 170 in diameter 7 2 mm 0 285 in diameter 230 C 450 F Barrel temperature C F Zone 1 Zone 2 Zone 3 Zone 4 Zone 5 180 360 190 370 195 380 200 390 205 400 Line Conditions Motor Line speed Air pressure 24 Amps 128 m min 420 ft min 0 1 MPa 1 1 4 psig Head pressure 210 MPa 3 000 psig The above settings produce tubing with an i d of 4 1 4 2 mm 0 161 0 165 in an o d of 5 1 5 2 mm 0 200 0 205 in and a wall thickness of 0 46 0 56 mm 0 018 0 022 in oriented by drawing down the outside surface by a factor of 1 7 1 and blowing up the inside surface by a factor of approximately 1 6 1 For example to make brake cable with an o d of 5 84 mm 0 230 in and wall of 0 64 mm 0 025 in use a die with an opening of 10 2 mm 0 400 in and a 2 8 mm 0 1
42. dditional deformation 3 4 Annealing Annealing of Celcon acetal copolymer is sometimes useful to relax molded in stresses and stabilize dimensions The amount of shrinkage that may occur primarily depends on cooling rate that is affected by part thickness and mold temperature Table 3 2 shows examples of the shrinkage resulting from annealing two different thicknesses of an 17 Fig 3 2 Shrinkage Due to Heat Aging for 9 0 Standard Melt Flow Grade of Celcon Acetal Shrinkage Due to Heat Aging 3 2 mm Thick Specimen Flow Direction a e N fo N a Shrinkage N N ient T iS Time Months unfilled C elcon acetal test specimen A nnealing molded parts will lead to dimensional changes so that allowances must be made for any additional shrinkage The decision on whether to anneal parts of Celcon acetal copolymer in a post molding operation should be made while the part and mold design are in the initial planning stage and certainly prior to machining the mold cavities and cores to size As parts of Celcon acetal copolymer will shrink in annealing allowance must be made for the additional shrinkage in determining the mold shrinkage which should be used for the mold cavities and cores Asa guide the typical values of mold shrinkage and total shrinkage after annealing obtained on 1 8 inch 3 12 mm and 1 2 inch 12 7 mm thick test bars 1 2 inch 12 7 mm wide by 5 i
43. des fast plastication and a homogeneous melt which will permit molding parts at reduced melt temperatures and pressures as well as decreased cycle time A single stage reciprocating screw injection molding machine is most commonly used with C elcon acetal 4 1 1 Barrel and Screw W hile the standard metering screw available in commercial reciprocating screw injection molding machines can be used it is not totally satisfactory Problems such as excessive oxidative deterioration poor thermal homogeneity unmelted resin pellets and or lower productivity rates can sometimes occur A screw such as the one shown in Figure 4 1 having the following characteristics is recommended for optimum results M TheL D length to diameter ratio should preferably be no less than 16 1 and no greater than 24 1 The flight clearance should be approximately 0 13 mm 0 005 in M The flight width w should be approximately 10 of the screw diameter For unfilled C elcon resins the screw should be hard faced or coated with a corrosion resistant material such as chrome or Stellite 6 H For filled reinforced Celcon resins the screw and barrel should be hard faced or coated with a corrosion and abrasion resistant material such as tungsten carbide CPM 9V or Colmonoy 56 for screws CPM 10V Bimex or X aloy 101 or 306 for barrels Thescrew should be fitted with a non return valve to prevent back flow of resin in the screw channel as the
44. e thermal properties of the resin filler type and level part design especially wall thickness gate size and resin flow direction M olding conditions including melt and mold temperature injection speed and pressure are particularly important Variations in mold surface temperature and mold injection pressure for example can cause shrinkage in test bars made from one specific grade C elcon M 90 ranging from 1 2 to 3 7 A s a result it is difficult to predict the exact mold shrinkage of a specific part Typical effects of processing conditions on part shrinkage are summarized in Table 3 1 Shrinkage of standard unfilled C elcon acetal products measured on laboratory test specimens cover a range from 1 2 2 4 Mold shrinkage for an actual part has been observed as high as 3 7 Consult the C econ Short Term Properties Brochure CE 4 for typical values of specific C elcon grades This information should be used only as a guide in estimating shrinkage for tool construction Additional guidance is provided in Table 3 1 Effect of Processing Conditions on Part Shrinkage Molding Effect on Part Shrinkage Wall thickness increases Increases Gate size increases Decreases Pressure increases Decreases Mold Temperature Increases increases Decreases for parts up to 3 2 mm No effect for parts 3 2 9 5 mm thick Resin melt viscosity Increases with increasing increases viscosity when molded under similar processin
45. e and numerous other variables It is advisable to begin processing using the general start up conditions outlined earlier in this chapter and ensure that the selected C elcon resin will provide adequate melt strength which is critical for this technique Ticona Some complex contours can be more easily made by extruding a simple shape tube or sleeve and continuously post forming after the die with specially designed sizing plates sleeves or simple brass fingers Examples of post forming parts are shown in Figure 6 2 This procedure though versatile and using less expensive dies requires more care and design to achieve dimensional control and may eventually be more costly Fig 6 2 Miscellaneous Post Formed Profiles shapes such as these can be made by extruding a simpler shape and forming it into a more complex contour while the extrudate is still pliable ORIGINAL INTERMEDIATE FINAL PROFILE POSTFORM POSTFORM O EP SEAL Table 6 3 High Speed Tubing Extrusion Troubleshooting Guide SYMPTOMS POSSIBLE SOLUTIONS Brittleness Increase land length Decrease head temperature Decrease screw speed Biaxially orient tubing Concentricity poor Increase take off speed Increase die temperature Decrease cooling water temperature Surface roughness inside Increase land length Increase take off speed Decrease screw speed Dry resin Heat mandrel Surface roughne
46. east equal to the diameter of the die or the lands should be 10 to 20 times the thickness of the extruded section An approach angle of 20 30 to the die lands is recommended for most types of dies Accurate heat control at all points on the die and die head as well as the ability to determine the stock temperature in the head via a melt thermocouple are essential A pressure gauge should be mounted on the die head to help establish and maintain proper operating conditions The gauge is also a safety feature to alert the operator if excessive pressure build up should develop 6 1 6 Hopper The hopper on the extruder should be large enough to hold pellets for about a half hour s production If the hopper is manually loaded it should be equipped with a hinged or tight fitting lid to avoid resin contamination While C elcon acetal grades normally can be used directly from their original shipping containers without drying a hopper drier with a three hour capacity can be beneficial where pick up of excess moisture has inadvertently occurred The use of a magnetic screen or metal detector is advised to minimize the risk of contamination or equipment damage due to foreign material 6 2 High Speed Tubing Extrusion 6 2 1 Equipment Extruder Size There is no strict requirement for size but the extruder must be able to deliver the required resin output at a constant temperature properly plasticated and without surging For example to
47. er to allow fill of the mold cavities while preventing freeze off Avoid the use of sharp corners turns should be curved to promote streamline flow and minimize stagnant areas Full round half round and trapezoidal cross section runners are all acceptable but full round runners are preferred Suggested dimensions for full round runners are shown in Table 4 2 page 4 5 Runners should be made thicker than the maximum wall thickness of the molded part When a multi cavity mold is used the runner system should be balanced i e the flow paths from the sprue to the far end of each cavity should be equivalent 22 Celcon acetal copolymer Ticona Table 4 2 Runner Size Recommendations for Celcon Acetal Copolymer Part thickness diameter mm in Runner length mm in Minimum runner diameter mm in Less than 0 51 0 020 Up to 50 8 2 3 18 0 125 0 51 1 52 0 020 0 060 Greater than 50 8 2 4 78 0 188 1 52 3 81 0 060 0 150 Up to 101 6 4 4 78 0 188 1 52 3 81 0 060 0 150 Greater than 101 6 4 6 35 0 250 3 81 6 35 0 150 0 250 Up to 101 6 Up to 4 6 35 0 250 3 81 6 35 0 150 0 250 4 2 7 Runnerless Molding In comparison with cold runner molding runnerless molding can reduce the amount of resin per molding cycle shorten production cycle time enhance productivity and improve part quality It is estimated that approximate
48. erature time as needed Rough inner surface Surface pitting Increase temperature and or time for adequate melting of powder Use less or no mold release Clean mold surfaces 49 Uneven wall thickness Warpage Remove excess metal which may be acting as heat sinks from flanges and arms Balance speed of each axis for uniform polymer flow Improve air flow path around mold Ensure continuous mold rotation from heating through cooling cycle Clear vents to prevent vacuum formation Use slower rate of cooling Celcon acetal copolymer Ticona NOTICE TO USERS To the best of our knowledge the information contained in this publication is accurate however we do not assume any liability whatsoever for the accuracy and completeness of such information Any values shown are based on testing of laboratory test specimens and represent data that fall within the standard range of properties for natural material Colorants or other additives may cause significant variations in data values Any determination of the suitability of this material for any use contemplated by the users and the manner of such use is the sole responsibility of the users who must assure themselves that the material as subsequently processed meets the needs of their particular product or use It is the sole responsibility of the users to investigate whether any existing patents are infringed by the use of the materials mentione
49. es sizing sleeves etc 6 1 2 Extruder Barrel A barrel with an L D length to diameter ratio of 16 1 and preferably up to 24 1 is recommended to allow sufficient residence time for proper melting In conjunction with a properly designed screw longer barrels tend to improve melt homogeneity and reduce melt temperature and pressure fluctuations Ticona If a shorter barrel L D less than 16 1 must be used the use of a high resistance die with either long lands or asmall die opening is recommended to generate the necessary back pressure to work the material With low resistance dies the use of a valve in the extruder head is suggested to increase the back pressure level in the barrel 6 1 3 Screw Design A metering screw as shown in Figure 6 1 is recommended for extruding C elcon acetal Characteristics of this screw are AnL D ratio of 16 1 minimum up to 24 1 preferred M The flight clearance should be approximately 0 13mm 0 005 in The flight width w should be approximately 10 of the screw diameter E For unfilled Celcon acetal the screw should be hard faced or coated with a corrosion resistant material such as chrome or Stellite 6 M For filled Celcon acetal the screw and barrel should be hard faced or coated with a corrosion and abrasion resistant material such as tungsten carbide CPM 9V or Colmonoy 56 for screws and CPM 10V Bimex or X aloy for barrels Fig 6 1 Recommended Metering S
50. ess and in thick parts with increasing gate size The precise shrinkage for a given part may be obtained by initially designing the mold cavities with oversized cores and undersized cavities Following this parts should be molded at equilibrium molding conditions which provide the best overall results for mold cycle time and part quality for production Parts should then be conditioned at room temperature for at least 24 hours preferably 48 hours Dimensions of critical areas can then be measured and the cavity and core machined if necessary to bring the molded parts within dimensional tolerances Celcon Ticona acetal copolymer Fig 3 1 The Effect of Molding Conditions and Wall Thickness on Mold Shrinkage for Celcon M90 Acetal Part Wall Thickness 4 1 mm 5 1 mm 3 8 3 8 3 6 3 6 Mold Surface Temp 29 C 3 2 3 2 2 8 2 8 2 4 2 4 Shrinkage CB HO 2 0 1 6 1 6 1 2 f 1 2 8 12 16 20 8 12 16 20 8 12 16 20 oe D e as wn 8 12 16 20 8 12 16 20 8 12 16 20 8 12 16 20 Mold Surface Temp 125 C Shrinkage 8 12 16 20 16 20 8 12 16 20 8 12 16 20 Injection Pressure psi x 103 NOTE Melt temperature 190 204 C Shrinkage measured in the direction of material flow 16 Celcon Ticona acetal copolymer 3 2 Part Warpage Wall thickness should be as uniform as possible Differences in cooling rates of thick and thin sections isake
51. ethylene or acrylic must first be used to thoroughly clean the machine The new material should be introduced to the machine only after proper cleaning and adjustment to the appropriate processing conditions 2 8 Processing Shutdown To shut down a machine with C elcon acetal the same precautions must be taken against blockage of the nozzle as when starting up the machine The nozzle should be the last part of the heating cylinder assembly to cool L eave the screw in the forward position M Set the nozzle temperature at 204 221 C 400 420 F Turn off the cylinder heaters Shut off the feed to the cylinder Purge and run the barrel dry L eave the screw in the forward position Shut off the power to the machine 2 9 Use of Regrind C elcon acetal can be reprocessed a number of times without significant change in physical properties or processing characteristics Tables 2 1 and 2 2 show the effect of remolding unreinforced and fiberglass reinforced C elcon acetal copolymer W hen remolding 13 Ticona glass fiber reinforced C elcon acetal some loss in mechanical properties may be seen due to breakage of the glass fiber reinforcement Knife type grinders with a 5 16 in 8mm screen are recommended for grinding resin sprues runners and off test pieces As with other thermoplastics regrinding may produce enough dust to cause discomfort to the operator In addition acetal copolymer dust can present an explosi
52. g conditions i e Celcon M270 has lower shrinkage than Celcon M25 Melt Temperature Ticona Figure 3 1 which shows the effects of molding conditions and wall thickness on mold shrinkage and Chapter 4 of this manual which details mold design for injection molding O f the process variables injection hold or packing pressure and time injection speed and mold temperature are the most significant and about equal in importance in controlling mold shrinkage M aterial temperature is also significant but to a lesser degree O f the part and mold variables wall thickness has the most significant effect on part shrinkage followed by gate size G ate location is of lesser importance but still significant and is highly dependent on part geometry Parts which are relatively long and narrow and gated at the narrow end will have material flow predominantly in one direction This will result in anisotropic shrinkage For unreinforced C elcon acetal there will be less shrinkage in the width transverse than in the length flow direction For reinforced C elcon acetal the opposite will occur Shrinkage will be less in the length direction than in the width direction The reason for this is that reinforcing glass fibers align themselves in the direction of the material flow and when the part cools and the material solidifies the fibers inhibit shrinkage in this direction Anisotropic shrinkage increases with increasing wall thickn
53. g Conditions on Mechanical Properties C elcon acetal can be satisfactorily molded over a broad range of conditions H owever since molding conditions influence aesthetics structural integrity mold shrinkage and physical properties of the molded part it is important to identify the major requirements for a given application to select the appropriate molding conditions Among the important molding conditions that can influence physical properties are melt and mold temperature injection pressure hold time injection speed and screw rotational speed Slightly different molding conditions are required to optimize individual key properties Because every application is unique and new in some way the molding conditions discussed below should be used as a guide only Some adjustments will generally be necessary to ensure that the optimum cycle time and part performance are obtained The data shown in Table 4 6 were measured in a carefully controlled DOE using standard ASTM test specimens 3 2 mm thick M olds had generous gates runners and vents Restrictive gates runners of varying thickness parts with varying wall thicknesses and weld lines in parts may significantly affect these results 30 Celcon Ticona acetal copolymer Table 4 6 Effects of Molding Conditions on Mechanical Properties Unreinforced Grades Injection Melt Mold Pressure Temperature Injection Temperature Speed 77 104 C 170 200 F
54. gt 104 C gt 200 F To optimize tensile strength 188 199 C 370 390 F To optimize modulus 188 199 C 370 390 F To optimize notched bar impact To decrease mold shrinkage To increase mold shrinkage T Increase Decrease No significant effect Occasionally part design criteria or processing equipment parameters may require deviation from the guidelines M oreover actual parts are usually more complex in shape than laboratory test specimens To maximize engineering performance the processor should work closely with the part designer to specify molding parameters based on actual part performance 4 7 1 Unreinforced Celcon Acetal Grades Tensile strength increases with increasing injection pressure and is optimum with a melt temperature in the range of 188 199 C 370 390 F and mold temperature of 77 104 C 170 220 F Screw rotational speed and injection speed have little effect on tensile strength N otched bar impact strength is highest with low injection pressure and medium injection speed It increases with decreasing mold temperature melt temperature and screw rotational speed M odulus is most significantly affected by both mold temperature and melt temperature and increases with an increase in both temperatures M aximum modulus is obtained with a melt temperature of 188 199 C 370 390 F and mold temperature of 104 C 220 F D ecreasing injection
55. he molded parts on ejection 4 5 3 Decompression Settings Most commercially available injection molding machines are equipped with a decompression suck back feature This is used to relieve pressure on the resin after plastication in the heating cylinder and prevent nozzle drool during the mold open time prior to beginning the next cycle Usually about 0 2 0 6 seconds decompression time is satisfactory for C elcon acetal copolymer 29 Ticona 4 5 4 Screw Speed Screw rotational speed and back pressure should be kept to a minimum preferably 20 40 rpm and zero back pressure respectively The screw speed should be set such that the screw fully retracts just prior to the expiration of the mold closetime Excessive screw speed and back pressure can cause severe overheating of the resin and in the case of fiberglass reinforced products increase glass fiber breakage leading to a significant reduction in mechanical properties 4 5 5 Cycle Time Cycle time depends primarily on wall thickness which governs the rate of cooling and to some extent on part design dimensional tolerance molding equipment mold design etc Some approximate cycle times versus wall thickness are shown in Table 4 5 for unfilled C elcon acetal copolymer F aster cycles than indicated may be obtained with grades of resins that are reinforced filled and those with higher melt flow rates 4 5 6 Process Optimization Conducting a Design of Experiments
56. hrinks lead ing to ashiny areain an otherwise matte surface M The gates should be large enough that the cavity pressure is adequate to completely fill the part If necessary increase the gate size relocate the gate or add additional gates M Ensure that the injection hold time is adequate to prevent loss of cavity pressure before resin freeze off in the gate Pit marks in the surface are a clear indication of low cavity pressure For part configuration Ensure that the resin melt flow path is not too long or too complex M Check the fill rate to ensure adequate cavity pressure For wall thickness Injection fill pressure should be adequate especially where a part has a thick wall thin wall configuration O therwise a too low cavity pres sure may result M Wall thickness should not be too thick in relation to gate size otherwise jetting or tumbling of the melt may occur creating fold over lines and inadequate surface definition M Gate size should not be too small for the wall thickness otherwise sink marks may occur U se a relatively coarse grain on the mold surface and a rib thickness 50 of the adjoining wall surface in high shrink resins to assure sink free parts For resin melt viscosity M Melt viscosity may in some cases be too high to allow adequate packing of the cavity runners and gates may have to be enlarged to assure ade Ticona quate fill Increasing the melt temperature and
57. ic additives to prevent irreversible thermo oxidative depolymerization of the polymer backbone during processing 1 2 General Characteristics Celcon acetal copolymer is a high strength crystalline engineering thermoplastic material having an unusual and desirable balance of properties It is an ideal candidate to replace metals and thermosets because of its predictable long term performance over a wide range of in service temperatures and harsh environments C elcon acetal retains properties such as high strength creep resistance fatigue endurance wear resistance and solvent resistance under very demanding service conditions Celcon acetal can be easily converted from pellet form into parts of different shapes using a variety of processes such as injection molding blow molding extrusion rotational casting and compression molding Rod slab and sheet stock which can be readily machined into desired shapes are also available 7 Ticona 1 3 Product Types Both standard and special grades of C elcon acetal copolymer are designed to provide a wide range of properties to meet specific applications Standard and custom grades of C elcon acetal copolymer can be obtained in pre compounded color form or color concentrates which may be blended with other grades All colorants used in C elcon resins are lead mercury and cadmium free and all C elcon acetal products conform to current environmental O SH A regulations for these meta
58. ich allows control of the thickness of the parison 5 2 7 Hopper The hopper on the extruder should be large enough to hold Celcon pellets for about a half hour s production If the hopper is manually loaded it should be equipped with a hinged or tightly fitted lid to avoid contamination While C elcon resins normally can be used directly from their original shipping container without drying a hopper drier with a three hour capacity can be beneficial where pick up of excess moisture has inadvertently occurred The use of a magnetic screen or metal detector placed in the hopper is advised to minimize the risk of contamination or equipment damage due to foreign metal 5 2 8 Molds A choice of materials for molds depends on the anticipated volume of production complexity of the mold piece technique used and number of molds required Aluminum and zinc are commonly used to make the low cost molds for short runs Beryllium copper a harder material is sometimes used despite its higher cost because it has excellent heat transfer characteristics Steel dies are relatively expensive and are mostly used for long production runs Fig 5 2 Parison Die with Adjustable Core Pin for Control of Parison Thickness lt Push Rod Manual Alignment Die Heater Zone 1 Flow Divider Die Heater J Zone 2 Pin Adapter Die Heater x Zone 3 Tapered Die Pin Die Bushin Replaceable iReelaceakslel
59. in Table 1 1 N ot all grades are covered by all regulatory listings C all Product Information Services at 1 800 833 4882 for further information on which grades are approved under the various regulations 1 5 Product Support In addition to our technical publications experienced design and application development engineers are available for assistance with part design moldflow characterization materials selection specifications and molding trials Call your local Ticona Polymers sales representative or Product Information Services at 1 800 833 4882 for further help Celcon acetal copolymer Ticona Table 1 1 Regulatory Listings Plumbing Code Bodies International Association of Plumbing Mechanical Officials IAPMO Building Officials Conference of America BOCA Southern Standard Building Code Plumbing fixtures and specific plumbing and mechanical applications covered in the various codes Canada Standards Association CSA Plumbing fixtures fittings and potable water contact items UL ratings in Canada Plastic Pipe Institute PPI Recommended Hydrostatic Design Stress RHDS rating of 1 000 psi at 23 C 73 F as an injection molded plumbing fitting Food and Drug Administration FDA Repeated use food contact applications including food machinery components conforming to 21 CFR 177 2470 United States Pharmacopoeia USP Class VI Compliant NSF International Standards 14 51
60. ith C elcon 2 6 Changing from Another Resin to Celcon Acetal If the other resin in the processing equipment requires a higher melt temperature than C elcon acetal copolymer e g nylon polycarbonate etc or isa resin such as PVC see note p 2 1 which can chemically react with C elcon and cause degradation the cylinder must first be thoroughly purged clean of these resins Celcon acetal copolymer H igh density polyethylene or acrylic is suitable for purging and should be put in the machine directly behind the resin already in the cylinder and kept at the same temperature settings A fter all traces of the other resin are removed the temperature should be set at 188 199 C 370 390 F After the temperature has stabilized C elcon acetal copolymer can then be placed in the machine to remove the purge compound The machine settings can be adjusted to the desired production conditions 2 7 Changing from Celcon Acetal Copolymer to Another Resin In changing from Celcon acetal copolymer to another resin similar considerations as described earlier will apply When the machine is started up with C elcon acetal copolymer in the cylinder the proper procedure outlined in Start U p must be followed before changing over to another resin If the new resin requires a higher or lower temperature or is one that can chemically react with C elcon acetal copolymer such as PVC an intermediate purging compound such as poly
61. l prevent jetting and blush marks For round or cylindrical parts which must be concentric a center sprue gate a diaphragm gate disk gate or a set of three gates spaced at 120 intervals around the part is recommended 4 2 11 Vents Vents With all plastics cavities should be well vented to allow the escape of trapped gases and air Inadequate venting can cause burn marks short shots dimensional problems surface defects and blushing Proper venting on the other hand will help to lower injection and clamp pressures reduce cycle times eliminate or reduce molded in stress and minimize shrinkage and warpage It is advisable to have as much venting as possible without allowing the resin to flow out of the mold Size Vents should be 0 0254 mm 0 001 in maximum deep by 3 175 6 35 mm 0 125 0 250 in wide To prevent blockage of the vents they should be deepened to 1 59 mm 1 16 in at a distance of 3 175 4 76 mm 1 8 3 16 in from the cavity to the outside Peripheral venting is preferred whenever possible Location Vents should preferably be located at the last point to fill Vents should be placed in other locations as well including the runner system weld line regions and other areas of possible gas entrapment Ticona N atural vents can be built into the parting line of the tool and at the interface of the pieces of metal used to build up the cavities Ejector pins can also provide some venting but should not be
62. lish a descending profile The highest temperature should be at the feed zone If extrusion rates are relatively low and there is danger of bridging in the feed zone the temperature in the feed zone should be kept low and the second zone made the high point for the decreasing temperature profile Barrel temperatures can range from 165 C 329 F to as high as 216 C 420 F depending on the blown part geometry dimensions blowing cycle and many other factors Typical operating conditions used in preparing various blown parts are shown in Table 5 3 Table 5 3 Typical Blow Molding Conditions Aerosol Container 3 5 oz Bullet shape Aerosol Container Float 5 oz Barrel shape Approx size diam x ht mm x mm in x in 40 6 x 132 1 1 6 x 5 2 55 9 x 83 8 2 2 x 3 3 63 5 x 25 4 2 5 x 1 Weight grams 29 44 27 Blow molding method Extrusion Multi station rotary Extrusion Fixed Mold accumulator Extrusion Multi station rotary Extruder size mm in 64 2 5 64 2 5 64 2 5 L D 24 1 24 1 20 1 Compression ratio B25 3 5 1 2 5 1 Die busing ID mm in 14 9 0 588 17 8 0 700 Die mandrel OD mm in 8 0 0 313 5 6 0 220 Land length mm in 12 7 0 500 25 4 1 00 Temperature C F Mold 93 200 93 200 88 190 Barrel Zone 1 149 300 Zone 2 Zone 3 Zone 4 171 340 216 420 171 340 166
63. lity 4 5 Cushion A 3 2 6 4mm 1 8 1 4 in cushion is recommended The check ring non return valve must function properly and ensure that the recommended cushion is held constantly throughout the molding cycle M alfunctioning check rings may not seat tightly and will usually result in inconsistent parts short shots poor control of dimensional tolerances and weak weld lines 4 5 1 Injection Speed Rapid fill of the mold cavities is preferred for most moldings and can be accomplished by opening the flow control valve to the maximum and adjusting the first stage so that it stays on for the full injection stroke If flash occurs the injection speed should be reduced preferably by reducing the injection velocity set point as little as is necessary to eliminate flash The injection hold time should be adequate to completely fill the mold cavities with the screw coming to a complete stop in the fully forward position and then allow the gate to seal under pressure before the pressure is released This is particularly important to prevent suck back of resin through the gate when the screw is withdrawn If surface imperfections such as splay or flow marks are encountered reduce the injection speed in small increments 4 5 2 Solidification Time Solidification time should be adequate for the resin to properly set up in the mold and maintain dimensional tolerance and geometry without distortion warpage or ejector pin penetration of t
64. ll as molding by using control feedback loops on molding equipment and using a minimum number of tooling cavities will help to achieve this objective Careful consideration should be given to the need for very tight tolerances to avoid excessive mold and processing costs Also it may be unreasonable to specify extremely close tolerances on a part which will be exposed to a wide temperature range 3 6 Moisture Absorption Some dimensional change is seen when C elcon acetal copolymer is exposed to moist environments The changes are usually lower than those observed for other engineering thermoplastics Figures 3 3 and 3 4 show that after one year of continuous exposure to high humidity or immersion at various water temperatures dimensional changes are minimal See page 5 5 of the Celcon Design M anual for additional data on the effects of water on material properties Fig 3 4 Change in Linear Dimensions at 23 C 73 F and 50 Relative Humidity Contraction Extension Time Seconds 18 Celcon Ticona acetal copolymer 4 Injection Molding 4 1 Equipment Injection molding is the most widely used method for processing acetals C elcon acetal can be successfully processed in all types of commercially available injection molding machines designed for thermoplastics These may be single or two stage reciprocating and stationary screw injection types Screw injection provi
65. ls C onsult our brochure C elcon A cetal Copolymer Short Term Properties C E 4 for information on specific grades The most common categories of C elcon resins are described below General Purpose G eneral purpose M series products are identified by melt flow rate Divide the grade number by 10 to obtain the melt flow rate For example C elcon M 90 has a melt flow rate of 9 0 grams per 10 minutes per ASTM D 1238 or ISO 1133 190 C and 2 16 Kg load In comparison to M 90 products designated by a higher melt flow rate i e M 270 fill thinner walls and complex shapes more readily maintain the same strength and stiffness but exhibit a slight decrease in toughness Products with lower melt flow rates i e Celcon M 25 exhibit increased toughness compared to C elcon M 90 but may be more difficult to mold into parts with thin walls or long flow paths Glass Fiber Coupled G lass fiber coupled GC products provide higher strength stiffness and creep resistance than the unfilled grades T hese products are identified with a number indicating the percentage of short glass fiber in the product and are based on general purpose C elcon polymers The glass fibers are chemically coupled to the polymer matrix Glass Bead Filled These glass bead G B filled grades contain glass beads for lower shrinkage better dimensional tolerances and warp resistance and are especially helpful when molding large flat and thin walled par
66. ly 25 of all Celcon acetal molding jobs are currently being performed using runnerless molds Celcon acetal copolymer is well suited to the demands of hot runner molding C elcon acetal copolymer has better thermal stability important because of the longer heat history during runnerless molding C elcon acetal copolymer processes at least 10 C 18 F lower than some other acetals reducing heating requirements and producing faster molding cycles Some applications are natural fits with runnerless tooling i e applications such as medical parts where regrind cannot be used H ot runners can also be justified because they eliminate scrap and the need for auxiliary equipment such as sprue pickers and granulators A nother suitable application is in high volume jobs where the same material is run for a long time without switching grades or colors Finally where parts with very precise surface appearance are required zero vestige gates can be used to virtually eliminate gate marks Practically all commercial hot runner systems work well with C elcon acetal copolymer with the possible exception of insulated runner systems In general melt flow channels should be large and streamlined with generous radii and no sharp corners This will prevent resin hangup facilitate resin melt flow and reduce pressure loss A full range of drops are available for runnerless molding Either bushings or hot runner nozzles can be used successfully
67. mperatures 360 390 F 182 199 C Avoid strong acids and oxidizing agents Do not allow the end use application to come in contact with acidic solutions of pH 4 or less especially mineral acid solutions like hydrochloric sulfuric hydrofluoric perchloric nitric or phosphoric Do not use with strongly acid salts such as zinc chloride or other Lewis acids Do not use with chlorinated water solutions which are not typical of domestic potable water Celcon acetal copolymer 2 4 Drying Celcon acetal copolymer does not readily absorb moisture and can normally be fed to the extruder or molding machine without drying H owever if the material has adsorbed moisture due to improper handling or storage drying may be necessary to prevent splay and odor problems during processing It is good practice and preferable for processing consistency to dry the resin before processing to avoid potential production problems due to moisture Celcon acetal copolymer should be dried ina dehumidifying oven or ahopper dryer For oven drying the C elcon pellets should be spread evenly in less than one inch deep layers on trays and placed in the oven for three to four hours at 82 C 180 F For a hopper dryer a three hour residence time at 82 C 180 F is sufficient Caution Formaldehyde fumes may be released good ventilation is required in the area 2 5 Processing Start Up To start up a machine which was shut down with C el
68. nches 127 mm long are shown in Table 3 2 The specimens were end gated molded using a mold temperature of 200 F 93 C and annealed in oil at 305 F 152 C In many cases properly molded parts will exhibit satisfactory dimensional stability especially at continuous service temperatures of 82 C 180 F or below A high 82 120 C 180 250 F mold temperature will optimize the dimensional stability of an as molded part for service temperatures up to 82 C 180 F Table 3 2 Shrinkage Before and After Annealing Part Thickness Annealed Flow Transverse mm in 152 C 305 F Direction Direction 3 18 0 125 No 2 2 1 8 3 18 0 125 Yes 2 7 2 0 12 7 0 500 No 2 6 2 0 12 7 0 500 Yes 3 0 2 0 Celcon Ticona acetal copolymer Table 3 3 Recommended Annealing Procedure Recommendation Required Service Temperature Generally properly molded parts will not require annealing In service temperature of 82 C 180 F or below In service temperature higher than 82 C 180 F Annealing may be necessary to improve the dimensional stability of the molded part Annealing Parameters Recommendation Time As a general rule anneal for 15 minutes for each 3 2 mm 1 8 in of wall thickness if using an annealing liquid longer if annealing in an air oven Temperature 152 C 2 C 305 F 5 F Medium Any refined or silicone oil which is not acidic Oil is preferred over air because it is a better c
69. nd Sheet Troubleshooting Guide Table 6 6 Profile Extrusion Troubleshooting G uide Table 7 1 Rotational C asting Troubleshooting Guide 12 13 14 15 17 18 20 23 24 28 29 31 32 32 35 36 39 42 43 45 46 46 47 49 Ticona Celcon acetal copolymer List of Figures Fig Fig Fig Fig Fig Fig Fig Fig Fig Fig Fig Fig Fig Fig Fig Fig Fig Fig Fig Fig 3 1 3 2 3 3 3 4 4 1 4 2 4 3 4 4 4 5 5 1 Effect of M olding C onditions and Wall Thickness on M old Shrinkage Shrinkage due to H eat A ging for 9 0 Standard M elt Flow Grade of C elcon A cetal Water A bsorption by U nfilled C elcon A cetal U nder Various C onditions Dimensional C hanges due to Water Absorption by Unfilled Celcon A cetal Typical Screw Profile for Injection M olding C elcon A cetal Recommended C heck Valve D esign Recommended M olded in Insert D esigns O utsert M olded M oving Parts G ear Cam and Spring Some Basic Gate D esigns Suitable for Celcon A cetal Flow Pin Divider to Promote Smooth Flow and Avoid Weld Lines Parison Die with A djustable C ore Pin for Control of Parison Thickness Effect of M old Temperature on Shrinkage Effect of Cooling Time on Shrinkage Effect of Blow Pressure on Shrinkage Effect of Wall Thickness on Shrinkage Landed Pinch O ff for Improved Impact Strength Blow Molded Fluidics C ontainer Recommended M etering Screw for Extrusion Miscellaneous Post F
70. nducting aDOE you reduce the number of experiments substantially while still being able to measure the effects that each variable has on the process and parts Additionally you are able to see clearly the influence of interactions among variables For example what effects do the interaction of injection pressure and melt temperature a two factor interaction have on the part properties Similarly what effects does the interaction of injection pressure melt temperature and mold temperature a three factor interaction have on part properties Thus the information that you obtain from the DOE provides avery detailed map of the whole process for optimizing the molding parameters 1 Box G E P H unter W G and H unter J S Statistics for Experimenters John Wiley amp Sons Inc N ew York 1978 4 6 Quality Control of Molded Parts 4 6 1 Part Weight M olded parts most often have to meet very exacting specifications such as dimensions color appearance or some measurement of performance capabilities mechanical strength an end use tests etc Such QC tests are usually done many hours 24 or 48 after the parts are molded O ver the years we have found that one of the best and most timely Quality C ontrol Q C test that a molder can perform to predict just how apart will measure up against the established release specifications or end use performance criteria for the part is part weight A major feature of part weight is th
71. oduce a variety of patterns on the finished sheet 6 3 2 Processing Extrusion conditions depend on the gauge and width of the film or sheet being produced A careful balance between material temperature and roll temperature is necessary for good surface finish and to prevent sticking to the roll Typical conditions for film and sheet of various gauges are shown in Table 6 4 on the following page Die lip to roll take up distance must be kept as small as possible for heavy sheets but can be greater for thinner films Slabs of up to 3 2 mm 1 8 in may be prepared by extrusion Thicker slabs up to 25 4 mm or 1 in have been prepared by stock shape manufacturers using compression molding 6 3 3 Troubleshooting Table 6 5 lists potential problems during film and sheet extrusion and possible corrective actions Ticona 6 4 Profile Extrusion 6 4 1 Equipment Extruder Extruders with barrel diameter of 35 60 mm 1 1 2 2 1 2 in and 5 15 H P drives are typical The extruders used for the profile extrusion of Celcon acetal are generally small because most profile cross sections are small and output rates are low Screw Metering screws as described in Figure 6 1 and Table 6 1 are recommended for profile extrusion of Celcon acetal Best results are obtained for screws with L D ratio 20 1 to 24 1 Pitch diameter ratio 1 1 Compression ratio 3 1 to 4 1 Length of metering section screw 25 length of 5 to 6 diameters
72. old can be used to reduce the propensity for wear Inserts should be considered where wear may be a concern and long production runs are anticipated For example P 20 tool steel can be successfully used for unfilled C elcon acetal copolymer grades where a limited production run is anticipated and Rc 58 60 tool steel may be required for molding a highly glass filled grade where an extensive production campaign is anticipated Beryllium copper cavities are also satisfactory for manufacturing good parts and offer the advantage of high thermal conductivity for good heat transfer and 21 Ticona prevention of hot spots in the mold H obbed cavities will work but lack the inner toughness of the alloy steels and are more susceptible to collapse under localized stress For prototyping or short production runs pre hardened steel Re 30 35 zinc alloys or aluminum are acceptable but may not be durable enough for long or high volume production 4 2 4 Mold Surface Finish A wide variety of surface finishes can be used with Celcon acetal as the resin exhibits excellent mold definition Various surface finishes designs script etc can be obtained by using standard techniques such as sand blasting vapor honing embossing and engraving the mold cavities and cores Flash chroming is recommended to prevent rust and preserve a highly polished surface condition M atte finishes are also achievable with an appropriate metal surface treatment
73. on acetal containers depends primarily on mold finish and mold temperature and partly on the ability to prevent air entrapment in the cavity Air entrapment can be prevented by proper venting Where air entrapment is a problem and ahigh glossy surface is not required a textured finish on the cavity is recommended Ticona 5 4 3 Impact Strength Impact strength in a blown container is dependent primarily upon the general design and wall thickness of the part but mold temperature can have a significant effect O ptimum impact strength is often achieved by maintaining the mold at 93 C 200 F or higher In some items where impact failures have occurred in the pinch off area because of poor welding a landed pinch off i e the use of a flat rather than a knife edge pinch off as shown in Figure 5 7 will yield significant improvement in impact strength Table 5 4 Blow Molding Troubleshooting Guide SYMPTOMS POSSIBLE SOLUTIONS Die Lines Polish die lip Increase mold temperature Increase air volume Eliminate hang up area in die Modify die to eliminate weld lines Internal Roughness Increase material temperature Dry air supply Increase length of land Low Gloss Increase mold temperature Increase stock temperature Increase parison thickness Improve mold surface finish Poor Definition Increase mold temperature Increase air volume Increase air pressure
74. on hazard N ormal safety precautions such as the use of a dust mask and adequate ventilation are highly recommended D ust can be minimized by keeping knife blades sharp and using proper clearances and screen size Surface absorption of water on regrind tends to be slightly higher than for pellets because of the larger surface area As aresult it is essential that regrind be properly dried before use to avoid any moisture related production problems To insure maximum retention of mechanical properties regrind usage should be limited to no greater than 25 for most applications Special care should be taken to prevent contamination by other resins especially PVC other halogenated polymers and partially and or fully crosslinkable thermoplastic elastomers For any processing technique it is particularly important to also avoid dirt and other impurities which can create surface blemishes or plug flow paths Because of the possibility of contaminating regrind with metal such as from the regrinder knife blade the use of a hopper magnet in the feedstream is strongly recommended Table 2 2 Effect of Remolding on the Properties of Unreinforced Celcon Acetal Ist 5th 11th Property Molding Molding Molding Tensile yield strength MPa Value 59 3 59 6 SYP percent retention 101 97 Notched Izod impact strength 23 C 73 F J m Value 62 5 66 8 percent retention 107 Celcon acetal copolymer Table 2 3 Effect
75. onductor of heat and provides a blanket to minimize or prevent oxidation Cooling Cool annealed parts slowly one hour for every 3 2 mm of wall thickness In some cases however because of in service temperatures annealing may be required especially where dimensional stability is of critical importance Some general guidelines for annealing are given in Table 3 3 Circulating air ovens and oil baths capable of providing a uniform temperature of 152 2 C 305 5 F are recommended for annealing C elcon acetal W hile equipment offering lower annealing temperatures may be suitable for some applications it is not preferred because the annealing time to obtain best results can be expected to increase significantly with decreasing temperature 3 5 Tolerances Dimensional tolerance can be defined as a variation above and below anominal mean dimension If recommendations for part mold design and proper molding are followed typical tolerances expected are M 0 2 up to the first 25 mm 1 inch or less M 0 1 for additional length over 25 mm 1 inch Fig 3 3 Water Absorption by Unfilled Celcon Acetal Under Various Conditions Boiling Water Immersion 100 C Water Immersion Water Immersion dL Tel O 2 2 ta lt 2 e Part Exposure 23 C 93 Rel Hum Part Exposure 23 C 50 Rel Hum 40 60 80 Time Days In cases where tighter tolerances are required precision tooling as we
76. ormed Profiles Rotational M olding Process 16 17 18 18 19 20 24 24 25 35 Ticona Overview General Guidelines Dimensional Stability Injection Molding Blow Molding Extrusion Rotational Casting Celcon acetal copolymer 1 Overview 1 1 Chemistry of Acetal Polymers Acetal polymers are chemically known as polyoxymethylenes PO M Two types of acetal polymers are commercially available H omopolymer is prepared by polymerizing anhydrous formaldehyde to form a polymer composed of oxymethylene repeating units CH 0 Acetal homopolymer products have somewhat better short term mechanical properties than the copolymer Copolymers including C elcon acetal copolymer are prepared by copolymerizing trioxane a cyclic trimer of formaldehyde with a cyclic ether usually containing an ethoxy or other oxyalkylene group to form a polymeric chain composed of oxymethylene CH 0 and oxyethylene CH CH 5 O or similar repeating units Copolymers have a wider processing window better long term mechanical properties and superior chemical resistance compared to homopolymers and are inherently more stable and resistant to thermal degradation during service life This is because the randomly dispersed comonomer units block polymer unzipping under thermal stress or exposure to hot water or hot alkaline solutions Both the homopolymer and copolymer are end capped and also contain specif
77. perature in the range of 82 93 C 180 200 F is used to provide a good balance of properties reduce molded in stress provide a high surface gloss and assure good part dimensional stability F or fiberglass reinforced C elcon acetal a slightly higher mold temperature of 93 127 C 200 260 F is recommended to provide a resin rich high gloss surface M olding with this higher mold temperature will also facilitate resin flow into the cavities reduce molded in stress and provide parts with improved dimensional stability in end use Uniform mold temperature control is extremely important especially for molding parts consistently within precision tolerances 4 4 4 Injection Pressure Injection pressure will vary with several factors including geometry of the molded part length of flow design of the runner and gates melt temperature clamp capacity of the molding machine condition of the mold with regard to flash etc While most moldings of C elcon acetal are successfully produced in the range of 76 138 M Pa 11 20 Kpsi 82 C 180 F for 3 hrs generally adjustments must be made to adequately fill the mold without flash To avoid back flow before the gate seals first and second stage injection pressures should be the same for most moldings In some cases it may be advantageous to reduce second stage injection pressure e g to reduce molded in stress in the gate area 4 4 5 Cavity Pressure Measurement
78. r control over melt temperature 4 2 8 Molded in Inserts A wide variety of molded in inserts have been successfully mated with various grades of C elcon acetal Because of the resin s high strength and excellent creep resistance characteristics retention of the inserts is good even after exposure to severe temperature and moisture cycling tests Recom mended designs for molded in inserts are shown in Figure 4 3 Celcon Ticona acetal copolymer Fig 4 3 Recommended Molded In Insert In addition to standard insert molding inserts can be assembled in pilot holes of molded C elcon acetal parts using press fits spin welding or ultrasonic welding in a post molding operation Designs for Celcon Acetal Recommended oe 4 2 9 Outsert Molding devices Insert molding is a well established process for mating _ plastics and metals but outsert molding extends its Ga Flat advantages to produce entire subassemblies with resistance multifunctional acetal copolymer parts The process is claimed to eliminate assembly steps and improve quality and productivity O ne company for example molds over 100 acetal components onto a specially designed galvanized steel baseplate for a video recorder All of the parts are formed from a single shot of an acetal copolymer sent to four levels via a mold with 25 pinpoint gates per level C elcon acetal is ideal for injection molding both fixed and movable parts onto a metal plate clamped into
79. resin is injected into the mold The valve should have large clearances and well radiused corners when open to ensure that the melt flows freely is not hung up and is not overheated Thechannel depth ratio i e the ratio of the channel depth in the feed zone to that in the metering zone h h2 should be between 3 and 4 5 A channel depth ratio of 4 1 is recommended for optimum results M The feed section should occupy about 40 of the screw length the transition zone about 30 and the metering section about 30 Typical screw dimensions for injection molding screws are given in Table 4 1 A diagram of arecommended check valve is shown in Figure 4 2 and indicates the need for flats to be machined at the mating joints B and C The flutes in the screw tip A and the flow path through the check Fig 4 1 Typical Injection Molding Screw for Plasticizing Celcon Acetal Copolymer Metering Transition Celcon acetal copolymer Ticona Table 4 1 Typical Screw Dimensions for Plasticizing Celcon Acetal Copolymer for Injection Molding Screw Diameter in Metering in Feed in Channel Depth Zone Length as of screw length Ratio Feed Transition Metering 11 2 0 083 0 29 3 5 40 50 30 20 30 2 0 089 0 30 3 4 40 30 30 21 2 0 097 0 32 33 40 30 30 31 2 0 108 0 35 3 2 40 30 30 41 2 0 119 0 38 ring D should be generously proportioned and well radiused to
80. s not done moisture in the air may condense on the surface of the pellets and lead to surface defects on molded or extruded plastic parts This is especially critical with impact modified grades which can be moisture sensitive and deteriorate during processing The use of ahopper magnet in the feedstream is highly recommended to insure against any form of metallic contamination which could occur while transporting the resin and cause equipment damage Every effort should be made to avoid pellet spills or loss Spilled pellets can be very slippery and may result in employee accidents Pellet loss to the environment could lead to fines or other penalties under Storm Water Regulations issued by the Environmental Protection A gency Ticona Polymers supports the Society of the Plastics Industry O peration C lean Sweep program 2 2 Safety and Health Information The usual precautions must be observed as when processing any hot and molten thermoplastic CAUTION Normal processing temperatures and residence times should not be exceeded Celcon acetal copolymer should never be heated above 238 C 460 F nor be allowed to remain above 193 C 380 F for more than 15 minutes without purging Excessively high temperature or long residence time in a heated chamber can cause the resin to discolor and in time degrade to release formaldehyde a colorless and irritating gas This gas can be harmful in high concentrations so proper ven
81. speed tends to increase modulus but to alesser degree than both melt and mold temperatures Injection pressure and screw rotational speed have little effect on modulus M old shrinkage in both the flow and transverse directions decreases with increasing injection pressure increasing injection speed decreasing mold temperature and a melt temperature in the lower range of 171 182 C 340 360 F Conversely mold shrinkage increases with decreasing injection pressure decreasing injection speed increasing mold 31 temperature and a melt temperature in the range of 182 210 C 360 410 F By minimizing shrinkage during molding you will maximize the potential level of post molding shrinkage Thus during use the part could shrink out of dimensional tolerance It is recommended to adjust initially the mold cavity dimensions for maximum shrinkage and mold at conditions of maximum shrinkage Typical molding conditions for providing maximum mold shrinkage are shown in Table 4 7 4 7 2 Glass Mineral Coupled and Filled Celcon Acetal Grades The effect of molding conditions on physical properties should be used only as a guide Some variation in conditions will most likely be required to achieve optimum performance for each part While screw rotational speed back pressure and injection speed have little or no effect on the properties of unreinforced C elcon acetal except where they cause severe overheating of the resin these
82. ss outside Increase die temperature Decrease screw speed Control volume in the head Wall thickness variation Maintain minimum volume in the head Increase land length Use a lower melt flow rate material Decrease screw temperature 45 Celcon acetal copolymer Table 6 4 Parameter Ticona Typical Conditions for Film and Sheet Extrusion Film Sheet Gauge mm mils Materials Extruder Screen pack mesh size lt 0 25 lt 10 Celcon M25 M90 Special grades 90 mm 3 1 2 in 20 40 60 80 100 0 25 2 5 10 100 Celcon M25 M90 Special grades 90 mm 3 1 2 in 20 40 60 80 100 Barrel temperature C F Zone rear Zone 2 rear Zone 3 rear Zone 4 rear Zone 5 Front 185 365 195 383 195 383 200 392 200 392 185 365 195 383 195 383 200 392 200 392 Gate temperature C F Die temperature C F Casting roll temperature C F Roll 1 Roll 2 Screw speed rpm Back pressure MPa psi Compression ratio Die Type Coat hangar or Y Dimensions cm in Land length cm in Production rate mpm fpm 195 383 190 374 195 383 190 374 115 239 115 239 115 239 115 239 56 80 3 4 500 3 4 500 3 25 1 3 25 1 Coat hangar or Y 40 16 3 8 1 5 6 12 20 40 AO 16 5 1 2 0 0 6 6 2 20 Using a reverse temperature profile for the extruder barrel ma
83. such as air circulation rate mold material of construction and mold wall thickness all of which affect heat transfer In no case should melt temperature be allowed to exceed 238 C 460 F nor should the resin be allowed to remain above 193 C 380 F for more than 30 minutes For thick walled parts greater than 6 4 mm or 1 4 in a melt temperature of 216 C 420 F is recommended Ticona 7 3 3 Part Cooling Rates Air cooling at a moderate rate is recommended for best finished part properties Rapid cooling will induce brittleness For that reason water cooling should be avoided 7 4 Troubleshooting Table 7 1 lists some potential problems that may be encountered during rotational casting and possible solutions Table 7 1 Rotational Casting Troubleshooting Guide SYMPTOMS POSSIBLE SOLUTIONS Bubbles on outer wall Dry resin Discolored part Flash excessive Reduce time temperature to prevent degradation Ensure proper venting Confirm good parting line seal Long oven cycle Improve air circulation Increase temperature or check temperature calibration Reduce mass of rotation arms for better heat transfer Low density less than 1 37 g cm Optimize temperature time to prevent degradation Dry resin Poor mold filling Poor properties Increase rotation speed Increase radii or width of mold recesses Increase cooling time Check part density and reduce temp
84. the mold Parts can include gears pins bushings wall sections springs cams and other shapes T he plates themselves are usually 0 040 0 080 in thick with tolerances of 0 002 in In addition to holes used for mounting parts two holes secure the plate in the mold The distance between them must be controlled to a tolerance of 0 001 in per 4 0 in length to ensure precise molding locations Fine knurl Either three plate or hot runner molds with multiple pin gates can be used U nlike conventional parts shrinkage allowance in outsert molding is determined from the center of each individual molding on the plate Fig 4 4 Outsert Molded Moving Parts Gear left cam center and spring right For more information on this technique call us at i 1 800 833 4882 4 2 10 Gating Standard Injection Molding Gate Type Parts made from C elcon acetal have been successfully made with a variety of gate types Figure 4 5 gives examples of common gate types suitable for molding Celcon acetal parts Table 4 3 Recommended Gate Dimensions for Rectangular Edge Gates mm in Part thickness mm in Gate width mm in Gate depth mm in Land length mm in 0 76 2 29 0 030 0 090 0 51 2 29 0 020 0 090 0 51 1 52 0 020 0 060 1 02 0 04 2 29 3 18 0 090 0 125 2 29 3 30 0 090 0 130 1 51 2 16 0 060 0 085 1 02 0 04 3 18 6 35 0 125 0 250 3 30 6 35 0 130 0 250 2 16 4 19 0 08
85. the general purpose type having a few flights of uniform depth in the feed zone a tapered compression zone and several flights of uniform depth in the metering zone Screw pitch should be uniform and equal to the screw diameter Compression ratio should be in the range of 3 1 4 1 A typical 2 1 2 in diameter 20 1 L D screw for extruding C elcon acetal should have M 3to 5 flights of uniform depth in the feed zone M 10 12 flights of increasing root diameter in the compression transition zone and M 5 flights of uniform depth in the metering zone 34 Celcon acetal copolymer Table 5 1 Equipment Costs Comparison of Injection and Extrusion Blow Molding Extrusion Injection Ticona Total equipment cost Lower Higher Parison die cost Lower Higher Blow mold cost Approx equal Approx equal Processing Production rates Lower Higher Finishing required Considerable Little or none Waste and regrind Some Little or none Product Overall quality achievable Good Excellent Sizes obtainable Small medium large Small to medium Wall thickness control Good Excellent Tolerance on neck finish Fair Excellent Flight depth in the feed zone would be 0 44 in and 0 11 in in the metering zone Recommended screw characteristics to plasticize C elcon acetal for extrusion blow molding are summarized in Table 5 2
86. the nozzle If the nozzle is blocked for any reason such as by malfunction of the positive shut off device or resin freeze off in the nozzle sufficient pressure could develop to cause blow back of the resin through the feed zone and hopper or create other hazardous conditions A nozzle heating band with independent temperature control is recommended for fine tuning nozzle temperature to prevent nozzle drool or freeze off of resin in this area W hile a powerstat rheostat can be satisfactorily used for temperature control indicating type temperature controllers are preferred 4 1 3 Plasticizing Capacity Aswith other engineering plastics C elcon acetal should not be exposed to excessive temperatures or very long residence times The shot weight for C elcon acetal should be in the range of 50 75 of the rated machine capacity for best results 4 1 4 Clamping Force Clamping force should be high enough to prevent the mold from opening during resin injection at maximum pressure and speed U sually 5 10 tons clamping force per square inch of projected area including molded parts sprue and runners is adequate for molding C elcon acetal copolymer The clamping force must exceed the projected area times the second stage pressure 20 Celcon acetal copolymer 4 2 Mold Design 4 2 1 General Criteria Standard industry principles for good mold design and construction apply to the design of molds for processing C elcon acetal
87. ticking in Cavities Decrease injection pressure Decrease screw forward time Minimize cushion Increase mold close time Lower mold temperature Check for undercuts or insufficient draft Sticking in Core Decrease injection pressure Decrease hold pressure Minimize cushion Decrease mold close time Decrease core temperature Check for undercuts or insufficient draft Unmelted Pellets Machine plasticizing capacity too small for shot and cycle speed required Temperatures too low particularly at rear zone Increase screw speed Increase back pressure Check for proper compression screw Check for wear of the screw or barrel Check for proper screw flight depth Increase injection pressure Increase screw forward time Decrease cushion Increase mold temperature Increase feed gate runner sprue Check for leaking check ring Warpage part distortion Equalize temperature in both halves of mold Check uniformity of ejection Check handling of parts after ejection from mold Increase hold time Alter injection pressure Increase cooling time Reduce material temperature Try differential mold temperatures to counteract warp Fixture parts in a cooling jig 33 Celcon acetal copolymer 5 Blow Molding 5 1 Blow Molding Methods Blow molding Celcon acetal copolymer allows for the production of hollow and irregular shapes through either extrusion or injection techniq
88. tilation is essential If venting is inadequate high pressures could develop in the equipment which may lead to blow back through the feed area If no exit is available for these gases the equipment may rupture and endanger personnel Ticona Consult the current C elcon acetal copolymer M aterial Safety D ata Sheets M SD S for health and safety data for specific grades of C elcon acetal copolymer prior to processing or otherwise handling these products Copies are available by calling your local Ticona sales office or C ustomer Services at 1 800 526 4960 or from the internet site www ticona com 2 3 Flammability When ignited C elcon acetal copolymer burns with little or no smoke and with a barely visible blue flame Combustion products are carbon dioxide and water If C elcon acetal copolymer burns with a muffled flame and combustion is incomplete carbon monoxide and some formaldehyde may be released Exposure to high concentrations especially in a poorly ventilated area can be harmful For more detailed information on worker exposure limits for formaldehyde refer to the M aterial Safety D ata Sheet for C elcon acetal copolymer WARNING Avoid flame Do not allow mixing of this material with PVC other halogen containing materials and partially and or fully crosslinkable thermoplastic elastomers Do not heat above 460 F 238 C Avoid prolonged heating at or above the recommended processing temperature Recommended melt te
89. ts Low Wear Low wear LW grades are chemically modified to provide low coefficient of friction and enhanced wear resistance and are exceptional for demanding applications requiring good sliding properties reduced gear and bearing noise and enhanced lubricity Celcon acetal copolymer Mineral Coupled These mineral coupled MC products contain chemically coupled mineral fillers in varying percentages The mineral filled grades are recommended whenever resistance to warpage especially in thin sections and dimensional stability are key application parameters They are generally tougher than the glass bead filled polymers but are more difficult to color uniformly Ultraviolet Resistant The ultraviolet resistant UV products are available with various melt flow rates in natural and a wide variety of colors and are lead mercury and cadmium free T hey are specially formulated for improved resistance to color shift and mechanical degradation from ultraviolet light both sunlight and fluorescent lighting C onsult the Ticona brochure C elcon U Itraviolet R esistant G rades Extend Part Life in H arsh Environments C E UV for further information about these products Weather Resistant Weather resistant WR products are formulated for maximum outdoor weathering resistance Several different melt flow rate grades are offered They are available in black color only Antistatic These antistatic A S products are chemi
90. ues While most commercial applications are for relatively small components such as cosmetic bottles dishwasher spray arms and automotive hydraulic fluid reservoirs blow molders have produced plenums measuring 50 in long and 16 in wide in developmental runs The two general methods for blow molding plastics articles are extrusion blow molding and injection blow molding Both methods may be used to produce items made from C elcon acetal The two methods differ primarily in the method of preparation of the parison i e the tube of molten resin from which the molded article is formed Both techniques have much in common and information on recommended grades of C elcon acetal for both blow molding processes can be obtained by calling Product Information Services at 1 800 833 4882 5 1 1 Extrusion Blow Molding Extrusion blow molding is more extensively used than injection blow molding and parts are made either by a continuous or discontinuous intermittent extrusion method In the continuous method there is no interruption in parison extrusion When the parison reaches the appropriate length a mold closes around it and the parison is cut Air is introduced and pressurized to create the blow molded part The mold moves and a new mold closes around the continuously moving parison M ore than half of all C elcon acetal blow molded parts are made using the continuous method due to its lower cost and shorter processing cycle
91. ure Decrease cooling rate Lines Clean die to remove any hard particles Remove any nicks or burrs on take off system Pits Bottom surface only Cool the part more thoroughly before placing on conveyor belt Pits All over surface Dry resin Check for contamination and eliminate Surface roughness Increase die temperature Dry resin Decrease exit speed at die Use die with larger openings or longer lands Use a smaller machine Surging Check for broken gear tooth worn belts controller and repair Remove likely causes for any variation in temperature pressure screw speed or motor load Increase pressure lower rate Decrease temperature Use a die with longer lands Use better mixing screw Warpage Ensure uniform cooling Support warping section until cool enough to hold shape Provide more gradual cooling Lower line speed Wrong shape Too large Increase pull on the contour Change take off speed or material temperature either way may help Use longer lands 47 Wrong shape Too small Decrease pull on the contour Change take off speed or material temperature either way may help Use shorter lands Celcon acetal copolymer 7 Rotational Casting Rotational casting is also referred to as rotational molding or rotomolding and is a process for manufacturing hollow and seamless products of all sizes and shapes t offers
92. used as the primary means of venting Ejector and core pins used for venting should be flattened 0 0254 mm 0 001 in on one side Blind holes where gases may become trapped can be vented by drilling a small 3 175 6 35 mm 1 8 1 4 in hole at the bottom of the cavity and inserting a small diameter pin flattened to 0 0254 mm 0 001 in on one side When using these techniques we recommend that mold temperatures be kept in excess of 180 F to avoid gas condensation on the pins and prevent corrosion 4 2 12 Cooling Channels The actual mold temperature as well as temperature uniformity is extremely important in ensuring good quality molded parts Each mold must contain cooling channels to help maintain uniform heat distribution throughout the tool The cooling channels should be as large in diameter as is practical at least 14 3 mm or 9 16 in and located in areas directly behind the cavities and the cores C hannels should be uniformly spaced to prevent localized hot spots N on uniform cooling can lead to surface blemishes sink marks excessive molded in stresses warpage and poor dimensional control with a possibility of excessively long cycle times 4 2 13 Draft Plastic parts are almost always designed with a taper in the direction of mold movement to ease ejection from the mold This is commonly referred to as draft in the line of draw The deeper the draw the more draft will be required Some C elcon acetal parts h
93. void Weld Lines To further weld the material passing the die pin and discourage streaks an annular restriction choke should be placed downstream from the flow divider The choke serves to increase back pressure in the melt stream by reducing the cross sectional area between the flow divider and the die The higher pressure which is achieved tends to smooth out the weld and reduce the chance of streaking Celcon acetal copolymer Ticona Table 5 2 Typical Screw Characteristics to Plasticize Celcon Acetal Copolymer for Blow Molding Screw Diameter mm in Channel Depth Zone Length as of screw length Metering mm in Feed mm in 38 1 1 5 2 11 0 083 7 37 0 29 Feed Transition Metering 40 50 30 20 30 50 8 2 0 2 26 0 089 7 62 0 30 40 30 30 63 5 2 5 2 46 0 097 8 13 0 32 40 30 30 88 9 3 5 2 74 0 108 8 89 0 35 40 30 30 114 3 4 5 3 02 0 119 9 65 0 38 40 30 30 Type Metering general purpose type constant pitch L D 20 1 to 24 1 preferred 16 1 minimum Compression ratio 3 1 to 4 1 The die should be kept hot to avoid freeze off of the molten resin after it has passed through the spider 5 2 6 Die The parison die is a key element in blow molding because it controls material distribution in the finished item and in turn influences the economics of the final product Figure 5 2 shows a parison die with an adjustable core pin wh
94. water by volume will boil at 113 C 235 F an 80 ethylene glycol solution boils at 138 C 280 F With a pressurized water circulating unit maximum temperatures of 93 99 C 200 210 F are attainable in most molds when the unit is operated at the extreme high limit of its temperature range For those moldings where mold temperatures higher than 99 C 210 F are needed a pressurized oil circulating unit is normally required For flexible temperature control the oil reservoir in the unit should be equipped with a suitable heat exchanger for lowering the oil temperature when required An alternate system for mold temperature control is to install cartridge heaters of an appropriate size in the cooling channels of the mold and control temperature via a powerstat rheostat The cartridge heaters should tightly fit the diameter of the cooling channel to prevent premature burn out of the heaters and prolong their life While this is a relatively simple and inexpensive system for mold temperature control it is not recommended as it could cause localized hot spots and lead to various molding problems 27 Ticona It is prudent to check the mold temperature after the machine has been operating for about 30 minutes If necessary readjust the mold heater temperature settings to maintain the desired temperature 4 3 2 Process Control Where it is important to maintain tight dimensional tolerances a closed loop system should be
95. xcellent surface quality but are much more expensive rock and roll or shuttle Ticona 7 2 1 Particle size Plastic powders must be used for rotational casting to ensure rapid and adequate melting Resin particles greater than 30 mesh size will significantly increase cycle time 7 3 Processing Parameters 7 3 1 Resin Drying Conditions Powders used for rotational casting will have a higher surface area than the pellets normally used for other processing methods As a result a higher content of adsorbed water is expected To avoid poor finished part surface due to moisture proper drying is necessary Six hoursin an air circulating oven at 82 C 180 F is recommended Using a dehumidifying hopper dryer adds extra insurance against adsorbed moisture C aution should be exercised in the design of the dryer to handle the dusting of the powder clogging of filters 7 3 2 Part Heating Oven Parameters O ven temperature and time must be balanced to yield a satisfactory part while avoiding thermal degradation of the polymer Time is a key parameter in obtaining a smooth surface finish Too little oven time will cause inadequate melting of the plastic powder too long oven time may lead to resin degradation Fig 7 1 Rotomolded Acetal Copolymer Parts Production M PNE gt 48 Celcon acetal copolymer The optimum temperature for each situation will vary and will be influenced by processing parameters
96. y be helpful when maximum melt strength is required especially for large diameter and or heavy wall profile extrusion Call us at 1 800 833 4882 for further information Table 6 5 Film and Sheet Troubleshooting Guide SYMPTOMS POSSIBLE SOLUTIONS Dull surface Increase melt temperature Increase roll temperature If cloudy dry resin If problem persists use vented barrel Lines Across direction of extrusion Decrease roll temperature Eliminate chatter due to take off Blow air between rolls and sheet film Lines Curved Improve mixing by e increasing back pressure and or e increasing screw speed or e using higher compression screw Lines in direction of extrusion Pock marks spots Eliminate die nicks Eliminate lip build up Dry resin Increase roll temperature Improve mixing as for curved lines Decrease roll temperature if sticking Dry resin Use vented barrel Warping Increase roll temperature Increase tension at take off 46 Celcon acetal copolymer SYMPTOMS Ticona Table 6 6 Profile Extrusion Troubleshooting Guide POSSIBLE SOLUTIONS Distortion Ensure uniform temperature at all points on the die Change location of sizing plates and fingers Modify design Gloss Low in strips Eliminate cold or rough spots in die or sizing devices Gloss low all over Increase die temperature Increase melt temperat
97. y contributor to warping O ther factors affecting warpage are Gate size G ate location M old temperature Filler type level Orientation of fillers HM Mold cooling 3 3 Post Molding Shrinkage After mold shrinkage has occurred and the part reaches ambient temperature further mold shrinkage may occur as time passes This post molding shrinkage is usually related to stress relaxation and additional crystallization within the molded part resulting in a permanent shrinkage of the part and it is affected by cooling rate i e mold temperature and part thickness H ot mold temperatures of 82 120 C 180 250 F reduce post mold shrinkage to nearly negligible values At ambient temperatures this shrinkage is relatively small on the order of 0 1 0 2 for the standard unfilled 9 0 melt flow grade of C elcon acetal copolymer C ontinuous exposure of the molded parts to high temperatures accelerates both the rate and magnitude of the post molding shrinkage Figure 3 2 illustrates the shrinkage behavior of the standard unfilled 9 0 melt flow grade of Celcon acetal copolymer after a six month exposure at several temperatures in the flow direction for a part 3 2 mm thick It is extremely important to utilize mold surface temperature in excess of the anticipated maximum temperature that the part may seein end use since stress is frozen into the part at the mold temperature The frozen in stresses will be relieved by annealing causing a

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