Analyzing and Troubleshooting Single
Contents
1. The standard array of diagnostic equipment is required for the troubleshooting of a process that is flow surging These tools include screw measuring devices pyrometers and devices to calibrate sensors in the process These devices are dis cussed in Chapter 10 Often it is very difficult to impossible to determine a cause and effect relationship from process displays that are attached to typical extrusion 544 12 Flow Surging lines However a portable data acquisition system that is capable of collecting pro cess data as a function of time is highly useful in determining the cause and effect relationships between process parameters In all of the cases presented here the extrusion line was either equipped with a data acquisition system or a temporary acquisition system was connected to the machine during the trial E 12 3 Barrel Zone and Screw Temperature Control Improper selection of process temperatures poor temperature control and inopera tive temperature control devices are common causes for flow surging As stated earlier temperatures for the metal surfaces in the solids conveying zone must be within a specific range for an application This temperature range will depend on the resin equipment design placement of the temperature control sensor and rate Thermocouple placement on extruders is not standard and thus they can be positioned at different axial positions for the zones and at different depths into the barrel wal2. Processes that flow surge will often cause a higher level of degradation products to occur in the extrudate For these cases the unsteady nature of the flows in the screw channels will tend to break off small levels of degradation products adhering to the screw The degradation products could occur at the flight radii and regions with long residence times and they may not contaminate the extrudate under normal conditions But the unsteady state nature of the flow surge will tend to break them away from the screw surface E 12 1 An Overview of the Common Causes for Flow Surging Improper process temperatures and poor temperature controls are common root causes for flow surging For example solids conveying depends on a balance of the forwarding forces at the barrel wall and the pushing flight and the retarding forces at the screw surface These forces depend mainly on the geometry of the channel and are directly proportional to the coefficient of dynamic friction for temperatures less than the melting or devitrification temperature and on viscous forces for higher temperatures 6 Since the coefficient depends on temperature pressure and velocity 7 surface temperature changes for the barrel and screw in the feeding section will strongly affect the performance of the extruder If the surface tempera tures become too different from the optimal values flow surging and loss of spe cific rate will occur If the solids conveying section of the
3. 160 200 Screw Speed 120 0 80 0 10 20 30 40 50 60 Time minutes Figure 12 15 Screw speed and motor current for the screw cooling experiment 40 y 200 35 Cooling On gt Cooling On E 160 T 25 D o Screw Speed 120 5 20 i i o 8 45 P1 i 80 z A i b 10 Extruder Discharge AO 2 5 0 0 Time minutes Figure 12 16 Screw speed pressure at the entry to the first stage meter P1 and discharge pressure for the screw cooling experiment Based on the above data the cause of the extrusion instability was identified as high temperatures on the screw surfaces of the feed section These high surface temperatures caused the coefficients of dynamic friction to increase increasing the retarding forces on the solids at the screw surface Since solids conveying depends on a combination of forwarding forces at the barrel wall and pushing flight and retarding forces at the screw root and trailing flight an increase in the retarding forces will cause a reduction in the solids conveying rate The instability appeared to be random due to the complicated interactions of cooling water flow rate and temperature and due to changes in bulk density of the feedstock Several technical solutions were considered to increase the cooling level to the feed section of the screw including increased water flow and the use of chilled water The best technical solution and quickest to implement was to increase the 561 562 12
4. 3 1 and is shown in Fig 4 11 Since the coefficient of dynamic friction is only defined for solid state processes the friction values are reported here as stress at the interface because the stress can be described from ambient temperatures up to processing temperatures The shear stress at the inter face for HIPS resin is shown in Fig 12 17 at a pressure of 0 7 MPa As indicated by this figure the shear stress was nearly constant from ambient temperature up to about 110 C increased to a maximum stress near 150 C and then decreased as the temperature was increased further Optimal performance of the solids conveying section for this resin would be such that the forwarding forces are maximized with metal surface temperatures near 150 C where the stress is a maximum and the retarding forces minimized with metal surface temperatures of 110 C or lower Thus optimal solids conveying for HIPS resin would occur with a feed zone barrel inner surface temperature near 150 C and a screw surface temperature in the feed section no higher than 110 C In practice screw temperatures less than 90 or 100 C are preferred such that melting of the resin does not happen if an emer gency shutdown should occur For the solid state temperature region the shear Stress at the interface can be converted to the coefficient of dynamic friction by the following f r P 12 4 where fis the coefficient of dynamic friction 7 is the shear stress at the polymer me
5. Flow Surging length of the cooling hole in the screw The length of the cooling hole was increased from 3 8 diameters into the flighted section to 7 diameters up to the end of the feed section After the screw modification the extruder has not experienced instabili ties of this type and the rate has increased to 100 of its maximum potential rate Cooling on the screw and feed casing are often limited by the water pressure at the supply and discharge sides That is if the water pressure on the discharge header is nearly the same as that of the supply side then the water flow rate will be very low due to the lack of a pressure driving force Thus if the driving pres sure for water flow is not available then adequate cooling to the screw and casing may not exist A simple way to test if the cooling water flow is acceptable is to dis connect the discharge water line from the header and either flow this water to a drain or the parking lot using a temporary hose The discharge water flow should be high and the temperature should be warm to the touch A permanent arrange ment might consist of a water pump and a rotameter in line upstream of the rotary union attached to the screw To aid in the understanding of this solids conveying problem the coefficient of dynamic friction was measured for the resin as a function of temperature and sliding velocity at a pressure of 0 7 MPa The equipment used to make the measurement is described in Section 4
6. caused by a combination of frictional heating of the solids on the wall and also by conduction from the first heated zone of the barrel It is estimated that temperatures as high as 170 C occurred in the feed casing As presented in Section 12 7 3 optimal solids conveying will occur when the stress at the polymer metal interface at the barrel is a maximum and for HIPS resin this surface temperature is near 150 C Surface temperatures higher than 150 C in the feed section will reduce conveying and lead to starving of the screw Channels and ultimately flow surging Corrosion inside of the cooling chan nels of the feed casing prevented the flow of cooling water Cleaning the cooling channels and adding a larger cooling water recirculation pump reduced the tem perature of the feed casing and eliminated the flow surging problem 563
7. required a nearly steady supply of molten polymer For this case the 551 552 12 Flow Surging 230 Set Point Water Temperature ual On 220 210 Barrel Temperature C 200 0 30 60 90 120 150 Time minutes Figure 12 7 Barrel temperature data for a 203 2 mm diameter extruder running GPPS resin and with water cooling on the barrel heating and cooling This extruder was configured with a water cooling capability that was too high for the process barrel zones were electrically heated and water cooled The barrel zone tempera ture is shown in Fig 12 7 The barrel heater for this zone was only used during startup Once the extruder was operating the energy dissipation from the screw to the resin was more than enough to keep the section hot In fact in order to main tain the zone at the set point temperature of 215 C the extruder was operated with a very small amount of barrel cooling At the low temperature portions of the cycle both heating and cooling were off The small amount of excess energy dissi pated in the screw channel was causing the barrel temperature to increase slightly with time When the temperature exceeded 220 C the control algorithm took action and opened the solenoid valve on the water line upstream of the heating and cooling barrel jacket as shown in Fig 12 8 a The controller opened the solenoid valve for the minimum amount of time sending a short burst of water to the zone The water wou
8. returns to normal until the next solid fragment blocks the restricted region Repeated blocking and clearing of the restricted region creates the flow surging To eliminate surging due to solid blockages the troubleshooter must eliminate the defect that caused the solid bed to break up and must also mitigate the restric tion in the downstream section of the screw It is preferred to correct both defects to permanently eliminate surging from the process E 12 7 Case Studies for Extrusion Processes That Flow Surge Numerous case studies are presented in the next sections that show some common flow surging problems In these case studies the problem is presented in a manner that the troubleshooter would encounter during a trial or information gathering ses sion Incomplete data and erroneous data are often presented to the troubleshooter These data were not included here because including them may mislead the reader The troubleshooter however must be able to separate the actual facts of the process from misleading perceptions In each case study the modifications required to fix the process are detailed along with supporting fundamental information In all cases the rate of the process was limited and the cost to manufacture was high 12 7 1 Poor Barrel Zone Temperature Control A 203 2 mm diameter plasticating extruder was running GPPS resin and dis charging to a specialty downstream process Like most processes the downstream equipment
9. At about 16 minutes into the run the extruder was switched from automatic to manual screw control the screw speed was held constant at 67 rpm As shown by the data in Fig 12 5 the motor current variation was unchanged indicating that the screw speed control algorithm was not induc ing the variation in the motor current During the period that the screw speed was held constant the pressure to the inlet of the pump slowly increased as shown in Fig 12 6 This pressure was increasing because the screw was operating at a speed that delivered a rate slightly higher than that needed by the pump When the con trol was placed back into automatic mode the screw speed was decreased initially to compensate for the higher than desired inlet pressure This type of analysis is recommended when minor levels of flow surging are observed with a process where the screw speed is controlled from the inlet pressure of a gear pump 549 550 12 Flow Surging 800 Motor Current 750 Q lt G 700 O Cc D D 2 F 650 3 3 5 i Screw Speed 600 dp gt Automatic Screw Manual Automatic Screw 590 Control 1 Control i Control 500 0 5 10 15 20 25 30 35 40 Time minutes Figure 12 5 An extrusion process with a downstream gear pump with the screw operating in inlet pressure control and followed by the screw in manual operation constant screw speed The large level of variation in the motor current during constant screw spee
10. Gear Pump Control process is shown in Fig 12 4 If the pressure to the inlet of the pump is less than the set point value then the control system will increase the screw speed of the extruder Conversely if the inlet pressure is too high the control system will decrease the screw speed Thus processes that use a gear pump downstream of an extruder can show large variations in the screw speed in an attempt to compensate for an extruder that is flow surging Hopper Vent Figure 12 4 Schematic of a two stage extruder with a downstream gear pump A poor control algorithm for the pump can cause some variation in the extruder screw speed causing large variations in the inlet pressure to the pump This type of control induced surging can occur even though the process as designed is inherently stable To determine if the control algorithm is inducing the surging the screw speed of the extruder should be operated in a manual mode and at a constant speed If the controller is inducing the surging placing the process in manual control mode will stabilize the process Transient process data were col lected for an extruder with a downstream gear pump as shown in Fig 12 4 For this case the control algorithm was controlling the speed of the screw such that the inlet pressure to the pump was maintained at 8 MPa Although the variation in screw speed was not excessive at 67 1 5 rpm the variation in motor current seemed quite high at 540 90 A
11. HANSER Elri A L Troubleshooting and Anahsis m Singie Sicrew Extnuders Sample Pages Gregory A Campbell Mark A Spalding Analyzing and Troubleshooting Single Screw Extruders ISBN Buch 978 3 446 41371 9 ISBN E Book 978 3 446 43266 6 For further information and order see http www hanser fachbuch de 978 3 446 41371 9 or contact your bookseller Carl Hanser Verlag Munchen Flow Surging Flow surging is defined as the oscillatory change in the rate of the extruder while maintaining constant set point conditions Flow surging can originate from many different sources including improper solids conveying melting instabilities flow restrictions and improper control algorithms 1 5 Surging in most cases results in lower production rates higher scrap rates higher resin consumption material degradation and higher labor costs In mild cases flow surging will cause plant personnel to set the product at the low end of the dimension setting at the low rate portion of the surge At the high rate portion of the surge the dimensions of the product will be oversized Oversized products will use more resin than necessary adding cost to the product and decreasing the profitability of the plant Obviously a process that is very steady has the capability of minimizing resin usage and thus maximizing the profitability of the process For a profile process where the dimen sion of the cross section is critical to downstream ass
12. a constant depth metering channel that is completely filled with molten resin a screw speed of 60 rpm and a positive downstream pressure gradient OP Oz gt 0 five rotations are shown The pres sure is the highest at the pushing side of the channel and the lowest at the trailing side of the channel The pressure typically increases nearly linearly with rotation from the trailing side of the channel to the pushing side As the flight tip passes underneath the transducer the pressure decreases quickly to that of the trailing side of the channel Figure 12 2 b shows a similar pressure profile with rotation in a conventional melting section For this case the solid bed extends across about 50 of the channel The pressure profile is similar to that for the metering channel case except that the pressure gradient in the region over the solid bed is higher than that for the melt pool The width of the molten resin can be estimated by the time fraction that the transducer spends over the melt pool and solid bed 547 548 12 Flow Surging The pressure profiles with rotation shown in Fig 12 2 are ideal In practice the pressure profiles contain a level of measurement error and unsteady state behavior Pressure in an actual channel operating at a screw speed of 30 rpm for an ABS resin is shown in Fig 12 3 Metering Section Melting Section Pressure MPa 100 104 108 112 116 120 Time s Figure 12 3 Measured pressure profiles wi
13. d control suggests that the extruder process is unstable and the control algorithm is not the root cause for the variation in the motor current 90 85 z g z 80 D Q Pa ag T5 5 70 A Wp O Ol Automatic Screw Manual Control i Control i Control Ji O 0 5 10 15 20 25 30 35 40 Time minutes Figure 12 6 Pressure at the inlet to the gear pump for the data presented in Fig 12 5 The pressure increased during manual control because the flow rate of the extruder was slightly higher than the rate of the pump 12 7 Case Studies for Extrusion Processes That Flow Surge E 12 6 Solids Blocking the Flow Path Compacted solid polymer fragments can block and restrict the flow in a process In order for this to occur two defects typically exist in the process The first defect causes the compacted solid to fragment and flow downstream in the screw chan nels The second defect is a restriction in the channel where the fragments are trapped and accumulated As the restriction builds the local pressure just upstream of the restriction will increase while the pressure downstream will decrease As the downstream pressure decreases the pressure and rate at the discharge of the extruder will also decrease The local and high pressure just upstream of the restric tion will cause the melting rate of the fragments to increase temporarily clearing the blockage 2 When the blockage is removed the rate of the process
14. e barrel temperatures for each extruder using the technique described in Section 10 9 12 7 3 Flow Surging Due to High Temperatures in the Feed Section of the Screw A severe and random flow surging problem limited the production rate for a large diameter two stage vented extruder If it were not for a gear pump positioned between the extruder and die this extrusion line would not have been operable The surging did however limit the output of the line to about 70 of its potential rate The maximum potential rate is the rate that the extruder can run at high screw speeds and with proper operation The extruder was 203 2 mm in diameter and had a 40 L D barrel A schematic for the extruder and gear pump arrange ment is shown in Fig 12 11 and the screw channel dimensions are provided in 12 7 Case Studies for Extrusion Processes That Flow Surge Table 12 1 The specific rotational flow rate for the first stage metering section was calculated at 20 0 kg h rpm The extruder was fed a mixture of fresh HIPS resin with 30 to 60 recycled ground sheet from a downstream thermoforming process The level of recycle affected the bulk density of the feedstock entering the extruder The HIPS resin had an MER of 3 9 dg min 230 C 5 0 kg The screw was single flighted and typical of what is used for HIPS resins Screw temperature control was accomplished by flowing cooling water through a rotary union into and out of a hole cut into the feed end of th
15. e screw as shown in Fig 12 1 This hole extended 3 8 diameters into the feed section Pressure sensors were positioned in the barrel wall at the end of the first stage transition section P1 at the end of the first stage metering section just before the vent P2 and at the discharge Additional pres sure sensors were positioned at the discharge of the extruder and at the inlet suc tion side to the gear pump A screen filtering system was positioned between these pressure sensors as shown in Fig 12 11 A commercial control scheme adjusted the screw speed to maintain a constant pressure of 9 MPa to the inlet of the gear pump The gear pump was operated at constant speed in order to maintain a constant flow rate of material to the die Table 12 1 Screw Channel Dimensions for a 203 2 mm Diameter Two Stage Vented Screw Running HIPS Resin Depth mm Length diameters Notes Feed section 28 6 7 The compression ratio was 4 0 and the compression First stage transition 10 rate was 0 0032 First stage meter Ai 8 Vent section 31 9 4 5 The pump ratio was 1 7 Second stage transition Do Second stage meter 12 3 6 Lead length flight width and flight clearance were 203 2 23 9 and 0 20 mm respectively in all sections of the screw A 28 7 mm diameter screw cooling hole was drilled in the shank end of the screw and it extended 3 8 diameters into the feed section The first 2 5 diameters of the screw were inside a water cooled feed casing The speci
16. embly processes the extreme ends of the rate surges may result in a product that is outside of the specification and some profiles will need to be scrapped or recycled In this chapter numerous case Studies along with diagnostic methods are presented for processes that flow surge The additional cost of producing products from a line that is flow surging can be substantial If the flow surge is not too large and the line can be operated the insta bility of mass flow at the die can cost the converter from 5 to 15 added costs in resins The higher resin costs are incurred because the dimensions of the articles are larger or thicker than needed For example if a line is producing sheet for a downstream thermoforming process and it is operating unstably then some ther moformed parts will have an acceptable mass while others will have a higher mass costing the plant more in resin Often the instability occurs only at a high rate while at lower rates the process is stable In this case the plant may miss shipment dates since the line can only be operated at a fraction of its capacity or the plant may incur higher labor costs because the line will need to operate over weekends In severe cases flow surging can cause the line to be incapable of producing pro duct at any rate Thus in order to produce product at the lowest possible cost the line must be operating stably so that the rate and product quality are maximized 542 12 Flow Surging
17. extruder is controlling rate not the metering section as designed then a portion of the screw channel between the sections will be partially filled at the low rate swing of the cycle and most often will be completely filled at the high rate region of the cycle Improper design and operation of the melting section of the screw can lead to extrusion instabilities For example solid bed breakup 3 can cause solids to migrate downstream These solids can wedge into other sections of the screw and cause the extruder to flow surge 2 4 or cause the extrudate to have periodic changes in temperature Periodic changes in discharge temperature will cause some level of flow surging at the die 8 12 1 1 Relationship Between Discharge Pressure and Rate at the Die Dies are shaping devices that operate at a rate that is directly proportional to the upstream pressure Thus if the pressure to the die is not constant then a variable rate will occur at the die opening causing the dimensions of the product to vary Rate surges at the die can be estimated from the pressure surges using the 12 2 Troubleshooting Flow Surging Processes following equations for flow through a cylindrical restriction or die for a power law fluid 4 AQ 0 0 0 1 1 AP 12 1 AP P P P 12 2 0 p 1 n TA 12 3 Q E where n is the power law index Q and P are the rate and discharge pressure at condition 1 and Q and P are the rate and pre
18. fic rotational rate of the first stage metering section was calculated at 20 kg h rpm Hopper Gear Vent Pump Screw E Cooling sea Hole P P Screen Figure 12 11 Schematic of the 203 2 mm diameter extrusion process for HIPS resin 557 558 12 Flow Surging In order to diagnose the problem a data acquisition system was temporarily con nected to the extrusion panel All available sensor outputs were connected in parallel with the acquisition system Electronic data were collected at a frequency of once every 9 s Steady state operation of the extruder is shown by the first 400 minutes in Figs 12 12 12 13 and 12 14 The data for these figures were from the same production run The extruder was running at 2250 kg h and a screw speed of 99 rpm for a specific rate of 22 7 kg h rpm This specific rate is about 14 higher than the specific rotational flow rate calculated for the first stage metering section indicating that a negative pressure profile exists in the section The negative pressure gradient is expected for a first stage metering section of a vented screw that is operating properly that is the first stage metering section was full of resin To maintain the stability the extruder screw speed was reduced such that the extruder was operating at about 70 of its potential maximum rate That is at screw speeds higher than 99 rpm the extruder was more likely to transi tion from a stable to an unstable operati
19. g section In general the temperature of the screw in this section needs to be less than the 7 for amorphous resins or less than the melting tempera ture for semicrystalline resins Small diameter screws will typically operate at feed 12 3 Barrel Zone and Screw Temperature Control zone screw temperatures that are low enough without the need for special cooling For screws 150 mm in diameter and higher the temperature of the screw however can become too hot for optimal solids conveying In these cases the temperature of the screw can be decreased by flowing water into the screw using a rotary union and piping assembly as shown in Fig 12 1 Cool process water flows through the union and into a pipe that extends up to within 10 cm of the end of the cooling hole The water then flows back out of the screw through a section of cast pipe The cast pipe is attached by threads to the screw shank and rotary union The length of the cooling hole and the flow rate of water are used to maintain the screw tempera ture in an optimal range In general the cooling hole is drilled into the screw up to the end of the feed section Two case studies are presented that show flow surging processes that had poor temperature cooling on the feed section of the screw Figure 12 1 Diagram showing a rotary union piping assembly for cooling the feed section of a screw Two zone temperature control of the screw has been utilized to mitigate process instabilities
20. in the solids conveying zone and carbonaceous material buildup on the screw root in the melting zone for polyvinylidene chloride PVDC resins 9 Two zone screw temperature control can also be used to control the temperature of the solids conveying zone and energy removal in the metering zone The control device is similar to that shown in Fig 12 1 except that a second rotary union is required for the second fluid and a sealing device 10 is needed to isolate the cooling fluids 12 3 1 Water and Air Cooled Barrel Zones Heating and cooling of the barrel zones is typically done using modules that are equipped with electrical heaters and either water or air cooling These modules are then clamped onto the outside of the barrel Water cooling has the capability of removing more energy from the process and it is well suited for extruders larger 546 12 Flow Surging than 150 mm in diameter where the cooling demand is high that is where water flows to the modules for 10 or more of the time If lower levels of cooling are required however water cooling can create temperature oscillations in the zone For example when the zone becomes too hot the controller will open the solenoid valve to the water flow line for the shortest possible duration If this minimum amount of water flow is too large then the cooling on the zone can be too much causing the temperature of the zone to undershoot the set point temperature 11 The control scheme wi
21. into the run the extruder started to operate unstably as indicated in Figs 12 12 12 13 and 12 14 The processing change that caused the extruder to go from a stable operation to an unstable one was not known but it could have been due to minor changes in the bulk density of the feedstock or cooling water fluctuations to the screw As indicated by these figures the event started when the P1 pressure decreased slightly causing the rate and the P2 pressure to decrease This decreased pressure transmitted down the extrusion system even tually decreasing the pressure at the inlet to the gear pump To correct for the lower pressure the controller on the gear pump increased the speed of the screw from 99 rpm to about 160 rpm Next the P1 pressure increased due to the higher 560 12 Flow Surging screw speed and higher flow rate as indicated in Fig 12 12 As the pressure increased at the gear pump inlet the gear pump controller decreased the screw speed back to about 100 rpm causing the extruder to flow surge Flow surging caused the screw speed controller to oscillate about once every 25 minutes As indicated in Fig 12 12 the screw speed controller was able to provide a relatively stable pressure to the pump inlet allowing the process to run at reduced rates The barrel zone temperatures as indicated in Fig 12 14 were extremely oscillatory As indicated in Fig 12 12 the P1 pressure was considerably lower during the period of unstab
22. l Because of these extruder and process differences barrel tempera tures typically need to be optimized for the machine and application Optimization of barrel temperatures was presented in Section 10 9 Equipment devices that are not functioning properly can cause a process to flow surge For example the feed casing of the extruder is typically cooled with water such that the outside temperature of the casing is about 50 C or less If the cooling water flow is turned off or is not flowing at a high enough rate then the tempera ture of the inside wall of the casing will become too hot to convey solids into the machine As a general rule for most resins the outside temperature of the feed casing will be too hot to touch if the inside wall becomes too hot to convey solids that is at temperatures higher than 50 C At high casing temperatures the rate limiting step of the process is the solids conveying of resin from the casing to the barrel and not the metering channel of the screw Thus the specific rate will decrease and flow surging is very likely to occur For specialty PE resins with very low solid densities the temperature of the feed casing may need to be less than 35 C High temperatures on the feed casing can also cause the resin to bridge over the feed opening such that pellet flow to the extruder is severely or completely restricted Flow surging can occur if the temperature of the screw becomes too high in the solids conveyin
23. ld flash evaporate in the unit and then quickly cool the barrel to about 210 C Since the solenoid was opened for the shortest possible amount of time the level of cooling that was utilized was the minimum It was very obvious that the level of cooling water to this barrel zone was too high for this process The barrel temperature oscillations shown in Fig 12 7 were enough to cause a small variation in the product dimensions Although the variations in the product dimen sions were acceptable the variations did reduce the profitability of the process by causing too much resin to be used in the final product 12 7 Case Studies for Extrusion Processes That Flow Surge Control On Off Solenoid Valves Figure 12 8 Heating and cooling system on the barrel a schematic of the original configuration that created the temperature oscillations in Fig 12 7 and b a better configuration that minimized the temperature oscillations In order to reduce the cooling level to the barrel zone a metering valve was placed in the water line upstream of the solenoid valve as shown in Fig 12 8 b Now when the controller opens the solenoid valve a much lower quantity of water and thus cooling is available to the barrel zone Prior to this modification the barrel temperatures oscillated 10 C about the set point temperature After the modifica tion the temperature oscillations were reduced to about 3 C and the profitability of the process was improved due t
24. le operation This result indicates that the cause of the problem originated in the first stage of the screw before the first stage metering section At a screw speed of 160 rpm the extruder was still operating at a rate of 2250 kg h but the specific rate decreased to 14 kg h rpm This specific rate is considerably less than the specific rotational flow rate of 20 kg h rpm indicating that the first stage metering section was operating improperly and only partially filled The most likely reason for a partially filled or starved metering section was poor solids con veying from the feed section to the transition section Poor solids conveying was likely due to improper temperature control of the metal surfaces in the feed section of the extruder and screw Barrel feed zone heaters controllers and the feed casing were examined and determined to be operating properly at set point temperatures typically used for HIPS resin Based on this information the investigation was focused on the temperature control of the screw It was hypothesized that the screw temperature in the feed section was too hot to convey solids effectively to downstream sections of the screw To test this hypothe sis the effect of internal screw cooling was determined during a period when the extruder was operating stably For this period cooling water was flowing to the screw cooling device and the extruder was operating stably and properly at a rate of 2360 kg h and a screw
25. ll cause the zone temperature to oscillate Variation in tem perature for the barrel zones can affect the rate and discharge temperature The oscillations can be mitigated by installing metering or needle valves in the water flow lines to reduce the water flow rate to the module An in line water filter is typically installed in the cooling line so that the needle valves do not get plugged with particulates Air cooled zone modules do not have the ability to remove as much energy as do water cooled units For processes that only require a low level of cooling air cooled units will provide a more stable control of the temperature Recent innovations in air cooling using high flow fan systems 12 have allowed the replacement of some water cooled systems with less costly and lower maintenance air cooled systems 11 E 12 4 Rotation and Geometry Induced Pressure Oscillations Pressure transducers that are positioned in the barrel can be extremely useful for troubleshooting a process Common positions include midway into the melting sec tion and at the entry to the metering section For two stage screws positioning of a transducer at the entry to the second stage metering section provides information on the degree of fill of the stage and provides knowledge on the likelihood of vent flow The pressures measured from these transducers provide three types of infor mation 1 the average pressure in the channel 2 the pressure variation in the ang
26. nded on the screw speed and the temperature of the feed resin Moreover the processor indicated that flow surging was experienced for some extruders at zone T1 barrel temperatures as low as 148 C 995 556 12 Flow Surging 25 180 20 B 170 amp n a 15 7 g Ul 160 8 DB 10 S gt gt le uh a m Aah A il M 0 140 30 40 50 80 90 100 Time minutes Figure 12 10 Barrel pressures and temperatures for the 38 1 mm diameter extruder operating at a zone 1 barrel temperature condition that caused the extruder to flow surge The temperature profiles are in red while the pressures are black Based on the data collected the Maddock solidification experiment and the numerical calculations the problem was diagnosed as poor solids conveying from improper temperatures in the section Slight differences between extruders such as the axial and radial position of the zone T1 thermocouple barrel zone controller tuning screw geometry variations and thermocouple accuracy likely caused con ditions such that some of the extruders flow surged while others did not These minor variations could influence the temperature of the inside barrel wall of the solids conveying section Moreover different rate requirements for different pro ducts required that the extruder be operated at different screw speeds which further complicated the solids conveying problem The problem could have been avoided if plant personnel had optimized th
27. o the minimization of resin consumption This temperature control problem occurred due to the implementation of a high performance type screw The original screw was fabricated with a relatively shal low metering channel The shallow channel had a low specific rate and also dissipated a relatively high level of energy The excess energy was easily removed through the barrel wall with the water cooling using the configuration shown in Fig 12 8 a That is the solenoid valve was in the open position enough to main tain cooling while not causing the barrel temperature to undershoot the set point temperature The high performance screw however was designed with a deeper metering section had a considerably higher specific rate and dissipated less energy For this screw less excess energy needed to be conducted through the bar rel wall Since the cooling system was designed for a process with a high heat flux through the barrel the temperature became very oscillatory when the energy flux was reduced when the high performance screw was implemented 553 554 12 Flow Surging This simple case study shows the importance of verifying the control algorithms before proceeding with a troubleshooting trial Before any testing or equipment modifications are performed it is extremely important to have a deep understand ing of the process and have all process controls and sensors in acceptable opera tion Ifthe sensors and controls are not func
28. on The barrel pressure at the end of the first stage transition section P1 had variations of about 3 MPa about the average pressure This pressure variation was considerably higher than expected and suggests that the extruder although running stably was on the verge of unstable operation Some of the variation was due to the movement of the flight tip past the sensor Barrel zone temperatures tracked the set point values and were stable 1400 Current Current My 1200 AM oo fo a a La 800 S g 5 3 fi 3 2 600 g AT AOL AT Pump Inlet iN 400 4 200 0 200 400 600 800 1000 Time minutes Figure 12 12 Barrel discharge and pump inlet pressures and motor current for stable and unstable extrusion for a large diameter extruder running HIPS resin 12 7 Case Studies for Extrusion Processes That Flow Surge 559 280 1400 1200 240 Motor Current S 1000 5 ca s00 a 3 600 a 160 5 D 400 120 Screw Speed 200 80 0 0 250 500 750 1000 Time minutes Figure 12 13 Screw speed and motor current for a large diameter extruder running stably and unstably 300 320 Extrudate 280 250 T8 7 S ARK T9 240 5 E rift T5 T6 F 200 0 D 150 T2 T3 z 160 5 100 Screw Speed 120 50 80 0 200 400 600 800 1000 Time minutes Figure 12 14 Screw speed extrudate temperature and barrel zone temperatures for a large diameter extruder running stably and unstably At about 410 minutes
29. speed of about 104 rpm The metal surface temperatures of the pipes used to flow water into and out of the screw were measured at 29 and 37 C respectively At about 28 minutes into the run the cooling water flow to the screw was turned off as indicated in Figs 12 15 and 12 16 At about 30 minutes the pressure at the end of the first stage transition section P1 started to decrease as shown in Fig 12 16 indicating that solids conveying was significantly reduced Like before the reduced solids flow caused the downstream pressures to decrease and ultimately to cause the extruder to flow surge At about 36 minutes into the run cooling water flow was turned on and within about four minutes the extruder operation became stable as indicated in Figs 12 15 and 12 16 The surface tem perature of the pipe for water flow out of the screw was measured at 81 C just after the cooling water was turned on a temperature change of 44 C As will be presented later in this section solids conveying of HIPS resin becomes difficult or unstable at screw temperatures of about 150 C and higher The temperature of the screw surface was unknown but it likely increased by at least 44 C and possibly approached 150 C 12 7 Case Studies for Extrusion Processes That Flow Surge 1400 360 1200 Motor Current 390 T 1000 280 S F jel 800 240 5 Q O 09 _ 600 no T l 200 gt 9 Cooling On Cooling Cooling On S 400 OTT
30. ss data were collected from each sensor at a frequency of once every 10 seconds using a portable data acquisition system For barrel zone temperatures of 150 163 and 174 C for zones T1 through T3 respectively the extruder was operating stably and at rates that were consistent with numerical simulations and it was producing a high quality product Process data for steady State operation are shown in Fig 12 9 for a screw speed of 50 rpm As indicated by this figure the barrel zone temperatures were steady and only small variations occurred for the P2 and P3 pressure sensors Slight pressure variations were expected for this extruder because the sensors were positioned in the barrel and were measuring pressure in different regions of the channel as the screw rotated The pressure patterns are not periodic like those in Fig 12 3 due to the screw Speed and acquisition rate used For this case pressure samples were collected every 8 3 rotations A faster data collection rate would have shown a periodic oscil 12 7 Case Studies for Extrusion Processes That Flow Surge 25 180 T3 170 15 T2 5 g AVY AANA AWW 160 5 P 10 5 Ti 150 P UMM ern Vn 0 140 0 5 10 15 20 Time minutes Figure 12 9 Barrel pressures and temperatures for the 38 1 mm diameter extruder operating stably The temperature profiles are in red while the pressures are black lation of the pressure These data indicate that conditions exist for the stable pro ce
31. ssing of the resin For a second experiment the extruder was operated at barrel set point tempera tures of 160 163 and 174 C for zones T1 through T3 the zone T1 temperature was increased by 10 C and zones T2 and T3 temperatures were unchanged This increase in the T1 temperature caused the extruder to flow surge and decreased the rate by about 20 The process data for the unstable conditions are shown in Fig 12 10 As indicated by this figure the pressure for the midbarrel pressure sen sor P2 was zero during the low pressure swing of the cycle indicating that this portion of the channel was operating partially filled or starved Later in the experiment the pressure sensor responses were checked when the pressure was known to be zero in the channels The pressure was measured by the sensors at 1 4 MPa when the pressure was actually zero explaining the offset pressure at the bottom of the pressure cycle in Fig 12 10 Numerical calculations and a Maddock solidification experiment confirmed that the midsection of the extruder was operating partially filled Thus a small 10 C increase in the first barrel zone tem perature was enough to cause the extruder to go from operating as a Stable process producing high quality product to one that was unstable with reduced rates and having a product with unacceptable product dimensions Numerous other experi ments had shown that the first barrel zone temperature needed for stable extru sion depe
32. ssure at condition 2 The pressure at the die lip is assumed to be zero For example a 5 variation in the discharge pres sure AP 0 05 for a polymer with a power law index of 0 3 will cause a 16 change in the instantaneous rate AQ 0 16 An instantaneous rate change of this magnitude is unacceptable for most processes The flow relationship with pressure is much more complicated than this for a commercial die but the trend is the same E 12 2 Troubleshooting Flow Surging Processes The analysis and troubleshooting of a process that is flow surging can be a difficult task especially when the line is required to run production The analysis can often be complicated by the operation of equipment downstream from the die For exam ple if a pulling system is not operating at a constant speed then variations in velocity can cause the product to vary in dimension even though the extruder is operating stably Worn components on a calendering roll stack can cause the speed of the rolls to vary or cause the gap between the rolls to change during a revolu tion Both conditions will cause the product to change dimensions in the down stream direction Unit operations downstream from the die must be checked to determine if they are the root cause of the product variation The troubleshooter must be diligent to set a hypothesis and then test the hypothesis If some problem other than the root cause is fixed then the process will continue to flow surge
33. tal interface and P is the pressure 0 7 MPa in this case 12 7 Case Studies for Extrusion Processes That Flow Surge oO Oo 7 6 cm s a 15 2 cm s a 30 5 cm s oO NO oO Shear Stress MPa oO _ 0 50 100 300 150 200 250 Temperature C Figure 12 17 Shear stress between HIPS resin and a metal surface at a pressure of 0 7 MPa and as a function of temperature and sliding velocity 12 7 4 Flow Surging Due to High Temperatures in the Feed Casing The extruder described in Fig 12 11 on a different occasion started to flow surge but with a slightly different frequency as shown in Figs 12 18 and 12 19 As indi cated in these figures there were short time periods when the discharge pressure and screw speed were stable and the motor current was high During these periods the extruder was operating well but at a reduced production rate During periods of unstable operation the motor current decreased by about 20 the screw speed increased and the discharge pressure became extremely oscillatory Like the pre vious case as the motor current decreased solids conveying decreased causing the controller to increase the speed of the screw During the trial the feed casing to the extruder had an outside surface tempera ture of about 80 C Although not measured the inside cylinder wall of the feed casing for the first 1 5 diameters downstream of the feed opening was considerably hotter These higher temperatures were
34. th rotation for a 63 5 mm diameter extruder running an ABS resin at 30 rpm a conventional single flighted screw and with a positive downstream pressure gradient in the metering section OP Oz gt 0 As shown in Fig 12 3 for the metering channel the highest pressure is at the pushing side of the channel and the lowest is at the trailing side of channel The angular pressure profile in the melting section was typical and very similar to the ideal profile shown in Fig 12 2 because properly operating melting sections have positive pressure gradients in the downstream direction The data in Fig 12 3 clearly shows that a level of measurement noise and unsteady state activity is occurring in the process E 12 5 Gear Pump Control Gear pumps are often positioned between the extruder and the die and they pro vide several processing advantages These advantages include the mitigation of pressure and flow surges from the extruder a decrease in the discharge tempera ture by generating the pressure for the die by the pump instead of the extruder and by decreasing the discharge pressure via the pump a capacity increase is pos Sible 13 For gear pump assisted extrusion the extruder control algorithms are set to maintain a constant pressure to the inlet side of the pump The pump is operated at a constant rotational speed and thus it delivers molten polymer at a very steady and controlled rate A schematic of a gear pump assisted extrusion 12 5
35. tioning properly then the troubleshooter may modify the wrong section of the process and obtain little to no improvement in the process 12 7 2 Optimization of Barrel Temperatures for Improved Solids Conveying Numerous complaints were logged by a single processor from several different manufacturing plants on flow surging and reduced rates for a specialty resin The flow surging caused unacceptable variations in the final product In all cases small diameter extruders were used but the operating conditions reported were differ ent in the plants In several of the plants there were some extruders that did not flow surge yet the design of these machines appeared to be identical to those that experienced flow surging It was not apparent why some of the extruders were operating well while others were surging An extrusion trial was performed at the processor s plant using a 38 1 mm dia meter production extruder a proprietary screw design and resin that had pre viously exhibited flow surging and reduced rate The extruder was equipped with three barrel zone heaters with control thermocouples labeled T1 T2 and T3 and two pressure sensors One pressure sensor was located in the midsection zone 2 of the barrel P2 and the other at the end of the barrel near the tip of the screw P3 Both transducers were positioned over the top of the screw such that a pressure variation due to screw rotation would be observed During the trial proce
36. ular direction due to the rotating screw and 3 the stability of the process by comparing the pressure oscillations during several screw rotations The pressure in the angular direction is composed of two pressure components 1 a pressure component in the downstream direction OP Oz and 2 the cross channel pres sure gradient OP Ox The shape of the angular pressure profile depends on the magnitudes of the components In order to measure the pressures during rotation high speed data acquisition equipment is required For example a screw that is rotating at a speed of 60 rpm will require a data acquisition frequency of at least 12 4 Rotation and Geometry Induced Pressure Oscillations 20 Hz providing 20 pressure measurements per rotation Typical pressure measure ments for transducers positioned in melting sections and metering sections that are filled with molten resin are shown in Fig 12 2 Pressure Sensor Pushing side A Sa a g V V 2 A S Pushing side 2 g b 7 D Trailing side Solid bed 0 1 2 3 4 5 Time s Figure 12 2 Typical pressure measurements for transducers positioned in the barrel for a screw speed of 60 rpm and a positive downstream pressure gradient OP Oz gt 0 a for a transducer positioned in a metering section where the resin is completely molten and b for a transducer positioned in a single flighted melting section The pressure profile shown in Fig 12 2 a is for
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