Hytrel®

polyester elastomer

Extrusion Guide

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DescriptionHytrel is the DuPont registered trademark for itsfamily of engineering thermoplastic elastomers.

Hytrel combines many of the most desirablecharacteristics of high-performance elastomers aflexible plastics. It features: exceptional toughnesand resilience; high resistance to creep, impact, aflex fatigue; flexibility at low temperatures; andgood retention of properties at elevated tempera-tures. In addition, it resists deterioration from manindustrial chemicals, oils, and solvents.

Hytrel grades are grouped into four categories:

• Standard grades exhibit versatile processingcharacteristics, are lowest in cost, and are suitafor many extrusion applications;

• High-performance grades generally provide anextra margin of mechanical properties for themore demanding applications;

• Specialty grades provide special properties orprocessing characteristics for particular applications;

• Concentrates contain relatively high concentra-tions of specific property-enhancing additives foblending with other grades of Hytrel.

Extrusion Applications andGrade SelectionThe excellent properties and processing charactetics of Hytrel engineering thermoplastic elastomequalify it for many demanding applications. Suchproperties as mechanical strength, dynamic flexperformance, fluid and chemical resistance, andwide service temperature range, have led to the uof Hytrel in many extruded products, includinghose and tubing, belts, extruded profiles, rope ancable covers, sheeting, and films.

Most Hytrel grades are suitable for extrusionprocesses. Selection of the most appropriate grafor a particular application, in terms of properties

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and serviceability, should be made by referring tothe general Hytrel product literature that is avail-able from your local DuPont representative.

Performance of different grades of Hytrel inparticular extrusion processes depends to someextent on composition.

• The softer grades are more suitable for freeextrusion of solid profiles. These are the Hytreltypes having Shore D hardness up to and including 55D, in particular those with low melt flowrates.

• Profile extrusion using calibration dies is not geerally possible with Hytrel, although it may bepossible to achieve acceptable results with theharder grades.

• The harder grades are best for vacuum calibratof tubing. Those grades of Shore D hardness 4and above give excellent results with this proce

Melt PropertiesThe ability to process Hytrel by a specific extrusiotechnique depends largely on the characteristics the melt, which are determined by the grade ofHytrel selected, and by processing conditions.

Some general melt properties should be considerin the extrusion of Hytrel:

• All grades have a sharp crystalline melting pointhat increases (and becomes sharper) with in-creasing hardness and crystallinity.

• Melt viscosity is strongly dependent upon melttemperature, and this dependency increases wiincreasing hardness.

• Crystallization rate increases with increasinghardness; therefore, the ability to supercool themelt without onset of crystallization decreaseswith increasing hardness.

Melt viscosity as a function of temperature andshear stress for Hytrel extrusion grades is shownFigures 1 and 2.

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Hytrel polymers have relatively flat viscosityversus shear rate curves, especially at low shearrates that are typical for extrusion. This means, foexample, that high-shear screw and die designs not reduce the melt viscosity of Hytrel as much awith some other polymers, but may cause undesable local temperature increase.

From Figure 2 it can be seen that a significantchange in melt viscosity can result from a relativesmall change in melt temperature. The extrusionmelt temperature can therefore be decreased toprovide greater melt strength for improved stabilitof the extrudate. It also means that good control melt temperature is an important factor in succesful extrusion of Hytrel resins.

Basic Extrusion EquipmentGeneral Design of ExtruderExperience has shown that the best results withHytrel are obtained with a single screw extruderdesign. Twin screw extruders tend to generateexcessive shear heating effects and are not recomended. Vented machines can not be used to drHytrel.

The emphasis in equipment selection should be othe uniformity and quality of melt produced. Aconstant delivery of homogeneous melt of uniformtemperature, with the ability to maintain the desirmelt temperature over a wide range of screwspeeds, should be the objective for good extrusio

Materials of ConstructionHytrel engineering thermoplastic elastomer in themolten state is noncorrosive to metals. Screws

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102

1010 102 103 1

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Hytrel 4056 HTR6108

Hytrel 5526

Hytrel G4074 Hytrel G4774 Hytrel G5544 HTR8171

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Figure 1. Melt Viscosity at Processing Temperature

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should have hardened (nitrided) surfaces but neenot be made of corrosion-resistant alloys, althougmany standard extruder barrels and screws aremade from such materials in order to process awide range of materials.

Extruder DriveD.C. motors with thyristor drive are recommendesince they provide good speed control and infinitvariable adjustment of speed over a large range.Such drives normally provide automatic currentlimitation to prevent screw breakage as a result oexcessive torque. Whatever type of drive is usedis essential that some form of overload safetydevice is incorporated in the drive system. Addi-tional protection should be provided by means ofrupture disc installed in the zone between theextruder screw and breaker plate/screen pack, opressure transducer with high pressure cut-outinterlocked to the extruder drive.

Material Hopper and Feed ThroatOverhead or tangential-type feed throats, as nor-mally provided on single screw extruders, workwell with Hytrel. Water-cooling of the throat isrecommended to prevent excessive heating of thresin entering the screw and to serve as protectiofor the drive bearings.

Hopper drying is not essential, but should be usewhere available, to protect the polymer fromadditional moisture pickup. A hopper drier is alsorecommended to ensure the resin is supplied to extruder at a constant temperature and moisturecontent, which can help to guarantee regularfeeding and melting characteristics.

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trel 3078 trel 4069 trel 4556 trel 5556 trel 6356 trel 7246

HTR8068

ProcessingType of Hytrel Temperature, °C (°F)

Hytrel G3548W 190 (374)Hytrel G4074 200 (392)Hytrel G4774 230 (446)Hytrel G5544 230 (446)Hytrel 3078 190 (374)Hytrel 4056 190 (374)Hytrel 4069 230 (446)Hytrel 4556 230 (446)Hytrel 5526 230 (446)Hytrel 5556 230 (446)Hytrel 6356 230 (446)Hytrel 7246 240 (464)HTR4275BK 230 (446)HTR5612BK 230 (446)HTR6108 190 (374)HTR8068 190 (374)HTR8171 190 (374)

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Figure 2. Shear Melt Viscosity versus Temperatureat a Shear Rate of 100 sec–1

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240 (464)

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HTR4275BK HTR5612BK

Hytrel 5526

Hytrel G3548W

Hytrel G4774 Hytrel G5544

Hytrel 4556 Hytrel 5556 Hytrel 6356 Hytrel 7246

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A moisture content of 0.10% or less is required inorder to maintain extrusion tolerances and tominimize degradation during processing. If themoisture content is uncertain because regroundscrap is being used, a suitable drier or oven shoube used. See “Extruder Operation” for furtherdetails on drying.

Extruder BarrelExtruder barrels that are suitable for use withcommon thermoplastics such as plasticized nylonPVC, or polyolefins are usually suitable for extru-sion of Hytrel. Length-to-diameter (L/D) ratios ofat least 24:1 or higher provide the best melt qualitfor precision extrusion.

Small clearances between the screw flights andthe barrel wall are important to prevent backflowof molten resin and possible surging in extruderoutput. It is suggested that radial clearances of<0.1 mm (0.004 in) are maintained for extruders uto 64 mm (2.5 in) diameter. These clearancesshould be checked from time to time, and refurbisment of the screw or barrel carried out whennecessary.

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Extruders with an intensively cooled barrel feedzone containing axial “grooves” to encouragepositive feeding of granules have been found tocause excessive shear with Hytrel during themelting/compression stage, with consequent rapitemperature buildup and high motor drive currentSuch barrels are unsuitable for processing Hytrel

It is recommended that the barrel be equipped wiat least four heat control zones, and the temperatof each zone controlled by a separate thermocouand proportional control instrument. Efficientcooling should also be provided by water or airblowers, independently controlled for each zone bthe temperature controllers.

Screw DesignThe most important element of an extruder is thescrew.

In general for Hytrel, good results can be obtainewith simple 3-zone screws having approximatelyequal length feed, transition (compression), andmetering zones. The length-to-diameter ratio shobe a minimum of 24:1 for good uniformity ofextrudate (i.e., minimum temperature and pressuvariations). Compression ratios should be betwee2.5 and 3.5 to 1, as determined by the depth of thfeed zone channel divided by the depth of themetering zone channel (“apparent compressionratio”). The depth of channel in both feed andmetering sections is important; if the feed channeis too deep and not sufficiently long, particularlywith large diameter screws, it can cause poorfeeding and loss of output with some harder Hytretypes. If the metering channel is too deep, it canresult in nonuniform temperature distributionthrough the melt, while a metering channel that istoo shallow can result in overheating of the meltdue to shear.

A typical screw of the 3-zone, gradual transitiontype, for processing Hytrel is shown in Table 1.

The use of complex designs incorporating highshear zones or intensive mixing devices, decom-pression zones, etc., are not recommended forHytrel. They cause excessive local heat buildup dto intensive shearing action, and usually causedifficulty in achieving the desired melt temperaturas well as possible high motor drive torque.

However, certain designs of “barrier” screw havebeen found to be very successful with Hytrel,particularly in achieving constant melt characteristics for critical extrusion operations (such as high-speed tubing extrusion).

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Internal screw cooling is not recommended forHytrel, although limited cooling of the screw in thefeed zone has been found to be successful ineliminating problems of irregular feeding (fall-offin output) in larger extruders.

Screens and Breaker PlateA breaker plate of streamlined design (e.g.,counterbored on both sides) is usually placedbetween the end of the screw and the adaptor tosupport the screen pack.

A screen pack is used for two purposes: to removany impurities or unmelted material from the meltstream, and also to ensure sufficient back pressuat the end of the screw to help create a homoge-neous melt and constant output pressure.

The screen pack should consist of two 80 meshscreens, supported by 40 mesh screen on thedownstream side, next to the breaker plate. Forcritical applications, where ultimate melt cleanli-ness is required, finer screens (120 or 150 mesh)can be used.

Good external heating is essential in the breakerplate area of the extruder. Sufficient heatingcapability should be provided to rapidly raise thetemperature in this region to the normal processinsetting, in order to ensure that any residual polymis thoroughly melted before start-up. Similarly,because the breaker plate (head clamp) area isusually one where a large amount of heat is lost tthe surrounding air, the heater design in this zonecritical.

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TablGradual Tran

Channel DepthDiameter (D) Pitch (P) Feed Section (in mm in mm in mm

1.5 38.1 1.5 38.1 0.250 6.2.0 50.8 2.0 50.8 0.310 7.2.5 63.5 2.5 63.5 0.385 9.3.5 88.9 3.5 88.9 0.455 11.4.5 114 4.5 114 0.525 13.

Feed Section

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Adaptor, Head, and DieBoth the adaptor and the head must be of stream-lined design. Flow channels should not containsudden changes in cross section, surface interruptions (caused by mismatched assembly joints ordamage, for example), or other “dead spots.” Areaof flow stagnation can give rise to localized poly-mer degradation and subsequent release of particof degraded resin into the melt stream.

Adequately sized heaters must be provided forthe adaptor, since it is generally a large piece ofmetal. It is especially important to control thetemperature of the adaptor and head separately,since they usually differ greatly in size and energyrequirements.

The die, where it extends beyond the extruder heashould also have its own thermocouple and tem-perature controller.

For head and die designs for specific extrusionprocesses, such as tubing extrusion, please refer the appropriate process in “Extrusion Processes”section (pages 9–17) of this bulletin.

InstrumentationThe function of an extruder is to pump moltenthermoplastic at a constant rate and temperature.Sophisticated instrumentation is a prerequisite forquality production. To gauge extruder performanceit is important to determine the pressure andtemperature of the melt, as well as to provideadequate methods of control.

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of Channel Depth ofh1) Metering Section (h2) Land Width (W)

in mm in mm

45 0.080 2.0 0.150 3.818 0.095 2.4 0.200 5.088 0.110 2.8 0.250 6.356 0.130 3.3 0.350 8.893 0.150 3.8 0.450 11.4

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Metering Section

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Pressure GaugesMelt pressure should be monitored during extru-sion, particularly at start-up. Recording and monitoring of melt pressure during start-up will indicatif there is proper flow of the material or if a bridg-ing or freeze-off situation exists. During produc-tion, pressure changes will also indicate output aviscosity changes of the molten plastic.

For accurate measurement and rapid response, adiaphragm-type transducer with electronicindicator is recommended. The most commonlocation for the transducer is immediately beforethe screen pack, since this is where high pressurmost likely to be generated. However, it may alsobe advantageous to install a second transducer ithe extruder head area to indicate output pressurcloser to the die.

Long- or short-term fluctuations may influence thquality and uniformity of the product, and for thisreason it may be beneficial to continuously monitmelt pressure by linking the transducer output to chart recorder.

Temperature ControllersRelatively small temperature fluctuations in theextruder, particularly in the front end and die, cangreatly influence extrudate quality when extrudingHytrel engineering thermoplastic elastomer,because the melt viscosity of Hytrel is stronglytemperature-dependent. Therefore, the type oftemperature control device used is of considerabimportance.

To maintain optimum temperature control and athermally homogeneous melt, the controller shoube of the proportioning or variable voltage type. Aon-off controller is not recommended for use withHytrel. A temperature fluctuation of 3°C (38°F) isnot uncommon with this type of controller; tem-perature variation of this magnitude can produceexcessive viscosity fluctuations in the melt andunacceptable dimensional variability in criticalextrusion processes.

Temperature feedback to the controllers is providby suitably positioned thermocouples in eachtemperature zone of the barrel, adaptor, head, andie. It is important that these thermocouples arepositioned close enough to the melt stream toaccurately register the temperature of the metalimmediately surrounding the melt.

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Melt ThermocouplesThermocouples that indicate the actual melt tem-perature of the extrudate are useful when extrudinHytrel. For rapid response they should be of theunshielded variety and located either in the adaptplate or, preferably, in the die, or as close to it aspossible.

The use of a handheld needle pyrometer to checkthe actual melt temperature before start-up shoulbe encouraged. Measurements should be made anormal running screw speed, after purging forsufficient time to allow temperatures to stabilize.

Extruder OperationGeneral Resin HandlingHytrel is normally supplied in 25 kg (55 lb) mois-ture resistant, sealed bags. 500 kg (1,102 lb) boxwith moisture barrier lining are also available forlarge volume usage.

In extrusion processing operations, the highestpossible level of cleanliness should be maintainedin the preparation, processing, and reworking of tmaterial, in order to prevent dust particles or otheforms of contamination from entering the extruderAutomatic granule conveying systems, sealedhoppers, and careful opening of bags and handlinof regrind material will contribute greatly to theoverall cleanliness and quality of the finishedproduct.

Safety PrecautionsAll safety practices normally followed in thehandling and processing of thermoplastic polymeshould be followed for Hytrel polyester elastomerThe polymer is not hazardous under normal ship-ping and storage conditions.

During processing, if recommended temperaturesand holdup times are exceeded to any great degrHytrel may degrade and decompose with evolutioof gaseous products. Usually, under normal procetemperature and throughput conditions, the amouof decomposition of these resins is minimal.However, potential hazards from these gaseousdecomposition products include “blow-back,” fire,and exposure to toxic vapors (principally tetrahy-drofuran). Also, as with all thermoplastics, thermaburns from contact with molten polymer are apotential hazard during processing. Before procesing Hytrel, see bulletin “Handling and ProcessingPrecautions for Hytrel,” and observe the precau-tions recommended therein.

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Moisture Pickup and DryingHytrel granules are supplied in moisture-resistantpackaging. However, when exposed to air, thegranules pick up moisture. Moisture levels above0.10% may seriously impair an extrusion operatiocausing highly variable melt pressure, varyingextruder output, degradation of the resin, and,possibly, bubbles in the melt as it exits from thedie.

At temperatures above the melting point, excessivmoisture causes hydrolytic degradation of thepolymer. Such degradation results in poor physicaproperties and brittleness, particularly at lowtemperatures.

Equilibrium moisture levels depend on the grade,and are shown in Table 2 (ASTM D570 method).Rate of moisture absorption for a typical Hytrelextrusion grade (5556) is shown in Figure 3.

Table 2Equilibrium Moisture Levels of Hytrel

Equilibrium MoistureType of Hytrel Level, % after 24 hr

High ProductivityG3548W 5.0G4074 2.1G4078W 3.0G4774, G4778 2.5G5544 1.5

High Performance4056 0.64069 0.74556 0.65526 0.55556 0.56356 0.37246 0.38238 0.3

Specialty3078 3.05555HS 0.7HTR4275BK 0.3HTR5612BK 0.4HTR6108 0.2HTR8068 1.9HTR8139LV 0.7HTR8171 54HTR8206 30

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Figure 3. Moisture Absorption at AmbientTemperature—Hytrel 5556

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DryingHytrel polyester elastomer must be dried prior toprocessing. It is critical to ensure that the resin isdry during processing to make quality parts thatwould give good service performance.

Also, in the case of critical extrusion operations,such as vacuum calibration of tubes to smalltolerances, it has been found that extruder outpumay fluctuate slightly with changing moisturelevels and temperature of the granules in thehopper. For this reason, drying of Hytrel granulesin a dessicant (dehumidifying) drier underconditions of fixed temperature and time isrecommended.

Drying time and temperature will depend on theinitial moisture level in the material, as well as thetype of drier or oven used. However, generalguidelines for drying Hytrel, which are based onlaboratory and industrial experience, are shown iFigure 4.

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Figure 4. Recommended Guidelines for DryingHytrelDrying Time versus Temperature

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Time, hr1 32 4 5 6 7 8

Max. (low melting point grades)

Max. (most grades)

300

275

250

225

200

Tem

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Start-up, Shutdown, and PurgingProceduresStart-upStart-up technique is important, as it involves thesafety of both operating personnel and the equip-ment. Start-up techniques vary depending onwhether or not the machine is clean.

Clean MachineTo start-up a clean, empty extruder, set the tem-perature controllers for the die, head, adaptor, anbarrel zones at the operating temperatures for theparticular resin being used (refer to processing daon page 22).

At this time the operation of the heaters and con-trollers should also be checked. When all zonesreach their operating temperatures, they should ballowed to “soak” for 30 to 60 minutes beforefeeding resin to the screw. Turn on the feed throacooling water. Cooling of the entire screw is notrecommended but, if cooling of the feed section othe screw is provided, it may be used, and issometimes effective in solving feed problems.

When all heating zones have been at their settemperatures for 30 to 60 minutes, turn on thescrew at slow speed (5 to 10 rev/min) and startfeeding resin through the hopper. When the meltappears at the die it should become “clear” after afew minutes, and both the melt temperature andhead pressure should level out.

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It is desirable to use a handheld needle pyrometemonitor temperature at start-up. Reference shoulalso be made to the machine’s ammeter and pressure gauge, if fitted. For maximum safety of theequipment and the operator, the pressure gaugeshould be located between the end of the screw athe breaker plate.

Full MachineSometimes the extruder has been shut downwhen filled or partially filled with Hytrel or otherpolymer. In this case, the melting point of theresidual material should be considered duringstart-up.

Be careful when starting a full machine to prevenbridging in the feed section, localized overheating(which may result in polymer degradation), andcold spots (plugs of unmelted resin occurringprimarily in the adaptor or barrel extension). Ensuthe die and head reach temperature before theflange and barrel to allow for thermal expansion opolymer as it melts.

Set all controllers (with the exception of the feedsection zone which can be set to its normal runnintemperature) 10–20°C (50–70°F) above the nomi-nal melting point of the grade to be processed (reto processing data on page 22). If the residualmaterial in the barrel has a melting point that issubstantially above that of the new resin, then thecontrollers should be set higher than the meltingpoint of the old material.

When the controllers have reached these tempertures and have been held for 20 to 30 minutes,slowly increase screw speed up to about 10 rev/muntil molten polymer flows from the die. It isimportant at this time to check for any excessive dpressure or amperage that may indicate a plug ofunmelted resin. Start feeding fresh resin through hopper, again checking for excessive pressures omotor amperage. Run the extruder slowly whilepurging with fresh resin for up to 30 minutes, oruntil a flow of clear molten polymer is obtained.

During this process, the screw speed should occasionally be increased to normal operating speedsabove, for short periods. This will help release anold degraded or unmelted resin from the internalsurfaces of the extruder.

When smooth flow of clear molten polymer hasbeen obtained, all temperatures should be returneto normal running settings, and slow purgingcontinued until temperatures have restabilized.

At this point, melt temperature should be checkedafter setting screw speed to desired running spee

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Shutdown and Purging ProceduresFor brief shutdowns of 30 minutes or less, no actiis required, other than a short purge with new resafter start-up.

If the extruder is to be shut down for longer peri-ods, empty the barrel and turn off the heat controlers. If the extruder is so equipped, barrel coolingmay be used to cool the residual melt rapidly andprevent polymer degradation. During the next staup, any material contained in the screw should beexpelled and not used. Venting of gases that maygenerated should be considered (see “SafetyPrecautions”).

Purging with polyethylene or other polymers is nonormally advised, except when other resins fromprevious running need to be removed, or just prioto strip-down of the equipment for cleaning (see“Equipment Cleaning”). It can take a long periodafter start-up to completely eliminate traces ofpolyethylene from the Hytrel.

Any purging should be done with temperatures seto a value that is above the melting point of theresin being purged.

Special purging compounds (e.g., cast-acrylicresins) may be used to purge the extruder. Sincethese cross-linked materials do not melt but onlysoften, it is necessary to remove the die, barrel,screens, and breaker plate before purging. If this not done, unsafe amperages and pressures mayresult that may damage the machine or injure theoperators.

Equipment CleaningOccasional dismantling and cleaning of theextruder screw, adaptor, head, and die componenis recommended. The optimum frequency of thesstrip-downs will depend on the number of start-upshutdown operations and the number of differentresin changes that have occurred.

Cleanout procedure consists of purging the extrudwith polyethylene or polystyrene and then remov-ing the die, adaptor, and head from the extruder.With the head removed, the screw and barrel maythen be cleaned using a cast acrylic type of purgecompound.

Complete removal of the screw for thoroughcleaning is essential from time to time, since it isthe only way to ensure that hard particles ofdegraded polymer and other residues are properlremoved from the screw and barrel surfaces.

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The larger quantities of resin can be removed frothe screw and other components by scraping whistill hot. This may be followed by wire brushing orthe use of special scouring pads. A rotating wirebrush attached to an extension rod on a suitablepower tool should be used to clean along the fulllength of the extruder barrel.

Stubborn residue may be removed from head andie parts by burning with a propane torch; thismethod, however, is not generally encouraged siflammable and toxic gases may be formed. A musuperior method is to immerse parts in a hotfluidized bed that is specially designed for thepurpose. Such equipment uses hot air fluidizedaluminum oxide, into which the parts to be cleaneare lowered in a wire basket. With suitable fumeextraction, this method is fast and thorough, andparts are left ready for reinstallation.

Recycling of ScrapThe unusually good thermal stability and com-pletely thermoplastic nature of Hytrel allow reuseof scrap from the extrusion process. Hytrel can bground and blended with virgin polymer at a leveup to approximately 50%, assuming the polymerhas been correctly processed. At all times exerciscare to ensure that reground polymer has not beedegraded, and is free from foreign matter.

Chop scrap into chips approximately the same sias the original pellets. Use a scrap grinder with wadjusted, sharp knives to produce clean, sharpregrind. Dry all regrind and blend well with virginpolymer to ensure uniform quality.

Melt flow rate check is a practical way to monitorquality of regrind on representative samples, anda useful quality control tool for both finishedproducts and regrind. Melt flow rate, in effect,measures restricted flow of molten polymer. Thehigher the index, the lower the viscosity and,therefore, molecular weight—thus an indication opolymer degradation.

As a general rule, scrap material should not bereused if the melt index check shows a value thamore than about 50% higher than the value for thvirgin material.

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Extrusion ProcessesGeneralMany different extrusion processes have beensuccessfully used with Hytrel. Along with theprocessing conditions for individual Hytrel grades(see page 22), there are some general points thaapply to all grades and to most extrusion process

The melt viscosity of the Hytrel extrudate dependon the melt temperature. The melt temperaturesslightly above the nominal melting point give thehighest viscosity, and therefore the easiest handlin most extrusion processes. Typically, the actuamelt temperature should be 5 to 15°C (41 to 60°F)above the nominal melting point, when measuredwith a needle pyrometer held in the melt flow, witthe extruder running at normal operating speeds.

The temperature profile used to achieve this willdepend on the individual extruder, and to a largeextent on the screw design and throughput. However, it is important that temperatures in the adaptor, head, and die zones should at all times be sea value that is at least equal to the nominal meltinpoint for the grade of Hytrel being processed,otherwise cold slugs and skins of unmelted matemay accumulate on the internal metal surfaces aresult in the release of lumps of cold or degradedHytrel into the melt flow. In the extreme, a coldplug can form that leads to a potentially dangeroumelt pressure condition.

Because of the viscosity/temperature relationshipis important that good control of temperature ineach zone is maintained. This depends, not only having properly maintained and calibrated instru-mentation, but also on good operating practices. example, a large fluctuation in ambient air tempeture caused by opening an adjacent door caninfluence extruder head temperatures and therefomelt viscosity.

Attention to detail in other parts of the extrusionprocess can also help prevent potential problemsSuch factors as changing cooling water tempera-ture, variations in haul-off speed, vibrations inmechanical equipment, and fluctuations in electrical supply voltage or plant water pressure, have been known to cause problems that may then bewrongly attributed to the material or extruder.

ProfilesSolid and hollow profiles may be extruded succesfully with Hytrel grades, with various complexityand shape requirements.

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By Free ExtrusionFor simple shapes, such as solid round or “V” beltprofiles, a free extrusion technique is the best. Thtechnique employs an inexpensive die that can, inmany cases, be made as a flat plate die (usuallyaluminum) that fixes to the front of the extruderhead. Although these plate dies have the disadvatage of encouraging a buildup of stagnant polymeat the rear of the plate, which will eventuallydegrade and cause problems during long extrusioruns, they are nevertheless useful for prototypingand short production runs (provided they aredismantled and cleaned after each run).

Plate dies should have a thickness of 6 to 12 mm(0.25 to 0.5 in), depending on the profile size, andshould be reduced locally to 3–5 mm (0.12–0.2 inwhere the profile may have a section that is signifcantly thinner than the main profile. The actual cuout dimensions of the die orifice should be approxmately twice the dimensions of the finished profileto provide enough draw-down of the melt duringthe extrusion process. This draw-down is necessato produce sufficient tension in the extrudate toprevent sagging and fluctuations in dimensions.Normally, some modifications to the die orifice arenecessary after initial trials, in order to achieve therequired distribution of material. In particular, it isusual to open out, or chamfer any internal cornersat the back of the die opening by careful hand filinor grinding, to encourage more flow in these area

Where necessary, the design of more permanentstreamlined dies can be developed from the shapof the plate die prototype. Note, however, thatchanging from one grade of Hytrel to another, orchanging the melt temperature or extrusion speedmay affect the final profile shape.

While free extrusion is acceptable for simple“solid” shapes, it may not be possible to achieve tdesired profiles with this technique where compleshapes or sudden changes in section are required(e.g., “U” channels or ribbed profiles).

In such cases, it may be possible to arrange suitaguides or supports within the water bath that “holdthe shape of the extrudate until sufficiently solidi-fied. However, this technique may still not beadequate, particularly with the harder Hytrel gradewhere differential shrinkage can cause warping andistortion of certain sections of a profile. Vacuumcalibration may therefore be more successful forcomplex profiles with the harder Hytrel types.

Note: The best grades of Hytrel for free extrusionhave been found to be the higher viscosity, softergrades, particularly when extruded at the lowestpossible melt temperature.

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By Vacuum CalibrationThis technique, which is well established for morerigid polymers such as rigid PVC, has been foundto be moderately successful with the harder Hytregrades. The softer types exhibit too much frictionagainst the calibration die surfaces and tend tostick.

For the harder grades, a “wet” calibration diesystem should be used, where the extrudate islubricated by a film of water supplied through aseries of small (0.5 to 1.0 mm [0.020 to 0.040 in]diameter) holes drilled close together around theentrance to the calibration die.

The entrance should be radiused (3 to 5 mm [0.1to 0.2 in]) and the whole internal surface finishedby sandblasting. Alternatively, a Teflon®

fluoropolymer resin coating may be applied tocalibration surfaces to reduce the tendency for thextruded profile to stick to the die.

Extruder die design should follow the principlesoutlined for free extrusion although less draw-dowshould be used. Here again, plate dies can be usfor prototype trials and short production runs.Where more streamlined machined dies are usedthe die land length should be around 5–10 times profile thickness.

MonofilamentsMonofilament extrusion is straightforward with themedium viscosity Hytrel extrusion grades.

Equipment normally used with resin such as nyloand PBT may also be used for Hytrel. Processingtemperatures may be somewhat higher formonofilament extrusion than for other extrusionprocesses. Typically, melt temperatures of about15–20°C (60–70°F) above the nominal meltingpoint of the particular Hytrel grade should be useThe draw-down ratio, as measured by the diediameter relative to the monofilament diameter

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Figure 5. Free Extrusion of Tubing

Die Adjustment Bolts

Cool

ExtruderControlled Air Supply S

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leaving the first water quench bath, should bebetween 4:1 and 10:1.

The amount of orientation and temperatures forstretching and annealing should be determined fothe specific Hytrel grade being used, and by theend-use property requirements.

Free Extrusion of TubingSmall size tubing can be prepared from Hytrelpolyester elastomer by free extrusion.

The method is most generally used for tubing up about 6 mm (0.25 in) O.D. Larger sizes can bemade by the differential pressure sizing method(also called vacuum-sizing—see “Vacuum Calibrtion of Tubing”).

A typical setup for free extrusion is shown inFigure 5. The process consists of extruding a tubof resin and pulling it through a trough of coldwater. One or more precisely sized metal rings mbe installed under the water surface to assist inmaintaining tube shape. The process is inexpenssince a variety of sizes can be made from one dieand mandrel combination by simply varying thedimension of the sizing plate(s), screw speed, takoff rate, and internal air pressure (if used).

In order to prevent collapse of the tube during freextrusion, it is sometimes necessary to supply airunder regulated pressure through the mandrel toas internal support. A very sensitive control valveor manostat must be used to minimize smallpressure variations that can result in variations intubing diameter.

Selection of extruder die and pin (mandrel) diam-eters for a required tube dimension depend on thamount of draw-down to be used. For most Hytregrades, satisfactory results have been obtainedwhen the die and pin are chosen with diametersapproximately twice those of the tube outside andinside diameters, respectively.

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Take-off Uniting Trough Tubing Windup

izing Rings optional)

pump

sump tank

To Drain

Extruder

air water

Bleed Valve Vacuum Tank

Caterpillar Haul-off

Windup Unit

Cooling Trough

Sizing Die

Differential Pressure Sizing Unit

Figure 6. Tubing Extrusion with Differential Pressure Sizing

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Vacuum Calibration of TubingThe free extrusion method is not generally used fHytrel tubing of about 6 mm (0.25 in) diameter orabove because roundness is difficult to control. Tmost popular procedure for sizing larger tubing isthe differential pressure or vacuum sizing tech-nique. A typical setup is shown in Figure 6. Thismethod requires no internal gas pressure to suppthe tube, permitting the tubing to be cut to anylength without disrupting the process.

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Figure 7. Tubular Sizing Die

Front Plate

Water Inlet

Annular Water Outlet

Slit

Water Channel

Tube Exit

Flange for Fixing to

Vacuum Tank

Vacuum Tank

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One method of calibration is by means of a stackaluminum or brass plates each containing a pre-cisely drilled hole, but the preferred system is atubular sizing die generally fabricated of brass witperipheral holes drilled in the tube wall to allow thsurrounding vacuum to act on the extruded tube.The inside should be sandblasted and containshallow grooves or “rifling” to decrease the surfacdrag of the extruded tube. Both the plate die and tubular former are normally made between 3 and15% oversize to compensate for shrinkage of theextrudate. A die that has been found to work wellwith all Hytrel grades over 40D hardness is showin Figure 7.

Suitable lubrication must be provided between theextruded polymer and the metal surface of thecalibrator. Usually, this can be accomplished by afine water flow through small holes at the front ofthe tubular die or through an annular water ringdevice at the entrance to the die.

Typically, calibration dies should be 3 to 5% largefor tubes that are to be extruded at moderate linespeeds (up to about 25 m/min [80 ft/min]), whilesmall diameter tubes (under approximately 12 mm[0.5 in] diameter) that are to be extruded at highespeeds will require a calibration die 10–15%oversize.

In general, 35 and 40 Shore D Hytrel grades cannbe run successfully by the differential pressuresizing method using either the plate die or thetubular die. Their very rubbery nature and slowercrystallization rate causes the polymer to bind or“grab” in the sizing die. In certain cases, howeverit has been possible to extrude large diameter, thwall tube in these grades, using a calibration dieunder very low vacuum.

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The correct extruder die and pin should be selecas follows.

1. The extruder die should be between 2 and2.5 times the required tube outside diameterfor tubes up to about 20 mm (0.8 in) diameteextruded at moderate line speeds (up to25 m/min [80 ft/min]).

Where larger diameters are to be made, orwhere small tubes are to be extruded at highspeed (over 25 m/min [80 ft/min]), then theextruder die should be between 1.5 and 2 timthe tube outside diameter.

2. The extruder pin should then be selected sothat the pin should be larger than the requiretube inside diameter by a factor that is about90% of that used to determine die diameter.

For example, to extrude a tube having 8 mm(0.3 in) outside diameter and 6 mm (0.2 in) insiddiameter, at a line speed of 30 m/minute:

Extruder die diameter = 8 × 2.0 = 16 mmTherefore extruder pin = 6 × 1.8 = 11 mm

Selection of pin and die according to these guidelines will result in a draw-down ratio of about4 to 1 (draw-down ratio is defined as the ratio ofthe cross-sectional area of the extrudate as it leathe extruder die to the cross-sectional area of thefinished tube). Draw-down ratios of between 3 toand 6 to 1 have been found to be optimum forHytrel tubing extrusion using the vacuum sizingtechnique.

In all vacuum sizing operations there should beprovision for fine adjustment of the vacuum, inorder to accurately control and maintain the extenal diameter of the extruded tube. Vacuum of2–10 in/Hg has been used successfully to maintaaccurate dimensional control of small diameterHytrel tubes.

Other factors that are worth special attentioninclude:

• Optimum screw design to give constant outputrate and minimum variation in melt temperaturThis is particularly important for high speedtubing extrusion, and where accurate dimensiomust be maintained over long lengths.

• Constant pressure lubricating water supply to tsizing die, preferably from a header tank situatat least 1.5 m (4.9 ft) above the die.

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• Constant water temperature in the vacuum bath,either by use of a heat exchanger and temperatucontroller in a closed loop system, or provision oadequate discharge and fresh water makeup in aopen system.

• Uniform granule feed temperature and moisturecontent. This is best achieved by the use of ahopper drier system or a drier with automatictransfer to a closed hopper.

Cover Extrusion (“Cross-Heading”)Most Hytrel grades have been used to cover variotypes of product, including hoses, ropes, cables, awires. The basic equipment required includes anextruder, fitted with a suitably designed cross-heaa pay-off (or unwinding) system, incorporating abrake or tensioning device, a water cooling bath,and a variable speed haul-off or capstan, followedby an automatic windup or coiling system.

The cross-head and die arrangement can be one two types:

• Cross-head with pressure (packing) die, or• Cross-head with tubing (sleeving) die.

Pressure die extrusion involves the extrudedmaterial coming in contact with the core (e.g., hoswire, etc.) within the extruder head, which results isome pressure being applied to the melt so that ittends to penetrate any interstices in the core material. This technique is preferred where good adhe-sion is required, or a smooth, regular, outer coverdiameter is required over an irregular, or rough,inner core material.

Figure 8 shows a typical pressure die arrangemen

The diameter of the die should be approximately5% greater than the required cover diameter. Theland length (E) should be equal to the final coverdiameter, but considerably less for very thin cover(below 0.5 mm [0.020 in]).

The clearance between the core to be covered, anthe bore of the “torpedo” or mandrel, should bebetween 1 and 5% of the core diameter (dependinon the material and regularity of the core surface).

The distance between mandrel tip and die entranc(F) should be adjustable, but is normally set to beequal to or greater than the cover thickness to beapplied.

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Figure 8. Extruder Head for Pressure-Die Covering

A B C

F E

A

A

B

B

C

C

D

= = =

Die Mandrel Melt

D E F

= = =

Core material (hose, wire, etc.) Land length Distance between mandrel and die

Figure 9. Extruder Head for Tubing-Die Covering

A B C

A

A

B

B

C

C

D

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Die Mandrel Melt

D E

= =

Core material (hose, wire, etc.) Land length

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Tubing die (sleeving) extrusion is illustrated inFigure 9. In this technique, Hytrel is extruded inthe form of a tube, and is “drawn-down” to meetthe surface of the core material. This is oftenachieved with the assistance of a vacuum that isapplied to the inside of the torpedo and acts throuthe bore of the mandrel (pin). The optimum draw-down ratio (DDR) is in the range of 5 to 20:1, andthe die diameter and mandrel diameter can becalculated from the selected draw-down ratio asfollows:

DD2 – DM2

DC2 – DW2

Where DD = Die diameterDM = Mandrel diameterDC = Diameter of covered core

(rope, cable, etc.)DW = Diameter of uncovered core

The advantages of tubing die extrusion are bettercontrol of cover wall thickness and easier to “stripcover from core (e.g., wire coating applications).

Some other points that may be important in crosshead extrusion with Hytrel:

• If a thermally conductive core material is beingcovered (e.g., electrical conductor, steel braidhose, etc.), it may be necessary to preheat thecore by passing through a flame or hot-air tunnebefore entering the cross-head. This will help toprevent too rapid freeze-off of the melt when it

DDR =

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comes in contact with the metal core, which maygive insufficient penetration of the core materialSimilarly, fibrous cores (e.g., textile rope orbraiding) may need to be dried by passingthrough a hot-air tunnel, or storing in a warm areprior to covering, in order to avoid moistureblisters appearing through the Hytrel cover.

• For high speed extrusion, particularly with thinwall covering, it may be necessary to raise thehead and die temperatures, and possibly also thbarrel temperatures, to achieve sufficient flowrate at acceptable melt pressure. Melt tempera-tures up to 40° above the nominal melting pointof most Hytrel grades can be safely used toreduce melt viscosity, providing the design ofadaptor, head, and associated parts does not girise to any hold-up spots where thermal degradation might take place.

• Although water temperature in the cooling bath generally not critical (10–20°C [50–68°F] isnormally used), there may be cases where a mogradual cooling of the Hytrel cover by using hotwater (e.g., 60°C [140°F]) may be beneficial.This is sometimes the case with optical fibersheathing, or small diameter electrical conduc-tors, where fast crystallization caused by coldwater may result in undesirable stresses in Hytrwhen it cools.

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Cast Film, Sheeting, and FabricCoatingCast FilmIn the cast film process, molten Hytrel is extrudedthrough a slit die onto a polished metal roll—usually known as the “quench roll” or “chill roll”—which serves to quench the hot melt. From thequench roll, the film passes around a series of othrolls designed to guide and keep it wrinkle-free atwindup. Figure 10 illustrates a typical cast filmline. This process provides a high degree of controover film properties, particularly optical clarity.

Film thickness is controlled by the relationshipbetween the extruder output and surface speed ofthe quench roll.

Films of Hytrel engineering thermoplastic elas-tomer as thin as 0.013 mm (0.0005 in) have beenmade by this method.

The quench roll is normally cooled internally bywater or water/glycol mixtures. With Hytrel, hotwater or hot oil may be used to control quenchtemperature for some applications (see discussionof effect of quench temperature on film properties)Tension control must be precise, and very light witthe more flexible grades of Hytrel, in order toproduce wrinkle-free film with good roll conforma-tion (flatness across the roll). Air jets can be used pin the edges of the melt web to the chill roll, inorder to minimize edge weaving and reduce neck-in. Take care that none of the air flow reaches thedie lips, since this can result in uneven cooling ofthe die and poor dimensional uniformity in the film

14

Figure 10. Typical Cast Film Unit Setup

Extruder

Chill Roll

Slit

Chill Roll Tangent

Strip Roll

Sheeting Die

Water Cooling (individual)

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The processing conditions of melt temperature,quench temperature, air gap, and extrusion rate influence film properties, but quench temperatureand air gap have the greatest effect. The followingeneral statements indicate how each of theseconditions affects film properties; the degree towhich the property is affected depends on the grof Hytrel used.

• Increasing melt temperature– Increases transparency– Increases gloss– Decreases haze– Decreases modulus and yield strength

• Increasing quench temperature– Increases modulus and yield strength– Increases haze– Decreases gloss– Decreases transparency

• Increasing air gap– Increases haze– Decreases gloss and transparency– Increases modulus and yield strength

• Increasing extrusion rate– Decreases haze– Increases transparency and gloss

To obtain maximum film clarity, a quench rolltemperature below 0°C (32°F) may be necessary.Film having high modulus and high haze can beobtained by increasing quench roll temperature.

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Rubber Pull Rolls

Idler

Turret Wind

Dancer- Lay-On

Trim Removal

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To avoid sticking, quench temperature should notexceed 50°C (122°F) for soft Hytrel types (35Dand 40D hardness grades), 80°C (176°F) for 47Dto 63D hardness types, or 100°C (212°F) for 72Dand harder Hytrel grades.

SheetingThe term “sheeting” normally describes materialthat is 0.25 mm (0.010 in) or greater in thickness.Sheeting of Hytrel up to about 0.5 mm (0.02 in)thick can be produced on the same equipment usto make cast film, depending upon the die angleand the ability to achieve a slight, controlledsticking to the quench roll. Sheeting of greaterthickness is made on a three-roll finisher, as showin Figure 11. Both mechanical and air pressuresystems for applying roll force have been used.

As in other extrusion processes, die design andtemperature control are critical.

The die lip opening should be 10 to 20% greaterthan the thickest sheet to be produced; a thinnersheet can be made with the same die opening byincreasing roll speed. Keep melt temperature as loas possible, consistent with uniform extruderoutput. The air-gap should be the minimum permited by equipment geometry. The melt puddle, orbank of surplus resin between the roll “nips,”should also be kept small to minimize oxidativedegradation, but if the bank is too small, resultant“starving” of the rolls will cause erratic dimen-sional variations in the sheet.

Roll temperatures should be individually con-trolled. Typical roll temperatures for Hytrel sheet-ing are as follows:

Type of Hytrel(Shore D Hardness) Roll Temperature, °C (°F)

35D and 40D 15 to 30 (59 to 86)47D to 82D 40 to 70 (104 to 158)

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Figure 11. Extrusion of Sheeting

Die

Individually Heated or Cooled

Extruder

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The temperature of the upper roll will be limited tothe temperature at which the sheet sticks to the roand is normally kept as low as possible.

Fabric CoatingFabric coating with Hytrel can be achieved on athree- or four-roll coating line that is similar inprinciple to the sheeting extrusion line shown inFigure 11. The fabric is introduced above theextruded Hytrel, between the top and middle rolls.A more common setup is the standard extrusioncoating line shown in Figure 12. In this process themelt is extruded through a slit die onto the fabric oother substrate in the nip between the chill andpressure rolls.

The melt contacts the substrate just prior to meetithe chill roll that solidifies or quenches the melt.The melt web usually extends beyond the edges othe substrate slightly, and therefore, contacts boththe chill and pressure rolls. To avoid any tendencyto stick on the pressure roll, it has proved effectiveto cover rolls with a coating of Teflon FEP.

A chill roll of 300 to 600 mm (12 to 24 in) indiameter is usually used in extrusion coating. Thetemperature of the chill roll is normally maintainedbetween 20 and 40°C (68 and 104°F).

The stability of the melt web before it contacts thechill roll depends greatly on having the correctdesign of extrusion die, as well as on the melttemperature, the air gap between die and roll nip,and on the line speed.

As for cast film extrusion, careful control of tensionin the windup is very important to minimizewrinkling and ensure good roll conformation.

A successful coating operation depends on manyfactors, not least of which is the skill and experi-ence of the line operator.

5

Pull Rolls

Trim Roll Tension Rolls

Windup

Trim Knife

Tension Nip

LaminatorPreheatFloat RollTurret Unwind Platform Turret Winder

Chill Roll

Chilled Stripper Roll

Unwind-Laminator

Backup Roll

Pressure Roll

Figure 12. Typical Extrusion Coating Line (Substrate is fed from the turret unwind at left. After preheating, itis coated by extruder at laminator station (center). After cooling, stock is wound at right.)

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Blown FilmHytrel may be processed with standard film blowing equipment such as that shown in Figure 13.The higher melt strength of Hytrel blow moldinggrades make them easier to handle and allow filmup to 250 microns (0.01 in) to be produced withblow-up ratios of up to 3:1. Other Hytrel grades,particularly the softer 35D and 40D hardnesstypes, can also be blown into films up to about150 microns (0.006 in) thick, at blow-up ratios upto approximately 2.8:1, but antiblocking agentssuch as Kemamide B1 or Crodamide SR2 (or similarproducts) may be required to prevent sticking of film to itself and to the rolls.

CoextrusionCoextrusion of Hytrel with several other resins habeen successfully evaluated for applications, sucas hose and tubing, profiles, sheeting, and film. Ithis technique, Hytrel is brought together with thesecond resin within a single die, where the twomaterials are fused to form distinct, well-bondedlayers in a single extruded product.

1Product of Humko Sheffield Chemical Division, P.O. Box 398,Memphis, Tennessee, U.S.A.

2Product of Croda Universal Limited, North Humberside, DN14 9AAEngland.

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Figure 13. Extrusion of Blown Film

Blown Tube

Collapsing PlateWindup

Gusset Bars

Guide Rollers

Mandrel

Cooling Ring

Air Inlet

Adjustable Section of Die

Die

Air Supply

Extruder

Driven Pinch Rolls

Valve

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By using coextrusion, the finished product cancombine the advantages of two or more polymersa cost-effective way.

The successful processing of Hytrel in coextrusiodepends on the following factors.

• Compatibility of the two resins in terms of fusionor “weldability.”

• Closeness of melting points or normal processintemperatures.

• Similarity of flow properties within the die, andof shrinkage and rate of crystallization effectsafter leaving the die.

• Design of the coextrusion die.

Experience has shown that Hytrel is extremelycompatible with most rigid and flexible PVCcompounds, and equipment normally used tocoextrude rigid and flexible PVC has given goodresults with Hytrel. The lower melting point gradesof Hytrel generally give best results.

1

Other polymers that have been successfullycoextruded with Hytrel include Alcryn® syntheticrubber, Rynite® thermoplastic polyester resin, PBT,and PET. Resins that are not normally consideredbe compatible may still be coextrudable, providedthat a suitable intermediate or “tie” layer is usedthat provides a bond between the other materials

The use of a multimanifold die design, where thedifferent melt streams enter the die separately anjoin just before the final die orifice, has been founto give maximum freedom in terms of choice ofpolymers. Such dies usually result in less distortioat the material interface, and better control ofindividual layer thicknesses, particularly whenusing polymers having large differences in flowproperties.

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18

Troubleshooting Guide for Extrusion of Hytrel

• Preheat or dry substrate• Clean substrate• Increase extrusion rate• Decrease air-gap• Apply vacuum through mandrel

• Reduce temperatures• Increase extrusion rate• Check for stagnation (dead spots)

in extruder or die• Check operation of heaters, controllers,

thermocouples• Use smaller extruder to decrease holdup

time• Use proper screw• Allow extruder to run for several minutes

after start-up• Increase rear barrel temperature• Use proper screw• Increase back pressure• Check controllers• If resin has become wet, dry it before

extruding

Problem Probable Causes Suggested Solutions

Blisters (on surface) • Substrate contains volatilewhen coating substrates substances (water, oil, etc.)

(e.g., when covering textilematerials)

Bubbles • Resin degradation due to hightemperatures or long holduptime, perhaps after extruderstoppage

• Air entrapment

• Moisture in resin

Buckled extrudate See Out of roundness—folded or buckled extrudate

Concentricity, lack of See Out of roundness—poor concentricity or deformed extrudate

Cone breaks See Pinholes, lumps, tears, splits, or cone breaks

Contamination • Improper resin handling• Extruder not properly cleaned

• Poor regrind quality• Inadequate screen pack

• Uneven/insufficient melt tension

• Burr or other imperfection ondie face or pin

• Draw rate too high (wire covering)

• Draw-down ratio too high (tubes)• Uneven shrinkage (profiles)

Deformed/folded/warped extrudate

• Keep resin clean• Clean extruder, remove all traces of

other resins or degraded Hytrel• Use only clean dry regrind

• Fit proper screen pack

• Increase draw-down ratio• Reduce melt temperature• Adjust die centering (tubes)• Hole in mandrel may be too large and

should be made smaller (cross-headcovering)

• Remove imperfection

• Reduce draw rate by lengthening cone(reduce vacuum)

• Reduce draw-down ratio• Improve support of profile in water

bath• Reduce melt temperature• Change to low shrinkage/slow

crystallizing grades of Hytrel

(continued)

19

Problem Probable Causes Suggested Solutions

• Extruder output surging• Variations in take-off speed

• Temperature cycling

• Draw-down ratio too low• Variations in diameter of

substrate (covering)• Excessive drag on sizing die or

sizing plates (tubing extrusion)

Frozen particles in See Unmelted or frozen particlesextrudate in extrudate

• Too rapid cooling

• Cone too long (cools beforedrawn down onto core)

• Die out of round, pin/mandrelout of round or bent

• Covering sags before freezingor extrudate sags before enteringwater bath or sizing die (tube)

• Pressure applied by haul-off belts orother take-up equipment is too highand causes deformation of theextrudate

• Uneven cooling rate

• Die out of adjustment• Pin or mandrel too flexible

(this will generally give variableconcentricity)

• Hole in mandrel too large for coreor wire being covered

Diameter variationsalong the extrudedlength (roundprofiles, tubing, orcross-head covering)

• See Surging• Check speed control of haul-off• Increase pressure on caterpillar haul-off

belts• Use proportioning controllers• Check operation of controllers including

setting or proportional bandCheck constant material temperature inhopper (use of hopper drier/pre-heatermay help)

• Increase draw-down ratio• Check substrate

• Check design of sizing die• Reduce vacuum on sizing die• Adjust air gap before entering sizing die• Increase lubricating water flow to front

of die

Troubleshooting Guide for Extrusion of Hytrel (continued)

Loose coatings(cross-head covering)

• Lengthen air-gap• Reduce extrusion rate• Shorten cone by increasing (or apply-

ing) vacuum through pin/mandrel

Out of roundness—poor concentricity ordeformed extrudate(tubes or cross-headcovering)

• Replace or remachine die or pin

• Generally, increase melt tension– Reduce temperature of melt– Increase the rate of draw-down by

increasing the extrusion rate, short-ening the cone length (more vacuumthrough mandrel), or increasing thedraw-down ratio

– Reduce air-gap between die andwater trough or sizing die

• Reduce belt pressure• Reduce payoff tension in cross-head

extrusion• Increase cooling capacity to ensure that

the extrudate is cold before it contactshaul-off, capstans, etc.

• Adjust die centering• Ensure uniform water cooling around

extrudate• Adjust die centering• Redesign pin• Use a shorter pin

• Reduce size of hole in mandrel

(continued)

20

Problem Probable Causes Suggested Solutions

Overloading the extruder • Rear temperature too low

• Incorrect screw design• Restrictions in head or

adaptor/screen pack area

• Contamination• Excessively high draw-down ratio• Temperature of extrudate too low• Poor dispersion of fillers or

pigments or too high a loadingthereof

• Lumps of degraded resin releasedfrom within head or die

Shrinkback, excessive • Orientation too great during(wire covering and other draw-downcross-head operations)

Splits See Pinholes, lumps, tears, splits, or cone breaks

Surface roughness • Melt fracture (“sharkskin” orroughness on surface)

• Die imperfections

• Contamination

• Wire or core vibration(cross-head covering)

• Core/substrate not smooth• Buildup on die face

• Resin degradation(To determine if roughness is dueto melt fracture or degradation,stop the screw momentarily. If thereis melt fracture, it will disappearwhen the screw is stopped.Symptoms of degradation [bubblesand discoloration] will persist orbecome worse)

• Moisture in resin

• Die temperature too low

• Raise rear temperature• Check thermocouple and controller of

rear zone• Use correct design screw• Check for cold plugs, etc.• Increase adaptor/head temperatures• Check design of head

Pinholes, lumps, tears,splits, or cone breaks

• See Contamination• Reduce draw-down ratio• Raise melt and die temperature• Improve blending procedures

• Clear head and die• Ensure head design is streamlined with

no “hold-up” spots or damage tosurfaces

• Decrease draw-down ratio• Decrease quench rate (lengthen air-

gap/use hot water quench)• Preheat wire/core material• Raise melt and die temperature

• Generally, reduce shear in die by– Reducing extrusion rate– Increasing die temperature– Increasing melt temperature– Increasing die opening/increasing draw-down ratio

• Check for burrs, etc., and remove• Check for good finish on die and pin• See Contamination• Use guides or pads to dampen

vibration

• Check core/substrate• Keep die face cleaned off• Increase die temperature• See resin degradation as a cause of

Bubbles

• See Bubbles• Increase die temperature

Troubleshooting Guide for Extrusion of Hytrel (continued)

(continued)

21

Problem Probable Causes Suggested Solutions

Surging • Inadequate melt back pressure

• Temperature variations• Bridging in feed section

(intermittent)

• Bridging in transition section

• Slippage in haul-off, or speedvariation

• Barrel or screw wear

Tears See Pinholes, lumps, tears, splits, or cone breaks

Unmelted or frozen • Barrel temperature setting tooparticles in extrudate low

• Heater watt density too low

• Compression ratio of screw toolow

• Inadequate screen pack• Cold spot(s) in extruder

Voids (in center of section) • Use of high shrinkage/fast• Too rapid cooling crystallizing grade• Section too thick

• Use higher screw speed• Check screw design• Check temperature controllers• Prevent bridging by reducing rear

temperatures• Check controllers in feed zone• Use cooling water in feed throat• Make rapid screw speed changes to

dislodge bridge• Increase temperature in rear and

center-rear barrel zones• Use screw with longer feed section• Tighten belts, check speed

• Refurbish barrel and screw

• Raise controller settings

• Change heater bands to increasewattage

• Decrease extrusion rate• Change screw to recommended design• Increase screen pack density• Increase screen pack density

• Check operation of heaters, controllers,and thermocouples; recalibrate ifnecessary

• Raise temperatures or supply additionalheater capacity to barrel extension,adaptor, breaker plate area, or die; useseparate controllers for these areas

• Increase back pressure with screenpack

• Reduce die opening• Change temperature profile—raising

rear temperature, reducing fronttemperatures may help

• Reduce cooling rate (use air cooling oralternate water/air)

• Redesign to reduce thickness• Use lower shrinkage or slower crystal-

lizing grade

Troubleshooting Guide for Extrusion of Hytrel (continued)

22

NominalType of Melting Center CenterHytrel Point** Rear Rear Front Front Head Die Melt

G3548W 156°C (313°F) 180°C 180°C 180°C 180°C 180°C 180°C 180°C4056 150°C (302°F) (355°F) (355°F) (355°F) (355°F) (355°F) (355°F) (355°F)

3078 170°C (338°F) 195°C 195°C 195°C 195°C 195°C 195°C 195°CG4074 170°C (338°F) (385°F) (385°F) (385°F) (385°F) (385°F) (385°F) (385°F)G4078W 170°C (338°F)

4069 193°C (379°F) 220°C 220°C 220°C 220°C 220°C 220°C 220°C4556 193°C (379°F) (430°F) (430°F) (430°F) (430°F) (430°F) (430°F) (430°F)

5556 203°C (397°F) 225°C 225°C 225°C 225°C 225°C 225°C 225°C5555HS 203°C (397°F) (435°F) (435°F) (435°F) (435°F) (435°F) (435°F) (435°F)

G4774 208°C (406°F) 230°C 230°C 230°C 230°C 230°C 230°C 230°CG5544 215°C (419°F) (445°F) (445°F) (445°F) (445°F) (445°F) (445°F) (445°F)6356 211°C (412°F)

7246 218°C (424°F) 240°C 240°C 240°C 240°C 240°C 240°C 240°C8238 223°C (433°F) (465°F) (465°F) (465°F) (465°F) (465°F) (465°F) (465°F)

HTR6108 168°C (334°F) 195°C 195°C 195°C 195°C 195°C 195°C 195°C(385°F) (385°F) (385°F) (385°F) (385°F) (385°F) (385°F)

HTR8139LV 192°C (378°F) 220°C 220°C 220°C 220°C 220°C 220°C 220°C(430°F) (430°F) (430°F) (430°F) (430°F) (430°F) (430°F)

HTR4275 196°C (385°F) 225°C 225°C 225°C 225°C 225°C 225°C 225°CHTR5612BK 196°C (385°F) (435°F) (435°F) (435°F) (435°F) (435°F) (435°F) (435°F)

Typical Temperature Profiles of Hytrel

Typical Extruder Temperatures*

**The processing conditions presented here are representative of those typically used or preferred. Rounded numbers areshown for both English and SI units.

**Differential scanning calorimeter (DSC) peak of endotherm.

23

(3/94) 236385A Printed in U.S.A.[Replaces: E-80327]Reorder No.: E-80327-1

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