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thermoplastic polyester resinCrastin PBT

Molding Guide

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Cra

stin

PBT

Copyright © 1999 E.I. duPont de Nemours and Company. All rights reserved.

StartwithDuPont

Table of ContentsIntroduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1

Crastin® PBT Thermoplastic Polyester Resin . . . . . . . . . . . . . . . . . . . 1

Safety and Handling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3Thermal Effects . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3Off-Gases and Particulates . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3Resin Handling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3

Drying . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4Conditions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4Equipment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4Vented Barrels . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4

Molding Equipment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5Screw Injection Molding Machines . . . . . . . . . . . . . . . . . . . . . . . . . . . 5Screw Design . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5Screw Check Ring Assembly . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5Wear . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5Nozzles . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5Pressure Control . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6

Molding Conditions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7Cylinder Temperatures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7Hold-Up Time . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7Mold Temperature . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7Fill Rate . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8Screw Speed and Back Pressure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8Purging . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8

Resin Characteristics and Part Design . . . . . . . . . . . . . . . . . . . . . 9Amorphous versus Crystalline Materials . . . . . . . . . . . . . . . . . . . . . . . 9Flow Properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9Shrinkage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9Dimensional Stability . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10

Mold Design . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13Sprues and Runners . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13Gate Position . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13Gates . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13Venting . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14Draft . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14Undercuts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15Runners Molds . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15Temperature Control . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15Nozzle Manifolds . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15Nozzles . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15Mold Temperature Control . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16Cores . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16

Regrind . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17

Troubleshooting . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18Moisture Content . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18Freezing Rate . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18Shrinkage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18

Introduction

Crastin PBT Thermoplastic Polyester ResinCrastin® PBT thermoplastic polyester resins are polybutylene terephthalate (PBT) resins modified to offerexceptional performance in a variety of demanding applications. Crastin® PBT resins are readily processedon conventional molding machines using standard industry practices. They are crystalline materials withsharp melting and freezing points, which have good flow characteristics and mold in fast cycles. Theprocessing details in this manual will enhance part quality and productivity.

While all types of Crastin® PBT are based on PBT, the product line can be divided into groups with com-mon performance advantages. By adding reinforcing agents and tougheners, the inherent characteristicsof PBT polymers like low creep, dimensional stability, and good electrical properties are augmented toprovide a family of molding resins with exceptional strength, stiffness, and toughness. Alloying with otherpolymers and selecting the proper reinforcement technology can improve dimensional stability and surfaceappearance. Most types of Crastin® PBT are also available in flame-retarded versions that are recognizedby Underwriters Laboratories as UL 94 V-0. Key features of the Crastin® PBT product line are describedbelow, along with some illustrative examples. For more detailed information on specific types, see theCrastin® PBT Product and Properties Guide.

General-Purpose ResinsCrastin® PBT formulations and compounding technologies are carefully selected to produce high-qualityinjection molding resins that generally outperform other PBT resins in strength, stiffness, and toughness.These resins are used in mechanical and electrical goods that require low creep, good stiffness at elevatedtemperatures, dimensional stability in humid environments, or good electrical properties. Typical examplesinclude the following:

Crastin® S610 Unreinforced. General-purpose molding resin, lubricated for mold release andgood feeding characteristics in molding screws. Available in colors. UL 94 HB.

Crastin® SK602 Glass-reinforced for additional stiffness and strength. UL 94 HB.Crastin® SK605 Lubricated and available in colors.

Crastin® S660FR Unreinforced. Flame-retarded version of Crastin® S610. UL 94 V-0 at 0.032 in.Lubricated and available in colors.

Crastin® SK652FR1 Glass-reinforced. Flame-retarded versions of Crastin® SK602 and SK605.Crastin® SK655FR1 UL 94 V-0 at 0.032 in. Lubricated and available in colors.

Toughened ResinsThe resilience and impact resistance of Crastin® PBT can be increased by employing proprietary tougheningtechnologies. Products range from Tough grades, with improved elongation and impact strength, to SuperTough products designed for severe mechanical abuse. Tough resins are especially well suited to partdesigns that require additional extensibility, e.g., snap fits. Super Tough resins are designed for high abuseapplications, especially those where ductile failure modes are required. Typical examples include thefollowing:

Crastin® ST820 Super Tough. Unreinforced. “No Break” in Notched Izod Impact tests. Use inCrastin® ST830FR applications where mechanical abuse is anticipated. Good flexibility and

recovery allow large deflection snap fit designs.

Crastin® HR5015F Glass-reinforced. Tough resins with high ultimate elongation. Can be used toCrastin® HR5030F solve cracking problems with glass-reinforced resins. Also flow well.

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Low Warp ResinsThe high shrinkage and glass fiber orientation effects that cause PBT parts to warp or distort are alleviatedin these resins. Polymer alloys are designed to afford low shrinkage while preserving the mechanicalproperties expected of PBT. Glass/mineral compositions rely on glass reinforcement for strength andmineral reinforcement for stiffness.

Crastin® LW9320 Glass-reinforced alloys. Combine improved flatness with good surfaceCrastin® LW9330 aesthetics. Especially useful in parts where the strength and stiffness of PBT

are required, but dimensional control is a problem. Also low specific gravity.

Crastin® LW685FR 30% glass/mineral-reinforced. Flame-retarded resin. UL 94 V-0 at 0.032 in andUL 94 5VA at 0.084 in. An excellent choice for box shapes or parts with largeflat sections.

High Flow ResinsProprietary technology allows dramatic increases in flow and reduced pressure drops in cavities whilemaintaining the strength, stiffness and toughness expected of glass-reinforced PBT resins. These resins willflow up to 50% further than standard PBT resins at comparable injection pressures. Especially useful in partswith delicate cores where reduced injection pressures and lower pressures in cavities can extend tool life.

Crastin® HF672FR Glass-reinforced and flame-retarded. Enhanced flow without losses inCrastin® HF675FR mechanical properties. UL 94 V-0 at 0.032 in with a 50% regrind rating.

Crastin® HR5015F Combine high flow with improved toughness and hydrolytical stability. SolveCrastin® HR5030F breakage problems with standard PBT resins. Especially well suited for snap fit

designs.

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Safety and Handling

While processing Crastin® PBT is ordinarily asafe operation, all of the potential hazards associ-ated with injection molding of thermoplastic resinsmust be anticipated and either eliminated orguarded against by following established industryprocedures. Consideration should be given to thefollowing.

Thermal EffectsBecause Crastin® PBT resins are molded at hightemperatures, the molten resin can inflict severeburns. Furthermore, above the melting point,moisture and other gases may generate pressure inthe cylinder, which, if suddenly released, can causethe molten polymer to be violently ejected throughthe nozzle.

Be particularly alert during purging and wheneverthe resin is held in the machine at higher than usualtemperatures or for longer than usual periods oftime—as in a cycle interruption. Pay particularattention to the section “Molding Conditions.”

In purging, be sure that the high volume (booster)pump is off and that a purge shield is in place.Reduce the injection pressure, and “jog” theinjection forward button a few times to minimizethe possibility that trapped gas in the cylinder willcause “spattering” of the resin. Immerse the purgedmaterial immediately in cold water in a metalcontainer to decrease the evolution of gaseousfumes.

If polymer decomposition is suspected at any time,a purge shield should be positioned, the carriage(nozzle) retracted from the mold, and the screwrotated to empty the barrel. After the screw starts torotate, the feed throat should be closed, and then asuitable purge compound should be introduced. Thetemperature can then be gradually lowered, and themachine shut down. If jogging the injection orscrew rotation buttons does not produce melt flow,the nozzle may be plugged. In this case, shut off thecylinder heats, and follow your established safepractices.

Always assume that gas at high pressure could betrapped behind the nozzle and that it could bereleased unexpectedly. A face shield and protectivelong-sleeved gloves should be worn at such times.

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In the event molten polymer does contact the skin,cool the affected area immediately with cold wateror an ice pack, and get medical attention for ther-mal burn. Do not attempt to peel the polymer fromthe skin.

Off-Gases and ParticulatesSmall amounts of gases and particulate matter (i.e.,low molecular weight modifiers) may be releasedduring the molding, purging, or drying of Crastin®

PBT. As a general principle, local exhaust ventila-tion is recommended during processing of allthermoplastic resins. For Crastin® PBT, a ventila-tion rate of 75 ft3 of air per minute per pound ofresin processed per hour will keep the concentrationof particulates below the OSHA limit (15 mg/m3)for nuisance dusts and will also be sufficient toremove any gaseous products if the resin is beingprocessed at or below the recommended maximummelt temperatures and residence times.

Crastin® PBT resins, like all thermoplastic poly-mers, can form gaseous decomposition productsduring long hold-up times at high melt tempera-tures. This is especially true of flame-retardedresins. Purged material should be immersed in coldwater to reduce evolution of volatiles.

Adequate exhaust ventilation should also be pro-vided during regrind operations and equipmentburn-out procedures.

Resin HandlingGranules of Crastin® PBT present a slipping hazardif spilled on the floor. They are cylindrical and havea low coefficient of friction. Spills should be sweptup immediately.

Care should be taken to prevent static dischargewhen resin is conveyed in pneumatic materialshandling systems.

Drying

Crastin® PBT should be dried prior to processingto remove moisture that can reduce the strengthand toughness of finished parts. In order to ensuregood performance of molded parts, the moisturecontent of resin must be less than 0.04 wt% duringprocessing.

ConditionsDrying to moisture level less than 0.04 wt% canbest be accomplished by using desiccant typedryers. Dryers should be sized to afford a residencetime of 2–4 hr at 250°F (120°C). Dryers should becapable of maintaining a dew point below –4°F(–20°C) with an air flow of at least 1 cfm per poundper hour. Typical drying curves are presented inFigure 1. Alternatively, resin can be dried at lowertemperatures using longer residence times (220°F[105°C]) for 4–8 hr.

Figure 1. Drying Times in Dehumidifying Dryers.Air Temperature 250°F (120°C) with DewPoint Under –4°F (–20°C)

0.3

0.2

0.1

00 1 2 3 4

Drying Time, hr

Mo

istu

re C

on

ten

t, w

t%

4

EquipmentDesiccant type dryers are required to achieveoptimum molding performance and part qualitywhen processing Crastin® PBT. These dryersphysically remove the moisture from the resin andtransfer it to a desiccant bed. Circulating air ovensrely on temperature to drive the moisture out of theresin and are only effective when the humidity ofambient air is very low.

Because the temperatures required to dry Crastin®

PBT are relatively high, dryers and air transferlines should be well insulated to ensure driereffectiveness and to conserve energy. Designs thatincorporate an after cooler (which lowers thetemperature of return air before it enters thedesiccant bed) are preferred because desiccants canremove and hold more moisture at lower tempera-tures. After coolers are required at drying tempera-tures over 250°F (120°C). Dryers should becapable of maintaining a dew point below –4°F(–20°C) with an air flow of at least 1 cfm perpound per hour.

Drier maintenance is very important to productionof high quality parts in a robust process. Connec-tions should be air tight and filters should bechecked regularly to maintain proper air flow. Airflow, temperature, and dew point should be moni-tored continuously (downstream of the desiccantbeds) to ensure the drier is functioning properly.

Vented BarrelsWhile injection cylinders fitted with vented barrelscan be used with Crastin® PBT, they are not asuitable substitute for dehumidifying drier systems.Moisture reacts very quickly with PBT resins atmelt processing temperatures, and venting ofmoisture in these systems occurs after the resin ismolten.

Molding Equipment

Crastin® PBT can be processed in all standardscrew injection molding machines. Both toggle andhydraulic clamp machines fitted with either electri-cal or hydraulic drives can be used. Clamp pressureof 3 to 5 ton/in2 (40 to 70 MPa) of projected shotarea is desirable.

Screw Injection MoldingMachinesIn order to obtain good melt homogeneity and toensure good quality parts, the length to diameterratio of the screw should be at least 18 to 1. Three-zone heating control of the barrel should be pro-vided for close temperature control and high outputrates. In all cases, the temperature of the nozzleshould be independently and precisely controlled.

Plunger injection molding machines are not recom-mended for Crastin® PBT.

Screw DesignThe general-purpose, gradual transition screws thatare installed in molding machines as originalequipment are usually suitable for molding Crastin®

PBT resins. Length to diameter (l/d) ratio should beat least 18 to 1.

At high output rates, specific screw designs willprovide better melt temperature uniformity andeliminate unmelt. Screw l/d should be 20 to 1 with10 diameters in feed length, 5 in transition and 5 inmetering length.

Table 1Suggested Screw Design for Improved

Melt Quality*

Screw Feed Section Meter Section Diameter Depth Depth(in/mm) (in/mm) (in/mm)

1.5 (38.1) 0.300 (7.62) 0.085 (2.17)

2.0 (50.8) 0.320 (8.13) 0.105 (2.67)

2.5 (63.5) 0.380 (9.65) 0.120 (3.05)

3.5 (88.9) 0.440 (11.18) 0.140 (3.56)

4.5 (114.3) 0.500 (12.7) 0.150 (3.81)

*Square pitch screw with 20:1 l/d.

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Screw Check Ring AssemblyCheck valves (non-return valves) are necessaryduring injection to ensure constant cavity pressureand part weight uniformity from shot to shot. Ringcheck valves are preferred over ball check valves. Ifused, ball check valves must have carefully stream-lined flow passages to avoid holdup.

WearGlass-reinforced resins are abrasive and tend towear the lands and edges of screw flights. Eventu-ally, the root of the screw in the transition andmetering sections will wear. Use of heat treated andstress relieved alloy steel with a hard surface (e.g.,Colmonoy® 56, UCAR WT-1) is necessary. Nitrid-ing is not recommended.

Sliding type ring check valves will undergo rapidwear when used with glass-reinforced resins. Evenwhen properly hardened, these valves should beconsidered expendable and replaced after three tofour months of operation. Nitriding has been founduseful in extending the life of check rings. A typicalmaterial of construction is Nitralloy 135M. Thevalve seat is generally hardened more than thesleeve; seat Rc55 and sleeve Rc45 are typical.

Screw tips should be made of a stress-relieved AISI4140 steel, hardened to Rc55 with an abrasionresistant surface coat (e.g., “Borafuse”). Thistreatment will lead to an appreciably harder surfacethan that of the check ring.

NozzlesStraight through reverse taper nozzles should beused when molding Crastin® PBT. Melt decompres-sion (suck-back) at the end of plastification(screw retraction) can be used to minimize drool.Positive shut-off nozzles are not recommended.

The nozzle must be equipped with an independenttemperature controller. Crastin® PBT is a verycrystalline resin with a sharp melting point. Un-heated nozzles will not draw sufficient heat from thebarrel to prevent freeze-off. Trying to maintainnozzle temperature by increasing the settings of thethird (metering section) barrel controller will resultin excessive melt temperature and resin degradation.

Pressure ControlBecause Crastin® PBT thermoplastic polyesterresins are very crystalline, they solidify veryrapidly. Glass-reinforced grades must be processedwith high injection speeds to allow complete fillingof the mold cavity. Therefore, it is essential that themachine have a sufficiently high injection capacity.To achieve short injection times, high injectionpressures are usually necessary to overcome flowresistance in areas of small cross section.

Precise switching from injection pressure to hold-ing pressure will avoid over-injection and over-packing of molded parts or damage to the tool. Aswitch from injection pressure to holding pressureinitiated by screw position has proved to be advan-tageous in conventional molding machines. Aprerequisite for this technique is that metering iscarried out with back pressure applied, to ensurethat variations in the metered quantity are small.

If time-controlled switching is used, adjustablesteps of 0.1 s are too large for the short injectiontimes needed with Crastin® PBT, because the screwtravel rate is too great. Good results can be ob-tained with adjustment steps of 0.01 s.

Using cavity pressure to control switching canfurther improve shot-to-shot uniformity in theproduction of precision parts. Recording the cavitypressure/injection time curve simplifies optimiza-tion of the process.

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Molding Conditions

Crastin® PBT has good thermal stability at process-ing temperatures; however, like most thermoplasticresins, excessive residence times and/or tempera-tures can cause degradation. Processing tempera-tures should be matched to shot size. Fast injectionrates provide the best flow in thin sections, andmold temperature must be regulated to controlshrinkage.

Cylinder TemperaturesTypical melt temperatures are 480°F (250°C) inparts using 30–70% of machine rated capacityoperating on cycles under 60 s. Cylinder tempera-tures usually increase from 455°F (235°C) in therear zone to 475°F (245°C) in the center and frontzones. Melt temperature should be measured with aneedle pyrometer after the machine has beencycling for at least 15 min. The front and centerbarrel zones can be adjusted to achieve the desiredmelt temperature. The nozzle, which should becontrolled independently, is generally set at thedesired melt temperature. For parts demanding veryhigh melting capacity, e.g., running over 80% ofrated capacity, it is usually advisable to raise thetemperature of the first (feed section) zone.

Table 2Suggested Temperatures for Parts Using

30–70% of Machine-Rated Capacity

Temperature, °F (°C)

Cylinder ZoneRear (Feed) 455 (235)Center 475 (245)Front (Meter) 475 (245)Nozzle 480 (250)

Melt TemperaturesTypical 480–500 (250–260)Maximum 520 (270)Minimum 455 (235)

Higher melt temperatures are sometimes used toreduce resin melt viscosity and improve flow in themold. Melt temperatures can be raised to 500°F(260°C) for parts that use at least 60% of ratedcapacity and run in cycles of less than 45 s. Thisassumes the resin, including any regrind, is dried tobelow 0.04 wt% water. Higher moisture contentwill result in hydrolytic degradation, even at lowermelt temperatures. See “Drying.” Melt tempera-tures over 520°F (270°C) should be avoided,especially for flame-retarded compositions.

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Because all types of Crastin® PBT are semicrystal-line materials with very sharp freezing points, melttemperatures below 455°F (235°C) should beavoided to guard against premature freeze-off thatcan lead to dangerous over-pressure situations.

Hold-Up TimeFigure 2 illustrates the relationship between maxi-mum recommended cycle time and machine capa-city for several melt temperatures. For example, forparts using 40% of machine capacity melt tempera-tures up to 500°F (260°C) can be used if overallcycle time is less than 50 seconds. This chart isbased on the assumption that moisture content ofthe resin is below 0.04 wt%.

Figure 2. Permissible Cycle Times for Different ShotWeights at Several Melt Temperatures

Maximum Cycle Time, s

Melt Temperature

Mac

hin

e C

apac

ity,

%

100

80

60

40

20

00 50 100 150 200

500°F(260°C)

480°F(250°C)

460°F(240°C)

Mold TemperatureIn order to achieve optimum mechanical propertiesand post-molded dimensional stability in partsmolded out of Crastin® PBT, mold temperaturemust be controlled to produce a uniform degree ofcrystallinity in the resin as it solidifies. Althoughdimensional stability is determined to a largedegree by part design, differences in cavity surfacetemperature will result in differential shrinkage andproduce parts with a tendency to warp. In addition,overall shrinkage depends on absolute mold tem-perature. See “Resin Characteristics and PartDesign” for more information on these subjects.

Because Crastin® PBT is a very crystalline polyes-ter resin, it freezes rapidly and can be successfullymolded over a broad range of mold temperatures

(85 to 265°F [30 to 130°C]). Mold temperaturesunder 150°F (65°C) will produce short cycles withunreinforced resins, while mold temperatures over150°F (65°C) will produce better surface aesthetics,flow, and dimensional stability with reinforcedresins. Higher mold temperatures should be usedfor parts with critical dimensional stability require-ments to reduce the risk of dimensional changescaused by post-shrinkage. Generally, a moldtemperature of 175°F (80°C) is sufficient to obtainparts with good dimensional stability. For precisionparts that will be subject to high temperatures inservice, a mold temperature over 200°F (93°C) maybe required.

Fill RateCrastin® PBT resins exhibit fast set-up in the mold.To prevent premature surface freezing (whichresults in poor surface appearance and weak weldlines), fast fill rates (1 to 4 s) are recommended.To accommodate these fast injection rates, the moldmust be adequately vented. See “Mold Design.”

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Screw Speed and Back PressureIn general, screw speed should be adjusted toachieve a screw retraction time that is 75% of theavailable mold closed time, because slower screwspeeds produce more uniform melt. Back pressurewill also improve melt uniformity and is particu-larly useful in obtaining good mixing uniformitywhen adding concentrated colorants. With glass-reinforced resins, high screw speeds and/or highback pressure can cause glass fiber breakage andreduce mechanical properties of parts.

PurgingIf the cycle must be interrupted for a time equal toseveral times the normal cycle, the cylinder shouldbe purged with fresh resin. Failure to purge aftersuch interruptions can result in defective parts dueto thermal degradation of the resin. This is espe-cially important in hot runner molds.

The cylinder (and hot runner manifolds) should bepurged when switching to or from materials thatprocess at temperatures over 550°F (288°C). Ingeneral, it is good molding practice to purge themachine when changing to a different resin family.

The best purging materials are high-densitypolyethylene and polystyrene. Cast acrylic purgecompounds are very effective, but the nozzle mustbe removed during purging. See “Safety andHandling” for important suggestions on purgingprocedures.

Resin Characteristics and Part Design

Crastin® PBT thermoplastic polyester resins areinjection molding resins based on polybutyleneterephthalate (PBT). PBT is a semi-crystallinethermoplastic resin.

Amorphous versus CrystallineMaterialsThermoplastic materials can be broken up into twogeneral categories: amorphous resins and crystal-line resins. Amorphous materials have no realstructure, but rather gain their mechanical andphysical properties from loose associations ofpolymer molecules. Because the strength of theseassociations decreases at high temperatures, thepolymer will flow and can be formed into a newshape. Crystalline materials, on the other hand,owe their mechanical and physical properties toa regular structural framework as well as toamorphous-like molecular associations. To molda crystalline resin, sufficient heat must be added toboth overcome this regular structure and to reducemelt viscosity to a point where the resin will flowinto a mold.

The regular structural framework of crystallineresins is responsible for their advantages in creep,chemical resistance, toughness, and fatigue proper-ties, and retention of strength and stiffness atelevated temperatures. It also influences processingcharacteristics such as flow, shrinkage, and dimen-sional stability.

Flow PropertiesCrastin® PBT has very good flow properties. It willreadily fill molds with long or complicated flowpaths with good replication of mold surface details.However, because it is crystalline, it solidifiesrapidly and fast injection rates are necessary.

Because the melt viscosity of Crastin® PBT de-creases with increasing temperature, higher melttemperatures are suggested for hard to fill parts.The effect of injection pressure and melt tempera-ture on flow length of glass-reinforced resins isillustrated in Figure 3. High flow grades ofCrastin® PBT typically flow 50% further thanstandard grades at the same injection pressure.They are well suited for parts with thin sections.Flow of standard and high flow resins is comparedin Figure 4.

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Injection Pressure, psi

Flow in 0.100 in Channels

Flo

w L

eng

th, i

n

25

20

15

10

5

02500 5000 7500 10,000 12,500 15,000 17,500

500°F (260°C)

480°F (250°C)

Figure 3. Effect of Melt Temperature and InjectionPressure on Flow of 30% Glass-ReinforcedResins. Endless Flow Channel 0.5 in Wide,0.100 in Thick

Figure 4. Effect of Injection Pressure on Flow of 30%Glass-Reinforced Standard and High FlowResins. Endless Flow Channel 0.5 in Wide,0.040 in Thick. Melt Temperature 480°F(250°C).

ShrinkageIn amorphous thermoplastics, shrinkage is causedprimarily by contraction of the molded part as itcools to room temperature. With semicrystallinethermoplastics, like Crastin® PBT, shrinkage is alsoinfluenced greatly by the crystallization of thepolymer. The degree of crystallization depends onthe transient and local temperature changes in themold. High mold temperatures and heavy wallthicknesses (high heat content of the melt) promotecrystallization and increase shrinkage. These effectsare illustrated in Tables 3 and 4.

Injection Pressure, psi

Flow in 0.040 in Channels

Flo

w L

eng

th, i

n

765432102500 5000 7500 10,000 12,500 15,000 17,500

High Flow Resins

Standard Resins

Table 3Effect of Mold Temperature on Shrinkage

(3″ × 5″ × 0.125″ Plaques)

Mold Shrinkage, %

Mold Temperature 100°F 150°F 200°F(38°C) (66°C) (93°C)

Unfilled Resins 1.4 1.6 1.8

30% Glass-reinforced ResinsFlow (length) 0.17 0.19 0.21Transverse (width) 0.65 0.75 0.85

Super Tough Resins 2.0 2.2 2.4

Table 4Effect of Part Thickness on Shrinkage

(Mold Temperature 150°F [66°C])

Mold Shrinkage, %

Part Thickness 0.125 in 0.250 in

Unfilled Resins 1.6 2.0

30% Glass-reinforced ResinsFlow (length) 0.19 0.35Transverse (width) 0.75 0.85

Super Tough Resins 2.0 —

In addition to mold temperature effects, bothholding pressure and holding time affect theshrinkage of Crastin® PBT. These effects areillustrated in Figures 5 and 6 for unreinforced andglass-reinforced grades, respectively.

Figure 5. Shrinkage versus Holding Pressure forUnfilled (Top Line) and Glass-Reinforced(Bottom Line) Resins. Measured on a FlatQuadrant with 100-mm Radius and 4-mmThickness

5000 10,000 15,000 20,000

2.7

2.2

1.7

1.2

0.7

0.2

Unfilled

Glass-reinforced

Holding Pressure, psi

Sh

rin

kag

e, %

1

Figure 6. Shrinkage Versus Screw Forward Time forUnfilled (Top Line) and Glass-Reinforced(Bottom Line) Resins. Measured on a FlatQuadrant with 100-mm Radius and 4-mmThickness

0 10 20 30 40

2.7

2.2

1.7

1.2

0.7

0.2

Unfilled

Glass-reinforced

Screw Forward Time, s

Sh

rin

kag

e, %

Part and mold design features can also influenceshrinkage. For example, if undersized gates areused, much higher shrinkage will be observed thanis shown in the illustrations due to premature gatefreeze-off that reduces effective holding (pack out)pressure. Mold features such as cores, slides, andinserts can restrict shrinkage, leading to valueslower than expected. In glass-reinforced resins,shrinkage depends on the direction in which theglass fibers are oriented. As a result, there is adifference in shrinkage parallel to (longitudinal)and perpendicular to (transverse) the direction offlow (see tables), and this makes accurate predic-tion of shrinkage difficult. Depending on the fiberorientation, which is determined by the flow patternin the mold, shrinkage values that lie between thelongitudinal and transverse values reported in thetables may be observed.

Dimensional StabilityDimensions and shape of plastic parts may changeafter they are ejected from the mold. These changesare usually due to differences in shrinkage causedby differential cooling rates or, with reinforcedresins, to glass fiber orientation effects. In additionto dimensional changes, these effects can lead tointernal stresses in parts that may lead to breakageproblems.

0

Differential CoolingBecause the shrinkage of a crystalline plasticdepends on the rate at which it is cooled, part ormold designs that lead to differential cooling ratesmay warp.

Parts should be designed with uniform wall thick-ness where possible. Figure 7 illustrates routes toachieving uniform wall thickness in part design.Where non-uniform sections cannot be avoided,wall intersections should be blended gradually.

Figure 7. Uniform Wall Thickness in Part Design

Use this . . . . . . not this

Molds should be capable of delivering uniformcooling across the entire cavity surface. It isadvisable to provide independent temperaturecontrol of the fixed and movable halves of themold, and to check and adjust surface temperaturesafter the machine is on cycle. Provision for coolinglarge or deep cores must be included in the molddesign. See “Mold Design.”

Uniform cooling is especially important withunreinforced resins or resins filled or reinforcedwith low aspect ratio (i.e., round) materials, such asglass beads or most minerals. The effects are sopronounced that differential cooling of mold halvesis sometimes used to control overall flatness in partdesigns that are prone to distortion.

1

Fiber OrientationIn glass fiber reinforced resins, shrinkage is influ-enced by the direction in which the glass fibers areoriented. This anisotropic shrinkage of reinforcedresins probably contributes more to part distortionthan any other factor.

Parts molded out of glass-reinforced resins shouldbe designed and gated with this shrinkage differ-ence in mind. Any condition that will create arandom distribution of glass fibers will reduce thetendency of the part to warp. An alternative strategyis to encourage fiber orientation. For example, long,thin parts are best gated at the end to encouragefibers to orient along the length of the part (seeFigure 8). The mold can then be cut to accommo-date different shrinkage values along the length andacross the width of the cavity. Round parts areoften center-gated to achieve radial orientation ofthe fibers. See “Mold Design.”

Figure 8. Gate Positions for Glass-ReinforcedCrastin® PBT Resins

Poor

Poor

Good

Good

1

Material SelectionOften the realities of part function conflict withthe theories of part design. In these cases, using aCrastin® PBT resin especially designed to be lesssusceptible to differential cooling or fiber orienta-tion effects can produce parts with little or nodistortion.

By alloying with polymers with inherently lowshrinkage, the overall shrinkage of a Crastin® PBTresin can be reduced. Thus, while glass fiberorientation effects are still present, part distortion isreduced, because the differences in shrinkage alongand across the fiber orientation direction arereduced. An example is Crastin® LW9330, a 30%glass-reinforced polymer alloy, which molds flatterthan 30% glass-reinforced PBT resins whileretaining strength and stiffness.

Glass fibers are added to Crastin® PBT resins toincrease both strength and stiffness. Low-aspectratio minerals, which raise stiffness but do little toimprove strength, are used in place of some of theglass in some Crastin® PBT resins. This combina-tion reduces fiber orientation and leads to lessdifferential shrinkage. An example is Crastin®

LW685FR, a flame-retarded, 30% glass/mineral-reinforced resin, which is especially well suited tobox shapes or parts with large flat areas. Crastin®

LW685FR is listed by Underwriters Laboratories asUL 94 V-0 at 0.032 in and UL 94 5VA at 0.084 in.

See the Crastin® PBT Product and Properties Guidefor other low warp resin choices.

Post-ShrinkagePost-shrinkage is shrinkage associated with thereorganization of crystalline structure to alleviatestresses in molded parts. It is accelerated by condi-tioning parts at high temperatures (annealing). Ingeneral, parts molded at higher mold temperaturesachieve a more stable crystalline structure andexhibit less post shrinkage. This is illustrated inFigures 9 and 10.

1

Figure 9. Effects of Conditioning Temperature,Mold Temperature, and Part Thickness onPost-Shrinkage of Unreinforced Resins

Figure 10. Effects of Conditioning Temperature,Mold Temperature, and Part Thicknesson Post-Shrinkage of 30% Glass-Reinforced Resins

0.2

0.1

0

16(60)

21(70)

27(80)

32(90)

38(100)

43(110)

49(120)

Conditioning Temperature, °F (°C)

Po

st-S

hri

nka

ge,

% 2 mm Thick4 mm Thick

176°F (80°C)Mold

212°F (100°C)Mold

0.4

0.3

0.2

0.1

016

(60)21

(70)27

(80)32

(90)38

(100)

176°F (80°C)Mold

212°F (100°C)Mold

43(110)

49(120)

Conditioning Temperature, °F (°C)

Po

st-S

hri

nka

ge,

%

2 mm Thick4 mm Thick

2

Mold Design

Sprues and RunnersSprues and runners for Crastin® PBT are similar indesign to those used for any other semicrystallinematerial, such as nylon or acetal. The entrancediameter of the sprue should be in the range of 0.15to 0.28 in (3.8 to 7.1 mm). The smallest diametersprue should be used where possible. Hot spruebushings can be used.

Runners should have a full round or trapezoidalcross section. Normally, a runner diameter about0.060 in (1.5 mm) larger than the thickness of thegated wall is sufficient. Runners should be as shortas possible to minimize rework and to reducepressure drop to the cavity. In multicavity tools,flow paths to individual cavities should be of equallength.

Gate PositionIf possible, the gate should be located in the thick-est wall section of the part to ensure that holdingpressure remains effective throughout the time thatsolidification is taking place.

With glass-reinforced resins, the number andposition of the gates have a significant effect on theorientation of the glass fibers and, therefore, on thestrength and warpage of the part. Where possible,the mold should be filled “in a straight line.” Forexample, flat plates should use a fan or flash gate,and long thin parts should be gated at an end ratherthan from the side. In parts subject to mechanicalloads, gate positions that avoid orientation of theglass fibers parallel to the applied load should beselected.

GatesRound or Rectangular GatesTypically, round gates should have a diameter ofabout 50% of wall thickness and rectangular gatesshould have a thickness greater than 50% of partthickness with a gate width of 1.5 to 2 times gatethickness. For both round and rectangular gates, theland should be short (0.030 to 0.060 in [0.76 to1.52 mm]).

1

In three-plate molds, the gate diameter should beless than 0.090 in (2.29 mm) to ensure automaticdegating.

Center or Sprue GatesDirect injection with a sprue gate is effective foraxially symmetrical parts and thick-walled parts.It is particularly useful for precision engineeringcomponents with narrow tolerances. If the gate iscorrectly dimensioned and located, holding pres-sure will remain effective during the entire solidifi-cation time, producing parts free of voids or sinks.The sprue should be kept as short as possible, andthe diameter of the gate should be only as large asneeded to allow sufficient pack time. A 1° taperangle on the sprue is sufficient for ejection.

Tunnel GatesFor small parts with wall thicknesses up to 0.080 in(2 mm), tunnel gates may be used, provided thegate diameter is greater than 0.020 in (0.5 mm).Land lengths should be kept short. Clean separationof the runner is generally achieved with both un-reinforced and glass-reinforced types of Crastin®

PBT. With glass-reinforced types, increased wearmust be expected in the gate area. It is good prac-tice to design the gate area as an insert (block) thatcan be easily replaced (Figure 11).

Figure 11. Tunnel Gate

Replaceable Insert forGlass-Reinforced Resins

3

Fan and Flash GatesIn parts with a large surface area (e.g., flat plates)molded out of glass-reinforced resins, it is oftenadvantageous to use flash gates (Figure 12) or fangates (Figure 13). These gates provide a uniformflow front and even glass fiber orientation. Thisensures that distortion of parts is reduced to aminimum.

Diaphragm GatesDiaphragm gates (Figure 14) are useful in axiallysymmetrical parts that must have close dimensionaltolerances and high strength. They provide uniformand symmetrical filling of the mold without weldlines and are particularly useful in processing glass-reinforced resins.

Figure 12. Flash Gate

Gate Runner

Figure 13. Fan Gate

Flat Part

14

Figure 14. Diaphragm Gate

Cylindrical Part

Runner or Sprue

Gate

Disk

VentingInjection molds must be vented in order to alloweasier filling of the cavity. Poor venting can causelocalized burning of the part, poor weld linestrength, incomplete filling, and sinks. Becausetrapped air insulates the plastic part from the mold,shrinkage may vary and parts may warp. Insuffi-cient venting can also result in damage to the moldand excessively high injection pressures.

Vents for unreinforced grades of Crastin® PBTshould be less than 0.001 in (0.025 mm) deep,while those for reinforced grades should be 0.0010to 0.0015 in (0.025 to 0.019 mm) deep. In bothcases, after a distance of about 0.030 in (0.76 mm)from the cavity, the vents should be deepened to atleast 0.020 in (0.5 mm) and extended to the edge ofthe mold. Vent width is not critical, but should beat least 0.25 in (6.4 mm). Vents should be locatedliberally around the entire parting line.

Areas of the mold that are filled after they aresealed off from the parting line must be ventedthrough clearances between moving cores, sleeves,or ejector pins.

DraftMold surfaces that are perpendicular to the partingline should be tapered to allow easy part ejection.These include ribs, bosses, and sides. A taper (draftangle) of 0.5 to 1° is usually satisfactory withCrastin® PBT.

UndercutsUndercuts should be avoided with unreinforcedresins and are not recommended with glass-rein-forced resins.

If undercuts are used with unreinforced resins, theyshould be designed to minimize distortion of thepart as it is stripped from the mold. The part mustbe free to deflect sufficiently in the area of theundercut to clear the mold. A radius or bevel on theundercut will reduce the force required to strip it.Mold temperature should be selected to ensure thepart is warm enough to deflect, yet cool enough toprevent permanent distortion. The amount ofundercut depends on wall thickness and partdiameter.

Runners MoldsWhen processing Crastin® PBT on hot runnermolds, careful attention must be paid to the melttemperature and melt residence time. As a generalrule of thumb, at melt temperatures below 500°F(260°C), residence times should be less than 10 min(less than 5 min for flame-retarded resins). Aminimum melt temperature of 455°F (235°C) isneeded to prevent material from freezing in the hotrunner nozzle. These suggestions are illustratedschematically in Figure 15.

Figure 15. Temperature Suggestions in Hot RunnerMolds

Temperaturein Manifold464–500°F(240–260°C)

Mold SurfaceTemperature176°F (80°C)

Temperatureat Gate500°F (260°C) min.

NozzleTemperature500°F (260°C)max.

1

Temperature ControlAs for most semicrystalline resins, the need tomaintain melt temperature in a fairly narrow rangemust be addressed in the design of the system. Thehot runner nozzle must be well insulated from themold steel; this is accomplished by designing forminimum contact and incorporating large air gaps.Separate and accurate (power proportionating)temperature controllers for each nozzle in themanifold are recommended to accomplish accuratetemperature control and to avoid localized over-heating of the melt. Fitting the nozzle area withheating/cooling channels that are independent fromthe main mold cooling circuits makes it easier tocontrol nozzle temperature without affecting theoverall mold temperature.

Nozzle ManifoldsIn order to maintain uniform pressure drop and meltresidence time to each cavity, nozzle manifoldsmust be designed so that flow paths to the variousinjection paths have equal length. In conventionalmolding, differences in flow length to each cavitywill produce poor part-to-part uniformity, becauseof differences in effective injection pressure at thegates. In hot runner manifolds, there is the addedcomplication of different melt residence timesleading to part-to-part differences in resin quality.Of course, manifold systems must be carefullystreamlined to avoid hold-up spots.

NozzlesExternally heated nozzles with a free flow orificeare preferred over those heated internally with acartridge. With internal heaters, additional heatenergy must be passed through the melt to compen-sate for heat losses to the surrounding tool steel.External heaters can lose heat directly to tool steel,while maintaining the melt at the appropriatetemperature. If internally heated nozzles are used,they should not be mounted directly, but should usean antechamber bushing. A large gate cross sectionshould be used. See Figures 16 and 17.

Direct injection of parts is possible if a short tip or alarge gate diameter (0.060 to 0.080 in [1.5 to 2.0mm]) can be used. With direct injection, it is oftendifficult to obtain parts with clean separation thatrequire no finishing operations. With indirectinjection, where the hot runner nozzle injects into ashort runner system, the gate diameter should belarge (0.080 to 0.120 in [2 to 3 mm]). Because theparts in indirect injection are fed by a conventionalrunner system, part separation is not usually aproblem.

5

Figure 16. Good design of internally heated hotrunner nozzles. Bushing with air gapinsulates melt from tool steel.

Large Annular Gap

Torpedo

Air Gap

Torpedo

Figure 17. Poor design of internally heated hotrunner nozzles. Direct mount leads tohigh heat loss to surrounding tool steel.

Needle-type, self-closing nozzles can experiencefunctional problems in the sealing system whenrunning glass-reinforced resins.

Mold Temperature ControlThe location and efficiency of the cooling system ina mold affects both the properties of the finishedpart and the cycle time. As with all semicrystallinethermoplastics, the dimensional accuracy anddistortion of parts molded out of Crastin® PBT arealso influenced by mold temperature. Temperaturecontrol must be part of the overall design conceptfor molds.

Cooling circuits should be designed and located toensure a rapid removal of heat from the mold, whilemaintaining a uniform temperature across the entiremold. Inserts, ejector pins, splits, and cores mustalso be considered in the design if uniform cavitysurface temperature is to be achieved.

16

Flat PartsGood temperature distribution can usually beachieved in flat parts with either spiral or drilledcooling channels. For large parts, it may be neces-sary to use several separate cooling channels.

Multicavity MoldsMulticavity molds can be designed with annularcooling channels. In order to avoid variation incavity surface temperatures, several circuits mustbe used in large molds.

CoresWhen molding sleeves or long cylindrical sections,most of the heat must be removed through the toolcore. The ratio of cooling surface to the amount ofheat to be removed becomes less favorable as thecore diameter is reduced. Figure 18 shows somepossibilities for cooling cores.

Figure 18. Some Methods for Cooling Cores

Coolant Flow Path

Regrind

Crastin® PBT is a thermoplastic resin that can bereground and molded repeatedly, as long as care istaken to minimize degradation during processing.Both virgin resin and regrind must be dry (less than0.04 wt% water), and excessive melt temperaturesand residence times should be avoided. If the rec-ommendations in “Drying and Molding Condi-tions” are followed, up to 25% of regrind can beused without significant losses in strength andtoughness.

The actual amount of regrind that can be addedmust be determined for each part by a suitable end-use test, because parts have differing functional re-quirements. Parts subject to high mechanical loadsmay require 100% virgin resin, while parts requir-ing only stiffness or electrical properties maytolerate regrind levels greater than 25%.

Grinder screens should have a hole size of about 5⁄16

in and yield particles with a diameter of about 1⁄8 in.Cutting blades should be kept sharp to reduce fines.With glass-reinforced resins, strength and tough-ness depend on glass fiber length that can bereduced in grinding operations. Scrap parts, sprues,and runners should be ground while warm to reducefiber breakage.

It is common industry practice to use a melt viscos-ity or melt flow rate test to monitor regrind quality.The value determined for regrind is compared tothe value for a virgin resin control, and this ratio, orpercentage, is correlated with a target value basedon part performance testing or previous experience.In the case of melt viscosity tests, regrind will havelower melt viscosity than virgin resin; in melt flowrate tests, regrind will have higher melt flow ratesthan virgin resin. In either case, the change ofregrind relative to virgin resin control must becorrelated to end-use performance before a decisionregarding regrind quality can be made. However, asgeneral rules of thumb, it is typical to see changesup to 25% when virgin resin is processed under

1

good conditions (low moisture, reasonable combi-nations of melt temperature/residence time); regrindthat differs from virgin resin by greater than 50% islikely to be severely degraded and should bediscarded.

Melt viscosity tests run on piston rheometers arerecommended over melt flow rate tests run on meltindexers. Piston rheometers operate at constantshear rate that can be set to a value typical of theinjection molding process. Melt indexers operate atvery low shear rates (typically well under 100reciprocal seconds) and are better suited to monitor-ing resins in extrusion processes. In addition, theshear rates in melt flow tests depend on the flowcharacteristics of the material being tested (shearrate is the volume of material passing through agiven orifice per unit time; melt flow rate is theweight of material passing through a given orificeper unit time, i.e., 10 min (under a given load) andare inherently less accurate, because the flowcharacteristics of polymer melts depend on shearrate. Thus, melt viscosity by piston rheometer givesmore accurate values measured at shear ratesmore representative of those actually observed inthe mold.

Melt viscosity or melt flow rate changes can beused with Crastin® PBT to monitor regrind quality;however, the test must be performed in a mannerthat ensures consistent results. Because moisturewill degrade molten Crastin® PBT very quickly, themoisture content of regrind must be controlledduring testing. Typically, regrind and a virgin resincontrol are dried (usually overnight) in a vacuumoven at 248°F (120°C) until moisture content isbelow a target level (e.g., 0.003 wt%). A drynitrogen blanket may be required to attain this levelof dryness. Typical test conditions are 482°F(250°C) at a shear rate of 1000 reciprocal secondsfor melt viscosity and 482°F (250°C) with a weightof 2160 g for melt flow rate (ASTM D 1238Condition T).

7

Troubleshooting

Although Crastin® PBT thermoplastic polyesterresins are generally easy to mold in trouble-free,robust processes, occasional problems will arise.The following Troubleshooting Guide offers somesuggested remedies for common problems. Mostproblems can be related to three characteristics ofthe resin.

Moisture ContentCrastin® PBT resins must be processed with amaximum moisture content of less than 0.04 wt%to avoid hydrolytic degradation. Processing wetmaterial may contribute to excessive flow (flash)and will produce brittle parts. Most moistureproblems can be traced to dryer maintenanceproblems—clogged filters, inaccurate instrumenta-tion, air leaks. If regrind is being used, it must bedry and of good quality. Parts, sprues, and runnersfrom resin that was processed wet will be degradedand may produce brittle parts if mixed with virginresin and properly dried before molding. For moreinformation, see “Drying and Regrind.”

Freezing RateCrastin® PBT resins are very crystalline and freezevery rapidly. Fast injection times are required togive the resin a chance to fill the mold before itfreezes. Short shots, sink, and excessive (or vari-able) shrinkage can often be traced to prematurefreezing as can weak or very visible weld lines.Fast injection rates mean the mold must be wellvented and call for careful control of the transitionfrom injection to holding pressure. Gates must belarge enough to allow complete packing of the part.For more information, see “Molding Conditions”and “Mold Design.”

1

ShrinkageBecause Crastin® PBT is crystalline, it shrinks onfreezing. Apart from the obvious impact on controlof dimensional tolerances, shrinkage also plays amajor role in dimensional stability. Parts that areundersize or are variable in dimension (or containsink) may benefit from longer screw forward timesor larger gates to allow more resin to flow into thecavity to compensate for the volume change thatoccurs during shrinkage. Parts may warp, especiallyin unreinforced resins, if mold temperature varies inthe cavity area, because shrinkage depends on moldtemperature. Because thick sections cool moreslowly than thin sections, part design may also leadto differential shrinkage that causes parts to distort.For more information, see the sections “MoldingConditions” and “Resin Characteristics and PartDesign.”

8

Troubleshooting Guide

Problem

WeakBrittle Short Voids Parts Parts Weld Sprue Part Poor

Suggested Remedies* Parts Shots in Part Sinks Burned Warp Lines Sticks Sticks Surface

Ensure Resin Is Dry 1 1

Change Injection Pressure 2 2 2 2 3 2 2 2

Increase Injection Speed 3 4 2 5

Decrease Injection Speed 4 3 3 4

More Screw Forward Time 3 3

Less Screw Forward Time 3

Check Melt Temperature 2 4 5 5 1 6 6 3

Increase Mold Temperature 5 5 3 1 1

Increase Nozzle Temperature 1

Increase Gate Size 7 6 6 5 8

Increase Vent Size 6 7 2 4 7

Use Reverse Taper Nozzle 4

Decrease Hold-up Time 3

Change Cycle 4 4 4 6

Check Pad Size (Cushion) 1 1 1

Repair Mold 5 5

Increase Taper 6 6

Change Gate Location 6 5 7 9

Reduce Regrind Level 4

Balance Mold Temperature 1

Check Puller Design 7

Check for Contamination 5

Check for Voids 6

Decrease Mold Temperature 7

*Try in order listed.

19

Identity and Trademark Standards

Guidelines for Customer Use—Joint ventures and authorizedresellersOnly joint ventures and resellers who have signed special agreements with DuPont to resell DuPontproducts in their original form and/or packaging are authorized to use the Oval trademark, subject to theapproval of an External Affairs representative.

Guidelines for Customer Use—All other customersAll other customer usage is limited to a product signature arrangement, using Times Roman typography,that allows mention of DuPont products that serve as ingredients in the customer’s products. In this signa-ture, the phrase, “Only by DuPont” follows the product name.

Crastin® PBTOnly by DuPont

A registration notice ® or an asterisk referencing the registration is required. In text, “Only by DuPont” mayfollow the product name on the same line, separated by two letter-spaces (see above example). When aDuPont product name is used in text, a ® or a reference by use of an asterisk must follow the product name.For example, “This device is made of quality DuPont Crastin* PBT thermoplastic polyester resin.”

*Crastin® is a DuPont registered trademark.

Crastin® PBT Only by DuPont or

thermoplastic polyester resinCrastin PBT®

e

Printed in U.S.A.[Replaces H-64532] (96.6)Reorder No.: H-81100 (99.11) DuPont Engineering Polymers

e

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The data listed here fall within the normal range of properties, but they should not be used to establish specification limits nor used alone as the basis ofdesign. The DuPont Company assumes no obligations or liability for any advice furnished or for any results obtained with respect to this information.All such advice is given and accepted at the buyer’s risk. The disclosure of information herein is not a license to operate under, or a recommendation toinfringe, any patent of DuPont or others. DuPont warrants that the use or sale of any material that is described herein and is offered for sale by DuPontdoes not infringe any patent covering the material itself, but does not warrant against infringement by reason of the use thereof in combination with othermaterials or in the operation of any process.

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