27257774 molding guide
TRANSCRIPT
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a
Delrin Performance Resin
In Applications:
Consistent Part Performance
Chemical Resistance
Toughness
Abrasion ResistanceColorfast
Lubricity
Fatigue Resistance
In Processing:
Fast Cycling
Good Melt Stability
Low Mold Deposit
Improved Equipment UtilityImproved Productivity
Molding Guide
acetal resins
Delrin
StartwithDuPont
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Table of Contents
Page
General Information . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1Description . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1
Melt Properties of Delrin
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1Packaging . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1
Equipment for Injection Molding of Delrin . . . . . . . . . . . . . . . . . . 5Barrel Heating Capacity and Screw Design . . . . . . . . . . . . . . . . . . 5
Nozzle . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5Non-return Valve and Cylinder Adaptor . . . . . . . . . . . . . . . . . . . 6Screw Design . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
Cylinder Temperature Control . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8Injection Rate . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8Cycling Rate . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8Clamping Force . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
Operating the Molding Machine . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9Start-up and Shutdown Procedures . . . . . . . . . . . . . . . . . . . . . . . . 9
Start-up with Resin Change . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9Reheating a Cylinder Containing Delrin . . . . . . . . . . . . . . . . 9Start-up after Emergency Shutdown . . . . . . . . . . . . . . . . . . . 9Normal Shutdown . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10Temporary Shutdown . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
Operating Conditions for Delrin . . . . . . . . . . . . . . . . . . . . . . . . . . 10Cylinder Temperature . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10Thermal Stability of Delrin . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10Back Pressure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11Injection Pressure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
Injection Rate . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12Molding Cycle . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13Mold Temperature . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16
Techniques for Optimum Productivity Molding . . . . . . . . . . . . . . 16Handling Precautions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18
Molds . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19Ability to Fill . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19Gates . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20
Gate Location . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20Gate Design . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21
Runners . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24
Vents . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24Undercuts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26Runnerless Molds . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26Mold Maintenance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27Mold Deposit . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27
(continued)
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Table of Contents
Page
Dimensional Considerations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28Fundamentals of Dimensional Control . . . . . . . . . . . . . . . . . . . . . 28
Mold Shrinkage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28Estimating Mold Shrinkage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28Shrinkage Around Inserts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31Mold Shrinkage of Filled Compositions . . . . . . . . . . . . . . . . . . . 31
Post-Molding Shrinkage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31Annealing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32
When to Anneal . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32Annealing Procedures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33Cooling Procedure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33
Environmental Changes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33Tolerances . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33
Auxiliary Operations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37Material Handling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37Reground Resin . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37Drying . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 38Coloring . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 38Disposal . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 38
Troubleshooting Guide . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39
Molding Data Record . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 43
Mold Inspection and Repair Record . . . . . . . . . . . . . . . . . . . . . . . . . 44
Index . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 45
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General Information
DescriptionDelrin acetal resins are thermoplastic polymersmade by the polymerization of formaldehyde. Theyhave gained widespread recognition for reliability inmany thousands of engineering components all overthe world. Since commercial introduction in 1960,
Delrin has been used in the automotive, appliance,construction, hardware, electronics, and consumergoods industries, among others.
Delrin is noted for:
High strength and rigidity in a wide temperaturerange
Fatigue endurance unmatched by most otherplastics
Excellent resistance to moisture, gasolines,solvents, and many other neutral chemicals
Ease of fabrication
High resistance to repeated impacts
Resilience
Good wear resistance and low coefficient offriction
FDA acceptability in many food contactapplications
Delrin acetal resins are available in a variety ofcompositions to meet different end-use and process-ing requirements. The effect of processing variableson physical properties is discussed in Operating theMolding Machine. A summary of the compositions
is shown in Table 1.Unless otherwise noted, Delrin will be usedthroughout this publication to refer to both standardDelrin and Delrin II.
Melt Properties of Delrin
Delrin acetal resins are available in compositionscovering a broad range of melt viscosities. Theresins having lower melt viscosity, Delrin 900 and1700, are usually chosen for injection moldingapplications with hard-to-fill molds. The intermedi-ate melt viscosity Delrin 500 is used for general-
purpose injection molding applications. The highesviscosity composition, Delrin 100, is often moldedwhen maximum toughness properties are needed.The melt viscosities of various compositions ofDelrin are compared in Figure 1.
Delrin is a crystalline polymer that requires agreater heat input for melting than amorphousresins do. This additional heat is needed to destroythe ordered crystalline structure of the solid, and itis called the heat of fusion. The heat of fusion andtotal heat required for processing Delrin arecompared with those of other crystalline and
amorphous resins in Table 2. The heat require-ments of crystalline resins impose special consider-ations on screw design for injection moldingmachines when high melt output is needed. Screwdesign for Delrin is discussed in Equipment forInjection Molding of Delrin.
PackagingDelrin acetal resin is supplied as spherical pelletsapproximately 2.5 mm (0.1 in) in diameter orcylindrical pellets, 3 mm (0.125 in) in diameter by2.3 mm (0.09 in) in length. They are packaged in1,000 kg (2,200 lb) net weight bulk corrugated
boxes and 25 kg (55.1 lb) moisture protected, tearresistant polyethylene bags. The bulk density of theunfilled resin granules is about 0.8 g/cm3 (50 lb/ft3)
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Extruded, InjectionMolded
100,1,2 100P1 High viscosity resinused in easy-to-fillmolds. Surfacelubricated. Delrin
100P has superiorprocessing charac-teristics.
Maximum tough-ness without modi-fication.
General mechani-cal partsgears,fasteners, cams.Mill shapes forproduction ma-chining.
500,1,2 500P1 Extruded, InjectionMolded
General-purpose,surface lubricatedresin. Delrin 500Phas superior pro-cessing charac-teristics.
Good balance ofproperties.
Highly stressedparts; mill shapes,sheet, rod tubing.
Multicavity molds
and thin sectionsthat are difficultto fill.
900,1,2 900P1 Low viscosity high
flow surface lubri-cated resin. Delrin
900P has superiorprocessing charac-teristics.
Similar to Delrin
500 with slightlylower tensileelongation andimpact resistance.
Injection Molded
1700P Injection Molded Very low viscositysuitable for specialpurpose molding.Delrin 1700P hassuperior processingcharacteristics.
Balance of proper-ties lower thangeneral-purposeDelrin.
Table 1Compositions of Delrin Acetal Resins
Standard Delrin Resin Products
Melt Flow Process ProductRates Processing Method Characteristics Characteristics Applications
Parts with com-plex shapes, thinwalls, long flowpaths or multi-cavity tools.
Specialty Delrin Resin Products
Process ProductGrade Processing Method Characteristics Characteristics Applications
100AF1 Extruded, InjectionMolded
High viscosity resinused in easy-to-fillmolds. Teflon3
PTFE fibers addedto resin.
Maximum unmodi-fied toughness; ex-tremely low frictionand wear.
Bearings, bush-ings, cams, andother anti-frictiondevices where ex-tra toughness isneeded.
107 Extruded, InjectionMolded
High viscosity resinused in easy-to-fillmolds with UVstabilizer.
High toughnesswith weatherability.
Highly stressedparts for outdooruse.
150SA1,2 Extruded High viscosity resin
with low die de-posit.
Maximum tough-
ness in an extrusionresin without modi-fication.
Highly stressed
sheet, rod, andtubing.
150E1,2 Extruded High viscosity resinwith low die de-posit.
Toughness withreduced centerlineporosity.
Exclusively stockshapes that aregreater than 0.25in thick.
(continued)
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500AF1
100ST Extruded, InjectionMolded
High viscosity resinused in easy-to-fillmolds. Surfacelubricated.
500CL
General-purposesurface lubricatedresin with 20%Teflon3 PTFE fibersadded to resins.
General-purposechemically lubri-cated resin.
Super tough acetal.
Balanced propertieswith extremely lowfriction and wear.
Low friction andwear against metal.Highest PV limit
of all Delrin
acetalresins.
Highly stressedparts where out-standing tough-ness is essential.
Bearings, bush-ings, cams, andother antifrictionuses.
Mechanical partsrequiring im-proved wear re-
sistance.
Extruded, InjectionMolded
Extruded, InjectionMolded
Table 1Compositions of Delrin Acetal Resins
Specialty Delrin Resin Products
Process ProductGrade Processing Method Characteristics Characteristics Applications
(continued)
500T
507
Injection Molded
Injection Molded
General mechani-cal parts whereimproved tough-ness is needed.
Balanced propertiesresistant to UL light.
General-purposeoutdoor applica-tions.
General-purposesurface lubricatedresin.
General-purposesurface lubricatedresin with UV stabi-lizer.
Toughness similarto Delrin 100, lowwear slightly re-duced stiffness andstrength properties.
570 Injection Molded General mechani-cal parts where
improved stiff-ness is needed.
Very high stiffness,low warpage, low
creep for superiorperformance atelevated tem-peratures.
General-purposesurface lubricated
20% glass-filledresin.
577
Extruded
General mechani-cal parts wherestiffness andweatherability arerequired.
Very high stiffness,low warpage, lowcreep for superiorperformance atelevated temper-atures with UVstabilizer.
Injection Molded
Stock shapes formachining partsincluding rod,
sheet, and tube.
Excellent balancedproperties in resinproducing uniform
rod stock.
550SA2
500TL2 Injection Molded General-purposesurface lubricatedresin with 1.5%Teflon3 micro-powder.
Low coefficient offriction applica-tions such as con-veyor chain.
General-purposesurface lubricatedUV stabilizer 20%glass-filled resin.
General-purposeextrusion resin withadditive system that
allows fast cyclingwithout voids orwarpage.
Balanced propertiessuitable for highspeed, low frictionapplications.
1 Many grades of Delrin meet NSF potable water contact requirements for Standard 14 and Standard 61. Contact DuPont for acurrent NSF official listing.
2 May comply with FDA regulations for use as an article or components of articles intended for food-contact use on a repeatedbasis. Contact DuPont for current FDA status.
3 Teflon is the registered trademark for DuPont fluoropolymer resin, films, enamels, and fibers.
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Figure 1. Melt Viscosity of Thermoplastics
Temperature, F
Temperature, C
10,000
5,000
1,000
500
100
Viscosity,
Pas
Delrin100,150SA
Delrin500
350 400 450 500 550 600
200 225 250 275 300
100,000
50,000
10,000
5,000
1,000
Zytel*42
Viscosity,
P
Shear rate: 100 sec1
Delrin900
Delrin1700 Zytel101
Table 2Heats Required for Processing
Melt Temperature Heat of Fusion Total Heat Required
Resin Structure C F kJ/kg Btu/lb kJ/kg Btu/lb
Polystyrene Amorphous 260 500 0 0 370 160
Acetal Resin Crystalline 210 410 160200 7087 435475 190206
Polyethylene(Low Density) Crystalline 227 440 130 56 635 274
Polyethylene(High Density) Crystalline 227 440 240 104 720 310
Nylon 66 Crystalline 285 545 75105 3346 756786 329342
*Zytel is the registered trademark for DuPont nylon resin.
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Equipment for Injection Moldingof Delrin
Delrin acetal resins are molded throughout theworld in a wide variety of makes and designs ofinjection and extrusion equipment. To obtain thebest moldings and production efficiency, machinesused for molding Delrin should be evaluated for:
Barrel heating capacity and screw design
Cylinder temperature control
Injection rate
Cycling rate
Clamping force
Barrel Heating Capacity andScrew DesignTo deliver the necessary amount of uniformlymelted resin of good quality, the adaptor, non-return valve (screw check valve), screw, and
cylinder must be properly designed to ensuremaximum streamlining. This is necessary to avoidholdup of resin, which will lead to resin degrada-tion. Relatively insignificant holdup spots cancause degradation and a reduction in quality of theparts molded. Poor seating or matching of diam-eters, as well as poor maintenance, can also causeholdup spots.
NozzleTwo nozzles of the open design (not shutoff) thatare suitable for use with Delrin are shown inFigures 2 and 3. Like other crystalline polymers,
Delrin
may either tend to drool from the nozzle
between shots if the nozzle is too hot, or it mayfreeze if too much heat is lost to the sprue bushing.The nozzle designs illustrated control these prob-lems by acting as thermal shutoffs.
The heater bands should extend as close to thenozzle tip as possible and cover as much of theexposed surface as practical to counteract heat loss,
especially to the sprue bushing. It is particularlyimportant that heater bands cover the small orificewhere material separation occurs. If a thermocoupleis used, the well should be located over this orifice,as shown in Figure 2. Improper location of heaterbands or thermocouple will result in overheatingof certain areas in an effort to prevent freezing atthe tip.
Screw decompression or suck back is frequentlyused to make control of drool easier with these opennozzles. This feature is available in most machines.
The nozzle shown in Figure 2 has a reverse tapersection in front to move the separation point awayfrom the tip to a place where temperature is easierto control. In both designs, the reduction in diam-eter from the adaptor to the small bore of the nozzlemust be a gradual taper to avoid resin holdup. Thetaper should not exceed 60 (30 per side). Thetaper should also terminate at a short cylindricalsection at the rear of the nozzle that joins thecylinder adaptor, so that minor repair of the sealingsurface will not alter the diameter joining theadaptor. The inside diameter of the cylindricalsection must exactly match that of the adaptor to
avoid resin holdup at this location.
Figure 2. Reverse Taper Nozzle
Radius to suit
R0.25 mm(0.010 in)
MinimumTaper Length
510 mm(0.20.4 in)
4
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Figure 3. Straight Bore Nozzle for Machines Without Screw Decompression
d
L
L = 10dL = 40 mm (1.6 in), maximum
Although shutoff nozzles have been used success-fully with Delrin, they tend to cause holdup of
resin that results in brown streaks or gassing,especially after some wear has occurred in themoving parts of the nozzle. These nozzles are notgenerally recommended for Delrin. The use of thegood open nozzles described above with screwdecompression makes shutoff nozzles unnecessaryin most cases. Shutoff nozzles are, however,required if it is necessary to continue screw rotationduring the time the mold is opening or in somevertical cylinder molding machines.
Non-return Valve and Cylinder AdaptorThe adaptor and non-return valve shown in Figure
4 are designed to avoid holdup areas. Note that theadaptor has short cylindrical sections (A and B)where it joins both the nozzle and the cylinder tomaintain accurate matching of these diameters,even as it becomes necessary to reface the matingsurfaces. The mating surfaces (C) should be narrowenough to develop a good seal when the nozzle oradaptor is tightened and yet wide enough to avoidpeening.
In addition to its mechanical function of reducingthe diameter, the adaptor acts to isolate the nozzlethermally from the front of the cylinder for better
control of nozzle temperature. A separate adaptor,which is easier to repair or replace than a cylinder,also protects the cylinder from damage due tofrequent changing of nozzles.
The non-return valve or check ring shown inFigure 4 prevents melt from flowing backwardduring injection. This unit is frequently not prop-erly designed to avoid holdup of resin and flowrestrictions. Malfunctioning that allows resin
backflow is also a common experience and iscaused by poor design or maintenance. A leaking
non-return valve will add to screw retraction time,which can increase cycle, and it will also causepoor control of packing and tolerances.
The non-return valve must meet the followingrequirements:
No holdup spots
No flow restrictions
Good seal
Control of wear
These requirements are provided for in the non-return valve shown in Figure 4. The slots or flutes
(D) in the screw tip are generously proportioned,and the space (E) between the check ring and tip issufficient for resin flow. The seat ring is cylindricalwhere it joins both the end of the screw (F) and thescrew tip (G) to permit accurate matching of thesediameters and avoid holdup. The screw tip threadhas a cylindrical section (H) ahead of the threadsthat fits closely in a matching counterbore forsupport and alignment of the screw tip and seatring. The free space (J) behind the threaded shankof the screw tip is kept to a minimum to avoidaccumulation of degraded resin from any leakage
that may occur. Gases generated from prolongeddegradation of resins trapped in this area can causethe screw tip to be ejected violently during equip-ment disassembly. The screw tip and check ringseat should be harder (about Rc 52) than the ring(Rc 44), because it is less expensive to replace thering when wear occurs.
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Figure 4. Design of Adaptor and Non-return Valve
Screw Tip
C
E
B
CG F H
Screw
Check Ring Seal
Check Ring
A
Nozzle
Adaptor
J
Heating Cylinder
>1.5D
Screw DesignImprovements in both modern injection moldingmachines and the resins they use have increased thedemand for good quality melt at high output.Although general-purpose screws are widely usedfor molding Delrin, optimum productivity mayrequire that other designs be used. Exceeding theoutput capability of an inadequately designed screwwill cause wide temperature variations and unmeltedparticles, resulting in loss of toughness, warping,plugged gates, or other molding problems.
The quality of the melt can sometimes be judged bymaking purge shots equal to the shot weight and atthe production cycle. Particles of unmelt or irregularflow caused by these particles are frequently visible
in the melt stream from the nozzle, especiallytoward the end of the shot.
A screw designed to process a crystalline resin likeDelrin at high output must provide a greater heatinput than is required for amorphous resins. Thisadditional heat, called the heat of fusion, is neededto destroy the ordered crystalline structure. Theheats required for processing Delrin are comparedto those of other resins in Table 2.
In general, a screw designed for optimum pro-cessing of a crystalline material such as Delrin wilhave shallow flight depths in the metering sectionand a slightly higher compression ratio than ageneral-purpose screw. Compression ratio is theratio of flight depth or volume of one turn in the feesection to that in the metering section. Specificsuggestions are given for various screw diametersand compositions of Delrin acetal resin in Table 3This table also includes a screw diagram to identifythe terms and dimensions used in the table and inthis discussion.
The length of the screw will also affect the meltquality and consequently the output usable for goodparts. The preferred length is about 20 times the
screw diameter or 20 turns when the pitch anddiameter are equal. The screw should be divided asfollows: 4550% (910 turns) feed section, 2030%(46 turns) transition, and 2530% (56 turns)metering section. Screws with 20 turns are com-monly divided into 10 turns feed, 5 turns transition,and 5 turns metering. In screws less than 16 diam-eters (turns) long, it may be necessary to depart fromthese distributions, although the feed section shouldnever be less than 7 or 8 turns.
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Table 3Screw Design for Delrin Acetal Resins
Delrin 500, 900, 500T, 1700 Delrin 100, 150SA, 100ST
Nominal Depth of Depth of Depth of Depth ofDiameter (D) Feed Section (h
1) Metering Section (h
2) Feed Section (h
1) Metering Section (h
2)
mm mm mm mm mm
30 5.4 2.0 5.2 2.6
45 6.8 2.4 6.5 2.8
60 8.1 2.8 7.5 3.0
90 10.8 3.5 8.7 3.6
120 13.5 4.2
(in) (in) (in) (in) (in)
(1
1
2
) (0.240) (0.087) (0.230) (0.105)(2) (0.290) (0.100) (0.270) (0.115)
(212) (0.330) (0.110) (0.300) (0.120)
(312) (0.420) (0.140) (0.340) (0.140)
(412) (0.510) (0.160)
The shallow metering, high compression ratioscrews suggested for Delrin are designed toincrease the heat input by mechanical working ofthe resin. Because the energy for this increasecomes from the screw motor, additional horse-power must be available if an increase in meltingcapability is to be realized. The suggested screw forDelrin will also have to be operated at a greaterrotational speed (revolutions per minute) to deliverequal or greater melt output during a specifiedscrew retraction time, because the amount dis-placed by one turn is less. If a general-purposescrew is operating at maximum horsepower orrotational speed, a screw designed for Delrin willnot be helpful. Neither a general-purpose nor highcompression screw should be operated at rotationalspeeds so high that unmelted particles or degrada-tion occurs. For Delrin, this rotational speed limitwill be higher for the high compression screw than
the general-purpose one.Usually, the molding machine injection unit ischosen so that the shot size is 50% or less of theinjection stroke. For thin parts molded at more thanfour shots per minute, as little as 1020% ofinjection stroke capacity may be used to obtainadequate melting capacity. On the other hand, forthick parts molded on long cycles more than 50%of the stroke may be used. These considerationsdetermine the average holdup time of the resin inthe molding cylinder. Holdup time is discussed indetail in Operating the Molding Machine.
In some cases, it may be preferable to use aninjection molding machine with a larger cylinderthan to change to a special screw designed forDelrin.
Cylinder Temperature ControlThree-zone temperature control should be used, and
thermocouples should be placed near the center ofeach zone. Burnout of one or more heater bandswithin a zone may not be apparent from the tem-perature controllers, so some molders have usedammeters in each zone to detect changes.
Injection RateThe molding machine should be able to inject itsrated capacity in less than 2 sec with lower viscos-ity Delrin acetal resins. Delrin 100, which has ahigher viscosity, may have slower fill rates. Controlof injection rate, however, will often be necessary
to correct certain molding problems. For example, aslower injection rate may be required to preventwarping or jetting of resin into some cavities. Otherapplications of this control feature are suggested inTroubleshooting Guide. Also, two-stage injectionis useful for more complete control of the moldingprocess.
Dh1
FEED SECTION
Pitch h2
METERING
SECTIONTRANSITION
(20/1 Length/Diameter Ratio)
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Cycling RateFor optimum productivity of good quality parts, themolding machine must be capable of a rapid andreliable cycling rate. The cycle should be limited bythe shape and thickness of the part being molded,not by unnecessary delays in the action of themolding machine. This is particularly important for
fast cycling resins like Delrin.
Clamping ForceThe force required to hold a mold closed againstinjection pressure depends on:
Injection pressure to fill the mold and pack-outparts
Injection speed required for filling
The dimensions of parts and runners at the moldparting line (projected area of parts and runners)
Mold detailslateral slide actions, core pullrequirements, etc.
Precision and tolerance requirements
Most well-built molds can be adequately clampedif the machine has 4869 MPa (3.55.0 tons/in2)based on the projected area, because injectionpressures are seldom over 100 MPa (14 kpsi) andfill rates are moderate. Higher available clampingpressure is preferred and will give more freedom inchoosing the most suitable molding conditions forsome molds. Very thin, long flow parts may requireclamp force as high as 100 MPa (7 tons/in2),because extremely fast fill rates and high injectionpressures may be required.
Operating the Molding MachineInjection molding of Delrin acetal resin is similarto that of other plastic resins. The engineeringapplications for which Delrin is used, however,frequently require tight specifications on strength,dimensions, and surface condition, so that controlof the molding operation becomes more critical.
The information discussed in this section includessuggestions for:
Start-up and shutdown procedures
Operating conditions for Delrin
Thermal stability of Delrin
Techniques for optimum productivity molding
Handling precautions
Start-up and Shutdown Procedures
Start-up with Resin ChangeThe suggested start-up procedure with Delrin isdesigned to prevent overheating of the resin andcontamination in the cylinder with material fromprevious runs.
In starting up a machine that contains another resin,the cylinder must be purged at temperatures abovethe melting temperature for the other resins with aclear polystyrene or natural color low melt indexpolyethylene (MI 1 to 2) until the cylinder has beencleared. The cylinder temperatures should then beadjusted to about 205C (400F) while continuing
to process the purge resin through the machine.When the cylinder has reached 205C (400F),Delrin can be added to the hopper. After the purgeresin has been cleared from the cylinder, themolding operation with Delrin may begin.
Both polystyrene and polyethylene are chemicallycompatible with Delrin, whereas even a trace ofpolyvinyl chloride is not. Contamination of Delrin
with such foreign material can cause objectionableodor or even a violent blowback. Very thoroughpurging is required to remove the last traces ofpolyvinyl chloride from a cylinder.
In unusual circumstances, an intermediate purgewith a harsher compound may be required toremove adherent deposits from the screw andcylinder. Special purge compounds are used for thispurpose.
These purge compounds must also be removedfrom the cylinder by purging with polyethyleneor polystyrene before Delrin is introduced. In theworst cases, e.g., after use of glass-reinforcedresins or severe degradation of previous material,it may be necessary to pull the screw and clean theequipment manually to prevent contamination of
moldings.
Reheating a Cylinder Containing Delrin
After the normal shutdown procedure, described inthe next section, the screw and cylinder will beessentially empty. In this case, the nozzle tempera-ture and then the cylinder temperatures should beset at the normal operating temperatures. TheDelrin in the nozzle and adaptor should meltbefore the cylinder zones reach operating tempera-tures. If the heater bands on the nozzle are ad-equate, this usually does not require any advanceheating time for the nozzle. When all temperatures
are in the operating range, the hopper can be filledor opened, and molding can begin after a briefpurge with Delrin.
Start-up after Emergency ShutdownA different procedure should be used after anemergency shutdown due to loss of power or othercauses. In this case, the screw may be full ofDelrin that cooled slowly and was exposed to melttemperatures for a prolonged period. The screw
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(400440F),* as measured with a needle pyrom-eter in the melt from two or three consecutive airshots on cycle. The actual cylinder temperaturesettings to achieve this melt temperature willdepend on the design of the heating cylinder andscrew, screw back pressure, screw rotation rate,shot size and cycle. Typical settings are provided in
Table 4 for several types of Delrin
as a guide. Thesettings will frequently be 510C (1020F) lowerthan the melt temperature, because of the mechani-cal heat input from the screw. When the cycle isshort and melt output is high, higher than normalcylinder settings may be required. Similarly, whencycle is long and melt output is low, lower settings,especially in the rear zone, may be used. Becausegeneralization of cylinder temperature settings isdifficult, it is often wise to begin with a levelprofile and adjust as needed.
The nozzle temperature is adjusted to control drooland freezing, although it is often operated near themelt temperature.
The proper design and operation of the heatingcylinder components, as discussed here and inEquipment for Injection Molding of Delrin, areintended to provide a homogeneous melt of uni-form temperature. This uniform melt crystallizesuniformly, as can be shown by microstructuralanalysis, and produces a structure with minimuminternal stress. Uniform melt also promotes uni-form mold shrinkage.
Thermal Stability of Delrin
Delrin has very good thermal stability for normalprocessing, but as with all plastic resins, degrada-tion can occur under unusual conditions. If theresin is overheated, decomposition to formaldehydecan cause splay marks on the moldings and bubblesin the melt. Degradation may also cause yellow orbrown discoloration.
The rate of decomposition of Delrin depends onboth temperature and time. Although the resin willdecompose rapidly at extremely high temperatures,it can be decomposed at normal processing tem-peratures after very long periods of time. The
relationship between temperature and maximumholdup time in an injection molding cylinder isshown in Figure 5. These data are based on ourexperience with a variety of cylinders, but will varysomewhat for specific equipment. Holdup time is afunction of cycle time, shot weight, and the weight
may even be in the retracted position with a largequantity of Delrin in front of the screw. In orderto vent gases from resin that may be degraded, it isessential that the nozzle be open and heated tooperating temperature and the Delrin in this areabe completely melted before the cylinder reachesmelt temperature. The cylinder zones should be
heated to an intermediate temperature below themelting point of Delrin and the machine allowedto equilibrate at that temperature. Cylindertemperatures of 150175C (300350F) aresuggested. After all zones have been at this tem-perature for 30 min, cylinder temperatures shouldbe raised to 195C (380F). As soon as the Delrin
has melted, it should be purged from the cylinderwith fresh Delrin. The partly degraded, hot purgeresin should be placed in a pail of water if it emitsan odor. When the old resin is purged from thecylinder, the cylinder temperatures may be raisedto normal production settings.
Normal ShutdownThe shutdown of a molding operation with Delrin
is very simple. The best practice is to shut off thehopper feed gate, turn off the cylinder heaters(leaving the nozzle heater on), and continuemolding or purging until the cylinder is evacuated.The machine is then turned off with the screw inthe forward position and is ready for reheating bythe procedure just outlined. If another resin will bemolded next, Delrin should be purged from thecylinder with polyethylene or polystyrene asdescribed in Start-up Procedure. This can be
done during shutdown or subsequent start-up withthe other resin.
Temporary ShutdownA molding machine with Delrin in the cylinder atoperating temperatures should not be allowed tostand idle. When it is necessary to interrupt thecycle for a minor repair, it is good practice to takeperiodic air shots or shut the hopper feed gate andempty the cylinder and screw if the interruptionwill be more than a few minutes. The cylindertemperatures should be reduced to about 150C(300F) if the interruption is prolonged.
Operating Conditions for Delrin
Cylinder TemperatureDelrin acetal resin is a crystalline resin with amelting point of 175C (347F). The pre-ferred melt temperature range is 205225C
*The preferred melt temperature range for Delrin 100ST and 500T is193220C (380430F) for impact strength retention.
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Table 4Typical Cylinder Temperature Settings
Temperature Setting, C (F)
Resin Type Nozzle Front Center Rear Comments
All Other Types 190 (375) 205 (400) 205 (400) 205 (400)
100 190 (375) 190 (375) 190 (375) 218 (425) Control shear* heat generationin rear zone, force center/frontzone controls to cycle on/off.
100ST, 500T 190 (375) 190 (375) 190 (375) 218 (425) Maintain melt residence timeabove 198 (390) at a minimum.
Colors, HighRegrind Percentage 190 (375) 210 (410) 193 (380) 193 (380) Move melt generation forward to
reduce residence time as melt.
*It may be necessary to reduce the screw rpm in order to reduce shear heat generation in the rear zone. Excess shear heat wilbe evident if the center zone heater control does not cycle, calling for heat input. In this case, the measured melt temperaturewill be significantly above the set temperature of the nozzle.
of resin in the heating cylinder. It can be determined
experimentally by observing the time for a fewcolored resin particles to move from the back of thescrew at the hopper throat to the nozzle, while themachine is operating on cycle. A rough estimate canbe based on the assumption that the heating cylindercontains a quantity of Delrin equal to two times therated capacity. A formula for calculating holduptime from this estimate is included in Figure 5.
Stability problems with Delrin are frequentlycaused by inappropriate or malfunctioning moldingequipment. A heating cylinder, non-return valve, ornozzle with poor streamlining will expose at least
some of the resin to prolonged periods of heating.Holdup areas also result from improperly seatednozzles or adaptors. Poorly controlled cylinderheating zones or heater bands may also causeproblems. Equipment problems are discussed indetail in Equipment for Injection Molding ofDelrin.
Certain contaminants and impurities have an adverseeffect on the thermal stability of Delrin. For ex-ample, careful cleaning of equipment after moldingpolyvinyl chloride (PVC), flame retardant, or otheracid-generating resins is required before moldingDelrin acetal resin to avoid degradation. Degrada-tion can even result from the use of the wrongpigment or molding powder lubricant. Suggestionsfor choosing pigments and lubricants are given inAuxiliary Operations, and a thorough discussionof handling precautions are given at the end of thissection on page 18.
Back Pressure
Increasing back pressure increases the work doneby the screw on the melt. This, in turn, increasesthe melt temperature and its uniformity. Higherback pressure may be used to eliminate unmeltedparticles and to improve color mixing when colorconcentrates are used. Increasing back pressure,however, decreases the output of the screw, whichtends to reduce glass fiber length and changeproperties of filled resins such as Delrin 570. Backpressure should be used, therefore, only whenincreasing cylinder temperature or other changesare not effective or possible.
For low viscosity (high flow) resins, such asDelrin 1700, some suckback (no back pressure)may be required to control drool at the end of eachpressure cycle.
Injection PressureNormal injection pressures for Delrin acetal resinslie in a range of 70112 MPa (1016 kpsi), al-though higher and lower pressures are sometimesused. High injection pressures are used to fill longflow, thin sections, to decrease mold shrinkage, andto correct some of the problems discussed inTroubleshooting Guide. To obtain maximum part
toughness and elongation, an injection pressure of80100 MPa (1114 kpsi) may be necessary.Delrin 100may require an injection pressure of100 MPa (14 kpsi) or more to fill adequately,because of its higher melt viscosity and resultinggreater pressure loss in the nozzle, runner, etc. Foroptimum packing pressure, the length of the nozzleand runner system must be a minimum so as tominimize associated pressure loss.
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Figure 5. Effect of Temperature on Holdup Time of Delrin500
250
240
230
220
210
200
0 20 40 60 80 100
380
400
420
440
460
480
MeltTemperature
,C
MeltTe
mper a
ture
,F
Maximum Holdup Time, min
weight of resin in heating cylinder
shot weight
cycle (sec)
60xAverage Holdup Time, min =
Note:
Injection pressure may be limited by the force of theclamp when the projected area of the mold cavitiesand runners is large. The clamp force must be atleast equal to the product of projected area and theeffective cavity pressure, which will be less thaninjection pressure and dependent upon such factorsas length of flow, resin viscosity, and injection rate.
Exceeding this force will open the mold and flashthe parts. Injection pressure also may be limited bymold construction. Insufficient support will causemold deformation and will also result in flash.
Although single-stage injection pressure is fre-quently used for molding Delrin, two-stage and/orprogrammed injection are useful to accomplish bothcomplete filling and avoid flash, sticking in themold, etc. Misuses of multistage injection, such asexcessively high and prolonged first-stage pressure,can cause filling flaws as well as molded-in stressesnear the gate.
Injection RateThe optimum fill rate for a part depends on itsgeometry, the size and location of the gate, the melttemperature, and the mold temperature. Whenmolding thin section parts, high injection rates areusually required to fill the part before it freezes.Higher and more uniform surface gloss can beobtained if the injection rate is fast enough to allowthe cavity to be filled before the resin begins to
solidify, although localized surface flaws, such asjetting and gate smear, are often reduced bydecreasing the injection rate. Warping is sometimescorrected by careful adjustment of injection ratealong with mold temperature.
When molding parts with thick sections and
relatively small gates, it is sometimes desirable toreduce injection rate to delay freezing of the gateand allow packing for the maximum time. This isparticularly true for Delrin 100 type high viscosityresins. Slower fill will minimize melt shear stressand produce parts with lower residual crystallinestresses. Mold temperatures approaching 105C(220F) will also aid in the effort to complete thepack out. Use of an insulation material between thenozzle and sprue bushing frequently helps retainheat.
Injection rate and injection pressure are indepen-
dently controlled, although they are interrelated.The injection pressure is controlled by a reliefvalve that limits the maximum oil pressure on thescrew. The injection rate is controlled by a flowcontrol valve that regulated the rate of flow of oilto the injection hydraulic cylinder. In older injec-tion machines, the separation of injection pressurecontrol from injection rate control was less clear,resulting in processing constraints.
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The effect of screw forward time on part weightand mold shrinkage of a 3 mm (0.12 in) test bar isshown in Figure 6. Maximum part weight andminimum mold shrinkage occur at about 27 sec forthis thickness, which is the time when the gatefreezes in this case. The discussion of screwforward time and its effects assumes that the gate
will be adequately sized as discussed in Molds,so that the gate will not freeze before the cavity isfilled. It also assumes that the non-return valvefunctions properly and maintains a cushion of meltin front of the screw.
The relationship between part thickness and timefor the gate to freeze is shown in Figure 7. If thescrew forward times (equivalent to gate freezetimes) indicated in this figure are used, the moldshrinkage of Delrin 500 will be largely inde-pendent of part thickness and near 2% in all cases.
The elongation of 3 mm (0.12 in) test bars de-
creases as screw forward time is decreased belowgate seal time. This same property loss is notobserved in testing 1.5 mm (0.06 in) bars, whichare representative of a greater portion of commer-cial application of Delrin acetal resins than thethicker parts. Other toughness tests, such as Izodimpact, do not show the same sensitivity to screwforward and gate seal time, even at 3 mm (0.12 in)thickness.
Delrin can also be molded on cycles that areshorter than can be achieved by using a screwforward time equal to the time to freeze the gate,
although the screw forward time should not bereduced to the point where sinks and voids occur.Mold shrinkage will increase. Other end-userequirements should be checked to be certain thatquality is satisfactory.
Other factors that can influence cycle significantlyinclude:
Gate configuration and location
Mold cooling capacity or design
Molding machine, slow action, or insufficientplastifying capacity
Molding conditions selected
The cycle estimation graph in Figure 8 shows arange of total cycle times that have been used forgood quality molding of Delrin in parts of variousthicknesses. The cycle will be close to the lowerlimit when a high productivity resin such as Delrin
Molding CycleThe molding cycle is made up of several parts asshown in Figure 9. The major elements of cycleare:
Screw forward time
Hold time
Mold open timeScrew forward time involves injection time plus thetime that the screw is held forward by hydraulicpressure. During hold time, the screw rotates andretracts, and the moldings cool sufficiently to beejected when the mold opens.
In an ideal case, the molding cycle will be deter-mined largely by part thickness. The screw forwardtime, which is a function of part thickness andchanges slightly with mold temperature, will belong enough for the gate to freeze. This results inmaximum part weight, minimum mold shrinkage,
and minimum tendency to form sinks or voids. Insome cases, as described below, mechanicalproperties, especially elongation, will be greatestwhen screw forward time equals gate freeze time.When screw forward time equals or exceeds gatefreezing time, there is no need for additional partcooling time. The hold portion of the cycle needs tobe only long enough for screw retraction. However,when cooling time must be longer, screw retractiontime should be extended by slowing screw rotationto occupy most of the available cooling time. Thistends to give better melt and color uniformity.
Note: Before undertaking corrective action forpart quality, be sure the part is fullypacked out.
Measure and record the weight of several shots(parts only). Add time, nominally 510 sec, to thescrew forward time. Then repeat weighing andrecording several shots. If there is no weightchange, you have either a maximum weight or apremature gate freeze off. Premature gate freezingwould be evident by the presence of voids near thegate of a part cut in half through the gate area.Adjustment of the gate dimensions to retardfreezing would be needed. If a weight increase isobserved, repeat incremental increases in screwforward time until there is no further weightincrease. Part weight is now optimized, and othercorrective actions may be considered if they arestill needed.
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Figure 6. Screw Forward Time vs. Part Weight and Mold Shrinkage of Delrin 500
17.4
17.2
17.0
16.8
16.6
16.4
16.2
16.0
15.8
3.2
3.0
2.8
2.6
2.4
2.2
2.0
0 5 10 15 20 25 30 35
PartWeight,g
MoldShrinkage,
%
Screw Forward Time, sec
Test Bar Thickness,3 mm (0.12 in)
Figure 7. Gate Freeze Time vs. Part Thickness of Delrin 500
6
5
4
3
2
1
0
0.24
0.18
0.12
0.06
10080604020
PartThickness,mm
PartThickness,in
Screw Forward Time for Gate to Freeze, sec
Mold Temperature,93C (200F)
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Figure 8. Estimation of Cycles for High Quality Molding of Delrin Acetal Resins
160
140
120
100
80
60
40
20
00 1 2 3 4 5 6
0 0.040 0.080 0.120 0.160 0.200 0.240
Average Wall Thickness of Part, in
Average Wall Thickness of Part, mm
To
talCycleTime,sec
Spec
ialCa
ses
NormalR
ange
shrinkage and post-molding shrinkage, which is thesubject of a detailed discussion in DimensionalConsiderations. In fast cycling operations, it maybe necessary to use a lower mold coolant tempera-ture to maintain the mold surface temperature in therecommended range. Chilled water is often used forvery short cycles or to cool core pins and othermold sections that tend to run very hot. Occasion-ally, a mold surface temperature well below therecommended range is used to achieve minimum
cycle time when dimensional stability is notimportant. Mold surface temperatures as low as10C (50F) are also required to obtain maximumtoughness in the exceptional case of molding verythin sections, about 0.8 mm (0.03 in), from Delrin
100. This is not necessary for other Delrin resinsor for thicker sections of Delrin 100.
1700 is used and when end-use requirements areless stringent. The cycle will approach the upperlimit of the shaded area when Delrin 100 or 500isused and when requirements are more severe. Thelight band above this represents a small fraction ofthe molding operations where equipment is se-verely limiting or an unusual situation prevails, forexample, some very large parts or complex moldaction.
Mold TemperatureA mold surface temperature of 80105C(180220F) is recommended for molding Delrin
in order to obtain maximum dimensional stability,surface gloss, flow, and minimum molded-in stress.The preferred mold temperature range for Delrin
100ST and 500T is 1071C (50160F). Moldtemperature has a major effect on both mold
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Techniques for OptimumProductivity MoldingThe operating conditions for Delrin discussed inthe previous section cover the normal range ofmolding conditions, including those used to obtainmaximum mechanical properties and minimummold shrinkage. Although most applications for
Delrin
have demanding requirements, in many ofthese cases the specific requirements can be metusing molding conditions that will also optimizeproductivity.
The step-by-step procedure outlined below isintended as a guideline to aid in determining theoptimum molding cycle. It assumes a basic knowl-edge of molding Delrin as presented in other partsof this Molding Guide. Although the procedure isintended primarily for the high productivity resins,it can be used to develop optimum cycle for otherDelrin acetal resins as well.
A blank run sheet is included at the back of thisguide for recording cycle changes in following thisprocedure.
1. Obtain the Production Records on SalableParts Produced on the Present Cycle
This serves as a basis to judge productivityimprovements.
2. Determine the Critical Quality Parametersof the Part
This is a necessary step in order to determinewhen standard and nonstandard parts are
produced. Critical quality parameters can bepart weight, dimensions, tolerances, finish,flatness, etc. For practical reasons, it is desir-able to check the parts shortly after they are outof the mold to determine the relative effects ofmolding variables with a minimum time loss.However, it should be noted that as-moldeddimensions will change as the part is cooled,and, in a part where dimensions are critical,they should be measured about 24 hr aftermolding.
An approximate idea of what the part dimen-sions will be after standing 24 hr can be ob-tained by comparison of parts molded atdifferent conditions after accelerated coolingwhen the part is ejected. This can be done byimmersing the parts in cold water until com-pletely cooled before measuring dimensions.
It is important to keep accurate records andsave properly labeled parts as each moldingchange is made. If you attain a very fast cycle,but parts are found to be out of specificationafter standing for a day, your records mayindicate an intermediate cycle that may beoptimum, and you may not have to return to
your original cycle. For example, fine tuning ofprocess variables, such as injection pressure,may enable you to bring a part within dimen-sional tolerance at a faster cycle than seemedpossible. Also, relatively simple and inexpen-sive mold alterations, such as changing the sizeof a core pin, may be used to further shortenoverall cycle and still get satisfactory parts.
3. Check Equipment Operation
To maximize the benefits of using Delrin,especially high-yield grades such as Delrin
1700, all parts of the injection molding opera-
tion must be functioning properly. Frequently,an equipment problem can be easily corrected,which will allow you to attain the optimumlevel of productivity.
4. Begin Molding on the Usual Cycle
Begin molding on the established cycle, usingyour old production records to establish con-ditions. Usually, the initial cylinder temperatureis 204C (400F) average, and the mold cavitysurface temperature range is 66121C (150250F). Higher resin temperature will berequired as the cycle is reduced, so slightly
higher cylinder temperature or higher backpressure settings may be required. If the higherflow resins, Delrin 1700, are being tried, it isgood practice to reduce the injection pressureby 1020% on the first series of shots to avoidflashing the mold. Pressure can be readjustedlater to obtain optimum part quality. Steps 510refer to Figure 9, which schematically illus-trates the components of the molding cycle.
5. Keep Mold Open Short Time
The mold should remain open just long enoughto allow the parts and runner to drop. Low
pressure close protection should always beused. The opening stroke should be as short aspossible, and techniques, such as air jets, can beused to accelerate the fall of light parts. Rubberbumpers or springs should be used in a three-plate mold to prevent banging of the floatingplate. A mold with internal action (cams, cores,etc.) will have a greater travel time.
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10. Minimize Screw Forward Time
When the hold time has been minimized, and ifthe parts are still within specification, injectionhold time can be reduced in steps. Again, care-ful records should be kept and samples taken.The minimum injection hold is the shortest timein which acceptable parts are still ejecting
smoothly. Further reduction is usually limitedby deformation or penetration of the parts onejection or failure of the parts to hold desireddimensions. Mold temperature control mayagain have to be readjusted to maintain thedesired cavity temperature. The critical qualityparameters should be checked as each change ismade, because a too short screw forward timemay increase mold shrinkage or, under someconditions, reduce toughness properties.
11. Operate on the New Cycle
Run at this new cycle for 1 hr and periodically
check the critical quality parameters of theparts.
12. Enter New Cycle in Records
After several shifts on this new cycle, comparethe number of salable parts produced with thenumber obtained on the original cycle with theother material. Salable parts per shift can bedefined as:
Total Parts Produced Rejects
Number of Shifts
In order to take maximum advantage of the
improved processibility, it may be advanta-geous to consider some of the following areas:
Resin Supply SystemThis system must beable to keep the machine full at the higherproduction rates.
Mold Temperature ControlMold tempera-ture control is critical to obtain parts with gooddimensional control. A mold chiller is desirableto ensure rapid solidification of parts. Fastcycling will produce hot cavities even with coldmold coolant.
Mold OperationThe maximum advantage ofhigh productivity molding can best be attainedwith automatic mold operation.
MOLD OPENTIME
SCREWFORWARDTIME
HOLDTIME
CycleStart
ScrewStopped
Screw Rotatingand
Retraction InjectionHold
Part Cooling in Mold
Inject
Figure 9. Molding Cycle
6. Minimize Injection Time
The gate will start to freeze as soon as melt hasslowed or stopped flowing; therefore, a fastinjection contributes to minimum gate freezetime. The booster, or high-volume pump,should normally be used to fill the cavityrapidly. The screw travel time usually dependson the booster time and the first stage injectionpressure. Booster time should end just beforethe cavity is filled.
7. Raise Injection Pressure
The injection pressure should be raised to thestandard level that does not cause the mold toflash, provided that correct part dimensions aremaintained.
8. Minimize Hold Time
Hold time can frequently be reduced to equalthe screw retraction time. Screw rpm can beincreased to minimize screw retraction and holdtime. With a well-designed and maintainedshutoff nozzle, hold time might be reducedfurther, because the mold can be opened andthe parts dropped while the screw is stillretracting.
9. Adjust Mold Temperature Controller
The temperature of the fluid used to control themold temperature may have to be reduced inorder to maintain the desired surface tempera-ture as the cycle is decreased and resin through-put is increased.
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Mold VentingVenting of the mold is criticalto part quality. As cycles are reduced, fasterinjection puts a greater load on the vents. Thetrapped air must rush out freely as the melt fillsthe cavity. Because Delrin acetal resin burnscleanly, poor venting is not easily detected. Amethod of detecting venting problems is
described in the next section.Proper venting should also help prevent moldcorrosion and deposits.
Mold Runner SystemAnother way to reducecycle time is to reduce sprue and runner size tothe absolute minimum that will do the job.Look for ways to make the sprue as short aspossible. Perhaps you can use an extendednozzle into the mold. Reducing the sprue andcold slug diameter should reduce the cycle,because a smaller cold slug at the sprue pullerwill freeze sooner and allow sprue pull sooner.
Runner diameters can often be smaller whenusing fast-filling, fast-freezing crystalline resinsor the low viscosity Delrin 1700 acetal resin.
Part HandlingPart handling and finishingequipment must be capable of handling thehigher production rate. If part handling be-comes the limiting factor, you can find a list ofthe suppliers of parts handling systems inpublications such as theModern PlasticsEncyclopedia.
Handling PrecautionsTo avoid decomposition of Delrin, the resinshould not be exposed to melt temperatures nearthe holdup time limits shown in Figure 5,forexample, no more than 20 min at 230C (450F).If there is any doubt about the actual melt tem-perature, a reading should be taken with a hand
pyrometer.
See current Material Safety DataSheet (MSDS) for health and safetyinformation. To obtain a currentMSDS, call Dial DuPont First at(302) 999-4592 or DuPont ProductInformation at (800) 441-7515.
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Molds
Delrin acetal resins have been used in many typesof molds, and molders have a wealth of knowledgein mold design for Delrin. Molds for Delrin arebasically the same as molds for other thermoplas-tics. The parts of a typical mold are identified inFigure 10.
This section will focus on the elements of molddesign that deserve special consideration forprocessing Delrin and can lead to higher produc-tivity and lower cost for the molder. These topicsare:
Ability to fill Undercuts
Gates Runnerless molds
Runners Mold maintenance
Vents
Mold shrinkage and other aspects of moldsizing are discussed in Dimensional Consider-ations.
Ability to FillMelt viscosity largely governs the ability of a resinto fill a mold. Delrin acetal resins range in melt
viscosity from Delrin
1700, the lowest in viscosityor most fluid, to Delrin 100, the highest. Themelt viscosity of Delrin resins are compared inFigure 1. The viscosity of Delrin does not de-crease rapidly as melt temperature increases, incontrast to some other thermoplastic resins, suchas acrylic resin. Increasing melt temperature willnot greatly improve the ability of Delrin to fill athin section.
Figure 10. Exploded View of Mold
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In addition to the properties of the resin, themolding conditions and cavity thickness determinethe distance of flow. Figure 11 shows the maxi-mum flow distances that can be expected at twocavity thicknesses for Delrin acetal resins as afunction of injection pressure. These comparisonswere made in an open-ended snake flow mold with
no gate restriction. Obstructions in the flow path,such as sudden changes in flow direction or corepins, can significantly reduce the flow distance.
GatesThe gates of a mold can play a major role in thesuccess or failure of a molding job. The location,design, and size of a gate are all important.
Figure 11. Maximum Flow Distance of DelrinAcetalResins
60 80 100 120
10 15 20
F
lowDistance,
in
Injection Pressure, MPa
500CL500
900
140
Injection Pressure, 103 psi
50
40
30
20
10
0
FlowDistance,mm
1200
1000
800
600
400
200
0
1700
100, 150SA, 100ST
570, 500AF
500CL500, 500T
900
150SA100
500AF
570
1700
Melt Temperature, 210C (410F)Mold Temperature, 96C (205F)
2.5 mm (0.100 in)1 mm (0.040 in)
Gate LocationThe gate should be located where finishing will beunnecessary or, if that is not possible, wheretrimming or finishing will be easiest. In general,when a part is not uniform in wall thickness, itshould be gated into the thickest section forgood packing.
An area where impact or bending will occur shouldnot be chosen as the gate location, because the gatearea may have residual stress and be weak. Simi-larly, the gate should not cause a weld line to occurin a critical area.
The gate should be positioned so that the air will beswept toward a parting line or ejector pinwhereconventional vents can be located. For example, aclosed-end tube such as a pen cap should be gatedat the center of the closed end, so air will be ventedat the parting line. An edge gate will cause airtrapping at the opposite side near the closed end.
When weld lines are unavoidable, for examplearound cores, an escape for gases must be providedto avoid serious weakness and visual flaws. Spe-cific recommendations for venting are given later inthis section.
Another consideration in choosing a gate locationfor Delrin is surface appearance. Gate smear orblush, as well as jetting, are minimized by locatingthe gate so that the melt entering the cavity im-pinges against a wall or core pin. Surface appear-ance is sometimes better when a part of variablethickness is gated into the thin section.
This may lead, however, to sinks and voids in thethick sections, especially if they are twice as thickor more. Specific remedies for these surfaceproblems, as well as sinks and voids, are given inTroubleshooting Guide.
A central gate location is often necessary to con-trol roundness of gears and other critical circularparts. Multiple gates, usually two to four, arecommonly used when there is a central hold toavoid a difficult-to-remove diaphragm gate.
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Gate DesignBoth rectangular (edge) and round (tunnel or three-plate) gates are commonly used in molds forDelrin. Gates should be flared at the entrance tothe cavity to improve surface appearance. Flaring isnot possible in a tunnel gate, and it may cause anunacceptable finishing problem or gate vestige in
other cases. Gates more complex than those men-tioned, such as ring, diaphragm, flash and fan gates,are sometimes employed in molds for Delrin
acetal resins when special hardware limitations orpart properties dictate.
The gate thickness or diameter of rectangular orround gates should be equal to one-half of partthickness or slightly greater to achieve goodpacking, minimum shrinkage, and optimum me-chanical properties.
Both impinging and non-impinging rectangulargates are shown in Figure 12. The overlap modifi-
cation of a rectangular gate has been used where animpinging gate location is not available. Thefundamentals of gate design are described briefly inFigure 13. Details of the rectangular gate areshown in Figure 14.
In rectangular gates, the area can be controlledindependently of gate thickness by changing width.Consequently, gate area can be enlarged to increaseflow without changing thickness and alteringfreezing time. Gate width is frequently 1.5 timesthe gate thickness or greater, depending on thevolume of melt that must flow through the gate.
Gates several times as wide as the gate thicknessare often used for large parts.
Figure 15 illustrates the common types of roundgates: three-plate and tunnel (submarine) gates.A detailed drawing of the three-plate gate is shownin Figure 16. In the case of round gates, increasinggate area for greater flow necessitates an increase indiameter or gate thickness, which will increase thescrew forward time necessary for the gate to freeze.It may be preferable to use multiple gates.
The maximum diameter of these round gates islimited by the need to break when the mold opens
and parts are ejected. Tunnel gates as large as 2.2mm (0.085 in) and three-plate gates as large as 2.8mm (0.110 in) in diameter have been used and ejectsatisfactorily.
Figure 12. Rectangular Gates
IMPINGING EDGE GATE
Side View
Bottom View
NON-IMPINGING EDGE GATE
Side View
Bottom View
Side View
Bottom View
OVERLAP GATE
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Figure 13. Gate Design Principles Tunnel gates of two general varieties are used, shorttunnels and long tunnels, as shown in Figure 17.Generally, the short tunnel is preferred. When along tunnel is used, the angle between the part andthe tunnel is important. With a mold temperature inthe range of 6593C (150200F), the angleshould not exceed 30, to ensure gate shearing.
Thinner gates than those suggested above have beenused in some cases when maximizing productivityis of primary importance and some loss of mechani-cal properties and increase in mold shrinkage areacceptable. On the other hand, gates thicker thanone-half of part thickness may be required for verythin parts, less than 1.5 mm (0.06 in) thick, espe-cially long flow, thin parts. Larger gates may alsobe needed for Delrin 100 acetal resins, becausehigher molecular weight reduces flow. Because oftheir increased toughness, gating for Delrin 100STand 500T may need special consideration whenrapid degating is desired.
Figure 14. Detail of Rectangular Gate
Runner
SIDE VIEW
z = Max. 1 mm (0.04 in)
T = Part ThicknessT
x = 0.5T
T
TOP VIEWALTERNATE DESIGNS
y = 0.5T
Delrin 100 may require z = Min. 0.25 mm (0.01 in).NOTE:
Oval Gate Cross Section
x = Gate Minor Dimensiony = Gate Major Dimensionz = Land Length
Area (x y) and gate thickness (x)control melt flow volume
Keeping x constant and increasing y, allows more melt volume to pass through the gate without affecting freeze time.
Gate thickness (x) and land (z) control gate freezing, and both have a minor effect on pressure drop.
y
x
CAVITY
GATE
RUNNER
MELTFLOWDIRECTION
z
x
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Figure 15. Round Gates
Figure 16. Detail of Three-Plate Gate
D
Radius
D
d
T
0.8 mm (0.03 in) Max.
D1
Gate diameter, d = 0.50.6 of part thicknessD > D1
21
Figure 17. Types of Tunnel Gates
D1
LongTunnel
Max.30
T
0.5 T
ShortTunnel
D
D1
Max.30
T
0.5 T
D
D > DD > 1.2T
1
D > DD 1.2T
1
Tunnel Gate
Three-Plate Gate
PL3
PL2
PL1
PL
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RunnersKey guidelines to follow when designing a runnersystem include the following:
Plan a layout to transmit pressure uniformly to allcavities.
Make large enough for adequate flow and mini-mum pressure loss.
Keep size and length to the minimum consistentwith previous guidelines.
Each factor can affect the quality of molded parts.Rework in moldings should be kept to a minimumto avoid contamination and reduce costs. On theother hand, runners must be large enough for rapidand controlled filling of parts with optimumproperties.
The optimum runner cross section is the full round,which minimizes both heat and pressure losses for agiven runner weight. Trapezoidal runners are
usually easier to machine and almost as good. Theeffective cross section of a trapezoidal runner isequal to the diameter of the full round that can beinscribed in it. One case shown in Figure 18, inwhich the full round runner cannot be used, is thatof the mold with sliding action across the partingline where the runner would be involved.
Figure 18. Mold with Sliding Action
TrapezoidalRunner
For parts of Delrin with the best physical proper-ties, the runners next to the gate should have anequal or slightly greater thickness as the wallsection at the gate. When the moldings are verythin, however, this runner cannot be less than about1.5 mm (0.06 in) in thickness. The runner thicknessis usually increased at each of the first one or two
turns from the cavity. These increases can often bekept very small when the parts are small. Because itis easy to increase runners that prove to be too thin,runners should be designed to minimize size. Theamount of rework and the pressure drop alsodepend on runner length or layout. A balancedlayout where the flow distance between each cavityand the sprue can be made essentially equal isgood, but it should not be used when the number ofcavities makes it too complex and long. A lateralrunner is quite satisfactory in most cases. Bothbalanced and lateral runner systems are shown inFigure 19.
VentsVenting a mold for Delrin is particularly impor-tant, and special attention should be given to thisfactor during both the design of the mold and itsinitial trial. This attention is required becauseburning of parts caused by inadequate venting isnot easily observed with Delrin. With other resins,poor venting results in a blackened and burned spoton the part. With Delrin, however, there may beeither no visible flaw or an inconspicuous whitishmark on the molding.
Venting problems with Delrin
acetal resins may bemade more obvious by spraying the mold with ahydrocarbon or kerosene-based spray just beforeinjection. If venting is poor, the hydrocarbon willcause a black spot where the air is trapped. Thistechnique is particularly useful for detecting poorvents in multicavity molds. A convenient source ofa hydrocarbon spray is a rust preventative spraysuch as DME Mold Saver1 or Rust Veto.2
1 Detroit Mold Engineering Co., 29111 Stephenson Highway,Madison Heights, MI 48071.
2
Rust Veto is the registered trademark of E. F. Houghton & Co.,Madison and Van Buren Avenues, Valley Forge, PA 19482.
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Balanced Runner
Lateral Runner
Figure 19. Runner Systems
Inadequate venting of molds for Delrin may cause agradual buildup of mold deposit where vents shouldbe located and in mold crevices through which
limited venting has taken place. These depositsconsist of a white solid material formed from thetraces of gas evolved during normal molding. Goodvents allow this gas to escape with the air.
Poor venting may also cause pitting or corrosion ofthe metal surfaces of the mold. This is the result ofrepeated brief exposure to very high temperaturesfrom rapid compression of air and gases. In somecases, when adequate venting cannot be provided, itmay prove desirable to use stainless steel for cavitiesor to plate them with chrome or electroless nickel.
Venting problems may be aggravated by high melt
temperature, long holdup time, or holdup areas in themolding cylinder, which will generate more thannormal amounts of gas (see Equipment for Injec-tion Molding of Delrin and Operating the Mold-ing Machine). Fast injection speed will alsoaggravate these problems. Remedies for molddeposit problems are listed in TroubleshootingGuide.
Venting usually occurs through the parting line of amold. It is common practice to extend the cavities
about 0.025 mm (0.001 in) above the cavity plates.This should ensure that the cavities will seal andthat venting at the parting line will be easier. Better
venting at the parting line will be provided bymilling channels in the cavity plate and inserts. Thevent depth from the cavity should be no more than0.03 mm (0.0013 in). It is important to keep thevent land short, about 1 mm (0.04 in), and the ventchannel depth beyond this land should be 0.8 mm(0.03 in).
In some cases, venting may be accomplishedaround a knockout pin. This vent will also beimproved by milling flats on the pin and relievingthe vent after a short land. Pins that do not movewith the ejection system tend to clog and no longer
provide venting after a short run.Venting the runner system may be helpful inreducing the amount of air that must be ventedthrough the cavities. Because flash is unimportanton the runner, these vents can be slightly deeperthan cavity vents, for example, 0.06 mm(0.0025 in).
An example of correct venting of cavities andrunners is illustrated in Figure 20.
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Figure 20. Venting
Part Part
Main Vent Channel
To Mold Edge
Part VentLand Length
Runner VentLand Length
Main Runner
All vent channels 0.8 mm (0.03 in) deepRunner vent land length 1 mm (0.04 in); Max. depth 0.06 mm (0.0025 in)Part vent land length 1 mm (0.04 in); Max. depth 0.03 mm (0.0013 in)
UndercutsGeneral suggestions for stripping undercuts withDelrin acetal resins are:
The undercut part must be free to stretch orcompress, that is, the wall of the part opposite theundercut must clear the mold or core beforeejection is attempted.
The undercut should be rounded and well-filletedto permit easy slippage of the plastic part over themetal and to minimize stress concentration duringthe stripping action.
Adequate contact area should be providedbetween the knockout and plastic part to preventpenetration of the molded part or collapse of thinwall sections during the stripping action.
The length of the molding cycle should bestudied to determine the best time for ejection.Sufficient part rigidity must be developed withoutcausing binding, due to excessive in-the-moldshrinkage around pins forming an internalundercut. Ejection of parts with undercuts on theoutside diameter will be aided by in-the-moldshrinkage.
Higher mold temperature, which keeps the parthotter and more flexible when the mold opens,may aid ejection from an undercut.
Generally, parts of Delrin acetal can be moldedwith a maximum 5% undercut. Calculation ofallowable undercut is illustrated in Figure 21.The allowable undercut varies somewhat withboth wall thickness and diameter.
Runnerless MoldsThe term runnerless molding refers to any injectionmolding process in which the sprue and runners arenot removed from the mold along with the partsduring each cycle. Runnerless molds have beenmade in a variety of designs.
In one type of design, the melt distribution systemor runner manifold is operated at a temperatureabove the melting point or flow point of the resinbeing molded. The content of the entire manifold iskept molten. This design is generally not satisfac-tory for use with Delrin acetal resins, because it isdifficult to avoid holdup of molten resin and theresulting degradation.
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(A B) 100
B= % UNDERCUT
(A B) 100
C= % UNDERCUT
Figure 21. Calculations for % Undercut
A
B
A
B
A
B
C
A
B
COUTSIDE
MOLDED PART
INSIDE
An aerosol cleaner called Slide* Resin Remover(The Stripper) has been used to remove molddeposit before serious buildup occurs duringproduction runs. This spray is applied betweenshots and a few shots are discarded.
The deposit often can be vaporized easily by heat.Gentle flaming with a propane torch or heating witha hot-air gun has been used. Other methods ofremoval depend upon a combination of chemicalaction and mechanical rubbing. Automotive chrome
cleaners and heavy-duty cleaning compounds, suchas trisodium phosphate cleaners, have provenuseful. The best solutions for mold deposit are toprevent it with better venting, milder processingconditions, and a well-maintained machine or byusing an alternate grade of Delrin.
Another possible type of mold deposit is a thinbrown substance that may occur near gates andother areas of high shear in the mold. This can beremoved by cleaning with isopropyl alcohol. Aminor accumulation of this deposit can sometimesbe removed by spraying the affected area with
isopropyl alcohol between shots.
* Slide is the registered trademark of Percy Harms Corp., 430 S.Wheeling Rd., Wheeling, IL 60090.
A runnerless mold design particularly suitable forDelrin has been developed by DuPont. Moredetails may be obtained by contacting yourDuPont representative through the sales officeslisted on the back of this bulletin.
Mold MaintenanceAs a general rule, molds for processing Delrin
require the same care as those for processing otherthermoplastic materials. Simple wiping down of the
mold and application of a rust-preventing solutionis usually adequate after a production run. Ofcourse, the local storage conditions and the antici-pated period of storage largely govern the preven-tive maintenance that should be given to the mold.A sample maintenance record sheet is shown at theend of this guide.
Mold DepositHowever, a few molds for Delrin acetal resins dorequire special care, especially if they are notadequately vented. In such cases, a white depositoccasionally forms in the mold. It may provenecessary to remove it during a production run, andit certainly should be removed at the end of a run.
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Dimensional ConsiderationsParts molded of Delrin acetal resins have gooddimensional stability over a wide range of environ-mental conditions. As with all materials, however,there are factors affecting the dimensional stabilityof parts of Delrin that must be considered whenclose tolerances are essential. These factors are
discussed in this section: Fundamentals of dimensional control
Mold shrinkage
Post-molding shrinkage
Annealing
Environmental changes
Tolerances
Fundamentals of DimensionalControlThe dimensions of a molded part are determined
primarily by those variables that affect cavitypacking and cooling rate. Both injection pressureand screw forward time (until the gate freezes)have an important effect on cavity packing (andpart weight). The cooling rate is largely a functionof mold temperature and part thickness.
Cooling rate affects the dimensions of moldings,because Delrin is a crystalline resin. In the melt,the polymer chains are in a disordered mass. Whenthe polymer solidifies, the chains form a moreorderly and dense crystalline arrangement. Com-plete crystallization is not possible, but the slower
the cooling, the greater the degree of crystalliza-tion. With increasing crystallinity, density andshrinkage also increase.
The dimensional changes that may occur aftermolding can be either reversible or irreversible.Reversible changes result from the thermal expan-sion or contraction and from absorption of water orother solvents. These are discussed later in thissection, under Environmental Changes.
Irreversible changes in dimension occur whenpolymer chains frozen in an unstable conditionmove toward a more stable state, for example,
relief of molded-in stresses or increase in crystal-linity. These changes are discussed under Post-Molding Shrinkage and Annealing.
Sizing a cavity for Delrin requires allowance forboth mold shrinkage and post-molding shrinkage.Mold shrinkage is the difference in size betweenthe cavity at room temperature and the molded partsoon after it has cooled to room temperature,usually within one hour. Post-molding shrinkagemay continue indefinitely after that time.
Mold ShrinkageThe mold shrinkage of Delrin acetal resins isdependent on such factors as:
Mold temperature
Injection pressure
Screw forward time
Melt temperature
Gate size
Part thickness
Composition (i.e., glass, filler)
Good molding practices, however, lead to choicesamong these variables so that the mold shrinkagesof Delrin 500 and several other compositions arevery often near 2.0% (0.020 mm/mm [in/in]). Thissection provides a detailed discussion of thesefactors and a procedure for estimating mold shrink-age that will usually result in parts with the requireddimensions.
Estimating Mold ShrinkageThe nomograph in Figures 22 and 23 is used forestimating mold shrinkage and was derived from a
dimensional analysis of moldings of Delrin
. Itrelates mold shrinkage to the combined effects ofpart thickness and gate area at these approximatemolding conditions: a mold temperature of 93C(200F), injection pressure of 112 MPa (16 kpsi),and melt temperature of 210C (410F).
There are two sets of values on the mold shrinkageline, marked optimum and typical. Both sets ofvalues are based on the molding conditions statedabove. For the optimum values, it is assumed thatthe gate is designed as described in Molds, with aminimum gate thickness equal to one-half of thewall thickness. It is also assumed that screwforward time is at least equal to the time requiredfor the gate to freeze. These optimum conditionsrepresent good molding practice for Delrin,
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Figure 22. Estimating Mold Shrinkage for DelrinAcetal Resin (Delrin 500, 500CL, 900)
Nomograph in SI UnitsPart
Thickness Gate Area
Mold Shrinkage (mm/mm)
optimum typical
mm mm2
(3)(2)
(1)
0.6
0.4
0.8
1.0
1.5
2
3
4
6
8
10
15
20
0.035
0.030
0.025
0.020
0.015
0.010
0.015
0.020
0.025
0.030
0.035
0.8
1.5
3
6
10
20
40
80100
60
30
15
8
4
2
1.0
0.6
Figure 23. Estimating Mold Shrinkage for DelrinAcetal Resin (Delrin 500, 500CL, 900)
Nomograph in English UnitsPartThickness
Gate Area
Mold Shrinkage (in/in)
optimum typical
in in2
(3)(2)
(1)
0.035
0.030
0.025
0.020
0.015
0.010
0.020
0.025
0.030
0.035
0.015
0.002
0.004
0.008
0.015
0.030
0.060
0.001
0.003
0.006
0.010
0.020
0.040
0.0800.100
1.00
0.80
0.60
0.40
0.30
0.20
0.15
0.10
0.08
0.06
0.04
0.03
0.02
Example:A straight lineconnecting a part thickness of0.12 in on Line (1) to a gatearea of 0.0072 in2. A 0.06 inthick 0.12 in wide gate on Line(3) intersects Line (2) at 0.020or 0.025 in/in for optimum ortypical mold shrinkage, respec-tively, at a mold temperatureof 200F.Important:See Mold Shrink-age for additional discussion ofmolding conditions, limitations,
and correction factors for lessthan optimum mold design,cycle settings, and use of othercompositions.
Example:Draw a straight lineconnecting a part thickness of3 mm on Line (1) to a gate areaof 4.5 mm2. A 1.5 mm thick 3 mm wide gate on Line (3)intersects Line (2) at 0.020 or0.025 mm/mm for optimum ortypical mold shrinkage, respec-tively, at a mold temperatureof93C.Important:See Mold Shrink-age for additional discussion ofmolding conditions, limitations,and correction factors for lessthan optimum mold design,cycle settings, and use of other
compositions.
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although correction factors can be used for somevariation from them. A factor of 0.001 mm/mm(in/in) should be added to mold shrinkage estimatesfor an increase of 14C (25F) in mold temperature(i.e., cavity surface temperature) or subtracted for asimilar decrease. A factor of 0.0004 mm/mm (in/in)should be subtracted from the mold shrinkage
estimate for each 7 MPa (1 kpsi) increase in injec-tion pressure or added for a similar decrease ininjection pressure. Pressure adjustment is recom-mended, because it does not affect post-moldshrinkage.
When molding Delrin with a screw forward timeless than the time required for gate seal,the shrinkage values are closer to those shownas typical values. These shrinkage values areusually larger than the optimum values. Thismolding technique is sometimes used when maxi-mum properties are not required and/or dimen-sional tolerances are large. But it is not possible togive a specific correction factor. The increase inshrinkage may range from 0.001 to 0.010 mm/mm(in/in) depending on the amount of reduction inscrew forward time, although the increase is oftenfrom 0.002 to 0.005 mm/mm (in/in). The shrinkagebecomes constant beyond the recommended screwforward time that is required for gate seal. Gate sealtime is discussed in greater detail under MoldingCycle in Operating the Molding Machine, andan estimate of gate freezing time for variousthicknesses is presented in Figure 7.
The shrinkage estimates from the nomographin Figures 22 and 23 may be used for Delrin 500,500CL, and 900. Delrin 100 will be 0.002to 0.004 mm/mm (in/in) higher than the nomographestimate. Certain colored compositions of Delrin
have shown a lower mold shrinkage than theirnatural counterparts. Shrinkages of Delrin 500AF
and 570 are discussed on page 31 and in Table 5.For parts of uniform wall thickness, mold shrink-age tends to be uniform throughout. For partshaving variable thickness, sh