Celcon POM Processing and Troubleshooting Guide - Hi Polymers

Processing Celcon ®acetal copolymerProcessing Celcon ® acetal copolymerCE-6

Celcon ®acetal copolymerForewordThe Celcon ® Acetal Copolymer Processing and Troubleshooting guide is written for plastics processors whorequire specific information on start-up, processing techniques and troubleshooting using this versatile group ofproducts. Material handling techniques, resin drying conditions and health and safety issues are also included.Chapters 1 and 2 cover an introduction to Celcon acetal copolymer grades and physical characteristics,regulatory and flammability listings, start-up and shutdown procedures, and the safety and health aspectspertaining to handling Celcon acetal copolymer. This information is pertinent to all processing methods.These two chapters should be read before attempting to process any grade of Celcon acetal copolymer.Chapter 3 is devoted to the important topic of molded part dimensional stability, including part shrinkage,annealing, dimensional tolerances and the effect of moisture absorption on part dimensions. Each of the finalfour chapters of the manual describes a specific processing technique: injection molding, extrusion, blow moldingand rotational casting, and includes a troubleshooting section. Information on machine settings, mold design, and(where appropriate) screw design is also included.For more information on material characteristics and part and mold design, consult the following manuals:Celcon acetal copolymer Short Term Properties (CE-4), Designing with Plastic: The Fundamentals (TDM-1)and Designing with Celcon acetal copolymer (CE-10). They are available by contacting your local Ticonasales representative, by calling our Technical Information Hotline at 1-800-833-4882, or on our web site,www.ticona.com.Comments and suggestions for improving this and other Ticona literature are always welcome, and may be sent tous at the above phone number, by writing to us at the address shown on the back cover or by e-mailing thewebmaster on our internet site.1

Celcon ®acetal copolymerTable of ContentsIntroduction1. Overview 71.1 Chemistry of Acetal Copolymers 71.2 General Characteristics 71.3 Product Types 71.4 Regulatory Codes and Agency Listings 81.5 Product Support 82. General Guidelines 112.1 Storage and Handling 112.2 Safety and Health Information 112.3 Flammability 112.4 Drying 122.5 Processing Start-Up 122.6 Changing from Another Resin 122.7 Changing from a Different Grade of 13Celcon ® Acetal Copolymer2.8 Processing Shutdown 132.9 Use of Regrind 132.10 Secondary Operations 142.10.1 Finishing 142.10.2 Surface Treatment 143. Dimensional Stability 153.1 Shrinkage Caused by Processing 15(Injection Molding)3.2 Part Warpage 173.3 Post-Molding Shrinkage 173.4 Annealing 173.5 Tolerances 183.6 Moisture Absorption 184. Injection Molding 194.1 Equipment 194.1.1 Barrel and Screw 194.1.2 Nozzles 204.1.3 Plasticizing Capacity 204.1.4 Clamping Force 204.2 Mold Design 214.2.1 General Criteria 214.2.2 Mold Bases 214.2.3 Mold Cavities and Cores 214.2.4 Mold Surface Finish 21OverviewGeneralGuidelinesDimensionalStabilityInjectionMoldingBlow MoldingExtrusionRotationalCasting12345672

Celcon ®acetal copolymer4.2.5 Sprue Bushings 224.2.6 Runners 224.2.7 Runnerless Molding 234.2.8 Molded-In Inserts 234.2.9 Outsert Molding 244.2.10 Gating 244.2.11 Vents 264.2.12 Cooling Channels 264.2.13 Draft 264.2.14 Parting Line 264.3 Auxiliary Equipment 274.3.1 Mold Temperature Control Units 274.3.2 Process Control 274.4 Processing 274.4.1 Typical Molding Conditions 274.4.2 Melt Temperature 274.4.3 Mold Surface Temperature 284.4.4 Injection Pressure 284.4.5 Cavity Pressure Measurement 28(CPM) Technology4.5 Cushion 294.5.1 Injection Speed 294.5.2 Solidification Time 294.5.3 Decompression Settings 294.5.4 Screw Speed 294.5.5 Cycle Time 294.5.6 Process Optimization: Conducting a Design 29of Experiments (DOE)4.6 Quality Control of Molded Parts 304.6.1 Part Weight 304.6.2 Part Dimensions 304.7 Effect of Molding Conditions on 30Mechanical Properties4.7.1 Unreinforced Celcon ® Acetal Grades 314.7.2 Glass/mineral Coupled Celcon Acetal Grades 314.8 Molding Problems 324.8.1 Deposits in Mold 324.8.2 Troubleshooting 325. Blow Molding 345.1 Blow Molding Methods 345.1.1 Extrusion Blow Molding 345.1.2 Injection Blow Molding 345.2 Equipment 345.2.1 Extruder 345.2.2 Screws 345.2.3 Screen Pack 355.2.4 Breaker Plate 355.2.5 Die Head 353

Celcon ®acetal copolymer5.2.6 Die 365.2.7 Hopper 365.2.8 Molds 365.3 Processing Parameters 375.3.1 Barrel Temperature 375.3.2 Mold Temperature 385.3.3 Blowing Pressure 385.4 Effects of Process Variables on Part 38Dimensional Stability and Part Quality5.4.1 Mold Shrinkage 385.4.2 Surface Appearance 395.4.3 Impact Strength 396. Extrusion 416.1 Equipment 416.1.1 Materials of Construction 416.1.2 Extruder Barrel 416.1.3 Screw Design 416.1.4 Screen Pack 426.1.5 Head and Die Design 426.1.6 Hopper 426.2 High Speed Tubing Extrusion 426.2.1 Equipment 426.2.2 Processing Parameters 436.3 Profile Extrusion 446.3.1 Equipment 446.3.2 Processing Parameters 446.3.3 Troubleshooting 446.4 Profile Extrusion 446.4.1 Equipment 446.4.2 Processing 45OverviewGeneralGuidelinesDimensionalStabilityInjectionMoldingBlow MoldingExtrusionRotationalCasting12345677. Rotational Casting 487.1 Equipment 487.2 Molds 487.2.1 Particle Size 487.3 Processing Parameters 487.3.1 Resin Drying Conditions 487.3.2 Part Heating Oven Parameters 487.3.3 Part Cooling Rates 497.4 Troubleshooting 494

Celcon ®acetal copolymerList of TablesTable 1.1 Regulatory Listings 9Table 2.1 Typical Start-Up Conditions 12Table 2.2 Effect of Remolding (Regrind) on the 13Properties of UnreinforcedCelcon ® AcetalTable 2.3 Effect of Remolding (Regrind) on the 14Properties of Glass-CoupledCelcon AcetalTable 3.1 Effect of Processing Conditions 15on Part ShrinkageTable 3.2 Shrinkage Before and After Annealing 17Table 3.3 Recommended Annealing Procedure 18Table 4.1 Typical Injection Molding Screw for 20Plasticizing Celcon Acetal CopolymerTable 4.2 Typical Runner Size Recommendations 23for Celcon Acetal CopolymerTable 4.3 Recommended Gate Dimensions for 24Rectangular Edge GatesTable 4.4 Typical Start-Up Conditions 28Table 4.5 Approximate Cycle Times as a Function 29of Wall Thickness – Unreinforced GradesTable 4.6 Effect of Molding Conditions on 31Mechanical Properties -Unreinforced GradesTable 4.7 Typical Molding Conditions for 32Shrinkage Range - Unreinforced GradesTable 4.8 Troubleshooting Guide - Injection 32Molding Celcon Acetal CopolymerTable 5.1 Comparison of Injection and Extrusion 35Blow Molding ProcessesTable 5.2 Typical Screw Characteristics to Plasticize 36Celcon Acetal Copolymer Blow MoldingTable 5.3 Typical Blow Molding Conditions 37Table 5.4 Troubleshooting Guide - Blow molding 39Table 6.1 Recommended Metering Screw 42Dimensions for Extruding Celcon AcetalTable 6.2 Typical Conditions for Tubing Extrusion 43Table 6.3 High Speed Tubing Extrusion 45Troubleshooting GuideTable 6.4 Typical Conditions for Film 46and Sheet ExtrusionTable 6.5 Film and Sheet Troubleshooting Guide 46Table 6.6 Profile Extrusion Troubleshooting Guide 47Table 7.1 Rotational Casting Troubleshooting Guide 495

Celcon ®acetal copolymerList of FiguresFig. 3.1 Effect of Molding Conditions and 16Wall Thickness on Mold ShrinkageFig. 3.2 Shrinkage due to Heat Aging for 9.0 17Standard Melt Flow Grade of Celcon ® AcetalFig. 3.3 Water Absorption by Unfilled Celcon 18Acetal Under Various ConditionsFig. 3.4 Dimensional Changes due to Water 18Absorption by Unfilled Celcon AcetalFig. 4.1 Typical Screw Profile for Injection 19Molding Celcon AcetalFig. 4.2 Recommended Check Valve Design 20Fig. 4.3 Recommended Molded-in Insert Designs 24Fig. 4.4 Outsert Molded Moving Parts: Gear, 24Cam and SpringFig. 4.5 Some Basic Gate Designs Suitable 25for Celcon AcetalFig. 5.1 Flow Pin Divider to Promote Smooth 35Flow and Avoid Weld LinesFig. 5.2 Parison Die with Adjustable Core 36Pin for Control of Parison ThicknessFig. 5.3 Effect of Mold Temperature on Shrinkage 38Fig. 5.4 Effect of Cooling Time on Shrinkage 38Fig. 5.5 Effect of Blow Pressure on Shrinkage 38Fig. 5.6 Effect of Wall Thickness on Shrinkage 38Fig. 5.7 Landed Pinch-Off for Improved 39Impact StrengthFig. 5.8 Blow Molded Fluidics Container 39Fig. 6.1 Recommended Metering Screw for Extrusion 41Fig. 6.2 Miscellaneous Post-Formed Profiles 45Fig. 7.1 Rotational Molding Process 48OverviewGeneralGuidelinesDimensionalStabilityInjectionMoldingBlow MoldingExtrusionRotationalCasting12345676

Celcon ®acetal copolymer1. Overview1.1 Chemistry of Acetal PolymersAcetal polymers are chemically known aspolyoxymethylenes (POM). Two types of acetalpolymers are commercially available:Homopolymer is prepared by polymerizinganhydrous formaldehyde to form a polymercomposed of oxymethylene repeating units(-CH 2 O-). Acetal homopolymer products havesomewhat better short term mechanical propertiesthan the copolymer.Copolymers, including Celcon ® acetal copolymer,are prepared by copolymerizing trioxane (a cyclictrimer of formaldehyde) with a cyclic ether (usuallycontaining an ethoxy or other oxyalkylene group) toform a polymeric chain composed of oxymethylene(-CH 2 O-) and oxyethylene (-CH 2 -CH 2 -O-) orsimilar repeating units. Copolymers have a widerprocessing window, better long term mechanicalproperties and superior chemical resistance comparedto homopolymers, and are inherently more stable andresistant to thermal degradation during service life.This is because the randomly dispersed comonomerunits block polymer “unzipping” under thermalstress, or exposure to hot water or hot alkalinesolutions.Both the homopolymer and copolymer are endcapped,and also contain specific additives to preventirreversible thermo-oxidative depolymerizationof the polymer backbone during processing.1.2 General CharacteristicsCelcon acetal copolymer is a high strength, crystallineengineering thermoplastic material having an unusualand desirable balance of properties. It is an idealcandidate to replace metals and thermosets becauseof its predictable long-term performance overa wide range of in-service temperatures and harshenvironments. Celcon acetal retains properties suchas high strength, creep resistance, fatigue endurance,wear resistance and solvent resistance under verydemanding service conditions.Celcon acetal can be easily converted from pelletform into parts of different shapes using a variety ofprocesses such as injection molding, blow molding,extrusion, rotational casting and compressionmolding. Rod, slab and sheet stock which can bereadily machined into desired shapes are also available.1.3 Product TypesBoth standard and special grades of Celcon acetalcopolymer are designed to provide a wide range ofproperties to meet specific applications. Standard andcustom grades of Celcon acetal copolymer can beobtained in pre-compounded color form or colorconcentrates which may be blended with othergrades. All colorants used in Celcon resins are lead,mercury, and cadmium-free, and all Celcon acetalproducts conform to current environmental(OSHA) regulations for these metals. Consult ourbrochure: “Celcon Acetal Copolymer Short TermProperties” (CE-4) for information on specificgrades. The most common categories of Celconresins are described below.General Purpose: General purpose M-series productsare identified by melt flow rate. Divide the gradenumber by 10 to obtain the melt flow rate. Forexample, Celcon M90 has a melt flow rate of 9.0(grams per 10 minutes, per ASTM D 1238 or ISO1133, @ 190°C and 2.16 Kg. load). In comparison toM90, products designated by a higher melt flow rate,i.e. M270, fill thinner walls and complex shapes morereadily, maintain the same strength and stiffness, butexhibit a slight decrease in toughness. Products withlower melt flow rates, i.e. Celcon M25, exhibitincreased toughness compared to Celcon M90 butmay be more difficult to mold into parts with thinwalls or long flow paths.Glass Fiber Coupled: Glass fiber coupled (GC)products provide higher strength, stiffness and creepresistance than the unfilled grades. These products areidentified with a number indicating the percentage ofshort glass fiber in the product and are based ongeneral purpose Celcon polymers. The glass fibers arechemically coupled to the polymer matrix.Glass Bead Filled: These glass bead (GB) filledgrades contain glass beads for lower shrinkage,better dimensional tolerances and warp resistance,and are especially helpful when molding large, flatand thin-walled parts.Low Wear: Low wear (LW) grades are chemicallymodified to provide low coefficient of friction andenhanced wear resistance, and are exceptional fordemanding applications requiring good slidingproperties, reduced gear and bearing noise andenhanced lubricity.7

Celcon ®acetal copolymerMineral Coupled: These mineral coupled (MC)products contain chemically coupled mineral fillers invarying percentages. The mineral filled grades arerecommended whenever resistance to warpage(especially in thin sections) and dimensional stabilityare key application parameters. They are generallytougher than the glass bead filled polymers but aremore difficult to color uniformly.Ultraviolet Resistant: The ultraviolet resistant (UV)products are available with various melt flow rates innatural and a wide variety of colors and are lead,mercury and cadmium-free. They are speciallyformulated for improved resistance to color shift andmechanical degradation from ultraviolet light (bothsunlight and fluorescent lighting). Consult the Ticonabrochure, “Celcon ® Ultraviolet-Resistant GradesExtend Part Life in Harsh Environments” (CE-UV)for further information about these products.Weather Resistant: Weather resistant (WR) productsare formulated for maximum outdoor weatheringresistance. Several different melt flow rate grades areoffered. They are available in black color only.Antistatic: These antistatic (AS) products arechemically modified to decrease static build-up forapplications such as conveyer belt links and audio andvideo cassette hubs and rollers.Electrically Conductive: These electrically conductive(EC) grades are used for applications requiring lowelectrical resistance and/or rapid dissipation of staticbuild-up. Some electrically conductive grades containcarbon fibers and exhibit high strength and stiffness.Impact Modified: The impact modified (TX)products are formulated to provide moderate to highlevels of improvement in impact strength and greaterflexibility compared to the standard product.Laser Markable: The laser markable (LM) grades ofCelcon acetal copolymer have been developed withenhanced capability for laser printing. These blackproducts produce extremely robust white markingsfor applications such as bar codes, graphic oralphanumeric characters, and 2-D symbology.One of the grades has good ultraviolet light stability.Both grades have excellent toughness and dimensionalstability for many applications includingautomotive parts.Colors: Most grades are available in natural, blackand custom colors. There are standard colorconcentrates available for use in all natural grades.Custom color concentrates can also be made.1.4 Regulatory Codes and Agency ListingsMany grades of Celcon acetal are in compliance with,or approved under a variety of agency specificationsand regulatory standards as shown in Table 1.1. Notall grades are covered by all regulatory listings. CallProduct Information Services at 1-800-833-4882 forfurther information on which grades are approvedunder the various regulations.1.5 Product SupportIn addition to our technical publications, experienceddesign and application development engineers areavailable for assistance with part design, moldflowcharacterization, materials selection, specificationsand molding trials. Call your local Ticona Polymerssales representative, or Product Information Servicesat 1-800-833-4882 for further help.18

Celcon ®acetal copolymerTable 1.1 · Regulatory ListingsPlumbing Code Bodies:International Association of PlumbingMechanical Officials (IAPMO)Building Officials Conference of America (BOCA)Southern Standard Building CodeCanada Standards Association (CSA)Plastic Pipe Institute (PPI)Food and Drug Administration (FDA)United States Pharmacopoeia (USP)NSF InternationalStandards 14, 51, 61Underwriters Laboratories (UL)International Association of Food Industry Suppliers (IAFIS)United States Department of Agriculture (USDA)ASTM D 4181 [Supersedes ASTM D2133,Military Specification LP-392-A, Mil-P-6137A(MR)]SAE J2274ISO 9988-1, -2Plumbing fixtures and specific plumbing and mechanicalapplications covered in the various codesPlumbing fixtures, fittings and potable water contact items,“UL” ratings in CanadaRecommended Hydrostatic Design Stress (RHDS) ratingof 1,000 psi at 23°C (73°F) as an injection moldedplumbing fittingRepeated-use food contact applications including foodmachinery components conforming to 21 CFR 177.2470Class VI CompliantItems including plumbing components, beveragedispensers, etc., for contact with potable waterVarious UL ratings for flammability, electrical and thermalservice useSanitary Standards 3AApproved for direct contact use with meat and poultryproductsGeneral Material SpecificationSociety of Automotive Engineers Global SpecificationsDesignation system processing conditions andtesting protocols for comparative properties.9

Celcon ®acetal copolymerPower Seat BeltTransmission Gear Set1Celcon ® M90 Acetal CopolymerProvides:■■■■■■■■LubricityLow Coefficient of FrictionResistance to LubricantsMechanical StrengthFatigue EnduranceSurface HardnessDimensional StabilityEase of Molding10

Celcon ®acetal copolymer2. General Guidelines2.1 Storage and HandlingCelcon ® acetal copolymer should be stored in itsoriginal container on pallets in a dry place. Opencontainers should be carefully resealed beforereturning to storage. In the winter, containers of resinshould be brought into the warm processing area atleast 24 hours prior to use and allowed to come toroom temperature before opening. If this is not done,moisture in the air may condense on the surface ofthe pellets and lead to surface defects on molded orextruded plastic parts. This is especially critical withimpact modified grades which can be moisturesensitive and deteriorate during processing.The use of a hopper magnet in the feedstream ishighly recommended to insure against any form ofmetallic contamination which could occur whiletransporting the resin and cause equipment damage.Every effort should be made to avoid pellet spills orloss. Spilled pellets can be very slippery and mayresult in employee accidents. Pellet loss to theenvironment could lead to fines or other penaltiesunder Storm Water Regulations issued by theEnvironmental Protection Agency.Ticona Polymers supports the Society of the PlasticsIndustry Operation “Clean Sweep” program.2.2 Safety and Health InformationThe usual precautions must be observed as whenprocessing any hot and molten thermoplastic.CAUTION: Normal processing temperaturesand residence times should not be exceeded.Celcon acetal copolymer should never be heatedabove 238° C (460° F) nor be allowed to remainabove 193° C (380° F) for more than 15 minuteswithout purging. Excessively high temperature orlong residence time in a heated chamber can causethe resin to discolor and, in time, degrade to releaseformaldehyde, a colorless and irritating gas. Thisgas can be harmful in high concentrations, so properventilation is essential. If venting is inadequate, highpressures could develop in the equipment which maylead to blow back through the feed area. If no exit isavailable for these gases, the equipment mayrupture and endanger personnel.Consult the current Celcon acetal copolymer MaterialSafety Data Sheets (MSDS) for health and safety datafor specific grades of Celcon acetal copolymer priorto processing or otherwise handling these products.Copies are available by calling your local Ticona salesoffice or Customer Services at 1-800-526-4960 orfrom the internet site, www.ticona.com.2.3 FlammabilityWhen ignited, Celcon acetal copolymer burns withlittle or no smoke, and with a barely visible blueflame. Combustion products are carbon dioxide andwater. If Celcon acetal copolymer burns with amuffled flame and combustion is incomplete, carbonmonoxide and some formaldehyde may be released.Exposure to high concentrations, especially in apoorly ventilated area, can be harmful. For moredetailed information on worker exposure limits forformaldehyde, refer to the Material Safety Data Sheetfor Celcon acetal copolymer.WARNING: Avoid flame. Do not allow mixingof this material with PVC, other halogen-containingmaterials, and partially and/or fully crosslinkablethermoplastic elastomers. Do not heat above460°F (238°C). Avoid prolonged heating at orabove the recommended processing temperature.Recommended melt temperatures 360 - 390°F(182 - 199°C).Avoid strong acids and oxidizing agents. Do notallow the end use application to come in contactwith acidic solutions of pH = 4 or less, especiallymineral acid solutions like hydrochloric, sulfuric,hydrofluoric, perchloric, nitric or phosphoric. Do notuse with strongly acid salts such as zinc chloride orother Lewis acids. Do not use with chlorinatedwater solutions which are not typical of domesticpotable water.11

Celcon ®acetal copolymer2.4 DryingCelcon ® acetal copolymer does not readily absorbmoisture and can normally be fed to the extruder ormolding machine without drying. However, if thematerial has adsorbed moisture due to improperhandling or storage, drying may be necessary toprevent splay and odor problems during processing.It is good practice, and preferable for processingconsistency, to dry the resin before processing toavoid potential production problems due to moisture.Celcon acetal copolymer should be dried in adehumidifying oven or a hopper dryer. For ovendrying, the Celcon pellets should be spread evenly inless than one-inch deep layers on trays and placedin the oven for three to four hours at 82°C (180° F).For a hopper dryer, a three hour residence time at82°C (180°F) is sufficient.Caution: Formaldehyde fumes may be released;good ventilation is required in the area.2.5 Processing Start-UpTo start up a machine which was shut downwith Celcon acetal copolymer in the cylinder, thenozzle must not be blocked. This is one of the mainreasons a nozzle heater band is recommended. SeeTable 2.1 for the typical start-up conditions.The procedure for starting a machine with Celconacetal copolymer already in the cylinder is as follows:Table 2.1 · Typical Start-Up ConditionsCylinder Temperature:Rear 360°F 182°CCenter 370°F 188°CFront 380°F 193°CNozzle 390°F 198°CMelt Temperature 360 - 390°F 182 - 199°C(measured by “air shot”)Mold Temperature 180 - 250°F 82 - 121°C1st Stage Injection Pressure11,000 - 20,000 psi(75-138 MPa)2nd Stage (Hold) Pressure11,000 -20,000 psi(75 - 138 MPa)1st Stage Injection Fill Time2 - 5 seconds1st Stage Injection SpeedmoderateScrew Speed20 - 40 rpmBack Pressure0 - 50 psi (Higher endif using concentrates)Cushion1/8 - 1/4 in.Drying180°F (82°C) for 3 hrs.(Usually not necessary)Set the nozzle temperature at 400-420°F (204-216°C)and cylinder temperatures to 250-275°F (121-135°C).As the nozzle and cylinder come up to temperature,the nozzle orifice should be watched for signs ofdrooling. When drooling occurs (indicating that thematerial in the nozzle is fully molten), cylindertemperatures may be raised to 370-380°F (188-193°C). A few purge shots should then be taken atreduced injection pressure and speed with no booster.If there is no blockage, cylinder and nozzletemperatures as well as other conditions may beadjusted as desired, the heating cylinder closed to themold, and molding started on cycle.When Celcon acetal copolymer is started in anempty cylinder, nozzle temperature should be set at400-420°F (204-216°C) and cylinder temperatureat 370-390°F (188-199°C). Using low pressure(approximately 5,000 psi) and slow injector speed,pack Celcon resin into the cylinder with several short,deliberate strokes of the screw (plunger). After a fewminutes, commence another series of strokes andrepeat this procedure.As the material melts, successive packing strokes willwork the material farther into the cylinder until it isfull. A few air shots should then be taken to clear thecylinder of any air bubbles which may have beenentrapped during the packing process. Cylinder andnozzle temperatures, cycle time and other conditionsmay then be adjusted as required and moldingmay proceed.In all cases, once Celcon acetal copolymer isintroduced into the cylinder, it should be keptmoving to prevent overheating. If a delay of morethan 15 minutes is anticipated, the cylinder should bebacked away from the mold and the machine purged(on cycle or manually) every few minutes. If a longerdelay is expected, it is recommended that the machinebe shut down entirely, following the procedureoutlined under “Shutting Down a Machinewith Celcon.”2.6 Changing from Another Resin toCelcon AcetalIf the other resin in the processing equipmentrequires a higher melt temperature than Celcon acetalcopolymer (e.g. nylon, polycarbonate, etc.) or is aresin such as PVC (see note p. 2-1) which canchemically react with Celcon and cause degradation,the cylinder must first be thoroughly purged clean ofthese resins.212

Celcon ®acetal copolymerHigh density polyethylene or acrylic is suitable forpurging and should be put in the machine directlybehind the resin already in the cylinder and kept atthe same temperature settings. After all traces of theother resin are removed, the temperature should beset at 188-199°C (370-390°F). After the temperaturehas stabilized, Celcon ® acetal copolymer can thenbe placed in the machine to remove the purgecompound. The machine settings can be adjusted tothe desired production conditions.2.7 Changing from Celcon Acetal Copolymer toAnother ResinIn changing from Celcon acetal copolymer to anotherresin, similar considerations as described earlier willapply. When the machine is started up with Celconacetal copolymer in the cylinder, the properprocedure outlined in “Start-Up” must be followedbefore changing over to another resin. If the newresin requires a higher or lower temperature or is onethat can chemically react with Celcon acetalcopolymer (such as PVC), an intermediate purgingcompound such as polyethylene or acrylic must firstbe used to thoroughly clean the machine. The newmaterial should be introduced to the machine onlyafter proper cleaning and adjustment to theappropriate processing conditions.2.8 Processing ShutdownTo shut down a machine with Celcon ® acetal, thesame precautions must be taken against blockage ofthe nozzle as when starting up the machine. Thenozzle should be the last part of the heating cylinderassembly to cool. Leave the screw in the forwardposition.■ Set the nozzle temperature at 204 - 221°C(400 - 420°F)■■■■■Turn off the cylinder heaters.Shut off the feed to the cylinder.Purge and run the barrel dry.Leave the screw in the forward position.Shut off the power to the machine.2.9 Use of RegrindCelcon acetal can be reprocessed a number of timeswithout significant change in physical properties orprocessing characteristics. Tables 2.1 and 2.2 show theeffect of remolding unreinforced and fiberglassreinforced Celcon acetal copolymer. When remoldingglass fiber reinforced Celcon acetal, some loss inmechanical properties may be seen due to breakage ofthe glass fiber reinforcement.Knife-type grinders with a 5/16 in. (8mm) screen arerecommended for grinding resin sprues, runners andoff-test pieces. As with other thermoplastics,regrinding may produce enough dust to causediscomfort to the operator. In addition, acetalcopolymer dust can present an explosion hazard.Normal safety precautions such as the use of a dustmask and adequate ventilation are highlyrecommended. Dust can be minimized by keepingknife blades sharp, and using proper clearances andscreen size.Surface absorption of water on regrind tends to beslightly higher than for pellets because of the largersurface area. As a result, it is essential that regrind beproperly dried before use to avoid any moisturerelated production problems.To insure maximum retention of mechanicalproperties, regrind usage should be limited to nogreater than 25% for most applications. Special careshould be taken to prevent contamination by otherresins, especially PVC, other halogenated polymersand partially and/or fully crosslinkable thermoplasticelastomers. For any processing technique, it isparticularly important to also avoid dirt and otherimpurities which can create surface blemishes orplug flow paths. Because of the possibility ofcontaminating regrind with metal (such as from theregrinder knife blade), the use of a hopper magnet inthe feedstream is strongly recommended.Table 2.2 · Effect of Remolding on the Propertiesof Unreinforced Celcon ® Acetal1st 5th 11thProperty Molding Molding MoldingTensile yield strength, MPaValue 59.3 59.6 57.2percent retention — 101 97Notched Izod impactstrength @ 23°C (73°F), J/mValue 62.5 66.8 68.3percent retention — 107 10913

Celcon ®acetal copolymerTable 2.3 · Effect of Remolding on the Propertiesof Fiberglass Reinforced Celcon ® Acetal1st 3rd 5thProperty Molding Molding MoldingTensile yield strength, MPaValue 110 92.5 85.6percent retention — 81.7 75.6Tensile modulus, MPaValue 8,280 7,660 6,970percent retention — 92.6 84.2Flexural modulus, MPaValue 7,250 6,830 6,350percent retention — 94.1 87.62.10 Secondary Operations2.10.1 FinishingCelcon ® acetal can be readily machined, drilled,punched, buffed, sawed, sanded and routered bymethods commonly used on soft metals such as brassand aluminum. It is a good idea to direct ajet of cool, compressed air on the machined areato prevent overheating and sticking of the shavings tothe molded part. High tool speed and slow feed isrecommended. Avoid excessive speeds and pressures.Standard metal working tools are satisfactory formachining Celcon acetal.2.10.2 Surface TreatmentCelcon parts can be surface treated by laser marking,printing, labeling and hot stamping.Black, laser markable grades are available which canalso possess good ultraviolet light stability if required,such as for many automotive applications. Theseproducts produce extremely robust white markingsfor applications such as bar codes, graphic oralphanumeric characters, and 2-D symbology.Printing by conventional silk screen dry-offset, directtechniques, etc., require special inks for satisfactoryadhesion. Conventional surface adherent inks areavailable in addition to special ones which penetratethe material and offer outstanding abrasion resistance.Printing inks usually require a high temperature bakefor best adhesion.Transfer labels exhibit moderate to good adhesion toCelcon acetal. As many as four colors can be appliedusing this method. Decoration with transfer labels isusually less costly than silk screening for multiplecolor decoration, and when a very large number(millions) of parts are involved.Paper labels, the lowest cost decorating method, areavailable in many standard types including some withheat or pressure sensitive adhesives which giveunusually good bonding to Celcon acetal surfaces.Hot stamping can be used as long as recommendedprocedures and foil laminates are used to obtainsatisfactory adhesion.For information on machining, consult Chapter 12,“Machining and Surface Operations” of Designingwith Celcon Acetal Copolymer (CE-10), or callProduct Information Services at 1-800-833-4882.214

Celcon ®acetal copolymer3. Dimensional StabilityWhen manufacturing parts from Celcon ® acetal, itis important to understand the factors which may causedimensional changes. The dimensional effects ofshrinkage (both in-mold and post-molding), annealingand moisture absorption are discussed in this chapter.3.1 Shrinkage Caused by Processing(Injection Molding)Many factors influence mold shrinkage. They includethermal properties of the resin, filler type and level,part design (especially wall thickness), gate size, andresin flow direction. Molding conditions, includingmelt and mold temperature, injection speed andpressure are particularly important. Variations inmold surface temperature and mold injectionpressure, for example, can cause shrinkage in test barsmade from one specific grade (Celcon M90 ) rangingfrom 1.2 to 3.7%. As a result, it is difficult to predictthe exact mold shrinkage of a specific part.Typical effects of processing conditions on partshrinkage are summarized in Table 3.1.Shrinkage of standard unfilled Celcon acetalproducts measured on laboratory test specimenscover a range from 1.2 - 2.4%. Mold shrinkage for anactual part has been observed as high as 3.7%.Consult the Celcon Short Term PropertiesBrochure (CE-4) for typical values of specificCelcon grades. This information should be used onlyas a guide in estimating shrinkage for toolconstruction. Additional guidance is provided inMoldingTable 3.1 · Effect of Processing Conditionson Part ShrinkageWall thickness increasesGate size increasesPressure increasesMold TemperatureincreasesMelt TemperatureResin melt viscosityincreasesEffect on Part ShrinkageIncreasesDecreasesDecreasesIncreasesDecreases (for partsup to 3.2 mm)No effect (for parts 3.2 - 9.5mm thick)Increases with increasingviscosity when moldedunder similar processingconditions; i.e., CelconM270 has lower shrinkagethan Celcon M25Figure 3.1 which shows the effects of moldingconditions and wall thickness on mold shrinkage,and Chapter 4 of this manual which details molddesign for injection molding.Of the process variables, injection hold (or packing)pressure and time, injection speed and moldtemperature are the most significant, and about equalin importance in controlling mold shrinkage. Materialtemperature is also significant, but to a lesser degree.Of the part and mold variables, wall thickness has themost significant effect on part shrinkage followed bygate size. Gate location is of lesser importance, butstill significant, and is highly dependent on partgeometry. Parts which are relatively long and narrowand gated at the narrow end will have material flowpredominantly in one direction. This will result inanisotropic shrinkage. For unreinforced Celconacetal, there will be less shrinkage in the width(transverse) than in the length (flow) direction. Forreinforced Celcon acetal, the opposite will occur.Shrinkage will be less in the length direction than inthe width direction.The reason for this is that reinforcing glass fibersalign themselves in the direction of the material flowand, when the part cools and the material solidifies,the fibers inhibit shrinkage in this direction.Anisotropic shrinkage increases with increasing wallthickness and, in thick parts, with increasing gate size.The precise shrinkage for a given part may beobtained by initially designing the mold cavities withoversized cores and undersized cavities. Followingthis, parts should be molded at equilibrium moldingconditions, which provide the best overall results formold cycle time and part quality for production.Parts should then be conditioned at roomtemperature for at least 24 hours (preferably 48hours). Dimensions of critical areas can then bemeasured and the cavity and core machined, ifnecessary, to bring the molded parts withindimensional tolerances.15

Celcon ®acetal copolymerFig 3.1 · The Effect of Molding Conditions and Wall Thickness on Mold Shrinkage for Celcon ® M90 AcetalPart Wall ThicknessMoldSurfaceTemp.29°CShrinkage (%)3.83.63.22.82.42.0 CB AG1.61.22.0 mm3.83.63.22.82.42.01.61.24.1 mmABC,DE,F,G3.83.63.22.82.42.01.61.25.1 mmABC,DF,G3.83.63.22.82.42.01.61.210.2 mmABC,DE,F,G38 12 16 208 12 16 208 12 16 208 12 16 20MoldSurfaceTemp.79°CShrinkage (%)3.83.63.22.82.42.01.6A,B,DG3.83.63.22.82.42.01.6GABD3.83.63.22.82.42.01.6ABDG3.83.63.22.82.42.01.6ABDG1.21.21.21.28 12 16 208 12 16 208 12 16 208 12 16 203.83.63.83.63.83.63.83.6MoldSurfaceTemp.125°CShrinkage (%)3.22.82.42.0ABD3.22.82.42.0ABD3.22.82.42.0AB,D3.22.82.42.0AB,D1.61.61.61.61.21.21.21.28 12 16 208 12 16 208 12 16 208 12 16 20Injection Pressure (psi x 10 3 )NOTE: Melt temperature: 190-204°C.Shrinkage measured in the direction ofmaterial flow.Gate Area (mm 2 )A 1.9B 3.9C 7.7D 12.2E 18.1F 23.9G 31.316

Celcon ®acetal copolymer3.2 Part WarpageWall thickness should be as uniform as possible.Differences in cooling rates of thick and thin sectionsis a key contributor to warping. Other factorsaffecting warpage are:■■■■■■17Gate sizeGate locationMold temperatureFiller type/levelOrientation of fillersMold cooling3.3 Post-Molding ShrinkageAfter mold shrinkage has occurred and the partreaches ambient temperature, further mold shrinkagemay occur as time passes. This post-moldingshrinkage is usually related to stress relaxation andadditional crystallization within the molded partresulting in a permanent shrinkage of the part and it isaffected by cooling rate, i.e. mold temperature andpart thickness. Hot mold temperatures of 82 - 120°C(180 - 250°F) reduce post-mold shrinkage to nearlynegligible values. At ambient temperatures, thisshrinkage is relatively small, on the order of 0.1 –0.2% for the standard unfilled 9.0 melt flow grade ofCelcon ® acetal copolymer.Continuous exposure of the molded parts to hightemperatures accelerates both the rate and magnitudeof the post-molding shrinkage. Figure 3.2 illustratesthe shrinkage behavior of the standard unfilled 9.0melt flow grade of Celcon acetal copolymer after a sixmonth exposure at several temperatures in the flowdirection for a part 3.2 mm thick.It is extremely important to utilize mold surfacetemperature in excess of the anticipated maximumtemperature that the part may see in end use sincestress is frozen into the part at the mold temperature.The frozen-in stresses will be relieved by annealingcausing additional deformation.3.4 AnnealingAnnealing of Celcon acetal copolymer issometimes useful to relax molded-in stresses andstabilize dimensions. The amount of shrinkage thatmay occur primarily depends on cooling rate that isaffected by part thickness and mold temperature.Table 3.2 shows examples of the shrinkage resultingfrom annealing two different thicknesses of anShrinkage, %Fig 3.2 · Shrinkage Due to Heat Aging for 9.0Standard Melt Flow Grade of Celcon ® Acetal3.02.82.62.42.2Shrinkage Due to Heat Aging, 3.2 mm Thick SpecimenFlow Direction115°C Oven82°C Oven23°C (Ambient)2.00 1 2 3 4 5Time, Monthsunfilled Celcon acetal test specimen. Annealingmolded parts will lead to dimensional changes so thatallowances must be made for any additionalshrinkage. The decision on whether to anneal parts ofCelcon acetal copolymer in a post molding operationshould be made while the part and mold design are inthe initial planning stage and certainly prior tomachining the mold cavities and cores to size. Asparts of Celcon acetal copolymer will shrink inannealing, allowance must be made for the additionalshrinkage in determining the mold shrinkage whichshould be used for the mold cavities and cores.As a guide, the typical values of mold shrinkage andtotal shrinkage after annealing obtained on 1/8 inch(3.12 mm) and 1/2 inch (12.7 mm) thick test bars, 1/2inch (12.7 mm) wide by 5 inches (127 mm) long areshown in Table 3.2. The specimens were end-gated,molded using a mold temperature of 200°F (93°C)and annealed in oil at 305°F (152°C).In many cases, properly molded parts will exhibitsatisfactory dimensional stability, especially atcontinuous service temperatures of 82°C (180°F) orbelow. A high, 82 -120°C (180 - 250° F) moldtemperature will optimize the dimensional stability ofan as-molded part for service temperatures up to82°C (180°F).Table 3.2 · Shrinkage Before and After AnnealingPart Thickness, Annealed Flow Transversemm (in.) 152°C (305°F) Direction, % Direction, %3.18 (0.125) No 2.2 1.83.18 (0.125) Yes 2.7 2.012.7 (0.500) No 2.6 2.012.7 (0.500) Yes 3.0 2.06

Celcon ®acetal copolymerTable 3.3 · Recommended Annealing ProcedureRequired Service TemperatureIn-service temperature of 82°C (180°F) or belowIn-service temperature higher than 82°C (180°F)Annealing ParametersTimeTemperatureMediumCoolingRecommendationGenerally, properly molded parts will not require annealingAnnealing may be necessary to improve the dimensional stability of the molded partRecommendationAs a general rule, anneal for 15 minutes for each 3.2 mm (1/8 in.) of wall thickness if usingan annealing liquid; longer if annealing in an air oven152°C ± 2°C (305°F ± 5°F)Any refined or silicone oil which is not acidic. Oil is preferred over air because it is a betterconductor of heat and provides a blanket to minimize or prevent oxidationCool annealed parts slowly (one hour for every 3.2 mm of wall thickness).In some cases, however, because of in-servicetemperatures, annealing may be required, especiallywhere dimensional stability is of critical importance.Some general guidelines for annealing are givenin Table 3.3.Circulating air ovens and oil baths capable ofproviding a uniform temperature of 152 ± 2°C(305 ± 5°F) are recommended for annealing Celconacetal. While equipment offering lower annealingtemperatures may be suitable for some applications, itis not preferred because the annealing time to obtainbest results can be expected to increase significantlywith decreasing temperature.3.5 TolerancesDimensional tolerance can be defined as a variationabove and below a nominal mean dimension. Ifrecommendations for part/mold design and propermolding are followed, typical tolerances expected are:■■± 0.2% up to the first 25 mm (1 inch or less)± 0.1% for additional length over 25 mm (1 inch)In cases where tighter tolerances are required,precision tooling as well as molding by using controlfeedback loops on molding equipment, and using aminimum number of tooling cavities will help toachieve this objective.Careful consideration should be given to the need forvery tight tolerances to avoid excessive mold andprocessing costs. Also, it may be unreasonable tospecify extremely close tolerances on a part whichwill be exposed to a wide temperature range.3.6 Moisture AbsorptionSome dimensional change is seen when Celcon acetalcopolymer is exposed to moist environments. Thechanges are usually lower than those observed for otherengineering thermoplastics. Figures 3.3 and 3.4 showthat after one year of continuous exposure to highhumidity or immersion at various water temperatures,dimensional changes are minimal. See page 5.5 of theCelcon Design Manual for additional data on the effectsof water on material properties.3Fig 3.3 · Water Absorption by Unfilled Celcon ®Acetal Under Various ConditionsFig 3.4 · Change in Linear Dimensions at23°C (73°F) and 50% Relative HumidityWater Absorbed, %2.0Boiling Water Immersion @ 100°C1.5Water Immersion @ 82°C1.0Water Immersion @ 23°C0.5 Part Exposure @ 23˚C/93% Rel.Hum.Part Exposure @ 23˚C/50% Rel.Hum.00 20 40 60 80 100Time, Days(%) Contraction(%) Extension0.30.20.100.1Wet SpecimenLosing WaterDry SpecimenGaining Water0.21 Day 1 Yr.0.310 4 10 5 10 6 10 7 10 8Time, Seconds18

Celcon ®acetal copolymer4. Injection Molding4.1 EquipmentInjection molding is the most widely used method forprocessing acetals. Celcon ® acetal can be successfullyprocessed in all types of commercially availableinjection molding machines designed forthermoplastics. These may be single or two stage,reciprocating and stationary screw injection types.Screw injection provides fast plastication and ahomogeneous melt which will permit molding partsat reduced melt temperatures and pressures, as well asdecreased cycle time.A single stage reciprocating screw injection moldingmachine is most commonly used with Celcon acetal.4.1.1 Barrel and ScrewWhile the standard metering screw available incommercial reciprocating-screw injection moldingmachines can be used, it is not totally satisfactory.Problems such as excessive oxidative deterioration,poor thermal homogeneity, unmelted resin pelletsand/or lower productivity rates can sometimes occur.A screw such as the one shown in Figure 4.1 havingthe following characteristics is recommended foroptimum results:■■■The L/D (length-to-diameter) ratio shouldpreferably be no less than 16/1 and no greaterthan 24/1.The flight clearance should be approximately0.13 mm (0.005 in.).The flight width (w) should be approximately10% of the screw diameter.■■■■■For unfilled Celcon resins, the screw should behard faced or coated with a corrosion resistantmaterial such as chrome or Stellite 6.For filled reinforced Celcon resins, the screw andbarrel should be hard faced or coated with acorrosion and abrasion resistant material such astungsten carbide, CPM-9V or Colmonoy 56 forscrews (CPM-10V, Bimex, or Xaloy 101 or 306for barrels).The screw should be fitted with a non-returnvalve to prevent back flow of resin in the screwchannel as the resin is injected into the mold. Thevalve should have large clearances and wellradiusedcorners when open to ensure that themelt flows freely, is not “hung-up” and is notoverheated.The channel depth ratio, i.e., the ratio of thechannel depth in the feed zone to that in themetering zone, (h 1 /h 2 ), should be between 3 and4.5. A channel depth ratio of 4:1 is recommendedfor optimum results.The feed section should occupy about 40% of thescrew length, the transition zone about 30% andthe metering section about 30%.Typical screw dimensions for injection moldingscrews are given in Table 4.1.A diagram of a recommended check valve is shown inFigure 4.2 and indicates the need for flats to bemachined at the mating joints B and C. The flutes inthe screw tip, A, and the flow path through the checkFig 4.1 · Typical Injection Molding Screw for Plasticizing Celcon ® Acetal CopolymerMetering Transition Feedh 2h 1wD19

Celcon ®acetal copolymerTable 4.1 · Typical Screw Dimensions for Plasticizing Celcon ® Acetal Copolymer for Injection MoldingScrew Diameter (in.) Channel Depth Zone Length as % of screw lengthMetering (in.) Feed (in.) Ratio Feed Transition Metering1 1/2 0.083 0.29 3.5 40-50 30-20 302 0.089 0.30 3.4 40 30 302 1/2 0.097 0.32 3.3 40 30 303 1/2 0.108 0.35 3.2 40 30 304 1/2 0.119 0.38 3.2 40 30 30ring, D, should be generously proportioned and wellradiused to ensure minimum flow restriction. Themating surfaces between the screw tip, A, and thecheck ring seal, E, and the check ring seal and thescrew, F, should be cylindrical and machined flush toensure no projections into the flow. All surfacescontacting the flow should have a surface finish betterthan 16 r.m.s. and all flow channels should be freefrom sharp turns.Although standard unopened containers of Celcon ®acetal usually do not have to be dried, a vented barrelmachine with a two stage extraction screw may beused if the level of moisture encountered duringmolding is high enough to warrant removal. Whenimproved melting is required to reduce the cycle time,a barrier flight may be introduced in the first stage.The barrier flight clearance should be 1.02 - 1.52 mm(0.040 - 0.060 in.)4.1.2 NozzlesConventional free flow and reverse taper nylon-typenozzles fitted with a heater band for temperatureNozzleBFig 4.2 · Recommended Check ValveAdapterScrew TipCBarrelDCheck RingECheck Ring SealFScrewcontrol of the nozzle are recommended for Celconacetal copolymer.Caution: Nozzle designs with positive shut-offdevices are not recommended for safety reasons,although they have been successfully used.Formaldehyde gas may be released from Celconacetal in the molding process, particularly if left atelevated temperature in the heated barrel for anextended period. This gas must be free to escapethrough the nozzle. If the nozzle is blocked for anyreason such as by malfunction of the positive shut-offdevice or resin freeze-off in the nozzle, sufficientpressure could develop to cause blow-back of theresin through the feed zone and hopper or createother hazardous conditions.A nozzle heating band with independent temperaturecontrol is recommended for fine tuning nozzletemperature to prevent nozzle drool or freeze-off ofresin in this area. While a powerstat (rheostat) can besatisfactorily used for temperature control, indicatingtypetemperature controllers are preferred.4.1.3 Plasticizing CapacityAs with other engineering plastics, Celcon acetalshould not be exposed to excessive temperatures orvery long residence times. The shot weight for Celconacetal should be in the range of 50-75% of the ratedmachine capacity for best results.4.1.4 Clamping ForceClamping force should be high enough to prevent themold from opening during resin injection atmaximum pressure and speed. Usually 5-10 tonsclamping force per square inch of projected area(including molded parts, sprue and runners) isadequate for molding Celcon acetal copolymer. Theclamping force must exceed the projected area timesthe second stage pressure.420

Celcon ®acetal copolymer4.2 Mold Design4.2.1 General CriteriaStandard industry principles for good mold designand construction apply to the design of molds forprocessing Celcon ® acetal copolymer. Conventional2-plate, 3-plate and runnerless molds may allbe used.4.2.2 Mold BasesMold bases should be fabricated in a suitable steelgrade and be made sturdy enough with pillars toadequately support the cavities and the cores withoutbuckling of the retainer plates during injectionmolding. They should also be large enough toaccommodate water cooling channels to provideuniform mold temperature. This operation is essentialto produce acceptable parts.4.2.3 Mold Cavities and CoresThe selection of steels for the mold can be critical toits successful performance. Just as resins areformulated to satisfy processing and performancerequirements, steels are alloyed to meet the specificneeds for mold fabrication, processing and itsintended use. There are many different parts to themold, e.g. cavity, gates, vents, pins, cores, slides, etc.,and these may have different requirements. Forexample, some applications may require a mold steelwith high hardness to resist wear and abrasion at theparting line while another application may requiretoughness to resist mechanical fatigue. Usually, steelswith higher hardness and wear resistance propertiestend to be more brittle and steels with highertoughness will show less wear resistance. Theselection process of the tool steels should includeinput from the tool steel supplier, the mold designerand mold fabricator in addition to the resin supplier.Post-treatment of the mold can be used to reduce thepropensity for wear. Inserts should be consideredwhere wear may be a concern and long productionruns are anticipated. For example, P-20 tool steel canbe successfully used for unfilled Celcon acetalcopolymer grades where a limited production run isanticipated and Rc 58 -60 tool steel may be requiredfor molding a highly glass filled grade where anextensive production campaign is anticipated.Beryllium-copper cavities are also satisfactory formanufacturing good parts and offer the advantage ofhigh thermal conductivity for good heat transfer andprevention of hot spots in the mold. Hobbed cavitieswill work but lack the inner toughness of the alloysteels and are more susceptible to collapse underlocalized stress.For prototyping or short production runs, prehardenedsteel (R C 30-35), zinc alloys or aluminumare acceptable but may not be durable enough forlong or high volume production.4.2.4 Mold Surface FinishA wide variety of surface finishes can be used withCelcon acetal, as the resin exhibits excellent molddefinition. Various surface finishes, designs, script,etc., can be obtained by using standard techniquessuch as sand blasting, vapor honing, embossing andengraving the mold cavities and cores. Flash chromingis recommended to prevent rust andpreserve a highly polished surface condition. Mattefinishes are also achievable with an appropriatemetal surface treatment.Several factors affect surface finish, includingcondition of the mold surface itself, moldtemperature, cavity pressure, part configuration, wallthickness, resin melt viscosity and flow pattern. Acheck list of the key parameters is shown below:For mold surface condition and surface temperature,■ Check mold surfaces for nicks, blemishes, etc.■■■Check for worn surfaces from glass-reinforcedresins.Make sure the melt temperature is not on the lowside; this can lead to abrasion from reinforcedand filled resin grades.Mold surface temperatures should be highenough to prolong freezing of the melt in thecavity and gate, allowing better pressure transmissionto the part extremities. Surface pit marksand visible flow lines are indications of low moldsurface temperature.■ A minimum mold surface temperature of 82°C(180°F) is recommended for thin-walled parts(< 1.5 mm or 0.06 in. or less). Lower surfacetemperatures may be satisfactory for thickerwalledparts, but precautions should be takenagainst increased post-molding shrinkage.21

Celcon ®acetal copolymerFor cavity pressure,■■■■Packing pressure must be adequate to force themelt against the mold surface and keep it thereuntil a cooled surface film has formed to insureadequate reproduction of the surface. If the pressuredrop from the gate to the furthest point offill is too high, the frozen skin may pull awayfrom the mold surface as the resin shrinks, leadingto a shiny area in an otherwise matte surface.The gates should be large enough that the cavitypressure is adequate to completely fill the part.If necessary increase the gate size, relocate thegate or add additional gates.Ensure that the injection hold time is adequate toprevent loss of cavity pressure before resinfreeze-off in the gate.Pit marks in the surface are a clear indication oflow cavity pressure.For part configuration,■ Ensure that the resin melt flow path is not toolong or too complex.■Check the fill rate to ensure adequate cavitypressure.For wall thickness,■■■Injection fill pressure should be adequate,especially where a part has a thick wall-thin wallconfiguration. Otherwise a too low cavity pressuremay result.Wall thickness should not be too thick in relationto gate size; otherwise jetting or tumbling of themelt may occur, creating “fold-over lines” andinadequate surface definition.Gate size should not be too small for the wallthickness; otherwise sink marks may occur. Use arelatively coarse grain on the mold surface and arib thickness 50% of the adjoining wall surface inhigh-shrink resins to assure sink-free parts.For resin melt viscosity,■Melt viscosity may in some cases be too high toallow adequate packing of the cavity; runnersand gates may have to be enlarged to assure adequatefill. Increasing the melt temperature andusing a faster fill rate may marginally increasepacking pressure and eliminate the problem.Be careful not to exceed the critical melt shearrate, which may lead to resin flow lines, splay andpit marks. Refer to the discussion on excessivemelt shear during runnerless molding (page 4-5)for further comments.4.2.5 Sprue BushingsStandard sprue bushings with a taper of 2 1/2° perside perform satisfactorily with Celcon ® acetal.The sprue diameter should be larger than the matingend of the molding machine nozzle to prevent anundercut and facilitate ejection of the sprue.The end of the sprue bushing which mates with therunner should be larger than the diameter of therunner and be radiused at the junction. Opposite thejunction of the sprue bushing and the runner,provision should be made for a cold slug well and astandard “Z” (or other design) sprue puller. The spruepuller pin should be kept below the runner system toprevent interference with resin flow.Secondary sprues used for gating in 3-plate moldsshould have a taper of 2° - 3° included angle andshould also be radiused where they join the runner.The sprue size must be larger than the maximum wallthickness of the molded part.4.2.6 RunnersIn designing a runner system, it is preferable torestrict the length and diameter to minimize theamount of material that has to be recycled. Runnersshould be as short as possible and adequate in crosssectionaldiameter to allow fill of the mold cavitieswhile preventing freeze-off. Avoid the use of sharpcorners; turns should be curved to promotestreamline flow and minimize stagnant areas. Fullround, half round and trapezoidal cross-sectionrunners are all acceptable, but full round runners arepreferred. Suggested dimensions for full roundrunners are shown in Table 4.2, page 4-5. Runnersshould be made thicker than the maximum wallthickness of the molded part.When a multi-cavity mold is used, the runnersystem should be balanced, i.e., the flow pathsfrom the sprue to the far end of each cavity shouldbe equivalent.422

Celcon ®acetal copolymerTable 4.2 · Runner Size Recommendations for Celcon ® Acetal CopolymerPart thickness diameter mm (in.) Runner length mm (in.) Minimum runner diameter mm (in.)Less than 0.51 (0.020) Up to 50.8 (2) 3.18 (0.125)0.51 - 1.52 (0.020 - 0.060) Greater than 50.8 (2) 4.78 (0.188)1.52 - 3.81 (0.060 - 0.150) Up to 101.6 (4) 4.78 (0.188)1.52 - 3.81 (0.060 - 0.150) Greater than 101.6 (4) 6.35 (0.250)3.81 - 6.35 (0.150 - 0.250) Up to 101.6 (Up to 4) 6.35 (0.250)3.81 - 6.35 (0.150 - 0.250) Greater than 101.6 (Up to 4) 7.92 (0.312)4.2.7 Runnerless MoldingIn comparison with cold-runner molding, runnerlessmolding can reduce the amount of resin per moldingcycle, shorten production cycle time, enhanceproductivity and improve part quality. It is estimatedthat approximately 25% of all Celcon ® acetalmolding jobs are currently being performed usingrunnerless molds.Celcon acetal copolymer is well suited to the demandsof hot runner molding. Celcon acetal copolymer hasbetter thermal stability, important because of thelonger heat history during runnerless molding. Celconacetal copolymer processes at least 10°C (18°F) lowerthan some other acetals, reducing heatingrequirements and producing faster molding cycles.Some applications are natural fits with runnerlesstooling; i.e. applications such as medical parts, whereregrind cannot be used. Hot runners can also bejustified because they eliminate scrap and the need forauxiliary equipment such as sprue pickers andgranulators. Another suitable application is in highvolumejobs, where the same material is run for along time without switching grades or colors. Finally,where parts with very precise surface appearance arerequired, zero vestige gates can be used to virtuallyeliminate gate marks.Practically all commercial hot runner systems workwell with Celcon acetal copolymer, with the possibleexception of insulated runner systems. In general,melt flow channels should be large and streamlined,with generous radii and no sharp corners. This willprevent resin hangup, facilitate resin melt flow andreduce pressure loss.A full range of drops are available for runnerlessmolding. Either bushings or hot runner nozzles canbe used successfully, as can partial systems such as hotsprues. A wide variety of drop designs are acceptable,including hot tip, hot edge, angle gate, torpedo, angletip, multi-tip and E-type nozzles. Machine systemsuppliers can provide extensive design services todetermine the best drops for a specific application.A variety of gate configurations can be used forprocessing Celcon acetal in hot runners, includingsystems which provide thermal freeze-off. Valvegates, especially hydraulic designs, work well withparts requiring zero vestiges. Generally, gates shouldbe relatively unrestricted and should not subject themelt to shear rates higher than 1,500 - 2,000/sec atpolymer melt temperatures. Excessive shear mayresult in melt fracture. Gate design and locationinfluence mold filling patterns and affect mechanicalproperties, dimensions and surface finish. The gateland should be a minimum 1 mm (0.040 in.).Tips should be hardened to reduce wear, especiallywith reinforced or filled systems, and should bedesigned to be easily replaced when excessively worn.Temperatures need to be accurately controlled in allmelt channels. Thermocouple placement is critical. Itis recommended that control systems based onproportional-integral-derivative (PID) algorithms beused. These systems anticipate temperaturefluctuations and account for thermal inertia whenregulating heaters. The result is much finer controlover melt temperature.4.2.8 Molded-in InsertsA wide variety of molded-in inserts have beensuccessfully mated with various grades of Celconacetal. Because of the resin’s high strength andexcellent creep resistance characteristics, retention ofthe inserts is good, even after exposure to severetemperature and moisture cycling tests. Recommendeddesigns for molded-in inserts are shownin Figure 4.3.23

Celcon ®acetal copolymerFig 4.3 · Recommended Molded-In InsertDesigns for Celcon ® AcetalIn addition to standard insert molding, inserts can beassembled in pilot holes of molded Celcon ® acetalparts, using press fits, spin welding or ultrasonicwelding in a post-molding operation.RecommendedCoarseknurl;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;Radii onall cornersRound onblind endTorqueretentiondevicesFlutePull-outresistanceFlat4.2.9 Outsert MoldingInsert molding is a well established process for matingplastics and metals, but outsert molding extends itsadvantages to produce entire subassemblies withmultifunctional acetal copolymer parts. The process isclaimed to eliminate assembly steps and improvequality and productivity. One company, for example,molds over 100 acetal components onto a speciallydesigned galvanized steel baseplate for a videorecorder. All of the parts are formed from a singleshot of an acetal copolymer, sent to four levels via amold with 25 pinpoint gates per level.PoorSharp corners;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;Fine knurlCelcon acetal is ideal for injection molding both fixedand movable parts onto a metal plate clamped into themold. Parts can include gears, pins, bushings, wallsections, springs, cams and other shapes. The platesthemselves are usually 0.040 - 0.080 in. thick withtolerances of ± 0.002 in. In addition to holes used formounting parts, two holes secure the plate in themold. The distance between them must be controlledto a tolerance of ± 0.001 in. per 4.0 in. length toensure precise molding locations.4Fig 4.4 · Outsert Molded Moving Parts: Gear(left), cam (center), and spring (right)Either three-plate or hot runner molds with multiplepin gates can be used. Unlike conventional parts,shrinkage allowance in outsert molding is determinedfrom the center of each individual molding on the plate.For more information on this technique, call us at1-800-833-4882.4.2.10 Gating - Standard Injection MoldingGate Type: Parts made from Celcon acetal have beensuccessfully made with a variety of gate types. Figure4.5 gives examples of common gate types suitable formolding Celcon acetal parts.Table 4.3 · Recommended Gate Dimensions for Rectangular Edge Gates, mm (in.)Part thickness mm (in.) Gate width mm (in.) Gate depth mm (in.) Land length mm (in.)0.76 - 2.29 (0.030 - 0.090) 0.51 - 2.29 (0.020 - 0.090) 0.51 - 1.52 (0.020 - 0.060) 1.02 (0.04)2.29 - 3.18 (0.090 - 0.125) 2.29 - 3.30 (0.090 - 0.130) 1.51 - 2.16 (0.060 - 0.085) 1.02 (0.04)3.18 - 6.35 (0.125 - 0.250) 3.30 - 6.35 (0.130 - 0.250) 2.16 - 4.19 (0.085 - 0.165) 1.02 (0.04)24

Celcon ®acetal copolymerFig 4.5 · Some Basic Gate Designs Suitable for Celcon ® AcetalSprueSide or EdgeSecondary Sprue(3 plate mold)Restricted or PinTabDiaphragmInternal RingExternal RingCut runner as closeas possible to part45°FlashParting LinePart0.040-0.060 in. diameter(1.00-1.50 mm)20° – 30°Submarine25

Celcon ®acetal copolymerGate Size: Gate size should be selected so that themolten plastic in the gate freezes before the secondstage pressure is released, thereby preventingbackflow of the plastic. Recommended gate sizesfor rectangular edge gates are given in Table 4.3 forvarious ranges of thickness. The smaller gatedimension should be one-half to two-thirds of themaximum part wall thickness.The minimum diameter recommended for a roundgate is 1.0 mm (0.040 in.), preferably greater than 1.5mm (0.060 in.). Although parts have been successfullyproduced with gates as small as 0.5 mm (0.020 in.).These gate sizes should be restricted to very smallparts weighing less than 1 gram wall thicknesses of lessthan 0.5 mm (0.020 in.).Gate Location: Gating in areas of the molded partswhich will be subjected to high stress, bending orimpact during use should be avoided. Gates shouldgenerally be located in the thickest cross-section ofthe part and be in a position so that the initial flow ofplastic into the mold impinges on a wall. This willprevent jetting and blush marks.For round or cylindrical parts which must beconcentric, a center sprue gate, a diaphragm gate, diskgate or a set of three gates spaced at 120° intervalsaround the part is recommended.4.2.11 VentsVents: With all plastics, cavities should be well ventedto allow the escape of trapped gases and air.Inadequate venting can cause burn marks, short shots,dimensional problems, surface defects and blushing.Proper venting, on the other hand, will help to lowerinjection and clamp pressures, reduce cycle times,eliminate or reduce molded-in stress, and minimizeshrinkage and warpage. It is advisable to have asmuch venting as possible without allowing the resinto flow out of the mold.Size: Vents should be 0.0254 mm (0.001 in.) maximumdeep by 3.175 - 6.35 mm (0.125-0.250 in.) wide. Toprevent blockage of the vents, they should be deepenedto 1.59 mm (1/16 in.) at a distance of 3.175-4.76 mm(1/8-3/16 in.) from the cavity to the outside. Peripheralventing is preferred whenever possible.Location: Vents should preferably be located at the lastpoint to fill. Vents should be placed in other locationsas well including the runner system, weld line regions,and other areas of possible gas entrapment.Natural vents can be built into the parting line of thetool and at the interface of the pieces of metal used tobuild up the cavities. Ejector pins can also providesome venting but should not be used as the primarymeans of venting.Ejector and core pins used for venting should beflattened 0.0254 mm (0.001 in.) on one side. Blindholes where gases may become trapped, can be ventedby drilling a small (3.175 - 6.35 mm; 1/8 -1/4 in.) holeat the bottom of the cavity and inserting a smalldiameter pin flattened to 0.0254 mm (0.001 in.) onone side. When using these techniques, werecommend that mold temperatures be kept in excessof 180°F to avoid gas condensation on the pins andprevent corrosion.4.2.12 Cooling ChannelsThe actual mold temperature as well as temperatureuniformity is extremely important in ensuring goodquality molded parts. Each mold must containcooling channels to help maintain uniform heatdistribution throughout the tool. The coolingchannels should be as large in diameter as is practical(at least 14.3 mm or 9/16 in.) and located in areasdirectly behind the cavities and the cores. Channelsshould be uniformly spaced to prevent localized hotspots. Non-uniform cooling can lead to surfaceblemishes, sink marks, excessive molded-in stresses,warpage and poor dimensional control with apossibility of excessively long cycle times.4.2.13 DraftPlastic parts are almost always designed with a taperin the direction of mold movement to ease ejectionfrom the mold. This is commonly referred to as draftin the line of draw. The deeper the draw, the moredraft will be required.Some Celcon ® acetal parts have been successfullydesigned with no draft and have exhibited littleproblem with part ejection. However we suggest aminimum draft of 1/2 - 1° per side for best results.4.2.14 Parting LineParting lines should be located away fromaesthetically important areas but should notcomplicate mold construction. Where appearance isimportant, the parting line should be placed in an areawhere the line will be concealed, such as aninconspicuous edge of the part, an area of changinggeometry or on a shoulder.426

Celcon ®acetal copolymer4.3 Auxiliary Equipment4.3.1 Mold Temperature Control UnitsThree types of mold temperature control units arecommercially available and suitable for molding partsof Celcon ® acetal:1. Non-pressurized water circulating units in whichthe reservoirs are open to the atmosphere.2. Pressurized water-circulating units.3. Pressurized oil-circulating units.To maintain a mold surface temperature of 93°C(200°F), the mold temperature control unit mustusually be operated in the range of 104 - 110°C (220 -230°F) to compensate for heat loss in water lines,platens, etc. In a non-pressurized unit using wateronly, these temperatures cannot be attained becausethe water will boil off. If the heaters, gaskets, etc. insuch a unit are operable at these high temperatures,the boiling point of the water may be safely raisedby the addition of ethylene glycol. A solution of 60%ethylene glycol/water (by volume) will boil at 113°C(235°F); an 80% ethylene glycol solution boils at138°C (280°F).With a pressurized water-circulating unit, maximumtemperatures of 93-99°C (200-210°F) are attainablein most molds when the unit is operated at theextreme high limit of its temperature range.For those moldings where mold temperatures higherthan 99°C (210°F) are needed, a pressurized oilcirculatingunit is normally required. For flexibletemperature control, the oil reservoir in the unitshould be equipped with a suitable heat exchanger forlowering the oil temperature when required.An alternate system for mold temperature control isto install cartridge heaters of an appropriate size inthe cooling channels of the mold and controltemperature via a powerstat (rheostat). The cartridgeheaters should tightly fit the diameter of the coolingchannel to prevent premature burn-out of the heatersand prolong their life. While this is a relatively simpleand inexpensive system for mold temperature control,it is not recommended as it could cause localized hotspots and lead to various molding problems.It is prudent to check the mold temperature after themachine has been operating for about 30 minutes. Ifnecessary, readjust the mold heater temperaturesettings to maintain the desired temperature.4.3.2 Process ControlWhere it is important to maintain tight dimensionaltolerances, a closed-loop system should beconsidered. This will automatically adjust moldingconditions to provide consistently satisfactory parts.Open-loop systems which are less costly can be setup to stop the molding cycle or sound an alarm toindicate that molding conditions to providesatisfactory parts have changed and should bereadjusted to prevent rejects. Both types of systemsare suitable for molding Celcon acetal.4.4 ProcessingBefore placing Celcon acetal in a molding machine, itis highly recommended that you refer to Chapter 2 ofthis publication,“General Guidelines” for safehandling and processing information. The MaterialSafety Data Sheets for specific grades of Celcon acetalwill also provide additional information related tosafety, handling and use, and may be obtained bycalling 1-800-526-4960.4.4.1 Typical Molding ConditionsTypical processing conditions for injectionmolding Celcon acetal are shown in Table 4.4.The recommendations should be used as an initialguide, and “fine-tuned” as necessary for eachspecific application.4.4.2 Melt TemperatureMost moldings are made using a melt temperature inthe range 182 - 199°C (360 - 390°F) to facilitateprocessing and provide good quality parts atminimum cycle. Melt temperatures substantiallyabove 199°C (390°F) should be accompanied by acorresponding decrease in residence time to avoidoverheating and possible degradation of the resin.This can be achieved by using a smaller capacitymachine relative to the shot size, decreasing theoverall cycle time, or, if practical, increase the numberof cavities in the mold.Celcon acetal should never be processed above238°C (460°F). If overheating is observed orsuspected, lower the cylinder temperature and purge27

Celcon ®acetal copolymerproduced or are caught when they are made, theamount of post molding QC can be reduced. Becauseprocess “fine tuning” can usually be done through thejudicious use of the data generated at the time ofoperation, it is often possible to reduce the variabilityof molded part product quality.4.5 CushionA 3.2 - 6.4 mm (1/8 - 1/4 in.) cushion is recommended.The check ring non-return valve must functionproperly and ensure that the recommended cushionis held constantly throughout the molding cycle.Malfunctioning check rings may not seat tightly andwill usually result in inconsistent parts, short shots,poor control of dimensional tolerances and weakweld lines.4.5.1 Injection SpeedRapid fill of the mold cavities is preferred for mostmoldings and can be accomplished by opening theflow control valve to the maximum and adjusting thefirst stage so that it stays on for the full injectionstroke. If flash occurs, the injection speed should bereduced (preferably by reducing the injection velocityset point) as little as is necessary to eliminate flash.The injection hold time should be adequate tocompletely fill the mold cavities, with the screwcoming to a complete stop in the fully forwardposition and then allow the gate to seal underpressure before the pressure is released. This isparticularly important to prevent suck-back of resinthrough the gate when the screw is withdrawn.If surface imperfections such as splay or flow marksare encountered, reduce the injection speed in smallincrements.4.5.2 Solidification TimeSolidification time should be adequate for the resinto properly set up in the mold and maintaindimensional tolerance and geometry withoutdistortion, warpage or ejector pin penetration of themolded parts on ejection.4.5.3 Decompression SettingsMost commercially available injection moldingmachines are equipped with a decompression(suck-back) feature. This is used to relieve pressureon the resin after plastication in the heating cylinderand prevent nozzle drool during the mold open timeprior to beginning the next cycle. Usually about 0.2 -0.6 seconds decompression time is satisfactory forCelcon acetal copolymer.4.5.4 Screw SpeedScrew rotational speed and back pressure should bekept to a minimum, preferably 20-40 rpm and zeroback pressure, respectively. The screw speed should beset such that the screw fully retracts just prior to theexpiration of the mold close time. Excessive screwspeed and back pressure can cause severe overheating ofthe resin and, in the case of fiberglass reinforcedproducts, increase glass fiber breakage leading to asignificant reduction in mechanical properties.4.5.5 Cycle TimeCycle time depends primarily on wall thicknesswhich governs the rate of cooling, and to some extenton part design, dimensional tolerance, moldingequipment, mold design, etc. Some approximate cycletimes versus wall thickness are shown in Table 4.5for unfilled Celcon acetal copolymer. Faster cyclesthan indicated may be obtained with grades of resinsthat are reinforced, filled and those with higher meltflow rates.4.5.6 Process Optimization: Conducting a Designof Experiments (DOE) 1Process optimization can be handled in a number ofways, but the technique that is recognized as thepreferred one by injection molding experts is calledthe DOE, an abbreviation for Design of Experiments.Before the DOE made its entry into the field ofprocess optimization, the most widely used techniquewas to conduct a long series of experiments whereone process variable was changed at a time (e.g., thetemperature, or the pressure, or the speed) and theeffect noted. So, for example, mold temperaturemight be raised from an initial value of 80°F to 200°Fin steps while keeping all the other moldingparameters fixed. Samples would be collected at eachstep for testing. Then, after setting mold temperatureat “the best” value, the next variable (e.g. injectionpressure) would then be studied in a similar way.Table 4.5 · Approximate Cycle Times as a Functionof Wall Thickness – Unreinforced GradesWall thickness, mm (in.)Approximate total cycletime (secs.)1.6 (1/16) 203.2 (1/8) 306.4 (1/4) 5012.7 (1/2) 7529

Celcon ®acetal copolymerThe steps repeated until all variables were studied.Thus, the process optimization would be studied onevariable at a time. Obviously, the whole process was along and involved one, and in most cases its use neverreally led to the “best” conditions. The big shortcomingwas that the interactions among injection moldingparameters would not be studied directly.In conducting a DOE, you reduce the number ofexperiments substantially while still being able tomeasure the effects that each variable has on theprocess and parts. Additionally, you are able to seeclearly the influence of interactions among variables.For example, what effects do the interaction ofinjection pressure and melt temperature (a two-factorinteraction) have on the part properties? Similarly,what effects does the interaction of injection pressure,melt temperature and mold temperature (a threefactorinteraction) have on part properties? Thus, theinformation that you obtain from the DOE providesa very detailed map of the whole process foroptimizing the molding parameters.1 Box, G.E.P., Hunter, W.G. and Hunter, J.S.,Statistics for Experimenters, John Wiley & Sons, Inc.,New York (1978).4.6 Quality Control of Molded Parts4.6.1 Part WeightMolded parts most often have to meet very exactingspecifications, such as dimensions, color/appearance,or some measurement of performance capabilities(mechanical strength, an end-use tests, etc.). SuchQC tests are usually done many hours (24 or 48) afterthe parts are molded. Over the years, we have foundthat one of the best and most timely Quality Control(QC) test that a molder can perform to predict justhow a part will measure up against the establishedrelease specifications or end-use performance criteriafor the part is part weight. A major feature of partweight is that it can be done almost contemporaneouswith the part’s molding, i.e., the weight can bedetermined just seconds after the part is ejected fromthe mold. Thus, the molder can obtain almostimmediate feedback, thus allowing very timelycorrective actions to be taken.The procedure that is normally followed is todetermine the average weight of known “good” parts,along with the range of deviations (+/-) from theaverage (e.g., 10.6 +/- 0.03 grams). Then, a systematicweight check would be made on all production partsas they are produced. Any part over or under therange (10.57 to 10.63g) would be set aside, whilethose within the range would be kept for the usualsubsequent QC. Causes could be identified andcorrective actions taken to improve productionefficiencies.4.6.2 Part DimensionsIn the same way that part weight was determineddirectly after molding, part dimensions can be used asa QC check just as soon as the part is ejected fromthe mold. While we know that dimensions changewith time after molding, this could be factored intothe analysis. What one would do is to measure theparticular dimension(s) as a function of time aftermolding, e.g., 0.50, 1, 4, 8, 12, 18 and 24 hours. Next,plot these data, and extrapolate the plotted line backto “zero” time. This would then give you adimension for parts “right off the mold”, and indicatewhat the dimension would be 24 hours later.4.7 Effect of Molding Conditions onMechanical PropertiesCelcon ® acetal can be satisfactorily molded over abroad range of conditions. However, since moldingconditions influence aesthetics, structural integrity,mold shrinkage and physical properties of the moldedpart, it is important to identify the majorrequirements for a given application to select theappropriate molding conditions.Among the important molding conditions that caninfluence physical properties are melt and moldtemperature, injection pressure, hold time, injectionspeed and screw rotational speed. Slightly differentmolding conditions are required to optimizeindividual key properties. Because every application isunique and new in some way, the molding conditionsdiscussed below should be used as a guide only. Someadjustments will generally be necessary to ensure thatthe optimum cycle time and part performanceare obtained.The data shown in Table 4.6 were measured in acarefully controlled DOE using standard ASTM testspecimens 3.2 mm thick. Molds had generous gates,runners and vents. Restrictive gates, runners ofvarying thickness, parts with varying wall thicknesses,and weld lines in parts may significantly affectthese results.430

Celcon ®acetal copolymerTable 4.6 · Effects of Molding Conditions on Mechanical Properties – Unreinforced GradesInjection Melt Mold Screw InjectionPressure Temperature Temperature Speed SpeedTo optimize tensile strength 188 - 199 °C 77 - 104 °C — —(370 - 390 °F) (170 - 200 °F)To optimize modulus — 188 - 199 °C > 104 °C — (370 - 390 °F) (> 200 °F)To optimize notched bar impact —To decrease mold shrinkage — To increase mold shrinkage — Increase Decrease — No significant effectOccasionally, part design criteria or processingequipment parameters may require deviation from theguidelines. Moreover, actual parts are usually morecomplex in shape than laboratory test specimens.To maximize engineering performance, the processorshould work closely with the part designer tospecify molding parameters based on actualpart performance.4.7.1 Unreinforced Celcon ® Acetal GradesTensile strength increases with increasing injectionpressure and is optimum with a melt temperature inthe range of 188 - 199°C (370 - 390°F ) and moldtemperature of 77 - 104°C (170 - 220°F). Screwrotational speed and injection speed have little effecton tensile strength.Notched bar impact strength is highest with lowinjection pressure and medium injection speed. Itincreases with decreasing mold temperature, melttemperature and screw rotational speed.Modulus is most significantly affected by both moldtemperature and melt temperature and increases withan increase in both temperatures. Maximum modulusis obtained with a melt temperature of 188 - 199°C(370 - 390°F) and mold temperature of 104°C (220°F).Decreasing injection speed tends to increase modulusbut to a lesser degree than both melt and moldtemperatures. Injection pressure and screw rotationalspeed have little effect on modulus.Mold shrinkage, in both the flow and transversedirections, decreases with increasing injectionpressure, increasing injection speed, decreasing moldtemperature and a melt temperature in the lowerrange of 171 - 182°C (340 - 360°F). Conversely, moldshrinkage increases with decreasing injection pressure,decreasing injection speed, increasing mold31temperature and a melt temperature in the range of182 - 210°C (360 - 410°F). By minimizing shrinkageduring molding, you will maximize the potential levelof post-molding shrinkage. Thus, during use, the partcould shrink out of dimensional tolerance. It isrecommended to adjust initially the mold cavitydimensions for maximum shrinkage and mold atconditions of maximum shrinkage. Typical moldingconditions for providing maximum mold shrinkageare shown in Table 4.7.4.7.2 Glass-Mineral Coupled and Filled CelconAcetal GradesThe effect of molding conditions on physicalproperties should be used only as a guide. Somevariation in conditions will most likely be required toachieve optimum performance for each part.While screw rotational speed, back pressureand injection speed have little or no effect on theproperties of unreinforced Celcon acetal except wherethey cause severe overheating of the resin, theseconditions can have a significant effect on themechanical properties of fiberglass reinforced gradesdue to glass breakage. Increased glass breakage can beexpected to occur mostly with increasing screwrotational speed and to a lesser extent with increasinginjection speed and back pressure.Tensile strength and modulus will decrease withincreasing screw rotational speed due to breakage ofglass fibers.Mold shrinkage will increase with increasing screwrotational speed for the same reason. When moldingfiberglass reinforced Celcon acetal, screw speed andback pressure should be kept to a minimum, 20-30rpm and 0 psi, respectively. Injection speed should bemedium consistent with adequate fill of the moldcavities with flash.

Celcon ®acetal copolymerTable 4.7 · Typical Molding Conditions for MaximumShrinkage of Unreinforced Celcon ® Acetal CopolymerMelt temperature, °C (°F) 180 - 210 (360 – 410)Mold temperature, °C (°F) 121 (250)Injection pressure, MPa (psi) 35 (5,000)Injection speedMinimumScrew rotational speed has little effect on mold shrinkageof unreinforced Celcon grades.4.8 Molding Problems4.8.1 Deposits on MoldsIn those rare instances when deposits may buildup on mold cavity and runner surfaces, the mostlikely causes are inadequate venting, mold surfacesthat are colder than recommended, or excessive shearheating by injecting at high rates through smalldiameter runners and tiny gates. To prevent depositbuild-up, assure adequate venting of the cavities andrunners, use mold surface temperatures of 82°C(180°F) or higher, make sure runners have thickercross sections than the maximum wall thickness of themolded part, and gates larger than 1 mm (0.040 in.).To remove the deposits, we recommend heatingin a hot 82°C (180°F) detergent solution for severalhours followed by rigorous scrubbing. An alternateapproach for small cavity blocks would be to immerseinverted in a hot 82°C (180°F) detergent solutionwithin an ultrasonic bath for several hours. Afterdeposit removal, the cavities should be dried andtreated with a rust preventative.4.8.2 Troubleshooting Injection MoldingTable 4.8 lists common injection molding problemswith possible corrective actions in the preferred orderof implementation.4Table 4.8 · Troubleshooting Guide for Injection Molding Celcon ® Acetal CopolymerSYMPTOMSBrittlenessBurn marksDiscolorationNozzle droolPoor Weld LinesPOSSIBLE SOLUTIONS- Lower material temperature- Check for contamination- Decrease regrind level- Decrease injection speed- Decrease injection pressure- Improve venting- Increase gate size- Purge machine- Lower cylinder temperature- Decrease back pressure- Lower screw speed- Decrease nozzle temperature- Reduce injection speed- Eliminate contamination from feed- Examine nozzle and cylinder for hold up points- Ensure molding machine has free flowing check ring- Move mold to machine where shot size is 50 - 75% of capacity- Lower nozzle temperature- Lower material temperature- Minimize cushion- Delay sprue break time- Decrease mold open time- Use nozzle with smaller diameter orifice- Use decompression- Increase injection pressure- Increase screw forward time- Raise mold temperature- Vent cavity at area of weld line- Raise melt temperature- Increase injection speed- Relocate gate to alter flow pattern- Provide overflow well at weld area- Check for leaking check ring32

Celcon ®acetal copolymerTable 4.8 · Troubleshooting Guide for Injection Molding Celcon ® Acetal Copolymer (continued)SYMPTOMSShort Shots, Pit Marks, and Surface RipplesSink marksSplay Marks/DelaminationSticking in CavitiesSticking in CoreUnmelted PelletsVoidsWarpage/part distortionPOSSIBLE SOLUTIONS- Increase feed- Increase injection pressure- Increase mold temperature- Decrease cushion- Increase injection speed- Increase injection time- Add vents- Use larger machine- Check for leaking check ring- Increase injection pressure- Increase screw forward time- Maintain proper cushion- Reduce tool temperature- Increase injection speed- Increase feed- Enlarge gate or runner- Decrease injection speed- Reduce injection rate- Enlarge gate size- Reduce cylinder temperature- Increase mold temperature- Dry material- Decrease injection pressure- Decrease screw forward time- Minimize cushion- Increase mold close time- Lower mold temperature- Check for undercuts or insufficient draft- Decrease injection pressure- Decrease hold pressure- Minimize cushion- Decrease mold close time- Decrease core temperature- Check for undercuts or insufficient draft- Machine plasticizing capacity too small for shot and cyclespeed required- Temperatures too low, particularly at rear zone- Increase screw speed- Increase back pressure- Check for proper compression screw- Check for wear of the screw or barrel- Check for proper screw flight depth- Increase injection pressure- Increase screw forward time- Decrease cushion- Increase mold temperature- Increase feed, gate, runner, sprue- Check for leaking check ring- Equalize temperature in both halves of mold- Check uniformity of ejection- Check handling of parts after ejection from mold- Increase hold time- Alter injection pressure- Increase cooling time- Reduce material temperature- Try differential mold temperatures to counteract warp- Fixture parts in a cooling jig33

Celcon ®acetal copolymer5. Blow Molding5.1 Blow Molding MethodsBlow molding Celcon ® acetal copolymer allows forthe production of hollow and irregular shapesthrough either extrusion or injection techniques.While most commercial applications are for relativelysmall components such as cosmetic bottles,dishwasher spray arms and automotive hydraulicfluid reservoirs, blow molders have producedplenums measuring 50 in. long and 16 in. wide indevelopmental runs.The two general methods for blow molding plasticsarticles are extrusion blow molding and injectionblow molding. Both methods may be used to produceitems made from Celcon acetal. The two methodsdiffer primarily in the method of preparation of the“parison,” i.e., the tube of molten resin from whichthe molded article is formed. Both techniques havemuch in common, and information on recommendedgrades of Celcon acetal for both blow moldingprocesses can be obtained by calling ProductInformation Services at 1-800-833-4882.5.1.1 Extrusion Blow MoldingExtrusion blow molding is more extensively usedthan injection blow-molding and parts are madeeither by a continuous or discontinuous (intermittent)extrusion method.In the continuous method, there is no interruption inparison extrusion. When the parison reaches theappropriate length, a mold closes around it, and theparison is cut. Air is introduced and pressurized tocreate the blow-molded part.The mold moves and anew mold closes around the continuously movingparison. More than half of all Celcon acetal blowmoldedparts are made using the continuous method,due to its lower cost and shorter processing cycle.The discontinuous method is suitable only forprocessing resins which are not heat sensitive, and isnot recommended for Celcon acetal.5.1.2 Injection Blow MoldingInjection blow-molding is a two-stage process formanufacturing completely finished thermoplasticcontainers.Advantages of injection over extrusion blow moldingare the ability to mold a finished neck on a containerwith good dimensional control and betterdimensional control of wall thickness. This results inbetter product quality, less material usage, and aminimum of waste material to be reworked.The major advantages and disadvantages of eachmethod are summarized in Table 5.1.5.2 Equipment5.2.1 ExtruderAny conventional commercially available extrudercan be used to plasticize/melt Celcon acetal with littledifficulty for use in blow molding. It is highlyrecommended that the selected machine have a screwwith an L/D (length-to-diameter) ratio of at least16:1, and preferably higher (20:1 or 24:1). The use ofa machine with a higher L/D ratio allows moreuniform mixing of the molten resin, eliminates resinmemory and provides a more uniform melttemperature.In selecting an extruder, it is recommended thatparticular attention be given to the quality of thetemperature controls on the machine. Parisontemperature control is critical for trouble-freeblow molding.5.2.2 ScrewsScrews for the extrusion of Celcon acetal parisonsshould be of the general purpose type, having a fewflights of uniform depth in the feed zone, a taperedcompression zone, and several flights of uniformdepth in the metering zone. Screw pitch should beuniform and equal to the screw diameter.Compression ratio should be in the range of 3:1 - 4:1.A typical 2 1/2 in. diameter, 20:1 L/D screw forextruding Celcon acetal should have:■■■3 to 5 flights of uniform depth in the feed zone,10 - 12 flights of increasing root diameter in thecompression (transition) zone, and5 flights of uniform depth in the metering zone.534

Celcon ®acetal copolymerTable 5.1 · Comparison of Injection and Extrusion Blow MoldingEquipment Costs Extrusion InjectionTotal equipment cost Lower HigherParison die cost Lower HigherBlow mold cost Approx. equal Approx. equalProcessingProduction rates Lower HigherFinishing required Considerable Little or noneWaste and regrind Some Little or noneProductOverall quality achievable Good ExcellentSizes obtainable Small, medium, large Small to mediumWall thickness control Good ExcellentTolerance on neck finish Fair ExcellentFlight depth in the feed zone would be 0.44 in., and0.11 in. in the metering zone. Recommended screwcharacteristics to plasticize Celcon ® acetal forextrusion blow molding are summarized in Table 5.2.5.2.3 Screen PackA screen pack of assorted screen sizes (e.g 20-60-80-20 mesh) should be placed immediately in front of thescrew to filter out any unmelted particulates whichcould lead to defective products or abrasion of theinner head and die surfaces. The screen pack alsoserves to maintain back pressure which preventsuneven parison flow due to surging. The screen packshould be replaced at regular intervals.5.2.4 Breaker PlateA breaker plate should be used to hold the screenpack firmly in place while interfering as little aspossible with a smooth flow of the melt stream. Abreaker plate is a steel plate usually about 6.4 mm(1/4 in.) thick and perforated with closely-spaced 6.4(1/4 in.) holes. The holes in the breaker plate arefrequently chamfered on the side facing the screw toimprove melt flow.5.2.5 Die HeadA die head and adapter is generally placed betweenthe breaker plate and the parison die to guide the meltstream to the die entrance. Many assemblies requirethat the melt stream turn 90° as it approaches the die.To encourage smooth flow and avoid undesirableweld lines, flow “pin spiders” with helical deflectorsmachined to a knife edge at the point of convergenceare often used. The melt stream can flow freelyaround such a die pin without dead spots wherematerial could hang up. A flow divider of this type isshown in Figure 5.1.Fig 5.1 · Flow Pin Divider to Promote SmoothFlow and Avoid Weld LinesApprox.74°To further weld the material passing the die pin anddiscourage streaks, an annular restriction (“choke”)should be placed downstream from the flow divider.The choke serves to increase back pressure in the meltstream by reducing the cross-sectional area betweenthe flow divider and the die. The higher pressurewhich is achieved tends to smooth out the weld andreduce the chance of streaking.35

Celcon ®acetal copolymerTable 5.2 · Typical Screw Characteristics to Plasticize Celcon ® Acetal Copolymer for Blow MoldingScrew Diameter mm (in.) Channel Depth Zone Length as % of screw lengthMetering mm (in.) Feed mm (in.) Ratio Feed Transition Metering38.1 (1.5) 2.11 (0.083) 7.37 (0.29) 3.5 40 - 50 30 - 20 3050.8 (2.0) 2.26 (0.089) 7.62 (0.30) 3.4 40 30 3063.5 (2.5) 2.46 (0.097) 8.13 (0.32) 3.3 40 30 3088.9 (3.5) 2.74 (0.108) 8.89 (0.35) 3.2 40 30 30114.3 (4.5) 3.02 (0.119) 9.65 (0.38) 3.2 40 30 30Type - Metering general purpose type; constant pitch L/D - 20:1 to 24:1 preferred; 16.1 minimum Compression ratio - 3:1 to 4:1The die should be kept hot to avoid freeze-offof the molten resin after it has passed throughthe “spider”.Fig 5.2 · Parison Die with Adjustable Core Pinfor Control of Parison Thickness5.2.6 DieThe parison die is a key element in blow moldingbecause it controls material distribution in thefinished item, and, in turn, influences the economicsof the final product. Figure 5.2 shows a parison diewith an adjustable core pin which allows control ofthe thickness of the parison.5.2.7 HopperThe hopper on the extruder should be large enoughto hold Celcon ® pellets for about a half hour’sproduction. If the hopper is manually loaded, itshould be equipped with a hinged or tightly fitted lidto avoid contamination. While Celcon resinsnormally can be used directly from their originalshipping container without drying, a hopper drierwith a three hour capacity can be beneficial wherepick-up of excess moisture has inadvertentlyoccurred. The use of a magnetic screen or metaldetector placed in the hopper is advised to minimizethe risk of contamination or equipment damage dueto foreign metal.Die Heater(Zone 1)Die Heater(Zone 2)Die Heater(Zone 3)Die Bushing(Replaceable)Push Rod(Manual Alignment)Flow DividerPin AdapterTapered Die Pin(Replaceable)55.2.8 MoldsA choice of materials for molds depends on theanticipated volume of production, complexity of themold piece, technique used and number of moldsrequired. Aluminum and zinc are commonly used tomake the low cost molds for short runs. Berylliumcopper,a harder material, is sometimes used despiteits higher cost because it has excellent heat transfercharacteristics. Steel dies are relatively expensive andare mostly used for long production runs.If a mold cavity is not properly vented, trapped aircan hold the plastic material away from the cavitysurface, interfere with heat transfer in the cavity andresult in a poor finish on the molded part. Vents0.051-0.102 mm (0.002-0.004 in.) deep should beprovided at the mold parting line in areas where airentrapment is likely. Molds can be vapor honed orsand blasted to provide a matte surface whichprevents sticking and facilitates part ejection.36

Celcon ®acetal copolymer5.3 ProcessingThe extruder should be started up by the proceduresoutlined in Chapter 6 of this publication. As theextrudate issues from the die, the cold parison willappear rough and translucent. Raising thetemperature produces a well converted, cleartransparent parison. If parison temperature is toohigh, bubbles and slight discoloration will appear.Each different blow molding job will require somevariation in operating conditions to optimize theproduction process. Usually the lowest material melttemperature should be used consistent with obtaininga fully plasticized melt (no unmelted pellets) toprovide a satisfactory parison with maximummelt strength.5.3.1 Barrel TemperatureInitially, barrel temperature profiles should berelatively flat but after operation is well underway,some benefit will be derived from raising thetemperature in the feed section and lowering thetemperature of the succeeding zones to establish adescending profile. The highest temperature shouldbe at the feed zone. If extrusion rates are relativelylow and there is danger of bridging in the feed zone,the temperature in the feed zone should be kept lowand the second zone made the high point for thedecreasing temperature profile.Barrel temperatures can range from 165°C (329°F) toas high as 216°C (420°F) depending on the blownpart geometry, dimensions, blowing cycle, and manyother factors. Typical operating conditions used inpreparing various blown parts are shown in Table 5.3.Table 5.3 · Typical Blow Molding ConditionsItem Aerosol Container Aerosol Container Float3.5 oz. Bullet shape 5 oz. Barrel shapeApprox. size diam x ht., 40.6 x 132.1 (1.6 x 5.2) 55.9 x 83.8 (2.2 x 3.3) 63.5 x 25.4 (2.5 x 1)mm x mm (in. x in.)Weight, grams 29 44 27Blow molding method Extrusion Multi-station rotary Extrusion Multi-station rotary Extrusion, Fixed MoldaccumulatorExtruder size, mm (in) 64 (2.5) 64 (2.5) 64 (2.5)L/D 24:1 24:1 20:1Compression ratio 3.5:1 3.5:1 2.5:1Die busing ID, mm (in.) 14.9 (0.588) 17.8 (0.700) —Die mandrel OD, mm (in.) 8.0 (0.313) 5.6 (0.220) —Land length., mm (in.) 12.7 (0.500) 25.4 (1.00) —Temperature, °C (°F)Mold 93 (200) 93 (200) 88 (190)Barrel Zone 1 149 (300) 171 (340) 216 (420)Zone 2 171 (340) 166 (330) 210 (410)Zone 3 182 (360) 171 (340) 204 (400)Zone 4 166 (330) 171 (340) 204 (400)Adapter 188 (370) 166 (330) 204 (400)Die Zone 1 188 (370) 166 (330) 204 (400)Zone 2 204 (400) 166 (330) 210 (410)Zone 3 210 (410) 191 (375) —Melt 210 (410) 196 (385) 199 (390)Screw, rpm 30 45 70Current, Amps 100 95 12Back pressure, MPa (psi) 8.3 (1,200) 11.0 (1,600) —Blow pressure, MPa (psi) 0.83 (120) 0.83 (120) 0.45 (65)37

Celcon ®acetal copolymer5.3.2 Mold TemperatureA mold temperature of as high as 138°C (280°F) isoften used to achieve the optimum in part quality.A mold temperature below 93°C (200°F) is notrecommended.5.3.3 Blowing PressureTypical blowing pressure for Celcon ® acetal is in therange of 0.69 - 0.90 MPa (100 - 130 psi). Pressuresbelow 0.55 MPa (80 psi) can be used but are notrecommended. Blow pressures for Celcon acetal arehigher than required for polyethylene and,consequently, require higher clamping forces onthe mold.5.4 Effects of Process Variables on Part Dimensionand Quality5.4.1 Mold ShrinkageMold shrinkage for blow molded Celcon acetalgenerally ranges from 2-5%. Mold shrinkage isdependent on such factors as mold temperature,cooling time, blow pressure and wall thickness.The effect of these variables on shrinkage are shownin Figures 5.3 through 5.6. Average shrinkage valuesobtained in a round 3.4 ounce container with anominal 1.27mm (0.050 in.) wall thickness are shownin Figures 5.3, 5.4 and 5.5 while the effect of wallthickness on shrinkage in a relatively hot(127°C/260°F) mold is seen in Figure 5.6.Fig 5.3 · Effect of Mold Temperature on ShrinkageFig 5.4 · Effect of Cooling Time on Shrinkage0.0500.0400.0500.0405Shrinkage, in/./in0.0300.020Melt Temperature: 188°C (370°F)Cooling Time: 20 Seconds0.010Blow Pressure: 85 psiWall Thickness: 1.27 mm (0.050 in.)080 120 160 200 240 280Mold Temperature, °FShrinkage, in/./in0.0300.0200.0100Mold Temperature: 127°C (260°F)Melt Temperature: 188°C (370°F)Blow Pressure: 85 psiWall Thickness: 1.27 mm (0.050 in.)0 10 20 30 40Cooling Time, SecondsFig 5.5 · Effect of Blow Pressure on ShrinkageFig 5.6 · Effect of Wall Thickness on Shrinkage0.0500.0400.0500.040Shrinkage, in/./in0.0300.0200.010Mold Temperature: 127°C (260°F)Melt Temperature: 188°C (370°F)Cooling Time : 20 SecondsWall Thickness: 1.27 mm (0.050 in.)Shrinkage, in/./in0.0300.0200.010Mold Temperature: 127°C (260°F)Melt Temperature: 188°C (370°F)Cooling Time : 20 SecondsBlow Pressure: 85 psi00 30 60 90 120 150Blow Pressure, psi00 0.020 0.040 0.060 0.080 0.100Wall Thickness, in.38

Celcon ®acetal copolymerIn general, a blown container will shrink slightlymore in its length than in its diameter, and slightlymore in the neck area than in other sections.5.4.2 Surface AppearanceThe surface appearance of blown Celcon ® acetalcontainers depends primarily on mold finish andmold temperature, and partly on the ability toprevent air entrapment in the cavity. Air entrapmentcan be prevented by proper venting. Where airentrapment is a problem and a high glossy surface isnot required, a textured finish on the cavityis recommended.5.4.3 Impact StrengthImpact strength in a blown container is dependentprimarily upon the general design and wall thicknessof the part, but mold temperature can have asignificant effect. Optimum impact strength is oftenachieved by maintaining the mold at 93°C (200°F )or higher. In some items where impact failures haveoccurred in the pinch-off area because of poorwelding, a landed pinch-off, i.e., the use of a flatrather than a knife-edge pinch-off as shown inFigure 5.7 will yield significant improvement inimpact strength.Table 5.4 · Blow Molding Troubleshooting GuideSYMPTOMSDie LinesInternal RoughnessLow GlossPoor DefinitionPoor Pinch-Off WeldWalls too ThickWalls too ThinPOSSIBLE SOLUTIONS- Polish die lip- Increase mold temperature- Increase air volume- Eliminate hang-up area in die- Modify die to eliminate weld lines- Increase material temperature- Dry air supply- Increase length of land- Increase mold temperature- Increase stock temperature- Increase parison thickness- Improve mold surface finish- Increase mold temperature- Increase air volume- Increase air pressure- Increase stock pressure- Increase parison wall thickness- Reduce blowing pressures- Use wider pinch-off blades- Decrease screw speed- Change die dimensions as required- Increase screw speed- Change die dimensions as requiredFig 5.7 · Landed Pinch-off For ImprovedImpact StrengthFig 5.8 · Blow Molded Fluidic CapacitorMOLD CAVITY0.60 - .100" Land39

Celcon ®acetal copolymer6. ExtrusionHigher molecular weight grades of Celcon ® acetalcopolymers (higher viscosity than for injectionmolding, such as Celcon M25) are recommended forextrusion. Sheets up to about 6.4 mm (0.25 in.) thickand tubes with wall thicknesses 0.38 to 1.0 mm (0.015to 0.040 in.) can be extruded.Call Product Information Services at 1-800-833-4822for recommendations on specific grades of Celconacetal for each type of job requirement.The information on equipment given below appliesgenerally to all types of extrusion. Additional specificdetails for high speed tubing, film and sheet, andprofile extrusion are also covered in this Chapter.6.1 Equipment6.1.1 Materials of ConstructionAt extrusion temperatures, Celcon acetal copolymeris not affected by contact with copper, zinc, iron,nickel, brass or bronze. The designer therefore hasfreedom in selecting the most cost effective materialfor a specific application for dies, sizing sleeves, etc.6.1.2 Extruder BarrelA barrel with an L/D (length-to-diameter) ratio of16:1 (and preferably up to 24:1) is recommended toallow sufficient residence time for proper melting. Inconjunction with a properly designed screw, longerbarrels tend to improve melt homogeneity and reducemelt temperature and pressure fluctuations.If a shorter barrel (L/D less than 16:1) must be used,the use of a high resistance die with either long landsor a small die opening is recommended to generatethe necessary back pressure to “work” the material.With low resistance dies, the use of a valve in theextruder head is suggested to increase the backpressure level in the barrel.6.1.3 Screw DesignA metering screw as shown in Figure 6.1 isrecommended for extruding Celcon acetal.Characteristics of this screw are:■ An L/D ratio of 16:1 (minimum); up to 24:1preferred.■■■■The flight clearance should be approximately0.13mm.(0.005 in).The flight width, (w), should be approximately10% of the screw diameter.For unfilled Celcon acetal, the screw should behard faced or coated with a corrosion resistantmaterial such as chrome or Stellite 6.For filled Celcon acetal, the screw and barrelshould be hard faced or coated with a corrosionand abrasion resistant material such as tungstencarbide, CPM-9V or Colmonoy 56 for screws,and CPM-10V, Bimex or Xaloy for barrels.Fig 6.1 · Recommended Metering Screw for ExtrusionMetering Transition Feedh 2wh 1D41

Celcon ®acetal copolymer■■The channel depth ratio, i.e., the ratio of thechannel depth in the feed zone to that in themetering zone, (h 1 /h 2 ), should be between 3 and4.5. A channel depth ratio of 4 is recommendedfor optimum results.The feed section should occupy about 35% of thescrew length, the transition zone about 30% andthe metering zone about 35%.Recommended dimensions for extrusion screws aregiven in Table 6.1.6.1.4 Screen PackA 20-60-100-60-20 mesh screen pack is generallyrecommended preceding the breaker plate whenextruding unfilled resins. This will remove mostunmelted contaminants. It is especially important touse a screen pack when regrind is used. The screenpack also helps to increase back pressure andminimize surging while improving mixing. If the resincontains a filler or reinforcement, a screen pack isnot used.6.1.5 Head and Die DesignStraight-through, crosshead or offset dies may beused to extrude Celcon ® acetal copolymer. All insidesurfaces should be highly polished and streamlined. Ifthere are any areas of stagnation or hold-up, resin insuch areas could degrade and result in discoloredstreaks in the extrudate.Low resistance dies may not provide sufficient backpressure. In such cases, relatively long lands arerecommended. As a rule-of-thumb, land length forcircular cross sections should be at least equal to thediameter of the die, or the lands should be 10 to 20times the thickness of the extruded section. Anapproach angle of 20-30° to the die lands isrecommended for most types of dies.Accurate heat control at all points on the die and diehead, as well as the ability to determine the stocktemperature in the head via a melt thermocouple areessential. A pressure gauge should be mounted on thedie head to help establish and maintain properoperating conditions. The gauge is also a safetyfeature to alert the operator if excessive pressurebuild-up should develop.6.1.6 HopperThe hopper on the extruder should be large enoughto hold pellets for about a half hour’s production.If the hopper is manually loaded, it should beequipped with a hinged or tight-fitting lid to avoidresin contamination. While Celcon acetal gradesnormally can be used directly from their originalshipping containers without drying, a hopper drierwith a three hour capacity can be beneficial wherepick-up of excess moisture has inadvertentlyoccurred.The use of a magnetic screen or metal detector isadvised to minimize the risk of contamination orequipment damage due to foreign material.6.2 High Speed Tubing Extrusion6.2.1 EquipmentExtruder Size: There is no strict requirement for size,but the extruder must be able to deliver the requiredresin output at a constant temperature, properlyplasticated and without surging. For example, tomake brake cable tubing with OD 5.8mm. (0.230 in)and wall thickness 0.64mm (0.025 in.) at a rate of 91m/min (300 ft/min), a machine must deliver 82 kg/hr(180 lb/hr) of molten resin. Since this is the upperlimit for a 2 1/2 in. or 60mm extruder, a 3 1/2 in. or90 mm extruder is necessary.6Table 6.1 · Recommended Metering Screw Dimensions for ExtrusionScrew Diameter mm (in.) Channel Depth Zone Length as % of screw lengthMetering mm (in.) Feed mm (in.) Feed Transition Metering38.1 (1.5) 1.78 (0.070) 7.11 (0.28) 35 30 3563.5 (2.5) 2.79 (0.110) 11.18 (0.44) 35 30 3588.9 (3.5) 3.18 (0.125) 12.70 (0.50) 35 30 35114.3 (4.5) 3.56 (0.140) 14.72 (0.56) 35 30 3542

Celcon ®acetal copolymerWater Bath: A three axis positioning water bath isneeded to properly align the bath to the die andextrusion path. This alignment is critical in high speedextrusion for good surface finish. Bath length mayvary from 1.8 - 3.7 meters (6 - 12 ft.) withtemperature maintained typically between 16°C(60°F) and 38°C (100°F).6.2.2 ProcessingDie and Head Temperature: The die and headtemperatures should be in the range of 190 - 230°C(374 - 446°F) for Celcon ® acetal copolymer.A temperature profile with a die temperature of10 - 38°C (50 - 100°F) greater than the headtemperature gives the best results.In practice, for every 6°C (10°F) increase in melttemperature, surface roughness is reduced by about10 micro-inches R.M.S. Although surface roughnessdecreases with increasing die temperature, a dietemperature of 230°C (446°F) usually yields the bestbalance of minimum surface roughness and goodappearance. Typical processing conditions are shownin Table 6.2.Sizing Techniques: An external sizing sleevesubmerged in a water bath will provide good results.A series of plates or rollers should follow the sleeveto keep the tube below the water level in line with theexternal sleeve. This will prevent “chatter marks” onthe tube.In conjunction with the air pressure injected throughthe mandrel, tube diameter is controlled by the filmof water trapped between the tube and the externalsizing sleeve. Tubing with diameter 2.5 - 5.8 mm(0.100 - 0.230 in.) can be made using a 6.2mm (0.245in.) diameter sleeve. Increasing the inside diameter ofthe tube is accomplished using air pressure of 0.07 -0.35 kg/cm 2 (1 - 5 psig) transmitted into the tube viathe die mandrel.Vacuum sizing is generally conducted at higher linespeeds and may also be used for tubing extrusion ofCelcon acetal. A draw down ratio of 1.5 to 2.1 isrecommended. The sizing plates should graduallydecrease in diameter in the direction of extrusion. Thelast plate should be 1 to 5% oversize, depending onthe line speed, to allow recrystallization and shrinkdown to the desired final diameter.Orientation: When producing tubing, biaxialorientation of the extrudate is the best way to reducebrittleness. Heavy walled tubing can be biaxially43Table 6.2 · Typical Conditions for Tubing ExtrusionMachine3 1/2 in.L/D Ratio 24:1Size of Motor75 Hp (56 kW)Amp Rating 192Screw typeMeteringCompression ratio 3:1Metering zone depth 3.6 mm (0.14 in.)Screw speed33 RPMScreen pack20-40-80-40-20 meshSizingFour plates, 6.4 mm (2 1/2 in.)apart:#1. 7.1 mm (0.281 in.) diameter#2. 6.2 mm (0.242 in) diameter#3. 5.6 mm (0.220 in) diameter#4. 5.2 mm (0.204 in) diameterDie dimensionsMandrel4.3 mm (0.170 in.) diameterBushing7.2 mm (0.285 in.) diameterDie temperature230°C (450°F)Barrel temperature °C (°F)Zone 1 180 (360)Zone 2 190 (370)Zone 3 195 (380)Zone 4 200 (390)Zone 5 205 (400)Line ConditionsMotor24 AmpsLine speed128 m/min (420 ft/min)Air pressure0.1 MPa (1 1/4 psig)Head pressure210 MPa (3,000 psig)The above settings produce tubing with an i.d. of 4.1 - 4.2 mm (0.161 -0.165 in.); an o.d. of 5.1 - 5.2 mm (0.200 - 0.205 in.) and a wall thicknessof 0.46 - 0.56 mm (0.018 - 0.022 in.)oriented by drawing down the outside surface by afactor of 1.7:1 and blowing up the inside surface by afactor of approximately 1.6:1. For example, to makebrake cable with an o.d. of 5.84 mm (0.230 in.) andwall of 0.64 mm (0.025 in.), use a die with an openingof 10.2 mm (0.400 in.) and a 2.8 mm (0.110 in.)mandrel.o.d. 10.2mm/5.84mm = 1.74 drawdownratio(0.400 in./0.230 in.)i.d. 4.57mm/2.79mm = 1.6 blow-up ratio(0.180 in./0.110 in.)The tubing produced in this manner will be lessbrittle than that extruded without biaxial orientation.

Celcon ®acetal copolymerTroubleshooting: Table 6.3 lists some typicalproblems for high speed tubing extrusion andsuggested solutions.6.3 Film and Sheet Extrusion6.3.1 EquipmentFor highest efficiency, a long barrel extruder (L/D24:1) is recommended. This should have a meteringscrew with at least a five - turn metering section toensure melt homogeneity.Standard center feed dies may be used. The diemanifold can be in a straight line or bent to form a“Y”, but the latter is preferred because it provides amore uniform flow from the die.An adjustable choker bar, which acts as a valve, canbe used to regulate the thickness across the sheet. Thechoker bar combines with the die lip to give enoughback pressure to force the plastic out to the ends ofthe manifold. Thick or wide sheets require longerlands and more choking to ensure die fill out. Dielands should be 38-51 mm (1.5 - 2.0) long, dependingon the thickness of the die opening.Either a two- or three-rolled stack system may beused. The three-roll system is preferred because itprovides greater precision and control and provides aglossy finish on both sides of the product. Rolls mayalso be textured to produce a variety of patterns onthe finished sheet.6.3.2 ProcessingExtrusion conditions depend on the gauge and widthof the film or sheet being produced. A careful balancebetween material temperature and roll temperature isnecessary for good surface finish, and to preventsticking to the roll. Typical conditions for film andsheet of various gauges are shown in Table 6.4 on thefollowing page.Die lip to roll take-up distance must be kept as smallas possible for heavy sheets, but can be greater forthinner films. Slabs of up to 3.2 mm (1/8 in.) may beprepared by extrusion. Thicker slabs (up to 25.4 mmor 1 in.) have been prepared by stock shapemanufacturers using compression molding.6.3.3 TroubleshootingTable 6.5 lists potential problems during film andsheet extrusion and possible corrective actions.6.4 Profile Extrusion6.4.1 EquipmentExtruder: Extruders with barrel diameter of 35 - 60mm. (1 1/2- 2 1/2 in. ) and 5 -15 HP drives aretypical. The extruders used for the profile extrusionof Celcon ® acetal are generally small because mostprofile cross sections are small and output ratesare low.Screw: Metering screws as described in Figure 6.1and Table 6.1 are recommended for profile extrusionof Celcon acetal. Best results are obtained forscrews with:L/D ratio 20:1 to 24:1Pitch/diameter ratio 1:1Compression ratio 3:1 to 4:1Length of metering sectionscrew25% length of(5 to 6 diameters)Tip shape Conical (150°included angle)Die: Steel is commonly used for die construction.Chrome plating of all internal surfaces isrecommended for long production runs. Brass andberyllium copper are sometimes used for short runsbecause they are easy to machine and are good heatconductors.Both straight through and cross-head dies may beused, with straight through dies preferred in mostcases. Because of the melt elasticity and strength ofCelcon acetal, the die should be 15-20% oversize inwidth and 10% in thickness. It is recommended thatland length to thickness ratio should be 8:1 - 15:1 toprovide the correct level of exit velocity, backpressure and mixing to give a smooth, lustroussurface to the extruded part.Sizing and cooling: Air from an air ring or “profiled”copper tubing should be blown on the surface of thepart. Water is generally not recommended forcooling Celcon profiles.644

Celcon ®acetal copolymerFor sizing of simple thin shapes, the extrudate shouldbe passed through a set of brass or aluminum sizingplates with the first plate 5-8% oversize and theothers progressively smaller. Complex shapes can besized by passing through a number of adjustable brassfingers which are appropriately positioned to producethe desired cross section.6.4.2 ProcessingProfile extrusion processing conditions can varysignificantly depending on the geometry of the part,thickness, equipment, resin grade and numerous othervariables. It is advisable to begin processing using thegeneral start up conditions outlined earlier in thischapter and ensure that the selected Celcon ® resin willprovide adequate melt strength, which is critical forthis technique.Some complex contours can be more easily made byextruding a simple shape - tube or sleeve - andcontinuously post-forming after the die with speciallydesigned sizing plates, sleeves or simple brassfingers. Examples of post-forming parts are shown inFigure 6.2. This procedure, though versatile andusing less expensive dies, requires more care anddesign to achieve dimensional control and mayeventually be more costly.Fig 6.2 · Miscellaneous Post-Formed Profiles –shapes such as these can be made by extrudinga simpler shape and forming it into a more complexcontour while the extrudate is still pliable.ORIGINALPROFILEINTERMEDIATEPOSTFORMFINALPOSTFORMSEALSEALTable 6.3 · High Speed Tubing Extrusion Troubleshooting GuideSYMPTOMSBrittlenessConcentricity (poor)Surface roughness (inside)Surface roughness (outside)Wall thickness variationPOSSIBLE SOLUTIONS- Increase land length- Decrease head temperature- Decrease screw speed- Biaxially orient tubing- Increase take-off speed- Increase die temperature- Decrease cooling water temperature- Increase land length- Increase take-off speed- Decrease screw speed- Dry resin- Heat mandrel- Increase die temperature- Decrease screw speed- Control volume in the head- Maintain minimum volume in the head- Increase land length- Use a lower melt flow rate material- Decrease screw temperature45

Celcon ®acetal copolymerTable 6.4 · Typical Conditions for Film and Sheet ExtrusionParameter Film SheetGauge, mm (mils)

Celcon ®acetal copolymerTable 6.6 · Profile Extrusion Troubleshooting GuideSYMPTOMSDistortionGloss - Low in stripsGloss - low all overLinesPits - Bottom surface onlyPits - All over surfaceSurface roughnessSurgingWarpageWrong shape - Too largeWrong shape - Too smallPOSSIBLE SOLUTIONS- Ensure uniform temperature at all points on the die- Change location of sizing plates and fingers- Modify design- Eliminate cold or rough spots in die or sizing devices- Increase die temperature- Increase melt temperature- Decrease cooling rate- Clean die to remove any hard particles- Remove any nicks or burrs on take-off system- Cool the part more thoroughly before placing on conveyor belt- Dry resin- Check for contamination and eliminate- Increase die temperature- Dry resin- Decrease exit speed at die- Use die with larger openings or longer lands- Use a smaller machine- Check for broken gear tooth, worn belts, controller and repair- Remove likely causes for any variation in temperature, pressure,screw speed or motor load- Increase pressure/lower rate- Decrease temperature- Use a die with longer lands- Use better mixing screw- Ensure uniform cooling- Support warping section until cool enough to hold shape- Provide more gradual cooling- Lower line speed- Increase pull on the contour- Change take-off speed or material temperature (either way may help)- Use longer lands- Decrease pull on the contour- Change take-off speed or material temperature (either way may help)- Use shorter lands47

Celcon ®acetal copolymer7. Rotational CastingRotational casting is also referred to as rotationalmolding or rotomolding and is a process formanufacturing hollow and seamless products of allsizes and shapes. It offers significant advantagescompared to other molding techniques for thefollowing reasons:■■■■Low equipment and mold costsLittle or no scrapEasy adaptability for short production runsMultiple products and multiple colors can bemolded simultaneouslyCelcon ® acetal copolymer has been rotomolded intoparts up to 20 feet long and weighing hundreds ofpounds. For information on specific grades andprocessing parameters, please call ProductInformation Services at 1-800-833-4882.One disadvantage of rotational casting is the potentialfor weak points in the rotomolded part since nopressure is applied to promote complete melding ofall part sections. For applications where parts will beexposed to one or more impact, rigorous testing isrecommended to ensure adequate strength over thetotal part surface.7.1 EquipmentCelcon acetal can be readily rotomolded using anytype of commercially available machine, including a“carousel”, “clamshell”, “rock-and-roll”, or “shuttle”.The “shuttle” and “rock-and-roll” type machines areused most often to produce longer and heavier parts.7.2 MoldsFor small and medium-sized parts, cast aluminum iscommonly used because of its good heat transfercharacteristics and cost-effectiveness. One majordrawback of using cast aluminum is that its surface iseasily damaged.Sheet metal molds are normally used for prototypingand for extremely large parts. Electro- or vaporformednickel molds give excellent surface quality butare much more expensive.7.2.1 Particle sizePlastic powders must be used for rotational casting toensure rapid and adequate melting. Resin particlesgreater than 30 mesh size will significantly increasecycle time.7.3 Processing Parameters7.3.1 Resin Drying ConditionsPowders used for rotational casting will have a highersurface area than the pellets normally used for otherprocessing methods. As a result, a higher content ofadsorbed water is expected. To avoid poor finishedpart surface due to moisture, proper drying isnecessary. Six hours in an air circulating oven at82°C (180°F) is recommended. Using adehumidifying hopper dryer adds extra insuranceagainst adsorbed moisture. Caution should beexercised in the design of the dryer to handle thedusting of the powder clogging of filters.7.3.2 Part Heating Oven ParametersOven temperature and time must be balanced to yielda satisfactory part while avoiding thermal degradationof the polymer. Time is a key parameter in obtaininga smooth surface finish.Too little oven time will cause inadequate melting ofthe plastic powder; too long oven time may lead toresin degradation.Fig 7.1 · Rotomolded Acetal CopolymerParts Production748

Celcon ®acetal copolymerThe optimum temperature for each situation will varyand will be influenced by processing parameters suchas air circulation rate, mold material of construction,and mold wall thickness, all of which affect heattransfer. In no case should melt temperature beallowed to exceed 238°C (460°F) nor should the resinbe allowed to remain above 193°C (380°F) for morethan 30 minutes. For thick walled parts (greater than6.4 mm or 1/4 in.), a melt temperature of 216°C(420°F) is recommended.7.3.3 Part Cooling RatesAir cooling at a moderate rate is recommended forbest finished part properties. Rapid cooling willinduce brittleness. For that reason, water coolingshould be avoided.7.4 TroubleshootingTable 7.1 lists some potential problems that may beencountered during rotational casting and possiblesolutions.Table 7.1 · Rotational Casting Troubleshooting GuideSYMPTOMSBubbles on outer wallDiscolored partFlash excessiveLong oven cycleLow density (less than 1.37 g/cm 3 )Poor mold fillingPoor propertiesRough inner surfaceSurface pittingUneven wall thicknessWarpagePOSSIBLE SOLUTIONS- Dry resin- Reduce time/temperature to prevent degradation- Ensure proper venting- Confirm good parting line seal- Improve air circulation- Increase temperature or check temperature calibration- Reduce mass of rotation arms for better heat transfer- Optimize temperature/time to prevent degradation- Dry resin- Increase rotation speed- Increase radii or width of mold recesses- Increase cooling time- Check part density and reduce temperature/time as needed- Increase temperature and/or time for adequate melting of powder- Use less or no mold release- Clean mold surfaces- Remove excess metal which may be acting as heat sinks fromflanges and arms- Balance speed of each axis for uniform polymer flow- Improve air flow path around mold- Ensure continuous mold rotation from heating through cooling cycle- Clear vents to prevent vacuum formation- Use slower rate of cooling49

Celcon ®acetal copolymerNOTICE TO USERS: To the best of our knowledge,the information contained in this publicationis accurate; however, we do not assume any liabilitywhatsoever for the accuracy and completeness of suchinformation.Any values shown are based on testing of laboratorytest specimens and represent data that fall within thestandard range of properties for natural material.Colorants or other additives may cause significantvariations in data values.Any determination of the suitability of this materialfor any use contemplated by the users and the mannerof such use is the sole responsibility of the users, whomust assure themselves that the material assubsequently processed meets the needs of theirparticular product or use.It is the sole responsibility of the users to investigatewhether any existing patents are infringed by the useof the materials mentioned in this publication.Please consult the nearest Ticona Sales Office, or callthe numbers listed above for additional technicalinformation. Call Customer Services for theappropriate Materials Safety Data Sheets (MSDS)before attempting to process these products.Celcon ® acetal copolymer is not intended for use inmedical or dental implants.50

Products offered by TiconaCelcon ® and Hostaform ® acetal copolymer (POM)GUR ® ultra-high molecular weight polyethylene (UHMWPE)Celanex ® thermoplastic polyesterImpet ® thermoplastic polyesterVandar ® thermoplastic polyester alloyRiteflex ® thermoplastic polyester elastomerVectra ® liquid crystal polymer (LCP)Celstran ® long fiber reinforced thermoplasticsFortron ® polyphenylene sulfide (PPS)Celanese ® Nylon 6/6 (PA 6/6)Topas ® cyclic-olefin copolymer (COC)Encore ® recycled thermoplastic molding resinsNorth AmericaTicona90 Morris AvenueSummit, NJ 07901Technical Information1-800-833-4882Customer Service1-800-526-4960EuropeTicona GmbHIndustriepark Höchst,Building C65765926 FrankfurtTechnical Information+49(0)69-305-4653Customer Service+49(0)69-305-31949Visit our Internet Site at http://www.ticona.comfor further information.Asia PacificPolyplastics Co., Ltd.Kasumigaseki Bldg (6th Fl.)2-5 Kasumigaseki 3-chomeChiyoda-ku, Tokyo, 100JapanTelephone Number+(81) 3-3593-2411Fax Number+(81) 3-3593-2455TiconaA business of Celanese AG© 2000 Ticona Printed in USA 00-303/3M/0700Processing Celcon ® acetal copolymer