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Acetal (Acetal) Troubleshooting - Common Acetal processing problems.
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B C D E F J M P S V W  Top Black Specks Check for contaminated material, particularly if regrind is being used. Decrease injection pressure, decrease booster time Decrease stock temperatures to minimize polymer and additive degradation Increase stock temperatures to reduce trapped air Increase stock temperatures and purge to loosen degraded resin Examine nozzle seat, cylinder walls and valves for dead spots Examine screw , screw tip and end cap for charred material Examine screw , screw tip and end cap for rough surfaces Examine heater bands and controls for malfunctions. Locate hot spots. Correct mismatch nozzle Be sure runner system is streamline Clean screw when molding homopolymer behind copolymer Blisters Reduce screw rpm Increase injection back pressure Raise mold temperature Relocate gate Material needs additional regrinding Regrind not mixed sufficiently with Virgin material Material contaminated Lower injection speed Properly dry material Blush Relocate gate Increase mold temperatures Reduce injection fill speed Analyze and adjust melt temperature accordingly Analyze and adjust nozzle temperature accordingly Pre-dry material Increase the number and / or size of cold well slug Increase injection pressure Increase gate diameter Break edges around gate Brittleness Increase mold temperatures Check regrind levels Improve cooling channels to prevent heat build up Be certain material is dry If gate is to thin, increase gate size Adjust injection pressure Part designed with abrupt wall section transitions, sharp fillets, or radii Bubbles Check for clogged vents Increase injection pressure Be certain material is dry Note: Bubbles and Voids are two different problems Increase the number of vents Improve the location of vents Increase injection forward time Increase feed zone temperature Minimize or eliminate suck back Match nozzle and sprue Burns Check for clogged vents Examine heater bands and controls for malfunctions. Locate hot spots. Relocate gate Decrease injection speeds Decrease injection pressures Increase mold temperature of a cold mold Analyze screw rpm and adjust accordingly Decrease back pressures Analyze and adjust stock temperatures Check residence time of polymer in the barrel Vent cavity at point of air entrapment Vent runner system Increase gate dimensions Increase nozzle diameter Use a lower compression ratio screw Top Cracks Increase mold temperatures Reduce fill rates Lower pack pressures Slow ejector speeds Slow mold open speeds Use lubricated material grades if possible Minimize sharp corners on part Put parts in an insulated container to cool Top Delamination / Lamination Be certain material is dry Examine resin for contamination and foreign materials. Check for dissimilar resins being mixed, particularly if regrind is being used. Increase stock temperatures Use higher back pressure, change screw speed, use a higher compression screw Increase injection speed if inadequate Remove sharp corners at gate Toughened grades are more susceptible to delamination Set mold temperatures to recommended conditions Discoloration Examine heater bands and controls for malfunctions. Locate hot spots. Reduce stock temperature Lower nozzle temperatures Decrease screw speeds Reduce back pressures Check for contamination Decrease residence time, Shorten overall cycle Check for sufficient venting Top Excessive Shrinkage Increase injection pressure Increase injection hold time, and /or pressure Increase length of cooling cycle Lower melt temperatures Decrease mold temperatures Analyze gate location and relocate as needed Decrease gate land length to improve cavity pack Check for a worn check valve assembly, barrel or screw Ensure there is a proper cushion Top Flow Lines Increase mold temperatures Increase melt temperature Increase runner size Increase gate size and/or relocate as needed Increase cold slug well size or number Increase back pressure Check for color concentrate carrier compatibility Top Jetting Increase mold temperatures Decrease injection speeds Analyze and adjust stock temperatures Decrease gate land length Increase sprue, runner or gate size Check nozzle heater band Change gate location so the material will impinge off the opposite wall Flare gate Top Mold Flash Be certain material is dry Decrease injection pressures Slow fill speed Transfer to hold pressure sooner Adjust shot size Check for parting line seal, steel may be collapsed Adjust clamp pressure to maximum for that mold / press combination. Go to a larger press if the machine is undersize Use a higher flow material if the end use requirements permits Check mold supports Decrease size of vents if oversized, increase number of vent if necessary Balance cavities to within 90 % of fill Check parting line for contamination and / or build up Top Parts stick in mold Decrease injection pressures Decrease injection rate Decrease screw forward time Analyze and adjust mold temperature Correct mold defects Use alternative mold releases (Internal, external) Increase draft angles on part Top Short Shots Increase mold temperatures Increase injection pressure Use a higher flow material if the end use requirements permits Increase feed to maintain a consistent cushion Increase injection speed Increase boost or high pressure time Increase melt temperature if to low (Check spec.) Use a short sprue with an extended nozzle Enlarge gates, sprue and runners Provide additional mold vents Relocate gates Check for worn check ring, screw and barrel Check for bridging at the throat, reduce rear zone temp., check cooling in throat area Insure the nozzle orifice is not too small Silver Streaks Dry material to eliminate any surface moisture or condensation Sink Marks Increase injection pressure Analyze and adjust mold temperature Provide additional mold vents Increase injection hold time, and /or pressure Increase length of cooling cycle Lower melt temperatures Check for a worn check valve assembly, barrel or screw Increase shot size Enlarge runner and gates to prevent freeze-off before part can be packed properly Decrease land length of gate and sprue Relocate gates on or as near as possible to thick sections Decrease rib thickness to approx. 75 % or less of adjoining wall Differential mold temperatures; Lower mold surface temperature opposite rib side Adjust feed to increase cushion Splay Marks / Silver Streaking Check for contamination Be certain material is dry, test for moisture Reduce residence time Reduce decompression Decrease screw rpm Adjust injection back pressure Decrease melt temperature Reduce nozzle temperature and /or go to larger nozzle Go to larger sprue Increase gate dimension, and/or reduce gate land Check for burrs in runner or gate Provide additional mold venting Check for water leaks in the tool Surface Finish (Low Gloss) Increase mold temperatures Increase mold temperatures Increase injection pressure Increase injection forward time Increase melt temperature Increase melt temperature Increase injection speed Be certain material is dry, test for moisture Prepare mold surface to give the gloss expected for the molded item Vent cavity at the point of air entrapment Relocate gate so that melt flow impinges on the a mold surface, not directly into cavity Analyze and adjust injection pressure Analyze and adjust injection speed Check for material contamination Check that sprue, runner and gate are a sufficient size Clean mold surface Check for sufficient feed, increase cushion accordingly Increase gate size Provide adequate venting Remove and clean mold Top Voids Raise mold temperature Increase injection pressure Increase injection forward time Increase feed allotment if the cushion is too small Decrease the feed allotment for to much cushion Reduce rib thickness Relocate gate near heavy sections Eliminate heavy wall sections Top Warping Analyze and adjust injection speed Increase mold close time Increase packing time Analyze and adjust holding pressure Increase melt temperature and reduce injection pressure to reduce molded in stress Adjust melt temperatures to match the temperature profile suggested for specific grade Adjust mold temperatures to the requirements for your specific grade Equalize temperatures of both halves of the mold Correct mold temperature variations Reduce variation in wall thickness Fixture parts during post molding cooling Relocate gates to avoid thin sections, ribs or curved sections Adjust gate dimension to improve flow of resin Be sure gates are balanced on multiple gated parts Check for parts hanging up in the tool on mold open and part ejection Weld Lines Increase melt temperature Increase injection pressure and speed Provide adequate venting in cavities and runner system Vent cavity at point of weld / air entrapment Analyze and adjust gate, nozzle and sprue size Decrease distance from gate to weld line by using multiple gates Change gate location to alter flow pattern Use a smaller amount or change type of mold release, additives and colorants Increase the thickness of the weld line area or add a rib

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