Injection Molding
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General-purpose polyethylene injection molded products burnt marks/burnt spots（Variant: Strengthened Measures）
Problembeschreibung
Black or brown burnt spots appear locally on the product, usually located at the flow end or at parts with difficult venting.
The material at the burnt marks has undergone thermal degradation, mechanical properties are severely reduced, and the product is scrapped.In particular, extra attention is needed on top of the original basis, this problem is more prominent and requires key attention.
Ursachenanalyse
（1） Gas in the cavity is compressed and heated, the temperature exceeds the material ignition point causing local carbonization and burning\
（2） Injection speed too fast, gas in the cavity has no time to escape and is compressed producing high temperature\
（3） Poor mold venting, trapped gas is compressed at the flow end\
（4） Barrel temperature too high, material has partially degraded in the barrel\
（5） Screw speed too fast, shear overheating occurs in the screw\
（6） On top of the original basis, extra attention is needed: process parameter fluctuations or equipment status changes aggravate this problem
Lösung
（1） Reduce injection speed, especially the speed in the later filling stage, leaving time for gas to escape\
（2） Improve mold venting, add vent grooves, vent pins or vent inserts at the burnt mark location\
（3） Appropriately lower the barrel temperature to a reasonable value within the 160-260°C range\
（4） Reduce screw speed, reduce back pressure, prevent shear overheating\
（5） Check the gate position to avoid the melt forming closed air pockets\
（6） And strengthen process monitoring; on top of the original basis, extra attention is needed, paying special attention to parameter stability
Wichtigste Erkenntnis
Burnt marks are the result of trapped gas in the cavity being compressed and heated causing material carbonization; the core solution is to improve venting and reduce injection speed.Targeted preventive measures need to be taken in different production scenarios.
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