Injection Molding
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Burnt marks/burnt spots appear on general-purpose polyethylene injection molded products
Fenómeno do problema
Black or brown burnt spots appear locally on the product, usually located at the flow end or in areas where venting is difficult.
The material at the burnt mark has been thermally degraded, the mechanical properties are severely reduced, and the product is scrapped.
Análise das causas
（1）The gas in the cavity is compressed and heated, and the temperature exceeds the ignition point of the material, resulting in local carbonization and burning
（2）The injection speed is too fast, and the gas in the cavity has no time to be discharged and is compressed to generate high temperature
（3）The mold venting is poor, and the trapped air is compressed at the flow end
（4）The barrel temperature is too high, and the material has been partially degraded in the barrel
（5）The screw speed is too fast, and shear overheating occurs in the screw
Solução
（1）Reduce the injection speed, especially the speed in the later stage of mold filling, to allow time for the gas to be discharged
（2）Improve the mold venting, and add venting grooves, venting pins, or venting inserts at the burnt mark position
（3）Appropriately lower the barrel temperature to a reasonable value within the range of 160-260°C
（4）Reduce the screw speed and back pressure to prevent shear overheating
（5）Check the gate position to avoid the melt forming a closed air pocket
Conclusão principal
Burnt marks are the result of trapped air in the cavity being compressed and heated to carbonize the material, and the core solution is to improve venting and reduce the injection speed.
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