Injection Molding
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Burn Marks/Burnt Spots on High-Density Polyethylene Injection-Molded Products
Fenómeno del problema
Black or brown burnt spots appear locally on the product, usually located at the flow end or parts where venting is difficult.
The material at the burn marks has undergone thermal degradation, the mechanical properties are seriously reduced, and the product is scrapped.
Análisis de la causa
(1) The gas in the cavity is compressed and heated, and the temperature exceeds the ignition point of the material, causing 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 gas is compressed at the flow end
(4) The barrel temperature is too high, and the material has partially degraded in the barrel
(5) The screw speed is too fast, and shear overheating occurs in the screw
Solución
(1) Reduce the injection speed, especially the speed in the later stage of filling, to allow time for gas discharge
(2) Improve mold venting, and add vent grooves, vent pins, or vent inserts at the burn mark positions
(3) Appropriately reduce the barrel temperature to a reasonable value within the range of 180-260°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
Conclusión clave
Burn marks are the result of the trapped gas in the cavity being compressed and heated, causing the material to carbonize; the core solutions are to improve venting and reduce the injection speed.
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