Injection Molding
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Burn marks/burnt spots on low density polyethylene injection products
Fenómeno del problema
Black or brown burnt spots appear locally on the product, usually located at the flow end or at locations where venting is difficult.
The material at the burnt marks has undergone thermal degradation, mechanical properties are severely reduced, and the product is scrapped.
Análisis de la causa
(1) Gas in the cavity is compressed and heated; the temperature exceeds the ignition point of the material, causing local carbonization and burning
(2) Injection speed too fast; gas in the cavity has no time to escape and is compressed, generating high temperature
(3) Poor mold venting; trapped air is compressed at the flow end
(4) Barrel temperature too high; the material has partially degraded in the barrel
(5) Screw rotation speed too fast, shear overheating occurs in the screw
Solución
(1) Reduce injection speed, especially in the later stage of mold filling, to allow time for gas to escape
(2) Improve mold venting; add vent grooves, vent pins, or vent inserts at the burn mark location
(3) Appropriately reduce barrel temperature to a reasonable value within the 160-220°C range
(4) Reduce screw rotation speed, reduce back pressure, prevent shear overheating
(5) Check gate position to avoid the melt forming enclosed air pockets
Conclusión clave
Burn marks are the result of trapped air in the cavity being compressed and heated, causing material carbonization; the core solution is to improve venting and reduce injection speed.
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