Injection Molding
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Burnt marks/burn spots appearing in random copolymer polypropylene injection molded products
Fenómeno del problema
Black or brown burnt spots appear locally on the product, usually at the flow end or positions with difficult venting.
The material at the burnt mark has thermally degraded, mechanical properties have severely declined, and the product is scrapped.
Análisis de la causa
(1) Gas in the cavity is compressed and heated; the temperature exceeds the material's ignition point, causing local carbonization and burning
(2) Injection speed is too fast; gas in the cavity has no time to escape and is compressed to produce high temperature
(3) Poor mold venting; trapped air is compressed at the flow end
(4) Barrel temperature is too high; the material has partially degraded in the barrel
(5) Screw speed is too fast; shear overheating occurs in the screw
Solución
(1) Reduce injection speed, especially in the later filling stage, allowing time for gas to escape
(2) Improve mold venting, adding vent grooves, vent pins or vent inserts at burnt mark positions
(3) Appropriately lower barrel temperature to a reasonable value within the 190-260°C range
(4) Reduce screw speed and back pressure to prevent shear overheating
(5) Check gate position to avoid the melt forming enclosed air pockets
Conclusión clave
Burnt marks are the result of trapped air in the cavity being compressed and heated, causing material carbonization; the core solutions are improving venting and reducing injection speed.
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