Injection Molding
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Burnt Marks/Burnt Spots Appearing on Block Copolymer Polypropylene Injection Molded Products
Fenómeno del problema
Black or brown burnt spots appear locally on the product, usually located at the flow end or at parts where venting is difficult.
The material at the burnt marks has thermally degraded, the mechanical properties are severely 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 produce 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 rotation speed is too fast, and shear overheating is generated in the screw
Solución
（1）Reduce the injection speed, especially the speed in the later stage of mold filling, to allow time for gas discharge
（2）Improve the mold venting, and add vent grooves, vent pins, or vent inserts at the burnt mark positions
（3）Appropriately reduce the barrel temperature to a reasonable value within the 200-270°C range
（4）Reduce the screw rotation speed and reduce the back pressure to prevent shear overheating
（5）Check the gate position to avoid the melt forming a closed air pocket
Conclusión clave
Burnt marks are the result of trapped gas in the cavity being compressed and heated, causing carbonization of the material; the core solution measures are to improve venting and reduce the injection speed.
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