Injection Molding
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PP Trapped Gas Burning
Problem Phenomenon
Black/brown burn marks appear at the last-filled parts of the cavity, rib ends, or weld lines, possibly with slight burnt odor, at fixed positions and repeatedly.
Cause Analysis
During filling, air in the cavity is rapidly compressed and cannot escape, causing 'diesel effect' adiabatic heating that ablates plastic and forms burn marks; vent grooves blocked by oil/material debris, excessive end injection speed, and excessive clamping force closing vent gaps all induce it.
Solution
①Open or deepen vent grooves at burn positions (generally 0.02~0.04mm for PP, subject to no flash); ②regularly clean vents blocked by oil; ③reduce end injection speed to give air time to escape; ④use moderate clamping force to avoid closing parting line vent gaps; ⑤reduce mold release agent usage to prevent vent contamination.
Key Takeaway
Repeated burning at fixed positions is 90% a venting problem: first clean/open vent grooves, then adjust end speed—don't just blindly lower melt temperature; burn points are often the last-filled points of the cavity and can be precisely located with the short shot method.
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