Material Drying
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PP Part Internal Bubble
Problem Phenomenon
Voids appear inside thick-walled PP parts, visible when cut open, often accompanied by surface sink marks, sometimes appearing at the moment of demolding or after storage.
Cause Analysis
At thick walls the surface layer cools and shrinks first, and internal melt forms vacuum bubbles due to insufficient holding pressure feeding; another case is regrind/masterbatch containing moisture or additive volatilization producing gas bubbles—the two mechanisms differ and must be distinguished (vacuum bubbles have smooth walls, gas bubbles may accompany silver streaks).
Solution
①For thick-walled parts, prioritize increasing holding pressure and time and place the gate at the thick wall to solve vacuum bubbles; ②strengthen drying of regrind and masterbatch and control ratios to reduce moisture and volatiles; ③appropriately lower melt temperature to reduce volatilization; ④improve mold venting to avoid entraining air; ⑤structurally reduce material at thick walls to reduce shrinkage from the source.
Key Takeaway
For PP bubbles, first distinguish 'vacuum bubble' from 'gas bubble': vacuum bubbles rely on feeding (holding pressure + gate position), gas bubbles rely on venting and volatiles control (drying + cooling + venting)—failure to distinguish leads in the opposite direction.
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