Molding Process
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PVA extruded film thick at edges and thin in the middle
Problem Phenomenon
PVA extruded film edges 20-30% thicker than the middle, thickness showing a U-shaped distribution
Cause Analysis
Improper die flow channel design, low resistance and large flow at edges; PVA melt elasticity swells more at edges; air ring cooling has different effects at edges
Solution
1) Die adopts coat-hanger design to balance resistance between middle and edges; 2) Fine-tune die gap, slightly larger in the middle and slightly smaller at edges for compensation; 3) Slightly reduce air ring airflow at edges; 4) Add restrictor bar to adjust edge flow
Key Takeaway
U-shaped thickness is caused by die flow channel and swelling differences; coat-hanger die and gap fine-tuning are the key
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