Injection Molding
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Fast gate freezing of low density polyethylene injection mold
Problem Phenomenon
The gate solidifies too early during holding, holding pressure cannot be transmitted into the cavity, and shrinkage voids or depressions appear in thick-wall areas of the product.
Gate freezing time is shorter than holding time, resulting in insufficient shrinkage compensation.
Cause Analysis
(1) Gate size too small; cross-sectional area insufficient to maintain the melt channel
(2) Mold temperature too low, gate area cools too fast
(3) Melt temperature too low; melt at the gate has high viscosity and solidifies quickly
(4) Improper gate shape design, e.g., pin gate diameter too small
(5) Holding pressure set too low, insufficient melt flow driving force
Solution
(1) Enlarge gate size; gate thickness at least 60-70% of product wall thickness
(2) Appropriately increase mold temperature to the higher value within the 30-60°C range
(3) Appropriately increase melt temperature to the higher value within the 160-220°C range
(4) Optimize gate shape; avoid overly small pin gates, use edge gates or fan gates instead
(5) Increase holding pressure, increase the driving force of melt through the gate
Key Takeaway
Gate freezing too fast is caused by small gate size or low temperature; freezing time needs to be extended by enlarging the gate and increasing temperature.
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