Injection Molding
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Sink depressions in thick-wall areas of homopolymer polypropylene injection products
Fenómeno del problema
Obvious depressions appear in thick-wall areas of the product (such as rib roots, backs of bosses, wall thickness transitions), with uneven surface, affecting appearance and dimensional accuracy.
Depression depth is usually 0.3-1.5mm, especially obvious under side lighting; in severe cases, the depression can be felt by finger touch.
Análisis de la causa
(1) Insufficient holding pressure or holding time too short; melt not fully compensated into thick-wall areas
(2) High crystallinity of homopolymer polypropylene; large volume shrinkage during cooling (shrinkage rate 1.5-2.5)
(3) Mold gate size too small or position improper; gate solidifies too early, holding pressure cannot be effectively transmitted
(4) Uneven product wall thickness; thick-wall areas cool slowly and shrink more
(5) Mold temperature too high (exceeding the 40-90°C upper limit); slow cooling rate aggravates shrinkage
Solución
(1) Increase holding pressure to 80-120MPa, extend holding time to 15-30s, ensure sufficient shrinkage compensation in thick-wall areas
(2) Appropriately reduce mold temperature to the lower value within the 40-90°C range, speed up cooling to reduce shrinkage
(3) Enlarge gate cross-sectional area; gate thickness at least 60-70% of product wall thickness, extend gate freezing time
(4) Optimize product design; wall thickness as uniform as possible, use slopes or arc transitions at thickness transitions, thickness ratio not exceeding 2:1
(5) Appropriately reduce melt temperature to the lower value within the 200-280°C range, reduce volume shrinkage
Conclusión clave
Sink depressions are a typical defect of '2079851475616104452 crystalline materials; the root cause is local volume shrinkage not being effectively compensated, requiring coordinated solutions from holding pressure parameters, gate design, and cooling conditions.
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