Injection Molding
11 조회수
Sink marks appearing at thick-wall areas of random copolymer polypropylene injection molded products
문제 현상
Obvious depressions appear at thick-wall areas of the product (such as the roots of reinforcing ribs, the back of bosses, and abrupt wall thickness changes); the surface is uneven, affecting appearance and dimensional accuracy.
The depression depth is usually 0.3-1.5mm, especially obvious under side light, and in severe cases the depression can be felt by fingers.
원인 분석
(1) Holding pressure is insufficient or holding time is too short; the melt cannot fully compensate for shrinkage into the thick-wall area
(2) Random copolymer polypropylene has high crystallinity, and volume shrinkage during cooling is large (shrinkage rate 1.0-2.0)
(3) The mold gate size is too small or improperly positioned; premature gate solidification prevents holding pressure from being effectively transmitted
(4) Product wall thickness is uneven; thick-wall areas cool slowly and shrink more
(5) Mold temperature is on the high side (exceeding the 40-80°C upper limit); slow cooling rate aggravates shrinkage
해결 방법
(1) Increase holding pressure to 80-120MPa and extend holding time to 15-30s to ensure full shrinkage compensation in thick-wall areas
(2) Appropriately lower mold temperature to the lower value within the 40-80°C range to accelerate cooling and reduce shrinkage
(3) Increase gate cross-sectional area; gate thickness is at least 60-70% of product wall thickness to extend gate freezing time
(4) Optimize product design; keep wall thickness as uniform as possible, use slopes or arcs at thick-thin transitions, with a thick-thin ratio not exceeding 2:1
(5) Appropriately lower melt temperature to the lower value within the 190-260°C range to reduce volume shrinkage
핵심 요약
Sink marks are a typical defect of crystalline material '2079851475616104452; the root cause is that local volume shrinkage is not effectively compensated, and they need to be solved coordinately from three aspects: holding pressure parameters, gate design and cooling conditions.
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