Sink marks appearing at thick-wall areas of random copolymer polypropylene injection molded products (variant: strengthened measures)

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. Especially, additional attention is needed on top of the original basis; this problem is more prominent and requires focused attention.

(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 (6) On top of the original basis, additional attention is needed as process parameter fluctuations or equipment condition changes aggravate this problem

(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 (6) And strengthen process monitoring; on top of the original basis, additional attention is needed to pay special attention to parameter stability

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. In different production scenarios, targeted preventive measures need to be taken.