Sink Marks at Thick-Wall Sections of High-Density Polyethylene Injection-Molded Products

Obvious depressions appear at thick-wall sections of the product (such as the roots of reinforcing ribs, the backs of bosses, and sudden wall thickness changes); the surface is uneven, affecting appearance and dimensional accuracy. The depression depth is usually 0.3-1.5mm, which is especially obvious under side-light illumination, and in severe cases the depression can be felt by fingers.

(1) The holding pressure is insufficient or the holding time is too short, and the melt fails to fully compensate for shrinkage into the thick-wall area (2) High-density polyethylene has a high crystallinity, and the volume shrinkage during cooling is large (shrinkage 1.5-3.0) (3) The mold gate size is too small or the position is improper, and the gate solidifies too early, so the holding pressure cannot be effectively transmitted (4) The wall thickness of the product is uneven, and the thick-wall sections cool slowly and shrink greatly (5) The mold temperature is too high (exceeding the upper limit of 30-80°C), and the slow cooling rate aggravates shrinkage

(1) Increase the holding pressure to 80-120MPa and extend the holding time to 15-30s to ensure full shrinkage compensation in the thick-wall area (2) Appropriately reduce the mold temperature to the lower value within the range of 30-80°C to accelerate cooling and reduce shrinkage (3) Increase the gate cross-sectional area; the gate thickness should be at least 60-70% of the product wall thickness to prolong the gate freezing time (4) Optimize the product design with as uniform wall thickness as possible; use inclined surfaces or arc transitions at thick-thin transitions, with a thick-to-thin ratio not exceeding 2:1 (5) Appropriately reduce the melt temperature to the lower value within the range of 180-260°C to reduce the volume shrinkage

Sink marks are a typical defect in the '2079851475616104452 of crystalline materials; the root cause is that local volume shrinkage is not effectively compensated, and it needs to be solved coordinately from three aspects: holding parameters, gate design, and cooling conditions.