Sink Marks at Thick-Walled Parts of Homopolymer Polypropylene Injection Molded Products (Variant: Strengthened Measures)

Obvious depressions appear at the thick-walled parts of the product (such as the root of reinforcing ribs, the back of bosses, and places where the wall thickness changes abruptly), the surface is uneven, affecting the appearance and dimensional accuracy. The depression depth is usually 0.3-1.5mm, particularly obvious under side light, and in severe cases the depression can be felt by fingers. In particular, extra attention is needed on the original basis; this problem is more prominent and requires key attention.

（1）The holding pressure is insufficient or the holding pressure time is too short, and the melt cannot fully compensate the shrinkage into the thick-walled area （2）The crystallinity of homopolymer polypropylene is high, and the volume shrinkage during cooling is large (shrinkage rate 1.5-2.5) （3）The mold gate size is too small or the position is improper, and the gate solidifies prematurely, so the holding pressure cannot be effectively transmitted （4）The product wall thickness is uneven, and the thick-walled parts have a slow cooling speed and a large shrinkage （5）The mold temperature is high (exceeding the 40-90°C upper limit), and the slow cooling rate aggravates the shrinkage （6）On the original basis, extra attention is needed: process parameter fluctuations or equipment status changes have aggravated this problem

（1）Increase the holding pressure to 80-120MPa and extend the holding pressure time to 15-30s to ensure sufficient shrinkage compensation in the thick-walled area （2）Appropriately reduce the mold temperature to a lower value within the 40-90°C range to accelerate cooling and reduce shrinkage （3）Increase the gate cross-sectional area, with the gate thickness being at least 60-70% of the product wall thickness, to extend the gate freezing time （4）Optimize the product design, make the wall thickness as uniform as possible, use slopes or arcs for transitions between thick and thin parts, and keep the thick-to-thin ratio not exceeding 2:1 （5）Appropriately reduce the melt temperature to a lower value within the 200-280°C range to reduce the volume shrinkage （6）And strengthen process monitoring; on the original basis, extra attention is needed, with special attention to parameter stability

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