Sink marks appearing at thick-walled parts of polypropylene injection molded products

Obvious sink marks appear at thick-walled parts of the product (such as the roots of reinforcing ribs, the backs of bosses, and abrupt wall thickness changes), the surface is uneven, affecting appearance and dimensional accuracy. The sink mark depth is usually 0.3-1.5mm, particularly obvious under side light, and in severe cases the sink mark 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-walled area （2）Polypropylene has high crystallinity and large volume shrinkage during cooling (shrinkage rate 1.0-2.5) （3）The mold gate size is too small or the position is improper, and the gate solidifies prematurely so that the holding pressure cannot be effectively transmitted （4）The product wall thickness is uneven, and the thick-walled parts cool slowly and shrink greatly （5）The mold temperature is too high (exceeding the 40-90°C upper limit), and the slow cooling rate aggravates shrinkage

（1）Raise the holding pressure to 80-120MPa and extend the holding time to 15-30s to ensure sufficient shrinkage compensation in the thick-walled area （2）Appropriately lower the mold temperature to the lower value within the range of 40-90°C to speed up cooling and reduce shrinkage （3）Increase the gate cross-sectional area; the gate thickness is 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 inclined surfaces or arc transitions at thick-thin transitions, with the thick-thin ratio not exceeding 2:1 （5）Appropriately lower the melt temperature to the lower value within the range of 200-280°C to reduce the volume shrinkage

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