Sink Mark / Depression

Local depressions appear on the thick-wall surface of the part. The surface gloss of the sink mark area is obviously lower than that of the surroundings, it feels collapsed to the touch, and appears as a circular or elliptical shallow pit, usually appearing on the back of the root of the rib, at the position corresponding to the BOSS post, or at sudden wall thickness changes. For parts requiring appearance (such as home appliance housings, automotive interior parts), sink marks are unacceptable defects; for transparent parts, the uneven density caused by sink marks also causes optical distortion.

1. Process parameter factors: insufficient holding pressure or insufficient time, insufficient cooling, excessive wall thickness difference, excessive melt temperature. 2. Mold comprehensive factors: in addition to venting, mold design parameters such as cooling water channel layout, gate position and size, and demolding draft angle all have a systematic impact on part quality, and optimization is required from the source of mold design.

1. Process parameter optimization and adjustment: increase holding pressure and holding time, extend cooling time, optimize wall thickness design, appropriately lower the melt temperature. 2. Systematic troubleshooting method: troubleshoot item by item in the order of "parameters -> material -> mold -> equipment". First adjust the parameters that are easiest to change, such as injection pressure and temperature, and observe the effect; if there is no improvement, check the drying and fluidity of the raw material; further check mold venting and gate design; finally check the mechanical condition of the equipment (check ring, screw). Change only one variable at a time to determine the root cause. 3. Preventive management: establish standardized process parameter cards and quality control limits for each mold/product, and regularly conduct process capability index (CPK) evaluation. For recurring problems, use fishbone diagram or 5Why analysis to trace the root cause, and fix the countermeasures as PFMEA control points.

The essence of sink marks is that local volume shrinkage cannot be compensated by feeding. The core countermeasure is to establish and maintain an effective feeding channel — set the gate near the thick-wall area, extend the holding time until after the gate freezes, and at the same time reduce the absolute shrinkage value of the thick-wall area by optimizing cooling.