Shenzhen Alu Rapid Prototype Precision Co., Ltd.

Sink marks are surface depressions that occur in injection-molded parts due to uneven cooling and shrinkage of the polymer as it solidifies. Thicker sections cool and shrink more than thinner ones, pulling the surface inward. Preventing them involves a combination of design optimizations, material choices, and process adjustments

Key Strategies to Avoid Sink Marks

1.Maintain Uniform Wall Thickness

Design parts with consistent wall thicknesses (typically 2-4 mm for most thermoplastics) to ensure even cooling and minimize differential shrinkage. Avoid thick sections greater than 6 mm, as they are prone to voids and sinks. If thicker areas are necessary (e.g., for structural reasons), incorporate gussets or ribs instead.

2.Optimize Rib and Boss Design

Keep ribs no thicker than 60-70% of the nominal wall thickness and space them at least twice their thickness apart to reduce heat buildup. For bosses, taper walls near the base or add fillets to distribute stress evenly, preventing sinks at attachment points.

3.Select Appropriate Materials

Choose resins with low shrinkage rates, such as amorphous polymers (e.g., ABS or polycarbonate) over semi-crystalline ones (e.g., PP or nylon). Add fillers like glass fibers to reduce overall shrinkage if compatible with part requirements.

4.Adjust Gate Location and Design

Position gates closer to thick sections to ensure complete filling before solidification. Use multiple or fan gates for larger parts to promote uniform flow and packing, delaying gate freeze-off and reducing sinks.

5.Control Process Parameters 

Increase Pack and Hold Pressure/Time: Apply higher second-stage pressure (80-100% of first-stage) for longer durations to compensate for shrinkage.  

Lower Melt Temperature: Reduce it slightly (e.g., 10-20°C) to slow cooling in thick areas without underfilling.  

Enhance Cooling: Use conformal cooling channels or balanced coolant flow to achieve uniform mold temperatures, avoiding hot spots.