Shenzhen Alu Rapid Prototype Precision Co., Ltd.

Sink marks are surface depressions that occur on plastic parts due to uneven cooling and shrinkage, particularly in thicker sections like ribs or bosses, where the material contracts more than surrounding areas. They can be minimized through a combination of design, process, mold, and finishing adjustments. Below are proven strategies, categorized for clarity.

1. Optimize Part Design

Design changes are often the most effective long-term solution, as they address the root cause of differential shrinkage.

a.Maintain uniform wall thickness: Aim for consistent thicknesses (e.g., 2-4 mm for most resins) and avoid thick sections by coring out non-structural areas or using multiple thinner layers. Round corners to promote even flow and cooling. 

b.Refine ribs and bosses: Limit rib thickness to 50-60% of the nominal wall thickness (0.5-0.6T), with a height no more than 3 times the wall thickness (3T). For bosses, keep the base 60% of wall thickness (0.6T), add fillets with a 0.25T radius, and include a triangular groove around the base (30% wall depth at a 30° angle) to reduce material buildup. 

c.Strategic gate placement: Position gates closer to thick sections to ensure even filling and delay gate freeze-off, preventing premature pressure loss. Use design for manufacturability (DFM) simulations (e.g., in SOLIDWORKS Plastics) to predict and iterate on potential sink areas early. 

2. Adjust Process Parameters

Fine-tuning machine settings can compensate for design limitations without major changes.

a.Increase packing/holding pressure and time: Boost pressure to force more material into thick areas (compensating for shrinkage) and extend hold time until the gate freezes, ensuring even cooling. Start with 80-100% of injection pressure and adjust incrementally to avoid overpacking stresses. 

b.Control temperatures: Keep melt temperature within the resin manufacturer's range (avoid excess to prevent prolonged cooling) and lower mold temperature in thick areas for faster, more uniform solidification. Verify coolant flow to maintain balance and prevent distortion. 

3. Improve Mold Design

Enhance the tool to support better flow and cooling.

a.Uniform cooling channels: Place water lines close to thick sections (especially internals) for even heat extraction; consider conformal cooling or high-conductivity inserts (e.g., beryllium copper) for complex geometries. 

b.Adequate venting and gating: Add vents to release trapped air and use appropriately sized gates (or multiple gates for long flow paths) to maintain pressure and achieve full cavity fill. 

4. Material Selection and Surface Finishing

These can mitigate visibility or inherent shrinkage tendencies.

a.Choose low-shrinkage materials: Opt for resins with lower shrinkage rates (e.g., amorphous polymers like ABS over crystalline ones like PP) to reduce overall contraction. 

b.Apply matte or textured finishes: Use mold texturing (e.g., MT-11010 or SPI C3-D3 matte) to diffuse light and hide minor sinks. Lighter colors also reduce visibility compared to glossy dark ones.