In injection molding part design, sink marks are one of the most common surface defects, primarily caused by slower cooling in thicker wall areas, where internal shrinkage pulls the surface inward, creating depressions. Good design can dramatically prevent or eliminate sink marks at the source, resulting in more attractive appearance, more stable quality, and significantly reduced costs for mold modifications or rework.
To effectively avoid sink marks, follow these professional design principles:
- Uniform wall thickness: Maintain consistent wall thickness throughout the part, avoiding localized thick sections (ideal range: 1.0–3.0 mm, adjusted per material); use gradual transitions or large radii (R ≥ 0.5 × wall thickness) at thickness changes to prevent sharp shrinkage gradients.
- Rib design optimization: Keep rib thickness at 40–60% of the main wall (most commonly 50%); limit rib height to no more than 3× wall thickness; prefer multiple thin ribs over a single thick one, and add a 7° gradual taper or generous fillets at the rib base to avoid “hot spots” that cause prominent sink marks.
- Boss optimization: Limit boss base thickness to ≤60% of the main wall; core out (deepen) the inner hole as much as possible, or add a “moat” (recess) around the boss to reduce local mass; apply at least 0.5° draft on both inner and outer surfaces for better filling and ejection.
- Avoid material concentration in thick areas: Core out (hollow) thick sections wherever possible, or replace solid masses with grid/honeycomb structures; apply generous radii to all intersecting features to minimize heat buildup.
- Additional aids: Select appropriate materials (high-flow, low-shrinkage options like PP or ABS perform better than PC); add texture to cosmetic surfaces to mask minor sink marks; for stubborn cases, consider gas-assisted injection or micro-foaming processes to fully resolve persistent issues.
Great design eliminates sink marks before they even occur! ATC-Mould specializes in injection molding part DFM optimization and high-quality mold manufacturing. With professional Moldflow analysis and extensive hands-on experience, we’ve helped hundreds of clients eliminate over 90% of sink mark risks during the design phase, achieving zero-defect appearance and defect rates below 0.3%, while greatly boosting mass production efficiency and market competitiveness.
Facing sink mark issues with your product? Feel free to share your 3D files or requirements—we offer free DFM review and sink mark risk assessment to help you create flawless, high-efficiency “zero-sink” part solutions! Make your injection molded products perfect from the start and more competitive than ever!