Injection Molding Sink Marks: Causes, Solutions & Prevention
What Are Sink Marks in Injection Molding?
Sink marks are localized surface depressions or dimples that appear on molded parts, typically in thicker wall sections, ribs, or boss areas. They occur when the outer surface solidifies faster than the inner material, causing the still-molten core to shrink inward as it cools.
These cosmetic defects are among the most common injection molding quality issues, affecting both appearance and part dimensions. Sink marks are classified as a Class B defect per ISO 294-1 and can render parts non-compliant for visible surfaces.
Root Causes of Sink Marks
1. Uneven Wall Thickness
Abrupt transitions from thick to thin walls are the #1 cause. The thicker section takes longer to cool, creating a differential shrinkage that pulls the surface inward. Ideal wall thickness ratio should not exceed 1.5:1 between adjacent sections.
2. Insufficient Packing Pressure
When packing pressure is too low or hold time too short, the molten material in the cavity continues to shrink after the gate freezes. Holding pressure should typically be 50-70% of injection pressure.
3. Inadequate Cooling Time
If cooling time is insufficient, the core temperature remains above the material glass transition temperature when ejected. The skin contracts as it continues cooling outside the mold.
4. Gate Freeze-Off Too Early
Gates that are too small or positioned poorly freeze prematurely, cutting off the packing phase. For sink mark-prone parts, the gate must remain open long enough to allow packing material into thick sections.
Material-Specific Susceptibility
| Material | Shrinkage % | Sink Mark Risk | Recommended Fix |
|---|---|---|---|
| PP (Polypropylene) | 1.5-2.5% | High | Increase hold pressure + longer hold time |
| ABS | 0.4-0.7% | Moderate | Optimize wall thickness + rib design |
| Nylon 6 (PA6) | 0.5-1.5% | High | Use glass-filled grade; control moisture |
| PC (Polycarbonate) | 0.5-0.7% | Low-Moderate | Target 240-260°C melt temp |
| HDPE | 1.5-4.0% | Very High | Thin walls < 3mm; slow injection speed |
| POM (Acetal) | 1.8-2.5% | High | Use optimized gate design |
Mold Design Solutions
Optimize Rib and Boss Design
Ribs should be no more than 60% of the adjacent wall thickness. For a 3mm wall, rib base should be max 1.8mm. Use a radius at the base (R = 0.5-1.0mm) to reduce stress concentration while minimizing material accumulation.
Strategic Gate Placement
Place gates near thick sections to ensure these areas receive maximum packing pressure. For multiple cavities, consider sequential valve gating to control flow front and packing sequence.
Cooling Channel Optimization
Position cooling channels within 2-3 diameters of the cavity surface. Use conformal cooling (3D-printed inserts) for complex geometries to achieve uniform heat extraction, reducing sink marks by up to 40%.
Process Parameter Optimization
- Increase holding pressure to 60-80% of injection pressure
- Extend hold time to 1.5x gate freeze time
- Reduce melt temperature by 10-15°C (within material spec)
- Increase cooling time by 20-30% for sink-sensitive parts
- Reduce injection speed in the final 10-20% of fill