Sink marks occur when the outer surface of the molded part cools and solidifies faster than the inner material, leading to internal voids or surface depressions. In glass-filled ABS, this happens due to the interaction of glass fibers and resin shrinkage during cooling. Uneven cooling, insufficient packing pressure, or thick sections exacerbate the problem.
Think of it like baking a cake: if the edges cool and set too quickly while the center remains soft, you might end up with a sunken middle. Similarly, sink marks form when the material inside the mold doesn't solidify uniformly.
Common Causes of Sink Marks in Glass-Filled ABS and How to Fix Them
Cause 1: Uneven Wall Thickness
Uneven wall thickness causes material to cool at different rates, leading to sink marks in thicker sections.
- Solution: Design parts with uniform wall thickness wherever possible. If thickness variations are unavoidable, use gradual transitions between thick and thin areas to minimize cooling disparities.
Cause 2: Insufficient Packing Pressure
Low packing pressure fails to compensate for material shrinkage, resulting in sink marks. When the mold cavity isn't properly filled or compacted, voids form as the material contracts.
- Solution: Increase packing pressure during the hold phase of the injection cycle. This ensures that additional material fills the cavity as the part cools and shrinks.
Cause 3: Inadequate Cooling Time
If cooling time is too short, the material doesn’t solidify uniformly, leading to sinks. Glass-filled ABS parts require sufficient cooling to ensure the fibers and resin set properly.
- Solution: Optimize cooling time based on part thickness and mold temperature. Use conformal cooling channels to achieve even heat dissipation across the mold.
| Parameter | Effect on Sink Marks | Suggested Adjustment |
|---|---|---|
| Packing Pressure | Reduces voids and material shrinkage | Increase during hold phase |
| Cooling Time | Promotes uniform solidification | Extend for thicker parts |
| Wall Thickness | Minimizes cooling rate differences | Ensure uniform design |
Cause 4: Improper Gate Location
A poorly placed gate can lead to uneven flow and packing, causing sink marks. In glass-filled ABS, flow orientation matters because fibers tend to align with the flow direction, affecting shrinkage.
- Solution: Position gates strategically to ensure even material flow and packing pressure. For thick sections, use multiple gates or a larger gate size to reduce shrinkage-related issues.
Cause 5: Material Selection and Fillers
Incorrect material grades or filler content can exacerbate sink marks. Low glass content or improper resin grades may shrink unevenly, increasing the likelihood of sinks.
- Solution: Use a higher glass-fiber content ABS grade to reduce shrinkage. Consult with material suppliers to choose the best resin for your application.
How to Prevent Sink Marks in Glass-Filled ABS Parts
Here’s a summary of the best practices to avoid sink marks:
- Design Optimization: Ensure uniform wall thickness and use fillets at transitions to avoid sharp changes in thickness.
- Optimize Process Parameters: Increase packing pressure and extend hold time during the injection cycle to compensate for material shrinkage.
- Enhance Cooling Systems: Use conformal cooling channels to ensure even cooling across the mold cavity.
- Material Selection: Choose a glass-filled ABS grade with higher filler content to reduce shrinkage and improve dimensional stability.
- Gate Placement: Position gates strategically to promote even flow and packing pressure.
Conclusion
Sink marks in glass-filled ABS parts are often caused by uneven cooling, insufficient packing pressure, or poor design. To fix these issues, focus on optimizing mold design, adjusting process parameters, and using the right materials.
For more guidance on troubleshooting injection molding defects, check out our resource center or contact us. Together, we can help you achieve flawless, high-quality molded parts.
