Sink Mark Prevention in Boss Features
Key Takeaway: Sink marks at bosses are a design problem first, process problem second. Fix the design (wall ratio 50–60%, cored-out boss with longer core pin) before tuning the process. Process adjustments alone can reduce but not eliminate structural sink marks.
Prevention Strategies Ranked by Effectiveness
| Strategy | Effectiveness | Cost | When to Use |
|---|---|---|---|
| Reduce boss wall to 50–60% | ★★★★★ | Design change (before tooling) | Always — primary design rule |
| Cored-out boss (longer core pin) | ★★★★ | Longer core pin or modification | When wall ratio cannot be reduced further |
| Increase packing pressure/time | ★★★ | Free (process change) | Mild sinks, wall ratio already at 60% |
| Gas-assist at boss | ★★★★ | $5,000–15,000 (system) | Large bosses, critical show surfaces |
| Texture opposite surface | ★★ | $200–500 (texturing) | Cosmetic masking only — doesn't fix root cause |
Cored-Out Boss Design
Using a longer core pin for boss that extends through the full boss height creates a hollow boss:
- Material volume reduced 50–70% at the boss base
- Core pin tip should end 0.5–1.0 mm from the part surface (residual wall)
- Pin must be long enough to core the full boss height plus plate engagement
- Gas release feature recommended if cored depth > 3× core diameter
Process Adjustments
- Increase packing pressure: 10–15% above current setting. More material forced into shrinking boss compensates for volumetric shrinkage.
- Extend packing time: Hold packing until the gate freezes. Premature packing cutoff allows the boss to shrink freely.
- Increase cooling time: Allow the boss to solidify more completely before ejection. Under-cooled bosses continue to shrink after ejection.
- Optimize gate location: Locate gate near the boss to ensure packing pressure reaches the boss effectively.
Frequently Asked Questions
What causes sink marks at boss features?+
Thick material at the boss base shrinks more than the surrounding thin wall during cooling, pulling the show surface inward. Severity depends on wall ratio, resin shrinkage, and packing.
Can process adjustments alone fix sink marks?+
Partially — packing pressure can reduce sink 40–60%. But if boss wall exceeds 60% of adjacent wall, design changes are required for full elimination.
How does a cored-out boss reduce sinks?+
A longer core pin creates a hollow boss, reducing material volume at the base by 50–70%. Less material = less shrinkage = less sink.
Related
Fighting Sink Marks?
Send photos of the sink marks and your boss dimensions — our engineers will recommend design modifications and core pin changes.
✓ Free sink analysis✓ Boss pins in stock✓ MOQ 1 piece