Defect guide · Surface quality
How to solve pinhole defects in PDCPD-RIM molding
Pinholes are clusters of tiny bubbles on the part surface. They spoil the finish and can weaken the skin. The fix has three parts: cavity surface, sealing and venting.
1. Improve the cavity surface
- Mirror polish. A class-A surface needs a high-grade mirror polish. Microscopic roughness traps air or moisture, which expands during the exothermic reaction and leaves pinholes.
- Aluminium ages. Aluminium molds such as 6061-T6 are cost-effective, but over time the surface oxidizes and forms microscopic pits.
- Hard chrome on steel. Where the budget allows, a steel cavity with hard chrome plating (HRC 62) resists oxidation and stays smooth, with far fewer pinholes than an aging aluminium surface.
2. Seal the mold completely
- Air suction is a main cause. RIM is a low-pressure process, but any gap at the parting line acts like a venturi and draws air into the cavity as the resin flows.
- Maintain seals. Inspect and replace O-rings regularly, and check that interlocks keep the two halves aligned and airtight during injection.
3. Size vents for cavity pressure
- Vent depth around 0.1 mm. Deeper vents cause flash; shallower vents trap air.
- Hold back-pressure. Placing and sizing vents to keep some resistance holds dissolved gas such as nitrogen in solution during polymerization, so it cannot nucleate into bubbles. The result is a dense, pinhole-free skin.
Summary
Pinhole-free PDCPD parts come from a smooth, stable cavity surface, an airtight mold and vents sized to keep pressure in the cavity.
Seeing this defect on your parts? Send photos and the 3D data. We will tell you whether the fix is in the process, the part design or the mold. Contact us.