Defect guide · Surface quality
How to eliminate sink marks in PDCPD-RIM molding
Sink marks are shallow depressions on the visible surface. They appear when internal shrinkage during polymerization pulls the skin inward. Two levers fix them: structural design and mold temperature.
1. Optimize rib and structural design
Reinforcement features are the most important factor. Extra mass where a rib meets the main wall creates a hot spot that shrinks more than the area around it.
- Control rib thickness. Keep the ratio of rib thickness (D) to main wall thickness (T) below 2/3. Where ribs are closely spaced, reduce it to 1/2 so heat does not concentrate.
- Use moderate root radii. Sharp corners at the rib root cause stress and flow resistance; very large fillets add local thickness. A moderate fillet smooths the flow and spreads shrinkage evenly.
- Add transition slopes. Sudden changes in wall thickness are a main cause of sink marks. A gradual taper keeps the reaction front steady and removes the local mass that triggers depressions.
2. Use a mold temperature split
In PDCPD molding you can steer the pull of shrinkage away from the cosmetic side.
- Recommended settings: cavity (female) about 90 °C, core (male) about 60 °C.
- Why it works: the hotter cavity increases adhesion between resin and cavity surface, so shrinkage pulls from the core side, the back of the part.
- Limit: do not drop the core below 50 °C to widen the gap. Too cold a core slows the DCPD reaction and hurts physical properties and surface quality.
Summary
A rib-to-wall ratio of 2/3 or less, combined with a 90 °C / 60 °C cavity-core split, removes most sink marks on PDCPD parts.
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.