10 critical points for Overmold /Two-Shot / Insert Mold Design

1. Ensure Material Compatibility, make sure chemical bonding or mechanical interlock between substrate and overmold layer is the first step. But if materials don’t adhere perfectly, undercuts or holes are needed for strong physical bonding.

2. When chosing resign, shrinkage difference should be considered.Calculate different shrinkage rates between materials to prevent warpage or stress.


3. Design Shut-Off surface seal tightly, make sure precise metal-to-metal contact to prevent flash during second injection.

4. Control wall thickness, typically 1.5mm-3.0mm, to prevent sink or short shots.

5. Design gates properly to ensure uniform flow of resin.

6. Design independent cooling circuits to ensure a balanced cooling rate for the two types of materials.

7. Core Rotation/Indexing: Design precise turntable alignment for accurate second-shot registration.

8. Ensure good venting so that adequate air escape at flow fronts, especially in encapsulated areas.

9. When design ejection system, should avoid the possibility of damaging soft-touch material. Use large ejectors or stripper plates would be a good option.

10. The product should be designed with a reasonable draft angle to prevent the steel from re-inserting into the product during the second injection molding process, which could cause damage.