Thermoplastic Elastomer Molding Guidelines

Overmolding Design Considerations

Proper shut off design is critical to stop the TPE flow in precise geometry location and prevent undesirable random edge flash

Design shut off to minimize potential for edge peeling of TPE
General geometry needs to be a very sharp transition area between molded TPE edge and supporting substrate.
Resulting TPE geometry must be designed to vent the cavity properly

Provide interference fit of 0.002 to 0.004 in (0.050 to 0.101 mm) - somewhat dependant on cosmetic needs
Specific plastic substrate ductility
Heat treat shut off steel to a minimum 54 Rockwell hardness
Consider substrate edge design to "hide" TPE edge from consumer.
Pre-dry hygroscopic substrate and TPE pellets to avoid porous surfaces nearest substrate interface
Where appropriate, have the actual shut-offs employed as inserts (helps downstream injection mold maintenance)

Avoid using rounded/radiused shut offs
Component designs where TPE geometry lies high above the substrate (e.g., cliff wall)
Placing shut offs directly in the mold base
Building the tool without developing a clear shut off strategy first
Placing the vent directly at shut off edges - can actually encourage flashing

Mechanical combination of TPE/substrate geometry (very difficult to separate in use)
Method to greatly improve component abrasion resistance
Can be used across a substrate geometry - multiple locations