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.

Approach for Good Functionality

  • 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
Shut-Off Design to Reduce Peeling
Shut-Off Designs to Reduce Peeling

Shut-Off Recommendations

  • 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)

Shut-Offs to Avoid

  • 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

Fundamental “lock” of TPE to Substrate: Three Basic Methods

  • Chemical adhesion
  • Mechanical design techniques
  • Interlocks

Mechanical Interlocks Offer Major User Benefits

  • 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