Thermoplastic Elastomer Molding Guidelines:
Overmolding Design Considerations

  • Select ejection mechanism(s) to minimize mechanical damage to surface.
  • Consider using pneumatic air poppets as a means to create initial molded TPE surface release from tool steel – excellent method for very thin wall components. Use pneumatic poppets (air) release approach where feasible.
  • Determine the most cosmetically important requirements from end user to prioritize surface needs which will guide tool details.
  • Use stripper plate to spread stress across molded TPE surface.
  • Where feasible, have stripper plate push against the exposed thermoplastic surfaces which are slightly less prone to surface damage.
  • Avoid using “sharp” (small diameter) ejector pins in component ejection system.
  • Slow ejector speed to minimize the “punch through effect” into the newly molded TPE surface.
  • Use texture on mold surface to promote ejection/release.
  • If clarity is required, use SPI/SPE #2 finish and use air poppets and a stripper sleeve for ease of ejection.

Surface Texture

Basic Feel of a soft over-molded surface is affected by three primary factors.

  • RTP Company TPE product selection (modulus vs. hardness)
  • TPE component wall thickness coupled with modulus
  • Structural geometry of plastic substrate under the TPE

The Benefits of Using Custom TPE Surface Textures

  • Can provide a softer feel when using a harder RTP Company TPE grade
  • Can decrease total cycle time (vs. using thicker TPE across section)
  • Help reduce scuff and mar
  • Can assist in shedding fluid from TPE surface
  • Can help “hide” gate marks and cosmetic surface defects
  • Can actually increase total surface area that consumer uses with finished product
  • Select appropriate RTP Company TPE to maximize bond strength to selected RTP Company thermoplastic compound