Overmolding: Troubleshooting

The most common problems encountered in overmolding are:

  • Poor adhesion
  • Incomplete filling of the substrate or overmold
  • Flashing of the overmold

Corrective actions for many of these problems are presented below.


Possible Causes

Corrective Actions

Flash (over substrate or on periphery of part)

  • Poor mold fit
  • Inadequate molding machine tonnage
  • Poor shutoff design
  • Substrate shrinkage
  • Check by bluing the tool
  • Increase tonnage or decrease injection and pack pressure
  • Re-cut tool to obtain complete shutoff
  • Check for substrate sinks and re-cut tool

Short shots

  • Not enough material
  • Not enough injection pressure
  • Not enough fill speed
  • Melt too cold
  • Poor venting
  • Increase shot size
  • Increase injection pressure
  • Increase injection speed
  • Increase melt temperature
  • Decrease the clamp tonnage and re-cut vents

Poor quality knit lines

  • Gas trapped between polymer fronts
  • Low melt temperature
  • Improve vents
  • Increase injection speed and melt/mold temperature

Overmold breaks or impinges through hollow substrate

  • Improperly supported substrate
  • High injection pressure and melt temperature
  • Wrong location of gate
  • Fully support the substrate to resist hydraulic injection pressures and melting
  • Lower injection pressure and melt temperature
  • Relocate gate

Warped parts

  • Post mold shrinkage
  • Lower the molded in stress in both the substrate and the elastomer
  • Increase the stiffness of the substrate by including glass or increasing thickness of ribs

Poor adhesion

  • Incompatible materials
  • Contamination
  • Gate freeze-off too early
  • Re-select correct grade of TPE
  • Check for color concentrate compatibility and/or the use of a lubricated grade of substrate
  • Increase the process temperatures and/or mold temperature

Surface sinks

  • Material shrinkage causes non-uniform release from tool surface
  • Gate freeze-off too early
  • Increase the pack pressure/hold time and decrease material temperature
  • Increase gate size

Non-uniform color

  • Incompatible color concentrate
  • Inadequate melting and dispersion of color concentrate
  • Check for concentrate compatibility
  • Increase rear zone temperature, back pressure, screw RPM

Surface splaying (silver streaking)

  • Moisture in TPE
  • Dry materials properly

Parts stick in the "A" side of the tool

  • Not enough draft
  • Part forms a vacuum in the "A" cavity
  • "A" cavity too shiny
  • "A" cavity too hot
  • Increase the draft
  • Provide air assist release
  • Sandblast the cavity
  • Run the "A" half cooler

Elastomer is sticky or smells

  • Material degradation
  • Check for material residence time and reduce temperature in rear zones if possible