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          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