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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