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.
Observation
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Possible Causes
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Corrective Actions
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Flash (over substrate or on periphery of part)
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- Poor mold fit
- Inadequate molding machine tonnage
- Poor shutoff design
- Substrate shrinkage
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- 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
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Short shots
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- Not enough material
- Not enough injection pressure
- Not enough fill speed
- Melt too cold
- Poor venting
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- Increase shot size
- Increase injection pressure
- Increase injection speed
- Increase melt temperature
- Decrease the clamp tonnage and re-cut vents
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Poor quality knit lines
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- Gas trapped between polymer fronts
- Low melt temperature
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- Improve vents
- Increase injection speed and melt/mold temperature
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Overmold breaks or impinges through hollow substrate
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- Improperly supported substrate
- High injection pressure and melt temperature
- Wrong location of gate
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- Fully support the substrate to resist hydraulic injection pressures and melting
- Lower injection pressure and melt temperature
- Relocate gate
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Warped parts
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- 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
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Poor adhesion
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- Incompatible materials
- Contamination
- Gate freeze-off too early
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- 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
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Surface sinks
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- Material shrinkage causes non-uniform release from tool surface
- Gate freeze-off too early
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- Increase the pack pressure/hold time and decrease material temperature
- Increase gate size
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Non-uniform color
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- Incompatible color concentrate
- Inadequate melting and dispersion of color concentrate
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- Check for concentrate compatibility
- Increase rear zone temperature, back pressure, screw RPM
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Surface splaying (silver streaking)
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Parts stick in the "A" side of the tool
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- Not enough draft
- Part forms a vacuum in the "A" cavity
- "A" cavity too shiny
- "A" cavity too hot
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- Increase the draft
- Provide air assist release
- Sandblast the cavity
- Run the "A" half cooler
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Elastomer is sticky or smells
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- Check for material residence time and reduce temperature in rear zones if possible
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