Overmolding: Troubleshooting

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