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.
| Observation
| 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 |
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