Key principles.
Bond chemistry
Some material pairs bond well (TPE on PP, TPE on ABS); others need mechanical lock. Verify compatibility chart.
For non-bonding
When materials don't chemically bond, use mechanical lock features — undercuts, holes, ribs.
Both layers
Substrate wall ≥ 1.5 mm. Overmold thickness 0.5-2 mm typical. Avoid extreme thickness ratios.
First-shot stays in mold
First-shot substrate must stay in tool when second injection occurs. Design supports this.
Avoid bond face
Gate position affects flow and stress. Don't gate where overmold-substrate bond critical.
Different materials
Different shrinkage rates between substrates. Compensate in mold design.
FAQ
TPE or TPU overmolded on rigid substrate. Toothbrush handles, tool grips, electronics grips. Standard production approach.
Two-shot: same machine, sequential injection, automated. Insert molding: substrate in separately, manually placed. Two-shot more efficient at volume.
Different colors achievable. Specify Pantone for each material. Verify colorfast across operating conditions.
Two-shot tool 1.5-2× single-shot tool. Insert molding tool similar to single-shot but secondary operation.
Bond per material pair. TPE on PP excellent. Verify with pull tests during process development.
Two-shot adds 50-80% to single-shot cycle. Per-part cost optimized at production volumes.
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