Overmoulding bonds a soft-touch or functional material onto a rigid substrate in one part. Material compatibility, bond design, shut-offs and process sequence decide whether it bonds, seals and looks good.
A power tool handle was designed with a soft-touch TPE grip overmoulded onto a nylon substrate. The prototypes looked and felt excellent. In production, the grip began to peel at its edges after a few months of use. The cause was a combination of factors: the TPE grade was not designed to bond to nylon, the overmould edges finished flush on the surface with no lock, and the substrate was not dried before the second shot.
Overmoulding can create excellent products: comfortable grips, integrated seals, multiple colours and functional combinations. It also has its own design rules.
Methods
- Two-shot (multi-shot) moulding: both materials moulded in one machine, using a rotating tool or transfer. Excellent bonding and cycle efficiency, highest tool cost.
- Insert overmoulding: a pre-moulded substrate, or metal insert, placed in a second tool and overmoulded. Lower tool cost, more handling.
Material compatibility
Bonding depends on chemical compatibility between the materials:
- Many TPE grades are formulated to bond to specific substrates, such as PP, ABS, PC, PC/ABS or nylon. Choose a grade designed for your substrate.
- Some combinations do not bond chemically at all. They need mechanical interlocks.
- Processing temperatures must be compatible: the overmould melt must activate bonding without deforming the substrate.
Check supplier compatibility data, then test peel strength on real parts.
Design guidelines
- Mechanical interlocks: holes, undercuts or grooves in the substrate that the overmould flows through or around, especially at edges and where bonding is weak.
- Edge design: avoid overmould edges ending flush on a surface where they can be peeled. Recess the overmould edge into a groove in the substrate.
- Overmould thickness: typically 1–3 mm for TPE grips. Too thin can be hard to fill; too thick can cause sink and long cycles.
- Uniform overmould thickness, for consistent feel and filling, following the uniform wall principle.
- Shut-offs: the tool must seal against the substrate to stop the overmould flashing onto it. Design clear, robust shut-off surfaces with sufficient compression.
- Substrate support: the substrate must withstand overmould injection pressure without deforming. Add ribs or support where needed.
- Venting: trapped air at the end of the overmould flow causes poor bonding and short shots.
Process considerations
- Substrate temperature and condition: a warm, clean substrate usually bonds better. Dry hygroscopic substrates before overmoulding.
- Avoid contamination from mould release agents or handling.
- Gate the overmould so that flow pushes towards the edges and interlocks, without displacing or deforming the substrate.
- Validate peel strength and seal performance across the process window.
Testing
- Peel testing on production parts, including after environmental ageing.
- Chemical exposure tests, such as sweat, sunscreen, oils and cleaning agents, for grips.
- Functional testing for seals.
Overmoulding works best when the substrate, overmould material, tool and process are designed together from the start, which is exactly the role of a Design for Manufacture review.
