Key takeaways
- An overmould holds by chemical bond, mechanical interlock or both. Decide which one the design relies on.
- The substrate and the overmould material must be chosen as a pair, not separately.
- A seal needs a controlled compression and a clean gland, not just a soft material.
- Strain relief works through a gradual transition, not a thick lump at the cable entry.
What overmoulding adds
Overmoulding moulds a second material over a rigid substrate. The substrate is usually a moulded housing, a connector body or a cable assembly, and the overmould is usually a soft thermoplastic elastomer. The result can be a grip that is comfortable to hold, a seal that keeps water out of a joint or a strain relief that stops a cable bending sharply at the entry.
The process is used for three different jobs, and each one puts a different demand on the design. A grip needs surface feel and resistance to wear. A seal needs controlled compression and chemical compatibility. A strain relief needs a gradual change in stiffness along the cable.
Choose the substrate and the overmould as a pair
The substrate is often PP, ABS, PC/ABS, PA or PBT. The overmould is often a styrenic block copolymer, a thermoplastic vulcanisate or a thermoplastic polyurethane. The pair decides whether the two materials bond chemically or only grip mechanically.
Some pairs bond well because the polymers are compatible at the interface. Others need a compatibiliser, a primer or a mechanical interlock. A soft TPE over PP is a common example: the bond depends on the grade and the process, not on the fact that both are thermoplastic.
Mechanical interlock is the fallback and often the more reliable route. Holes, grooves, undercuts, a textured substrate surface or a flange that the overmould wraps around all give the soft material something to hold. The interlock should be designed so the overmould cannot peel from an edge, because peeling starts at a free edge and travels.
Shore hardness is the next choice. A softer grade feels better and seals with less force, but tears more easily. A harder grade is more durable but feels less like a grip. The right number comes from the function and the test.
Grip geometry that survives use
A grip is a surface with a shape. The shape should give the hand a location, not just a texture: a slight waist, a thumb rest or a raised ridge does more for control than random bumps. Mould the texture into the overmould and design it so the tool can release it.
Thickness matters. A very thin overmould wears through and telegraphs the substrate; a very thick one adds weight, costs material and can sink. A soft grip is often 1.5 to 3 mm over the substrate, with local ribs where the hand loads it.
The edge of the overmould is the weak point. A feathered edge peels; a square edge catches and lifts. Terminate the overmould in a groove, under a flange or in a recess so the edge is protected. If the edge is visible, radius it and keep it away from the highest wear area.
Seals need compression, not just softness
A seal works because the elastomer is compressed against a mating surface. The compression is a percentage of the free section, and the right value depends on the material, the gland and the pressure. Too little leaks; too much causes set, cracking or a joint that will not close.
The gland has to contain the material. A seal with somewhere to flow will flow there instead of pressing on the mating face. The groove should accept the compressed section and leave the required squeeze. Radius the gland corners, because a sharp corner is a leak path.
Chemical compatibility is part of the seal design. A material that swells in the fluid changes its compression and may jam the joint; one that shrinks loses contact. Check the fluid, temperature and duration, not just the chart at room temperature.
Flash is a seal problem as well as a cosmetic one. A flash line across a sealing face is a leak path. The tool needs a clean shut-off and enough pack pressure to fill the seal without overpacking.
Strain relief is a stiffness transition
A cable fails where it bends most sharply. A strain relief should move that point away from the connector and spread the bend over a longer length. A short, stiff sleeve creates a new hard edge at its end, and the cable bends there instead.
A good strain relief tapers. It is stiff at the connector and gradually more flexible towards the cable. The taper can come from a change in section, a change in material hardness or a series of ribs that act as flexible hinges. The transition length is usually several cable diameters, and the exact length depends on the cable stiffness and the bend radius the cable can tolerate.
Support the cable without clamping it hard. A tight clamp concentrates stress and can crush the insulation. A moulded-in relief with a slight interference is better than a rigid collar, and it should allow movement without transferring it to the termination.
Two-shot or insert overmould
In a two-shot process, the substrate is moulded in one cavity and the tool then rotates or transfers to a second cavity where the overmould is injected. The substrate is still in the tool, so the bond is formed while the surface is fresh and the part is located precisely. Two-shot gives the best bond and the shortest cycle, but the tool is more complex and the volume has to justify it.
In insert overmoulding, the substrate is moulded separately, then loaded into a second tool. The tool is simpler and more flexible for low volume, but the substrate has to be handled, cleaned and located. The bond depends on the surface and the overmould temperature, and the cycle includes loading time.
Both routes need a dry, clean substrate. Dust, release agent or handling oil on the bond surface is a common cause of delamination. The overmould material also needs drying: processed wet, it can splay, form weak weld lines or fail at the interface.
| Failure | Likely cause | Check before tooling |
|---|---|---|
| Delamination | Weak chemical bond or no mechanical interlock | Material pair, surface prep and undercuts |
| Flash at the seal | Poor shut-off or too much pack pressure | Gland geometry and tool shut-off |
| Leak | Insufficient compression or a damaged sealing face | Squeeze, groove depth and surface finish |
| Cable pull-out | Short transition or a hard clamp | Relief length, taper and cable support |
| Grip wear | Overmould too thin at the contact point | Local thickness and hardness |
Failure modes overlap. A leak can start as a flash line, and a delamination can start at a feathered edge.
What to validate
For a grip, test abrasion, adhesion and resistance to cleaning agents, with a peel test at the edge to check the bond. For a seal, test leak tightness at the specified pressure and temperature, and check compression set. For a strain relief, bend the cable through the specified cycle and inspect the conductor.
Condition the samples before testing. A seal that has absorbed fluid or a grip that has been cleaned may behave differently from a new part. If the product is used outdoors, add UV exposure and a temperature cycle. The test plan should state the condition, the acceptance limit and the sample size.
A section through the bond line is a useful supplement to the mechanical tests. It shows voids, contamination and the actual interlock. It is also a good way to check that the overmould has filled the undercut rather than bridging over it.
What to put in the RFQ
Specify the substrate material and grade, the overmould material and hardness, the bond method, the grip or seal function, the cable type and the service environment. Add the adhesion, leak and bend requirements with the test method. If the overmould is decorative, state the colour, texture and gloss.
Send the assembly model, not just the overmoulded part. The cable, the mating housing and the user's hand all affect the geometry. A grip that looks right on the part can be uncomfortable in the hand, and a strain relief can still leave a sharp bend at the cable exit.
Ask for a DFM review of the bond line, the seal gland, the overmould edges and the tool shut-offs. Those are the features that decide whether the process works, and they are much cheaper to change before the tool is cut.
Frequently asked questions
What is the difference between two-shot moulding and insert overmoulding?
Two-shot moulding forms the substrate and the overmould in one tool with a rotation or transfer, so the bond is made while the substrate is still located in the tool. Insert overmoulding moulds the substrate first and loads it into a second tool. Two-shot gives a better bond and a shorter cycle but needs a more complex tool.
How do I make a TPE bond to a PP housing?
Use an overmould grade formulated for PP, or add a mechanical interlock such as grooves, holes or undercuts. The bond depends on the specific grade and the process, not just on the polymer pair. Test peel strength at the edge rather than assuming adhesion.
How much compression does an overmoulded seal need?
It depends on the elastomer, the gland and the pressure. The gland must contain the material and leave a controlled squeeze, with radiused corners and a clean sealing face. Confirm the value with a leak test at the service temperature and pressure.
How long should a strain relief be?
Long enough to spread the bend over several cable diameters, with a taper from stiff at the connector to flexible at the cable. A short, uniform sleeve creates a new hard edge. Test the assembly through the specified bend cycle rather than relying on a length rule alone.
Sources and further reading
- ISO 20457:2018, Plastics moulded parts, tolerances and acceptance conditions
- ISO 37, Rubber, vulcanised or thermoplastic, determination of tensile stress strain properties
- Qumek overmoulding DFM reviews and assembly validation records
Qumek Engineering Desk
Engineering team
The engineering team behind Qumek drawings, DFM notes, tolerances and inspection standards. Every guide is reviewed against the drawings and processes used on live programmes.
Review an overmoulded grip or seal
Send the substrate, the overmould material and the assembly model. The engineering desk will check the bond line, the gland and the strain relief before tooling.
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