Overmoulding a TPE onto a rigid substrate removes an assembly step and eliminates the water-ingress path a mechanically fitted grip leaves behind. The selection problem is a four-way constraint: hardness for feel, melt flow for the tool, base polymer for service life, and substrate compatibility for the bond.
Key takeaways
- SEBS-based compounds bond well to polyolefin substrates such as polypropylene without adhesive or primer.
- Bonding to polar substrates such as ABS or polycarbonate is compound-specific and must be trialled.
- Soft-touch overmoulds typically land between 40 and 70 Shore A.
- A thin overmould section needs high melt flow; the injection grades here reach 367 g/10 min at 230C.
- Bond strength depends on substrate grade, tool temperature and interface geometry, so it has to be verified on the real part.
Why overmould at all
A two-shot or insert-moulded TPE layer is formed directly against the rigid part. Compared with a separately moulded grip that is assembled afterwards, that removes an assembly operation, removes the tolerance stack between two parts, and removes the seam where water and dirt would otherwise enter.
It also lets you put softness exactly where the hand contacts the product and stiffness everywhere else, which is difficult to achieve any other way.
The bond
SEBS-based thermoplastic elastomers bond to polyolefin substrates - polypropylene above all - through interdiffusion at the interface while the TPE is still molten. The compound and the substrate are chemically compatible, so no adhesive or primer is needed.
Polar substrates behave differently. ABS, polycarbonate and polyamide do not share that chemical compatibility, and bonding to them depends on the specific compound formulation. Some TPE grades are formulated with compatibilisers for polar substrates; a general-purpose SEBS grade may not adhere adequately.
Three process variables move bond strength substantially: substrate temperature at the moment of overmoulding, melt temperature of the TPE, and interface geometry. A mechanical interlock - an undercut, a through-hole, a textured interface - provides a safety margin that does not depend on chemistry at all, and is worth designing in on any load-bearing grip.
The bond must be verified on your actual part. Datasheet values cannot predict it because it depends on your substrate grade, your tool and your geometry.
Hardness for the overmould
| Application | Typical hardness | Candidate grades |
|---|---|---|
| Soft cushioned pads and bumpers | 15 - 30 Shore A | FARRPRENE IEU150A, FARRPRENE IU204A, FARRPRENE IU250A |
| Soft-touch surfaces on housings | 40 - 55 Shore A | FARRPRENE IF402A, FARRPRENE IU450A, FARRPRENE IF500A, FARRPRENE IU550A |
| Ergonomic handle and tool grips | 55 - 70 Shore A | FARRPRENE IU601A, FARRPRENE IF602AU, FARRPRENE IF658AS, FARRPRENE IF700A |
| Hard-wearing working grips | 70 - 85 Shore A | FARRPRENE IF753A, FARRPRENE IF801A, FARRPRENE IF850A |
Melt flow for the overmould section
Overmould layers are usually thin, and often have to travel a long way around a handle from a single gate. That combination demands flow. The highest-flow injection grades in this catalogue are FARRPRENE IF900A at 367 g/10 min, FARRPRENE IF700A at 347 and FARRPRENE IF751A at 256, all measured at 230°C/2.16 kg.
Where the overmould section is thicker, a moderate-flow grade such as FARRPRENE IF753A at 61 g/10 min often gives better dimensional consistency and less flash around the interface.
A worked selection
A power-tool handle grip: polypropylene substrate, 60 Shore A target, 2 mm overmould wall, indoor and jobsite use, black.
- Substrate is polypropylene, so a SEBS grade bonds without primer.
- 60 Shore A narrows to FARRPRENE IU601A (unfilled) or FARRPRENE IF602AU (filled).
- 2 mm wall over a long flow path favours the higher flow: FARRPRENE IF602AU at 193 g/10 min against FARRPRENE IU601A at 56.
- Black pigment means translucency is irrelevant, so the filled grade's dimensional stability is the better trade.
That points to FARRPRENE IF602AU, with FARRPRENE IU601A as the alternative if colour fidelity later becomes a requirement. Trial the bond on the real substrate before committing.
All values quoted are typical average values from a limited sample set, not a formal specification. Request the signed datasheet for current values.
Scope note. The guidance on this page is based on published mechanical properties - hardness, tensile strength, elongation, melt flow, specific gravity, base polymer and stabilisation. Food contact, skin contact, biocompatibility, flame retardancy, halogen-free formulation and electrical performance are not characterised in the public catalogue and require a separately qualified compound and testing. Contact us to qualify a grade against any of those.
Frequently Asked Questions
Does TPE bond to polypropylene without primer?
SEBS-based compounds bond to polyolefin substrates such as polypropylene through interdiffusion at the interface, so no adhesive or primer is normally required. Actual bond strength still depends on the substrate grade, tool temperature and interface geometry, so it should be verified on your own part before production.
Can TPE be overmoulded onto ABS or polycarbonate?
It depends on the compound. ABS and polycarbonate are polar substrates and do not share the chemical compatibility that makes polyolefin bonding straightforward. Some TPE grades are formulated with compatibilisers for polar substrates; a general-purpose SEBS grade may not adhere adequately. Trial it, and design in a mechanical interlock as a safety margin.
What hardness should a soft-touch overmould be?
Between 40 and 70 Shore A for most products. Below about 40 Shore A the surface marks and picks up dirt too easily for a visible grip; above about 80 Shore A users stop perceiving it as soft-touch. Ergonomic handle grips most often land between 55 and 70 Shore A.
What melt flow do I need for a thin overmould?
High. Thin walls combined with the long flow paths typical of a wrap-around grip need a compound that reaches the far end before freezing off. The highest-flow injection grades in this range run 256 to 367 g/10 min at 230C. Thicker overmould sections do better on a moderate-flow grade, which reduces flash at the interface.
How can I improve the bond strength of an overmoulded grip?
Raise the substrate temperature at the moment of overmoulding, keep the TPE melt temperature within its recommended window, and design a mechanical interlock into the interface - an undercut, a through-hole or a textured surface. The interlock gives a margin that does not depend on chemical adhesion at all.
Related guides
- TPE Melt Flow Index: Why Injection and Extrusion Grades Are Not Interchangeable
- How to Choose a TPE Grade: A Specifier's Guide
- TPE Shore Hardness: A Selection Guide From 15 Shore A to 52 Shore D
- Filled vs Unfilled TPE: What Specific Gravity Tells You
About the FARRPRENE range
FARRPRENE is the thermoplastic elastomer (TPE) compound range of Farr Polychem FZCO, a polymer compounder based in Jebel Ali Free Zone, Dubai, United Arab Emirates, supplying the UAE, the wider MENA region and Europe. The range covers injection moulding and extrusion compounds from soft gel-like Shore A grades through rigid Shore D grades, in unfilled translucent and filled natural variants, colour-matchable to a customer reference, with free samples and technical datasheets available on request. Browse the full grade catalogue, see every grade side by side on the comparison table, or request a sample.
FARRPRENE GRADES
Grades commonly specified for industrial applications
| GRADE | HARDNESS | TENSILE (MPa) | TYPICAL USE |
|---|---|---|---|
| IEU150AInjection / Extrusion / Unfilled | 15 Shore A | 3 | Vibration-damping pads and soft mounts |
| IF752AInjection / Filled | 75 Shore A | 5.5 | Bushings, spacers and wear components |
| IF900AInjection / Filled | 90 Shore A | 5.5 | Hard bushings and near-rigid flexible parts |
Typical average values from a limited sample set; not a formal specification. Request the signed datasheet for current values.
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