Overmolding
Overmolding molds a second material, usually a soft elastomer, directly onto an already-molded rigid substrate so the two bond permanently.
- Part
- Forming
- Revised
- 2026-08-11
At a glance
- Family
- Plastics and Rubber
- Typical tolerances
- Substrate features hold normal injection molding tolerances, about ±0.005 in (±0.127 mm). Dimensions taken over the elastomer are looser and compressible; in insert-transfer overmolding, add the substrate loading clearance to the position tolerance of the second shot.
- Surface finish
- Substrate takes any SPI finish; the elastomer is normally textured or matte, since a gloss soft-touch surface shows fingerprints and wear quickly
- Typical volumes
- 5,000–1,000,000+ parts; two-shot tooling pays back above roughly 100,000
- Lead time
- 8–14 weeks for a two-shot rotating tool; 6–10 weeks for a pair of insert-transfer tools; cycles are one conventional molding cycle plus the elastomer shot
- Materials
- Plastic, Metal
What it is
Overmolding injects a second material onto an already-molded rigid substrate so the two become one part — almost always a soft thermoplastic elastomer over a rigid engineering plastic. Power tool grips, toothbrush handles, sealing gaskets molded onto housings, cable strain reliefs, and soft-touch instrument bezels are all overmolded.
It runs two ways. In two-shot (multishot) molding, one machine with two injection units and a rotating or indexing tool molds the substrate and the overmold back to back without the part ever leaving the mold. In insert-transfer overmolding, the substrate is molded separately, then reloaded into a second tool — slower and more labor-intensive, but with far cheaper tooling and no requirement for a two-shot press.
The design question that decides everything is material compatibility: the elastomer must bond chemically to that specific substrate, or the joint has to be mechanical.
How it works
- Mold the substrate. The rigid first shot is molded conventionally, in ABS, PC, PC/ABS, PP, nylon, or PBT. Its surface must be clean and dry — mold release, fingerprints, and absorbed moisture all degrade the bond.
- Position for the second shot. In two-shot molding the tool rotates the core carrying the substrate into a second cavity, so position is fixed by the tool and repeatable to molding tolerances. In insert-transfer the substrate is reloaded by hand or robot into a separate tool, and the joint now carries the substrate's tolerance plus the locating clearance.
- Inject the elastomer. TPE melt temperatures typically run 355–445 °F (180–230 °C). The hot second shot momentarily softens the substrate skin, and the two polymer chains interdiffuse at the interface. That interdiffusion is the bond — it is not adhesion in the glue sense, and it only happens between compatible chemistries.
- Cool and eject. The elastomer shrinks more than the substrate, which is why a thin or asymmetric substrate can bow after overmolding. A substrate stiff enough to resist that differential is a design requirement, not a nicety.
Which elastomers bond to which substrates?
The compatibility rule follows chemical family. SEBS-based TPEs bond to polypropylene, polyethylene, and polystyrene. TPU bonds to ABS, PC, PC/ABS, nylon, and PBT. TPV bonds to polypropylene. Standard silicone bonds to essentially nothing without a self-bonding grade or a primer. Bond quality is verified with a 90° peel test, and a good result fails cohesively — tearing within the elastomer — rather than adhesively, cleanly separating at the interface.
What if the materials do not bond?
Design a mechanical interlock: through-holes the elastomer flows into and rivets over, undercut grooves, dovetails, or a wrap-around edge. Any structural or sealing overmold should carry mechanical retention regardless, since chemical bonds degrade with heat aging, UV, and chemical exposure.
Design guidelines
Overmold wall thickness
0.020–0.080 in (0.5–2.0 mm) covers most soft-touch and grip layers, with 0.030–0.060 in (0.75–1.5 mm) the common target. Below about 0.020 in the elastomer will not fill reliably at typical grip lengths. Above about 0.120 in (3 mm) the soft layer feels spongy and its own shrinkage starts pulling on the substrate.
Keep the elastomer layer uniform
The same rule as any molded part, but it matters more here because thick regions shrink more and drag the substrate. Where the grip must vary in thickness, put the variation in the substrate and keep the overmold constant.
Substrate design
Design the substrate as a proper molded part first — the wall, rib, boss, corner, and draft rules on /charts/injection-molding-design-guidelines all apply to it unchanged. Then add stiffness where the overmold will pull: a thin, flat substrate with a heavy elastomer on one face will bow.
Terminate the edge properly
Never end an overmold on a flat, exposed edge — it becomes a lifting point that a fingernail or a cleaning cloth will start peeling. Terminate into a step, a groove, or a shallow recess in the substrate so the elastomer edge is captured and sits flush.
Gate the second shot carefully
The elastomer is being injected against a plastic substrate rather than steel. Gate so the flow front does not impinge directly on a thin substrate wall, or it will deflect or wash. Gate location also determines where the elastomer knit lines land, and knit lines in a soft grip are visible as well as weak.
Bond area and mechanical backup
Give the bond meaningful surface area — a narrow strip of TPE on a small footprint will peel regardless of chemistry. For any overmold that seals, carries load, or is exposed to heat, solvents, or UV, add through-holes or undercut grooves as mechanical retention rather than relying on interdiffusion that degrades over the product's life.
| Feature | Recommended | Limit | Why |
|---|---|---|---|
| Overmold wall | 0.030–0.060 in (0.75–1.5 mm) | 0.020–0.120 in (0.5–3 mm) | Thin will not fill; thick feels spongy and pulls the substrate |
| Overmold uniformity | Constant thickness | — | Thick regions shrink more and bow the substrate |
| Edge termination | Into a step or groove | Never a flat exposed edge | An exposed edge is a peel initiation point |
| Substrate stiffness | Ribbed or thick enough to resist bow | — | Elastomer shrinkage loads the substrate asymmetrically |
| Bond verification | 90° peel, cohesive failure | Adhesive failure | Cohesive failure means the bond exceeds the material |
| Mechanical interlock | Through-holes or undercut grooves | — | Required where chemistry does not bond or will age |
| Substrate condition | Clean, dry, release-free | — | Contamination is the most common bond failure cause |
Cost drivers
The route decides the cost structure. Two-shot molding needs a multi-material press and a rotating or indexing tool, so tooling runs well above a pair of single-shot molds — but the cycle is one operation, position is fixed by the tool, and there is no handling between shots. Insert-transfer needs only two conventional tools and a standard press, but adds a load-and-unload step to every part and stacks the substrate's tolerance onto the joint.
Volume breakpoints:
- Under 5,000 parts: insert-transfer with two simple tools, or a mechanically assembled elastomer part.
- 5,000–100,000: insert-transfer with a robot loader, or a two-shot tool if the program justifies it.
- Over 100,000: two-shot, essentially always. The eliminated handling step dominates.
Cost reduction:
- Pick a compatible material pair first. Choosing a TPE that bonds to the chosen substrate avoids primers, plasma treatment, or a redesign around mechanical interlocks — all of which are more expensive than the material decision would have been.
- Reduce overmold coverage. Soft material is usually the more expensive resin, and a grip that covers only the contact zones costs less than one that wraps the whole housing.
- Design a single, simple parting for the second shot. Complex second-shot shutoffs against a molded substrate are a common source of flash and rework.
- Consider an assembled elastomer part at low volume. A separately molded sleeve or a mechanically retained pad can beat overmolding entirely below a few thousand parts.
- Keep the overmold off tight-tolerance features. The soft layer shrinks more and is compressible, so any dimension taken over it is inherently loose.
Questions
7 questionsWhich TPE bonds to which plastic substrate?
Compatibility follows chemical family. SEBS-based TPEs bond to polypropylene, polyethylene, and polystyrene; TPU bonds to ABS, PC, PC/ABS, nylon, and PBT; TPV bonds to polypropylene. Standard silicone bonds to essentially nothing without a self-bonding grade or a primer, so pick the pair before finalizing either material.
How thick should an overmolded grip be?
0.030–0.060 in (0.75–1.5 mm) is the usual target, within a practical range of 0.020–0.120 in (0.5–3 mm). Below about 0.020 in the elastomer will not fill reliably over a normal grip length; above about 0.120 in it feels spongy and its own shrinkage begins pulling the substrate out of shape.
What is the difference between two-shot molding and insert-transfer overmolding?
Two-shot molds both materials in one machine with a rotating or indexing tool, so the part never leaves the mold and position is fixed by the tooling. Insert-transfer molds the substrate separately and reloads it into a second tool — much cheaper tooling and no multi-material press, but an added handling step and a looser positional tolerance.
How do I test whether an overmold bond is good?
A 90° peel test. A good bond fails cohesively, tearing within the elastomer itself, which means the interface is stronger than the material. Adhesive failure — the elastomer separating cleanly from the substrate — indicates incompatible chemistry, a contaminated substrate, or too low a second-shot melt temperature.
Do I need a mechanical interlock if the materials bond chemically?
For anything structural, sealing, or exposed to heat, solvents, or UV, yes. Chemical bonds degrade with age and environmental exposure, while a through-hole the elastomer rivets over or an undercut groove does not. Treat the interlock as insurance on a joint that has to last the product's life.
Why does an overmolded part bow?
The elastomer shrinks more than the rigid substrate, and if it sits on one face of a thin, flat part that differential pulls the substrate into a curve. Stiffen the substrate with ribs or additional wall, keep the overmold thickness uniform, and where possible balance the soft material across the neutral axis.
Where should an overmold edge terminate?
Into a step, groove, or shallow recess in the substrate, never on a flat exposed edge. An exposed elastomer edge is a peel initiation point that a fingernail, a cleaning cloth, or normal handling will eventually lift, and once peeling starts the bond unzips along the interface.