Front page / What is the difference between overmoulding and insert moulding?
What is the difference between overmoulding and insert moulding?
Overmoulding moulds one plastic over another, most often a soft elastomer (TPE) over a rigid plastic part to make a grip or seal. Insert moulding places a pre-made piece, usually a metal threaded insert, bushing or pin, into the mould and moulds plastic around it. Both join two materials in one part without glue or assembly, but they need the materials and the design to be chosen for it.
In both cases the first piece is called the substrate. For overmoulding, the substrate is itself a moulded part, made in a separate mould or in the first stage of a two-shot machine. For insert moulding, Protolabs describes the substrate as a preformed part, often metal, loaded into the mould before the shot.
How they compare
| Overmoulding | Insert moulding | |
|---|---|---|
| Substrate | A moulded plastic part | A pre-made part, usually metal |
| Common uses | Soft-touch grips, seals, buttons, colour accents | Threaded inserts, bushings, pins, electrical contacts |
| Main risk | Poor adhesion between the two plastics | Insert movement, poor retention or flash around the insert |
| Tooling | Two moulds, or one two-shot mould | One mould, with locations to hold the insert |
Bonding: chemistry and mechanics
Protolabs notes that a chemical bond between overmoulded materials is possible but depends on compatibility, and strongly recommends adding a mechanical lock, such as an undercut, if the bond is critical. Plastics Technology's guide to hard-soft overmoulding shows how sensitive the chemistry is: with one first-generation TPE, adhesion dropped sharply on impact-modified, heat-stabilised nylon 6, while a newer adhesion grade of the same TPE family performed much better. Even the carrier resin in a colour concentrate can change adhesion.
Published design figures for soft-touch overmoulds
- A uniform TPE layer of about 0.04 in (1 mm) is described as desirable for good adhesion.
- Below about 0.060 in (1.5 mm), the soft feel can be lost because the hard substrate shows through.
- Avoid feathering the TPE to a thin edge; use a sharp shut-off step so the edge does not curl or fray.
- Vent depths of about 0.0005 to 0.001 in are typical for TPE overmoulding, because trapped air weakens the bond at the end of flow.
- Draft of 0.5 to 1 degree per side usually helps ejection, but stickier materials such as TPU can need 5 to 6 degrees.
- Moisture-sensitive TPEs should be dried; the guide suggests no more than 0.05 percent moisture at moulding.
- In one example, adhesion of a 65 Shore A TPE on polycarbonate improved from 370 °F to 400 °F melt temperature, but fell again at 430 °F, so hotter is not always better.
These figures come from the Plastics Technology article and describe general TPE practice; each TPE supplier publishes its own bonding table and processing guide.
Insert moulding points
- Use standard moulding inserts from recognised suppliers; Protolabs lists the brands it accepts and stocks.
- Threaded inserts and bushings are the most common reason for insert moulding, adding durable threads or wear surfaces to plastic parts that are assembled repeatedly.
- Protolabs keeps its normal substrate tolerance of about ±0.003 in plus resin tolerance for thermoplastic overmoulds, but widens it to 0.025 in/in when the overmould is liquid silicone rubber.
- Inserts loaded by hand add cycle time, which matters more at higher volumes.
How to choose
- If you need a metal feature such as a strong thread, consider insert moulding, or a post-moulded heat-set insert at low volume.
- If you need grip, sealing or a second colour, consider overmoulding, and choose the TPE grade for your specific substrate.
- Design a mechanical lock wherever the bond carries a load.
- Ask for adhesion testing on first samples, such as a peel test, and agree the pass level in advance.
The figures here are published general guidance. The TPE or insert supplier's data and the toolmaker's review take priority for a real part.
Questions buyers ask
Is two-shot moulding the same as overmoulding?
Two-shot moulding is one way to overmould: a single machine with two injection units moulds the substrate and the overmould in sequence. It suits high volumes; lower volumes often use two separate moulds instead.
Will any TPE stick to any plastic?
No. TPE grades are formulated for particular substrates, and the Plastics Technology data show that additives in the substrate can change adhesion a lot. Always check the TPE supplier's compatibility table for your exact substrate grade.
Can I overmould silicone?
Liquid silicone rubber can be overmoulded onto suitable substrates, and Protolabs offers it, with wider tolerances than thermoplastic overmoulds.
Is insert moulding cheaper than pressing inserts in later?
It depends on volume. Moulding the insert in avoids a secondary operation, but loading inserts adds cycle time. Compare both routes at your quantity.
Sources
- Protolabs, Overmolding and insert molding design guidelines
- Plastics Technology, How to optimize adhesion in hard-soft overmolding