Overmoulding and Insert Moulding: A Practical Guide
Overmoulding bonds a second polymer shot onto a previously moulded substrate. Insert moulding encapsulates a pre-placed component — almost always metal — inside a single plastic shot. Both processes eliminate downstream assembly steps, improve product performance, and are available from Nordmould as part of its secondary-capabilities offering.
What is overmoulding and when should you use it?
Overmoulding injects a second material over a rigid substrate to create a multi-material part in a single finished component. The classic example is a tool handle: a rigid ABS or PP core provides structural stiffness, while a soft TPE layer is overmoulded onto grip zones to provide tactile feel, vibration damping, and colour contrast.
Use overmoulding when a part must combine structural rigidity with soft touch, sealing, grip, or acoustic damping — and when the two performance requirements cannot be met by a single material.
Overmoulding is also appropriate when two colours must be bonded rather than assembled, or when a gasket must be integrated into a housing rather than inserted as a separate component.
What is insert moulding and when should you use it?
Insert moulding places a pre-formed component — typically a threaded brass insert, a metal pin, a terminal, or a spring — into the open mould cavity. Plastic is then injected around the insert, encapsulating it permanently.
The result is a metal-in-plastic assembly with better thread strength, pull-out resistance, and dimensional stability than either a moulded-in plastic thread or an ultrasonically pressed heat insert fitted downstream.
Use insert moulding when a part needs durable metal threads, electrical contacts embedded flush, or structural reinforcement that plastic alone cannot provide. It is the standard approach for plastic housings that are opened and closed repeatedly in service.
How do overmoulding and insert moulding compare?
| Attribute | Overmoulding | Insert moulding |
|---|---|---|
| Primary purpose | Multi-material surfaces, grip, sealing, aesthetics | Embed metal or pre-formed components |
| Second material | Thermoplastic elastomer, second rigid polymer | Metal insert, electrical terminal, printed circuit |
| Tooling needed | Substrate tool + overmould tool (or 2-shot tool) | Single mould + insert-loading fixture |
| Assembly eliminated | Bonding, gasket fitting, grip attachment | Post-mould heat insertion, press fitting, threading |
| Adhesion mechanism | Chemical bond + mechanical interlock | Mechanical encapsulation |
| Typical volume | Low–production (same range as standard moulding) | Low–production (same range as standard moulding) |
| Tooling cost premium | +80–150% over single-material part | +20–40% over standard tool |
Which materials are compatible for overmoulding?
Chemical adhesion between the substrate and overmould depends on polarity and surface energy. When chemical adhesion is weak, mechanical interlocking through holes, slots, and undercuts in the substrate provides a reliable supplement.
| Substrate material | Compatible overmould materials | Notes |
|---|---|---|
| PP (polypropylene) | TPE (SEBS-based), TPU | PP is low-polarity; mechanical interlock strongly recommended |
| ABS | TPE, TPU, soft PVC | Good chemical adhesion to SEBS-TPE and TPU |
| PA (nylon, PA6/PA66) | TPE, TPU | PA absorbs moisture; pre-dry substrate before overmoulding |
| PC (polycarbonate) | TPU, some TPE grades | High heat resistance; check processing temperature compatibility |
| PC/ABS blend | TPU, TPE | Good all-round compatibility |
| POM (acetal) | Very limited — low adhesion to most elastomers | Mechanical interlock essential; consult Nordmould before specifying |
| PMMA (acrylic) | Limited; optical-grade applications require specialist grades | Surface preparation critical |
Soft-to-soft overmoulding (e.g., TPE over TPU) is generally not recommended unless the two materials are chemically matched by the material manufacturer. Nordmould reviews material pairings at the DFM stage and will flag incompatible combinations before tooling is cut.
What design rules apply to overmoulded parts?
Good overmould geometry ensures reliable adhesion, complete fill, and dimensional accuracy in the finished part.
Substrate wall thickness. The substrate must be rigid enough not to flex under injection pressure of the overmould shot. A minimum substrate wall of 1.5 mm is typical; consult your material data sheet for the specific polymer.
Overmould layer thickness. Recommended overmould thickness is 1.5–4 mm. Thin layers risk short shots and adhesion failures; thick layers cause sink marks in the substrate and extend cooling time.
Mechanical interlock features. For low-adhesion pairings (particularly PP substrates), design the substrate with through-holes, grooves, or undercuts in the bond zone. These give the overmould mechanical purchase independent of chemical adhesion.
Gate placement. The overmould gate must be positioned to fill the bond area uniformly and avoid weld lines in visible or functional zones. Nordmould's DFM review includes gate placement for both shots.
Parting lines. The overmould parting line should not fall on a sealing or grip surface. Plan parting line location at the design stage, not after tooling is ordered.
What design rules apply to insert moulding?
Inserts must be positioned, retained, and encapsulated reliably across every production cycle.
Insert location. Inserts are placed by hand or by robot into the open mould. They must self-locate positively — a counterbore, pin, or ledge prevents the insert from tilting or floating under injection pressure.
Plastic wall around the insert. The plastic wall surrounding the insert must be thick enough to resist splitting under torque. For brass threaded inserts, a minimum wall thickness of 1.5–2× the insert outer diameter is typical.
Insert geometry. Knurled or hex-profile inserts provide superior pull-out and torque resistance compared to smooth-barrel inserts. Standard DIN or PEM-style brass inserts are the most commonly used; Nordmould can accept customer-supplied inserts or source standard types.
Material compatibility. The plastic must be compatible with insert temperature during moulding. High-temperature-resistant materials such as PA, POM, and PC are generally well-suited; high-shrinkage materials may cause residual stress around the insert and should be reviewed case by case.
What are typical applications for each process?
Overmoulding applications:
- Tool and instrument handles requiring soft-touch grip
- Wearable device housings with integrated seals or skin-contact surfaces
- Consumer electronics covers with two-colour or soft-feel exterior
- Automotive interior trim with integrated tactile zones
- Medical device grips requiring sterilisation-compatible soft surfaces
Insert moulding applications:
- Plastic enclosures with brass threaded inserts for repeated assembly
- PCB connector housings with integrated metal terminals
- Cable assemblies with moulded strain-relief bodies
- Mechanical linkages with moulded-in pivot pins or bushings
- Sensor housings with encapsulated electrical contacts
When should you choose overmoulding versus insert moulding?
Choose overmoulding when the goal is a multi-material surface — grip, sealing, aesthetics, or damping. The second material is a polymer, and the bond zone is a surface, not a single embedded component.
Choose insert moulding when a non-plastic component must be permanently and precisely embedded inside the part. The insert provides a function — thread, conductivity, stiffness, or retention — that the surrounding plastic cannot replicate.
Both processes are available through Nordmould and can be combined: a part can contain moulded-in metal inserts and also receive a TPE overmould in the same production programme.
Frequently asked questions
What is the difference between overmoulding and insert moulding? Overmoulding bonds a second plastic layer onto a moulded substrate — typically a soft TPE grip over a rigid PP or ABS body. Insert moulding encapsulates a pre-formed component, most often a metal insert, inside a plastic part during the moulding cycle. Nordmould supports both processes.
What materials can be overmoulded onto each other? TPE overmoulds well onto PP, ABS, and PA. TPU bonds to ABS and PC. Soft-to-soft overmoulding is rarely successful. Chemical adhesion depends on polarity and surface energy; mechanical interlocking (through-holes, undercuts) provides a reliable backup. Nordmould advises on compatibility at the DFM stage.
Do I need two separate moulds for overmoulding? Yes, typically. The substrate is moulded in a first tool, then transferred to a second tool where the overmould shot is injected. A 2-shot (two-component) mould can combine both steps in one machine cycle, but requires a more complex and expensive rotating tool. Nordmould can quote both approaches.
What is a common application for insert moulding? Threaded brass inserts in plastic enclosures are the most common use case — producing a durable, re-usable threaded hole in an otherwise plastic housing without a heat-insertion step downstream. Metal-to-plastic bushings, electrical contacts, and stiffening ribs are also typical.
How does overmoulding affect tooling cost? Overmoulding requires two separate tools: one for the substrate and one for the overmould. Total tooling investment is typically 1.8–2.5× a single-material part. At Nordmould, overmould tooling starts from €3,000 per tool, and the DFM review covers both tools together.
What wall thickness is recommended for overmoulded parts? The overmould layer is typically 1.5–4 mm thick. Too thin risks incomplete fill and adhesion failures; too thick causes sink marks on the substrate and excessive cooling time. Nordmould checks wall stack-up during the free DFM review.
Can overmoulding replace adhesive bonding or press-fitting? In many cases, yes. Overmoulding creates a permanent, precise bond with no adhesive migration, no cure time, and no assembly step. It is particularly effective where grip texture, sealing, vibration damping, or aesthetic soft-touch surfaces are required.
Send your STEP file or drawing to Nordmould for a free DFM review covering substrate geometry, overmould compatibility, and insert placement.
Last reviewed: 2026-05