A client arrives with a tool handle that slips. Another with a connector that vibrates and breaks. A third with an electronics housing that is not watertight. In all three cases, the technical answer is often the same: plastic overmoulding. It is a two-material injection process that solves problems that secondary assembly cannot durably resolve. Here is how it works, which applications it fits, and how to get a fast feasibility assessment.
What is plastic overmoulding?
Overmoulding consists of injecting a second plastic material (or elastomer) onto an already-moulded part, called the insert. The two materials bond mechanically or chemically depending on the combination chosen. The result is a single-body part with different properties in different zones: a rigid core for structural strength, and a soft zone for grip, sealing or vibration damping.
Overmoulding eliminates secondary assembly steps. No bonding, no crimping, no mechanical fastening. The part exits the second mould as a single functional unit. For the affected production runs, this time and step saving is direct and measurable.
Overmoulding versus 2K injection: what is the difference?
Two-shot (2K) injection injects both materials in the same machine cycle, using a rotary mould or a turning platen. Overmoulding uses two separate moulds and two separate cycles: the insert is moulded first in mould one, transferred to mould two, and the second material is injected.
2K injection is faster in high-volume production, but the mould is more complex and tooling investment is significantly higher. For small and medium runs, overmoulding is often more relevant: two simpler moulds, better flexibility to evolve each material's design independently.
Common materials for overmoulding
For the rigid body (insert): PP, ABS, PA6, PA66, PC, POM depending on the mechanical and thermal requirements. The choice depends on structural constraints, required chemical resistance and any regulatory requirements (food contact, biocompatibility).
For the soft overmould: TPE (thermoplastic elastomer) for standard grip, TPU (thermoplastic polyurethane) for high abrasion resistance, SEBS for high-flexibility applications, LSR silicone for sealing or biocompatibility requirements. Chemical compatibility between the two materials is verified before any mould design. Some combinations do not bond without mechanical anchoring (undercuts, through-holes, locking grooves).
A poor material choice is a frequent cause of delamination after a few thousand cycles. Compatibility analysis is always carried out before project validation.
Sector applications of overmoulding
Automotive: door and control handles with soft grip on PA or PC body, integrated seals on connectors, two-material pedal covers, cable protectors with elastomer sheathing.
Medical: surgical instrument handles with soft anti-slip grip zone, device covers with integrated seal, single-body parts replacing multi-component assemblies. In medical, materials must comply with biocompatibility standards (ISO 10993): grade selection is critical.
Electronics: watertight housings with integrated TPE or silicone seal, shells with overmoulded metal insert (antenna, shielding), keys and buttons with soft tactile surface.
Tools and equipment: hand tool handles (rigid PA or PP body, TPE grip), sports equipment handles, impact guards on industrial equipment.
Cosmetics and packaging: two-material closures with rigid base and soft cap, bottles with grip, pumps with soft pressure surface. So Fresh, one of our clients, uses overmoulding for premium packaging parts in their high-end cosmetic range.
Mould design for overmoulding: key considerations
Manufacturing an overmoulding mould is more complex than a standard mould. You need to plan the precise positioning of the insert in the second mould (placement tolerances drive interface quality), the contact zones between the two materials (fusion or mechanical anchoring interface), and the parting lines without flash on the overmoulding side.
Our moulds are manufactured in P20 or H13 steel depending on required hardness and target cycle life. A standard mould runs between 300,000 and 500,000 cycles. On request, heat treatment can extend life beyond one million cycles for long runs.
Before any mould manufacture, we carry out a DFM analysis (Design for Manufacturing) of the client's 3D file: insufficient draft angles, bonding zones between the two materials, parting lines and gate positioning. This step prevents costly modifications during manufacture.
Metal insert overmoulding: the most technical variant
Overmoulding is not limited to plastic-on-plastic. It is also possible to overmould onto a metal insert (steel, aluminium, brass) to create a hybrid metal-plastic part. Common applications: screw or threaded insert embedded in a plastic part, electrical connector with overmoulded seal, structural part with metal reinforcement and plastic cladding.
The bond between metal and plastic is purely mechanical (the plastic conforms to the insert geometry, often knurled or grooved). Anchoring quality depends directly on the contact zone geometry and injection pressure. A poorly positioned insert in the mould or insufficient anchoring geometry produces an overmould that delaminates in service.
Our offer: DFM analysis, tooling and production in Belgium
We handle the project end to end: DFM analysis of the 3D file, mould design and manufacture, machine setting and trials, small-to-medium-run production, quality control and delivery. Engineering is managed in partnership with LGR Design Studio in Wavre, production runs in Ath, Belgium.
We work in small and medium runs. The conversation is honest about economic feasibility based on quantities. For large runs, the model can adapt depending on the project.
9. Overmoulding versus bonding: why overmoulding wins long-term
Secondary bonding or ultrasonic welding are alternatives to overmoulding. They add production steps, require fixture tooling and can fail in service as adhesive ages or bond fatigue accumulates. Overmoulding creates a single-body part where the two materials are permanently bonded at the molecular or mechanical level from the first cycle.
For parts subject to repeated stress (grip zones, seals in pressure cycles, connectors in vibrating assemblies), overmoulding consistently outperforms secondary bonding in lifetime testing. The added complexity at tooling stage pays back in reduced warranty costs and field failures.
10. Get your feasibility analysis
Send your 3D file, specify the two materials you have in mind (or describe the desired function), and indicate the estimated annual volume. We assess feasibility at no charge, identify DFM risks and come back with a technical proposal. Response within 5 working days, no commitment required.
FAQ
FAQ
What is the difference between overmoulding and two-shot (2K) injection?
Overmoulding uses two separate moulds and two separate cycles: the insert is moulded first, then transferred to a second mould for the second material to be injected. 2K injection injects both materials in the same cycle in a rotary mould. 2K is faster in high-volume production but requires a more complex mould and higher tooling investment. Overmoulding is often more suitable for small and medium runs.
Which materials can be overmoulded onto an ABS or PA body?
Onto ABS: TPE (typically Shore A 40 to 80) bonds well through chemical adhesion. Onto PA: TPE-PA (specially graded for polyamide adhesion) or TPU. Onto PP: chemical adhesion is difficult without mechanical anchoring - prefer a locking geometry (through-holes, grooves). In all cases, compatibility is confirmed by testing before project launch.
Can you overmould onto a metal insert?
Yes. Overmoulding onto metal inserts (steel, aluminium, brass) is common for creating hybrid metal-plastic parts. The bond is mechanical: the plastic conforms to the insert geometry, typically knurled or grooved to create the anchor. Anchoring quality depends on the contact zone geometry and injection pressure.
Is overmoulding feasible for small production runs?
Yes. Overmoulding is often better suited to small and medium runs than 2K injection, because two simpler moulds are less costly than a complex 2K mould. Economic feasibility depends on volumes: we discuss this honestly during the feasibility analysis.
How do I get a quote for an overmoulding project?
Send your 3D file (STEP or IGES), the two materials you have in mind or a description of the desired function (grip, sealing, vibration damping), and the estimated annual volume. We assess feasibility at no charge and come back with a DFM analysis and a technical basis for costing. Response within 5 working days.
