Overmoulding of Plastic-Rubber and Plastic-Metal Components: When It Pays Off and How It Works

Multi-material components produced in a single production cycle, without downstream bonding, screws or welding. Overmoulding (also known as co-moulding) is the technological answer to an increasingly common industrial requirement: integrating different mechanical, functional and aesthetic properties into a single part, reducing assembly steps and increasing the reliability of the finished product.

This article examines the technical aspects of overmoulding that make the difference between a project that works and one that creates problems: chemical compatibility between materials, mechanical anchoring techniques, the design constraints of multi-material components, and the industrial applications where this technique delivers the greatest value.

Material Combinations: The Heart of the Project

Not all material combinations work. The success of an overmoulded component largely depends on the bond that can be achieved between the substrate and the overmoulded material. This bond can be chemical (adhesion between compatible polymers), mechanical (geometric anchoring through undercuts and dedicated shapes) or, in the best cases, both.

Rigid Plastic + Thermoplastic Elastomer (TPE)

This is the most common combination in industrial practice. A rigid substrate in PP, ABS or PC is overmoulded with a thermoplastic elastomer to achieve soft-touch grips, integrated seals and non-slip finishes. TPE manufacturers formulate grades specifically compatible with each type of substrate: grades are available for PP, PA, PC, SAN, PS, POM, PBT, ASA and ABS, with increasing levels of chemical adhesion. Selecting the correct grade is the first and most important success factor.

Plastic + Metal Insert

Encapsulating metal inserts in plastic is used to produce electrical connectors, integrated threaded bushings, sensors with moulded housings, and components that combine the mechanical strength of metal with the electrical insulation or lightness of plastic. In this case, chemical adhesion is almost absent: anchoring is primarily mechanical, achieved through geometric features (knurling, grooves, holes) on the metal that the plastic wraps around during moulding.

Plastic + Textile Element or Cord

A less well-known but industrially relevant category is the overmoulding of plastic onto ropes, chains, technical fabrics and webbing. The flexible substrate is positioned in the mould and plastic is injected around it, creating terminals, protective sheaths and permanent joints. Applications range from lifting systems to technical awnings, from safety devices to automotive components.

Rigid Plastic + Rigid Plastic (Two-Component)

Less frequent but technically refined is the combination of two different rigid plastics: for example, transparent PC over coloured ABS for electronic device housings with decorative elements, or PA combined with glass-filled PA for structural components with aesthetic surfaces. This requires careful attention to differential shrinkage and to the compatibility of thermal expansion coefficients.

Sequential or Two-Shot Overmoulding: Two Different Approaches

From a production standpoint, there are two main approaches to overmoulding, each with very different economic and operational characteristics.

Sequential Overmoulding (Insert Moulding)

The substrate is produced in a first moulding cycle (or supplied already finished, in the case of metal inserts) and subsequently positioned manually or by robot inside a second mould, where the second material is overmoulded. This is the most flexible and least capital-intensive approach: it requires a standard mould for each stage, using conventional presses. It is ideal for medium-volume production or for projects where the material combination may change over time.

Two-Shot Overmoulding (2K)

The two materials are injected in sequence within the same press, equipped with two independent injection units. The part is formed through mould rotation or cavity translation between the two injection positions. It requires dedicated presses and moulds, with significantly higher initial investment, but it eliminates intermediate handling, ensures faster cycle times, and improves chemical adhesion quality because the second material is injected while the first is still hot. It is the reference approach for high-volume production and components with demanding aesthetic quality.

Design Constraints to Know

Designing a component for overmoulding requires specific attention that goes beyond good design practice for a single-material part. Some key constraints deserve consideration from the concept stage onward.

Differential shrinkage is the first critical point. Two different materials have different shrinkage coefficients and behave differently during cooling. If the substrate and the overmoulded layer are rigidly constrained to each other, the stresses generated during cooling can cause deformation, cracks at the material interface or loss of adhesion. The part geometry must allow a degree of “give” between the two materials.

The thickness of the overmoulded layer is equally important. Layers that are too thin may not fill correctly or may solidify too quickly, compromising adhesion. Layers that are too thick can create areas of heat accumulation and excessive shrinkage. As a general guideline, a thickness of 1.5 to 3 mm is recommended for most TPE-on-rigid-plastic combinations.

The position of the injection points in the second mould must be studied to ensure the material flows over the substrate without displacing it, without leaving unfilled areas, and without creating visible weld lines in aesthetic zones. A preliminary flow simulation is particularly useful when designing two-shot moulds.

The injection temperature of the second material must be compatible with the heat resistance of the substrate: injecting PA at 280°C onto a PP substrate can deform it. For this reason as well, preliminary DFM analysis is particularly important in overmoulding projects: it allows incompatibilities to be identified before investing in mould construction.

Industrial Applications Where Overmoulding Makes the Difference

Overmoulding finds its most compelling applications where the alternative — downstream assembly of separate components — would be more expensive, less reliable or technically inferior.

In the electrical and electronics industry, connector and cable housings combine rigid plastic for structure with TPE or elastomers for sealing gaskets, ensuring dust and water resistance without separate O-rings. Overmoulded cables feature integrated strain reliefs that solve the problem of accidental pull-out at the base.

In the automotive sector, handles, control buttons and technical components in contact with the user often combine structural plastic with soft-touch TPE finishes, both for functional reasons (grip, vibration reduction) and for perceived quality.

In medical devices, overmoulding allows ergonomic grips to be integrated onto surgical instruments, sealing gaskets onto infusion set connectors, and valves with integrated elastomeric seats, reducing the component count and simplifying sterilisation.

In lifting, anchoring and safety systems, overmoulding of plastic onto metal or fibre ropes creates permanent terminals, abrasion protection and integrated colour identifiers.

When Overmoulding Is Not the Right Choice

Overmoulding is not always the most economical or most suitable choice. For very low volumes, the investment in dedicated overmoulding tooling may not be justified compared to traditional assembly solutions. When the substrate is subject to frequent design changes during the project, the flexibility of mechanical assembly may be preferable to the rigidity of an overmoulding tool.

In some contexts where repair or separate replacement of one of the two parts is required, overmoulding creates a permanent bond that cannot be undone without damaging the component. The complexity of end-of-life recycling should also be considered, as it can be compromised by the integration of dissimilar materials that are not easily separated.

A Real Case: Overmoulding of Ropes and Chains

An Akron project that clearly illustrates the potential of this technique is the overmoulding of polypropylene onto ropes and chains. The project required precise positioning of the load-bearing element (rope or chain) within the mould, the definition of injection points so as not to displace the substrate during filling, and the optimisation of injection pressure to achieve an adherent coating without deforming the internal element. The result is a component that combines the mechanical strength of the load-bearing element with the surface protection and manageability of the plastic coating.

Our Approach to Overmoulding

Overmoulding is one of the techniques we master with particular specialisation. The integration between our technical department, in-house mould shop and production department allows us to approach overmoulding projects with a complete perspective, from assessing material compatibility to mould design and process set-up.

We work with plastic-rubber, plastic-metal and plastic-rope combinations, with the capability to manage both sequential processes and two-shot moulding. Our in-house mould design capability allows us to fine-tune the geometry of mechanical anchoring areas and the distribution of injection points in detail — elements that make the difference between a reliable component and a problematic one.

If your project involves a multi-material component and you want to assess whether overmoulding is the most suitable technical solution, contact us for a preliminary feasibility analysis with a technical-economic proposal.