Rubber-to-metal overmoulding, also known as co-moulding, insert moulding, or insert overmoulding, is an advanced and highly engineered manufacturing technology that enables the production of composite components in which rubber and metal are permanently integrated. 

The process involves the insertion of a preformed metal insert, i.e. one that has been suitably machined and shaped, into the mould, onto which a layer of rubber is then overmoulded. 

This technique is particularly valued as it allows the combination of the complementary properties of the two materials, ensuring high performance, reliability, durability, and versatility. 

In the following sections, we will analyse in detail how the process works, the main advantages over other traditional solutions, and its industrial applications. 

How does rubber-to-metal overmoulding work? 

The first stage of the rubber-to-metal overmoulding process involves preparing the metal insert, which is essential to ensure optimal adhesion of the elastomer. The metal surface is thoroughly cleaned and sandblasted to remove any impurities and to create a surface roughness that promotes mechanical bonding. A specific adhesive, known as a primer, is then applied, selected based on the type of metal and elastomer used. 

In parallel, the rubber is precisely weighed and processed into strips. The metal insert is then positioned inside the mould, and the polymer is distributed around or above it and compressed. The heat and pressure of the heated mould activate the vulcanisation process, allowing complete cross-linking of the elastomer’s polymer chains and the formation of a stable chemical bond between the rubber and metal. The production cycle typically lasts from 10 to 90 minutes, at temperatures between 140°C and 170°C, depending on the compound used and the dimensions of the unit. 

At the end of the cycle, the overmoulded component is extracted from the mould and undergoes appropriate trimming to remove any excess material. 

The finished overmoulded component features a rubber-to-metal chemical bond thanks to the use of the primer and the cross-linking of the polymer. Additionally, any irregularities or undercuts deliberately made on the insert surface enhance rubber interlocking, creating a mechanical bond. The result is a composite part that is permanently integrated, offering robustness, resistance, and durability. 

Compression overmoulding is performed on all thermosetting elastomers, such as EPDM, NBR, and silicones. In the case of thermoplastic elastomers, such as TPE, TPU, and SEBS, injection overmoulding is used: the molten material is injected into the mould around the metal insert, and solidifies upon cooling, enclosing the metal surface. This method ensures a stable mechanical bond between elastomer and metal, making use of undercuts and surface geometries designed to maximise polymer interpenetration. 

Advantages of rubber-to-metal overmoulding 

Rubber-to-metal overmoulding offers numerous technical and economic benefits, making the process highly competitive when compared to traditional assembly techniques. Below are the key advantages of this solution. 

Structural adhesion between rubber and metal 

The process ensures a strong and permanent structural bond between rubber and metal, guaranteed by the chemical and/or mechanical bonding formed during the cycle. 

This results in superior performance compared to manual bonding, significantly reducing the risk of detachment even under mechanical stress, pressure, and vibration. 

Production process optimisation 

The process delivers a pre-assembled component at the end of the cycle, eliminating manual bonding and assembly phases. This integration reduces operating costs and production times, ensuring greater efficiency on automated lines. 

Functional material integration 

The rubber-to-metal solution combines materials with differing mechanical properties in a single component. Metal provides rigidity and structural resistance, ensuring dimensional precision and stability under mechanical loads. Rubber, on the other hand, provides elasticity, flexibility, impact and vibration insulation, and sealing. The result is a high-performance, engineered component. 

Advanced design flexibility 

The process supports the creation of complex geometries, the integration of technical inserts, and the use of different materials to optimise functional performance. In general, its versatility enables the development of customised solutions tailored to various sectors and operating conditions. 

Component reliability and durability 

By eliminating mechanical joints and external adhesives, the overall robustness of the component is increased. It maintains its integrity even under extreme thermal, chemical, or operational conditions, thereby extending service life and reducing the need for maintenance. 

Production efficiency and component lightness 

The functional combination of rubber and metal allows for the design of lighter components, using metal only where structural rigidity is required. This reduces the component’s weight without compromising performance, improving production efficiency through optimal material usage. 

Applications of rubber-to-metal overmoulding in various industrial sectors 

Rubber-to-metal overmoulding is used in numerous industrial sectors. At Corti Srl, we apply the process in the production of components for the automotive, railway, hydraulic, manufacturing, furniture, lighting, marine, and textile industries. Below are some application examples. 

Automotive and railway 

In the automotive and railway sectors, the process is used to manufacture anti-vibration mounts, silent blocks, bushings, flexible joints, and gaskets – key components for vibration and noise dampening, as well as for ensuring stability and safety. 

Hydraulic and manufacturing 

In the hydraulic and manufacturing sectors, overmoulding is applied to valves, pumps, membranes, and sealing systems, where the rubber-to-metal bond ensures tightness, mechanical resistance, and long service life. It is also used in damping elements, functional supports, inserts, and conveyor belt wheels, which demand both durability and vibration damping capabilities. 

Furniture 

In the furniture sector, the technology is used for the production of feet, caps, spacers, and technical supports with overmoulded metal inserts that ensure stability, load-bearing capacity, and wear resistance. 

Lighting and marine 

In the lighting and marine sectors, the solution is used for insulation and protection components often exposed to harsh environmental conditions, such as humidity, vibrations, or thermal changes.