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Overmoulding and insert moulding for injection moulding

Overmoulding and insert moulding are established part of the wider injection moulding process.  In this short article, we’ll take a look at the use of each technique. 

Overmoulding process

Overmoulding is typically used when the mechanical or chemical properties required cannot be achieved from a single material. Typically a substrate material is placed into an injection moulding tool in which the overmould material is placed into, onto, or around the substrate. When the overmould material solidifies, the two join together as a single part.

This process is utilised for various reasons that vary according to project requirements. For example, where you need the hand-grip on a tool, such as a drill or other electric hand tool, to have a relatively soft finish, while retaining a robust structure for the tool housing. There are some limitations on what materials are suitable for overmoulding as it can cause compatibility issues. 

Overmoulding is most commonly used for adding additional functionality, such as tactile feel, or for enhancing the aesthetic appearance of an injection moulded part or product. 

Common overmoulding applications are as follows:

  • Plastic over plastic: Overmoulding one plastic resin over or around another is a common technique for creating two-coloured components at a competitive cost, without the need for assembly operations.
  • Plastic over metal: Overmoulding plastic over or around a metal substrate is a common technique where, for example, a plastic handle needs to be added to a metal tool part, or a threaded insert incorporated into a plastic enclosure.
  • Rubber over plastic: Overmoulding an elastomer or rubber over a plastic substrate offers the opportunity to combine materials with different characteristics in a single component, such as a rubber handgrip on a hard plastic moulding for a power tool.
  • Rubber over metal: Overmoulding rubber over or around a metal substrate allows a tactile or textured finish to be added to a component at a competitive cost, in a single production operation.

Keep in mind, there are limitations to consider between materials as it can cause compatibility issues during the overmoulding process. Also, the process is not limited to two materials, in fact, to achieve colour breaks and grip surfaces multiple materials can be used.

Overmoulding injection moulding

Overmoulding for injection moulding: factors to consider 

Perhaps the most important factor to consider when specifying overmoulding for injection moulded parts is the following: 

  • Compatibility of the different substrate and overmoulding materials, to create an effective bond and eliminate the potential for an unwanted chemical reaction between opposing surfaces. 

In applications where a metal substrate and plastic or rubber overmoulding material are used, then it will be necessary to machine recesses or holes within the metal into which the overmoulding material can flow; this allows it subsequently to form a locking shape once it cools. 

The choice of materials will be determined by other factors, including the mechanical or physical requirements of the part. These can include:

  • An elastomer handle overmoulded onto a rigid resin shell will need sufficient thickness to provide comfort and vibration absorption in use, while providing a high degree of friction to ensure a safe grip. 

One of the advantages of overmoulding is the ability to reduce production costs, as it is possible to create multi-functional parts, with different materials in a simple production operation.  This can be far more cost-effective than conventional assembly techniques. To be effective, however, it may be necessary to review the design and shape of the required part, to enable it to be injection moulded efficiently. 

Insert moulding process

Insert moulding also provides an effective method of combining dissimilar materials to create a finished part with a unique set of characteristics.   

With insert moulding, a metal component, such as a threaded insert, is placed in the mould prior to injection moulding, so that the liquid resin subsequently flows around and encapsulates the insert. A typical example is the addition of threaded inserts into a twopart enclosure, which is designed to be screwed or bolted together. 

Unlike overmoulding, which is essentially a two-stage process, insert moulding is carried out in a single shot. 

One of the advantages of insert moulding is that it provides a cost-effective alternative to allmetal components, especially in volume, while offering the opportunity to reduce overall part weight. 

Insert moulding for injection moulding: factors to consider 

With insert moulding, issues of material compatibility are of less concern than with overmoulding.   

Nonetheless, careful consideration should be given to the design of both the insert part and the injection moulded encapsulation.  For example, engineering undercuts or bosses into the insert part will ensure that it remains locked firmly in place.  Similarly, the insert part should not be embedded too deeply within the moulding. 

The insert part should also be designed into the overall moulding so that it can easily be inserted into the mould tool; this is normally achieved using an automated handling system.

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