Often considered inferior to the more sophisticated two-shot process, the reality is that over-moulding has its own set of idiosyncrasies than can yield beneficial results in some scenarios.
In simple terms, over moulding describes the process of taking a conventional single-shot moulded part (which has already been produced) and loading it into a second injection mould tool, where another polymer or elastomer is injected over the substrate.
This results in a single component, comprised of two materials.
The substrate is often a stiff grade of polymer in one colour, where the over-mould could be chosen as a soft-touch elastomer in another colour.
In this novel example, two parts are pressed together, before being combined by an elastomer over-mould.
Over-moulding a component can provide benefits for marketeers, designers, engineers, production operations and quality teams alike.
Some examples of the benefits include; adding an in-moulded brand or logo, adding a secondary colour or localised soft-touch surfaces, including a water-tight seal or removing the need to assemble a secondary component by hand. As shown here, a separate light-guide can be produced and combined within the finished article.
When applied correctly, over-moulding is preferable for building-up a secondary material on both sides of a component.
There are lots of examples Hymid can provide to provoke ideas on how to incorporate over-moulding to yield the best results.
Whilst it’s an exciting prospect to use over moulding to add value to an end part or product, there are limitations to using the process which need to be understood before any capital investment is made.
Over-moulded parts can be susceptible to a high degree of post-mould-distortion. This relates to a phenomenon known as ‘differential shrinkage’.
In addition to geometric deviations, over-moulded parts do not reach the same cosmetic finesse as the two-shot process, owing to the transition from one tool core to another.
Hymid’s design and project staff are on-hand to advise on these areas in more detail.
The type of application where over-moulding is suitable might be:
complex components that require elastomer on both sides of the substrate (core and cavity)
parts with multiple items (or bodies) to be be combined within the over-moulding cycle; as displayed here
Larger parts where a single two-shot tool may become too large to be safely handled within the injection press
Various reasons specific to part geometry such as feed locations, material processing requirements or undercut geometry
If you’re looking to take your conventional injection moulded part to the next level, then considering over-moulding is a sensible first step.
If you’re looking for the ultimate solution, then two-shot should also be taken into account – and don’t be fooled into assumptions on price; the capital cost of each process is comparable, whereas over-moulded parts are invariably more expensive to manufacture.
Let us know what you’re considering and we’ll provide all the technical and commercial support to ensure you’re on the best path possible for a successful product launch.