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Over-moulding at the prototype stage

07 March 2017

RP Technologies are approached on what seems to be a daily basis to discuss over-moulding as a process for prototype tooling and moulding, and whether it is suitable for low volumes.

ABS substrate with TPE over-mould

The over-moulding process has been around for many years but with the market trend to improve the feel and functionality of co-moulded parts growing at an alarming rate RP Technologies have seen more enquiries over the last few years. The continued development of soft polymers such as TPE’s and TPV’s also plays a part to the increase of over-moulded components.

Over-moulding is a moulding process in which two materials are combined to produce a single part. Typically, the part seamlessly binds a rigid plastic (the substrate) with a TPE elastomer (the over-moulding). To produce an over-moulded component two mould tools are manufactured, a substrate tool which produces a rigid component, and an over-mould tool in which the rigid component is manually inserted into so that the TPE can be over-moulded.

With continuous in-house development and a close partnership with its TPE material supplier, Abbey Polymers, RP Technologies have gained experience with over-moulding projects and have seen good results. This knowledge enables the company to provide advice to customers when considering over-moulding at prototype stage.

TPEs are extremely versatile and can be supplied to cover a wide range of product areas. They can be offered with medical approvals, for toys and sporting items, for automotive interior & exterior parts, and where flame-retarded or WRAS-listed materials are required. 

One of the most important things to consider with over-moulding is how the over-moulded material will attach itself to the substrate. There are two different options to consider:

Chemical bond 

The most successful option is chemical bond, however it’s vital to select two materials that have the correct compatibility to ensure the materials don’t come apart once cooled. As standard, TPEs will bond successfully to Polypropylene (PP) (including talc or glass filled) but modified grades are also readily available which allow over-moulding onto a much wider range of substrates. Bond strengths on PP are very high, and are also very good on PC and PC/ABS but some other materials have lesser bond strengths in over-moulding. It’s difficult to give precise guidelines as parts need to be considered on a case-by-case basis. Over-mould thickness and flow length have some influence in substrates that are more difficult to bond to – so this is an area where advice is particularly useful.

If the customer’s application requires materials that don’t have a good chemical bond, then mechanical attachment is another option.

Mechanical attachment 

This involves designing the component with suitable undercuts (holes, keyways, ribs etc.) within the substrate to allow the over-moulding to grip to. Soft TPEs are tolerant of quite large undercuts and have good flow behaviour so flowing into holes in the component is not usually problematic. RP Technologies and Abbey Polymers are always on hand to consider flow paths in parts of this type in order to avoid gas traps.

Being able to produce over-moulded components at the prototype stage ensures that components are fully fit and functional for product testing and development. RP’s ‘Without Limits’ philosophy means that its sure to find a solution for your project, no matter how complex the over-moulding design, ensuring the product development stage is made easier. 

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