Can I use overmolding in prototypes?

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Becky Carter/Product Manager/ProtoLabs

Overmolding is the process of molding two or more materials together to create a single part. The process is used in medical devices, for example, to create soft-touch grip handles for surgical instruments. The technology can improve functionality and comfort for users. Overmolding can also be used to mold multiple colors on a given product for aesthetics or branding. One benefit of overmolding is that it eliminates manufacturing costs by removing the need for a secondary assembly step. If you need to have two or more materials molded together, overmolding is a good option to consider.

The process of designing an overmolded part is very much like designing any other injection-molded part. In order to produce a good over-molded part, you need to follow general molding guidelines, such as making sure to add sufficient draft to vertical surfaces so the part can be ejected from the mold. You also need to pay close attention to thick areas to avoid sink and watch out for thin areas that may make the part difficult to fill.

An additional consideration for overmolding is ensuring that the design has adequate shut-off between the overmold and the substrate, in order to minimize potential for flashing. Overmolding typically uses an elastomeric material as the overmold and those tend to be more likely to flash, so it’s a bigger consideration for overmolding than it is for general injection molding.

Historically there have been very few options for cost-effective prototyping or low-volume production of overmolded parts. Typically what engineers have to do during prototyping is separately produce the substrate and overmold parts then use a secondary method such as adhesive or fasteners to join them together for testing purposes. They might also rely on urethane casting using RTV or silicone molds as an alternative.

Although these methods give the engineer a general idea of how well the finished products will perform, the only way to truly validate the properties and functional performance of an overmolded part is to produce the prototype in the actual material and production method using rapid overmolding.

With the introduction of rapid overmolding engineers can now resolve potential manufacturing issues much earlier in their product development process, and they can minimize the risk of unpleasant surprises down the road.

In the past, designers needed to have tens of thousands or hundreds of thousands of parts in order for overmolding to be a feasible manufacturing option. But e-commerce practices and companies willing to focus on low-volume production have changed that. For example, ProtoLabs has its clients upload CAD files via its website, using software automation that helps accelerate the front-end experience. It enables us to analyze parts very quickly and get our quotes turned around within a few hours, complete with manufacturability feedback. Because everything is automated, once the customer finalizes their order, everything flows through the system and ProtoLabs starts making that mold immediately.

Software automation allows us to really speed up the process and also keep costs down, making overmolding for prototypes and low-volume production a possibility. Designers can have finished overmolded parts in-hand in 15 business days or less, and companies can order as few as 25 and up to tens of thousands of parts. Engineers and designers now have easy access to overmolding so they can experiment with the process in a cost effective manner.

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