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An In-Depth Look at Overmolding and the Overmolding Process


Plastic injection molding has been used for over 150 years in every aspect of manufacturing. As the world of plastic molding grew, so did the need for additional molding practices to fill in the gaps where traditional injection molding falls short. Overmolding is one such process that addresses the need for combining materials into a single assembly. To help you understand how this manufacturing process can be used in your design, we have put together this overview on overmolding and the overmolding process. 


What is overmolding?

 Overmolding is a type of plastic injection molding that combines multiple parts or materials into one finished assembly during the molding process. Unlike insert molding that involves two materials molded at the same time, overmolding involves two separately molded parts. The result is two parts assembled and permanently joined together.


What is overmolding used for?

 Overmolding is useful for combining new layers of material over a previously molded part, resulting in a single assembly. Some typical applications include plastic over plastic, rubber over plastic, plastic over metal, and lastly rubber over metal. This technique is most useful in applications where the plastics are either too thick or too thin to be molded in a single shot. Additionally, in situations where complex geometry needs to be preserved during the molding process, overmolding is the preferred method. A common application where overmolding is used is adding rubber grips or handles to different devices from toothbrushes to cordless drills.


How does the overmolding process work?

During overmolding, the new plastic part is “molded over” an existing part, referred to as the substrate. The pieces are bonded to each other either chemically or mechanically.

The mechanical method requires the substrate being overmolded to have specific grooves or pass-throughs where the overmold material can fill in and hold the two components together.

Chemical bonding requires selecting resins and substrates that are chemically compatible and typically require the substrate to be plastic. During the molding process, molten resin is injected over the substrate causing the materials to bond to each other once cooled.


Design considerations for overmolding

Given the complex nature of overmolded parts, several considerations need to be thought through during the design for manufacturing phase of the product development process. If the part experiences large amounts of mechanical stresses, the part retention features are required to prevent separation of the two materials. Another way to prevent separation is to use both mechanical and chemical bonding methods when creating the overmolded part. Tool design must account for any shift to the substrate caused by the injection molding process. If the substrate were to shift inside the tool this could cause significant damage.


About Synectic Product Development: Synectic Product Development is an ISO 13485 compliant, full-scale product development company. Vertically integrated within the Mack Group, our capabilities allow us to take your design from concept all the way to production. With over 40 years of experience in design, development, and manufacturing, we strive for ingenuity, cost-effectiveness, and aesthetics in our designs.  Learn more about our contract manufacturing services and see how we can help your next project.

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