When to Use Overmolding with Injection Molding

Maybe you have know injection molding process already. In the following article, we’ll take a close look at overmolding, a specific process within the injection molding area that is used across many industries to create parts made from multiple materials. Read on to learn the what, how, why and when of overmolding as well as why you should partner with rapid prototyping service RapidDirect for your overmolding and injection molding needs.

What is overmolding?

Traditional injection molding, as we can see in more detail here, consists of injecting molten materials into a mold cavity to create a solid product. However, there are different approaches within the realm of injection molding, which are used to make more complex products, such as multi-material components. One of the ways in which these types of products are manufactured is overmolding.

Overmolding is an injection molding process wherein an existing part or structure is used as a substrate and an additional material is added to the substrate. The overmolding process typically uses a rigid material as the substrate (such as metal, plastic or glass) and rubber or thermoplastic as the overmold material.

The overmolding process combines two types of material:

• Substrate: Metal, plastic, glass, etc.
• Overmold material: rubber or thermoplastic such as TPE

How does injection molding work with overmolding?

If an injection molded part is to be overmolded, the process is fairly straightforward. The substrate is molded first, using conventional injection molding. Alternatively, the substrate can be made using CNC Machining. When the substrate is complete, it is then placed into a new mold. The overmold material, often a resin, is then injected into the new mold, bonding with the substrate.

Bonding between the two materials is achieved in two key ways. The substrate and the overmold material can become chemically bonded in the injection molding process. Or, the materials can become mechanically bonded. Product designers or manufacturers can enhance mechanical bonding by integrating features such as undercuts into the design, which cause the substrate and overmold to lock into place.

Substrates can be fully covered with the overmold material or partially covered to add visual or functional features to the final product. A good example to visualize overmolding is the common hammer. The metal tool, often made from steel, usually has an overmolded plastic layer on the handle, resulting in better handling and grip.

What are overmolding applications?

The applications for overmolding are practically limitless, and we encounter overmolded products every day. From your plastic toothbrush with a rubber grip, to your scissors with plastic handles, to the knobs on your kitchen appliances.

Injection molding with overmold materials is not only used for consumer products, it also has many industrial applications, including electronic casings and enclosures, medical devices and instruments, and products for the military, aerospace and automotive sectors.

Overmolding is commonly used for production applications requiring up to tens of thousands of parts. For customers requiring mass production in the millions of parts, overmolding can also offer an effective prototyping process.

Choosing the right injection molding and overmolding materials

In each application area, overmolding is used in a few basic configurations:

• Plastic substrate with plastic overmold
• Metal substrate with plastic overmold
• Plastic substrate with rubber overmold and
• Metal substrate with rubber overmold.

In each case, many material combinations can be used, however some are more compatible than others. Compatibility is important to create the chemical and mechanical bonds between the CNC machined or injection molded substrate and the overmolded layer. Incompatible materials can result in improper bonding or product deformation.

When working with a metal substrate, there are many compatible plastic or rubber materials for overmolding. When working with a plastic substrate, however, there are some incompatible combinations to watch out for. Commonly used combinations of plastics for overmolding include a substrate made from ABS (acrylonitrile butadiene styrene) or PC (polycarbonate) and an overmolded feature made from TPU (thermoplastic polyurethane) or TPC (thermoplastic copolyester).

To determine whether a combination of polymers is well suited for overmolding, it is important to consult with a professional service, many of which have close ties to material suppliers. This knowledge will enable you to know whether a pair of materials are compatible or not before moving into production.

The benefits of overmolding

When working with injection molding, there are many benefits to overmolding. Some of these advantages come into play during the manufacturing process and others are related to the quality and function of the end-use product.

Looking at the former category, the overmolding process can reduce the need for secondary operations when making a multi-material component. For instance, rather than produce two separate parts made from different materials and assemble them manually using glue or screws, overmolding enables you to seamlessly create a single part from multiple materials. The need for fewer steps has a knock-on effect on production costs, and overmolding is considered to be a highly cost-effective injection molding process.  

In terms of post-manufacturing benefits, overmolding can improve the look, feel and function of products. It can, for example, add cushioned, soft or non-slip grips to handheld products, improving ergonomics and overall function. These soft coatings can also offer safety benefits, absorbing shock or vibrations. Companies can also take advantage of overmolding to visually improve products, adding different colors, including brand signature hues, to their products. Overmold aesthetic features can boost consumer appeal for products and provide a competitive edge.

RapidDirect, for your injection molding needs

At RapidDirect we have extensive knowledge of the injection molding process and can help you to achieve the best results for your product. Among our injection molding services are rapid prototype tooling, production tooling, family molds, multi-cavity molds and, of course, overmolding.

On a tight schedule? Get in touch with us about your injection molding project and get a online quote to you in under two hours. If you like what you see, we will deliver your parts in as little as five days. Our sweet spot for production capacity is between 5,000 and 10,000 prototype molds.

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