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Plastic Injection Molding: Overmolding

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What is Overmolding?

The overmolding process involves creating a single part using a combination of two or more plastic or elastomer materials. During the plastic overmolding process, the base layer part is molded first, then the additional plastic layer(s) are molded over and around the original part. One or more plastic resins may be used depending on material compatibility to achieve a specific texture or look.

Overmolding is used for a range of applications including medical, automotive, military and more. A common example of injection overmolding is a toothbrush with a stiff frame and rubberized grips on the handle. Different overmolding material options are also used to add color contrast for aesthetics or to create flexible areas on rigid components. Benefits of plastic overmolding include:

  • Improve grip and ergonomics
  • Provide electrical insulation
  • Create a waterproof seal
  • Dampen vibrations or absorb sound

Plastic overmolding can help reduce production time because the second piece is molded directly onto the original part rather than having to completely manufacture the two pieces separately and then assemble them.  Over molding is also a very difficult process, requiring adherence to the very tightest of tolerances.

The Difference Between Overmolding vs. Insert Molding

The general concept of overmolding vs. insert molding is quite similar but there are distinct differences between the two processes. Overmolding is a two-step process where two separately molded parts are combined to enhance a product’s features. They are permanently joined together but the process does not involve full encapsulation. First, a plastic component is produced using plastic injection molding. After the part has cooled, it is placed into an overmolding tool where it is then coated with molten thermoplastic or rubber.

The process for insert molding is different because it involves encapsulating a product in molded plastic.  Insert molding is a slightly faster process because the two plastic materials are molded at the same time. During this process, the melted plastic resin is injected into a mold and a plastic piece is inserted into the mold to produce an encapsulated finished product.

Stack Plastics' plastic overmolding processes create parts specifications as tight as 0.001”. We utilize the most advanced injection molding equipment for overmolding to ensure the highest possible quality and superb repeatability. Different types of overmolding injection molding we provide include:

  • Plastic over metal
  • Rubber over metal
  • Plastic over plastic
  • Rubber over plastic

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What Material Combinations Can You Use for Overmolding?

Overmolding is often used to create a textured or comfort grip surface on metal components so an initial part of the material selection process is reviewing different material properties to see which will produce the desired results. This become a bit trickier when using more than one material because material properties can change when two or more resins are combined.

Compatibility is key for the overmolding process, so there may be some limitations as to which overmolding material combinations you can use. There are also important considerations regarding thickness effects, hardness, and coefficient or friction to ensure compatibility between your overmolding materials.

We have extensive knowledge of plastic materials including thermoplastics and exotic resins to help you select the right materials for your specific part. Some of the more common materials used for injection overmolding are:

  • ABS: a common and versatile plastic that provides good resistance heat, oils, acids and alkalis. Other features include high impact strength and toughness, good dielectric properties, excellent ductility and ability to be alloyed with other polymers. 
  • PEEK: this high-performance plastic provides high chemical and temperature resistance without degradation and provides excellent fatigue and stress-crack resistance. It is FDA-approved and suitable for use in medical device applications.
  • Acrylic Thermoplastics: provide excellent weatherability and resistance to UV fading. They feature high tensile strength and are scratch-resistant. Acrylics are also available in FDA-approved grades for use in food and medical applications.

Specialized Plastic Overmolding to Meet Your Design Requirements

Stack Plastics has the expertise to make sure your overmolding project is a success. We provide guidance throughout every step of the process from design to final fabrication and assist with troubleshooting if issues arise.

Since the materials we use shrink as they cool following the overmolding process, special considerations are made during the tooling development process to accommodate for changes in dimensional tolerance. Our mold designers will perfectly match their mold specs to the unique needs of your overmolding project. And because we use several different materials in overmolding with different cooling/shrinking properties, we create our molds to match the specific properties of the material(s) being used.

The variety of materials we use, the precise capabilities of our equipment, and our mold design services make our overmolding process extremely customizable. Stack Plastics will work with you to create the ideal solution for your exact needs. No matter what the specifications of your project, no matter how low- or high-volume your production run will be, Stack Plastics will meet or exceed the expectations of your next overmolding project.

Contact Stack Plastics for Injection Overmolding

Contact Stack Plastics to discuss material selection and tooling requirements for your overmolding project.