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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. it 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. Plastic overmolding is also used to add color contrast for aesthetics or 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 over molding 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 presents far more opportunities for mistakes and a much smaller margin of error.

Over molding involves molding one material over another, where insert molding involves incapsulating 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. The result is 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


Plastic Over Molding Materials

Compatibility is key for the overmolding process, so there may be some limitations as to which materials you can use. 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

Because the materials we use shrink as they cool following the injection overmolding process, special consideration must be also made in the creation of our molds. And, because we use a number of different materials in overmolding, all of which have different cooling/shrinking properties, it is crucial that we create our molds to match the specific properties of the material(s) being used. Our mold designers will perfectly match their mold specs to the unique needs of your overmolding project.

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 put our overmolding services to work for you today.