Overmolding is the process of adding an additional layer of resin to the existing plastic part. This process adds characteristics that a single piece of plastic can’t provide.

One common reason to use this technique is to add a soft, functional, ergonomic layer of rubber-like material (typically a thermoplastic elastomer) over a hard surface. This improves the grip of a hand-held item.

One example of this is toothbrushes. Toothbrushes are often made from a hard plastic center with a soft plastic grip around the handle.

Another use for over-molding is to change or enhance the appearance of a part by overmolding material of a different color or finish.

There are two primary methods of overmolding: 

Two-shot molding uses a single production mold.

Pick-n-place molding uses two production molds where an entire batch of parts are molded. Then, they are manually placed into a second mold where the overmold resin is injected to produce the completed parts.

Thousands of possible combinations exist for over-molded material. Resins have to be adhesive and compatible with each other in order for the process to work.

If the goal of using overmolding is to enhance grip or increase cushioning in your product, make sure your injection molding company knows those goals. Factors like cushioning, flexibility, and friction will play into the type of resins that are used in the product.

Benefits of Overmolding:

It can provide a soft, non-slip grip to your product.

It acts as an environmental barrier to shock, vibrations, and noise.

It creates colorful, visually attractive surfaces.

It reduces the number of secondary steps and costs associated with them, in turn reducing the complexity of assembly.

It can provide adhesion between different materials and eliminate the need to assemble different materials by hand.