What is Overmolding?
Overmolding is a two-shot plastic injection molding process that creates a single part by combining two separate but complimentary thermoplastic materials.
The first shot, called the substrate, is usually composed of a more rigid plastic and is designed to accept the second shot, called the overmold. The overmold is typically composed of a softer, more flexible plastic-like rubber.
By mechanically and/or chemically bonding two plastic materials, the overmolding process can be used to help product designers achieve additional functionality or aesthetics that would otherwise not be practical.
Why use the Overmolding process in plastic injection molding?
One of the more common reasons to use the overmolding process in injection molding is to create a soft grip.
Pulling from the example used above, the green part is used functionally as a twistable nozzle that will help direct the flow of a liquid. However, in this case, the plastic chosen to facilitate the flow and to keep its chemical resistance for lasting part integrity happens to be very rigid and hard to the touch; not ideal for tightening and loosening with human hands.
The solution is to design for an overmolded rubber-like grip to aid the user in the twisting of this product.
But, tactile functionality is not limited to human grip. It can also be cleverly used to add rubber-like grips to clips designed to grab inanimate objects.
Here is another tactile example where a softer, rubber-like grip has been overmolded onto a rigid plastic part. In this case, though, the rubber-like grips are overmolded in two separate areas, which is a feasible use of the overmolding process. That is, it can be utilized in more than one area of the same part to achieve even more functionality.