Typically the substrate material or part is placed into an injection molding tool at which point the overmold material is shot into, onto, or around the substrate. When the overmold material cures or solidifies, the two materials become joined together as a single part.
Free Tip: It is usually a good idea to have your substrate and overmold material interlock in some mechanical capacity. This way, the two materials will not only be bonded together chemically, they will also be held together physically.
Specifics For Prototype Overmolding
At Creative Mechanisms, we do prototype overmolding. More often than not, we are overmolding a soft rubber onto a plastic or metal substrate. We typically create plastic molds to hold the substrate in place and then mix polyurethane casting rubbers to be injected into the molds. Once cured, the rubber and substrate are removed as one part. Of note, we can control the hardness and the color of the overmold material. You can watch the process here [EMBED VIDEO]. In mass production, with TPEs, overmolding can be done in 30-60 seconds. However, in prototyping, it takes several hours because we use 2-part casting rubbers that need to cure over time.
Why Would you Want to Overmold?
There are a lot of reasons to overmold. Among the most common are the following:
As a means to break up color (aesthetic impacts).
To provide a soft grip surface around a part of separate material.
To add flexible areas to a rigid part.
To eliminate assembly line time. Instead of manufacturing a metal tool and a plastic hand grip separately and then joining the two together manually or with automation, you can just overmold the metal tool with a plastic hand grip and eliminate the need for assembly altogether.
To capture one part inside of another without having to use fasteners or adhesives.