What is Overmolding?
Overmolding is a process where a single part is created using two or more different materials in combination. Typically the first material, sometimes referred to as the substrate, is partially or fully covered by subsequent materials (overmold materials) during the manufacturing process. We have a great video that illustrates the process and should help the reader to make sense of the following explanation. You can watch it here:
What is the Substrate?
The substrate could really be anything; a machined metal part, a molded plastic part, or even an existing product like threaded inserts, screws, or electrical connectors. It is the first piece in what will eventually become a single continuous part composed of chemically bonded and often mechanically interlocked materials of separate types.
What is the Overmolding Material?
Overmold materials (typically plastic) start off in pellet form. These pellets are mixed with additives like colorants, foaming agents, and other fillers. They’re then heated to their melting point and injected into the mold tooling as a liquid. There are some limitations on what materials are suitable for overmolding. If you are overmolding a metal part with plastic, you can really use any plastic. If you are overmolding a plastic part with another plastic (or a rubber or TPE), then there can be compatibility issues. The material manufacturer typically publishes a compatibility chart for overmolding.
What is Overmolding Used For?
The overmolding process is utilized for a number of reasons that vary according to the specifics of the particular project. Common materials include toothbrushes, tool handgrips (e.g. cordless drills and screw drivers), and personal care products (e.g. shampoo bottles and shaving razors).
Here are some examples of typical overmolding applications:
Plastic Over Plastic - First a rigid plastic substrate is molded. Then another rigid plastic is molded onto or around the substrate. The plastics could differ in color and/or resin.
Rubber Over Plastic - First a rigid plastic substrate is molded. Then a soft rubber or TPE is molded onto or around the substrate. This is often used to give a soft grip area to a rigid part.
Plastic Over Metal - First a metal substrate is machined, cast or formed. Then, the substrate is inserted into an injection molding tool and the plastic is molded onto or around the metal. This is often used to capture metal components in a plastic part.
Rubber Over Metal - First a metal substrate is machined, cast, or formed. Then the substrate is inserted into an injection molding tool and the rubber or TPE is molded onto or around the metal. This is often used to provide a soft grip surface
A few things to note:
There are limitations and compatibility issues to consider between different materials.
You are not limited to only two materials. We have seen some products with three different materials on one part in order to achieve color breaks and grip surfaces. Here is a simple example with a product you will be intimately familiar with: scissors.
How is the Process of Overmolding Typically Accomplished?
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.
In summary, overmolding is regularly used to manufacture parts, sub-sections of parts, and for prototype development. To receive a quote or start a discussion about your overmolding project please visit Creative Mechanisms here.