Plastic molding has revolutionised industrial design and manufacturing in the past 100 years. Overmolding is a process that is very effective in specific applications, due to the low cost of the molding materials and the manufacturing process. These properties usually come at a price – low mechanical strength. In many cases, the efficiency of the molding process needs to be combined somehow with the superior strength of machined, cast or forged materials.
Overmolding is a subcategory of injection molding, where the molding material is applied onto another material. While material would be injected into an “empty” mold during conventional injection molding, an object is inserted into the mold beforehand during overmolding.
Molding one material on top of another material (or substrate) has merit in certain cases. Consider a common screwdriver, for example. The shaft is manufactured from hard stainless steel to endure the wear and torsion, while the handle is made from plastic or rubber. In most cases, this plastic or rubber handle is molded over the steel shaft, hence the name “overmolding”.
The humble toothbrush is a good example of overmolding. In common toothbrushes, rubber is molded over plastic. This provides grip for the user, but also provides the aility to use more than one colour in the design.
This is a good example of where more than one instance of overmolding is used. Scissor blades are usually made from stainless steel, which is molded over with a structural plastic, often followed by rubber step for grip.
Overmolding is often used here to combine the mechanical effects of certain steels with the ergonomic and economical appeal of plastics and rubbers. Plastic-over-plastic or rubber-over-plastic is common here for similar reason as with the toothbrush and scissors.
Overmolding is not only used in light duty objects. In some instances, pumps may use an impellers manufactured from a steel-polymer overmold. Heavy duty slurry pumps (which convey irregular and granular material) often use such impellers. The shaft and core structure is still manufactured from steel, whereas the outer areas of the impeller consist of an overmolded material such as rubber. The rubber can absorb shock loads from the pumped material, and is a much cheaper wear material than the steel.
Overmolding is effectively used in the electronics manufacturing industry. Plastics are used for structural components, mainly due to their low cost. Plastic does not conduct electricity, so the need for a conductive material remains unchanged. The marriage between copper and overmolded plastic addresses this need perfectly.
Overmolding is often used for the sheer purpose of providing more than one colour; something which is not possible using a single molding step
Especially in hand tools, certain areas may be molded over with softer materials for prolonged use, or to increase the traction of the user’s hand.
Parts can be locked into other parts, bypassing the need for other methods of retaining them. Think of a polymer bushing being molded into a bearing area of a steel component.
Molding two different subsections of a component in two steps may eliminate the need for a liquid seal in the interface of those two surfaces.
In some cases, additional assembly steps may be avoided. Imagine a component consisting of a steel substrate and a plastic user interface. Instead of having to manufacture the steel part, mold the plastic interface and then assembling them in a separate step, the molding and assembly could be combined into one super step with the use of overmolding.