For mass production of simple parts, there are few manufacturing processes more suitable than injection molding, the most widely used plastic manufacturing process in the world. Injection molding is used to make everything from food packaging to aircraft parts, while its speed and cost-effectiveness — especially at scale — make it the top choice of many manufacturers.
There are, however, limitations to the basic injection molding process. The envelope of an injection molding machine limits the size of molded parts, and the process itself — in which molten plastic is forced into a metal mold — imposes geometrical restrictions that could be sidestepped with a process like 3D printing or even CNC machining.
Moreover, injection molding is generally only suited to simple plastic parts that need to be made in huge quantities.
That is, unless we use the process of overmolding or insert molding to create complex, multi-material molded parts. This article looks at how overmolding works and what it can be used for, giving you an idea of whether you could incorporate overmolding into your production cycle.
What is overmolding?
As its name suggests, overmolding is a manufacturing process in which the basic injection molding process is carried out over the top of another molded component.
That other molded component, known as the substrate, is placed within a metal mold. So when we inject the molten plastic into the mold, the liquid material surrounds the substrate or in some way attaches to it. When the molten plastic cools down, we end up with a complex part made up of two components: the substrate, and the newly injected plastic on top of it.
Overmolding allows a manufacturer to create a molded part consisting of more than one material, without having to manually assemble two or more separate components.
Why use overmolding?
Overmolding is an ideal solution in many manufacturing situations. For inexpensive, mass-produced parts that require two (or more) components made from different materials, overmolding is often faster and more affordable than the manual assembly of multiple components.
And in many cases, overmolding allows for designs that would not be possible to manually assemble. (Injecting a second material within a partially hollow plastic component, for example.)
Overmolding can be ideal in the following situations:
- Injecting rubber over a rigid plastic substrate
- Injecting rigid plastic over a rigid plastic substrate
- Injecting rubber over a metal substrate (insert molding)
- Injecting rigid plastic over a metal substrate (insert molding)
Overmolding vs. insert molding
Overmolding and insert molding are very similar processes. Both involve injecting molten plastic over another material to create a multi-material, multi-component part.
The difference lies with the substrate — the component over which the new material is added.
With overmolding, the substrate is itself always a molded component. So the overmolding process essentially comprises two consecutive rounds of injection molding (using two separate metal molds). First, the substrate is created in its own mold; then, the overmolding is carried out over the previously molded substrate in a different mold.
Insert molding is different. With insert molding, the substrate does not have to be a molded component. It can be a machined metal component, for example, and the molding process can be used to add plastic or rubber to the metal component.
Example applications of overmolding
Rubber over rigid plastic
Adding rubber components to plastic is a common process in the production of consumer products, and is widely used to add soft, grippy handles to parts. Toothbrushes, for example, are often made using overmolding, with a rubber handle section added to the rigid plastic stem.
Rigid plastic over rigid plastic
Combining two different rigid plastics is useful for creating multicolor parts or parts with partially translucent sections. Children’s toys, consisting of two or more colorful plastic sections, are often made with overmolding.
Rubber over metal
Insert molding is ideal for adding a rubber component to a preformed metal part. This may be useful for adding a grip, or even a watertight seal or gasket, to the metal part. Rubber can also act as a shock absorber.
Rigid plastic over metal
Insert molding with rigid plastic is widely used in the manufacture of products like handheld tools and safety razors, where a solid plastic handle is molded around a metal tool head or blade for ergonomic reasons.
Overmolding material compatibility & bonding
The purpose of overmolding and insert molding is to create a complex part whose components are in some way sealed together. If the two materials easily separate from one another, then the overmolding has not succeeded.
However, there are two primary ways in which the substrate and the overmolding material can be bonded together.
One way is chemical bonding, which involves the overmolding material fusing with and sticking to the substrate material. Chemical bonding is only possible if the two materials are chemically compatible, like TPU (rubber) over ABS (rigid plastic), but it allows for a great degree of design freedom.
If the substrate material and overmolding material are not chemically compatible, they can still be bonded if the engineer designs a mechanical bond for the part. A mechanical bond may take the form of an undercut or reverse taper in the substrate: once the overmolding material flows around and under the undercut, it becomes locked in place and cannot separate from the substrate.
In fact, mechanical bonds are a unique design feature of overmolding and insert molding. A fully enclosed, interlocking connection between the two components is often impossible to achieve using manual assembly. (Unless, for example, a rubber component is stretched around or compressed into the solid component.)
Overmolding with 3ERP
3ERP has many years of experience in injection molding, and can execute overmolding and insert molding projects of all shapes and sizes.