Overmolding is a special form of plastic injection molding. Unlike conventional molding, where a finished product is made in one cycle, overmolding requires at least one additional injection cycle using a dissimilar material. This method is also known as 2K overmolding, Two-Shot Molding, or even multiple material molding.
Overmolding starts with a base substrate, on top of which a relatively softer TPE or TPU material is bonded. The base can be metal or plastic, although, in this article, we will focus on plastic applications primarily.
If the substrate is a plastic base, then it’s customary to mold it in the same production cycle. This is done because it’s more cost-effective to make the entire finished piece at one time, rather than paying to transport and store semi-finished goods. But the more important reason is that the elastomer will adhere much better to a still-warm, semi-cured, and “sticky” substrate.
Overmolding uses a special type of plastic injection molding tool, one that has a cavity for the substrate portion and another separate cavity representing the overmolded section.
The first conventional injection cycle forms the substrate, using a standard thermoforming resin. This resin fills the gate, runner, and cavity system of only one part of the mold, while the section representing the overmold is blocked.
Once the base cools and is semi-hardened, the mold is then rotated to a new position and connected to a separate nozzle, gate, and runner system. The remaining void of the mold tool cavity is filled with TPE or TPU elastomer, creating the overmold that bonds to the substrate.
The process is not limited to just two materials, and can theoretically be expanded to include an indefinite number of additional overmolds. However, in practice, more than two becomes increasingly complex and expensive.
Insert molding involves placing a pre-manufactured component, such as a threaded fitting or an electrical contact, onto a mounting pin or other fixture in a plastic injection molding tool. During the molding cycle, thermoforming resin surrounds this insert and solidifies, thereby permanently sealing it into place.
In this video, Gordon Styles, the founder and President of Star Rapid, goes into greater detail explaining how these processes work, and he gives examples of each. Be sure to check it out for more useful information.
Inserts can be either male or female. Female inserts are used to make circular fittings inside the surrounding plastic. When these types of inserts have internal threads, they’re called nutserts. There are many advantages to using both male and female inserts to enhance a product’s appearance and utility.
Read the 6 advantages of insert molding:
The nutserts themselves add the expense to the build, but that cost is offset by the fact that they help to avoid secondary drilling and tapping, which also takes time and money.
There is no need to invest in equipment upgrades.
This is a great way to protect electrical connections, for example.
This helps to create a complete finished part in one quick and economical operation.
For example, consider a common toggle switch or lever. By using metal inside the body of the switch only where it’s needed to make a mechanical contact, and substituting the remainder of the switch for plastic, the weight of the entire assembly is thereby reduced. This approach, not incidentally, also decreases the reliance on a more expensive, full metal switch.
Now, if that design logic is applied to a fleet of aircraft or millions of automobiles on an assembly line, then the reduction in weight and cost becomes substantial.