Overmolding is a multi-step injection molding process where two or more components are molded over top of one another. Overmolding is sometimes referred to as two-shot molding because it is a two-step process.

First, a base component (otherwise known as a substrate) is molded and allowed to cure. Overmolded substrates are often made of plastic. Then, a second layer is molded directly on top of the first to create a single solid piece. Overmolding is commonly used to manufacture plastic parts that feature a rubber handle. The two-shot process of overmolding a toothbrush, for example, consists of forming a base layer for the plastic handle and a top layer of rubber (to make the toothbrush less slippery to hold).

Overmolding is an injection molding process used to mold one plastic (commonly a rubber-like plastic called TPE) over top of another component (substrate). The substrate is usually an injection-molded plastic part, but it could also be made from various other materials. Overmolding is used across several industries and serves a wide array of purposes. Below are a few examples of overmolding applications:

v Grip – Generally speaking, components with handles often need something to help the operator maintain grip. Most importantly, overmolded grips are safer for operators by providing increase grip, especially in wet conditions.

v Vibration Dampening – Rubber bumpers or mounts can protect components (like electronics) from vibrations.

v Comfort – Since the grip material is injection overmolded, it can be formed in any shape, making it great for ergonomics.

v Sealing – Overmolding can be used to create water-resistant seals on electronics and other devices.

v Sound Absorption – Much like vibration dampening, sound waves can be absorbed by overmolding.

Designing for Overmolding

Designing for overmolding can be difficult. However, ensuring your part is correctly designed for the overmolding process is critical. The design process starts with understanding the function of the part. Here are some good questions to ask upfront:

v What does your part accomplish? – Because the overmolding process has a wide array of applications, you need to understand the core function of your part. For example, if you are molding a seal on a water-resistant case, proper sealing is likely the most important function.

v What will the part be exposed to? – Exposure to UV light from the sun or harsh chemicals can damage certain kinds of plastics. If you understand where the part goes and what it’s exposed to, you can select materials that are more resistant to UV light than others.

v Why is the part overmolded? – If you are adding a TPE (a rubber-like plastic) to a handle, it could be for grip, ergonomics, comfort, or vibration dampening (see above). Answering this question helps you figure out which material is best for your purposes. For example, you may choose a certain grade TPE (there are many) to yield a high grip in wet conditions and another for high vibration dampening.

v What are the long-term production needs? – Part design factors, such as wall thickness, can affect cycle time and piece price. An example is a high vibration absorbing grip. The TPE could be thicker to allow better vibration absorption, but that may cause a longer cycle time. The long-term cost of the added material and cycle time could be huge at scale; however, it could be “worth it” in a lower volume application.

Once you clearly understand the function of your part, you want to look at the design of the part and make sure it’s optimized for the manufacturing process. Other design considerations include wall thickness, gate location, shrink, sealing features, and mechanical interlock features.

² Wall Thickness – Like any injection molded part, the wall thickness throughout the part should be consistent and thick areas should be avoided. Consistent wall thickness creates even plastic flow. Having plastic too thick can cause shrink marks and voids

² Gate Location – Plastic needs to get from the molding machine into the desired area of the part. When the plastic enters the part (at the gate), a small mark may be formed. For cosmetic parts, it’s essential to plan for that.

² Sealing Features – The mold must seal against the surface of the substrate properly to obtain clean edges between the substrate and overmold. If the substrate is plastic, some interference can be designed into the overmold tooling to compress and seal against it.

² Shrink – Like any other plastic, the overmold will shrink based on the material manufacturers’ specifications and part design. Since more shrink is present with larger wall thicknesses, it’s important to make sure that the overmold does not shrink away from the substrate.

Overmolding can be used with a wide range of materials, including:

• ABS (Acrylonitrile Butadiene Styrene)

• HDPE (High-density Polyethylene)

• PEEK (Polyether Ether Ketone)

• Nylon (Polyamide)

• PC (Polycarbonate)

• PE (Polyethylene)

• PEI (Polyetherimide)

• PBTR (Polybutylene Terephthalate)

• PMMA (Acrylic)

• POM (Polyoxymethylene)