What Is Overmolding?

A wide variety of household items and commercial equipment are made with overmolding. From toothbrushes to culinary utensils, mirrors to portable electric fans, shampoo bottles to reusable food containers, overmolded products improve user-friendliness, ease of use, and overall lifespan. Overmolding is an injection molding process that involves a base plastic part with an additional layer of a soft, pliable thermoplastic elastomer (TPE) added over it. Manufacturers may also use other types of plastics, depending on the specific project requirements.

The overmolded material is bonded to the substrate using either a chemical or mechanical process. Chemical bonding depends on the interaction of the two materials at the molecular level and is influenced by a number of factors, including wetness; overmold and substrate temperature and viscosity; substrate texture and porosity; and surface modification. Mechanical bonding is influenced by the physical interlock between the two components and is dependent on the overmold and substrate being properly designed.

Injection mold design is critical to successful overmolding. Proper draft angles and wall thickness, as well as a uniform transition between the substrate and overmold, are important to a successful bond. In addition, the materials used must be compatible in order to achieve a desired product outcome. Generally, the overmold should be softer than the substrate in order to provide a good grip. In addition, the overmold should be able to withstand the forces of usage without tearing or breaking.

A TPE layer’s flexural modulus, the resistance to bending, also influences how well it performs in overmolding applications. High modulus materials tend to feel stiffer than thinner ones, which reduces gripping comfort. In addition, the overmold’s inherent softness is also a major factor in how it feels to the end-user.

Overmolding is also a popular solution in the automotive industry to add protection and comfort to components that are exposed to harsh conditions such as shock, vibration, and continual flexing. For example, car seats are overmolded with a cushioning material that reduces impact and vibration for the safety of passengers.

Many hardware tools are a great illustration of overmolding, including blades, pocket knives, pliers, wrenches, hammers, and tape measures. Overmolding helps manufacturers make the tools easier to hold, grip, and operate and lessens fatigue during use. In addition, overmolding can be used to incorporate corporate logos and other brand identification into the tool. This information can help buyers decide whether a particular tool is right for their business.

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