Overmolding overview

Editor's note: The following information on overmolding was taken from a report on the subject created by Lonnie Tustison, tooling manager of Tooling Express Inc. (Bellevue, ID).

Just as it can help a product differentiate itself, overmolding or insert molding can help a molding shop stand out in the plastics processing crowd as well. Molding softer materials like TPE over metals or other plastics can enhance the grip and ergonomics of a product as well as make it visually appealing without sacrificing structural strength. Despite the many potential benefits of adopting the process, there are several areas processors should examine before they decide to take on overmolding themselves.

Material compatibility
Virtually any material can be overmolded onto another one, but that doesn't mean they'll stay that way. If chemical reactions occur between the different materials that affect adhesion, one material will eventually begin to peel away from the other. Make sure the materials you're looking to join are indeed compatible.

Consistent insert tolerances
If there are any inconsistencies with the size and shape of the inserts, this will adversely affect the end product. In the case of machined steel or metal inserts, any variation in insert dimensions will affect how it is seated in the mold prior to resin injection. In the case of one thermoplastic being overmolded onto another, inconsistencies in the size and shape of the inner component can also affect the end product. Molds are designed to compensate for material shrinkage, but if the final tolerances for a part are in the area of ±.005 inch, there isn't much room for error. Molders should remember that some materials take several days to cure. In addition to not overmolding these parts until they've reached their final dimensions, care needs to be taken in their handling and storage.

Potential applications
The ability to enhance the ergonomics of a wide variety of products has made the overmolding of soft-grip materials over hard structural materials standard practice in everyday items like toothbrushes and kitchen utensils. Not only is greater functionality added, but product aesthetics are usually increased as well. In terms of product assembly, the insert molding of metal threads can increase the life span of components that require screws for disassembly. Metal threads also provide greater strength and durability.

In truth, any application in which a contrast of colors and/or textures would help draw consumer interest to a product is a likely candidate for insert or overmolding. It's up to molders to examine their business and account for all the different variables.