Overmolding is an injection molding process that creates parts from two or more materials. It is sometimes referred to as “in-mold assembly.” Overmolding can be roughly divided into two distinct processes, insert molding and multiple-shot molding. Insert molding (sometimes spelled insert moulding) is commonly used to add metal features to plastic parts such as threaded bosses, but it also describes the technique of coating any preformed object with plastic via injection molding. Multiple-shot molding is used to create plastic parts from multiple materials, such as polypropylene and silicone rubber, during a single molding process to improve external characteristics such as impact resistance and feel. This article looks at both kinds of processes and discusses applications where one process might be selected over the other. If you’d like to learn more about other types of molding, feel free to check out our guide on the different methods of plastic fabrication.

What is Insert Molding?
Insert molding, also known as metal insert molding or plastic insert molding, is commonly used to add metal parts to injection molded parts during molding rather than after the parts have cured, cutting down on post-molding manufacturing/assembly operations. Inserts are placed manually or automatically onto mold cores cast into the molds themselves. After the mold halves are closed, plastic pellets are plasticized in the injector barrel and shot into the mold, the plastic material flowing around the captured inserts. Once the plastic hardens, the pieces are ejected from the mold, with the inserts now encapsulated in the parts. Insert molding is a single-shot process. In many instances, inserts are threaded nuts that will be used for later part assembly, but they can as easily be bushings, sleeves, pins, blades, or even non-metallic parts. Standard inserts are available from a number of manufacturers. Many inserts have knurled outer surfaces to increase their adhesion to the plastic.

Insert molding is also the term used to describe the overmolding technique of adding a layer of plastic or rubber material onto an existing metal or plastic part. Here, too, the metal or plastic part is placed into a single-shot injection mold and the overmold material is injected around it. Single-shot overmolding uses a preformed substrate and a single-barrel injection molding machine to add an additional layer of material to the product.

Read more: Insert Molding vs. Overmolding – What’s the Difference?