Insert Molding vs. Overmolding – What’s the Difference?
Unsure of the difference between insert molding and overmolding? Here is an explanation.
According to Thomas Net:
“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 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.
“Insert molding is used to add usually 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, etc. 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.
Double Shot Overmolding
“Plastic overmolding, sometimes called two-shot or multi-shot molding, adds additional material such as thermoplastic elastomer (TPE) to plastic substrates to improve the aesthetics, handling characteristics, grip, impact resistance, environmental resistance, etc. of injection molded parts. Multi-shot molding is usually performed by way of multi-barrel injection molding machines, the process adding additional material shortly after the substrate is formed to create strong bonds between materials. Plastic overmolding serves to eliminate steps in the manufacturing process while creating enhanced injection molded products.
“Depending on the materials selected for the substrate and overmold, materials may be bonded chemically or mechanically (via undercuts, for example). Various published charts detail the compatibility of a range of substrate and overmold materials for chemical adhesion. In many instances, mechanical bonding is recommended to fortify any chemical bonding that may take place. Among the many substrate materials that are suited to overmolding, polycarbonate, ABS, HDPE, and nylon are common. Overmold materials include thermoplastic polyurethane (TPU), styrene-ethylene/butylene-styrene copolymer (SEBS), thermoplastic rubber (TPR), thermoplastic vulcanite (TPV), etc.
“Insert molding is a common manufacturing method for producing connectors, electronic sockets, etc. in addition to its pervasive application in making parts that are assembled with threaded fasteners. Insert molding is used to provide soft grips on many hand tools such as screwdrivers, surgical implements, etc.
“Multi-shot overmolding has many applications in high volume personal products such as toothbrushes and disposable razors, in addition to being useful for many higher-value products such as medical instruments where it can provide enhanced grip, cleanability, moisture protection, and so on. Delicate electronic circuits can be overmolded using a two-step process that first encapsulates the circuit in low-melt-temp resin and then adds a higher temperature protective shell. Automobile manufacturers use overmolding to reduce the need for additional assembly steps that would require using adhesives. A typical application might be a plastic engine part with an overmolded elastomer seal.
“Many injection molding companies provide both insert molding and two-shot overmolding in their service repertoires. Because of the additional cost involved in special multi-shot machinery, two-shot overmolding is normally used for high volume production where the high capital costs are offset by reduced labor expenses. Generally, multi-shot molding makes economic sense at parts volumes of 250,000+ per year. Sometimes, too, two-shot molding is the only way to create an acceptable bond between different materials.
“A common misunderstanding in applying overmolding to achieve soft grips is that the softness of the overmold material plays the only role in the feel of the final product. The thickness of the applied layer plays a big part also as too thin a layer will permit the harder substrate to be felt through the softer top coat.
“Overmolding is a complex endeavor, especially in light of the many variables that come into play and the possibility of reproducing mistakes in countless injection-molded copies. Companies that provide overmolding services should be consulted in the early stages of part design as their expertise in the various processes can help avoid the many pitfalls that are ready and able to hamper the realization of a successful part.
“In designing for overmolded parts, a number of rules of thumb apply: Overmold edges should not feather but should end abruptly and at full thickness against a stop or into an indentation. Surface texture on the overmold can significantly aid in releasing the part from the mold. It also gives the impression that the part feels softer than it actually is while disguising any flaws from the molding process. Ample draft angles help also in mold release. Mold designers need to give careful attention to gate and vent design.
“This article presented a brief discussion of the differences between insert molding and multi-shot overmolding. For more information on related products, consult our other guides or visit the Thomas Supplier Discovery Platform to locate potential sources of supply or view details on specific products.”