Overmolding describes a form of injection molding creating components from two or more different materials. Some manufacturers refer to the process as in-mold assembly. Two distinct elements or processes are involved with overmolding techniques – multiple-shot and insert molding. Insert molding usually features in production tasks to add steel features to plastic parts, such as threads, to give them more strength. It also describes coating preformed objects with plastic surfaces using injection molding techniques. Multi-shot molding creates plastic components from several materials, such as silicone rubber and polypropylene, to improve component characteristics regarding texture and impact resistance.

In this post, we’ll cover both processes and discuss applications for each technology.

What is Insert Molding?

Insert molding is also known as plastic or metal insert molding. Manufacturers use this production technique to add metal to components during molding rather than waiting for the part to cure. The result is a reduction in operational costs for post-molding manufacturing and assembly.

Manufacturers place the inserts automatically or manually into mold cores. After closing the halves, the technician plasticizes pellets in the machine injector barrel, shooting them into the mold.

After the plastic cures and hardens, the machine ejects the parts with the inserts encapsulated in its final product form. Thus, we say insert molding is a “single-shot” manufacturing process.

Insert molding also describes an overmolding technique adding layers of rubber or plastic material onto existing plastic or metal parts. This part is also placed in a single-shot injection mold, with the overmolded materials injected around the component.

Single-shot overmolding applications require using a single-barrel injection molding machine and preformed substrate to add additional layers of material to the final product.

What is Overmolding?

The plastic overmolding technique is called multi-shot, two-shot, or double-injection plastic molding. This process adds materials like thermoplastic elastomers (TPEs) to plastics to enhance molded parts’ aesthetics, grip, handling, and impact or environmental resistance.

Overmolding is usually performed by a multi-barrel injection molding machine, with the process of adding material to the component shortly after forming the substrate to create strong material bonds.

Plastic injection overmolding eliminates steps in manufacturing processes while producing complex and durable injection-molded products.

Manufacturers can bond materials mechanically or chemically, depending on the materials chosen for the process. Manufacturers sometimes recommend mechanical bonding techniques to reinforce chemical bonding agents.

Some of the common substrate materials used in overmolding include the following.

• Polycarbonate
• ABS
• HDPE
• Nylon

Overmolded materials include thermoplastic rubber (TPR), thermoplastic elastomers like thermoplastic polyurethane (TPU), styrene-ethylene/butylene-styrene copolymer (SEBS), and thermoplastic vulcanite (TPV). Silicone overmolding is used when plastic parts require soft rubber covers for protection against damage.

Insert Molding vs. Overmolding

Insert and overmolding involve incorporating plastic or metal materials with liquid plastics to create products featuring construction with several materials. However, insert molding is only one of two overmolding techniques.

Double-shot molding involves producing a plastic injection molded part and placing it in a secondary mold for covering with a different color plastic or another grade of plastic. This process occurs during the same manufacturing run.

What are the Applications for Overmolding?

There are plenty of applications for insert molding. It’s a common manufacturing technique used in the production of electronic sockets, connectors, and many other plastic parts requiring incorporation with other materials. Insert molding provides soft grips on hand tools such as hammers and screwdrivers.

Multi-shot overmolding features in applications for high-volume personal hygiene products such as toothbrushes, combs, and disposable razors. Multi-shot overmolded products also feature in hospitals in medical instruments to provide enhanced moisture protection, cleanability, grip, and more.

Manufacturers can overmold delicate electronic circuits using the two-step molding process to encapsulate them in a low-melt-temperature resin, then add a protective shell. Vehicle manufacturers use overmolding to reduce assembly steps requiring adhesives.

For instance, they might use multi-hot overmolding on plastic motor vehicle parts featuring an overmolded rubber or elastomer seal.

Due to the extra cost involved with specialized multi-shot molding machines, the process is reserved for high-volume production applications where labor expenses can offset capital costs.

Typically, multi-shot overmolding produces economies of scale at around the 250,000+ unit mark per year. Occasionally, two-shot molding offers the only solution for creating lasting bonds between two or more materials.

One of the more common misunderstandings in applying the overmolding technique in production runs to produce a soft grip is that the overmolded material has the final role in determining the feel of the finished product.

However, this is not the case. The applied layers’ thickness plays a significant role in the strength of the final product. If the layer is too thin, the user will feel the harder substrate through the soft topcoat.

Wrapping Up