Injection molding is an excellent method of producing plastic parts. It’s capable of very high-volume production, and part-to-part variation is practically zero, but did you know it can also perform assembly?
Assembly in injection molding is done by overmolding and insert molding. These two processes are similar but also have some significant differences. By choosing the right one and designing for the process, it’s possible to lower costs and make better products.
More specifically, overmolding and insert molding can save on assembly while making higher-quality products with features that increase customer appeal. However, resin selection, part design, and tooling are more complex than regular injection molding. So, overmolding vs. insert molding—what’s the difference? Here’s what you should know about the two processes:
What is overmolding?
The term describes the process. It’s a method of injection molding a thermoplastic elastomer (TPE) over a previously manufactured substrate. Think of it as applying a soft, flexible outer covering to something molded in a hard plastic.
The key to overmolding is having the outer cover bond chemically to the substrate. To achieve this, the two materials should be compatible, which is why the substrate is almost always some type of polymer resin.
If there’s little or no compatibility between the cover and substrate, the bond
Between the two, it will be low. A designer can address this by incorporating mechanical keys into the substrate for the TPE to lock into. These can range from a textured surface to holes and undercuts. For maximum strength, use a combination of chemical and mechanical bonding.
Overmolding is performed in two ways. There’s two-shot overmolding and pick-and-place overmolding.
Two-Shot Overmolding
Two-shot overmolding produces both the substrate and the outer cover with the same tool on the same machine. The substrate is injected and formed first, then either the tool is adjusted or the part moved to a second cavity, and the TPE is injected. This requires very specialized equipment and complex, expensive tools but can be cost-effective for large production quantities. It results in a stronger bond than pick-and-place because the substrate is always warm and clean. 2-Shot Overmolding
Pick-and-Place Overmolding
In pick-and-place, the substrate is molded on a different tool and brought to the overmolding machine. When the mold opens, the substrate is placed into the cavity, which is secured by pins. The mold then closes, and the TPE is injected around the substrate. The process is labor-intensive and needs two separate molds. In addition, cooling the substrate before molding reduces bond strength. For the same reason, it’s also very important to keep the substrates clean.
What is insert molding?
Similar to overmolding, in insert molding, a piece or pieces are placed in the cavity of an injection molding tool so plastic can be molded around it or them. The big difference is in the materials used.
In insert molding, the inserts are usually metal, although they could be other hard materials. The overmolded plastic is typically a rigid polymer resin. While some material combinations may form a bond, insert molding generally relies on the inserts being mechanically captured by the plastic. To aid this, inserts are textured or have features for the resin to engage with.
Insert molding entails placing the inserts into the mold tool, where they are held in place by pins. In low-volume production, this placement is done manually. For high-volume production, robotic placement is faster, cheaper, and more reliable. On some machines, there may be a camera to verify the correct inserts are in the right places before the mold closes. Insert Molding
Overmolding vs. Insert Molding: What’s the Difference?
From the process descriptions above, it should be clear that overmolding is a specialized form of insert molding. The differences are more in the materials and product function than the process.
Reasons for performing overmolding include:
- Applying a soft, textured cover to a harder substrate to improve feel or grip
- Combining two materials with different properties to attain the characteristics of both
- Eliminating an assembly step
- Provide a decorative feature, such as surrounding raised lettering in the substrate with a softer TPE.
- Create contrast in appearance and/or feel between two surfaces.
From this list, you might notice that overmolding is more about appearance and feel than actual function. In contrast, insert molding is done primarily to provide a specific function.
For example:
- to incorporate threaded metal inserts to accept screws, which increases fastener security and facilitates opening and repair.
- holding and sealing electrical components
- Adding strength in specific locations with reinforcement or mounting brackets
- Providing a hard, wear-resistant surface
- Incorporating heat sinks
Overmolding and Insert Molding Industry Applications
Overmolding and insert molding are used to produce a very wide range of products in many different industries. Here are a few examples:
Industrial
Overmolding in the industrial industry is used to add TPE to metal carriers for seals and gaskets.
Medical & Healthcare
Medical overmolding provides soft-feel grips that can survive rigorous cleaning and sterilization regimens. Toothbrushes are similar, with overmolding producing a soft and attractive grip over a more rigid substrate.
Electronics
Overmolding produces plastic housings around cables where they enter and exit components like connectors and sensors in electronic applications. Insert molding produces housings with encapsulated metal thread inserts for attaching covers and brackets.
Consumer and Recreational
Hand tools and kitchen equipment are examples of products where overmolding is often used to provide cushioning and enhance grip.
Automotive
Overmolded and insert molded exterior and interior trim pieces with logos and reinforcement or mounting brackets.
Designing for Overmolding and Insert Molding
Designing parts for production by one of these processes is more complicated than designing a single material piece for injection molding. There’s more to consider during resin selection: the product must be designed to suit the process, and the molding tools are more complex. For these reasons, it’s advisable to work with an expert in resins and molding at the design stage.
Learn how overmolding and insert molding could benefit your business.
Overmolding and insert molding extend the capabilities of injection molding. In particular, they can perform assembly functions and eliminate secondary operations in the process. This is either done by applying a soft TPE over the top of a hard substrate or by incorporating inserts of metal and other hard materials into an injection-molded part. The benefits are improved product appearance and feel, coupled with lower assembly costs.
If you’d like to explore whether overmolding or insert molding could help you make better products at a lower cost, V1 Plastics can help. Contact us to discuss your application.
