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.
Assembly in injection molding can also be achieved through overmolding and insert molding. These two processes are similar, but have some significant differences. By choosing the right one, and designing for the process, it’s possible to lower costs and assemble two or more components in one step.
Overmolding and insert molding can save on assembly costs while making higher quality products with features that increase customer appeal. Resin selection, part design and tooling tend to be 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.
Overmolding is a method of injection molding where one layer of plastic, typically a thermoplastic elastomer (TPE) or similar material, is molded over a previously manufactured substrate. Think of it as applying an outer covering to a part molded in a hard plastic, creating a seamless combination of multiple materials into a single part.
The key to overmolding is to have 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 cover and substrate material, the bond between the two will be low. A designer can address this by incorporating mechanical keys into the substrate, ranging from a textured surface to holes and undercuts. For maximum strength, engineers design with a combination of chemical and mechanical bonding.
Overmolding is performed in two ways: two-shot overmolding and pick-and-place overmolding.
Two-shot overmolding produces both the substrate and the outer cover in the same tool on the same machine. The substrate is injected and formed first. Next, either the tool is adjusted, or the part is moved to a second cavity, and then the second material is injected. This takes very specialized equipment and complex, expensive tools, but can be cost-effective for large production quantities. It can result in a stronger bond than a pick-and-place process.
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, where it is located by pins in the mold. The mold then closes and the second material is injected around the substrate. The process inquiries two two separate molds, and the substrate must be cooled before molding to increase bond strength.
Similar to overmolding, insert molding requires a part to be placed in the cavity of an injection molding tool so plastic can be molded around it. The big difference here is the materials used.
In insert molding, the inserts are usually metal components, 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 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 are also cameras installed to verify the correct insert placement before the mold closes.
To sum it up, overmolding is a specialized form of insert molding. The differences are more in the materials and product function than the process.
Reasons for overmolding include:
From this list, you might notice that overmolding can be more about appearance and feel than actual function. In contrast, insert molding is done primarily to provide a specific function.
Reasons for insert molding include:
Overmolding and insert molding are used to produce a very wide range of products in many different industries. Here are a few examples:
Overmolding is used to add a TPE to metal carriers for seals and gaskets in a variety of HVAC, renewable energy and building efficiency applications.
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.
Overmolding produces plastic housings around cables where they enter or exit components like connectors and sensors in electronic applications. Insert molding produces housings with encapsulated metal thread inserts for attaching covers and brackets.
Hand tools and kitchen equipment are examples of products where overmolding is often used to provide cushioning and enhance grip.
Exterior and interior trim pieces containing logos and with reinforcement/mounting brackets are both overmolded and insert molded. Functional gears for window and sunroof mechanisms are also insert molded.
Designing parts for production with 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. PMT has niche expertise in both under molding and overmolding, and our experienced team can help ensure products are optimized for these processes.
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 by applying a plastic material 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 how overmolding or insert molding can help you make better products at a lower cost, PMT can help. Contact us today to discuss your next project.