Companies that want to use injection molding processes and similar strategies for complex parts development should know about some of the most popular techniques in today's industry. Two of these are called insert molding and over molding. Both of these processes have gotten quite a bit of attention as manufacturing companies innovate their processes to fit the technologies of the 21st century.

These two slightly different processes often get confused with each other -- there is a lot of back and forth about which one to use for any particular project, and how they differ. Knowing a little more about how the industry treats these two terms can help busy project managers to understand what's best for a certain assembly line or manufacturing process.

Insert Molding

The idea in insert molding is that one part or piece is specifically manufactured to fit inside a larger injection molding design, as an inner piece or component.

So the original part or “insert” is handed over to another assembly space, and it gets integrated into the bigger injection molded component. The plastic molding flows around the original piece to encapsulate or connect with it in some way. The finished product is composed of that original piece, or multiple pieces, precision molded into the larger plastic product.

Experts commonly cite examples such as metal knives or scalpels with injection molded plastic exterior pieces, or items like screws or fasteners molded into the surface of a plastic design.

Over Molding

In over molding, the process is very similar in some ways. Over molding happens when one original piece is injection molded, and a secondary injection molding process goes over top.

One way to think about this is in a project that involves manufacturing of a plastic utensil or consumer product with more than one plastic surface. The original part is injection molded first, and the secondary part is poured over it. That's why they call this over molding.

Think of a plastic item like a mop, toothbrush or snow scraper for your car, with an original solid plastic body and a soft grip injection molded piece around the handle. This is one of the most common uses of over molding, and is a good example of how this process works. This article from Composite World also goes into detail on modern over molding processes and how valuable these have become to the manufacturing community.

Judging by Project Materials

Some experts tend to simplify the use of the terms insert molding and over molding, saying that in general, insert molding is for plastic on metal, and over molding is for plastic on plastic.

However, the reality is a bit more nuanced. Individual products may be made with multiple plastics, or a combination of plastic and metal, composite, or different types of polymers or acrylic materials. So it really make sense to evaluate insert molding and over molding on a case-by-case basis.

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Evaluating Assembly Processes

One detail that may affect the use of insert molding or over molding processes is the specific assembly line approach that a company uses to create a finished product.

If the original molded part has to go through entirely separate line assemblies, that may suggest that insert molding rather than over molding is going to be the desired technique.

Think about whether the original part must go through multiple product checks, evaluations or ancillary processes before being molded into a larger piece. For example, going back to the metal screw or fastener, you might see that this metal piece goes to a different department for processes like polishing, deburring, heat sealing or annealing before receiving the additional injection molding. This is also a good example of the insert molding technique.

By contrast, products made of plastic components may be able to be made in a streamlined way, where over molding is very simply and practically applied to the initial product. Again, the example of a molded grip or soft touch contoured surface being placed over an original hard body is an excellent example of an over molding process.

More Examples

For other examples of insert molding processes, think about all of the plastic parts and pieces of equipment for industries like automotive, medical, home improvement or tool assembly, where small pieces like bushings, clips, serrated edges or fasteners are inserted into a plastic framework or structural design, where again, the plastic is poured around the small pieces to create a complex product.

Over time, companies that pursue both of these techniques will start to learn what a typical insert molding project looks like, and how to fine tune these processes to make production more precise and ensure higher quality results.

As for over molding, companies will often use this technique on products with multiple plastic components that complement each other well.

For example, a two-part process involving a rubber-based silicone product or heavy thermoplastic molded over an interior plastic material constitutes an example of over molding. Another example is the process of applying two or more layers of plastic to cables, connectors or other manufactured product.

Another way to think about the use of over molding is to consider the utility of the secondary over molded product. For example, over molding is often done simply to weatherize or ruggedize some particular product, to make it chemical resistant or weather resistant, or to add layers without adding a lot of weight -- which is evident when you look at the move away from metal-based design in previous decades to lightweight plastic builds today. In fact, the lightweight nature of the build is something that drives many over molding designs.

Another typical use of over molding is for a textured feel or soft grip as mentioned above, or to insulate electrical conduits for safety purposes.

Think about all of these aspects of insert molding and over molding when designing a manufacturing process. Weiss-Aug offers a lot of skill and experience in general parts production processes. Ask about what this company can do to help you build a quality supply chain and serve customers effectively.