Exploring the Overmolding Process: Definition, and Techniques

A subset of injection molding, the overmolding process involves a number of steps: the substrate must first be produced, after which the material to overmold it is injected. But a lot more goes into overmolding parts than the mentioned steps, as you’ll find out below. Apart from explaining the overmolding process step by step, well also see example applications of the process.

Let’s begin with the overmolding definition. After that, well delve into the steps that the process takes, before looking at the process’s applications examples and, finally, comparing overmolding with insert molding.

What is Overmolding in Injection Molding?

Overmolding is the process where, using the injection molding technique, an already molded part is covered or overmolded with another plastic material. Other names given to the process, and which you might have come across, include 2K overmolding and two-shot overmolding.

The aim of the injection overmolding process is to introduce a material of different properties than the base material or substrate, either for decorative or functional reasons. Think about this; a hand tool must offer a firm grip, and the plastic covering its handle may not provide that. So a different material, typically rubber, is used to overmold it.

After the overmolding meaning, let’s now see what materials are needed, or used, to overmold parts. Also, the different methods of the process that your manufacture may use to cover inserts with resin or rubber.

Overmolding Materials

To overmold parts, manufacturers may use different material. These are materials that can be injection molded and usually thermoplastics. Here is a list of overmolding materials that are commonly being utilized today. Note that one or more of these may be used at the same time, depending on the required outcomes.

• Acrylonitrile Butadiene Styrene (ABS)
• Nylon (Polyamide)
• Polypropylene (PP)
• High-density Polyethylene (HDPE)
• Polycarbonate (PC)
• Polyether Ether Ketone (PEEK)
• Polyethylene (PE)
• Thermoplastic Polyurethane (TPU)
• Thermoplastic Rubber (TPR)
• Silicone (SI)

Overmolding Techniques

Overmolding can be done in two different ways: using a pick and place or manual method, and using what’s commonly called a two-shot overmolding process. Let’s see how the two overmolding techniques differ.

In the pick and place method, two different molds are used, one for the substrate and another the overmolding process. The item or product is first injection molded in the first mold, then transferred to the second mold where similar steps are used to cover it with resin.

In the two shot method, everything happens in the same mold. This can be done in these 3 different ways: rotational, transfer, and core back overmolding. The two-shot overmolding technique, although quicker and more effective, requires an expensive mold and other machinery.

Overmolding Process Step by Step

The overmolding process can be divided into 3 distinct steps: injected molding the part to be overmolded, placing the part into a mold, injecting the molten overmolding plastic, and ejecting the overmolded part.

Step 1: Molding Insert Part

Overmolding is an injection molding subset. So the part to be overmolded is first normally molded using conventional injection molding steps. This involves injecting a resin melt, under pressure, into a preformed mold that contains the shape of the required product.

Depending on the type of overmold process being used, the molded part is left in the mold to be covered with another plastic, or removed and placed in another mold as an insert before being covered with the secondary molding material.

Step 2: Injecting Plastic Behind Part

The earlier molded part, while still warm, is injection molded again, this time with a different material. As said before, this can happen without having to remove it from the mold, using a different gate and runner system to introduce the melted plastic, or by first removing it and inserting it into another mold.

After the parts have been overmolded, they’re then allowed to cool. The bond formed between the two dissimilar resins is then a tough one, seeing the two materials are still warm and sticky when they bond. If needed, a mechanical bond may also be made, usually using interlocks.

Step 3: Ejecting Overmolded Part

Once the plastic has cooled, it’s time to eject the overmolded part. This is the last major stage of plastic overmolding process. Final touches are then performed on the finished product, just as they do when carrying out the injection molding process. These can involve trimming the part to remove excess plastic, or examining it for defects.

Overmolding Examples

Overmolding injection molding is done using different materials for both the substrate and the overmolding material, usually depending on the required results. These include plastic over plastic and rubber to plastic for either visual appeal or to provide specific product characteristics.

Plastic Overmolding

In plastic overmolding, a plastic substrate or part is overmolded with a different type of plastic. An example is when you want a product to have different colors, or when a soft plastic is used to cover another for functional and usability reasons.

Overmolding Rubber to Plastic

This involves injection molding rubber to cover a plastic substrate with rubber done using the two-shot molding technique. Although reasons may vary, overmolding rubber to plastic is mostly done to make a part easier to grip.

This can be the handle of a hand tool, for example, or the handle of a toothbrush. Other examples of rubber overmolding include:

• Electrical cable connectors
• Personal care products such as razors
• Medical devices
• Automotive industry parts
• Consumer electronics

Insert Molding vs. Overmolding

The overmolding process, as we have seen, concerns covering an injection molded part with a secondary plastic material, mostly with reason to decorate it or make it more usable. But how is it different from the insert molding process?

While both are injection molding techniques, insert molding and injection overmolding differ in a number of ways. First, insert molding involves covering a part with plastic and using the injection molding method, and this can be a plastic, metal, or even wood.

The injection overmolding process involves using a plastic part as the substrate, which is usually initially made using the molding plastic. To fully understand what that means and the steps involved, check out our insert molding process guide.

Conclusion

The injection overmolding process is one of the most common ways of using injection molding to make parts. The aim is usually to include different characteristics than those off the base material, which can range from making the product more attractive to making it more functional. We hope this article has helped you understand the overmolding process more deeply.