What is the difference between insert molding and two-shot overmolding?

Do you know the difference between insert molding and two-shot overmolding? Prepare to amaze your friends and baffle your enemies as we take a deep breath and plunge right in.


Hello. This is Gordon Styles, the founder and CEO of Star Rapid. I’ve been involved in rapid prototyping and new product development for, oh, I don’t know, a long time, and I want to welcome you to another episode of Serious Engineering for serious engineers.

There are a couple of ways that plastic injection molding can be used to combine two (or more) materials into a single part. How can this help a product developer? Let’s take a closer look at the advantages of insert molding and overmolding.

What is insert molding?
Insert molding means to place one component – the insert – onto a pin or other holding fixture inside of a plastic injection molding tool. When the mold closes, thermoplastic resin is then molded around the insert, permanently sealing it into place. The insert is usually a small metal or plastic piece like a threaded screw fitting or an electrical contact. By the way, threaded fittings are called ‘nutserts’ – and no, we’re not going to make a pun about that.

Using inserts in this way, especially for screw fittings, makes sense because it avoids the need for extra drilling and tapping which takes time and money. And inserts can also be used on thin-walled cases that otherwise wouldn’t be thick enough to allow for a tapped hole. Most importantly, though, is the fact that insert molding works with conventional injection molding machines so there’s no need to invest in equipment upgrades.

Are there drawbacks to insert molding?
There are a couple of concerns when using inserts. First, if they’re installed by hand, it slows down production time which in turn increases labor costs.

Second, if holding strength is especially a concern then the plastic case might need to be designed so that it locks the insert in place.

And finally, it’s possible that cracks can form around the insert due to stress. This is because the resin shrinks while the metal does not. This is especially a problem with polycarbonate plastic.

As a general rule of thumb, hand-loaded inserts make sense if the annual production volume is about 50K ~100K or so. That’s because it’s cost effective to use manual labor for lower volumes, but beyond that it probably makes more sense to invest in automation.

What is overmolding?

Overmolding, also known as two-shot, 2K or multiple material molding, is a type of plastic injection molding where a relatively soft rubber or elastomer such as TPU or TPE is permanently bonded to a more rigid plastic substrate. Rubber can also be bonded to metal parts, for example in caster wheels or machine mounts.

But that technique is not exactly injection molding so we’re going to stick with plastic. Stick, get it? Overmolding? Ah, never mind.

Ok, why do product developers use overmolding? Well, there are many potential advantages and applications. For example, overmolding can improve the appearance of many products. This is helped by the fact that the overmolded material doesn’t need to be the same color as the substrate but instead can be a contrasting or complementary color. You’ll often find this on toothbrush handles, for example.

Overmoldings provide texture that improves the grip and feel for the user. This is common on power tools or cooking utensils.

Overmoldings can also reproduce a designer’s logo or trademark image on the product.

Functionally speaking, overmoldings offer electrical and thermal insulation and they make air- and watertight seals for cases. And the process can be used to encapsulate other components, holding them firmly in place to prevent mechanical vibration.

How does overmolding work?

Overmolding is done with a special injection molding machine that has two barrels. One barrel typically holds a rigid thermoplastic plastic, and the other has some form of elastomer such as urethane or silicone.

But it’s not just the machine that’s unique – so is the design of the tools.

In a conventional mold tool, there might be one cavity and one core, with the two halves together forming the final shape of the part. But with overmolding we only want to create a partial shape of the rigid plastic substrate in the first cycle. Therefore, multi-cavity tools must be made to accomodate two different injection cycles.

In cycle #1, thermoplastic resin is injected to form the shape of the substrate. After the mold opens, it’s possible to either automatically rotate the B-side of the mold (which is the core half), or the partially finished piece can also be moved by hand from one core to another. In either case it’s necessary to maintain the shape of the substrate on a rigid core because it’s still warm and therefore soft and easily damaged.

Then the mold is closed a second time and rubber resin is injected to fill the gaps in the substrate.

What are the advantages of two-shot molding?
The main advantage of two-shot molding is that the bond between the materials is very strong, often exceeding the shear strength of the TPE. This is helped by the fact that both materials are still hot and not fully cured when they’re joined so they’re effectively glued together.

Also, different Shore hardnesses and colors can be mixed to achieve a variety of effects. And the process is largely automated so it’s ideal for large production runs.

What are the drawbacks of two-shot molding?
The design of mold tools is more complex, so it takes more time and costs more money. Also, dedicated equipment is necessary, although it is possible to add an auxiliary “piggyback” molder to convert a conventional machine to two-shot molding.

The most important consideration, however, is to make sure that your choice of elastomer is compatible with the thermoplastic substrate. For example, nylon, PEEK, ABS and some other engineering grades can be difficult to form a good bond with. To deal with this it’s possible to use plasma surface preparation and other treatments that help improve bonding, and we’ll talk about those in another video.

Remember, to get the best results you need to work closely with your manufacturing partner in the early planning stages to optimize tool designs while also ensuring that you use the right materials and process parameters.

Ok, that’s all the time we have for today. Remember to ring the bell, like us and subscribe. We’ll be back soon with another fascinating and fun-filled episode of Serious Engineering for serious engineers. See you then!