A Complete Guide to Overmolding vs Insert Molding

In this article, we discuss insert molding and overmolding, the advantages of each, and the applications they’re best suited for. We hope these manufacturing insights, along with useful design tips, will help you get great results on your next plastic injection molding project.

Navigate to these sections in the article:

• What is Overmolding?
• What is Insert Molding?
• Insert Molding vs Overmolding Video Explanation
• 6 Advantages of Insert Molding
• Drawbacks of Insert Molding
• Advantages of Overmolding
• Drawbacks of Overmolding
• How to Choose Between Insert Molding or Overmolding for Your Project
• When to use Insert Molding
• When to use Overmolding
• A Brief Guide to Designing for Insert Molding and Overmolding
• Overmolding Design Guide
• Insert Molding Design Guide

What is Overmolding?

Overmolding is a special form of plastic injection molding. Unlike conventional molding, where a finished product is made in one cycle, overmolding requires at least one additional injection cycle using a dissimilar material. This method is also known as 2K overmolding, two-shot molding, or even multiple material molding. 

Overmolding starts with a base substrate, on top of which a relatively softer TPE or TPU material is bonded. The base can be metal or plastic, although, in this article, we will focus on plastic applications primarily.

If the substrate is a plastic base, then it’s customary to mold it in the same production cycle. This is done because it’s more cost-effective to make the entire finished piece at one time, rather than paying to transport and store semi-finished goods. But the more important reason is that the elastomer will adhere much better to a still-warm, semi-cured, and “sticky” substrate.

Overmolding uses a special type of plastic injection molding tool, one that has a cavity for the substrate portion and another separate cavity representing the overmolded section. 

The first conventional injection cycle forms the substrate, using a standard thermoforming resin. This resin fills the gate, runner, and cavity system of only one part of the mold, while the section representing the overmold is blocked.

Once the base cools and is semi-hardened, the mold is then rotated to a new position and connected to a separate nozzle, gate, and runner system. The remaining void of the mold tool cavity is filled with TPE or TPU elastomer, creating the overmold that bonds to the substrate.

The process is not limited to just two materials, and can theoretically be expanded to include an indefinite number of additional overmolds. However, in practice, more than two becomes increasingly complex and expensive.

What is Insert Molding?

Insert molding involves placing a pre-manufactured component, such as a threaded fitting or an electrical contact, onto a mounting pin or other fixture in a plastic injection molding tool. During the molding cycle, thermoforming resin surrounds this insert and solidifies, thereby permanently sealing it into place.

Insert Molding vs Overmolding Video Explanation

In this video, Gordon Styles, the founder and President of Star Rapid, goes into greater detail explaining how these processes work, and he gives examples of each. Be sure to check it out for more useful information.

6 Advantages of Insert Molding

Inserts can be either male or female. Female inserts are used to make circular fittings inside the surrounding plastic. When these types of inserts have internal threads, they’re called nutserts. There are many advantages to using both male and female inserts to enhance a product’s appearance and utility. 

Read the 6 advantages of insert molding:

1. Use threaded inserts to create strong and reusable connections. The nutserts themselves add the expense to the build, but that cost is offset by the fact that they help to avoid secondary drilling and tapping, which also takes time and money.
2. Use inserts for thin-walled cases that cannot be tapped. 
3. Use inserts in conventional machines. There is no need to invest in equipment upgrades. 
4. Seal components permanently into place to protect against pull-out, vibration, moisture, and dust. This is a great way to protect electrical connections, for example.
5. Make strong overmolded hand grips, such as found on hand tools. This helps to create a complete finished part in one quick and economical operation.
6. Reduce weight and save costs in complex assemblies. For example, consider a common toggle switch or lever. By using metal inside the body of the switch only where it’s needed to make a mechanical contact, and substituting the remainder of the switch for plastic, the weight of the entire assembly is thereby reduced. This approach, not incidentally, also decreases the reliance on a more expensive, full metal switch.

Now, if that design logic is applied to a fleet of aircraft or millions of automobiles on an assembly line, then the reduction in weight and cost becomes substantial.

Drawbacks of Insert Molding

While there are many advantages to insert molding, there are some potential drawbacks that designers should be aware of.

• Labor costs are higher when inserted by hand.
• Cycle times will vary when handloading. Why is this important? For plastic injection molding, the single most important variable that must be carefully controlled for maximum cost savings is machine cycling time or the time it takes to make each part. If this time is variable, it becomes impossible to control costs or accurately predict product ship dates. For relatively smaller volumes, say below 20K per year, it still makes economic sense to use handloading rather than invest in expensive automation. But as volumes increase it eventually becomes more cost-effective to use automation, which will pay for itself by effectively decreasing the cost of labor.
• Resin can shrink around the insert, causing it to crack. This is especially a problem with polycarbonate plastic. Later, we will discuss ways to fix this.
• Resin can be forced inside the insert because of molding pressure. When this happens, the excess flash must be cleaned out in a separate step.
• The tool design must be compatible with the insert. If they don’t fit together correctly when the mold is closed and under injection pressure, the mold can be damaged or destroyed.

Advantages of Overmolding

There are many reasons why product designers choose to add overmolding to plastic parts. Here are the main advantages.

• Add a comfortable grip to bare plastic. But this is done not just for comfort. Some elastomers also absorb unwanted vibration while protecting users from excessive heat or electrical shock.
• Improve the appearance of your parts. The elastomer material can be molded with a complementary or contrasting color to enhance the appearance of the overall product. Creative designers can take advantage of overmolding to make embossed or debossed areas that feature a company’s unique name, insignia, or logo for branding purposes.
• Make strong, permanent hand grips. It creates a strong bond with the substrate, often stronger than the shear strength of the TPE itself. This means that overmolded grips won’t just randomly fall or slip off. That’s why you’ll find them on all types of sporting goods and power tools, for example, because overmolds withstand hard use and wet conditions without separating from the base.

Drawbacks of Overmolding

To get the best results from this process, product developers should be aware of the following limitations and potential disadvantages.

• The process requires special molds or two separate tools. In both cases, this increases the development costs and it takes more time to build these tools compared to conventional single-shot molding.
• Overmolding requires specialized equipment. Many suppliers may not have access to this type of equipment or will charge more for this service.
• The process is slower than conventional molding. It may take more manual labor, and it ties up two pieces of production equipment to make a one finished part. But it’s still cost-effective when working with relatively small volumes.
• Some elastomers don’t adhere to some types of base plastic. We urge you to work closely with your supplier well in advance of production to confirm the suitability of your material choices or ask for their help in finding alternatives.

How to Choose Between Insert Molding or Overmolding for Your Project

Which process is right for you depends entirely on your application. Keep in mind that they aren’t mutually exclusive molding methods because they serve different purposes. In fact, it’s not uncommon to find both used on a single product. Think, for example, of a power tool that has overmolded grips on the handle, but that also uses nutserts to bolt the case together.

When to Use Insert Molding

You should consider using insert molding for a few main reasons. The first is that your product application needs strong mechanical fittings to hold two or more parts together, typically for an enclosure or housing. Threaded nutserts are great for this but there are many types of snap connectors as well. 

Insert molding is used to put a rubber or plastic handle on a metal part, such as you might find on hand tools or kitchen knives. And insert molding is ideal for sealing wires and electrical connectors into permanent plastic housings that block out dust and moisture.

Inserts should be used whenever a plastic case needs to be occasionally opened for service and then locked tight again, such as when performing routine maintenance or changing batteries. They cost a little more but that cost is offset by making a much more durable end product.

When to Use Overmolding

Product developers should consider overmolding in order to improve the grip and texture on parts that need to be held in the hand or to protect the end user against vibration, heat, or electricity. Overmolding is also a great way to permanently bond rubber to metal, as is found on wheels and casters.

Overmolding also offers more opportunities to be creative by applying colorful designs to improve the appearance of the part as well as its performance.

And overmolding adds cushioning and shock absorption to many common household items that might protect users from accidental injury.

A Brief Guide to Designing for Insert Molding and Overmolding

In this section, we highlight important points between designing for overmolding and insert molding.

Overmolding Design Guide

Remember that overmolding is rarely used to cover the entire base substrate. Instead, apply overmolding in sections. For each of these, keep in mind these useful design tips:

• Work with your manufacturing partner to ensure the compatibility between the substrate and the TPE or TPU.
• Use a TPE or TPU that has a melt temperature lower than that of the base plastic.
• Apply an overmold that is slightly thinner than the substrate which supports it.
• Design the overmold to sit just below the surface of the substrate.
• If you need more holding power, design undercuts, keyways, and other mechanical features to lock the materials together.

Insert Molding Design Guide

There are some unique engineering challenges that designers might face when using insert molding. The advantages of added strength and versatility must be weighed against the need for a more careful design for manufacturing review. Here are some useful guidelines to consider:

• The single biggest concern is the shrinkage of the resin. This creates hoop stress around the fitting that can cause cracking over time, especially if the part is under mechanical tension. This is how you can counteract this:
  • Use resin material with a relatively low shrinkage rate;
  • Use resin material that has been strengthened with fillers;
  • Surround the insert with a larger area of plastic;
  • Support the insert with bosses and ribs;
  • Preheat the inserts before molding. This allows the resin and the insert to cool and shrink in tandem, thus relieving some of the stress between the materials.

• Use pre-manufactured inserts that have knurled surfaces to help lock them in place.
• Avoid sharp corners and use rounded profiles instead to reduce stress.
• Design the inserts so that they’re slightly recessed. This helps to avoid damaging the tool.

Common Applications of Overmolding and Insert Molding

Neither overmolding nor insert molding is restricted in their use to any particular industry or product type. Because both methods are so useful and adaptable, new practical applications are being found all the time that take advantage of their benefits. Here are a few examples:

• Hand and power tools
• Lawn and garden equipment
• Agricultural equipment
• Kitchen utensils
• Sporting goods
• Electrical switches
• Health and beauty products
• Home appliances
• Automotive / transportation
• Furniture
• Computers and electronics
• Toys and games
• Medical instruments

This is just a partial overview of insert molding and overmolding. Are they the right solutions for you? Not sure how to apply them to your next project? Just contact our technical experts and we can offer helpful advice about how to get the best results from both processes.

If you liked reading this article, we recommend the following content:

• Tips for Moving Your Product from Prototype to Production
• Surface Finishes for Your Product
• For more on high volume manufacturing, take a look at Production Tooling