What Is Insert Molding?

Sometimes the problem isn’t the part. It’s the process.

Years ago, I kept getting complaints from customers about loose connections in one of our plastic assemblies. I’d tested the design. I trusted the supplier. But something was off.

Turned out, we needed to rethink how those parts were made not just how they fit.

That’s how I learned about insert molding.

If you’re here, you might be wondering: What is insert molding? And will it fix the problems I’m facing?

This article will help you answer that. We’ll walk through what insert molding actually is, when it helps, and how to decide if it’s the right choice for your product or process.

Let’s begin!

1. What Is Insert Molding?

Insert molding is one of those processes you might not think about—until you need it. That’s what happened to me.

I was working on a product that needed a metal part set inside a plastic casing. At first, we used glue. Then we tried press-fitting. Neither held up. After too many failures, someone suggested insert molding. I had no idea what that meant at the time.

Here’s what I learned:

Insert molding is a manufacturing process. It combines two materials into a single part. You take a pre-made item, called an insert, and place it inside a mold. Then, molten plastic is injected around it. Once the plastic cools, you’re left with one solid piece.

The insert is usually made of metal. But sometimes it’s ceramic or even another type of plastic.

This is different from regular injection molding. In traditional molding, you start from scratch. Plastic is injected into a mold to create a full part, no insert, no mix of materials.

Insert molding works well for:

• Metal threads inside plastic housings
• Electrical connectors
• Tools with plastic handles and metal cores
• Medical and automotive parts

So why does this matter to you?

Because insert molding can help you reduce assembly steps, avoid weak glue joints, and make your parts stronger. But it’s not right for every job.

That’s what we’ll explore in the next sections. Is it right for your product? Let’s find out.

2. How Insert Molding Works

Insert molding might sound a bit technical at first, but once you see how it works, it’s actually pretty easy to understand. I still remember the first time I watched it on the shop floor. One minute there was a metal insert sitting in a mold the next, a finished plastic part with that metal locked inside. Simple. Fast. Clean.

So how does that happen?

Let’s break it down into four clear steps, so you can see exactly what’s going on.

Preparing the Insert

Every insert molding job starts with a pre-made part. This insert is usually metal, like brass or stainless steel, but sometimes it’s ceramic or hard plastic. The insert must be clean, dry, and properly sized to fit inside the mold cavity. In some cases, the surface might be roughened or coated to help it grip better once the plastic is molded around it.

This step matters more than you might think. A dirty or misaligned insert can lead to poor bonding or weak spots in the final part.

Placing the Insert into the Mold

Once the insert is ready, it’s placed into the mold. For small batches, this might be done by hand. But in high-volume production, robotic arms often take over. They’re faster and more precise, which helps maintain consistency.

Positioning is critical. If the insert shifts during molding, it can ruin the part or throw off your tolerances.

Injecting Plastic Around the Insert

Now the plastic comes in. Heated resin is injected into the mold at high pressure. It flows around the insert and fills the remaining space, taking on the shape of the cavity.

The type of plastic you use matters too. Some materials bond better to metal, while others are chosen for strength, flexibility, or resistance to heat.

Cooling and Ejection of the Final Part

Once filled, the mold is left to cool. As the plastic hardens, it locks the insert firmly in place. When the cooling is done, the mold opens and the finished part is ejected.

What you’re left with is a single piece: strong, sealed, and ready for use.

Insert molding might involve a few steps, but once you understand the rhythm, it starts to make sense. It’s about combining two materials into one part—in a way that saves time, strengthens the product, and keeps your assembly line moving.

3. Common Materials Used

Now that you understand how insert molding works, you might be wondering: What materials are actually used in this process?

That’s a good question and the answer depends on what your part needs to do. Insert molding always involves two materials: one for the insert, and one for the plastic resin that surrounds it. Picking the right combination is a big part of getting the results you want.

Let’s start with inserts.

These are the solid parts that get placed into the mold before the plastic is injected. Most often, they’re metal. Why? Because metal gives strength, conductivity, or thread support where plastic alone would wear out or fail.

Common insert materials include:

• Brass: often used for threaded inserts
• Stainless steel: great for strength and corrosion resistance
• Aluminum: lightweight but strong
• Copper: used when electrical conductivity is important

Now for the plastics. The molten resin flows around the insert and becomes the outer shell of the finished part. Here are some plastics that work well:

• ABS: strong, impact-resistant, and easy to mold
• Nylon: tough and flexible, used in many industrial parts
• Polycarbonate: clear and strong, good for housings
• PEEK: high-performance plastic for medical or aerospace uses

Some typical pairings include:

• Brass insert + ABS plastic: used in consumer electronics
• Stainless steel insert + nylon: ideal for automotive clips or fasteners
• Copper insert + polycarbonate: often seen in electrical devices

Choosing the right combo is about more than just strength. It’s about how the part functions and how it holds up over time.

4. When Should You Use Insert Molding?

Insert molding isn’t always the answer. But when it works, it can solve problems you didn’t even realize you were stuck with.

I used to spend hours figuring out how to join metal inserts to plastic parts. Adhesives failed. Press fits slipped. We kept tweaking the design, but the root problem was the process itself. Insert molding fixed it.

So how do you know if it’s right for you?

Let’s break it down.

Key Advantages

Insert molding brings several benefits especially if you’re looking to improve efficiency or part strength.

• Combines parts into one: This means fewer components to track, store, or assemble.
• Reduces assembly time: There’s no need to glue, weld, or screw parts together after molding.
• Creates stronger bonds: Plastic flows around the insert, forming a solid hold that lasts longer than glue or friction fits.
• Improves durability: Parts stay intact longer, even under stress or vibration.
• Cuts labor and material costs: Less assembly means lower labor needs and fewer fasteners.
• Allows more compact designs: Combining materials in one shot can free up space inside your part.

Ideal Applications

Insert molding works best in parts where strength, precision, or multi-material function matters.

• Electrical components with embedded contacts
• Plastic handles with metal threaded inserts
• Medical devices with small internal features
• Automotive parts that include fasteners or bushings
• Consumer electronics like USB plugs, ports, and buttons

If your part needs to be strong, efficient, and built to last, insert molding might be worth looking into.

5. Insert Molding vs. Overmolding: What’s the Difference?

If you’re researching insert molding, you’ve probably seen the term overmolding pop up too. At first, I assumed they were the same. They both involve plastic. They both involve more than one material. But after seeing both processes in action, I realized they’re actually quite different.

Let’s walk through each one so you know exactly what you’re dealing with.

Feature / Factor	Insert Molding	Overmolding
Process Start	Uses a pre-formed insert (e.g., metal)	Starts with a molded plastic base part
Insert Placement	Insert is placed into mold before injection	Base part is placed into second mold
Number of Steps	Usually a single-shot process	Often a two-shot or multi-step process
Common Uses	Electrical components, metal threads	Soft grips, seals, ergonomic overlays
Material Bonding Type	Mechanical and thermal bonding around insert	Adhesion between two plastic layers
Tooling Complexity	Moderate (needs insert holding features)	Higher (multi-shot mold or multiple stages)
Cost Consideration	Cost-effective for insert integration	Better for dual-material parts

6. Common Insert Molding Challenges (and How to Avoid Them)

Insert molding can solve a lot of problems but it can also create new ones if it’s not done right. I’ve seen great designs fail because of small mistakes in setup or material choice. That’s why it’s important to know what can go wrong, and more importantly, how to avoid it.

Let’s look at a few common issues you might run into.

Insert Misalignment

This one’s easy to miss until it’s too late. If the insert shifts even a little during molding, your final part could come out crooked, or worse, unusable.

How do you fix that?

• Use well-designed molds with alignment features
• Add jigs or guides to hold the insert in place
• Consider robotic placement for consistency in high-volume runs

Poor Bonding Between Insert and Plastic

Have you ever had a part that looked perfect until the plastic started peeling off the metal?

That usually means the bond wasn’t strong enough. This happens when the materials don’t grip well or when the insert is too smooth.

To avoid this:

• Use materials that naturally bond well together
• Add surface treatments like knurling or texturing
• Clean inserts thoroughly before molding

Insert Movement or Float

Sometimes, inserts don’t stay where they’re supposed to during the plastic injection. They float or shift under pressure.

To stop this:

• Add mechanical locks or undercuts to the insert design
• Use mold pressure wisely to avoid shifting
• Test a few early samples before ramping up production

The good news? These problems are fixable. With the right setup, your insert molded parts can come out clean, strong, and reliable every time.

7. What to Ask a Manufacturer Before Choosing Insert Molding

Once you’ve decided that insert molding might be a good fit, the next step is finding the right partner to do the work. This part can make or break your results. I’ve learned that the right questions, asked up front, can save weeks of back-and-forth later.

Not all manufacturers offer insert molding. And even fewer are experienced with your type of part. So how do you know who’s right for the job?

Start with the basics.

Do you have experience with insert molding for my type of insert?

This tells you if they’ve worked with similar materials, tolerances, or functions before.

Can you help with DFM support for insert placement and part strength?

DFM stands for Design for Manufacturing. If your supplier offers it, they can help you avoid costly mistakes before production starts.

What are the tooling costs and lead times?

Insert molding may need custom molds. Ask what those will cost and how long they’ll take to make.

What insert loading methods are available: manual or robotic?

Manual is fine for small runs. Robotic is better for larger batches.

Can you handle both prototyping and mass production?

Some shops do one but not the other. Make sure they can grow with your needs.

If you’re looking for a manufacturer who can walk you through both prototyping and mass production, MachMaster can help. We offer insert molding, CNC machining, and injection molding all under one roof.

Conclusion

Insert molding isn’t just a process. It’s a decision that affects cost, quality, and your daily workflow.

You now know what insert molding is, how it works, and where it fits best. We’ve covered the materials, the benefits, and even the risks.

So here’s the next step: try it with just one part.

Start small. Test results. Then build from there.

What’s one piece you could simplify or strengthen right now?

Got a design in mind?

Contact us today and let’s see if insert molding makes sense for you.