How to Choose Metal Inserts for Molded Plastics


John Wesley Hyatt succeeded in creating celluloid in 1870 and only two years later also invented injection molding, though it was very limited.

It wasn’t until WWII came with unique challenges that James R W Hendry stepped up to deliver the world’s first screw injection machine. With this invention, metal components and plastic ones finally melded as one.

But it’s not easy to combine molded plastics with metal inserts. Not just any metal insert will do.

How can you choose the right one for your project? Keep reading to find out.

When to Use Metal Inserts

Metal inserts are used on a variety of molded plastics and other soft materials. Other applications for metal inserts are thin-wall products and brittle products.

Whenever you have a base material that permanently deforms if overtightened, metal inserts are a good way around the problem.

Threaded inserts are the most common form of metal insert used. 

Threaded Metal Inserts for Molded Plastics

Common materials used in metal inserts for molded plastic are brass, aluminum, and stainless steel. They also are ultrasonic, thermal, molded-in, and press-in types.

What is remarkable about threaded inserts, aside from their versatility, is the range of sizes, textures, and shapes they can come in to suit almost any need.

E-Z LOK threaded inserts for plastic are some of the best quality, size range, type, shape, and material inserts around.

What Type of Insert Should I Use?

Every project is different, so you should weigh the pros and cons of using particular insert types. They’re made for specific purposes, applications, and plastic materials.

Press-in inserts are installed in pre-drilled or cored holes. They’re designed for both thermoplastics and thermoset. This versatility makes them ideal in situations like 3D printing.

Aside from molded-in inserts, press-in inserts tend to be a good choice with thermoset.

Ultrasonic inserts are installed using a high-frequency vibration tool that causes the plastic to soften. Once it cools, the insert is permanently fixed.

Similar to this is “heat staking.” The insert goes into the material using a mounting hole with a thermal press. Like with ultrasonic inserts, when the plastic cools it’s permanently in place.

There are pros and cons between the two, despite their similarity in reforming plastic using heat.

The Difference Between Thermoset and Thermoplastic

Thermoset plastics include polyurethane, melamine, polyester, fiberglass, and other resin-injected materials. Common thermoplastics include acrylic, polypropylene, polyethylene, PVC (polyvinyl chloride), and nylon.

For thermoset plastics, press-in, self-tapping, and mold-in metal inserts are best. Heat staking and ultrasonic aren’t suitable for thermoset plastics. On the other hand, they may be the best option for thermoplastic.

Thermoset plastic burns rather than melts, which is the main problem between thermal insertion and thermoset plastic.

Metal Inserts for Molded Plastics: The Right Choice

The world of molded plastics needs metal inserts. Plastic may be one of the most useful inventions of the 19th, 20th, and 21st centuries, but it can’t do everything.

Now that you know how to choose the right metal insert for your plastic injection molding project, you may have other questions.

Don’t worry, we have you covered with more manufacturing tips. Keep browsing to find more articles to get you through whatever you’re facing in your project.


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BDC 317 : Jun 2024