What Does the Polymer Injection Molding Process Look Like?

What Does the Polymer Injection Molding Process Look Like?

When you picture the biggest manufacturing industries in the world, you probably think of the automotive, medical, food and beverage, and aviation industries. All of these, and many more, rely on plastics to create their final products.

With its roots in an American entrepreneurial spirit, injection molding has come a long way over the last 150 years. What began as an attempt to mass-produce billiard balls has spiraled out into one of the most efficient, consistent, and cost-effective manufacturing processes in the world.

Today, injection molding companies can create just about anything, whether it’s a durable, large object or an intricate, small part.

With options ranging from food-safe materials to heat-resistant ones, injection molding fabricates millions of parts every year—but only a few manufacturers actually understand how their parts are produced. We’re here to demystify the process and explain some of the benefits of injection molding. Read on to learn what the polymer injection molding process looks like.

Benefits of Injection Molding

Plastic injection molding is the go-to process for manufacturing plastic components. Explore the benefits of this process.


After your mold is designed, plastic injection companies can replicate the mold and fabricate thousands of parts every hour. There isn’t a more efficient process if you require your parts to be produced in bulk.


Aside from the physical flexibility that any plastic part can have, injection molding is also a highly customizable process. You have a say in the mold design process in addition to the type of thermoplastic for your final part.

There are dozens of types of thermoplastics, each with their own unique properties. This means that injection molding can provide a variety of parts based on your specific needs.


The best injection molding manufacturers keep strict control of their facilities. Once you’ve made your mold, they can produce thousands of identical pieces with a consistent high quality.


The mold is the most expensive part of the equation, as it takes time and careful planning to design it. Once the mold exists, however, the remainder of the process is the picture of cost-effectiveness. Of course, the cost of production per component is reduced more and more for every additional part you need.


You may be looking for a product with high tensile strength or fine detail—whatever you need, injection molding can provide it!

The Process

Now that you have a better understanding of why an industry might want to use injection molding in their pipeline, let’s learn a little more about how the process works.

Creating a Mold

The first and most important thing to do is to create your mold. Each mold has two halves: a cavity and a core. Before you can create the mold in a physical form, it’s standard procedure to design a mold with 3D modeling software. This helps iron out any kinks before you prototype.

Once the designer has a 3D model of a mold they think will work well, the mold is usually tested with 3D printing technology. After the designer is confident that the mold is functional, they can create the final mold that will be used for injection molding. Work with custom injection molding companies to get the parts you need.

Molds are usually made of steel or aluminum—both materials are well known for their ability to stand up well against high temperatures and pressure.

Choosing a Thermoplastic

The choice of steel or aluminum can come down to the thermoplastic you’re planning to use for your part. Because all thermoplastics have different properties, some mold materials may function better than others when the injection molding machine needs to remove the part from the mold.

While there are dozens (if not hundreds) of thermoplastics to choose from, there are a few go-to options that are commonly used. These include:

  • Acrylonitrile-Butadiene-Styrene (ABS): ABS is commonly used for parts that need high stability and tensile strength.
  • Polypropylene (PP): PP is resistant to heat and fatigue and is also a food-safe material.
  • Polycarbonate (PC): PC is a great mixture of high strength with a light weight.

Your specifications are the most important factor when choosing a thermoplastic. Determine whether you need a lightweight product or something that can withstand high temperatures. No matter what you need, there’s a thermoplastic that can accommodate.

Melting the Thermoplastic

Thermoplastics are not stored in a liquid state. Connected to the injection molding machine is a funnel, into which pellets of thermoplastic are poured. These pellets are melted down into a liquid form, which is then carried through the injection molding machine by a specially designed screw.

Injecting the Thermoplastic

When the molten plastic reaches the end of the screw, the core and cavity of the mold come together, sealing it. Then, the plastic is injected into the mold through a gate in the mold at high pressure. Small holes in the injection pins allow air to escape—these holes are large enough to facilitate air’s escape, but too small for the thermoplastic to leak through.

A set amount of plastic then enters the mold, filling it completely.

Cooling the Part

Once the plastic is in the mold, the machine waits for a set amount of time for the part to cool. This cooling time may take milliseconds, or it may take minutes—it all depends on the complexity of your part and the thermoplastic you’re working with. During this time, the screw in the injection molding machine retracts, preparing more molten plastic for the next part.

Ejecting the Part

Once the part has cooled, the mold opens up and pins or plates push the finished part out of the mold. The parts then drop onto a belt or into a compartment and are ready for any last steps.

Finally, your part may require a few finishing touches before it’s ready to be used. Some molds have extra plastic that needs to be removed from the final part, while others may need drying or finishing. Depending on the part and the processes, these steps may be completed by operators or a machine.

Now that you understand what the polymer injection molding process looks like, consider contacting an injection molding company the next time you need a plastic part made.

What Does the Polymer Injection Molding Process Look Like?

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