What Is Plastic Injection Molding? A Complete Guide

Plastic injection molding, toys
Plastic injection molding, toys
Resource: https://www.youtube.com/watch?v=eoaTl8Q0PQc

For many years, plastic injection molding has been the most popular way to mass-produce plastic products. It still is! According to a report by Fortune Business Insights, the global plastic injection molding market size is expected to reach $476.46 billion by 2028. This represents a CAGR of 4.2% from 2021 to 2028.

Why is this process so popular? Also, what plastic types are used? This plastic injection molding guide explores the engineering principles behind the manufacturing method, the process itself, and much more.

What is Plastic Injection Molding?

The plastic injection molding definition to start with: Plastic injection molding is used to mean the manufacturing process where plastic or polymer, in the form of granules or pellets, is melted and injected under high pressure into a mold.

The mold is designed in the profile of the final part or product. When cooled, the plastic hardens and takes this shape. The process of injection molding plastic is extremely versatile, allowing parts with a wide range of shapes and sizes to be produced.

In addition to versatility, this manufacturing method is also relatively quick and efficient, which makes it ideal for mass production. And, because there is very little waste generated during the process, it’s also environmentally friendly.

Pellets for plastic injection molding
Pellets for plastic injection molding
Resource: https://www.plasticstoday.com

Plastic Injection Molding Materials

Plastic injection molding raw materials can be different types of plastics, depending on the required properties for the end product. The three most common types of plastic materials for injection molding are thermoplastics, elastomers, and thermosets.

Thermoplastic Injection Molding

As the name suggests, thermoplastics can be melted and cooled multiple times without changing their properties. This makes them the best plastics for injection molding. They are also softer and more flexible, in addition to having a low melting point. Common examples include:

  • Polyethylene (PE) – water bottles, packaging containers, chairs
  • Polypropylene (PP) – prosthetics, automotive parts, housings
  • Polystyrene (PS) – disposable cutlery, packaging material, pens
  • Polyvinyl chloride (PVC) – shoes soles. Plastic tubing, electrical insulation
  • Acrylonitrile Butadiene Styrene (ABS) – toys, auto parts, Lego bricks
  • Polyamide (PA) – gears, bushings, electronic housings
  • Acrylics – lenses, signs, trays, light housings, displays

Elastomer Injection Molding

Elastomers are a type of plastic that can be stretched or compressed without permanently changing their shape. They are also known as rubber-like plastics. The most common elastomer injection molding materials are:

  • Polyurethane (PU) – insulation soles of shoes, toys
  • Silicone – seals and gaskets, gloves,

Thermosetting Plastic Injection Molding

Thermosets are plastics that can be molded only once. That’s because they set or cure when heated and cannot be melted and reformed like thermoplastics. So they’re not as commonly used. Examples of thermosetting plastic injection molding are:

  • Epoxy – fiber-reinforced composites, electrical laminates
  • Unsaturated polyesters – boat hulls, shower stalls
Plastic injection molding machine
Plastic injection molding machine
Resource: https://mag.turkishplastics.net

How Does Plastic Injection Molding Work?

Plastic material or materials (normally in pellet form) is melted and injected, under high pressure, into a mold cavity. The melt then cools and solidifies into the final required shape.

To better understand how plastic injection molding works, we will first look at the type of machine used and its various parts. After that, we will see how the entire plastic injection molding process works step-by-step.

Plastic Injection Molding Machine

Let’s take a closer look at the parts of the machine that makes it all possible. There are three main parts to a plastic injection molding machine: the hopper, the barrel, the mold, and ejection unit.

Hopper: The injection molding machine hopper is where the plastic pellets are poured. The size of the hopper will depend on the size of the machine and the amount of plastic that needs to be melted.

Barrel: This is where the plastic pellets are mixed, melted, and transformed into molten plastic. The injection molding machine barrel is usually surrounded by heating bands that serve to heat the pellets.

Inside the barrel is also what’s called a reciprocating screw. The screw’s function is to mix the plastic pellets and melt them evenly.

Mold: The mold is the negative of the desired shape that needs to be created. It is usually made out of metal, usually steel or aluminum, and may contain one or more cavities.

Ejection Unit: This part of the plastic injection molding machine helps to remove the finished product from the mold.

Inject molding process flowchart
Inject molding process flowchart
Resource: https://www.researchgate.net

Plastic Injection Molding Process

Once inside the machine, the plastic must now undergo the injection molding process. The plastic injection molding process steps are divided into the following: loading of the plastic pellets into the hopper, melting them, feeding the melt into the mold, cooling and ejection.

Loading: The plastic pellets are gravity fed from the hopper into the barrel. This is the first stage when injection molding plastic to form different parts.

Melting: The pellets are melted in the barrel, partially heaters that wrap around it and mostly by the pressure and friction created by the screw.

Injection: The molten plastic is injected into the mold. This is done by opening up the nozzle and injecting the molten plastic under high pressure.

Cooling: The plastic is cooled in the mold until it solidifies and takes on the shape of the mold cavity. Most often, the mold has a system of channels that help to circulate a coolant (usually water) around the mold

Ejection: The last step of the plastic injection molding process is to eject the finished product from the mold. This is done by opening up the mold, usually with the help of ejector pins. To make sure that the product doesn’t stick to the mold, a release agent may be used.

During the final touches to the ejected parts, trimming is done to remove any excess plastic that may have been left behind. The waste from the trimming process may be recycled and used to make new pellets, or collected and sold to other plastic injection molding companies.

Plastic injection molding chairs
Plastic injection molding chairs
Resource: https://youtu.be/USg7An6OIPE

Plastic Injection Molding Benefits

As we saw before, plastic injection molding is the most popular way to manufacture plastic products. There are many reasons for this, but we will just mention a few of the main benefits.

Mass Production: One of the main benefits of plastic injection molding is that it’s a fast way to create large numbers of identical plastic parts. Once the mold is created, the process is very quick and can be easily automated. This makes it great for mass production.

Minimal Wastage: With injection molding, there is very little waste created during the process. The excess plastic, especially when using thermoplastics, can be recycled and used to make new pellets or even new products.

Cost-Effectiveness: Another of the many plastic injection molding benefits is that it’s also a very cost-effective way to manufacture plastic parts. The initial setup costs may be high, but once the mold is created, the cost per unit is very low.

Precision: The plastic injection molding process is also a very precise way to manufacture plastic parts. The process allows for very tight tolerances and can create parts with very intricate designs.

How Much Does Plastic Injection Molding Cost?

The plastic injection molding cost will depend on several factors, such as the complexity of the part, the size of the part, the number of parts, and the material used. Usually, tooling takes up the majority of the cost, as the mold must be made to specific dimensions and tolerances

  • For a simple part, the cost to produce a mold can be as low as $100 to $500. For a more complex part, the cost can be $100,000 or higher. If the mold is very large and requires more time to produce, it will cost more, too.
  • Different plastic injection companies will also have different pricing structures, so it’s important to compare quotes from several vendors before making a decision.

Although the upfront cost of plastic injection molding can be high, the long-term savings are significant: the parts are very consistent in quality and there is little waste or scrap.

In addition, the process is automated, so it can be completed quickly and with little labor. That means you getting your products to market faster and at a lower overall cost.


Plastic injection molding is the most used manufacturing method for plastic parts, from small components like buttons to large items like car components. As we have seen, it offers many benefits. But there are also some challenges, and these must be considered, such as the higher initial costs. With the right approach, though, injection molding can be the best way to produce your plastic parts

Table of Contents
Latest Posts
Contact Us