What Is Injection Molding and How Does It Work?

If something is made from plastic, there’s a high chance it was made with injection molding. From contact lens cases to bottle caps, plastic injection molding is used in almost every industry to create thousands of products we use in our daily lives.

Because this process can quickly create identical components from multiple types of materials, injection molding is especially well-suited for mass production runs. In short, it’s an efficient, reliable way to manufacture high-precision components at scale. 

What is Injection Molding?
Injection molding is a popular manufacturing process for producing plastic parts and components in high volumes. The process works by injecting molten resin into a mold, where it sits under pressure to cool until it’s ready to be released. 
This manufacturing process is so versatile that it’s used far and wide in almost every industry imaginable. Basically, if you can design a mold for it, you can make it with plastic injection molding. To put it into perspective, companies use injection molding to make parts smaller than a grain of rice or as large as a plane’s airframe. 

Types of Injection Molding
There are multiple types of injection molding processes, but the most common types are:

Thermoplastic injection molding: This is the most common process you’ll see in injection molding. It involves hot plastic resin entering a mold before cooling to create the finished product.

Insert Molding: A specialized process that involves inserting a component (usually metal) into the mold before liquid resin is added. This creates a single, integrated part that is typically stronger and more durable than those made without an insert.

Overmolding: Overmolding allows parts to be made with multiple materials. It’s commonly used to make products more flexible or easier to grip.

Two-shot molding: Like overmolding, two-shot molding makes it possible to manufacture products with multiple materials. In terms of production volume, two-shot molding is best for high-volume runs, while overmolding may be preferred for low-volume runs. 

Common Injection Molding Materials
One benefit of this type of manufacturing is that it works with multiple types of materials, including:

High-density Polyethylene (HDPE): HDPE is extremely durable due to its high strength-to-density composition and ability to withstand chemical exposure. It’s used to make everything from industrial parts to food packaging.

Polypropylene (PP): This material is impact-resistant and can withstand wear and tear over time. It’s commonly used for snap-over lids and medical pipette tubing.

Polyethylene Terephthalate (PET): This semi-crystalline resin provides excellent stability, along with UV-resistance, and a natural high-gloss finish. It’s ideal for food and beverage packaging, and is often used in electrical products. 

How Does Injection Molding Work?
There are six steps to the injection molding process: Clamping, injection, dwelling, cooling, mold opening, and ejection.

Most injection molds are made in two metal clamshell pieces. Once a mold is designed and ready to start producing parts, the injection molding machine will push both sides of the mold against each other. Next, liquid plastic is injected into that mold until full. During the dwelling phase, direct pressure is applied to the mold to make sure the material covers every cavity. 

After this, the mold is set aside to cool, a process that can take anywhere from a few seconds to a few minutes. When the part is ready, a clamping motor will open the mold, allowing an ejector bar to gently push the finished part out. Oftentimes, a fabricator will then use cutters to trim away excess material that can then be melted back down and reused for other parts, reducing waste that other molding methods can generate.

What Are the Advantages of Injection Molding?
High-volume injection molding is a popular manufacturing method for good reason. Some of its biggest advantages include:

High efficiency: If you need to produce parts fast, this is the way to do it. Cycle times for injection molding can be as short as two seconds, depending on the product, and can produce tens of thousands of identical parts in a short amount of time. You may also be able to use a multi-cavity mold, which allows multiple parts to be manufactured from the same press.

Repeatability: The injection molding process ensures every part will come out identical, so you can repeat production runs as often as needed. This is helpful when your parts need to have specific tolerances or resistances. 

What Are the Disadvantages of Plastic Injection Molding?
Injection molding is powerful, but it’s not a perfect fit for every application. While you’re considering your options, keep these disadvantages in mind:

Longer initial lead times: Because molds are complex to design and produce, it can take several weeks before a mold is finalized for production. A good partner will help accelerate this process, so you can move to production faster. 
Potential design constraints: Molds need to be designed with multiple considerations in mind, including uniform wall thickness, rounded corners, and draft angles. If you get the initial design wrong, it will need to be redone entirely. 

Plastic Injection Molding Made to Last 
Even if you’re not familiar with Alltrista, you’ve probably used our products before. Our bottles and rigid plastic packaging can be found in medicine cabinets and stores across the world. And with over 200 molding machines across the US, UK, and Puerto Rico, we’re able to tailor custom injection molding solutions to meet your company’s exact needs.

We are committed to making consistent, precise, high-quality products across a wide range of industries. Let us help you move from concept to commercialization faster.