Injection molding is a manufacturing process that melts plastic, pushes it into a metal mold under pressure, and cools it into a finished part. It is the most common way to make plastic parts in volume, from small clips to large industrial components, because once the mold exists, every part comes out fast, consistent, and at a low cost per piece.

This guide covers how the process works, what plastics it uses, what it is good and not so good at, and the main variations, including the low-pressure methods used for large parts. We are a custom injection molder at 20/20 Custom Molded Plastics, and this is the plain-English version we would give an engineer sizing up the process for a new part.

How injection molding works

Every injection molded part goes through the same basic cycle:

• Clamp. The two halves of the mold close and are held shut with heavy force. That clamping force has to be greater than the pressure of the incoming plastic, or the mold would push open and let material leak out along the seam.
• Inject and hold. Plastic pellets are melted in a heated barrel and pushed into the mold by a screw. Because plastic shrinks as it cools, the machine keeps pressure on for a moment after filling, packing a little more material in to hold the part’s dimensions.
• Cool and eject. The part cools and hardens against the mold, then the mold opens and pins push it out. Cooling is usually the longest part of the cycle, and it is where problems like warping or sink marks show up if the part or mold is not designed well.

That whole cycle can take anywhere from seconds to a couple of minutes depending on the part’s size and wall thickness, and it repeats for every part.

What plastics are used

Most injection molding uses thermoplastics, which melt and re-solidify cleanly. They fall into two rough groups:

• Commodity plastics like HDPE and polypropylene are affordable, chemical-resistant, and easy to run. They are common in material handling, packaging, and durable everyday parts.
• Engineering resins like polycarbonate, ABS, and nylon cost more but offer higher strength, impact resistance, or heat tolerance for demanding parts.

Each material shrinks a little differently as it cools, and the mold has to be built for the specific resin. Design a mold for one plastic and run another, and the parts can come out the wrong size.

What injection molding is used for

Injection molding shows up almost everywhere plastic parts are made in quantity. On the industrial side, that includes material handling parts like pallets, bins, and dunnage; utility and infrastructure parts like electrical enclosures and underground boxes; construction and outdoor products; consumer products; and agricultural components. If a plastic part is made more than a few thousand times, it was probably injection molded.

When injection molding is the right fit

The big advantage is cost at volume. The mold is a real upfront investment, but once it exists, the cost per part drops sharply, and the process can produce complex shapes and built-in features without extra machining. The parts are also highly repeatable, so part number ten thousand matches part number one. Additionally, if a superior part finish is required, this is likely the right process. 20/20 CMP can mold extremely large parts with this process, with weights of over 300 pounds.

The trade-off is that the initial tooling costs can be high. A mold takes time to build and is not cheap, so injection molding pays off for production volumes, not one-off prototypes. For a handful of parts, methods like machining or 3D printing usually make more sense. The other thing to get right early is the part design, which is where a molder’s feedback before the mold is cut saves the most money.

There is more than one kind of injection molding

“Injection molding” is really a family of processes. Standard high-pressure molding is the default and suits smaller, detailed, thin-walled parts. When parts get large and thick-walled, low-pressure methods like structural foam take over, producing big, rigid, lighter parts without the sink marks and huge presses that high-pressure molding would need at that size. There are also variations like structural web, gas-assist, and multi-shot for specific jobs.

Working with a custom injection molder

A custom molder takes your part design and produces it to spec at volume. The things worth checking in a partner are the range of processes they run, the press sizes they have, and whether they will review your design before tooling. We run the full range, high-pressure and low-pressure, structural foam, structural web, and gas-assist, on presses from smaller tonnage up to 8,000 tons across plants in Holiday City, Ohio; Bluffton, Indiana; and Covington, Georgia. We give design-for-manufacturability feedback early, before tooling is committed, so problems get caught on the screen rather than in steel.

If you have a part in mind, send us the design. Our engineers will review the geometry, recommend the right process, and put together a molding quote. Contact 20/20 Custom Molded Plastics to start.

Frequently Asked Questions

What is injection molding used for?

It is used to make plastic parts in volume across almost every industry, from small components to large industrial parts like pallets, bins, and enclosures. It is the standard choice whenever the same plastic part is needed in large quantities.

How does injection molding work in simple terms?

Plastic is melted and pushed into a metal mold under pressure, held while it packs out, then cooled until it hardens and ejected. The mold opens, the part drops out, and the cycle repeats.

Is injection molding expensive?

The mold is a significant upfront cost, but the cost per part is low once production starts. That makes injection molding economical for volume production and a poor fit for one-off parts, where machining or 3D printing is cheaper.

What plastics can be injection molded?

Most thermoplastics, including HDPE, polypropylene, polycarbonate, ABS, and nylon. The right one depends on the part’s strength, impact, temperature, and cost requirements.