What Is Injection Molding?

The basic idea

Mechanically it's simple: a machine closes a two-part metal mold, injects molten plastic into the cavity, lets it cool and harden, then opens the mold and drops out a finished part. The magic is repetition. Making the mold is expensive and slow, but once it exists, each part comes out in seconds for pennies of plastic, which is why injection molding makes everything from bottle caps to LEGO bricks.

How the cycle works

Every molded part comes from the same repeating sequence:

1. Clamp. The two mold halves close and lock under enough force to resist the plastic pushing back.
2. Inject. Molten plastic is pushed into the cavity at high pressure.
3. Hold (pack). Pressure is held to compensate as the plastic shrinks (usually 1 to 3 percent).
4. Cool. The part solidifies in the mold. Cooling is the longest step, often 60 to 80 percent of the cycle.
5. Open and eject. The mold opens and ejector pins push the part out. Then it closes and repeats.

The step-by-step detail, and why each stage matters for quality, is in injection molding process steps.

The parts of a mold

The mold is where the cost and the engineering live. Beyond the cavity that forms the part, it contains gates and runners (the channels the plastic flows through), cooling channels, and ejector pins that push the finished part free. A mold is a precision-machined block of metal, usually cut on a CNC machine, and its complexity is the main reason tooling is so expensive.

Common materials

Injection molding uses thermoplastics, picked for the properties your part needs:

• ABS is tough and molds easily, a common default.
• Polypropylene (PP) is flexible and chemical-resistant, great for lids and living hinges.
• Polycarbonate (PC) is clear and strong.
• Nylon is tough and wear-resistant.
• HDPE is strong and slightly flexible.

Designing a part to be moldable

A part has to be designed for the mold, or it won't come out cleanly. The core rules:

• Uniform wall thickness, usually 1.2 to 3 mm. Thick sections cool slowly and cause sink and warping, so keep walls even and use smooth chamfers or fillets at transitions.
• Draft angle, a slight taper (1 to 2 degrees, more for textured surfaces) on vertical walls so the part releases from the mold instead of dragging or sticking.
• Rounded corners, because sharp internal corners concentrate stress and disrupt flow.
• Avoid undercuts, features that block the part from ejecting in one straight pull. If you can't design them out, the mold needs costly side-actions or lifters.
• Ribs for strength instead of thick solid sections, typically around 60 percent of the wall thickness.

When injection molding is the right process

Injection molding front-loads cost into the mold and then makes each part cheap, so it wins at volume, typically once you're making thousands of a frozen design with proven demand. Below that, other processes like CNC machining or 3D printing are cheaper because they skip the tooling. The usual path is to prove the product with a low-volume run first, then commit to a mold once the design is locked and the numbers justify it. For that full journey, see how to manufacture a product.