A Practical Overview of Plastic Injection Molding
Table of Contents
What is Injection Molding?
At its core, injection molding is about making a lot of identical plastic parts quickly and consistently. We melt plastic pellets, push the liquid plastic into a custom steel mold under high pressure, let it cool, and pop the finished part out. It’s the most cost-effective way to make thousands or millions of the exact same piece, whether it's a tiny internal gear or a tough equipment housing.
Picking the Right Plastic
The plastic you choose depends entirely on what your part needs to do. Here are a few common materials we run every day:
- ABS: Tough and inexpensive. Great for everyday consumer goods and electronic housings.
- PC (Polycarbonate): Clear and incredibly strong. We use this for clear covers or parts that need to take a hit.
- PP (Polypropylene): Bends without breaking. Perfect for living hinges or parts exposed to chemicals.
- POM (Delrin/Acetal): Slick and stiff. Our go-to material when making gears or parts that slide against each other.
Note: Need something tougher? We also frequently run engineering grades like PA+GF or PEEK for high-heat industrial applications.
Going Beyond Basic Parts
Sometimes a simple plastic part isn't enough. If you want to skip manual assembly, we use these setups:
Overmolding (2-Shot)
Shooting a soft rubbery grip (like TPU) directly over a hard plastic base in the same machine so they permanently stick together.
Insert Molding
Molding the plastic right around metal pieces—like brass threaded nuts or electrical pins—so they never pull out.
Does This Process Make Sense for You?
The biggest reason to choose injection molding is the price drop at scale. Yes, paying for the steel mold upfront is an investment. But once that mold is built, making each plastic part costs pennies. It's the best route if you need large volumes and you need every single part to be identical.
Basic Design Rules to Remember (DFM)
Before we cut any steel, we always check your 3D files for a few basic rules. Catching these early prevents warped parts and saves you money:
- Keep Wall Thickness Even: If walls go from thick to thin abruptly, the plastic cools unevenly and the part will warp or show sink marks.
- Add Draft Angles: Think of a baking pan. If the sides are perfectly straight, the cake sticks. Adding a slight angle (1° to 2°) helps the part slide out of the mold cleanly.
- Round the Corners: Sharp internal corners are weak points where parts tend to crack. Adding a small curve (radius) makes the part much stronger.
Our engineering team at GBM Mold Technology runs a free DFM check on every project to make sure your design is mold-ready before any steel is cut.