Key Plastic Injection Molding Terms to Know Before Starting Your Next Project

Learning plastic injection molding terms can lead to greater knowledge of your product, a better understanding of how it functions, and in some cases, cost savings.

Learn the definitions of key terms and their importance with Metro Plastics.

Core Terms

These key plastic injection molding terms are central tenets of overall injection molding production. Knowing these terms can be helpful even in first conversations with a manufacturer.

Injection Molding Process: A cyclical manufacturing process where plastic pellets are melted, injected into a closed mold, cooled, and then ejected as a finished part.
Resin: Another term for material, and in this case, plastic. Resin can be interchanged with pellets. This term is used to describe what’s being melted during the injection molding process.
Mold: Made up of steel or aluminum, the mold is the tool that shapes the melted material, and once it’s cooled, it will match the shape of the mold.

Mold Design Terms

In injection molding, there are two components that go through a design phase: the mold and the part itself. These plastic injection molding terms are geared toward understanding mold design terminology.

Cavity and Core: Molds are like vinyl records in that they have an ‘a-side’ and a ‘b-side.’ The cavity forms as the a-side, which is mainly cosmetic. The core forms the b-side, which the part sticks to and eventually ejects from.
Single vs. Multi Cavity: Injection molding can happen with a single cavity or multiple cavities. In a single cavity, only one component is made per cycle, whereas in a multi-cavity solution, multiple components of the same shape can be made at once.
Family Mold: A family mold is a mold that can create different parts that end up matching. As an example, a left and a right part can be made at the same time in a family mold.

Part Design Terms

Starting with a good part design will ensure tight tolerance repeatability, improved manufacturability, and reduced overall part costs.

Wall Thickness: The nominal cross section of the part walls. Manufacturers work to keep the walls as thin as possible to reduce potential defects.
Draft Angle: Taper added to vertical faces so parts release from the mold without causing any damage.
Ribs and Bosses: Ribs are thin reinforcing features that allow greater strength and stiffness in your injection molded parts. Bosses are used for locating, mounting, and assembly.

Flow and Gating Terms

A properly designed gate is essential for a high-quality part because it controls the material’s flow into the mold.

Gate: The small opening where melted plastic enters the part from the runner system.
Runner and Sprue: These two terms coincide. Runners are what connect the sprue to the gate. The sprue is the main channel where melted plastic travels to the gate and ultimately enters the mold.
Flow and Weld Lines: Visual artifacts where material fronts meet or change direction during filling.

Quality Terms

When it’s time to ask questions about your product’s quality, you can use these plastic injection molding terms as a reference point.

Cycle Time: The total time spent manufacturing. This includes filling, packing, holding, cooling, opening, and ultimately ejection.
Shrink and Warp: Shrink is what’s used to describe the material’s contraction as it cools. It is a natural part of the injection molding process, but should be minimized. Warp is a distortion caused by non-uniform shrink, ultimately a defect.
Flash, Sink, and Short Shot: All three of these terms are product defects, but with proper design and manufacturing, can be avoided. Flash is where the component leaks excess plastic along parting lines. Sink is where local depressions are seen over thick areas or ribs. Short shots are simply parts that aren’t fully filled.

Learn More Than Plastic Injection Molding Terms by Partnering With Metro Plastics

Since 1975, Metro Plastics has produced millions of injection molded components. We believe serving our customers means more than just creating high-quality products; it’s also about building relationships. Educating and sharing knowledge helps ensure customers have a greater and deeper understanding of their products and our process.

These plastic injection molding terms are only the beginning of what we want to share. Metro Plastics is here to answer your questions before we get to work on your next project. Contact us today to learn more about what we can do for you.