Injection molding is the process of forcing melted plastic into a mold cavity. Once the plastic has cooled, the part can be ejected. It is useful when the parts are too complex or cost prohibitive to machine. With this process many parts can be made at the same time out of the same mold.

During the injection phase, plastic materials, usually in the form of pellets, are loaded into a hopper on top of the injection unit. The pellets feed into the cylinder where they are heated until they reach molten form (think of how a hot glue gun works here). Within the heating cylinder there is a motorized screw that mixes the molten pellets and forces them into the end of the cylinder.

Once enough material has accumulated in front of the screw, the injection process begins. The molten plastic is inserted into the mold through a sprue, while the pressure and speed are controlled by the screw. Next, pressure is applied to make sure all of the mold cavities are filled. The plastic is then allowed to cool to its solid form within the mold. The clamping unit is opened and the finished piece is ejected from the mold.

Injection molding is a good choice when a high production rate and low labor costs are needed. With repeatable higher tolerances, minimal scrap losses, and little or no need for additional finishing of part after the molding process, injection molding is usually the lower cost option for parts, offset by higher tooling costs for start-up.

Injection molding is a more complex process than extrusion because more variables come into play. These variables include mold design and cooling times, as well as the temperature of the molten plastic and the pressure at which it is injected into the mold.

Even the type of plastic being molded makes a difference. This is because of the different molding characteristics of different plastics. In fact, because of one of these characteristics, shrink rate, a mold that easily produces a part from one type of plastic may not be able to be used for another. This is because, as different types of plastic cool inside a mold, they contract or shrink differently. This changes the dimension of the finished part.

The major advantages of injection molding are speed of production; minimal requirements for post molding operations, such as machining to final dimensions; and the ability to mold several parts at one time with a multi-cavity mold.

A wide range of materials can be used in injection molding which is sometimes limited in other processes. Nylons, Acetals, ABS, PP, PE, PPO, Polyesters, PVC, PC, and Acrylics are just some of the more common thermoplastics used.