The plastic injection mold consists of two primary components, the cavity half of the mold (A half) and the ejector half of the mold (B half). These mold halves are designed to work in conjunction as follows:

1. Plastic resin from the molding machine enters through a “sprue” or “gate” on the A half.
2. A sprue bushing seals tightly against the nozzle of the injection barrel of the molding machine. This allows the molten plastic to flow from the barrel into the mold (or “cavity”).
3. The sprue bushing directs the molten plastic through channels (called “runners”) that are machined into the faces of the A and B halves of the mold.
4. The molten plastic flows through the runner and enters one or more specialized gates and into the cavity to form the desired part.

A mold is usually designed so that the molded part reliably remains on the B half of the mold when it opens. The runner and the sprue are drawn out of the A half. The molded part then falls freely when ejected from the B half.

A single plastic injection mold can have one cavity, producing one part at a time, to multiple cavities for extremely high-production molds (like those for bottle caps) that can have 100 plus cavities.