To put is basically, injection molding is the process of forcing melted plastic in to 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
The mold defines the shape of the part, as well as the path by which the molten plastic flows from the barrel. A simple mold has several features:
Fixed and Moving half – These are rectangular blocks of steel into which the shape of the part is cut.
Cavity – When the fixed and moving half are touching, the space formed by the cut-out portions, called the cavity, defines the shape of the part.
Sprue – The sprue is a hole cut into the center of the fixed platen. Molten plastic flows from the sprue to fill the cavity.
Runners – Runners are channels cut into the platens that direct molten plastic from the sprue to the gates.
Gates – Gates are small openings between runners and cavities. These are the points at which plastic enters the cavity. They are generally small so that the finished part may be easily broken away from the useless sprue and runner material.