The injection molding process works by adding plastic pellets or metal powder to the injection molding machine’s hopper. Next, the hopper pours the plastic into the barrel, where it is heated into a liquid form. The molten plastic or metal enters the mold from the barrel through the nozzle.
A nozzle in the extruder presses tightly into a recess in the stationary half of the mold. The two halves of the mold, with a vertical parting surface, are made from heavy steel and mounted on thick steel tie-bars. Within the injection mold is a system of runners that distributes melt from the sprue to the mold cavities that determine the dimensions of the molded product. Using clamp plates, the mold halves clamp together with high pressure from a hydraulic piston.
Cooling channels passing near the cavities keep the mold cold, well below the solidification range of the plastic or other material. Next, the screw moves forward, forcing the melt through the runners and into the cavities, and maintains melt pressure (5,000-15,000 psi, 34-103 MPa) while the newly injection molded part cools and solidifies.
Then, the far half of the injection mold is withdrawn, activating ejector pins that push the molded part and runners out of the mold halves. At the same time, the extruder retracts and generates more hot melt.
Typically, the process is mostly automated with computer controls. Injection machines are rated by their melting capacity, shot size, and clamping force. They range from machines shooting two ounces of melt into a family mold that makes the parts for a hobbyist’s model airplane to machines that can mold 40-gallon trash cans in injection molds with two cavities—one for the can, the other for its lid. Injection-molded parts come in many shapes and sizes, meaning mold components come in various types.