Molding starts with the closure of the mold cavity. The molding machine nozzle is seated into the sprue bushing. After contact, a premeasured quantity of molten feedstock is forced into the cavity. Pressure is sustained on the cooling feedstock until the gate freezes, and after complete cooling the component is ejected. Between the time when the gate freezes and the part is ejected while it is still cooling, the screw is turning to plasticize and meter the next shot of hot feedstock.
The pressure in the molding cavity increases rapidly as the molten feedstock enters the cavity and that pressure is held until the gate freezes. Once the gate has frozen the molding machine no longer controls the cavity pressure. Cavity pressure determines the final part mass and dimensions. A shot-to-shot mass variation is a means to monitor the success of proper control.
After mold filling, as the feedstock cools, a natural pressure reduction occurs due to binder thermal contraction. At the point of ejection the residual pressure in the cavity should be very low. Otherwise, the component will stick in the mold cavity. For ejection, pins move from flush positions on the tool walls and push the component from the cavity.
Robots are often used to automate part removal from the mold/press. Modern molding machines integrate the robot control and coordination with the molding cycle. Mass and/or sizes are monitored after molding to ensure proper quality.