In working with this method, product designers must account for both the mold and the part itself. The overall design task should take into account every aspect of process from molding machine to processing conditions to resin. Molding machines have an injection section that plasticizes and pushes the resin into the mold, and a clamping mechanism that opens and closes the mold. The process is usually worked with thermoplastics rather than thermosets, so only thermoplastics are considered here. The plastic melts and collects in front of the screw, which retracts until enough molten plastic is present to fill the mold.
Shear generated by the flights of the screw mixes the melt so it is uniform.
This occurs at pressures as high as to 30,000 psi. Water, or another fluid circulating through channels cut in the mold, removes heat so that the plastic cools and solidifies from the walls of the cavity inward. When parts are rigid enough, the mold is opened and they are removed. The next shot is melted in the barrel as parts cool in the mold. Cooling time generally controls mold cycle time and thus production rate.