There are three main units which make up an injection molding machine - the feed hopper, the heater barrel, and the ram. The plastic in the hopper is in granular or powder form, though some materials such as silicone rubber may be a liquid and may not require heating.
Once in hot liquid form, the ram ('screw') forces the liquid into the tightly clamped mold and the liquid sets. More viscous molten plastics require higher pressures (and higher press loadings) to force the plastic into every crevice and corner. The plastic cools as the metal mold conducts heat away and then the press is cycled to remove the molding.
However, for thermosetting plastics, the mold will be heated to set the plastic.