Injection molding is a precise method for manufacturing complex three-dimensional products. The injection machine has two main parts: the injection unit and the clamping unit.
Resin pellets are poured into the feed hopper, feeding the granules down to a screw, which depth decreases towards its end nearest the mold, compressing the heated plastic. As the screw rotates, the pellets are moved forward, undergoing extreme pressure and friction which generates most of the heat needed to melt the pellets. Heaters on either side of the screw assist in the heating and temperature control during the melting process. The melt accumulates at the front of the sleeve as there is no opening allowing for the free flow of material. When the amount of melt reaches a predetermined portion, the screw acts like a piston pushing the molten resin into pattern molds under high pressure. When the filling is completed, the opening closes up and the screw begins to prepare a new batch of material.
The function of the clamping unit is to see that the mold is closed and to maintain the closed position at high pressure to withstand the forces developing during the injection of melt into the mold. The melt cools and hardens inside the mold, and when it reaches an appropriate mechanical strength, the clamping unit opens the mold to release the finished product and closes it anew for a new injection.