Injection molding is an intricate and complicated process. In reality, the injection molding machine has three main parts: the injection unit, the mold, and the clamp. Plastic pellets are fed into the barrel through the hopper. Inside the barrel, stress implied by a screw pushes the pellets forward. Heater bands are wrapped around the barrel. Due to the movement and heating of these bands, gradually, the pellets are melted. Once enough plastic pellets are melted, the amount of molten plastic material gets collected near the front of the screw; the screw pushes the molten plastic material into the empty part of the mold called the cavity image. The molten material solidifies within a minute. The mold opens, and the part is ejected, the mold again closes, and the process repeats.
Before the mid 20th century, injection molding machines only used the external heating barrel to melt the plastic before a plunger injected the material to mold the plastic. But the temperature remained uneven in the barrel, the middle portion was too cool to be melted, and the outer regions were too hot and degraded the plastic. The solution to this was the reciprocating screw, which was regarded as the most crucial contribution to revolutionizing the plastic industry in the 20th century.
In the earlier plunger injection molding machines, plastics only intended to pass through a cylindrical barrel. But in modern times, the reciprocating screw solved the problem as well as the design improved. For example, modern machine plastics are wrapped only around the shaft region; this eradicates the problem of uneven cooling and extra heating of the plastic pellets inside the barrel. Secondly, also the screw has flights wrapped around the shaft. As the screw rotates, the beam transfers the raw material across the barrel, and the flights help in mixing the raw materials across the plastic. Thirdly, the action of the screw itself heats the plastic. This heating temperature ranges from 60 to 90 degrees, which includes the rest of the heating come from the heater bands to melt the plastic pellets. In order to cool the plastic pellets and let them solidify, water as a coolant is used. As the material solidifies, the mold opens. In this step, firstly, the mold opens only a few millimeters to evacuate the pressure and then quickly opens to remove the solidified part quickly. This step may be difficult as, during the removal of the plastic material, plastic might get stuck to the wall of the core. Molds have built in ejector pins that push off the parts from the mold away.
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