Plastic injection molds are metal forms created to produce a specific plastic item. These molds are the primary way that all plastic parts are manufactured. Heating and melting granules or pellets of plastic begins the process. The liquefied plastic is then forced, injected, into the closed split-die mold under high pressure and allowed to cool. The plastic injection mold is then opened to release the product.
Once a product has been designed, plastic injection molds are made, usually of either steel or aluminum, and precision machined to form the smallest feature of the plastic product. Often several plastic injection molds are used for a product that has numerous parts. Plastic injection molds have both core and cavity inserts, which will correspond to the shapes desired.
The material used for injecting into the plastic injection molds is derived from two different basic plastics: thermoplastic and thermosetting plastic. Most familiar plastic items are made from thermoplastics because these meltable plastics can be shaped and reshaped easily. Thermoplastics are used to make such common items as milk containers, soft drink bottles, pantyhose, styrofoam cups, plastic toys, etc. This type of plastic is easily recyclable as used products can be melted, injected into molds, and reformed into new items.
Thermosetting, or thermoset, plastics are rigid and set in a one-time pattern when they come out of the plastic injection mold. They cannot by recycled or melted for further use. Examples of thermoset plastics are automobile tires, appliance cases, electrical and electronic circuit boards, and electrical switches.
John Wesley Hyatt developed the first plastic injection mold, and the injection molding process, in 1868. He successfully injected hot, liquid celluloid, often called the first thermoplastic, into a split-die mold by the plunger method, a process that was little changed until James Hendry built the first screw plastic injection mold in 1946. Today, nearly all plastic injection molding uses the screw method.
To inject liquid plastic into a mold using the screw method, the screw below the molten plastic hopper is moved back to allow the material to flow into the mold. The screw assembly is then advanced to close the injection entry, using a non-return assembly to keep the material from backing up into the screw area. After allowing enough time for the plastic to cool, the mold is opened and the part removed, either by hand or by machine. The mold is then closed, the screw opened, and it is prepared to receive more material.