Plastic injection molds are metal shapes used to produce plastic items by injecting melted plastic into a precision-machined mold. Thermoplastics are commonly used and easily recyclable, while thermosetting plastics cannot be recycled. The screw-type method is now widely used for injection molding.
Plastic injection molds are metal shapes created to produce a specific plastic item. These molds are the primary way all plastic parts are produced. Heating and melting plastic granules or pellets begins the process. The liquefied plastic is then forced, injected, into the closed split 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, usually steel or aluminum, are made 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 match the desired shapes.
The material used for injection into plastic injection molds is derived from two different basic plastics: thermoplastic and thermosetting plastic. The most common plastic items are made from thermoplastics because these fusible plastics can be shaped and reshaped easily. Thermoplastics are used to make common items such as milk containers, soda bottles, pantyhose, Styrofoam cups, plastic toys, etc. This type of plastic is easily recyclable as used products can be melted down, injected into molds and transformed into new objects.
Thermosetting or thermosetting plastics are stiff and set in a one-time pattern when they come out of the plastic injection mold. They cannot be recycled or melted down for further use. Examples of thermosetting plastics are automobile tires, appliance cases, electrical and electronic circuits, and electrical switches.
John Wesley Hyatt developed the first plastic injection mold and injection molding process, in 1868. He successfully injected hot, liquid celluloid, often called the first thermoplastic, into a split mold mold by the plunger method, a process that was little changed until James Hendry built the first screw-type plastic injection mold in 1946. Today, nearly all plastic injection molding uses the screw-type method.
To inject liquid plastic into a mold using the screw method, the screw under the hopper of molten plastic is moved back to allow the material to flow into the mold. The screw assembly is then advanced to close off the injection port, using a check valve to prevent material from flowing back into the screw area. After allowing the plastic to cool sufficiently, the mold is opened and the part removed, either by hand or by machine. The mold is then closed, the screw opened and prepared to receive more material.
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