Injection Molding 101: The Process Of Plastics

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Recommended: An essay on plastics

Christian Hampel
Shinabarger
English 101
21 Apr. 2014
Injection Molding 101
All across America there are certain funny shaped objects in a variety of shape, size, color, texture, hardness, and even smell, that are seen or used daily. These materials are commonly called plastic. Plastics are seen all throughout cars as dashboards and bumpers, in banks as plexiglas, and even is people’s hearts as pacemakers (Marmo). If the plastic part isn’t ziplock® bag thin or hollow on the inside like a milk jug, chances are it can be injection molded. The process of injection molding is a complicated multi step cycle composed of developing a shot, closing the mold, injecting the mold, an ejecting the part.
Injection molding machines seem incredibly complex when looking at them as a whole so it’s best to think of it as two units; An injection unit that melts, mixes and pushes plastic, and a clamp unit which is what holds the plastic and gives it the desired shape (Lokensgard). The process starts out by plastic pellets being loaded into an injection molding machines hopper. This piece of equipment is always located on the top of the machine above the screw. This acts like a tank for the plastic as well as a funnel guiding it to the correct place while gravity pulls it down.
Once gravity brings the pellets downward to the screw, they begin to mix and melt together. A temperature of 400-450⁰F is usually required to melt the plastic (Lyondell). Thankfully William Willert invented the reciprocating screw. The reciprocating screw is a screw in which the depth between inner and outer parts changes to increase friction levels, more friction causes more heat. The beginning of the screw has the deepest depth, referred to as the feed section. The screw sta...

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...damage the part. All of the pins are connected to the ejector pin plate which is located in between the clamping unit and “B” mold back. Molds are designed so when they open the part is carried away from the injector nozzle to prevent any cooling and blockage. When the cooling time has completed the mold opens up and the ejector plate is pushed towards the mold causing the pins to press against the part and push it out. The part falls into a bucket or onto a conveyor belt to be dealt with elsewhere. As for as the injection molding machine is concerned, its job is done.
The entire process of making a shot, injecting it and ejecting it is called a cycle. The cycle time is done very fast pace ranging from 2 seconds to several minutes again depending on the part size (Beganovic). Many machines are completely automated to increase the efficiency and production of parts.

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