The Industry of turning plastic resin into everyday items has greatly changed the world of products we know today. This change came about with the invention of a process called injection molding. It was invented in the early thirties in the U.S. and was responsible for filling the world with "made in the USA" products. One of the major advantages of injection molding is the amount of choices it brings to manufacturing because of it's versatility. Excess and unused pieces of plastic can be melted down and reused, so this process has also greatly reduced loss. The major distinction that separates injection molding from metal casting, is that where you must pour metal into a mold, molten plastic can be forcibly injected into the mold.
The products that are manufactured using the extrusion method, which uses huge injection machines, vary from small toy rings found in gum ball machines, to large side panels for cars.
The two elements of these injection molding machines are the injection units and the clamping units.
The procedure all begins when a mold is pressure clamped during the injecting and cooling-down steps. The plastic resin is put in a machine called an injection barrel and then it is brought up to the temperature where it melts. It is then forced directly in to the molding.
The following step is called dwelling, and it is where a hydraulic pump forces the melted plastic into all of the crevices within the molding. The next step is the cooling process, where the plastic hardens while in the molding.
The last phase of the process is where the two halves of the molding are separated and the product is forced out by the injection pins. The parts of the product that stick out and are extra are called runners, and are simply cut off, melted down and used again. This entire process only takes anywhere from ten seconds to two minutes, and is repeated over and over again.
This time difference is due to the fact that different companies have different policies how long the plastic should cool. Each type of plastic resin will shrink a different amount during the cooling process, and the manufacturer must know before hand exactly by how much. Plastic sometimes has the tendency to warp, and this another major problem to avoid.
This warpage happens when the temperature of the plastic is too elevated when it's injected into the mold, and it sets unevenly. As well as warping, this elevated temperature can hinder the plastic from reaching every part of the molding, so heat control is a big issue. The plastic moldings used in this process can be rather expensive, because all of the high quality ones are all made in America, so purchasing in larger orders is the best way to keep your overhead down.