Injection molding is a process of melting plastic and injecting it into a mold. It offers benefits such as minimal scrap losses and minimal finishing requirements. The process uses large machines with two basic elements: the injection unit and the clamping unit. The completed part may contain extraneous pieces called runners, which are cut up and recycled. The injection molds themselves can be expensive, but the resulting plastic parts are reasonably priced. This molding method can be used with a variety of plastic resins, including polypropylene, polyethylene, and ABS.
Injection molding is a process of shaping plastic by melting it and injecting it into a pre-designed mold. The process was first designed in the 1930s and was originally based on metal die casting designs. It offers many benefits to alternative manufacturing methods, including minimal scrap losses (since scrap pieces can be melted down and recycled) and minimal finishing requirements. This process differs from metal die casting in that molten metals can simply be poured; plastic resins must be injected with force.
The process uses large injection molding machines, which advance resins through six major processes to produce everything from computer parts to plastic Halloween spiders. Although this machine is a complex piece of equipment, it is made up of two basic elements: the injection unit and the clamping unit.
The process begins with a mold, which is clamped under pressure to accommodate the injection and cooling process. Then, the pelletized resins are fed into the machine, followed by the appropriate dyes. The resins then fall into an injection barrel, where they are heated to their melting point and then injected into the mold via a screw or ramming device.
Then follows the indwelling stage, wherein the molten plastic is contained within the mold, and hydraulic or mechanical pressure is applied to ensure that all cavities within the mold are filled. The plastic is then allowed to cool inside the mold, which is then opened separating the two halves of the mold. In the final stage, the plastic part is ejected from the mold with ejector pins. The completed part may contain extraneous pieces called runners, which are cut up and recycled. The entire process is cyclical, with cycle times ranging from ten to 100 seconds, depending on the cooling time required.
The injection molding process requires some complex calculations. Each different type of resin has a shrinkage value that must be taken into account and the mold must compensate for this. If this value is not accurately determined, the final product will be incorrectly sized or may contain defects. Typically, this is compensated for by first filling the mold with resin, holding it under pressure, and then adding more resin to compensate for the shrinkage. Other complications can include burnt parts resulting from setting the melt temperature too high, warping resulting from uneven surface temperature, or incomplete filling due to too slow an injection stroke.
The injection molds themselves can be surprisingly expensive, sometimes as much as $100,000 US Dollars (USD). If the quantity of parts desired is large enough, however, the cost of the mold becomes relatively insignificant and the resulting plastic parts are very reasonably priced. Some molds are made with more than one cavity; these multi-cavity molds cost more than their single-cavity counterparts, but due to increased manufacturing efficiency, the cost per part is minimized.
This molding method can be used with a variety of plastic resins. The most popular resins for this type of molding include polypropylene (PP), polyethylene (PE), and ABS. Each has its own set of advantages and disadvantages, and they are chosen based on the desired characteristics of the final part.
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