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Unit_8_-_Plastic_Packaging (7)

Unit_8_-_Plastic_Packaging (7) - through the die while...

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8.8.7 Injection Molding Cups, bowls, tubs and similar containers, closures, and many other objects are formed by injection molding. To form a tub or similar shaped container, a mold is made in the shape and with the dimensions of the outside of the container. A second part of the mold, a moveable piece called a plug, is inserted into the outside mold, leaving a thin space between the two parts. Melted plastic from an extruder is forced into the space between the two parts of the mold, flowing from a single feed point called a sprue. The sprue generally feeds plastic into the mold from a location at the center of the bottom. The type of extruder used to melt plastic for injection molding is usually called an injection molding machine. It is very similar to the extruder described above; the main difference is that extruders used for extrusion are usually designed to flow plastics continuously
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Unformatted text preview: through the die, while those used for injection molding are designed to accumulate a bunch of melted plastic, called the “shot”, and then suddenly push (inject) it into the mold. After the mold has been filled, it is held closed long enough for the plastic to cool into the shape of the mold. The plug is then pulled out and the finished object is pushed out of the outer mold. As it is pushed out, a small piece of plastic from the sprue is broken off, leaving a tiny rough point at the center of the bottom of the container. This rough point can be used to identify a container made by the injection molding process. In a commercial facility, an injection molding machine may make only one large object at a time or as many as 150 smaller items, such as closures. Cycle time can range from a few seconds to as long as half a minute. 8. 20...
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