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Name: Sarah
Status: Student
Age: 20s
Location: N/A
Country: N/A
Date: N/A


Question:
how are those fashionable plastic "containers" we see sold in stores like the container store and target made? The items, from chinese take-out boxes which are marketed as gift giving items, and pseudo-briefcases, are made out of one piece of plastic folded in>various ways. I wondered if they were cut out from sheets?



Replies:
Most "one piece" plastic items are manufactured by a process called: "injection molding". The parts/procedure consist of a 3-D hollow template or mold the shape of the article being produced. The mold is usually made of steel. The plastic is formulated in advance to have the desired properties such as color, viscosity at elevated temperature [You'll see why in a moment.], flexibility, and so on. The list of properties can be long. The plastic is heated to a temperature above its glass transition temperature. That is the temperature where it becomes molten -- the viscosity of honey or even higher, depending on the equipment. The molten plastic is then squirted [injected] into the mold by a piston or by high pressure air. The mold has been heated to a temperature somewhat lower than the glass transition temperature. The molten plastic fills all the nooks and crannies of mold, but cools below the glass transition temperature, and hardens,so it no longer flows. The mold is snapped open, and the plastic piece pops out and goes on down the assembly line, cooling more as it goes along.

These are big, fast machines that can produce hundreds or thousands of articles per minute, depending upon the size and complexity of the article. If you look at the bottom of a soda bottle, or some inconspicuous place on some other plastic article, and see a little "nib", it was probably formed by injection molding.

Vince Calder



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