Ask A Scientist© Environmental Earth Science Archive

 
 


>
> > >   name        sarah
> > >   status      student
> > >   age         20s
>
> > >   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?
> >
>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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