denatured
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Melting in plastic processing machinery
How plastic is melt inside a plastic processing machine [injection - extrusion] is done without burning/oxidizing it? do they vacuum the heating
cylinder or replace the air with an inert gas?
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watson.fawkes
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Quote: Originally posted by denatured  | | How plastic is melt inside a plastic processing machine [injection - extrusion] is done without burning/oxidizing it? do they vacuum the heating
cylinder or replace the air with an inert gas? |
The use of a thermostat prevents overheating.
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denatured
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Whenever I melt PET, it never melts to clear plastic. Its color is white-yellow or white with black dots or yellow, never white. I set the oven to
480-500F.
Anyone knows which book(s) discusses the design of plastic machinery?
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watson.fawkes
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I can't provide a title, but there a
few of the thick industrial "encyclopedia" volumes from the usual-suspect publishers that give a complete overview of plastics technology, including
fabrication machines. I've looked through one, but don't recall a title.
The machines you're referencing tend to run on prilled feedstock. There's a combination auger-piston feeding the melting chamber. I do recall
thermostatic control to preserve the material being important. I don't recall inert gas shields, but that doesn't mean they're not used. One
difference with your account about an oven is that prilled stock doesn't have much gas to begin with and the gas doesn't really flow anywhere, as the
prills melt and exclude any surrounding gas. If there's degradation through oxygen, it might only be a small amount at the beginning of a run. After
than, you'd have a nitrogen atmosphere. By contrast, there's significant air flow in a consumer oven (to vent moisture) and so you wouldn't run out of
oxygen.
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Broken Gears
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The reciprocating screw in an injection machine is conical, which means that the plastic chips that travels along the screw not only is heating to its
melting point, but also highly compressed. Its shoots the liquid plastic into the mold at high pressure and fills the mold almost instantly.
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bbartlog
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In the case of extrusion, there is generally no contact with air until the plastic comes out of the die; if conditions are such that this would result
in combustion it could be either precooled by a cold section at the end of the die or even spray-cooled with liquid just as it emerged. But I think
that for most plastics there is a softening/melting temperature that is well below the ignition temperature, so mostly this just comes down to
temperature control. In the early 1990s I worked at a company that made machines which would weld the corners on PVC window frames. Pretty primitive
really; heat an aluminum plate to 260C (IIRC), mash the mitered ends of the extruded PVC bits against the plate for a little while, quickly withdraw
plate and mash the softened ends together. Some browning was generally observed in the weld seam but there were subsequent cleanup steps...
The less you bet, the more you lose when you win.
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Texium
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Thread Moved
20-11-2023 at 12:23 |
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