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[*] posted on 1-9-2012 at 01:43
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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[*] posted on 1-9-2012 at 05:53


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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[*] posted on 1-9-2012 at 15:48


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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[*] posted on 2-9-2012 at 07:09


Quote: Originally posted by denatured  
Anyone knows which book(s) discusses the design of plastic machinery?
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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[*] posted on 2-9-2012 at 09:47


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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[*] posted on 2-9-2012 at 19:29


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...



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