Plastic extrusion process
In the extruded plastic, the top loading
hopper is added to the barrel of the extruder.
Material enters through the feed throat
and comes into contact with the screw.
The rotating screw forces plastic beads
forward into the barrel.
The barrel is heated to the desired
melting temperature of the molten plastic (which may be 200°C/400°F to
275°C/530°F depending on the polymer).
In most of the processes, a heating
curve is set for the bucket, in which three or more independent PID-controlled
heater zones gradually increase the temperature of the bucket from the rear
(where the plastic enters).
This allows the plastic beads to
gradually melt as they are pushed through the cartridge and reduce the risk of
overheating that may lead to polymer degradation.
The extra heat is caused by the intense
pressure and friction in the barrel.
In fact, if the extrusion line runs a
material at a sufficiently fast speed, the heater can be turned off and the
melt temperature maintained within the barrel by separate pressure and
friction.
After passing through the crush plate,
molten plastic enters the mold. Almost any conceivable shape can be created as
long as it is a continuous outline.
The product must now be cooled, usually
by pulling the extrudate through a water bath. Plastics are very good thermal
insulators and therefore difficult to cool quickly. Compared to steel, plastics
have a slower cooling rate of 2000 times. In a tube or tube extrusion line, a
carefully controlled vacuum is applied to the sealed water bath to keep the
newly formed and still molten tube or tube from collapsing. For products such
as plastic sheets, cooling is achieved by pulling through a set of chill rolls.
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