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Plastic Extruder is essentially a machine used to melt the plastic
and deliver the melted plastic under pressure to a die. Typically,
extrusion begins with granular material, gravity-fed via a hopper
to a rotary screw.
The
screw is a raised flighted helix that traps material and moves it
forward through an enclosed, heated barrel where it is first melted,
and then pressurized.
Extruders are manufactured for horizontal or vertical operation,
and are available in many sizes and shapes. Individual product
requirements determine the type of extruder needed.
Two basic designs are common:
The
illustration below shows the current industry standard — a vented
Plastic Extruder having a two-stage screw.
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Typical
Vented Plastic Extruder
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In the two-stage screw design, the barrel is vented to a vacuum
pump at approximately 55 to 60 percent of cylinder length, or typically
after 17 flights in a 32:1 L/D (length to diameter) extruder.
Vented extrusion allows better release of entrapped volatiles and
moisture than does non-vented extrusion — leading to better product
appearance, uniformity and properties.
For maximum effectiveness, the vent should be connected to a vacuum
pump to remove volatiles more completely. Because the vent is located
in a decompression zone of the screw, the polymer melt continues
toward the die rather than going out the vent. However, when high
back pressure is encountered at the die, some material may ooze
out of the vent. This problem can be corrected by:
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Decreasing
the head pressure or screen pack resistance, or
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Slightly
raising the rear zone (hopper) barrel temperature, which also
decreases the first-stage pumping rate, or
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Sometimes
lowering second stage barrel temperatures.
The
head pressure valve is used for melt pressure control. It can
be a significant factor in eliminating surging, non-uniform
melt temperature, or non-uniform additive dispersion. The valve
can be opened or closed, depending on overall operating needs.
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