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dbanig@prflexbag.com.
Best regards
David Banig
http://www.prflexbag.com Polyethylene Film Extrusion Intro
An extruder is a screw turning cylinder or extrusion
barrel. There is a feed hopper at one end of the barrel and
a specially shaped hole or die at the other, where the
product comes out. Plastics for extrusion are thermoplastics they get soft
when heated and harden again on cooling. When such a
material is fed into the hopper it is caught by the screw
and pushed through the barrel where it gets hot and softens
enough to continue out through the die. The heat is
generated by friction as the screw turns in the plastic
mass. The energy to melt really comes from the motor as it
turns the screw. Sometimes more heat is provided by
external barrel heaters and pre heated feed throat. As the hot soft plastic comes out of the die it takes shape
of the hole it passes through a long slit makes a film or
sheet, a circular opening makes pipe many small holes make
filaments, etc. Once out of the die the plastic must be
cooled quickly by air, water or contact with metal and
pulled away to be rolled or cut up to the desired
dimensions. The turning screw can be imagined as trying to unscrew
itself backward out of the barrel full of material. It
can't go back because a bearing holds it in place but its
pushed against the material forces that material out the
other end. The die at the other end acts as a resistance. The longer
and smaller it is the more screw must work to push the
material out (horsepower required of the motor that turns
the screw). The extruder operator controls the temperatures of the
screw and the material that enters. The barrel is divided
into zones. each with its own heating and cooling controls.
Barrel temperatures do have to match material temperatures
but are selected as needed in each zone. The rear feed zone
is especially important as it affects the feed rate and may
control production rate. In the head and die there are
heaters and controls also and metal temperatures are
usually close to material temperatures. Material melt temperature is measured at the output end of
the screw just before the plastic enters the die. It is
typically between 350 to 450 degrees F. but may be more for
certain plastics. If it gets to high there maybe cooling
problems of the material or perhaps chemical breakdown of
the plastic. Melt pressure is also measured at the output and of the
screw and reflects the resistance of the head and die
assembly. Typical pressure are between 500 psi and 5,000
psi and for tiny dies or very viscous material it may be
even more. Screw speed is selected usually as high as long as it
produces good product. Typical screw speeds are between 50
and 150 RPM. Motor amps are measured as a warning to stay below the
danger limits and to show up surging which causes thickness
variation in the material and unexpected changes in
material viscosity (ease of flow). Amps and horsepower vary
greatly with machine size from small machines 5 to 50
horsepower up to huge lines for compounding which uses
1,000 horsepower or more.
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The extrusion of polyethylene materials is really a complex
issue.
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Best, regardsDavid, Banighttp://www.prflexbag.comPolyethylen,
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