Transmission Efficiency of plastic Films Part 1

TRANSMISSION EFFICIENCY OF PLASTIC FILMS
volume of one flight of the screw at the hopper end to the volume of one flight at the
die end. L/D ratios is most commonly used for single screw extruders are between
about 15:1 to 30:1 while compression ratios can vary from 2:1 to 4:1. An extruder
screw is usually divided into three sectors, namely, feed, compression and metering.
The feed section transports the material from under the hopper mouth to the hotter
portion of the barrel. The compression section is that section where the diminishing
depth of thread causes a volume compression of the melting granules. The main
effect of this is an increase in the shearing action of the molten polymer due to the
relative motion of the screw surface with respect to the barrel wall.
This improves the mixing and also leads to an increase in frictional heat and a more
uniform heat distribution throughout the melt. The function of the final section of the
screw is to homogenize the melt further, meter it uniformly through the die and
smooth out pulsations.
Multi-screw extruders, particularly twin-screw extruders also have their own
advantages and limitations. In general, multiple-screw machines are more expensive
and because of their more complicated construction are likely to be less sturdied. A
more positive transport of the molten polymer is possible, and better mixing is
obtained. By virtue of their positive pumping action, multiple-screw extruders produce
less shear heat and this makes them very suitable for materials that are heat
sensitive, have low coefficients of friction or must leave the die at low extrusion
temperatures.
2.2 COEXTRUSION:
This method of producing composite films differs in one important detail from all
those already described, As it can be adapted to produce a composite film in either
lay-flat tubing form or as a flat film. The process for making tubular composite films is
often referred to as coaxial extrusion.
Basically the process consists in coupling two or more extruders to a single die head.
It is essential that precise control is available over screw speed, melt pressures and
temperatures and power capacity of the extruder drives. Size of extruders used will
depend on the ratio of the components in the laminate. Extruders of similar capacity
are usually employed when a wide range of composite film thicknesses is required,
but for composites of widely different polymer content, different sizes of extruders are
usually combined. So minimizing capital cost.
However, the success of the technique lies chiefly in the design of the die and the
way in which the individually extruded melts are brought together, prior to being
extruded as a multi-layer film. There are, essentially, two different methods of
carrying out this operation. In one system, each separate melt is extruded into a
separate manifold and then brought together at a common point inside the die. The
channels within the manifold are normally calculated to match the flow properties of
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