Spun Laying
Spun laying includes extrusion of the filaments from the polymer raw material, drawing the filaments and laying them into a batt. As laying and bonding are normally continuous, this process represents the shortest possible textile route from polymer to fabric in one stage Spun laying starts with extrusion. Virtually all commercial machines use thermoplastic polymers and melt extrusion.
Spun laying includes extrusion of the filaments from the polymer raw material, drawing the filaments and laying them into a batt. As laying and bonding are normally continuous, this process represents the shortest possible textile route from polymer to fabric in one stage Spun laying starts with extrusion. Virtually all commercial machines use thermoplastic polymers and melt extrusion.
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| Spun Laying |
Polyester and polypropylene are by
far the most common, but polyamide and polyethylene can also be used. The
polymer chips are fed continuously to a screw extruder which delivers the
liquid polymer to a metering pump and thence to a bank of spinnerets, or
alternatively to a rectangular spinneret plate, each containing a very large
number of holes.
The liquid polymer pumped through
each hole is cooled rapidly to the solid state but some stretching or drawing
in the liquid state will inevitably take place. In spun laying the most common
form of drawing the filaments to obtain the correct modulus is air drawing, in
which a high velocity air stream is blown past the filaments moving down a long
tube, the conditions of air velocity and tube length being chosen so that
sufficient tension is developed in the filaments to cause drawing to take
place. The laying of the drawn filaments must satisfy two criteria; the batt
must be as even as possible in mass per unit area at all levels of area,
and the distribution of filament orientations must be as desired, which may not
be isotropic. Taking the regularity criterion first, the air tubes must direct
the filaments onto the conveyor belt in such a way that an even distribution is
possible. However, this in itself is not sufficient because the filaments can
form agglomerations that make ‘strings’ or ‘ropes’ which can be clearly seen in
the final fabric.
A number of methods have been
suggested to prevent this, for instance, charging the spinneret so that the
filaments become charged and repel one another or blowing the filaments from
the air tubes against a baffle plate, which tends to break up any
agglomerations.
Air Laying
The air-laying method produces the
final batt in one stage without first making a lighter weight web. It is also
capable of running at high production speeds but is similar to the parallel-lay
method in that the width of the final batt is the same as the width of the
air-laying machine, usually in the range of 3–4m.
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| Air-Laying |
The degree of fibre opening it is
very much lower than in a card. As a consequence of this more fibre opening
should be used prior to air laying and the fibres used should be capable of
being more easily opened, otherwise the final batt would show clumps of
inadequately opened fibre.
The diagram in Fig. 5 shows the
principle of an air-lay machine, although the actual machines may vary
considerably from this outline. Opened fibre from the opening/blending section
is fed into the back of hopper A, which delivers a uniform sheet of fibres to
the feed rollers. The fibre is then taken by the toothed roller B, which is
revolving at high speed.
There may or may not be worker and
stripper rollers set to the roller B to improve the opening power. A strong air
stream C dislodges the fibres from the surface of roller B and carries them
onto the permeable conveyor on which the batt is formed. The stripping rail E
prevents fibre from recirculating round the cylinder B. The air flow at D helps
the fibre to stabilise in the formation zone.
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