Needle
Punching
Needle
punching or needle-felting relies on frictional forces and fibre entanglements,
and are known as mechanical bonding. The basic concept of needle-felting is
apparently simple; the batt is led between two stationary plates, the bed and
stripper plates, as shown in Diagram. While between the plates the batt is
penetrated by a large number of needles, up to about 4000 per meter width of
the loom. The needles are usually made triangular and have barbs cut into the three
edges. When the needles descend into the batt the barbs
catch some fibres and pull them through the other fibres. When the needles
return upwards, the loops of fibre formed on the down stroke tend to remain in
position, because they are released by the barbs. This downward pressure
repeated many times makes the batt much denser, that is, into a needle-felt.
The
above description illustrates how simple the concept seems to be. Without going
into too much detail it may be interesting to look at some of the complication.
First, the needles can only form vertical loops or ‘pegs’ of fibre and increase
the density of the batt. This alone does not form a strong fabric unless the
vertical pegs pass through loops already present in the horizontal plane of the
batt. It follows from this that parallel-laid fabric is not very suitable for needling
since there are few fibre loops present, so most needling processes are carried
out with cross-laid, air-laid and spun-laid batts. Second, the amount of
needling is determined partly by the distance the drawing rollers move between each
movement of the needle board, the ‘advance’, and partly by the number of
needles per meter across the loom. If the chosen advance happens to be equal
to, or even near the distance between needle rows, then the next row of needles
will come down in exactly the same position as the previous row, and so on for
all the rows of needles. The
result
will be a severe needle patterning; to avoid this, the distance between each
row of needles must be different. There are several different types of needle
loom. The machine shown in Figure 1 is called a ‘down-punch’ because it is pushing
the fibres downwards. Similarly an ‘up-punch’ pushes the fibres upwards. There
is some advantage in combining an up-punch with a down-punch when trying to
make a dense felt, rather than to punch continually in the same direction. For
this reason some looms are made ‘double-punch’, that is, one board is punching
down and one board punching up.
Needle
felts have a high breaking tenacity and also high tear strength but the modulus
is low and the recovery from extension is poor. For these last two reasons any
needle felt which is likely to be subjected to a load has to have some form of
reinforcement to control the extension. Needled carpets, for instance, may be
impregnated with a chemical binder that gives better dimensional stability and
increases the resistance to wear.
Needle
felts are used widely in gas filtration media and in some wet filtration.
Needle felts are also used in geotextiles, but in view of the low modulus their
application is mainly in removing water rather than as reinforcement. Many
makers of synthetic leather have taken the view that the structure should be
similar to natural leather. In these cases the backing or foundation of the
synthetic leather is a needle felt. Needle felts are also widely used in home
and commercial carpeting.
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