Carding
is the last major stages of opening and . It is also where separated
fibers are converted into the rope-like sliver form.
The
design of the card developed in the nineteenth century. According to GUNTER there was little basic change over the next
century except for the introduction of ‘revolving
flats’ that move slowly over the surface of a rotating cylinder. The word ‘revolving’
does not mean that the flats revolve about their own axes but merely that they
move around a specified path. The teeth on the active elements have to be very fine
because they have to be capable of handling single fibers. The order of
magnitude of a
typical dimension is 0.1inch. It
also means that they are vulnerable to
damage from foreign objects.
- Embrace opening
- Cleaning
- Blending
- Condensation
- Removal of short fiber and trash
- Removal of nep
- Fibre straightened
- Fibre orientation
CARDING ACTION
Two
alternative arrangements of the carding elements exist; in one, moving flats
cooperate with
the cylinder and in the other, fixed plates or segments are used. In the first
case, some 40 flats are linked together and move slowly over an arc of the rotating
cylinder. The surface speed of the cylinder is usually in the range of 457
m/min. There is a small clearance between the teeth, the setting of which can be
varied from 0.2 to 0.5 mm according to the fiber being processed.
Thus the shear rate is very high and a tuft of fibers caught by one set of
teeth
is wrenched apart by the opposing set. There is very little time for the
fragments of the
tuft to relax until the next division is applied to them. Consequently the
fibers within
the tuftlets retain some orientation in the direction of shear, i.e. in the
direction of
movement of the surface of the cylinder. This permits a carding action.
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| FLAT TOP CARD |
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| FIXED TOP CARD |
It is
possible to replace the flats and their cleaning apparatus by a simple curved plate
with fixed teeth when carding clean fibers of
relatively even length. Some designs exist in which trash can be evacuated from between
the segments by interposing small wedge shaped plates that deflect the flow of air.
The distances between the tips of the teeth on the fixed tops and tips
of the teeth on the cylinder have to be
carefully adjusted. Also care has to
be taken with trash evacuation systems to ensure that they do not choke. Such chokes
might not be detected immediately but cause deterioration in quality that might
not be diagnosed in the early stages. Worn
teeth give trouble and it is customary to test the card output
for nep on a regular basis to provide a control. When the nep levels exceed a level determined by
experience, grinding or rewiring become necessary.Excessive wear would
require regrinding to remove the metal between AA’
and BB’ before an adequately sharp edge could be attained. (Of course, the clearance
would be restored to AB but the tips become wider.) The teeth are case hardened
and consequently there is only a limited number of regrinds that can be carried
out under normal conditions before rewiring becomes necessary. Case hardening means
that the body metal has a thin skin of harder metal. For the extreme case portrayed,
it would then be likely that the case hardening had been ground away, in which
case there would be very rapid wear when the wire was put in to service again. Also
it would be questionable whether a sufficient degree of fiber penetration could be
achieved with the wide tooth tips. In regrinding, too heavy a cut with the in situ apparatus
used to grind the tips of the wire causes burrs to form for a view
as seen with a pocket microscope). This condition might give good nep performance
at the start but the performance deteriorates rapidly thereafter. If problems persist,
it might be time to investigate other designs of card wire. The
fibers leaving the flats on the surface of the cylinder are sometimes exposed to
another carding and/or cleaning process. Carding segments somewhat similar to, but
larger than, the flats carry out the carding at this stage and further cleaning
may be
carried out by installing a knife edge with proper air pressure control and
waste removal
facilities. A cleaning edge is an effective way of removing pepper trash but care
has to be taken to monitor the condition of the knife edge. Hard particles and abrasive
material tend to nick and wear the vulnerable edge that then creates nep and causes operational
problems.Merenyi
reported the sensitivity of the plate-to-cylinder and flat-to-cylinder
settings.
With a 0.008 inch flat setting the mass of flat strip removed increased by 150%
as the plate setting was changed from 0.017 to 0.019 inch. The work was probably
carried out with wire1 clothing but
it still has some relevance.The
reason can be imagined when it is realized that the ingoing nip of two large cylinders
rotating in proximity creates a considerable pressure especially along the line A–A
. Unless the pressure is controlled and contained, it tends to blow out in
the direction of the gray arrows and carry dust and lint with it. There is low pressure
under the cylinder/doffer nip and the flow of air from the high to the low pressure
zone affects the fiber orientation in the fiber transfer zone. The air pressure gradient
in this zone affects air leakage as well as the fiber transfer between the cylinder and doffer.
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| CARD WIRE |
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| CARD WIRE |
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| MICROSCOPIC VIEW OF CARD WIRE |