Page 283 - Handbook of Properties of Textile and Technical Fibres

Tensile properties of cotton fibers: importance, research, and limitations 257

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fibers. The volume density of the bale may range from 25 to 30 lb/ft (or 0.4e0.5 g/
3
cm ). Throughout the spinning preparation process, the tensile behavior of cotton
fibers is manifested in numerous points of fiberemachine interaction due to the
need to convert a massive bulk of fibers into a linear strand (sliver or yarn). A detailed
discussion of these interaction points is outside the scope of this chapter. Nevertheless,
Fig. 7.12 illustrates two common examples of fiberemachine interaction in which the
fibers can be strained along their axes.
During opening and cleaning, cotton fibers are either freely fed to the different
opening and cleaning units (air fed) or through a pair of feeding roll that are momen-
tarily gripping one end of the fibers at random points while an opening roller is
working on the fibers to perform the opening and cleaning action (Fig. 7.12(a)). For
this reason, the setting between the feed roll and the opening roll is critically important
and optimization of this setting is one of the key design criteria in spinning preparation.
In the drafting zone, pairs of drafting rolls are used and fibers flowing through the
drafting zone may typically exhibit four possible fiber positions (Fig. 7.12(b)): some
fiber ends may be caught at the nip of the front roller (n 1 ), other fiber ends may be
caught at the nip of the back roller (n 2 ), some may be caught by the nips of both
the front and the back rolls (n 3 ), and some fibers may be floating freely in the drafting
zone (n 4 ). These four positions are associated with different degrees of pulling along
the fiber axis. The most serious position is the n 3 -position as it can result in significant
stretching and breaking of the fibers. The ideal flow of fibers in the drafting zone is
when the leading end of a fiber is caught by the nip of the front roller, the moment
the trailing end of the fiber is released from the nip of the back roller in the drafting
zone. This ideal flow cannot be monitored or checked in the current technology and
all possible positions may be occurring simultaneously. For this reason, the drafting
zone is set to be slightly larger than the staple fiber length to avoid floating fibers in
the drafting zones or fiber breakage under tensile forces as a result of the front roll
speed being higher than the speed of back roll to cause drafting.

(a) (b) Fibers gripped at one end
Feed rolls by the nip of the front roller
Pressure rolls
The main
cleaning unit
n 3 n 1
Feed lattice
n 2
Fiber
Fiber
gripping point Back Front
roll roll
Carding
segment
Mote knifes with Fibers gripped at one end
suction hoods by the nip of the back roller
Fibers gripped at both
ends by draft rollers
Figure 7.12 Points of fiber strain in spinning preparation. (a) Straining points during opening
and cleaning; (b) Straining points during drafting.