Twist has effect on the following
properties:-
1)
Bending behavior.
2)
Resistance to creasing.
3) Drape ability.
4) Strength
5) Resistance
to abrasion.
6) Elasticity.
7) Impact
Strength.
8) Stress
distribution and analysis.
More twist cause hard yarn production. Excessive
twist causes breakage of yarn. With the increasing twist yarn strength also increases up to a certain limit. After that the strength of yarn decreases with the further increasing of twist.
twist causes breakage of yarn. With the increasing twist yarn strength also increases up to a certain limit. After that the strength of yarn decreases with the further increasing of twist.
STRENGTH VS TWIST CURVE:
With the increase
of twist, stretch on fiber increases. Strength vs twist curve can be
divided into 3 sections.
1) SECTION
a. It is the low twist region. In this region yarn strength increases with the
increase of twist.
2) SECTION
b. It is the medium twist region. In this region the yarn strength increases
with some fiber breakages.
3) SECTION
c. It is the high twist region. In this region the strength does not increase
with the increase of twist. In this region the fiber breakage is high.
EFFECT OF TWIST ON YARN STRENGTH:
1) Twist
increases the strength of staple yarn up to an optimum limit.
2) Finer
the yarn higher the twist per inch (TPI).
3) Higher
TPI lower the production.
4) Higher
the TPI higher the cost of production.
5) Higher
the TPI, the more the spirality and thus the obliquity is higher.
ANGLE OF TWIST AND
TWIST FACTOR
Although the amount of twist in a yarn is usually expressed
in turns per inch (t.p.i.), this is useless for comparing yarns of different
counts. What matters is not the number of turns but the angle of twist. This is
the angle which fibers on the surface of the yarn make with the axis of the
yarn, 0 in
In this diagram, yarns A and B have the same
angle of twist, but yarn B is twice as thick and has half as many t.p.i. as
yarn A. Their relative degree, or hardness, of twist cannot be compared in
terms of their t.p.i., but it can by comparing their angle of twist. Since this
is the same, their properties will also be similar.
If t = t.p.i., then 1/t is the
length occupied by one complete turn, or spiral, of twist. Now imagine the
surface of yarn B to have been peeled off, in figure C. The figure shown in
figure B will now form the diagonal of a rectangle of length l / t and
width π d. So
tan Ө = π d
¸
l / t = π dt.
The constant k is called the twist factor or the twist
multiplier.
We can now use the twist factor
either to calculate the t.p.i. required to give the desired degree of twist in
any counts, or to compare the degree of twist in yarns of different counts. It
is to noted that different counting systems will give different twist factors
for the same angle of twist, and therefore to be compared must their counts expressed
in the same system
EXAMPLE:- Compare the degree of twist in the following
yarns:
a) 36s
cotton, 27 t.p.i., and b) 16s cotton, 16 t.p.i.
The two yarns have the same
degree of folding twist.
Twist factor is not often used in connection with
continuous filament yarns, but with the introduction of the tex system it will
be necessary to recalculate twist factors for spun yarns whose yarn number is
expressed in the tex system.
Table below gives the angles of
twist, based on the a specific volume of 1.1, corresponding to the normal range
of twist factors for single cotton yarns.
It also shows the corresponding tex twist factors, derived as above.
TWIST EFFECT ON STRENGTH
Table
Angle of
twist Cotton twist factor Tex twist factors
t.p.i. t.p.m.
18
36 3.0 72.9 2,870
24
10 4.0 97.2 3,827
28
18 5.0 121.5 4,784
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