Info Teknologi Tekstil: Drafting arrangement

In most spinning mills today, the first intermediate product is a card sliver. It contains about 20 000-40 000 fibers in cross-section.

This number must be reduced in several operating stages to about 100 in the yarn cross section. The reduction can be effected in two ways:

· through the draft, i.e. the distribution of an approximately constant total number of fibers over a greater length of the product ; or
· through elimination of fibers (loss) into waste (p).

Elimination is not an intentional reduction of the number of fibers but arises as an unavoidable side effect of the necessity for cleaning; it occurs in the blowroom, in carding, and in combing. (Fiber loss is intentional in combing, as the aim is to remove short fibers.)

However, since drafting takes place simultaneously here, the term “attenuation” is used. This is defined by:

$Attenuation = Draft \times \frac {100}{ \left(100 -p \right)}$

p is the waste percentage.

The reduction of the number of fibers in the cross-section logically leads to a reduction in diameter of the strand. In terms of fineness, the following relationship is obtained:

$\frac {d_A}{d_Z} = \frac { \sqrt{tex_A}}{ \sqrt{tex_Z}}$

where
d_A = diameter of delivered product;
d_Z = diameter of infeed product.

Fig.43 Draft trough rollers

During drafting, the fibers must be moved relative to each other as uniformly as possible by overcoming the cohesive friction. Uniformity implies in this context that all fibers are controllably rearranged with a shift relative to each other equal to the degree of draft.

However, such regularity is utopian as regards both the fiber material and the mechanical means available. Drafting operations always run irregularly, and each draft stage will therefore always lead to an increase in unevenness.

Drafting is effected mostly on roller-drafting arrangements (Fig. 43). The fibers are firmly nipped between the bottom steel rollers and the weighted top pressure rollers. If the rollers are now rotated in such a way that their peripheral speed in the throughflow direction increases from roller pair to roller pair, then the drawing apart of the fibers, i.e. the draft, takes place. This is defined as the ratio of the delivered length (L_D) to feed length (L_F), or the ratio of the corresponding peripheral speeds:

$V = \frac{L_D}{L_F} = \frac{V_D}{V_F}$