Multiple-sliver feed improves evenness but also leads to high costs and the need for a very high degree of opening.
Opening is performed as for rotor spinning. In this case also, straightening of the released fibers and the degree of longitudinal orientation are problematic, but exert a strong influence on yarn characteristics.
The fibers can move to the collecting device in free flight (airborne) with (Platt Saco Lowell Masterspinner) or without (Dref- 2000) guidance by a duct. Free flight of the fibers without guidance in a duct leads to fiber disorientation, which affects not only the yarn characteristics but also the spinning limits.
The fibers are drawn by a suction airstream toward the collecting surface and the open yarn end (Fig. 5 (a), (b), and (c)). In rotor spinning, the fibers are additionally accelerated during collection and are thereby straightened, but in friction spinning the opposite happens. The fibers come into contact with a surface that is moving more slowly than they are. The result is fiber-buckling and deterioration in fiber orientation. The fibers are bound into the yarn in a loop form ; this effect is clearly visible in the yarn product and is more marked with longer fibers. The strength of friction-spun yarn is therefore lower than that of rotor-spun yarns.
In terms of flow direction, the fibers meet the drums and the open yarn end at right angles to the direction of yarn withdrawal (Dref), in the same direction, or in the opposite direction. In accordance with the system described by Luenenschloss and Brockmanns , reference is made to forward (Fig. 5 (b)) or backward (Fig. 5 (c)) spinning. In general, fiber guidance can perhaps be classified into (refer to Fig. 5):
- right-angle guidance (a);
- forward guidance (b); and
- backward guidance (c).
Back doubling is obtained in friction spinning as in rotor spinning, but the degree of doubling in friction spinning is smaller.
Imparting twist presents problems as great as those of collecting and binding-in. A strand of loose fibers must take up twist by means of friction on the drums but without the aid of high contact pressure on the drums. The transfer of rotation to the yarn is dependent on the coefficient of friction and the contact pressure; both these quantities are difficult to keep constant between spinning positions and over time. The apparent slip is variable. A notable characteristic of friction-spun yarn is therefore uncertainty about the rate of imparting twist. Nevertheless, from the technical and economic points of view, this method of imparting twist exhibits remarkable advantages. In practically all other twisting assemblies, one revolution of the twisting element is needed to impart one turn of twist to the yarn. In friction spinning, one revolution of the twisting element can generate several turns of twist. This result is obtained because of the large difference in diameter between the drums and the yarn.
With reference to Fig. 6 (a) and (b), drum (1) has to rotate through a fraction of a revolution to cause the yarn to rotate once, i.e. one full drum revolution generates 100 and more yarn turns. The illustration also shows that the transmission ratio is still greater for fine yarns (with a smaller yarn diameter) than for coarse yarns. In the course of one drum revolution, the fine yarn therefore takes up more turns of twist than the coarse yarn. This remains true even though the smaller zone of contact of the finer yarn on the drums leads to greater apparent slip. This is the only spinning method in which the delivery speed is practically independent of yarn count .
The high transmission ratio (up to 200:1) has the further advantage that a lower rate of drum revolutions suffices, although, when considered in relation to the diameter ratio, the yarn takes up only 15 - 40% of drum rotation . Delivery speeds can be made correspondingly high. Spinning speeds of 500 m/min or even higher are conceivable. Unfortunately, the spinning speed is limited in practice by yarn quality to some 200 m/min. In fact, a higher fiber throughput rate leads to a deterioration in yarn quality.
Withdrawal and winding up
In contrast to most other spinning processes, yarn tension (and hence end break frequency) is very low during withdrawal from the spinning zone. Tension therefore has no influence on the spinning limit. The yarn is wound up onto cross-wound packages so that, in comparison with conventional spinning, rewinding is eliminated.