Open-end spun yarns


Fig. 53 – Binding-in of the fibers in open-end spinning

Fig. 54 – Yarn formation in the rotor

In contrast to ring spinning, twisting during rotor spinning takes place from the inside outwards. The rotating, brush-like open yarn end (C, Fig. 53) first catches fibers in the core and then with further rotation gradually takes up fibers towards the periphery. In the interior, where the fibers cannot avoid the twist, the strand becomes more compact but also somewhat harder. On the other hand, towards the exterior, compactness and hardness fall off to an increasing degree, since here the fibers are able partially to avoid twisting-in.

Typical characteristics of this so-called core-twist are therefore a harder handle accompanied by a lower strength than is obtained with sheath-twist, since the outer layers have relatively little twist and can thus contribute little to strength. However, abrasion-resistance is often better. Removal of outer fibers due to abrasion has little effect, since these fibers did not create much strength anyhow. In rotor-spun yarns, this outer layer exhibits other peculiarities. One of these is the presence of wrap fibers. These are fibers which fly directly onto the fully created yarn as the rotor passes under the feed passage. By the further rotation of the yarn in the rotor they are wrapped around the already spun yarn like the band on a cigar. This is a typical characteristic of rotor-spun yarn.

Another peculiarity is a thin outer layer of fibers with hardly any twist, or even with twist in the reverse sense. This arises from the false twist between the navel (Fig. 54, T) and the binding-in zone (A). In the latter, during each rotation of the rotor, new fibers join on to the already well twisted fiber strand. These latecomers receive only a fraction of the desired twist level. If this low twist is less than the false-twist effect, the fibers are twisted in the reverse sense during cancellation of the false twist (reverse twisting) at the navel, and are thus wrapped around the other fibers with reverse twist.

A further disadvantage of the loose outer layers is their sensitivity to axial rubbing. Since these open layers are not firmly secured in the core, they tend to accumulate in small knots during passage of the yarn over edges, guide elements, etc. As far as possible, open-end spun yarns should not be rewound.