Rieter

When it is removed from the rotor, the yarn is diverted virtually at right angles by the draw-off nozzle protruding into the rotor and guided out by the draw-off tube immediately following it. Meanwhile the yarn rolls continuously on the surface of the draw-off nozzle. During the rolling motion the yarn is repeatedly raised briefly in rapid succession from the nozzle surface – due to the design of the draw-off nozzle surface. This high-frequency vibration – together with the false-twist effect created by the unwinding motion – promotes twist propagation into the rotor groove. The greater the false-twist effect and the more intensive the creation of twist in the rotor groove, the lower the genuine yarn twist that can be selected and the bulkier and softer the yarns that can be spun. Spinning stability also improves with the increasing false-twist effect, of course.
In this context the positioning of the top edge of the nozzle relative to the rotor groove is also of some importance. Normally, the draw-off nozzle protrudes far enough into the rotor cup for ends extracted from the rotor groove to be diverted virtually at right angles at the draw-off nozzle. The position of the draw-off nozzle relative to the rotor groove can be changed by means of washers of differing thickness. The further the draw-off nozzle protrudes into the rotor, the larger the yarn‘s angle of wrap at take-off, the more false twist is created and the longer the binding zone in the rotor groove. In some cases this can help to increase yarn tenacity. If the washers are removed, the false twist effect is reduced, the binding zone becomes shorter and thus the positive impact on yarn tenacity is also reduced.

Draw-off nozzles are made of either ceramics or steel. Drawoff nozzles usually consist of two parts, a wear-resistant ceramic nozzle head and a metal nozzle holder (Fig. 78). Ceramic or metal draw-off nozzles are also in use in which nozzle head and nozzle holder are produced in one piece. There are no technological differences, except that solid ceramic draw-off nozzles feature very low heat dissipation (ceramics are used as insulators in electrical installations) and can therefore hardly be considered for processing manmade fibers. By contrast, metal draw-off nozzles feature excellent heat dissipation, would therefore also be ideally suitable for processing man-made fibers, but due to short service lives are only used in certain cases for processing very temperature- sensitive man-made fibers, i.e. fibers with very low melt and softening point.

The use of appropriate types of ceramic and the combination of ceramic head and metal holder create conditions with regard to heat dissipation that enable most common man-made fibers and their blends to be processed successfully. The service life of ceramic nozzles can be several years, depending on raw material and material throughput, and they are the most long-lived spinning element compared with the service lives of opening rollers and rotors. Only the processing of cottons with a high mineral sand content and man-made fibers containing too much delustring agent (> 0.15% TiO₂) can appreciably reduce the service life of a ceramic nozzle. If these restrictions are observed, the service life of a ceramic nozzle is between 10 000 hours (PES, CV, PAN) and 20 000 hours (CO), although in mill operations service lives of between 20 000 and 40 000 can certainly be achieved with these materials. Service lives with blends of cotton and man-made fibers are about in the middle of these ranges.

If the rotor groove makes a crucial contribution to yarn quality and bulk in technological terms, the structure and design of the nozzle surface exert a decisive influence on surface structure and hairiness.

Fig. 79 to Fig. 85 illustrate the different surface designs of draw-off nozzles (surface in contact with the yarn).

Essentially, the following types of nozzle are used in manufacturing the wide range of rotor-spun yarns:

• Nozzles with a smooth surface (Fig. 79) are suitable for producing smooth warp yarns with low hairiness. This type of nozzle is rarely used, since very high yarn twist has to be imparted due to the low level of false twist created. Yarn values are not better than with other nozzle types in every case. The use of a TWISTstop draw-off tube (refer to section Draw-off tubes with and without ceramic insert (TWISTstop)) is recommended for stable running conditions.
• Nozzles with a spiral surface (Fig. 80) are ideally suitable for compact and fine warp yarns in 100% cotton with low hairiness and good yarn values. High spinning stability.
• Nozzles with 3, 4, 6, 8 or more notches (Fig. 81) are universally applicable both for cotton and also for man-made fibers and their blends. The nozzle with 4 – mostly short – notches is the universal nozzle with the widest range of application: suitable for both warp and weft yarns (e.g. 4 notches) or knitting yarns (4 - 8 notches, depending on the required hairiness). Notched nozzles usually offer high spinning stability – the more notches, the higher the false-twist effect and the higher the spinning stability – but the higher also the yarn hairiness and the tendency to generate fly in downstream processing. Furthermore, the higher the number of notches, the more aggressive their effect and the greater their influence on yarn quality.
• Spiral or notched nozzle surfaces combined with an eddy insert in the nozzle throat (Fig. 85) are used solely – but very successfully – for very hairy, bulky and very soft-twist knitting yarns. The nozzles also offer very good spinning stability. However, yarn quality is not first priority with these nozzles!
• Externally knurled draw-off nozzles with additional notches in the nozzle radius (Fig. 82) and an eddy insert in the nozzle throat are recommended solely for manufacturing extremely hairy, very bulky, soft-twist yarns. „Yarn quality“ corresponds to the yarn structure.
• Nozzle surfaces with a small nozzle radius and 4 short notches (Fig. 83 and Fig. 84) feature a smaller contact surface compared with the standard nozzle radius and are therefore especially suitable for processing PES and its blends at speeds up to over 100 000 rpm. Rotor speeds are therefore up to 15% higher than those for other draw-off nozzles.