The drawframe is of crucial importance for the quality of the yarn and thus ultimately also for the quality of woven and knitted fabrics. Defects which are not leveled out on the drawframe reappear undiminished in the yarn. An essential task of modern drawframes is to deliver defect- free drawframe slivers of maximum regularity to the rotor spinning machine. This is assured nowadays by highly effective leveling devices on the drawframe (especially through the open control loop leveling principle) (mill readings, see Table 12).
Modern high-performance drawframes are currently equipped with highly efficient extraction systems which reliable remove a substantial proportion of the dust still present in the fiber material. Dust, fiber fragments and trash are effectively separated from the fibers by fiber/fiber friction during the drafting process in the drawframe and can thus very easily be removed by the extraction system. In contrast to ring spinning, where in principle 2 drawframe passages, when processing blends even 3 drawframe passages are used, rotor spinning operates with one or no more than two drawframe passages (even with blends). In rotor spinning the effect of fiber hooks is of secondary importance on the one hand, and additional blending takes place in the rotor due to back-doubling on the other. Only 2 drawframe passages are therefore used, even when manufacturing blended yarns, without loss of quality. Directly leveled carded sliver can also be fed to the rotor spinning machine in certain applications.
Fig. 71 compares rotor spinning systems with different sliver preparation, which are used depending on the required yarn quality.
Two drawframe passages (leveling in the 2nd passage):
- for rotor-spun yarns in the fine count range (finer than Nm 34 / Ne 20 / 30 tex) and high demands on yarn count constancy (e.g. for single jersey); the 2nd passage also serves for additional de-dusting;
- for rotor-spun blends with drawframe sliver and stock blending in the medium and fine count range;
- for rotor-spun denim yarns (branded goods) with high standards in terms of tenacity, elongation and yarn purity.
One drawframe passage (with leveling):
- for rotor-spun yarns in the medium and coarse count range without very high demands on yarn quality;
- for rotor-spun denim yarns (low-price products) without particular quality specifications by garment manufacturers;
- for rotor-spun yarns with a high short-fiber content, where a second drawframe passage can even result in a deterioration in sliver regularity („floating“ clumps of short fibers in the drafting zone);
- combed rotor-spun yarns (only one leveled drawframe passage after the comber, also for ring-spun yarns!). Excellent parallelization is already achieved in the combed sliver due to combing and the high doubling number up to and including the comber. With each additional drawframe passage sliver cohesion would decline so steeply that false drafts are inevitable in downstream processing.
Direct processing of carded sliver (leveled card):
- for rotor-spun yarns in the count range coarser than Nm 20 / Ne 12 / 50 tex without particular demands on yarn quality;
- for rotor-spun yarns with a very high short-fiber content (e.g. cotton waste, recycled weaves or knits).
Special case: card with drawframe module (with leveling):
- Range of application as for one drawframe passage with leveling. Exception: combed rotor-spun or ring-spun yarns, since doubling cannot be dispensed with in this application.