Rotor spinning technology offers considerable flexibility as regards raw material selection. Rotor spinning machines are capable of processing successfully fiber lengths between 10 and 60 mm in virtually all natural and man-made fibers. Rotor spinning thus offers a range of application that no other spinning process, with the exception of ring spinning, can even approach. The following section will explain which raw materials and raw material properties are especially suitable for the rotor spinning process, and how the different characteristic values and properties of the fibers affect the quality of the yarn and end product and the spinning process.
Fig. 67 shows the individual raw materials as a proportion of the total volume of rotor-spun yarns. Even recycled cotton waste and noil are processed successfully on rotor spinning machines. In mill operations the rotor spinning process has earned the reputation of being especially „cotton-friendly“. This is also the reason why predominantly carded rotor-spun yarns of 100% cotton or blends of cotton and man-made fibers are currently produced worldwide. Table 4 shows the preferred cotton qualities for the rotor spinning process.
A special application is the manufacture of combed rotor spun yarns. Although this application has not established itself on a broad basis – due to the manufacturing costs for the additional combing passage – the results that can be achieved by using combed feed sliver in mill applications are entirely convincing. These advantages have an impact both on yarn quality (higher tenacity, better regularity, fewer imperfections), on running properties on the rotor spinning machine and on downstream processing (fewer stoppages, less fiber fly generated in weaving and knitting), and thus also on the quality of the end product (e.g. softer hand in knitted fabrics).
In addition to cotton, man-made fibers and/or their blends are successfully processed on rotor spinning machines – and have been since this spinning process was launched. Especially yarns made from polyester fibers and their blends with cotton (in rare cases in blends with wool, bast fibers and angora), are used in a wide range of end products. The reasons for this remarkable development are in particular:
- the outstanding physical and chemical properties of polyester fibers for use in clothing;
- the low production costs for manufacturing polyester fibers and the resulting economical raw material costs;
- and the limited availability of cotton in light of growing global fiber consumption; the annual increase in fiber consumption of some 3% is now accounted for almost entirely by man-made fibers, and here mostly by polyester fibers.
Viscose fibers, also known as rayon in the US and Asia, account for an appropriate proportion of the total volume of rotor- spun yarns, either pure or in blends with cotton and polyester. However, viscose fibers are heavily exposed to variations in fashion, so that their share of total yarn volume can vary from virtually zero up to 10%, depending on fashion trends.
Table 5 a) shows the man-made fibers and their blends which can be considered for rotor spinning, including a list of fiber criteria (Table b)) that have to be borne in mind when processing them on rotor spinning machines.
Since both wool and bast fibers display low inherent strength, these fibers are usually processed in blends with man-made fibers or cotton, which act as stabilizing fibers.
The raw materials listed in Table 6 are rarely processed on rotor spinning machines. Special process expertise is required for processing them and the raw materials listed under 1) 2) in Table 5 and under 3) 4) in Table 6; this has been developed by the spinning mills in question, often in cooperation with the machinery manufacturer or with textile institutions. This starts already with the selection of raw materials and the pre-treatment that is necessary in most cases. Processing in the spinning mill usually calls for specifically adapted machine speeds, delivery speeds and setting parameters, especially at nip and fiber transfer points, and in most cases necessitates an appropriate modification of the spinning climate, in terms of both temperature and humidity. In certain cases special spinning elements are adapted by yarn manufacturers on their own initiative, and these take into account the gentle treatment usually required for these fibers.
We will therefore not deal in greater detail with these special applications, since in most cases the fiber- and process- specific features are not revealed, i.e. are regarded as the intellectual property of the individual spinning mills.