In cotton spinning the cost of the raw material is equal to 50% of the total processing cost.
It is therefore essential that the cotton type selection is based on a wide knowledge of the fibre properties and on the end-item to be produced.
In the last decade the so-called testing lines, which were initially used only by cotton producers and exporters, have been steadily developed and improved, so that they have now been adopted also by the spinning mills; these are driven in this direction by the need to control the quality of the various fibre sources and the reliability of the suppliers, moreover they are attracted by the effective advantages offered by these integrated testing systems, which ensure a closed loop control of fibre characteristics, yarn quality and production process and are focused on the spinning mill's profit. The initials HVI (High Volume Instruments), now well known to all cotton dealers, stand for the kind of instruments which compose the cotton testing lines. These lines offer several advantages:
- measurement of the main characteristics on a cotton fibre bundle: span length as measure of the fibre length, length uniformity, strength, elongation, micronaire as a fineness measure, colour and reflectance, foreign matter (trash) contents. The spinners have asked to add also the SFC (Short Fibre Contents) value which, as already mentioned, has a great impact on the quality of carded yarns. In fact short fibres increase yarn breakages while reducing the yarn tenacity and regularity. The combing process removes the short fibres and originates a quality yarn, but at the expense of a larger quantity of wastes. At least one manufacturer of HVI systems has already the software suited to provide the SFC value from the fibrogram produced by the HVI system. Although the standard error is still about 2%, this procedure allows to identify cotton batches which went through an excessive beating or drying or were too intensively cleaned in the ginning machine.
- possibility of testing up to 180 specimen/h and of checking each single bale. - attainment of reliable test results.
The American cotton growers, under the guidance of USDA (U.S. Department of Agriculture), are making considerable efforts to improve the quality and to reduce the damages caused to the fibre by mechanical picking and ginning. Therefore they were first to equip themselves with these HVI instruments, so that they are in a position to deliver controlled cotton bales with every information which can help the spinner to plan in the best way his production through:
- cotton bale management
- raw material optimisation
- process optimisation with consequent cost reduction.
By bale management we mean the ideal selection of the cotton bales to obtain acceptable technical and economic performance during their processing as well as consistent yarn quality.
By raw material optimisation we mean the result of following operations:
- consistent blending with the support of bales management;
- selection of fibre characteristics according to the requirements of the end-product and based on yarn structure;
- purchase of the most convenient raw material suited to meet the requirements of the end-product.
The process optimisation, on the other hand, depends on following factors:
- selection of the best setting for the drafting rolls;
- optimisation of the processing speeds through comparative trials.
