316 (#410) ############################################ WOOLLEN AND WORSTED SPINNING 1 (a) To stretch the yarn as far as it can be stretched, or is likely to be stretched in the subsequent manufacturing processes; and (6) To boil the yarn in this stretched state to take out the marked irregular strains usually present, and also to dry in this stretched form. Why boiling takes out the strains and why they come in again on additional tension being applied is obviously a fascinating physical problem for research. It is evident that the drying time and temperature should be carefully taken cognizance of. Recent researches have suggested that the presence or absence of a medulla and the relative proportions of cuticle and cortex must also be taken into account. The physical behaviour of wool fibres appears, at least in part, to depend upon internal structure. Lustre also depends, in part, upon internal structure- hence the importance of the work at present being undertaken by Professor Cossar Ewart, F.R.S., of Edinburgh University. As shown in Chapter XII., p. 292, it is possible to so balance single twist and twofolding twist that a limp non-kinking yarn is directly taken off the spinning frame. How far such a yarn will serve the purpose of a “set” yarn is worth considering. It may а. also be well to consider which type of twofold twist --balanced, straight-fibre, or square-twist-best lends itself to “setting on the lines described above. But under any circumstances irregular fibre strains, fibre curvature and twist curvature must be taken into account, especially in cross-breds, in which materials irregularities are much in evidence. Certain hosiery fabrics are made unshrinkable by 316
194 195 Textile Machine