About 25% of the total migration observed in commercial yarns come from the tension mechanism and the other 75% or more from geometric and random causes, states Dr S K Basu, who provides key findings from a critical review of fibre/filament migration in a twisted yarn.
Yarn structure depends, to a large extent, on the way in which the individual filaments or fibres are arranged within the yarn. The simplest theoretical model of a single yarn, as shown in Figure 1 is essentially based on the assumption that all the filaments lie on a set of coaxial helices, of constant twist or pitch. However, this idealised structure would be useless, if it existed at all, because no matter how much twist was present, no frictional cohesion or fibre binding force in the yarn would be generated to prevent the successive layers from peeling off at the slightest surface abrasion.
Considering that the paths followed by the fibres in the yarn vary in length according to their distance from the axis, it is evident that in order to produce such a structure, each filament would have to be delivered for twisting at the rate according to the position it is destined to occupy in the yarn structure. This structure, for both staple and continuous filament yarns, could not be made by any of the exiting methods of yarn production. In all twisting methods, all fibres are delivered at the same average rate.
That such a structure is far from realistic was first recognised by Peirce, who supposed that the "fibres appearing on the surface have their ends tucked inside". He also suggests the "theory of random tangle". However, the process of yarn formation in front of the rollers of the spinning frame suggests that, while there must be tangle, it is not necessarily random. As Peirce, Morton and others who have quoted their works give a very clear explanation of migration phenomena; it is acknowledged that parts of the following paragraphs are quoted directly from these sources.
What is Migration?
The word migration was first used by Morton and Yen. This describes the variation of radial position of a single fibre or filament as it moves along and around the yarn axis. However, the same term was, afterwards, used by other workers to describe the preferential radial distribution of dimensionally different fibres (in the case of blend spun yarns) within the whole body of the yarn.
Migration (used in the former sense) happens as a consequence of the twisting mechanism and is not controllable during the textile process. Studies of the migrational behaviour of fibres provide detailed information on the configuration of fibres or filaments in the yarn and how the filaments are packed in concentric shells of the yarn.
In this chapter, migration of the type as defined by Morton will be discussed in greater detail. But to make the discussion complete, a very brief review of the migration, as defined by preferential radial distribution of fibres within the whole body of the yarn, and changes in this distribution, will be made. To avoid confusion migration under this second definition will be designated movement.
Theories and mechanism
Migration due to tension variation
The theory of migration due to tension variation has been postulated by Morton. Since the length of the fibre path in a yarn increases from the core to the surface, the tension in the yarn cross-section will experience a different level of tension depending upon the radial position they occupy, ie, fibres on the surface will have higher tension than those remaining in the core. Now if the external restraint is withdrawn (ie, spinning tension becomes zero), the resultant yarn would be one in which the surface fibres remain under slight tension and th