It is common practice to wax staple-spun yarns for knitting applications, given the problems of friction associated with the many thread line deflection points of the thread guides and the knitting needles on knitting machines. For optimal running of yarns during knitting, there needs to be a uniform wax distribution along yarns and a minimum of wax rub-off.
The amount of wax deposited on the yarn has a marked influence on the dynamic frictional characteristics of the yarn. Figure 1a shows that, at a given running speed, the yarn coefficient of friction decreases to a minimum with increasing wax deposited and then increases with further yarn waxing. Of the three zones indicated, clearly, Zone II is the preferred range of deposition, usually 0.5 to 1.0 g/kg of yarn. Wax grades vary according to melting point, oil content, microstructure, and hardness. Little has been published in the way of selecting a wax grade for optimal running performance. However, it would seem that selection depends on many factors such as fiber type; yarn structure, count, and moisture content; as well as room temperature and humidity in the winding area and during storage and shipment.
The preference with the commonly used wax disc is for a coarse microcrystalline structure, which allows small wax particles to be removed and held onto the yarn surface as shown in Figure 7.17b, as this should enable a uniform distribution of deposition. Steaming or high-humidity conditioning of wax yarns can result in an increased friction coefficient. Steaming will melt the wax particles and also give a partial penetration of wax into the yarn. If the yarn has to be relaxed in this way, then the deposition should be increased to offset the effect.
FIGURE 1 Yarn thread line friction with increased wax take-up. (Courtesy of W. Schlafhorst AG & Co.)