False-twist Compact Spinning Method For Cotton Ring Spinning And Its Yarn Structures And Properties

The compact spinning technology could improve yarn quality to a certain extent and be regarded as the most representative new ring spinning technology. In this dissertation, principles and techniques of fiber strand condensing in the ring spinning frame were studied, the compact spinning and various related methods, devices and technics were analyzed. The false-twist compact spinning method for cotton ring spinning was proposed and expounded, and then, the device used for this method was designed and produced. Afterwards, the morphology, structure and properties of the yarns were measured and analyzed. At the same time, the twist changing and distribution of each section of the strand in the false twist area were studied in detail. Finally, a series of experiments were designed and conducted to analysis and optimize the parameters of false twister and spinning process.The relationships between drafting form, rotational direction and rotational speed ratio of the false twister and the properties of four kinds of yarns were studied in the preliminary experiment. These properties include breaking strength (cN/tex), extension at break (%), irregularity (CVm %) and hairiness index (≥3mm). The results showed that the apron drafting system could get higher yarn quality than the three-roller drafting system. The choose of rotational direction of false twister should be based on the twist level of roving, and the determination of the rotational speed ratio of the false-twister should take the rotational direction into consideration, too low or much higher speed ratio will do harm to the resultant yarn quality.The response surface methodology was used to investigate the influence of rotational speed ratio of false twister, the position of twister and the minimum curvature radius of false twist channel on yarn properties, after that the optimization work of these spinning parameters was conducted. The results showed that the minimum curvature radius of false twist channel is the most important factor for yarn irregularity, the hairiness index was influenced by rotational speed ratio of false twister and minimum curvature radius of false twist channel. All of the three parameters have main effect on yarn breaking strength and extension at break. The result using the desirability function to optimize the multi-response showed that when rotational speed ratio of false twister lies between 0.367-0.374, the distance between back end of the false twist and apron nip is 11mm, and the minimum curvature radius of false twist channel was set between 18.2and 18.3mm, the yarn properties could achieve overall optimal quality.The morphology of yarns, the forms and twist angles of the fibers were observed and recorded under microscope, and the appearance average diameters and profiles were measured by EIB-S system. The results indicated that when the twist factor of yarn kept constant, the higher the rotational speed ratio of false twister, the lower the spun yarn quality. Compared with the conventional ring spinning yarns with same twist, the yarns produced under lower rotational speed ratio have better evenness and fewer hairs in short segment. The yarns with higher twist factor usually have even appearance and fewer wrapping fibers despite of the spinning method. The inclination angles of fibers in the surface layer of yarns were accord with the values calculated by theoretical formula.Yarn cross sections were observed by using microtomy methods and the packing densities of different yarns were calculated. The changing pattern of fiber packing density to yarn radius showed that all of the yarns are more compact in the core area than the outside layers and this trend could be depict by a cubic function. Moreover, the yarns with higher twist factor have higher compact density. Compared with the conventional yarn with the same twist factor, the yarns produced by false twist compact method have higher density near the core of the yarn and lower density at the outer layers.The testing results of yarn properties showed that when yarn twist factor is kept 280, the evenness of yarns produced by false twist compact method inferior to that of conventional yarns, and the hairiness index, breaking strength, extension at break will better than the conventional yarns only if the rotational speed ratio of the false twister was suitable. If the twist factor was raised to 360, the evenness of false twist compact yarn could be better than conventional yarn at certain rotational speed ratio, but other three properties of yarn may overmatch that of the conventional yarns in a rather wider range of rotational speed ratio of the false twister.The theoretical analysis on the twist distribution of strand in the false twist area showed there are two different propagation ways for the false twist. One is the false twist propagated from the apron nip to front roller, and each strand section will revolved around its own axis in succession, this way could be referred as "sequential propagation" mode. The other way is the false twists with different direction generated in two sides of the false twister will propagate to the center of false twister and then converge at certain point of the false twist channel, this way could be referred as "end-to-center" mode. Which mode the twist propagation will follows was depend on the twist difference between two adjacent strand sections, but this difference was determined by strand tension and structures, physical properties of the fibers and curve of the false twist channel. Moreover, the twist changing rules of each strand section at different propagation mode will be varied, this variation mainly reflected at the first seconds of spinning starts and then is balanced promptly. Finally, the steady twist in the strand will be same for both of the propagation modes.The dual response surfaces robust parameter design methodology was used to study and optimize the false-twist compact spinning process. The breaking strength (cN/tex), extension at break (%), irregularity (CVm%) and hairiness index (≥3mm) were set as responses. First, the fractional factor design was employed to screen the important factors from twist factor, breaking drafting, breaking gauge and rotational speed ratio of false twister. The result indicated that the twist factor and rotational speed ratio of false twister were significant factors, and then, the central design point of dual response surfaces robust parameter design was located by steepest ascent method. In the dual response surfaces robust parameter design, the cross array was constructed, the inner array was constituted by different levels using the central composite design. The outer array was the three levels of compound noise, which combined three different levels of the noise factors (temperature, humidity and mechanical state of the false twister). Systematical experiments were conducted to study the relationship between factors and responses. The regression equations for the mean and standard deviation of each response were obtained. These equations displayed the effect and significance of each factor and their interaction. The results of ANOVA showed that these equations fit well and could reflect the relationships between factors and responses. By virtue of the response surface and contour plots, the relationships aforementioned were analysised in detail. The result of optimization by desirability function showed that when twist factor is 344 and the rotational speed ratio of the false twister is 0.34, the spun yarn would have the best quality. The verification experiment also confirmed the optimization and indicated the models were properly and suitable.