Spinning dynamics and performances of modified ring spun yarns

Abstract

Spinning is a fundamental method to produce long strands from staple raw fibers of cotton, wool, flax, or other material. Twisting is a vital process to determine the staple yarn structure and performances like strength (tenacity), elongation, evenness and hairiness. Till now, ring spinning continues to predominate in yarn manufacturing industry due to its high yarn quality and flexibility in materials and yarn counts. With the increasing demand of novel features or improving qualities, many modifications have been developed, such as Compact, Siro and Solo. In recent years, a novel spinning technology, named Nu-Torque, has been developed by introducing a false-twisting unit into the conventional ring frame for producing low twist and soft handle single yarns. The modified cotton yarns and fabrics have significant advantages in terms of soft handle, higher yarn strength at lower twist factor, lower residual torque and low knitted fabric spirality after washing and tumble-dry cycles. Among over ten mills using the technology, 10 to 40% increment in production rate has been achieved for cotton ring yarns with various versions of the technology. In addition, a significant average energy saving of 337Kwh/ton was reported by a mill producing Ne 30 and 1100 Kwh/ton for another mill for spinning Ne 80 yarns. Although the low twist and soft handle single yarns have been achieved, several urgent issues relating to spinning dynamics and yarn performances need to be addressed. Firstly, twist generation and propagation are of particular interest because they answer several key questions including the amount of false-twist generated by the false-twister, false-twisting efficiency and blockage rate, as well as the relationships between false-twisting efficiency and system parameters. The false-twisting unit employed in this study is single friction-belt with circular cross-section. It has been identified that the moving belt generates the false-twists into yarn, meanwhile traps the upward propagation of the real twist as well as congests the downward propagation of the false twists. In this study, a steady-state model of yarn dynamics in the modified ring spinning system has been proposed, which deals with two important phenomena simultaneously, that is, twist generation and twist propagation. The model was validated by experiments and a good agreement has been demonstrated. For the first time, influences of several parameters on the twisting process have been revealed in terms of false-twisting efficiency, propagation coefficients of twist trapping and congestion. Based on the proposed model, a systematic simulation has been carried out to evaluate effects of system parameters on the false-twisting efficiency as well as propagation coefficients. In order to minimize the number of experiments, response surface methodology involving a central composite design in three factors of twist multiplier, speed ratio and wrap angle was successfully employed for the study and analysis. The significant terms of the models were studied and it reveals that the speed ratio and wrap angle are statistically significant for the responses of twist efficiency, propagation coefficients of twist trapping and congestion. And more importantly, linear relationships are found among the three responses. Secondly, Stability is the basic requirement for any practical applications. Unstable twist generation and propagation arising from the vibration of the false-twister, forced variation of the electric motor as well as electric interference or malfunction may results in poor spinnability as well as uneven features or imperfections of the resultant yarns, such as strength deterioration, diameter irregularity and wrapping fibers along yarn length. On the other hand, for a stable process or product, it should permit a certain tolerance for the system variation or error. However, little work has been found in the literature thus remains elusive. In this study, twist stability and robustness is concerned and their relations to the belt spatial position and properties are elaborated. Based on twist kinematics, a transient model has been proposed to study twist variations during yarn formation process. Three idealized cases that cause twist variations are investigated, namely step function, rectangular function and periodic function changes in false twist. Even though several assumptions have been proposed to make the analysis manageable, the findings have provided a clear clue of the significant twist levels that can generate in the zones of the machine. It has been verified by the experiments that the resultant yarn properties within 30% periodic change in false twist demonstrated no obvious difference in properties when compared to the yarn without variation. From model simulation, it has been found that twist variations in different zones are different and the effect on final yarns is limited. It is also suggested by the model that zone length has a significant influence on the twist redistributions. Besides, reducing the twist efficiency and the twist blockage rate leads to decreasing twist variation and balancing time in three zones. In particular, belt oscillation has little influence on twist variations and wool-like fiber is easier to cause large twist variations in spinning process than that of cotton-like fiber at the same condition. Thirdly, in previous study, our group has found severe deterioration of yarn neps for production of high count yarns. The blackboard evenness also proves that the produced yarns have one grade worse than that of control yarns. Although optimizations have been carried out to adjust the spinning parameters in order to control the yarn evenness, the results are less satisfied. So far, the mechanism behind the fact has not been fully explored yet. In order to solve the severe deterioration of yarn neps in the modified system, in this study systematic investigation of Ne 40 cotton yarns has been carried out. The study includes three aspects, namely belt spatial positions, system parameters, and friction surface of the belt. Belt spatial installation position is an important issue with special interest for mill applications. It is not only restricted by the space of different spinning machines, but also affects the stability and quality of the yarns. As a key component, the belt friction surface may have large influences on false-twisting process, and determine the final yarn performances. Moreover, the belt friction surface and their interplay with yarn properties have been investigated systematically. Based on the preliminary analysis of belt friction surface and geometry, potential influencing factors were identified, and the fractional factorial methodology was employed to find out the statistically significant factors on yarn tenacity, evenness and hairiness. According to the experimental results, twist factor, speed ratio and wrap angle were confirmed as the significant parameters on yarn properties in terms of tenacity, evenness, neps (+140%), and hairiness, therefore, these three parameters were selected for further study using response surface methodology to find the optimal value for yarn production. The second-order equations were obtained to examine and estimate the relationships between the factors and responses, and the overlaid contour plot was employed to find the optimal results. The optimized yarn with 11.1% twist reduction apparently outweighed the conventional yarn in hairiness and has slight improvements in yarn evenness and thick places (+50%); whereas the neps (+140%) of the optimized yarn is still 50% worse than that of the corresponding ring yarn. In order to overcome the occurrence of neps, the mechanism of neps generation has been studied and promising solutions to alleviate or diminish such yarn imperfections have also been put out. Neps generated in this system are mainly caused by rearrangement of the yarn surface fiber along its axis. The yarn neps can be successfully suppressed by either shortening interactive path or arranging the fibers in a completely parallel and close position before untwisting is imparted, which have been verified by the experiments. It has also been found from the experiments that the belt hardness is independent of the occurrence of neps (+140%) and the belt with rough surface morphology even deteriorates the situation. With the combination of compact device and belt-type false-twister, the wrap angle can be further enlarged by introducing a rotatable guide installed between the front roller and the belt to enhance the yarn tenacity and the twist factor can be further decreased from 3.2 to 2.8 to achieve a low residual torque and soft handle feeling yarns. Finally, this study explores the surface characteristics and properties of knitted fabrics produced by the modified yarns and compares with the fabrics made from the conventional ring yarns. For preparing fabric samples, quantities of the Ne 40 modified yarns as well as conventional yarns were spun, respectively. Then, the prepared cop yarns were wound on the cone before knitting. After that, three single yarns were fed into the gauge of flat knitting machine and the interlock structure was adopted to avoid the fabric spirality. Properties and performances of the knitted fabrics were evaluated and compared, including fabric weight, thickness, loop length, bursting strength, air permeability, thermal property, pilling resistance, etc. The knitted fabrics made from the modified yarns show a 6.28% higher in bulkiness than that of the conventional yarns, resulting to a better capacity of thermal insulation and warmer feeling. Moreover, these two fabrics show similar bursting strength and same pilling grade.