Abstract:

Objective In order to address the snarl problem of non-elastic covered yarns caused by the internal stress generated by the outer yarn wrapping, a real-time core yarn reverse twisting method is proposed, aiming at offsetting the reverse twisting moment generated by the outer yarn wrapping through the reverse twisting moment generated by the reverse twisting of the core yarn, so as to achieve a stable structure of snarl-free covered yarns. The core tasks include the verification of the feasibility of two core yarn twisting technologies, i.e. hollow spindle mechanism and doubling twisting mechanism, the clarification of the principle of twist direction matching, and the exploration of the influence of the reverse twisting twist of the core yarn on the snarl index of the covered yarn with a given wrapping twist.

Method Two technologies for realizing the twisting of the core yarn on the hollow spindle covering machine were adopted. The first is twisting by the hollow spindle mechanism, where based on the technical feature of the hollow spindle mechanism of wrapping when in the presence of a core, and twisting when in the absence of a no core, the original hollow spindle components of the equipment were utilized, and the reverse twisting of the core yarn was achieved through the series configuration of the upper-row and lower-row. The second is twisting by the doubling twisting mechanism. The doubling twisting mechanism was utilized to replace the lower-row of hollow spindle mechanisms to achieve the real-time twisting of the core yarn. The advantages and disadvantages of the two twisting methods were analyzed. Polyester filament covered yarns with different levels of core yarn twisting were spun using the hollow spindle twisting mechanism (core yarn 167 dtex, outer wrapping yarn 83 dtex, wrapping twist 500 twists/m); wool/polyester covered yarns with different core yarn twists were spun using the doubling twisting mechanism (28 tex wool yarn as the core yarn, 111 dtex polyester filament as the outer wrapping yarn, and the wrapping twist of 800 twists/m). The closed-loop method (GB/T 7960.6—2013) was adopted to measure the snarl index of the covered yarns, and the influence of twist matching on torque offset was analyzed. The tensile properties were tested by CRE method (GB/T 3916—2013) to investigate the influences of the core yarn twist on covered yarns.

Results For polyester filament covered yarns, when the reverse twist of the core yarn increased from 0 to 400 twists/m, the average value of the snarl index decreased from 140.0 twists/m to 11.6 twists/m, representing a decrease of over 90%. The increase in the twist significantly reduced the self-twisting snarl, and the yarn tended to be stable. For wool/polyester mixed-color covered yarns, as the reverse twist of the core yarn increased, the snarl index of the covered yarn decreased. When the core yarn twist reached 200 twists/m, the snarl index is close to zero (-0.2 twists/m), indicating that the reverse twisting moment was completely offset. For polyester filament covered yarns, the breaking strength increased first and then decreased with the increase of twist of the core yarn, and the elongation at break of the covered yarn was higher than that of the core yarn without twisting, but the change with the twist of the core yarn was not significant. For wool/polyester mixed-color covered yarns, the breaking strength decreased with the increase of twist of the core yarn, and the elongation at break of the covered yarn was lower than that of the core yarn without twisting, but no significant change occurred with the increasing twist of the core yarn.

Conclusion The anti-twisting torque generated by the outer wrapping yarn when wrapping the core yarn causes the non-elastic single-covered yarn to exhibit a snarling phenomenon similar to yarn twisting. By using a twisting mechanism to first apply real-time twisting to the core yarn in a direction opposite to the wrapping twist direction, followed by the covering process, the anti-twisting torque from the core yarn twisting can offset the anti-twisting torque produced by the outer wrapping yarn during covering. This achieves torque balance inside the covered yarn and produces non-snarling covered yarn. Core yarn twisting can be achieved using a hollow spindle mechanism, leveraging its characteristic of wrapping when in the presence of core and twisting when in the absence of a core, or it can be realized with a general-purpose double-twisting mechanism. The former has a smaller core yarn package capacity and is more suitable for core yarns with smaller linear density. The latter, however, requires the core yarn to have higher strength to overcome processing tension, but it can achieve high twist at low speed. For filament core yarns, the twisting direction of the core yarn and the wrapping twist direction only need to meet the requirement of being in opposite configurations. For staple fiber core yarns, due to their inherent initial twist, the reverse twisting direction of the core yarn must be the same as its initial twist direction, while the wrapping twist direction must be opposite to the initial twist direction of the core yarn. The snarl index of the covered yarn decreases as the reverse twist level of the core yarn increases, until the non-snarling state is achieved. The required reverse twist level of the core yarn to achieve non-snarling covered yarn varies depending on the configurations of the core yarn and outer wrapping yarn, as well as the wrapping twist level of the outer wrapping yarn. It must be determined through experiments on the variation of the core yarn's reverse twist level. Twisting the core yarn changes the structure of the covered yarn, which in turn leads to changes in its properties. For different combinations of core yarn and outer wrapping yarn, as well as different wrapping twist configurations, the variation trends of the covered yarn's performance indicators will differ with changes in the core yarn's twist level. Specific analysis through experiments is therefore required.

Key words: covered yarn, snarl-free, reverse twisting of core yarn, hollow spindle mechanism, doubling twisting mechanism, twist direction matching, snarl index