Center marks are vertical line defects which are invariably visible in spandex plated fabrics knitted by a circular knitting machine. Center marks result in unnecessary efforts and waste fabric. Therefore, solutions to eliminate the occurrence of these defects are necessary. It is not clear whether center marks are a product of knitting or the dyeing process. The marks are usually inspected visually by the operator at the production line using front and back lighting. In this study, we examined the shapes and color distributions of the loops in dyed and finished fabrics and we assessed the state of these visual inspection methods using different lighting conditions. Consequently, the exact shape of the center marks was found to be caused by the bulging of successive vertical loops and the center marks are not caused by dyeing. More exactly, the bulging shape consisted of narrowed loops and two neighboring loops. Furthermore, it became clear that center marks were visually recognized (1) by the shadow casted by the bulging due to the side light, or (2) by the reduction of light transmission by the reduced loop space when using back light. Finally, we concluded that the prevention of the formation of the bulging shape and prevention of the reduction of loop space are the two counter-measures necessary to eliminate center marks in the future.

For the purpose of making clear the mechanism of interlacing process in a slit-type interlacer, an effect of supplied air pressure on both filaments velocity and its characteristics was studied using a high-speed video camera and particle image velocimetry. Results obtained are as follows: (1) Filaments must produce an opening state of twin spirals to generate tangling parts. It was confirmed that the tangling parts were generated at smaller amount of filaments movement at high supplied air pressure and the opening state became longer time at low supplied air pressure. In addition, filaments in an opening state in left and right regions of the yarn passage need to change their relative positions by crossing the symmetric plane of yarn passage to different region above 300 to 400 filaments. This series of processes produces an interlaced yarn owing to twists accumulated at both ends of opening part. (2) Either fast filament motion or long opening state time can generate tangling parts even when the twin spiral motion of filaments was asymmetric. (3) As the supplied air pressure increased, the position-time-mean angular velocity became large, thus an increment of supplied air pressure was effective to generate tangling parts. (4) In the tangling state, the timeposition-mean of magnitude of velocity increased with increasing supplied air pressure, but the time-position-mean of magnitude of its deviation was almost constant, because filaments formed one bundle. On the other hand, in the opening state, the time-position-mean of magnitude of velocity increased with increasing supplied air pressure from 0.1 MPa to 0.3 MPa, but indicated almost the same value in the supplied air pressures between 0.3 MPa and 0.5 MPa. However, the time-position-mean of deviation of magnitude of velocity was independent of the supplied air pressure. (5) This and our previous studies are available for design guideline of an optimum interlacer.