Air-jet spinning is a pneumatic method which consists of passing a drafted strand of fibers through one or two fluid nozzles located between the front roller of a drafting system and a take up a device.Look at the figure which demonstrates the Murata principle of producing fasciated yarns with two nozzles N1 and N2. This has been shown to be superior to methods employing a single nozzle to spin the yarn.
The drafting system S drafts the input material into a ribbon like form with parallel fibers.Air is injected into the two nozzles N1 and N2 at high pressures, which cause swirling air streams in opposite directions.
Sliver is fed into the machine and is further drawn out to the final count and twist is inserted by means of a rotating vortex of high pressured air. The resultant yarn is cleared of any defects and wound onto packages ready for use in fabric formation.
Some fibers, particularly those at the edges of the ribbon, will not be subjected to the full twisting action imparted to the main body of fibers by the downstream air-jet. Hence they receive less twist than those fibers in the main bundle. When the yarn gets untwisted in the downstream of the twister, the low twist edge fibers get untwisted to a greater degree than their original twist. Therefore they are given a true twist in the direction opposite to that of the upstream twist. The main body of the strand will be untwisted into parallel fibers, forming the core and these will be wrapped around by the edge fibers forming the wrapper fiber layer or the sheath, thus forming a fasciated yarn. The function of the back nozzle is to enhance the cohesion of the strand thus giving greater yarn strength.Since the direction of airflow in the back nozzle is opposite to that of the front nozzle, the back nozzle tends to untwist the wrapper fibers as they are formed inside the front nozzle. This increases the length of wrapping, thus improving the yarn cohesion.
The air-jet spun yarn consists of an untwisted core of parallel fibers and a surface wrapping of fibers. The core fibers account for approximately 85-95% of the yarn mass.
The surface wrapper fibers are helical in nature unlike the wrapper fibers in the rotor yarn. The wrapper fibers are not uniformly distributed over the length; sometimes they
are more on the surface and sometimes very few are on the surface. Their frequency and tightness being influenced by the fiber physical properties and the spinning process
parameters. “The high level of constriction of the straight core fibers by the surface wrapper fibers results in high bending modulus of air-jet yarns”.
The tensile strength is lower than that of rotor and ring yarns. More wrapping turns give better yarn strength, but at the same time higher wrapping frequency leads to higher bending rigidity and lower compressive softness. The untwisted core fibers contribute to very low snarling tendency of the air-jet spun yarns. Surface fibers twisted lightly around the core cause the yarn to be well suited for use as filling in air-jet weaving machines, as it can be propelled across the shed more quickly. The lack of twist in the air-jet yarn core is believed to contribute to the low pilling propensity of these yarns. The pills can still be created from abrasion, but they are not locked into the structure because of the absence of the twist. Pills break away easily after they are formed. Also it has been shown that as the number of the core fibers increase, the proportion of the protruding fibers is reduced, resulting in lower yarn hairiness. The typical yarn properties exhibited by the air-jet yarns are good tenacity, good evenness, low snarling tendency, and low tendency to pilling, high stiffness and high shrinkage.
The production rate of air jet/vortex spinning is 3-5 times higher than rotor spinning and 10-20 times that of ring spinning and, like rotor spinning, air-jet spun yarn is a lot cheaper to produce as it also uses fewer production stages. As is the case with rotor spun yarns, air jet yarns are more even, but weaker and have a harsher feel than ring spun yarns. Airjet spun yarns are mainly produced in the medium count (30 Ne, 20 tex) range and are mainly polyester/cotton blended yarns. End uses include woven sheeting and knitted lightweight shirting.