As already mentioned, air-jet yarns have a fasciated (core / sheat) structure. In fact, air-jet spun yarns consist of a core of essentially parallel fibers without any twist, which is surrounded and bound together by wrapping fibers. These wrapping fibers provide compression forces in the core and thereby the necessary fiber friction in order to achieve the desired yarn strength. As the wrapping fibers in airjet yarns account for 15 to 30% of the total yarn mass, the core fibers are virtually completely covered by the wrapping fibers, so that air-jet spun yarns look very much like a fully twisted yarn, such as a ring-spun yarn.
This particular structure of the air-jet spun yarns influences the yarn properties, of course. These properties are mainly determined by 2 parameters:
- the percentage of wrapping fibers,
- the twist level of the wrapping fibers.
The percentage of wrapping fibers can be influenced by the spinning draft and by the distance L (Fig. 35), and it is also a function of the yarn count.
The spinning draft is the ratio between the speed of the take-up roller and the speed of delivery roller of the drafting unit. This ratio is usually slightly below 1, which means that the yarn take-up speed is slightly lower than the delivery speed of the drafting unit. If the spinning draft is reduced, the number of wrapping fibers increases.
L is the distance between the outlet nip of the drafting unit and the spindle. With an increase in distance L, more fiber ends have the chance to be separated from the main fiber flow, and thus more wrapping fibers are generated.
Experience has shown that with coarser yarn count, the number of wrapping fibers increases, but not at the same rate as the tex count. The percentage of wrapping fibers therefore tends to decrease as the yarns become coarser. While fine count yarns reach a level of up to 30% of wrapping fibers, this percentage drops to 15% or even below for coarse yarns.
The second parameter of great importance for the yarn properties is the wrapping twist. This twist can be influenced by the spinning speed and the flow rate of the compressed air.
For given nozzle conditions, the fiber sun rotates at virtually constant speed. An increase in the yarn delivery speed must therefore lead to a reduction in the wrapping twist level. This is in fact the case, as is shown by the spinning results in Fig. 39.
The wrapping twist level is furthermore a function of the flow rate of the compressed air. This flow rate depends primarily on the air pressure and on the cross-section of the injection holes. When the pressure of the compressed air increases, the wrapping twist level increases virtually proportionally (Fig. 40). Similarly, a larger cross-section of the injection bores leads to a higher wrapping twist.
In air-jet spinning, it is therefore easily possible to accurately control the level of the wrapping twist.
Fig. 39 – Yarn twist as a function of spinning speed (Rieter)
Fig. 40 – Yarn twist as a function of air pressure (Rieter)
