Bangladesh has been showing continuous growth in woven product export over the years in the major export destinations. In fact more than fifty percent of the RMG exports come from woven products; shirts, denims, trousers, jackets and suits. But unfortunately more than 60% of the woven fabrics are imported from foreign countries due to inability of the local producers to weave quality fabrics. As a result, lead time for woven garments are too high (120-140 days) and the currency retention is also very low (due to bulk imports of fabric). Weaving is a complex fabric manufacturing process and it requires precision settings in the machines. One of the major problem lies in identifying the correlation among warp tension and crimp for different types of fabrics. Thus, loom setting requires detailed understanding of the dynamics of loom operation and fabric properties. Recently, Mr. Toufique Ahmed, faculty of NITER has completed a comprehensive research on determining the correlations among warp tension, crimp of fabric, and loom settings as a part of his Master’s thesis. This article is a synopsis of his research for the readers who are passionate about weaving and the technical aspects of loom setting.
Toufique Ahmed, Assistant Professor of Fabric Manufacturing, National Institute of Textile Engineering and Research (NITER)
The most important fabric manufacturing processes is weaving. For producing good quality fabric proper warp tension is a vital need. For ensuring proper warp tension electronic warp control is used in modern looms. But changing of different loom setting points can play an important role in assuring proper warp tension which may be an important tool for eliminating starting mark, weaving damage etc. as tension varies with the change of fabric structure, the tension should be readjusted just after changing any design.
Many researchers across the world have investigated on different scenario of warp tension. Among loom components, the backrest roller is a crucial part which adjusts the tension of warp yarns and therefore it could affect fabric properties [1, 2]. The backrest roller motion has also significant effect in improving the efficiency of the weaving performance, especially when the loom moves faster . The properties of denim fabric is affected by the tension of warp which is studied by Adanur and Jing [4, 5] the swinging motion of backrest roller has an influencing effect on variation in warp yarn tension. During the weaving process variation in tension become smaller by the swinging motion of backrest roller’s. It could be possible to obtain a suitable relationship between movements of the backrest roller and tension of warp during running at high speed . Sheikhzadesh et al. investigated the relationship between the ration of the force applied on the warp yarn by the backrest roller to the warp yarn . They also find out the vertical and horizontal displacement of the backrest roller with the variation in warp beam radius during the weaving process as the beam diameter decreases with the advancement of weaving process. Weinsdorfer et al.  studied the relation between shed geometry and warp tension. The distribution of warp tension over the warp width connected with the changes in shed geometry (backrest positions). They found that by changing the shed geometry, the warp tension also varies. By lifting the backrest, the elongation of warp yarn in the lower shed increases, as a result the warp tension in the lower shed also increases. On the other hand, the warp tension in the upper shed decreases.
Measuring Warp tension:
Warp tension can be measured by digital warp tension. In this meter tension is measured in cN. The digital warp tension meter has a numerous advantages of measuring warp tension. it is very user-friendly and easy to operate.
Tension can also be measured by using load cell integrated in backrest roller. The measurements are displayed in the console panel.
Warp tension and Crimp:
The low tension along the warp will give more crimp. Which will require more warp yarn consumption. On the other hand, warp yarn at high tension will require less yarn and produce lower crimp. But the fabric will shrink more. In figure 3, when the warp is at high tension the crimp is comparatively low.
In figure 4, it is seen that in fabric the high tensioned warp will create a lower degree of crimp and the lower tension warp will create a higher degree of crimp. With the high tension in yarn the crimp in the warp dirention will try to be straighten and therefore crimp will be lower.
Factors affecting warp tension:
There are many factors of affecting warp tension in the loom.
- Beam diameter:
Warp tension varies with various things, for example if the beam diameter is decreased then the warp tension will certainly not similar to that of the initial stage.
With the advancement of time the beam diameter is decreased, the angle of lap between warp yarn and beam diameter. As we know, where is the angle of lap increases with the decreases of beam.
- Loom settings:
In the following figure the loom settings are depicted. It consists backrest roller position, dropper height, dropper depth etc. The change in any settings will cause variation in warp tension. For example if backrest position or backrest height change then the warp tension also change. Similarly, if the dropper position or dropper height changes the warp tension also change. For all settings the warp tension follow a specific pattern. In the figure 6 the loom settings are shown.
2.1 Backrest position on warp tension
Backrest roller is a crucial part of weaving machine. Proper positioning of backrest roller may give good quality of fabric it also helps to achieve good efficiency. It is investigated earlier that upper position of backrest roller increase tension of lower shed and lower position of backrest roller increase the tension of upper shed. Several looms were analyzed in order to establish a relationship between backrest position and tension.
The effect of Backrest position is influenced by the position of dropper line. At the nearest backrest position (i,e 11 cm to 16cm at Leonardo loom) increasing backrest position will put the dropper line at the middle of the warp yarn. The dropper impose more downward tension by its weight when it is at the middle so more tension will be required to make the warp yarn proper level. But for the higher backrest position (i,e above 17cm at Leonardo loom) where dropper line is kept constant position from backrest (i,e dropper line comes relatively closer to the backrest) then the backward movement of the backrest roller will reduce the required tension. (Figure 7)
2.2 Backrest height on warp tension
The effect of backrest height is illustrated in figure 8 . Here the digit having (-) ve sign indicates downward position and the digit without sign indicates upward position of backrest. The required warp tension will be low when the backrest is moved downward but it increases with the increase of backrest height. From the figure 7 it is seen that downward position of backrest roller will require low tension but upward position of backrest.
2.3 Dropper height on warp tension
The effect of dropper height is illustrated in figure 9 with the increase of dropper height (lifting the dropper line) warp tension tends to decrease provided that the dropper line is situated at the middle of the warp. But if the dropper line is situated closer to the heald frame then this effect will be nullified by the effect of dropper position. From the figure 8 and table 6 it is observed that with the increase of dropper height the tension reduces if the dropper lies at the middle of the warp. Because increase of dropper height means lifting of dropper, which will decrease the downward tension imposed on the yarn.
The quality of woven fabric depends on the quality of warp and weft yarn, proper sizing, drawing-denting and proper loom setting. Loom setting for different types of fabrics are different. Again, different types of looms (like airjet, rapier and projectile) require different settings. Hence, the understanding of the warp tension mechanics and the appropriate loom settings are essential for production engineers. This study will be helpful for the engineers who are directly involved in the weaving floor operating airjet or rapier looms for production of different types of woven fabric.
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