Sunday, December 30, 2018

BOWING OR SKEWING IN THE FABRIC, A WEAVING DEFECT, A FABRIC DEFECT, CAUSES, AND REMEDIES

BOWING IN THE FABRIC, A WEAVING DEFECT, A FABRIC DEFECT, CAUSES AND REMEDIES:

 Bowing effect in the fabric: 

The bowing effect cannot be defined in few words. This is a very common fabric defect. This defect occurs during weaving or dyeing. Normally, bowing effect generates during the dyeing process. Since we know that end and pick intersect to each other in the woven fabric at a right angle, then the intersecting angle of warp and weft should be identical at every chosen or desired place on the fabric surface. We also know that every grey fabric has 1” to 1.5” bowing at both the selvedge of the fabric. This is known as residual bowing.





This amount of bowing can not be avoided during weaving because the fabric near both the selvedge comes out later than main body fabric. It results in the form of bowing at both the selvedge. It can be rectified easily and completely during processing.
The main possible cause generating bowing are being given below:

Excessing bowing in grey fabric: 

This problem mainly occurs due to faults in weaver’s beam. This kind of situation can reflect in the beams to be manufactured on the sectional warping machine as well as the direct warping machine.

When the warping of the beam is done on the sectional warping machine, then the warper needs to be careful about the selection of required cone height or warping drum traverse speed according to warp beam parameters.

The cone height on the variable cone height warping machine is set precisely to avoid the bowing problem. If the warper selects improper cone height, the beam diameter on both flanges side will not be equal, this will result in the form of bowing in the grey fabric.




If we mark at one complete pick across the full fabric width and marks at another one complete pick after one-meter interval across the full width of the fabric and now we measure the fabric length at both the selvedge. There will be a difference at both places. Actually, this difference is bowing due to improper setting of cone height.  

If the warping machine is equipped with a fixed cone height drum, all beam parameters are configured in the machine software. The machine software automatically selects the required drum traverse speed. This is not a full proof system at all. The machine software calculates the drum traverse speed according to beam parameters ie. Warp count, flange to flange distance, the total number of ends in the beam, standard warp tension and type of material (cotton, polyester, others). 

We know, the nominal count and actual count of warp yarn always differ to each other. This difference makes the difference between actual warp yarn diameter and calculated diameter by machine software. The amount of twist (turns per inch) also affects the actual yarn diameter, this factor is also not taken into consideration by machine software. 

The warp tension is also a very important factor in this case. In normal practice, warper does not change the warping creel tension for every warp count, he changes tension only when the warp count increases or decrease highly from previous warp count. This practice is also responsible for bowing effect occurring in the grey fabric. All above-stated reasons cause the bowing effect in the fabric. Apart from warp beam parameters configured in the machine software, machine manufacturer provides another solution to avoid the bowing effect. A special wrap reel is provided with the warping machine. The suggested number of ends are wound on the wrap reel, the number of wrappings is counted. This number of wrappings are fed into the software manually. This process neutralises all causes which can generate bowing in the fabric. If the warper follows this process precisely, the beam diameters at both flanges result equal to each other. Thus the possibility of bowing is avoided.

If the warping is being done on the direct warping machine, the warping creel tension plays a very important role to avoid bowing effect. If warper places equal dead weight washers on each warp end throughout the warping creel, then a tension difference results between front warp end and rare warp end. The tension of rare warp ends is decreased slightly. This decreased tension causes bowing in the grey fabric at both the selvedge sides.




If warper observes this kind of situation, then he should adjust the creel tension by following below method:

First of all, divide the creel into three sections, say front section, middle section and rare section, now adjust the creel tension in such a way that front section ends should have the lowest tension, the middle section ends should have more tension than front ends, and the rare section ends should have the highest tension. This tension arrangement avoids the chances of bowing in the grey fabric.

Bowing during processing:  

If there is no bowing in the fabric but bowing appears after finishing. This may happen due to the wrong practice adopted during processing. The main possible causes generating bowing effect during processing are given below:

Since we know that a dyeing lot is completed by joining together many pieces of grey fabric by stitching, we also know that every grey fabric has residual bowing, then there is always need of a high degree of care ness during piece joining. The tailor should look carefully at the direction of residual bowing.



The proper method of joining the fabric pieces during lot preparation


He should always stitch all the pieces in one direction. He should also confirm that the piece cutting is proper at both the ends of the fabric piece.


The fabric loading on the jigger should be carried out correctly. Improper loading can cause bowing in the fabric.

Now the final role of the stenter machine comes into play in bowing effect. The machine operator checks the direction of residual bowing in the fabric and selects to feed the fabric in the right direction. Now, you can ask, what is the correct direction of fabric feeding? The feeding direction in which the bowing is decreased by pulling manually at both the selvedge is called correct feeding direction. The bowing rollers are also activated. These rolls help to improve the bowing effect in the fabric.

Please note: if our machine is equipped with automatic weft straightening device, it monitors the bowing direction automatically and adjust the feeding tension at centre and selvedge according to the situation.   
                               
Related articles:

Pinholes in the fabric

Wrong end, wrong drawing and wrong denting in the fabric

Broken pick in the fabric

Warp floats in the fabric

Fabric defects due to yarn faults

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