Fabric defects and problems of machine regulation

The finished fabrics can show various kind of faults which can be ascribed to the operations which follow one another till the realization of the finished fabric. The most common defects which appear in more or less extended areas of the fabric are:

• knot;
• crease, mark;
• abrasion or hole;
• tear;
• stain;
• dirt, contamination;
• moirè = presence of vawy areas in periodical sequence, reflecting the light and due to a different compression of weft or also of warp.
• grain = presence of designs with streaked and sinuous lines.

The most common fabric defects due to warp are:- Faulty thread = a thread or pieces of thread which are coarse, fine, irregular owing to higher or lower twist or to other twist direction, of different colour, with two or three ends;
- missing thread = a thread or pieces of ground or effect threads which are missing in the fabric weave;
- tight/slack thread = a thread or pieces of thread which are tighter or slacker than the other pieces/threads;
- incorrectly woven yarn = a thread which in some parts only of the fabric is not interlaced in the standard way
- broken warp = small pieces of cut or missing warp thread
- reversed thread = crossed, exchanged threads or thread pieces;
- warp stripes = one or more faulty threads giving rise to zones of different aspect; it can be due to scraping or rubbing from members of production machines or to inaccurate reeding;

The most common fabric defects due to weft are:• Faulty weft = a weft or pieces of weft which are coarse, fine, irregular (slubs, etc.), twisted, reversed, with different twist, of different colour, double weft;
• missing weft = weft or pieces of weft missing in the fabric weave;
• tight/slack weft = a weft or pieces of weft which are tighter or slacker than the other pieces/wefts;
• incorrectly woven weft = a weft which in some parts only of the fabric is not interlaced in the standard way;
• cut wefts = short pieces of cut wefts;
• weft bars (starting marks) = visual light/dark effect in weft direction due to higher or lower weft density caused by the weaving machine.

The quality control on the fabrics is carried out on a special inspecting machine, equipped with special lamps which facilitate the defect detection by the operator, marks them with labels of different colours according to the fault type and importance.

Depending on the number of faults and on their importance, the fabric pieces can be classified as standard (in respect to quality specifications) or can be subjected to a more or less serious degrading with consequent compensations to the customers or with the sale of the fabric at a reduced price.

Various defects can arise during the stages of weaving preparation (warping, sizing, threading-in into the heddles and into the reed) as well as during weaving itself. It is therefore important to regulate accurately the various devices of the weaving machine and to understand how to act in case of anomalous operating situations which create defects and/or reduce weaving efficiency.Let us see in the following which practical effects some of the most common regulations might have.

Warp tension:

The warp must be under tension to permit weft insertion and fabric construction. The increase in the tension avoids stressing heavily the yarns during the reed beat-up, reduces their sticking together during shedding especially when weaving yarns with poor elasticity and with low airiness, facilitates the separation of the interlaced or glued yarns and the passage of the knots through the reed. The tension might however increase the tensile stress on the warp threads and consequently lead to a higher number of broken ends. On the other hand the reduction in the tension results into a lower yarn breakage rate and also into a lower friction of the threads against the heald frames. In certain cases it could cause however difficulties in obtaining the desired weft density owing to the less effective stroke.

Position of the back rest roller:• horizontal regulation: it is suggested to move the back rest roller away from the harness to reduce the elongation of the single threads, particularly when using yarn with low elastic recovery or when weaving with a high number of heald frames. The back rest roller can be however brought near to the harness when you want to increase the elongation of the single yarns with the purpose of reducing the sticking of the threads together; at the same time an adequate distance from the warp stop motion should be maintained in order to favour the lining up of the threads with the respective drop wires and to facilitate the repair operations;

• vertical regulation: with back rest roller positioned in the centre to get a symmetric shed and thus to reduce the stress on the threads during shed opening (normal condition); with back rest roller moved upwards to loosen the threads of the upper shed and to favour the insertion of the wefts in very dense fabrics; with back rest roller moved downwards to reduce the stress on there lease springs of the heald frames in the Jacquard machines or when weaving with the warp effect of greatly unbalanced weaves turned upside down;

• locking position: the locking of the back rest roller is carried out when stiff warp yarns are used in order to reduce the oscillations, or when snarls arise owing to the twist of the beam threads;
• free rotation: the back rest roller rotates when delicate warps, elastic warps or warps with high elongation are used or when only few heald frames are in motion (limited oscillations).

Warp stop motion:The selection of the type of drop wire, of the weight and density of each contact rail must be made with great care on basis of the yarn count and composition, following the indication of the manufacturers. The responsiveness of the warp stop motion can be increased by reducing the drop height of the drop wires towards the contact rail, in case of threads which are prone to getentangled or which show very difference counts or twists. This responsiveness can be reduced in case of loose threads or false stops.

Shedding:The centring of the shed towards the weft insertion tool used plays an important role, to avoid abrasion risks, weave defects, thread cutting, selvedge trimming and other faults. An increase in the shed dimension reduces the possibility of mistakes and thread breakage caused by their sticking together, whereas a decrease in the shed dimension reduces the stress on the threads.Sometimes it can be necessary to offset the heald frames to favour the separation of the threads or to avoid placing threads with too different tension close to each other.

Timing of the dobby:It might be convenient to advance the shed closing time of the dobby when using very dense and hairy warps, to improve the clearness of the shed; this way the possibility of producing loose wefts after the opening of the pulling rapier is reduced and the possibility of blocking the wefts during the stroke is increased. The closing of the shed is on the contrary delayed to obtain a better extension of the weft and to facilitate its insertion.

Take-up coatings:The take-up coating plays an important role to prevent fabric gliding during its taking-down,which would cause unavoidably streakiness. In general the friction coefficient should grow with the increasing of the warp tension. The maximum adhesion of the fabric is obtained using emery cloth coatings, but sometimes this kind of coating can result in abrasion spots on delicate fabrics.In these cases surfaces coated with rough or smooth rubber, or with resin are used.

Anti-streakiness cycles:The modern machines equipped with electrically connected electronic warp let-off and cloth takeup motions which are managed by the microprocessor system of the controller permit to carry out maintenance cycles aimed at avoiding the formation of stripes (continuous stripes and loom starting marks) after machine stops, while taking into account, at loom re-starting, the different reed beat-up speed in respect to the running speed, the plastic deformations of the threads and of the fabric, as well as possible displacements of the fabric formation edge during the stop. To avoid different initial beat-up conditions, it is also possible to carry out idle strokes.

Other interventions:Many other regulations are possible: on weft feeding and braking mechanisms, on selvedge formation devices, on temples, on weft cutting, on insertion mechanisms used. The fact of being in a position to produce the best suited regulations and corrections contributes in a decisive way to he improvement of the fabric quality and of the weaving efficiency.

Posted in: Weaving