Traditional denim weaving had been done with the rapier looms and projectile looms for a long time. But with the development of the air jet weaving technology, the main flow of the denim weaving has been changed to Air Jet Loom due to its suitability to mass production.
The general guidelines in selection of various types of shuttle less weaving machines are as follows:
Rapier is known for its versatility in weaving fancy fashion materials of different constructions like suiting, upholstery, furnishing etc.
Air Jet weaving machine is most suitable for mass production.
Projectile is suited for dress material, industrial fabrics, heavy denim and geotextiles in single or multiple widths.
An evaluation on the application of various methods of weft insertion carried out by the Textile Machinery Society of Japan is given in Table 5.1
As can be seen from the Table that Air Jet Weaving scores in high productivity and labour savings however suffers in high energy costs and yarn wastage. Projectile scores well on both parameters, that is energy saving and fabric quality. More over, running cost will be the lowest for projectile during extended machine’s economic life beyond capital life. However, these ratings may be different in Indian context and for specific denim applications.
ON AIR JET LOOM
The important features of Denim weaving on Air Jet Loom are as follows:
a. High Productivity: As an actual production in the mill, 900 rpm is achieved already with the latest models of different makes.
b. Start Marks: The rush motor emits 1200% torque at the time of start and avoids start marks. Electronic let off, ensures the even warp tension from the full beam to the empty beam. Electronic let off is also equipped with the programmable kick back function which controls the cloth fell position at the start according to the loom stop duration. The second feeler is equipped to detect blow off of weft yarn.
c. Special arrangements for the Denim Weaving of Coarse Yarns:
I. Reinforced loom structure for heavy duty weaving.
II. Reinforced let-off motion for heavy warp.
III. Intermediate rocking supporter to make a strong beat without bending for heavy fabric.
IV. Positive cam motion for high speed operation for the heavy load of warp.
d. Stop Market Prevention:
i) Automatic Leveling Device: It automatically closes the shed after a loom stop by leveling all the shafts and prevents warp strain which may create corrugate marks.
ii)One Pick Insertion System: To minimize stop marks, a special weft insertion system functioning at the time of restart can be equipped with. When the loom is started first pick is automatically inserted just before the first beat. By avoiding the beat- up without yarn at reverse rotation, this system prevents the corruption of the fabric construction and prevents corrugate marks.
e. Devices for a coarse count weft:
The following are some of the arrangements equipped to use a coarse count yarn for weft.
i) Balloon Breaker reduces the weft tension due to ballooning.
ii) Auxiliary main nozzle is installed before the main nozzle to insert a coarse weft with less air pressure.
iii)Stretch nozzle is furnished to give adequate stretch to the weft for a perfect insertion of a coarse weft.
f. Labour Saving:
This is realized with a large size packages and loom automation. Automation includes automatic pick finding for easier weft repair and automatic defective pick remover.
REQUIREMENTS FOR HIGH INSERTION RATES IN AIR JET WEAVING
weaving profitability is the result of weaving productivity. The higher the weaving machine speed and efficiency the higher the productivity. As soon as speed is increased however, weaving machine efficiency, which is affected by loom down time on warp and product changing on repairs and yarn break repair times and also quit markedly by the number of warp and weft stoppages falls a rule.
The standards for loom stops per hour is given in Table-5.2
In air jet weaving, the proportion of weft stoppages to total is high, and their reduction is therefore of great importance for increased efficiency in air jet weaving processes. In addition, fewer warp yarn breaks, their faster repair and short setting times are the key for high production. It is not the speed of the machine, but the number of picks actually produced that is the deciding factor in modern weaving. The important points are i)the factors which guarantee high insertion rates ii) the humidity levels iii) the factors which lead to a high fabric quality.
Influence of Yarn:
The yarn breaks in spinning are correlated to the yarn breaks on the weaving machine. It is important to maintain count and strength variability to the lowest. The standards for Count CV% and Strength CV% is given in Table- 5.3.
The count CV% will be higher by +0.5 where auto levelers are not existing on finisher draw frame. Therefore it is necessary to use two passage draw frames where finisher draw frame is equipped with auto levelers particularly fine, ring and characteristic yarns.
In case of OE spinning there is an interesting correlation between high residual trash- content and increased yarn breaks in weaving. The norms for residual trash content at feed sliver are 0.5%.
The weft insertion performance depends to a large extent on the level of weak spots. The standards for Uster Tensojet Tensile properties are given in Table-5.4. Percentile 0.1% values given in Col 5 and 6 for Elongation% and Force in cN is a tool for identifying the weak spots in the yarn lot and estimated performance on loom.
Influence of Weaving Preparatory on the Cloth Production System:
Tight ends, lost ends, pieces of yarn or lint dragged onto the warper beam are performance killers in air jet weaving. Therefore the accurate functioning of the stop motion on the creel plays a key role.
Warper beams should not contain any grooves, high edges or crossed ends. They should be made with bobbins of adjusted length and run off the creel without crossing ends and at uniform tension. In this manner yarn breaks on the sizing machine can be almost eliminated.
During sizing, the lengths run at creep speed should present less than 4% and a moderate size application control system should be used. For air jet weaving, two size boxes and real wet splitting should be used in case of warps with more than 70% cover density, in order to reduce the hairiness of the yarn.
The use of size add on control seems to be essential when operating with two size boxes, because there are no two size boxes, because there are no two size boxes which, in spite of exactly identical setting, produce exactly the same size add on. The differences measured may reach as much as 4% in absolute figures.
Over drying, particularly when sizing cotton yarns should absolutely be avoided.
After waxing, (0.2 to 0.5%) of the warp ends generally improves its performance.
Finally the warp ends should be fixed by tapes in such a way that they can be entered into the harness without crossing.
Factors influencing the performance of the warp in the weaving machine:
The shed geometry in front of the reed is to a large extent designed by the builder of the machine. When setting the back shed and harness stroke the weaver is required to choose the proper setting, also when determining the warp tension. These settings have an important impact on the performance of the machine. It is therefore essential that these settings are optimized with utmost care and by using modern measuring instruments.
Following two factors have a particular influence on the behaviour of the warp:
i) yarn traction force
ii) yarn hairiness
A too high warp yarn traction force is leading to overloading the warp ends. As the yarn traction force is not constant during weaving, the peak tensions which generally appear in the bottom shed, particularly in the middle of the warp should be taken into consideration. These peak values should not exceed 5 to 6 cN/tex, depending on the quality of the yarn.
The importance of the setting of the warp stop motion is often under estimated. Its position has a direct influence on the back shed and thus on the yarn traction forces in the bottom and top sheds. The vertical position of the warp stop motion should therefore be set very precisely. It changes the asymmetry of the shed.
Relative humidity have a great impart on the performance of the weaving machines. The optimal dissipation of temperature and humidity over the machines, that is the warp, is generally not reached, because the sources of heat within the weaving machine disturb the climatic conditions. Numerous yarn breaks are caused by dust, lint and fibre accumulations. The best experiences have been made with air conditioning systems whereby humidified air is conducted over the machine, whilst the dust – loaded exhaust air is evacuated through floor opening under the back shed, and because the descending flow of the conditioned air is assisting the sedimentation of the lint. At the same working conditions for the personnel are improved. It is surprising how much cleaner the machines and the whole weave room are, compared with systems whereby the exhaust air is evacuated in the alleys, or worse, through the outside walls.
Table -7.6: Weaving – Typical Denim Constructions
TABLE-7.7 : PRODUCTION CALCULATION
The following information is an estimated performance for weaving 14.5 ozs indigo denim.
a)Fabric : Indigo Denim 14.5 ozs/square yard
Warp Yarn : 100% Cotton Ne 7s Indigo Dyed
Weft Yarn : 100% Cotton Ne 6s Grey
Warp Density: 60 ends/inch
Weft Density : 40 picks/inch
Width : 66.5 inches
Weave : 3/1
b)Reed Space : 68.5 inches
2 Colour Weft Mixing
Positive cam shedding Motion, 4 shafts
-Denim Weaving Arrangement
c)Estimated Loom Speed : 900 rpm
d)Estimated Production Per Loom:
i)Daily Production (linear length)
= 900 rpm x 60min x 24h x 0.92 eff x 0.0254 / 40 ppi
= 757 metres/day/loom
ii)Monthly Production (linear length)
= 757 m x 30 days