Friday, 7 November 2008
A float or Jala is formed when there is no proper interlacement of the warp and weft yarns over a certain area. The remedial measures are the use of overall high warp tension and keeping the lease rods close to the heald shafts.
Weft Crack (Jerki)
A srip in the fabric where the pick density is lower than normal is calld a weft crack or jerki
Crammed Pick (Patti)
A strip in the fabric where the pick density is more than normal is called a patti. The defect is caused by improper setting of the anti crack motion.
The defect is caused when many ends break consequent to a shuttle trap. The important causes of shuttle trap are : wrong timing of shedding, soft picking, insufficient checking of shuttle in the boxes, severe slough off, and damaged or broken picking accessories.
A weft slough of one or two coils. the main causes of slough are softly wound pirn, overfilled pirns, high density of coils, short chase length etc.
Improper Weft Threading (Single Moti)
The defect is characterised by a loose and uneven selvedge consisting of weft and warp protroduing loosly beyong the true selvedge line.
Hardened fluff, as well as foreign matter such as piece of leather accessories or wood chips, woven into the texture of the fabric is called a gout.
In this defect, the yarns are distorted from their true paths and fine holes are caused near the selvedges. Unsuitable choice of temples and poor mechanical condition of temples are the main causes of this defect.
The defect is characterised by uneven and distorted appearance of the fabric, because of hard gummed spots in the warp. Use of cold size or keeping the immesion roller dipped in size during a long machine stop causes such spots.
Stains are caused by lubricants and rust. Most of the stains can be traced back to poor manintenance and material handling.
Thursday, 6 November 2008
Common Fabric Defects
1. Weft Streaks
These are faint or sometimes prominant lines along the lenght of the fabric. Use of good quality reeds will largly reduce streaks due to uneven end spacing. Streaks can also be avoided through careful housekeeping and ensuring that different batches of yarns do not get mixed up.
This defect is characterised by a general grouping of warp ends drawn through individual dents of the reed with a fine crack showing up in between such groups. This is caused by
- insufficient tension difference between the shed lnes during beat up.
- excessive warp tension
- late shedding
3. Irregular reppiness
It is characterised by the prominance of alternate picks over small areas on one face of the fabric. The following precautions can help to minimise the occurrence of reppiness.
a. Worn out tappets and tradle bowls should be replaced
b The back rest should be raised by about 4 cm with respect to the fell line.
c. The emery rolles should be in good condition.
d. An increase in warp tension within workable limits helps to reduce this defects.
4. Small weft loops ( phurkies)
A phurki is a weft loom protruding from the cloth surface. Generally, the length of the yarn in the loop is not sufficient to permit snarling. These loops may protrude from both faces of the fabric. Early shedding, incresing warp tension within workable limits, use of good temples and adequate size pick of about 12% or higher - these measures can be used to prevent the occurrence of phurkies.
5. Curled and folded selvedges
This defect is characterised by the appearance of curls and folds in the fabric selvedges. The following points merit attention:
a. Denting order- the number of ends per reed dent in the selvedge should be 50 to 100% more than that in the body.
b. Warp Tension- An increase in warp tension results in a tighter selvedge.
c. Selvedge construction- The selvedge yarns should be preferably two ply and should be drawn one end per heald eye.
6.Missing Interlacement of weft at selvedges
In this defect, a group of extreme warp ends in the selvedges do not interlace with the weft over a fabric length of 1 to 4 cm. The main cause of this defect is excessive tension in the weft yarn.
7. Weft snarls
A weft snarl in a fabric is caused by a short length of three fold weft yarn of which two folds are intertwisted.
The incidence of such snarls can be reduced by ensuring the process setting of twist and by minimising the possibility of severe rubbing of yarn between the shuttle and the box front plate.
8. Weft Bar
A weft bar is dark or light band across the width of the fabric which shows up prominantly particularly in dyed fabrics.
Medium to long term periodic irregularity in yarn will produce regular weft bars in the fabric.
The remedial measures can be:
a. Better process control such as:
i. uniformity of count on groups of ring frames spinning the same count.
ii. Prevention of periodic irregularity in the yarn.
iii. Control of winding and pirning tension.
iv. Proper maintenance of let off and take up motion
v. Control over blend proportion.
vi. Better housekeeping to prevent mixing up of yarn from frames.
9. Broken Picks
In plain woven fabrics, this defect materialises by the presence of two picks in the same shed for a part of the width of the fabric.
The main cause of weft breaks are rough surfaces of shuttle, shuttle box, rough or incorrect placement of shuttle eye, loose fitting of pirn in the shuttle, incorrect alignment of pirn with shuttle eye and low yarn strength.
Lashing in is the term used to describe bits of extra weft yarn found tucked into the selvedge of the fabric.
Remedial measures are:
a. Proper maintenance of accessories and loom parts.
b. Adjusting the pattern of changes of the boxes.
c. The use on drop-box looms of a suitable brush fixed at the end of the temple rod.
11. Missing End (Chira)
A defect where one or more warp ends are missing in the fabric is called a chira. This is the most frequently occurring defect in Indian fabrics and constitutes 40-50% of the total defects. Most chiras are caused by broken that are not mended immediately.
It can be prevented by minimising missing ends in the beam and efficient maninteance of the warp stop motion.
Effective supervision to ensure that a broken end is not left unmended for long can also minimise the occurrence of chira.
Wednesday, 5 November 2008
Maximising Loom Efficiency at Loom Shed
Control of Stops:
The various stops can be classified as
1. Stops due to warp breaks and weft faults
2. Stops due to weft breaks
3. Stops due to shuttle changes
4. Stops due to mechanical failures
1. Stops due to warp breaks and warp faults
Given a quality of yarn, the end breaks during weaving are increased by
- Preparatory deficiencies such as uncleared yarn faults, unsized beams, excessively starched yarns etc.
- faulty loom settings
- defective loom parts and accessories
-unsatisfactory atmospheric conditions
The stops due to warp faults such as slack ends, sticky ends, cross ends, missing ends etc. are due to preparatory deficiencies.
2. Stops due to weft Breaks
Given the quality of yarn, breaks are mainly influenced by unwinding tension and package faults.
The unwinding tension in turn depends upon
- The condition of the surface of the pirn
- shape and dimension of the pirn
- condition of the shuttle accessories, such as shuttle tongue, shuttle jaw
3. Stops due to shuttle changes
4. Stops due to Mechanical Failures
The more common mechanical failures are: faulty working of the warp-stop-motion, weft-stop-motion, pirn transfer mechanical, fast read motion, bang off and shuttle trap.
5. Miscellaneous Stops
All other stops no caused by the earlier four categories such as stops due to beam gaiting, repairs, oiling and cleaning, and delays due to shortage of pirns, beam accessories are included here.
6. Quality of Yarn
It is proven that in mills where there is a consistently low warp breakage rate, the yarn CSP is higher and also the number of thin places is less than the certain minimum level.
7. Loom Performance
In order to determine whether the prevailing breakage rate is really different from the norm and to compare the effect of two treatments say two different size mixings, the appropriate tests are:
((O-N)^2)>=4 and ((A-B)^2)/(A+b) >=4
where O are observed breaks and N are norms for breaks. A and B are breaks with different treatments
Control of Loss of efficiency by Snap Reading
Snap reading is a technique that helps both in listing the various causes of loss in efficiency and in estimating the percentage loss due to each cause. For taking such reading, one walks down the loom alleys making the tally of looms that are stopped against a list of the causes of stoppage.
For example, in a shed of 800 looms, after taking 25 rounds of snap readings, if 4000 looms are found stopped, then the loss of loom efficiency is (4000x100)/(800x25) = 20%
If out of the stopped looms, 1600 looms are found stopeed due to end breaks and warp faults, the loss in efficiency due to this cause is 8%.
OPTIMUM LOOM ALLOCATION
Generally when we answer this question "How many looms per weaver", we should taken into account both operative and machine efficiencies simultaneously.