In textile industry, one of the most common and perplexing quality control problems is barre (repetitive yarn direction streaks). The factors which can cause or contribute to barre are varied and diverse. Barre is defined as “unintentional, repetitive visual pattern of continuous bars or stripes usually parallel to the filling of woven fabric or to the courses of circular knit fabric.” Barre can be caused by physical, optical or dye differences in the yarns, geometric differences in the fabric structure or by any combination of these differences. Barre is basically a visual phenomenon and any property of yarn which makes it ‘look’ different from the adjacent yarn in a fabric would result in this defect.
Barre can be due to the following
- Fibre properties
- Yarn characteristics
- knitting parameters
In weft knit fabrics Barre is taken to include only those fabric defects characterized by course wise (widthwise) repeating bars or stripes. In warp knits, the warp (or length) direction. This is symptomatic of the way in which the fabrics are produced.
CAUSES OF BARRE
All barre is the consequence of subtle differences in yarn reflectance between individual yarn in the knit structure. Any mechanism that can change the reflectance of a yarn in a knit structure is a potential barre source. Barre can be caused by physical, optical, or dye differences in the yarns, geometric differences in the fabric structure, or by any combination of these differences. A barre streak can be one course or end wide or it can be several – a “shadow band”. It is not the inadequacy of the raw material property which results in Barre, It is the inconsistency or the variability of the particular property which results in Barre.
The properties which are the causes of Barre are given below.
- Fibre Micronaire variation
- Fibre color variation
- Yarn linear density variation
- Yarn twist variation
- Yarn hairiness variation
- Knitting tension variation
- Improper mixing of cotton from different origin
- Improper mixing of cotton from different varieties
- Improper mixing of cotton grown in different seasons
Zellweger Uster has published the following details regarding Barre
- Fibre : 70%
- Yarn Count Variation: 10%
- Twist variation: 10%
- Yarn hairiness and its variation: 10%
Micronaire:
The difference in Micronaire average of the mixings of the entire lot should not be more than 0.2. The range of the Micronaire of the individual bales used in the mixings should be same. the C.V.% of Micronaire of individual bales within the mixing should be less than 12 % . Same micronaire bales should not be placed side by side and a group should be formed with the different micronaire bales and it should be repeated in the bale laydown.
The difference in micronaire value increases, the intensity of the barre effect becomes more serious. The cause of barre is not the difference in dye uptake between the thin cell walls of the low micronaire fibres and the thicker cell walls of the high micronaire fibres. dye exhaustion / fixation is essentially the same for the low micronaire and high micronaire fibres and also illustrated that at high micronaire values, both the maturity and fineness registered an increase. By proper blending of the cottons, the occurrence of barre due to differences of even upto 1.6 micronaire can be eliminated.
Fluorescence:
- The difference in UV reading average of the mixings of the entire lot should be same
- Variation in UV readings within the bale should be less
- Out side storage of cotton should be avoided
- UV readings increase over time if it is stored for a long time
- Should not mix high and low UV bales
Colour has been one of the primary factors of cotton quality for quite a long time. Colour is particularly important as a measure of how well a yarn or fabric will dye or bleach. Colour in general is expressed in trichromatic terms, such as L, a and b (Reflectance, Redness/greenness and Yellowness / blueness). The significance of these components with regard to cotton has been extensively studied and is generally agreed upon that only reflectance and degree of yellowness are important in assessing cotton colour. The influence of cotton colour on the dyeability characteristics of fabrics have been studied and reported by the U.S. Department of Agriculture , which revealed that a significant correlation exists between the colour characteristics of raw cotton and the colour of washed and wash-dyed cotton fabrics. Since much of the barre effects are due to the variations in dyeability characteristics within a fabric, difference in colour properties could be expected to influence the seriousness of any barre incidences in the fabric.
Yarn properties:
It has been widely accepted that it is the inconsistency or the variation aspect of the yarn properties which is a prime cause for ‘Barre”. Of the various quality characteristics tested, variation in hairiness count and twist are considered to be three important properties which need proper control to avoid barre. Slippage of spindle tape is the main reason for the TPI variations. If the TPI is more in yarn then the yarn diameter will reduce adn number of helical angle will increase. If the diameter of the yarn is low then more light will pass through that region of cloth because the gap between the two yarn is more.When more ridges are present, then more light reflects from the surface of the yarn. Hence regions with high TPI yarn appear light coloured after dying.
knitting:
- A bar or stripe may be caused by several variables shown below
- Tight loops: This may take the form of a shadow (several courses involved) or a discreet line (one course involved). It will normally show up as a dark or dense line or shadow
- Slack loop: Similar to above, but it shows up as a sheer or light line.
- improper stich length at a feed
- improper tension at a feed variation in fabric take-up from loose to tight
- Worn needles, which generally produce length direction streaks
- Uneven cylinder height needles (wavy barre)