IMPORTANCE OF RAWMATERIAL IN YARN MANUFACTURING:
Raw material represents about 50 to 70% of the production cost of a short-staple yarn. This fact is sufficient to indicate the significance of the rawmaterial for the yarn producer. It is not possible to use a problem-free raw material always , because cotton is a natural fibre and there are many properties which will affect the performance. If all the properties have to be good for the cotton, the rawmaterial would be too expensive. To produce a good yarn with this difficulties, an intimate knowledge of the raw material and its behaviour in processing is a must.
Fibre characteristics must be classified according to a
certain sequence of importance with respect to the end product
and the spinning process. Moreover, such quantified
characteristics must also be assessed with reference to the
- what is the ideal value?
- what amount of variation is acceptable in the bale material?
- what amount of variation is acceptable in the final blend
Such valuable experience, which allows one to determine the most suitable use for the raw material, can only be obtained by means of a long, intensified and direct association with the raw material, the spinning process and the end product.
Low cost yarn manufacture, fulfilling of all quality
requirements and a controlled fibre feed with known fibre
properties are necessary in order to compete on the world's
textile markets. Yarn prodcution begins with the rawmaterial in
bales, whereby success or failure is determined by the fibre
quality, its price and availability. Successful yarn producers
optimise profits by a process oriented selection and mixing of
the rawmaterial, followed by optimisation of the machine
settings, production rates, operating elements, etc.
Simultaneously, quality is ensured
by means of a closed loop control system, which requires the application of supervisory system at spinning and spinning preparation, as well as a means of selecting the most sutable bale mix.
BASIC FIBRE CHARACTERISTICS:
A textile fibre is a peculiar object. It has not truly fixed length, width, thickness, shape and cross-section. Growth of natural fibres or prodction factors of manmade fibres are responsible for this situation. An individual fibre, if examined carefully, will be seen to vary in cross-sectional area along it length. This may be the result of variations in growth rate, caused by dietary, metabolic, nutrient-supply, seasonal, weather, or other factors influencing the rate of cell development in natural fibres. Surface characteristics also play some part in increasing the variablity of fibre shape. The scales of wool, the twisted arrangement of cotton, the nodes appearing at intervals along the cellulosic natural fibres etc.
Following are the basic chareteristics of cotton fibre
- fibre length
- fibre friction
- structural features
STANDARD ATMOSPHERE FOR TESTING:
The atmosphere in which physical tests on textile materials are performed. It has a relative humidity of 65 + 2 per cent and a temperature of 20 + 2° C. In tropical and sub-tropical countries, an alternative standard atmosphere for testing with a relative humidity of 65 + 2 per cent and a temperature of 27 + 2° C,
may be used.
The "length" of cotton fibres is a property of commercial value as the price is generally based on this character. To some extent it is true, as other factors being equal, longer cottons give better spinning performance than shorter ones. But the length of a cotton is an indefinite quantity, as the fibres, even in a small random bunch of a cotton, vary enormously in length. Following are the various measures of length in use in different countries
- mean length
- upper quartile
- effective length
- Modal length
- 2.5% span length
- 50% span length
It is the estimated quantity which theoretically signifies the arithmetic mean of the length of all the fibres present in a small but representative sample of the cotton. This quantity can be an average according to either number or weight.
Upper quartile length:
It is that value of length for which 75% of all the observed values are lower, and 25% higher.
It is difficult to give a clear scientific definition. It may be defined as the upper quartile of a
numerical length distribution
eliminated by an arbitrary construction. The fibres eliminated are shorter than half the effective length.
It is the most frequently occurring length of the fibres in the sample and it is related to mean and median for skew distributions, as exhibited by fibre length, in the follwing way.
(Mode-Mean) = 3(Median-Mean)
Median is the particular value of length above and below which exactly 50% of the fibres lie.
2.5% Span length:
It is defined as the distance spanned by 2.5% of fibres in the specimen being tested when the fibres are parallelized and randomly distributed and where the initial starting point of the scanning in the test is considered 100%. This length is measured using "DIGITAL FIBROGRAPH".
50% Span length:
It is defined as the distance spanned by 50% of fibres in the specimen being tested when the fibres are parallelized and randomly distributed and where the initial starting point of the scanning in the test is considered 100%. This length is measured using "DIGITAL FIBROGRAPH".
The South India Textile Research Association (SITRA) gives the following empirical relationships to estimate the Effective Length and Mean Length from the Span Lengths.
Effective length = 1.013 x 2.5% Span length + 4.39
Mean length = 1.242 x 50% Span length + 9.78
FIBRE LENGTH VARIATION:
Eventhough, the long and short fibres both contribute towards the length irregularity of cotton, the short fibres are particularly responsible for increasing the waste losses, and cause unevenness and reduction in strength in the yarn spun. The relative proportions of short fibres are usually different in cottons having different mean lengths; they may even differ in two cottons having nearly the same mean fibre length, rendering one cotton more irregular than the other.It is therefore important that in addition to the fibre length of a cotton, the degree of irregularity of its length should also be known. Variability is denoted by any one of the following attributes
- Co-efficient of variation of length (by weight or number)
- irregularity percentage
- Dispersion percentage and percentage of short fibres
- Uniformity ratio
Uniformity ratio is defined as the ratio of 50% span length
to 2.5% span length expressed as a percentage. Several
instruments and methods are available for determination of
length. Following are some
- shirley comb sorter
- Baer sorter
- A.N. Stapling apparatus
uniformity ration = (50% span length / 2.5% span length) x
uniformity index = (mean length / upper half mean length) x 100
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