It is an assembly of substantial length and a relatively small cross-section of fibres and or / filaments with or without a twist. Yarn occurs in the following forms :
a. A number of fibres twisted together;
b. A number of filaments laid together without twist (a zero-twist yarn);
c. A number of filaments laid together with a degree of a twist;
d. A single filament with or without twist (a monofilament); or
e. A narrow strip of material, such as paper, plastic film, or metal foil, with or without a twist, intended for use in textile construction.
The yarn which consists of staple fibres held together by twist is known as spun yarn. The yarns produced on ring spinning, open-end rotor spinning and air-jet spinning systems, all are the spun yarns.
The number which shows the fineness or coarseness of yarn is called yarn number.
There are two systems of yarn numbering.
i. Direct system
ii. Indirect system
In a direct system, yarn number is called the linear density of yarn with units of tex, denier and dtex, etc. Similarly, in the indirect system, yarn number is called the yarn count with units of NEC, Nm and N woollen, etc.
Following is the detailed explanation of these two systems:
It is used for the measurement of length per unit weight of the yarn. In this system, weight is kept constant while the length is variable.
In the indirect system, yarn thickness and yarn number are inversely proportional. This means that as the yarn count increases the yarn weight decreases and hence yarn becomes finer.
Basic Preparatory Processes for Spinning Operations
Fibres in the bale form are not suitable to start the yarn manufacturing. There is a number of processes to make them suitable for spinning. Following subsections describe the basic preparatory processes which may be used as per the end products requirements
Preparation of Cotton to Feed the Blow Room
After opening the strips of selected bale / bales and cleaning the sides, small tufts from the bales are taken and spread on the floor selected area for making the layers of the heap, as shown in Figure 3.1. A number of horizontal layer upon layers are made till
the end of bales. This heap of cotton is left for 24 hours to release the packing pressure and condition the material so that moisture in the material becomes homogeneous.
Blow room line consists of different machines and each manufacturer provides its own line of machines. The sequence of machines in a typical blow room line.
Objectives of a blow room line are as follows:
Opening: To open the compressed fibres up to very small tufts
Cleaning: To remove the impurities like seed fragments, stem pieces, leaf particles, neps, short fibres, dust and sand
Mixing and blending: To make homogenous mixture of the material
De-dusting: To extract the dust if present
Uniform feed for the card: To convert the mass of fibres into a thick sheet called lap which should be uniform length and width wise or to provide output in the form of tufts of optimum size
Following is the blow room operation summary:
Cotton is fed manually on the feed belt of the blending feeder. The opening, cleaning and blending are carried out by the inclined lattice and evener roller. The stripper roller transports this material to the feed lattice of fine opener-I. the material flow through the blending as well as feeder machine. The beater of fine opener-I beats it against the grid bars for cleaning. This material is sucked by a condenser transport fan through two rollers cleaner, which performs the opening, cleaning and dust extraction of cotton.
The cage of the condenser separates the dusty air from the material as well as delivers it on the control tower feed rollers. A beater in the control tower beats the material against the grid bars and transfers to the spiked rollers of super cleaner. The six spiked rollers of super cleaner perform as well as the opening and cleaning of material and deliver it to the feed lattice of fine opener-II. Where a Krishnor beater with steel pins further reduce the tuft size and help in cleaning.
Opening and Cleaning Material
The opened and cleaned material from the fine opener-II is sucked by cage condenser fan and then it is delivered to the control tower. At the bottom of the control tower, the material is fed to a beater with the help of a pair of feed rollers. The beater treats the material against the grid bars as well as for cleaning. And transfers to the feed lattice of the Hopper feeder. A spiked lattice and an evener roller in the Hopper feeder perform the operation of blending, opening and cleaning. A stripper roller delivers the material as well as to the feed lattice of the scutcher through a condensing box.
The Scutcher consists of a regulating feed unit, pin beater, lap forming cage, calendering unit and lap winding unit. The incoming feeding material in the form of a bulky sheet is checked length-wise and width-wise by a regulating feeding system. Thus a uniform amount of material is fed to the beater.
Material and Delivers
The beater further opens and Cleans the material and delivers it to the cage, which makes a lap sheet. The calender rollers compact this lap sheet while shell rollers wind it on a lap rod. Thus a compact roll of lap sheet is delivered by the scutcher, which is transferred manually on a trolley to the next machine called the card.
In the latest blow room lines, the material from fine openers / cleaners is transferred to the card directly by a fan through a chute feed system which is attached as well as at the back of the card. The latest blow room line in which the scutcher is excluded and fine cleaner delivery pipes are directly connected to the cards.