Chapter 2 - Cotton value addition - Textile processing
Yarn formation is the process of converting loose cotton fibre into a yarn structure, involving a progression of distinctly different and separate processes. The primary functions of these processes are:
Whatever the end result desired, proper fibre selection is the foundation of any successful spinning operation.
The requirements of the end-product, or of the consumer of the yarn, will be the dictating forces in determining the fibre quality and properties that are best suited for the most economic situation. Using fibre that is of better quality than required will prove unprofitable. Likewise, using fibre that is of poorer quality than required will result in losses. Therefore, correct decisions regarding the most suitable fibre properties for a given operation are paramount for maintaining profitability.
Opening breaks down compressed layers or clumps of fibre into small tufts, facilitating transport and efficient cleaning (see figure 2.31).
Figure 2.31: A bale plucker
A bale plucker feeds fibre to the spinning mill from an assemblage of individual cotton bales (a laydown). This step is considered part of the opening process. The plucker takes a small layer of fibre from the top of all the bales on each pass. Fibre is then transferred to the cleaning line. The inset shows how the plucker head removes a small layer of fibre from each bale as it travels back and forth along the laydown.BlendingBlending brings together fibre tufts from many bales to form a consistent, homogenous mix.CleaningCleaning removes extraneous matter from desirable fibre.There are four basic principles of cleaning:
Carding aligns, parallels, cleans and condenses fibre into sliver (see figure 2.32). Other important capabilities of carding are:
Figure 2.32: Diagram of a card
DrawingDrawing blends, straightens, and levels (see figure 2.33).Figure 2.33: Diagram of drawing sliver
Lap preparationLap preparation combines a number of slivers into a wound, flat ribbon (lap), needed for combing.CombingCombing removes short fibres, straightens and blends.RovingRoving is an intermediate drafting processrequired for ring spinning that also places sliveron to a bobbin (see figure 2.34).
SpinningThe insertion of twist into the fibre strand is necessary to give integrity and strength to the fibre bundle. The methods employed for inserting this twisting action are distinctly different depending on the spinning technology used. Because the methods for inserting twist are different, the resulting yarn structures also display their own unique forms.There are three main technologies available for inserting this twist for the purpose of creating a yarn structure. These are ring spinning, open end (or rotor) spinning, and air jet (vortex) spinning.Ring spinningRing spinning inserts twist by means of a rotating spindle (see figure 2.35). Ring spinning is both the slowest spinning method and the most expensive because of the additional processes required (roving and winding).Figure 2.35: Diagram of a ring spinning operation
Ring spinning produces the strongest, finest, and softest yarn (see figure 2.36). It is also the most mature spinning technology.Figure 2.36: Ring spun yarn
This SEM image clearly shows the helix angle of twist that is responsible for holding the individual cotton fibres together. (M.J. Grimson)Open end (rotor) spinningOpen end or rotor spinning inserts twist by means of a rotating rotor (see figure 2.37).Figure 2.37: Diagram of an open end (rotor) spinning operation (K. Charlton)
Open end spinning has a high production capability. It has a low cost due to its high production rate and the elimination of processing steps. Open end spinning produces a weaker yarn than ring spinning, has a limited count range, and produces a yarn that is ‘dryer’ or harsher in hand (see figure 2.38).Figure 2.38: Open end (rotor) yarn
In comparison with the ring spun yarn (figure 37), the difference in yarn structure is very evident in this SEM image of an open end yarn. Note particularly the wrapper fibres that are perpendicular to the yarn form. (M.J. Grimson)Air jet (vortex) spinningAir jet (vortex) spinning (see figure 2.39) inserts twist (see figure 2.40) by means of a rotating vortex of compressed air. Air jet spinning has a high production capability and a low cost due to its high production rate and the elimination of processing steps. Air jet spinning produces a weaker yarn than ring or rotor spinning (for 100% cotton) and has a limited range of yarn counts.Figure 2.39: Air jet (vortex) spinning (K. Charlton)
Figure 2.40: Air jet (vortex) spun yarn
This SEM image of a vortex yarn shows a high degree of similarity to the ring yarn structure. (M.J. Grimson)As the yarn count gets finer, the yarn strength improves over open end spun yarns of the same count. Vortex yarn is appropriate for medium to fine yarn counts. The softness of fabrics made from vortex spun yarns usually falls between similar open end and ring fabrics.