DRAFTING

 Historically, the term ‘drawing’ was used in connection with the drawframe in staple spinning. ‘Drafting’ was used regarding roller drafting systems in roving and ring spinning. Upon the appearance of man-made fibers, the term ‘drawing’ was also used to describe the elongational process to improve the molecular orientation of the filaments. Custom still insists on the use of the historically founded words but in essence there is little fundamental difference between drafting and drawing. Linear density is defined as mass per unit length of a strand or along the flow path of a stream of fibers.

Purposes of Drafting

Drafting occurs when a stream of fibers passes through an acceleration zone5. The place where the acceleration occurs is called a ‘draft zone’ and it is necessary to control the fiber flowing through it. The solutions to the problem of fiber control are diverse and only a few examples can be given to illustrate the importance of mass flow control by passive devices.There are two major reasons for drafting or drawing, which are 

1. To better orient the molecules or fibers in the strand, 
2.  To change the cross-sectional area of the strand. 

In the drawing of polymers, one very important objective is to orient the long-chain molecules to give the filament better properties. In staple processing, an important objective is to orient the fibers within the strand by causing them to slide over one another to give the strand better properties. It should be noted that improved orientation can only be achieved by drafting the strand to give a smaller output crosssection.

There are cases that are not always regarded as drawing but which really are. For example, in extrusion, the linear density of the molten polymer approaching the spinneret is higher than the sum of the linear densities of the output filaments even before conventional drawing. The speed of the output material is faster than that of

the input. While an extruder is not regarded as a drawing machine, it always is.

Control of flowing material

Both polymer and staple drawing and drafting have instabilities in flow. Control is exercised by imposing restraints on the systems. With polymer in the solid state, control is exercised by hot pins or the like. Heat flow from the control surface permits control of the local visco-elastic constants of the polymer in such a way as to promote stability. In the case of staple processing, the variable frictional forces between the flowing fibers are a strong factor in producing the instability, which reduces their value in both yarn and fabric forms. These instabilities produce quasi-random errors in the product. The addition of an external retarding force to the flowing fiber reduces the instability.

Principle of drafting or drawing

Consider a sample of the input material before and after discontinuous drafting or drawing. If there were no losses in the process, the mass of the input sample would be the same as it is after drawing. Let  be the packing density (not to be confused with linear density), a the cross-sectional area, l the sample length, iaili be the mass in the input sample, and oaolo be the mass after drafting.

Drawing in staple fiber processing

In staple spinning, the material flows through the drafting or drawing zones of the equipment. (The term ‘drawing’ is often used to describe the particular overall process but it is common to refer to the components that carry it out with the adjective ‘drafting’. Thus we speak of drafting rolls and draft in a drawframe which seems odd, but that is the common usage.) Fibers are accelerated as they pass through each zone. Also fibers can, and do,migrate with respect to one another along the direction of flow. Conventional theory

has been mainly restricted to roller drafting, in which there are fiber acceleration zones within the spaces between two consecutive sets of rollers. (A similar idea applies to filament drawing but godets are used rather than rollers. Godets are cylinders about which a yarn is wrapped to grip the yarn for the purpose of elongating it.) However, fundamentals merely require that the exit material moves at a greater velocity

than the entry material. The theory in Appendix 8 seeks to include the case where fibers are drafted by toothed rolls.

 

Drafting Rollers

Error During Drafting

Periodic errors

Roller or godet defects such as those previously described translate into periodic errors in yarn, roving, sliver or tow, which are sharply defined. Not only does the linear density of the material vary in consequence but so also does the structure of the material strand.

Random errors

Textile strands also contain random errors with a very wide spectrum of errors.Staple fiber drafting,Fluted,bottom rolls,Reaction,Single sliver output,Reaction,Weighting Weighting,Multiple,sliver input,Rubber-covered top rolls,Rubber-covered top rolls

Cumulative effects of drafting

Where there is a number of drafting stages, the results are cumulative and the range of error wavelengths can be very large. Yarns show not only an extremely large range of error but these errors translate into faults in the fabric. The end result of these irregularities is that the fabrics made from the yarns show undesirable patterning known as moiré or barré, which reduces their value.