Drawing

Drawframe

Within the sequence of m/c in cotton spinning mill, the drawframe is the definitive compensation point for elimination errors. Inadequacies in the product leaving the drawframe not only pass into the yarn, they are actually reinforced by drafting effects following the draw frame. The yarn is never better than the draw frame sliver.

At drawing stage, material passes not only one m/c but usually 2, arranged 1 after the other & combined to form a group. Processing in 2 passages is necessary completely to fulfill the requirements. The 2nd passage is often superfluous after combing m/c because then it does not normally generate any improvement in quality. In old machinery 3 drawings were used, breaker, intermediate & finisher. But now in latest m/c only 2 drawings, breaker & finisher are used & in D-35 only one drawing is used.

Object Of Drawing

• Through doubling the slivers are made even
•  Doubling results in homogenization(blending)
• Through draft fibres get parallelised
• Hooks created in the card are straightened
• Through the suction ,intensive dust removal is achieved
• Autoleveller maintains absolute sliver fineness

Task Of The Drawframe  

• Drafting
• Equalizing
• Parallelizing
• Blending
• Dust removal

Drafting Arrangement: Drafting Arrangement is the heart of the Draw frame. The Drafting Arrangement should be:-

• Simple
• Stable design with smooth running of rollers
• Able to run at higher speeds and produce high quality product
• Flexible i.e suitable to process different materials , fibre lenths and sliver hanks
• Able to have good fibre control easy to adjust

Rotary Screen Printing

In rotary-screen printing, continuous rotation of a cylindrical screen while in contact with the fabric ensures genuinely continuous printing. Print paste is fed into the inside of the screen, and during printing is forced out through the design areas with the aid of a stationary squeegee. Figure 2.9 illustrates some of the squeegee types in use.

In the case of flexible-blade squeegees, the rotation of the screen in contact with the stationary blade builds up the pressure to force the paste through the screen. This is, of course, the converse of flat-screen printing, where the screen remains stationary while the squeegee moves. A typical rotary-screen printing machine is shown in Figure 2.10. The design of most machines follows the pattern established for fully automatic flat-screen machines: an endless driven blanket, screen positions along the top, and blanket washing and drying effected underneath during the return passage. Provision for the use of a thermoplastic adhesive is common on rotary machines, with a curved-surface heating plate to heat the fabric before it is pressed on to the blanket. The cylindrical screens can be much closer together than is possible with flat screens and so the blanket is shorter (for a given number of colours). The fabric dryer, however, must be longer to enable the printed fabric to be adequately dried at higher running speeds. Typically, speeds of 30–70 m min-1 are used depending on the design and the fabric quality. It is quite possible to run the machine faster than this, the limitations often being the length and efficiency of the cloth and blanket dryers and the difficulty of observing printing faults at high running speeds. Print paste is often poured into flat screens by hand, even in fully automatic machines, but the continuous movement of a cylindrical screen and the restricted access necessitates automation of this operation. The print paste is pumped into the screen through a flexible pipe from a container at the side of the machine; inside the screen, the paste pipe has a rigid structure as it also acts as a support for the squeegee. Holes in the pipe allow the paste to run down into the bottom of the screen; since the paste is pumped in from one end, the holes need to be larger at the end furthest from the pump to achieve an even spread across the full width of the screen. A sensor (level control) actuates the pump when the paste level falls below a preset height

Flat Screen Printing

Flat screens are not suitable coloration units for a truly continuous process, and in all the successful machines for fully automatic flat-screen printing the colour is applied through the screens while the fabric is stationary.

The main features of a typical automatic flat-screen machine are illustrated in Figure 2.5 and shown diagrammatically in Figure 2.6. All the screens for the design (one screen for each colour) are positioned accurately along the top of a long endless belt, known as a blanket. A machine intended to print traditional furnishing designs might have space for 15 or more screens. The width of the gap between the areas printed by any two adjacent screens must be a whole number of lengthways design
repeats. This need not necessarily be the same as the lengthways screen repeat as there may be several design repeats per screen repeat; for example, where there are three design repeats per screen repeat, the gap between adjacent screens need only be one-third of a screen repeat. The fabric is gummed to the blanket at the entry end and moves along with the blanket in an intermittent fashion, one screen-repeat distance at a time. All the colours in the design are printed simultaneously while the fabric is stationary; then the screens are lifted and the fabric and blanket move on. When the fabric approaches the turning point of the blanket, it is pulled off and passes into a dryer. The soiled blanket is washed and dried during its return passage on the underside of the machine.

Screen Printing

The printing process consists of forcing a viscous print paste through the open areas of the screen with a flexible, synthetic rubber squeegee. The rubber blade, which is contained in a wooden or metal support, is drawn steadily across the screen at a constant angle and pressure. If the screen is too wide to allow one operator to reach all the way across it, two operators may work together, one on either side of the table. The pressures exerted by the two must be as similar as possible.

Before printing begins, the screens must be carefully positioned on the fabric. The area printed by a screen (screen repeat) must fit exactly alongside the adjacent one, a slight overlap being preferable to a gap. With flat-screen printing this is not automatically achieved (as is the case with rotary printing). The differently coloured areas must be in register and, again, a small overlap is usually allowed. To achieve accurate registration it is common practice to attach to the frame a bracket which locates against fittings, known as ‘stops’, on a guide rail along one edge of the table. The stops are spaced exactly one (lengthways) screen repeat apart along the whole length of the table. Two adjustable screws set the distance of the frame from the rail.

  

Controlling Amount Of Print paste

The amount of print paste passing through the screen can be controlled in several ways. Factors affecting this are:

• The ‘mesh’ (threads per inch) or ‘raster’ (threads per cm) of the screen fabric; generally a coarse mesh allows more paste to pass through than a fine one

• The fraction of open area in the screen fabric; this depends not only on the mesh butalso on the yarn diameter and the effect of subsequent treatments, such as calendering

• The hardness and cross-section of the squeegee blade; a hard rubber squeegee with a sharp cross-section is suitable for outlines, whereas a soft, rounded blade applies more paste and is suitable for blotches

• The hardness of the printing table; if the top of the table is firm a soft squeegee is probably necessary, whereas with a resilient table surface a harder squeegee is preferable

• The viscosity of the print paste; within the constraint of the requirement for good definition, the viscosity can be varied, thinner pastes passing through the screen pores more readily than viscous ones

Classification Of Screen Printing
Screen Printing Can be classified in two process. They are:-

1. Flat-Screen Printing
2. Rotary Screen Printing