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