Screen printing

Screen printing:

Screen printing uses the principle of stencil. It is a resist method of printing in the sense that the screen is made to resist penetration of the print paste in areas where printing is not desired. Dye or pigment in a thickened formulation is forced by a blade or roller called a “squeeze” through a permeable screen onto a fabric under math the screen. Screen printing may be done with either flat or cylindrical screens.

Centuries ago, Japanese developed the stenciling technique for textile printing as a fine art, by the 19^th century this printing method spreaded worldwide.

In 1850, French printers introduced the use of silk fabric to provide a continuous support for the paper stencil and that was stretched across a frame. This combination becomes known as a screen. At first screen fabric were made from hydrophilic yarn like cotton, silk, rayon etc afterward from hydrophobic ones like nylon and polyesters.

Strong developed mechanized screen printing process by the introduction of a movable carriage, in which the screens are mounted one at a time. The squeeze was driven by a motor attached to the carriage.

Fully automatic flat screen printing technique was developed by Buser, Stork and Johannes Zimmer in1950’s. These machines print all the colours in a design simultaneously along the top of an endless blanket conveyor belt.

Fully continuous printing by rotary screen was manufactured by AJC de O Barros in 1954. But this rotary screen had a seam line along its circumference.

The invention of seamless rotary screen of electrodeposited nickel was introduced by Zimmer in 1961 and latter by stork in 1963.

Machines using rod or roller squeeze manufactured by Zimmer was very successful in printing wider substrates e.g. carpets.

Currently rotary screen printing is the major printing method which replaced copper roller printing. These have also been used to print paper for transfer printing process.

Classification of screen printing:

There are four types of screen printing machines/processes-

1.      Hand screen printing.

2.      Semi-automatic screen printing.

3.      Fully automatic screen printing.

4.      Rotary screen printing.

Hand screen printing:

Hand screen printing is restricted in the college of art, small-scale unit and high fashion industries as it is a craft rather than a productive method of printing.

Here the number of color in a design is equal to the no. of screens used in that design. For every color, different print paste is made. Again in case of two or more color design the motif which covers more area in printed portion is applied first and so on. For hand screen printing the following things are required:-

        i.            Screen.

      ii.            Table

    iii.            Bed on the printing table.

    iv.            Back grey.

      v.            Fabric to be printed.

    vi.            Squeeze system.

As here the squeezer is manually driven by hand on the screen, it is called hand screen printing.

Printing is carried out on a flat, solid table covered with a large of resilient felt and a washable blanket (coated with neoprene rubber)

Fabric movement or shrinkage is avoided to maintain registration of the pattern. The fabric to be printed is laid on the table and stuck to be the blanket directly using either a water soluble adhesive. Alternatively the fabric is combined with a back-grey. In the latter  instance an absorbent fabric is stuck to the blanket and the fabric to be printed is pinned down on top of it, Sometimes fabric and back-grey are combined before fixing to the table using an adhesive and a special padding mangle.

Before printing the screens must be positioned carefully on the fabric. The area printed by a screen (screen repeat) must fit exactly alongside the adjacent one. Now print paste is taken on the screen. The printing process consists of forcing this viscous print paste through the open areas of the screen, with a flexible synthetic rubber squeeze. The rubber blade contained in a wooden or metal support is drawn steadily across the screen at a constant angle (about 45°) and pressure. Two strokes of squeezing gives better result. For very even print, 4 strokes are also done.

Advantages:

       I.            less investment cost.

    II.            Not so much necessity of dryer.

 III.            Multicolor design can be produced.

 IV.            Less floor space required.

    V.            Suitable for small scale production.

Disadvantages:

       I.            Slow production.

    II.            Small scale production.

 III.            Labour intensity is large.

 IV.            Fastness properties are not of international standard.

    V.            It is difficult to maintain even penetration of print paste through the screen.

 VI.            Not suitable for large scale production.

Controlling the amount of print paste passing:

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

2)      The fraction of open area in the screen fabric, this depends not only on the mesh but also on the yarn diameter and the effect of subsequent treatments e.g. calendering.

3)      The hardness and the cross section of the squeeze blade; a hard rubber squeeze with a sharp cross section is suitable for outlines whereas a soft rounded blade applies more paste and is suitable for blotches.

4)      The hardness of the printing table. If the top of the table is firm a soft squeeze is suitable.

5)      The viscosity of the print paste, thinner paste passes through the screen more readily.

6)      The number of squeeze strokes generally 2-4 strokes are applied.

7)      The squeeze angle and pressure.

8)      The speed of squeeze strokes.

All these variables are taken into consideration along with the nature of design, when printing the chosen fabric.