ROLLER SCREEN PRINTING
Definition: A mechanised process of printing where rollers engraved with patterns are used to print designs on fabric. Good to produce mass quantity.
The Roller printing machine consists of a large central cylinder (1) wrapped with layers of cloth called lapping(2) around which an endless (30-50mm) woollen blanket (3) a backgrey (4) and the cloth tobe printed (5) pass in contact with each other . Against the fabric and the cylinder press a number of engraved rollers (6) each of which is mounted on a steel shaft or mandrel (7), these rollers revolve in contact with the central cylinder. Each roller contributes 1colour, hence if colours increase in the print then the engraved rollers also have to increase correspondingly. Each engraved roller receives its colour from a colour furnishing roller (8) which rotates in a small trough or colour box (9) containing the print paste. A long fine sharp edged steel blade called the doctor (10) presses against one side of the engraved roller and a brass blade called the lint doctor (11) rests on the other side of the roller.
1. Central Cylinder is made of steel or cast iron. It rotates moving the fabric. It is wound with 8-12layers of lapping (linen/woollen) cloth. This helps in resisting the pressure to which the cylinder is subjected during printing. As the cast iron/steel cylinder has very little resilience, it is covered with a endless woollen felt cloth or blanket to give elasticity to the fabric being printed. To prevent the blanket from soiling by the colour being printed, a cheap greigh cotton fabric called backgrey is run between the blanket and fabric being printed.
2. Printing Rollers: The colour printing rollers are made of copper and the design according to the colour is engraved on it. The length is 200cms and dia of 12.5 – 30cms or 12.5-75cms. The impression gets filled by the colour and transfers the colour to cloth. 2 specialised rollers can be used in co-ordination to get tints, shaded effect or outlines. Thin lines and dots also can be achieved.
3. Mandrels: are steel shafts working as axels for rollers mounted on then.
4. The Colour Box: is a shallow trough through which a wooden roller covered with fabric rolls carrying print paste from the trough to the engraved roller.
5. The Doctor Blade is made of steel and has a razor like edge which rests on the engraved roller. It scrapes off excess paste.
6. The Lint Doctor - is made of brass. It picks up loose cotton threads or fibres attached to the engraved roller. This also removes the excess paste of the preceding roller, helping maintain purity of colour.
The fabric tobe printed is fed to the machine and as the driving motor is switched on, it moves forward around the central cylinder which also starts rotating. At the same time, the engraved rollers and the colour furnishing roller start rotating at suitable speeds to keep pace with the fabric. The colour furnishing roller picks-up the colour paste from the colour box in which it rotates and transfers it to the engraved rollers and fills up its hollows or the sunk in patterns. The engraved roller has no coour or other parts of its surface. As the engraved roller comes in contact with the dry fabric and is pressed against it, the colour is transferred and the particular part of the design is impressed on the cloth. The doctor blade scrapes off excesss colour paste and the lint doctor removes any lint, loose fibers etc.. from the cloth being printed. Each engraved roller continues to impress its part of the pattern on the fabric as long as the machine is operating. The engraved rollers have tobe adjusted very carefully so that they register or contribute their part correctly at the correct time. This adjustment is done by various devices in the machine. When the cloth is fully printed, it moves forward and leaves the cylinder. It is then dried in a drying chamber and given appropriate after treatment, to fix colours. The back-grey is removed for washing when it is soiled. The woollen blanket which goes round the cylinder is removed for washing when soiled.
In roller printing, designs with upto 16 colours can be printed. But generally designs with 4-6 colours are printed for economical and practical reasons.