ROTARY SCREEN PRINTING
The rotary screen printing machine very much resembles the automatic flat-bed screen printing machine in all respects except the rotary screen section. In rotary screen printing, perforated circular or cylindrical metal screens are used for printing instead of the flat screens.
The first rotary printing machine was introduced by Stork of Holland in 1963. This machine has a hollow perforated thin shell of nickel without a repeat joint. The squeegee lies inside the circular screen, it is rod type or flexible blade type activated by an electro-magnate.
This machine is built for 6,9,12,15 or 18 colours with maximum width of 160/180/240/320cms. Suitable for printing on any type of fabric construction including knits.
The main features of the machine are:
- Each screen is independently driven,
- printing paste is fed by special pumps individually to the different screens,
- the print paste level inside the rotary screen is automatically and constantly controlled
- the transfer of printing paste through the perforations of the screen on to the cloth is governed by specially designed squeegee blades
- the pressure of the squeegee on the inside of the screen as well as the individual pressure and positioning of the different rotary screens can be controlled at will.
- The screen fits automatically whenever the machine stops, so that no stop marks are produced
- The printing is effected on a conveyor blanket.
- Upto 24 colours and minimum of 100mtrs of fabric can be printed.
- The printing machine has a feed arrangement, a selvedge alignment unit, a continuous gluing unit for gumming down the fabric.
- The print paste is pumped into the screen as required and its level is regulated by automatic level controls.
- For cleaning, each rotary screen can be dismantled and washed and a change of pattern can be achieved in about 3minutes per screen.
- A high efficiency washing and drying unit is provided below the printing table.
- The drying unit uses hot air, the temperature is thermostatically controlled.
PREPARATION OF ROTARY SCREEN
1. A matrix of steel cylinder is first prepared, then coated with a chemical which will withstand the action of electroplating.
2. This is them electroplated with nickel with hexagonal perforations of the required size and mesh.
3. The electroplated shell of required thickness is them removed by immersing the steel matrix in a solution, whereby the shell is loosened from the surface of matrix.
4. The perforated shell is then cut to required width and fitted with patented end rings.
5. The shell so prepared is then lacquered and engraved and is ready for use as a rotary screen.
Nickel is the most suitable metal due to its high tensile strength, flexibility and good resistance to most of chemicals being used in the textile industry. However nickel is expensive, hence most of rotary printers have a shell manufacturing also.
ENGRAVING ON THE ROTARY SCREENS (GALVANO SCREEN)
The pattern is transferred to the screen by the photochemical method using a photo-sensitive and screen hardening lacquer.
It is possible to produce screens of 250mesh, due to which a very fine line effect, half-tone effect and non -serrated line effects can be produced.
In another type of the screens, both sides of fabric can be printed simultaneously by screens placed on opposite sides. It is also possible to print 2 different patterns as well as the same pattern on both sides of fabric. Apart from this one can print a pattern on one side and have a plain dyed surface on the other side.