ROTARY SCREEN PRINTING

               Fully automatic screen printing achieved excellent results, but it only associated with intermittent printing process, printers and designers were still looking for a method which would offer continuous printing. This was achieved in mid-1963, when European manufacturers brought out successful units producing continuous screen prints, by utilizing cylindrical engraved rotary screens.

Operations involved in Rotary Screen Printing

   Fabric Feed:-

·         The fabric to be printed must be introduced in the machine in a crease less, faultless and tension less manner.

·         In order to have prolonged printing without stoppages, it should be preferably fed from big batches.

·         The fabric is to be fed over a nip roller, according to the speed of the printing machine.

·         For delicate or warp-knit fabrics, selvedge uncurls are to be provided.

  Fixing the Cloth on endless conveyor Blanket:-

·         Standard two-roller gluing device and a glue holding trough are used for fixing the cloth.

·         Now a days, coating of synthetic adhesive is introduced and used widely, which does not require gluing trough.

·         CMC and PVA are most widely used adhesives for fixing the cloth on blanket.

 Working of Rotary Screen Printing:-

·         The printing effect on the cloth, in the rotary machines, is achieved by causing a proper and adequately pressurized contact between the fabric, which is stuck on the printing conveyor blanket and the cylindrical surface of the electro-deposited nickel screens.

·         This contact can be brought about by:

v  Bringing the revolving rotary screens down onto the endless printing conveyor blanket, or

v  By lifting the printing blanket to meet the rotary screens.

The former system is usually preferred.

·         The contact of the rotary screens and the printing blanket is also effected by two alternate systems:

v  Bringing all the rotary screens down in contact with the printing blanket simultaneously,  or

v  Bringing about this contact in a sequence according to the order of the colours arranged one screen after the other.

·         It is important to ensure that, once all the colours in the pattern are properly set to produce the correct impression of the design.

·         In the system with the individual screen lowering and lifting, it is possible to register the entire pattern without any unnecessary loss of printing paste and printing times in the initial setting of the design will alter.

·         The dyestuff paste is supplied to the interior of revolving screen, distributed in the entire screen body and is pressed on the inner side of the screen, pressed through perforations onto the fabric.

·         It is essential that the running of rotary unit be very smooth.

·         The assembly of machine, along with screens and squeegees, are well supported by frames on both the sides.

·         The height of the printing assembly from the blanket is adjusted with utmost accuracy, which is made to suit the thickness of the fabric being printed.

·         Improper height adjustments may lead to insufficient or excess screen pressure, which results into inaccurate printing.

·         The lower part of the printing unit contains the counter pressure system for the printing point and also the controls for repeat setting.

·         These controls help adjustments of repeat in the longitudinal and diagonal directions.

·         The top part contains the screen tensioning system, the screen driving shaft and the traverse repeat setting system.

Colour Distribution System:-

·         The pumps used for supplying the printing paste should work automatically by making use of accurate sensing instruments.

·         The supply of printing paste is to be arranged in such a manner, that a specific minimum quantity should always remain in the squeegee system.

·         This quantity is maintained by means of the colour probe. The working of probe is based on an electrical contact system or a pneumatic contact system.

·         Further distribution of paste in the squeegee body is achieved through smaller distribution tubes positioning at regular interval to supply equal amount of paste throughout the screen.

·         The colour pump can be run both in forward and reverse direction, to ensure pumping back the colour in case of sudden stoppage of the machine.

 Squeegee Unit:-

·         The squeegee units should to have the necessary sturdy and rigid supports from the sides, where squeegee blades are located.

·         The pressure applied on the squeegee blade is to be uniform from side to side and also in middle.

·         The units should be free of any vibrations and the colour application should be precisely controlled.

·         Squeegee blade pressure systems are:

v  Pressing system making use of the squeegee blades themselves as he pressure generating devices.

v  Pressing system in which independent pressure rollers are used to produce the printing pressure (electromagnetic rollers)

 Drives of Rotary Screens:-

·         In order to cause and maintain perfect synchronization between each screen, screen bearing assemblies are provided, which draw the revolving movement from then main drive of the printing machine.

·         The rotary screens are driven by the rotary screen driving motors.

Repeat adjustment of the Patterns:-

·         The pattern should be produce exactly according to the printing plan chosen, for that rotary screen can be revolved in both the forward and reverse direction.

·         These screen adjustment is possible in three possible directions:

v  In the sidewise, i.e. longitudinal direction

v  In the rotational, i.e. forward or reverse direction

v  In the diagonal direction

Repeat Size:-

·         The most common repeat size internationally recognized is 641 mm. i.e. about 25.25”

·         Other repeats are also favour of some manufacturers like 820 mm (32”) and 1018 mm (40”).

·         An exceptionally larger repeat size is 1395 mm (55”).

Number of colours:-

·         Rotary screen printing machines are suitable for 12, 14 or even 16 coloured designs.

·         Usually, only 5 or 6 coloured patterns are preferred.

 Printing Width:-

·         The most commonly used machine printing width happens to be 1620 mm (63”), although the width of designs may vary from 950 to 1620 mm.

·         Some manufacturers also offered wider width rotary screen printing machines, e.g. 1850 mm (72”), 2400 mm (96”), 2800 mm (110”) or even 320 0mm (126”).

Rotary Screen Printing Speed:-

·         The speed of rotary screen printing machine, available from 4 meters to 100 meters per minute.

·         Printing speed is also depend on various important factor:-

ð  Chemical nature of the fibre (in the substrate)

ð  Twist on yarn and texture of the fabric

ð  Chemical nature of dye (used for printing)

ð  Drying and post printing facilities

ð  Quantity of material to be printed

·         In the case of normal cotton, the speed could be high as 60 meters per minute, while for synthetic, it should be lower as 20-25 meters per minute. For blended material, speed will be governed by the blend percentage, texture etc.

 Cleaning of Printing Conveyor Blanket:-

·         The fabric is stuck on blanket using adhesive at entry side.

·         Blanket is running endless, so the portion of the blanket on which cloth is pasted, returns to the same position after the return passage.

·         During the return passage of the blanket from the delivery end back to the entry end below the printing table, the blanket is thoroughly washed, cleaned and wiped dry.

·         For the purpose of cleaning, a very efficient cleaning system is provided, which is known as the conveyor blanket cleaning unit.

·         The unit is provided with a water-spraying system, along with motorized scrubbing brush rollers.

·         Subsequent wiping offers a very clean and dry blanket surface.

 Colour Paste Pumps:-

·         The paste pumps perform the duty of supplying the printing paste to the colour distribution system of the rotary assembly.

·         The pumps have adequate capacity to supply the dyestuff paste even under conditions of maximum printing coverage, say 100 %.

·         The pumps are reversible and can draw back the printing paste when necessary.

·         The material used for construction of the pump is such that it is not affected by any of the ingredients used while preparing the paste.

Screen Washing Unit:-

·         After completion of printing, screens should be washed and cleaned as early as possible.

·         Cleaning is to be done by means of a good water spray.

·         The cleaning unit should be such a design that water can be sprayed from inside the screen, as well as from outside.

·         The washing unit should be located near the printing plant for minimizing the handling of the screen and to execute the washing operations without any loss of time.

 Drying the Printed Fabric:-

·         Machine is provided with a suitable jet dryer.

·         The jet dryer should have enough drying capacity, so that, even at the maximum machine speed, the printed cloth is well dried during the passage through the dryer.

     Reference:- (Book)

1.      Handbook of Textile Processing Machinery by R.S. Bhagwat

2.      Textile Printing by LWC Miles

3.      Technology of Printing by Dr. V.A. Shenai

4.      Technology of Printing by R.S. Prayag