Rotary screen printing is now widely used for printing in our country. We can produce different intricate patterns using rotary screen printing machines.
In this article, we will know about the rotary screen printing technique along with the screen quality that is used here.
What is Rotary Screen Printing?
In rotary screen printing, colored paste is forced through the open areas of a cylindrical printing screen that rotates as the fabric moves beneath it.
We have different screen printing methods like falt bed screen printing technique, rotary screen printing technique, manual screen printing technique. In contrast to intermittent flat screen printing, rotary screen printing is fully continuous with fabric speeds up to 50 m/min, if adequate drying capacity is available. Rotary screen printing is now by far the major printing method for textile fabrics.
Requirements of Rotary Screen Printing
Open (design) area
Print paste feed
Level control (from end of screen)
Rotary Screen Printing Process
The printing blanket is shorter than in flat screen printing because cylindrical screens occupy less fabric length. Their diameter is 3.14 times less than their circumference. The use of a thermoplastic adhesive on a neoprene coated blanket is common since this simplifies washing
A pump delivers paste from a container into a central pipe that runs inside the full length of the screen. This pipe also supports the squeegee.
The paste runs down onto the rotating screen from holes in the pipe. These are larger towards the far end to give even paste distribution. The paste collects under the stationary squeegee, which forces it out through the holes in the rotating screen.
This is the opposite of flat screen printing where the movement of the squeegee forces the paste onto the fabric. The squeegee is a flexible stainless steel blade, since rubber squeegees wear away too quickly.
The blade curvature depends on the applied pressure and can be varied by adjustment of the side bearings. Uniform blade pressure across the width of the screen is essential to ensure even transfer of the paste to the fabric.
The screens are strong enough for rotation provided they are under tension along the cylindrical axis. They fit onto light aluminum end rings that can be driven from either one or both sides.
The blanket and screen drives are usually linked. Once the screens are in place the paste delivery tubes and squeegee assemblies are fitted.
On starting to print, the screen height can be adjusted and the screens can be advanced or retarded to give correct registration of the pattern.
Direct rotary screen printing does not allow the large repeats of flat screens. Large repeats can, however, be printed by automated intermittent raising and lowering of the rotating screens.
In this way, two or three different rotating screens print the same color to give a large repeat. Only a portion of the screen surface is open so that when the screens are raised the paste is held in a closed area and does not drip onto the fabric.
Lacquered Rotary Screens
Originally from Stork, usually have a pattern of hexagonal holes. The holes have sloping walls, being larger on the outside of the screen than on the inside.
They are manufactured by electro-deposition of nickel onto an engraved mandrel. The mandrel is the cathode and the anodes are pieces of nickel submerged in the plating bath of a nickel salt.
Hexagonal recesses are first carved into the thin copper layer on the mandrel surface and filled with an insulating polymer. The exposed copper is then lightly nickel plated and the recesses again filled in with insulating polymer.
The nickel layer may then be oxidized with chromic acid to produce an oxide layer. This avoids adhesion of the nickel deposit to the mandrel in the final plating step that follows. No nickel deposits in the areas around the insulating polymer.
This is where the hexagonal holes will be. The 0.1 mm thick screen of nickel can then be removed from the mandrel and rinsed. Very fine mesh screens cannot be produced because of the problem of bridging across the insulating polymer zones and filling-in of the holes as the nickel layer becomes thicker. Typical screens are 60 mesh (holes per linear inch) for blotches and 80 mesh for outlines.
Photo-patterns are produced similarly to those for flat screens but using a single full-scale positive film for each color. The color positives are prepared by the step-and-repeat process using multiple exposures from a negative of the original repeat.
Screens may be hand-coated with several layers of light-sensitive polymer or single coated with a special annular coating machine. The types of polymers used, such as melamine-formaldehyde condensates, are also different from those used for flat screens since good adhesion to the nickel surface is essential.
The polymer-coated screen is placed on an inflatable rubber tube and wrapped with the dispositive film, great care being taken to place it in exactly the correct position. The seam is usually helical to avoid sideways slippage.
The inflated rubber tube holds the screen onto the film and the whole rotates during exposure to ultraviolet light. Washing then removes unexposed soluble polymer from what will be the open areas
After drying, any pin holes are touched up with lacquer and the residual polymer is heat cured to completely harden it. Finally the end rings are carefully fitted.
Galvano screens from Zimmer have solid electrodeposited nickel layers in the filled zones in place of lacquer and are therefore much stronger and less susceptible to pin holes.
A thin inflatable nickel tube is coated with photosensitive polymer solution and dried. A full-size negative of the color separation is wrapped around this followed by a negative of the mesh pattern.
Inflation of the mandrel provides good contact between the layers during exposure to the ultraviolet light. After washing and drying, only the non-pattern areas and the areas corresponding to the supporting mesh in the pattern areas have no coating of hardened polymer.
The latter acts as an insulator when the mandrel is subsequently nickel plated. The nickel builds up in those areas where there is no hardened polymer.
The result is a thin nickel sheet with screen holes only in the areas of the design. Bridging across the tiny insulated zones is a problem with this type of screen and they do not have a mesh as fine as lacquered screens.
The limit is about 80 mesh, in contrast to 100 mesh for lacquered screens. Lacquered screens are preferred for finer work and narrow fabrics. The stronger galvano screens are better for wider fabrics and for the more mechanically rigorous printing of carpets where more robust, thicker screens (0.4 mm) are needed.
Salma Hasin Shila, the author of this site completed her BSc. in Textile Engineering (Wet Processing Engineering) from Bangladesh University of Textiles www.butex.edu.bd (BUTEX). She has a passion for textile technology and love to write about it. She wants to share her knowledge to help the students in their studies and businessman & entrepreneurs in their business in making wise decisions fast.