The screen printing is the most commonly used printing method at industrial scale. In this printing technique a woven mesh is used to support an ink-blocking stencil. The attached stencil forms open areas of mesh that transfer ink or other printable materials which can be pressed through the mesh as a sharp-edged image onto a substrate. A roller or squeegee is moved across the screen stencil, forcing or pumping ink past the threads of the woven mesh in the open areas. This type of printing has increased enormously in its use in recent years because of its versatility and the development of rotary screen printing machines which are capable of very high rates of production.
There are two main types of screen printing: flat-bed screen printing and rotary screen printing. Flat-bed screen printing can be manual or automatic. Rotary screen printing is usually automatic and gives the highest printing productivity. Screen printing involves passing the print paste onto a fabric through a mesh or screen which has some open and some blocked areas according to the desired print pattern. The print design obtained on the fabric depends on the pattern of the open areas of the screen. In this article I will discuss about rotary screen printing.
Rotary Screen Printing:
The rotary screen is a screen in a cylindrical form. The color is applied from inside while the rotary screen is revolving. Rotary screen printing uses cylindrical screens as opposed to flat screens. Again, a separate screen is required for each color of the design being printed. More complex designs require the application of many different colors, and typical rotary screen printing machines have the capacity for up to 20 screens. The screens rotate in contact with the substrate and the print paste is fed from inside the screens. The paste is forced from out of the inside of the screen by means of a metal squeegee blade. Again the fabric is adhered to a continuous washable rubber blanket, although in rotary screen printing the fabric and substrate run continually through the machine as opposed to a start stop motion in flat screen printing. The printed fabric is again taken off the end of the machine and dried whilst the rubber blanket is washed and re‐gummed. Rotary screen printing is widely used for a variety of textile applications including printed apparel and printed interior fabrics. Currently, rotary screen printing dominates the textile printing industry. Introduced in the 1960s, it now accounts for about 65% of printed textiles.
In rotary-screen printing, continuous rotation of a cylindrical screen while in contact with the fabric ensures genuinely continuous printing. Print paste is fed into the inside of the screen, and during printing is forced out through the design areas with the aid of a stationary squeegee. A diagram of a rotary screen with squeegee blade is shown in Figure 1. A magnetic roller can also be used.
Figure 2 shows a typical rotary screen printing machine in operation. The design being printed is not especially complex, so not all screen locations are required.
In basic operation, rotary screen and flat screen-printing machines are very similar. Both use the same type of in-feed device, glue trough, rotating blanket (print table), dryer, and fixation equipment. The process involves initially feeding fabric onto the rubber blanket. As the fabric travels under the rotary screens, the screens turn with the fabric. Print paste is continuously fed to the interior of the screen through a color bar or pipe. As the screen rotates, the squeegee device pushes print paste through the design areas of the screen onto the fabric. As in flat-bed screen printing, only one color can be printed by each screen. After print application, the process is the same as flat screen printing.
By converting the screen-printing process from semi-continuous to continuous, higher production speeds are obtained. Typical speeds are from 50-120 ypm (45-100 mpm) for rotary screen printing depending upon design complexity and fabric construction. Initially, no continuous patterns such as stripes were available with this method due to the seams in the rotary screens. However, with the development of seamless screens, continuous patterns such as linear stripes or plaids became possible. Rotary screen machines are more compact than flat screen machines for the same number of colors in the pattern. Therefore, they use less plant floor space.
Also with rotary screens, the size of the design repeat is dependent upon the circumference of the screens. This was initially seen as a disadvantage, because the first rotary screens were small in diameter. However, with today’s equipment, screens are available in a range of sizes and are no longer considered design limited. The fact is that today’s rotary screen machines are highly productive, allow for the quick changeover of patterns, have few design limitations, and can be used for both continuous and discontinuous patterns. Estimates indicate that this technique controls approximately 65% of the printed fabric market worldwide. The principle disadvantage of rotary screen printing is the high fixed cost of the equipment. The machines are generally not profitable for short yardages of widely varying patterns, because of the clean-up and machine down time when changing patterns. Flat screen printing is much more suitable for high pile fabrics, because only one squeegee pass is available with rotary screen. However, rotary machines are used for carpet and other types of pile fabrics.
Advantages and Disadvantages of Rotary Screen Printing:
There are three main advantages possessed by rotary printing over other flat screens: first, its high productivity, 10,000–12,000 yd being commonly printed in one shift of 8 hr; second, by its capacity of being applied to the reproduction of every style of design; and third, the wonderful exactitude with which each portion of an elaborate multicolor pattern can be fitted into its proper place without faulty joints at its points of repetition.
The main disadvantage is that it is not economical if the orders are small and we need to change the design very frequently. It is very difficult to get orders in same design in lakhs of metres.
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Founder & Editor of Textile Learner. He is a Textile Consultant, Blogger & Entrepreneur. He is working as a textile consultant in several local and international companies. He is also a contributor of Wikipedia.