Flatbed Screen Printing Method in Textile Industry

Screen production for printing is the culminating step in a process involving many important aesthetic and technical decisions. These include:

1. The selection of the fabric and the printed design.
2. The choice of the size and arrangement of the design’s repeat rectangle.
3. The necessary color separation of the design.

These steps are common to all printing methods. Color separation involves reproduction of the pattern for each color or separate clear films (diapositive). Mush of the design work is now computerized. The designer must ensure printing of colors with a light overlap to avoid white gaps between them. In some cases, printing of one color on top of another produces a particular color effect. This is known as a fall-on.

The same technique of screen production is used for both flat and rotary screens. The flat screen, mounted on its frame, is located with a solution of a photo-sensitive polymer that will crosslink and harden on exposure to ultraviolet light. It will then be insoluble. A typical polymer is polyvinyl alcohol, its crosslinking sensitized by ammonium dichromate. The coated screen is dried and then exposed beneath the diapositive for the given color.

All the operations take place in a darkroom. The inked zones, corresponding to the particular color pattern, do not transmit the ultraviolet light. The layer of polymer on the screen beneath the pattern is thus not exposed and does not crosslink while the rest hardens. The non-exposed polymer remains soluble and can be washed out, leaving the screen open in those areas. The print paste will pass through the screen where the unexposed polymer has hardened and become insoluble.

Any small pin holes in the hardened areas on the screen are painted over. A coasting of cellulose or polyurethane lacquer reinforces the screen, the excess being sucked out of the open areas. In some cases, thermal or rediative curing of the polymer makes it harder and more resistant.

A flat screen may be produced in a single coating and exposure operation. Alternatively, in the step and repeat process, one repeat design is produced per exposure, the remainder of the screen being protected by dark paper. The pattern or the screen is then moved by one repeat distance and the process repeated.

The screen mesh varies from 60-150 holes per linear inch depending upon the type of pattern and the fabric. There is always a tendency for the edges of printed areas to have a serrated appearance. This comes from the arrangement of the spaces between the thread along the edge of an open area of the screen. The spreading of the paste below the screen threads reduces this, aided by using fine screens and lower viscosity pastes.