Screen-printing is the most flexible printing process. It can be used to print on a wide variety of substrates, including paper, paperboard, plastics, glass, metals, fabrics, and many other materials including paper, plastics, glass, metals, nylon and cotton. Some common products from the screen-printing industry include posters, labels, decals, signage, and all types of textiles and electronic circuit boards. The advantage of screen-printing over other print processes is that the press can print on substrates of any shape, thickness and size.
An important characteristic of screen-printing is that a greater thickness of ink can be applied to the substrate as compared to the other printing techniques. This allows for various interesting effects that cannot be achieved through the other printing methods. Because of the simplicity of the application process, a wider range of inks and dyes are available for use in screen-printing than for use in any other printing process.
Utilization of screen printing presses has begun to increase because production rates have improved. This has been a result of the development of the automated and rotary screen printing press, improved dryers, and U.V. curable ink. The major chemicals used in screen-printing include screen emulsions, inks, and solvents, surfactants, caustics and oxidizers used in screen reclamation.
Overview of the Screen Printing Process
Screen-printing consists of three elements
- The screen which is the image carrier
- The squeegee
The screen printing process uses a porous mesh stretched tightly over a frame made of wood or metal. Proper tension is essential for accurate color registration. The mesh is made of a porous fabric or stainless steel. A stencil is produced on the screen either manually or photo chemically. The stencil defines the image to be printed. In other printing technologies this is referred to as the image plate.
Screen printing ink is applied to the substrate by placing the screen over the material. Ink with a paint-like consistency is placed onto the top of the screen. Ink is then forced through the fine mesh openings using a squeegee that is drawn across the screen, applying pressure thereby forcing the ink through the open areas of the screen. Ink will pass through only in areas where no stencil is applied, thus forming an image on the printing substrate. The diameter of the threads and the thread count of the mesh will determine how much ink is deposited onto the substrates.
Many factors such as composition, size and form, angle, pressure, and speed of the blade (squeegee) determine the quality of the impression made by the squeegee. At one time most blades were made from rubber, which, however, had a tendency to warp and distort. While blades continue to be made from rubber such as neoprene, most of them are now made from polyurethane, which can produce as many as 25,000 impressions without significant degradation of the image.
If the item is to be printed on a manual or automatic screen press the printed product will be placed on a conveyor belt, which carries the item into the drying oven or through the UV curing system. Rotary screen presses feed the material through the drying or curing system automatically. Air-drying of certain inks, though rare in the industry, is still sometimes utilized.
The rate of screen-printing production was once dictated by the drying rate of the screen print inks. As a result of improvements and innovations in the printing technology, the production rate has greatly increased. Some specific innovations, which affected the production rate and have also increased screen press popularity include:
- Development of automatic presses versus hand operated presses, which have comparatively slow production time.
- Improved drying systems, which significantly improve production rate.
- Development and improvement of U.V. curable ink technologies
- Development of the rotary screen press, which allows continuous operation of the press. This is one of the recent technology developments.
Screen (or image transfer) preparation includes a number of steps. First the customer provides the screen printer with objects, photographs, text, ideas, or concepts of what they wish to have printed. The printer must then transfer a "picture" of the artwork (also called "copy") to be printed into an "image" (a picture on film) which can then be processed and eventually used to prepare the screen stencil.
Once the artwork is transferred to a positive image that will be chemically processed onto the screen fabric (applying the emulsion or stencil) and eventually mounted onto a screen frame that is then attached to the printing press and production begins.