Printing Secrets from Print Acumen

Silkscreen Printing

The most versatile of all printing processes

Applications

Silkscreen printing is arguably the most versatile of all printing processes. This process can be used to print on a wide variety of stocks including –

• Paper
  
• Cardboard
  
• Plastics
  
• Glass
  
• Metals
  
• Fabrics including nylon & cotton

Common products produced by the silkscreen printing process include –

• Posters
• Labels
  
• Signage
  
• Textiles and electronic circuit boards

The advantage of silkscreen printing over other print processes is that the press can print on stocks of any shape, thickness and size.

A significant characteristic of silkscreen printing is that a greater thickness of the ink can be applied to the stock than is possible with other printing techniques. This allows for some very interesting effects that are not possible using other printing methods.

Because of the simplicity of the application process, a wider range of inks and dyes are available for use in silkscreen printing than for use in any other printing process.
Utilization of silkscreen printing presses has begun to increase because production processes have become more efficient.

This has been a result of the development of the automated and rotary silkscreen printing press, improved dryers, and U.V. curable ink. The major chemicals used include screen emulsions, inks, and solvents, surfactants, caustics and oxidizers used in screen reclamation. The inks used vary dramatically in their formulations.

Silkscreen printing consists of three elements –

• The screen which carries the image to be printed
  
• The squeegee
  
• The ink

The silkscreen printing process uses a porous mesh stretched tightly over a frame made of wood or metal. Proper tension is essential for accurate colour registration. The mesh is made of porous fabric or stainless steel mesh. A stencil is produced on the screen either manually or photochemically.

The stencil defines the image to be printed in other printing technologies this would be referred to as the image plate.

Silkscreen printing ink is applied to the stock by placing the screen over the material. Ink with a paint-like consistency is placed onto the top of the screen and 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 stock. The diameter of the threads and the thread count of the mesh will determine how much ink is deposited onto the stocks.

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, is prone to wear and edge nicks and has a tendency to warp and distort. While blades continue to be made from rubbers such as neoprene, most are now made from polyurethane which can produce as many as 25,000 impressions without significant degradation of the image.

If the item was 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 utilised.
The rate of silkscreen printing production was once dictated by the drying rate of the screen print inks. Do to improvements and innovations the production rate has greatly increased. Some specific innovations which affected the production rate and has also increased screen press popularity include -

• Development of automatic presses versus hand operated presses which have comparatively slow production times.

• Improved drying systems which significantly improves 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 more recent technology developments.

Screen Preparation

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.

Silkscreen printing Presses

There are three types of silkscreen printing presses –

• The flat-bed (probably the most widely used)
  
• The cylinder
  
• The rotary.

Until relatively recently all silkscreen printing presses were manually operated. Now, however, most commercial and industrial silkscreen printing is done on single and multicolour automated presses.

Screen Reclamation (post-press)

Why reclaim screens?

Polyester fabric costs $10 to $40 per square metre.
Failure to reclaim screens and ruined screens cost on average $5,000-$10,000 per year. The average monthly fabric cost $360. One study showed chemical reclamation cost between $2 and $10 per average screen, while screen disposal cost just shy of $50.

The process of reclaiming screens generates solvent waste and waste water. Solvent waste generated from screen cleaning and waste water is generated through the process of emulsion removal. The waste water will contain particulates comprised of ink pigment, emulsion and emulsion remover (periodate).

Silkscreen printing Inks

Silkscreen printing inks are moderately viscous inks which exhibit different properties when compared to other printing inks such as offset, gravure and flexographic inks though they have similar basic compositions (pigments, solvent carrier, toners, and emulsifiers). There are five different types of screen ink to include solvent, water, and solvent plastisol, water plastisol, and UV curable.

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