Four colour screen-

Four colour screen-process printing

What is Screen-printing?

screen-process printing, commonly known as silk-screen due to it's original use of silk based stencils, has become important in the production of a wide array of manufactured items, including decorative panels, printed circuit boards, touch sensitive switches, plastic containers and printed garments.

Presses for screen-printing range from simple manual devices for the small scale printing of T-shirts and banners, to large sheet-fed presses for multicolour commercial applications.
The process is distinguished by it's ability to print finely detailed images and thick deposits of rich coloured inks onto practically any surface, including paper, plastics, metals and even three dimensional surfaces.

It is also the only major printing process that is routinely used to produce images that are not meant to be viewed.
The circuit patterns in touch-sensitve switch panels, for example, are screen printed with special conductive inks.

How is it done?

The image for impression is carried on a stencil.
The stencils for commercial printing are usually produced by photo-mechanical means.

A fine synthetic fabric or, in certain cases, a metal mesh is stretched over a rectangular frame, and a photo-polymer coating is applied to the entire surface. Exposure of the photo-polymer through a film positive causes it to harden in the areas not intended to print.
The unexposed photo-polymer areas are then washed away to create the open areas of the stencil.

During the screen-printing process, the screen is pressed against the surface to be impressed, and ink is forced through the open areas of the stencil with a rubber squeegee, transferring the ink onto the substrate.

Four colour process is the method for printing full colour jobs (Four colour sets) with tonal values, such a photographs and paintings.
This process can simulate vrtually all colours by combining red (also called magenta), blue (cyan) yellow, and black, which provides greater detail and density as well as shades of grey and type.

The original artwork is scanned by a special machine and the colours are separated into the four process colours (CMYK).
Computer software is then used to convert each colour into a series of dots, outputting them onto clear films. They are then, if required, projected up onto larger films and are ready for use.

In the printing process, the four colours are laid down one by one on top of each other, as mentioned earlier. The process inks are transparent and blend with each other, producing an almost exact reproduction of the original artwork.

So what are the problems?

With each print run, there will always be wastage of both time and materials.
With each colour, time must be allowed for the sheets to be loaded into the sheet feeder, for the sheet feeder and press to be set up, and for colour balance and registration to be adjusted.

Many print processes other than screen-printing , have presses capable of printing more than one colour at a time.
The most common machines are four colour presses, consisting of four presses linked together, with one printing cyan, the next printing magenta, etc...
The result is a four colour job, printed in one pass, with minimal amount of setting up time.

Screen machines, by and large, still use the single colour format, making four colour printing an expensive and time consuming business.
Because of the relatively slow speeds of screen presses, the printing of posters may only be viable for shorter runs, with the higher volumes going almost exclusively to the litho industry.

In addition to this Litho/screen problem, there has been much debate over the appearance of non-impact digital presses, similar to ink jet printers, but designed for short run, large format posters, normally a profitable job for the screen printer.
Many say that these machines will never take over from screen printing, clinging to their old, slow and outdated presses, while others rush into digital revolution, throwing away large sums on early, and therefore almost certainly, out dated technology.

Few can argue against the fact that, just as computers have revolutionised the pre-press industry, the constant improvements in computer technology, combined with the downward spiralling in costs, make the digital printing of short run, large format posters very attractive to clients who may normally go screen.

The threat from digital printing is a very serious one that should not be underestimated.

If a truly competitive digital press comes onto the market place, without the screen industry in readiness, it could be snapped up by the powerful few, cornering the market and starving out the smaller businesses from the short run market.
These machines will inevitably become faster, leaving screen printing squashed between digital and litho, begging for the scraps.

And what is the answer?

If the digital threat is realized, one answer would be for screen-printing to break into the litho market.

This has already been attempted with the introduction of four colour screen presses, a forward move, but not though out correctly.
As stressed earlier, screen-printers still tend to use single colour machines, making four-colour printing a time consuming business. There have been attempts to produce four colour presses but they have one major drawback.

The majority of screen presses, including four colour presses, still use flat bases.

However, over the last two decades, cylinder presses have come onto the market. While a cylinder press cannot print on thick material, board for example, it is capable of speeds many times that of a flat bed press. The problem of thick stock is overcome by printing the job onto paper and then having the sheets mounted to the correct stock, using special finishing machines.

The manufacturers of these four colour flatbed presses are of the opinion that current cylinder technology makes a four colour cylinder press unviable.

There is a history of the screen industry sticking with what seems to work, regardless of technological advances.

In my humble opinion, this has been one of the many reasons for it's rather slow development over the last thirty years.