Mark Evans of Exile Technologies discusses the rise of computer-to-screen and summarises the different systems on the market

Although some garment decorators may be still somewhat in the dark about computer-to-screen (CTS, aka direct-to-screen or DTS), the technology has in fact been around for many years.

Lüscher’s large format direct inkjet stencil systems were introduced in the late 1990s, and at the Fespa Expo in Munich in 2005 the number of CTS systems being shown by manufacturers and suppliers was nearly into double figures.

What is surprising, then, is how long it has taken for this technology to actually reach tipping point and become a commercially viable and well accepted method for screen stencil production.

Part of the problem has been affordability. The early systems were very expensive and also aimed at the large format graphics market. The cheapest inkjet base technology shown at Fespa Munich back in 2005 was in excess of £60,000 – and the direct exposure based systems were all priced north of £100,000.

In the last 18 months, however, there has been a significant upsurging CTS installations within the UK textile screen printing industry. Since Exile’s Spyder II system was launched early last year there have been 15 Spyder systems installed in the UK with an additional three or four systems due for installation by the end of 2016.

So let’s take a look at the CTS technologies that are available…

CTS inkjet imaging systems

  • Water- or liquid-based inkjet, such as the M&R i-Image and Exile’s FreeStyler
    Water-based inkjet systems use a conventional inkjet process to print a stencil image directly on a normal emulsion coated screen using a high density, water-based, black ink with UV blocking capabilities.
  • Wax- or resin-based solid inkjet, such as the Exile (and MHM) Spyder II Direct to Screen
    Wax-based inkjet has a specially modified inkjet system that uses heat to ‘melt’ a solid wax- or resin-based, UV-blocking ink during the imaging process.

All CTS inkjet systems print onto normal screen emulsions, replacing the film positive with the printed ink stencil. The image is digitally positioned and printed directly onto the screen. The screen is then exposed in the normal way, using UV light, although the absence of film means that the exposure process no longer requires a vacuum.

The main difference between these two types of ink jet process is that water-based inkjet systems will get varying results depending on the emulsion being used as emulsions naturally absorb water. A water-based ink could be susceptible to this problem, thereby reducing the UV-blocking capabilities of the ink.

At its worst this problem will show up as a cross contamination between the ink and the emulsion, whereby the black ink dot gets ‘trapped’ inside the emulsion prior to exposure. It is important, therefore, to make sure that water-based inkjet systems are compatible with the emulsions you want to use.

With the wax/resin-based inkjet systems, the ink returns to its natural solid state as soon as it is printed, resulting in a very clean dot with minimal dot gain and no ‘overspray’. This solid wax ink will work with all emulsions as it dries immediately on the surface of the screen and is not absorbed in any way by the emulsion.

Direct emulsion exposure systems

In direct emulsion exposure systems (DMD), such as those from CST Systems and Signtronic, the emulsion is exposed using laser or LEDfocused lights to directly image the screen without the need of a UV-blocking mask or stencil.

These systems produce high resolution images (up to 2540 dpi) and are commonly used in industrial screen print applications. The advantage of this process is that they use no consumables other than the screen emulsion, although the process does require a fast exposure emulsion to work properly. These DMD systems are considerably more expensive than most inkjet CTS technologies and therefore are less likely to appeal to most garment decorators.

Direct thermal imaging

Direct thermal imaging systems, such as the Riso Goccopro, use a special silicon-coated screen mesh. The stencil is then imaged by a thermal print head, which melts the silicon coating and opens up the mesh to allow the ink to pass through. This eliminates the need for a darkroom as well as the exposure and washout processes, and makes the need for screen reclaim completely redundant. It is a simple, fast and clean process.

It does, however, mean that screens cannot be re-used – instead the mesh needs to be re-stretched for each print job.

Direct thermal imaging is sometimes called ‘digital screen making’ as it removes some of the ‘analogue’ screen making processes, namely screen exposure and washout. But it would be more accurate to describe all CTS technology as a ‘digital’ process as the image is digitally transposed directly onto the screen and image positioning is digitally performed by computer technology.

CTS versus film

All CTS methods offer significant advantages compared with using film positives.

Labour and consumable cost-savings are often cited as the main reasons for making the jump to CTS. The image is digitally placed on the screen and multicolour job registration from screen to screen is performed automatically. The ink consumables used by the inkjet systems are generally much cheaper than the cost of polyester film, and direct exposure systems eliminate the consumable element completely. Only the direct thermal imaging system is more expensive than using conventional film, but this approach has other savings as screen reclaim is no longer required.

Cost savings aside, the biggest reason to make the change to CTS is improved print quality. No matter how good a film positive, it will never produce the highlight dot quality that can be achieved by a good CTS stencil. What few screen printers appreciate is how much fine detail is being lost during the exposure process when using a film positive. Under-cutting, whereby the UV light ‘creeps’ around small highlight dots, thereby exposing the emulsion underneath, is virtually eliminated with a good CTS stencil.

Do I need it?

For small print shops and hobbyists, CTS is probably overkill. Textile screen printers have been using film positives and other more basic stencil solutions for years, and for many smaller customers CTS may not be worth the investment outlay.

However, for medium size and larger screen shops, the convenience of CTS and the way in which it simplifies the workflow will be of great importance. These labour and time savings costs combined with cheaper screen production consumables can add up to considerable savings. Also, for screen printers doing complicated halftone work, the print quality improvements are immediate and difficult to ignore.

The M&R i-Image and Exile/MHM Spyder II inkjet systems require a minimum screen production of 40-50 screens per day to be able to make a sensible return on investment. These systems will be the ‘heavy lifters’ for larger customers with two or more fully automatic presses for whom productivity and production cost per screen are the key issues.

The more affordable Goccopro and FreeStyler systems are aimed at screen printers who may only be making 10, 20 or 30 screens per day.

Implementing a computer to screen system is a big decision, but one that has significant cost and quality benefits. The combination of fast high end systems such as the i-Image and Spyder II for the bigger textile screen shops combined with the introduction of more affordable alternatives such as the Goccopro and FreeStyler means that CTS has finally arrived in the mainstream and is now worthy of serious consideration.

www.exiletech.co.uk