Computer-to-screen equipment is transforming the way in which screen printers work. Tony Palmer examines the different ways in which these labour-saving devices can benefit your print shop
The start of the process when printing any garment is the creation of a reliable stencil or screen to consistently pass the ink through. In the past this has been achieved by many different methods, but the basic process consists of coating a screen with light-sensitive emulsion and then creating a positive (‘shadow’) image on a sheet of film and then holding this film as close to the emulsion as possible while shining ultraviolet rays through it to expose (harden) the emulsion in the non-image areas.
The film creation part of the process has changed over time. The early days of cameras, darkrooms and endless hours carefully cutting Rubylith film while setting curved text using a pencil line from a coffee cup outline and rub-off Letraset letters are well behind us now, as is the use of highly technical thermo-driven in-house imagesetters or outsourcing to expensive reprographics houses – the introduction of inkjet printers and polyester-coated carrier film has allowed most of us the freedom now to create positive films using a normal inkjet printer.
But even the revolutionary move to inkjets has had some drawbacks: the film would stretch as the roll was used up, and the print heads would fail to work on a Monday morning no matter how many expletives were thrown in their direction. Moreover, the filing of used films was always a thing to behold in any print shop: after the press operator had glued the film to the platen and used it to mix three different colours on, and foreign bodies that were not of human origin had stuck themselves to the film forming a bond that was stronger than any man-made adhesive, only then would the film be filed… in the wrong job bag under the wrong customer name with the wrong date in the wrong drawer of on old filing cabinet that the sales department no longer used.
Take it from me, you’ll never win back that time spent trawling through all the films printed over the past decade, only to find five films from a six-colour job filed under ‘G’ for ‘ginger guy that comes in the shop’.
Once we have our precious positive film we need to position it onto the screen. This needs to be done with precision as it is imperative that all screens are made in at least a similar area to each other – crosses, tick marks, crosshairs and registration marks are all important in image placement.
This is a common failure point as consistency is passed to the guy who didn’t get home until 7.30am and looks like he needs a bucket of Red Bull just to function.
And so we enter the next generation of screen imaging: modern computer-to- screen (CTS) systems all use ‘electronic films’ in the form of data files that are automatically named, can be searched and are stored on a drive the size of a packet of tissues. (They still get stored under ‘D’ for ‘Dave’s-final-new-final-revised-approved-final.tif’ but we know exactly what we mean.)
CTS has made a significant impact on today’s garment decorator. The term broadly describes the process of imaging screens directly, removing the requirement for carrier film entirely.
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The Douthitt system is a wax printer system that delivers high accuracy and image resolution
There are three basic types of CTS, all using direct-to-screen imaging techniques:
Inkjet This process uses a high-quality inkjet print head to directly image onto an emulsion-coated screen using a UV-blocking water-based ink.
Wax printer This process uses a phase-change print head to take a solid block of ink (wax) and pass it through a print head.
Direct laser exposure This process uses a high-resolution laser to expose and image the screen in one process.
All these processes are more efficient than using a film carrier for your image as they remove at least one step from the imaging process.
A major advantage of all CTS systems is the elimination of the ‘human’ element in the positioning process. All units use a set of co-ordinates inside a pre-set page size to determine where the image is produced. This is invaluable when using a positioning system such as a pin system or tri-lock system to hold screens in the press. When you image the screen in the same position as you hold the screen in the press, you can virtually eliminate the need for registration.
Inkjet systems These have greatly improved and, with the use of extra opaque UV- blocking ink, can produce consistent results, although a small amount of dot gain can occur as the wet ink is fired onto the emulsion surface of the screen. They can typically image a full size T-shirt screen in under a minute. The combination of removing the handling process, no longer having to tape films onto a screen and the process of sorting and organising workflow greatly decrease the time required to image a screen. Some units even have a built-in exposure unit so the screen can be imaged and exposed in one operation.
Wax systems These use a solid block of UV-blocking ink and melt it down to a liquid that passes through the print head. This can give resolution up to and above 85lpi with virtually no dot gain as the ink solidifies on contact with the cold screen. These systems are very popular and have revolutionised many print shops with their accuracy and high image resolution.
The removal of the film process, along with no need for glass or vacuum, has eliminated the risk of contaminants such as dust and other particles getting onto the film. This greatly reduces, and in most cases eliminates, the ‘pin-hole’ phenomenon that has plagued all printers, whereby the shadows of small biscuit crumbs and dog hairs that were not part of the original design somehow make their way onto your lovingly prepared screens.
Laser exposure systems These are the emerging technology. This process is in widespread use in the graphics industry, and the units are now becoming affordable for the garment industry. The process is almost opposite to what we currently associate with making a screen. Traditionally we print or create a shadow (reversed) image and shine UV rays onto light-sensitive emulsion. Where light hits the emulsion it becomes water-resistant and where light is blocked (by the shadow image) the emulsion stays water-soluble.
Laser systems use only UV light in the shape of a targeted wavelength laser that can shine UV light in only the places where we need it and turn it off in the places that do not need exposing, all in super-high speed and giving high-resolution images. The major advantages of this system are the complete lack of consumables –no ink, no wax, no film–and the fact it doesn’t require a separate exposure process. With the laser head’s long (10,000 hours) lifespan, these systems also promise an excellent return on investment, especially if you image two screens at once.
All systems have small advantages and disadvantages. For example, each emulsion has a specific wavelength that must be compatible with the exposure system you are using, and the surface of the emulsion must also be receptive to the inkjet brand’s ink to avoid any spread of images. Long periods of inactivity can also risk the usability of some print heads.
Nevertheless, the introduction of CTS has revolutionised screen-making departments. The repeatability of images without the distortion associated with film, and the computer-placed positioning of images using a clamping system along with the removal of any contaminants on a carrier film, have all greatly improved the consistency of one of the most important parts of the garment decoration process.
The throughput of a print shop’s stencil department is critical to the business’s commercial success – if it is too slow and can’t keep up with the presses it can easily lead to a bottleneck. The predictable and consistent imaging of stencils is an important part of this process. A system that can eliminate downtime due to faster set-up times can make any investment worthwhile in itself. We only make money when the squeegees are moving, so the less time spent setting up a job and the more time spent printing is always a positive move.
Tony Palmer has been in the garment decoration industry for over 30 years and is now an independent print consultant working closely with print shops to get the most from existing processes and techniques. Tony is passionate about keeping and enhancing production skill levels within in the industry. He is the owner and consultant at Palmprint Consultants, offering practical help and assistance to garment decorators all over the globe.