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Automated Screen Production : Embracing new production methods
DLP technology contains a rectangular array of up to two million hinge-mounted microscopic mirrors
Recently I have been involved extensively in digital printing technology and I am concerned with what I see. Potential users do not understand their options and consequently can make uneducated buying decisions. Once installed machines become prey to a ‘push-button’ culture where the electronics will ‘sort it out’.
Substrate management, regular maintenance, RIP operation, colour control and other issues that are taken for granted, create problems for all involved.
Regular basic maintenance is being missed on the grounds of ‘we don’t have time’. Calibration for different substrates is eschewed in favour of an ‘it will do’ culture. The signs of decay that pervaded some less than professional screen printing shops in the past (and unfortunately in some cases the present) are now creeping into digital printing.
Historically the biggest problem with the successful use of screen printing has been a lack of understanding of key issues by users. Acceptance of poor quality, high levels of rejects and corrosive down-time has been the norm, particularly when screen printing companies have been faced with unreasonable delivery demands from their clients. This, in turn, has created unnecessary problems for suppliers to the industry. Suppliers have had to devote considerable resources resolving problems caused by this lack of understanding, not just with the printers but also within their own ranks.
Printing is a manufacturing business
Printing is a manufacturing business where understanding and controlling costs are crucial to success. So often the estimate prior to quotation bears little relationship to manufacturing costs. Once the order gets to the production floor it becomes a disorder!
The adoption of digital printing by screen printers has given them an insight into what is possible when it comes to process measurement and control: that is digital workflow. Thanks to the relatively slow pace of digital printing and scary ink costs, justifying its use has to be quantified. Firstly, with return-on-investment calculations, then continuing and refining the business model once digital print is in production.
There is no reason why screen printers should not use the same techniques. Their Achilles Heel is often stencil production particularly generating the image on the stencil. Producing, using and storing photopositives is an increasing problem. The total costs of using them can be unreasonable, particularly in large format or applications where dozens of stencils are produced in a day. It is even worse when printing the same image on different substrates because the printer will need to apply different profiles to colour separations to deal with dot gain variations. A cost of Ł1,200 is not unusual for a set of photopositives so, very quickly, annual costs of Ł100,000.00 have to be swallowed.
There is an alternative: computer-to-screen. Openings to the technique are increasing as the technology used in wide format flatbed digital printers is incorporated into imaging stencils. Options come in several forms. Initially, a screen coated with emulsion had an image printed onto it with either ink or wax using a piezo digital printing head. Luscher is the best-known manufacturer of this equipment and has sold hundreds of units worldwide. The Luscher JetScreen uses wax that it fires through piezo heads. It can produce dots down to 60µm. These techniques have proved successful, particularly in large format printing. This technique immediately removed the need for expensive photopositives and meant there was no undercutting when the imaged screen was exposed. With traditional exposure, this often occurred if the emulsion on the photopositive and photosensitive emulsion of the coated screen was not in intimate contact due to poor vacuum or a dirty blanket. Exposure of a direct imaged screen is also faster as there is no need for exposure unit glass between the UV source and emulsion.
Last minute images changes
Even in the best organised facilities errors or last minute images changes must be accommodated. Image can quickly be adjusted in the computer-to-screen system without the time and costs of remaking the photopositives. Suitable for many applications, the savings in the production of photopositives gave a fast return on capital.
When creating images directly on coated screens, piezo head resolution meant line rulings above 85 lines per inch were, to say the least, ambitious. Resolution limitations of the printed image on the coated screen meant fine applications were beyond its scope. In large format PoS applications, printers preferred to work around 50 lines per inch. However, it was possible to have a wide tonal range from two to 98 per cent. This is quite acceptable at viewing distances of 2m. For images that were viewed at arms length, or closer, it was not acceptable.
Why some clients insist on high resolution images on bus sides is beyond me but if that is what they want, that is what they must have. People do not understand that lower resolution prints have a thicker ink film which, in turn, gives a more vibrant print. What is needed on a large format poster is impact not nose hairs. The inability to exceed 85 lines per inch satisfactorily ruled out the creation of high resolution images (100/120 or even 150 lines per inch) used, for example, in printing CDs or high quality ceramics. In spite of this, printing direct onto the coated mesh with wax or ink has and is proving an efficient, cost effective method of creating the image digitally.
Direct exposure using focussed ultra violet light was a further improvement and is likely to take over from digital printing onto a coated mesh where its slower speed and higher capital cost can be tolerated. This technology is accelerating in screen printing with equipment from Kiwo, Signtronic, CST and soon Luscher whose thermal wax based direct-to-screen system has held sway in large format stencil applications for several years.
The co-operation between Signtronic and Proditec is particularly interesting. Proditec has the long established Proka range of direct projection cameras. The Signtonic system will sit nicely in the Proditec automation system. The Signtronic focussed UV overcomes the need to use white mesh and special fast acting emulsions. Also 60l/cm 150lpi line rulings are attainable. The Signtronic system will not yet reach direct projection camera production speeds of 18 large format frames per hour but 28m2/hr still gives a reasonable throughput for high resolution stencils.
Digital light processing chip
The heart of the direct exposure systems is a Digital Micromirror Device or digital light processing chip. This is a remarkable component developed by Texas Instruments. DLP technology is probably the world's most sophisticated light switch. It contains a rectangular array of up to 2 million hinge-mounted microscopic mirrors. Each of these micro-mirrors measures less than one-fifth the width of a human hair. Focussing ultra violet light is one of many uses for this technology. System reliability is proven since its invention in 1987.
For very fine exposures, systems that use lasers to expose coated screens are now becoming available. Sefar has brought out the Laser Direct exposure System (LDS) for high quality stencils. The system is targeted at printers of optical discs and similar small-format applications such as labels and solid objects. It can be used with Sefar pre-coated mesh to further reduce potential stencil production anomalies. Another interesting factor is that Sefar is working with Signtronic and selling its JetScreen large format systems to augment the LDS system.
These examples of co-operation between major suppliers demonstrate the way forward. Companies like Sefar have a clear picture of what is happening to the industry through their mesh sales. Who would have imagined 10-years ago that a mesh manufacturer of its standing would be selling digital equipment, both CTS and digital printing onto containers. Times are changing fast.
Embracing digital technology in all aspects of screen printing is essential. Ranging from managed workflow, computer-to-screen and digital instrumentation for measuring the process. In the digital workplace the same system can deliver data to CTS, flatbed digital, roll-fed digital and provide management data to verify costings. What is the point of running a business if you don’t know where, or even if, you are making a profit?
Fingerprinting their presses
Remarkably many screen printers who print four colour process may own but do not use a densitometer. Why bother, they haven’t finger-printed their presses. They use the same separations on paper as they do on PVC. The same separations on a flatbed press as on a cylinder press. Isn’t it about time they stepped into the real world?
No wonder print buyers don’t want screen printing, they have probably never seen a decent example. In a conversation with an engineer who uses graphics screen printing as a production method it was described to me as process from a land that time had forgot. When it was explained to him the real capabilities of the process his enthusiasm was rekindled. Let it be the same for the screen printing industry.
