Computer-to-Plate: Violet, Thermal or Processless?

(NPES International E-commerce Website, Sep 03 2008) Direct transfer from computer to offset plate has transformed the whole of prepress right up to the press since Drupa 1995. The elimination of film processing, manual sheet assembly and plate setting in print down frames brought prepress departments and printers such great savings that the purchase of a digital platesetter and the doubling of the price of high sensitivity offset presses could be quickly recouped.

The development of high sensitivity offset plates and the corresponding setters has resulted in today's choice of several alternative technologies. At Drupa 2008, these will probably be lined up alongside each other and the statements of their various suppliers may well create some confusion. In the 4- and 8-page format it is thermal technology that dominates the market but it is being pressurized by violet laser technology. Both technologies have seen the appearance of processless plates, whilst new suppliers are seeking to image plates directly with inkjet. In addition, new, high powered violet laser diodes now offer the possibility of setting the conventional UV sensitive plates that have been around for nearly 50 years.

Which technology is the best? Each technology has different properties and pros and cons. The plate format, the run length and the volume of plates required each day are the key criteria and it is only by working from these that one can determine, the most suitable setter. This is because whilst the setter constitutes the biggest investment on entering CtP, the sum of the plate costs over the amortization period for the setter wilt generally be several times that of the machine itself.
Therefore, when entering CtP or acquiring a new setter, it is the plate technology and its associated costs that should be the deciding factor.

Violet laser technology:
Out of the dark room
and into yellow light
In the 90s imposition programs appeared that allowed assembled pages to be exposed on film that was the same size as the plate. For this step, known as Computer-to-Film, 70 x 100 cm filmsetters were developed. Most of these were internal drum setters, because this offered better register of the four colour separations than capstan setters did. For the step to the direct setting of offset plates, most suppliers therefore also opted for internal drum setters.
There were, however, no lasers powerful enough to expose a conventional, UV sensitive plate in a digital, internal drum setter, and therefore the plate manufacturers had first to develop highly sensitive plates. The first generation of such plates included silver halide plates, which required as little as 0.010mj /cm2 of energy. This allowed a low energy laser to image the plate and the reaction it triggered was terminated by the developer, which made developer consistency important. Alternatively, this first generation also included photopolymer plates, but these were only a tenth as sensitive and required 0.10 mj/cm2. The weak imaging source only delivered a maximum of 10% of the required energy to the plate, with the remaining 90% coming from an additional pre-heat process. Here too, great demands were placed on the consistency of the pre-heating.
Depending on the plate type an argon ion, ND:YAG or He-Ne laser was used as a light source. All of these emit in the visible spectrum and therefore required plates that were processed with red lighting alone or in complete darkness. As a result, platesetters needed to be built that were totally light proof from plate feed to finished processing, which was very expensive, or the entire operation needed to be carried out in a darkroom.

In the years up to Drupa 2000 it appeared as if the thermal technology being developed around that time would completely supplant the first generation of light sensitive plates. However, at Drupa 2000 Agfa solved the problem of the internal drum setter with silver plates that were sensitive in the UV range at 405 nm, as had previously been the case with repro film, and this allowed them to be processed under bright yellow safety lighting. A single 0.5 mW violet laser diode was used as a light source. These were highly economical as a result of their use in mass market CD players and they offered a very long lifespan. This brought the manufacturers of internal drum setters back into the game. In 2002 Fujifilm released a photopolymer plate for the 30 mW violet diodes that were by then available, and some market observers forecast that these would completely supersede thermal technology. So far, however, this has not happened because these plates behave in the same, highly sensitive way as their predecessors of the first generation and the high demands placed on the consistency of chemical developing and pre-heating remain.

That being said, the manufacture of internal drum setters is considerably simpler than that of external drum setters and so, in the 2- and 4-page plate format, violet platesetters are clearly more attractive. In the 4- and 8-page format violet plates struggle against the widespread thermal plates and in the even larger VLF formats there are no violet plates but only thermal ones.

Thermal technology:
Market leader thanks to
non-linear behaviour
In 1991 the Canadian development company Creo Products was commissioned by Dainippon Screen to develop a platesetter for less sensitive offset plates. It opted for the external drum principle, because this allowed any number of laser beams separated by up to a few millimeters to be directed at the plate. The largest printing group in America showed an interest in this development, the first fruit of which was the Creo 3244 platesetter. This used a FD:YAG solid state laser whose beam was split into 480 sub-beams that could be modulated individually to set a half centimeter wide band with every rotation of the drum at a resolution of 2400 dpi. In the 81.3 x 111.8 cm eight-page format this equated to an output of as plates per hour and it imaged an industry standard photopolymer plate using green light at 532 nm in the visible spectrum.

Creo thus succeeded in demonstrating that the external drum setter was a fundamentally better design for a platesetter, in having to be lightproof, it did, however, have the fault of being very large and expensive. Therefore, together with Kodak, Creo developed a second setting head that could image the same photopolymer plates but with laser diodes in the infrared range at 830 nm and with 240 individual beams. During exposure these heated the plates to around 110℃ at the printing points and then, following exposure, the plates were not chemically developed but passed through an oven and heated to around 140℃. This pre-heat process resulted in the polymers interlinking at the exposed points and forming an extremely stable resin. Finally, the plates were stripped. Runs of 200,000 could be guaranteed and, with baking, in excess of one million.

Known today as Kodak DITP Gold, this negative working plate allowed the creation of a semi-automatic setter with manual plate feed and delivery, which was shown for the first time in the autumn of 1995 as the.
Another unique feature of the thermal process was that with the interlinking of the polymers a threshold was reached beyond which exposure or processing fluctuations had very little effect; the consequence of which was a hitherto unknown degree of processing stability. This advantage of thermal technology gave thermal setters an surprising fillip. All plate manufacturers now added thermal plates to their arsenal and new suppliers of external drum setters entered the market in the years up to 2000. Nearly all, however, were positive rather than negative plates, and these worked completely differently. Here the non-printing areas of the image were exposed by the laser beam so that during the stripping the coating was removed from these areas and the hydrophilic aluminium was exposed. The unexposed, ink friendly parts of the plate carried the image.

These plates too could be baked to achieve very tong runs and before stripping required no pre-heating in an oven. However, in order for it to be possible to set these plates at an adequate speed they had to be stored in heated rooms for several weeks after coating in order to absorb sufficient energy and then had to be transported to the customer under climate-controlled conditions. The replacement of the pre-heat process by these difficult production and storage conditions meant that positive thermal plates, like violet plates, required careful monitoring to control the uncertainties, and this is why the Kodak negative plate is still seen as the most reliable plate for long run, web offset printing.

How will we loose the process? In 1991 Heidelberg launched an offset press in which each printing unit imaged the plate digitally. The creator of the technology was Presstek, which had developed plates that could be imaged in daylight by means of ablation with thermal laser diodes, both on the press and in stand alone platesetters. Up until 2000 nearly all the press manufacturers sought to implement this principle but it failed to prevail against the CtP wave.
En route to non-ablative plates, Agfa Graphics developed its Thermofuse technology, in which a latex coating was cured with thermal laser diodes. Today, Agfa offers two plates: Azura and Amigo. For runs of up to l00000 the Azura is cleaned with gum arabic and can then be directly print ed. This is primarily used in jobbing printing but for longer web offset runs the Amigo plate is used. This is cleaned with a cleaning agent in order to prevent scumming over long runs, and it can then also be baked.

In 2006 Kodak and Fujifilm each launched a thermal processless plate, which is cleaned on the press as the damping unit is running up. Here, the unexposed coating is removed with the ink on the initial start up sheets. After exposure, both plates display almost no image contrast, which is something their users, who are mostly small jobbing printers, criticize. Neither firm is prepared to give the trade press figures for the distribution of these plates, which does not suggest that they have been a success.
Agfa Graphics has announced a processless plate for Drupa 2008 that will be the first to be set with violet laser diodes and which, according to Agfa, is intended for the newspaper sector.

Back to conventional UV plates
In 2000 the German firm Basysprint launched a p[atesetter that could image conventional UV sensitive offset plates of the kind that had previously been exposed in print down frames with UV lamps and assembled films. It used a patented micro mirror system from Texas Instruments that exposed the plates with UV lamps in step and repeat mode.
Productivity at first was tow but this was accepted by many offset printers because by continuing to image conventional UV plates it meant that the move to CtP did not entail a switch to using new, highly sensitive plates on the press - and all the more so as these plates were considerably cheaper. With over 800 machines in use around the world, it is clear that they work.

The development of higher and higher output violet laser diodes has now resulted in the first manufacturer of thermal platesetters now also offering its setters with 405 nm violet laser diodes for conventional UV plates. Launched at Ipex 2006 by Lnscher, the first machines were delivered at the end of 2006 and a5 months later the medium-sized manufacturer had already installed the hundredth setter and sold 200. Since these violet laser diodes are still considerably more expensive than thermal ones, the UV machines are around 30 % more expensive but this is quickly recouped because the plates are some 35 % cheaper. Had these 130 mW violet laser diodes been available back in 1995, the printing industry would have been spared the move to highly sensitive and expensive silver and photopolymer plates.
Although whatever new setter technologies that make their debut at Drupa 2008 may welt find a market niche, they will not supplant the current installed base of ptatesetters. At best, they will make headway over the investment cycle of today's almost saturated CtP market.