International conference on Computer Aided Design and Manufacturing CADAM'05

Mahović S.; Agić, D. & Gojo, M.

Computer to Plate (CtP) technology has been developed in the last ten years and is today the subject of investigations of a large number of manufacturers. On the graphic product market, there is a considerable number of producers of output CtP devices and printing plates, but their products have not yet been standardized. They offer different platesetter surface geometries for plate imaging, different imaging laser technologies and different active coatings on the printing plates.

In this work, the most represented CtP device architectures of the renowned manufacturers have been investigated and the quality of the plates and prints was compared. The investigations we made are of significance in the field of graphic reproduction because they define the advantages or disadvantages of observed technologies. The analysis of the investigation results was made by measuring the relevant parameters for monitoring the reproduction quality. Results have shown that there were some measurable mechanical and optical differences on the printing plates and prints, but visual assessment of reproduced motives has shown insignificant changes in colour appearance.
Keywords: CtP systems, offset printing, prints quality.

The development of electronic hardware and software, data manipulation and the usage of new materials and laser technology has resulted in systems that image directly from computer to printing plates (CtP). Like the majority of the apparently revolutionary inventions CtP technology is the product and result of the investigations in several different fields. Spreading of the usage of the printing plates, laser technology, photopolymer chemistry and DTP influenced the creating of the existing CtP systems [1]. This kind of technology has improved and accelerated printing process, because it eliminates the film making phase from graphic reproduction (Fig. 1). This has enabled achieving more positive economic effect in the context of saving graphic material and chemical substances. From the ecological point of view this technology contributes to maintaining the quality of the environment.

Fig. 1. Conventional (A) and CtP (B) workflow

Three types of platesetter architectures are available: internal and external drum geometry and flat bed geometry. The choice of these geometries can significantly aff ect the quality of the image output [2]. Other aspects of imaging surface geometry also affect the image quality: writing beam configuration, beam positioning system, vacuum hold down system and material docking system. If there are any variationsin any of these aspects the changes in printing plates and prints quality may occur. Nowadays, the prevalent geometries of the CtP device in graphic reproduction are internal and external drum.

The aim of this paper was the comparison of the quality of the printing elements on the printing plates and screening elements on the prints with the internal and external drum imaging system workflow.

2.1 Axis assignments
Internal drum, external drum and flat bed surface geometry make use of three axes of motion to create an image: X, Y and Z [3] . The X axis is arbitrary assigned to the “slow scan axis”, the Y axis is assigned to the “fast scan axis” and the Z axis represents the modulation of the data (on/off information) (Fig. 2)

Fig. 2. X, Y and Z axis

Two critical components of internal and external drum system that affect the image quality are runout and cylindricity [2] . Runout is the measure of positional error relative to a defined center of axis motion and causes drum wobbling. Cylindricity is a measure for roundness and is the problem in which all points on the surface are equidistant from common axis. Wobbling of the drum and variations in cylindricity change the writing beam depth of focus and creates artifacts on the printing plates.

2.2 Internal drum surface geometry
CtP device with internal drum geometry has stationary printing plate which is placed in a cylindrical trough [3]. The rotating mirror is located precisely in the geometrical axis of the drum (Fig. 3). During the imaging, writing beam is reflected from the mirror and hits the optics with the rotating mirror which deflected the beam onto the printing plate surface. The writing beam moves over the material surface in the fast scan axis and in the same time moves down the slow scan axis and forms an image. As a result of rotating process, mechanical runout and cylindricity errors are possible. Even a very small error will be visible on the plates and consequentially on the prints.

Fig. 3. Internal drum surface geometry

2.3 External drum surface geometry
In the CtP device with external drum geometry, plate is mounted on a drum, on the same principle as with the plate cylinder of a printing process (Fig. 4). The slow scan axis is located outside the drum and the plate rotates with the drum in the fast scan axis. Optical head with multiple writing beams moves in the slow scan axis of motion.

Fig. 4. External drum surface geometry

The crucial advantage of the external drum imaging geometry over the internal drum is that both structurally and optically it is relatively easy to focus several laser beams on the plate surface simultaneously. This leads to a corresponding reduction in imaging time for the entire plate. During the imaging, external drum wobbles slightly due to runout as it rotates which can cause focus errors. Focus problems could be also caused by higher speeds when plate can throw the drum out of balance.

Computer generated original has been reproduced by the computer led CtP systems directly onto the thermal printing plates [4] and has been printed out in offset. Original consisted of the control wedge for instrumental measurement and characteristic motives for visual assessment.

The measurings were performed on the tone patches of the primary colours of the subtractive synthesis (cyan, magenta, yellow and black). Device with the CCD camera for measuring the mechanical deformations of the printing and screen elements and densitometer for measuring the optical deformations of screen elements have been used. Before the experiment the CtP systems characterization was made. T he printing process has been led in controlled conditions concerning ink and fountain solution, and the printing substrate has not been changed.

Mechanical characteristic of the printing elements on the printing plates and screening elements on the prints were measured with device with the CCD camera. Results point at relatively uniform values (Fig. 5, 6, 7, 8). For cyan, magenta, yellow and black mechanical characteristics of the surface coverage on the plates and on the prints were similar.

Fig. 5. Surface coverage on the printing plates and prints (cyan)

These results were expected because they resulted from adequate and frequent platesetter and printing machine characterisation. Platesetter characterisation include: laser power controlling, focus, zoom and rotation speed adjustment which is indispensable for optimal reproduction [5] . Two CtP systems used in this research as well as the print machine have been regularly maintained according to the types of printing plates, paper and inks used in production.

Fig. 6. Surface coverage on the printing plates and prints (magenta)

Fig. 7. Surface coverage on the printing plates and prints (yellow)

Fig. 8. Surface coverage on the printing plates and prints (black)

Second measurings included optical analysis of the prints for both types of CtP imaging drum geometry. The results of the optical surface coverage are shown in figures 9, 10, 11, 12. It is visible that measured results for cyan and black are similar, but there are significant differences for magenta and yellow prints.

Fig. 9. Optical characteristics of the prints (cyan)

Fig. 10. Optical characteristics of the prints (magenta)

On the basis of the measured values the dot gain has been defined. It is well-known from the offset printing process that the reproduction of tone values is associated with an increase in tone value (dot gain) in the finished print [6]. The greatest dot gain on the prints is between 50% and 60% surface coverage. This is the consequence of the mechanical and optical changes of the screen elements on the printing substrate [7], [8]. It is obvious from figure 13 that the dot gain is slightly bigger on the yellow and ma genta prints which were maid with the plates imaged in the internal drum geometry.

We were interested to see if the results of the optical measuring could influence the visual appearance of colour. Thirty six observers have participated in this survey. All of them have normal colour vision and have passed the Ishihara Colour Blindness test directly prior to the experiment [9].

Fig. 11. Optical characteristics of the prints (yellow)

Fig. 12. Optical characteristics of the prints (black)

Fig. 13. Dot gain measured on the 50% of surface coverage

The visual observations have been made in neutral daylight environment [10]. The eight examinees declared that the prints reproduced by the CtP internal drum geometry were of considerably better quality. These participants obviously preferred prints which appear more saturated. The other observers thought that the prints are similar and they did not see any differences at all.

During the imaging in the internal drum geometry system the printing plate is stationary and in the external drum system printing plate rotates with the drum. Results obtained in this research have shown that differences in the mechanical characteristics of the printing elements on the printing plates and on the prints with internal and external CtP systems are small and insignificant. For the optimal reproduction in regard to the used input and output devices and used materials the application of the colour management is suggested [11]. Differences are measurable, but the analysis of the visual assessment has shown that they do not have any influence on the final visual appearance of colour. We can assume that these differences have resulted from different position of the plate during the imaging. In the internal drum geometry the plate is mounted inside the drum which is opposite to the way in which the plate is positioned during the printing process. This can be an interesting hypothesis and will be the subject of our further investigations.

[1] Adams R. M. & Romano F. (1996). Computer to Plate: Automating the Printing Industry , Graphic Arts Technical Foundation , USA
[2] Goulet R. G. (1996). Drum Technology: The Influence of Imaging Surface Geometry on Image Quality , TAGA Proceedings , pp. 3-15, Rochester (NY)
[3] Kipphan H., Handbook of Print Media, Springer, Berlin , (2001), 503-626.
[4] Van hunsel J., Van Damme M., Vermeersch J., Elsässer A., (1998), Thermostar: A new Thermal Litho Printing Plate Technology for CtP Recording, TAGA Proceedings, pp. 395-409, Rochester (NY)
[5] Dlott D. D., Focus fluctuations in laser-materials interactions , Optics & Photonics News 13(9), pp. 34-57, 2002.
[6] Nilsson, C.M.; Malmqvist, L.; Busk, H., Kristiansson, P. (1997). Optical Enhancement of Closely Positioned Screen Dots, Proceedings of TAGA, Taga office, pp. 11-21, ISBN 14623-5604, Rochester, NY, 1997, USA
[7] Agic, D.; Mandic, L., Gojo, M. (1999). Influence of Some Parameters on the Change of Printing Elements Dimension, Proceedings of 10 th International Daaam Symposium, Katalinic, B., pp. 5-6, ISBN 3-901509-10-0, Vienna, October 1999, DAAAM International Vienna, Austria
[8] Mahović S., Agić D., Gojo M., Mechanical and Optical Differences in Long Run Printing in Conventional and CtP Offset Systems, Proceedings of the 30th International Iarigai Research Conference, Croatia, (2003), 219-227.
[9] ISO 3664:2000(E); Viewing Conditions – Graphic technology and photography
[10] G.Wyszecki, W.S. Stiles, Color Science: Concepts and Methods, Quantitative Data and Formulae , 2nd Edition (John Wiley&Sons, Canada, 1982).
[11] Fraser B., Murphy C., Bunting F., Real World of Colour Management , Second Edition, Peachpit Press, 2005.

Authors: Mr.Sc. Mahović Sanja, Ph.D. Agić Darko, Ph.D. Gojo Miroslav, University of Zagreb Faculty of Graphic Arts, Getaldićeva 2, 10000 Zagreb, Croatia, phone: +385 1 2371 080, fax: +385 1 2371 077, e-mail: