Analysis of the silver halide printing plates after printing process.

Cigula, Tomislav ; Mahovic Poljacek, Sanja ; Gojo, Miroslav 等

1. INTRODUCTION

The quality of the plate making process and the stability of the printing plates during the printing process have an essential role in the final product quality. Digital plate making process, Computer to Plate (CtP), which has been observed in this paper, enables greater control and standardization in the plate making process in a contrast to analogue process (Schmitt, 2005). As the Computer to Plate technology is introduced into the graphic reproduction in the last ten years is important to observe all the factors and aspects of this process which could cause instability of the printing process. The most important factors are definitely the consistency and quality of the photoactive and the aluminium oxide layers. According to this fact, the aim of this research includes defining and monitoring the stability of the photoactive layer during the printing process.

2. BACKGROUND

The lithography printing plates are mainly made of aluminium foils covered with a thin photoactive layer. It has a significant role in the printing process as it represents the printing areas and must attract oil based printing inks. The plate making process starts with the exposure of the photoactive layer with defined electromagnetic radiation that causes physical and chemical changes in photoactive layer. After exposure printing plate must be immersed in alkaline solution where photoactive layer become soluble and is removed from nonprinting areas (Urano et al., 2004; Fiebag & Savariar-Hauck, 2003).

Printing plates with silver halide coating, which have been observed in this study, are often used in newspaper reproduction. During the printing process, silver halide layer is chemically and mechanically distorted and therefore changed in its optical characteristics (Mahovic et al., 2003). As within the lithography it is possible to print only one tone of the colour, the halftones are made with screening and presented in different coverage values (MacPhee, 1998). Screening elements are generally small sized and greatly influenced by changes in the characteristics of the photoactive layer. Those changes can cause different degradations of prints quality and consequently, degradation of the coverage values on prints (Milos et al., 2008). In accordance with these facts, the aim of this paper was to observe the changes of the coverage values on the printing plates after the printing process is finished and to define what has happened with screening elements in a case that changes occur.

3. EXPERIMENTAL

In this research the printing plates samples were made in a digital plate making process and automatically developed in an alkaline solution (pH [approximately equal to] 12). There were made two plates for four main colours (cyan, magenta, yellow and black). The first one plate was used for printing and the second one was used for measuring the starting values of coverage value before printing. To avoid possible differences in coverage values in plate making process, the exposure and developing parameters were equal for all samples (Cigula et al., 2007).

For precise measuring results a special control wedge was designed for this research. This wedge contained 26 control fields. The difference between two neighbour fields was 5% of the coverage. For detail observation in lower (0-20%) and higher (80-100%) coverage areas more control fields were defined.

The printing process was carried out on a web offset printing press on a light weighted paper (45 g[m.sup.-2]) with a cold set printing ink. This kind of ink is melted on heated ink rollers and printing plate and it hardens in contact with cold paper.

The measuring was performed by a Gretag Macbeth iCPlate II Platereader. This kind of measuring unit enables coverage value measurement, dot radius measurement and gives a preview of the measured area.

4. RESULTS AND DISCUSSION

In Fig. 1. the coverage values measured on the printing plates of four main colours before printing process are presented. One can see that there is no significant difference in coverage values on the printing plates for observed colours. This result is implicating that the Platesetter unit and the developing unit are functioning without any fluctuations.

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The results of the coverage values measured on the printing plates after the printing process are shown in Fig. 2. One can see that there is not significant change of the coverage values at lower coverage areas on the printing plates after the printing process. In the medium and higher coverage area changes of the coverage values can be seen. On cyan printing plate it is visible that difference between coverage values and reference curve are increasing with increasing nominal coverage value. In opposition, the smallest difference is accomplished between coverage value on magenta printing plate and the reference curve.

In Fig. 3. the difference between coverage values on a printing plate before and after the printing process is shown. It is visible that the most significant differences are in the area from 30 to 50 percent coverage value and are decreased after the printing process. From 70 to 90 percent coverage value, values are increased after the printing process. The changes of the coverage values are inside 2%. It can also be seen that coverage values on cyan plate is most changeable.

In Figs. 4 and 5 the results of the screening elements--dots radius before and after the printing process are presented. C1, Ml, Y1, B1 are dot radius values on the printing plate before the printing process. C2, M2, Y2, B2 are values of dot radius on the plate after the printing process. One can see in Fig. 4 that in the lower coverage area the dot radius is decreased which causes the decreasing of the coverage values. Decreased values are smallest on yellow. It can be seen in Fig. 5 that in the higher coverage area the greatest increase of dot is accomplished by cyan printing plate. These results indicate that yellow ink has smallest influence on wearing of photoactive layer and consequently causes minimal decrease of coverage value.

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5. CONCLUSION

Computer to plate process with automatic developing unit produces printing plates of the same quality with no significant changes in coverage values. In addition, results obtained by this research also declined presumption that there will be significant changes in coverage values caused by printing process. One could say that changes had occurred, but not causing decrease in print quality. The silver halide printing plates proved to have large endurance and could be used for longer printing run with no significant decrease in print quality. This paper has shown that with correct maintenance it is possible to standardize the silver halide plate making process. Our further research will be point at the thermal CtP plate making processes, endurance and possibility of standardization.

6. REFERENCES

Cigula, T.; Mahovic Poljacek, S. & Gojo, M. (2007). The Significance of Laser Exposition in CtP Plate Making Process, Annals of DAAAM for 2007 & Proceedings ofthe 18th International DAAAM Symposium, Katalinic, B. (Ed.), pp. 143144, ISBN 3-901509-58-5, ISSN 1726-9679, Zadar, October 2007, DAAAM International Vienna, Vienna

Fiebag U. & Savariar-Hauck C., Aqueous Developer for Lithographic Printing Plates, Patent No.: US 6,649,324 B1, 2003.

MacPhee, J. (1998). Fundamentals of Lithography Printing, Volume I, Mechanics of Printing, GATFPress, ISBN 088362-214-9, Pittsburg

Mahovic, S.; Agic, D. & Gojo, M. (2003). Mechanical and Optical Differences in Long Run Printing in Conventional and CtP Offset Systems, Proceedings of IARIGAI, Lovrecek, M. (Ed.), pp. 213-221, ISBN 953-96276-8-0, Dubrovnik-Cavtat, September 2003, Acta Graphica Publishers, Croatia

Milos, S.; Cigula, T.; Mahovic Poljacek, S. & Gojo, M. (2008). Contribution to the Printing Ink--Printing Plate--Run Length Interactions, Proceedings of MATRIB, Grilec, K.; Maric, G.; Jakovljevic, S. (Ed.), pp. 205-210, ISBN 978953-7040-14-7, Vela Luka, June 2008, Croatian Society for Materials and Tribology, Zagreb

Schmitt U. (2005). Korrekter Einsatz der FOGRA-CtP--und der FOGRA-Prozessor-Testform, FOGRA Symposium: Computer to Plate, Munchen

Urano T., Kohori K. & Okamoto H., Photosensitive Lithographic Printing Plate and Method for making a Printing Plate, Patent No.: US 6,689,537 B2, 2004.