The significance of laser exposition in CTP plate making process.

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

Abstract: In this study the quality control of the CtP (Computer to Plate) printing plates was observed by analysing the differences in surface coverage caused by the laser power and the developing time oscillations. These two parameters were selected in the printing plate making process because it was assumed that they have large impact on printing plate quality. The results have showed that changes in laser power during the printing plate exposition and differences in chemical developing time have a great influence on coverage values and consequently on the printing plate quality.

Key words: CtP plates, quality control, laser power, chemical developing

1. INTRODUCTION

Developing and reproduction through the CtP systems bring out a number of novelties, especially in the area of quality control and standardization of the plate making process. Nowadays, when different models for quality control of conventional printing plates are well known and used, CtP plate making process cause and require development of new quality control techniques and standards. Exposition of the printing plate by laser diodes is only one of the novelties in graphic reproduction which cause possible disturbances in the plate making process. Focus and zoom oscillations could occur as a consequence of different photoactive coating thickness of different printing plate manufactures, centrifugal oscillations during the drum rotation or vibrations of the CtP device (Koulikov & Dlott 2000, Dlott 2002). Adjustment of the laser power and drum rotation speed will influence on the printing plate imaging time and consequently on the quality level of the printing plates (DeBoer 1995).

Chemical processing is one of the crucial parameters which could cause disturbances in the quality level of the printing plates (Mahovic Poljacek et al., 2006)

2. BACKGROUND

The incursion of the technical revolution into all the areas of the communication industry pushed the traditional reproduction processes into the history. The boundaries among the prepress departments, photographic laboratories, photographs processing and printing departments disappear gradually. The simultaneous development of the devices, computer systems and program applications influenced numerous novelties in the processes of the graphic industry. The tendency is to minimalize the devices and to eliminate the greatest possible number of intermediate processors as well as the human factor, an important problem in the need of the production standardization. As the majority of the apparently revolutionary developments, CtP technology is the product and the result of the investigations in several different fields. The spreading of the usage of the printing forms, laser technology, photopolymer chemistry and DTP influenced the creating of the existing CtP systems.

In order to achieve the offset printing process, the printing and nonprinting areas on the printing plate must differ in their physical--chemical properties. The nonprinting areas are hydrophilic and they attract water. On the contrary, the printing areas are oleophilic i.e. hydrophobic because they absorb printing ink which is produced on the basis of oil and resins. Stability of the interaction between the fountain solution and the printing ink during the printing process is very important because any kind of their emulsification could cause scumming on the imprints.

3. EXPERIMENTAL

Investigation included the imaging and developing of CtP thermal aluminium printing plates. In the printing plate making process, after irradiation exposure, photoactive coating became soluble in the alkaline solution (Shriver & Atkins, 1999). Commercially used alkaline solution (pH[approximately equal to] 13) is used to remove this coating from nonprinting parts of the printing plates without significantly affecting the under-aluminium oxide coating structures. According to the recommended laser power information, the printing plate's samples were exposed by lower and higher power values. Consequently, the exposed plate samples were chemically developed in the automated developing process by lowering and magnifying the speed of the printing plates through developing unit.

Computer generated control strip with values from 10% to 100% surface coverage was reproduced on each CtP printing plate samples. Mechanical properties of the printing elements on the printing plates were measured by the device with the CCD camera.

4. RESULTS AND DISCUSSION

Results of the surface coverage values obtained with different developing time (expressed in mm/min) are shown in Fig. 1 to 3. Sample 1 is the printing plate sample processed with speed of 720 mm/min, sample 2 with speed of 900 mm/min and sample 3 with speed of 1080 mm/min. The results are presented partially from surface coverage of 0% to 30% (Fig. 1), 30% to 70% (Fig. 2) and 70% to 100% (Fig. 3).

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It is clearly visible on the Fig. 1 to 3 that different developing speed of the printing plates has a influence on the surface coverage of the printing elements. The differences are mostly visible in the middle surface coverage area (30% 70%). One can see that measured values of the surface coverage on the printing plate processed with 720 mm/min (sample 1) are lower than on the plate processed with 1080 mm/min (sample 3). From those results it is obviously that increasing developing speed will cause higher values of surface coverage.

Further analysis included comparison of the surface coverage on the printing plates exposed with different laser power. Sample 4 is the printing plate sample exposed with laser power of 176W, sample 2 with 220W and sample 3 with 264W. The results are presented partially from surface coverage of 0% to 30% (Fig. 4), 30% to 70% (Fig. 6) and 70% to 100% (Fig. 6).

Results presented in Fig. 4 to 6 show significant influence of the laser power on the printing plate quality. Lower laser power values (sample 4) cause higher surface coverage values. Higher laser power values (sample 6) influence on the decreasing values of surface coverage. From these results one can concluded that laser power has an important role in controlling the quality level of printing plates. By lowering laser power the photo-chemical reaction in the photoactive coating will be uncompleted and wil cause the higher values of surface coverage.

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5. CONCLUSION

Performed investigations were based on the fact that the oscillations in laser exposure and developing speed in the printing plate making process could cause differences in surface coverage values on the printing plates. The measurements were made on CtP thermal plates by measuring the mechanical properties of the printing elements. Results have shown significant differences in quality level caused by variations of both parameters. Higher differences were measured in the middle surface coverage values and were mostly expressed by variations in laser power irradiation.

According to there results one can conclude that in the starting process of CtP systems and in the adjustment time specific consideration must be directed to the two crucial points: laser power adjustment and processing time of the printing plates. Results have shown that by optimal balance between laser power exposure and processing speed of the printing plate it is possible to determinate stability and functional properties of the printing plate surface in reproduction process.

6. REFERENCES

Dlott D. D. (2002). Focus fluctuations in laser-materials interactions, Optics & Photonics News, Vol. 13, No. 9, 34-57

DeBoer C. (1995). Graphic arts applications of laser thermal printing, TAGA Proceedings, Rochester (NY), 29-43

Koulikov S. G. & Dlott D. D. (2000). Focus fluctuations in laser photothermal imaging, Journal of Imaging Science and Technology, Vol. 44, No. 1, 111-117

Mahovic Poljacek, S.; Agic, D. & Gojo, M. (2006). Influence of the Chemical Processing on the CtP Printing Forms, Annals of Daaam for 2006. & Proceedings of 17th International DAAAM Symposium, Katalinic, B. (Ed.), pp. 231-232, ISBN 3-901509-57-7, ISSN 1726-9679, Austria, November 2006, Vienna

Shriver, D. F. & Atkins, P. W. (1999). Inorganic Chemistry, 3rd Edition, W. H. Freeman and Company, NY