New approach to the printing forms microsurface characterisation.

Mahovic Poljacek, Sanja ; Gojo, Miroslav ; Mahovic, Sanjin 等

Abstract: The surface structure of the offset printing forms has the key factor in functioning of the conventional offset printing. Following characteristics are most important: physical-chemical surface properties of the printing forms and surface geometry of the printing forms. One of the greatest causes of physical-chemical changes on the surface is in most cases the processing of the printing forms, i.e. the development of the printing forms. Because of that physical-chemical as well as geometrical changes in the surface microstructure of the printing forms have been observed, caused by the processing conditions of the printing forms and the composition of the developing solution. The changes of the properties of the nonprinting areas by measuring the surface roughness and by SEM analysis have been exclusively observed. The investigations showed that determined physical--chemical changes as well as the geometrical ones appeared. These changes can have considerable influence on the application of the wetting solution on the printing form and on the correct water ink balance during the printing process.

Key words: offset printing, CtP plates, nonprinting elements, roughness, SEM analysis

1. Introduction

The production of the printing forms and especially their processing after imaging has an important role in the reproduction process. The processing of ctp (computer to plate) of thermal printing forms used in this investigation includes chemical developing process of the printing forms in alkaline developer. By the development of the direct computer controlled imaging of the printing forms (ctp) and thermal active layers (adams & romano, 1996), there is the trend of creating the "processless" printing forms, i.e. Creating of such forms which do not require the chemical developing process (walls, 1994). The reason for that is a great number of parameters that can easily vary in the developing process and that can influence the obtaining of printing forms of lower quality. It is the question of keeping the constant processing conditions, keeping the correct concentration of the developer components, developing speed, temperature etc. Nevertheless the most represented printing forms today in the graphic reproduction are ctp thermal printing plates which demand the chemical development processes (hare et al., 1997). Because of that they are the topic of our investigation. During the development, chemical interaction between the developer and the surface of the printing form can influence the difference in shape and sharpness of the printing elements. If the processing conditions of the printing form are not satisfactory it is supposed that the developer solution can aggressively act on the surface of the [al.sub.2][O.sub.3]. This could influence the decrease of hydrophilic ability of the nonprinting areas ([al.sub.2][O.sub.3] surface) and the appearance of scumming in printing.

2. Background

2.1 Offset printing technique--basis

The technique of the planographic printing is based on the selective wetting of the nonprinting areas by the fountain solution and the wetting of the printing elements by the printing ink. In order to achieve the quality imprints in the printing process, the printing and nonprinting areas must differ in their physical--chemical properties. The nonprinting areas are hydrophillic, they attract water. The printing areas are oleophilic i.e. Hydrophobic because they absorb printing ink which is produced on the basis of oil and resins. Hydrophobic ability is not much expressed and that is the reason why the fountain solution is always firstly applied on the printing form during the printing process. Because of that the fountain solution will be adsorbed onto the nonprinting areas and the printing ink will be adsorbed on the printing areas which will prevent scumming on the imprints.

2.2 Printing form production

In the offset printing process most of the printing forms are based on aluminium substrate. They are mostly pre-coated aluminum foils with the medium arithmetic roughness profile aberration Ra =1.5[micro]m (Zivkovic et al., 2000). The use of aluminium substrates as a standard is because of the attractive mechanical properties of aluminium, e.g. its strength and ability for bending, needed for the high quality work and plate handling on printing presses; the high quality printing performance achieved with a grained and anodised aluminium surface, e.g. high run length, printing latitude, scratch resistance, etc. (Vander et al., 2005).

Aluminium forms are mostly used as for lithographic printing process. These are typically 0.15 to 0.51 mm thick and are coated with a thin photo-sensitive layer. During the printing form production the surface of the aluminium substrate is electrochemically treated in acidic aqueous solutions using an electrolyte of hydrochloric acid or nitric acid to impart to the substrate a <<pits-within-a-pit>> gain structure uniformly over its entire surface (US Patent 4,087,341). Aluminium surface is during graining anodized and the layer of [Al.sub.2][O.sub.3] is formed. Because of its polarity [Al.sub.2][O.sub.3] has more expressed hydrophilic properties than aluminum. Such oxide coating on metals essentially changes their properties, because metal takes over the surface properties of the oxide coating which has in a rule the polar character. Anodizing of aluminium produced fine porous microstructure which will insure the absorption of the polar molecules of water. Roughening process must be carried out to enhance the adhesion of a photo-sensitive layer on aluminium and to improve fountain solution retention properties of the uncoated surface. The most important one is that it enlarges the contact area what causes better ink adhesion. Why is hydrophilic ability of the nonprinting areas important? By absorption of the salts for hydrophilization from the fountain solution, the hydrophilic ability of the nonprinting areas on the printing form is kept, which influences the stability of the imprints quality. The quality of the absorbed liquid depends on the technological quality of fountain solution, on the specific surface which is it proportional to and generally on the surface topography phenomena. Nevertheless, the surface should not be to rough because it might result with the reduction of sharpness of the screening element on the paper surface, which in turn might cause the decrease of the imprints and reproduction quality.

2.3 Printing form surface characterisation

The standardization of the printing process will become simpler by predicting the possible changes of the imprints and by following the technological parameters which could have influence on their quality. These steps could bring certain advantages to the printing houses in the sense of reduction of the reproduction time, as well as the planning of the stable financial dimension.

Surface topography is one of the critical factors which could cause the instability in the quality performance and the durability of the printing forms during printing process. It has a very complex quantification and its estimation demands necessary simplification. It is revealed through the quantification system of surface roughness condition by one-dimensional parameters based on shot of two-dimensional profile on the part of the investigated surface. In regard to the amplitude and the horizontal characteristics of the profile, there are horizontal surface parameters, vertical ones and hybrid ones.

The modern equipment for measuring the surface roughness enables measurements of great numbers of parameters, each describing a single characteristic of the surface roughness (Mahovic & Marosevic, 1997). The choice of the roughness parameters which will give the optimal characteristics of the surface depends firstly on the process of its elaboration and the function of investigated surface (Dimogerontakis et al., 2006).

In this paper, the printing plate surface topography was evaluated through following amplitude parameters:

* Rz--mean peak-valley height in 10 dots. It describes the differences between middle height of the five highest peaks and the five lowest valleys inside the reference length.

* Ra--arithmetical mean of the roughness, (roughness average)

* Rp--the highest peak inside the reference length;

* and through following hybrid parameters:

* Rk--core roughness depth, working surface which will influence the consistency of the material (printing plate)

* Rpk--reduced peak height, main part of the surface which will be worn out through the processing (printing process)

* Rvk--reduced valley depth.

Investigated roughness parameters are defined according to the ISO/DIS 13565-2 (1994) standard on the curve of relative length carrying capacity, so called Abbott's curve (Drevs & Weniger, 1989). Abbott's curve gives the relative share of the material as a function of the line high cross section and describes relative growth of the material share with the increasing profile. Abbott's curve with determined values of the quoted parameters for a characteristic profile inside the referent length value is shown in figure 1.

[FIGURE 1 OMITTED]

3. Experimental

Investigations were performed on ctp printing forms with the thermal active layer. The activity of ir radiation results in thermal disintegration in the thermal active layer and it becomes soluble in the developer. By removing the imaged areas during the developing process, the hydrophilic area of [al.sub.2][o.sub.3] which makes the nonprinting areas and the not imaged areas of polymer form the printing areas.

Performed investigations are based on the fact that the changes in physical chemical properties of the nonprinting areas as well as the changes in the surface microstructure of the printing forms directly influence the quality of the reproduction (lovrecek et al, 1999). Physicochemical parameter, which greatly influences the whole printing process, is ph value of the fountain solution, electrical conductivity, contact angle and surface tension. Narrow area of ph value is caused by corrosion stability of the bohemitic structure [al.sub.2][o.sub.3] as the material forming the non-printing areas, steel parts and chromium coatings on the machine parts (gojo et al., 2004). Except the influence on damping of the printing plate, ph value considerably influences the oxypolymerization, viscosity, tackiness and tinctorial strength of the ink as well as corrosion of the machine parts.

The samples of the damping solution are prepared by mixing the concentrate with the demineralized water in the concentration of 2.5 vol%. Measurements of the contact angle were performed by goniometer npl c.a. model n a-100 of the firm rame-hart. The solution samples were measured by means of the immersing method (dragcevic et al., 2002). For measuring the surface roughness mechanical-chemical measuring instrument perthometer s8p with feeler was used, which enabled the measurements of the material surfaces, graphic presentation, data processing and surface profile protocoling. Based on the results the abbot's curve was defined.

4. Results and discussion

The imaging of the ctp forms was done in the same defined conditions, but the surface processing of the printing forms was different. The samples of the printing form were developed in the alkaline developer of high (sample 1) and low (sample 2) ph value. Depending on concentration of the wetting solution their physical chemical properties were investigated, such as: ph value, surface tension and electric conductivity, and the contact angle were determined on the nonprinting areas of the thermal printing forms. The results of the changes in the contact angle are visible in figure 2.

[FIGURE 2 OMITTED]

Geometrical changes in microstructure of the nonprinting areas of the printing forms caused by the developing conditions were measured by determination of roughness parameters. The surface roughness parameters were measured on three different spots on the samples. In the table 1. And 2. One can see the results of all measured parameters, their average values and standard deviation (fig. 3).

[FIGURE 3 OMITTED]

By determination the roughness parameters relevant for the description of nonprinting areas one can see the decrease of Rpk values from 0.418 to 0.364 (Table 1. and 2.; Fig. 4). These results point at the negative activity of the developer on the surface [Al.sub.2][O.sub.3]. Namely, Al and [Al.sub.2][O.sub.3] are amphotheric, they equally dissolve in acids and alkalines forming soluble salts (Shriver & Atkins 1999). It is obvious that the alkaline character of the developer influences the dissolution of the peaks in the structure [Al.sub.2][O.sub.3], and the change of the microstructure of the nonprinting areas of the printing form which is visible on the surface photos of the SEM analysis (Fig. 5).

[FIGURE 4 OMITTED]

Decreasing of roughness parameter values and by removing all the active points on the surface topography the weaker absorption of the wetting solution appeared. This resulted with the smaller values of the contact angle as the measure of the successful wetting of the printing forms.

[FIGURE 5 OMITTED]

5. Conclusion

The geometry of the printing surface and the changes on the surface during reproduction, present an important segment in achieving prints of satisfying quality. Obtained results show that in dependence on the developer quality, i.e. on its pH value considerable changes of the nonprinting areas appear.

It is visible in the decrease of the values of the roughness parameters caused by the dissolving of the anodic layer of [Al.sub.2][O.sub.3]. This dissolving leads to the decrease of the active surface for absorption and to the smaller quantity of the wetting solution. The increase of the contact angle can cause weaker wetting of the nonprinting areas and additional problems in the planographic printing process. These problems are most often expressed in disturbing the optimal balance of wetting solution and printing ink during the printing process and in appearance of scumming on prints.

6. References

Adams, R. M. & Romano, F. (1996). Computer to Plate: Automating the Printing Industry, Graphic Arts Technical Foundation, ISBN 0-88362-191-6, USA

Dimogerontakis, Th.; Van Gils, S.; Ottevaere, H.; Thienpont, H. & Terryn H. (2006). Quantitative topography characterization of surfaces with asymmetric roughness induced by AC-graining on aluminium. Surface and Coatings Technology, Vol. 201, No. 3-4, 5 (October 2006), 918-926

Dragcevic, K.; Gojo, M. & Agic, D. (2002). Investigations of Physicochemical Properties of Fountain Solution in the Function of Printing Quality Prediction, Proceedings of 13th International DAAAM Symposium, Katalinic, B. (Ed.), pp. 141142, ISBN 3-901509-29-1, Austria, October 2002, Daaam Int., Vienna

Drevs, P. & Weniger, R. (1989). Redisovering the Abbott-Firestone Curve. Quality, Vol. 15, No. 3, (1989), 50-53.

Gojo, M., Mahovic, S., Agic, D. & Mandic, L. (2004). The Influence of Paper on Physical-Chemical Characteristics of Fountain Solution, Chapter 22, In: DAAAM International Scientific Book, B. Katalinic (Ed.), pp. 219-230, Published by DAAAM International, ISBN: 3-901509-38-0, Vienna, Austria

Hare, D. E., Dlott, D. D., D'Armato, R. J. & Lewis T. E. (1997). Fundamental Mechanisms of Lithographic Printing Plate Imaging by near--Infrared Lasers. J. Imag. Sci. Tech. Vol. 41, No. 3, (1997), 291-300

ISO/DIS 13565 1, 2, 3 (1994). Characterisation of Surfaces Having Stratified Functional Properties

Lovrecek, M., Gojo, M. & Dragcevic, K. (1999). Advances in Printing Science and Technology, Vol. 25, Pira International, Surrey UK

Mahovic, S. & Marosevic, G. (1997). Surface Roughness of the Offset Rubber Blanket, Acta Graphica, Vol. 9, No. 1, (march, 1997) 1-14, ISSN 0353-4707

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

Vander, A. J., Vermeersch, J. & Van hunsel, J. (2005). Thermofuse Digital Plate Technology, TAGA Proceedings, pp. 135-151, Canada, April 2005, TAGA Office, Rochester (NY)

Sewickley, Pennsylvania Walls, J. E. (1994). Unconventional Printing Plate exposed by IR (830) Laser Diodes, TAGA Proceedings, pp. 259-267, SAD, 1994, TAGA Office, Rochester (NY)

Zivkovic, P. M., Jovanovic, S., Popov, K. I. & Ilic, N. (2000). Modification of the aluminium for making offset printing plates. J. Serb. Chem. Soc., Vol. 65, No. 12, 2000, 935-9382

This Publication has to be referred as: Mahovic Poljacek, S.; Gojo, M. & Mahovic, S. (2006). New Approach to the Printing Forms Microsurface Characterisation, Chapter 32 in DAAAM International Scientific Book 2006, B. Katalinic (Ed.), Published by DAAAM International, ISBN 3-901509-47-X, ISSN 1726-9687, Vienna, Austria

DOI: 10.2507/daaam.scibook.2006.32

Authors' data: M. Sc. Mahovic Poljacek S.[anja]*, Ph.D. Gojo M.[iroslav]*, Ph.D. Mahovic S.[anjin]**, *Faculty of Graphic Arts, University of Zagreb, Croatia, ** Faculty of Mechanical Engineering and Naval Architecture, University of Zagreb, Croatia, smahovic@grf.hr, mgojo@grf.hr

Table 1. Results of the measured roughness parameters on sample 1.

Parameters 1 2 3 Avarage s

[R.sub.max] 4,779 4.712 5,413 4,968 0,386835
 [R.sub.z] 4,239 4,285 4,305 4,276333 0,033843
 [R.sub.a] 0,539 0,522 0,525 0,528667 0,009074
 [R.sub.k] 1,53 1,495 1,464 1,496333 0,03302
[R.sub.pk] 0,342 0,396 0,353 0,363667 0,028537
[R.sub.vk] 1,101 1,117 1,117 1,111667 0,009238
[M.sub.R1] 6,3 7,2 7,9 7,133333 0,802081
[M.sub.R2] 84 85,1 83,5 84,2 0,818535
 [A.sub.1] 10,84 14,46 14,1 13,13333 1,994225
 [A.sub.2] 87,69 83,14 91,93 87,58667 4,395911

Table 2. Results of the measured roughness parameters on sample 2.

Parameters 1 2 3 Avarage s

[R.sub.max] 4,762 5,019 5,373 5,051333 0,306781
 [R.sub.z] 4,147 4,314 4,582 4,347667 0,219446
 [R.sub.a] 0,534 0,523 0,558 0,538333 0,017898
 [R.sub.k] 1,446 1,495 1,652 1,531 0,107615
[R.sub.pk] 0,391 0,364 0,499 0,418 0,071435
[R.sub.vk] 1,148 1,164 1,153 1,155 0,008185
[M.sub.R1] 8,7 6,8 8 7,833333 0,960902
[M.sub.R2] 83 84,3 85 84,1 1,014889
 [A.sub.1] 17,22 12,44 20,15 16,60333 3,891816
 [A.sub.2] 97,53 91,27 86 91,6 5,772079