The influence of dot gain mid tone spread on print quality.

Zjakic, Igor ; Milcic, Diana ; Bolanca, Stanislav 等

Abstract: Through the development and the market organization, the development of electronics, robotics and technology the new standards of the printing quality and the production of the printed products have been set. As the printing quality is one of the most important factors which define the final quality of the printed products the researches in science lead to conclusion that it was necessary to satisfy the quality which had less and less permitted tolerance of quality deviations. The quality of the printed product is influenced by a series of factors which, depending on the conditions, can be or cannot be controlled during the printing of the production. Because of that the aim of each graphic production is to control and standardize the mentioned parameters according to the newest standards.Researches in this paper were performed on prints printed in real graphic production by caclulating mid tone spread. It was confirmed in the performed researches that the printing quality is variable depending on the quantity of prints. The mentioned oscillation of the printing quality is mostly seen in the dot gain which is in the researched conditions dependent on the optical dot gain.

Key words: Mid Tone Spread, Graphic Quality, Dot Gain, Ink, Density

1. Introduction

1.1 Standardisation of the graphic product quality

Through the development and the market organization, the development of electronics, robotics and technology the new standards of the printing quality and the production of the printed products have been set. As the printing quality is one of the most important factors which define the final quality of the printed products the researches in science lead to conclusion that it was necessary to satisfy the quality which had less and less permitted tolerance of quality deviations (Kipphan, 2001).

The quality of the printed product is influenced by a series of factors that, depending on the conditions, can be or cannot be controlled during the printing of the production. Because of that the aim of each graphic production is to control and standardize the mentioned parameters according to the newest European and world standards. As the offset printing is one of the most representative printing technique which could satisfy the mentioned standards the defined printing tolerances are set to satisfy the relation among the quality and economic demands of the market. Such definition of quality refers to the defining of inking density, dot gain, relative printing contrast, hue error grey error, ink adherence etc. However, one of the mort important parameters of the quality printing among all the mentioned ones is the defined and controlled dot gain values which appeared because of the deformations of the screen element during the whole printing production.

1.2 Definition of dot gain

Factors, which can deform the screen element during the production, influence the screen element so that it increases (positive deformation) or decreases (negative deformation) in relation to the theoretical surface.

Deformation of the screen element is mostly positive, except in cases when the screen element is influenced preventively in some part of the production and when the negative deformation is enabled which will become the positive one later during the production.

Deformations of the round screen element are possible to determine by the difference of radiuses between the theoretical and real screen element size. When the real size of the screen element is greater than the theoretical one ([r.sub.1]>[r.sub.0]) the positive deformation appears and when the theoretical size of the screen element is greater than the real one ([r.sub.1]<[r.sub.0]) the negative deformation appears.

[FIGURE 1 OMITTED]

Because the deformation of the screen elements is inevitable in the production process no matter if it is positive or negative, the aim of each reproduction is to avoid these deformations as much as possible.

Precondition for that is the qualitative determination and quantification of the formed deformations. Deformation of the screen element is mostly positive except in cases when the screen element is influenced preventively in some part of the production and when the negative deformation is enabled which will become the positive one later during the production.

During the hue reproduction by the screening system, the printed surface depends on the screen elements that have no theoretical coverage identical with the real coverage. The increase of real coverage in relation to the theoretical coverage is called dot gain.

[FIGURE 2 OMITTED]

The total dot gain value for a single colour reproduction is possible to calculate by the Yule -Nielsen equation (Gustavson S, 1997):

R(a) = [(a[R.sup.1/n.sub.S] + (1 - a)[R.sup.1/n.sub.0]).sup.n] (1)

for 1 [less then or equal to] n [less then or equal to] 2, reflection of the printing [R.sub.s] and nonprinting [R.sub.0] elements.

Because the printing material has a determined reflection, the previous equation has limitation; that is the area in which 1 [less then or equal to] n [less then or equal to] 2. Because of that it is necessary to consider the relation of the printing surface, reflection from the printing substrate and the influence from environment. If the printing layer is constant and the light from the surface is diffuse, the reflection can be expressed in the following way:

R = Ks + x(1 - s)r[(1 - a + at).sup.2] / 1 - r(1 - x)(1 - a + [at.sup.2]) (2)

Ks is reflection from the upper layer of the paper surface and (1-s) is the share of light which passes through the paper, and a surface covered with ink, while t is the ink transparency. The value x represents the light which comes out of the printing material and which has already been reflected in the inner layers of the material (Wyszecki & Stiles, 2000).

As the light reflection is very difficult to calculate which depends directly on the mentioned factors, the calculation of the dot gain (DG) can be done by Murray-Davies equation in which one must know the inking density of the screen patch [D.sub.iR] and the inking density of the solid tone. [D.sub.iS]:

[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] (3)

The obtained value is subtracted from the theoretical value and the difference of these values gives the dot gain value.

As the dot gain depends on the printing substrate, viscosity of ink, chemical characteristics of the dampening solution, pressure among cylinder etc., the aggravating case of achieving the uniform printing quality is the fact that each of the mentioned parameters is different on each printing unit. Because of that it is very important to achieve the least possible dot gain difference among particular colours. Deviations between the proof and the real print, as well as the variation during the whole printing production are given by the following table:

2. Experimental

Researches in this paper were performed on prints printed in real graphic production. The products were printed in offset printing technique on the machine Heidelberg Speedmaster 102F in edition of 8000 prints. The proof was made on calibrated device Epson 4000 by InkJet technology and the values obtained on the strip of the proof served for characterization of the printing system. Printing forms were made by thermal CtP technology on the device Creo. The dampening solution was demineralized and it was artificially hardened H2O with the following pH values--pH = 5,2, dH = 8,5[degrees] and the conductivity of 1010 mS, The quantity of II propanol was 14%.

Control strips were printed on prints on which there were [D.sub.R] patches from 10-100% screen value in steps of 10%, as well as the solid tone patches served for calculation of [D.sub.PP]. The control strip contained the mentioned patches for primary printing colours.

After printing the patches were measured by spectral photometer X-Rite on 8 prints taken in steps of 1000 prints in order to calculate the mid tone spread from the production print variation tolerance. After printing and measurement the following results of the dot gain were obtained.

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3. Discussion

By construction of dot gains of the researched prints it can be seen that the dot gain curves are different depending on the edition, and they are different for each of the primary printing colours (Green,1999). Graph 3 presents the dot gain curves of prints appeared after 1000 prints. The greatest dot gain value was seen in cyan, then in magenta, then yellow and black. Such different dot gain in edition of 1000 products is caused by great number of parameters are not sufficiently controlled in small run production. One of the most important parameters that cause the mentioned problem is the temperature of the inking system that must be about 33[degrees]C in optimal conditions.

Temperature of the inking system is smaller than the optimal one at the beginning and it is in direct correlation with the printing ink viscosity. Viscosity should not theoretically influence the dot gain but different viscosity does not only change the geometrical dot gain but also the optical dot gain which defines the light passing through the screen element and the reflection in the eye or measuring device. Such optical dot gain that is bound to the viscosity of the printing ink can be caused by parameters from the figure 11.

[FIGURE 11 OMITTED]

where:

A--reflection from the surface of the printing substrate

B--thickness, density and the structure of the printing substrate

C--ink layer permeability of the recurrent light

D--ink layer absorption

E--ink layer reflection

F--light absorption on the ink layer

G--permeability of the incident permeability and recurrent light

H--adsorption of the incident permeability and recurrent light

I--reflection of partly absorbed light through the ink layer and the printing material

By calculation of the mid tone spread of the researched sample it can be seen that the value of the mid tone spread is greatest between cyan and black. Calculation of the mid tone spread was performed on the patch of 40% screen dot value. The mentioned mid tone spread of these two colour is 9 which is much greater value that the permitted tolerance according to ISO standards. Mid tone spread is within the value 4 among other colours. When observing the curve of dot gain in figure 4 it can be seen that the dot gain difference is smaller in edition of 2000 prints. It is also visible that the dot gain in cyan is somewhat smaller and the dot gain of black somewhat greater than the pervious one. Such trend of dot gain happens because of the viscosity decrease of the printing ink and it tends to get approximately the same values. Such values are defined by ISO standards where the optimal dot gain for the printing substrate (in this work) is the one where 50% patch does not come over the dot gain value of 17%. In connection with this a small shift in magenta and yellow happens and the dot gain values are shifted towards the values from the ISO standard. Calculating the mid tone spread for the edition of 2000 prints the value 7 is obtained for the relation between cyan and black which is also outside the permitted tolerances, while the mid tone spread for other colours are within the permitted tolerances. Analyzing the dot gain results after 3000 prints it can be seen that the dot gain difference is much smaller than the dot gain difference in the edition of 2000 prints. Mid tone spread between cyan and black is 4,8 that is near the permitted tolerances. Contrary to the recent research of prints within the edition of 3000 prints and analyzing the research results of the edition of 4000 prints, the results follow the positive trend of mid tone spread decrease and the values. The mid tone spread value in the edition of 4000 prints is 3,5 which is within the permitted tolerances. Observing the mid tone spread of other colours the decrease of the mid tone spread can be seen in relation to the previous prints. Because of that it can be said that the print made in the edition of 4000 prints is within the permitted tolerances independent which colour is observed. It is important to mention that the dot gain values of this print tend to achieve the value less than 17% which is the aim of each graphic reproduction because of the repeatability of the process within the permitted tolerances of the printing quality. All other prints up to the edition of 8000 prints are within the permitted tolerances of the mid tone spread as well. The value of the mid tone spread for 5000 prints is 2,5, for 6000 is 2 and for 7000 and 8000 prints is also 2. All other values of the mid tone spread have the tendency of decreasing the value that is another condition of printing quality increase of the graphic product as well as the repeatability of the process.

4. Conclusion

It was confirmed in the performed researches that the printing quality is variable depending on the quantity of prints. The mentioned oscillation of the printing quality is mostly seen in the dot gain which is in the researched conditions dependent on the optical dot gain, which is conditioned primarily by the changes of the characteristics of the printing ink. Except the optical dot gain the geometrical deformation of the screen element influences the total dot gain, but such deformation has not great influence on the change of the dot gain in different parts of edition (Berns, 2000). From the research results it can be concluded that the dot gain in all the researched prints was the greatest in cyan, then in magenta, yellow and the smallest one in black. Such difference of the starting dot gain can be caused by the characteristics of the printing units because the starting dot gain was also caused by the geometrical deformations of the screen elements which have not great influence on the dot gain in later parts of the edition. The mid tone value among the investigated prints is also different depending on the edition and it decreases with the increase of the edition. As the repeatability of the printing process with the same qualitative characteristics of prints is one of the important parameters for the quality of the graphic reproduction, it can be concluded, by means of the mid tone spread, after which part of the edition it is possible to repeat the process without great influence on the printing quality, by means of the mid tone spread. In the performed researches it was concluded that the mid tone spread was outside the permitted tolerances of the printing quality in the editions smaller than 3000 prints. The consequence of that can be the problem in defining the printing quality of smaller editions in which the edition of 3000 prints occupies the greatest part of the total edition. When observing the mid tone spread through printing the whole edition, the permitted tolerance of the mid tone spread according to the international standards must satisfy 68% of the total edition. When the obtained research results and the data of satisfying the 68% of edition are put into correlation, it can be concluded that in the edition greater than 12500 prints the value of the mid tone spread can be the parameter of the printing quality.

5. References

Berns, R. S. (2000). Principles of Color Technology, John Wiley&Sons, 047119459-X, USA

Gustavson S. (1997). Dot Gain in Colour Halftones, Doctor Disertation--Linkoping University, Linkoeping

Green P. (1999). Understanding Digital Color, GATF Press, ISBN 1-85802-450-1, Pittsburgh

Kipphan H. (2001). Handbook of Print Media, Springer, ISBN 3-540-67326-1, Heidelberg

Wyszecki G. & Stiles W. S. (2000). Color Science: Concepts and Methods, Quantitative Data and Formulae, Wiley Classic Library, 0-471-02106-7, USA

Authors' data: PhD. Ass. Prof. Zjakic I.[gor], PhD. Prof. Milcic D.[iana], PhD. Prof. Bolanca S.[tanislav], Faculty of Graphic Arts Zagreb, Croatia, zjakic@grf.hr, diana.milcic@grf.hr, sbolanca@grf.hr

This Publication has to be referred as: Zjakic, I.; Milcic, D. & Bolanca S. (2006). The Influence of Dot Gain Mid Tone Spread on Print Quality, Chapter 54 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.2006.54

Table 1. Maximum mid tone spread for proof
and production printing (in percent)

Film tone Proof print OK print Production
values deviation deviation print variation
 tolerance tolerance tolerance

40 or 50 3 4 4
75 or 80 2 3 3
Max mid tone
spread 4 5 5