International Journal of Graphics and Multimedia (IJGM), ISSN (Print), INTERNATIONAL JOURNAL OF GRAPHICS AND

Transcription

1 INTERNATIONAL JOURNAL OF GRAPHICS AND MULTIMEDIA (IJGM) ISSN (Print) ISSN (Online) Volume 4, Issue 2, May - December 2013, pp IAEME: Journal Impact Factor (2013): (Calculated by GISI) IJGM I A E M E IMPROVEMENT OF COLOR REPRODUCTION ACCURACY OF FLEXO PRINTING TECHNOLOGY FOR DIFFERENT PRINTING SUBSTRATES TATYANA BOZHKOVA, TANYA STEFANOVA, ALEXANDER GANTCHEV University of Chemical Technology and Metallurgy, 8, St. Kliment Ohridski Blvd., 1756, Sofia, Bulgaria ABSTRACT Analysis of color shift for spot colors and specification of the lowest color difference on client s samples in flexo printing on various printing substrates is carried out. The analyses are based on two main stages. The first stage is focused on a laboratory research, and based on it are selected the specifications of anilox rollers. The goal is to invest less time, cut production costs and diminish the hazardous impact on the environment by reducing waste. The second stage spreads over the complete preparation of the printing press and results in a real printing process. An additional task is to analyze if the results received in the lab will be confirmed in a real production conditions. Keywords: Color Differences, Flexo Printing, Printing Presses, Optimization of Printing Conditions, Inks for Flexo Printing and Color Reproduction. INTRODUCTION With the appearance of writing, as a tool to reflect human thoughts, arose the necessity to protect knowledge. With time passing, the need not only to protect the information, but also to make it available for more people appears strongly. This accelerates the development of methods for reproduction of the available information. The development in graphic arts is in the direction of satisfying the need to reproduce information. Printing is an important branch of the industry, which is serving the different needs of the cultural, economic and political life of society. The printing industry includes production of books, 11

2 newspapers, magazines, brochures, catalogues, packaging and many others, but the requirements for color consistency in the printing process is different. Various printing methods are developed to reproduce identical copies on many various materials with flat or relief forms. There are different printing methods, such as letter press, litho offset, gravure, screen or digital printing, with a lot of varieties [1]. What is very important when talking about packaging is the color consistency, especially when we are focused on the printing of spot colors for different registered trademarks of print buyers. During the years, all these methods have been developed and improved, with the goal to increase their quality and reliability, while in the same time reduce the hazardous impact on the environment. In parallel to the new development of printing technologies, there are more and more advance methods to control the process [2, 3, 4], which lead to the optimization of the printing process and predictable and repeatable results as well, especially in colors [ 5, 6, 7 ]. EXPERIMENTAL Web flexo printing press MPS Systems bv Maximum printing width cm. Number of colors - 8 nb. UV driers 8 nb. The printing process for the experiment was at fixed printing speed and tension of the print substrate. Ink For the test trial Pantone 354 ink was used. The ink is mixed according to the given ink formulation and contains no solvents. It is appropriate for UV-drying inks and for the used substrate. Print substrate S 2000 N (self-adhesive papir) HGW (self adhesive paper - glossy) PE on a self-adhesive paper. Anilox rollers Anilox rollers, with characteristics shown in Table 1, were used for the test trial, as these are used the most in the printing house. Used tapes and test targets In the research two different type of adhesive tapes were used soft and hard, and two kinds of solid areas were tested solids 100% and screened solids of 98%. 12

3 Table 1: Anilox rollers with different volumes used in the research Ink volume [сm 3 /m 2 ] Screen ruling of the anylox rollers [lpi] RESULTS AND DISCUSSION The analyses are carried out on two basic stages. The first stage is focused on a laboratory research, used to specify the anilox rollers. The goal is to use less time, resources, as well as to reduce the impact on the environment, and to check if the results achieved in lab conditions will correspond to the real printing process. The second stage includes the entire preparation of the printing press and printing the samples in working conditions. Laboratory investigations The spot color is prepared by mixing two colors, in accordance with the ink formulation for Pantone. Then tests are printed on a proofing press Orange Proofer. Four different screen rulings and three different aniloxes with different ink volumes were used. Samples are printed for each combination, as the samples are chosen according to a preliminary defined model and each tenth sample is accepted as a specimen and is measured with a spectrophotometer. The next step is evaluation of the print tests. The values for the color differences for the different substrates are shown on Table 2 and Figure 1. The color differences were calculated according the following formula: Eab= { a 2 + b }½ Where: = 1-2 ( 1 - coordinates of the unknown color, 2 - coordinates of the real color) a = a 1 - a 2 (a 1 - coordinates of the unknown color, a 2 - coordinates of the real color) b = b 1 b 2 (b 1 - coordinates of the unknown color, b 2 - coordinates of the real color) Lab tests show that the best results are achieved with anilox rollers with volume 6,and when Top was used as a print substrate. Table 2: Color differences of print samples printed on Orange Proofer Anilox volume [сm 3 /m 2 ] Eab 100% of Top Eab100% of Eab 100% of HGW

4 Figure 1: Comparison of color differences for three kind of substrates, printed at the same conditions with pantone 345 ink RESEARCHES IN WORKING PRINTING CONDITIONS During the second stage, test trials were done at the operational parameters used in daily working conditions. During this stage of the test trials, two different tapes were used, soft and hard, in addition to the different aniloxes and test images,. Different combinations are shown in Table 3. Table 3: Materials and equipment used in the experiment Anilox volume lpi; SB lpi; SB lpi; SB lpi; SB lpi; HB lpi; HB lpi; HB lpi; HB Substrate HGW HGW HGW HGW HGW HGW HGW HGW 14

5 The color shift and the color differences Еab are measured after printing all samples and careful selection. The samples are selected according to a preliminary defined model, as each tenth sample is selected as a specimen and 10 samples from each combination are selected (for a combination of a substrate anilox roller tape). Еab is measured for 100 % and for 98 % screened solids and the results of the average Еab are shown in Table 4. Table 4: Average Еab for different print conditions Anilox volume Form SB HB SB HB SB HB SB HB Top Еab, 100% Еab, 100% HGW Еab, 100% Top Е, 98% 2,30 1,45 1,67 1,67 6,02 7,2 13,11 13,31 Еab, 98% HGW Еab, 98% The measured Еab and the average color differences achieved during test trials are shown in Fig. 2 and 3. The analyses show that the results achieved in the laboratory conditions are confirmed and correspond to what was expected. Therefore, that they may be used to preliminary define the values for real working conditions. With the help of the laboratory data, the waste materials and the working time are significantly reduced and lead to optimization of the production process. 15

6 Figure 2: Comparison of Еab for the three kinds substrate with the use of soft tape 16

7 Figure 3: Comparison of Еab for the three kinds of substrates with the use of hard tape 17

8 CONCLUSIONS Based on the test trials and print samples evaluations, the following conclusion may be given for the tested Pantone 354 ink with flexo printing of client s samples for defining the smallest color differences: For the Top substrate the results received in the lab show that the use of anilox rollers with volume of 5 is recommended. When the real printing process is used, it is confirmed that those with 5 are the most suitable. The differences in Еab at both aniloxes are insignificant. It is observed that at 98% tone value and anilox 6,5, the values of Еab for both hard and soft tapes are approximately the same. According to the results, the lowest Еab is achieved at 98% tone value and 5, 5 volume aniloxes. At substrate - the results from the lab show that none of the anilox rollers is suitable for this job. In all four cases the value of Е surpasses the recommended values. The laboratory results are confirmed during real printing. Regardless of the used combination, the lowest values for Е are in the range of 3,80, which means that such color shift will be visible by a naked eye. Such color difference is not an exception for spot colors, but it exceeds the requirements of critical print buyers. For HGW substrate when using HGW in lab conditions, the results are, in general, acceptable for the color difference with anilox volume of 6. Real printing process shows good results for Еab when using 6,5 anilox volume and 100% solid tone. The color difference increases at 98% tone value. Increase of Еab is observed when hard tape is used and that is why the use of soft tape is recommended. From all conclusions listed above, it can be summarized that the best results are achieved with РЕ Top substrate. The color differences received with the substrate prove in practice that good color reproduction is difficult. At HGW good color reproduction may be achieved by using aniloxes with volume 6, 5 and 98% tone value. Here, the tape does not have a significant influence on the print results. REFERENCES 1. H.Toliver-Nigro, Printing technologies, Moscow, E. Petkova, I. Spiridonov, I. Lozanova-Doncheva, R. Boeva-Spiridonova, Investigation of thermal ageing and influence of conservation treatment on colour characteristics and physical and mechanical properties of offset imprints on glossy ced paper, 5th International GRID symposium,11-12 November, Novi Sad, Serbia, pp I. Spiridonov, Comparison of Optimal Inking Obtained by Using the Method of Maximum Print Contrast and Inking Obtained by Achieving the Color Values in CIE LAB, 42nd Conference of the International Circle of Educational Institutes for Graphic Arts Technology and Management,19-20 October, Moscow, 2010, pp I.Spiridonov, N.Kachin, Determination of the Deviations Tolerances of the Process- Colour Solids from the OK Print in Offset Printing Method, 42nd Conference of the International Circle of Educational Institutes for Graphic Arts Technology and Management, October, Moscow, 2010, pp Helmut Kipphan,. Hanbook of Print Media, Springer, Germany, 2001, pp R. Piers, Handbook of printing inks, Springer, ISO , 2004, Graphic technology. 18

9 INFORMATION ABOUT THE AUTHORS Dr. Tatyana Bozhkova is working for the University of Chemical Technology and Metallurgy Sofia, Bulgaria, as an assoc. professor of Department of Printing Arts, Pulp and Paper. Ing. Alexander Gantchev is working for the University of Chemical Technology and Metallurgy Sofia, Bulgaria, as an assistant of Department of Printing Arts, Pulp and Paper. Mrs. Tanya Stefanova is a student in the University of Chemical Technology and Metallurgy Sofia, Bulgaria, as an assistant of Department of Printing Arts, Pulp and Paper 19