Digital printing technique is used for a wide variety of substrates, one of which are PVC foils. Samples used in this research were printed by digital ink jet printing technique using Mimaki JV22 printing machine and J-Eco Subly Nano inks. As printing substrates, two different types of materials were used (ORACAL 640 - Print Vinyl and LG Hausys LP2712). A test card consisting of fields of CMYK colours was created and printed, varying the number of ink layers applied. Samples were exposed to light after the printing process. Spectrophotometric measurements were conducted before and after the light treatment. Based on spectrophotometricaly obtained data, colour differences ΔE2000 were calculated. Results showed that increasing number of layers, as well as the right choice of substrates, can improve the behaviour of printed product during exploitation

Dragoljub Novaković

Mladen Stančić

Nemanja Kašiković

Rastko Milošević

Stefan Đurđević

English

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51 original scientific paperacta graphica 248Print Quality of  Ink Jet Printed PVC FoilsAutors1Nemanja Kašiković*, 1Rastko Milošević, 1Dragoljub Novaković, 2Mladen Stančić, 1Stefan Đurđević 1University of  Novi Sad, Faculty of  Technical Sciences, Graphic Engineering and Design, Novi Sad, Serbia2University of  Banja Luka, Faculty of  Technology, Graphic Engineering, Banja Luka, Bosnia and Herzegovina*E-mail: knemanja@uns.ac.rsAbstract:Digital printing technique is used for a wide variety of substrates, one of which are PVC foils. Samples used in this research were printed by digital ink jet printing technique us-ing Mimaki JV22 printing machine and J-Eco Subly Nano inks. As printing substrates, two different types of materials were used (ORACAL 640 - Print Vinyl and LG Hausys LP2712). A test card consisting of fields of CMYK colours was created and printed, varying the number of ink layers applied. Samples were exposed to light after the printing process. Spectrophotometric measurements were conducted before and after the light treatment. Based on spectrophotometricaly obtained data, colour differences ΔE2000 were calcu-lated. Results showed that increasing number of layers, as well as the right choice of sub-strates, can improve the behaviour of printed product during exploitation.Keywords:PVC foils, Digital printing, Printing quality, Lightfastness 1 IntroductionDigital printing technology is well known for its possibility to print on various substrate materials such as: paper, plastics, textiles, ceramics, metal, wood, glass and many other materials (Adam and Robertson, 2001). Substrates with irregular surfaces and with different thicknesses can be successfully printed using this print-ing process that contributes to its wide applicability. The most frequently used plastic material within this print-ing technology is PVC, self-adhesive foils that are com-mercially available either in rolls or sheets of different formats and colors. However, it can be successfully printed onto hard PVC material as well. Beside men-tioned substrate, digital printing technology can be used for printing on polyethylene material, which is used for plastic bags manufacturing.One of the numerous applications of digital printing technique is for car windshield labels. These prints are exposed to light and other envi-ronmental influences, which has a great impact on product quality, mostly because it causes print-ed color change. This can impose big problems N. Kašiković et al: Print quality of  ink jet printed PVC foils, acta graphica 26(2015)3, 51-5652both printed with Mimaki JV22 – 160 digital ink jet printer and J-Eco Subly nano inks. The used inks are dispersed digital ink dyes for direct ink jet material printing, which have good fastness properties, which makes them suitable for outdoor products printing. Two different types of PVC foils were selected as printing substrates, where both foils have identical composition and color (transparent), but differ-ent thickness (Avery 2920QM - 60 µm; LG Hausys LP2712 – 80 µm). It is important to point out that both substrates are intended for outdoor exploita-tion, where according to the manufacturer speci-fications, the usability of these materials is up to two years for outdoor application. For the purpose of light influence treatment investigation, custom test chart was developed (40 × 10 cm). The test cart consisted of four fields sized 10 × 10 cm; first field – 100 % cyan, second field – 100 % magenta, third filed – 100 % yellow and fourth filed – 100 % black.Digital ink jet printing machine Mimaki JV22 – 160 provides the possibility to print multiple ink lay-ers, used to print test charts in five different number of ink layers. Therefore, the analysis included these ink volume variations in combination with artificial light exposure effects on printed colour changes.Printed samples were subjected to the environ-mental factors in Xeno Test chamber Alpha from Atlas, to obtain light and environmental impacts on colourfastness. The exposure and other process parameters (temperature, relative humidity, irra-diance), used to provide data for printed materi-als behaviour under simulated accelerated ageing, were kept constant and programmed according to the standard values for these types of tests. The accelerated ageing and weathering tests were fol-lowed by instrumental evaluations with the aim to determine the colourfastness. For instrumental measurements, Techkon SpectroDens (Illumination type D50, standard observer angle 2º, measurement geometry 0o/45o, measurement aperture 3 mm) was used. Measur-ing was repeated five times for each sample and results shown are the mean values. CIE L, a and b colour coordinates were determined for each sam-ple, as well as the colour differences values using CIE ΔE2000 formula, between samples after print-ing and after light exposing. Forty samples were analysed altogether, twenty samples of each mate-rial varying colours and number of ink layers (af-ter printing process as well as after light exposing).because product’s final quality cannot be predicted (Herascu et al., 2008). For determining light fast-ness these standards can be used: ISO105 – B02, ISO105-B01-1999, ISO105-B03 -1997, ISO105-B04-1997, ISO105-B05-1996, ISO105-B06-1999, ISO105-B07:2009, ISO105-B08:1999.A lot of researchers use ISO105 – B02 stand-ard (Vizárová, et al., 2011; Varesano and Tonin, 2008; Goren¬šek, et al., 2008; Erkan, et al., 2011; Zarkogianni et al., 2010; Riva et al., 1999; Gun and Tiber, 2011). The color differences between sam-ples, exposed and unexposed to light, are usually determined visually, by the blue wool reference. However, visual judgment cannot provide infor-mation accurate enough in all circumstances. The more objective way to measure colour difference changes for different colour fastness test (light, wash, rub) is a spectrophotometric method that was used by several researchers (Kan, et al., 2011; Rat, et al., 2011; Mikuž, et al., 2005).With spectrophotometric measurements, col-our difference can be calculated based on col-our characteristics measured by instrument after printing process and after light exposure. Numer-ous formulas were developed for the purpose of color differences estimation, such as basic ΔE76, more advanced CMC (l : c) (Clarke, McDonald and Rigg, 1984), BFD (l: c) (Luo and Rigg, 1987), CIE 94 (CIE, 1995) and the most recent one CIE ΔE2000 (Kim and Nobbs, 1997). All color dif-ference formulas use the Euclidean distance in a device independent color space (ΔL*, Δa*, Δb*) (Luo, Cui and Rigg, 2001). Newer color differ-ence formulas are extended to address perceptual non-uniformities while retaining the L*a*b* color space. There is an ongoing debate that of the for-mulas gives the best results. Vast majority of the researchers is claiming that the CIE ΔE2000 to be the best one (Kočevar, 2006; Xu, Yaguchi and Shiori, 2001; Gabrijelčić and Dimitrovski, 2007; Gabrijelčić and Dimitrovski, 2009).The aim of this research is to determine how PVC materials char-acteristics and a different number of printed ink layers affect light fastness, as well as print quality.2 Material and methodsIn this research two types of PVC foils were used (Avery 2920QM and LG Hausys LP2712), N. Kašiković et al: Print quality of  ink jet printed PVC foils, acta graphica 26(2015)3, 51-56533 Results and discussionAfter printing process the colour coordinates CIE L (lightness), a, b, were determined for printed samples with different ink layers number. Results in Table 1 show  color differences (calculated us-ing colour difference ΔE2000 formula) between the samples printed using five ink layers (reference Table 1. Spectrophotometric measurement results after printing processSample L a b ΔE2000A-C-1 51.25 -36.30 -52.98 26.02A-C-2 40.10 -26.65 -57.72 15.29A-C-3 33.85 -18.30 -58.26 8.93A-C-4 29.13 -12.23 -57.52 4.13A-C-5 25.47 -6.75 -56.91 -A-M-1 46.00 75.65 9.62 18.98A-M-2 39.67 74.24 29.20 8.93A-M-3 36.42 71.45 36.45 4.52A-M-4 33.80 69.22 39.60 1.86A-M-5 31.61 66.80 38.78 -A-Y-1 88.21 -3.73 94.46 10.40A-Y-2 85.67 3.84 101.50 6.10A-Y-3 83.27 9.55 101.26 2.71A-Y-4 81.34 12.84 100.59 0.59A-Y-5 81.18 14.02 101.61 -A-K-1 8.93 2.38 -2.32 4.95A-K-2 4.55 -0.39 -3.15 0.76A-K-3 4.35 -0.48 -2.79 0.55A-K-4 4.01 -0.36 -2.59 0.26A-K-5 3.69 -0.24 -2.56 -L-C-1 49.44 -33.17 -56.59 25.85L-C-2 38.53 -21.16 -61.00 15.10L-C-3 32.44 -12.72 -61.05 8.72L-C-4 27.93 -6.26 -60.54 3.76L-C-5 24.64 -1.61 -59.63 -L-M-1 44.87 75.96 4.74 19.62L-M-2 38.37 73.56 23.21 9.81L-M-3 35.40 70.76 32.07 4.89L-M-4 32.52 67.78 36.33 1.47L-M-5 30.96 65.88 36.91 -L-Y-1 88.07 -7.55 78.97 9.68L-Y-2 85.52 -1.98 92.65 5.33L-Y-3 84.32 2.07 96.44 3.00L-Y-4 82.65 5.12 97.77 1.05L-Y-5 81.94 6.87 97.66 -L-K-1 8.21 3.10 -4.52 5.35L-K-2 3.97 -0.14 -2.66 0.30L-K-3 3.80 -0.26 -2.52 0.34L-K-4 3.72 -0.25 -2.41 0.32L-K-5 3.60 -0.04 -2.49 -Note: letters A and L represent materials, C, M, Y and K – colours, 1, 2, 3, 4 and 5 – number of  layersN. Kašiković et al: Print quality of  ink jet printed PVC foils, acta graphica 26(2015)3, 51-5654sample) and the other samples, printed with four, three, two and one ink layer.Results presented in Table 1 show that increas-ing number of ink layers during print process leads to decreasing of lightness in all 40 samples. Therefore, the colour difference value will rise with a change of ink layer number. The biggest values of ΔE2000 were calculated for samples print-ed with cyan and magenta. Lower values of ΔE2000 were calculated for yellow printed samples, and the lowest values of ΔE2000 were calculated for black colour.Figure 1. ΔE2000 values between unexposed and light exposed samples - AveryFigure 2. ΔE2000 values between unexposed and light exposed samples - LGN. Kašiković et al: Print quality of  ink jet printed PVC foils, acta graphica 26(2015)3, 51-5655Based on the linear tendency of lightness it can be assumed that further increase of an ink layer number will increase value ΔE2000 also.Those samples had been exposed to light ac-cording to ISO105-B02 standard (method 2) (tem-perature, lightness, moisture). After light expo-sure, the spectrophotometric measurements were conducted to determine how the exposure to light influenced printed ink colour.Values of ΔE2000 are grouped according to sub-strates and presented in Figure 1 and 2.Both figures indicate that increased number of ink layers during printing cause better light fast-ness compared to samples printed with a lower number of ink layers. This can be concluded since the minimum value of ΔE2000 was marked after light exposure of the sample printed with five ink layers, while the highest ΔE2000 value was recorded after light exposure of the sample printed using one ink layer (for all samples). Also, colour differ-ences values are slightly lower for samples printed on LG substrates.4 ConclusionThe influence of light on printed materials has been a research topic for many researchers. The use of digital ink jet printers enables multi-pass printing with a different number of ink layers. This research focused on a spectrophotometric assess-ment of the influence of multiple printed ink layers and light fastness of samples printed on PVC foils. Exposure to light of PVC printed samples showed that increased number of ink layers increases light fastness. The lowest ΔE2000 values were recorded on light exposed samples with five ink layers while the highest ΔE2000 values were obtained for sam-ples printed by only one ink layer. This can be explained by a greater number of ink particles on the surface. Moreover, colour differences between samples printed with five ink layers and all other samples were calculated. The greatest colour dif-ference was noted between samples with one and five ink layers, and the smallest one was obtained between samples printed with four and five ink layers, for each process color.Characteristics of the substrate materials have been proven to have an important role for light fastness as well and, therefore, cannot be ignored. Samples printed on LG substrates showed lower colour differences values which is in relation to material thickness.According to results of the research, it is rec-ommended to take into consideration substrates as well as a number of ink layers when choosing digital printing techniques in order to achieve high print quality. If extended colourfastness is needed, additional layers of the ink can be applied to the substrate material.5 Acknowledgements This work was supported by the Serbian Min-istry of Science and Technological Development, Grant No.: 35027 "The development of software model for improvement of knowledge and pro-duction in graphic arts industry"6 ReferencesAdam, R. and Robertson, C., 2001. Screen printing Paperback. W. W. Norton and Co Inc.Erkan, G., Sengul, K., Kaya, S., 2014. Dyeing of white and indigo dyed cotton fa¬brics with Mimosa tenuiflora extract. Journal of Saudi Chemical Society, 18(2), pp. 139-148.CIE. Industrial color-difference evaluation. CIE Publication 116, 1995. Vienna: CIE Central Bureau.Clarke, F. J. J., McDonald, R., and Rigg, B., 1984. Modification to the JPC79 color-difference formula. Journal of The Society of Dyers and Colourists, 100, pp. 128–132.Gabrijelčić, H., and Dimitrovski, K., 2007. 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