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The influence of heat treatment on print mottle of screen printed textile knitted fabrics
Applied Thermal Engineering 90 (2015) 215e220
Contents lists available at ScienceDirect
Applied Thermal Engineering journal homepage: www.elsevier.com/locate/apthermeng
Research paper
The influence of heat treatment on print mottle of screen printed textile knitted fabrics Ivana Juri c a, *, Nemanja Kasikovi c a, Mladen Stan ci c b, Dragoljub Novakovi c a, cc Gojko Vladi c a, Igor Majnari a b c
University of Novi Sad, Faculty of Technical Sciences, Department of Graphic Engineering and Design, Serbia University of Banja Luka, Faculty of Technology, Bosnia and Herzegovina University of Zagreb, Faculty of Graphic Arts, Croatia
h i g h l i g h t s Textile can be printed with screen and digital (Inkjet) print technologies. Heat can not only change the reproduced colour, but also affects the print mottle. Results induce the use of 130 C temperature for ironing.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 5 May 2015 Accepted 4 July 2015 Available online 14 July 2015
Printed fabric is widely used, and it is exposed to various influences. One unavoidable impact is heat treatment during subsequent maintenance e ironing. Hence, many printed textile materials are exposed to different temperatures. Textile can be printed with conventional (screen) and digital (Inkjet) print technologies. In this paper, we analyzed screen printed textile and its sustainability after heat treatment. Heat can not only change the reproduced colour, but also affects the print uniformity (print mottle). The goal of this paper was to find what could be controlled and changed in order to get long-lasting printed textile. We used one natural textile material e cotton that was printed with screen print technology. Constant parameter during the experiment was a time of heat treatment. Ironing temperature and screen mesh count were varied. Results showed that samples printed with smaller screen mesh count maintain print uniformity after heat treatment. © 2015 Elsevier Ltd. All rights reserved.
Keywords: Textile material Screen printing Heat treatment Image quality analysis
1. Introduction Nowadays, print on substrate such as textile is widely applied. These materials can be printed using various techniques, but the most common in use are conventional screen and digital Inkjet printing. Screen printing is the most important conventional printing technology in the textile world [1e3]. The advantages of this technique are less cost and higher productivity for large print runs [4,5]. It is developed to carry out mass production [6]. On the other side, due to the physical and chemical nature of the substrate, Inkjet is the leading and the most relevant digital printing technology for textiles [6]. In comparison to screen printing, Inkjet has
* Corresponding author. Trg Dositeja Obradovi ca 6, 21000 Novi Sad, Serbia. E-mail address: [email protected] (I. Juri c). http://dx.doi.org/10.1016/j.applthermaleng.2015.07.013 1359-4311/© 2015 Elsevier Ltd. All rights reserved.
no limitation in format size, and its use reduces the time of making the product, from design to the press, accelerating the production itself [7]. Although, the better visual effect can be achieved with liquid UV Inkjet inks [8,9], in this paper focus is on screen printing. Many factors, which are closely related to each other, influence on final print quality [10]. Print speed, squeegee hardness, squeegee pressure and the distance between the screen and printing substrate (snap-off distance) affect print quality. These parameters all affect the quality, but Pan et al. [11] pointed out that the greatest impact on the quality has squeegee hardness and printing speed. In addition to these parameters, printing screen mesh counts and fiber thickness affects the final ink density and tone value reproduction [12]. Ingram in his work [13] stated that printing form, ink and substrate influence on the reproduction quality of basic parameters, lines and dots. To achieve the appropriate quality, all the parameters mentioned above must be controlled.
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The most commonly used textile materials in screen printing technology are made of cotton [14]. Cotton material is widely used in textile industry because of its excellent properties, such as: air permeability, diffusion of moisture and heat, softness, hypoallergenic and antistatic properties [15], and also great thermal properties [16e18]. Printed textiles are often exposed to various influences such as light, washing, chemical reagents, and heat [19]. One of the most common impacts is the effect of heat. The consequence of the heat treatment is the change of reproduced colours [20]. The heat leads to changes in the applied ink and also in the structure of the material itself [21]. In the papers [22e24] is noted that the temperature can be transferred to the textile material in three ways: conduction, convection and by electromagnetic radiation. Test for colour fastness to heat treatment could be carried out on the basis of international standard ISO 105-X11:1994 [25e29]. According to this standard, the gray scale (from one to five) is used for quality judgment. The quality of the treated prints can be also controlled using the spectrophotometric measurement [20] or image quality analysis. Print quality analysis beside colour includes other print quality attributes, including line quality, blurriness, raggedness, line width, darkness, micro (graininess) and macro (print mottle) uniformity, etc. [30]. The image quality analysis is performed using automated print quality systems to quantify quality attributes. The study [7] demonstrated the application of the image quality analysis for quality control of printed textile. Besides this automated system, there are also other image quality methods. For assessing print uniformity, GLCM [31] and Histogram Mottle Macro [32] methods could be used. Based on a review of current literature and research we set a goal of this study. We investigated the influence of heat treatment on print uniformity (print mottle) of the screen printed textile materials. In order to quantify this parameter (print uniformity e print mottle), we used image quality analysis. 2. Materials and method One textile knitted fabric with a single weave type was used. Material characterization was done according to the following standards: material composition (ISO 1833), fabric weight (ISO 3801) and thread count (ISO 7211-2). These properties are presented in Table 1. Test chart used for the experiment consisted of one black square 25 25 mm for obtaining print uniformity. It was printed using screen printing technique, M&R Sportsman E-Series six-colour printing machine. Pan et al. [11] found that four main parameters have a crucial effect on screen print quality. These parameters were kept constant during the printing. Printing speed was 15 cm/s; squeegee hardness was 80 Shore Type A, printing pressure 275.8 103 Pa and 4 mm snap-off distance. Sericol Texopaque Classic OP Plastisol inks were used. Plastisol inks require additional fixation after printing. Ink fixation was done at a temperature of 160 C, exposure time 150 s with thermal press MAGNET RV. Printing form was made using three different printing screen mesh count: 90, 120 and 140 threads per cm, on aluminum tubing frames. Conventional exposure was used with linear positive films
Table 2 Exposure time of stencils. Thread count (threads/cm) Light exposure time (min)
90 3
120 2.6
140 1.6
with optical density Dmin ¼ 0.3 and Dmax ¼ 4.1, measured with densitometer Viptronic150. Film screening was five times smaller than printing screen mesh count. Photosensitive Sericol Dirasol 915 emulsion was used. Light exposure was done using metal-halogen UV lamp (1000 W) at a 1 m distance from the mesh. Exposure time for each stencil was calculated using control tape Autotype Exposure Calculator by Sericol Company. Light exposure time for each stencil is represented in Table 2. Samples were heat treated according to standard ISO 105X11:1994, using thermal press MAGNET RV. Time (15s) and contact pressure (850 daN) were constant during the experiment. Samples were exposed to three different temperatures: 110 C, 130 C and 150 C. In this way, the ironing process is simulated, which is an inevitable part of the use of any of printed textile material. The heating element was tested using thermovision camera to minimize or completely eliminate temperature variations. The surface of the heating element was cleaned and prepared for measurement. Fig. 1 represents characteristic values of temperatures used in this experiment. It can be seen that there are some variations in temperature of the heating element surface. In order to eliminate the possibility of variation in thermal load applied, all the samples were positioned in the same spot so they can be in contact with the center of the heating element where the temperature was constant. Print uniformity was characterized according to two different methods, Histogram Mottle Macro [32] and Gray level cooccurrence matrix (GLCM) [31]. For assessing print mottle with image analysis method, printed samples need to be digitized. Therefore, after printing and every heat treatment, samples were scanned by scanner Canon CanoScan5600F at 600 spi. Scanned samples were transformed from RGB to Lab colour space and only L* channel was used for evaluation. Histogram Mottle Macro was applied in ImageJ and for GLCM calculations we used MATLAB software with a code proposed by Uppuluri [33]. Histogram Mottle Macro is a method developed by Maja Stani c, Tadeja Muck and Ales Hladnik, and it measures the print mottle, or the amount of non uniformity in the grayscale image. The amount of non uniformity is calculated on the basis of the histogram of the image. It measures the difference between the upper Ux and lower Lx values of the image histogram (NU ¼ Ux Lx). Ux is calculated as the mean of the intensities between median and maximum values of the histogram. Lx is calculated as the mean of the intensities between minimum and median values of the histogram. The amount of the non uniformity is expressed as the non uniformity index (NU). The greater the NU index, the surface is non-uniform, i.e. there is more amount of mottle [32]. GLCM e also known as the gray-level spatial dependence matrix e is a matrix that keeps track of how often different combinations (pairs) of pixel intensity (gray level) values in a specific spatial relationship and distance, occur in an image [31]. When building
Table 1 Characteristics of the material used in the experiment. Type of waves
Material Method
Single
Material composition (%)
Cotton ISO 1833
Fabric weight (g/m2)
138 ISO 3801
Thread count (cm1) Warp
Weft
14 ISO 7211-2
19
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Fig. 1. Thermovision images of heating element: 110, 130 and 150 C.
the GLCM, parameters like number of grey levels, the distance between two pixels of the GLCM (d) and orientation (q) should be taken into account. In this experiment a 256 grey level image (L* channel) was used. The distance (d) between two pixels whose repetition was examined, was selected to 1 pixel (there are no other specifications related to the distance, so we used 1 pixel). For the orientation (q) the average of the possible four (0 , 90 , 45 and 45 ) was taken into account. With this code, it is possible to obtain 22 parameters. From those, as parameters of importance for print mottle we take into account only Entropy [34]. Entropy in any system represents disorder, where in the case of texture analysis entropy is a measure of spatial disorder in an image. This parameter is calculated according to Eq. (1):
Entropy ¼
X
pði; jÞlogðpði; jÞÞ
(1)
i;j
where p(i, j) stands for (i, j)th entry or value in a normalized GLCM. Additional microscopic analysis (topographical data) of the sample, after printing and after heat treatment (150 C) was made by SEM (Scanning Electron Microscope) JEOL JSM 6460 LV. The images were taken at 1000 magnification for samples printed with printing screen mesh count 120 threads/cm. 3. Results Results obtained in this experiment are presented in Figs. 2 and 3. They show changes in print uniformity of one material before and after exposure to three ironing temperatures (110 C, 130 C and 150 C). The same trend was obtained, regardless of the used method. Only the scale of values is different, depending on the method used. Generally, the largest non-uniformity (largest print mottle) was achieved on samples printed with highest printing screen mesh (140 threads/cm). This can be also determined
according to the enlarged patches, as shown in Figs. 4e7. Non uniformity of these patches is clearly visible for all applied temperatures. The increase of the temperature does not necessarily increase the non uniformity of printed patches. The same relations between values were obtained for 90 and 140 threads/cm screen mesh while for middle screen mesh we noted different relation. Heat treatment changes the uniformity of print, distribution of ink particles on the material, but we did not obtain a linear increase. In the first case, heat treatment (110 C) increased print mottle, but already the second heat treatment (130 C) resulted in a reduction of print mottle. The highest heat temperature (150 C) totally increased print mottle in comparison with a sample after printing. Samples obtained with 120 threads/cm screen mesh showed an entirely different trend. During the process of printing, ink is partly applied to the surface of the textile and partly into the textile itself. Using printing form with higher mesh count, a form of the mesh is also projected to the print, which can lead to the ink concentration. According to that fact, it is possible to explain the higher values of non uniformity on prints printed with a denser screen mesh. During the thermal effect, under the influence of heat, a part of the printed ink from the surface of the material penetrates into the material, between the fibers. Thereby, fibers are connected and surface become more smooth, which can be clearly noticed in the SEM images. This means that a certain amount of ink that was on the fibers should be reduced. In addition, due to the uneven penetration of ink there is a change in surface uniformity of print. In the case of using screen mesh of 90 and 140 threads/cm under first temperature treatment, there was an increase in the non uniformity value. There was a fall in the non uniformity when using higher temperatures for ironing. According to this we can conclude that in the case of the thermal effects of lower temperature was a smaller amount of the ink which
NU index GLCM_entropy
40
1.9
35
1.8 1.7
30
1.6
25
1.5 1.4
20
1.3
15
1.2
T0
T1 (110°C) T2 (130°C) T3 (150°C)
T0
T1 (110°C)
T2 (130°C)
T3 (150°C)
90 threads/cm
16.99
21.23
19.52
19.2
90 threads/cm
1.403
1.519
1.443
1.442
120 thredas/cm
22.59
20.9
23.34
19.49
120 thredas/cm
1.531
1.47
1.575
1.361
140 thredas/cm
26.92
36.67
27.97
33.41
140 thredas/cm
1.568
1.818
1.59
1.736
Fig. 2. Results of print mottle measured with GLCM method, parameter entropy.
Fig. 3. Results of print mottle measured with Histogram Mottle Macro method, NU index.
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Fig. 4. Scanned samples (600 spi) before temperature treatment: a) 90, b) 120 and c) 140 (threads/cm).
Fig. 5. Scanned samples (600 spi) after the first temperature treatment (110 C): a) 90, b) 120 and c) 140 (threads/cm).
Fig. 6. Scanned samples (600 spi) after second temperature treatment (130 C): a) 90, b) 120 and c) 140 (threads/cm).
Fig. 7. Scanned samples (600 spi) after third temperature treatment (150 C): a) 90, b) 120 and c) 140 (threads/cm).
was melted on the surface of the material, and thus the uneven penetration of the ink into the material. By increasing the temperature, the ink is melted more and ink is penetrated evenly into the material. Completely different results were obtained for prints
printed with screen mesh of 120 threads/cm. This inconsistency could probably be explained as a consequence of various interactions between the structure of the mesh and the structure of the material.
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Fig. 8. SEM images (enlargement 1000): a) before printing, b) after printing (mesh count of 120 threads/cm), c) after heat treatment (150 C).
Obtained results can be confirmed from the enlarged images (Figs. 4e7) where more or less are visible cotton threads. Using the densest screen (140 threads/cm), minimum ink is transferred to the substrate and thus the heat treatment left the greatest effect on these samples. Fig. 8 presents the topography (SEM pictures) of material before, after printing and heat treatment (150 C) for only one screen mesh (120 threads/cm). From the SEM picture before printing (Fig. 8a) is clearly visible the structure of cotton. After printing (Fig. 8b), using Plastisol screen ink, the effect of fixing ink temperature is noticeable. The hot ink penetrates into the structure of cotton fabrics and some particles are also clearly visible that remain on the surface. After heat treatment (ironing), it can be seen that part of the ink particles were removed from the surface under the influence of heat (Fig. 8c), and the surface is more smooth. 4. Conclusions Printed textile is often exposed to many factors that can reduce the quality of the print. These prints are exposed to the sunlight, washing or ironing temperature. All this can indeed reduce the quality. As the temperature is an inevitable thing, this paper examines how to minimize quality degradation due to the heat treatment (ironing process). Based on the obtained results, it can be concluded that if we want to achieve better quality, we need to use printing form with the smaller screen mesh count. As well, it is impossible to make a global conclusion and declare that the increment of heat will increase print mottle. A possible reason for this is the structure of materials and ink adhesion into the material during heat treatment. However, if we want to maintain good print uniformity of textiles over an extended period, it is necessary to use a print form with fewer dense screens. Also, these results induce the use of 130 C temperature for ironing that to the lowest extent changes the print quality (print uniformity). The values of print mottle are very similar to the values of print mottle only after printing (third and first dot on curves in Figs. 2 and 3). Presented research needs to be conducted on materials of different raw composition with different surface structure, in order to examine in more detail the interaction phenomena between printing inks, screens and structure of materials. Acknowledgements This work was supported by the Serbian Ministry of Science and Technological Development, Grant No.: 35027 “The development of software model for improvement of knowledge and production in graphic arts industry”. References [1] N. Kasikovi c, D. Novakovi c, I. Karlovi c, G. Vladi c, Influence of ink layers on the quality of ink jet printed textile materials, Tekst. ve Konfeksiyon 22 (2) (2012) 115e124.
[2] S. Kiatkamjornwong, P. Putthimai, H. Noguchi, Comparison of textile print quality between inkjet and screen printings, Surface Coatings Inter. Part B Coatings Trans. 88 (1) (2005) 25e34. € [3] D. Onar Çatal, A.T. Ozgüney, E.P. Akçakoca Kumbasar, The influence of rheological properties of the pretreatment thickeners on ink-jet printing quality, Tekst. ve Konfeksiyon 22 (4) (2012) 309e316. [4] T.M. Lee, Y.J. Choi, S.Y. Nam, C.W. You, D.Y. Na, H.C. Choi, D.Y. Shin, K.Y. Kim, K.I. Jung, Color filter patterned by screen printing, Thin Solid Films 516 (21) (2008) 7875e7880. [5] F. Krebs, M. Jørgensen, K. Norrman, O. Hagemann, J. Alstrup, T. Nielsen, J. Fyenbo, K. Larsen, J. Kristensen, A complete process for production of flexible large area polymer solar cells entirely using screen printing e first public demonstration, Sol. Energy Mater. Sol. Cells 93 (4) (2009) 422e441. [6] P. Chong, The role of digital printing and color technology in the digital revolution for the textile world, in: AIC Color 01 Rochester: The 9th Congress of the International Colour Association June 24e29, 2001, Rochester, New York, 2001. [7] M. Tse, J.C. Briggs, Y.K. Kim, A.F. Lewis, Measuring print quality of digitally printed textiles, in: IS&T's NIP14 International Conference on Digital Printing Technologies October 18e23, 1998, Toronto, Ontario, Canada, 1998. [8] C.H. Xue, M.M. Shi, H.Z. Chen, Preparation and application of nanoscale micro emulsion as binder for fabric inkjet printing, Colloids Surfaces A Physicochem. Eng. Aspects 287 (1e3) (2006) 147e152. [9] P. Owen, Digital printing: a world of opportunity from design to production, AATCC Rev. 3 (9) (2003) 10e12. [10] M. Stan ci c, D. Novakovi c, I. Tomi c, I. Karlovi c, Influence of substrate and screen thread count on reproduction of figure elements in screen printing, Acta Graph. 23 (1e2) (2012) 1e12. [11] J. Pan, G. Tonkay, A. Quintero, Screen printing process design of experiments for fine line printing of thick film ceramic substrates, in: Proceedings of International Symposium on Microelectronics, San Diego, California, USA, 1998, pp. 264e269. €rgyvo €lgy, A. Borbe ly, Printability of PVC and PS substrates by [12] R. Szentgyo screen printing, Obuda Univ. e-Bull. 2 (1) (2011) 293e300. [13] S. Ingram, Screen Printing Primer, GATF Press, Sewickley, 1999, pp. 99e110. [14] S. Gupta, Ink jet printing e a revolutionary ecofriendly technique for textile printing, Indian J. Fibre Text. Res. 26 (1e2) (2001) 156e161. [15] Y.L. Lam, C.W. Kan, C.W.M. Yuen, Physical and chemical analysis of plasmatreated cotton fabric subjected to wrinkle-resistant finishing, Cellulose 18 (2) (2011) 493e503. [16] T. Gobi Kannan, C. Mou Wu, K. Bing Cheng, C. Yu Wang, Effect of reinforcement on the mechanical and thermal properties of flax/polypropylene interwoven fabric composites, J. Ind. Text. 42 (4) (2013) 417e433. [17] M.A. Lopez Manchado, M. Arroyo, J. Biagiotti, et al., Enhancement of mechanical properties and interfacial adhesion of PP/EPDM/Flax fiber composites using maleic anhydride as a compatibilizer, J. Appl. Polym. Sci. 90 (2003) 2170e2178. [18] P.A. De Arcaya, A.A. Retegi, A. Arbelaiz, et al., Mechanical properties of natural fibers/polyamides composites, Polym. Compos 30 (2008) 257e264. [19] N. Kasikovi c, Razvoj modela pra cena procesnih parametara stampe tekstilnih materijala (PhD thesis), University of Novi Sad, Faculty of Technical Scineces, Graphic engineering and Design, 2012. [20] N. Kasikovi c, D. Novakovi c, G. Vladi c, M. Klan cnik, Influence of heat treatment on characteristics of Inkjet print on textile material, J. Graph. Eng. Des. 2 (1) (2011) 24e30. [21] M. Michalak, M. Felczak, B. Wie˛ cek, Evaluation of the thermal parameters of textile materials using the thermographic methods, Fibres Text. East. Eur. 17 (3) (2009) 84e89. [22] N. Mao, S.J. Russell, The Thermal insulation properties of spacer fabrics with a mechanically integrated wool fiber surface, Text. Res. J. 77 (2007) 914e922. [23] C. Bankvall, Heat transfer in fibrous material, J. Test. Eval. 1 (3) (1973) 235e243. [24] M. Bomberg, S. Klarsfeld, Semi-empirical model of heat transfer in dry mineral fiber insulations, J. Therm. Insul. 6 (1) (1983) 157e173. [25] E.V. Solina, V.V. Safonov, V.B. Ivanov, Optimization of oligourethane methacrylate based UVpaints composition for printing on cotton fabrics, Text. Res. J. 81 (1) (2011) 67e80. nchez-Fernandez, A. Romero, J.F. Rodríguez, L. S [26] P. S anchez, M.V. Sa anchezSilva, Development of thermo-regulating textiles using paraffin wax microcapsules, Thermochim. Acta 498 (2010) 16e21.
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[27] Z. Mohammadkhodaei, J. Mokhtari, M. Nouri, Novel anti-bacterial acid dyes derived from naphthalimide: synthesis, characterisation and evaluation of their technical properties on nylon 6, Color. Technol. 126 (2010) 81e85. [28] H.F. Qian, X.Y. Songb, Structure-property relationships for azo disperse dyes on polyurethane fibre, Color. Technol. 125 (2009) 146e150. [29] S. Oprea, The effect of polyhydroxyurethane on dyeing of cellulosic fabrics with direct and reactive dyes, Color. Technol. 123 (2007) 329e332. [30] A. Dhopade, Figure Quality Assessemnet According to ISO 13660 and ISO 19751, Test Targets 9.0, Published by RIT School of Print Media, Rochester, New York, 2009, pp. 43e50. [31] A. Hladnik, M. Debeljak, D. Gregor-Svetec, Assessment of paper surface topography and print mottling by texture analysis, in: Conference: ImageJ
User and Developer Conference, 2010, http://dx.doi.org/10.13140/ 2.1.3068.6400 (Luxembourg). [32] Image J, Slikovna analiza primeri primjene u grafi ckoj tehnologiji, 2008 [Online] Available at: http://materijali.grf.unizg.hr/media/PGM_slikovna%20analiza.pdf (accessed 25.06.13). [33] A. Uppuluri, GLCM Texture Features, 2008 [Online] Available at: http://www. mathworks.com/matlabcentral/fileexchange/22187-glcm-texture-features (accessed 10.02.15). [34] A. Gebejes, I. Tomi c, R. Huertas, M. Stepani c, A preliminary perceptual scale for texture feature parameters, in: Proceedings of the 6th International Symposium on Graphic Engineering and Design, Faculty of Technical Science, 2012, ISBN 978-86-7892-294-7, pp. 195e202.
Contents lists available at ScienceDirect
Applied Thermal Engineering journal homepage: www.elsevier.com/locate/apthermeng
Research paper
The influence of heat treatment on print mottle of screen printed textile knitted fabrics Ivana Juri c a, *, Nemanja Kasikovi c a, Mladen Stan ci c b, Dragoljub Novakovi c a, cc Gojko Vladi c a, Igor Majnari a b c
University of Novi Sad, Faculty of Technical Sciences, Department of Graphic Engineering and Design, Serbia University of Banja Luka, Faculty of Technology, Bosnia and Herzegovina University of Zagreb, Faculty of Graphic Arts, Croatia
h i g h l i g h t s Textile can be printed with screen and digital (Inkjet) print technologies. Heat can not only change the reproduced colour, but also affects the print mottle. Results induce the use of 130 C temperature for ironing.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 5 May 2015 Accepted 4 July 2015 Available online 14 July 2015
Printed fabric is widely used, and it is exposed to various influences. One unavoidable impact is heat treatment during subsequent maintenance e ironing. Hence, many printed textile materials are exposed to different temperatures. Textile can be printed with conventional (screen) and digital (Inkjet) print technologies. In this paper, we analyzed screen printed textile and its sustainability after heat treatment. Heat can not only change the reproduced colour, but also affects the print uniformity (print mottle). The goal of this paper was to find what could be controlled and changed in order to get long-lasting printed textile. We used one natural textile material e cotton that was printed with screen print technology. Constant parameter during the experiment was a time of heat treatment. Ironing temperature and screen mesh count were varied. Results showed that samples printed with smaller screen mesh count maintain print uniformity after heat treatment. © 2015 Elsevier Ltd. All rights reserved.
Keywords: Textile material Screen printing Heat treatment Image quality analysis
1. Introduction Nowadays, print on substrate such as textile is widely applied. These materials can be printed using various techniques, but the most common in use are conventional screen and digital Inkjet printing. Screen printing is the most important conventional printing technology in the textile world [1e3]. The advantages of this technique are less cost and higher productivity for large print runs [4,5]. It is developed to carry out mass production [6]. On the other side, due to the physical and chemical nature of the substrate, Inkjet is the leading and the most relevant digital printing technology for textiles [6]. In comparison to screen printing, Inkjet has
* Corresponding author. Trg Dositeja Obradovi ca 6, 21000 Novi Sad, Serbia. E-mail address: [email protected] (I. Juri c). http://dx.doi.org/10.1016/j.applthermaleng.2015.07.013 1359-4311/© 2015 Elsevier Ltd. All rights reserved.
no limitation in format size, and its use reduces the time of making the product, from design to the press, accelerating the production itself [7]. Although, the better visual effect can be achieved with liquid UV Inkjet inks [8,9], in this paper focus is on screen printing. Many factors, which are closely related to each other, influence on final print quality [10]. Print speed, squeegee hardness, squeegee pressure and the distance between the screen and printing substrate (snap-off distance) affect print quality. These parameters all affect the quality, but Pan et al. [11] pointed out that the greatest impact on the quality has squeegee hardness and printing speed. In addition to these parameters, printing screen mesh counts and fiber thickness affects the final ink density and tone value reproduction [12]. Ingram in his work [13] stated that printing form, ink and substrate influence on the reproduction quality of basic parameters, lines and dots. To achieve the appropriate quality, all the parameters mentioned above must be controlled.
I. Juric et al. / Applied Thermal Engineering 90 (2015) 215e220
216
The most commonly used textile materials in screen printing technology are made of cotton [14]. Cotton material is widely used in textile industry because of its excellent properties, such as: air permeability, diffusion of moisture and heat, softness, hypoallergenic and antistatic properties [15], and also great thermal properties [16e18]. Printed textiles are often exposed to various influences such as light, washing, chemical reagents, and heat [19]. One of the most common impacts is the effect of heat. The consequence of the heat treatment is the change of reproduced colours [20]. The heat leads to changes in the applied ink and also in the structure of the material itself [21]. In the papers [22e24] is noted that the temperature can be transferred to the textile material in three ways: conduction, convection and by electromagnetic radiation. Test for colour fastness to heat treatment could be carried out on the basis of international standard ISO 105-X11:1994 [25e29]. According to this standard, the gray scale (from one to five) is used for quality judgment. The quality of the treated prints can be also controlled using the spectrophotometric measurement [20] or image quality analysis. Print quality analysis beside colour includes other print quality attributes, including line quality, blurriness, raggedness, line width, darkness, micro (graininess) and macro (print mottle) uniformity, etc. [30]. The image quality analysis is performed using automated print quality systems to quantify quality attributes. The study [7] demonstrated the application of the image quality analysis for quality control of printed textile. Besides this automated system, there are also other image quality methods. For assessing print uniformity, GLCM [31] and Histogram Mottle Macro [32] methods could be used. Based on a review of current literature and research we set a goal of this study. We investigated the influence of heat treatment on print uniformity (print mottle) of the screen printed textile materials. In order to quantify this parameter (print uniformity e print mottle), we used image quality analysis. 2. Materials and method One textile knitted fabric with a single weave type was used. Material characterization was done according to the following standards: material composition (ISO 1833), fabric weight (ISO 3801) and thread count (ISO 7211-2). These properties are presented in Table 1. Test chart used for the experiment consisted of one black square 25 25 mm for obtaining print uniformity. It was printed using screen printing technique, M&R Sportsman E-Series six-colour printing machine. Pan et al. [11] found that four main parameters have a crucial effect on screen print quality. These parameters were kept constant during the printing. Printing speed was 15 cm/s; squeegee hardness was 80 Shore Type A, printing pressure 275.8 103 Pa and 4 mm snap-off distance. Sericol Texopaque Classic OP Plastisol inks were used. Plastisol inks require additional fixation after printing. Ink fixation was done at a temperature of 160 C, exposure time 150 s with thermal press MAGNET RV. Printing form was made using three different printing screen mesh count: 90, 120 and 140 threads per cm, on aluminum tubing frames. Conventional exposure was used with linear positive films
Table 2 Exposure time of stencils. Thread count (threads/cm) Light exposure time (min)
90 3
120 2.6
140 1.6
with optical density Dmin ¼ 0.3 and Dmax ¼ 4.1, measured with densitometer Viptronic150. Film screening was five times smaller than printing screen mesh count. Photosensitive Sericol Dirasol 915 emulsion was used. Light exposure was done using metal-halogen UV lamp (1000 W) at a 1 m distance from the mesh. Exposure time for each stencil was calculated using control tape Autotype Exposure Calculator by Sericol Company. Light exposure time for each stencil is represented in Table 2. Samples were heat treated according to standard ISO 105X11:1994, using thermal press MAGNET RV. Time (15s) and contact pressure (850 daN) were constant during the experiment. Samples were exposed to three different temperatures: 110 C, 130 C and 150 C. In this way, the ironing process is simulated, which is an inevitable part of the use of any of printed textile material. The heating element was tested using thermovision camera to minimize or completely eliminate temperature variations. The surface of the heating element was cleaned and prepared for measurement. Fig. 1 represents characteristic values of temperatures used in this experiment. It can be seen that there are some variations in temperature of the heating element surface. In order to eliminate the possibility of variation in thermal load applied, all the samples were positioned in the same spot so they can be in contact with the center of the heating element where the temperature was constant. Print uniformity was characterized according to two different methods, Histogram Mottle Macro [32] and Gray level cooccurrence matrix (GLCM) [31]. For assessing print mottle with image analysis method, printed samples need to be digitized. Therefore, after printing and every heat treatment, samples were scanned by scanner Canon CanoScan5600F at 600 spi. Scanned samples were transformed from RGB to Lab colour space and only L* channel was used for evaluation. Histogram Mottle Macro was applied in ImageJ and for GLCM calculations we used MATLAB software with a code proposed by Uppuluri [33]. Histogram Mottle Macro is a method developed by Maja Stani c, Tadeja Muck and Ales Hladnik, and it measures the print mottle, or the amount of non uniformity in the grayscale image. The amount of non uniformity is calculated on the basis of the histogram of the image. It measures the difference between the upper Ux and lower Lx values of the image histogram (NU ¼ Ux Lx). Ux is calculated as the mean of the intensities between median and maximum values of the histogram. Lx is calculated as the mean of the intensities between minimum and median values of the histogram. The amount of the non uniformity is expressed as the non uniformity index (NU). The greater the NU index, the surface is non-uniform, i.e. there is more amount of mottle [32]. GLCM e also known as the gray-level spatial dependence matrix e is a matrix that keeps track of how often different combinations (pairs) of pixel intensity (gray level) values in a specific spatial relationship and distance, occur in an image [31]. When building
Table 1 Characteristics of the material used in the experiment. Type of waves
Material Method
Single
Material composition (%)
Cotton ISO 1833
Fabric weight (g/m2)
138 ISO 3801
Thread count (cm1) Warp
Weft
14 ISO 7211-2
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217
Fig. 1. Thermovision images of heating element: 110, 130 and 150 C.
the GLCM, parameters like number of grey levels, the distance between two pixels of the GLCM (d) and orientation (q) should be taken into account. In this experiment a 256 grey level image (L* channel) was used. The distance (d) between two pixels whose repetition was examined, was selected to 1 pixel (there are no other specifications related to the distance, so we used 1 pixel). For the orientation (q) the average of the possible four (0 , 90 , 45 and 45 ) was taken into account. With this code, it is possible to obtain 22 parameters. From those, as parameters of importance for print mottle we take into account only Entropy [34]. Entropy in any system represents disorder, where in the case of texture analysis entropy is a measure of spatial disorder in an image. This parameter is calculated according to Eq. (1):
Entropy ¼
X
pði; jÞlogðpði; jÞÞ
(1)
i;j
where p(i, j) stands for (i, j)th entry or value in a normalized GLCM. Additional microscopic analysis (topographical data) of the sample, after printing and after heat treatment (150 C) was made by SEM (Scanning Electron Microscope) JEOL JSM 6460 LV. The images were taken at 1000 magnification for samples printed with printing screen mesh count 120 threads/cm. 3. Results Results obtained in this experiment are presented in Figs. 2 and 3. They show changes in print uniformity of one material before and after exposure to three ironing temperatures (110 C, 130 C and 150 C). The same trend was obtained, regardless of the used method. Only the scale of values is different, depending on the method used. Generally, the largest non-uniformity (largest print mottle) was achieved on samples printed with highest printing screen mesh (140 threads/cm). This can be also determined
according to the enlarged patches, as shown in Figs. 4e7. Non uniformity of these patches is clearly visible for all applied temperatures. The increase of the temperature does not necessarily increase the non uniformity of printed patches. The same relations between values were obtained for 90 and 140 threads/cm screen mesh while for middle screen mesh we noted different relation. Heat treatment changes the uniformity of print, distribution of ink particles on the material, but we did not obtain a linear increase. In the first case, heat treatment (110 C) increased print mottle, but already the second heat treatment (130 C) resulted in a reduction of print mottle. The highest heat temperature (150 C) totally increased print mottle in comparison with a sample after printing. Samples obtained with 120 threads/cm screen mesh showed an entirely different trend. During the process of printing, ink is partly applied to the surface of the textile and partly into the textile itself. Using printing form with higher mesh count, a form of the mesh is also projected to the print, which can lead to the ink concentration. According to that fact, it is possible to explain the higher values of non uniformity on prints printed with a denser screen mesh. During the thermal effect, under the influence of heat, a part of the printed ink from the surface of the material penetrates into the material, between the fibers. Thereby, fibers are connected and surface become more smooth, which can be clearly noticed in the SEM images. This means that a certain amount of ink that was on the fibers should be reduced. In addition, due to the uneven penetration of ink there is a change in surface uniformity of print. In the case of using screen mesh of 90 and 140 threads/cm under first temperature treatment, there was an increase in the non uniformity value. There was a fall in the non uniformity when using higher temperatures for ironing. According to this we can conclude that in the case of the thermal effects of lower temperature was a smaller amount of the ink which
NU index GLCM_entropy
40
1.9
35
1.8 1.7
30
1.6
25
1.5 1.4
20
1.3
15
1.2
T0
T1 (110°C) T2 (130°C) T3 (150°C)
T0
T1 (110°C)
T2 (130°C)
T3 (150°C)
90 threads/cm
16.99
21.23
19.52
19.2
90 threads/cm
1.403
1.519
1.443
1.442
120 thredas/cm
22.59
20.9
23.34
19.49
120 thredas/cm
1.531
1.47
1.575
1.361
140 thredas/cm
26.92
36.67
27.97
33.41
140 thredas/cm
1.568
1.818
1.59
1.736
Fig. 2. Results of print mottle measured with GLCM method, parameter entropy.
Fig. 3. Results of print mottle measured with Histogram Mottle Macro method, NU index.
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Fig. 4. Scanned samples (600 spi) before temperature treatment: a) 90, b) 120 and c) 140 (threads/cm).
Fig. 5. Scanned samples (600 spi) after the first temperature treatment (110 C): a) 90, b) 120 and c) 140 (threads/cm).
Fig. 6. Scanned samples (600 spi) after second temperature treatment (130 C): a) 90, b) 120 and c) 140 (threads/cm).
Fig. 7. Scanned samples (600 spi) after third temperature treatment (150 C): a) 90, b) 120 and c) 140 (threads/cm).
was melted on the surface of the material, and thus the uneven penetration of the ink into the material. By increasing the temperature, the ink is melted more and ink is penetrated evenly into the material. Completely different results were obtained for prints
printed with screen mesh of 120 threads/cm. This inconsistency could probably be explained as a consequence of various interactions between the structure of the mesh and the structure of the material.
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Fig. 8. SEM images (enlargement 1000): a) before printing, b) after printing (mesh count of 120 threads/cm), c) after heat treatment (150 C).
Obtained results can be confirmed from the enlarged images (Figs. 4e7) where more or less are visible cotton threads. Using the densest screen (140 threads/cm), minimum ink is transferred to the substrate and thus the heat treatment left the greatest effect on these samples. Fig. 8 presents the topography (SEM pictures) of material before, after printing and heat treatment (150 C) for only one screen mesh (120 threads/cm). From the SEM picture before printing (Fig. 8a) is clearly visible the structure of cotton. After printing (Fig. 8b), using Plastisol screen ink, the effect of fixing ink temperature is noticeable. The hot ink penetrates into the structure of cotton fabrics and some particles are also clearly visible that remain on the surface. After heat treatment (ironing), it can be seen that part of the ink particles were removed from the surface under the influence of heat (Fig. 8c), and the surface is more smooth. 4. Conclusions Printed textile is often exposed to many factors that can reduce the quality of the print. These prints are exposed to the sunlight, washing or ironing temperature. All this can indeed reduce the quality. As the temperature is an inevitable thing, this paper examines how to minimize quality degradation due to the heat treatment (ironing process). Based on the obtained results, it can be concluded that if we want to achieve better quality, we need to use printing form with the smaller screen mesh count. As well, it is impossible to make a global conclusion and declare that the increment of heat will increase print mottle. A possible reason for this is the structure of materials and ink adhesion into the material during heat treatment. However, if we want to maintain good print uniformity of textiles over an extended period, it is necessary to use a print form with fewer dense screens. Also, these results induce the use of 130 C temperature for ironing that to the lowest extent changes the print quality (print uniformity). The values of print mottle are very similar to the values of print mottle only after printing (third and first dot on curves in Figs. 2 and 3). Presented research needs to be conducted on materials of different raw composition with different surface structure, in order to examine in more detail the interaction phenomena between printing inks, screens and structure of materials. Acknowledgements This work was supported by the Serbian Ministry of Science and Technological Development, Grant No.: 35027 “The development of software model for improvement of knowledge and production in graphic arts industry”. References [1] N. Kasikovi c, D. Novakovi c, I. Karlovi c, G. Vladi c, Influence of ink layers on the quality of ink jet printed textile materials, Tekst. ve Konfeksiyon 22 (2) (2012) 115e124.
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