Analiza neravnomjerne pokrivenosti otiska na papirnatim podlogama s vlakancima ječma

Irena Bates1, Ivana Plazonić1, Željka Barbarić-Mikočević1, Vesna Džimbeg-Malčić1

1Sveučilište u Zagrebu Grafički fakultet, Getaldićeva 2, Zagreb, Hrvatska

Sažetak

Neravnomjerna pokrivenost otiska prema standardu ISO 13660 promatra se na dvije razine: u mikro razini kao zrnatost i u makro razini kao nejednoličnost punog tona. Ovim parametrima se može opisati neujednačenost nanosa boje na tiskovnoj podlozi. Neravnomjerna pokrivenost otiska je jedan od važnijih faktor u pogledu vizualnog dojma tiskanih grafičkih proizvoda. Ovaj fenomen uglavnom se javlja kao rezultat karakteristika površine tiskovne podloge, tiskarskog stroja te boje. U ovom istraživanju, fokus je stavljen na adheziju između papira s promjenjivim sadržajem vlakanaca ječma i dvije različite vrste tiskarskih boja (novinska i UV inkjet boja). Temeljem izvedenih analiza moguće je zaključiti kako se tiskanjem s konvencionalnom tehnikom tiska postiže ravnomjernija reprodukcija punih tonova u odnosu na digitalno tiskanje.
Ključne riječi: vlakanca ječma, novinski tisak, inkjet tisak, zrnatost, nejednoličnost punog tona

The non-uniformity of prints analysed on paper substrate with barley fibres

Abstract

Non-uniformity of prints by ISO 13660 is defined in small-scale as graininess and large-scale as mottling. With these two parameters unevenness of ink on the printing substrate could be described. Non-uniformity of prints is most important factor regarding visual impression of printed graphic products. Usually is resulted from the surface characteristics of the substrate, the setting of the printing machines and the characteristics of the printing ink. In this study, the focus was put on the adhesion between the paper with variable content of barley fibres and two different type of printing inks (coldset offset ink and UV inkjet ink). Based on conducted analysis it could be concluded that more uniform reproduction of solid tone could be achieved by conventional printing then by digitally printing technique.
Keywords: barley fibres, coldest offset printing, inkjet printing, graininess, mottling

Introduction
Uvod

Print quality of graphic products is a complex term that implies desired colour reproduction and satisfactory reproduction of image elements [1]. One of the basic area attributes for controlling the deterioration of print quality is non-uniformity of prints. Usually it occurs in the manner of systematically structured patterns, which the human eye perceived easily. Therefore characterization and evaluation of print non-uniformity is a crucial step in the assessment of the quality of prints [2,3].
Commonly used objective measurement methods for analysing unevenness of prints is defined by standard ISO 13660:2001 [4]. Uneven ink layer applied on different substrates, by mentioned standard, is monitored in small-scale (micro) as graininess and large-scale (macro) non-uniformity as mottling. Usually is resulted from the surface characteristics of the substrate, the setting of the printing machines and the characteristics of the printing ink.
The aim of this research is to analyse non-uniformity of coldset offset and UV inkjet ink layer applied on paper substrates with variable content of barley fibres. Coldset lithographic printing is the most widely used technique for newspaper printing where ink is transferred from printing plate to the rubber blanket cylinder which transfers ink onto paper [5,6]. In digital, unlike the conventional printing (offset, flexography, gravure, screen printing, letterpress) a finite amount of liquid (ink or dye) from print-heads is placed directly onto a printing substrate. Therefore image carrier, like printing plate, is no needed in this process [7].  In comparison with traditionally used printing processes this relatively new type of printing process requires minimum production space, can be mounted on the production line and can be used on many different substrates. For achieving optimal quality reproduction it is necessary to find out paper characteristics and possibilities of printing techniques.

Materials and Methods
Materijali i metode

This research included three steps: 1. Forming laboratory papers with variable content of barley fibres as a printing substrate; 2. Printing of prepared substrates by two types of printing technique in full-tone black; 3. Analysing of quality reproduction by observing the non-uniformity of printed area.

2.1. Forming laboratory paper substrates with variable content of barley fibres

The printing substrates (42.5 g/m2, 20 cm diameter) with variable content of barley pulp were laboratory produced by Rapid Köthen Sheet Machine. For that purpose straw of barley grown in the continental Croatia was cut manually into 1 to 3 cm long pieces and converted to a semi chemical pulp according to the Soda method (Figure 1).

Figure1 Obtaining the barley pulp for laboratory papers production

Obtained unbleached barley pulp was mixed with recycled newsprint, Commercial UPM News C paper, in different weight ratios for forming laboratory papers (Figure 2).

Figure 2 Schematic view of forming laboratory paper substrates and their marks

Laboratory paper substrates formed of newsprint pulp were used as control samples (marked as N).

2.2. Printing paper substrates

All laboratory made papers were printed on the felt side by two types of printing technique in full-tone black. In the simulation of coldset offset printings by Prufbau Printability Testing Machine a carbon black ink was used. The ink dosage of 2.00 cm3 was distributed evenly over the printing cylinder and the ink was transferred to the laboratory paper samples with a width of 40 mm, an applied load of 150 N cm-3 and a speed of 1 m s-1 [8]. Digital prints were made by AGFA, Anapurna M1600, UV-curable piezo inkjet printer. Thickness of ink layer in UV piezo inkjet technique is in range from 5 to 15 µm, which depends on the printing substrate, while the dynamic viscosity of these inks is from 1 to 30 mPa s. In piezo inkjet technique if drop frequency is defined between 10 and 20 kHz with drop volume of 14 pl, the drop diameter is approximately 30 µm [5]. The printer used in this research has the print-heads of 1024 nozzles with a droplet volume of 12 pl for colours with viscosity from 10 to 15 mPa s, which produce high quality solids and tonal rendering at up to 720 x 1440 dpi. The smallest element of a print generated by an ink jet printer is a dot [9].

2.3. Analysing of quality reproduction

The analysis of quality reproduction on printed substrates was based on observing non-uniformity of printed area. The evaluation of prints non-uniformity was performed by digital microscope PIAS-II using software which is built on international print quality standards ISO-13660. ISO-13660 is the international standard that provides quantitative assessment of wide range of print quality parameters.
According to the Standard, prints non-uniformity defines in small-scale (>42μm and <1270μm) non-uniformity as graininess and large-scale (>1270μm) non-uniformity as mottling. The printed area which is examined is divided into 100 uniform tiles (1.27×1.27mm). Within each tile, 900 measurements of reflectance are made in small non-overlapping square areas (42.3×42.3μm). In each tile the mean (mi) and standard deviation (σi) of the 900 reflectance measurements are calculated.
Graininess is defined by the following equation:

                                                                                               (1)

where σi is the standard deviation within cell i; n is the total number of cells.

Mottling is defined as the standard deviation of the mean reflectance values of the tiles, i.e. how much variation in density are from one tile to the next tile.

                                                            (2)

where mi  is the mean value of the reflection coefficient; n is the total number of tiles [4].

Namely the greater the variation of coefficient value, the more obvious the unevenness.

Results and Discussion
Rezultati i diskusija

The graphs shown below present non-uniformity values of prints achieved on uncoated laboratory papers with variable content of barley fibres by two different types of printing.

Figure 3. a) the graininess and b) the mottling values of printed substrate
obtained by conventional and digital printing

The results of graininess and mottling values presented in Figure 3 have proven that different printing techniques provide significantly different quality of prints on the same printed substrates. It can be noticed that values of graininess (Fig. 3a) and values of mottling (Fig.3b) are considerably lower for all printed samples achieved by conventional printing technique. Gained results are in correlation with other studies where systematic noise i.e. non-uniformity area is more common in prints made by digital than by conventional printing technique [10]. This occurs in digital technique because of way that printer makes a print. Namely, inkjet printers print a dot at a given area of paper substrate creating a pattern of black dots which have a non-uniform appearance [11].
Comparing a composition of paper substrates used for making prints (control and those which contain barley pulp), it is clearly visible that values of graininess are similar or equal for all prints made by conventional printing (0.70 - 0.78). The mottling values have the same trend (1.08 - 1.10). In prints made by digital printing both non-uniformity attribute values are increasing by increasing the content of barley pulp in printing substrates (graininess = 2.24 - 2.46; mottling = 1.74 -2.16). Obtained higher graininess on digital prints causes a resolution and contrast reduction of print. The graininess values less than 0.8 units shown no visual by naked eye but under magnification can be extremely variable spreading of ink on paper [12].
In order to more visually indicate the non-uniformity of the printed substrates, in Table 1, are compared 3D surface plots diagrams of all printed areas applied by conventional printing and digital printing on paper substrates. Substrates with different topography reflect light in various manner which change the print quality [1]. It can be noticed that prints made by conventional printing possesses the most uniform solid-tone ink surface, which is in correlation with low values of print graininess and mottling (Figure 3a and 3b). A lower quality reproduction have prints made by digital printing, which possess grainy surface structure.

Table 1 3D surface plots diagrams of all printed areas applied by conventional printing and digitally printing on paper substrates

It is well known that commercial papers gained by the industrial production have better optical, physical and mechanical properties compared to laboratory made papers. Accordingly, the printability of commercial paper and achieved quality reproduction will be improve. However, the results gained in this research on laboratory papers are the first indication how barley fibres have a potential to be used in newsprints printed by coldset offset technique.

Conclusion
Zaključak

The aim of the research was to point out the influence of substrates with variable content of barley fibres on print quality in conventional and digital printing technology.
Taking into account all obtained results, the following could be concluded:

• digital printing provides higher non-uniformity on printed area, than conventional printing process
• barley fibres have a potential to be used in printing substrates for coldset offset printing as share of those virgin fibres have no significant influence on mottling and graininess values

Literature
Literatura

[1] Novaković D.; Stančić M.; Karlović I.; Kašiković N.;Vukmirović V.; Milošević R.; Influence of surface roughness on print quality on digitally printed self adhesive foils; Journal of Print Media Technology Research, Vol.2., No.2, ISSN: 2223-8905; pp: 67-76; 2013.
[2] Anayath, R.; Baral A. K.; Critical Analysis of Mottling and its Impact on Various Grades of Paper Substrates Printed under Conventional Sheet Fed Offset, Dry Toner & Liquid Toner Based Digital Print Engines; Acta Graphica, Vol.27., No 1 ISSN: 0353-4707; pp: 7-14; 2016.
[3] Dubé M.; Mairesse F.; Boisvert J. P.; Voisin Y.; Wavelet analysis of print mottle; IEEE Transactions on Image Processing; 2005.
http://www.uqtr.ca/~dubma/double.pdf , 21.01.2017.
[4] ISO 13660, 2001 – ISO/IEC 13660 Information Technology - Office Equipment –
Measurement of image quality attributes - Binary Monochrome text and graphic Images
[5] Kipphan H.; Handbook of Print Media; Springer, ISBN 978-3-540-67326-2; p.221-223; Berlin, 2001.
[6] Lundströma J.; Verikasa A.; Tullanderc E.; Larssond B.; Assessing, exploring, and monitoring quality of offset colour prints; Measurement, Vol.46., Issue 4; ISSN: 0263-2241; pp: 1427–1441; 2013,. 
[7] Lundberg A.; Örtegren J.; Alfthan E.; On the Effect of Variations in Paper Composition on Inkjet Print Quality; Proceedings of 25th International Non Impact Printing Conference, pp: 316-319, ISBN 978-1-62276-618-5; Kentucky, Louisville, 2009.
[8] Plazonić I.; Barbarić-Mikočević Ž; Džimbeg-Malčić V.; Bates I.; The rub resistance of printed papers with variable content of wheat pulp; Proceedings of Natural resources, green technology and sustainable development/2, pp: 76-79., ISBN 978‐953‐6893‐04‐1; Zagreb, 2016.
[9]  Plazonić I.; Bates I.; Barbarić-Mikočević Ž; The Effect of Straw Fibers in Printing Papers on Dot Reproduction Attributes, as Realized by UV Inkjet Technology; BioResources, Vol. 11., No. 2; ISSN 1930-2126; pp: 5033-5049; 2016.
[10] Chrisstoffersson J.; Evaluation of Systematic & Colour Print Mottle / Master thesis. Norrkoping, Sweden: Linkopings University, 2014, 30 p.
[11] Briggs J.C.; Application Note: Graininess Measurements on Halftones, Quality Engineering Associates, Inc.; 2002.
www.qea.com/wp-content/uploads/2015/04/AppNote-QEA-Graininess2.pdf, 19.01.2017.
[12] Dhopade A.; Image Quality Assessment Acording to ISO 13660 and ISO 19751, 2009.
http://cias.rit.edu/~gravure/tt/pdf/pc/TT9_image_quality_assessment.pdf, 27.02.2011.