The influence of the toner structure on the quality of black-white digital printing.

Majnaric, Igor ; Bolanca, Stanislav ; Bolanca Mirkovic, Ivana 等

1. INTRODUCTION

In recent times, the majority of the digital printing machines development has been reduced to the investigation of the new formulation and the finest possible toner structures which result in better printing properties and higher productivity. The aim of this work is the analysis of the quality of the black-white digital printing and recognizing how different toner types adhere to the non absorbent printing substrate. Indirect determination of thickness of the ink film is based on the optical density measurement. Optical density (D) is defined in form of logarithmic ratio: D = log 1/[beta] = log [I.sub.o]/I. Reflectance factor ([beta]) is the ratio of light intensity, (I) of light remitted by the ink film in relation to the intensity of light ([I.sub.o]) remitted by the blank paper.

2. THEORETICAL PART

The digital printing techniques are characterized by the unique digital input data which define the contents of the future printing form, i.e. the elements of the final print. The digital printing has been developed into two directions: Computer to Press and Computer to Print (Goldmann, 2004). The characteristic of the black-white digital printing, is achieving the coloration by different types of coloration carriers. They are polymerised toners and new colorants, pigment dispersion, encapsulated pigment technology and dye. (Williams, 2001).

In the digital waterless offset black pigments are tiny particles of carbon black or pure elementary carbon (20-150 [micro]m) whose share is 10-30%. The basis of black-white digital screen printing ink is soy oil (>6%), petroleum solvent (<10%), glycerol (<5%), black pigment (<10%), alkid resins and water. The particles of the carbonate black toner are less than 1[m. The basic principle of EP (electrohotoraphy) is founded on photoelectric effect (Bolanca et al., 2005). In 80% of the cases the toners are powders. Two component toner of 1st generation is a mixture which contains: resins (80-90%), coloured pigments (5-15%), substances for charge control (1-3%) and additives (Thompson, 2004). Toner of the 2nd generation is based on process EA (emulsion aggregation). The process of EA enables exact position control of waxes within the toner particles. The printer fusers melt the toner particles by heating, making it sticky and able for adhering to paper even on the lower fusing temperatures (Suzuki et al, 2003). The pigment particles of the liquid EP toners have star like shapes (1-2[im). For directed movement of ElectroInk, the uniformly dispersed agents (polar molecules) for increasing the electric conductivity are responsible (Majnaric et al., 2007). Ink Jet more recent pigment dispersions are made on the basis of uncapsulated technology where each pigmented particle is comprised with the oil coating adapted for good penetration into the printing substrate. In this way the corresponding surface smooth coating is obtained which increases the optical reflectance of the printed layer (Larson et al., 2002).

3. EXPERIMENTAL PART

In this work, the printing quality of black-white digital printing systems which is applied in office business and editing has been investigated. The digital printing machines have been compared for that: EP (Hp Indigo Turbostream, Xerox 250, HP LaserJet 9050), Piezo Ink Jet-a (Epson 2400 photo), digital screen printing (RISO RZ 970 E) and digital offset printing (Heidelberg Quickmaster 46 DI). They all use especially composed ink. For the needs of investigation, the special printing form in PDF was generated. It contains: the standard achromatic illustration, wedge with 99 patches (in the range from 1 to 100%) positive and negative micro elements, and positive and negative lines of the standard thickness of 650 [im. By means of the densitometer X-rite DTP 41 the optical inking density of the printed ink layer was determined, which directly showed the binding of toner to the printing substrate, i.e. the possibility of printing the broadest possible range of the achromatic inking. With the system for image analysis (Personal IAS) the dimensional exactness of the printed reproduction. The calendering natural paper substrate with the high degree of whiteness, designed for digital printing was used for printing (Splendogel EW 115 g/m2).

4. RESULTS AND DISCUSSION

In figure 2 the curves of the dependence of the inking density (D) on total screen value (1-100% screen value), have been presented. All the curves have the increasing form which is uniform. The adapted printing substrate for the digital printing has the greatest inking density on Xerox ([D.sub.MAX] = 2,14). Indigo TS in greatest part (1-90% screen value) has the smallest inking density. The exception is Epson 2400 which gets the maximal inking density at 85% screen value ([D.sub.85%RTV] = 0,89) but it does not succeed to keep it in higher tones.

[FIGURE 1 OMITTED]

[FIGURE 2 OMITTED]

The liquid pigment inks (Epson 2400) in great layers cause the opposite effect, i.e. too big ink penetration into the substrate which causes the capillary blooding and closing the darkest screen values. The quality of the surface layer is emphasized which is visible at low screen values. The greatets inking density is achieved by HP 9050 ([D.sub.10%RTV] = 0,26), after that follow Riso, Epson, Xerox and Heidelberg. In these printers, the growing trend in dependence curves [D/RTV.sub.1-10%] is visible, which means that there are differences in creation of the lighter tones. On Indigo there is no difference in inking density and the areas 1-15% screen value are unchanged. Such characteristic movement in value can be explained by the usage of the software linear LUT, by which propositions the dot gain is eliminated. In reproduction of high screen values, the impressions of Xerox stand out more considerably. The nearest to it are the impressions of HP 9050 DN. It is characteristic fot the darker tone areas, that the smallest inking density has Epson 2400. On the formed layer in the zone of the medium quality there are the digital machines Riso, Indigo, Heidelberg QM, which have the maximal inking densities in the area between D=1,23 and D=1,36. In relation to the linear curves, only Xerox realizes the increase, i.e. the possiblity of the more qualitative reprodcution of darker tones. By the image analyses Personal IAS positive and negative lines are increased and measured. Results are presented in table 1.

In relation to the desired positive original line, the increase of reproduction was noticed at: Indigo TS ([DELTA]=35,35 [micro]m) and Epson 2400 ([DELTA]d = 40,81 [micro]m). The other printers generate the decrease of the e.g. Xerox ([DELTA]d = -1,41 [micro]m) Heidelberg QM 46 ([DELTA]d = -17,87 [micro]m), HP 9050 ([DELTA]d = -23,89 [micro]m) and RISO ([DELTA]d = 83,44 [micro]m). For the reproduction of the negative lines the decrease of the desired size is applicable. The smallest change appears at Heidelberg QM 46 ([DELTA]d = -33,31 [micro]m), while the greatest one at Epson 2400 ([DELTA]d = -127,93 [micro]m), Riso ([DELTA]d = -136 [micro]m) and Indigo TS ([DELTA]d = -173,95 [micro]m).

5. CONCLUSION

The investigations have shown their justification in the scientific sense and in application. The different types of digital black white machines generate different ink layers on the adapted non coated printing substrate for digital printing by the application of the specific toners. The liquid toner based on the uncapsulated pigment, will realize the different deviations in the inking density on different screen values (in the range from 10% screen value). For the same screen areas, the black liquid EP ink realizes minimal deviations within the light and medium tones ([DELTA][D.sub.1-10%] = 0,01; [DELTA][D.sub.45-55%] = 0,04), but great deviations in dark areas as well ([DELTA][D.sub.91-100] = 0,36). Paste offset ink realizes identical deviation in inking density in lighter and medium tones ([DELTA][D.sub.1-10%] = 0,06; [DELTA][D.sub.45-55%] = 0,06), while in the darker areas it grows for 100% ([DELTA][D.sub.91-100%] = 0,12). The inking densities of the water-oil black emulsion toner behave according to the whole screen value. With the increase of the screen value, the range of inking densities constantly grows; for the light areas ([DELTA][D.sub.1-10%] = 0,10), medium areas ([DELTA][D.sub.45-55%] = 0,13) and for the dark areas ([DELTA][D.sub.91-100] = 0,21). In relation to the tested toners, the powder two component toner will oscillate greatly in its light areas ([DELTA][D.sub.1-10%] = 0,15). In the middle area, the oscillation is also important ([DELTA][D.sub.45-55%] = 0,19), while in the darkest area the saturation and constancy appear ([DELTA][D.sub.91-100] = 0,02). The toner obtained by EA enables the highest quality in black white printing. The inking density deviation grows proportionally with the increase of the screen value ([DELTA][D.sub.1-10%] = 0,07; [DELTA][D.sub.45-55%] = 0,24; [DELTA][D.sub.91-100%] = 0,41). In this way the thickest ink coating is achieved ([DELTA][D.sub.max] = 2,14) while the tone values remain open. The densitometric results are confirmed by the image analysis. Liquid inks do not succeed the preciseness of the defined printing elements, so the small gain of the positive printed elements was noticed, i.e. the greater decrease of negative printing elements. The positive elements printed with powder toner reproduce smaller line, and only the toner obtained by emulsion aggregation corresponds most closely to ideal width. Algorithm of printing elements formation influences that movement. The constructions of simple single coloured units have lower resolution in relation to the multicoloured ones. Riso printed lines oscillate multi dimensionally. The tolerance threshold in quality is very low which enables the printing of black white applications with lower demands.

6. REFERENCES

Bolanca, S.; Majnaric, I. & Pigac, S. (2005) Digital Printing with Increased Ink Layers, Graphic Arts Technology design communications: Z. Bolanca, M. Mikota, (Ed.) 131- 142, University of Zagreb Faculty of Graphic Arts, ISNB 953-96020-3-3, Zagreb.

Goldmann, G. (2004) The World of Printers, Oce Printing Systems GmbH, ISBN 3-00-001081-5, Dusseldorf.

Larson, J. R.; Gipson, G. A. & Schmidt, P. S. (2002) Liquid Toner Materials, HandBook of Imaging Materials, 2nd edition, A.D. Diamond and D. S. Weiss (Ed.). ISNB 08247-8903-2

Majnaric, I.; Bolanca, S. & Golubovic, K. (2007). The Influence of ElectriInk Pigmentation on the Quality of Indirect Digital Printing, Proceedings of 11th International Conference on Printing, Design and Graphic Communications, Z. Bolanca (Ed.), p.p. 85-89, ISBN 978953-96020-7-7, Zadar, Croatia.

Suzuki, C.; Takagi, M. & Inoue, S. (2003) Toner Characteristics and Xero Interactive Performance of EA Particles with Specific External Additives, Proceedings of International Conference on Digital Printing Technologies NIP 19, IS&T, p.p.134-137, ISBN 0-89208-247, New Orleans, Louisiana, USA.

Thompson, B. (2004). Printing Materials: Science and Technology, Pira International, ISBN 1 85802 981 3, Surrey.

Williams, C. (2001). Printing Inks, Digital Demand: The Journal of Printing and Publishing Technology 6(1), (Pira International) 40-47, ISBN 1471-5694

Tab. 1. The thickness of the reproduced positive and negative
650 [micro]m lines

PRINTERS Measured width
 of 650 [micro]m
 line

EPSON 2400 POZ 690.81 [micro]m
 NEG 523.07 [micro]m

HP 9050 dn POZ 626.11 [micro]m
 NEG 555.56 [micro]m

HEIDELBERG QM46 POZ 632.17 [micro]m
 NEG 616.69 [micro]m

HP INDIGO POZ 685.35 [micro]m
 NEG 474.05 [micro]m

RISO RZ970 E POZ 567.56 [micro]m
 NEG 514.00 [micro]m

XEROX DC 250 POZ 648.59 [micro]m
 NEG 519.02 [micro]m