Influence of paper surface characteristics on digital printing quality

Abstract

Today's demands for high quality digital printing emphasise the importance of paper surface. This work demonstrates the effects of surface topography and appearance on printing quality. Samples were printed in black and white with a laser jet printer under identical conditions, using different types of paper in order to have different surface properties. The papers and printed samples were evaluated with atomic force microscopy (AFM), scanning electron microscopy (SEM), gloss meter, spectrophotometer, and densitometer. Results showed that the decrease in surface roughness increased optical density and print quality. Moreover, the gloss of the papers significantly influenced print quality by decreasing it. The effect of the colourimetric values of the papers on the colourimetric values of the final printed samples were less significant in higher optical density printed samples.

Keywords

Surface property Print quality Atomic force microscopy Optical microscopy Scanning electron microscopy

Introduction

Digital printing is the newest generation within the printing system used in the publishing and printing industry. Electrophotographic digital printing like laser jet printer is currently a popular choice among digital printing technologies for applications that require high quality print, low cost, fast printing process and when a wide range of substrates are of critical importance.^1,2 In laser jet printing as well as other printing technologies, print quality is influenced by the characteristics of the substrate, the ink (toner), and the specific technology.³ Toner, considered as the ink in laser printing, is composite of polymer, colourant, charge control agent, magnetite, and other additives.¹ The exact use of electrophotography can vary from one manufacturer to another, but the basic principle remains the same. Electrophotography relies upon six main steps:⁴ first and second, the charging of a photoconductor whereby the photoconductor is exposed to the image; third, the development of the latent image on the photoconductor; fourth, transfer of the toner from the photoconductor to paper or other printing substrate; fifth, fusing the toner onto the printing substrate; and sixth, cleaning the photoconductor for later use. Understanding, controlling, and optimizing of each step improves print quality.⁵

In order to achieve high quality print, electrophotographic printing paper and toner requires controlled characteristics. Smaller size toner produce smaller pixels on the image and a high quality print.¹ Paper properties in electrophotographic printing like surface roughness, gloss, whiteness are also more important in print quality and specially more important than other printings technique.^6,7 The effects of substrate on print quality using conventional printing methods have been reported.⁸ This subject has been addressed to a lesser extent in digital electrophotographic printing, although the negative effect that substrate roughness has on the quality of electrophotographic colour prints, especially in high-speed printing, is well-known in the industry.⁹ Wright and et al.¹⁰ also reported analogous qualitative observations in this area. Concluded from Provatas and et al.¹¹experimental studies, print density is dependent on filler distribution in paper and on variations in thickness, but they did not indicate whether there were any differences in roughness between their trial papers. Both quantitative and qualitative effects of paper roughness on print density in monochrome printing have been reported, based on sets of uncoated commercial paper products.¹²

As mentioned before, the correlation between print appearance and paper characteristics in printing are well known; however, few attempts have been made to study how print appearance changes according to paper characteristics in electrophotographic digital printing.^7–13 Thus, the aim of the present work was to investigate the effect of paper characteristics (including gloss, roughness, and whiteness and colour properties) on digital printing quality.

Experiment

Samples and printing trials

Six paper samples with different surface structures, the selected papers differed in gloss, whiteness, or texture, were printed in a controlled environment [23°C, 50% relative humidity (RH)] using a monochrome laser-jet printer (HP 1320, Laser-jet printer). This printer has a hot roll fusing system containing two metal rolls covered with silicone rubber and heated from inside the rolls. The papers are specially selected and supplied from different manufacturers to present various properties.

Print quality and paper property measurement

The specular gloss of each paper and printed sample was measured using a Novo-Gloss IQ Goniophotometer manufactured by Rohpoint in two angles of 60 and 85°. For each paper and printed sample, the spectral and gloss measurements were taken five times on different sections.

The optical density of the printed samples was measured with a spectrophotometer S900 (Ihara, USA). Optical density is the percentage of absorption by the ink film of the light falling upon it. The logarithm mathematically represents the relationship between the amounts of light reflected from a point on the ink film to the amount of light falling on the same point (equation (1))¹⁴ (1) where L_W is the intensity of light reflected from the white region of the paper, and where L_R is the intensity of light reflected from the process ink region printed on the paper.

The printed paper was evaluated with Dinocature optical microscopy (Twain) at a magnification of 200.

The spectral data and whiteness of the paper and printed samples were measured between the range of 400 and 800 nm with 10 nm intervals with a GretagMacbeth (now XRite) Macbeth Colour Eye 7000A spectrophotometer (USA), which is an instrument with 8/d geometry. The spectral reflectance factor of all samples was determined and then transformed into CIELAB colourimetric coordinates (L*, a*, b*) using CIE standard illuminant D65 and a CIE 1964 standard colourimetric observer. An increase in L* indicates lightening of the sample. A positive Δa* signifies a colour shift toward red; a negative Δa* signifies a colour shift toward green. Similarly, a positive Δb* signifies a colour shift toward yellow; a negative Δb* signifies a colour shift toward blue. The paper whiteness is calculated from the measured colour spectra of the paper using D65 illuminant (equation (2))^1,14 (2) Y₁₀ is the Y-value, x₁₀ and y₁₀ are the chromaticity coordinates of the samples and x_n10 and y_n10 are the chromaticity coordinates for D65 illuminant with standard observer.

Paper surface roughness was characterised using an atomic force microscopy (DME 2401, DualScope TM Microscope, Denmark) over a fixed area of 10×10 mμ and scanning electron microscopy (SEM, KYKY-EM3200, China).

Results and discussion

Colour characteristics of paper and printed sample

Table 1 shows the gloss value of the papers and the printed samples. The selected papers covered a wide range of gloss from 3·2 to 32 units in 60° and from 1·8 to 58·8 units in 85°. Table 1 also shows the optical density of the printed samples. As indicated, the trend of gloss values for printed samples was similar to the trend of gloss values for papers, but with a reduction. The results showed that paper 5 with the highest gloss showed the lowest optical density and low printing quality. The optical microscopy images of the printed sample confirmed this observation (Fig. 1). It seems that the low quality of printing on paper 5 was caused by the high temperature needed to fuse the toner on this paper. While the fusing temperature is fixed for all samples, this phenomenon causes low quality printing and optical density.¹⁵ Clearly, one difference in paper 5 is the additional energy that is needed to soften the toners. This is one possible explanation for the higher fusing temperature needed to achieve high quality and high optical density printing on paper 5.

Table 1

Gloss value and optical density of papers and printed papers

		Paper number
		1	2	3	4	5	6
Paper	Gloss 60° (GU)	4·2	4·6	3·8	3·2	32	3·1
	Gloss 85° (GU)	6·9	3·3	4·5	1·8	58·8	0·9
	Whiteness	111·88	45·70	68·12	113·14	87·37	114·15
Printed paper	Gloss 60° (GU)	1·6	1·6	1·3	1·8	7·8	1·3
	Gloss 85° (GU)	20·3	11·4	3·8	1·7	15·3	4
	Optical density	1·14	1·12	0·70	0·47	0·36	0·97

The rather low quality optical densities of the printed sample also observed in papers 3 and 4 were confirmed by the optical microscopy image results (Fig. 1). The reason for this observation is the high roughness of papers 3 and 4 which complicated the fusing of the toner into the paper (see the section on ‘Morphology of paper and printed sample’). If the toner is less uniformly distributed on the paper, it is harder to make the toner flow and form a smooth, high quality surface.¹⁶

Figure 2 shows SEM images of printed papers. The results show that on the surface of papers 1, 2 and 6 with highest optical density and highest print quality toners are completely fused and create a uniform film. While on the surface of papers 3−5 with less optical density and lower print quality, incomplete fusing and film formation of toners because of high roughness or high gloss of papers can be seen. The textures of papers behind the toners are noticeable in those images. White dots in the SEM image of paper 5 are created because of low heat resistance of this paper in front of the SEM electron beam.

The low quality of printing and low optical density, which resulted from the bad toner coverage in papers 3−5 caused the detection of papers reflectance from the printed samples. Figures 3 and 4 show the reflectance diagram of the papers and printed samples. The results show that in papers 1 and 2, which had the highest optical density (∼1·1), the paper reflectance was fully covered (see, for example, peak at 480 wavelength nm). While in the low optical density papers, paper reflectance was totally clear. The peaks in 480 wavelength nm come from the optical brightener which is used in paper production.¹⁷ The optical brightener in the papers 1, 4 and 6 increases those paper's whiteness upper than 100, and results in blue−white appearance (negative b* value in Table 2).

Table 2

Colour properties (CIELAB coordinates) of papers and printed papers

		Paper number
		1	2	3	4	5	6
Paper	L*	91·322	92·269	92·332	93·463	93·061	93·744
	a*	1·572	−0·99	1·369	2·459	−0·021	2·603
	b*	−10·893	6·949	−0·333	−9·564	−1·003	−10·039
Printed paper	L*	27·053	29·237	46·666	59·937	56·134	35·684
	a*	0·493	0·004	0·723	1·602	−0·056	0·963
	b*	−1·784	0·827	−1·317	−6·622	−1·265	−3·522

Morphology of paper and printed sample

Figure 5 illustrates the surface properties of the papers and the printed samples in 3D topographies. The roughness parameters calculated from the 2D image are also summarized in Table 3. The definition, abbreviation, and formulation of each parameter are as follow:^18,19

Table 3

Surface roughness parameters of papers and printed papers

	Paper number
Parameter	1	2	3	4	5	6
S _y	1·56 μm	1·81 μm	1·98 μm	2·68 μm	989 nm	31·3 nm
S _z	931 nm	1·51 μm	1·52 μm	1·68 μm	922 nm	26·0 nm
S _a	141 nm	275 nm	279 nm	254 nm	109 nm	3·61 nm
S _q	182 nm	236 nm	336 nm	323 nm	138 nm	4·47 nm
S _c	4·36 nm	5·08 nm	5·55 nm	43·9 nm	2·77 nm	87·6 pm

(i)

range (S_y): the Z value range in the specified area

(ii)

ten point height (S_z): the range from the mean height of the five tallest summits to the mean depth of the five deepest valleys

(iii)

arithmetic mean deviation (S_a): the mean of the deviation from the area mean Z value

(iv)

root mean square deviation (S_q): a measure of the dispersion of the data from the area mean

(v)

core void volume (S_c): this indicates the ability for fluid retention in the core zone of the area.

The results showed that, except for a little change, the variations in the papers surface roughness happen in the same manner. The high roughness of papers 3 and 4 make them inappropriate for printing; thus the optical density and quality of the printed samples are low. Contrarily, the low roughness of papers 1, 2, and 6 increases the quality and optical density of the printed samples (see Table 1 and Fig. 1). The low surface roughness of paper 5, however, does not have the same effect on printing quality as other paper with low surface roughness (papers 1, 2, and 6) due to its high gloss and low porosity and texture.

The SEM micrograph (Fig. 6) illustrate the surface textures of the papers. The results showed that the higher texture and porosity of surface papers, if it does not lead to a very high level of surface roughness, cause better fusing and printing quality. The SEM images (Fig. 6) show that paper 5 has a very non-porous surface in addition to its high gloss, which makes fusing difficult. Paper 6, on the other hand, had a very smooth surface based on the roughness parameters, but it also has high texture and porosity, leading to high optical density and good printing quality (Fig. 6).²⁰

Conclusion

In this study, the effects of paper appearance (gloss, whiteness and coloirimetric properties) and paper morphology (roughness and texture) on electrophotographic printed samples were investigated. The study was applied to 6 different types and brands of papers using a black and white laser jet printer. Coloirimetric and optical properties such as gloss, whiteness, and colour values were measured. The results showed that high gloss paper (58 GU in °85) requires a higher temperature for fusing and toner flowing. Therefore, in comparison, to other papers, it shows less print quality and optical density (0·36). The coloirimetric properties of the papers show a significant effect in low optical density printed samples. AFM and SEM results show a significant influence of the papers surface topography and textures on the printing quality. The effect of a paper's roughness on the final printed samples is important, and increasing the surface roughness parameter S_y from 31·3 nm to 1·56 μm causes a 70% decrease in optical density.

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