Flash Fusing in Electrophotography AL-Rubaiey, H.*, Hartus, T.**, Oittinen, P.* *Aalto University, School of Science (Aalto SCI), Department of Media Technology, P.O.Box 15500, FIN-00076 Aalto, Finland, http://media.tkk.fi **Aalto University, School of Chemical Technology (Aalto CHEM), Department of Forest Products Technology, P.O.Box 16300, FIN-00076 Aalto, Finland, E-mail: alrubaiey@gmail.com, Timo.Hartus@aalto.fi, Pirkko.Oittinen@aalto.fi ABSTRACT Fusing or fixing is the final stage in electrophotographic printing processes. In this stage, the transferred toner is sintered, spread and penetrated into paper. Flash fusing is one of several non-contact fusing methods available. It could be implemented alone as the main fusing method or in combination with any other fusing technique, depending on the desired quality level of given applications. Commercially, flash fusing is usually combined with pressure fixing as a post fusing method in high speed black and white printing, to remove the gloss and improve the adhesion quality. It has been used alone to enable high-speed, high-quality digital colour printing of about 400 pages per minute (ppm) to compete with newspaper printing in the on demand printing market /1,2/. This study focuses on the mechanism of Near-Infra-Red (NIR) flash fusing. At the outset, the hypothesis was that the method would be very efficient due to lack of absorption of NIR radiation by paper. Also, the spherical shaped toner used requires less fixing energy than irregular toner. Experiments were carried out in a separate flash fusing unit, which was built to be operated at controllable flashing energy, pulse width and damping parameter. The influences of flashing energy levels on irregular and spherical shaped toners with different thermal properties were determined. Unfused images at different grey scale levels were transferred by electrostatic means to coated and uncoated paper grades by a commercial laser printer. Image adhesion was evaluated by a modified controllable tape test. The results show that the fixing quality, in terms of image density after fusing and image adhesion, was higher on coated paper than on rougher uncoated paper, irrespective of their thickness. A spherical toner was better than irregular toners. This was interpreted to be due to the even property distributions arising from the regular shape of single particles. Initially, flashing energy is consumed by the toner particles to cohere to each other, and the rest of the energy -if any remains- for melting and penetrating into the paper. Hence, the pulse width needs to be optimized for an effective fixing process. The optimization between the effective flashing energy (J/cm 2 ) and the pulse width should be related to the toner composition that determines the fluidity of the toner /3/. This article is an extended research paper of the two pages summary that was published under the title of "The Influence of Flash Fusing Variables on Image Fixing Quality" /4/, in which the main research idea was introduced. Also, some results were discussed in the doctoral thesis titled TKK-ME-D-3 "THE ROLE OF PAPER AND PROCESS TECHNOLOGIES FOR MECHANISMS AND IMAGE QUALITY IN DIGITAL ELECTROPHOTOGRAPHY", published by the Department of Media Technology-TKK, on 18.12.2009 /5/. 1
INTRODUCTION Additional to print evenness and normal optical quality such as gloss and density, toner adhesion to paper is one of the most important quality parameters in electrophotographic prints /6/. This paper studies toner adhesion by using flash fusing technology. Print quality as a function of toner adhesion is influenced by adjustable fusing parameters, and toner and paper properties. In the fusing stage, the energy is applied as conductive heat transfer in a roller nip or as radiation from a Xenon flashing unit. The main process variables in contact roller fusing are nip width, pressure and temperature. Dwell time is another variable; its magnitude is determined by nip width and printing process speed. In addition to speed limitations, problems in contact fusing can also arise due extreme heat applied to the whole thickness of paper, which leads to drying of the paper and which may cause difficulties in paper handling. The pressure may also cause image deformation and paper curling /7,8/. Toner adhesion takes place when penetrating into the paper at melting phase. Flash fusing has been integrated with electrostatic jumping toner transfer as the main fusing method in high speed electrophotographic machines for high image quality reproduction /2, 8-13/. In such technology the toner image does not come in contact with any surface during the printing process stages. In combination with some other fusing method, flash fusing is usually used with Electron Beam Imaging (EBI), also called ionography. In this technology /9/, the paper is pre-heated before the magnetic toner is transferred to it under high pressure (transfixing). Gloss is produced by high pressure. The flashing unit is used to remove the unwanted gloss and improve the fixing strength. In this paper, flash fusing is introduced as the main fusing method in the normal electrophotographic process. In this case, the parameters of the fusing unit and the fusing process are different from those in contact fusing methods. Because papers reflect the wavelengths in the visible and NIR range, most of flashing energy is absorbed by toner particles, in a mechanism similar to that of Black Body. If the toner is of irregular shape, then the first part of the flashing energy will be consumed to change the toner to the regular spherical shape /10/. This means that the use of spherical toner in electrophotography would require less fusing energy than irregular shaped toner when other conditions are the same. FLASH FUSING VARIABLES There are three groups of variables which influence image fixing quality. The first group of variables is associated with the use of different substrates or different paper grades such as, smoothness or roughness, surface chemistry, coating materials, and the colour of the paper. Second, the variables are associated with toners, which have different properties such as NIR absorption, shape and particle size, viscosity, melting energy, temperature of glass transition, surface energy, thermal conductivity, heat capacity, colorants and carbon black concentration, the polymer resin selected, and additives. The third group of variables is associated with the flashing unit and include the pulse width and the intensity of energy supplied. These three groups of variables should be optimized in connection with other printing process parameters to produce high image quality at high speeds. 2
The optimization between the effective flashing energy and the pulse width values should be done in relation to the toner composition that determines the fluidity of the toner /3/. An energy of about 0.93 J/cm 2 supplied by a short pulse (from 0.5 to 1 ms) tends to produce excess surface temperature on the toner, which causes the toner to vaporize. The same energy delivered to the toner by a longer pulse (2 to 5 ms) means that the toner material at the toner-paper interface remains in a very viscous state. This condition prevents wetting and spreading of the toner on the paper, but allows for coalescence of the toner particles, which at the end is seen as image fading. The net effect of applying a longer pulse is to allow a toner pile to remain in the coalescence stage of fusing at energies which, if applied in shorter pulses, would have driven the toner to a much higher temperature and resulted in good image enhancement. All flashing systems feeding energy with shorter pulses ( 1 ms) have one common disadvantage, their power supplies require capacitors, which add considerable cost, weight, and size to the system. Longer pulse energy (> 3-5 ms) can be drawn directly from a 117 volt alternating current line which would eliminate the need for capacitors /3/ and enable the flash fusing system to be installed in small desktop machines. DESIGN OF FLASHING UNIT The output energy of the experimental flashing unit is determined by many factors. Some of them are constant, such as Xenon lamp specification, input voltage V, triggering, UV filtering, fusible area and optical reflecting system. Figure 1 shows the distribution of flashing energy resulting from the configuration of the optical system in our experimental flashing system. Other factors are variables, such as the capacitance C and the inductance L in the electric system, used to control the output energy E and the pulse width t 1/3 according to Equations 1 and 2: E = ½ CV 2 (Eq. 1) t 1/3 = 3 (LC) 1/2 (Eq. 2) The time constant t of the electric system is defined as: t = 3t 1/3 or t = 9(LC) 1/2 (Eq. 3) For a specific flash lamp, pulse width, and energy, there is only one value for C, L, and V that will result in critical damping parameter a, which is equal to 0.8 in this experimental flashing unit. The objective is that the substrate, the paper should reflect NIR radiation whereas the printed area should absorb as much as possible. This means that a small image area will be fused better than a large image. 3
Figure 1 Ray tracing of flashing energy in the experimental set-up. EXPERIMENTS Four different black toners were used in this experiment; three of them have irregular shapes, with fairly different thermal properties. Table 1 and Figures 7 and 8 characterise the properties. The tones have been originally designed to be used with three different printers employing different sets of electrophotographic process parameters. The fourth toner differs from the others as it is a chemically produced spherical toner and contains about 15 % wax of low molecular weight. All these toners were transferred to two different paper grades to produce unfused samples under constant electrostatic transfer conditions. The papers were 80 g/m 2 uncoated (base paper), and 100 g/m 2 double-layer pigment coated paper. About 13 % of styrene-butadiene (SB) latexes were used as binders in the coating colour formulation of the paper. SB latexes are modified copolymers of styrene latex (hard monomer) and butadiene latex (soft monomer) at suitable ratio /14/. The latex particle sizes range from 50 nm to 300 nm, and their maximum glass transition temperature (T g ) is about 20 C less than the minimum (T g ) of toner. The purpose of the binders are to bind the pigment particles to each other and to base paper, and partly filling of voids between pigment particles to create a porous coating structure. Consequently, the binders improve the functional printability of the coating layer in terms of toner adhesion, print evenness and toner-coating layer interaction for high quality visual appearance of printed image. The thermal compatibility between toners and latexes has led some toner manufacturers to produce chemical toners using macro-monomer latexes. This is a clear improvement in toner production allowing the control of toner particle shape and size within narrow size distribution, with consequent high performance and better print quality /15/. The test image of the experiment is composed of one solid print patch (100%) and five grey areas of constant percentages; 75 %, 50 %, 25 %, 10 % and 5 % as shown in Figure 2 from left to right respectively. The samples were fused under five different energy levels with different pulse widths; E 1, E 2, E 3, E 4 and E 5 listed in decreasing order. One region was left unfused. This is indicated by (NO) in Figure 2. 4
Figure 2 The experimental design for each test print. The optical density was measured from all fused and unfused regions. Image enhancement occurs if the density is improved after flash fusing for a given image area, otherwise the image is de-enhanced. Since the enhancement evaluation is an optical tool, it does not show how strongly the toner has adhered to the paper. Calculation of adhesion as percentage (adhesion %) was measured by using a controllable tape test in which the tape is first pressed against the fused image by controlled and repeatable pressure. In the second test the tape is pulled from the paper by controlled and repeatable force, speed and angle. This was achieved by a device designed according to ASTM D 1894. Radiant energy absorption and surface energy were measured by established means for toners and papers. Moreover, toner thermal properties such as viscosity, melting energy, glass transition and the change of specific heat were measured as well. RESULTS AND DISCUSSION The radiation spectra illustrated in Figure 3 were obtained by a diffuse reflectance measurement method. The figure presents the visible and NIR radiations spectra of the 100 g/m 2 paper, black and yellow toners. The reflectance spectrum of the paper lies at a significantly higher level than that of the black toner at both visible and NIR radiation regions. The radiation energy is mostly absorbed by the black toner. It is important to mention that different white paper grades have nearly the same spectra of visible and NIR radiation. This is also true for different black toners. The colour of any material is the dominating factor to determine the absorbed radiation in this range. Also, Figure 3 shows that a yellow toner reflects almost all the radiation in the ralevant range. Magenta and cyan toners produce nearly identical NIR radiation spectra. 5
Figure 3 NIR radiation spectrum of 100 g/m 2 paper, and black and yellow toners. Flash fusing for colour electrophotography is possible by adding a certain proportion of NIR absorbing additives to the colour toners to reach roughly the same energy absorption level. High heat sensitivity of NIR additives is required in flash fusing to ensure fast thermal transfer through toner layers to the substrate surface /13/. The achromatic nature of those additives will produce high darkness level of the coloured image /16/. To avoid that, one solution would be to add as small a portion as possible of those additives and increase the flashing energy at the same time. Density measurements in Figure 4 show the relation between unfused and fused image densities. The image enhancement is different on the two papers. It is higher on the smoother coated paper than on the rough uncoated paper. Figure 4 Enhancement phenomena from fused and unfused image densities for two paper grades. 6
Image enhancement occurs (optical image density is raised) after fusing the halftone images in the range from 0.15 to 0.75 unfused image densities. In this range, the toner particles tend to cohere to each other creating a network across partially separated printed dots to cover the empty area in halftone images. Even though the density increase after flash fusing is lower compared to contact hot roll-pressure fusing due to the absence of contact and deformation effects, it is still higher at the halftone part than the solid part of the image fused by flash fusing. This is obviously due to the spreading of the melted toner horizontally in a parallel direction to the paper surface to cover part of non printed area in the halftone structure of the image, so the unfused toner area at the halftone structure of an image will consume high fusing energy for toner spreading and the area becomes larger after fusing. And if the halftone image is in such structures that enable the printed dots to be all connected after fusing as a thin film with no more energy available to penetrate into paper, the removal of this thin film image from the paper will be easier than the unfused image, which means the adhesion % is very low. The adhesion % curve of Toner C (cf. Table 1) is an example of this condition in Figure 5. It shows a reduction in adhesion % as the toner amount increases and halftone structure changes to a solid image (toner C requires higher melting energy compared to toners A & B as shown in Table 1). This mechanism disappears in the sold image, or is limited only to the edge areas. The spreading of toner in solid image does not take place horizontally, but mostly vertically in relation to the paper surface causing toner adhesion. So, most of the energy consumed by toner spreading at halftone areas is consumed by toner adhesion at the solid areas of the image. This explains the high adhesion % of spherical toner as well as Toners A and B grey level about 75 %, as illustrated infigure 5. If the pulse width is short and the time is not enough for the toner to penetrate, then certain degree of coalescence will occur. Therefore, the reduction of the fused image density in Figure 4 above the density level of 0.75 is usually due to coalescence /10/ and after the adhesion of a dense toner image (solid image), the optical density is not increased in spite of a further increase of toner amount /17/. These are the two boundary cases which influence the fusing latitude. Another boundary case is one where the flashing energy is so small that it can melt only the upper surface of the toner layer. In this situation, there will not be significant change in optical image density, and the melted layer can be removed easily by the adhesion test, producing adhesion % values around 50 %. The image produced by Toner B at the grey level of 50 % is an example of this situation as shown in Figure 5. If the fusing energy is lower the same value of adhesion % is expected to appear at higher grey levels of the image printed by the same toner (toner B) and the same paper grade. That was exactly the experimental case, when the energy was reduced from level E2 = 2.64 J/cm 2 to the level E3 = 2.30 J/cm 2. These three boundary conditions of fusing quality as a function of flashing energy, pulse width and toner properties, could also be used to explain the rest of adhesion % results presented in Figure 5. 7
Figure 5 Adhesion % for the black toner samples fused under the same flashing conditions. The three cases where discussed above considering no change in the paper. If the paper is replaced by another grade such as coated paper, then the surface profile and chemical composition of the coating layer lead different thermal properties which will result in different toner-paper interaction. Subsequently, the limitations of the three boundaries move upward in the quality-scale for better fusing quality and image appearance. This forward shift in the quality scale was examined by the coated paper used in this experiment. The range of fusing latitude and the boundary limitations can move downward for worst fusing quality if the coating materials are inadequate for the toner printing technology. In this regard, the result in Figure 3 is evidence to prove that the paper grade has an influence on flash fused image quality. In this figure, a given toner printed in the same process conditions has produced two levels of optical densities because two different paper grades were used to print the image. In the use of flash fusing where no pressure is applied, it is important to consider the smoothness and chemical compatibility of the substrate surface, such as the thermal reaction between the toner and the binders in coated paper at the early stage of fusing, near by the glass transition temperature. Figure 5 is an example of five plots of adhesion % vs. grey scale % for the toners fused under five energy levels. All of the data are governed by the boundaries of the fusing mechanism described above. As the effective fusing energy increased the fusing quality and toner adhesion % moved upward on the quality scale. All data suggest that the spherical toner has better adhesion than the others. Microscopic observation reveals that the toners consume the first part of the energy to become spherical which explains the difference. Figure 6 shows two irregular toner particles turned into spherical shape after flashing them with a small amount of energy. In the second step the toner particles cohere to each other consuming energy to create a thin film of toner layer, and the rest of the energy -if any remains- consumed by melting, spreading and penetrating into the paper. 8
Figure 6 Microscopic observation of toner particles turned into spherical shape after applying suitable flashing energy. It was also clear from thermal analysis and viscosity results presented in Table 1 and Figure 7 that the spherical shaped toner, which was produced by chemical method, has a lower melting energy value and different viscosity behaviour than the irregular toners. The wax of low melting temperature inside the toner particles causes melting of the toner at low melting energy. Also wax helps to wet the paper during the final phase of the fusing process, which is very important for toner penetration into paper. Table 1 Thermal properties of the toners (DSC analysis). Toners Melting energy, J/g at a range of 100-200 C Tg, C Change of specific heat, J/(Kg) Toner A 526 67.6 0.18 Toner B 360 64.9 0.31 Toner C 881 65.6 0.37 Spherical toner 333 66.6 0.32 The values are from the analysis made with a closed crucible. Figure 7 Viscosity measured in oscillation mode (10 Hz) of black toners at different temperatures. 9
Figure 8 underlines the importance of a significant difference between the surface energy of paper or any other substrate and the surface energy of toner to ensure good toner adhesion. Usually, surface energy of any toner is designed according to the fusing temperature to avoid both cold set and heat set offset. In general the toner is designed to have a surface energy less than 35 mn/m (35 mj/m 2 ) at 150 C and in contact fusing the silicon oil usually gives rise to much lower surface energy than that of the toner to ensure efficient toner releasing from fixing roll /18,19/. From the substrate or paper side on which the toner image will be fixed, the surface energy should be sufficient for toner adhesion. Therefore, paper surface modification is required to obtain high surface energy to improve the print quality and toner adhesion. This is done with electrical corona discharge treatment /20/, especially for coated paper and packaging board used in toner based printing technology. Figure 8 Surface energies of paper and toner samples, calculated from contact angles of two references at various temperatures using a novel method. CONCLUSION The results suggest that flash fusing has a potential for high speed and high quality digital electrophotography, especially with rapid development of chemically produced toner. By optimizing the paper, toner and the process variables with respect to one another, desired fixing quality can be achieved. Overall, the flash fusing energy should be sufficient for heat transfer from the upper surface of the toner layer to the toner-paper interface to obtain a temperature equilibrium condition between toner and paper or any other substrate. Spherical toner containing a low molecule wax and produced by a chemical method showed better performance in the printing process than conventional toners, particularly at the fusing stage. The smaller particle size, narrow size distribution and an even spherical shape are required parameters in toner charging, toner transfer as well as toner fusing in contact and non contact methods, to reproduce high digital image quality compatible with offset print quality. 10
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