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THE EFFECT OF COATING COLOUR ON TONER TRANSFER
Coating is meant to improve the printability of paper. This can be achieved directly or indirectly through the improvement of related paper properties to suit one or more printing methods. In digital electrophotographic printing where dry toner is used, the first interaction between toner, paper and printing process parameters is the electrostatic interaction at the stage called toner transfer. At the transfer stage a sufficient charge density is accumulated on the paper surface and an electric field is created between the paper and the toner particles. As a result, the charged toner is electrostatically transferred from the photoconductor (PC) to the paper. Since the effect of the paper as a resistive layer is essential to create the surface charge density, it would be important to understand the influence of different coating colours on the electrical behaviour of digital paper under different conditions.
The experiments for this study were carried out in a real printing process by printing a base paper, coated by different coating colours and then calendered under similar conditions. The transfer current and toner amounts were measured to characterize the transfer situation under given humidity levels. The transfer efficiency represented by transfer current was examined as a function of physical paper properties. The experimental results show that the amount of toner transferred to the paper and the transfer current for a certain coating colour under certain humidity conditions are directly related to each other and are functions of the transfer voltage. Overall, the relative humidity (RH %) is a very important variable even within fairly narrow ranges. In addition, the results show that coating colour formulation controls the electrical behaviour of coated paper.
Toner transfer is one of the seven stages in the electrophotographic printing process. At this significant stage; the toner, printer and the paper interact with each other for the first time during the printing process to produce the final printed-paper. The developed image on the PC can be transferred to the paper surface by using electrostatic forces, adhesive forces, thermal energy, mechanical forces or a combination of different energies [1, 2]. The electrostatic transfer method is the common one in commercial colour laser and LED printers. This method can be implemented by using different marking engine technologies such as roller transfer, belt transfer, an intermediate transfer drum, a transfer corona, and transfer drum (TD). Irrespective of the transfer technology used, the basic principle is to generate a sufficient electric field across the paper to attract and transfer the charged toner particles of the developed image from the PC to the paper. There are two ways of electric field generation for all electrostatic marking technologies. One is ion emission from a corona charger directly onto the paper or across a paper carrier element such as a belt or a drum. The other is to apply a DC voltage via the TD or a transfer roller directly to the paper or across the carrier element.
Fig 1: Configuration of toner transfer zone
This experiment is based on the use of TD technology with such transfer configuration illustrated in Fig 1. In the transfer stage the paper is clamped and attracted to the TD by another element called an attraction roller. Electrical attraction occurs between the paper and the TD when a sufficient transfer voltage Vt is applied to the TD in opposite polarity to that of the toner charge. Part of the transfer voltage is lost because of the resistance of the TD layers and the resistance of the contact region between the TD and the paper. Another part of the voltage is lost because of the resistance of the paper; this part is important for toner transfer and it must be high enough to cause effective polarization of paper and to accumulate a sufficient charge density on the paper surface to allow an electric field to be created between the paper and charged toner particles. The electrostatic force of this field will overcome the adhesion of toner particles on the PC and transfer them to the paper. So the transfer voltage must be high enough to overcome and transfer a large proportion of toner particles to the paper.
Toner Transfer Efficiency
Transfer efficiency η is common term determined as the ratio of the toner amount transferred to the paper, to the toner amount developed on the PC. From the imaging science point of view, the term "transfer efficiency" is more complicated than just a ratio of toner amount; it should also include other efficiency factors such as grey scale reproduction , detail rendering and the accuracy of colour registration, otherwise the toner amount ratio is no more than just "Transfer Coefficient", only. In some researches, the transfer efficiency is given as an evaluation function defined by the ratio of the optical image density transferred to the paper, to the optical density of the same image developed on the PC [1, 3 ]. It is more meaningful to measure the density of the image transferred to the paper before the fusing stage, so that it will be relevant and comparable to the image density measured on the PC. It was found from the result of pre-experimental test presented in Fig 2 that the optical density is not always a good indicator of the toner amount; it reaches a saturation level at a certain limit of the toner amount where the halftone image becomes solid. After this limit, any additional amount of toner will not influence the image density. Instead, the thickness of the image will be increased. Therefore the transfer efficiency can be given as an evaluation function determined by the ratio of the thickness of the image transferred to the paper, to the thickness of the image developed on the PC. The thickness of the image is usually in proportion to the number of toner layers and the layer thickness is determined by toner particle size. So this assumption requires a uniform shape and size of toner particles, and for further theoretical analysis, the toner layer has to be considered as a homogeneous slab, which can be split into two layers at any thickness point [1, 4, 5].
Fig 2: The optical density is almost equal when the toner amount increases steps of overprint (T = toner layer), so the density cannot represent the toner amount.
From fundamental physics, the electric current flows in the electrical circuit across the transfer zone called transfer current It=Vt/RT. Clearly, the current depends on the total resistance RT of the TD layers, the paper, and the contact region between them. This simple idea can be used in many approaches. First, printing an image on different paper grades, under similar printing process parameters and environmental conditions (RH% and temperature), in which case the resistance of the paper grade will influence the transfer current. For example, different paper resistivities will produce different transfer current. Therefore, this approach can be used to study the influence of coating colour, calendering and basic properties on the electrical properties of paper. Second, printing a similar image under similar environmental conditions on a certain paper grade by adjusting the transfer voltage for each print, in which case the total resistance RT will remain the same each time, but the toner amount and transfer current will increase with an increase in the transfer voltage. This approach is used to examine the transfer efficiency as a function of transfer voltage. More than one trial for different paper samples will examine the influence of different papers on the toner transfer coefficient. And finally the third approach was to print a similar image under many different levels of RH% and keeping all other variables constant. The same paper grade has a higher moisture content (m.c.%) at a higher level of RH%, so the volume and surface resistivities will be reduced [3, 5, 6]. In other words, the paper becomes more conductive and cannot be polarized effectively, and the surface charges leak through the paper thereby decreasing the surface charge density. This approach is very important to be able to understand the behaviour of paper and the transfer process under different environmental conditions.
Multi-pass desktop colour laser printer with a transfer drum configuration was equipped with two devices for adjusting the transfer voltage and monitoring the transfer process by measuring the transfer current. Fig 3 illustrates the levels of optimum transfer voltages and two other cases where certain levels of the voltage were adjusted. The system allows the flow of current in the transfer zone to be recorded as the photoconductor revolves. Fig 4 shows two different currents resulted from the two adjusted voltages.
The experiments were carried out in a real printing process by applying different transfer voltages to print different commercial paper grades under given humidity levels. The paper grades were selected to fit the purpose of this research. They are uncoated, coated by different coating colours and calendered. The coating materials are same in but different proportions were used to coat each sample. The transfer current was monitored and measured for each case. The transferred toner amount was weighed for each print before the fusing stage.
Other electrical properties such as volume and surface resistivities, and potential surface were measured for the same samples under controlled conditions.
Fig 3 monitors three different input settings of transfer voltages during the transfer stage. The dark grey series is the optimum transfer voltage and two other adjusted voltage levels. In the real printing process of the experimental printer, there are nine optimum levels of transfer voltage. Four levels for four different colours; they are V2, V4, V6 and V8, for magenta, cyan, yellow and black respectively. The other five levels are for pre and post transfer of each single colour. The figure shows that the transfer voltage is increased rapidly after each colour print to overcome the extra resistance of the previous toner layer. Each of these voltage levels generates a relevant transfer current.
Fig 3. Adjusting and monitoring transfer voltage levels
Fig 4. Tr. current resulting from two adjusted voltages
The figure also shows the other two cases were the transfer voltage adjusted to 710V for whole nine levels presented by light grey series, and the adjustment of the fourth level (V4) to zero was shown by the black series. It might be important to mention that in the cases where the voltage and the current have zero values, the figures show that there are slightly small values of each of them. These values represent the minimum requirements by the measurement devices to close the electric circuits and operate properly, and they are taken into account if the experimental approaches allow them to influence the results, otherwise they can be neglected, such as in comparable investigation between different current levels of Fig 5.
The flexibility of adjusting the transfer voltage is at any level needed within the latitude of zero to 2000V. It is very important to study the toner transfer efficiency as a function of both; transfer voltage and the resulted transfer current .
Transfer current monitoring provides a lot of valuable information for examining four-colour transfer. Fig 4 shows the current data of the transfer processes relevant to two adjustable cases shown in Fig 3. The real current data were shifted up by 12μA to monitor small and negative values. The black series of transfer current data was resulted from the black series of transfer voltage presented in Fig 3. It was mentioned that the transfer voltage at level-4 was adjusted to have zero value (V4=0) and the other eight levels were 710V. Level-4 meant to transfer the second colour, which in this kind of engine is the cyan colour. The image designed on the computer was cyan rectangle 16cm x 10cm. The result was clearly presented in Fig 4 by zero transfer current at the cyan region. Also, the cyan rectangle was not printed on the paper. This is simply because there is no transfer voltage that can generate enough surface charge on the paper to move and transfer the cyan toner developed already on the photoconductor, instead the developed image was cleaned as wasted toner during the cleaning stage which usually takes place harmonically after the post transfer and before the new development.
Many cleaning techniques are used to remove the residual toner and charges from the PC; one of them is electrostatic cleaning, which means another electrical interaction is going on at the same time with others at different process stages. This is just one example of several phenomena, to sense the complicity of the dry electrophotographic process.
At the light grey series in Fig 3 all the transfer voltages were set to 710V including the V4. In this case the cyan rectangle was printed, and the relevant grey current data in Fig 4 presented both the printed image and the passing of empty part of the paper through the transfer zone. In general, the current is equal to Vt/RT and it will be a function of Vt through its adjustment, and RT due to any changes in paper grades or RH%, because the rest of resistances in the transfer zone are constant such as the resistances of the insulator layers in the transfer drum which they can be effected only by changing the environmental conditions.
In examining the change in the current due to toner transfer, the electrical interaction between PC and TD circuits should be considered. The electricity flows in the PC circuit is to develop a negatively charged toner to be adhered and carried by the PC, and the electricity flows in the TD circuit is to obtain a sufficient surface charge on the paper surface opposite to that of the toner to ensure efficient toner transfer.
RESULTS AND DISCUSSION
Transfer current data shown in Fig 5, were collected during the printing process of four different paper grades at sampling frequency of 10 Hz.
Fig 5. Current data before and after toner transfer
The transfer current data presents two situations in toner transfer stage. First is the current data before the toner is transferred to paper which indicates the surface charge density on each paper grade. This condition is essential as prior to toner transfer. This part of the data shows that the higher the grammage the better the surface charge density generated on the top of the paper, and then the higher amount of opposite charged toner particles, attracted and transferred to the paper. Second part of the data presents the current levels after the toner is transferred to each paper. This level is resulted from the electrostatic interactions between the positive surface charge of the paper and the negative charge of the toner particles.
Obviously, the positive surface charge accumulated on the paper was higher than the negative charge of the toner, which is strategically designed in every laser printer to ensure high efficiency of toner transfer. It is also clear that the difference between the first current level and the second one is the amount of surface charge required for electrostatic neutralisation of opposite toner charge per certain time. Therefore this difference can be called "Toner Transfer Current", which is the best indicator for the surface charge density. It might be important to mention that the neutralisation process of toner transfer usually takes place in image plane area of x-axis and y-axis. If the y-axis dimension of the image is considered to be parallel to the photoconductor cylinder, then the other the x-axis dimension will be time dependent variable. So the charge carried by toner particles measured by (μC), moved and transferred to the plane image area (cm2) per certain time (sec.), and that was called "Transfer Current Density" measured by (μA.cm-2). Because of the dynamic behaviour of the transfer current density, it could be called "Dynamic Transfer Current Density" [2, 6].
In Fig 6, different component of transfer current are shown according to the law of energy conservation. The transfer current provided by the power supply of the printer used in this experiment is at nine different levels and they are in range between 45 to 70μA.
Fig 6. Transfer current components
Fig 7. Toner amount as function of transfer voltage
These levels are generated by nine different values of optimum transfer voltage shown in Fig 3. But most of the current will be damped by many resistances, and the final average value reached in the transfer zone was measured and it was around 17μA. This value is shown in Fig 6 as "total current". The total current across the transfer zone splits into two components; current-1 is the component flow due to the conductive path to close the transfer loop, and current-2, which is the component through the insulating behaviour of the paper. This is the one presents the surface charge density on the paper surface and it was shown as the current data before toner transfer in Fig 6. The total sum of these two current components is always 17μA in this experimental printer, and if one is increased, the other one is decreased by the same value. The reasons of the fluctuation can be different paper grades, coating colour, calendering, environmental conditions and any other variables within the paper and transfer stage parameters. At this situation the toner will be transferred and neutralise some of the surface charge (current-2), and the result after this electrostatic interaction is named "current-2.1". The difference between them is clearly the net value of the charge carried by the toner particles during the transfer stage. This value is called transfer current (Tr. current), which is the only component, can be calculated whereas the rest were measured directly during the printing process by the equipped measuring device.
Fig 7 shows the toner amount transferred to two paper grades as a function of transfer voltage. The toner amount transferred to 100 g/m2 coated paper is more than the amount transferred to 80 g/m2 uncoated grade. Previous result has shown same conclusion in terms of transfer current when the higher grammage paper has obtained higher surface charge density, which in turn can transfer more toner amount.
The rest of the figures show that the grammage or any other bulking properties are not influencing the printability of digital paper separately from other factors. For instance coating and calendering are not just to improve the grammage and other bulking properties of the paper, they can also improve other printability influencing properties such as electrical and thermal properties in the case of dry toner technology. Fig 8 shows the toner amount transferred to 10 paper samples. All the samples were prepared from one base paper, coated by same coating materials but with five different proportions that means five different coating colours were used (A, B, C, D, and E) and the they were calendered by the same conditions. The result shows that each coating colour has provided new identity to the paper in terms of printability, which is, reflected here by volume and surface resistivities and the toner amount. The paper density was increased by the calendering treatment and that improve the toner transfer, but not necessarily good for the rest of printing stages of dry electrophotography. If the hypothesis of this paper is that the transfer current is a good indicator of the toner amount transfer to paper, then the relationship between the transfer current and the volume resistivity in Fig 9 is supporting this hypothesis and the results of Fig 8 as well.
Fig 8. The influence of different coating and calendaring on the toner amount and resistivities.
Fig 9. Transfer current as a function of volume resistivity.
From Fig 8, it is clear that the volume resistivity is directly related to the surface resistivity of the paper (the surface resistivity was measured from both sides of the paper and the average between them was plotted in Fig 8). The papers with high volume resistivity require more energy to be polarised and as a result, high surface charge will be obtained on the surface of these papers to transfer the toner. Therefore the transfer current shown in Fig 9 (the charges carried by the toner particles) is higher and it is clearly related to the volume and surface resistivities of paper.
All the paper properties are somehow interrelated to each other, and they are all influenced by the environmental conditions, especially the moisture content m.c.%. Fig 10 shows two levels of surface resistivities obtained by two conditions (RH=20% and RH=50%). At lower relative humidity, the resistivity is higher and the surface potential of the paper is far better to introduce better toner transfer condition. At higher relative humidity (RH=50%), the moisture content of paper is increased and it will be very difficult to polarise the paper prior to toner transfer. The paper will act as a conductive material more than what it should be as insulating material.
Fig 10. The influence of RH% on the electrical properties.
The toner transfer efficiency was studied from the viewpoint of variables influencing the interactions between toner, paper, printing process parameters and the environmental conditions in which the real printing process is handling. Transfer voltage, transfer current, toner amount, paper grades, coating colour, calendaring, electrical properties and environmental conditions, were the variable considered in this study. Toner properties were not considered directly, but they were involved in terms of charge to mass ratio, which is essential in toner transfer stage. The efficiency of toner transfer to different paper grades was presented as a function of transfer voltage in terms of toner amount. The transfer current was calculated as a difference between the current data before the toner transfer and the current data after the toner transfer, which both were measured with the total current across the transfer zone:
Total Current = (current-1) + (current-2)
The component (current-1) is due to conductive behaviour of the materials involved in transfer zone including the paper during the printing process. The other one (current-2); is the component due to the insulator behaviour of the paper, and its value present the surface charge density accumulated on the paper prior to toner transfer. After the transfer of the toner, this component will be two parts; one is the transfer current (Tr. current) resulted from the neutralisation between the toner charges and the opposite surface charge of the paper. Second component (current-2.1) is the residual charge remains on the tope of printed image. According to this study, the Tr. current responds sensitively to any change in the materials, such as coating and calendaring, and electrical properties of the paper and the q/m of the toner. It is also sensitive to image and process variables such as halftone, size and colour of the image and transfer voltage, and relative humidity. So, under the electrostatic treatment in the transfer zone, the paper behaves as a conductor and as insulator at the same time. The current component consumed by the insulating behaviour to polarise the paper is the most important in transfer stage. This property will be improved by suitable coating colour, or it will be lost from the paper if the moisture content is increased.
The relationship between Tr. current and toner amount was found to be in good quantitative agreement with the ideal transfer profile , and good representative of other electrical properties of paper. So, the Tr. current can be used as a tool to study the electrical behaviour of the paper and all other variables involved in characterising the transfer situation in dry toner technology.
According to the findings of this study, the transfer voltage creates a surface charge density on the paper, which determines the extent of transfer through the net current over the total resistance. When the surface charge is changed for some reason such as the moisture content, the net flow of the Tr. current will change in the opposite direction 3.
1. J. W. May and T. N. Tombs, Electrostatic toner transfer model, NIP 13, pp. 71-76 (1997).
2. Jerome L. Johnson, Principles of Non Impact Printing, Palatino Press (1986).
3. Kumasaka et al., U.S. Pat. No. 5,291,253 (1994).
4. Chen I., Tse M. K., The role of dielectric relaxation in media for electrophotography (1), Modelling of electrostatic transfer, NIP 15, pp 155-158 (1999).
5. Al-Rubaiey H., Oittinen, P., Transfer Current and Efficiency in Toner Transfer to Paper, NIP 17, (2001).
6. Fletcher C. M., Simple and Complex Relationships between "DYNAMIC CURRENT" and the Applied Fields in Electrophotographic Transfer System, NIP 8, pp 82-85 (1992).
The authors express their acknowledgements to Sappi Technology Centre in South Africa, and Helsinki University of Technology for their cooperation.