Electrostatic recording system using a dielectric belt

Abstract: An electrostatic recording system using an endless dielectric belt (10) performs electrification of a sheet of paper (3) and the belt (10) so that paper is not wound and caught by the photoconductor drums (12a - 12d) as the paper passes between each drum and its associated transfer roller (20a - 20d). The transfer voltage (V TB , V TC , V TM , V TY ) can be lowered so as to smoothly separate the sheet of paper (3) from the belt (10). The electrostatic recording system includes first electrifying means (30) for applying a voltage (HV1) to only the dielectric belt (10) at a stage before the sheet of paper (3) is laid on the belt and second electrifying means (40) for applying a voltage (HV2) to the sheet of paper (3) and the dielectric belt (10) after the sheet of paper has been laid on the belt.

Description: This invention relates to printing by means of an electrophotographic process and more particularly to an electrostatic recording system using a dielectric belt. Electrophotographic colour printing methods for printing a plurality of toner images having different colours such as black, yellow, magenta and cyan, on a sheet of paper in superposition can be broadly classified into the following two kinds. The first kind is a system or method which uses four colour developing devices for one photoconductor drum or belt, develops and transfers toner images, one colour by one, and repeats this procedure for each colour, in other words, four times in all. Transfer methods from the photoconductor drum or belt to the sheet of paper in this case include a method which transfers the toner to the sheet of paper through an intermediate transfer belt or drum. Another transfer method transfers the toner to the sheet of paper without using such an intermediate member. In either of these transfer methods, the transfer operation must be repeated four times for each sheet of paper, so that the printing speed is 1/4 of that of a comparable monochrome printing process. The second kind is a so-called "tandem" printing system or method which sequentially aligns four developing devices and four photoconductor drums for the four colours and prints the colour image by the single conveying operation of the sheet of paper. Transfer of the toners from the photoconductor drums to the sheet of paper is carried out for the respective photoconductor drums while the sheet of paper passes them once. The sheet of paper is then conveyed to a fixing device after the transfer of the four colours, and the toners are thereafter fixed to the sheet of paper. An example of a prior art method using a transfer belt is shown in Fig. 1 of the accompanying drawings. In Fig. 1, reference numeral 1 denotes a photoconductor drum, reference numeral 2 denotes a transfer belt, reference numeral 3 denotes a sheet of paper, reference numeral 4 denotes a corona electrifier for attraction, reference numeral 5 denotes an electrifying brush, reference numeral 6 denotes a -corona electrifier and reference numeral 7 denotes an electrifier for de-electrification. The sheet of paper 3 is conveyed by the transfer belt 2. At the entrance portion to the photoconductor drum 1, both the transfer belt 2 and the sheet of paper 3 are electrified by the corona electrifier 4 disposed below the transfer belt 2 and the electrifying brush 5 disposed above the transfer belt 2, respectively, so that the sheet of paper 3 is attracted to the transfer belt 2. At the exit portion of the sheet of paper 3, the charge of the sheet of paper 3 is removed by the corona discharge from the electrifier 7 for de-electrification, and the sheet of paper 3 is separated from the transfer belt 2. According to this method, both the sheet of paper 3 and the transfer belt 2 are electrified to the same polarity. This serves to prevent the sheet of paper 3 from being wound up by the photoconductor drum 1 and to execute the transfer of the toner from the photoconductor drum 1 to the sheet of paper 3 while the sheet of paper 3 is kept attracted to the transfer belt 2. When the surface of the photoconductor drum 1 is electrified to negative polarity, the corona electrifier 4 at the sheet entrance portion applies an electrifying voltage of positive polarity so as to electrify the surface of the transfer belt 2 and the sheet of paper 3 with negative charge when the voltage is applied from the back of the transfer belt 2. Therefore, the transfer voltage by the corona electrifier 6 must be elevated and consequently the attraction force between the sheet of paper 3 and the transfer belt 2 becomes so high that when the sheet of paper 3 is separated from the transfer belt 2 the charge of both the transfer belt 2 and the sheet of paper 3 must be removed by the electrifier 7 for de-electrification. Because de-electrification by the electrifier 7 must be conducted by a corona discharge having opposite polarity to that of the sheet of paper 3 and the toner, the toner on the sheet of paper 3 is attracted and scattered by the corona electrifier 7 for de-electrification, thereby lowering the image quality. Accordingly, the sheet of paper 3 has a tendency to wind into the photoconductor drum and/or make separation of the sheet of paper from the transfer belt difficult, at least if image quality is to be preserved. US 5585906 discloses an image forming apparatus having a pre-charging roller which charges the conveyor belt when there is no paper on it, and an adhering roller which applies a bias voltage to the paper on the conveyor belt. In the apparatus disclosed therein, the pre-charging roller charges the conveyor belt at the opposite end of the apparatus to that at which the paper is collected by the conveyor belt. As a consequence, the conveyor belt carries a charge during its passage across the apparatus. A problem with this arrangement is that the charge on the belt may decay during its passage across the apparatus. A further problem is that the charged belt may interfere with other components in the apparatus. According to a first aspect of the invention there is provided an electrostatic recording system comprising: an image carrying body on a surface of which a toner image is developed; a dielectric belt for conveying a sheet of paper while electrostatically 5 attracting the same, and bringing said sheet of paper into contact with the surface of said image carrying body during conveying; a transfer electrifier for applying a transfer voltage to said image carrying body from a side of said dielectric belt opposite to said image carrying body and transferring said toner image on the surface of said image carrying body to said sheet of paper; first electrifying means for applying a voltage to only said dielectric belt at a stage before said sheet of paper is attracted to said dielectric belt; and second electrifying means for applying a voltage to said sheet of paper conveyed on said dielectric belt, and to said dielectric belt, while said sheet of paper is on the belt; characterised in that said first and second electrifying means are arranged to oppose a common conductive roller so that the dielectric belt passes between the conductive roller and the first electrifying means, and between the conductive roller and the second electrifying means. The electrostatic recording system can appropriately transfer the toner image to the sheet of paper while preventing the sheet of paper from being wound into the image carrying body such as the photoconductor drum, and can control the first and second electrifying means and the transfer electrifier so that the sheet of paper can be smoothly separated from the dielectric belt. The electrostatic recording system may be arranged to control the first and second electrifying means so that the potential of the sheet of paper immediately before it passes through the transfer electrifier has the same polarity as that of the surface potential of the image carrying body. In this way, the likelihood that a sheet of paper is wound into the image carrying body, such as the photoconductor drum, can be reduced. When the sheet potential immediately before the sheet of paper passes through the transfer electrifier has a polarity opposite to that of the surface potential of the image carrying body, the first and second electrifying means and the transfer electrifier may be controlled so that the sheet potential immediately after the passage of the sheet through the transfer electrifier has the same polarity as that of the surface potential of the image carrying body. In this way, the likelihood that a sheet of paper is wound into the image carrying body, such as the photoconductor drum can be reduced and the necessary transfer of the toner image to the sheet of paper can be carried out. The electrostatic recording system may comprise a plurality of image carrying bodies on the surfaces of which toner images are developed, the dielectric belt being arranged to bring a sheet of paper sequentially into contact with the surfaces of a plurality of image carrying bodies during conveying; and a plurality of transfer electrifiers for applying a transfer voltage to each image carrying body from the side of the dielectric belt from the side opposite to each image carrying body, and sequentially transferring the toner images on the surfaces of a plurality of image carrying bodies to the sheet of paper. According to this construction, transfer from each image carrying body to the sheet of paper can be carried out sequentially and appropriately without the sheet of paper being caught by each image carrying body (photoconductor drum), and the first and second electrifying means and each transfer electrifier can be controlled so that the sheet of paper can be smoothly separated from the dielectric belt after the sheet of paper passes through the last image carrying body. The transfer electrifier corresponding to the last image carrying body may be controlled in such a manner as to lower the transfer voltage at only the distal end portion of the sheet of paper when the toner image is transferred from the last image carrying body to the sheet of paper. In this way, the smoothness of the separation of sheets of paper from the dielectric belt after they pass through the last image carrying body can be improved. For full colour printing, four image carrying bodies may be arranged in parallel, so that yellow, magenta, cyan and black toners may be sequentially supplied to these image carrying bodies, respectively, and the toner images of these colours developed on the surfaces of the image carrying bodies and sequentially transferred to sheets of paper. When the polarity of the potential of the sheets of paper immediately prior to passing through the transfer electrifier is the same as that of the surface potential of the corresponding image carrying body, or when the polarity of the potential of the sheets of paper immediately prior to passing through the transfer electrifier is opposite to that of the surface potential of the corresponding image carrying body, the first and second electrifying means and each transfer electrifier may be controlled so that the potential of a sheet immediately after it passes through the transfer electrifier is the same as the potential on the surface of the image carrying body concerned. In full colour printing, therefore, transfer from one image carrying body to the sheet of paper can be carried out smoothly and sequentially in a manner which reduces the possibility of catching of the sheet at each image carrying body (e.g. photoconductor drum). At least one of the first and second electrifying means may comprise an electrifying roller. In this case, at least the surface of the electrifying roller may comprise a porous body. In this way the structures of both the first and second electrifying means can be simplified. The electrifying roller may be pressed onto, and brought into contact with, the dielectric belt, and freely rotate due to movement of the dielectric belt in such a manner as to follow this movement. Therefore, a driving mechanism for the electrifying roller itself is not necessary, and the construction of the driving portion can be simplified. The first and second electrifying means may comprise electrifying rollers and the common conductive roller, may be grounded. At least the first electrifying means may be 5 disposed so as to come into contact with the dielectric belt. In this case, the first electrifying means may comprise a rotary brush, the rotating surface of which comes into contact with the dielectric belt. According to this construction, the first electrifying means not only serves to execute electrification for attracting the sheet of paper to the dielectric belt, but also serves to clean the dielectric belt. Further, the peripheral speed of the rotary brush and the peripheral speed of the image carrying body may be arranged to be different. In this case, the cleaning effect of the dielectric belt can be further improved. The volume resistivity of the electrifying roller or the rotary brush is preferably 10 3 to 10 7 ohm-cm. The volume resistivity of the dielectric belt is preferably at least 10 13 ohm-cm. In this way, electrification of the dielectric belt can be kept within a predetermined range. A rotary brush cleaner for keeping a predetermined voltage so as to remove excessive charge accumulated in the dielectric belt may be disposed immediately before the position of the first electrifying means. Accordingly, the dielectric belt can be de-electrified before the dielectric belt is electrified by the first electrifying means for attracting the sheet of paper. The rotary brush cleaner may be disposed in such a manner as to come into contact with the inside surface of the dielectric belt opposite to the sheet conveying surface. According to this construction, the dielectric belt can be de-electrified without being affected by the residual toner adhering to the sheet conveying surface of the dielectric belt, or the like. The voltage applied to the rotary brush cleaner may be alternately changed for at least a predetermined time at the initial stage before the actual transfer operation is started. In this way, the dielectric belt can be cleaned, in advance, before the start of the transfer operation. According to a second aspect of the present invention, there is provided an electrostatic recording system comprising: an image carrying body on a surface of which a toner image is developed; a dielectric transfer belt for electrostatically attracting a sheet of paper, conveying said sheet of paper and bringing said sheet of paper into contact with a surface of said image carrying body while it is conveyed; a transfer electrifier for applying a transfer voltage to said image carrying body from a side of said dielectric belt opposite to said image carrying body, and transferring the toner image on the surface of said image carrying body to said sheet of paper; electrifying means for applying a voltage to said dielectric belt at a stage before said sheet of paper is attracted to said dielectric belt; and electrostatic attraction means for electrostatically attracting said sheet of paper to the dielectric belt, after electrification of the belt by the electrifying means, said electrostatic attraction means comprising a conductive roller which has a peripheral portion coming into contact with a surface of said sheet of paper conveyed on said dielectric belt on an opposite side to said dielectric belt; characterised in that the core of said conductive roller is grounded, and an absolute value of a voltage applied to said electrifying means is higher than a threshold breakdown voltage for discharge in the conductive roller, and in that said electrifying means and said electrostatic attraction means are arranged to oppose a common conductive roller so that the dielectric belt passes between the conductive roller and the electrifying means, and between the conductive roller and the electrostatic attraction means. Because the sheet electrostatic attraction means comprises the grounded conductive roller and the absolute value of the voltage applied to the electrifying means is higher than the discharge start voltage by this conductive roller, when the sheet of paper is conveyed to the dielectric belt, discharge is started simultaneously by the conductive roller, so that the potential of the sheet of paper can be electrified to the polarity opposite to that of the dielectric belt. Therefore, the sheet of paper is electrostatically attracted by the dielectric belt and can be stably conveyed by a simple construction. The conductive roller comprises a grounded conductive metal core and a flexible or resistive member disposed round the metal core. The resistive member serves to electrostatically attract a sheet of paper to the dielectric belt. Accordingly, when the resistive member is brought into contact with the dielectric belt, discharge is effected from the side of the dielectric belt through the sheet of paper, through the resistive member and to the metal core. The electric resistance value of the resistive member is preferably from 10 3 to 10 7 ohm-cm. Discharge can be effected through the sheet of paper and this paper is electrified to the polarity opposite to that of the dielectric belt. The resistive member may be made of a rubber having a hardness of at least 20 degrees by JIS-A and/or a frictional coefficient of 0.3 to 1.2 on the outer peripheral surface thereof. UV treatment or resin coating may be applied to the resistive member so as to lower the frictional coefficient of the rubber surface. The resistive may alternatively be constituted by a porous sponge in place of rubber. In this case, the sponge preferably has physical and electrical properties similar to those of the rubber. The electrifying means for applying the voltage to the dielectric belt may be a stationary or rotary brush having a resistance value of 10 3 to 10 7 ohms. A felt-like dielectric belt cleaning device coming into contact with the dielectric belt can be disposed adjacent to this stationary or rotary brush. If the brush is a rotary brush, it can be arranged to rotate in the opposite direction to the sheet conveying direction of the dielectric belt. The electrifying means for applying the voltage to the dielectric belt may be made of a conductive porous material, may have the shape of a roller, and may be arranged to rotate in the same direction as the sheet conveying direction of the dielectric belt. An A.C. voltage may be applied to the electrifying means for applying the voltage to the dielectric belt, and this A.C. voltage may be a sine wave or rectangular wave having a D.C. offset voltage. The polarity of the offset voltage may be opposite to the polarity of the surface potential of the image carrying body. According to this arrangement, the sheet of paper will be electrified to the same polarity as that of the image carrying body and the sheet of paper can be easily separated from the image carrying body. Certain preferred embodiments of the present invention can suitably conduct the transfer of the toner image to the sheet of paper without catching of the sheet of paper by the photoconductor drum, can smoothly separate the sheet of paper from the dielectric belt and can improve printing quality. For a better understanding of the invention, and to show how the same may be carried into effect, reference will now be made, by way of example, to the accompanying drawings, in which:- Fig. 1 is a schematic view showing a prior art example of an electrostatic electronic recording apparatus for effecting conveying and transfer of a sheet of paper by using a dielectric belt; Fig. 2 is a schematic view of an electrostatic electronic recording apparatus using a dielectric belt according to an embodiment of the present invention; Fig. 3 is a schematic view showing a conveying and transfer portion of a sheet of paper by the dielectric belt according to an embodiment of the present invention; Fig. 4 is a schematic view useful for explaining a method of measuring the potentials of the sheet of paper and the transfer belt, and attraction force; Fig. 5 is diagram showing the relationship between V 1 - V 2 and a paper potential; Fig. 6 is a diagram showing the relationship between V 1 - V 2 ; Fig. 7 is a diagram showing the relationship between a transfer voltage (V TY ) and a paper voltage; Fig. 8 is a diagram showing the relationship between a transfer voltage (V TY ) and the paper voltage; Fig. 9 is a diagram showing the relationship between the position of the sheet of paper on the transfer belt and the paper voltage; Fig. 10 is a diagram showing the relationship between the transfer voltage (V TY ) and the paper attraction force; Fig. 11 is a schematic view showing an electrostatic electronic recording apparatus using a dielectric belt according to another embodiment of the present invention; Fig. 12 is a schematic view showing a rotary brush cleaner used in the embodiment shown in Fig. 11; Fig. 13 is a schematic view showing an electrostatic electronic recording apparatus using a dielectric belt according to still another embodiment of the present invention; Fig. 14 is sectional view of a conductive roller used in the embodiment shown in Fig. 13; Fig. 15 is a diagram showing an electrified state of the dielectric belt immediately after electrification with respect to the applied voltage of electrifying means; and Fig. 16 is a diagram showing the electrified state of each portion with respect to the applied voltage of the electrifying means. Referring to Fig. 2, there is shown a multi-colour electrostatic recording apparatus according to one embodiment of the present invention which includes an endless transfer belt 10 for conveying a recording medium (sheet of paper 3). The transfer belt 10 comprises an endless belt 10a made of a flexible dielectric material such as a suitable synthetic resin material. The endless belt 10a is spread around a plurality of rollers 10b to 10f. The roller 10b functions as a driving roller, and the driving roller 10b drives the endless belt 10a by a suitable driving mechanism not shown in the drawing. The roller 10c functions as a follower roller. Both of the rollers 10d and 10e function as the guide rollers, and are disposed in the proximity of the driving roller 10b and the follower roller 10c. The tension roller 10f is interposed between the follower roller 10c and the guide roller 10e, and applies a suitable tension to the endless belt 10a. The upper driving portion of the endless belt 10a, that is, the driving portion defined between the driving roller 10b and the follower roller 10c, defines a paper moving path. Sheets of paper 3 are introduced from the side of the follower roller 10c and discharged from the side of the driving roller 10b. When introduced from the side of the follower roller 10c, a sheet of paper 3 is electrostatically attracted to the endless belt 10a due to electrification of the endless 5 belt 10a as will be later described. The multi-colour electrostatic recording apparatus is equipped with four electrostatic recording units Y, C, M and B disposed in series from the upstream side to the downstream side along the upper travelling portion of the endless belt 10a. The electrostatic recording units Y, C, M and B all have the same construction and record the toner images in yellow, cyan, magenta and black respectively on the sheet of paper moving along the upper travelling portion of the endless belt 10a. The electrostatic recording units Y, C, M and B have respective image carrying bodies in the form of photoconductor drums 12a to 12d respectively. Each photoconductor drum 12 is driven for rotation in a direction indicated by an arrow at the time of the recording operation. Respective pre-electrifiers 14, constituted as corona chargers or scorotron chargers, are disposed above the respective photoconductor drums 12 and sequentially and uniformly electrifies the rotating surface of the respective photoconductor drums 12. An electrostatic latent image is written to the electrification area of the respective photoconductor drum 12 by optical write means such as a laser beam LB emitted from a laser beam scanner. In other words, the laser beam LB is turned ON and OFF on the basis of digital image data obtained from a computer, a word processor and the like, so that the electrostatic latent image is written as a dot image. The electrostatic latent image is written onto the photo-electrostatically developed with a predetermined colour toner as an electrified toner image by a developing device 18, which is disposed on the upstream side of the paper passage portion with respect to the photoconductor drum 12. The electrified toner images on the photoconductor drums 12a to 12d of the electrostatic recording units Y, C, M, B are transferred electrostatically and sequentially to the sheet of paper 3 by respective transfer electrifiers disposed below the photoconductor drum concerned, the transfer electrifiers being conductive transfer roller 20a to 20d. The conductive transfer rollers 20a to 20d are disposed facing their respective photoconductor drums 12a to 12d on opposite sides of the upper travelling portion of the endless belt 10a. The transfer rollers 20 serve to apply charge having opposite polarity to that of the electrified toner image, so that the electrified toner image is electrostatically transferred from the photoconductor drums 20a to 20d to the sheet of paper 3. The transfer by the conductive transfer rollers 20a to 20d will be described in more detail below. According to the construction described above, when the sheet of paper 3 is introduced from the follower roller 10c of the transfer belt 10 and passes in turn through the electrostatic recording units Y, C, M and B, the toner images of the four colours are superposed to thereby form a full colour image. Next, the sheet of paper is fed from the side of the driving roller 10b of the transfer belt 10 to a heated roller type heat fixing device 22, where the full colour image is thermally fixed on the sheet of paper. The heated roller type heat fixing device 22 comprises a heated roller 22a and a backup roller 22b. During the printing operation, the heated roller 22a and the backup roller 22b are driven in the direction indicated by the arrows in Fig. 2. A sheet of paper discharged from the side of the driving roller 10b of the transfer belt 10 is carried between the nip formed by the rollers 22a and 22b. At this time, the transferred toner image on the surface of paper is thermally fused, so that the transferred toner image is thermally fixed on the sheet of paper. In each of the electrostatic units Y, C, M and B, residual toner which has not been transferred to the sheet of paper remains adhered to the surface of the photoconductor drum 12 and is removed by respective cleaners 24 disposed on the downstream side of the sheet moving path with respect to the respective photoconductor drum 12. Reference numeral 26 denotes a light emitting member, such as a light emitting diode array, for removing charge from the surface of the photoconductor drum 12 after the transferring process. Reference numeral 28 denotes a developer supplementing container for suitably supplementing the toner component to the developing device 28. The apparatus further comprises a first electrifying means 30 for applying a voltage V 1 to only the dielectric belt 10, at a stage before the sheet of paper 3 is attracted to the dielectric belt 10, and a second electrifying means 40 for applying a voltage V 2 to a sheet of paper 3 and the dielectric belt while they are mutually superposed on the belt. Fig. 3 is a schematic view for explaining the conveyance and transfer of the sheet of paper by the dielectric transfer belt. The dielectric transfer belt 10 for conveying the sheet of paper 3 and transferring the toner from the photoconductor drum 12 is rotated in the direction of the arrow by the driving roller 10b. The transfer belt 10 is provided with the electrifying means 30 and the electrifying means 40. The transfer belt 10 is de-electrified by an AC de-electrifier 32 prior to the electrifying means 30. The electrifying means 30 electrifies the transfer belt 10 at a stage before the sheet of paper 3 is supplied to the transfer belt 10. The transfer voltage of the transfer means is V 1 . The electrifying means 40 then performs a second electrification of the transfer belt 30 at a stage when the sheet of paper 3 is held on the transfer belt 10 (which is already electrified by virtue of the electrification by the electrifying means 30). The transfer voltage of the transfer means 40 is V 2 . The sheet of paper 3 is thus electrostatically attracted to the transfer belt 10. The sheet of paper 3 is then fed to the first photoconductor drum 12a. These electrifying means 30 and 40 may be for example sponge-like rollers, fixed brushes or rotary brushes. A cleaning operation can be carried out on the dielectric belt 10 by making the peripheral speed of the rotary brush different from that of the dielectric belt. The volume resistivity of the surface of the dielectric belt 10 is preferably at least 10 13 ohm-cm, and the volume resistivity of the electrifying roller or the rotary brush is preferably 10 3 to 10 7 ohm-cm. In this case, electrification is effected to the dielectric belt 10 under the transfer condition described above. As shown in Fig. 3, the constructions of the electrifying means 30 and 40 can be simplified, since the electrifying means 30 and 40 are made as electrifying rollers and a common conductive roller 34 is disposed to oppose these two electrifying rollers, with the dielectric belt 10 passing between the conductive roller 34 and the two electrifying means 30 and 40. The conductive roller 34 is connected to the ground. As described above, the yellow, cyan, magenta and black toner images are developed on the surface of the photoconductor drums 12a to 12d, respectively. When the transfer voltages are applied to the conductive transfer rollers 20a to 20b, the respective toner images are transferred to the sheet of paper 3. After passing through the four photoconductor drums 12a to 12d, the sheet of paper 3 is separated from the transfer belt 10, as described above, and is fed to the fixing device 22, where the toner is fused and solidified, and thus fixed to the sheet of paper 3. The electrifying roller 20d used for the last photoconductor drum 12d is controlled in such a manner that the transfer voltage to only the distal end portion of the sheet of paper 3 is lowered when the black toner image is transferred from the last photoconductor drum 12d to the sheet of paper 3. Therefore, even if the cancelling electrifier 7 which has been used in the prior art example shown in Fig. 1 is not provided, the sheet of paper 3 can be smoothly separated from the dielectric belt 10 by the curved path by the driving roller 10b of the electrifying belt 10 after the sheet of paper 3 passes through the last photoconductor drum 12d. Fig. 4 is a schematic view for explaining the method of measuring the potential in the conveying/transferring apparatus of the sheet of paper whose outline is shown in Fig. 3. First, the method of measuring the potentials of the sheet of paper 3 and the transfer belt 10 will be explained. A fixed electrode 50, which is connected to the ground, is disposed inside the transfer belt 10. The transfer belt 10 and the sheet of paper 3 attracted to this transfer belt 10 are allowed to pass over this fixed electrode 50. A surface potential sensor 52 is disposed facing the fixed electrode 50 above the transfer belt 10, and serves to measure the surface potential V TY of the sheet of paper 3. A further surface potential sensor 54 is provided for measuring the potential V B of the transfer belt after the point at which sheets of paper are separated from the transfer belt 10. The paper potential V P after paper is separated from the transfer belt 10 has a value obtained by the following equation: V P = V PB - V B Means are also provided for measuring the paper attraction force acting between the sheet of paper 3 and the transfer belt 10 and is operable by stopping the transfer belt 10 before the sheet of paper 3 separates from the transfer belt 10, pulling the sheet of paper 3 by a spring balance (not shown) in the paper conveying direction indicated by an arrow in the drawing and measuring the maximum load at the time when the sheet of paper 3 separates from the transfer belt 10. Fig. 5 shows the result of measurement of the potentials electrified by the electrifying means 30 and 40 at a position ahead of the first photoconductor drum 12 shown in Fig. 3 (position 3 of Fig. 3) and the paper potential. That is, Fig. 5 shows the relationship between the difference (V 1 - V 2 ) between the potential V 1 of the transfer belt 10 electrified by the electrifying means 30 and the potential V 2 on the sheet of paper 3 from the electrifying means 40 and the paper potential V at the position 2 in Fig. 3. The relationship is such that when V 1 is lower than V 2 , the sheet of paper 3 is electrified to a positive polarity, and when V 1 is higher than V 2 , the sheet of paper 3 is electrified to a negative polarity. Fig. 6 shows the relationship between the potential difference (V 1 - V 2 ) and the paper attraction force (g) by the transfer belt 10 at this time. The eater the potential difference, the greater becomes the paper attraction force irrespective of the polarity of the potential of the sheet of paper. An explanation will now be given of the stage during which the sheet of paper 3 passes by the first photoconductor drum 12a. Here, the relationship of the potential is determined at the position (position 3 ) between the photoconductor drum 12a and the photoconductor drum 12b in Fig. 3, assuming that the surface of the photoconductor drum 12a has the negative charge (surface potential: -600V) and a positive voltage (V TY ) is applied to the transfer roller 20a. At this time, V 2 is kept constant at +400V, the transfer voltage V TY is kept constant at +1600V, and V 1 is changed. If V 1 is increased, the paper voltage (V) at the position in Fig. 3 becomes lower. When the sheet of paper 3 is in the positive charge (when V 1 is lower than -0.3 kV), catching of the paper into the photoconductor drum 12a is likely to occur as indicated by "Paper Caught Region" in Fig. 7. This is because when the charge of the sheet of paper 3 has a positive charge, this positive charge mutually attracts the negative charge on the surface of the photoconductor drum 12a after transfer and consequently the sheet of paper 3 is wound on the photoconductor drum 12a. The paper potential changes also with the value of the transfer voltage. Fig. 8 shows their relationship. Here, both of V 1 and V 2 are set to +400V and the transfer voltage (V TY ) given by the transfer roller 20a is changed from 0 to 3.5 KV. Catching of the sheet of paper on the photoconductor drum 12a occurs below the transfer voltage of 1.0 KV at which the paper potential becomes positive. Therefore, in order to prevent this catching of the sheet of paper 3 on the photoconductor drum 12a, the relationship between V 1 and V 2 and also between the transfer voltage (V TY ) must be selected so that the potential of the sheet of paper which has just passed through the photoconductor drum has at least the same potential as that of the surface potential of the photoconductor drum 12a. If setting is preferably made by only the relationship between V 1 and V 2 so that the paper potential has the same polarity as that of the surface potential of the photoconductor drum 12a, catching of the sheet of paper to the photoconductor drum 12a does not occur. It has also been confirmed, by experiment that, when settings are chosen so that the paper potential has the same polarity as that of the surface potential of the photoconductor drum 12a, or when the transfer belt 10 is electrified to +800V by the electrifying means 30 and the sheet of paper to +400v by the electrifying means 40, catching of the sheet of paper 3 on the photoconductor drum 12a does not occur at a temperature and humidity of from 5°C and 208 to 35°C and 80%. The absolute value of the potential of the paper as it passes in turn through the second photoconductor drum 12b, the third photoconductor drum 12c and the fourth photoconductor drum 12d becomes progressively larger due to the voltages of the respective transfer rollers 12b, 12c and 12d. This is illustrated in Fig. 9 which shows the paper potentials at the position in front of the photoconductor drum 12a in Fig. 3 (position 2 ), the position between the photoconductor drums 12a and 12b (position 3 ), the position between the photoconductor drums 12b and 12c (position 4 ), the position between the photoconductor drums 12a and 12b (position 5 ) and the position at the back of the photoconductor drum 12d (position 6 ), respectively. As explained above, catching of the sheet of paper on the photoconductor drum 12 can be prevented over a broad environmental range by electrifying the paper potential to the same polarity as that of the surface potential of the photoconductor drum. The relationship between the potential V 2 at the electrifying means 40 and the transfer voltage V TY will now be explained. The relationship between V 2 and the transfer voltage V TY at which the toner transfer efficiency by the first photoconductor drum 12a becomes at least 85% was examined. It was found that V TY becomes lower when V 2 becomes higher and particularly when V 2 comes to have the opposite polarity to that of the photoconductor drum. That is, when V 2 becomes positive, V TY becomes even lower (Fig. 8 shows V TY when V 2 is +400V). Because the toner has the same polarity as that of the surface of the photoconductor drum, the toner is more likely to be transferred to the sheet of paper as it is attracted by charge having opposite polarity on the surface of the sheet of paper. The voltage necessary for transfer is influenced by three factors, namely the transfer belt 10, the photoconductor drum 12a and the toner. In the following it is assumed to be the case that: V TY = 2000 - V 2 According to the equation given above, the transfer voltage V TY can be lowered by making V 2 higher. However, when V 2 exceeds 1300 to 1400V, the toner is attracted by the sheet of paper and scatters before it adheres to the photoconductor drum. Therefore, V 2 must be set below these voltages. The relationship between the transfer voltage V TY . and the paper potential is shown in Fig. 8. The relationship between the transfer voltage V TY and the paper attraction force is shown in Fig. 10. As can be seen clearly from Fig. 8, when the transfer voltage V TY is made higher, the absolute value of the paper potential becomes higher in the negative direction. Consequently, as shown in Fig. 10, the attraction force becomes stronger. This indicates that the higher the transfer voltage, the more difficult it becomes to separate the sheet of paper from the transfer belt. It can therefore be understood that the sheet of paper can be separated more easily by setting the potential V2 of the electrifying means to a voltage as high as possible so as to lower the transfer voltage. Fig. 11 is a schematic view showing another embodiment of the present invention, wherein same or corresponding reference numerals are used to identify same or corresponding constituent elements as in Figs. 2 and 3, the explanation of such elements being omitted. In this embodiment, a rotary brush cleaner 36 is provided immediately before the first electrifying means 30. A voltage is applied for de-electrifying the excessive charge accumulated in the dielectric belt 10. This rotary brush cleaner 36 is disposed in such a manner as to come into contact with the inside surface of the dielectric belt 10 opposite to the sheet conveying surface, and a conductive roller 38 which is grounded is so disposed as to oppose the rotary brush cleaner 36 while interposing the dielectric belt 10 between them. A voltage for removing the excessive charge accumulated in the dielectric belt 10 is applied to this rotary brush cleaner 36. The polarity of this voltage is alternately changed for at least a predetermined time at the initial stage before the practical transfer operation is started. In this way, cleaning of the rotary brush cleaner 36 itself can be carried out. Fig. 12 shows a preferred example of the rotary brush cleaner 36. Reference numeral 36a denotes a core portion, and reference numeral 36b denotes a conductive brush portion disposed around the core portion. This conductive brush portion 36b is greater than at least the width of the dielectric belt 10 and can de-electrify the entire width of the dielectric belt 10. Fig. 13 is a schematic view showing still another embodiment of the present invention, wherein the same or corresponding reference numerals are used to identify the same or corresponding constituent members as in Figs. 2 and 3, the explanation of such elements being omitted. The difference of this embodiment from the embodiment shown in Fig. 2 is that a conductor roller 60, a part of which is grounded, is disposed as means for electrostatically attracting the sheet of paper 3 to the dielectric belt 10 in place of the second electrifying means 40 in Fig. 2. As shown in Fig. 14, the conductive roller 60 comprises a conductive metal core 61 which is grounded, and a resistive member 62, made for example of rubber or from a porous sponge, disposed around the metal core 61. The resistance value of the resistive member 62 is 10 3 to 10 7 ohms, the hardness is at least 20 degree in terms of JIS-A and the frictional coefficient of the outer peripheral surface is preferably from 0.3 to 1.2. If the resistive member 62 is of rubber, it is possible to apply a known UV treatment or resin coating to the outer peripheral surface thereof and, in this way, the frictional coefficient can be adjusted to such a low value. The resistive member is preferably flexible. The electrifying means for applying the voltage to the dielectric belt 10 has fundamentally the same construction as the first electrifying means 30 shown in Fig. 2. In this embodiment, however, the absolute value of the voltage applied to the electrifying means 30 must be set to a higher voltage than the breakdown threshold voltage for discharge in the conductive roller 60. The electrifying means 30 may be constituted by a conductive stationary or rotary brush having a resistance value of 10 3 to 10 7 ohms or a roller-like conductive porous material rotating in the same direction as the sheet conveying direction by the dielectric belt 10. The dielectric transfer belt 10 is rotated by the driving roller 10b in the direction indicated by an arrow in the drawing so as to convey the sheet of paper 3 and to transfer the toner from the photoconductor drums 12a to 12d. After de-electrification by the de-electrifying device 32, the dielectric belt 10 is electrified by the electrifying means 30. D.C. voltage or A.C. voltage is applied to this electrifying means 30. When the A.C. voltage is applied, a sine wave having a D.C. offset voltage or a rectangular wave is preferably used. The polarity of this offset voltage is opposite to the polarity of the surface potential of the photoconductor drums 12a to 12d as the image carrying body members. Because the absolute value of the voltage applied to the electrifying means 30 is greater than the threshold voltage for discharge in the conductive roller 60, discharge occurs from the side of the sheet of paper 3 to the metal core 61 grounded through the resistive member 62 of the conductive roller 60 simultaneously when the sheet of paper 3 is conveyed to the conductive belt 10 which is electrified. In consequence, the sheet of paper 3 keeps a predetermined potential and moreover has a polarity opposite to the polarity of the potential applied to the electrifying means 30. Accordingly, when the surface potential of the photoconductor drums 12a to 12d is negative, for example, electrification by the electrifying means 30 to the dielectric belt 10 is made positive, and the sheet of paper 3 can be thus electrified to be negative. When such polarities are used, the sheet of paper 3 and the photoconductor drums 12a to 12d repel one another, and catching of the sheet of paper 3 on the photoconductor drums 12a to 12d can be prevented. Since the potential of the dielectric belt 10 is positive in this case, the transfer voltage for transferring the sheet of paper 3 from the photoconductor drums 12a to 12d can be lowered. When the conductive roller 60 which does not at all have any voltage application means but is merely grounded is disposed, catching of the sheet of paper 3 on the photoconductor drums 12a to 12d can be prevented, and the sheet of paper 3 can be stably conveyed on the dielectric belt 10. At the same time, the transfer voltage can be lowered, and the voltage for de-electrification by the corona discharge can also be lowered with the drop of the transfer voltage, and the generation of ozone due to the corona discharge can be restricted. A felt-like dielectric belt cleaner which comes into contact with the dielectric belt 10 can be disposed in the proximity of the electrifying means 30 in the same way as in the embodiment shown in Fig. 11. In such a case, it is possible to constitute integrally the electrifying means 30 comprising the fixed brush, etc, and the felt-like cleaner, and to removably fit the integral assembly to the apparatus. It is possible further to removably arrange a cleaner (not shown) for removing contamination of the surface of the rubber of the conductor roller 60 itself. Fig. 15 shows the electrified state of the dielectric belt 10, immediately after being electrified, when the voltage applied to the electrifying means 30 is changed. If the electrifying means 30 is constituted by the electrifying roller (10 -4 ohms) ( ▪ ), if it comprises the electrifying roller (10 -7 ohms) ( ⋄ ), and if it comprises the electrifying brush ( Δ ), the respective electrified state is shown. Fig. 16 shows the electrified state of each portion, in which the electrifying brush is used as the electrifying means 30, when the offset voltage applied to this electrifying brush is changed. The drawing shows the electrified voltage of the surface of the sheet of paper ( ▪ ), the electrified voltage of the surface of the dielectric belt 10 immediately after being electrified ( ⋄ ), the electrified voltage of the surface of the dielectric belt 10 on the upstream side of the sheet of paper ( Δ ), and the electrified voltage of the surface of the dielectric belt 10 on the downstream side of the sheet of paper ( □ ). As shown in this diagram, it can be understood that the electrified voltage on the surface of the sheet of paper 3 has a polarity opposite to that of the impressed voltage to the electrifying means 30 by the corona discharge by the conductive roller 10. Although the invention has been explained in terms of sheets of paper as the recording medium, systems embodying the invention may employ other kinds of recording medium, such as other paper configurations or sheets other than sheets of paper.

Claims: An electrostatic recording system comprising: an image carrying body (12) on a surface of which a toner image is developed; 5 a dielectric belt (10) for conveying a sheet of paper (3) while electrostatically attracting the same, and bringing said sheet of paper into contact with the surface of said image carrying body (12) during conveying; a transfer electrifier (20) for applying a transfer voltage to said image carrying body (12) from a side of said dielectric belt (10) opposite to said image carrying body (12) and transferring said toner image on the surface of said image carrying body (12) to said sheet of paper (3) ; first electrifying means (30) for applying a voltage (V 1 ) to only said dielectric belt (10) at a stage before said sheet of paper is attracted to said dielectric belt (10) ; and second electrifying means (40) for applying a voltage (V 2 ) to said sheet of paper conveyed on said dielectric belt, and to said dielectric belt, while said sheet of paper is on the belt (10); characterised in that said first and second electrifying means (30, 40) are arranged to oppose a common conductive roller (34) so that the dielectric belt (10) passes between the conductive roller (34) and the first electrifying means (30), and between the conductive roller (34) and the second electrifying means (40). An electrostatic recording system according to claim 1, wherein said first and second electrifying means (30, 40) are controllable so that the potential of said sheet of paper immediately before it passes said transfer electrifier (20) has the same polarity as that of the potential on the surface of said corresponding image carrying body (12). An electrostatic recording system according to claim 1, wherein, when the polarity of the potential of said sheet of paper immediately before it passes through said transfer electrifier (20) is opposite to 5 the polarity of the potential on the surface of said image carrying body (12), said first and second electrifying means (30, 40) and said transfer electrifier (20) are controllable so that the paper potential immediately after passing through said transfer electrifier (20) becomes the same polarity as that of the potential of the surface of said associated image carrying body (12) . An electrostatic recording system according to any of the preceding claims, comprising: a plurality of image carrying bodies (12) on a surface of which toner images are developed, said dielectric belt (10) being arranged to bring said sheet of paper sequentially into contact with the surfaces of a plurality of said image carrying bodies (12); and a plurality of transfer electrifiers (20) for applying respective transfer voltages to said image carrying bodies (12) from a side of said dielectric belt opposite to said image carrying body and sequentially transferring said toner images on the surfaces of a plurality of said image carrying body (12) to said sheet of paper. An electrostatic recording system according to claim 4, wherein, when the potential of said sheet of paper immediately before it passes through each said transfer electrifier (20) has the same polarity as that of the potential on the surface of the corresponding one of said image carrying bodies (12) or is an opposite polarity, said first and second electrifying means and each of said transfer electrifiers are controlled so that a potential of said sheet of paper immediately after it passes through said transfer electrifier has the same polarity as that of the surface of said image carrying body. An electrostatic recording system according to any one of the preceding claims, wherein said or the last one of said transfer electrifiers is controllable 5 in such a manner as to lower a transfer voltage of only the distal end portion of a sheet of paper when said toner image is transferred from said or the last one of said image carrying bodies to said sheet of paper. An electrostatic recording system according to any one of the preceding claims and comprising four image carrying bodies for yellow, magenta, cyan and black toner, the image carrying bodies being arranged to transfer toner images from their surfaces sequentially onto a sheet of paper being conveyed on 5 the dielectric belt. An electrostatic recording system according to any one of the preceding claims, wherein said first electrifying means (30) is a first electrifying roller. An electrostatic recording system according to claim 8, wherein a surface of said first electrifying roller comprises a porous body. An electrostatic recording system according to any of the preceding claims, wherein said second electrifying means (40) is a second electrifying roller. An electrostatic recording system according to claim 10, wherein a surface of said second electrifying roller comprises a porous body. An electrostatic recording system according to any of the preceding claims, wherein the common conductive roller is grounded An electrostatic recording system according to any one of claims 8 to 12, wherein the or each electrifying roller is arranged to be pressed against and brought into contact with said dielectric belt and to freely rotate on movement of said dielectric belt while following the movement. An electrostatic recording system according to any one of the preceding claims, wherein said first electrifying means (30) is disposed so as to come into contact with said dielectric belt (10). An electrostatic recording system according to claim 14, wherein said first electrifying means (30) comprises a rotary brush whose rotating surface comes into contact with said dielectric belt (10). An electrostatic recording system according to any of the preceding claims, wherein said second electrifying means (40) is so disposed as to come into contact with said dielectric belt (10) . An electrostatic recording system according to claim 16, wherein said second electrifying means (40) comprises a rotary brush whose rotating surface comes into contact with said dielectric belt (10). An electrostatic recording system according to claim 15 or 17 and operable such that a peripheral speed of the or each rotary brush and a peripheral speed of the or said image carrying bodies are different. An electrostatic recording system according to any one of claims 8 to 18, wherein the volume resistivity of the or each electrifying roller or the or each rotary brush is 10 3 to 10 7 ohm-cm. An electrostatic recording system according to any of the preceding claims, wherein the volume resistivity of said dielectric belt is at least 10 13 ohm-cm. An electrostatic recording system according to any one of the preceding claims, wherein a rotary brush cleaner for keeping a predetermined voltage to remove excessive charge accumulated in said dielectric belt is disposed immediately before said first electrifying means. An electrostatic recording system according to claim 21, wherein said rotary brush cleaner is disposed so as to come into contact with an inside surface of said dielectric belt (10) on the opposite side to the paper conveying surface of the dielectric belt (10) . An electrostatic recording system according to claim 21 or 22, wherein the polarity of a voltage applied to said rotary brush cleaner can be changed alternately for at least a predetermined time at an initial stage before a actual transfer operation is started. An electrostatic recording system comprising: an image carrying body (12) on a surface of which a toner image is developed; a dielectric transfer belt (10) for electrostatically attracting a sheet of paper, conveying said sheet of paper and bringing said sheet of paper into contact with a surface of said image carrying body while it is conveyed; a transfer electrifier (20) for applying a 20 transfer voltage to said image carrying body (12) from a side of said dielectric belt (10) opposite to said image carrying body (12), and transferring the toner image on the surface of said image carrying body to said sheet of paper; electrifying means (30) for applying a voltage to said dielectric belt (10) at a stage before said sheet of paper is attracted to said dielectric belt (10) ; and electrostatic attraction means (60) for electrostatically attracting said sheet of paper to the dielectric belt, after electrification of the belt by the electrifying means (30), said electrostatic attraction means (60) comprising a conductive roller which has a peripheral portion coming into contact with a surface of said sheet of paper conveyed on said dielectric belt on an opposite side to said dielectric belt (10) ; characterised in that the core of said conductive roller (60) is grounded, and an absolute value of a voltage applied to said electrifying means (30) is higher than a threshold breakdown voltage for discharge in the conductive roller (60), and in that said 5 electrifying means (30) and said electrostatic attraction means (60) are arranged to oppose a common conductive roller (34) so that the dielectric belt (10) passes between the conductive roller (34) and the electrifying means, and between the conductive roller (34) and the electrostatic attraction means (60). An electrostatic recording system according to claim 24, wherein said conductive roller (60) comprises a grounded conductive core (61) of metal and an electrically resistive member (62) disposed around said metal core. An electrostatic recording system according to claim 25, wherein the electrical resistance of the resistive member (62) is 10 3 to 10 7 ohm-cm. An electrostatic recording system according to claim 25 or 26, wherein the hardness of the resistive member (62) is at least 20 degree by JIS-A. An electrostatic recording system according to claim 25, 26 or 27, wherein the resistive member (62) comprises rubber having a friction coefficient of 0.3 to 1.2. An electrostatic recording system according to claim 28, wherein a surface of the rubber has a UV treatment or resin coating for lowering the friction coefficient of said surface. An electrostatic recording system according to claim 25, 26 or 27, wherein the resistive member (62) is a porous sponge. An electrostatic recording system according to any one of claims 25 to 30, wherein said electrifying means (30) for applying a voltage to said dielectric belt comprises a conductive porous material and has a roller shape and is arranged to rotate in the same direction as the paper conveying direction with respect to said dielectric belt (10). An electrostatic recording system according to any one of claims 25 to 31 and having means for applying an A.C. voltage signal to said first-mentioned electrifying means (30) for applying a voltage to said dielectric belt (10), said A.C. voltage signal being a sine wave or rectangular wave having a D.C. offset voltage. An electrostatic recording system according to claim 32, wherein the polarity of said offset voltage is opposite to the polarity of the surface potential of said image carrying body (12). An electrostatic recording system according 5 to any one of the preceding claims, wherein the electrifying means (30) for applying a voltage to said dielectric belt (10) comprises a conductive fixed brush or rotary brush having a resistance value of 10 3 to 10 7 ohms. An electrostatic recording system according to claim 34 and comprising a felt-like dielectric belt cleaning device arranged for coming into contact with said dielectric belt (10) and disposed adjacent to said fixed brush or said rotary brush. An electrostatic recording system according to claim 34 or 35, wherein said first-mentioned electrifying means (30) is a rotary brush arranged to rotate in an opposite direction to the paper conveying direction with respect to said dielectric belt (10).

Inventors: FUKE KENJI, KAWABATA KAZUMI, MIZUNO TSUNEO, NAKASHIMA TETUROU, SHIMATSU KATSUYA, SUGIMOTO KATSUMI, SUZUKI EIJI, TAKIZAWA YOSHIMI, TANAKA TOMOAKI, WAKABAYASHI YASUHIRO, YOSHII HITOSHI, FUKE, KENJI, KAWABATA, KAZUMI, MIZUNO, TSUNEO, NAKASHIMA, TETUROU, SHIMATSU, KATSUYA, SUGIMOTO,KATSUMI, SUZUKI, EIJI, TAKIZAWA, YOSHIMI, TANAKA, TOMOAKI, WAKABAYASHI, YASUHIRO, YOSHII, HITOSHI
Applicants: FUJITSU LTD, FUJITSU LIMITED
Application year: 1997
Main classification: G03G15
IPC codes: G03G15