An image forming apparatus

Abstract: An image forming apparatus includes a movable image bearing member (1); an image forming device (3) for forming an image on the image bearing member (1); a charging member (5) press-contacted to or faced to the image bearing member (1) to transfer the image from the image bearing member (1) onto the transfer material (P) passed therebetween; a bias voltage applying device (8) for applying a bias voltage to the charging member (5), wherein the voltage applying device constant-voltage-controlled the charging member (5) in a period when an image region of the image bearing member (1) is present in the transfer position, and constant-current-controls the charging member (5) in another period, and wherein a voltage applied to the charging member (5) during the constant voltage control is a sum of a voltage provided during the constant current control and a predetermined voltage.

Description: FIELD OF THE INVENTION AND RELATED ART The present invention relates to an image forming apparatus such as an electrophotographic machine or an electrographic printer, more particularly to an image forming apparatus having an image transfer means in the form of a transfer roller. Document EP-A-0 323 226 discloses an image forming apparatus, comprising a movable image bearing member and image forming means for forming an image on said image bearing member. A charging member press-contacted to or faced to the image bearing member transfers the image from the image bearing member onto a transfer material passed therebetween, while bias voltage applying means apply a bias voltage to the charging member. In a known image forming apparatus wherein a toner image is formed on an image bearing member, and the toner image is transferred onto a transfer material, an image transfer means having a contact type transfer roller or a transfer belt is contacted to an image bearing member to form an image transfer position in the form of a nip formed therebetween. Into the nip, the toner image and the transfer material are inserted with the transfer means supplied with an image transfer bias, by which the toner image is transferred from the image bearing member onto the transfer material. In such an image forming apparatus, it is desirable in order to effect proper image transfer operation that the transfer current per unit area of the transfer material during the transfer operation is within a predetermined range irrespective of the resistance or width of the transfer material so as to apply proper amount of the electric charge onto the transfer material. Generally, an image forming apparatus such as a copying machine is usable with different kinds of transfer materials. Depending on the size of the transfer material used the area in which the image bearing member or the transfer roller is in contact with the transfer material is different. Under the circumstances, if a constant current control is effected when the transfer bias is applied, the area in which the transfer means is directly contacted to the image bearing member is different because the size of the transfer material is different. Therefore, it is difficult to flow substantially constant current only through the transfer material irrespective of the transfer materials. The problem also arises from the thickness or the material of the transfer material as well as the size of the transfer material. When, for example, the size of the transfer material supplied is small, and therefore, the contact area between the transfer roller and the image bearing member is too large, the constant current control through the transfer material is not satisfactory because the amount of charge applied to the transfer material is sometimes insufficient with the result of improper image transfer and the smaller toner retaining power onto the transfer material leading to the scattering of the toner or the disturbance of the image. When, on the contrary, the size of the transfer material supplied is so large that the contact area between the transfer roller and the image bearing member is too small, the constant current control through the transfer material is also unsatisfactory because the amount of electric charge applied onto the transfer material is sometimes too large, with the result that the toner is electrically charged to the polarity opposite to the right polarity, which leads to local image transfer void. In order to avoid the defects in the constant current control, it is considered that the transfer bias is constant-voltage-controlled. However, the roller or the belt used for the transfer roller or the transfer belt, has a resistance significantly changing depending on the ambient condition, particularly the humidity, although the change is different depending on the materials. In addition, paper which is most frequently used as the transfer material has the resistance significantly dependent on the ambient condition. Therefore, it is difficult to effect the stabilized image transfer operation irrespective of the size or the material of the transfer material or the ambient condition. In order to solve these problems, U.S. Patent No. 5179397 which has been assigned to the assignee of this application has proposed that for the purpose of simultaneously compensating for the variations in the resistances of the transfer material and the transfer roller or belt depending on the ambient condition, the constant current control is carried out when the transfer material is absent in the image transfer position, and when there is the transfer material in the transfer position, a constant voltage control is carried out with a voltage level provided by multiplying a coefficient and a voltage in the constant current control period. However, such a combination of the constant current control and the constant voltage control is still not completely satisfactory. For example, when the resistance of the transfer roller is small, the voltage level obtained by the constant current control is also small. If, at this time, the resistance of the transfer material is large, the bias voltage required during the constant voltage control is large, because it is a sum of a voltage across the transfer drum, the voltage across the transfer roller and the voltage across the transfer material. Because, however, the bias voltage provided in the constant current control is small, a required bias voltage is not provided. This results in short of electric charge on the transfer material, and therefore, improper image transfer or disturbance of the image. This stems from the fact that the voltage level of the transfer bias is obtained on the premise that the ambient conditions in the constant current control is the same as those in the constant voltage control, in other words, the variation in the resistance of the roller or the like depending on the ambience is detected, the change in the resistance of the transfer material is simultaneously corrected on the assumption that the resistance of the transfer material is that under the same ambience. Therefore, the proper voltage is not applied to the transfer material not yet saturated in the ambience, a high resistance transfer material such as OHP sheet or the like or the dried transfer material immediately after being subjected to the image forming operation in the case of duplex copying machine. SUMMARY OF THE INVENTION Accordingly, it is a principal object of the present invention to provide an image forming apparatus wherein stabilized images can be obtained irrespective of resistance change of the transfer roller depending on the ambience. It is another object of the present invention to provide an image forming apparatus wherein stabilized image can be provided irrespective of the resistance change of the transfer material depending on the ambience change or the like. These objects are achieved by the measures featured in claim 1. These and other objects, features and advantages of the present invention will become more apparent upon a consideration of the following description of the preferred embodiments of the present invention taken in conjunction with the accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is a side view of a major part of an image forming apparatus according to an embodiment of the present invention. Figure 2 is a timing chart illustrating operation of the apparatus of Figure 2. Figure 3A is a sectional view in which a transfer material is present between the photosensitive member and the transfer roller. Figure 3B shows an equivalent circuit of the state of Figure 3A. Figure 4 is a graph of a voltage-current characteristics of the transfer roller under a low temperature and low humidity condition. Figure 5 is a graph showing a voltage-current characteristics of an OHP sheet under the same ambient condition. Figure 6 is a graph of a voltage-current characteristics of an OPC photosensitive member under the same ambient condition. Figure 7 is a graph of a voltage-current characteristics under the same ambient condition when the sheet is not present there and when the OHP sheet is present there. Figure 8 is a graph of a current-voltage characteristics of a transfer roller under a high temperature and high humidity condition. Figure 9 is a graph of a voltage-current characteristics of an OPC photosensitive member under the same ambient condition. Figure 10 is a graph of a voltage-current characteristics under the same ambient condition when the sheet is not present there and when an OHP sheet is present there. DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring to Figure 1, there is shown an image forming apparatus having a cylindrical image bearing member (photosensitive member) 1 rotatable in a direction indicated by an arrow about an axis perpendicular to the sheet of the drawing. The photosensitive member 1 is electrically charged to a negative polarity by charging means the form of a charging roller 2 contacted to the photosensitive member 1 (discharging wire is usable in place of the charging roller 2). The charged photosensitive member 1 is exposed to light L in accordance with image information by an exposure means 3, by which an electrostatic latent image is formed on the photosensitive member 1 in accordance with the image information. A developing means 4 develops the electrostatic latent image thus formed, with toner having been charged to a positive polarity into a visualized image. The transfer roller having an axis extending in the same direction as that of the photosensitive member 1 is made of EPDM and is contacted to the photosensitive member 1. By the transfer roller 5, the toner image described above is transferred from the photosensitive member 1 onto the transfer material P in the transfer position which is the region between the photosensitive member 1 and the transfer roller 5. The transfer material P has been fed along a conveyance passage 6. To the transfer roller 5, the transfer bias of the negative polarity is applied. Between the transfer roller 5 and the photosensitive member 1, a clearance may be provided if it is smaller than the thickness of the transfer material P. In such a case, the transfer material P is pressed between the photosensitive member 1 and the transfer roller 5. Thereafter, the transfer material P is separated from the photosensitive member 1, and is conveyed along the passage 6 to an unshown image fixing station wherein the toner image is fixed on the transfer material P. On the other hand, the photosensitive member from which the image has been transferred, is cleaned by a cleaning device 7, so that the toner remaining on the photosensitive member is removed to be prepared for the repeated image forming operation. The transfer roller 5 is supplied with a predetermined bias voltage at predetermined timing in accordance with an image signal from a CPU (central processing unit) 9 by a voltage source (bias applying means) 8 which is capable of effecting constant voltage control and constant current control (ATVC: active transfer voltage control). Figure 2 shows the operational timing of the constant current control and the constant voltage control. As will be understood from this Figure, the constant current control is carried out when the sheet is absent from the transfer region, and a constant voltage control is effected when the sheet is present in the transfer region. In other words, when the image region of the image bearing member (the region having the toner image) is in the transfer position, the voltage source 8 effects the constant voltage control to the transfer roller 5, and the voltage Source 8 effects the constant current control during part of the other period. Thus, the transfer roller 5 is constant-current-controlled when the non-image area of the photosensitive member 1 not having the toner image is in the transfer position, that is, when the transfer material is absent from the transfer position (non-passage period), as shown in Figure 2. Such a period occurs after the CPU 9 receives the image-on signal, more particularly, upon the pre-rotation period for the purpose of warming the fixing device up or in the sheet interval period between end of one sheet and a start of the next sheet when plural images are continuously formed. In this embodiment, the constant current through the transfer roller was -1 micro-ampere during the non-passage period. During this period, a basic datum for obtaining a voltage level required during the constant voltage control is obtained (V D ). Then, the voltage V D across the transfer roller 5 is stored in an unshown storing means. When the image region of the photosensitive member, in other words, the transfer material is present in the transfer positionl the CPU 9 supplies a constant voltage control signal to the voltage source 8. Then, the voltage source 8 effects the constant voltage control to the transfer roller 5 with a constant voltage level which is provided by adding a predetermined constant voltage to the stored voltage V D . By the constant voltage control, the toner image is transferred from the photosensitive member 1 to the transfer material P. Here, the voltage V D may be an average of plural samples voltages during the constant current control period. The voltage V actually applied to the transfer roller 5 during the constant voltage control (transfer operation) is determined in the following manner. Figure 3A schematically shows the state wherein the transfer material P is present in the transfer position. Figure 3B shows an equivalent circuit thereof. In Figure 3B, the transfer roller is represented as a resistor, and the transfer material P and the photosensitive member 1 is parallel circuit constituted by a resistor and a capacitor. The voltage V applied to the transfer roller 5 during the period for transferring the toner image from the photosensitive member 1 to the transfer material P for effecting the proper image transfer operation is: V = Vroll + Vpaper + Vdrum When a certain density of electric charge is applied to a unit area of the toner image on the photosensitive member in the transfer operation, that is, when a certain current flows through a unit area of the toner image, the efficiency of the toner transfer is maximum, and the image quality of the copy image is optimum. The current level should not be larger and should not be smaller. If the voltage across the transfer roller is constant, the electric charge (current) of the toner image per unit area is constant. In order to provide the good quality of the image under different ambient conditions and with different kinds of the transfer material, it is desired that the optimum current flows per unit area of the toner image. Then, it is desirable that a transfer current providing it is obtained. Therefore, the characteristics of the voltages Vroll, Vpaper, Vdrum will be analyzed. Figures 4, 5 and 6 show the voltage-current characteristics (V-I characteristics) of the transfer roller, the transfer material (OHP sheet) and the OPC photosensitive member under the low temperature and low humidity (15 o C and 10 %RH). Figure 7 shows the V-I characteristics of all of them combined. More particularly, the line 0 V in Figure 6 shows the V-I characteristics of the photosensitive member in the non-passage period, that is, when the photosensitive member is not electrically charged. The line represented by "non-toner" represents the V-I characteristics of the charged photosensitive member without toner image (exposed area), and the line indicated by "toner" represents the V-I characteristics of the charged photosensitive member in the image portion (the portion deposited with the toner) (sheet passage period). In Figure 7, the curvature (1) represents the V-I characteristics during the non-passage period when the photosensitive member has the potential of 0 V (that is, during the constant current control), and the curve (2) shows the V-I characteristics when the OHP sheet is passed through the transfer station. In other words, the curve (1) in Figure 7 is the V-I characteristics during the constant current control, and therefore, it is a sum of the V-I characteristics of the transfer roller shown in Figure 4 and the V-I characteristics of the photosensitive member shown by 0 V curve of Figure 6. The curve (2) of Figure 7 represents the V-I characteristics during the constant voltage control, and therefore, it is a sum of the curve (1), the V-I characteristics of the transfer material shown in Figure 5 and the V-I characteristics "toner" of Figure 6. Assuming that the optimum current during the transfer action (sheet passage period) is -1 micro-ampere, the optimum voltage in is selected so that the current of -1 micro-ampere flows through the transfer material P or through the toner image on the photosensitive member. To accomplish this, the transfer roller 5 is constant-current controlled during the sheet-non-passage period in which the transfer material is absent from the transfer position. The voltage V D in Figure 7 is a voltage when the constant current of -1 micro-ampere flows during the non-passage period. That is, it is the voltage (Vroll + V'drum ) provided when a constant current flows through the circuit shown in Figure 3B without paper with the charge potential of the photosensitive member being, when the constant current control is effected during non-passage period. Therefore, the voltage V D is the voltage during the constant voltage control providing the optimum current during the transfer operation. During this, V D = -1.7 (KV) was provided. The voltage V D is held for the purpose of the subsequent constant voltage control during the transfer action. On the basis of the stored voltage V D the optimum voltage level V during the constant voltage control is provided. In the constant voltage control, a toner image is on the photosensitive member or drum 1. In addition, since the transfer material P is present between the photosensitive member 1 and the transfer roller 5, the voltage V1 corresponding thereto is added to the voltage V D . The potential in the toner deposited portion is predetermined in the apparatus. As the voltage through the transfer material is, in this embodiment, a voltage corresponding to the resistance of a highest resistance used in this apparatus (OHP sheet, for example) under the low temperature and low humidity condition (5 o C, 10 %RH, for example). When any transfer material is used in the apparatus, the voltage corresponding to the highest resistance of the transfer material is used. Therefore, no higher resistance of the transfer material is present in the apparatus, and the resistance of the transfer material is not higher than that in any used condition of the apparatus. Therefore, the voltage added to the detected voltage V D is constant. By adding the constant voltage V1 to the voltage V D , a good voltage V can be obtained even if the resistance of the used transfer material does not correspond to the ambient condition under which the constant current control is carried out. From Figures 5 and 6, a voltage inpaper = -1.5 KV and Vdrum = -0.8 KV were obtained as the voltages to be added to the voltage V D . Therefore, the voltage V to be applied during the constant voltage control is V D + V1, more particularly, in = -4.0 KV. The current through the transfer material at this time will be under,stood to be -1 micro-ampere from Figure 7, and therefore, good image transfer operation is provided. Figures 8 and 9 shows the V-I characteristics of the transfer roller and the OPC photosensitive member under the high temperature and high humidity ambience (32.5 °C and 90 %RH). Figure 10 is the V-I characteristics when the transfer roller is combined with Figure 5. Under a high humidity condition, the V-I characteristics of the transfer roller 5 are as shown in Figure 8, which is different from that under the low humidity condition (Figure 4). However, the voltage across the photosensitive member 1 is as shown in Figure 9, and therefore, is not hardly different from that during under the low humidity (Figure 6). With this characteristics, if the current of -1 micro-ampere flows during the constant current control, V' D (= V'roll + V'drum) can be provided. Thereafter, during the constant voltage control, a constant voltage V1 (= Vpaper + Vdrum) under any ambient conditions, and the transfer roller is supplied with the transfer bias voltage of V D ′ + V1. The current through the transfer material at this time is -1 micro-ampere as will be obtained from Figure 10, and therefore, good image transfer operation is possible. As described in the foregoing, according to the present invention, the change in the resistance of the transfer roller can be, in effect, detected, and the good image transfer operation is possible irrespective of the resistance of the transfer material and the kinds of the transfer material. In the foregoing embodiment, the surface potential of the photosensitive member 1 during the constant current control is 0 V, but it is not limited to 0 V. It is considered that the constant current control is carried out when the surface potential of the photosensitive member 1 is that after a blank exposure or that after the development. In this case, the constant voltage added during the constant voltage control is only Vpaper. While the invention has been described with reference to the structures disclosed herein, it is not confined to the details set forth and this application is intended to cover such modifications or changes as may come within the purposes of the improvements or the scope of the following claims.

Claims: An image forming apparatus, comprising : a movable image bearing member (1); image forming means (2,3,4) for forming a toner image on said image bearing member (1); a charging member (5) press-contacted to or faced to said image bearing member (1) to transfer the toner image from said image bearing member (1) onto a transfer material (P) passed therebetween; bias voltage applying means (8) for applying a bias voltage (V) to said charging member (5), said image forming apparatus being characterized in that: said voltage applying means (8) is adapted to constant-voltage-control said charging member (5) in a first period when an image region of said image bearing member (1) is present in a transfer position, and to constant-current-control said charging member (5) at an optimum current in a second period when an image region of said image bearing member is not present in the transfer position, in that said image forming apparatus is adapted to detect and store the voltage (V D ) arising across the charging member during constant-current-control; and in that said bias voltage (V) applied to said charging member (5) during the constant-voltage-control is a sum of said detected and stored voltage (V D ) and of a predetermined constant voltage (V₁) based at least on a resistance of a transfer material. An apparatus according to claim 1, characterized in that said charging member (5) and said transfer material (P) have resistances variable depending on ambient conditions. An apparatus according to claim 2, characterized in that the ambient condition includes an ambient humidity and an ambient temperature. An apparatus according to claim 1, characterized in that said image forming means (2, 3, 4) includes charging means (2) for charging said image bearing member (1), exposure means (3) for exposing said image bearing member (1) to light in accordance with an image signal, and means (4) for forming a toner image by depositing toner to a latent image provided by said exposure means (3). An apparatus according to claim 4, characterized in that said first voltage (V D ; V' D ) provided during the constant current control corresponds to a sum of a voltage (V roll ; V' roll ) across said charging member (5) and a voltage (V' drum ) across said image bearing member (1) not charged by said charging means (2) during the constant current control. An apparatus according to claim 4, characterized in that said first voltage (V D ; V' D ) corresponds to a sum of a voltage (V roll ; V' roii ) across said charging member (5) during the application of the constant current, and a voltage (V' drum ) across said image bearing member (1) during application of the constant current. An apparatus according to claim 4 or 5, characterized in that said predetermined voltage (V₁) corresponds to a sum of a potential difference (V drum ) and a voltage (V paper ) produced in the case the constant current flows through said transfer material (P) left under 15°C and 10% relative humidity, said potential difference (V drum ) being a difference between a potential of a toner image portion of said image bearing member (1) and a potential of a portion of said image bearing member (1) being in the transfer position during the constant current control. An apparatus according to claim 4 or 5, characterized in that said predetermined voltage (V₁) corresponds to a sum of potential difference (V drum ) and a voltage (V paper ) produced in the case the constant current flows through the transfer material which is in the form of an OHP transparent sheet and which is left under 15°C and 10% relative humidity, said potential difference (V drum ) being a difference between a potential of the toner image portion of said image bearing member (1) and a potential of said image bearing member (1) at the transfer position during the constant current control. An apparatus according to claim 1, characterized in that said charging member (5) is contacted to said image bearing member (1). An apparatus according to claim 4, characterized in that the image region is a region in which said image bearing member (1) has the toner image. An apparatus according to claim 1, characterized in that said transfer position is a position where said image bearing member (1) and said charging member (5) are faced to each other. An apparatus according to claim 1, characterized in that the constant current control is carried out before image transfer. An apparatus according to claim 4, characterized in that the constant current control is carried out before said image bearing member (1) is charged by said charging means (2). An apparatus according to claim 1, characterized in that said charging member (5) is in the form of a roller. An apparatus according to claim 1, characterized in that said charging member (5) is in the form of a belt. An apparatus according to claim is characterized in that said charging member (5) includes EPDM. An apparatus according to any one of the preceding claims, characterized in that the potential of said image bearing member (1) at the transfer position during the constant current control is substantially zero. An apparatus according to any one of the preceding claims, characterized in that in said first period, said transfer material (P) is in the transfer position, and in said second period, said transfer material (P) is not in the transfer position.

Inventors: ASAI JUN, ASAI, JUN
Applicants: CANON KK, CANON KABUSHIKI KAISHA
Application year: 1990
Main classification: G03G15
IPC codes: G03G15