An image forming apparatus

Abstract: An image forming apparatus includes an image bearing member for carrying toner images of different colors; a rotatable intermediary transfer member to which the toner images are superimposedly transferred from said image bearing member onto said intermediary transfer member at a first transfer position, wherein the toner images are then transferred all together from said intermediary transfer member onto the transfer material at a second transfer position; wherein said intermediary transfer member includes an elastic layer having a thickness of 0.5-2(mm), a coating layer, on said elastic layer, having a volume resistivity which is larger than that of said elastic layer, and said intermediary transfer member satisfies: T≤τ≤500 (sec) where τ (sec) is time required for a potential V of said intermediary transfer member at one second after a surface of said intermediary transfer member charged to a predetermined potential rotates at a rotational speed of 10 (cm/s) to become V/e (e: base of natural logarithm, e=2.71828...); and T (sec) is a rotation period of said intermediary transfer member when the toner images on said image bearing member are sequentially and superimposedly transferred onto said intermediary transfer member at the first transfer position.

Description: FIELD OF THE INVENTION AND RELATED ART The present invention relates to an image forming apparatus wherein a toner image formed on an image bearing member is transferred onto an intermediary transfer member, which in turn is transferred onto a transfer material. Conventionally, in an image forming apparatus of an electrophotographic type, an intermediary transfer member is provided in addition to a photosensitive drum as an image bearing member. In such an apparatus, the primary transfer operation of transferring a toner image formed on the image bearing member onto an intermediary transfer member, is repeated to superimpose the toner images on the intermediary transfer member, and then, the toner images are transferred all together onto a transfer material(secondary transfer). Figure 11 shows an example of an image forming apparatus using the intermediary transfer member. The image forming apparatus shown in this Figure is provided with a photosensitive drum 101 as the image bearing member. Around the photosensitive drum 101 supported for rotation in the direction of an arrow R1, four developing devices 105, 106, 107, 108 containing black (BK), magenta (M), cyan (C) and yellow (M) toner materials. Among the developing devices, the one operated for the development of the electrostatic latent image on the photosensitive drum 101 is brought into contact to the photosensitive drum 101 by toward and away from means (unshown). The photosensitive drum 101 is charged uniformly by a charger 102, and is exposed to a scanning light (laser beam) 104 through a laser exposure optical system 103 or the like so that an electrostatic latent image is formed. The electrostatic latent image is developed by the developing device 105 or the like into a toner image, and is transferred (primary transfer) onto the intermediary transfer belt 109 (intermediary transfer member) sequentially by a primary transfer roller 110. The development of the electrostatic latent image, the development thereof and the primary transfer thereof is carried out for the four color toner materials by the developing devices 105-108 or the like sequentially, by which superimposed color toner image is formed on the intermediary transfer belt 109. Then, the toner image is transferred (secondary transfer) all together onto the transfer material 118 fed by a secondary transfer roller 111 and an intermediary transfer belt 109. The primary transfer and the secondary transfer will be further described. When the photosensitive drum 101 is an OPC (organic photoconductor) photosensitive member having a negative charging property, for example, negative property toner is used in the development by the developing devices 105-108 to deposit the toner to the exposure portion (laser beam 104). Therefore, the primary transfer roller 110 is supplied with a transfer bias voltage by a bias voltage source 120. Here, the intermediary transfer belt 109 is normally an endless resin film of PVdF (polyvinylidene fluoride), Nylon, PET (polyethylene terephthalate), polybarbonate or the like material ((resistance adjustment is carried out if necessary) having a thickness of 100-200µm and having a volume resistivity of 10 11 -10 16 Ω, cm approx., and is extended around a rear surface roller 112, driving roller 115, tension roller 116 or the like. Usually, the primary transfer roller 110 is of a low resistance roller having a volume resistivity of not more than 10 5 Ω, cm. By using a thin film as the intermediary transfer belt 109, a large electrostatic capacity such as several 100 to several 1000pF can be provided at the primary transfer nip N 1 , and therefore, stabilized transferring current can be provided. In the foregoing, the primary transferring means is constituted by the primary transfer roller 110 and the bias voltage source 120. Then, the toner image is transferred onto the transfer material 118 by secondary transferring means including the secondary transfer roller 111, rear roller 112, bias voltage source 121 or the like. In the secondary transfer station, the rear roller 112 having a low resistance and supplied with a proper bias or electrically grounded is provided inside the intermediary transfer belt 109 as an opposite electrode, and the intermediary transfer belt 109 is sandwiched by the rear roller 112 and the secondary transfer roller 111 having a low resistance and disposed outside to form a secondary transfer nip N 2 . A transfer bias of the positive is applied to the secondary transfer roller 111 by a bias voltage source 121, and the secondary transfer roller 111 is contacted to the back side of the transfer material 118. The photosensitive drum 101 having subjected to the primary transfer is cleaned by a cleaner 119 for removing the primary untransferred toner from the surface thereof, and then, the residual charge is removed by an exposure device 117 so that it can be used for the next image formation. On the other hand, the surface of the intermediary transfer belt 109 which has been subjected to the secondary transfer, is cleaned by a cleaner 113 so that secondary untransferred toner is removed, and thereafter, is electrically discharged by a (discharging means) 114. The discharging 114 is an AC corona charging in many cases. Usually an opposite electrode is provided inside the intermediary transfer belt 109 to increase the discharging efficiency. In the conventional system, there are the following problems. (1) when the intermediary transfer belt 109 has a high surface hardness, central void tends to occur in the toner image on the intermediary transfer belt 109 after the primary transfer. (2) the transferring current is determined mainly by an electrostatic capacity of the intermediary transfer belt 109, and therefore, the secondary transfer tends to become insufficient if the toner amount per unit area is large. (3) when the attracting electrostatic force of the toner to the intermediary transfer belt is small, the intermediary transfer belt may repeatedly be bent at the outer surface of the rollers 112, 115 and 116 or the like around which the intermediary transfer belt is stretched, as shown in Figure 11, or the surface expansion and contraction are repeated, the unfixed Y, M, C, BK toner images superposed on the intermediary transfer belt surface can be disturbed. The disturbance of the toner image occurs remarkably when the amounts of the toners constituting the toner image are large, and a full-color letter or the like is formed by superimposing a plurality of colors of toners on the intermediary transfer belt 109. This is because when the toner images are superimposed on the intermediary transfer belt 109, the toner of the toner image on the surface part (the toner image transferred afterward) scatters. On the other hand, US-A-5,243,392 discloses that in order to improve the secondary transfer efficiency, a charge relaxation time τ of the intermediary transfer belt is made 0.3-200 (sec). The charge relaxation time τ is the one theoretically determined. However, the theoretical charge relaxation time τ is significantly different from the charge relaxation time measured actually. SUMMARY OF THE INVENTION Accordingly, it is a principal object of the present invention to provide an image forming apparatus wherein scattering of the toner on the intermediary transfer member due to the weakness of the electrostatic attraction force is prevented. It is another object of the present invention to provide an image forming apparatus wherein the reduction of the transfer efficiency of the toner image from the intermediary transfer member onto the transfer material due to the property of the layer structure of the intermediary transfer member, is suppressed, while preventing the toner scattering. 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 an illustration of an image forming apparatus according to Embodiment 1 of the present invention. Figure 2 is a longitudinal sectional view showing a layer structure of the intermediary transfer belt. Figure 3 is an illustration of a measuring method of a charge relaxation time τ. Figure 4 shows a change of the surface potential of the intermediary transfer belt with time. Figure 5, (a) shows a state in which M toner is superimposed on a Y toner on a conventional intermediary transfer belt surface, and (b) shows a state in which the M toner on the Y toner is scattered when the intermediary transfer belt is bent by a roller outer surface. Figure 6, (a) shows a state in which M toner is superimposed on Y toner on the intermediary transfer belt surface in the apparatus according to Embodiment 1, and (b) shows a state in which the M toner on the Y toner does not scatter even if the intermediary transfer belt is bent by the roller outer surface. Figure 7 is an illustration of a second image bearing member in Embodiment 2 of the present invention. Figure 8 is an illustration of a second image bearing member in Embodiment 3 of the present invention. Figure 9 shows timing of primary transfer, secondary transfer and discharging during a continuous printing operation. Figure 10 is an illustration of a second image bearing member according to Embodiment 6 of the present invention. Figure 11 is an illustration of a conventional image forming apparatus. Figure 12 shows a relation between the charge relaxation time τ and line scattering and secondary transfer property. Figure 13 shows a relation among a coating thickness, charge relaxation time τ, surface potential V 0 , line scattering and a secondary transfer property. Figure 14 shows a bias waveform in Embodiment 5 of the present invention. DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, the embodiments of the present invention will be described with reference to drawings. Embodiment 1 Figure 1 is a schematic drawing which illustrates the general structure of the full-color image forming apparatus in the first embodiment of the present invention. First, the overall structure and operation of the image forming apparatus will be described with reference to the drawing. The image forming apparatus illustrated in the drawing is a full-color image forming apparatus based on four primary colors, and comprises the following seven essential structural members (means): an image bearing member 1, visible image forming means 2, 3 and 4, an intermediary transfer member 5, a first transferring means 6, and a secondary transferring means 7. The general operation of the image forming apparatus, which is carried out by these essential structural members (means), is as follows. A visible image is formed on the image bearing member 1 by the visible image forming means 2, 3 and 4, and the visible image is first transferred onto the intermediary transfer member 5 by the primary transferring means. Thereafter, the visible image on the intermediary transfer member 5 is transferred onto a transfer medium P such as paper by the secondary transferring means 7. Next, the steps in the image forming operation will be described following the normal sequence. The image bearing member illustrated in the drawing is an electrophotographic photosensitive member 1 (hereinafter, "photosensitive drum") in the form of a drum having a diameter of approximately 46 mm. The photosensitive drum 1 comprises a cylindrical base member of aluminum, and a photosensitive layer, for example, an organic photoconductor layer, which covers the surface of the cylindrical base member. The photosensitive drum 1 is rotatively driven in the direction of an arrow mark R 1 by a driving means (unillustrated). The visible image forming means comprises a charging means 2, an exposing means 3, a developing means 4, and the like. The charging means 2 is provided with a charge roller 21, which is placed in contact with the photosensitive drum 1, and an electrical power source (unillustrated) for applying charge bias to the charge roller 21. In this embodiment 1, the surface of the photosensitive drum 1 is uniformly charged to negative polarity by the electrical power source through the charge roller 21. The exposing means 3 is provided with a laser based optical system 31. The surface of the photosensitive drum 1 is exposed to a scanning laser beam 32 projected according to image data. As a result, charge is removed from the exposed portions; in other words, an electrostatic latent image is formed. The developing means 4 comprises a rotary member 41, and four developing devices, that is, developing devices 4M, 4C, 4Y and 4B containing magenta, cyan, yellow and black developer (toner), correspondingly, which are mounted on the rotary member 41. In developing an electrostatic latent image, a developing device which contains specific color toner for developing an electrostatic latent image on the photosensitive drum 1 is positioned at a developing point at which the developing device is caused to face the surface of the photosensitive drum 1 by rotating the rotary member 41. Since, the four developing devices are the same in structure, they are described with reference to the magenta color developing device 4M. The developing device 4M comprises a rotatable sleeve 4a, a coating roller 4b for coating toner on the surface of the development sleeve 4a, an elastic blade 4c for regulating the thickness of the toner layer formed on the development sleeve 4a, and the like. In an image developing operation, non-magnetic, single component, negatively chargeable, magenta color toner in a toner container 4d is uniformly coated, while being triboelectrically charged, on the development sleeve 4a. Then, as development bias is applied so that the potential level of the development sleeve 4a becomes negative, relative to that of the photosensitive drum 1, the magenta toner is adhered to the latent image on the photosensitive drum 1; the latent image is developed in reverse. The main structural component of the intermediary transfer member 5 is an intermediary transfer belt 51 (intermediary transfer member). The intermediary transfer belt 51 is basically an approximately 0.5 - 2.0 mm thick flexible endless belt, and is stretched around a driving roller 52, a follower roller 53, and an auxiliary secondary transfer roller 72, which will be described later, and the like, and is rotatively driven in the direction of an arrow mark R 5 . The intermediary transfer belt 51 is pinched by the aforementioned photosensitive drum 1 disposed on the outward surface side of the belt, and a primary transfer roller 61 disposed on the inward surface side of the belt, which will be described later. The contact area between the surface of the intermediary transfer belt 51 and the surface of the photosensitive drum 1 constitutes the primary transfer nip N 1 (primary transfer point), which is in the form of a narrow rectangle elongated in the direction of the generatrix of the surface of the photosensitive drum 1. The primary transferring means 6 comprises a primary transfer roller 61 and an electrical power source 62. The primary transfer roller 61 is 14 mm in diameter, and is composed of electrically conductive sponge rubber having an electrical resistance of no more than 10 5 ohm/cm. It is placed in contact with the inward surface of the intermediary transfer belt 51. The power source 62 applies the primary transfer bias to the primary transfer roller 61. As the primary transfer bias in a range of +100 to +1000 V is applied, with gradual increase, to the primary transfer roller 61 by the power source 62, the magenta toner image formed on the photosensitive drum 1 is transferred onto the intermediary transfer belt 51 (primary transfer). After the primary transfer, the photosensitive drum 1 is cleaned by a cleaner 8; the toner remaining on the photosensitive drum 1 after the primary transfer is moved by the cleaner 8. Then, the cleaned photosensitive drum 1 is subjected to the following image formation. The above described image formation sequence comprising the charging, exposing, developing, transferring (first), and cleaning processes is carried out for remaining three colors, that is, cyan, yellow and black colors. As a result, four color toner images are superposed on the intermediary transfer belt 51. The secondary transferring means 7 comprises a secondary transfer roller 71 disposed on the outward surface side of the intermediary transfer belt 51, and an auxiliary secondary transfer roller 72 disposed on the inward surface side of the intermediary transfer belt 51 to oppose the secondary transfer roller 71. The contact area between the surface of the secondary transfer roller 71 and the surface of the intermediary transfer belt 51 constitutes a narrow rectangular secondary transfer nip N 2 (secondary transfer point). To the secondary transfer roller 71, an electrical power source 73 which applies the secondary transfer bias to the secondary transfer roller 71 is connected, and the auxiliary secondary transfer roller 72 is floated. The four color toner images transferred (primary transfer) onto the intermediary transfer belt 51 are transferred (secondary transfer) all at once onto a transfer medium P such as paper as the secondary transfer bias is applied to the secondary transfer roller 71 from the power source 73. After the secondary transfer, the intermediary transfer belt 51 is cleared of the charge remaining on the surface thereof, by discharging means 9. The discharging means 9 comprises a discharge roller 91, a housing 92 movable in the direction of an arrow mark K 9 , and an auxiliary roller 93 which opposes the discharge roller 91, with interposition of the intermediary transfer belt 51. In discharging the intermediary transfer belt 51, the housing 92 is moved in the direction of the arrow mark K 9 so that the intermediary transfer belt 51 is pinched between the discharge roller 91 and the auxiliary roller 93, and a predetermined bias voltage is applied by the power source 94. As a result, the residual charge on the intermediary transfer belt 51 is removed; in other words, the intermediary transfer belt 51 is initiated. The intermediary transfer belt 51 can be discharged by a contact type charging means, which does not depend on corona discharge; as one of the effects of using low resistance rubber material as the material for the base layer of the intermediary transfer belt 51, which will be described later, the residual charge can be removed with the use of a contact type discharging means. The transfer medium P onto which the four color toner images have been transferred (secondary transfer) by the secondary transferring means 7 is heated and pressed by a fixing apparatus (unillustrated) so that the toner images are fixed to the surface of the transfer medium P. Thereafter, it is discharged from the main assembly of the image forming apparatus. In an image forming operation comprising the aforementioned sequence of processes, a process speed Vp is set at 10.0 cm/sec, and the transfer medium P is conveyed in the direction of an arrow mark Kp by a transfer medium conveying means (unillustrated). Next, the second image bearing member 5, the secondary transferring means 7, and the discharging means 9, which characterize the present invention, will be described in detail. Referring to Figure 2, the intermediary transfer belt 51 comprises a base layer 51a and a layer 51b coated on the base layer 51a. The base layer 51a is in the form of a seamless cylinder which is 1 mm in thickness, 220 mm in width, and approximately 140xπ mm in peripheral length. It is formed of nitrile butadiene rubber, ethylene propylene rubber, or the like, which has a hardness of 60 deg. in JIS-A scale, and the volumetric resistivity of which has been adjusted to approximately 1x10 4 ohm.cm with admixture of carbon, titanium oxide, tin oxide, and the like. One of the methods for forming the base layer 51a is as follows: the rubber is extruded in a manner to cover a reinforcement fiber core, and is hardened. This method produces a very strong base layer 51a which stretches or shrinks very little. As for the high resistance layer 51b coated on the base layer 51a, urethane binder, or the like, in which a mold releasing agent such as Teflon or the like has been dispersed is coated on the base layer 51a to a thickness of approximately 50 µm. As for the coating method, spraying, dipping, and the like can be used. In this embodiment, six intermediary transfer belts were made, the charge relaxation times τ of which were set at no more than 1 second, 2 seconds, 5 seconds, 50 seconds, 500 seconds, and no less than 1000 seconds by adjusting the resistance value of the material for the layer 51b, and were subjected to an evaluation, which will be described later. Next, a method for measuring the charge relaxation time τ of the base layer 51a will be described. The length of the charge relaxation time τ is generally determined by the resistance R and capacitance C of the intermediary transfer belt: τ = R.C. The resistance of the intermediary transfer belt 51 in this embodiment is rendered ignorably small, compared to the resistance of the coated layer 51b, to yield a sufficient amount of transfer current (volumetric resistivity is desired to be in a range of 10 2 - 10 7 ohm.cm), and therefore, the values of the aforementioned R and C of the intermediary transfer belt 51 are determined by the coated layer 51b, or the surface layer. However, in reality, even if each parameter is individually measured, and the charge relaxation time τ is calculated according to the formula: τ = R.C, the calculated value does not completely match the actual charge relaxation time. Therefore, it is desirable that the charge relaxation time τ is directly measured with the use of a jig. As the resistance of the base layer 51a becomes unignorably large, the apparent charge relaxation time τ of the intermediary transfer belt becomes large, but scattering of toner is not reduced since the capacitance of the intermediary transfer belt 51 is small. Therefore, the secondary transfer performance also deteriorates. As for the method for measuring the resistance of the base layer 51a, it is simplest to measure the resistance before the layer 51b is coated. For example, it can be measured in the following manner. The base layer 51a is molded as an endless belt which is approximately 140xπ mm in peripheral length, and 220 mm in width. Then, a piece having a predetermined size is cut from the molded belt, and the resistance of this piece is measured by a high resistance meter 8340A of Advantest Co. (probe electrode diameter: 50 mm; guard electrode diameter: 70 mm in internal diameter and 80 mm in external diameter; opposing electrode: one in conformity with JIS-K6911). In measuring the resistance of the piece of the belt, the piece is pinched from the top and bottom, and a voltage of 500 V is applied. It should be noted here that, if necessary, the voltage to be applied may be lowered since breakdown may occur depending on the amount of the resistance. Next, referring to Figure 3, a method for measuring the charge relaxation time τ will be described. In Figure 3, the intermediary transfer belt 51 is stretched around a driving roller 207 and a metallic tension roller 206 of a measuring jig, and is rotated in the direction of an arrow mark at a speed of 10.0 cm/sec. The intermediary transfer belt 51 is pinched by a charge roller 201 (made of the same material as the discharge roller 91, which will be described later) and an opposing metallic auxiliary roller 208, at a charging point, and is charged by an AC power source 202, the output of which is approximately 3 kV in peak-to-peak voltage Vpp, and a DC power source 203, the output of which is +500 V. The intermediary transfer belt 51 charged by the charge roller 201 is measured for surface potential by a surface potentiometer 205, the probe 204 of which is positioned at a point which is one second away from the charging point in terms of the rotational time of the belt. After the surface potential of the intermediary transfer belt 51 is measured, the driving roller 207 is stopped, and then, the relaxation of the surface potential of the belt is measured. When measured actually, the surface potential of the intermediary transfer belt 51 attenuated as shown in Figure 4, in which V 0 represents the surface potential of the intermediary transfer belt 51 at the moment when the intermediary transfer belt 51 is stopped, and τ represents the time which elapsed before the surface potential of the intermediary transfer belt 51 attenuated to V 0 /e, e being the base of natural logarithm (e = 2.71828...). In order to make the aforementioned six intermediary transfer belts different in charge relaxation time τ (no more than one second to no less than 1000 seconds), six different materials were selected for the layer 51b from among the materials, the volumetric resistivities of which were in an approximate range of 10 12 - 10 16 ohm.cm. Since the volumetric resistivity of the coated layer 51b is very high, the measured volumetric resistivity of the intermediary transfer belt 51 is very dependent on the voltage at the time of the measurement, and the thickness of the coated layer 51b. Therefore, it is desirable that the charge relaxation time τ is directly measured by the method described above. In this embodiment, the measurement was made in an environment with normal temperature (23 °C) and humidity (50 %RH). The secondary transfer roller 71 of the secondary transferring means 7 is a rubber roller which is 18 mm in diameter, and is made of foamed EPDM which is approximately 40 deg. in hardness (ASCA-C scale), and approximately 10 4 ohm.cm in volumetric resistivity. As for the material for the secondary transfer roller 71, low resistance urethane rubber, chloroprene rubber, NBR, or the like may be used, in addition to the material used in this embodiment. To the transfer bias power source 73, a voltage in a range of approximately +1000 to +2000 V was applied while adjusting the voltage, so that a transfer current of approximately 10 pA flowed while a transfer medium was passed. The discharge means comprised a discharge roller 91 made of the same material as the material for the charge roller 21. The charge roller 21 was a well-known contact type charge roller. It was a cylindrical member having an overall diameter of approximately 12 mm, and comprised: an approximately 3 mm thick bottom layer of electrically conductive elastic rubber; a 100 - 200 µm thick middle layer having a medium volumetric resistivity of approximately 10 6 ohm.cm; and an adhesion preventive top layer (nylon resin or the like), the thickness of which was no less than 10 µm and no more than 100 µm. To the charge roller 91, a combination of an AC voltage having a peak-to-peak voltage Vpp of approximately 3 kV, and a DC voltage in a range of +100 - +1000 V was applied from an electrical power source 94, and the opposing auxiliary roller 93 was kept floated. Under the above described conditions, images were actually formed for evaluation. Generally speaking, the depth of a recorded image is improved in proportion to the amount of the toner contained in the image, that is, the amount of the toner contained in an image formed on the photosensitive drum 1, and also, the amount of the toner which is scattered greatly changes depending on the amount of the toner contained in the image formed on the photosensitive drum 1. Therefore, the amount of the toner to be adhered to the photosensitive drum 1 was adjusted in consideration of the above fact. More specifically, the amount of the toner to be adhered to the photosensitive drum 1 was adjusted so that the amount of the toner contained in a solid image of yellow, magenta, cyan or black color became approximately 0.7 mg/cm 2 , and under this condition, letters of compound colors (blue, green, red, or the like) were printed and were evaluated in terms of the scattering of toner from the letters, that is, the images formed of lines. The amount of the toner scattered under the above described condition was assumed to be greater by 10 - 50 %, compared to the amount of the toner scattered in an average image. All the toners employed in this embodiment were non-magnetic, single component, negatively chargeable toners. Figure 12 shows the results of the evaluations of the toner scattering and the secondary transfer, regarding the aforementioned intermediary transfer belts which were different in charge relaxation time τ. Among the results given in Figure 12, the scattering of the toner from the lines (line washout) seems to be caused by the following mechanism. Referring to Figure 5, (a), when a red letter, for example, is formed by the toners, a yellow toner layer and a magenta toner layer are transferred (primary transfer), or superposed, onto the intermediary transfer belt 51 in this order. While the four color toner images are superposed on the intermediary transfer belt 51, a given point of the intermediary transfer belt 51 passes the rollers 52, 72 and 53 a number of times, and each time the given point of the intermediary transfer belt 51 passes the rollers, it is bent; in other words, the outward portion of the belt is stretched, and the inward portion of the belt is compressed, compared to a straight portion of the belt. As this bending occurs to the given point of the belt, the magenta toner superposed on the yellow toner is subjected to the shock from the bending, that is, the stretching and compressing, of the intermediary transfer belt 51, and the electrical repulsion from the yellow toner at the same time. As a result, the scattering of the magenta toner as illustrated in Figure 5, (b) occurs. In this embodiment in which a reversal development system is employed, when the charge relaxation time τ of the intermediary transfer belt 51 is long, the surface potential of the photosensitive drum 1 correspondent to the background region of an image (dark portion potential) is greater in terms of negativity than the surface potential of the photosensitive drum 1 correspondent to the actual image portion (light portion potential), that is, the region to which the toner is to adhere. Therefore, the amount of negative charge which transfers from a photosensitive drum region with less toner is more than that from a photosensitive drum region with more toner. As a result, "walls" of negative charge are formed on the intermediary transfer belt 51 as illustrated in Figure 6, (a), due to the potential difference between the two regions. More specifically, the aforementioned walls are formed due to the difference in the light region potential and dark region potential after the primary transfer (positive polarity). It is thought that these walls prevent the magenta toner (negatively charged) on the yellow toner layer from being scattered in the adjacencies. In this first embodiment, the time it took for the intermediary transfer belt 51 to be rotated once was approximately 5 seconds. In the case of an intermediary transfer belt with a charge relaxation time τ longer than 5 seconds, the magenta toner is electrostatically prevented from scattering, and in the case of an intermediary transfer belt with a charge relaxation time τ of less than 5 seconds, the scattering of the magenta toner could not be prevented. This is thought to be due to the following reason. That is, the intermediary transfer belt with the longer charge relaxation time τ could prevent the magenta toner from scattering throughout a full rotation of the intermediary transfer belt, whereas in the case of the intermediary transfer belt with the shorter charge relaxation time τ, the charge on the background region completely attenuates before the intermediary transfer belt is rotated a full turn and charged again by the primary transfer nip N 1 , and therefore, the scattering of the magenta toner cannot be prevented electrostatically. Further, this phenomenon, that is, the scattering of the toner, is more apparent when the diameters of the rollers 52, 53, and 72 in contact with the inward surface of the intermediary transfer belt 51 (in this embodiment, the diameters are 30 mm, 16 mm, and 30 mm, correspondingly) is smaller. Therefore, in order to effectively prevent the scattering of the toner, it is necessary to make the charge relaxation time τ of the intermediary transfer belt 51 longer than the time T (second) it takes for the belt 51 to be rotated one full turn. Also, the magnitude of the shock, to which the magenta toner is subjected as the intermediary transfer belt 51 is bent, that is, as the portions thereof are stretched or compressed, is affected by the thickness of the base layer 51a of the intermediary transfer belt 51; the thicker the base layer 51a, the worse the shock. This is the reason why the upper limit in the thickness of the base layer 51a in this embodiment 1 was set at 2 mm, whereas the lower limit was set at 0.5 mm to provide the intermediary transfer belt 51 with sufficient strength. On the other hand, in the case of the secondary transfer, if the charge relaxation time τ is too long, such a phenomenon that the toner cannot be entirely attracted onto a transfer medium P when the amount of the toner is large (toner fails to be entirely transferred through the secondary transfer process) occurs. This seems to be due to the following reason. In the case of an intermediary transfer belt 51 with a long charge relaxation time τ, the toner on the intermediary transfer belt 51 (in particular, yellow toner which passes the primary transfer point more times than the other color toners) is charged to a higher level of negative polarity as the primary transfer process is repeated. This high level charge is not neutralized by the positive charge during the secondary transfer process, because the resistance of the coated layer 51b of the intermediary transfer belt 51 is too high. In other words, the negative triboelectric charge of the toner becomes too much, interfering the transfer (secondary transfer) of the toner onto the transfer medium P. As a result, a certain amount of the toner remains on the intermediary transfer belt 51. According to the evaluation in this embodiment, the charge relaxation time τ of the intermediary transfer belt 51 is desired to be no more than 500 seconds. In addition, the effect of the thickness of the coated layer 51b of the intermediary transfer belt 51 was evaluated. In this test, seven intermediary transfer belts 51 having thicknesses of 1 µm, 2 µm, 5 µm, 20 µm, 50 µm, 80 µm, and 100 µm were made using the same material that was coated to a thickness of 50 µm to give the intermediary transfer belt 51 a charge relaxation time τ of 50 seconds. Then, these seven intermediary transfer belts 51 were used to form the aforementioned images, and the formed images were comparatively evaluated. The results of the evaluation are given in Figure 13. According to Figure 13, in order to prevent the occurrence of the line washout, the thickness of the coated layer 51b (hereinafter, "coat thickness") is desired to be no less than 2 µm, whereas from the standpoint of secondary transfer performance, it is desired to be no more than 80 µm. Also, it is evident from Figure 3 that between the two concerns, the line washout is greatly affected by the potential level V 0 , described regarding the method for measuring the charge relaxation time τ, to which the intermediary transfer belt 51 is charged, in addition to the charge relaxation time τ. The reason why the rate of the charge attenuation is drastically greater in the case of an intermediary transfer belt 51 having a coat thickness of no more than 5 µm than in the case of an intermediary transfer belt 51 having a coat thickness of no less than 20 µm is due to the fact that the electrostatic capacity of the intermediary transfer belt 51 increases as the coat thickness decreases, and the charging performance of the charge roller 201 illustrated in Figure 3 is not sufficient to accommodate the increase. It should be noted here that the fact that the potential level V 0 is low means that the walls created by the regions with no toner, which were illustrated in Figure 6, (a), are also low. Further, the charge relaxation time τ is not supposed to change, in view of the relationship (τ = R.C) among the charge relaxation time τ, the capacitance C, and resistance R, according to which increase in the capacitance C is offset (canceled) by decrease in the resistance R. Yet, Figure 13 shows that the thinner the coat thickness, the shorter the actually measured charge relaxation time τ. This contradiction is thought to be caused because the change in coat thickness and the change in resistance are not proportional to each other. In other words, as the coat thickness is reduced, the apparent resistance of the intermediary transfer belt 51 increases at a rate far greater than the rate of the coat thickness reduction, due to such phenomenons as leak, tunnel effect, and the like, and therefore, the charge relaxation time τ decreases. Further, Figure 13 indicates that as the coat thickness increases, the secondary transfer performance declines. This is thought to occur because the capacitance of the intermediary transfer belt 51 becomes so small that the secondary transfer current does not flow in an amount sufficient to transfer a large amount of toner. As described above, in this embodiment 1, the intermediary transfer belt 51 comprising the base layer 51a and a surface layer 51b was employed, wherein the base layer 51a was a 0.5 - 2.0 mm thick elastic rubber belt with a low resistance (10 2 - 10 7 ohm.cm in volumetric resistivity), and the surface layer 51b was a 2 - 80 µm thick coated layer with a high resistance. The charge relaxation time τ of the intermediary transfer belt 51 was rendered no less than the time a single rotational cycle of the intermediary transfer belt 51 takes (5 seconds in this embodiment 1), and no more than 500 seconds. As a result, the intermediary transfer belt 51 in this embodiment produced the following effects. (1) The usage of the highly strong and yet flexible rubber as the base layer of an intermediary transfer belt made it possible to produce an intermediary transfer belt which is very durable, and does not cause central void transfer during the primary transfer process (durability can be further increased with addition of a reinforcement core such as a fabric core). (2) The high resistance layer 51b was coated on the low resistance rubber base layer 51a to adjust the charge relaxation time τ of the intermediary transfer belt to a proper length, and therefore, even when a large amount of toner was transferred onto the intermediary transfer belt 51, the toner was prevented from being scattered by the deformation of the intermediary transfer belt 51 which occurs as the intermediary transfer belt 51 was rotated, and as a result, each toner image on the intermediary transfer belt 51 could be held in a desirable condition. (3) The high resistance coated layer 51b of the intermediary transfer belt 51 was rendered thin, being in a range of 2 - 80 µm, and therefore, a larger capacitance than that of a resin belt, or a belt or a prior type, could be realized, and the larger capacitance could generate a larger amount of secondary transfer current. As a result, the toner was very efficiently transferred from the intermediary transfer belt 51 onto the transfer medium P; a desirable secondary transfer performance was realized. Figure 7 depicts the second embodiment. Since the base layer 51a of the intermediary transfer belt 51 in this embodiment is extremely low in electrical resistance, the voltage on the inward facing surface of the intermediary transfer belt 51 remains virtually stable. Therefore, it is possible to apply DC voltage from a secondary transfer roller 71 and a discharge roller 91 simply by providing a primary transfer roller 61 with voltage while floating other rollers 53, 72, and 93, as illustrated in Figure 1. However, the AC voltage applied to the discharge roller 91 sometimes attenuates between the discharging point and the primary transfer point if the resistance of the base layer 51a of the intermediary transfer belt 51 is higher than a certain level. More specifically, if the volumetric resistivity of the rubber material for the base layer 51a is increased to a value in a range of 10 5 - 10 7 ohm.cm, there is a tendency that when a combination of an AC bias in the form of a sine wave having a voltage of 2.5 kVpp and a frequency of 2 kHz, and a DC bias having an approximate voltage of +100 V, is applied to the discharge roller 91 by the high voltage power source 74, the AC voltage applied in the thickness direction of the coated layer 51b is liable to attenuate, and hence discharge efficiency is liable to deteriorate. On the other hand, if the resistance of the rubber material for the base layer 51a is reduced, it becomes necessary to provide sufficient withstand voltage between the base layer 51a and the surrounding members. In other words, in terms of affording more latitude in apparatus design, it is better to set the resistance of the rubber material for the base layer 51a as high as possible. The problem described above can be reduced in magnitude by connecting the rollers 53, 61, 72, 93, and the like, disposed on the inward facing side of the intermediary transfer belt 51, to the primary transfer power source, as illustrated in Figure 7. In particular, in this second embodiment, an opposing roller 93 to a discharge roller 91 was rendered electrically conductive and was connected to the primary transfer power source. The results were very desirable (in this embodiment, the surface of the driving roller 52 was covered with insulative rubber to provide it with friction, and therefore, it was left floated). The above described structure sometimes displays its effectiveness in stabilizing the DC voltage applied to each bias roller, provided that the length of the intermediary transfer belt 51, and the positioning of the rollers 53, 61, 72 and 93, disposed on the inward facing side of the intermediary transfer belt 51, are properly adjusted. Embodiment 3 Figure 8 depicts the third embodiment of the present invention. In the preceding embodiments, the discharging AC current which is flowed through the discharge roller 91 flows to the ground through the primary transfer power source 62. Therefore, if the AC impedance of the primary transfer power source 62 itself is unignorably high compared to that of the intermediary transfer belt 51, the AC voltage applied by the discharge power source 94 is divided between the intermediary transfer belt 51 and the power source 62. As a result, the high AC voltage divided by the power source 62 is applied to the low resistance base layer 51a of the intermediary transfer belt 51. In the above described case, insertion of a bypass condenser 63 between the power source 62 and the ground makes it possible to accurately apply the AC voltage generated by the power source 94, between the discharge roller 91 and the opposing roller 93. As for the aforementioned bypass condenser 63, when a bypass condenser having a capacity of approximately 1x10 4 pF or more was used, desirable results could be obtained. For example, when a bypass condenser having a capacity of 10 pF was used, effective results could not be obtained. Embodiment 4 In the preceding embodiments, the arrangement in which a voltage in the form of a sine wave having a 2.5 Vpp and a frequency of 2 kHz was used as the discharging AC bias applied to the discharge roller 91 was described. The arrangement is definitely very effective if the secondary transfer efficiency is 100 %, but when there remains toner on the intermediary transfer belt 51 after a secondary transfer process, unillustrated cleaning means must be separately provided. In such a case, the cleaning means must be disposed on the upstream side of the discharge roller 91, relative to the rotational direction of the intermediary transfer belt 51, because if toner remains on the surface of the intermediary transfer belt 51 after a transfer process, problems such as the scattering of toner in the adjacencies occurs as AC bias is applied by placing the discharge roller 91 in contact with the belt 51 to discharge the belt 51. However, if the AC bias in the form of a sine wave applied to the discharge roller 51 is changed to a bias in the form of a rectangular wave having 60 - 90 % of the wave components on the positive side, and 40 - 10 % on the negative side, as illustrated in Figure 14, the aforementioned scattering of toner can be prevented; the residual charge on the intermediary transfer belt 51 can be removed; and in addition, the polarity of the post-transfer residual toner can be reversed (from negative to positive). Therefore, the aforementioned cleaning means becomes unnecessary. This is due to the following reason. As the polarity of the residual toner on the intermediary transfer belt 51 is reversed to positive, it becomes possible to transfer normally (negatively) charged toner from the photosensitive drum 1 onto the intermediary transfer belt 51 through a primary transfer process, while recovering the residual toner on the intermediary transfer belt 51, onto the photosensitive drum 1; it becomes possible to carry out "simultaneous toner swapping". In other words, the residual toner on the intermediary transfer belt 51 from the secondary transfer process is ultimately recovered by a photosensitive drum cleaner 8. As is evident from this explanation, the apparatus in accordance with the present invention can be simplified with the use of asymmetrical AC bias as the bias to be applied to the discharge roller 9. More specifically, a bias comprising an AC voltage having a frequency of 2 kHz, a duty ratio of 80 % on the positive side, and a peak-to-peak ratio of 2.5 kV, and a DC voltage which sets the middle voltage Vmid of the bias at approximately +100 V, was applied to the discharge roller 91. The results were desirable: charge was removed from the intermediary transfer belt 51 at the same time as positive charge was given to the post-secondary transfer residual toner on the intermediary transfer belt 51, without scattering the toner. Embodiment 5 Since the rubber of the base layer 51a of the intermediary transfer belt 51 in this embodiment is extremely low in electrical resistance, the voltage on the inward facing surface of the intermediary transfer belt 51 remains virtually stable. Therefore, it is possible to apply DC voltage from a secondary transfer roller 71 and a discharge roller 91 simply by providing only a primary transfer roller 61 with voltage while keeping other rollers floated. Further, with the additional provision of the structure described in the second and third embodiments, desirable conditions for the application of the discharge AC voltage can be established. As for the DC current which flows through the secondary transfer roller 71, the discharge roller 91, and the like, its level is greatly affected by the potential of the opposing rollers 72, 93, and the like, that is, the primary transfer voltage. Therefore, in order to flow stable DC current for the secondary transfer and the discharge, the voltage value of the primary transfer bias must be kept at a predetermined level while the secondary transfer process, the charge removal process, or the like, is carried out. Figure 9 presents timing for continuous printing. First, yellow, magenta, cyan and black color toner images (first to fourth color images) are sequentially transferred onto the intermediary transfer belt 51 (primary transfer). Immediately after the completion of the primary transfer of the fourth color toner image, the primary transfer bias value is switched back to a value which is the same as the value of the primary transfer bias for the first color toner image. In other words, the value of the bias to be applied during the period between the completion of the primary transfer of the fourth color toner image for any given page, and the beginning of the primary transfer of the first color toner image for the following page, and the value of the bias to be applied for the primary transfer of the first color toner image for the following page, are rendered the same. With this arrangement, the value of the primary transfer bias can be prevented from fluctuating while the charge is removed from the intermediary transfer belt 51, and during a secondary transfer process, hence, the DC current values in the secondary transfer process, and the discharge, can be kept stable. In order to do so, it is necessary only to make the distance between the primary transfer nip N 1 and the secondary transfer nip N 2 measured in the rotational direction of the intermediary transfer belt 51 longer than the length of a printed image (length of a transfer medium P measured in the conveyance direction thereof). Embodiment 6 In the preceding fifth embodiment, if the distance between the primary transfer nip N 1 and the secondary transfer nip N 2 is shorter than the length of an image to be printed, it is necessary either to render the primary transfer bias value for the first color toner image equal to that for the fourth color toner image, or to form the image for the following page after rotating the intermediary transfer belt 51 an extra distance after the completion of the primary transfer of the fourth color toner image. However, the former is impossible when an intermediary transfer belt coated with a high resistance layer is employed as in the present invention (proper primary transfer bias value for the first color toner image is in a range of +100 to +200 V, whereas the proper primary transfer values for the second color toner image and thereafter, must be increased in stages; the proper primary transfer bias value for the fourth color toner image must be in a range of +600 to +1000 V). On the other hand, the latter has a problem in that throughput declines in continuous printing. Figure 10 depicts the sixth embodiment, according to which even if the primary transfer bias value fluctuates, the current is not affected during the secondary transfer and the discharge. In the drawing, in addition to a secondary transfer power source 73 and a discharge power source 94, an electrical power source 212 for a post charger (charging means) 211, and the like, are also connected to the output terminal of a primary transfer power source 62. In this case, the post charger 211 is used by applying, for example, an AC voltage having a peak-to-peak voltage Vpp of 8 kV, and a DC voltage of -500 V. It is disposed on the upstream side, for example, immediately before the secondary transfer point, to equalize the amount of the charge carried by the toner particles in the four color toner images formed on the intermediary transfer belt 51, so that the secondary transfer process can be carried out with better results. With the provision of the structure illustrated in Figure 7, even if the distance between the primary transfer nip N 1 and the post charger 211 is shorter than the length of an image, the process carried out by the post charger is prevented from being affected by the fluctuation of the primary transfer bias (the same is true with the secondary transfer process, the discharge process, and the like). The sixth embodiment can be used in conjunction with the second embodiment or the like, with no problem. As described above, according to the present invention, in order to prevent toner from scattering, during the image forming rotation of an intermediary transfer belt, from the full-color image regions composed of superposed toner images of primary color, an intermediary transfer belt is structured as described above, so that the charge relaxation time τ of the intermediary transfer belt can be adjusted to satisfy the following requirement: T ≤ τ ≤ 500 sec T: time necessary to rotate the intermediary transfer belt a full turn. Therefore, very desirable full-color images which do not suffer from central transfer void can be produced. Desirable efficiency can be realized for the secondary transfer even in the case of an image composed of a large amount of toner. Also, according to the present invention, the low resistance base layer of an intermediary transfer belt is utilized as a counter electrode, and therefore, the intermediary transfer member can be easily discharged with the use of a simple contact type discharge roller; the structure can be simplified. Further, the voltage for primary transfer is used as the reference potential for the post discharger as charging means disposed to face the intermediary transfer medium, the reference potential for a roller for secondary transfer, and the reference potential for a discharge roller, and the like. Therefore, images are not affected even if the voltage for primary transfer fluctuates. Further, such an arrangement is effective to reduce image formation time. 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 scope of the following claims.

Claims: An image forming apparatus comprising: an image bearer member (1) for carrying toner images of different colors; a rotatable intermediary transfer member (51) having an image-receiving surface to which successive toner images are superimposedly transferable from said image bearer member (1) to said intermediary transfer member at a first transfer position (N 1 ) during respective rotations of said intermediary transfer member, and wherein the toner images are then transferable all together from said intermediary transfer member onto a transfer medium (P) at a second transfer position (N 2 ); wherein said intermediary transfer member (51) includes an elastic layer (51a) having a thickness of 0.5-2mm, a coating layer (51b), on said elastic layer, having a volume resistivity which is larger than that of said elastic layer, and said intermediary transfer member satisfies T≤τ≤500s; where T is the period of rotation of said intermediary transfer member and τ is the charge relaxation time required for a potential V which is a potential of said image-receiving surface of said intermediary transfer member at one second after the start of charging of said intermediary transfer member, to become V/e (where e is the base of natural logarithms, e=2.71828...). An image forming apparatus according to claim 1, wherein the following is satisfied; 1.005≤V 2 /V 1 ≤1.02, and 0.985≤V 2 /V 1 ≤0.995 where V 1 is a surface speed of said intermediary transfer member (51) at the second transfer position (N 2 ) and V 2 is a surface speed of the transfer medium (P) when it passes through the second transfer position (N 2 ). An apparatus according to claim 1 or claim 2, wherein the volume resistivity of said elastic layer (51a) is 10 2 -10 7 Ω.cm. An apparatus according to any one of claims 1 to 3 wherein the thickness of said coating layer (51b) is 2-80µm. An apparatus according to any of claims 1 to 4, further comprising discharging means (9) for electrically discharging said intermediary transfer member (51), said discharging means (9,91) being movable toward and away from said image-receiving surface, wherein said discharging means (91) is arranged to be brought into contact with said intermediary transfer member to discharge said intermediary transfer member after the toner images are transferred all together from said intermediary transfer member (51) onto the transfer medium (P) at the second transfer position (N 2 ). An apparatus according to claim 5, further comprising developing means (4) for developing electrostatic images on said image bearer member (1) into the toner images, wherein said discharging means (9,91) is operable to charge residual toner remaining on said intermediary transfer member (51) after the toner images are transferred all together from said intermediary transfer member onto the transfer medium (P) at the second transfer position (N 2 ), to a polarity opposite from a regular charging polarity of the toner in said developing means (4), whereby the charged residual toner on said intermediary transfer member is transferred back onto said image bearer member at the first transfer position (N 1 ). An apparatus according to claim 6, wherein the image bearer member (1) is operable to transfer a next toner image from said image bearer member onto the intermediary transfer member (51) substantially simultaneously with the back-transfer of the charged residual toner from said intermediary transfer member (51) onto said image bearer member (1) at said first transfer position (N 1 ). An apparatus according to any preceding claim, wherein said intermediary transfer member is in the form of a belt.

Inventors: ENOMOTO NAOKI, FUNATANI KAZUHIRO, KOBAYASHI TATSUYA, MIYASHIRO TOSHIAKI, TAKEUCHI AKIHIKO, TSURUYA TAKAAKI, ENOMOTO, NAOKI, FUNATANI, KAZUHIRO, KOBAYASHI, TATSUYA, MIYASHIRO, TOSHIAKI, TAKEUCHI, AKIHIKO, TSURUYA, TAKAAKI
Applicants: CANON KK, CANON KABUSHIKI KAISHA
Application year: 1997
Main classification: G03G15
IPC codes: G03G21, G03G15