JP2012133039A - Image forming apparatus - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an image forming apparatus forming an image by sequentially overlapping each toner image formed on a plurality of photosensitive drums on a conductive belt, in which a primary transference property and a secondary transference property are not deteriorated even when the image forming apparatus has a constitution in which residual toner is charged by toner charging means.SOLUTION: An image forming apparatus comprises a current supply member contacting with an intermediate transfer belt 8 which is conductive. A current supplied by the current supply member flows in a circumferential direction of the intermediate transfer belt 8 allowing photosensitive drums to primarily transfer toner images on the intermediate transfer belt 8 in a plurality of primary transfer parts. Further, a Zener diode or a varistor is connected to a plurality of extending members to allow the intermediate transfer belt 8 to maintain a prescribed electrical potential even when the current is supplied from toner charging means.

Description

  The present invention relates to an image forming apparatus such as a copying machine or a laser printer.

  In an electrophotographic color image forming apparatus, in order to print at high speed, each color image forming unit is independently provided, and a toner image is sequentially transferred from the image forming unit of each color to an intermediate transfer belt. A configuration is known in which toner images are transferred to a transfer material all at once.

  Each color image forming unit has a photosensitive drum as an image carrier. Each image forming unit further includes a charging member that charges the photosensitive drum and a developing device that develops the toner image on the photosensitive drum. The charging member of each image forming unit is brought into contact with the photosensitive drum with a predetermined pressure contact force, and the surface of each photosensitive drum is uniformly set to a predetermined polarity and potential by a charging voltage applied from a charging voltage source (not shown). Is charged. The developing device of each image forming unit attaches a toner image to the electrostatic latent image formed on the photosensitive drum, and develops (visualizes) the toner image.

  The toner image developed on the photosensitive drum of each image forming unit is primarily transferred to the intermediate transfer belt by a primary transfer roller that is a primary transfer member facing the photosensitive drum via the intermediate transfer belt. Each primary transfer roller has a voltage source dedicated to primary transfer. The toner image primarily transferred to the intermediate transfer belt is secondarily transferred to the transfer material by the secondary transfer member. A secondary transfer roller, which is a secondary transfer member, is connected to a voltage power source dedicated to secondary transfer.

  Patent Document 1 discloses a configuration in which four primary transfer rollers are connected to a voltage power source dedicated to primary transfer, and four voltage power sources dedicated to primary transfer are provided. In Patent Document 2, the transfer voltage to be applied to each primary transfer roller is changed according to resistance variation due to endurance of the intermediate transfer belt and the primary transfer roller and environmental variations before the image forming operation. Control to do.

  Further, as in Patent Document 3, an image forming apparatus is known in which residual toner is charged on an intermediate transfer belt with a charging member and then transferred to each image forming unit at a primary transfer unit and collected.

JP 2003-35986 A JP 2001-125338 A JP 2009-205012 A

  However, the conventionally known primary transfer voltage setting has the following problems. Since it is necessary to set an appropriate primary transfer voltage in each image forming unit, a plurality of voltage power supplies are required, which leads to an increase in the size of the image forming apparatus and an increase in voltage power supply. In addition, since an appropriate primary transfer voltage is calculated before image formation in consideration of resistance variation of the primary transfer member, it may take time to start image formation. Further, if the primary transfer member presses the photosensitive drum with a predetermined pressure via the intermediate transfer belt in each image forming unit, there is a possibility that the photosensitive drum is loaded and the consumption of the photosensitive drum is accelerated. Further, in the configuration as disclosed in Patent Document 3, since current is supplied from the toner charging unit, the potential may be increased in an intermediate transfer belt having low electrical resistance characteristics.

  The present invention has been made in view of the above problems, and provides an image forming apparatus capable of collecting toner remaining on an intermediate transfer belt at each image forming unit while having an appropriate primary transfer property. For the purpose.

  In order to solve the above problems, the present invention has the following configuration.

A plurality of image carriers that can rotate and carry a toner image; a rotatable endless intermediate transfer belt; and a plurality of stretching members that carry the intermediate transfer belt. The upper toner image is primarily transferred to the intermediate transfer belt by a plurality of primary transfer portions corresponding to the plurality of image carriers, and the intermediate transfer is performed by a secondary transfer portion formed by the current supply member and the intermediate transfer belt. In an image forming apparatus that collectively transfers toner images on a belt to a transfer material,
The intermediate transfer belt has electrical conductivity, and the current supplied from the current supply member flows in the circumferential direction of the intermediate transfer belt, so that the plurality of primary transfer units transfer the image carrier to the intermediate transfer belt. A toner charging unit that primarily transfers a toner image and charges the toner that is not secondarily transferred to the transfer material and remains on the intermediate transfer belt; and the toner charging unit converts the residual toner to a normal charging polarity of the toner. The residual toner charged to a reverse polarity is charged in the intermediate transfer belt at the primary transfer portion formed with the image carrier that comes into contact with the intermediate transfer belt by a current flowing in the circumferential direction of the intermediate transfer belt. The plurality of stretching members are connected to Zener diodes or varistors that maintain the intermediate transfer belt at a predetermined potential. And wherein the Rukoto.

  In order to solve the above-described problem, another configuration of the present invention has the following.

A plurality of image carriers that can rotate and carry a toner image; a rotatable endless intermediate transfer belt; and a plurality of stretching members that carry the intermediate transfer belt. A toner image carried by the toner image is primarily transferred to the intermediate transfer belt by a plurality of primary transfer portions corresponding to the plurality of image carriers, and the intermediate transfer belt is formed by a secondary transfer portion formed by a current supply member and the intermediate transfer belt. In an image forming apparatus for secondary transfer of toner images on a transfer belt to a transfer material at once,
The intermediate transfer belt has electrical conductivity, and the current supplied from the current supply member flows in the circumferential direction of the intermediate transfer belt, so that the plurality of primary transfer units transfer the image carrier to the intermediate transfer belt. A toner charging unit that primarily transfers a toner image and charges the toner that is not secondarily transferred to the transfer material and remains on the intermediate transfer belt; and the toner charging unit converts the residual toner to a normal charging polarity of the toner. The residual toner charged to a reverse polarity is charged in the intermediate transfer belt at the primary transfer portion formed with the image carrier that comes into contact with the intermediate transfer belt by a current flowing in the circumferential direction of the intermediate transfer belt. To the image carrier, and the plurality of stretching members are connected to varistors.

  As the current supplied by the current supply member flows in the circumferential direction of the intermediate transfer belt, the toner image is primarily transferred from the photosensitive drum to the intermediate transfer belt at a plurality of primary transfer portions. Further, by connecting Zener diodes or varistors to the plurality of stretching members, the potential of the intermediate transfer belt 8 can be maintained at a predetermined potential even when current is supplied from the toner charging unit.

1 is a schematic cross-sectional view illustrating an image forming apparatus according to Embodiment 1 of the present invention. FIG. 3 is a schematic cross-sectional view showing an image forming apparatus having a voltage power source dedicated to primary transfer in each image forming unit. FIG. 3 is a diagram for explaining a circumferential resistance measurement method for an intermediate transfer belt according to a first embodiment of the invention. FIG. 4 is an explanatory diagram for explaining a circumferential resistance measurement result of the intermediate transfer belt according to the first embodiment of the invention. FIG. 4 is an explanatory diagram illustrating a method for measuring the potential of the intermediate transfer belt according to the first embodiment of the invention. FIG. 5 is an explanatory diagram for explaining a result of potential measurement of the intermediate transfer belt according to the first embodiment of the invention. FIG. 3 is an explanatory diagram for explaining primary transfer according to Embodiment 1 of the present invention. FIG. 3 is an explanatory diagram illustrating a belt potential and a secondary transfer voltage according to the first exemplary embodiment of the present invention. FIG. 3 is an explanatory diagram illustrating a residual toner recovery method according to Embodiment 1 of the invention. FIG. 3 is an explanatory diagram that marks the influence of toner charging means on the potential of the intermediate transfer belt according to the first exemplary embodiment of the present invention. The figure explaining the Zener diode electric potential or varistor electric potential which concerns on Example 1 of this invention.

Example 1
In the following, preferred embodiments of the present invention will be described in detail with reference to the drawings. However, the components described in this embodiment are merely examples, and are not intended to limit the scope of the present invention.

  FIG. 1 is a configuration diagram of a color image forming apparatus of an inline type (4-drum system). The image forming apparatus forms an image forming unit 1a that forms a yellow image, an image forming unit 1b that forms a magenta image, an image forming unit 1c that forms a cyan image, and a black image. The image forming unit 1d is provided with four image forming units (image forming units). These four image forming units are arranged in a line at regular intervals.

  In each of the image forming units 1a, 1b, 1c, and 1d, photosensitive drums 2a, 2b, 2c, and 2d, which are image carriers, are arranged, respectively. In the present embodiment, the photosensitive drums 2a, 2b, 2c, and 2d are negatively charged organic photosensitive members having a photosensitive layer (not shown) on a drum base (not shown) such as aluminum, and a driving device (not shown). ) Is rotated at a predetermined process speed.

  Around each of the photosensitive drums 2a, 2b, 2c, and 2d, charging rollers 3a, 3b, 3c, and 3d, which are charging members, and developing devices 4a, 4b, 4c, and 4d are arranged, respectively. Further, drum cleaning devices 6a, 6b, 6c and 6d are installed around the photosensitive drums 2a, 2b, 2c and 2d, respectively. Further, exposure devices 7a, 7b, 7c, and 7d are installed above the respective photosensitive drums 2a, 2b, 2c, and 2d. Each developing device 4a, 4b, 4c, 4d contains yellow toner, cyan toner, magenta toner, and black toner, respectively.

An endless intermediate transfer belt 8 that is an intermediate transfer member and is rotatable is provided at a position facing each image forming unit. The intermediate transfer belt 8 is stretched by a drive roller 11, a secondary transfer counter roller 12, and a tension roller 13 (the above three rollers are referred to as “stretch members”). The intermediate transfer belt 8 is rotated (moved) in the direction of the arrow (counterclockwise) by driving of the driving roller 11 to which a motor (not shown) is connected. Hereinafter, the rotation direction of the intermediate transfer belt 8 is defined as the circumferential direction of the intermediate transfer belt 8. The driving roller 11 is provided with a high-friction rubber layer on the surface layer for driving the intermediate transfer belt 8, and the rubber layer has conductivity with a volume resistivity of 10 ⁵ Ωcm or less. The secondary transfer counter roller 12 is in contact with the secondary transfer roller 15 via the intermediate transfer belt 8 to form a secondary transfer portion. The secondary transfer counter roller 12 was provided with a rubber layer on the surface layer, and the rubber layer was made conductive with a volume resistivity of 10 ⁵ Ωcm or less. The tension roller 13 is made of a metal roller, applies a tension of a total pressure of about 60 N to the intermediate transfer belt 8, and rotates following the intermediate transfer belt 8.

  The driving roller 11, the secondary transfer counter roller 12, and the tension roller 13 are grounded via resistance members having the same resistance value. Since the resistance of each rubber layer of the driving roller 11 and the secondary transfer counter roller 12 is sufficiently smaller than that of the resistance member, the electrical influence can be ignored.

As the secondary transfer roller 15, an elastic roller having a volume resistivity of 10 ⁷ to 10 ⁹ Ωcm and a rubber hardness of 30 ° (Asker C hardness meter) was used. The secondary transfer roller 15 is pressed against the secondary transfer counter roller 12 via the intermediate transfer belt 8 with a total pressure of about 39.2N. Further, the secondary transfer roller 15 is driven to rotate as the intermediate transfer belt 8 rotates. Further, a voltage of −2.0 to 7.0 kV can be applied to the secondary transfer roller 15 from the voltage power supply 19.

On the outer surface side of the intermediate transfer belt 8, toner charging means for removing and collecting the transfer residual toner remaining on the surface of the intermediate transfer belt 8 is disposed. The cleaning brush 71 as a toner charging unit is a brush configured so that nylon fibers having conductivity of 10 ⁶ to 10 ⁹ Ωcm are substantially dense. The tip position of the cleaning brush 71 is set so that the amount of penetration with respect to the surface of the intermediate transfer belt 8 is 1.0 mm. The length of the cleaning brush 71 in the longitudinal direction (direction intersecting the moving direction of the surface of the intermediate transfer belt 8) is substantially the same as the width in the same direction of the image formable area on the intermediate transfer belt 8. As described above, the cleaning brush 71 positioned on the upstream side in the moving direction of the surface of the intermediate transfer belt 8 rubs the surface of the intermediate transfer belt 8 as the intermediate transfer belt 8 moves. A voltage of −2.0 to +2.0 kV can be applied to the cleaning brush 71 from a power source (voltage power source) 72 as a voltage supply unit. Further, in the rotation direction of the intermediate transfer belt 8, a fixing device 17 having a fixing roller 17a and a pressure roller 17b on the downstream side of the secondary transfer portion where the secondary transfer counter roller 12 and the secondary transfer roller 15 are in contact with each other. Is installed.

  Next, an image forming operation will be described. When a start signal for starting an image forming operation is issued from the controller, a transfer material (recording medium) is sent one by one from a cassette (not shown) and conveyed to a registration roller (not shown). At that time, the registration roller (not shown) is stopped, and the leading edge of the transfer material stands by just before the secondary transfer portion. On the other hand, in each of the image forming units 1a, 1b, 1c, and 1d, when a start signal is issued, the photosensitive drums 2a, 2b, 2c, and 2d start to rotate at a predetermined process speed. The photosensitive drums 2a, 2b, 2c, and 2d are uniformly charged by the charging rollers 3a, 3b, 3c, and 3d, respectively, with negative polarity in this embodiment. The exposure devices 7a, 7b, 7c, and 7d scan and expose the laser beams on the photosensitive drums 2a, 2b, 2c, and 2d, respectively, to form electrostatic latent images.

First, yellow toner is adhered to the electrostatic latent image formed on the photosensitive drum 2a by the developing device 4a to which a developing voltage having the same polarity as the charging polarity (negative polarity) of the photosensitive drum 2a is applied. Visualize as a toner image. The potential of the photosensitive drum is adjusted so that the potential after being charged by the charging roller is −500 V, and the potential (image portion) after being exposed by the exposure device is −100 V, and the developing bias is adjusted. -300V. The process speed is set to 250 mm / sec. The image formation width, which is the length in the direction perpendicular to the conveyance direction (rotation direction), is set to 215 mm, the toner charge amount is -40 μC / g, and the toner amount on the photosensitive drum in the solid image portion is set to 0.4 mg / cm ^2. doing.

The yellow toner image is primarily transferred onto the rotating intermediate transfer belt 8. Here, a portion where the toner image is transferred from each photosensitive drum facing each photosensitive drum is defined as a one-time transfer portion. There are a plurality of primary transfer portions on the intermediate transfer belt corresponding to a plurality of image carriers. In this embodiment, primary transfer is performed by a current flowing in the circumferential direction of the intermediate transfer belt 8 from a current supply member that is in contact with the outer surface of the intermediate transfer belt 8. (The principle of primary transfer will be described later.)
The area where the yellow toner image is transferred moves to the image forming unit 1 b side by the rotation of the intermediate transfer belt 8. In the image forming unit 1b as well, a magenta toner image formed on the photosensitive drum 2b in the same manner is superimposed on the yellow toner image on the intermediate transfer belt 8 and transferred. In the same manner, cyan and black toner images formed by the photosensitive drums 2c and 2d of the image forming units 1c and 1d are sequentially superimposed on the yellow and magenta toner images superimposed and transferred onto the intermediate transfer belt 8 in the same manner. In addition, a full-color toner image is formed on the intermediate transfer belt 8.

  Then, the transfer material is conveyed to the secondary transfer portion by a registration roller (not shown) in accordance with the timing when the front end of the full color toner image on the intermediate transfer belt 8 is moved to the secondary transfer portion. A full-color toner image on the intermediate transfer belt 8 is collectively applied to a transfer material by a secondary transfer roller 15 which is a secondary transfer member to which a secondary transfer voltage (a voltage having a polarity opposite to that of toner (positive polarity)) is applied. Secondary transferred. The transfer material on which the full-color toner image is formed is conveyed to the fixing device 17. At the fixing nip formed by the fixing roller 17a and the pressure roller 17b, the full-color toner image is heated and pressurized, thermally fixed on the surface of the transfer material P, and then discharged to the outside. In this embodiment, the residual toner remaining on the intermediate transfer belt 8 without being transferred to the transfer material is charged by the cleaning device 75 and collected on each photosensitive drum at the primary transfer portion.

  In this embodiment, primary transfer for transferring a toner image from the photosensitive drums 2a, 2b, 2c, and 2d to the intermediate transfer belt 8 is performed, and a voltage power source 9 is applied to the primary transfer rollers 55a, 55b, 55c, and 55d shown in FIG. Therefore, the present invention is characterized in that it is executed without adopting a configuration in which a voltage is applied. Hereinafter, the volume resistivity, surface resistivity, and circumferential resistance of the intermediate transfer belt 8 will be described in order to explain the characteristics of the present embodiment. The definition and measurement method of the circumferential resistance will be described later.

[Volume resistivity and surface resistivity of the intermediate transfer belt 8 used in this embodiment]
The intermediate transfer belt 8 of this embodiment has a base layer in which carbon is dispersed in a polyphenylene sulfide (PPS) resin having a thickness of 100 μm to adjust electric resistance. The resin used may be polyimide (PI), PVdF, nylon, PET, PBT, polycarbonate, PEEK, PEN or the like. Further, the intermediate transfer belt 8 has a multilayer structure. Specifically, a surface layer of an acrylic resin having a thickness of 0.5 to 3 μm and a high resistance is provided on the outer surface of the base layer. This is because the high resistance layer on the surface layer reduces the current difference between the sheet passing area and the non-sheet passing area in the longitudinal direction of the secondary transfer portion, thereby obtaining the effect of improving the secondary transferability of the small size sheet.

  Next, a method for manufacturing the belt will be described. In this embodiment, a manufacturing method using an inflation molding method is used. PPS serving as a base material and blending components such as carbon black as conductor powder are melt-kneaded by a biaxial kneader. A belt is manufactured by extruding the obtained kneaded material with an annular die.

  The surface coat layer is formed by spray-coating an ultraviolet curable resin on the surface of the molded endless belt, drying, and curing by ultraviolet irradiation. If the coat layer is too thick, it tends to break, so the coating amount is adjusted to be in the range of 0.5 to 3 μm.

  In this embodiment, carbon black is used as the conductor powder. The additive to be mixed for adjusting the electric resistance value of the intermediate transfer belt 8 is not particularly limited. Examples of the conductive filler for adjusting the resistance include carbon black and various conductive metal oxides. Non-filler resistance modifiers include low molecular weight ionic conductive materials such as various metal salts and glycols, antistatic resins containing ether bonds and hydroxyl groups in the molecule, or organic polymer compounds exhibiting electronic conductivity, etc. .

  Increasing the amount of carbon added will lower the resistance of the belt, but if it is increased too much, the strength of the belt itself will be insufficient and it will be easily broken. In this embodiment, the resistance of the belt is reduced so that the belt strength is within a range that can be used in the image forming apparatus. The intermediate transfer belt of this embodiment has a Young's modulus of about 3000 MPa. The Young's modulus E measurement was based on the tensile modulus measurement method of JIS-K7127, and the thickness of the measurement sample was 100 μm.

  Table 1 shows belts in which the relative ratio of the carbon amount to the substrate is changed.

  Table 1 shows the amount of added carbon and the presence or absence of a surface coat layer. For example, belt B has a carbon amount 1.5 times that of belt A, and belt C has a carbon amount twice that of belt A. Further, the belt A, the belt B, and the belt C are provided with a surface layer, and the belt D and the belt E are single-layer belts. The relative ratio of the carbon amount of the belt B and the belt D is the same, and the relative ratio of the carbon amount of the belt C and the belt E is also the same.

Moreover, the comparative example belt of the polyimide which adjusted resistance by changing the relative ratio of the carbon amount was manufactured as a comparative belt. The comparative belt has a relative carbon amount ratio of 0.5 and a volume resistivity of 10 ¹⁰ to 10 ¹¹ Ωcm. This comparative belt is a belt having a general resistance value as a belt employed as an intermediate transfer belt.

  The measurement results of volume resistivity and surface resistivity of the comparative belt and belts A to E are shown below. First, it measured using the resistivity meter Hiresta UP (MCP-HT450) by Mitsubishi Chemical Analytech Co., Ltd. with respect to the above-mentioned comparative example belt and belts A to E. Table 2 shows the measured volume resistivity and surface resistivity (outer surface of the belt). The measuring method was based on JIS-K6911, and after obtaining favorable contact property between the electrode and the surface of the belt by using conductive rubber as an electrode. The measurement conditions are an application time of 30 seconds and an applied voltage of 10V and 100V.

The comparative belt has a volume resistivity of 1.0 × 10 ¹⁰ Ωcm and a surface resistivity of 1.0 × 10 ¹⁰ Ω / □ when an applied voltage of 100 V is applied. However, in the comparative belt, when the applied voltage is 10 V, the flowing current is too small to measure the volume resistivity, and “over” is displayed.

On the other hand, when 100 V is applied to belts B, C, and D, the value of the flowing current is too large because the belt resistance is low, and an under is displayed indicating that volume resistivity cannot be measured. The belt B had a surface resistivity of 2.0 × 10 ⁸ Ω / □ when 100 V was applied, but the belts C and D were displayed as under when 100 V was applied.

  In Table 2, each volume resistivity and surface resistivity of the applied voltage 10 V of the belt A cannot be measured. Further, when the surface resistivity of the belt A and the comparative belt when 100 V is applied is compared, the belt A is higher. This is due to the influence of the coating layer, and it can be seen that the resistance of the belt A having a high-resistance surface coating is higher than that of the comparative belt having no surface coating.

  Further, by comparing the belt B and the belt D and the belt C and the belt E, it can be seen that the resistance value is increased due to the presence of the coat layer. Further, by comparing the belt B and the belt C, and the belt D and the belt E, it can be seen that when the carbon amount is increased, the resistance value is lowered. In belt E, the resistance is too low to measure all items.

  In this embodiment, it is necessary to use an intermediate transfer belt in a range indicated as “under” in Table 2. Therefore, the resistance of the intermediate transfer belt defined by other than the volume resistivity and surface resistivity was measured. The resistance of the intermediate transfer belt 8 according to another rule is the electrical resistance in the circumferential direction of the intermediate transfer belt described above.

[How to determine the circumferential resistance of the intermediate transfer belt]
In this embodiment, the resistance value of the belt whose resistance is lowered is measured by the method shown in FIG. In FIG. 3A, when a constant voltage is applied from the voltage power source 19 to the outer surface roller 15M, the current flowing to the ammeter connected to the photosensitive drum 2dM of the image forming unit 1d is detected. A method is used in which the electric resistance of the intermediate transfer belt 8 between the position where the outer surface roller 15M is in contact and the position where the photosensitive drum 2dM is in contact is obtained from the detected current value. That is, the resistance of the belt is calculated by measuring the current flowing in the circumferential direction (rotation direction) of the intermediate transfer belt 8 by this method. At this time, in order to eliminate the influence of resistance other than the intermediate transfer belt, the outer roller 15M and the photosensitive drum 2dM are made of only metal. In the drawing, M (Metal) is added to the reference numeral to indicate that it is a metal roller. In this embodiment, the distance between the contact portion of the outer roller 15M and the photosensitive drum 2dM is 370 mm on the upper surface side of the intermediate transfer belt and 420 mm on the lower surface side of the intermediate transfer belt.

  FIG. 4A shows the result of measuring the belts A to E by changing the applied voltage with the above measurement method. In this measurement method, the resistance in the circumferential direction, which is the rotational direction of the intermediate transfer belt, is measured. Therefore, in this embodiment, the resistance of the intermediate transfer belt measured by this measuring method is referred to as circumferential resistance [Ω].

  As the applied voltage is increased for all belts, the circumferential resistance tends to decrease little by little. This is a characteristic of a belt in which carbon is dispersed in a resin.

  Note that FIG. 3B is different from FIG. 3A only in the position of the ammeter. The resistance measurement result at this time is almost the same as that shown in FIG. 4, and the measurement method of this embodiment does not vary depending on the position of the ammeter.

  In the belts A to E, the circumferential resistance can be measured by the method shown in FIG. 3, but the circumferential resistance cannot be measured in the comparative belt. This is because the comparative belt is a belt used in an image forming apparatus having a configuration in which a voltage power source is connected to each primary transfer roller 55a, 55b, 55c, and 55d as shown in FIG. .

  In the image forming apparatus having the configuration shown in FIG. 2, the volume resistance and area resistance of the intermediate transfer belt are set so that adjacent voltage power supplies 9 are not affected by currents flowing into each other via the intermediate transfer belt (so as not to interfere with each other). Highly designed. The comparative belt is a belt having a resistance that does not interfere with each primary transfer portion even when a voltage is applied to each primary transfer roller 55a, 55b, 55c, 55, and the current hardly flows in the circumferential direction. Designed as a belt with. A belt such as a comparative belt is defined as a high resistance belt, and a belt in which a current flows in the circumferential direction such as belts A to E is defined as a conductive belt that is a low resistance belt.

  FIG. 4B is a plot of the current measured by the measurement method of FIG. The vertical axis (resistance [Ω]) of FIG. 4A described above is a value obtained by dividing the measured current value of FIG. 4B by the applied voltage.

As shown in FIG. 4B, in the comparative belt, no current flowed in the circumferential direction even when 2000 V was applied. However, it can be seen that the belts A to E flow 50 μA or more at 500 V or less. In this embodiment, the belt used as the intermediate transfer belt has a circumferential resistance of 10 ⁴ to 10 ⁸ Ω.

Next, the belt surface potential of the intermediate transfer belt 8 having the circumferential resistance of 10 ⁴ to 10 ⁸ Ω will be described. FIG. 5 shows a method for measuring the belt surface potential. In the figure, four surface potential meters are used to measure potential at four locations. In the figure, 5dM and 5aM are metal rollers for measurement. The surface potential meter 37a and the measurement probe 38a measure the potential of the primary transfer roller 5aM (metal roller) of the image forming unit 1a. The measuring instrument used was a surface potential meter MODEL344 manufactured by Trek Japan. Since the metal roller has the same potential as the inner surface of the intermediate transfer belt, the inner surface potential of the intermediate transfer belt can be measured by this method. Similarly, the surface potential meter 37d and the measurement probe 38d measure the potential on the inner surface of the intermediate transfer belt based on the potential of the primary transfer roller 5dM (metal roller) of the image forming unit 1d.

  The surface potential meter 37e and the measurement probe 38e measure the intermediate transfer belt outer surface potential with the drive roller 11M facing each other, and the surface potential meter 37f and the measurement probe 38f face the tension roller 13 with the intermediate transfer belt outer surface potential facing each other. Is measuring. In addition, an electric resistance member is connected to each of the driving roller 11M, the secondary transfer opposing roller 12, and the tension roller 13.

  When the potential of the intermediate transfer belt was measured by this measurement method, it was found that there was almost no difference depending on the measurement location, and the belt potential was almost the same in the intermediate transfer belt. In other words, the belt used in this example can be considered as a conductive belt although it has a certain resistance value.

  FIG. 6 shows the measurement result of the intermediate transfer belt potential. Here, the resistance member connected to the drive roller is Re, the resistance member connected to the secondary transfer counter roller 12 is Rg, and the resistance member Rf connected to the tension roller 13. FIG. 5A shows the result when a resistance element of Re = Rf = Rg = 1 GΩ is used. The horizontal axis represents the voltage (applied voltage) applied to the voltage power supply 31 for transfer, and the vertical axis represents the potential of the intermediate transfer belt, showing the results for belts A to E.

  Similarly, FIG. 6B shows the result when Re = Rf = Rg = 100 MΩ, and FIG. 6C shows the result when Re = Rf = Rg = 10 MΩ.

  As the applied voltage is increased for any belt, the belt potential also increases. Further, when the resistance value is decreased to 1 GΩ, 100 MΩ, and 10 MΩ, the belt potential decreases. Here, the resistance values of Re, Rf, and Rg are all the same. However, it is known that when any one of the resistance values is lowered, the belt potential is lowered according to the resistance.

  Further, the belt potential cannot be measured by the above-described method with an intermediate transfer belt having a resistance value in which a current hardly flows in the circumferential direction like the comparative example belt. In a configuration in which a voltage is applied to each primary transfer roller by a dedicated voltage power source 9 as shown in FIG. 2, a potential measuring probe cannot be arranged. In addition, since the potential varies depending on the position in the circumferential direction of the belt, it is meaningless to measure by placing a potential measuring probe with the stretching roller (stretching member) facing each other.

  Next, the reason why the toner image can be transferred from the photosensitive drum to the intermediate transfer belt with the configuration of this embodiment will be described with reference to FIG. FIG. 7A illustrates the potential relationship of the primary transfer portion. The potential of the photosensitive drum is -100 V in the toner portion (image portion), and +200 V is shown as the surface potential of the intermediate transfer belt. The toner having the charge amount q developed on the photosensitive drum is primarily transferred by receiving the force F in the direction of the intermediate transfer belt by the electric field E formed by the photosensitive drum potential and the intermediate transfer belt potential.

  Next, FIG. 7B is a diagram illustrating multiple transfer. The multiple transfer is a primary transfer in which toner of another color is further superimposed on the toner that has been primary transferred once onto the intermediate transfer belt. FIG. 7B shows an example in which the toner is negatively charged, and the toner surface potential is +150 V due to the toner transferred once. In this case, the toner on the photosensitive drum is primarily transferred by receiving the force F 'in the direction of the intermediate transfer belt by the electric field E' formed by the photosensitive drum potential and the toner surface potential. FIG. 7C shows that the multiple transfer has been completed.

  As described above, the primary transfer of the toner is related to the charge amount of the toner and the potential difference between the photosensitive drum and the intermediate transfer belt, and the intermediate transfer belt potential needs to be a certain level or more to ensure the primary transfer performance. I know that there is.

  Examining the intermediate transfer belt potential necessary for primary transfer of the toner developed on the photosensitive drum under the above-described conditions of this embodiment, it was found that 200 V or more is necessary.

  FIG. 7D is a graph in which the horizontal transfer belt potential is plotted on the horizontal axis and the transfer efficiency is plotted on the vertical axis. The transfer efficiency is a transferability index indicating how much of the toner developed on the photosensitive drum is transferred onto the intermediate transfer belt, and it is determined that the transfer can be normally performed when the transfer efficiency is 95% or more. The figure shows that the intermediate transfer belt potential is 200 V or more and good transfer of 98% or more is performed.

  At this time, the potential differences between the photosensitive drum and the intermediate transfer belt in the image forming units 1a, 1b, 1c, and 1d are all the same. That is, a potential difference of 300 V between the photosensitive drum potential of −100 V and the intermediate transfer belt potential +200 V is formed in the primary transfer portions of the image forming portions 1 a, 1 b, 1 c, and 1 d. This potential difference is a potential difference necessary for multiple transfer of the above three toner colors (single color solid is 100% and 300%), and a primary transfer voltage is applied to each primary transfer roller in a conventional primary transfer configuration. Is almost the same as A normal image forming apparatus does not form a 400% image even if it has four colors, and a full toner image can be sufficiently formed with a maximum toner amount of about 210 to 280%.

Therefore, in this embodiment, primary transfer is enabled by flowing a current in the circumferential direction of the intermediate transfer belt so that the surface potential of the intermediate transfer belt becomes a predetermined potential. In this embodiment, by applying a voltage to the secondary transfer roller 15 which is a secondary transfer member, it is possible to perform primary transfer and secondary transfer with one voltage power source. That is, the secondary transfer roller 15 also serves as a current supply member for flowing a current in the circumferential direction of the intermediate transfer belt 8. In the secondary transfer, the toner that has been primarily transferred onto the intermediate transfer belt 8 is moved onto the transfer material by the Coulomb force as in the case of the primary transfer. Under the conditions of this embodiment, high-quality paper (basis weight 75 g / m ² ) is used as a recording material, and the secondary transfer voltage required for secondary transfer is 2 kV or more. The current supply member is not necessarily used as the secondary transfer roller 15, and may be a contact member that is provided with a separate voltage power source and contacts the outer surface of the intermediate transfer belt 8. As the contact member, a roller, a charging brush, a rotating brush, or the like can be considered. The contact member can be used as a current supply member by applying a voltage from a voltage power source.

  FIGS. 8A, 8B, and 8C are obtained by adding conditions for primary transfer and secondary transfer to the intermediate transfer belt potential of FIG. 8A shows the result when a resistance element of 1 GΩ is used for the resistance member, FIG. 8B shows the result when a resistance element of 100 MΩ is used for the resistance member, and FIG. A dotted line A in FIG. 8 is a line of an intermediate transfer belt potential necessary for primary transfer. FIG. 8B shows the secondary transfer setting range. As shown in FIGS. 8A and 8B, in the case of 1 GΩ and 100 MΩ, primary transfer and secondary transfer are OK by applying a secondary transfer voltage of a certain level or more. As shown in FIG. 8C, in the case of 10 MΩ, it can be seen that more secondary transfer voltage is required. If the secondary transfer voltage is increased even at 10 MΩ, the secondary transfer is OK. However, since a current is actually passed through the stretching roller, a larger capacity power source is required. While the transfer material is passing through the secondary transfer portion, the potential of the intermediate transfer belt 8 decreases due to the resistance of the transfer material, but the voltage applied to the current supply member is set by assuming the decrease in advance. Just keep it.

  If the primary transfer portion and the secondary transfer portion are sufficiently separated from each other, if necessary, a transfer voltage optimum for the primary transfer is applied to the secondary transfer roller 15 during the primary transfer. When the primary transfer is completed and the secondary transfer timing is reached, it is possible to switch to a transfer voltage optimum for the secondary transfer.

  The toner remaining on the intermediate transfer belt 8 is transferred to each photosensitive drum and is collected from the photosensitive drum by the drum cleaning device 6. This will be described in detail with reference to FIG. The toner that has not been transferred onto the transfer material in the secondary transfer step is charged by a cleaning brush 71 that is a toner charging unit of the belt cleaning device 75. A voltage having a polarity opposite to the normal charging polarity of the toner is applied to the cleaning brush from the power supply voltage 72, and the residual toner is charged to a polarity opposite to the normal charging polarity of the toner. The charged residual toner is transferred to the photosensitive drum 2 at the primary transfer portion. The toner transferred onto the photosensitive drum 1a is collected by the drum cleaning device 6a.

  Further, during continuous image formation, primary transfer for primary transfer of the toner image from each photosensitive drum to the intermediate transfer belt 8 for the next print and transfer of residual toner from the intermediate transfer belt to the photosensitive drum are simultaneously performed. In the present embodiment, the toner is negatively charged in the developing device 4a, and has a polarity opposite to the polarity of the residual toner after passing through the cleaning brush 71.

  In this embodiment, when the residual toner is transferred from the intermediate transfer belt to the photosensitive drum, the current supply member 15 is used to set the potential of the intermediate transfer belt to a positive polarity (a polarity opposite to the normal charging polarity of the toner). A voltage is applied to A positive voltage is also applied to the cleaning brush 71 in order to charge the residual toner positively. Therefore, a current flows in the circumferential direction of the intermediate transfer belt 8 by the cleaning brush 71 to which a voltage is applied. FIG. 10 shows the relationship between the voltage applied to the current supply member and the belt potential. A broken line A shows a case where a voltage is applied only to the current supply member without applying a voltage to the cleaning brush, and a broken line B shows a case where a voltage is applied to both the current supply member and the cleaning brush. As shown in FIG. 10, when the voltage is applied to the cleaning brush even with the same applied voltage, the intermediate transfer belt potential rises. That is, since the potential of the intermediate transfer belt 8 is increased by the cleaning brush 71 for charging the residual toner, the primary transfer efficiency and the residual toner transfer efficiency in the primary transfer portion may be reduced. is there. Further, when the potential becomes higher than desired, the secondary transfer efficiency of the toner image from the intermediate transfer belt 8 to the transfer material is lowered.

  In consideration of such a decrease in primary transferability and secondary transferability, in this embodiment, the resistance members Rg, Re, and Rf connected to the plurality of stretching rollers are replaced with a specific voltage threshold value instead of the resistance elements. Ground through a Zener diode or varistor with FIG. 11 shows the potential of the intermediate transfer belt when a voltage is simultaneously applied to the current supply member 15 and the cleaning brush 71 when grounded via a Zener diode or a varistor. In addition, the case where a resistance element is connected to the stretching member is shown by a dotted line for comparison. When a resistance element is connected, if the applied voltage is increased, the intermediate transfer belt potential also increases in proportion to the applied voltage. However, when a Zener diode or a varistor is connected, when the Zener potential or the varistor potential is exceeded, current flows out to the tension roller side. Therefore, the potential of the intermediate transfer belt 8 maintains a Zener potential or a varistor potential. For this reason, even if the applied voltage is increased, the belt potential does not rise above the Zener potential or the varistor potential, the belt potential can be kept constant, and the primary transferability can be stabilized. The action of keeping this potential constant is the same when the voltage applied to the cleaning brush is increased.

  The Zener potential or the varistor potential may be set to a predetermined potential of 200 V in consideration of environmental influences. With this configuration, the voltage applied to the current supply member 15 and the voltage applied to the cleaning brush 71 can be optimized independently while stabilizing the primary transfer property and the secondary transfer property.

  In the present embodiment, a brush is used as the toner charging means of the intermediate transfer belt 8, but the present invention is not limited to this. Good.

  Further, the intermediate transfer belt has conductivity by adding carbon to PPS. However, the present invention is not limited to this, and the same effect can be obtained if other resins, metals, etc. have the same conductivity as this example. Can be expected. Further, although a single-layer and two-layer intermediate transfer belt is used, a similar effect can be expected even with a belt having three or more layers, such as an elastic layer, provided that the circumferential resistance is used.

  The two-layer intermediate transfer belt is manufactured by coating after forming the base layer. However, the manufacturing method is not limited to this as long as the resistance value satisfies the above-described conditions such as integral molding.

1a to 1d Image forming unit 2a to 2d Photosensitive drum (image carrier)
8 Intermediate transfer belt 15 Current supply member 19 Voltage power supply 75 Belt cleaning device 71 Toner charging means

Claims (4)

A plurality of image carriers that can rotate and carry a toner image; a rotatable endless intermediate transfer belt; and a plurality of stretching members that carry the intermediate transfer belt. A toner image carried by the toner image is primarily transferred to the intermediate transfer belt by a plurality of primary transfer portions corresponding to the plurality of image carriers, and the intermediate transfer belt is formed by a secondary transfer portion formed by a current supply member and the intermediate transfer belt. In an image forming apparatus for secondary transfer of toner images on a transfer belt to a transfer material at once,
The intermediate transfer belt has electrical conductivity, and the current supplied from the current supply member flows in the circumferential direction of the intermediate transfer belt, so that the plurality of primary transfer units transfer the image carrier to the intermediate transfer belt. The toner image is primarily transferred,
A toner charging unit that charges the toner that is not secondarily transferred to the transfer material and remains on the intermediate transfer belt; the residual toner is charged to a polarity opposite to a normal charging polarity of the toner by the toner charging unit; The residual toner is transferred from the intermediate transfer belt to the image carrier at the primary transfer portion formed with the image carrier that contacts the intermediate transfer belt by a current flowing in the circumferential direction of the intermediate transfer belt. And
An image forming apparatus, wherein the plurality of stretching members are connected to Zener diodes.   2. The Zener diode has a predetermined voltage threshold value, and the electric potential of the intermediate transfer belt is maintained at a predetermined electric potential even when a current is supplied to the intermediate transfer belt from the toner charging unit. The image forming apparatus described in 1. A plurality of image carriers that can rotate and carry a toner image; a rotatable endless intermediate transfer belt; and a plurality of stretching members that carry the intermediate transfer belt. A toner image carried by the toner image is primarily transferred to the intermediate transfer belt by a plurality of primary transfer portions corresponding to the plurality of image carriers, and the intermediate transfer belt is formed by a secondary transfer portion formed by a current supply member and the intermediate transfer belt. In an image forming apparatus for secondary transfer of toner images on a transfer belt to a transfer material at once,
The intermediate transfer belt has electrical conductivity, and the current supplied from the current supply member flows in the circumferential direction of the intermediate transfer belt, so that the plurality of primary transfer units transfer the image carrier to the intermediate transfer belt. The toner image is primarily transferred,
A toner charging unit that charges the toner that is not secondarily transferred to the transfer material and remains on the intermediate transfer belt; the residual toner is charged to a polarity opposite to a normal charging polarity of the toner by the toner charging unit; The residual toner is transferred from the intermediate transfer belt to the image carrier at the primary transfer portion formed with the image carrier that contacts the intermediate transfer belt by a current flowing in the circumferential direction of the intermediate transfer belt. And
A varistor is connected to the plurality of stretching members.   4. The varistor according to claim 3, wherein the varistor has a predetermined voltage threshold value, and the potential of the intermediate transfer belt maintains a predetermined potential even when current is supplied to the intermediate transfer belt from the toner charging unit. The image forming apparatus described.