JP5693203B2 - Image forming apparatus - Google Patents

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 images are sequentially transferred from the image forming unit of each color to the intermediate transfer belt, and further from the intermediate transfer belt. A configuration is known in which an image is transferred to a transfer material 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 toner 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.

JP 2003-35986 A JP 2001-125338 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 sources are required, resulting in an increase in the size of the image forming apparatus and an increase in high-voltage power source. In addition, when the photosensitive drum is excessively charged by the primary transfer member, the surface of the photosensitive drum becomes a positive potential, and there is a case where the positive potential cannot be eliminated by subsequent charging by the charging member or exposure. . When the primary transfer is performed in a state where the positive potential of the photosensitive drum cannot be eliminated, the primary transferability may be deteriorated. 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.

  The present invention has been made in view of the above problems, and an image forming apparatus having appropriate primary transfer property and secondary transfer property while reducing the number of voltage power sources for applying a voltage to the primary transfer member. The purpose is to provide.

In order to solve the above problems, the present invention has the following configuration.
(1) An image carrier that carries a toner image, a charging member that charges the image carrier, a first voltage application unit that applies a voltage to the charging member, and an endless and rotatable conductivity. An intermediate transfer belt; a current supply member that contacts an outer peripheral surface of the intermediate transfer member; and a second voltage application unit that applies a voltage to the current supply member. A voltage is applied from a voltage application unit to cause a current to flow in the circumferential direction of the intermediate transfer belt, thereby causing a primary transfer of a toner image from the image carrier to the intermediate transfer belt, and the current supply member to the second transfer member. A toner image is secondarily transferred from the intermediate transfer belt to a transfer material at a secondary transfer portion formed by the intermediate transfer belt and the current supply member by applying a voltage from a voltage applying unit;
Image forming characterized in that after the second voltage applying means stops applying a voltage to the current supply member , the first voltage applying means stops applying a voltage to the charging member. apparatus.

By supplying current from the current supply member in the circumferential direction of the intermediate transfer belt, it is not necessary to provide each voltage power source to the plurality of primary transfer members, and primary transfer and secondary transfer are performed with one current supply member. It becomes possible. Further, when a current flows in the circumferential direction of the intermediate transfer belt, the image carrier is charged by the charging member, so that excessive charging of the image carrier due to the current flowing in the circumferential direction is suppressed.

1 is a schematic cross-sectional view illustrating an image forming apparatus according to an embodiment of the present invention. FIG. 3 is a schematic cross-sectional view illustrating an image forming apparatus having a transfer 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. 3 is an explanatory diagram for explaining the relationship between the potential measurement result of the intermediate transfer belt and the applied voltage according to Embodiment 1 of the invention. FIG. 3 is an explanatory diagram for explaining primary transfer according to Embodiment 1 of the present invention. FIG. 4 is an explanatory diagram for explaining a relationship between a potential measurement result of the intermediate transfer belt and a secondary transfer voltage according to the first exemplary embodiment of the present invention. FIG. 3 is a diagram illustrating voltage application timing of each image forming unit according to the first exemplary embodiment of the present invention. FIG. 6 is a schematic cross-sectional view showing an image forming apparatus having another configuration applicable to the present invention. FIG. 6 is a schematic cross-sectional view showing an image forming apparatus having another configuration applicable to the present invention. FIG. 6 is a schematic cross-sectional view showing an image forming apparatus having another configuration applicable to the present invention. FIG. 6 is a schematic cross-sectional view showing an image forming apparatus having another configuration applicable to the present 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 plurality of photosensitive drums 2a, 2b, 2c, and 2d are negatively charged organic photoreceptors, each having a photosensitive layer (not shown) on a drum base (not shown) such as aluminum, and a driving device ( (Not shown) is driven to rotate at a predetermined process speed.

  Around each of the photosensitive drums 2a, 2b, 2c, and 2d, a plurality of charging rollers 3a, 3b, 3c, and 3d, and developing devices 4a, 4b, 4c, and 4d are arranged. The charging rollers 3a, 3b, 3c, and 3d as charging members are connected to a power source (not shown) as first voltage applying means. 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 the connected driving roller 11. 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 forms a secondary transfer portion with a secondary transfer roller 15 as a transfer member via the intermediate transfer belt 8. 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 electrical resistance members (resistance elements) having the same resistance value. In this embodiment, the resistance member has three resistance values of 1 GΩ, 100 MΩ, and 10 MΩ. Since the resistance of each rubber layer of the driving roller 11 and the secondary transfer counter roller 12 is sufficiently smaller than 1 GΩ, 100 MΩ, and 10 MΩ, the electrical influence can be ignored.

As the secondary transfer roller 15 in contact with the outer surface of the intermediate transfer belt 8, 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. The voltage power source 19 is a second voltage applying unit that applies a voltage to the secondary transfer roller.

  Outside the intermediate transfer belt 8, a belt cleaning device 75 for removing and collecting the transfer residual toner remaining on the surface of the intermediate transfer belt 8 is installed. 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 transport direction (rotation direction), is 215 mm, the toner charge amount is -40 μC / g, and the toner amount on the photosensitive drum (image carrier) of the solid image portion is 0.4 mg / It is set to be cm ² .

  The yellow toner image is primarily transferred onto the rotating intermediate transfer belt 8. The configuration for primary transfer of the yellow toner image on the intermediate transfer belt 8 in this embodiment will be described later. In FIG. 1, opposed members 5 a, 5 b, 5 c, and 5 d are provided at positions facing each image forming unit via the intermediate transfer belt 8. By forming the nip with the opposing members 5a, 5b, 5c, and 5d, it is possible to stabilize the nip width widely. In this embodiment, the opposing members 5a, 5b, 5c, and 5d are not applied members that are connected to a voltage power source dedicated to primary transfer.

  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. Similarly, cyan and black toner images formed on 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 on 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 reaches the secondary transfer portion. The full-color toner image on the intermediate transfer belt 8 is secondarily transferred onto the transfer material collectively by a secondary transfer roller 15 to which a secondary transfer voltage (a voltage having a polarity opposite to that of toner (positive polarity)) is applied. 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 primary transfer for transferring the toner image from the photosensitive drums 2a, 2b, 2c, and 2d to the intermediate transfer belt 8 is applied to the primary transfer rollers 55a, 55b, 55c, and 55d as shown in FIG. It is characterized by not performing by applying. 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 is easy 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 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 roller 15M, a current flowing to an 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 the resistance other than the intermediate transfer belt, the outer surface roller 15M and the photosensitive drum 2dM are made of only metal, and M (Metal) is added to the reference symbol 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 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 resistance can be measured by the method shown in FIG. 3, but the 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 surface resistance of the intermediate transfer belt are set such that adjacent voltage power sources are not affected by current flowing into each other via the intermediate transfer belt. 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 current flows in the circumferential direction such as belts A to E is defined as a conductive 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, no current flowed in the circumferential direction even when 2000 V was applied to the comparative belt. However, as shown in FIG. 3B, 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. FIGS. 5A and 5B show 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. Re, Rf, and Rg, which are electrical resistance members (resistance elements), are connected to the driving roller 11M, the secondary transfer counter roller 12, and the tension roller 13, respectively.

  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. FIG. 6A shows the result when a resistance element of Re = Rf = Rg = 1 GΩ is used. The horizontal axis represents the voltage applied to the voltage power supply 19 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 no current flows in the circumferential direction as in the comparative belt. In a configuration in which a voltage is applied to each primary transfer roller by a dedicated transfer power supply 9 as shown in FIG. 2, a potential measurement probe cannot be arranged. In addition, since the electric potential varies depending on the position in the belt circumferential direction, it is meaningless to perform measurement by arranging the electric potential measuring probe with the tension roller 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. 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, the secondary transfer roller 15 as a secondary transfer member is used as a transfer member (current supply member) for supplying a current in the circumferential direction of the intermediate transfer belt 8. A voltage power supply 19 for applying a voltage to the transfer member is a common power supply unit for performing primary transfer and secondary transfer.

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 example, high-quality paper (basis weight 75 g / m ² ) is used as a transfer material, and the secondary transfer voltage necessary for secondary transfer is 2 kV or more.

  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. Further, when there is a transfer material in the secondary transfer portion, the surface potential of the intermediate transfer belt tends to be lower than when there is no transfer material. The method for measuring the surface potential when there is a transfer material is shown in FIG. Therefore, if the magnitude of the applied voltage is determined in accordance with the transfer material in the secondary transfer portion, the surface potential of the intermediate transfer belt 8 is set to a potential at which primary transfer is possible (here, 200 V or more). Is possible.

  In addition, when the primary transfer portion and the secondary transfer portion are sufficiently separated from each other, if necessary, a voltage optimum for the primary transfer is applied to the secondary transfer roller 15 as a transfer member during the primary transfer. When the primary transfer is completed and the secondary transfer timing is reached, the voltage can be switched to the optimum voltage for the secondary transfer.

  Next, the voltage application timing for the secondary transfer roller as a transfer member and the voltage application timing for the charging roller 3 as a charging member for charging the photosensitive drum 2 will be described. The charging rollers 3a, 3b, 3c, and 3d are applied with voltages having the same polarity (negative polarity) as the charging polarity of the photosensitive drum 2 from a power source (not shown). FIG. 9 shows the voltage application timing of each power source to the charging member 3, the developing device 4, and the transfer member 15. Here, FIG. 9A shows the voltage application timing of the image forming apparatus using the high resistance belt described in FIG. 2, and FIG. 9B shows the image formation using the conductive belt of this embodiment. The voltage application timing of the apparatus is shown. In the figure, ON indicates that a voltage is being applied, and OFF indicates that voltage application has been stopped. Here, a case where a start signal for starting a continuous image forming operation on two transfer materials is received from the controller is described.

  In FIG. 9A, when image formation is started, a charging voltage is first applied, that is, the charging voltage is turned on, and then the developing voltage is turned on. The primary transfer voltage and the secondary transfer voltage are turned on in accordance with the timing at which the toner image developed on the photosensitive drum 2 reaches the primary transfer portion and the secondary transfer portion. When the primary transfer and the secondary transfer are completed, they are turned off. In FIG. 9A, the charging voltage is kept on until the primary transfer voltage for the second transfer material is turned off. However, the charging voltage is between the first transfer material and the second transfer material. The voltage may be turned off.

  As shown in FIG. 9A, when the conventional high resistance belt is used, the timing of turning off the charging voltage is not related to the timing of turning off the secondary transfer voltage. When the charging voltage is turned off at the timing shown in FIG. 9A, the photosensitive drum is excessively charged when the conductive belt of this embodiment is used. Since current flows in the circumferential direction of the conductive belt, current flows in the primary transfer portion even at the timing when the secondary transfer is performed after the completion of the primary transfer, and this current charges the photosensitive drum to a positive polarity. .

  The photosensitive drum used in this embodiment is negatively charged, and the surface corresponding to the image portion is exposed. Further, in order to remove the residual charge on the photosensitive drum after the primary transfer and make the potential uniform, the entire surface of the photosensitive drum may be exposed by an exposure device after the primary transfer and before charging. Thus, the negatively charged photosensitive drum can keep the drum potential within a predetermined potential range by exposure. However, if the photosensitive drum is charged to the positive polarity, even if it is exposed, it cannot be within the predetermined potential range, and even if it is intended to be charged to the negative polarity by the charging member when the next image is formed, it is sufficiently negative. Can not be charged, causing image defects.

  Therefore, in this embodiment, as shown in FIG. 9B, the charging voltage is turned on at a timing before the timing at which the voltage is applied to the transfer member and the voltage is applied to the transfer member. Continue to turn on. When the voltage-on state for the transfer member is completed, the charging voltage is turned off. FIG. 9B shows only the voltage on / off timing. The magnitude of the voltage actually applied to the charging member is the voltage applied for image formation and the photosensitive drum 2 other than image formation. The voltage may be changed depending on the voltage applied in order to suppress the positive charge. Comparing FIG. 9A and FIG. 9B, since the voltage is applied to the transfer member until the timing at which the secondary transfer ends, the timing at which the voltage is turned on for the charging member is as shown in FIG. The case of b) is longer.

  In this embodiment, a resistance element having a resistance value of 100 MΩ to 1 GΩ is used as the resistance member, but a Zener diode or a varistor may be connected to ground instead of the resistance element. In the case of the resistance element, the belt potential increases as the secondary transfer voltage is increased. However, in the case of a Zener diode or varistor, a current flows when the Zener potential or the varistor potential is exceeded, and the Zener potential or the varistor potential is maintained. For this reason, even if the secondary transfer voltage is increased, the belt potential does not rise above the Zener potential or the varistor potential. For this reason, the belt potential can be kept constant, and the primary transferability can be stabilized. Further, the setting range of the secondary transfer voltage is widened, and the degree of freedom in setting the secondary transfer voltage is increased.

  In this embodiment, the Zener potential or the varistor potential may be set to 220 v in consideration of the influence of the environment. With this configuration, it is possible to optimize the secondary transfer setting independently of the primary transfer while stabilizing the primary transfer property. (This is because the surface potential of the intermediate transfer belt for primary transfer can be determined by the zener potential or the varistor potential, so that the range of setting of the secondary transfer voltage is widened.) Thus, according to the configuration of the embodiment, A conductive intermediate transfer belt is used, a resistance element having a predetermined value or higher, a Zener diode or a varistor maintaining a predetermined potential or higher is connected to the resistance member, and a voltage is applied from a voltage power source. With this configuration, the surface potential of the intermediate transfer belt can be maintained at a predetermined potential or higher regardless of the resistance of the transfer material.

  It is possible to further reduce the size of the main body by using two tension rollers for tensioning the intermediate transfer belt 8 described in the first embodiment. Further, as shown in FIGS. 10 to 13, it is possible to eliminate the opposing members 5a to 5d that form the primary transfer portions with the respective photosensitive drums via the intermediate transfer belt 8. As another example of forming the primary transfer portion other than the facing members 5a to 5d, primary transfer rollers 40a, 40b, and 40c are arranged between the photosensitive drums in the intermediate transfer belt as shown in FIG. The belt may be pushed up toward the photosensitive drum. Further, as shown in FIG. 11, only one primary transfer roller 40d can be arranged between the image forming units 1b and 1c.

  Further, as shown in FIG. 12, it is possible to contact the photosensitive drum only with the tension of the intermediate transfer belt. In this case, the primary transfer roller is eliminated by causing the image forming portions 1a, 1b, 1c, and 1d to enter slightly below the belt surface on the primary transfer side formed by the secondary transfer counter roller and the drive roller. It is also possible. In some cases, the photosensitive drum and the intermediate transfer belt can be brought into contact with each other more reliably by increasing the amount of penetration of the image forming units 1b and 1c than the amount of penetration of the image forming units 1a and 1d.

  In FIG. 13, the image forming units 1c and 1d are arranged on the lower surface of the intermediate transfer belt. In this case, it is preferable that the image forming units 1a and 1b enter below the intermediate transfer belt surface and the image forming units 1c and 1d enter above the intermediate transfer belt surface. By arranging the image forming units in this way, the image forming apparatus main body may be further miniaturized.

  Further, the voltage supplied to the secondary transfer roller 15 may be any of constant voltage control, constant current control, and a combination of both controls as long as primary transfer and secondary transfer performance can be sufficiently exhibited.

  Further, the photosensitive drum potential, toner charge amount, intermediate transfer belt potential, process speed, Zener potential, varistor potential, and the like are not limited thereto. By using a conductive intermediate transfer belt and setting the belt potential to a predetermined value or more by resistance connected to the stretching roller, it can be established under other conditions.

  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)
5a-5d Opposing member 8 Intermediate transfer belt 9a-9d Voltage power supply dedicated for primary transfer 12 Secondary transfer opposing roller 15 Current supply member 19 Voltage power supply for transfer

Claims (6)

An image bearing member for carrying a toner image, a charging member for charging the image bearing member, a first voltage applying means for applying a voltage to the charging member, and an endless and rotatable intermediate transfer belt A current supply member that contacts an outer peripheral surface of the intermediate transfer member; and a second voltage application unit that applies a voltage to the current supply member, and the second voltage application unit is applied to the current supply member. By applying a voltage to the intermediate transfer belt, a toner image is primarily transferred from the image carrier to the intermediate transfer belt by causing a current to flow in the circumferential direction of the intermediate transfer belt, and the second voltage applying unit is applied to the current supply member. A toner image is secondarily transferred from the intermediate transfer belt to a transfer material at a secondary transfer portion formed by the intermediate transfer belt and the current supply member by applying a voltage from
Image forming characterized in that after the second voltage applying means stops applying a voltage to the current supply member , the first voltage applying means stops applying a voltage to the charging member. apparatus.   The image forming apparatus according to claim 1, wherein the intermediate transfer belt has a multilayer structure, and a surface layer has a higher resistance than other layers.   The polarity of the voltage applied by the first voltage applying unit is opposite to the polarity of the voltage applied by the second voltage applying unit. Image forming apparatus. A tension member that contacts the inner peripheral surface of the intermediate transfer belt and faces the current supply member ; the tension member is a resistance element for maintaining the surface potential of the intermediate transfer belt at a predetermined potential or higher; The image forming apparatus according to claim 1, wherein the image forming apparatus is connected. The contact with the inner peripheral surface of the intermediate transfer belt has a tension member which is opposed to the current supply member, said tension member is a Zener diode to maintain the surface potential of the intermediate transfer belt than the predetermined potential or The image forming apparatus according to any one of claims 1 to 3, wherein a varistor is connected. The first voltage application unit starts applying a voltage to the charging member before the second voltage application unit starts applying a voltage to the current supply member. The image forming apparatus according to any one of claims 1 to 5.