Image forming device

Abstract

To make it possible to set an appropriate transfer voltage according to a gap generated by a medium.SOLUTION: An image forming apparatus 100 comprises: a photoconductor drum 102 that carries a toner image; a transfer voltage application unit 147 that applies a transfer voltage for transferring the toner image to a medium; a transfer roller 120 that receives application of the transfer voltage and transfers the toner image carried on the photoconductor drum 102 to the medium; and a transfer control unit 160 that controls the transfer voltage applied by the transfer voltage application unit 147. The transfer control unit 160 calculates the transfer voltage according to a first current being a current in a medium inside area and a second current being a current in a medium outside area not including a gap generated by the medium.SELECTED DRAWING: Figure 2

Description

[Technical Field]

[0001] The present disclosure relates to an image forming apparatus. [Background technology]

[0002] Conventionally, a technique has been known in which, when transferring a developer image to a medium, the magnitude of the transfer voltage is determined by calculating the amount of current proportional to the medium width and the width outside the medium (see, for example, Patent Document 1). [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Japanese Patent Application Laid-Open No. 2014-106413 Summary of the Invention [Problem to be solved by the invention]

[0004] However, when the medium is thick, the current amount calculated using the above method does not match the actual situation, and it may not be possible to set an appropriate transfer voltage, which may result in poor printing results.

[0005] Therefore, one or more aspects of the present disclosure aim to make it possible to set an appropriate transfer voltage depending on the gap caused by the medium. [Means for solving the problem]

[0006] An image forming apparatus according to a first aspect of the present disclosure includes an image carrier that carries a developer image, which is an image formed by a developer; a transfer voltage application unit that applies a transfer voltage for transferring the developer image to a medium; a transfer unit that receives the transfer voltage and transfers the developer image carried on the image carrier to the medium; and a transfer control unit that controls the transfer voltage applied by the transfer voltage application unit, wherein the transfer control unit controls a first current that is a current in an area within the medium through which the medium passes between the image carrier and the transfer unit when the developer image is transferred; and the area within the medium and a second current which is a current in an area outside the medium, which is an area excluding the area between the image carrier and the transfer unit. [Effects of the Invention]

[0008] According to one or more aspects of the present disclosure, an appropriate transfer voltage can be set depending on the gap caused by the medium. [Brief explanation of the drawings]

[0009] [Figure 1] 1 is a longitudinal sectional view schematically illustrating a configuration of an image forming apparatus according to first and second embodiments. [Figure 2] 2 is a block diagram schematically illustrating the configuration of a control system in the image forming apparatus according to the first embodiment. FIG. [Figure 3] 1A and 1B are block diagrams showing an example of the hardware configuration of a control system in an image forming apparatus. [Figure 4] 6 is a flowchart showing a process for calculating a transfer voltage when the image forming apparatus according to the first embodiment performs printing. [Figure 5] FIG. 10 is a schematic diagram showing a voltage data table for measuring transfer current. [Figure 6] FIG. 4 is a schematic diagram showing a transfer current table. [Figure 7] 5 is a schematic diagram showing the state of a transfer nip portion between a photosensitive drum and a transfer roller when a transfer current is measured. FIG. [Figure 8] FIG. 10 is a schematic diagram showing a transfer setting data table. [Figure 9] 10 is a schematic diagram showing a state in which a gap occurs in a transfer nip portion between a photosensitive drum and a transfer roller during transfer. FIG. [Figure 10] 1 is a graph showing the relationship between media thickness and media width and gap width. [Figure 11] 1 is a table showing the relationship between media thickness and media width and gap width. [Figure 12] FIG. 10 is a schematic diagram showing a gap width table. [Figure 13] 10 is a graph showing a relationship in which the values ​​of the slope and intercept change around a certain threshold medium width. [Figure 14] 10 is a schematic diagram showing the state of a transfer nip portion between a photosensitive drum and a transfer roller when a toner image is transferred to thin paper. FIG. [Figure 15] 10 is a schematic diagram showing the state of a transfer nip portion between a photosensitive drum and a transfer roller when a toner image is transferred onto very thick paper. FIG. [Figure 16] 10 is a graph showing the dependency of transfer current density on the voltage between the transfer shafts in the case of high-quality paper. [Figure 17] 10 is a graph showing the dependency of transfer current density on the voltage between the transfer shafts in an OHP. [Figure 18] 10 is a graph showing the calculation results of transfer current for thin plain paper and very thick plain paper. [Figure 19] FIG. 10 is a block diagram schematically illustrating the configuration of a control system in an image forming apparatus according to a second embodiment. [Figure 20] 10A and 10B are schematic diagrams showing a state in which a gap is generated in the transfer nip portion between the photosensitive drum and the transfer roller in the pressure-up mode and the pressure-down mode. [Figure 21] 10 is a graph showing gap width in combination with media width and pressure mode. [Figure 22] 10 is a table showing gap width values ​​for combinations of media widths and pressure modes. [Figure 23] 10 is a graph showing an optimum control current when the transfer pressure is variable. [Figure 24] 10 is a flowchart showing a process for calculating a transfer voltage when an image forming apparatus according to a second embodiment performs printing. DETAILED DESCRIPTION OF THE INVENTION

[0010] FIG. 1 is a vertical cross-sectional view schematically showing the configuration of an image forming apparatus 100 according to the first embodiment. The image forming apparatus 100 shown in FIG. 1 is a monochrome image forming apparatus that forms black and white images by electrophotography.

[0011] In the image forming apparatus 100, a printing mechanism 101, which is an independent image drum unit, is disposed in a transport path extending from the medium insertion side to the medium ejection side.

[0012] The printing mechanism 101 is an electrophotographic LED printing mechanism (image forming mechanism) for forming a black (K) image. The printing mechanism 101 includes a photosensitive drum 102, a charging roller 103, a developing roller 104, a developing blade 105, a supply roller 106, a discharging light irradiator 107, and a toner cartridge 108.

[0013] The photosensitive drum 102 is an image carrier (image carrier) that carries an image. For example, the photosensitive drum 102 carries an electrostatic latent image and a toner image, which is a developer image formed by attaching a developer to the electrostatic latent image. The charging roller 103 is a charging unit that uniformly charges the surface of the photosensitive drum 102 . The developing roller 104 is a developing unit that adheres toner, which is a developer, to the electrostatic latent image formed on the photosensitive drum 102, thereby forming a toner image as a developer image.

[0014] The developing blade 105 is a regulating portion that thins the toner on the surface of the developing roller 104 . The supply roller 106 is a supply unit that supplies the toner stored in the toner cartridge 108 to the developing roller 104 .

[0015] The charge-removing light irradiating unit 107 irradiates the photosensitive drum 102 with charge-removing light, thereby removing electricity from the photosensitive drum 102 . The toner cartridge 108 is a toner container (developer container) that contains toner. The toner cartridge 108 contains black toner.

[0016] An LED (Light Emitting Diode) head 109 is disposed above the photosensitive drum 102 of the printing mechanism 101. The LED head 109 is an exposure unit that forms an electrostatic latent image by exposing the surface of the photosensitive drum 102. Specifically, the LED head 109 includes an LED array (not shown), a board (not shown) that mounts a drive IC that drives the LED array and a group of registers that hold data, and a SELFOC lens array (not shown) that focuses light from the LED array.

[0017] The LED head 109 then causes the LED array to emit light in accordance with signals representing image data input from an external host computer (not shown). The light emitted by the LED head 109 exposes the surface of the photosensitive drum 102, forming an electrostatic latent image on the surface of the photosensitive drum 102. Toner on the circumference of the developing roller 104 adheres to this electrostatic latent image by electrostatic force, forming a toner image.

[0018] The image forming apparatus 100 also includes a paper feed mechanism (medium supply mechanism) 110 for supplying paper as a medium. The paper feed mechanism 110 includes a paper cassette 111, a hopping roller 112, and a group of transport rollers 113 and 114.

[0019] The paper storage cassette 111 is a paper storage unit (medium storage unit) that stores paper. The hopping roller 112 picks up the sheets of paper stored in the paper storage cassette 111 one by one.

[0020] A group of conveying rollers 113 and 114 corrects the skew of the sheet of paper picked up by the hopping roller 112 and conveys the sheet of paper to the printing mechanism 101 . In addition, sensors 115, 116, and 117 are arranged in the paper transport path to detect the position of a single sheet or a jam.

[0021] Below the photosensitive drum 102 of the printing mechanism 101, a transfer roller 120 is disposed as a transfer unit. A transfer voltage is applied to the transfer roller 120, and the toner image formed on the surface of the photosensitive drum 102 is transferred by the transfer roller 120 onto a sheet of paper that has been conveyed.

[0022] After the toner image is transferred, the sheet of paper separated from the printing mechanism 101 is sent to a fixing mechanism 130 serving as a fixing unit. The fixing mechanism 130 includes a heat roller 131 , a pressure roller 132 that presses the heat roller 131 , a heater 133 that heats the heat roller 131 , and a thermistor 134 that detects the temperature of the heat roller 131 .

[0023] The fixing mechanism 130 heats and melts the toner on the conveyed sheet of paper, thereby fixing the toner image to the sheet of paper. The heat roller 131 is driven by a heater motor 158 (described later), and the pressure roller 132 rotates in accordance with the rotation of the heat roller 131 .

[0024] The heater 133 is disposed inside the heat roller 131 and is a halogen lamp as a heat source. A thermistor 134 is disposed near the surface of the heat roller 131 . A sensor 1135 is also disposed downstream of the heat roller 131 to detect the position of the paper or a jam.

[0025] The sheet of paper on which the toner image has been fixed by the fixing mechanism 130 is discharged to the outside of the image forming apparatus 100 by a group of discharge rollers 136 and 137 .

[0026] FIG. 2 is a block diagram schematically showing the configuration of a control system in image forming apparatus 100 according to the first embodiment. The host interface unit (hereinafter referred to as the host I / F unit) 140 is an interface that communicates with a host computer (not shown). The host I / F unit 140 is an interface on the physical layer, and is made up of a connector and a communication chip.

[0027] The command image processing unit 141 processes commands and print data (image formation data) from the host computer. For example, the command image processing unit 141 interprets the commands and develops the image data included in the print data into a bitmap to generate processed image data. Furthermore, the command image processing unit 141 controls the entire processing in the image forming apparatus 100 .

[0028] The LED head interface unit (hereinafter referred to as the LED head I / F unit) 142 is an interface that processes the processed image data from the command image processing unit 141 in accordance with the interface of the LED head 109. The LED head I / F unit 142 is composed of a semi-custom LSI, RAM, etc. (not shown).

[0029] The high voltage control unit 143 controls the charging voltage application unit 144, the developing voltage application unit 145, the supply voltage application unit 146, and the transfer voltage application unit 147, thereby controlling the charging voltage, the developing voltage, the supply voltage, and the transfer voltage in the printing mechanism 101.

[0030] The charging voltage application unit 144 applies or stops the application of the charging voltage to the charging roller 103 . The developing voltage application unit 145 applies or stops the application of the developing voltage to the developing roller 104 . The supply voltage application unit 146 applies or stops the application of the supply voltage to the supply roller 106 .

[0031] The transfer voltage application unit 147 applies or stops the application of a transfer voltage for transferring the toner image from the photosensitive drum 102 to the medium. The magnitude of the transfer voltage applied by transfer voltage application unit 147 is determined by calculation unit 154 (to be described later) depending on the magnitude of the resistance of transfer roller 120 .

[0032] The charging voltage application section 144, the developing voltage application section 145, the supply voltage application section 146, and the transfer voltage application section 147 can be configured, for example, by a power supply circuit (not shown).

[0033] The magnitude of the resistance of transfer roller 120 is determined from the magnitude of the current flowing through transfer roller 120 and the magnitude of the voltage applied from transfer voltage application unit 147. Since the resistance is detected from the magnitude of the current in this way, this method of detecting resistance is called current detection.

[0034] Specifically, a fixed resistor 148 is provided between the transfer voltage application unit 147 and the transfer roller 120, and the current flowing through the fixed resistor 148 is calculated by the transfer current measurement unit 149. Here, the transfer current measurement unit 149 calculates the current from the voltage applied to the fixed resistor 148 in accordance with Ohm's law.

[0035] The mechanism control unit 150 controls the printing mechanism 101, the paper feed mechanism 110, the fixing mechanism 130, various sensors 115, 116, 117, 135, and the discharge roller groups 136, 137, and controls printing (image formation) in the image forming apparatus 100. For example, the mechanism control unit 150 controls the print mechanism 101. The mechanism control unit 150 also controls various sensors 115, 116, 117, 135, the thermistor 134, the heater 133, and the discharge roller groups 136, 137. Furthermore, the mechanism control unit 150 controls a hopping motor 156 that drives the hopping roller 112, a conveying group motor 157 that drives the conveying roller groups 113 and 114, a heater motor 158 that drives the heat roller 131, and a drum motor 159 that drives the photosensitive drum 102.

[0036] The mechanism control unit 150 includes a medium type specifying unit 151 , a medium width specifying unit 152 , a medium thickness specifying unit 153 , and a calculation unit 154 .

[0037] The medium type identification unit 151 identifies the medium type from the medium setting information included in the print data sent from the host computer. The medium width determination unit 152 determines the medium width from the medium setting information included in the print data sent from the host computer. The medium thickness determination unit 153 determines the medium thickness from the medium setting information included in the print data sent from the host computer.

[0038] Before the printing operation (image forming operation), the calculation unit 154 calculates the magnitude of the transfer voltage to be applied during transfer according to the medium type, medium width, and medium thickness set by the user. The method of calculating the transfer voltage will be described later.

[0039] The storage unit 155 stores data and programs necessary for processing by the mechanism control unit 150. The storage unit 155 can be configured, for example, by a non-volatile memory.

[0040] As described above, the transfer voltage applied to the transfer roller 120 is controlled by the mechanism control unit 150 and the high voltage control unit 143, and therefore the mechanism control unit 150 and the high voltage control unit 143 constitute a transfer control unit 160.

[0041] Some or all of the command image processing unit 141, high voltage control unit 143, transfer current measurement unit 149, and mechanism control unit 150 described above can also be configured as a processing circuit 10 such as a single circuit, a composite circuit, a programmed processor, a parallel programmed processor, an ASIC (Application Specific Integrated Circuit), or an FPGA (Field Programmable Gate Array), as shown in FIG. 3(A).

[0042] Furthermore, as shown in FIG. 3B, some or all of the above-described command image processing unit 141, high voltage control unit 143, transfer current measurement unit 149, and mechanism control unit 150 can be configured with a memory 11 and a processor 12 such as a CPU (Central Processing Unit) that executes a program stored in the memory 11. Such a program may be provided over a network or may be provided by being recorded on a recording medium. That is, such a program may be provided as, for example, a program product. As described above, the command image processing unit 141, the high voltage control unit 143, the transfer current measuring unit 149, and the mechanism control unit 150 can be configured as a processing circuit network. The storage unit 155 can be realized by a volatile or non-volatile memory.

[0043] Next, the operation of the image forming apparatus 100 will be described. FIG. 4 is a flowchart showing a process for calculating the transfer voltage when the image forming apparatus 100 performs printing (image formation). First, when the host I / F unit 140 receives print data sent from the host computer (S10), the transfer control unit 160 starts calculating the transfer voltage.

[0044] The medium type identification unit 151 identifies the medium type by reading information about the medium type from the medium setting information included in the print data sent from the host computer (S11). The medium type can be set when the user sends print data including image data that the user wants to print to the host I / F unit 140.

[0045] Based on the medium type identified by medium type identification unit 151, high voltage control unit 143 reads out the corresponding transfer current measurement voltage Vm from the transfer current measurement voltage information stored in memory unit 155 (S12). Transfer current measurement voltage Vm is a transfer voltage applied to transfer roller 120 by transfer voltage application unit 147 when measuring the transfer current flowing through transfer roller 120.

[0046] FIG. 5 is a schematic diagram showing a transfer current measurement voltage data table 170, which is an example of transfer current measurement voltage information. Transfer current measurement voltage data table 170 stores transfer current measurement voltages Vmji according to the medium type and the number of measurements. Here, symbol j is an identification code indicating the medium type, with j=1 indicating fine paper, j=2 indicating plain paper, and j=3 indicating film for overhead projectors (OHPs) (hereinafter simply referred to as OHPs). Symbol i is an identification code indicating the number of measurements, with i=1 indicating the first measurement, i=2 indicating the second measurement, and i=3 indicating the third measurement.

[0047] In step S12 of FIG. 4, high voltage control unit 143 only needs to read out transfer current measurement voltage Vmji for all the times corresponding to medium type j. The voltage for measuring the transfer current may be determined by conducting an experiment in advance. Conditions for determining the voltage for measuring the transfer current will be described later with reference to FIGS.

[0048] 4, high voltage control unit 143 sequentially supplies transfer current measurement voltage Vmji to transfer roller 120 from transfer voltage application unit 147 according to the number of measurements, and causes transfer current measurement unit 149 to sequentially measure transfer current Im flowing through transfer roller 120 (S13). Then, transfer current measurement unit 149 stores the measured transfer current Im in memory unit 155 as transfer current information.

[0049] FIG. 6 is a schematic diagram showing a transfer current table 171 as an example of transfer current information. As shown in FIG. 6, the transfer current Imji corresponding to the medium type j and the number of measurements i is also stored in the transfer current table 171 for the transfer current Im. For example, the transfer current Im11 is the value obtained when the transfer current Im for fine paper is measured for the first time. In this case, the voltage Vm11 for measuring the transfer current is applied to the transfer roller 120.

[0050] Returning to FIG. 4, high voltage control unit 143 determines the electrical characteristics of transfer roller 120 based on the measurement results in step S13 (S14).

[0051] Here, the electrical characteristics of the transfer roller 120 will be described. FIG. 7 is a schematic diagram showing the state of the transfer nip portion between the photosensitive drum 102 and the transfer roller 120 when the transfer current is measured.

[0052] The photosensitive drum 102 includes a surface layer 102a as a photosensitive member and a conductive metal shaft 102b. The transfer roller 120 also includes an elastic surface layer 120a and a conductive metal shaft 120b. At the transfer nip portion, the surface layer 102a of the photosensitive drum 102 and the surface layer 120a of the transfer roller 120 are in contact with each other.

[0053] The electrical characteristics of the transfer roller 120 are expressed by the linear approximation formula shown in the following formula (1), which is obtained from the relationship between the transfer shaft voltage Vs applied between the metal shaft 120b of the transfer roller 120 and the metal shaft 102b of the photosensitive drum 102 and the transfer current density Jm per unit length. Vs=a×Jm+b (1) where a is the slope and b is the intercept.

[0054] Here, the transfer shaft voltage applied between the metal shaft 120b of the transfer roller 120 and the metal shaft 102b of the photosensitive drum 102 when a transfer current measurement voltage Vm [V] is applied is defined as Vs [V], the transfer current density per unit length when a transfer current Im flows through the transfer roller 120 is defined as Jm [μA / mm], the resistance value of the fixed resistor 148 is defined as R [MΩ], and the length of the transfer roller 120 in a direction perpendicular to the medium transport direction is defined as L [mm].

[0055] Here, the slope a and intercept b of the linear approximation equation are determined from the relationship between the voltage Vs between the transfer shafts and the transfer current density Jm when the type of medium detected by the medium type identification unit 151 is "high quality paper" or "plain paper" and when it is "OHP film."

[0056] First, we will explain the case where the medium type is "high-quality paper" or "plain paper." Note that the method for calculating the slope a and intercept b is the same for both the case where the medium type is "high-quality paper" and the case where the medium type is "plain paper." Therefore, the following explanation will focus on the case where the medium type is "high-quality paper."

[0057] In the following, the "1" in the "1i" symbol, which is the last letter and the second-to-last number of the symbols indicating voltage, current, transfer current density, and potential, is an identification code indicating fine paper, and "i" is an identification code indicating the number of measurements. Here, i = 1, 2, and "i" may be represented as "1" or "2."

[0058] The relationship between the voltage Vs1i [V] between the transfer shafts, the surface potential Vr1i [V] on the transfer roller, and the surface potential Vd1i [V] on the photosensitive drum can be expressed by the following equation (2). Vs1i=Vr1i+Vd1i (2)

[0059] Here, if the surface potential Vd1i [V] on the photosensitive drum is uniformly charged to, for example, −500 V, it is expressed by the following equation (3). Vd1i=-500 (3)

[0060] Substituting equation (3) into equation (2) gives the following equation (4). Vs1i=Vr1i-500 (4) Although Vd1i=-500 is used here, Vd1i is not limited to -500 V, and may be any potential magnitude that uniformly charges the surface of the photosensitive drum 102 depending on the control. This value may be a constant or calculated based on the time evolution of a parallel plate capacitor model.

[0061] Next, the relationship between the transfer roller surface potential Vr1i [V], the transfer current measurement voltage Vm1i [V], the transfer current Im1i [μA], and the resistance value R [MΩ] can be expressed by the following equation (5). Vr1i = Vm1i - Im1i × R (5)

[0062] Formula (4) and formula (5) give the following formula (6). Vs1i = Vm1i - 500 - Im1i × R (6) Furthermore, from the definition of the transfer current density Jm1i, the following equation (7) is obtained. Jm1i=Im1i / L (7)

[0063] From the above, the slope a and intercept b of the linear approximation equation to be obtained are given by the following equations (8) and (9). a=(Vs12-Vs11) / (Jm12-Jm11) (8) b=(Vs11×Jm12-Vs12×Jm11) / (Jm12-Jm11) (9)

[0064] The slope a and intercept b are calculated in the above manner for fine paper and plain paper of a variety of thicknesses. Note that the number of measurements is not limited to two, and the same calculations can be performed any number of times.

[0065] Next, the case where the medium type is "OHP" will be described. In the following description, the "3" in "3i," which is the last letter and the second-to-last number of the symbols indicating voltage, current, transfer current density, and potential, is an identification code indicating the OHP, and "i" is an identification code indicating the number of measurements. Here, i = 1, 2, 3, and "i" may be represented as "1," "2," or "3."

[0066] The relationship between the voltage Vs3i [V] between the transfer shafts, the surface potential Vr3i [V] on the transfer roller, and the surface potential Vd3i [V] on the photosensitive drum can be expressed by the following equation (10). Vs3i=Vr3i+Vd3i (10) Further, if the surface potential Vd3i [V] of the photosensitive drum is uniformly charged to, for example, −500 V, it is expressed by the following formula (11). Vd3i=-500 (11)

[0067] Substituting equation (11) into equation (10) gives the following equation (12). Vs3i=Vr3i-500 (12) Here, Vd3i=-500 is used, but it is not limited to -500 V, and any magnitude of potential may be applied depending on the control to uniformly charge the surface of the photosensitive drum 102. This value may be a constant or calculated based on the time evolution of a parallel plate capacitor model.

[0068] Next, the relationship between the transfer roller surface potential Vr3i [V], the transfer current measurement voltage V3mi [V], the transfer current Im3i [μA], and the resistance value R [MΩ] can be expressed by the following equation (13). Vr3i = Vm3i - Im3i × R (13)

[0069] From the above, the following equations (14) to (16) are obtained. Vs3i=Vr3i-500 (14) Vs3i = Vm3i - 500 - Im3i × R (15) Jm3i=Im3i / L (16)

[0070] In the case of an OHP, the slope a1 and intercept b1 obtained in the first and second measurements, and the slope a2 and intercept b2 obtained in the second and third measurements are calculated separately. The reason for this will be described later with reference to FIG. 17. When Vs≦Vs32 (Jm≦Jm32), the following equations (17) and (18) hold. a1=(Vs32-Vs31) / (Jm32-Jm31) (17) b1=(Vs31×Jm32-Vs32×Jm31) / (Jm32-Jm31) (18)

[0071] Furthermore, when Vs>Vs32 (Jm>Jm32), the following equations (19) and (20) hold. a2=(Vs33-Vs32) / (Jm33-Jm32) (19) b2=(Vs32×Jm33-Vs33×Jm32) / (Jm33-Jm32) (20)

[0072] For OHPs of various thicknesses, the slope a1, intercept b1, slope a2, and intercept b2 can be calculated in the above manner. Note that the number of measurements is not limited to three, and similar calculations can be performed for any number of measurements.

[0073] Returning to FIG. 4, the medium width determination unit 152 determines the medium width from the medium setting information included in the print data sent from the host computer, and the medium thickness determination unit 153 determines the medium thickness from the setting information (S15).

[0074] Here, the medium width is the width of the medium in a direction perpendicular to the medium transport direction. For example, if the medium is A3 size, it is 297 mm, if the medium is A4 size, it is 210 mm, or if the medium is A5 size, it is 148 mm. Note that the medium width is not limited to these examples and may be other values.

[0075] Next, the calculation unit 154 reads out the transfer reference current density Jb and transfer reference voltage Vb corresponding to the medium type identified in step S11 from the transfer setting information stored in the storage unit 155 (S16).

[0076] FIG. 8 is a schematic diagram showing a transfer setting data table 172, which is an example of transfer setting information. As shown in FIG. 8, the transfer setting data table 172 stores the transfer reference current density Jb and the transfer reference voltage Vb for each medium type. The transfer reference current density Jb and transfer reference voltage Vb corresponding to the medium type were experimentally determined. Specifically, the transfer current density flowing through the region within the medium necessary for good transfer on each type of medium and the transfer voltage applied to the medium at that time were experimentally determined.

[0077] 4, the calculation unit 154 determines whether the medium thickness determined in step S15 is equal to or greater than a predetermined threshold (S17). If the medium thickness is smaller than the predetermined threshold (No in S17), the process proceeds to step S18, and if the medium thickness is equal to or greater than the predetermined threshold (Yes in S17), the process proceeds to step S19.

[0078] In step S18, the calculation unit 154 calculates the transfer voltage without taking into consideration the gap between the photosensitive drum 102 and the transfer roller 120 that occurs due to the thickness of the medium. On the other hand, in step S19, the calculation unit 154 calculates the transfer voltage taking into consideration the gap between the photosensitive drum 102 and the transfer roller 120 that occurs due to the thickness of the medium.

[0079] The following describes how the method for calculating the transfer voltage changes depending on the thickness of the medium. The intra-medium region in the first embodiment refers to the region where the medium is interposed between the transfer roller 120 and the photosensitive drum 102 when a toner image is transferred onto the medium. The outer medium area refers to the area where the transfer roller 120 and the photosensitive drum 102 are in contact with each other in a direction perpendicular to the medium transport direction when a toner image is transferred onto the medium. Furthermore, the gap refers to an insulating region where the transfer roller 120 and the photosensitive drum 102 do not come into contact when a toner image is transferred to the medium. The gap occurs between the inside-medium region and the outside-medium region.

[0080] FIG. 9 is a schematic diagram showing a state in which a gap occurs in the transfer nip between the photosensitive drum 102 and the transfer roller 120 during transfer. When printing on a medium Pa having a thickness d [μm], the photosensitive drum 102 and the transfer roller 120 apply pressure to the medium Pa, bringing the medium Pa, the transfer roller 120, and the photosensitive drum 102 into sufficient contact with each other, so that the toner image on the photosensitive drum 102 is transferred onto the medium Pa by the electrical force of the transfer voltage and the physical force of the contact.

[0081] At this time, the transfer roller 120 is pressed in by an amount corresponding to the thickness d of the medium Pa, and a gap width K(d) [mm] is required in the direction perpendicular to the conveying direction until the transfer roller 120 comes into contact with the photosensitive drum 102 again due to the restoring force of the transfer roller 120. The gap width K(d) is the width of the gap in the direction perpendicular to the conveying direction of the medium Pa.

[0082] In the first embodiment, a combination of a transfer roller 120 having a hardness of 38±4 degrees and a rigid photosensitive drum 102 will be described. Note that, for example, if the transfer roller is a metal roller, the combination will be rigid and the gap width K(d) will be more pronounced, but in the first embodiment, at least one of the transfer roller 120 and the photosensitive drum 102 must be an elastic body.

[0083] As described above, the gap width K(d) [mm] is determined by the pressure between the transfer roller 120 and the photosensitive drum 102 and the medium thickness d [μm], so there is a correlation between the medium thickness d [μm] and the gap width K(d) [mm]. In addition to this relationship, it is known that the gap width K(d) also varies depending on the medium width W [mm] of the recording medium, because the load on the metal shaft 102b is not uniform. Therefore, K(d) [mm] can be calculated using the following equation (21). K(d)=p×W+q (21) Here, p is the slope determined by the medium width W [mm] and the medium thickness d [μm], and q is the intercept determined by the medium width W [mm] and the medium thickness d [μm].

[0084] There are situations where the gap width K(d) can be ignored. When the medium thickness d [μm] becomes thin enough, the amount by which the medium presses the transfer roller 120 decreases, and the gap width K(d) [mm] becomes zero due to the restoring force of the transfer roller. The medium thickness d [μm] at the boundary where the gap width K(d) becomes zero is specifically referred to as the threshold value dth [μm]. In this embodiment, dth = 60 [μm].

[0085] Here, the correspondence relationship between the medium width W [mm] and the gap width K(d) [mm] is shown in FIG. FIG. 11 is a table showing the values ​​of the gap width K(d) [mm] when the medium width W [mm] is changed for two types of medium with a thickness d [μm].

[0086] FIG. 12 is a schematic diagram showing a gap width table 173 for specifying the slope p and intercept q in equation (21). Each column in the gap width table 173 stores the slope and intercept used to calculate the gap width, and these values ​​change around a certain threshold media width. Figure 13 is a graph showing this relationship. The gap width table 173 is assumed to be stored in the storage unit 155.

[0087] During actual printing operation, when calculating the amount of transfer current in the area outside the medium, the calculation unit 154 reads out the slope p and intercept q corresponding to the medium thickness d [μm] identified by the medium thickness identification unit 153 and the medium width W [mm] identified by the medium width identification unit 152.

[0088] In step S18 or S19 of FIG. 4, the calculation unit 154 calculates the transfer voltage Vtr based on the electrical characteristics of the transfer roller 120 calculated in step S14, the medium width and medium thickness identified in step S15, and the transfer reference current density Jb and transfer reference voltage Vb read in step S16.

[0089] A specific example of a method for calculating the transfer voltage Vtr will be described below. First, the case where the medium thickness d<threshold value dth in step S18 of FIG. 4 will be described. FIG. 14 is a schematic diagram showing the state of the transfer nip between the photosensitive drum 102 and the transfer roller 120 when a toner image is transferred onto thin paper where the medium thickness d<threshold value dth.

[0090] First, the calculation unit 154 calculates the transfer voltage Vc [V] applied to areas other than the medium Pa in the media intra-media area when the current density in the medium Pa is the transfer reference current density Jb, using the following equation (22) from the electrical characteristics of the transfer roller 120 calculated in step S14 of Figure 4. Vc=a×Jb+b (22)

[0091] When the medium Pa is present in the transfer nip portion, the transfer shaft voltage Vs [V] applied between the metal shaft 120b of the transfer roller 120 and the metal shaft 102b of the photosensitive drum 102 can be calculated using the following equation (23): Vs = Vb + Vc + Vd (23)

[0092] Here, for example, the surface potential Vd of the photosensitive drum 102 is set to -50 [V]. Therefore, equation (23) becomes equation (24) below. Vs=Vb+Vc-50 (24)

[0093] Then, the calculation unit 154 calculates the transfer current density Jout [μA / mm] as the density of the current flowing in the outside-of-medium area from the electrical characteristics of the transfer roller 120 calculated in step S14 of Fig. 4. This is expressed by the following equation (25). Jout=(Vs-b) / a (25) The value a in equation (25) can be a1 or a2 depending on the medium type and the voltage between the transfer shafts, and the value b can be b1 or b2 depending on the medium type and the voltage between the transfer shafts.

[0094] Here, if the width of the medium Pa in the direction perpendicular to the transport direction is W [mm], the transfer current Itr [μA] flowing through the transfer roller 120 required for good transfer is the sum of the transfer currents flowing through the medium area and the medium outside area, and can be calculated using the following equation (26). Itr=Jb×W+Jout×{(LW―K(d)} (26)

[0095] Since we are considering thin paper as a condition, K(d) is infinitely close to 0, so equation (26) becomes equation (27) below. Itr=Jb×W+Jout×(LW) (27)

[0096] Then, when a transfer current Itr [μA] flows through the transfer roller 120, the applied voltage VR [V] across the fixed resistor 148 is expressed by the following equation (28) according to Ohm's law. VR = Itr × R (28)

[0097] The transfer voltage Vtr [V] supplied by the transfer voltage application unit 147, which is required to pass the transfer current Itr through the transfer roller 120, is the sum of the voltage Vs between the transfer shafts and the applied voltage VR [V] across the fixed resistor 148, and is expressed by the following equation (29). Vtr = Vs + VR (29) In this manner, the calculation unit 154 can determine the transfer voltage Vtr for thin paper.

[0098] Next, the case where the medium thickness d≧threshold value dth in step S19 of FIG. 4 will be described. FIG. 15 is a schematic diagram showing the state of the transfer nip between the photosensitive drum 102 and the transfer roller 120 when a toner image is transferred onto very thick paper where the medium thickness d≧threshold value dth. The above calculation formulas (22) to (26) are also applicable to very thick paper. However, for very thick paper, the medium thickness d [μm] becomes large, and as a result, the gap width K(d) becomes non-negligible. For this reason, for very thick paper, equation (26) is used instead of equation (27).

[0099] Hereafter, similarly to the case of thin paper, when a transfer current Itr [μA] flows through the transfer roller 120, the applied voltage VR [V] to the fixed resistor 148 is expressed by the following equation (30) according to Ohm's law. VR = Itr × R (30)

[0100] The transfer voltage Vtr [V] supplied by the transfer voltage application unit 147, which is required to pass the transfer current Itr through the transfer roller 120, is the sum of the voltage Vs between the transfer shafts and the applied voltage VR [V] across the fixed resistor 148, and is expressed as the following equation (31). Vtr = Vs + VR (31) In this way, the calculation unit 154 can obtain the transfer voltage Vtr even in the case of very thick paper.

[0101] Here, Vs in equation (31) is a value determined according to the type of medium Pa, and is a fixed value for each type of medium Pa. Therefore, in equation (26), if the current calculated by Jb×W is the first current and the current calculated by Jout×{(LW−K(d)} is the second current, the calculation unit 154 calculates the second current by dividing the first current, which is the current in the area within the medium where the medium passes between the photosensitive drum 102 and the transfer roller 120 when the toner image is transferred, and the second current, which is the current in the area within the medium where the medium passes between the photosensitive drum 102 and the transfer roller 120 when the toner image is transferred. and areas within the mediaA second current, which is the current in the outer medium region, which is the region excluding the region between the photosensitive drum 102 and the transfer roller 120, is used to calculate the transfer voltage.

[0102] Since the size of the gap increases as the thickness of the medium increases, the transfer voltage decreases as the size of the gap increases.

[0103] As described above, the calculation unit 154 calculates the transfer voltage by adding a second voltage (VR) calculated by Ohm's law to a value obtained by adding a first current that increases as the width of the medium increases to a first voltage (Vs) determined by the type of the medium, and a second current that decreases as the size of the gap increases.

[0104] FIG. 16 is a graph showing the dependence of the transfer current density on the voltage between the transfer shafts in the case where the medium type is high-quality paper. As described above, when the transfer reference current density, which is the density of the current flowing through the medium used when calculating the transfer voltage Vtr, is the transfer reference current density Jb1, the transfer voltage Vc1 applied to the outside of the medium in the inner medium region, and the transfer current density Jout flowing through the outer medium region when the voltage between the transfer shafts is Vp are obtained from the electrical characteristics of the transfer roller 120 determined in step S14 of FIG. 4.

[0105] In order to accurately obtain the transfer voltage Vc and the transfer current density Jout, the measurement range of the electrical characteristics of the transfer roller 120 determined in step S14 of FIG. 4 must include at least the transfer voltage Vc1 and the transfer current density Jout.

[0106] From the above, in Embodiment 1, it is necessary to create the transfer current measurement voltage data table 170 shown in FIG. 5 so that Vc1>Vs11 and Jout<Jm12. The same applies when the medium type is plain paper.

[0107] FIG. 17 is a graph showing the dependence of the transfer current density on the voltage between the transfer shafts when the medium type is OHP. Similar to the case where the medium type is high-quality paper, when calculating the transfer voltage Vtr, the transfer reference current density, which is the density of the transfer current flowing through the medium, is the transfer reference current density Jb3. The transfer voltage Vc3 applied to the outside of the medium in the medium region and the transfer current density Jout flowing through the outside region of the medium when the voltage between the transfer shafts is Vp are obtained from the electrical characteristics of the transfer roller 120 obtained in step S14 of FIG. 4.

[0108] When the medium type is OHP, compared with the case where the medium type is high-quality paper, the transfer reference voltage Vb3 is high, so the voltage Vp between the transfer shafts also tends to be high. That is, in terms of the voltage between the transfer shafts and the transfer current density, the electrical characteristics of the transfer roller 120 in a wider range are required.

[0109] In order to accurately obtain the transfer voltage Vc and the transfer current density Jout, the measurement range of the electrical characteristics of the transfer roller 120 obtained in step S14 of FIG. 4 must include at least the transfer voltage Vc3 and the transfer current density Jout. However, generally, the electrical characteristics of the transfer roller 120 are often non-linear rather than linear.

[0110] Therefore, unlike the voltage data table 170 for transfer current measurement shown in FIG. 5, when two transfer current measurement voltages Vm for OHP are used, the difference between the linear approximation formula obtained from the electrical characteristics of the transfer roller 120 obtained in step S14 of FIG. 4 and the actual electrical characteristics of the transfer roller 120 becomes large.

[0111] For this reason, in Embodiment 1, when the medium type is OHP, by creating the voltage data table 170 for transfer current measurement with three transfer current measurement voltages Vm so that Vc3>Vs31 and Jm32<Jout<Jm33, the transfer voltage Vc and the transfer current density Jout can be obtained more accurately than with two transfer current measurement voltages Vm.

[0112] In the first embodiment, the transfer current measurement voltage data table 170 is created with at least two or more transfer current measurement voltages Vm depending on the medium type, but the first embodiment is not limited to this example. The number of transfer current measurement voltages Vm may be changed as needed.

[0113] As described above, the calculation unit 154 calculates the transfer voltage Vtr using actual values ​​according to the medium type and medium width.

[0114] FIG. 18 is a graph showing the calculation results of the transfer current Itr for thin plain paper Pa1 (thickness 70 μm) and very thick plain paper Pa2 (thickness 220 μm). Here, the hardness of the transfer roller 120 was set to 38±4°, the length L of the transfer roller 120 in the direction perpendicular to the conveying direction was set to 320 mm, and the resistance value R of the fixed resistor 148 was set to 20 MΩ.

[0115] As shown in the above equation (31), the transfer voltage Vtr is calculated as the sum of the voltage Vs between the transfer shafts and the applied voltage VR [V] applied to the fixed resistor 148. Since the voltage Vs between the transfer shafts is a fixed value for each type of medium, if the medium type is the same, the transfer voltage Vtr is determined by the applied voltage VR [V] applied to the fixed resistor 148. The applied voltage VR is calculated by the formula (30), and since the resistance value R of the fixed resistor 148 is fixed, the applied voltage VR is determined by the transfer current Itr flowing through the transfer roller 120. From the above, the transition of the transfer voltage Vtr is similar to the transition of the transfer current Itr shown in FIG.

[0116] Therefore, similar to the progression of the very thick paper Pa2 shown in Figure 18, when the thickness of the medium is equal to or greater than a predetermined thickness, if the width of the medium is smaller than the predetermined width, the calculation unit 154 decreases the transfer voltage as the width of the medium increases, and when the width of the medium is equal to or greater than the predetermined width, the calculation unit 154 increases the transfer voltage as the width of the medium increases.

[0117] As described above, according to the first embodiment, even when a gap occurs outside the medium due to the medium thickness, the amount of current outside the medium can be optimized by determining the gap width according to the thickness. Therefore, the first embodiment can determine the magnitude of the transfer voltage more appropriately than conventional control. As a result, good print results can always be obtained. Moreover, in the first embodiment, unnecessary voltage is not applied, which also contributes to energy conservation.

[0118] Embodiment 2 In the first embodiment, the pressure applied between the transfer roller 120 and the photosensitive drum 102 is described as being constant, but in the second embodiment, the case where the pressure is variable will be described. When the pressure between the transfer roller 120 and the photosensitive drum 102 is variable, it becomes possible to deal with various printing defects that occur with any medium.

[0119] As shown in FIG. 1, the schematic configuration of the image forming apparatus 200 according to the second embodiment is similar to the schematic configuration of the image forming apparatus 100 according to the first embodiment. However, as shown in FIG. 19, the image forming apparatus 200 according to the second embodiment includes a pressure changing unit 270, a display unit 271, and an input unit 272.

[0120] FIG. 19 is a block diagram schematically showing the configuration of a control system in an image forming apparatus 200 according to the second embodiment. The control system in image forming apparatus 200 according to embodiment 2 is also substantially the same as the control system in image forming apparatus 100 according to embodiment 1. However, the control system in embodiment 2 differs from the control system in embodiment 1 in that mechanism control unit 250 in transfer control unit 260 is provided with pressure control unit 273 and in the processing in calculation unit 254.

[0121] The pressure changing unit 270 changes the pressure between the transfer roller 120 and the photosensitive drum 102 by changing the position of the transfer roller 120. For example, to increase the pressure, the transfer roller 120 is moved toward the photosensitive drum 102, and to decrease the pressure, the transfer roller 120 is moved away from the photosensitive drum 102.

[0122] The display unit 271 displays various screen images. The input unit 272 accepts input of instructions from the user. For example, the input unit 272 accepts input of a mode related to the pressure of the transfer roller 120. Specifically, the input unit 272 accepts input of a selection from a normal pressure mode in which the pressure is normal, a pressure-up mode in which the pressure is higher than in the normal pressure mode, and a pressure-down mode in which the pressure is lower than in the normal pressure mode. The pressure in the normal pressure mode is the same as the pressure in the image forming apparatus 100 in the first embodiment. In other words, the pressure in the normal pressure mode is the pressure used when the pressure between the transfer roller 120 and the photosensitive drum 102 is not variable.

[0123] The pressure control unit 273 controls the pressure between the transfer roller 120 and the photosensitive drum 102 by controlling the pressure changing unit 270 . Here, the pressure control unit 273 switches the pressure between the transfer roller 120 and the photosensitive drum 102 between normal pressure, pressure increase, and pressure decrease according to the mode selected by the user.

[0124] Here, the situation in which the pressure-up mode or the pressure-down mode is selected will be described with reference to FIG. FIG. 20A is a schematic diagram showing a state in which a gap occurs in the transfer nip portion between the photosensitive drum 102 and the transfer roller 120 in the pressure-up mode. The pressure-up mode is selected, for example, when the user seeks stability in density. In this case, the pressure change unit 270 presses the transfer roller 120 more firmly. As a result, as shown in FIG. 20(A), the gap width becomes smaller than in the case of normal pressure shown in FIG.

[0125] The gap width becomes smaller because the photosensitive drum 102 and the transfer roller 120, which are separated by the thickness d of the medium Pa, are pressed against each other more strongly than under normal pressure, making them more likely to come into contact outside the medium area.

[0126] FIG. 20B is a schematic diagram showing a state in which a gap occurs in the transfer nip portion between the photosensitive drum 102 and the transfer roller 120 in the pressure down mode. The pressure-down mode is selected to prevent the occurrence of wrinkles in the medium. In this case, the pressure change unit 270 weakens the pressure of the transfer roller 120. As a result, as shown in FIG. 20(B), the gap width becomes larger than in the case of normal pressure shown in FIG.

[0127] In contrast to the pressure-up mode, the gap width becomes larger in the pressure-down mode because the photosensitive drum 102 and the transfer roller 120, which are separated by the thickness d of the medium Pa, are pressed against each other more weakly than under normal pressure, making it difficult for them to come into contact outside the medium area.

[0128] Furthermore, even for the same medium width, the gap width changes depending on the transfer pressure, which is the pressure between the photosensitive drum 102 and the transfer roller 120. For example, even for the same medium width of 70 mm, the gap widths that occur in the normal pressure mode, the pressure-up mode, and the pressure-down mode are different.

[0129] As described above, the lower the pressure between the photosensitive drum 102 and the transfer roller 120, the longer the gap width becomes, and the larger the size of the gap itself becomes.

[0130] FIG. 21 is a graph showing the gap width in combination with the media width and pressure mode. FIG. 22 is a table showing the gap width values ​​for combinations of medium widths and pressure modes. 21 and 22 show the results of experimental measurements of the gap width by the inventors in each pressure mode. The hardness of the transfer roller 120 here is 38±4 degrees.

[0131] In reality, it is expected that the image forming device 200 will print sizes other than the media widths shown in Figures 21 and 22, so the memory unit 155 stores a gap width table 173 such as that shown in Figure 12 for each pressure mode.

[0132] Since the value of the K(d) term in equation (26) described in embodiment 1 fluctuates, the calculation unit 254 also changes the magnitude of the applied voltage accordingly. Therefore, the calculation unit 254 in embodiment 2 calculates the magnitude of the transfer voltage to be applied during transfer according to the pressure of the transfer roller 120, the medium type, the medium width, and the medium thickness set by the user before the printing operation. In embodiment 2 as well, the larger the gap, the smaller the transfer voltage.

[0133] Specifically, once a pressure mode is selected and the medium thickness d and medium width W are determined, the calculation unit 254 can read the slope p and intercept q from the gap width table corresponding to the pressure mode. This allows the calculation unit 254 to calculate the gap width K(d) using equation (21) described in the first embodiment.

[0134] FIG. 23 is a graph showing the optimum control current when the transfer pressure is variable. As shown in Figure 23, in the pressure-up mode, the gap is smaller, so the transfer current is smaller than in the normal pressure mode for the same media width. On the other hand, in the pressure-down mode, the gap is larger, so the transfer current is larger than in the normal pressure mode for the same media width.

[0135] The display unit 271 described above can be realized by a display, and the input unit 272 can be realized by an input device such as an input key. The display unit 271 and the input unit 272 may be realized by a touch panel.

[0136] FIG. 24 is a flowchart showing a process for calculating the transfer voltage when the image forming apparatus 200 according to the second embodiment performs printing. In FIG. 24, steps that perform the same processing as steps included in the flowchart shown in FIG. 4 are given the same reference numerals as in FIG.

[0137] First, in step S20, the input unit 272 accepts a selection of a pressure mode from the user. For example, a selection screen image for selecting a pressure mode is displayed on the display unit 271, and the user can select a pressure mode via the selection screen image. The selected pressure mode is notified to the calculation unit 254 and the pressure control unit 273.

[0138] The processing of steps S10 to S18 in Fig. 24 is the same as the processing of steps S10 to S18 in Fig. 4. However, if the medium thickness is equal to or greater than a predetermined threshold in step S17 (Yes in S17), the processing proceeds to step S29.

[0139] In step S29, the calculation unit 254 calculates the transfer voltage Vtr based on the pressure mode selected in step S20, the electrical characteristics of the transfer roller 120 calculated in step S14, the medium width and medium thickness identified in step S15, and the transfer reference current density Jb and transfer reference voltage Vb read in step S16.

[0140] As described above, according to the second embodiment, even when there are multiple pressure modes, the amount of current outside the medium can be optimized by determining the gap width according to the medium thickness and pressure mode. Therefore, the second embodiment can determine the magnitude of the transfer voltage more appropriately than conventional control. As a result, good print results can always be obtained.

[0141] In the above-described first and second embodiments, image forming apparatus 100 has been described using a monochrome printing apparatus as an example, but the first and second embodiments are not limited to monochrome printing apparatuses. For example, image forming apparatus 100 according to the first embodiment may be a color printing apparatus. In this case, the same processing as described above may be performed in the image drum units of each color.

[0142] Although the first and second embodiments described above show examples in which a toner image is transferred to a medium by a direct transfer method, the first and second embodiments are not limited to such examples. For example, the image forming apparatus 100 may include an intermediate transfer belt as an image carrier, and the toner image that has been primarily transferred from the photosensitive drum 102 to the intermediate transfer belt may be secondarily transferred to the medium. In this case, the transfer voltage may be calculated as described above when the secondary transfer is performed. [Explanation of symbols]

[0143] 100,200 Image forming apparatus, 101 Printing mechanism, 102 Photosensitive drum, 103 Charging roller, 104 Developing roller, 105 Developing blade, 106 Supply roller, 107 Discharge light irradiation unit, 108 Toner cartridge, 109 LED head, 110 Paper feed mechanism, 111 Paper storage cassette, 112 Hopping roller, 113,114 Conveying roller group, 115,116,117 Sensor, 120 Transfer roller, 130 Fixing mechanism, 131 Heat roller, 132 Pressure roller, 133 Heater, 134 Thermistor, 140 Host I / F unit, 141 Command image processing unit, 142 LED head I / F unit, 143 High voltage control unit, 144 Charging voltage application unit, 145 Development voltage application unit, 146 supply voltage application unit, 147 transfer voltage application unit, 148 fixed resistor, 149 transfer current measurement unit, 150, 250 mechanism control unit, 151 media type identification unit, 152 media width identification unit, 153 media thickness identification unit, 154, 254 calculation unit, 155 memory unit, 156 hopping motor, 157 transport group motor, 158 heater motor, 159 drum motor, 160, 260 transfer control unit, 270 pressure change unit, 271 display unit, 272 input unit, 273 pressure control unit.

Claims

1. an image carrier that carries a developer image, which is an image formed by the developer; a transfer voltage applying section that applies a transfer voltage for transferring the developer image onto a medium; a transfer section that transfers the developer image carried on the image carrier to the medium upon application of the transfer voltage; a transfer control unit that controls the transfer voltage applied by the transfer voltage application unit, the transfer control unit calculates the transfer voltage in accordance with a first current, which is a current in an area within the medium where the medium passes between the image carrier and the transfer unit when the developer image is transferred, and a second current, which is a current in an area outside the medium, which is an area obtained by excluding a gap generated when the medium passes between the image carrier and the transfer unit when the developer image is transferred, and the area within the medium from the area between the image carrier and the transfer unit. An image forming apparatus comprising:

2. The size of the voids increases as the thickness of the medium increases, The larger the size of the gap, the smaller the transfer voltage.

2. The image forming apparatus according to claim 1, wherein:

3. a pressure changing unit that changes the pressure between the image carrier and the transfer unit; The size of the void increases as the pressure decreases, The larger the size of the gap, the smaller the transfer voltage.

2. The image forming apparatus according to claim 1, wherein:

4. The transfer control unit calculates the transfer voltage by adding a second voltage calculated by Ohm's law from a value obtained by adding the first current, which increases as the width of the medium increases, and the second current, which decreases as the size of the gap increases, to a first voltage determined depending on the type of the medium.

4. The image forming apparatus according to claim 1, wherein:

Images