Image forming apparatus

Abstract

To provide an image forming apparatus capable of making the density of a toner image appropriate regardless of temperature and humidity environment.SOLUTION: The controller 100 of the image forming apparatus 1 executes a first process, a second process, and a third process in the developing bias setting process. In the first process, a patch image is formed on the transfer belt 73, and a first developing bias Vb1, which is a developing bias such that the concentration of the formed patch image becomes a predetermined target concentration, is set. In the second process, a second developing bias Vb2, which is a lower limit value of the developing bias, is set. In the third process, the developing bias Vb is set to the first developing bias Vb1 when the first developing bias Vb2 is equal to or greater than the second developing bias Vb1, and the developing bias Vb is set to the second developing bias Vb1 when the first developing bias Vb2 is less than the second developing bias Vb2. In the second process, the controller 100 sets the second developing bias Vb2 according to the temperature-humidity detected by the temperature-humidity sensor 14.SELECTED DRAWING: Figure 1

Description

[Technical Field]

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

[0002] Conventionally, there is known an image forming apparatus that forms a patch image on a belt, measures the density of the formed patch image with a sensor, and sets a development bias based on the measured density of the patch image (for example, Patent Document 1). [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Japanese Patent Application Publication No. 2023-028869 Summary of the Invention [Problem to be solved by the invention]

[0004] However, even if an appropriate developing bias is set based on the density of a patch image, the density of the toner image when printed may not be appropriate depending on the temperature and humidity environment. Therefore, it is desirable to be able to make the density of a toner image appropriate regardless of the temperature and humidity environment. [Means for solving the problem]

[0005] In a first disclosure of the present application, an image forming apparatus includes an image forming unit, a density sensor, a temperature and humidity sensor, and a control unit. The image forming unit has a developing roller and is capable of forming a toner patch image on a transfer target. The density sensor is capable of measuring the density of a patch image formed on a transfer target. The temperature and humidity sensor detects the temperature and humidity. The control unit can execute a developing bias setting process for setting a developing bias to be applied to the developing roller during printing. The control unit executes a first process, a second process, and a third process in the development bias setting process. The first process is a process in which a patch image is formed on a transfer object, the density of the formed patch image is measured using a density sensor, and a first development bias is set based on the measured density of the patch image so that the density of the patch image becomes a predetermined target density. The second process is a process for setting a second developing bias, which is the lower limit of the magnitude of the developing bias to be applied to the developing roller. The third process is a process in which, if the magnitude of the first developing bias is equal to or greater than the magnitude of the second developing bias, the developing bias applied to the developing roller during printing is set to the first developing bias, and, if the magnitude of the first developing bias is less than the magnitude of the second developing bias, the developing bias applied to the developing roller during printing is set to the second developing bias. In the second process, the control section sets the second developing bias in accordance with the temperature and humidity detected by the temperature and humidity sensor.

[0006] In the second process of the developing bias setting process, the control unit sets the second developing bias according to the temperature and humidity detected by the temperature and humidity sensor, so that the developing bias can be set to an appropriate value according to the temperature and humidity, thereby making it possible to optimize the density of the toner image regardless of the temperature and humidity environment.

[0007] In the second disclosure of the present application, in the image forming apparatus of the first disclosure, the magnitude of the second developing bias in the first environment is greater than the magnitude of the second developing bias in the second environment. The first environment is an environment in which the temperature and humidity detected by the temperature and humidity sensor are within a preset temperature and humidity range. The second environment is an environment in which at least one of the temperature and humidity detected by the temperature and humidity sensor is lower than the temperature and humidity range.

[0008] By making the magnitude of the second developing bias in the first environment greater than the magnitude of the second developing bias in the second environment, it is possible to ensure the density of the toner image when solid printing is performed using toner with low charging performance and glossiness.

[0009] In a third disclosure of the present application, in the image forming apparatus of the second disclosure, the developing rollers include a first developing roller that carries black toner and at least one second developing roller that carries a toner other than black. When setting the developing bias to be applied to the first developing roller during printing, the control unit sets the second developing bias in the second process according to the temperature and humidity detected by the temperature and humidity sensor. When setting the developing bias to be applied to the second developing roller during printing, the control unit sets the second developing bias in the second process, which does not change depending on the temperature and humidity detected by the temperature and humidity sensor.

[0010] In the second process for setting the developing bias to be applied to the first developing roller, the second developing bias is set according to the temperature and humidity detected by the temperature and humidity sensor, thereby ensuring the density of the black toner image when toner with low charging performance and glossiness is used.In the second process for setting the developing bias to be applied to the second developing roller, the second developing bias is set so as not to change depending on the temperature and humidity detected by the temperature and humidity sensor, thereby ensuring the density of the toner image other than black when toner with high charging performance and low glossiness is used.

[0011] In the fourth disclosure of the present application, in the image forming apparatus of the second or third disclosure, the control unit sets the second developing bias in the second process by referring to a table showing the relationship between the temperature and humidity and the second developing bias.

[0012] In a fifth disclosure of the present application, in the image forming apparatus of the first disclosure, the control unit executes the following process in the first process. A process of forming a patch image on a transfer target by applying a first reference developing bias to a developing roller, and forming a patch image on a transfer target by applying a second reference developing bias different from the first reference developing bias to the developing roller. A process in which the density of a patch image formed on a transfer target is measured by a density sensor. The first developing bias is set based on the density of a patch image formed by applying a first reference developing bias to the developing roller and the density of a patch image formed by applying a second reference developing bias to the developing roller.

[0013] In a sixth disclosure of the present application, in the image forming apparatus of the first disclosure, the image forming apparatus includes an endless belt as a transfer object. [Effects of the Invention]

[0014] The density of the toner image can be made appropriate regardless of the temperature and humidity environment. [Brief explanation of the drawings]

[0015] [Figure 1] FIG. 1 is a cross-sectional view showing an image forming apparatus. [Figure 2] FIG. 2 is a perspective view showing a transfer unit and a density sensor. [Figure 3] FIG. 10 is a diagram illustrating a first process for setting a first developing bias. [Figure 4] FIG. 10 is a diagram showing an example of a table for setting a second developing bias. [Figure 5] 10 is a flowchart illustrating an example of the operation of a control unit. DETAILED DESCRIPTION OF THE INVENTION

[0016] Next, an embodiment will be described. As shown in FIG. 1, the image forming device 1 is a color printer and includes a device main body 10, a cover 11, a sheet supply unit 20, an image forming unit 30, a sheet discharge unit 90, a concentration sensor 13, a temperature and humidity sensor 14, and a control unit 100.

[0017] The device body 10 has an opening 10A. The cover 11 opens and closes the opening 10A. The cover 11 is movable between a closed position where the opening 10A is closed and an open position where the opening 10A is open. Specifically, the cover 11 is rotatable between the closed position and the open position.

[0018] The sheet supply unit 20 includes a sheet tray 21 and a sheet supply mechanism 22. The sheet tray 21 stores sheets S. The sheet supply mechanism 22 supplies the sheets S in the sheet tray 21 to the image forming unit 30.

[0019] The image forming section 30 includes four exposure units 40, four process cartridges 50, a transfer unit 70, and a fixing unit 80.

[0020] The exposure unit 40 has a plurality of LEDs. The exposure unit 40 exposes the photosensitive drum 53 to light emitted from the LEDs. The exposure unit 40 is held by the cover 11.

[0021] The process cartridges 50 include a process cartridge 50Y corresponding to yellow, a process cartridge 50M corresponding to magenta, a process cartridge 50C corresponding to cyan, and a process cartridge 50K corresponding to black.

[0022] The process cartridges 50 are arranged in the order of process cartridges 50Y, 50M, 50C, and 50K from upstream to downstream in the conveying direction of the sheet S. In this specification and drawings, when distinguishing between the colors yellow, magenta, cyan, and black, Y, M, C, and K are added to the reference symbols.

[0023] Each process cartridge 50 includes a drum cartridge 51 and a developing cartridge 52. The developing cartridge 52 is detachably mounted to the drum cartridge 51.

[0024] The drum cartridges 51 include drum cartridge 51Y, drum cartridge 51M, drum cartridge 51C, and drum cartridge 51K. Each drum cartridge 51 has a photosensitive drum 53 and a charger 54. The photosensitive drums 53 include photosensitive drum 53Y on which a yellow toner image is formed, photosensitive drum 53M on which a magenta toner image is formed, photosensitive drum 53C on which a cyan toner image is formed, and photosensitive drum 53K on which a black toner image is formed.

[0025] The developing cartridges 52 include a developing cartridge 52Y that contains yellow toner, a developing cartridge 52M that contains magenta toner, a developing cartridge 52C that contains cyan toner, and a developing cartridge 52K that contains black toner. Each developing cartridge 52 has a developing roller 55 and a toner storage section 56 that stores toner.

[0026] The developing rollers 55 include a developing roller 55Y that carries yellow toner, a developing roller 55M that carries magenta toner, a developing roller 55C that carries cyan toner, and a developing roller 55K that carries black toner. The developing roller 55K corresponds to the "first developing roller." The developing rollers 55Y, 55M, and 55C correspond to "second developing rollers" that carry toners other than black.

[0027] The process cartridge 50 is detachably attached to the apparatus main body 10. Specifically, the process cartridge 50 is detachably attached to the apparatus main body 10 through the opening 10A with the cover 11 open. The developing cartridge 52 is detachably attached to the apparatus main body 10 with the developing cartridge 52 attached to the drum cartridge 51.

[0028] The transfer unit 70 includes a drive roller 71, a driven roller 72, a transfer belt 73 as a transfer object, and four transfer rollers 74. The drive roller 71 is a roller that rotates the transfer belt 73. The transfer belt 73 is stretched between the drive roller 71 and the driven roller 72. The transfer belt 73 is an endless belt that transports the sheet S between the transfer belt 73 and the photosensitive drum 53. The outer peripheral surface of the transfer belt 73 is in contact with the photosensitive drum 53. The transfer roller 74 sandwiches the transfer belt 73 between itself and the photosensitive drum 53.

[0029] The fixing unit 80 includes a heating roller 81 and a pressure roller 82. The heating roller 81 includes a heater 81A therein. The pressure roller 82 sandwiches the sheet S between itself and the heating roller 81.

[0030] When printing is performed, the charger 54 charges the surface of the rotating photosensitive drum 53. The exposure unit 40 exposes the surface of the charged photosensitive drum 53 to light. As a result, an electrostatic latent image is formed on the photosensitive drum 53. The developing roller 55 supplies toner to the photosensitive drum 53. As a result, a toner image is formed on the photosensitive drum 53.

[0031] The photosensitive drum 53 and the transfer roller 74 transport the sheet S supplied from the sheet supply unit 20. At this time, the transfer roller 74 transfers the toner image formed on the photosensitive drum 53 onto the sheet S. The heating roller 81 and the pressure roller 82 fix the toner image onto the sheet S while transporting the sheet S onto which the toner image has been transferred. In this way, an image is formed on the sheet S.

[0032] The sheet discharge section 90 conveys the sheet S on which the image has been formed toward the outside of the apparatus body 10. The sheet discharge section 90 includes a conveying roller 91 and a discharge roller 92. The conveying roller 91 conveys the sheet S toward the discharge roller 92. The discharge roller 92 discharges the sheet S onto the discharge tray 12.

[0033] As shown in FIGS. 1 and 2, the density sensor 13 can measure the density of a toner patch image P formed on the transfer belt 73. The patch image P is a test toner image for correcting the density of the toner image. The density sensor 13 faces the surface of the transfer belt 73. As an example, the density sensor 13 is an optical sensor having a light-emitting element and a light-receiving element. The light-emitting element emits light toward the transfer belt 73. The light-receiving element receives reflected light of the light emitted from the light-emitting element.

[0034] The image forming unit 30 is capable of forming a patch image P on the transfer belt 73. When forming the patch image P, the image forming unit 30 transfers a toner image formed on the photosensitive drum 53 to the transfer belt 73 by the transfer roller 74. The image forming unit 30 is capable of forming patch images PY, PM, PC, and PK corresponding to the respective colors of yellow, magenta, cyan, and black on the transfer belt 73. The patch images P formed on the transfer belt 73 are collected by the belt cleaner 75 after the density is measured by the density sensor 13.

[0035] The temperature and humidity sensor 14 detects temperature and humidity. In this embodiment, the temperature and humidity sensor 14 detects the temperature and humidity inside the device main body 10. The temperature and humidity sensor 14 may also detect the temperature and humidity outside the device main body 10. The temperature and humidity sensor 14 may also be configured to detect both temperature and humidity, or may be configured to include a temperature sensor that detects temperature and a humidity sensor that detects humidity separately.

[0036] The control unit 100 includes a CPU, RAM, ROM, input / output circuits, etc. The control unit 100 executes control by performing arithmetic processing based on programs and data stored in the ROM, etc. The control unit 100 is capable of executing a development bias setting process.

[0037] The developing bias setting process is a process for setting the developing bias Vb to be applied to the developing roller 55 during printing. The control unit 100 can execute the developing bias setting process for each of the developing rollers 55Y, 55M, 55C, and 55K, and sets the developing biases VbY, VbM, VbC, and VbK.

[0038] As an example, the control unit 100 executes the developing bias setting process every time printing is performed on a predetermined number of sheets S (for example, 1,000 sheets). The control unit 100 also executes the developing bias setting process when the drum cartridge 51 is replaced. The control unit 100 also executes the developing bias setting process when the user performs an operation to instruct the setting of the developing bias Vb. The control unit 100 also executes the developing bias setting process when the temperature has changed by more than a predetermined threshold value after executing the developing bias setting process.

[0039] In the developing bias setting process, the control unit 100 executes a first process, a second process, and a third process. In this embodiment, the toner is a positively charged dry toner, and the developing bias Vb is a positive bias.

[0040] 3, in the first process, the control unit 100 sets a first developing bias Vb1. The first developing bias Vb1 is a developing bias that causes the density of the patch image P formed on the transfer belt 73 to become a predetermined target density Dr. In the first process, the control unit 100 executes a patch image forming process, a density measurement process, and a first developing bias setting process.

[0041] In the patch image forming process, the control unit 100 forms a patch image P on the transfer belt 73. More specifically, the control unit 100 applies a first reference developing bias V1 to the developing roller 55 to form the patch image P on the transfer belt 73. The control unit 100 also applies a second reference developing bias V2 to the developing roller 55 to form the patch image P on the transfer belt 73.

[0042] The second reference developing bias V2 is a developing bias different from the first reference developing bias V1. As an example, the first reference developing bias V1 is a value obtained by subtracting a predetermined value ΔV from the reference developing bias V0. The second reference developing bias V2 is a value obtained by adding a predetermined value ΔV to the reference developing bias V0. The control unit 100 stores in advance the reference developing bias V0 and the predetermined value ΔV for each of the colors yellow, magenta, cyan, and black.

[0043] In the density measurement process, the control unit 100 measures the density of the patch image P formed on the transfer belt 73 using the density sensor 13. In this embodiment, the control unit 100 acquires a first density Dm1 and a second density Dm2. The first density Dm1 is the density of the patch image P formed by applying a first reference developing bias V1 to the developing roller 55. The second density Dm2 is the density of the patch image P formed by applying a second reference developing bias V2 to the developing roller 55.

[0044] In the first developing bias setting process, control unit 100 sets first developing bias Vb1 based on the measured densities Dm1 and Dm2 of patch image P. In this embodiment, control unit 100 sets first developing bias Vb1 based on the first density Dm1 and the second density Dm2. Specifically, control unit 100 calculates a straight line L from the first reference developing bias V1 and the first density Dm1, and the second reference developing bias V2 and the second density Dm2. Then, control unit 100 sets the first developing bias Vb1 to a developing bias that makes the density of patch image P equal to the target density Dr based on the calculated straight line L.

[0045] In the first process, the first reference developing bias V1 may be applied to the developing roller 55 to form a patch image P, the density Dm1 of this patch image P may be measured by the density sensor 13, and then the second reference developing bias V2 may be applied to the developing roller 55 to form the patch image P. Alternatively, the first reference developing bias V1 may be applied to the developing roller 55 to form a patch image P, and then the second reference developing bias V2 may be applied to the developing roller 55 to form the patch image P, and then the densities Dm1 and Dm2 of the patch image P may be measured by the density sensor 13. Alternatively, the second reference developing bias V2 may be applied to the developing roller 55 first to form the patch image P, and then the first reference developing bias V1 may be applied to the developing roller 55 to form the patch image P.

[0046] In the second process, the control unit 100 sets the second developing bias Vb2, which is the lower limit of the magnitude of the developing bias applied to the developing roller 55.

[0047] In this embodiment, when setting the developing bias VbK to be applied to the black developing roller 55K during printing, the control unit 100 sets the second developing bias Vb2 in the second process in accordance with the temperature and humidity detected by the temperature and humidity sensor 14. Specifically, when setting the developing bias VbK, the control unit 100 sets the second developing bias Vb2 in the second process by referring to a table such as that shown in FIG.

[0048] The table shows the relationship between the temperature and humidity and the second developing bias Vb2. The control unit 100 sets the second developing bias Vb2 based on the table shown in FIG. 4 and the temperature and humidity detected by the temperature and humidity sensor 14.

[0049] The table has a preset temperature and humidity region R, which is indicated by dotted hatching in Fig. 4. In the embodiment, an environment in which the temperature and humidity detected by the temperature and humidity sensor 14 are within the temperature and humidity region R is referred to as a high-temperature, high-humidity environment, which is a first environment. An environment in which at least one of the temperature and humidity detected by the temperature and humidity sensor 14 is lower than the temperature and humidity region R is referred to as a normal-temperature, low-humidity environment, which is a second environment.

[0050] The magnitude of the second developing bias Vb2 in a high-temperature, high-humidity environment is greater than the magnitude of the second developing bias Vb2 in a normal-temperature, low-humidity environment. For example, the second developing bias Vb2 in a high-temperature, high-humidity environment is 260 V, and the second developing bias Vb2 in a normal-temperature, low-humidity environment is 200 V.

[0051] In the table of FIG. 4, a temperature of "up to 15°C" indicates that the temperature is 15°C or less. A temperature of "up to 18°C" indicates that the temperature is higher than 15°C and lower than 18°C. A temperature of "over 42°C" indicates that the temperature is higher than 42°C. Similarly, a humidity of "up to 9%" indicates that the humidity is 9% or less. A humidity of "up to 14%" indicates that the humidity is higher than 9% and lower than 14%. A humidity of "over 89%" indicates that the humidity is higher than 89%.

[0052] For example, when the temperature detected by the temperature and humidity sensor 14 is 25° C. and the humidity detected by the temperature and humidity sensor 14 is 50%, the control unit 100 refers to the table and sets the second developing bias Vb2 to 200 V. When the temperature detected by the temperature and humidity sensor 14 is 35° C. and the humidity detected by the temperature and humidity sensor 14 is 70%, the control unit 100 refers to the table and sets the second developing bias Vb2 to 260 V.

[0053] When setting the developing biases VbY, VbM, and VbC to be applied to developing rollers 55Y, 55M, and 55C other than black during printing, the control unit 100 sets, in the second process, a second developing bias Vb2 that does not change depending on the temperature and humidity detected by the temperature and humidity sensor 14. In this embodiment, when setting the developing biases VbY, VbM, and VbC, the control unit 100 sets, in the second process, the second developing bias Vb2 to a predetermined fixed value that does not change depending on the temperature and humidity detected by the temperature and humidity sensor 14. As an example, the fixed value is 200V.

[0054] In the developing bias setting process, the second process may be executed after the first process, or the first process may be executed after the second process. Also, the first process and the second process may be executed in parallel.

[0055] In the third process, the control unit 100 sets the developing bias Vb to be applied to the developing roller 55 during printing. Specifically, if the magnitude of the first developing bias Vb1 set in the first process is equal to or greater than the magnitude of the second developing bias Vb2 set in the second process, the control unit 100 sets the developing bias Vb to be applied to the developing roller 55 during printing to the first developing bias Vb1. Furthermore, if the magnitude of the first developing bias Vb1 set in the first process is less than the magnitude of the second developing bias Vb2 set in the second process, the control unit 100 sets the developing bias Vb to be applied to the developing roller 55 during printing to the second developing bias Vb2 (lower limit value).

[0056] Next, an example of the operation of the control unit 100 will be described with reference to a flowchart. 5, in the developing bias setting process, the control unit 100 applies a first reference developing bias V1 to the developing roller 55 to form a patch image P on the transfer belt 73 (S11). Then, the control unit 100 measures the density of the patch image P formed on the transfer belt 73 with the density sensor 13 to obtain a first density Dm1 (S12).

[0057] Furthermore, the control unit 100 applies a second reference developing bias V2 to the developing roller 55 to form a patch image P on the transfer belt 73 (S13). Then, the control unit 100 measures the density of the patch image P formed on the transfer belt 73 with the density sensor 13 to obtain a second density Dm2 (S14).

[0058] Then, the control unit 100 calculates a straight line L (see Figure 3) based on the first reference developing bias V1, the first density Dm1, the second reference developing bias V2, and the second density Dm2, and sets a first developing bias Vb1 from the calculated straight line L so that the density of the patch image P becomes the target density Dr (S15).

[0059] Further, the control section 100 determines whether or not it is time to set the developing bias Vb (VbK) to be applied to the black developing roller 55K (S21).

[0060] When setting the developing bias VbK (S21, Yes), the control unit 100 detects the temperature and humidity using the temperature and humidity sensor 14 (S22). Then, the control unit 100 sets the second developing bias Vb2 (200V or 260V) by referring to the detected temperature and humidity and a table (see FIG. 4) (S23).

[0061] In step S21, when setting the development bias Vb (VbY, VbM, VbC) to be applied to the development rollers 55Y, 55M, 55C other than black (No), the control unit 100 sets the second development bias Vb2 to a fixed value (200V) (S24).

[0062] After setting the first developing bias Vb1 and the second developing bias Vb2, the control section 100 determines whether the first developing bias Vb1 is equal to or greater than the second developing bias Vb2 (S31).

[0063] If the first developing bias Vb1 is equal to or greater than the second developing bias Vb2 (S31, Yes), the control unit 100 sets the first developing bias Vb1 to the developing bias Vb to be applied to the developing roller 55 during printing (S32), and ends the developing bias setting process. If the first developing bias Vb1 is less than the second developing bias Vb2 (S31, No), the control unit 100 sets the second developing bias Vb2 to the developing bias Vb to be applied to the developing roller 55 during printing (S33), and ends the developing bias setting process.

[0064] Next, the effects of the embodiment will be described. In the second process of the developing bias setting process, the control unit 100 sets the second developing bias Vb2 according to the temperature and humidity detected by the temperature and humidity sensor 14, so that the developing bias Vb can be set to an appropriate value according to the temperature and humidity. This makes it possible to make the density of the toner image appropriate regardless of the temperature and humidity environment.

[0065] The magnitude of the second developing bias Vb2 (260 V) in a high temperature and high humidity environment is greater than the magnitude of the second developing bias Vb2 (200 V) in a normal temperature and low humidity environment, thereby ensuring the density of the toner image when solid printing is performed using toner with low charging performance and glossiness.

[0066] Additionally, if a toner with low chargeability and glossiness is used, the density of the patch image P will be higher than the target density Dr in a high-temperature, high-humidity environment. Therefore, if the developing bias Vb is set to a bias that achieves the target density Dr, the magnitude of the developing bias Vb may be smaller than necessary. This may result in a failure to ensure the density of the toner image when solid printing is performed using a toner with low chargeability and glossiness. Therefore, by setting the magnitude (lower limit) of the second developing bias Vb2 in a high-temperature, high-humidity environment to be larger than in a normal-temperature, low-humidity environment, the developing bias Vb can be set to an appropriate magnitude, thereby ensuring the density of the toner image when solid printing is performed using a toner with low chargeability and glossiness.

[0067] In the second process for setting the black developing bias VbK, the density of the black toner image can be ensured when toner with low charging performance and low glossiness is used by setting the second developing bias Vb2 according to the temperature and humidity detected by the temperature and humidity sensor 14. In the second process for setting the non-black developing biases VbY, VbM, and VbC, the second developing bias Vb2 is set to a fixed value (200 V) that does not change depending on the temperature and humidity detected by the temperature and humidity sensor 14, so that the density of the non-black toner image can be made appropriate when toner with high charging performance and low glossiness is used.

[0068] Although the embodiment has been described above, the image forming apparatus can be modified as appropriate as exemplified below.

[0069] In the above-described embodiment, in the first process, two different reference developing biases are applied to the developing roller 55 to form two types of patch images on the transfer belt 73, and the first developing bias Vb1 is set based on the densities of the two types of patch images. However, for example, in the first process, three or more different reference developing biases may be applied to the developing roller to form three or more types of patch images on the transfer belt, and the first developing bias may be set based on the densities of the three or more types of patch images.

[0070] In the above embodiment, when setting the developing biases VbY, VbM, and VbC for colors other than black, the second developing bias Vb2, which does not change depending on temperature and humidity, is set in the second process. However, for example, when setting the developing biases VbY, VbM, and VbC for colors other than black, the second developing bias Vb2 may be set depending on temperature and humidity in the second process, similar to when setting the developing bias VbK for black.

[0071] In the above embodiment, the developing roller 55 includes three second developing rollers (developing rollers 55Y, 55M, and 55C), but the number of second developing rollers is arbitrary. For example, the number of second developing rollers may be one.

[0072] In the above embodiment, the transfer belt 73 that transports the sheet S between the photosensitive drum 53 and the endless belt is exemplified, but the endless belt may be, for example, an intermediate transfer belt.

[0073] In the above-described embodiment, an endless belt is exemplified as the transfer object, but the transfer object may also be, for example, a sheet S. That is, the image forming apparatus may form a patch image P on the sheet S, measure the density of the patch image P formed on the sheet S, and set the first developing bias Vb1 based on the measured density of the patch image P.

[0074] In the above-described embodiment, the exposure unit 40 of the image forming section 30 exposes the photosensitive drum 53 with light emitted from an LED, but for example, the exposure unit may expose the photosensitive drum with a beam emitted from a semiconductor laser or the like. Also, in the above-described embodiment, the fixing unit 80 of the image forming section 30 includes the heating roller 81 and the pressure roller 82, but for example, the fixing unit may be a belt fixing type fixing unit in which at least one of the heating section and the pressure section has an endless belt.

[0075] In the above embodiment, the image forming apparatus 1 is a printer, but the image forming apparatus may be, for example, a copying machine or a multifunction peripheral.

[0076] In the above embodiment, the image forming apparatus 1 uses a positively charged toner, but the image forming apparatus may use a negatively charged toner, for example. In addition, when the image forming apparatus uses a negatively charged toner, the positive and negative of the developing bias are reversed from those in the above embodiment.

[0077] The elements described in the above-described embodiment and modifications may be implemented in any combination. [Explanation of symbols]

[0078] 1. Image forming device 13 Concentration sensor 14 Temperature and humidity sensor 30 Image forming unit 55 Developing roller 73 Transfer belt 100 control section P patch image

Claims

1. an image forming unit having a developing roller and capable of forming a toner patch image on a transfer target; a density sensor capable of measuring the density of a patch image formed on a transfer target; a temperature and humidity sensor for detecting temperature and humidity; a control unit capable of executing a developing bias setting process for setting a developing bias to be applied to the developing roller during printing; The control unit, in the developing bias setting process, a first process of forming a patch image on a transfer target, measuring the density of the formed patch image by the density sensor, and setting a first developing bias, which is a developing bias that makes the density of the patch image a predetermined target density, based on the measured density of the patch image; a second process for setting a second developing bias, which is a lower limit value of the magnitude of the developing bias to be applied to the developing roller; a third process for setting the developing bias to be applied to the developing roller during printing to the first developing bias when the magnitude of the first developing bias is equal to or greater than the magnitude of the second developing bias, and setting the developing bias to be applied to the developing roller during printing to the second developing bias when the magnitude of the first developing bias is less than the magnitude of the second developing bias; The image forming apparatus according to claim 1, wherein the control unit sets a second developing bias in accordance with the temperature and humidity detected by the temperature and humidity sensor in the second process.

2. The image forming apparatus according to claim 1, characterized in that the magnitude of the second developing bias in a first environment in which the temperature and humidity detected by the temperature and humidity sensor are within a predetermined temperature and humidity range is greater than the magnitude of the second developing bias in a second environment in which at least one of the temperature and humidity detected by the temperature and humidity sensor is lower than the temperature and humidity range.

3. the developing rollers include a first developing roller carrying black toner and at least one second developing roller carrying a toner other than black; The control unit When setting a developing bias to be applied to the first developing roller during printing, the second process sets a second developing bias in accordance with the temperature and humidity detected by the temperature and humidity sensor; An image forming apparatus as described in claim 2, characterized in that when setting a development bias to be applied to the second development roller during printing, in the second process, a second development bias is set that does not change depending on the temperature and humidity detected by the temperature and humidity sensor.

4. 4. The image forming apparatus according to claim 2, wherein the control unit sets the second developing bias in the second process by referring to a table showing the relationship between the temperature and humidity and the second developing bias.

5. In the first process, the control unit a process of applying a first reference developing bias to the developing roller to form a patch image on a transfer object, and applying a second reference developing bias different from the first reference developing bias to the developing roller to form the patch image on the transfer object; a process of measuring the density of the patch image formed on the transfer target by the density sensor; 2. The image forming apparatus according to claim 1, further comprising: a process for setting a first developing bias based on the density of a patch image formed by applying a first reference developing bias to the developing roller and the density of a patch image formed by applying a second reference developing bias to the developing roller.

6. 2. The image forming apparatus according to claim 1, further comprising an endless belt as the transfer object.

Images