Image forming device

Abstract

To provide an image forming apparatus that can prevent the occurrence of transfer residual ghost while preventing a reduction in print density.SOLUTION: An image forming apparatus 1 comprises: a photoconductor drum 4Y; a developing roller 72Y; a humidity sensor 11; and a control unit 12. The control unit 12 applies a first developing bias to the developing roller 72Y in single-sided print processing (S3). In double-sided print processing (S6), when the humidity detected by the humidity sensor 11 is higher than a threshold (S4: NO), the control unit 12 applies the first developing bias to the developing roller when performing printing on a second surface S2 of a sheet S. On the other hand, when the humidity detected by the humidity sensor 11 is equal to or less than the threshold (S4: YES), the control unit applies a second developing bias smaller than the first developing bias to the developing roller 72Y when performing printing on the second surface S2 of the sheet S.SELECTED DRAWING: Figure 3

Description

[Technical field]

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

[0002] Conventionally, there is known an image forming device that can be switched between single-sided mode and double-sided mode, which applies a first developing bias to a developing roller in single-sided mode, and applies a second developing bias lower than the first developing bias to the developing roller when printing on the second side of a sheet of paper in double-sided mode (see Patent Document 1 below). [Prior art documents] [Patent documents]

[0003] [Patent Document 1] JP 2009-288550 A Summary of the Invention [Problem to be solved by the invention]

[0004] According to the image forming apparatus described in Patent Document 1, the reduction rate of the second developing bias relative to the first developing bias (developing bias reduction rate) is set by a coefficient according to the relative humidity. More specifically, the developing bias reduction rate is set as a linear function of the developing bias reduction rate relative to the relative humidity.

[0005] Therefore, even in a humidity environment where there is no need to reduce the developing bias, the developing bias may be reduced, resulting in a decrease in print density.

[0006] Therefore, an object of the present disclosure is to provide an image forming apparatus capable of suppressing the occurrence of transfer residual ghosts while suppressing the decrease in print density. [Means for solving the problem]

[0007] (1) The image forming apparatus of the present disclosure includes a photosensitive drum, a developing roller, a humidity sensor, and a control unit. The humidity sensor detects humidity. The control unit is capable of executing single-sided printing processing and double-sided printing processing. When the control unit executes single-sided printing processing, the image forming apparatus forms an image only on a first side of a sheet. When the control unit executes double-sided printing processing, the image forming apparatus forms an image on both the first side of the sheet and the second side of the sheet.

[0008] The control unit applies a first developing bias to the developing roller in a single-sided printing process.

[0009] In a double-sided printing process, if the humidity detected by the humidity sensor is higher than a threshold value, the control unit applies a first developing bias to the developing roller when printing on the second side of the sheet, and if the humidity detected by the humidity sensor is equal to or lower than the threshold value, the control unit applies a second developing bias smaller than the first developing bias to the developing roller when printing on the second side of the sheet.

[0010] According to this configuration, in double-sided printing processing, if the humidity is higher than the threshold value, the developing bias is not reduced even when printing on the second side of the sheet, and if the humidity is equal to or lower than the threshold value, the developing bias is reduced when printing on the second side of the sheet.

[0011] This makes it possible to ensure the print density of the second side without lowering the development bias in an environment where the humidity is higher than the threshold and transfer residual ghosts are unlikely to occur.

[0012] On the other hand, in an environment where the humidity is below the threshold and transfer residual ghosts are likely to occur, the development bias can be lowered to suppress the occurrence of transfer residual ghosts in printing on the second side.

[0013] As a result, it is possible to suppress the occurrence of transfer residual ghosts while suppressing the decrease in print density. (2) The control unit may be configured to reduce the difference between the first developing bias and the second developing bias as the humidity detected by the humidity sensor increases.

[0014] According to this configuration, the second developing bias can be set higher as the humidity increases and the occurrence of transfer residual ghost becomes more difficult.

[0015] As a result, the decrease in print density can be further suppressed.

[0016] (3) The difference between the first developing bias and the second developing bias may be calculated by the following formula 1.

[0017] 1st equation: ΔVb=D×b' (In the first formula, ΔVb is the difference between the first developing bias and the second developing bias, D is the target value of the print density, and b' is the slope calculated based on the density of the actually measured toner patch.) According to this configuration, the second developing bias can be set based on ΔVb calculated from the actually measured density of the toner patch and the target value of the print density.

[0018] Therefore, ΔVb can be corrected according to the actually measured density of the toner patch.

[0019] As a result, the decrease in print density can be further suppressed.

[0020] (4) In double-sided printing processing, if the humidity detected by the humidity sensor is equal to or lower than a threshold value, the control unit may apply a third developing bias smaller than the first developing bias to the developing roller when printing on the first side of the sheet.

[0021] With this configuration, the print density of the first side can be reduced to match the print density of the second side.

[0022] As a result, it is possible to prevent the difference in print density between the first and second sides from becoming excessively large. Effect of the Invention

[0023] According to the image forming apparatus of the present disclosure, it is possible to suppress the occurrence of transfer residual ghosts while suppressing the decrease in print density. [Brief description of the drawings]

[0024] [Figure 1] FIG. 1 is a schematic diagram of an image forming apparatus. [Diagram 2] FIG. 2 is an explanatory diagram showing electrical connections between the humidity sensor, the control unit, and a plurality of developing rollers. [Diagram 3] FIG. 3 is a flowchart for explaining the control of the image forming apparatus. [Figure 4] FIG. 4 is a graph for explaining b' in the first formula. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0025] 1. Overview of image forming apparatus 1 An outline of an image forming apparatus 1 will be described with reference to FIG.

[0026] The image forming apparatus 1 includes a main body housing 2, a sheet storage section 3, multiple photosensitive drums 4Y, 4M, 4C, 4K, multiple charging devices 5Y, 5M, 5C, 5K, an exposure device 6, multiple developing cartridges 7Y, 7M, 7C, 7K, a transfer device 8, and a fixing device 9.

[0027] 1.1 Main unit 2 The main body housing 2 houses a sheet storage section 3, photosensitive drums 4Y, 4M, 4C, and 4K, charging devices 5Y, 5M, 5C, and 5K, an exposure device 6, developing cartridges 7Y, 7M, 7C, and 7K, a transfer device 8, and a fixing device 9.

[0028] 1.2 Sheet storage section 3 The sheet storage unit 3 stores the sheets S. The sheets S in the sheet storage unit 3 are transported from the sheet storage unit 3 toward the photosensitive drum 4Y. The sheets S are, for example, printing paper. The sheet storage unit 3 may be a sheet cassette.

[0029] 1.3 Photosensitive drums 4Y, 4M, 4C, 4K The photosensitive drums 4Y, 4M, 4C, and 4K are aligned in the direction in which the sheet S is conveyed by the belt 81. The belt 81 will be described later.

[0030] The photosensitive drum 4Y extends in the axial direction. The photosensitive drum 4Y is rotatable about a drum axis A1. The drum axis A1 extends in the axial direction.

[0031] The description of each of the photosensitive drums 4M, 4C, and 4K is the same as the description of the photosensitive drum 4Y. Therefore, the description of the photosensitive drums 4M, 4C, and 4K will be omitted. 1.4 Charging devices 5Y, 5M, 5C, 5K The charging device 5Y charges the photosensitive drum 4Y. The charging device 5M charges the photosensitive drum 4M. The charging device 5C charges the photosensitive drum 4C. The charging device 5K charges the photosensitive drum 4K. In this embodiment, each of the charging devices 5Y, 5M, 5C, and 5K is a scorotron type charging device. Each of the charging devices 5Y, 5M, 5C, and 5K may be a charging roller.

[0032] 1.5 Exposure Equipment 6 The exposure device 6 exposes the photosensitive drum 4Y charged by the charging device 5Y. In this embodiment, the exposure device 6 is a laser scan unit. The exposure device 6 can expose not only the photosensitive drum 4Y but also the photosensitive drums 4M, 4C, and 4K. The exposure device 6 may be an exposure head having an LED array.

[0033] 1.6 Developer cartridges 7Y, 7M, 7C, 7K Each of the developing cartridges 7Y, 7M, 7C, and 7K can be attached to the main body housing 2.

[0034] The developing cartridge 7Y includes a developing housing 71Y and a developing roller 72Y. In other words, the image forming apparatus 1 includes the developing roller 72Y.

[0035] The developing casing 71Y contains toner. The toner is a non-magnetic one-component toner that can be charged by friction. In this embodiment, the toner is positively charged by friction.

[0036] The developing roller 72Y is supported by the developing housing 71Y. When the developing cartridge 7Y is mounted in the main housing 2, the developing roller 72Y can supply the toner in the developing housing 71Y to the photosensitive drum 4Y. The developing roller 72Y extends in the axial direction. The developing roller 72Y can rotate about the developing shaft A2. The developing shaft A2 extends in the axial direction.

[0037] The description of each of the developing cartridges 7M, 7C, and 7K is the same as the description of the developing cartridge 7Y, so the description of the developing cartridges 7M, 7C, and 7K will be omitted.

[0038] 1.7 Transfer device 8 The transfer device 8 includes a belt 81 and a plurality of transfer rollers 82K, 82Y, 82M, and 82C.

[0039] The belt 81 comes into contact with the photosensitive drums 4Y, 4M, 4C, and 4K. The belt 81 transports the sheet S from the sheet storage unit 3 toward the fixing device 9. The belt 81 transports the sheet S from the photosensitive drum 4Y toward the photosensitive drum 4K.

[0040] The transfer roller 82Y transfers the toner on the photosensitive drum 4Y to the sheet S being transported by the belt 81. The transfer roller 82M transfers the toner on the photosensitive drum 4M to the sheet S being transported by the belt 81. The transfer roller 82C transfers the toner on the photosensitive drum 4C to the sheet S being transported by the belt 81. The transfer roller 82K transfers the toner on the photosensitive drum 4K to the sheet S being transported by the belt 81.

[0041] 1.8 Fixing device 9 The fixing device 9 applies heat and pressure to the sheet S onto which the toner has been transferred, thereby fixing the toner to the sheet S. The sheet S that has passed through the fixing device 9 is discharged onto the top surface of the main body housing 2.

[0042] 2. Details of Image Forming Apparatus 1 Next, the image forming apparatus 1 will be described in detail with reference to FIG.

[0043] As shown in FIG. 2, the image forming apparatus 1 includes a humidity sensor 11 and a control unit 12.

[0044] 2.1 Humidity sensor11 The humidity sensor 11 detects humidity. In this embodiment, the humidity sensor 11 detects humidity outside the main body housing 2. "Humidity outside the main body housing 2" refers to, for example, the humidity in a room in which the image forming apparatus 1 is installed. Note that the humidity sensor 11 may also detect humidity inside the main body housing 2.

[0045] 2.2 Control Unit 12 The control unit 12 is electrically connected to the humidity sensor 11. This allows the control unit 12 to acquire the humidity detected by the humidity sensor 11. The control unit 12 is also electrically connected to each of the developing rollers 72Y, 72M, 72C, and 72K. The control unit 12 controls each of the developing bias Vb(Y) applied to the developing roller 72Y, the developing bias Vb(M) applied to the developing roller 72M, the developing bias Vb(C) applied to the developing roller 72C, and the developing bias Vb(K) applied to the developing roller 72K.

[0046] 3. Control of Image Forming Apparatus 1 Next, the control of the image forming apparatus 1 by the control unit 12 will be described with reference to FIGS.

[0047] When the control unit 12 acquires the print job, the control unit 12 executes a print process. As the print process, the control unit 12 is capable of executing a single-sided printing process (S3) and a double-sided printing process (S6).

[0048] 3.1 Single-sided printing process (S3) As shown in FIG. 3, if the print job is not set for double-sided printing (S1: NO), the control unit 12 sets the target value of the developing bias Vb(Y) to the first developing bias Vb(Y1), sets the target value of the developing bias Vb(M) to the first developing bias Vb(M1), sets the target value of the developing bias Vb(C) to the first developing bias Vb(C1), and sets the target value of the developing bias Vb(K) to the first developing bias Vb(K1) (S2).

[0049] Next, the control unit 12 executes a single-sided printing process (S3).

[0050] When the control unit 12 executes the single-sided printing process (S3), the image forming apparatus 1 forms an image only on the first surface S1 of the sheet S, as shown by the solid line in Fig. 1. At this time, the control unit 12 adjusts the developing bias Vb(Y) to the first developing bias Vb(Y1), adjusts the developing bias Vb(M) to the first developing bias Vb(M1), adjusts the developing bias Vb(C) to the first developing bias Vb(C1), and adjusts the developing bias Vb(K) to the first developing bias Vb(K1). That is, the control unit 12 applies the first developing bias Vb(Y1) to the developing roller 72Y in the single-sided printing process (S3).

[0051] 3.2 Double-sided printing process (S6) 3, when the print job is set to double-sided printing (S1: YES), the control unit 12 executes double-sided printing processing (S6). When the control unit 12 executes double-sided printing processing (S6), the image forming apparatus 1 forms an image on the first side S1 of the sheet S and the second side S2 of the sheet S.

[0052] More specifically, the image forming apparatus 1 first forms an image on a first surface S1 of a sheet S as indicated by a solid line in FIG.

[0053] Next, the image forming apparatus 1 conveys the sheet S that has passed through the fixing device 9, starting from the rear end of the sheet S, to the photosensitive drum 4Y as indicated by the dashed line in FIG.

[0054] Here, taking the photosensitive drum 4Y and the transfer roller 82Y as an example, when the sheet S that has passed through the fixing device 9 passes through the nip between the photosensitive drum 4Y and the transfer roller 82Y, discharge may occur between the sheet S and the photosensitive drum 4Y.

[0055] Specifically, in a low humidity environment where the relative humidity is 40% or less, the sheet S is likely to be charged. As a result, discharge may occur between the sheet S and the photosensitive drum 4Y. In particular, in a low humidity environment where the relative humidity is 40% or less, discharge is likely to occur when an image is formed on the second surface S2 of the sheet S that has passed through the fixing device 9. When discharge occurs, the charged state of the toner on the photosensitive drum 4Y changes, and transfer residual toner occurs that is not transferred to the sheet S and remains on the photosensitive drum 4Y. If the generated transfer residual toner adheres to the sheet S without being cleaned, image quality may deteriorate. The deterioration of image quality caused by the generated transfer residual toner adhering to the sheet S without being cleaned is defined as a transfer residual ghost.

[0056] In addition, discharges may occur between the transfer roller 82M and the photosensitive drum 4M, between the transfer roller 82C and the photosensitive drum 4C, and between the transfer roller 82K and the photosensitive drum 4K, just as between the transfer roller 82Y and the photosensitive drum 4Y.

[0057] 3, when performing double-sided printing processing (S6) in a low humidity environment (S4: YES), the control unit 12 reduces the developing biases Vb(Y), Vb(M), Vb(C), and Vb(K) when printing on the second side S2 of the sheet S (S7). This will be described in detail below.

[0058] 3.2.1 Double-sided printing process in a high humidity environment (S4:NO) If the print job is set for double-sided printing (S1: YES), the control unit 12 sets target values ​​of the developing biases Vb(Y), Vb(M), Vb(C), and Vb(K) for printing on the first side S1 of the sheet S, and target values ​​of the developing biases Vb(Y), Vb(M), Vb(C), and Vb(K) for printing on the second side S2 of the sheet S.

[0059] If the print job is set to double-sided printing (S1: YES), the control unit 12 acquires the humidity from the humidity sensor 11 (S4).

[0060] Next, if the humidity detected by the humidity sensor 11 is higher than the threshold value (S4: NO), the control unit 12 sets the target values ​​of the developing biases Vb(Y), Vb(M), Vb(C), and Vb(K) for printing on the first side S1 and the target values ​​of the developing biases Vb(Y), Vb(M), Vb(C), and Vb(K) for printing on the second side S2 to the same values ​​as when performing the single-sided printing process (S3) (S5).

[0061] Specifically, the control unit 12 sets the target value of the developing bias Vb(Y) for printing on the first surface S1 to the first developing bias Vb(Y1), sets the target value of the developing bias Vb(M) for printing on the first surface S1 to the first developing bias Vb(M1), sets the target value of the developing bias Vb(C) for printing on the first surface S1 to the first developing bias Vb(C1), and sets the target value of the developing bias Vb(K) for printing on the first surface S1 to the first developing bias Vb(K1).

[0062] In addition, the control unit 12 sets the target value of the developing bias Vb(Y) for printing on the second surface S2 to the first developing bias Vb(Y1), sets the target value of the developing bias Vb(M) for printing on the second surface S2 to the first developing bias Vb(M1), sets the target value of the developing bias Vb(C) for printing on the second surface S2 to the first developing bias Vb(C1), and sets the target value of the developing bias Vb(K) for printing on the second surface S2 to the first developing bias Vb(K1).

[0063] In this embodiment, the threshold is 40%.

[0064] Next, the control unit 12 executes the double-sided printing process (S6) as described above.

[0065] When the image forming apparatus 1 forms an image on the first surface S1 of the sheet S, the control unit 12 adjusts the developing bias Vb(Y) to a first developing bias Vb(Y1), adjusts the developing bias Vb(M) to a first developing bias Vb(M1), adjusts the developing bias Vb(C) to a first developing bias Vb(C1), and adjusts the developing bias Vb(K) to a first developing bias Vb(K1).

[0066] When the image forming apparatus 1 forms an image on the second side S2 of the sheet S, the control unit 12 adjusts the developing bias Vb(Y) to the first developing bias Vb(Y1), adjusts the developing bias Vb(M) to the first developing bias Vb(M1), adjusts the developing bias Vb(C) to the first developing bias Vb(C1), and adjusts the developing bias Vb(K) to the first developing bias Vb(K1). In other words, when the humidity detected by the humidity sensor 11 is higher than the threshold value (S4: NO), the control unit 12 applies the first developing bias Vb(Y1) to the developing roller 72Y when printing on the second side S2 of the sheet S in the double-sided printing process (S6).

[0067] 3.2.2 Double-sided printing process in low humidity environment (S4: YES) If the humidity detected by the humidity sensor 11 is below the threshold value (S4: YES), the control unit 12 sets the target value of the developing bias Vb(Y) for printing on the second side S2 to the second developing bias Vb(Y2), sets the target value of the developing bias Vb(M) for printing on the second side S2 to the second developing bias Vb(M2), sets the target value of the developing bias Vb(C) for printing on the second side S2 to the second developing bias Vb(C2), and sets the target value of the developing bias Vb(K) for printing on the second side S2 to the second developing bias Vb(K2) (S7).

[0068] The second developing bias Vb(Y2) is smaller than the first developing bias Vb(Y1). Since the second developing bias Vb(Y2) is smaller than the first developing bias Vb(Y1), the amount of toner supplied to the photosensitive drum 4Y can be reduced. This reduces the amount of residual toner after transfer, and suppresses deterioration of image quality.

[0069] Here, the difference ΔVb between the first developing bias Vb(Y1) and the second developing bias Vb(Y2) is calculated by the following first formula.

[0070] 1st equation: ΔVb=D×b' (In the first formula, D is the target value of the print density, and b' is the slope calculated based on the density of the actually measured toner patch.) The print density target value D is stored as a data table that takes into account the printing environment in the memory of the control unit 12. Examples of the printing environment include temperature, humidity, and type of sheet S. The control unit 12 reads out a specific target value D according to the printing environment.

[0071] Next, b' will be described with reference to FIG. 4. As shown in FIG. 4, in the density correction process, the image forming apparatus 1 forms a plurality of toner patches T1, T2, and T3 on the belt 81 (see FIG. 1) and detects the respective densities d1, d2, and d3 of the plurality of toner patches T1, T2, and T3. The density correction process is executed, for example, every time the cumulative number of printed sheets reaches a predetermined number, while the image forming apparatus 1 is not forming images. The cumulative number of printed pages is the cumulative number of pages printed after the replacement work of the developing cartridge 7Y is performed. The toner patch T1 is a toner patch when the developing bias Vb1 is applied to the developing roller 72Y, the toner patch T2 is a toner patch when the developing bias Vb2 is applied to the developing roller 72Y, and the toner patch T3 is a toner patch when the developing bias Vb3 is applied to the developing roller 72Y.

[0072] The control unit 12 derives a function f(d) of the development bias and the density of the toner patch from the density d1 of the toner patch T1, the density d2 of the toner patch T2, and the density d3 of the toner patch T3.

[0073] Next, the control unit 12 calculates b' by differentiating the function f(d) with respect to the target value D. In other words, b' is the gradient of the tangent t(d) of the function f(d) at the target value D.

[0074] Next, the control unit 12 calculates the difference ΔVb between the first developing bias Vb(Y1) and the second developing bias Vb(Y2) from the above first equation.

[0075] Next, the control unit 12 calculates the second developing bias Vb(Y2) by subtracting ΔVb from the first developing bias Vb(Y1).

[0076] Each of the second developing biases Vb(M2), Vb(C2), and Vb(K2) is calculated in the same manner as the second developing bias Vb(Y2). That is, the second developing bias Vb(M2) is smaller than the first developing bias Vb(M1), the second developing bias Vb(C2) is smaller than the first developing bias Vb(C1), and the second developing bias Vb(K2) is smaller than the first developing bias Vb(K1).

[0077] Also, as shown in FIG. 3, if the humidity detected by the humidity sensor 11 is below the threshold value (S4: YES), the control unit 12 sets the target value of the developing bias Vb(Y) for printing on the first side S1 to the third developing bias Vb(Y3), sets the target value of the developing bias Vb(M) for printing on the first side S1 to the third developing bias Vb(M3), sets the target value of the developing bias Vb(C) for printing on the first side S1 to the third developing bias Vb(C3), and sets the target value of the developing bias Vb(K) for printing on the first side S1 to the third developing bias Vb(K3) (S7).

[0078] The third developing bias Vb(Y3) is smaller than the first developing bias Vb(Y1). Preferably, the third developing bias Vb(Y3) is the same as the second developing bias Vb(Y2). By making the third developing bias Vb(Y3) smaller than the first developing bias Vb(Y1), the amount of toner supplied to the photosensitive drum 4Y can be reduced. This allows the print density of the first surface S1 to match the print density of the second surface S2.

[0079] The third developing bias Vb(M3) is smaller than the first developing bias Vb(M1), the third developing bias Vb(C3) is smaller than the first developing bias Vb(C1), and the third developing bias Vb(K3) is smaller than the first developing bias Vb(K1).

[0080] Next, the control unit 12 executes a double-sided printing process (S6).

[0081] When the image forming apparatus 1 forms an image on the first side S1 of the sheet S, the control unit 12 adjusts the developing bias Vb(Y) to the third developing bias Vb(Y3), adjusts the developing bias Vb(M) to the third developing bias Vb(M3), adjusts the developing bias Vb(C) to the third developing bias Vb(C3), and adjusts the developing bias Vb(K) to the third developing bias Vb(K3). In other words, when the humidity detected by the humidity sensor 11 is equal to or lower than the threshold (S4: YES), the control unit 12 applies the third developing bias Vb(Y3) to the developing roller 72Y when printing on the first side S1 of the sheet S in the double-sided printing process (S6).

[0082] When the image forming apparatus 1 forms an image on the second side S2 of the sheet S, the control unit 12 adjusts the developing bias Vb(Y) to the second developing bias Vb(Y2), adjusts the developing bias Vb(M) to the second developing bias Vb(M2), adjusts the developing bias Vb(C) to the second developing bias Vb(C2), and adjusts the developing bias Vb(K) to the second developing bias Vb(K2). In other words, when the humidity detected by the humidity sensor 11 is equal to or lower than the threshold (S4: YES), the control unit 12 applies the second developing bias Vb(Y2) to the developing roller 72Y when printing on the second side S2 of the sheet S in the double-sided printing process (S6).

[0083] When there are no more print jobs, the control unit 12 ends the print process.

[0084] 4. Effects (1) According to the image forming apparatus 1, as shown in FIG. 3, in the double-sided printing process (S6), if the humidity is higher than the threshold value (S4: NO), the developing bias is not reduced even when printing on the second side S2 of the sheet S, and if the humidity is equal to or lower than the threshold value (S4: YES), the developing bias is reduced when printing on the second side S2 of the sheet S.

[0085] Specifically, in the double-sided printing process (S6), if the humidity is higher than the threshold value (S4: NO), the first developing bias is applied even when printing on the second side S2 of the sheet S, and if the humidity is below the threshold value (S4: YES), a second developing bias lower than the first developing bias is applied when printing on the second side S2 of the sheet S.

[0086] As a result, in an environment where the humidity is higher than the threshold (S4: NO) and transfer residual ghosts are unlikely to occur, the print density of the second surface S2 can be ensured without lowering the developing bias.

[0087] On the other hand, in an environment where the humidity is equal to or lower than the threshold (S4: YES) and transfer residual ghost images are likely to occur, the developing bias can be lowered to suppress the occurrence of transfer residual ghost images in printing on the second side.

[0088] As a result, it is possible to suppress the occurrence of transfer residual ghosts while suppressing the decrease in print density. (2) According to the image forming apparatus 1, the difference ΔVb between the first developing bias and the second developing bias is calculated by the following first formula.

[0089] 1st equation: ΔVb=D×b' 4, in Equation 1, D is the target value of the print density, and b' is the slope of the tangent t(d) of the quadratic function f(d) derived from the actually measured density of the toner patch, which passes through the target value D of the print density.

[0090] Therefore, the second developing bias can be set by ΔVb calculated from the actually measured toner patch density and the target print density value D. In other words, ΔVb can be corrected according to the actually measured toner patch density.

[0091] As a result, the decrease in print density can be further suppressed.

[0092] (3) According to the image forming apparatus 1, as shown in FIG. 3, in the double-sided printing process (S6), if the humidity is equal to or lower than the threshold (S4: YES), the developing bias is lowered when printing on the first side S1 of the sheet S.

[0093] Specifically, in the double-sided printing process (S6), if the humidity is equal to or lower than the threshold value (S4: YES), a third developing bias lower than the first developing bias is applied when printing on the first side S1 of the sheet S.

[0094] This allows the print density of the first surface S1 to be reduced to match the print density of the second surface S2.

[0095] As a result, it is possible to prevent the difference in print density between the first surface S1 and the second surface S2 from becoming excessively large.

[0096] 5. Variations Next, a modified example will be described. In the modified example, the same members and processes as those in the above-described embodiment are denoted by the same reference numerals, and the description thereof will be omitted.

[0097] (1) The control unit 12 may reduce the difference ΔVb between the first developing bias and the second developing bias as the humidity detected by the humidity sensor 11 increases.

[0098] According to this modified example, the second developing bias can be set higher as the humidity increases and the occurrence of transfer residual ghost becomes more difficult.

[0099] As a result, the decrease in print density can be further suppressed.

[0100] (2) When b' exceeds a predetermined upper limit, the control unit 12 may set b' to the upper limit.

[0101] According to this modification, it is possible to prevent ΔVb from becoming excessively large.

[0102] As a result, it is possible to prevent the print density on the second surface S2 from decreasing excessively.

[0103] (3) The image forming apparatus 1 may be an image forming apparatus dedicated to monochrome printing that does not have the photosensitive drums 4Y, 4M, and 4C.

[0104] (4) The image forming apparatus 1 may have a drum unit having photosensitive drums 4Y, 4M, 4C, and 4K.

[0105] The image forming apparatus 1 may have a drum cartridge having a photosensitive drum 4Y, a drum cartridge having a photosensitive drum 4M, a drum cartridge having a photosensitive drum 4C, and a drum cartridge having a photosensitive drum 4K.

[0106] The image forming device 1 may have a process cartridge having a photosensitive drum 4Y and a developing roller 72Y, a process cartridge having a photosensitive drum 4M and a developing roller 72M, a process cartridge having a photosensitive drum 4C and a developing roller 72C, and a process cartridge having a photosensitive drum 4K and a developing roller 72K. [Explanation of symbols]

[0107] 1. Image forming device 4Y Photosensitive drum 11 Humidity Sensor 12 Control section 72Y Developing roller S Seat S1 1st page S2 side 2

Claims

1. A photosensitive drum; A developing roller; A humidity sensor for detecting humidity; a control unit capable of executing a single-sided printing process for forming an image only on a first side of a sheet, and a double-sided printing process for forming images on the first side of the sheet and on a second side of the sheet; Equipped with The control unit is In the single-sided printing process, a first developing bias is applied to the developing roller; In the double-sided printing process, applying the first developing bias to the developing roller when printing on the second side of the sheet if the humidity detected by the humidity sensor is higher than a threshold value; applying a second developing bias smaller than the first developing bias to the developing roller when printing on the second side of the sheet if the humidity detected by the humidity sensor is equal to or lower than the threshold value; The image forming apparatus according to claim 1, wherein a difference between the first developing bias and the second developing bias is calculated by the following first formula: First equation: ΔVb=D×b' (In the first formula, ΔVb is the difference between the first developing bias and the second developing bias, D is the target value of the print density, and b′ is the slope calculated based on the density of the actually measured toner patch.)

2. 2. The image forming apparatus of claim 1, wherein the control unit applies a third developing bias smaller than the first developing bias to the developing roller when printing on the first side of the sheet during the double-sided printing process when the humidity detected by the humidity sensor is below the threshold value.

Images