Image forming apparatus

Abstract

This method suppresses excessive transfer current when forming an image on the second surface of a sheet during double-sided printing. [Solution] When forming an image on both sides of the sheet, the control unit applies a second development bias to the second side. When the second development bias is set to a second reference value, the control unit sets a reference transfer current value as the transfer bias when transferring toner to the second side. When the second development bias is set to a value smaller than the second reference value (S4, Yes), the control unit performs a transfer bias reduction process (S7) which sets a reference transfer current value as the transfer bias when transferring toner to the first side and sets a value smaller than the reference transfer current value as the transfer bias when transferring toner to the second side.

Description

Technical Field

[0001] The present invention relates to an image forming apparatus capable of performing double-sided printing by reversing the front and back of a sheet.

Background Art

[0002] Conventionally, an image forming apparatus capable of double-sided printing that performs printing on the second side by reversing the front and back of the sheet after printing on the first side of the sheet has been known (see Patent Document 1). In this image forming apparatus, in order to suppress transfer defects associated with excessive discharge in the apparatus, when performing double-sided printing, the second developing bias for forming an image on the second side of the sheet is set lower than the first developing bias for forming an image on the first side. The second developing bias is calculated by subtracting the amount of decrease according to humidity from the first developing bias.

Prior Art Documents

Patent Documents

[0003]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0004] By the way, when forming an image on the second side of the sheet in double-sided printing, if the second developing bias when forming an image on the second side is set to a value smaller than the first developing bias, the amount of toner on the photosensitive drum decreases, so the transfer current may become excessive. There was a problem that transfer defects are likely to occur when the transfer current becomes excessive.

[0005] Therefore, an object of the present invention is to suppress the transfer current from becoming excessive when forming an image on the second side of the sheet in double-sided printing.

Means for Solving the Problems

[0006] To solve the aforementioned problems, the image forming apparatus according to the present invention is capable of performing double-sided printing by reversing the front and back sides of a sheet, and comprises a photosensitive drum, a developing roller, a developing bias application circuit, a transfer device, a transfer bias application circuit, a fixing device, a re-transport device, and a control unit. The developing roller supplies toner to the photoreceptor drum. The developing bias application circuit applies a developing bias to the developing roller. The transfer device transfers the toner from the photoreceptor drum to the sheet. The transfer bias application circuit applies a transfer bias to the transfer device. The fuser device fixes the toner to the sheet. The re-transport device re-transports the sheet that has passed through the fuser device back to the transfer device. When forming an image on both sides of a sheet, the control unit applies a first development bias to the developing roller when forming an image on the first side of the sheet, and then applies a second development bias to the developing roller when forming an image on the second side opposite to the first side. When the second development bias is set to the second reference value, the control unit sets the reference transfer current value as the transfer bias when transferring toner to the second surface. If the control unit sets the second development bias to a value smaller than the second reference value, it performs a transfer bias reduction process to set the transfer bias to a value smaller than the reference transfer current value when transferring toner to the second surface.

[0007] When the control unit sets the second development bias to a value smaller than the second reference value when forming an image on the second surface, it performs a transfer bias reduction process to set the transfer bias to a value smaller than the reference transfer current value when transferring toner to the second surface, thereby suppressing excessive transfer current.

[0008] Furthermore, the control unit may be further equipped with a humidity sensor to detect humidity, and in the transfer bias reduction process, the amount by which the transfer current is reduced may be reduced as the humidity increases.

[0009] In the transfer bias reduction process, the higher the humidity, the smaller the reduction in transfer current. This ensures that the transfer current necessary to maintain print density is secured, except in excessively low-humidity environments.

[0010] Furthermore, the system may be further equipped with a humidity sensor for detecting humidity. When the humidity is above a first threshold, the control unit may set the first development bias to a first reference value and the second development bias to a second reference value. When the humidity is below the first threshold, the control unit may set the first development bias to a first reference value and the second development bias to a value less than the second reference value by a first correction value. The first correction value may be set to a larger value as the humidity decreases.

[0011] In low-humidity environments, transfer defects due to excessive discharge in the transfer device are likely to occur when forming the image on the second side of double-sided printing. In this invention, when the humidity is below the first threshold, the second development bias for forming the image on the second side is set to a value smaller than the second reference value by the first correction value, thereby suppressing transfer defects due to excessive discharge.

[0012] Furthermore, the first reference value may be the same as the second reference value.

[0013] Furthermore, if the first correction value is greater than or equal to the second threshold, the control unit may set the first development bias to a value smaller than the first reference value when forming an image on the first surface.

[0014] If the second correction value is greater than or equal to the second threshold, the difference in development bias between the first and second sides during image formation can be reduced by making the first development bias of the first side of double-sided printing smaller than the first reference value (K1).

[0015] Furthermore, the control unit may be configured to control the transfer bias application circuit so that a predetermined transfer current flows when the image-forming area of ​​the sheet passes between the photoreceptor drum and the transfer device.

[0016] By controlling the transfer bias application circuit so that a predetermined transfer current flows, the control unit can suppress significant differences in transfer capability between the first and second sides of the sheet, even when the impedance of the sheet changes between the first and second sides during double-sided printing.

[0017] Also, a plurality of photosensitive drums may be arranged in the sheet conveyance direction. And the transfer device may include a belt that contacts the plurality of photosensitive drums and conveys the sheet together with the photosensitive drums, and a transfer roller that sandwiches the belt between the photosensitive drums.

[0018] Further, the control unit may be selectively executable to perform an image forming mode that executes transfer bias reduction processing and an image forming mode that does not execute transfer bias reduction processing according to a setting.

Advantages of the Invention

[0019] According to the present invention, when forming an image on the second side of the sheet in double-sided printing, it is possible to suppress the transfer current from becoming excessive.

Brief Description of the Drawings

[0020] [Figure 1] It is a cross-sectional view of an image forming apparatus according to an embodiment. [Figure 2] It is a block diagram showing the connection of a control unit, a developing bias application circuit, and a transfer bias application circuit. [Figure 3] It is a table (a) showing a reference correction value according to humidity and a table (b) showing a temperature correction coefficient according to temperature. [Figure 4] It is a table showing a second correction value corresponding to a first correction value. [Figure 5] It is a flowchart showing an example of a procedure in which a control unit sets a developing bias and a transfer current value. [Figure 6] It is a table showing a first correction value corresponding to temperature and humidity. [Figure 7] It is a table (a) of a first mode and a table (b) of a second mode in a form in which a plurality of modes can be selected. [Figure 8] It is a table (a) of a third mode and a table (b) of a fourth mode in a form in which a plurality of modes can be selected. [Figure 9]This is a table of the fifth mode in a configuration where multiple modes can be selected. [Modes for carrying out the invention]

[0021] Next, embodiments of the present invention will be described in detail with reference to the drawings as appropriate. As shown in Figure 1, the image forming apparatus 1 is a color printer. The image forming apparatus 1 comprises a main body housing 10, a supply unit 20, an image forming unit 30, and a transport unit 90.

[0022] The supply unit 20 comprises a supply tray 21 and a sheet supply mechanism 22. The supply tray 21 is a tray that holds the sheets S. The sheet supply mechanism 22 transports the sheets S in the supply tray 21 toward the image forming unit 30.

[0023] The image forming unit 30 comprises four LED units 40, four process cartridges 50, a belt unit 70, and a fixing device 80.

[0024] The LED unit 40 has multiple LEDs. The LED unit 40 exposes the photosensitive drum 51, which will be described later.

[0025] The process cartridge 50 contains toners of black, cyan, magenta, and yellow, respectively, and is indicated by the symbols 50K, 50C, 50M, and 50Y. These cartridges are arranged in the order of 50K, 50C, 50M, and 50Y from downstream to upstream in the conveying direction of the sheet S. The process cartridge 50 includes a photoreceptor drum 51, a charger 52, a developer roller 53, a supply roller 54, and a toner storage chamber 55. Multiple photoreceptor drums 51, chargers 52, developer rollers 53, supply rollers 54, and toner storage chambers 55 are arranged in the conveying direction of the sheet.

[0026] The charger 52 charges the surface of the photoreceptor drum 51. The developing roller 53 supplies toner to the photoreceptor drum 51. The supply roller 54 supplies toner from the toner storage chamber 55 to the developing roller 53. The toner storage chamber 55 stores the toner.

[0027] The belt unit 70 is an example of a transfer device. The belt unit 70 transfers toner from the photoreceptor drum 51 to the sheet S. The belt unit 70 comprises a drive roller 71, a driven roller 72, a belt 73, and four transfer rollers 74.

[0028] The drive roller 71 and the driven roller 72 are rollers that rotate the belt 73. The drive roller 71 and the driven roller 72 are in contact with the inner circumferential surface of the belt 73.

[0029] The belt 73 conveys the sheet S together with each photoreceptor drum 51. The outer surface of the belt 73 is in contact with each photoreceptor drum 51. The transfer roller 74 forms a transfer nip by sandwiching the belt 73 between itself and the photoreceptor drum 51.

[0030] The fixing device 80 includes a heating roller 81 and a pressure roller 82. The heating roller 81 has a halogen heater 81A inside. The pressure roller 82 sandwiches the sheet S between itself and the heating roller 81.

[0031] In the image forming unit 30, first, the surface of the photoreceptor drum 51 is charged by the charger 52 and then exposed by the LED unit 40. This forms an electrostatic latent image on the photoreceptor drum 51. Subsequently, toner is supplied to the electrostatic latent image from the developing roller 53, forming a toner image on the photoreceptor drum 51.

[0032] Next, the sheet S supplied onto the belt 73 passes between the photoreceptor drum 51 and the transfer roller 74, transferring the toner image formed on the photoreceptor drum 51 onto the sheet S. Then, the sheet S passes between the heating roller 81 and the pressure roller 82, thermally fixing the toner image transferred onto the sheet S.

[0033] The transport unit 90 is configured to transport the sheet S discharged from the fixing device 80 to the outside of the main housing 10, or back towards the image forming unit 30. The transport unit 90 includes a discharge path 91, a re-transport path 92, a first transport roller 93, a switchback roller 94, a discharge roller 95, and a plurality of re-transport rollers 96.

[0034] The image forming apparatus 1 is equipped with a re-transport device, which allows for double-sided printing by reversing the front and back sides of the sheet S. The re-transport path 92, switchback roller 94, discharge roller 95, and multiple re-transport rollers 96 are examples of the re-transport device.

[0035] The discharge path 91 is a path that guides the sheet S discharged from the fixing device 80 toward the discharge tray 11 outside the main housing 10. The discharge path 91 extends upward from the outlet of the fixing device 80, then curves forward and extends toward the discharge tray 11.

[0036] The re-transport path 92 is a path that guides the sheet S, whose first surface has been image-formed, back towards the image-forming unit 30. The re-transport path 92 branches off from the discharge path 91, extends downward, then curves forward, then extends forward passing under the supply tray 21, curves upward at the front of the supply tray 21, and extends towards the sheet supply mechanism 22.

[0037] The first transport roller 93 is provided on the fixing device 80. The first transport roller 93 transports the sheet S on which the toner image has been heat-fixed toward the switchback roller 94.

[0038] The switchback roller 94 is located at the point where the discharge path 91 and the re-transport path 92 diverge. The switchback roller 94 and the discharge roller 95 are rollers that can rotate in both forward and reverse directions. When rotating in the forward direction, the switchback roller 94 and the discharge roller 95 transport the sheet S guided in the discharge path 91 toward the discharge tray 11 outside the main housing 10. When rotating in the reverse direction, the switchback roller 94 and the discharge roller 95 transport the sheet S located in the discharge path 91 toward the re-transport path 92.

[0039] Multiple re-transport rollers 96 are provided along the re-transport path 92. The re-transport rollers 96 transport the sheets S within the re-transport path 92 toward the sheet supply mechanism 22.

[0040] When the image forming apparatus 1 performs double-sided printing, the first side of the sheet S is image-formed, and the sheet S discharged from the fuser 80 is drawn into the discharge path 91 by the forward rotation of the switchback roller 94 and discharge roller 95. After this, when the rear end of the sheet S reaches the vicinity of the switchback roller 94, the switchback roller 94 and discharge roller 95 are rotated in the reverse direction to transport the sheet S from the discharge path 91 to the re-transport path 92. The sheet S transported to the re-transport path 92 is transported to the sheet supply mechanism 22 by the re-transport roller 96. The sheet S transported to the sheet supply mechanism 22 is flipped over, so the second side is facing upwards. After the second side is image-formed, the sheet S is discharged from the discharge tray 11 via the discharge path 91.

[0041] As shown in Figure 2, the image forming apparatus 1 further comprises a control unit 100, an optical sensor 110, a humidity sensor 120, a temperature sensor 130, a development bias application circuit 140, and a transfer bias application circuit 150.

[0042] The control unit 100 includes a CPU (Central Processing Unit), RAM (Random Access Memory), ROM (Read Only Memory), and input / output circuits. It controls the image forming apparatus 1 by performing various calculations based on programs and data stored in the ROM, etc.

[0043] The optical sensor 110 is positioned opposite the surface of the belt 73 (see Figure 1). The optical sensor 110 detects the position and density of the toner patch, which is a detection toner image formed on the surface of the belt 73.

[0044] When the control unit 100 detects the location and density of a patch, it forms a patch on the surface of the belt 73 (not shown). The optical sensor 110 detects the brightness of the specular and diffuse reflection components in the reflected light of the "part with the patch" on the surface of the belt 73, and the brightness of the specular and diffuse reflection components in the reflected light of the "part without the patch". Based on the detection results of the optical sensor 110, the control unit 100 determines the location and density of the patch.

[0045] The control unit 100 performs density correction control to determine a reference value for the development bias so that the image to be formed has an appropriate density, based on the detection result of the optical sensor 110 and the density of the determined patch. Density correction control is performed when a predetermined number of images have been formed, when a predetermined amount of toner has been consumed, or when the humidity or temperature has changed by more than a predetermined value.

[0046] The humidity sensor 120 detects humidity. Specifically, the humidity sensor 120 is located inside the main unit housing 10 and outputs a signal corresponding to the relative humidity of the atmosphere. The signal output by the humidity sensor 120 is sent to the control unit 100.

[0047] The temperature sensor 130 detects the temperature. Specifically, the temperature sensor 130 is located inside the main housing 10 and outputs a signal corresponding to the ambient temperature. The signal output by the temperature sensor 130 is sent to the control unit 100.

[0048] The development bias application circuit 140 applies a development bias Vb to the development roller 53. The development bias Vb is a value corrected from the reference potential by density correction control, and is an absolute value. In this embodiment, a positively charged toner is used as the developer. Corresponding to the positively charged toner, the development bias application circuit 140 applies a positive voltage as a bias. The development bias Vb of the development bias application circuit 140 is controlled by the control unit 100.

[0049] The transfer bias application circuit 150 applies a transfer bias to the transfer roller 74. The transfer bias application circuit 150 is controlled by the control unit 100. The control unit 100 applies a negative voltage to the transfer roller 74 via the transfer bias application circuit 150 and performs constant current control. Constant current control is a control method that controls the flow of a predetermined transfer current between the photoreceptor drum 51 and the transfer roller 74 when the image forming area of ​​the sheet S passes between the photoreceptor drum 51 and the transfer roller 74.

[0050] Next, we will describe the operations performed by the control unit 100 when it receives a print command and forms an image. Here, we will describe the operations in which the control unit 100 sets the development bias Vb and the transfer current value TR.

[0051] The control unit 100 determines whether the received print command is a command to form an image on only one side or a command to form an image on both sides.

[0052] When printing on one side of the sheet S, that is, when forming an image only on the first side of the sheet S, the control unit 100 applies a first development bias Vb1 to the development roller 53. The control unit 100 sets Vb1 to a first reference value K1. The first reference value K1 is a value calculated by density correction control.

[0053] The control unit 100 sets the first transfer current value TR1 as a reference transfer current value TRs as a transfer bias when transferring toner to the first surface. The reference transfer current value TRs is a value corresponding to the first reference value K1. The reference transfer current value TRs is determined when the first reference value K1 is calculated by density correction control.

[0054] When printing on both sides of the sheet S, that is, when forming images on the first side and the second side opposite the first side of the sheet S, the control unit 100 applies a first development bias Vb1 to the development roller 53 when forming an image on the first side of the sheet S, and then applies a second development bias Vb2 to the development roller 53 when forming an image on the second side.

[0055] When the humidity is above a first threshold, the control unit 100 sets the first development bias Vb1 to the first reference value K1 and the second development bias Vb2 to the second reference value K2. The first development bias Vb1 for the first surface is the same value as the development bias when forming an image on only one side. The first reference value K1 may be the same as the second reference value K2, or it may be a different value, but in this embodiment it is the same value.

[0056] When printing on both sides of the sheet S, the control unit 100 sets the first transfer current value TR1 to the reference transfer current value TRs as the transfer bias when transferring toner to the first side, just as when printing on one side.

[0057] When the control unit 100 sets the second development bias Vb2 to the second reference value K2, it sets the second transfer current value TR2 to the reference transfer current value TRs as the transfer bias when transferring toner to the second surface.

[0058] When the humidity is below the first threshold, the control unit 100 sets the first development bias Vb1 to the first reference value K1 and sets the second development bias Vb2 to a value smaller than the second reference value K2. In this embodiment, when the humidity is below the first threshold, the control unit 100 sets the first development bias Vb1 to the first reference value K1 and sets the second development bias Vb2 to a value smaller than the second reference value K2 by the first correction value C1. The second development bias Vb2 can be expressed by the following formula. Vb² = K² - C²

[0059] The first correction value C1 is larger as humidity decreases, and smaller as temperature increases at the same humidity. The first correction value C1 is determined by multiplying the reference correction value Cs by the temperature correction coefficient α. The first correction value C1 can be expressed by the following formula. C1 = Cs × α

[0060] In this embodiment, the reference correction value Cs is determined from the table shown in Figure 3(a). The reference correction value Cs is larger the lower the humidity and smaller the higher the humidity. For example, the reference correction value Cs is "50" when the humidity is less than 25% and "0" when the humidity is 40% or higher.

[0061] In this embodiment, the temperature correction coefficient α is determined from the table shown in Figure 3(b). The temperature correction coefficient α is a positive number less than or equal to 1, and is larger at lower temperatures and smaller at higher temperatures. For example, the temperature correction coefficient α is "1" when the temperature is less than 15°C, and "0.2" when the temperature is 30°C or higher.

[0062] When the control unit 100 determines that the first correction value C1 is greater than or equal to the second threshold TH2, it sets the first development bias Vb1 to a value less than the first reference value K1 when forming an image on the first surface. For example, when the first correction value C1 is greater than or equal to the second threshold TH2, the first development bias Vb1 is set to a value less than the first reference value K1 by the amount of the second correction value C2. The first development bias Vb1 can be expressed by the following formula. Vb1 = K1 - C2

[0063] The second correction value C2 is determined, for example, from the table shown in Figure 4. For example, when the first correction value C1 is 50 or greater, as an example of the second threshold TH2, the second correction value C2 becomes "15". In this case, the first development bias Vb1 is 15 less than the first reference value K1. On the other hand, when the first correction value C1 is less than 50, the second correction value C2 becomes "0". In this case, the first development bias Vb1 becomes the first reference value K1.

[0064] If the control unit 100 sets the second development bias Vb2 to a value smaller than the second reference value K2, it performs a transfer bias reduction process to set the second transfer current value TR2 to a value smaller than the reference transfer current value TRs.

[0065] Specifically, in the transfer bias reduction process, the control unit 100 calculates the second transfer current value TR2 by multiplying the reference transfer current value TRs by the ratio of the second development bias Vb2 to the first development bias Vb1. The second transfer current value TR2 can be expressed by the following formula. TR2 = TRs × Vb2 / Vb1

[0066] Next, an example of an operation performed by the control unit 100 will be described according to the flowchart shown in Figure 5.

[0067] As shown in Figure 5, the control unit 100 determines whether to form an image on both sides based on the received print command (S1).

[0068] In step S1, if the control unit 100 does not determine that an image should be formed on both sides (S1, No), it is sufficient to print only on the first side of the sheet S, so it sets the first development bias Vb1 to the first reference value K1 (S11).

[0069] After step S11, the control unit 100 sets the first transfer current value TR1 to the reference transfer current value TRs (S12).

[0070] In step S1, if the control unit 100 determines that images should be formed on both sides (S1, Yes), it obtains humidity from the humidity sensor 120 and temperature from the temperature sensor 130 to determine the first correction value C1 of the second development bias Vb2 (S2). The first correction value C1 is calculated by multiplying the reference correction value Cs by the temperature correction coefficient α. The reference correction value Cs is determined according to the humidity obtained by referring to the table in Figure 3(a). The temperature correction coefficient α is determined according to the temperature obtained by referring to the table in Figure 3(b).

[0071] After step S2, the control unit 100 sets the second development bias Vb2 (S3). The second development bias Vb2 is set to a value that is less than the second reference value K2 by the first correction value C1.

[0072] After step S3, the control unit 100 determines whether the first correction value C1 is greater than or equal to the second threshold TH2 (S4).

[0073] In step S4, if the control unit 100 does not determine that the first correction value C1 is greater than or equal to the second threshold TH2 (S4, No), it sets the first development bias Vb1 to the first reference value K1 (S13).

[0074] After step S13, the control unit 100 sets the first transfer current value TR1 to the reference transfer current value TRs (S14).

[0075] After step S14, the control unit 100 sets the second transfer current value TR2 to the reference transfer current value TRs (S15).

[0076] In step S4, if the control unit 100 determines that the first correction value C1 is greater than or equal to the second threshold TH2 (S4, Yes), it sets the first development bias Vb1 to a value less than the first reference value K1 by the second correction value C2 (S5).

[0077] After step S5, the control unit 100 sets the first transfer current value TR1 to the reference transfer current value TRs (S6).

[0078] After step S6, the control unit 100 sets the second transfer current value TR2 (S7). The second transfer current value TR2 is calculated by multiplying the reference transfer current value TRs by the value obtained by dividing the second development bias Vb2 by the first development bias Vb1.

[0079] After step S7, step S12, or step S15, the control unit 100 terminates the process and starts image formation.

[0080] Based on the above, the following effects can be obtained with the image forming apparatus 1. In low-humidity environments, excessive discharge is likely to occur at the transfer nip during image formation on the second side of sheet S in double-sided printing, leading to transfer defects such as some toner not being transferred. In this embodiment, when the humidity is below the first threshold, the second development bias Vb2 for image formation on the second side is set to a value smaller than the second reference value K2 by the first correction value C1. As a result, the amount of toner on the photoreceptor drum 51 decreases, and the amount of charge on the toner on the photoreceptor drum 51 decreases. This suppresses the occurrence of excessive discharge at the transfer nip, thereby suppressing transfer defects. Furthermore, at high temperatures, even if the humidity detected by the humidity sensor 120 is the same, the amount of moisture in the atmosphere is greater, thus suppressing the occurrence of excessive discharge in the transfer nip. In this embodiment, the first correction value C1 is set to be larger as humidity decreases and smaller as temperature increases at the same humidity. Therefore, at high temperatures where it is not necessary to reduce the second development bias Vb2, the second development bias Vb2 does not become unnecessarily small. As a result, a decrease in print density can be suppressed.

[0081] Furthermore, if the second correction value C2 is greater than or equal to the second threshold TH2, the difference in development bias between the first and second sides during image formation can be reduced by making the first development bias Vb1 of the first side of double-sided printing smaller than the first reference value K1. By reducing the difference in development bias between the first and second sides during image formation, the difference in print density between the first and second sides can be suppressed.

[0082] Furthermore, in double-sided printing, the sheet S is heat-fixed in the fixing device 80 when the image is formed on the first side, so the impedance may change depending on the material of the sheet S. However, by performing constant current control in which the control unit 100 controls the transfer bias application circuit 150 so that a predetermined transfer current flows, it is possible to suppress a large difference in transfer capability between the first and second sides in double-sided printing, even if the impedance of the sheet S changes between the first and second sides.

[0083] Furthermore, if the second development bias Vb2 is set to a value smaller than the second reference value K2 when forming an image on the second surface, the amount of toner on the photoreceptor drum 51 decreases, which can lead to excessive transfer current. However, when the second development bias Vb2 is set to a value smaller than the second reference value K2, the control unit 100 performs a transfer bias reduction process, which suppresses excessive transfer current and thus prevents transfer defects.

[0084] The present invention is not limited to the embodiments described above, and can be used in various forms as illustrated below.

[0085] In the embodiment described above, the first correction value C1 was determined by multiplying the reference correction value Cs by the temperature correction coefficient α. However, the control unit 100 may obtain the first correction value C1 by referring to a table. For example, the control unit 100 stores the table shown in Figure 6. The control unit 100 may obtain the first correction value C1 by referring to the table. The table in Figure 6 is a table that associates temperature and humidity with the first correction value C1. Similar to the embodiment described above, the first correction value C1 is set to be larger as humidity decreases and smaller as temperature increases at the same humidity. The control unit 100 obtains the first correction value C1 from the table in Figure 6.

[0086] In the embodiment described above, the reference correction value Cs was determined by one table in Figure 3(a), but as shown in Figures 7 to 9, a configuration may be used in which multiple tables are stored and multiple tables can be selected. For example, the control unit 100 may store multiple tables and determine the reference correction value Cs, the second correction value C2, the correction coefficient for the first transfer current value TR1, and the correction coefficient for the second transfer current value TR2 based on the table corresponding to the settings from among the multiple tables.

[0087] Specifically, the control unit 100 stores five tables. These five tables are the first mode table shown in Figure 7(a), the second mode table shown in Figure 7(b), the third mode table shown in Figure 8(a), the fourth mode table shown in Figure 8(b), and the fifth mode table shown in Figure 9. The control unit 100 can selectively execute each mode according to the settings. The selection of each mode can be entered by a user or service technician from the operation panel (not shown) of the main unit housing 10, or it can be entered from an external terminal via a printer driver or application.

[0088] As shown in Figures 7 to 9, each table has a standard correction value Cs, a second correction value C2, a correction coefficient β for the first transfer current value TR1, and a correction coefficient γ for the second transfer current value TR2, all set according to humidity. The first correction value C1 is calculated by multiplying the reference correction value Cs determined in each table by the temperature correction coefficient α shown in Figure 3(b). Similar to the embodiment described above, the first correction value C1, the second development bias Vb2, and the first development bias Vb1 can be expressed by the following equations. C1 = Cs × α Vb² = K² - C² Vb1 = K1 - C2

[0089] The first transcription current value TR1 is calculated by multiplying the reference transcription current value TRs by the correction coefficient β of TR1. The first transcription current value TR1 can be expressed by the following formula. TR1 = TRs × β The correction coefficient β for the first transcription current value TR1 is a positive number less than or equal to 1, and when multiplied by the reference transcription current value TRs, it is a coefficient that reduces the first transcription current value TR1. The amount of reduction D1 in the first transcription current value TR1 due to the correction can be expressed by the following equation. D1 = TRs × (1 - β)

[0090] The second transcription current value TR2 is calculated by multiplying the reference transcription current value TRs by the correction coefficient γ of TR2. The first transcription current value TR1 can be expressed by the following formula. TR1 = TRs × γ The correction coefficient γ for the second transcription current value TR2 is a positive number less than or equal to 1, and multiplying it by the reference transcription current value TRs reduces the second transcription current value TR2. The amount of reduction D2 in the second transcription current value TR2 due to the correction can be expressed by the following equation. D2 = TRs × (1 - γ)

[0091] The correction coefficient β for the first transfer current value TR1 and the correction coefficient γ for the second transfer current value TR2 are set so that the reduction amounts D1 and D2 that reduce the transfer current decrease decrease as humidity increases. In this way, in the transfer bias reduction process, by reducing the reduction amounts D1 and D2 that reduce the transfer current decrease as humidity increases, it is possible to secure the transfer current necessary to maintain print density in environments that are not excessively low in humidity.

[0092] The first mode is a mode in which image formation is performed using the second development bias as the second reference value, regardless of temperature and humidity. The first mode is a mode in which image formation is always performed without performing a transfer bias reduction process. Specifically, in the first mode, the reference correction value Cs is always 0 regardless of temperature and humidity, so the first correction value C1 is always 0. Also, in the first mode, the second correction value C2 is always 0 regardless of temperature and humidity. Furthermore, in the first mode, the correction coefficient β for TR1 and the correction coefficient γ for TR2 are always 1.00 regardless of temperature and humidity, so the reduction amounts D1 and D2 that reduce the transfer current are always 0.

[0093] Modes 2 through 5 are modes in which the second development bias Vb2 is made smaller than the second reference value K2 depending on the temperature and humidity to form an image. Modes 2 through 5 are modes in which the transfer bias reduction process is performed to form an image.

[0094] The third mode is a table of standard setting values. The second mode is set to a smaller reference correction value Cs for the same humidity than the third mode. For example, if the table for the third mode is called the first table and the table for the second mode is called the second table, then in the first table, the reference correction value Cs corresponding to the first humidity (e.g., 0 ≤ H < 15) is the first predetermined value (50), whereas in the second table, the reference correction value Cs corresponding to the first humidity (0 ≤ H < 15) is the second predetermined value (30), which is smaller than the first predetermined value (50). The control unit 100 is configured to selectively refer to either the first table or the second table to obtain the reference correction value Cs, depending on the setting.

[0095] Mode 4 is set to a higher standard correction value Cs at the same humidity than Mode 3. Mode 5 is set to a higher standard correction value Cs at the same humidity than Mode 4.

[0096] In each mode, the second correction value C2 is set based on the reference correction value Cs. For example, when the reference correction value Cs is "less than 50", the second correction value C2 is set to "0". When the reference correction value Cs is "50", the second correction value C2 is set to "15". When the reference correction value Cs is "75", the second correction value C2 is set to "40". When the reference correction value Cs is "100", the second correction value C2 is set to "50".

[0097] In modes 3 through 5, the correction coefficient γ is set to reduce the second transfer current value TR2 when the humidity is below a predetermined threshold. The correction coefficient γ for the second transfer current value TR2 is set to a larger value as the humidity increases. As a result, in modes 3 through 5, the amount of reduction D2 in the second transfer current value TR2 decreases as the humidity increases. In the transfer bias reduction process, by reducing the reduction amounts D1 and D2 that reduce the transfer current as the humidity increases, it is possible to secure the transfer current necessary to maintain print density in environments that are not excessively low in humidity.

[0098] In the fifth mode, a correction coefficient β is set to reduce the first transfer current value TR1 when the humidity is lower than a predetermined threshold. The correction coefficient β for the first transfer current value TR1 is set to a larger value as the humidity increases. As a result, in the fifth mode, the amount of decrease D1 of the first transfer current value TR1 decreases as the humidity increases. In the transfer bias reduction process, by reducing the amount of decrease D1 and D2 that reduces the transfer current as the humidity increases, it is possible to secure the transfer current necessary to maintain print density in environments that are not excessively low in humidity.

[0099] In this way, by storing multiple tables that refer to reference correction values ​​such as Cs, image formation can be performed according to the user's selection. For example, if you do not want to lower the print density regardless of temperature and humidity, you can select mode 1 or mode 2. On the other hand, if you want to reduce transfer defects due to excessive discharge, you can select modes 3 to 5.

[0100] In the embodiment described above, when the second development bias Vb2 was set to the second reference value K2, the first transfer current value TR1 and the second transfer current value TR2 were set to the same value. However, the first transfer current value TR1 and the second transfer current value TR2 may be set to different values.

[0101] In the embodiment described above, a positive voltage was applied as the development bias in accordance with the positively charged toner. However, a negative voltage may be applied as the development bias when a negatively charged toner is used.

[0102] In the above-described embodiment, a belt unit 70 was exemplified as an example of a transfer device, but the transfer device may also be a transfer roller that directly sandwiches the sheet between itself and the photoreceptor drum.

[0103] In the embodiments described above, the image forming apparatus 1 was a color printer, but it may also be a monochrome printer. Furthermore, the present invention may be applied to copiers, multifunction printers, and the like.

[0104] The elements described in the above-mentioned embodiments and modifications may be implemented in any combination. [Explanation of symbols]

[0105] 1. Image forming apparatus 53 Developing roller 74 Transfer Roller 100 Control Unit 120 Humidity Sensor 130 Temperature Sensor 140 Developing bias application circuit 150 Transfer bias application circuit C1 First Correction Value C2 Second Correction Value K1 First Reference Value K2 Second Reference Value Vb1 First development bias Vb2 Second development bias

Claims

1. An image forming apparatus capable of performing double-sided printing by reversing the front and back sides of a sheet, Photosensitive drum and A developing roller that supplies toner to the photosensitive drum, A developing bias application circuit for applying a developing bias to the developing roller, A transfer device for transferring toner from the photosensitive drum to a sheet, A transfer bias application circuit for applying a transfer bias to the transfer apparatus, A fixing device that fixes toner to the sheet, A retransport device that retransports the sheet that has passed through the fixing device to the transfer device, It includes a control unit, The control unit, When forming an image on both sides of a sheet, a first development bias is applied to the developing roller when forming an image on the first side of the sheet, and then a second development bias is applied to the developing roller when forming an image on the second side opposite to the first side. When the second development bias is set to the second reference value, the reference transfer current value is set as the transfer bias when transferring toner to the second surface. An image forming apparatus characterized in that, when the second development bias is set to a value smaller than the second reference value, a transfer bias reduction process is performed to set the transfer bias when transferring toner to the second surface to a value smaller than the reference transfer current value.

2. It also features a humidity sensor to detect humidity, The image forming apparatus according to claim 1, characterized in that the control unit reduces the amount by which the transfer current is reduced in the transfer bias reduction process as humidity increases.

3. It also features a humidity sensor to detect humidity, The control unit, When the humidity is above the first threshold, the first development bias is set to the first reference value, and the second development bias is set to the second reference value. When the humidity is less than the first threshold, the first development bias is set to the first reference value, and the second development bias is set to a value less than the second reference value by the first correction value. The image forming apparatus according to claim 1, characterized in that the first correction value is set to a larger value as the humidity decreases.

4. The image forming apparatus according to claim 3, characterized in that the first reference value is the same as the second reference value.

5. The image forming apparatus according to claim 3, characterized in that when the control unit is greater than or equal to the second threshold value, it sets the first development bias to a value smaller than the first reference value when forming an image on the first surface.

6. The image forming apparatus according to claim 1, characterized in that the control unit controls the transfer bias application circuit so that a predetermined transfer current flows when the image forming region of the sheet passes between the photoreceptor drum and the transfer apparatus.

7. Multiple photosensitive drums are arranged in the direction of sheet transport. The transfer apparatus is, A belt that contacts multiple photoreceptor drums and transports the sheet together with the photoreceptor drums, The image forming apparatus according to claim 1, further comprising a transfer roller that sandwiches the belt between itself and the photosensitive drum.

8. The control unit, A mode in which the aforementioned transfer bias reduction process is performed to form an image, A mode in which an image is formed without performing the aforementioned transfer bias reduction process, The image forming apparatus according to claim 1, characterized in that it can be selectively executed according to the settings.

Images