Image forming apparatus

Abstract

To prevent toner fogging while preventing density unevenness.SOLUTION: When voltage having the same polarity as the charging polarity of toner in AC voltage with respect to DC voltage is defined as Vgo, voltage having the opposite polarity to the charging polarity of toner with respect to the DC voltage as Vre, and the peak-to-peak voltage of the AC voltage as Vgo+Vre, a developing bias application unit applies a developing bias to a developer carrier so that the peak-to-peak voltage when humidity detected by a humidity sensor is first humidity is smaller than the peak-to-peak voltage when humidity detected by the humidity sensor is second humidity lower than the first humidity, and Vgo when humidity detected by the humidity sensor is the second humidity is the same as Vgo when humidity detected by the humidity sensor is the first humidity.SELECTED DRAWING: Figure 13

Description

[Technical field]

[0001] The present invention relates to an image forming apparatus that applies a developing bias, which is a superimposed DC voltage and AC voltage, to a developer carrier in order to develop an electrostatic latent image formed on the image carrier with a developer containing toner. [Background technology]

[0002] In an image forming apparatus that forms a toner image on a recording material, a configuration is known in which, when an electrostatic latent image formed on an image carrier is developed with toner, a developing bias in the form of a superimposed DC voltage and AC voltage is applied to a developer carrier carrying a developer.

[0003] Patent document 1 proposes a configuration for such a developing bias in which the duty ratio of the AC component that has the opposite polarity (positive polarity) to the normal charging polarity (negative polarity) of the toner is increased to 50% or more, thereby achieving excellent image graininess.

[0004] In recent years, electrophotographic image forming apparatuses have begun to spread in the printing industry, and the demand for high-speed output and high image quality is rapidly increasing. Among the demands for high image quality, there is a strong demand for uniform image density within a page, and it is important to minimize uneven image density within a page.

[0005] There are various factors that cause such density unevenness. One of the factors is that the development gap fluctuates due to insufficient processing precision, such as eccentricity, in the photosensitive drum or the developing sleeve.

[0006] The image density unevenness in the image process direction (hereinafter referred to as sub-scanning density unevenness) as described above occurs significantly when the toner charge amount is high, particularly in a low humidity environment, etc. Therefore, Patent Document 2 proposes a technology to reduce image density unevenness regardless of the toner charge amount by changing the duty ratio and Vpp of the development bias based on the patch detection density on the intermediate transfer body. [Prior art documents] [Patent documents]

[0007] [Patent Document 1] JP 2016-11981 A [Patent Document 2] JP 2016-21043 A Summary of the Invention [Problem to be solved by the invention]

[0008] However, if the peak-to-peak voltage Vpp of the AC voltage component of the developing bias is increased in a low-humidity environment where the toner charge amount is large in order to improve sub-scanning density unevenness, the potential difference Vgo between the development-side potential and the DC voltage component of the AC voltage component of the developing bias will become large, and there is a risk of toner fogging occurring.

[0009] The present invention has been made in view of the above problems, and an object of the present invention is to provide an image forming apparatus capable of suppressing toner fogging while suppressing density unevenness. [Means for solving the problem]

[0010] In order to achieve the above object, an image forming apparatus according to one aspect of the present invention has the following configuration: That is, the image forming apparatus includes an image carrier, a charging unit that charges the surface of the image carrier to a predetermined charging potential, a latent image forming unit that forms an electrostatic latent image on the surface of the image carrier charged to the predetermined charging potential by the charging unit, a developer carrier that carries a developer containing toner, a developing bias application unit that applies a developing bias in which a DC voltage and an AC voltage are superimposed to the developer carrier in order to develop the electrostatic latent image formed on the image carrier by the latent image forming unit with the developer carried on the developer carrier, and a humidity sensor that detects humidity, wherein a voltage of the AC voltage having the same polarity as the charging polarity of the toner with respect to the DC voltage is defined as Vgo, and among the AC voltages, the DC voltage is defined as Vc, and among the AC voltages, the DC voltage is defined as Vdc. the voltage on the side of the polarity opposite to the toner charge polarity is Vre, and the peak-to-peak voltage of the AC voltage is Vgo+Vre, the developing bias application unit applies the developing bias to the developer carrier so that the peak-to-peak voltage when the humidity detected by the humidity sensor is a second humidity lower than the first humidity is greater than the peak-to-peak voltage when the humidity detected by the humidity sensor is a first humidity, and so that Vgo when the humidity detected by the humidity sensor is the second humidity is the same as Vgo when the humidity detected by the humidity sensor is the first humidity.

[0011] In order to achieve the above object, an image forming apparatus according to another aspect of the present invention has the following configuration: That is, the image forming apparatus includes an image carrier, a charging unit that charges the surface of the image carrier to a predetermined charging potential, a latent image forming unit that forms an electrostatic latent image on the surface of the image carrier charged to the predetermined charging potential by the charging unit, a developer carrier that carries a developer containing toner, a developing bias application unit that applies a developing bias in which a DC voltage and an AC voltage are superimposed to the developer carrier in order to develop the electrostatic latent image formed on the image carrier by the latent image forming unit with the developer carried on the developer carrier, and a humidity sensor that detects humidity, wherein, of the AC voltages, a voltage on the same polarity as the charging polarity of the toner with respect to the DC voltage is defined as Vgo, and, of the AC voltages, a voltage on the same polarity as the charging polarity of the toner with respect to the DC voltage is defined as Vc, and, of the AC voltages, a voltage on the same polarity as the charging polarity of the toner with respect to the DC voltage is defined as Vd. When a voltage on the polarity and opposite polarity side is Vre and a duty ratio in one cycle of the AC voltage is Vgo / (Vgo+Vre), the developing bias application unit applies the developing bias to the developer carrier so that the peak-to-peak voltage when the humidity detected by the humidity sensor is a second humidity lower than the first humidity is greater than the peak-to-peak voltage when the humidity detected by the humidity sensor is a first humidity, and so that the duty ratio when the humidity detected by the humidity sensor is the second humidity is smaller than the duty ratio when the humidity detected by the humidity sensor is the first humidity. Effect of the Invention

[0012] According to the present invention, it is possible to suppress toner fogging while suppressing density unevenness. [Brief description of the drawings]

[0013] [Figure 1] 1 is a schematic configuration diagram of an electrophotographic image forming apparatus according to a first embodiment of the present invention. [Diagram 2] 2 is a schematic cross-sectional view of the configuration of the developing device and the photosensitive drum of FIG. 1 according to the first embodiment of the present invention. [Diagram 3]2 is a plan view of the developing device of FIG. 1 according to the first embodiment of the present invention, with a portion thereof omitted. [Figure 4] 3 is a graph showing a change over time in a developing bias applied to the developing sleeve of FIG. 2 according to the first embodiment of the present invention. [Diagram 5] FIG. 4 is an explanatory diagram showing the direction of electrostatic force acting on the toner and carrier when a developing bias is applied according to the first embodiment of the present invention. [Figure 6] 4 is a graph showing the relationship between periodic density unevenness and toner charge amount according to the first embodiment of the present invention. [Figure 7] 4 is a graph showing the relationship between the toner charge amount and environmental humidity according to the first embodiment of the present invention. [Figure 8] 5 is a graph showing the relationship between the peak-to-peak voltage of the developing bias and periodic density unevenness according to the first embodiment of the present invention. [Figure 9] 5 is a graph showing the relationship between fog and the potential difference between the development side potential and the DC voltage component of the AC voltage component of the developing bias according to the first embodiment of the present invention. [Figure 10] 5 is a graph showing a set voltage and a set duty ratio of a developing bias with respect to environmental humidity according to the first embodiment of the present invention. [Figure 11] 5 is a table showing the configuration of the developing bias according to the first embodiment of the present invention and the effects of the configuration on periodic density unevenness and fogging. [Figure 12] FIG. 1 is a control block diagram of an image forming apparatus according to a first embodiment of the present invention. [Figure 13] 13 is a flowchart of the CPU in FIG. 12 showing a developing bias determination flow according to the first embodiment of the present invention. [Figure 14] 10 is a graph showing the relationship between Vback and carrier adhesion for different duties of the developing bias according to the second embodiment of the present invention. [Figure 15] 10 is a graph showing a set voltage and a set duty ratio of a developing bias with respect to environmental humidity according to a second embodiment of the present invention. [Figure 16]13 is a table showing the configuration of a developing bias according to a second embodiment of the present invention and the effects of the configuration on periodic density unevenness, fogging, and the number of adhered carriers. [Figure 17] 13 is a graph showing a change in density for each tone when the developing bias setting is changed according to a third embodiment of the present invention. [Figure 18] 13 is a graph showing a setting duty ratio of a developing bias with respect to environmental humidity according to a third embodiment of the present invention. [Figure 19] 13 is a graph showing a change in density of an image on a recording material when a developing bias setting is changed according to a third embodiment of the present invention. [Figure 20] 13 is a flowchart of the CPU in FIG. 12 according to the fourth embodiment of the present invention. [Figure 21] 13 is a graph showing the timing of changing the developing bias setting and image density adjustment control according to the fourth embodiment of the present invention. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0014] Hereinafter, preferred embodiments of the present invention will be described in detail with reference to the drawings. Note that the components described in the following embodiments are merely examples, and various conditions such as the configuration, function, dimensions, materials, shape, and relative arrangement of the device to which the present invention is applied can be appropriately modified or changed without departing from the spirit of the present invention, and the present invention is not limited to the following embodiments.

[0015] [First embodiment] FIG. 1 is a schematic diagram of an electrophotographic image forming apparatus according to a first embodiment of the present invention.

[0016] In Fig. 1, the image forming apparatus 200 is an electrophotographic full-color printer having four image forming units PY, PM, PC, and PK provided corresponding to the four colors of yellow, magenta, cyan, and black. In this embodiment, the image forming units PY, PM, PC, and PK are arranged in tandem along the rotation direction of an intermediate transfer belt 10 described later. The image forming apparatus 200 forms a toner image (image) on a recording material in response to an image signal from a document reading device (not shown) connected to the image forming apparatus main body or a host device such as a personal computer connected to the image forming apparatus main body so as to be able to communicate with the image forming apparatus main body. As the recording material, a sheet material such as paper, plastic film, or cloth is used.

[0017] To give an overview of the image forming process of this image forming apparatus 200, first, each image forming station PY, PM, PC, and PK forms a toner image of each color on the photosensitive drum 13Y, 13M, 13C, and 13K, respectively. The toner image of each color thus formed is transferred onto the intermediate transfer belt 10, and then transferred from the intermediate transfer belt 10 onto a recording material. The recording material onto which the toner image has been transferred is transported to a fixing device 11, where the toner image is fixed onto the recording material.

[0018] The configuration of the image forming apparatus 200 will be described in detail below. The four image forming units PY, PM, PC, and PK included in the image forming apparatus 200 have substantially the same configuration except for the difference in the developing color. Therefore, the image forming unit PY will be described below as a representative, and the configurations of the other image forming units will be shown by replacing the suffix "Y" of the reference numerals attached to the components of the image forming unit PY with M, C, and K, respectively, and the description thereof will be omitted.

[0019] In the image forming section PY, a cylindrical photosensitive member, i.e., a photosensitive drum 13Y, is disposed as an image carrier. Around the photosensitive drum 13Y, a charging roller 12Y as a charging device (charging section), a developing device 1Y as a developer container, a primary transfer roller 17Y, and a cleaning device 15Y are disposed. Below the photosensitive drum 13Y in the figure, a laser scanner 14Y as an exposure device (latent image forming section) is disposed.

[0020] The charging roller 12Y rotates in accordance with the photosensitive drum 13Y during image formation. The charging roller 12Y is biased toward the photosensitive drum 13Y by a pressure spring (not shown). A charging bias is applied to the charging roller 12Y from a high-voltage power supply. As a result, the photosensitive drum 13Y is almost uniformly charged by the charging roller 12Y.

[0021] An intermediate transfer belt 10 is disposed opposite the photosensitive drums 13Y, 13M, 13C, and 13K.

[0022] Primary transfer rollers 17Y, 17M, 17C, and 17K are arranged as primary transfer members at positions facing the photosensitive drums 13Y, 13M, 13C, and 13K across the intermediate transfer belt 10, forming primary transfer sections T1Y, T1M, T1C, and T1K that transfer the toner images formed on the photosensitive drums 13Y, 13M, 13C, and 13K to the intermediate transfer belt 10.

[0023] The intermediate transfer belt 10 is tensioned by a plurality of tension rollers and rotates by being driven by a drive roller, which is one of the tension rollers. A secondary transfer outer roller 16 is disposed as a secondary transfer member at a position facing an inner secondary transfer roller 18 of the tension rollers across the intermediate transfer belt 10, forming a secondary transfer portion T2 that transfers a toner image on the intermediate transfer belt 10 to a recording material. A fixing device 11 is disposed downstream of the secondary transfer portion T2 in the recording material transport direction.

[0024] A feeding section (not shown) is disposed below the image forming apparatus 200, and the recording material fed from the feeding section when the image forming operation starts is transported to the secondary transfer section T2 at a predetermined timing.

[0025] Next, a process of forming an image by the image forming apparatus 200 configured as described above will be described.

[0026] First, when the image forming operation is started, the surface of the rotating photosensitive drum 13Y is uniformly charged by the charging roller 12Y. Next, the photosensitive drum 13Y is exposed to a laser beam corresponding to an image signal emitted from the exposure device 14Y. As a result, an electrostatic latent image corresponding to the image signal is formed on the photosensitive drum 13Y. The electrostatic latent image on the photosensitive drum 13Y as an electrostatic latent image carrier is visualized by the toner contained in the developing device 1Y, becoming a visible image (toner image).

[0027] The toner image formed on the photosensitive drum 13Y is primarily transferred to the intermediate transfer belt 10 at the primary transfer portion T1Y. Toner remaining on the surface of the photosensitive drum 13Y after the primary transfer (transfer residual toner) is removed by the cleaning device 15Y. This operation is performed in sequence at each of the magenta, cyan, and black image forming portions, and the four color toner images are superimposed on the intermediate transfer belt 10.

[0028] Thereafter, in accordance with the timing of forming the toner image, the recording material stored in a recording material storage cassette (not shown) of the feeding section is conveyed to the secondary transfer section T2, and the four color toner images on the intermediate transfer belt 10 are secondarily transferred collectively onto the recording material. Any toner that was not transferred completely at the secondary transfer section T2 and remains on the intermediate transfer belt 10 is removed by an intermediate transfer belt cleaner 19.

[0029] Next, the recording material is conveyed to the fixing device 11. The fixing device 11 includes a fixing roller 20 having a heat source such as a halogen heater inside, and a pressure roller 21, and a fixing nip portion is formed by the fixing roller 20 and the pressure roller 21. The recording material conveyed to the fixing device 11 passes through the fixing nip portion, whereby the toner image is fixed onto the recording material, and the recording material is discharged outside the machine.

[0030] This completes a series of image forming processes. It is also possible to form a single-color image or a multi-color image of desired colors by using only desired image forming units.

[0031] Here, the two-component developer used in this embodiment will be described. The developer is a mixture of a non-magnetic toner with a negative charge polarity and a magnetic carrier with a positive charge polarity. That is, in this embodiment, the charge polarity of the normally charged toner is negative. The non-magnetic toner is a resin such as polyester or styrene acrylic that contains colorants, wax components, etc., and is pulverized or polymerized to form a powder, to which fine powders such as titanium oxide and silica are added on the surface. The magnetic carrier is a core made of resin particles kneaded with ferrite particles or magnetic powder, and a resin coating is applied to the surface layer.

[0032] Next, the detailed configuration of the developing device 1Y will be described. The developing devices 1M, 1C, and 1K have the same configuration.

[0033] FIG. 2 is a schematic cross-sectional view of the developing device 1Y and the photosensitive drum 13Y in FIG. 1, and FIG. 3 is a plan view of the developing device 1Y in FIG. 1 with a portion thereof omitted.

[0034] 2 and 3, the developing device 1Y has a developing container 2 that contains a developer consisting of a magnetic carrier and a non-magnetic toner, and a developing sleeve 54 as a developer carrier that carries and transports the developer in the developing container. The developing sleeve 54 has a surface that is rotatably supported, while a magnet roll 54a consisting of multiple magnetic poles (S1, S2, S3, N1, N2) is non-rotatably arranged inside.

[0035] The developing container 2 is divided by a partition wall 51 into a first transport path (mixing chamber) 52 as a first chamber and a second transport path (developing chamber) 53 as a second chamber, and the first transport path 52 and the second transport path 53 communicate with each other through communication ports at both ends. As a result, the first transport path 52 and the second transport path 53 form a circulation path for the developer.

[0036] The developing container 2 is provided with two screw members as transport members for transporting the developer while stirring it. That is, the first transport path 52 is provided with a first transport screw (first transport section) 58, and the second transport path 53 is provided with a second transport screw (second transport section) 59. The first and second transport screws 58 and 59 have rotation shafts 58a and 59a, respectively, and blades 58b and 59b spirally provided around (on) the rotation shafts 58a and 59a.

[0037] When the first conveying screw 58 rotates around the rotation shaft 58a, the spiral blade 58b conveys the developer in the first conveying path 52 in the direction of the arrow α (first direction) which is one side of the longitudinal direction of the developing device 1Y (axial direction of the rotation shaft 58a). When the second conveying screw 59 rotates around the rotation shaft 59a, the spiral blade 59b conveys the developer in the second conveying path 53 in the direction of the arrow β (second direction) which is the other side of the longitudinal direction of the developing device 1Y (axial direction of the rotation shaft 58a). This causes the developer to circulate between the first conveying path 52 and the second conveying path 53.

[0038] The developer in the second transport path 53 is pumped up into the magnetic force range of the S2 pole by a second transport screw 59 installed below the developing sleeve 54 inside the second transport path 53, and is carried on the surface of the developing sleeve 54. The carried developer is carried as the surface of the developing sleeve 54 rotates. A regulating blade 55 is disposed near the magnetic pole N1 of the developing sleeve 54, with a predetermined gap therebetween, as a member for forming a thin layer of developer. The gap between the developing sleeve 54 and the regulating blade 55 is generally set to about 200 to 500 μm, and the wider the gap, the greater the amount of developer carried on the developing sleeve 54.

[0039] The transported developer forms a magnetic brush at the N1 pole, and a desired amount of developer is formed into a thin layer on the surface of the developing sleeve 54 by a regulating blade 55 installed at a predetermined interval from the developing sleeve 54. Thereafter, the developer transported to the opposing portion of the photosensitive drum 13Y forms a magnetic brush again at the S1 pole, and a development nip is formed between the photosensitive drum 13Y and the developer.

[0040] As described above, the surface of the photosensitive drum 13Y is charged to a constant potential by the charging roller 12Y, and the image portion is exposed by the exposure device 14Y to form an exposure potential. Meanwhile, a developing bias is applied to the developing sleeve 54 through a high-voltage circuit (not shown). The developing bias applied to the developing sleeve 54 will be described in detail later.

[0041] The toner charged in the developing device 1Y receives a driving force due to the potential difference between the developing bias and the drum surface in the developing nip, and adheres to the exposed portion, completing the developing process.

[0042] The carrier and undeveloped toner are further transported downstream in the rotational direction of the developing sleeve 54, where they lose their magnetic binding force in the zero gauss zone (the area where the radial magnetic flux density is zero) formed between the S2 pole and the S3 pole, and are again collected in the second transport path 53.

[0043] At this time, the image forming apparatus 200 according to this embodiment is configured to develop the electrostatic latent image by AC development, in which a developing bias consisting of a direct current (DC) voltage, which is a direct current voltage, and an alternating current (AC) voltage, which is an alternating current (AC) voltage, is superimposed on the direct current (DC) voltage, is applied to the developing sleeve 54.

[0044] Here, the principle of AC development when the image forming apparatus 200 according to this embodiment develops the electrostatic latent image formed on the photosensitive drum 13Y will be described with reference to FIGS.

[0045] FIG. 4 is a graph showing the change over time of the developing bias applied to the developing sleeve 54 in FIG. 2. The image forming apparatus 200 according to this embodiment is configured to apply an AC voltage having a maximum voltage value of V2 (V) and a minimum voltage value of V1 (V) as a developing bias to the developing sleeve 54 so that the peak-to-peak voltage (hereinafter referred to as "Vpp"), which is the potential difference between the maximum voltage value and the minimum voltage value of the AC voltage, is V2-V1 volts (V). As described above, in this embodiment, the charge polarity of the normally charged toner is negative. Therefore, in this embodiment, the voltage value of the AC voltage on the opposite polarity side to the charge polarity of the toner with respect to the charge potential (Vd) of the photosensitive drum (i.e., the surface potential of the non-image portion, the predetermined charge potential) is the maximum voltage value (V2) of the AC voltage. Also, in this embodiment, the voltage value on the same polarity side as the charge polarity of the toner with respect to the charge potential (Vd) of the photosensitive drum is the minimum voltage value (V1) of the AC voltage.

[0046] In addition, the ratio of the application time of the maximum voltage value (V2) in one cycle of the AC voltage (hereinafter referred to as the "Duty ratio") is calculated as T2 / (T1+T2), where T1 (s) is the application time of the minimum voltage value (V1) and T2 (s) is the application time of the maximum voltage value (V2).

[0047] At this time, the potential difference (hereafter referred to as "Vgo") between the minimum voltage value (V1) of the AC voltage and the DC voltage (Vdc), the potential difference (hereafter referred to as "Vre") between the maximum voltage value (V2) of the AC voltage and the DC voltage (Vdc), the duty ratio, and Vpp satisfy the following two equations in order to offset the effect on the DC voltage even when Vpp is changed. Note that the application time (T1) of the minimum voltage value (V1) of the AC voltage corresponds to the application time of Vgo in one cycle of the AC voltage. Also, the application time (T2) of the maximum voltage value (V2) of the AC voltage corresponds to the application time of Vre in one cycle of the AC voltage.

[0048] Duty ratio = Vgo / Vpp (Formula 1) Vpp = Vgo+Vre (Equation 2) FIG. 5(A) is an explanatory diagram showing the direction of electrostatic force acting on the toner when a developing bias is applied, and FIG. 5(B) is an explanatory diagram showing the direction of electrostatic force acting on the carrier when a developing bias is applied.

[0049] As shown in FIG. 5(A), when the developing bias is at a minimum voltage value (V1) which is a development side potential (a potential having the same polarity as the toner's charging polarity with respect to the charging potential (Vd) on the surface of the photosensitive drum 13Y), the toner is subjected to an electrostatic force toward the photosensitive drum 13Y, and an electrostatic latent image is developed. On the other hand, when the developing bias is at a maximum voltage value (V2), the toner is subjected to an electrostatic force toward the developing sleeve 54, and some of the toner that had adhered to the photosensitive drum 13Y is pulled back to the developing sleeve 54.

[0050] On the other hand, as shown in FIG. 5(b), when the developing bias is at the minimum voltage value (V1), the carrier is subjected to an electrostatic force in the direction of the developing sleeve 54, whereas when the developing bias is at the maximum voltage value (V2), the carrier is subjected to an electrostatic force in the direction of the photosensitive drum 13Y.

[0051] As for the duty ratio, by reducing Vre, the force pulling back the toner adhering to the photosensitive drum 13Y is weakened, which improves the reproducibility of the shallow highlight portion of the latent image and improves roughness, so a duty ratio of 50% or more is preferable. Also, if the duty ratio is too large, there is a concern that the fogging may worsen significantly, so a duty ratio of 50% or more and 90% or less is more preferable.

[0052] Usually, the distance between the surface of the developing sleeve 54 and the surface of the photosensitive drum 13Y in the developing area (hereinafter referred to as the "developing gap") periodically varies with the rotation of the developing sleeve 54 and the photosensitive drum 13Y due to eccentricity of the developing sleeve 54 and the photosensitive drum 13Y, etc. When the developing gap varies in this way, the electric field due to the developing bias applied to the developing sleeve 54 becomes stronger when the developing gap is small, and on the other hand, becomes weaker when the developing gap is large.

[0053] Therefore, in an image forming apparatus in which the development gap fluctuates in this way, when the development gap is small, the amount of toner that moves to the electrostatic latent image increases, and the density of the image corresponding to that portion becomes dense, whereas when the development gap is large, the amount of toner that moves to the electrostatic latent image decreases, and the density of the image corresponding to that portion becomes thin. As a result, in an image forming apparatus in which the development gap fluctuates in this way, there is a problem in that periodic image density unevenness (hereinafter referred to as "periodic density unevenness") occurs in association with the fluctuation of the development gap.

[0054] Figure 6 shows the relationship between the density difference ΔD of this type of cyclical density unevenness and the toner charge amount (toner tribo) μC / g. Note that the density difference ΔD represents the difference between the maximum and minimum density when the development gap fluctuation amount is 15 μm. As shown in the graph in Figure 6, which shows the relationship between cyclical density unevenness and toner charge amount, the higher the toner charge amount, the greater the density difference ΔD becomes, and the worse the cyclical density unevenness becomes. This is because when the toner charge amount is high, the electrostatic adhesion force acting between the toner and carrier becomes large, making it difficult for the toner to separate from the carrier when the development gap is large and the electric field due to the development bias is small.

[0055] On the other hand, the toner charge amount tends to increase as the environmental humidity decreases, as shown in the graph of Fig. 7, which shows the relationship between the toner charge amount and the environmental humidity. In other words, even if the amount of fluctuation in the development gap is the same, periodic density unevenness tends to occur more easily in a low humidity environment.

[0056] As a technique for suppressing such periodic density unevenness that becomes evident in a low humidity environment, it is common to make the Vpp of the developing bias larger in a low humidity environment than in a high humidity environment.

[0057] FIG. 8 is a graph showing the relationship between the Vpp of the developing bias and the density difference ΔD of the periodic density unevenness. As shown in FIG. 8, the periodic density unevenness tends to decrease as the Vpp of the developing bias increases. This is because increasing Vpp makes it easier for the toner to separate from the carrier. In this way, since the Vpp of the developing bias contributes to the separation of the toner from the carrier, as shown in the following formulas (3) and (4), the force F1 (N) acting on the toner when Vpp is applied must be greater than the toner's adhesive force F2 to the carrier. Note that q and d respectively indicate the charge amount of the toner and the development gap.

[0058] F1>F2 (Formula 3) F1=Vpp×q / d (Formula 4) On the other hand, when the developing bias Vpp is increased, Vgo also increases accordingly, and the fogging worsens, as shown in the graph of Fig. 9, which shows the relationship between the developing bias Vgo and fogging. In other words, under conditions where the duty ratio is constant, when Vpp is increased in a low humidity environment to improve periodic density unevenness, Vgo increases accordingly, and the problem of fogging in a low humidity environment becomes apparent.

[0059] In this embodiment, as shown in the graph of Fig. 10, which shows the voltage and duty ratio of the developing bias set with respect to the environmental humidity, the Vpp and duty ratio of the developing bias are simultaneously controlled according to the environmental humidity to keep (maintain) Vgo constant, thereby simultaneously suppressing periodic density unevenness and fogging. In this embodiment, the duty ratio setting can be changed in units of 0.1%.

[0060] The configuration of the developing bias of this embodiment as described above and the effect of this configuration on periodic density unevenness and fog are shown in Fig. 11. Table 1 in Fig. 11(A) shows the configuration of the developing bias of this embodiment, and Table 2 in Fig. 11(B) shows the effect of the developing bias configuration of this embodiment shown in Fig. 11(A) on periodic density unevenness and fog, together with Comparative Examples 1 and 2.

[0061] 11A and 11B, in Comparative Example 1, the duty ratio and Vpp of the developing bias are fixed at 75% / 1400 V regardless of the environmental humidity. Therefore, at an environmental humidity of 5%, where the toner charge amount increases, the periodic density unevenness worsens.

[0062] In Comparative Example 2, the duty ratio is fixed at 75% and only Vpp is controlled according to the environmental humidity, and the lower the environmental humidity, the larger Vpp becomes. Therefore, unlike Comparative Example 1, periodic density unevenness is suppressed regardless of the environmental humidity, but in a low humidity environment, Vgo becomes large and fogging worsens.

[0063] On the other hand, in this embodiment, the lower the environmental humidity, the larger Vpp is made but the smaller the duty ratio is made, and Vgo is controlled to a constant 1050 V regardless of the environmental humidity. This simultaneously suppresses periodic density unevenness and fog.

[0064] FIG. 12 is a control block diagram of the image forming apparatus 200. As shown in FIG.

[0065] 12, image forming apparatus 200 includes a control unit 70 for performing various controls such as the image forming operation described above. The operation of each part of image forming apparatus 200 is controlled by control unit 70 provided in image forming apparatus 200. A series of image forming operations is controlled by control unit 70 in accordance with each input signal input from an operation unit on the top surface of the apparatus body or via a network.

[0066] The control unit 70 has a CPU (Central Processing Unit) 71 as an arithmetic control means, a ROM (Read Only Memory) 72, a RAM (Random Access Memory) 73, etc. The CPU 71 controls each part of the image forming apparatus 200 while reading a program corresponding to a control procedure stored in the ROM 72. Working data and input data are stored in the RAM 73, and the CPU 71 performs control by referring to the data stored in the RAM 73 based on the above-mentioned program, etc.

[0067] In addition, the control unit 70 processes image information in the image processing unit 74 to generate drive signals for each unit, controls the operation of each unit in the image formation control unit 75, and controls toner supply to the developing devices 1Y, 1M, 1C, and 1K in the supply control unit 76.

[0068] The control unit 70 is also connected to a temperature and humidity sensor 79 as a humidity detection means for detecting the environmental humidity, and a bias power supply 80 for supplying a development bias to the developing devices 1Y, 1M, 1C, and 1K. The temperature and humidity sensor 79 is provided, for example, at a part of the wall of the first conveying path (mixing chamber) 52 on the downstream side in the toner conveying direction in order to detect information related to the temperature and humidity inside the developing device 1. The control unit 70 calculates the absolute moisture amount inside the developing device 1 based on information related to the temperature and humidity inside the developing device 1Y, which is the detection result of the temperature and humidity sensor 79. That is, the temperature and humidity sensor 79 detects information related to the absolute moisture amount inside the developing container 2.

[0069] In this embodiment, the control unit 70 calculates information on volumetric absolute humidity as information on the absolute moisture content. However, the information on the absolute moisture content is not limited to information on volumetric absolute humidity, and information on weight absolute humidity may be calculated.

[0070] The control unit 70 determines the developing bias based on the information on the calculated absolute water content. The bias power supply 80 outputs the developing bias determined by the control unit 70.

[0071] The control unit 70 is also connected to a density detection sensor 22 as a density detection means that forms a toner image for adjusting image density on the intermediate transfer belt 10 and detects the density of the formed toner image. The density detection sensor 22 irradiates each patch image formed on the intermediate transfer belt 10 in the image forming units PY, PM, PC, and PK with measurement light and detects the amount of reflected light from the patch image. The detection result is transmitted to the control unit 70 as a received light output signal.

[0072] The density detection sensor 22 is generally an optical sensor equipped with a light-emitting element such as an LED and a light-receiving element such as a photodiode. When measuring the amount of toner adhesion on the intermediate transfer belt 10, the light-emitting element emits measurement light toward the toner image, and the measurement light is reflected by the toner and the belt surface and enters the light-receiving element. When the amount of toner adhesion is large, the light reflected from the belt surface is blocked by the toner, so the amount of light received by the light-receiving element decreases. On the other hand, when the amount of toner adhesion is small, the light reflected from the belt surface increases, and as a result, the amount of light received by the light-receiving element increases.

[0073] As a result, the density of the patch image of each color is detected based on the output value of the light reception signal based on the amount of reflected light received, and the setting value of the exposure device 14 is adjusted by comparing it with a predetermined reference density, thereby performing density correction for each color. Image density adjustment using the density detection sensor 22 is performed every time a predetermined number of images are output. In this embodiment, image density adjustment control is performed every time 500 A4 size images are output.

[0074] The flow of determining the developing bias in this embodiment will be described with reference to the flow chart of the CPU 71 shown in Fig. 13. Note that the following describes the flow when an image forming operation signal is input as an example.

[0075] When an image forming operation signal is input, in step S101, the temperature and humidity sensor 79 detects information relating to the absolute moisture amount inside the developing container 2, and in step S102, the CPU 71 calculates information relating to the volumetric absolute humidity based on that information.

[0076] Next, in step S103, based on the information regarding the calculated volumetric absolute humidity, the Vpp and duty ratio of the development bias are determined as shown in Table 1 of Figure 11(A) so that Vgo is constant regardless of the environmental humidity, and in step S104, the information is stored in RAM 73.

[0077] Then, in step S105, an instruction is given to the bias power supply 80 to output a developing bias consisting of the duty ratio and Vpp stored in the RAM 73 in step S104, and in step S106, an image forming operation is performed.

[0078] In the above embodiment, an example was shown in which a two-component developer was used as the developer, but the present invention can be similarly applied to a one-component developer.

[0079] Second Embodiment Next, a second embodiment of the present invention will be described.

[0080] In this embodiment, in addition to changing the Vpp and duty ratio of the developing bias so that Vgo is kept constant according to the environmental humidity in the first embodiment, the potential difference between the photosensitive drum charging potential (Vd) (i.e., the surface potential of the non-image portion) and Vdc (i.e., the absolute value of the difference between Vd and Vdc) (hereinafter referred to as Vback) is also changed at the same time. Note that other configurations are the same as those in the first embodiment, and therefore these are to be used.

[0081] During application of the developing bias, a phenomenon may occur in which the carrier detaches from the developing sleeve 54 and adheres to the photosensitive drum 13Y (hereinafter, referred to as carrier adhesion). Since the occurrence of carrier adhesion can have an adverse effect on durability deterioration due to wear of the photosensitive drum 13Y, it is desirable that carrier adhesion does not occur.

[0082] Carrier adhesion is closely correlated with the duty ratio and Vback of the developing bias. Fig. 14 is a graph showing the relationship between Vdc and carrier adhesion for different duty ratios. Note that the Vgo of each developing bias is 1050V. In addition, the carrier adhesion value is shown as a value obtained by visually measuring the number of carriers per unit area after collecting the carriers adhered to the photosensitive drum 13Y with an adhesive tape.

[0083] As shown in Fig. 14, the smaller the duty ratio is and the larger Vback is, the worse the carrier adhesion tends to be. Therefore, when the duty ratio is controlled according to the environmental humidity as in the first embodiment, the carrier adhesion worsens in a low humidity environment where the duty ratio is small. Therefore, in this embodiment, as shown in the graph of Fig. 15 which shows the developing bias set according to the environmental humidity, the developing bias Vpp, the duty ratio and Vdc are simultaneously controlled according to the environmental humidity, thereby suppressing periodic density unevenness, fogging and carrier adhesion.

[0084] It is preferable to set Vback in the range of 50 V to 200 V. The reason why Vback is set to 50 V or more is that, in consideration of the stability of the potential of the non-image area, fogging may easily occur if it is less than 50 V (for example, 0 V). On the other hand, the reason why Vback is set to 200 V or less is that there is a concern about carrier adhesion as described above if it is set to more than 200 V.

[0085] The configuration of the developing bias of this embodiment as described above and its effect on periodic density unevenness, fogging, and the number of carrier adhesions are shown in Fig. 16. Table 3 in Fig. 16(A) shows the configuration of the developing bias of this embodiment, and Table 4 in Fig. 16(B) shows the effect on periodic density unevenness, fogging, and the number of carrier adhesions of the developing bias configuration of this embodiment shown in Fig. 16(A) together with Comparative Examples 1 and 2 and the first embodiment. Note that, since the effects on periodic density unevenness and fogging overlap with those of the first embodiment, a description thereof will be omitted below.

[0086] As described above, the smaller the duty ratio, the greater the number of carrier adhesions. Therefore, in the first embodiment in which the duty ratio decreases as the environmental humidity decreases, the lower the environmental humidity, the worse the carrier adhesion becomes. In contrast, in this embodiment, the CPU 71 in FIG. 12 reduces Vback as the environmental humidity decreases, thereby suppressing periodic density unevenness, fogging, and carrier adhesion at any environmental humidity.

[0087] Third Embodiment Next, a third embodiment of the present invention will be described.

[0088] In the first and second embodiments, even if the settings of the Vpp, duty ratio, and Vdc of the developing bias are changed in response to changes in the environmental humidity, the charge amount of the toner does not change immediately, but is gradually changed to a value according to the environment as it is stirred and transported in the developing device. Therefore, when the developing bias setting is changed, the toner charge amount does not change, and only the development characteristics change, resulting in a change in the density of each tone.

[0089] For example, if the humidity inside the developing device changes from 50% to 30% while the developing device 1 is out of operation, the Vpp and duty ratio will be changed when setting the developing bias during the next operation, but since the toner inside the developing device while it is out of operation is not agitated, the toner charge amount will remain the same as it was at the 50% humidity before the humidity change.

[0090] Therefore, since the toner charge amount remains unchanged and only the developing bias setting is changed, the developability of the toner on the photosensitive drum 13 changes, and the density of each tone of the output recording material changes, as shown in the graph of Fig. 17, which shows the density change for each tone when the developing bias setting is changed. At this time, the density change ΔD becomes large especially near low densities (0.3 to 0.6), and if this occurs in the developing devices 1Y, 1M, 1C, and 1K, a variation in the color tone on the image of the four colors combined occurs.

[0091] In view of this, this embodiment aims to suppress the occurrence of color variations when the developing bias setting is changed. Note that other configurations are similar to those of the first and second embodiments, and therefore these are to be used herein.

[0092] In the first and second embodiments, for example, if an image forming apparatus is operated for a long time in a room with a humidity of 50%, the temperature inside the apparatus rises, the humidity inside the developing device changes little by little from 50% to 30%, and the developing bias setting is also changed little by little in accordance with the change in humidity, so the density change ΔD when the bias is changed is small. However, if the main body is not operated for a while, for example, by leaving it overnight, the humidity inside the developing device returns to 50%, and if the bias setting is changed significantly all at once at the next operation, the density change ΔD becomes large because the toner charge amount during the operation stoppage did not change.

[0093] In this embodiment, the change range of the Vpp and duty ratio of the developing bias per change is set to a maximum value, and when the change range of the developing bias due to a change in humidity becomes larger than the maximum value, the change is made multiple times to reduce the density change ΔD when the developing bias is changed.

[0094] In this embodiment, the CPU 71 in FIG. 12 sets the maximum change range of the duty ratio per change of the developing bias to a predetermined value of 0.5% (Vpp=10V), and the next change is performed after a predetermined number of image formations, such as 100 image formations, since the previous change.

[0095] Fig. 18 is a graph showing the duty ratio of the developing bias set against the environmental humidity. Fig. 18 shows the duty ratio settings of the developing bias in this embodiment and in embodiment 2 against the humidity change when 3000 sheets are continuously output in a room with a humidity of 50%, operation is stopped until the next day, and then continuous output is started again. Note that in this embodiment, Vpp and Vback are also changed at the same time as the duty ratio, as in embodiment 2.

[0096] In the second embodiment, the duty ratio is changed significantly in response to the change in humidity the day after 3,000 sheets are output, but in the third embodiment, the duty ratio is switched in stages. The graph in Fig. 19, which shows the change in density of an image on a recording material when the developing bias setting is changed, shows the change in density of an image on a recording material with a density of 0.4 at this time. The maximum value of ΔD when the developing bias is changed is ΔD=0.06 in the second embodiment, but it was reduced to ΔD=0.012 in the third embodiment, and color variation could be suppressed.

[0097] [Fourth embodiment] Next, a fourth embodiment of the present invention will be described.

[0098] In this embodiment, the change of the developing bias setting according to the environmental humidity is executed only immediately before the image density adjustment control by the density detection sensor, thereby suppressing the color variation when the developing bias setting is changed. Note that, since the other configurations are the same as those of the first to third embodiments, they are used here.

[0099] This embodiment will be described with reference to the flow chart of the CPU 71 in FIG. 12 shown in FIG.

[0100] In step S201, one image is output, and then in step S202, the humidity inside the developing device is checked by the temperature and humidity sensor 79. In step S203, it is determined whether or not the developing bias setting needs to be changed according to the humidity inside the developing device detected by the temperature and humidity sensor 79, and if it is determined in step S203 that the developing bias setting needs to be changed, it is determined in step S204 whether or not image density adjustment control, which is performed once every 500 sheets, is about to be executed. If it is determined in step S204 that image density adjustment control is about to be executed, the developing bias setting is changed in step S205, and then image density adjustment control is executed in step S206.

[0101] On the other hand, if it is determined in step S204 that the image density adjustment control is not about to be executed, the developing bias is not changed and the process returns to S202, where the temperature and humidity sensor 79 checks the humidity inside the developing device.

[0102] Furthermore, if it is determined in step S203 that a change in the development bias setting is not necessary, the process proceeds to step S207, where it is determined whether or not image density adjustment control is about to be executed. If it is determined that the timing is about to be immediately before execution of image density adjustment control, the process proceeds to step S206, where only image density adjustment control is performed.

[0103] Fig. 21 is a graph showing the timing of changing the developing bias setting and executing image density adjustment control in this embodiment and embodiment 2. In Fig. 21, in embodiment 2, if the humidity changes significantly, the developing bias is changed frequently, and color variations occur each time the change is made. In this embodiment, even if the humidity changes, the developing bias is not changed until the next image density adjustment control, and the developing bias is changed immediately before the image density adjustment control. In this case, the change range of the developing bias per time may be larger than in embodiment 2, but since the image density adjustment control is executed according to the changed developing bias, color variations can be reduced.

[0104] Fifth embodiment Next, a fifth embodiment of the present invention will be described.

[0105] Although the third and fourth embodiments can reduce color variations when the developing bias is changed, they do not completely eliminate the effects of changing the developing bias. The above-mentioned problem of periodic density unevenness becomes noticeable when outputting images with a high image coverage. On the other hand, when many images with a low image coverage are taken, there is little need to change the developing bias.

[0106] Therefore, in this embodiment, a color variation priority mode in which the developing bias is not changed even if the humidity in the developing device changes, and an in-plane density unevenness priority mode in which the developing bias is changed are provided, and these modes can be freely selected. Note that other configurations are the same as those of the first to fourth embodiments, and therefore these are to be used.

[0107] In this embodiment, a button for selecting a color variation priority mode or an in-plane density unevenness priority mode is provided on the operation unit of the image forming apparatus 200, so that the user can freely select. Then, the CPU 71 in Fig. 12 switches between the color variation priority mode and the in-plane density unevenness priority mode in response to the selection operation using the button.

[0108] In each of the above embodiments, the image forming apparatus is a printer. However, the present invention is also applicable to a copying machine, a facsimile machine, or a multifunction machine having a plurality of these functions.

[0109] In addition, in each of the above embodiments, an example was described in which Vgo was 1050 V, but similar effects can be expected if Vgo is constant regardless of the environmental humidity, even if it is not 1050 V. Note that Vgo being constant regardless of the environmental humidity may include cases in which Vgo fluctuates up and down within a range of ±5% relative to the reference depending on the environmental humidity, as long as the relationship is satisfied in which Vpp is increased while the duty ratio is decreased as the environmental humidity decreases.

[0110] For example, in the first embodiment described above, when the environmental humidity is 80%, an example is described in which Vpp is 1400V, Vgo is 1050V, and the duty ratio is 75% (see FIG. 11(A)), but this is not limited thereto. A modified example may be used in which Vpp is 1400V, Vgo is 1000V, and the duty ratio is 71.4% when the environmental humidity is 80%. In this modified example, when the environmental humidity is 50%, Vpp is 1475V, Vgo is 1050V, and the duty ratio is 71.2%, as in the first embodiment described above. In such a modified example, although Vgo is not the same value when the environmental humidity is 80% and when it is 50%, the relationship of increasing Vpp and decreasing the duty ratio is satisfied compared to when the environmental humidity is 80%.

[0111] In addition, in each of the above embodiments, the combinations of the environmental humidity, Vpp, duty ratio, and Vback are not limited to those shown in Figures 10, 11(A), 15, and 16(A). For example, the developing bias in the second embodiment at an environmental humidity of 50% may be Vpp 1500V, duty ratio 70%, Vgo 1050V, and Vback 140V. [Explanation of symbols]

[0112] 1Y, 1M, 1C, 1K...Developing device 12Y, 12M, 12C, 12K...Charging roller 13Y, 13M, 13C, 13K...Photosensitive drum 14Y, 14M, 14C, 14K...Exposure equipment 54…Developing sleeve 70...Control unit 71…CPU 79...Temperature and humidity sensor 80...Bias power supply 200...Image forming apparatus

Claims

1. An image carrier; a charging section for charging the surface of the image carrier to a predetermined charging potential; a latent image forming unit that forms an electrostatic latent image on the surface of the image carrier that has been charged to the predetermined charging potential by the charging unit; a developer carrier that carries a developer containing a toner; a developing bias application unit that applies a developing bias, which is a superposition of a DC voltage and an AC voltage, to the developer carrier in order to develop the electrostatic latent image formed on the image carrier by the latent image forming unit with a developer carried on the developer carrier; A humidity sensor that detects humidity; Equipped with Among the AC voltages, a voltage having the same polarity as the toner charge polarity with respect to the DC voltage is denoted as Vgo, among the AC voltages, a voltage having a polarity opposite to the toner charge polarity with respect to the DC voltage is denoted as Vre, and a peak-to-peak voltage of the AC voltage is denoted as Vgo+Vre, The developing bias application unit applies the developing bias to the developer carrier so that the peak-to-peak voltage when the humidity detected by the humidity sensor is a second humidity lower than the first humidity is greater than the peak-to-peak voltage when the humidity detected by the humidity sensor is a first humidity, and so that the Vgo when the humidity detected by the humidity sensor is the second humidity is the same as the Vgo when the humidity detected by the humidity sensor is the first humidity.

1. An image forming apparatus comprising:

2. The developer carried on the developer carrier contains a toner and a carrier, When the predetermined charging potential is Vd, the DC voltage is Vdc, and the absolute value of the difference between Vd and Vdc is Vback, The developing bias application unit applies the developing bias to the developer carrier so that the Vback when the humidity detected by the humidity sensor is the second humidity is smaller than the Vback when the humidity detected by the humidity sensor is the first humidity.

2. The image forming apparatus according to claim 1,

3. When the duty ratio in one period of the AC voltage is Vgo / (Vgo+Vre), the duty ratio is 50% or more and 90% or less when the humidity detected by the humidity sensor is the first humidity; When the humidity detected by the humidity sensor is the second humidity, the duty ratio is 50% or more and 90% or less.

2. The image forming apparatus according to claim 1,

4. a developer container for accommodating the developer, The humidity sensor detects the humidity inside the developing container.

2. The image forming apparatus according to claim 1,

5. An image carrier; a charging section for charging the surface of the image carrier to a predetermined charging potential; a latent image forming section that forms an electrostatic latent image on the surface of the image carrier that has been charged to the predetermined charging potential by the charging section; a developer carrier that carries a developer containing a toner; a developing bias application unit that applies a developing bias, which is a superposition of a DC voltage and an AC voltage, to the developer carrier in order to develop the electrostatic latent image formed on the image carrier by the latent image forming unit with a developer carried on the developer carrier; A humidity sensor that detects humidity; Equipped with Among the AC voltages, a voltage having the same polarity as the toner charge polarity with respect to the DC voltage is denoted as Vgo, and among the AC voltages, a voltage having a polarity opposite to the toner charge polarity with respect to the DC voltage is denoted as Vre, and a duty ratio in one cycle of the AC voltage is denoted as Vgo / (Vgo+Vre), The developing bias application unit applies the developing bias to the developer carrier so that the peak-to-peak voltage when the humidity detected by the humidity sensor is a second humidity lower than the first humidity is greater than the peak-to-peak voltage when the humidity detected by the humidity sensor is a first humidity, and so that the duty ratio when the humidity detected by the humidity sensor is the second humidity is smaller than the duty ratio when the humidity detected by the humidity sensor is the first humidity.

1. An image forming apparatus comprising:

6. The developing bias application unit applies the developing bias to the developer carrier so that the Vgo when the humidity detected by the humidity sensor is the second humidity is the same as the Vgo when the humidity detected by the humidity sensor is the first humidity.

6. The image forming apparatus according to claim 5,

7. The developer carried on the developer carrier contains a toner and a carrier, When the predetermined charging potential is Vd, the DC voltage is Vdc, and the absolute value of the difference between Vd and Vdc is Vback, The developing bias application unit applies the developing bias to the developer carrier so that the Vback when the humidity detected by the humidity sensor is the second humidity is smaller than the Vback when the humidity detected by the humidity sensor is the first humidity.

6. The image forming apparatus according to claim 5,

8. When the humidity detected by the humidity sensor is the second humidity, the duty ratio is 50% or more.

6. The image forming apparatus according to claim 5,

9. When the humidity detected by the humidity sensor is the first humidity, the duty ratio is 90% or less.

6. The image forming apparatus according to claim 5,

10. a developer container for accommodating the developer, The humidity sensor detects the humidity inside the developing container.

6. The image forming apparatus according to claim 5,

Images