Image forming apparatus

Abstract

To provide an image forming apparatus that can prevent occurrence of an image defect due to an external additive added to toner.SOLUTION: A control unit calculates an average image ratio obtained by averaging image ratios from the start of use of a developing device to an image forming operation on the current recording material (S2). The control unit sets a developing voltage to be applied to a developing bias power supply on the basis of the average image ratio (S3). At this time, when the average image ratio is a first ratio, the peak-to-peak voltage of the AC voltage is set to a first voltage, and when the average image ratio is a second ratio larger than the first ratio, the peak-to-peak voltage of the AC voltage is set to a second voltage smaller than the first voltage. Consequently, when the average image ratio is large, the peak-to-peak voltage is reduced and the amount of an external additive supplied to a photoconductor drum 1Y in a developing period is reduced, and thereby the occurrence of a ghost image due to the external additive remaining on the photoconductor drum can be prevented.SELECTED DRAWING: Figure 6

Description

[Technical field]

[0001] The present invention relates to an image forming apparatus such as a printer, a copier, a facsimile, or a multifunction machine that uses electrophotographic technology. [Background technology]

[0002] In an electrophotographic image forming apparatus, an image is formed on a recording material using a two-component developer in which a non-magnetic toner and a magnetic carrier are mixed. To ensure the fluidity and chargeability of the toner, an external additive having the same charge polarity as the toner is added. The developer is supplied from the developing container to the photosensitive drum by the developing sleeve in response to application of a developing voltage in which an AC voltage and a DC voltage are superimposed, and the toner contained in the developer develops the electrostatic latent image formed on the surface of the photosensitive drum. After development, the toner remaining on the surface of the photosensitive drum is removed from the photosensitive drum by a drum cleaner.

[0003] Since toner is consumed during development and external additives can be separated from the toner by stirring and transporting the developer in the developer container, the concentration of the external additives in the developer container increases as the image formation on the recording material proceeds. In order to maintain the concentration of the external additives appropriately, the device described in Patent Document 1 discloses that the concentration of the external additives in the developer container is reduced by mainly discharging the external additives charged with the same polarity as the toner from the developer container to the photosensitive drum when the image formation operation, that is, the job is completed or interrupted. In the device described in Patent Document 1, after the formation of an image that reaches a preset image density, the peak-to-peak voltage of the AC voltage is made higher than that during image formation to rotate the developing sleeve, thereby discharging the external additives in the developer container to the photosensitive drum. The external additives discharged to the photosensitive drum are removed from the photosensitive drum together with the toner by a drum cleaner. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] JP 2019-66547 A Summary of the Invention [Problem to be solved by the invention]

[0005] However, in the past, when development was performed before the external additive was discharged and the concentration of the external additive in the developer container was relatively high, some of the external additive on the photosensitive drum was not removed by the drum cleaner and remained on the photosensitive drum, and was carried around by the photosensitive drum. The external additive forms an electric field that attracts toner due to its own charge, and areas on the photosensitive drum where a large amount of external additive remains are more likely to attract toner than other areas. Therefore, even though an image of uniform density is normally formed, there is a risk of image defects, so-called ghost images, having density differences due to the external additive.

[0006] The present invention has been made in view of the above problems, and has an object to provide an image forming apparatus capable of suppressing image defects caused by external additives added to toner. [Means for solving the problem]

[0007] According to an embodiment of the present invention, an image forming apparatus for forming an image on a recording material includes a rotating image carrier, a charging section for charging the surface of the image carrier, an exposure section for exposing the charged surface of the image carrier based on image data to form an electrostatic latent image, a developing device including a developing container for accommodating a developer containing a toner and an external additive having the same charging polarity as the toner, and a developer carrier that rotates while carrying the developer contained in the developing container and develops the electrostatic latent image formed on the image carrier into a toner image with the developer, and a developing device for developing the electrostatic latent image formed on the image carrier into a toner image by the developer. the developing device includes a developing power source that applies a superimposed voltage of a DC voltage and an AC voltage to the developer carrier in order to develop an image formed on a recording material with a developer; and a setting unit that, when performing an image forming operation on a recording material, sets a peak-to-peak voltage of the AC voltage to a first voltage if an average image ratio obtained by averaging image ratios based on the image data from when the developing device was first used until a current image forming operation is a first ratio, and sets the peak-to-peak voltage of the AC voltage to a second voltage smaller than the first voltage if the average image ratio is a second ratio larger than the first ratio. Effect of the Invention

[0008] According to the present invention, it is possible to suppress the occurrence of image defects caused by external additives added to toner. [Brief description of the drawings]

[0009] [Figure 1] 1 is a schematic diagram showing a configuration of an image forming apparatus according to an embodiment of the present invention. [Diagram 2] FIG. [Diagram 3] FIG. 4 is a schematic diagram showing a circulation path of a developer. [Figure 4] FIG. 4 is a control block diagram showing a control system related to toner image formation. [Diagram 5] FIG. 4 is a diagram showing the waveform of a developing voltage. [Figure 6] 4 is a flowchart showing an image forming process according to the first embodiment. [Figure 7]1A and 1B are diagrams showing toner images formed on a recording material to obtain experimental results, in which (a) is a vertical solid image, and (b) is a vertical solid image and a horizontal halftone image. [Figure 8] 6 is a graph showing the reflection density difference of the present embodiment and the reflection density difference of the comparative example. [Figure 9] 10 is a flowchart showing an image forming process according to a second embodiment. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0010] [First embodiment] <Image forming device> Hereinafter, an embodiment of the present invention will be described with reference to the drawings. First, the configuration of an image forming apparatus of this embodiment will be described with reference to FIG. 1. The image forming apparatus 100 shown in FIG. 1 is an intermediate transfer tandem type color image forming apparatus in which four color image forming units PY, PM, PC, and PK are arranged facing an intermediate transfer belt 5. The image forming apparatus 100 forms a toner image on a recording material S based on image data sent from an external device such as a document reading device (not shown) connected to the image forming apparatus 100 or a personal computer.

[0011] The image forming apparatus 100 includes four image forming units PY, PM, PC, and PK that form toner images of yellow, magenta, cyan, and black. The image forming units PY, PM, PC, and PK have photosensitive drums 1Y, 1M, 1C, and 1K, respectively, and form toner images of each color on these photosensitive drums 1Y to 1K. The toner images carried on the photosensitive drums 1Y to 1K are transferred to a recording material S via an intermediate transfer belt 5. Examples of the recording material S include various types of sheet materials such as plain paper, thick paper, rough paper, textured paper, coated paper, and other papers, plastic films, and cloth.

[0012] Since the image forming units PY to PK have the same configuration except for the color of the toner used for development, the following description will be given taking the yellow image forming unit PY as an example. In addition to the photosensitive drum 1Y, the image forming unit PY has a charging roller 2Y, an exposure device 3Y, a developing device 4Y, a primary transfer roller 6Y, and a drum cleaner 7Y.

[0013] The photosensitive drum 1Y as an image carrier is rotated in the rotation direction of the intermediate transfer belt 5 by a drive motor (not shown). A charging roller 2Y as a charging section uniformly charges the surface of the photosensitive drum 1Y by applying a charging voltage. The contact point between the charging roller 2Y and the photosensitive drum 1Y is the charging position a. An exposure device 3Y as an exposure section irradiates the photosensitive drum 1Y with laser light L modulated based on image data, and forms an electrostatic latent image on the surface of the photosensitive drum 1Y. The irradiation position of the laser light L on the photosensitive drum 1Y is the exposure position b.

[0014] The developing device 4Y contains a developer containing toner, and supplies the toner to the photosensitive drum 1Y by application of a developing voltage, thereby developing the electrostatic latent image into a toner image. The developing device 4Y will be described in detail later (see Figs. 2 and 3). The toner image formed on the photosensitive drum 1Y is primarily transferred to the intermediate transfer belt 5 at the primary transfer nip T1Y by a primary transfer roller 6Y as a transfer member to which a primary transfer voltage is applied. Toner remaining on the photosensitive drum 1Y without being transferred to the intermediate transfer belt 5 at the primary transfer nip T1Y is removed by a drum cleaner 7Y. The drum cleaner 7Y as a cleaning member comes into contact with the photosensitive drum 1Y to remove the toner remaining on the photosensitive drum 1Y after the toner image is transferred to the intermediate transfer belt 5.

[0015] The intermediate transfer belt 5 as another image carrier is driven to rotate in the direction of the arrow R2 in the figure. The image forming operation described above is performed in parallel in each image forming portion PY to PK, and four color toner images are superimposed and multi-transferred to the intermediate transfer belt 5 in the primary transfer nip portions T1Y to T1K to form a full-color toner image. This toner image is carried by the intermediate transfer belt 5 and conveyed to the secondary transfer nip portion T2 formed by the secondary transfer outer roller 64 and the secondary transfer inner roller 62. A secondary transfer voltage is applied to the secondary transfer outer roller 64, so that the toner image carried by the intermediate transfer belt 5 is secondarily transferred to the recording material S. In this embodiment, the primary transfer rollers 6Y to 6K, the intermediate transfer belt 5, the secondary transfer outer roller 64, and the secondary transfer inner roller 62 constitute a transfer device 190 that transfers the toner images formed on the photosensitive drums 1Y to 1K to the recording material S. The toner remaining on the intermediate transfer belt 5 after the secondary transfer is removed by the belt cleaner 18. The recording material S onto which the toner image has been transferred is conveyed to the fixing device 16, and is pressurized and heated by the fixing device 16. As a result, the toner image is fixed onto the recording material S.

[0016] <Developing device> Next, the developing device 4Y will be described with reference to Fig. 2 and Fig. 3 along with Fig. 1. The developing device 4Y includes a developing container 201, a developing sleeve 203, a conveying screw 205, a stirring screw 206, and a regulating blade 208, as shown in Fig. 2.

[0017] 2, the developing container 201 has an opening 300 formed in a region facing the photosensitive drum 1Y. The developing sleeve 203 as a developer carrier is rotatably disposed in the developing container 201 such that a part of its outer circumferential surface is exposed from the opening 300 of the developing container 201. The developing position c is a position downstream of the exposure position b in the rotation direction of the photosensitive drum 1Y (downstream in the direction of arrow R1) and where the developing sleeve 203 faces the photosensitive drum 1Y. The developing sleeve 203 is rotationally driven by a motor (not shown) so that the surface of the developing sleeve 203 moves in the same direction as the rotation direction of the photosensitive drum 1Y at the developing position c.

[0018] A regulating blade 208 is disposed upstream of the opening 300 of the developing container 201 in the rotation direction (arrow R3 direction) of the developing sleeve 203. The regulating blade 208 regulates the height of the magnetic brush of the developer formed on the developing sleeve 203 by the magnetic force of the magnet 202, thereby forming a developer layer on the developing sleeve 203. The developer layer on the developing sleeve 203 rubs against the surface of the photosensitive drum 1Y at the development position c.

[0019] The developing sleeve 203 is made of a non-magnetic material, and rotates with the developer carried on its surface by the magnetic force of a magnet 202 arranged non-rotatably inside, to transport the developer to a developing position c. The developer is supplied from the developing sleeve 203 to the photosensitive drum 1Y at the developing position c, so that the electrostatic latent image formed on the photosensitive drum 1Y is developed into a toner image. That is, the toner in the developer transported to the rotating developing sleeve 203 selectively adheres to the electrostatic latent image on the photosensitive drum 1Y by the electric field due to the developing voltage, so that the electrostatic latent image on the photosensitive drum 1Y is developed into a toner image. At that time, the developer that has passed the developing position c without being supplied to the photosensitive drum 1Y returns to the developing container 201 as the developing sleeve 203 rotates, and is separated from the developing sleeve 203 in the developing chamber 215 and collected.

[0020] The inside of the developing container 201 is partitioned into a developing chamber 215 capable of accommodating a developer and an agitating chamber 216 by a partition wall 207 extending in the direction perpendicular to the drawing (longitudinal direction) at approximately the center. A conveying screw 205 is rotatably disposed in the developing chamber 215, and an agitating screw 206 is rotatably disposed in the agitating chamber 216.

[0021] As shown in FIG. 3, in the developing container 201, partition openings (207a, 207b) for transferring developer between the developing chamber 215 and the stirring chamber 216 are formed at one end side and the other end side in the longitudinal direction of the partition 207 provided between the conveying screw 205 and the stirring screw 206. The developing chamber 215 and the stirring chamber 216 are communicated with each other through the partition openings (207a, 207b), thereby forming a circulation path for the developer circulating between the developing chamber 215 and the stirring chamber 216. The conveying screw 205 and the stirring screw 206 convey the developer in the opposite directions to each other along the rotation axis direction of the developing sleeve 203. Therefore, the developer is transferred from the developing chamber 215 to the stirring chamber 216 through the partition opening 207a, and is transferred from the stirring chamber 216 to the developing chamber 215 through the partition opening 207b.

[0022] The transport screw 205 transports while stirring the developer in the developing chamber 215. When the transport screw 205 rotates, the developer in the developing chamber 215 is transported along the transport screw 205, and at that time, a part of the developer is supplied to the developing sleeve 203. On the other hand, the stirring screw 206 transports while stirring the replenishment agent containing toner that is timely replenished from a toner replenishing device (not shown) and the developer in the stirring chamber 216, thereby making the toner concentration of the developer contained in the developing container 201 uniform.

[0023] The developer container 201 contains a two-component developer containing a negatively charged non-magnetic toner and a magnetic positively charged carrier (hereinafter referred to as a magnetic carrier) as a developer. The non-magnetic toner is a resin such as polyester or styrene acrylic that contains colorants and wax, and is pulverized or polymerized to form a powder. The magnetic carrier is a core made of resin particles kneaded with ferrite particles and magnetic powder, and a resin coating is applied to the surface layer. The developer is stirred and conveyed inside the developer container 201 by a conveying screw 205 and a stirring screw 206, so that the toner contained in the developer is negatively charged. In this embodiment, a reversal development method is adopted in which a toner charged with the same polarity as the charging polarity of the photosensitive drum 1Y is supplied to an electrostatic latent image, and the electrostatic latent image is developed into a toner image. In addition, particulate external additives such as silica and titanium oxide are added to the toner in order to ensure fluidity and chargeability. The external additives added to the toner and the external additives separated from the toner are stirred and transported inside the developing container 201, and are thereby charged to the same polarity as the toner (here, negative polarity).

[0024] <Control Unit> As shown in Fig. 1, the image forming apparatus 100 has a control unit 101 that controls the image forming apparatus 100. The control unit 101 will be described with reference to Fig. 4. Note that in addition to those shown in the figure, various devices such as a motor and a power supply that operate the image forming apparatus 100 are connected to the control unit 101, but since this is not the main point of the invention, illustration and description of these devices will be omitted here.

[0025] 4, the control unit 101 has a CPU (Central Processing Unit) 102 and memory 103 such as ROM and RAM, and controls various motors and power sources based on image data and input from various sensors. The memory 103 stores various programs such as "image forming processing" and various data such as a "correction table" used to correct the development voltage. The CPU 102 can execute the various programs stored in the memory 103, and executes the various programs to operate the image forming apparatus 100.

[0026] The control unit 101 also has an average image ratio calculation unit 104. When performing an image forming operation on the recording material S, the average image ratio calculation unit 104 calculates an average image ratio by averaging image ratios based on image data from the start of use of the developing device 4Y to the current image forming operation. The method of calculating the average image ratio will be described later.

[0027] The control unit 101 is connected to a charging bias power supply 81, a developing bias power supply 82, a primary transfer bias power supply 83, and a secondary transfer bias power supply 84. The charging bias power supply 81 applies a charging voltage to the charging roller 2Y to uniformly charge the surface of the photosensitive drum 1Y to a negative charging potential (Vd). In this embodiment, the charging bias power supply 81 applies a DC charging voltage, so that the surface of the photosensitive drum 1Y is uniformly charged to a charging potential of "-700V". The exposure device 3Y exposes the charged surface of the photosensitive drum 1Y to a laser scanning. This laser scanning exposure reduces the potential of the area on the photosensitive drum 1Y irradiated with the laser light, so that an electrostatic latent image is formed on the photosensitive drum 1Y. When the charged photosensitive drum 1Y is exposed by the exposure device 3Y in this way, the potential of the image area where the electrostatic latent image is formed on the photosensitive drum 1Y (also called the exposed area potential) changes to, for example, "-150V". The potential of the non-image portion of the photosensitive drum 1Y, which is not exposed by the exposure device 3Y and has no electrostatic latent image formed thereon, remains at the charging potential of "-700V."

[0028] The developing bias power supply 82 as a developing power supply can apply a developing voltage to the developing sleeve 203. The developing voltage applied to the developing sleeve 203 is a superimposed voltage in which a DC voltage (Vdc) and an AC voltage (Vac) are superimposed. The developing bias power supply 82 has a DC power supply 82a and an AC power supply 82b. The developing bias power supply 82 applies a superimposed voltage in which, for example, a DC voltage of "-550V" output from the DC power supply 82a and an AC voltage of a frequency of "11.0kHz" and a peak-to-peak voltage of "1.4kV" output from the AC power supply 82b are superimposed as a reference developing voltage to the developing sleeve 203. However, in this embodiment, as will be described later, a different peak-to-peak voltage is set by the control unit 101 as a setting unit according to the average image ratio. The DC voltage (Vdc) of the developing voltage is a potential between the charging potential (Vd) and the exposed portion potential. A primary transfer bias power supply 83 as a transfer power supply applies a positive polarity primary transfer voltage to the primary transfer roller 6Y for performing the primary transfer. The primary transfer bias power supply 83 applies a primary transfer voltage so that a primary transfer current of, for example, 22.0 μA as a reference flows through the primary transfer nip T1Y. The secondary transfer bias power supply 84 applies a positive polarity secondary transfer voltage to the secondary transfer outer roller 64 for performing the secondary transfer.

[0029] FIG. 5 shows the waveform of the developing voltage applied by the developing bias power supply 82. As described above, the developing bias power supply 82 applies a developing voltage in which a DC voltage is superimposed on an AC voltage to the developing sleeve 203. The AC voltage of the developing voltage is, for example, a square wave with a frequency of "11.0 kHz". As shown in FIG. 5, the developing voltage has blank portions in which the AC voltage is intermittently thinned out to leave only a DC voltage. In this specification, the square wave pulse that exists in the portion corresponding to the blank portion when the AC voltage is not thinned out, in other words, the pulse that has become blank (the pulse that no longer exists) due to the thinning out of the AC voltage, is referred to as a "blank pulse". In addition, the portion that has become a DC voltage by intermittently thinning out the AC voltage is referred to as a "blank portion".

[0030] Therefore, the developing voltage applied by the developing bias power supply 82 has a waveform in which, as shown in FIG. 5, one period includes an AC bias portion in which a DC voltage (Vdc) is superimposed on an AC voltage, and a blank portion consisting of only a DC voltage (Vdc) following this AC bias portion.

[0031] In this embodiment, a double blank pulse waveform (hereinafter referred to as WBP) in which a blank portion is provided after an AC bias portion including a square wave of two cycles (four pulses) is used as the developing voltage. In this specification, the number of pulses of the square wave is counted as one pulse for half a cycle of the square wave. The blank portion time in one cycle of the developing voltage is defined as blank time t1. The total time during which the electric field (pulse) of the developing side of the AC bias portion is generated in one cycle of the developing voltage is defined as developing time t2, and the total time during which the electric field of the recovery side is generated is defined as recovery time t3. Furthermore, the ratio (duty ratio) of the electric field of the developing side to the electric field of the recovery side in the AC bias portion is set to "50%". Here, the electric field of the developing side is an electric field in which the toner moves from the developing sleeve 203 to the photosensitive drum 1Y by the AC bias portion during one cycle of the developing voltage. The electric field of the recovery side is an electric field in which the toner is pulled back from the photosensitive drum 1Y to the developing sleeve 203 by the AC bias portion during one cycle of the developing voltage.

[0032] During development, the sum of the voltages (corresponding to the amplitude of the AC bias section) on the development side (developer application side) and the recovery side (developer return side) in the AC bias section is set to "Vpp", and here, as an example, "Vpp" is set to "1.4 kV". During development, a potential difference (hereinafter, referred to as Vback) is set between the potential of the non-image portion on the photosensitive drum 1Y where no electrostatic latent image is formed and the potential of the developing sleeve 203, and a development voltage according to "Vback" is applied. "Vback" is the absolute value of the potential difference between the potential of the non-image portion, that is, the charged potential (Vd) of the photosensitive drum 1Y, and the potential of the developing sleeve 203. As an example, when the charged potential of the photosensitive drum 1Y is "-700V", "Vback" is set to "150V", and the DC voltage of the development voltage is "-550V". "Vback" forms an electric field (electric field on the recovery side) that draws toner back from the photosensitive drum 1Y to the developing device 4Y, more specifically, to the developing sleeve 203. This is to prevent the occurrence of image defects called fog images caused by toner adhering to non-image areas of the photosensitive drum 1Y during development.

[0033] As described above, the developer used in this embodiment is a two-component developer containing non-magnetic toner, magnetic carrier, and external additives added to the toner. When the image portion on which the electrostatic latent image is formed on the photosensitive drum 1Y is developed with this developer, the toner of the developer carried on the developing sleeve 203 in the developing device 4Y moves to the photosensitive drum 1Y for development. At this time, the external additives added to the toner also move to the photosensitive drum 1Y. In addition, a part of the external additives added to the toner may separate from the toner during stirring and transport.

[0034] The external additives transferred to the photosensitive drum 1Y are transferred to the intermediate transfer belt 5 together with the toner when the toner is primarily transferred to the intermediate transfer belt 5, but because the particle size of the external additives is small and the non-electrostatic adhesive force is strong, some of the external additives remain on the image portion of the photosensitive drum 1Y after the primary transfer. In addition, the toner remaining on the photosensitive drum 1Y after the primary transfer is removed by the drum cleaner 7Y (see FIG. 1), but because the particle size of the external additives is small compared to the toner and the non-electrostatic adhesive force is strong, the external additives are likely to remain on the photosensitive drum 1Y without being completely cleaned.

[0035] When a charging voltage is applied by the charging roller 2Y to the photosensitive drum 1Y on which the external additive remains, the negatively charged external additive attracts more toner to the areas on the photosensitive drum 1Y where the external additive remains than to the areas where the external additive does not remain. Therefore, when developing the next recording material S, the amount of toner moving from the developing sleeve 203 to the photosensitive drum 1Y may be greater than intended in the areas where the external additive remains. In particular, when the same or similar image patterns are continuously formed on multiple sheets of recording material S, a large amount of external additive accumulates in the areas where the external additive remains in the image area. In this case, when an image such as a halftone image is formed, there is a risk that a density difference will occur in the image areas of the same or similar image patterns, resulting in a ghost image.

[0036] As described above, the reason why ghost images are caused by the external additive is that the external additive remaining on the photosensitive drum 1 attracts more toner. When image formation is performed continuously on a plurality of recording materials S at a high image ratio, the amount of external additive in the developing container 201 is likely to increase compared to when image formation is performed continuously at a low image ratio. When image formation is performed continuously at a high image ratio, the concentration of the external additive in the developing container 201 may excessively increase because the toner-containing replenishment agent is frequently replenished into the developing container 201 by a toner replenishing device (not shown). In this case, the external additive is likely to be supplied to the photosensitive drum 1Y together with the toner during development, increasing the risk of ghost images. Furthermore, since the toner contained in the replenishment agent is introduced into the developing container 201 in a low-charged state, the charge amount of the toner in the developing container 201 decreases after replenishment. When the charge amount of the toner is low, even if the development contrast, which is the potential difference between the potential of the exposed portion on the photosensitive drum 1Y and the DC voltage of the development voltage, is the same, more toner is used to develop the electrostatic latent image compared to when the charge amount of the toner is high. In other words, the developability of the toner becomes excessively high as the replenishment agent is replenished, and more toner containing external additives can be supplied to the photosensitive drum 1Y.

[0037] Thus, the main causes of ghost images are an increase in the amount of external additives in the developing container 201 and an excessive improvement in the developability of the toner due to a decrease in the charge amount. Therefore, in this embodiment, the developability of the toner is kept appropriate by changing the developing voltage according to the average image ratio, and the amount of external additives supplied to the photosensitive drum 1Y during development is reduced. The image forming process of this embodiment that achieves this will be described below using FIG. 6 with reference to FIG. 4. The "image forming process" shown here is started in response to acquisition of a start command for an image forming job by the control unit 101.

[0038] <Image formation process> As shown in FIG. 6, when the control unit 101 receives a command to start an image forming job from an operation unit (not shown) or an external device (for example, a personal computer), the control unit 101 calculates an image ratio of the recording material S on which the "n"th toner image is formed, which starts counting from the start of use of the developing device 4Y (S1). The control unit 101 obtains a video count value of an output image to be formed on the "n"th recording material S, and uses this to calculate an image ratio of the current recording material S on which the toner image is to be formed. The video count value is an integrated value obtained by integrating the level (0 to 255 level) of each pixel of the input image data for one surface of the output image. Then, the control unit 101 calculates an average image ratio by averaging the image ratios from the start of use of the developing device 4Y to the image forming operation on the current (nth) recording material S (S2).

[0039] A method for calculating the average image ratio by the average image ratio calculation unit 104 will be described. The average image ratio calculation unit 104 calculates the average image ratio (Ave_Duty(n)) according to the following formula 1. In formula 1, "Ave_Duty(n-1)" is the average image ratio obtained by averaging the image ratios from the start of use of the developing device 4Y to the image forming operation on the "n-1"th sheet of recording material S, and "New_Duty" is the image ratio on the "n"th sheet of recording material S.

number

[0040] Then, the control unit 101 sets the developing voltage to be applied to the developing sleeve 203 by the developing bias power supply 82 based on the calculated average image ratio (S3). At this time, the control unit 101 determines the Vpp correction amount according to the average image ratio by referring to a correction table. Table 1 shows the correction table used when correcting the peak-to-peak voltage (Vpp) of the AC voltage based on the average image ratio. [Table 1]

[0041] As shown in Table 1, when the average image ratio is "7%," the Vpp correction amount is "-0.04 kV", so the control unit 101 changes the setting of the peak-to-peak voltage from the reference peak-to-peak voltage "1.4 kV" to "1.36 kV (1.4-0.04)". When the average image ratio is "21%," the Vpp correction amount is "-0.10 kV", so the control unit 101 changes the setting of the peak-to-peak voltage from the reference peak-to-peak voltage "1.4 kV" to "1.30 kV (1.40-0.10)". In this way, according to the correction table in Table 1, when the average image ratio is the first ratio (7%), the control unit 101 sets the peak-to-peak voltage of the AC voltage to the first voltage (1.36 kV), and when the average image ratio is the second ratio (21%) that is larger than the first ratio, the control unit 101 sets the peak-to-peak voltage of the AC voltage to the second voltage (1.30 kV) that is smaller than the first voltage. In addition, the correction amount between the average image ratios and the average image ratios described in the correction table, including other correction tables described later, may be calculated by linear interpolation based on the correction amount described corresponding to each average image ratio.

[0042] Thereafter, the control unit 101 applies to the developing bias power supply 82 a superimposed voltage, which is a DC voltage superimposed with an AC voltage having a peak-to-peak voltage set according to the average image ratio described above, as a developing voltage, and performs image formation on the "n"th sheet of recording material S (S4). Each time image formation on one sheet of recording material S is completed, the control unit 101 determines whether there is a recording material S on which image formation will be performed following the current recording material S (S5). If there is no recording material S on which image formation will be performed (NO in S5), the control unit 101 ends this image formation process. On the other hand, if there is a recording material S on which image formation will be performed (YES in S5), the control unit 101 returns to the process of step S1.

[0043] Next, the effect of this embodiment will be described in comparison with the experimental results of a comparative example. Figures 7(a) and 7(b) are diagrams showing toner images formed on a recording material S to obtain the experimental results. Figure 8 is a graph showing the experimental results of this embodiment and the comparative example. As described below, the inventors conducted an experiment to examine the difference in reflection density between the area where a solid image and a halftone image formed on a recording material S using the image forming unit PK overlap, and the reflection density of the area where the solid image and the halftone image do not overlap.

[0044] In the experiment, first, the image forming unit PK outputs 200 black images with an image ratio of 5% on A4-sized recording material S. The developing voltage at this time is set to the standard developing voltage. After that, the image forming unit PK continuously forms a vertical solid image with an image ratio of 100% with a main scanning width of 30 mm and a sub-scanning width of 410 mm as shown in FIG. 7(a) on five sheets of A3-sized recording material S. Next, continuously, an image in which a horizontal band of 30HT black halftone is formed at the rear end of the vertical solid image is formed on one sheet of recording material S as shown in FIG. 7(b). Then, the reflection density of the overlapping portion G of the area surrounded by a dotted line immediately after the vertical solid image in the 30HT halftone image of the image shown in FIG. 7(b) and the non-overlapping portion where no solid image is formed immediately before the 30HT halftone image were measured, and the reflection density difference between the overlapping portion G and the non-overlapping portion was obtained. In the above experiment, the image ratio of the black image output to the recording material S for 200 sheets before forming the images shown in Fig. 7(a) and Fig. 7(b) was set to "5%, 10%, 20%, 30%", and the density step between the overlapping portion G and the non-overlapping portion for each image ratio was measured. At this time, the developing voltage when forming the image on the recording material S for each image ratio was set to the peak-to-peak voltage developing voltage set and changed based on the Vpp correction value shown in Table 1 above, and the density step between the overlapping portion G and the non-overlapping portion for each image ratio was compared with the state where the Vpp correction as shown in Table 1 above was not performed as a comparative example. Fig. 8 shows the experimental results. In Fig. 8, the comparative example is indicated by a dotted line, and this embodiment is indicated by a solid line.

[0045] As shown in Fig. 8, when the average image ratio is small, the reflection density difference is smaller in both this embodiment and the comparative example than when it is large. And, in all cases where the average image ratio is "5%, 10%, 20%, 30%, the reflection density difference in this embodiment is lower than the reflection density difference in the comparative example. Also, as the average image ratio increases, the difference between the reflection density in this embodiment and the reflection density in the comparative example increases. That is, in this embodiment, by changing the setting of the peak-to-peak voltage based on the average image ratio as described above, the reflection density difference can be reduced, that is, the occurrence of ghost images can be suppressed.

[0046] As described above, in this embodiment, the peak-to-peak voltage (Vpp) of the AC voltage is changed based on the average image ratio with respect to the developing voltage applied to the developing bias power supply 82 to develop a toner image. In consideration of the fact that the amount of toner and external additives moving from the developing container 201 to the photosensitive drum 1Y via the developing sleeve 203 increases as the average image ratio increases, the peak-to-peak voltage when the average image ratio is large is set lower than the peak-to-peak voltage when the average image ratio is small. When the average image ratio is large, the peak-to-peak voltage is lowered to reduce the amount of toner and external additives moving from the developing container 201 to the photosensitive drum 1Y via the developing sleeve 203. This reduces the amount of external additives supplied to the photosensitive drum 1Y during development, thereby suppressing the occurrence of ghost images due to the external additives remaining on the photosensitive drum 1Y.

[0047] Note that lowering the peak-to-peak voltage (Vpp) of the AC voltage may normally result in a decrease in the amount of toner supplied to the photosensitive drum 1Y, resulting in effects such as a decrease in image density. However, when image formation with a high image ratio continues, the number of times the toner is replenished by the toner replenishment device (not shown) increases, and the amount of uncharged toner in the developing container 201 increases. As a result, the charge of the toner in the developer is low and the electrostatic adhesion to the carrier is reduced, making the toner easier to develop. Therefore, even if the peak-to-peak voltage (Vpp) is lowered when the average image ratio is high, the effect on the image density is very minor.

[0048] [Second embodiment] In the first embodiment described above, the development voltage is changed according to the average image ratio, and the amount of external additives supplied to the photosensitive drum 1Y during development is reduced to suppress the occurrence of ghost images due to the external additives remaining on the photosensitive drum 1Y. In addition, when the toner image is primarily transferred from the photosensitive drum 1Y to the intermediate transfer belt 5, if the external additives remaining on the photosensitive drum 1Y are moved to the intermediate transfer belt 5 to reduce the amount of external additives on the photosensitive drum 1Y, the occurrence of ghost images due to the external additives can be further suppressed. The "image forming process" of the second embodiment that realizes this will be described with reference to FIG. 9. In the "image forming process" shown in FIG. 9, the same processes as those in the "image forming process" of the first embodiment described above (see FIG. 6) are assigned the same step numbers, and the description will be simplified or omitted.

[0049] As shown in FIG. 9, the control unit 101 calculates the image ratio of the recording material S on which the "n"th toner image is formed (S1), and calculates an average image ratio by averaging the image ratios from the start of use of the developing device 4Y to the image forming operation on the current (nth) recording material S (S2). Then, the control unit 101 refers to a peak-to-peak voltage correction table (see Table 1) and sets the developing voltage to be applied to the developing bias power source 82 based on the average image ratio (S3). Also, the control unit 101 sets the primary transfer current to be applied to the primary transfer nip T1Y in association with the application of a DC voltage by the primary transfer bias power source 83 based on the calculated average image ratio (S11). At this time, the control unit 101 refers to a correction table for the primary transfer current and determines the primary transfer current correction amount according to the average image ratio. Table 2 shows a correction table used when changing the setting of the primary transfer current based on the average image ratio. [Table 2]

[0050] As shown in Table 2, for example, when the average image ratio is "15%," the primary transfer current correction amount is "+2.0 μA", so the control unit 101 changes the setting of the primary transfer current from the reference primary transfer current "22.0 μA" to "24.0 μA (22.0 + 2.0)". For example, when the average image ratio is "45%," the primary transfer current correction amount is "+5.0 μA", so the control unit 101 changes the setting of the primary transfer current from the reference primary transfer current "22.0 μA" to "27.0 μA (22.0 + 5.0)". In this way, according to the correction table in Table 2, when the average image ratio is the first ratio (15%), the control unit 101 sets the primary transfer current to the first current (24.0 μA), and when the average image ratio is the second ratio (45%) that is greater than the first ratio, the control unit 101 sets the primary transfer current to the second current (27.0 μA) that is greater in absolute value than the first voltage. Thus, as shown in Table 2, as the average image ratio increases, the first transfer current is changed from the reference current by a larger "+" in absolute value.

[0051] Thereafter, the control unit 101 performs image formation on the "n"th sheet of recording material S (S12). At that time, the control unit 101 controls the developing bias power supply 82 to apply, as a developing voltage, a superimposed voltage obtained by superimposing an AC voltage having a peak-to-peak voltage set according to the average image ratio described above on a DC voltage. The control unit 101 also controls the primary transfer bias power supply 83 to apply, as a primary transfer voltage, a DC voltage through which a primary transfer current flows, set according to the average image ratio described above.

[0052] Then, each time image formation on one sheet of recording material S is completed, the control unit 101 determines whether there is a recording material S on which image formation will be performed next after the current recording material S (S5). If there is no recording material S on which image formation will be performed (NO in S5), the control unit 101 ends this image forming process. On the other hand, if there is a recording material S on which image formation will be performed (YES in S5), the control unit 101 returns to the process of step S1.

[0053] As described above, in the second embodiment, the setting of the primary transfer current is changed in addition to the peak-to-peak voltage (Vpp) of the AC voltage based on the average image ratio. As described above, when the average image ratio is large, the peak-to-peak voltage is lowered, so that the toner and external additives moving from the developing container 201 to the photosensitive drum 1Y via the developing sleeve 203 can be reduced. Furthermore, when the average image ratio is large, the absolute value of the primary transfer current is increased, so that the external additives can be moved from the photosensitive drum 1Y to the intermediate transfer belt 5. This reduces the amount of external additives remaining on the photosensitive drum 1Y, so that the occurrence of ghost images due to the external additives remaining on the photosensitive drum 1Y can be suppressed.

[0054] <Frequency> In the above embodiment, the peak-to-peak voltage of the AC voltage and the primary transfer current are changed based on the average image ratio, but this is not limited to the above. In addition to changing the peak-to-peak voltage or the peak-to-peak voltage and the primary transfer current, the control unit 101 may also change the frequency, duty ratio, and blank time (see FIG. 5) of the AC voltage in an appropriate combination based on the average image ratio. Table 3 shows a correction table used when changing the frequency of the AC voltage based on the average image ratio. [Table 3]

[0055] As shown in Table 3, for example, when the average image ratio is "3%," the frequency correction amount is "-0.30 kHz", so the control unit 101 changes the frequency setting from the reference frequency "11.0 kHz" to "10.7 kHz (11.0-0.30)". For example, when the average image ratio is "55%," the frequency correction amount is "-1.80 kHz", so the control unit 101 changes the frequency setting from the reference frequency "11.0 kHz" to "9.20 kHz (11.0-1.80)". In this way, according to the correction table in Table 3, the control unit 101 changes the frequency of the AC voltage to the first frequency (10.7 kHz) when the average image ratio is the first ratio (3%), and sets the frequency of the AC voltage to the second frequency (9.20 kHz) lower than the first frequency when the average image ratio is the second ratio (55%) which is greater than the first ratio. In this way, when the frequency is lowered, the number of times that the toner and external additives move between the developing sleeve 203 and the photosensitive drum 1Y is reduced, and the amount of external additives supplied to the photosensitive drum 1Y during development is further reduced, thereby making it possible to better suppress the occurrence of ghost images caused by external additives remaining on the photosensitive drum 1Y.

[0056] <Duty ratio> Table 4 shows a correction table used when changing the duty ratio of the AC voltage based on the average image ratio. In this specification, the duty ratio is the ratio of the application time of the maximum voltage to the application time of one cycle, which is the sum of the application time of the maximum voltage and the application time of the minimum voltage, in the AC bias section shown in FIG. [Table 4]

[0057] As shown in Table 4, for example, when the average image ratio is "5%," the duty ratio correction amount is "-1.5%, so the control unit 101 changes the duty ratio setting from the reference duty ratio "50%" to "48.5% (50-1.5)". For example, when the average image ratio is "20%," the duty ratio correction amount is "-4.50%, so the control unit 101 changes the duty ratio setting from the reference duty ratio "50%" to "45.5% (50-4.5)". In this way, according to the correction table in Table 4, when the average image ratio is the first ratio (5%), the control unit 101 sets the duty ratio to the first duty ratio (48.5%), and when the average image ratio is the second ratio (20%) which is larger than the first ratio, the control unit 101 sets the duty ratio to the second duty ratio (45.5%) which is lower than the first duty ratio. In this way, when the duty ratio of the AC voltage is reduced, the time (development time) for the toner and the external additive to move from the developing container 201 to the photosensitive drum 1Y side is reduced. Therefore, the amount of the external additive supplied to the photosensitive drum 1Y during development is further reduced, and the occurrence of ghost images due to the external additive remaining on the photosensitive drum 1Y can be further suppressed.

[0058] <Blank Time> Table 5 shows a correction table used when changing the setting of the blank time of the AC voltage based on the average image ratio. [Table 5]

[0059] As shown in Table 5, for example, when the average image ratio is "3%," the blank time correction amount is "0.03 μsec", so the control unit 101 changes the setting of the blank time from the reference blank time "0.27 μsec" to "0.30 μsec (0.27+0.03)". For example, when the average image ratio is "10%," the blank time correction amount is "0.06 μsec", so the control unit 101 changes the setting of the blank time from the reference blank time "0.27 μsec" to "0.33 μsec (0.27+0.06)". In this way, according to the correction table in Table 2, when the average image ratio is the first ratio (3%), the control unit 101 sets the blank time to the first time (0.30 μsec), and when the average image ratio is the second ratio (10%) that is larger than the first ratio, the control unit 101 sets the blank time to the second time (0.33 μsec) that is longer than the first time. In this way, by extending the blank time, the time (development time) for toner and external additives to move from the developing container 201 to the photosensitive drum 1Y side is reduced, thereby further reducing the amount of external additives supplied to the photosensitive drum 1Y during development, thereby further suppressing the occurrence of ghost images caused by external additives remaining on the photosensitive drum 1Y.

[0060] In the above-described embodiment, the image forming apparatus 100 is described as an intermediate transfer type in which the toner image is primarily transferred from the photosensitive drums 1Y to 1K of each color to the intermediate transfer belt 5, and then the toner image is secondarily transferred from the intermediate transfer belt 5 to the recording material S, but the present invention is not limited to this. The above-described embodiment is also applicable to a direct transfer type image forming apparatus in which the toner image is directly transferred to the recording material S from the photosensitive drums 1Y to 1K of each color that rotate while carrying the toner image.

[0061] In the above-described embodiment, the image forming apparatus 100 is provided with the developing devices 4Y to 4K in which the developer containing the non-magnetic toner, the magnetic carrier, and the external additive having the same charge polarity as the toner (so-called two-component developer) is contained in the developing container 201, but the present invention is not limited thereto. The above-described embodiment is also applicable to an image forming apparatus provided with a developing device in which the developer containing the toner and the external additive having the same charge polarity as the toner (so-called one-component developer) is contained in the developing container. [Explanation of symbols]

[0062] 1Y (1M, 1C, 1K)...image carrier (photosensitive drum), 2Y (2M, 2C, 2K)...charging section (charging roller), 3Y (3M, 3C, 3K)...exposure section (exposure device), 4Y (4M, 4C, 4K)...developing device, 5...another image carrier (intermediate transfer belt), 6Y (6M, 6C, 6K)...transfer member (primary transfer roller), 7Y (7M, 7C, 7K)...cleaning member (drum cleaner), 82...developing power supply (developing bias power supply), 83...transfer power supply (primary transfer bias power supply), 101...setting section (control section), 201...developing container, 203...developer carrier (developing sleeve), S...recording material

Claims

1. An image forming apparatus for forming an image on a recording material, A rotating image carrier; a charging unit that charges a surface of the image carrier; an exposure section for exposing the charged surface of the image carrier to light based on image data to form an electrostatic latent image; a developing device including a developer container that contains a developer including a toner and an external additive having the same charge polarity as the toner, and a developer carrier that rotates while carrying the developer contained in the developer container and develops an electrostatic latent image formed on the image carrier into a toner image with the developer; a developing power source that applies a superimposed voltage, which is a DC voltage and an AC voltage, to the developer carrier in order to develop the electrostatic latent image formed on the image carrier with a developer; a setting unit which sets a peak-to-peak voltage of the AC voltage to a first voltage when an average image ratio obtained by averaging image ratios based on the image data from when the developing device was first used until a current image forming operation is performed is a first ratio when an image forming operation is performed on a recording material, and sets the peak-to-peak voltage of the AC voltage to a second voltage smaller than the first voltage when the average image ratio is a second ratio larger than the first ratio, 1. An image forming apparatus comprising:

2. the setting unit sets a frequency of the AC voltage to a first frequency when the average image ratio is the first ratio, and sets the frequency of the AC voltage to a second frequency lower than the first frequency when the average image ratio is the second ratio.

2. The image forming apparatus according to claim 1,

3. the setting unit sets a duty ratio during which a maximum voltage is applied in one cycle of the AC voltage to a first duty ratio when the average image ratio is the first ratio, and sets a duty ratio to a second duty ratio lower than the first duty ratio when the average image ratio is the second ratio.

2. The image forming apparatus according to claim 1,

4. the developing power source outputs a waveform having one cycle including an AC bias portion in which an AC voltage and a DC voltage are superimposed, and a blank portion consisting of only a DC voltage following the AC bias portion; the setting unit sets a time of the blank portion in one cycle to a first time when the average image ratio is the first ratio, and sets a time of the blank portion in one cycle to a second time longer than the first time when the average image ratio is the second ratio.

2. The image forming apparatus according to claim 1,

5. a transfer member for transferring the toner image formed on the image carrier to another image carrier; a transfer power source that applies a transfer current to the transfer member so as to transfer a toner image from the image carrier to the other image carrier; the setting unit sets the transfer current to a first current when the average image ratio is the first ratio, and sets the transfer current to a second current whose absolute value is greater than that of the first current when the average image ratio is the second ratio.

2. The image forming apparatus according to claim 1,

6. a transfer member for transferring the toner image formed on the image carrier to another image carrier; a cleaning member that comes into contact with the image carrier and removes toner remaining on the image carrier after the toner image is transferred by the transfer member; 2. The image forming apparatus according to claim 1,

Images