Image forming apparatus

Abstract

To provide an image forming apparatus that can prevent the occurrence of image defect due to an external additive added to toner.SOLUTION: A control unit, when receiving an instruction to start a continuous image formation job, controls a charging bias power supply to start application of a charging voltage to adjust the electric potential of a photoconductor drum to a charging electric potential in a developing period (S2), and controls a developing bias power supply to start application of a DC voltage in the developing period and application of an AC voltage in the developing period (S3, S4). When there is a subsequent recording material (YES in S8), the control unit controls the developing bias power supply to increase the peak-to-peak voltage of an AC voltage and "Vback" in a non-developing period compared to the developing period, and thereby starts application of the AC voltage in the non-developing period (S15) and application of the DC voltage in the non-developing period (S16). Consequently, in the non-developing period of the continuous image formation job, recovery of an external additive from the photoconductor drum by an electric field on a recovery side resulting from "Vback" and recovery of the external additive from the photoconductor drum by a magnetic carrier resulting from the peak-to-peak voltage, are performed.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, 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 to form an electrostatic latent image, a developing container for accommodating a developer containing a non-magnetic toner, a magnetic carrier, and an external additive having the same charging polarity as the toner, and a developing device including a developer carrier that rotates while carrying the developer contained in the developing container and develops an electrostatic latent image formed on the image carrier into a toner image with the developer at a developing position facing the image carrier, a developing power source for applying a superimposed voltage of a DC voltage and an AC voltage to the developer carrier in order to develop an electrostatic latent image formed on the developer carrier with the developer, and a developing device for recording the toner image formed on the image carrier, the developing device including a developing power source for applying a superimposed voltage of a DC voltage and an AC voltage to the developer carrier, the ... the control unit controls the developing power supply, and during a continuous image forming job in which images are formed continuously on a plurality of recording materials, when an image area on the surface of the image carrier corresponding to the recording material to which a toner image is transferred passes through the developing position, the control unit sets the peak-to-peak voltage of the AC voltage to a first peak-to-peak voltage and the DC voltage to a first DC voltage, and during a non-developing time when a non-image area on the surface of the image carrier located between two successive image areas in the rotational direction of the image carrier passes through the developing position, the control unit sets the peak-to-peak voltage of the AC voltage to a second peak-to-peak voltage greater than the first peak-to-peak voltage and the DC voltage to a second DC voltage smaller in absolute value than the first DC voltage. 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. 4A is a diagram showing the waveform of a developing voltage applied during development, and FIG. 4B is a diagram showing the waveform of a developing voltage applied during non-development. [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] 13 is a graph showing the reflection density difference of the experimental results. [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.

[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 rotationally driven 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 application of 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 according to image information, 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 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 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 the 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 on 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 on 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] As shown in Fig. 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 information and inputs from various sensors. The memory 103 stores various programs such as "image forming process" (see Fig. 6 described later), and various data such as development voltages (DC voltage value and AC voltage value described later) to be applied during development and non-development. The CPU 102 is capable of executing the various programs stored in the memory 103, and executes the various programs to operate the image forming apparatus 100.

[0026] In this specification, the developing time is a period during which an image area on the surface of the photosensitive drum 1Y corresponding to the recording material S to which the toner image is transferred passes through the developing position c in a continuous image forming job in which images are formed continuously on multiple recording materials S. The non-developing time is a period during which a non-image area located between two continuous image areas on the surface of the photosensitive drum 1Y in the rotation direction of the photosensitive drum 1Y passes through the developing position c in a continuous image forming job in which images are formed continuously on multiple recording materials S. In other words, the non-developing time is a period during which a non-image area on the surface of the photosensitive drum 1Y corresponding to the area between the preceding recording material S and the succeeding recording material S (so-called paper gap) passes through the developing position c.

[0027] A continuous image forming job is a period from the start of image formation to the completion of image forming operation based on a print signal for continuously forming images on multiple recording materials S. Specifically, it refers to the period from pre-rotation (preparatory operation before image formation) after receiving a print signal (input of an image forming job) to post-rotation (operation after image formation), and includes the image formation period and the interval between sheets. For example, if one job is followed by another job, these are collectively determined as one continuous image forming job.

[0028] The control unit 101 is connected to a temperature and humidity sensor 110, 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 temperature and humidity sensor 110 as a humidity detection unit is disposed inside the image forming apparatus main body (see FIG. 1), and detects the temperature and relative humidity (hereinafter simply referred to as humidity) inside the image forming apparatus main body.

[0029] 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 charging potential (Vd) of negative polarity. 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 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 on the photosensitive drum 1Y where the electrostatic latent image is formed (also called the exposed area potential) changes to, for example, "-150V". The potential of the non-image area on the photosensitive drum 1Y where the electrostatic latent image is not formed because it is not exposed by the exposure device 3Y remains at the charging potential "-700V".

[0030] 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 "11kHz" and a peak-to-peak voltage of "1.4kV" output from the AC power supply 82b are superimposed as a developing voltage during development to the developing sleeve 203. The DC voltage (Vdc) of the developing voltage is a potential between the charging potential (Vd) and the exposed portion potential. The primary transfer bias power supply 83 applies a positive polarity primary transfer voltage for performing primary transfer to the primary transfer roller 6Y. The secondary transfer bias power supply 84 applies a positive secondary transfer voltage to the outer secondary transfer roller 64 for performing secondary transfer.

[0031] 5(a) and 5(b) show waveforms of the developing voltage applied by the developing bias power supply 82. Fig. 5(a) shows the waveform of the developing voltage applied during development, and Fig. 5(b) shows the waveform of the developing voltage applied during non-development.

[0032] 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 a square wave with a frequency of "11 kHz". As shown in Figs. 5(a) and 5(b), the developing voltage has blank portions in which the AC voltage is intermittently thinned out and only the DC voltage is present. 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". Also, the portion that has become a DC voltage by intermittently thinning out the AC voltage is referred to as a "blank portion".

[0033] Therefore, the developing voltage applied by the developing bias power supply 82 has a waveform in which one period consists of 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, as shown in Figures 5(a) and 5(b).

[0034] 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 a half cycle of the square wave. The blank portion time in one cycle of the developing voltage is defined as blank time t1. The total application time of the maximum voltage in one cycle of the developing voltage in which an electric field (pulse) on the developing side of the AC bias portion is generated is defined as developing time t2, and the total application time of the minimum voltage in which an electric field on the recovery side is generated is defined as recovery time t3. Furthermore, the ratio (duty ratio) of the electric field on the developing side to the electric field on the recovery side in the AC bias portion is set to "50%". Here, the electric field on 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 on 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.

[0035] During development, the sum of the voltages (corresponding to the amplitude of the AC bias section) on the development side (developer application side) and recovery side (developer return side) in the AC bias section is set to "Vpp1", and here, as an example, "Vpp1" 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" during development forms an electric field (electric field on the recovery side) that pulls the toner back from the photosensitive drum 1Y to the developing device 4Y, specifically, to the developing sleeve 203. This is to suppress the occurrence of image defects called fog images caused by so-called fogging, in which the toner adheres to the non-image portion of the photosensitive drum 1Y during development.

[0036] 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.

[0037] 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.

[0038] 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.

[0039] Therefore, in this embodiment, the developing voltage applied during non-development is made higher in peak-to-peak voltage (Vpp) than the developing voltage applied during development, and "Vback" is also made larger, so that the external additive can be recovered from the photosensitive drum 1Y to the developing device 4Y. Hereinafter, the image forming process of this embodiment that realizes this will be described 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 continuous image forming jobs by the control unit 101.

[0040] <Image formation process> As shown in FIG. 6, when the control unit 101 receives a start command for a continuous image forming job from an operation unit (not shown) or an external device (for example, a personal computer), the control unit 101 controls a drive motor (not shown) to start rotating the photosensitive drum 1Y (S1). The control unit 101 controls the charging bias power supply 81 to start applying a charging voltage so that the charging potential (charging potential Vd1 during development) of the photosensitive drum 1Y becomes, for example, "-700V" (S2). Then, the control unit 101 sets the DC voltage to, for example, "-550V" (first DC voltage, DC voltage Vdc1 during development) and the peak-to-peak voltage to, for example, "1400V" (first peak-to-peak voltage, AC voltage Vpp1 during development) as the development voltage to be applied during development, and causes the development bias power supply 82 to apply them (S3, S4). At this time, "Vback1" during development becomes "150V (700V-550V)" (see FIG. 5(a)). Then, the control unit 101 controls a motor (not shown) to start rotating the developing sleeve 203 (S5). Then, the control unit 101 starts conveying the recording material S to form an image on the recording material S (S6). Every time image formation on one sheet of the recording material S is completed (S7), the control unit 101 determines whether there is a subsequent recording material S on which image formation is to be performed continuously (S8).

[0041] If there is no subsequent recording material S (NO in S8), the control unit 101 stops the rotational driving of the developing sleeve 203 (S9). The control unit 101 also stops the application of AC and DC voltages by the developing bias power supply 82 (S10, S11). Then, the control unit 101 stops the application of the charging voltage by the charging bias power supply 81 (S12), and thereafter stops the rotational driving of the photosensitive drum 1Y (S13), thereby completing this image forming process.

[0042] On the other hand, if there is a subsequent recording material S (YES in S8), the control unit 101 switches the developing voltage to the developing voltage applied during non-development (S14). The control unit 101 sets the peak-to-peak voltage to, for example, "1.8 kV" (second peak-to-peak voltage, AC voltage Vpp2 during non-development) and the DC voltage to, for example, "-490 V" (second DC voltage, DC voltage Vdc2 during non-development) as the developing voltage applied during non-development, and applies them to the developing bias power supply 82 (S15, S16). At this time, "Vback2" during non-development becomes "210 V (700 V-490 V)" (see FIG. 5(b)). After that, the control unit 101 returns to the process of step S3 to return to the developing voltage during development for the subsequent recording material S.

[0043] The effects of this embodiment will be described in comparison with the experimental results of a conventional example and a comparative example. Figures 7(a) and 7(b) are diagrams showing toner images formed on a recording material S to obtain experimental results. Figure 8 is a graph showing the experimental results of this embodiment and the experimental results of the conventional example 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 30% onto A4-sized recording material S. The development voltage at this time was set to the same as the development voltage (Vpp, Vback) of the conventional example shown in Table 1. This increases the concentration of the external additive in the developer container 201. After that, the image forming unit PK continuously formed 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. The development voltage when forming the solid image was set to the same as the development voltage (Vpp, Vback) of the conventional example shown in Table 1. Next, as shown in FIG. 7(b), a horizontal image with a black halftone of 30HT formed at the rear end of the vertical solid image was continuously formed on one sheet of recording material S. When the images of Figures 7(a) and 7(b) were formed continuously, the development voltages during non-development were set to the development voltages (Vpp2, Vback2) of the conventional example, this embodiment, Comparative Example 1, and Comparative Example 2 shown in Table 1. [Table 1]

[0045] As shown in Table 1, in the conventional example, the development voltage during non-development (Vpp2, Vback2) is the same as the development voltage during development (Vpp1, Vback1). In comparative example 1, the peak-to-peak voltage during non-development (Vpp2) is greater than the peak-to-peak voltage during development (Vpp1). In comparative example 2, "Vback2" during non-development is greater than "Vback1" during development. In this embodiment, as described above, the development voltage during non-development is greater than the development voltage during development. In this embodiment, the electric field on the recovery side needs to be increased to accommodate the increase in the amount of toner flying that accompanies the increase in peak-to-peak voltage (Vpp2), so the set value of Vback2 also increases.

[0046] Then, the reflection density of the overlapping portion G of the area surrounded by the dotted line immediately after the vertical solid image in the halftone image of 30HT in the image shown in FIG. 7(b) and the non-overlapping portion where no solid image is formed immediately before the halftone image of 30HT were measured, and the reflection density difference between the overlapping portion G and the non-overlapping portion was obtained. FIG. 8 shows the reflection density difference of the experimental result. As shown in FIG. 8, the reflection density difference in the conventional example was "0.031". In this embodiment, the reflection density difference was "0.019". In the comparative example 1, the reflection density difference was "0.037". In the comparative example 2, the reflection density difference was "0.027". That is, by increasing both the peak-to-peak voltages "Vpp2=1.8kV" and "Vback2=210V", the reflection density difference can be reduced compared to the conventional example, comparative example 1, and comparative example 2. In other words, the occurrence of ghost images can be suppressed.

[0047] In Comparative Example 1, the reason why the density difference becomes large is because the electric field on the development side becomes stronger by increasing only the peak-to-peak voltage (Vpp2). By increasing the electric field on the development side, even during non-development, external additives can move from the developing device 4K together with the toner and accumulate on the photosensitive drum 1K. As the external additives accumulate on the photosensitive drum 1K, the absolute value of the potential becomes smaller than the exposure potential in the range where the electrostatic latent image formed during development of the next recording material S and the portion where the external additives have accumulated overlap, and more toner adheres, resulting in a larger density difference.

[0048] In Comparative Example 2, the electric field on the recovery side is strengthened by increasing only "Vback2", and the external additive is recovered from the photosensitive drum 1K to the developing device 4K, so the concentration difference is smaller than in the conventional example and Comparative Example 1. However, in order to obtain the same effect as in this embodiment by increasing only "Vback2", it is necessary to increase "Vback2" to about "250V". However, in such a case, the electric field in the direction in which the magnetic carrier, which has a charge polarity opposite to that of the toner, moves from the developing device 4K to the photosensitive drum 1K becomes stronger, and the magnetic carrier moves to the photosensitive drum 1K and the adhesion amount increases. If the adhesion amount of the magnetic carrier to the photosensitive drum 1K increases, a large amount of the magnetic carrier may remain on the drum cleaner 7K, which recovers the toner remaining on the photosensitive drum 1K after the primary transfer. This may cause damage to the cleaner blade (not shown) of the drum cleaner 7K. Alternatively, the surface of the photosensitive drum 1K may be scratched by the magnetic carrier accumulated between the cleaner blade and the photosensitive drum 1K, and image defects may occur due to the scratches. Therefore, it is difficult to adopt a method of decreasing the density difference by increasing only "Vback2".

[0049] In contrast, in this embodiment, "Vback2" during non-development is set to, for example, "210V or more and 230V or less." That is, in this embodiment, "Vback2" during non-development is suppressed to about "230V," and in addition to increasing "Vback2," the peak-to-peak voltage "Vpp2" is also increased. By doing so, the amount of magnetic carrier that contacts the surface of the photosensitive drum 1K from the developing sleeve 203 increases. Since the magnetic carrier has a polarity opposite to that of the external additive, the external additive remaining on the photosensitive drum 1K adheres to the magnetic carrier and is collected into the developing container 201 by the rotation of the developing sleeve 203. In this way, in this embodiment, in addition to strengthening the electric field on the collection side at "Vback2" compared to Comparative Example 2, the peak-to-peak voltage "Vpp2" is also increased, so that the contact amount of the magnetic carrier with the photosensitive drum 1K increases, and the collection of the external additive remaining on the photosensitive drum 1K is actively performed. In addition, the scattering of toner and external additives caused by increasing the peak-to-peak voltage "Vpp2" is substantially suppressed by the strengthening of the electric field on the recovery side by "Vback2". Therefore, in this embodiment, the amount of external additives remaining on the photosensitive drum 1K does not increase.

[0050] As described above, in this embodiment, the peak-to-peak voltage (Vpp) and "Vback" are made larger during non-development than during development. In order to make "Vback" larger, the DC voltage (Vdc2) of the development voltage is made smaller in absolute value than during development. By doing so, the recovery of the external additive from the photosensitive drum (1Y to 1K) by the electric field on the recovery side caused by "Vback" and the recovery of the external additive from the photosensitive drum (1Y to 1K) by the magnetic carrier caused by the peak-to-peak voltage "Vpp" are performed during non-development during an image forming job that forms an image on the recording material S. This makes it possible to suppress the occurrence of image defects caused by the external additive added to the toner.

[0051] [Second embodiment] Incidentally, the inventors have confirmed through experiments that the external additives are more likely to remain on the photosensitive drums (1Y to 1K) during development when the humidity in the image forming apparatus body is low than when the humidity is high. This is because the charge amount of the external additives is affected by humidity. That is, when the humidity is low, the charge amount of the external additives is larger than when the humidity is high, and the amount of the external additives remaining on the photosensitive drums (1Y to 1K) tends to increase. In view of this, it is conceivable to more reliably recover the external additives on the photosensitive drums (1Y to 1K) by changing the development voltages (Vpp and Vback) depending on the humidity. Hereinafter, the "image forming process" of the second embodiment for achieving 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. In addition, the image forming unit PY will be described below as an example.

[0052] 9, when the control unit 101 receives a command to start a continuous image forming job from an operation unit (not shown) or an external device, the control unit 101 identifies the humidity inside the image forming apparatus body based on a detection signal transmitted from the temperature and humidity sensor 110 (S21). Then, the control unit 101 determines the development voltages (Vpp1, Vback1) to be applied during development and the development voltages (Vpp2, Vback2) to be applied during non-development based on the environment table shown in Table 2 (S22).

[0053] Table 2 shows the environment table. In the environment table shown in Table 2, the values ​​of the peak-to-peak voltages "Vpp1, Vpp2" and "Vback1, Vback2" are specified for each humidity level determined based on the detection signal (detection result) of the temperature and humidity sensor 110. This environment table is stored in advance in the memory 103 (see FIG. 4). In Table 2, for example, when the humidity is less than 5%, the peak-to-peak voltages "1.60 kV, 1.80 kV" and Vback "145 V, 230 V" are specified, and when the humidity is equal to or greater than 5% and less than 20%, the peak-to-peak voltages "1.53 kV, 1.80 kV" and Vback "147 V, 220 V" are specified. [Table 2]

[0054] As shown in Table 2, the environment table specifies that the peak-to-peak voltages of the development voltages "Vpp1, Vpp2" and "Vback1, Vback2" are larger on the low humidity side than on the high humidity side, regardless of whether they are in development or non-development. "Vback2" during non-development is, for example, "210V or more and 230V or less."

[0055] The control unit 101 controls the drive motor to start the rotational drive of the photosensitive drum 1Y (S1). The control unit 101 controls the charging bias power supply 81 to start the application of the charging voltage (S2). Then, the control unit 101 controls the developing bias power supply 82 to start the application of the developing voltage during development (S3, S4). At this time, the control unit 101 makes the developing bias power supply 82 apply the developing voltage during development according to the peak-to-peak voltages "Vpp1" and "Vback1" determined according to the humidity. For example, when the humidity is "52%," the developing voltage is applied according to the environmental table of Table 2 such that the peak-to-peak voltages "Vpp1" are "1.37 kV" and "Vback1" are "158 V". The control unit 101 starts the rotational drive of the developing sleeve 203 (S5), and then starts the transport of the recording material S to form an image on the recording material S (S6). Every time image formation on one sheet of recording material S is completed (S7), the control section 101 determines whether or not there is a subsequent recording material S on which image formation is to be performed successively (S8).

[0056] If there is no subsequent recording material S (NO in S8), the control unit 101 stops the rotational driving of the developing sleeve 203 (S9). The control unit 101 also stops the application of AC and DC voltages by the developing bias power supply 82 (S10, S11). Then, the control unit 101 stops the application of the charging voltage by the charging bias power supply 81 (S12), and thereafter stops the rotational driving of the photosensitive drum 1Y (S13), thereby completing the image forming process.

[0057] On the other hand, if there is a succeeding recording material S (YES in S8), the control unit 101 switches the developing voltage to the developing voltage applied during non-development (S14). At this time, the control unit 101 causes the developing bias power supply 82 to apply the developing voltage during non-development according to the peak-to-peak voltages "Vpp2" and "Vback2" determined according to the humidity (S23, S24). For example, when the humidity is "52%," a developing voltage is applied in which the peak-to-peak voltage "Vpp2" is "1.77 kV" and the peak-to-peak voltage "Vback2" is "210 V" according to the environment table of Table 2. When the humidity is "33%," a developing voltage is applied in which the peak-to-peak voltage "Vpp2" is "1.80 kV" and the peak-to-peak voltage "Vback2" is "215 V" according to the environment table of Table 2. That is, when the relative humidity is the second humidity (33%) lower than the first humidity (52%), the peak-to-peak voltage is set to "1.77 kV" (third peak-to-peak voltage) which is higher than "1.37 kV" (second peak-to-peak voltage). Also, the DC voltage is set to a DC voltage (third DC voltage) with a smaller absolute value at which "Vback2" is "215 V" than the DC voltage (second DC voltage) at which "Vback1" is "158 V". Thereafter, the control unit 101 returns to the process of step S3 to return to the developing voltage at the time of developing the subsequent recording material S.

[0058] In this way, in the second embodiment, the peak-to-peak voltage (Vpp2) and "Vback2" applied during non-development can be controlled according to the detection result of the temperature and humidity sensor 110. That is, the peak-to-peak voltage (Vpp2) and DC voltage (Vdc2) are controlled to be appropriate for recovering the external additive from the photosensitive drum 1Y according to the amount of the external additive remaining on the photosensitive drum 1Y, which can change depending on the humidity. This makes it possible to efficiently suppress the occurrence of image defects caused by the external additive, regardless of the humidity inside the image forming apparatus main body.

[0059] <Other embodiments> In the above-described embodiment, the peak-to-peak voltage of the AC voltage is controlled, but this is not limiting. In addition to the peak-to-peak voltage, for example, the duty ratio of the AC voltage may be further controlled. The case where the duty ratio of the AC voltage is controlled is shown in Table 3 below. In this specification, the duty ratio is the proportion of the application time of the maximum voltage to the 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. 5(a). [Table 3]

[0060] As shown in Table 3, during development, the duty ratio of the AC voltage is set to a first duty ratio (e.g., 50%), and during non-development, the duty ratio of the AC voltage is set to a second duty ratio (e.g., 45%) lower than the first duty ratio. When the duty ratio of the AC voltage is lowered, the time during which the magnetic carrier contacts the photosensitive drum 1Y side from the developing container 201 increases. Therefore, during non-development, it becomes easier to collect the toner via the magnetic carrier, and it becomes easier to remove the external additives on the photosensitive drum 1Y.

[0061] 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. [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, 7Y (7M, 7C, 7K)...cleaning member (drum cleaner), 82...developing power source (developing bias power source), 101...control section, 110...humidity detection section (temperature and humidity sensor), 190...transfer device, 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 to form an electrostatic latent image; a developing device including a developing container that contains a developer containing a non-magnetic toner, a magnetic carrier, 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 developing container and develops an electrostatic latent image formed on the image carrier into a toner image with the developer at a developing position facing the image carrier; 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 developer carrier with a developer; a transfer device that transfers the toner image formed on the image carrier onto a recording material; A control unit that controls the developing power source, The control unit, in a continuous image forming job for continuously forming images on a plurality of recording materials, during development when an image area on the surface of the image carrier, which corresponds to a recording material onto which a toner image is to be transferred, passes through the development position, a peak-to-peak voltage of the AC voltage is set to a first peak-to-peak voltage, and the DC voltage is set to a first DC voltage, during a non-development period in which a non-image area located between two consecutive image areas in a rotation direction of the image carrier passes through the development position on the surface of the image carrier, the peak-to-peak voltage of the AC voltage is set to a second peak-to-peak voltage that is greater than the first peak-to-peak voltage, and the DC voltage is set to a second DC voltage that is smaller in absolute value than the first DC voltage, 1. An image forming apparatus comprising:

2. A humidity detector for detecting relative humidity is provided. The control unit, during the non-development period, when the relative humidity detected by the humidity detection unit is a first humidity, the peak-to-peak voltage of the AC voltage is set to the second peak-to-peak voltage and the DC voltage is set to the second DC voltage, when the relative humidity detected by the humidity detection unit is a second humidity lower than the first humidity, the peak-to-peak voltage of the AC voltage is set to a third peak-to-peak voltage higher than the second peak-to-peak voltage, and the DC voltage is set to a third DC voltage lower in absolute value than the second DC voltage.

2. The image forming apparatus according to claim 1,

3. the control unit sets a duty ratio during which a maximum voltage is applied in one cycle of the AC voltage to a first duty ratio during the developing operation, and sets a duty ratio during the non-developing operation to a second duty ratio lower than the first duty ratio.

2. The image forming apparatus according to claim 1,

4. 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 device; 2. The image forming apparatus according to claim 1,

Images