Image forming device

Abstract

To suppress occurrence of an image defect due to faulty recovery of toner.SOLUTION: During execution of image forming operation to form an image on a recording material, an image forming apparatus recovers, with a developing member, a toner not transferred by transfer means and remaining on the surface of an image carrier. With a difference between a developing voltage and a surface potential of the image carrier in a developing unit as Vbc, before start of the image forming operation, control means executes preparation operation to start rotation of the image carrier and gradually raise an electrification voltage and a developing voltage, and in the preparation operation, causes electric charge application means to apply electric charges, and controls the electrification voltage and the developing voltage so that the value of Vbc at the time point when a surface area of the image carrier located at the developing unit at the start of rotation of the image carrier reaches the developing unit again becomes larger than the value of Vbc in the image forming operation.SELECTED DRAWING: Figure 7

Description

[Technical Field]

[0001] The present invention relates to an image forming apparatus that forms an image on a recording material. [Background technology]

[0002] In electrophotographic image forming devices, the surface of an image carrier such as a photosensitive drum is uniformly charged by a charging member, an exposure unit exposes the surface of the image carrier to light to write an electrostatic latent image, and a developing member develops the electrostatic latent image into a toner image using toner (developer). The phenomenon of thin toner adhering to areas of the image carrier surface where no image is to be formed is called fogging. To prevent fogging, the potential difference (back contrast, fogging removal contrast) between the image carrier and the developing member in the developing unit where the image carrier and developing member face each other is controlled within an appropriate range.

[0003] Patent Document 1 describes a method for suppressing the occurrence of fogging at startup by applying a voltage of opposite polarity to the normal charging polarity of toner to the developing member simultaneously with the startup of the motor that drives the image carrier. Patent Document 2 describes a method for maintaining the potential difference between the developing sleeve and the photoconductor in the development unit within a predetermined range that can suppress the adhesion of a large amount of toner to the photoconductor by gradually increasing the voltage applied to the charger and the voltage applied to the developing sleeve when the photoconductor starts to rotate.

[0004] On the other hand, Patent Document 3 describes a cleaner-less image forming apparatus in which, after a toner image on an image carrier is transferred to a recording material, the untransferred toner remaining on the image carrier is collected by a developing member and reused, rather than collected by a cleaning member. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Japanese Patent Application Laid-Open No. 2005-345915 [Patent Document 2] Japanese Patent Application Publication No. 7-253693 [Patent Document 3] Japanese Patent Application Publication No. 59-133573 Summary of the Invention [Problem to be solved by the invention]

[0006] However, even when the configurations described in Patent Documents 1 and 2 are used, fogging may occur when an image forming operation is performed after the image forming apparatus has been in standby mode for a long period of time. That is, when the charge of the toner carried on the developing member decays and the charge amount becomes very small during a long stop period, the toner non-electrostatically adheres to the image carrier, causing fogging, with almost no effect from the potential difference between the developing member and the image carrier in the developing unit.

[0007] In a cleanerless image forming apparatus, if the fogging toner adhering to the image carrier cannot be sufficiently collected before the image forming operation starts, there is a possibility that a toner image will adhere to an area of ​​the recording material where an image is not to be formed, resulting in an image defect (white background staining).

[0008] SUMMARY OF THE INVENTION It is therefore an object of the present invention to provide an image forming apparatus that can prevent the occurrence of image defects due to insufficient toner collection. [Means for solving the problem]

[0009] One aspect of the present invention is a method for manufacturing a toner image forming apparatus comprising: a rotatable image carrier; a charging member that forms a charging section between the image carrier and the charging member and charges the surface of the image carrier; a developing member that is arranged in contact with the image carrier and forms a developing section between the image carrier and the charging member and supplies toner to the developing section to form a toner image on the image carrier; transfer means that transfers the toner image from the image carrier to a transfer material; charge applying means that is provided opposite the image carrier; and control means that controls a charging voltage that is applied to the charging member and a developing voltage that is applied to the developing member, and during an image forming operation that forms an image on a recording material, toner that remains on the surface of the image carrier and that has not been transferred by the transfer means is transferred by the developing member. and the control means, before the start of the image forming operation, starts the rotation of the image carrier and executes a preparatory operation of gradually increasing the charging voltage and the developing voltage, where the difference between the surface potential of the image carrier in the developing unit and the developing voltage is Vbc, causes the charge applying means to apply a charge in the preparatory operation, and controls the charging voltage and the developing voltage in the preparatory operation so that the value of Vbc at the time when the surface area of ​​the image carrier that was located in the developing unit at the time the image carrier started rotating reaches the developing unit again becomes larger than the value of Vbc in the image forming operation. [Effects of the Invention]

[0010] SUMMARY OF THE INVENTION An object of the present invention is to provide an image forming apparatus that can prevent the occurrence of image defects due to insufficient toner collection. [Brief explanation of the drawings]

[0011] [Figure 1] 1 is a schematic diagram of an image forming apparatus according to a first embodiment. [Figure 2] FIG. 4 is a graph showing the relationship between the back contrast and the amount of fogging toner in the first embodiment. [Figure 3] 3A to 3C are operation process diagrams of the image forming operation in the first embodiment. [Figure 4] 4 is a timing chart of the pre-rotation operation in the first embodiment. [Figure 5] 5A and 5B are diagrams showing changes in the surface potential of the photosensitive drum and the development voltage in the development unit during pre-rotation in the first embodiment. [Figure 6] 10A and 10B are diagrams showing the transition of the back contrast during the forward rotation operation in the first embodiment. [Figure 7] 5 is a timing chart of a drive motor and back contrast in a forward rotation operation of the image forming apparatus according to the first embodiment. [Figure 8] 4A and 4B are diagrams showing the state of the image forming apparatus during pre-rotation in the first embodiment. [Figure 9] FIG. 10 is a diagram showing a state of the image forming apparatus at time t3 during the pre-rotation operation in the first embodiment. [Figure 10] FIG. 10 is a diagram showing a state of the image forming apparatus at time t3′ during the pre-rotation operation in the first embodiment. [Figure 11] FIG. 10 is a diagram showing a state of the image forming apparatus at time t10 during the pre-rotation operation in the first embodiment. [Figure 12] FIG. 10 is a diagram showing a state of the image forming apparatus at time t11 during the pre-rotation operation in the first embodiment. [Figure 13] 10 is a timing chart of a drive motor and back contrast in a forward rotation operation of an image forming apparatus in Comparative Example 1. [Figure 14] 10 is a timing chart of a drive motor and back contrast in a forward rotation operation of an image forming apparatus in Comparative Example 2. [Figure 15] 10 is a timing chart of a drive motor and back contrast in a forward rotation operation of an image forming apparatus in Comparative Example 3. [Figure 16] 10 is a timing chart of a drive motor and back contrast in a forward rotation operation of an image forming apparatus according to a second embodiment. [Figure 17] FIG. 10 is a diagram showing a state of the image forming apparatus at time t12 during the pre-rotation operation in the second embodiment. [Figure 18] FIG. 11 is a diagram showing a state of the image forming apparatus at time t13 during the pre-rotation operation in the second embodiment. [Figure 19] 10A and 10B are schematic diagrams showing modified examples of the image forming apparatus. [Figure 20] FIG. 10 is a diagram showing an evaluation image of an E character evaluation. [Figure 21] FIG. 10 is a graph showing the relationship between the time when the device is left standing and the density of fog at startup. [Figure 22] 10A and 10B are diagrams showing changes in the charge amount of fog toner before and after the charging roller. [Figure 23] 10A and 10B are diagrams showing the difference in characteristics between toner adhering to a collection area and toner adhering to a non-collection area on a developing roller. DETAILED DESCRIPTION OF THE INVENTION

[0012] Hereinafter, embodiments of the present disclosure will be described with reference to the drawings.

[0013] First Embodiment The overall configuration of an image forming apparatus according to the first embodiment will be described with reference to Figure 1. Figure 1 is a schematic diagram showing the cross-sectional configuration of an image forming apparatus 100 according to this embodiment. The image forming apparatus 100 is a monochrome laser printer that forms an image on a recording material (recording medium) R based on image information received from an external computer. A variety of sheet materials of different sizes and materials can be used as the recording material R, including paper such as plain paper and cardboard, surface-treated sheet materials such as plastic film, cloth, and coated paper, and sheet materials with special shapes such as envelopes and index paper.

[0014] The image forming apparatus 100 includes a process cartridge 10 as an image forming unit that creates a toner image. The process cartridge 10 includes a photosensitive drum 1 as an image carrier, a charging roller 2 as a charging member, a developing device 20, and a pre-exposure LED 6 as a pre-exposure device (discharge device). The image forming apparatus 100 also includes an exposure unit 3 as an exposure device, a transfer roller 5 as transfer means, the pre-exposure LED 6 as a pre-exposure device, a fixing device 7 as fixing means, and a control unit 50 as control means that controls the image forming apparatus 100.

[0015] In the following description of this embodiment, toner 44 having a normal negative charging polarity is used as the developer, and a reversal development method is adopted; however, the charging polarity of each component can be changed depending on the normal charging polarity of the toner and the development method.

[0016] The photosensitive drum 1 is a cylindrical electrophotographic photosensitive member. A specific example of the photosensitive drum 1 has a drum-shaped substrate made of, for example, aluminum, and a photosensitive layer formed of a negatively chargeable organic photosensitive member on the substrate. The photosensitive drum 1 is driven by a drive motor as a drive source mounted in the image forming apparatus 100, and is rotatable in the direction of the arrow in the figure (clockwise direction).

[0017] In this embodiment, the photosensitive drum has a diameter of 24 mm and is rotated at a peripheral speed of 139 mm / sec. The distance from the developing portion P4 to the charging portion P2 in the peripheral direction of the photosensitive drum 1 is 54 mm.

[0018] The charging roller 2 is a contact charging type charging member that is disposed in contact with the photosensitive drum 1 and forms a charging portion P2 (contact portion between the charging roller 2 and the photosensitive drum 1) between the charging roller 2 and the photosensitive drum 1. The charging roller 2 in this embodiment is biased by a biasing means such as a spring member and is pressed against the photosensitive drum 1 with a predetermined pressure. A predetermined charging voltage (charging voltage) is applied to the charging roller 2 from a charging power source PW1, which is a voltage generating circuit mounted on the image forming apparatus 100, causing a proximity discharge at the charging portion P2. Note that the "charging roller 2 disposed in contact with the photosensitive drum 1" is not limited to cases where the surfaces of the photosensitive drum 1 and the charging roller 2 are in direct contact with each other, but also includes cases where there is a minute gap where the surfaces of the members can come into contact with each other with the toner 44 carried on one of the members sandwiched therebetween.

[0019] The exposure unit 3 irradiates the surface of the photosensitive drum 1 with laser light at an exposure section P3 located downstream of the charging section P2 and upstream of the developing section P4 described below in the rotation direction of the photosensitive drum 1. The exposure unit 3 irradiates the photosensitive drum 1 with laser light via a polygon mirror or the like based on an image signal (video signal) transmitted from the control section 50 of the image forming apparatus 100, thereby scanning and exposing the surface of the photosensitive drum 1. Note that the exposure unit 3 is not limited to a laser scanner device, and may, for example, be an LED exposure device having an LED array in which multiple LEDs are arranged along the longitudinal direction (rotation axis direction, main scanning direction) of the photosensitive drum 1.

[0020] The developing device 20 has a developing roller 41 as a developing member or developer carrier, a supply roller 42 as a developer supply member, a regulating blade 43 as a regulating member, and a developing container 45 as a container for accommodating the developer. The developing roller 41 and the supply roller 42 are rotatably supported by the developing container 45 that constitutes the frame of the developing device 20. The developing roller 41 is disposed at the opening of the developing container 45 so as to face the photosensitive drum 1.

[0021] The developing roller 41 is disposed in contact with the photosensitive drum 1, and forms a developing portion P4 (a contact portion between the developing roller 41 and the photosensitive drum 1, a developing area) between the photosensitive drum 1 and the developing roller 41. Note that "the developing roller 41 disposed in contact with the photosensitive drum 1" is not limited to cases where the surfaces of the photosensitive drum 1 and the developing roller 41 are in direct contact with each other, but also includes cases where there is a minute gap where the surfaces of the members can come into contact with each other, sandwiching toner 44 carried on one of the members. The developing roller 41 rotates while carrying toner 44, and supplies the toner 44 to the developing portion P4. In this embodiment, the developing roller 41 rotates at a rotational speed (circumferential speed) that is 1.4 times the rotational speed (circumferential speed) of the photosensitive drum 1.

[0022] The developing device 20 of this embodiment uses a contact development method. That is, a layer of toner 44 carried on the developing roller 41 comes into contact with the photosensitive drum 1 at the developing portion P4. A predetermined developing voltage is applied to the developing roller 41 from a developing power supply PW2, which is a voltage generating circuit mounted on the image forming apparatus 100. In this embodiment, a DC developing voltage is used.

[0023] In this embodiment, the developing roller 41 is in constant contact with the photosensitive drum 1 at least during the image forming operation and the associated pre-rotation and post-rotation operations when the process cartridge 10 is attached to the image forming apparatus 100. The image forming apparatus 100 may be configured without a contact / separation mechanism that brings the developing roller 41 into contact with and separates it from the photosensitive drum 1.

[0024] The supply roller 42 is disposed in contact with the developing roller 41 and rotates in a direction opposite to the rotation of the developing roller 41 (a direction in which the circumferential surfaces of the opposing rollers move in the opposite direction). Note that the supply roller 42 is not necessarily required as long as the configuration can supply a sufficient amount of toner to the developing roller 41.

[0025] In this embodiment, toner 44 having an average particle size of 6 μm and a normal negative charge polarity is used. Polymerized toner produced by polymerization is used as the toner 44, for example. The toner 44 is a so-called non-magnetic single-component developer that does not contain a magnetic component and is supported on the developing roller 31 mainly by intermolecular forces and electrostatic forces (image forces). However, a single-component developer containing a magnetic component may be used instead of the toner 44. In addition, the single-component developer may contain additives (e.g., wax or silica particles) in addition to the toner particles to adjust the fluidity and charging performance of the toner. Alternatively, a two-component developer composed of non-magnetic toner and a magnetic carrier may be used. When a magnetic developer is used, a cylindrical developing sleeve with a magnet disposed inside is used as the developing member (developer carrier).

[0026] The regulating blade 43 is an elastic member that is disposed in contact with the developing roller 41 and is installed in a state where it is bent against the reaction force received from the developing roller 41. The regulating blade 43 regulates the thickness of the layer of toner 44 carried on the developing roller 41, and also frictionally charges the toner 44 by rubbing it against the toner 44 that passes through the space between the regulating blade 43 and the developing roller 41.

[0027] An agitating member 45a serving as agitating means is provided inside the developing container 45. The agitating member 45a is driven by the drive motor and rotates in conjunction with the rotation of the developing roller 41, agitating the toner 44 in the developing container 45 and sending the toner 44 toward the developing roller 41 and the supply roller 42. Note that the agitating member 45a is not limited to a rotating type. For example, an agitating member having an oscillating type may be employed.

[0028] The transfer roller 5 is disposed opposite the photosensitive drum 1 at a transfer section P5 (transfer position) located downstream of the development section P4 and upstream of the pre-exposure section P6 described below in the rotation direction of the photosensitive drum 1. A transfer nip section (hereinafter, the transfer nip section will also be referred to as the transfer section P5) where a toner image is transferred from the image carrier to the recording material R is formed as a nip section between the transfer roller 5 and the photosensitive drum 1. A predetermined transfer voltage (bias voltage) is applied to the transfer roller 5 from a transfer power supply, which is a voltage generation circuit mounted in the image forming apparatus 100.

[0029] Instead of applying a transfer voltage to the transfer roller 5 (transfer member), an electric field for transferring the toner image at the transfer portion may be formed by other voltage application means. For example, the transfer roller 5 is connected to the earth potential, and a charging roller 2 to which a charging voltage of the same polarity as the normal charging polarity of the toner 44 is applied applies a voltage to the photosensitive drum 1, thereby forming such an electric field at the transfer portion.

[0030] The pre-exposure LED 6 is disposed opposite the photosensitive drum 1 at a pre-exposure section P6 located downstream of the transfer section P5 and upstream of the charging section P2 in the rotation direction of the photosensitive drum 1. The pre-exposure LED 6 irradiates light onto an area of ​​the surface of the photosensitive drum 1 that has passed through the transfer section P5.

[0031] The fixing device 7 employs a thermal fixing system that heats and melts the toner on the recording material R to fix the image. The fixing device 7 includes, for example, a flexible, cylindrical fixing film, a heater such as a ceramic heater that heats the fixing film, a thermistor that measures the heater temperature, and a pressure roller that is pressed against the heater via the fixing film. The control unit 50 of the image forming apparatus 100 controls the power supply to the heater based on a detection signal from the thermistor to maintain the surface of the fixing film at a predetermined temperature suitable for fixing the image. The fixing device 7 is not limited to this configuration. For example, a pair of rollers may be used as a pair of rotating bodies that rotate while sandwiching the recording material, and a halogen lamp or an induction heating mechanism may be used instead of the ceramic heater as a heating means.

[0032] The control unit 50 includes at least one processor and a computer-readable, non-transitory storage medium storing a program for controlling the operation of the image forming apparatus 100. The control unit 50 includes, for example, a non-volatile memory for storing the program, a CPU for reading and executing the program from the memory, and a volatile memory that serves as a workspace when the program is executed. The control unit 50 also includes a drive circuit for driving the actuators (drive motors, etc.) of the image forming apparatus 100, a network interface for connecting to an external computer, etc. The CPU is connected to the other elements of the control unit 50 via a bus and issues instructions to the drive circuit, etc. in accordance with the program, thereby realizing operations such as image formation by the image forming apparatus 100.

[0033] (Image formation operation) Next, a description will be given of the image forming operation of the image forming apparatus 100. When an image formation command (print job) is input to the image forming apparatus 100, an image forming process is started in the process cartridge 1B based on image information input from an external computer or reading device connected to the image forming apparatus 100.

[0034] First, the charging roller 2 uniformly charges the surface of the rotating photosensitive drum 1 at a charging portion P2 to the same polarity (negative polarity in this embodiment) as the normal charging polarity of the toner 44. The exposure unit 3 irradiates the exposure portion P3 of the photosensitive drum 1 with laser light modulated according to an image signal generated based on input image information. This forms an electrostatic latent image on the photosensitive drum 1. When the electrostatic latent image reaches a developing portion P4, the toner 44 supplied from the developing roller 41 adheres to the photosensitive drum 1 according to the potential distribution on the photosensitive drum 1, thereby developing (visualizing) the electrostatic latent image as a toner image.

[0035] In parallel with the formation of the toner image on the photosensitive drum 1, recording material R is fed one sheet at a time from a recording material R stacker provided at the bottom of the image forming apparatus 100. The recording material R is transported to the transfer portion P5 by a pair of registration rollers (not shown) so that the leading edge of the toner image reaches the transfer portion P5 and the leading edge of the recording material R enters the transfer portion P5 at approximately the same time. Then, at the transfer portion P5, the toner image carried on the photosensitive drum 1 is transferred onto the recording material R by a transfer roller 5 to which a transfer voltage is applied.

[0036] The recording material R that has passed through the transfer portion P5 is transported to the fixing device 7. The fixing device 7 conveys the recording material R while sandwiching it in a nip portion (fixing nip portion) between a fixing film and a pressure roller, and heats and pressurizes the toner image on the recording material R using the fixing film heated by a heater. This melts the toner particles and then solidifies them, thereby fixing the toner image to the recording material R. The recording material R that has passed through the fixing device 7 is discharged outside the image forming apparatus 100 by a pair of discharge rollers serving as a discharge means, and is stacked on a discharge tray serving as a stacking portion formed on the top of the printer main body.

[0037] When the surface area of ​​the photosensitive drum 1 that has passed through the transfer section P5 reaches the pre-exposure section P6, the electrostatic latent image on the photosensitive drum 1 is erased by light irradiated from the pre-exposure device 6. This makes the surface area available for use again in the image formation process.

[0038] (cleanerless system) Next, the cleanerless system employed in this embodiment will be described. In the cleanerless system, the developer remaining on the image carrier without being transferred from the image carrier to the transfer material (recording material or intermediate transfer body) at the transfer portion P5 is not collected by a cleaning device, but is collected by a developing member and reused. In this embodiment, the residual toner remaining on the photosensitive drum 1 after passing through the transfer portion P5 is collected into the developing device 20 by the developing roller 41 when it reaches the developing portion P4 via the pre-exposure portion P6, the charging portion P2, and the exposure portion P3.

[0039] During image formation, residual toner is typically removed through the following process. Residual toner includes a mixture of toner charged positively (opposite to the normal charge polarity) and toner that is negatively charged but not sufficiently charged. Therefore, the pre-exposure device 6 neutralizes the photosensitive drum 1 after transfer, and then the charging roller 2 uniformly discharges the residual toner, causing it to be negatively charged again. The residual toner, negatively charged again at the charging station P2, reaches the developing station P4 as the photosensitive drum 1 rotates. The surface area of ​​the photosensitive drum 1 that has passed through the charging station P2 is then exposed by the exposure unit 3, with the residual toner still adhering to its surface, and an electrostatic latent image is written onto it.

[0040] Here, the behavior of the transfer residual toner that has reached the development unit P4 will be explained separately for the exposed area (bright area) and the non-exposed area (dark area) of the photosensitive drum 1. The transfer residual toner adhering to the non-exposed area of ​​the photosensitive drum 1 is transferred to the development roller 41 at the development unit P4 due to the potential difference between the potential of the non-exposed area of ​​the photosensitive drum 1 (dark area potential) and the development voltage, and is then collected in the development container. This is because when the normal charge polarity of the toner 44 is negative, the development voltage applied to the development roller 41 is set to be positive relative to the potential of the non-exposed area. The toner collected in the development container 45 is stirred and homogenized with the toner in the development container 45 by the stirring member 45a, and is then carried by the development roller 41 and used again in the development process.

[0041] On the other hand, the transfer residual toner adhering to the exposed area of ​​the photosensitive drum 1 does not transfer from the photosensitive drum 1 to the developing roller 41 at the developing portion P4, but remains on the drum surface. This is because, when the normal charging polarity of the toner 44 is negative, the developing voltage applied to the developing roller 41 has a potential that is even more negative than the potential of the exposed area (light area potential). In other words, when the surface area of ​​the photosensitive drum 1 to which the transfer residual toner adhered becomes the exposed area at the exposure portion P3, the transfer residual toner forms a new toner image together with other toner that transfers from the developing roller 41 to the exposed area, and is transferred to the recording material R at the transfer portion P5.

[0042] Such a cleaner-less configuration eliminates the need for a cleaning member or collection container for collecting residual toner, etc., and enables further miniaturization of image forming apparatus 100. Furthermore, by reusing residual toner in subsequent image formations, it is possible to reduce the operating costs of image forming apparatus 100 (reduce toner consumption).

[0043] Example 1 (1. Voltage setting during image formation) Next, the potential difference between the photosensitive drum 1 and its surrounding members during the image forming operation in one example (Example 1) of this embodiment will be described.

[0044] During the image forming operation, a charging voltage of −1240 V is applied to the charging roller 2, and the surface of the photosensitive drum 1 is charged to a uniform charging potential Vd (dark area potential: −740 V) by discharge at the charging section P2. Of the surface of the photosensitive drum 1 charged to the charging potential Vd, the potential of the exposure area exposed by the exposure unit 3 changes to a post-exposure potential Vl (light area potential: −50 V). In this embodiment, the exposure dose E0 that forms Vl is 0.35 μJ / cm 2 A developing voltage Vdc (developing potential: -380V) is applied to the developing roller 41. The exposed area (image forming area) and the non-exposed area (non-image forming area) are formed within an image formable area on the surface of the photosensitive drum 1. The image formable area is an area in the main scanning direction where toner 44 can be supplied from the developing roller 41 to the surface of the photosensitive drum 1. The image formable area can also be said to be an area where toner 44 can be carried on the surface of the developing roller 41.

[0045] From the above, the development contrast Vcont, which is the potential difference between the light area potential on the photosensitive drum 1 and the development voltage Vdc, is 330 V, and the back contrast Vbc, which is the potential difference between the dark area potential on the photosensitive drum 1 and the development voltage Vdc, is 360 V. By setting the potentials in this way, it becomes possible to appropriately output images such as solid black images, halftone images, and white text.

[0046] Here, the development contrast Vcont and the back contrast Vbc are determined by the surface potential of the photosensitive drum 1 at the development unit P4 and the development voltage Vdc applied to the development roller 41. If an image formation operation is performed without appropriate potential settings, a defective image will occur on the recording material R. Specifically, if the development contrast Vcont is excessively small, the amount of toner adhering to the exposed area (image forming area) on the photosensitive drum 1 will be reduced, which may result in a defective image (low density) where the image density is low. On the other hand, if the development contrast Vcont is excessively large, the amount of toner adhering to the exposed area (image forming area) on the photosensitive drum 1 will be increased, which may result in a fixing defect where the toner does not melt sufficiently in the fixing process and is not fixed sufficiently to the recording material R. Therefore, the development contrast Vcont needs to be adjusted appropriately taking these factors into consideration.

[0047] Furthermore, the voltage in this embodiment is expressed as a potential difference with respect to the earth potential (0 V). Therefore, the development voltage Vdc=-380 V means that the potential difference of the development voltage applied to the core of the development roller 41 with respect to the earth potential is -380 V. The same applies to the charging voltage, etc.

[0048] (2. Back contrast and fogging) Next, the reason for controlling the back contrast Vbc will be explained. By appropriately controlling the back contrast Vbc, it is possible to suppress excess toner from adhering to the non-image forming area (white area), which is the surface area on the photosensitive drum 1 where no image is formed. This excess toner is called fogging toner, and the phenomenon in which fogging toner occurs is called fogging.

[0049] When fogging occurs, toner adheres to the non-image forming areas of the photosensitive drum 1 and is transferred to the recording material R, causing color to appear in the areas on the recording material R where no image is supposed to be formed (white areas), which may result in the user not being able to achieve the quality of the finished product they desire.

[0050] If the back contrast Vbc is too small, the electric field that keeps the toner 44, which is negatively charged, which is the normal charging polarity in this embodiment, on the developing roller 41 is weakened, and such toner 44 is likely to adhere to the non-image forming areas on the photosensitive drum 1 as fog toner. On the other hand, if the back contrast Vbc is too large, toner 44, which is positively charged, which is the opposite polarity to the normal charging polarity on the developing roller 41, is likely to adhere to the non-image forming areas on the photosensitive drum 1, causing fog.

[0051] Fog caused by toner 44 charged to the normal charging polarity adhering to the non-image forming area on the photosensitive drum 1 is called normal fog. Fog caused by toner 44 charged to the polarity opposite to the normal charging polarity adhering to the non-image forming area on the photosensitive drum 1 is called reverse fog. Therefore, to simultaneously suppress normal fog and reverse fog, it is sufficient to set the back contrast Vbc within an appropriate range.

[0052] Furthermore, in a cleanerless configuration, a sufficient back contrast Vbc must be set in order to efficiently collect fog toner and transfer residual toner at the development unit P4. This is because most of the fog toner and transfer residual toner are charged with the normal charging polarity. When these negatively charged toners reach the development unit P4 while adhering to non-image forming areas (dark potential), a certain level of back contrast Vbc is required to transfer (recover) them from the photosensitive drum 1 to the development roller 41 using an electric field. If toner collection at the development unit P4 is not sufficient, the fog toner may pass through the development unit P4 while still adhering to the photosensitive drum 1 and may be transferred to the recording material R at the transfer unit P5, resulting in a defective image (white background smearing).

[0053] It is also known that the density of one dot and the line width change depending on the settings of the back contrast Vbc and the development contrast Vcont. Therefore, the development contrast Vcont is set to be suitable for one dot and the line width while setting the back contrast Vbc to be suitable for suppressing fog. The output voltages of the charging power supply PW1 and the development power supply PW2 and the exposure intensity of the exposure unit 3 are set to satisfy the above conditions.

[0054] Figure 2 shows the relationship between back contrast Vbc and the amount of fogging toner. The horizontal axis of the graph represents back contrast Vbc, and the vertical axis represents the amount of fogging toner. The amount of fogging toner was measured by tracing the toner on the photosensitive drum 1 with Mylar tape, attaching the tape to a reference paper, and then measuring the density with a Tokyo Denshoku Corporation reflection densitometer (TC-6DS / A). The amount of fogging toner was calculated by performing an image formation operation using the image forming apparatus 100, varying the back contrast Vbc without using the recording material R, and calculating the amount of toner adhering to the surface area of ​​the photosensitive drum 1 that passed through the development unit P4. The amount of fogging toner is invisible below a certain value, so it does not affect image quality. However, as the amount of fogging toner increases, it becomes visible and results in poor image quality. The range in which the amount of fogging toner falls below the threshold at which it is visible is the appropriate range for back contrast Vbc.

[0055] In the first embodiment, which employs a cleanerless system, it is desirable to control the back contrast Vbc within an appropriate range as described above in order to suppress uneven charging caused by adhesion of fogging toner to the charging roller 2 and poor collection of fogging toner at the development section P4.

[0056] In this embodiment, as shown in Figure 2, setting the back contrast Vbc in the range of 130V to 550V is preferable because the amount of fog toner becomes invisible to the naked eye and toner consumption during non-image formation is also suppressed. However, the back contrast Vbc represents the potential difference in which the photosensitive drum 1 side is negative as positive. In this embodiment, setting the back contrast Vbc to 360V, which is within the above range, suppresses fog during image formation and toner consumption during non-image formation.

[0057] (3. Operational Process of Image Forming Device) Next, the operation of image forming apparatus 100, including the processes before and after the image forming operation, will be described. Fig. 3 is a diagram showing the operation process of image forming apparatus 100 from the power-off state to the end of image formation. Fig. 4 is a diagram showing the operation process of image forming apparatus 100 from the standby state before the start of image forming operation to the end of jam recovery when a jam occurs during image forming operation.

[0058] First, the operational process from the power-off state to the end of image formation will be explained in order with reference to FIG.

[0059] (1) Stopped state When the power supply of the image forming apparatus 100 is OFF (the main power switch is OFF), or when the door is open and the door switch is OFF, the power supply to the main control circuit of the image forming apparatus 100 is cut off, and the image forming apparatus 100 is held in a stopped state in which it cannot perform image formation operations.

[0060] (2) Initial rotation operation (pre-rotation operation) The initial rotation operation is a start-up operation that is executed when power is applied (power ON) to the image forming apparatus 100 (A in the figure). That is, when power is applied to the image forming apparatus 100, the drive motor is started and the initial rotation operation is an operation that warms up a plurality of process devices that are involved in the rotational driving of the photosensitive drum 1.

[0061] The image forming apparatus 100 is powered on when the main power switch is turned from OFF to ON while the door switch is on (the door is closed), or when the door switch is turned from OFF to ON while the main power switch is on. In either case, power supply to the main control circuit is started, and the image forming apparatus 100 is maintained in a state where it can perform image formation operations. The door switch is a switch or sensor that detects the opening and closing of a door that is openably provided on the front side of the main body of the image forming apparatus 100 to allow access to the inside of the image forming apparatus 100.

[0062] The initial rotation operation is a preparatory operation for enabling the image forming apparatus 100 to perform stable image formation. For example, the controller 50 detects the state of the process cartridge and performs control to determine appropriate settings for charging, developing, and transfer voltages in accordance with that state. Alternatively, process control is performed such as applying a constant charging voltage from a charging power supply to make the surface potential of the photosensitive drum 1 uniform, or irradiating the photosensitive drum 1 with laser light from the exposure unit 3.

[0063] (3) Standby When the initial rotation operation is completed, the driving of the drive motor is stopped, and the image forming apparatus 100 is maintained in a standby state until an image formation start signal S is input.

[0064] (4) Forward rotation Based on the input of the image formation start signal S, the drive motor is driven again to execute a predetermined pre-image formation operation involving the rotational driving of the photosensitive drum 1. More specifically, (5) preparations for executing the image formation operation are made in the following order: (a) the control unit 50 receives the image formation start signal S, (b) the formatter develops the image, and (c) the pre-rotation operation is started.

[0065] The pre-rotation operation is a preparatory operation that is executed immediately before the image forming operation when an instruction (job) to execute image formation is input to the image forming apparatus 100. The pre-rotation operation includes start-up control that starts rotation of the image carrier and gradually or continuously raises the charging voltage and developing voltage to the voltage values ​​for the image forming operation. In the start-up control of this embodiment, the charging voltage and developing voltage are gradually or continuously raised in order to prevent fogging from occurring at the developing unit P4 during the pre-rotation operation. Details of the start-up control will be described later.

[0066] The development time for the above step (b) varies depending on the amount of image data and the processing speed of the formatter. If the image formation start signal S is input during the initial rotation operation of step 2), after the initial rotation operation is completed, the pre-rotation operation of step (4) is executed immediately without entering the standby state of step (3).

[0067] (5) Image formation operation After the pre-rotation operation is completed, an image forming operation is performed to output one image (monoprint) or to continuously output a predetermined number of images (continuous image forming job, multi-print), and the recording material R on which the images have been formed is output. Figure 3 shows an image forming operation to continuously output n images. The paper interval shown in the figure is the interval from when the trailing edge of the preceding recording material R passes through the transfer portion P5 to when the leading edge of the next recording material R reaches the transfer portion P5 in the case of a continuous image forming job.

[0068] (6) Rear rotation operation In the post-rotation operation, even after the image forming operation for one sheet or a predetermined number of sheets is completed, the drive motor continues to be driven for a predetermined time, and a predetermined finishing operation accompanied by the rotational driving of the photosensitive drum 1 is executed.

[0069] (7) Standby When the post-rotation operation is completed, the drive motor is stopped, and the image forming apparatus 100 is maintained in a standby state until the next image formation start signal S is input. When the next image formation start signal S is input, the operation proceeds to (4) the pre-rotation operation.

[0070] (4. Start-up control during pre-rotation operation) The pre-rotation operation in the first embodiment and the charging voltage and developing voltage ramp-up control performed during the pre-rotation operation will be described in detail with reference to Figures 4 and 5. Figure 4 is a timing chart of the drive motor, charging voltage, and developing voltage during the pre-rotation operation. Figure 5 shows the transition of the surface potential of the photosensitive drum 1 at the developing unit P4 and the developing voltage during the pre-rotation operation. Note that the waveform of the surface potential of the photosensitive drum 1 in Figure 5 is delayed (shifted to the right in the figure) compared to the waveform of the charging voltage in Figure 4 by the amount of time required for a point on the photosensitive drum 1 to move from the charging unit P2 to the developing unit P4 between Figures 4 and 5.

[0071] Hereinafter, the developing voltage and the surface potential of the photosensitive drum 1 are expressed by adding a sign indicating the polarity of the voltage (potential) to variables (Va to Vg) that represent the absolute value of the voltage (potential).

[0072] When the image formation start signal S is input at time t1 shown in FIGS. 4 and 5, the control unit 50 causes the development power supply PW2 to start applying a positive development voltage +Va to the development roller 41. At this time, the surface potential of the photosensitive drum 1 is approximately 0V, so a back contrast Vbc (=+Va) of approximately the same value as the development voltage is formed in the development unit P4. For this reason, the positive development voltage +Va is set to a value that puts the back contrast Vbc into an appropriate range (see FIG. 2) for a surface potential of 0V. In this embodiment, the positive development voltage (+Va) used at the beginning of the pre-rotation operation is +150V.

[0073] After that, at time t2, the control unit 50 starts (ON) the rotation of the drive motor. When the drive motor is turned ON, both the photosensitive drum 1 and the developing roller 41 start rotating.

[0074] Next, at time t3, as shown in FIG. 4, the control unit 50 starts applying a charging voltage to the charging roller 2 from the charging power supply PW1. The charging voltage applied at this time is a value (-Ve') set as Stage 1 of the start-up control so that the surface potential of the photosensitive drum 1 becomes -Ve. Thereafter, at time t4, the control unit 50 switches the voltage applied to the charging roller 2 by the charging power supply PW1 from a value (Stage 1, -Ve') that makes the surface potential of the photosensitive drum 1 -Ve to a value (Stage 2, -Vf') that makes the surface potential of the photosensitive drum 1 -Vf, which is higher than -Ve. Furthermore, at time t5, the control unit 50 switches the voltage applied to the charging roller 2 by the charging power supply PW1 from a value (Stage 2, -Vf') that makes the surface potential of the photosensitive drum 1 -Vf to a value (Stage 3, -Vg') that makes the surface potential of the photosensitive drum 1 -Vg, which is higher than -Vf. The charging voltage (corresponding to the surface potential −Vg of the photosensitive drum 1) in the final stage (stage 3) of the start-up control is equal to the charging voltage in the image forming operation.

[0075] In parallel with this stepwise increase in the charging voltage, the control unit 50 also stepwise increases the developing voltage, as shown in FIG. 6. That is, at time t6, which is after time t3′ when the surface region of the photosensitive drum 1 charged to a surface potential −Ve by the charging voltage in stage 1 reaches the developing unit P4, the developing voltage is switched from a positive voltage +Va to a negative voltage −Vb (stage 1). At time t7, which is after time t4′ when the surface region of the photosensitive drum 1 charged to a surface potential −Vf by the charging voltage in stage 2 reaches the developing unit P4, the developing voltage is switched from the negative voltage −Vb to a negative, higher voltage −Vc (stage 2). At time t8, which is after time t5′ when the surface region of the photosensitive drum 1 charged to a surface potential −Vg by the charging voltage in stage 3 reaches the developing unit P4, the developing voltage is switched from the negative voltage −Vc to a negative, higher voltage −Vd (stage 3). The development voltage (-Vd) in the final stage (stage 3) of the start-up control is equal to the development voltage Vdc in the image forming operation.

[0076] When preheating in the fixing device 7 is completed at time t9, the control unit 50 determines that the pre-rotation operation is completed, and proceeds to the image forming operation.

[0077] Here, the timings (t6, t7, t8) for switching the development voltage from Va to Vb, Vb to Vc, and Vc to Vd are set so that they occur after the surface potential of the photosensitive drum 1 at the development unit P4 has risen to Ve, Vf, and Vg, respectively, and stabilized. The timings (t3, t4, t5) for switching the charging voltage from 0 V to Ve', Ve' to Vf', and Vf' to Vg' are set so that the surface potential of the photosensitive drum 1 at the development unit P4 has switched to Ve, Vf, and Vg, respectively, after the voltage value has stabilized after the development voltage has been switched. Specifically, these timings are set taking into consideration the rise characteristics (response time) of the charging power supply PW1 and the development power supply PW2 in response to a command to switch the voltage value, the time required for a point on the surface of the photosensitive drum 1 to move from the charging unit P2 to the development unit P4, and the like.

[0078] In this embodiment, the charging voltage and developing voltage are controlled in three stages, passing through two intermediate values ​​(stages 1 and 2) and then increasing to the same voltage value as during image formation (stage 3). However, the number of stages may be less than or greater than three. Furthermore, the charging voltage and developing voltage in the final stage of the start-up control during the pre-rotation operation may be different from the voltage values ​​used in the image formation operation. For example, the charging voltage and / or developing voltage may be increased to a voltage value lower (smaller in absolute value) than the voltage value used for image formation during the start-up control, and then increased to the voltage value used for image formation when the image formation operation is started. Alternatively, the charging voltage and / or developing voltage may be increased to a voltage value higher (larger in absolute value) than the voltage value used for image formation during the start-up control, and then decreased to the voltage value used for image formation when the image formation operation is started.

[0079] The change in the back contrast Vbc during the forward rotation operation in this embodiment will be described with reference to FIG.

[0080] When the image formation start signal S is input at time t1, output of a positive developing voltage (Va) begins. At this time, Vbc = 150 V. Subsequently, when the surface area of ​​the photosensitive drum 1, which has been charged to a surface potential Ve by the start of application of the charging voltage, reaches the developing unit P4 at time t3', Vbc changes to 500 V. After that, when the developing voltage is switched from positive to negative (Vb) at time t6, Vbc = 300 V. After that, when the surface area of ​​the photosensitive drum 1, which has been charged to a surface potential Vf by the charging voltage of stage 2, reaches the developing unit P4 at time t4', Vbc = 500 V. After that, when the developing voltage is switched to the voltage of stage 2 (Vc) at time t7, Vbc = 300 V. After that, when the surface area of ​​the photosensitive drum 1, which has been charged to a surface potential Vg by the charging voltage of stage 3, reaches the developing unit P4 at time t5', Vbc = 500 V. Thereafter, when the development voltage is switched to the voltage value (Vd) of stage 3 at time t8, Vbc becomes 360 V. Vbc (360 V) after time t8 is the same value as Vbc during image formation.

[0081] In this way, the charging voltage and the developing voltage are switched in stages so that the back contrast Vbc, which is the difference between the surface potential of the photosensitive drum 1 and the developing voltage, falls within a certain range, and the process of waiting for one voltage or potential to rise before raising the other voltage or potential is repeated.

[0082] At each stage of the charging voltage and developing voltage ramp-up control, the charging voltage and developing voltage are set so that the surface potential (Ve to Vg) of the photosensitive drum 1 formed by the charging voltage is negative relative to the developing voltages (Vb to Vd). Therefore, as shown in FIG. 5, from time t1, when application of a positive developing voltage (Va) begins before the photosensitive drum 1 starts to rotate, to time t9, when the pre-rotation operation is completed, a back contrast Vbc is formed at the developing portion P4, where the photosensitive drum 1 side is negative. In other words, at the developing portion P4, the photosensitive drum 1 side has the same polarity as the normal charging polarity of the toner 44, and the developing roller 41 side has the opposite polarity, so that an electric field is continuously formed that electrostatically attracts the normally charged toner 44 to the developing roller 41.

[0083] It is also desirable to set the values ​​of the charging voltage and developing voltage (Va to Vd, Ve' to Vg') at each stage of the start-up control so that the possible values ​​of the back contrast Vbc in the process of gradually increasing the charging voltage and developing voltage are within the appropriate range shown in Fig. 2. In other words, it is desirable for the control unit 50 to control the charging voltage and developing voltage during the pre-rotation operation (preparatory operation before the image forming operation) so that the surface potential of the image carrier in the development unit with respect to the developing voltage becomes the same polarity as the normal charging polarity of the toner, and so that the potential difference between the developing voltage and the surface potential of the image carrier is maintained within a predetermined range.

[0084] This makes it possible to prevent the occurrence of fogging caused by negatively charged toner particles on the developing roller 41 electrostatically adhering to the photosensitive drum 1 during the pre-rotation operation. Also, it is possible to prevent toner consumption during the pre-rotation operation due to the occurrence of fogging.

[0085] In the first embodiment, the charging voltage and the developing voltage are controlled in the pre-rotation operation so that a back contrast Vbc higher than the back contrast Vbc (360 V) during image formation and a back contrast Vbc lower than the back contrast Vbc during image formation alternately appear (FIG. 6).

[0086] (5. State of the device during pre-rotation) Control of the back contrast Vbc during the pre-rotation operation in this embodiment will be described using Figures 7 to 10. Figure 7 is a timing chart of the drive motor and back contrast during the pre-rotation operation, illustrating the period (t2 to t10) from when the fog start point Pa on the photosensitive drum 1 moves from the development portion P4 to when it reaches the development portion P4 again after one rotation of the photosensitive drum 1. Figures 8 to 10 are schematic diagrams showing the state of the image forming apparatus 100 at each point in time during the pre-rotation operation.

[0087] The "fog start point Pa" is the start point of the range on the photosensitive drum 1 where fog occurs at startup, which will be described below. The "fog end point Pb" is the end point of the range on the photosensitive drum 1 where fog occurs at startup.

[0088] 8(a) shows the state of the image forming apparatus 100 at the time when the drive motor starts rotating at time t2 (FIG. 7) during the pre-rotation operation. At this time, in the developing unit P4, the toner 44 carried on the developing roller 41 is in contact with the photosensitive drum 1. If the toner 44 on the developing roller 41 is sufficiently negatively charged, the above-described control of the back contrast Vbc can prevent the occurrence of start-up fog caused by the toner 44 electrostatically adhering to the photosensitive drum 1 during the pre-rotation operation.

[0089] However, if the standby state continues for a long time before the pre-rotation operation, the charge amount of the toner 44 on the developing roller 41 decays to a low value. In this case, even if the back contrast Vbc is formed in advance at the time (t2) when the drive motor is started, the toner 44 cannot be electrostatically retained on the developing roller 41 because the charge amount of the toner 44 is low, and some of the toner 44 non-electrostatically adheres to the photosensitive drum 1. In this way, when the photosensitive drum 1 starts to rotate after an image formation operation has not been performed for a long time, toner 44 with a low charge amount non-electrostatically adheres to the photosensitive drum 1 at the developing portion P4, causing start-up fogging (start-up fogging after a long period of non-use).

[0090] 8(a), the start-up fog occurs from the portion of the surface of the photosensitive drum 1 that was in contact with the toner 44 on the developing roller 41 at time t2 when the drive motor starts to rotate. In other words, the fog start point Pa is the surface area of ​​the photosensitive drum 1 that was located at the developing portion P4 at the time (t2) when the photosensitive drum 1 started to rotate.

[0091] On the other hand, as shown in FIG. 8(b), the start-up fog ends when the toner 44 carried by the developing roller 41 and supplied to the developing portion P4 becomes sufficiently charged. This is because if the charge amount of the toner 44 is large, it is electrostatically held to the developing roller 41 by the control of the back contrast Vbc described above. In this embodiment, the charge amount of the toner 44 is mainly increased by friction with the regulating blade 43. Therefore, the fog end point Pb is the surface area of ​​the photosensitive drum 1 where the toner 44, which was located at the contact portion 43a between the tip of the regulating blade 43 and the developing roller 41 at the time when the photosensitive drum 1 started to rotate (t2), comes into contact with the toner 44 at the developing portion P4.

[0092] In other words, the start-up fog occurs when the toner 44 that reaches the developing portion P4 without passing through the regulating blade 43 adheres to the photosensitive drum 1 after the developing roller 41 starts to rotate.

[0093] 9 shows the state of the image forming apparatus 100 at time t3 (FIG. 7) when, after the drive motor starts rotating, the application of the first-stage charging voltage (-Ve') to the charging roller 2 begins. At this time, the fogging start point Pa on the photosensitive drum 1 is located upstream of the charging point P2 in the rotation direction of the photosensitive drum 1.

[0094] 10 shows the state of the image forming apparatus 100 at time t3' when the leading edge of the surface area of ​​the photosensitive drum 1, which has been charged to a surface potential of -Ve by the start of application of the charging voltage to the charging roller 2, reaches the developing unit P4. At this point, the surface potential of the photosensitive drum 1 at the developing unit P4 has changed to -Ve, and the back contrast Vbc at the developing unit P4 becomes 500V.

[0095] 11 shows the state of the image forming apparatus 100 at time t10 when the fog start point Pa reaches the developing station P4 again. At this time, the back contrast Vbc at the developing station P4 remains at 500V.

[0096] 12 shows the state of the image forming apparatus 100 at time t11 when the fog end point Pb reaches the developing station P4 again. At this time, the back contrast Vbc at the developing station P4 remains at 500V.

[0097] 11, in this embodiment, at least at the time when the leading edge of the fog at startup (fog start point Pa) reaches the developing portion P4 again, the value of the back contrast Vbc is a value (500 V) larger than the value during image formation. In other words, the charging voltage and developing voltage are controlled so that the value of Vbc at the time when the surface area of ​​the image carrier that was located at the developing portion P4 at the time when the image carrier started to rotate (fog start point Pa) reaches the developing portion P4 again is larger than the value of Vbc during image formation operation.

[0098] 9, before the fog start point Pa reaches the charging portion P2, application of a charging voltage equal to or greater than the discharge start voltage is started to the charging roller 2 as a charge applying means. As a result, the start-up fog that reaches the developing portion P4 again at time t10 is in a state in which a charge is applied (injected) by the discharge at the charging portion P2. Then, the start-up fog formed at the developing portion P4 is efficiently collected by the developing roller 41 due to the back contrast Vbc that is greater than the back contrast Vbc during image formation.

[0099] Preferably, as shown in Figures 11 and 12, while the leading edge to trailing edge of the fog at startup (fog start point Pa to fog end point Pb) passes through development zone P4 again, the back contrast Vbc is maintained in a state greater than the value during image formation. In other words, the charging voltage and developing voltage are controlled so that the value of Vbc is greater than the value of Vbc during image formation over the period from the time when the surface area (Pa) of the image carrier that was located in the development zone at the time the image carrier started rotating reaches development zone P4 again until the time d / Vd has elapsed. Here, the peripheral speed of the image carrier is Vd (mm / sec), and the moving distance of the surface of the developing member from the contact point (43a) between the developing member and the regulating member to the developing unit is d (mm).

[0100] In this embodiment, the photosensitive drum 1 has a diameter of 24 mm and is rotated at a speed of 139 mm / sec. Therefore, the circumference of the photosensitive drum 1 is 24 × π = 75.36 [mm], and the time (t10-t2) from the start of rotation of the photosensitive drum 1 (t2) to the time when the fog start point Pa reaches the development portion P4 again (t10) in the pre-rotation operation is calculated as follows: t10-t2=(75.36 / 139)×1000=542[msec]

[0101] 8(a) shows, the movement distance (d) of the surface of the developing roller 41 from the regulating blade 43 and 43a of the developing roller 41 to the developing portion P4 is 12 mm. Furthermore, the peripheral speed (Vd) of the developing roller 41 in this embodiment is 195 mm / sec, which is 1.4 times the peripheral speed of the photosensitive drum 1. The time (t11-t10) required for the leading edge to trailing edge of the start-up fog (from the fog start point Pa to the fog end point Pb) to pass through the developing portion P4 again is equal to the time width for the start-up fog to be formed, and can be expressed as follows: t11-t10=d / Vd=(12 / 195)×1000=62[mec]

[0102] From the above, the period from t10 to t11 in Example 1 is from the point when 542 msec has elapsed since the start of the drive motor (t2) to the point when 604 (=542+62) msec has elapsed. 7, in this embodiment, the value of the back contrast Vbc at the developing station P4 is maintained at least during this period, being greater than the value (360 V) during image formation.

[0103] <Comparative Example 1> Comparative Example 1 will be described with reference to Fig. 13. Fig. 13 shows a timing chart of the drive motor and charging voltage in the pre-rotation operation for Comparative Example 1, illustrating the period until the fog start point Pa on the photosensitive drum 1 reaches the development portion P4 again.

[0104] Comparative Example 1 differs from Example 1 in that the value of the back contrast Vbc at the developing unit P4 is maintained at 360 V, the same as during image formation, throughout the entire period of the pre-rotation operation. In other words, the charging voltage and developing voltage during the pre-rotation operation are controlled so that the change in the surface potential of the photosensitive drum 1 at the developing unit P4 is synchronized with the change in the developing voltage. The other configurations are the same as those of Example 1.

[0105] <Comparative Example 2> Next, Comparative Example 2 will be described with reference to Fig. 14. Fig. 14 shows a timing chart of the drive motor and charging voltage in the pre-rotation operation for Comparative Example 2, illustrating the period until the fog start point Pa on the photosensitive drum 1 reaches the development portion P4 again.

[0106] In Comparative Example 2, similar to Example 1, the charging voltage and developing voltage in the pre-rotation operation are controlled so that a back contrast Vbc higher than the back contrast Vbc (360 V) during image formation and a back contrast Vbc lower than that during image formation appear alternately. However, unlike Example 1, during the period (t10 to t11) when the start-up fog passes through the developing section P4 again, the back contrast Vbc is a value (300 V) lower than that during image formation. The other configurations are the same as in Example 1.

[0107] <Comparative Example 3> Next, Comparative Example 3 will be described with reference to Fig. 15. Fig. 15 shows a timing chart of the drive motor and charging voltage in the pre-rotation operation for Comparative Example 3, illustrating the period until the fog start point Pa on the photosensitive drum 1 reaches the development portion P4 again.

[0108] In Comparative Example 3, the back contrast Vbc is 150 V until time t3' when the surface area of ​​the photosensitive drum 1, which has been charged to a surface potential -Ve by the start of application of the charging voltage, reaches the development unit P4, and after time t3', Vbc is kept constant at 500 V. Here, in Comparative Example 3, Vbc is kept constant at 500 V after time t3', including during the execution of the image forming operation. The other configurations are the same as in Example 1.

[0109] <Example 2> Next, a second embodiment will be described with reference to Fig. 16. Fig. 16 shows a timing chart of the drive motor and charging voltage in the pre-rotation operation of the second embodiment, illustrating the period until the fog start point Pa on the photosensitive drum 1 reaches the development portion P4 again, and the period required for one rotation of the development roller 41.

[0110] In Example 2, similarly to Example 1, the charging voltage and developing voltage in the pre-rotation operation are controlled so that a back contrast Vbc higher than the back contrast Vbc (360 V) during image formation and a back contrast Vbc lower than that during image formation appear alternately. Also, during the period (t10 to t11) in which the start-up fog passes through the developing section P4 again, the back contrast Vbc is a value (500 V) higher than that during image formation.

[0111] However, in the second embodiment, the back contrast Vbc is a smaller value (300 V) than during image formation during the period from time t12 to time t13, which is after time t11 when the fog end point Pb of the start-up fog passes through the development portion P4.

[0112] Times t12 and t13 will be described with reference to Figures 11, 12, 17, and 18. As shown in Figure 11, the surface area of ​​the developing roller 41 located at the developing station P4 at the time (t10) when the fog start point Pa of the startup fog reaches the developing station P4 again is defined as Pa'. Also, as shown in Figure 12, the surface area of ​​the developing roller 41 located at the developing station P4 at the time (t11) when the fog end point Pb of the startup fog reaches the developing station P4 again is defined as Pb'. In other words, the range on the developing roller 41 from point Pa' to point Pb' is the range to which the fog toner collected from the photosensitive drum 1 adheres to the developing roller 41 while the startup fog passes through the developing station P4 again.

[0113] 17, time t12 is the time when point Pa', which is the leading end of the range in which fog toner adheres to developing roller 41, reaches developing unit P4 again after one rotation of developing roller 41. Also, time t13 is the time when point Pb', which is the trailing end of the range in which fog toner adheres to developing roller 41, reaches developing unit P4 again after one rotation of developing roller 41, as shown in FIG. 18. In other words, the period from time t12 to time t13 is the period during which fog toner from startup that has been collected by developing roller 41 may pass through developing unit P4 again after one rotation of developing roller 41.

[0114] In the second embodiment, a configuration is adopted in which, during such a period (t12 to t13), the back contrast Vbc is set to a value (300 V) smaller than that during image formation.

[0115] In other words, the second embodiment employs the following configuration. The value of Vbc at a first time point (t10) when the surface area (Pa) of the image carrier, which was located at the developing station P4 when the image carrier started to rotate, reaches the developing station again is defined as Vk1. The value of Vbc at a second time point (t12) when the surface area (Pa') of the developing member, which was located at the developing station P4 at the first time point (t10), reaches the developing station P4 again after one rotation of the developing member is defined as Vk2. The control unit then controls the charging voltage and developing voltage in the preparatory operation so that Vk2 is smaller than Vk1. In this embodiment, Vk1 is set to 500 V and Vk2 is set to 300 V.

[0116] Preferably, the following configuration is adopted: the peripheral speed of the developing member is Vd (mm / sec), and the moving distance of the surface of the developing member from the contact point between the developing member and the regulating member to the developing unit is d (mm). The control means controls the charging voltage and the developing voltage so that the value of Vbc is Vk1 during the period from the first time point until the time d / Vd has elapsed (t10 to t11), and so that the value of Vbc is Vk2 during the period from the second time point until the time d / Vd has elapsed (t12 to t13). The advantages of this second embodiment will be described later.

[0117] Second Embodiment The second embodiment of the present disclosure will be described below. Elements with the same reference numerals as those in the first embodiment have substantially the same configurations and functions as those described in the first embodiment unless otherwise specified, and differences from the first embodiment will be mainly described.

[0118] The process cartridge 10 of this embodiment does not include a supply roller 42 (FIG. 1). By not using the supply roller 42, it is possible to reduce the size and cost of the developing container 45. In this embodiment, the agitating member 45a rotates at a predetermined speed in conjunction with the rotation of the developing roller 41, agitating the toner 44 in the developing container and supplying the toner 44 directly to the developing roller 41.

[0119] In the first embodiment, the supply roller 42 scrapes off any remaining toner on the developing roller 41 that has not been used in image formation by relative movement of the peripheral surfaces or by a voltage difference, thereby refreshing the toner 44 on the developing roller 41. However, in this embodiment, there is no supply roller 42, so when there is a lot of toner 44 on the developing roller 41 or when the toner has a large charge amount and adheres strongly to the developing roller 41, the toner amount regulating function of the regulating blade 43 decreases, and as a result, the charge amount of the toner 44 tends to decrease.

[0120] That is, in this embodiment, the effect of increasing the charge amount of the startup fogging toner collected by the developing roller 41 is relatively low, and compared to the first embodiment, the collected toner is more likely to cause fogging again the next time it reaches the developing unit P4. In other words, although this embodiment can be made smaller and more cost-effective than the first embodiment, it can be said that this configuration is prone to poor collection of startup fogging toner by the developing roller 41 under severe conditions.

[0121] <Examples and Comparative Examples of the Second Embodiment> For the second embodiment, Example 1-2 is an example in which the same control as Example 1 of the first embodiment is applied to the control of charging voltage and developing voltage during the pre-rotation operation. For the second embodiment, Example 2-2 is an example in which the same control as Example 2 of the first embodiment is applied to the control of charging voltage and developing voltage during the pre-rotation operation. Similarly, for the second embodiment, Example 1-2, Comparative Example 2-2, and Comparative Example 3-2 are examples in which the same control as Comparative Example 1, Comparative Example 2, and Comparative Example 3 of the first embodiment is applied to the control of charging voltage and developing voltage during the pre-rotation operation.

[0122] <Other embodiments> In the above-described embodiment, in order to effectively collect the startup fogging toner in the developing unit P4, a voltage equal to or higher than the discharge start voltage is applied to the contact charging roller 2 in the charging unit P2 to impart a charge to the startup fogging toner. In other words, the charging roller 2 also serves as a charge imparting means for imparting a charge to the toner that adheres to the photosensitive drum 1 during the pre-rotation operation. However, the method for imparting a charge to the startup fogging toner is not limited to this, and for example, the configurations shown in Figures 19(a) and 19(b) may be used.

[0123] 19(a), a corona discharge type charger 2A is used instead of the charging roller 2. The charger 2A is both a charging member and a charge applying means. In this case, as in the first embodiment, application of a charging voltage at a voltage value equal to or greater than the discharge start voltage can be started before the fog start point Pa reaches the charging portion P2 (the discharge position of the charger 2A).

[0124] In the example of Figure 19(b), in addition to the charging roller 2, a conductive brush 2B is arranged in contact with the photosensitive drum 1. The conductive brush 2B is an example of a charge applying means provided separately from the charging member. In this case, before the fogging start point Pa reaches the contact point of the conductive brush 2B, a voltage of the same polarity as the normal charging polarity of the toner is applied to the conductive brush 2B, thereby applying a charge to the fogging toner at startup.

[0125] As another example, in the first and second embodiments, the voltage applied to the contact charging roller 2 to charge the start-up fogging toner may be the same polarity as the normal charging polarity of the toner, but less than the discharge start voltage of the charging roller 2. This is because even when a voltage smaller than the discharge start voltage is applied, the charging roller 2 will still charge the start-up fogging toner on the photosensitive drum 1.

[0126] As yet another example, the transfer roller 5 may be used as the charge applying means. In this case, a transfer voltage equal to or greater than the discharge start voltage of the transfer roller 5 is applied to the transfer roller 5 before the fog start point Pa reaches the transfer portion P5 during the pre-rotation operation. In other words, application of a transfer voltage equal to or greater than the discharge start voltage of the transfer member is initiated before the surface area of ​​the image carrier that was located at the development portion P4 when the image carrier started to rotate (the fog start point Pa) reaches the transfer portion P5. Note that while the transfer voltage during image formation has a polarity opposite to the normal charging polarity of the toner, the transfer voltage applied to the transfer roller 5 as the charge applying means during the pre-rotation operation has the same polarity as the normal charging polarity of the toner.

[0127] Any other configuration may be used as long as it can impart a charge of the same polarity as the normal charging polarity to the toner on the photosensitive drum 1, regardless of whether or not a voltage is applied. For example, a sheet-like member that comes into contact with the surface of the photosensitive drum 1 may be provided as a charge imparting means downstream of the transfer portion P5 and upstream of the charging portion P2 in the rotation direction of the photosensitive drum 1. In this case, the sheet-like member can rub against the fogging toner on the photosensitive drum 1, thereby imparting a charge by frictional charging.

[0128] <Evaluation Methods for Examples and Comparative Examples> In the first and second embodiments, image evaluations were carried out for Examples 1 and 2 and Comparative Examples 1, 2, and 3. Details of the image evaluations are described below.

[0129] (1) Evaluation of fogging after leaving The fogging in this evaluation refers to an image defect (white background staining) that appears as background staining due to a small amount of toner adhering to a white background area on the recording material where no image is supposed to be formed, caused by fogging on the photosensitive drum 1. The amount of fogging was evaluated as follows.

[0130] The image forming apparatus 100 was stopped while forming an all-white image (an image based on image information that leaves the entire recording material blank). The door of the image forming apparatus 100 was opened, and the toner adhering to the surface area of ​​the photosensitive drum 1 located between the development unit P4 and the transfer unit P5 was transferred onto transparent adhesive tape, which was then affixed to a recording sheet. A tape with no toner attached was also affixed to the same recording sheet. The optical reflectance of the tape attached to the recording sheet was measured using a green filter with an optical reflectance measuring instrument (TC-6DS manufactured by Tokyo Denshoku Co., Ltd.). This was subtracted from the reflectance of the tape with no toner attached to determine the reflectance of the fog toner, which was then used as the fog amount. The fog amount was measured at three or more points on the tape and the average value was calculated. The fog amount and the visual inspection results for image defects were evaluated according to the following criteria. A: The amount of fogging is less than 1.0%, and is not visible on the image. B: The amount of fogging is less than 1.0 to 3.0%, and is not visible on the image. C: The amount of fog is 3.0 to less than 5.0%, and white background stains are visible in parts of the image. D: The amount of fogging is 5.0 or more, and white background stains are visible on the entire image.

[0131] The fog evaluation was performed after the image forming apparatus 100 was left for 24 hours in a test environment of 32.5°C and 80% RH to acclimate to the environment and then printed 20,000 test images. During the test, an image consisting of a horizontal line pattern with an image ratio of 5% was repeatedly output. After the test, an all-white image was printed and output on one sheet for image evaluation. Specifically, the horizontal line pattern with an image ratio of 5% is a pattern in which a 1-dot line image and a 19-dot blank area are repeated. After the test, the image was evaluated after leaving the apparatus for 48 hours without performing image formation.

[0132] (2) E letter rating For the E character evaluation, the image forming apparatus 100 was left in a test environment of 32.5°C and 80% RH for 24 hours to acclimate to the environment, and then 100 horizontal line images were output. The horizontal line images were images consisting of horizontal line patterns with an image ratio of 5%. Evaluation images were then output and evaluated.

[0133] The evaluation image had the letter "E" printed across the entire image. Specifically, the image was an image in which 4-point size "E" letters were arranged so that they covered 4% of the area of ​​an A4-sized sheet of paper. A schematic diagram of the evaluation image is shown in Figure 20. The evaluation image was observed and evaluated according to the following criteria. The horizontal line image and evaluation image were monochrome and output in normal paper mode (139 mm / sec) at 400 dpi x 400 dpi. A: Even when observed under a microscope, the letter "E" is clearly visible and no chips are visible at all. B: When visually inspected, the letter "E" is clear and no chips are visible, but when observed under a microscope, chips can be seen in part of the letter "E." C: The letter "E" is unclear to the naked eye, and when observed under a microscope, chips are observed in the letter "E". D: The letter "E" is unclear when visually inspected, and chips are observed in the letter "E."

[0134] Table 1 shows the evaluation results of (1) fogging after standing and (2) E character evaluation for Examples 1, 2, 1-2, and 2-2 and Comparative Examples 1, 2, 3, 1-2, 2-2, and 3-2.

[0135] [Table 1]

[0136] <Advantages over Comparative Example 1> First, we will discuss the advantages of each example over Comparative Example 1. In the pre-rotation operation of Comparative Example 1, the charging voltage and developing voltage are increased in stages while the back contrast Vbc, which is the potential difference between the surface potential of the photosensitive drum 1 at the developing unit P4 and the developing voltage, is controlled within a certain range. In this way, even though the back contrast Vbc is controlled within an appropriate range (FIG. 2) for suppressing fogging on the photosensitive drum 1 through the pre-rotation operation, Comparative Example 1 had a low fogging evaluation after leaving it alone (C).

[0137] The reason why Comparative Example 1 had a low evaluation of fog after being left unused will be explained using FIGS. 8 and 12. When left in standby mode for a long period of time, the amount of charge on the toner 44 carried on the developing roller 41 decays over time. If the pre-rotation operation is started when the amount of charge on the toner 44 on the developing roller 41 is very low, some of the toner 44 adheres non-electrostatically to the photosensitive drum 1, causing startup fog, as shown in FIG. 8. As described above, even if a positive voltage +Va is applied to the developing roller 41 before the photosensitive drum 1 starts to rotate to form back contrast, it is difficult to prevent startup fog if the amount of charge on the toner 44 is very low.

[0138] Startup fog occurs from the time when the photosensitive drum 1 starts to rotate (t2) until the toner 44, which was in a position just after passing the tip of the regulating blade 43 at the time when the photosensitive drum 1 starts to rotate (t2), reaches the developing portion P4. In the period thereafter, the toner 44, which has been rubbed against the regulating blade and given a sufficient charge, reaches the developing portion P4, so that fog can be suppressed by an appropriate back contrast Vbc.

[0139] Figure 21 shows the relationship between the time left in standby mode and the toner concentration of startup fog. This evaluation was performed in an environment of 32.5°C and 80% RH, and the fog toner concentration on the photosensitive drum 1 immediately after rotation began was measured using the aforementioned method of tracing it with Mylar tape. The results in Figure 21 show that startup fog is more likely to occur suddenly when the standby time is longer than 10 minutes, and that the degree of startup fog continues to worsen over time. On the other hand, if the standby time is very short, the toner charge does not decay and the impact of startup fog is small.

[0140] From this, it is thought that startup fogging occurred at the developing unit P4 during the pre-rotation operation after a long period of non-use, causing a large amount of fogging toner to adhere to the photosensitive drum 1. Then, as shown in FIG. 11, when the startup fogging toner reaches the developing unit P4 again as the photosensitive drum 1 rotates and passes through the developing unit P4, sufficient back contrast Vbc is not formed in Comparative Example 1 (t10 to t11 in FIG. 13). Here, the back contrast Vbc in Comparative Example 1 is constant at 360 V, the same as during image formation. For this reason, it is thought that in Comparative Example 1, the fogging toner that adhered to the photosensitive drum 1 due to startup fogging after a long period of non-use could not be sufficiently collected by the developing device 20, and the fogging toner remaining on the photosensitive drum 1 caused white background smearing during the next image formation.

[0141] In the case of Comparative Example 1, which is the configuration of the first embodiment having the supply roller 42, the fogging evaluation after leaving On the other hand, in the case of Comparative Example 1-2, which is the configuration of the second embodiment without the supply roller 42, the evaluation of fogging after leaving it was D. This is thought to be because in Comparative Example 1-2, where the reset action of the supply roller 42 cannot be obtained, poor collection of fogging toner at startup is more likely to occur.

[0142] Next, the reason why Example 1 had a good fog evaluation after standing will be explained by comparing it with Comparative Example 1. As shown in Fig. 13, in Comparative Example 1, the back contrast Vbc during the period (t10 to t11) when the fog at startup passed through the developing unit P4 was 360 V, the same as during image formation. On the other hand, as shown in Fig. 7, in Example 1, the back contrast Vbc during the same period (t10 to t11) was 500 V, which was higher than during image formation.

[0143] As described above, the toner charge amount on the developing roller 41 decays during the pre-rotation operation after a long period of inactivity, and startup fogging occurs in both Comparative Example 1 and Example 1. Figure 22 shows the toner charge amount on the photosensitive drum 1 before and after passing through the charging section P2. The charge amount "before passing the charging roller" in Figure 22 indicates the charge amount of the startup fogging toner at the timing (t3) shown in Figure 9. In contrast, the charge amount "after passing the charging roller" in Figure 22 indicates the charge amount of the startup fogging toner at the timing (t3') shown in Figure 10. As can be seen from the results in Figure 22, the startup fogging toner is given a charge when it passes through the charging section P2.

[0144] Next, when the amount of toner (fog amount) adhering to the surface area on the photosensitive drum 1 where start-up fogging occurred was checked after the surface area had passed through the developing unit P4, the amount of fogging in Example 1 was found to be less than that in Comparative Example 1. From this, it can be seen that in Example 1, the back contrast Vbc when the start-up fogging toner reaches the developing unit P4 again (t10) is a larger value (500 V) than during image formation, and therefore the fogging toner can be efficiently collected by the developing roller 41. It is believed that the reduction in the amount of fogging toner remaining on the photosensitive drum 1 resulted in the suppression of white background smearing.

[0145] On the other hand, in Comparative Example 1, when the start-up fog toner reached the developing unit P4 again (t10), the back contrast Vbc was the same value (360 V) as during image formation, which is thought to have been insufficient to efficiently collect the start-up fog toner. In particular, the amount of toner adhering to the photosensitive drum 1 due to start-up fog can be significantly greater than the fog that occurs during image formation, so it is thought that collection was insufficient with the same back contrast Vbc as during image formation. It is thought that the start-up fog toner remained on the photosensitive drum 1 without being collected by the developing roller 41, resulting in white background smearing. The same explanation applies to Example 1-2 and Comparative Example 1-2 according to the second embodiment.

[0146] <Superiority over Comparative Examples 2 and 3> Next, Comparative Examples 2 and 3 will be compared with Example 1. In the evaluation of fogging after standing, the staining on the white background was more noticeable in Comparative Example 2 than in Example 1. The reason for this will be explained below.

[0147] In Comparative Example 2, as shown in Figure 14, during the period (t10 to t11) when the start-up fog from the leading edge to the trailing edge (Pa to Pb) passes through the developing unit P4 again, the back contrast Vbc was set to a value (300 V) lower than that during image formation. That is, in Comparative Example 2, the back contrast Vbc during the period (t10 to t11) was even lower than in Comparative Example 1, where the start-up fog was not sufficiently collected. This is thought to have further reduced the recovery rate of the start-up fog toner, causing white background smearing due to the fog toner that was not collected by the developing roller 41 and remained on the photosensitive drum 1.

[0148] In contrast, in Example 1, the back contrast Vbc during the period (t10 to t11) is greater than that during image formation, which prevents poor collection of fogging toner and suppresses the occurrence of white background smearing, as described above. This also applies to Example 1-2 and Comparative Example 2-2 according to the second embodiment.

[0149] Next, a description will be given of Comparative Example 3. While no significant difference was observed in the E character evaluation between the above-mentioned Example 1 and Comparative Examples 1 and 2, the E character evaluation decreased in the Comparative Example. The reason for this will be explained.

[0150] 15, in Comparative Example 3, during the period (t10 to t11) when the start-up fog from the leading edge to the trailing edge (Pa to Pb) passes through the developing section P4 again, the back contrast Vbc was set to 500 V. Furthermore, after the timing (t3') when the surface area on the photosensitive drum 1 charged by the start of application of the charging voltage reaches the developing section P4, the back contrast Vbc was controlled to a constant value (500 V) including during the image forming operation.

[0151] As described above, since the back contrast Vbc is sufficiently large during the period (t10 to t11) when the fog toner at startup passes through the development section P4 again, poor recovery of the fog toner did not occur in Comparative Example 3, and it is thought that white background smearing was suppressed to the same extent as in Example 1.

[0152] On the other hand, in the E character evaluation, the characters were unclear and some characters were missing in Comparative Example 3. This is thought to be because the developability was reduced because the same high back contrast Vbc as during the pre-rotation operation was set during image formation, and the thin-line electrostatic latent image was not sufficiently developed.

[0153] In contrast, in Example 1, the back contrast Vbc is set high during the period (t10 to t11) when the start-up fogging toner passes through the developing unit P4 again, and the back contrast Vbc during image formation is set low. This is thought to have suppressed white background smearing due to insufficient collection of the start-up fogging toner while also enabling clear development of thin-line images, resulting in good results in both the fogging evaluation after leaving the toner and the E character evaluation. The above explanation also applies to Comparative Example 3-2 and Example 1-2 according to the second embodiment.

[0154] <Regarding Example 2> Next, Example 2 will be described. The evaluation of fog after leaving Example 2 was good, with no visible staining on the white background. This was also true for Example 2-2, in which the reset action of the supply roller 42 was not obtained, and better results were obtained than in Example 1-2. The reasons for this will be explained below.

[0155] As described above, the range from point Pa' to point Pb' on the developing roller 41 shown in FIGS. 11 and 12 is the range in which fog toner collected from the photosensitive drum 1 adheres to the developing roller 41 while the start-up fog passes through the developing station P4 again. Time t12 is the time when point Pa', which is the leading edge of the range in which fog toner adheres to the developing roller 41, reaches the developing station P4 again in one rotation of the developing roller 41, as shown in FIG. 17. Time t13 is the time when point Pb', which is the trailing edge of the range in which fog toner adheres to the developing roller 41, reaches the developing station P4 again in one rotation of the developing roller 41, as shown in FIG. 18. In other words, the period from time t12 to time t13 is the period in which the fog toner from the start-up fog collected by the developing roller 41 may pass through the developing station P4 again in one rotation of the developing roller 41.

[0156] In the configuration of the second embodiment (configuration without a supply roller) according to Examples 1-2 and 2-2, compared to the configuration with a supply roller, some of the fog toner collected by the developing roller 41 continues to be retained on the developing roller 41 and is more likely to reach the developing unit P4 again. Therefore, the amount of toner (coating amount) adhering to the area of ​​the developing roller 41 from point Pa' to point Pb' where the collected fog toner adheres is greater than in other areas. If the coating amount on the developing roller 41 is large, it becomes difficult to impart a sufficient charge to the toner by passing the regulating blade 43 only once, and the proportion of toner with a small charge amount or a charge opposite to the normal charging polarity increases.

[0157] As a result, in the configuration of the second embodiment, reverse fog is likely to occur during the period (t12 to t13) when the fog toner collected by the developing roller 41 passes through the developing unit P4 again. In other words, as shown in FIG. 23, the fog characteristics of the toner adhering to the surface area (collection area) on the developing roller 41 where the fog toner has been collected differ from the fog characteristics (same as in FIG. 2) of the toner adhering to the surface area (non-collection area) where the fog toner has not been collected. The threshold value of the back contrast Vbc at which a visible image defect (white background stain) occurs due to reverse fog is lower for the toner adhering to the collection area than for the toner adhering to the non-collection area.

[0158] In other words, in the second embodiment, during the period (t12 to t13) when the collection area on the developing roller 41 where the fog toner has been collected passes through the developing unit P4 again, image defects due to inverted fog are more likely to occur even at a lower back contrast Vbc than in other periods. As a result, it is thought that the fog evaluation after leaving for Example 1-2 belonging to the second embodiment was lower than the fog evaluation after leaving for Example 1 belonging to the first embodiment.

[0159] Here, Example 2-2, which belongs to the second embodiment, showed an improved evaluation of fog after standing compared to Example 1-2, and obtained evaluations at the same level as Examples 1 and 2.

[0160] In Example 2-2, during the period (t10 to t11) when the fog toner at startup passes through the developing unit P4, the back contrast Vbc is set to a value (500 V) greater than that during image formation. After that, during the period (t12 to t13) when the recovery area on the developing roller 41 where the fog toner has been recovered passes through the developing unit P4 again, the back contrast Vbc is set to a value (300 V) smaller than that during the period (t10 to t11) (see Table 1).

[0161] In this way, during the period (t10 to t11) when the start-up fog passes through the developing section P4, by setting the back contrast Vbc to a value (500 V) greater than that during image formation, it is possible to suppress the occurrence of image defects due to poor collection at the developing section P4, as in Example 1-2.

[0162] In addition, in Example 2-2, the occurrence of reverse fog can be suppressed by lowering the back contrast Vbc during the period (t12 to t13) when the collection area on the developing roller 41 where the fog toner has been collected passes through the developing unit P4 again. In other words, according to this example, the occurrence of image defects can be suppressed more effectively.

[0163] (Other embodiments) In the above embodiment, the control in the pre-rotation operation or jam recovery rotation operation before the start of the image forming operation has been described. However, the same control can be applied when the rotation of the stopped image carrier in the image forming apparatus is started and the charging voltage and the developing voltage are increased.

[0164] In the above-described embodiment, the image forming apparatus has been described as a so-called monochrome image forming apparatus having only one image carrier. However, the same control can be applied to a full-color image forming apparatus having multiple image carriers and forming images using multiple developers with different toner colors. The full-color image forming apparatus may be of an intermediate transfer type in which single-color toner images formed on multiple image carriers are primarily transferred to an intermediate transfer medium and then transferred to a recording material all at once, or may be of a sequential transfer type in which single-color toner images are sequentially transferred to a recording material.

[0165] In the case of the intermediate transfer method, the term "transfer means" refers to, for example, a transfer roller (primary transfer roller) that performs primary transfer of a toner image from a photosensitive drum 1 serving as an image carrier to an intermediate transfer material serving as a transfer receiving material. The term "transfer portion" refers to the portion where the image carrier and the intermediate transfer material face each other. The intermediate transfer material may be, for example, an endless belt member stretched over multiple rollers. The toner image that has been primarily transferred onto the intermediate transfer material is then secondarily transferred from the intermediate transfer material to a recording material by a secondary transfer means, such as a secondary transfer roller, that forms a secondary transfer nip between the intermediate transfer material and the intermediate transfer material. Even in this type of intermediate transfer method configuration, the same effects as those of the above-described embodiments can be achieved by replacing the transfer roller in the above-described embodiments with a primary transfer roller. [Explanation of symbols]

[0166] 1... image carrier (photosensitive drum) / 2... charging member, charge applying means (charging roller) / 5... transfer means (transfer roller) / 41... developing member (developing roller) / 50... control means (control section) / P2... charging section / P4... developing section

Claims

1. a rotatable image carrier; a charging member that forms a charging portion between the image carrier and the charging member and charges the surface of the image carrier; a developing member disposed in contact with the image carrier, forming a developing section between the image carrier and the developing member, and supplying toner to the developing section to form a toner image on the image carrier; a transfer means for transferring the toner image from the image carrier to a transfer material; a charge applying means provided opposite the image carrier; a control means for controlling a charging voltage applied to the charging member and a developing voltage applied to the developing member; and recovering, by the developing member, toner remaining on the surface of the image carrier without being transferred by the transfer unit during an image forming operation of forming an image on a recording material, The difference between the surface potential of the image carrier in the developing unit and the developing voltage is Vbc, The control means Before the start of the image forming operation, the rotation of the image carrier is started, and a preparatory operation is performed in which the charging voltage and the developing voltage are increased in stages; In the preparatory operation, the charge applying means is caused to apply a charge; controlling the charging voltage and the developing voltage in the preparatory operation so that the value of Vbc at a time when the surface area of ​​the image carrier that was located at the developing unit at the time when the image carrier started to rotate reaches the developing unit again becomes larger than the value of Vbc in the image forming operation; An image forming apparatus characterized by:

2. the control unit, in the preparatory operation, causes the charge applying unit to start applying charge before a surface area of ​​the image carrier that was located in the developing unit at the time when the rotation of the image carrier started reaches the charge applying unit.

2. The image forming apparatus according to claim 1, wherein the image forming apparatus is a recording medium.

3. the control means, in the preparatory operation, starts applying a voltage to the charge applying means before a surface area of ​​the image carrier that was located in the developing unit at the time when the rotation of the image carrier started reaches the charge applying means.

3. The image forming apparatus according to claim 2, wherein the image forming apparatus is a recording medium.

4. the control means, in the preparatory operation, causes the charge applying means to start applying a voltage equal to or higher than a discharge start voltage before a surface area of ​​the image carrier that was located in the developing unit at the time when the rotation of the image carrier started reaches the charge applying means.

4. The image forming apparatus according to claim 3, wherein the image forming apparatus is a recording medium.

5. the charge applying means is the charging member, the control unit starts application of the charging voltage to a surface area of ​​the image carrier that was located in the developing unit at the time when the rotation of the image carrier started, before the surface area reaches the charging unit, in the preparatory operation.

2. The image forming apparatus according to claim 1, wherein the image forming apparatus is a recording medium.

6. the control means starts application of the charging voltage at a voltage value equal to or higher than a discharge start voltage of the charging member before the surface area of ​​the image carrier that was located in the developing unit at the time when the rotation of the image carrier started reaches the charging unit in the preparatory operation.

6. The image forming apparatus according to claim 5,

7. the charge applying means is the transfer means, the control means, in the preparatory operation, starts application of a transfer voltage to the transfer means at a voltage value equal to or higher than a discharge start voltage of the transfer means before a surface area of ​​the image carrier that was located in the development unit at the time when the rotation of the image carrier started reaches a transfer unit where transfer by the transfer means is performed.

2. The image forming apparatus according to claim 1, wherein the image forming apparatus is a recording medium.

8. the control means controls the charging voltage and the developing voltage during the preparatory operation so that the surface potential of the image carrier in the developing unit becomes the same polarity as the normal charging polarity of the toner with respect to the developing voltage, and so that the potential difference between the surface potential of the image carrier and the developing voltage is maintained within a predetermined range.

8. The image forming apparatus according to claim 1, wherein the image forming apparatus is a recording medium.

9. The predetermined range is 130V or more and 550V or less.

9. The image forming apparatus according to claim 8,

10. the control means controls the charging voltage and the developing voltage in the preparatory operation so that, during the preparatory operation, a state in which the Vbc is higher than the value of the Vbc in the image forming operation and a state in which the Vbc is lower than the value of the Vbc in the image forming operation are alternately repeated.

10. The image forming apparatus according to claim 1, wherein the image forming apparatus is a recording medium.

11. a regulating member disposed in contact with the developing member and frictionally charging the toner while regulating the amount of the toner carried by the developing member and transported to the developing section; The peripheral speed of the developing member is Vd (mm / sec), The distance traveled by the surface of the developing member from the contact portion between the developing member and the regulating member to the developing portion is defined as d (mm), the control means controls the charging voltage and the developing voltage in the preparatory operation so that the value of Vbc is greater than the value of Vbc in the image forming operation over a period from when the surface area of ​​the image carrier that was located at the developing unit at the time when the image carrier started to rotate reaches the developing unit again until a time d / Vd has elapsed.

11. The image forming apparatus according to claim 1, wherein the image forming apparatus is a recording medium.

12. The control means a value of Vbc at a first time point when the surface area of ​​the image carrier that was located at the developing unit at the time when the image carrier started to rotate reaches the developing unit again is defined as Vk1; the value of Vbc at a second time point when the surface area of ​​the developing member, which was located at the developing portion at the first time point, reaches the developing portion again after one rotation of the developing member, is defined as Vk2; The charging voltage and the developing voltage in the preparatory operation are controlled so that the Vk2 is smaller than the Vk1.

11. The image forming apparatus according to claim 1, wherein the image forming apparatus is a recording medium.

13. a regulating member disposed in contact with the developing member and frictionally charging the toner while regulating the amount of the toner carried by the developing member and transported to the developing section; The peripheral speed of the developing member is Vd (mm / sec), The distance traveled by the surface of the developing member from the contact portion between the developing member and the regulating member to the developing portion is defined as d (mm), the control means controls the charging voltage and the developing voltage in the preparatory operation so that the value of Vbc becomes Vk1 during a period from the first time point until a time d / Vd has elapsed, and so that the value of Vbc becomes Vk2 during a period from the second time point until a time d / Vd has elapsed.

13. The image forming apparatus according to claim 12.

14. The toner is a non-magnetic one-component developer.

14. The image forming apparatus according to claim 1, wherein the image forming apparatus is a recording medium.

15. the transfer material is the recording material, 15. The image forming apparatus according to claim 1, wherein the image forming apparatus is a recording medium.

16. an intermediate transfer member; a secondary transfer unit that transfers the toner image from the intermediate transfer body to the recording material; Further provided with the transfer material is the intermediate transfer body; 15. The image forming apparatus according to claim 1, wherein the image forming apparatus is a recording medium.

Images