Image forming apparatus

Abstract

To prevent toner scattering brought about by toner that has caused fogging at start-up.SOLUTION: In an image forming apparatus 100, rotation of a photoreceptor 1 and a developing member 4a is started in a state where the photoreceptor 1 and the developing member 4a are in contact with each other. When an area on the photoreceptor 1 located at a developing part Nd during stop is defined as a first area, and an area on the photoreceptor 1 brought into contact at the developing part Nd first with an area on the developing member 4a located at a regulation part Nb during stop as a second area, control means 50 controls transfer voltage application means 18 to, in a first period including at least part of a period from when the first area reaches a transfer part Nt first until when the second area reaches the transfer part Nt first, apply voltage having a polarity opposite to the normal electrification polarity of toner to a transfer member 5, and in a second period after the first period and before a toner image reaches the transfer part Nt, apply, to the transfer member 5, at least a first voltage having the same polarity as the normal electrification polarity, and a second voltage having the same polarity as that of the first voltage and having an absolute value larger than the absolute value of the first voltage.SELECTED DRAWING: Figure 3

Description

Technical Field

[0001] The present invention relates to an image forming apparatus such as a laser beam printer, a copying machine, or a facsimile machine using an electrophotographic system. Background Art

[0002] Conventionally, in an image forming apparatus such as a laser beam printer using an electrophotographic system, for example, the surface of a photosensitive drum as an image bearing member is uniformly charged to a dark portion potential by a charging unit. Thereafter, the surface of the charged photosensitive drum is exposed by an exposure unit, and a bright portion potential is formed on the surface of the photosensitive drum. Accordingly, an electrostatic latent image is formed on the surface of the photosensitive drum by the contrast between the dark portion potential and the bright portion potential. The electrostatic latent image formed on the photosensitive drum is supplied with toner by a developing unit, so that a toner image is formed on the photosensitive drum. A developing roller is generally used as a developing member constituting the developing unit. The developing roller abuts against the photosensitive drum to form a nip portion, and supplies toner in a developing container onto the photosensitive drum while rotating.

[0003] The toner image formed on the photosensitive drum is transferred onto a recording material by a transfer unit. A transfer roller is generally used as a transfer member constituting the transfer unit. The transfer roller abuts against the photosensitive drum to form a nip portion (hereinafter referred to as "transfer nip portion"), and transfers the toner on the photosensitive drum onto the recording material while rotating to convey the recording material. At this time, a transfer voltage having a polarity opposite to a normal charging polarity of toner is applied to the transfer roller, and the toner image on the image bearing member is electrostatically transferred onto the recording material. Thereafter, the toner image transferred onto the recording material is fixed onto the recording material by being heated and pressurized by a fixing unit.

[0004] Although the recording material is sometimes referred to as "paper," the recording material may be made of materials other than paper, or materials containing materials other than paper, such as synthetic paper or film made primarily of synthetic resin, or metallized paper (special paper) with a metal layer. Also, for convenience, unless otherwise specified, the magnitude (high or low, strength) of potential, voltage, or current shall refer to the magnitude (high or low, strength) when compared in absolute value. Furthermore, with respect to the recording material, "front end" and "rear end" refer to the front and rear ends of the recording material in the transport direction, respectively.

[0005] In the image forming apparatus described above, a phenomenon called "startup fogging" may occur. Startup fogging is a phenomenon that occurs when the image forming apparatus is started up after a long period of time has elapsed since the end of the previous print operation, and even though there is no electrostatic latent image on the photosensitive drum, some of the toner that was held on the developing roller is transferred to the photosensitive drum. This phenomenon occurs because the surface potential of the photosensitive drum decreases over time, making it impossible to maintain the potential difference between the photosensitive drum and the developing roller, and also because the charge of the toner on the developing roller decreases over time.

[0006] A common method to suppress "startup fogging" is to separate the developing roller from the photosensitive drum when the image forming machine is started. If the developing roller is separated from the photosensitive drum, the toner on the developing roller will not transfer to the photosensitive drum. However, equipping the image forming machine with a contact / separation mechanism to switch between contact and separation of the developing roller from the photosensitive drum would lead to an increase in the size and cost of the machine.

[0007] Patent Document 1 proposes that when an image forming apparatus is started, a voltage of the opposite polarity to the normal voltage is applied to the developing roller to generate a potential difference between the photosensitive drum and the developing roller, thereby reducing the amount of toner transferred from the developing roller to the photosensitive drum. [Prior art documents] [Patent Documents]

[0008] [Patent Document 1] Japanese Patent Publication No. 2013-117591 [Overview of the Initiative] [Problems that the invention aims to solve]

[0009] However, after a long period of time has passed since the last print operation, the toner on the developing roller contains a large amount of toner with zero charge or toner whose polarity has reversed to the opposite of its normal charge polarity. Therefore, it is difficult to completely prevent fogging at startup using only the potential difference between the photosensitive drum and the developing roller.

[0010] When toner that has experienced this type of startup fouling (hereinafter also referred to as "fouling toner") passes through the transfer nip, toner scattering occurs from the transfer nip. This toner scattering causes toner to adhere to components located downstream of the transfer nip in the direction of recording material transport, such as static elimination needles and transport guides. As startup fouling occurs repeatedly, this toner contamination accumulates.

[0011] Therefore, the objective of the present invention is to suppress toner scattering caused by toner fouling during startup. [Means for solving the problem]

[0012] The above objective is achieved by the image forming apparatus according to the present invention. In summary, the present invention comprises a rotatable photoreceptor, a charging means for charging the photoreceptor in a charging unit, a rotatable developing member in a developing unit that contacts the photoreceptor and supplies toner onto the photoreceptor charged by the charging means, a restricting member in a restricting unit that contacts the developing member and applies an electric charge to the toner on the developing member, a transfer member that contacts the photoreceptor to form a transfer unit and transfers toner from the photoreceptor to a recording material passing through the transfer unit, a transfer voltage applying means for applying a voltage to the transfer member, and a control means capable of controlling the transfer voltage applying means, wherein the photoreceptor and the developing member are in contact. In an image forming apparatus in which the rotation of the photoreceptor and the developing member is started in a certain state, when the region on the photoreceptor that was located in the developing section when the photoreceptor and the developing member stopped is defined as the first region, and after the rotation of the photoreceptor and the developing member has started, the region on the developing member that was located in the regulating section when the photoreceptor and the developing member stopped and the region on the photoreceptor that first comes into contact with the developing section are defined as the second region, the control means controls the period from when the rotation of the photoreceptor and the developing member has started and the first region has reached the transfer section until the second region has first reached the transfer section. interval The image forming apparatus is characterized by controlling the transfer voltage application means to apply, during a first period including, no voltage to the transfer member or a voltage with the opposite polarity to the normal charging polarity of the toner, after the first period and during a second period before the toner image formed on the photoreceptor reaches the transfer section, a first voltage having at least the same polarity as the normal charging polarity and a second voltage having the same polarity as the first voltage and whose absolute value is greater than the absolute value of the first voltage. [Effects of the Invention]

[0013] According to the present invention, it is possible to suppress toner scattering caused by toner fouling during startup. [Brief explanation of the drawing]

[0014] [Figure 1]It is a schematic cross-sectional view of an image forming apparatus. [Figure 2] It is a schematic diagram for explaining the generation mechanism of toner scattering caused by fogging at startup. [Figure 3] It is a timing chart showing the transition of transfer voltage in transfer voltage control at startup of the image forming apparatus of Example 1 (V0=+1000V). [Figure 4] It is a schematic diagram for explaining the mechanism by which toner scattering caused by fogging at startup is suppressed. [Figure 5] It is a graph showing the charge distribution of toner on the developing roller, photosensitive drum and transfer roller at startup of the image forming apparatus. [Figure 6] It is a timing chart showing the transition of transfer voltage in transfer voltage control at startup of the image forming apparatus of Example 1 (V0=0V). [Figure 7] It is a timing chart showing the transition of transfer voltage in transfer voltage control at startup of the image forming apparatus of Comparative Example 1. [Figure 8] It is a timing chart showing the transition of transfer voltage in transfer voltage control at startup of the image forming apparatus of Comparative Example 2. [Figure 9] It is a schematic cross-sectional view of another embodiment (cleaner-less configuration) of the image forming apparatus of Example 1. [Figure 10] It is a graph showing the charge distribution of toner on the developing roller at startup of the image forming apparatus for each elapsed time from the end of the previous printing operation. [Figure 11] It is a flowchart of the control of Example 2. [Figure 12] It is a schematic block diagram showing the control configuration of the image forming apparatus. DETAILED DESCRIPTION OF EMBODIMENTS

[0015] Hereinafter, the image forming apparatus according to the present invention will be described in further detail with reference to the drawings.

[0016] Example 1 (1) Image Forming Apparatus Figure 1 is a schematic cross-sectional view of the image forming apparatus 100 of this embodiment. The image forming apparatus 100 of this embodiment is an electrophotographic laser printer. The image forming apparatus 100 forms an image (monochromatic black image) on a recording material P according to image information input from an external device (not shown), such as a host computer.

[0017] The image forming apparatus 100 has a photosensitive drum 1, which is a rotatable drum-shaped (cylindrical) electrophotographic photoreceptor (photoreceptor) as an image carrier, inside the apparatus body M. The photosensitive drum 1 is made of a photosensitive material such as OPC (organic photoconductive material), amorphous selenium, or amorphous silicon, placed on a cylindrical drum base made of aluminum or nickel. In this embodiment, the photosensitive drum 1 is a negatively charged OPC photoreceptor with an outer diameter of φ24 mm, and has a photosensitive layer on the surface of a conductive base made of an aluminum cylinder, in which a charge generation layer and a charge transport layer are laminated in that order from the conductive base side. The photosensitive drum 1 is driven to rotate in the direction of arrow Rd (clockwise direction) in the figure.

[0018] The following means are arranged around the photosensitive drum 1. First, a charging roller 2, which is a roller-shaped charging member, is arranged as a charging means. Also, an exposure device 3 is arranged as an exposure means. Also, a developing device 4 is arranged as a developing means. Also, a transfer roller 5, which is a roller-shaped transfer member, is arranged as a transfer means. Also, a static elimination needle 20, which is a static elimination member, is arranged as a static elimination means. Also, a cleaning device 6 is arranged as a cleaning means. Along the rotation direction Rd of the photosensitive drum 1, the charging section by the charging roller 2 (charging position), the exposure section by the exposure device 3 (exposure position), the developing section by the developing device 4 (development position), the transfer section by the transfer roller 5 (transfer position), and the cleaning section by the cleaning device 6 (cleaning position) are arranged in order.

[0019] The charging roller 2 is composed of, for example, a conductive base shaft (core metal) that also serves as a power supply electrode, and an elastic layer that cylindrically surrounds its outer surface. In this embodiment, the charging roller 2 is an elastic roller with an outer diameter of φ10 mm, a core metal diameter of φ5 mm, and an elastic layer thickness of 2.5 mm. In this embodiment, SUS is used for the core metal of the charging roller 2, and a mixed rubber material of NBR and epichlorohydrin is used for the elastic layer of the charging roller 2. The charging roller 2 is positioned in contact with the photosensitive drum 1. The charging roller 2 charges the surface (outer surface) of the photosensitive drum 1. The charging section (charging position) is the position where the charging treatment by the charging roller 2 is performed in the rotational direction of the photosensitive drum 1. The charging roller 2 charges the surface of the photosensitive drum 1 by discharge generated in at least one of the minute gaps between the photosensitive drum 1 and the charging roller 2 that are formed on the upstream and downstream sides of the contact area between the photosensitive drum 1 and the charging roller 2 in the rotational direction of the photosensitive drum 1. However, in this explanation, the contact area between the photosensitive drum 1 and the charging roller 2 will be considered as the charged area (charged nip area) Nc.

[0020] In this embodiment, the exposure device 3 is composed of a laser scanner device (laser optical system). The exposure device 3 exposes the photosensitive drum 1 to form an electrostatic latent image (electrostatic image). The position where exposure by the exposure device 3 is performed in the rotational direction of the photosensitive drum 1 is the exposure section (exposure position).

[0021] The developing device 4 includes a developing container 4c for containing toner as a developer, a developing roller 4a as a developing member (developer carrier) for transporting toner to the part opposite the photosensitive drum 1, and a developing blade 4b as a regulating member. The developing roller 4a is rotated in the direction of the arrow in the figure (counterclockwise direction), that is, in a direction in which the direction of movement of the developing roller 4a and the direction of movement of the photosensitive drum 1 are in the forward direction at the part opposite to the photosensitive drum 1. The developing blade 4b contacts the developing roller 4a on the side opposite to the side of the developing roller 4a that faces the photosensitive drum 1. The developing blade 4b regulates the amount of toner carried on the developing roller 4a and imparts an electric charge to the toner on the developing roller 4a. The developing blade 4b is a plate-shaped member having a predetermined length in the longitudinal direction, which is arranged along the rotation axis direction of the developing roller 4a, and in the short direction, which is substantially perpendicular to the longitudinal direction. The developing blade 4b is positioned such that its free end in the short direction faces upstream of the developing roller 4a in the rotational direction, and the vicinity of its tip contacts the outer circumferential surface of the developing roller 4a. In this embodiment, the image forming apparatus 100 does not have a contact / separation mechanism to switch between contact and separation of the developing roller 4a with respect to the photosensitive drum 1. That is, in this embodiment, the developing roller 4a and the photosensitive drum 1 are always driven in contact. By not providing a contact / separation mechanism in the image forming apparatus 100 in this way, the size of the apparatus body M is reduced. In this embodiment, the photosensitive drum 1 and the developing roller 4 are driven by a common drive source, for example, the drive unit 60 (Figure 12), and rotate and stop synchronously. The position where toner is supplied by the developing roller 4a of the developing apparatus 4 in the rotational direction of the photosensitive drum 1 is the developing section (developing position). In this embodiment, the contact point between the developing roller 4a and the photosensitive drum 1 is the developing section (developing nip section) Nd. Furthermore, the contact portion between the developing roller 4a and the developing blade 4b in the rotational direction of the developing roller 4a is defined as the blade nip portion (regulating portion) Nb.

[0022] The transfer roller 5 is positioned opposite the photosensitive drum 1. The transfer roller 5 is biased (pressed) toward the photosensitive drum 1 by a transfer pressure spring (not shown), which is a biasing member acting as a biasing means, and is in contact with the photosensitive drum 1. This forms a transfer nip portion (transfer portion, transfer clamping portion) Nt, which is the contact point between the photosensitive drum 1 and the transfer roller 5. The transfer roller 5 rotates in accordance with the rotation of the photosensitive drum 1. In this embodiment, the image forming apparatus 100 does not have a contact / separation mechanism to switch between contact and separation of the transfer roller 5 with respect to the photosensitive drum 1. The transfer roller 5 clamps the recording material P between itself and the photosensitive drum 1, and when a voltage is applied, it transfers a toner image from the photosensitive drum 1 to the recording material P passing through the transfer nip portion Nt. The transfer roller 5 is composed of, for example, a conductive base shaft (core metal) that also serves as a power supply electrode, and an elastic layer that cylindrically surrounds its outer circumferential surface. Generally, semiconducting rubber materials such as EPDM, NBR, urethane rubber, epichlorohydrin, and silicone rubber are used as the elastic layer of the transfer roller 5. In this embodiment, the transfer roller is an elastic roller with an outer diameter of φ14 mm, a core diameter of φ5 mm, and an elastic layer thickness of 4.5 mm. In this embodiment, SUS is used for the core of the transfer roller 5, and a mixed rubber material of NBR and epichlorohydrin is used for the elastic layer of the transfer roller 5. In this embodiment, the contact pressure of the transfer roller 5 against the photosensitive drum 1 is 9.8 N (1 kgf). In this embodiment, the electrical resistance value of the transfer roller 5 is 4.0 × 10⁻¹⁰ when the transfer roller 5 is pressed onto an aluminum cylinder with a force of 9.8 N, rotated at 50 mm / sec, and +1000 V is applied. 7 It is Ω. The transfer position is the position where toner is transferred by the transfer roller 5 in the rotational direction of the photosensitive drum 1 (the position corresponding to the transfer nip portion Nt mentioned above).

[0023] The static elimination needle 20 eliminates excess charge on the surface of the recording material P after transfer. In this embodiment, the static elimination needle 20 is made of a thin metal plate material such as a SUS plate or an aluminum plate, which has a sawtooth-shaped pointed end and good conductivity. This static elimination needle 20 is positioned downstream of the transfer roller 5 in the transport direction of the recording material P, and the needle tip faces the surface of the photosensitive drum 1. In other words, the static elimination needle 20 is positioned so that the needle tip faces the transport path of the recording material P (facing the recording material P being transported along the transport path). In this embodiment, the static elimination needle 20 is electrically grounded via a resistive element (not shown). Alternatively, a voltage (for example, a voltage of the same polarity as the normal charging polarity of the toner) may be applied to the static elimination needle 20.

[0024] The cleaning device 6 cleans the surface of the photosensitive drum 1 by removing any toner remaining on it. In this embodiment, the cleaning device 6 has a cleaning blade 6a as a cleaning member and a cleaning container 6b for storing the collected toner. The cleaning section (cleaning position) is the position in the rotational direction of the photosensitive drum 1 where the cleaning device 6 removes the toner (the contact point between the cleaning blade 6d and the photosensitive drum 1).

[0025] Furthermore, a recording material cassette 7, which stores recording materials (transfer material, recording medium, paper, sheet) P such as paper, is located at the bottom of the main body M in the diagram. Along the transport path from the recording material cassette 7 to the recording material P, a feed roller 8, a transport roller 9, a top sensor 10, a pre-transfer guide 24, a transfer-fixing transport guide 11, a fixing device 12, an discharge roller 13, and an discharge tray 14 are arranged in order.

[0026] As shown in Figure 12, the main body M of the image forming apparatus 100 is equipped with a control unit 50, which serves as a control means for controlling the overall operation of the image forming apparatus 100. The control unit 50 is configured to include, for example, a CPU 51 as an arithmetic control means, a memory 52 such as ROM, RAM, or non-volatile memory as a storage means, and an input / output unit (not shown) that controls the exchange of information (signals) between the control unit 50 and external devices. The CPU 51 executes predetermined arithmetic processing. A predetermined control program is stored in the ROM of the memory 52. ​​Data is temporarily stored in the RAM of the memory 52. ​​The usage history of each part is stored in the non-volatile memory of the memory 52. ​​For example, a charging power supply 21, a charging current detection circuit 22, a transfer power supply 18, a transfer current detection circuit 19, and an exposure device 3 are connected to the control unit 50. In addition, a drive device 60 is connected to the control unit 50 as a driving means for driving, for example, a photosensitive drum 1, a developing roller 4a, a feeding roller 8, a transport roller 9, etc. The control unit 50 then controls each part of the image forming apparatus 100 according to the control program stored in the ROM, using information stored in the RAM and non-volatile memory as appropriate.

[0027] Furthermore, the photosensitive drum 1, the charging roller 2 acting as a process means thereon, the developing device 4, and the cleaning device 6 may be integrated into a process cartridge that can be attached to and detached from the main body M of the image forming apparatus 100.

[0028] Next, the image forming operation of the image forming apparatus 100 in this embodiment will be described. The photosensitive drum 1 is driven to rotate at a peripheral speed (process speed) of 320 mm / sec in the direction of the arrow Rd in the figure by the drive source of the drive device 60. The surface of the rotating photosensitive drum 1 is uniformly charged by the charging roller 2 to a desired potential (dark area potential) with the same polarity as the normal charging polarity (negative polarity in this embodiment) of the toner. During the charging process, a charging voltage (charging bias), which is a DC voltage with the same polarity as the normal charging polarity (negative polarity in this embodiment) of the toner, is applied to the charging roller 2 from the charging power supply (high voltage power supply) 21, which is a means for applying a charging voltage, via the charging current detection circuit 22. In this embodiment, a voltage of -1100V is applied to the charging roller 2 as the charging voltage, and the dark area potential of the photosensitive drum 1 becomes -500V. The surface of the charged photosensitive drum 1 is exposed to image information L by the exposure device 3, and the charge in the exposed area is removed to become the bright area potential (-100V in this embodiment). As a result, an electrostatic latent image (electrostatic image) is formed on the photosensitive drum 1 by the contrast between the dark area potential and the bright area potential.

[0029] The electrostatic latent image formed on the photosensitive drum 1 is developed (visualized) by the developing device 4 when toner is supplied, and a toner image is formed on the photosensitive drum 1. The toner on the developing roller 4a is given a negative charge by being rubbed by the developing blade 4b. During development, the developing roller 4a is subjected to a developing voltage (developing bias), which is a DC voltage with the same polarity (negative in this embodiment) as the normal charge polarity of the toner, from the developing power supply (high voltage power supply) 23, which serves as a developing voltage application means. In this embodiment, a voltage of -350V is applied to the developing roller 4a as the developing voltage. The toner, which has been charged by the developing blade 4b, adheres to the image portion of the electrostatic latent image on the photosensitive drum 1, forming a toner image. Thus, in this embodiment, toner charged with the same polarity (negative in this embodiment) as the charge polarity of the photosensitive drum 1 adheres to the exposed portion (image portion) on the photosensitive drum 1, where the absolute value of the potential has decreased after uniform charging treatment and exposure (reverse developing method). In this example, the normal charge polarity of the toner, which is the primary charge polarity of the toner during development, is negative polarity.

[0030] The toner image formed on the photosensitive drum 1 is transferred to a recording material P, such as paper, at the transfer nip section Nt by the action of the transfer roller 5. During transfer, a transfer voltage (transfer bias), which is a DC voltage with the opposite polarity (positive polarity in this embodiment) to the normal charging polarity of the toner, is applied to the transfer roller 5 from a transfer power supply (high voltage power supply) 18, which serves as a means for applying a transfer voltage, via a transfer current detection circuit 19. As a result, the toner image on the photosensitive drum 1 is electrostatically transferred to a predetermined position on the recording material P. The recording material P is stored in a recording material cassette 7 and is fed out one sheet at a time by a feed roller 8. This recording material P is transported by a transport roller (resist roller) 9 in time with the toner image on the photosensitive drum 1 and supplied to the transfer nip section Nt along a pre-transfer guide 24, which serves as a guide member.

[0031] The recording material P onto which the toner image has been transferred at the transfer nip section Nt has excess charge removed from its surface by the static elimination needle 20. The recording material P that has passed through the static elimination needle 20 is transported along the transfer-fixing transport guide (hereinafter also simply referred to as the "transport guide") 11, which serves as a guide member, to the fixing device 12, which serves as a fixing means. The fixing device 12 has a fixing roller 12a with a built-in heater, and a pressure roller 12b that presses against the fixing roller 12a. The fixing device 12 applies heat and pressure to the recording material P as it passes through the fixing section (fixing nip section) formed by the fixing roller 12 and the pressure roller 12b, fixing (melting, solidifying) the unfixed toner image onto the recording material P. The recording material P on which the toner image has been fixed by the fixing device 12 is discharged (output) by the discharge roller 13 onto the discharge tray 14 formed on the upper surface of the device body M in the figure.

[0032] On the other hand, toner that remains on the surface of the photosensitive drum 1 without being transferred to the recording material P (transfer residue toner) is removed from the surface of the photosensitive drum 1 by the cleaning blade 6a of the cleaning device 6 and collected in the cleaning container 6b.

[0033] By repeating the above operations, images can be formed one after another. The image forming apparatus 100 of this embodiment can print at a print speed of 60 frames per minute.

[0034] The image forming apparatus 100 executes a print operation (print job), which is a series of operations that start with a single start instruction and form and output an image on one or more recording materials P. A print operation generally includes an image forming process (image forming operation), a pre-rotation process, an inter-paper process when forming an image on multiple recording materials P, and a post-rotation process. The image forming process is the period during which the electrostatic latent image, toner image, and toner image transfer of the image to be actually formed and output on the recording material P are performed, and this period is referred to as the image forming time (image forming operation time). More specifically, the timing of the image forming time differs depending on the position in which each of the above processes of electrostatic latent image formation, toner image formation, and toner image transfer is performed, and corresponds to the period during which the image forming area on the photosensitive drum 1 passes through each of the above positions. The pre-rotation process is the period during which preparatory operations are performed before the image forming process, from when a start instruction is input until the image is actually formed. The inter-paper process (inter-image process, inter-recording material process) is the period between recording materials P when image formation is performed on multiple recording materials P in succession (continuous image formation, continuous printing). The post-rotation process is the period during which tidying operations (preparation operations) are performed after the image formation process. Non-image formation time is the period other than the image formation time, and includes the pre-rotation process, inter-paper process, post-rotation process, and pre-multi-rotation process, which is the preparation operation when the image forming apparatus 100 is powered on or recovers from sleep mode. More specifically, the timing of non-image formation time corresponds to the period during which the non-image formation area on the photosensitive drum 1 passes through the positions where each of the processes of electrostatic latent image formation, toner image formation, and toner image transfer is performed. Here, the image formation area on the photosensitive drum 1 is the area where a toner image can be formed, which is transferred to the recording material P and output from the image forming apparatus 100, and which is set in advance according to the size of the recording material P, etc., and the non-image formation area is the area other than the image formation area.

[0035] (2) Mechanism of toner scattering due to overflow at startup Next, the mechanism of toner scattering due to startup fogging will be explained. Figure 2 is a schematic diagram of the area around the photosensitive drum 1 to illustrate the mechanism of toner scattering due to startup fogging. Figure 2 shows the case where a conventional transfer voltage control is used as the transfer voltage control at startup of the image forming apparatus, using an image forming apparatus 100 which has substantially the same basic configuration as the image forming apparatus 100 of this embodiment.

[0036] Figure 2(a) shows the state after a long period of time has elapsed since the end of the previous print operation. In this embodiment, "long period of time" refers to 12 hours or more, during which the average charge of the toner T on the developing roller 4a approaches 0 μC / g (zero tribotoner). Furthermore, after such a long period of time, the surface potential of the photosensitive drum 1 has decayed to near 0V.

[0037] When a drive instruction such as a print signal is given to the image forming apparatus 100, the photosensitive drum 1 is rotated (pre-rotation) with no recording material P on the transfer nip Nt, in order to remove any remaining toner on the photosensitive drum 1 with the cleaning device 6 before moving to the image forming operation. Almost simultaneously with the start of rotation of the photosensitive drum 1, the application of a charging voltage to the charging roller 2 and the application of a developing voltage to the developing roller 4a, similar to that during normal image forming, begin. The developing voltage at this time does not have to be the same as the developing voltage during normal image forming. For example, a developing voltage with the same polarity as the developing voltage during normal image forming but with a smaller absolute value may be used, or a voltage with the opposite polarity to the normal polarity may be applied instead of a negative polarity voltage, which is the normal polarity of toner. Figure 2(b) shows the state immediately after the drive. As shown in Figure 2(b), the toner T that was attached to the developing roller 4a when stopped is transferred to the photosensitive drum 1 as start-up fogging. There are two main reasons for this.

[0038] The first reason is that in the section on the developing roller 4a that carries the toner T, there is a section that passes through the developing section Nd without passing through the blade nip section Nb when the image forming apparatus 100 is started (the section from the blade nip section Nb to the developing section Nd in the rotational direction of the developing roller 4a). The toner T in that section enters the developing section Nd as "zero tribo toner". Therefore, regardless of the potential difference between the photosensitive drum 1 and the developing roller 4a in the developing section Nd, the toner T is transferred from the developing roller 4a to the photosensitive drum 1 due to the force relationship of the non-electrostatic adhesion force between the toner T and each component (photosensitive drum 1, developing roller 4a).

[0039] The second reason is that there is a section on the photosensitive drum 1 where the surface potential remains attenuated when the image forming apparatus 100 is started, and this section passes through the developing section Nd (the section from the charged section Nc to the developing section Nd in the rotational direction of the photosensitive drum 1). In that section, the surface of the photosensitive drum 1 cannot generate a sufficient potential difference between the developing section Nd and the developing roller 4a. Therefore, regardless of the charge of the toner T, the toner T is transferred from the developing roller 4a to the photosensitive drum 1 due to the force relationship of the non-electrostatic adhesion force between the toner T and each component (photosensitive drum 1, developing roller 4a).

[0040] For the two reasons described above, under conditions where fogging occurs at startup, for example under relatively unfavorable conditions, fogging toner T that resembles a solid image with one or more layers of toner T overlapping will transfer from the developing roller 4a to the photosensitive drum 1. As described above, in the rotational direction of the developing roller 4a, toner T in the section from the area on the developing roller 4a that was located at the blade nip section Nb when stopped to the area on the developing roller 4a that was located at the developing section Nd when stopped is likely to transfer to the photosensitive drum 1 when the image forming apparatus 100 starts up. Also, as described above, in the rotational direction of the photosensitive drum 1, toner that has passed through the blade nip section Nb may transfer to the section from the area on the photosensitive drum 1 that was located at the charging section Nc when stopped to the area on the photosensitive drum 1 that was located at the developing section Nd when stopped, even if it has passed through the blade nip section Nb, when the image forming apparatus 100 starts up.

[0041] When toner T is transferred from the developing roller 4a onto the photosensitive drum 1, it first passes through the transfer nip Nt, rubbing against the transfer roller 5 and scattering downstream of the transfer nip Nt in the direction of transport of the recording material P (Figure 2(c)). This scattered toner T adheres to the static elimination needle 20 and the transport guide 11. Normally, during the forward rotation, no transfer voltage is applied (0V), or a transfer voltage with the same polarity as the normal charge polarity of the toner T (negative polarity in this embodiment) is applied to suppress toner adhesion to the transfer roller 5. When the transfer voltage is 0V, there is no potential difference between the photosensitive drum 1 and the transfer roller 5 at the transfer nip Nt. Therefore, the holding force of toner T on both the photosensitive drum 1 and the transfer roller 5 weakens, and toner scattering occurs from the transfer nip Nt due to the inertia caused by the rotational drive of the photosensitive drum 1. On the other hand, when a negative polarity transfer voltage is applied, the zero-tribo toner becomes slightly negatively polarized due to friction in the developing section Nd. As a result, this slightly negatively polarized toner T repels the transfer roller 5 to which the negative polarity transfer voltage is applied, and tries to remain on the photosensitive drum 1. Therefore, due to this repulsion and the inertia caused by the rotational drive of the photosensitive drum 1, toner scattering occurs from the transfer nip section Nt.

[0042] Due to non-electrostatic adhesion between the toner T and the transfer roller 5, some of the toner T transferred from the photosensitive drum 1 to the transfer roller 5 is repeatedly rubbed against the transfer nip Nt, causing repeated toner scattering from the transfer nip Nt (Figure 2(d)).

[0043] Furthermore, if toner scattering as described above is repeated each time a problem occurs during startup, for example, toner T will accumulate on the static elimination needle 20 and the transport guide 11. This accumulated toner T can manifest as paper trailing edge stains caused by the trailing edge of the recording material P rubbing against the transport guide 11 during printing, or as poor transfer image quality due to a decrease in the function of the static elimination needle 20.

[0044] In addition to the above-mentioned case of overprinting during startup, toner T, which resembles a solid image, can enter the transfer nip Nt without passing through the recording material P, it can also occur when the image forming apparatus 100 is started up (recovered) after jamming due to a paper jam or the like. However, in this case, the image forming apparatus 100 is almost always started up (recovered) relatively quickly after jamming. Therefore, the charge of the toner T on the photosensitive drum 1 remains, and the surface potential of the photosensitive drum 1 has not decreased, so the retention force of the toner T on the photosensitive drum 1 remains strong. Consequently, in this case, toner scattering from the transfer nip Nt is unlikely to occur.

[0045] In other words, toner scattering from the transfer nip Nt is a phenomenon specific to startup fogging that occurs when the developing roller 4a does not separate from the photosensitive drum 1 during startup of the image forming apparatus 100, resulting in the transfer of charge-attenuated toner T from the developing roller 4a to the photosensitive drum 1.

[0046] (3) Toner scattering suppression control in this embodiment Next, the transfer voltage control at startup of the image forming apparatus 100 (also referred to here as "toner scattering suppression control") to suppress toner scattering due to startup fogging in this embodiment will be described. Figure 3 is a timing chart showing the transition of the transfer voltage in the transfer voltage control at startup of the image forming apparatus 100 in this embodiment. The horizontal axis of Figure 3 represents time, and the vertical axis represents the transfer voltage (t1, t2, and t3 in Figure 3 represent timings (points in time)). Figure 4 is a schematic diagram of the area around the photosensitive drum 1 to explain the mechanism by which toner scattering from the transfer nip Nt is suppressed when the transfer voltage control at startup of the image forming apparatus 100 in this embodiment is performed. Figure 5 is a graph showing the measurement results of the charge distribution of toner T on the developing roller 4a in the state of Figure 4(a), toner T on the photosensitive drum 1 in the state of Figure 4(b), and toner T on the transfer roller 5 in the state of Figure 4(d), which will be described later. The charge distribution of toner T was measured using an EST-G (manufactured by Hosokawa Micron Corporation).

[0047] Figure 4(a) shows the state 12 hours after the last print operation ended. This state in Figure 4(a) is the same as the state in Figure 2(a). The results of measuring the charge distribution of toner T on the developing roller 4a in the state shown in Figure 4(a) are shown in the "On the developing roller" plot in Figure 5. Normally, toner T has a negative charge. However, due to the effect of charge decay over a long period of time, the median value of the charge distribution of toner T is 0 μC / g.

[0048] Figure 4(b) shows the state immediately after a drive instruction, such as a print signal, is given to the image forming apparatus 100 and the rotational drive of the photosensitive drum 1 is started in the pre-rotation operation. This state in Figure 4(b) is the same as the state in Figure 2(b). In this embodiment, almost simultaneously with the start of the rotational drive of the photosensitive drum 1, the application of a charging voltage to the charging roller 2, similar to that during normal image forming operation, and the application of a developing voltage to the developing roller 4a, similar to that during normal image forming operation, are started. As mentioned above, the developing voltage at this time does not have to be the same as the developing voltage during normal image forming. For example, a developing voltage with the same polarity as the developing voltage during normal image forming but with a smaller absolute value may be used, or a voltage with the opposite polarity to the normal polarity may be applied instead of a negative polarity voltage, which is the normal polarity of toner. As described above, the toner T transferred from the developing roller 4a to the photosensitive drum 1 becomes slightly negatively charged due to friction in the developing section Nd. The results of measuring the charge distribution of the toner T on the photosensitive drum 1 in the state of Figure 4(b) are shown in the "On the Drum" plot in Figure 5. As shown in the "On the Developing Roller" plot in Figure 5, the median charge distribution of toner T on the developing roller 4a was 0 μC / g, whereas as shown in the "On the Drum" plot in Figure 5, the median charge distribution of toner T on the photosensitive drum 1 was -3 μC / g.

[0049] Figure 4(c) shows the state when the fouling toner T transferred from the developing roller 4a to the photosensitive drum 1 first passes through the transfer nip section Nt. At this time, as shown in Figure 3, a transfer voltage V0 with the opposite polarity (positive polarity in this embodiment) to the normal charging polarity of the toner T is applied from t1 to t2. In this embodiment, t1 is set to the timing when the toner T that was present in the developing section Nd at the time of stopping reaches the transfer nip section Nt. In other words, in this embodiment, t1 is the timing when the image forming apparatus 100 is started up, and the area on the photosensitive drum 1 that was located in the developing section Nd at the time of stopping first reaches the transfer nip section Nt. Also, in this embodiment, t2 is set to the timing when the toner T that was present in the blade nip section Nb at the time of stopping reaches the transfer nip section Nt. In other words, in this embodiment, t2 is the timing when the image forming apparatus 100 is started up, and the area on the photosensitive drum 1 that first comes into contact with the developing section Nd and the area on the developing roller 4a that was located in the blade nip section Nb at the time of stopping first reach the transfer nip section Nt. This allows toner T located in the area of ​​the developing roller 4a that was located at the blade nip section Nb when stopped, and the area of ​​the developing roller 4a that was located at the developing section Nd when stopped, which is easily transferred onto the photosensitive drum 1 when the image forming apparatus 100 is started, to be transferred onto the transfer roller 5, as described later.

[0050] Furthermore, t1 and t2 should be set so that they include the timing when some or all of the areas on the photosensitive drum 1 where startup fogging occurs first pass through the transfer nip section Nt. For example, t2 may be set to the timing when the area on the photosensitive drum 1 that was in the charged section Nc at the time of stopping, where the surface potential of the photosensitive drum 1 is undefined, first passes through the transfer nip section Nt. In other words, t2 may be the timing when the area on the photosensitive drum 1 that was located in the charged section Nc at the time of stopping first reaches the transfer nip section Nt. This allows toner T in the section from the area on the photosensitive drum 1 that was located in the charged section Nc at the time of stopping to the area on the photosensitive drum 1 that was located in the developing section Nd at the time of stopping, where toner T may be transferred from the developing roller 4a when the image forming apparatus 100 is started up, to be transferred to the transfer roller 5 as described later. In this embodiment, when the image forming apparatus 100 is started up, the area on the developing roller 4a that was located in the blade nip section Nb at the time of stopping reaches the developing section Nd before the area on the photosensitive drum 1 that was located in the charged section Nc at the time of stopping. Furthermore, taking into consideration the responsiveness of the transfer power supply 18, the interval from t1 to t2 may be wider than the interval from t1 to t2 in this embodiment.

[0051] As can be seen from the charge distribution of toner T on the photosensitive drum 1 shown in the "On the Drum" plot in Figure 5, the toner T on the photosensitive drum 1 is slightly negatively polarized. Therefore, when a positive polarity transfer voltage is applied, a large amount of toner T is transferred from the photosensitive drum 1 to the transfer roller 5. In this embodiment, when V0 was set to +1000V, approximately 80% of the fouled toner T on the photosensitive drum 1 was transferred to the transfer roller 5. In this way, an electrostatic force acts in the direction that attracts the negatively polarized fouled toner T to the transfer roller 5, thus suppressing toner scattering.

[0052] Figure 4(d) shows the state in which toner T transferred from the photosensitive drum 1 to the transfer roller 5 is gradually transferred from the transfer roller 5 to the photosensitive drum 1 at the transfer nip Nt. At this time, as shown in Figure 3, the absolute value of the transfer voltage is gradually increased from t2 to t3 (V1 to V8). This gradually transfers the toner T on the transfer roller 5 to the photosensitive drum 1. t3 is the start timing of a normal print operation. In this embodiment, in a normal print operation, the photosensitive drum 1 is rotated for a predetermined period of time with a charging voltage and a developing voltage applied, and no transfer voltage applied (0V), after which the exposure device 3 starts image exposure and the application of a positive polarity transfer voltage.

[0053] Table 1 shows the values ​​of the transfer voltages V1 to V8 in this embodiment. Furthermore, the results of measuring the charge distribution of toner T on the transfer roller 5 in the state shown in Figure 4(d) are shown in the "On Transfer Roller" plot in Figure 5. As shown in the "On Transfer Roller" plot in Figure 5, the median charge distribution of toner T on the transfer roller 5 is -4 μC / g, which is slightly higher on the negative side than the median charge distribution of toner T on the photosensitive drum 1 due to friction at the transfer nip portion Nt. Because the toner T on the transfer roller 5 has this charge distribution, when a negative polarity transfer voltage with a low absolute value, such as V1, is applied, only the toner T with a high charge on the negative polarity side of the transfer roller 5 is transferred to the photosensitive drum 1, while the other toner T remains on the transfer roller 5. By gradually increasing the absolute value of this negative polarity transfer voltage from V1 to V3, and from V3 to V5, the toner T with the highest charge on the negative polarity side is transferred from the transfer roller 5 to the photosensitive drum 1 in order. Furthermore, by changing the polarity of the transfer voltage, such as from V1 to V2 and from V3 to V4, toner T whose polarity has been reversed to positive polarity, which is present within the toner charge distribution shown in the plot "On the transfer roller" in Figure 5, can also be transferred from the transfer roller 5 to the photosensitive drum 1. For the positive polarity transfer voltage, the absolute value is also gradually increased, such as from V2 to V4 and from V4 to V6. In this embodiment, the absolute value of the voltage for each polarity is gradually increased while alternating between the polarities of the transfer voltage, such as from V1 to V2, from V2 to V3, and from V3 to V4. However, it is also possible to continuously apply transfer voltages of the same polarity in multiple stages, for example, changing from V1 to V3, from V3 to V2, and from V2 to V4, while gradually increasing the absolute value. In addition, depending on the charge distribution of toner that has been transferred from the photosensitive drum 1 to the transfer roller 5 due to fouling at startup, it is also possible to apply only a negative polarity voltage between t2 and t3, while gradually increasing its absolute value.

[0054] By performing this type of control, it is possible to suppress toner scattering by gradually transferring a small amount of toner T from the transfer roller 5 to the photosensitive drum 1. If a high negative polarity transfer voltage is applied from the beginning to transfer the toner T on the transfer roller 5 to the photosensitive drum 1 all at once, the inertial force applied to the toner T will be large, and repulsion between the toner T and the transfer roller 5 may be induced, potentially worsening toner scattering.

[0055] Furthermore, it is preferable that the transfer voltages applied between t2 and t3 for each value are applied for at least the duration of one rotation of the transfer roller 5 (more than the duration of one rotation, and for example, less than or equal to five rotations, typically less than or equal to three rotations). This ensures that the toner T is transferred from the transfer roller 5 to the photosensitive drum 1 without bias in the circumferential direction of the transfer roller 5 using each transfer voltage value.

[0056] [Table 1]

[0057] Here, Figure 6 is a timing chart similar to Figure 3, showing the transition of the transfer voltage in a modified example of the transfer voltage control at startup of the image forming apparatus 100 according to this embodiment. As shown in Figure 6, even when no transfer voltage is applied between t1 and t2 (V0=0V), the effects of this embodiment can be obtained. As described above using Figure 2(c), when no transfer voltage is applied, toner scattering occurs, and some of the toner T is transferred from the photosensitive drum 1 to the transfer roller 5 by non-electrostatic adhesion force. In the configuration of this embodiment, when V0=0V, approximately 30% of the toner T was transferred from the photosensitive drum 1 to the transfer roller 5. If the transfer voltage control at startup of the image forming apparatus 100 according to the present invention is not performed, the toner T transferred to the transfer roller 5 is repeatedly rubbed against the transfer nip Nt, worsening toner scattering. However, by performing the transfer voltage control at startup of the image forming apparatus 100 according to the present invention as shown in Figure 6, the toner T on the transfer roller 5 is gradually transferred to the photosensitive drum 1, thereby suppressing toner scattering from the transfer nip Nt from the second time onward. However, as shown in Figure 3, applying a transfer voltage with the opposite polarity (positive polarity in this embodiment) to the normal charging polarity of the toner between t1 and t2 results in a larger amount of toner being transferred from the photosensitive drum 1 to the transfer roller 5 when the fouled toner T first passes through the transfer nip Nt. Therefore, this is more preferable from the viewpoint of suppressing toner scattering.

[0058] (4) Effects of this embodiment The evaluation experiment to confirm the effects of this embodiment will now be described. The evaluation experiment was conducted using this embodiment and Comparative Examples 1 and 2, in which the toner scattering suppression control in this embodiment was not implemented.

[0059] In this embodiment, the transfer voltage control (toner scattering suppression control) described with reference to Figures 3 and 6 was performed during the pre-rotation operation when the image forming apparatus 100 was started. Specifically, in this embodiment, 0V or +1000V was used as V0, and the values ​​in Table 1 were used as V1 to V8.

[0060] In Comparative Example 1, the transfer voltage control shown in Figure 7 was performed during the pre-rotation operation when the image forming apparatus 100 was started. In other words, in Comparative Example 1, no transfer voltage was applied during the pre-rotation operation when the image forming apparatus 100 was started, and the transfer voltage between t1 and t3 was set to 0V.

[0061] In Comparative Example 2, the transfer voltage control shown in Figure 8 was performed during the pre-rotation operation when the image forming apparatus 100 was started. Specifically, in Comparative Example 2, a negative polarity transfer voltage V9 was applied during the pre-rotation operation when the image forming apparatus 100 was started, and the transfer voltage between t1 and t3 was set to V9 = -1000V.

[0062] Figures 7 and 8 are timing charts showing the transition of the transfer voltage, similar to Figure 3. Furthermore, the configurations of the image forming apparatus 100 in Comparative Examples 1 and 2 are substantially the same as those of the image forming apparatus 100 in this embodiment, except that the transfer voltage control at startup of the image forming apparatus 100 is different.

[0063] The evaluation experiment was conducted as follows: The recording material P was LTR size with a basis weight of 75 g / m². 2 Vitality (manufactured by Xerox) was used. The image forming apparatus 100 was installed in an environment with a temperature of 23°C and a humidity of 50%, and printing operations were performed. For each of this example, Comparative Example 1, and Comparative Example 2, a cycle of printing two sheets 12 hours after the end of the previous printing operation was repeated 10 times. By repeating these small prints after a long period of time, contamination of the static elimination needle 20 and the transport guide 11 by toner ("static elimination needle / transport guide contamination") and contamination of the paper's trailing edge due to toner adhering to the trailing edge of the recording material P of the printed material ("toner adhesion to the trailing edge of the paper") were confirmed.

[0064] Table 2 shows the results of the evaluation experiment. For static elimination needle and transport guide contamination, × (poor) was used if contamination occurred, △ (fairly good) if slight contamination was present, and ○ (good) if no contamination occurred. Similarly, for toner adhesion at the paper's trailing edge, × (poor) was used if contamination occurred, △ (fairly good) if slight toner adhesion was present, and ○ (good) if no toner adhesion occurred.

[0065] [Table 2]

[0066] In Comparative Example 1, both static elimination needle / transport guide contamination and toner adhesion to the paper's trailing edge occurred. This is because the fouled toner, whose charge has decayed over time, is scattered by friction at the transfer nip section Nt. Similarly, in Comparative Example 2, both static elimination needle / transport guide contamination and toner adhesion to the paper's trailing edge occurred. This is due to the friction of the fouled toner at the transfer nip section Nt, as well as the toner, which has become slightly negatively polarized in the developing section Nd, repelling the negatively polarized transfer roller 5, causing the toner to scatter.

[0067] In this embodiment, when V0=0V, slight contamination of the static elimination needle and transport guide, as well as toner adhesion to the trailing edge of the paper, occurred, but the results were better than those of Comparative Examples 1 and 2. This is because, although toner scattering occurs when the toner is rubbed for the first time at the transfer nip Nt, the effect of this embodiment is evident in that, when rubbed for the second time and beyond, the toner on the transfer roller 5 gradually transfers to the photosensitive drum 1 and decreases.

[0068] In this embodiment, when V0 = +1000V, neither static elimination needle / transport guide contamination nor toner adhesion to the paper's trailing edge occurred, resulting in the best results in this evaluation experiment. This is because, even when the fouled toner is rubbed for the first time at the transfer nip section Nt, the fouled toner on the photosensitive drum 1, which has become slightly negatively polarized at the developing section Nd, is attracted to the transfer roller 5 by an electrostatic force and is transferred to the transfer roller 5, making toner scattering less likely. Furthermore, even when the fouled toner is rubbed for the second time and beyond at the transfer nip section Nt, the effect of this embodiment is evident in that the toner on the transfer roller 5 gradually transfers to the photosensitive drum 1 and decreases.

[0069] As described above, in this embodiment, the image forming apparatus 100 includes a rotatable photoreceptor 1, a charging means 2 for charging the photoreceptor 1 with a charging unit Nc, a rotatable developing member 4a for supplying toner onto the photoreceptor charged by the charging means 2 by contacting the photoreceptor 1 with a developing unit Nd, a regulating member 4b for contacting the developing member 4a with a regulating unit Nb to impart charge to the toner on the developing member, a transfer member 5 for contacting the photoreceptor 1 to form a transfer unit Nt and transferring toner from the photoreceptor 1 to a recording material P passing through the transfer unit Nt, a transfer voltage application means 18 for applying a voltage to the transfer member 5, and a control means 50 capable of controlling the transfer voltage application means 18. The rotation of the photoreceptor 1 and the developing member 4a is started when the photoreceptor 1 and the developing member 4a are in contact. In this embodiment, when the photoreceptor 1 and developing member 4a are stopped, the region on the photoreceptor 1 that was located on the developing unit Nd is defined as the first region, and after the rotation of the photoreceptor 1 and developing member 4a begins, the region on the developing member 4a that was located on the regulating unit Nb when the photoreceptor 1 and developing member 4a were stopped and the region on the photoreceptor 1 that first comes into contact with the developing unit Nd are defined as the second region. The control means 50 then controls the first region to reach the transfer unit Nt after the rotation of the photoreceptor 1 and developing member 4a begins, and the second region first comes into contact with the transfer unit Nt. During a first period that includes at least a portion of the time until t is reached, no voltage is applied to the transfer member 5, or a voltage with the opposite polarity to the normal charging polarity of the toner is applied. After the first period, and during a second period before the toner image formed on the photoreceptor 1 reaches the transfer section Nt, the transfer voltage application means 18 is controlled to apply to the transfer member 5 a first voltage with the same polarity as the normal charging polarity, and a second voltage with the same polarity as the first voltage but with an absolute value greater than the absolute value of the first voltage.

[0070] In one preferred embodiment, the control means 50 controls the transfer voltage application means 18 to apply a voltage to the transfer member 5 that is opposite in polarity to the normal charging polarity during the first period. In another preferred embodiment, the first period is set to include the period from when the rotation of the photoreceptor 1 and the developer member 4a is started and the first region first reaches the transfer section Nt until the second region first reaches the transfer section Nt. Furthermore, if the region on the photoreceptor 1 that was located at the charging section Nc when the photoreceptor 1 and the developer member 4a are stopped is defined as the third region, the first period may be set to include the period from when the rotation of the photoreceptor 1 and the developer member 4a is started and the first region first reaches the transfer section Nt until the third region first reaches the transfer section Nt. In this embodiment, the control means 50 controls the transfer voltage application means 18 to apply to the transfer member 4a a third voltage which is opposite in polarity to the normal charging polarity, and a fourth voltage which is the same polarity as the third voltage but whose absolute value is greater than the absolute value of the third voltage, during the second period. In this embodiment, the control means 50 controls the transfer voltage application means 18 to alternately apply to the transfer member 5 a voltage which is the same polarity as the normal charging polarity, and a voltage which is opposite in polarity to the normal charging polarity, during the second period.

[0071] As described above, according to this embodiment, it is possible to suppress toner scattering from the transfer nip section Nt due to startup fogging when the image forming apparatus 100 is started with the developing roller 4a and the photosensitive drum 1 in contact after a long time has elapsed since the end of the previous print operation. As a result, even in an image forming apparatus 100 that does not have a contact / separation mechanism to switch between contact and separation of the developing roller 4a and the photosensitive drum 1, it is possible to suppress soiling of the static elimination needle 20 and transport guide 11, as well as soiling of the paper's trailing edge by toner. In other words, according to this embodiment, it is possible to suppress toner scattering caused by toner fogging at startup.

[0072] (5) Other embodiments of this model Figure 9 is a schematic cross-sectional view of another embodiment of the image forming apparatus 100 in this model. The difference from the image forming apparatus 100 in Figure 1 is that it does not have a cleaning device 6, and the developing device 4 also serves as the cleaning mechanism (cleanerless configuration). The other configurations of the image forming apparatus 100 in Figure 9 are substantially the same as those of the image forming apparatus 100 in Figure 1.

[0073] Even in an image forming apparatus 100 with a cleanerless configuration as shown in Figure 9, the toner scattering suppression control (transfer voltage control at startup of the image forming apparatus 100) in this embodiment can be used.

[0074] In other words, when using the transfer voltage control at startup of the image forming apparatus 100 in this embodiment, the toner transferred from the photosensitive drum 1 to the transfer roller 5 is gradually returned to the photosensitive drum 1. This toner returned to the photosensitive drum 1 includes not only negative polarity toner, but also zero-tribo toner and toner whose polarity has been reversed to positive polarity.

[0075] In the cleanerless image forming apparatus 100 shown in Figure 9, negatively polarized toner adhering to the dark potential areas on the photosensitive drum 1 is collected by the developer unit Nd and transferred to the developer unit 4. However, zero-tribo toner and positively polarized toner cannot be collected at this time, so it is necessary to make such toners negatively polarized. In the image forming apparatus 100 shown in Figure 9, during the pre-rotation operation (when the photosensitive drum 1 is being cleaned), the charging roller 2 is subjected to the same charging voltage of -1100V as during normal image forming operation, and a discharge occurs between the charging roller 2 and the photosensitive drum 1. This discharge makes the polarity of the fouled toner on the photosensitive drum 1 negative, allowing the fouled toner to be collected by the developer unit Nd.

[0076] Thus, the toner scattering suppression control in this embodiment can be used regardless of the cleaning method of the image forming apparatus 100.

[0077] [Example 2] Next, other embodiments of the present invention will be described. The basic configuration and operation of the image forming apparatus in this embodiment are the same as those of the image forming apparatus in Embodiment 1. Therefore, in the image forming apparatus of this embodiment, elements having the same or corresponding functions or configurations as those of the image forming apparatus 100 in Embodiment 1 are denoted by the same reference numerals as in Embodiment 1, and detailed descriptions are omitted.

[0078] In this embodiment, the image forming apparatus 100 decides whether or not to perform the toner scattering suppression control described in Embodiment 1, depending on the history of the previous print operation. Performing the toner scattering suppression control described in Embodiment 1 every time the image forming apparatus 100 is started up may increase the startup time or affect the lifespan of the components. Therefore, in this embodiment, these possibilities are reduced by performing the toner scattering suppression control only when there is a high need to do so.

[0079] Table 3 shows the evaluation results of toner adhesion at the trailing edge of the paper for each elapsed time since the end of the previous print operation. The transfer voltage control at startup of the image forming apparatus 100 in both Example 1 (V0 = +1000V) and Comparative Example 1 described above was performed each time the image forming apparatus 100 was started up. At that time, the elapsed time since the end of the previous print operation was varied to three levels: 1 hour, 6 hours, and 12 hours. Other conditions of the evaluation experiment were the same as those of the evaluation experiment that obtained the results in Table 2, but here only toner adhesion at the trailing edge of the paper was evaluated. If toner adhesion occurred at the trailing edge of the paper, it was marked as × (poor), if a small amount of toner was adhered, it was marked as △ (fairly good), and if no toner adhesion occurred, it was marked as ○ (good).

[0080] [Table 3]

[0081] In Example 1, when the transfer voltage control at startup of the image forming apparatus 100 is performed every time the image forming apparatus 100 is started, good toner adhesion to the trailing edge of the paper is achieved regardless of the elapsed time since the end of the previous print operation.

[0082] On the other hand, when the transfer voltage control at startup of the image forming apparatus 100 in Comparative Example 1 was performed every time the image forming apparatus 100 was started, the toner adhesion to the trailing edge of the paper was poor, as described above, when 12 hours had elapsed since the end of the previous print operation. However, the results improved as the elapsed time since the end of the previous print operation decreased, to 6 hours and 1 hour. This is because, when the elapsed time since the end of the previous print operation is short, the charge of the toner on the developing roller 4a remains negative, and in addition, the surface potential of the photosensitive drum 1 has not completely decayed, so there is less toner fouling at startup. Figure 10 is a graph showing the measurement results of the toner charge distribution on the developing roller 4a when the elapsed time since the end of the previous print operation is varied. From Figure 10, it can be seen that the shorter the elapsed time, the longer the toner charge is maintained on the negative polarity side. In this way, by reducing the amount of toner fouling at startup, the scattering of toner from the transfer nip Nt is also reduced. Therefore, in such cases, toner adhesion to the trailing edge of the paper improves even without performing the toner scattering suppression control described in Example 1.

[0083] Table 4 shows the evaluation results of toner adhesion to the paper's trailing edge for each number of sheets passed through in the previous print operation. The transfer voltage control at startup of the image forming apparatus 100 was performed each time the image forming apparatus 100 was started up, as described above for Example 1 (V0 = +1000V) and Comparative Example 1. At that time, the number of sheets passed through in the previous print operation was varied to three levels: 2 sheets, 5 sheets, and 10 sheets. Other conditions of the evaluation experiment were the same as those of the evaluation experiment that obtained the results in Table 2, but here only toner adhesion to the paper's trailing edge was evaluated. If toner adhesion occurred at the paper's trailing edge, it was marked as × (poor), if a small amount of toner was adhered, it was marked as △ (fairly good), and if no toner adhesion occurred, it was marked as ○ (good).

[0084] [Table 4]

[0085] In Example 1, when the transfer voltage control at startup of the image forming apparatus 100 was performed every time the image forming apparatus 100 was started, good toner adhesion to the trailing edge of the paper was achieved regardless of the number of sheets passed through in the previous print operation. On the other hand, in Comparative Example 1, when the transfer voltage control at startup of the image forming apparatus 100 was performed every time the image forming apparatus 100 was started, the toner adhesion to the trailing edge of the paper was poor when the number of sheets passed through in the previous print operation was 2, as described above. However, the results improved as the number of sheets passed through in the previous print operation increased to 5 sheets and 10 sheets. This is because the more sheets passed through, the less toner is deposited on the transport guide 11 due to toner scattering caused by toner overflow at startup, and the less toner is deposited on the transport guide 11. Thus, even if toner scattering occurs, if the toner contamination is gradually removed from the transport guide 11 by passing paper, toner adhesion to the trailing edge of the paper will improve even without performing the toner scattering suppression control described in Example 1.

[0086] Next, the procedure for controlling the transfer voltage at startup of the image forming apparatus 100 in this embodiment, taking into account the results in Tables 3 and 4, will be described. Figure 11 is a flowchart showing the procedure for controlling the transfer voltage at startup of the image forming apparatus 100 in this embodiment.

[0087] When the control unit 50 receives a print signal from the host computer, it determines whether the elapsed time since the end of the previous print operation is 6 hours or more (above a predetermined threshold) before starting the startup operation of the image forming apparatus 100 (S101). The control unit 50 can, for example, measure the time from the end of the print operation to the reception of the print signal and make the determination in S101 based on the measurement result. Alternatively, the control unit 50 can store the date and time of the end of the print operation in a storage means such as a non-volatile memory, and make the determination in S101 based on the elapsed time since the end of the previous print operation, which is calculated from the stored date and time and the date and time the print signal was received.

[0088] If less than 6 hours have elapsed since the last print operation, even if a normal startup operation is performed, the amount of toner overflow at startup will be small, and the risk of toner scattering from the transfer nip Nt is low. Therefore, if the control unit 50 determines in S101 that less than 6 hours have elapsed ("No"), it will perform a normal print operation (S104) without executing toner scattering suppression control, and will terminate the print operation once the specified image forming operation is completed.

[0089] If more than 6 hours have elapsed since the end of the previous print operation, the amount of overprinting at startup increases, which raises the risk of toner scattering from the transfer nip Nt. Therefore, if the control unit 50 determines in S101 that more than 6 hours have elapsed ("Yes"), it then determines whether the number of sheets of paper fed (number of prints) in the previous print operation was 5 or less (below a predetermined threshold) (S102).

[0090] If the number of sheets fed in the previous print operation was 6 or more, even if a normal startup operation is performed, the toner contamination on the transport guide 11 is gradually discharged by the paper feeding, so the risk of contamination at the trailing edge of the paper is low. Therefore, if the control unit 50 determines in S102 that the number of sheets fed is more than 5 ("No"), it performs a normal print operation (S104) without executing toner scattering suppression control, and terminates the print operation when the specified image forming operation is completed.

[0091] Then, if the control unit determines in S102 that the number of sheets to be fed is 5 or less ("Yes"), it executes the toner scattering suppression control described in Example 1 during the pre-rotation operation when the image forming apparatus 100 is started (S103). After that, the control unit 50 executes a normal print operation (S104), and terminates the print operation when the specified image forming operation is completed.

[0092] In this embodiment, both S101, which determines whether toner scattering suppression control is necessary based on the elapsed time since the end of the previous print operation, and S102, which determines whether toner scattering suppression operation is necessary based on the number of sheets of paper passed through in the previous print operation, were performed. However, in this embodiment, only one of the decisions in S101 and S102 may be performed (for example, only the decision in S101 may be performed). In that case as well, compared to Embodiment 1, it is possible to reduce the possibility of increased startup time and impact on the lifespan of components.

[0093] Thus, in this embodiment, the control means 50 can selectively execute, based on information regarding the operation history of the image forming apparatus 100, a first control that controls the transfer voltage application means 18 (toner scattering suppression control) during the first and second periods described above, during the operation period from when the rotation of the photoreceptor 1 and the developing member 4a starts until the toner image formed on the photoreceptor 1 reaches the transfer unit Nt, and a second control in which the control of the transfer voltage application means 18 (toner scattering suppression control) during the first and second periods described above is not performed during the operation period, and the operation period is shorter than when the first control is performed. In this embodiment, in the second control, the control means 50 controls the transfer voltage application means 18 so that the voltage initially applied to the transfer member 5 during the operation period is a voltage with polarity opposite to the normal charging polarity of the toner. Furthermore, the information regarding the operation history may be information regarding the stop time of the photoreceptor 1 since the end of the previous print operation. Furthermore, the information regarding the operation history may be information regarding the number of prints in the previous print operation.

[0094] As explained above, in this embodiment, the toner scattering suppression control described in Embodiment 1 is performed only when there is a high need for it, at the time the image forming apparatus 100 is started up. As a result, it is possible to shorten the time of the pre-rotation operation when the image forming apparatus 100 is started up, and reduce the possibility of affecting the lifespan of the components (such as surface abrasion of the photosensitive drum 1 and increased resistance of the transfer roller 5) by applying the transfer voltage when there is no recording material P on the transfer nip Nt.

[0095] [others] Although the present invention has been described above with reference to specific embodiments, the present invention is not limited to the embodiments described above.

[0096] The transfer member is not limited to a roller-shaped member, but may also be a block (pad)-shaped member, a brush-shaped member, or a sheet-shaped (film-shaped) member.

[0097] Furthermore, the image forming apparatus may be an image forming apparatus for various purposes, such as a printer, various printing machines, photocopiers, fax machines, or multifunction devices. [Explanation of Symbols]

[0098] 1 Photosensitive drum 2 Charging rollers 4. Developing device 5 Transfer roller 6. Cleaning device 11. Transport Guide 12 Fixing device 18 Transfer power supply 20 Static Eliminator Needle 50 Control Unit

Claims

1. A rotatable photoreceptor, A charging means for charging the photoreceptor in the charging section, A rotatable developing member that contacts the photoreceptor in the developing unit and supplies toner onto the photoreceptor which has been charged by the charging means, A restricting member that contacts the developing member at the restricting section and applies an electric charge to the toner on the developing member, A transfer member that contacts the photoreceptor to form a transfer portion and transfers toner from the photoreceptor to a recording material that passes through the transfer portion, A transfer voltage application means for applying a voltage to the transfer member, Control means capable of controlling the transfer voltage application means, It has, In an image forming apparatus in which the rotation of the photoreceptor and the developing member is started while the photoreceptor and the developing member are in contact, When the photoreceptor and the developing member are stopped, the region on the photoreceptor that was located in the developing unit is defined as the first region, and after the rotation of the photoreceptor and the developing unit begins, the region on the developing unit that was located in the regulating unit when the photoreceptor and the developing unit were stopped, and the region on the photoreceptor that first comes into contact with the developing unit are defined as the second region. The image forming apparatus is characterized in that the control means controls the transfer voltage application means to apply to the transfer member either no voltage is applied or a voltage with the opposite polarity to the normal charging polarity of the toner during a first period including the period from when the rotation of the photoreceptor and the developing member is started until the first region first reaches the transfer section until the second region first reaches the transfer section, and after the first period and before the toner image formed on the photoreceptor reaches the transfer section during a second period, a first voltage having at least the same polarity as the normal charging polarity and a second voltage having the same polarity as the first voltage and whose absolute value is greater than the absolute value of the first voltage.

2. The image forming apparatus according to claim 1, characterized in that the control means controls the transfer voltage application means to apply a voltage with the opposite polarity to the normal charging polarity to the transfer member during the first period.

3. When the area on the photoreceptor that was located in the charged portion when the photoreceptor and the developing member were stopped is defined as the third area, The image forming apparatus according to claim 1, characterized in that the first period includes the period from when the rotation of the photoreceptor and the developing member is started and the first region first reaches the transfer section until the third region first reaches the transfer section.

4. The image forming apparatus according to claim 1, characterized in that the control means controls the transfer voltage application means to apply to the transfer member during the second period a third voltage having the opposite polarity to the normal charging polarity, and a fourth voltage having the same polarity as the third voltage and whose absolute value is greater than the absolute value of the third voltage.

5. The image forming apparatus according to claim 4, characterized in that the control means controls the transfer voltage application means to alternately apply to the transfer member a voltage having the same polarity as the normal charging polarity and a voltage having the opposite polarity to the normal charging polarity during the second period.

6. The image forming apparatus according to any one of claims 1 to 5, characterized in that the control means can selectively execute, based on information regarding the operation history of the image forming apparatus, a first control that performs the control of the transfer voltage application means in the first period and the second period during the operation period from when the rotation of the photoreceptor and the developing member is started until the toner image formed on the photoreceptor reaches the transfer section, and a second control in which the control of the transfer voltage application means in the first period and the second period is not performed during the operation period, and the operation period is shorter than when the first control is performed.

7. The image forming apparatus according to claim 6, characterized in that the control means controls the transfer voltage application means in the second control such that the voltage initially applied to the transfer member during the operating period is a voltage with polarity opposite to the normal charging polarity.

8. The image forming apparatus according to claim 6, characterized in that the information relating to the operation history is information relating to the stop time of the photoreceptor since the end of the previous print operation.

9. The image forming apparatus according to claim 6, characterized in that the information relating to the operation history is information relating to the number of prints in the previous print operation.

Images