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, that uses an electrophotographic system. [Background technology]

[0002] In a conventional image forming apparatus such as a laser beam printer using an electrophotographic method, the surface of a photosensitive drum serving as an image carrier is uniformly charged to a dark potential by a charging means. The charged surface of the photosensitive drum is then exposed by an exposure means to form a bright potential on the surface of the photosensitive drum. As a result, an electrostatic latent image is formed on the surface of the photosensitive drum with a contrast between the dark potential and the bright potential. Then, toner is supplied to the electrostatic latent image formed on the photosensitive drum by a developing means, and a toner image is formed on the photosensitive drum. A developing roller is generally used as a developing member constituting the developing means. The developing roller contacts 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 to the recording material by a transfer means. A transfer roller is generally used as a transfer member constituting the transfer means. The transfer roller contacts the photosensitive drum to form a nip portion (hereinafter referred to as a "transfer nip portion"), and transfers the toner on the photosensitive drum onto the recording material while rotating and transporting the recording material. At this time, a transfer voltage of a polarity opposite to the normal charging polarity of the toner is applied to the transfer roller, and the toner image on the image carrier is electrostatically transferred onto the recording material. The toner image transferred to the recording material is then fixed onto the recording material by being heated and pressed by a fixing means.

[0004] Incidentally, 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 of materials mainly composed of synthetic resin, or metal-deposited paper (specialty paper) having a metal layer. For convenience, unless otherwise specified, the magnitude (high / low, strength) of electric potential, voltage, or current refers to the magnitude (high / low, strength) when compared in absolute values. Furthermore, the "leading edge" and "rear edge" of a recording material refer to the leading edge and rear edge, respectively, in the conveying direction of the recording material.

[0005] In the image forming apparatus described above, a phenomenon called "start-up fog" may occur. "Start-up fog" is a phenomenon in which, when the image forming apparatus is started up a long time after the end of the previous print operation, a part of the toner held on the developing roller is transferred onto the photosensitive drum even though there is no electrostatic latent image on the photosensitive drum. This phenomenon occurs because the surface potential of the photosensitive drum decreases over a long period of time, making it impossible to maintain the potential difference between the photosensitive drum and the developing roller, and because the charge of the toner on the developing roller decreases over a long period of time.

[0006] A common method for suppressing "start-up fog" is to separate the developing roller from the photosensitive drum when the image forming apparatus is started. If the developing roller is separated from the photosensitive drum, the toner on the developing roller will not be transferred onto the photosensitive drum. However, providing an abutment / separation mechanism for switching the developing roller between contact and separation with the photosensitive drum in the image forming apparatus leads to an increase in size and cost of the apparatus.

[0007] Patent document 1 proposes that when an image forming device is started up, a voltage of opposite polarity to normal 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] JP 2013-117591 A Summary of the Invention [Problem to be solved by the invention]

[0009] However, the toner on the developing roller after a long time has passed since the end of the previous printing operation contains a lot of toner with zero charge or toner with a polarity reversed to the normal charged polarity, so it is difficult to completely prevent fog at startup only by the potential difference between the photosensitive drum and the developing roller.

[0010] When toner that has caused such start-up fogging (hereinafter also referred to as "fogging toner") passes through the transfer nip, toner scattering occurs from the transfer nip. This toner scattering causes toner stains to adhere to members, such as the charge removal needle and the conveyance guide, that are disposed downstream of the transfer nip in the conveyance direction of the recording material. As start-up fogging occurs repeatedly, this toner stain accumulates.

[0011] SUMMARY OF THE PRESENTLY PREFERRED EMBODIMENTS An object of the present invention is to suppress toner scattering caused by toner that has caused fog at the time of startup. [Means for solving the problem]

[0012] The above object is achieved by an image forming apparatus according to the present invention. In summary, the present invention provides an image forming apparatus having a rotatable photoconductor, a charging means for charging the photoconductor in a charging section, a rotatable developing member that contacts the photoconductor in a developing section and supplies toner onto the photoconductor that has been charged by the charging means, a regulating member that contacts the developing member in a regulating section and applies a charge to the toner on the developing member, a transfer member that contacts the photoconductor to form a transfer section and transfers the toner from the photoconductor to a recording material passing through the transfer section, a transfer voltage applying means that applies a voltage to the transfer member, and a control means that can control the transfer voltage applying means, in which the image forming apparatus starts rotating the photoconductor and the developing member while the photoconductor and the developing member are in contact with each other, a region on the photoconductor that was located in the developing section when the photoconductor and the developing member stopped is referred to as a first region, and a region on the photoconductor that was located in the developing section when the photoconductor and the developing member stopped is referred to as a second region, and a region on the photoconductor and the developing member that was located in the developing section after the rotation of the photoconductor and the developing member started is referred to as a second region. When the area on the developing member that was located in the regulating section when the developing member was stopped and the area on the photosensitive member that first comes into contact in the developing section are defined as a second area, the control means controls the transfer voltage application means to apply no voltage to the transfer member or to apply a voltage of an opposite polarity to the normal charging polarity of the toner during a first period that includes at least a portion of the period from when the rotation of the photosensitive member and the developing member starts and the first area first reaches the transfer section to when the second area first reaches the transfer section, and to apply to the transfer member at least a first voltage of the same polarity as the normal charging polarity and a second voltage of the same polarity as the first voltage and having an absolute value greater than that of the first voltage during a second period after the first period and before the toner image formed on the photosensitive member reaches the transfer section. Effect of the Invention

[0013] According to the present invention, it is possible to suppress toner scattering caused by toner that has caused fogging at the time of startup. [Brief description of the drawings]

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

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

[0016] [Example 1] (1) Image forming device 1 is a schematic cross-sectional view of an image forming apparatus 100 according to this embodiment. The image forming apparatus 100 according to this embodiment is an electrophotographic laser printer. The image forming apparatus 100 forms an image (black monochrome image) on a recording material P in accordance with 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-type (cylindrical) electrophotographic photosensitive member (photoconductor) serving as an image carrier, inside an apparatus main body M. The photosensitive drum 1 is a photosensitive material such as an OPC (organic photoconductor), amorphous selenium, or amorphous silicon provided on a cylindrical drum base made of aluminum, nickel, or the like. In this embodiment, the photosensitive drum 1 is a negatively charged OPC photosensitive member having an outer diameter of φ24 mm, and has a photosensitive layer in which a charge generating layer and a charge transport layer are laminated in this order from the conductive base side on the surface of a conductive base made of an aluminum cylinder. The photosensitive drum 1 is driven to rotate in the direction of the arrow Rd in the figure (clockwise direction).

[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 charge eliminating needle 20, which is a charge eliminating member, is arranged as a charge eliminating means. Also, a cleaning device 6 is arranged as a cleaning means. Along the rotation direction Rd of the photosensitive drum 1, a charging portion (charging position) by the charging roller 2, an exposure portion (exposure position) by the exposure device 3, a development portion (developing position) by the development device 4, a transfer portion (transfer position) by the transfer roller 5, and cleaning (cleaning position) by the cleaning device 6 are arranged in this 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 the outer circumferential surface of the base shaft. In this embodiment, the charging roller 2 is an elastic roller with a roller outer diameter of φ10 mm, a core metal diameter of φ5 mm, and a thickness of the elastic layer of 2.5 mm. In this embodiment, the core metal of the charging roller 2 is made of SUS, and the elastic layer of the charging roller 2 is made of a mixed rubber material of NBR and epichlorohydrin. The charging roller 2 is disposed in contact with the photosensitive drum 1. The charging roller 2 charges the surface (outer circumferential surface) of the photosensitive drum 1. The position where the charging roller 2 performs charging in the rotation direction of the photosensitive drum 1 is the charging portion (charging position). The charging roller 2 charges the surface of the photosensitive drum 1 by discharging generated in at least one of the small gaps between the photosensitive drum 1 and the charging roller 2 formed on the upstream side and downstream side of the contact portion between the photosensitive drum 1 and the charging roller 2 in the rotation direction of the photosensitive drum 1. However, in this case, the contact portion between the photosensitive drum 1 and the charging roller 2 is regarded as a charging portion (charging nip portion) Nc.

[0020] In this embodiment, the exposure device 3 is configured with 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 in the rotation direction of the photosensitive drum 1 where exposure by the exposure device 3 is performed is the exposure section (exposure position).

[0021] The developing device 4 has a developing container 4c that contains toner as a developer, a developing roller 4a as a developing member (developer carrier) that conveys the toner to a portion facing the photosensitive drum 1, and a developing blade 4b as a regulating member. The developing roller 4a is driven to rotate in the direction of the arrow in the figure (counterclockwise direction), that is, in a direction in which the moving direction of the developing roller 4a and the moving direction of the photosensitive drum 1 are forward directions at the portion facing the photosensitive drum 1. The developing blade 4b abuts against the developing roller 4a on the side opposite to the side facing the photosensitive drum 1. The developing blade 4b regulates the amount of toner carried by 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 that has a predetermined length in the longitudinal direction arranged along the rotation axis direction of the developing roller 4a and in the transverse direction approximately perpendicular to the longitudinal direction. The developing blade 4b is provided so that the tip of the free end side in the short side direction faces the upstream side in the rotation direction of the developing roller 4a, and the vicinity of the tip abuts on the outer circumferential surface of the developing roller 4a in a surface contact state. In this embodiment, the image forming apparatus 100 does not have a contact / separation mechanism that switches 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 a contact state. In this way, by not providing a contact / separation mechanism in the image forming apparatus 100, the device main body M is made compact. In this embodiment, the photosensitive drum 1 and the developing roller 4 are driven by a driving force transmitted from a common driving source of, for example, the driving device 60 (FIG. 12), and rotate and stop in synchronization. The position where the toner is supplied by the developing roller 4a of the developing device 4 in the rotation direction of the photosensitive drum 1 is the developing portion (developing position). In this embodiment, the contact portion between the developing roller 4a and the photosensitive drum 1 is the developing portion (developing nip portion) Nd. Further, a contact portion between the developing roller 4a and the developing blade 4b in the rotation direction of the developing roller 4a is defined as a blade nip portion (regulating portion) Nb.

[0022] The transfer roller 5 is disposed 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 as a biasing means, and is in pressure contact with the photosensitive drum 1. As a result, a transfer nip portion (transfer portion, transfer nip portion) Nt, which is a contact portion between the photosensitive drum 1 and the transfer roller 5, is formed. The transfer roller 5 rotates following the rotation of the photosensitive drum 1. In this embodiment, the image forming apparatus 100 does not have a contact / separation mechanism for switching between contact and separation of the transfer roller 5 with respect to the photosensitive drum 1. The transfer roller 5 sandwiches the recording material P between itself and the photosensitive drum 1, and transfers a toner image from the photosensitive drum 1 to the recording material P passing through the transfer nip portion Nt by applying a voltage. The transfer roller 5 is composed of, for example, a conductive base shaft (core metal) which also serves as a power supply electrode, and an elastic layer cylindrically surrounding the outer circumferential surface of the conductive base shaft. The elastic layer of the transfer roller 5 is generally made of semiconductive rubber such as EPDM, NBR, urethane rubber, epichlorohydrin, silicone rubber, etc. 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, the core of the transfer roller 5 is made of SUS, and the elastic layer of the transfer roller 5 is made of a mixed rubber material of NBR and epichlorohydrin. 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 of the transfer roller 5 is 4.0×10 when the transfer roller 5 is pressed against an aluminum cylinder with a force of 9.8 N, rotated at 50 mm / sec, and +1000 V is applied. 7 The position in the rotation direction of the photosensitive drum 1 where the toner is transferred by the transfer roller 5 (the position corresponding to the transfer nip portion Nt) is the transfer position.

[0023] The charge eliminating needle 20 eliminates excess charge on the surface of the recording material P after transfer. In this embodiment, the charge eliminating needle 20 is made of a metal thin plate material such as a SUS plate or an aluminum plate having a sawtooth-shaped sharp tip and good conductivity. This charge eliminating needle 20 is disposed downstream of the transfer roller 5 in the conveying direction of the recording material P and such that the needle tip faces the surface of the photosensitive drum 1. In other words, the charge eliminating needle 20 is disposed such that the needle tip faces the conveying path of the recording material P (faces the recording material P conveyed through the conveying path). In this embodiment, the charge eliminating needle 20 is electrically grounded via a resistance element (not shown). Note that a voltage (for example, a voltage of the same polarity as the normal charging polarity of the toner) may be applied to the charge eliminating needle 20.

[0024] The cleaning device 6 removes the toner remaining on the photosensitive drum 1 to clean the surface of the photosensitive drum 1. In this embodiment, the cleaning device 6 has a cleaning blade 6a as a cleaning member and a cleaning container 6b that contains the collected toner. The position in the rotation direction of the photosensitive drum 1 where the toner is removed by the cleaning device 6 (the contact portion between the cleaning blade 6d and the photosensitive drum 1) is the cleaning portion (cleaning position).

[0025] A recording material cassette 7 that stores recording materials (transfer materials, recording media, paper, sheets) P ​​such as paper is disposed at the bottom of the device main body M in the figure. Also, from the recording material cassette 7, a feed roller 8, a conveying roller 9, a top sensor 10, a pre-transfer guide 24, a transfer / fixing conveying guide 11, a fixing device 12, discharge rollers 13, and a discharge tray 14 are disposed in this order along the conveying path of the recording material P.

[0026] As shown in FIG. 12, the main body M of the image forming apparatus 100 is provided with a control unit 50 as a control unit for controlling the overall operation of the image forming apparatus 100. The control unit 50 is configured to have, for example, a CPU 51 as an arithmetic control unit, a memory 52 such as a ROM, RAM, or nonvolatile memory as a storage unit, and an input / output unit (not shown) for controlling the exchange of information (signals) between the control unit 50 and an external device. The CPU 51 executes a 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 nonvolatile memory of the memory 52 stores the usage history of each unit. For example, a charging power source 21, a charging current detection circuit 22, a transfer power source 18, a transfer current detection circuit 19, and an exposure device 3 are connected to the control unit 50. For example, a driving device 60 as a driving unit for driving the photosensitive drum 1, the developing roller 4a, the feeding roller 8, the conveying roller 9, and the like is connected to the control unit 50. The control unit 50 controls each unit of the image forming apparatus 100 according to a control program stored in the ROM, using information appropriately stored in the RAM or non-volatile memory.

[0027] The photosensitive drum 1 and the charging roller 2, developing device 4, and cleaning device 6 acting as process means for the photosensitive drum 1 may be integrally formed as a process cartridge that is detachably mountable to the main body M of the image forming apparatus 100.

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

[0029] The electrostatic latent image formed on the photosensitive drum 1 is developed (visualized) by the development device 4, and a toner image is formed on the photosensitive drum 1. The toner on the development roller 4a is given a negative charge by being rubbed by the development blade 4b. During development, a development voltage (development bias) which is a DC voltage of the same polarity (negative in this embodiment) as the normal charging polarity of the toner is applied to the development roller 4a from a development power source (high voltage power source) 23 as a development voltage application means. In this embodiment, a voltage of -350V is applied to the development roller 4a as the development voltage. The toner given a charge by the development blade 4b adheres to the image portion of the electrostatic latent image on the photosensitive drum 1 to form a toner image. Thus, in this embodiment, the toner charged to the same polarity (negative in this embodiment) as the charging polarity of the photosensitive drum 1 adheres to the exposed portion (image portion) on the photosensitive drum 1, the absolute value of the potential of which has been reduced by being exposed after being uniformly charged (reverse development method). In this embodiment, the normal charging polarity of the toner, which is the main charging polarity of the toner during development, is negative.

[0030] The toner image formed on the photosensitive drum 1 is transferred to a recording material P such as paper at the transfer nip Nt by the action of the transfer roller 5. During the transfer, a transfer voltage (transfer bias), which is a DC voltage of 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 source (high voltage power source) 18 as a transfer voltage application means 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 sent out one sheet at a time by a feed roller 8. The recording material P is transported by a transport roller (registration roller) 9 in synchronization with the toner image on the photosensitive drum 1, and is supplied to the transfer nip Nt along a pre-transfer guide 24 as a guide member.

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

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

[0033] By repeating the above operations, image formation can be performed one after another. The image forming apparatus 100 of this embodiment can perform printing at a print speed of 60 sheets per minute.

[0034] The image forming apparatus 100 executes a print operation (print job) which is a series of operations for forming and outputting an image on one or more recording materials P, which is started by a single start instruction. The print operation generally includes an image forming process (image forming operation), a pre-rotation process, a paper-interval process in the case of forming images on multiple recording materials P, and a post-rotation process. The image forming process is a period during which an electrostatic latent image of an image to be actually formed on the recording material P and output is formed, a toner image is formed, and the toner image is transferred, and this period is referred to as the time of image formation (time of image forming operation). More specifically, the timing during image formation differs depending on the positions at which the steps of forming the electrostatic latent image, forming the toner image, and transferring the toner image are 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 a period during which a preparatory operation is performed before the image forming process, from when a start instruction is input until the image actually starts to be formed. The sheet interval process (inter-image process, inter-recording material process) is a period corresponding to the interval between recording materials P when image formation is continuously performed on a plurality of recording materials P (continuous image formation, continuous printing). The post-rotation process is a period in which a rearrangement operation (preparatory operation) is performed after the image formation process. The non-image formation time is a period other than the image formation time, and includes the pre-rotation process, the sheet interval process, the post-rotation process, and the pre-multiple rotation process which is a preparatory operation when the image forming apparatus 100 is turned on or when the image forming apparatus 100 returns from a sleep state. More specifically, the timing of the non-image formation time corresponds to a period in which the non-image formation area on the photosensitive drum 1 passes through each position where each process of forming the electrostatic latent image, forming the toner image, and transferring the toner image is performed. Here, the image formation area on the photosensitive drum 1 is an area in which a toner image can be formed that is transferred to the recording material P and output from the image forming apparatus 100, which is preset according to the size of the recording material P, and the like, and the non-image formation area is an area other than the image formation area.

[0035] (2) Mechanism of toner scattering caused by fog at startup Next, the mechanism by which toner scattering occurs due to fog at startup will be described. Fig. 2 is a schematic diagram of the periphery of the photosensitive drum 1 to explain the mechanism by which toner scattering occurs due to fog at startup. Fig. 2 shows a case where an image forming apparatus having substantially the same basic configuration as the image forming apparatus 100 of this embodiment is used, and a conventional transfer voltage control is used as the transfer voltage control at the startup of the image forming apparatus.

[0036] 2(a) shows a state in which a long time has passed since the end of the previous print operation and the print operation has been stopped. In this embodiment, "a long time has passed" 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 tribo toner). After such a long time has passed, the surface potential of the photosensitive drum 1 has decayed to nearly 0 V.

[0037] When a drive instruction such as a print signal is given to the image forming apparatus 100, the photosensitive drum 1 is rotated without the recording material P in the transfer nip Nt in order to remove the remaining toner on the photosensitive drum 1 by the cleaning device 6 before moving to the image forming operation (pre-rotation operation). Almost simultaneously with the start of the rotation drive of the photosensitive drum 1, application of the same charging voltage as in the normal image forming operation to the charging roller 2 and application of the same developing voltage as in the normal image forming operation to the developing roller 4a are started. The developing voltage at this time does not have to be the same developing voltage as in the normal image formation. For example, a developing voltage with the same polarity as the developing voltage in the normal image formation but with a small absolute value may be applied, or a voltage of the opposite polarity to the normal polarity may be applied instead of a voltage of the negative polarity which is the normal polarity of the toner. FIG. 2(b) shows the state immediately after the drive. As shown in FIG. 2(b), the toner T that was attached on the developing roller 4a at the time of stopping is transferred onto the photosensitive drum 1 as fog at the time of starting. There are two main reasons for this.

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

[0039] The second reason is that there is a section on the photosensitive drum 1 where the surface potential passes through the developing unit Nd while attenuating when the image forming apparatus 100 is started (the section from the charging unit Nc to the developing unit Nd in the rotation direction of the photosensitive drum 1). The surface of the photosensitive drum 1 in that section cannot generate a sufficient potential difference between the developing roller 4a at the developing unit Nd. 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 between the toner T and each member (the photosensitive drum 1, the developing roller 4a).

[0040] For the above two reasons, under conditions where fog occurs at startup, for example, in relatively bad conditions, fog toner T that resembles a solid image in which one or more layers of toner T overlap is transferred from the developing roller 4a to the photosensitive drum 1. As described above, in the rotation direction of the developing roller 4a, the toner T in the section from the area on the developing roller 4a that was located at the blade nip Nb when stopped to the area on the developing roller 4a that was located at the development section Nd when stopped is likely to transfer onto the photosensitive drum 1 when the image forming apparatus 100 is started. Also, as described above, even toner that has passed through the blade nip Nb at the time of starting the image forming apparatus 100 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 development section Nd when stopped in the rotation direction of the photosensitive drum 1.

[0041] The toner T transferred from the developing roller 4a onto the photosensitive drum 1 scatters toward the downstream side of the transfer nip Nt in the conveying direction of the recording material P by rubbing against the transfer roller 5 when passing through the transfer nip Nt for the first time (FIG. 2(c)). This scattered toner T adheres to the charge removing needle 20 and the conveying guide 11. Normally, during the pre-rotation operation, either no transfer voltage (0V) is applied, or a transfer voltage of the same polarity as the normal charging polarity of the toner T (negative polarity in this embodiment) is applied to suppress the adhesion of the toner 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 the toner T on the photosensitive drum 1 and the holding force of the toner T on the transfer roller 5 are both weakened, and the toner scatters 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 transfer voltage is applied, the zero tribo toner is slightly negatively polarized due to friction in the developing section Nd, and the toner T that has been slightly negatively polarized repels the transfer roller 5 to which a negative transfer voltage is applied and tends 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 from the transfer nip Nt occurs.

[0042] A portion of the toner T that is transferred from the photosensitive drum 1 to the transfer roller 5 due to non-electrostatic adhesion between the toner T and the transfer roller 5 is repeatedly rubbed in the transfer nip portion Nt, causing the toner to repeatedly scatter from the transfer nip portion Nt (Figure 2(d)).

[0043] If the above-described toner scattering is repeated whenever fog occurs during startup, for example, toner T accumulates (builds up) on the charge eliminating needles 20 and the conveying guide 11. This accumulated (accumulated) toner T may manifest itself as paper rear end stains caused by the trailing edge of the recording material P rubbing against the conveying guide 11 during printing operation and the toner T adhering to the trailing edge of the recording material P, or as a transfer image defect caused by a decrease in the function of the charge eliminating needles 20.

[0044] Examples of cases in which toner T for a solid image enters the transfer nip Nt without the recording material P being interposed include when the image forming apparatus 100 is started (restored) after clearing a jam caused by a paper jam or the like, in addition to when the start-up fog described above occurs. In most cases, however, the image forming apparatus 100 is started (restored) in a relatively short time after clearing the jam. Therefore, the charge of the toner T on the photosensitive drum 1 remains held, and the surface potential of the photosensitive drum 1 has not decayed, so the holding force of the toner T on the photosensitive drum 1 remains strong. In this case, therefore, toner scattering from the transfer nip Nt is unlikely to occur.

[0045] In other words, toner scattering from the transfer nip portion Nt is a phenomenon specific to startup fogging, in which toner T with attenuated charge is transferred from the developing roller 4a to the photosensitive drum 1 when the developing roller 4a does not separate from the photosensitive drum 1 when the image forming apparatus 100 is started up.

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

[0047] Figure 4(a) shows the state 12 hours after the previous print operation was stopped. The 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 developing roller 4a in the state in Figure 4(a) are shown in the "On developing roller" plot in Figure 5. Toner T should normally have 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] FIG. 4(b) shows a state immediately after a drive instruction such as a print signal is given to the image forming apparatus 100 and the photosensitive drum 1 starts to rotate in a pre-rotation operation. The state of FIG. 4(b) is the same as the state of FIG. 2(b). In this embodiment, the application of a charging voltage similar to that in a normal image forming operation to the charging roller 2 and the application of a developing voltage similar to that in a normal image forming operation to the developing roller 4a are started almost simultaneously with the start of the rotational drive of the photosensitive drum 1. As described above, the developing voltage at this time does not have to be the same as that in a normal image forming operation. For example, a developing voltage having the same polarity as the developing voltage in a normal image forming operation but a small absolute value may be applied, or a voltage of the opposite polarity to the normal polarity may be applied instead of a voltage of the negative polarity which is the normal polarity of the toner. The toner T transferred from the developing roller 4a to the photosensitive drum 1 as described above is charged slightly to the negative polarity side by friction in the developing section Nd. The result of measuring the charge distribution of the toner T on the photosensitive drum 1 in the state of FIG. 4(b) is shown in the plot "on drum" in FIG. 5. As shown in the "On the developing roller" plot in Figure 5, the median of the 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 of the charge distribution of toner T on the photosensitive drum 1 was -3 μC / g.

[0049] FIG. 4(c) shows a state when the fog toner T transferred from the developing roller 4a onto the photosensitive drum 1 passes through the transfer nip portion Nt for the first time. At this time, as shown in FIG. 3, a transfer voltage V0 having a polarity opposite to the normal charging polarity of the toner T (positive polarity in this embodiment) is applied between t1 and t2. In this embodiment, t1 is set to the timing when the toner T present in the developing portion Nd at the time of stopping reaches the transfer nip portion Nt. That is, in this embodiment, t1 is the timing when the area on the photosensitive drum 1 located in the developing portion Nd at the time of stopping at the start of the image forming apparatus 100 first reaches the transfer nip portion Nt. Also, in this embodiment, t2 is set to the timing when the toner T present in the blade nip portion Nb at the time of stopping reaches the transfer nip portion Nt. That is, in this embodiment, t2 is the timing when the area on the developing roller 4a located in the blade nip portion Nb at the time of stopping and the area on the photosensitive drum 1 that first abuts at the developing portion Nd at the start of the image forming apparatus 100 first reach the transfer nip portion Nt. This allows the toner T, which is likely to be transferred onto the photosensitive drum 1 when the image forming apparatus 100 is started, in the section from the area on the developing roller 4a that was located at the blade nip portion Nb when the image forming apparatus 100 was stopped to the area on the developing roller 4a that was located at the development portion Nd when the image forming apparatus 100 was stopped to be transferred onto the transfer roller 5 as described below.

[0050] It should be noted that t1 and t2 may be set so as to include the timing when a part or all of the portion on the photosensitive drum 1 where fog occurs at the time of starting passes through the transfer nip portion Nt for the first time. For example, the timing when the portion on the photosensitive drum 1 that was at the charging portion Nc at the time of stopping, where the surface potential of the photosensitive drum 1 becomes indefinite, passes through the transfer nip portion Nt for the first time, may be set as t2. In other words, t2 may be the timing when the area on the photosensitive drum 1 that was at the charging portion Nc at the time of stopping reaches the transfer nip portion Nt for the first time. This allows the toner T in the section from the area on the photosensitive drum 1 that was at the charging portion Nc at the time of stopping to the area on the photosensitive drum 1 that was at the developing portion Nd at the time of stopping, to which the toner T may be transferred from the developing roller 4a at the time of starting the image forming apparatus 100, to be transferred onto the transfer roller 5 as described later. It should be noted that in this embodiment, when the image forming apparatus 100 is started, the area on the developing roller 4a that was at the blade nip portion Nb at the time of stopping reaches the developing portion Nd earlier than the area on the photosensitive drum 1 that was at the charging portion Nc at the time of stopping. In addition, in consideration of the responsiveness of the transfer power source 18, the interval from t1 to t2 may be set wider than the interval from t1 to t2 in this embodiment.

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

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

[0053] The values ​​of the transfer voltages V1 to V8 in this embodiment are shown in Table 1. The results of measuring the charge distribution of the toner T on the transfer roller 5 in the state of FIG. 4(d) are shown in the plot "on the transfer roller" in FIG. 5. As shown in the plot "on the transfer roller" in FIG. 5, the median of the charge distribution of the toner T on the transfer roller 5 is -4 μC / g, which is slightly higher on the negative polarity side than the median of the charge distribution of the toner T on the photosensitive drum 1 due to friction at the transfer nip Nt. Since the toner T on the transfer roller 5 has such a charge distribution, when a negative transfer voltage with a low absolute value such as V1 is applied, only the toner T having a high charge on the negative polarity side on the transfer roller 5 is transferred onto the photosensitive drum 1, and the other toner T remains on the transfer roller 5. By gradually increasing the absolute value of this negative transfer voltage from V1 to V3, and from V3 to V5, the toner T is transferred from the transfer roller 5 to the photosensitive drum 1 in order of the charge on the negative polarity side. Also, by changing the polarity of the transfer voltage from V1 to V2 and from V3 to V4, the toner T whose polarity is inverted to positive polarity in the charge distribution of the toner shown in the plot "on the transfer roller" in FIG. 5 can also be transferred from the transfer roller 5 to the photosensitive drum 1. The absolute value of the positive transfer voltage is also gradually increased from V2 to V4 and from V4 to V6. In this embodiment, the absolute value of the voltage of each polarity is gradually increased while the polarity of the transfer voltage is alternately changed from V1 to V2, from V2 to V3, and from V3 to V4. However, it is also possible to continuously apply a transfer voltage of the same polarity in multiple stages with gradually increasing absolute values, for example, by changing from V1 to V3, from V3 to V2, and from V2 to V4. Also, depending on the charge distribution of the toner that has been transferred from the photosensitive drum 1 to the transfer roller 5 due to fog at the time of startup, only the negative polarity voltage may be applied while gradually increasing the absolute value between t2 and t3.

[0054] By performing such control, it becomes 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 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 force of inertia applied to the toner T may become large, or a repulsion between the toner T and the transfer roller 5 may be induced, which may worsen toner scattering.

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

[0056] [Table 1]

[0057] FIG. 6 is a timing chart similar to FIG. 3, showing the transition of the transfer voltage in a modified example of the transfer voltage control at the start of the image forming apparatus 100 according to the present embodiment. As shown in FIG. 6, even if the transfer voltage is not applied between t1 and t2 (V0=0V), the effect of this embodiment can be obtained. As described above with reference to FIG. 2(c), when the transfer voltage is not applied, toner scattering occurs and a part of the toner T is transferred from the photosensitive drum 1 to the transfer roller 5 by non-electrostatic adhesion. In the configuration of this embodiment, when V0=0V, about 30% of the toner T is transferred from the photosensitive drum 1 to the transfer roller 5. If the transfer voltage control at the start 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 at the transfer nip portion Nt, and toner scattering becomes worse. However, by performing the transfer voltage control at the start of the image forming apparatus 100 according to the present invention as shown in FIG. 6, the toner T on the transfer roller 5 is gradually transferred onto the photosensitive drum 1, and it becomes possible to suppress toner scattering from the transfer nip portion Nt from the second time onwards. 3, applying a transfer voltage of the opposite polarity (positive polarity in this embodiment) to the normal charging polarity of the toner during the period from t1 to t2 increases the amount of toner transferred from the photosensitive drum 1 to the transfer roller 5 when the fog toner T first passes through the transfer nip portion Nt. Therefore, this is more preferable from the viewpoint of suppressing toner scattering.

[0058] (4) Effects of this embodiment An evaluation experiment for confirming the effect of this embodiment will be described below. The evaluation experiment was carried out for this embodiment, as well as for Comparative Examples 1 and 2 in which the toner scattering suppression control of this embodiment was not executed.

[0059] In this embodiment, the transfer voltage control (toner scattering suppression control) described with reference to Fig. 3 and Fig. 6 was executed in the pre-rotation operation at the start of the image forming apparatus 100. That is, in this embodiment, 0V or +1000V was used as V0, and the values ​​in Table 1 were used as V1 to V8.

[0060] 7 was executed during the pre-rotation operation at the start of the image forming apparatus 100. That is, in the comparative example 1, the transfer voltage was not applied during the pre-rotation operation at the start of the image forming apparatus 100, and the transfer voltage was set to 0 V from t1 to t3.

[0061] In Comparative Example 2, the transfer voltage control shown in Fig. 8 was executed in the pre-rotation operation at the start-up of image forming apparatus 100. That is, in Comparative Example 2, a negative transfer voltage V9 was applied during the pre-rotation operation at the start-up of image forming apparatus 100, and the transfer voltage from t1 to t3 was set to V9 = -1000V.

[0062] 7 and 8 are timing charts showing the transition of the transfer voltage similar to that of Fig. 3. The configurations of the image forming apparatuses 100 in Comparative Examples 1 and 2 are substantially the same as the configuration of the image forming apparatus 100 in this embodiment, except that the transfer voltage control at the start-up of the image forming apparatus 100 is different.

[0063] The evaluation experiment was carried out as follows. The recording material P was LTR size with a basis weight of 75 g / m 2 A Vitality (manufactured by Xerox) of 100 was used. The image forming apparatus 100 was installed in an environment with a temperature of 23°C and a humidity of 50%, and a printing operation was performed. For each of this embodiment, 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 a small number of prints after a long period of time in this manner, the staining of the static elimination needle 20 and the transport guide 11 by toner ("static elimination needle / transport guide staining"), and the staining of the paper rear end caused by toner adhering to the rear end of the recording material P of the printed matter ("toner adhesion to paper rear end").

[0064] The results of the evaluation experiment are shown in Table 2. Regarding the static electricity removal needle and transport guide stains, if they occurred they were rated as × (bad), if there was only a small amount of staining they were rated as △ (fairly good), and if there was no staining they were rated as 〇 (good). Regarding toner adhesion at the rear edge of the paper, if it occurred they were rated as × (bad), if there was only a small amount of toner adhesion they were rated as △ (fairly good), and if there was no toner adhesion they were rated as 〇 (good).

[0065] [Table 2]

[0066] In Comparative Example 1, both the static elimination needle / conveyance guide stains and the toner adhesion at the rear end of the paper occur. This is because the fog toner, whose charge has decayed over time, scatters due to friction in the transfer nip Nt. Also, in Comparative Example 2, both the static elimination needle / conveyance guide stains and the toner adhesion at the rear end of the paper occur. This is due to the toner scattering due to the toner being slightly negatively polarized in the development unit Nd repelling the transfer roller 5 to which a negative polarity has been applied, in addition to the friction of the fog toner in the transfer nip Nt.

[0067] In this embodiment, when V0=0V, slight staining of the charge eliminating needle / transport guide and toner adhesion to the rear edge of the paper occur, but the results are better than those of Comparative Example 1 and Comparative Example 2. This is because the effect of this embodiment is that, when the fog toner is rubbed at the transfer nip portion Nt for the first time, toner scattering occurs, but when rubbed for the second time and thereafter, the toner on the transfer roller 5 is gradually transferred onto the photosensitive drum 1 and reduced.

[0068] In this embodiment, when V0=+1000V, neither the static elimination needle / transport guide stain nor the toner adhesion to the rear end of the paper occurred, and this was the best result in this evaluation experiment. This is because, even when the fog toner is rubbed for the first time at the transfer nip Nt, the fog toner on the photosensitive drum 1 that has been slightly negatively polarized at the development unit Nd is transferred to the transfer roller 5 while being subjected to an electrostatic force that attracts it to the transfer roller 5, and toner scattering is unlikely to occur. Also, even when the fog toner is rubbed for the second or subsequent times at the transfer nip Nt, the effect of this embodiment is that the toner on the transfer roller 5 is gradually transferred to the photosensitive drum 1 and reduced.

[0069] Thus, in this embodiment, the image forming apparatus 100 has a rotatable photoconductor 1, a charging means 2 which charges the photoconductor 1 at a charging section Nc, a rotatable developing member 4a which contacts the photoconductor 1 at a developing section Nd and supplies toner onto the photoconductor charged by the charging means 2, a regulating member 4b which contacts the developing member 4a at a regulating section Nb and applies a charge to the toner on the developing member, a transfer member 5 which contacts the photoconductor 1 to form a transfer section Nt and transfers the toner from the photoconductor 1 to a recording material P passing through the transfer section Nt, a transfer voltage application means 18 which applies a voltage to the transfer member 5, and a control means 50 which can control the transfer voltage application means 18, and rotation of the photoconductor 1 and the developing member 4a begins when the photoconductor 1 and the developing member 4a are in contact with each other. In this embodiment, the area on the photoconductor 1 that was located at the development portion Nd when the photoconductor 1 and the developing member 4a stopped is defined as the first area, and the area on the developing member 4a that was located at the regulating portion Nb when the photoconductor 1 and the developing member 4a stopped after the photoconductor 1 and the developing member 4a started to rotate and the area on the photoconductor 1 that first abuts at the development portion Nd are defined as the second area. The control means 50 determines whether the first area reaches the transfer portion Nt first after the photoconductor 1 and the developing member 4a start to rotate and whether the second area reaches the transfer portion Nt first. During a first period including at least a part of the period until the toner image reaches the transfer portion Nt, the transfer voltage application means 18 is controlled so as to apply to the transfer member 5 at least a first voltage having the same polarity as the normal charging polarity and a second voltage having the same polarity as the first voltage and an absolute value greater than that of the first voltage during a second period after the first period and before the toner image formed on the photoconductor 1 reaches the transfer portion Nt.

[0070] In a preferred embodiment, the control unit 50 controls the transfer voltage application unit 18 to apply a voltage of a polarity opposite to the normal charging polarity to the transfer member 5 during the first period. In a preferred embodiment, the first period is set to include a period from when the rotation of the photoconductor 1 and the developing member 4a starts and the first region first reaches the transfer portion Nt until the second region first reaches the transfer portion Nt. When the region on the photoconductor 1 that was located at the charging portion Nc when the photoconductor 1 and the developing member 4a are stopped is defined as a third region, the first period may be set to include a period from when the rotation of the photoconductor 1 and the developing member 4a starts and the first region first reaches the transfer portion Nt until the third region first reaches the transfer portion Nt. In this embodiment, the control means 50 controls the transfer voltage application means 18 to apply to the transfer member 4a, during the second period, a third voltage having a polarity opposite to the normal charging polarity, and a fourth voltage having the same polarity as the third voltage and an absolute value greater than that of the third voltage. In this embodiment, the control means 50 controls the transfer voltage application means 18 to alternately apply to the transfer member 5, during the second period, a voltage having the same polarity as the normal charging polarity, and a voltage having a polarity opposite to the normal charging polarity.

[0071] As described above, according to this embodiment, it is possible to suppress toner scattering from the transfer nip Nt due to fogging at startup when the image forming apparatus 100 is started in a state in which the developing roller 4a and the photosensitive drum 1 are in contact with each other a long time after the end of the previous print operation. As a result, even in the image forming apparatus 100 that does not have a contact / separation mechanism that switches between contact and separation of the developing roller 4a with the photosensitive drum 1, it is possible to suppress staining of the charge removal needle 20, the conveyance guide 11, and the paper trailing end caused by toner. In other words, according to this embodiment, it is possible to suppress toner scattering caused by toner that has caused fogging at startup.

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

[0073] The toner scattering suppression control (transfer voltage control at the start-up of the image forming apparatus 100) in this embodiment can also be used in the image forming apparatus 100 having a cleaner-less configuration as shown in FIG.

[0074] That is, when the transfer voltage control at the start-up of the image forming apparatus 100 in this embodiment is used, the toner transferred from the photosensitive drum 1 to the transfer roller 5 is gradually returned to the photosensitive drum 1. The toner returned to the photosensitive drum 1 includes, in addition to the negative toner, zero tribo toner and toner whose polarity has been reversed to positive.

[0075] In the image forming apparatus 100 with a cleanerless configuration shown in FIG. 9, negative toner adhering to the dark potential portion on the photosensitive drum 1 is collected in the developing device 4 by the developing unit Nd. At this time, zero-tribo toner and positive toner cannot be collected, so such toner must be made negative. In the image forming apparatus 100 shown in FIG. 9, during the pre-rotation operation (during the cleaning operation of the photosensitive drum 1), the charging roller 2 is applied with −1100V, which is the same charging voltage as during normal image forming operation, and discharge occurs between the charging roller 2 and the photosensitive drum 1. This discharge makes the polarity of the fog toner on the photosensitive drum 1 negative, and the fog toner can be collected in the developing unit Nd.

[0076] In this way, 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, another embodiment of the present invention will be described. The basic configuration and operation of the image forming apparatus of this embodiment are the same as those of the image forming apparatus of 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 of embodiment 1 are given the same reference numerals as those of embodiment 1, and detailed explanations are omitted.

[0078] In this embodiment, the image forming apparatus 100 determines whether or not to perform the toner scattering suppression control described in the first embodiment, depending on the history of the previous print operation. If the toner scattering suppression control described in the first embodiment is performed every time the image forming apparatus 100 is started, there is a possibility that the start-up time will increase and that the lifespan of the components will be adversely affected. Therefore, in this embodiment, the toner scattering suppression control is performed only in situations where it is highly necessary, thereby reducing these possibilities.

[0079] Table 3 shows the evaluation results of toner adhesion at the rear end of paper for each time elapsed since the end of the previous print operation. The transfer voltage control at the start of the image forming apparatus 100 in each of the above-mentioned Example 1 (V0 = +1000V) and Comparative Example 1 was executed every time the image forming apparatus 100 was started. In this case, the time elapsed since the end of the previous print operation was set to three levels: 1 hour, 6 hours, and 12 hours. The other conditions of the evaluation experiment were the same as the evaluation experiment that obtained the results in Table 2, but here only toner adhesion at the rear end of paper was evaluated. The case where toner adhesion at the rear end of paper occurred was marked x (bad), the case where there was a slight amount of toner adhesion was marked △ (slightly good), and the case where no toner adhesion occurred was marked ◯ (good).

[0080] [Table 3]

[0081] When the transfer voltage control at the start-up of the image forming apparatus 100 in the first embodiment is performed every time the image forming apparatus 100 is started up, good results are obtained for toner adhesion at the rear end of the paper, regardless of the time that has elapsed since the end of the previous printing operation.

[0082] On the other hand, when the transfer voltage control at the start of the image forming apparatus 100 in Comparative Example 1 is executed every time the image forming apparatus 100 is started, the toner adhesion at the rear end of the paper was poor as described above after 12 hours had passed since the end of the previous print operation. However, the results improved as the time elapsed from the end of the previous print operation was shortened to 6 hours and 1 hour. This is because, if the time elapsed from 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 yet decayed, so that the amount of fog toner at the start is reduced. FIG. 10 is a graph showing the measurement results of the charge distribution of the toner on the developing roller 4a when the time elapsed from the end of the previous print operation is varied. From FIG. 10, it can be seen that the charge amount of the toner is maintained on the negative polarity side as the elapsed time is shorter. In this way, the amount of fog toner at the start is reduced, and the amount of toner scattering from the transfer nip Nt is also reduced. Therefore, in such a case, toner adhesion at the rear end of the paper is improved even if the toner scattering suppression control described in the first embodiment is not executed.

[0083] Table 4 shows the evaluation results of toner adhesion at the rear end of paper for each number of sheets passed in the previous printing operation. The transfer voltage control at the start of the image forming apparatus 100 in each of the above-mentioned Example 1 (V0 = +1000V) and Comparative Example 1 was executed every time the image forming apparatus 100 was started. At that time, the number of sheets passed in the previous printing operation was set to three levels: 2 sheets, 5 sheets, and 10 sheets. The other conditions of the evaluation experiment were the same as the evaluation experiment that obtained the results in Table 2, but here only the toner adhesion at the rear end of paper was evaluated. The case where toner adhesion occurred at the rear end of paper was marked x (bad), the case where a small amount of toner adhered was marked △ (slightly good), and the case where no toner adhesion occurred was marked ◯ (good).

[0084] [Table 4]

[0085] When the transfer voltage control at the start of the image forming apparatus 100 in the first embodiment is executed every time the image forming apparatus 100 is started, the toner adhesion at the rear end of the paper is good regardless of the number of sheets passed in the previous printing operation. On the other hand, when the transfer voltage control at the start of the image forming apparatus 100 in the first comparative example is executed every time the image forming apparatus 100 is started, the toner adhesion at the rear end of the paper is poor as described above when the number of sheets passed in the previous printing operation was two. However, the result improves as the number of sheets passed in the previous printing operation increases to five sheets, ten sheets, and so on. This is because the more sheets passed, the less toner accumulates on the conveying guide 11, since the toner that adheres to the conveying guide 11 due to toner scattering caused by fog at the start is gradually discharged after adhering to the recording material P in an amount that cannot be visually confirmed. In this way, even if toner scattering occurs, if the situation is such that the dirt caused by toner is gradually removed from the conveying guide 11 by the passing of the paper, the toner adhesion at the rear end of the paper is improved without executing the toner scattering suppression control described in the first embodiment.

[0086] Next, a procedure for transfer voltage control at the start of the image forming apparatus 100 in this embodiment will be described taking into consideration the results of Tables 3 and 4. Fig. 11 is a flow chart showing the procedure for transfer voltage control at the start of the image forming apparatus 100 in this embodiment.

[0087] When the control unit 50 receives a print signal from the host computer, it judges whether or not the time elapsed since the end of the previous print operation is 6 hours or more (a predetermined threshold or more) before starting the startup operation of the image forming apparatus 100 (S101). The control unit 50 can measure the time from the end of the print operation to the reception of the print signal, for example, and make the judgment in S101 based on the measurement result. Alternatively, the control unit 50 can store the date and time when the print operation is completed in a storage means such as a non-volatile memory, and make the judgment in S101 based on the time elapsed since the end of the previous print operation calculated from the stored date and time and the date and time when the print signal was received.

[0088] If the time that has elapsed since the end of the previous printing operation is less than six hours, even if a normal startup operation is performed, the amount of startup fog is small, and the risk of toner scattering from the transfer nip Nt is low. Therefore, if the control unit 50 determines in S101 that the time that has elapsed is less than six hours ("No"), it executes a normal printing operation without executing toner scattering suppression control (S104), and ends the printing operation when the specified image forming operation is completed.

[0089] If the time that has passed since the end of the previous printing operation is six hours or more, the amount of fog that occurs at startup increases, increasing the risk of toner scattering from the transfer nip Nt. Therefore, when the control unit 50 determines in S101 that the time that has passed is six hours or more ("Yes"), it next determines whether the number of sheets passed (number of printed sheets) in the previous printing operation is five or less (a predetermined threshold or less) (S102).

[0090] If the number of sheets passed in the previous printing operation was six or more, the risk of paper rear end stains occurring is low even if normal startup operations are performed, because toner stains on the transport guide 11 are gradually discharged as the sheets pass. Therefore, if the control unit 50 determines in S102 that the number of sheets passed is greater than five ("No"), it executes normal printing operations (S104) without executing toner scattering suppression control, and ends the printing operations when the specified image forming operations are completed.

[0091] If the control unit determines in S102 that the number of sheets passed is five or less ("Yes"), it executes the toner scattering suppression control described in the first embodiment in the pre-rotation operation at the start-up of the image forming apparatus 100 (S103). Thereafter, the control unit 50 executes a normal print operation (S104), and ends the print operation when the specified image forming operation is completed.

[0092] In this embodiment, both the determination of whether toner scattering suppression control should be performed based on the elapsed time since the end of the previous print operation in S101 and the determination of whether toner scattering suppression operation should be performed based on the number of sheets passed in the previous print operation in S102 were performed. However, only one of the determinations in S101 and S102 in this embodiment may be performed (for example, only the determination in S101 is performed). Even in this case, it is possible to reduce the possibility of an increase in start-up time and an impact on the lifespan of components compared to the first embodiment.

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

[0094] As described above, in this embodiment, the toner scattering suppression control described in the first embodiment is executed only when it is highly necessary, at the start-up of the image forming apparatus 100. As a result, it is possible to shorten the time of the pre-rotation operation at the start-up of the image forming apparatus 100, and to reduce the possibility of affecting the lifespan of the members (such as scraping of the surface of the photosensitive drum 1 and an increase in the resistance of the transfer roller 5) by applying a transfer voltage in a state where there is no recording material P in the transfer nip portion Nt.

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

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

[0097] The image forming apparatus may be an image forming apparatus for various purposes, such as a printer, various printing machines, a copying machine, a FAX machine, or a multifunction machine. [Explanation of symbols]

[0098] 1 Photosensitive drum 2. Charge roller 4. Developing device 5 Transfer roller 6 Cleaning device 11 Transport guide 12 Fixing device 18 Transfer power supply 20. Static electricity removal needle 50 Control section

Claims

1. a rotatable photoreceptor; a charging unit for charging the photosensitive member in a charging section; a rotatable developing member that contacts the photosensitive member in a developing section and supplies toner onto the photosensitive member that has been charged by the charging means; a regulating member that contacts the developing member at a regulating portion and applies a charge to the toner on the developing member; a transfer member that contacts the photosensitive member to form a transfer portion and transfers toner from the photosensitive member to a recording material that passes through the transfer portion; a transfer voltage applying means for applying a voltage to the transfer member; a control means capable of controlling the transfer voltage application means; and In an image forming apparatus, the rotation of the photosensitive member and the developing member is started in a state where the photosensitive member and the developing member are in contact with each other, When an area on the photosensitive member that was located in the developing section when the photosensitive member and the developing member stopped is defined as a first area, and an area on the developing member that was located in the regulating section when the photosensitive member and the developing member stopped after the photosensitive member and the developing member started to rotate and an area on the photosensitive member that first abuts in the developing section are defined as a second area, an image forming apparatus characterized in that the control means applies no voltage to the transfer member or applies a voltage of a polarity opposite to the normal charging polarity of the toner to the transfer member during a first period including a period from when the rotation of the photosensitive member and the developing member starts and the first region first reaches the transfer unit until the second region first reaches the transfer unit, and controls the transfer voltage application means to apply to the transfer member at least a first voltage of the same polarity as the normal charging polarity and a second voltage of the same polarity as the first voltage but with an absolute value greater than that of the first voltage during a second period after the first period and before the toner image formed on the photosensitive member reaches the transfer unit.

2. 2. The image forming apparatus according to claim 1, wherein the control means controls the transfer voltage application means so as to apply a voltage of a polarity opposite to the normal charging polarity to the transfer member during the first period.

3. When the area on the photosensitive member that was located in the charging section when the photosensitive member and the developing member were stopped is defined as a third area, 2. The image forming apparatus according to claim 1, wherein the first period includes a period from when the rotation of the photosensitive member and the developing member starts and the first region first reaches the transfer portion to when the third region first reaches the transfer portion.

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

5. 5. The image forming apparatus according to claim 4, wherein the control means controls the transfer voltage application means so that, during the second period, a voltage of the same polarity as the normal charging polarity and a voltage of the opposite polarity to the normal charging polarity are alternately applied to the transfer member.

6. The image forming apparatus of any one of claims 1 to 5, characterized in that the control means is capable of selectively executing, based on information regarding the operating history of the image forming apparatus, a first control that controls the transfer voltage application means during the first period and the second period during the operating period from when the rotation of the photosensitive member and the developing member starts to when the toner image formed on the photosensitive member reaches the transfer section, and a second control that does not control the transfer voltage application means during the first period and the second period during the operating period, and the operating period is shorter than when the first control is executed.

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

8. 7. The image forming apparatus according to claim 6, wherein the information about the operation history is information about the stop time of the photosensitive member from the end of the previous printing operation.

9. 7. The image forming apparatus according to claim 6, wherein the information relating to the operation history is information relating to the number of prints in the previous printing operation.

Images