Image formation device

Abstract

To suppress a peel-off discharge between an image carrier and a recording material when a rear end of the recording material exits a transfer nip part.SOLUTION: An image formation device 1 has a recording material charging member 14 that is arranged on an upstream side relative to a transfer nip part NT in a conveyance direction of a recording material P to be clipped and conveyed by the transfer nip part NT, is contactable with an image formation surface serving as a surface of the recording material P to be conveyed toward the transfer nip part NT to which a toner image is transferred, and enables charging same polarity as charging polarity of an image carrier 2 by a friction charging. A contact part 14b of the recording material charging member 14 with the recording material P is configured to be movable to a direction crossing the surface of the recording material P in a position facing the recording material charging member 14.SELECTED DRAWING: Figure 1

Description

[Technical field]

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

[0002] Conventionally, in an image forming apparatus using an electrophotographic method, the surface of a photoconductor as an image carrier is charged, and the charged surface of the photoconductor is exposed according to image information to form an electrostatic latent image on the photoconductor. Charged toner is attached to the electrostatic latent image formed on the photoconductor to form a toner image on the photoconductor, and the toner image formed on the photoconductor is transferred onto a sheet-like recording material such as paper. The recording material onto which the toner image has been transferred is separated from the photoconductor and conveyed to a fixing device. A rotatable photoconductor drum is often used as the photoconductor. A transfer roller is often used as a transfer member for transferring the toner image from the photoconductor to the recording material. An image forming apparatus having a photoconductor drum and a transfer roller will be described below as an example. Furthermore, the leading edge and trailing edge of the recording material refer to the leading edge and trailing edge in the conveying direction of the recording material when passing through the transfer nip, even if not specified otherwise.

[0003] The transfer roller contacts the photosensitive drum to form a transfer nip between the photosensitive drum and the transfer roller. A transfer voltage of a polarity opposite to the normal charging polarity of the toner is applied to the transfer roller, so that a charge is applied to the recording material that is sandwiched between the photosensitive drum and the transfer roller at the transfer nip, and the toner image on the photosensitive drum is transferred onto the recording material. In this configuration, when the trailing end of the recording material passes through the transfer nip, a peeling discharge may occur between the photosensitive drum and the recording material. When this peeling discharge occurs, a potential difference occurs between a portion of the photosensitive drum that is affected by the peeling discharge and a portion of the photosensitive drum that is not affected by the peeling discharge. When an image to be transferred to the subsequent recording material is formed, the potential difference on the photosensitive drum cannot be completely canceled by the charging process, and a phenomenon called "rear end memory" may occur, which appears as a black streak (a streak-like density unevenness extending in the direction of the rotation axis of the photosensitive drum).

[0004] Therefore, Patent Document 1 discloses a method for suppressing the above-mentioned trailing end memory by applying a weak voltage (trailing end voltage) that is lower than the transfer voltage applied in the image forming area to the non-image forming area (margin area) on the trailing end side of the recording material (Patent Document 1). [Prior art documents] [Patent documents]

[0005] [Patent Document 1] JP 2002-55542 A Summary of the Invention [Problem to be solved by the invention]

[0006] In recent years, there has been a trend toward demand for image forming apparatuses with higher productivity and higher image quality than ever before.

[0007] As a means for increasing the productivity of an image forming apparatus, the conveying speed of the recording material may be increased. In this case, it is necessary to transfer a larger amount of toner from the photosensitive drum to the recording material per unit time, and it is likely that a stronger transfer voltage needs to be applied to the transfer roller. In general, it is known that when a strong transfer voltage is applied to the transfer roller, the amount of charge on the non-image forming surface (non-printing surface, back surface) of the recording material increases, which is the opposite polarity to the normal charging polarity of the toner, i.e., the opposite polarity to the charging polarity of the photosensitive drum, and the amount of peeling discharge described above also increases accordingly.

[0008] In addition, as a means for improving the image quality of the image forming apparatus, the charge amount of the toner may be increased. By increasing the charge amount of the toner, the amount of toner with insufficient charge can be reduced, and the toner can be attached to the electrostatic latent image portion more faithfully. This reduces the disturbance of the toner image, and achieves higher image quality. In this case, the total charge amount of the toner present on the recording material is greater. Therefore, in order to electrostatically hold this toner image on the recording material, it is necessary to apply a stronger transfer voltage to the transfer roller and charge the non-image forming surface of the recording material with a polarity opposite to the normal charging polarity of the toner, that is, the polarity opposite to the photosensitive drum. In this case, the charge amount of the non-image forming surface of the recording material increases, and the amount of peeling discharge described above also increases.

[0009] In this way, for example, when attempting to increase the productivity or image quality of an image forming apparatus, it may be difficult to sufficiently suppress the trailing edge memory with the conventional configuration. Therefore, there is a demand for a method that can suppress the trailing edge memory in place of or in addition to the conventional method.

[0010] SUMMARY OF THE PRESENTLY PREFERRED EMBODIMENTS It is therefore an object of the present invention to suppress separation discharge between the image carrier and the recording material when the trailing edge of the recording material passes through the transfer nip portion. [Means for solving the problem]

[0011] The above object is achieved by an image forming apparatus according to the present invention. In summary, the present invention comprises a rotatable photoconductor, a charging device that charges the surface of the photoconductor to a predetermined polarity, a developing device that supplies toner charged to the same polarity as the predetermined polarity to the surface of the photoconductor charged by the charging device to form a toner image on the surface of the photoconductor, a transfer member that contacts the surface of the photoconductor to form a transfer nip between the photoconductor and the transfer member and transfers the toner image from the photoconductor to a recording material passing through the transfer nip by applying a transfer voltage, and an application unit that applies the transfer voltage of a polarity opposite to the predetermined polarity to the transfer member. The image forming apparatus has a recording material charging member that is positioned upstream of the transfer nip portion in the transport direction of the recording material that is sandwiched and transported by the transfer nip portion, is capable of contacting an image forming surface onto which the toner image of the recording material transported toward the transfer nip portion is transferred, and is capable of charging the image forming surface to the same polarity as the specified polarity by frictional charging, and the contact portion of the recording material charging member with the recording material is capable of moving in a direction intersecting the surface of the recording material at a position opposite the recording material charging member. Effect of the Invention

[0012] According to the present invention, it is possible to suppress separation discharge between the image carrier and the recording material when the trailing edge of the recording material passes through the transfer nip portion. [Brief description of the drawings]

[0013] [Figure 1] FIG. 1 is a schematic cross-sectional view of an image forming apparatus according to a first embodiment. [Diagram 2] 3 is a schematic cross-sectional view of the vicinity of a transfer nip portion in the first embodiment. FIG. [Diagram 3] FIG. 4 is a schematic diagram for explaining transfer voltage control. [Figure 4] 1A and 1B are schematic diagrams for explaining a mechanism by which a trailing-end memory occurs; [Diagram 5] 11A and 11B are schematic diagrams for explaining a mechanism for suppressing a trailing memory. [Figure 6]3 is a schematic cross-sectional view of an upper transfer guide in Example 1 and Comparative Examples 1 and 2. FIG. [Figure 7] 13 is a schematic cross-sectional view of a transfer upper guide according to a modified example of the first embodiment. FIG. [Figure 8] FIG. 11 is a schematic cross-sectional view of another modified example of the first embodiment. [Figure 9] FIG. 11 is a schematic cross-sectional view of the vicinity of a transfer nip portion in an image forming apparatus according to a second embodiment. [Figure 10] FIG. 11 is a schematic cross-sectional view of the vicinity of a transfer nip portion in an image forming apparatus according to still another embodiment. [Figure 11] FIG. 13 is a schematic diagram of a recording material charging member and a moving mechanism in an image forming apparatus according to still another embodiment. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0014] Hereinafter, the image forming apparatus according to the present invention will be described in more detail with reference to the drawings.

[0015] [Example 1] (1) Image forming device First, the overall configuration of the image forming apparatus 1 of this embodiment will be described. Fig. 1 is a schematic cross-sectional view of the image forming apparatus 1 of this embodiment (a cross-sectional view showing a cross section substantially perpendicular to the direction of the rotation axis of a photosensitive drum 2 described later). The image forming apparatus 1 of this embodiment is a monochrome laser printer using an electrophotographic method, and can form a black monochrome image on a sheet-shaped recording material P according to image information input from an external device (not shown) such as a host computer.

[0016] The image forming apparatus 1 has a photosensitive drum 2, which is a rotatable drum-type (cylindrical) photosensitive member (electrophotographic photosensitive member) serving as an image carrier. When a print command is input from an external device to the image forming apparatus 1, the photosensitive drum 2 is rotated in the direction of the arrow R1 in the figure (clockwise direction) at a predetermined peripheral speed (process speed). In this embodiment, the photosensitive drum 2 is an organic photosensitive drum with an outer diameter of 30 mm, in which an OPC (organic photoconductor) layer having a 20 μm-thick CT layer (charge transfer layer) mainly made of a polycarbonate-based binder is formed on an aluminum cylinder.

[0017] The surface (outer circumferential surface) of the photosensitive drum 2 is uniformly charged to a predetermined potential of the same polarity (negative polarity in this embodiment) as the normal charging polarity of the toner by the charging roller 3, which is a roller-type charging member. The charging roller 3 constitutes a charging device as a charging means. In this embodiment, the charging roller 3 is a single-layer elastic roller in which a conductive elastic layer is coated on a conductive core metal. The charging roller 3 is disposed so that its surface (outer circumferential surface) contacts the surface of the photosensitive drum 2, and both ends of the conductive core metal in the direction of the rotation axis are pressed toward the photosensitive drum 2 by pressing means (not shown). The charging roller 3 rotates in accordance with the rotation of the photosensitive drum 2. During charging, a predetermined 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 3.

[0018] The surface of the charged photosensitive drum 2 is scanned and exposed by a laser scanner (exposure device) 4 as an exposure means, and an electrostatic latent image (electrostatic image) is formed on the photosensitive drum 2. That is, image information input from an external device to the image forming apparatus 1 is converted by a video controller 110 into image creation information for the image forming apparatus 1 to form an image. Thereafter, the laser scanner 4 receives a light emission instruction based on the image creation information from the control unit 120, and outputs a laser beam L modulated according to a time-series electric digital pixel signal based on the image creation information. Then, the laser scanner 4 scans and exposes the charged surface of the photosensitive drum 2 with the laser beam L. As a result, an electrostatic latent image according to the image information is formed on the surface of the photosensitive drum 2.

[0019] The electrostatic latent image formed on the photosensitive drum 2 is developed (visualized) by supplying toner as a developer by the developing device 5 as a developing means, and a toner image (toner image, developer image) is formed on the photosensitive drum 2. The developing device 5 has a developing container 52 in which toner is accommodated, and a developing roller 51 as a developer carrier rotatably supported by the developing container 52. The developing roller 51 carries the toner in the developing container 52, transports the toner to a developing section which is an opposing portion between the photosensitive drum 2 and the developing roller 51, and supplies the toner to the electrostatic latent image on the photosensitive drum 2. In this embodiment, the developing roller 51 abuts against the photosensitive drum 2 during development. In addition, a predetermined developing voltage (developing bias), which is a DC voltage having the same polarity as the normal charging polarity of the toner (negative polarity in this embodiment), is applied to the developing roller 51 during development. The developing device 5 deposits toner charged with the same polarity as the charge polarity of the photosensitive drum 2 (negative polarity in this embodiment) onto an exposed portion (image portion) on the photosensitive drum 2, the absolute value of the potential of which has been reduced by exposure after being uniformly charged (reverse development method). In this embodiment, the normal charge polarity of the toner, which is the main charge polarity of the toner during development, is negative polarity.

[0020] A transfer roller 8, which is a roller-type transfer member serving as a transfer means, is disposed facing the photosensitive drum 2. The transfer roller 8 is disposed so that its surface (outer circumferential surface) abuts against the surface of the photosensitive drum 2, and is pressed toward the photosensitive drum 2 to form a transfer nip portion (transfer portion) NT between the surface of the photosensitive drum 2 and the surface of the transfer roller 8. The toner image formed on the photosensitive drum 2 is sent to the transfer nip portion NT by the rotation of the photosensitive drum 2. In this embodiment, an image forming unit 19 is configured by the photosensitive drum 2, the charging roller 3, the laser scanner 4, the developing device 5, etc.

[0021] Sheet-like recording materials (recording media, transfer materials) P such as paper are stacked and stored in a feed cassette 9 serving as a feeding section. The recording materials P are picked up one by one by a feed roller 10 serving as a feeding member driven at a predetermined control timing, and are sent out from the feed cassette 9. The recording materials P are sent to a registration section (hereinafter also referred to as a "registration section") 7 by a pair of conveying rollers 11 serving as a conveying member. The registration section 7 is provided with a pair of registration rollers (hereinafter also referred to as a "registration roller pair") 12 serving as a synchronous conveying member. The recording material P is once received at a registration nip section (hereinafter also referred to as a "registration nip section") NR formed by the pair of registration rollers 12, and skew correction of the recording material P is performed. In addition, a registration sensor (hereinafter also referred to as a "registration sensor") 13 serving as a recording material detection means is provided in the registration section 7. The registration sensor 13 detects the arrival timing of the leading edge and the trailing edge of the recording material P. The recording material P is transported to the transfer nip NT by a pair of registration rollers 12 so as to be synchronized with the toner image on the photosensitive drum 2. After passing through the registration section 7, the recording material P is transported to the transfer nip NT while being guided by an upper transfer guide 14 and a lower transfer guide 15, which are guide members serving as a guide means.

[0022] The recording material P supplied to the transfer nip NT is sandwiched between the photosensitive drum 2 and the transfer roller 8 and transported. In this embodiment, the transfer roller 8 is an elastic roller with an outer diameter of 14 mm, in which a sponge elastic layer (foamed elastic layer) made of NBR and hydrin with a thickness of 4.5 mm is formed on a core metal made of SUS (stainless steel) with an outer diameter of 5 mm. A predetermined 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 8 during the process in which the recording material P is transported through the transfer nip NT. As a result, the toner image on the photosensitive drum 2 is transferred to the image forming surface (printing surface, front surface) of the recording material P.

[0023] The recording material P onto which the toner image has been transferred and separated from the surface of the photosensitive drum 2 is transported to a heat fixing device 16 serving as a fixing means. The heat fixing device 16 heats and pressurizes the recording material P carrying the unfixed toner image while transporting it in a fixing nip portion NF formed by a pair of fixing rollers, thereby fixing (melting and bonding) the toner image onto the recording material P. After the toner image has been fixed, the recording material P is discharged from the fixing nip portion NF of the heat fixing device 16 and transported to a discharge roller 17 serving as a transport member. The recording material P is discharged (output) by the discharge roller 17 onto a discharge tray 18 serving as a discharge portion provided outside the device body 1a of the image forming apparatus 1.

[0024] Furthermore, toner remaining on the surface of the photosensitive drum 2 after the recording material P has been separated (residual toner after transfer) is removed and collected by a cleaning device 6 serving as a cleaning means. The cleaning device 6 has a cleaning blade 61 serving as a cleaning member arranged so as to come into contact with the surface of the photosensitive drum 2, and a cleaning container 62. The cleaning device 6 scrapes off the residual toner after transfer from the surface of the rotating photosensitive drum 2 with the cleaning blade 61, and stores it in the cleaning container 62. In this way, the surface of the photosensitive drum 2 is cleaned and is repeatedly used for image formation.

[0025] Incidentally, the image forming apparatus 1 of this embodiment has a print speed of 55 sheets / minute (letter size paper) and a process speed (circumferential speed of the photosensitive drum 2) of approximately 300 mm / s.

[0026] In this embodiment, the photosensitive drum 2 and the charging roller 3, developing device 5 and cleaning device 6 acting as process means thereon are integrally configured as a process cartridge that is detachably attached to the main body 1a of the image forming apparatus 1.

[0027] (2) Configuration of the Transfer Nip Area Next, the configuration of the vicinity of the transfer nip NT in this embodiment will be described in more detail. Fig. 2 is a schematic cross-sectional view (a cross-sectional view showing a cross section substantially perpendicular to the rotation axis direction of the photosensitive drum 2) showing the configuration of the vicinity of the transfer nip NT in this embodiment.

[0028] Here, the leading edge and trailing edge of the recording material P refer to the leading edge and trailing edge in the conveying direction of the recording material P when passing through the transfer nip NT, even if not specified otherwise. Furthermore, the upstream and downstream of elements arranged on the conveying path of the recording material P refer to the upstream and downstream in the conveying direction of the recording material P, even if not specified otherwise. Furthermore, with respect to the image forming apparatus 1 and its elements, the front side of the paper in FIG. 1 (the front side in the direction of the arrow A) is the "front (front)" side, and the rear side of the paper (the rear side in the direction of the arrow A) is the "rear (rear)" side, and the front-rear direction connecting the front side and the rear side is approximately parallel to the direction of the rotation axis of the photosensitive drum 2 (the direction approximately perpendicular to the moving direction of the surface). Furthermore, with respect to the image forming apparatus 1 and its elements, the up and down refers to the up and down of the gravity direction (vertical direction), but does not mean only directly above and directly below, but also includes the upper and lower sides of a horizontal plane passing through the element or position of interest.

[0029] To the charging roller 3, a DC charging voltage power supply 24 is connected as a charging voltage application means (charging voltage application section) for charging the photosensitive drum 2.

[0030] Further, to the developing roller 51, a direct current developing voltage power supply 25 is connected as a developing voltage application means (developing voltage application section) for attaching the toner to the image portion of the electrostatic latent image on the photosensitive drum 2.

[0031] A DC positive transfer voltage power supply 20 as a transfer voltage application means (transfer voltage application section) and a DC negative transfer voltage power supply 21 as a reverse transfer voltage application means (transfer reverse voltage application section) are connected to the transfer roller 8. A current detection section (current detection circuit) 22 as a current detection means for performing a transfer constant current control, which will be described later, is also connected to the transfer roller 8. The positive transfer voltage power supply 20 and the negative transfer voltage power supply 21 each also function as a voltage detection means (voltage detection section).

[0032] Further, a charge eliminating needle 23, which is a charge eliminating member serving as a charge eliminating means for the recording material P, is provided downstream of the transfer nip NT. This charge eliminating needle 23 is provided for the purpose of eliminating at least a part of the charge applied to the recording material P from the transfer roller 8 and suppressing the trailing end memory.

[0033] Additionally, an upper transfer guide 14 and a lower transfer guide 15, which are guide members serving as guide means for guiding the recording material P to the transfer nip NT, are provided upstream of the transfer nip NT. The upper transfer guide 14 is disposed downstream of the pair of registration rollers 12 (further downstream of the registration sensor 13) and upstream of the lower transfer guide 15. The lower transfer guide 15 is disposed downstream of the upper transfer guide 14 and upstream of the transfer nip NT.

[0034] The upper transfer guide 14 comes into contact with the image forming surface of the recording material P (the surface that comes into contact with the surface of the photosensitive drum 2 when it passes through the transfer nip NT for the first time after passing through the resin nip NR) to regulate the transport trajectory of the recording material P. In this embodiment, no member that comes into contact with the image forming surface of the recording material P is provided downstream of the upper transfer guide 14 and upstream of the transfer nip NT. Therefore, in this embodiment, the image forming surface of the recording material P comes into contact with the surface of the photosensitive drum 2 after the upper transfer guide 14. In addition, the lower transfer guide 15 comes into contact with the non-image forming surface of the recording material P (the surface opposite to the surface that comes into contact with the surface of the photosensitive drum 2 when it passes through the transfer nip NT for the first time after passing through the resin nip NR) to regulate the transport trajectory of the recording material P. That is, with respect to a straight line L1 passing through the registration nip portion NR and the transfer nip portion NT in the cross section shown in FIG. 2, the upper transfer guide 14 is inserted below (the transfer roller 8 side) in the figure, and the lower transfer guide 15 is inserted above (the photosensitive drum 2 side) in the figure. As a result, after passing through the registration portion 7, the recording material P is conveyed to the transfer nip portion NT while being guided by rubbing against the upper transfer guide 14 and the lower transfer guide 15. In this way, the upper transfer guide 14 guides the recording material P downward once in the figure, and the lower transfer guide 15 guides the recording material P upward in the figure toward the transfer nip portion NT (the conveying path of the recording material P is approximately S-shaped in the cross section shown in FIG. 2). As a result, the recording material P is conveyed to the transfer nip portion NT while being deformed into a substantially constant shape by the upper transfer guide 14 and the lower transfer guide 15, so that the recording material P can be appropriately guided to the transfer nip portion NT regardless of the type or state of the recording material P. The upper transfer guide 14 will be described in more detail later.

[0035] (3) Transfer voltage control Next, the transfer voltage control in the image forming operation will be described. In this embodiment, the image forming apparatus 1 performs a combination of ATVC (Auto Transfer Voltage Control), constant voltage control, and trailing edge voltage control as the transfer voltage control. Fig. 3 is a schematic diagram for explaining the transfer voltage control in this embodiment. Fig. 3 shows a graph (lower diagram) showing the transition of the transfer voltage, and a schematic diagram (upper diagram) showing the position of the recording material P passing through the transfer nip NT at each point in time shown on the horizontal axis of this graph.

[0036] First, the ATVC will be described. The ATVC is a control executed before the recording material P is conveyed to the transfer nip NT. When the ATVC is executed, the control unit 120 as a control means controls the transfer positive voltage power source 20 to apply an initial voltage to the transfer roller 8 and waits until the output of the initial voltage is stabilized. After that, the control unit 120 samples the current detection result by the current detection unit 22 for a predetermined time and calculates the average current value. Then, the control unit 120 compares this average value with the target current of the ATVC and changes the voltage to be applied to the transfer roller 8 next so that the difference becomes small. By repeating the above, the control unit 120 controls so that the average value of the detection result of the current detection unit 22 converges to the target current of the ATVC (transfer constant current control). By performing this control, it is possible to grasp the base voltage V required to pass a constant current through the transfer nip NT. As a result, it is possible to grasp the electric resistance value R of the transfer roller 8 (transfer nip NT) based on the following formula (1) from the target current I of the ATVC and the base voltage V. R=V / I...Equation (1)

[0037] The transfer voltage during image formation (transfer) can be determined based on the electrical resistance value R. In this embodiment, the initial voltage is set to 500 V, the stabilization waiting time for the initial voltage is set to 100 ms, the current sampling time is set to 50 ms, and the target current I of the ATVC is set to 10 μA. Note that instead of using the electrical resistance value R, the transfer voltage during image formation (transfer) may be determined using the base voltage V required to pass the target current I.

[0038] Next, the constant voltage control will be described. The constant voltage control is a control executed mainly to transfer toner to the recording material P. Before the recording material P is conveyed to the transfer nip NT, the control unit 120 controls the transfer positive voltage power source 20 to apply a voltage to the transfer roller 8, similar to the ATVC. The voltage value applied to the transfer roller 8 can be determined by referring to the result of the ATVC, the type of the recording material P, the range of the toner image formed on the photosensitive drum 2, and the like. For example, the control unit 120 can determine the transfer voltage during image formation (transfer) by adding a recording material allotted voltage that is set in advance for each type of recording material P to a base voltage V required to pass the target current I obtained in the ATVC. The control unit 120 can obtain the recording material allotted voltage based on information regarding the type of recording material P included in a print command input from an external device, for example. In this embodiment, a DC voltage of, for example, +1800 V, which is opposite in polarity to the normal charging polarity of the toner (i.e., the charging polarity of the photosensitive drum 2), is supplied from the transfer positive voltage power supply 20 to the transfer roller 8 so that a current of about 20 μA flows.

[0039] The type of recording material includes any information that can distinguish the recording material, such as attributes based on general characteristics such as plain paper, high-quality paper, recycled paper, glossy paper, coated paper, thick paper, thin paper (so-called paper type category), numerical values ​​or numerical ranges of basis weight and thickness, or brand (including manufacturer, product number, etc.). The basis weight of thin paper is, for example, 52 to 63 g / m 2 For example, the basis weight of plain paper is 64 to 105 g / m 2 ), and the basis weight of cardboard is, for example, 106 to 256 g / m 2 It is.

[0040] Next, the trailing edge voltage control will be described. The trailing edge voltage control is a control executed to suppress the trailing edge memory. In this embodiment, the control unit 120 controls the trailing edge voltage control so that a voltage of the same polarity as the normal charging polarity of the toner (i.e., the charging polarity of the photosensitive drum 2) is applied to the transfer roller 8 during a predetermined period from before the trailing edge of the recording material P is discharged from the transfer nip NT to after the recording material P is discharged. Specifically, in this embodiment, the predetermined period corresponds to a section from 5 mm before the trailing edge of the recording material P is discharged from the transfer nip NT (a region corresponding to the margin of the trailing edge of the recording material P) to 10 mm after the recording material P is discharged, that is, a total section of 15 mm. In this embodiment, the control unit 120 controls the output of the transfer positive voltage power source 20 to be OFF and the output of the transfer negative voltage power source 21 to be ON during this predetermined period, so that a voltage of -200 V (trailing edge voltage) is applied to the transfer roller 8. By reducing the amount of positive charge at the trailing edge of the recording material P through the trailing edge voltage control, it is possible to reduce peeling discharge between the photosensitive drum 2 and the recording material P when the trailing edge of the recording material P leaves the transfer nip NT. In addition, the trailing edge voltage control is executed at a timing determined by taking into consideration the detection results of the leading edge and trailing edge of the recording material P by the registration sensor 13 and the transport distance of the recording material P between the registration sensor 13 and the transfer nip NT. This makes it possible to control the transfer voltage in synchronization with the timing at which the recording material P passes through the transfer nip NT.

[0041] (4) Mechanism of rear-end memory Next, the mechanism by which the trailing edge memory occurs due to the peeling discharge will be described. Fig. 4 is a schematic diagram for explaining the mechanism by which the trailing edge memory occurs. The "+" and "-" symbols in Fig. 4 typically indicate the charged state of the surface of the photosensitive drum 2 and the surface of the recording material P.

[0042] FIG. 4(a) shows a state in which the recording material P is passing through the transfer nip NT. As described above, a positive transfer voltage is applied to the transfer roller 8 during image formation (transfer). As a result, a positive charge is applied to the non-image forming surface of the recording material P, making it possible to electrostatically transfer the toner image on the photosensitive drum 2 to the recording material P. When the transfer of the toner is completed in the transfer nip NT, the charge applied to the recording material P is discharged to a certain extent by the charge discharging needle 23. Also, as described above, the surface of the photosensitive drum 2 is negatively charged by the charging roller 3 in order to form an image.

[0043] FIG. 4B shows the state after the trailing edge of the recording material P has left the transfer nip NT. In this state, the non-image forming surface of the recording material P is given a positive charge after being neutralized to a certain extent by the neutralizing needle 23. Also, in this state, the surface of the photosensitive drum 2 is negatively charged by the charging roller 3, as in the state of FIG. 4A. However, this state differs from the state of FIG. 4A in that the trailing edge of the recording material P has left the transfer nip NT, and the vicinity of the trailing edge of the recording material P separated from the photosensitive drum 2 is close to the surface of the photosensitive drum 2. Here, the non-image forming surface of the recording material P is positively charged, and the surface of the photosensitive drum 2 is negatively charged, so that even before the recording material P is separated from the photosensitive drum 2, a relatively large potential difference is generated between the non-image forming surface of the recording material P and the photosensitive drum 2. Due to this potential difference, charges are stored in the recording material P, and it functions like a capacitor. When the recording material P is separated from the photosensitive drum 2 in this state where charges are stored in the recording material P, that is, when the recording material P is charged, the apparent capacitance decreases abruptly, and the potential difference between the recording material P and the photosensitive drum 2 increases abruptly. When this potential difference exceeds the discharge threshold, a peeling discharge occurs, and a positive charge moves abruptly to the photosensitive drum 2. If this peeling discharge is significant, the effect of the peeling discharge cannot be canceled by the charging process by the charging roller 3, and even after passing through the charging roller 3, the area affected by the peeling discharge is in a state where the charge amount is partially small compared to the surrounding area (a state where the absolute value of the potential is small). Therefore, in the developing section, more toner adheres to the area with the small charge amount. As a result, the area on the photosensitive drum 2 that has been subjected to the peeling discharge becomes apparent as a black streak-like image defect (a streak-like density unevenness extending in the direction of the rotation axis of the photosensitive drum 2) after one rotation of the photosensitive drum 2.

[0044] The above is the mechanism by which the trailing edge memory (transfer memory) caused by the peeling discharge occurs.

[0045] The reason for changing the transfer voltage near the trailing end of the recording material P by the aforementioned trailing end voltage control of the recording material P is to reduce the amount of charge near the trailing end of the recording material P as much as possible, as explained in relation to the mechanism by which the trailing end memory occurs, and to suppress peeling discharge.

[0046] (5) Recording material charging member and trailing edge memory suppression mechanism Next, the upper transfer guide 14 of this embodiment will be described in more detail with reference to Fig. 2. In this embodiment, the upper transfer guide 14 is used as a recording material charging member (recording material charging means) to suppress the rear end memory.

[0047] In this embodiment, the upper transfer guide 14, which functions as a recording material charging member, is a thin plate-like member made of PC (Polycarbonate) resin. The upper transfer guide 14 has a predetermined length in the longitudinal direction, which is disposed approximately parallel to the rotation axis direction of the photosensitive drum 2, and a predetermined thickness in the lateral direction, which is approximately perpendicular to the longitudinal direction. In this embodiment, the upper transfer guide 14 is not substantially deformed by contact with the recording material P. In this embodiment, the material constituting the upper transfer guide 14 is a material having a volume resistivity of 3×10 14 The upper transfer guide 14 is made of a material having a hardness of Ω (PC resin). The upper transfer guide 14 has a function of being triboelectrically charged by rubbing against the recording material P (i.e., triboelectrically charging the recording material P). Specifically, in the image forming apparatus 1 of this embodiment, the recording material P is, for example, recording paper (plain paper) CS-068 (basis weight 68 g / m 2 , manufactured by Canon) is conveyed, the surface potential of the upper transfer guide 14 becomes positive due to frictional charging, and the surface potential of the image forming surface of the recording material P becomes negative. The surface potential of the upper transfer guide 14 and the surface potential of the recording material P were measured using a HIGH SPEED ELECTROSTATIC VOLT METER (manufactured by Trek Corporation) as a measuring device and a MODEL3800S-2 (manufactured by Trek Corporation) as a probe.

[0048] The upper transfer guide 14 is configured to be rotatable in the direction of the arrow R2 in FIG. 2 (clockwise direction) and the opposite direction around the guide rotation shaft 14a. The rotation axis of the guide rotation shaft 14a is substantially parallel to the rotation axis direction of the photosensitive drum 2 (the longitudinal direction of the upper transfer guide 14), that is, the front-rear direction of the image forming apparatus 1. In this embodiment, the guide rotation shaft 14a is provided at the end of the upper transfer guide 14 on the upstream side (the resin nip NR side). In this embodiment, the upper transfer guide 14 has a tip 14b on the downstream side (the transfer nip NT side) that contacts the recording material P. When the upper transfer guide 14 is not rotated in the direction of the arrow R2, the tip 14b is disposed below (the transfer roller 8 side) in the figure with respect to a straight line L1 passing through the resin nip NR and the transfer nip NT in the cross section shown in FIG. 2. By rotating the upper transfer guide 14 about the guide rotation shaft 14a, the tip 14b of the upper transfer guide 14, which is the contact portion with the recording material P, can move in a direction intersecting the surface of the recording material P (the surface of the recording material P transported from the resin nip portion NR to the transfer nip portion NT) at the position facing the upper transfer guide 14. The width of the upper transfer guide 14 in the rotation axis direction of the photosensitive drum 2 is wider than the image forming area (area where a toner image can be formed) on the photosensitive drum 2 in the same direction (the image forming area fits within the width of the upper transfer guide 14). The width of the image forming area (the maximum width corresponding to the maximum recording material P on which the image forming device 1 can form an image) is, for example, 210 mm. The upper transfer guide 14 is not limited to contacting the entire image forming surface in the transport direction of the recording material P, and may be capable of contacting only a part of the image forming surface in the transport direction of the recording material P depending on the setting of the transport trajectory of the recording material P. However, it is preferable that the upper transfer guide 14 is capable of coming into contact with at least the rear end of the recording material P in the conveying direction of the recording material P and the vicinity thereof (rear end portion).

[0049] In addition, the transfer upper guide 14 is pressed by a pressure spring 14c, which is an elastic member (urging member) serving as an urging means, in a direction in which the tip 14b of the transfer upper guide 14 rotates downward in the figure (in the direction opposite to the direction of the arrow R2) so as to contact the recording material P. The image forming apparatus 1 is configured such that, when the recording material P is in contact with both the transfer upper guide 14 and the transfer nip NT, a force acts on the tip 14b of the transfer upper guide 14 to push it upward in the figure (in the direction of the arrow R2) by the recording material P. Specifically, the position of the pair of registration rollers 12, the discharge angle of the recording material P from the pair of registration rollers 12, the position of the transfer lower guide 15, and the like are adjusted so that the above-mentioned force acts. In particular, when a paper having a large basis weight (high stiffness) generally called thick paper is transported as the recording material P, a stronger force acts on the transfer upper guide 14 compared to when a paper generally called plain paper or a paper generally called thin paper is transported as the recording material P. In this embodiment, due to the weight of the upper transfer guide 14 and the pressure of the pressure spring 14c, the upper transfer guide 14 is set to rotate upward in the figure (in the direction of arrow R2) when a force of 0.59 N acts on the tip 14b of the upper transfer guide 14. This is to prevent excessive frictional charging of the upper transfer guide 14 when a thick paper with high stiffness is transported, as will be described in detail later.

[0050] Next, the mechanism for suppressing the trailing end memory in this embodiment will be described. FIG. 5 is a schematic diagram for explaining the mechanism for suppressing the trailing end memory in this embodiment. The "+" and "-" symbols in FIG. 5 typically indicate the charged state of the surface of the photosensitive drum 2 and the surface of the recording material P. As described above, the tip portion 14b of the upper transfer guide 14 comes into contact with the recording material P and frictionally charges the recording material P. Specifically, in the image forming apparatus 1 of this embodiment, for example, CS-068 (basis weight 68 g / m2), which is recording paper (plain paper), is used as the recording material P. 2When a recording material P (manufactured by Canon) is conveyed, the upper transfer guide 14 is charged to a positive polarity due to frictional charging, and the image forming surface of the recording material P is charged to a negative polarity. To express this effect, in FIG. 5, the image forming surface of the recording material P (upper side in the figure) on the upstream side of the transfer nip NT (right side in the figure) is shown with a "-" symbol to indicate that the recording material P is typically charged to a negative polarity.

[0051] As described above, after the recording material P passes through the transfer nip NT, a positive charge is applied to the non-image forming surface of the recording material P due to the effect of the positive transfer voltage applied to the transfer roller 8. Then, a charge is transferred from the recording material P to the photosensitive drum 2 due to a peeling discharge that occurs when the trailing end of the recording material P passes through the transfer nip NT and separates from the photosensitive drum 2. The amount of charge transferred by this peeling discharge (herein also referred to as the "peeling discharge amount") is determined by the charge amount of the recording material P (specifically, the charge amount of the positive polarity that is the opposite polarity to the charge polarity of the photosensitive drum 2). Here, in this embodiment, the image forming surface of the recording material P is negatively charged by the transfer upper guide 14. Due to this action, the negative polarity of the image forming surface and the positive polarity of the non-image forming surface contribute to the charge amount of the recording material P in a direction that cancels out each other. In other words, in this embodiment, it is possible to reduce the charge amount of the recording material P when the recording material P is separated from the photosensitive drum 2, compared to the case where the action of the transfer upper guide 14 does not exist. This reduces the amount of discharge caused by separation when the recording material P is separated from the photosensitive drum 2, and as a result, it is possible to suppress the occurrence of trailing edge memory.

[0052] (6) Materials for charging the recording material Next, the material of the recording material charging member will be further described. First, a method for evaluating the triboelectric charging performance of the recording material charging member will be described. The triboelectric charging performance of the recording material charging member can be roughly evaluated by selecting a specific recording material P, rubbing the recording material P against an evaluation member made of the material to be evaluated with a certain load, and then measuring the surface potential of the evaluation member. In this example, CS-068 (basis weight 68 g / m), which is recording paper (plain paper), was used as the recording material P for evaluating the triboelectric charging performance of the recording material charging member. 2A 15mm×15mm square contact probe was selected. The friction conditions between the recording material P and the evaluation member were as follows. The recording material P for evaluation was subjected to a 20mm wide reciprocating motion five times with a load of about 0.98N on a square contact probe having a contact area of ​​15mm×15mm with the evaluation member. Thereafter, the surface potential of the evaluation member was measured using a HIGH SPEED ELECTROSTATIC VOLT METER (Trek) as a measuring device connected to a MODEL3800S-2 (Trek). The evaluation member was a substantially planar member with substantially no unevenness on the surface. The evaluation member was also used as a member having a width sufficiently wider than the reciprocating width of the contact probe, for example, a square member having a size of 50mm×50mm.

[0053] Table 1 shows examples of materials used in evaluation of the recording material charging member and the evaluation results.

[0054] [Table 1]

[0055] For example, the evaluation result of the PC resin used as the material of the upper transfer guide 14 in this embodiment will be described. As shown in Table 1, the surface potential of the PC resin evaluation member in the above-mentioned evaluation method was about +1.5 to +2.0 KV, and it was confirmed that the evaluation member was positively charged due to friction with the recording material P. It is generally known that when frictional charging occurs, one member is charged with the polarity opposite to the polarity of the other member. Therefore, in this case, the recording material P is negatively charged due to friction with the evaluation member. As described above, the feature of this embodiment is that the trailing end memory is suppressed by charging the recording material P with the same polarity (negative polarity in this embodiment) as the charging polarity of the photosensitive drum 2 by the recording material charging member. From this viewpoint, it is sufficient that the recording material charging member is configured to frictionally charge the recording material P to negative polarity, that is, that the recording material charging member is configured to frictionally charge the recording material P to positive polarity. Therefore, PC resin is suitable as a material for the recording material charging member.

[0056] Similarly, as shown in Table 1, ABS (Acrylonitrile-Butadiene-Styrene) copolymer resin, PC+ABS alloy resin, POM (Polyoxymethylene) resin, PET (Polyethylene Terephthalate) resin, resin obtained by mixing PET resin with glass fiber, and PBT (Poly Butylene Terephthalate) resin with glass fiber are suitable materials for the recording material charging member.

[0057] The material of the recording material charging member is not limited to the above-mentioned materials as long as it can triboelectrically charge the recording material P to the same polarity as the charging polarity of the photosensitive drum 2. The recording material charging member is not limited to being entirely made of a material mainly made of resin such as the above-mentioned materials. The recording material charging member may have a material on its surface that contacts the recording material P that can triboelectrically charge the recording material P to the same polarity as the charging polarity of the photosensitive drum 2. For example, the same effect can be obtained by applying a substance or component having the above-mentioned triboelectric charging performance to the surface of a base body made of any material (e.g., resin) that contacts the recording material P, or by attaching a member having the above-mentioned triboelectric charging performance. Furthermore, in this embodiment, the recording material charging member is a member that triboelectrically charges the recording material P to a negative polarity, but is not limited to this. For example, when the charging polarity of the photosensitive drum 2 is positive, the recording material charging member may be a member that triboelectrically charges the recording material P to a positive polarity.

[0058] (7) Image output experiment results The results of an image output experiment conducted for the configurations of this embodiment and a comparative example described later will be described below. In the image output experiment, the occurrence degree of the above-mentioned "rear end memory" and the below-mentioned "black dot image" was evaluated.

[0059] First, the method of evaluating the "trailing end memory" will be described. The mechanism of occurrence of the "trailing end memory" has been described above. The "trailing end memory" was evaluated by continuously printing two sheets of the recording material P under a normal temperature and humidity environment. During image formation (transfer), a voltage of +1800V was supplied from the transfer positive voltage power source 20 to the transfer roller 8 so that a current of about 20 μA would flow. The image patterns formed in the continuous printing were a character image with a relatively low printing rate (printing rate of about 5%) for the first sheet, and a halftone image (printing rate of about 50%) for the second sheet, which makes it easy to determine the occurrence of the "trailing end memory". The recording material P was CS-068 (basis weight 68 g / m), which is recording paper (plain paper). 2 The presence or absence of "rear end memory (black streak-like image defect)" on the recording material P output by continuous printing was visually confirmed.

[0060] Next, the mechanism of occurrence of "black dot images" and the method of evaluation will be described. "Black dot images" are image defects that occur when excessive frictional charging of the recording material charging member is performed, and specifically, are a phenomenon in which the density becomes darker in dots in parts when a halftone image or the like is formed. The amount of charge that the recording material charging member is charged with friction with the recording material P varies depending on the operating environment of the image forming apparatus 1, the type of recording material P, or the number of prints.

[0061] Regarding the operating environment of the image forming apparatus 1, when the image forming apparatus 1 is operated in a relatively low humidity environment, the amount of charge of the recording material charging member tends to be large. This is because, when the image forming apparatus 1 is used in a low humidity environment, the frictional charge accumulated in the recording material charging member is unlikely to gradually discharge into the atmosphere, and the recording material charging member is likely to maintain the charged state. In addition, when the image forming apparatus 1 is used in a low humidity environment, the recording material P itself is also likely to be dry and have a high electrical resistance, and the recording material charging member tends to be more highly charged even when it rubs against the same number of recording materials P. Regarding the type of recording material P, when a recording material P with a large basis weight, that is, thick paper with a relatively high stiffness, is used, the amount of charge of the recording material charging member tends to be large. As will be described in detail later, this is because, when thick paper or the like is transported as the recording material P, the recording material charging member and the recording material P tend to rub against each other strongly. This is particularly noticeable when the transfer upper guide 14 functioning as the recording material charging member is not configured to be rotatable as in this embodiment, but is configured not to be rotatable as in Comparative Example 1 described later. Furthermore, regarding the number of prints, when images are formed on a large number of sheets of recording material P in a short period of time, the amount of charge on the recording material charging member tends to increase. This is because the proportion of the time that the recording material P rubs against the recording material charging member per unit time increases, and the amount of charge on the recording material charging member increases.

[0062] In this way, when the charge amount of the recording material charging member increases due to the operating environment of the image forming apparatus 1, the type of recording material P, or the number of prints, a discharge phenomenon occurs from the recording material charging member to the recording material P in addition to the frictional charging of the recording material P by the recording material charging member. When this discharge phenomenon is significant, the image forming surface of the recording material P may have charge unevenness due to this discharge phenomenon. For example, in the case where the recording material charging member is positively charged as in this embodiment, positive discharge is performed on the recording material P, so that non-uniform dot-like positively charged parts are generated on the surface of the recording material P. In this embodiment, since the toner is negatively charged, when the recording material P is transported to the transfer nip NT in a state where it is non-uniformly dot-like positively charged as described above, when the toner image is transferred to the recording material P, some of the toner gathers at the positively charged parts, and the image becomes locally darker than other parts, which appears as a "black dot image".

[0063] The "black dot image" was evaluated by continuously printing 200 sheets of recording material P in a relatively low humidity environment with a relative humidity of 15%. During image formation (transfer), a voltage of +4200 V was supplied from the transfer positive voltage power source 20 to the transfer roller 8 so that a current of about 15 μA would flow. The image pattern formed during the continuous printing was a halftone image (print rate of about 50%) that makes it easy to distinguish the occurrence of the "black dot image". Springhill Digital Index (basis weight 199 g / m2), which is a recording paper (thick paper), was used as the recording material P. 2 The presence or absence of "black dot images" on the recording material P output by continuous printing was visually confirmed.

[0064] Table 2 shows the performance evaluation results of this example and the comparative example.

[0065] [Table 2]

[0066] First, the evaluation results of this embodiment will be described. FIG. 6(a) is a schematic cross-sectional view of the upper transfer guide 14 that functions as a recording material charging member in this embodiment. In this embodiment, the upper transfer guide 14 performs negative frictional charging of the recording material P, so no trailing edge memory occurs. In this embodiment, the tip 14b of the upper transfer guide 14 can rotate and retreat due to the force received from the recording material P, so no black dot images occur. In this way, this embodiment can suppress the "rear edge memory" and the "black dots", making it possible to form an image of good quality.

[0067] Next, the evaluation result of Comparative Example 1 will be described. FIG. 6(b) is a schematic cross-sectional view of the upper transfer guide 14 functioning as a recording material charging member of Comparative Example 1. In Comparative Example 1, the guide rotation shaft 14a and the pressure spring 14c are not provided, and the upper transfer guide 14 is fixed so as not to rotate (move). The other configurations of the upper transfer guide 14 of Comparative Example 1 are substantially the same as those of the upper transfer guide 14 of this embodiment. Therefore, the position of the upper transfer guide 14 of Comparative Example 1 is the same as the position of the upper transfer guide 14 of this embodiment when it is not rotated by the recording material P. In Comparative Example 1, the upper transfer guide 14 performs negative frictional charging of the recording material P as in this embodiment, so no trailing edge memory occurred. However, in Comparative Example 1, since the upper transfer guide 14 does not rotate, the upper transfer guide 14 and the recording material P rub against each other more strongly than in this embodiment, especially when thick paper is conveyed. Therefore, in Comparative Example 1, a slight black dot image caused by excessive charging of the upper transfer guide 14 occurred. That is, in this embodiment, when thick paper is transported, the transfer upstream upper guide 14 can rotate and retreat in response to the weight received from the recording material P. Therefore, in this embodiment, it is possible to prevent the recording material P and the transfer upper guide 14 from strongly rubbing against each other due to an excessive weight. This action can prevent the transfer upper guide 14 from being excessively charged. On the other hand, in Comparative Example 1, the transfer upper guide 14 does not rotate. Therefore, in Comparative Example 1, the recording material P and the transfer upper guide 14 rub against each other with a relatively strong force, and the amount of charge on the transfer upper guide 14 is likely to become large.

[0068] Next, the evaluation result of Comparative Example 2 will be described. FIG. 6(c) is a schematic cross-sectional view of the upper transfer guide 14 of Comparative Example 2. In Comparative Example 2, a conductive coating member 14d made of SUS, which is a conductive material, is provided at the tip 14b, which is the contact portion of the upper transfer guide 14 with the recording material P, and the coating member 14d is further electrically grounded. The other configurations of the upper transfer guide 14 of Comparative Example 1 are substantially the same as those of the upper transfer guide 14 of this embodiment. In Comparative Example 2, the upper transfer guide 14 is rotatable as in this embodiment, and the conductive coating member 14d is electrically grounded in the first place, so that the upper transfer guide 14 is not excessively charged. Therefore, in Comparative Example 2, no black dot image was generated. However, in Comparative Example 2, frictional charging of the recording material P by the upper transfer guide 14 was not performed, and a slight trailing edge memory was generated.

[0069] As described above, in this embodiment, the upper transfer guide 14 functioning as the recording material charging member can frictionally charge the recording material P to the same polarity as the charging polarity of the photosensitive drum 2. This can suppress peeling discharge between the photosensitive drum 2 and the recording material P when the recording material P passes through the transfer nip NT, thereby suppressing the rear end memory. In addition, in this embodiment, the upper transfer guide 14 functioning as the recording material charging member can rotate and retreat due to the force received from the recording material P when, for example, thick paper is transported as the recording material P. This can suppress excessive frictional charging of the upper transfer guide 14, thereby suppressing black dot images. In this way, according to this embodiment, it is possible to suppress both the rear end memory and the black dot images.

[0070] In this embodiment, in the trailing end voltage control, a voltage (trailing end voltage) of negative polarity (same polarity as the charging polarity of the photosensitive drum 2) is applied from the viewpoint of reducing the amount of charge of the recording material P as much as possible, but this is not limited to this. For example, if the image forming apparatus 1 does not have a power source that outputs a negative voltage, in the trailing end voltage control, a voltage of positive polarity (opposite polarity to the charging polarity of the photosensitive drum 2) whose absolute value is smaller than that of the transfer voltage applied in the constant voltage control during image formation (transfer) may be applied. This makes it possible to reduce the amount of charge of the recording material P as much as possible. In other words, when the trailing end of the recording material P in the conveying direction of the recording material P passes through the transfer nip portion NT, a voltage of opposite polarity to the normal charging polarity of the toner (i.e., the charging polarity of the photosensitive drum 2) and whose absolute value is smaller than that of the voltage applied to the transfer roller 8 when the center of the recording material P in the conveying direction of the recording material P and its vicinity (center portion) pass through the transfer nip portion NT may be applied to the transfer roller 8. Furthermore, in the case where the effect of suppressing the trailing end memory by the recording material charging member is high, it is also possible to set the voltage applied during a period similar to the period during which the trailing end voltage control in this embodiment is executed to the same voltage as the transfer voltage applied under constant voltage control during image formation (transfer).

[0071] In this embodiment, the specific recording paper (plain paper, thick paper) was used for the evaluation. However, according to the inventor's research, the same effect can be obtained with any paper other than the specific recording paper exemplified above, as long as the paper is used in an electrophotographic image forming apparatus.

[0072] As described above, in this embodiment, there are a rotatable image carrier (photosensitive drum) 2, a charging device 3 which charges the surface of the image carrier 2 to a predetermined polarity, a developing device 5 which supplies toner charged to the same polarity as the predetermined polarity to the surface of the image carrier 2 charged by the charging device 3 to form a toner image on the surface of the image carrier 2, a transfer member 8 which comes into contact with the surface of the image carrier 2 to form a transfer nip NT between the image carrier 2 and the developing device 5, and which transfers the toner image from the image carrier 2 to a recording material P passing through the transfer nip NT by applying a transfer voltage to the transfer member 8, and a transfer voltage which is opposite to the predetermined polarity to the transfer member 8. The image forming apparatus 1 has the application units 20 and 21 that apply the voltage to the recording material P, which is sandwiched and conveyed by the transfer nip NT, and is arranged upstream of the transfer nip NT in the conveying direction of the recording material P, and has a recording material charging member 14 that can contact the image forming surface, which is the surface to which the toner image of the recording material P conveyed toward the transfer nip NT is transferred, and can charge the image forming surface to the same polarity as the predetermined polarity by frictional charging, and a contact portion 14b of the recording material charging member 14 with the recording material P can move in a direction intersecting with the surface of the recording material P at a position facing the recording material charging member 14. In this embodiment, the recording material charging member 14 is arranged downstream of a conveying member (a pair of registration rollers) 12 that conveys the recording material P at a position closest to the transfer nip NT on the upstream side of the transfer nip NT in the conveying direction. In this embodiment, the recording material charging member 14 has a function of guiding the recording material P conveyed toward the transfer nip NT. In this embodiment, the image forming surface of the recording material P comes into contact with the surface of the image carrier 2 after the recording material charging member 14. In this embodiment, the contact portion 14b of the recording material charging member 14 moves due to the force received from the recording material P. Particularly, in this embodiment, the contact portion 14b of the recording material charging member 14 moves due to the rotation of the recording material charging member 14. In this embodiment, the image forming apparatus 1 has a biasing means (pressure spring) 14c that biases the recording material charging member 14 in a direction in which the contact portion 14b of the recording material charging member 14 moves from the image forming surface side toward the surface opposite to the image forming surface.In this embodiment, the biasing means 14c is configured so that the contact portion 14b of the recording material charging member 14 can be located downstream in the direction from the surface side opposite to the image forming surface toward the image forming surface side when the second type of recording material P, which has a basis weight larger than that of the first type of recording material P, is transported toward the transfer nip NT, rather than when the first type of recording material P is transported toward the transfer nip NT. Here, it is preferable that the recording material charging member 14 can contact at least the trailing end of the recording material P in the transport direction. It is also preferable that the recording material charging member 14 can contact the recording material P in substantially the entire image forming area in the direction perpendicular to the transport direction. In this embodiment, the application portions 20 and 21 apply a voltage to the transfer member 8 when the trailing end of the recording material P in the transport direction passes through the transfer nip NT, which is different from the voltage applied to the transfer member 8 when the center of the recording material P in the transport direction passes through the transfer nip NT. When the trailing end of the recording material P in the transport direction leaves the transfer nip NT, the application units 20 and 21 can apply to the transfer member 8 a voltage of opposite polarity to the predetermined polarity and having an absolute value smaller than the absolute value of the voltage applied to the transfer member 8 when the central portion of the recording material P in the transport direction passes through the transfer nip NT. Furthermore, the application units 20 and 21 can apply to the transfer member 8 a voltage of the same polarity as the predetermined polarity when the trailing end of the recording material P in the transport direction leaves the transfer nip NT.

[0073] (8) Modification of the First Embodiment Next, a modification of the first embodiment will be described.

[0074] (8-1) Contact range First, a modified example regarding the contact area of ​​the upper transfer guide 14 with the recording material P will be described.

[0075] The left diagrams of Fig. 7(a) to Fig. 7(d) are schematic cross-sectional views of the upper transfer guide 14 of the first embodiment and the modified example, and the right diagrams of Fig. 7(a) to Fig. 7(d) are front views seen from the direction of the arrow α in each of the left diagrams.

[0076] FIG. 7(a) shows the upper transfer guide 14 of the first embodiment. From the viewpoint of suppressing the trailing end memory by frictionally charging the recording material P to the same polarity as the charging polarity of the photosensitive drum 2 by the upper transfer guide 14, it is desirable that the upper transfer guide 14 contacts the recording material P uniformly in the longitudinal direction without unevenness. In addition, it is desirable that the contact width between the upper transfer guide 14 and the recording material P in the longitudinal direction (hereinafter, simply referred to as the "contact width") is at least wider than the image forming area (the image forming area fits within the contact width). Therefore, as shown in FIG. 7(a), it is desirable that the upper transfer guide 14 contacts the recording material P uniformly in the longitudinal direction without providing an uneven shape to the part of the upper transfer guide 14 that contacts the recording material P. In addition, as shown in FIG. 7(a), it is desirable that the contact width in the longitudinal direction is wider than the image forming area so that the frictional charging of the recording material P is performed in a sufficient range.

[0077] On the other hand, there may be cases where it is difficult to implement the configuration shown in FIG. 7(a) due to various constraints, for example, as described below.

[0078] First, it is assumed that the upper transfer guide 14 is divided in the longitudinal direction, for example, to improve assembly. Alternatively, it is assumed that when the upper transfer guide 14 is molded using a mold, a recessed groove is partially formed in the upper transfer guide 14, for example, to ensure molding stability. In either case, it is assumed that a portion of the upper transfer guide 14 that does not come into contact with the recording material P will be generated in a predetermined width ("b" in FIG. 7(b)) in the longitudinal direction, as shown in FIG. 7(b). The upper transfer guide 14 having a recessed groove as shown in FIG. 7(b) is referred to as Modification Example 1.

[0079] It is also assumed that a rib shape is provided on a part of the upper transfer guide 14, for example, for the purpose of improving the dimensional accuracy for conveying the recording material P. In this case, it is assumed that, as shown in Fig. 7(c), for example, the upper transfer guide 14 is provided with ribs protruding toward the surface of the recording material P at predetermined intervals in the longitudinal direction ("c" in Fig. 7(c)). The upper transfer guide 14 having the ribs as shown in Fig. 7(c) is referred to as Modification Example 2.

[0080] Furthermore, for example, in order to reduce warping of the transfer upper guide 14, it is assumed that the width of the transfer upper guide 14 in the longitudinal direction is narrower than the image formation area. In this case, it is assumed that, for example, as shown in Fig. 7(d), an area where the transfer upper guide 14 does not exist will be generated within a predetermined range in the longitudinal direction ("d" in Fig. 7(d)) ​​in the image formation area (typically both ends). The transfer upper guide 14 whose contact width in the longitudinal direction is narrower than the image formation area as shown in Fig. 7(d) is referred to as Modification 3.

[0081] Table 3 shows the performance evaluation results of the above-mentioned modified example. The performance evaluation item is the back-end memory, and the evaluation method is the same as that described above.

[0082] [Table 3]

[0083] First, the evaluation results of the modified examples 1-1 to 1-3 having different groove widths b as modified example 1 having a groove shape will be described. The transfer upper guide 14 of modified example 1-1 has a groove with a relatively narrow width b=2 mm. In the configuration of modified example 1-1, the groove width b is narrow, so the transfer upper guide 14 has a sufficient effect of suppressing the peeling discharge. Alternatively, in the configuration of modified example 1-1, even if a small peeling discharge occurs, the trailing end memory is not visually confirmed. Therefore, in the configuration of modified example 1-1, the trailing end memory does not substantially occur. In the configuration of modified example 1-2 (b=5 mm), the trailing end memory does not substantially occur, as in modified example 1-1. The transfer upper guide 14 of modified example 1-3 has a groove with a relatively wide width b=10 mm. In the configuration of modified example 1-3, the groove width b is wide, so there is an area where the contact of the transfer upper guide 14 with the recording material P is insufficient, and the trailing end memory occurs slightly. As described above, when the upper transfer guide 14 has a groove as shown in FIG. 7(b), for example, it is desirable that the width b of the groove is about 5 mm or less.

[0084] Next, the evaluation results of modified examples 2-1 to 2-3, which have different rib widths c as modified example 2 having a rib shape, will be described. The transfer upper guide 14 of modified example 2-1 has ribs at a relatively wide interval of c=60 mm. In the configuration of modified example 2-1, the rib interval c is wide, so the effect of suppressing the peeling discharge by the transfer upper guide 14 was sufficient. Alternatively, in the configuration of modified example 2-1, even if the peeling discharge was generated in a small amount, the trailing end memory was not visually confirmed. Therefore, in the configuration of modified example 2-1, the trailing end memory did not substantially occur. In the configuration of modified example 2-2 (c=30 mm), similarly to modified example 2-1, the trailing end memory did not substantially occur. The transfer upper guide 14 of modified example 2-3 has ribs at a relatively narrow interval of c=10 mm. In the configuration of modified example 2-3, the rib interval c is narrow, so there is an area where the contact of the transfer upper guide 14 with the recording material P is insufficient, and the trailing end memory occurs slightly. As described above, when the upper transfer guide 14 has ribs as shown in FIG. 7(c), for example, it is desirable that the rib spacing c is approximately 30 mm or more.

[0085] Further, the evaluation results of modified examples 3-1 to 3-3, which have different widths d of the region where the transfer upper guide 14 does not exist at both ends of the image forming region as modified example 3, in which the contact width in the longitudinal direction is narrower than the image forming region, will be described. The transfer upper guide 14 of modified example 3-1 has a region where the transfer upper guide 14 does not exist at both ends of the image forming region in the longitudinal direction within a relatively narrow range of d = 5 mm. In the configuration of modified example 3-1, the region where the transfer upper guide 14 does not exist is narrow, so that the effect of suppressing peeling discharge by the transfer upper guide 14 is sufficient. Alternatively, in the configuration of modified example 3-1, even if a small peeling discharge occurs, the rear end memory is not visually confirmed. Therefore, in the configuration of modified example 3-1, the rear end memory does not substantially occur. In the configuration of modified example 3-2 (d = 10 mm), similarly to modified example 3-1, the rear end memory does not substantially occur. In modified example 3-3, the transfer upper guide 14 has a region where the transfer upper guide 14 does not exist at both ends of the image forming region in the longitudinal direction within a relatively wide range of d = 20 mm. In the configuration of modified example 3-3, the area where the transfer upper guide 14 does not exist is wide, so there is a wide area where the contact of the transfer upper guide 14 with the recording material P is insufficient, and slight trailing end memory occurs especially in the area at the end in the longitudinal direction. As described above, for example, when the contact width in the longitudinal direction is narrower than the image formation area as shown in Fig. 7(d), it is desirable that the width d of the area in the image formation area where the transfer upper guide 14 does not exist is about 10 mm or less.

[0086] Although the configuration of the modified example has been described using specific numerical values, the present invention is not limited to the numerical values ​​exemplified above. It is sufficient that the recording material charging member performs a predetermined frictional charging by contacting substantially the entire area of ​​the recording material P in the longitudinal direction and suppresses the trailing end memory. Therefore, even outside the numerical range exemplified above, it is possible to adopt a configuration in which the shape and dimensions of the recording material charging member are changed as long as the trailing end memory can be suppressed within an acceptable range.

[0087] (8-2) Evacuation configuration Next, a modified example of the retracting configuration of the upper transfer guide 14 will be described.

[0088] As described above, in the first embodiment, the guide rotation shaft 14a is disposed at the end of the upper transfer guide 14 on the upstream side (the resin nip portion NR side), and the tip portion 14b on the downstream side (the transfer nip portion NT side) of the upper transfer guide 14 rotates and retreats. However, it is assumed that it is difficult to dispose the guide rotation shaft 14a at the position as in the first embodiment, for example, from the viewpoint of the arrangement of various members and the smooth conveyance of the recording material P. In such a case, the configuration described below may be used. The left diagrams of Figs. 8(a) and (b) are schematic cross-sectional views of other modified examples of the upper transfer guide 14. The right diagram of Fig. 8(a) is a front view seen from the direction of the arrow α in the left diagram of Fig. 8(a), and the right diagram of Fig. 8(b) is a top view seen from the direction of the arrow β in the left diagram of Fig. 8(b).

[0089] 8(a), the guide rotation shaft 14a can be provided at the end of the upper transfer guide 14 on the downstream side (the transfer nip NT side). In this case, the end of the upper transfer guide 14 on the upstream side (the resin nip NR side) comes into contact with the recording material P, and this end rotates to retreat. The guide rotation shaft 14a may be provided at a position between the upstream end and the downstream end of the upper transfer guide 14.

[0090] Moreover, the upper transfer guide 14 is not limited to being rotatable, and may be slidably moved as shown in FIG. 8(b). In the configuration shown in FIG. 8(b), the image forming apparatus 1 is provided with a guide retraction groove 14e that supports and guides the upper transfer guide 14 so that it can slide. In this configuration, the upper transfer guide 14 is provided with an engagement portion 14f that is loosely fitted into the guide retraction groove 14e. In the illustrated example, two engagement portions 14f are provided at each of the four corners of the upper transfer guide 14, that is, at both longitudinal ends of the upstream end and at both longitudinal ends of the downstream end. In this configuration, the upper transfer guide 14 is pressed downward in the figure by a pressure spring (not shown) so as to come into contact with the recording material P. When the upper transfer guide 14 receives a reaction force from the recording material P, it can be slid upward in the figure along the guide retraction groove 14e and retracted. Even with this configuration, the tip portion 14b, which is the contact portion of the transfer upper guide 14 with the recording material P, can be moved in a direction intersecting the surface of the recording material P at the position opposite the transfer upper guide 14 (the surface of the recording material P transported from the resin nip portion NR to the transfer nip portion NT).

[0091] [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 of embodiment 1 are given the same reference numerals as those of embodiment 1, and detailed explanations are omitted.

[0092] In the first embodiment, the upper transfer guide 14 is configured to be able to rotate and move in a direction in which the tip 14b, which is the contact portion of the upper transfer guide 14 with the recording material P, intersects with the surface of the recording material P at the position facing the upper transfer guide 14. In contrast, in the present embodiment, the upper transfer guide is configured to be a deformable sheet-like member. In the present embodiment, the upper transfer guide is configured to be able to deform and move in a direction in which the tip 14b, which is the contact portion of the upper transfer guide 14 with the recording material P, intersects with the surface of the recording material P at the position facing the upper transfer guide 14.

[0093] FIG. 9 is a schematic cross-sectional view (cross-sectional view showing a cross section substantially perpendicular to the rotation axis direction of the photosensitive drum 2) of the vicinity of the transfer nip NT in this embodiment. In this embodiment, the image forming apparatus 1 has a sheet-like transfer upper guide 30. This transfer upper guide 30 has a predetermined length in the longitudinal direction disposed substantially parallel to the rotation axis direction of the photosensitive drum 2 and a predetermined thickness in the lateral direction substantially perpendicular to the longitudinal direction. In this embodiment, this transfer upper guide 30 is made of a thin resin film (sheet, film). In this embodiment, the material of the transfer upper guide 30 is PC, and the thickness is 350 μm. In this embodiment, the tip portion 30b of the transfer upper guide 30 on the downstream side (transfer nip NT side) contacts the recording material P. In an undeformed state, the downstream tip 30b of the upper transfer guide 30 is disposed below (toward the transfer roller 8) a straight line L1 passing through the resin nip portion NR and the transfer nip portion NT in the cross section shown in Fig. 9. In this embodiment, the sheet-like upper transfer guide 30 functions as a recording material charging member (recording material charging means). That is, in this embodiment, the sheet-like upper transfer guide 30 has a function of frictionally charging the recording material P to the same polarity as the charging polarity of the photosensitive drum 2, similar to the upper transfer guide 14 in the first embodiment.

[0094] The upper transfer guide 30 is held (supported) by a guide holding member 30a. In this embodiment, the upper transfer guide 30 is fixed by attaching the upstream end to the guide holding member 30a so that the downstream end protrudes from the guide holding member 30a by about 15 mm. This allows the tip 30b of the downstream side (transfer nip NT side) of the upper transfer guide 30 to move upward in the figure by deformation. The force required for deformation can be adjusted by adjusting the thickness of the sheet constituting the upper transfer guide 30 and the protrusion amount from the guide holding member 30a. In this embodiment, similar to the first embodiment, the tip 30b of the upper transfer guide 30 is set to deform upward in the figure when a force of 0.59 N is applied to the tip 30b of the upper transfer guide 30.

[0095] As described above, in this embodiment, the upper transfer guide 30 can frictionally charge the recording material P to the same polarity as the charging polarity of the photosensitive drum 2. This reduces peeling discharge when the recording material P passes through the transfer nip NT, thereby suppressing rear end memory. Also, in this embodiment, when, for example, thick paper or the like is transported as the recording material P, the upper transfer guide 30 deforms and retreats. This makes it possible to suppress excessive charging of the upper transfer guide 30 and suppress black dot images. That is, the effect achieved by the rotation of the upper transfer guide 14 in the first embodiment can be achieved by the deformation of the upper transfer guide 30 in this embodiment.

[0096] Table 4 shows the performance evaluation results of the upper transfer guide 30 of this embodiment. The performance evaluation items were the rear end memory and the black dot image, and the evaluation method was the same as that described in the first embodiment.

[0097] [Table 4]

[0098] In the configuration of this embodiment, similarly to Example 1, frictional charging of the recording material P was performed by the transfer upper guide 30, so no trailing edge memory occurred. Also, in the configuration of this embodiment, when thick paper is conveyed, the tip 30b of the transfer upper guide 30 can be deformed and retracted, so similarly to Example 1, excessive charging of the transfer upper guide 30 was suppressed, and no black dot images were generated.

[0099] In this embodiment, the material of the upper transfer guide 30 is PC, but it is not limited to this. As described in the first embodiment, any material that can triboelectrically charge the recording material P to a predetermined polarity can be used.

[0100] As for the shape of the upper transfer guide 30, as explained as a modified example of the first embodiment, it is preferable that the upper transfer guide 30 contacts substantially the entire area of ​​the recording material P in the longitudinal direction to perform a predetermined frictional charging and suppress the rear end memory. Therefore, as long as the rear end memory can be suppressed within an allowable range, the shape of the upper transfer guide 30 can be appropriately changed. For example, as explained as a modified example of the first embodiment, a groove shape (divided in the longitudinal direction), a rib shape, or a region where the upper transfer guide 30 does not exist at the end of the image forming region by shortening the longitudinal width may be provided.

[0101] In this manner, in this embodiment, similarly to the first embodiment, the contact portion 14b of the recording material charging member 14 moves due to the force received from the recording material P. Particularly, in this embodiment, the contact portion 14b of the recording material charging member 14 moves due to the deformation of the recording material charging member 14. In this embodiment, the recording material charging member 14 is configured so that the contact portion 14b can be located downstream in the direction from the surface side opposite to the image forming surface of the recording material P toward the image forming surface side when a second type of recording material P having a larger basis weight than the first type of recording material P is transported toward the transfer nip NT, compared to when the first type of recording material P is transported toward the transfer nip NT.

[0102] [Other Examples] Although the present invention has been described above with reference to specific embodiments, the present invention is not limited to the above-mentioned embodiments.

[0103] In the above-mentioned embodiment, the guide member for guiding the recording material has the function of the recording material charging member, but the image forming apparatus may have a recording material charging member separate from the guide member. FIG. 10 is a schematic cross-sectional view of the vicinity of the transfer nip portion in an image forming apparatus having a recording material charging member separate from the guide member. In FIG. 10, elements having the same or corresponding functions as those in the above-mentioned embodiment are given the same reference numerals as in the first embodiment. The image forming apparatus 1 shown in FIG. 10 has a recording material charging member 31 that contacts the image forming surface of the recording material P downstream of the upper transfer guide 14 and upstream of the lower transfer guide 15. The recording material charging member 31 contacts the recording material P at a contact portion 31b formed by a surface facing the recording material P. The recording material charging member 31 is pressed downward in the figure by a pressure spring 31a so as to contact the recording material P. Even with such a configuration, the same effect as in the above-mentioned embodiment can be obtained. In the image forming apparatus 1 shown in FIG. 10, the upper transfer guide 14 may be fixedly disposed in the same manner as that shown in FIG. 6(b).

[0104] In the above embodiment, when a recording material having high rigidity such as thick paper is transported, the recording material charging member is retracted in a state of contact with the recording material, thereby suppressing strong rubbing between the recording material and the recording material due to excessive load. In contrast, when a recording material having high rigidity such as thick paper is transported, the recording material charging member may be prevented from contacting the recording material. FIG. 11 is a schematic diagram of a main part of an image forming apparatus capable of moving the recording material charging member so as not to contact the recording material. In FIG. 11, elements having the same or corresponding functions as those in the above embodiment or FIG. 10 are given the same reference numerals as those in the first embodiment. The image forming apparatus 1 shown in FIG. 11 has a recording material charging member 31 and a pressure spring 31a similar to the configuration shown in FIG. 10. The image forming apparatus 1 shown in FIG. 11 has a movement mechanism 40 capable of moving the recording material charging member 31 upward in the figure (in a direction away from the transport trajectory of the recording material P) against the urging force of the pressure spring 31a. The moving mechanism 40 has a cam 41 that can rotate by contacting with the receiving portions 31c provided at both ends of the recording material charging member 31 in the longitudinal direction, and a driving unit 42 that drives the cam 41 to rotate. The driving unit 42 is configured to have a motor as a driving source, a drive transmission member, and the like. The control unit 120 can control the driving unit 42 to place the recording material charging member 31 at a first position (contact position) where the recording material charging member 31 can contact the recording material P, and at a second position (separated position) where the recording material charging member 31 does not contact the recording material P. The cam 41 can push the recording material charging member 31 upward in the figure against the biasing force of the pressure spring 31a, thereby moving the recording material charging member 31 from the first position to the second position. In addition, by releasing the pressure of the cam 41, the recording material charging member 31 can be allowed to move from the second position to the first position by its own weight and the pressure force of the pressure spring 31a.

[0105] For example, the control unit 120 can control the recording material charging member 31 to be placed at a second position where it does not contact the recording material P when a predetermined type of recording material P is conveyed based on information about the type of recording material P included in a print command input from an external device. For example, when a recording paper having a basis weight equal to or greater than a predetermined threshold value is conveyed, the recording material charging member 31 can be placed at the second position. This can prevent the recording material P from being triboelectrically charged by the recording material charging member 31 when a recording material P having high rigidity, such as thick paper, is conveyed. Note that, for example, when thick paper is used as the recording material P, the conveying speed of the recording material P may be lowered compared to when plain paper is used as the recording material P in order to improve fixability. When the conveying speed of the recording material P is low, the transfer voltage can be lowered compared to when the conveying speed of the recording material P is high, and peeling discharge may be less likely to occur when the recording material P passes through the transfer nip NT. In such a case, even if frictional charging of the recording material P by the recording material charging member 31 is not performed when, for example, thick paper is used as the recording material P as described above, the problem of the trailing end memory is unlikely to occur. In this manner, the image forming apparatus 1 may have a moving mechanism 40 capable of moving the recording material charging member 31 to a first position where the contact portion 31b can contact the recording material P and a second position where the contact portion 31b does not contact the recording material P. The moving mechanism 40 can place the recording material charging member 31 at the first position when a first type of recording material P is transported toward the transfer nip NT, and can place the recording material charging member 31 at the second position when a second type of recording material P having a basis weight larger than that of the first type of recording material P is transported toward the transfer nip NT.

[0106] In addition, in Example 1 and other examples, a configuration has been described in which the recording material charging member is urged in the direction of contact with the recording material by a pressure spring. However, depending on the configuration (weight, etc.) of the recording material charging member, the recording material charging member may be urged only by gravity (its own weight).

[0107] Furthermore, the image carrier is not limited to a drum type, but may be an endless belt type or the like.

[0108] Furthermore, the transfer member is not limited to a roller-shaped one, but may be a brush-shaped, blade-shaped, pad-shaped, film-shaped or the like. [Explanation of symbols]

[0109] 1. Image forming device 2 Photosensitive drum 3. Charging roller 4. Laser Scanner 5. Developing device 6 Cleaning device 8 Transfer roller 14 Transfer top guide 14a Guide pivot shaft

Claims

1. A rotatable image carrier, A charging device for charging the surface of the image carrier to a predetermined polarity, A developing apparatus that supplies toner charged with the same polarity as the predetermined polarity to the surface of the image carrier which has been charged by the charging device, thereby forming a toner image on the surface of the image carrier, A transfer member that contacts the surface of the image carrier to form a transfer nip between itself and the image carrier, and transfers the toner image from the image carrier to the recording material passing through the transfer nip when a transfer voltage is applied, The transfer member is provided with an application unit that applies the transfer voltage having the opposite polarity to the predetermined polarity, In an image forming apparatus having, The recording material charging member is positioned upstream of the transfer nip in the transport direction of the recording material being held and transported by the transfer nip, and is capable of contacting the image-forming surface, which is the surface on which the toner image is transferred, of the recording material being transported toward the transfer nip, and is capable of charging the image-forming surface to the same polarity as the predetermined polarity by frictional charging, The contact portion of the recording material charging member with the recording material is movable in a direction intersecting the surface of the recording material at a position facing the recording material charging member. The contact portion moves due to the force received from the recording material. An image forming apparatus characterized in that the contact portion has a biasing means that biases the recording material charging member in a direction from the image forming surface side of the recording material toward the side opposite to the image forming surface.

2. The image forming apparatus according to claim 1, characterized in that the recording material charging member is positioned upstream of the transfer nip portion in the transport direction, closest to the transfer nip portion, and downstream of the transport member that transports the recording material.

3. The image forming apparatus according to claim 1, characterized in that the recording material charging member has the function of guiding the recording material that is conveyed toward the transfer nip portion.

4. The image forming apparatus according to claim 1, characterized in that the image forming surface of the recording material comes into contact with the surface of the image carrier after the recording material charging member.

5. The image forming apparatus according to claim 1, characterized in that the contact portion moves by rotation or sliding movement of the recording material charging member.

6. The biasing means is configured such that when a second type of recording material having a larger basis weight than the first type of recording material is conveyed toward the transfer nip portion, the contact portion can be positioned downstream in the direction from the side of the recording material opposite to the image forming surface toward the image forming surface, compared to when a first type of recording material is conveyed toward the transfer nip portion. This is the image forming apparatus according to claim 1.

7. The image forming apparatus according to claim 1, characterized in that the contact portion moves due to the deformation of the recording material charging member.

8. The image forming apparatus according to claim 7, characterized in that the recording material charging member is configured such that the contact portion can be positioned downstream in the direction from the side of the recording material opposite to the image forming surface toward the image forming surface when a second type of recording material having a larger basis weight than the first type of recording material is conveyed toward the transfer nip portion, compared to when a first type of recording material is conveyed toward the transfer nip portion.

9. The image forming apparatus according to claim 1, characterized in that it has a moving mechanism capable of moving the recording material charging member to a first position in which the contact portion can contact the recording material and a second position in which the contact portion does not contact the recording material.

10. The image forming apparatus according to claim 9, characterized in that the moving mechanism positions the recording material charging member at the first position when the first type of recording material is transported toward the transfer nip section, and positions the recording material charging member at the second position when the second type of recording material having a larger basis weight than the first type of recording material is transported toward the transfer nip section.

11. The image forming apparatus according to any one of claims 1 to 10, characterized in that the recording material charging member is capable of contacting at least the rear end of the recording material in the transport direction.

12. The image forming apparatus according to any one of claims 1 to 10, characterized in that the recording material charging member is able to contact the recording material over substantially the entire area of ​​the image forming region in a direction perpendicular to the transport direction.

13. The image forming apparatus according to any one of claims 1 to 10, characterized in that the application unit applies a voltage to the transfer member that is different from the voltage applied to the transfer member when the central part of the recording material is passing through the transfer nip in the transport direction, when the rear end of the recording material in the transport direction passes through the transfer nip.

14. The image forming apparatus according to claim 13, characterized in that the application unit applies a voltage to the transfer member when the rear end of the recording material in the transport direction passes through the transfer nip, the voltage being the opposite polarity to the predetermined polarity and having an absolute value smaller than the absolute value of the voltage applied to the transfer member when the central part of the recording material in the transport direction passes through the transfer nip.

15. The image forming apparatus according to claim 13, characterized in that the application unit applies a voltage of the same polarity as the predetermined polarity to the transfer member when the rear end of the recording material in the transport direction passes through the transfer nip unit.