Image forming device

Abstract

To transfer a toner image stably regardless of the type or basis weight of the recording material. [Solution] An image forming apparatus 100 includes image forming units 10y, 10m, 10c, and 10k that form images on an intermediate transfer belt 107, a secondary transfer unit TR2 that transfers the image formed on the intermediate transfer belt 107 to a recording material S, a pre-transfer contact unit 200 that is provided upstream of the secondary transfer unit TR2 in the conveyance direction of the recording material S and has a contact member 201 for grounding the recording material S being conveyed to the secondary transfer unit TR2, and a control unit 121. The control unit 121 controls the pre-transfer contact unit 200 to move the contact member 201 between a first position where it does not contact the recording material S and a second position where it does contact the recording material S. The control unit 121 positions the contact member 201 at either the first position or the second position depending on the characteristics of the recording material S.

Description

[Technical Field]

[0001] The present invention relates to an image forming apparatus such as a printer, a copying machine, or a multifunction machine. [Background technology]

[0002] Image forming devices such as electrophotographic devices and electrostatic recording devices form toner images by developing electrostatic latent images formed on photosensitive drums. The toner images are then transferred to recording materials. Transfer to the recording materials is performed either directly from the photosensitive drum or indirectly from the photosensitive drum via an intermediate transfer belt. Here, we will explain the case where transfer is performed indirectly via an intermediate transfer belt.

[0003] The transfer section that transfers the toner image from the intermediate transfer belt to the recording material includes an inner transfer roller provided inside the intermediate transfer belt and an outer transfer roller provided outside the intermediate transfer belt. During transfer, a predetermined bias voltage is applied to the inner transfer roller. The bias voltage is controlled to be constant during transfer. The constant voltage control adjusts the current flowing through the recording material to be constant during transfer.

[0004] Metal-metallized paper with a metal layer is sometimes used as a recording material. Stable transfer of toner images onto metal-metallized paper is desirable. However, metal-metallized paper has higher conductivity and lower impedance than paper recording materials without a metal layer. Therefore, even if the transfer unit is controlled to a constant voltage, the current flowing through the metal-metallized paper may be unstable, making it difficult to stably transfer a toner image. Patent Document 1 discloses an image forming apparatus that applies a stable current to the metal-metallized paper during transfer. This image forming apparatus has a contact member near the transfer unit, and the recording material is grounded by contacting the contact member. The contact member actively forms a current path that was not previously necessary, stabilizing the current flowing through the recording material. This results in good transfer of the toner image. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Japanese Patent Application Laid-Open No. 2018-60072 Summary of the Invention [Problem to be solved by the invention]

[0006] In addition to metallized paper, image forming apparatuses may also form images on recording materials such as thin paper with a small basis weight. The transfer unit sandwiches and conveys the recording material between an inner transfer roller and an outer transfer roller during transfer. Because a recording material with a small basis weight has a smaller weight than a recording material with a large basis weight, being sandwiched between the inner transfer roller and the outer transfer roller can cause the trailing edge of the recording material to warp. This warping causes the recording material with a small basis weight to separate from the contact member during transfer. When the recording material separates from the contact member, the current flowing through the transfer unit fluctuates, potentially preventing stable transfer of the toner image.

[0007] Although a contact member provided near the transfer unit in this manner is effective for stable transfer of a toner image to a recording material with low impedance, such as metallized paper, it is not suitable for stable transfer of a toner image to a recording material with low basis weight, such as thin paper. For this reason, there is a demand for an image forming apparatus that can stably transfer a toner image regardless of the type or basis weight of the recording material. [Means for solving the problem]

[0008] The image forming apparatus of the present invention is characterized by comprising an image carrier, an image forming means for forming an image on the image carrier, a transfer means for transferring the image formed on the image carrier to a recording material, a transport means for transporting the recording material to the transfer means, a contact means arranged upstream of the transfer means in the transport direction of the recording material and having a contact member for grounding the recording material transported to the transfer means, a moving means for moving the contact member between a first position where it does not contact the recording material transported to the transfer means and a second position where it does contact the recording material, and a control means for positioning the contact member at either the first position or the second position depending on the characteristics of the recording material. [Effects of the Invention]

[0009] According to the present invention, a stable transfer of a toner image is possible regardless of the type or basis weight of the recording material. [Brief explanation of the drawings]

[0010] [Figure 1] FIG. 1 is a diagram illustrating the configuration of an image forming apparatus. [Figure 2] FIG. 4 is a diagram illustrating the configuration of a pre-transfer contact portion. [Figure 3] FIG. 4 is a diagram of the secondary transfer unit as seen from the side of the inner secondary transfer roller. [Figure 4] FIG. [Figure 5] An explanatory diagram of metallized paper. [Figure 6] 10 is a diagram illustrating an unstable current in a paper passing portion. [Figure 7] 10 is a flowchart showing a process during image formation. [Figure 8] 5 is an explanatory diagram of a current when a toner image is transferred onto a recording material. [Figure 9] 10 is a flowchart showing a process during image formation. DETAILED DESCRIPTION OF THE INVENTION

[0011] Hereinafter, preferred embodiments of the present invention will be described with reference to the accompanying drawings.

[0012] 1 is a diagram showing the configuration of an image forming apparatus according to this embodiment. The image forming apparatus 100 according to this embodiment includes four image forming units. The first image forming unit 10y forms yellow images, the second image forming unit 10m forms magenta images, the third image forming unit 10c forms cyan images, and the fourth image forming unit 10k forms black images. The image forming units 10y, 10m, 10c, and 10k have the same configuration except for the color of the toner used as the developer.

[0013] 1, the suffix y indicates a configuration for forming a yellow image, the suffix m indicates a configuration for forming a magenta image, the suffix c indicates a configuration for forming a cyan image, and the suffix k indicates a configuration for forming a black image. Here, the configuration of the first image forming unit 10y that forms a yellow image will be described, and descriptions of the configurations of the second, third, and fourth image forming units 10m, 10c, and 10k that form images of other colors will be omitted.

[0014] The first image forming unit 10y includes a photosensitive drum 101y, a charging roller 102y, an exposure unit 103y, a developing unit 104y, a primary transfer roller 105y, and a photosensitive cleaner 106y. The photosensitive drum 101y is a first image carrier that rotates in the direction of the arrow and has its surface uniformly charged to a predetermined polarity and potential by the charging roller 102y. The exposure unit 103y scans the charged surface of the photosensitive drum 101y with a laser beam Ey modulated based on image data representing the image to be formed. This forms an electrostatic latent image on the surface of the photosensitive drum 101y. The developing unit 104y develops the electrostatic latent image using yellow toner to form a toner image on the surface of the photosensitive drum 101y. Similarly, a magenta toner image is formed on the photosensitive drum 101m of the second image forming unit 10m, a cyan toner image is formed on the photosensitive drum 101c of the third image forming unit 10c, and a black toner image is formed on the photosensitive drum 101k of the fourth image forming unit 10k.

[0015] An intermediate transfer belt 107, which is a second image carrier, is disposed below the first to fourth image forming units 10y, 10m, 10c, and 10k. The intermediate transfer belt 107 is driven to rotate by an inner secondary transfer roller 109, which functions as a drive roller. An outer secondary transfer roller 108 is disposed opposite the inner secondary transfer roller 109 across the intermediate transfer belt 107. The inner secondary transfer roller 109 and the outer secondary transfer roller 108 form a secondary transfer unit TR2. An intermediate transfer belt cleaner 110 is disposed upstream of the first image forming unit 10y in the rotation direction of the intermediate transfer belt 107.

[0016] The toner images formed on the photosensitive drums 101y, 101m, 101c, and 101k are transferred onto the intermediate transfer belt 107 by primary transfer rollers 105y, 105m, 105c, and 105k in a superimposed manner in synchronization with the rotation of the intermediate transfer belt 107. Toner remaining on the photosensitive drums 101y, 101m, 101c, and 101k after transfer is removed by photosensitive cleaners 106y, 106m, 106c, and 106k. The toner images transferred onto the intermediate transfer belt 107 are transported toward the inner secondary transfer roller 109 by the rotation of the intermediate transfer belt 107.

[0017] The image forming apparatus 100 includes a paper feed cassette 111 that stores recording materials S, such as paper, on which an image is formed. To transport the recording materials S stored in the paper feed cassette 111, the image forming apparatus 100 also includes a guide member 115, a pickup roller 112, a pair of paper feed rollers 113, a pair of registration rollers 114, pairs of conveyance rollers 117 and 118, and a discharge roller 119. The pickup roller 112 feeds the recording materials S stored in the paper feed cassette 111 to a conveyance path. The pair of paper feed rollers 113 separates the recording materials S fed by the pickup roller 112 into individual sheets and conveys them to the pair of registration rollers 114. The pair of registration rollers 114 corrects skew of the recording materials S. The pair of registration rollers 114 conveys the recording materials S to the secondary transfer unit TR2 via the guide member 115 in accordance with the timing at which the toner image transferred to the intermediate transfer belt 107 is conveyed to the secondary transfer unit TR2.

[0018] A pre-transfer contact section 200 including a contact member 201 and a switching section 400 is provided upstream of the secondary transfer section TR2 in the conveyance direction of the recording material S. The pre-transfer contact section 200 will be described in detail later.

[0019] The four-color toner image carried on the intermediate transfer belt 107 is transferred to a recording material S passing between a secondary transfer inner roller 109 and a secondary transfer outer roller 108. A predetermined high voltage (bias voltage) is applied to either the secondary transfer inner roller 109 or the secondary transfer outer roller 108, thereby transferring the toner image from the intermediate transfer belt 107 to the recording material S. Any toner remaining on the intermediate transfer belt 107 after transfer is scraped off and collected by an intermediate transfer belt cleaner 110.

[0020] The image forming apparatus 100 includes a fixing device 116 that fixes the toner image onto the recording material S. The recording material S onto which the toner image has been transferred is transported to the fixing device 116 by a secondary transfer outer roller 108. The fixing device 116 fixes the toner image onto the recording material S by applying heat and pressure. The recording material S onto which the toner image has been fixed is discharged to the outside of the image forming apparatus 100 via a pair of conveying rollers 117 and 118 and a discharge roller 119. This results in a recording material S (a product) onto which a color image has been formed.

[0021] The image forming apparatus 100 is provided with a control unit 121 for controlling the operation of each of the components described above. The image forming apparatus 100 also has an operation unit 120 as a user interface having an input interface and an output interface. The operation unit 120 has various keys and a touch panel as an input interface. The operation unit 120 has a display and a speaker as an output interface. The control unit 121 accepts instructions from the user via the operation unit 120 or an external device via a network (not shown). When the control unit 121 accepts a print job command (image formation command), it controls the operation of each component as described above to perform a print operation (image formation operation) on the recording material S.

[0022] 2 is an explanatory diagram of the configuration of the pre-transfer contact portion 200. The solid arrows in Fig. 2 indicate the rotation direction of the intermediate transfer belt 107 and the rotation directions of the outer secondary transfer roller 108 and the inner secondary transfer roller 109 when the toner image is transferred to the recording material S. The dashed arrow indicates the conveyance direction of the recording material S.

[0023] As described above, the pre-transfer contact section 200 includes the contact member 201 and the switching section 400. The contact member 201 is a conductive member that is always grounded. The switching section 400 positions the contact member 201 either at a position where it contacts one side (the side on which the toner image is not transferred) of the recording material S conveyed from the guide member 115, or at a position where it does not contact the recording material S, based on a switching signal. The switching signal is transmitted from the control section 121. The detailed configuration of the switching section 400 will be described later. The contact member 201 is positioned so that it contacts the recording material S at one or more points when the recording material S contacts the outer secondary transfer roller 108.

[0024] The outer secondary transfer roller 108 and inner secondary transfer roller 109 of the secondary transfer portion TR2 form a nip portion 210. At the nip portion 210, the intermediate transfer belt 107 and the recording material S are sandwiched and conveyed during transfer. In this embodiment, a bias voltage of the same polarity as the toner (negative potential in this embodiment) is applied to the inner secondary transfer roller 109, thereby transferring the toner image from the intermediate transfer belt 107 to the recording material S. In order to stabilize the current flowing through the recording material S during transfer, the bias voltage is controlled to be a constant voltage. The outer secondary transfer roller 108 and the contact member 201 are disposed at a distance equal to or greater than a predetermined distance so that discharge does not occur when a bias voltage is applied during transfer.

[0025] The lengths of the outer secondary transfer roller 108 and the inner secondary transfer roller 109 in a direction intersecting the conveyance direction of the recording material S (hereinafter referred to as the "width direction") are longer than the width direction of the recording material S. As a result, in the nip portion 210, a current flows through the portion where the recording material S does not pass (non-paper passing portion current) and a current flows through the portion where the recording material S passes (paper passing portion current). The current value of the current flowing through the secondary transfer portion TR2 is the sum of the non-paper passing portion current and the paper passing portion current. The ratio of the non-paper passing portion current to the paper passing portion current varies depending on the resistance value, which is determined by the size and material of the recording material S. Therefore, constant current control, which keeps the current flowing through the secondary transfer portion TR2 constant, makes it difficult to stabilize the paper passing portion current. For these reasons, constant voltage control is used for the secondary transfer portion TR2.

[0026] 3 is a view of the secondary transfer portion TR2 as seen from the side of the inner secondary transfer roller 109. The contact member 201 is cylindrical, with the width direction as its axial direction. In this case, the contact member 201 and the recording material S come into linear contact in the width direction. In order to suppress fluctuations in the load impedance of the secondary transfer portion TR2, it is preferable that the length of the contact member 201 in the width direction be longer than the length of the recording material S of the maximum size (A3 size) allowed for use in this image forming apparatus. This suppresses fluctuations in the length of the contact portion with the recording material S, even if the recording material S is conveyed with an expected amount of deviation in the width direction.

[0027] The material of the contact member 201 needs to be a material with lower impedance than the guide member 115, and in this embodiment, it is made of, for example, a metal material. With this configuration, it is possible to stabilize the load impedance seen from the secondary transfer portion TR2 regardless of the conveying position of the recording material S.

[0028] 4 is an explanatory diagram of the switching unit 400. The switching unit 400 has a guide frame 401 with a support member 402 on the inside, and a movement mechanism that moves the contact member 201 using the support member 402. The movement mechanism has a cam 403, a power transmission belt 404, a driven gear 405, a drive gear 406, a motor 407, and a motor drive circuit 408.

[0029] The guide frame 401 has two walls that are arranged to sandwich the support member 402 in the conveying direction of the recording material S. The support member 402 supports the contact member 201 between the two walls of the guide frame 401. The support member 402 is movable toward the conveying path (in the vertical direction in the drawing) along the guide frame 401. The movement of the support member 402 allows the contact member 201 to move in a direction toward or away from the conveying path.

[0030] The motor 407 is a drive source for moving the contact member 201 and is driven by a motor drive circuit 408. The drive force output from the motor 407 is transmitted to the cam 403 via a drive gear 406, a driven gear 405, and a power transmission belt 404. The cam 403 is a rotating cam member, and its cross section perpendicular to its rotation axis is elliptical. A rotation axis 403A is provided parallel to the longitudinal direction of the contact member 201. The rotation axis 403A is provided at a position offset from the center C of the ellipse of the cam 403. The cam 403 is a rotating member that rotates around the rotation axis 403A in response to the transmitted drive force. The cam 403 abuts against the lower surface of the support member 402, and by rotating, causes the support member 402 to reciprocate up and down along the guide frame 401. The up and down movement of the support member 402 controls the contact state (contact / non-contact) of the contact member 201 with the recording material S.

[0031] The control unit 121 moves the contact member 201 between the contact position and the separated position by sending a switching signal to the motor drive circuit 408. The contact position is a position where the contact member 201 comes into contact with the recording material S being conveyed along the conveyance path, and the separated position is a position where the contact member 201 is retracted from the conveyance path and does not come into contact with the recording material S. The switching signal takes two different levels. When the switching signal is one value (high level), the motor drive circuit 408 drives the motor 407 to position the contact member 201 at the contact position. This allows the contact member 201 to come into contact with the recording material S. When the switching signal is the other value (low level), the motor drive circuit 408 drives the motor 407 to position the contact member 201 at the separated position. This prevents the contact member 201 from coming into contact with the recording material S.

[0032] 4A shows a state in which the support member 402 is separated from the conveyance path with the recording material S sandwiched in the nip portion 210, and the contact member 201 is disposed at a position (separated position A) where it does not contact the recording material S. FIG. 4B shows a state in which the support member 402 is approached to the conveyance path with the recording material S sandwiched in the nip portion 210, and the contact member 201 is disposed at a position (contact position B) where it contacts the recording material S. In this way, the contact member 201 can be disposed at a first position (separated position A) where it does not contact the recording material S, and a second position (contact position B) where it contacts the recording material S, depending on the rotation of the cam 403.

[0033] Note that, as long as the cross section of cam 403 in a direction perpendicular to the rotation axis is elliptical, rotation central axis 403A may be at the same position as center C. In this case, when the long axis portion of cam 403 abuts against guide frame 401, contact member 201 is located at abutment position B, and when the short axis portion of cam 403 abuts against guide frame 401, contact member 201 is located at separation position A. Also, cam 403 may be configured so that the cross section in a direction perpendicular to the rotation axis is circular and rotation central axis 403A is provided at a position shifted from center C. In this case, when the position farthest from rotation central axis 403A of cam 403 abuts against guide frame 401, contact member 201 is located at abutment position B, and when the position closest to rotation central axis 403A of cam 403 abuts against guide frame 401, contact member 201 is located at separation position A. Note that cam 403 does not have to be configured to rotate, but may be configured to move in one dimension.

[0034] The image forming apparatus 100 configured as described above can form an image on a recording material S having a metal layer, such as metal-deposited paper. FIG. 5 is an explanatory diagram of metal-deposited paper. FIG. 5 shows a cross-sectional view of metal-deposited paper. Metal-deposited paper generally consists of three layers. In the example of FIG. 5, it consists of three layers: a plain paper layer Pp, a metal layer Pm, and a coating layer Pc. The second metal layer Pm is made of a metal such as aluminum, and is vapor-deposited on the first plain paper layer Pp. Because the metal layer Pm is made of metal, it has better conductivity and much lower impedance than the plain paper layer Pp.

[0035] It is known that when metallized paper passes through the nip portion 210, the current at the paper passing portion is unstable if the contact member 201 is not provided, even if the secondary transfer portion TR2 is under constant voltage control. FIG. 6 is an explanatory diagram showing why the current at the paper passing portion is unstable. FIG. 6 shows a conventional configuration that does not include the contact member 201, and guide members A and B are provided upstream of the secondary transfer portion TR2 in the conveying direction of the recording material S. The metallized paper is conveyed to the secondary transfer portion TR2 via the guide members A and B.

[0036] Guide member A is made of a material with lower impedance than the outer secondary transfer roller 108 and is grounded. Guide member B is made of a material with a predetermined impedance (here, resistor 601). The solid arrows in Figure 6 indicate the paths of currents (non-paper passing section current, paper passing section current) that flow through secondary transfer unit TR2. The dashed arrows indicate the transport direction of recording material S (metal-deposited paper).

[0037] 6(a) shows the state where the leading edge of the metal-deposited paper reaches the nip portion 210 of the secondary transfer portion TR2. Because the impedance of the guide member A is smaller than the impedance of the outer secondary transfer roller 108, the current flows from the grounded guide member A through the metal layer Pm of the metal-deposited paper to the inner secondary transfer roller 109.

[0038] At this time, the load impedance seen from the secondary transfer unit TR2 is the sum of the impedances of the guide member A, the metallized paper, the intermediate transfer belt 107, and the inner secondary transfer roller 109. In other words, the load impedance is the sum of the impedances below. Since the secondary transfer unit TR2 is under constant voltage control, a current corresponding to the load impedance flows. Contact impedance at the point where guide member A and metallized paper come into contact Impedance across each layer of metallized paper Contact impedance at the contact point between the intermediate transfer belt 107 and the inner secondary transfer roller 109 Impedance of the inner secondary transfer roller 109

[0039] Figure 6(b) shows the state in which the metal-deposited paper has been transported further from the state in Figure 6(a). The trailing edge of the metal-deposited paper has passed through guide member A, so it is not in contact with guide member A. Therefore, the current flowing through secondary transfer unit TR2 flows from guide member B, which is grounded via resistor 601, to inner secondary transfer roller 109 via metal layer Pm of the metal-deposited paper. At this time, the load impedance seen from secondary transfer unit TR2 is the sum of the following impedances: Resistance 601 of guide member B Contact impedance at the point where guide member B and metallized paper come into contact Impedance across each layer of metallized paper Contact impedance at the contact point between the intermediate transfer belt 107 and the outer secondary transfer roller 108 Impedance of the inner secondary transfer roller 109

[0040] Because the secondary transfer unit TR2 is subjected to constant voltage control, a current corresponding to the load impedance flows. The load impedance in FIG. 6(b) is smaller than the load impedance in FIG. 6(a) because the resistor 601 of the guide member B is involved, resulting in a smaller current at the paper-passing section. Due to this mechanism, the member that comes into contact with the metal-deposited paper changes depending on the transport position of the metal-deposited paper, causing fluctuations in the current at the paper-passing section that flows through the toner. This causes the transferability of the toner image to be impaired, resulting in transfer defects. Therefore, in this embodiment, a contact member 201 is provided between the guide member B and the outer secondary transfer roller 108 to stabilize the current flowing through the recording material S, thereby maintaining the transferability of the toner image.

[0041] 7 is a flowchart showing the process during image formation. This process is for transferring a toner image onto the recording material S without causing transfer defects, regardless of the type of recording material S.

[0042] When the image forming apparatus 100 starts operation, the control unit 121 checks whether a print job command has been received from the operation unit 120 of the image forming apparatus 100 or from an external device via a network (S501). If no command has been received (S501: N), the control unit 121 waits until a command is received.

[0043] When a command is acquired (S501: Y), the control unit 121 checks the paper type of the recording material S stored in the paper feed cassette 111 specified in the print job, and determines whether the recording material S is metallized paper (S502). The paper type of the recording material S is checked, for example, from a setting value set by the user using the operation unit 120 when the recording material S is stored in the paper feed cassette 111. The setting value includes, for example, characteristic information of the recording material S (paper type, size, etc.).

[0044] If the recording material S is other than metallized paper (S502: Y), the recording material S does not need to be grounded by the contact member 201. To this end, the control unit 121 sends a first-level (low-level) switching signal to the switching unit 400, causing the switching unit 400 to move the contact member 201 to the separated position A (see FIG. 4(a)) (S503, S504). The first-level switching signal is a control signal for positioning the contact member 201 at the separated position A. The recording material S is not grounded because the contact member 201 does not come into contact with it.

[0045] If the recording material S is metallized paper (S502: N), the recording material S needs to be grounded by the contact member 201. To this end, the control unit 121 sends a second-level (high-level) switching signal to the switching unit 400, causing the switching unit 400 to position the contact member 201 at contact position B (see FIG. 4(b)) (S505, S506). The second-level switching signal is a control signal for positioning the contact member 201 at contact position B. The recording material S is grounded because the contact member 201 comes into contact with it.

[0046] After moving the contact member 201 to the separated position A or the abutting position B, the control unit 121 starts a print operation to form an image in accordance with the print job (S507). When the print operation instructed by the print job is completed, the control unit 121 ends a series of processes for image formation in accordance with the print job.

[0047] In this manner, the image forming apparatus 100 of this embodiment switches the position of the contact member 201 in the pre-transfer contact section 200 between a separation position A and a contact position B depending on the paper type of the recording material S. Specifically, when the recording material S is a first paper type (e.g., metallized paper), the contact member 201 is located at a contact position B and contacts the recording material S. When the recording material S is a second paper type having a higher impedance than the first paper type, the contact member 201 is located at a separation position A and does not contact the recording material S. Therefore, when the recording material S is of the first paper type, current flows through the contact member 201, and fluctuations in the paper-passing section current are suppressed. When the recording material S is of the second paper type, current flows through the outer secondary transfer roller 108, and fluctuations in the paper-passing section current are suppressed. Because fluctuations in the paper-passing section current are suppressed in this manner, stable transfer of a toner image is possible regardless of the paper type of the recording material S being passed.

[0048] (Variation) The above example is a configuration in which fluctuations in the current at the paper passing section are suppressed by switching whether or not the contact member 201 is in contact with the recording material S depending on the paper type of the recording material S. In this example, fluctuations in the current at the paper passing section are suppressed by switching whether or not the contact member 201 is in contact with the recording material S depending on the basis weight of the recording material S.

[0049] In addition to metallized paper, the image forming apparatus 100 may also form images on thin paper or other recording materials S with a small basis weight. FIG. 8 is an explanatory diagram of the current when a toner image is transferred to a recording material S with a small basis weight, such as plain paper or thin paper. The solid arrows in FIG. 8 indicate the paths of the currents (currents in non-paper-passing areas and paper-passing areas) that flow through the secondary transfer unit TR2. The dashed arrows indicate the transport direction of the recording material S (plain paper or thin paper). Thin paper has a smaller weight than recording materials with a large basis weight, such as metallized paper. For this reason, when plain paper or thin paper is used as the recording material, being sandwiched in the nip portion 210 may cause the recording material S to warp near the trailing edge in the transport direction.

[0050] 8(a) is a diagram of a conventional configuration in which no contact member 201 is provided. In this configuration, even if the recording material S is warped, the current path is uniquely determined in the direction indicated by the arrow, so fluctuations in the current in the sheet passing section are small.

[0051] FIG. 8B shows a configuration in which the contact member 201 is provided as in this embodiment. When the recording material S warps, it loses contact with the contact member 201. As a result, the current path at the secondary transfer portion TR2 changes depending on whether the recording material S is in contact with the contact member 201 or in a separated state. In other words, the load impedance seen from the secondary transfer portion TR2 changes. As a result, the fluctuation in the current at the paper passing portion increases, and the transferability of the toner image onto the recording material S decreases.

[0052] That is, when a toner image is transferred to a recording material having a basis weight greater than a predetermined value, transferability is maintained better in a configuration that includes the contact member 201. When a toner image is transferred to a recording material having a small basis weight, transferability is maintained better in a configuration that does not include the contact member 201. Therefore, in this embodiment, the configuration shown in FIG. 2 is used, which maintains the transferability of the toner image regardless of the paper type or basis weight of the recording material S.

[0053] Fig. 9 is a flowchart showing the processing during image formation. This processing differs from the processing in Fig. 7 in the criteria by which the control unit 121 inputs a signal to the switching unit 400. This processing is processing for transferring a toner image onto the recording material S without causing transfer defects, regardless of the basis weight of the recording material S.

[0054] When the image forming apparatus 100 starts operation, the control unit 121 checks whether a print job command has been received from the operation unit 120 of the image forming apparatus 100 or from an external device via a network (S601). If no command has been received (S601: N), the control unit 121 waits until a command is received.

[0055] When a command is received (S601: Y), the control unit 121 checks the basis weight of the recording material S stored in the paper feed cassette 111 specified in the print job and compares the basis weight with a predetermined value (S602). Based on the comparison result between the basis weight and the predetermined value, the control unit 121 controls the input of a switching signal to the switching unit 400. The basis weight of the recording material S is checked, for example, from a setting value set by the user using the operation unit 120 when the recording material S is stored in the paper feed cassette 111. The setting value includes, for example, characteristic information of the recording material S (basis weight, size, etc.). An example of a recording material S with a basis weight equal to or less than the predetermined value is thin paper. An example of a recording material S with a basis weight greater than the predetermined value is metallized paper.

[0056] As a result of the comparison, for example, if the basis weight of the recording material S is equal to or less than a predetermined value (S602: Y), the recording material S does not need to be grounded by the contact member 201. For this reason, the control unit 121 transmits a switching signal of the first level (low level) to the switching unit 400, similar to the processing of S503 and S504 in Fig. 7, and causes the switching unit 400 to position the contact member 201 at the separated position A (see Fig. 4(a)) (S603, S604).

[0057] As a result of the comparison, for example, if the basis weight of the recording material S is greater than a predetermined value (S602: N), the recording material S needs to be grounded by the contact member 201. To achieve this, the control unit 121 transmits a switching signal of the second level (high level) to the switching unit 400, similar to the processing of S505 and S506 in Fig. 7, and causes the switching unit 400 to position the contact member 201 at the contact position B (see Fig. 4(b)) (S605, S606).

[0058] After positioning the contact member 201 at the separated position A or the abutting position B, the control unit 121 starts a print operation to form an image in accordance with the print job (S607). When the print operation instructed by the print job is completed, the control unit 121 ends a series of processes for image formation in accordance with the print job.

[0059] As described above, the image forming apparatus 100 of this embodiment switches the position of the contact member 201 in the pre-transfer contact section 200 between a separation position A and a contact position B depending on the basis weight of the recording material S. Specifically, when the basis weight of the recording material S is greater than a predetermined value, the contact member 201 is located at a contact position B, so that the contact member 201 contacts the recording material S. When the basis weight is equal to or less than the predetermined value, the contact member 201 is located at a separation position A, so that the contact member 201 does not contact the recording material S. Therefore, when the basis weight of the recording material S is greater than the predetermined value, current flows through the contact member 201, and fluctuations in the current at the paper passing section are suppressed. When the basis weight of the recording material S is equal to or less than the predetermined value, current flows through the outer secondary transfer roller 108, so that fluctuations in the current at the paper passing section are suppressed. Because fluctuations in the current at the paper passing section are suppressed in this way, stable transfer of a toner image is possible regardless of the basis weight of the recording material S being passed.

[0060] As described above, the image forming apparatus 100 can stabilize the transfer of the toner image to the recording material S by controlling the position of the contact member 201 based on the characteristics of the recording material S, such as the paper type and basis weight. In the above example, the contact member 201 is configured to contact one side of the recording material S (the side to which the toner image is not transferred), but it may also be configured to contact the opposite side of the recording material S (the side to which the toner image is transferred). The contact member 201 may be a cylindrical metal member or a metal wire such as a wire or copper wire. The contact member 201 is configured to contact the recording material S at one or more points when the recording material S is sandwiched in the nip portion 210. The contact portion may be a point, a line, or a surface.

[0061] 4, the switching unit 400 includes a support member 402, a guide frame 401, a cam 403, a motor 407, a motor drive circuit 408, and a driving force transmission mechanism, but is not limited to this. The switching unit 400 may be configured to be able to switch the position of the contact member 201 between the separation position A and the contact position B.

[0062] Furthermore, in this embodiment, a configuration in which a toner image is transferred onto the recording material S by the secondary transfer unit TR2 has been described, but this embodiment is also effective when a toner image is transferred directly from the photosensitive drums 101y, 101m, 101c, and 101k onto the recording material S. For example, some image forming apparatuses that form monochrome images are configured so that a toner image is transferred directly from a photosensitive drum onto the recording material S. Also, some image forming apparatuses that form color images are configured so that toner images of different colors are formed sequentially on a single photosensitive drum and then transferred directly onto the recording material S. The toner image is transferred onto the recording material S at a nip portion that is configured between the photosensitive drum and a primary transfer roller.

[0063] In this case, a contact member 201 is provided upstream of the photosensitive drum in the conveying direction of the recording material S. If the recording material S is of a type that would affect the transfer of the toner image if it were not grounded by the contact member 201, the control unit 121 will use a switching signal to make the switching unit 400 conductive, thereby grounding the recording material S.

Claims

1. an image carrier; an image forming means for forming an image on the image carrier; a transfer means for transferring the image formed on the image carrier onto a recording material; a conveying means for conveying the recording material to the transfer means; a contact means provided upstream of the transfer means in the conveyance direction of the recording material, the contact means having a contact member for grounding the recording material being conveyed to the transfer means; a moving means for moving the contact member between a first position where the contact member does not contact the recording material conveyed to the transfer means and a second position where the contact member does contact the recording material; a control unit that positions the contact member at either the first position or the second position depending on the characteristics of the recording material; characterized in that it comprises Image forming device.

2. the control means disposes the contact means at the second position when the recording material is of a first paper type, and disposes the contact means at the first position when the recording material is of a second paper type having a larger impedance than the first paper type.

2. The image forming apparatus according to claim 1.

3. the control means is characterized in that the first paper type is metal-deposited paper having a metal layer.

3. The image forming apparatus according to claim 2.

4. the control unit disposes the contact member at either the first position or the second position based on the basis weight of the recording material.

2. The image forming apparatus according to claim 1.

5. the control means disposes the contact member at the second position when the basis weight of the recording material is greater than a predetermined value, and disposes the contact member at the first position when the basis weight of the recording material is less than the predetermined value.

5. The image forming apparatus according to claim 4.

6. the moving means comprises a support member that contacts the contact member, a drive source that generates a drive force, and a cam member that rotates in response to the drive force transmitted from the drive source to move the support member between the first position and the second position, 2. The image forming apparatus according to claim 1.

7. the conveying means conveys the recording material to the transfer means via a guide member; The contact member is a member having a lower impedance than the guide member.

2. The image forming apparatus according to claim 1.

8. the contact means contacts the surface of the recording material onto which the image is transferred by the transfer means, 2. The image forming apparatus according to claim 1.

9. the contact means contacts a surface of the recording material opposite to a surface onto which the image is transferred by the transfer means, 2. The image forming apparatus according to claim 1.

Images