JP5548585B2 - Transfer device and image forming apparatus having the same - Google Patents

Description

  The present invention relates to a transfer device that transfers a toner image on an image carrier onto a sheet, and an image forming apparatus including the transfer device.

Image forming apparatuses such as copiers, printers, and facsimiles that use electrophotography are
The image forming apparatus includes a transfer device that transfers a toner image formed on the photosensitive drum onto a sheet. The transfer device includes a transfer belt on which a toner image is primarily transferred from a photosensitive drum, a driving roller and a driven roller on which the transfer belt is wound, and a secondary for secondary transfer of the toner image on a sheet from the transfer belt. And a transfer roller.

  Many techniques have been proposed for satisfactorily transferring a toner image onto a sheet from a transfer belt. In the transfer device of Patent Document 1, as the paper basis weight (weight per unit area) of the paper increases, the linear velocity difference between the transfer roller and the transfer belt is increased by increasing the linear velocity of the transfer roller. Further, the transfer device of Patent Document 1 reduces the linear velocity difference between the transfer roller and the transfer belt by decreasing the linear velocity of the transfer roller as the paper basis weight of the paper decreases. As described above, the linear velocity of the paper and the linear velocity of the transfer roller are set to be equal by appropriately changing the linear velocity difference between the transfer roller and the transfer belt in accordance with the paper basis weight. As a result, color misregistration during secondary transfer is suppressed.

JP 2008-289311 A

  However, in a transfer apparatus such as Patent Document 1, if a conductive tube is used as the surface layer of the transfer roller, wrinkles are generated when the paper passes through the transfer nip portion between the transfer roller and the transfer belt. Since the conductive tube is made of a resin or rubber having a high surface resistance, it has a characteristic of suppressing white point generation during double-sided printing of paper by suppressing dielectric breakdown inside and outside the transfer nip. . Although the conductive tube has such characteristics, when a sheet, particularly a sheet having a small paper basis weight, passes through the transfer nip portion, the sheet is easily adsorbed by the conductive tube. When the outer diameter of the transfer roller has a variation in the axial direction, the sheet conveyance speed varies along the axial direction of the transfer roller when the sheet passes through the transfer nip portion while being attracted to the conductive tube. As a result, the paper is wrinkled.

  Accordingly, in view of the above circumstances, the present invention provides a transfer device capable of suppressing the occurrence of paper wrinkles even when a conductive tube is used as the surface layer of the transfer roller, and an image forming apparatus including the transfer device. provide.

  In order to achieve the above object, a transfer apparatus according to the present invention has an image carrier that carries a toner image while rotating in a predetermined direction, and a surface layer made of a conductive tube. A transfer roller that forms a transfer nip portion between the image carrier and the image carrier, wherein the transfer roller rotates in a predetermined direction and conveys the paper together with the image carrier, while the transfer nip portion A transfer roller for transferring the toner image onto the paper, a first drive source for rotating the image carrier at a first rotation speed, and a second drive source for rotating the transfer roller at a second rotation speed. A bias applying unit that applies a bias to the transfer roller to cause a potential difference between the transfer roller and the image carrier, thereby transferring the toner image on the image carrier onto the paper. Of the paper Comprising a basis weight acquisition unit for acquiring a paper basis weight representing the weight per unit area, and a control unit for controlling the first drive source and the second drive source based on the paper basis weight. The controller drives the first drive so that a speed difference is generated between the first rotation speed of the image carrier and the second rotation speed of the transfer roller when the paper basis weight is a predetermined value or less. One of the power source and the second drive source.

  According to the transfer device of the present invention, the control unit controls one of the first drive source and the second drive source when the paper basis weight of the paper is a predetermined value or less, for example, when the paper is thin. Then, a speed difference is generated between the first rotation speed of the image carrier and the second rotation speed of the transfer roller. For this reason, the conveying force that the image carrier acts on the sheet to convey the sheet is different from the conveying force that the transfer roller acts on the sheet to convey the sheet. As a result, the surface of the paper facing the image carrier and the surface of the paper facing the transfer roller receive different conveying forces, so that the paper is nipped due to the characteristics of the conductive tube forming the surface layer of the transfer roller. Is prevented from being adsorbed to the transfer roller when passing through. As a result, even when the outer diameter of the transfer roller varies in the axial direction, it is possible to suppress the occurrence of paper wrinkles when the paper passes through the nip portion.

  In a preferred embodiment of the present invention, the transfer device further includes a pressing mechanism that presses the transfer roller against the image carrier in order to form the transfer nip portion. The pressing mechanism includes a first state in which the transfer roller is pressed against the image carrier with a first pressing value, and a second pressing value that is smaller than the first pressing value with respect to the image roller. It is possible to switch between the second state of pressing with. The control unit controls the pressing mechanism to switch the pressing mechanism to the first state when the paper basis weight is larger than a predetermined value, and when the paper basis weight is equal to or less than the predetermined value, The mechanism is controlled to switch the pressing mechanism to the second state.

  According to this configuration, the control unit switches the pressing mechanism to the first state when the paper basis weight of the paper is larger than a predetermined value, for example, when the paper is thick paper. When in the first state, the pressing mechanism presses the transfer roller against the image carrier with a first pressing value that is greater than the second pressing value. Therefore, the air gap between the sheet and the image carrier is reduced, and a strong electric field is formed between the image carrier and the transfer roller. As a result, the transferability of the toner image onto the paper is improved.

  On the other hand, the control unit switches the pressing mechanism to the second state when the paper basis weight of the paper is a predetermined value or less, for example, when the paper is thin paper. When in the second state, the pressing mechanism presses the transfer roller against the image carrier with a second pressing value smaller than the first pressing value. As the second pressing value is smaller than the first pressing value, the force with which the sheet is pressed against the image carrier is reduced, so that the generation of sheet wrinkles is further suppressed.

  In another preferred embodiment of the present invention, the transfer roller has a rotating shaft. The pressing mechanism includes a biasing member that applies a biasing force to the transfer roller in a direction in which the transfer roller is separated from the image carrier, and a first direction and a second direction opposite to the first direction. A contact portion that is rotatable and contacts the rotation shaft of the transfer roller, and a third drive source that rotates the contact portion. The control unit controls the third drive source to rotate the contact portion in the first direction while resisting the urging force of the urging member, thereby bringing the pressing mechanism into the first state. On the other hand, the pressing mechanism is switched to the second state by controlling the third drive source and rotating the contact portion in the second direction while following the urging force of the urging member.

  According to this configuration, the pressing mechanism can be switched between the first state and the second state with a simple configuration in which the biasing force of the biasing member is adjusted by the rotation of the contact portion.

  In still another preferred embodiment of the present invention, the contact portion is a cam.

  According to this configuration, the pressing mechanism can be switched between the first state and the second state with a simple operation of appropriately adjusting the rotation range of the cam.

  An image forming apparatus according to the present invention includes a photosensitive drum, an image forming unit that forms a toner image on the photosensitive drum, and a transfer device that transfers the toner image on the photosensitive drum onto a sheet. The transfer unit having the above-described configuration is used as the transfer device.

  According to the transfer device and the image forming apparatus of the present invention, it is possible to suppress the occurrence of paper wrinkles even when a conductive tube is used as the surface layer of the transfer roller.

1 schematically shows an internal structure of an image forming apparatus according to an embodiment of the present invention. FIG. 2 is a partially enlarged view of FIG. 1 and shows a transfer device. It is a perspective view of the secondary transfer roller of the transfer device. It is sectional drawing of a secondary transfer roller. It is a schematic diagram showing a drive system and a bias system of the transfer device. It is a schematic diagram which shows the structure of a press mechanism, and shows the time when a press mechanism exists in a 1st state. It is a schematic diagram which shows the structure of a press mechanism, and shows the time when a press mechanism exists in a 2nd state. It is a schematic diagram for demonstrating the control according to the paper basic weight of the paper by a control part. It is a figure which shows the setting and result of the experiment which were performed about generation | occurrence | production of a paper wrinkle, generation | occurrence | production of a white spot, and generation | occurrence | production of fluff.

  Hereinafter, embodiments according to the present invention will be described with reference to the drawings.

  FIG. 1 schematically shows an internal structure of an image forming apparatus 10 according to the present embodiment. FIG. 2 is a partially enlarged view of FIG. The image forming apparatus 10 is, for example, a tandem color printer, and includes an apparatus main body 11 having a box shape. The apparatus main body 11 includes an image forming unit 12 that forms a toner image based on image information transmitted from an external device such as a computer, and a transfer that transfers the toner image formed by the image forming unit 12 onto the paper P. The apparatus 20 includes a fixing unit 13 that performs a fixing process on an image transferred onto the paper P, and a paper storage unit 14 that stores the paper P for transfer. A paper discharge unit 15 for discharging the paper P after the fixing process is provided on the upper portion of the apparatus main body 11.

  The image forming unit 12 includes a magenta unit 12M that uses magenta toner and a cyan unit that uses cyan toner, which are sequentially arranged from the upstream side (to the right of the sheet of FIG. 1) toward the downstream side. 12C, a yellow unit 12Y that uses yellow toner, and a black unit 12K that uses black toner.

  Each unit 12M, 12C, 12Y, 12K includes a photosensitive drum 121 and a developing device 122. The photosensitive drum 121 forms an electrostatic latent image and a toner image along the electrostatic latent image on the peripheral surface, and is rotatable counterclockwise in FIG. The developing device 122 supplies toner to the electrostatic latent image on the circumferential surface of the photosensitive drum 121 to form a toner image.

  Each unit 12M, 12C, 12Y, 12K further includes a charging device 123 provided immediately below each photoconductor drum 121. An exposure device 124 is provided below the charging device 123. The charging device 123 uniformly charges the peripheral surface of the photosensitive drum 121. The exposure device 124 irradiates the peripheral surface of the charged photosensitive drum 121 with laser light corresponding to each color based on image data input from a computer or the like. Thereby, an electrostatic latent image is formed on the peripheral surface of the photosensitive drum 121. When toner is supplied to the electrostatic latent image from the developing device 122, a toner image is formed on the peripheral surface of the photosensitive drum 121.

  The transfer device 20 transfers the toner image formed on the photosensitive drum 121 by the image forming unit 12 onto the paper P conveyed from the paper storage unit 14. The transfer device 20 includes a transfer belt 125 (image carrier), a driving roller 16, a driven roller 17, a primary transfer roller 126, a tension roller 125 c, a secondary transfer roller 21, and a pressing mechanism 30.

  The transfer belt 125 is an endless belt that is wound between the driving roller 16 and the driven roller 17. In FIG. 1, the driving roller 16 is configured to rotate clockwise, and the transfer belt 125 circulates between the driving roller 16 and the driven roller 17 as the driving roller 16 rotates. The primary transfer roller 126 is provided for each photosensitive drum 121 and presses the transfer belt 125 against the peripheral surface of the corresponding photosensitive drum 121. Accordingly, the transfer belt 125 rotates in a state where the peripheral surface of the photosensitive drum 121 is pressed.

  The tension roller 125c is provided between the driving roller 16 and the driven roller 17 at a position close to the driven roller 17 side. The tension roller 125c applies tension to the transfer belt 125 and is biased upward by a biasing force of a biasing member (not shown).

  The secondary transfer roller 21 is rotatably disposed at a position facing the driving roller 16 in a sheet conveyance path 111 described later. The secondary transfer roller 21 is configured to be able to rotate counterclockwise in FIG. The secondary transfer roller 21 forms a transfer nip TN with the transfer belt 125. The paper P from the paper storage unit 14 is sandwiched between the transfer belt 125 and the secondary transfer roller 21, and the transfer nip is rotated by the rotation of the transfer belt 125 (that is, the rotation of the driving roller 16) and the rotation of the secondary transfer roller 21. Pass through part TN.

  The pressing mechanism 30 presses the secondary transfer roller 21 against the transfer belt 125. Accordingly, the sheet P passing through the transfer nip TN is pressed against the transfer belt 125 by the secondary transfer roller 21. As will be described later, the pressing mechanism 30 can adjust the pressing value for pressing the secondary transfer roller 21 against the transfer belt 125 in three stages (30N, 70N, and 0N).

  FIG. 3 is a perspective view of the secondary transfer roller 21, and FIG. 4 is a cross-sectional view of the secondary transfer roller 21. The secondary transfer roller 21 is a roller member having a three-layer structure including a shaft 22, a sponge layer 23, and a tube layer 24. The shaft 22 is a member made of a conductive material such as stainless steel and extending in parallel to the drive roller 16. The shaft 22 functions as a rotation shaft for rotating the secondary transfer roller 21. Both end portions of the shaft 22 protrude from the end surfaces of the sponge layer 23 and the tube layer 24.

The sponge layer 23 is laminated on the outer peripheral surface of the shaft 22 and is formed of a conductive sponge material, for example, a material containing carbon in NBR. The resistance of the sponge layer 23 is 10 ⁶ to 10 ⁹ Ω, and the Asker C hardness is 25 to 45 degrees. The thickness of the sponge layer 23 is about 6 mm.

The tube layer 24 is laminated on the sponge layer 23 to form the surface layer of the secondary transfer roller 21, and is made of a conductive tube material such as resin or rubber having high resistance, for example, a material containing carbon in nylon. Is formed. The surface resistance of the tube layer 24 is 10 ⁹ to 10 ¹³ Ω, and the thickness thereof is 60 to 180 μm. The transfer nip portion TN is formed between the tube layer 24 and the surface of the transfer belt 125. The tube layer 24 has a characteristic that suppresses dielectric breakdown inside and outside the transfer nip portion TN and suppresses the generation of so-called white spots during double-sided printing of the paper P.

  FIG. 5 is a schematic diagram showing a drive system and a bias system of the transfer device 20. The transfer device 20 further includes a first drive source M1 and a second drive source M2. The first drive source M1 is connected to the drive shaft 16a of the drive roller 16 and rotates the drive roller 16. As a result, the transfer belt 125 rotates at the first rotation speed V1. The second drive source M2 is connected to the shaft 22 of the secondary transfer roller 21, and rotates the secondary transfer roller at the second rotation speed V2.

  The transfer device 20 further includes a bias applying unit B. The bias application unit B is connected to the secondary transfer roller 21 and applies a bias to the secondary transfer roller 21. By applying a bias to the secondary transfer roller 21 by the bias application unit B, a predetermined potential difference is formed between the secondary transfer roller 21 and the transfer belt 125 in the transfer nip portion TN.

  The transfer device 20 further includes a control unit U that controls the first drive source M1, the second drive source M2, and the bias application unit B. The control unit U performs primary transfer of the toner image formed on the photosensitive drum 121 onto the transfer belt 125 and control for secondary transfer of the toner image on the transfer belt 125 onto the paper P. Next, primary transfer and secondary transfer by the control unit U will be described.

  The control unit U controls the first drive source M1 to rotate the drive roller 16, thereby rotating the transfer belt 125 and controlling the second drive source M2 to rotate the secondary transfer roller 21. Then, the control unit U applies a bias to the primary transfer roller 126 from a bias application unit (not shown) to generate a potential difference between the primary transfer roller 126 and the photosensitive drum 121. As a result, the toner image on the photosensitive drum 121 is attracted to the primary transfer roller 126 side according to the potential difference and is primarily transferred onto the transfer belt 125.

  Specifically, as the transfer belt 125 circulates, first, a primary transfer of a magenta toner image on the photosensitive drum 121 of the magenta unit 12M is performed on the surface of the transfer belt 125. Subsequently, the cyan toner image on the photosensitive drum 121 of the cyan unit 12C is transferred to the transfer position of the magenta toner image on the transfer belt 125 while being overcoated. In the same manner, the transfer of the yellow toner image on the photosensitive drum 121 of the yellow unit 12Y and the transfer of the black toner image on the photosensitive drum 121 of the black unit 12K are performed in an overcoating state. As a result, a color toner image is formed on the surface of the transfer belt 125, and the primary transfer ends.

  The toner image primarily transferred onto the transfer belt 125 is conveyed to the transfer nip portion TN as the transfer belt 125 rotates. At this time, the control unit U controls the bias application unit B to apply a bias to the secondary transfer roller 21. As a result, a predetermined potential difference is formed between the secondary transfer roller 21 and the transfer belt 125 in the transfer nip portion TN. Then, when the paper P from the paper storage unit 14 is conveyed by the rotation of the transfer belt 125 and the secondary transfer roller 21 and passes through the transfer nip TN, the toner image on the transfer belt 125 follows the potential difference according to the potential difference. Is attracted to the side and is secondarily transferred onto the paper P. Thereby, the secondary transfer is completed.

  Returning to FIGS. 1 and 2, the image forming unit 12 will be further described. Each unit 12M, 12C, 12Y, 12K further includes a drum cleaning device 127 disposed at the left position of the photosensitive drum 121 in FIG. The drum cleaning device 127 cleans the peripheral surface by removing residual toner remaining on the peripheral surface of the photosensitive drum 121 after the primary transfer. The peripheral surface of the photosensitive drum 121 cleaned by the drum cleaning device 127 is uniformly charged again by the charging device 123 to form a new toner image.

  The sheet conveyance path 111 extends in the vertical direction at the left position in FIG. 1 of the image forming unit 12 and conveys the sheet P from the sheet storage unit 14 in the order of the transfer nip unit TN and the fixing unit 13. A plurality of conveyance roller pairs 113 are provided at appropriate positions on the sheet conveyance path 111. The paper P from the paper storage unit 14 is conveyed toward the transfer nip TN by driving of the conveyance roller pair 113.

  In addition, a registration roller pair 112 is provided on the sheet conveyance path 111 at a position upstream of the transfer nip portion TN. The registration roller pair 112 adjusts the timing at which the paper P is conveyed toward the transfer nip TN and corrects the oblique feeding of the paper P.

  The sheet storage unit 14 includes a manual feed tray 141 that can be freely opened and closed on the right side wall of the apparatus main body 11 in FIG. 1, and a paper tray 142 that is removably mounted in the apparatus main body 11 below the exposure device 124. including. A sheet bundle P 1 formed by stacking a plurality of sheets P is stored in the sheet tray 142. The uppermost sheet P of the sheet bundle P1 is sent to the sheet conveyance path 111 one by one by driving the pickup roller 143.

  The fixing unit 13 performs a fixing process on the toner image secondarily transferred onto the paper P. The fixing unit 13 includes a heating roller 131 having an energized heating element as a heating source therein, a fixing roller 132 disposed to face the heating roller 131, and a fixing belt 133 stretched between the fixing roller 132 and the heating roller 131. And a pressure roller 134 disposed to face the fixing roller 40 with the fixing belt 133 interposed therebetween. Since the paper P supplied to the fixing unit 13 receives heat from the fixing belt 133 while passing between the pressure roller 134 and the fixing belt 133, the toner image on the paper P is fixed on the paper P. . The paper P for which the fixing process has been completed is discharged toward a paper discharge tray 151 of the paper discharge unit 15 through a paper discharge conveyance path 114 extending from the upper part of the fixing unit 13.

In the transfer device 20 configured as described above, the tube layer 24 is used as the surface layer of the secondary transfer roller 21. The tube layer 24 has the following problem, although it has a characteristic capable of suppressing the generation of white spots. That is, when the paper P, in particular, a thin paper having a paper basis weight (paper weight per unit area) of 64 g / m ² or less passes through the transfer nip portion TN, it is easily electrostatically attracted to the tube layer 24. When the outer diameter of the secondary transfer roller 21 varies in the axial direction, that is, as shown in FIG. 3, one end of the secondary transfer roller 21 has an outer diameter D1, and the other end is outside. When the outer diameter D2 is different from the diameter D1, the conveyance speed of the sheet P varies along the axial direction of the secondary transfer roller 21 when the sheet P passes through the transfer nip portion TN while being attracted to the tube layer 24. For this reason, wrinkles are likely to occur at the rear end of the paper P, particularly the paper P.

  Therefore, in this embodiment, in order to perform secondary transfer control according to the paper basis weight of the paper P and suppress the occurrence of paper wrinkles when the paper P is thin paper, the transfer device 20 is as shown in FIG. The paper basis weight acquisition unit 50 is further included. The paper basis weight acquisition unit 50 acquires the paper basis weight of the paper P before the paper P enters the transfer nip TN.

  The paper basis weight acquisition unit 50 acquires the paper basis weight of the paper P based on the paper type designated by the user through an operation panel (not shown) arranged in the image forming apparatus 10. The paper basis weight is determined according to the type of the paper P, for example, A4 paper and B5 paper, and data regarding the paper basis weight is stored in the paper basis weight acquisition unit 50. Therefore, the paper basis weight acquisition unit 50 can acquire the paper basis weight of the paper P by referring to the paper basis weight data every time the user designates the type of the paper P. In addition, when configured so that the user can selectively specify thick paper and thin paper through the operation panel, the paper basis weight acquisition unit 50 stores data on the paper basis weight of the thick paper and the paper basis weight of the thin paper. The paper basis weight of the paper P is acquired by referring to the paper basis weight data every time the user designates thick paper or thin paper.

  Furthermore, the paper basis weight acquisition unit 50 may include an actuator and an optical sensor. That is, by arranging an actuator at an appropriate position on the paper transport path 11, irradiating the actuator with light, and obtaining an angle of reflected light that changes according to the amount of movement of the actuator when the paper P contacts, The type of P may be determined. As the paper basis weight of the paper P increases (that is, as the thickness of the paper P increases), the amount of movement of the actuator increases and the angle of the reflected light increases. The configuration of the paper basis weight acquisition unit 50 is not particularly limited.

  Further, in this embodiment, the pressing mechanism 30 is configured as shown in FIGS. 6 and 7 in order to suppress the occurrence of paper wrinkles. FIG. 6 is a schematic diagram showing the configuration of the pressing mechanism 30 and shows the pressing mechanism 30 in the first state. FIG. 7 is a schematic diagram showing the configuration of the pressing mechanism 30 and shows the state when the pressing mechanism 30 is in the second state. The pressing mechanism 30 is configured to be switchable between a first state and a second state. When the pressing mechanism 30 is switched to the first state, the secondary transfer roller 21 is pressed against the transfer belt 125 with the first pressing value, and when the pressing mechanism 30 is switched to the second state, the secondary transfer roller 21 is transferred. The belt 125 is pressed with a second pressing value smaller than the first pressing value. The first pressing value is 70N, and the second pressing value is 30N.

  In order to enable switching between the first state and the second state, the pressing mechanism 30 includes a bearing 31, a swing member 32, a biasing member 35, and a cam 36.

  The bearing 31 is a rolling bearing that is fitted around the outer peripheral portion of the end portion of the shaft 22 of the secondary transfer roller 21. The swing member 32 includes a swing piece 33 and a swing shaft 34. The swing piece 33 includes a first portion 33a and a second portion 33b. The first portion 33 a is provided integrally with the bearing 31 and extends from the outer peripheral surface of the bearing 31 in the radial direction of the secondary transfer roller 21. The second portion 33b extends at a position substantially opposite to the first portion 33a with the swing shaft 34 interposed therebetween. The swing shaft 34 supports the swing piece 33 so as to be swingable, and is positioned above the first portion 33a.

  The urging member 35 is a spring member attached to the second portion 33 b of the swing piece 33. The urging member 35 applies an urging force to the secondary transfer roller 21 in a direction in which the secondary transfer roller 21 is separated from the transfer belt 125. Specifically, the urging member 35 always urges the second portion 33b upward as indicated by an arrow S. Therefore, the swing piece 33 is always biased to rotate clockwise in FIGS. Therefore, the secondary transfer roller 21 having the shaft 22 on which the bearing 31 is fitted is moved away from the transfer belt 125 by the urging force of the urging member 35 transmitted through the swing piece 33 and the bearing 31. Always energized.

  The cam 36 is a member for switching the pressing mechanism 30 between the first state and the second state. The cam 36 has a cam surface 36 a that contacts the bearing 31 from below, and is configured to be rotatable about the cam shaft 37 in a state where the cam surface 36 a is in contact with the bearing 31. Therefore, when the cam 36 rotates, the secondary transfer roller 21 is pressed against the transfer belt 125, and the pressing force with which the secondary transfer roller 21 is pressed against the transfer belt 125 is determined according to the rotation amount of the cam 36. A third drive source M3 that rotates the cam shaft 37 is connected to the cam shaft 37. Therefore, when the cam shaft 37 is rotated by the third drive source M3, the cam 36 is rotated. The third drive source M3 is controlled by the control unit U. Next, switching between the first state and the second state of the pressing mechanism 30 by the cam 36 will be described.

  First, switching of the pressing mechanism 30 from the first state to the second state will be described. When the pressing mechanism 30 is in the first state, the cam 36 is fitted on the bearing 31 in a state in which the cam 36 resists the urging force of the urging member 35 (the force that separates the secondary transfer roller 21 from the transfer belt 125). The secondary transfer roller 21 is pressed against the transfer belt 125 with the first pressing value (70N) via the shaft 22. In this first state, when the control unit U controls the third drive source M3 to rotate the cam 36 counterclockwise (first direction) R1 in FIG. Slide along the outer peripheral surface. As the cam surface 36a slides on the outer peripheral surface of the bearing 31, the pressing force with which the cam 36 presses the secondary transfer roller 21 against the transfer belt 125 gradually decreases. The secondary transfer roller 21 moves in the direction W1 away from the transfer belt 125 according to the urging force of the urging member 35 as the cam surface 36a slides. Thereby, the pressing mechanism 30 is switched from the first state to the second state. The amount of rotation of the cam 36 at this time, that is, the sliding range of the cam surface 36a is such that the pressing value at which the secondary transfer roller 21 is pressed against the transfer belt 125 by the cam 36 is changed from the first pressing value (70N) to the second pressing value ( 30N).

  Next, switching of the pressing mechanism 30 from the second state to the first state will be described. When the pressing mechanism 30 is in the second state, the cam 36 presses the secondary transfer roller 21 against the transfer belt 125 with the second pressing value (30N). In this second state, when the control unit U controls the third drive source M3 to rotate the cam 36 in the clockwise direction (second direction) R2 in FIG. 7, the cam surface 36a of the cam 36 is a biasing member. It slides along the outer peripheral surface of the bearing 31 while resisting the urging force of 35. As the cam surface 36a slides on the outer peripheral surface, the pressing force with which the cam 36 presses the secondary transfer roller 21 against the transfer belt 125 gradually increases. The secondary transfer roller 21 moves in the direction W2 close to the transfer belt 125 against the urging force of the urging member 35 as the cam surface 36a slides. Thereby, the pressing mechanism 30 is switched from the second state to the first state.

  As described above, the pressing mechanism 30 can be switched between the first state and the second state with a simple configuration in which the adjustment is performed by the rotation of the cam 36. The slope of the cam surface 36a is appropriately set so that the pressing value can be smoothly switched between the first pressing value and the second pressing value.

  Although not shown, when the cam 36 is further rotated counterclockwise from the state shown in FIG. 7, the cam 36 does not press the secondary transfer roller 21 against the transfer belt 125 at all (the pressing value is 0). That is, the secondary transfer roller 21 moves according to the urging force of the urging member 35 and is separated from the transfer belt 125.

  Next, control according to the paper basis weight of the paper P by the control unit U will be described with reference to FIG. As described above, the transfer belt 125 is rotated at the first rotation speed V1 by the first drive source M1, and the secondary transfer roller 21 is rotated at the second rotation speed V2 by the second drive source M2. That is, the transfer belt 125 is driven at the first linear velocity L1 in the transfer nip portion TN, and the secondary transfer roller 21 is driven at the second linear velocity L2 in the transfer nip portion TN.

  The first linear velocity L1 of the transfer belt 125 is the linear velocity of the surface portion of the transfer belt 125 that contacts the paper P, and the direction of the first linear velocity L1 is the tangential direction of the surface portion. The second linear velocity L2 of the secondary transfer roller 21 is the linear velocity of the surface portion of the secondary transfer roller 21 that contacts the paper P, and the direction of the second linear velocity L2 is the tangential direction of the surface portion. is there.

When the paper basis weight of the paper P acquired by the paper basis weight acquisition unit 50 is larger than a predetermined value (64 g / m ² ) (when the paper P is thick paper), the control unit U performs the first linear velocity of the transfer belt 125. The first drive source M1 and the second drive source M2 are controlled so that L1 and the second linear velocity L2 of the secondary transfer roller 21 become equal linear velocity. That is, the control unit U sets the linear velocity ratio L2 / L1 between the second linear velocity L2 and the first linear velocity L1 to 1 when the paper P is thick paper.

  Further, the control unit U controls the third driving source M3 in accordance with the control of the first driving source M1 and the second driving source M2, and switches the pressing mechanism 30 to the first state. Further, the control unit U controls the bias application unit B in accordance with the control of the first drive source M1 to the third drive source M3, and generates a predetermined potential difference between the secondary transfer roller 21 and the transfer belt 125. Let As described above, the secondary transfer is performed when the sheet P passes through the transfer nip TN.

  When the paper P is a thick paper, even if the paper P is pressed against the transfer belt 125 with the first pressing value, the paper P is hardly electrostatically attracted to the tube layer 24 of the secondary transfer roller 21. On the contrary, when the paper P is a thick paper, the paper P is pressed against the transfer belt 125 with a strong load (first pressing value 70N), so that the surface of the paper P and the transfer belt 125 due to the unevenness of the paper P The air gap between them becomes smaller. Thereby, a strong electric field is formed between the transfer belt 125 and the secondary transfer roller 21. As a result, the secondary transfer property of the toner image onto the paper P is improved.

On the other hand, the control unit U performs the first rotation of the transfer belt 125 when the paper basis weight of the paper P acquired by the paper basis weight acquisition unit 50 is a predetermined value (64 g / m ² ) or less (when the paper P is thin paper). Specifically, the first linear speed L1 of the transfer belt 125 and the second linear speed of the secondary transfer roller 21 are set so that a speed difference is generated between the speed V1 and the second rotational speed V2 of the secondary transfer roller 21. The first drive source M1 and the second drive source M2 are controlled so that a linear velocity difference occurs with respect to L2. In the present embodiment, the control unit U controls the second drive source M2 so that the second linear velocity L2 of the secondary transfer roller 21 is higher than the first linear velocity L1 of the transfer belt 125, thereby providing the second drive source M2. A linear velocity difference is generated between the linear velocity L2 and the first linear velocity L1.

  Further, the control unit U controls the third drive source M3 in accordance with the control of the first drive source M1 and the second drive source M2, and switches the pressing mechanism 30 to the second state. Further, the control unit U controls the bias application unit B in accordance with the control of the first drive source M1 to the third drive source M3, and generates a predetermined potential difference between the secondary transfer roller 21 and the transfer belt 125. Let As described above, the secondary transfer is performed when the sheet P passes through the transfer nip TN.

  In the present embodiment, the linear velocity ratio (L2 / L1) between the second linear velocity L2 and the first linear velocity L1 is set to 1.03. Therefore, the first conveyance force F1 that the transfer belt 125 acts on the paper P in order to convey the paper P, and the second conveyance that the secondary transfer roller 21 acts on the paper P in order to convey the paper P. The force F2 is different. Accordingly, the first surface PA of the paper P facing the transfer belt 125 and the second surface PB of the paper P facing the secondary transfer roller 21 receive different conveying forces. As a result, the sheet P is prevented from being attracted to the tube layer 24 when it passes through the transfer nip TN due to the characteristics of the conductive tube layer 24 that forms the surface layer of the secondary transfer roller 21. As a result, even when the outer diameter of the secondary transfer roller 21 varies in the axial direction as shown in FIG. 3, the occurrence of paper wrinkles when the paper P passes through the transfer nip TN is suppressed. it can.

The control unit U switches the pressing mechanism 30 to the second state when the paper basis weight of the paper P is equal to or less than a predetermined value (64 g / m ² ). The second pressing value is smaller than the second pressing value. Since the force with which the paper P is pressed against the transfer belt 125 is reduced by the amount that the second pressure value is smaller than the first pressure value, the occurrence of paper wrinkles is further suppressed. Since the paper P is a thin paper, even if the load pressing the paper P against the transfer belt 125 is small, the formation of an air gap between the paper P and the transfer belt 125 is sufficiently suppressed. Therefore, a strong electric field is formed between the transfer belt 125 and the secondary transfer roller 21. As a result, the secondary transfer property of the toner image onto the paper P is maintained.

Next, a description will be given of experiments conducted on the generation of paper wrinkles, the generation of white spots, and the generation of roughness. In the experiment, Examples 1 and 2 and Comparative Examples 1 to 5 were used as shown in FIG. In the experiment, the paper basis weight of the paper is set to one of 64 g / m ² (thin paper) and 80 g / m ² (thick paper), and the secondary transfer roller 21 is configured to use the tube layer 24. In addition, the pressing force (load) by which the pressing mechanism 30 presses the secondary transfer roller 21 against the transfer belt 125 is set to one of 70N (that is, the first state) and 30N (that is, the second state). Further, the linear velocity ratio between the second linear velocity L2 of the secondary transfer roller 21 and the first linear velocity L1 of the transfer belt 125 was set to one of 1 and 1.03.

  Example 1 and Example 2 and Comparative Example 1 to Comparative Example 5 are set as shown in FIG. 9, and then a toner image is formed on the paper. Example 1, Example 2, Comparative Example 1 to Comparative Example 1 For each Example 5, the occurrence of paper wrinkles, white spots, and cracks was confirmed visually. The evaluation criteria for the occurrence of paper wrinkles, white spots, and crackling are as follows. That is, when generation | occurrence | production was not confirmed, it evaluated as (circle), when generation | occurrence | production was confirmed, but it was in tolerance level, it evaluated as (triangle | delta), and when the generation | occurrence | production degree was out of tolerance range, it evaluated as x. Note that the “pasaki” means that a strong electric field is not formed between the paper and the transfer belt 125 due to an air gap between the paper and the transfer belt 125, and as a result, the toner does not move well on the paper. This is the density unevenness that occurs.

In Example 1, a toner image was formed using a paper having a paper basis weight of 64 g / m ² , but the linear velocity ratio (L2 / L1) was set to 1.03 and the load was set to 30N. As a result, the paper was not attracted to the tube layer 24 of the secondary transfer roller 21 and no paper wrinkles occurred. In addition, since the paper is thin, even if the load is 30 N, the paper can be sufficiently pressed against the transfer roller, and the formation of an air gap between the paper and the transfer belt 125 can be suppressed. It was. As a result, no cracking occurred. In Example 1, since the secondary transfer roller 21 having the tube layer 24 was used, white spots did not occur.

In Example 2, the linear velocity ratio was set to 1, but the paper was thick paper with a paper basis weight of 80 g / m ² , so no paper wrinkles occurred. In the second embodiment, since the load is set to 70 N, even when the paper is a thick paper, the paper can be sufficiently pressed against the secondary transfer roller 21, and the paper is between the paper and the transfer belt 125. The formation of an air gap could be suppressed. As a result, no cracking occurred. Also in Example 2, the secondary transfer roller 21 having the tube layer 24 was used, so no white spots were generated.

In Comparative Example 1, the linear velocity ratio was set to 1 and the load was set to 70 N even though the paper was a thin paper having a paper basis weight of 64 g / m ² . For this reason, the sheet is attracted to the tube layer 24 of the secondary transfer roller 21 when the secondary transfer is performed, and as a result, the sheet is wrinkled.

  In Comparative Example 2, although generation of paper wrinkles and roughness was not confirmed, white spots were generated because the secondary transfer roller 21 that does not have the tube layer 24 and the sponge layer 23 was exposed was used.

  In Comparative Example 3, the linear velocity ratio was set to 1.03 suitable for thin paper, but the load was set to 70 N, so the paper was pressed more strongly than necessary to the transfer belt 125, resulting in paper wrinkles. did.

  In Comparative Example 4, although the load was set to 30 N suitable for thin paper, the linear velocity ratio was set to 1, so that the sheet was adsorbed to the tube layer 24 of the secondary transfer roller 21 when the secondary transfer was performed. As a result, although the paper wrinkle was within the allowable range, it was confirmed.

In Comparative Example 5, the load was set to 30 N even though the paper was a thick paper with a paper basis weight of 80 g / m ² . As a result, an air gap is formed between the sheet and the transfer belt 125, and as a result, there is a looseness.

DESCRIPTION OF SYMBOLS 10 Image forming apparatus 16 Drive roller 20 Transfer apparatus 21 Secondary transfer roller 24 Tube layer 30 Press mechanism 36 Cam 50 Paper basis weight acquisition part 125 Transfer belt (image carrier)
F1 First transport force F2 Second transport force L1 First linear velocity L2 Second linear velocity M1 First drive source M2 Second drive source M3 Third drive source P Paper PA First surface PB Second surface TN Transfer nip portion U Control part V1 1st rotational speed V2 2nd rotational speed

Claims (4)

An image carrier that carries a toner image while rotating in a predetermined direction;
A transfer roller having a surface layer made of a conductive tube and forming a transfer nip portion between the image carrier and a sheet passing through the paper while being conveyed, and rotates in a predetermined direction together with the image carrier. A transfer roller for transferring the toner image on the image carrier onto the paper at the transfer nip portion while conveying the paper;
A first drive source for rotating the image carrier at a first rotational speed;
A second drive source for rotating the transfer roller at a second rotational speed;
A bias applying unit that applies a bias to the transfer roller to cause a potential difference between the transfer roller and the image carrier, thereby transferring the toner image on the image carrier onto the paper;
A basis weight acquisition unit for acquiring a paper basis weight representing a weight per unit area of the paper;
A control unit for controlling the first drive source and the second drive source based on the paper basis weight;
A pressing mechanism that presses the transfer roller against the image carrier to form the transfer nip portion;
With
The pressing mechanism includes a first state in which the transfer roller is pressed against the image carrier with a first pressing value, and a second pressing value that is smaller than the first pressing value with respect to the image roller. It is possible to switch between the second state pressed with
The controller is
When the paper basis weight is larger than 64 g / m ^2, while controlling the pressing mechanism to switch the pressing mechanism to the first state,
When the paper basis weight is less than or equal to 64 g / m ² , the first drive source and the first drive source so that a speed difference is generated between the first rotation speed of the image carrier and the second rotation speed of the transfer roller. while controls of the second drive source, and toggle its transfer device push pressure mechanism by controlling the pressing mechanism in the second state. The transfer device according to claim 1 ,
The transfer roller has a rotation shaft,
The pressing mechanism is
A biasing member that applies a biasing force to the transfer roller in a direction to separate the transfer roller from the image carrier;
A contact portion that is rotatable in a first direction and a second direction opposite to the first direction, and is in contact with the rotation shaft of the transfer roller;
A third drive source for rotating the contact portion;
The control unit controls the third drive source to rotate the contact portion in the first direction while resisting the urging force of the urging member, thereby bringing the pressing mechanism into the first state. On the other hand, the transfer device that switches the pressing mechanism to the second state by controlling the third drive source and rotating the contact portion in the second direction while following the biasing force of the biasing member. The transfer device according to claim 2 ,
The transfer device, wherein the contact portion is a cam. An image forming unit having a photosensitive drum and forming a toner image on the photosensitive drum;
A transfer device for transferring the toner image on the photosensitive drum onto a sheet;
With
An image forming apparatus in which the transfer device according to any one of claims 1 to 3 is used as the transfer device.

Images