Image forming apparatus

Abstract

To prevent the occurrence of an image quality defect at a portion of a sheet located at a transfer nip at a timing when a leading end of the sheet enters a fixing nip.SOLUTION: An image forming apparatus comprises: a transfer changing mechanism that moves at least one of an image carrier and a transfer member 34 to change a pressure-contact state between the image carrier and the transfer member 34; a fixing changing mechanism that moves at least one a pair of fixing members 27, 29 to change a pressure-contact state between the pair of fixing members 27, 29; and control means that controls the transfer changing mechanism to variably control a transfer nip load at a transfer nip NT, and controls the fixing changing mechanism to variably control a fixing nip load at a fixing nip NF. By the time a leading end of a sheet P enters the fixing nip NF, the control means increases the transfer nip load from a first transfer nip load to a second transfer nip load larger than the first transfer nip load, and decreases the fixing nip load from a first fixing nip load to a second fixing nip load smaller than the first fixing nip load.SELECTED DRAWING: Figure 3

Description

[Technical Field]

[0001] The present invention relates to an image forming apparatus. [Background technology]

[0002] Electrophotographic image forming apparatuses are known. These image forming apparatuses include an image carrier (transfer belt, photosensitive drum, or the like) that carries an image on its surface, a transfer member that is pressed against the image carrier to form a transfer nip and transfers the image on the image carrier to paper, and a pair of fixing members that are disposed downstream of the transfer member on the paper transport path and are pressed against each other to form a fixing nip and perform a fixing process on the paper. Patent Documents 1 to 3 exemplify electrophotographic image forming apparatuses. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Japanese Patent Application Laid-Open No. 2013-72908 [Patent Document 2] Japanese Patent Application Laid-Open No. 2010-151983 [Patent Document 3] Japanese Patent Application Laid-Open No. 2003-54783 Summary of the Invention [Problem to be solved by the invention]

[0004] In an electrophotographic image forming apparatus, when the leading edge of a sheet of paper enters the fixing nip, the sheet may be subjected to an impact, which may result in poor image quality in the portion of the sheet that is positioned in the transfer nip.

[0005] An object of the present invention is to prevent poor image quality from occurring in the portion of the paper positioned in the transfer nip when the leading edge of the paper enters the fixing nip. [Means for solving the problem]

[0006] The invention of claim 1 includes an image carrier that carries an image on its surface, a transfer member that is pressed against the image carrier to form a transfer nip and transfers the image on the image carrier to paper, a pair of fixing members that are disposed downstream of the transfer member in a paper transport path and are pressed against each other to form a fixing nip and perform a fixing process on the paper, a transfer change mechanism that moves at least one of the image carrier and the transfer member to change the pressure state between the image carrier and the transfer member, and a mechanism that moves at least one of the pair of fixing members to change the pressure state between the pair of fixing members. The image forming device comprises a fixing change mechanism, and a control means for variably controlling the transfer nip load at the transfer nip by controlling the transfer change mechanism, and for variably controlling the fixing nip load at the fixing nip by controlling the fixing change mechanism, wherein the control means increases the transfer nip load from a first transfer nip load to a second transfer nip load that is larger than the first transfer nip load, and decreases the fixing nip load from the first fixing nip load to the second fixing nip load that is smaller than the first fixing nip load, before the leading edge of the paper enters the fixing nip.

[0007] The invention of claim 2 is an image forming apparatus as described in claim 1, wherein the control means changes at least one of the amount of increase, which is the difference between the first transfer nip load and the second transfer nip load, and the amount of decrease, which is the difference between the first fixing nip load and the second fixing nip load, based on the type of paper to be printed.

[0008] The invention of claim 3 is an image forming apparatus as described in claim 2, wherein the control means reduces the amount of increase in the transfer nip load when the type of paper to be printed is coated paper, the surface of which has been coated, compared to when the type of paper is not coated.

[0009] The invention of claim 4 is an image forming apparatus according to claim 2, wherein the control means reduces the decrease in the fixing nip load when the type of paper to be printed is embossed paper having an embossed surface, compared to when the type of paper is not embossed paper.

[0010] The invention of claim 5 is an image forming apparatus as described in claim 1, further comprising a conveying device having a suction mechanism that sucks air to generate negative pressure and that conveys the paper while adhering the paper to a conveying member using the suction mechanism, wherein the conveying device is arranged between the transfer nip and the fixing nip, and the control means increases the suction force of the suction mechanism from a first suction force to a second suction force that is greater than the first suction force before the leading edge of the paper enters the fixing nip, and decreases the suction force of the suction mechanism from the second suction force to the first suction force after the leading edge of the paper enters the fixing nip, and when the thickness of the paper to be printed is the first thickness, the difference between the first suction force and the second suction force is larger than when the thickness is the thinner second thickness.

[0011] The invention of claim 6 is an image forming apparatus as described in claim 1, wherein the control means increases the fixing nip load from the second fixing nip load to the first fixing nip load while maintaining the transfer nip load at the second transfer nip load after the leading edge of the paper enters the fixing nip.

[0012] The invention of claim 7 is the image forming apparatus of claim 6, wherein the control means reduces the transfer nip load from the second transfer nip load to the first transfer nip load before the trailing edge of the paper exits the transfer nip.

[0013] The invention of claim 8 is an image forming apparatus as described in claim 6 or 7, further comprising a conveying device having a suction mechanism that sucks air to generate negative pressure and that conveys paper while adsorbing the paper to a conveying member using the suction mechanism, wherein the conveying device is arranged between the transfer nip and the fixing nip, and the control means increases the suction force of the suction mechanism from a first suction force to a second suction force that is greater than the first suction force before the leading edge of the paper enters the fixing nip, and decreases the suction force of the suction mechanism from the second suction force to the first suction force after the leading edge of the paper enters the fixing nip.

[0014] The invention of claim 9 is an image forming apparatus comprising: an image carrier that carries an image on its surface; a transfer member that is pressed against the image carrier to form a transfer nip and transfers the image on the image carrier to paper; a pair of fixing members that are arranged downstream of the transfer member on the paper transport path and are pressed against each other to form a fixing nip and perform a fixing process on the paper; a conveying device that has a suction mechanism that sucks air to generate negative pressure and transports the paper while adsorbing it to the conveying member using the suction mechanism; and a control means, wherein the conveying device is arranged between the transfer nip and the fixing nip, and the control means increases the suction force of the suction mechanism from a first suction force to a second suction force that is greater than the first suction force before the leading edge of the paper enters the fixing nip, and reduces the suction force of the suction mechanism from the second suction force to the first suction force after the leading edge of the paper enters the fixing nip. [Effects of the Invention]

[0015] According to the invention of claim 1, it is possible to prevent poor image quality from occurring in the portion of the paper that is positioned in the transfer nip at the timing when the leading edge of the paper enters the fixing nip.

[0016] According to the invention of claim 2, at least one of the amount of increase in the transfer nip load and the amount of decrease in the fixing nip load is set according to the type of paper.

[0017] According to the invention of claim 3, poor image quality is prevented from occurring when using coated paper.

[0018] According to the invention as defined in claim 4, the occurrence of poor image quality on embossed paper is suppressed.

[0019] According to the invention as set forth in claim 5, poor image quality caused by relatively thick paper is suppressed.

[0020] According to the invention of claim 6, the change in the posture of the paper when the fixing nip load changes is suppressed.

[0021] According to the seventh aspect of the present invention, an increase in the speed of the image carrier when the trailing edge of the paper leaves the transfer nip is suppressed.

[0022] According to the invention of claim 8, fluctuations in the speed of the paper at the timing when the leading edge of the paper enters the fixing nip are suppressed.

[0023] According to the invention of claim 9, it is possible to prevent poor image quality from occurring in the portion of the paper that is positioned in the transfer nip at the timing when the leading edge of the paper enters the fixing nip. [Brief explanation of the drawings]

[0024] [Figure 1] 1 is a diagram illustrating an image forming apparatus according to an embodiment. [Figure 2] FIG. 2 is an enlarged view of a transfer unit and a fixing unit. [Figure 3] FIG. 10 is a diagram showing a flow of control of a transfer nip load and a fixing nip load. [Figure 4] FIG. 10 is a diagram showing a flow of control of a transfer nip load and a fixing nip load. [Figure 5] 10A and 10B are diagrams for explaining the variable amounts of the transfer nip load and the fixing nip load according to the type of paper; [Figure 6] 10A and 10B are diagrams for explaining image quality improvement according to the amount of change in the transfer nip load and the fixing nip load. [Figure 7]10A and 10B are diagrams illustrating a flow of control of a transfer nip load, a fixing nip load, and a suction force according to another embodiment. DETAILED DESCRIPTION OF THE INVENTION

[0025] Hereinafter, embodiments of the present invention will be described in detail with reference to the accompanying drawings. The configurations described below are examples for the purpose of explanation and can be modified as appropriate. Furthermore, when multiple embodiments or variations are included below, it is assumed from the outset that their characteristic parts will be used in appropriate combination. In all drawings, identical or similar elements are designated by the same reference numerals, and redundant explanations will be omitted.

[0026] 1 is a diagram showing an image forming apparatus 10 according to an embodiment. The image forming apparatus 10 includes an image forming section 12, a storage section 14, a conveying section 16, and a control device 18.

[0027] The image forming unit 12 forms a toner image by, for example, electrophotography. The storage unit 14 stores paper P, which is an example of a recording medium. The transport unit 16 transports the paper P stored in the storage unit 14 along a transport path 20 toward the image forming unit 12. The transport unit 16 also transports the paper P transported along the transport path 20 along a reversing path 22 to invert the paper P and transport it again toward the image forming unit 12.

[0028] The toner image formed by the image forming unit 12 is formed on the surface of the paper P transported along the transport path 20. The paper P on which the toner image has been formed is discharged to the outside of the housing 10a of the image forming apparatus 10.

[0029] When a toner image is formed on the back side of the paper P, the paper P with the toner image formed on the front side is transported along the reverse path 22, and the toner image is again formed on the back side of the paper P by the image forming unit 12. The paper P is then ejected to the outside of the housing 10a. Note that, in the example shown in FIG. 1, the image forming device 10 has the function of forming a toner image on both sides of the paper P, but it may also have the function of forming a toner image on only one side of the paper P.

[0030] As an example, the image forming unit 12 includes image forming units 24Y, 24M, 24C, and 24K, a transfer device 25, and a fixing device 26. The image forming unit 24Y forms a yellow (Y) toner image using yellow (Y) toner. The image forming unit 24M forms a magenta (M) toner image using magenta (M) toner. The image forming unit 24C forms a cyan (C) toner image using cyan (C) toner. The image forming unit 24K forms a black (K) toner image using black (K) toner. The transfer device 25 transfers the toner images formed by the image forming units 24Y, 24M, 24C, and 24K to paper P. The fixing device 26 fixes the toner images transferred to paper P by the transfer device 25 to paper P. Although four colors of toner are used in the example shown in FIG. 1, this is merely an example, and five or more colors of toner may be used. For example, other spot colors may be used in addition to yellow, magenta, cyan, and black. Alternatively, only black toner may be used.

[0031] The image forming units 24Y, 24M, 24C, and 24K have basically the same configuration except for the toner used. For example, the image forming units 24Y, 24M, 24C, and 24K each include a rotating cylindrical photosensitive drum, a charger that charges the photosensitive drum, an exposure device, and a developing device. The exposure device irradiates the charged photosensitive drum with light to form an electrostatic latent image. The developing device develops the electrostatic latent image into a toner image using a developer containing toner.

[0032] The transfer device 25 includes a transfer belt 30, a primary transfer roll 32, a secondary transfer roll 34, and a roll 36. The transfer belt 30 is wound around multiple rolls including the roll 36 and rotates in the direction of the arrow in FIG. 1. A secondary transfer portion NT (i.e., a transfer nip) that transfers a toner image to paper P is formed between the secondary transfer roll 34 and the transfer belt 30. Hereinafter, the secondary transfer portion NT will be simply referred to as the transfer portion NT.

[0033] The fixing device 26 is disposed downstream of the transfer unit NT in the conveyance direction of the paper P. The fixing device 26 includes a fixing roll 27, a pressure roll 29, and a fixing belt 28. The fixing roll 27 and the pressure roll 29 are cylindrical rolls attached to rotating shafts and rotatable. The fixing belt 28 is wound around the fixing roll 27 and other rolls. The fixing roll 27 is rotated by a motor (not shown). The fixing roll 27 also incorporates a heater such as a halogen lamp and heats the paper P via the fixing belt 28. The pressure roll 29 is disposed opposite the fixing roll 27 across the conveyance path of the paper P. The pressure roll 29 is pressed against the fixing roll 27 via the fixing belt 28 by an elastic member such as a spring. As a result, the pressure roll 29 presses the paper P toward the fixing roll 27. A fixing unit NF (i.e., a fixing nip) for fixing the toner image to the paper P is formed between the fixing belt 28 and the pressure roll 29. The paper P transported to the fixing unit NF is heated by the fixing roll 27 via the fixing belt 28, and is pressed toward the fixing belt 28 (fixing roll 27) by the pressure roll 29. This fixes the toner image onto the paper P. Note that the fixing device 26 may be configured without the fixing belt 28, and the fixing unit NF may be formed between the fixing roll 27 and the pressure roll 29, so that the toner image is fixed onto the paper P.

[0034] The storage section 14 includes a storage member 38 and a feed roll 40. The storage member 38 stores the paper sheet P. The feed roll 40 feeds the paper sheet P stored in the storage member 38 to the transport path 20.

[0035] The transport section 16 includes a plurality of transport rolls and a transport device 42. The paper P sent out from the storage section 14 is transported along the transport path 20 by the plurality of transport rolls. The transport device 42 is disposed downstream of the transfer section NT and upstream of the fixing section NF in the paper transport direction. The transport device 42 transports the paper P onto which the toner image has been transferred while sucking it, and delivers it to the fixing device 26.

[0036] The conveying device 42 includes a conveying belt 44, a support roll 46, a drive roll 48, and a suction mechanism 50. The conveying belt 44 is a belt for conveying the paper P. The conveying belt 44 is wound around the support roll 46 and the drive roll 48. The support roll 46 is a cylindrical member and is attached to a rotating shaft to be rotatable. The support roll 46 is disposed closer to the transfer unit NT (i.e., upstream) than the suction mechanism 50. The drive roll 48 is a cylindrical member and attached to a rotating shaft to be rotatable when driven by a motor (not shown). The drive roll 48 is disposed closer to the fixing device 26 (i.e., downstream) than the suction mechanism 50. The conveying belt 44 is an endless belt. The conveying belt 44 has multiple holes formed therein that penetrate from the inside to the outside. The suction mechanism 50 draws air inside the conveying belt 44 to generate negative pressure. For example, an intake fan is disposed inside the suction mechanism 50. When the intake fan is operated, the suction mechanism 50 sucks in air above the conveyor belt 44 through multiple holes in the conveyor belt 44. As a result, the paper P is adsorbed to the conveyor belt 44. When the drive roll 48 is rotated, the conveyor belt 44 rotates to transport the paper P from the transfer unit NT to the fixing unit NF. Furthermore, when the suction mechanism 50 sucks in air above the conveyor belt 44, the paper P is adsorbed to the conveyor belt 44 and transported from the transfer unit NT to the fixing unit NF. Note that the conveyor belt 44 is an example of a transport member.

[0037] The control device 18 controls each part of the image forming apparatus 10. The control device 18 is a controller including a processor. The control device 18 is an example of a control means.

[0038] According to the image forming apparatus 10, an image is formed on the paper P as will be described below.

[0039] First, in each of the image forming units 24Y, 24M, 24C, and 24K, the surface of the photosensitive drum is charged by a charger, and the surface of the photosensitive drum is exposed to light by an exposure device to form an electrostatic latent image. The electrostatic latent image is then developed by a development device. As a result, a toner image is formed on the surface of the photosensitive drum. The toner images of each color are transferred in order to the transfer belt 30 by a primary transfer roll 32.

[0040] The paper P is sent out from the storage member 38 to the transport path 20 by the delivery roll 40, and is sent along the transport path 20 to the transfer unit NT. At the transfer unit NT, the paper P is transported between the transfer belt 30 and the secondary transfer roll 34, so that the toner image transferred to the transfer belt 30 is transferred onto the surface of the paper P.

[0041] The paper P onto which the toner image has been transferred is transported by the transport device 42 to the fixing device 26. The toner image transferred onto the surface of the paper P is fixed to the paper P by the fixing device 26. The paper P onto which the toner image has been fixed is ejected to the outside of the housing 10a.

[0042] When a toner image is also formed on the back side of the paper P, the transport unit 16 transports the paper P that has passed through the fixing device 26 along the reversal path 22 to invert the paper P, and transports the inverted paper P along the transport path 20 to the transfer unit NT. The toner image is transferred to the back side of the paper P at the transfer unit NT, and the toner image is fixed to the paper P by the fixing device 26. The paper P with the fixed toner image is discharged from the housing 10a to the outside.

[0043] 2 is an enlarged view of the transfer unit NT and the fixing unit NF, and shows a simplified view of the transfer device 25. The image forming apparatus 10 includes a transfer change mechanism 60 and a fixing change mechanism 62.

[0044] The transfer change mechanism 60 is a mechanism that moves the secondary transfer roll 34 to change the pressure state between the secondary transfer roll 34 and the transfer belt 30. The transfer change mechanism 60 may be configured to move the transfer belt 30 by moving the roll 36 to change the pressure state between the secondary transfer roll 34 and the transfer belt 30. Alternatively, the transfer change mechanism 60 may be configured to move both the secondary transfer roll 34 and the transfer belt 30 (roll 36) to change the pressure state between the secondary transfer roll 34 and the transfer belt 30. The transfer change mechanism 60 may be realized by known technology. The control device 18 controls the transfer change mechanism 60 to variably control the transfer nip load at the transfer nip NT. The transfer nip load is the load applied to the paper P at the transfer nip NT.

[0045] The fixing force change mechanism 62 is a mechanism that moves the pressure roll 29 to change the pressure contact state between the pressure roll 29 and the fixing belt 28. The fixing force change mechanism 62 may be configured to move the fixing belt 28 by moving the fixing roll 27 to change the pressure contact state between the pressure roll 29 and the fixing belt 28. Alternatively, the fixing force change mechanism 62 may be configured to move both the pressure roll 29 and the fixing belt 28 (fixing roll 27) to change the pressure contact state between the pressure roll 29 and the fixing belt 28. The fixing force change mechanism 62 may be realized by known technology. The control device 18 controls the fixing force change mechanism 62 to variably control the fixing nip load at the fixing nip NF. The fixing nip load is the load applied to the paper P at the fixing nip NF.

[0046] 3 and 4 are diagrams for explaining the control flow of the transfer nip load and the fixing nip load. In Fig. 4, the horizontal axis represents "time" and the vertical axis represents "position," and the changes in the paper position and the changes in the transfer nip load and the fixing nip load over time are shown. In Figs. 3 and 4, the following steps S1 to S4 are shown. S1 is a stage in which the paper P is passing through the transfer nip NT and before the leading edge of the paper P enters (enters) the fixing nip NF. S2 is the stage after the leading edge of the paper P enters the fixing nip NF. S3 is the stage before the trailing edge of the paper P exits the transfer nip NT. S4 is the stage after the trailing edge of the paper P has exited the transfer nip NT.

[0047] In S1, the control device 18 controls the fixing nip load changing mechanism 62 to reduce the fixing nip load. Specifically, the control device 18 reduces the fixing nip load from the first fixing nip load to a second fixing nip load, which is smaller than the first fixing nip load. This reduces the paper reaction force and paper speed fluctuations when the leading edge of the paper enters the fixing nip NF.

[0048] Also, in S1, the control device 18 controls the transfer change mechanism 60 to increase the transfer nip load. Specifically, the control device 18 increases the transfer nip load from the first transfer nip load to a second transfer nip load that is larger than the first transfer nip load. This makes it possible to prevent slippage between the paper and the transfer belt at the transfer nip NT when the leading edge of the paper enters the fixing nip NF.

[0049] In this way, by changing the transfer nip load and the fixing nip load in S1, the control device 18 suppresses image quality defects in the portion of the sheet P located at the transfer nip NT when the leading edge of the sheet P enters the fixing nip NF. The right-hand side of FIG. 2 (a top view of the sheet P) shows a smear, which may occur in the portion of the sheet P located at the transfer nip NT, as shown by a dashed line. This smear is caused by the sheet P momentarily decelerating when its leading edge enters the fixing nip NF, and is also known as impulse banding (or fixing entry impulse). According to this embodiment, the fixing nip load is reduced and the transfer nip load is increased in S1, thereby suppressing this smear. Changing only the fixing nip load or only the transfer nip load alone may result in limited improvement in image quality due to the narrow range of nip load changes. However, changing both the fixing nip load and the transfer nip load, as in this embodiment, can be expected to effectively improve image quality defects.

[0050] In S1, the timing of lowering the fixing nip load and the timing of increasing the transfer nip load may or may not be simultaneous. Also, if they are not simultaneous, either the timing of lowering the fixing nip load or the timing of increasing the transfer nip load may occur first. Note that the expression "increase the transfer nip load and decrease the fixing nip load" includes both cases where they are performed simultaneously and cases where they are performed at different times.

[0051] In S2, after the leading edge of the paper P enters the fixing nip NF, the control device 18 returns the fixing nip load to the load required for the fixing process. Specifically, the control device 18 increases the fixing nip load from the second fixing nip load to the first fixing nip load, which is larger than the second fixing nip load. At this time, the control device 18 keeps the transfer nip load increased, i.e., maintains it at the second transfer nip load (a relatively high load). This suppresses changes in the paper P's posture and speed fluctuations when the fixing nip load changes.

[0052] In S3, before the trailing edge of the paper P leaves the transfer nip NT, the control device 18 reduces the transfer nip load. Specifically, the control device 18 reduces the transfer nip load from the second transfer nip load to a smaller first transfer nip load. While the paper P is passing through the transfer nip NT, the drive torque of the transfer belt 30 (see FIG. 2) is high. After the trailing edge of the paper P leaves the transfer nip NT, the drive torque changes to a low state. The amount of change in this drive torque increases as the transfer nip load increases. When the trailing edge of the paper P leaves the transfer nip NT while the transfer nip load remains high, the drive torque changes significantly, causing the transfer belt 30 to rotate faster. This may result in image defects in the areas where the photosensitive drums of the image forming units 24Y, 24M, 24C, and 24K (see FIG. 2) are in contact with the transfer belt 30, i.e., the areas where primary transfer is occurring. That is, there is a possibility that a defect will occur in the toner image on the transfer belt 30, causing an image defect on the next sheet of paper P to which it is transferred at the transfer section NT (secondary transfer section). However, in this embodiment, as described above, the transfer nip load is reduced before the sheet of paper P exits the transfer nip NT, thereby reducing load fluctuations (driving torque fluctuations) when the sheet of paper P exits the transfer nip NT and suppressing speed fluctuations of the transfer belt 30. Therefore, it is possible to prevent image quality defects from occurring on the next sheet of paper P.

[0053] At S4, the rear end of the paper P has exited the transfer nip NT.

[0054] FIG. 5 is a diagram illustrating the variable amounts of the transfer nip load and the fixing nip load depending on the type of paper. The control device 18 may vary the amount of reduction in the fixing nip load and the amount of increase in the transfer nip load at S1 based on the type of paper being printed. The control device 18 includes a storage device, and a table such as that shown in FIG. 5 is stored in the storage device. The table associates, for example, the type of paper or paper identification information, the design nominal value of the transfer nip load, the design nominal value of the fixing nip load, the amount of increase in the transfer nip load, and the amount of decrease in the fixing nip load. The design nominal value indicates the normal load before the load is increased or decreased. Note that in FIG. 5, the design nominal value column displays symbols (A1), (A2), . . . , and (D2) instead of specific numerical values, and the columns for the amount of increase in the transfer nip load and the amount of decrease in the fixing nip load display relative magnitudes instead of specific numerical values. Note that the paper types shown in FIG. 5 are merely examples.

[0055] The control device 18 may obtain information about the paper to be printed, and obtain from the table the design nominal value of the transfer nip load, the design nominal value of the fixing nip load, the amount of increase in the transfer nip load, and the amount of decrease in the fixing nip load that are associated with the paper to be printed, and use these to set the transfer nip load and the fixing nip load at S1.

[0056] Here, because coated paper has a coated surface, increasing the transfer nip load can easily cause toner to scatter on the surface of the paper. Therefore, when the paper to be printed is coated paper, the control device 18 may reduce the increase in the transfer nip load at S1 compared to when the paper is not coated. This reduces toner scattering on the surface of the coated paper. In this case, the control device 18 may also increase the decrease in the fixing nip load by the amount that the increase in the transfer nip load is reduced. This reduces image quality defects in the portion of the coated paper located at the transfer nip NT when the leading edge of the coated paper enters the fixing nip NF.

[0057] Furthermore, because embossed paper has an embossed surface, the design nominal value of the fixing nip load is relatively high to prevent poor fixing. When printing on embossed paper, if the fixing nip load is significantly reduced before the leading edge of the paper enters the fixing nip NF, the amount of movement required to return the fixing nip load to its original state after the leading edge of the paper enters the fixing nip NF becomes large, which could result in poor fixing, for example, near the leading edge of the paper. Therefore, when the paper to be printed is embossed paper, the control device 18 may reduce the amount of reduction in the fixing nip load at S1 compared to when the paper is not embossed. In this case, the control device 18 may also increase the amount of increase in the transfer nip load by the amount of reduction in the fixing nip load. Note that with embossed paper, toner is less likely to scatter even if the transfer nip load is increased. This prevents poor image quality from occurring in the portion of the embossed paper located at the transfer nip NT when the leading edge of the embossed paper enters the fixing nip NF.

[0058] Some image forming apparatuses 10 are equipped with a glossy mode in which printing is performed by increasing the fixing nip load regardless of the type of paper being printed on. In this glossy mode, as with printing on embossed paper, if the fixing nip load is significantly reduced before the leading edge of the paper enters the fixing nip NF, the amount of movement required to return the fixing nip load to its original state after the leading edge of the paper enters the fixing nip NF increases. Therefore, when printing in the glossy mode, the control device 18 may reduce the amount of reduction in the fixing nip load at S1 compared to when printing in a mode other than the glossy mode (referred to as the normal mode). In this case, the control device 18 may increase the amount of increase in the transfer nip load by the amount of reduction in the fixing nip load.

[0059] FIG. 6 is a diagram illustrating image quality improvement according to the amount of change in the transfer nip load and the fixing nip load. This diagram relates to image quality defects that occur in the portion of the paper located at the transfer nip NT (hereinafter referred to as the target portion) when the leading edge of the paper enters the fixing nip NF. The upper part of FIG. 6 shows a schematic graph with the horizontal axis representing the amount of increase in transfer nip load and the vertical axis representing the amount of image quality improvement in the target portion. This graph is obtained, for example, by linking the transfer nip load with the amount of image quality improvement in the target portion of the printed paper (e.g., the amount of image quality improvement detected visually or by a device) while sequentially changing the transfer nip load. The graph provides a slope α, which indicates the sensitivity of the target portion to image quality improvement.

[0060] Also, at the bottom of Figure 6, a schematic graph is shown with the horizontal axis representing the "reduction in fixing nip load" and the vertical axis representing the "image quality improvement in the target area." This graph is obtained, for example, by linking the fixing nip load with the image quality improvement in the target area of ​​the printed paper (for example, the image quality improvement detected by visual inspection or a device) while sequentially changing the fixing nip load. The graph provides a slope β, which indicates the sensitivity to the improvement in the image quality of the target area.

[0061] The image quality improvement sensitivities α and β may be obtained from a graph such as that shown in Figure 6, and then the "amount of increase in transfer nip load" and the "amount of decrease in fixing nip load" may be determined using these. Specifically, the "amount of increase in transfer nip load (referred to as Δtransfer nip load)" and the "amount of decrease in fixing nip load (referred to as Δfixing nip load)" may be determined using the following formulas.

[0062] Target improvement G<α×Δ Transfer nip load + β×Δ Fixing nip load (1)

[0063] For example, if the image quality of the target area is G5 when the transfer nip load is not increased or decreased before the leading edge of the paper enters the fixing nip NF, and the target image quality of the target area is G2, then an improvement of G5 - G2 = G3 is required. Note that G2, G3, and G5 indicate the image quality levels. Therefore, the following formula is established.

[0064] G3<α×Δ Transfer nip load + β×Δ Fixing nip load (2)

[0065] Using such formulas, the "amount of increase in transfer nip load" and the "amount of decrease in fixing nip load" may be determined. As explained using FIG. 5, for coated paper, embossed paper, etc., there are limitations on the amount of increase in transfer nip load or the amount of decrease in fixing nip load, so the "amount of increase in transfer nip load" and "amount of decrease in fixing nip load" that can be selected vary depending on the paper type and running mode (glossy mode, normal mode). Therefore, it is advisable to determine the "amount of increase in transfer nip load" and the "amount of decrease in fixing nip load" taking these factors into consideration and create a table like that shown in FIG. 5.

[0066] Next, another embodiment will be described. Fig. 7 is a diagram showing the flow of control of the transfer nip load, fixing nip load, and suction force according to another embodiment. Fig. 7 is a diagram corresponding to Fig. 4. In this another embodiment, in addition to changing the transfer nip load and fixing nip load in the above embodiment, the suction force of the suction mechanism 50 of the conveying device 42 (see Fig. 2) is changed.

[0067] 7, in step S1, before the leading edge of the paper enters the fixing nip NF, the control device 18 increases the suction force of the suction mechanism 50 of the transport device 42 from a first suction force to a second suction force, which is greater than the first suction force. This causes the paper to adhere more closely to the transport belt 44. In addition to decreasing the fixing nip load and increasing the transfer nip load, increasing the suction force of the suction mechanism 50 in this manner further reduces image quality defects in the portion of the paper located at the transfer nip NT when the leading edge of the paper enters the fixing nip NF.

[0068] In S1, the timing of increasing the suction force of the suction mechanism 50 may be simultaneous with the timing of decreasing the fixing nip load or the timing of increasing the transfer nip load, or may not be simultaneous. If they are not simultaneous, either one may occur first.

[0069] Furthermore, in S2, after the leading edge of the paper enters the fixing nip NF, the control device 18 reduces the suction force of the suction mechanism 50 of the transport device 42 from the second suction force to the smaller first suction force. As a result, after the leading edge of the paper enters the fixing nip, the suction force of the suction mechanism 50 becomes relatively small, making it easier to transport the paper. The timing for reducing the suction force of the suction mechanism 50 may or may not be simultaneous with the timing for increasing the fixing nip load. If not simultaneous, either of them may occur first.

[0070] In the embodiment described above, the transfer nip load and the fixing nip load are changed while the suction force of the suction mechanism 50 is changed. However, the transfer nip load and the fixing nip load may not be changed. That is, the control device 18 may increase the suction force of the suction mechanism 50 from a first suction force to a second suction force, which is greater than the first suction force, while keeping the transfer nip load and the fixing nip load constant, and then decrease the suction force of the suction mechanism 50 from the second suction force to the first suction force after the leading edge of the paper enters the fixing nip NF. Even in this configuration, poor image quality in the portion of the paper located at the transfer nip NT is suppressed when the leading edge of the paper enters the fixing nip NF.

[0071] Furthermore, when the paper to be printed is relatively thick, the control device 18 may increase the suction force relatively greatly before the leading edge of the paper enters the fixing nip NF. That is, when the paper to be printed is thick, the control device 18 may increase the difference between the first suction force and the second suction force compared to when the paper is thin. Thick paper may experience a relatively large impact when its leading edge enters the fixing nip NF. In this configuration, when the paper is relatively thick, the suction force of the suction mechanism 50 is relatively large, so the paper adheres more closely to the conveyor belt 44, and fluctuations in the paper speed are suppressed when the paper enters the fixing nip. This suppresses poor image quality in the portion of the paper located at the transfer nip NT.

[0072] In the above embodiment, the transfer belt 30 is an example of an image carrier, and the secondary transfer roll 34 is an example of a transfer member. In one known embodiment of the image forming apparatus 10, the transfer belt 30 is omitted and the toner image on the photosensitive drum is directly transferred to the paper P. In this embodiment, a transfer nip NT is formed between the photosensitive drum and the transfer roll, and the paper P passes through the transfer nip NT. In this embodiment, the photosensitive drum is an example of an image carrier, and the transfer roll is an example of a transfer member.

[0073] The pressure roll 29 and the fixing belt 28 are an example of a pair of fixing members. In a configuration in which the fixing device 26 is configured without the fixing belt 28, a fixing section NF is formed between the pressure roll 29 and the fixing roll 27, and the pressure roll 29 and the fixing roll 27 are an example of a pair of fixing members.

[0074] In the above embodiment, the term "processor" refers to a processor in a broad sense, and includes general-purpose processors (e.g., CPU: Central Processing Unit, etc.) and dedicated processors (e.g., GPU: Graphics Processing Unit, ASIC: Application Specific Integrated Circuit, FPGA: Field Programmable Gate Array, programmable logic device, etc.).

[0075] Furthermore, the operations of the processors in the above embodiments may not only be performed by a single processor, but may also be performed by multiple processors located at physically separate locations working together. Furthermore, the operations of the processors are not limited to the order described in the above embodiments, and may be changed as appropriate.

[0076] (Addendum) (((1))) an image carrier that carries an image on its surface; a transfer member that is pressed against the image carrier to form a transfer nip and transfers the image on the image carrier to paper; a pair of fixing members disposed downstream of the transfer member in a paper transport path, and pressed against each other to form a fixing nip and perform a fixing process on the paper; a transfer change mechanism that changes the pressure contact state between the image carrier and the transfer member by moving at least one of the image carrier and the transfer member; a fixing change mechanism that moves at least one of the pair of fixing members to change the pressure contact state between the pair of fixing members; a control unit that controls the transfer change mechanism to variably control a transfer nip load at the transfer nip and also controls the fixing change mechanism to variably control a fixing nip load at the fixing nip, The control means Before the leading edge of the paper enters the fixing nip, increasing the transfer nip load from a first transfer nip load to a second transfer nip load that is greater than the first transfer nip load, and decreasing the fusing nip load from the first fusing nip load to a second fusing nip load that is less than the first fusing nip load; Image forming device. (((2))) The image forming apparatus according to (((1))), The control means changing at least one of an increase amount, which is a difference between the first transfer nip load and the second transfer nip load, and an decrease amount, which is a difference between the first fixing nip load and the second fixing nip load, based on the type of paper to be printed; Image forming device. (((3))) The image forming apparatus according to (((2))), The control means When the type of paper to be printed is coated paper, the amount of increase in the transfer nip load is made smaller than when the type of paper is not coated paper. Image forming device. (((4))) The image forming apparatus according to (((2))), The control means When the type of paper to be printed is embossed paper having an embossed surface, the amount of decrease in the fixing nip load is made smaller than when the type of paper to be printed is not embossed paper. Image forming device. (((5))) An image forming apparatus according to any one of (((1))) to (((4))), a conveying device having a suction mechanism that sucks air to generate negative pressure, and conveying the paper while adsorbing the paper to the conveying member by the suction mechanism; the conveying device is disposed between the transfer nip and the fixing nip, The control means increasing the suction force of the suction mechanism from a first suction force to a second suction force greater than the first suction force before the leading edge of the paper enters the fixing nip; After the leading edge of the paper enters the fixing nip, the suction force of the suction mechanism is reduced from the second suction force to the first suction force; When the thickness of the paper to be printed is a first thickness, the difference between the first suction force and the second suction force is made larger than when the thickness of the paper to be printed is a second thickness that is thinner than the first thickness. Image forming device. (((6))) An image forming apparatus according to any one of (((1))) to (((5))), The control means After the leading edge of the paper enters the fixing nip, increasing the fixing nip load from the second fixing nip load to the first fixing nip load while maintaining the transfer nip load at the second transfer nip load; Image forming device. (((7))) The image forming apparatus according to (((6))), The control means Before the trailing edge of the paper exits the transfer nip, reducing the transfer nip load from the second transfer nip load to the first transfer nip load; Image forming device. (((8))) An image forming apparatus according to any one of (((1))) to (((7))), a conveying device having a suction mechanism that sucks air to generate negative pressure, and conveying the paper while adsorbing the paper to the conveying member by the suction mechanism; the conveying device is disposed between the transfer nip and the fixing nip, The control means increasing the suction force of the suction mechanism from a first suction force to a second suction force greater than the first suction force before the leading edge of the paper enters the fixing nip; After the leading edge of the paper enters the fixing nip, the suction force of the suction mechanism is reduced from the second suction force to the first suction force. Image forming device. (((9))) an image carrier that carries an image on its surface; a transfer member that is pressed against the image carrier to form a transfer nip and transfers the image on the image carrier to paper; a pair of fixing members disposed downstream of the transfer member in a paper transport path, and pressed against each other to form a fixing nip and perform a fixing process on the paper; a conveying device having a suction mechanism that sucks air to generate negative pressure, and conveying the paper while adsorbing the paper to a conveying member by the suction mechanism; a control means; the conveying device is disposed between the transfer nip and the fixing nip, The control means increasing the suction force of the suction mechanism from a first suction force to a second suction force greater than the first suction force before the leading edge of the paper enters the fixing nip; After the leading edge of the paper enters the fixing nip, the suction force of the suction mechanism is reduced from the second suction force to the first suction force. Image forming device.

[0077] According to the invention related to (((1))), it is possible to prevent poor image quality from occurring in the portion of the paper that is positioned in the transfer nip at the timing when the leading edge of the paper enters the fixing nip. According to the invention related to (((2))), at least one of the amount of increase in the transfer nip load and the amount of decrease in the fixing nip load is set according to the type of paper. According to the invention (((3))), the occurrence of poor image quality on coated paper is suppressed. According to the invention related to (((4))), the occurrence of poor image quality on embossed paper is suppressed. According to the invention related to (((5))), the occurrence of poor image quality when using relatively thick paper is suppressed. According to the invention related to (((6))), the change in the position of the paper when the fixing nip load changes is suppressed. According to the invention related to (((7))), the speed increase of the image carrier when the trailing edge of the paper leaves the transfer nip is suppressed. According to the invention related to (((8))), fluctuations in the speed of the paper are suppressed when the leading edge of the paper enters the fixing nip. According to the invention (((9))), it is possible to prevent poor image quality from occurring in the portion of the paper that is positioned in the transfer nip at the timing when the leading edge of the paper enters the fixing nip. [Explanation of symbols]

[0078] 10 image forming apparatus, 10a housing, 12 image forming unit, 14 storage unit, 16 conveying unit, 18 control device, 20 conveying path, 22 reversing path, 24Y, 24M, 24C, 24K image forming unit, 25 transfer device, 26 fixing device, 27 fixing roll (fixing member), 28 fixing belt, 29 pressure roll (fixing member), 30 transfer belt, 32 primary transfer roll, 34 secondary transfer roll (transfer member), 36 roll, 38 storage member, 40 delivery roll, 42 conveying device, 44 conveying belt, 46 support roll, 48 drive roll, 50 suction mechanism, 60 transfer change mechanism, 62 fixing change mechanism, NT transfer unit (transfer nip), NF fixing unit (fixing nip), P paper.

Claims

1. an image carrier that carries an image on its surface; a transfer member that is pressed against the image carrier to form a transfer nip and transfers the image on the image carrier to paper; a pair of fixing members disposed downstream of the transfer member in a paper transport path, and pressed against each other to form a fixing nip and perform a fixing process on the paper; a transfer change mechanism that changes the pressure contact state between the image carrier and the transfer member by moving at least one of the image carrier and the transfer member; a fixing change mechanism that moves at least one of the pair of fixing members to change the pressure contact state between the pair of fixing members; a control unit that controls the transfer change mechanism to variably control a transfer nip load at the transfer nip and also controls the fixing change mechanism to variably control a fixing nip load at the fixing nip, The control means Before the leading edge of the paper enters the fixing nip, increasing the transfer nip load from a first transfer nip load to a second transfer nip load that is greater than the first transfer nip load, and decreasing the fusing nip load from the first fusing nip load to a second fusing nip load that is less than the first fusing nip load; Image forming device.

2. 2. The image forming apparatus according to claim 1, The control means changing at least one of an increase amount, which is a difference between the first transfer nip load and the second transfer nip load, and an decrease amount, which is a difference between the first fixing nip load and the second fixing nip load, based on the type of paper to be printed; Image forming device.

3. 3. The image forming apparatus according to claim 2, The control means When the type of paper to be printed is coated paper, the amount of increase in the transfer nip load is made smaller than when the type of paper is not coated paper. Image forming device.

4. 3. The image forming apparatus according to claim 2, The control means When the type of paper to be printed is embossed paper having an embossed surface, the amount of decrease in the fixing nip load is made smaller than when the type of paper to be printed is not embossed paper. Image forming device.

5. 2. The image forming apparatus according to claim 1, a conveying device having a suction mechanism that sucks air to generate negative pressure, and conveying the paper while adsorbing the paper to the conveying member by the suction mechanism; the conveying device is disposed between the transfer nip and the fixing nip, The control means By the time the leading edge of the paper enters the fixing nip, the suction force of the suction mechanism is increased from a first suction force to a second suction force that is greater than the first suction force; After the leading edge of the paper enters the fixing nip, the suction force of the suction mechanism is reduced from the second suction force to the first suction force; When the thickness of the paper to be printed is a first thickness, the difference between the first suction force and the second suction force is made larger than when the thickness of the paper to be printed is a second thickness that is thinner than the first thickness. Image forming device.

6. 2. The image forming apparatus according to claim 1, The control means After the leading edge of the paper enters the fixing nip, increasing the fixing nip load from the second fixing nip load to the first fixing nip load while maintaining the transfer nip load at the second transfer nip load; Image forming device.

7. 7. The image forming apparatus according to claim 6, The control means Before the trailing edge of the paper exits the transfer nip, reducing the transfer nip load from the second transfer nip load to the first transfer nip load; Image forming device.

8. 8. The image forming apparatus according to claim 6, wherein: a conveying device having a suction mechanism that sucks air to generate negative pressure, and conveying the paper while adsorbing the paper to the conveying member by the suction mechanism; the conveying device is disposed between the transfer nip and the fixing nip, The control means By the time the leading edge of the paper enters the fixing nip, the suction force of the suction mechanism is increased from a first suction force to a second suction force that is greater than the first suction force; After the leading edge of the paper enters the fixing nip, the suction force of the suction mechanism is reduced from the second suction force to the first suction force. Image forming device.

9. an image carrier that carries an image on its surface; a transfer member that is pressed against the image carrier to form a transfer nip and transfers the image on the image carrier to paper; a pair of fixing members disposed downstream of the transfer member in a paper transport path, and pressed against each other to form a fixing nip and perform a fixing process on the paper; a conveying device having a suction mechanism that sucks air to generate negative pressure, and conveying the paper while adsorbing the paper to a conveying member by the suction mechanism; a control means; the conveying device is disposed between the transfer nip and the fixing nip, The control means By the time the leading edge of the paper enters the fixing nip, the suction force of the suction mechanism is increased from a first suction force to a second suction force that is greater than the first suction force; After the leading edge of the paper enters the fixing nip, the suction force of the suction mechanism is reduced from the second suction force to the first suction force. Image forming device.

Images