JP5810800B2 - Image forming apparatus - Google Patents

Description

  The present invention relates to an image forming apparatus.

  Conventionally, electrophotographic image forming apparatuses are known as printers, copiers, and the like. For example, a tandem color image forming apparatus forms a transfer nip between an intermediate transfer belt and a transfer roller, and transfers an image to a sheet in the process of passing through the transfer nip, and then fixes the sheet. The image is formed on the paper through a series of processes of applying the process. By the way, in such an image forming apparatus, there is a problem that an impact is generated when the leading edge of the paper enters the transfer nip, resulting in a decrease in image quality such as primary transfer deviation. Such a phenomenon becomes more prominent in a sheet having high rigidity such as a sheet having a large basis weight. In view of this, there is known a method for mitigating the impact at the time of entering the paper by relaxing the pressure contact state of the transfer nip in response to the entrance of the leading edge of the paper.

  For example, Patent Document 1 includes means for releasing the support of the transfer roller by the support mechanism due to the thickness of the paper that has entered between the transfer nips when the leading edge of the paper having a predetermined thickness or more enters the transfer nip. An image forming apparatus is disclosed. The image forming apparatus is designed to increase the pressure state of the transfer nip again after releasing the support of the transfer roller by the support mechanism.

JP 2007-10840 A

  However, according to the technique disclosed in Patent Document 1, a plurality of springs are combined and the support of the transfer roller is mechanically released by using the force relationship between these springs, or the pressure state of the transfer nip is mechanically changed. Or increase it. Therefore, the support of the transfer roller is released after the leading edge of the paper enters the transfer nip and the thickness of the paper acts between the transfer nips. For this reason, when the leading edge of the paper enters, the pressure in the transfer nip remains high, and there is still a concern about the occurrence of the impact described above. Further, even when the pressure state of the transfer nip is increased again, it is merely mechanically restored, so that the time concept such as by which timing the pressure state of the transfer nip is increased again. Is not necessarily adopted. Therefore, when the pressure contact state of the transfer nip is relaxed, there is a possibility that the transfer performance is deteriorated due to various impacts accompanying the conveyance of the paper.

  The present invention has been made in view of such circumstances, and its purpose is to suppress a decrease in transfer performance due to various impacts accompanying the conveyance of the paper while reducing the impact caused by the entry of the paper into the transfer nip. There is to do.

In order to solve this problem, the present invention provides an image carrier that carries an image on the surface, a transfer member that presses against the image carrier to form a transfer nip, and transfers an image on the image carrier to paper. An image including: a movable mechanism that moves the transfer member to set a pressure contact state between the image carrier and the transfer member; and a control unit that variably controls the nip load in the transfer nip by controlling the movable mechanism. A forming apparatus is provided. Here, the control unit controls the image on the image carrier that controls the nip load when the leading edge of the paper enters the transfer nip based on the conveyance position of the paper after the leading edge of the paper passes through the transfer nip. Is controlled to a second load larger than the first load, which is the nip load that can be transferred to the sheet .

  Here, the present invention further includes a fixing device that is disposed downstream of the transfer member in the paper conveyance path and performs fixing processing on the paper by a pair of fixing members that are pressed against each other to form a fixing nip, and is controlled. The section preferably controls the nip load to the second load before the leading edge of the paper enters the fixing nip.

  In addition, the present invention further includes a registration roller that is disposed upstream of the transfer member in the paper conveyance path and includes a pair of conveyance rollers that are in pressure contact with each other. It is preferable to control the nip load to the second load before passing the roller.

  Further, in the present invention, the control unit applies the nip load until the earlier timing of the timing when the leading edge of the sheet enters the fixing nip and the timing when the trailing edge of the sheet passes the registration roller, whichever is earlier. It is preferable to control to the load.

  In addition, the present invention further includes a registration roller that is disposed upstream of the transfer member in the paper conveyance path, and includes a pair of conveyance rollers that can be pressed against or separated from each other. It is preferable to perform switching control for switching the pair of transport rollers, which are the registration rollers, from the press-contact state to the separated state during transport of the nip roller, and to control the nip load to the second load by the start timing of the switching control. .

  In the present invention, the control unit controls the nip load to the second load by the earlier timing of the timing at which the leading edge of the sheet enters the fixing nip and the timing for starting execution of the switching control. It is preferable to do.

  Further, in the present invention, the control unit sets the nip load from the second load between the sheets after the trailing edge of the preceding sheet passes through the transfer nip until the leading edge of the succeeding sheet enters the transfer nip. It is preferable to control to a small third load.

  Moreover, in this invention, it is preferable that a control part sets a 3rd load to a load smaller than a 1st load.

  In the present invention, it is preferable that the transfer member is configured to rotate following the rotation of the image carrier that is rotationally driven.

  The present invention preferably further includes a power supply unit that supplies a transfer current for applying a transfer voltage to the transfer member, and the control unit variably controls the transfer current in accordance with the nip load.

According to the present invention, after the leading edge of the paper passes, the impact caused by the paper entering the transfer nip is reduced by controlling the nip load to the second load based on the paper transport position. The nip load can be increased more than the first load set as Thereby, even if the impact accompanying conveyance of a sheet | seat arises, the fall of transfer performance can be suppressed. Further, as a premise that the sheet enters the transfer nip, the impact caused by the entry of the sheet into the transfer nip can be reduced by controlling the nip load to the first load.

Configuration diagram schematically showing an image forming apparatus Explanatory drawing schematically showing the configuration of the secondary transfer unit Block diagram schematically showing a control system of an image forming apparatus Explanatory drawing showing the correspondence between nip load and secondary transfer current Flow chart showing the procedure of variable control of nip load by the image forming apparatus Explanatory drawing showing transition of nip load in variable control Explanatory drawing showing transition of nip load in variable control

(First embodiment)
FIG. 1 is a configuration diagram schematically illustrating an image forming apparatus according to the first embodiment. This image forming apparatus is an electrophotographic image forming apparatus such as a copying machine, and forms a full-color image by arranging a plurality of photoconductors facing one intermediate transfer belt in a vertical direction. A tandem type color image forming apparatus.

  The image forming apparatus mainly includes a document reading device SC, four sets of image forming units 10Y, 10M, 10C, and 10K, and a fixing device 30, and these are housed in one casing.

  The document reader SC scans and exposes an image of the document with the optical system of the scanning exposure device, reads the reflected light with a line image sensor, and obtains an image signal. The image signal is subjected to processing such as A / D conversion, shading correction, and compression, and then input to the control unit 50 as image data. Note that the image data input to the control unit 50 is not limited to data read by the document reading device SC, and is, for example, data received from a personal computer connected to the image forming apparatus or another image forming apparatus. Also good.

  The four image forming units 10Y, 10M, 10C, and 10K include an image forming unit 10Y that forms a yellow (Y) image, an image forming unit 10M that forms a magenta (M) image, and a cyan (C) image. An image forming unit 10 </ b> C to be formed and an image forming unit 10 </ b> K that forms a black (K) image.

  The image forming unit 10Y includes a photosensitive drum 1Y and a charging unit 2Y, an optical writing unit 3Y, a developing device 4Y, and a drum cleaner 5Y arranged around the photosensitive drum 1Y. Similarly, the image forming units 10M, 10C, and 10K also include the photosensitive drums 1M, 1C, and 1K and the charging units 2M, 2C, and 2K disposed in the periphery thereof, the optical writing units 3M, 3C, and 3K, and the developing devices 4M, 4C, 4K and drum cleaners 5M, 5C, 5K.

  The surfaces of the photosensitive drums 1Y to 1K are uniformly charged by the charging units 2Y to 2K, and latent images are formed on the photosensitive drums 1Y to 1K by scanning exposure by the optical writing units 3Y to 3K. Is done. The developing devices 4Y to 4K develop the latent images on the photosensitive drums 1Y to 1K by developing with toner. As a result, toner images corresponding to yellow, magenta, cyan, and black are formed on the respective photosensitive drums 1Y to 1K. The toner images formed on the photosensitive drums 1Y to 1K are sequentially transferred to predetermined positions on the intermediate transfer belt 6 rotated by the primary transfer rollers 7Y, 7M, 7C, and 7K. The intermediate transfer belt 6 is rotatably spanned around a plurality of rollers including the opposing roller 8 and functions as an image carrier that carries an image on the surface.

  The toner image composed of each color transferred onto the intermediate transfer belt 6 is transferred onto the paper P by the secondary transfer unit 20 when the paper P is conveyed at a predetermined timing as will be described later.

  FIG. 2 is an explanatory diagram schematically showing the configuration of the secondary transfer unit 20. The secondary transfer unit 20 is mainly composed of a secondary transfer roller 21 and a movable mechanism.

  The secondary transfer roller 21 is disposed so as to be pressed against the opposing roller 8 via the intermediate transfer belt 6, thereby forming a transfer nip by pressing against the intermediate transfer belt 6, and the sheet P in the transfer nip. The image on the intermediate transfer belt 6 is transferred onto the paper P in correspondence with the passage. A first rotating shaft 22 is disposed in the vicinity of the secondary transfer roller 21, and a support metal plate 23 is rotatably attached to the first rotating shaft 22. Further, the support metal plate 23 rotatably supports the secondary transfer roller 21, whereby the secondary transfer roller 21 rotates around the first rotation shaft 22 together with the support metal plate 23. Note that the secondary transfer roller 21 is configured to rotate following the rotation of the intermediate transfer belt 6 that is rotationally driven.

  The movable mechanism is a mechanism for setting the pressure contact state between the intermediate transfer belt 6 and the secondary transfer roller 21 by moving the secondary transfer roller 21. Specifically, the movable mechanism includes a movable arm 24 bent in an L shape, and the movable arm 24 is attached to the second rotating shaft 26 so as to be rotatable. A compression coil spring 25 is disposed between one end of the movable arm 24 and the support sheet metal 23, and the secondary transfer roller 21 causes the intermediate transfer belt 6 ( It is pressed to the opposite roller 8) side. Further, a cam 27 connected to the rotation shaft is provided at the other end of the movable arm 24 so as to be in contact therewith. The cam 27 displaces the position of the other end of the movable arm 24 according to the rotational position. When the movable arm 24 rotates about the second rotation shaft 26 through such displacement, the relative distance between the movable arm 24 and the support metal plate 23 is displaced. Depending on the displacement of the relative distance and the pressing force of the compression coil spring 25, the pressure contact state between the intermediate transfer belt 6 and the secondary transfer roller 21, that is, the nip load at the transfer nip can be increased or decreased.

  In addition, a high voltage power supply unit 29 is connected to the secondary transfer roller 21. The high-voltage power supply unit 29 supplies a secondary transfer current to the secondary transfer roller 21, whereby a voltage (secondary transfer voltage) necessary for the secondary transfer is applied to the secondary transfer roller 21.

  Referring again to FIG. 1, the paper feed unit 40 includes a plurality of paper feed trays 41 each containing paper P. The paper P stored in the paper feed tray 41 has one sheet on the top surface. It is taken in and sent out to the transport path. In this transport path, a plurality of transport rollers including a registration roller 42 are arranged on the upstream side of the transfer nip, that is, the transfer position where the toner image is transferred to the paper P. After the paper P sent out from the paper feed tray 41 is transported by a plurality of transport rollers, the image on the intermediate transfer belt 6 is registered with a registration roller 42 located upstream (directly above) the transfer nip in the transport path of the paper P. It is conveyed to the transfer nip at a predetermined timing so as to be synchronized.

  The sheet P on which the toner image has been transferred is conveyed to a fixing device 30 located downstream (directly below) the transfer nip in the sheet P conveyance path. The fixing device 30 is a device that fixes an unfixed toner image on the paper P, and includes, for example, a pair of fixing rollers 31 and 32 that form a fixing nip and a heater that heats one fixing roller 31. Has been. In the fixing device 30, the sheet P passes through the fixing nip in the process of transporting the sheet P, so that the sheet P is applied to the sheet through the action of the pressure of the pair of fixing rollers 31 and 32 and the heat of the fixing roller 31. The toner image is fixed.

  The paper P on which the fixing process has been performed by the fixing device 30 is discharged by a paper discharge roller to a paper discharge tray 43 attached to the outer side surface of the housing. When image formation is also performed on the back surface of the paper P, the paper P on which image formation has been completed on the front surface of the paper is conveyed by the guide member 45 to the reversal conveyance unit 46 located below. The reverse conveyance unit 46 reverses the conveyed paper P and sends it out to the refeed conveyance path. The paper P sent out to the re-feed transport path is transported by a plurality of transport rollers, thereby returning the paper P to the transport path upstream of the transfer nip.

  FIG. 3 is a block diagram schematically showing the configuration of the control system of the image forming apparatus according to the present embodiment. The control unit 50 has a function of controlling the image forming apparatus in an integrated manner, and a microcomputer including a CPU, a memory such as a ROM and a RAM, a communication I / F, and the like can be used.

The control unit 50 controls each unit of the image forming apparatus to execute the following processes and form a toner image on the paper P.
(1) Charge the photosensitive drums 1Y to 1K (2) Form the electrostatic latent images on the photosensitive drums 1Y to 1K by the optical writing units 3Y to 3K (3) Toner on the formed electrostatic latent images (4) The toner images on the photosensitive drums 1Y to 1K are primarily transferred to the intermediate transfer belt 6. (5) The toner images on the intermediate transfer belt 6 are secondarily transferred to the paper P. (6) The fixing device 30. To fix the toner image transferred onto the paper P

Further, the control unit 50 controls the cam drive motor 28 in relation to the present embodiment. The cam drive motor 28 is connected to the rotation shaft of the cam 27 via a power transmission mechanism such as a gear, and the cam 27 can be set to an arbitrary rotation position according to the drive amount of the cam drive motor 28. . Therefore, the control unit 50 can variably control the nip load in the transfer nip by controlling the cam drive motor 28 as necessary. As one of the features of this embodiment, after the leading edge of the paper P passes through the transfer nip, the control unit 50 determines the nip load based on the transport position of the paper P, and the leading edge of the paper P enters the transfer nip. The purpose is to control the maximum load (second load) larger than the initial load (first load) which is the nip load to be controlled.

  Furthermore, as one of the features of the present embodiment, the control unit 50 can variably control the secondary transfer current supplied to the secondary transfer roller 21 by controlling the high-voltage power supply unit 29. As shown in FIG. 4, the control unit 50 holds a map or an arithmetic expression that defines the correspondence between the nip load and the secondary transfer current, and controls the secondary transfer current according to the nip load. .

  Various signals are input to the control unit 50 in order to perform the above-described control. The sheet detection sensor 51 is a sensor that detects the sheet P conveyed on the conveyance path, and is disposed, for example, between the registration roller 42 and the transfer nip. As the paper detection sensor 51, for example, a reflective optical sensor having a light emitting element and a light receiving element can be used. The paper detection sensor 101 normally outputs an off signal, and outputs an on signal while the paper P passes the detection position of the sensor. The passage of the leading edge and the trailing edge of the paper P can be determined in accordance with the signal rise and fall of the paper detection sensor 101. As a result, the control unit 50 can recognize the passage of the paper P at the sensor position based on the detection signal of the paper detection sensor 51, and the conveyance speed of the paper P, the size of the paper P, and the sensor of the paper P The transport position of the paper P in the transport path can be recognized on the basis of the position passage timing.

  The secondary transfer current sensor 52 is disposed in a current path between the high voltage power supply unit 29 and the secondary transfer roller 21, and detects the secondary transfer current from the high voltage power supply unit 29. Based on the detection signal of the secondary transfer current sensor 52, the control unit 50 can feedback-control the secondary transfer current supplied from the high voltage power supply unit 29 to a desired value.

  The operation unit 60 is disposed on the upper part of the image forming apparatus (see FIG. 1), and outputs various information set by the user to the control unit 50. As the operation unit 60, for example, a touch panel capable of performing an input operation according to information displayed on the display can be used. Through the operation unit 60, the user can set printing conditions, that is, the type (for example, size and basis weight) of the paper P to be printed, the number of copies, and the like. The type of paper P may be set by specifying the paper feed tray 41 by associating the paper feed tray 41 with the type of paper P accommodated therein in advance. Further, the control unit 50 can display various messages to the user via the operation unit 60 by controlling the operation unit 60. Various information set by the user may be output to the control unit 50 from a personal computer connected to the image forming apparatus or another image forming apparatus without using the operation unit 60.

  FIG. 5 is a flowchart showing a procedure for variable control of the nip load by the image forming apparatus according to the present embodiment. The processing shown in this flowchart is called according to a job execution command from the user and executed by the control unit 50. When this process is executed, the control unit 50 can recognize the print information of the job to be executed based on the information output from the operation unit 60. Further, the control unit 50 can control the next transfer current according to the nip load by controlling the high voltage power supply unit 29 in response to the control of the nip load described later.

  First, in step 10 (S10), the control unit 50 determines whether or not the paper P to be printed is a target paper for variably controlling the nip load. The variable control of the nip load according to the present embodiment is performed for the highly rigid paper P. Therefore, the control unit 50 determines whether or not the paper P to be printed is a highly rigid paper P.

  Specifically, the control unit 50 determines whether the basis weight of the paper P is equal to or greater than a determination threshold value. This is because the rigidity of the paper P has a large correlation with the basis weight of the paper P, and the rigidity of the paper P tends to increase as the basis weight of the paper P increases. Therefore, an optimum value is set in advance as a basis value of the determination threshold as a basis weight of the paper P that determines whether variable control is possible or not by experiment or simulation. For example, a value such as 130 g / m ^ 2 can be used as the determination threshold. If a negative determination is made in step 10, that is, if the basis weight of the paper P is smaller than the determination threshold value, the process proceeds to step 11 (S11). On the other hand, if an affirmative determination is made in step 10, that is, if the basis weight of the paper P is greater than or equal to the determination threshold, the process proceeds to step 12 (S12).

  In step 11, the control unit 50 normally controls the nip load of the transfer nip. In this normal control, the nip load is constantly applied to all the sheets P from the first sheet P corresponding to the job to the final sheet P (final sheet) based on a normal load set in advance. It is a technique to control. Here, the normal load is a load set from the viewpoint of maintaining good transfer performance, and the optimum value is set in advance through experiments and simulations. For example, a value such as 6 kg can be used as the normal load.

  In step 12, the controller 50 determines a change point (hereinafter referred to as “control change point”) for changing the nip load prior to the execution of the variable control of the nip load. As will be described later, when the nip load is controlled to the maximum load, it is necessary to execute this until the sheet conveyance position reaches a predetermined point. That is, this control change point defines the sheet transport position where the control of the nip load is completed to the maximum load as a predetermined timing. In the present embodiment, the control change point is a timing at which an impact accompanying the sheet conveyance occurs while the sheet P passes through the transfer nip. Specifically, the timing at which the leading edge of the sheet P enters the fixing nip. Or, the timing at which the trailing edge of the paper P passes the registration roller is applicable. Further, in the present embodiment, these events occur while the paper P passes through the transfer nip, so the earlier one of these timings corresponds to the control change point. When determining this control change point, the size of the paper P (the length in the paper conveyance direction), the distance between the transfer nip and the fixing nip, and the distance between the transfer nip and the registration roller 42 are considered. Is done.

  In step 13 (S13), the control unit 50 starts conveying the paper P from the paper supply unit 40.

  In step 14 (S14), the control unit 50 controls the nip load to the initial load. This initial load is set to the smallest load in the nip load range in which the toner image can be transferred to the paper P, and the optimum value is set in advance through experiments and simulations. This initial load is a value smaller than the aforementioned normal load, and for example, a value of 3 kg can be used.

  In step 15 (S15), the control unit 50 determines whether or not the leading edge of the paper P has passed through the transfer nip. If an affirmative determination is made in step 15, that is, if the leading edge of the paper P has passed through the transfer nip, the process proceeds to step 16 (S16). On the other hand, if a negative determination is made in step 15, that is, if the leading edge of the paper P has not passed through the transfer nip, the process returns to step 15.

In step 16, the control unit 50 performs nip load increase control. This increase control refers to control in which the nip load is increased from the initial load to the maximum load until the control change point determined in step 12 is reached, and thereafter the nip load is maintained at the maximum load. Here, the maximum load can be the same value as the normal load described above. Further, when increasing the nip load from the initial load to the maximum load, the control unit 50 can use a method of increasing the nip load in proportion to the increase in time, for example. However, other methods are not excluded, and it may be increased according to a quadratic function of time, or an instantaneous increase in nip load is periodically repeated to increase the nip load stepwise. May be.

  In step 17 (S17), the control unit 50 determines whether the trailing edge of the paper P has passed through the transfer nip. If an affirmative determination is made in step 17, that is, if the rear end of the paper P has passed the transfer nip, the process proceeds to step 18 (S18). On the other hand, if a negative determination is made in step 17, that is, if the trailing edge of the paper P has not passed the transfer nip, the process returns to step 17.

  In step 18, the control unit 50 controls the nip load to the minimum load (third load). This minimum load is a load that is set in consideration of reducing wear and improving durability of the secondary transfer roller 21, and an optimum value is set in advance through experiments and simulations. This initial load is smaller than the above-described initial load, and a value such as 1 kg can be used.

  In step 19 (S19), the control unit 50 determines whether or not the sheet P that has passed through the transfer nip is the final sheet in the job. If the determination in step 19 is affirmative, that is, if the paper P is the final paper, this routine is terminated. On the other hand, if a negative determination is made in step 19, that is, if the paper P is not the final paper, the process proceeds to step 20 (S20).

  In step 20, the control unit 50 determines whether or not the paper P of the next page has reached the transfer nip, that is, whether or not the leading edge of the paper P has reached a position a predetermined distance before the transfer nip. If the determination in step 20 is affirmative, that is, if the paper P of the next page reaches the transfer nip, the process returns to step 14 described above. On the other hand, if a negative determination is made in step 20, that is, if the paper P of the next page has not reached the transfer nip, the process returns to step 20.

  FIG. 6 is an explanatory diagram showing the transition of the nip load in the variable control. FIG. 4A shows the transition of the nip load on the A3 size paper P, and FIG. 4B shows the transition of the nip load on the A4 size paper P. FIG. Here, “ta” is the timing when the leading edge of the paper P reaches the transfer nip, “tb” is the timing when the leading edge of the paper P reaches the fixing nip, and “tc” is the trailing edge of the paper P passing through the registration roller 42. “Td” indicates the timing at which the trailing edge of the paper P passes through the transfer nip.

As described above, in the present embodiment, the image forming apparatus controls the nip load to a maximum load larger than the initial load based on the conveyance position of the paper P after the leading edge of the paper P passes through the transfer nip. Here, the initial load corresponds to a nip load that is controlled when the leading edge of the paper P enters the transfer nip.

According to such a configuration, after passing the leading edge of the paper P, the nip load can be increased more than the initial load by controlling the nip load to the maximum load based on the transport position of the paper P. As a result, the pressure contact state in the transfer nip can be strengthened before an impact caused by the conveyance of the paper P occurs. Therefore, even when the impact occurs, a paper shift or the like is suppressed, and the transfer performance is deteriorated. Can be suppressed. Further, as a premise that the paper P enters the transfer nip, the impact caused by the paper P entering the transfer nip can be reduced by controlling the nip load to the initial load. As a result, deterioration in image quality can be suppressed, and high-quality image formation can be realized.

  In the present embodiment, the image forming apparatus controls the nip load to the maximum load before the leading edge of the paper P enters the fixing nip. When the leading edge of the paper P enters the fixing nip, an impact is likely to occur. When the nip load is small, the paper P may be displaced due to the generated impact, which may cause a transfer failure. In this respect, in the present embodiment, by controlling the nip load to the maximum load before the leading edge of the paper P enters the fixing nip, it is possible to suppress a decrease in transfer performance.

  In this embodiment, the image forming apparatus controls the nip load to the maximum load until the trailing edge of the paper P passes the registration roller 42. When the rear end of the paper P passes the registration roller 42, an impact is likely to occur. When the nip load is small, the paper P may be displaced due to the generated impact, which may cause a transfer failure. In this regard, in the present embodiment, by controlling the nip load to the maximum load before the trailing edge of the paper P passes the registration roller 42, it is possible to suppress a decrease in transfer performance.

Further, in the present embodiment, the image forming apparatus has the earlier timing among the timing when the leading edge of the paper P enters the fixing nip and the timing when the trailing edge of the paper P passes the registration roller 42, whichever is earlier. The nip load is controlled to the maximum load. According to such a configuration, the nip load can be controlled to the maximum load when both events occur by controlling the nip load in conjunction with the factor that causes the early occurrence of the impact among the factors of the impact. Thereby, the fall of transfer performance can be suppressed.

  Further, in the present embodiment, the image forming apparatus maximizes the nip load between the sheets after the leading end of the preceding sheet P passes through the transfer nip and before the leading end of the succeeding sheet P enters the transfer nip. The minimum load that is smaller than the load is controlled. As a result, the nip load can be reduced before the subsequent paper P enters the fixing nip, so that the impact caused by the paper P entering the transfer nip can be reduced.

  In this embodiment, the image forming apparatus sets the minimum load to a load smaller than the initial load. According to this configuration, it is possible to reduce the wear of the secondary transfer roller 21 and improve durability.

  In the present embodiment, the secondary transfer roller 21 is configured to rotate following the rotation of the intermediate transfer belt 6 that is rotationally driven. The merits of such a configuration are as follows.

  For example, when power is transmitted to the secondary transfer roller 21 from the drive mechanism of the intermediate transfer belt 6 via a gear, the crushing amount of the secondary transfer roller 21 changes according to the change in the nip load. For this reason, the distance between the centers of the gears changes, power is not transmitted normally, and image defects may occur. Even when the secondary transfer roller 21 is equipped with a drive motor alone, the roller linear velocity changes due to the change in the amount of crushing of the secondary transfer roller 21, so that it cannot be synchronized with the intermediate transfer belt 6. Image defects may occur. For the above reasons, when the nip load of the transfer nip is variably controlled, it is desirable that the secondary transfer roller is driven.

  In this embodiment, the image forming apparatus variably controls the transfer current according to the nip load. The merits of such a configuration are as follows.

  When the crushing amount of the secondary transfer roller 21 changes according to the change in the nip load, the nip area also changes at the same time. The nip area affects the electrical resistance value between the rollers. For example, when the nip load is gradually increased, the resistance value is decreased, and as a result, the transfer efficiency is increased and the image is gradually darkened. Therefore, the occurrence of such a phenomenon can be suppressed by variably controlling the transfer current according to the nip load.

  In this embodiment, the rigidity of the paper P is determined from the basis weight of the paper P. However, in addition to this, the rigidity of the paper P is determined in consideration of the manufacturer of the paper P and the direction of the paper. Also good.

(Second Embodiment)
The image forming apparatus according to the second embodiment will be described below. The image forming apparatus according to the second embodiment is different from that of the first embodiment in that the control change point is determined in consideration of the separation timing of the registration roller 42. The description of the same configuration as that of the first embodiment will be omitted, and the following description will focus on the differences.

In this embodiment, the resist roller 42, while being configured to be example cut conversion in a pair of rollers that constitute this press contact state and the separated state, a direction orthogonal to the transport direction of the sheet P (hereinafter "sheet width It is configured to be swingable with respect to the “direction”. Switching between the pressure contact state and the separation state and the swinging state with respect to the registration roller 12 is controlled by the control unit 50.

  When the image forming apparatus is actually operated, there may occur a situation in which a deviation occurs in the conveyance position of the paper P in the paper width direction and the paper P deviates from the original conveyance position. When the paper P is transported over a relatively long distance, such as when the paper P is transported to the transfer nip via the reverse transporting section 46, the paper P is offset. It can be caused by the influence of an error. In addition, the initial storage state of the paper P stored in the paper feed tray 41 can be a factor of the deviation of the paper P.

  Therefore, the control unit 11 adjusts the transport position of the paper P by swinging the transport of the paper P by the registration roller 42. By this resist swing, the toner image is formed on the paper P in a state where the positions of the paper P and the toner image on the intermediate transfer belt 6 are appropriately matched.

  When performing such registration swinging, when performing registration swinging, the registration roller 42 swings in accordance with the offset amount of the paper P after being set at a prescribed home position, and then Is returned to the home position toward the resist swinging with respect to the paper P. Although such a return to the home position is performed between papers, there is a limit to the operation between papers in order to meet the recent demand for higher speeds. Therefore, the registration roller 42 is swung according to the deviation amount, and when the paper P enters the transfer nip, the switching control is performed to switch the registration roller 42 from the pressure contact state to the separation state. The registration roller 42 is returned to the home position. Then, the registration rollers 42 are brought into pressure contact with each other between the sheets, and then conveyed and prepared for the sheet P.

In the image forming apparatus having such a configuration, a shock is transmitted to the paper P due to the operation of separating the registration rollers 42. Therefore, in the present embodiment, the control unit 50 controls the nip load to the maximum load before the switching control is executed. That is, the control unit 50 as control changes, the timing of the leading end of the sheet P enters the fixing nip, and, among the timing of the start of the execution of switching control of the registration roller 42, be determined whichever comes It becomes.

  FIG. 7 is an explanatory diagram showing the transition of the nip load in the variable control. FIG. 4A shows the transition of the nip load on the A3 size paper P, and FIG. 4B shows the transition of the nip load on the A4 size paper P. FIG. Here, “ta” is the timing when the leading edge of the paper P reaches the transfer nip, “tb” is the timing when the leading edge of the paper P reaches the fixing nip, and “td” is the trailing edge of the paper P passing through the transfer nip. The timing, “te”, indicates the timing for starting the registration roller 42 switching control.

  According to this configuration, the image forming apparatus controls the nip load to the maximum load before the start timing of the switching control. When the registration roller 42 is switched from the pressure contact state to the separation state, an impact is likely to occur. When the nip load is small, the sheet P may be displaced due to the generated impact, which may cause a transfer failure. In this respect, in the present embodiment, by controlling the nip load to the maximum load before the start of execution of the switching control of the registration roller 42, it is possible to suppress a decrease in transfer performance.

Further, the image forming apparatus controls the nip load to the maximum load by the earlier timing among the timing when the leading edge of the paper P enters the fixing nip and the timing when the execution of the switching control starts. According to such a configuration, the nip load can be controlled to the maximum load when both events occur by controlling the nip load in conjunction with the factor that causes the early occurrence of the impact among the factors of the impact. Thereby, the fall of transfer performance can be suppressed.

  The image forming apparatus according to the embodiment of the present invention has been described above, but the present invention is not limited to the above-described embodiment, and various modifications can be made within the scope of the invention. .

SC Document Reading Device 6 Intermediate Transfer Belt 8 Opposing Rollers 10Y to 10K Image Forming Unit 20 Secondary Transfer Unit 21 Secondary Transfer Roller 24 Movable Arm 25 Compression Coil Spring 27 Cam 28 Cam Drive Motor 29 High Voltage Power Supply Unit 30 Fixing Device 31, 32 Fixing roller 40 Paper feed unit 42 Registration roller 43 Paper discharge tray 45 Guide member 46 Reverse conveyance unit 50 Control unit 51 Paper detection sensor 52 Secondary transfer current sensor 60 Operation unit

Claims (12)

An image carrier that carries an image on the surface;
A transfer member that presses against the image carrier to form a transfer nip and transfers an image on the image carrier to paper;
A movable mechanism for moving the transfer member to set a pressure contact state between the image carrier and the transfer member;
A control unit that variably controls the nip load in the transfer nip by controlling the movable mechanism,
Wherein, after the leading end of the sheet passes through the transfer nip, based on the conveying position of the sheet, the nip load, the leading end of the sheet on the image bearing member to control the time of entering the transfer nip An image forming apparatus that controls a second load larger than a first load that is a nip load capable of transferring an image onto a sheet . A fixing device that is disposed downstream of the transfer member in the paper conveyance path and that performs fixing processing on the paper by a pair of fixing members that are pressed against each other to form a fixing nip;
The image forming apparatus according to claim 1, wherein the control unit controls the nip load to the second load before a leading edge of a sheet enters the fixing nip. A registration roller that is disposed upstream of the transfer member in the paper conveyance path and includes a pair of conveyance rollers that are pressed against each other;
The image forming apparatus according to claim 1 , wherein the control unit controls the nip load to the second load before a rear end of the sheet passes the registration roller. A fixing device that is disposed downstream of the transfer member in a paper conveyance path and performs a fixing process on the paper by a pair of fixing members that are pressed against each other to form a fixing nip;
A registration roller that is disposed upstream of the transfer member in the paper conveyance path and includes a pair of conveyance rollers that are in pressure contact with each other;
The controller is configured to apply the nip load to the second timing by the earlier timing among the timing at which the leading edge of the sheet enters the fixing nip and the timing at which the trailing edge of the sheet passes the registration roller. The image forming apparatus according to claim 1 , wherein the image forming apparatus is controlled by a load. In the transport path of the paper is disposed upstream of said transfer member further comprises a registration roller composed of mutually pressed or separable pair of conveying rollers,
The control unit performs switching control for switching a pair of transport rollers, which are the registration rollers, from the press-contact state to the separated state when the paper is transported by the registration rollers, and the nip load before the start timing of the switching control. The image forming apparatus according to claim 1 , wherein the second load is controlled. A fixing device that is disposed downstream of the transfer member in a paper conveyance path and performs a fixing process on the paper by a pair of fixing members that are pressed against each other to form a fixing nip;
A registration roller that is disposed upstream of the transfer member in the paper conveyance path, and includes a pair of conveyance rollers that can be pressed against or separated from each other;
The control unit performs switching control to switch the pair of conveyance rollers, which are the registration rollers, from the pressure contact state to the separation state when the paper is conveyed by the registration rollers , and the timing at which the leading edge of the paper enters the fixing nip, and of the timing of starting execution of said switching control, image forming apparatus according to the nip load by whichever comes in claim 1, wherein the controller controls the second load.   The control unit sets the nip load to be greater than the second load between the sheets after the trailing end of the preceding sheet passes through the transfer nip and before the leading end of the succeeding sheet enters the transfer nip. The image forming apparatus according to claim 1, wherein the image forming apparatus is controlled to have a small third load.   The image forming apparatus according to claim 7, wherein the control unit sets the third load to a load smaller than the first load.   9. The image forming apparatus according to claim 1, wherein the transfer member is configured to rotate following the rotation of the image carrier that is driven to rotate. A power supply for supplying a transfer current for applying a transfer voltage to the transfer member;
The image forming apparatus according to claim 1, wherein the control unit variably controls the transfer current according to the nip load.   An image carrier that carries an image on the surface;
  A transfer member that presses against the image carrier to form a transfer nip and transfers an image on the image carrier to paper;
  A movable mechanism for moving the transfer member to set a pressure contact state between the image carrier and the transfer member;
  A registration roller that is disposed upstream of the transfer member in the paper conveyance path, and includes a pair of conveyance rollers that can be pressed against or separated from each other;
  A control unit that variably controls the nip load in the transfer nip by controlling the movable mechanism,
  The control unit performs switching control to switch the pair of conveyance rollers, which are the registration rollers, from the press contact state to the separation state during conveyance of the sheet by the registration roller, and after the leading end of the sheet has passed the transfer nip, The second nip load is larger than the first load, which is the nip load that is controlled when the leading edge of the paper enters the transfer nip before the start of execution of the switching control based on the transport position of An image forming apparatus controlled to a load.   An image carrier that carries an image on the surface;
  A transfer member that presses against the image carrier to form a transfer nip and transfers an image on the image carrier to paper;
  A movable mechanism for moving the transfer member to set a pressure contact state between the image carrier and the transfer member;
  A fixing device that is disposed downstream of the transfer member in a paper conveyance path and performs a fixing process on the paper by a pair of fixing members that are pressed against each other to form a fixing nip;
  A registration roller that is disposed upstream of the transfer member in the paper conveyance path, and includes a pair of conveyance rollers that can be pressed against or separated from each other;
  A control unit that variably controls the nip load in the transfer nip by controlling the movable mechanism,
  The control unit performs switching control to switch the pair of conveyance rollers, which are the registration rollers, from the press contact state to the separation state during conveyance of the sheet by the registration roller, and after the leading end of the sheet has passed the transfer nip, The leading edge of the sheet enters the fixing nip and the timing to start execution of the switching control, whichever is earlier, the leading edge of the sheet An image forming apparatus that controls a second load that is larger than a first load that is a nip load to be controlled when entering a transfer nip.

Images