JP2007304190A - Image forming apparatus - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To stabilize recording paper conveying speed, in a tandem type color image forming apparatus. <P>SOLUTION: An attraction roll 34 is arranged between a transfer roll 31Y arranged on the uppermost stream side in the moving direction of a conveying belt 30 and a registration roll 53, configured to press the paper conveyed by the conveying belt 30 toward the conveying belt 30. The pressing force of the attraction roll 34 pressing the paper conveyed by the conveying belt 30 is set larger than the pressing force of the registration roll 53 pressing the paper. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to an image forming apparatus using an electrophotographic system such as a copying machine, a printer, a facsimile machine, or a multifunction machine of these.

  In an image forming apparatus such as a copying machine or a printer using an electrophotographic system, for example, a photosensitive member (photosensitive drum) formed in a drum shape is uniformly charged with a predetermined potential by a charging device, and this is used as image information. An electrostatic latent image is formed by exposure with controlled light. Then, the electrostatic latent image is developed by a developing device to form a toner image, and the toner image is further transferred and fixed on the recording paper, thereby forming a toner image on the recording paper.

In such a color image forming apparatus using an electrophotographic system, a configuration called “tandem system” suitable for high speed is often employed. In the tandem color image forming apparatus, Y (yellow), M (magenta), C (cyan), and K (black) toner images are formed on different photosensitive drums, and are formed on the respective photosensitive drums. Each color toner image is directly transferred onto a recording sheet, or sequentially transferred onto an intermediate transfer member, and then transferred onto a recording sheet at a time, thereby forming a full color image on which the respective color toner images are superimposed.
In particular, when a configuration in which each color toner image is sequentially transferred directly to the recording paper is adopted, it is necessary to control the conveyance member that conveys the recording paper along each photosensitive drum so as to always move at a constant speed. is there. Accordingly, the recording paper conveyance speed is set to be constant during image formation to stabilize color registration (color matching) (see, for example, Patent Document 1).

JP-A-9-114348 (page 4-5)

However, the transport member for transporting the recording paper is provided with another transport means for transporting the recording paper, such as a registration roll and a fixing device, before and after the recording paper transport direction. For this reason, when the recording paper is carried into the conveyance member and when the recording paper is carried out of the conveyance member, a situation occurs in which the recording paper receives the conveyance force from the other conveyance means simultaneously with the conveyance member.
Therefore, even when the moving speed of the conveying member is controlled to be constant, when the trailing edge of the recording paper comes out of the other conveying means described above or when the leading edge of the recording paper enters the other conveying means described above. In some cases, the recording paper conveyance speed fluctuates due to the influence of the conveyance force from these conveyance means. As a result, image disturbance such as color misalignment and image misalignment may occur.

  Accordingly, the present invention has been made to solve the above technical problems, and an object of the present invention is to stabilize the conveyance speed of recording paper in a tandem color image forming apparatus. It is in.

  For this purpose, the image forming apparatus of the present invention includes a plurality of image carriers, a belt member that conveys the recording material along the arrangement direction of the plurality of image carriers, and a recording material that is conveyed by the belt member. In addition, each transfer member for transferring each toner image formed by the plurality of image carriers, a carry-in roll member for carrying a recording material into the belt member, and the most moving member in the moving direction of the belt member among the transfer members. A pressure roll member disposed between the transfer member disposed on the upstream side and the carry-in roll member and pressing a recording material conveyed by the belt member toward the belt member; The pressing force for pressing the recording material conveyed by the belt member is set to be larger than the pressing force for the carry-in roll member to press the recording material.

  Here, the pressing roll member can be characterized in that it is disposed at a position for pressing the stretching roll member that stretches the belt member. Further, the pressing roll member can be characterized by following the belt member. Further, the pressing roll member can supply charges to the recording material conveyed by the belt member. In addition, the carry-in roll member can carry the recording material into the belt member substantially parallel to the traveling direction of the belt member.

  In addition, the transfer member disposed on the most downstream side in the moving direction of the belt member in the transfer member is opposed to the other transfer member by the pressing force that acts on the image holding member facing the belt member. It can be characterized in that it is set to be larger than the pressing force applied to the image carrier. In addition, the image carrier includes a contact charging member that charges the image carrier, a developing member that develops the electrostatic latent image formed on the image carrier, and a cleaning member that cleans the surface of the image carrier. It can also be characterized in that it is arranged in an image forming unit that is configured in a general manner.

  The image forming apparatus of the present invention includes a plurality of image carriers, a belt member that conveys the recording material along the arrangement direction of the plurality of image carriers, and a plurality of recording materials that are conveyed by the belt member. Each transfer member for transferring each toner image formed on the image carrier and a fixing member for fixing the toner image transferred to the recording material are provided on the most fixing member side among the transfer members. In the transfer member, the pressing force that acts on the opposite image carrier via the belt member is set to be larger than the pressing force that the other transfer members act on the opposite image carrier. It is characterized by.

  Here, the belt member and the fixing member may be driven by the same drive source. The belt roll may be further provided with a belt charging member that charges the belt member and the recording material conveyed to the belt member at a position where the tension roll member that stretches the belt member is wrapped by the belt member. it can.

  According to the present invention, since the accuracy of color registration (color matching) can be improved, a high-quality color image can be formed.

Embodiments of the present invention will be described below in detail with reference to the accompanying drawings.
FIG. 1 is a diagram illustrating an overall configuration of an image forming apparatus to which the exemplary embodiment is applied. The image forming apparatus 1 shown in FIG. 1 is a so-called tandem type in which four image forming units (process cartridges) 10Y, 10M, 10C, and 10K are arranged in parallel at regular intervals in the vertical direction (substantially vertical direction). It is configured. The process cartridges 10Y, 10M, 10C, and 10K include a photosensitive drum 11 as an image carrier, a charging roll 12 that uniformly charges the surface of the photosensitive drum 11 at a predetermined potential, toner (negative charging characteristics), and a carrier (magnetic). A developing device 13 of a two-component developing system that develops the electrostatic latent image formed on the photosensitive drum 11 using a developer composed of particles), and a drum cleaner 14 that cleans the surface of the photosensitive drum 11 after transfer. It is configured as a unit.

Here, the process cartridges 10Y, 10M, 10C, and 10K are configured in substantially the same manner except for the toner stored in the developing device 13. Each of the process cartridges 10Y, 10M, 10C, and 10K forms yellow (Y), magenta (M), cyan (C), and black (K) toner images.
The process cartridges 10Y, 10M, 10C, and 10K are configured to be detachable from the main body of the image forming apparatus 1. For example, when the toner in the developing device 13 is consumed, the process cartridges 10Y, 10Y, It is comprised so that it can replace | exchange separately for every 10M, 10C, and 10K.

  In the image forming apparatus 1 of the present embodiment, as an exposure system, a laser exposure apparatus 20 that exposes the photosensitive drums 11 disposed in the process cartridges 10Y, 10M, 10C, and 10K, and the photosensitive drum 11 after transfer. A static elimination lamp 21 for neutralizing the static electricity is provided.

  In addition, the image forming apparatus 1 according to the present embodiment includes a conveyance belt (belt) that conveys a sheet of recording material (recording paper) while being in contact with the photosensitive drums 11 of the process cartridges 10Y, 10M, 10C, and 10K. Member) 30 is arranged. The conveyor belt 30 is formed of a film-like endless belt capable of electrostatically adsorbing paper. Then, the paper roll M1 is stretched around the drive roll 32 and the idle roll 33 and is circulated to form a paper transport path M1 through which the paper is transported.

Transfer rolls 31Y, 31M, 31C, and 31K, which are examples of transfer members, are disposed on the inner side of the conveyance belt 30 and facing the photosensitive drums 11, respectively. Each of the transfer rolls 31 (31Y, 31M, 31C, 31K) forms a transfer electric field between the transfer drums 31 (31Y, 31M, 31C, 31K), and thereby the process cartridges 10Y, 10M are formed on the paper held and transported by the transport belt 30. , 10C, and 10K, the respective color toner images are sequentially transferred.
Further, an adsorption roll 34 as an example of a belt charging member that charges the conveyance belt 30 is disposed at the most upstream portion of the conveyance belt 30 on the photosensitive drum 11 side. Since the conveyance belt 30 and the paper are charged by the suction roll 34, the paper can be stably electrostatically adsorbed on the conveyance belt 30.
In addition, a fixing device 40 as an example of a fixing member that performs fixing processing by heat and pressure on an unfixed toner image on the paper is disposed on the downstream side of the paper conveyance path M1 of the conveyance belt 30.

Further, the image forming apparatus 1 according to the present embodiment includes a charging power source 61 that applies a predetermined charging bias voltage to the charging rolls 12 of the process cartridges 10Y, 10M, 10C, and 10K as power sources (voltage supply means). A developing power source 62 that applies a predetermined developing bias voltage to the developing devices 13 of the process cartridges 10Y, 10M, 10C, and 10K, a transfer power source 63 that applies a predetermined transfer voltage to each transfer roll 31, and a predetermined suction voltage to the suction roll 34 Is provided with an adsorption power source 64 for applying.
The charging power supply 61 of the present embodiment is configured to be able to variably output a negative DC voltage. The developing power source 62 is configured to output a voltage obtained by superimposing a predetermined value of an alternating voltage on a negative variable direct current voltage. Further, the transfer power supply 63 and the suction power supply 64 of the present embodiment are configured to output a predetermined positive DC voltage.

  The control unit 60 performs output control such as output voltage values and output timings in the charging power supply 61, the developing power supply 62, the transfer power supply 63, and the suction power supply 64. Moreover, lighting control of the static elimination lamp 21 is performed. Further, the entire image forming apparatus 1 including the drive control of the conveyance belt 30 is also controlled.

  In the image forming apparatus 1 of the present embodiment, under the operation control by the control unit 60, the laser exposure apparatus 20 generates a laser beam (exposure beam) modulated based on the image information, and each process cartridge An electrostatic latent image is formed on the photosensitive drum 11 of 10Y, 10M, 10C, and 10K. For example, in the black (K) process cartridge 10K, the surface of the photosensitive drum 11 uniformly charged at a predetermined potential by the charging roll 12 is scanned and exposed with the exposure beam generated by the laser exposure device 20, and the photosensitive member is detected. An electrostatic latent image is formed on the drum 11. The formed electrostatic latent image is developed by the developing device 13, and a black (K) toner image is formed on the photosensitive drum 11. Similarly, Y, M, and C color toner images are formed in the process cartridges 10Y, 10M, and 10C, respectively.

When the formation of each color toner image is started in each of the process cartridges 10Y, 10M, 10C, and 10K, the paper taken out from the paper cassette 50 is electrostatically attracted onto the conveyance belt 30 that circulates and moves in the direction of the arrow in FIG. In this state, the paper is conveyed on the paper conveyance path M1. Then, the respective color toner images are sequentially and superimposedly transferred onto the paper by the transfer electric field formed by the transfer rolls 31 (31Y, 31M, 31C, 31K).
The sheet on which the toner image has been electrostatically transferred is peeled from the conveyance belt 30 downstream of the process cartridge 10K and conveyed to the fixing device 40. When the sheet is conveyed to the fixing device 40, the unfixed toner image on the sheet is fixed to the sheet by being subjected to a fixing process by heat and pressure. The paper on which the fixed image is formed is stacked on a paper discharge unit 70 provided in the discharge unit of the image forming apparatus 1. On the other hand, during double-sided printing, the same transfer process is performed again via the double-sided conveyance path M2 (see FIG. 1), and then the paper is stacked on the paper discharge unit 70.

Next, the paper transport mechanism in the image forming apparatus 1 of the present embodiment will be described. FIG. 2 is a diagram for explaining the paper transport mechanism of the present embodiment.
As shown in FIG. 2, in the image forming apparatus 1 according to the present embodiment, as a paper transport mechanism, a paper cassette 50 for storing paper on the paper feeding side, and papers accumulated in the paper cassette 50 are provided on a predetermined basis. A pickup roll 51 that is picked up and conveyed at timing, a conveyance roll 52 that conveys the sheet fed by the pickup roll 51, and a registration roll 53 as an example of a carry-in roll member that sends the sheet to the conveyance belt 30 in accordance with the image forming operation are arranged. It is installed.

Further, a conveyance belt 30 that conveys the sheet while being in contact with each of the photosensitive drums 11 is disposed in an area where the process cartridges 10Y, 10M, 10C, and 10K are disposed. The conveyor belt 30 is stretched between a drive roll 32 that is driven to rotate at a constant speed by a drive motor (not shown) controlled by the controller 60 and an idle roll 33 that is driven to rotate.
At the most upstream portion of the conveyance belt 30 on the photosensitive drum 11 side, an adsorption roll 34 for charging the conveyance belt 30 and the sheet is disposed so as to press the idle roll 33 with a predetermined pressure via the conveyance belt 30. Yes. When the sheet conveyed from the sheet feeding side to the conveyance belt 30 passes between the conveyance belt 30 and the suction roll 34, a predetermined voltage (for example, applied to the suction roll 34 from the suction power source 64 (see FIG. 1)). , + 3000V). Thereby, the sheet is stably electrostatically adsorbed on the surface of the conveyance belt 30. Then, the conveyance belt 30 is circulated and driven while the sheet is electrostatically attracted, so that the sheet is conveyed from the lower side to the upper side in a substantially vertical direction while contacting the photosensitive drum 11 at the transfer position. M1 is formed.

  Further, on the paper discharge side, a paper discharge roll 54 that conveys the paper fixed by the fixing device 40, and in the case of single-sided printing, the paper is discharged toward a paper discharge unit 70 provided in the upper part of the apparatus main body. In the case of double-sided printing, by reversing in the reverse direction at a predetermined timing from the rotation direction toward the paper discharge unit 70, the reversal for sending out the paper on which one side is fixed by the fixing device 40 toward the double-sided conveyance path M2. A roll 55 is provided. In addition, a plurality of conveyance rolls 56 are disposed along the double-side conveyance path M2 in the double-side conveyance path M2.

In the paper transport mechanism of the present embodiment, when the formation of each color toner image is started in each process cartridge 10Y, 10M, 10C, 10K, the pickup roll 51 rotates at a predetermined timing, and the paper is fed from the paper cassette 50. Pull out. The fed paper is conveyed to the registration roll 53 by the conveyance roll 52 and is supplied to the conveyance belt 30 by the registration roll 53 in accordance with the toner image formation timing.
The paper supplied to the conveyance belt 30 first passes between the conveyance belt 30 and the suction roll 34. At that time, a predetermined voltage (for example, +3000 V) is applied to the suction roll 34 from the suction power source 64. As a result, the sheet is charged with a positive charge and is strongly and electrostatically adsorbed to the transport belt 30.
Then, the sheet is conveyed through the sheet conveying path M1 while being electrostatically attracted onto the conveying belt 30 that circulates and moves in the direction of the arrow in FIG.

Each color toner image formed by each process cartridge 10Y, 10M, 10C, 10K is formed by each transfer roll 31 on the sheet conveyed through the sheet conveyance path M1 while being electrostatically attracted onto the conveyance belt 30. The images are sequentially transferred in a superimposed manner by the transfer electric field. Therefore, a full-color transfer toner image is formed on the paper when it passes through the process cartridge 10K.
After the sheet passes through the process cartridge 10K, the sheet is peeled off from the conveyance belt 30 and conveyed to the fixing device 40 disposed on the downstream side of the sheet conveyance path M1 of the conveyance belt 30. The fixing device 40 heats and pressurizes the full-color transfer toner image formed on the paper, thereby forming a full-color fixed image on the paper.

In the case of single-sided printing, the paper subjected to the fixing process by the fixing device 40 is stacked on the paper discharge unit 70 by the paper discharge roll 54 and the reverse roll 55. On the other hand, in the case of double-sided printing, after the fixing process in the fixing device 40, the reversing roll 55 reversely rotates (reverses) from the rotation direction toward the paper discharge unit 70 to the rotation direction opposite to that at a predetermined timing. To do. As a result, the paper on which one side is fixed by the fixing device 40 is sent out toward the double-sided conveyance path M2. The sheet sent out to the double-sided conveyance path M2 is conveyed to the registration roll 53 by a plurality of conveyance rolls 56 arranged along the double-sided conveyance path M2.
Thereafter, the toner is again supplied to the transport belt 30 by the registration roll 53 in accordance with the toner image formation timing. The paper supplied to the transport belt 30 is similarly electrostatically attracted onto the transport belt 30 and is transported through the paper transport path M1. At that time, the toner image is electrostatically adsorbed on the conveyance belt 30 so that the back surface of the surface on which the toner image was previously formed faces the photosensitive drum 11 side. Then, after a fixed image is formed on this back surface, it is stacked on the paper discharge unit 70.

Here, the conveyance force acting on the sheet conveyed to the conveyance belt 30 of the present embodiment will be described. FIG. 3 is a diagram showing a configuration from the registration roll 53 to the transfer position of the process cartridge 10Y.
As shown in FIG. 3, when an image is formed, there is a state in which a sheet (denoted as “P” in the following drawings) is conveyed across both the registration roll 53 and the conveyance belt 30. In this state, the sheet P is conveyed by receiving the conveyance force F1 from the conveyance belt 30 by being electrostatically attracted to the conveyance belt 30, but as described below, the conveyance force of the registration roll 53 is reduced. It will also be affected.

  There is usually a slight speed difference between the moving speed of the conveying belt 30 when conveying the paper P and the peripheral speed (roll surface speed) of the registration roll 53 that carries the paper P into the conveying belt 30. Yes. That is, by design, the moving speed of the conveyor belt 30 and the peripheral speed of the registration roll 53 are set to coincide. However, in reality, there are manufacturing tolerances (so-called dimensional variations) of parts such as the resist roll 53 and the idle roll 33, and there are subtle differences in rotational speed when, for example, the drive source (drive motor) is different. For this reason, it is extremely difficult to make the moving speed of the conveyor belt 30 and the peripheral speed of the registration roll 53 exactly match. Therefore, a speed difference is generated between the moving speed of the conveyance belt 30 and the peripheral speed of the registration roll 53. For example, when the peripheral speed of the registration roll 53 is slower than the moving speed of the conveyance belt 30, the frictional force generated between the registration roll 53 and the sheet is drag in the direction opposite to the conveyance force F <b> 1 of the conveyance belt 30. It acts on the paper P as F2. In that case, when the trailing edge of the paper P has finished passing through the registration roll 53, the transport of the paper P by the transport belt 30 is slightly affected.

FIG. 4 is a diagram illustrating a transport state of the paper P when the rear end of the paper P has passed the registration roll 53. In the state shown in FIG. 3 described above, the sheet P is subjected to the conveyance force F1 due to the conveyance belt 30 and the drag F2 caused by the speed difference between the movement speed of the conveyance belt 30 and the peripheral speed of the registration roll 53. The paper P is conveyed by receiving a conveying force F ′ (= F1−F2). However, as shown in FIG. 4, the drag force F <b> 2 becomes 0 at the moment when the trailing edge of the paper P has passed the registration roll 53, so that the transport force F ′ acting on the paper P is equal to the transport belt 30. Only the conveying force F1 due to the above, and rapidly increases from F ′ to F1 (> F ′). At that time, a phenomenon in which a minute slip (change in moving speed) occurs between the paper P and the conveying belt 30 due to such a change in conveying force is observed. When the slip occurs, the movement speed of the paper P may change at the transfer positions of the process cartridges 10Y, 10M, 10C, and 10K, and the color registration (color matching) may shift.
Since the image density in the recent image forming apparatus 1 is formed with extremely high definition, for example, 1200 dpi (dot per inch), a slight color registration shift lowers the color reproducibility and decreases the image quality. Will be allowed to.

Therefore, in the image forming apparatus 1 of the present embodiment, the suction roll 34 is disposed on the paper transport path M1 between the registration roll 53 and the transfer roll 31Y so as to be driven to rotate by the transport belt 30 and the suction roll 34. Is installed so as to press the idle roll 33 with a predetermined pressing force via the conveyor belt 30. Then, the pressing force between the suction roll 34 and the idle roll 33 is configured to increase the frictional force between the paper P and the transport belt 30 at the position where the suction roll 34 is disposed. Accordingly, the suction roll 34 here functions as a pressing roll member that presses the sheet toward the transport belt 30.
In that case, the suction roll 34 is disposed in a region where the idle roll 33 is wrapped by the transport belt 30 and the transport belt 30 transports the paper. Thereby, the idle roll 33 becomes a backup roll, and the pressing force from the suction roll 34 can be set high. In addition, since the adhesion between the paper and the suction roll 34 can be improved, the charging efficiency can also be improved.

  As a result, when the drag F2 changes to 0 at the moment when the trailing edge of the paper P has passed through the registration roll 53, the frictional force between the paper P and the conveyance belt 30 at the position of the suction roll 34 is reduced. It is possible to mitigate the influence of the movement speed of the paper P on the downstream side of the suction roll 34 due to the force that opposes the force that advances P forward in the transport direction. Therefore, the change in the moving speed of the paper P at the transfer positions of the process cartridges 10Y, 10M, 10C, and 10K can be made extremely slight. As a result, it is possible to extremely reduce the occurrence of color registration misalignment.

  In this case, the pressing force between the suction roll 34 and the idle roll 33 is set larger than the pressing force between the resist rolls 53. By setting in this way, at the position where the suction roll 34 and the idle roll 33 are arranged, the suction force between the paper P and the transport belt 30 is in addition to the electrostatic suction force and the suction roll 34 and the idle roll 33. A large frictional force is applied due to the pressing force between. Therefore, in the region between the suction roll 34 and the idle roll 33, the conveyance force by the conveyance belt 30 becomes large. For example, when the peripheral speed of the registration roll 53 is slower than the moving speed of the conveyance belt 30, the sheet P is loaded. It slips when passing the resist roll 53. Therefore, the drag F2 caused by the speed difference between the moving speed of the conveying belt 30 and the peripheral speed of the registration roll 53 is reduced, and the drag F2 is 0 at the moment when the trailing edge of the paper P has finished passing through the registration roll 53. The impact caused by the change can be made extremely small. As a result, occurrence of color registration misalignment can be greatly reduced.

  Here, FIG. 5 is a diagram showing the relationship between the pressing force (suction roll load) between the suction roll 34 and the idle roll 33 and the color registration displacement amount. In this case, the load between the resist rolls 53 is 300 g. As shown in FIG. 5, it can be confirmed that as the pressing force between the suction roll 34 and the idle roll 33 is increased, the color registration shift amount is decreased. In the present embodiment, for example, if the allowable value of the color registration shift amount is set to 150 μm, the color registration shift amount is set to 500 g or more as the suction roll load from the result of FIG. It can be reduced to an acceptable level.

  On the other hand, the arrangement configuration of the registration rolls 53 for feeding the paper to the conveyance belt 30 is preferably configured so that the method for advancing the paper P and the method for advancing the conveyance belt 30 match. That is, it is preferable to set the nip direction of the registration roll 53 so that the paper P carried out from the registration roll 53 enters substantially straight with respect to the traveling direction of the conveyance belt 30. As a result, the paper P carried between the suction roll 34 and the idle roll 33 travels in parallel with the nip formed between the suction roll 34 and the idle roll 33, so that the angle with respect to the nip The contact area between the paper P and the transport belt 30 around the suction roll 34 is set larger than the case of entering with a squeeze. Therefore, since the sheet P is effectively given a positive charge from the suction roll 34, the electrostatic attraction force between the sheet P and the transport belt 30 can be increased. Thereby, the conveyance force by the conveyance belt 30 can be increased, and the above-described impact can be further reduced. As a result, the amount of color registration misalignment can be further reduced.

  Further, the suction roll 34 is disposed so as to be driven and rotated by the conveyance belt 30. Therefore, the peripheral speed of the suction roll 34 is always set to the same speed as the transport belt 30. As a result, the suction roll 34 does not have a speed difference from the conveyance belt 30, so that it does not resist the conveyance force F <b> 1 of the conveyance belt 30.

  Subsequently, FIG. 6 is a diagram showing a configuration from the transfer position of the process cartridge 10Y to the fixing device 40. As shown in FIG. 6, when the image is formed, the paper P may be transported across both the transport belt 30 and the fixing device 40. In that case, the sheet P is conveyed by receiving a frictional force Fr between the sheet P and the conveying belt 30 by being electrostatically attracted to the conveying belt 30, but as described below, the sheet P is conveyed by the fixing device 40. It is also affected by the force and the pressing force between each photosensitive drum 11 and the transfer roll 31.

  As shown in FIG. 6, when the leading edge of the paper P passes through the fixing device 40 during the image forming process, the rear end of the paper P is still arranged on the most upstream side of the paper transport path M1. A state of passing through the 10Y transfer position (transfer roll 31Y) may occur. In that case, the paper P has a frictional force Frb due to electrostatic attraction with the conveying belt 30, a conveying force F3 with the fixing device 40, and a pressing force between each photosensitive drum 11 and each transfer roll 31. Is working. Here, the pressing force between each photosensitive drum 11 and each transfer roll 31 is the frictional force Fr (FrY, FrM, between the paper P and the conveyance belt 30 at each transfer roll 31 position (transfer position). FrC, FrK) can be generated.

  When the paper P is transported to the fixing device 40, the paper P receives a transport force F3 by a nip formed between a fixing roll disposed in the fixing device 40 and the pressure member. For example, the thicker the basis weight, the larger the nip amount of the fixing device 40, the larger the conveyance force F3 tends to increase. When the paper P receives a large conveying force F3 from the fixing device 40, a tensile force acts on the paper P toward the fixing device 40. In that case, when the paper P is transported on the transport belt 30, the frictional force Frb due to the electrostatic adsorption force with the transport belt 30 and the friction between the paper P and the transport belt 30 at each transfer position. The force Fr (FrY, FrM, FrC, FrK) acts as a resistance against the conveying force F3 by the fixing device 40.

In a state where the rear end of the sheet P is still passing the transfer position (transfer roll 31Y) of the process cartridge 10Y, the drag force against the conveyance force F3 by the fixing device 40 is a friction caused by the electrostatic adsorption force with the conveyance belt 30. The force Frb and the total amount of frictional force Fr between the paper P and the conveying belt 30 at each transfer position (= FrY + FrM + FrC + FrK). However, the total amount of the frictional force Fr at the transfer position is gradually reduced every time it passes through each transfer roll 31.
Here, FIG. 7 is a diagram illustrating an example of a state in which the paper P is transported on the transport belt 30. As shown in FIG. 7, when the paper P passes through the process cartridge 10Y, the total amount of the frictional force Fr is FrM + FrC + FrK, and the paper P passes through the process cartridge 10M (FIG. 7 shows this state). ), The total amount of the frictional force Fr is FrC + FrK, and when the paper P passes through the process cartridge 10C, the total amount of the frictional force Fr is only FrK. Further, the frictional force Frb caused by the electrostatic attraction force with the transport belt 30 also decreases in the same manner because the contact area between the paper P and the transport belt 30 gradually decreases.
For this reason, the drag force decreases as the paper P is transported on the transport belt 30, so that the transport force F <b> 3 by the fixing device 40 becomes dominant, and the phenomenon that the moving speed of the paper P gradually increases occurs. For this reason, the amount of color registration shift gradually increases.

  Therefore, in the image forming apparatus 1 of the present embodiment, the pressing force between the photosensitive drum 11K and the transfer roll 31K in the process cartridge 10K disposed at the most downstream portion of the sheet conveyance path M1 is another process. The pressure is set so as to be larger than the pressing force between the photosensitive drums 11 and the transfer rolls 31 in the cartridges 10Y, 10M, and 10C. As a result, the frictional force FrK between the paper P and the transport belt 30 at the transfer position in the process cartridge 10K can be set large, so that even if the paper P passes through the process cartridges 10Y, 10M, and 10C, the fixing device 40 The force that opposes the conveying force F3 can be maintained at a predetermined value or more by the frictional force FrK. As a result, it is possible to suppress the amount of color registration from gradually increasing as the paper P is transported on the transport belt 30.

  That is, the pressing force between the photosensitive drum 11K and the transfer roll 31K is set to be larger than the pressing force between the photosensitive drums 11 and the transfer rolls 31 in the other process cartridges 10Y, 10M, and 10C. By doing so, FrY, FrM, FrC << FrK is realized. As a result, the friction force FrK at the transfer position in the process cartridge 10K can be independently countered with the conveyance force F3 by the fixing device 40. Therefore, as the sheet P is conveyed on the conveyance belt 30, FrY, Even if FrM, FrC = 0 and Frb≈0, it is possible to suppress an increase in the amount of misregistration due to the conveyance of the paper P.

  In this case, it is preferable that the drive source for rotationally driving the fixing device 40 and the drive source for circulatingly driving the conveying belt 30 are configured by the same drive source. By using the same drive source in this way, the sheet conveying speed of the fixing device 40 and the conveying belt 30 can be set to the same speed as much as possible. Therefore, the fixing device 40 acting on the sheet P conveyed on the conveying belt 30. It can suppress that the pulling force from the side becomes large. Therefore, the amount of color registration shift can be stably reduced.

  Here, FIG. 8 is a diagram showing the relationship between the pressing force between the photosensitive drum 11K and the transfer roll 31K (load of the transfer roll 31K) and the amount of color registration deviation. In this case, the pressing forces between the photosensitive drums 11Y, 11M, and 11C and the transfer rolls 31Y, 31M, and 31C are both 200 g. As shown in FIG. 8, it can be confirmed that as the pressing force between the photosensitive drum 11K and the transfer roll 31K is increased, the color registration shift amount is decreased. In the present embodiment, for example, if the allowable value of the color registration shift amount is set to 150 μm, the load of the transfer roll 31K is set to 550 g or more from the result of FIG. The amount can be reduced to an acceptable level.

As described above, in the image forming apparatus 1 of the present embodiment, the suction roll 34 is disposed on the paper transport path M1 between the registration roll 53 and the transfer roll 31Y, and the suction roll 34 holds the transport belt 30. The idle roll 33 is installed so as to be pressed with a predetermined pressing force. Accordingly, the pressing force between the suction roll 34 and the idle roll 33 is configured to increase the frictional force between the paper P and the transport belt 30 at the position where the suction roll 34 is disposed.
Therefore, when the trailing edge of the paper P has finished passing through the registration roll 53, it is possible to mitigate the influence of the movement speed of the paper P on the downstream side of the suction roll 34. As a result, it is possible to extremely reduce the occurrence of color registration misalignment.

Further, in the process cartridge 10K arranged at the most downstream portion of the sheet conveyance path M1, the pressing force between the photosensitive drum 11K and the transfer roll 31K causes each photosensitive member in the other process cartridges 10Y, 10M, and 10C. It is set to be larger than the pressing force between the drum 11 and each transfer roll 31. Thereby, the frictional force FrK between the paper P and the conveyance belt 30 at the transfer position in the process cartridge 10K is set to be large.
Therefore, even if the paper P passes through the process cartridges 10Y, 10M, and 10C, the force that opposes the conveyance force F3 by the fixing device 40 can be maintained at a predetermined value or more, and thus the amount of color registration deviation is allowed. The level can be reduced.

1 is a diagram illustrating an overall configuration of an image forming apparatus to which the present invention is applied. FIG. 6 is a diagram illustrating a paper transport mechanism. FIG. 5 is a diagram illustrating a configuration from a registration roll to a transfer position of a process cartridge (10Y). FIG. 5 is a diagram illustrating a paper conveyance state when the trailing edge of the paper has passed through a registration roll. It is the figure which showed the relationship between the adsorption | suction roll load and the deviation | shift amount of a color registration. FIG. 3 is a diagram illustrating a configuration from a transfer position of a process cartridge (10Y) to a fixing device. FIG. 6 is a diagram illustrating an example of a state in which a sheet is conveyed on a conveyance belt. It is the figure which showed the relationship between the load of a transfer roll (31K), and the deviation | shift amount of a color registration.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 ... Image forming apparatus, 10Y, 10M, 10C, 10K ... Image forming unit (process cartridge), 11 (11Y, 11M, 11C, 11K) ... Photosensitive drum, 12 (12Y, 12M, 12C, 12K) ... Charging roll , 13 (13Y, 13M, 13C, 13K) ... developer, 14 (14Y, 14M, 14C, 14K) ... drum cleaner, 20 ... laser exposure device, 21 ... static discharge lamp, 31 (31Y, 31M, 31C, 31K) ... Transfer roll, 40 ... Fixer, 60 ... Control unit, 61 ... Charging power source, 62 ... Developing power source, 63 ... Transfer power source, 64 ... Suction power source

Claims (10)

A plurality of image carriers;
A belt member for conveying a recording material along an arrangement direction of the plurality of image carriers;
Each transfer member that transfers each toner image formed by the plurality of image carriers to a recording material conveyed by the belt member;
A carry-in roll member for carrying a recording material into the belt member;
Among the transfer members, a recording material disposed between the transfer member disposed on the most upstream side in the moving direction of the belt member and the carry-in roll member and transported by the belt member is used as the belt member. A pressing roll member that presses toward the
The image forming apparatus according to claim 1, wherein the pressing roll member is configured such that a pressing force for pressing the recording material conveyed by the belt member is set larger than a pressing force for pressing the recording material by the carry-in roll member.   The image forming apparatus according to claim 1, wherein the pressing roll member is disposed at a position for pressing the stretching roll member that stretches the belt member.   The image forming apparatus according to claim 1, wherein the pressing roll member follows the belt member.   The image forming apparatus according to claim 1, wherein the pressing roll member supplies a charge to a recording material conveyed by the belt member.   The image forming apparatus according to claim 1, wherein the carry-in roll member carries a recording material into the belt member substantially parallel to a traveling direction of the belt member.   The transfer member disposed on the most downstream side in the moving direction of the belt member in the transfer member is opposed to the other transfer member by a pressing force that acts on the opposite image carrier through the belt member. 2. The image forming apparatus according to claim 1, wherein the pressure is set to be larger than a pressing force applied to the image carrier.   The image carrier includes a contact charging member that contacts and charges the image carrier, a developing member that develops an electrostatic latent image formed on the image carrier, and a cleaning member that cleans the surface of the image carrier. The image forming apparatus according to claim 1, wherein the image forming apparatus is disposed in a configured image forming unit. A plurality of image carriers;
A belt member for conveying a recording material along an arrangement direction of the plurality of image carriers;
Each transfer member that transfers each toner image formed by the plurality of image carriers to a recording material conveyed by the belt member;
A fixing member for fixing the toner image transferred to the recording material,
Among the transfer members, the transfer member disposed closest to the fixing member has a pressing force that acts on the image holding member facing through the belt member, so that the other transfer members face each other. An image forming apparatus characterized in that the pressure is set to be larger than a pressing force acting on a body.   The image forming apparatus according to claim 8, wherein the belt member and the fixing member are driven by the same drive source.   A belt charging member for charging the belt member and a recording material conveyed to the belt member is further provided at a position where the stretching roll member for stretching the belt member is wrapped by the belt member. The image forming apparatus according to claim 8.

Images