JP2015060030A - Image forming apparatus - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an image forming apparatus capable of improving image quality by suppressing the speed fluctuations of an intermediate transfer belt, when a cardboard enters a secondary transfer part.SOLUTION: The image forming apparatus including at least one image carrier on which a toner image is formed, an intermediate transfer belt stretched to be wound between a driving roller and a driven roller and rotationally driven, primary transfer means which is primary transfer means pressing the intermediate transfer belt and forming a nip between the primary transfer means and the image carrier through the intermediate transfer belt and sequentially transfers the toner images formed on the respective image carriers to the intermediate transfer belt, and secondary transfer means transferring the toner image on the intermediate transfer belt to a recording medium includes nip pressure adjusting means capable of adjusting the pressure of the nip. The nip pressure adjusting means can perform adjustment according to the thickness of the recording medium.

Description

  The present invention relates to a fixing device used in an image forming apparatus such as a copying machine, a printer, and a facsimile, and an image forming apparatus including the same.

  In recent years, as an example of an image forming apparatus, a plurality of image forming units are provided, and toner images of different colors (yellow (Y), cyan (C), magenta (M), black (K), etc.) are provided in each image forming unit. Tandem-type electrophotography that forms a color image by forming toner images formed on the photosensitive drums and transferring the toner images formed on the photosensitive drums onto an intermediate transfer member or recording medium conveyed at a constant speed. A type of image forming apparatus is known.

  In such an image forming apparatus, when the thick paper enters the secondary transfer portion, the speed of the intermediate transfer belt fluctuates, and there is a relative speed difference between the intermediate transfer belt of the primary transfer portion and the photosensitive drum. There is a problem in that it causes a local transfer position shift.

Specifically, as shown in FIG. 12, when the paper P enters the secondary transfer nip portion 580, the intermediate transfer belt 502 varies in its running speed. FIG. 11 is a measurement graph in which the speed fluctuation of the intermediate transfer belt 502 is measured in the area 1 between the K-color photosensitive drum 501K and the M-color photosensitive drum 501M. This variation in running speed is noticeable in thick paper with thick paper. The thick paper referred to here means paper having a continuous weight of 220 kg and a basis weight of 256 g / m ² or more.

  On the other hand, the photosensitive drum 501 is driven to rotate at substantially the same speed without being affected by the speed fluctuation of the intermediate transfer belt 502. For this reason, when the running speed of the intermediate transfer belt 502 varies, a speed difference occurs between the photosensitive drum 501 and the intermediate transfer belt 502. As a result, the transfer position shifts in the primary transfer portion 570.

  As shown in FIG. 13, the toner image transferred to the intermediate transfer belt 502 when the transfer position shift occurs is represented by FIG. 13 where the travel distance from the K primary transfer portion 570K to the secondary transfer portion 580 is L. Then, it is transferred and fixed at a position L from the front end of the sheet. Then, a final image having local density unevenness of a portion A having a low toner density and a portion B having a high toner density is discharged.

  Such a shift of the transfer position that occurs when the sheet enters the secondary transfer nip portion can be prevented to some extent by the primary transfer roller pressing the intermediate transfer belt (see, for example, Patent Document 1). .

  In the image forming apparatus described in Patent Document 1, the contact pressure of the image carrier on the most downstream side with respect to the subject in the traveling direction of the intermediate transfer belt is set higher than the contact pressure of the other image carrier with respect to the subject.

  However, in the image forming apparatus described in Patent Document 1, when the thick paper enters the secondary transfer unit, when the photosensitive drum on the most downstream side is close to the driving roller of the intermediate transfer belt in the intermediate transfer belt traveling direction, The contact pressure at which the uppermost photosensitive drum comes into contact with the intermediate transfer belt is lowered. For this reason, when thick paper enters the secondary transfer portion, the effect of suppressing the speed fluctuation of the intermediate transfer belt in the vicinity of the most upstream photosensitive drum is reduced.

  In particular, the effect of suppressing the speed fluctuation of the intermediate transfer belt is significantly reduced during monochrome printing than during full color printing compared to full color printing.

  Specifically, in full-color printing, as shown in FIG. 14, the primary transfer rollers 524 Y, C, and M of the Y, C, and M colors are transferred to the intermediate transfer belt 502 via the link mechanism 521 by the action of the eccentric cam 523. Press. The K primary transfer roller 524K presses the intermediate transfer belt 502 independently. All the primary transfer rollers 524Y, C, M, and K press the intermediate transfer belt 502. The toner images of the respective colors are transferred to the intermediate transfer belt 502. On the other hand, in monochrome printing, the Y, C, M primary transfer rollers 524Y, 524, and M and the intermediate transfer belt 502 are separated from each other via the link mechanism 521 by the action of the eccentric cam 523, as shown in FIG. To do. Then, the K primary transfer roller 524K is brought into pressure contact with the intermediate transfer belt 502, and the K color toner image is transferred to the intermediate transfer belt 502.

  Therefore, compared to full color printing where the intermediate transfer belt is restrained at the four primary transfer portions, the driving force obtained from the photosensitive drum is less during monochrome printing where the intermediate transfer belt is restrained at one primary transfer portion, The belt speed fluctuation is not suppressed and an image defect occurs.

  An image forming apparatus capable of solving such image defects during monochrome printing is known (for example, see Patent Document 2).

  The image forming apparatus described in Patent Document 2 can set the nip pressure and the nip width of the transferred member with respect to the image bearing member large when the full color mode is switched to the subtractive color mode.

However, it is required to handle thicker paper, for example, paper having a basis weight of 350 g / m ² or more. In such a paper thickness, image defects occur not only in monochrome printing but also in full-color printing. There is a risk that.

  SUMMARY OF THE INVENTION An object of the present invention is to provide an image forming apparatus that suppresses fluctuations in the speed of an intermediate transfer belt when cardboard enters a secondary transfer section and can improve image quality.

In order to solve the above-described problem, an image forming apparatus according to claim 1 of the present invention is provided.
At least one image carrier on which a toner image is formed;
An intermediate transfer belt that is wound around a driving roller and a driven roller and is driven to rotate;
Primary transfer means that presses the intermediate transfer belt and forms a nip with the image carrier via the intermediate transfer belt, and a toner image formed on each image carrier is transferred to the intermediate transfer belt Primary transfer means for transferring sequentially;
In the image forming apparatus having secondary transfer means for transferring the toner image on the intermediate transfer belt to a recording medium,
A nip pressure adjusting means capable of adjusting the nip pressure;
The nip pressure adjusting means can be adjusted according to the thickness of the recording medium.

  According to the present invention, it is possible to provide an image forming apparatus capable of suppressing the speed fluctuation of the intermediate transfer belt when the thick paper enters the secondary transfer portion and improving the image quality.

1 is a configuration diagram schematically illustrating an image forming apparatus according to a first embodiment of the present invention. FIG. 2 is a block diagram schematically showing drive control of an eccentric cam of the image forming apparatus shown in FIG. 1. FIG. 2 is a flowchart for explaining control of nip pressure in accordance with the paper thickness of the image forming apparatus shown in FIG. 1. It is explanatory drawing explaining the conventional image forming apparatus. 6 is a graph for explaining a belt speed variation of a conventional image forming apparatus. 3 is a graph for explaining a belt speed variation of the image forming apparatus shown in FIG. 1. It is a block diagram which shows schematically the image forming apparatus which concerns on 2nd Embodiment of this invention. It is explanatory drawing explaining the conventional image forming apparatus. 6 is a graph for explaining a belt speed variation of a conventional image forming apparatus. It is a graph explaining the speed fluctuation of the belt of the image forming apparatus shown in FIG. FIG. 10 is an explanatory diagram illustrating a conventional fixing device. FIG. 10 is an explanatory diagram illustrating a conventional fixing device. FIG. 10 is an explanatory diagram illustrating a conventional fixing device. FIG. 10 is an explanatory diagram illustrating a conventional fixing device. FIG. 10 is an explanatory diagram illustrating a conventional fixing device.

  Next, a first embodiment of an image forming apparatus to which the present invention is applied will be described below with reference to the drawings. In each drawing for explaining the present embodiment, constituent elements such as members or components having the same function or shape are given the same reference numerals as much as possible, and once explained, the explanation will be given. Is omitted.

  As shown in FIG. 1, the printer as the image forming apparatus according to the present embodiment forms four sets of images using toners of four different colors (yellow: Y, cyan: C, magenta: M, black: K). The units are provided in parallel, and an intermediate transfer belt 2 as an intermediate transfer member for transferring the toner image formed by these image forming units is provided below the units. That is, the printer is a tandem type image forming apparatus in which four sets of image forming units are arranged in parallel along the moving direction of the intermediate transfer belt 2. Since these image forming units have the same configuration except for the difference in developer (toner) color, in the following description, the subscripts Y, C, M, and K in the reference numerals are omitted as appropriate. To do.

  Each image forming unit includes a photosensitive drum 1 as an image carrier and a charging device (not shown) that charges the surface of each photosensitive drum with a charging roller. Further, the charged surface of each photosensitive drum 1 is exposed to a laser beam based on image information obtained by reading an original document conveyed by ADF with an image scanner unit or image information transmitted from a PC. An image information exposure device (not shown) for forming a latent image is provided. Further, a developing device (not shown) as image forming means for converting the latent image on each photoconductive drum 1 into a toner image, and a photoconductor cleaning device (not shown) for cleaning the surface of each photoconductive drum 1 are provided. It is provided around each photosensitive drum 1.

  The photosensitive drums 1 of the four sets of image forming units are rotationally driven in the direction of the arrow in the figure by a photosensitive drum driving device (not shown). Note that the black photosensitive drum 1K and the color photosensitive drums 1Y, 1M, and 1C may be driven to rotate independently, so that, for example, when forming a monochrome image, the black photosensitive drum 1K may be rotated. Only the photosensitive drum 1K can be rotated, and when forming a color image, the four photosensitive drums 1Y, 1M, 1C, and 1K can be simultaneously rotated.

  The intermediate transfer belt 2 is wound around a plurality of support rollers such as a secondary transfer counter roller 5 and support rollers 3 and 7. The support roller 3 is a drive roller 3 that moves the intermediate transfer belt 2 endlessly in the clockwise direction in the drawing by being rotationally driven by a drive motor (not shown). On the other hand, the support roller 7 is a driven roller 7 that rotates as the intermediate transfer belt 2 moves. The roller 6 is a tension roller 6 that contacts the outer peripheral surface of the intermediate transfer belt 2.

  At a primary transfer position where the toner image is transferred from each photosensitive drum 1 to the intermediate transfer belt 2, a primary transfer roller 24 serving as a primary transfer unit faces the respective photosensitive drums 1 with the intermediate transfer belt 2 interposed therebetween. Is provided. The primary transfer roller 24 constituting the intermediate transfer unit is separated from the color photosensitive drums 1Y, 1M, and 1C when forming a monochrome image, and from all the photosensitive drums when the image formation itself is not performed. A contact / separation mechanism is provided so as to be separated.

  The intermediate transfer belt 2 as a transfer member is pressed against the photosensitive drum 1 by being pressed by the primary transfer roller 24, and a primary transfer nip is formed at a portion facing each of the photosensitive drums 1. .

  A secondary transfer roller 8 as a secondary transfer unit is disposed below the intermediate transfer belt 2 in the drawing, and comes into contact with the secondary transfer counter roller 5 via the intermediate transfer belt 2 to form a secondary transfer nip. Is forming.

  The secondary transfer unit 10 having the secondary transfer roller 8 has a rotation shaft 43 on the downstream side in the traveling direction of the intermediate transfer belt 2, and is configured to be rotatable around the rotation shaft 43. The secondary transfer roller 8 is arranged to be pressed against the secondary transfer counter roller 5 via the intermediate transfer belt 2 by a tension spring load of the secondary transfer pressure spring. Then, the image on the intermediate transfer belt is transferred onto the paper P by the pressure of the secondary transfer roller 8 and a transfer bias (not shown).

  A transport belt 11 that transports the paper P to the fixing device 15 is provided on the downstream side of the secondary transfer unit P in the paper transport direction, and on the further downstream side, fixing that fixes an unfixed image transferred onto the paper. A device 15 is provided. In the conventional example shown here, the secondary transfer roller 8 is driven to rotate by a motor (not shown), but it may be configured to rotate around the intermediate transfer belt.

  The fixing device 15 is configured by pressing the pressure roller 12 against a fixing belt stretched between a fixing roller 13 and a heating roller 14. The fixing belt is heated by an IH coil (not shown) in the heating roller, and is heated to a temperature necessary for image fixing. On the other hand, the pressure roller 12 also incorporates a heater (not shown) and is used for preheating during standby. The unfixed image on the paper is fixed on the paper P by applying heat and pressure at the nip portion between the fixing belt and the pressure roller 12. The heater of the fixing device 15 does not have to use an IH coil, and may be a system constituted by a pair of heat rollers.

  An image forming operation in the image forming apparatus configured as described above will be briefly described. When a color image is formed by the printer having the above-described configuration, each photosensitive drum 1 is rotationally driven in the direction of the arrow in the figure, and at this time, the surface of each photosensitive drum 1 has a predetermined polarity by a charging device. For example, it is charged to negative polarity. Next, a light-modulated laser beam emitted from the image information exposure apparatus is irradiated on the charging surface of each photoconductive drum 1, thereby forming an electrostatic latent image on the surface of each photoconductive drum 1. That is, the portion where the absolute value of the potential on the surface of the photosensitive member is lowered by the laser beam becomes an electrostatic latent image (image portion), and the portion where the absolute value of the potential is kept high without being irradiated with the laser beam is the background portion. It becomes. Next, the electrostatic latent image is developed with toner stored in the developing device and charged to a predetermined polarity, and visualized as a toner image.

  Each color toner image formed on each photosensitive drum 1 is formed by applying a bias voltage having a polarity opposite to the polarity of the toner (minus in this example) to the primary transfer roller 24 at each primary transfer nip portion. Are transferred onto the intermediate transfer belt in a superimposed manner by both the action of the transfer electric field and the clamping pressure between the primary transfer roller 24 and the photosensitive drum 1 via the intermediate transfer belt 2. As a result, a full color toner image composed of four color toner images is formed on the intermediate transfer belt.

  Untransferred toner remaining on each photosensitive drum without being transferred to the intermediate transfer belt 2 is scraped off by a photosensitive member cleaning device (not shown), and the surface of each photosensitive drum 1 is cleaned. The toner removed from each photosensitive drum 1 can be transported to a developing device using a toner recycling device (not shown) for toner recycling.

  On the other hand, the paper feed roller 18 rotates and the paper P fed from the paper feed cassette 20 is separated one by one by the separation roller 17. Then, the separated paper P is transported by the transport roller 16 between the intermediate transfer belt 2 and the secondary transfer roller 8 at a predetermined timing. Then, the sheet P conveyed by the conveying roller 16 abuts against the registration roller (positioning roller) 9 and stops. Alternatively, the paper P transported from a manual feed tray (manual feed portion) (not shown) via the manual feed path similarly hits the registration roller 9 and stops. Then, the registration roller 9 rotates in synchronization with the full-color toner image superimposed on the intermediate transfer belt, and the sheet P is a secondary transfer formed between the secondary transfer roller 8 and the secondary transfer counter roller 5. It is fed into the nip part. Then, the full color toner image is collectively transferred onto the paper P sent to the secondary transfer nip portion.

  The paper P on which the full-color toner image is transferred is heated and pressurized by the fixing device 15 so that the toner image is fixed on the paper. Thereafter, the sheet on which the toner image is fixed is discharged from the paper discharge unit to a paper discharge tray (not shown). Alternatively, the image is again guided to the transfer position by a double-side reversing mechanism (not shown), and an image is recorded on the back surface, and is discharged to a paper discharge tray after fixing.

  Next, the nip pressure adjusting means 50 will be described in detail below. As shown in FIG. 1, the primary transfer roller 24 </ b> Y includes a link mechanism 34 that can rotate around a rotation shaft 35 disposed on the upstream side in the belt traveling direction. The link mechanism 34 is configured such that the primary transfer roller 24Y presses in the direction of the photosensitive drum 1Y by the urging force of the compression spring 36 held at a predetermined position. The link mechanism 34 is provided with an eccentric cam 33 at a predetermined location, and the rotation of the link mechanism 34 can be controlled by the rotation of the eccentric cam 33. Specifically, when the pressing force that the primary transfer roller 24Y presses in the direction of the photosensitive drum 1Y is increased, the eccentric cam 33 is abutted against the link mechanism 34 to the maximum, and the compression spring 36 is compressed. On the other hand, when lowering the pressing force that the primary transfer roller 24Y presses in the direction of the photosensitive drum 1Y, the eccentric cam 33 is driven by a predetermined angle, and the compression of the compression spring 36 is relaxed.

  On the other hand, the primary transfer rollers 24 </ b> C and 24 </ b> M are provided with a link mechanism 21 that can rotate around a rotation shaft 22 disposed on the upstream side in the belt traveling direction. The C and M primary transfer rollers 24 press the intermediate transfer belt 2 through the link mechanism 21 by the action of the eccentric cam 23. Further, the primary transfer roller 24K presses the intermediate transfer belt 2 by the biasing force of the biasing spring. In addition, the nip pressure of the Y primary transfer portion can be controlled independently.

  Next, the paper thickness detection means 60 will be described in detail below. The primary transfer roller 24 </ b> Y is provided with a paper thickness detection unit 60 on the opposite roller 40 that faces the conveyance roller 16. The paper thickness detection unit 60 includes a link mechanism 38 and a distance measuring sensor 37. The link mechanism 38 is L-shaped in the axial direction of the opposing roller 40, and a part of the long side is supported by the rotating shaft of the opposing roller 40. Further, a biasing spring is provided on a part of the short side of the link mechanism 38, and the opposing roller 40 pressurizes the conveying roller 16 by the biasing force. The link mechanism 38 includes a rotation shaft 39 at a portion where the long side and the short side intersect, and the link mechanism 38 rotates around the rotation shaft 39 when the paper P passes through the transport roller 16. It has become. And the displacement of the link mechanism can be detected by the distance measuring sensor 37 provided on the end side of the long side of the link mechanism. Further, by preparing a table of the displacement and paper thickness of the link mechanism 38 in advance, the paper thickness corresponding to the detected displacement of the link mechanism 38 can be detected.

  Then, as shown in FIG. 2, when the detection information from the paper thickness detection means 60 is input to the drive command unit 27 via the external interface (I / F), the microprocessor 28 preliminarily reads the nip pressure and the cam rotation. Information on an appropriate rotation angle is received from the memory 29 in which the table value of the angle is recorded. The microprocessor 28 outputs a command voltage corresponding to the rotation angle information to the drive driver 31 of the eccentric cam 33 to the cam position switching mechanism 30, and the rotation drive driver 31 drives the cam drive motor 32. Then, the eccentric cam 33 is rotated by a predetermined angle by the drive of the cam drive motor 32. With such a configuration, the primary transfer nip pressure can be controlled.

  Note that the nip pressure setting is not limited to a mode in which detection information from the paper thickness detection means 60 is input via the external interface (I / F), and the nip pressure is switched according to the paper thickness set by the user. You may be allowed to.

When the paper P enters the secondary transfer portion, the paper thickness of the paper to enter and the speed fluctuation of the intermediate transfer belt are substantially proportional. Therefore, when the paper thickness is such that the speed fluctuation at the time of paper entry is small and there is no problem in image quality, the nip pressure of the primary transfer portion of Y color is changed to the nip pressure of the primary transfer portion of other colors (for example, 8N). Set to the same as. On the other hand, in the case of a paper thickness (for example, a paper having a basis weight of 300 g / m ² or more) having a large speed fluctuation at the time of entering the paper and affecting the image quality, the nip pressure of the primary transfer portion of the Y color is set to other colors. A nip pressure (for example, 20 N) larger than the nip pressure (for example, 8 N) of the primary transfer portion is set.

  Next, the control of the nip pressure according to the paper thickness will be described in detail below with reference to the flow of FIG. The user turns on the power, selects the full color mode from the operation panel, and starts a full color printing operation (step S10). Next, information is input from an external interface or an operation panel operated by a user (step S11). If the paper is thick paper, drive control of the eccentric cam 33 is executed so as to correspond to the thick paper, and the nip pressure of the Y primary transfer portion is increased (step S12). Next, it is determined whether printing has started (step S13). If printing has not started, the process returns to step S13. On the other hand, when printing is started, a printing process is executed (step S14). Next, it is determined whether or not printing has been completed (step 15). If printing has not ended, the process returns to step S15. On the other hand, when printing is completed, drive control of the eccentric cam 33 is executed so as to correspond to the normal paper thickness, and the nip pressure of the primary transfer portion of Y color is equal to the nip pressure of the primary transfer portion of other colors. (Step S16). Then, the full color printing operation is terminated (step 17).

  On the other hand, if the paper is not thick paper, it is determined whether printing has started (step S21). If printing has not started, the process returns to step S21. On the other hand, when printing is started, a printing process is executed (step S22). Next, it is determined whether or not printing has been completed (step S23). If printing has not ended, the process returns to step S23. On the other hand, when the printing is finished, the full-color printing operation is finished (step 17).

  Next, the relationship between the position of the primary transfer portion and the speed fluctuation of the intermediate transfer belt 2 will be described below. The nip pressure of the Y-color primary transfer portion arranged farthest from the drive roller 3, that is, the nip pressure formed by the primary transfer roller 24Y pressing the photosensitive drum 1Y via the intermediate transfer belt 2, For example, 20N is set. On the other hand, the nip pressure of the primary transfer portion of C, M, K colors, that is, the nip formed by the primary transfer rollers 24C, M, K pressing the photosensitive drums 1C, M, K via the intermediate transfer belt 102. Is set lower than the nip pressure of the Y primary transfer portion, for example, 8N.

  By setting the nip pressure of the primary transfer portion of Y color located farthest from the drive roller 3 to be higher than the nip pressure of the primary transfer portion of other colors, the speed fluctuation of the intermediate transfer belt 2 is suppressed, and the image Defects can be prevented.

  In the conventional image forming apparatus, as shown in FIG. 4, the nip pressures of the primary transfer portions of Y, C, M, and K are all set to the same pressure value in the full color mode. Here, fluctuations in the running speed of the intermediate transfer belt 502 that occurred when the paper P entered the secondary transfer portion were measured in three areas according to the distance from the drive roller 503. Area 1 is a position at which the variation in travel speed of the intermediate transfer belt 502 between the primary transfer portion of K color and the primary transfer portion of M color is measured at the position closest to the drive roller 503. An area 2 is a position where the variation in the traveling speed of the intermediate transfer belt 502 between the M primary transfer portion and the C primary transfer portion is measured at a position closest to the drive roller 503 next. An area 3 is a position at which the fluctuation of the running speed of the intermediate transfer belt 502 between the C primary transfer portion and the Y primary transfer portion is measured at the farthest position from the drive roller 3. The nip pressure at each primary transfer portion was 8N.

  As shown in FIG. 5, the magnitude of the fluctuation in the running speed of the intermediate transfer belt 502 in each area is area 3> area 2> area 1. That is, the further away from the drive roller 503, the larger the fluctuation in the traveling speed of the intermediate transfer belt 502 becomes. This is presumably because the greater the distance from the driving roller 503, the less the driving force from the driving roller 503 is transmitted, and the greater the speed fluctuation of the intermediate transfer belt 502. For this reason, in the conventional image forming apparatus, the largest transfer position shift occurs in the primary transfer portion of Y color, and there is a possibility that an image defect may be caused depending on the paper size and the paper interval.

  On the other hand, also in the image forming apparatus according to the present embodiment, the fluctuation in the traveling speed of the intermediate transfer belt 2 that occurs when the paper P enters the secondary transfer nip portion is changed in accordance with the distance from the drive roller 3. The same measurement was performed in the area. Area 1 is a position at which the variation in the running speed of the intermediate transfer belt 2 between the primary transfer portion of K color and the primary transfer portion of M color is measured at the position closest to the drive roller 3. An area 2 is a position where the variation in the traveling speed of the intermediate transfer belt 2 between the M primary transfer portion and the C primary transfer portion is measured at a position closest to the drive roller 3 next. An area 3 is a position at which the variation of the traveling speed of the intermediate transfer belt 2 between the C primary transfer portion and the Y primary transfer portion is measured at the farthest position from the drive roller 3. The nip pressure at the Y primary transfer portion was 20 N, and the nip pressure at the C, M, K primary transfer portions was 8 N. As shown in FIG. 6, the fluctuation of the running speed of the intermediate transfer belt 2 in each area is suppressed in any area as compared with the conventional image forming apparatus. In particular, it can be seen that the suppression effect in area 3 is significant.

  Note that the nip pressure of each primary transfer portion is not limited to the above-described mode, and can be appropriately adjusted depending on the layout, process conditions, and the like. Specifically, when the nip pressure of the Y primary transfer portion is set high, local density unevenness can be eliminated in the final image, but white streaks or character image formation Sometimes, an image formation defect called so-called hollow out, in which the image at the center of the character image is lost, may occur. The degree of nip pressure at which these side effects occur depends on the temperature and humidity environment, process conditions, and the like. For this reason, it is desirable to be able to make appropriate adjustments while judging the transfer deviation due to speed fluctuations and the image quality of white and medium stripes.

  The nip pressure adjusting means is not limited to the above-described mode, and the primary transfer rollers 24Y, 24C, 24M, and 24M may include a link mechanism that can rotate around a rotation shaft that is disposed upstream in the belt traveling direction. good. Due to the action of the eccentric cam, the Y, C, M primary transfer rollers 24 press the intermediate transfer belt 2 via the link mechanism. In this case, the urging force of the compression spring gripped by each primary transfer roller 24Y, C, M can be adjusted by a known adjusting means, so that the nip pressure of each primary transfer roller 24Y, C, M with respect to the photosensitive drum. Can be controlled. Therefore, each nip pressure can be controlled according to the detection value of the paper thickness detection means or the paper thickness set by the user.

  The operation of the image forming apparatus according to the present embodiment will be described. Since the nip pressure adjusting unit 50 can control the nip pressure with respect to the image carrier in accordance with the paper thickness based on the detection information from the paper thickness detecting unit 60, fluctuations in the belt speed of the intermediate transfer belt 2 can be suppressed. In particular, by setting the nip pressure for the image carrier farthest from the drive roller to be higher than the nip pressures for other image carriers, fluctuations in belt speed can be effectively suppressed. Further, since the contact pressure is not increased when the paper thickness is thin and the speed fluctuation is small, side effects such as white streaks caused by increasing the contact pressure can be suppressed. The K-color image carrier is normally arranged on the most downstream side of the primary transfer portion in the belt traveling direction in order to give priority to the first print time. For this reason, by disposing the drive roller on the downstream side, it is possible to suppress the displacement of the primary transfer of the K color when the paper enters.

  Next, a second embodiment of the present invention will be described in detail with reference to the drawings. In addition, about the structure equivalent to embodiment mentioned above, a corresponding code | symbol is attached | subjected and detailed description is abbreviate | omitted. The image forming apparatus of the present embodiment is different in the configuration of the drive roller compared to the image forming apparatus of the first embodiment. As shown in FIG. 7, the driving roller 103 according to the present embodiment is disposed on the upstream side of the primary transfer unit in the traveling direction of the intermediate transfer belt 102.

  Next, the nip pressure adjusting means 150 will be described in detail below. As shown in FIG. 7, the primary transfer roller 124 </ b> K includes a link mechanism 134 that can rotate about a rotation shaft 135 that is disposed on the upstream side in the belt traveling direction. The link mechanism 134 is configured such that the primary transfer roller 124K presses in the direction of the photosensitive drum 101K by the urging force of the compression spring 136 held at a predetermined position. The link mechanism 134 is provided with an eccentric cam 133 at a predetermined location, and the rotation of the link mechanism 134 can be controlled by the rotation of the eccentric cam 133.

  On the other hand, the primary transfer rollers 124M, C, and Y are provided with a link mechanism 121 that can rotate around a rotation shaft 122 disposed on the downstream side in the belt traveling direction. Due to the action of the eccentric cam 123, the primary transfer rollers 124 M, C, and Y of M, C, and Y colors press the intermediate transfer belt 102 through the link mechanism 121. In addition, the nip pressure of the Y primary transfer portion can be controlled independently.

  Next, the relationship between the nip pressure of the primary transfer portion and the speed fluctuation of the intermediate transfer belt 102 will be described below. The nip pressure of the primary transfer portion of the K color arranged farthest from the driving roller 103, that is, the nip pressure formed by the primary transfer roller 124K pressing the photosensitive drum 101K through the intermediate transfer belt 102, For example, 20N is set. On the other hand, the nip pressure of the primary transfer portion of the M, C, Y colors, that is, the nip formed by the primary transfer rollers 24M, C, Y pressing the photosensitive drums 1M, C, Y via the intermediate transfer belt 102. Is set lower than the nip pressure of the K primary transfer portion, for example, 8N.

  By setting the nip pressure of the primary transfer portion of the K color that is disposed farthest from the driving roller 103 to be higher than the other nip pressures, fluctuations in the speed of the intermediate transfer belt 102 are suppressed, and image defects can be prevented.

  In the conventional image forming apparatus, as shown in FIG. 8, the nip pressures of the primary transfer portions of the K, M, C, and Y colors are all set to the same pressure value in the full color mode. Here, fluctuations in the running speed of the intermediate transfer belt 602 that occurred when the paper P entered the secondary transfer nip portion were measured in three areas according to the distance from the drive roller 603. The position closest to the drive roller 603 is the area 1 where the variation in the running speed of the intermediate transfer belt 602 between the Y primary transfer portion and the C primary transfer portion is measured. An area 2 is a position at which the variation in the traveling speed of the intermediate transfer belt 602 between the C primary transfer portion and the M primary transfer portion is measured at a position closest to the drive roller 603 next. An area 3 is a position at which the variation in the traveling speed of the intermediate transfer belt 602 between the primary transfer portion of K color and the primary transfer portion of M color is measured at the position farthest from the driving roller 3. The nip pressure at each primary transfer portion was 8N.

  As shown in FIG. 9, the magnitude of the fluctuation in the running speed of the intermediate transfer belt 602 in each area is as follows: area 3> area 2> area 1. That is, the further away from the driving roller 603, the greater the fluctuation in the traveling speed of the intermediate transfer belt 602. This is presumably because the greater the distance from the driving roller 603, the less the driving force from the driving roller 603 is transmitted, and the greater the speed fluctuation of the intermediate transfer belt 602. For this reason, in the conventional image forming apparatus, the largest transfer position shift occurs in the primary transfer portion of K color, and there is a possibility that an image defect may be caused depending on the paper size and the paper interval.

  On the other hand, also in the image forming apparatus according to the present embodiment, the fluctuation in the traveling speed of the intermediate transfer belt 102 that occurs when the paper P enters the secondary transfer nip portion is changed in accordance with the distance from the driving roller 103. The same measurement was performed in the area. Area 1 is a position at which the variation in the traveling speed of the intermediate transfer belt 102 between the Y primary transfer portion and the C primary transfer portion is measured at a position closest to the drive roller 103. Area 2 is a position at which the variation in the traveling speed of the intermediate transfer belt 102 between the primary transfer portion of C color and the primary transfer portion of M color is measured at a position closest to the driving roller 103 next. An area 3 is a position at which the variation in travel speed of the intermediate transfer belt 102 between the primary transfer portion of K color and the primary transfer portion of M color is measured at the position farthest from the driving roller 103. The nip pressure of the primary transfer portion for K color was 20 N, and the nip pressure of the primary transfer portion for M, C, and Y colors was 8 N. As shown in FIG. 10, the fluctuation of the running speed of the intermediate transfer belt 102 in each area is suppressed in any area compared to the conventional image forming apparatus. In particular, it can be seen that the suppression effect in area 3 is significant. As described above, even if the driving roller is arranged on the upstream side of the primary transfer unit, it is possible to suppress the positional deviation of the primary transfer unit.

  Note that the nip pressure of each primary transfer portion is not limited to the above-described mode, and can be appropriately adjusted depending on the layout, process conditions, and the like. Specifically, when the nip pressure of the primary transfer portion of K color is set high, local density unevenness can be eliminated in the final image, but white streaks or character image image formation occurs. Sometimes, an image formation defect called so-called hollow out, in which the image at the center of the character image is lost, may occur. The degree of nip pressure at which these side effects occur depends on the temperature and humidity environment, process conditions, and the like. For this reason, it is desirable to be able to make appropriate adjustments while judging the transfer deviation due to speed fluctuations and the image quality of white and medium stripes. In addition, the comparison by this layout is a comparison after aligning various conditions such as the secondary transfer nip pressure, the roller diameter, the belt tension, and the sheet feeding linear speed. Needless to say, the magnitude relationship may change.

  As mentioned above, although embodiment of this invention was described, this invention is not limited to this. The above-described embodiments can be carried out singly or in combination of at least two. It should be noted that the materials and dimensions of each component introduced in the above-described embodiment are merely examples, and it is needless to say that various materials and dimensions can be selected within a range where the effects of the present invention can be exhibited.

1 Photosensitive drum (an example of an image carrier)
2 Intermediate transfer belt 3 Drive roller 4 Driven roller 8 Secondary transfer roller (an example of secondary transfer means)
24 primary transfer roller (an example of primary transfer means)
50 Nip pressure adjusting means 60 Paper thickness detecting means N Nip

JP 2008-304780 A Japanese Patent No. 4701900 Japanese Patent No. 5206269

Claims (5)

At least one image carrier on which a toner image is formed;
An intermediate transfer belt that is wound around a driving roller and a driven roller and is driven to rotate;
Primary transfer means that presses the intermediate transfer belt and forms a nip with the image carrier via the intermediate transfer belt, and a toner image formed on each image carrier is transferred to the intermediate transfer belt Primary transfer means for transferring sequentially;
In the image forming apparatus having secondary transfer means for transferring the toner image on the intermediate transfer belt to a recording medium,
A nip pressure adjusting means capable of adjusting the nip pressure;
The image forming apparatus, wherein the nip pressure adjusting means can be adjusted according to the thickness of the recording medium. In the recording medium conveyance direction, a paper thickness detection means is provided upstream of the secondary transfer means,
The paper thickness detection means detects the thickness of the recording medium conveyed to the secondary transfer means,
The image forming apparatus according to claim 1, wherein the nip pressure adjusting unit is capable of adjusting the nip pressure in accordance with information detected by the paper thickness detecting unit.   3. The nip pressure adjusting means can be adjusted so that a nip pressure in an image carrier farthest from the drive roller is higher than a nip pressure in another image carrier. The image forming apparatus described in 1.   The image forming apparatus according to claim 1, wherein the driving roller is disposed downstream of the primary transfer unit in the intermediate transfer belt traveling direction.   The image forming apparatus according to claim 1, wherein the driving roller is disposed upstream of the primary transfer unit in the intermediate transfer belt traveling direction.

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