JP2013109045A - Image forming apparatus - Google Patents

Abstract

PROBLEM TO BE SOLVED: To transfer a toner image without trouble even in the case where an intermediate transfer body is flipped up by a transfer material in the vicinity of a nip part formed by an image carrier and a conveying member where a toner image is transferred to a transfer material.SOLUTION: Control means controls movement of a nip width adjusting member that adjusts a nip width that is a length in a conveyance direction of a nip part on the basis of a basis weight of a transfer material.

Description

  The present invention relates to an image forming apparatus having a transfer member that abuts an image carrier and transfers an image formed on the image carrier to a transfer material at a nip portion with the image carrier.

The toner image formed on the photosensitive drum is primarily transferred to the intermediate transfer belt in the primary transfer portion, and the toner image on the intermediate transfer belt is transferred to a sheet as a transfer material in the secondary transfer portion to form an image. Such an image forming apparatus is widely known.
As such an image forming apparatus, a configuration is known in which a sheet is held and conveyed in a secondary transfer unit while electrostatically transferring a toner image to the sheet at a nip between an intermediate transfer belt and a secondary transfer member. ing.
In the conventional image forming apparatus, as shown in FIG. 9A, a guide member 39 for allowing the paper P to enter is provided upstream of the secondary transfer unit 38 in the transport direction, and the paper P guided by the guide member 39. Has entered the nip N between the intermediate transfer belt 31 and the secondary transfer belt 38b.
In this case, when the rear end of the paper P entering the nip N between the intermediate transfer belt 31 and the secondary transfer belt 38b passes through the guide member 39, a phenomenon occurs in which the rear end of the paper P jumps up and jumps up. When the trailing edge of the paper P hits the intermediate transfer belt 31, the intermediate transfer belt 31 may be pushed up.
The phenomenon that the trailing edge of the sheet P as described above pushes up the intermediate transfer belt 31 is likely to occur particularly when an image is formed on a sheet having high rigidity such as a thick sheet. When the intermediate transfer belt 31 is pushed up, as shown in FIG. 9B, the nip width, which is the length of the nip portion N in the sheet conveyance direction, becomes narrower. A void portion is formed between P and P.

Generally, the toner image on the intermediate transfer belt 31 is applied to one of the secondary transfer roller 38a and the opposing roller 37d that is in pressure contact with the secondary transfer belt 38b via the intermediate transfer belt 31. A high voltage is applied for transfer. Therefore, if the nip width becomes narrow and a gap is formed in the vicinity of the nip as described above, a discharge phenomenon is likely to occur in the vicinity of the nip N.
When a discharge phenomenon occurs in the vicinity of the nip portion N during image transfer onto the paper P, there is a problem in that the transfer of the image onto the paper P is poor and the quality of the image is degraded.
As a technique for solving the problem of deterioration in image quality caused by jumping up of the trailing edge of the paper, for example, Patent Document 1 discloses a position where an intermediate transfer member and a stretching member that stretches the intermediate transfer member start contact. Image formation capable of preventing the trailing edge of the recording medium conveyed to the secondary transfer unit from jumping up by configuring the secondary transfer unit and the intermediate transfer member to contact further downstream in the conveyance direction. An apparatus is described.

JP 2011-64917 A

In the invention described in Patent Document 1, a configuration is employed in which the secondary transfer portion is provided on the downstream side in the transport direction from the conventional one so that the trailing edge of the recording medium does not jump up. However, by providing the secondary transfer portion on the downstream side in the transport direction, the nip width where the secondary transfer of the image is performed becomes narrower than before, and as a result, the discharge phenomenon described above occurs near the nip width, and the image There was a problem that the quality of the product deteriorated.
The present invention has been made in view of the above problems, and an object thereof is to provide an image forming apparatus capable of forming a high-quality image by preventing a problem that occurs when an image is transferred to a sheet. .

  As means for solving the above-mentioned problems, the invention according to claim 1 is characterized in that a belt-like image bearing member on which an image is carried, and the image bearing member are in contact with each other while a transfer material is sandwiched between the image bearing member. A conveying member that conveys; a transfer unit that electrostatically transfers an image on the image bearing member to a transfer material at a nip portion where the image bearing member and the conveying member are in contact; and a leading end in the conveying direction A guide member provided on the opposite side of the nip portion from the image carrier and upstream of the nip portion in the transport direction, and a guide member for guiding a transfer material to the nip portion; The nip width adjusting member that adjusts the nip width that is the length in the transfer material conveyance direction, the basis weight acquisition means that acquires the basis weight of the transfer material, and the basis weight of the transfer material that is acquired by the basis weight acquisition means, The nip width is widened when larger than the first predetermined value. As an image forming apparatus characterized by and a control means for controlling the operation of the nip width adjustment member.

  The invention according to claim 2 further includes a transfer material detection unit that detects a transfer material upstream of the nip portion in the conveyance direction, and the control unit is configured such that the transfer material detection unit has a trailing end of the transfer material. 2. The image forming apparatus according to claim 1, wherein the operation of the nip width adjusting member is controlled so as to widen the nip width when it is detected that the nip has passed through the guide member. 3.

The invention described in claim 3 includes a fixing unit that transports the transfer material while fixing the image on the transfer material, and a paper size acquisition unit that acquires the length of the transfer material in the transport direction. And
In the image forming apparatus, a transfer material transport speed by the fixing unit is different from a transfer material transport speed by the transport member, and the control unit is configured to transfer the transfer material in the transport direction acquired by the paper size acquisition unit. Based on the basis weight of the transfer material acquired by the basis weight acquisition unit when the length is longer than the length of the transfer material conveyance path between the fixing unit and the nip portion, The image forming apparatus according to claim 1, wherein the operation is controlled.

  According to a fourth aspect of the present invention, there is provided an image forming apparatus in which a transfer material conveyance speed in the fixing unit is slower than a transfer material conveyance speed by the conveyance member, and the control unit includes the basis weight acquisition unit. 4. The image forming apparatus according to claim 3, wherein an operation of the nip width adjusting member is controlled so as to widen the nip width when the basis weight of the transfer material acquired by the printer is larger than a second predetermined value. It is.

  The invention according to claim 5 further includes friction coefficient acquisition means for acquiring a friction coefficient on the surface of the transfer material, and the control means has a friction coefficient on the surface of the transfer material acquired by the friction coefficient acquisition means. 5. The image forming apparatus according to claim 3, wherein an operation of the nip width adjusting member is controlled so as to widen the nip width when the nip width is smaller than a predetermined value of 3.

  According to a sixth aspect of the present invention, there is provided the image forming apparatus in which the transfer material transport speed in the fixing unit is faster than the transfer material transport speed by the transport member, and the friction for obtaining the friction coefficient of the transfer material surface. A coefficient acquisition unit, and the control unit is configured to increase the nip width so as to widen the nip width when the friction coefficient of the transfer material surface acquired by the friction coefficient acquisition unit is smaller than a fourth predetermined value. 6. The image forming apparatus according to claim 5, wherein the operation is controlled.

  According to a seventh aspect of the present invention, the control unit is configured to reduce the nip width when the basis weight of the transfer material is smaller than a fifth predetermined value smaller than the first predetermined value. The image forming apparatus according to claim 1, wherein an operation of the width adjusting member is controlled.

  The invention according to claim 8 is characterized in that the nip width adjusting member is a roller-shaped member, and the nip width is adjusted by displacing the conveying member in contact with the conveying member. An image forming apparatus according to claim 1.

  The invention according to claim 9 is characterized in that the nip width adjusting member is a belt-like member, and the nip width is adjusted by displacing the conveying member in contact with the conveying member. An image forming apparatus according to claim 1.

  The invention according to claim 10 is the image forming apparatus according to any one of claims 1 to 9, wherein the nip width adjusting member is a conductive member.

  The invention according to claim 11 is the image according to any one of claims 1 to 10, wherein the nip width adjusting member is a member having a volume resistivity lower than that of the conveying member. Forming device.

  According to the image forming apparatus of the present invention, the nip width in which the image is transferred to the paper can be adjusted to an appropriate size, so that a problem in transferring the image to the paper can be prevented and high image quality can be achieved. An image can be formed.

1 is a cross-sectional view illustrating a configuration of an entire image forming apparatus according to a first embodiment of the present invention. 1 is a block diagram illustrating an electrical configuration of an image forming apparatus according to a first embodiment of the present invention. It is a figure which shows an example of a structure around the secondary transfer part in the 1st Embodiment of this invention. It is a flowchart which shows the control procedure of operation | movement of the nip width adjustment member by CPU50 in the 1st Embodiment of this invention. It is a figure which shows another example of a structure around the secondary transfer part in the 1st Embodiment of this invention. It is a figure which shows an example of a structure of the secondary transfer part periphery in the 2nd Embodiment of this invention. It is a flowchart which shows the control procedure of operation | movement of the nip width adjustment member by CPU50 in the 2nd Embodiment of this invention. It is a figure which shows another example of a structure around the secondary transfer part in the 2nd Embodiment of this invention. FIG. 3 is a diagram illustrating a configuration around a secondary transfer unit of the image forming apparatus.

Hereinafter, a first embodiment of the present invention will be described with reference to the drawings. FIG. 1 is a cross-sectional view showing the overall configuration of the image forming apparatus according to the present embodiment.
The image forming apparatus 20 includes a scanner unit 22, a printer unit 24, a paper feed unit 32, an operation display unit 23, and the like.
The scanner unit 22 includes an automatic document feeder 21, an optical system 22a, a CCD image sensor 22b, and the like.
The printer unit 24 includes image forming units 40Y, 40M, 40C, and 40K that form toner images corresponding to the colors yellow, magenta, cyan, and black, and intermediate transfer onto which the toner images formed by the image forming units are transferred. An intermediate transfer belt 31 as a body, a secondary transfer unit 38 that transfers a toner image on the intermediate transfer belt 31 to a sheet, a fixing unit 34 that fixes a toner image transferred to the sheet, and the like.

The paper feed unit 32 includes paper feed trays 32a to 32c and the like, and appropriate paper is fed from the paper feed unit 32 at an appropriate timing.
The operation display unit 23 includes, for example, a touch panel display, and the user can perform settings related to image formation, an instruction to start image formation, and the like by touching an icon or the like displayed on the screen. .

The above is the outline of each part of the image forming apparatus 20. Next, a series of operations will be described by taking a case where a full color image is formed on a sheet by the image forming apparatus 20 as an example.
When an instruction to start image formation is given on the operation display unit 23, the document J placed on the document table 21a of the automatic document feeder 21 is conveyed to the document reading unit. Then, the reflected light of the light irradiated on the document from a light source (not shown) is read by the CCD image sensor 22b through the optical system 22a.
Various kinds of image processing are performed on the signal read by the CCD image sensor 22b. Then, based on the image signal subjected to the image processing, the image forming units 40Y to 40K form toner images of the respective colors.
The image forming unit 40Y for forming a yellow toner image includes a photosensitive drum 41Y as an image carrier, a charging unit 42Y, an exposure unit 45Y, a developing unit 43Y, and a cleaning unit 44Y. When forming an image, the photosensitive drum 41Y is rotated in the direction of arrow A in FIG. 1 by a driving unit (not shown), and the surface thereof is uniformly charged by the charging unit 42Y. The exposure unit 45Y irradiates the uniformly charged photosensitive drum 41Y with laser light based on the image signal, thereby forming an electrostatic latent image corresponding to the yellow image.

Next, the electrostatic latent image on the photosensitive drum 41Y is developed by the developing unit 43Y, and a yellow toner image is formed on the photosensitive drum 41Y. The formed toner image is electrostatically transferred to the intermediate transfer belt 31 by a transfer voltage applied to the primary transfer roller 46Y by a voltage application unit (not shown).
The toner remaining on the surface of the photosensitive drum 41Y without being completely transferred to the intermediate transfer belt 31 is removed by the cleaning unit 44Y to prepare for the next toner image formation.
The image forming unit 40M that forms a magenta toner image, the image forming unit 40C that forms a cyan image, and the image forming unit 40K that forms a black image are all image forming units that form a yellow image. Since it has the same configuration as 40Y, its detailed description is omitted.
The intermediate transfer belt 31 is an endless belt wound around rollers 37a to 37e as support members, and is rotatably supported. As the intermediate transfer belt 31, a resin belt represented by polyimide or a belt provided with an elastic layer on a resin material is generally used.

When image formation is performed, the intermediate transfer belt 31 is rotated in the direction of arrow B in FIG. 1 by the rotational drive of the drive roller 37a.
When forming a full-color image, the primary transfer rollers 46Y, 46M, 46C, and 46K respectively transfer the toner images to the intermediate transfer belt 31 at the timing when the toner images of the respective colors overlap each other in order from the yellow toner image. By transferring, an appropriate full-color toner image corresponding to the original image is formed on the intermediate transfer belt 31.
On the other hand, the paper fed from any of the paper feed trays 32a to 32c is transported through the transport path 33a and reaches the registration roller pair 33c. The skew of the sheet conveyance direction is corrected by the registration roller pair 33c, and the sheet is conveyed to the secondary transfer unit 38 so as to be synchronized with the color toner image on the intermediate transfer belt 31.
In the secondary transfer unit 38, the full-color toner image on the intermediate transfer belt 31 is electrostatically transferred to the paper. Here, the secondary transfer portion 38 is provided with a nip width adjusting member 38d. Details of the configuration of the secondary transfer unit 38 and the nip width adjusting member 38d will be described later.

The sheet on which the full-color toner image has been transferred is conveyed toward the fixing unit 34 and the toner image is fixed. On the other hand, the toner remaining on the intermediate transfer belt 31 without being completely transferred to the paper in the secondary transfer unit 38 is removed by the intermediate transfer belt cleaning unit 35. In addition, the toner transferred to the secondary transfer belt 38b without being transferred to the paper in the secondary transfer unit 38 is removed by the secondary transfer belt cleaning unit 38e.
The fixing unit 34 includes a fixing roller 34a, a pressure roller 34b, and the like. Each of the fixing roller 34a and the pressure roller 34b is heated by a heating unit (not shown) so that the surface thereof has a temperature suitable for fixing the toner image. Here, the surface temperature of the fixing roller 34a is set in advance so as to be higher than the surface temperature of the pressure roller 34b.
The color toner image is fixed on the paper by applying heat and pressure to the paper on which the toner image has been transferred by the fixing roller 34a and the pressure roller 34b.
The sheet on which the color toner image is fixed is transported through the transport path 33 a and is discharged out of the image forming apparatus 20.

When images are formed on both sides of a sheet, when the toner image is fixed on the surface of the sheet by the fixing unit 34, the sheet is conveyed toward the reverse path 33b by the branching unit 36, and the leading and trailing ends in the conveying direction are set. Inverted. The sheet with the leading and trailing ends reversed is conveyed to the registration roller pair 33c through the reversing path 33b, and the skew in the conveying direction is corrected.
Thereafter, the color toner image corresponding to the image formed on the back side of the sheet is transferred as described above in the secondary transfer unit 38, and the color toner image on the back side is fixed by the fixing unit 34, and is out of the image forming apparatus 20. Discharged.
The above is a description of a series of operations when the image forming apparatus 20 forms a color image on a sheet.

  FIG. 2 is a block diagram showing an electrical configuration of the image forming apparatus according to the present embodiment. The same components as those shown in FIG. 1 are denoted by the same reference numerals as those shown in FIG. In FIG. 2, only the configuration deeply related to the present invention is shown, and the other configurations are not shown.

The image forming apparatus 20 includes a scanner unit 22, a printer unit 24, a paper feed unit 32, an operation display unit 23, a CPU 50, a ROM 51, a RAM 52, an HDD 53, and the like.
The CPU 50 serves as a control unit that controls the operation of each unit of the image forming apparatus 20. The CPU 50 controls the operation of each unit of the image forming apparatus 20 based on the input content in the operation display unit 23.
Various programs or data are stored in the ROM 51, and the CPU 50 reads out the programs or data from the ROM 51 and controls each unit of the image forming apparatus 20.
The RAM 52 is a place for temporarily storing a program or data necessary for the CPU 50 to control the operation of each unit.
The HDD 53 is a place for storing image data. Document image data read by the scanner unit 22 is stored in the HDD 53.

The printer unit 24 includes a nip width adjusting member 38d, a transfer power source HV, a sensor S1 that detects paper passage, and the like. As will be described later, the CPU 50 controls the operation of the nip width adjusting member 38d so as to adjust the nip width between the intermediate transfer belt 31 and the secondary transfer belt 38b as necessary based on the sheet detection signal from the sensor S1. To do.
Further, a terminal 60 is connected to the image forming apparatus 20 via an I / F unit 61 via a network. The terminal 60 can input information regarding a job in which image formation is performed, and can transmit the information to the image forming apparatus 20. Further, it is possible to instruct image formation start from the terminal 60.

FIG. 3 is a diagram showing the configuration of the secondary transfer unit 38 in the present embodiment. The secondary transfer unit 38 includes a roller 37d via a secondary transfer belt 38b wound around the secondary transfer roller 38a and the stretching roller 38c, and an intermediate transfer belt 31 wound around the rollers 37a to 37e. And the secondary transfer roller 38a are in pressure contact with each other, and a nip portion where the intermediate transfer belt 31 and the secondary transfer belt 38b are in contact with each other is formed.
As the secondary transfer belt 38b, a belt whose surface is coated with a fluorine coating is generally used, but a belt similar to the intermediate transfer belt 31 may be used.

In the present embodiment, the power supply HV is connected to the roller 37d, and the secondary transfer roller 38a is electrically grounded. From the power source HV, a voltage having the same polarity as the toner is applied to the roller 37d in order to transfer the toner image to the paper. As a result, the toner image on the intermediate transfer belt 31 is transferred to the sheet entering the nip portion between the intermediate transfer belt 31 and the secondary transfer belt 38b.
The power supply HV may be connected to the secondary transfer roller 38a. In this case, a voltage having a polarity opposite to that of the toner is applied from the power source HV to the secondary transfer roller 38a, and the roller 37d needs to be electrically grounded.
The nip width adjusting member 38d is a roller-like conductive member, and is configured to be movable in the vertical direction in FIG. 3 when the power of the motor M1 in FIG. 2 is transmitted by a power transmission mechanism (not shown). . The position of the nip width adjusting member 38d shown in FIG.
In the present embodiment, as shown in FIG. 3B, the secondary transfer belt 38b is lifted by operating the motor M1 so as to move the nip width adjusting member 38d upward, and the nip width of the nip portion N is increased. It can be expanded.

The operation of the nip width adjusting member 38d will be described with reference to FIG. The color toner images are transferred onto the intermediate transfer belt 31 by the image forming units 40Y to 40M for the respective colors, while the paper fed from any of the paper feed trays 32a to 32c is timed by the registration roller pair 33c. Then, it is guided by the guide member 39 and enters the secondary transfer portion 38.
Here, as shown in FIG. 3A, a sensor S <b> 1 is provided in the vicinity of the front end of the guide member 39 in the sheet conveyance direction. Detection information of the sensor S1 is output to the CPU 50.
As shown in FIG. 3A, the sheet that has entered the secondary transfer portion 38 enters a nip portion N where the intermediate transfer belt 31 and the secondary transfer belt 38b are in contact with each other. The color toner image on 31 is electrostatically transferred to the paper.

Here, when the sheet on which the toner image is transferred is a sheet having a large basis weight such as a thick sheet, the sheet rear end is located above the upper side of FIG. 3A when the sheet rear end passes through the guide member 39. In some cases, the paper jumped up and the paper that jumped up contacted the intermediate transfer belt 31.
As described above, when the paper that is flipped up on the intermediate transfer belt 31 comes into contact, the intermediate transfer belt 31 is also lifted up following the paper, and as a result, the paper is conveyed at the nip portion N as shown in FIG. The nip width which is the width in the direction is narrowed, and a discharge phenomenon occurs in the vicinity of the narrowed nip portion N, resulting in a problem that the image quality is deteriorated.
Therefore, in the present embodiment, when the basis weight of the paper is larger than a predetermined value, the CPU 50 shows the nip width adjusting member 38d as shown in FIG. By controlling the motor M1 to move upward, the nip width of the nip portion N is maintained.

In the present embodiment, the CPU 50 is configured to control the operation of the nip width adjusting member 38d based on the detection information of the sensor S1 when the basis weight of the paper is larger than, for example, 300 g / m ² . Here, the basis weight as a reference may be appropriately set based on factors such as the structure of the apparatus and the conveyance speed of the paper.

A specific operation will be described below with reference to FIGS. FIG. 4 is a flowchart showing a control procedure of the operation of the nip width adjusting member 38d by the CPU 50.
When image formation is started, the CPU 50 first acquires information on the basis weight of the paper on which the image is formed (step S1). Here, the basis weight information of the paper is acquired from, for example, information regarding the job transmitted from the terminal 60.

Next, the CPU 50 determines whether or not the acquired paper basis weight is greater than 300 g / m ² (step S2).
When the basis weight of the sheet is 300 g / m ² or less (step S2: No), the CPU 50 ends the operation control process of the nip width adjusting member 38d. As a result, the toner image is secondarily transferred to the sheet without increasing the nip width of the nip portion N.
On the other hand, when the basis weight of the paper on which the image is formed is larger than 300 g / m ² (step S2: Yes), the nip width of the nip portion N is set at a predetermined timing when the paper passes through the secondary transfer portion 38. Spreading control is performed.
Specifically, it is determined whether or not the trailing edge of the sheet passing through the secondary transfer portion 38 has passed the leading edge of the guide member 39 (step S3). This determination is made based on whether or not the detection information that the rear end of the sheet has passed by the sensor S1, that is, the information that the sheet is no longer detected by the sensor S1 is received.
The CPU 50 waits until the rear end of the sheet passes through the front end of the guide member 39 (step S3: Yes), and if the CPU 50 determines that it has passed through the nip width adjusting member 38d from the standby position to the position shown in FIG. Control is made to move (step S4).
Then, the nip width adjusting member 38d is controlled to be held at the position shown in FIG. 3B for a predetermined period until the trailing edge of the sheet passes through the nip portion N. The CPU 50 determines whether or not the predetermined period has elapsed (step S5). If the CPU 50 determines that the predetermined period has elapsed (step S5: Yes), the CPU 50 controls the nip width adjusting member 38d to move to the standby position (step S5). Step S6).

Here, the predetermined period is obtained from the distance of the sheet conveyance path between the sensor S1 and the spread nip portion N at the most downstream side in the conveyance direction, and the sheet conveyance speed in the secondary transfer unit 38. The minimum period required as the predetermined period is a value obtained by dividing the distance of the sheet conveyance path by the sheet conveyance speed. However, the minimum necessary period is considered in consideration of a change in the sheet conveyance path when the jump occurs. A period slightly longer than the period may be set as the predetermined period.
Further, the amount of movement of the nip width adjusting member 38d from the standby position takes into account the factors such as the configuration of the image forming apparatus 20, the sheet conveyance speed, and the basis weight of the sheet. The nip width should be set appropriately so that the discharge phenomenon does not occur near the portion N.

By controlling as described above, the nip width of the nip portion N can be set to a desired level so that the toner image can be stably transferred to the paper without causing a discharge phenomenon even when the trailing edge of the paper is flipped up. It becomes possible to maintain the size. Accordingly, it is possible to prevent the image quality from being deteriorated due to the discharge phenomenon.
According to the present embodiment, when a sheet having a large basis weight such as thick paper that can cause the trailing edge to jump up passes through the secondary transfer unit 38, the trailing edge of the sheet starts from the point when the trailing edge of the sheet occurs. The nip width adjusting member 38d is operated so as to widen the nip width only during a period until it passes through the nip portion N of the secondary transfer portion 38.
As a result, even if the nip width becomes narrow due to the trailing edge of the paper being raised, the nip width is widened by the nip width adjusting member 38d, so that the nip width necessary for transferring the toner image is always secured. can do. Therefore, the discharge phenomenon in the vicinity of the nip portion N can be prevented, and a good image can always be formed.

In the first embodiment, whether or not the basis weight of the paper on which an image is formed is greater than a predetermined value is determined by acquiring the basis weight of the paper from the information regarding the job transmitted from the terminal 60. CPU50 was doing. However, the present invention is not limited to this. For example, a sensor for detecting the basis weight of the sheet is provided for the sheet conveyance path upstream of the secondary transfer unit 38 in the sheet conveyance direction, and based on detection information of the sensor. The CPU 50 may make a determination. In this case, the sensor for detecting the basis weight of the sheet may be provided separately from the sensor S1, or a sensor capable of detecting the basis weight is used as the sensor S1, and the sensor S1 detects the passage of the sheet and the basis weight. You may make it double. Further, the CPU 50 may make the above determination by acquiring the basis weight of the sheet from the input content on the operation display unit 23.
In the present embodiment, the sensor S1 is provided in the vicinity of the front end of the guide member 39 in the transport direction as shown in FIG. 3, but the present invention is not limited to this, and the transport is performed more than the position shown in FIG. It may be provided at any location as long as it is upstream of the direction and can detect the sheet.

In the present embodiment, the example in which the nip width adjusting member 38d is controlled to move when the basis weight of the paper is larger than 300 g / m ² is shown. It is preferable to set an appropriate value in consideration of factors such as the structure of the forming apparatus 20 and the sheet conveyance speed. Further, a plurality of basis weights as the reference may be set in consideration of the degree of jumping of the trailing edge of the paper. By setting multiple basis weights to subdivide the basis weight range and setting the movement amount of the nip width adjusting member 38d for each subdivided range, more precise control over the nip width is possible. As a result, the secondary transfer portion 38 has an effect that better transfer is performed.
In the present embodiment, a roller-shaped member as shown in FIG. 3 is used as the nip width adjusting member 38d. However, as shown in FIG. 5A, the belt-like member 38f may be wound around the roller-like member 38d and the secondary transfer roller 38a inside the secondary transfer belt 38b.
The roller-shaped member 38d is configured to be movable in the vertical direction in the figure, similarly to the nip width adjusting member 38d shown in FIG. When increasing the nip width of the nip portion N, as shown in FIG. 5B, the roller-shaped member 38d is moved upward in the drawing. As a result, the secondary transfer belt 38b is pushed up by the belt-like member 38f, and the nip width of the nip portion N is widened. In FIG. 5, the roller-shaped member 38d and the belt-shaped member 38f function as a nip width adjusting member.

A material having conductivity is used for the belt-shaped member 38f in FIG. Thus, when the nip width is widened, the belt-like member 38f and the secondary transfer belt 38b are continuously in contact with each other on the downstream side in the transport direction from the original nip portion N. Uniformity is improved, and better transfer is performed.
Furthermore, by using a conductive material having a volume resistivity lower than that of the material used for the secondary transfer belt 38b as the belt-shaped member 38f, the uniformity of the electric field is further improved.

The timing at which the nip width adjusting member 38d in FIG. 3 or the roller-shaped member 38d shown in FIG. 5 is moved upward in each figure may be the timing at which the trailing edge of the paper rises as described above. It is not limited.
When the basis weight of the paper is larger than the predetermined value, the nip width of the nip portion N only needs to be widened at least from the time when the trailing edge of the paper occurs to the time when the trailing edge of the paper passes through the nip portion N. . Therefore, for example, the nip width adjusting member 38d or the roller-shaped member 38d may be moved upward in the drawing at a timing before the time when the trailing edge of the sheet is raised.
Further, when the rear end of the sheet passes through the nip portion N, the nip width adjusting member 38d or the roller-shaped member 38d may be moved downward in the drawing, or the nip width is widened without moving. It may be left as it is. In particular, when image formation is continuously performed on a sheet having a large basis weight, it is preferable to leave the nip width widened, such as the durability of the driving means for driving the nip width adjusting member 38d. It is preferable from the viewpoint.

The above is the description of the first embodiment of the present invention. In this embodiment, the CPU 50 controls the nip width to change at an appropriate timing while the sheet passes through the nip portion N of the secondary transfer portion 38.
Next, a second embodiment of the present invention will be described with reference to the drawings. The configuration of the image forming apparatus 20 in the present embodiment is substantially the same as the configuration of the image forming apparatus 20 in the first embodiment except for the timing at which the nip width adjusting member 38d is operated. Description is omitted.

FIG. 6 is a diagram illustrating the configuration of the peripheral portion of the secondary transfer portion 38 and the fixing portion 34. The intermediate transfer belt 31 is rotationally moved in the direction of arrow R1 in FIG. 6A at the speed v1 by driving of the driving roller 38a (see FIG. 1). The secondary transfer roller 38a and the secondary transfer belt 38b that are in pressure contact with the intermediate transfer belt 31 also move in the direction of arrow R2 in FIG. 6A at the speed v1.
Further, the fixing roller 34a is rotated in the direction of the arrow R3 in FIG. 6A at the speed v2 by driving of a driving means (not shown). The pressure roller 34b in pressure contact with the fixing roller 34a also rotates in the direction of arrow R4 in FIG. 6A at the speed v2.
That is, the sheet conveyance speed when the toner image on the intermediate transfer belt 31 is transferred to the sheet in the secondary transfer unit 38 is v1, and the sheet conveyance speed when the toner image transferred to the sheet is fixed in the fixing unit 34. Is v2.

In general, the sheet conveyance speed v2 in the fixing unit 34 takes into consideration the amount of heat to be given to the sheet during fixing and the surface temperature of the fixing roller 34a (usually around 200 ° C.), and the sheet conveyance speed in the secondary transfer unit 38. It is set slower than the speed v1 (v1> v2). Then, as shown in FIG. 6B, when the leading end of the paper P that has passed through the secondary transfer unit 38 enters the fixing unit 34, the trailing end of the paper P passes through the secondary transfer unit 38 at that time. If not, the sheet P may bend in the region between the secondary transfer unit 38 and the fixing unit 34 due to the difference in the sheet conveyance speed between the secondary transfer unit 38 and the fixing unit 34.
When the sheet P is bent in the region between the secondary transfer unit 38 and the fixing unit 34, a force F2 is applied to the sheet P to return to the upstream side in the transport direction. On the other hand, a force F <b> 1 transported to the downstream side in the transport direction by the secondary transfer unit 38 acts on the paper P.
In particular, when the paper is a strong paper such as a thick paper, the force F2 may exceed the force F1. When the force F2 exceeds the force F1, as shown in FIG. 6C, the phenomenon that the deflection of the paper P is eliminated and the paper P is returned to the upstream side in the transport direction occurs upstream of the fixing unit 34. .
When this phenomenon occurs, the sheet P moves in the direction opposite to the transport direction regardless of whether the toner image is being transferred to the sheet P in the secondary transfer unit 38, and therefore, the transfer deviation occurs. There was a problem that it would occur.
In order to solve the above-described problem, in the present embodiment, when image formation is performed on a sheet having a basis weight larger than a predetermined value, as shown in FIG. The CPU 50 controls the nip width adjusting member 38d to move a predetermined amount upward in the drawing so as to widen the nip width of the portion N.

  When image formation is performed on a strong paper such as thick paper by the above control, the nip width of the nip portion N of the secondary transfer unit 38 is widened so that the paper can be firmly held. The force for conveying the sheet to the fixing unit side by the transfer unit 38 is increased. As a result, even if the sheet is bent due to the difference in sheet conveyance speed between the secondary transfer unit 38 and the fixing unit 34, the phenomenon that the sheet is returned to the upstream side in the conveyance direction can be prevented, and the toner image can be appropriately displayed. It becomes possible to transfer.

The operation of the nip width adjusting member 38d by the CPU 50 in the present embodiment will be described. FIG. 7 is a flowchart showing a control procedure of the operation of the nip width adjusting member 38d by the CPU 50.
When image formation is started, the CPU 50 first acquires information on the length in the transport direction of the paper on which the image is formed (step S11). This information is acquired from the information regarding the job transmitted from the terminal 60, for example.
Next, the CPU 50 determines that the length of the acquired sheet in the conveyance direction is the length L of the sheet conveyance path between the nip portion between the fixing roller 34 a and the pressure roller 34 b and the nip portion N in the secondary transfer unit 38. It is determined whether it is longer (step S12).
When the length of the paper in the transport direction is shorter than L (step S12: No), the above-described problem does not occur, so the CPU 50 operates the nip width adjusting member 38d as shown in FIG. Without processing, the process is terminated.

On the other hand, when the length of the paper in the conveyance direction is longer than L (step S12: Yes), the above-described problem may occur. Therefore, the CPU 50 controls the operation of the nip width adjusting member 38d based on the basis weight of the paper.
In this case, the CPU 50 acquires information on the basis weight of the paper (step S13). This information is acquired from the information regarding the job transmitted from the terminal 60, for example.

Next, the CPU 50 determines whether or not the basis weight of the acquired paper is larger than a predetermined value set in advance (step S14). When the basis weight of the paper is smaller than the predetermined value (step S14: No), the CPU 50 ends the process without operating the nip width adjusting member 38d as shown in FIG.
On the other hand, when the basis weight of the sheet is larger than the predetermined value (step S14: Yes), the CPU 50 turns the motor M1 so that the nip width adjusting member 38d moves upward by a predetermined amount as shown in FIG. Operate (step S15).
The motor M1 may be operated at any timing as long as the leading edge of the sheet is before the paper enters the nip portion between the fixing roller 34a and the pressure roller 34b in the fixing unit 34.
Moreover, as a predetermined value of the said basic weight, 300 g / m < ² > is used, for example. The amount of movement of the nip width adjusting member 38d is determined based on the basis weight of the paper on which an image is formed and the values of the paper transport speeds v1 and v2 in the secondary transfer unit 38 and the fixing unit 34, respectively.
The sheet onto which the toner image has been transferred in the secondary transfer unit 38 is conveyed toward the fixing unit 34 while being nipped by the nip N having a wide nip width, and the toner image is fixed in the fixing unit 34. At this time, since the nip width of the nip portion N is widened, the rear end of the paper is not returned to the upstream side in the transport direction even after the front end of the paper enters the fixing portion 34, and a toner image is appropriately formed up to the rear end of the paper. Transcribed.

  Here, the CPU 50 obtains the determination as to whether the basis weight of the paper on which the image is formed is greater than a predetermined value from the information regarding the job transmitted from the terminal 60. However, the present invention is not limited to this, and a sensor for detecting the basis weight of the sheet may be provided upstream of the secondary transfer unit 38 in the sheet conveyance direction, and the sensor may be acquired based on the detection result of the sensor. Further, it may be acquired from information of input in the operation display unit 23.

According to the present embodiment, when the sheet conveyance speed in the fixing unit 34 is smaller than the sheet conveyance speed in the secondary transfer unit 38 and the basis weight of the sheet is larger than a predetermined value, the secondary transfer unit 38 Since the CPU 50 controls to widen the nip width of the nip portion N, it is possible to prevent transfer deviation due to the sheet being returned to the upstream side in the transport direction.
In the present embodiment, the example in which the nip width adjusting member 38d is controlled to move when the basis weight of the paper is larger than 300 g / m ² has been described, but the basis weight value serving as a reference is the image forming apparatus. It is preferable to set an appropriate value in consideration of the 20 structure and the factors such as the sheet conveyance speeds v1 and v2. Further, a plurality of basis weights serving as the reference may be set, and the amount of movement of the nip width adjusting member 38d may be set for each basis weight range.

In this embodiment, an example in which the CPU 50 controls to widen the nip width in accordance with the basis weight of the paper is shown. However, the basis for determining whether to widen the nip width is only the basis weight of the paper. Is not limited.
For example, when an image is formed on a sheet having a small surface friction coefficient, the sheet surface is slippery when the intermediate transfer belt 31 and the secondary transfer belt 38b come into contact with each other, so the surface friction coefficient is large. Compared with the case of paper, the phenomenon shown in FIG. Accordingly, when image formation is performed on a sheet having a surface friction coefficient smaller than a predetermined value, the CPU 50 may control to move the nip width adjusting member 38d to widen the nip width of the nip portion N. .
Whether the friction coefficient of the paper surface is larger than a predetermined value is determined by, for example, storing information in which the paper type and the friction coefficient of the surface are associated with each other in advance in the ROM 51, and the CPU 50 displays the information on the operation display unit 32. This is done by acquiring the friction coefficient of the paper surface based on the input paper type information and the stored information.
Further, the CPU 50 may determine whether or not to widen the nip width of the nip portion N based on both the basis weight of the paper and the friction coefficient.

Also in the present embodiment, as in the first embodiment, the basis weight and friction coefficient of the reference sheet may be provided in a plurality of stages.
Also in this embodiment, the nip width of the nip portion N may be adjusted using a belt-like member 38f as shown in FIG. By using the belt-shaped member 38f, particularly when the nip width is widened, the electric field distribution in the widened portion becomes more uniform, and better secondary transfer is performed.
Similarly to the first embodiment, a better secondary transfer can be performed by using a member having a volume resistivity lower than that of the secondary transfer belt 38b for the belt-like member 38f.

In the present exemplary embodiment, as a general example, an example in which a problem that occurs when the sheet conveyance speed in the fixing unit 34 is slower than the sheet conveyance speed in the secondary transfer unit 38 is shown. If the paper transport speed in the fixing unit 34 is faster than the paper transport speed in the secondary transfer unit 38, a phenomenon occurs in which the paper is pulled in the transport direction when the leading edge of the paper enters the fixing unit 34. In this case, for example, if the friction coefficient of the surface of the paper is small, there is a possibility that the paper that is being pulled is not grasped at the nip portion N of the secondary transfer unit 38 and transfer deviation occurs.
Therefore, in the above case, the present invention is applied, and the CPU 50 controls the nip width of the nip portion N so that the above problem can be solved.

The above is the description of the second embodiment of the present invention. In the present embodiment, the occurrence of transfer deviation is prevented by controlling the nip width to be changed when the paper is sandwiched between both the secondary transfer portion and the fixing portion in accordance with the characteristics of the paper. .
In the first and second embodiments, the example in which the CPU 50 controls to widen the nip width of the nip portion N when the basis weight of the sheet is larger than a predetermined value has been described. Control may be performed.
When the basis weight of the paper is large, there is a possibility that transfer deviation occurs as described above. However, when the basis weight of the paper is small, the paper is wrinkled when the paper is carried in the nip portion N. Such a problem may occur.

Therefore, when the basis weight of the paper is smaller than a predetermined value, the CPU 50 can also control the nip width of the nip portion N to be small.
In order to reduce the nip width of the nip portion N, for example, as shown in FIG. 8, when the nip width adjusting member 38d moves downward in the figure, the secondary transfer belt 38b is interlocked and pulled downward in the figure. Keep it as
When the basis weight of the sheet is smaller than a predetermined value, for example, 50 g / m ^{2, the} CPU 50 controls the motor M1 to be driven in the opposite direction to the case where the nip width is increased. As a result, the nip width adjusting member 38d moves downward in the drawing, and the secondary transfer belt 38b is pulled downward accordingly, whereby the nip width of the nip portion N is reduced.

By controlling the nip width of the nip portion N to be small when forming an image on a sheet having a small basis weight, it is possible to form an image without causing wrinkles on the sheet.
The timing for moving the nip width adjusting member 38d downward in the drawing is preferably the timing before the leading edge of the paper P enters the nip portion N. For example, when the leading edge of the paper P is detected by the sensor S1, the CPU 50 controls the motor M1 to start rotating.
Further, the control by the CPU 50 shown in the first embodiment and the control by the CPU 50 shown in the second embodiment can be used in combination. Accordingly, it is possible to prevent a transfer phenomenon due to a discharge phenomenon in the vicinity of the nip portion N and a difference in conveyance speed from the fixing portion, and to form a good image.
The present invention is not limited to the above-described embodiment, and different embodiments can be adopted without departing from the present invention.

20 image forming apparatus 22 scanner unit 23 operation display unit 24 printer unit 31 intermediate transfer belt 32 paper feed unit 34 fixing unit 38 secondary transfer unit 38a secondary transfer roller 38b secondary transfer belt 38d nip width adjusting member 39 guide member 40Y ~ 40K image forming means (Y to K)
50 CPU
60 Terminal M1 Motor HV Transfer power supply S1 Sensor

Claims (11)

A belt-like image carrier on which an image is carried;
A conveying member that is in contact with the image carrier and conveys a transfer material while sandwiching the image carrier;
Transfer means for electrostatically transferring an image on the image carrier to a transfer material at a nip portion where the image carrier and the conveying member are in contact with each other;
A guide member for guiding a transfer material to the nip portion, the leading end portion in the conveyance direction being opposite to the image carrier relative to the nip portion and upstream of the nip portion in the conveyance direction;
A nip width adjusting member for adjusting a nip width which is a length of the nip portion in the transfer material conveyance direction;
A basis weight obtaining means for obtaining a basis weight of the transfer material;
Control means for controlling the operation of the nip width adjusting member to widen the nip width when the basis weight of the transfer material acquired by the basis weight acquisition means is larger than a first predetermined value. An image forming apparatus. A transfer material detecting means for detecting a transfer material on the upstream side in the transport direction from the nip portion;
The control means controls the operation of the nip width adjusting member so as to widen the nip width when the transfer material detecting means detects that the rear end of the transfer material has passed through the guide member. The image forming apparatus according to claim 1. A fixing unit that conveys the transfer material while fixing the image to the transfer material;
Paper size acquisition means for acquiring the length of the transfer material in the conveyance direction,
An image forming apparatus in which the transfer speed of the transfer material by the fixing unit is different from the transfer speed of the transfer material by the transfer member,
The control unit acquires the basis weight when the length in the transfer direction of the transfer material acquired by the paper size acquisition unit is longer than the length of the transfer material transfer path between the fixing unit and the nip unit. The image forming apparatus according to claim 1, wherein the operation of the nip width adjusting member is controlled based on the basis weight of the transfer material acquired by the unit. An image forming apparatus in which the transfer speed of the transfer material in the fixing unit is slower than the transfer speed of the transfer material by the transfer member,
The control unit controls the operation of the nip width adjusting member to widen the nip width when the basis weight of the transfer material acquired by the basis weight acquisition unit is larger than a second predetermined value. The image forming apparatus according to claim 3. It has a friction coefficient acquisition means for acquiring the friction coefficient of the transfer material surface,
The control means controls the operation of the nip width adjusting member to widen the nip width when the friction coefficient of the transfer material surface acquired by the friction coefficient acquisition means is smaller than a third predetermined value. The image forming apparatus according to claim 3 or 4. An image forming apparatus in which the transfer speed of the transfer material in the fixing unit is faster than the transfer speed of the transfer material by the transfer member,
It has a friction coefficient acquisition means for acquiring the friction coefficient of the transfer material surface,
The control means controls the operation of the nip width adjusting member to widen the nip width when the friction coefficient of the transfer material surface acquired by the friction coefficient acquisition means is smaller than a fourth predetermined value. The image forming apparatus according to claim 5. The control means controls the operation of the nip width adjusting member to narrow the nip width when the basis weight of the transfer material is smaller than a fifth predetermined value which is smaller than the first predetermined value. The image forming apparatus according to any one of claims 1 to 6. The nip width adjusting member is a roller-shaped member, and adjusts the nip width by displacing the conveying member in contact with the conveying member. The image forming apparatus described. The nip width adjusting member is a belt-shaped member, and adjusts the nip width by displacing the conveying member in contact with the conveying member. The image forming apparatus described. The image forming apparatus according to claim 1, wherein the nip width adjusting member is a conductive member. The image forming apparatus according to claim 1, wherein the nip width adjusting member is a member having a volume resistivity lower than that of the conveying member.

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