JP2006071838A - Image forming apparatus, loop control method, and conveyance control method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an image forming apparatus preventing image problems from arising by stabilizing loop control over a transfer material at the entrance to a fixing means without reference to the kind of the transfer material. <P>SOLUTION: After the front end of the transfer material rushes into a fixation nip part formed as a couple of rotary body for fixing an image transferred to the transfer material comes into contact, the control width of the conveying speed of the transfer material at the fixation nip part is varied when at least one or more predetermined positions of the transfer material pass through the fixation nip part. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to an image forming apparatus such as a copying machine, a printer, and a facsimile machine that forms an image by electrophotography.

  Conventionally, as a color image forming apparatus that forms a color image at high speed, a photosensitive drum is arranged for each color of yellow (Y), magenta (M), cyan (C), and black (Bk) along a conveying belt, and conveyed. Inline that sequentially transfers the image of each color by transferring the transfer material by belt, or transferring the toner image of each color sequentially on the intermediate transfer belt so as to overlap, and finally transferring the toner image to the transfer material collectively There is a so-called method.

  A main problem in this in-line type color image forming apparatus is a color shift between colors of a color image. One of the factors that cause this color misregistration is a speed difference between the speed of the transport belt that transports the transfer material and the transport speed of the fixing unit (fixing means). This is because the speed of the transfer material is governed by the conveyance speed inside the fixing unit after entering the fixing unit, and therefore the transfer material cannot be fed at a constant speed after entering the fixing unit. Therefore, generally, as in Patent Document 1, a loop is formed on the transfer material before the entrance of the fixing unit, and the loop amount at that time is made constant (hereinafter referred to as “loop control”). And the conveyance speed difference during fixing is minimized.

  Further, in Patent Document 2, a conveyance guide for conveying a transfer material from a transfer portion toward a fixing nip portion of a fixing portion is disposed so that the conveyance surface is inclined upward toward the fixing nip portion, and the transfer material faces downward. An image forming apparatus that enables fine speed control that makes a loop amount constant by detecting a loop amount and switching the speed of a motor that drives a pressure roller in stages. Proposed.

JP-A-7-234604 JP 2000-344385 A

However, each of the above conventional examples has the following problems.
As means for detecting the loop amount, the movement of the swingable flag provided in the transport guide is detected by whether or not the photo interrupter provided on the other side is shielded from light. In such a loop amount detection means, the transfer material is located in the fixing nip portion of the fixing portion downstream of the flag, and the transfer material is supported by the transfer conveyance belt (electrostatic conveyance belt) upstream of the flag. When the transfer material forms a loop, the loop amount can be detected stably.

However, in the first stage after the leading edge of the transfer material enters the fixing nip part and until the trailing edge of the transfer material peels off from the transfer conveyance belt and enters the fixing part, in particular, the thickness of the transfer material, Detection of the loop amount has become unstable because the amount of pressing of the flag is affected by the difference in paper strain due to the paper type, the second side during duplex printing, and the like.
As a result, loop control becomes unstable, and image problems such as image rubbing, gloss unevenness, and trailing edge splash may occur at the leading and trailing edges of the transfer material.

  The present invention solves the above-described problems, and an object of the present invention is to provide an image forming apparatus that stabilizes the loop control of the transfer material at the entrance of the fixing unit regardless of the type of the transfer material and prevents the occurrence of an image problem. To do.

  In order to achieve such an object, an image forming apparatus according to the present invention includes a transfer unit that transfers a toner image formed on an image carrier to a transfer material, and an image transferred to the transfer material. A fixing unit that conveys the transfer material while sandwiching the transfer material at a fixing nip formed by contact of a pair of rotating bodies, and a loop amount that detects a loop amount formed on the transfer material by the transfer unit and the fixing unit In the image forming apparatus, comprising: a detection unit; and a loop amount control unit that executes a loop control that switches a conveyance speed of the transfer material in the fixing nip portion according to a detection result of the loop amount detection unit. The means is configured so that at least one or more predetermined positions of the transfer material pass through the fixing nip portion after the leading edge of the transfer material enters the fixing nip portion. Changing the control range of the conveying speed of the transfer material in the nip portion, characterized in that.

  The loop control method of the present invention is a loop control method in an image forming apparatus, wherein the transfer material is fixed to a fixing nip formed by contacting a pair of rotating bodies to fix an image transferred onto the transfer material. The control width of the transfer material transport speed in the fixing nip portion is changed at a timing when at least one or more predetermined positions of the transfer material pass through the fixing nip portion after the leading end of the transfer material has entered. And

  The image forming apparatus of the present invention includes a transfer unit that transfers an image formed on an image carrier to a transfer material, a fixing unit for fixing the image transferred to the transfer material, and the transfer unit. A detecting unit for detecting a loop amount formed in the transfer material conveyed to the fixing unit; and a transfer material by switching a transfer speed of the transfer material in the fixing unit based on a detection result of the detecting unit. A control unit that controls a loop amount formed on the transfer material, wherein the control unit controls a switching width of a transfer speed of the transfer material based on a transfer position of the transfer material in the fixing unit. Features.

  The transfer material conveyance control method of the present invention is a transfer material conveyance control method in an image forming apparatus, and includes a transfer unit that transfers an image to a transfer material and a fixing unit that fixes an image transferred to the transfer material. Detecting a loop amount formed on the transfer material conveyed to the fixing member, switching a transfer speed of the transfer material in the fixing unit based on the detected loop amount, and the fixing unit of the transfer material And a step of controlling a change width of the transfer speed of the transfer material on the basis of the transfer position.

  According to the present invention, by switching the control range of the driving speed when controlling the loop amount at the leading end or the trailing end of the transfer material, the loop control is stabilized regardless of the type of the transfer material, and an image problem occurs. It is possible to provide an image forming apparatus that prevents the above.

  Embodiments of the present invention will be described below with reference to the accompanying drawings.

[First Embodiment]
(overall structure)
FIG. 1 is a sectional view schematically showing a color image forming apparatus according to an embodiment of the present invention.
The color image forming apparatus shown in FIG. 1 includes four photosensitive drums 1 (1a, 1b, 1c, 1d), and the photosensitive drums are sequentially arranged around each photosensitive drum 1 in accordance with the rotation direction. Charging means 2 (2a, 2b, 2c, 2d) for uniformly charging the surface of the drum 1, and exposure means 3 (3a, 2a) for forming an electrostatic latent image on the photosensitive drum 1 by irradiating a laser beam based on image information 3b, 3c, 3d), developing means 4 (4a, 4b, 4c, 4d) for attaching the toner to the electrostatic latent image to visualize the toner image, and transferring the toner image on the photosensitive drum 1 to a transfer material (recording) Medium) Transfer means (transfer rollers) 5 (5a, 5b, 5c, 5d) for transfer to S, and cleaning means 6 (6a, 6b, 6c, 6) for removing post-transfer toner remaining on the surface of the photosensitive drum 1 after transfer. 6d) etc. are arranged to form an image. (Image forming portion) is constituted.

Here, the photosensitive drum 1, the charging unit 2, the developing unit 4, and the cleaning unit 6 for removing the toner are integrally formed into a cartridge to form a process cartridge 7 (7 a, 7 b, 7 c, 7 d).
Further, the transfer material S fed from the feeding unit 8 is conveyed to the image forming unit by a conveying unit 9 constituted by an electrostatic conveying belt 9a, and each color toner image is sequentially transferred to record a color image. Then, the image is fixed by the fixing unit (fixing unit) 10 and discharged to the discharge unit 13 by the discharge rollers 11 and 12.

  In the case of double-sided printing, the transfer material S is fixed by the fixing unit 10 and is conveyed to the double-sided conveyance path 15 by reversing the discharge rollers 11 and 12 before being discharged by the discharge rollers 11 and 12. (Direction of arrow A). The transfer material S conveyed to the double-sided conveyance path 15 passes through the oblique feeding roller 16 on the front surface of the color image forming apparatus main body, and is conveyed vertically downward to the U-turn roller 17. The transfer material S is conveyed by the U-turn roller 17 and the registration roller 8 d. It is conveyed to the image forming unit.

  Next, the configuration of each unit will be described sequentially.

(Transfer material feeding configuration)
The feeding unit 8 includes a feeding cassette 8a, a multi feeding tray 8b that is a multi feeding device, a multi feeding unit 8c, and a registration roller 8d.
The feeding cassette 8a stores a plurality of transfer materials S and is loaded on the inner bottom of the color image forming apparatus main body. At the time of image formation using the transfer material S of the feeding cassette 8a, the transfer material S is separated and conveyed one by one by the cassette pickup roller 8a1, and conveyed to the image forming unit by the cassette conveyance roller 8a2 and the registration roller 8d.

Next, the multi-feed tray 8b is normally stored in front of the color image forming apparatus main body, but when used, the multi-feed tray 8b is rotated and opened from the apparatus main body to set a plurality of transfer materials S. To do. At the time of image formation using the transfer material S on the multi-feed tray 8b, the transfer materials S are separated and conveyed one by one by the multi-pickup roller 8c1, and conveyed to the image forming unit by the multi-conveyance roller 8c2 and the registration roller 8d.
Separation and conveyance of sheets in the feeding cassette 8a and the multi-feeding unit 8c are performed through a gear drive train by a feeding motor (not shown) in the feeding unit 8.

(Image formation configuration)
The photosensitive drum 1 as an image carrier is configured by applying an organic optical conductor layer (OPC) to the outer peripheral surface of an aluminum cylinder. Both ends of the photosensitive drum 1 are rotatably supported by flanges, and a driving force is transmitted from one driving motor (not shown) to one end in a counterclockwise direction indicated by an arrow in FIG. Driven by rotation.
Each charging means 2 is a conductive roller formed in the shape of a roller. The roller is brought into contact with the surface of the photosensitive drum 1 and a charging bias voltage is applied by a power source (not shown) to thereby surface the surface of the photosensitive drum 1. Is uniformly charged.

The exposure means 3 has a polygon mirror, and this polygon mirror is irradiated with image light corresponding to an image signal from a laser diode (not shown).
The developing means 4 is a toner storage section storing toners of black, cyan, magenta, and yellow, respectively, and is adjacent to the surface of the photosensitive member, and is driven to rotate by a drive section (not shown). It is composed of a developing roller that performs development by applying a developing bias voltage.

  In addition, transfer means 5a, 5b, 5c, and 5d that are in contact with the electrostatic conveyance belt 9a so as to oppose the four photosensitive drums 1a, 1b, 1c, and 1d are disposed inside the electrostatic conveyance belt 9a described later. It is attached. These transfer means 5 are connected by a power supply for transfer bias (not shown), and a positive charge is applied from the transfer means 5 to the transfer material S via the electrostatic conveyance belt 9a. Then, the negative color toner images on the photosensitive drum 1 are sequentially transferred to the transfer material S in contact with each other to form a color image.

(Transfer material transfer configuration)
The transfer material S is conveyed from the feeding unit 8 to the image forming area by the conveying unit 9.
An electrostatic conveyance belt 9a as a transfer material carrier constituting the conveyance means 9 is stretched and supported by four rollers of a driving roller 9b and driven rollers 9c, 9d, and 9e, and all the photosensitive drums 1a, 1b, 1c and 1d are arranged facing each other. By this drive roller 9b, the transfer material S is conveyed to the transfer position by the electrostatic conveyance belt 9a, and the toner image on the photosensitive drum 1 is transferred.

  Further, an adsorption roller 9f that holds the transfer material S together with the belt 9a and attracts the transfer material S to the electrostatic conveyance belt 9a is disposed at the most upstream position of the electrostatic conveyance belt 9a. When the transfer material S is transported, an electric field is formed between the electrostatic roller belt 9a and the transfer material S by applying a voltage to the attracting roller 9f to form an electric field with the opposing grounded roller 9c. Polarization is generated to generate an electrostatic attracting force on both.

(Fixing configuration)
The fixing unit 10 as a fixing unit applies heat and pressure to the image formed on the transfer material S and fixes the image.
Reference numeral 10a denotes a cylindrical fixing belt having an electromagnetic induction heat generating layer, which is guided by a belt guide member 10c having a built-in magnetic field generating means including an exciting coil and a T-shaped magnetic core. An elastic pressure roller 10b sandwiches the fixing belt 10a and forms a fixing nip portion N having a predetermined width with a predetermined pressure contact force with the belt guide member 10c.
The elastic pressure roller 10b is rotationally driven by a driving means (not shown), and the cylindrical fixing belt 10a rotates accordingly, and electromagnetic induction heat generation of the fixing belt 10a is performed by feeding power from an excitation circuit (not shown) to the excitation coil. The

  In a state where the fixing nip N rises to a predetermined temperature and is adjusted in temperature, the transfer material S on which the unfixed toner image conveyed from the image forming unit is formed is between the fixing belt 10a and the elastic pressure roller 10b. The image surface is introduced upward, that is, facing the fixing belt surface. The image surface is in close contact with the outer surface of the fixing belt 10a at the fixing nip N, and is nipped and conveyed together with the fixing belt 10a. In the process in which the transfer material S is nipped and conveyed through the fixing nip N together with the fixing belt 10a, the unfixed toner image on the transfer material S is heated and fixed by the electromagnetic induction heat generated by the fixing belt 10a.

(Loop formation configuration)
Next, a configuration in which the fixing unit 10 forms a loop on the transfer material S will be described.
When the image on the transfer material S is fixed, the transfer material clamping force at the fixing nip N is stronger than the clamping force of the transfer unit 5, so that the conveyance speed at the fixing unit 10 and the conveyance speed at the transfer unit 5 are different. The transfer material S is conveyed at the conveyance speed in the fixing unit 10. For this reason, the color image forming apparatus of the present embodiment is provided with a loop detection unit for detecting the loop amount of the transfer material S between the fixing unit 10 and the transfer unit 5 as shown in FIG.

The loop detection means is configured such that the lever member 23 is rotatable by contacting the transfer material S, and whether or not the flag 21 of the rotation source portion shields the detection sensor 22 constituted by an optical sensor. Then, it is detected whether or not the loop amount of the transfer material S has become a certain amount or more. In order to keep the loop amount of the transfer material S constant, the CPU 26 calculates an appropriate speed of the motor that drives the elastic pressure roller 10b from the signal detected by the detection sensor 22, and controls the speed of the motor.
The driving speed of the elastic pressure roller 10b is switched stepwise by at least four steps by the speed switching means (CPU 26), and the driving speed of the elastic pressure roller 10b is determined according to the detection result of the detection sensor 22. Is switched to make the loop amount of the transfer material S constant.

Next, loop control in the present embodiment will be described with reference to FIGS.
FIG. 3 is a loop control flow in this embodiment.

When image formation is started, as shown in the figure, at step 1, the drive speed V of the elastic pressure roller 10b is set to R3 (here, R3 is set as an initial value).
Next, at step 2, after the leading edge of the transfer material S enters the fixing nip portion N of the fixing portion 10, the first predetermined position of the transfer material S has passed through the fixing nip portion N (that is, the fixing nip portion N). If it has not passed, the process proceeds to step 3, and if it has passed, the process proceeds to step 6.
Here, the CPU 26 determines whether or not the first predetermined position of the transfer material S has passed through the fixing nip N. The CPU 26 manages the operation timing of each unit from the start of the image forming operation, and the timing after the predetermined time from the timing when the image formation is started (paper is fed) is the first predetermined position of the transfer material S. It is determined that the timing has passed through the fixing nip N.

  In step 3, it is determined whether the flag signal (the output signal of the detection sensor 21; hereinafter the same) is on or off. If it is on, the driving speed V of the elastic pressure roller 10b is set to R2 and off in step 4. If there is, the driving speed V of the elastic pressure roller 10b is set to R3 at step 5, and the process returns to step 2.

On the other hand, in step 6, it is determined whether the flag signal is on or off. If it is on, the driving speed V of the elastic pressure roller 10b is set to R1 in step 7, and if it is off, the driving speed V of the elastic pressure roller 10b is determined in step 8. The drive speed V is set to R4.
In step 9, it is determined whether or not the image formation is completed. If not completed, the process returns to step 6. If completed, the series of processes shown in FIG.
Note that R1>R2>R3> R4, and R1 and R2, which are driving speeds of the elastic pressure roller 10b, indicate that the transfer speed of the transfer material S at the fixing nip N is higher than the transfer speed of the transfer material S by the transfer roller 5. R3 and R4 make the transfer speed of the transfer material S at the fixing nip N slower than the transfer speed of the transfer material S by the transfer roller 5.

  FIG. 4 is a diagram showing the relationship between the flag signal and the driving speed (fixing speed) of the elastic pressure roller 10b in the loop control described above.

  Until the leading edge of the transfer material S enters the fixing nip N and the detection of the loop amount is stabilized, that is, the first predetermined position of the transfer material S (here, the position where the detection of the loop amount is stable) For example, the loop control is performed by switching the driving speed of the elastic pressure roller 10b to R2 or R3 until the position 2 to 3 cm from the front end of the transfer material S passes through the fixing nip N. On the other hand, after the timing when the first predetermined position of the transfer material S has passed through the fixing nip N, loop control is performed by switching the driving speed of the elastic pressure roller 10b to R1 or R4.

  That is, until the leading edge of the transfer material S enters the fixing nip portion N and the first predetermined position passes through the fixing nip portion N, if the flag signal is turned on, the amount of loop is reduced, so that the elasticity is increased. The driving speed of the pressure roller 10b is switched from R3 to R2 having a higher speed. On the other hand, when the flag signal is turned off, in order to increase the loop amount, the driving speed of the elastic pressure roller 10b is switched from R2 to R3 having a slower speed, and as a result, the desired loop amount is controlled.

  Next, after the timing at which the first predetermined position of the transfer material S has passed through the fixing nip N, when the flag signal is turned on, the loop speed is reduced, so that the driving speed of the elastic pressure roller 10b is changed from R4. Switch to the faster R1. On the other hand, when the flag signal is turned off, in order to increase the loop amount, the driving speed of the elastic pressure roller 10b is switched from R1 to R4 having a slower speed, and the resulting loop is controlled to a desired amount.

As shown in FIG. 4, when the flag signal is in the ON state and when the drive speed control width switching timing (fixing speed control width change timing) of the elastic pressure roller 10b is reached, the drive speed V of the elastic pressure roller 10b is reached. Is switched from R2 to R1 having a higher speed. On the contrary, when the control signal of the driving speed of the elastic pressure roller 10b is switched when the flag signal is OFF, the driving speed V of the elastic pressure roller 10b is switched from R3 to R4 having a lower speed ( Not shown).
As for the switching timing of the control range of the driving speed of the elastic pressure roller 10b, the first predetermined position of the transfer material S is fixed from the transfer conveyance belt 9a when the transfer material S is conveyed by a desired loop amount. It is determined by the time required to reach the portion 10 (fixing nip portion N).

  Here, until the first predetermined position of the transfer material S reaches the fixing nip portion N, the drive speed V of the elastic pressure roller 10b is switched to R2 or R3 and controlled (the control width is set small). For the following reasons.

In the first stage after the leading edge of the transfer material enters the fixing nip portion, the transfer material loop formed by the transfer material supported and supported by the fixing nip portion and the transfer conveyance belt depends on the thickness of the transfer material and the type of paper. It becomes unstable due to the difference in paper strain. At this stage, if the driving speed V of the elastic pressure roller 10b is switched to R1 or R4 and controlled (the control width is set large), the control amount becomes large, and the loop amount control of the transfer material becomes unstable. This is because image defects occur due to control oscillation.
For this reason, in a region where the loop is unstable, switching the driving speed V to R2 or R3 and controlling it reduces the loop control width of the transfer material and stabilizes the loop amount control of the transfer material, resulting in image defects. There is nothing to do.

  By the way, even after the first predetermined position of the transfer material S has passed through the fixing nip portion N, the switching speed of the loop-controlled elastic pressure roller 10b is switched only in two stages instead of four stages as described above. When R2 and R3 having a small difference are used, the following problem occurs.

  The conveyance speed at the fixing unit 10 is determined by the driving speed of the elastic pressure roller 10b and the outer diameter of the elastic pressure roller 10b. The outer diameter of the elastic pressure roller 10b changes depending on manufacturing tolerances and fixing temperatures. For this reason, if the outer diameter of the elastic pressure roller 10b is small and the driving speed R2 of the elastic pressure roller 10b is too small, the loop of the transfer material S cannot be reduced, image rubbing, and the rear end of the transfer material S. This causes image defects such as splashing. On the other hand, if the outer diameter of the elastic pressure roller 10b is large and the driving speed R2 of the elastic pressure roller 10b is too large, the number of loops of the transfer material S cannot be increased. Image defects such as pulling will occur.

As described above, at the initial stage where the detection of the loop amount becomes unstable and the leading edge of the transfer material S enters the fixing nip portion N, the control range of the driving speed of the elastic pressure roller 10b is reduced, and the transfer material is transferred. The leading end of S is allowed to enter the fixing unit 10 stably. On the other hand, after the first predetermined position of the transfer material S has passed through the fixing nip portion N, the loop amount can be detected stably. Therefore, the control range of the driving speed of the elastic pressure roller 10b is increased, and an appropriate amount is detected. The amount of loop can be controlled. As a result, it is possible to prevent the occurrence of image problems such as image rubbing, uneven gloss, and trailing edge splashes.
In the present embodiment, a difference is provided between the driving speeds R2 and R3 of the elastic pressure roller 10b, but it goes without saying that the same effect can be obtained even at a constant speed (the speed difference is 0).

[Second Embodiment]
Next, a second embodiment of the present invention will be described with reference to FIG.
Since the basic configuration is the same as that of the first embodiment described above, the same reference numerals are used for the same parts as those in the first embodiment, and the description thereof is omitted, and only the case different from the first embodiment is described. explain.

The present embodiment is characterized in that the control width of the driving speed of the elastic pressure roller 10b is different from the timing until the trailing end of the transfer material S is peeled off from the electrostatic conveyance belt 9a and passes through the fixing unit 10. It is said.
Here, the loop control in this embodiment will be described with reference to FIGS.
FIG. 5 is a loop control flow in this embodiment.

When image formation is started, the driving speed of the elastic pressure roller 10b is set to R8 at step 1 as shown in FIG.
Next, in step 2, it is determined whether or not the rear end of the transfer material S has been peeled off from the electrostatic conveyance belt 9a (here, whether or not the second predetermined position of the transfer material S has passed through the fixing nip portion N). If it is not peeled off, the process proceeds to step 3. If it is peeled off, the process proceeds to step 6.

Here, the CPU 26 determines whether or not the rear end of the transfer material S is peeled off from the electrostatic transfer belt 9a. The CPU 26 manages the operation timing of each part from the start of the image forming operation. The timing after a predetermined time from the timing when the image formation is started (paper is fed) is the electrostatic transfer at the rear end of the transfer material S. It is determined that it is the timing of peeling from the belt 9a.
At step 3, it is determined whether the flag signal is on or off. If it is on, the driving speed V of the elastic pressure roller 10b is set to R5 at step 4, and if it is off, the driving speed of the elastic pressure roller 10b is set at step 5. Set V to R8 and return to step2.

  As for the switching timing of the control width of the driving speed of the elastic pressure roller 10b, that is, the timing at which the rear end of the transfer material S is peeled off from the electrostatic conveyance belt 9a, for example, the transfer material S with a desired loop amount. Is the time until the second predetermined position reaches the fixing nip N after the first predetermined position (described above) of the transfer material S has passed through the fixing unit 10 (fixing nip N). Determined by.

On the other hand, in step 6, it is determined whether the flag signal is on or off. If it is on, the driving speed V of the elastic pressure roller 10b is set to R6 in step 7, and if it is off, the elastic pressure roller 10b is turned on in step 8. The drive speed V is set to R7.
In step 9, it is determined whether or not the image formation is completed. If not completed, the process returns to step 6. If completed, the series of processes shown in FIG.
Note that R5>R6>R7> R8, and R5 and R6, which are the driving speeds of the elastic pressure roller 10b, indicate that the transfer speed of the transfer material S at the fixing nip N is higher than the transfer speed of the transfer material S by the transfer roller 5. R7 and R8 make the transfer speed of the transfer material S at the fixing nip N slower than the transfer speed of the transfer material S by the transfer roller 5.

FIG. 6 is a diagram showing the relationship between the flag signal and the driving speed (fixing speed) of the elastic pressure roller 10b.
Until the rear end of the transfer material S is peeled off from the conveying belt 9a, the driving speed V of the elastic pressure roller 10b is controlled by switching between R5 and R8. Further, after the timing immediately before the rear end of the transfer material S is peeled off from the conveying belt 9a, the loop control is performed by switching the driving speed V of the elastic pressure roller 10b between R6 and R7.

  That is, until the trailing edge of the transfer material S is peeled off from the conveying belt 9a, when the flag signal is turned on, the loop speed is reduced, so that the driving speed V of the elastic pressure roller 10b is increased from R8. When switching to fast R5 and when the flag signal is turned off, the driving speed V of the elastic pressure roller 10b is switched from R5 to R8 having a slower speed in order to increase the loop amount, so that the desired loop amount is obtained. To be controlled.

  Next, after the timing immediately before the trailing edge of the transfer material S is peeled off from the conveying belt 9a, when the flag signal is turned on, the loop speed is reduced, so that the driving speed V of the elastic pressure roller 10b is increased from R7. When the speed is switched to R6 and the flag signal is in the off state, the driving speed V of the elastic pressure roller 10b is switched from R6 to R7, which is a slower speed, in order to increase the loop amount. It is controlled to become.

  As shown in FIG. 6, when the flag signal is in the ON state and when the drive speed control width switching timing (fixing speed control width change timing) of the elastic pressure roller 10b is reached, the drive speed V of the elastic pressure roller 10b is reached. Is switched from R5 to R6, which is slower. On the contrary, when the control signal of the driving speed of the elastic pressure roller 10b is switched when the flag signal is OFF, the driving speed V of the elastic pressure roller 10b is switched from R8 to R7 having a higher speed ( Not shown).

  Here, after the timing just before the rear end of the transfer material S is peeled off from the transfer belt 9a, the drive speed V of the elastic pressure roller 10b is switched to R6 or R7 and controlled (the control width is set small). For the following reasons.

After the timing just before the trailing edge of the transfer material is peeled off from the transfer belt, the transfer material loop formed by the transfer material supported and supported by the fixing nip portion and the transfer conveyance belt is formed by the thickness of the transfer material and the paper type. It becomes unstable due to differences in the strain. At this stage, if the driving speed V of the elastic pressure roller 10b is switched to R5 or R8 and controlled (the control width is set to a large value), the loop amount control of the transfer material becomes unstable due to the large control width. This is because image defects occur due to control oscillation.
For this reason, in a region where the loop is unstable, switching the drive speed V to R6 or R7 and controlling it reduces the loop control width of the transfer material and stabilizes the loop amount control of the transfer material, resulting in image defects. There is nothing to do.

  By the way, even before the rear end of the transfer material S is peeled off from the conveying belt 9a, the switching speed of the loop-controlled elastic pressure roller 10b is switched only in two stages instead of four stages as described above. When R6 and R7 having a small variation width are used, the following problem occurs.

  As described above, the conveyance speed in the fixing unit 10 is determined by the driving speed of the elastic pressure roller 10b and the outer diameter of the elastic pressure roller 10b. The outer diameter of the elastic pressure roller 10b changes depending on manufacturing tolerances and fixing temperatures. For this reason, if the outer diameter of the elastic pressure roller 10b is small and the driving speed R6 of the elastic pressure roller 10b is too small, the loop of the transfer material S cannot be reduced. This causes image defects such as splashing. On the other hand, if the outer diameter of the elastic pressure roller 10b is large and the driving speed R7 of the elastic pressure roller 10b is too high, the number of loops of the transfer material S cannot be increased, and finally the loop disappears. Image defects such as pulling will occur.

As described above, the control width of the driving speed of the elastic pressure roller 10b in the loop control is switched at the timing immediately before the rear end of the transfer material S is peeled off from the conveying belt 9a, thereby the rear end portion of the transfer material S. Can stably enter the fixing unit 10 to prevent image defects.
In the present embodiment, a difference is provided between the driving speeds R6 and R7 of the elastic pressure roller 10b, but it goes without saying that the same effect can be obtained even at a constant speed (the speed difference is 0).

[Third Embodiment]
Next, a third embodiment of the present invention will be described with reference to FIG.
Since the basic configuration is the same as that of the first embodiment described above, the same reference numerals are used for the same parts as those in the first embodiment, and the description thereof is omitted, and only the case different from the first embodiment is described. explain.

  In this embodiment, the difference from the first and second embodiments is that only the second surface when the transfer material S is printed on both sides is the first after the leading edge of the transfer material S enters the fixing nip portion N of the fixing portion 10. The timing at which the predetermined position (described above) passes through the fixing nip N, or the timing immediately before the rear end of the transfer material S peels from the electrostatic conveyance belt 9a (that is, the second predetermined position is at the fixing nip portion). The timing of changing the driving speed of the elastic pressure roller 10b is changed at any one of the timings of (passing through N).

  In particular, for the second side on which the transfer material S is printed on both sides, the moisture content of the transfer material S decreases because the transfer portion S passes through the fixing unit 10 once when the image on the first side is formed. For this reason, the amount of pressing of the flag 21 is affected by the change in the strength of the transfer material S or the increase in the suction force with the transport belt 9a when the second image is formed. Detection of the loop amount at the rear end becomes unstable.

  As described above, in the present embodiment, the first predetermined position after the leading edge of the transfer material S enters the fixing nip N on the second side of double-sided printing, in which loop control is likely to become unstable, makes this fixing nip N By changing the control range of the driving speed of the elastic pressure roller 10b at any one of the timing of passing or the timing immediately before the trailing end of the transfer material S is peeled from the conveying belt 9a (described above), Since the leading end portion or the trailing end portion of the transfer material S stably enters the fixing unit 10, image defects can be prevented.

  In each of the above embodiments, control for switching the control range of the driving speed of the elastic pressure roller 10b may be performed according to the type of transfer material. Here, the type of transfer material includes whether or not the transfer material is once fixed. Further, the type of transfer material includes the thickness of the transfer material.

1 is a schematic cross-sectional view of a color image forming apparatus using loop control according to an embodiment of the present invention. It is explanatory drawing of a loop detection means. It is a figure which shows the flow of loop control based on 1st Embodiment of this invention. It is a figure which shows the relationship between a flag signal and fixing speed based on 1st Embodiment of this invention. It is a figure which shows the flow of loop control based on 2nd Embodiment of this invention. It is a figure which shows the relationship between a flag signal and fixing speed based on 2nd Embodiment of this invention.

Explanation of symbols

N: fixing nip S: transfer material 1 ... photosensitive drum 2 ... charging means 3 ... exposure means 4 ... developing means 5 ... transfer roller 6 ... cleaning means 7 ... process cartridge 8 ... feeding section 8a ... feeding cassette 8a1 ... Cassette pickup roller 8a2 ... cassette transport roller 8b ... multi feed tray 8c ... multi feed section 8c1 ... multi pickup roller 8c2 ... multi transport roller 8d ... registration roller 9 ... conveying means 9a ... electrostatic transport belt 9b ... drive roller 9c, 9d, 9e ... driven roller 9f ... suction roller 10 ... fixing unit 10a ... fixing belt 10b ... elastic pressure roller 10c ... belt guide members 11, 12 ... discharge roller 13 ... discharge unit 15 ... double-sided conveyance path 16 ... skew feeding roller 17 ... U-turn roller 21 ... Flag 22 ... Detection sensor 23 ... Lever member 26 ... C U

Claims (15)

A transfer unit that transfers a toner image formed on the image carrier to a transfer material, and a fixing nip formed by contacting a pair of rotating members to fix the image transferred to the transfer material. A fixing unit that conveys the material while pinching the material, a loop amount detection unit that detects a loop amount formed on the transfer material by the transfer unit and the fixing unit, and the fixing nip according to a detection result of the loop amount detection unit. An image forming apparatus comprising: loop amount control means for executing loop control for switching a conveyance speed of the transfer material in a section;
The loop amount control means is configured such that at least one or more predetermined positions of the transfer material passes through the fixing nip portion after the leading end of the transfer material enters the fixing nip portion. An image forming apparatus characterized by changing a control width of a transfer speed of a transfer material.   The control width of the transfer material transport speed in the fixing nip portion until the first predetermined position of the transfer material passes through the fixing nip portion is smaller than the control width of the transport speed after the passage. The image forming apparatus according to claim 1, wherein:   The control range of the transfer speed of the transfer material in the fixing nip portion until the first predetermined position of the transfer material passes through the fixing nip portion is 0. Image forming apparatus.   The control range of the transport speed after the second predetermined position following the first predetermined position of the transfer material has passed through the fixing nip portion is the second predetermined position passing through the fixing nip portion. 4. The image forming apparatus according to claim 1, wherein the image forming apparatus is smaller than a control range of a conveyance speed before the start.   5. The image forming apparatus according to claim 4, wherein a control width of the conveyance speed after the second predetermined position of the transfer material passes through the fixing nip portion is 0. 6.   The image forming apparatus according to claim 1, wherein the control range of the transport speed is switched according to a type of the transfer material.   The image forming apparatus according to claim 6, wherein the type of the transfer material is whether or not the transfer material has been fixed once.   The image forming apparatus according to claim 6, wherein the type of the transfer material is a thickness of the transfer material. A loop control method in an image forming apparatus,
After the leading end of the transfer material has entered the fixing nip formed by the contact of a pair of rotating bodies to fix the image transferred to the transfer material, at least one or more predetermined positions of the transfer material are fixed to the fixing material. A loop control method characterized by changing a control width of a transfer speed of the transfer material in the fixing nip portion at a timing of passing through the nip portion.   The control width of the transfer material transport speed at the fixing nip portion until the first predetermined position of the transfer material passes through the fixing nip portion is smaller than the control width of the transport speed after the passage. The loop control method according to claim 9.   The control range of the conveyance speed at the fixing nip portion where the second predetermined position following the first predetermined position of the transfer material has passed through the fixing nip portion is such that the second predetermined position is the fixing position. The loop control method according to claim 9 or 10, wherein the loop control method is smaller than a control width of a conveyance speed before passing through the nip portion. A transfer portion for transferring an image formed on the image carrier to a transfer material;
A fixing unit for fixing the image transferred to the transfer material;
A detection unit that detects a loop amount formed in the transfer material conveyed to the transfer unit and the fixing unit;
A control unit for controlling a loop amount formed in the transfer material by switching a transfer speed of the transfer material in the fixing unit based on a detection result of the detection unit;
Have
The image forming apparatus according to claim 1, wherein the controller controls a switching width of a transfer speed of the transfer material based on a transfer position of the transfer material in the fixing unit.   The control unit is configured to change a transfer width of the transfer material until the predetermined position of the transfer material passes through the fixing unit, and the conveyance speed after the predetermined position of the transfer material passes through the fixing unit. The image forming apparatus according to claim 12, wherein the switching width is controlled to be different. A transfer material conveyance control method in an image forming apparatus,
Detecting a loop amount formed in the transfer material conveyed to a transfer unit that transfers an image to a transfer material and a fixing unit that fixes the image transferred to the transfer material;
Switching the transfer speed of the transfer material in the fixing unit based on the detected loop amount;
Controlling a transfer width switching width of the transfer material based on a transfer position of the transfer material in the fixing unit;
A transfer material conveyance control method characterized by comprising: Further, a transfer width switching width of the transfer material until the predetermined position of the transfer material passes the fixing portion, and a change width of the transport speed after the predetermined position of the transfer material passes the fixing portion, And a step of controlling so as to be different from each other.

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