JP6905044B2 - Image forming device - Google Patents

Description

The present invention relates to an image forming apparatus such as a copier or a printer that employs an electrophotographic recording method or an electrostatic recording method.

Conventionally, the transfer nip portion that conveys the recording material while transferring the unfixed toner image on the image carrier to the recording material and the unfixed toner image (unfixed image) on the recording material are heated and pressurized to be used for the recording material. There is known an electrophotographic image forming apparatus provided with a fixing nip portion that conveys a recording material while fixing to. In such an image forming apparatus, when a difference in transfer speed of the recording material occurs between the transfer nip portion and the fixing nip portion, an image defect may occur.
That is, since the temperature of the fixing roller or the pressure roller of the fixing nip portion fluctuates depending on the number of sheets of the recording material and the number of times of paper passing, the amount of thermal expansion changes depending on the situation and appears as a change in the outer diameter. Therefore, the transfer speed of the recording material in the fixing nip portion changes, and a difference occurs between the transfer speed and the transfer speed of the recording material in the transfer nip portion.
When the transfer speed of the fixing nip is faster than the transfer speed of the transfer nip, the recording material is pulled by the fixing nip between the fixing nip and the transfer nip, and the transfer nip is applied to the recording material on the image carrier. When transferring the landing image, the image may be distorted.
On the other hand, when the transfer speed of the recording material by the fixing nip portion is slower than the transfer speed of the recording material of the transfer nip portion, the recording material starts to loosen between the fixing nip portion and the transfer nip portion. Hereinafter, such a state in which the recording material is loose is described as "the recording material forms a loop". When the recording material forms a loop as described above, if the loop becomes excessive, the recording material may be strongly pressed and rubbed against the transfer guide that guides the transfer of the recording material between the transfer nip portion and the fixing nip portion. Further, if the loop is excessively formed, the recording material may undulate between the transfer nip portion and the fixing nip portion, and may rub against the ceiling of the transport path or the like. As a result, the unfixed image on the transferred recording material is disturbed, and image defects occur.
Therefore, the following proposals have been made to suppress image defects by forming a predetermined loop between the transfer nip portion and the fixing nip portion to prevent the recording material from being pulled or loosened too much. ing.
That is, the transport guide between the fixing nip portion and the transfer nip portion is provided with a loop detection sensor for detecting the loop amount of the recording material and a loop detection flag for turning on / off the loop detection sensor. Then, according to the detection result of the loop detection sensor, the transfer speed of the fixing nip portion is switched between a first speed slower than the transfer speed of the transfer nip portion and a second speed faster than the transfer speed of the transfer nip portion, and predetermined. Maintain the loop amount within the range. By this loop control, the image can be formed while keeping the loop amount substantially constant while the recording material is sandwiched and conveyed by both the transfer nip portion and the fixing nip portion. (Patent Document 1, Patent Document 2)
However, since the amount of loop between the transfer nip portion and the fixing nip portion is always substantially constant, for example, when the size of this loop is set to a large value, the recording material is recorded when the rear end of the recording material passes through the transfer nip portion. The rear end of the recording material may bounce due to the stiffness of the recording material. The bounce of the recording material may cause the image surface of the recording material to rub against the ceiling of the transport path or the like, resulting in poor image quality.
In order to solve the problem of the rear end bouncing of the recording material, an image is formed by making the loop size of the recording material constant, and recording is performed at a predetermined timing before the rear end of the recording material passes through the transfer nip portion. A technique has been proposed in which the transfer speed of the fixing nip portion is switched so that the loop of the material is reduced. As a result, the rear end of the recording material passes through the transfer nip portion in a state where the loop of the recording material is small, and the rear end of the recording material is prevented from bouncing. (Patent Document 3)

Japanese Unexamined Patent Publication No. 05-107966 Japanese Unexamined Patent Publication No. 07-234604 Japanese Patent No. 4136392

However, depending on the needs of the market, various media support is required, and while image formation support for thicker thick paper is also required, miniaturization of the image formation device is also required. New challenges are emerging.
FIG. 8 shows an explanatory view of the posture of the recording material P between the transfer nip portion N1 and the fixing nip portion N2. FIG. 8 is for explaining the subject of the present invention, and does not show a conventional example. Therefore, the same components as those in the examples of the present invention are designated by the same reference numerals. In order to advance the miniaturization of the device, it is necessary to bring the transfer nip portion N1 and the fixing nip portion N2 closer to each other. When the transfer nip portion N1 and the fixing nip portion N2 are brought closer to each other, the recording material tip is stably and smoothly introduced into the fixing nip portion N2 when the recording material P conveyed from the transfer nip portion N1 is curled. It becomes difficult to do.
In order to stably introduce the curled recording material P or the like into the fixing nip portion, it is necessary to design the angle A formed by the nip tangent Lt of the transfer nip portion N1 and the nip tangent Lf of the fixing nip portion N2 to be larger than before. I came. By doing so, the tip of the recording material can easily follow the transport guide 73, and the tip of the recording material can be stably introduced into the fixing nip portion N2. In such a device, when passing thick paper, the rear end Pe of the recording material momentum toward the nip tangent Lf due to the rigidity of the recording material P at the moment when the rear end Pe of the recording material passes through the transfer nip portion N1. The loop is often eliminated (see FIG. 8 (a)). As a result, the rear end Pe of the recording material is struck by the transport guide 73, and the unfixed image in the vicinity of the rear end of the recording material is disturbed, which may cause image defects (see FIG. 8B).
Therefore, the thicker the recording material P, the higher the rigidity, so that the force for eliminating the loop of the recording material becomes stronger, and image defects are likely to occur. Further, the closer the distance between the transfer nip portion N1 and the fixing nip portion N2, and the larger the angle A formed by the nip tangent Lt of the transfer nip portion N1 and the nip tangent Lf of the fixing nip portion N2, the greater the force for eliminating the loop of the recording material. There is a problem that the image becomes stronger and image defects are more likely to occur.
Further, as in Patent Document 3, even when the rear end Pe of the recording material is controlled to reduce the loop of the recording material before passing through the transfer nip portion N1, in the case of thick paper, the nip tangent Lf of the fixing nip portion N2 The loop is eliminated by the rigidity of the recording material in the tangential direction. Therefore, the rear end Pe of the recording material is struck by the transport guide 73, and the occurrence of image defects cannot be suppressed.

An object of the present invention is to provide an image forming apparatus capable of stabilizing the behavior at the moment when the rear end of the recording material passes through the transfer nip portion and suppressing the occurrence of image defects.

In order to achieve the above object, the present invention uses an image carrier and
A transfer member that forms a transfer nip portion together with the image carrier and transfers the toner image carried by the image carrier to the recording material while transporting the recording material through the transfer nip portion.
A fixing unit that is arranged downstream of the transfer nip portion in the transport direction of the recording material and that fixes the toner image to the recording material while sandwiching and transporting the recording material at the fixing nip portion.
A guide member provided between the transfer nip portion and the fixing nip portion to guide the recording material to the fixing nip portion, and
A loop provided between the transfer nip portion and the fixing nip portion to detect the loop amount of the recording material in a state where the recording material being conveyed straddles both the transfer nip portion and the fixing nip portion. Amount detection means and
A control unit that controls the transport speed of the recording material in the fixing nip portion based on the detection signal of the loop amount detecting means, and in the fixing nip portion so that the loop amount of the recording material is maintained within a predetermined range. A control unit that controls the transport speed of the recording material,
In the image forming apparatus having
When the information regarding the rigidity of the recording material is higher than the reference value, the control unit finishes the operation of maintaining the loop amount of the recording material being conveyed within the predetermined range, and then the rear end of the recording material is transferred. until leaves the nip, for a period of, as a part of the recording material comes into contact with the guide member between the loop amount detecting means and the transfer nip portion, the conveying speed of the recording material in front Symbol fixing nip portion The deceleration control is executed, and when the information is lower than the reference value, the deceleration control is not executed .

According to the present invention, even in an image forming apparatus for which miniaturization has been promoted, it is possible to stabilize the behavior at the moment when the rear end of the recording material passes through the transfer nip portion and suppress the occurrence of image defects.

FIG. 3 is a cross-sectional view of an image forming apparatus according to an embodiment of the present invention. The schematic diagram of the cross section between the transfer nip part and the fixing nip part of the image forming apparatus which concerns on Example. The schematic diagram of the loop detection flag and the loop detection sensor of the image forming apparatus which concerns on an Example. The block diagram of the control system of the image forming apparatus which concerns on Example. The timing chart which shows the fixing motor control of the image forming apparatus which concerns on Example. The explanatory view of the posture of the paper at the time of transporting thin paper / plain paper of the image forming apparatus which concerns on Example. The explanatory view of the posture of the paper at the time of transporting the thick paper of the image forming apparatus which concerns on an Example. The behavior explanatory view of the rear end of the recording material between a transfer nip part and a fixing nip for explaining the subject of this invention.

Hereinafter, the best mode for carrying out the present invention will be described in detail exemplarily based on the embodiment with reference to the drawings.
[Example 1]
(1) Image forming apparatus FIG. 1 is a configuration diagram of an image forming apparatus according to an embodiment of the present invention. This image forming apparatus is an electrophotographic laser printer, and forms an image corresponding to image information input from an external device (not shown) such as a host computer on a recording material.
In the image forming apparatus 60 shown in this embodiment, when a print command is input from an external device, a drum-shaped electrophotographic photosensitive member (hereinafter referred to as a photosensitive drum) 61 as an image carrier is subjected to a predetermined speed (process) in the arrow direction. It is driven to rotate at (speed). The outer peripheral surface (surface) of the photosensitive drum 61 is uniformly charged to a predetermined polarity and potential by the charger 62. Image information is written on the charged surface of the surface of the photosensitive drum 61 by a laser scanner 63 as an exposure means. The laser scanner 63 outputs the laser beam L modulated according to the time-series electric digital pixel signal of the image information input from the external device to the printer. Then, the laser scanner 63 scans and exposes the charged surface of the photosensitive drum 61 with the laser light L.

As a result, an electrostatic latent image corresponding to the image information is formed on the surface of the photosensitive drum 61. The electrostatic latent image is developed as a toner image (developer image) by the developer 64 using toner (developer). The toner image on the surface of the photosensitive drum 61 (hereinafter referred to as a toner image) includes the surface of the photosensitive drum 61 and the outer peripheral surface (surface) of the transfer roller 67 arranged to face the surface of the photosensitive drum 61 due to the rotation of the photosensitive drum 61. It is sent to the transfer nip portion N1 between. The above is the configuration of the image forming unit.
On the other hand, the recording materials P loaded on the sheet loading platform 68a of the feeding cassette 68 are picked up one by one by the feeding rollers 69 driven at a predetermined control timing, and are picked up one by one by the feeding rollers pair 70 to the registration unit. Is sent. In the registration section, the tip of the recording material P is temporarily received by the nip portion between the registration roller 71 and the roller 71a to correct the skew of the recording material P. Further, a registration sensor 72 is arranged in the registration unit to detect the arrival timing of the front end and the rear end of the recording material and the length of the recording material. After that, the recording material P is fed from the registration section to the transfer nip section N1. By detecting the tip of the recording material by the register sensor 72, the image so that the tip of the toner image on the photosensitive drum 61 reaches the transfer nip N1 at the timing when the tip of the recording material P reaches the transfer nip N1. Perform formation.
The recording material P fed to the transfer nip portion N1 is sandwiched and conveyed by the photosensitive drum 61 and the transfer roller 67. Then, the toner image on the surface of the photosensitive drum 61 is transferred to the recording material P by the transfer bias applied to the transfer roller 67 in the transfer process of the recording material P. The recording material P separated from the surface of the photosensitive drum 61 is transported to the heat fixing device 74 along the transport guide 73.

As shown in FIG. 2, in the heat fixing device 74, a ceramic heater, which is a plate-shaped heat generating source, is pressed against a pressure roller 82 by a predetermined force via a thin-walled endless belt-shaped fixing film 83, and a fixing nip portion is provided. It forms N2. By applying heat and pressure to the recording material P carrying the unfixed toner image at the fixing nip portion N2, the unfixed toner image is heat-fixed to the recording material P, and the recording material P is discharged from the fixing nip portion N2. The recording material P discharged from the fixing nip portion N2 of the heat fixing device 74 is conveyed to the discharge roller 75. Then, the discharge roller 75 discharges the recording material P onto the discharge tray 76. After the recording material P is separated, the surface of the photosensitive drum 61 is repeatedly subjected to image formation after the transfer residual toner is removed by the cleaner 65.
The image forming apparatus 60 of this embodiment integrates the photosensitive drum 61, the charger 62, the developer 64, and the cleaner 65 into a process cartridge 66. The process cartridge 66 is detachably attached to the image forming apparatus 60 constituting the housing of the printer.
The image forming apparatus 60 is provided with a cooling fan 77. The cooling fan 77 is appropriately rotated to take in outside air into the image forming apparatus 60 to cool a temperature rising portion such as an image forming portion and an electrical substrate. An environment detection member 78 is provided in the vicinity of the cooling fan 77, and when the air outside the machine is taken in by the cooling fan 77, the temperature and humidity of the environment in which the image forming apparatus is installed are detected. Then, the detection result is fed back to the temperature control sequence of the heat fixing device 74.
The seat loading platform 68a of the feed cassette 68 is provided with a movable regulation guide (not shown) for loading various recording materials of different sizes. By displacing the regulation guide according to the size of the recording material P and loading the recording material P on the sheet loading platform 68a, various recording materials of different sizes are loaded one by one from the feeding cassette 68 by the feeding roller 69. Can be picked up.
The image forming apparatus of this embodiment is an image forming apparatus compatible with A4 size paper and has a printing speed of 63 sheets / minute (A4).

(2) Between the transfer nip portion and the fixing nip portion Next, the configuration between the transfer nip portion N1 and the fixing nip portion N2 will be described with reference to FIG. The basic configuration of FIG. 2 is the same as that of FIG. 8 used for the subject of the background technology.
As described above, the transfer nip portion N1 conveys the recording material P while transferring the toner image carried by the photosensitive drum 61 as the image carrier to the recording material P. Further, the fixing nip portion N2 is arranged on the downstream side of the recording material P in the transport direction with respect to the transfer nip portion N1, and the toner image is fixed to the recording material P while sandwiching and transporting the recording material P.
A transfer guide 73 and a fixing inlet guide 79 are provided between the transfer nip portion N1 and the fixing nip portion N2 as guide members for guiding the recording material P to the fixing nip portion N2. The transfer guide 73 is arranged on the outlet side of the transfer nip portion N1, and smoothly guides the recording material P separated from the transfer nip portion N1 to the fixing nip inlet. Further, the transport guide 73 and the fixing inlet guide 79 are formed in a gently curved shape so that the recording material P can accommodate the loop formed between the transfer nip portion N1 and the fixing nip portion N2. There is. As shown in FIG. 8, the transfer nip portion N1 and the fixing nip portion N2 are brought closer to each other in order to advance the miniaturization. Then, in order to stably introduce the curled recording material P or the like into the fixing nip portion N2, the nip tangent Lt of the transfer nip portion N1 and the nip tangent Lf of the fixing nip portion N2 intersect at a predetermined angle. The angle A formed by Lt and the nip tangent Lf is large. By doing so, the tip of the recording material can easily follow the transport guide 73 and the fixing inlet guide 79, and the tip of the recording material can be stably introduced into the fixing nip portion N2.

As shown in FIG. 3, a loop amount detection mechanism 80A as a loop amount detecting means is provided on the transport surface of the transport guide 73. The loop amount detection mechanism 80A detects the amount of loop formed in the recording material P when the recording material P is conveyed while being sandwiched between the transfer nip portion N1 and the fixing nip portion N2.
The loop amount detection mechanism 80A includes a loop detection flag 80 and a loop detection sensor 81. The loop detection flag 80 is composed of a rod-shaped member that can swing around one end thereof, and the other end is arranged so as to project from the transport surface of the transport guide 73. The loop detection flag 80 is urged by a spring member (not shown) and swings according to the amount of loop formed in the recording material P. One end of the loop detection flag 80 blocks / opens the detection position S (optical path) of the loop detection sensor 81 including a photo interrupter for detecting whether or not the loop amount of the recording material P exceeds a predetermined value. That is, the loop detection sensor 81 turns on / off according to the swinging motion of the loop detection flag 80.
The loop amount is the distance between the loop formed by the recording material P between the transfer nip portion N1 and the fixing nip portion N2 and the transfer guide 73. When the transfer speed of the fixing nip portion N2 is slower than the transfer speed of the transfer nip portion N1, the loop amount increases, and when the transfer speed of the fixing nip portion N2 is faster than the transfer speed of the transfer nip portion N1, the loop amount decreases. Here, the minimum loop amount Lmin indicates a state in which the recording material P is linearly stretched between the transfer nip portion N1 and the fixing nip portion N2. Further, the maximum loop amount Lmax is such that the loop formed by the recording material P between the transfer nip portion N1 and the fixing nip portion N2 becomes large, and a part of the recording material P comes into contact with the transport guide 73 or the fixing inlet guide 79. Indicates that you are.

(3) Loop Control Next, the control system for loop detection control according to the present embodiment will be described with reference to FIG. Note that FIG. 4 is a block diagram showing a control system according to the present embodiment.
The control unit 40 provided in the image forming apparatus 60 includes a timer 41, a CPU 42, and a memory 43. A registration sensor 72, a loop detection sensor 81, and a fixing motor M2 as timing detection means are connected to the control unit 40.
The registration sensor 72 is arranged on the upstream side of the recording material P in the transport direction with respect to the transfer nip portion N1 and detects the passing timing of the recording material P. The timer 41 counts each timing required for the control of the control unit 40. The CPU 42 executes various operations and the like necessary for controlling the control unit 40. The memory 43 records information necessary for controlling the recording material information, the passing timing of the front and rear ends of the recording material P detected by the registration sensor 72, and the like.
Information on the rigidity of the recording material P, that is, recording material information such as size and weight (basis weight) is input to the control unit 40 from the information input unit 44 provided in the image forming apparatus 60, or is not input by the user. It is input by the detection information from the illustrated detection means.
The control unit 40 determines from the recording material information input from the information input unit 44 whether the recorded material P to be conveyed is a high-rigidity recording material or a low-rigidity recording material according to a predetermined standard. do. In this embodiment, the recording material information is information on the basis weight of the recording material P, and when the basis weight is equal to or more than a predetermined value, the recording material has high rigidity, and when the basis weight is less than the predetermined value. Is judged to be a low-rigidity recording material.
That is, based on the input information on the weight (basis weight) of the recording material P, the recorded material P to be conveyed is (1) low-rigidity recording material "thin paper / plain paper" or (2) high-rigidity recording material. Determine if it is "thick paper". In the present embodiment, ^{the recording material P having a basis weight of less than 90 (g / m 2} ) is (1) a low-rigidity recording material “thin paper / plain paper”, and the recording material having a basis weight of 90 (g / m ² ) or more is recorded. Material P is determined to be (2) high-rigidity recording material "thick paper". However, in the present embodiment, the threshold value of the basis weight used for determining "thin paper / plain paper" and "thick paper" is 90 (g / m ² ), but the present invention is not limited to this.
Also, the basis weight of the same, since the strength of the stiffness is different, even previously placed as information about the type of recording medium may be a basis weight information for each type.

Next, loop control will be described with reference to FIGS. 2 and 4.
The basic control in the control unit 40 is to control the transfer speed of the recording material of the fixing nip unit N2 with respect to the transfer speed of the transfer nip unit N1 based on the detection signal of the loop amount detection mechanism 80A, and the recording material P. Keep the loop amount in a certain range.
The photosensitive drum 61 and the transfer roller 67 are rotationally driven by the drum motor M1. The transfer roller 67 may be driven and rotated by a frictional force with the photosensitive drum 61. The pressure roller 82 of the heat fixing device 74 is driven by the fixing motor M2, and the fixing film 83 is driven to rotate by the frictional force with the pressure roller 82. The drum motor M1 and the fixing motor M2 are each driven and controlled by the CPU 42.
Here, the CPU 42 drives the photosensitive drum 61 and the transfer roller 67 so that the recording material P is conveyed at a predetermined transfer speed v0 (mm / s) at the transfer nip portion N1. Further, the CPU 42 sets the transfer speed v (mm / s) at the fixing nip portion N2 in order to keep the loop amount of the recording material P between the transfer nip portion N1 and the fixing nip portion N2 within a predetermined range. It is controlled by switching the rotation speed of M2.
Specifically, the rotation speed v of the recording material P at the fixing nip portion N2 is controlled by switching to either the rotation speed R1 or R2 of the fixing motor M2 according to the detection signal of the loop detection sensor 81. The outer diameter of the pressure roller 82 changes greatly depending on the type of recording material P, the paper passing condition, the thermal expansion according to the environmental temperature, and the solid variation. In consideration of the variation in the outer diameter of all the pressure rollers 82 described above, in the rotation speed R1 of the fixing motor M2, the transport speed v of the recording material P at the fixing nip portion N2 is higher than the transport speed v0 at the transfer nip portion N1. It is the number of revolutions that becomes faster. At this time, the transport speed of the recording material P at the fixing nip portion N2 is set to v1 (mm / s). Further, in the rotation speed R2 of the fixing motor M2, the transfer speed v of the recording material P at the fixing nip portion N2 is the transfer speed v0 at the transfer nip portion N1 in consideration of the variation in the outer diameters of all the pressure rollers 82 described above. The rotation speed is slower than. At this time, the transport speed of the recording material P at the fixing nip portion N2 is v2 (mm / s). By controlling the rotation speed of the fixing motor M2 with R1 and R2 according to the detection signal of the loop detection sensor 81 in this way, the loop amount of the recording material P can be controlled.

(3-1) Loop control of thin paper With reference to FIGS. 5A and 6 for loop control when the recording material P conveyed by the CPU 42 by the information input unit 44 is determined to be "thin paper / plain paper". explain. FIG. 5A is a timing chart of the rotation speed switching control of the fixing motor M2 when passing thin paper / plain paper. FIG. 6 shows the posture of the recording material at each timing when passing thin paper or plain paper.
That is, when passing thin paper or plain paper, the loop amount is maintained within a certain range for a predetermined time after the tip of the recording material P reaches the fixing nip portion N2. During this loop forming section, the transfer speed of the fixing nip portion N2 is controlled so that the loop amount is such that the recording material P does not come into contact with the transfer guide 73.
Then, after the loop forming section elapses and ends, the recording material P does not come into contact with the guide member between the loop detection sensor 81 and the transfer nip portion N1 before the rear end Pe of the recording material P passes through the transfer nip portion N1. As described above, the transfer speed of the fixing nip portion N2 is controlled to be increased.
The timing for increasing the transfer speed of the fixing nip portion N2 is set after a predetermined time has elapsed after the registration sensor 72 detects the passage of the front end or the rear end of the recording material P.

Hereinafter, a more specific description will be given.
After the tip of the recording material P passes through the registration sensor 72 (= t0), loop control is started from t1 which rushes into the fixing nip portion N2. Immediately after the recording material P enters the fixing nip portion N2, the amount of loop formed by the recording material is small. Therefore, the loop detection flag 80 does not block the detection position S of the loop detection sensor 81, and the loop detection sensor 81 detects that it is off. do. The CPU 42 sets the rotation speed of the fixing motor M2 to R2 in order to increase the loop amount. Since the transfer speed v2 at the fixing nip portion N2 at the rotation speed R2 is slower than the transfer speed v0 at the transfer nip portion N1, the loop amount gradually increases. When the loop amount increases, the loop detection flag 80 blocks the detection position S of the loop detection sensor 81. Then, since the loop detection sensor detects the on, the CPU 42 switches the rotation speed of the fixing motor M2 to R1. Since the transfer speed v1 in the fixing nip portion N2 corresponding to the rotation speed is faster than the transfer speed v0 in the transfer nip portion N1, the loop amount is reduced. By repeating the above, as shown in FIG. 6A, one end of the loop detection flag 80 can keep the vicinity of the detection position S of the loop detection sensor 81, and the loop of the recording material P can be stably formed.
Then, the CPU 42 stops the loop control at the timing before the rear end Pe of the recording material P exits the transfer nip portion N1 based on the size information of the recording material P and the passing information of the front and rear ends of the recording material P by the registration sensor 72. Set t2. When the set t2 arrives, the CPU 42 stops the loop control, and the rotation speed of the fixing motor M2 is set to R3. The rotation speed R3 is set so that the transfer speed v3 (mm / s) of the fixing nip portion N2 becomes larger than the transfer speed v0 of the transfer nip portion N1. Further, t1 and R3 are set so that the loop amount does not become Lmin.

Here, the rotation speed R3 may be a fixed value rotation speed. However, the outer diameter of the pressure roller 82, which is a roller constituting the fixing nip portion N2, changes due to thermal expansion or the like. Therefore, in order to stabilize the loop amount at the moment when the rear end Pe of the recording material passes through the transfer nip portion N1, it is preferable to use the rotation speed R3 which changes according to the outer diameter of the pressure roller 82. For example, the rotation speed R0 at which the transfer speed v in the fixing nip portion N2 is substantially equal to the transfer speed v0 in the transfer nip portion N1 is calculated from the respective duty ratios of the rotation speeds R1 and R2 of the fixing motor M2 in the loop control section. Then, R0 may be used for R3 in which the rotation speed is increased at a constant rate.
Further, the temperature of the pressurizing roller 82 is detected, the outer diameter of the pressurizing roller 82 is predicted from the temperature, and the transport speed v of the fixing nip portion N2 is faster than the transport speed v0 of the transfer nip portion N1 by a predetermined speed. It is also possible to select the rotation speed R3 so as to be. By using the rotation speed R3 of the fixing motor M2 as described above, it is less likely to be affected by the thermal expansion of the pressurizing roller 82, and the behavior of the rear end of the recording material can be stabilized.
By performing such acceleration control, the loop amount at the moment (= t3, FIG. 6 (b)) when the rear end Pe of the recording material passes through the transfer nip is set to the loop in the loop control section (FIG. 6 (a)). Can be smaller than quantity. As a result, it is possible to suppress image defects due to the rear end bounce of the recording material at the moment (FIG. 6C) when the rear end Pe of the recording material passes through the transfer nip portion N1.

(3-2) Loop control of thick paper Next, loop control when the recording material P conveyed by the CPU 42 based on the recording material information input from the information input unit 44 is determined to be (2) “thick paper” is shown in FIG. This will be described with reference to b) and FIG. 7. FIG. 5B is a timing chart of the rotation speed switching control of the fixing motor M2 when passing thick paper. FIG. 7 shows the posture of the recording material at each timing when the thick paper is passed.
From the recording material information input from the information input unit 44, the control unit 40 executes the mode for thick paper when the recording material to be conveyed is a high-rigidity recording material determined by a predetermined standard, that is, thick paper. ..
That is, when passing thick paper, the loop amount is maintained within a certain range for a predetermined time after the tip of the recording material P reaches the fixing nip portion N2. During this loop forming section, the transfer speed of the fixing nip portion N2 is controlled so that the loop amount is such that the recording material P does not come into contact with the transfer guide 73. This loop forming section is the same as that of thin paper and plain paper.
Then, after the loop forming section has elapsed, before the rear end Pe of the recording material P passes through the transfer nip portion N1, the transport speed of the fixing nip portion N2 is reduced to increase the loop amount of the recording material P, and the recording material P is subjected to. A part of it touches the transport guide 73. The position where the transport guide 73 is touched is configured to be in contact with the recording material P on the upstream side in the transport direction of the loop detection sensor 81.
The timing at which the control unit 40 decelerates and controls the transfer speed of the fixing nip portion N2 before the rear end Pe of the recording material P passes through the transfer nip portion N1 is such that the registration sensor 72 passes through the front end or the rear end of the recording material P. It is set after a predetermined time has passed since it was detected.

Hereinafter, a more specific description will be given.
In thick paper, the CPU 42 uses the size information of the recording material P and the passing information (= t0) of the tip of the recording material P by the registration sensor 72 at the timing before the rear end Pe of the recording material P passes through the transfer nip portion N1. Set t5 to stop the loop control. Until the set t5 arrives, the CPU 42 performs the same loop control as from t1 when the tip of the recording material rushes into the fixing nip portion N2 when passing "thin paper / plain paper" to t2 when the loop control is stopped. Execute (Fig. 7 (a)).
When the predetermined timing t5 arrives, the CPU 42 stops the loop control and changes the rotation speed R4 of the fixing motor M2 so that the transfer speed v4 of the fixing nip portion N2 becomes slower than the transfer speed v0 of the transfer nip portion N1. At the timing t5 and the rotation speed R4 of the fixing motor M2, the loop amount is Lmax at the moment when the rear end Pe of the recording material P passes through the transfer nip portion N1 (= t3, FIG. 7B), that is, a part of the recording material is A value is set so as to be in contact with the transport guide 73.
Here, the rotation speed R4 of the fixing motor M2 may be a fixed value, but the rotation speed that changes according to the thermal expansion of the pressurizing roller 82 should be used in the same manner as the rotation speed R3 when "thin paper / plain paper" is conveyed. Is preferable. More specifically, using the duty ratio of the rotation speeds R1 and R2 of the fixing motor M2 during the loop control period, the detection result of the pressure roller temperature, and the like, the transfer speed v of the fixing nip portion N2 is the transfer speed of the transfer nip portion N1. It is preferable to use a rotation speed R4 that is slower than v0 by a predetermined speed. By using such a rotation speed R4, a part of the recording material P can be stably brought into contact with the transport guide 73 without being affected by the thermal expansion of the pressure roller.

Immediately after the rear end Pe of the recording material has passed through the transfer nip portion N1, the deceleration control attempts to eliminate the loop with a strong force due to the high rigidity of the thick paper, so that the rear end Pe of the recording material is struck against the transport guide 73. This is for suppressing image defects that occur as a result. Due to the deceleration control, a part of the recording material P is in contact with the transfer guide 73 at the moment (= t3, FIG. 7B) when the rear end Pe of the recording material passes through the transfer nip portion N1. Therefore, even after the rear end Pe of the recording material has passed through the transfer nip portion N1 (FIG. 7 (c)), the recording material P cannot change its posture more rapidly with the contact point as a fulcrum. The rear end Pe can smoothly land on the transport guide 73.
Therefore, the position of the transport guide 73 that a part of the recording material P touches at the moment when the recording material P passes through the transfer nip portion N1 is preferably as close to the transfer nip portion N1 as possible, and contacts on the upstream side of the loop detection flag 80. The transport guide 73 and the like are designed so as to be used.
As described above, during the transfer of "thin paper / plain paper", the recording material P is conveyed to the transfer nip portion N1 and the fixing nip portion N2 so as not to come into contact with the transfer guide 73 during the loop formation. On the other hand, during the transfer of "thick paper", the rear end Pe of the recording material is conveyed so as to be in contact with a part of the transfer guide 73 immediately before passing through the transfer nip portion N1. By controlling the posture of the rear end Pe of the recording material when the transfer nip portion N1 is pulled out according to the weight (basis weight) of the recording material P in this way, each of "thin paper / plain paper" and "thick paper" is used. The behavior of the rear end Pe of the recording material can be controlled, and image defects can be prevented.

In this embodiment, as the heat fixing device 74, an endless belt-shaped fixing film 83, a heater 84 in contact with the inner surface of the fixing film 83, and a fixing nip portion N2 are formed together with the heater 84 via the fixing film 83. A device having a pressure roller 82 and a pressure roller 82 is shown. However, the heat fixing device is not limited to this as long as the rotating body driven by the fixing motor M2 has an elastic layer, and the rotating body is heated to cause thermal expansion of the elastic layer. For example, an endless belt-shaped fixing film, a heater contained in the fixing film that heats the inner surface of the fixing film with radiant heat, a nip portion forming member that contacts the inner surface of the fixing film, and a nip through the fixing film together with the nip portion forming member. A device having a pressure roller forming a portion may be used. Further, a device having an endless belt-shaped belt that generates heat by itself, a nip portion forming member that contacts the inner surface of the endless belt-shaped film, and a pressure roller that forms a nip portion via the film together with the nip portion forming member. But it's okay.
Further, in the present embodiment, the timings t2 and t5 for stopping the loop control have been described by a method of calculating the timings t2 and t5 from the passing timing of the tip of the recording material by the register sensor 72, but the rear end Pe of the recording material has passed by the registration sensor 72. It may be calculated starting from the timing.
Further, in this embodiment, the monochrome image forming apparatus using the process cartridge has been described, but a color image forming apparatus using an intermediate transfer belt or a transfer transfer belt method may be used.
Further, in this embodiment, the loop detection flag 80 and the loop detection sensor 81 are combined to detect the loop amount of the recording material P, but the means for detecting the loop amount is not limited to this, and for example, an optical sensor may be used. It may be used to detect the loop amount.

40 Control unit 44 Information input unit 61 Photosensitive drum (image carrier)
67 Transfer roller 72 Registration sensor (timing detection means)
73 Transport guide (guide member)
79 Fixing entrance guide (guide member)
80A loop amount detection mechanism (loop amount detection means)
80 Loop detection flag 81 Loop detection sensor N1 Transfer nip part N2 Fixing nip part S Loop detection sensor detection position P Recording material Pe Recording material rear end

Claims (8)

Image carrier and
A transfer member that forms a transfer nip portion together with the image carrier and transfers the toner image carried by the image carrier to the recording material while transporting the recording material through the transfer nip portion.
A fixing unit that is arranged downstream of the transfer nip portion in the transport direction of the recording material and that fixes the toner image to the recording material while sandwiching and transporting the recording material at the fixing nip portion.
A guide member provided between the transfer nip portion and the fixing nip portion to guide the recording material to the fixing nip portion, and
A loop provided between the transfer nip portion and the fixing nip portion to detect the loop amount of the recording material in a state where the recording material being conveyed straddles both the transfer nip portion and the fixing nip portion. Amount detection means and
A control unit that controls the transport speed of the recording material in the fixing nip portion based on the detection signal of the loop amount detecting means, and in the fixing nip portion so that the loop amount of the recording material is maintained within a predetermined range. A control unit that controls the transport speed of the recording material,
In the image forming apparatus having
When the information regarding the rigidity of the recording material is higher than the reference value, the control unit finishes the operation of maintaining the loop amount of the recording material being conveyed within the predetermined range, and then the rear end of the recording material is transferred. until leaves the nip, for a period of, as a part of the recording material comes into contact with the guide member between the loop amount detecting means and the transfer nip portion, the conveying speed of the recording material in front Symbol fixing nip portion It executes the control of decelerating, when the information is lower than the reference value, the image forming apparatus characterized by not executing the control to the reduction. Wherein, when the information about the rigidity of the record material is lower than the reference value, after completing the operation for maintaining the loop amount of the recording material which is the conveyance in the predetermined range, the trailing edge of the recording medium Control to increase the transfer speed at the fixing nip portion so that the recording material does not come into contact with the guide member between the transfer nip portion and the loop amount detecting means during the period until the recording material exits the transfer nip portion. The image forming apparatus according to claim 1, wherein the image forming apparatus is performed. The image forming apparatus according to claim 1 or 2 , wherein the information regarding the rigidity of the recording material is the weight or basis weight of the recording material. The image forming apparatus according to any one of claims 1 to 3, wherein the predetermined range is set so that the recording material does not come into contact with the guide member. The image forming according to any one of claims 1 to 4, wherein the apparatus is configured such that the nip tangent of the transfer nip portion and the nip tangent of the fixing nip portion intersect. Device. The fixing unit includes an endless fixing film and a pressure roller that comes into contact with the outer surface of the fixing film and forms the fixing nip portion between the fixing films. 5. The image forming apparatus according to any one of 5. The image forming apparatus according to claim 6, wherein the fixing unit further has a heater that comes into contact with the inner surface of the fixing film. The image forming apparatus according to claim 7, wherein the fixing nip portion is formed by applying pressure between the heater and the pressurizing roller via the fixing film.

Images