JP4933191B2 - Image forming apparatus - Google Patents

Description

  The present invention relates to an image forming apparatus, and more particularly to control of sheet deflection.

  As a conventional image forming apparatus, when a sheet is conveyed using a plurality of conveyance units such as rollers, the plurality of conveyance units are controlled so that a certain degree of deflection (loop) is formed on the sheet by the plurality of conveyance units. An image forming apparatus is known.

  Here, as an example of the image forming apparatus, the image forming apparatus includes a transfer unit that transfers the toner image on the photosensitive drum to the sheet, and a fixing unit that fixes the toner image on the sheet sent from the transfer unit by, for example, a heat roller method. An electrophotographic image forming apparatus is known.

  An example of such an electrophotographic image forming apparatus will be described. The transfer speeds applied to the sheet by the transfer unit and the fixing unit differ depending on the change in the roller diameter of the fixing roller or the like constituting the fixing unit due to heat. Then, image degradation may occur due to the sheet being pulled between the transfer unit and the fixing unit, or an unnecessarily large loop formed in the sheet.

In order to solve this problem, a detection unit that detects a loop formed on the sheet from the transfer unit and the fixing unit is provided, and a roller is formed so that the loop amount formed on the sheet by the transfer unit and the fixing unit is maintained at a constant amount. A method of controlling a plurality of transport units such as the above is used (see Patent Document 1). Specifically, the loop amount detection unit outputs a loop presence signal or no loop signal according to the presence or absence of a certain loop, and the fixing conveyance is faster than the transfer conveyance speed at which the transfer unit conveys the sheet in response to the loop presence signal. Control to be speed. Further, control is performed so that the fixing conveyance speed is lower than the transfer conveyance speed in response to the loop non-signal. In this way, a constant loop amount is maintained.
Japanese Patent Laid-Open No. 5-107966

However, the following problems occur in the conventional image forming apparatus.
In other words, in order to keep the loop amount constant, the fixing conveyance speed at the time of no-loop signal output is slower than the transfer conveyance speed to increase the loop amount, and the fixing conveyance speed at the time of looped signal output decreases the loop amount. Therefore, it is necessary to set a speed higher than the transfer conveyance speed.

  At this time, in order to reduce the fluctuation of the loop amount from the loop amount target value and the noise generated by the motor that drives the fixing device when the fixing conveyance speed is switched, it is preferable to set the speed as close to the transfer conveyance speed as possible.

  However, even if the rotation speed (rps) of the fixing drive motor is kept constant, the diameter of the fixing roller or the like expands as the temperature rises, and the fixing conveyance speed increases. For this reason, it is necessary to give a large difference between the fixing conveyance speed when the loop no signal is output and when the loop presence signal is output in consideration of the fixing conveyance speed fluctuation.

  For example, if the fixing conveyance speed is increased by 4% by performing continuous printing from the start of printing (changes from -2% to + 2% of the transfer conveyance speed), the fixing conveyance speed at the time of output with a loop is transferred. It must be set to + 2% or more of the transport speed. Further, it is necessary to set the fixing conveyance speed at the time of no loop signal output to -2% or less of the transfer conveyance speed.

  As described above, in the conventional loop control method, the difference in fixing conveyance speed between when no loop signal is output and when a loop signal is output is increased. For this reason, the variation of the loop amount from the loop amount target value becomes large, and image deterioration may occur due to temporary pulling or pushing of the sheet. Further, there has been a problem that periodic changes in the fixing drive motor driving sound caused by repeatedly switching the fixing conveyance speed become annoying noise.

SUMMARY An advantage of some aspects of the invention is that it provides an image forming apparatus having the following effects a, b, and c.
a Reduce the difference in fixing conveyance speed between when a loop presence signal is output and when no loop signal is output. b Does not cause image degradation due to sheet pulling. c. Periodic noise generated by repeatedly switching the motor rotation speed is reduced.

  In order to solve the above problems, in the present invention, the image forming apparatus is configured as described in (1) and (2) below.

(1) transfer means for transferring an image formed on the image carrier to a recording medium;
Fixing means for fixing the image transferred to the recording medium;
In order to form a loop in the recording medium in a state where the recording medium is sandwiched between the transfer means and the fixing means, the driving speed of the fixing means is higher than the transfer speed at which the recording medium is conveyed by the transfer means . Control means for switching between a first speed and a second speed slower than the transfer speed;
A loop detection means for detecting the loop formed on the recording medium , and an image forming apparatus comprising:
The control means switches the driving speed of the fixing means to the first speed or the second speed according to the detection result of the loop detection means, and further continues the image formation, the first speed and the second speed. The first speed and the first speed are changed so that the speed difference between the first speed and the transfer speed and the speed difference between the second speed and the transfer speed do not exceed a predetermined speed difference. The speed difference between the lower limit speed and the transfer speed is more than the speed difference between the lower limit speed when driving at the second speed and the upper limit speed when driving at the second speed. an image forming apparatus comprising that you control the second speed so more becomes large.

(2) transfer means for transferring an image formed on the image carrier to a recording medium;
Fixing means for fixing the image transferred to the recording medium;
In order to form a loop in the recording medium in a state where the recording medium is sandwiched between the transfer means and the fixing means, the driving speed of the fixing means is higher than the transfer speed at which the recording medium is conveyed by the transfer means . Control means for switching between a first speed and a second speed slower than the transfer speed;
A loop detection means for detecting the loop formed on the recording medium, and an image forming apparatus comprising:
The control means, in response to a detection result of said loop detecting means, said driving speed of the fixing means switches to the first speed or the second speed, further wherein the temperature of the fixing unit increases Then the first speed and the The first difference between the first speed and the transfer speed, which changes as the second speed increases, and the first speed so that the speed difference between the second speed and the transfer speed does not exceed a predetermined speed difference. The lower limit speed and the transfer speed are set to be smaller than the speed difference between the lower limit speed when driving at the second speed and the upper limit speed when driving at the second speed. image forming apparatus towards the speed difference is characterized that you control the second speed so as to increase.

According to the present invention, the loop amount can be kept constant at a fixing conveyance speed closer to the process speed as compared with the conventional example. Thereby, the effect shown to the following a, b, and c is acquired, without requiring the cost increase and the space which adds components.
a Reduce the difference in fixing conveyance speed between when a loop presence signal is output and when no loop signal is output. b Sheet degradation and sheet loops are unnecessarily large, and image deterioration is not caused. c. Periodic noise generated by repeatedly changing the motor rotation speed is reduced.

  The best mode for carrying out the present invention will be described in detail with reference to an embodiment of a color image forming apparatus.

FIG. 1 is a cross-sectional view illustrating a schematic configuration of a “color image forming apparatus” according to the first embodiment.
The color image forming apparatus according to the present embodiment is configured so that the process cartridges 10m, 10c, 10y, and 10k for four colors can be attached to and detached from the corresponding development forming stations. An optical unit 20 capable of irradiating a laser beam, an electrostatic adsorption conveyance belt (hereinafter referred to as a conveyance belt) 31, a sheet conveyance unit 40, a fixing unit 50, and the like are provided.

  Each of the process cartridges 10m, 10c, 10y, and 10k has the same structure, and a photosensitive drum 12, which is an electrophotographic photosensitive member, a charging unit 13, and a developing device 14 are integrally configured.

  The electrostatic attraction / conveyance belt unit 30 includes a conveyance belt 31 that is an endless belt body, two rollers 32 and 33 that rotatably support the conveyance belt 31, and a spring that is disposed on the roller 33 side and stretches the conveyance belt 31. It consists of The roller 32 is rotated by the driving force obtained by the drive coupling device from the drive unit configured coaxially with the drive unit (not shown) and the roller 32, and the conveying belt 31 and the roller 33 are rotated. .

  The sheet S is conveyed by being electrostatically attracted to the conveyance belt 31, and includes a primary transfer roller (not shown) that transfers the toner image formed on each photosensitive drum 12 to the sheet S conveyed on the conveyance belt 31.

  The sheet conveying unit 40 includes a pickup roller 42 that conveys the sheet S from the sheet feeding cassette 41 to the conveying belt 31, a sheet feeding roller 43, a registration roller 44, and a sheet detection sensor 45 that detects the presence or absence of a sheet conveyed by the registration roller 44. Etc.

  The fixing unit 50 includes a fixing roller 51 and a pressure roller 52, and performs fixing by applying heat and pressure to the toner image on the sheet S.

  Further, after the loop of the sheet S is detected by the fixing loop sensor lever 55 so that the sheet S is sent between the fixing unit 50 and the electrostatic adsorption conveyance belt unit 30, the conveyance speed of the fixing unit 50 is changed. It is configured to be controllable.

In the above-described configuration, first, when the sheet S fed by the sheet transport unit 40 is transported, the feeding of the sheet S is detected and image writing is started.
In writing the image, in the process cartridge 10m of the first color, for example, magenta, the photosensitive drum 12 is uniformly charged by the charging means 13, and then a latent image is formed by the laser light 22 emitted from the optical unit 20. . The latent image is developed by the developing device 14 to form a toner image.

The toner image formed on the photosensitive drum 12 is primarily transferred to the sheet S that is electrostatically attracted to the transport belt 31 and transported. After completion of the primary transfer, the photosensitive drum 12 is cleaned by a cleaning device and used for the next image formation.
A similar image forming process is performed in each of the process cartridges 10c, 10y, and 10k to form toner images of the respective colors, and are sequentially transferred onto the previously formed toner images.
The optical unit 20 is configured as one unit, and is configured to irradiate each process cartridge of each image forming process mcyk with laser light so that an image can be sequentially formed.

  On the other hand, the sheet S is conveyed from the sheet feeding cassette 41 to the conveying belt 31 by the sheet conveying unit 40, and the toner image formed on the photosensitive drum 12 is transferred to the sheet S by the action of the primary transfer roller. The sheet S to which the toner image has been transferred is conveyed to the fixing unit 50, where the toner image is fixed at the nip portion between the fixing roller 51 and the pressure roller 52, and is discharged by the paper discharge roller 54.

  FIG. 2 is an enlarged view of a main part of FIG. 1, and a cartridge driving motor 60 for driving the cartridges 10m to 10k, a fixing unit driving motor 61 for driving the pressure roller 52 of the fixing unit 50, and a controller for controlling the driving thereof. 62 is shown. Counters that count pulses from drivers 63 and 64 for driving the motors 60 and 61 in response to control commands from the controller 62 and pulse generators (such as encoders and MR sensors) that generate pulses according to the rotation of the motors 65 is shown. Further, a loop detection sensor 56 that detects the operation of the fixing loop sensor lever 55 is shown. The controller 62 is constituted by a CPU or the like, and the counter 65 may be provided separately from the CPU, or may be constituted by a built-in counter 65 in the CPU. Further, the cartridge drive motor 60 may be provided in each of the cartridges 10m to 10k, or may be provided in common to all the cartridges. Further, as a loop amount detection device, in addition to a contact type composed of a sensor lever that contacts a sheet being conveyed and a sensor that detects the operation of the sensor lever, a non-contact type that detects the position of the sheet with a laser beam or the like A thing may be used.

Next, the operation of this embodiment will be described with reference to the flowchart of FIG. The processing in this flowchart is performed by the CPU in the controller 62.
In step S301, the controller 62 monitors whether image data transmitted from a video controller (not shown) or the like has been received.
In step S302, an image forming operation is started in response to the reception of the image data (Yes in step S301), and the image data is loaded in the paper feed cassette 41 that is a paper feed port designated from the video controller or the operation panel. The sheet S is fed by the feed roller 42.

  In step S303, the rotation speed of the fixing unit drive motor 61 corresponding to the conveyance speeds Vpu and Vpd of the fixing device is set in the controller 62 according to the number of prints and the print interval. Here, Vpu and Vpd are fixing conveyance speeds that are switched according to a signal output from the loop detection sensor 56 (see FIG. 2) when controlling the loop amount of the sheet formed by the transfer unit and the fixing unit. Vpu is a fixing conveyance speed when the loop detection sensor 56 outputs a loop presence signal, and is faster than the process speed Vp (transfer conveyance speed) that is the conveyance speed of the image forming unit in order to reduce the loop amount. Set as follows. Vpd is a fixing conveyance speed used when the loop detection sensor 56 outputs a no-loop signal, and is set to be slower than the process speed Vp in order to increase the loop amount.

In step S304, the output of the loop detection sensor 56 is monitored. If the loop detection sensor 56 is ON, the process proceeds to step S305, and if it is OFF, the process proceeds to step S306.
In step S305, it is determined that a loop has been formed, and the controller 62 transmits an acceleration command to the motor driver 64 to increase the rotation speed of the fixing unit drive motor 61. That is, the conveyance speed by the fixing unit 50 is set to Vpu that is higher than the process speed Vp, and the loop amount is controlled to decrease.

  In step S306, it is determined that the loop amount has decreased, and the controller 62 transmits a deceleration command to the motor driver 64 to reduce the rotation speed of the fixing unit drive motor 61. That is, the conveyance speed by the fixing unit 50 is set to Vpd that is lower than the process speed Vp, and control is performed so that the loop amount increases.

Steps S307, S308, S309, and S310 are steps executed in parallel with steps S304, S305, and S306. In step S307, the controller 62 determines whether or not the sheet S fed from the sheet cassette 41 has been transported and reached the sheet detection sensor 45, and the output of the sensor 45 has been turned on.
In step S308, in response to the leading edge of the sheet S being detected by the sheet detection sensor 45 (Yes in step S307), timing by the timer 66 is started.
In step S309, the controller 62 determines whether the trailing edge of the sheet S fed from the sheet feeding cassette 41 has reached the sheet detection sensor 45 and the output of the sensor 45 has been turned off.
In step S310, the length of the sheet S is calculated from the time required from the start of timing in step S308 until the output of the sheet detection sensor 45 is turned off in step S309, and the process speed Vp. From the result, the image of the sheet S is calculated. Set the formation end time. Note that the image formation end time is the time required from the start of timing by the timer 66 until the trailing edge of the sheet S is discharged from the fixing nip portion formed by the fixing roller 51 and the pressure roller 52.
In step S311, it is determined whether or not the trailing edge of the sheet S has been discharged from the fixing nip by monitoring whether or not the timer 66 started and timed in step S308 has reached the image formation end time set in step S310. To do.

If the image formation end time has not been reached, or if the trailing edge of the sheet S has not reached the sheet detection sensor 45 and the image formation end time has not been set, the process returns to step S304. The loop amount detection and the fixing conveyance speed control are repeated, and when the timer 66 reaches the image formation end time, the process proceeds to step S312.
In step S312, it is determined whether there is a page on which image formation is to be continued. If there is no page on which image formation continues, the process proceeds to step S313, the timer 66 and the image formation counter are reset, and the image formation operation is terminated. If there are pages to continue image formation, the number of image formations is counted in step S314, the timer 66 is reset in step S315, and the process returns to step S302.

  Next, referring to FIGS. 4 to 9, setting of the fixing conveyance speeds Vpu and Vpd when continuous image formation is performed when the temperature of the pressure roller 52 is room temperature (hereinafter referred to as a cold start). This method will be described in detail.

FIG. 4 shows the transition of the fixing conveyance speed when continuous image formation is performed from the cold start in this embodiment, assuming that the conveyance speed at the start of image formation is 100%. As the continuous image formation is performed from the cold start, the fixing conveyance speed increases due to the thermal expansion of the pressure roller 52, and the number of image formations is approximately saturated at about 100 sheets. The fixing conveyance speed to be reached varies depending on the basis weight of the sheet, and the conveyance speed is increased by about 3.0%, 2.6%, and 1.8% at 75 g / m ² , 105 g / m ² , and 176 g / m ² , respectively. Saturates at.

Hereinafter, taking a case where the basis weight of the sheet S is 75 g / m ² as an example, a conventional loop control method and a loop control method of this embodiment will be described.

As described above, in the configuration in which the fixing conveyance speed changes according to the continuous image formation, the transfer unit and the fixing unit can be set with the same setting without changing the rotational speed of the fixing unit driving motor 61 according to the number of images formed as usual. The amount of loop formed by and is kept constant. For this purpose, the following two conditions must be satisfied.
Condition 1: The fixing conveyance speed Vpu when the loop detection sensor 56 is ON is set to be higher than the process speed Vp even at a cold start.
Condition 2: The fixing conveyance speed Vpd when the loop detection sensor 56 is OFF is set to be slower than the process speed Vp even when the fixing conveyance speed is saturated by continuous image formation.

  In order to satisfy the above two conditions, for example, when the rotation speed of the fixing unit drive motor 61 is set so as to have a value as shown in FIG. 5 in the cold start state, the actual fixing conveyance speed changes as shown in FIG. To do. In such a setting, since the fixing conveyance speed Vpu is 100.5% of the process speed Vp and the fixing conveyance speed Vpd is 96.5% of the process speed Vp at the start of image formation, the difference between Vpu and Vp is The difference between Vp and Vpd is greater. Therefore, even if the loop amount is controlled to be constant by the output signal of the loop detection sensor 56, the overshoot that occurs on the side of increasing the loop amount is large, so the loop amount temporarily becomes too large. When the sheet S is pushed into the transfer portion, the transfer conveyance speed may be reduced and image deterioration such as color misregistration may occur.

  In the fixing conveyance speed saturation state, the fixing conveyance speed Vpu is 103.5% of the process speed Vp and the fixing conveyance speed Vpd is 99.5% of the process speed Vp. Therefore, Vpu with respect to the difference between Vp and Vpd. And the difference between Vp becomes larger. Therefore, even if the loop amount is controlled to be constant by the output signal of the loop detection sensor 56, the overshoot that occurs on the side of reducing the loop amount is large, so the loop amount becomes temporarily too small, and the transfer unit and the fixing unit are in the sheet. S will be pulled together. Therefore, the transfer conveyance speed increases, and image degradation such as color misregistration may occur in the same manner.

  Further, since the difference between Vpu and Vpd becomes large, the periodic change in the motor driving sound caused by repeatedly switching the rotation speed of the fixing unit driving motor 61 makes the operating sound of the entire image forming apparatus unpleasant. .

  As described above, conventionally, the rotation speed of the fixing unit driving motor 61 is always constant regardless of the number of images formed, whereas in this embodiment, the fixing unit driving motor 61 has three stages as shown in FIG. The rotation speed is switched. That is, the rotational speed of the motor 61 corresponding to the process speed Vp, which is the transport speed at the transfer portion, is set to 100%, and the rotational speed of the motor 61 corresponding to the fixing transport speed Vpu and Vpd in the cold start state according to the number of images formed. Set to the following values.

First step (number of images formed 1 to 10) Vpu: 100.5% Vpd: 98.5%
Second step (11 to 30 images) Vpu: 99.5% Vpd: 97.5%
Third step (31 images or more) Vpu: 98.5% Vpd: 96.5%
Thus, by setting the rotation speed of the fixing unit drive motor 61, the actual fixing conveyance speed changes as shown in FIG. The fixing conveyance speeds Vpu and Vpd fluctuate between 100.5 to 101.5% and 98.5 to 99.5%, respectively, and the difference from the process speed Vp is 1.5% at the maximum. It becomes smaller than the way. In addition, since the difference between Vpu and Vpd is also reduced, it is possible to prevent the operating sound of the image forming apparatus from being deteriorated in the sense of hearing due to the periodic motor driving sound generated by repeatedly switching the rotation speed of the fixing unit driving motor 61. .

As described above, in this embodiment, the case where the basis weight of the sheet S is 75 g / m ² has been described as an example. However, when the sheet types are different, or images are continuously formed on a plurality of different types of sheets. In this case, the same effect can be obtained. That is, the same effect can be obtained by changing the setting of the fixing unit drive motor 61 at a timing and rotation speed suitable for the transition of the fixing conveyance speed when each sheet is continuously formed.

  As a method for discriminating the type of the sheet S, a method for selecting a print mode according to the type of the sheet used in advance by the user, or a method for providing a media detection sensor for detecting the type of the sheet conveyed into the image forming apparatus. Etc. can be used.

  In this embodiment, the rotation speed of the fixing unit drive motor 61 is switched stepwise in accordance with an increase in the number of image formations. However, as shown in FIG. 9, in a stepless manner as the number of image formations increases. You may control so that the rotational speed of the drive motor 61 may be reduced gradually.

  In this embodiment, the case where continuous image formation is performed from a cold start (the temperature of the pressure roller 52 is room temperature) has been described. However, the present invention is not particularly limited, and image formation is intermittently performed. In this case, the same effect can be obtained by the following control method. For example, during intermittent image formation, the interval between image formations is measured by a timer or the like, and when the image formation interval is equal to or less than a predetermined value, the number of intermittent image formations is counted as the number of continuous image formations. The same fixing unit driving motor 61 is controlled. When the image forming interval is equal to or greater than the predetermined value, the number of continuous image forming is counted from the state in which the number of continuous image forming is reduced by an amount corresponding to the image forming interval in consideration of the decrease in the fixing conveyance speed due to the heat radiation of the fixing unit 50. After that, the same control as in the continuous image formation is performed. In this way, the same effects as in the present embodiment can be obtained even during intermittent image formation.

  As described above, according to the present exemplary embodiment, when the loop amount control of the sheet is performed, the first type of fixing conveyance speed that is faster than the transfer conveyance speed and the second speed that is slower than the transfer conveyance speed according to the number of formed images. The two types of fixing conveyance speed are changed over time. Therefore, the difference between the fixing conveyance speed for reducing the loop amount and the fixing conveyance speed for increasing the loop amount and the sheet conveyance speed of the transfer unit can be minimized. As a result, the fluctuation of the loop amount from the target value of the loop amount of the sheet is suppressed, and the transfer unit and the fixing unit temporarily pull the sheet, or a loop larger than necessary is formed so that the conveyance speed of the sheet being transferred is increased. By fluctuating, it is possible to prevent image deterioration such as color shift. Furthermore, it is possible to reduce periodic noise caused by repeatedly changing the rotation speed of the fixing drive motor.

A “color image forming apparatus” that is Embodiment 2 will be described.
Since the operations from sheet feeding to transfer and fixing of the image forming apparatus in this embodiment are substantially the same as those in the first embodiment, description thereof is omitted. Further, since the mechanical configuration is almost the same as that of the first embodiment, the same components as those shown in the first embodiment are denoted by the same reference numerals in FIG. 10 as those in FIG. 2, and the description of these components is omitted. To do.

  FIG. 10 shows a configuration of a main part of the second embodiment. A difference from the first embodiment is that a thermistor 57 is provided inside the fixing unit 50 as an element for detecting the temperature of the pressure roller 52. That is. The thermistor 57 is provided in contact with a ceramic heater (not shown) that heats the fixing roller 51, and detects the temperature of the ceramic heater. The temperature of the pressure roller 52 in contact with the fixing roller 51 can be estimated from the detection result.

  In this embodiment, a contact type thermistor is used, but a method of directly detecting the surface temperature of the pressure roller 52 by a non-contact type thermistor may be used.

  The operation of the second embodiment will be described using the flowchart of FIG. The processing shown in this flowchart is performed by the CPU in the controller 62.

  Steps S401 to S402 and steps S405 to S412 are the same as steps S301 to S302 and steps S304 to S311 in the first embodiment, and thus description thereof is omitted.

  In step S403, the thermistor 57 detects the ceramic heater temperature inside the fixing unit 50.

  In step S404, the thermal expansion state of the pressure roller 52 is determined from the ceramic heater temperature detected in step S403, and the rotation speed of the fixing unit drive motor 61 corresponding to the fixing conveyance speeds Vpu and Vpd is determined in accordance with the determination result. Set to.

  Thereafter, image formation is performed in the same flow as in the first embodiment. In step S413, the timer 66 in the controller 62 is reset. In step S414, it is determined whether there is a page for which image generation is to be continued. If there is, the process returns to step S402. If not, the image forming operation is terminated.

  A method for setting the fixing conveyance speeds Vpu and Vpd and the rotation speed of the fixing unit drive motor 61 will be described in detail with reference to FIGS.

  FIG. 12 shows the relationship between the ceramic heater temperature, which is the detection result of the thermistor 57, and the rotation speed of the fixing unit drive motor 61 set corresponding to the temperature. The thermistor detects the rotation speed of the motor 61 corresponding to the fixing conveyance speed Vpu and Vpd in the cold start state where the rotation speed of the motor 61 corresponding to the process speed Vp is 100% and the temperature of the pressure roller 52 is the same as the room temperature. Depending on, the following values are set.

First step (thermistor detection temperature less than 65 ° C.) Vpu: 100.5% Vpd: 98.7%
Second step (thermistor detection temperature 65 to 75 ° C.) Vpu: 99.9% Vpd: 98.3%
Third step (thermistor detection temperature: 75 to 90 ° C.) Vpu: 99.5% Vpd: 97.7%
Fourth step (thermistor detection temperature 90 to 130 ° C.) Vpu: 99.0% Vpd: 97.1%
5th step (thermistor detection temperature 130 ° C. or higher) Vpu: 98.5% Vpd: 96.6%
In this way, by setting the rotation speed of the fixing unit driving motor 61, the actual fixing conveyance speed increases as the number of continuous image formations increases, and the temperature of the fixing device increases as shown in FIG. . That is, the loop amount can be controlled at a speed at which the fixing conveyance speeds Vpu and Vpd are close to the process speed Vp. Therefore, the fluctuation of the loop amount from the loop amount target value is reduced, the sheet S is pulled between the transfer unit and the fixing unit, or the conveying speed of the sheet S during transfer fluctuates because the loop becomes too large. Deterioration can be prevented. In addition, since the difference between Vpu and Vpd can be reduced, it is possible to reduce the generation of periodic noise caused by repeatedly switching the rotation speed of the fixing unit drive motor 61.

  With the above configuration, the correspondence between the thermistor detection temperature and the fixing transport speeds Vpu and Vpd is set more finely, and the number of transport speed switching steps is increased to make Vpu and Vpd closer to Vp. Alternatively, the loop amount control for switching the fixing conveyance speed in accordance with the output signal of the loop amount detection sensor is not performed, and the fixing conveyance speed is always equal to Vp according to the thermistor detection temperature, thereby maintaining a more appropriate loop amount. It is also possible to do.

  However, the thermistor detection temperature and the fixing conveyance speed relative to the transfer conveyance speed due to the outer diameter tolerance of the driving roller 32 and the pressure roller 52 of the conveyance belt 31 that determines the conveyance speed of the transfer unit, or the environmental fluctuation around the image forming apparatus. There is an error in the relationship with the ratio. For this reason, it is difficult to always control the transfer conveyance speed and the fixing conveyance speed to be constant only by temperature detection by the thermistor 57, and Vpu and Vpd are given a certain width to respond to the output signal of the loop detection sensor. It is necessary to switch speed.

  In this embodiment, the rotation speed of the fixing unit drive motor 61 is switched stepwise in accordance with the detection result of the thermistor 57, but it may be controlled gradually and steplessly.

  As described above, according to this embodiment, when controlling the loop amount of the sheet, the first type of fixing conveyance speed that is faster than the transfer conveyance speed and slower than the transfer conveyance speed, depending on the temperature of the fixing device. The second type fixing conveyance speed is changed over time. Therefore, it is possible to minimize the difference between the fixing conveyance speed for reducing the loop amount and the fixing conveyance speed for increasing the loop amount and the sheet conveyance speed of the transfer unit. As a result, the fluctuation of the loop amount from the target value of the loop amount of the sheet is suppressed, and the transfer unit and the fixing unit temporarily pull the sheet, or a loop larger than necessary is formed so that the conveyance speed of the sheet being transferred is increased. By fluctuating, it is possible to prevent image deterioration such as color shift. Furthermore, it is possible to reduce periodic noise caused by repeatedly switching the rotation speed of the fixing drive motor.

The figure which shows the whole structure of Example 1. FIG. Main part enlarged view of Example 1 The flowchart which shows the process of Example 1. Diagram showing the relationship between the number of continuous images formed and the fixing conveyance speed, using the sheet basis weight as a parameter Explanatory drawing of conventional fixing conveyance speed setting The figure which shows the relationship between the number of conventional continuous image formation and fixing conveyance speed FIG. 6 is an explanatory diagram of fixing conveyance speed setting in the first embodiment. FIG. 10 is a diagram illustrating a relationship between the number of continuous image formation and the fixing conveyance speed in the first embodiment. Explanatory drawing of the deformation | transformation of Example 1. Main part enlarged view of Example 2 The flowchart which shows the process of Example 2. Explanatory drawing of fixing conveyance speed setting in Example 2 FIG. 10 is a diagram illustrating the relationship between the number of continuous image formations and the fixing conveyance speed in Example 2.

Explanation of symbols

40 Transfer material transport unit 50 Fixing unit 51 Fixing roller 52 Pressure roller 55 Fixing loop sensor lever 56 Loop detection sensor 61 Fixing unit drive motor 62 Controller 65 Image formation counter

Claims (7)

Transfer means for transferring an image formed on the image carrier to a recording medium;
Fixing means for fixing the image transferred to the recording medium;
In order to form a loop in the recording medium in a state where the recording medium is sandwiched between the transfer means and the fixing means, the driving speed of the fixing means is higher than the transfer speed at which the recording medium is conveyed by the transfer means . Control means for switching between a first speed and a second speed slower than the transfer speed;
A loop detection means for detecting the loop formed on the recording medium , and an image forming apparatus comprising:
The control means switches the driving speed of the fixing means to the first speed or the second speed according to the detection result of the loop detection means, and further continues the image formation, the first speed and the second speed. The first speed and the first speed are changed so that the speed difference between the first speed and the transfer speed and the speed difference between the second speed and the transfer speed do not exceed a predetermined speed difference. The speed difference between the lower limit speed and the transfer speed is more than the speed difference between the lower limit speed when driving at the second speed and the upper limit speed when driving at the second speed. an image forming apparatus comprising that you control the second speed so more becomes large. The control means in response to an increase of the images formed sheets, an image forming apparatus according to claim 1, characterized in that to decelerate the first speed and the second speed stages. It said control means, such that the first speed and images formation amount of the second speed is decelerated from the first number becomes the second number is further decelerated from second number to a third number of 2. The number of image formations from the second number to the third number is larger than the number of image formations from the first number to the second number. Or the image forming apparatus according to 2; Transfer means for transferring an image formed on the image carrier to a recording medium;
Fixing means for fixing the image transferred to the recording medium;
In order to form a loop in the recording medium in a state where the recording medium is sandwiched between the transfer means and the fixing means, the driving speed of the fixing means is higher than the transfer speed at which the recording medium is conveyed by the transfer means . Control means for switching between a first speed and a second speed slower than the transfer speed;
A loop detection means for detecting the loop formed on the recording medium, and an image forming apparatus comprising:
The control means, in response to a detection result of said loop detecting means, said driving speed of the fixing means switches to the first speed or the second speed, further wherein the temperature of the fixing unit increases Then the first speed and the The first difference between the first speed and the transfer speed, which changes as the second speed increases, and the first speed so that the speed difference between the second speed and the transfer speed does not exceed a predetermined speed difference. The lower limit speed and the transfer speed are set to be smaller than the speed difference between the lower limit speed when driving at the second speed and the upper limit speed when driving at the second speed. image forming apparatus towards the speed difference is characterized that you control the second speed so as to increase. A temperature detecting means for detecting the temperature of the fixing means;
The image forming apparatus according to claim 4 , wherein the control unit decelerates the first speed and the second speed in a stepwise manner in response to a rise in temperature detected by the temperature detection unit. The control means decelerates the first speed and the second speed when the number of image formation sheets changes from the first number to the second number, and further decreases when the number of image formation changes from the second number to the third number. 5. The method according to claim 4 , wherein control is possible in multiple stages, and the number of image formations from the second number to the third number is larger than the number of image formations from the first number to the second number. The image forming apparatus according to 5 . A basis weight detecting means for detecting the basis weight of the recording medium is provided.
Wherein, as the basis weight of the recording medium detected by the grammage detection means is large, an image forming apparatus according to claim 1 or 4, characterized in that to decelerate the first speed and the second speed .

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