JP2007310129A - Image forming apparatus - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an image forming apparatus in which the influence of a sequence of countermeasures against fixation slippage on an image forming operation can be reduced. <P>SOLUTION: The image forming apparatus includes: a fixing unit having a heater, a fixing film rubbing the heater, a pressure roller that forms a fixation nip part where the pressure roller comes in press contact with the heater through the fixing film, and a motor for rotationally driving the pressure roller, configured to convey a recording material formed with an unfixed image and fix the unfixed image at the fixation nip part between the fixation film and the pressure roller by heating by the heater; a transfer part that is provided upstream of the fixing unit and forms a transfer nip part; a loop amount detecting means for detecting the loop amount of the recording material formed between the transfer part and the fixing unit; a slippage preventing means for preventing the slippage of the recording material at the fixation nip part; and a control means, wherein the control means controls the operation of the slippage preventing means based on the detection result by the loop amount detecting means (S105 and S108). <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

The present invention relates to an image forming apparatus such as a printer, a copying machine, and a facsimile.
More specifically, an image forming apparatus using a film heating type fixing device (hereinafter referred to as a film type fixing device) as a fixing device (also referred to as a fixing device) for fixing an unfixed image formed and supported on a recording material. In particular, it relates to the fixing slip countermeasure.

  Many image forming apparatuses such as electrophotographic system and electrostatic recording system are contact heating type heat roller fixing devices with good thermal efficiency and safety as fixing devices for fixing unfixed images formed and supported on recording materials. In addition, energy-saving and on-demand film-type fixing devices are used.

  The film-type fixing device has a heater (heating body), a heat-resistant film (fixing film) that slides with the heater, and a pressure member that forms a fixing nip portion in pressure contact with the heater through the film. . Then, the recording material on which the unfixed image is formed is sandwiched and conveyed between the film in the fixing nip portion and the pressure member, and the unfixed image is applied by the heat from the heater applied through the film and the pressing force of the fixing nip portion. This is an apparatus for fixing an unfixed image on a recording material. This kind of film type fixing device is described in Patent Documents 1, 2, 3, and 4.

  As described above, various image forming apparatuses such as printers using the above-described heat fixing apparatus have many features such as no need for preliminary heating during standby due to high heating efficiency and rising speed, and elimination of waiting time. Has advantages. In particular, a method of driving a cylindrical film with a conveying force of a pressure roller can be realized at low cost, and therefore, it has been widely adopted from a small low speed machine to a large high speed machine.

  The fixing device will be described in more detail with reference to FIG. In the figure, an image formed by toner 17 formed on a recording material P is conveyed to a nip portion between a pressure roller 11 made of heat-resistant rubber and a cylindrical fixing film 12, and is heated by a heater 13 through the fixing film 12. It is heated and pressurized and fixed. The cylindrical fixing film 12 is pressurized to a pressure of about 4 to 15 kgf between the pressure roller 11 and is rotated and conveyed along the heater holder 16 that also serves as a film guide member together with the rotation of the pressure roller 11 by friction force.

  On the other hand, the heater 13 has a heating element pattern formed on a heater substrate made of a heat-resistant insulating material such as ceramic, the surface is protected by a heat-resistant glass 15, and the temperature detection element 14 is disposed on the back surface of the substrate. Is done. The temperature control of the fixing device is performed by temperature detection by the temperature detection element 14 on the back surface of the substrate.

(Fixed slip problem)
However, the method of driving the cylindrical film to be driven by the conveying force of the pressure roller has an adverse effect on speeding up. In other words, in a relatively humid environment, when paper that has absorbed moisture above a certain amount passes through the fixing nip, the water in the paper evaporates by receiving heat energy in the nip and becomes water vapor around the nip. Is released (see FIG. 6). At this time, if the temperature of the pressure roller 11 is not sufficiently high, the water vapor around the pressure roller surface forms a thin water vapor layer on the pressure roller surface, and this layer causes the interface between the back of the paper and the pressure roller. The frictional force is drastically reduced, and the paper conveying force due to the rotation of the pressure roller is greatly reduced. If the paper P is merely driven and conveyed by the frictional force with the pressure roller 11, the pressure roller 11 eventually turns idle, making it impossible to convey the paper, causing a fixing slip, and conveying the paper. A phenomenon that a defect such as a jam or an image dragging occurs.

  Such a fixing slip problem is caused by an increase in the amount of water vapor generated per unit time and an increase in water vapor pressure by increasing the temperature of the heater to increase the speed of the device or by increasing the recording paper conveyance speed. It becomes easy to do. Furthermore, it becomes easy to generate | occur | produce by the increase in the generation | occurrence | production area | region of water vapor | steam by reducing roller hardness and widening nip width, and has become a bad effect of the high-speed apparatus.

Conventionally, as the configuration of the image forming apparatus for preventing the above-described fixing slip, for example, a width of the pressure roller and the fixing film is increased, and a sufficient nip portion is formed even outside the recording paper size.
b The fixing film is driven not by a driven drive by a pressure roller but by a separate drive source (separate motor).
The occurrence of fixing slip has been avoided by using the above configuration.

  However, the constructions a and b have led to an increase in the size of the apparatus, and a fixing slip prevention method that avoids the increase in the size of the apparatus has been desired.

  With respect to the occurrence of fixing slip, it is known that if the surface temperature of the pressure roller is high, a water vapor layer is not formed on the surface of the pressure roller, and the occurrence of fixing slip is suppressed. However, while the pressure roller heated by the propagation of heat from the fixing film is supplied with heat from the fixing film between the paper, if the recording paper is in the nip, the recording paper loses heat and pressurizes. The temperature of the roller surface does not increase rapidly with the start of printing. In particular, in a high-speed machine, since the interval between the sheets is short, it often takes time for the pressure roller surface temperature to reach a predetermined value. Therefore, several sheets from the start of printing are in a state where fixing slip is likely to occur.

The following countermeasure sequence is effective as an example of a fixing slip prevention method that avoids the enlargement of the apparatus from the above situation.
c A wind is sent around the fixing nip during paper feeding, and the water vapor around the fixing nip is blown off to prevent the formation of a water vapor layer on the pressure roller surface.
d The formation of a water vapor layer on the surface of the pressure roller is prevented by increasing the gap between the recording sheets and preventing the temperature of the pressure roller from decreasing.

  However, it is not preferable for the image forming apparatus to always take the above measures. For example, c applies wind to the fixing film together with the pressure roller, and more power is required to maintain the temperature of the fixing film at a sufficient temperature for fixing the unfixed image. Further, widening the sheet interval with d affects the PPM (pages / minute, throughput) of the image forming apparatus. Therefore, it is preferable that the above-described countermeasure sequence is operated only when fixing slip is likely to occur.

  As described above, it is well known that fixing slip is likely to occur when the surface temperature of the pressure roller is low. On the other hand, in the above-described film-type on-demand type fixing device, a thermistor for detecting the temperature is provided on the back side of a heater (heating body) pressed against the fixing film, and usually the surface of the pressure roller. There is no thermistor to detect the temperature. For this reason, there is no conventional method for determining the surface temperature of the pressure roller, and there has been no determination data that can determine whether the fixing slip countermeasure sequence such as c or d is driven or not based on the temperature of the pressure roller. .

(Loop amount detection sensor before fixing)
On the other hand, with the recent miniaturization of image forming apparatuses, an increasing number of devices are equipped with sensors for detecting the loop amount of recording paper before fixing. The purpose and configuration of the sensor will be described below.
With the recent miniaturization of image forming apparatuses, it is inevitable that the paper path length between the fixing unit and the separation unit of the transfer conveyance unit is shorter than the recording paper length. At this time, the paper conveyance speed of the fixing roller needs to be set so as not to be at least faster than the conveyance speed of the transfer conveyance unit. This is because if the recording paper is being transported by the fixing roller and the trailing edge of the recording paper is still adsorbed onto the transfer belt of the transfer transport unit, the recording paper will be stretched and this will pull the transfer belt. This is because it adversely affects image transfer.

  On the other hand, since the pressure roller expands and contracts due to the temperature change, the fixing conveyance speed changes depending on the temperature of the pressure roller. That is, as the temperature of the pressure roller increases, the roller expands, so that the outer circumference of the roller increases, and the conveyance speed of the fixing unit increases even if the rotation speed of the pressure roller shaft is the same. For this reason, the recording paper is pulled between transfer and fixing, and as described above, the image is adversely affected during transfer. If the fixing conveyance speed is sufficiently lower than the transfer conveyance speed to prevent the pulling, a large recording paper deflection (loop) is formed between transfer and fixing. In addition, there is a case where the arrangement component is contacted between the transfer and the fixing, and the transfer image surface on the recording paper is rubbed. Further, at the moment when the trailing edge of the recording paper is separated, the deflection energy of the recording paper formed so far is released, so that the image on the recording paper is disturbed or the force acting on the transfer belt is suddenly applied. It will change. As a result, there may be a problem such as giving a shock to the image transferred on the next recording sheet.

  In order to solve such a problem, the loop amount of the recording paper is detected by the sensor having the configuration shown in FIG. 7, and the rotation speed of the motor that drives the fixing conveyance unit is changed so that the loop amount is maintained at a predetermined amount. Control is performed.

  The control will be further described with reference to FIG. The figure shows a loop sensor for detecting a loop amount of a recording sheet between a fixing nip portion N1 formed by the fixing film 12 and the pressure roller 11 and a transfer nip portion N2 formed by the photosensitive drum 117 and the transfer roller 115. 1 shows a configuration in which 1 is provided. In this configuration, the flag of the loop sensor 1 is pushed down when the loop amount between the nips is a predetermined value or more, and rotates around the shaft 2. It can be determined whether or not the loop amount is greater than or equal to a certain value based on whether or not the other flag of the loop sensor shields the detection sensor 3 composed of an optical sensor.

  When the temperature of the pressure roller 11 is low, the recording paper has a loop shape A shown in the figure, and the loop sensor is sufficiently pushed. On the other hand, when the temperature of the pressure roller 11 is increased, the roller expands as described above, and the conveyance speed of the fixing unit is increased even at the same rotational speed, so that the loop shape B shown in the figure is obtained. At this time, the flag of the loop sensor 3 enters the optical axis of the detection sensor 3, and puts the detection sensor 3 in a light-shielded state. Here, it can be determined that the paper is pulled. Conventionally, the control for decreasing the fixing conveyance speed when detecting the light shielding of the detection sensor 3 based on the detection result and repeating the control for increasing the fixing conveyance speed when the transmission of the detection sensor 3 is subsequently detected is repeated. It is controlled to keep the amount of loop in between.

The loop sensor 3 described above is mounted on the image forming apparatus for the purpose of being completely unrelated to the above-described problem of fixing slip, and there is no sequence for associating them with the conventional image forming apparatus.
JP-A-63-313182 Japanese Patent Laid-Open No. 2-157878 JP-A-4-44075 JP-A-4-204980

  However, as described above, it is not preferable for the image forming apparatus to always perform the fixing slip countermeasure sequence such as the fixing slip countermeasure sequences c and d for preventing the fixing slip.

  The present invention has been made under such circumstances, and an object of the present invention is to provide an image forming apparatus capable of reducing the influence of the fixing slip countermeasure sequence on the image forming operation.

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

(1) A heater, a fixing film that slides with the heater, a pressure roller that forms a fixing nip portion that is in pressure contact with the heater via the fixing film, and a motor that rotationally drives the pressure roller. A fixing device that sandwiches and conveys a recording material on which an unfixed image is formed between the fixing film and the pressure roller in the fixing nip portion, and fixes the unfixed image by the heat of the heater;
A transfer portion provided upstream of the fixing device and forming a transfer nip portion;
A loop amount detecting means for detecting a loop amount of a recording material formed between the transfer unit and the fixing device;
Anti-slip means for preventing slip of the recording material at the fixing nip,
Control means;
In an image forming apparatus comprising:
The image forming apparatus that controls the operation of the slip prevention unit based on a detection result of the loop amount detection unit.

  According to the present invention, it is possible to provide an image forming apparatus capable of reducing the influence of the fixing slip countermeasure sequence on the image forming operation.

  The best mode for carrying out the present invention will be described in more detail with reference to an embodiment of a laser beam printer.

A “laser printer” that is Embodiment 1 will be described with reference to FIGS.
First, the overall configuration will be described with reference to FIG.
In FIG. 1, a laser beam printer 101 has a paper cassette 102 that stores recording paper P. The recording paper P is fed out of the paper cassette 101 by a pickup roller 103 and sent to a paper transport path by a paper feed roller 104. At this time, in order to prevent double feeding of the recording paper P, a retard roller 105 is provided in a pair with the paper feed roller 104.

  The recording paper P sent to the transport path further passes the downstream pre-registration sensor 107 by the paper feed transport roller 106 and is transported toward the registration roller pair 108. Then, it is conveyed to the process cartridge 109 at a predetermined timing by driving the registration roller pair 108.

  The process cartridge 109 contains a developer (toner) necessary for a known electrophotographic process, and includes a toner container 110 that is a developer container including a developing sleeve 111 that is a developer carrier. The process cartridge 109 includes a toner container 110, a charging roller 112 as a charger, a cleaning element 116, and a photosensitive drum 117 as an electrophotographic photosensitive member, and is detachably attached to the main body 101. The

  On the photosensitive drum 117, image exposure based on an image signal is performed by a laser beam from a laser scanner unit 114 described later, and an electrostatic latent image is formed. This latent image is developed on the photosensitive drum 117 as a toner image by a developing device including the developing sleeve 111. The toner image is transferred onto the recording paper P conveyed to the nip portion between the photosensitive drum 117 and the transfer roller 115 by a transfer roller 115 which is a transfer device. Further, the transfer roller 115 is rotationally driven at a predetermined peripheral speed (process speed) by transmission from the main drive motor 120, and the photosensitive drum 117 is driven by the same drive source.

  The recording paper P on which the image is transferred at the transfer position is conveyed to a fixing nip portion in the fixing device 10 for heat fixing. A loop sensor 1 for detecting the loop amount of the recording paper is provided upstream of the fixing device, and the laser printer performs predetermined control based on ON / OFF information of the sensor 1. As the predetermined control, one is the control of the rotational speed of the fixing drive motor described later, and the other is the control of the fixing slip countermeasure sequence according to the present invention.

  The fixing device 10 includes a ceramic heater 13 as a heating body, a fixing film 12 that slides with the heater, and a pressure roller 11 as a pressure member, and is the same as the fixing device described in FIG. The pressure roller 11 is configured to be rotationally driven by a fixing drive motor 121, and the recording paper P passes through a fixing nip formed by the fixing film 12 and the pressure roller 11, thereby unfixed on the recording paper. The toner image is heated and fixed by heat from the heater 13.

  The recording paper P that has been subjected to the heat fixing process is discharged out of the laser beam printer 101 by a pair of paper discharge rollers 118. A series of these recording processes is controlled by a controller (not shown).

  The scanner unit 114 includes a laser unit 114a that emits a laser beam modulated based on an image signal sent from an external device (not shown). The scanner unit 114 includes a laser unit 114a, a polygon mirror 114b for scanning the laser light from the laser unit 114a onto the photosensitive drum 117, a scanner motor 114c, an imaging lens group 114d, and a mirror 114e.

  Reference numeral 119 denotes a fan that sucks wind into the laser beam machine, and has a configuration in which an air passage is provided around the pressure roller.

  The fixing device 10 in this embodiment is the same as the configuration of the conventional example described with reference to FIG. 6. In a relatively high humidity environment, the fixing device 10 fixes when paper that has absorbed moisture over a certain amount passes through the fixing nip. Slip is likely to occur. In order to avoid this, the sequence is such that the fan 119 is rotationally driven at the start of printing and the water vapor around the pressure roller 11 is blown away.

  On the other hand, the detection result of the loop sensor described in the above-described conventional example depends on the temperature of the pressure roller, and can be an element for determining the temperature of the pressure roller.

  Next, a configuration for forming a loop when the recording paper P is nipped and conveyed to the fixing device 10 will be described with reference to FIGS. A loop sensor 1 is provided between the fixing nip portion N1 formed by the fixing film 12 and the pressure roller 11 and the transfer nip portion N2 formed by the photosensitive drum 117 and the transfer roller 115 to detect the loop amount of the recording paper. It has been. The flag of the loop sensor 1 in this embodiment is pushed down when the loop amount between the nips is equal to or larger than a certain value, and rotates around the shaft 2. Whether or not the loop amount is greater than a certain value can be determined by whether or not the other flag of the loop sensor 1 shields the detection sensor 3 composed of an optical sensor. In this embodiment, the creepage distance X1 along the conveyance guide between the nips is 124 mm, and the linear distance X2 between the nips is 123 mm. That is, even when paper such as A3, A4, Letter, etc. is nipped in the front end fixing nip N1, the rear end is a distance that still remains in the transfer nip N2.

  In this configuration, if the conveyance speed at the fixing unit and the conveyance speed at the transfer unit are different, the recording paper P may be slackened or pulled. This will be described with reference to FIG. The recording paper P passes through the transfer nip N2 and is conveyed to the position of the loop sensor 1. At that time, as shown in FIG. 3A, the recording paper P pushes down the loop sensor 1 to bring the detection sensor 3 into a transmissive state. Thereafter, as shown in FIG. 3B, the recording paper is conveyed while being pushed down the loop sensor 1 in a state along the guide to some extent. On the other hand, when the printing operation is continuously performed, the outer periphery of the pressure roller 11 becomes larger due to thermal expansion. As a result, even when the rotation speed is the same, the conveyance speed of the fixing portion of the recording paper increases, and the recording paper is pulled at a conveyance speed different from the conveyance speed of the transfer portion. Further, when the expansion of the pressure roller exceeds a predetermined value, the recording paper does not touch the loop sensor 1 as shown in FIG. 3C, and the detection sensor 3 is in a light-shielded state.

  A CPU (not shown) mounted on the printer constantly monitors the output of the loop detection sensor 1 during printing, and at the time when “paper pulling” is first detected after printing is started, the pressure roller It is determined that the temperature has become sufficiently high, and the fixing slip countermeasure sequence is stopped. In this embodiment, the fixing slip countermeasure sequence is to rotate the fan 119. Further, control is performed so as to always keep the paper loop amount constant by switching the conveyance speed of the fixing unit.

  By doing so, it is possible to perform control that does not wastefully drive the fan in a state where the fixing slip does not occur and the pressure roller temperature is high.

  FIG. 4 shows a flowchart for explaining the processing of this embodiment. The control process shown in this flowchart is executed by the CPU according to a program stored in advance in the ROM.

  First, when image formation (printing) is started (S101), the fan (FAN) 119 is driven to prevent fixing slip (S102). Subsequently, the CPU detects that the recording paper that has passed through the transfer nip hits the loop sensor 1 and puts the detection sensor 3 into a transmissive state (S103). Thereafter, the loop sensor in the transmissive state is shielded from light (S104). There are two states, that is, the recording paper has passed through the position of the loop sensor and is in the state between the papers, or the recording paper is being pulled. For this reason, the CPU determines whether or not the time corresponding to the paper size length scheduled for printing has elapsed (S105), and if the time corresponding to the paper size length has elapsed, determines whether or not the job is to be finished next. (S106). If the job is ended, the fan is stopped (S107) and all printing operations are stopped, and the process is ended. On the other hand, if the next sheet is being conveyed because the job continues, the transmission and shading of the loop sensor are continuously monitored.

  If it is determined in S105 that the timing when the loop sensor 1 is in the light-shielded state has not passed for the length of the paper size and the paper is being pulled, driving of the fan as a countermeasure for fixing slip is stopped (S108). Then, in order to reduce the pulling of the paper, the fixing unit transport motor 121 is decelerated (S109). Thereafter, based on the detection result of the loop sensor 1 so as to keep the loop amount at a predetermined value, the rotation speed of the fixing conveyance motor 121 is always controlled until the end of the job (S110, S111).

  As described above, in this embodiment, when it is determined that the paper is pulled based on the detection result of the loop sensor, the temperature of the pressure roller is sufficiently high, and the subsequent continuous printing operation is performed. Determines that fixing slip does not occur in the fixing unit. Then, the fan drive, which is a fixing slip countermeasure sequence, is stopped. This makes it possible to prevent fixing slip when it is likely to occur, and to control the fan to not wastefully drive when the pressure roller temperature is high so that fixing slip does not occur. It becomes effective against. Note that the same effect can be expected not only by stopping the fan but also by reducing the driving of the fan (for example, by setting the driving speed to half the fan driving speed at the time of anti-slip).

  A “laser beam printer” that is Embodiment 2 will be described below. The hardware configuration of the laser beam printer in this embodiment is the same as that in the first embodiment, but the fixing slip countermeasure sequence is different. For the hardware configuration, the description of the first embodiment is cited.

  In this embodiment, the loop sensor detects the pulling of the paper, and increases the conveyance interval between the recording papers (hereinafter referred to as the “inter-paper interval”) until it is determined that the temperature of the pressure roller is high. The control for increasing the conveyance interval is control for increasing the interval more than the normal conveyance interval. As a result, a greater amount of heat is supplied from the fixing film to the pressure roller between the sheets, and the occurrence of fixing slip is suppressed.

The characteristic part of the present embodiment will be described below with reference to FIG.
First, when image formation (printing) is started, the first sheet is fed (S201, 202). Subsequently, feeding of the second sheet starts 600 ms after the trailing edge of the first sheet leaves the cassette sheet feeding unit (S203 to 205). This is 100 mm in distance (a state where the space between the papers is wide). Thereafter, the CPU detects that the first recording sheet that has passed through the transfer nip reaches the loop sensor 1 and sets the detection sensor 3 in the transmissive state (S206). Thereafter, the loop sensor 1 in the transmissive state is shielded from light (S207). As in the first embodiment, it is determined whether the recording paper is being pulled (S209). When it is determined that the recording sheet is pulled, the sheet interval is changed to 360 ms (normal sheet interval) from the next sheet feeding operation. This is a distance of 60 mm between the distances, and the distance between the sheets is narrower than the above-mentioned 100 mm (S210 to 212).

  In this way, when it is determined that the paper is being pulled based on the detection result of the loop sensor, the temperature of the pressure roller is sufficiently high, and fixing at the fixing unit is performed during the subsequent continuous printing operation. Judge that slip does not occur. By narrowing the space between the remaining print jobs, it is possible to reliably prevent the fixing slip when the fixing slip is likely to occur, and print without reducing the image forming ability of the apparatus in the state where the fixing slip does not occur. The operation can be performed.

Sectional drawing which shows schematic structure of Example 1. FIG. The figure explaining the structure and operation | movement of a loop sensor in Example 1. FIG. The figure explaining operation | movement of the loop sensor in Example 1. FIG. Flowchart showing processing in the first embodiment Flowchart showing processing in the second embodiment. Sectional drawing which shows the structure of the fixing device used for a prior art example and an Example. The figure explaining the structure and operation | movement of a loop sensor in a prior art example

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Loop sensor 10 Fixing apparatus 12 Cylindrical fixing film 13 Heater 101 Image forming apparatus 115 Transfer roller 121 Fixing drive motor

Claims (5)

A heater, a fixing film that slides with the heater, a pressure roller that forms a fixing nip portion that is in pressure contact with the heater via the fixing film, and a motor that rotationally drives the pressure roller, A fixing device that sandwiches and conveys a recording material on which an unfixed image is formed between the fixing film and the pressure roller in the fixing nip portion and fixes the unfixed image by the heat of the heater;
A transfer portion provided upstream of the fixing device and forming a transfer nip portion;
A loop amount detecting means for detecting a loop amount of a recording material formed between the transfer unit and the fixing device;
Anti-slip means for preventing slip of the recording material at the fixing nip,
Control means;
In an image forming apparatus comprising:
The image forming apparatus, wherein the control unit controls the operation of the slip prevention unit based on a detection result of the loop amount detection unit. The image forming apparatus according to claim 1.
The control unit stops the operation of the slip prevention unit when the loop amount detection unit first detects that the loop amount is equal to or less than a predetermined value from the start of printing. The image forming apparatus according to claim 2.
The image forming apparatus characterized in that the control unit controls the loop amount to be constant by switching the rotation speed of the motor according to a detection result of the loop amount detection unit after the slip prevention unit is stopped. apparatus. The image forming apparatus according to claim 1.
The slip prevention means is a fan that cools the fixing nip portion, and the control means first detects that the loop amount is equal to or less than a predetermined value by the loop amount detection means after starting printing. The image forming apparatus is controlled so as to stop or weaken the driving of the fan. The image forming apparatus according to claim 1.
The slip prevention means is a means for setting the recording material conveyance interval to a first interval, and the control means detects that the loop amount is less than or equal to a predetermined value by the loop amount detection means before the start of printing. In this case, the image forming apparatus controls the conveyance interval to be set to a conveyance interval narrower than the first interval.