JP2005338362A - Image forming apparatus - Google Patents

Abstract

[PROBLEMS] To realize stable temperature control by using mode switching temperature control different from normal temperature control at the time of mode switching.
In an image forming apparatus that performs so-called fixing mode switching for switching the conveyance speed and fixing temperature in a fixing unit for each medium, when mode switching occurs during printing due to media detection or a command, what is normal temperature control? Performs temperature control for switching between different modes.
[Selection] Figure 1

Description

  The present invention relates to an image forming apparatus such as a printer, and more particularly to an image forming apparatus having a plurality of image forming modes and performing image formation under processing conditions suitable for each image forming mode.

  Conventionally, the processing conditions of the image forming apparatus are constant, and only the physical properties of the paper change. Therefore, if only the paper type and environment (temperature, humidity) at room temperature are considered, a good image is formed. It was possible. However, in recent image forming apparatuses, the resolution has been increased in the same way as the printing speed has been increased. In designing and manufacturing such an apparatus, instead of the conventional one type of image forming mode, one apparatus is used. An apparatus has been proposed in which a plurality of types of image forming modes are provided to perform image formation that meets the user's request.

  For such a difference in image forming mode, for example, in the fixing unit, the fixing performance is determined by the pressure and the amount of heat applied when one sheet passes through the fixing unit. However, when a plurality of process speeds are used, it is obvious that the time for one sheet to pass through the fixing unit is different, and it is also obvious that the fixing performance is different at each process speed.

  For example, if fixing is performed at high speed while maintaining the low speed mode, which is the conventional process speed, fixing is performed at high speed, that is, in a short time, at a relatively low fixing temperature suitable for the low speed mode. Therefore, the amount of heat for melting the toner is insufficient, the unmelted toner adheres to the fixing roller, melts during one rotation, and adheres to the sheet at the next fixing, so-called low temperature offset occurs.

  On the other hand, if fixing is performed at a low speed, that is, for a long time with a relatively high fixing temperature suitable for the high speed mode, the toner is completely melted and adheres to the fixing roller, and the density of the image is increased. So-called high temperature offset occurs.

  Therefore, in order not to cause such a problem, a fixing temperature suitable for each process speed is set in advance, and when the process speed is selected at the time of printing request, the fixing temperature is set to a value suitable for the process speed. The setting method is used.

  In the above-mentioned method, the transition from the low temperature setting to the high temperature setting can be easily performed by turning on the fixing heater lamp. However, the transition from the high temperature setting to the low temperature setting is performed by idling the device and the temperature due to heat radiation from the fixing unit. It was necessary to wait for the drop, and it was impossible to improve the efficiency of the print job.

  Therefore, for example, in Patent Document 1, when an image is formed on both sides of a sheet, the first fixing is performed as an incomplete fixing temperature, and the second fixing is performed as a completed fixing temperature, so that a high-temperature sheet passes through the photosensitive drum. An image forming apparatus is described in which the shortening of the life of the photosensitive drum due to the operation is suppressed.

  Patent Document 2 describes a fixing device that suppresses overshoot and undershoot by providing a large number of threshold temperatures. Patent Document 3 describes a temperature lower than a target temperature when power is turned on. When the temperature reaches the fixing temperature range, the heating control is performed based on the difference between the target temperature and the detected temperature, so that the temperature of the thermal fixing device can be shifted to the fixing temperature range with a preferable target curve that suppresses overshoot with a simple configuration. A control method is described.

Patent Document 4 describes an image forming apparatus capable of improving the efficiency of a print job by performing processing condition transition processing for switching image forming modes in a short time.
JP-A-5-173384 Japanese Patent Laid-Open No. 5-232728 JP-A-8-328423 JP 2002-116676 A

  However, any prior art describes only the case where the image forming mode is known in advance, and does not describe switching of the image forming mode from the middle of image formation.

  For example, in recent years, a proposal has been made to mount a so-called media sensor that automatically determines the type of recording medium in an image forming apparatus. According to this media sensor, it is possible for the image forming apparatus to automatically determine the type of the recording medium and perform image formation under optimum process conditions according to the result without any particular action by the user. .

  However, there is an image forming apparatus in which the media determination is made only after the recording medium is picked up and transported to the media sensor position after the start of image formation.

  In the case of the image forming apparatus having such a configuration, even if the process condition is changed after the medium is determined, the image formation has already progressed. For example, when the fixing temperature is changed, the fixing temperature is changed. It is impossible to wait until the target temperature is reached.

  When using a heat fixing device of a type having poor heat responsiveness, for example, the structure of the heat fixing device is a film formed in a cylindrical body, a heater arranged inside the film and in contact with the film, A film fixing device comprising an elastic roller pressed against the heater through the film, the film being configured to be driven to rotate by the rotation of the elastic roller, and a thermistor for detecting the fixing temperature, In the case where the configuration is arranged so as to contact the inner surface of the film instead of the heater, the temperature control is often performed using predictive control. This is because there is a delay time until the heat of the heater is transferred to the film, that is, the film heated by the heater reaches the position of the thermistor, so the power supplied to the heater and the rotation speed of the film, that is, the process speed Therefore, it is necessary to perform control while predicting how the surface temperature of the film changes. In this case, the rotational speed of the film is an important condition for predictive control, and if the recording medium transport speed, that is, the rotational speed of the film is switched by sudden mode switching, a correct prediction cannot be made and the control becomes unstable. Become.

  As a result, there has been a problem that the fixing failure of the image is caused or the fixing device is erroneously detected.

  The present invention has been made in view of such circumstances, and an image forming apparatus that does not detect an image fixing failure or an error in fixing device error even when an image forming mode is switched after image forming is started. The purpose is to provide.

  In the present invention, means for solving the above-described problems are configured as follows.

In the first configuration of the present invention,
An image forming apparatus for fixing an unfixed image formed on a recording medium as a permanent image by passing it through a thermal fixing device, having a plurality of image forming modes, and heat for image formation under processing conditions suitable for each mode. In an image forming apparatus that performs fixing control and thermally fixes an image, if mode switching occurs after the start of image formation, the mode switching heat is processed under processing conditions different from the image forming thermal fixing control. Fixing control is performed.

  The image forming apparatus includes a media discriminating warning that discriminates the type of the recording medium, and mode switching occurs according to the result of media discrimination of the recording medium using the media discriminating means.

  Here, the mode switching is to switch the conveyance speed of the recording medium through the thermal fixing device, and the mode switching thermal fixing control is configured to turn off the energization to the thermal fixing device.

In the second configuration of the present invention,
The mode switching is configured to be arbitrarily generated by the user by a command from a host computer connected to the image forming apparatus or a control panel provided in the image forming apparatus.

In the third configuration of the present invention,
The mode-switching thermal fixing control is configured to keep the predetermined power supplied to the thermal fixing device.

In the fourth configuration of the present invention,
During the mode switching thermal fixing control, the abnormality detection condition of the thermal fixing device is set to a different condition from that during normal image forming thermal fixing control.

In the fifth configuration of the present invention,
While the mode fixing thermal fixing control is being performed, the abnormality detection condition of the thermal fixing unit is not performed.

  According to the first configuration, after the image formation is started, the type of the recording medium is determined by the media sensor, and when the mode switching, that is, the switching of the recording medium conveyance speed occurs in the middle, Since the energization is turned off, it is possible to avoid problems such as poor fixing due to unstable control of the thermal fixing unit and erroneous detection of a fixing unit failure.

  According to the second configuration, the mode can be switched according to the user's wish, and an image forming apparatus with excellent usability can be provided.

  According to the third configuration, when the mode is switched, a certain amount of electric power is continuously supplied to the thermal fixing device, so that it is possible to shorten the restart time of the thermal fixing device after the mode switching is completed.

  According to the fourth configuration, while the mode fixing thermal fixing control is being performed, the abnormality detection condition of the thermal fixing device is set to a different condition from that during normal image forming thermal fixing control. Sometimes, it is possible to prevent erroneous detection of an abnormality of the heat fixing device.

  According to the fifth configuration, during the mode switching thermal fixing control, the abnormality detection condition of the thermal fixing device is not performed. Can be prevented, and the fixing control process at the time of mode switching is reduced.

  In more detail, the present invention can solve the above problems by the following configuration.

  (1) An image forming apparatus that fixes an unfixed image formed on a recording medium as a permanent image by passing it through a thermal fixing device, and has a plurality of image forming modes and images under processing conditions suitable for each mode. In an image forming apparatus that performs thermal fixing control for formation and performs thermal fixing of an image, if mode switching occurs after the start of image formation, the mode is changed under processing conditions different from the thermal fixing control for image formation. An image forming apparatus that performs thermal fixing control for switching.

  (2) The image forming apparatus according to (1), wherein the mode switching is performed by switching a conveyance speed of the recording medium passing through the thermal fixing device.

  (3) The image forming apparatus according to (1) or (2), wherein the processing condition is a fixing temperature.

  (4) The image forming apparatus according to any one of (1) to (3), wherein in the thermal switching control for mode switching, power supply to the thermal fixing device is turned off.

  (5) The image forming apparatus according to any one of (1) to (3), wherein the mode-fixing thermal fixing control continues to supply a constant power to the thermal fixing device.

  (6) Any one of the above (1) to (5), wherein the abnormality detection method of the thermal fixing device is performed by a method different from that at the time of execution of the thermal fixing control for image formation during execution of the thermal switching control for mode switching The image forming apparatus according to the item.

  (7) The image forming apparatus according to any one of (1) to (5), wherein abnormality detection of the thermal fixing device is not performed during execution of the mode-switching thermal fixing control.

  (8) The recording medium according to any one of (1) to (7), further including a medium determining unit that determines a type of the recording medium, wherein mode switching is generated according to a detection result of the medium determining unit. Image forming apparatus.

  (9) The image forming apparatus according to any one of (1) to (7), wherein the mode switching can be arbitrarily executed by the user after the start of image formation.

  (10) The heat fixing device includes a film formed in a cylindrical body, a heater disposed inside the film, and in contact with the film, and an elastic roller pressed against the heater through the film, The image forming apparatus according to any one of (1) to (9), wherein the film fixing device is configured such that the film is driven to rotate by rotation of the elastic roller.

In the present invention,
In the first configuration of the present invention,
An image forming apparatus for fixing an unfixed image formed on a recording medium as a permanent image by passing it through a thermal fixing device, having a plurality of image forming modes, and heat for image formation under processing conditions suitable for each mode. In an image forming apparatus that performs fixing control and thermally fixes an image, if mode switching occurs after the start of image formation, the mode switching heat is processed under processing conditions different from the image forming thermal fixing control. Fixing control is performed.

  The image forming apparatus includes a media discriminating warning that discriminates the type of the recording medium, and mode switching occurs according to the result of media discrimination of the recording medium using the media discriminating means.

  Here, the mode switching is to switch the conveyance speed of the recording medium through the thermal fixing device, and the mode switching thermal fixing control is configured to turn off the energization to the thermal fixing device.

In the second configuration of the present invention,
The mode switching is configured to be arbitrarily generated by the user by a command from a host computer connected to the image forming apparatus or a control panel provided in the image forming apparatus.

In the third configuration of the present invention,
The mode-switching thermal fixing control is configured to keep the predetermined power supplied to the thermal fixing device.

In the fourth configuration of the present invention,
During the mode switching thermal fixing control, the abnormality detection condition of the thermal fixing device is set to a different condition from that during normal image forming thermal fixing control.

In the fifth configuration of the present invention,
While the mode fixing thermal fixing control is being performed, the abnormality detection condition of the thermal fixing unit is not performed.

Because it is configured as above,
According to the first configuration, after the image formation is started, the type of the recording medium is determined by the media sensor, and when the mode switching, that is, the switching of the conveyance speed of the recording medium occurs in the middle, the heat fixing device is energized. Since it is turned off, it is possible to avoid problems such as fixing failure and erroneous detection of failure due to unstable control of the thermal fixing device.

  According to the second configuration, the mode can be switched according to the user's wish, and an image forming apparatus with excellent usability can be provided.

  According to the third configuration, when the mode is switched, a certain amount of electric power is continuously supplied to the thermal fixing device, so that it is possible to shorten the next startup time of the thermal fixing device.

  According to the fourth configuration, while the mode fixing thermal fixing control is being performed, the abnormality detection condition of the thermal fixing unit is set to a condition different from that during normal image forming thermal fixing control. Sometimes, it is possible to prevent erroneous detection of an abnormality of the heat fixing device.

  According to the fifth configuration, during the mode switching thermal fixing control, the abnormality detection condition of the thermal fixing device is not performed. Can be prevented, and the fixing control process at the time of mode switching is reduced.

  Hereinafter, the best mode for carrying out the present invention will be described in detail with reference to examples.

(First embodiment)
FIG. 1 is a sectional view of a color laser printer showing a first embodiment of the present invention, and FIG. 2 is a sectional view of a fixing device of the color laser printer.

  In FIG. 1, reference numerals 115, 116, 117, and 118 denote process cartridges that combine a photosensitive drum, a charging roller, a developing device, and the like, which will be described later. These are configured to be detachable from the laser printer main body 136. 115 for black (BK), 116 for cyan (C), 117 for magenta (M), and 118 for yellow (Y). For convenience, 115 black (BK) will be described. However, the same applies to the other three colors.

  Inside the process cartridge 115 is a drum-shaped image carrier, that is, a photosensitive drum 101, which has a structure in which a photosensitive material such as OPC or amorphous Si is formed on a cylindrical substrate such as aluminum or nickel. The photosensitive drum 101 is rotationally driven in the direction of the arrow at a constant speed. First, its surface is charged to a uniform potential by a charging roller 105 as a charging device. Next, the laser beam L1 that is ON / OFF controlled according to the image information is irradiated from the optical scanning unit 109, whereby scanning exposure is performed and an electrostatic latent image is formed. The electrostatic latent image is developed by the developing device 111 and visualized as a black toner image.

  Similarly, a cyan toner image is formed on the photosensitive drum 102, a magenta toner image is formed on the photosensitive drum 103, and a yellow toner image is formed on the photosensitive drum 104.

  The visualized four-color toner images are superimposed on an intermediate transfer belt (hereinafter abbreviated as ITB: Intermediate Transfer Belt) 123 conveyed at a constant speed by primary transfer rollers 119 to 122 as transfer devices. Transcribed (primary transfer). That is, a yellow toner image is first transferred onto the ITB, then the toner image is transferred so as to overlap sequentially, such as magenta, further cyan, and then black, and the color image is transferred onto the ITB. It is formed. The ITB on which the color image is formed is conveyed in the direction of the arrow in the figure.

  The conveyance speed of the ITB 123 matches the peripheral speed of the photosensitive drum described above, and also coincides with the conveyance speed of the recording material at the time of transferring and fixing a toner image onto the recording material described later. This speed is a speed common to a series of image forming processes starting from charging to exposure, development, transfer, and fixing, and is called process speed.

  On the other hand, the recording material in the cassette 124 is picked up by the pick-up roller 125 at the position of the secondary transfer roller 126 so that the timing just matches the color image formed on the ITB 123. A color image on the ITB 123 is transferred (secondary transfer) to the picked-up recording material by the secondary transfer roller 126.

  Reference numeral 128 denotes a media detection sensor that determines the type of recording material. When the picked-up recording material passes over the media sensor, the media sensor determines whether the recording material is plain paper or OHT which is a transmissive sheet. Some media sensors can be used to determine whether the recording material transmits light and whether it is OHT or not. Others that have higher functionality can be used for the surface of the recording medium such as a CCD or CMOS. There are some that can detect glossy paper by taking it in with an image sensor and processing it. In the present embodiment, description will be made assuming that plain paper or OHT is discriminated.

  The recording material on which the color image is transferred is conveyed to the fixing device 127. The recording material conveyance speed at this time is the process speed described above.

  FIG. 2 shows a partial schematic configuration of the fixing device 127 of the present embodiment. In FIG. 2, reference numeral 201 denotes a fixing film having a small heat capacity. In order to enable quick start, it is made of polyimide having heat resistance and thermoplasticity with a thickness of 20 to 100 μm or less. Further, in order to prevent offset and ensure separation of the recording material, the surface layer is coated with a heat-resistant resin having a good releasability such as a silicone resin.

  Reference numeral 202 denotes a heater for heating provided inside the fixing film 201. Along the longitudinal direction on the surface of a highly insulating ceramic substrate, for example, an energization heating resistance layer such as Ag / Pd (silver palladium) is applied in a linear shape having a thickness of about 10 μm and a width of about 1 to 5 mm by screen printing or the like. Further, the heating resistance layer forming surface is covered with a thin glass protective layer. The temperature rise of the heater 202 is detected by temperature detection means 205 installed on the back surface of the heater. Further, the temperature rise of the film 201 is detected by the temperature detecting means 209 arranged in contact with the inner surface, and fed back to an energization control unit (not shown). The energization control unit controls power supply to the heater 202 so that the temperature of the film 201 detected by the temperature detection unit 209 is maintained at a predetermined substantially constant temperature (fixing temperature). That is, the film 209 is heated and adjusted to a predetermined fixing temperature.

  Reference numeral 203 denotes an insulating stay holder for holding the heater 202, which is formed of a liquid crystal polymer, phenol resin, or the like. The fixing film 201 is loosely fitted with a margin and is arranged to be rotatable in the direction of the arrow. ing. Further, since the fixing film 201 rotates while rubbing against the internal heating heater 202 and the heat insulating stay holder 203, it is necessary to reduce the frictional resistance between the heating heater 202 and the heat insulating stay holder 203 and the fixing film 201. . For this reason, a small amount of lubricant such as heat-resistant grease is interposed on the surfaces of the heater 202 and the heat insulating stay holder 203. As a result, the fixing film 201 can rotate smoothly.

  Reference numeral 204 denotes a pressure roller as a pressure member that presses against the heater 202 via the fixing film 201 to form a pressure nip N. The structure is composed of an elastic layer 207 formed by foaming heat-resistant rubber such as silicon rubber or fluorine rubber on the outside of the core metal 206, and a release layer 208 such as fluorine resin such as PFA or PTFE is formed thereon. Yes. The pressure roller 204 is sufficiently pressed from both ends in the longitudinal direction to form a nip portion necessary for heat fixing, and is driven to rotate counterclockwise as indicated by an arrow in FIG. Along with the rotation of the pressure roller 204, the cylindrical fixing film 201 is guided by the stay holder 203 and rotated in the clockwise direction indicated by the arrow.

  When a recording material on which an unfixed image is formed is introduced between the fixing film 201 and the pressure roller 204 in the fixing nip N, the recording material is brought into close contact with the fixing film 201 together with the film. The nip portion N is nipped and conveyed, and the unfixed image is heated and fixed on the recording material by the heat from the film 209 adjusted to a predetermined fixing temperature and the pressure contact force of the nip portion N.

  The recording material that has passed through the fixing device 127 is discharged to a discharge tray 129 at the top of the printer 136.

  Usually, when a toner image is fixed to OHT, the transparency of the image is regarded as important.

  In order to obtain a transparent image, it is necessary to sufficiently dissolve the toner and smooth the surface of the toner image. This will be described with reference to FIG. 5 and FIG. In FIG. 5, reference numeral 502 denotes a recording material, and reference numeral 501 denotes a toner image fixed on the recording material. Since the surface of 501 is not smooth, reflected light and transmitted light are irregularly reflected, resulting in poor transparency. On the other hand, in FIG. 6, reference numeral 602 denotes a recording material, and reference numeral 601 denotes a toner image fixed on the recording material.

  Since 601 has a smooth surface, it does not cause irregular reflection of light and exhibits excellent transparency. In order to increase the OHT permeability, the toner fixing property as shown in FIG. 6 is required rather than that shown in FIG.

  The reason why the surface of the toner image does not become smooth as shown in FIG. 5 is that the heat of the fixing device is not sufficiently transmitted to the entire toner image, and a portion where the toner is melted and a portion where the toner is not melted are mixed. .

  For this reason, in order to fix a medium such as OHT, in order to sufficiently dissolve the toner, the time for passing through the fixing device is lengthened. This is referred to as an OHT mode, and a control different from the normal paper mode, which is a normal plain paper printing mode, specifically, a control that lowers the process speed from the normal process speed. In this OHT mode, in order to prevent the amount of heat applied to the recording material from being excessive due to the reduced process speed, control for lowering the fixing temperature is often performed together.

  That is, in the case of plain paper, fixing is performed at a normal process speed. In the case of OHT, the process speed is reduced to, for example, a half speed, and fixing is performed at a lower fixing temperature.

  If printing of OHT is designated in advance as a recording medium, the printing operation is started in the OHT mode from the beginning. However, there is a case where the user does not particularly specify a recording medium and sets plain paper or OHT to the printer and starts printing. In this case, the printer determines whether or not the recording medium is OHT with the above-described media detection sensor, and performs printing in the OHT mode if the recording medium is OHT, or in the plain paper mode if the plain paper is used. However, the type of the recording medium is not known until the recording medium is conveyed to the media detection sensor. Until the media is conveyed to the media detection sensor, the printer is normally started in the plain paper mode. That is, immediately after the start of the printing operation, the process speed is a normal speed (1/1 speed), and the fixing temperature is set to the target temperature of plain paper. After that, when the media detection sensor determines that the recording medium is OHT, at that time, the process speed is set to, for example, 1/2 speed, and the target temperature of the fixing device is also changed to the target temperature for OHT.

  If the recording medium is unknown, the reason for starting in the plain paper mode will be briefly described. The normal usage frequency is often in the case of plain paper. Image formation can proceed without changing the control. This is because the time until the end of printing can be made the same as when the plain paper mode was previously set.

  FIG. 7 is a flowchart of this embodiment, and FIG. 4 is a timing chart. The above-described switching control from the plain paper mode to the OHT mode will be described in detail using this flowchart and a timing chart.

  S1 in FIG. 7 and t0 in FIG. 4 indicate the start of the printing operation. At this time, since the printer side still does not know what the recording medium is, printing starts in the plain paper mode. That is, the process speed is set to 1/1 speed which is the normal speed (s2), and the fixing temperature is set to 180 degrees which is the fixing target temperature of plain paper (s3).

  In FIG. 4, a solid line 401 indicates the temperature of the fixing device, and a broken line 402 indicates the target temperature. A solid line 403 indicates the conveyance speed of the recording medium. When the recording medium is conveyed and reaches the position of the medium detection sensor (s4), it is determined whether or not the medium is OHT (s5). If the medium is OHT, the energization to the fixing device is turned off (s6). At the same time, the process speed, which is the conveyance speed of the recording medium, is changed from 1/1 speed to 1/2 speed (s7). This is the timing at time t1 in FIG. Then, it waits for the process speed to be switched from 1/1 speed to 1/2 speed (s8). Usually, a stepping motor is often used as a means for conveying a recording medium. In order to change the speed of the stepping motor, the speed is not changed suddenly, but so-called slow-up / slow-down control is performed in which the speed is gradually increased or decreased. This is because if the speed of the stepping motor is suddenly changed, the motor will step out. FIG. 4 shows that the stepping motor is slowed down from the 1/1 speed to the 1/2 speed between times t1 and t2.

  The period from t1 to t2 indicates that the mode is switched from the plain paper mode to the OHT mode, and mode switching is being executed. At this time, the fixing device is controlled by energizing the fixing device. It is off. For this reason, the temperature of the fixing device indicated by 401 in FIG. 4 falls between t1 and t2. Eventually, when the switching of the process speed to the 1/2 speed is completed, the target temperature of the fixing device is set to 150 degrees which is the target temperature of OHT (s9, t2). The temperature of the fixing device starts to rise again after this time t2, and is controlled to 150 ° C. which is the target temperature of OHT.

  In this embodiment, energization to the fixing device is turned off during mode switching, that is, from t1 to t2. FIG. 3 illustrates what happens when the target temperature of the fixing device is suddenly changed to 150 ° C., which is the target temperature of OHT, when the medium is determined to be OHT without being turned off (t1).

  In FIG. 3, as in FIG. 4, 301 indicates the temperature of the fixing device, 302 indicates the target temperature of the fixing device, and 303 indicates the process speed that is the conveyance speed to the recording medium. The target temperature was suddenly changed from 180 degrees to 150 degrees at time t1 in FIG. As described above, when the fixing device has a poor thermal response and the temperature control of the fixing device is performed by predictive control, the conveyance speed, which is a precondition for predictive control, has changed. As a result, the temperature control of the fixing device becomes unstable due to the fact that it cannot respond to a sudden temperature change, and as shown by 301, the temperature of the fixing device largely fluctuates between t1 and t2. there is a possibility. If such a state continues for a long time, the fixability is adversely affected and image defects are caused. In this embodiment, the period between t1 and t2 is not suitable for continuing the predictive control as it is, and the energization to the fixing is turned off, and the fixing control using the predictive control is not performed. By doing so, the temperature instability of the fixing unit as shown in FIG. 3 is eliminated, and the image can be fixed without causing an image defect.

  In this embodiment, the case where the recording medium is determined to be OHT has been described. However, it is not limited to OHT. For example, in the case of a printer having a media detection sensor capable of discriminating glossy paper, the same processing may be performed even when glossy paper is discriminated.

(Second embodiment)
A second embodiment will be described with reference to FIG. The same parts as those in FIG.

  This embodiment is different from the first embodiment in that the media detection sensor 128 is not provided and an operation panel 901 and a host computer 902 are provided.

  In this embodiment, OHT is detected by the media detection sensor and mode switching does not occur, but mode switching can be generated by the user operating the operation panel 901 or the host computer 902 after starting printing. It has a configuration.

  Even if the printer does not have a media detection sensor as in this embodiment, mode switching is accepted after printing has started, so the user has set the OHT to the printer but forgot to set the OHT mode. In such a case, mode switching can be accepted even after printing is started.

  Accordingly, it is possible to prevent the OHT from being fixed in the plain paper mode as it is, and it is possible to provide a printer with excellent usability.

(Third embodiment)
A third embodiment will be described with reference to FIG. This embodiment is different from the first embodiment in that energization to the fixing device is not turned off from t1 to t2 and a certain minute electric power is continuously applied.

  The temperature control of the fixing device will be described with reference to FIG.

  FIG. 12 is a block diagram of a heater drive control circuit as temperature control means of the fixing device. The power supply electrodes c and c of the heater 21 are connected to the heater drive control circuit via a power supply connector (not shown).

  In the heater drive control circuit, 30 is an AC power supply, 31 is a triac, 32 is a zero cross generation circuit, and 33 is a control circuit unit (CPU). The triac 31 is controlled by the control circuit unit 33. The triac 31 energizes and shuts off the heating resistor layer b of the heater 21.

  The AC power supply 30 sends a zero cross signal to the control circuit unit 33 via the zero cross detection circuit 32. The control circuit unit 33 controls the triac 31 based on this zero cross signal. By energizing the heating resistor layer b of the heater 21 from the heater drive circuit, the entire heater 21 is rapidly heated.

  The first and second thermistors TH1 and TH2 detect the temperatures of the film (not shown) and the heater 21, respectively, and their outputs are taken into the control circuit unit (CPU) 33 via the A / D converters 34 and 35.

  Based on the heater temperature information from the first thermistor TH1, the control circuit unit 33 controls the heater energization power by controlling the AC voltage energized to the heater 21 by the triac 31 by phase, wave number control, etc. Control is performed so that the control target temperature (set temperature) is maintained.

  That is, the temperatures of the first and second thermistors TH1 and TH2 are monitored as voltage values by the control circuit unit 33, and thereby the heater temperature (paper passing portion temperature) is maintained at a predetermined set temperature. Control of the energization power to 21 is performed.

  PI control is used as a typical temperature control method. The power control method includes wave number control, phase control, and the like. Here, description will be given using phase control.

  That is, the control circuit unit 33 detects the temperature of the first thermistor TH1 every 2 μsec, and determines the power supply amount to the heater 21 by PI control so that the control circuit unit 33 controls the temperature to a desired temperature control temperature. . For example, in order to designate power in 5% increments, it is generally performed using energization angles in 5% increments for one half wave of an AC waveform supplied from a power source. The energization angle is obtained as the timing when the triac 31 is turned on starting from the time when the zero cross signal is detected by the zero cross generation circuit 32.

  In the present embodiment, during the period from t1 to t2, the heater is continuously supplied with, for example, 10% power.

  In this way, the degree of temperature decrease between t1 and t2 becomes gentler than that in the first embodiment, as indicated by 801 in FIG. As in the first embodiment, the predictive control is not performed during this period, but the energization is not completely turned off, and a small electric power of 10% is continuously energized to the heater. The temperature slowly goes down. 10% is a value shown as an example, and an optimal value may be determined according to the configuration and specifications of the fixing device.

  Therefore, when the fixing device is raised again to 150 degrees after the speed is reduced to 1/2 speed, the temperature at the initial t2 does not drop so much, so that the fixing device can be quickly started up.

  In this embodiment, the fixing device has a very small heat capacity and the specification of the fixing device in which the temperature rapidly decreases when the energization is turned off even for a short period, or the time t1 to t2 required for mode switching. It is effective when is long.

(Fourth embodiment)
In this embodiment, the abnormality detection method for the fixing device during t1 to t2, that is, at the time of mode switching, is controlled differently from the normal abnormality detection method. For example, normally, the abnormally low temperature of the fixing device is 100 degrees or less, but when the mode is switched, the abnormal low temperature is 50 degrees or less. As shown in FIG. 4, the temperature gradually decreases between t1 and t2. During this time, since it is known in advance that the temperature will drop, the abnormally low detection temperature is not set at the usual 100 degrees, but is set to 50 degrees or less.

  This will be described based on the flowchart of FIG. First, at a2, the abnormal low temperature detection temperature is set to 100 degrees. When the temperature of the fixing device is detected to be 100 degrees or less, it is determined that the fixing device is abnormal, and processing for immediately stopping the operation of the printer is performed. If it is determined that the recording medium is OHT, the abnormal low temperature detection temperature is set to 50 degrees in a7. When the switching of the process speed is completed, the abnormally low temperature detection temperature is returned to 100 degrees again at a12.

  By doing so, it is possible to eliminate the problem of erroneous detection of abnormality of the fixing device at the time of mode switching.

(Fifth embodiment)
In this embodiment, the abnormal low temperature detection of the fixing device is not performed between t1 and t2.

  If abnormal low temperature detection is not performed between t1 and t2, erroneous detection of abnormality during this time is completely eliminated. Further, during this time, it is not necessary to perform abnormally low temperature detection, so that control of the printer can be reduced. The printer simultaneously performs various processes in parallel. Since the abnormal low temperature detection process is not performed, time can be spent on other processes.

  This will be described with reference to the flowchart of FIG. First, at b2, the abnormal low temperature detection temperature is set to 100 degrees. If the recording medium is determined to be OHT, the abnormal low temperature detection is not performed in b7. When the switching of the process speed is completed, the abnormal low temperature detection temperature is returned to 100 degrees again at b12.

  In this embodiment, the period from t1 to t2 is short, and even if there is a fixing abnormality during this period, if it can be detected after t2, it is effective when the printer has a configuration that has no practical problem.

The figure of the color laser printer which shows 1st Example of this invention The figure which shows the structure of the fixing device of this invention. Timing chart explaining when fixing temperature fluctuates up and down during mode switching Timing chart for explaining the first embodiment of the present invention Timing chart explaining a toner image with poor transparency Timing chart for explaining a highly transparent toner image Flowchart for explaining the first embodiment of the present invention. Timing chart for explaining a third embodiment of the present invention The figure of the color laser printer which shows 2nd Example of this invention The flowchart which shows 4th Example of this invention. Flowchart showing the fifth embodiment of the present invention. Block diagram of heater drive control circuit

Explanation of symbols

101 Photosensitive drum (Black)
102 Photosensitive drum (cyan)
103 Photosensitive drum (magenta)
104 Photosensitive drum (yellow)
105 Charging roller (black)
106 Charging roller (cyan)
107 Charging roller (magenta)
108 Charging roller (yellow)
109 Scanner unit (black, cyan)
110 Scanner unit (magenta, yellow)
111 Developer (Black)
112 Developer (Cyan)
113 Developer (Magenta)
114 Developer (Yellow)
115 Process cartridge (black)
116 Process cartridge (cyan)
117 Process cartridge (magenta)
118 Process cartridge (yellow)
119 Primary transfer roller (Black)
120 Primary transfer roller (cyan)
121 Primary transfer roller (magenta)
122 Primary transfer roller (yellow)
123 Intermediate transfer belt 124 Paper cassette 125 Pickup roller 126 Secondary transfer roller 127 Fixing device 128 Media detection sensor 129 Paper discharge tray 136 Laser printer 201 Film 202 Heater 203 Heat insulation stay holder 204 Pressure roller 205 Thermistor 206 Core metal 207 Elastic layer 208 Release layer 301 Fixing device temperature 302 Fixing device target temperature 303 Process speed 401 Fixing device temperature 402 Fixing device target temperature 403 Process speed 501 Toner image with uneven surface 502 Recording medium 601 Toner image with flat surface 602 Recording medium 901 Operation panel 902 Host computer

Claims (10)

  An image forming apparatus for fixing an unfixed image formed on a recording medium as a permanent image by passing it through a thermal fixing device, having a plurality of image forming modes, and heat for image formation under processing conditions suitable for each mode. In an image forming apparatus that performs fixing control and thermally fixes an image, if mode switching occurs after the start of image formation, the mode switching heat is processed under processing conditions different from the image forming thermal fixing control. An image forming apparatus that performs fixing control.   The image forming apparatus according to claim 1, wherein the mode switching is performed by switching a conveyance speed of the recording medium passing through the thermal fixing device.   The image forming apparatus according to claim 1, wherein the processing condition is a fixing temperature.   4. The image forming apparatus according to claim 1, wherein the mode fixing thermal fixing control cuts off the power supply to the thermal fixing device. 5.   4. The image forming apparatus according to claim 1, wherein the mode-switching heat fixing control continues to supply a constant power to the heat fixing device. 5.   6. The image according to claim 1, wherein, during execution of the thermal switching control for mode switching, the abnormality detection method of the thermal fixing device is performed by a method different from that during execution of the thermal fixing control for image formation. Forming equipment.   6. The image forming apparatus according to claim 1, wherein abnormality detection of the heat fixing device is not performed during execution of mode fixing heat fixing control.   The image forming apparatus according to claim 1, further comprising a medium determining unit that determines a type of the recording medium, wherein mode switching occurs according to a detection result of the medium determining unit.   The image forming apparatus according to claim 1, wherein the mode switching can be arbitrarily executed by a user after the start of image formation.   The heat fixing device includes a film formed in a cylindrical body, a heater disposed inside the film and in contact with the film, and an elastic roller pressed against the heater through the film, and the elastic roller The image forming apparatus according to claim 1, wherein the film fixing device is configured such that the film is driven to rotate by rotation of the film.

Images