JP5578349B2 - Fixing apparatus, image forming apparatus, and temperature control method for fixing apparatus - Google Patents

Description

  The present invention relates to a fixing device that fixes an image on a recording medium, a temperature control method therefor, and an image forming apparatus including the fixing device.

  In an electrophotographic image forming apparatus such as a copying machine, a printer, a facsimile, or a composite machine of these, a heat fixing type is mainly used as a fixing device for fixing a toner image transferred onto a recording medium such as paper. ing. This type of fixing device generally has a fixing member heated by a heating source and a facing member facing the fixing member, and allows the recording medium to pass through a fixing nip formed in contact with each other. Then, the unfixed toner image on the recording medium is fixed with heat and pressure. The fixing member is provided with contact-type or non-contact-type temperature detection means. Based on the temperature detected by the temperature detection means, the temperature of the fixing member includes the optimum temperature for fixing the toner image. Energization / non-energization of the heating source is automatically controlled so as to be maintained within a predetermined temperature range.

  However, even if the temperature of the fixing member is controlled within a predetermined range as described above, the fixing performance depends on the outside air temperature and the outside air humidity. That is, when the outside air temperature is low, the temperature of the paper or the like as a recording medium is low, and when the outside air humidity is high, the paper etc. absorbs moisture and the moisture content becomes high, so that the fixing performance tends to deteriorate. . This is because the temperature rise of the recording medium at the fixing nip differs even when the temperature of the heating surface is the same.

  In order to prevent such deterioration of the fixing performance, in the conventional image forming apparatus, a fixing device that controls the temperature of the heating surface according to the means for detecting the outside air temperature and the outside air humidity and the output thereof (for example, Patent Document 1) 2), or various image forming apparatuses that detect the moisture content of paper and control the temperature of the fixing member have been proposed (see, for example, Patent Document 3).

  Further, it is known that when the moisture content of the paper increases, the paper tends to curl toward the opposing member having a low temperature, and such curling of paper becomes prominent when the temperature difference between the fixing member and the opposing member increases. It has been. In particular, when the curl amount of the paper increases, the paper passing performance deteriorates to cause a jam, or the stacking state deteriorates even if the paper is discharged onto the paper discharge tray.

  Therefore, conventionally, as a means for reducing the curling of the paper after fixing, for example, an image forming apparatus in which both the fixing member and the opposing member are provided with a heating source so as to reduce the temperature difference between the fixing member and the opposing member has been proposed. (See, for example, Patent Documents 1, 2, 3, and 4).

  FIG. 5 is a diagram for explaining a temperature control method for a conventional fixing device. (A) of the figure shows the surface temperature of the fixing member, (b) shows the surface temperature of the opposing member, and (c) shows an ON / OFF timing chart of energization to the heating source provided inside the opposing member. The fixing member and the opposing member are each provided with temperature detecting means for detecting the surface temperature. Based on the temperature detected by each temperature detecting means, the fixing member and the opposing member have a predetermined target temperature (fixing target temperature, (Opposed target temperature). In FIG. 5, the heat storage amount of the facing member is indicated by a one-dot chain line.

  Generally, the temperature of the fixing member is set higher than the temperature of the opposing member. For this reason, when the fixing member and the opposing member start to rotate at time t1 shown in FIG. 5, heat is transmitted from the fixing member to the entire circumferential direction of the opposing member via the fixing nip, and the surface temperature of the opposing member rises. After that, when the paper is passed through the fixing nip between t2 and t3 shown in FIG. 5, the heat of the opposing member is taken away by the paper, so the surface temperature of the opposing member is lowered. Then, from the time point t3 in FIG. 5 when the paper passes through the fixing nip to the time t4 in FIG. 5 where the next paper is passed, the opposing member comes into contact with the fixing member again, and the fixing member changes to the opposing member. Heat is transferred and the surface temperature of the opposing member rises. Thereafter, when continuously passing paper, similarly to the above, the temperature decrease at the time of passing through the opposing member and the temperature increase at the time of non-passing paper are repeated.

  As described above, when the surface temperature of the facing member rises between t1 and t2 in FIG. 5, the surface temperature exceeds the target temperature, so that the heating source is turned off and the facing member is not heated. As a result, even if the surface temperature of the opposing member is high, the amount of heat storage becomes insufficient (see the portion indicated by symbol α in FIG. 5). For this reason, if heat is taken away by paper passing after that, the temperature of an opposing member will fall rapidly. When the surface temperature of the opposing member falls below the target temperature, the heating source is turned on, but the heating member has an elastic layer with a certain thickness for the purpose of securing the width of the fixing nip, so that the heat capacity is large. It takes time for the heated heat to be transferred to the surface. Furthermore, since the capacity of the heat source given to the opposing member must be reduced due to the limitation of the usable power of the apparatus, the thermal response of the opposing member is not good. For this reason, the surface temperature of the opposing member continues to decrease (see the portion indicated by symbol β in FIG. 5).

  After that, the heat is transferred to the surface of the opposing member and the surface temperature rises. However, when this heat is transferred to the surface temperature, the amount of stored heat is excessive, and the heating source of the opposing member is turned off. However, the surface temperature rises and becomes too high (see the portion indicated by symbol γ in FIG. 5). Thereafter, this is repeated, and the temperature of the opposing member is not stable.

  As described above, in the conventional temperature control method, since the surface of the facing member is heated by the heat of the fixing member, the facing member cannot be heated when necessary in anticipation of a temperature drop due to subsequent sheet passing. The temperature of the member is not stable, and as a result, there is a possibility that problems such as curling of the recording medium and fixing failure may occur.

  SUMMARY OF THE INVENTION In view of such circumstances, the present invention is directed to a fixing device capable of suppressing temperature fluctuation of a facing member, reducing curling of a recording medium, improving fixing properties, and the like, a temperature control method therefor, and an image forming apparatus including the fixing device. The device is to be provided.

The fixing device according to the present invention includes a fixing member, an opposing member that contacts the fixing member to form a fixing nip, a fixing member heating unit that heats the fixing member, and an opposing member heating unit that heats the opposing member. A fixing device configured to pass a recording medium through the fixing nip and fix an image on the recording medium, the fixing device configured to control the surface temperature of the facing member to a predetermined target temperature. As the target temperature of the counter member, a first target temperature that is a temperature of the counter member suitable for fixing an image and a second target temperature higher than the first target temperature are set, and the surface temperature of the counter member is exceeds the first target temperature, and, when a predetermined condition is not present recording medium at least in the fixing nip, change to the second target temperature from the first target temperature, the opposing member heating means With generating heat, the case surface temperature of the fixing member and the temperature difference between the surface temperature of the counter member is large, is set high the second target temperature, if the temperature difference is small, the second target temperature It is characterized by being set low .

The fixing device according to the present invention includes a fixing member, a facing member that contacts the fixing member to form a fixing nip, a fixing member heating unit that heats the fixing member, and a facing member that heats the facing member. A fixing device that fixes the image on the recording medium by passing the recording medium through the fixing nip, and detects each surface temperature on the upstream side of the fixing nip and the downstream side of the fixing nip of the opposing member. A member temperature detecting means, and based on a temperature difference obtained by subtracting the surface temperature on the nip upstream side from the surface temperature on the nip downstream side of the opposing member, the surface temperature increase of the opposing member due to heat generated by the opposing member heating means; A relationship between the surface temperature rise of the facing member due to heat supplied from the fixing member is predicted, and the facing member heating unit generates heat based on the prediction.

The fixing device according to the present invention includes a fixing member, a facing member that contacts the fixing member to form a fixing nip, a fixing member heating unit that heats the fixing member, and a facing member that heats the facing member. A fixing device that fixes the image on the recording medium by passing the recording medium through the fixing nip, and detects each surface temperature on the upstream side of the fixing nip and the downstream side of the fixing nip of the opposing member. A temperature difference obtained by subtracting the surface temperature on the upstream side of the nip from the surface temperature on the downstream side of the nip when the recording medium is not present in the fixing nip and having a recording temperature not less than a predetermined threshold. The counter member heating means generates heat at a heating duty greater than the heating duty set when the temperature difference is less than the threshold and is close to the threshold. And characterized in that.

A temperature control method for a fixing device according to the present invention includes a fixing member, a facing member that contacts the fixing member to form a fixing nip, a fixing member heating unit that heats the fixing member, and heats the facing member. A fixing device for fixing the image on the recording medium by passing the recording medium through the fixing nip, wherein the surface temperature of the opposing member is controlled to a predetermined target temperature. In the temperature control method, the surface temperature of the facing member exceeds a first target temperature that is a temperature of the facing member suitable for fixing an image, and at least a predetermined condition where no recording medium exists in the fixing nip. And changing the first target temperature to a second target temperature higher than the first target temperature to cause the counter member heating means to generate heat, and to fix the surface temperature of the fixing member and the surface temperature of the counter member. If the temperature difference is large between the high sets the second target temperature, if the temperature difference is small, and sets lower the second target temperature.

The fixing device temperature control method according to the present invention includes a fixing member, a facing member that contacts the fixing member to form a fixing nip, a fixing member heating unit that heats the fixing member, and the facing member. In the temperature control method for a fixing device, which includes a heating member for heating the opposing member and fixes the image on the recording medium by passing the recording medium through the fixing nip, the surface temperature on the downstream side of the nip of the opposing member is upstream of the nip. Based on the temperature difference obtained by subtracting the surface temperature on the side, the relationship between the surface temperature increase of the counter member due to heat generated by the counter member heating means and the surface temperature increase of the counter member due to heat supplied from the fixing member is Predicting and heating the counter member heating means based on the prediction .

The fixing device temperature control method according to the present invention includes a fixing member, a facing member that contacts the fixing member to form a fixing nip, a fixing member heating unit that heats the fixing member, and the facing member. A temperature control method for a fixing device that includes an opposing member heating unit that heats the recording medium and fixes the image on the recording medium by passing the recording medium through the fixing nip. When the temperature difference obtained by subtracting the surface temperature on the upstream side of the nip from the surface temperature on the downstream side of the nip of the facing member is equal to or greater than a predetermined threshold, the temperature difference is less than the threshold. And generating heat with a heating duty larger than the heating duty set when the threshold is approaching .

  According to the present invention, the counter member heating unit can generate heat in anticipation of a drop in the surface temperature of the counter member due to the recording medium passing through the fixing nip. Thereby, since the opposing member can sufficiently store heat, the temperature drop due to the passage of the recording medium can be suppressed as compared with the conventional case, and the surface temperature of the opposing member can be stabilized close to the target temperature. It becomes possible. As a result, the temperature difference between the fixing member and the opposing member can be reduced, and curling of the recording medium can be reduced and fixing performance can be improved.

1 is a schematic configuration diagram of a color laser printer according to an embodiment of the present invention. FIG. 2 is a schematic configuration diagram of a fixing device mounted on the printer. FIG. 5 is a diagram for explaining a temperature control method of the fixing device of the present invention. It is a figure which shows an example of a determination table. It is a diagram for explaining a temperature control method of a conventional fixing device.

  Hereinafter, embodiments according to the present invention will be described with reference to the drawings. In addition, in each figure, the same code | symbol is attached | subjected to the part which is the same or it corresponds, The duplication description is simplified or abbreviate | omitted suitably.

  FIG. 1 is a schematic configuration diagram of a color laser printer (hereinafter simply referred to as “printer”) which is an image forming apparatus according to an embodiment of the present invention. However, the present invention is not limited to the printer shown in FIG. 1, but can be applied to other printers, copiers, facsimiles, or image forming apparatuses such as multifunction peripherals thereof.

  The printer 100 shown in FIG. 1 includes a tandem type image forming unit in which four process units 1Y, 1C, 1M, and 1Bk as image forming units are arranged side by side. Each of the process units 1Y, 1C, 1M, and 1Bk is configured to be detachable from the printer body, and contains toners of different colors of yellow, cyan, magenta, and black corresponding to the color separation components of the color image. Has the same configuration. Specifically, each of the process units 1Y, 1C, 1M, and 1Bk includes a drum-shaped photoreceptor 2 as a latent image carrier, a charging device 3 as a charging unit that charges the surface of the photoreceptor 2, and a photoreceptor. 2 is provided with a developing device 4 as a developing means for forming a toner image on 2 and a cleaning device 5 as a cleaning means for removing toner on the surface of the photoreceptor 2. In FIG. 1, only the charging roller 3, the developing device 4, and the cleaning device 5 included in the yellow process unit 1 </ b> Y are denoted by reference numerals, and the other process units 1 </ b> C, 1 </ b> M, and 1 </ b> Bk are omitted. .

  In the upper part of the printer 100, toner bottles 6Y, 6C, 6M, and 6Bk filled with toner of each color are provided. The toner in each of the toner bottles 6Y, 6C, 6M, 6Bk is transferred to the corresponding developing device 4 via a toner transfer pipe (not shown).

  Under each process unit 1Y, 1C, 1M, 1Bk, an exposure device 8 for exposing the surface of each photoconductor 2 is disposed. The exposure device 8 includes a light source, a polygon mirror, an f-θ lens, a reflection mirror, and the like, and irradiates the surface of each photoconductor 2 with laser light based on image data.

  A transfer device 9 is disposed between each process unit 1Y, 1C, 1M, 1Bk and each toner bottle 6Y, 6C, 6M, 6Bk. The transfer device 9 has an intermediate transfer belt 11 formed of an endless belt as a transfer body. The intermediate transfer belt 11 is stretched around a plurality of support rollers 12, 13, 14, and 15. One of the plurality of support rollers 12, 13, 14, 15 is a drive roller, and when the drive roller rotates, the intermediate transfer belt 11 travels in the direction indicated by the arrow in the figure. Yes.

  Four primary transfer rollers 16 as primary transfer means are disposed at positions facing the four photoconductors 2. A primary transfer nip is formed at a location where the intermediate transfer belt 11 is sandwiched between each primary transfer roller 16 and each photoconductor 2. Further, a secondary transfer roller 17 as a secondary transfer unit is in contact with the outer peripheral surface on the right side of the intermediate transfer belt 11 in the drawing. A secondary transfer nip is formed at a position where the intermediate transfer belt 11 is sandwiched between the secondary transfer roller 17 and the support roller 12 facing the secondary transfer roller 17. A belt cleaning device 18 for cleaning the surface of the intermediate transfer belt 11 is disposed on the outer peripheral surface at the left end of the intermediate transfer belt 11 in the drawing.

  Further, at the lower part of the printer 100, a paper feed tray 19 that stores recording paper P as a recording medium, a paper feed roller 20 that carries out the recording paper P from the paper feed tray 19, and the like are provided. In addition, a conveyance path R for guiding the recording paper P upward from the paper feed tray 19 is formed in the printer 100. In the transport path R, a pair of registration rollers 21 for measuring the transport timing of the recording paper P is disposed midway from the position where the paper feed roller 20 is disposed to the position where the secondary transfer roller 17 is disposed. Has been. A fixing device 22 for fixing the image on the recording paper P is disposed above the position where the secondary transfer roller 17 is disposed. Further, above the fixing device 22, a pair of paper discharge rollers 24 for discharging the recording paper P to a stock portion 23 formed by denting the upper surface of the printer 100 is disposed.

The basic operation of the image forming apparatus will be described below with reference to FIG.
When the image forming operation is started, the photoconductors 2 of the process units 1Y, 1C, 1M, and 1Bk are rotationally driven clockwise by a driving device (not shown), and the surface of each photoconductor 2 is predetermined by the charging device 3. It is uniformly charged to the polarity. The surface of each charged photoconductor 2 is irradiated with laser light from the exposure device 8 to form an electrostatic latent image on the surface of each photoconductor 2. At this time, the image information to be exposed on each photoconductor 2 is monochromatic image information obtained by separating a desired full-color image into color information of yellow, cyan, magenta, and black. By supplying toner to the electrostatic latent image formed on the photoreceptor 2 in this way by each developing device 4, the electrostatic latent image is visualized (visualized) as a toner image.

  When the driving roller that stretches the intermediate transfer belt 11 is driven to rotate, the intermediate transfer belt 11 travels in the direction indicated by the arrow in the figure. Further, a constant voltage having a polarity opposite to the charging polarity of the toner or a voltage controlled by a constant current is applied to each primary transfer roller 16. As a result, a transfer electric field is formed in the primary transfer nip between each primary transfer roller 16 and each photoreceptor 2. Then, the toner images of the respective colors formed on the respective photoreceptors 2 are sequentially superimposed and transferred onto the intermediate transfer belt 11 by the transfer electric field formed in the primary transfer nip. Thus, the intermediate transfer belt 11 carries a full-color toner image on its surface. Further, the lubricant is applied to the surface of each photoconductor 2 after the transfer of the toner image by the lubricant applying device 6, and then the toner remaining on the surface of each photoconductor 2 is removed by the cleaning device 5.

  When the image forming operation is started, rotation of the paper feed roller 20 is started, and the recording paper P stored in the paper feed tray 19 is sent out to the transport path R. The recording paper P sent to the transport path R is temporarily stopped by the registration roller 21. Thereafter, the driving of the registration roller 21 is resumed, and the recording paper P is placed in the secondary transfer nip between the secondary transfer roller 17 and the intermediate transfer belt 11 in synchronization with the toner image on the intermediate transfer belt 11. send. At this time, a transfer voltage having a polarity opposite to the toner charging polarity of the toner image on the intermediate transfer belt 11 is applied to the secondary transfer roller 17, thereby forming a transfer electric field in the secondary transfer nip. When the recording paper P and the toner image on the intermediate transfer belt 11 reach the secondary transfer nip, the toner image on the intermediate transfer belt 11 is transferred onto the recording paper P by the transfer electric field formed in the secondary transfer nip. Are collectively transferred. Further, the toner remaining on the intermediate transfer belt 11 after the transfer is removed by the belt cleaning device 18. The recording paper P onto which the toner image has been transferred is conveyed to the fixing unit 22 where the toner image is fixed on the recording paper P. Thereafter, the recording paper P is discharged to the stock unit 23 by the paper discharge roller 24.

  The above description is an image forming operation when a full-color image is formed on a recording sheet. A single-color image can be formed using any one of the four process units 1Y, 1C, 1M, and 1Bk. Two or three process units may be used to form a two or three color image.

FIG. 2 is a schematic configuration diagram of the fixing device mounted on the printer of the present invention.
As shown in FIG. 2, the fixing device 22 heats a fixing roller 30 as a fixing member, a pressure roller 31 as an opposing member provided facing the fixing roller 30, and the fixing roller 30 (fixing member). A heater 32 as a fixing member heating means for heating, a heater 33 as a facing member heating means for heating the pressure roller 31 (opposing member), and a fixing member temperature detecting means for detecting the surface temperature of the fixing roller 30 (fixing member). And two temperature sensors 35 and 36 as opposed member temperature detecting means for detecting the surface temperature of the pressure roller 31 (opposed member), and the detected temperature of each of the temperature sensors 34, 35 and 36. And a heating control means 37 for controlling the heat generation of the heaters 32 and 33.

  The fixing roller 30 is configured by a metal core 30a formed in a cylindrical shape. A heater 32 is disposed in the metal core 30a, and the fixing roller 30 is heated when the heater 32 generates heat.

  The pressure roller 31 includes a metal core 31a formed in a cylindrical shape and an elastic layer 13b made of silicon rubber or the like provided on the surface of the core 31a. Also in the pressure roller 31, a heater 33 is disposed in the cored bar 31b, and the pressure roller 31 is heated when the heater 33 generates heat. The pressure roller 31 is pressed against the fixing roller 30 by a biasing member such as a spring (not shown), and a fixing nip N is formed at the pressed position. Further, the pressure roller 31 is configured to be rotatable by a driving source (not shown), and the fixing roller 30 is driven to rotate by the rotating pressure roller 31.

  In addition to the roller-shaped fixing member and the opposing member, various shapes such as a belt shape and a plate shape are applicable. In addition, the fixing member and the opposing member are not limited to being in pressure contact with each other, and may be configured to simply contact each other to perform pressurization.

  A temperature sensor 34 that detects the surface temperature of the fixing roller 30 is disposed upstream of the fixing nip N in the recording medium conveyance direction (the arrow X direction in FIG. 2). Further, temperature sensors 35 and 36 for detecting the surface temperature of the pressure roller 31 are disposed upstream and downstream in the recording medium conveyance direction with respect to the fixing nip N. In the present embodiment, each temperature sensor 34, 35, 36 is a non-contact type temperature sensor, but may be a contact type temperature sensor.

  The heating control means 37 is constituted by a CPU or the like provided in the printer main body. The heating control unit 37 controls energization / non-energization of the heaters 32 and 33 based on the detected temperatures of the temperature sensors 34, 35, and 36, so that the fixing roller 30 and the pressure roller 31 are respectively predetermined. It is designed to be heated to the target temperature.

The fixing device 22 configured as described above operates as follows.
When the power switch of the printer main body is turned on, an AC voltage is applied (powered) from the AC power source to each of the heaters 32 and 33, and the pressure roller 31 is rotationally driven by a drive motor (not shown), and at the same time, the fixing roller. 30 is driven to rotate. Thereafter, the recording paper P is fed from the paper feed tray 19 and a toner image is carried on the recording paper P at the position of the secondary transfer nip. As shown in FIG. 2, the recording paper P carrying unfixed toner T (toner image) is conveyed to the fixing device 22 and fed into the fixing nip N. Then, the toner T is fixed on the surface of the recording paper P by the heat and pressing force of the fixing roller 30 and the pressure roller 31. Thereafter, the recording paper P is sent out from the fixing nip N by the rotating fixing roller 30 and the pressure roller 31, and is discharged to the stock unit 23 by the paper discharge roller 24.

Hereinafter, a temperature control method for the fixing device, which is a characteristic part of the present invention, will be described with reference to FIG.
3, (a) is the surface temperature of the fixing roller 30 as a fixing member, (b) is the surface temperature of the pressure roller 31 as an opposing member, and (c) is a heater provided inside the pressure roller 31. 33 shows an ON / OFF timing chart of energization to 33.

  As shown in FIG. 3, in the temperature control method of the present invention, the first target temperature and the second target temperature higher than the first target temperature are set as the target temperature of the pressure roller 31. The first target temperature here is a final target temperature that is a surface temperature of the pressure roller 31 suitable for fixing an image, and the second target temperature is a surface temperature of the pressure roller 31 that is the first target temperature. It is a temporary target temperature to approach This second target value is determined by the heat capacity of the fixing device, the printing speed, the thickness of the recording paper, and the like.

  In addition, the printer of the present invention includes a determination unit that determines whether or not the recording sheet P exists in the fixing nip N, that is, whether the sheet is passing or not. The determination of whether or not to pass paper can be easily calculated from, for example, the paper feed timing from the paper feed tray, the conveyance speed, and the distance to the fixing nip N. In FIG. 3, the time zone indicated by the symbol A indicates the time of paper passing, and the time zone indicated by the symbol B indicates the time of non-paper passing.

  Referring to FIG. 3 as an example, before the time indicated by t1 in FIG. 3, the fixing device is in a standby state (a state where immediate printing is possible). At this time, the surface temperature of the fixing roller 30 is equal to the fixing target temperature. The surface temperature of the pressure roller 31 is maintained to be the first target temperature. When the printing operation is started, first, rotation of the fixing roller 30 and the pressure roller 31 is started at time t1 in the figure. Since the fixing target temperature is set to be higher than the first target temperature, when rotation of the fixing roller 30 and the pressure roller 31 is started, the circumference of the pressure roller 31 from the fixing roller 30 through the fixing nip N is started. When the heat is transmitted in the entire direction, the surface temperature of the pressure roller 31 increases. At this time, the target temperature of the pressure roller 31 is changed from the first target temperature to the second target temperature.

  The surface temperature of the pressure roller 31 exceeds the first target temperature due to the increase in the surface temperature of the pressure roller 31 after the start of the rotation, but by changing the target temperature to the second target temperature, The surface temperature is determined to be lower than the changed target temperature (second target temperature). As a result, energization to the heater 33 in the pressure roller 31 is turned on, and heat storage to the pressure roller 31 is performed.

  Thereafter, when the recording paper P reaches the fixing nip N at time t2 shown in FIG. 3, the target temperature of the pressure roller 31 is returned from the second target temperature to the first target temperature. As a result, it is determined that the surface temperature of the pressure roller 31 is higher than the changed target temperature (first target temperature), and energization to the heater 33 is turned off. Further, during the time t2 to t3 in FIG. 3 in which the recording paper P passes through the fixing nip N, the heat of the pressure roller 31 is taken away by the recording paper P and the surface temperature of the pressure roller 31 is lowered. Since the surface temperature of the pressure roller 31 exceeds the first target temperature, the energization to the heater 33 is kept OFF.

  After that, when the recording paper P is discharged from the fixing nip N as indicated by t3 in FIG. 3, the target temperature of the pressure roller 31 is changed again from the first target temperature to the second target temperature. Since the recording sheet P does not exist in the fixing nip N between t3 and t4 in FIG. 3 until the next recording sheet P reaches the fixing nip N, the fixing roller 30 and the pressure roller 31 are in contact with each other. Then, heat is transmitted from the fixing roller 30 to the pressure roller 31, and the surface temperature of the pressure roller 31 rises. In this case, even if the surface temperature of the pressure roller 31 rises, the surface temperature is lower than the changed target temperature (second target temperature), so energization to the heater 33 in the pressure roller 31 is turned on. Then, heat storage to the pressure roller 31 is performed.

  When the next recording sheet P reaches the fixing nip N (time t4 in FIG. 3), the target temperature of the pressure roller 31 is returned from the second target temperature to the first target temperature. In this case, the surface temperature of the fixing roller 31 decreases due to heat being taken away by the recording paper P, but is higher than the first target temperature until the time indicated by t5 in FIG. It becomes. However, after that, since the temperature of the pressure roller 31 is lower than the first target temperature, the energization of the heater 33 is turned on.

  When the second recording sheet P is discharged from the fixing nip N at time t6 shown in FIG. 3, the target temperature of the pressure roller 31 is changed from the first target temperature to the second target temperature. Thereafter, temperature control is performed in the same manner as described above.

  As described above, according to the temperature control method of the present invention, when the surface temperature of the pressure roller 31 exceeds the first target temperature and the recording paper P does not exist in the fixing nip N, that is, t1 in FIG. By turning the target temperature of the pressure roller 31 to the second target temperature in the shaded time zones indicated by ˜t2, t3 to t4, and t6 to t7, the energization to the heater 33 in the pressure roller 31 is turned on. It is possible to do. This time zone is a time zone in which the heater cannot be turned on because the surface temperature of the pressure roller (opposing member) exceeds the target temperature in the conventional control method. As described above, according to the present invention, the heater can be turned on even in a time zone in which the heater 33 could not be turned on conventionally, so that the pressure roller according to the control method of the present invention indicated by the two-dot chain line in FIG. The amount of heat storage 31 increases as compared with the amount of heat stored by the conventional control method indicated by the one-dot chain line in FIG. As a result of the increase in the amount of stored heat, the surface temperature of the pressure roller 31 of the present invention indicated by the solid line in FIG. 3 is suppressed from dropping due to the passage of paper compared to the conventional surface temperature indicated by the dotted line in FIG. Control is performed so as to approach the first target temperature, which is the final target value.

  In the example shown in FIG. 3, the timing of switching between the first target temperature and the second target temperature is performed in correspondence with the timing of sheet passing / non-sheet passing. It is not necessary to strictly correspond to the timing of non-sheet passing, and can be appropriately adjusted according to the result of an experiment of each fixing device.

  In the embodiment of the present invention, the first target temperature and the second target temperature are set. However, the surface temperature of the pressure roller 31 exceeds the first target temperature without setting the second target temperature. In addition, when the recording paper P is not present in the fixing nip N, the heater 33 may be forcibly heated. In this case, forcible heat generation timing, heat generation time, and other conditions are desirably individually adjusted for each fixing device.

  Incidentally, the degree of increase in the surface temperature of the pressure roller 31 during non-sheet passing is affected by the surface temperature difference between the pressure roller 31 and the fixing roller 30. That is, when the surface temperature difference between the rollers 30 and 31 is small, the amount of heat transfer from the fixing roller 30 to the pressure roller 31 is small. Is large, the amount of heat transfer from the fixing roller 30 to the pressure roller 31 increases, so that the surface temperature rise of the pressure roller 31 increases. In particular, when the surface temperature of the pressure roller 31 greatly increases due to the influence of heat from the fixing roller 30, the increase in the surface temperature of the pressure roller 31 is higher than the increase in the amount of stored heat, and thus there is a possibility that insufficient heat storage will occur. There is.

  Therefore, the surface temperature on the nip upstream side of the fixing roller 30 is detected by the temperature sensor 34, and the surface temperature on the nip upstream side of the pressure roller 31 is detected by the temperature sensor 35. Set the second target temperature high. Thereby, heat storage to the pressure roller 31 can be sufficiently and reliably performed. On the other hand, when the surface temperature difference is small, the second target temperature is set low. As a result, unnecessary heat storage in the pressure roller 31 can be prevented, and excessive temperature rise thereafter can be suppressed.

Next, a method for controlling the temperature of the fixing device using the determination table shown in FIG. 4 will be described.
In the determination table shown in FIG. 4, the vertical axis indicates a value obtained by subtracting the target temperature T0 from the surface temperature T1 upstream of the nip of the pressure roller 31, and the horizontal axis indicates the surface temperature downstream of the nip of the pressure roller 31. A value obtained by subtracting the surface temperature T1 upstream of the nip from T2. Here, the target temperature T0 is the first target temperature. In this embodiment, the heating duty determined by the values on the vertical axis and the horizontal axis of the determination table is displayed as a percentage.

  Specifically, the temperature is detected at each control cycle by the temperature sensors 35 and 36 provided on the pressure roller 31, and the values on the vertical and horizontal axes of the determination table are calculated using the detected temperatures. Then, the calculated value is applied to the vertical and horizontal axes of the determination table to determine the heating duty, and the heater 33 in the pressure roller 31 is caused to generate heat based on the determined heating duty. For example, when the heating duty is 50%, the heater 33 is caused to generate heat for 50% of the control period. Note that the heating control means 37 calculates the values of the vertical and horizontal axes of the determination table and fits the calculated values to the determination table.

  As shown in FIG. 4, in this embodiment, the lower the value on the vertical axis, the greater the heating duty. That is, as the surface temperature T1 on the upstream side of the nip of the pressure roller 31 is lower, the amount of stored heat decreases, so the heating duty is increased to ensure the stored amount of heat.

  In FIG. 4, except for the case where the horizontal axis is +3 or more, the heating duty is set to be larger as the value of the horizontal axis is lower. In the horizontal axis, in particular, when the value of the horizontal axis is negative, the surface temperature T2 on the downstream side of the nip of the pressure roller 31 is lower than the surface temperature T1 on the downstream side of the nip. The case where the heat of the pressure roller 31 is taken is shown. On the other hand, when the value on the horizontal axis is positive, the surface temperature T2 on the downstream side of the nip of the pressure roller 31 is higher than the surface temperature T1 on the downstream side of the nip. The case where heat is transmitted to the pressure roller 31 is shown. At the time of passing and non-passing the paper on the horizontal axis, the lower the value on the horizontal axis, the smaller the amount of heat stored in the pressure roller 31, so the heating duty is increased to ensure the amount of stored heat. .

  However, when the horizontal axis is greater than or equal to +3, it can be determined that the surface temperature of the pressure roller 31 has increased particularly greatly due to heat being transferred from the fixing roller 30 to the pressure roller 31 when paper is not being passed. In such a case, unless the amount of stored heat is increased, it is difficult to suppress a drop in temperature due to subsequent sheet passing. Therefore, when the horizontal axis is +3 or more, the heating duty is set to be equal to or more than the most recent heating duty set when the horizontal axis is less than +3 (heating duty when the horizontal axis is +2). As a result, the amount of heat stored in the pressure roller 31 can be increased to suppress a drop in temperature due to the sheet passing. The threshold for setting the heating duty in this way may be a value other than +3 and can be changed according to various conditions.

  In FIG. 4, a range E surrounded by a one-dot chain line indicates a case where the surface temperature T1 on the nip upstream side of the pressure roller 31 exceeds the target temperature T0. Conventionally, the heater could not be turned on within this range E, but in this embodiment, the heating duty is set under a predetermined condition within the range E so that the heater 33 can generate heat. As described above, also in this embodiment, since the heater can be turned on under the condition that the heater could not be turned on conventionally, the pressure roller 31 can store heat at a desired timing. It becomes possible to control the surface temperature so as to approach the target temperature as compared with the conventional case. In FIG. 4, when the vertical axis is +2 or more, the heating duty is set to 0% so that the temperature of the pressure roller 31 does not rise excessively.

Further, in the control method using the determination table shown in FIG. 4, it is also possible to perform control in consideration of the surface temperature of the fixing roller 30.
Specifically, when the horizontal axis in the determination table of FIG. 4 is +3 or more, the value of the vertical axis is calculated by the following equation (1).

  In the above formula (1), T1 is the surface temperature on the nip upstream side of the pressure roller 31, T2 is the surface temperature on the downstream side of the nip of the pressure roller 31, T3 is the surface temperature on the upstream side of the nip of the fixing roller 30, and K is This is a constant determined by experiment.

  When the value is calculated by using this equation (1), when the difference between the temperature T3 on the nip upstream side of the fixing roller 30 and the temperature T2 on the nip downstream side of the pressure roller 31 is large, the value is applied to the vertical axis in FIG. Since the value is corrected in a decreasing direction, the heating duty in the lower column of the determination table is selected. Thereby, especially when the surface temperature of the pressure roller 31 is greatly increased by the heat from the fixing roller 30 (when the horizontal axis is +3 or more), a large heating duty can be selected. Sufficient heat storage can be performed to suppress a drop in temperature due to paper passing. In addition, in this case, finer control can be performed by using the surface temperature of the fixing roller 30.

  When the horizontal axis in FIG. 4 is less than +3, the overheating duty may be determined by the above-described control method without using the above equation (1). Further, the threshold value for determining whether to use Equation (1) may be a value other than +3, and can be changed according to various conditions.

  As described above, according to the present invention, the heater in the pressure roller can generate heat in anticipation of a drop in the surface temperature of the pressure roller due to paper passing. As a result, the pressure roller can sufficiently store heat, so that a drop in temperature due to the sheet passing can be suppressed as compared to the conventional case, and the surface temperature of the pressure roller can be stabilized close to the target temperature. It becomes possible. As a result, the temperature difference between the fixing roller and the pressure roller can be reduced, and the curling of the recording paper can be reduced and the fixing property can be improved.

  In addition, this invention is not limited to the above-mentioned embodiment, Of course, a various change can be added in the range which does not deviate from the summary of this invention.

22 Fixing device 30 Fixing roller (fixing member)
31 Pressure roller (opposing member)
32 Heater (fixing member heating means)
33 Heater (opposing member heating means)
34. Temperature sensor (fixing member temperature detection means)
35 Temperature sensor (opposing member temperature detection means)
36 Temperature sensor (opposing member temperature detection means)
37 Heating control means N Fixing nip P Recording paper (recording medium)

Japanese Patent No. 4201181 JP 2005-345488 A Japanese Patent No. 3443716 JP-A-10-254281

Claims (8)

A fixing member; a counter member that contacts the fixing member to form a fixing nip; a fixing member heating unit that heats the fixing member; and a counter member heating unit that heats the counter member. A fixing device for fixing an image on the recording medium by passing the recording medium,
In the fixing device configured to control the surface temperature of the facing member to a predetermined target temperature,
As the target temperature of the counter member, a first target temperature that is a temperature of the counter member suitable for fixing an image and a second target temperature higher than the first target temperature are set, and the surface temperature of the counter member is The counter member heating means generates heat by changing from the first target temperature to the second target temperature under a predetermined condition that exceeds the first target temperature and at least a recording medium does not exist in the fixing nip. along with the cause,
When the temperature difference between the surface temperature of the fixing member and the surface temperature of the opposing member is large, the second target temperature is set high, and when the temperature difference is small, the second target temperature is set low. fixing device being characterized in that the. A fixing member; a counter member that contacts the fixing member to form a fixing nip; a fixing member heating unit that heats the fixing member; and a counter member heating unit that heats the counter member. In a fixing device that passes a recording medium and fixes an image on the recording medium,
A counter member temperature detecting means for detecting surface temperatures of the counter member on the upstream side and the downstream side of the fixing nip;
Based on the temperature difference obtained by subtracting the surface temperature upstream of the nip from the surface temperature downstream of the nip of the counter member, the surface temperature of the counter member rises due to heat generated by the counter member heating means, and is supplied from the fixing member. A fixing device characterized by predicting a relationship with an increase in surface temperature of the opposing member due to heat, and causing the opposing member heating means to generate heat based on the prediction . A fixing member; a counter member that contacts the fixing member to form a fixing nip; a fixing member heating unit that heats the fixing member; and a counter member heating unit that heats the counter member. In a fixing device that passes a recording medium and fixes an image on the recording medium,
A counter member temperature detecting means for detecting surface temperatures of the counter member on the upstream side and the downstream side of the fixing nip;
When there is no recording medium in the fixing nip and the temperature difference obtained by subtracting the surface temperature on the nip upstream side from the surface temperature on the nip downstream side of the facing member is equal to or greater than a predetermined threshold value, A fixing device , wherein the counter member heating unit generates heat with a heating duty larger than a heating duty set when the temperature difference is less than the threshold value and close to the threshold value . The fixing device is set so that the heating duty of the counter member heating unit increases as the surface temperature on the nip upstream side of the counter member decreases.
When there is no recording medium in the fixing nip and the temperature difference obtained by subtracting the surface temperature on the nip upstream side from the surface temperature on the nip downstream side of the facing member is equal to or greater than a predetermined threshold value, 4. The fixing device according to claim 3 , wherein the surface temperature on the upstream side of the nip of the facing member is a temperature calculated using the following formula (1), and the heating duty is determined based on the calculated temperature .

In the formula (1), T1 is the surface temperature of the opposing member on the nip upstream side, T2 is the surface temperature of the opposing member on the nip downstream side, T3 is the surface temperature of the fixing member on the nip upstream side, and K is an experiment. It is a fixed constant. An image forming apparatus comprising the fixing device according to claim 1 . A fixing member; a counter member that contacts the fixing member to form a fixing nip; a fixing member heating unit that heats the fixing member; and a counter member heating unit that heats the counter member. A fixing device for fixing an image on the recording medium by passing the recording medium,
In the fixing device temperature control method for controlling the surface temperature of the facing member to a predetermined target temperature,
When the surface temperature of the facing member exceeds a first target temperature, which is a temperature of the facing member suitable for fixing an image, and the recording medium is not present at least in the fixing nip, the first temperature is set. The target temperature is changed from the target temperature to a second target temperature higher than the first target temperature to cause the counter member heating means to generate heat,
When the temperature difference between the surface temperature of the fixing member and the surface temperature of the opposing member is large, the second target temperature is set high, and when the temperature difference is small, the second target temperature is set low. A temperature control method for a fixing device . A fixing member; a counter member that contacts the fixing member to form a fixing nip; a fixing member heating unit that heats the fixing member; and a counter member heating unit that heats the counter member. In a temperature control method of a fixing device that passes a recording medium and fixes an image on the recording medium,
Based on the temperature difference obtained by subtracting the surface temperature upstream of the nip from the surface temperature downstream of the nip of the counter member, the surface temperature of the counter member rises due to heat generated by the counter member heating means, and is supplied from the fixing member. A method of controlling a temperature of a fixing device, wherein a relationship with a rise in the surface temperature of the opposing member due to heat is predicted, and the opposing member heating unit generates heat based on the prediction . A fixing member; a counter member that contacts the fixing member to form a fixing nip; a fixing member heating unit that heats the fixing member; and a counter member heating unit that heats the counter member. In a temperature control method of a fixing device that passes a recording medium and fixes an image on the recording medium,
When there is no recording medium in the fixing nip and the temperature difference obtained by subtracting the surface temperature on the nip upstream side from the surface temperature on the nip downstream side of the facing member is equal to or greater than a predetermined threshold value, A temperature control method for a fixing device, characterized in that the opposing member heating means generates heat with a heating duty larger than a heating duty set when the temperature difference is less than the threshold value and close to the threshold value .

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