JP2006276531A - Image forming apparatus - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an image forming apparatus capable of speedily decreasing the temperature of the surface of a thermal fixing member despite a simple configuration. <P>SOLUTION: The image forming apparatus includes a mode discrimination means 8 for discriminating whether a detection temperature detected by a detecting means 7 meets a condition in which the temperature exceeds a down mode execution temperature that is above the permissible range of a set temperature, such that it is discriminated to be a down mode when the temperature is above the down mode execution temperature, and it is discriminated to be a regular mode when the temperature does not meet the condition. The apparatus also includes an image formation control means 9 which exerts control such that in the regular mode, an image processing by an image forming engine 3 and a thermal fixing process by a fixing device 4 are immediately performed for a recording medium 2 and in the down mode, the recording material 2 is idly run to the fixing device 4 without being subjected to the image forming process by the image forming engine 3 and returned to an area before the transfer section of the image forming engine 3 via a bypass conveyance path 6, and thereafter the image forming process by the image forming engine 3 and the thermal fixing by the fixing device 4 are performed. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to an image forming apparatus including a fixing device that fixes an unfixed toner image formed on a recording material by contact heating with a heat fixing member in an image forming apparatus such as a copying machine, a printer, and a facsimile machine.

  In an image forming apparatus such as a copying machine employing an electrophotographic system, a fixing device is used to fix and fix an electrostatically formed toner image on a recording material such as paper. Various fixing devices have been proposed. The fixing device includes a heated heat fixing member and a pressure fixing member pressed against the heated fixing member, and electrostatically carries a toner image on the pressure nip portion of both rolls. Such a recording material that is inserted into the recording material and fixed and fixed to the recording material is effective from the viewpoint of thermal efficiency, safety, miniaturization, and the like, and is widely used.

In such an image forming apparatus, particularly in the case of a black and white color multifunction peripheral, as shown in FIG. 16, the fixing temperature region of the heat fixing member capable of performing stable toner fixing without image defects is larger than that of black and white printing. The color print is narrower.
This figure shows the relationship between the fixing temperature in color printing and black-and-white printing and the so-called offset toner amount at which the toner on the recording material is transferred to the heat fixing member side. In the figure, the non-visual offset is a small amount of offset that is constantly generated and does not affect the image. The hot offset is an offset that occurs when the surface of the heat fixing member is higher than the appropriate temperature, and the cold offset is an offset that occurs when the surface of the heat fixing member is lower than the appropriate temperature.

Here, in black and white printing, the temperature at which hot offset occurs is a, the temperature at which cold offset occurs is b, and the fixing temperature region for black and white printing is within the range of temperature b to a.
On the other hand, in color printing, while the hot offset occurrence temperature is A2, the maximum temperature in the color fixing temperature region is A1, which is lower than that. This is because the amount of toner in color printing requires a larger amount of heat at the time of fixing, and in order to obtain the amount of heat, the fixing speed is lowered while the maximum temperature is A1, and the fixing time is lengthened. A predetermined amount of heat is secured by (not shown). For this reason, if the surface temperature of the heat fixing member is set within the range of A1 to A2, offset does not occur, but the amount of heat applied to the toner becomes too high, causing image defects.
In addition, the cold offset occurrence temperature B for color printing is higher than the cold offset occurrence temperature b for monochrome printing. This indicates that the minimum amount of heat required itself increases due to an increase in the amount of toner in color printing, and a predetermined amount of heat cannot be secured only by increasing the fixing time, and the minimum fixing temperature is also increased.

As described above, the fixing temperature region of the heat fixing member is narrower in color printing than in black and white printing, and in particular, the maximum fixing temperature A1 in color printing is lower than the fixing maximum temperature a in black and white printing. In printing, the set temperature during standby is inevitably different.
On the other hand, the control temperature at the time of waiting for monochrome printing and color printing is common, and if priority is given to continuous productivity of monochrome printing, the temperature is set at a temperature corresponding to monochrome printing. Therefore, if a print command for color printing is received from such a standby state, the fixing temperature is too high for color printing, and there is a concern about the occurrence of image defects.

In addition, such a problem occurs not only at the time of transition from black and white printing to color print printing, but also at the time of so-called overshoot in which the surface temperature of the heat fixing member temporarily increases after completion of the fixing job by continuous printing. There is a possibility that it may occur even if it starts.
In particular, as shown in FIG. 17, for example, when color printing is started at the time of overshoot immediately after paper fixing for black and white printing, the print job starts at a temperature higher than the maximum temperature A1 in the fixing temperature region, and image defects occur. Is remarkable. In other words, as shown in the region VII in the figure, during the job, the fixing material is fixed to the recording material sent as needed, so that the heat fixing member loses heat to the recording material, and the surface temperature is stabilized. Yes. However, when the job is completed, as shown in the region VIII in the figure, the surface of the heat fixing member temporarily becomes high due to the residual heat, and exceeds the maximum temperature A1 in the fixing temperature region of the color print. In particular, if a color print job is started as the next job at that time, an image defect occurs as shown in the area IX in the figure.

  In order to solve such a technical problem, there is a technique in which printing is not executed until the temperature of the surface of the heat fixing member is lowered to a predetermined temperature. However, it takes several minutes to lower the temperature to the predetermined temperature. The waiting time until the start becomes long, which is not a preferable solution from the viewpoint of speeding up the apparatus.

JP-A-11-174897 (Embodiment of the Invention, FIG. 1)

Therefore, in order to solve such a technical problem, Patent Document 2 discloses a prior art in which a cooling roll provided with a latch mechanism is appropriately brought into contact with a heat fixing member.
However, in this type of prior art, it is inevitable that the fixing device main body becomes large and complicated, resulting in an increase in cost, and if the surface of the cooling roll becomes dirty, the dirt is transferred to the heat fixing member. As a result, there arises a technical problem that an image defect is generated.

  The present invention has been made to solve the above technical problem, and when color printing is started from a state in which the control temperature of the fixing device during standby is set to monochrome printing, An image forming apparatus capable of quickly reducing the temperature of the surface of a heating and fixing member while having a simple configuration in order to prevent an image defect that occurs when the next job starts during overshoot immediately after printing. It is to provide.

  That is, according to the present invention, as shown in FIG. 1, an image forming engine 3 that transfers an unfixed image formed on an image carrier 1 onto a sheet 2 and a heat fixing in which the surface is heated and contacts the recording material 2. An image forming apparatus having a member 5 and a fixing device 4 for heating and fixing an unfixed image on the recording material 2 formed by the image forming engine 3 is provided separately from a normal recording material conveyance path. In addition, a bypass conveyance path 6 for returning the recording material 2 to the front of the transfer site of the image forming engine 3 after passing through the fixing device 4, a detection means 7 for detecting the surface temperature of the heat fixing member 5 of the fixing device 4, and the detection means 7 Mode discrimination that determines whether the detected temperature is above the down mode execution temperature that exceeds the set temperature tolerance range, determines that the down mode is higher than the down mode execution temperature, and otherwise determines the normal mode Mean 8 In the normal mode, the image forming process by the image forming engine 3 and the heat fixing process by the fixing device 4 are immediately executed on the recording material 2, and in the down mode, the image forming process by the image forming engine 3 is not performed. Then, the recording material 2 is idled and returned to the front of the transfer portion of the image forming engine 3 via the bypass conveyance path 6, and then image forming processing by the image forming engine 3 and heat fixing processing by the fixing device 4 are executed. Control means 9 is provided.

In such technical means, the image forming method may be appropriately selected as long as the toner image is visualized, and it does not matter whether it is an electrophotographic method or an electrostatic recording method.
The transfer method may be a method in which an unfixed image formed on the image carrier 1 is directly transferred to the recording material 2 or a method in which the transfer is performed collectively via an intermediate transfer member. Further, the image carrier 1 may be one or plural.
Furthermore, the image creation engine 3 is not limited to the one that creates a full-color image, and may be one that creates a multi-color image such as two colors or three colors, or a single color image.

The fixing device 4 only needs to have a pair of heat fixing members 5 for heat fixing and nip-conveying the recording material 2, and the mode is a combination of a belt and a roll in addition to a roll pair configuration. There may be a belt pair configuration.
In addition, the present invention is not limited to single-sided printing or double-sided printing. If the recording material 2 has a bypass conveyance path 6 that is conveyed before the transfer site of the image forming engine 3 after passing through the fixing device 4, a tray is provided in the middle. In addition to a mode capable of supporting double-sided printing such as a mode in which recording material 2 is reversed and a mode in which recording material 2 is reversed, a mode for only single-sided printing that transports while keeping the printing surface of recording material 2 constant is appropriately used. You can choose.

The detection means 7 may be appropriately selected as long as the surface temperature of the heat fixing member 5 can be detected, and mainly includes a temperature sensor.
Based on the detection information of the detection means 7, it is determined whether or not the mode determination means 8 is under the condition of the down mode execution temperature exceeding the allowable range of the set temperature. In other cases, the normal mode is determined.

Here, in the normal mode, the surface of the recording material 2 is printed by immediately executing the image forming process by the image forming engine 3 and the heat fixing process by the fixing device 4 without using the bypass conveyance path 6.
On the other hand, in the down mode, after the recording material 2 is idled in the fixing device 4 without performing the image forming process by the image forming engine 3, the recording material 2 is returned to the front of the transfer portion of the image forming engine 3 via the bypass conveyance path 6. Then, the image forming process by the image forming engine 3 and the heat fixing process by the fixing device 4 are performed (hereinafter referred to as “carrying on the transport path dummy”). As described above, when the recording material 2 runs idle in the fixing device 4, the recording material 2 absorbs excess heat on the surface of the heating and fixing member 5, and the temperature is lowered.
Further, the image forming control means 9 controls whether the image forming engine 3 controls the presence or absence of image transfer to the surface of the recording material 2 or the bypass conveying path 6 according to the mode determined by the mode determining means 8. In addition to selecting whether or not to convey, the surface temperature of the heat fixing member 5 is controlled.

Further, the bypass conveyance path 6 is preferably configured by a recording material reversing return path that reverses the recording material 2 that has been recorded on one side in the double-sided recording mode and then returns the recording material 2 to the front of the transfer portion of the image forming engine 3.
For such a bypass conveyance path 6, an existing apparatus employing a so-called intermediate trayless system that reverses the recording material 2 that has passed through the fixing device 4 and immediately returns it to the sheet transfer portion can be used. Further, by adopting such a recording material reversal return path, it is not necessary to provide an intermediate tray or the like for temporarily storing the recording material 2, which is preferable from the viewpoint of downsizing the apparatus.

Furthermore, it is preferable that the down mode execution temperature is variably set according to the image forming conditions. That is, the down mode execution temperature is variably set by monochrome printing or color printing, and the down mode execution temperature is set according to each image forming condition.
Further, the down mode execution temperature is preferably set variably by the recording material 2. In this case, the down mode execution temperature is variably set according to the paper type or size of the recording material 2.
This is based on the fact that the hot offset generation temperature differs for each recording material 2. Specifically, the recording material 2 having a large basis weight, such as thick paper, has a larger heat capacity than that of plain paper, and requires a large amount of heat to fix the toner on the paper. As a result, the upper limit of the hot offset area is increased, and hot offset is less likely to occur, so the down mode execution temperature is higher than that of plain paper.

  Further, the image forming control means 9 is in a period from when the recording material 2 enters the fixing device 4 due to idle running until the image is transferred to the first surface via the bypass conveyance path 6 and enters the fixing device 4 again. The temperature can be lowered by turning off the heater of the heat fixing member 5.

Further, in this case, the image forming control means 9 enters the fixing device 4 because the recording material 2 is idle, and then the image is transferred to the first surface via the bypass conveyance path 6 and enters the fixing device 4 again. By controlling the time until this, the surface of the heat fixing member 5 can be kept at a more appropriate temperature when the recording material 2 enters the fixing device 4 again.
In this aspect, since the amount of decrease in the surface temperature of the heat fixing member 5 varies depending on the recording material 2 due to the difference in basis weight and process speed, the time for which the heater is turned off is set for each recording material 2. As a result, it is possible to prevent the surface temperature of the heat fixing member 5 from being lowered or excessively lowered, and the temperature of the heat fixing member 5 can be controlled corresponding to each recording material 2.

Furthermore, the image forming control means 9 starts from the time when the recording material 2 enters the fixing device 4 due to idle running until the image is transferred to the first surface via the bypass conveyance path 6 and sent again to the fixing device 4. It is preferable to control the surface temperature of the heat-fixing member 5 by turning on the heater when the surface temperature of the heat-fixing member 5 falls to a certain temperature within the heat-control time. .
That is, when the recording material 2 runs idle in the fixing device 4 and the surface temperature of the heat fixing member 5 is lowered to a certain temperature, the heater is turned on with the temperature as the heating control temperature. This prevents the decrease.
Furthermore, in this case, the time from when the sheet enters the fixing device 4 until the fixing device 4 is sent out again after the dummy travel is set as the heating control time, and by controlling this heating control time, the optimum temperature can be set more effectively. can do.
According to this aspect, it is possible to prevent an excessive decrease in temperature when the recording material 2 re-enters the fixing device 4 or an excessive increase in temperature when the heater is turned on.

Further, in this case, the heating control temperature and the heating control time are preferably set according to the size of each recording material 2.
In this case, for example, when the recording material 2 having a large size is used, the surface temperature of the fixing member during idling can be further lowered as compared with the recording material 2 having a small size. There is a possibility that the surface temperature of the liquid will be lower than the appropriate temperature. In this aspect, in order to avoid such a situation, the heating control temperature and the heating control time are set according to the size of the recording material 2.

Furthermore, the control temperature and control time are preferably set according to the paper type of the recording material 2.
In this case, for example, when the recording material 2 having a large basis weight such as cardboard is used, the temperature is greatly reduced as compared with the plain paper, and the surface temperature of the heat fixing member 5 may be lower than the appropriate temperature due to idle running. is there. In this aspect, in order to avoid such a situation, the heating control temperature and the heating control time are set according to the paper type.

  According to the present invention, when color printing is started from a state in which the control temperature of the fixing device during standby is set to monochrome printing, or when the next job starts at the time of overshoot immediately after continuous printing, Since the temperature of the surface of the heat fixing member can be reduced by running the recording material idly in the fixing device without performing image forming processing, the temperature of the surface of the heat fixing member can be rapidly reduced while having a simple configuration. Therefore, it is an object of the present invention to provide an image forming apparatus that can prevent image defects.

Hereinafter, the present invention will be described in detail based on embodiments shown in the accompanying drawings.
Embodiment 1
FIG. 2 shows Embodiment 1 of an image forming apparatus to which the present invention is applied. In the figure, the image forming apparatus of the present embodiment is provided with an apparatus main body 10 having an image forming unit 20 capable of forming a color image, and an image reading unit 11 for reading a document on the upper part of the apparatus main body 10. At the same time, a document feeding device 12 for feeding a document to the image reading unit 11 is disposed above, while a multi-stage feed for supplying paper as a recording material is provided below the image forming unit 20 of the apparatus body 10. The paper cassettes 51 to 53 are arranged so that they can be pulled out.

  The image forming unit 20 used in the present embodiment adopts, for example, an electrophotographic system, and is 4 (yellow (Y color), magenta (M color), cyan (C color), and black (K color)). This is a so-called tandem configuration in which the color image forming units 21 (21 a to 21 d) are arranged in parallel on the intermediate transfer belt 30. Therefore, the toner images of the respective colors formed by the respective image forming units 21 (21a to 21d) are sequentially primary-transferred and multiplexed on the intermediate transfer belt 30, and the multiplexed toner images are secondary-transfer device 35. Are collectively transferred onto the sheet conveyed from the sheet feeding cassettes 51 to 53 and guided to the fixing device 40. The color arrangement of the four-color image forming units 21 is not limited to this order, and may be in other orders.

  The image forming unit 21 (21a to 21d) in the present embodiment includes a photosensitive drum 22 that forms and supports each color component toner image, a charging device 23 such as a charging roll that charges the photosensitive drum 22, and a charged photosensitive member. A laser exposure device 24 that forms a latent image on the photosensitive drum 22, a developing device 25 that visualizes the electrostatic latent image on the photosensitive drum 22, and a toner image on the photosensitive drum 22 are primarily transferred onto the intermediate transfer belt 30. For example, a primary transfer device 26 including a primary transfer roll and a cleaning device 27 for cleaning residual toner remaining on the photosensitive drum 22 are configured. In the present embodiment, the laser exposure device 24 exposes the entire four-color image forming unit 21 with one laser exposure device 24.

  The intermediate transfer belt 30 is stretched around a plurality of stretching rolls 31 to 33, and is circulated and conveyed using, for example, the stretching roll 31 as a driving roll. For example, a secondary transfer device 35 including a secondary transfer roll. However, the tension roll 33 is opposed to the backup roll. Further, a belt cleaning device 36 for removing residual toner on the intermediate transfer belt 30 is disposed at a position facing the tension roll 32 of the intermediate transfer belt 30.

Here, the paper transport system in the present embodiment is as follows. A vertical conveyance path 61 extending substantially vertically upward from each of the paper feed cassettes 51 to 53, a main conveyance path 62 extending substantially horizontally on the downstream side of the vertical conveyance path 61, and substantially in the vicinity of the most downstream of the main conveyance path 62. A retreat conveyance path 63 extending downward in a Y shape and a reversal conveyance path 64 extending substantially horizontally from the retreat conveyance path 63 and leading to the vertical conveyance path 61 are configured.
In the main conveyance path 62, a registration roll 54 that regulates the positioning of the paper, a secondary transfer device 35 that collectively transfers the toner images multiplexed on the intermediate transfer belt 30, and a paper on which the toner images are collectively transferred. A conveying belt 55 that conveys the toner, a fixing device 40 that fixes the toner image on the sheet, a discharge roll 56 that discharges the sheet to a storage tray 57 that is provided on the side of the apparatus body 10 and stores the sheet, and the like are provided.
In these conveyance paths 61 to 64, conveyance members such as a conveyance roll and a conveyance guide for ensuring the conveyance of the paper are appropriately disposed.

  Further, the fixing device 40 of the present embodiment is as shown in FIG. In the figure, a fixing roll 41 provided with a heater 42 such as a halogen lamp, an endless belt 43 that rolls with the fixing roll 41 and sandwiches and conveys paper, and a back surface side of the endless belt 43 are arranged in contact with each other. A separation nip member 47 that forms a separation nip region between the fixing roll 41 and the endless belt 43, and a separation nip member 47 that is disposed upstream of the separation nip member 47 and forms a fixing nip region between the fixing roll 41 and the endless belt 43. The pre-nip member 46 is a main component.

The fixing roll 41 in this embodiment includes a cylindrical core 41a made of metal such as aluminum having excellent mechanical strength and good thermal conductivity, and an elastic layer such as silicone rubber formed on the surface of the core 41a. 41b and a release layer 41c that covers the surface of the elastic layer 41b and is provided to prevent the offset of the unfixed toner image on the paper.
Here, the material of the core 41a is not particularly limited as long as it has mechanical strength and good thermal conductivity. For example, a metal such as stainless steel, steel, brass, or an alloy can be used.
Further, the elastic layer 41b is not limited to silicone rubber, and may be, for example, fluorine rubber as long as it has heat resistance. The method of forming the elastic layer 41b on the surface of the core 41a is not particularly limited, An injection molding method or a coating method can be employed.
Further, the release layer 41c may be any material that has heat resistance and appropriate release properties for the toner. For example, fluorine rubber or fluorine resin is used.
Furthermore, the heater 42 in the fixing roll 41 is not particularly limited as long as it has a shape and structure that can be accommodated in the core 41a, and may be appropriately selected according to the purpose.

  In the present embodiment, a temperature sensor 49 that measures the surface temperature of the fixing roll 41 is provided around the fixing roll 41, and the heater is used to control the surface temperature of the fixing roll 41 by the temperature sensor 49. 42 temperature control is performed. The temperature sensor 49 is not particularly limited as long as it can measure the surface temperature of the fixing roll 41. For example, a temperature sensitive element such as a thermistor or a posistor can be used.

Then, on the back side of the endless belt 43, a peeling nip member 47 and a prenip member 46 constituting a fixing pressure member are mounted in a concave portion of a support body 44 made of a metal having a substantially concave shape in cross section, respectively. It comes into contact with the endless belt 43. In FIG. 4, symbol S indicates a sheet on which an unfixed toner image is transferred.
Further, in the present embodiment, a belt traveling guide 45 made of a rigid and low thermal conductive resin is provided in the endless belt 43 below the support body 44 in order to smoothly rotate the endless belt 43. .

In particular, as shown in FIG. 4, in the present embodiment, the control device 80 (corresponding to the mode discriminating means and the image forming control means) is based on each detection signal, in addition to the image forming process and the fixing device 41, and the recording material The conveyance system 60 is controlled.
That is, in addition to the operation signal from the paper type changeover switch 90, the size sensor 91 and the image forming mode changeover switch 92 for switching between black and white printing or color printing, the control device 80 is connected so that the detection signal from the temperature sensor 49 can be input. ing. On the basis of this input signal, the control device 80 performs mode discrimination for discriminating between the normal mode and the down mode, and at the same time, controls the temperature switching of the heater of the fixing roll 41 and selects the recording material conveyance system 60 as well as image forming processing. Is determined and executed, and a control signal is sent to each device.

Here, more specifically, the operation signals from the changeover switches 90 and 92 and the size sensor 91 include, for example, a selection button shown on the screen of the user interface 100, which will be described with reference to FIG. As shown in FIG. 5A, the normal screen of the user interface 100 includes settings of “basic copy” 101, “image quality adjustment” 102, “reading method” 103, “output format” 104, and “job editing” 105. The screen is displayed in such a manner that it can be changed. In FIG.
Further, on this screen, buttons for the settings of “magnification selection” 111, “paper selection” 112, and “color mode” 113 are displayed, and in the same figure, “magnification selection” 111 is set to “100%” 121. “Paper selection” 112 is selected as “A3” 122, and “Color mode” 113 is selected as “Monochrome” 123.
Here, in particular, in the setting of “color mode” 113, the selected button is sent to the control device 80 as an operation signal from the above-described image forming mode switch 92.

Furthermore, for example, when “tray 5” (not shown) is selected from “other tray” 124 of “paper selection” 112, the screen is switched to the screen of FIG. 5B, and “paper size” 131 and “paper type” 132 are set. A button is displayed so that it can be changed.
In this screen, “automatic size detection” 141, “standard size” 142, and “non-standard size” 143 are displayed as buttons for the “paper size” 131 setting. The changeover switch 90 is set to a predetermined paper size. At this time, an operation signal corresponding to the sheet size is sent to the control device 80. Here, when “automatic size detection” 141 is selected, the size of the paper is detected by the size sensor 91, and the detection signal is sent to the control device 80.
Similarly, “plain paper” 145, “recycled paper” 146, “thick paper 1” 147, “thick paper 2” 148, and “OHP film” 149 are displayed as buttons in the “paper type” 132 setting. When a button to be selected is pressed, a predetermined paper type is set. Also in this case, an operation signal corresponding to the paper type is sent to the control device 80 as the paper type changeover switch 90 described above. Note that the five types of paper displayed on the screen shown in the figure are the above five types, but when the next page selection button 150 is pressed, coated paper or other paper can be selected as appropriate.

Next, regarding the operation in the present embodiment, first, mode discrimination between the normal mode and the down mode is shown in FIG.
In the present embodiment, in particular, setting information selected from the paper size selection button 151, the paper type selection button 152, or the color mode selection button 153 of the user interface 100 is sent to the control device 80 as an operation signal. Based on this, the control device 80 determines the down mode execution temperature. At the same time, a detection signal from a temperature sensor 49 that measures the surface temperature of the fixing roll 41 is input to the control device 80. At this time, based on the determined down mode execution temperature and the input information from the temperature sensor 49, the control device 80 determines whether or not the sheet can be reduced to a predetermined fixing temperature by running the dummy. If it is not, it is determined to be down mode, and otherwise it is determined to be normal mode.

  In this case, the down mode execution temperature determined by the control device 80 varies depending on the selection of the “paper type” 132 of the user interface 100. For example, as shown in FIG. 7, the execution mode temperature of plain paper is 195 ° C., and the temperature of thick paper is set to 200 ° C. The heat capacity is large, and a large amount of heat is required to fix the toner on the paper. As a result, the upper limit of the hot offset area is increased and hot offset is less likely to occur, so the down mode execution temperature is set to be higher than that of plain paper. In the table below, the down mode execution temperature (190 ° C.) of the coated paper is described as a reference value. A curve indicating the surface temperature of the fixing roll shown in FIG.

Further, for example, as shown in FIG. 8, when the down mode execution temperature in the monochrome mode is set to 195 ° C., the down mode execution temperature in the color mode is set to 190 ° C. Depending on the image forming mode. This is because, as described above, the fixing temperature for color printing is set lower than that for monochrome printing from the viewpoint of preventing image defects. A curve indicating the surface temperature of the fixing roll shown in FIG.
7 and 8, the control temperature indicates a reference temperature for keeping the surface temperature of the fixing roll 41 properly, and the control device 80 performs temperature control based on the control temperature.
As described above, since the down mode execution temperature differs in each paper type and each mode, the control device 80 determines whether the down mode or the normal mode based on the determined down mode execution temperature and the detection information by the temperature sensor 49. is there.

Next, the operation of the paper transport system in the normal mode in the present embodiment will be described with reference to FIG.
The sheet sent out from any of the sheet feeding cassettes 51 to 53 advances along the vertical conveyance path 61, is positioned and regulated by a registration roll 54 provided on the main conveyance path 62, and then proceeds to the secondary transfer device 35. .
On the other hand, the color toner images transferred onto the intermediate transfer belt 30 by the image forming engine 21 (21 a to 21 d) are conveyed on the intermediate transfer belt 30 and reach the secondary transfer device 35.
Then, in the secondary transfer device 35, the multiplexed toner images on the intermediate transfer belt 30 are collectively transferred onto the paper. The sheet on which the toner images are collectively transferred is conveyed by the conveying belt 55 and reaches the fixing device 40. Then, the toner image is fixed on the sheet by the fixing device 40, proceeds on the main conveyance path 62 as it is, and is discharged from the discharge roll 56 to the storage tray 57.

  Here, when performing double-sided recording on a sheet, the sheet that has been fixed by the fixing device 40 is temporarily retreated from the main conveyance path 62 to the retreat conveyance path 63 side. Then, after sufficiently proceeding through the retreat conveyance path 63, when the rear end of the sheet exceeds the entrance of the reversal conveyance path 64, a predetermined conveyance roll in the retreat conveyance path 63 performs a reversing operation, and The sheet is reversed and conveyed to the reverse conveyance path 64 side. The reversely transported sheet is transported again to the main transport path 62 with the front and back sides of the sheet reversed, and a new toner image is formed. Thereafter, the toner is discharged from the fixing device 40 to the storage tray 57 via the discharge roll 56.

Next, the operation of the paper transport system in the down mode in the present embodiment will be described with reference to FIG.
FIG. 9A shows a sheet conveyance path in the down mode.
In other words, the sheet fed from the sheet feeding cassettes 51 to 53 advances along the vertical conveyance path 61 and is conveyed to the main conveyance path 62. Further, the sheet enters the fixing device 40 in a state of a blank sheet without a toner image being transferred onto the surface of the sheet in the secondary transfer device 35, and runs idle in the fixing device 40, and then enters the reverse conveyance path 64. Led. Then, the sheet is reversed and conveyed on the retreat conveyance path 63 of the same path, and is conveyed again to the front of the secondary transfer device 35 with the front and back sides of the sheet reversed. Hereinafter, such traveling is referred to as dummy traveling.
On the other hand, each color toner image that has been multiplex-transferred onto the intermediate transfer belt 30 by the image forming engine 21 (21a to 21d) while the paper is running in a dummy is conveyed on the intermediate transfer belt 30 to be a secondary transfer device. 35.
In the secondary transfer device 35, the multiplexed toner images on the intermediate transfer belt 30 are collectively transferred with the back surface of the paper as the first surface, and then reach the fixing device 40. Thereafter, the toner image is fixed on the paper by the fixing device 40 and then discharged (see FIG. 9B).

  Here, although the sheet conveyance path in the case of single-sided printing is shown in the figure, in the case of double-sided printing, the reverse conveyance path 64 is inserted again after the first-side printing, and the retreat conveyance path 63 of the same path. The sheet is reversed and conveyed, and the sheet is conveyed again up to the front of the secondary transfer device with the front and back sides reversed. In other words, when the paper is reversed and conveyed in the retreat conveyance path 63, a new toner image is formed with the front side of the paper turned upside down and the paper surface as the second side. Thereafter, the image is fixed by the fixing device 40 and then discharged.

Furthermore, FIG. 10 shows the fixing temperature appropriate temperature setting process in the down mode in the present embodiment.
In the present embodiment, after the sheet enters the fixing device 40, the fixing roll 41 transfers the toner image by the secondary transfer device 35 via the reverse conveyance path 64, and again in the fixing device 40. The heater is turned off until it enters.
In this case, for example, the surface temperature of the fixing roll 41 shows a curve as shown in FIG. This figure shows a temperature curve when the surface temperature of the fixing roll 41 raised by overshooting is lowered by turning off the heater of the fixing roll 41 and performing dummy running.
That is, in the figure, the region I indicates a region where the surface temperature of the fixing roll 41 is rising due to overshoot. Further, the area II shows a state in which the surface temperature of the fixing roll 41 is lowered by turning off the heater from the time when the paper enters the fixing device 40 to the time when the paper again enters after the dummy running. . Further, a region III shows a temperature curve when the sheet is re-entered into the fixing device 40 and the heater is turned on at the same time and is kept at a temperature close to the control temperature.

As shown in the figure, the heater is turned off from the time when the paper enters the fixing device 40 until it re-enters through dummy running, and the paper runs idle and absorbs excess heat on the surface of the fixing roll 41. By doing so, the surface temperature of the fixing roll 41 is lowered. As described above, when the paper that has undergone the dummy run again enters the fixing device 40, the temperature is kept close to the control temperature.
Further, in the present embodiment, since the process speed and the temperature reduction amount of the fixing roll 41 are different for each sheet, the time during which the heater of the fixing roll 41 is turned off (region II in the figure) can be appropriately changed for each sheet. According to the set mode, the temperature of the fixing roll 41 can be kept better.

  In Example 1, the image forming apparatus according to Embodiment 1 is used, and in the control of the fixing temperature of the control device in the down mode, the time from when the sheet enters the fixing device until it is sent out again is the heating control time. The surface temperature of the fixing roll 41 when the temperature control and the time control were performed so that the heater was turned on as the heating control temperature when the surface temperature of the fixing roll fell to a certain temperature within the heating control time was measured. Is. The fixing temperature appropriate temperature setting process in this embodiment is as shown in the flowchart of FIG.

In this embodiment, the control temperature is set to 190 ° C., the heating control temperature is set to 185 ° C., and the heating control time is set to 5 seconds. That is, the time from when the paper enters the fixing device to when the fixing device is re-delivered after the dummy run is set to 5 seconds, and during that time, the heater is turned off while the surface temperature of the fixing roll is the heating control temperature 185. When the temperature falls to 0 ° C., the temperature of the heater is turned on.
FIG. 13 shows the surface temperature of the fixing roll in this embodiment.
In the figure, when the sheet enters the fixing device due to idling, the heater is turned off and the temperature is lowered. Then, when the surface temperature of the fixing roll decreases to a constant temperature (185 ° C.) for 5 seconds after entering the fixing device again after passing through the dummy run, the heater is turned on and the fixing roll is heated. According to this embodiment, the surface temperature of the fixing roll can be maintained at a temperature close to the control temperature of 190 ° C. while the sheet travels again in the fixing device for toner fixing after the dummy traveling.

Example 2 uses the image forming apparatus according to Embodiment 1 and the surface temperature of the fixing roll when the heating control temperature and the heating control time are variably set according to the paper size in the control of the fixing temperature of the control device in the down mode. Is measured.
In this example, while maintaining the control temperature at 185 ° C. for plain paper, the heating control temperature is set to 180 ° C. and the heating control time is set to 3.5 seconds for the A4 paper, and the heating control temperature is 175 ° C. for the A3 paper. The heating control time is set to 5 seconds.

FIG. 14 shows the surface temperature of the fixing roll in this example.
As shown in the figure, when comparing the A3 and A4 sheets, the temperature drop after the heater is turned off is larger for the A3 sheet. This is because the amount of temperature reduction increases with the size of the paper.
Here, for the A4 sheet, the heating control temperature is set to a high temperature and the heating control time is set to be short because the temperature reduction amount is small. Thus, by setting both values in consideration of the balance between the heating control temperature and the heating control time, the surface temperature of the fixing roll is set to an appropriate temperature when the paper enters the fixing device again and is fixed with toner. Can keep.
On the other hand, for the A3 sheet, the heating control temperature is set to a low temperature and the heating control time is set to be long as the fixing roll temperature reduction amount increases. As described above, the surface temperature of the fixing roll is maintained at an appropriate temperature during toner fixing by setting the A3 sheet in consideration of the balance between the heating control temperature and the heating control time.
Therefore, as shown in the figure, although the temperature curve differs depending on the paper size, if the heating control temperature and the heating control time are set according to each paper size, the toner temperature is closer to the control temperature at the time of toner fixing after the dummy run. It is possible to keep the temperature.

Example 3 uses the image forming apparatus according to Embodiment 1, and the surface of the fixing roll when the heating control temperature and the heating control time are variably set depending on the paper type in the control of the fixing temperature of the control device 80 in the down mode. The temperature is measured.
In this example, for plain paper, each process speed is set to 220 mmsec, the control temperature is set to 185 ° C., the down mode execution temperature is set to 195 ° C., the heating control temperature is set to 180 ° C., and the heating control time is set to 3.5 seconds. Yes. On the other hand, for cardboard, each process speed is set to 110 mmsec, the control temperature is set to 190 ° C., the down mode execution temperature is set to 200 ° C., the heating control temperature is set to 185 ° C., and the heating control time is set to 7 seconds.

FIG. 15 shows the surface temperature of the fixing roll in this embodiment.
As shown in the figure, when comparing plain paper and thick paper, the temperature drop after turning off the heater is larger for thick paper. This is because the temperature reduction amount increases as the basis weight of the paper type increases.
Here, for plain paper, the heating control temperature is set to a high temperature because the temperature reduction amount is small, and the heating control time is set short because the process speed is high. By setting both values in consideration of the balance between the heating control temperature and the heating control time in this way, the surface temperature of the fixing roll is kept at an appropriate temperature when the paper enters the fixing device again and is fixed with toner. be able to.
On the other hand, for thick paper, the heating control temperature is set to a low temperature because the temperature reduction amount is large, and the heating control time is set to be long because the process speed is slow. As described above, even for thick paper, by setting the balance between the heating control temperature and the heating control time, it is possible to keep the surface temperature of the fixing roll at an appropriate temperature during toner fixing.
As shown in the figure, although the temperature curve varies depending on the paper type, if the heating control temperature and heating control time are set according to each paper type, the temperature will be closer to the control temperature during toner fixing after the dummy run. It is possible to keep.
In the table below, each set value for the coated paper is shown as a reference value.

1 is an explanatory diagram showing an outline of an image forming apparatus according to the present invention. 1 is an explanatory diagram showing an embodiment of an image forming apparatus to which the present invention is applied. FIG. 2 is an explanatory diagram illustrating a fixing device according to a first embodiment. 3 is a block diagram showing a control system used in Embodiment 1. FIG. (A) (b) is explanatory drawing which shows the user interface used in Embodiment 1. FIG. 6 is an explanatory diagram showing a flowchart of image formation control processing according to Embodiment 1. FIG. 6 is a graph showing a difference in down mode execution temperature depending on the paper type in the image forming apparatus according to the first embodiment. 6 is a graph illustrating a difference in down mode execution temperature depending on an image forming mode in the image forming apparatus according to the first embodiment. (A) and (b) are explanatory drawings showing the operation in the down mode of the image forming apparatus according to the first embodiment. FIG. 6 is an explanatory diagram showing a flowchart of a fixing temperature appropriate temperature process according to the first embodiment. 4 is a graph showing the surface temperature of the fixing roll in the image forming apparatus according to Embodiment 1; FIG. 6 is an explanatory diagram illustrating a flowchart of fixing temperature appropriate temperature processing according to the first exemplary embodiment. 3 is a graph showing the surface temperature of the fixing roll of Example 1. 6 is a graph showing the surface temperature of the fixing roll of Example 2. 6 is a graph showing the surface temperature of the fixing roll of Example 3. 5 is a graph showing a relationship between a fixing temperature and an offset toner amount in a fixing device. 5 is a graph showing the surface temperature of a heat fixing member during overshoot in the fixing device.

Explanation of symbols

  DESCRIPTION OF SYMBOLS 1 ... Image carrier, 2 ... Recording material, 3 ... Image forming engine, 4 ... Fixing device, 5 ... Heat fixing member, 6 ... Bypass conveyance path, 7 ... Detection means, 8 ... Mode discrimination means, 9 ... Image formation control means

Claims (9)

An image forming engine for transferring an unfixed image formed on an image carrier onto a sheet, and a heating fixing member whose surface is heated to come into contact with the recording material on the recording material formed by the image forming engine In an image forming apparatus including a fixing device that heat-fixes an unfixed image,
A bypass conveyance path that is provided separately from the normal recording material conveyance path and returns the recording material to the front of the transfer portion of the image forming engine after passing through the fixing device;
Detection means for detecting the surface temperature of the heat fixing member of the fixing device;
It is determined whether or not the detected temperature by the detecting means is a condition that is equal to or higher than the down mode execution temperature that exceeds the allowable range of the set temperature. When the temperature is equal to or higher than the down mode execution temperature, the down mode is determined. Mode discriminating means for discriminating;
In the normal mode, the image forming process by the image forming engine and the heat fixing process by the fixing device are immediately executed on the recording material.
In down mode, the recording material is idled without performing the image forming process by the image forming engine, and after returning to the front of the image forming engine transfer section via the bypass conveyance path, the image forming process by the image forming engine is performed. And an image forming control means for performing heat fixing processing by the fixing device. The image forming apparatus according to claim 1.
The bypass conveyance path is an image forming apparatus configured with a recording material reversing return path that reverses a recording material that has been recorded on one side in the double-sided recording mode and then returns it to the front of the transfer portion of the image forming engine. The image forming apparatus according to claim 1.
An image forming apparatus, wherein the down mode execution temperature is variably set according to image forming conditions. The image forming apparatus according to claim 1.
An image forming apparatus, wherein the down mode execution temperature is variably set depending on a recording material. The image forming apparatus according to claim 1.
The image forming control means sets the heater of the heating and fixing member between the time when the recording material enters the fixing device due to idle running and before the image is transferred to the first surface via the bypass conveyance path and again enters the fixing device. An image forming apparatus which is turned off. The image forming apparatus according to claim 5.
The image forming control means controls the time from when the recording material enters the fixing device due to idle running to when the image is transferred to the first surface via the bypass conveyance path and again enters the fixing device. Image forming apparatus. The image forming apparatus according to claim 1.
The image forming control means uses the time from when the recording material enters the fixing device as it travels idle to the time when the image is transferred to the first surface via the bypass conveyance path and sent out from the fixing device again as the heating control time. An image forming apparatus, wherein a heater is turned on as a heating control temperature when the surface temperature of the heat fixing member decreases to a certain temperature during the heating control time. The image forming apparatus according to claim 7.
An image forming apparatus, wherein the heating control temperature and the heating control time are set according to each recording material size. The image forming apparatus according to claim 7.
An image forming apparatus, wherein the heating control temperature and the heating control time are set according to a paper type of a recording material.

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