JP4350664B2 - Setting method of fixing condition in fixing device - Google Patents

Description

  The present invention relates to a fixing device that heats and fixes an unfixed toner image on a recording sheet in an image forming apparatus such as a copying machine, a printer, or a facsimile machine.

  2. Description of the Related Art Conventionally, an electrophotographic image forming apparatus (printing apparatus) such as a copying apparatus, a printer, or a facsimile apparatus is provided with a photosensitive drum, a charging device, an exposure device, a developing device, a transfer device, a fixing device, and the like. .

  In such an image forming apparatus, the exposure surface exposes the photosensitive surface (electrostatic latent image surface) of the photosensitive drum charged by the charging device to form an electrostatic latent image. Then, the formed electrostatic latent image is developed with toner (developer) in a developing device to generate a toner image (visible image). Then, the toner image is set to be fixed by a fixing device after being transferred onto a recording paper (recording material; a printing medium such as plain paper or OHP recording paper) by a transfer device.

  The fixing device fixes toner onto a recording medium such as recording paper by heating or pressing. The fixing device is a roller pair type fixing in which fixing is performed by passing an unfixed toner image through a pressure contact area formed between a pair of heated rollers (between the fixing roller and the pressure roller). The device is common.

  As the fixing roller, for example, an elastic layer is formed on the surface of a hollow metal core made of metal such as aluminum, and a halogen lamp is disposed as a heating source inside the core metal, and the temperature provided on the surface of the fixing roller. A temperature control circuit (not shown) is controlled to turn on / off the halogen lamp based on the sensor signal, and the temperature of the fixing roller surface is kept constant.

  As the pressure roller, a roller provided with a heat-resistant elastic layer such as silicon rubber as a coating layer on a cored bar is used. When the pressure roller and the fixing roller are pressed against each other, the elastic layer is elastically deformed to form a predetermined nip region (a nip portion or a pressing portion). Then, fixing is performed by passing a recording sheet on which an unfixed toner image is formed in the nip region and thermally melting the toner image.

  In the roller pair type fixing device, the heat to be applied to the recording paper for fixing the toner image is determined by the thermal characteristics of the toner. In other words, the toner image cannot be fixed unless given heat is applied. For this reason, for example, when fixing at a higher speed, it is necessary to increase the nip width (the width of the nip region in the recording paper conveyance direction) according to the fixing speed.

  In order to increase the nip width, there are methods such as increasing the load (pressing force) between both rollers, increasing the thickness of the elastic body, and increasing the roller diameter. However, when the load between the rollers is increased, the rollers are bent, and the nip width is not formed uniformly in the axial direction (the rotational axis direction of the rollers), which affects the transportability and image quality. Increasing the thickness of the elastic body or increasing the roller diameter has no problem in image quality, but there is a problem that the warm-up time becomes longer as the apparatus becomes larger and the heat capacity increases.

  In order to solve these problems and cope with higher speeds, for example, Patent Document 1 proposes a fixing device in which a plurality of fixing nip portions are formed by using a plurality of pressure rollers. Yes. Further, Patent Document 2, Patent Document 3 and the like propose a fixing device in which a plurality of wide fixing nip portions are formed by using a plurality of pressure rollers and a pressure belt stretched over them. . Further, Patent Document 4, Patent Document 5, and the like propose a fixing device in which a plurality of wide fixing nip portions are formed by using a fixing belt suspended on a plurality of rollers.

In the case of these fixing devices, compared to the conventional roller pair type fixing device, a plurality of wide fixing nip portions are used to heat the unfixed toner image on the recording paper for a long time, thereby sufficiently It melts and supports high speed.
JP-A-3-267975 (Publication date: November 28, 1991) JP-A-5-158365 (Publication date: June 25, 1993) JP 10-307493 A (publication date: November 17, 1998) JP-A-9-138598 (Publication date: May 27, 1997) Japanese Patent Laying-Open No. 2003-76202 (Publication date: March 14, 2003)

  However, the above-described conventional technique has a problem that an image defect occurs when the process speed (fixing process speed) is increased.

  Actually, the inventors of the present invention conducted a fixing experiment using a fixing device having a plurality of fixing nips as described above. In any fixing device, when the process speed is increased, In particular, in a solid image portion (so-called solid image portion) on which a 100% toner image is printed, an image defect due to micro unevenness in density occurs.

  Further, when this image defect was analyzed in detail, it was found that the image defect was caused by a positional deviation (image deviation) of the toner image partially generated on the recording paper. In addition, when this positional deviation is severe, the underlying recording paper is exposed at the portion where the toner image is displaced, and this may cause a decrease in density (in the following description, these image defects will be referred to as image deviation). .)

  Further, in the above-described conventional technique, there is a problem that an image defect due to a high temperature offset (image defect such as a high temperature offset) occurs when the interval between the plurality of fixing nip portions is widened. That is, a fixing nip portion (first nip portion) formed most on the paper entering side by the heating member and the first pressing member (pressure member), and on the downstream side in the paper transport direction from the first nip portion. The width in the paper conveyance direction of the portion (virtual nip portion) where the pressing member is not in contact with the fixing nip portion (second nip portion) formed by the heating member and the second pressing member is increased. As a result, image defects due to high temperature offsets occur.

  The present invention has been made in view of the above-described conventional problems, and an object of the present invention is to increase the speed of a fixing device having a plurality of fixing nip portions without causing image misalignment or image defects due to high temperature offset. A fixing method and a fixing device are provided.

  In order to solve the above-described problem, the fixing device of the present invention provides a fixing member heated to a predetermined temperature and a plurality of pressing forces in the direction of the fixing member to a recording paper on which an unfixed toner image is formed. A fixing device that fixes the unfixed toner image by passing the recording paper through each fixing nip portion formed between the fixing member and each pressing member. A fixing temperature that is a temperature of a surface of the fixing member facing the recording paper and a first nip portion that is a fixing nip portion located on the most upstream side in the recording paper conveyance direction among the fixing nip portions are set as the unfixed toner. The first nip passage time, which is the time for the image to pass, is set to a condition in which the unfixed toner image is heated to a temperature at which no low temperature offset occurs. The recording paper is not limited to paper, but means any sheet-like print medium on which a toner image is formed, and includes, for example, an OHP sheet.

  When a plurality of fixing nip portions are formed by the fixing member and the plurality of pressing members, a first nip portion formed by the first pressing member located on the most upstream side when viewed from the sheet passing direction of the recording paper; The fixing nip formed on the downstream side is not formed, or even if the fixing nip is apparently formed, the nip is in a state where almost no fixing pressure is applied (virtual nip) Nip portion) is formed. Here, in the first nip portion, the fixing member, the toner, and the recording paper are in close contact with each other by the pressing member, but the region from the first nip portion to the downstream nip portion (virtual nip portion) Then, since there is no pressing member, a slight gap is generated between the fixing member and the recording paper. At this time, if the melting state of the toner at the first nip portion is insufficient, a part of the toner image adheres to the fixing member side to cause a so-called low temperature offset phenomenon, and after passing through the first nip portion, If a slight misalignment occurs between the fixing member and the recording paper in the area up to the nip (virtual nip), the image displacement will occur if the fixing member and the recording paper come into close contact again at the next nip. It will occur.

  On the other hand, according to the above configuration, the fixing temperature of the fixing member and the nip passage time (first nip passage time) of the first nip portion heat the unfixed toner image to a temperature at which no low temperature offset occurs. The condition is set to Accordingly, it is possible to prevent the low temperature offset after passing through the first nip portion, that is, unfixed toner from adhering to the fixing member after passing through the first nip portion. Therefore, for example, even when the process speed is increased, it is possible to reliably prevent the image shift.

Further, the fixing temperature and the first nip passage time are such that the temperature closest to the recording paper in the unfixed toner image is equal to or higher than the toner outflow start temperature of the unfixed toner image at the exit of the first nip portion. It may be set to be. Here, the toner outflow start temperature refers to a heat melting characteristic measuring device, for example, a flow tester (CFT-500 manufactured by Shimadzu Corporation), and the toner powder is set in the device, the die hole diameter: diameter 1 mm, height 1 mm, It is defined as a value at which a toner starts to melt and a piston that applies a load to the toner starts to descend when measured with a load of 20 kgf / cm ² and a constant temperature increase at 6 ° C./min. In the same measurement, the temperature when the piston drops by 4 mm is defined as the 4 mm drop temperature.

  According to the above configuration, the unfixed toner image is heated to a temperature equal to or higher than the toner outflow start temperature at the first nip portion. For this reason, it can prevent reliably that low temperature offset arises after 1st nip passage. Therefore, it is possible to prevent the occurrence of image shift.

  Further, when the fixing temperature and the first nip passage time are tn1 (second), the toner outflow start temperature is Ti (° C.), and the fixing temperature is T, tn1 ≧ You may set so that 0.0006Ti-0.00024T + 0.0015 may be satisfy | filled.

  According to the above configuration, the unfixed toner image can be heated to a temperature equal to or higher than the toner outflow start temperature at the first nip portion. For this reason, it is possible to prevent the occurrence of low-temperature offset after passing through the first nip, so that it is possible to prevent the occurrence of image shift. Further, by setting the fixing temperature and the first nip passage time at which the unfixed toner image is equal to or higher than the toner outflow start temperature using the above formula, the fixing temperature and the first nip passage time are not obtained by experiments or analysis. , Image displacement can be prevented appropriately.

  Further, the fixing member and the pressing member may be rollers that rotate about a direction substantially perpendicular to the recording paper conveyance direction as a rotation axis.

  As described above, by configuring the fixing member and the pressing member with rollers, a fixing device having a plurality of nip portions can be realized with a simple configuration.

  The pressing member that forms the first nip portion may be an elastic roller that has a core layer covered with an elastic layer and rotates about a direction substantially perpendicular to the recording sheet conveyance direction as a rotation axis.

  According to said structure, the nip width | variety of a 1st nip part can be set sufficiently wide by using the press member which forms a 1st nip part as an elastic roller. For this reason, it is possible to easily realize the fixing condition (nip passing time) required in the first nip portion.

  And a plurality of support rollers that rotate about a direction substantially perpendicular to the recording paper conveyance direction and a belt suspended from the support rollers, and the pressing members press the belt against the fixing member. Accordingly, the fixing nips may be formed between the belt and the fixing member.

  According to the above configuration, since the recording paper discharged from the first nip portion is sandwiched and conveyed between the belt and the fixing member, it is possible to prevent a conveyance failure from occurring. In other words, when there is no belt, there is nothing that regulates the conveyance direction of the recording paper between the first nip portion and the nip portion formed by the pressing member located on the downstream side thereof. For this reason, after the recording paper passes through the first nip portion, the conveyance direction is changed before the recording paper enters the nip portion formed by the pressing member located on the downstream side thereof, which may cause a conveyance failure. . However, by providing the belt as described above, the recording paper discharged from the first nip portion is conveyed while being sandwiched between the belt and the fixing member, so that it is possible to prevent a conveyance failure from occurring.

  Further, at least one of the plurality of pressing members may be a non-rotating member. By making the pressing member a non-rotating member, that is, a pressure pad, the nip width, which is the width in the sheet passing direction of the fixing nip portion formed by the pressing member, can be made wider without increasing the size of the fixing device. can do.

  The non-rotating member may form the first nip portion. Thereby, the nip width (first nip width) of the first nip portion can be increased, and the first nip passage time can be extended. For this reason, for example, even when the process speed is high, it is easy to set the first nip passage time and the fixing temperature to a condition where the unfixed toner image is heated to a temperature at which a low temperature offset does not occur.

  The pressing member that forms the first nip portion is a roller that rotates about a direction substantially perpendicular to the recording paper conveyance direction as a rotation axis, and is a fixing nip that is formed by the pressing member made of the non-rotating member. A rotating nip portion is formed downstream of the first nip portion in the recording paper conveyance direction, and an end portion of the first nip portion on the downstream side of the recording paper conveyance direction and a recording paper conveyance direction of the non-rotating nip portion. The distance from the upstream end may be 3 mm or less.

  According to the above configuration, the first nip portion and the non-rotating nip portion are continuously or close to each other, so that the first nip portion and the non-rotating nip portion are substantially continuous. Acts as For this reason, image shift can be effectively prevented.

  Further, a guide member that regulates the recording paper conveyance direction between a pressing member that forms the first nip portion and a pressing member that is disposed downstream of the pressing member in the recording paper conveyance direction. May be provided.

  According to the above configuration, the recording paper discharged from the first nip portion is conveyed to the next nip portion while the conveyance direction is regulated by the guide member, so that it is possible to prevent a conveyance failure from occurring.

  In order to solve the above problems, another fixing device of the present invention applies a pressing force in the direction of the fixing member to a fixing member heated to a predetermined temperature and a recording paper on which an unfixed toner image is formed. A fixing device that fixes the unfixed toner image by passing the recording paper through each fixing nip formed between the fixing member and each pressing member. A first nip portion which is a fixing nip portion located on the most upstream side in the recording paper conveyance direction among the fixing nip portions, and a second nip located on the downstream side in the recording paper conveyance direction from the first nip portion. And a virtual nip portion that is an area between the first nip portion and the second nip portion, and a first time during which the unfixed toner image passes through the first nip portion and the virtual nip portion.・ Virtual nip passage time and above fixing And the fixing temperature is the temperature of the opposing surface between the paper at wood is, is characterized in that the unfixed toner image is set to a condition that does not cause high-temperature offset.

  Alternatively, in any one of the fixing devices described above, in a region between the second nip portion located downstream of the first nip portion in the recording sheet conveyance direction and the first nip portion and the second nip portion. A first imaginary nip passage time which is a time for the unfixed toner image to pass through the first nip portion and the virtual nip portion, and a surface of the fixing member facing the recording paper. The fixing temperature, which is the temperature, may be configured so that the unfixed toner image does not cause a high temperature offset.

  When a plurality of fixing nip portions are formed by the fixing member and the plurality of pressing members, a first nip portion formed by the first pressing member located on the most upstream side when viewed from the sheet passing direction of the recording paper; Even if the fixing nip portion is not formed between the second nip portion, which is the fixing nip portion formed on the downstream side, or the fixing nip portion is apparently formed, the fixing pressure is almost equal. A nip portion (virtual nip portion) that is not applied is formed.

  In such a virtual nip portion, there may be a speed difference between the rotation speed (circumferential speed) of the fixing member and the conveyance speed of the recording paper. At this time, if the toner of the unfixed toner image is heated more than necessary, the viscosity of the toner heated more than necessary is reduced, so that the toner surface is divided and image defects due to high temperature offset occur. End up. In other words, if the toner is heated more than necessary at the virtual nip, the toner melts to a temperature at which high temperature offset occurs and the viscosity decreases, so that a slight misalignment occurs between the fixing member and the recording paper. In this case, the toner is divided, and an image defect due to a high temperature offset (image defect such as a high temperature offset) occurs.

  On the other hand, according to the above configuration, the first / virtual nip passage time, which is the time during which the unfixed toner image passes through the first nip portion and the virtual nip portion, and the recording paper on the fixing member, The fixing temperature, which is the temperature of the opposite surface, is set to a condition in which the unfixed toner image does not cause a high temperature offset. In this case, the unfixed toner image on the recording paper is heated to such an extent that no high temperature offset occurs while passing through the first nip portion and the virtual nip portion. As a result, even when a slight misalignment occurs between the fixing member and the recording paper, it is possible to prevent the toner from being divided, and thus it is possible to prevent image defects due to high temperature offset.

In this case, the fixing temperature and the first / virtual nip passage time are set such that the temperature closest to the fixing member in the unfixed toner image is 4 mm of the toner in the unfixed toner image at the exit of the virtual nip portion. You may set so that it may become below temperature fall. Here, the 4 mm drop temperature of the toner is set at a constant speed by setting the toner powder in the apparatus using a thermal melting characteristic measuring apparatus, die hole diameter: diameter 1 mm, height 1 mm, load 20 kgf / cm ² , 6 ° C./min. This is the temperature at which the melting of the toner starts and the piston that applies a load to the toner drops by 4 mm when measured by heating.

  According to the above configuration, the temperature of the unfixed toner image is heated at the first nip portion and the virtual nip portion to the extent that the temperature does not exceed the 4 mm drop temperature of the toner. For this reason, it can prevent reliably that high temperature offset arises in a virtual nip part. Therefore, occurrence of image defects due to high temperature offset can be prevented.

Further, the fixing temperature and the first / virtual nip passage time are represented by tn2 (seconds), the 4 mm drop temperature of the toner is T _{4 mm} (° C.), and the fixing temperature is T. In this case, it may be set to satisfy tn2 ≦ 0.0023T _{4 mm−} 0.002T + 0.2078.

  According to the above configuration, the unfixed toner image is heated at the first nip portion and the virtual nip portion to the extent that the temperature does not exceed the 4 mm drop temperature of the toner. For this reason, since it is possible to prevent the high temperature offset from occurring in the virtual nip portion, it is possible to prevent the occurrence of image defects due to the high temperature offset. Further, by using the above formula, the fixing temperature and the first virtual nip passage time are set by setting the fixing temperature and the first / virtual nip passage time at which the temperature of the unfixed toner image at the exit of the virtual nip portion is equal to or lower than the 4 mm drop temperature of toner -It is possible to appropriately prevent image defects due to high temperature offset without obtaining the virtual nip passage time through experiments and analysis.

  In order to solve the above problems, the fixing method of the present invention provides a fixing member heated to a predetermined temperature and a plurality of pressurizing forces in the direction of the fixing member to a recording paper on which an unfixed toner image is formed. A fixing method for fixing the unfixed toner image by passing the recording paper through each fixing nip portion formed between the pressing member and the unfixed toner image. The first nip portion, which is the fixing nip portion located on the most upstream side in the recording paper conveyance direction, is heated to a temperature at which no low temperature offset occurs.

  According to the above method, the unfixed toner image is heated to a temperature at which no low temperature offset occurs in the first nip portion. As a result, it is possible to reliably prevent image displacement.

  Further, the unfixed toner image may be heated to a toner outflow start temperature or higher at the first nip portion.

  According to the above method, since the unfixed toner image is heated to the toner outflow start temperature or higher in the first nip portion, it is possible to prevent a low temperature offset from occurring after passing through the first nip. Therefore, it is possible to prevent the occurrence of image shift.

  Further, the temperature of the surface of the unfixed toner image closest to the recording paper is the temperature of the contact surface of the fixing member with the recording paper for heating the first nip portion to a toner outflow start temperature or higher. The fixing temperature and the first nip passage time, which is the time for which the unfixed toner image passes through the first nip portion, may be calculated by theoretical calculation using the fixing member and the unfixed toner image as an analysis model. .

  According to the above method, the temperature on the recording paper side in the unfixed toner image (toner lowest layer temperature) is heated at the first nip portion to a temperature equal to or higher than the toner outflow start temperature and the first nip passage. The time is calculated by theoretical calculation using the fixing member and the unfixed toner image as an analysis model. Thereby, the fixing temperature and the first nip passage time for suppressing image misalignment can be set easily and appropriately. Even when it is difficult to actually measure the temperature of the lowest toner layer at the outlet of the first nip, the first calculation is performed by theoretical calculation (for example, one-dimensional heat transfer analysis) using the fixing member and the unfixed toner image as models. The toner bottom layer temperature at the nip exit can be calculated.

  The fixing temperature, which is the temperature of the contact surface of the fixing member with the recording paper, and the first nip passage time, which is the time for the unfixed toner image to pass through the first nip portion, are passed through the first nip. When the time is tn1 (seconds), the toner outflow start temperature is Ti (° C.), and the fixing temperature is T, it may be set to satisfy tn1 ≧ 0.0006Ti−0.00024T + 0.0015.

  According to the above method, the fixing temperature and the first nip passage time for heating the unfixed toner image to the temperature equal to or higher than the toner outflow start temperature in the first nip portion can be easily and appropriately obtained without obtaining by experiment or analysis. Can be calculated. For this reason, it is possible to prevent the occurrence of low-temperature offset after passing through the first nip, so that it is possible to prevent the occurrence of image shift.

  According to another fixing method of the present invention, in order to solve the above-described problems, a fixing member heated to a predetermined temperature and a pressing force in the direction of the fixing member are applied to a recording sheet on which an unfixed toner image is formed. A fixing device that fixes the unfixed toner image by passing the recording paper through each fixing nip formed between the fixing member and each pressing member. The fixing nip portion located on the most upstream side in the recording paper conveyance direction among the fixing nip portions is defined as a first nip portion, and the fixing nip portion located on the downstream side in the recording paper conveyance direction from the first nip portion. When the second nip portion is used and the region between the first nip portion and the second nip portion is a virtual nip portion, the time required for the unfixed toner image to pass through the first nip portion and the virtual nip portion. When passing a certain first virtual nip When, a fixing temperature which is the temperature of the surface facing the said paper at the fixing member, the unfixed toner image is characterized by setting a condition that does not cause high-temperature offset.

  Alternatively, in any one of the fixing methods described above, the fixing nip portion located downstream of the first nip portion in the recording paper conveyance direction is set as the second nip portion, and the first nip portion and the second nip portion are A first imaginary nip passage time, which is a time during which the unfixed toner image passes through the first nip portion and the virtual nip portion, and the recording paper on the fixing member, when the area between them is a virtual nip portion. The fixing temperature, which is the temperature of the opposite surface, may be set to a condition in which the unfixed toner image does not cause a high temperature offset.

  According to the above method, the first and virtual nip passage time, which is the time during which the unfixed toner image passes through the first nip portion and the virtual nip portion, and the temperature of the surface of the fixing member facing the recording paper. Is set to a condition in which the unfixed toner image does not cause a high temperature offset. In this case, the unfixed toner image on the recording paper is heated to such an extent that no high temperature offset occurs while passing through the first nip portion and the virtual nip portion. As a result, even when a slight misalignment occurs between the fixing member and the recording paper, it is possible to prevent the toner from being divided, and thus it is possible to prevent image defects due to high temperature offset.

  Further, the fixing temperature and the first / virtual nip passage time are determined so that the temperature closest to the fixing member in the unfixed toner image is 4 mm lower than the toner in the unfixed toner image at the exit of the virtual nip portion. You may set so that it may become below temperature.

  In this case, the temperature of the unfixed toner image is heated in the first nip portion and the virtual nip portion to the extent that the temperature does not exceed the 4 mm drop temperature of the toner. For this reason, it can prevent reliably that high temperature offset arises in a virtual nip part. Therefore, occurrence of image defects due to high temperature offset can be prevented.

  Further, the fixing temperature and the temperature are set so that the temperature closest to the fixing member in the unfixed toner image is equal to or lower than the 4 mm drop temperature of the toner in the unfixed toner image at the exit of the virtual nip portion. The first / virtual nip passage time may be calculated by theoretical calculation using the fixing member and the unfixed toner image as an analysis model.

  According to the above method, the fixing temperature and the first / virtual nip passage time for preventing image defects due to high temperature offset can be set easily and appropriately. Even when it is difficult to actually measure the toner uppermost layer temperature at the virtual nip portion outlet, the virtual nip portion is calculated by theoretical calculation (for example, one-dimensional heat transfer analysis) using the fixing member and the unfixed toner image as models. The toner top layer temperature at the outlet can be calculated.

Further, the fixing temperature and the first / virtual nip passage time, the first / virtual nip passage time as tn2 (seconds), the 4 mm drop temperature of the toner as T _{4 mm} (° C.), and the fixing temperature as T, respectively. In this case, tn2 ≦ 0.0023T _{4 mm−} 0.002T + 0.2078 may be satisfied.

  According to the above method, the fixing temperature and the first / virtual nip passage time for heating the unfixed toner image in the first nip portion and the virtual nip portion within a range where the toner drop temperature is 4 mm or less are tested and analyzed. Can be calculated easily and appropriately. For this reason, since it is possible to prevent high temperature offset from occurring in the first nip portion and the virtual nip portion, it is possible to prevent occurrence of image defects due to high temperature offset.

  The image forming apparatus of the present invention includes any one of the fixing devices described above. For this reason, it is possible to reliably prevent the occurrence of image shift.

  As described above, the fixing device of the present invention has a fixing temperature which is the temperature of the surface of the fixing member facing the recording paper, and the fixing located on the most upstream side in the recording paper conveyance direction among the fixing nip portions. The nip passage time, which is the time during which the unfixed toner image passes through the first nip portion, which is the nip portion, is set to a condition where the unfixed toner image is heated to a temperature at which no low temperature offset occurs.

  Therefore, the low temperature offset after passing through the first nip portion, that is, the unfixed toner can be prevented from adhering to the fixing member after passing through the first nip portion, so that it is possible to reliably prevent image misalignment. it can.

  Another fixing device of the present invention includes a first nip portion which is a fixing nip portion located on the most upstream side in the recording paper conveyance direction among the fixing nip portions, and a recording paper conveyance direction from the first nip portion. A second nip portion located on the downstream side, and a virtual nip portion that is an area between the first nip portion and the second nip portion, wherein the first nip portion and the virtual nip portion are defined as the unfixed toner image. The first and virtual nip passage time, which is the time for which the toner passes, and the fixing temperature, which is the temperature of the surface of the fixing member facing the recording paper, are set under conditions that do not cause a high temperature offset in the unfixed toner image. ing.

  Therefore, the unfixed toner image on the recording paper is heated to such an extent that no high temperature offset occurs while passing through the first nip portion and the virtual nip portion. As a result, even when a slight misalignment occurs between the fixing member and the recording paper, it is possible to prevent the toner from being divided, and thus it is possible to prevent image defects due to high temperature offset.

  In the fixing method of the present invention, the unfixed toner image is heated to a temperature at which a low temperature offset does not occur in the first nip portion, which is the fixing nip portion located on the most upstream side in the recording paper conveyance direction among the fixing nip portions. Heat.

  Therefore, the low temperature offset after passing through the first nip portion, that is, the unfixed toner can be prevented from adhering to the fixing member after passing through the first nip portion, so that it is possible to reliably prevent image misalignment. it can.

  According to another fixing method of the present invention, the fixing nip portion located on the most upstream side in the recording paper transport direction among the fixing nip portions is defined as a first nip portion, and downstream of the first nip portion in the recording paper transport direction. When the fixing nip portion located on the side is the second nip portion, and the region between the first nip portion and the second nip portion is the virtual nip portion, the first nip portion and the virtual nip portion are determined to be indefinite. The condition that the unfixed toner image does not cause a high temperature offset is determined by the first / virtual nip passage time, which is the time for the contacted toner image to pass, and the fixing temperature, which is the temperature of the fixing member facing the recording paper. Set to.

  Therefore, the unfixed toner image on the recording paper is heated to such an extent that no high temperature offset occurs while passing through the first nip portion and the virtual nip portion. As a result, even when a slight misalignment occurs between the fixing member and the recording paper, it is possible to prevent the toner from being divided, and thus it is possible to prevent image defects due to high temperature offset.

  The image forming apparatus of the present invention includes any one of the fixing devices described above. For this reason, it is possible to reliably prevent the occurrence of image shift.

Embodiment 1
An embodiment of the present invention will be described with reference to the drawings.

(Image forming device)
FIG. 2 is a cross-sectional view showing the internal structure of the image forming apparatus provided with the fixing device according to the present embodiment. This image forming apparatus is a dry electrophotographic color image forming apparatus. For example, based on image data transmitted from a terminal device connected via a network, the image forming apparatus is multicolored or monochromatic on a predetermined recording sheet. Images are formed.

  As shown in FIG. 2, the image forming apparatus includes a visible image forming unit 10, a recording paper transport unit 30, a fixing device (heating device) 40, and a supply tray 20.

  The visible image forming unit 10 includes four visible image forming units 10Y, 10M, 10C, and 10B corresponding to the colors of yellow (Y), magenta (M), cyan (C), and black (B). It is installed side by side. That is, the visible image forming unit 10 includes four visible image forming units 10Y, 10M, 10C, and 10B. The visible image forming unit 10Y forms an image using yellow (Y) toner, and the visible image is formed. The forming unit 10M forms an image using magenta (M) toner, the visible image forming unit 10C forms an image using cyan (C) toner, and the visible image forming unit 10B is black (B). An image is formed using the toner. Specifically, four sets of visible image forming units 10Y, 10M, 10C, and 10B are arranged along a recording paper conveyance path that connects the supply tray 20 that feeds the recording paper and the fixing device 40. The so-called tandem type.

  The visible image forming units 10Y, 10M, 10C, and 10B have substantially the same configuration. That is, the photosensitive drum 11, the charger 12, the laser beam irradiation means 13, the developing device 14, the transfer roller 15, and the cleaner unit 16 are respectively provided, and each color toner is multiplex-transferred onto the conveyed recording paper. .

  The charger 12 uniformly charges the surface of the photosensitive drum 11 to a predetermined potential. The laser beam irradiation means 13 exposes the surface of the photosensitive drum 11 charged by the charger 12 according to the image data input to the image forming apparatus, and forms an electrostatic latent image on the surface of the photosensitive drum 11. Is. The developing device 14 visualizes the electrostatic latent image formed on the surface of the photosensitive drum 11 with toner of each color. The transfer roller 15 is applied with a bias voltage having a polarity opposite to that of the toner, and transfers the toner image formed on the photosensitive drum 11 onto the recording paper conveyed by the recording paper conveying means 30. The cleaner unit 16 removes and collects the toner remaining on the surface of the photosensitive drum 11 after the developing process by the developing device 14 and the transfer process by the transfer roller 15. The surface of the photosensitive drum 11 is charged by the charger 12, an electrostatic latent image is formed on the charged surface by the laser light irradiation unit 13, and the formed electrostatic latent image is developed by the developing device 14. The transferred toner image is transferred onto the recording paper by the transfer roller 15, and the toner image remaining on the surface after the transfer is removed and collected by the cleaner unit 16.

  Then, the transfer of the toner image onto the recording paper is performed in each color visible image forming unit. That is, since the toner image is transferred to the recording paper for the four colors, the same process is repeated four times.

  The recording paper conveying means 30 includes a driving roller 31, an idling roller 32, and a conveying belt 33, and conveys the recording paper so that the visible image forming unit 10 forms a toner image on the recording paper. The drive roller 31 and the idling roller 32 stretch the endless transport belt 33. The drive roller 31 is rotated by being controlled at a predetermined peripheral speed, thereby rotating the endless transport belt 33. Yes. The conveyance belt 33 generates static electricity on the outer surface, and conveys the recording paper while electrostatically adsorbing the recording paper.

  In this way, the recording sheet is transferred to the toner belt (unfixed toner image) while being transported to the transport belt 33, and then peeled off from the transport belt 33 by the curvature of the drive roller 31 and transported to the fixing device 40. . The fixing device 40 applies moderate heat and pressure to the recording paper, dissolves the toner transferred onto the recording paper, and fixes the toner on the recording paper, thereby forming a robust image on the recording paper.

(Fixing device)
Next, the fixing device 40 will be described with reference to FIG. FIG. 3 is a cross-sectional view illustrating a schematic configuration of the fixing device 40.

  As shown in this figure, the fixing device 40 includes a fixing roller 41, a plurality of pressing members (a pressure roller 42 and a peeling roller 43), and a belt 44.

  In the fixing roller 41, an elastic layer 41b and a release layer 41c are formed in this order on a cored bar 41a, and a halogen lamp (not shown) is provided inside the cored bar as a heating source (heating means). Is installed.

  The core metal 41a is made of hollow aluminum having an outer diameter of 47 mm. However, the material of the cored bar 41a is not limited to this, and may be made of, for example, iron metal. Further, the size of the cored bar 41a is not particularly limited.

  The elastic layer 41b is made of silicon rubber having a heat resistance of 1.5 mm in thickness. The release layer 41c is made of a PFA (copolymer of tetrafluoroethylene and perfluoroalkyl vinyl ether) tube having a thickness of about 30 μm. The material of the release layer 41c is not limited to PFA as long as it has excellent heat resistance and durability and excellent release properties from the toner, and other fluorine-based materials such as PTFE (polytetrafluoroethylene), for example. Materials may be used. The fixing roller 41 according to the present embodiment has an outer diameter of about 50 mm and a surface hardness of 75 ° (Asker C hardness).

  The pressure roller 42 is formed by laminating an elastic layer 42b and a release layer 42c on a cored bar 42a. The pressure roller 42 applies a pressing force toward the fixing roller 41 toward the fixing roller 41 by an elastic member (spring or the like) (not shown) to press the belt 44 against the fixing roller (heating roller) 41. A fixing nip portion (first nip portion) is formed between the fixing roller 41 and the fixing roller 41 via a belt 44. As a result, when the recording paper is passed through the first nip portion, the recording paper is pressed against the fixing roller 41 so that heat can be transmitted to the toner on the recording paper satisfactorily.

  Here, the core metal 42a of the pressure roller 42 is made of iron and has an outer diameter of 13 mm. However, the material and size are not limited to this, and are made of, for example, stainless steel or aluminum. May be.

  The elastic layer 42b of the pressure roller 42 is for uniformly pressing the recording paper and sufficiently transferring the heat of the fixing roller 41 to the recording paper to melt the toner on the recording paper. 8.5 mm heat-resistant silicone sponge rubber is used. However, the material and thickness are not particularly limited as long as such a function is achieved.

  Similar to the release layer 41c of the fixing roller 41, the release layer 42c of the pressure roller 42 is provided for the purpose of preventing adhesion of toner, and is made of, for example, PFA or PTFE. Note that the release layer 42c is not necessarily provided.

  Each layer of the pressure roller 42 is firmly adhered, and the outer diameter of the pressure roller 42 is 30.0 mm. Further, the nip width (the length in the recording paper conveyance direction) of the first nip portion formed by pressing the belt 44 against the fixing roller 41 by the pressing force of the pressure roller 42 changes the load of the elastic member. Can be adjusted.

  The peeling roller 43 presses the toner image on the recording paper heated at the first nip portion against the fixing roller 41 again via the belt 44, thereby further heating the toner image to sufficiently fix the toner image. At the same time as ensuring the strength, the recording paper is peeled off from the surface of the fixing roller 41, and a release core is coated on a stainless steel core. Note that the release layer is not necessarily provided. Similarly to the pressure roller 42, the peeling roller 43 applies a pressing force in the direction of the fixing roller 41 to the belt 44 by an unillustrated elastic member (spring), thereby pressing the belt 44 against the fixing roller 41 and fixing roller 41. A fixing nip portion (second nip portion) is formed between the belt 44 and the belt 44.

  In the present embodiment, a peeling roller 43 having an outer diameter of 15 mm is used. Moreover, you may provide an elastic layer between a metal core and a mold release layer. Note that the surface hardness of the peeling roller is preferably higher than the surface hardness of the elastic layer 41 b of the fixing roller 41. This is because the nip shape in the second nip portion formed by press-contacting the fixing roller 41 and the belt 44 can be made downward, so that when the color image is fixed, self-peeling (forced peeling assisting means such as a peeling nail) This is because it can be easily peeled off with a paper scraper without the necessity of peeling, and can be peeled off without relying on the peeling nails as much as possible. On the other hand, if the surface hardness of the peeling roller 43 is lower than the surface hardness of the elastic layer 41b of the fixing roller 41, the nip shape of the second nip portion is upward, so that the paper discharged from the second nip portion is the fixing roller. 41 is discharged along the surface. For this reason, sufficient self-peeling performance cannot be obtained.

  The belt 44 includes a belt base material and a release layer (both not shown), and is suspended (stretched) by the pressure roller 42 and the peeling roller 43. Similar to the release layer 41c of the fixing roller 41, the release layer of the belt 44 is provided for the purpose of preventing toner adhesion, and is made of, for example, PFA or PTFE. Further, the material of the belt base material is not particularly limited, but is made of, for example, PI (polyimide). The surface hardness of the belt 44 is preferably higher than the surface hardness of the elastic layer 41 b of the fixing roller 41, as with the peeling roller 43.

  The pressure roller 42 and the peeling roller 43 press the belt 44 against the surface of the fixing roller 41 by a spring pressure unit (not shown). As a result, as shown in FIG. 1, the first nip portion (fixing nip portion) where the fixing roller 41 and the belt 44 abut (pressure contact), and the second nip portion where the fixing roller 41 and the belt 44 abut (pressure contact). A virtual nip portion (fixing nip portion) that is a region where no pressure contact means (pressing member) is provided between the nip portion (fixing nip portion) and the first nip portion and the second nip portion is formed. A region from the front end of the first nip portion to the rear end of the second nip portion is defined as a total nip portion. The first nip portion is a fixing nip portion that is formed first from the paper entry side, and the second nip portion is formed on the paper discharge side relative to the first nip portion.

  In the fixing device 40 according to this embodiment, the nip width of the first nip portion formed by the pressure roller 42, the belt 44, and the fixing roller 41 is 7.3 mm, and the peeling roller 43, the belt 44, and the fixing roller The nip width of the second nip portion formed by 41 is 3 mm, and the nip width of the virtual nip portion formed between the first nip portion and the second nip portion is 9 mm. Therefore, the total nip width is 19.3 mm.

  In the virtual nip portion, the fixing roller 41 and the belt 44 are in contact with each other. For this reason, the conveyance failure of the recording paper can be prevented. In addition, although there is no pressing member in the virtual nip portion, since the fixing roller 41 and the belt 44 are in contact with each other, the heat of the fixing roller 41 can be appropriately transferred to the recording paper. Can be improved.

  In the fixing device 40, the surface of the fixing nip is not yet determined in each of the fixing nips, that is, the region sandwiched between the fixing roller 41 heated to a constant temperature by a heating unit (not shown) and the pressing member (the pressure roller 42 and the peeling roller 43). By passing the recording paper (heated material) P to which the adhered toner adheres, the toner is melted and pressed against the recording paper to fix the image on the recording paper. Note that after fixing, the toner is welded to the recording paper, and gloss is obtained.

(Experimental example 1: Image displacement evaluation experimental example)
Using the above-described fixing device 40, the fixing conditions such as the type of toner, the fixing temperature, and the nip passage time are changed, and other conditions are kept constant, and a recording paper (unfixed image sample) on which an unfixed image is placed is prepared. An experiment was conducted to pass the paper and evaluate the image shift of the fixed sample.

  As an unfixed image sample, an unfixed image formed on a recording paper using two kinds of color oilless toners (toner A: polyester resin, toner B: styrene acrylic resin) having different thermal characteristics was used.

As a result of measuring the thermal characteristics of the toner with a flow tester (fluidity evaluation apparatus), the outflow start temperature Ti of the toner A was 99 ° C., and the 4 mm drop temperature T _{4 mm} was 120 ° C. Toner B outflow start temperature Ti = 91 ° C., 4 mm drop temperature T _{4 mm} = 115 ° C.

The toner particle diameter is 6.5 μm for both toners A and B, and the amount of both toners adhered to the recording paper per unit area is 0.5 mg / cm ² . As the recording paper, 65 g of plain paper was used. The process speed was 350 mm / s.

  First, image deviation evaluation for each of toner A and toner B is performed using the surface temperature (fixing temperature) of the fixing roller 41 facing the pressure roller 42 and the nip width (first nip width) of the first nip portion as parameters. The experiment was conducted. More specifically, for each of toners A and B, the first nip width is changed in three stages of 5.6 mm, 6.4 mm, and 7.3 mm, and the fixing temperature is 160 ° C. for each first nip width. An evaluation test was performed by changing the temperature to 170 ° C and 180 ° C. The fixing temperature was measured by a temperature sensor (not shown) provided on the surface of the fixing roller 41. Further, the nip width (second nip width) of the second nip portion was constant at 3 mm in both cases of the toners A and B. The process speed was fixed at 350 mm / s.

  The results of an image shift evaluation experiment with toner A are shown in Table 1 below, and the results of an image shift evaluation experiment with toner B are shown in Table 2 below. The evaluation of image shift was performed by measuring the density of the sample after passing the unfixed image sample through the fixing device 40 and fixing it, and evaluating the image quality subjectively. More specifically, a sample fixed by a conventionally used roller pair type fixing device was prepared as a comparative example (standard sample), and a relative evaluation with respect to the standard sample was performed. In Tables 1 and 2, “○” if there is no problem in image density and image quality, there is no problem in image density, but the image quality is slightly inferior (there is no problem on the product, but when enlarged and observed, the comparison example The case where the density unevenness is present is indicated by “Δ”, and the case where the image density is lowered and the image quality is inferior (the image shift is clearly noticeable as compared with the comparative example) is indicated by [×].

  As shown in Table 1, in the toner A, when the first nip width was 5.6 mm, the evaluation was “x” at the fixing temperatures of 160 ° C. and 170 ° C., and “Δ” at the fixing temperature of 180 ° C. When the first nip width was 6.4 mm, the evaluation was “x” at a fixing temperature of 160 ° C., “Δ” at a fixing temperature of 170 ° C., and “◯” at a fixing temperature of 180 ° C. When the first nip width was 7.3 mm, the evaluation was “Δ” at a fixing temperature of 160 ° C., and “◯” at a fixing temperature of 170 ° C. and 180 ° C.

  On the other hand, as shown in Table 2, when the first nip width was set to 5.6 mm, toner B was evaluated as “Δ” at a fixing temperature of 160 ° C., and “◯” at fixing temperatures of 170 ° C. and 180 ° C. It was. When the first nip width was 6.4 mm and 7.3 mm, the evaluation was “◯” at any fixing temperature of 160 ° C. to 180 ° C.

  From this result, it can be seen that toner A is more likely to cause image displacement than toner B. It can also be seen that the image shift improves as the fixing temperature is higher and the first nip width is wider.

  Next, an experiment similar to the above was performed for the toner A in which image misalignment is likely to occur, with the first nip width fixed at 5.6 mm and the second nip width as a parameter instead. That is, as shown in Table 3 below, the second nip width is changed in three stages of 3 mm, 3.8 mm, and 4.6 mm, and the fixing temperature is 160 ° C., 170 ° C., and 180 ° C. for each second nip width. It was changed in three stages. The results are shown in Table 3. In this table, “◯”, “Δ”, and “×” represent evaluation results using the same evaluation criteria as in Tables 1 and 2.

  As shown in this table, when the second nip width is 3 mm, 3.8 mm, or 4.6 mm, the evaluation is “x” at the fixing temperatures of 160 ° C. and 170 ° C. The evaluation was “△”.

  From this result, it can be seen that even if the second nip width is widened, the image shift is not improved so much. Considering the results of Table 1 together, it is presumed that the main cause of image shift is not closely related to the second nip portion but closely related to the first nip portion.

  Therefore, in order to examine the cause of the image displacement in more detail, the peeling roller 43 and the belt 44 are removed from the fixing device 40 shown in FIG. 3 to obtain the same configuration (model) as that of the conventional roller pair type fixing device. The fixed image after passing through one nip portion was examined. More specifically, the peeling roller 43 and the belt 44 are removed from the fixing device 40, the fixing temperature is 160 ° C., 170 ° C., 180 ° C., the first nip width is 5.6 mm, 6 under the process speed of 350 mm / s. The unfixed image samples were passed through three stages of .4 mm and 7.3 mm, and the fixability was evaluated by a bending test. In this configuration, since the self-peeling property cannot be secured, oil is applied to the surface of the fixing roller 41 to secure the self-peeling property.

  As a method for evaluating the fixability, after lightly bending the fixed sample so that the printed surface is on the inside, a predetermined load is applied to the bent portion, and the width of the toner drop-off portion generated at the fold portion is measured. Based on the evaluation.

  The evaluation results are shown in Table 4. In this table, “×” is used when the low temperature offset occurs entirely, “△” is used when the low temperature offset is partially generated, and “◯” is used when the low temperature offset is not generated. . When the fixing strength is insufficient, “NG” is indicated, and when the fixing strength is sufficient, “OK” is indicated.

  As shown in Table 4, regarding the low temperature offset, when the first nip width was 5.6 mm, the evaluation was “x” at a fixing temperature of 160 ° C. and 170 ° C., and “Δ” at a fixing temperature of 180 ° C. . When the first nip width was 6.4 mm, the evaluation was “x” at a fixing temperature of 160 ° C., “Δ” at a fixing temperature of 170 ° C., and “◯” at a fixing temperature of 180 ° C. When the first nip width was 7.3 mm, the evaluation was “Δ” at a fixing temperature of 160 ° C., and “◯” at a fixing temperature of 170 ° C. and 180 ° C. Further, the fixing strength was insufficient (NG) in any measured case.

  From this result, although the low temperature offset occurs when the fixing temperature is low and the first nip width is narrow, the low temperature offset can be improved by increasing the fixing temperature or widening the first nip width. Recognize. It can also be seen that the fixing strength is insufficient (NG) even under conditions where low temperature offset does not occur. Further, comparing the results of Tables 1 and 4, it can be seen that the conditions for causing the image shift and the conditions for the low temperature offset completely coincide.

  From the above results, the cause of the image shift can be explained as follows. That is, a plurality of fixing nip portions (here, the first nip portion and the second nip portion) are formed on the fixing member (here, the fixing roller 41) by a plurality of pressing members (here, the pressure roller 42 and the peeling roller 43). In the case of the fixing device, a nip portion (virtual nip portion) in which almost no fixing pressure is applied is formed between the first nip portion and the second nip portion. That is, in the case where the belt 44 is not provided, there is no means for applying a fixing pressure, so that a fixing nip is not formed, and the belt (pressure belt) 44 is apparently fixed as in the fixing device 40 according to the present embodiment. Even if the nip is formed, only the pressure due to the tension of the belt 44 is applied, so that a nip portion (virtual nip portion) in which almost no fixing pressure is applied is formed.

  Accordingly, the toner image on the recording paper is in contact with the fixing member (fixing roller 41), the toner, and the recording paper by the pressing member at the first nip portion, but after passing through the first nip portion, the toner image on the second nip portion. Since there is no pressing member in the entire virtual nip portion, a slight gap is generated between the fixing roller 41 and the recording paper. At this time, if the melting state of the toner in the first nip portion is insufficient, a part of the toner image adheres to the fixing roller 41 side due to the low temperature offset phenomenon. Thereafter, when a slight misalignment occurs between the fixing roller 41 and the recording paper at the virtual nip portion, the fixing roller 41 and the recording paper are adhered to the fixing roller 41 again at the second nip portion. The position of the toner image with respect to the recording paper is displaced, and image displacement occurs.

  In other words, in order to prevent image misalignment in a fixing device having a plurality of fixing nip portions as shown in this embodiment, an unfixed image sample has a toner image that does not cause a low-temperature offset immediately after passing through the first nip portion. It can be seen that it is necessary to set the fixing conditions at the first nip portion so that is melted by heating.

  Note that the fixing conditions set at the first nip portion do not necessarily satisfy the fixing strength. In other words, the image sample immediately after passing through the first nip portion has insufficient fixing strength, but it is sufficient if heat is supplied while passing through the virtual nip portion and the second nip portion to satisfy the fixing strength.

(Analysis example 1)
Next, in order to examine the relationship between the toner temperature at the first nip exit and the image shift, the fixing device 40 shown in FIG. 3 is modeled as one-dimensional heat conduction as shown in FIG. 4 to analyze the heat transfer state. Went. That is, as shown in FIG. 4, the heating source Q provided inside the cored bar 41a of the fixing roller 41, the cored bar (aluminum) 41a of the fixing roller 41, the elastic layer (silicone rubber) 41b, the release layer (PFA). 41c, a first air layer (air layer) a1 formed between the release layer 41c and the toner on the recording paper, the toner, the recording paper, formed between the recording paper and the release layer 44a of the belt 44. A model in which a belt base material 44b composed of a second air layer (air layer) a2, a release layer 44a, and PI is arranged in this order is created, and a cross-sectional depth direction of the fixing roller 41 (pressed from the heating source Q) The heat transfer in the direction of the roller 42) was analyzed.

  In the above model, the air layers a1 and a2 are provided in the boundary region between the fixing roller 41 and the toner surface and in the boundary region between the release layer 44a of the belt 44 and the recording paper. This is because the surface of the recording paper has a certain degree of surface roughness, and the gap caused by the surface roughness is modeled as air layers a1 and a2. Also, considering the microscopic behavior, the temperature distribution is different among the layers, so that the layers of the elastic layer 41b, the release layer 41c, the toner, and the recording paper of the fixing roller 41 are finely divided in the heat transfer direction. Using this model, the lowermost layer temperature, which is the temperature of the layer (side) in contact with the recording paper, of the toner of the unfixed image (unfixed toner image) printed on the recording paper immediately before the first nip exit was calculated. .

  As analysis conditions, the thickness, initial temperature, specific heat, specific gravity, thermal conductivity, and other necessary physical properties of each layer shown in Table 5 were given. Regarding the elastic layer 41b of the fixing roller 41, the elastic layer 41b is divided into 15 layers. The initial temperature of each of the divided layers is 160 ° C. for the layer in contact with the release layer 41c and 188 ° C. for the layer in contact with the cored bar 41a. It was. Moreover, the initial temperature of the layer during the division was set to a value increased by 2 ° C. from the layer close to the release layer 41c.

  The analysis conditions shown in Table 5 are the settings (conditions) when the fixing temperature is 160 ° C. When calculating the fixing temperature at 170 ° C., the layers set by the fixing roller 41 (release layer 41c, elastic layer 41b). The initial temperature of the cored bar 41a) is increased by 10 ° C. Similarly, when calculating at a fixing temperature of 180 ° C., the temperature of each layer of the fixing roller 41 is further increased by 10 ° C.

  The calculation formula used for the analysis will be described. As shown in FIG. 5, in the layers adjacent in order of the a layer, the b layer, and the c layer, the temperature T (b, t + τ) after τ seconds from a certain time t of the b layer is expressed by the following equation.

T (b, t + τ) = T (b, t) + τ / (ρ _b × C _b × H _b ) × [{T (a, t) −T (b, t)} / (h _a / 2λ _a + h _{b / 2λ b) + {T} (c, t) -T (b, t)} / (h b / 2λ b + h c / 2λ c)] ··· equation (1)
Here, h is the layer thickness, T is the temperature, λ is the thermal conductivity, ρ is the specific gravity, and C is the specific heat. The subscripts a, b, and c are the values of the a layer, b layer, and c layer, respectively. It is shown that.

  Using this equation, the temperature of each layer after a minute time τ has been calculated, and the calculation is repeated until the time immediately before the completion of the first nip passage (the value obtained by dividing the nip width by the process speed).

  Table 6 shows the results of calculating the toner bottom layer temperature at the first nip outlet under the above analysis conditions using the fixing temperature and the first nip width as parameters.

  As shown in this table, when the first nip width was 5.6 mm, the lowest toner layer temperature was 87 ° C. at a fixing temperature of 160 ° C., 91 ° C. at a fixing temperature of 170 ° C., and 95 ° C. at 180 ° C. When the first nip width was 6.4 mm, the fixing temperature was 91 ° C. at 160 ° C., the fixing temperature was 170 ° C., 95 ° C., and 180 ° C. was 99 ° C. When the first nip width was 7.3 mm, it was 95 ° C. at a fixing temperature of 160 ° C., 99 ° C. at a fixing temperature of 170 ° C., and 103 ° C. at 180 ° C.

  From the results of Tables 1 and 6, it can be seen that when the toner A lowermost layer temperature at the first nip outlet reaches 99 ° C. or more in the toner A, a good image without image displacement is obtained. That is, it is understood from the thermal analysis that when the temperature of the lowermost layer of the toner at the first nip outlet reaches or exceeds the outflow start temperature of toner A (99 ° C.), no image shift has occurred.

  Similarly, from the results of Tables 2 and 6, when the toner bottom layer temperature at the first nip outlet reaches 91 ° C. or higher in the toner B, a good image without image displacement can be obtained. In other words, even if the physical property value of the toner is changed, the image shift can be improved by setting the fixing conditions (fixing temperature, first nip width) so as to be equal to or higher than the toner outflow start temperature in the first nip portion. it can.

  Incidentally, the amount of heat given to the toner in the first nip portion is determined by the fixing temperature and the nip passage time (nip passage time = nip width / process speed) calculated from the relationship between the first nip width and the process speed. Accordingly, if the nip passage time (here, the nip passage time at the first nip portion) is set so that the toner bottom layer temperature becomes equal to or higher than the toner outflow start temperature at a certain fixing temperature, the image shift is improved. It is also considered.

  Here, the toner A was used to evaluate image misregistration using the process speed and the first nip width as parameters at a fixed fixing temperature (170 ° C.). Further, the toner bottom layer temperature just before the completion of the first nip passage was calculated by thermal analysis. The results are shown in Table 7. Note that the evaluations “◯”, “Δ”, and “×” of image shift are based on the same evaluation criteria as in Tables 1 and 2.

  As shown in Table 7, the image shift has a correlation with the toner bottom layer temperature just before the completion of the first nip passage. The toner bottom layer temperature immediately before the completion of the first nip passage has a correlation with the first nip passage time. Further, it can be seen that image shift does not occur if the first nip passage time can be ensured such that the lowest temperature of the toner is equal to or higher than the outflow start temperature (99 ° C. for toner A).

  Note that the first nip passage time necessary for setting the lowest toner layer temperature immediately before the completion of the first nip passage to be equal to or higher than the toner outflow start temperature varies depending on the fixing temperature. FIG. 6 is a graph showing the relationship between the toner bottom layer temperature Tb immediately before passing through the first nip and the first nip passing time tn1 at each fixing temperature when the fixing temperatures are 160 ° C., 170 ° C., and 180 ° C. . In this graph, based on the data shown in Table 6, for each of the fixing temperatures described above, the horizontal axis represents the lowest temperature of the toner reached immediately before the completion of passing through the first nip, and the vertical axis represents the first nip at that time. The result plotted as the passage time and its approximate line are represented by a solid line.

  As described above, for each fixing temperature condition, there is a proportional relationship between the lowest layer temperature Tb of the toner immediately before passing through the nip and the first nip passing time tn1, and the primary order as shown in the following equation (2). It can be expressed as a function.

tn1 = a × Tb + b (2)
Note that a and b are constants.

  When considered as a linear function, since the graphs at the respective fixing temperatures are almost parallel, only the intercept b is different, and the inclination thereof is almost constant regardless of the fixing temperature (a = 0.006). is there. Further, in this graph, when the intercept b (value of tn1 when Tb = 0) is considered as a function f (T) having the fixing temperature T as a parameter, the first nip passage time tn1 has the following relationship. It can be approximated by the equation (equation (3)).

tn1 = 0.006 × Tb + f (T) (3)
FIG. 7 is a graph showing the relationship between the fixing temperature T and the intercept b (function f (T)). That is, when the approximate line of each fixing temperature shown in FIG. 6 is expressed by the function of the above formula (2), the horizontal axis is plotted as the fixing temperature T, the vertical axis is plotted as the intercept b, and the approximate line is represented. It is. Thus, the approximate line can be expressed as a linear function shown in the following equation (4).

b = −0.00024T + 0.0015 Formula (4)
From the above equations (3) and (4), the relationship between the first nip passage time tn1, the toner bottom layer temperature Tb, and the fixing temperature T can be expressed by the following equation (5).

tn1 = 0.006Tb−0.00024T + 0.0015 (5)
Here, as the first nip passage time tn1 becomes longer, the toner bottom layer temperature Tb immediately before the completion of the first nip passage becomes higher. Therefore, the first nip passage time (ie, the following inequality (6)) is satisfied. If the first nip width at a certain process speed is set, an image without image displacement can be obtained. That is, by setting the first nip passage time tn1 so as to satisfy the following expression (6), the toner bottom layer temperature Tb becomes higher than the toner outflow start time Ti, and image shift can be prevented.

tn1 ≧ 0.0006Ti−0.00024T + 0.0015 (6)
By using the above formula (6), it is possible to calculate the fixing temperature and the nip width (first nip width) necessary for preventing image misalignment in a toner having a certain thermal characteristic without obtaining it by experiment or analysis. . That is, by setting the fixing temperature T and the first nip passage time tn1 (first nip width when the process speed is constant) satisfying the above formula (6), image misalignment is prevented without performing an experiment. A fixing device can be realized.

  Based on the above results, in the present embodiment, the fixing device 40 is a fixing nip portion in which the conveyed recording paper first passes and is pressed into the pressing portion (fixing nip portion) with respect to the fixing roller 41. In the first nip portion, the toner image on the recording paper is heated to a state where no low temperature offset occurs. In other words, the surface temperature (fixing temperature) of the fixing roller 41, which is the fixing condition in the first nip portion, and the first nip passage time during which the recording paper passes through the first nip portion are the low-temperature offset of the toner image on the recording paper. It was set to be a condition that does not cause The first nip passage time is set by setting a nip width (first nip width) that is a width of the first nip portion in the recording sheet conveyance direction and a recording sheet conveyance speed (process speed). .

  In the fixing device 40, a plurality of fixing nip portions (first nip portion and second nip portion) are formed by the fixing member (fixing roller 41) and the plurality of pressing members (pressure roller 42 and peeling roller 43). . In this case, the fixing nip portion (first nip portion) formed by the first pressing member (pressure roller 42) and the second pressing member (peeling) as viewed from the paper passing direction (conveying direction) of the recording paper. No fixing nip is formed between the fixing nip portion (second nip portion) formed by the roller 43) or a state in which the fixing nip is formed but the fixing pressure is hardly applied. The nip portion (virtual nip portion) is formed. In this case, in the first nip portion, the fixing member (fixing roller 41), the toner, and the recording paper are in close contact with each other by the pressing member (pressure roller 42). In the virtual nip portion up to the nip portion, since there is no pressing member, a slight gap is generated between the fixing member (fixing roller 41) and the recording paper.

  At this time, if the melting state of the toner at the first nip portion is insufficient, a part of the toner image adheres to the fixing member (fixing roller 41) side (so-called low temperature offset phenomenon) and is fixed at the virtual nip portion. A slight misalignment occurs between the member (fixing roller 41) and the recording paper. When the fixing member (fixing roller 41) and the recording paper come into close contact with each other at the second nip portion, an image shift occurs.

  In contrast, in the fixing device 40, the toner image on the recording paper is heated to a state where no low temperature offset occurs in the first nip portion. Therefore, it is possible to reliably prevent image shift. Note that the heating of the toner image at the first nip portion is not necessarily sufficient in terms of fixing strength, and may be performed at least until the low-temperature offset does not occur. Thereby, image shift can be eliminated.

  In the fixing device 40, in order to set the fixing temperature and the first nip passage time, which are fixing conditions in the first nip portion, so that the toner image on the recording paper does not cause a low temperature offset, the toner on the recording paper is used. In the image, the fixing temperature and the first nip are set so that the temperature of the lowest toner layer (the lowest toner temperature), which is the layer closest to the recording paper, is equal to or higher than the toner outflow start temperature in the vicinity of the first nip outlet. The transit time is set. In this way, by heating the toner bottom layer temperature to the toner outflow start temperature or higher at the outlet of the first nip portion, low temperature offset in the first nip portion is prevented, and as a result, the occurrence of image shift can be suppressed. it can.

  Further, the toner lowermost layer temperature can be obtained by theoretical calculation using the fixing device 40 as a model. For example, the fixing device 40 is modeled as a one-dimensional heat transfer analysis model, and the theoretical analysis (primary analysis) using the fixing temperature and the first nip passage time (or process speed and first nip width) in the first nip portion as parameters. By performing the original heat transfer analysis), the toner bottom layer temperature in the vicinity of the outlet of the first nip portion can be calculated. Then, based on the result of this theoretical calculation, image misregistration is suppressed by setting the fixing temperature and the first nip passage time so that the toner bottom layer temperature near the outlet of the first nip portion becomes equal to or higher than the toner outflow temperature. Can do.

In the above theoretical calculation, the fixing temperature (surface temperature of the fixing roller 41 in the first nip portion) T (° C.), the first nip passage time (time for the recording paper to pass through the first nip portion) tn1 (s) When the toner outflow start temperature is Ti (° C.), the above formula (6), that is,
tn1 ≧ 0.0006Ti−0.00024T + 0.0015 (6)
What is necessary is just to set so that it may satisfy | fill.

  By setting the fixing temperature and the first nip passage time based on the above formula (6), for example, in the toner having an arbitrary thermal characteristic, the toner bottom layer temperature at the first nip portion outlet is equal to or higher than the toner outflow start temperature. It is possible to appropriately prevent image shift without obtaining the fixing conditions (fixing temperature and first nip passage time) obtained through experiments and analysis.

  The fixing device 40 uses a fixing roller 41 as a fixing member. The pressure roller 42 is used as the first pressing member. As described above, by configuring the fixing member and the pressing member with rollers, a fixing device having a plurality of nip portions can be realized with a simple configuration.

  Further, the pressure roller 42 as the first pressing member in the fixing device 40 is an elastic roller in which an elastic layer 42b is coated on a core metal 42a. Thus, by using the pressure roller 42 that forms the first nip portion as an elastic roller, the nip width of the first nip portion can be set sufficiently wide. Therefore, it is possible to easily realize the fixing conditions required at the first nip portion.

  In the fixing device 40, a belt 44 suspended from a plurality of support rollers (the pressure roller 42 and the peeling roller 43) is provided, and a plurality of nip portions are formed between the belt 44 and the fixing roller 41. Has been. That is, the belt 44 is pressed against the fixing roller 41 by a plurality of support rollers (pressing members; the pressure roller 42 and the peeling roller 43).

  When the belt 44 is not provided, there is a gap between the nip portion (first nip portion) formed by the first pressing member and the nip portion (second nip portion) formed by the second pressing member. Since there is nothing that regulates the conveyance direction, the conveyance direction of the recording paper changes until the recording paper enters the second nip portion, and the conveyance property such that the recording paper cannot enter the second nip portion. Problems can arise. Therefore, by providing the belt 44, the recording paper discharged from the first nip portion is conveyed while being sandwiched between the belt 44 and the fixing roller 41 (regulating the conveyance direction), thereby preventing conveyance failure. be able to.

  In the fixing device 40 according to the present embodiment, the belt 44 is suspended from the pressure roller 42 and the peeling roller 43. However, the present invention is not limited to this. For example, as shown in FIG. A configuration without 44 is also possible.

  However, in this case, if the space between the pressure roller 42 and the peeling roller 43 is too wide, there is nothing that suppresses the conveyance direction of the recording paper between the first nip portion and the second nip portion. By the time the recording paper enters the nip portion, the recording paper transport direction changes, which may cause a problem in transportability such as being unable to enter the second nip portion. For this reason, it is preferable to provide means for regulating the conveyance direction of the recording paper.

  For example, a guide member 45 as shown in FIG. 9 may be provided as a means for restricting the conveyance direction of the recording paper. The guide member 45 has a shape in which the surface of the guide member 45 faces the surface of the fixing roller 41 while maintaining a minute gap. Both ends (both ends of the guide member 45) are provided near the first nip portion outlet and the second nip portion inlet. ) Is located. As shown in this figure, by forming the shape of the guide member 45 in a crescent shape along the surface of the fixing roller 41, both ends are installed in the vicinity of the first nip portion outlet and the second nip portion inlet, respectively. Can do. Further, the recording paper that has come out of the first nip portion is conveyed while sliding on the surface of the guide member 45, and the leading edge of the recording paper enters the second nip portion. A low friction coefficient is preferable. For example, when the surface of the guide member 45 is coated with fluorine, the slip property is good and the paper transportability is good.

  The guide member 45 preferably retains a very small gap with respect to the fixing roller 41, the pressure roller 42, and the peeling roller 43. In order to improve the positional accuracy, for example, the guide member 45 is made of metal or the like. You may comprise with the formed block body.

  When it is desired to have heat insulation in the virtual nip portion, for example, a guide member composed of a plurality of layers such as a heat-resistant resin and a rubber material may be used on the surface of the guide member formed of a metal body.

  Further, for example, in addition to the pressure roller 42 and the peeling roller 43, one or more tension rollers (support rollers, not shown) may be provided for the purpose of maintaining the tension of the belt 44. The fixing device 40 includes a pressure roller 42 and a peeling roller 43 as support rollers for the belt 44, and the belt 44 is pressed against the fixing roller 41 by all the support rollers (the pressure roller 42 and the separation roller 43). However, the configuration is not limited thereto, and the belt 44 may be pressed (pressed) toward the fixing roller 41 by all or a part of the plurality of support rollers.

  Further, for example, as shown in FIG. 10, the fixing device 40 has a sponge-like structure in place of the pressure roller 42 that suspends a belt 44 from a peeling roller 43 and tension rollers 47a and 47b and forms a first nip portion. It is also possible to use a pressure pad (non-rotating member) 46.

  As the pressure pad 46, for example, an iron base (substrate) provided with an elastic layer made of heat-resistant silicone sponge rubber having a thickness of 5 mm and a rubber hardness of 25 ° (Asker C hardness) can be used. . The pressure pad 46 is pressed (pressed) against the surface of the fixing roller 41 via a belt 44 by an elastic member such as a spring (not shown) to form a first nip portion with the fixing roller 41. The length of the pressure pad 46 facing the fixing roller 41 in the recording paper conveyance direction is longer than the nip width of the first nip portion formed by the pressure roller 42 described above. Therefore, the nip width formed between the pressure pad 46 (the belt 44 applied with the pressing force by the pressure pad 46) and the fixing roller 41 is equal to the pressure roller 42 (the pressing force is applied by the pressure roller 42). The width of the nip formed between the belt 44) and the fixing roller 41 is wider.

  As described above, by using the pressure pad 46 as means (pressing member) for forming the first nip portion, a wide nip width (first nip width) can be ensured with a low load and in a narrow space. The device can be downsized. That is, the configuration using the pressure pad 46 can form a wider first nip width without increasing the size of the fixing device than the configuration using the pressure roller 42.

  For example, as shown in FIG. 11, a belt 44 is suspended from the pressure roller 42, the separation roller 43, and the tension roller 47c, and a pressure pad 46 that forms the second nip portion is added to the pressure nip 46 that forms the first nip portion. It may be arranged close to the downstream side of the pressure roller 42. That is, the nip portion (second nip portion, non-rotating nip portion) formed by the pressure pad 46 and the first nip portion formed by the pressure roller 42 are continuous or close to each other (for example, the first nip portion). The pressure pad 46 is supported by the support roller (pressure roller) forming the first nip portion so that the distance between the rear end of the nip and the tip of the second nip portion (pressure pad 46) is 3 mm or less. 42) in the vicinity of the downstream side in the recording paper conveyance direction.

  In this case, the second nip portion formed by the pressure pad 46 and the first nip portion formed by the pressure roller 42 are in a continuous or very close state. For this reason, since the nip portion obtained by adding the second nip portion to the first nip portion substantially acts as an integral nip portion, image displacement can be effectively prevented.

  In this embodiment, the fixing conditions (fixing temperature and first nip passage time) for setting the toner bottom layer temperature at the outlet of the first nip portion to be equal to or higher than the toner outflow start temperature are obtained by one-dimensional heat transfer analysis. Although the case has been described, the present invention is not limited to this. For example, an analysis considering a two-dimensional or three-dimensional shape may be performed, or an analysis considering a time change may be performed.

  In this embodiment, the unfixed toner image is heated to a temperature at which the low temperature offset does not occur at the first nip portion. However, at this time, the temperature is preferably set to a temperature at which the high temperature offset occurs. That is, if the toner becomes too high, image loss may occur due to so-called high temperature offset. Therefore, the temperature of the toner at the first nip portion outlet is set between each fixing nip portion and each fixing nip portion (each virtual nip portion). It is preferable to set the fixing temperature and the first nip passage time so that the low temperature offset does not occur and the high temperature offset does not occur.

[Embodiment 2]
Another embodiment of the present invention will be described. For convenience of explanation, members having functions similar to those of the members described in the first embodiment are denoted by the same reference numerals as those in the first embodiment, and description thereof is omitted.

  In the first exemplary embodiment, the fixing condition in the first nip portion, that is, the surface temperature (fixing temperature) of the fixing roller 41 and the first nip passage time during which the recording paper passes through the first nip portion are determined as the toner image on the recording paper. In the above description, the condition is set such that the low temperature offset does not occur. In the present embodiment, the virtual nip portion (the region between the first nip portion and the second nip portion), or the fixing condition of the virtual nip portion and the first nip portion is determined based on an image defect (high temperature offset (hot offset)). A case of setting so as to prevent image defects such as high temperature offset will be described.

  The image defect due to the high temperature offset (image defect such as the high temperature offset) is heated more than necessary when the toner is heated more than necessary in the virtual nip portion (or in the first nip portion and the virtual nip portion). Since the viscosity of the toner is reduced, when a slight positional deviation occurs between the fixing member (fixing roller 41) and the recording paper due to a speed difference generated between the fixing roller 41 and the belt 44, This is caused by the toner surface being divided.

(Experimental example 2: Image defect evaluation test)
First, in order to separate (distinguish) the image shift due to the low temperature offset and the image defect due to the high temperature offset described in the first embodiment, the conditions of the first nip portion for preventing the image shift described in the first embodiment. (In this experiment, the first nip width was set to 6 mm) was used to evaluate an image defect. In this experiment, similar to Experimental Example 1 of the first embodiment, using the fixing device 40 shown in FIG. 3, the toner type, the fixing temperature, the length of the virtual nip portion (that is, the virtual nip passage time), etc. An experiment was conducted in which the fixing conditions were changed and the other conditions were kept constant, and a recording paper on which an unfixed image was placed was passed through, and an image defect due to high temperature offset was evaluated for the fixed sample.

  Evaluation of image defects is performed by a relative evaluation (subjective image quality evaluation) with respect to the standard sample, using a sample in which an unfixed sample is fixed by a conventional roller-pair type fixing device as a comparative example (standard sample). It was. If there is no problem with the image quality, “◯” indicates that the image quality is slightly inferior (there is no problem in the product, but if the image is enlarged, the image quality unevenness is present in the comparative example) “△”, the image quality is inferior. A sample (a sample in which the image quality unevenness is clearly conspicuous with respect to the comparative example) is defined as [×]. The process speed was 175 mm / s, and unfixed samples and toners having the same conditions as in Experimental Example 1 in Embodiment 1 were used.

  First, an image defect evaluation experiment was performed using toner A and toner B, using the surface temperature (fixing temperature) of the fixing roller and the virtual nip passage time of the virtual nip portion as parameters. More specifically, the first and virtual nip width (first nip width + virtual nip width) was changed to three types of 21.8 mm, 24.8 mm, and 27 mm. In this case, the first and virtual nip passage time (first nip passage time + virtual nip passage time) changes to about 0.124 s, about 0.142 s, and about 0.155 s. In this experiment, the process speed was constant (175 mm / s), and the nip passage time was changed by changing the nip width (virtual nip width) of the virtual nip portion. In this experiment, the nip width (first nip width) of the first nip portion was constant (6 mm).

  The results when toner A is used are shown in Table 8, and the results when toner B is used are shown in Table 9.

  From Tables 8 and 9, image defects were more likely to occur when toner B was used than when toner A was used. Even when toner A was used, image defects were caused by increasing the fixing temperature or extending the first / virtual nip passage time (widening the first / imaginary nip width).

  From this result, when the toner is heated more than necessary at the virtual nip portion (at the first nip portion and the virtual nip portion), a fixing member (the fixing roller 41) is caused by a speed difference generated between the fixing roller 41 and the belt 44. ) And the recording paper, if the toner is heated more than necessary in the virtual nip, the viscosity of the toner is reduced, and the toner surface is divided, resulting in an image such as a high temperature offset. Defects (image defects due to high temperature offset) are considered to occur.

(Analysis example 2)
Next, in order to examine the relationship between the toner temperature at the virtual nip portion outlet (second nip portion inlet) and the image defect due to the high temperature offset, a one-dimensional model is used using the same model as the first analysis example in the first embodiment. Heat conduction analysis was performed. Using this model, the uppermost layer temperature (the temperature of the layer in contact with the fixing roller 41) of the toner of the unfixed image printed on the recording paper immediately before the exit of the virtual nip portion was calculated.

  As analysis conditions, the thickness, initial temperature, specific heat, specific gravity, thermal conductivity, and physical property values of each layer shown in Table 5 were used as in Analysis Example 1 in Embodiment 1. The elastic layer 41b of the fixing roller 41 is divided into 15 layers. The initial temperature of each of the divided layers was 160 ° C. for the layer in contact with the release layer 41c and 188 ° C. for the layer in contact with the core metal 41a. Moreover, the initial temperature of the layer during the division was set to a value increased by 2 ° C. from the layer close to the release layer 41c.

  The analysis conditions shown in Table 5 are the settings (conditions) when the fixing temperature is 160 ° C. When the fixing temperature is calculated at 170 ° C., each layer set by the fixing roller 41 (release layer 41c, elasticity) The initial temperatures of the layer 41b and the cored bar 41a) were each increased by 10 ° C. Similarly, when calculating at a fixing temperature of 180 ° C., the temperature of each layer of the fixing roller 41 is further increased by 10 ° C.

  Moreover, the calculation formula used for the analysis is the same as that of Analysis Example 1 in the first embodiment. Using this calculation formula, calculate the temperature of each layer after the minute time τ has elapsed, and the paper is the time immediately before the virtual nip exit from the first nip entrance (value obtained by dividing the first virtual nip width by the process speed) The calculation was repeated until.

  Table 10 shows the result of calculating the toner uppermost layer temperature at the virtual nip exit, using the fixing temperature and the first / virtual nip passage time as parameters under the above analysis conditions.

  From the experimental results in Table 8 and the analysis results in Table 10, when toner A is used, if the uppermost layer temperature of the toner at the virtual nip exit is 120 ° C. or lower, image defects due to high temperature offset do not occur and a good image is obtained. was gotten. Note that the 4 mm drop temperature of toner A is 120 ° C. That is, as a result of thermal analysis, when the temperature of the uppermost layer of the toner at the virtual nip exit has reached or exceeded the 4 mm drop temperature (120 ° C.) of toner A, an image defect due to high temperature offset occurred.

  Further, from the experimental results in Table 9 and the analysis results in Table 10, when toner B is used, if the lowest layer temperature of the toner at the virtual nip exit is 115 ° C. or less, image defects due to high temperature offset do not occur. A good image was obtained. The 4 mm drop temperature of toner B is 115 ° C. That is, when the toner B is used, as in the case of using the toner A, as a result of the thermal analysis, when the toner top layer temperature at the virtual nip outlet has reached the 4 mm drop temperature (115 ° C.) or more of the toner B. Image defects such as high temperature offset occurred.

  From these experimental results and analysis results, it was found that the image defect due to the high temperature offset occurs when the toner top layer temperature at the virtual nip exit is equal to or higher than the 4 mm drop temperature of the toner. The toner uppermost layer temperature at the virtual nip exit has a correlation with the first / virtual nip passage time (passing time from the first nip entrance to the virtual nip exit).

  Therefore, for example, when the fixing temperature is constant, the first / virtual nip passage time (virtual nip passage time + first nip passage time) such that the uppermost layer temperature of the toner at the exit of the virtual nip portion is 4 mm or lower. By setting, image defects due to high temperature offset can be prevented.

  The first / virtual nip passage time (virtual nip passage time + first nip passage time) for setting the toner uppermost layer temperature at the virtual nip outlet to be equal to or lower than the 4 mm drop temperature of the toner varies depending on the fixing temperature.

  FIG. 12 is a graph in which the data shown in Table 10 is plotted with the horizontal axis representing the toner top layer temperature (° C.) at the virtual nip exit and the vertical axis representing the first / virtual nip passage time (s). FIG. 12 shows the relationship between the toner top layer temperature (° C.) at the virtual nip exit and the first / virtual nip passage time (s) for each fixing temperature (160 ° C., 170 ° C., 180 ° C.). The approximate line is indicated by a solid line.

  As shown in FIG. 12, there is a proportional relationship between the uppermost layer temperature Tio of the toner at the virtual nip outlet and the first / virtual nip passage time tn2 for each fixing temperature. It can be expressed by a linear function shown in equation (7).

tn2 = α × Tio + β (α and β are constants) (7)
Further, for each fixing temperature, approximate lines indicating the relationship between the uppermost layer temperature Tio of the toner at the virtual nip outlet and the first / virtual nip passage time tn2 are substantially parallel to each other as shown in FIG. Therefore, the linear function shown in the above formula (7) is different in intercept only, and its slope is almost constant (α≈0.0023) regardless of the fixing temperature. In the graph shown in FIG. 12, when the intercept β is considered as a parameter of the fixing temperature T (when β = f (T) is considered), the above equation (7) is expressed by the following relational equation (equation (8 )).

tn2 = 0.0023 × Tio + f (T) (f (T) is a function of T) (8)
FIG. 13 is a graph showing the function f (T), that is, the relationship between the fixing temperature T and the intercept β. That is, when the approximate line for each fixing temperature shown in FIG. 12 is expressed by the above equation (8), the value of the intercept β (= f (T)) when Tio = 0 is FIG. 13 is a graph that plots the fixing temperature T on the axis and the intercept β on the vertical axis and represents the approximate line. As can be seen from this figure, the approximate line indicating the relationship between the fixing temperature T and the intercept β can be expressed as the following linear function (formula (9)).

β = f (T) = − 0.002T + 0.2078 (9)
From the above equations (8) and (9), The relationship between the virtual nip passage time (virtual nip passage time + first nip passage time) tn2, the toner uppermost layer temperature Tio at the virtual nip outlet, and the fixing temperature T can be expressed by the following equation (10).

tn2 = 0.0023 × Tio−0.002T + 0.2078 (10)
Here, the shorter the first / virtual nip passage time tn2, the lower the toner uppermost layer temperature Tio immediately before the virtual nip exit (just before (virtual nip + first nip) passage completion). Accordingly, when the 4 mm drop temperature of the toner is T _{4 mm} , the image defect due to the high temperature offset can be prevented by setting the first / virtual nip passage time tn2 so as to satisfy the following expression (11).

tn2 ≦ 0.0023 × T _{4 mm} −0.002T + 0.2078 (11)
By setting the first / virtual nip passage time tn2 using this equation (11), when using toner having a certain arbitrary thermal characteristic (4 mm drop temperature), fixing for preventing image defects due to high temperature offset The temperature and the first / virtual nip passage time (or the first / virtual nip width) can be appropriately set without performing experiments or analysis. That is, by setting the fixing temperature T and the first / virtual nip passage time tn2 (or the first / virtual nip width) satisfying the above formula (11), image defects due to high temperature offset can be prevented without performing an experiment. A fixing device can be designed.

  In addition, by setting each condition so as to satisfy both the expression (6) shown in the first embodiment and the above expression (11), a fixing device design that prevents image misalignment and image defects due to high-temperature offset is designed. It can be carried out. That is, by satisfying both of the above formulas (6) and (11), both image shift and image defects due to high temperature offset can be prevented.

  Based on the above results, in the present embodiment, a plurality of pressure contact portions (fixing nip portions) with respect to the fixing roller 41 and virtual nip portions formed in the respective fixing nip portions are positioned upstream in the recording paper conveyance direction. The toner image on the recording paper that has passed through the first nip portion (nip portion where the recording paper is first transported) that is the fixing nip portion and the subsequent virtual nip portion causes an image defect due to high temperature offset at the virtual nip portion outlet. The first / virtual nip passage time (first nip passage time + virtual nip passage time) and the fixing temperature were set so as not to be present.

  That is, the surface temperature (fixing temperature) of the fixing roller 41, which is a fixing condition in the first nip portion and the virtual nip portion, and the first / virtual nip passage time during which the recording paper passes through the first nip portion and the virtual nip portion. Was set to be a condition that does not cause image defects due to high temperature offset in the toner image on the recording paper. The first / virtual nip passage time is set by the first nip width that is the width of the first nip portion in the recording paper conveyance direction, the virtual nip width that is the width of the virtual nip portion in the recording paper conveyance direction, and the recording paper. This is done by setting the transport speed (process speed).

  In the fixing device 40, a plurality of fixing nip portions (first nip portion and second nip portion) are formed by the fixing member (fixing roller 41) and the plurality of pressing members (pressure roller 42 and peeling roller 43). . In this case, the fixing nip portion (first nip portion) formed by the first pressing member (pressure roller 42) and the second pressing member (peeling) as viewed from the paper passing direction (conveying direction) of the recording paper. No fixing nip is formed between the fixing nip portion (second nip portion) formed by the roller 43) or a state in which the fixing nip is formed but the fixing pressure is hardly applied. The nip portion (virtual nip portion) is formed.

  In this case, when the toner is heated more than necessary at the virtual nip (or at the first nip and the virtual nip), a fixing member (fixing) is caused by a speed difference generated between the fixing roller 41 and the belt 44. When a slight misalignment occurs between the roller 41) and the recording paper, the viscosity of the toner heated more than necessary at the virtual nip portion (or at the first nip portion and the virtual nip portion) decreases. As a result, the toner surface is divided and image defects due to high temperature offset occur.

  On the other hand, in the fixing device 40, the fixing conditions (fixing temperature and first / virtual nip passage time) are set so that the toner image on the recording paper does not cause an image defect due to high temperature offset at the exit of the virtual nip portion. Is set. Therefore, “image defects such as high temperature offset” can be reliably prevented.

  Further, in the fixing device 40, the fixing temperature and the first / virtual nip passage time as fixing conditions in the first nip portion and the virtual nip portion are set so as not to cause an image defect such as a high temperature offset in the toner image on the recording paper. Therefore, in the toner image on the recording paper, the temperature of the toner uppermost layer (the toner uppermost layer temperature) which is the layer closest to the fixing roller 41 is equal to or lower than the 4 mm drop temperature of the toner in the vicinity of the virtual nip exit. As described above, the fixing temperature and the first / virtual nip passage time are set.

  As described above, by setting the toner uppermost layer temperature at the virtual nip portion outlet to be equal to or lower than the 4 mm drop temperature of the toner, it is possible to prevent image defects due to the high temperature offset in the virtual nip portion.

  Further, the toner uppermost layer temperature at the virtual nip exit can be obtained by theoretical calculation using the fixing device 40 as a model. For example, the fixing device 40 is modeled as a one-dimensional heat transfer analysis model, and the fixing temperature and first / virtual nip passage time (or process speed and first / virtual nip width) at the first nip portion and the virtual nip portion By performing a theoretical analysis (one-dimensional heat transfer analysis) using as a parameter, the toner top layer temperature at the first nip exit can be calculated. Then, based on the result of this theoretical calculation, the fixing temperature and the first / virtual nip passage time are set so that the toner uppermost layer temperature in the vicinity of the exit of the first nip portion is 4 mm or less, thereby causing a high temperature offset. Image defects can be prevented.

In the above theoretical calculation, the fixing temperature (the surface temperature of the fixing roller 41 at the first nip portion and the virtual nip portion) T (° C.), the first / virtual nip passage time (the recording paper has the first nip portion and the virtual nip portion). Part time) tn2 (s), where the 4 mm drop temperature of the toner is T _{4 mm} (° C.), the above equation (11), that is,
tn2 ≦ 0.0023 × T _{4 mm} −0.002T + 0.2078 (11)
What is necessary is just to set so that it may satisfy | fill.

  By setting the fixing temperature and the first / virtual nip passage time based on the above formula (11), for example, when using toner having an arbitrary thermal characteristic, the toner uppermost layer temperature at the exit of the virtual nip portion is Image defects due to high-temperature offset can be appropriately prevented without obtaining fixing conditions (fixing temperature and first / virtual nip passage time) that are 4 mm drop temperature or less through experiments and analysis.

  In the fixing device 40 according to the present embodiment, the belt 44 is suspended from the pressure roller 42 and the peeling roller 43. However, the present invention is not limited to this. For example, as shown in FIG. A configuration without 44 is also possible. However, in this case, the virtual nip portion between the first nip portion and the second nip portion has no restriction on the recording paper conveyance direction, so that the recording paper can be moved before the recording paper enters the second nip portion. In some cases, the conveyance direction is changed, and there is a problem in conveyance such that the recording paper cannot enter the second nip portion. For this reason, it is preferable to provide means for regulating the conveyance direction of the recording paper. For example, the same means as in the first embodiment can be used as the means for regulating the recording paper conveyance direction.

  Further, as in the first embodiment, in addition to the pressure roller 42 and the peeling roller 43, one or more tension rollers (support rollers, not shown) may be provided for the purpose of maintaining the tension of the belt 44. Further, for example, as shown in FIG. 10 in the first embodiment, a belt 44 is suspended between the peeling roller 43 and the tension rollers 47a and 47b, and instead of the pressure roller 42 forming the first nip portion, a sponge-like shape is used. The pressure pad (non-rotating member) 46 may be used.

  In this embodiment, the fixing condition (fixing temperature and first / virtual nip passage time) for setting the toner uppermost layer temperature at the exit of the virtual nip portion to a temperature lower than 4 mm of the toner is one-dimensional heat transfer analysis. However, the present invention is not limited to this. For example, an analysis considering a two-dimensional or three-dimensional shape may be performed, or an analysis considering a time change may be performed.

  Also, fixing conditions (fixing temperature and first / virtual nip passage time) for setting the toner uppermost layer temperature at the exit of the virtual nip portion to 4 mm or lower of the toner, and the lowermost toner layer at the exit of the first nip portion Fixing conditions (fixing temperature, first nip passage time, first time) so as to satisfy both of the fixing conditions (fixing temperature and first nip passage time; refer to Embodiment 1) for setting the temperature to be equal to or higher than the toner outflow start temperature. It is preferable to set a virtual nip passage time).

  In this case, it is possible to prevent both the image shift due to the low temperature offset in the virtual nip portion and the image defect due to the high temperature offset.

  The concept of the present invention described in each of the above embodiments can be applied to any fixing device having a plurality of fixing nip portions. For example, fixing using a fixing belt as disclosed in Patent Document 4 or Patent Document 5 described above. It can also be applied to devices.

  In each of the above embodiments, a color of a dry electrophotographic system that forms a multicolor or single color image on a predetermined recording sheet based on image data transmitted from a terminal device connected via a network. Although the case where the image forming apparatus includes the fixing device of the present invention has been described, the present invention is not limited to this. For example, the present invention may be applied to a copying machine (image forming apparatus) that includes an image reading apparatus and forms a read image on a recording sheet, or an image forming apparatus that has multiple functions such as a facsimile machine, a copying machine, and a printer. Good.

  The present invention is not limited to the above-described embodiments, and various modifications can be made within the scope shown in the claims. That is, embodiments obtained by combining technical means appropriately modified within the scope of the claims are also included in the technical scope of the present invention.

  The present invention can be applied to any fixing device having a plurality of fixing nip portions and an image forming apparatus having the fixing device.

FIG. 4 is a cross-sectional view showing a nip portion formed by a fixing member and a pressing member in the fixing device according to the embodiment of the present invention. 1 is a cross-sectional view illustrating a main part of an image forming apparatus provided with a fixing device according to an embodiment of the present invention. 1 is a cross-sectional view illustrating a schematic configuration of a fixing device according to an embodiment of the present invention. It is a figure which shows the model of the one-dimensional heat transfer analysis performed in one Embodiment of this invention. It is a figure for demonstrating the heat transfer between several layers in the one-dimensional heat transfer analysis performed in one Embodiment of this invention. In the fixing device according to the embodiment of the present invention, when the fixing temperature is set to 160 ° C., 170 ° C., and 180 ° C., the toner bottom layer temperature immediately before passing through the first nip and the first nip passing time at each fixing temperature. It is a graph which shows a relationship. In the fixing device according to an embodiment of the present invention, the relationship between the intercept of the linear function and the fixing temperature when the first nip passage time is expressed as a linear function of the toner bottom layer temperature at the first nip exit is shown. It is a graph. FIG. 9 is a cross-sectional view showing a modification of the fixing device according to the embodiment of the present invention. FIG. 10 is a cross-sectional view showing another modification of the fixing device according to the embodiment of the present invention. FIG. 9 is a cross-sectional view showing still another modification of the fixing device according to the embodiment of the present invention. FIG. 9 is a cross-sectional view showing still another modification of the fixing device according to the embodiment of the present invention. In the fixing device according to another embodiment of the present invention, when the fixing temperatures are 160 ° C., 170 ° C., and 180 ° C., the toner uppermost layer temperature and the first / virtual nip passage time immediately before passing the virtual nip at each fixing temperature. It is a graph which shows the relationship. In the fixing device according to another embodiment of the present invention, when the first / virtual nip passage time is expressed as a linear function of the toner uppermost layer temperature at the exit of the virtual nip portion, the relationship between the intercept of the linear function and the fixing temperature. It is the graph which showed.

Explanation of symbols

40 Fixing device 41 Fixing roller (fixing member)
41a Metal core 41b Elastic layer 41c Release layer 42 Pressure roller (pressing member, roller, elastic roller)
42a Metal core 42b Elastic layer 42b Elastic layer 42c Release layer 43 Peeling roller (pressing member)
44 belt 44a release layer 44b belt base 45 guide member 46 pressure pad (pressing member)
47a, 47b, 47c Tension roller (support roller, pressing member)
Q Heating source tn1 First nip passage time tn2 First / virtual nip passage time T Fixing temperature T _{4 mm} toner 4 mm drop temperature Ti Toner outflow start temperature Tb Toner bottom layer temperature at first nip exit Tio Toner at virtual nip exit Top layer temperature a1 Air layer (first air layer)
a2 Air layer (second air layer)

Claims (1)

A fixing roller heated to a predetermined temperature, a pressure roller and a peeling roller for applying a pressing force in the direction of the fixing roller to a recording paper on which an unfixed toner image is formed, and the pressure roller and the peeling roller. And a second nip where the pressure roller and the fixing roller are pressed against each other via the belt, and a second nip where the peeling roller and the fixing roller are pressed against each other via the belt. A fixing condition setting method in a fixing device for fixing the unfixed toner image onto the recording paper by passing the recording paper between the fixing roller and the belt in a section,
The belt is in contact with the fixing roller in a virtual nip portion that is a region between the first nip portion and the second nip portion;
The fixing temperature, which is the temperature of the fixing roller facing the recording paper, and the first nip passage time, which is the time for the unfixed toner image to pass through the first nip portion, are determined in the unfixed toner image. The temperature of the recording paper in the direction perpendicular to the surface of the recording paper is set so that the temperature at the outlet of the first nip portion is equal to or higher than the starting temperature of the unfixed toner image.
The fixing temperature and the first / virtual nip passage time, which is the time for the unfixed toner image to pass through the first nip and the virtual nip, are perpendicular to the surface of the recording paper in the unfixed toner image. Is set so that the temperature closest to the fixing roller is equal to or lower than the 4 mm drop temperature of the toner in the unfixed toner image at the exit of the virtual nip portion.
The toner outflow start temperature is set when toner powder is set on a flow tester and heated at a constant speed under a load of 20 kgf / cm ² and 6 ° C./min using a die having a diameter of 1 mm × height of 1 mm. This is the temperature at which the toner starts to melt and the piston that applies a load to the toner starts to descend.
The fixing condition setting method, wherein the 4 mm drop temperature is a temperature at which the piston drops by 4 mm when the constant temperature increase is continued even after the piston starts to drop .

Images