JP2007199534A - Image forming apparatus and method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To reduce the occurrence of jamming due to curling when an edgeless printing mode is adopted. <P>SOLUTION: A determination means determines a ratio between an image forming region in which an image is formed and a non image forming region in which no image is formed in a region of cause becoming a cause for causing recording medium to curl among regions in which the image can be formed by an image forming part. A control means controls an action of an image forming means so that the occurrence of jamming due to curling is reduced according to the determined ratio. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to a technique for reducing a jam caused by winding of a recording medium in an image forming apparatus.

  In an image forming apparatus in which an image of a developer is fixed on a recording medium by applying pressure and heating, curling (warping) of the recording medium tends to be a problem. In particular, curling of the recording medium is not preferable because the recording medium is wound around the fixing film.

According to Patent Document 1, an invention for correcting curling of a recording medium that occurs during image fixing is proposed. Further, according to Patent Document 2, when the printing rate in a certain period from the writing start timing exceeds a predetermined amount, the leading edge margin is taken larger than the margin specified in advance, thereby preventing winding around the fixing roller. Has been proposed.
JP-A-5-341600 JP 2001-58450 A

  Incidentally, it is known that the curling of the recording medium occurs because the shrinkage rate of the recording medium itself and the shrinkage rate of the developer are different. Therefore, curling is more likely to occur on thin paper (basis weight 64 g / m 2 or less) than plain paper (basis weight 65 to 105 g / m 2) or thick paper (basis weight 106 to 300 g / m 2).

  FIG. 12 is a diagram illustrating an example of the relationship between the leading edge margin of the recording medium and the frequency of occurrence of winding. When the inventor of the present invention examined, when the margin at the tip was 5 mm or more, the frequency of winding the recording medium around the fixing film was zero. On the other hand, it has been found that when the tip margin is 3 mm or less, the occurrence frequency of winding increases rapidly.

  FIG. 13 is a diagram illustrating an example of the relationship between the gap from the fixing film to the separation guide and the frequency of occurrence of winding. When this gap was made as small as possible, if the leading edge margin was 2 mm or more, the occurrence frequency of wrapping when a solid image was formed on plain paper was zero.

  Recently, the demand for so-called borderless printing is expanding. In borderless printing, the leading edge margin of the recording medium is further smaller than 2 mm (the leading edge margin is 0 mm). In this case, the solid image at the leading end is not sufficiently fused to the recording medium. Therefore, the recording medium sticks to the fixing film side, and winding occurs. The winding of the recording medium not only causes a jam but also causes a failure of the fixing device, so it is desirable to prevent it beforehand.

  Therefore, an object of the present invention is to solve at least one of such problems and other problems. Other issues can be understood throughout the specification.

  The present invention is preferably realized in an image forming apparatus including an image forming unit, a determining unit, and a control unit. The image forming unit forms an image in any of a plurality of image forming modes. In these image forming modes, a first image forming mode in which margins are provided on four sides on a recording medium to form an image, and a second image in which no margins are substantially provided on four sides on a recording medium. At least the image forming mode. The determining unit determines a ratio between an image forming area where an image is formed and a non-image forming area where the image is not formed in a causal area which causes the recording medium to curl out of areas where an image can be formed by the image forming unit. . This determination is performed when an image is formed in the second image forming mode. The control unit controls the operation of the image forming unit so as to reduce the occurrence of the jam caused by the curl according to the determined ratio.

  According to the present invention, when an image is formed without providing a margin on the recording medium, the operation of the image forming unit is controlled according to a preset printing rate of the image forming area. This reduces the occurrence of jam in the fixing device.

  An embodiment of the present invention is shown below. Of course, the individual embodiments described below will be helpful in understanding various concepts such as the superordinate concept, intermediate concept and subordinate concept of the present invention. Further, the technical scope of the present invention is determined by the scope of the claims, and is not limited by the following individual embodiments.

[First Embodiment]
FIG. 1 is a schematic cross-sectional view of an image forming engine of the image forming apparatus according to the embodiment. The multicolor image forming apparatus 100 employs an electrophotographic system for the image forming unit 101. The media sensor 120 is a sensor for detecting the type of the recording medium P.

  The recording medium cassette 102 stores a plurality of recording media P. The paper feed roller 103 picks up and feeds the recording media P from the recording media cassette 102 one by one. The fed recording medium P is conveyed to the registration roller pair 104. Further, the recording medium P is fed to the image forming unit 101 by the registration roller pair 104 at a predetermined timing. The registration sensor 121 is a sensor for detecting the leading end of the recording medium P.

  The image forming unit 101 includes four image forming stations that form developer images of different colors on the recording medium P. In this example, yellow, magenta, cyan, and black toners are used as developers.

  The photosensitive drum 105 is a kind of image carrier, and its surface is uniformly charged by a charging roller 106 that receives power from a high-voltage power supply circuit. The exposure device 107 irradiates the surface of the photosensitive drum 105 with the light beam L. Thereby, an electrostatic latent image is formed on the surface of the photosensitive drum 105. Thus, the exposure apparatus 107 has a function of forming an electrostatic latent image.

  The developing roller 108 visualizes the electrostatic latent image using a developer such as toner and forms a toner image. The toner image on the photosensitive drum 105 is transferred to the recording medium P by the transfer roller 109.

  An electrostatic adsorption conveyance belt (hereinafter referred to as ETB) 110 is interposed between the photoconductive drum 105 and the transfer roller 109. The ETB 110 is stretched between the drive roller 111 and the tension roller 112. The ETB 110 is rotated by the drive roller 111. The ETB 110 conveys the recording medium P to each image forming station while adsorbing the recording medium P.

  The recording medium P on which the toner images of different colors are sequentially transferred at each image forming station is conveyed to a fixing device constituted by a pressure roller 113 and a heating film 114. An unfixed toner image is heated and pressurized by the fixing device and fixed on the recording medium P. The recording medium P is separated from the heating film 114 by the separation guide 116. The paper discharge roller 115 discharges the separated recording material P to the outside of the image forming apparatus 100.

  FIG. 2 is a diagram illustrating an example of the media sensor according to the embodiment. The light emitting element 201 is an LED for reflected light. The imaging element 202 is a photoelectric conversion element such as a photodiode. The condensing lens 203 is a lens that condenses the light from the light emitting element 201. The imaging lens 204 is a lens that forms an image of light on the imaging surface of the imaging element 210.

  The light condensed by the condenser lens 203 is applied to the surface of the conveyance guide 208 or the surface of the recording medium P. The reflected light from the recording medium P is condensed through the imaging lens 204 and imaged on the image sensor 202. Thereby, the image sensor 202 can read the surface image of the conveyance guide 208 or the recording medium P.

  In the present embodiment, the light from the light emitting element 201 is applied obliquely to the surface of the recording medium P. The light emitting unit 205 is disposed so as to face the media sensor 120. The light emitting element 206 is a transmitted light LED that emits light from the back side of the recording medium P. The condensing lens 207 is a lens for condensing the light from the light emitting element 206. The transport guide 208 is provided with a window for irradiating light from the back side of the recording medium. A light diffusion plate 211 made of polyacetal resin (POM) or the like diffuses light from the light emitting unit 205.

  When light is emitted from the light emitting element 206, the light is uniformed by the light diffusion plate 211. Therefore, the light diffusing plate 211 functions as a surface light source of light. That is, the light diffusing plate 211 prevents the light emission image of the light emitting element 206 from directly entering the imaging element 202. Therefore, since the image sensor 202 is not affected by the light emission image, it can detect the difference in the light reception level. Thereby, an OHT (overhead transparency) sheet or thin paper can be suitably identified.

In this manner, plain paper, an OHT sheet, glossy paper (glossy paper), and the like can be determined from information on reflected light from the recording medium irradiated from the light emitting element 201. That is, the type of recording material can be determined from the amount of reflected light. Of the light from the light emitting element 206, it is possible to discriminate between thin paper, plain paper, and thick paper from information on transmitted light that passes through the recording medium. That is, the thickness and basis weight of the recording medium can be determined from the transmitted light amount. Generally, thin paper refers to a recording medium having a basis weight of 64 g / m ² or less. Plain paper refers to a recording medium having a basis weight of 65 to 105 g / m ² . Cardboard refers to a recording medium having a basis weight of 106 to g / m ² .

  FIG. 3 is a diagram illustrating an example of the fixing device according to the embodiment. The fixing device includes a pre-fixing sensor 301 and a post-fixing sensor 302 for detecting that the recording medium P is wound around the fixing film 114. Both the pre-fixing sensor 301 and the post-fixing sensor 302 rotate counterclockwise when the leading edge of the recording medium P contacts, and output a signal indicating that the recording medium P has been detected. Usually, after the recording medium P is detected by the pre-fixing sensor 301, the recording medium P is detected by the post-fixing sensor 302 by a predetermined time. Therefore, if the recording medium P is not detected by the post-fixing sensor 302 after the recording medium P has been detected by the pre-fixing sensor 301, the winding has occurred. Reference numeral 303 denotes a heater for heating the fixing film 114. The amount of heat generated by the heater 303 can be adjusted by a control unit described later.

  FIG. 4 is a block diagram illustrating an example of a control unit according to the embodiment. The CPU 401 is a control circuit that comprehensively controls each unit of the image forming apparatus 100. When an image is formed in the borderless mode, the CPU 401 controls the image forming engine so as to reduce the occurrence of a jam in the recording medium due to curling. For example, in the area where an image can be formed by each station, there is a cause area that causes the recording medium to curl. Therefore, the CPU 401 determines a ratio between an image forming area where an image is actually formed and a non-image forming area where the image is not formed in the cause area. This ratio is, for example, a printing rate. Further, the CPU 401 executes jam reduction processing according to this ratio. The cause area does not necessarily exist only on the recording medium. The cause area may include an area on the transfer body larger than the size of the recording medium.

  The ROM 402 is a nonvolatile storage circuit, and stores a control program and various data. The RAM 403 is a volatile storage circuit and is mainly used as a work area. The laser lighting count circuit 404 is a circuit that counts the number of times the laser is turned on in accordance with a signal output from the video controller 410. Since a specific example of the laser lighting count circuit 404 is disclosed in Japanese Patent Application Laid-Open No. 2002-72770, etc., description thereof is omitted here. The CPU 401 calculates a printing rate from the number of lighting times acquired by the laser lighting counting circuit 404.

  The drive circuit 421 controls a motor group 422 that drives the pressure roller 113 of the fixing device and drives the registration roller 104 and other transport rollers. The heater control circuit 423 controls the power supplied to the heater 303 of the fixing device. Thereby, the amount of heat generated by the heater 303 is adjusted.

  The toner replenishment circuit 424 is a circuit that controls the amount of toner replenished to the developing roller 108 according to the toner consumption. The CPU 401 calculates the toner consumption amount from the laser lighting ratio information, the recording medium size information (area, etc.) and the M / S information. The information on the size of the recording medium may be acquired by a sensor for measuring the size, or may be determined based on information designated by the user from an operation unit (not shown). The M / S information is information representing a toner loading amount (M / S) for a toner image formed on the photosensitive drum 105 which is an image carrier. Generally, the applied toner amount is about 0.5 to 0.8 mg. Incidentally, the CPU 401 can calculate the toner consumption amount by the following equation.

    (Toner consumption per recording medium) = (Laser lighting ratio) × (Area of recording medium) × (M / S).

  When the registration sensor 121 detects the leading edge of the recording medium, the registration sensor 121 outputs leading edge registration information (referred to as a top signal) to the CPU 401. When a predetermined time elapses after the top signal is output, the image formation start position at the leading edge of the recording medium reaches the transfer position of the image forming unit 101. Therefore, the CPU 401 can specify the leading end portion of the recording medium. Therefore, the CPU 401 employs the printing rate when the leading end portion is specified as the leading end portion printing rate. Note that the tip portion is a predetermined portion with the side of the recording medium as the tip. This tip portion is also at least a part of a causal region that causes the winding.

  FIG. 5 is a diagram illustrating a recording medium, an image forming area, and a cause area according to the embodiment. FIG. 5A shows the recording medium P itself. FIG. 5B shows the recording medium P and the image forming area when performing borderless printing. In particular, the hatched portion 502 indicates an image forming area located outside the recording medium P. At the time of borderless printing, an image having a size larger than that of the recording medium is formed, so that an image is formed without substantially providing margins on the four sides of the recording medium P.

  FIG. 5C shows an example of a cause area where the printing rate is counted. The leading end portion 503 is a region having the upper side m of the recording medium P as the leading end. The rear end x of the front end portion 503 will be a few millimeters from the side m toward the center of the recording medium. Desirably, the position of the rear end x of the front end portion 503 is determined by experiment. This is because, for example, it is expected that the cause area (the image forming area set in advance) that causes the winding varies depending on the type and size of the recording medium.

  The rear end portion 504 is an area having the lower side n of the recording medium P as the rear end. The front end y of the rear end portion 504 will be at a position of about several mm from the side n toward the center of the recording medium. Desirably, the position of the tip y of the rear end portion 504 is determined by experiment. This is because, for example, the position of a causal area (an image forming area set in advance) that causes wrapping is expected to vary depending on the type and size of the recording medium.

  FIG. 5D shows an example of a cause area where the printing rate is counted. Two broken lines in the figure indicate the upper side m and the lower side n of the recording medium P, respectively. The leading edge portion 505 is an area having the leading edge at the image formation start position p in borderless printing. The rear end portion 506 is a region having the front end at the image formation end position q in borderless printing. It should be noted that the rear end of the front end portion 505 and the front end of the rear end portion 506 are desirably determined experimentally.

  FIG. 6 is a timing chart for counting the printing rate of the leading end portion of the recording medium. Here, it is assumed that the borderless print mode is applied as the image forming mode. As is well known, the borderless print mode is a mode in which an image is formed without substantially providing margins on four sides on a recording medium. This is in contrast to a normal image forming mode in which margins are provided on four sides on a recording medium to form an image.

  In FIG. 6, t1 is the time from the top signal to the image formation start position. t2 is the time from the image formation start position until the image is printed on the leading edge of the recording medium. In this embodiment, since the borderless print mode is applied, the image formation start position comes before the recording medium. t3 is the passage time of the recording medium. t4 is the time from the rear end of the recording medium to the image formation end position.

  t5 is the time from the top signal to the start of counting the printing rate at the tip portion 503. t6 is the time during which the leading end printing rate of the recording medium is counted. t7 is the time from the top signal to the start of image formation at the trailing edge portion 504 of the recording medium. t8 is the time for counting the printing rate at the trailing edge 504 of the recording medium.

  After detecting the top signal, the CPU 401 waits for (t1 + t2) and starts counting the leading edge printing rate. The CPU 401 counts the printing rate over a time t6 corresponding to the length of the cause area. The causal area is an area that causes the recording medium to curl. In general, the recording medium is jammed due to curling as described above.

  On the other hand, in the double-sided print mode, the CPU 401 waits for t7 after detecting the top signal, and then counts the printing rate at the rear end portion. The count of the printing rate at the rear end portion is executed over time t8.

  FIG. 7 is a flowchart illustrating an exemplary image forming process according to the first embodiment. This image forming process is started when an image signal is transmitted from the video controller 410.

  In step S <b> 701, the CPU 401 determines the type of the recording medium using the media sensor 120. In step S <b> 702, the CPU 401 determines whether the recording medium P is a recording medium that is relatively easily curled based on the determination result. The recording medium that easily causes curling is, for example, thin paper or an OHT sheet. On the other hand, the recording medium that hardly curls is plain paper or cardboard.

  If the recording medium is a thin paper or an OHT sheet, the process proceeds to step S703. On the other hand, if the recording medium is of a type other than thin paper or an OHT sheet, wrapping can be avoided by the separation guide 116 of the fixing device. Accordingly, the process proceeds to step S708, and the CPU 401 executes image formation in the normal print mode.

  In step S <b> 703, the CPU 401 calculates a period t <b> 6 for counting the printing rate at the leading end portion 503 of the recording medium from information such as the size and conveyance direction of the recording medium. In step S704, the CPU 401 counts the printing rate at the leading end portion 503 in accordance with the calculated period t6. Note that the printing rate at the tip portion 503 is calculated for each color. If necessary, the CPU 401 may count the printing rate in the rear end portion 504 according to the above-described period t8.

  In step S <b> 705, the CPU 401 determines whether a solid image for any color is formed on the leading end portion 503. For example, the CPU 401 determines whether any of the printing rates at the leading end portion 503 of each color is equal to or higher than the first threshold value. Needless to say, a complete solid image has a printing rate of 100%. If a solid image is included for one or more colors, there is a high possibility that the image will be wound around the fixing device. Therefore, the process advances to step S707, and the CPU 401 executes jam reduction processing. As a jam reduction process, for example, there is a method of decreasing the fixing speed, stopping the conveyance of the recording medium, or increasing the power supplied to the heater 303 in order to increase the amount of heat.

  On the other hand, if no solid image exists, the process proceeds to step S706. In step S706, the CPU 401 calculates the sum of the print ratios of the respective colors, and determines whether this sum is equal to or greater than a second threshold (eg, 200%). When the sum of the printing ratios exceeds the second threshold value, it is often difficult to avoid winding by adjusting the separation guide 116 alone. Therefore, the CPU 401 executes jam reduction processing in step S707. On the other hand, if the sum does not exceed the second threshold value, the process advances to step S708, and the CPU 401 executes image formation in the normal print mode.

  When changing the fixing speed as the jam reduction process, the CPU 401 sets the fixing speed to a lower speed than usual. When the fixing speed is high, an image with a high printing rate does not transmit sufficient heat to the toner layer. In this case, a part of the toner has a viscosity and becomes paste-like, so that it easily sticks to the fixing device. In the worst case, the recording medium is wound around the fixing film 114. Therefore, when the printing rate is high in the cause area such as the tip portion 503, it is preferable that the CPU 401 lowers the fixing speed to about ½ of the normal speed. As a result, sufficient heat is transmitted to the toner layer, so that the occurrence of jamming due to winding may be reduced.

  According to the present embodiment, the CPU 401 is a non-image that is not formed as an image forming area where an image is actually formed in a causal area that causes the recording medium to curl out of areas where an image can be formed by the image forming engine. The ratio (printing rate) with the formation area is determined. Then, the CPU 401 controls the image forming engine so as to reduce the occurrence of the jam of the recording medium due to the curl according to the determined ratio. This reduces the occurrence of jams in the recording medium due to curling.

  Further, the CPU 401 executes jam reduction processing when at least one of the ratios of the plurality of developers in the cause area is equal to or greater than the first threshold. If the ratio of one developer becomes an undesirable value, a jam occurs. Therefore, it is desirable to perform jam reduction processing.

  Such a first threshold may be, for example, 100% or a value close thereto. That is, when the solid image is formed in the cause area, the recording medium is easily wound around the fixing film 114. Therefore, when a solid image or the like that can substantially cause a jam is formed, it is desirable to perform a reduction process.

  Even when the sum of the ratios of the plurality of developers in the cause area is equal to or greater than the second threshold, jamming due to curling is likely to occur. Therefore, it is desirable for the CPU 401 to execute the reduction process. Such a second threshold is, for example, 200%.

  In the first embodiment, the reduction process is executed using the printing rate of the tip portion 503, but the printing rate of the tip portion 503 may be adopted instead. Of course, the printing rate of the tip portion 503 and the printing rate of the tip portion 503 may be considered.

[Second Embodiment]
FIG. 8 is a timing chart for counting the printing rate of the leading end portion 505 of the recording medium. In addition, the description is simplified by attaching | subjecting the same referential mark to the location already demonstrated in 1st Embodiment.

  In FIG. 8, t <b> 15 is the time from the top signal to the start of counting the printing rate at the tip portion 505. t16 is the time during which the leading end printing rate of the recording medium is counted. t18 is the time for counting the printing rate at the trailing edge 506 of the recording medium.

  Compared with FIG. 6, the printing rate counting periods t16 and t18 in FIG. 8 are set longer than the printing rate counting periods t6 and t8 in FIG. On the contrary, the time from the top signal to the start of counting the printing rate at the leading edge is shortened (t15 <t5). This is because the count of the printing rate is started from the time when the image formation in the sub-scanning direction is started by the vertical direction enable signal.

  As described above, in the borderless print mode, an image is formed over a size that is slightly larger than the size of the recording medium. for that reason. Image formation must be started before the leading edge of the recording medium reaches the transfer position. Therefore, in the second embodiment, counting of the printing rate at the tip portion is started with reference to the start timing of image formation. Accordingly, there is an advantage that the image formation control can be switched at an earlier stage than in the first embodiment.

  In general, the correlation between an image formed outside the four sides of the recording medium and an image formed inside the four sides of the recording medium is extremely high. Therefore, since the printing rate in the outer region and the printing rate in the inner region are almost equal, there is almost no problem in adding the printing rates in both regions.

  According to the second embodiment, the CPU 401 counts the printing rate at the tip portion. The count start and end timings are as shown in FIG. However, the count end timing may be set earlier than the timing shown in FIGS. This is because image formation control switching (S705 to S708) can be executed at an earlier timing by advancing the end timing.

  As described above, according to the second embodiment, if the printing rate of the leading end portion (e.g., period t16) with the image forming start position on the recording medium as the leading end is a value that can cause a jam, Perform jam reduction processing. As a result, jam reduction processing can be started at an earlier timing than in the first embodiment, so that the probability of jam occurrence can be reduced as compared with the first embodiment.

[Third Embodiment]
In the third embodiment, processing for reducing jamming caused by winding of a recording medium when image formation is performed on both sides of the recording medium will be described. When borderless printing is performed on both sides of the recording medium, the winding probability of the recording medium depends on the ratio (difference) in the printing rate of each side. For example, the front end portion 503 of the second surface exists behind the rear end portion 504 of the first surface. Therefore, if the printing rate A of the rear end portion of the first surface and the printing rate B of the front end portion of the second surface are substantially equal, the forces for curling on both surfaces will be balanced, and the winding around the fixing film will be Hard to occur. On the other hand, the greater the difference between the printing rate A of the rear end portion of the first surface and the printing rate B of the front end portion of the second surface, the higher the probability of winding. Therefore, in such a case, it can be said that it is desirable to execute the above-described jam reduction processing.

  FIG. 9 is a timing chart for counting the printing rate of the rear end portion 504 on the first surface according to the third embodiment. In addition, description is simplified by attaching | subjecting the same referential mark to the already demonstrated location. As apparent from comparison with FIG. 6, the printing rate is counted for the rear end portion 504 for the first surface. Instead of the rear end portion 504, a rear end portion 506 shown in FIG. 5D may be employed.

  FIG. 10 is a timing chart for counting the printing rate of the tip portion 503 on the second surface according to the third embodiment. In addition, description is simplified by attaching | subjecting the same referential mark to the already demonstrated location. As apparent from the comparison with FIG. 9, the printing rate is counted for the tip portion 503 for the second surface. Instead of the tip portion 503, the tip portion 505 shown in FIG. 5 (d) may be employed.

  FIG. 11 is a flowchart illustrating exemplary image forming processing according to the third embodiment. In addition, description is simplified by attaching | subjecting the same referential mark to the already demonstrated location. If it is determined in step S702 that the type of the recording medium is thin paper or an OHT sheet, the process proceeds to step S1103. If it is determined that the type of the recording medium is not thin paper or an OHT sheet, the process proceeds to step S115, and normal image formation is performed.

  In step S <b> 1103, the CPU 401 calculates a period t <b> 8 for counting the printing rate A in the rear end portion 504 of the first surface from information such as the size of the recording medium and the conveyance direction. Similarly, the CPU 401 calculates a period t6 for counting the printing rate B at the tip portion 503 of the second surface. In step S1104, the CPU 401 counts the printing rate A in the rear end portion 504 according to the calculated period t8. Note that the printing rate A in the rear end portion 504 is calculated for each color.

  In step S <b> 1105, the CPU 401 determines whether a solid image for any color is formed on the rear end portion 504 of the first surface. For example, the CPU 401 determines whether any of the printing rates in the rear end portion 504 of each color is equal to or higher than the first threshold value. Needless to say, a complete solid image has a printing rate of 100%. If a solid image is included for one or more colors, there is a high possibility that the image will be wound around the fixing device. Therefore, the process advances to step S1107, and the CPU 401 executes jam reduction processing.

  On the other hand, if there is no solid image, the process proceeds to step S1106. In step S1106, the CPU 401 calculates the sum of the printing ratios of the respective colors, and determines whether or not this sum is equal to or greater than a second threshold (eg, 200%). If the sum is equal to or greater than the second threshold value, the process advances to step S1108, and the CPU 401 executes image formation in the normal print mode.

  On the other hand, when the sum of the printing ratios exceeds the second threshold value, it is often impossible to avoid wrapping only by adjusting the separation guide 116. Therefore, the CPU 401 executes jam reduction processing in step S1107. Thereafter, in step S1108, the CPU 401 stores the printing rate A of the rear end portion 504 of the first surface in the RAM 403.

  In step S1110, the CPU 401 counts the printing rate B at the tip portion 503 of the second surface according to the calculated period t6. Note that the printing rate B in the rear end portion 504 is calculated for each color.

  In step S <b> 1111, the CPU 401 determines whether or not a solid image for any color is formed on the leading end portion 503 of the second surface. For example, the CPU 401 determines whether any of the printing rates B at the leading end portion 503 of each color is equal to or higher than the first threshold value. Needless to say, a complete solid image has a printing rate of 100%. If a solid image is included for one or more colors, there is a high possibility that the image will be wound around the fixing device. Therefore, the process advances to step S1114, and the CPU 401 executes jam reduction processing.

  On the other hand, if no solid image exists, the process proceeds to step S1112. In step S <b> 1112, the CPU 401 calculates the sum by summing up the printing ratios of the respective colors, and determines whether or not the sum exceeds a second threshold value (for example, 200%). When the sum of the print ratios is equal to or greater than the second threshold, the process proceeds to step S1114, and the CPU 401 executes jam reduction processing.

  On the other hand, if the sum is the second threshold, the process proceeds to step S1113. In step S <b> 1113, the CPU 401 determines whether or not a jam reduction process is necessary based on the ratio of the printing rate A of the rear end portion 504 of the first surface and the printing rate B of the front end portion 503 of the second surface. For example, if the printing rates on the first and second sides are similar (when the A / B ratio is close to 1), the difference in thermal shrinkage between the front and back of the recording medium is reduced, and curling is unlikely to occur. On the other hand, if the A / B ratio is sufficiently smaller than 1, it can be said that the printing rate on the first side is relatively small and the printing rate on the second side is relatively large. Accordingly, concave curls are likely to occur on the two surfaces. On the other hand, if the ratio of A / B is sufficiently larger than 1, it can be said that the printing rate of the first side is relatively large and the printing rate of the second side is small. Accordingly, a concave curl is likely to occur on one side. Generalizing these, when A / B <α or β <A / B (α << 1, 1 << β), jam reduction processing is required. The threshold values α and β are desirably obtained by experiments. This is because, for example, it is considered that the conditions for causing winding differ depending on the type, size, developer characteristics, and fixing device configuration of the recording medium.

  When the jam reduction process is necessary, in step S1114, the CPU 401 executes the jam reduction process. If jam reduction processing is not necessary, in step S1115, the CPU 401 executes image formation in the normal print mode.

  Although the example shown in FIG. 5C is adopted for the front end portion and the rear end portion in the third embodiment, the present invention is not limited to this. For example, the example shown in FIG. That is, instead of the printing rate A of the rear end part 504, the printing rate C of the rear end part 506 may be adopted. Further, instead of the printing rate B of the leading end portion 503, the printing rate D of the leading end portion 505 may be employed.

  As described above, according to the third embodiment, even when images are formed on both sides of the recording medium in the borderless print mode, it is possible to suitably suppress a jam that occurs due to curling.

  For example, if the difference between the printing rate A (C) of the rear end portion 504 (506) of the first surface and the printing rate B (D) of the front end portion 503 (505) of the second surface is large, a jam caused by curling is caused. It tends to occur. Therefore, in this case, it is preferable to form an image while performing jam reduction processing.

[Other Embodiments]
In the above-described embodiment, the printing rate is mainly counted using the laser lighting count circuit 404. However, the present invention is not limited by the printing rate acquisition method, and various acquisition methods can be employed. For example, the printing rate may be calculated from the toner consumption. Alternatively, the CPU 401 may determine the necessity of the jam reduction process based on the toner consumption amount at the front end portion or the rear end portion.

1 is a schematic cross-sectional view of an image forming apparatus according to an embodiment. It is a figure which shows an example of the media sensor which concerns on embodiment. 1 is a diagram illustrating an example of a fixing device according to an embodiment. It is a block diagram which shows an example of the control part which concerns on embodiment. FIG. 3 is a diagram illustrating a recording medium, an image forming area, and a cause area according to the embodiment. It is a timing chart at the time of counting the printing rate of the front-end | tip part of a recording medium. 4 is a flowchart illustrating exemplary image forming processing according to the first embodiment. 6 is a timing chart when counting the printing rate of the leading end portion 505 of the recording medium. It is a timing chart at the time of counting the printing rate of the rear-end part 504 in the 1st surface which concerns on 3rd Embodiment. FIG. 10 is a timing chart when counting the printing rate of the tip portion 503 on the second surface according to the third embodiment. 10 is a flowchart illustrating an exemplary image forming process according to a third embodiment. FIG. 6 is a diagram illustrating an example of a relationship between a leading edge margin of a recording medium and a winding occurrence frequency. It is the figure which showed an example of the relationship between the gap from a fixing film to a separation guide, and the frequency of winding generation | occurrence | production.

Claims (10)

An image forming apparatus for forming an image on a recording medium,
An image is formed by one of a first image forming mode in which a margin is provided on the recording medium to form an image and a second image forming mode in which an image is formed without providing a margin on the recording medium. Image forming means;
When an image is formed in the second image forming mode, a predetermined image forming area corresponding to the leading end portion or the trailing end portion of the recording medium among the regions where the image forming unit can form an image. A determining means for determining a printing rate;
An image forming apparatus comprising: a control unit that changes an image forming condition for forming an image in the image forming unit in accordance with the determined printing rate.   The printing rate is the printing rate at the leading end portion with the side of the recording medium as the leading end, the printing rate at the leading end portion with the image forming start position relating to the recording medium as the leading end, and 1 in the recording medium on which images are formed on both sides. The printing ratio of the rear end portion of the first surface and the front end portion of the second surface, or the image formation start on the rear end portion and the second surface with the image formation end position as the rear end on the first surface of the recording medium on which images are formed on both surfaces The image forming apparatus according to claim 1, wherein the print ratio is a printing rate of a tip portion having a position as a tip.   The control unit changes the image forming condition of the image forming unit when at least one ratio among the plurality of printing rates in the preset image forming region is equal to or greater than a first threshold value. The image forming apparatus according to claim 1.   4. The control unit according to claim 1, wherein the control unit changes an image forming condition of the image forming unit when a sum of a plurality of printing ratios in the preset image forming region is equal to or greater than a second threshold value. The image forming apparatus according to any one of the above.   The control means includes a printing rate A at the rear end portion with the first side of the recording medium on which the images are formed on both sides as a rear end, and a printing rate at the front end portion with the side of the recording medium on the second side as the front end. 3. The image according to claim 2, wherein the image forming condition of the image forming unit is changed when A / B <α or β <A / B (α << 1, 1 << β). Forming equipment.   The control means includes a print rate C of a rear end portion having an image formation end position as a rear end on the first surface of the recording medium on which images are formed on both sides and a front end portion having an image formation start position as a front end on the second surface. 3. The image forming condition of the image forming unit is changed when C / D <α or β <C / D (α << 1, 1 << β) with respect to the printing rate D. Image forming apparatus.   7. The image forming apparatus according to claim 1, wherein the control unit changes a fixing speed in the image forming unit as the image forming condition.   The image forming apparatus according to claim 1, wherein the control unit stops conveyance of the recording medium in the image forming unit.   The image forming apparatus according to claim 1, wherein the control unit increases a heat amount for heating the recording medium. A step of forming an image in one of a first image forming mode in which a margin is provided on a recording medium to form an image and a second image forming mode in which an image is formed without providing a margin on the recording medium. When,
When an image is formed in the second image forming mode, a preset image forming area corresponding to the leading end portion or the trailing end portion of the recording medium among the areas where the image can be formed by the image forming unit. A step of determining a printing rate;
And changing the image forming conditions for forming an image in accordance with the determined printing rate.

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