JP2011118238A - Image forming apparatus - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To obtain a favorable image without causing a hot offset or a fixation defect on the whole surface of a recording material even if the image is also formed at the end of the recording material. <P>SOLUTION: When the rear end of the recording material P is inserted into a fixation nip N, a heater is turned off (S106) to reduce a target heating temperature to be lower than that of a time when the center of the recording material P is inserted into the fixation nip N, and also, power supply to a motor M is turned off (S108) to make a fixation speed lower than a transfer conveying speed. <P>COPYRIGHT: (C)2011,JPO&INPIT

Description

  The present invention relates to an image forming apparatus that forms a permanent image by forming a toner image on a recording material and heating the recording material on which the toner image is formed. Examples of the recording material include paper, printing paper, transfer material sheet, OHT sheet, glossy paper, glossy film, electrofax paper, and electrostatic recording paper.

  An image forming apparatus used in a conventional electrophotographic printer will be described. In the image forming operation by the electrophotographic method, first, the surface of the photoreceptor having the photosensitive layer is uniformly charged. Next, the charged photoreceptor is exposed according to the image signal transmitted from the host computer to form an electrostatic latent image. The electrostatic latent image is visualized (toner image) with a developer (toner) and transferred to a recording material. The recording material on which the toner image is formed is fixed on the recording material by heating and melting the toner with a fixing device. Recently, it has been devised that the electrophotographic image forming apparatus realizes marginless printing (marginless printing) that has been put to practical use in an inkjet image forming apparatus (see, for example, Patent Document 1).

  In a fixing operation in an electrophotographic image forming apparatus, hot offset occurs when the heating temperature is too high, and fixing failure occurs when the heating temperature is too low. An appropriate fixed image can be obtained by fixing in an appropriate fixing temperature range in which these problems do not occur. As a fixing device that performs this fixing operation, a heat roller fixing device that uses a roller as a rotating member that transmits heat to a recording material and a surf fixing device that uses a sleeve are often used. In the heat roller fixing device, the heat capacity of the roller is large, and heat can be stored from a heat source that heats the roller. For this reason, when fixing the second half of the recording material, it has been devised to cut off the power supply to the heat source and maintain the appropriate fixing temperature of the roller with the remaining heat to suppress power consumption (see, for example, Patent Document 2).

JP 2004-45457 A JP-A-8-171307

  By the way, it has been found that in a borderless print that forms an image up to the end such as the trailing edge or the leading edge of the recording material, the proper fixing temperature is different between the terminal portion of the recording material and the other central portion. For this reason, when the end portion of the recording material is fixed, if the temperature is kept the same as that at the time of fixing the central portion of the recording material, a problem arises that hot offset of the end portion of the recording material occurs. In order to solve this problem, an attempt was made to lower the temperature at the time of fixing the end portion of the recording material by shutting off the energization of the heat source. However, when the temperature is lowered to a temperature at which no hot offset occurs at the trailing edge of the recording material, the temperature needs to be lowered during fixing of the central portion of the recording material, and the temperature at the central portion of the recording material is too low. There was a problem that fixing failure occurred. Therefore, there is a demand for an image forming apparatus that does not cause such hot offset or fixing failure.

  An object of the present invention is to obtain a good image without causing hot offset or fixing failure over the entire surface of the recording material even when an image is formed on the end portion of the recording material.

In order to solve the above problems, an image forming apparatus according to the present invention provides:
A transfer unit that transfers a toner image on the image carrier to a recording material at a predetermined transfer conveyance speed;
A heating nip is formed using a rotating body that is heated by a heat source and rotates at a fixing speed, and the toner on the recording material is heated and melted while the recording material on which the toner image is transferred is sandwiched and conveyed by the heating nip. A fixing unit for fixing
A member heated by the heat source or a temperature detecting member for detecting the temperature of the heat source;
A heating control unit that controls electric power supplied to the heat source so that the detected temperature detected by the temperature detecting member approaches the target heating temperature;
An image forming apparatus comprising:
In a mode in which an image is formed within a certain width up to the trailing edge of the recording material, when the trailing edge portion of the recording material passes through the heating nip, the target heating temperature is set at the central portion of the recording material. And the fixing speed is made slower than the transfer conveyance speed.

In order to solve the above problems, an image forming apparatus according to the present invention includes:
A transfer unit that transfers a toner image on the image carrier to a recording material at a predetermined transfer conveyance speed;
A heating nip is formed using a rotating body that is heated by a heat source and rotates at a fixing speed, and the toner on the recording material is heated and melted while the recording material on which the toner image is transferred is sandwiched and conveyed by the heating nip. A fixing unit for fixing
A member heated by the heat source or a temperature detecting member for detecting the temperature of the heat source;
A heating control unit that controls electric power supplied to the heat source so that the detected temperature detected by the temperature detecting member approaches the target heating temperature;
An image forming apparatus comprising:
In the mode of forming an image within a certain width up to the leading edge of the recording material, when the leading edge portion of the recording material passes through the heating nip, the target heating temperature is set to the target heating temperature, and the central portion of the recording material passes through the heating nip. And the fixing speed is made slower than the transfer conveyance speed.

  According to the present invention, even when an image is formed on the end portion of the recording material, a good image can be obtained without causing hot offset or fixing failure on the entire surface of the recording material.

Side sectional view of the image forming apparatus in Embodiment 1 Side sectional view of the fixing device in Embodiment 1. FIG. 6 is a diagram illustrating a fixing flow at the time of borderless printing in the first embodiment. The figure explaining the temperature transition in Example 1 FIG. 6 is a diagram for explaining a pressure distribution in a trailing end portion of a recording material in the first embodiment. FIG. 6 is a diagram for explaining an appropriate fixing area in the first embodiment. FIG. 6 is a diagram for explaining an appropriate fixing area in the first embodiment. FIG. 6 is a diagram illustrating a fixing flow at the time of borderless printing in the second embodiment. FIG. 6 is a diagram illustrating fixing speed and heating temperature control in the second embodiment. FIG. 6 is a diagram for explaining an appropriate fixing area in the second embodiment. Side sectional view of the image forming apparatus in Embodiment 3 FIG. 6 is a diagram for explaining a fixing flow at the time of borderless printing in the third embodiment. FIG. 6 is a diagram illustrating fixing speed and heating temperature control in Embodiment 3.

DESCRIPTION OF EMBODIMENTS Hereinafter, embodiments for carrying out the present invention will be exemplarily described in detail with reference to the drawings. However, the dimensions, materials, shapes, relative arrangements, and the like of the components described in this embodiment are not intended to limit the scope of the present invention only to those unless otherwise specified. .

<Example 1>
(1) Image forming apparatus (FIG. 1)
FIG. 1 is a schematic configuration diagram of an image forming apparatus. The image forming apparatus of this example is an inline electrophotographic full color printer, and can form 20 A4 size full color images per minute. At this time, the image forming speed and the transfer conveying speed are set to 120 mm / s. This configuration includes photosensitive drums 11a to 11d corresponding to yellow, magenta, cyan, and black color toners as photosensitive members as image carriers, and the transfer belt 20 is transferred to each of the photosensitive drums 11a to 11d. In contact. The transfer belt 20 has a thickness of 0.1 mm, in which carbon is dispersed in a polyimide resin and the volume resistivity is adjusted to 10 ⁸ Ω · cm. In each transfer portion, transfer rollers 15a to 15d, in which a core metal is coated with an elastic material having a medium resistance (actual resistance at the time of forming a nip when 500 V is applied, 10 ⁶ to 10 ¹⁰ Ω), together with the photosensitive drums 11a to 11d, are transfer belts. 20 are arranged in a sandwiched manner.

(2) Fixing device 10 (FIG. 2)
FIG. 2 is a side sectional view of the fixing device 10. A fixing device 10 as a fixing unit mainly includes a heating unit 1 and a pressure roller 3 forming a pair of rotating bodies. The pressure roller 3 is an elastic roller having an outer diameter of 20 mm, in which the outer periphery of the core metal 3a is covered with a 3 mm-thick silicone rubber layer 3b and the outer periphery thereof is further covered with a 50 μm-thick PFA resin 3c. The heating unit 1 has a heating sleeve 1a loosely fitted around a heater holder 1c that supports a ceramic heater 2, and is supported and fixed by a support stay 1b. The heating sleeve 1a corresponds to the rotating body of the present invention. The fixing nip N, which is a heating nip, is formed when the pressure roller 3 is pressed and pressed against the heating unit 1. The heating sleeve 1a is a stainless base layer having a thickness of 20 μm coated with a 300 μm silicone rubber (thermal conductivity 1.6 W / (m · K)), and a surface thereof is coated with a 20 μm fluororesin. .4 mm. The ceramic heater 2 is formed by printing a resistor as a heating element 2a as a heat source on an aluminum nitride 2b having a width of 6 mm and a thickness of 0.7 mm perpendicular to the recording material conveyance direction, and glass (not shown) on the resistor. ) With an output of 1200 W (when 100 V is input). A thermistor 5 serving as a temperature detection member is in contact with the back surface of the ceramic heater 2. The thermistor 5 is urged by a leaf spring and is in contact with the approximate center in the longitudinal direction perpendicular to the recording material conveyance direction of the ceramic heater 2. Thereby, the thermistor 5 detects the temperature of the ceramic heater 2 which is a member heated by the heat source. The thermistor 5 may directly detect the temperature of the heating element 2a that is a heat source, or may detect the temperature of the heating sleeve 1a that is a member heated by the heat source. The heating element 2a of the ceramic heater 2 is electrically connected to a power supply substrate controlled by a heating control device 100 as a heating control unit of the image forming apparatus, and the thermistor 5 is connected to the heating control device of the image forming apparatus by a signal line. 100 is electrically connected. The heating control device 100 compares the detected temperature of the thermistor 5 with a preset target heating temperature (heating temperature), and when the detected temperature is low, the power supplied to the ceramic heater 2 is increased and the detected temperature is high. In some cases, control is performed to reduce the power supply. That is, the heating control apparatus 100 controls the electric power supplied to the heating element 2a so that the detected temperature detected by the thermistor 5 approaches the target heating temperature. In this example, the heating temperature for the recording material of 90 gm ² paper is set to 180 ° C. The heating temperature may be changed according to the recording material. The pressure roller 3 is rotationally driven clockwise as indicated by an arrow in FIG. 2 by a motor M serving as a driving device, and the heating sleeve 1 a is driven to rotate by the pressure roller 3 due to friction in the fixing nip N. A recording material P carrying an unfixed toner image t as a material to be heated is introduced into a fixing nip N between the heating unit 1 and the pressure roller 3 so that the recording material P is placed on the outer surface of the heating sleeve 1a. It closely contacts and passes through the fixing nip N together with the heating sleeve 1a. Then, the toner of the toner image t is heated and melted by heat conduction from the heating sleeve 1a, and the toner image is fixed on the recording material. The recording material P that has passed through the fixing nip N is conveyed with its curvature separated from the outer surface of the heating sleeve 1a on the outlet side of the fixing nip N. As the fixing device 10, in addition to the surf heating method, a roller heating method, an external heating method, or the like can be used. In short, the fixing device 10 can change the speed at which the recording material P is conveyed in the fixing nip N, and can detect the temperature of the heat source or the temperature of the member heated by the heat source and supply the power supplied to the heat source. Anything that is controlled can be used.

(3) Image forming operation (FIG. 1)
The photosensitive drum 11a rotates in the direction of the arrow shown in the figure and is uniformly charged by the primary charger 12a. Image data transmitted from the host computer is converted into laser emission intensity and emission time by image data processing, and a laser beam is emitted from the scanner 13a to create an electrostatic latent image on the charged photosensitive drum 11a. The intensity of the laser beam and the irradiation spot diameter are appropriately set according to the resolution of the image forming apparatus and the desired image density. In the electrostatic latent image on the photosensitive drum 11a, the portion irradiated with the laser light is at the bright portion potential VL (about −100V), and the other portion is charged by the primary charger 12a and the dark portion potential VD (about −700V). It is formed by being held in. The electrostatic latent image reaches a portion facing the developing device 14a by the rotation of the photosensitive drum 11a, and is supplied with toner charged with the same polarity (minus polarity in this example) to be visualized (toner image). . In full-color image formation, toner images are similarly formed on the photosensitive drums 11b to 11d corresponding to the respective colors and sequentially transferred onto the recording material P conveyed at the transfer conveyance speed together with the transfer belt 20 in each transfer nip of each transfer unit. Each toner image is synthesized. At each transfer nip formed by the transfer belt 20 and the photosensitive drums 11a to 11d, the toner image is transferred by an electric field formed in the transfer nip region by a voltage (+500 to + 4000V) having a polarity opposite to that of the toner applied to the transfer rollers 15a to 15d. Is done. When the recording material P passes through the transfer nip with the photosensitive drum 11d, the full color image is carried on the recording material P, and the transfer process is completed. On the other hand, the surfaces of the photosensitive drums 11a to 11d after the transfer of the toner images are cleaned by the cleaning devices 16a to 16d, respectively, and then prepared for the next image forming process. The voltage (transfer voltage) supplied to the transfer rollers 15a to 15d is measured by the current detection circuit 18 when a predetermined voltage is applied to the transfer roller 15a before the recording material is supplied. Then, the control device 19 calculates and determines the resistance of the transfer members (transfer rollers 15a to 15d and transfer belt 20). By this control, it is possible to absorb a change in resistance of the transfer member due to the environment where the transfer member is placed, particularly moisture absorption, and to supply a constant transfer charge, thereby maintaining a stable image quality. After the transfer process, the recording material P is separated from the transfer belt 20 by the curvature of the driving roller 20a, reaches the fixing device 10, and is heated and pressed to obtain a permanent fixed image. Before the rear end of the recording material P is separated from the transfer belt 20, the recording material P straddles the fixing nip N, and therefore the fixing speed needs to match the transfer conveyance speed. . If the fixing speed is too fast or too slow, the speed of the recording material P being transferred becomes unstable, causing a problem that the color registration is not suitable. In this example, using a loop sensor (not shown) for detecting the loop state created by the recording material P, the speed of the motor M (FIG. 2) is adjusted in the range of -2 to + 2%, and the color registration is affected. It is controlled not to give.

(4) Borderless printing operation (Fig. 3)
In this image forming apparatus, in addition to normal printing in which margins having a constant width are provided on the four sides of the recording material, borderless printing without margins is performed. In this example, in the marginless printing mode, the front end portion where the accuracy of the registration mechanism is easily secured is limited to a margin of 0 to 0.5 mm, and the left and right ends and the rear end that are easily affected by paper conveyance are not sensitive. Electrostatic latent images are formed in areas larger than the recording material P on the drums 11a to 11d. The specific size of the electrostatic latent image is formed 2 mm outside each side of the recording material with respect to the left and right ends and the rear end in consideration of the registration variation of the recording material P. The electrostatic latent image is transferred to the recording material P through a process similar to the above-described image forming operation, so that a substantially marginless image is formed on the recording material P, although there is some margin at the tip. Formed on top. Since the toner image (excess toner) larger than the recording material P exists on the transfer belt 20 after the transfer process, these transfer toners are cleaned by the transfer cleaning device 21, and the transfer belt 20 performs the next image formation. Prepare for. The recording material P separated from the transfer belt 20 is heated and fixed while being nipped and conveyed by the fixing nip N of the fixing device 10.

  In the fixing operation in borderless printing, as shown in the flow of FIG. 3, the heating temperature is lowered and the fixing speed is lowered by lowering the power supply to the heater 2 during the fixing operation of one recording material P. The rear end portion of P is fixed. After the trailing edge of the recording material P is separated from the transfer belt 20, the fixing speed is made slower than the transfer conveyance speed at the timing when the trailing edge of the recording material P and its vicinity (rear edge portion) approach the fixing nip N. Even if the fixing speed is manipulated just after the trailing edge of the recording material P passes through the fixing nip N, it is difficult to reduce it to the target speed due to an electrical or mechanical delay. In this case, it becomes impossible to secure an interval (inter-paper) with the recording material P on the next page. Therefore, the rear end portion referred to in this example represents a time in which the above delay is taken into account and a range in which a space between sheets can be secured. In the control of this example, the rear end of the recording material P is separated from the transfer belt 20 at the timing when the position 70 mm from the rear end is sandwiched by the fixing nip N. Then, at the timing when the portion 50 mm from the rear end of the recording material P is sandwiched by the fixing nip N, the energization to the ceramic heater 2 is turned off (S106). In parallel with this energization OFF operation, the power supplied to the motor M is turned off at a timing of 35 mm from the rear end (S108), and the recording material P is conveyed to the rear end by inertia.

Next, the flow of FIG. 3 will be specifically described. FIG. 3 is a diagram for explaining a fixing flow when performing borderless printing of the recording material P. First, when the leading edge of the recording material P has not yet been inserted into the fixing nip N, energization of the ceramic heater 2 is turned on to control the temperature between sheets (S101). In the inter-paper temperature control, since the recording material P is not held in the fixing nip N, the target heating temperature of the ceramic heater 2 is maintained at a temperature that is lower than the proper fixing temperature but can be immediately raised to the proper fixing temperature. At the same time, the fixing speed for rotating the heating sleeve 1a is matched with the transfer conveyance speed (120 mm / s) (S102). Next, it is determined whether or not the leading end of the recording material P is inserted into the fixing nip N (S103). If a positive determination is made in S103, the process proceeds to S104. On the other hand, if a negative determination is made in S103, the determination in S103 is repeated. In S104, the fixing temperature is adjusted while energization of the ceramic heater 2 is ON (S104). In fixing temperature control, control is performed to maintain the target heating temperature of the ceramic heater 2 at an appropriate fixing temperature. Here, the fixing suitability temperature is a temperature at which the toner image formed on the central portion of the recording material P is satisfactorily fixed. Next, it is determined whether or not a portion 50 mm from the rear end of the recording material P is held by the fixing nip N (S105). If a positive determination is made in S105, the process proceeds to S106. On the other hand, if a negative determination is made in S105, the determination in S105 is repeated. In S106, the energization to the ceramic heater 2 is turned off (S106). Thereby, when the rear end portion of the recording material P passes through the fixing nip N, the target heating temperature can be lowered than when the central portion of the recording material P passes through the fixing nip N. In this example, since the surf heating method having a small heat capacity is used as the fixing device 10, the response speed of the heating temperature to the power supply is relatively high. When the energization is turned off at the timing of S106, the detected temperature of the thermistor 5 changes as shown in FIG. 4 and reaches about 160 ° C. when the trailing edge of the recording material P passes through the fixing nip N. Next, it is determined whether or not a portion 35 mm from the rear end of the recording material P is held by the fixing nip N (S107). If a positive determination is made in S107, the process proceeds to S108. On the other hand, if a negative determination is made in S107, the determination in S107 is repeated. In S108, the power supplied to the motor M is turned off (S108), and the recording material P is conveyed to the rear end by inertia. Accordingly, when the trailing end portion of the recording material P passes through the fixing nip N, the fixing speed can be made slower than the transfer conveyance speed when the central portion of the recording material P passes through the fixing nip N. In this example, when the power supply to the motor M is stopped at the timing of S108, the conveyance speed when the trailing edge of the recording material P passes through the fixing nip N is about 50 mm / s. Next, it is determined whether or not the rear end of the recording material P has come out of the fixing nip N (S109). If a positive determination is made in S109, the process proceeds to S110. On the other hand, if a negative determination is made in S109, the determination in S109 is repeated. When the trailing edge of the recording material P leaves the fixing nip N, power is again supplied to the motor M to make the fixing speed coincide with the transfer conveyance speed (120 mm / s) (S110). At the same time, energization of the ceramic heater 2 is turned on to adjust the temperature between the sheets (S111). Note that the timing of these operations used in this example is an example, and the timing of executing S106 may be adjusted by the heat capacity of the fixing device 10 and the timing of S108 is determined by the motor M, the pressure roller 3, and the like. It may be determined by the inertia force.

(5) Discussion Next, operational effects obtained by the operation shown in the flow of FIG. 3 will be described. In this example, the behavior of the rear end portion of the recording material P will be considered. FIG. 5 shows a state immediately before the trailing edge of the recording material P is discharged from the fixing nip N, and also shows the pressure distribution at that time. The pressure distribution can be obtained by inserting a pressure sensitive film together with the recording material P and measuring it. It is shown that a pressurizing force higher than usual is applied at a position corresponding to the rear end of the recording material P. This is presumably because the rear end of the recording material P is a portion that receives the deformation starting point of the silicone rubber layer 3b of the pressure roller 3, and thus a large pressure is applied locally.

  In general, it is considered that the fixing performance involves two elements, temperature and pressure. The temperature is an indispensable condition for heating and melting the toner, and the applied pressure is a condition for efficiently performing the heating and melting operation. Therefore, even if the heating sleeve 1a is kept at the same temperature, different fixing performance is obtained when the pressing force is different. In other words, the heating temperature optimally set for the normal pressure (average pressure in the figure) is the temperature that causes excessive heat supply to the end portion (terminal portion) that is locally high in pressure. It becomes. As a result, the toner is overmelted, and the affinity with the surface of the heating sleeve 1a is increased, and a hot offset phenomenon occurs in which the toner stains the surface of the heating sleeve 1a.

FIG. 6 shows a combination of the heating temperature and the applied pressure at which such hot offset occurs when the heating time is set to 85 ms. The curve in the figure shows the limit heating temperature at which hot offset does not occur under a predetermined pressure, and hot offset occurs on the region (I) side. If the heating temperature is set to T _1, although good fixation if pressure P ₁ obtained, the same at a heating temperature pressure P ₂ in the hot offset as indicated by the arrow (a) is generated. To avoid hot offset occurred at pressure P _2, as indicated by the arrow (b), it is necessary to lower the heating temperature to T _2. In the borderless printing that forms an image up to the end of the recording material P, since different pressurizing conditions coexist as shown in FIG. 5, the end portion and the central portion of the recording material P are at the same temperature. It has been found that it is difficult to obtain a good fixed image.

Next, attention is focused on the margin between hot offset and fixing failure when the fixing speed is lowered. FIG. 7 shows the heating temperature at which fixing failure occurs when the fixing speed is changed in a room temperature environment (15 to 27 ° C.) and the heating temperature at which hot offset occurs (at the center). It is expressed with respect to the applied pressure. In the region where the fixing speed is high, there is almost no margin for fixing failure and hot offset (F ₂ ), whereas when the fixing speed is lowered, a wide temperature region (F ₁ ) where neither occurs is secured. it can. This is probably because the fixing failure depends on the amount of heat for melting the entire toner image, whereas the hot offset is related to the maximum temperature of the toner image. That is, when the fixing speed is reduced and heating is performed for a long time, the entire toner image can be melted while suppressing local heating of the toner image. On the other hand, when heating at a high fixing speed in a short time, the toner surface reaches a high temperature at which the toner surface is locally hot offset. Such a tendency is applicable to a fixing operation that melts while performing heat transfer.

In this example, in order to use such a tendency, the operation shown in FIG. 3 described above is performed, the fixing speed is lowered while the heating temperature is lowered, and the hot offset of the rear end portion and the fixing failure other than the rear end portion are detected. A configuration that satisfies the above has been realized.

(6) Comparative Experiment this embodiment in order to make a comparison with the conventional method, while the heating temperature was kept at T ₁ as Comparative Example 1, a case of fixing to the recording material trailing end portion, recorded as Comparative Example 2 the heating temperature in the timber rear end experiments were performed by taking a case where the fixing is lowered from the heating temperature T ₁ of such that T _2. In the experiment, fixing failure when printing 10 pages of a red image on the entire surface of a recording material of 90 g / m ² and hot offset when printing 10 pages of a monochrome black belt having a width of 10 mm at the rear end were confirmed visually. Fixing failure was scored as “10” for pages that were lightly rubbed and peeled off, “0” for pages that were not peeled at all, and “5” in the middle. The hot offset was “10” for pages that could be seen at first glance, “0” for pages that were not found at all, and “5” for pages that were not clearly understood. As shown in Table 1, it can be seen that in this embodiment, neither hot offset nor fixing failure is seen, and the structure is compatible. Meanwhile, since the use of the heating temperatures T ₁ with respect to the rear end portion in Comparative Example 1, the hot offset occurs at a high frequency. Further, although it is possible to suppress the occurrence of Comparative Example 2, the hot offset, to lower left heating temperature T ₂ was kept the fixing speed, fixing failure occurs in the second half of the image by the insufficient heat.

  As described above, in this example, when an image is also formed on the rear end portion of the recording material P, when the rear end portion of the recording material P is inserted into the fixing nip N, an appropriate fixing operation is performed under a wide heating temperature condition. It can be performed. Therefore, even when an image is formed on the rear end portion of the recording material P, a good image can be obtained without causing hot offset or fixing failure over the entire recording material.

<Example 2>
The image forming apparatus of this example is the same as the apparatus of Example 1 except that the fixing speed and heating temperature control during borderless printing shown in FIG. 8 is performed. Hereinafter, this difference will be described. In the fixing operation in borderless printing, as shown in the flow of FIG. 8, the heating temperature is lowered and the fixing speed is lowered during the fixing operation of one recording material P, and the rear end portion of the recording material P is fixed. The rear end of the recording material P is separated from the transfer belt 20, and the heating temperature and the fixing speed are controlled according to the distance to the fixing nip N as shown in FIG. 9 (S206 and S207). As shown in FIG. 7 in the first embodiment, the heating temperature and the fixing speed are changed while selecting conditions under which neither high-temperature offset nor fixing failure occurs. Therefore, good fixing ability can be ensured throughout the area.

Next, the flow of FIG. 8 will be specifically described. FIG. 8 is a diagram for explaining a fixing flow when performing borderless printing of the recording material P. First, when the leading end of the recording material P has not yet been inserted into the fixing nip N, energization of the ceramic heater 2 is turned on to control the temperature between sheets (S201). At the same time, the fixing speed for rotating the heating sleeve 1a is matched with the transfer conveyance speed (120 mm / s) (S202). Next, it is determined whether or not the leading end of the recording material P is inserted into the fixing nip N (S203). If a positive determination is made in S203, the process proceeds to S204. On the other hand, if a negative determination is made in S203, the determination in S203 is repeated. In S204, the fixing temperature is adjusted while energization of the ceramic heater 2 is ON (S204). Next, it is determined whether or not a portion 60 mm from the rear end of the recording material P is held by the fixing nip N (S205). If a positive determination is made in S205, the process proceeds to S206. On the other hand, if a negative determination is made in S205, the determination in S205 is repeated. In S206, the energization amount to the ceramic heater 2 is decreased stepwise and the heating temperature is decreased stepwise as shown in FIG. 9 (S206). Thereby, when the rear end portion of the recording material P passes through the fixing nip N, the target heating temperature can be lowered than when the central portion of the recording material P passes through the fixing nip N. At the same time, the amount of power supplied to the motor M is decreased stepwise to decrease the speed of the motor M stepwise as shown in FIG. 9 (S207). Accordingly, when the trailing end portion of the recording material P passes through the fixing nip N, the fixing speed can be made slower than the transfer conveyance speed when the central portion of the recording material P passes through the fixing nip N. Next, it is determined whether or not the trailing edge of the recording material P has come out of the fixing nip N (S208). If a positive determination is made in S208, the process proceeds to S209. On the other hand, if a negative determination is made in S208, the determination in S208 is repeated. When the trailing edge of the recording material P leaves the fixing nip N, the amount of electric power supplied to the motor M is increased again to make the fixing speed coincide with the transfer conveyance speed (120 mm / s) (S209). At the same time, the temperature between sheets is adjusted with the amount of current supplied to the ceramic heater 2 substantially unchanged (S210). Note that the timing of these operations used in this example is an example and may be changed depending on the situation.

  The feature of this example is that a stable fixing operation can be ensured by changing the heating temperature and the fixing speed in accordance with a predetermined profile, and it is possible to flexibly cope with a change in the state of the image forming apparatus. The torque required for the motor M may increase as the total number of printed sheets of the image forming apparatus advances and the sliding portion wears. Even in such a case, in this example, the speed of the motor M is actively controlled, and it becomes possible to faithfully control according to the motor speed (fixing speed) shown in FIG. Further, even when the temperature and moisture content of the recording material change due to changes in environmental temperature and humidity, the heating temperature can be traced according to the profile shown in FIG. Furthermore, the optimum heating temperature and fixing speed shown in FIG. 7 may change depending on the temperature and moisture content of the recording material. FIG. 10 shows the optimum condition (colored area) at room temperature shown in FIG. 7 and the optimum condition (area sandwiched between broken lines) in an environment of room temperature 10 ° C. and humidity 50%. is there. In order to obtain a good fixing state with a recording material in a low temperature and high humidity environment, a larger amount of heat is required. For this reason, the optimum condition is shifted to the high temperature side as compared with the normal temperature environment. This example can maintain good fixing ability by adjusting the heating temperature or fixing speed against such environmental changes. The image forming apparatus having this configuration can exhibit the effect even for personal use apparatuses that are required to have higher environmental adaptability. As described above, in this example, when an image is also formed on the rear end portion of the recording material P, when the rear end portion of the recording material P is inserted into the fixing nip N, an appropriate fixing operation is performed under a wide heating temperature condition. It can be performed. Therefore, even when an image is formed on the rear end portion of the recording material P, a good image can be obtained without causing hot offset or fixing failure over the entire recording material.

<Example 3>
(1) Image forming apparatus (FIG. 11)
FIG. 11 is a schematic configuration diagram of the image forming apparatus. The image forming apparatus of this example is an inline electrophotographic full color printer, and can form 20 A4 size full color images per minute. At this time, the image forming speed and the transfer conveying speed are set to 120 mm / s. This configuration has photosensitive drums 11a to 11d corresponding to yellow, magenta, cyan, and black color toners as photosensitive members, and the intermediate transfer belt 22 contacts the photosensitive drums 11a to 11d at their primary transfer portions. is doing. The resistance of the intermediate transfer belt 22 is 0.1 mm thick with the volume resistivity adjusted to 10 ¹⁰ Ω · cm by dispersing carbon in polyimide resin. In each of the transfer portions, transfer rollers 15a to 15d that perform primary transfer in which a core metal is coated with an elastic material having a medium resistance (actual resistance in the nip formation when 500 V is applied is 10 ⁶ to 10 ¹⁰ Ω) are photosensitive drums 11a to 11a. 11d and the intermediate transfer belt 22 are disposed therebetween. The secondary transfer roller 23 secondarily transfers the toner image on the intermediate transfer belt 22 to the recording material P. The secondary transfer roller 23 covers the core metal with an elastic material having a medium resistance (actual resistance in nip formation when 500 V is applied is 10 ⁶ to 10 ¹⁰ Ω), and the surface layer is 10 ¹⁰ to 10 ¹³ Ω · cm. A conductive PFA tube having a volume resistivity of 5 mm is provided. The fixing device 10 as a fixing unit is equipped with the same configuration as that used in the first embodiment.

(2) Image forming operation (FIG. 11)
The photosensitive drum 11a rotates in the direction of the arrow shown in the figure and is uniformly charged by the primary charger 12a. Image data transmitted from the host computer is converted into laser emission intensity and emission time by image data processing, and a laser beam is emitted from the scanner 13a to create an electrostatic latent image on the charged photosensitive drum 11a. The intensity of the laser beam and the irradiation spot diameter are appropriately set according to the resolution of the image forming apparatus and the desired image density. In the electrostatic latent image on the photosensitive drum 11a, the portion irradiated with the laser light is at the bright portion potential VL (about −100V), and the other portion is charged by the primary charger 12a and the dark portion potential VD (about −700V). It is formed by being held in. The electrostatic latent image reaches a portion facing the developing device 14a by the rotation of the photosensitive drum 11a, and is supplied with toner charged with the same polarity (minus polarity in this example) to be visualized (toner image). . In full-color image formation, toner images are similarly formed on the photosensitive drums 11b to 11d corresponding to the respective colors, and are sequentially primary-transferred at each primary transfer nip onto the intermediate transfer belt 22 that is conveyed at the transfer conveyance speed to synthesize the toner images. . At each primary transfer nip formed by the intermediate transfer belt 22 and the photosensitive drums 11a to 11d, the toner image is primarily transferred by an electric field formed by a voltage (+100 to +1500 V) having a polarity opposite to that of the toner applied to the transfer rollers 15a to 15d. Is done. The voltage (primary transfer voltage) supplied to the primary transfer rollers 15a to 15d is measured by the current detection circuit 18 when a predetermined voltage is applied to the primary transfer roller 15a. Then, the control device 19 calculates and determines the resistance of the transfer members (the primary transfer rollers 15a to 15d and the intermediate transfer belt 22). By this control, it is possible to absorb a change in resistance of the transfer member due to the environment where the transfer member is placed, particularly moisture absorption, and to supply a constant transfer charge, thereby maintaining a stable image quality. At the stage of passing through the transfer nip with the photosensitive drum 11d, a full-color image is carried on the intermediate transfer belt 22, and the primary transfer process is completed. The composite toner image transferred onto the intermediate transfer belt 22 is transferred onto the recording material P conveyed at a transfer conveyance speed by a secondary transfer voltage (+500 to +5000 V) having a polarity opposite to that of the toner applied to the secondary transfer roller 23. Secondary transfer electrostatically. The secondary transfer residual toner existing on the intermediate transfer belt 22 is removed by the transfer cleaning device 21, and the intermediate transfer belt 22 prepares for the next image formation. The recording material P that has finished the secondary transfer process is transported by the transport belt 24 to reach the fixing device 10 and is heated and pressurized to obtain a permanent fixed image. In this example, the conveyance distance by the conveyance belt 24 is set to be longer than the length of the recording material P, and after the trailing edge of the recording material P is secondarily transferred, the leading edge of the recording material P enters the fixing nip N. Inserted. Therefore, the transfer belt 24 can be used as a speed buffer member so that the transfer conveyance speed and the fixing speed can be different.

(3) Borderless printing operation (FIGS. 12 and 13)
In this image forming apparatus, borderless printing is performed in addition to normal printing in which general margins are provided on the four sides of the recording material. When performing borderless printing, an electrostatic latent image larger than the recording material P is formed on the photosensitive drums 11a to 11d. The size of the electrostatic latent image is set to an image size obtained by adding 4 mm to the size of the recording material P in consideration of the registration variation of the recording material P, and is formed from each side of the recording material to 2 mm outside. With respect to this latent image, a toner image is transferred to the recording material P through the same process as the above-described image forming operation, whereby a borderless image is formed on the recording material P. In the secondary transfer step, a toner image (excess toner) larger than the recording material P is transferred onto the secondary transfer roller 23, so that the excess toner is removed by the transfer cleaning mechanism 25, and the recording material P The back side is prevented from being stained with this excess toner. After the trailing edge of the recording material P finishes the secondary transfer process, the speed of the transport belt 24 is reduced from the transfer transport speed, and the recording material P is nipped and transported to the fixing nip N.

  In the fixing operation in borderless printing, as shown in the flow of FIG. 12, after the leading end of the recording material P is inserted into the fixing nip N, the heating temperature is raised and the fixing speed is returned to the transfer conveyance speed. Fix it. Specifically, before the leading edge of the recording material P is inserted into the fixing nip N, the speed of the conveying belt 24 and the fixing speed are maintained at 50 mm / s. Then, at the timing when the leading edge of the recording material P is nipped by the fixing nip N, the heating temperature is changed simultaneously with returning to the transfer conveying speed while controlling the conveying belt 24 and the fixing speed as shown in FIG. 13 (S304). And S305). As shown in FIG. 7 in the first embodiment, the heating temperature and the fixing speed are changed while selecting conditions under which neither high-temperature offset nor fixing failure occurs. As a result, hot offset is not generated at the front end portion of the recording material P.

  Next, the flow of FIG. 12 will be specifically described. FIG. 12 is a diagram illustrating a fixing flow when performing borderless printing of the recording material P. First, when the leading end of the recording material P has not yet been inserted into the fixing nip N, energization of the ceramic heater 2 is turned on to control the temperature between sheets (S301). At the same time, the fixing speed for rotating the heating sleeve 1a is set to 50 mm / s, which is slower than the transfer conveyance speed (120 mm / s) (S302). Next, it is determined whether or not the leading end of the recording material P is inserted into the fixing nip N (S303). If a positive determination is made in S303, the process proceeds to S304. On the other hand, if a negative determination is made in S303, the determination in S303 is repeated. In S304, the energization amount to the ceramic heater 2 is increased from the inter-paper temperature control to the fixing temperature control, and the heating temperature is increased stepwise as shown in FIG. 13 (S304). As a result, when the leading end portion of the recording material P passes through the fixing nip N, the target heating temperature is increased from 160 ° C. of the inter-sheet temperature control, and the fixing temperature control when the central portion of the recording material P passes through the fixing nip N. The temperature can be lowered below 180 ° C. Further, when the central portion of the recording material P passes through the fixing nip N, the target heating temperature can be set higher than when the leading end portion of the recording material P passes through the fixing nip N by setting the target heating temperature to 180 ° C. of the fixing temperature control. it can. At the same time, the power supply amount of the motor M is increased stepwise to increase the fixing speed, which is the speed of the motor M, stepwise up to the transfer conveyance speed as shown in FIG. 13 (S305). Thereby, when the leading end portion of the recording material P passes through the fixing nip N, the fixing speed is increased from 50 m / s, and the transfer conveyance speed (120 m / s when the central portion of the recording material P passes through the fixing nip N). ) Can be kept slower. Further, when the central portion of the recording material P passes through the fixing nip N, the fixing speed can be matched with the transfer conveyance speed (120 m / s). Then, the fixing speed is matched with the transfer conveyance speed (120 m / s) (S306). At the same time, the fixing temperature is adjusted by maintaining the ceramic heater 2 at 180 ° C. (S307). Next, it is determined whether or not a portion 60 mm from the rear end of the recording material P is held by the fixing nip N (S308). If a positive determination is made in S308, the process proceeds to S309. On the other hand, if a negative determination is made in S308, the determination in S308 is repeated. In S309, the energization amount to the ceramic heater 2 is decreased stepwise and the heating temperature is decreased stepwise as shown in FIG. 13 (S309). Thereby, when the rear end portion of the recording material P passes through the fixing nip N, the target heating temperature can be lowered to 180 ° C. when the central portion of the recording material P passes through the fixing nip N. At the same time, the power supply amount of the motor M is decreased stepwise, and the fixing speed, which is the speed of the motor M, is decreased stepwise as shown in FIG. 13 (S310). Accordingly, when the trailing end portion of the recording material P passes through the fixing nip N, the fixing speed can be made slower than the transfer conveyance speed when the central portion of the recording material P passes through the fixing nip N. Next, it is determined whether or not the rear end of the recording material P has come out of the fixing nip N (S311). If a positive determination is made in S311, the process proceeds to S312. On the other hand, if a negative determination is made in S311, the determination in S311 is repeated. When the trailing edge of the recording material P leaves the fixing nip N, the amount of power supplied to the motor M is reduced to set the fixing speed to 50 m / s, which is slower than the transfer conveyance speed (120 mm / s) (S312). At the same time, the temperature control between the sheets is performed so as to maintain the energization amount to the ceramic heater 2 at 160 ° C. in the state where the amount of current is reduced most (S313). Note that the timing of these operations used in this example is an example and may be changed depending on the situation.

A feature of this example is that the heating temperature and the fixing speed are optimally controlled not only for the rear end portion of the recording material P but also for both end portions of the front end portion. As a result, in this example, when images are also formed on both end portions of the front and rear end portions of the recording material P, when the both end portions of the recording material P are inserted into the fixing nip N, it is appropriate under a wide heating temperature condition. Fixing operation can be performed. Therefore, even when images are formed on both end portions of the recording material P, a good image can be obtained without causing hot offset or fixing failure on the entire surface of the recording material.

  In this example, the heating temperature and the fixing speed are optimally controlled not only for the front end portion of the recording material P but also for both end portions of the rear end portion. However, it is not limited to this. The heating temperature and the fixing speed may be optimally controlled only for the front end portion of the recording material P. Further, in the mode for performing borderless printing in the above example, an image is formed with no borders on the recording material P other than the leading edge or all the edges. However, the mode to which the present invention can be applied is not limited to this. A mode in which an image is formed within a certain width that becomes a non-image forming area up to the leading end of the recording material P in the normal print mode, and a non-image forming area that reaches the rear end of the recording material P in the normal print mode. The present invention can be applied even in a mode in which an image is formed within the width. In addition, “within a certain width” includes “0” that forms an image without a border up to the front and rear ends.

DESCRIPTION OF SYMBOLS 1a ... Heating sleeve, 2a ... Heat generating body, 5 ... Thermistor, 10 ... Fixing device, 11a-11d ... Photosensitive drum, 100 ... Heating control device

Claims (3)

A transfer unit that transfers a toner image on the image carrier to a recording material at a predetermined transfer conveyance speed;
A heating nip is formed using a rotating body that is heated by a heat source and rotates at a fixing speed, and the toner on the recording material is heated and melted while the recording material on which the toner image is transferred is sandwiched and conveyed by the heating nip. A fixing unit for fixing
A member heated by the heat source or a temperature detecting member for detecting the temperature of the heat source;
A heating control unit that controls electric power supplied to the heat source so that the detected temperature detected by the temperature detecting member approaches the target heating temperature;
An image forming apparatus comprising:
In a mode in which an image is formed within a certain width up to the trailing edge of the recording material, when the trailing edge portion of the recording material passes through the heating nip, the target heating temperature is set at the central portion of the recording material. And an image forming apparatus wherein the fixing speed is made lower than the transfer conveyance speed. In the mode of forming an image within a certain width up to the leading edge of the recording material, when the leading edge portion of the recording material passes through the heating nip, the target heating temperature is set to the target heating temperature, and the central portion of the recording material passes the heating nip. And lowering the fixing speed slower than the transfer conveyance speed,
When the central portion of the recording material passes through the heating nip, the target heating temperature is raised higher than when the leading end portion of the recording material passes through the heating nip, and the fixing speed is made to coincide with the transfer conveyance speed. The image forming apparatus according to claim 1, wherein: A transfer unit that transfers a toner image on the image carrier to a recording material at a predetermined transfer conveyance speed;
A heating nip is formed using a rotating body that is heated by a heat source and rotates at a fixing speed, and the toner on the recording material is heated and melted while the recording material on which the toner image is transferred is sandwiched and conveyed by the heating nip. A fixing unit for fixing
A member heated by the heat source or a temperature detecting member for detecting the temperature of the heat source;
A heating control unit that controls electric power supplied to the heat source so that the detected temperature detected by the temperature detecting member approaches the target heating temperature;
An image forming apparatus comprising:
In the mode of forming an image within a certain width up to the leading edge of the recording material, when the leading edge portion of the recording material passes through the heating nip, the target heating temperature is set to the target heating temperature, and the central portion of the recording material passes through the heating nip. And an image forming apparatus characterized in that the fixing speed is lower than the transfer conveyance speed.

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