JP2009294312A - Image forming apparatus - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an image forming apparatus which can reduce the occurrence of shocking jitter in monochromatic image printing in a tandem type color image forming apparatus. <P>SOLUTION: In the tandem type image forming apparatus, in forming a monochromatic image, only a photoreceptor 1a for black is abutted on an intermediate transfer belt 3, and in forming a color image, all photoreceptors 1a-1d including the photoreceptor 1a for black are abutted on the intermediate transfer belt 3. A contact member 15 for selectively applying load to the intermediate transfer belt 3 is provided on the upstream side in the rotating direction of the intermediate transfer belt 3 from the contact part between the photoreceptor 1a for black and the intermediate transfer belt 3, and in forming a monochromatic image, the contact member 15 applies load to the intermediate transfer belt 3. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  The present invention relates to an image forming apparatus. In particular, it relates to a technique for reducing so-called shoulders.

  Image forming apparatuses that form color images are becoming mainstream toners in response to demands from the market. As a color image forming apparatus for forming a color image, for example, an image carrier such as a single photosensitive drum and each color obtained by developing each color electrostatic latent image sequentially formed on the image carrier. A toner image is sequentially superimposed on a transfer material such as an intermediate transfer belt or a recording material to obtain a color image. Hereinafter, this color image forming apparatus is referred to as a one-drum type image forming apparatus. Further, for example, a color image obtained by sequentially superimposing a plurality of image carriers corresponding to each color and each color toner image obtained by developing an electrostatic latent image formed on each image carrier on a transfer material. Is what you get. Hereinafter, this color image forming apparatus is referred to as a tandem type image forming apparatus.

  Since the one-drum type image forming apparatus has one image carrier, the apparatus can be reduced in size and cost can be reduced. However, since a single image carrier is used, it is necessary to repeat a series of processes from formation of a latent image, development processing, and transfer to a transfer material for the number of colors (usually four times). This is disadvantageous in terms of speeding up image formation. On the other hand, the tandem type image forming apparatus has the disadvantage that the apparatus becomes large and the cost is high, but since the image carrier is provided for each color, the series of processes described above is only required once. This is advantageous in terms of speeding up image formation. In recent years, the tandem type image forming apparatus has been attracting attention because the image forming speed (speed increase) of the color image forming apparatus is comparable to that of a monochrome image forming apparatus.

  In the tandem type image forming apparatus described above, from the viewpoint of the life of the photoconductor, in the monochrome mode for forming a monochrome image, only the black photoconductor is brought into contact with the intermediate transfer belt, and the color image forming photoconductor (magenta, cyan) is used. , Yellow) is separated from the intermediate transfer belt and stopped to increase the life of the color photoconductor.

JP 2003-57911 A

  As described above, in recent years, as color image formation has become the mainstream toner, the market demand for the thickness of paper has further increased. This means that there is a demand for an image forming apparatus that can provide a certain quality of image quality on any kind of paper. However, when image formation is performed on paper of a predetermined thickness or more, usually called “thick paper”, streak lines called so-called shock jitter appear on the image and image quality deteriorates. There is. This is due to the so-called density unevenness described below. In the indirect transfer type tandem image forming apparatus, when the thick paper enters the secondary transfer portion, a load fluctuation occurs on the intermediate transfer belt, and thereby a speed fluctuation occurs on the intermediate transfer belt. As a result, the image transfer position (primary transfer position) from the photosensitive drum to the intermediate transfer belt deviates from the target position. As a result, the image transferred from the photosensitive member to the intermediate transfer belt is expanded and contracted in the sub-scanning direction, and a phenomenon called density unevenness occurs. This problem is usually dealt with by slowing the paper transport speed during thick paper printing, and mitigating fluctuations in the load on the intermediate transfer belt caused by the thick paper entering the secondary transfer section. If the paper conveyance speed is slowed down, the productivity is lowered so much, which is not a preferable measure.

  In addition, as described above, in the tandem image forming apparatus, since the color photoconductor is separated from the intermediate transfer belt and only the black photoconductor is in contact with the intermediate transfer belt during monochrome printing from the viewpoint of the life of the photoconductor, Since the transfer belt rotates with a little load, the toner is very weak against an external load. When thick paper enters the secondary transfer section in this state, the load fluctuation of the intermediate transfer belt due to the entry of the paper becomes noticeable. Then, this appears as a toner that fluctuates the speed of the intermediate transfer belt, and ultimately causes a reduction in image quality called shock jitter.

  The present invention has been made in view of the above problems, and an object of the present invention is to reduce the occurrence of shock jitter during monochrome image printing in a tandem type image forming apparatus.

  In order to solve the above-described problems, the present invention is provided for each image carrier, a black image carrier, a plurality of color image carriers, an intermediate transfer member in contact with these image carriers. A primary transfer unit that primarily transfers the toner image formed on the image carrier to the intermediate transfer member; and a secondary transfer unit that transfers the toner image primarily transferred onto the intermediate transfer member to a recording medium. When forming a monochrome image, only the black image carrier is brought into contact with the intermediate transfer member, while when forming a color image, the entire image carrier including the black image carrier. In the image forming apparatus configured to contact the intermediate transfer member, load applying means for selectively applying a load to the intermediate transfer member is provided by the contact portion between the black image carrier and the intermediate transfer member. Is also provided upstream of the intermediate transfer member in the rotational direction. When forming a monochrome image, it proposes an image forming apparatus, wherein said load applying means applying a load to the intermediate transfer body.

  According to the present invention, it is advantageous that the load applying unit includes a contact member that contacts the surface of the intermediate transfer member, and a counter member that is provided at a position facing the contact member and the intermediate transfer member. It is.

  Further, according to the present invention, the black image carrier is located closest to the secondary transfer position toward the upstream side in the rotation direction of the intermediate transfer member, and includes the contact member and the counter member. The load applying means is advantageously provided between the color image carrier and the color image carrier closest to the black image carrier.

Furthermore, according to the present invention, when printing a color image, it is advantageous that at least one of the contact member and the facing member is separated from the intermediate transfer member.
Furthermore, according to the present invention, it is advantageous that the load applying means is any one of the plurality of color image carriers.

Furthermore, in the invention, it is advantageous that the load applying unit is an image carrier that is disposed closest to the black image carrier among the plurality of color image carriers. .
Furthermore, in the present invention, it is advantageous that the load applying means is all of the plurality of color image carriers.

Furthermore, according to the present invention, it is advantageous that the rotation speed of the color image carrier when a load is applied is slower than the rotation speed of the black image carrier.
Furthermore, according to the present invention, it is advantageous that the color image carrier when a load is applied applies a bias to the primary transfer means facing the color image carrier.

Furthermore, the present invention is advantageous when the bias value applied to the primary transfer unit is higher than the bias value applied to the primary transfer unit when printing a color image.
Furthermore, in the present invention, it is advantageous that the load applying unit is configured to apply a load to the intermediate transfer member only when the recording medium is of a specific type.

Furthermore, in the present invention, it is advantageous that the load applying unit is configured to apply a load to the intermediate transfer member only when the recording medium has a predetermined thickness or more.
Furthermore, in the present invention, it is advantageous that the load applying unit is configured to apply a load to the intermediate transfer member only when a conveyance speed of the recording medium is equal to or higher than a predetermined speed.

  Furthermore, in the present invention, it is advantageous that the image carrier is an image carrier that is held in a process cartridge in which charging means for charging at least the surface of the image carrier is integrated.

  According to the present invention, the speed fluctuation of the intermediate transfer member that occurs when the paper enters the secondary transfer portion during monochrome image printing of the tandem type image forming apparatus is reduced, and the occurrence of shock jitter due to the speed fluctuation is reduced. Can be suppressed.

Further, when the color image is formed, the contact member and the opposing member are separated from each other, so that the unfixed color toner image formed on the intermediate transfer member is not disturbed, and image deterioration can be suppressed.
Furthermore, by using a color photoconductor other than the black photoconductor as the load applying unit, it is possible to configure the load applying unit using an existing apparatus, so that the cost can be reduced.

  Furthermore, by increasing the load on the intermediate transfer member by the load applying means, it is possible to improve the stability of the intermediate transfer member against the load from the outside, and to further improve the shock jitter reduction effect.

  Furthermore, since the load applying means is operated only when a load that generates shock jitter acts on the intermediate transfer member, the wear of the intermediate transfer member and the photosensitive member is further prevented while maintaining the effect of reducing shock jitter. This makes it possible to extend the life of the device.

Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.
FIG. 1 is an explanatory view showing an embodiment of an image forming apparatus according to the present invention.
In FIG. 1, this embodiment is a tandem type image forming apparatus having four photoconductors 1a to 1d as image carriers, and from the right, a black photoconductor 1a, a magenta photoconductor 1b, a cyan photoconductor 1c, This is the yellow photoreceptor 1d. A latent image is formed on each of the photoconductors 1a to 1d by the exposure beams 5a to 5d of the respective colors irradiated from the writing unit (not shown in FIG. 1), and each of the photoconductors 1a is developed by the developing rollers 2a to 2d of the respective colors. A toner image is formed on ˜1d. Thereafter, the toner images formed on the yellow photoconductor 1d, the cyan photoconductor 1c, the magenta photoconductor 1b, and the black photoconductor 1a are sequentially bias rollers 6a to 6d (primary rollers) arranged at positions facing the photoconductors of the respective colors. Are sequentially transferred onto the intermediate transfer belt 3 by the transfer unit).

  On the other hand, a sheet as a recording medium is fed from a sheet feeding unit (not shown), conveyed from the registration roller pair 7 in synchronization with the toner image on the intermediate transfer belt 3, and the drive roller 11 and the intermediate transfer belt 3 The secondary transfer roller 9 formed by the secondary transfer roller 4 transfers the toner image on the intermediate transfer belt 3 by the action of the secondary transfer roller 4, and the fixing unit 8 fixes the toner image. The secondary transfer roller 4 is pressed against the intermediate transfer belt 3 with a predetermined pressure by an urging means (not shown) to form the secondary transfer portion 9.

  As shown in FIGS. 1 and 2, the intermediate transfer belt 3 is wound around a driving roller 11, driven rollers 10 and 13, and a tension roller 12. The drive source is a motor 15, and an output shaft gear (not shown) of the motor 15 and the drive transmission gear 14 are engaged with each other. Coupling means (not shown) for engaging with an engaged member provided at the tip of the rotation shaft of the drive roller 11 is provided at the tip of the rotation shaft of the drive transmission gear 14. The driving force of the motor 15 is transmitted to the driving roller 11 by this coupling means. The driven roller 10 is provided with an encoder 16 for detecting the rotational speed. The encoder 16 is provided with a control means 17 for detecting the rotational speed fluctuation of the driven roller 10 and feedback-controlling the motor 15 that is a driving source of the driving roller 10 so as to cancel the speed fluctuation. As a result, the intermediate transfer belt 3 is driven to rotate at a constant speed.

  In the indirect transfer type tandem color image forming apparatus as described above, a speed fluctuation occurs in the intermediate transfer belt 3 when a sheet enters the secondary transfer section 9, and accordingly, the intermediate transfer belt 3 and each photoreceptor 1a. There is a problem that the position of the image fluctuates at the primary transfer portion of ˜1d and density unevenness (so-called shock jitter) occurs. In particular, the phenomenon becomes more prominent as the thickness of the paper increases and the speed of entry of the paper increases.

Next, details of the phenomenon will be described with reference to FIGS. The mechanism of the speed fluctuation of the intermediate transfer belt 3 can be roughly divided into three phenomena.
First, the first phenomenon will be described with reference to FIG. As shown in FIG. 3A, the paper P is conveyed toward the secondary transfer portion by the registration roller pair 7. Next, as shown in FIG. 3B, when the leading edge of the paper P abuts against the intermediate transfer belt 3, the intermediate transfer belt 3 is pushed in the direction of arrow a. As a result, the portion of the intermediate transfer belt 3 that forms the primary transfer portion with each of the photoreceptors 1a to 1d is pulled in the direction of the arrow b, thereby increasing the speed of the intermediate transfer belt 3 (the state shown in FIG. 6A). There is a method in which the paper P is directly inserted into the secondary transfer portion nip formed by the drive roller 11, the intermediate transfer belt 3, and the secondary transfer roller 4, but a problem that transfer dust occurs as a side effect thereof. Not very good.

  A method of directly applying the paper P to the secondary transfer roller 4 is also conceivable. Usually, the secondary transfer roller 4 often employs a soft material such as sponge in order to increase the nip width of the secondary transfer portion. In this case, there is a problem that jamming is likely to occur because the paper P does not slip.

In order to solve these problems, the paper P is once applied to the intermediate transfer belt 3 and then entered into the secondary transfer unit 9.
Next, the second phenomenon will be described with reference to FIG. As shown in FIG. 4, the paper P once hits the intermediate transfer belt 3 and then is conveyed upward along the intermediate transfer belt 3 to be formed by the driving roller 11, the intermediate transfer belt 3, and the secondary transfer roller 4. Inserted into the next transfer section. At this time, since the secondary transfer roller 4 is pushed down in the direction of the arrow c, the secondary transfer roller 4 is pressed by the thickness of the paper by the action of the biasing means that biases the secondary transfer roller 4 toward the intermediate transfer belt 3. The pressing force of the transfer roller 4 increases and the load increases instantaneously. As a result, the speed of the intermediate transfer belt 3 is lowered (the state shown in FIG. 6A).

  Finally, the third phenomenon will be described with reference to FIG. As shown in FIG. 5, after the leading edge of the sheet P passes through the nip portion of the secondary transfer unit 9, the pressing force of the sheet P against the intermediate transfer 3 decreases, and the intermediate transfer belt 3 moves in the direction of arrow d. . As a result, the portions of the intermediate transfer belt 3 forming the primary transfer portions with the respective photoreceptors 1a to 1d move relatively in the direction of the arrow e, and the speed of the intermediate transfer belt 3 is further reduced ( FIG. 6C shows the state).

  FIG. 6 schematically shows the above three phenomena from the viewpoint of speed fluctuation (position fluctuation) of the intermediate transfer belt 3. In FIG. 6, the vertical axis represents position fluctuation, that is, speed fluctuation of the intermediate transfer belt 3, and the horizontal axis represents time. The position fluctuation in FIG. 6 is created based on the data detected by the encoder provided in the driven roller 10 described above. In the state of FIG. 6A, the speed of the intermediate transfer belt 3 is increased, and in the state of A, C, the intermediate transfer belt 3 is decreased. In addition, in the state of D, the aftermath of those fluctuations appears. As a result of the speed fluctuation (position fluctuation) occurring in the intermediate transfer belt 3 in this way, the toner transfer position on the intermediate transfer belt 3 deviates from the target position from the respective photoreceptors 1a to 1d in the primary transfer portion, so-called density. Unevenness occurs. Normally, the toner image becomes thinner (extends) when the intermediate transfer belt 3 becomes faster, and becomes darker (shrinks) when it becomes slower.

  The so-called shock jitter described above appears remarkably when only the black photosensitive member 1a is brought into contact with the intermediate transfer belt 3 and the remaining photosensitive members 1b to 1d are separated during monochrome printing. Further, as the thickness of the sheet increases, the shock at the time of entering the secondary transfer unit 9 increases, so that shock jitter tends to appear. In addition, when the sheet conveyance speed is high, it is likely to appear as well. FIG. 13 shows the result of experimental confirmation of this phenomenon.

  FIG. 13 shows the intermediate transfer belt when the black paper 1a is brought into contact with the intermediate transfer belt 3 when the monochrome image is printed on the thick paper (180K paper). 3 represents the speed fluctuation. The vertical and horizontal axes in FIG. 13 are the same as those in FIG. Further, the data shown in FIG. 13 is obtained by mounting the encoder 16 on the driven roller 10 that is moved by the movement of the intermediate transfer belt 3, and FV-converting and integrating the encoder waveform to measure the deviation amount from the ideal position. is there. As can be seen from the figure, at the timing when the paper P enters the secondary transfer portion 9, a large speed fluctuation indicated by the amplitude A occurs in the intermediate transfer belt 3. This speed fluctuation causes shock jitter on the image.

  On the other hand, the data shown in FIG. 14 indicates that when printing a monochrome image on plain paper, the plain paper was allowed to enter the secondary transfer unit 9 with only the black photoreceptor 1 a in contact with the intermediate transfer belt 3. In this case, the fluctuation of the speed of the intermediate transfer belt 3 is shown. When the paper P is plain paper, the speed fluctuation at the timing of entering the secondary transfer section 9 is clearly small and no occurrence of shock jitter was observed.

  Therefore, when black paper 1a is brought into contact with the intermediate transfer belt 3 in monochrome printing and the thick paper is moved into the secondary transfer portion 9, the speed fluctuation of the intermediate transfer belt 3 appears remarkably. I understand that.

  In view of this, the present invention has an object to provide the following means for reducing shock jitter that occurs when a monochrome image is printed on a sheet having a predetermined thickness or more, such as cardboard. Examples will be described below.

  As shown in FIG. 8, in an image forming apparatus having a monochrome mode using only the black photosensitive member 1a and a color mode using all the photosensitive members 1a to 1d, only the black photosensitive member 1a used when the monochrome mode is selected is intermediately transferred. A load applying means for applying a load to the intermediate transfer belt 3 by contacting the intermediate transfer belt 3 while the belt 3 is in contact is provided. As shown in FIG. 8, the load applying means is a backup member as a contact member 15 that contacts the intermediate transfer belt 3 and a contact member that is disposed at a position opposite to the contact member 15 and sandwiches the intermediate transfer belt 3 between the contact member 15. A roller 16 is provided.

  Further, by sandwiching the intermediate transfer belt 3 between the contact member 15 and the backup roller 16, shock jitter in monochrome can be reduced. FIG. 16 shows this reduction effect. FIG. 16 shows that the black photosensitive member 1a is brought into contact with the intermediate transfer belt 3 during monochrome printing, and the thick paper enters the secondary transfer unit 9 with the intermediate transfer belt 3 sandwiched between the contact member 15 and the backup roller 16. This shows the speed fluctuation of the intermediate transfer belt 3 in the case of the above. The speed fluctuation of the amplitude C occurs at the timing when the paper enters the secondary transfer unit 9. However, it can be seen that the speed fluctuation (amplitude A) is considerably reduced when only the black photosensitive member 1a shown in FIG. 13 is in contact with the intermediate transfer belt 3.

  Further, the abutting member 15 is separated from the intermediate transfer belt 3 during color printing as shown in FIG. 9 so as not to disturb the unfixed color toner image primarily transferred onto the intermediate transfer belt 3. . The contact member 15 is located on the intermediate transfer belt 3 at a position higher than the primary transfer position of the black photoreceptor 1a so as not to disturb the unfixed toner image primarily transferred from the black photoreceptor 1a onto the intermediate transfer belt 3 during monochrome printing. It is desirable to arrange it upstream in the rotational direction.

  The contact member 15 may be configured to contact the intermediate transfer belt 3 only when a sheet of a predetermined thickness, for example, a thick sheet, is conveyed during monochrome printing. This is because, as shown in FIG. 14, even during monochrome printing, in the case of plain paper, speed fluctuations that cause shock jitter on the intermediate transfer belt 3 do not occur. Specifically, as shown in FIG. 9, a paper thickness detection sensor 18 for detecting the thickness of the paper is provided on the paper conveyance path, and the sensor 18 detects that the paper is a predetermined thickness or more. Only in such a case, a drive control means may be provided so that the contact member 15 contacts the intermediate transfer belt 3. If there is a button for selecting a paper type on the operation panel of the image forming apparatus without providing the paper thickness detection sensor 18, the contact member 15 is intermediately transferred only when a specific paper type is selected. You may make it drive in the direction contact | abutted to the belt 3. FIG.

  In addition, the above-described shock absorber may occur even when the paper conveyance speed is equal to or higher than a predetermined speed. Therefore, the intermediate transfer belt 3 may be sandwiched between the contact member 15 and the backup roller 16 only when the transport speed of the transported paper is a predetermined value or more. Specifically, as shown in FIGS. 8 and 9, a speed sensor 26 for detecting the sheet conveyance speed is provided upstream of the secondary transfer unit 9 in the sheet conveyance direction. Then, the contact member 15 may be brought into contact with the intermediate transfer belt 3 only when the speed sensor 26 detects that the paper transport speed is equal to or higher than a predetermined value.

A second embodiment will be described with reference to FIGS. 10 and 11.
10 and 11 are schematic views of an indirect transfer tandem color image forming apparatus having a plurality of photoconductors. FIG. 10 shows a state during color printing in which all the photoconductors 1 a to 1 d are in contact with the intermediate transfer belt 3, and FIG. 11 shows a state in monochrome printing in which only the black photoconductor 1 a is in contact with the intermediate transfer belt 3. It is. The same members as those in FIG. 1 are denoted by the same reference numerals, and detailed description thereof is omitted.

  In FIG. 10, 19 is a writing device, 1a to 1d are photoconductors rotating in the clockwise direction, 20a to 20d are charging rollers for charging the surface of the photoconductor, 2a to 2d are developing rollers for attaching toner to the electrostatic image, and 6a to 6d. 6d is a bias roller, 3 is an intermediate transfer belt that rotates in the direction of arrow f, 11 is a moving roller that drives the intermediate transfer belt 3, 7 is a registration roller pair, 10 and 13 are driven rollers, 12 is a tension roller, and 4 is two The next transfer roller, 21 is a transfer member cleaning device, and 23 is a color bias roller holding frame for bringing the color bias rollers 6b to 6d into contact with and separating from the photoreceptors 1b to 1d. The color bias roller holding frame 23 is provided so as to be rotatable about a fulcrum 22, and the color bias rollers 6 b to 6 d are provided by springs 25 as urging means provided at the other end opposite to the fulcrum 22. Is urged in the direction of separating from the photosensitive members 1b to 1d. Reference numeral 24 denotes an eccentric cam for contact / separation for switching the contact / separation of the color bias rollers 6b-6d with respect to the photoreceptors 1b-1d. The contact / separation eccentric cam 24 is configured to be rotatable by a drive motor (not shown). As shown in FIG. 9, the eccentric cam 24 is rotated in a direction in contact with the color bias roller holding frame 23, so that the color bias holding frame 23 resists the urging force of the spring 25 and the direction of the color photoconductors 1 b to 1 d. Move to. As a result, the color bias rollers 6b to 6d are brought into contact with the photoreceptors 1b to 1d, and all the bias rollers 6a to 6d are brought into contact with the photoreceptors 1a to 1d. As a result, the intermediate transfer belt 3 is in toner in contact with all the photoreceptors 1a to 1d.

  On the other hand, when the eccentric cam 24 is rotated in the direction away from the color bias roller holding frame 23 as shown in FIG. 11, the color bias roller holding frame 23 is separated from the color photoreceptors 1b to 1d by the urging force of the spring 25. Moved in the direction. As a result, the color bias rollers 6b to 6d are separated from the color photoreceptors 1b to 1d, and only the black bias roller 6a is in contact with the black photoreceptor 1a. As a result, the intermediate transfer belt 3 is in toner in contact with only the black photoreceptor 1a.

  As described above, at the time of monochrome printing, only the black photoconductor 1a is brought into contact with the intermediate transfer belt 3, so that the color photoconductors 1b to 1d are not rotated excessively and the life of the photoconductor can be extended. Toner.

  However, as described above, as shown in FIG. 11, during black-and-white printing, only the black photosensitive member 1a is in contact with the intermediate transfer belt 3, so that the intermediate transfer belt is rotated with a little load. The toner is very weak against the external load. For this reason, when a sheet having a predetermined thickness or more such as thick paper enters the secondary transfer unit 9 or when the sheet enters at a conveyance speed equal to or higher than a predetermined speed, the intermediate transfer belt 3 is likely to fluctuate in speed. Therefore, shock jitter tends to occur.

  Therefore, in this embodiment, during black-and-white printing, only the black photoconductor 1a is normally brought into contact with the intermediate transfer belt 3 as shown in FIG. 11, but all the photoconductors are used as in color printing as shown in FIG. 1 a to 1 d are brought into contact with the intermediate transfer belt 3. Of course, during monochrome printing, toner images are not formed on the color photoconductors 1b to 1d, and are merely rotated in contact with the intermediate transfer belt 3. Usually, the surface speeds of the photoconductors 1a to 1d are set to be slower than the surface speed of the intermediate transfer belt 3 in order to prevent missing characters from being printed. Therefore, as described above, not only the black photoconductor 1a but also the color photoconductors 1b to 1d are brought into contact with the intermediate transfer belt 3 during monochrome printing. Stability against external loads is improved. As a result, it is possible to effectively reduce the occurrence of shock jitter.

  FIG. 15 shows this effect. FIG. 15 shows the speed fluctuation of the intermediate transfer belt 3 that occurs when a cardboard is entered into the secondary transfer unit 9 with all the photoconductors 1 a to 1 d in contact with the intermediate transfer belt 3 during monochrome printing. It is a thing. As can be seen from the figure, the fluctuation of the amplitude C occurs at the timing when the sheet is fed into the secondary transfer unit 9, but it is reduced to about half of the amplitude A in FIG. The mechanism for reducing the speed fluctuation amount will be described below in correspondence with the three phenomena described with reference to FIGS.

  First, in the first phenomenon, as shown in FIG. 3B, when the leading edge of the sheet P hits the intermediate transfer belt 3, the intermediate transfer belt 3 tries to be pushed in the direction of the arrow a. ˜1d is in contact with the intermediate transfer belt 3, so that it reduces the amount of pressing of the intermediate transfer belt 3 in the direction of the load (brake) toner arrow a. As a result, the amount by which the portions of the intermediate transfer belt 3 forming the primary transfer portions with the photoreceptors 1a to 1d are pulled in the direction of the arrow b is also reduced, so the increase in the speed of the intermediate transfer belt 3 is reduced. (The state of FIG. 7A ').

  Next, regarding the second phenomenon, although the load on the intermediate transfer belt 3 increases due to the entry of the sheet into the secondary transfer portion 9, all of the phenomenon occurs rather than bringing only the black photosensitive member 1 a into contact with the intermediate transfer belt 3. Since the load on the intermediate transfer belt 3 can be increased by bringing the photoreceptors 1a to 1d into contact with each other, the load fluctuation of the intermediate transfer belt 3 due to the entry into the secondary transfer portion 9 is relatively reduced. Can do. In other words, when the brake is further applied to the small brake force, the effect of the transfer belt 3 is less when the brake is further applied to the large brake force, and the speed fluctuation can be reduced (FIG. 7). B 'state). Finally, with respect to the third phenomenon, as shown in FIG. 5, after the leading edge of the paper P faces the nip portion of the secondary transfer portion, the pressing force of the paper P against the intermediate transfer 3 decreases, and the intermediate transfer belt. 3 moves in the direction of arrow d. However, since the amount of pushing in the direction of arrow a shown in FIG. 3B is reduced, the amount of movement in the direction of arrow d is reduced. Accordingly, the amount of movement of the portions of the intermediate transfer belt 3 forming the primary transfer portions with the respective photoreceptors 1a to 1d is also reduced, and as a result, the speed of the intermediate transfer belt 3 is reduced. The amount is also reduced (state in FIG. 7C).

  FIG. 7 schematically shows the above three phenomena from the viewpoint of speed fluctuation (position fluctuation) of the intermediate transfer belt 3. It can be seen that the fluctuations of A ′ to D ′ in FIG. 7 are all reduced as compared with FIG.

  The case where the color photoconductors 1b to 1d are rotated at the same speed as that of the black photoconductor 1a has been described above. However, from the viewpoint of improving the braking effect, the rotation speed of the color photoconductors 1b to 1d is higher than that of the black photoconductor 1a. You may make it slow. It is also possible to set the rotational speed of the color photoconductors 1b to 1d to be gradually slower from the side closer to the black photoconductor 1a, or to be gradually faster. In this case, it is preferable that the rotation speed of the photoconductor having the fastest rotation speed among the color photoconductors 1b to 1d is set slower than the rotation speed of the black photoconductor 1a.

  As in the case of the first embodiment, the color photoconductors 1b to 1d are configured to contact the intermediate transfer belt 3 only when a sheet of a predetermined thickness, for example, a thick sheet, is conveyed during monochrome printing. It is also possible. It is also possible to configure the color photoreceptors 1b to 1d to contact the intermediate transfer belt 3 only when the sheet conveyance speed is equal to or higher than a predetermined speed. The explanation of the operation in each case can be explained by replacing the contact member 15 in the first embodiment with the color photoreceptors 1b to 1d, and the explanation thereof is omitted.

Next, in FIGS. 10 and 11, all the color photoconductors 1b to 1d other than the black photoconductor 1a are brought into contact with the intermediate transfer belt 3 during monochrome printing. However, as shown in FIG. Any one of the photoconductors 1b to 1d may be brought into contact with the intermediate transfer belt 3. By doing so, the life of the photosensitive member can be extended as compared with the case where all the photosensitive members 1a to 1d are brought into contact with the intermediate transfer belt 3.
However, in this case, compared to the case where all the color photoconductors 1b to 1d are in contact with the intermediate transfer belt 3, the load applied to the intermediate transfer belt 3 is reduced, so that the effect of reducing the shock jitter can be reduced. .

  Therefore, a bias is applied to the color bias roller facing one color photoconductor that is in contact with the intermediate transfer belt 3, and the color photoconductor and the color bias roller facing it are sandwiched between the intermediate transfer belt 3 in a state where the adhesion force is increased. Like that. By doing so, it is possible to increase the load (braking force) on the intermediate transfer belt 3 and to suppress the decrease in the effect of reducing the shock jitter. When any one of the color photoconductors 1b to 1d is brought into contact with the intermediate transfer belt 3, it is preferable that the color photoconductor is closest to the black photoconductor 1a.

  As in the case of the first embodiment, only one of the black photoconductor 1a and the color photoconductors 1b to 1d is transferred only when a paper having a predetermined thickness or more, for example, a thick paper, is conveyed during monochrome printing. It is also possible to configure the color photoconductor so as to contact the intermediate transfer belt 3. It is also possible to configure such that any one of the black photosensitive member 1a and the color photosensitive members 1b to 1d is brought into contact with the intermediate transfer belt 3 only when the sheet conveyance speed is equal to or higher than a predetermined speed. It is. The explanation of the operation in each case can be explained by replacing the “contact member 15” in the first embodiment with “any one of the color photoconductors 1b to 1d”, and the explanation is omitted.

FIG. 17 is an external view of the process cartridge 27 including the photosensitive member 30, and FIG. 18 is a cross-sectional view showing the internal configuration of the process cartridge 27.
In FIG. 18, the process cartridge 27 includes a photoconductor unit 28 and a developing unit 29. The photosensitive unit 28 and the developing unit 29 may be integrally structured toner.

  In FIG. 18, the surface of the photoreceptor 30 that rotates clockwise is charged to a predetermined potential by the charging roller 36. The charging roller 36 is configured to be rotatable about its shaft 35. A cleaning roller 34 is in contact with the surface of the charging roller 36 so as to clean the surface of the charging roller 36. Then, the photoreceptor 30 uniformly charged by the charging roller 36 is exposed by the light beam L emitted from the writing device 19 to form a latent image.

  The developing unit 29 includes a developing roller 37 disposed so as to be partly exposed from the opening of the developing case 38, conveying screws 39 and 41, a developing doctor 42, and a toner concentration sensor 40. Toner is supplied. The developing case 38 contains a two-component developer (hereinafter simply referred to as a developer) containing a magnetic carrier and negatively charged toner. The developer is frictionally charged while being agitated and conveyed by the conveying screws 39 and 41 and then carried on the surface of the developing roller 37. Then, after the layer thickness is regulated by the developing doctor 42, the developer is conveyed to a developing position facing the photosensitive drum 30, where toner is attached to the electrostatic latent image on the photosensitive drum 30. By this adhesion, a predetermined color toner image is formed on the photosensitive drum 30. The developer that has consumed toner by development is returned to the developing case 38 as the developing roller 37 rotates. The toner density of the developer in the developing case 38 is detected by the toner density sensor 40, and toner is replenished from the toner storage container (not shown) to the space above the screw 39 as necessary.

It is explanatory drawing which shows one Embodiment of the image forming apparatus which concerns on this invention. FIG. 6 is a perspective view illustrating a driving unit of an intermediate transfer belt. (A) shows a state in which the paper is conveyed toward the secondary transfer portion by a pair of registration rollers, and (b) is an explanatory view showing a state in which the leading edge of the paper has abutted against the intermediate transfer belt. FIG. 6 is an explanatory diagram illustrating a state after a sheet once hits an intermediate transfer belt. FIG. 6 is an explanatory diagram illustrating a state in which a sheet is inserted into a secondary transfer unit. 6 is a graph schematically showing the speed of the intermediate transfer belt during monochrome printing of thick paper. 6 is a graph schematically showing the speed of the intermediate transfer belt during monochrome printing of plain paper. FIG. 3 is an explanatory diagram during monochrome printing showing an embodiment of an image forming apparatus according to the present invention. FIG. 9 is an explanatory diagram illustrating a state during color printing in the image forming apparatus of FIG. 8. It is explanatory drawing at the time of color printing which shows the other Example of the image forming apparatus which concerns on this invention. FIG. 11 is an explanatory diagram showing a state during monochrome printing in the image forming apparatus of FIG. 10. It is explanatory drawing which shows the modification of the Example shown in FIG. 6 is a graph showing the speed fluctuation of the intermediate transfer belt when only a black photosensitive member is brought into contact with the intermediate transfer belt and entering the secondary transfer portion when printing a monochrome image on thick paper. 6 is a graph showing the speed fluctuation of the intermediate transfer belt when only a black photosensitive member is brought into contact with the intermediate transfer belt and entering the secondary transfer portion when printing a monochrome image on plain paper. 6 is a graph showing the speed fluctuation of the intermediate transfer belt that occurs when a thick paper is entered into the secondary transfer portion with all the photoreceptors in contact with the intermediate transfer belt during monochrome printing. The black photosensitive member is brought into contact with the intermediate transfer belt during monochrome printing, and the speed fluctuation of the intermediate transfer belt when the thick paper enters the secondary transfer section with the intermediate transfer belt sandwiched between the contact member and the backup roller. It is the shown graph. 2 is an external view of a process cartridge including a photoreceptor. FIG. It is sectional drawing which shows the internal structure of the process cartridge shown in FIG.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Photoconductor 3 Intermediate transfer belt 4 Secondary transfer roller 6a-6d Bias roller 15 Contact member 16 Backup roller

Claims (14)

A black image carrier, a plurality of color image carriers, an intermediate transfer member in contact with these image carriers, and a toner image provided on each image carrier and formed on the image carrier Primary transfer means for primary transfer to the intermediate transfer body, and secondary transfer means for transferring the toner image primarily transferred onto the intermediate transfer body to a recording medium,
When forming a monochrome image, only the black image carrier is brought into contact with the intermediate transfer member, while when forming a color image, the entire image carrier including the black image carrier is removed. In the image forming apparatus that is brought into contact with the intermediate transfer member,
A load applying means for selectively applying a load to the intermediate transfer member is provided on the upstream side in the rotation direction of the intermediate transfer member with respect to a contact portion between the black image carrier and the intermediate transfer member;
An image forming apparatus, wherein the load applying unit applies a load to the intermediate transfer member when forming a monochrome image.   2. The image forming apparatus according to claim 1, wherein the load applying unit is provided at a position where the contact member that contacts the surface of the intermediate transfer member and the contact member and the intermediate transfer member are opposed to each other. An image forming apparatus comprising:   3. The image forming apparatus according to claim 2, wherein the black image bearing member is located closest to the secondary transfer position toward the upstream side in the rotation direction of the intermediate transfer member, and is opposed to the contact member. The image forming apparatus according to claim 1, wherein the load applying unit including a member is provided between the color image carrier and the color image carrier closest to the black image carrier.   4. The image forming apparatus according to claim 1, wherein when a color image is printed, at least one of the contact member and the opposing member is separated from the intermediate transfer member. 5. Image forming apparatus.   2. The image forming apparatus according to claim 1, wherein the load applying unit is one of the plurality of color image carriers.   6. The image forming apparatus according to claim 5, wherein the load applying unit is an image carrier that is disposed closest to the black image carrier among the plurality of color image carriers. An image forming apparatus.   2. The image forming apparatus according to claim 1, wherein the load applying unit is all of the plurality of color image carriers.   8. The image forming apparatus according to claim 5, wherein the rotation speed of the color image carrier when a load is applied is slower than the rotation speed of the black image carrier. Image forming apparatus.   9. The image forming apparatus according to claim 5, wherein the color image carrier when a load is applied applies a bias to the primary transfer means facing the color image carrier. An image forming apparatus.   10. The image forming apparatus according to claim 9, wherein a bias value applied to the primary transfer unit is higher than a bias value applied to the primary transfer unit when a color image is printed. apparatus.   11. The image forming apparatus according to claim 1, wherein the load applying unit is configured to apply a load to the intermediate transfer member only when the recording medium is a specific type. An image forming apparatus.   12. The image forming apparatus according to claim 11, wherein the load applying unit is configured to apply a load to the intermediate transfer member only when the recording medium has a predetermined thickness or more. apparatus.   11. The image forming apparatus according to claim 1, wherein the load applying unit applies a load to the intermediate transfer member only when a conveyance speed of the recording medium is equal to or higher than a predetermined speed. An image forming apparatus comprising:   14. The image forming apparatus according to claim 1, wherein the image carrier is held in a process cartridge in which charging means for charging at least a surface of the image carrier is integrated. An image forming apparatus.

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