JP4341601B2 - Image forming apparatus - Google Patents

Description

  The present invention relates to a tandem image forming apparatus.

Conventionally, it is directly (so-called) at a transfer position between a plurality of photoconductors arranged in a line in one direction, a plurality of transfer rollers respectively facing the photoconductors, and the transfer roller facing the photoconductors. An endless belt that conveys a developer image transferred in an intermediate transfer tandem type image forming apparatus) or indirectly (in the case of a so-called direct transfer tandem type image forming apparatus) via a recording medium; and an endless belt 2. Description of the Related Art A tandem color image forming apparatus including a driving roller that circulates is known.
By the way, in such an image forming apparatus, when the drive roller is assembled in an eccentric state due to variations in components such as a drive roller and a drive gear for driving the drive roller, the eccentricity of the drive roller is caused by this eccentricity. The movement speed of the outer peripheral surface may vary.

  Specifically, for example, when the driving roller has a cylindrical shape, if the roundness of the outer shape varies, even if the driving roller is rotated so that the rotational angular velocity is constant, the rotating shaft of the driving roller The part far from the moving speed is fast, and the part near the rotation axis is slow. Since the fluctuation of the moving speed has periodicity, the fluctuation of the moving speed is repeated for each rotation of the driving roller.

  Such a change in the moving speed of the driving roller causes a change in the conveying speed of the endless belt, and as a result, the transferred developer image is displaced. This is because, in a color image forming apparatus in which developer images of different colors are transferred in an overlapping manner, if the color misregistration of the colors transferred in an overlapping manner is different for each color, an image with color misregistration is formed. Will be.

Therefore, in order to prevent misalignment due to the eccentricity of the driving roller or the like, as shown in Patent Document 1, it was considered that the interval between adjacent photosensitive members is an integral multiple of the outer peripheral length of the driving roller. . As a result, even when the movement speed of the driving roller varies periodically, the positional deviation of the developer image for each color to be transferred becomes periodically equal, and an image with no color deviation as a whole is obtained. It becomes possible to obtain.
Japanese Unexamined Patent Publication No. 64-31173

  By the way, the cause of the actual position shift in the developer image is not limited to the above-described eccentricity of the driving roller or the like, but may be due to the eccentricity of the photosensitive member. Specifically, when the photoconductor is a drum shape, if the distance from the rotation axis to the outer peripheral surface is caused by variation in the roundness of the outer shape, even if the rotation angular velocity is rotated, A portion far from the rotation axis has a high moving speed, and a portion near the rotation axis has a low moving speed. In addition, for example, the rotational angular velocity of each photoconductor may change due to a pitch error of the drive gear of the rotary body, which may cause periodic fluctuations in the moving speed of the peripheral surface of the photoconductor. Therefore, in the configuration of the above-mentioned Patent Document 1, the positional deviation of the developer image that occurs at the time of transfer due to the change in the moving speed of the outer peripheral surface of the photosensitive member is not taken into account, and the color deviation can be sufficiently prevented. It was hard to say.

  The present invention has been completed based on the above-described circumstances, and an object thereof is to provide an image forming apparatus capable of reducing image color misregistration caused by a driving roller and a photoconductor.

  As means for achieving the above object, an image forming apparatus according to the invention of claim 1 includes a plurality of photoconductors arranged in alignment in one direction, and a plurality of transfer rollers facing each of the photoconductors. And an endless belt that conveys a developer image transferred directly or indirectly via a recording medium at a transfer position between the transfer roller facing the photoconductor, and a circulation drive of the endless belt In the tandem type color image forming apparatus including the driving roller, the interval P between the adjacent transfer positions is {(diameter Dd of the driving roller + thickness t of the endless belt + diameter Db of the photoreceptor) × ( It is characterized by being an integral multiple of 1/2) × π}.

  The diameter Dd of the driving roller means the diameter of the driving roller, and π means the circumference.

  A second aspect of the present invention is characterized in that, in the first aspect of the present invention, the shaft of the transfer roller facing the photoconductor is located substantially directly below the shaft of the photoconductor.

  The invention of claim 3 is characterized in that, in the invention of claim 1, the position of the shaft of the transfer roller facing the photoconductor is located downstream of the position of the shaft of the photoconductor. .

<Invention of Claim 1>
According to this configuration, not only the misalignment of the developer image that occurs during transfer due to the eccentricity of the drive roller, but also the misalignment of the developer image that occurs during transfer due to the eccentricity of the photosensitive member, etc. Since the position interval P is determined, color misregistration generated in the image due to the driving roller and the photosensitive member can be reduced.

  Further, by setting the transfer position interval P in consideration of the thickness t of the endless belt, it is possible to more reliably reduce the color shift that occurs in the image.

<Invention of Claim 2>
According to this configuration, the interval P between the adjacent transfer positions can be set to the interval between the centers of adjacent photoconductors (pitch between the photoconductors), so that color misregistration generated in an image can be reduced with a simple configuration. be able to.

<Invention of Claim 3>
Assuming that the shaft of the transfer roller facing the photoconductor is positioned upstream of the shaft of the photoconductor, the electric field generated by the transfer roller before the photoconductor is contacted with the endless belt or the recording medium transported by the endless belt. As a result, the developer carried on the photosensitive member is scattered on the endless belt or the recording medium on the endless belt, so-called pre-transfer phenomenon may occur, which may reduce the quality of image formation. On the other hand, according to this configuration, since the position of the shaft of the transfer roller is located on the downstream side of the position of the shaft of the photosensitive member, pre-transfer can be suppressed.

<Embodiment 1>
1. Overall Configuration A first embodiment of the present invention will be described with reference to FIGS. FIG. 1 is a central sectional view showing a schematic configuration of a laser printer (corresponding to an image forming apparatus of the present invention) 1. The laser printer 1 is a so-called direct tandem type color laser printer including four photosensitive drums 51B, 51C, 51M, and 51Y corresponding to the respective colors of black, cyan, magenta, and yellow. is there. In the present embodiment, the photosensitive drums 51B, 51C, 51M, and 51Y are arranged at regular intervals so as to be aligned in the depth direction of the apparatus. In the following description, the near side indicates the right side in FIG. 1, and the back side indicates the left side in FIG.

  A laser printer 1 includes a main body casing 2 in which a paper feeding unit 4 for feeding paper as a recording medium, a belt unit 5 for conveying paper, and image formation for forming an image on the fed paper. A unit 6 and a scanner unit 7 as an exposure unit are stacked in order from the bottom.

  The paper conveyance path L of the laser printer 1 is as indicated by a two-dot chain line in FIG. 1 and is inverted by about 180 degrees in a U shape from the front side to the rear side of the laser printer 1 at the upper front portion of the paper feed unit 4. Heading to the rear of the laser printer 1. Then, when reaching the rear end side, a path is formed that is reversed by about 180 degrees in a U-shape toward the front side to reach the paper discharge tray 87 provided on the upper surface wall 2A of the main casing 2.

  As will be described in detail below, a toner image (developer image) is formed on the sheet fed from the sheet feeding unit 4 in the process of being conveyed through the sheet conveying path L, and this is thermally fixed by the fixing unit 81. Thus, a desired color image is formed.

2. Configuration of Each Part (1) Configuration of Paper Feed Unit The paper feed unit 4 includes a paper feed tray 11, a paper feed roller 13, and a transport roller 15. The paper feed roller 13 takes out the sheets stacked on the paper feed tray 11 one by one, and the transport roller 15 transports the fed paper toward the belt unit 5.

(2) Basic Configuration of Belt Unit The belt unit 5 includes a driving roller 21, a driven roller 23, and four transfer rollers 31B, 31C, 31M, and 31Y. Each roller is an endless sheet conveying belt (the endless belt of the present invention). Equivalent to 25).

  The driving roller 21 is formed by covering a surface of a metal cylinder with a thin rubber material (a non-slip rubber having a high friction and a thickness of about 0.5 mm), and a driving force is transmitted from a motor (not shown). It is configured to be. The driven roller 23 is for applying an appropriate tension to the paper transport belt 25, and is urged rightward in FIG. 1 by a spring (not shown).

  The transfer rollers 31B, 31C, 31M, and 31Y are arranged to face the photosensitive drums 51B, 51C, 51M, and 51Y, and more specifically, the photosensitive drums 51B, 51C, 51M, and the like arranged in the depth direction of the apparatus. Just as with the photosensitive drum 51, the paper conveying belt 25 is sandwiched between them just below 51Y. Each of these transfer rollers 31B, 31C, 31M, and 31Y is an elastic roller in which a metal roller shaft is covered with a conductive sponge material (thickness, about 5 mm), and each of the transfer rollers 31B, 31C, 31M, and 31Y A constant current source (not shown) is connected to the roller shaft, and a transfer bias (negative voltage) is applied at a predetermined timing.

  In addition, a coil spring 32 is attached to each of the transfer rollers 31B, 31C, 31M, and 31Y. These coil springs 32 urge the transfer rollers 31B, 31C, 31M, and 31Y upward in the drawing so as to form an appropriate nip between the transfer roller 31 and the photosensitive drum 51.

  The paper transport belt 25 is made of, for example, a resin material such as polycarbonate, and the width dimension thereof is equal to or larger than the width dimension of the maximum printable paper size (for example, A4 size). When the driving force is transmitted from the motor (not shown) to the driving roller 21, the driving roller 21 starts to rotate. By the rotation of the drive roller 21, the paper transport belt 25 is driven to circulate and transport the paper horizontally from the right side to the left side in FIG.

(3) Configuration of Image Forming Unit and Scanner Unit The image forming unit 6 includes four photosensitive drums 51B, 51C, 51M, and 51Y corresponding to black, cyan, magenta, and yellow colors. These photosensitive drums 51B, 51C, 51M, and 51Y are arranged in a line at equal intervals in the depth direction of the apparatus (in the horizontal direction in FIG. 1) as described above.

  Developing units 55B, 55C, 55M, and 55Y are provided at obliquely upper right positions of the respective photosensitive drums 51B, 51C, 51M, and 51Y, and a charger 61 is provided at an obliquely upper left portion. A scanner unit 7 is disposed above the photosensitive drums 51B, 51C, 51M, and 51Y. The basic configurations of the photosensitive drums 51B, 51C, 51M, and 51Y, the developing units 55B, 55C, 55M, and 55Y, the charger 61, and the scanner unit 7 are the same and do not differ between the colors. In the description, the right end (black one) is taken as an example, and the explanation for the other ones is omitted.

  The developing unit 55B includes a developing roller 57, a supply roller 58, and a layer thickness regulating blade 59 at a lower portion of a casing that stores toner. The developing roller 57 and the supply roller 58 are arranged to face each other, and when the toner passes between them, the toner is supplied from the supply roller 58 to the developing roller 57, and the toner is rubbed to a positive polarity by friction accompanying rotation. Has the function of charging. The layer thickness regulating blade 59 makes the layer thickness of toner positively charged and carried on the developing roller 57 constant. The developing roller 57 is disposed so as to face the photosensitive drum 51B.

  The charger 61 is a so-called scorotron charger, and includes a charging wire 63 accommodated in a shield case 62. The charging wire 63 is configured to be applied with a high voltage, thereby causing corona discharge. In discharging, a constant voltage is applied to the grid to uniformly charge the surface of the photosensitive drum 51B to a positive polarity.

  The scanner unit 7 includes a laser light source (not shown), a polygon mirror 71, an fθ lens 73, a reflecting mirror 75, and the like. The polygon mirror 71 rotates by driving the polygon motor 79 and has a function of irradiating the surface of the photosensitive drum 51B with the laser light emitted from the laser light source through the path of the two-dot chain line shown in FIG.

  A fixing unit 81 is provided on the rear side of the belt unit 5. The fixing unit 81 includes a heating roller 83 and a pressing roller 82 that is opposed to the heating roller 83. The heating roller 83 includes a halogen lamp (not shown) for heating, and the color image transferred onto the sheet by each of the transfer rollers 31B, 31C, 31M, and 31Y is displayed on the sheet between the heating roller 83 and the pressing roller 82. It is heat-fixed while passing between.

  A series of image forming processes performed by the laser printer 1 configured as described above will be briefly described. First, the surfaces of the photosensitive drums 51B, 51C, 51M, and 51Y are made uniform by the charger 61 as they rotate. Is positively charged. Thereafter, when predetermined image data is input from, for example, a host device, control based on the image data is started, and laser light is emitted from each scanner unit 7 toward each photosensitive drum 51B, 51C, 51M, 51Y. Is done. As a result, a predetermined electrostatic latent image corresponding to the image data is formed on the surface of each photosensitive drum 51B, 51C, 51M, 51Y, that is, the positively charged photosensitive drums 51B, 51C, 51M, 51Y. Of the surface, the potential of the portion irradiated with the laser light decreases.

  Next, by the rotation of the developing roller 57, the toner carried on the developing roller 57 and positively charged is supplied to the electrostatic latent images formed on the surfaces of the photosensitive drums 51B, 51C, 51M, and 51Y. . As a result, the electrostatic latent images on the photosensitive drums 51B, 51C, 51M, and 51Y are visualized, and toner images by reversal development are carried on the surfaces of the photosensitive drums 51B, 51C, 51M, and 51Y.

  Further, in parallel with the above-described processing for forming the toner image, processing for transporting the paper is performed. That is, as the paper feed roller 13 rotates, the paper is fed from the paper feed tray 11 to the paper transport path L one by one. The sheet sent to the sheet conveyance path L is transferred by a conveyance roller 15 and a sheet conveyance belt 25 on a transfer position (on a line connecting the center of each photosensitive drum 51 and the center of each transfer roller 31 facing each photosensitive drum 51). The photosensitive drum 51 and the transfer roller 31 are brought into contact with each other). Then, the toner images of the respective colors carried on the surfaces of the photosensitive drums 51B, 51C, 51M, and 51Y are transferred by the transfer bias applied to the transfer rollers 31B, 31C, 31M, and 31Y when passing through the transfer position. The images are sequentially superimposed and transferred onto the surface of the paper (indirectly transferred via a recording medium). Thus, a color image is formed on the paper. Thereafter, when passing through the fixing unit 81, the transferred toner image (color image) is thermally fixed, and the sheet is discharged onto a discharge tray 87.

3. Next, a configuration for reducing color misregistration that occurs during transfer will be described.
FIG. 2 is a diagram for explaining the interval between adjacent transfer positions. FIG. 3 is a diagram for explaining the influence of the thickness of the paper transport belt 25 on the transport speed. FIG. 4 shows a periodic speed change that occurs for each of the drive roller 21 and the photosensitive drum 51 in the upper stage, and a case where a periodic speed change occurs in both the drive roller 21 and the photosensitive drum 51 in the lower stage. It is a figure which shows roughly the shift | offset | difference of a transfer position.

(1) Among the configurations of the laser printer described above, the drive roller 21 and the photosensitive drum 51 may be shaken during rotation due to eccentricity or the like.
Specifically, as shown in FIG. 2, when the driving roller 21 has a cylindrical shape, if the roundness of the outer shape varies, the driving roller 21 is rotated so that the rotational angular velocity is constant. However, the moving speed of the portion of the driving roller 21 far from the roller shaft 21a (rotating shaft) is fast, and the moving speed of the portion near the roller shaft 21a is slow. Since the fluctuation of the moving speed has periodicity, the fluctuation of the moving speed is repeated for each rotation of the driving roller 21 (see the dotted line in the upper part of FIG. 4).

  On the other hand, even when the photosensitive drum 51 has a cylindrical shape, if the distance from the rotation shaft 51a to the outer peripheral surface becomes close due to the variation in the roundness of the outer shape, the photosensitive drum 51 may be rotated so that the rotation angular velocity is constant. The part far from the rotating shaft 51a has a high moving speed, and the part near the rotating shaft 51a has a low moving speed. In addition, for example, the rotational angular velocity of each photosensitive drum 51 may change due to a pitch error of the driving gear of the rotating body, and the movement speed of the circumferential surface of the photosensitive drum 51 may periodically change (see the solid line in the upper part of FIG. 4). ).

  Therefore, by setting the interval P between adjacent transfer positions to be an integral multiple of the outer peripheral length of the driving roller 21 or the outer peripheral length of the photosensitive drum 51, the transfer is performed when the moving speed of the driving roller 21 varies periodically. It is conceivable to obtain an image with no color misregistration as a whole by periodically equalizing the developer image misregistration for each color (the distance conveyed in the cycle of T1 in the upper solid line in FIG. 4 is the transfer position). Interval P).

However, when the above-described periodic changes are different between the photosensitive drum 51 and the driving roller 21 (T1 ≠ T2 in the upper part of FIG. 4), as shown in the lower part of FIG. The positional deviation of the toner image affected by both of these occurs at a period T different from T1 and T2. Accordingly, when the transfer position interval P is set in accordance with the periodic change of one of the drive roller 21 and the photosensitive drum 51, the positional deviation of the developer image transferred for each photosensitive drum 51 is different. The color shift of the image as a whole becomes relatively large.
Specifically, for example, when the transfer position interval P is set in accordance with only the periodic change (change in each T1) of the photosensitive drum 51, a color shift corresponding to A in the lower part of FIG. When the transfer position interval P is adjusted only for the periodic change of the roller 21 (change for each T2), a color shift corresponding to B in the lower part of FIG. 4 occurs.

(2) Therefore, in this embodiment, the transfer position interval P for each color is set to
P = (distance that the sheet conveying belt 25 conveys the sheet while the drive roller 21 rotates once) + distance that the sheet conveying belt 25 conveys the sheet while the photosensitive drum 51 rotates once) × (1/2) ·・ ・ (I)
It is said.
Here, the “distance that the sheet conveying belt 25 conveys the sheet while the driving roller 21 rotates once” in the equation (i) is determined based on the diameter Dd of the driving roller 21 and the thickness t of the sheet conveying belt. be able to. The “distance that the sheet conveying belt 25 conveys the sheet while the photosensitive drum 51 rotates once” can be determined based on the diameter Db of the photosensitive drum 51.

Therefore, in other words, the transfer position interval P is:
P = {(Dd + 2tα + Db) × π × (1/2)} (ii)
It becomes.
Dd: diameter of the driving roller t: thickness of the paper conveying belt α: coefficient corresponding to the elastic expansion / contraction rate Db: diameter of the photosensitive drum π: circumference

  Of the 2tα in the equation (ii), the thickness t of the sheet conveying belt is equal to the outside of the sheet conveying belt 25 (the sheet conveying surface of the sheet conveying belt 25) and the inside of the sheet conveying belt 25 (drive) as shown in FIG. This is added because the moving speed of the sheet conveying belt 25 is different from the outer surface of the roller 21. The thickness t of the belt is doubled because the thickness of the sheet conveying belt 25 affects the conveying speed for each of the upper and lower sides of the driving roller 21.

  The coefficient α corresponding to the elastic expansion / contraction rate is a coefficient corresponding to the elastic expansion / contraction rate of the paper conveyance belt 25, and the coefficient α corresponding to the elastic expansion / contraction rate that varies depending on the material of the paper conveyance belt 25 is determined. It has become. For example, if the inner side of the paper conveying belt 25 is more easily stretched than the outer side, the speed of the paper conveying belt 25 approaches the speed indicated by X in FIG. If the material is easy to stretch, the speed of the paper transport belt 25 approaches the speed at Y. Therefore, by multiplying the belt thickness t by a coefficient α corresponding to the elastic expansion / contraction rate, the distance that the sheet conveying belt 25 moves while the drive roller 21 and the photosensitive drum 51 make one round can be obtained more accurately. It has become.

  Here, in the present embodiment, the material of the paper conveying belt 25 is the same material in the thickness direction and the elastic expansion / contraction rate is the same, and the speed of the paper conveying belt 25 is a half of the thickness of the paper conveying belt 25. (The position of Z in FIG. 3) (α = 1/2).

That is, the transfer position interval P =
{(Dd + t + Db) × π × (1/2)} (iii)
It has become.

Further, the distance (Dd + t) × π in equation (iii) that the sheet conveying belt 25 conveys the sheet while the driving roller 21 makes one rotation and the sheet conveying belt 25 while the photosensitive drum 51 rotates once. Is that (1/2) is added to the distance obtained by adding the distance (Db × π in the equation (iii)) to which the sheet is conveyed, as described above, simply as the driving roller 21 and the photosensitive element. When the interval P between adjacent transfer positions (the distance between adjacent photosensitive drums 51) is determined based on the distance that the sheet is conveyed while one of the drums 51 makes one rotation, A and B in the lower part of FIG. This is because there may be a color shift corresponding to.
Therefore, the interval P between adjacent transfer positions (the distance between the adjacent photosensitive drums 51) is half the distance (1 /) of the distance that the sheet is conveyed while the drive roller 21 and the photosensitive drum 51 rotate once. 2), a transfer that is relatively close to the periodic positional deviation that occurs as a whole when the positional deviation of both the drive roller 21 and the photosensitive drum 51 occurs (the positional deviation that occurs in the period T in the lower stage of FIG. 4). Since the position interval is P, color misregistration generated in the image due to the shake of the driving roller 21 and the photosensitive drum 51 can be reduced.

4). Effects of this Embodiment According to this embodiment, the transfer position interval P is determined based on the diameter Dd of the driving roller 21, the thickness t of the belt, and the diameter Db of the photosensitive drum 51 (photosensitive drum 51). In addition to the positional deviation of the toner image (developer image) generated at the time of transfer due to the eccentricity of the roller 21 or the like, the transfer position in consideration of the positional deviation of the toner image at the time of transfer due to the eccentricity of the photosensitive drum 51 or the like. Since the interval P is determined, the color misregistration generated in the image due to the driving roller 21 and the photosensitive drum 51 can be reduced.

  Further, the roller shafts 31 a of the transfer rollers 31 B, 31 C, 31 M, and 31 Y that face the photosensitive drum 51 are positioned almost directly below (substantially directly below) the rotating shaft 51 a of the photosensitive drum 51. Accordingly, since the interval P between the adjacent transfer positions can be set to the interval between the centers of the adjacent photosensitive drums 51 (pitch between the photosensitive drums 51), the color shift generated in the image can be reduced with a simple configuration. it can.

<Embodiment 2>
A second embodiment of the present invention (corresponding to claim 5) will be described with reference to FIGS. In addition, about the structure same as Embodiment 1, the same code | symbol is attached | subjected and description is abbreviate | omitted.
In the first embodiment, the roller shaft 31a of the transfer roller 31 is positioned directly below the rotation shaft 51a of the photosensitive drum 51. However, in the second embodiment, as shown in FIG. The photosensitive drums 51 and the transfer rollers 31 are arranged so as to be shifted from directly below the photosensitive drum 51 to the downstream side in the transport direction, and in the direction inclined with respect to the vertical direction (direction perpendicular to the paper transport direction) in the drawing. It is arrange | positioned so that it may be pressed.

  That is, the position of the roller shaft 31a of the transfer rollers 31B, 31C, 31M, and 31Y facing the photosensitive drum 51 is positioned downstream of the position of the rotating shaft 51a of the photosensitive drum 51.

As a result, as shown in FIG. 6, each photosensitive drum 51 can be brought into contact with the paper transport belt 25 (paper) faster than each transfer roller 31. Accordingly, since the paper transport belt 25 and the paper are in close contact with the surface of the photosensitive drum 51, scattering of the toner (developer) is suppressed and pre-transfer is prevented.
In the embodiment as shown in FIGS. 5 and 6, the transfer rollers 31 are not disposed directly below the photosensitive drums 51 but are shifted to the downstream side in the transport direction, so that the sheet transport belt 25 (transfer belt) is partially The vehicle travels while curving (see FIG. 6).
Therefore, the interval P between the adjacent transfer positions is not the distance between the adjacent transfer rollers 31 (linear distance) but the length (curved) of the sheet conveying belt 25 positioned between the transfer positions. That is, when the interval P between adjacent transfer positions is set to be the same as that in the first embodiment, the inter-axis distance (linear distance) between the adjacent transfer rollers 31 is slightly shorter than that in the first embodiment.

(Effect of Embodiment 2)
If the roller shaft 31a of the transfer roller 31 facing the photosensitive drum 51 is positioned upstream of the rotation shaft 51a of the photosensitive drum 51, the transfer roller before the sheet (recording medium) contacts the photosensitive drum 51. A so-called pre-transfer phenomenon occurs in which the developer carried on the photosensitive drum 51 is scattered on the paper due to the influence of the electric field by 31B, 31C, 31M, and 31Y, and there is a possibility that the quality of image formation is deteriorated. On the other hand, according to the second embodiment, the positions of the roller shafts 31a of the transfer rollers 31B, 31C, 31M, and 31Y are located on the downstream side of the position of the rotation shaft 51a of the photosensitive drum 51. Can do.

< Reference example >
The following describes the present onset Akira of the reference example. In addition, about the structure same as the said embodiment, the same code | symbol is attached | subjected and description is abbreviate | omitted.
In the above embodiment, the interval P between adjacent transfer positions is determined based on the diameter of the driving roller 21, the thickness of the paper transport belt 25, and the diameter of the photosensitive drum 51. The interval P between adjacent transfer positions is determined based on the rotation frequency Fd and the rotation frequency Fb of the photosensitive drum 51.

Specifically, the transfer position interval P is determined by the following equation (iv).
P = N × 2Vd ÷ (Fb + Fd) (iv)
Vd: speed of the paper transport belt Fd: rotational frequency of the driving roller Fb: rotational speed of the photosensitive drum N: integer The speed of the paper transport belt is an actual moving speed of the paper transport belt, and is a detection sensor (not shown). Or the like, the amount of movement of the sheet conveying belt per unit time may be obtained, and this amount of movement may be used as the moving speed of the sheet conveying belt. The rotation frequency is the number of rotations per unit time, and can be obtained from the rotation frequency of a motor (not shown).
In this way, the interval P between the adjacent transfer positions is determined not only by the rotation frequency Fd of the drive roller 21 but also by the transfer position interval P in consideration of the rotation frequency Fb of the photosensitive drum 51. Color misregistration that occurs in the image due to the roller 21 and the photosensitive drum 51 can be reduced.

  Further, since the interval P between the transfer positions is determined not by the diameters of the driving roller 21 and the photosensitive drum 51 but by the rotational frequency Fd of the driving roller 21 and the rotational frequency Fb of the photosensitive drum 51, the sheet conveying belt 25 as in the first embodiment. It is not necessary to use the coefficient of the thickness t.

<Other embodiments>
The present invention is not limited to the embodiments described with reference to the above description and drawings. For example, the following embodiments are also included in the technical scope of the present invention, and further, within the scope not departing from the gist of the invention other than the following. Various modifications can be made.

(1) In each of the above embodiments, a direct tandem system that performs direct transfer on a sheet is exemplified. However, as shown in FIG. 7, the present invention is an intermediate transfer member 110 (intermediate transfer belt) for a sheet. The present invention can also be applied to an intermediate transfer type color laser printer that performs transfer via the printer. The basic image formation process of an intermediate transfer type color laser printer, that is, the process from exposure to formation of a toner image on a photosensitive drum is the same as that of a direct tandem color printer. The format to transfer is different.
Specifically, in the direct tandem system, the toner images of each color are sequentially transferred onto the paper, whereas in the intermediate transfer tandem system, first, the intermediate transfer belt (corresponding to the endless belt of the present invention) 110 is used. In contrast, the toner images of the respective colors are sequentially transferred in the same manner as in the first embodiment (directly transferred to the intermediate transfer belt 110 at the transfer position between the photosensitive drum 51 and the transfer roller 31 facing the photosensitive drum 51). . Then, the color image formed on the intermediate transfer belt 110 (the image in which the toner images of the respective colors are superimposed) is collectively transferred onto the sheet by the secondary transfer backup roller 113 and the secondary transfer roller 115.

  (2) In the above embodiment, the transfer position interval P for each color is set to {(diameter Dd of the driving roller 21 + belt thickness t × α + diameter Db of the photosensitive drum 51) × (1/2) × π}. However, even when this interval is an integral multiple (2 times, 3 times,...), Color misregistration caused by the positional deviation of the transfer position that periodically changes can be prevented.

  (3) In each of the above embodiments, the number of the image forming units 6 is four, and the four-color image of CMYK is formed. However, the present invention is not limited to this. For example, the number of image forming units 6 may be three or less or five or more (N), and images of two colors (red and black), three colors, or five colors or more may be formed.

  (4) In the first embodiment, the transfer position interval P (the distance between adjacent photosensitive drums 51) is a distance obtained by adding the distance for conveying the sheet while the drive roller 21 and the photosensitive drum 51 make one rotation. Although it is half (1/2), it may not be half (1/2). Specifically, the actual misregistration period is obtained by experiments or the like according to the diameter of the driving roller 21, the diameter of the photosensitive drum 51, the rotation speed, the period, the rotation frequency, and the like, and the values that are most unlikely to cause color misregistration are integrated. You just have to do it.

1 is a cross-sectional view of the center side showing a schematic configuration of a laser printer according to Embodiment 1. Diagram showing the interval between adjacent transfer positions The figure explaining that conveyance speed changes with the thickness of a paper conveyance belt. The figure explaining the influence which it gives to a transfer position by shake of a driving roller and a photosensitive drum Central side sectional view showing a schematic configuration of the laser printer of the second embodiment Enlarging the vicinity of the transfer position Central sectional side view which shows schematic structure of the laser printer of the intermediate transfer system in other embodiment

DESCRIPTION OF SYMBOLS 1 ... Laser printer 21 ... Drive roller 25 ... Paper conveyance belt (endless belt)
31 (31B, 31C, 31M, 31Y) ... transfer roller 51 (51B, 51C, 51M, 51Y) ... photosensitive drum (photoconductor)
55 (55B, 55C, 55M, 55Y) ... Development unit Dd ... Diameter (diameter) of drive roller
Db: Diameter of photosensitive drum (diameter)
Fd: rotational frequency of the driving roller Fb: rotational frequency of the photosensitive drum α: coefficient corresponding to the elastic expansion / contraction rate t: thickness of the paper conveying belt

Claims (3)

A plurality of photoconductors arranged in alignment in one direction;
A plurality of transfer rollers facing each of the photoreceptors;
An endless belt for conveying a developer image transferred directly or indirectly via a recording medium at a transfer position between the transfer roller and the transfer roller facing the photoconductor;
In a tandem type color image forming apparatus comprising a driving roller for circulatingly driving the endless belt,
The interval P between adjacent transfer positions is:
An image forming apparatus, which is an integral multiple of {(diameter Dd of the driving roller + thickness t of the endless belt + diameter Db of the photoconductor) × (½) × π}. The image forming apparatus according to claim 1, wherein the shaft of the transfer roller facing the photoconductor is located substantially immediately below the shaft of the photoconductor. 2. The image forming apparatus according to claim 1, wherein the position of the shaft of the transfer roller facing the photoconductor is located downstream of the position of the shaft of the photoconductor.

Images