JPH0756575B2 - Image forming device - Google Patents

Description

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention generally relates to a color image forming apparatus, and in particular, a plurality of image forming stations for forming an image are arranged in parallel on an image carrier to form each image. The present invention relates to a color image forming apparatus that conveys a transfer member to a station via a conveyor belt that is moved by a driving roller, and superimposes and transfers an image on an image carrier to the transfer member.

The present invention can be embodied not only in the electrophotographic color copying apparatus but also in various color printers and the like, but in the present specification, the color electrophotographic copying apparatus will be described. Further, the image carrier used in the present invention may have any form such as a belt shape or a drum shape as long as it is an image carrier which moves endlessly, but in the present specification, it is described as a drum shape. To do.

2. Description of the Related Art Conventionally, several methods have been proposed as a color copying apparatus using an electrophotographic method, and one of the typical methods is illustrated in FIG.

According to the color electrophotographic copying machine shown in FIG. 5, image forming stations I, II, III and IV accommodating color toners such as cyan, magenta, yellow and black are arranged side by side in the main body of the apparatus. Image forming station I, II, II
I and IV are provided with image carriers, that is, photosensitive drums 11, 12, 13, and 14 supported by a support frame (not shown), and primary corona chargers 21, 22, 23, and 24 on the outer periphery thereof. Developing devices 31, 32 having color toners of cyan, magenta, yellow, black, etc.,
33, 34, the transfer corona chargers 41, 42, 43, 44 and the cleaners 51, 52, 53, 54 are arranged and formed, respectively.

The photosensitive drums 11, 12, 13, 14 are the primary corona chargers 21, 22,
After being uniformly charged by 23, 24, the photosensitive drums 11, 12, 13, 14 are irradiated with the light beams I ₁ , I ₂ , I ₃ , I ₄ whose light intensity is modulated by the information signal. An electrostatic latent image is formed by scanning above.

These electrostatic latent images are developed by developing devices 31, 32, 33, 34 to form toner images of different colors such as cyan, magenta, yellow and black.

The toner image is composed of a thin dielectric sheet or a screen mesh, and is a transfer member electrostatically adsorbed by an adsorbing charger 62 on a conveyor belt 61 suspended by a driving roller 63 and a driven roller 64 and moving in the arrow direction. P is a transfer corona charger 4
When passing through 1, 42, 43, and 44, multiple transfer is performed on the transfer member P, and a color image is formed on the transfer member P.

Problems to be Solved by the Invention In the image forming apparatus having the above-described configuration, if the drive roller that drives the conveyor belt 61 has eccentricity due to processing or assembly, the moving speed of the conveyor belt is not constant, and therefore multiple copying is performed. In order to prevent the image misalignment in (1), it is preferable that the conveyor belt is moved between the adjacent image forming stations, that is, between the adjacent photosensitive drums, by one rotation or an integral rotation of the driving roller.

The reason for this is that if the drive roller is eccentric due to processing or assembly, the moving speed of the conveyor belt will not be constant and will change sinusoidally as shown in FIG. If H is set to be equal to or equal to the peripheral length πD of the drive roller, the period T _{1 of the} sine wave is the time required for the transfer member P on the conveyor belt to move from one transfer position to the next transfer position. As shown in FIG. 3, the image transfer position changes sinusoidally compared to the ideal transfer position, but the phase angle of the eccentricity of the drive roller at the start of transfer of each color is always This is because the phase of the sine wave of the transferred image always matches because it is constant. Therefore, in FIG. 3, for example, a relative color shift of each color such as magenta (M) and yellow (Y) on the image does not occur.

However, in an actual copying apparatus, even if the above-mentioned configuration is adopted, image shift may occur on the transfer member. As a result of pursuing the cause, the inventors of the present invention have found that when the linear expansion coefficient in the diameter direction of the drive roller and the linear expansion coefficient in the transport direction of the transfer material of the support frame of the photosensitive drum are different, for example, the temperature in the room The peripheral length of the drive roller and the shape of the photosensitive drum support frame change due to environmental changes such as changes. Therefore, the transfer member on the conveyor belt moves between the adjacent photosensitive drums by one rotation or an integral rotation of the drive roller. It was found that they could not move and eventually an image shift occurred on the transfer member.

The present invention has been made based on such a new finding.

OBJECT OF THE INVENTION It is an object of the present invention to prevent image shift on a transfer member even if the ambient atmospheric temperature changes by improving the support belt body for supporting the image bearing member, especially the driving roller of the conveying belt means, An object of the present invention is to provide an image forming apparatus capable of obtaining a high quality image.

Means for Solving Problems The above object is achieved by the image forming apparatus according to the present invention. In summary, the present invention provides a plurality of image carriers that carry images, a common support frame that supports the plurality of image carriers,
A transfer belt that holds and transfers a transfer material to the transfer portion of the image carrier, and a drive roller that drives the transfer belt, and transfer between the transfer portions of the image carrier adjacent to each other with a peripheral length of the drive roller. In the image forming apparatus, which is an integral multiple of the distance along the material conveying path, sequentially transfers the images of the plurality of image carriers to the transfer material carried on the conveyor belt at the transfer unit, and transfers the support frame. The image forming apparatus is characterized in that the linear expansion coefficient in the material conveying direction and the linear expansion coefficient in the diametrical direction of the drive roller are made substantially equal.

Embodiment Hereinafter, the image forming apparatus according to the present invention will be described in more detail with reference to the drawings.

FIG. 1 shows an embodiment of the image forming apparatus according to the present invention. The apparatus of this embodiment has image forming stations I, II, III, and IV similar to the four-color electrophotographic copying apparatus described above with reference to FIG. 5, and each image carrier, that is, electrophotographic apparatus. The photosensitive drums 9 are supported by a common supporting frame (not shown) at equal intervals, and the photosensitive drums 9 are surrounded by the respective units necessary for electrophotographic image formation as in the image forming station shown in FIG. Means are provided, but they are omitted from FIG.

According to the present invention, a conveyor belt 2 made of, for example, a thin dielectric sheet or a screen mesh is disposed under each photosensitive drum 9 so as to be suspended by the conveyor rollers 5, 6, and 7.
The conveyance belt 2 is driven by being sandwiched between the driving roller 4 and the pinch roller 3 adjacent to the conveyance roller 5. More specifically, when the driving roller 4 is driven by a driving source (not shown) such as an electric motor, the conveying belt 2 is rubbed by the driving roller 4 and the pinch roller 3 to cause the conveying rollers 5, 6, 7 to move. Move around in the direction of arrow A.

The driving roller 4 has a distance H (distance of the conveyor belt 2 between the photosensitive drums 9 and 9) along the transfer material conveying path between the transfer portions of the two photosensitive drums 9 and 9 having the same circumference. They are equal. In other words, if the diameter of the drive roller 4 is D, the relationship of πD = H is established. In this case, H = nπD may be set. However, n is an integer.

As described above, with the above configuration, the expansion and contraction amount of the image transferred by the two photosensitive drums 9 and 9 (image forming stations II and III) is plotted as shown in FIG. That is, the transfer position of the image changes sinusoidally as compared with the ideal transfer position, but since the phase angle of the eccentricity of the drive roller 4 at the start of transfer of each color is always constant, the sine wave of the transferred image is changed. The phases of are always the same. For this reason,
Relative color shift of each color such as magenta (M) and yellow (Y) on the image does not occur.

However, if the support frame that supports the photosensitive drum 9 is made of, for example, iron and the drive roller 4 is made of, for example, aluminum,
The relationship between the two is πD = H or nπD = in an atmosphere at room temperature of 20 ° C.
Even if designed to be H, when the temperature changes by t ° C, the linear expansion coefficients of iron and aluminum are 11.7 × 10 ^-6.
/ deg and 23.9 × 10 ^-6 / deg, which are different from each other, there is a difference of X μm between the peripheral length (πD or nπD) of the driving roller and the distance (H) between the photosensitive drums.

When the expansion / contraction amount of the image in this case is plotted, as shown in FIG. A relative color shift (X) occurs.

According to the present invention, the support frame and the drive roller 4 are both formed of a material having the same linear expansion coefficient, that is, either iron or aluminum. That is, the linear expansion coefficient P of the support frame in the transfer material transport direction and the linear expansion coefficient P ′ of the drive roller 4 in the diameter direction are made substantially equal. Therefore, a toner image having no color misregistration can be obtained at each image forming station without being affected by a change in ambient temperature between the supporting frame and the driving roller 4.

Effects of the Invention The image forming apparatus according to the present invention configured as described above is
Even if the ambient atmosphere temperature changes, the linear expansion coefficient of the support frame and the drive roller can be made the same so that the peripheral length of the drive roller can always be kept the same as the center distance between two adjacent photosensitive drums. Therefore, it is possible to always obtain a high-quality image with no color shift, and it is possible to significantly improve the reliability of the image forming apparatus.

[Brief description of drawings]

FIG. 1 is a side view showing a main part of an image forming apparatus according to the present invention. FIG. 2 is an explanatory diagram of changes in the speed of the conveyor belt. FIG. 3 is a plot diagram of a transferred image in a stretched state. FIG. 4 is a plot diagram of the expansion / contraction state of the transferred image corresponding to the change of the ambient temperature. FIG. 5 is a side view of a conventional multicolor electrophotographic copying machine. I, II, III, IV: Image forming station 2: Conveyor belt 4: Drive roller 9: Image carrier H: Center distance between image carriers D: Drive roller diameter

Claims (1)

[Claims] 1. A plurality of image carriers that carry images, a common support frame that supports the plurality of image carriers, and a conveyor belt that carries and conveys a transfer material to a transfer portion of the image carriers. A driving roller that drives the conveyor belt, and the peripheral length of the driving roller is an integral multiple of the distance along the transfer material conveying path between the transfer portions of the adjacent image carriers, and is carried by the conveyor belt. In an image forming apparatus for sequentially transferring the images of the plurality of image carriers to a transfer material in a transfer portion, the linear expansion coefficient of the support frame in the transfer material transport direction and the linear expansion coefficient of the driving roller in the diameter direction. An image forming apparatus characterized in that and are substantially equal to each other.