JP2006018177A - Image forming apparatus - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an inexpensive image forming apparatus, without transfer scattering in a narrow line or a minute dot image by suppressing previous transfer without losing the color shift or the stability of transfer. <P>SOLUTION: A belt body 8 and a transfer member 4 are in contact with each other, in the area of a downstream side of a moving direction of the belt body 8 from among a contact area W1 of an image carrier 2 and the belt body 8 in the moving direction of the belt body 8, and the belt body 8 and the transfer member 4 are not in contact with each other in the area of an upstream side of the moving direction of the belt body 8. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to an electrophotographic or electrostatic recording type image forming apparatus such as a copying machine or a printer, and in particular, transfers a color image obtained by superimposing developer images (toner images) formed on a plurality of image carriers. BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an image forming apparatus provided with a belt body such as an intermediate transfer body or a transfer material transport body that moves in contact with an image carrier in a transfer apparatus for transferring to a material.

  2. Description of the Related Art Conventionally, for example, an electrophotographic full-color image forming apparatus has a plurality of photoreceptors as image carriers, each carrying a developer image (toner image) of different colors, and superimposing toner images from the photoreceptors. A so-called in-line image forming apparatus that forms a color image together is effective in increasing the speed and is widely used.

  In this in-line method, there are a direct transfer method and an intermediate transfer method as a method for transferring a color image formed by superimposing toner images from a plurality of photoconductors onto a transfer material. In the direct transfer method, the transfer material carrier carries and transfers the transfer material to a position where it comes into contact with each photoconductor, and directly transfers the toner image from the photoconductor to the transfer material. In the intermediate transfer method, the image is once transferred to the intermediate transfer member in the primary transfer portion, and the toner images superimposed on the intermediate transfer member are collectively transferred to a transfer material such as paper at the secondary transfer portion.

  As a medium that moves in contact with a plurality of photoconductors, such as a transfer material conveyance body in the direct transfer system or an intermediate transfer body in the intermediate transfer system, a belt-like medium (belt body) is like a drum. Compared with the case where a rigid cylinder is used, there is an advantage that the degree of freedom in arranging inside the image forming apparatus is increased, and the apparatus main body can be reduced in size and cost by effective use of space.

  Here, a configuration in which the intermediate transfer method is adopted in the inline-type full-color image forming apparatus will be described. FIG. 4 is a schematic diagram showing an example of a conventional electrophotographic inline-type full-color image forming apparatus.

  The color image forming apparatus includes a photosensitive member 2 adjacent to the intermediate transfer belt 8 in a plurality of image forming units of each color provided along the belt-shaped intermediate transfer member (intermediate transfer belt) 8 based on image input data. An electrostatic latent image is formed thereon, and a single color toner is developed on the electrostatic latent image to form a single color toner image. Then, the single color toner images formed in the respective image forming units are superimposed on the intermediate transfer belt 8 to form a multiple toner image, the multiple toner image is transferred to the transfer material P, and the multiple toner on the transfer material P is transferred. The image is fixed by the fixing device 21.

  Here, in each color image forming portion, drum-shaped photoreceptors (photosensitive drums) 2 (2a, 2b, 2c, 2d) are arranged in a line.

  Around each photosensitive drum 2, as a means for forming a toner image, a charging roller 7 (7a, 7b, 7c, 7d) as a primary charging means, an exposure device 1 (1a, 1b, 1c, 1d), a developing means The developing device 3 (3a, 3b, 3c, 3d) and the photosensitive drum cleaning blade 5 (5a, 5b, 5c, 5d) are disposed. Each developing device 3 accommodates toner cartridges 6 (6a, 6b, 6c, 6d) of the respective colors.

  In this embodiment, each color photosensitive drum 2 is an organic photoconductor, and has an OPC photosensitive layer on an aluminum drum base (not shown), and is driven to rotate at a predetermined process speed by a driving device (not shown). Is done.

  Below each color photosensitive drum 2, at each primary transfer nip portion (primary transfer portion) 13 (13a, 13b, 13c, 13d), foam rubber or the like is formed on the surface via an endless intermediate transfer belt 8 that is a belt body. Are in contact with a primary transfer roller 4 (4a, 4b, 4c, 4d) which is a transfer member coated with the elastic body.

  The intermediate transfer belt 8 is stretched by a driving roller 10, a tension roller 9, and a secondary transfer counter roller 24, and is rotated in the direction of the arrow by driving the driving roller 10.

  The transfer material P, which is a sheet conveyed from the sheet feeding cassette 20, is abutted against the nip portion between the registration roller 18 and the registration roller facing roller 17 in a stopped state, and forms a loop by the stiffness of the sheet P, whereby the leading end of the sheet P By correcting the skew of the portion and taking a timing with the registration sensor 16 signal, power is transmitted to the registration roller 18 via a registration clutch (not shown), and the paper P is fed again.

  The sheet P re-feeded from the registration roller 18 is guided to the nip formed by the secondary transfer roller 15 and the intermediate transfer belt 8 along the conveyance guide 19, and the toner image formed on the intermediate transfer belt 8. Is transferred onto the paper P, and a fixed toner image can be obtained on the paper P by being pressurized and heated by the fixing device 21.

  On the other hand, the toner image (waste toner) on the intermediate transfer belt 8 that could not be transferred to the paper P in the secondary transfer portion is scraped off from the intermediate transfer belt 8 by the intermediate transfer body cleaning blade 11, and then the waste toner feeding mechanism. 12 is sent to the waste toner container 14 and stored therein.

  Such an in-line intermediate transfer type image forming apparatus achieves high speed, and further uses an intermediate transfer member, so that the transfer target in the process of overlaying the developer is always the same object, that is, the intermediate transfer member, so that it is stable. A superimposed image can be obtained.

  However, in such an image forming apparatus including a belt body that moves in contact with the photosensitive member and has a transfer member disposed on the back side of the contact portion with the photosensitive member, for example, an intermediate transfer type image shown in FIG. In the forming apparatus, in the primary transfer portion 13 that is a contact portion between the intermediate transfer belt 8 and the photosensitive drum 2, a phenomenon of “scattering” in the transfer portion, which will be described in detail below, and image defects may occur due to this phenomenon. is there.

  This phenomenon causes a problem that when an image pattern composed of thin lines, characters, and small dots is printed, the image is significantly deteriorated.

  FIG. 5, FIG. 6, and FIG. 7 show the mechanism of occurrence of an image defect called “scattering” in the transfer section in this configuration, taking the primary transfer process in the image forming apparatus having the configuration shown in FIG. 4 as an example. The explanation will be given.

  FIG. 5 is an enlarged cross-sectional view of the primary transfer unit 13 in the image forming unit of the image forming apparatus. In the vicinity of the primary transfer portion 13 shown in FIG. 5, the width of the first contact nip (hereinafter referred to as “drum / belt nip”) formed by the photosensitive drum 2 and the intermediate transfer belt 8 in the belt 8 moving direction. When the width W1 and the width of the second contact nip (hereinafter referred to as “belt / roller nip”) formed by the intermediate transfer belt 8 and the primary transfer roller 4 are the width W2, the posture of the belt 8 at the time of transfer is determined. In order to stabilize, the belt / roller nip W2 on the intermediate transfer belt 8 protrudes to the upstream side in the surface movement direction of the photosensitive drum 2 relative to the drum / belt nip W1, and this region is hereinafter referred to as “ It is referred to as “upstream air gap nip”, and its width is indicated by a width W3.

  Usually, the toner image formed on the photosensitive drum 2 is transferred to the intermediate transfer belt 8 in an overlap region (hereinafter referred to as “overlap nip”) W between the drum / belt nip W1 and the belt / roller nip W2. Done.

  In the conventional image forming apparatus, the belt / roller nip W2 protrudes to the upstream side in the movement direction of the photosensitive drum 2 relative to the drum / belt nip W1 as described above. Exists.

  When the upstream air gap nip W3 exists, an electric field acts on the upstream air gap nip W3 existing on the upstream side of the photosensitive drum 2 surface movement direction from the overlap nip W where the transfer is actually performed, and upstream of the overlap nip W. The phenomenon that toner on the photosensitive drum 2 flies onto the intermediate transfer belt 8, that is, “pre-transfer” among the so-called scattering phenomenon occurs. This pre-transfer causes image deterioration such as transfer scattering and dot position deviation when an image composed of fine lines and fine dots is printed.

  For this reason, as shown in FIG. 6, the primary transfer roller 4 is offset to the downstream side with respect to the photosensitive drum 2, and the belt / roller nip W2 from the most upstream portion of the belt / drum nip W1 in the drum 2 surface movement direction (rotation direction). It is conceivable to prevent the “pre-transfer” from occurring by eliminating the upstream gap nip W3.

  However, in the configuration in which the upstream gap nip W3 is eliminated, since the overlap nip W is narrow and the primary transfer roller 4 is offset, the nip configuration in the primary transfer unit 13 is the tension of the intermediate transfer belt 8. The overlap nip W is destabilized due to the balance between the contact spring pressure of the primary transfer roller 4 and the restoring force due to the deflection of the primary transfer roller 4, and the transfer efficiency is also destabilized accordingly. End up. In addition, the running performance of the intermediate transfer belt 8 becomes unstable, so that speed unevenness and meandering are likely to cause color misregistration.

  Also in Patent Document 1, as shown in FIG. 7, the primary transfer roller 4 is arranged on the downstream side of the drum / belt nip W <b> 1 in the movement direction of the photosensitive drum 2 to suppress “pre-transfer”.

  However, here, since the photosensitive drum 2 and the primary transfer roller 4 are not in contact with each other via the intermediate transfer belt 8, the transfer bias applied to the primary transfer roller 4 is applied to the photosensitive drum 2 via the intermediate transfer belt 8. It will flow.

In this case, if the resistance of the intermediate transfer belt 8 is lowered, scattering occurs. The reason for this is that, according to the considerations of the present inventors, when the volume resistivity of the intermediate transfer belt 8 is less than 10 ⁵ Ωcm, the bias applied to the primary transfer roller 4 is low because the resistance of the intermediate transfer belt 8 is low. Then, leakage to a peripheral member or the like provided in an adjacent image forming unit via the intermediate transfer belt 8 makes it impossible to obtain a transfer current necessary for transfer.

On the other hand, when the volume resistivity of the intermediate transfer belt 8 is set to 10 ⁵ Ωcm or higher at which primary transfer is possible, the degree varies depending on the material of the intermediate transfer belt 8, but has an electric capacity component. Therefore, when the toner image on the drum 2 is transferred to the intermediate transfer belt 8 by the primary transfer unit 13, the non-image part on the surface of the intermediate transfer belt 8 is charged up and the potential fluctuates. In particular, at the boundary portion between the toner portion and the non-toner portion, the transfer current concentrates on the non-toner portion, so that charges are concentrated on the non-toner portion side of the image boundary portion. The charge concentration on the non-toner portion side at the boundary portion becomes a charge barrier, and the scattering of the toner from the toner portion side to the non-toner portion is suppressed by the Coulomb force.

  On the contrary, if the resistance of the intermediate transfer belt 8 is low, the toner pressing force due to the charge barrier becomes weak, so that the scattering is worsened. Accordingly, it is necessary to increase the resistance of the intermediate transfer belt 8 from the viewpoint of scattering.

  However, if the resistance of the intermediate transfer belt 8 is high, the impedance of the primary transfer unit 13 becomes high, so that a high transfer voltage is required to flow a current necessary for primary transfer, and the cost of the high voltage increases. There is.

In other words, in the transfer section where the photosensitive member of the image forming apparatus using the belt body and the belt body are in contact with each other, the primary transfer member is arranged on the downstream side of the image carrier in the moving direction of the intermediate transfer body, and pre-transfer It can be seen that preventing this causes color misregistration, lowering the stability of transfer, and causing the scatter to occur unless the volume resistivity of the intermediate transfer member is increased.
JP 2003-98800 A

  An object of the present invention is to provide an inexpensive image forming apparatus that suppresses pre-transfer and prevents transfer scattering in fine line and fine dot images without impairing color misregistration and transfer stability.

The above object is achieved by the image forming apparatus according to the present invention. In summary, the first aspect of the present invention provides an image carrier that carries a developer image, a belt body for transferring the developer image on the image carrier, and a transfer that contacts the back surface of the belt body. A member, and the image carrier is in contact with the surface of the belt body.
Of the contact area between the image carrier and the belt body in the moving direction of the belt body, the belt body and the transfer member are in contact with each other in a region downstream of the moving direction of the belt body, and the movement of the belt body An image forming apparatus is provided in which the belt body and the transfer member are not in contact with each other in a region upstream in the direction.

According to a second aspect of the present invention, there is provided an image carrier that carries a developer image, a belt body for transferring the developer image on the image carrier, and a transfer member that contacts the back surface of the belt body. In the image forming apparatus in which the image carrier is in contact with the surface of the belt body,
Of the contact area between the image carrier and the belt body in the moving direction of the belt body, a straight line A passing through the most upstream point in the belt body moving direction and orthogonal to the belt body is parallel to the straight line A. In addition, the image forming apparatus is characterized in that, in a straight line B passing through the rotation center point of the transfer member, the straight line B is located downstream of the straight line A in the belt body movement direction.

  The image forming apparatus of the present invention has an image carrier that carries a developer image, a belt body for transferring the developer image on the image carrier, and a transfer member that contacts the back surface of the belt body, In the image forming apparatus in which the image carrier is in contact with the surface of the belt body, the belt body and the transfer member are located in a downstream area in the moving direction of the belt body in a contact area between the image carrier and the belt body in the moving direction of the belt body. Since the belt body is not in contact with the transfer member in the upstream area in the moving direction of the belt body, the occurrence of an electric field on the upstream side of each primary transfer portion is suppressed, and color misregistration is achieved without cost. It is possible to prevent pre-transfer and scattering without generating any. Furthermore, by providing a resistance layer having a high friction coefficient on the surface of the transfer member, the transfer current can be made uniform over the entire length of the primary transfer portion, thereby preventing transfer ghosts. Further, by increasing the friction between the transfer member and the belt body, the rotational stability of the transfer member and the running stability of the belt body are improved, and it is possible to prevent color misregistration and to stabilize the transfer. Further, by providing an elastic layer on the surface layer of the belt body, damage to the image carrier when foreign matter enters the transfer portion can be minimized, and stable transfer can be performed at the transfer portion.

  The image forming apparatus according to the present invention will be described below in more detail with reference to the drawings.

Example 1
FIG. 3 shows the overall configuration of a color image forming apparatus as the image forming apparatus of this embodiment. In this embodiment, an inline intermediate transfer system, that is, a tandem color image forming apparatus that forms toner images of each color in a plurality of image forming units each having an image carrier, and a belt member as an intermediate transfer member. Use. Further, the image forming apparatus of this embodiment employs an electrophotographic system. The same members as those in the image forming apparatus shown in FIG. 4 of the conventional example are denoted by the same reference numerals, and redundant description is omitted. Also in this embodiment, image formation is performed in the same manner as the conventional image forming apparatus described above. In other words, a toner image is formed on the surface of the photosensitive drum 2 as an image carrier in each image forming unit by the same image forming process as that in the conventional example, and the intermediate transfer belt 8 as a belt body from each image forming unit. The toner image is transferred in a superimposed manner, is secondarily transferred to the transfer material P, and the transferred toner image fixed on the transfer material P is discharged as an image formed product.

  Each image forming unit in this embodiment that performs this image forming operation will be described in detail.

  In this embodiment, each photosensitive drum 2 in each color image forming unit uses a negatively chargeable photosensitive drum 2 having a diameter of 30.6 mm, and a charging bias in which an AC component is superimposed on a DC component is applied to a charging roller 7. The photosensitive drum 2 was uniformly charged to about -650V.

  The exposure apparatus 1 has a near-infrared laser diode (not shown) having a wavelength of 760 nm and a polygon scanner that scans the photosensitive drum 2 with laser light, and lowers the potential of the image portion to −250V.

  The developing device 3 is a jumping developing device using a non-magnetic one-component toner as a developer, and a polymer toner having a particle size of about 7 μm having a core / shell structure containing wax as a toner is applied to the developing device 3 (not shown). The toner is applied to the developing sleeve 31 by a roller, the toner layer thickness is regulated by an unillustrated elastic blade, and then the toner is jumped on the photosensitive drum 2 to the electrostatic latent image for development.

  In this embodiment, the primary transfer roller 4 serving as a transfer member is made of SUS having a diameter of 10 mm, and the hardness is 57 ° in Asker C hardness under a load of 500 g, which is equal to or greater than the measurement limit under the present measurement conditions.

  The primary transfer roller 4 is driven to rotate by friction with the back surface of the intermediate transfer belt 8, and both ends are supported by bearings. One of the bearings is a conductive bearing, and the transfer bias is applied to the core metal of the primary transfer roller 4 through the conductive bearing.

  In addition, there are springs below the bearings at both ends of the primary transfer roller 4, and the primary transfer roller 4 is pressed against the back side of the intermediate transfer belt 8 by this spring. In this embodiment, the total pressure is 10 N so that the photosensitive drum 2 and the primary transfer roller 4 are in contact with each other via the intermediate transfer belt 8 during image formation.

In this embodiment, the secondary transfer roller 15 is configured such that a core rubber having a diameter of 10 mm is coated on the outer periphery thereof with a conductive rubber layer covering a longitudinal direction (image forming width direction) of 310 mm so that the diameter becomes 17 mm. The resistance value was 10 ⁷ Ω. The measurement method is a result of measurement using an R8340 ultrahigh resistance meter (registered trademark) manufactured by Advantest while the roller to be measured is driven and rotated with respect to an aluminum cylinder having a diameter of 30 mm. The measurement conditions are applied voltage; 2 kV, application time: 30 seconds, contact pressure: 9.8 N, rotational peripheral speed: 117 mm / sec.

  A secondary transfer bias power source (not shown) is connected to the metal core of the secondary transfer roller 15 through a power supply spring. A secondary transfer bias that can be changed during secondary transfer is applied to the secondary transfer roller 15 from a connected secondary transfer bias power source.

  In this embodiment, the intermediate transfer belt 8 stretched by the tension of 4 kgf by three of the driving roller 9, the tension roller 10, and the secondary transfer counter roller 24 has a thickness of 75 μm, a circumferential length of 1115 mm, and a longitudinal direction (image formation). (Width direction) A single-layer endless (seamless) resin belt having a length of 310 mm, which is formed of polyimide whose resistance is adjusted by carbon dispersion.

The volume resistivity of the intermediate transfer belt 8 was 10 ⁹ Ωcm, and the surface resistance was 10 ¹² Ω / cm ² or more. The volume resistivity measurement method is based on JIS-K6911. In order to obtain good contact between the electrode and the surface of the belt 8, conductive rubber is used as the electrode. The volume resistivity of the belt 8 is set to 100V, 10 It is the result of measurement using an R8340 ultrahigh resistance meter (registered trademark) manufactured by Advantest under the condition of applying a second. The measured values on either the front or back side of the intermediate transfer belt 8 were the same.

  Note that the image forming conditions in this example were set to process speed: 117 mm / s, primary transfer bias: +300 V, and secondary transfer bias: +2.3 kV.

  In the above-described intermediate transfer type image forming apparatus as shown in FIG. 3, scattering phenomenon such as pre-transfer is caused by the positional relationship between the photosensitive drum 2 and the transfer roller 4 with respect to the intermediate transfer belt 8 in the primary transfer unit 13. May occur, or color misalignment or image defects may occur. In particular, in the tandem type so-called in-line type color image forming apparatus as in the present embodiment, since there are a plurality of primary transfer portions 13, inconveniences in the primary transfer portions 13 are formed in each photosensitive drum 2. When superposed toner images are superposed, there is a risk of causing noticeable image defects.

  In the primary transfer unit 13 in this embodiment, as shown in FIG. 1, the transfer roller 4 is arranged on the downstream side of the photosensitive drum 2 in the surface movement direction (rotation direction) of the photosensitive drum 2 in order to prevent the scattering phenomenon. I let you. That is, the transfer roller 4 is disposed downstream of the photosensitive drum 2 in the rotational direction from the most upstream point in the rotational direction of the photosensitive drum 2 at the contact portion between the photosensitive drum 2 and the intermediate transfer belt 8. By doing so, as described in the prior art, the upstream side gap nip W3 can be eliminated, and pre-transfer can be prevented.

  In order to find the optimum position of the primary transfer roller 4 in the present invention without causing the scattering phenomenon in the primary transfer portion 13, an experiment was performed while changing the position of the transfer roller 4.

Experimental Example In this experimental example, the primary transfer portion 13 shown in FIG. 2 passes through the most upstream point in the rotation direction of the drum 2 at the contact portion (nip) between the drum 2 and the intermediate transfer belt 8 and is orthogonal to the intermediate transfer belt 8. The distance L between the straight line A and the straight line B parallel to the straight line A and passing through the rotation center of the primary transfer roller 4 (hereinafter referred to as “offset amount”) L is changed to 1 mm, 3 mm, 5 mm, and 10 mm. The transfer efficiency and the scattering were evaluated, and whether the primary transfer roller 4 and the photosensitive drum 2 were in contact via the intermediate transfer belt 8 was visually checked.

  The experimental results and the visual confirmation results are shown in Table 1. ○ in the table indicates that good results were obtained. In other words, the transfer efficiency indicates that the toner image is sufficiently transferred with respect to the transfer bias, that is, the toner image is sufficiently transferred with respect to the transfer bias, and the scattering means that the scattering phenomenon including pre-transfer can be prevented. X means that the transfer efficiency does not have sufficient transfer efficiency, that is, the transfer is not sufficiently performed with respect to the transfer bias, and the scattering phenomenon occurs as a whole. It means that the level has not been reached.

  Table 1 shows the results of the present embodiment, that is, the conditions using the photosensitive drum 2 having a diameter of 30 mm, the primary transfer roller 4 having a diameter of 10 mm, the intermediate transfer belt 8 having a thickness of 75 μm and a longitudinal length of 310 mm. .

  Regarding the results, as shown in Table 1, when the photosensitive drum 2, the primary transfer roller 4, and the offset amount L are reduced to 0.5 mm, the scattering state is deteriorated. This is a portion sandwiched between the intermediate transfer belt 8 and the photosensitive drum 2 upstream of the transfer nip in the moving direction of the surface of the photosensitive drum 2 as indicated by the hatched portion in FIG. 1 because the offset amount L is not sufficient. This is because pre-transfer occurs because an electric field is formed in a certain gap C due to wraparound.

  In the region where the offset amount L is 1 mm to 5 mm, both transfer efficiency and scattering were good. As shown in FIG. 1, since the primary transfer roller 4 is arranged downstream of the photosensitive drum 2 surface movement direction, the belt / roller nip W2 moves downstream in the photosensitive drum 2 surface movement direction, and the upstream gap nip W3. Since no electric field exists in the gap C at the upstream side of the transfer nip, pre-transfer does not occur and the photosensitive drum 2 and the primary transfer roller 4 are in contact with each other. Since the W width was secured, the transfer efficiency was good.

  Conversely, when the offset amount L was increased to 10 mm, the scattering state was good, but image unevenness occurred and the transfer efficiency was remarkably deteriorated. The reason why the transfer unevenness occurs is that the contact portion between the photosensitive drum 2 and the transfer roller 4 via the intermediate transfer belt 8, that is, the overlap portion W shown in FIG. Therefore, it is considered that the transfer current flows non-uniformly due to uneven resistance in the surface of the intermediate transfer belt 8, which causes uneven transfer efficiency and image unevenness.

The reason why the transfer efficiency is deteriorated is that the overlap portion W between the primary transfer roller 4 and the photosensitive drum 1 is eliminated, so that the transfer bias flows to the photosensitive drum 2 via the intermediate transfer belt 8, and the intermediate This is because when the resistance of the transfer belt 8 is high, that is, when the volume resistivity is 10 ⁵ Ωcm or more, the impedance of the primary transfer portion 13 is increased and the transfer efficiency is lowered.

  As a countermeasure, it is possible to increase the transfer bias, but the high-voltage power source becomes complicated, which increases the complexity and cost of the apparatus. As a countermeasure, lowering the resistance of the intermediate transfer belt 8 lowers the impedance of the primary transfer unit 13, but lowering the resistance of the intermediate transfer belt 8 weakens the toner holding force of the intermediate transfer belt 8. On the contrary, the splatter situation will get worse.

  Therefore, in this embodiment, the offset amount L is set to be greater than 1 mm, and the primary transfer roller 4 and the photosensitive drum 2 are brought into contact with each other via the intermediate transfer belt 8, thereby preventing transfer splattering while maintaining transfer efficiency. It can be improved.

  In this embodiment, the offset amount is 1 mm or more under the condition that the photosensitive drum 2 having a diameter of 30 mm, the primary transfer roller 4 having a diameter of 10 mm, the intermediate transfer belt 8 having a thickness of 75 μm and a longitudinal length of 310 mm are used. However, it is adjusted as appropriate according to the size of the photosensitive drum and transfer roller and the thickness of the intermediate transfer belt. However, in any condition, it is preferable that the offset amount L is provided and is larger than zero.

  That is, in FIG. 2, the photosensitive drum 2 and the intermediate transfer belt 8 are in contact with each other in the moving direction of the intermediate transfer belt 8 and pass through the most upstream point in the moving direction of the intermediate transfer belt 8 and the intermediate transfer belt 8. In a straight line A that is perpendicular to the straight line A and a straight line B that is parallel to the straight line A and passes through the rotation center point of the primary transfer roller 4, the straight line B is located downstream of the straight line A in the moving direction of the intermediate transfer belt 8.

  Here, if the surface of the primary transfer roller 4 is covered with an elastic layer such as foamed rubber having a low hardness as in the conventional example, the amount of winding of the intermediate transfer belt 8 around the photosensitive drum 2 is reduced due to the hardness of the elastic layer. It becomes unstable, and a stable contact state between the primary transfer roller 4 and the photosensitive drum 2 cannot be obtained.

  Therefore, in this embodiment, the primary transfer roller 4 is made of metal, and the primary transfer roller 4 is deformed when the primary transfer roller 4 biased in the direction toward the intermediate transfer belt 8 is pressed against the drum 2. Therefore, a stable contact state between the primary transfer roller 4 and the photosensitive drum 2 is obtained, and the length of the intermediate transfer belt 8 between the adjacent photosensitive drums 2 is stabilized, so that the color misregistration state is improved.

  In this case, the hard primary transfer roller 4 and the hard aluminum photosensitive drum 2 come into contact with each other through the intermediate transfer belt 8, and when foreign matter is caught in the primary transfer unit 13, There is a risk of damaging the surface coating. Therefore, in the present invention, the primary transfer roller 4 is urged by an elastic body such as a spring in the direction toward the intermediate transfer belt 8 so as to contact the photosensitive drum 2.

  As a result, even when foreign matter is mixed into the primary transfer portion 13, damage to the surface of the photosensitive drum 2 can be minimized.

  As described above, according to the present invention, the transfer roller 4 is located in the contact area between the photosensitive drum 2 and the intermediate transfer belt 8 and on the downstream side in the moving direction of the surface of the intermediate transfer belt 8 relative to the photosensitive drum 2. The intermediate transfer belt 8 which is in contact with the belt 8 and is lifted by the transfer roller 4 is wound around the photosensitive drum 2, and the upstream side of the intermediate transfer belt 8 in the moving direction is between the transfer roller 4 and the intermediate transfer belt 8. By eliminating the contact portion, the transfer efficiency is maintained, and the current state of scattering including pre-transfer is prevented.

Example 2
The image forming apparatus of the present embodiment has the same configuration as that of the first embodiment except that the primary transfer roller 4 is coated with a resistance-adjusted coating.

  In this embodiment, the primary transfer roller 4 is a core metal having an outer diameter of 10 mm coated with a urethane coat whose resistance is adjusted with carbon black, the coat thickness is 50 μm, and the hardness is Asker C hardness at a load of 500 g. Was 57 ° or more than the measurement limit.

The actual resistance value of the primary transfer roller 4 was 10 ⁴ Ω. The measurement method was the same as the resistance measurement of the secondary transfer roller 15 described in Example 1, and the applied voltage was 50V.

  By providing a resistance layer on the surface of the primary transfer roller 4, even when toner partially exists in the primary transfer nip during the primary transfer, a transfer current flows evenly in a portion where the toner is not present and a portion where the toner is present. Therefore, transfer ghost can be prevented.

  The transfer ghost here means that the surface potential of the photosensitive drum 2 after the primary transfer is uneven because the primary transfer current is uneven in the longitudinal direction of the primary transfer portion 13 when the surface resistance of the primary transfer roller 4 is low. This is a phenomenon that appears as a ghost after one revolution of the photosensitive drum 2.

  Therefore, in this embodiment, a resistance layer is provided on the surface layer of the primary transfer roller 4 so that the transfer current flows uniformly over the entire length of the primary transfer nip.

Here, when the volume resistivity of the surface resistance layer of the primary transfer roller 4 is smaller than 10 ⁵ Ωcm, the above-described primary transfer current wraps around in the longitudinal direction of the surface of the primary transfer roller 4, thereby causing ghost and density unevenness. To do. On the other hand, when the volume resistivity is larger than 10 ¹² Ωcm, the charge-up on the surface of the transfer roller 4 cannot be ignored, and adverse effects such as discharge at the peeling portion of the transfer roller 4 and the intermediate transfer belt 8 occur.

For this reason, in the primary transfer roller 4, the required volume resistivity of the surface resistance layer varies depending on the resistance and configuration of the intermediate transfer belt 8, but the volume resistivity ranges from 10 ⁵ to 10 ¹² Ωcm. It is desirable to select a resistance range according to The measurement method is the same as the resistance measurement of the secondary transfer roller 15 described in the first embodiment, but the applied voltage is 50V.

  When the primary transfer roller 4 is driven to rotate by friction with the intermediate transfer belt 8, the primary transfer roller 4 rotates smoothly by adopting a material having a high frictional resistance for the surface layer of the primary transfer roller 4. The running of the intermediate transfer belt 8 is stabilized, the color misregistration is improved, and a stable primary transfer is obtained.

  In this embodiment, the urethane coating is applied, but the same effect can be obtained even if solid rubber such as EPDM, NBR, hydrin is used.

  In particular, when the surface layer of the primary transfer roller 4 is coated with solid rubber, the contact area with the intermediate transfer belt 8 is larger and the frictional resistance is larger than when the foam rubber is coated, so that the primary transfer roller 4 rotates stably. However, the color shift is improved. Further, since the deformation of the primary transfer roller 4 is small even when the primary transfer voltage is applied, the intermediate transfer belt 8 can be reliably wound around the drum 2 and the scattering can be stably improved.

  However, when the surface hardness in the state of the primary transfer roller 4 is smaller than 45 ° (Asker C hardness, 500 g load), the amount of deformation of the primary transfer roller 4 by the primary transfer increases, and the photosensitive drum 2 and the primary transfer roller 4 are intermediately transferred. A reliable contact state cannot be obtained via the belt 8, and color misregistration occurs due to deterioration in running stability of the intermediate transfer belt 8 and instability of the length of the intermediate transfer belt 8 between stations. Therefore, when the elastic layer is coated on the surface layer, it is necessary that the rubber hardness of the surface of the primary transfer roller 4 is 45 ° (Asker C hardness, 500 g load) or more.

Example 3
The image forming apparatus of this embodiment is the same as that of Embodiment 1 except that the intermediate transfer belt 8 is an elastic belt.

  The intermediate transfer belt 8 is obtained by coating a 100 μm resin sheet (PVDF in this example) as a base layer with a 200 μm elastic layer (chloroprene rubber in this example), and adjusting the resistance of both the base layer and the elastic layer with carbon black. It is a thing.

The volume resistivity of the intermediate transfer belt 8 was 10 ¹⁰ Ωcm, and the surface resistance was 10 ¹² Ω / cm ² or more. The volume resistivity measurement method is the same as that described in Example 1. The intermediate transfer belt 8 had a micro hardness of 75 °. The micro hardness here is the result of using MICROMETER (MD-1) manufactured by KOBUNSHIKEIKI.

  By covering the surface layer of the intermediate transfer belt 8 with an elastic layer, not only can the damage to the photosensitive drum 2 be prevented when foreign matter enters the primary transfer portion 13, but also the toner layer is uniformly formed at the primary transfer portion 13. Since the primary transfer pressure is applied, stable transfer can be obtained.

  In addition, the inventors' experiments have also revealed that the effect of the elastic layer cannot be obtained when the micro hardness is higher than 90 °. Therefore, the micro hardness of the surface of the intermediate transfer belt 8 needs to be 90 ° or less. However, when the micro hardness is less than 60 °, although the damage to the photosensitive drum 2 is small, the transportability of the intermediate transfer belt 8 is deteriorated. It's not good because it ends up. The deterioration of the transportability is because the micro hardness is too low, so that the intermediate transfer belt 8 is deformed too much with the movement of the intermediate transfer belt 8, and the intermediate transfer belt 8 is not transported at a constant surface speed. When the transportability of the intermediate transfer belt 8 deteriorates, a registration error occurs in which toner images of a plurality of colors are shifted at the transfer position.

  Regarding the thickness of the elastic layer, although it depends on the hardness of the elastic layer to be coated and the hardness of the base layer, an elastic layer of 30 μm or more is necessary. On the contrary, when the elastic layer is 500 μm or more, there arises a problem that wrinkles enter the surface of the elastic layer in the intermediate transfer belt 8 stretched portion.

Since the belt body may be scattered when the volume resistivity of the belt body is low, the present invention is capable of primary transfer of the volume resistivity of the belt body. It is particularly effective when it is 10 ⁵ or more and considering the cost or the like, it is 10 ¹² or less.

  Here, the overall configuration of the image forming apparatus of the present invention described in the first to third embodiments is not limited to that shown in FIG. For example, an electrostatic recording type may be used, and any number of image carriers may be used. The present invention is particularly effective in an in-line system having a plurality of image carriers, but a configuration in which a plurality of developing devices are provided for one image carrier may also be used, and a monochromatic image forming apparatus. It can also be applied to. Further, the present invention can also be applied to a so-called transfer / conveying belt system (direct transfer system) having a transfer material conveying body that electrostatically attracts and conveys a transfer material and superimposing a toner image on the transfer material in the conveying process. .

  In addition, the dimensions, materials, shapes, and relative positions of the components of the image forming apparatus described above are not intended to limit the scope of the present invention to those unless otherwise specified. Absent.

It is sectional drawing which shows an example near the primary transfer part which concerns on this invention. FIG. 6 is an explanatory diagram illustrating an arrangement of an image carrier and a transfer member in the vicinity of a primary transfer unit. 1 is a schematic configuration diagram illustrating an example of an image forming apparatus according to the present invention. It is a schematic block diagram which shows an example of the conventional image forming apparatus. It is sectional drawing which shows an example of the conventional primary transfer part vicinity. It is sectional drawing which shows the other example of the conventional primary transfer part vicinity. It is sectional drawing which shows the other example of the conventional primary transfer part vicinity.

Explanation of symbols

2 Photosensitive drum (image carrier)
4 Primary transfer roller (transfer member)
8 Intermediate transfer belt (belt body)
13 Primary transfer section

Claims (12)

An image carrier that carries a developer image, a belt member for transferring the developer image on the image carrier, and a transfer member that contacts a back surface of the belt member, and the image carrier Is an image forming apparatus in contact with the surface of the belt body,
Of the contact area between the image carrier and the belt body in the moving direction of the belt body, the belt body and the transfer member are in contact with each other in a region downstream of the moving direction of the belt body, and the movement of the belt body An image forming apparatus, wherein the belt body and the transfer member are not in contact with each other in a region upstream in the direction. An image carrier that carries a developer image; a belt member for transferring the developer image on the image carrier; and a transfer member that contacts a back surface of the belt member. Is an image forming apparatus in contact with the surface of the belt body,
Of the contact area between the image carrier and the belt body in the moving direction of the belt body, a straight line A passing through the most upstream point in the belt body moving direction and orthogonal to the belt body is parallel to the straight line A. An image forming apparatus characterized in that, in a straight line B passing through the rotation center point of the transfer member, the straight line B is located downstream of the straight line A in the belt body movement direction.   The plurality of image bearing members, arranged side by side along the moving direction of the belt member, and having a plurality of transfer members corresponding to the number of the image bearing members. 1 or 2 image forming apparatus.   4. The image forming apparatus according to claim 1, wherein the transfer member has a roller shape.   The image forming apparatus according to claim 1, wherein the belt body is endless.   The image forming apparatus according to claim 1, wherein the hardness of the transfer member measured from the contact surface direction with the belt body is 45 ° or more in AskerC.   The image forming apparatus according to claim 1, wherein a surface layer of a surface of the transfer member in contact with the belt body is formed of solid rubber.   The image forming apparatus according to claim 1, wherein the transfer member includes a rotatable metal roller. The image forming apparatus according to claim 1, further comprising a layer having a volume resistivity of 10 ⁵ to 10 ¹² Ωcm on a contact surface of the transfer member with the belt body. The image forming apparatus according to claim 1, wherein the belt body has a volume resistivity of 10 ⁵ to 10 ¹² Ωcm.   The image forming apparatus according to claim 1, wherein the belt body has an elastic layer as a surface layer, and the micro hardness of the surface layer is 60 ° or more and 90 ° or less.   The image forming apparatus according to claim 1, wherein the belt body has an elastic layer as a surface layer, and the thickness of the elastic layer is 30 μm or more and 500 μm or less.

Images