JP2011128468A - Image forming apparatus - Google Patents

Abstract

An image forming apparatus capable of eliminating a transfer defect in a primary transfer unit and maintaining the image quality of a transferred image at a certain level or higher.
A primary transfer means for primary transfer of a toner image carried on an image carrier to the intermediate transfer member is a roller having two or more layers, and the volume resistivity of the intermediate transfer member is R1, and the roller When the volume resistivity of the outermost layer of R2 is R2, and the volume resistivity of the layer inside the outermost layer is R3, R2 is the smallest.
[Selection] Figure 7

Description

  The present invention relates to an image forming apparatus such as a copying machine, a printer, and a facsimile machine that forms an image by electrophotography.

  Conventionally, an intermediate transfer body type image forming apparatus using an intermediate transfer belt is known. This system is a color image forming apparatus or multicolor image forming that sequentially transfers and stacks a plurality of component color images based on color image information and multicolor image information, and obtains an image formed product in which the color image and multicolor image are synthesized and reproduced. It is effective as a device (see, for example, Patent Document 1).

  By the way, in the color image forming apparatus as described above, the problem of “retransfer” in which the toner transferred to the intermediate transfer belt at the upstream transfer unit is collected on the photosensitive drum as the image carrier at the downstream transfer unit. There is. As one method for suppressing retransfer, a method of increasing the width of the transfer nip has been proposed. In order to increase the width of the transfer nip, there is a method of increasing the force for urging the transfer roller toward the photosensitive drum. However, when the pressing force per unit area is increased, the force with which the toner is pushed back to the photosensitive drum side is also increased, which may make it easier to re-transfer.

  Therefore, it is necessary to increase the nip width without increasing the pressing force per unit area by reducing the hardness of the transfer roller.

  Here, the configuration of a conventional transfer roller will be described with reference to FIG.

  FIG. 13A schematically illustrates an enlarged view of the roller surface cell shape and a surface shape of the roller having an Asker C hardness of 20 degrees, and the surface shape of the roller 31 by the intermediate transfer belt 31 and the sponge roller 47 of the transfer roller. It is a figure to do.

  FIG. 13B schematically illustrates an enlarged view of the roller surface cell shape, and the surface shape of the roller having an Asker C hardness of 30 degrees, and the surface shape by the back surface 31a of the intermediate transfer belt 31 and the sponge roller 47 of the transfer roller. It is a figure to do.

  When reducing the hardness of NBR, epichlorohydrin rubber, etc. by reducing the foam density, as shown in FIG. 13B, the Asker C hardness is reduced to about 30 degrees and the hardness is reduced while maintaining good surface properties. It is possible. When the hardness is further lowered to about 20 degrees, as shown in the enlarged image of the cell of FIG. 13A, the size of the foamed cell increases, the foamed state becomes uneven, and small bubbles are partially formed. It becomes a concentrated structure. The case where the foam having such a structure is molded by a polishing method such as pressing a rotating cylindrical grindstone against a rotating workpiece will be described. As shown in the structural schematic diagram of the surface of the foam as shown in FIG. 13A, only the portion of the pillars forming the foam, particularly where the plurality of pillars are concentrated, remains sparsely without being polished, resulting in large lumps 47. It becomes the existing surface state. Therefore, when the transfer nip is formed with the intermediate transfer belt, even if the nip width can be increased, the area that actually contacts the back surface 31a of the intermediate transfer belt 31 becomes very small.

  Another problem different from re-transfer is that the image is disturbed by electric discharge that occurs when the back surface of the belt peels from the transfer roller on the downstream side of the transfer nip. In order to deal with this problem, the following means are available.

  That is, the resistance of the transfer roller is kept as low as possible. As a result, the charge accumulated on the back surface of the belt at the transfer nip formed by the photosensitive drum and the transfer roller is quickly collected on the transfer roller side downstream of the transfer nip where only the transfer belt and the transfer roller are in contact. Good to do.

Conventionally, for this purpose, a configuration is used in which the actual resistance of the transfer roller is set to about 10 ⁵ to 10 ⁷ Ω and converted to the volume resistivity of the material, about 10 ⁴ to 10 ⁶ Ω · cm.

As described above, when the hardness of the transfer roller is lowered to about 20 degrees and the volume resistivity is lowered to 10 ⁶ Ω · cm or less, very good image quality is achieved in a state where the transfer current is accurately set. Was able to. However, when the transfer current becomes inadequate due to variations in the detection capability and output of current and resistance with a high-voltage power supply, as well as the state of toner related to image formation, variations in the shape and resistance of the parts, A transfer failure consistent with the above occurred. Especially in the case of high-speed and long-life image forming devices, the film thickness decreases by continuing to use the photosensitive drum, and the transfer current is insufficient without correcting for the increase in the electrical capacitance component on the surface of the photosensitive drum. Transfer defects occur.

  The cause of this transfer failure is considered as follows.

  When large blisters are scattered as shown in FIG. 13A, the hardness of the flakes protruding is overwhelmingly higher than other parts, so even if the roller is deformed at the transfer nip to form a sufficient nip width. Actually, the only contact with the back surface of the belt is the kerf. At that time, particularly when the resistance of the roller is low, the resistance of the marking portion is very low as compared with the resistance of the air layer where there is no marking, so that the transfer current selectively flows only in the marking portion. As an example, using a roller with a foam cell diameter of 300 μm, the transfer current was drastically lowered, and an experimental image of the contact of the mark was shown on the image, and the mark interval was about 1 to 2 mm. It was. For this reason, even if the transfer conditions are fluctuated minutely, a transfer defect in which an image appears to be missing in a portion without a blur is likely to be manifested.

Japanese Patent Laid-Open No. 9-152791

  SUMMARY OF THE INVENTION An object of the present invention is to solve the above problems, and an image forming apparatus capable of eliminating a transfer defect in a primary transfer unit and maintaining the image quality of a transferred image at a certain level or more. Is to provide.

The above object is achieved by the image forming apparatus according to the present invention. In summary, the present invention provides an image carrier that carries a toner image, a rotatable intermediate transfer member, and a primary transfer means that primarily transfers the toner image carried on the image carrier to the intermediate transfer member. An image forming apparatus comprising: a secondary transfer unit that transfers the toner image transferred to the intermediate transfer member to a transfer material;
The primary transfer means is a roller having two or more layers, the volume resistivity of the intermediate transfer member is R1, the volume resistivity of the outermost layer of the roller is R2, and the volume resistivity of the inner layer from the outermost layer. When R is R3, the image forming apparatus is characterized in that R2 is the smallest.

  According to the present invention, even in an image forming apparatus having an intermediate transfer member, even if it is a high-speed and long-life image forming apparatus, the transfer failure in the primary transfer portion is eliminated, and the image quality of the transferred image exceeds a certain level. Can be maintained.

1 is a schematic cross-sectional view illustrating a configuration of an image forming apparatus according to a first embodiment. FIG. 2 is a schematic cross-sectional view illustrating a configuration of an intermediate transfer member in the first embodiment. It is schematic structure sectional drawing of the roller resistance measuring jig in 1st Example. It is a figure which shows the method of observing the mode of flaking of the primary transfer roller in 1st Example. It is a figure which shows the support method of the primary transfer roller in 1st Example. It is a figure which shows an example of the structure using the metal roller in 1st Example. It is a figure which shows the layer structure of the primary transfer roller in 1st Example. FIG. 3 is a diagram illustrating a layer configuration of an intermediate transfer belt in the first embodiment. It is a figure explaining the state of the primary transfer roller surface in 1st Example. FIG. 10 is a diagram illustrating a layer configuration of an intermediate transfer belt in a third embodiment. It is a figure which shows the layer structure of the primary transfer roller in 4th Example. It is a figure which shows the nip shape of the primary transfer roller and photosensitive drum in 4th Example. It is a figure explaining the state of the surface of the primary transfer roller in the past.

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

Example 1
In this embodiment, the image forming apparatus is a color inline image forming apparatus using an electrophotographic process. An example of this embodiment is shown in FIG.

  The image forming apparatus according to the present exemplary embodiment includes four image forming stations of Y (yellow), M (magenta), C (cyan), and Bk (black). At each image forming station, a toner image is formed on a drum-shaped image carrier, i.e., a photosensitive drum, and the first transfer means forms an intermediate transfer member as a second image carrier, i.e., an intermediate transfer belt. Transfer the toner image. Thereafter, the toner image on the intermediate transfer belt is collectively transferred to the transfer material by the second transfer unit with respect to the transfer material conveyed by the feeding member, and finally fixed.

  More specifically, the image forming apparatus of this embodiment includes Y, M, C, and Bk photosensitive drums 2 (2a, 2b, 2c, and 2d). Around each photosensitive drum 2, there are a charger (charging means) 3 (3a, 3b, 3c, 3d), a developing device (developing means) 5 (5a, 5b, 5c, 5d), and a photosensitive drum 2. A cleaning unit (cleaning means) 6 (6a, 6b, 6c, 6d) for removing residual toner is provided. In addition, the photosensitive drum 2, the charger 3, the developing device 5, and the cleaning unit 6 are integrally configured, and four independent process cartridges 32 (32a, 32b, 32c, and 32d) that can be attached to and detached from the apparatus main body. Are juxtaposed in the vertical direction. In the present embodiment, the process cartridges 32 (32a, 32b, 32c, 32d) each form an image forming station.

  Then, toner images of different colors formed by these process cartridges (image forming stations) 32 are sequentially transferred onto the intermediate transfer belt 31 and then transferred onto the transfer material S to obtain a full color image. It has a configuration. The transfer material S is fed from the paper feed unit 15 and discharged to a paper discharge tray (not shown).

  Further, the image forming apparatus will be described. Each process cartridge 32 (32a, 32b, 32c, 32d) has the same configuration. Therefore, in the following description, the subscripts (a, b, c, d) for distinguishing the constituent members of the process cartridges 32a, 32b, 32c, 32d need to be described by distinguishing the process cartridges 32. Except for the case described above, the description is omitted and summarized.

  The photosensitive drum 2 is a rotary drum type electrophotographic photosensitive member that is repeatedly used, and is rotationally driven at a predetermined peripheral speed (process speed). In this embodiment, in order to be able to output 30 A4 size images per minute, the process speed is 180 mm / sec. Set to.

  The photosensitive drum 2 is uniformly charged to a predetermined polarity / potential (minus 500 V in this embodiment) by a primary charging roller (charger) 3. Then, the image exposure means 4 (4a, 4b, 4c, 4d) is subjected to image exposure (configured by a laser diode, a polygon scanner, a lens group, etc.). As a result, electrostatic latent images corresponding to the first to fourth color toner images (for example, yellow, magenta, cyan, and black component images) are formed. (For example, in the case of a solid image in which 100% of toner is developed, the potential after exposure is minus 150 V.)

  Next, the developing device 5 performs so-called development in which toner as a developer is attached to the electrostatic latent image formed on the photosensitive drum 2. The developing device 5 includes a toner container 51 (51a, 51b, 51c, 51d) that stores toner, and a developing roller 52 (52a, 52b, 52c, 52d) as a developer carrier that carries and transports the toner. ing.

  The developing roller 52 is made of elastic rubber whose resistance is adjusted. The developing roller 52 is in contact with the photosensitive drum 2 while rotating in the forward direction with respect to the photosensitive drum 2. By applying a high voltage of a predetermined polarity (minus in this embodiment) to the developing roller 52, the toner carried on the developing roller 52 in a state of being frictionally charged to the same polarity in each developing device, Development is performed by transferring to an electrostatic latent image on the photosensitive drum 2.

  The intermediate transfer belt 31 is rotationally driven by the action of the driving roller 8 at the same peripheral speed as the photosensitive drum 2 while being in contact with each photosensitive drum 2 at the primary transfer portion T1 (T1a, T1b, T1c, T1d). Yes. The intermediate transfer belt 31 is composed of an endless film member. In the primary transfer portion T1, a primary transfer roller (primary transfer means) 14 (14a, 14b, 14c, 14d) is disposed at a position facing the photosensitive drum 2 with the intermediate transfer belt 31 interposed therebetween. Separates and moves with respect to the follower.

  After passing through each process cartridge 32, toner images of different colors are transferred from the photosensitive drum 2 to the intermediate transfer belt 31 by the action of static electricity due to the high pressure applied to the primary transfer roller 14.

  The primary transfer residual toner remaining on the photosensitive drum 2 after the toner image is transferred from the photosensitive drum 2 to the intermediate transfer belt 31 is the cleaning blade 61 (61a, 61b, 61c, 61d) of the cleaning unit 6. Removed and recovered.

The transfer material S fed from the paper feed unit 15 by the paper feed roller 16 is transferred to the secondary transfer portion of the intermediate transfer belt 31 and the secondary transfer roller 35 by the registration roller pair 17 that is driven and rotated at a predetermined timing. That is, the sheet is fed toward the transfer nip T2. Subsequently, the toner image on the intermediate transfer belt 31 is transferred to the transfer material S by the action of static electricity due to the high pressure applied to the secondary transfer roller 35. The secondary transfer roller 35 has a configuration in which a 4 mm-thick sponge rubber is laminated on a metal core having a diameter of 8 mm, and the actual resistance of the roller is adjusted to 10 ⁸ Ω. As for the transfer material S, the full-color toner image is fixed to the transfer material S by heating and pressurization by the fixing device 18 and discharged outside the apparatus (outside the image forming apparatus main body). The secondary transfer residual toner remaining on the intermediate transfer belt 31 after the toner image is transferred from the intermediate transfer belt 31 to the transfer material S is removed and collected by a cleaning blade 33 as a cleaning unit.

FIG. 2 shows a configuration of the intermediate transfer member of the image forming apparatus. The intermediate transfer belt 31 is rotatably stretched by three stretching rollers 8, 10 and 34. The drive roller 8 has a structure in which a metal core is coated with silicon rubber with a thickness of 75 μm, and also serves as a counter roller of the optical detection sensor 40. The elastic roller 34 disposed opposite to the secondary transfer roller 35 includes a counter roller for forming a nip for transferring a toner image from the intermediate transfer belt 31 to the transfer material S at the secondary transfer portion T2, and a secondary roller. It also serves as a counter roller of the cleaning blade 33 for collecting the transfer residual toner. The elastic roller 34 has a configuration in which a metal core metal has a 2 mm thick ethylene propylene rubber surface layer, and has a hardness of 74 ± 5 ° (JIS-A) and an actual resistance of 10 ⁵ Ω or less. The tension roller 10 is a metal surface roller, and has a surface roughness Ra = 3.2 μm or less. By using the tension roller 10 and the intermediate transfer belt 31 having a three-axis tension configuration, the nip shape in the secondary transfer portion T2 and the distance relationship between the transfer material S and the intermediate transfer belt 31 can be optimized, and abnormal discharge occurs. It is possible to suppress deterioration in image quality due to scattering of images and toner.

  Here, in this embodiment, in order to explain the advantages of the implementation configuration in comparison with other configurations, in addition to the implementation configuration, a combination of four types of primary transfer rollers 14 and the intermediate transfer belt 31 was used as a comparative example. .

In the measurement of the volume resistivity of this example, the following two types of measuring devices were used according to the resistivity range.
・ Less than 10 ⁶ Ω · cm;
Loresta-GP ASP probe (trade name) manufactured by Dia Instruments Co., Ltd. (JISK7194)
・ 10 ⁶ Ω ・ cm or more;
Hiresta UP J box U type (trade name) manufactured by Dia Instruments Co., Ltd. (JISK6911)

  In addition, about the roller-shaped sample, the same material was shape | molded in the sheet form and the resistivity measurement was implemented using the URS probe.

  Further, a rotational resistance measuring jig shown in FIG. 3 was used for measuring the actual resistance value of the transfer roller 14 of this embodiment. With the jig, the measuring roller 14 is loaded from both ends of the core metal with a force F of 4.9 N on one side against the aluminum roller 60 rotating at 30 rpm (total load is 4.9 × 2 N), and from the bias power source 63 Apply voltage. At that time, the current flowing between the aluminum roller 60 and the ground is measured by measuring the voltage V applied to both ends of the resistor 61 with the voltmeter 62. In this embodiment, the voltage V applied from the bias power source 63 is 100 V, and the resistance R of the resistor 61 is 1 KΩ.

  Below, the detail of each structure is demonstrated.

(Comparative Example 1) 10 constituting this comparative example which is a combination of an intermediate transfer belt of ^5.6 Omega · cm of NBR / epichlorohydrin sponge rollers 10 ¹⁰ Omega · cm as intermediate transfer belt 31, the volume of 10 ¹⁰ Omega · cm An endless polyimide belt having a single-layer structure having a thickness of 65 μm and having resistivity was used.

In FIG. 7A, the primary transfer roller 14 used in this comparative example has an outer diameter of 14 mm, the core metal 14-1 has a diameter of 6 mm, and the sponge layer 14-2 has a thickness of 4 mm. The volume resistivity of the sponge material is adjusted to about 10 ^5.6 Ω · cm so that the hardness of the sponge layer 14-2 is 17 to 23 ° (Asker-C) and the actual resistance value of the roller is 10 ^6.0 Ω. . In order to achieve both low hardness and low resistance, NBR and epichlorohydrin rubber are blended as main components.

  The foamed cells on the roller surface were observed using a microscope, and the average of the outer diameters of the cells measured for 10 representative cells was about 300 μm. Further, the surface of the surface formed by polishing is rough, and as shown in FIG. 4, the transfer roller 14 is placed on a flat sheet metal 41 such as aluminum with no load, and leakage occurs from the portion where the roller 14 and the sheet metal 41 abut. The light 43 coming from the light source 42 is observed visually by the observer 44. Then, as shown in the enlarged schematic diagram 45 of the gap shown in FIG.

  As shown in FIG. 5, the transfer roller 14 is supported by bearings 22 at both ends, and a pressing force of about 500 gf is applied to the photosensitive drum as a total pressure in the left and right directions by a pressure spring 21 from a cored bar at both ends in the longitudinal direction. Is pressed. Therefore, there is no visible gap between the transfer roller 14 and the intermediate transfer belt 31 as observed with no load. However, when the transfer bias setting deviates from the optimum value, unevenness in the contact state occurs due to the fluffing, and therefore, transfer unevenness corresponding to the pitch of the flare occurs.

  Table 1 below shows that when the image is actually output using the transfer roller 14 of this comparative example, when the black transfer bias is swung up and down by 100 V with respect to 300 V using the black transfer bias as a standard value, respectively. It shows how the color image has changed. Each color image was a solid image in which a single color was printed at a density of 100%.

  In Table 1, when the black voltage setting is lowered, unevenness occurs in the center of the black image. This unevenness is unevenness that occurs in correspondence with the pitch of the marks on the transfer roller 14, and is generated because the transfer current selectively flows only in the defective part and the current in other parts is insufficient. . This occurs particularly at the center because the roller is bent by the contact pressure and the contact pressure at the center is low.

  On the other hand, in Table 1, when the black voltage setting is increased, unevenness occurs at the image edge other than black. This non-uniformity is manifested in the black transfer portion because re-transfer occurs in which other color toner is collected on the black photosensitive drum. This unevenness is a phenomenon in which a transfer current selectively flows excessively according to the pitch of the black transfer roller 14 and only a portion where retransfer occurs in a color other than black is thinned. What occurs particularly at the left and right edges of the image is that the contact pressure of the black transfer roller is higher at the left and right edges than at the center, so the transfer current becomes excessive, and retransfer is particularly noticeable. Because. The reason why the black image does not cause a problem even if the transfer current becomes excessive enough to cause retransfer is as follows.

  In other words, the black solid image has a low photosensitive drum potential when the solid image is exposed, and the difference from the transfer roller potential is small, so the transfer setting is excessive for the black solid image. This is because it must not. The retransfer of the other colors is severe because when the solid image of the other color comes to the black station, the black photosensitive drum is not exposed at all in the solid white state, so the potential remains high and the transfer is This is because the potential difference with the roller is large.

  In the description of this comparative example, image defects are reproduced by intentionally changing the transfer voltage. On the other hand, in an actual image forming apparatus, variations in manufacturing component resistance, variations in the film thickness of the photosensitive drum, differences in potential that change depending on usage conditions, and the like occur. In such an actual image forming apparatus, if the original optimum bias is a small value of 300V, it is very susceptible to the above-described fluctuation. For this reason, an image defect that occurs due to a difference in the contact state of the transfer roller is easily revealed.

(Comparative Example 2) Structure in which an intermediate transfer belt of 10 ¹⁰ Ω · cm is combined with a metal roller Next, as a comparative example, an aluminum roller having an outer diameter of 14 mm and a smooth surface is transferred as shown in FIG. A configuration used as the roller 14 will be described.

In this comparative example, an endless polyimide belt having a thickness of 65 μm and a volume resistivity of 10 ¹⁰ Ω · cm was used as the intermediate transfer belt 31 as in Comparative Example 1.

  If the surface is a smooth metal, the transfer failure of the central part and the retransfer of the end part as in Comparative Example 1 caused by the sponge roller scraping do not occur. Further, if the outer diameter is 14 mm and the pressure is 500 gf, the roller hardly bends, so that there is no difference in the contact state between the end and the center. However, since the surface of the metal roller is not deformed at all, the nip width formed between the photosensitive drum and the transfer roller becomes extremely narrow. For this reason, it is easily affected by slight misalignment and unevenness of the surface state of the belt, and the contact state becomes unstable, so that in reality there are many image unevennesses and only low quality images can be obtained.

  Therefore, when a metal roller is used as the primary transfer roller 14, as shown in FIG. 6, the position of the primary transfer roller 14 (14a, 14b) is moved significantly downstream. Further, the transfer belt 31 may be wound around the photosensitive drum 2 and the transfer roller 14 by drastically lifting the photosensitive drum 2 (2a, 2b). FIG. 6 shows only the primary transfer roller 14 (14a, 14b) and the photosensitive drum 2 (2a, 2b), but the other primary transfer roller 14 (14c, 14d) and the photosensitive drum 2 (2c, 2d) are illustrated. Is the same.

  In that case, the contact pressure of the transfer roller 14 needs to be significantly increased, and the intermediate transfer belt 31 also bends repeatedly, which is disadvantageous in terms of the life of the belt 31 and the photosensitive drum 2. In order to weaken the contact pressure of the transfer roller 14, there is a means for weakening the tension force that stretches the intermediate transfer belt 31. In this case, the grip force of the driving roller is also reduced, so that the intermediate transfer belt 31 slips. There is a risk that the belt conveyance stability is impaired.

(Comparative Example 3) Configuration in which a transfer roller comprising a 10 ^4.6 Ω · cm urethane sponge base layer and a 10 ⁵ Ω · cm tube surface layer is combined with a 10 ¹⁰ Ω · cm intermediate transfer belt 31 Next, as a comparative example, FIG. The transfer roller 14 having the configuration shown in FIG. The outer diameter of the transfer roller 14 was 14 mm. The transfer roller 14 has a hardness of 12 to 18 degrees (Asker-C) with respect to a core metal 14-1 having an outer diameter of 6 mm. Furthermore, it has a urethane sponge base layer 14-2 in which the volume resistivity is adjusted to about 10 ^4.6 Ω · cm so that the actual resistance is 10 ^5.0 Ω. Further, the heat-shrinkable tube 14-3 made of PFA having a volume resistivity adjusted to about 10 ⁵ Ω · cm is coated so that the actual resistance becomes 10 ^7.0 Ω with a thickness of 100 μm. A configuration using the transfer roller 14 will be described.

In this comparative example, an endless polyimide belt having a thickness of 65 μm and a volume resistivity of 10 ¹⁰ Ω · cm was used as the intermediate transfer belt 31 as in Comparative Examples 1 and 2.

  Since this roller 14 uses urethane sponge for the base layer 14-2, it is possible to make the hardness very low and also to make the resistance of the base layer 14-2 very low. . On the other hand, since the surface is covered with the PFA tube 14-3, there is an advantage that there is no fear of hydrolysis even if it is used for a long time.

Further, by covering the tube 14-3 having an actual resistance of 10 ⁷ Ω on the surface layer, the actual resistance of the entire roller has a high resistance value of 10 ⁷ Ω. Therefore, when outputting an image under the same conditions as in Comparative Example 1, the standard transfer bias setting is 1200V. When the transfer bias setting is 1200V, the absolute value of the transfer bias is caused by variations in manufacturing component resistance, variations in the film thickness of the photosensitive drum, and potential differences that change depending on usage conditions, which are problems in Comparative Example 1. This is a relatively small error compared to. Therefore, it becomes difficult to appear in the difference of images.

  However, when the resistance of the surface layer 14-3 increases, the charge generated on the back surface of the belt in the transfer nip hardly flows to the transfer roller on the downstream side of the transfer nip. For this reason, the potential on the back surface of the transfer belt becomes high, spark discharge from the back surface of the belt occurs, and the image on the belt surface is disturbed. This image disturbance is particularly noticeable in a halftone density range, and the discharge pattern becomes a very unsightly image defect that looks like a bird's footprint. In order to avoid such a discharge pattern, it is necessary to lower the setting of the transfer bias. On the other hand, if the transfer bias is set to be low, a low density occurs. Therefore, in the configuration of Comparative Example 3, there is no setting that achieves both suppression of the discharge pattern and density.

(Comparative Example 4) An intermediate transfer belt having a thickness of 67 μm, in which a 10 ^6.6 Ω · cm NBR / hydrin sponge roller is provided with a 10 ³ Ω · cm coating layer on the back surface of a 10 ¹⁰ Ω · cm base material Comparative Example 4 is a transfer roller similar to Comparative Example 1, except that the hardness is 15 to 21 ° (Asker-C) and the volume resistivity is 10 ^7.0 so that the actual resistance is 10 ^7.0 Ω. A transfer roller 14 made of NBR / hydrin sponge adjusted to about ^6.6 Ω · cm is used.

Further, in this comparative example, as shown in FIG. 8, an endless film-like member having a plurality of layers, in this example, a two-layer structure, was used as the intermediate transfer belt 31. That is, the 65 μm-thick polyimide transfer belt 31-1 used in Comparative Examples 1, 2, and 3 is made of PFA as the lowermost layer, that is, the back surface has a volume resistivity of 10 ³ Ω · cm and a thickness of 2 μm. This is an endless film-like member having a two-layer structure coated with the conductive layer 31-2.

In this comparative example, as in Comparative Example 3, the actual resistance of the transfer roller 14 is about 10 ⁷ Ω. For this reason, as in Comparative Example 3, the variation in component resistance in manufacturing, the variation in the film thickness of the photosensitive drum, and the difference in potential that varies depending on the usage state are relatively small errors compared to the absolute value of the transfer bias. Therefore, it is difficult to appear in the difference of images.

  Further, in this comparative example, a conductive layer 31-2 having a very low volume resistivity is provided on the back surface of the intermediate transfer belt 31. Therefore, it is difficult for charges to accumulate on the back surface of the belt in the transfer nip, and the charge on the back surface of the belt on the downstream side of the transfer nip becomes very small. As a result, it is possible to suppress the discharge pattern, which is a problem in Comparative Example 3 and the like.

  On the other hand, since the resistance of the back surface of the belt is very low, the transfer bias applied at the transfer portion flows into the ground through the rollers 8, 10, and 34 that stretch the intermediate transfer belt 31. Therefore, it is necessary to make the stretching rollers 8, 10, 34 be medium resistance members such as the transfer roller 14, or to insert a resistance member such as electrical resistance between the stretching rollers 8, 10, 34 and the ground. Further, when the resistance on the back surface of the intermediate transfer belt 31 is lowered, it becomes impossible to hold charges on the surface layer of the intermediate transfer belt 31, so that the charge of the toner layer on the belt surface layer easily escapes to the surroundings. As a result, when the intermediate transfer belt 31 supporting the toner layer is bent when the toner layer on the intermediate transfer belt 31 passes through the stretching rollers 8, 10, 34, the retention force due to electric charge does not act on the toner layer. Easily scatter. As a result, the halftone image formed by the fine dots is visible, or the fine character image is crushed, resulting in a significant reduction in image quality. Therefore, the configuration of this comparative example is not preferable for the image forming apparatus.

(Example 1) 10 ^6.6 Ω · cmNBR / Hydrin sponge base layer + 10 ³ Ω · cm tube transfer layer combined with a 10 ¹⁰ Ω · cm intermediate transfer belt Comparative Examples 1 and 2 described above As a configuration for solving all the problems 3 and 4, the configuration of the first embodiment will be described.

As in Comparative Example 3, the transfer roller of Example 1 has a plurality of layers as shown in FIG. That is, the base layer 14-2 made of NBR / hydrin sponge having a volume resistivity of about 10 ^6.6 Ω · cm is used so that the hardness is 17 ° (Asker-C) and the actual resistance is 10 ^7.0 Ω. did. The base layer 14-2 was covered with a PFA heat-shrinkable tube 14-3 having a volume resistivity adjusted to about 10 ³ Ω · cm so that the thickness was 100 μm and the actual resistance was 10 ^5.0 Ω. As the thickness of the tube layer 14-3, for example, if the thickness is reduced to about 20 μm, the durability is impaired. On the other hand, even if the material has elasticity such as PFA, if the thickness exceeds 500 μm, the hardness of the transfer roller increases, and the width of the transfer nip becomes narrow, which is disadvantageous for retransfer. Therefore, the thickness of the surface layer 14-3 is preferably set between 50 μm and 500 μm. In addition, as the surface layer 14-3 of this example, a PFA heat shrinkable tube having a surface roughness Rz (JIS B 0601 10-point average roughness Rz scale) of 1 to 2 μm was used.

In this embodiment, as in the case of Comparative Examples 1, 2, and 3, an endless film-like polyimide having a thickness of 65 μm and having a volume resistivity of 10 ¹⁰ Ω · cm is used as the intermediate transfer belt 31. It was. The roughness Rz of the back surface of the intermediate transfer belt 31 of this embodiment was 2 to 4 μm.

  The configuration of this embodiment will be described with reference to FIG.

  In FIG. 9, the transfer roller surface layer 14-3 of this example has a very smooth surface with Rz of 1 to 2 μm. Therefore, as shown in the schematic diagram C showing the state of the surface of FIG. 9, the surface layer 14-3 of the transfer roller 14 has very fine irregularities. Therefore, the back surface 31a of the intermediate transfer belt 31 and the front surface 52a of the transfer roller surface layer 14-3 are uniformly in contact with each other by countless points. On the other hand, since the sponge layer used as the base layer 14-2 has a very low hardness, the surface of the sponge surface is very large and rough. For this reason, the number of the contact surfaces 47 of the back surface 52b of the transfer roller surface layer 14-3 and the mark is limited. However, since the surface layer resistance of the transfer roller 14 is very low, the transfer current spreads in the area direction of the surface layer 14-3 as indicated by the arrow in FIG. 9, and the surface layer 14-3 and the back surface 31a of the belt 31 are in contact with each other. The transfer current is easy to flow uniformly. In the configuration of this embodiment, the thickness of the surface layer 14-3 of the transfer roller 14 is as thin as 100 μm, and the hardness of the sponge of the base layer 14-2 is as extremely low as 17 °. For this reason, the nip width formed by the front surface 52a of the transfer roller 14 and the back surface 31a of the belt can be made sufficiently wide as in the case of a conventional sponge roller single layer roller. Therefore, even if the transfer roller 14 is brought into contact with the photosensitive drum 2 with the same contact pressure as in the prior art, the contact pressure does not partially increase and retransfer does not occur.

  Table 2 below shows that when the image is actually output using the transfer roller 14 of the present embodiment and the image is actually output as in Comparative Example 1, it is 200 V up and down with respect to 1200 V using the black transfer bias as a standard value. It shows how the image of each color changed when shaken one by one. When the transfer via is raised and lowered in increments of 100 V as in Comparative Example 1, no noticeable image defect has occurred, so the increment is doubled.

  In Table 2, even when the black voltage setting was lowered, the black image remained OK at the 1000 V setting. In addition, when 800 V was set, a thin density occurred in the center of the black image. However, since the configuration of Example 1 is a uniform surface layer with no flare as in Comparative Example 1, unevenness due to partial current flow between the intermediate transfer belt 31 and the transfer roller 14 does not occur. For this reason, the image quality does not deteriorate extremely.

  On the other hand, in Table 2, when the black voltage setting was increased, the image other than black remained OK at the 1400 V setting. When the voltage was further increased to 1600 V, a thin density occurred at the edge of the image other than black. Similar to the first comparative example, the low density is manifested in the black transfer portion because retransfer occurs in which toner of other colors is collected on the black photosensitive drum. However, as in the case of the image when the transfer bias setting is lowered, only the density is lowered and no unevenness is generated, so that the image quality is not greatly lowered.

  Also in the image evaluation of the present embodiment, the image defect is reproduced by intentionally changing the transfer voltage as in the first comparative example. In this embodiment, the original voltage setting is as high as 1200 V, and the image does not change when the transfer voltage is increased or decreased by 200 V. Therefore, it is difficult to be affected by variations in manufacturing component resistance, variations in the film thickness of the photosensitive drum, potential differences that change depending on usage conditions, and the like, which occur in an actual image forming apparatus.

Furthermore, the surface layer 14-3 used in this example is characterized by a very low resistance. As described above, the actual resistance of this tube is 10 ^5.0 Ω, and the volume resistivity is a very low material of 10 ^3.0 Ω · cm. Therefore, similarly to the metal roller of Comparative Example 2, charges generated on the back surface of the belt in the transfer nip easily flow into the transfer roller on the downstream side of the transfer nip. Therefore, the potential on the back surface of the transfer belt is kept low, and spark discharge does not occur on the back surface of the belt. Therefore, in the configuration of the first embodiment, it is possible to achieve both suppression of the discharge pattern and high density.

  That is, in this embodiment, the volume resistivity of the intermediate transfer belt is R1, the volume resistivity of the outermost layer (surface layer) 14-3 of the transfer roller 14 is R2, and the volume of the inner layer (base layer) 14-2 from the outermost layer. When the resistivity is R3, R2 is minimized.

Further, between the volume resistivity R2 of the outermost layer of the transfer roller 14 and the volume resistivity R3 of the layer inside the outermost layer,
(R3) / (R2) ≧ 100
The relationship holds.

  In this example, an NBR / hydrin sponge roller (that is, a conductive foam from the coating layer 14-3) is used for the base layer 14-2, and a PFA tube (conductive elastic material) is used for the coating layer 14-3. The configuration using the body has been described. However, the base layer 14-2 can be made of a foamed urethane sponge or other rubber material, and the coating layer 14-3 can be made of a resin such as polyethylene or EVA, or a fluororesin such as PTFA or PVDF. You can use it.

  In this embodiment, in the transfer roller 14, the layer 14-2 inside the outermost layer 14-3 of the transfer roller 14 is a conductive foam, and the outermost layer 14-3 is a conductive elastic body. is important.

Example 2
In the present embodiment, the color image forming apparatus configuration of FIG. 1 is provided as in the first embodiment, and the description of the same configuration as in the first embodiment is omitted.

  In the second exemplary embodiment, the primary transfer roller 14 can be separated from and contacted with the intermediate transfer belt 31, and is separated from the intermediate transfer belt 31 during non-image formation, and the photosensitive drum 2 and the intermediate transfer belt 31 are also separated. The contact time of the photosensitive drum 2, the intermediate transfer belt 31, and the primary transfer roller 14 is set only when the toner image is primarily transferred from the photosensitive drum 2 to the intermediate transfer belt 31 during image formation. This is for suppressing adverse effects such as surface scraping caused by rubbing of the photosensitive drum 2 with the intermediate transfer belt 31 and shape change due to the primary transfer roller 14 being kept in contact. This configuration is particularly advantageous for a high-speed image forming apparatus in which a long life design is essential.

  In the intermediate transfer member having such a configuration, when both the back surface of the intermediate transfer belt and the surface of the primary transfer roller are smooth, the rotation of the transfer roller is delayed for a moment when the separation state shifts to the contact state. Therefore, foreign matters such as dust and discharge products adhering to the back surface of the belt are scraped off by the transfer roller, and as a result, there are cases where the transfer roller surface adheres as linear stains. When this linear stain appears as a horizontal line-shaped image unevenness on the image, it becomes apparent as a disturbance of the image particularly on a uniform halftone, and the quality of the image may be impaired.

  Therefore, in this embodiment, the surface roughness on the back surface of the belt and the surface roughness on the surface of the transfer roller are assigned to several stages, and an optimum combination of roughness is selected.

  As the intermediate transfer belt 31, a polyimide belt is used in the second embodiment as in the first embodiment. There are roughly two methods for manufacturing a polyimide belt. One is a method in which the surface property of the mold is transferred to the back surface of the belt by applying a polyimide varnish to the outer surface of the inner mold and drying and baking it. The other is a method in which the surface property of the mold is transferred to the surface of the belt by applying a polyimide varnish to the inner surface of the outer mold, drying and baking it. If the former method is used, the inner surface roughness of the belt can be controlled by controlling the surface property of the mold. With this method, the surface roughness of the back surface of the belt was controlled in this example.

  On the other hand, there are various methods for controlling the surface property of the tube layer 14-3 provided on the surface of the transfer roller 14. In this example, EVA (ethylene vinyl acetate copolymer) was used and formed into a tube shape as a material that can easily control the surface properties of the tube layer 14-3. Thereafter, the tube was sandwiched between a metal roller whose surface was processed, and the surface property was controlled by transferring the surface property of the metal roller to the tube.

  Table 3 below shows the results of varying the roughness (JIS B 0601 10-point average roughness Rz scale) of the belt back surface and the transfer roller surface in this example.

  In Table 3, when the surface roughness Rz is 1 μm on both the transfer roller surface and the belt back surface, slip occurs when the transfer roller contacts the belt. When the surface roughness of either the transfer roller or the back of the belt was 10 to 25 μm, no slip occurred and the image had no problem. When the surface roughness was further increased and the surface roughness of the transfer roller was set to 50 μm, image unevenness similar to that of a sponge-like transfer roller having a crack was generated. Even when the back surface of the belt was 50 μm, the same image unevenness as when the surface roughness of the transfer roller was 50 μm occurred. Further, in this embodiment, the elastic roller 34 is used to face the cleaning blade 33. However, when this elastic roller is changed to a metal roller as a test, the contact state between the belt 31 and the cleaning blade 33 becomes non-uniform, There was also a cleaning failure due to toner slipping.

  From the above results, it is desirable that the surface roughness of the transfer roller 14 and the roughness Rz of the back surface of the intermediate transfer belt 31 are 1 μm or more and 50 μm or less, preferably about 10 μm to 25 μm. When either surface is a very smooth surface having an Rz of about 1 μm, the roughness of the surface combined as the counterpart is preferably roughened.

  From the above results, by controlling the surface roughness of the surface layer of the primary transfer roller 14 and the roughness of the back surface of the intermediate transfer belt as described above, image deterioration at the primary transfer portion in the color image forming apparatus having the intermediate transfer belt 31 can be achieved. It can be suppressed and a good image quality can be obtained.

Example 3
This embodiment has the color image forming apparatus configuration of FIG. 1 as in the first and second embodiments, and the description of the same configuration as in the first and second embodiments is omitted.

  In Example 3, as shown in FIG. 10, the transfer belt 31 used an endless rubber belt member having a plurality of layers, in this example, a three-layer configuration, as the intermediate transfer belt 31. That is, the base rubber 31-3 made of 800 μm thick millable urethane was coated with a material in which an iron oxide filler was dispersed in a solvent-soluble fluorine material as the lowermost layer 31-4 and the uppermost layer 31-5. The lowermost layer and the uppermost layer were coated at the same time by the dipping method, and the thickness of each coating was 20 μm.

When the base rubber 31-3 has a thickness of 800 μm, it is very difficult to disperse a conductive agent such as carbon and uniformly finish the volume resistivity in a medium resistance region. Therefore, it is necessary to set the base rubber layer to a sufficiently low resistance value so that resistance unevenness does not occur. Therefore, in this example, the volume resistivity of the base rubber layer was adjusted to 10 ^5.5 Ω · cm.

On the other hand, as described above in Comparative Example 4, when the lowermost layer of the belt has a low resistance, it is necessary to make the tension roller an intermediate resistance or to insert a resistance member such as an electric resistance between the tension roller and the ground. . In addition, the quality of the image is greatly reduced. Therefore, in this example, the volume resistivity of the coat layer used for the lowermost layer 31-4 and the uppermost layer 31-5 was adjusted to 10 ^10.0 Ω · cm.

  In this embodiment, the transfer roller 14 uses an NBR / hydrin sponge roller (that is, a foam that is more conductive than the coating layer 14-3) as the base layer 14-2, as in the first and second embodiments. A PFA tube (conductive elastic body) was used for -3.

  By using a rubber belt as the intermediate transfer belt in this way, even if a rough surface paper called a thick paper or rough paper having a large surface unevenness is used as the transfer material, the surface property of the transfer material is improved by the elasticity of the belt. The surface property can follow. As a result, the adaptability of the material used as the transfer material can be greatly increased. On the other hand, since the surface of the rubber material is poorly releasable from the toner, the transferability of the toner is poor as it is. Although the cleaning property is poor, it is possible to improve the transfer property and the cleaning property by using a fluorine material on the surface. Furthermore, by coating the back surface of the belt simultaneously with the front surface with a material having sufficient resistance, it becomes possible to keep the resistance of the rubber layer low with no resistance unevenness. Further, no special configuration is added to the tension roller, and the uniformity of the image can be kept high.

Example 4
In the present embodiment, the color image forming apparatus configuration of FIG. 1 is provided in the same manner as in the first, second, and third embodiments, and the description of the same configuration as in the first, second, and third embodiments is omitted.

In Example 4, as shown in FIG. 11, the primary transfer roller 14 has a hardness of 15 to 21 ° (Asker-C) and a volume resistivity of 10 ^6.6 Ω · cm so that the actual resistance is 10 ^7.0 Ω. Heat-shrinkage made of PFA with a volume resistivity adjusted to about 10 ³ Ω · cm so that the actual resistance is 10 ^5.0 Ω with respect to the base layer 14-2 made of NBR / hydrin sponge adjusted to a degree Tube 14-3 was coated. Furthermore, as shown in FIG. 11, the heat-shrinkable tube 14-3 of the present embodiment has an outer diameter smaller than that of the base layer 14-2 at the end on the longitudinal side and overlaps with the base layer 14-2 on the longitudinal side. About the part, the outer diameter was comprised 1 mm larger than the outer diameter of the base layer 14-2.

  With this configuration, as shown in FIG. 12, when the primary transfer roller 14 is brought into contact with the intermediate transfer belt 31 and the photosensitive drum 2 in the transfer nip T1, the surface layer 14-3 is statically moved. Since it is attracted to the belt 31 by the electroadsorption force, the nip formed by the surface layer 14-3 and the belt 31 is extended further downstream than the nip formed by the base layer 14-2 and the belt 31. By the downstream nip 46 formed by the intermediate transfer belt 31 and the primary transfer roller surface layer 14-3, the charge on the belt back surface 31a formed in the transfer nip is more effectively collected on the surface layer 14-3 of the transfer roller 14. Is done. As a result, even when the image forming apparatus is used in an environment where abnormal discharge is likely to occur, particularly in a low-temperature and low-humidity environment, the occurrence of abnormal discharge can be suppressed and an intermediate transfer body with higher environmental adaptability can be configured. Become.

  Even if the gap g is provided between the base layer 14-2 and the surface layer 14-3, the shape of the end of the surface layer 14-3 is reduced to a smaller diameter than that of the base layer 14-2. It is not necessary to provide a member for regulating the orbit of the surface layer 14-3. With this configuration, the surface layer 14-3 can be rotated while following the base layer 14-2.

  As described above, by using the configuration of this embodiment, it is possible to provide an image forming apparatus that can obtain a stable density and image uniformity with a simple configuration under a wide range of environmental conditions.

2a to 2d Photosensitive drum (image carrier)
14a to 14d Primary transfer roller (primary transfer means)
31 Intermediate transfer belt (intermediate transfer member)
32a to 32d Process cartridge (image forming station, process unit)
35 Secondary transfer roller (secondary transfer means)
S transfer material

Claims (7)

An image carrier that carries a toner image, a rotatable intermediate transfer member, a primary transfer unit that primarily transfers the toner image carried on the image carrier to the intermediate transfer member, and a toner image that is transferred to the intermediate transfer member. An image forming apparatus having a secondary transfer means for transferring the toner image to a transfer material,
The primary transfer means is a roller having two or more layers, the volume resistivity of the intermediate transfer member is R1, the volume resistivity of the outermost layer of the roller is R2, and the volume resistivity of the inner layer from the outermost layer. An image forming apparatus, wherein R2 is the smallest when R3 is R3.   2. The image forming apparatus according to claim 1, wherein the volume resistivity R2 of the outermost layer of the roller and the volume resistivity R3 of a layer inside the outermost layer satisfy (R3) / (R2) ≧ 100.   3. The image forming apparatus according to claim 1, wherein a layer inside the outermost layer of the roller is a conductive foam, and the outermost layer is a conductive elastic body.   The surface of the intermediate transfer member in contact with the roller has a surface roughness of JIS B 0601 10-point average roughness Rz scale of 1 μm to 50 μm, and the roller has a surface roughness of JIS B 0601 10-point average roughness. The image forming apparatus according to claim 1, wherein the image forming apparatus has an Rz scale of 1 μm to 50 μm. 5. The image forming apparatus according to claim 1, wherein the intermediate transfer member includes a plurality of layers, and a volume resistivity of a lowermost layer is 10 ⁶ Ω · cm or more.   The image forming apparatus according to claim 1, wherein an outermost layer of the roller has a thickness of 50 μm or more and 500 μm or less.   The image forming apparatus according to claim 1, wherein a gap can be formed between an outermost layer of the roller and a layer inside the outermost layer.

Images