JP2005345912A - Image forming apparatus - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a color image forming apparatus of an intermediate transfer system which includes secondary transfer and prevents image quality degradation, regardless of environmental changes by of low-cost means and a means of increasing the set width for the secondary transfer current. <P>SOLUTION: In the color image forming apparatus, with which superimposed toner images obtained by primarily sequentially transferring a plurality of toner images onto an intermediate transfer belt 1 are secondarily transferred to a recording medium all at once by a secondary transfer means, a secondary transfer roller 2 is grounded, and a secondary transfer bias so controlled as to have constant current is applied to an opposite secondary transfer roller 3. In addition, a conductive member 4, whose resistance change with environment is small, is grounded near the opposite secondary transfer roller and at the side of the back of the intermediate transfer belt. Full-width resistance, from a secondary transfer nip to the grounded part of the conductive member, is set to 5E+7Ω to 2E+8Ω. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to an electrophotographic image forming apparatus used for a copying machine or a printer, and more particularly to setting of an electric resistance of a grounding member used at the time of secondary transfer of a color image forming apparatus having an intermediate transfer belt.

For example, in a color image forming apparatus having a conventional intermediate transfer belt as shown in FIG. 3, a plurality of image forming units including a photoconductor 7 as an image carrier and a primary transfer unit 6 on the outer periphery of the intermediate transfer belt 1. The toner images of the respective colors formed on the photoreceptor 7 of the image forming unit are sequentially primary transferred onto the intermediate transfer belt 1 to form a superimposed toner image, and the superimposed toner primarily transferred onto the intermediate transfer belt 1 is formed. In general, the image is sandwiched between the secondary transfer unit 2 and the secondary transfer counter unit 3 to which a bias is applied from the transfer bias applying unit 5 and is collectively transferred onto the recording medium P. Thereafter, the second transferred toner image is fixed by the post-transfer fixing unit 8, and the intermediate transfer belt 1 is cleaned by the cleaning unit 9.
In an electrophotographic image forming apparatus using such an intermediate transfer system, a conductive elastic roller is usually used as the secondary transfer unit 2. As a method of imparting conductivity to the conductive elastic roller as the secondary transfer means 2, a method of adjusting resistance by dispersing carbon in rubber or foamed rubber, a so-called ionic conductivity represented by a hydrin system, and the like. There is a method of adjusting the resistance by dispersing a conductive material.
In general, the carbon dispersion type has little resistance fluctuation (less than 0.5 digit), but has large resistance unevenness and resistance tolerance variation (about one digit). On the other hand, in the ion conductive type, resistance variation and resistance tolerance variation are small (0.3 digits or less), but resistance fluctuations are large (1 digit or more).
The roller, which is the secondary transfer unit 2 used in the electrophotographic image forming apparatus using the intermediate transfer method, transfers color images onto the recording medium P all at once. In many cases, an ion conductive type roller is used. Resistance fluctuations that cause problems in the ion conductive type can be used by feeding back the temperature and humidity environment to the transfer current value.
In addition, when the secondary transfer bias is constant voltage control, control for changing the voltage setting value according to the resistance value of the transfer roller or the recording medium, so-called resistance feedback control, is essential, but in the case of constant current control, the transfer roller In addition, since the load voltage is adjusted with respect to the resistance value of the recording medium, the current setting value does not greatly deviate, and the need for resistance feedback is reduced.

However, when the secondary transfer bias is controlled at a constant current and an ion conductive member is used for the secondary transfer roller, the transfer roller resistance increases in a low humidity environment, and the load voltage with respect to the transfer current increases rapidly. In addition, there is a problem that the setting range of the transfer current becomes extremely narrow.
In order to solve this problem, a method of switching from constant current to constant voltage control and a method of providing a plurality of voltage limiter values when the transfer roller becomes high resistance has been proposed. It is not reasonable.
Furthermore, in anticipation that the transfer roller resistance increases in a low humidity environment, even if the roller resistance is reduced, the roller resistance will decrease more than necessary in a high humidity environment, and transfer during printing on narrow media such as postcards. A large amount of current leaks from both ends of the transfer nip where there is no medium, the effective transfer voltage is lowered, and the image quality is extremely lowered. If an attempt is made to secure a current capacity to compensate for this, the power supply cost will increase significantly, which is not reasonable.
The present invention has been made in consideration of the above-described actual situation, and the image quality does not deteriorate even when the environment changes due to the following configuration by means of expanding the secondary transfer current setting range at low cost. An object is to provide an intermediate transfer type color image forming apparatus.

In order to solve the above-mentioned problem, the invention according to claim 1 is directed to a recording medium in which a plurality of toner images are sequentially transferred onto an intermediate transfer belt in sequence by a secondary transfer unit. In the color image forming apparatus that performs secondary transfer, the secondary transfer unit is composed of a secondary transfer roller and a secondary transfer counter roller, and the secondary transfer bias is grounded while the secondary transfer bias is constant current control. A conductive member that is applied to the secondary transfer counter roller side, is grounded near the secondary transfer counter roller and on the back side of the intermediate transfer belt, and has a small resistance fluctuation in the environment. The full width resistance is 5E + 7Ω to 2E + 8Ω.
According to a second aspect of the present invention, there is provided an image forming apparatus in which the conductive member according to the first aspect is in contact with a belt back surface on the upstream side of the secondary transfer counter roller.
According to a third aspect of the present invention, the conductive member according to the first aspect is brought into contact with the surface of the secondary transfer counter roller, and the full width resistance between the shaft core and the surface of the secondary transfer counter roller and the conductive member. The image forming apparatus is characterized in that the combined resistance of the full width resistance from the contact portion with the secondary transfer counter roller to the ground is 5E + 7Ω to 2E + 8Ω.
According to a fourth aspect of the present invention, the conductive member according to any one of the first to third aspects is an image forming apparatus having a sheet shape.

  According to the present invention, in a color image forming apparatus that secondary-transfers a superposed toner image obtained by sequentially transferring a plurality of toner images onto an intermediate transfer belt to a recording medium collectively by a secondary transfer unit. The secondary transfer bias with constant current control is applied to the secondary transfer counter roller, and a conductive member with little environmental fluctuation of resistance is grounded near the secondary transfer counter roller and on the back side of the intermediate transfer belt. By setting the total width resistance from the secondary transfer nip portion to the grounding portion of the conductive member to 5E + 7Ω to 2E + 8Ω, an ion conductive secondary transfer roller having a large resistance fluctuation is used, and the transfer bias Even when constant current control is used, it is possible to ensure a sufficiently wide transfer current setting range in a low humidity environment without increasing the cost of the power supply.

Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings. FIG. 1 shows a schematic diagram of a secondary transfer portion of the first embodiment according to the present invention. Details of the members used are as follows. Note that FIG. 3 showing the image forming unit is also referred to, and the configuration other than the secondary transfer unit is omitted in FIG.
The intermediate transfer belt 1 is a Gunze polyimide resin having a single layer surface resistance of 1E + 11Ω (measured by Hiresta-IP500V manufactured by Yuka Denshi).
The secondary transfer roller pair constituting the secondary transfer unit includes a secondary transfer roller 2 and a secondary transfer counter roller 3. The secondary transfer roller 2 is made of Sumitomo Rubber's ion conductive foam rubber having an outer diameter of φ20, a shaft core of φ13, and a shaft-surface resistance of 5E + 7Ω (measured at 23 ° C. 50% RH 500V 1 kg load full width). did. The secondary transfer counter roller 3 is an EP rubber made by Yamauchi, with an outer diameter of φ17.45 (hollow matt tube) and a shaft-to-surface resistance of 1E + 6Ω (23 ° C. 50% RH 500V 1 kg load full width measurement value) did.
The conductive member 4 which is a feature of the present invention is obtained by attaching a 3M ultra sheet (1 kΩ or less) to a 75 μm thick PET film. The secondary transfer bias was applied to the secondary transfer counter roller 3 as constant current control, and the axis of the secondary transfer roller 2 was grounded. The conductive member 4 was in contact with the back surface of the intermediate transfer belt 1 to 8 mm upstream from the secondary transfer nip portion N. Table 1 shows the results obtained by measuring the full width resistance from the secondary transfer nip N to the grounding portion of the conductive member 4 by applying 500V. This measurement was performed in a laboratory environment (normal temperature and humidity environment).

(Table 1)

The secondary transfer characteristics with and without the conductive member were evaluated. The evaluation conditions are as follows. The actual measurement value of the secondary transfer roller in this evaluation environment was 2E + 8Ω.
The environment was 10 ° C. and 15% RH, and the recording medium was Ricoh NBS180k cardboard (image evaluation on the back side of A4 width double-sided printing).
The secondary transfer characteristics were compared using the transfer current as a parameter in the range of the secondary color solid and halftone image quality good range. The comparison results are shown in Table 2.

(Table 2) Transferability evaluation in low temperature and low humidity environment

導電性部材４の設置位置を、転写ニップ部から当接部までの距離を１ｍｍまで狭めると、転写電流の大幅な上昇がみられた。以上の結果から、導電性部材の設置位置は、二次転写ニップ部から導電性部材の接地部までの全幅抵抗として、５Ｅ＋７Ω〜２Ｅ＋８Ωとすることで、転写電流の設定範囲を広く確保することができることが分かった。 As a side effect, we confirmed an increase in transfer current in a high humidity environment. The evaluation conditions are as follows. The actual measurement value of the secondary transfer roller in this evaluation environment was 9E + 6Ω. The environment was 32 ° C. and 80% RH, and the recording medium was RBS NBS 180k cardboard (image evaluation on the back of postcard size double-sided printing).
The secondary transfer characteristics were compared using the transfer current as a parameter in the range of the secondary color solid and halftone image quality good range. The comparison results are shown in Table 3. The upper limit value of the transfer current is set to 200 μA from the current capacity of the power source.









(Table 3) Transferability evaluation in high temperature and high humidity environment

When the installation position of the conductive member 4 was reduced to 1 mm from the transfer nip portion to the contact portion, a significant increase in transfer current was observed. From the above results, the setting position of the conductive member is set to 5E + 7Ω to 2E + 8Ω as the full width resistance from the secondary transfer nip portion to the grounding portion of the conductive member, thereby ensuring a wide transfer current setting range. I understood that I could do it.

FIG. 2 shows a schematic diagram of a secondary transfer portion of a second embodiment according to the present invention. Details of the members used are as follows. Note that the portions other than the secondary transfer portion are omitted.
As the intermediate transfer belt, a Gunze polyimide resin single layer having a surface resistance of 1E + 11Ω (measured by Hiresta-IP500V manufactured by Yuka Denshi) was used. The secondary transfer roller 2 is an ion conductive foam rubber made of Sumitomo Rubber, having an outer diameter of φ20, a shaft core of φ13, and a shaft-to-surface resistance of 5E + 7Ω (23 ° C. 50% RH 500V 1 kg load full width measurement value). .
The secondary transfer counter roller 3 is an EP rubber made by Yamauchi, and has an outer diameter of φ17.45 (hollow matt tube) and a shaft-to-surface resistance of 1E + 6Ω (23 ° C. 50% RH 500V 1 kg load full width measurement value). used. The conductive member 4 is the same as the intermediate transfer belt 1.
The secondary transfer bias was applied to the secondary transfer counter roller as constant current control, and the axis of the secondary transfer roller was grounded. The conductive member 4 was in contact with the surface of the secondary transfer counter roller 3 and the total width resistance from the axis of the secondary transfer counter roller to the grounding portion of the conductive member 4 was measured by applying 500 V, and was about 1E + 8Ω. . The secondary transfer characteristics with and without the conductive member were compared and evaluated. The evaluation conditions are as follows. The measured value of the secondary transfer roller resistance in this evaluation environment was 2E + 8Ω.
The environment was 10 ° C. and 15% RH, and the recording medium was NBS 180k thick paper (image evaluation on the back side of A4 width double-sided printing) manufactured by Ricoh. The secondary transfer characteristics were compared by using the transfer current as a parameter and the width of the good image quality range of the secondary solid color and halftone. When there is no conductive member, the good range of the transfer current is −13 μA to −15 μA, whereas when the conductive member is present, it is −15 μA to −20 μA. The settable range of the next transfer current could be expanded. Further, when the harmful effect in a high temperature and high humidity environment was confirmed, the settable range of the transfer current was −80 μA or more, and it was confirmed that there was almost no change with and without the conductive member.

FIG. 3 is a schematic diagram of a secondary transfer unit of the color image forming apparatus of the present invention. FIG. 6 is another schematic diagram of a secondary transfer unit of the color image forming apparatus of the present invention. Schematic of the color image forming apparatus of a prior art example.

Explanation of symbols

1 Intermediate transfer belt, 2 Secondary transfer roller, 3 Secondary transfer counter roller, 4 Conductive member

Claims (4)

In a color image forming apparatus in which a plurality of toner images are sequentially transferred onto an intermediate transfer belt in a primary transfer, and a secondary toner image is transferred to a recording medium.
The secondary transfer means is composed of a secondary transfer roller and a secondary transfer counter roller,
The secondary transfer bias, which is connected to the secondary transfer roller and grounded with constant current control, is applied to the secondary transfer counter roller side, and it is a conductive material with little environmental fluctuation of resistance near the secondary transfer counter roller and on the back side of the intermediate transfer belt. An image forming apparatus characterized in that a conductive member is grounded, and a full width resistance from a secondary transfer nip portion to a ground portion of the conductive member is 5E + 7Ω to 2E + 8Ω.   An image forming apparatus, wherein the conductive member according to claim 1 is in contact with the back surface of the intermediate transfer belt on the upstream side of the secondary transfer counter roller.   The conductive member according to claim 1 is brought into contact with the surface of the secondary transfer counter roller, and the full width resistance between the shaft core and the surface of the secondary transfer counter roller and the contact portion of the conductive member with the secondary transfer counter roller. An image forming apparatus characterized in that a combined resistance of full width resistance to ground is 5E + 7Ω to 2E + 8Ω. The image forming apparatus according to claim 1, wherein the conductive member has a sheet shape.

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