JP4585187B2 - Image forming apparatus - Google Patents

Description

  The present invention relates to an image forming apparatus such as an electrophotographic copying machine or a printer, and more particularly to a full color image forming apparatus using an intermediate transfer system.

  Conventionally, as a full-color image forming apparatus using this type of intermediate transfer method, for example, a plurality of photoconductors as an image carrier that carries a toner image for each color according to image information, and the plurality of photoconductors are pressed against each other. And an endless intermediate transfer belt that is installed on a plurality of rollers and rotates in a predetermined direction, and a primary transfer that sequentially superimposes and transfers a single-color toner image formed on each photoconductor onto the intermediate transfer belt. And a secondary transfer unit that batch-transfers toner images on an intermediate transfer belt to a recording medium are known. The primary transfer means uses a transfer electric field formed between the image carrier and the intermediate transfer belt, and the secondary transfer means uses a transfer electric field formed between the intermediate transfer belt and the recording medium. It has been.

  In such an image forming apparatus, the primary transfer unit is required to faithfully and stably transfer the toner image formed on the photosensitive member to the intermediate transfer belt, and the secondary transfer apparatus similarly performs the intermediate transfer. It is required that the toner image formed on the belt is faithfully and stably collectively transferred to a recording medium. In other words, both the primary transfer unit and the secondary transfer unit need to perform stable transfer with high transfer efficiency.

Incidentally, as one of the causes of image deterioration in the primary transfer portion, there is a transfer dust phenomenon in which a line image, a character image, or the like is blurred due to toner scattering. This phenomenon is caused by the toner moving from the photosensitive member to the intermediate transfer belt in the space portion upstream of the nip portion of the primary transfer portion, and it has been found that this phenomenon can be prevented by suppressing the transfer electric field upstream of the nip portion. Yes.
As a means for preventing such transfer dust, the surface resistivity on the back surface of the intermediate transfer belt is set high so that the range of the transfer electric field necessary for the transfer of the intermediate transfer belt is given and the transfer electric field outside the transfer nip is transferred. There is known an image forming apparatus in which the control is made small (see, for example, Patent Document 1).
JP 2003-57995 A

However, in such a conventional image forming apparatus, since the surface resistivity on the back surface of the intermediate transfer belt is set high, a current supplied from a voltage application member such as a transfer roller via the intermediate transfer belt is not supplied. It is difficult to flow, and it is difficult for electric charges to be supplied to the peeled portion between the photosensitive member and the intermediate transfer belt, and the potential balance between the front and back surfaces of the intermediate transfer belt tends to deteriorate. Therefore, there has been a problem that peeling discharge occurs when the photoreceptor and the intermediate transfer belt are separated from each other in the primary transfer portion, and the discharge trace easily disturbs the image.
The present invention has been made in view of the above points, and in the full-color image forming apparatus using the intermediate transfer system as described above, when the photosensitive member and the intermediate transfer belt are separated from each other in the primary transfer unit with a simple configuration. An object of the present invention is to provide an image forming apparatus capable of preventing the peeling discharge and forming a good image.

In order to achieve the above-described object, the present invention provides an image carrier on which a toner image corresponding to image information is formed, and an endless intermediate member that is in pressure contact with the image carrier at a nip portion and stretched between a plurality of rollers. A transfer belt; primary transfer means for transferring the toner image on the image carrier to the intermediate transfer belt; and secondary transfer means for collectively transferring the toner images transferred and held on the intermediate transfer belt to a recording medium. In a full-color image forming apparatus using an intermediate transfer system having
A voltage application member that supplies electric charges to the intermediate transfer belt is provided near the most downstream point of the nip portion.
The voltage application member is a transfer roller of the position primary transfer unit, and is disposed so as to be pressed against the image carrier via the intermediate transfer belt,
The intermediate transfer belt has a back surface resistivity of 10 ¹⁰ to 10 ¹² Ω / □,
A tension roller that forms a nip portion by pressing the intermediate transfer belt against the image carrier separately from the plurality of rollers that stretch the intermediate transfer belt is provided on the downstream side of the transfer roller.
Provided is an image forming apparatus in which a distance between a straight line passing through the center of the image carrier and the center of the transfer roller and a most downstream point of the nip portion is 2 mm or less.

Further, it is preferable that the transfer roller has an Asker C hardness of 50 or less.

In the full-color image forming apparatus using the intermediate transfer system according to the present invention, as described above, a transfer roller is provided as a voltage application member in the vicinity of the most downstream point of the nip portion of the primary transfer unit to supply charges to the intermediate transfer belt. Therefore, the potential balance between the front and back surfaces of the intermediate transfer belt at the most downstream point of the nip portion is adjusted, and the peeling discharge can be suppressed to effectively prevent the occurrence of discharge traces and transfer dust. In particular, by setting the surface resistivity of the back surface of the intermediate transfer belt to 10 ¹⁰ to 10 ¹² Ω / □, the generation of transfer dust is kept within a level where the image can be satisfied, and the occurrence of discharge traces is within the range where there is no problem. Can be paid.
Further, a tension roller is provided on the downstream side of the transfer roller as the voltage application member, in addition to the plurality of rollers for stretching the intermediate transfer belt, and presses the intermediate transfer belt against the image carrier to form a nip portion. As a result, the transfer roller can be disposed after the nip portion has been formed in advance by the tension roller, and the transfer roller can be pressed against the photoconductor with a stable pressure to prevent transfer unevenness and the pressure contact pressure. Can be suppressed to a minimum, and transfer omission due to pressure can also be prevented.

Embodiments of the present invention will be specifically described below with reference to the drawings.
FIG. 5 is an overall configuration diagram showing an image forming apparatus embodying the present invention. Here, a color copying machine is shown as an example. This color copying machine is a tandem type electrophotographic apparatus using an intermediate transfer belt 10, a paper feeding table 2 at the bottom, a copying machine main body 1 above it, and a scanner 3 and automatic document feeding at the top. A device (ADF) 4 is provided.
The copying apparatus main body 1 is provided with a transfer device 20 having an endless intermediate transfer belt 10 at the substantially center. The intermediate transfer belt 10 is stretched by a driving roller 9 and driven rollers 15 and 16, and is shown in the figure. The residual toner remaining on the surface after image transfer is removed by the cleaning device 17 that rotates clockwise and is provided on the left side of the driven roller 15, so that the transfer device 20 can prepare for another image formation.
Four image forming portions of yellow, cyan, magenta, and black are formed above the linear intermediate transfer belt 10 spanned between the driving roller 9 and the driven roller 15 along the moving direction. Drum-shaped photoconductors 40Y, 40C, 40M, and 40K (hereinafter simply referred to as photoconductors 40 unless otherwise specified) are provided so as to be rotatable counterclockwise in the figure, and around them are known charging devices. 60, a developing device 61, a primary transfer device 62 constituting a primary transfer means, a photoconductor cleaning device 63, and a charge eliminating device 64 are provided, and an exposure device 20 is provided above the photoconductor 40.

On the other hand, a secondary transfer device 22 constituting a secondary transfer unit is provided below the intermediate transfer belt 10. The secondary transfer device 22 has an endless secondary transfer belt 24 bridged between a pair of rollers 23, 23. The secondary transfer belt 24 is connected to the driven roller 16 via the intermediate transfer belt 10. It comes to come in pressure contact. The secondary transfer device 22 collectively transfers the toner images on the intermediate transfer belt 10 onto a sheet P as a recording medium fed between the secondary transfer belt 24 and the intermediate transfer belt 10.
A fixing device 25 for fixing the toner image formed on the sheet P is provided downstream of the secondary transfer device 22 in the sheet conveying direction, and a pressure roller 27 is in pressure contact with the endless fixing belt 26. The sheet P after the image transfer is conveyed to the fixing device 25 by the secondary transfer device 22. The secondary transfer device 22 may be a transfer device using a transfer roller or a non-contact charger. A sheet reversing device 28 for reversing the sheet P when an image is formed on both the front and back surfaces of the sheet is provided below the secondary transfer device 22.

  When making a color copy with the color copying machine having the above-described configuration, the document is usually set on the document table 30 of the automatic document feeder 4, but when the document is manually set, The automatic feeding device 4 is opened, an original is set on the contact glass 32 of the scanner 3, and the original is pressed against the contact glass 32 by closing the automatic original feeding device 4.

Next, when a start switch (not shown) is pressed, when the document is set on the automatic document feeder 4, the document is automatically fed onto the contact glass 32. When the document is manually set on the contact glass 32, the scanner is immediately scanned. 3 operates, and the first traveling body 33 and the second traveling body 34 start traveling. Thereby, the light from the light source of the first traveling body 33 is emitted toward the document, the reflected light from the document surface is reflected by the mirror of the first traveling body 33 toward the second traveling body 34, and further the second A direction of 180 degrees is changed by a pair of mirrors of the traveling body 34, passes through the imaging lens 35, and enters the reading sensor 36 to read the contents of the document.
Further, when the start switch is pressed, the intermediate transfer belt 10 starts rotating, and at the same time, the photosensitive members 40Y, 40C, 40M, and 40K also start rotating, and yellow, cyan, magenta, Each black monochrome image is formed. Each single-color image formed on each photoconductor in this way is superimposed and sequentially transferred onto each intermediate transfer belt 62 on the intermediate transfer belt 10 that rotates in the clockwise direction in the drawing, and is a full-color composite color image. Is formed.

  On the other hand, the sheet feeding roller 42 of the selected sheet feeding stage in the sheet feeding table 2 rotates, and the sheet P is fed out from the selected sheet feeding cassette 44 in the paper bank 43 and separated into one sheet by the separation roller 45. And conveyed to the paper feed path 46. The fed sheet P is conveyed by the conveying roller 47 to the sheet feeding path 48 of the copying machine main body 1 and comes into contact with the registration roller 49 to be temporarily stopped. In the case of manual sheet feeding, the sheet P set on the manual sheet feeding tray 51 is fed out by the rotation of the sheet feeding roller 50, separated into one sheet by the separation roller 52, and conveyed to the manual sheet feeding path 53. It comes into contact with the registration roller 49 and temporarily stops.

In any case, the registration roller 49 starts to rotate at an accurate timing according to the color image on the intermediate transfer belt 10, and the sheet P that has been stopped is placed between the intermediate transfer belt 10 and the secondary transfer device 22. The color image is transferred onto the sheet P by the secondary transfer device 22 described above. The sheet P on which the color image has been transferred is conveyed to the fixing device 25 by the secondary transfer device 22 having a conveying function, and is heated and pressed to fix the transferred image, and then guided to the discharge side by the switching claw 55. Then, the paper is discharged onto the paper discharge tray 57 by the discharge roller 56 and stacked.
When the double-sided copy mode is selected, the sheet P on which the image is formed on the front surface is conveyed to the sheet reversing device 28 side by the switching claw 55, reversed and guided again to the transfer position, and the image is formed on the back surface. After being formed, the paper is discharged onto a paper discharge tray 57 by a discharge roller 56.
Further, when a black monochrome image is formed on the intermediate transfer belt 10, the driven rollers 15 and 16 other than the driving roller 9 are moved so that the yellow, cyan, and magenta photoreceptors 40 Y, 40 C, and 40 M are transferred to the intermediate transfer belt 10. It is trying to keep away from. In the so-called one-drum type image forming apparatus having only one photoconductor, instead of the tandem type shown in FIG. 2, black image formation is performed first in order to increase the first copy speed. Generally, the remaining colors are imaged only when the original is in color.

In such a configuration, the registration roller 49 is usually used while being grounded, but a bias can be applied to remove the paper dust from the sheet P. For example, when a bias is applied using a conductive rubber roller having a diameter of 18 mm and a surface coated with a conductive NBR rubber having a thickness of 1 mm, the volume resistance of the rubber material is about 10 ⁹ Ωcm, and the toner transfer side (surface Side) and a voltage of about +200 V is applied to the back side of the sheet. In general, in the intermediate transfer system, paper dust is difficult to move to the photoconductor, so there is little need to consider paper dust transfer, and even if it is grounded. Although a DC bias is generally applied as the applied voltage, an AC voltage having a DC offset component can be applied to charge the sheet more uniformly.
Since the surface of the sheet after passing through the registration roller to which the bias is applied in this way is slightly charged to the negative side, the transfer from the intermediate transfer belt to the sheet is compared with the case where no voltage is applied to the registration roller. As a result, the transfer conditions change and the transfer conditions may be changed.

  By the way, in the intermediate transfer type image forming apparatus as described above, discharge traces may appear on the screen and the image quality may be impaired. This trace of discharge is caused by various causes, one of which is caused by the peeling discharge that occurs when the photoreceptor and the intermediate transfer belt are peeled off at the primary transfer portion, as described above. It has been found from the results of repeated experiments by the inventor that the problem tends to occur when the potential balance between the front and back surfaces of the intermediate transfer belt is poor. Hereinafter, a specific configuration for adjusting the potential balance between the front and back of the intermediate transfer belt will be described for each of the examples.

FIG. 1 is a block diagram showing details of the primary transfer device 62 constituting the primary transfer means which is the primary transfer unit in FIG.
The primary transfer device 62 is stretched around the photosensitive member 40 rotating in the counterclockwise direction and a plurality of rollers ( the driving roller 9 and the driven rollers 15 and 16 shown in FIG. 5 ) and moves in the arrow X direction. The intermediate transfer belt 10 is inscribed in a freely rotatable manner following the back side of the intermediate transfer belt 10 and is biased upward in the drawing to press the surface side (upper side in FIG. 1) of the intermediate transfer belt 10 to the photoreceptor 40. A tension roller 5 that forms a nip portion N, a transfer roller 6 as a roller member that contacts the back surface side of the intermediate transfer belt 10 at the most downstream point A of the nip portion and rotates in a clockwise direction, and a bias to the transfer roller 6 It comprises a transfer bias power supply 7 for applying a voltage. The transfer roller 6 has an Asker C hardness of 50 or less.
Further, as shown in FIG. 1, on the downstream side of the transfer roller 6, the intermediate transfer belt 10 is pressed against a photoconductor 40 as an image carrier separately from the plurality of rollers that stretch the intermediate transfer belt 10. A tension roller 5 for forming the portion is provided.

  1 shows a case where a straight line La connecting the center C40 of the photoconductor 40 and the center C6 of the transfer roller 6 passes through the most downstream point N of the nip portion N. However, FIG. In order to examine the occurrence state of the discharge trace caused by the above-described peeling discharge when the position is changed, the main part in the state where the transfer roller 6 is changed from the position shown in FIG. 1 to the downstream side or the upstream side is enlarged. Show. In FIG. 2, a straight line La passing through the center C6a of the transfer roller 6a (same position as the transfer roller 6 in FIG. 1) and the center C40 of the photoreceptor 40 (see FIG. 1) passes through the surface of the intermediate transfer belt 10 (nip). Transfer having centers C6b and C6c on straight lines Lb and Lc (arbitrary thing is called straight line L) shifted by distances Db and Dc (arbitrary thing is called distance D) on the downstream side and upstream side from the final point A) Rollers 6b and 6c are also shown. Regarding the signs of the above distances Db and Dc, 0 is obtained when the straight line La passes through the nip portion most downstream point A, and minus is obtained when the center C6b of the transfer roller 6b is on the straight line Lb downstream thereof (Db). (−), And (Dc) when the center C6c of the transfer roller 6c is on the straight line Lc upstream thereof is defined as plus (+).

Further, in this embodiment, the state of occurrence of discharge trace when the surface resistivity of the back surface of the intermediate transfer belt 10 is changed is also investigated.
As the material of the intermediate transfer belt, polyimide is used. Carbon black is dispersed in a polyamic acid solution, the dispersion is poured into a metal drum, dried, and then the film peeled off from the metal drum is heated to a high temperature. A polyimide film was formed by stretching under, and cut into an appropriate size to produce an endless belt made of polyimide resin. According to a general method, film formation was performed by injecting a polymer solution in which carbon black was dispersed into a cylindrical mold, rotating the cylindrical mold while heating at 100 to 200 ° C., and forming a film by centrifugal molding. The film thus obtained was demolded in a semi-cured state and covered with an iron core, and the polyimide transfer reaction was allowed to proceed at 300 to 450 ° C. for curing to obtain an intermediate transfer belt. At this time, the amount of carbon, the firing temperature, the curing rate, and the like were changed to adjust the surface resistivity of the back surface of the belt to 1 × 10 ¹⁰ to 1 × 10 ¹² Ω / □. The resistance value was measured with a Hiresta UP (MCP-HT450) high resistivity meter (Mitsubishi Chemical) in an environment of a temperature of 23 ° C. and a humidity of 50%. -HTP14) and the applied voltage was 500V.

In FIG. 3, the values of the distances Db and Dc in FIG. 2 are changed from 0 to 2.5 (mm) in absolute value, and the surface resistivity of the back surface of the intermediate transfer belt 10 is 1 × 10 ^9.3 Ω / □. 1 shows the results of evaluation of the occurrence of discharge traces when image formation was carried out while changing from 1 to 10 × 10 ¹³ Ω / □. In the figure, ○: no problem, Δ: acceptable limit, x: unacceptable Each rank is shown.
Referring to FIG. 3, when the surface resistivity of the back surface of the intermediate transfer belt 10 is in a logarithmic range of 9.3 to 13.0, the trace of discharge can be allowed by setting the distance D to 2 or less in absolute value. It is confirmed that it can fall within the range.
In FIG. 3, when the surface resistivity of the back surface of the intermediate transfer belt is 9.3 logarithmically, no discharge trace is generated even if the distance D is 2.5 in absolute value. When the surface resistivity on the back surface of the transfer belt is lowered, the electric field generated when the primary transfer bias is applied is likely to act on the toner even outside the nip portion, and transfer dust may be deteriorated. On the other hand, even if the surface resistivity is too high, discharge may occur and the discharge trace may disturb the image.

FIG. 4 shows the results of actual image formation using five types of intermediate transfer belts having different surface resistivity on the back surface, and evaluation of transfer dust and discharge traces on the obtained image. In the figure, the transfer dust is evaluated with a rating of 1 to 5, and the level at which the image is satisfactory is ranked 4 or higher. Similarly to the case of FIG. 3, the change in the discharge trace was performed in three ranks: ◯: no problem, Δ: acceptable limit, x: unacceptable. In FIG. 4, the evaluation result of close-up dust is shown at positions above and below the plotted point, and the evaluation of the discharge trace is shown below the plotted point.
Referring to FIG. 4, by setting the surface resistivity of the back surface of the intermediate transfer belt to 1 × 10 ¹⁰ Ω / □ or more, transfer dust can be kept within a satisfactory level of the image, and 1 × 10 ¹² Ω / □. It can be seen that the discharge trace can be kept within the range without problems by the following. Therefore, by setting the surface resistivity of the back surface of the intermediate transfer belt to 1 × 10 ¹⁰ to 1 × 10 ¹² Ω / □, the transfer dust is kept within a level where the image can be satisfied, and the occurrence of discharge traces is within the range where there is no problem. It becomes possible to fit in.

In this embodiment, as described above, since the hardness of the transfer roller 6 is 50 or less in Asker C hardness, the transfer roller 6 is slightly deformed in a state where it is pressed against the intermediate transfer belt. The contact area is enlarged, and charges can be supplied to the intermediate transfer belt over a wide range. Therefore, the electric charge is reliably supplied to the most downstream point of the nip portion, and peeling discharge can be effectively suppressed. At the same time, since the contact area is increased, the local concentration of the pressure is alleviated and the pressure applied to the toner layer is also dispersed, so that the transfer omission due to the concentrated pressure is prevented.
Further, since the voltage applying member is a roller member, there is no possibility of hindering the movement of the intermediate transfer belt, and image disturbance can be prevented. Since the roller member is the transfer roller of the primary transfer means, the number of rollers can be reduced and the primary transfer device can be downsized. By providing the transfer roller on the downstream side of the nip portion, The transfer electric field in front of the portion can be suppressed to a small level, and it is possible to effectively prevent the toner from being moved by the transfer electric field in front of the nip portion to cause transfer dust and image deterioration.
Further, since the tension roller is disposed separately from the driving roller 9 and the driven rollers 15 and 16 for stretching the intermediate transfer belt 10 on the downstream side of the transfer roller, a nip portion is formed in advance by the tension roller. A transfer roller can be installed later, and the transfer roller can be pressed against the photoconductor with a stable pressure to prevent transfer unevenness, and the pressure contact pressure can be minimized. It is also possible to prevent the transfer from being lost.

1 is a configuration diagram illustrating a primary transfer portion of an embodiment of an image forming apparatus according to the present invention. FIG. 2 is an explanatory diagram showing the arrangement positions of the transfer roller shown in FIG. 1 by enlarging the main part of FIG. 1. FIG. 4 is an explanatory diagram showing the results of evaluating the discharge trace occurrence state at each placement position of the transfer roller shown in FIG. 2 for images formed on a plurality of intermediate transfer belts having different surface resistivity on the back surface. FIG. 2 is a diagram showing the relationship between the surface resistivity on the back surface of the intermediate transfer belt shown in FIG. 1 is an overall configuration diagram illustrating an example of an image forming apparatus that implements the present invention.

5: Tension roller 6: Transfer roller 7: Transfer bias power supply
10: Intermediate transfer belt 40: Photoconductor

Claims (2)

An image carrier on which a toner image corresponding to image information is formed, an endless intermediate transfer belt that is pressed against the image carrier at a nip portion and stretched around a plurality of rollers, and a toner image on the image carrier A full-color image using an intermediate transfer system, comprising: a primary transfer unit that transfers the toner image to the intermediate transfer belt; and a secondary transfer unit that collectively transfers a toner image transferred and held on the intermediate transfer belt to a recording medium. In the forming device,
A voltage applying member for supplying electric charge to the intermediate transfer belt is provided near the most downstream point of the nip portion;
The voltage application member is a transfer roller of the primary transfer unit, and is disposed so as to be in pressure contact with the image carrier via the intermediate transfer belt.
The intermediate transfer belt has a back surface resistivity of 10 ¹⁰ to 10 ¹² Ω / □,
In addition to the plurality of rollers that stretch the intermediate transfer belt, a tension roller that presses the intermediate transfer belt against the image carrier to form a nip portion is provided on the downstream side of the transfer roller.
An image forming apparatus, wherein a distance between a straight line passing through a center of the image carrier and a center of the transfer roller and a most downstream point of the nip portion is 2 mm or less.   The image forming apparatus according to claim 1, wherein the transfer roller has an Asker C hardness of 50 or less.

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