JP2004191870A - Image forming apparatus - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an image forming apparatus which has no separation fault and poor transferring phenomenon of a latter half part even when an OHP sheet is used and to provide the image forming apparatus in which separation and transferability are compatible even when either of the OHP sheet and plain paper is used. <P>SOLUTION: The image forming apparatus is provided with a transfer means (24) for transferring a toner image onto a transfer sheet and a separation means (22) installed at the downstream side of the transfer means and separation voltage is applied to the separation means during transportation of the transfer sheet between the separation means and an image carrier. Further, the image forming apparatus has a first control mode in which application of the separation voltage to the separation means is turned on at the time point when a transfer sheet tip passes through a separation region between the separation means and the image carrier by at least first prescribed length and is turned off at the time point when more than second prescribed length of a non-passed part remains and a second control mode in which application of the separation voltage is turned off after the non-passed part disappears and either can be chosen. <P>COPYRIGHT: (C)2004,JPO&NCIPI

Description

[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to an image forming apparatus such as a copying machine and a printer, and more particularly to an image forming apparatus in which an OHP sheet is used as a transfer sheet and has improved separation characteristics from an image carrier. More particularly, the present invention relates to a color image forming apparatus that forms a color toner image by superposing at least two or more monochromatic toner images on an image carrier.
[0002]
[Prior art]
In an electrostatic copying machine, a toner image formed on the surface of an image carrier such as a photoconductor or an intermediate transfer body is transferred to a transfer sheet, and then the transfer sheet is separated from the image carrier and transported to a fixing device. We are fixing. Generally, plain paper is used for the transfer sheet. When the photoreceptor is a small-diameter drum, so-called curvature separation can be performed by the rigidity (stiffness, stiffness of paper or the like) of the paper without providing any separation means. However, in the case of a photoreceptor drum or an intermediate transfer drum whose diameter is not reduced, separation by curvature separation is not perfect, and winding may occur due to poor separation.
[0003]
In particular, in a full-color image forming apparatus, each color toner image formed on the surface of the photoreceptor is sequentially transferred to the intermediate transfer drum, and the toner image for one transfer sheet is carried on the outer peripheral surface of the intermediate transfer drum. However, it is difficult to separate the transfer material due to the rigidity of the transfer material because the diameter of the intermediate transfer drum is inevitably increased. In many cases, a separating means for separating the transfer sheet from the image carrier is separately provided because it is necessary to separate the transfer sheet from the image carrier.
[0004]
For example, Patent Literature 1 below uses an AC corona separation method as a separation unit. Because the electrostatic attraction between the transfer sheet and the image carrier is very strong for the reason described above, if the application of the AC voltage is stopped when the first half of the paper is separated from the image carrier, the transfer is separated from the middle. Since a defect occurs, the AC voltage is continuously applied while the paper is passing.
[0005]
On the other hand, an OHP sheet may be used as a transfer sheet for creating presentation materials. When an OHP sheet is used in an image forming apparatus provided with a separating unit, a transfer failure phenomenon such as transfer unevenness or image density failure occurs at the rear end of the OHP sheet. As a countermeasure against such a transfer failure, for example, Japanese Patent Application Laid-Open No. H11-163,873 discloses a control unit that determines whether a transfer material P is plain paper or an OHP sheet, and controls a bias value of a transfer roller according to a type of the transfer material. Is disclosed.
[0006]
[Patent Document 1]
Japanese Patent Application Laid-Open No. 5-129555 [Patent Document 2]
Japanese Patent Application Laid-Open No. 8-123221
[Problems to be solved by the invention]
However, as disclosed in Patent Document 2, even in the case of using a method of controlling the bias value of the transfer roller by determining the type of the transfer material (transfer sheet), the transfer in the latter half of the transfer material is performed. There has been a problem that the adhesion between the material and the image carrier is reduced, and the transferability is deteriorated. That is, when the OHP sheet is used as the transfer material, the rigidity of the OHP sheet is higher than that of the plain paper, so that the suction force between the OHP sheet and the image carrier tends to decrease as the rear end approaches. Regardless, since the voltage applied to the separation charger remains high, the separation is rapidly performed at the rear end of the OHP sheet, so that the adhesion to the image carrier becomes insufficient and the transferability of the toner deteriorates. It is considered.
[0008]
The inventor of the present application has conducted various studies based on the above-mentioned considerations in order to solve the above-mentioned problems. As a result, before the rear end of the OHP sheet reaches the separation charger, that is, the portion of the OHP sheet that has not passed through the separation charger section. It has been found that the transferability of the toner can be improved by turning off the voltage applied to the separation charger at a point in time when a predetermined length or more remains, thereby improving the adhesion to the OHP sheet image carrier, and completing the present invention. It has been reached.
[0009]
That is, an object of the present invention is to solve the above-described problems, and to provide an image forming apparatus free from separation failure and transfer failure in the rear half even when an OHP sheet is used. It is a second object of the present invention to provide an image forming apparatus in which the separation property and the transfer property are compatible both in the case of an OHP sheet and plain paper.
[0010]
[Means for Solving the Problems]
The above object of the present invention can be achieved by the following configurations. That is, the invention according to claim 1 of the present application provides a transfer unit that transfers a toner image formed on the surface of an image carrier to a transfer sheet, and a transfer unit that separates the transfer sheet onto which the toner has been transferred from the image carrier. Separation means installed downstream of the means,
An image forming apparatus that applies a separation voltage to the separation unit while transporting the transfer sheet between the separation unit and the image carrier,
The application of the separation voltage to the separation unit is turned on when the leading edge of the transfer sheet passes through the separation region between the separation unit and the image carrier for a first predetermined length or more, and the unpassed portion remains for a second predetermined length or more. A first control mode that is turned off at the point in time, and a second control mode that is turned off after the non-passed portion has disappeared,
It is characterized in that one of the first control mode and the second control mode can be selected.
[0011]
According to a second aspect of the present invention, in the image forming apparatus according to the first aspect, the first predetermined length is 5 mm and the second predetermined length is 35 mm. According to a third aspect, in the image forming apparatus according to the first aspect, the first control mode 1 is selected when the transfer sheet is an OHP sheet, and the first control mode is selected when the transfer sheet is plain paper. The second control mode 2 is selected.
[0012]
According to this configuration, in the case of plain paper, the separation voltage is continuously applied from the leading end to the trailing end, so that a separation failure can be prevented. Further, in the case of an OHP sheet, the separation voltage is applied at least at the time when the leading end of the OHP sheet passes through the separation area of 5 mm. , The application of the separation voltage is turned off, so that transfer failure due to abrupt separation of the rear end is prevented, and a good transfer image is transferred onto the OHP sheet.
[0013]
According to a fourth aspect of the present invention, there is provided the image forming apparatus according to any one of the first to third aspects, wherein the image bearing member has a plurality of color toner images formed on a surface of a photoreceptor. Are successively transferred to form an intermediate transfer drum that carries a toner image for one transfer sheet on the surface thereof. Further, the invention according to claim 5 of the present application is characterized in that the image according to claim 4 is In the forming apparatus, the intermediate transfer drum has an outer diameter of 134 mm or more.
[0014]
According to such a configuration, a high transfer current is applied to the transfer sheet in order to transfer a toner image in which a plurality of colors overlap, so that the adhesion between the transfer sheet and the image carrier is high, and the outer diameter of the intermediate transfer drum is large. Even in a color image forming apparatus in which curvature separation can hardly be expected, the OHP sheet is completely separated from the image carrier, and a transfer failure phenomenon at the rear end is prevented.
[0015]
BEST MODE FOR CARRYING OUT THE INVENTION
Hereinafter, embodiments of the present invention will be illustratively described in detail with reference to the drawings.
As will be described later, a secondary transfer roller 24 is used as a transfer unit for transferring the toner image on the surface of the intermediate transfer drum to the transfer sheet, and a separation charger 22 is used as a separation unit for separating the transfer sheet on which the toner has been transferred from the intermediate transfer drum. are doing.
[0016]
FIG. 1 is a schematic sectional view of an image forming apparatus showing an embodiment of the present invention. In FIG. 1, an image forming apparatus 1 includes a photoconductor 11, an exposure unit 13, a charger 12, a cleaning unit 19, developing units 15 to 18 disposed around the photoconductor 11, and a drum-shaped intermediate transfer body 20 (hereinafter referred to as intermediate transfer). And a secondary transfer roller 24, a separation charger 22, a cleaning unit 23, a sheet cassette 3, a pair of registration rollers 4, a transport belt 5, a vertical transport path 7, a discharge tray 8, and the like. , A fixing device 6 disposed between the conveyance belt 5 and the vertical conveyance path 7, a secondary transfer roller 24, and a die gel heater 30 installed below the separation charger 22. Are arranged in the housing 2 in the following positional relationship.
[0017]
The photoconductor 11 is a drum-shaped photoconductor having a photoconductor made of amorphous silicon (a-Si) (hereinafter, referred to as a photoconductor drum). A unit 13, developing units 15, 16, 17, and 18 having developing sleeves 15a, 16a, 17a, and 18a, an intermediate transfer drum 20, and a cleaning unit 19 are arranged. The developing sleeves 15a, 16a, 17a, and 18a include a carrier and Is a mixture of 5 parts by weight of black, yellow, magenta, and cyan toners having a positive triboelectric charge polarity and a volume average particle diameter of 8 μm and 100 parts by weight of a ferrite carrier having a weight average particle diameter of 70 μm. And the developing sleeves 15a, 16a, 17a, and 18a, the intermediate transfer drum 20, and the photosensitive drum 11. Each rotates in the direction of arrow A in the figure to form an image.
[0018]
The charging unit 12 uniformly charges the amorphous silicon (a-Si) photosensitive member of the photosensitive drum 11 to about +400 [V], and irradiates the exposure unit 13 with light 14 corresponding to image data. 11 is performed to form an electrostatic latent image. At this time, the surface potential of the a-Si photosensitive member after exposure is about +10 [V]. The intermediate transfer drum 20 has a three-layer structure of an aluminum tube as an inner layer, a conductive rubber layer as an intermediate layer, and a fluororesin layer as a surface layer, and has an outer diameter of 150 mm. The conductive rubber layer is a urethane rubber layer having a thickness of 8 mm, and the volume resistivity is adjusted to 3 × 10 7 Ω · cm by dispersing carbon. The surface fluororesin layer is PTFE having a thickness of 100 μm, and the volume resistivity is adjusted to 8 × 10 9 Ω · cm by dispersing carbon.
[0019]
An alternating electric field (Vdc = 160 [V], Vpp = 1.0 [kV], f = 3.6 [kHz]) acts between the photoconductor drum 11 and the developing sleeve 15a to expose the photoconductor. Develop the toner image on the part. Next, a bias of +350 V is applied to the tube of the photosensitive drum 11 to generate a potential difference between the drum and the intermediate transfer drum 20 that is grounded through the aluminum tube of the inner layer. Is transferred to the intermediate transfer drum 20. Similarly, the toner image by the developing device 16 is transferred to the intermediate transfer drum 20, and the developing device 17 and the developing device 18 are repeatedly transferred to the intermediate transfer drum 20, and then transferred to the paper P, so that the color toner is transferred. Form an image. Each time the transfer of the toner of each color to the intermediate transfer drum 20 is completed, the untransferred residual toner on the surface of the photosensitive drum 11 is removed by the cleaning unit 19 and the photosensitive drum 11 returns to a clean state.
[0020]
As described above, as the toner image is formed on the surface of the intermediate transfer drum 20 and the leading end thereof approaches the transfer position, the paper P stored in the paper feed cassette 3 is measured at 134 mm / sec. The paper is fed along the path indicated by the arrow B in the middle, and passes through the transfer area between the secondary transfer roller 24 and the intermediate transfer drum 20 with the timing adjusted by the registration roller pair 4. The secondary transfer roller 24 is applied with a negative transfer voltage, and is controlled by a constant current method so that a transfer current of 20 μA in the color mode and 15 μA in the monochrome mode flows in the sheet direction of the paper P.
[0021]
At that time, the sheet P is transferred the toner image on the surface of the intermediate transfer drum 20 and is separated into AC by the separation charger 22. The separation charger 22 is a shield case having an opening on the side of the intermediate transfer drum 20 and provided with two charger wires having an interval of 10 mm. This charger wire has a peak-to-peak DC voltage of + 400V. An AC voltage of 12.5 kV is superimposed and applied.
[0022]
The separated sheet P is transported on the transport belt 5 and reaches the fixing unit 6. The sheet P on which the toner image has been fixed by passing through the fixing unit 6 is discharged to a sheet discharge tray 8 via a vertical conveyance path 7. After the toner image is transferred to the sheet P, the untransferred toner is removed from the intermediate transfer drum 20 by the cleaning unit 23, and the intermediate transfer drum 20 returns to a clean state. The cleaning units 19 and 23 are composed of a cleaning blade or a fur brush or the like.
[0023]
Below the separation charger 22 and the secondary transfer roller 24, a planar diegel heater 30 is mounted in the axial direction of the photoconductor drum so as to have substantially the same length as the photoconductor drum. The die gel heater 30 is for preventing an image flow phenomenon due to moisture adsorption to the intermediate transfer drum 20 and the photosensitive drum 11, and has a rated power of 10 W and a surface temperature of about 70 ° C.
[0024]
Next, with reference to the timing chart of FIG. 2, the energization control (first control mode) of the separation unit and the transfer unit according to the present invention when the A4 size OHP sheet is used will be described in detail. 2A and 2B are timing charts showing energization control of each unit in the image forming apparatus according to the present invention when an OHP sheet is used, wherein FIG. 2A shows a registration clutch, FIG. 2B shows a separation charger 22, and FIG. The secondary transfer roller 24, (d) is a diagram showing the respective energization waveforms of the lifting clutch of the secondary transfer roller 24.
[0025]
The transfer area described below is a line portion where a plane including the center axis of the intermediate transfer drum 20 and the center axis of the secondary transfer roller intersects with the nip between the intermediate transfer drum 20 and the secondary transfer roller. The separation region is a line portion where a plane including the central axis of the image carrier and the charger line intersects the transfer sheet. Although not shown in the present embodiment, it has two charger wires. In this case, an intermediate position between the charger lines is set as a reference charger line position that defines the separation region. In the present embodiment, the distance between the transfer area and the separation area is 25 mm.
[0026]
When a registration sensor (not shown) detects the leading end of the A4-size OHP sheet fed from the sheet cassette 3, the rotation of the registration roller pair 4 is stopped, and the OHP sheet temporarily stops immediately before the registration roller pair 4. . When the toner image on the surface of the intermediate transfer drum 20 approaches the secondary transfer roller 24, a registration clutch (not shown) is turned on, the rotation of the registration roller pair 4 is resumed, and the OHP sheet starts to be conveyed. .
[0027]
Based on the ON signal of the registration clutch, the registration roller pair 4 starts to convey the OHP sheet, and after 100 msec, the secondary transfer lifting / lowering clutch operates to bring the secondary transfer roller 24 into contact with the intermediate transfer drum 20, and after 320 msec Starts applying a predetermined high voltage to the separation charger 22 and the transfer roller 24. Immediately after 321.6 msec, the leading end of the OHP sheet passes through the transfer area.
[0028]
As described above, since the transfer speed of the transfer sheet is 134 mm / sec and the distance between the transfer area and the separation area is 25 mm, the time required for the leading edge of the OHP sheet to pass between them is 25 mm / 134 mm / sec × 1000. = 186 msec. Therefore, the leading edge of the OHP sheet passes through the separation area after (321.6 + 186.6) = 508.2 msec. That is, the leading end of the OHP sheet passes through the transfer area after 321.6 mm / sec, and passes through the separation area after 508.2 msec.
[0029]
Since the length of the A4 landscape size in the paper passing direction (transport direction) is 210 mm, the time required for the rear end to reach the leading end position is 210/134 × 1000 = based on the paper passing speed of 134 mm / sec. 1567.2 msec. Therefore, based on the ON signal of the registration clutch, the rear end of the OHP sheet passes through the transfer area after 1567.2 + 321.6 = 1888.8 msec, and further the rear end of the OHP sheet after 1567.2 + 508.2 = 2075.4 msec. Pass through the separation region.
[0030]
Since the energization time of the high voltage to the separation charger is 1415 msec, the voltage off timing of the separation charger is after 1735 msec based on the ON signal of the registration clutch. Since the energization time of the high voltage to the secondary transfer roller is 1557 msec, the energization off timing to the secondary transfer roller based on the ON signal of the registration clutch is after 2077 msec. At the same time as the control of turning off the power supply to the secondary transfer roller, the secondary transfer lifting / lowering clutch operates to separate the secondary transfer roller from the intermediate transfer drum.
[0031]
According to the first control mode described above, when the leading end of the OHP sheet reaches the separation area, the power supply to the separation charger is turned on, and 134 mm / sec × 342 msec / 1000 msec = 45. The rear end of the OHP sheet is separated by 8 mm, and no charger voltage is applied. In other words, the separation charger is turned off when the OHP sheet is passed by a distance of 210 mm−45.8 mm = 164.2 mm from the leading end of the OHP sheet. The OHP sheet has only the A4 horizontal size, and the off timing of the separation and the secondary transfer is also controlled by the timing chart starting from the on signal of the registration clutch.
[0032]
On the other hand, since plain paper uses each size, the energization off control of separation and secondary transfer is not controlled by the timing chart starting from the ON signal of the registration clutch, but by passing through the rear end. The time is controlled by the registration sensor.
[0033]
With reference to the timing chart of FIG. 3, the control of energization (second control mode) to the separation unit and the transfer unit of the present invention when A4 size plain paper is used will be described in detail. FIGS. 3A and 3B are timing charts showing the energization control of each unit in the image forming apparatus according to the present invention when plain paper is used, wherein FIG. 3A shows a registration clutch, FIG. 3B shows a separation charger 22, and FIG. FIG. 7D is a diagram illustrating a current-carrying waveform of the secondary transfer roller 24, and FIG.
[0034]
When a registration sensor (not shown) detects the leading end of the A4-size plain paper sheet fed from the paper feed cassette 3, the rotation of the registration roller pair 4 is stopped, and the plain paper sheet is stopped immediately before the registration roller pair 4. Stop. As the toner image on the surface of the intermediate transfer drum 20 approaches the secondary transfer roller 24, a registration clutch (not shown) is turned on, the rotation of the registration roller pair 4 is restarted, and the conveyance of the plain paper sheet is started. You.
[0035]
On the basis of the ON signal of the registration clutch, the secondary transfer lifting / lowering clutch is operated 103 msec later to bring the secondary transfer roller into contact with the intermediate transfer drum 20. After 320 msec, application of a predetermined high voltage to the separation charger 22 and the transfer roller 24 is started. The leading end of the plain paper sheet passes through the transfer area after 321.6 msec. As described above, since the transfer speed of the transfer sheet is 134 mm / sec and the distance between the transfer area and the separation area is 25 mm, the time required for the leading end of the plain paper sheet to pass between them is 25 mm / 134 mm / sec × 1000 = 186 msec.
[0036]
Therefore, the leading edge of the plain paper sheet passes through the separation area after (321.6 + 186.6) = 508.2 msec. That is, the leading end of the plain paper sheet passes through the transfer area after 321.6 msec, and passes through the separation area after 508.2 msec.
[0037]
Since the length of the A4 landscape size in the paper passing direction is 210 mm, the time required for the rear end to reach the leading end position is 210/134 × 1000 = 1567.2 msec based on the paper passing speed of 134 mm / sec. . Therefore, based on the ON signal of the registration clutch, the rear end of the OHP sheet passes through the transfer area after 1567.2 + 321.6 = 1888.8 msec, and further, after 1567.2 + 508.2 = 2075.4 msec, after the plain paper sheet The edge passes through the separation area.
[0038]
110 msec after the registration sensor detects the passage of the rear end of the plain paper sheet, the registration clutch is turned off, and after 462 msec, the power to the secondary transfer roller and the separation charger is turned off. The trailing edge of the plain paper passes through the transfer area after 273.8 msec, and the power to the transfer roller is turned off thereafter. After passing through the separation region after 460 msec, the separation charger is turned off after that.
[0039]
According to the above-described second control mode, when the leading end of the plain paper reaches the separation area, the power supply to the separation charger is turned on and turned off after the rear end of the plain paper passes through the separation charger. . That is, in the second control mode, power is continuously supplied to the separation charger while the entire length of the plain paper passes through the separation region.
[0040]
The first control mode and the second control mode can be switched and selected between a case where the transfer sheet to be used is plain paper and a case where the transfer sheet is an OHP sheet. Alternatively, the transfer material can be determined and automatically switched and selected, or the switching material can be manually switched and selected when a transfer sheet is set.
[0041]
In the first control mode, in the case of an OHP sheet, various examinations were performed based on the dimensions of the respective mechanisms, and as a result, a separation voltage was applied at least when the leading end of the OHP sheet passed the separation region of 5 mm. It is preferable to turn off the application of the separation voltage when the passing portion has a predetermined length, for example, 35 mm or more, so that the separation failure at the front end is prevented while the transfer failure due to the rapid separation at the rear end is prevented. A good transfer image can be transferred onto the OHP sheet. Further, according to the second control mode, in the case of plain paper, the separation voltage is continuously applied from the leading end to the trailing end, so that separation failure can be prevented.
[0042]
【The invention's effect】
As described above in detail, according to the image forming apparatus of the present invention, in the case of plain paper, the separation voltage is continuously applied from the leading end to the trailing end, so that separation failure can be prevented. Further, in the case of the OHP sheet, the separation voltage is applied at least at the time when the leading end of the OHP sheet passes through the separation region, so that the separation failure can be prevented. Since the application is turned off, transfer failure due to abrupt separation of the rear end can be prevented, and a good transfer image can be transferred onto the OHP sheet.
[Brief description of the drawings]
FIG. 1 is a cross-sectional view illustrating a schematic configuration of an image forming apparatus according to an embodiment of the present invention.
FIGS. 2A and 2B are timing charts showing energization control of each unit when an OHP sheet is used, wherein FIG. 2A is a registration clutch, FIG. 2B is a separation charger 22, FIG. 2C is a secondary transfer roller 24, and FIG. FIG. 6 is a diagram showing respective energization waveforms of the lifting clutch of the secondary transfer roller 24.
3A and 3B are timing charts showing energization control of each unit when plain paper is used, wherein FIG. 3A shows a registration clutch, FIG. 3B shows a separation charger 22, FIG. 3C shows a secondary transfer roller 24, and FIG. FIG. 4 is a diagram illustrating a lifting / lowering clutch of the secondary transfer roller 24, and FIG.
[Explanation of symbols]
REFERENCE SIGNS LIST 1 image forming apparatus 2 housing 3 paper feed cassette 4 registration roller pair 5 transport belt 6 fixing unit 7 vertical transport path 8 Paper discharge tray 11 Photoconductor 12 Charger 13 Exposure unit 14 Exposure light 15, 16, 17, 18 Developing devices 15a, 16a, 17a, 18a Developing Sleeve 19: Cleaning unit 20: Intermediate transfer body 22: Separation charger 23: Cleaning unit 24: Transfer roller 30: Digel heater

Claims (5)

A transfer unit for transferring the toner image formed on the surface of the image carrier to the transfer sheet; and a separation unit provided downstream of the transfer unit for separating the transfer sheet on which the toner has been transferred from the image carrier. ,
An image forming apparatus that applies a separation voltage to the separation unit while transporting the transfer sheet between the separation unit and the image carrier,
The application of the separation voltage to the separation unit is turned on when the leading edge of the transfer sheet has passed at least the first predetermined length through the separation region between the separation unit and the image carrier, and the unpassed portion remains for the second predetermined length or more. A first control mode that is turned off at the point in time, and a second control mode that is turned off after the non-passed portion has disappeared,
An image forming apparatus, wherein either one of the first control mode and the second control mode can be selected. The image forming apparatus according to claim 1, wherein the first predetermined length is 5 mm, and the second predetermined length is 35 mm. The control mode 1 is selected when the transfer sheet is an OHP sheet, and the control mode 2 is selected when the transfer sheet is plain paper. 3. The image forming apparatus according to 2. 2. An image forming apparatus according to claim 1, wherein the image carrier is an intermediate transfer drum that successively transfers toner images of a plurality of colors formed on the surface of the photosensitive member, thereby carrying a toner image of one transfer sheet on the surface. The image forming apparatus according to claim 3. The image forming apparatus according to claim 4, wherein the intermediate transfer drum has an outer diameter of 134 mm or more.

Images