JP2004045826A - Image forming apparatus - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an image forming apparatus capable of retaining the degradation of image quality to the minimum and also capable of providing the satisfactory durability of a fixing belt. <P>SOLUTION: In the image forming apparatus which is provided with a fixing device for developing an unfixed toner image on a recording material P by passing the recording material P carrying the unfixed toner image through the nip formed between a fixing belt 24 on whose surface a high resistive release layer is coated and a roller, a means for applying a bias voltage to the fixing belt 24 is provided and the bias voltage to be applied by the bias applying means is changed according to an image processing method for forming an image. <P>COPYRIGHT: (C)2004,JPO

Description

[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to an image forming apparatus including a fixing device having a fixing belt.
[0002]
[Prior art]
2. Description of the Related Art In an image forming apparatus using toner for forming a visible image, a fixing device is provided for fixing a toner image as a permanent image on a recording material such as transfer paper. In such a fixing device, a recording material carrying an unfixed toner image is passed through a nip formed between a roller and an endless belt, and the unfixed toner image is applied to the recording material by the action of heat, pressure, or the like. A belt-fixing type device for fixing is also well known. In a fixing device using a belt of this type, since the fixing nip width can be widened even if the pressure roller or the like is a small-diameter roller, the device itself can be reduced in size, and is suitable for a full-color image forming apparatus that carries a large amount of toner. ing.
[0003]
However, although the belt fixing device can have a large fixing nip width, a problem has occurred that has not been generated in a normal roller-to-roller fixing device as described below. In the fixing device, the fixing member and the pressing member are charged by frictional charging and peeling charging between the fixing member and the pressing member and the transfer paper by passing the paper. The graph in FIG. 3 shows the potential of the fixing belt measured immediately before the fixing nip when one sheet of A3 size is printed. In FIG. 3, the transfer paper passes through the fixing nip, and the fixing belt (roller) ), A peeling discharge is generated, and the surface of the fixing belt (roller) is charged. When white paper is passed, the silicon surface is negatively charged, but when the rear end of the transfer paper is peeled off, as shown by A, it is positively charged. The charge amount increases as the linear speed increases, and also increases in a full-color fixing machine using silicon rubber on the surface of the fixing belt. Further, in the belt fixing device, since the nip width is wider than that of the conventional roller system, the charging potential tends to increase.
[0004]
Since the positive charge has a higher potential than other portions, it does not attenuate between sheets, and the next transfer sheet is fixed in a state where it remains on the fixing belt as horizontal charge position unevenness. At this time, the horizontal band plus potential on the belt disturbs the next image. In particular, this is remarkable when the next transfer paper is a halftone, and the dots are disturbed and occur as “horizontal black bands” in the fixing belt cycle (hereinafter, referred to as “fixing horizontal black bands”). This abnormal image is particularly remarkable in the case of a belt fixing type in which a preheat portion is provided before the fixing nip, and hardly occurs in a configuration close to conventional roller fixing. This is because the unfixed image is affected by the potential on the fixing belt in a state where sufficient pressure is not applied in the preheating portion, whereas the pressure is applied by the fixing nip as soon as the unfixed image approaches the belt in the roller type. Therefore, it is considered that the toner image does not move.
[0005]
In order to solve the black belt, it is necessary to remove the positive potential of the trace of the discharge of the transfer sheet on the fixing belt. However, unlike the Teflon (trade name) roller of the monochrome fixing device, in the case of silicon rubber used for color fixing, the potential on the belt cannot be removed with a charge removing brush. Therefore, the fixing horizontal black belt cannot be solved by a method in which a static elimination brush is brought into contact with a non-image surface of a Teflon roller and static elimination is performed on the roller surface.
[0006]
Further, in order to solve the problem of the fixing horizontal black belt, there is a method of making the material of the belt surface conductive. This is mainly made by dispersing a conductive substance such as carbon in silicone rubber, but has a large side effect such as hot offset because the releasability from the toner is impaired. As a method of applying a bias to the belt surface, there is a method using corona discharge. However, ozone is generated by the discharge, and there are many problems such as deterioration of rubber and necessity of an ozone filter.
[0007]
Incidentally, a method of applying a bias to a brush or a blade in contact with a fixing belt and a method of applying a bias to a roller in contact with the surface of a fixing belt are disclosed in JP-A-10-221983, JP-A-2000-305388, and the like. Have been. These methods can apply a bias uniformly on the belt, and are effective for a fixed horizontal black belt.
[0008]
[Problems to be solved by the invention]
However, in the technology described in the above publication, a high voltage bias (about 2 to 3 kV) is required to cancel the potential after the peeling discharge by applying a bias to the electrically high-resistance belt surface, and the high belt surface height is required. If the resistance layer has a flaw or a pinhole, a leak occurs. Initially existing scratches and pinholes can be removed by sorting when assembling the unit, but it is difficult to extract those that exist as bubbles inside the belt surface layer by visual inspection and leak inspection It is. When a high-voltage bias is continuously applied to such a belt, a dielectric breakdown occurs at a portion where the thickness of the surface layer is thin due to bubbles over time, and a leak occurs. Initially, a small leak is caused, but by repeating the leak, physical destruction of the surface layer and eventually destruction of the belt base material are started. That is, continuing to apply the high-voltage bias to the fixing belt is nothing but shortening the service life.
[0009]
An object of the present invention is to provide an image forming apparatus capable of minimizing deterioration of image quality and obtaining good durability of a fixing belt in view of the above-described conventional problems.
[0010]
[Means for Solving the Problems]
In order to solve the above-described problems, the present invention provides a recording medium carrying an unfixed toner image through a nip formed between a fixing belt and a roller coated with a high-resistance release layer. In an image forming apparatus provided with a fixing device for fixing a toner image to the recording material, means for applying a bias voltage to the fixing belt is provided, and the bias voltage applied by the bias applying means is applied to an image processing method during image formation. It is characterized in that it is changed according to.
[0011]
In addition, according to the present invention, when the image processing method at the time of image formation forms a halftone with independent dots, the bias voltage applied to the fixing belt is set to be high, and the image processing method at the time of image formation takes a large amount of halftone. When forming an image in a line pattern, it is effective to lower or turn off the bias voltage applied to the fixing belt.
[0012]
BEST MODE FOR CARRYING OUT THE INVENTION
Hereinafter, embodiments of the present invention will be described with reference to the drawings.
FIG. 1 is a configuration diagram schematically showing an image forming apparatus according to the present invention.
[0013]
The image forming apparatus of this embodiment is a tandem type full-color printer in which four image forming units 2 are arranged along a belt running side of a belt transfer device 1, and the image forming units 2 are yellow Y, magenta M, It is configured to form four color toner images of cyan C and black K. Each of the image forming units 2Y, 2M, 2C and 2K has a photosensitive drum 3 as an image carrier, and when the photosensitive drum 3 is rotated in a counterclockwise direction by a driving device (not shown), charging means is provided. The portion uniformly charged by the charging roller 4 is exposed to laser light 5 corresponding to the color image information for each unit. The electrostatic latent image formed by this exposure is developed by the developing unit 6 to become a toner image, and arrives at a position facing the belt transfer device 1.
[0014]
On the other hand, the transfer material P is fed from the paper supply unit 10, and the fed transfer material P is conveyed onto the belt transfer device 1 by the registration roller 13 in synchronization with the toner image, and is operated by the transfer roller 7. The toner image is transferred.
[0015]
In the case of full-color printing, the Y toner image formed by the first image forming unit 2Y is transferred to the transfer material P, and a latent image of M component color is formed by the second image forming unit 2M during the transfer. When the M toner image is obtained with the M toner by the means 6, the M toner image is transferred onto the transfer material P, which has been transferred by the first image forming unit 2Y, and is superimposed on the Y toner image. Hereinafter, image formation is performed on the C toner image and the K toner image in the same manner, and four color toner images are superimposed on the transfer material P. Then, the transfer material P on which the four color toner images are superimposed is separated from the belt transfer device 1 and conveyed to the fixing device 20. Note that the residual toner is removed from each of the photoconductors on which the transfer has been completed by the cleaning unit 8, and the photoconductors are prepared for the next image formation to be performed subsequently.
[0016]
As shown in FIG. 2, the fixing device 20 includes a fixing belt 24 wound around a fixing roller 21, a heating roller 22 and an auxiliary roller 23 as a belt support, and a fixing belt 24 around the fixing roller 21 and the auxiliary roller 23. The pressure roller 25 has a pressure roller 25 that is disposed to face through the pressure roller 25. The fixing roller 21 is covered with a silicon sponge having a diameter of 30 mm and a thickness of 6 mm on the surface. The heating roller 22 is formed of an aluminum pipe having a diameter of 30 mm and a thickness of 1 mm, and includes a heater 26 as heating means. The auxiliary roller 23 is a stainless roller having a diameter of 8 mm. The fixing belt 24 has a circumference of 60 mm, and the surface of a polyimide belt having a thickness of 90 μm is coated with silicon rubber having a thickness of 150 μm. The pressure roller 25 is coated with silicon rubber having a diameter of 40 mm and a thickness of 1.5 mm, and a heater 27 as a heating means is provided inside. Reference numeral 28 denotes a thermistor used for controlling the temperature of the heater 26, and reference numeral 29 denotes a thermistor used for controlling the temperature of the pressurizing heater 27.
[0017]
In the fixing device 20, the pressure roller 25 is driven to rotate clockwise in FIG. 2 by a driving unit (not shown), and the fixing roller 21 is driven to rotate by the pressure roller 25. The driven rotation of the fixing roller 21 causes the fixing belt 24 to rotate and run counterclockwise. The transfer material P carrying the untoner-fixed toner image enters the fixing nip formed between the auxiliary roller 23 facing the pressure roller 25 and the fixing roller 21 such that the toner image contacts the fixing belt 24. Then, the transfer material P that has been fixed is guided by a discharge guide (not shown), and is discharged out of the apparatus via a discharge roller 40.
[0018]
The fixing belt 24 is provided with a cleaning roller 30 for cleaning the surface at a position immediately after passing over the fixing nip, and the cleaning roller 30 is provided with a scraper 31 for removing dirt attached to the surface. The fixing belt 24 is provided with an oil application roller 32 following the cleaning roller 30 to uniformly apply silicone oil as a release agent to the surface of the fixing belt. The surface of the oil application roller 31 is coated with silicon rubber having a medium resistance, for example, 10 ^{8 to 9} Ω.
[0019]
By the way, in the fixing device using the fixing belt 24 as in the present embodiment, the transfer material P is separated from the fixing belt 24 and separation discharge occurs, and the surface of the fixing belt is positively charged. Since the positive charging generates a fixing horizontal black band and deteriorates image quality, applying a high-voltage bias (−2 to −3 kV) to the fixing belt 24 is effective in preventing the fixing horizontal black band.
[0020]
In the fixing device 20 of the present embodiment, a bias is applied via the oil application roller 32 described above. However, as described above, there is a problem that the life of the fixing belt 24 is shortened when a high-voltage bias is applied.
[0021]
Therefore, the present inventor has paid attention to the phenomenon of the above-mentioned fixed horizontal black band. Since this fixed black horizontal band appears as a density difference due to scattering of dots forming a halftone, it has been found that the degree of dot dust differs depending on an image processing method when forming a halftone. More specifically, if the halftone is formed by independent dots of 2 × 2 or 4 × 4, the fixed horizontal black band shows dust spread in all directions as a density difference as shown in FIG. 4B. This leads to a decrease in image quality. On the other hand, when the halftone is formed by a line pattern formed by vertically long thin lines, as shown by C in FIG. 5, the one in the vertical direction overlaps the vertically long pattern, so that only the one in the horizontal direction has a density difference. Appears as. Therefore, in the image processing method for forming a halftone in a line pattern, the influence of the fixed horizontal black band is small.
[0022]
Therefore, in the present invention, the bias applied to the fixing belt 24 is switched between -2.5 kV and -1.5 kV according to the image processing method. That is, a bias of -2.5 kV is applied in the image quality mode in which the halftone is formed by the independent dots, and a bias of -1.5 kV is applied in the image quality mode in which the halftone is formed by the line pattern. Make up. For the switching of the applied bias, a method of providing a power supply of -2.5 kV and -1.5 kV to switch the applied power supply itself, a method of controlling and changing the magnitude of the applied bias, and the like can be applied. Further, in the image quality mode in which the line pattern is formed, the bias may not be applied. In this case, the above switching is not required, and the on / off control of the applied bias is sufficient.
[0023]
By the way, a color image forming apparatus having an image quality mode in which a halftone is formed by independent dots and an image quality mode in which a halftone is formed by a line pattern is often found in an apparatus having a high image quality mode and a normal mode. At this time, in the high image quality mode, the halftone is formed by independent dots. Therefore, when the high quality mode is selected at the start of printing, a normal bias is applied to the fixing belt 24 to prevent the fixing horizontal black band. To obtain high quality prints. Further, in the normal mode, halftones are formed in a line pattern. Therefore, when the normal mode is selected at the start of printing, the bias applied to the fixing belt 24 is reduced or not applied, so that the fixing belt has a long service life. Is obtained. In addition, although a fixed horizontal black band occurs at this time, the influence of the fixed horizontal black band on the line pattern is small, so that the image quality is slightly reduced.
[0024]
As described above, the preferred embodiments of the present invention have been described, but the present invention is not limited to the above embodiments, and various modifications can be made.
For example, in the above-described embodiment, the fixing belt 25 is wound around three rollers, but the number of belt support members as rollers may be two, or four or more.
[0025]
Further, if the belt support is provided with one drive roller, the other belt support may be a fixed member having at least a belt sliding surface curved.
Furthermore, in the above-described embodiment, the bias is applied to the fixing belt 24 via the oil application roller 32, but the bias may be applied to the fixing belt using a brush, a blade, or the like.
[0026]
【The invention's effect】
According to the configuration of the first aspect, the bias voltage applied by the image processing method at the time of image formation is changed, so that the durability of the fixing belt and good image quality can be achieved at the same time.
[0027]
According to the configuration of the second aspect, when the image processing method at the time of image formation forms a halftone with independent dots, the bias voltage applied to the fixing belt is set high, so that the fixing horizontal black band is prevented and high quality is achieved. When the image processing method at the time of image formation forms a halftone in a line pattern, the bias voltage applied to the fixing belt is lowered or turned off, so that the life of the fixing belt is extended. Can be achieved.
[Brief description of the drawings]
FIG. 1 is a configuration diagram schematically showing an image forming apparatus according to the present invention.
FIG. 2 is an enlarged explanatory diagram illustrating a configuration of a fixing device according to the present invention.
FIG. 3 is a graph showing a potential of a fixing belt measured immediately before a fixing nip when one sheet of A3 size is printed.
FIG. 4 is an explanatory diagram showing a phenomenon of a fixed horizontal black band when a halftone is formed by independent dots.
FIG. 5 is an explanatory diagram showing a phenomenon of a fixed horizontal black band when a halftone is formed in a line pattern.
[Explanation of symbols]
Reference Signs List 20 fixing device 21 fixing roller 22 heating roller 23 auxiliary roller 24 fixing belt 25 pressure roller

Claims (2)

A fixing device for fixing an unfixed toner image to the recording material by passing a recording material carrying the unfixed toner image through a nip formed between a fixing belt and a roller having a high-resistance release layer coated on the surface thereof Image forming apparatus comprising: means for applying a bias voltage to the fixing belt, wherein the bias voltage applied by the bias applying means is changed in accordance with an image processing method at the time of image formation. apparatus. 2. The image forming apparatus according to claim 1, wherein when the image processing method at the time of image formation forms the halftone with independent dots, a bias voltage applied to the fixing belt is set high, and the image processing method at the time of image formation is performed. An image forming apparatus, wherein when forming a halftone in a line pattern, the bias voltage applied to the fixing belt is reduced or turned off.

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