Image forming device

Description: TECHNICAL FIELD [Detailed description of the invention]

【0001】

[Technical field to which the invention belongs]

The present invention relates to an image forming apparatus using an electrophotographic method, for example, a copier, a printer, a facsimile, or the like.

【0002】

[Conventional technology]

FIG. 13 shows an example of a conventional electrophotographic color image forming apparatus. According to this example, the color image forming apparatus includes a drum-shaped electrophotographic photosensitive member which is an image carrier, that is, a photosensitive drum 3. The photosensitive drum 3 is driven in the direction of the arrow by a driving means (not shown), and is uniformly charged by the primary charger 4, for example, to a negative electrode. Next, the exposure device 5 irradiates the photosensitive drum 3 with a laser beam according to the magenta image pattern, and an electrostatic latent image is formed on the photosensitive drum 3.

【0003】

A rotary developing device 6 is arranged so as to face the photosensitive drum 3. The rotary developing device 6 is composed of a rotating support 60 and a plurality of developers 6a, 6b, 6c, and 6d detachably mounted on the rotating support 60, in this example, four developing devices 6a, 6b, 6c, and 6d. ..

【0004】

When the photosensitive drum 3 on which the electrostatic latent image is formed advances in the direction of the arrow, the developing device 6a containing the negative magenta toner in the developing device supported by the rotating support 60 faces the photosensitive drum 3. The electrostatic latent image on the photosensitive drum 3 is visualized by so-called reverse development, that is, a toner image by the selected developer 6a.

【0005】

Located below the photosensitive drum 3, the intermediate transfer belt 2 as a belt-shaped intermediate transfer body in this example is stretched around the rollers 21, 22, 23, and 24, and at substantially the same speed as the photosensitive drum 3. It is rotated in the direction of the arrow. The toner image formed and supported on the photosensitive drum 3 is primarily transferred to the outer peripheral surface of the intermediate transfer belt 2 by the primary transfer bias applied to the primary transfer roller 22 in the primary transfer unit.

【0006】

By performing the above steps for cyan, yellow, and black, toner images of a plurality of colors are superimposed and transferred onto the intermediate transfer belt 2.

【0007】

Next, the transfer material 11 is fed from the transfer material cassette 13 by the pickup roller 12 at a predetermined timing. At the same time, a secondary transfer bias is applied to the secondary transfer roller 7 as the secondary transfer device, and the toner image is collectively transferred from the intermediate transfer belt 2 to the transfer material 11. The transfer material 11 is conveyed to the fixing device 10 by the transfer belt 9, and the transferred toner image is melted and fixed to obtain a color image.

【0008】

The transfer residual toner (secondary transfer residual toner) on the intermediate transfer belt 2 after the completion of the secondary transfer is reversed in polarity by a charging device 20 such as a contact charger, which is an intermediate transfer belt cleaning device, in this example. It is positively charged. The transfer residual toner charged in the opposite polarity is conveyed to the primary transfer section again by the movement of the intermediate transfer belt 2, and the photosensitive drum 3 which is the counter electrode thereof is transferred by the primary transfer bias applied to the primary transfer roller 22. Reverse transcription on top. The primary transfer residual toner and the reverse transfer toner (secondary transfer residual toner) on the photosensitive drum 3 are cleaned by the cleaning device 8.

【0009】

FIG. 14 shows another example of a conventional electrophotographic color image forming apparatus. This color image forming apparatus is called a tandem method or an in-line method, and unlike the color image forming apparatus shown in FIG. 13, photosensitive drums 3 (3a, 3b, 3c, 3d) are arranged for each color, and the photosensitive drums 3 (3a, 3b, 3c, 3d) are arranged therein. Correspondingly, each developer 6a, 6b, 6c, 6d is arranged. In the device of FIG. 14, members having the same configuration and operation as the members in the device shown in FIG. 13 are assigned the same reference numbers, and detailed description thereof will be omitted.

【0010】

Each of the photosensitive drums 3a, 3b, 3c, and 3d is arranged in series on the intermediate transfer belt 2, and a method of collectively forming a color image by one rotation of the intermediate transfer belt 2 is used. Therefore, there is a feature that higher speed printing is possible.

【0011】

Also in this method, the intermediate transfer belt can be cleaned by the contact charger 20 which is the intermediate transfer belt cleaning device described above. In this case, the cleaning device 8a provided attached to the photosensitive drum 3a of the first color is used. The next transfer residual toner will be collected.

【0012】

Further, unlike the one-drum method described above, the tandem type intermediate transfer belt cleaning device 20 is always in contact with the intermediate transfer belt 2 as an intermediate transfer body.

【0013】

Here, the mechanism of intermediate transfer belt cleaning will be described in more detail. Since the mechanism of the intermediate transfer belt cleaning is the same in the image forming apparatus of FIG. 13 and the color image forming apparatus of FIG. 14, it will be described in relation to the image forming apparatus of FIG.

【0014】

When the toner is transferred from the intermediate transfer belt 2 to the transfer material 11, the secondary transfer residual toner receives a strong electric field having a polarity opposite to that of the toner, and has a polarity opposite to the normal charging polarity (negative electrode property in this example). Most of the toner is charged (positive electrode in this example) and remains on the intermediate transfer belt 2. However, not all toners are reversed to positive electrode properties, and some toners are partially neutralized and have no electric charge, and some toners maintain negative electrode properties.

【0015】

Therefore, immediately after the secondary transfer, a contact charger is provided as the intermediate transfer belt cleaning device 20, and an AC component superimposed on the DC component is applied as the intermediate transfer belt cleaning bias. Due to the AC component, the secondary transfer residual toner repeats reciprocating motion and is charged more uniformly to the positive electrode.

【0016】

The secondary transfer residual toner charged with a uniform positive electrode property is reverse-transferred onto the photosensitive drum 3 at the primary transfer nip portion, and is collected by the cleaning device 8 on the photosensitive drum 3.

【0017】

Even during continuous printing, the charges between the back-charged secondary transfer residual toner on the intermediate transfer belt 2 and the regular toner primary-transferred from the photosensitive drum 3 are not canceled by short-term contact. Therefore, each toner can be transferred to the photosensitive drum 3 and the regular toner on the photosensitive drum 3 can be transferred to the intermediate transfer belt 2 at the primary transfer unit. Therefore, the secondary transfer residual toner is not transferred onto the transfer material 11 at the time of the next printing, and an appropriate image is output.

【0018】

By using the intermediate transfer belt cleaning device 20 described above, the waste toner container for collecting the transfer residual toner on the intermediate transfer belt 2 can be shared with the cleaning device 8 on the photosensitive drum 3, so that the device can be miniaturized and maintainability. There is a merit that can be improved.

【0019】

By the way, the volume resistance value of the intermediate transfer belt 2 used in the color image forming apparatus described above is approximately 10.⁵Ωcm ~ 10⁸A non-charge type that is made of Ωcm material and is hard to be charged by itself due to the transfer of electric charge or toner due to the primary transfer and secondary transfer bias.⁹Ωcm ~ 10¹⁶There is a charge type that is made of Ωcm material and is easily charged by itself.

【0020】

Many configurations of the charge type intermediate transfer belt 2 have been devised. For example, as shown in FIGS. 7 and 8, the intermediate transfer belt 2 is composed of 2 to 3 layers, and in the belt of FIG. 7, the surface layer 2A is formed. Also, in the belt shown in Fig. 8, the intermediate layer 2B (about 5 to 50 μm thick) is roughly 10¹⁰~10¹⁶Ωcm and base layer 2C is 10³~10⁸Consists of a material with a resistance value of Ωcm, for a total of 10⁹Ωcm ~ 10¹⁶A belt with Ω cm has also been devised.

【0021】

9 and 10 show the toner state on the intermediate transfer belt 2, but as shown in FIG. 10, when the charge type configuration is used as the intermediate transfer belt, it is compared with the case of the non-charge type shown in FIG. Since the absolute value of the potential of the superimposed toner image is lower than that of the non-image portion, there is an advantage that the toner scattering, which is a problem in the color overlay of the intermediate transfer belt 2, can be reduced, and the image quality can be improved.

【0022】

The above-mentioned intermediate transfer belt cleaning can be performed when any of the above types of intermediate transfer belts 2 is used, but when a charge type intermediate transfer belt is used, the intermediate transfer belt cleaning device 20 is used for intermediate cleaning. It is also possible to remove excess charge on the transfer belt 2.

【0023】

As the intermediate transfer belt cleaning bias including such removal ability, a rectangular wave having many high frequency components and high charging ability is used as the AC bias, and further, in order to achieve both the chargeability of the secondary transfer residual toner. A deflected waveform as shown in FIG. 11 is used. When such a waveform is used, the convergence potential is not the integrated mean value of the waveform, but the potential just in the middle of Vmax and Vmin, that is, Vmin + (Vmax-Vmin) / 2.

【0024】

To give an example of the intermediate transfer belt cleaning bias, if an AC bias of frequency 3kHz, AC component (Vp-p) = 2.4kVpp, + Duty80%, DC component (Vdc) = + 720V to 920V is applied, The charging potential is 0V to + 200V.

【0025】

[Problems to be Solved by the Invention]

As described above, the intermediate transfer belt cleaning device for the charge type intermediate transfer belt makes the polarity of the secondary transfer residual toner uniform to positive electrode (opposite to the normal charge polarity) and secondary. It also serves to initialize the surface potential of the intermediate transfer belt that is positively charged by the transfer bias. Therefore, the intermediate transfer belt cleaning device must satisfy the uniform chargeability of the toner as well as the uniform chargeability of the belt surface.

【0026】

However, when the bias applied to the intermediate transfer belt cleaning device is turned off, a non-uniform charging state exists for the reason described below, and there is a problem that uneven charging occurs on the intermediate transfer belt.

【0027】

The uneven charging on the intermediate transfer belt also causes unevenness in the primary transferability of the next image. In particular, in a uniform halftone image, the image becomes white due to a decrease in transfer efficiency, or conversely, the halftone image deviates from the optimum transfer electric field region, the halftone image scatters, and the density becomes uneven, resulting in a significant deterioration in image quality. This problem has been a problem when the intermediate transfer belt cleaning device is composed of a low-cost contact charger that generates a small amount of ozone, for example, a roller charger.

【0028】

The mechanism by which uneven charging occurs in the intermediate transfer belt is due to what kind of discharge is performed in the discharge region of the contact charger when the cleaning bias is turned off. Since the cleaning bias uniformly charges the intermediate transfer belt after the secondary transfer, it is necessary to reliably remove one round of the intermediate transfer belt from the starting point, but the discharge at the end of static elimination is as follows. Potential unevenness is created.

【0029】

FIG. 12 shows the waveform when the cleaning bias supplied to the intermediate transfer belt cleaning device is OFF, and what kind of potential unevenness is generated on the intermediate transfer belt before and after that.

【0030】

As can be understood from this figure, the potential on the intermediate transfer belt at that time depends on the position where the AC bias applied to the roller charger is interrupted when the cleaning bias is turned off, in other words, where the last potential was. Has been decided. When the potential difference between this last potential and the surface potential of the intermediate transfer belt in the discharge nip exceeds the discharge start voltage, the last discharge is performed, and the discharge start voltage (approximately 500V) is calculated from the last potential. It becomes the subtracted potential. Therefore, the worst case is when the AC bias is turned off at the apex, and the above problem becomes remarkable when a square wave is used.

【0031】

In order to solve such a problem, there is a method of setting the OFF timing of the cleaning bias to the non-image part on the intermediate transfer belt, but since the photosensitive drum corresponding to this non-image part is not always the non-image part. There is a problem that the potential unevenness on the intermediate transfer belt is traced as a transfer memory on the photosensitive drum by the primary transfer unit, and the potential unevenness on the photosensitive drum cannot be erased by the primary charging and reappears in the image as a primary charging defect. It will occur.

【0032】

An object of the present invention is to provide an image forming apparatus capable of preventing uneven charging of the intermediate transfer body after being charged by the charging means and always obtaining a high-quality image. ..

【0033】

Another object of the present invention is to provide an image forming apparatus capable of always obtaining a high-quality image by preventing potential unevenness on the surface of the image carrier and the phenomenon of toner ejection.

【0034】

Another object of the present invention is to provide an image forming apparatus capable of preventing uneven charging of an image carrier or an intermediate transfer body due to a charging member and always obtaining a high-quality image.

【0035】

Other objects of the invention will become apparent by reading the detailed description below.

【0036】

[Means for solving problems]

The above object is achieved by the image forming apparatus according to the present invention. In summary, according to the first invention, an image carrier, an image forming means for forming a toner image on the image carrier, and a toner image formed on the image carrier by the image forming means. Has a residual toner remaining on the image carrier and a charging member that charges the image carrier after being transferred to the transfer material, and the charging member can be brought into contact with and detached from the image carrier. When charging, the charging member has a control means for controlling a voltage applied to the charging member in an image forming apparatus that comes into contact with the image carrier, and when the charging member charges the image carrier, the charging member Discharge is performed between the charging member and the image carrier by a voltage obtained by superimposing a DC voltage and an AC voltage applied to the charging member, and the control means separates the charging member from the image carrier. Provided is an image forming apparatus characterized by controlling the surface potential of the charging member at the time of

【0037】

According to one embodiment of the present invention, when the charging member is separated from the image carrier, the control means starts discharging the potential difference between the surface potential of the charging member and the surface potential of the image carrier. It is controlled so that it is smaller than the voltage and the surface potential of the charging member has the same polarity as the surface potential of the image carrier and is large. According to another embodiment, the control means attenuates the peak-to-peak voltage of the voltage applied to the charging member and the AC voltage to the charging member before the charging member is separated from the image carrier. Stop the supply of.

【0038】

According to another embodiment of the present invention, the control means sets the inter-peak voltage so that the potential difference between the surface potential of the charging member and the surface potential of the image carrier is smaller than the discharge start voltage. Attenuate. Further, according to another embodiment, the control means attenuates the inter-peak voltage applied to the charging member so that the center value of the inter-peak voltage is maintained. Further, according to another embodiment, the control means independently controls the DC voltage and the peak-to-peak voltage. According to yet another embodiment, the DC voltage is zero.

【0039】

According to another embodiment of the present invention, the charging member charges the image carrier at least while the image carrier makes one revolution.

【0040】

According to another embodiment of the present invention, after the toner image on the image carrier is transferred to the transfer material, the charging member charges the residual toner remaining on the image carrier to a predetermined polarity. ..

【0041】

According to still another embodiment of the present invention, the image forming apparatus has a recovery member that electrostatically recovers residual toner on the image carrier charged by the charging member.

【0042】

According to the second invention, the intermediate transfer body, the image forming means for forming the toner image on the intermediate transfer body, and the toner image formed on the intermediate transfer body by the image forming means are used as the transfer material. After being transferred, it has residual toner remaining on the intermediate transfer body and a charging member that charges the intermediate transfer body, and the charging member can be brought into contact with and detached from the intermediate transfer body, and when charged, the charging member The charging member has a control means for controlling a voltage applied to the charging member in an image forming apparatus that comes into contact with the intermediate transfer body, and when the intermediate transfer body is charged by the charging member, the charging member is applied to the charging member. Discharge is performed between the charging member and the intermediate transfer body by a voltage obtained by superimposing the DC voltage and the AC voltage, and the control means is used when the charging member is separated from the intermediate transfer body. An image forming apparatus characterized by controlling the surface potential of a charged member is provided.

【0043】

According to one embodiment of the present invention, when the charging member is separated from the intermediate transfer body, the control means discharges the potential difference between the surface potential of the charging member and the surface potential of the intermediate transfer body. It is controlled so that it is smaller than the starting voltage and the surface potential of the charging member has the same polarity as the surface potential of the intermediate transfer body and is large. According to another embodiment, the control means attenuates the peak-to-peak voltage of the voltage applied to the charging member and alternating current to the charging member before the charging member is separated from the intermediate transfer body. Stop the voltage supply.

【0044】

According to another embodiment of the present invention, the control means has the inter-peak voltage so that the potential difference between the surface potential of the charging member and the surface potential of the intermediate transfer member is smaller than the discharge start voltage. To attenuate. According to another embodiment, the control means attenuates the peak voltage applied to the charging member so that the center value of the peak voltage is maintained. According to still another embodiment, the control means independently controls the DC voltage and the peak-to-peak voltage.

【0045】

According to another embodiment of the present invention, the control means is after the supply of the AC voltage to the charging member is stopped in the period before the charging member is separated from the intermediate transfer body. , The supply of the DC voltage applied to the charging member is stopped. According to another embodiment, the DC voltage is zero.

【0046】

According to another embodiment of the present invention, after the toner image on the intermediate transfer body is transferred to the transfer material, the charging member charges the residual toner remaining on the intermediate transfer body to a predetermined polarity. To do.

【0047】

According to another embodiment of the present invention, the charging member charges the residual toner on the intermediate transfer body with a polarity opposite to the normal charging polarity of the toner. According to another embodiment, the duty ratio of the voltage applied to the charging member is greater than 50%.

【0048】

According to another embodiment of the present invention, the image forming means includes an image carrier that supports a toner image, and the toner image on the image carrier is transferred to the intermediate transfer body at a transfer position. According to another embodiment, the residual toner on the intermediate transfer body charged by the charging member is electrostatically transferred to the image carrier at the transfer position. According to still another embodiment, at the transfer position, the residual toner on the intermediate transfer body charged by the charging member is transferred to the image carrier, and at the same time, the toner image on the image carrier is transferred. Is transferred to the intermediate transfer body to form an electric field.

【0049】

According to another embodiment of the present invention, the charging member charges the residual toner on the intermediate transfer body and the intermediate transfer body at least while the intermediate transfer body makes one rotation.

【0050】

According to another embodiment of the present invention, the volume resistivity of the intermediate transfer material is 10.⁹~10¹⁶It is Ωcm.

【0051】

According to another embodiment of the present invention, the image forming means includes an image carrier that supports a toner image, and toner images of a plurality of colors are sequentially transferred from the image carrier to the intermediate transfer body, and the toner image of a plurality of colors is sequentially transferred. The multi-color toner image on the intermediate transfer body is transferred to the transfer material.

【0052】

According to still another embodiment of the present invention, the image forming means includes a plurality of image carriers each supporting a plurality of color toner images, and the plurality of color toner images on the plurality of image carriers are provided. The toner images are sequentially transferred to the intermediate transfer body, and the toner images of a plurality of colors on the intermediate transfer body are transferred to the transfer material.

【0053】

According to a third aspect of the present invention, an image carrier, an image forming means for forming a toner image on the image carrier, a charging member that contacts the image carrier and charges the image carrier, and the like. A control means for controlling a voltage applied to a charging member, and a toner image on the image carrier formed by the image forming means is an image forming apparatus transferred to a transfer material, and the charging is performed. When the image carrier is charged by the member, a discharge is performed between the charging member and the image carrier by a voltage obtained by superimposing a DC voltage and an AC voltage applied to the charging member, and the control means is used. Provided is an image forming apparatus characterized in that the supply of an AC voltage to the charging member is stopped after the peak-to-peak voltage of the voltage applied to the charging member is attenuated.

【0054】

According to a fourth aspect of the present invention, the intermediate transfer body, an image forming means for forming a toner image on the intermediate transfer body, a charging member that contacts the intermediate transfer body and charges the intermediate transfer body, and the like. A control means for controlling a voltage applied to a charging member, and a toner image on the intermediate transfer body formed by the image forming means is an image forming apparatus transferred to a transfer material, and the charging is performed. When the intermediate transfer body is charged by the member, a discharge is performed between the charging member and the intermediate transfer body by a voltage obtained by superimposing a DC voltage and an AC voltage applied to the charging member, and the control means is used. Provided is an image forming apparatus characterized in that the supply of an AC voltage to the charging member is stopped after the peak-to-peak voltage of the voltage applied to the charging member is attenuated.

【0055】

BEST MODE FOR CARRYING OUT THE INVENTION

Hereinafter, the image forming apparatus according to the present invention will be described in detail with reference to the drawings.

【0056】

Example 1 The present invention will be described using the tandem type color image forming apparatus illustrated in FIG.

【0057】

The image forming apparatus of this embodiment includes four image carriers 3a, 3b, 3c, and 3d on which toner images of yellow, magenta, cyan, and black having a normal charge polarity of negative electrode are formed. .. In this embodiment, the image carriers 3a, 3b, 3c, and 3d are drum-shaped electrophotographic photosensitive members, so-called photosensitive drums, and are arranged in series. Primary chargers 4a, 4b, 4c, 4d, developers 6a, 6b, 6c, 6d and cleaning devices 8a, 8b, 8c, 8d are arranged around each photosensitive drum, and photosensitive drums 3a Above, 3b, 3c, 3d, exposure devices 5a, 5b, 5c, 5d are arranged.

【0058】

Each of the photosensitive drums 3a, 3b, 3c, and 3d is negatively charged by the charging rollers 4a, 4b, 4c, and 4d that come into contact with each other, and the color-separated optical images of each color of yellow, magenta, cyan, and black are exposed to the exposure device 5a, Exposure with 5b, 5c, 5d forms yellow, magenta, cyan, and black latent images on photosensitive drums 3a, 3b, 3c, and 3d, and inverts each latent image with developers 6a, 6b, 6c, and 6d. It is developed by development, and yellow, magenta, cyan, and black toner images are sequentially formed on the photosensitive drums 3a, 3b, 3c, and 3d.

【0059】

An intermediate transfer belt 2 as an intermediate transfer body (image carrier) is arranged below the photosensitive drums 3a, 3b, 3c, and 3d. The intermediate transfer belt 2 is stretched around the rollers 21, 22, 23, and 24, and is rotated in the direction of the arrow at substantially the same speed as the photosensitive drum 3. The toner image formed and supported on the photosensitive drums 3a, 3b, 3c, and 3d is intermediated by the primary transfer bias (positive electrode voltage) applied to the primary transfer rollers 22a, 22b, 22c, and 22d in the primary transfer section. The primary transfer is electrostatically performed on the outer peripheral surface of the transfer belt 2, whereby a toner image of a plurality of colors is formed on the intermediate transfer belt 2.

【0060】

Next, the transfer material 11 is fed from the transfer material cassette 13 by the pickup roller 12 at a predetermined timing. At the same time, a secondary transfer bias (positive electrode voltage) is applied to the secondary transfer roller 7 as the secondary transfer device, and the toner image is electrostatically transferred from the intermediate transfer belt 2 to the transfer material 11. The transfer material 11 is conveyed to the fixing device 10 by the transfer belt 9, and the transferred toner image is melted and fixed to obtain a color image.

【0061】

The transfer residual toner (secondary transfer residual toner) on the intermediate transfer belt 2 after the completion of the secondary transfer is charged with a polarity opposite to the normal charging polarity by the contact charger 1 which is an intermediate transfer belt cleaning device (charging means). Will be done. The transfer residual toner charged in the opposite polarity is conveyed to the primary transfer section again by the movement of the intermediate transfer belt 2, and is applied to the primary transfer roller 22 with a positive primary transfer bias (the normal charge polarity of the toner). It is reverse-transferred onto the photosensitive drum 3a, which is the counter electrode, by a voltage having the opposite polarity. In this case, the cleaning device 8a provided attached to the photosensitive drum 3a of the first color collects the secondary transfer residual toner.

【0062】

Further, in the intermediate transfer belt cleaning device 1, that is, the charging roller as the charging means is always in contact with the intermediate transfer belt 2.

【0063】

FIG. 1 shows a block diagram of an embodiment of the cleaning bias power supply 100. In this embodiment, the AC output value of the cleaning bias power supply 100 and the AC output value and the DC output value of the DC bias power supplies 101 and 102 can be controlled. As a control method, a PWM circuit 105 is connected to the zero-cross detection circuit 104 and the DC bias power supply 102, and the output voltage Vp-p (AC output value (inter-peak voltage)) and the output voltage Vp-p (AC output value (inter-peak voltage)) and the output voltage Vp-p (AC output value (inter-peak voltage)) and The method of controlling Vdc (DC output value) is used.

【0064】

The Xerox detection circuit 104 used in this embodiment is a circuit that operates when the potential difference between the output potential and a predetermined potential becomes 0, instead of setting the integrated mean value to 0.

【0065】

Further, to be described in detail, when a cleaning bias having a predetermined duty ratio is created by the cleaning bias power supply 100 as shown in FIG. 1, a waveform such that the integrated average value becomes 0 from the AC waveform generator 103. Is transmitted, and the target potential (center potential) to be charged is adjusted by superimposing the output value of the DC bias power supply 102 in consideration of the change in the waveform. In this embodiment, since it is desired to cut off the AC bias at the central potential, a circuit that operates with a potential difference from a predetermined voltage using a comparator circuit or the like is used instead of the Xerox detection circuit 104 that cuts off at the integrated mean value of the AC waveform. There was. Theoretically, for a square wave, such a circuit cannot take a signal, but there is a slight slew rate when the actual square wave is alternated, that is, the waveform actually goes from peak to peak. When heading, it does not rise vertically upward (downward) and has a slight inclination (it does not fall), so it can be captured by using a high-speed switching element.

【0066】

FIG. 2 shows the cleaning bias OFF sequence of this embodiment. According to this embodiment, in FIG. 2, the PWM signal from the CPU is transmitted from the AC and DC bias power supplies 101 and 102 as shown in FIGS. 2 (A) and 2 (B) from several tens of msec before the OFF timing. The data is changed so that the output voltages Vp-p and Vdc are gradually reduced.

【0067】

FIG. 2C is a conceptual diagram of the waveform immediately before the cleaning bias is turned off when such bias control is used.

【0068】

Further, FIG. 2D shows the surface potentials of the intermediate transfer belt 2 on the intermediate transfer belt 2 before and after the cleaning bias is turned off in this embodiment.

【0069】

As can be understood from FIG. 2, in this embodiment, the output Vp-p and Vdc of the AC bias of the cleaning bias power supply 100 are gradually narrowed down, and the center value of the AC bias waveform is changed while decreasing Vp-p. It is configured not to. As Vp-p is attenuated, the discharge region in the gaps upstream and downstream of the contact position between the charging roller and the intermediate transfer belt gradually becomes smaller, and when the potential difference between the two becomes smaller than the discharge start voltage, the discharge region disappears. When the cleaning bias (AC voltage) is OFF, the intermediate transfer belt is no longer abnormally charged. Therefore, as shown in FIG. 2C, the surface potential of the intermediate transfer belt 2 converges evenly to the DC output value, which is the target potential. The target potential may be zero.

【0070】

In this embodiment, the AC bias waveform is attenuated to its central potential by controlling it from the CPU, but the waveform always maintains a potential state substantially symmetric with respect to the central potential and converges to that central potential. If so, a self-attenuating bias power supply circuit may be used.

【0071】

Since the intermediate transfer belt 2 in the discharge region (charged region nip) when the AC bias is OFF has already been statically eliminated to the center potential of the cleaning bias one lap before, the Vp-p of the AC bias is about 1 kVpp (discharge start voltage). It does not discharge below (about twice the voltage of). Therefore, the variable AC and DC bias may be attenuated until Vp-p is 1 kVpp so that the central potential does not change.

【0072】

The present inventor sets the cleaning bias to Vp-p = 0Vpp by changing the data from the state where the frequency is 3kHz, Vp-p = 2.4kVpp, + duty ratio is 80%, and the AC bias of Vdc = + 820v is output. , Vdc = + 100V The data was changed so that it became.

【0073】

In this embodiment, the output values of AC bias and DC bias may be made variable, and the above-mentioned high-speed switching element may not be used. In this case, there is an advantage that it can be configured with a cheaper high-voltage power supply.

【0074】

In this example, since a waveform with a duty ratio of not 50% was used for the square wave, the DC bias value was also changed after the AC bias was turned off, but this is not necessary when a waveform with a duty ratio of 50% is used. It is clear that.

【0075】

Here, in this embodiment, when the residual toner in the form of an intermediate transfer material is charged to a predetermined polarity (in this embodiment, the polarity is opposite to the charging polarity of the toner), the waveform becomes larger than the effective value of the above waveform. The duty ratio is calculated by (t / T) × 100 (%), where t is the time that is closer to the predetermined polarity in one cycle and T is one cycle of the waveform.

【0076】

Example 2 In Example 1, the present invention has been described when it is embodied in a tandem type color image forming apparatus, but it can be similarly applied to the one drum type color image forming apparatus illustrated in FIG. Since the overall configuration of this color image forming apparatus is the same as that of the image forming apparatus described with reference to FIG. 13, detailed description thereof will be omitted.

【0077】

When the present invention is applied to a one-drum type color image forming apparatus, the contact charger 1, that is, the charging roller as a charging means, is separated from the intermediate transfer belt 2 in order to form a four-color full-color image. That is, they must be separated during image formation, that is, at least while the toner image passes through, and the following problems occur at the time of the separation.

【0078】

As described in the conventional example, most of the secondary transfer residual toner is a toner whose polarity is opposite to the normal polarity of the toner (positive electrode property in this example), but a part of the toner has the above-mentioned normal polarity (this example). Then, negative polarity) toner is also mixed. Most of the normal polarity toners are charged in the opposite polarity due to the cleaning bias, that is, in the present embodiment, they are positively charged, but some of the normal polarity toners are charged in the intermediate transfer cleaning device 1. Adheres to the roller surface. The adhered toner is always formed into a thin layer on the roller surface while maintaining the normal polarity. Then, the normal polarity toner on the roller surface adheres to the roller surface while the DC bias is applied to the roller.

【0079】

However, if the DC bias is turned off when the charging roller 1 and the intermediate transfer belt are separated from each other, the normal polar toner formed in a thin layer on the surface of the charging roller 1 is transferred to the intermediate transfer belt 2, so-called. , "Toner spitting" phenomenon occurs.

【0080】

This is because when the DC bias is turned off, the magnitude of each surface potential on the surface of the charging roller 1 and the surface of the intermediate transfer belt 2 is reversed, so that the normal polarity toner adhering to the roller surface moves electrically. is there. Since this discharged toner is polar in that it is not reverse-transferred from the intermediate transfer belt to the photosensitive drum at the primary transfer portion, it appears in the next image when an image is continuously formed on a plurality of transfer materials. There is.

【0081】

In response to such a problem, in the present invention, the average potential of the surface of the charging roller during attenuation after the AC bias is turned off is higher than the surface potential of the surface of the intermediate transfer belt, and the potential difference thereof is set. Was solved by setting the timing to be lower than the discharge start voltage. The details will be described below.

【0082】

FIG. 3 shows a timing chart (FIGS. 3 (A), (B), (C)) of the intermediate transfer belt cleaning device 1 of the present invention and a conceptual diagram of a bias waveform at that time (FIG. 3 (D)). Is shown.

【0083】

In FIG. 3A, the timing at which the intermediate transfer belt cleaning device 1 separates from the intermediate transfer belt 2 which is the counter electrode thereof is approximately 250 msec after the separation signal is transmitted from the CPU. This delay is the time required for the connection time of the electromagnetic clutch and the rotation time to the effective position of the eccentric cam, such as when a combination of the electromagnetic clutch eccentric cams is used for the attachment / detachment of the intermediate transfer belt cleaning device 1. Therefore, approximately 250 msec after receiving the signal of actual separation, the potential of the intermediate transfer belt cleaning device 1 determines whether it is an abnormal discharge or a toner discharge.

【0084】

In the present invention, by controlling the AC and DC bias cleaning bias prior to this actual separation timing, the bias waveform satisfy the following conditions by varying the respective data in Suyo block the bias under the following conditions , The actual separation timing was almost the same as this.

【0085】

1. The Vp-p of the AC bias waveform is 1 kVpp or less.

【0086】

2. The integrated mean value of the decay waveform is higher than the surface potential of the intermediate transfer belt with the same polarity, and the potential difference is 500 V or less.

【0087】

When the above conditions are satisfied, no discharge or toner exchange occurs between the intermediate transfer belt 2 and the charger 1. Therefore, the above problem can be avoided by separating the charger 1 from the intermediate transfer belt 2 in a state where the bias waveform is attenuated under this condition. Further, since the bias power supply 100 is turned off at the time of separation and does not actively apply the bias, it does not cause an abnormal discharge that significantly changes the surface potential even if the load fluctuates at the time of separation. ..

【0088】

The above-mentioned variable AC and DC bias will be described in more detail. By changing the data from the state where the cleaning bias, frequency 3kHz, Vp-p = 2.4kVpp, + Duty80%, Vdc = + 820v shown in the conventional example are output, Vp-p = 1.0kVpp, Vdc = + Set to 400v. In this state, AC discharge stops when Vp-p is 1 kVpp or less, but since the average potential of charger 1 is the integrated mean value of the AC waveform, discharge occurs between the charger 1 and the surface of the intermediate transfer belt 2. However, there is a potential difference (300v in this case). Since the cleaning bias must be applied at least once around the intermediate transfer belt, the cleaning bias is already applied to the surface of the intermediate transfer belt opposite to the timing at which the cleaning bias is turned off, and the surface is charged to approximately + 100 V. Therefore, the toner on the intermediate transfer belt cleaning device 1 is attracted to the charger side.

【0089】

Under the condition that the surface is charged to + 100v using the waveform as in this example, the AC bias is + 400V even at the time of Vp-p = 1.0kVpp, and there is no abnormal discharge. Therefore, if the AC and DC biases are turned off in this state and separated during the attenuation, the surface potential of the charger 1 is approximately higher than + 100v. Of course, of the AC and DC bias power supplies 101 and 102 used in this embodiment, at least the time constant of the DC bias power supply 102 is longer than the evacuation speed of the charger 1. In the DC bias power supply 102 used in this example, it took about 50 msec to attenuate from + 400 V to + 100 V, so the timing was set to turn off about 10 msec before the actual time when the charger 1 was separated. This 10 msec is to prevent the DC bias from being actively applied at the time of separation due to an error in the clutch engagement time.

【0090】

The above description has described the case where the AC bias and DC bias power supplies 101 and 102 can be changed by data and are forcibly controlled so as to satisfy the above two conditions. However, the internal impedance of each bias power supply circuit has been described. It is also possible to set it appropriately so that the attenuation state after the bias is turned off satisfies the above two conditions.

【0091】

In each of the above examples, the intermediate transfer belt 2 is used as the image carrier (intermediate transfer body) and the intermediate transfer belt cleaning device 1 is used as the charging means. However, as in the following Example 3, the image carrier is photosensitive. The same effect can be obtained by applying the present invention to the drums 3 or 3a to 3d and the charging rollers 4 or 4a to 4d as charging means.

【0092】

Further, the intermediate transfer body has been described as being an endless belt, but the present invention is not limited to this, and of course, a drum shape is also possible.

【0093】

Example 3 FIG. 4 shows an electrophotographic color image forming apparatus according to another embodiment of the present invention.

【0094】

The color image forming apparatus of this embodiment is called a multiple development method, and includes a drum-shaped electrophotographic photosensitive member which is an image carrier, that is, a photosensitive drum 3. The photosensitive drum 3 is driven in the direction of an arrow by a driving means (not shown), and is uniformly charged, for example, to a negative electrode by a primary charger 4A, which is a corona charger in this embodiment. Next, the exposure device 5 irradiates the photosensitive drum 3 with laser light, and an electrostatic latent image is formed on the photosensitive drum 3. Further, in this embodiment, four developing machines 6a, 6b, 6c, and 6d are arranged so as to face the photosensitive drum 3.

【0095】

In this embodiment, first, the exposure apparatus 5 irradiates the photosensitive drum 3 with a laser beam according to the image pattern of magenta, and the electrostatic latent image formed on the photosensitive drum 3 is formed in the direction of the arrow by the photosensitive drum 3. As it progresses, it is visualized by so-called reverse development by a developer 6a containing magenta toner having a normal charge polarity of negative polarity, that is, a toner image is obtained.

【0096】

By performing the above steps for cyan, yellow, and black, toner images of a plurality of colors are formed on the photosensitive drum 3.

【0097】

Next, the transfer material 11 is fed from the transfer material cassette 13 by the pickup roller 12 at a predetermined timing. At the same time, a transfer bias is applied to the transfer roller 7 as a transfer device, and the toner image is collectively transferred from the photosensitive drum 3 to the transfer material 11. The transfer material 11 is conveyed to the fixing device 10 by the transfer belt 9, and the transferred toner image is melted and fixed to obtain a color image.

【0098】

The transfer residual toner on the photosensitive drum 3 after the transfer is completed is removed by the cleaning device 8.

【0099】

In the photosensitive drum cleaning of the color image forming apparatus of this embodiment, the cleaning apparatus 8 is always separated from the contact because the exposure and development are sequentially repeated on the photosensitive drum.

【0100】

Conventionally, in mechanical cleaning using a blade, a complicated and costly contact separation mechanism has been used in order to solve problems such as blade contact setting and vibration due to contact. Further, in blade cleaning, it is necessary to use a cleaning auxiliary means such as a fur brush in order to prevent the toner streaks remaining on the blade contact portion generated at the time of separation from affecting the image.

【0101】

According to this embodiment, the cleaning device 8 includes a contact charger 1A as a charging means for charging the transfer residual toner on the photosensitive drum 3, and a recovery device 1B for collecting the transfer residual toner charged by the contact charger 1A. The configuration is such that the photosensitive drum 3 can be brought into contact with and detached from the photosensitive drum 3.

【0102】

The contact charger 1A used in this embodiment has substantially the same configuration as the contact charger 1 used in the intermediate transfer belt cleaning device described in the above embodiment, and is connected to a high voltage generating means, that is, a cleaning bias power supply 100. The bias blocking method also satisfies the same conditions as described in Example 2 above.

【0103】

The recovery device 1B includes a recovery roller 15, a cleaning blade 16 that scrapes the recovered transfer residual toner from the recovery roller 15, and a static elimination roller 17 that removes static electricity from the recovery roller surface. The recovery roller 15 and the static elimination roller 17 are connected to the positive electrode side of the power supply 18 in this embodiment. The surface of the recovery roller 15 is covered with an insulating film 15a, and the static elimination roller 17 holds the potential of the recovery roller 15 at a potential opposite to that of the transfer residual toner, which is about several hundred V larger than that of the opposing photosensitive drum 3.

【0104】

Therefore, the cleaning bias blocking method of the present invention as described above is also effective in this embodiment, and can prevent potential unevenness on the surface of the photosensitive drum 3 and the phenomenon of toner ejection. Furthermore, since the contact charger 1A and the like are composed of rollers that can simplify the contact / detachment mechanism, the device can be simplified.

【0105】

[Effect of the invention]

As described above, in the first invention, the image carrier, the image forming means for forming the toner image on the image carrier, and the toner image formed on the image carrier by the image forming means are transferred materials. It has a residual toner remaining on the image carrier and a charging member that charges the image carrier after being transferred to the image carrier, and the charging member can be brought into contact with the image carrier, and when charging, the charging member is an image. In an image forming apparatus that comes into contact with a carrier, it has a control means for controlling the voltage applied to the charging member, and when the image carrier is charged by the charging member, the DC voltage and the AC voltage applied to the charging member are controlled. A discharge is performed between the charging member and the image carrier by the superimposed voltage, and the control means is configured to control the surface potential of the charging member when the charging member is separated from the image carrier. It is possible to prevent potential unevenness on the carrier and the phenomenon of toner discharge, and it is possible to always obtain a high-quality image.

【0106】

In the second invention, after the intermediate transfer body, the image forming means for forming the toner image on the intermediate transfer body, and the toner image formed on the intermediate transfer body by the image forming means are transferred to the transfer material, It has residual toner remaining on the intermediate transfer body and a charging member that charges the intermediate transfer body. The charging member can be brought into contact with the intermediate transfer body, and when charging, the charging member comes into contact with the intermediate transfer body. The forming apparatus has a control means for controlling the voltage applied to the charging member, and when the intermediate transfer member is charged by the charging member, the charging member is charged by a voltage obtained by superimposing a DC voltage applied to the charging member and an AC voltage. A discharge is performed between the and the intermediate transfer body, and the control means is configured to control the surface potential of the charging member when the charging member is separated from the intermediate transfer body. After being charged, it is possible to prevent uneven charging of the intermediate transfer body and always obtain a high-quality image.

【0107】

The third invention is applied to an image carrier, an image forming means for forming a toner image on the image carrier, a charging member that contacts the image carrier and charges the image carrier, and a charging member. A control means for controlling a voltage, and a toner image on an image carrier formed by the image forming means is an image forming device that is transferred to a transfer material, and when the image carrier is charged by a charging member. , A discharge is performed between the charging member and the image carrier by a voltage obtained by superimposing the DC voltage and the AC voltage applied to the charging member, and the control means attenuates the peak-to-peak voltage of the voltage applied to the charging member. Since the supply of the AC voltage to the charging member is stopped after the charging is performed, uneven charging of the image carrier due to the charging member can be prevented, and a high-quality image can always be obtained.

【0108】

The fourth invention is applied to an intermediate transfer body, an image forming means for forming a toner image on the intermediate transfer body, a charging member that contacts the intermediate transfer body and charges the intermediate transfer body, and a charging member. When the toner image on the intermediate transfer body formed by the image forming means is an image forming apparatus which has a control means for controlling the voltage and is transferred to the transfer material, and the intermediate transfer body is charged by the charging member. , A discharge is performed between the charging member and the intermediate transfer member by a voltage obtained by superimposing the DC voltage and the AC voltage applied to the charging member, and the control means attenuates the peak-to-peak voltage of the voltage applied to the charging member. Since the supply of the AC voltage to the charging member is stopped after the charging, uneven charging of the intermediate transfer member due to the charging member can be prevented, and a high-quality image can always be obtained.

[Simple explanation of drawings]

[Figure 1]

It is a block diagram of the cleaning bias power source which is a high voltage generating means used in this invention.

[Figure 2]

The sequence and waveform when the cleaning bias is cut off in the present invention, and the surface potential of the intermediate transfer belt before and after the cleaning bias is cut off are shown.

[Fig. 3]

The other sequences and waveforms used in the present invention when the cleaning bias is blocked, and the surface potential of the intermediate transfer belt before and after the cleaning bias is blocked are shown.

[Fig. 4]

It is a schematic block diagram of one Example of the color image forming apparatus to which this invention is applied.

[Fig. 5]

It is a schematic block diagram of another Example of the color image forming apparatus to which this invention is applied.

[Fig. 6]

It is a schematic block diagram of another Example of the color image forming apparatus to which this invention is applied.

[Fig. 7]

The configuration of one example of the intermediate transfer belt is shown.

[Fig. 8]

The configuration of another embodiment of the intermediate transfer belt is shown.

[Fig. 9]

It is a conceptual diagram of the state of toner on a non-charge type intermediate transfer belt.

[Fig. 10]

It is a conceptual diagram of the state of the toner on the charge type intermediate transfer belt.

[Fig. 11]

An example of cleaning bias is shown.

[Fig. 12]

The waveform when the conventional cleaning bias is cut off and the surface potential of the intermediate transfer belt at that time are shown.

[Fig. 13]

It is a schematic block diagram of the conventional image forming apparatus.

[Fig. 14]

It is a schematic block diagram of the conventional image forming apparatus.

[Explanation of symbols]

1,1A cleaning device (contact charger) 2 Intermediate transfer belt (second image carrier) 3 Photosensitive drum (first image carrier) 4, 4A primary charger 5 Exposure device 6 Developer 7 Secondary transfer device 8 Cleaning equipment 100 High-voltage generating means 101 AC bias power supply 102 DC bias power supply 103 AC waveform generator 104 Zero cross detection circuit 105 PWM circuit