JP2000029287A - Image forming device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To prevent toner and carrier from being splashed by providing a device with an inner potential applying means applying inner potential whose polarity is negative or positive with respect to ground potential to a photoreceptor. SOLUTION: This device is provided with a high-voltage power source 61 for applying the potential (inner potential) whose polarity is negative or positive to a base material 11 in the image forming area of the photoreceptor drum 1 and the inner potential applying means 6 including a conductive body 62 for setting the surface potential of the drum 1 to be near the ground potential. By applying the inner potential whose polarity is negative or positive with respect to the ground potential to the photoreceptor 1 by the applying means 6, the image forming area of the surface of the photoreceptor 1 is uniformly electrified to the surface potential whose polarity is positive or negative with respect to the inner potential of the photoreceptor 1. Thus, since the change of the surface potential of the photoreceptor 1 is also reduced when the electrifying action is moved to the image forming area from the non-image forming area, developer is prevented from being unnecessarily attached to the surface of the photoreceptor without executing a complicated control action. Besides, the developer is prevented from being unnecessarily splashed toward peripheral members.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an image forming apparatus, and more particularly, to a copying machine for performing reversal development in which toner is adhered to an exposed portion of a uniformly charged photosensitive member surface. The present invention relates to an image forming apparatus such as a facsimile and a printer.

[0002]

2. Description of the Related Art For example, image forming apparatuses such as copying machines, facsimile machines, laser printers and the like perform reversal development in which toner is adhered to a uniformly charged surface of a photoreceptor exposed to light. In some cases, regular development is performed in which toner is adhered to a portion of a uniformly charged photoconductor surface that has not been exposed. In an analog machine, the reflected light from the original is directly radiated to the photoreceptor, so the above-mentioned regular development is often used. In a digital copying machine or the like, the above-mentioned reversal development is often used in order to attach toner to a laser emitting portion. Used. First, an image forming apparatus that performs the above-described reversal development will be described with reference to FIG. here,
FIG. 5 is a diagram showing an example of a schematic configuration of a copying machine. As shown in FIG. 5, in the copying machine, a voltage is applied between the photosensitive drum 1 having the photosensitive layer 12 formed on the base material 11 and the tungsten wire 21 and the base material 11 to apply a voltage to the surface of the photosensitive layer 12. A charger 2 for uniformly charging the image forming area of the photosensitive layer 12, a laser light source 3 for exposing the image forming area on the surface of the photosensitive layer 12 uniformly charged by the charger 2 in accordance with a document image, The image forming apparatus includes developing means 4 for attaching toner to a portion exposed by the light source 3 to perform development, and transfer means 5 for transferring a toner image formed by the development of the developing means 4 onto a sheet. The photosensitive drum 1 includes a grounded conductive substrate 11, a photosensitive layer 12 made of a photoconductor such as an organic photosensitive member, and the like, and is driven to rotate by a drive motor (not shown). The photoconductor is usually in an electrically insulated state, and the electric resistance of the irradiated portion is locally changed by light irradiation. The charger 2 includes, for example, a tungsten wire 21, a metal case 22, a high-voltage power supply 23, and the like, and uniformly charges the image forming area of the photosensitive layer 12 in an electrically insulated state. That is, a high voltage is applied between the grounded base material 11 and the tungsten wire 21 by the high-voltage power supply 23 to cause corona discharge, so that the photosensitive layer 12, in other words, the surface of the photosensitive drum 1 A charge of a predetermined polarity is supplied. When the laser light source 3 irradiates the image forming area of the photosensitive layer 12 uniformly charged by the charger 2 with light by the laser light source 3, the electric resistance of the irradiated portion changes, and the potential drops only in that portion. The exposure by the laser light source 3 is controlled on and off in accordance with the image of the document, and the rotation of the photosensitive drum 1 driven by the drive motor causes the document to be formed in an image forming area on the surface of the photosensitive drum 1. An electrostatic latent image corresponding to the image is formed. The developing means 4 includes a developer container 41, a developing roller 42, a high-voltage power supply 43, and the like.
The developer accommodating section 41 accommodates a two-component developer in which toner, which is insulating particles colored in a specific color, and carrier, which is magnetic particles, are mixed. By stirring the toner and the carrier in the developer container 41,
The toner is frictionally charged to a predetermined polarity. On the other hand, the carrier is frictionally charged to the opposite polarity to the toner by the stirring. The developing roller 42 is provided with a magnetic pole inside, and the toner is attached to the surface of the developing roller 42 together with the carrier. A developing bias having a predetermined polarity is applied to the developing roller 42 by a high-voltage power supply 43, and the toner on the surface of the developing roller 42 is electrically attached to the electrostatic latent image.
For example, if the toner is frictionally charged to a positive polarity by stirring with a carrier, in the conventional apparatus, the surface of the photosensitive layer 12 is also charged to a positive polarity by the charger 2.
The absolute value of the high voltage applied to the developing roller 42 by the high voltage power supply 43 is smaller than the surface potential of several hundred volts formed by applying the high voltage to the tungsten wire 21 by the high voltage power supply 23. Even in the image forming area, the toner does not adhere to a portion where the exposure is not performed. On the other hand, the potential of the portion where the potential is reduced by the exposure becomes lower than the developing voltage, and the toner adheres. Hereinafter, the potential of the portion where the toner adheres (the potential of the exposed portion in this case) is referred to as a bright potential, and the potential of the portion where the toner does not adhere (the potential of the non-exposed portion in this case) is referred to as a dark potential. As described above, the toner is adhered to the electrostatic latent image by the developing means 4, so that a toner image corresponding to the image of the document is formed on the photosensitive drum 1. And
This toner image is transferred to a sheet by the transfer unit 5.
Next, referring to FIG. 6, the timing of voltage application in each section of the copying machine will be described. 6A shows ON / OFF of the high voltage power supply 23 of the charger 2, FIG. 6B shows ON / OFF of the laser light source 3, FIG. 6C shows the drum potential on the surface of the photosensitive drum 1, and FIG. 6D shows ON / OFF of the high-voltage power supply 43 of the developing unit 4, FIG. 6E shows ON / OFF of the drive motor, FIG. 6F shows ON / OFF of a discharging lamp (not shown), and FIG. 5 on and off are shown in time series. When the user gives a copy instruction, the power supply of the drive motor for rotating the photosensitive drum 1 and the like is turned on, and is turned off after a predetermined time has elapsed (see FIG. 6F). After the power supply of the drive motor is turned on, the high voltage power supply 23 (see FIG. 6A) of the charger 2 and the high voltage power supply 43 of the developing roller 42 (see FIG. 6D) are turned on, and the drive motor is turned on. Before turning off the power, the high-voltage power supply 23 of the charger 3 and the high-voltage power supply 43 of the developing roller 42 are turned off. When turned on, the high-voltage power supply 23 of the charger 2 applies a voltage of, for example, 800 V between the tungsten wire 21 and the base 11. This allows 0-100V
The drum potential on the surface of the photosensitive drum 1 (see FIG. 6 (c)), which has been there, rises to 800V with a delay of a predetermined time. This delay is similar to the delay that occurs when a step voltage is input to the capacitance.
Also occurs when the high-voltage power supply 23 is turned off. The developing roller 4
The high-voltage power supply 43 is turned on in advance before the drive motor starts to be turned on, and applies a developing bias of, for example, −100 V to the developing roller 42. When the drive motor is turned on, the developing bias is increased stepwise by voltage control in accordance with the delay of the drum potential, and finally a developing bias of 600 V is applied to the developing roller. Further, when the drive motor is turned off from this state, the developing bias is reduced stepwise by voltage control in accordance with the delay of the drum potential, and the developing bias of -100 V is applied in the same manner as before the drive motor was turned on. The voltage is applied to the developing roller 42. The reason for applying the developing bias of -100 V before starting the drive motor or after turning off the drive motor is to prevent the developer from adhering to a non-image forming area other than the image forming area. This is because the bright potential becomes a value near the ground potential. The high voltage power supply 43 supplies the developing roller 42 with 60
While the developing bias of 0 V is applied (the dark potential portion), the positive polarity toner adhering to the surface of the developing roller 42 generates a repulsive force larger than the repulsive force received from the developing bias (600 V). The drum potential (80
0V), the toner does not transfer from the surface of the developing roller 4 to the surface of the photosensitive drum 1.
Further, the developing roller 42 includes not only the toner of positive polarity but also
Although the negative carrier is also attached, the developing roller 4
Since the voltage (difference between the developing bias and the drum potential) acting between the photosensitive drum 2 and the photosensitive drum 1 is about 200 V, there is almost no transfer of the carrier to the photosensitive drum 1. Further, while the developing bias of about 600 V is applied, the laser light source 3 is turned on and off in accordance with the image of the document. As a result, the surface of the photosensitive drum 1 is exposed, and the drum potential of the exposed portion decreases from 800 V to 100 V. At this time, the positive polarity toner adhering to the surface of the developing roller 4 receives a repulsive force from the developing bias (600 V) larger than the repelling force received from the drum potential (100 V), and the exposure is performed. The toner adheres from the surface of the developing roller 4 to the surface of the photosensitive drum 1 at the portion where the operation has been performed. Further, for the carrier, since the electric attraction force from the developing roller 4 is stronger, the carrier does not move to the photosensitive drum 1.

[0003]

In order to obtain an appropriate image density, it is necessary to secure a certain difference between the developing bias and the drum potential. This is because the amount of toner adhering from the developing roller 42 to the surface of the photosensitive drum 1 changes in accordance with the magnitude of the difference between the developing bias and the drum potential. However,
In a conventional image forming apparatus such as a copying machine that performs reversal development, since the drum potential is about 0 to 100 V, in order to obtain an appropriate image density, it is necessary to set the developing bias to a high potential of about 600 V. As a result, the developer may be scattered around the members at the ground potential. Also,
In order to prevent the toner from adhering to the unexposed area in the image forming area, the potential applied by the high-voltage power supply 23 of the charger 2, that is, the dark potential in the image forming area, It is necessary to set a value larger than the developing bias, for example, about 800 V. For this reason, when the drum potential shifts from the non-image forming area to the image forming area, a large change is caused from the vicinity of the ground potential to about 800 V. As already mentioned,
At this time, the drum potential does not change in steps but changes in a curve, and a delay occurs with respect to the developing bias that changes in steps. For this reason, for example, at the same time when the high-voltage power supply 23 of the charger 2 is turned on, the high-voltage power supply 4 of the developing roller 42 is turned on.
When step 3 is turned on in a stepwise manner, during the delay, the developing bias becomes larger than the drum potential, so that the toner unnecessarily adheres to the surface of the photosensitive drum 1. On the other hand, if the developing bias is turned on at the same time as the completion of the change of the drum potential, the toner is prevented from adhering to the photosensitive drum 1 as described above. Increases, the electric attraction force acting on the carrier to the photosensitive drum 1 increases, and the carrier unnecessarily adheres to the photosensitive drum 1. In order to prevent such unnecessary scattering of the developer when shifting from the non-image forming area to the image forming area, the developing bias is controlled stepwise in accordance with the change in the drum potential. Is required, and the configuration of the power supply system becomes complicated. Further, when shifting from the non-image forming area to the image forming area, the developing bias is changed over two positive and negative polarities. This is because a developing bias of, for example, about -100 V is applied by the high-voltage power supply 43 also in the non-image forming area in order to prevent unnecessary adhesion of the developer in the non-image forming area. as a result,
When shifting from the non-image forming area to the image forming area, the developing roller 42 is temporarily in an electrically floating state. Also at this time, especially in the contact developing method in which the development is performed by bringing the photosensitive drum 1 into contact with the developing roller 42, the developer may be unnecessarily transferred from the developing roller 42 to the photosensitive drum 1. There is. These problems are not limited to the above-described case in which the positively charged photoreceptor is subjected to reversal development using the toner of the positive polarity. This also occurs when reversal development is performed using a neutral toner. For reference, FIG. 7 shows, in chronological order, the timing of voltage application in each section of the image forming apparatus that performs reversal development using a negatively charged toner on a negatively charged photoconductor. The correspondence between (a) to (h) of FIG. 7 is the same as that of FIG. As shown in FIG. 7, the timing chart when the negative toner is used for the negatively charged photoreceptor is different from the case where the positive toner is used for the positively charged photoreceptor as described above. Problem that the developing bias must be set to a large value of negative polarity,
The problem that complicated voltage control is required when shifting from the non-image forming area to the image forming area, and the developing bias is changed across two negative and positive polarities when shifting from the non-image forming area to the image forming area The problem has arisen as well. Such various problems in reversal development do not occur in an image forming apparatus such as a copying machine that performs the above-described regular development. here,
FIGS. 8 and 9 are time charts for explaining the timing of voltage application in each section of the image forming apparatus performing the normal development. In the above-described regular development, which is often used in analog machines, exposure is performed by using a negatively or positively charged toner of opposite polarity to a photoreceptor uniformly charged at a positive or negative potential. The toner is adhered to the missing portion. FIG. 8 corresponds to a case where a negative toner is used for a positively charged photoconductor, and FIG. 9 corresponds to a case where a positive toner is used for a negatively charged photoconductor. In both figures, (a) is the on / off of the high voltage power supply of the charger, (b) is the on / off of the blank lamp, (c) is the on / off of the reflected light, (d) is the drum potential on the surface of the photosensitive drum, and (e) Indicates a developing bias, (f) indicates on / off of a drive motor of the photosensitive drum or the like, (g) indicates on / off of a discharging lamp, and (h) indicates on / off of a transfer unit in a time-series manner. In the case of reversal development, only a neutralizing lamp for resetting the photosensitive member is required, but in the case of regular development, a blank lamp is required to erase the potential of unnecessary portions (front and rear ends). As shown in FIG. 8 or FIG. 9, in the image forming apparatus for performing the regular development, the surface of the photosensitive drum is simultaneously turned on and off at the same time as the drive motor for rotating the photosensitive drum and the like is turned on and off (see (f) in both figures). A high-voltage power supply (see (a) in both figures) of a charger for charging the battery in a similar manner and a high-voltage power supply (see (e) in both figures) for applying a developing bias to the developing roller are turned on and off. The magnitude of the drum potential applied to the photosensitive drum surface by the charger is, for example, about 700V. On the other hand, the magnitude of the developing bias is about 200V. Then, in the image forming area, the surface of the photosensitive drum is exposed by the reflected light corresponding to the original image (see (c) in both figures), and only the portion corresponding to the original image 700
A drum potential of about V remains. The potential (dark potential) of other portions in the image forming area is 0 to 10 by the above exposure.
It becomes about 0V. In the portion where the drum potential of about 700 V remains, the toner having the polarity opposite to the charging polarity of the photosensitive drum receives a greater suction force from the photosensitive drum than the electrical suction force received from the developing roller. That is, at that portion, the toner is transferred from the developing roller to the photosensitive drum, and normal development is performed. Conversely, a carrier having a polarity opposite to that of the toner, that is, a carrier having the same polarity as the charging polarity of the photosensitive drum, receives a repulsive force from the photosensitive drum larger than the electric repulsive force received from the developing roller. There is no scattering from the developing roller to the photosensitive drum. Further, in the image forming region where the magnitude of the potential is reduced from 700 V to about 0 to 100 V by the exposure, the toner applies a larger suction force than the electric suction force received from the photosensitive drum from the developing roller. The photosensitive drum 1
Does not scatter. As for the carrier, the electric repulsion received from the developing roller is smaller than the electric repulsion received from the photosensitive drum. However, the toner and the carrier may be charged with opposite polarities. The difference between the drum potential and the developing bias is 200
If it is about V, it does not unnecessarily scatter from the developing roller to the photosensitive drum. As described above, even in the normal development, the toner is caused to adhere to the photosensitive drum from the developing roller due to the difference between the developing bias and the drum potential. However, the magnitude relationship between the developing bias and the drum potential is different from that of the reversal developing. The opposite is true. That is, in the normal development, the drum potential has a value smaller than the development bias. This is because the toner adheres to the portion where the drum potential is high, and does not adhere to the portion where the drum potential is low. In the case of the reversal development, the magnitude of the dark potential is about 800 V, whereas in the case of the normal development, the magnitude of the dark potential is about 0 to 100 V. Therefore, the developing bias is set to 600
Although it needs to be set to about V, the developing bias may be about 200 V in the normal development. By setting the drum potential higher than this developing bias by about 500 V, an appropriate image density can be obtained.
That is, in the case of the regular development, the developing bias is 200
Since it is a relatively small value of about V and the developer is also magnetically constrained, the developer does not scatter to surrounding members near the ground potential. Further, in order to prevent the toner from scattering in the non-image forming area, it is not necessary to apply the voltage before the start of the drive motor or after the drive motor is turned off. That is, since the drum potential and the dark potential in the non-image forming area hardly change, there is no need to change the developing bias stepwise, and there are two positive and negative polarities when shifting from the non-image forming area to the image forming area. It is not necessary to change the developing bias over the range. As described above, in the image forming apparatus that performs the normal development, the toner adheres to the portion where the magnitude of the drum potential is large and the toner does not adhere to the portion where the drum potential is small. Therefore, the problem of the image forming apparatus that performs the reversal development does not occur. However,
In a digital machine, laser irradiation must be performed on a blank portion to perform regular development, which shortens the life of the optical unit. In order to solve the problems in the conventional technique of performing the above-described reversal development, the present invention improves the image forming apparatus and sets the internal potential of the photosensitive drum to be negative or positive with respect to the ground potential. Accordingly, it is an object of the present invention to provide an image forming apparatus which solves the above-mentioned problem by performing reversal development by forming the surface potential of the photosensitive member to be positive or negative in view of the internal potential.

[0004]

In order to achieve the above object, the invention according to claim 1 is to reduce the image forming area on the surface of the photoconductor to a positive or negative surface potential when viewed from the internal potential of the photoconductor. Charging means, an exposure means for exposing the image forming area of the photoreceptor corresponding to the original image, and developing by attaching a positive or negative toner to a portion exposed by the exposure means. An image forming apparatus comprising: a developing means; and an internal potential applying means for applying a negative or positive internal potential to the photoconductor as viewed from the ground potential. ing. According to a second aspect of the present invention, in the image forming apparatus according to the first aspect, the internal potential applying means includes:
The gist of the invention is to provide the photoconductor with the internal potential higher than the surface potential. According to a third aspect of the present invention, in the image forming apparatus according to the first or second aspect, the charging means applies a surface potential near ground potential to the photoconductor. . According to a fourth aspect of the present invention, in the image forming apparatus of the third aspect, the charging means includes a conductor that contacts the surface of the photoconductor. According to a fifth aspect of the present invention, in the image forming apparatus of the fourth aspect, a DC bias near a ground potential is applied to the conductor. In addition, the invention according to claim 6 is based on claim 4 or 5 described above.
The gist of the present invention is that an AC bias is applied to the conductor. According to a seventh aspect of the present invention, in the image forming apparatus according to any one of the first to sixth aspects, the toner is contained in a two-component developer mixed with a carrier charged to the opposite polarity. It is the gist of what is. According to an eighth aspect of the present invention, in the image forming apparatus according to any one of the first to seventh aspects, the invention is characterized in that the photoconductor and the developing unit are in contact with each other. According to the image forming apparatus of any one of claims 1 to 8, the internal potential of the photoconductor drum is set to be negative or positive with respect to the ground potential by the internal potential applying means, so that the photoconductor is provided. Since the surface potential is formed to be positive or negative with respect to the internal potential and reversal development is performed, the potential of the exposed portion can be set higher than the potential of the unexposed portion. Become. That is, similarly to the normal development, the potential of the portion where the developer is attached can be set higher than the potential of the portion where the developer is not attached. For this reason, even when shifting from the non-image forming area to the image forming area, the change in the surface potential of the photoreceptor is reduced, and without complicated control,
Unnecessary adhesion of the developer can be prevented. Also,
Since the surface potential of the exposed portion of the photoreceptor has a large value, the developing bias can be set to a small value correspondingly, preventing the developer from being unnecessarily scattered to peripheral members. can do. In addition, since the developer adheres to a portion having a large potential, it is not necessary to apply a reverse bias for preventing unnecessary developer from being adhered even in a non-image forming area where no voltage is applied.
When shifting from the non-image forming area to the image forming area, there is no need to change the potential across the two polarities. Therefore, an electric floating state does not occur temporarily at the time of potential switching, and unnecessary adhesion of the developer in the floating state can be prevented even in the contact developing method in which the photosensitive member and the developing unit are in contact with each other. Further, since the same potential setting as in the normal development can be performed, the system can be shared between the normal development and the reversal development.

[0005]

Embodiments of the present invention will be described below with reference to the accompanying drawings to provide an understanding of the present invention.
The following embodiment is a specific example of the present invention and does not limit the technical scope of the present invention. First, a schematic configuration of an image forming apparatus according to an embodiment of the present invention will be described with reference to FIG. The image forming apparatus according to the present embodiment is embodied as, for example, a copying machine, and as shown in FIG. 1, a photosensitive drum 1 having a photosensitive layer 12 formed on a base material 11 and the photosensitive drum 1. Laser light source 3 for exposing an image forming area of body drum 1 in accordance with a document image
It is the same as a conventional image forming apparatus in that it includes a developing unit 4 for performing reversal development for attaching toner to a portion exposed by the laser light source 3. The image forming apparatus is particularly different from the conventional apparatus in that a high-voltage power supply 61 for applying a negative or positive potential (internal potential) to the substrate 11 in the image forming area of the photosensitive drum 1 and the photosensitive body And a conductor 62 for setting the surface potential of the drum 1 near the ground potential. The internal potential applying means 6 detects the negative or positive internal potential with respect to the ground potential from the ground potential. The difference is that the image forming area on the surface of the photoreceptor is uniformly charged to a positive or negative surface potential when viewed from the internal potential of the photoreceptor 1 when applied to the body 1. The photosensitive drum 1 is formed by forming a photosensitive layer 12 made of a photoconductor such as an organic photoconductor on a conductive base material 11, and is driven to rotate together with other members by a drive motor (not shown). The photoconductor is usually in an electrically insulated state, and the electric resistance of the irradiated portion is locally changed by light irradiation. A high-voltage power supply 61 of the internal potential applying means 6 is provided on the base 11 by a leaf spring,
They are connected via a conductive member such as a rotating shaft. The high-voltage power supply 61 of the internal potential applying means 6 applies a negative or positive potential of, for example, about 800 V to the base material 11 in the image forming area of the photosensitive drum 1. The surface of the photosensitive layer 12 is provided with the internal potential applying means 6.
Are in contact with each other. As shown in FIG. 2, the conductor 62 of the internal potential applying means 6 was provided so as to uniformly contact the photosensitive drum 1 in the axial direction.
For example, it is a conductive resin fiber brush or the like. This conductor 6
2 is connected to, for example, an AC power supply 621, and its potential is maintained near the ground potential. Therefore, the surface potential of the photosensitive drum 1 with which the conductor 62 contacts is set near the ground potential. In the image forming apparatus, the surface potential of the photosensitive drum 1 is set near the ground potential, and the internal potential of the photosensitive drum 1 (base
Is set to a positive or negative potential of, for example, about 800 V corresponding to the image forming area, so that the surface potential of the photosensitive drum 1 viewed from the internal potential becomes A negative or positive potential of about 700 V is set, and the image forming area of the photosensitive drum 1 is uniformly charged. When the laser light source 3 irradiates the image forming area of the photosensitive layer 12 uniformly charged by the internal potential applying means 6 with the laser light source 3, the electric resistance of the irradiated portion changes and the internal potential is changed from the internal potential. The potential of the surface of the photosensitive drum 1 as viewed decreases only in that portion. The exposure by the laser light source 3 is controlled on and off in accordance with the image of the document, and an electrostatic latent image is formed in an image forming area on the surface of the photosensitive drum 1 as the photosensitive drum 1 rotates. The developing means 4 includes a developer container 4
1, a developing roller 42, a high-voltage power supply 43, and the like. The developer accommodating section 41 accommodates a two-component developer in which toner, which is insulating particles colored in a specific color, and carrier, which is magnetic particles, are mixed. By stirring the toner and the carrier in the developer container 41, the toner is frictionally charged to a predetermined polarity. On the other hand, the carrier is frictionally charged to the opposite polarity to the toner by the stirring. The developing roller 42 is provided with a magnetic pole inside, and the toner is attached to the surface of the developing roller 42 together with the carrier. A developing bias having a predetermined polarity is applied to the developing roller 42 by a high-voltage power supply 43, and the toner on the surface of the developing roller 42 is electrically attached to the electrostatic latent image. For example, assuming that the toner is negatively frictionally charged by stirring with the carrier, in the image forming apparatus according to the present embodiment, the potential of the substrate 11 (the internal potential of the photosensitive drum 1) is changed to the internal potential. The potential is set to a positive polarity by the high voltage power supply 61 of the potential applying means 6. FIG. 3 shows the timing of voltage application in each section of the copying machine in chronological order. 3A shows the on / off state of the high voltage power supply 23 of the charger 2, FIG. 3B shows the on / off state of the high voltage power supply 61 of the internal potential applying means 6, FIG. 3C shows the on / off state of the laser light source 3, and FIG. 3D shows the drum potential on the surface of the photosensitive drum 1, FIG. 3E shows the on / off state of the high-voltage power supply 43 of the developing means 4, FIG. 3F shows the on / off state of the drive motor, and FIG.
FIG. 3 (g) shows the on / off state of the neutralization lamp (not shown), and FIG. 3 (h) shows the on / off state of the transfer means 5 in time series. When the user gives a copy instruction, the power of the drive motor for rotating the photosensitive drum 1 and the like is turned on, and is turned off after a predetermined time has elapsed (see FIG. 3F).
Almost simultaneously with turning on / off the power of the drive motor, the high-voltage power supply 61 of the internal potential applying means 6 (see FIG. 3B), the high-voltage power supply 43 of the developing roller 42 (see FIG. 3D), and the like are also turned on / off. . The charger 2 (see FIG. 3A) is set to GND. The high-voltage power supply 61 of the internal potential applying means 6 applies, for example, a voltage of 800 V to the substrate 11 of the photosensitive drum 1 in the image forming area of the photosensitive drum 1 when viewed from the ground potential. On the other hand, the potential of the surface of the photosensitive drum 1 is maintained near the ground potential by the conductor 62. As a result, the drum potential on the surface of the photosensitive drum 1 is set to a negative potential of about 700 V when viewed from the substrate 11 of the photosensitive drum 1. The high-voltage power supply 43 of the developing means 4 does not apply any developing bias to the developing roller 42 before the drive motor is turned on, that is, in the non-image forming area. Unlike the conventional reversal developing apparatus, in which a reverse bias is not applied in advance like the normal developing apparatus in which a developer is attached to a portion having a small potential value, the copying machine according to the present embodiment is different from the conventional reversing developing apparatus. This is because the potential of the exposed portion is higher than the non-image forming area and the dark potential, and unnecessary scattering of the developer is less likely to occur in the non-image forming area. When the driving motor is turned on, the high-voltage power supply 43 applies a potential of 200 V to the developing roller 42 as the developing bias almost simultaneously. Therefore, when shifting from the non-image forming area to the image forming area, the switching of the potential does not extend over two polarities as in the conventional apparatus for performing reversal development. Therefore, the developing roller 42 is temporarily prevented from being in an electrically floating state, and the photosensitive drum 1
Even when the contact developing method of performing development by bringing the developer into contact with the developing roller 42 is used, unnecessary adhesion of the developer is prevented. While a developing bias of about 200 V is applied to the developing roller 42 by the high-voltage power supply 43 (dark potential portion), the negative polarity toner adhering to the surface of the developing roller 4 removes the ground potential. The toner does not transfer from the surface of the developing roller 4 to the surface of the photosensitive drum 1 because the developing bias receives an electric attraction greater than the electric force received from the nearby drum potential. Further, not only the negative polarity toner but also the positive polarity carrier charged to the opposite polarity to the toner adheres to the developing roller 4.
Is about 200 V, and carriers hardly migrate due to the magnetic restraining force. Next, while the developing bias of about 200 V is applied, the laser light source 3 is turned on and off in accordance with the image of the document. As a result, the surface of the photosensitive drum 1 is exposed, and the drum potential of the exposed portion is changed from near the ground potential to 7
The potential changes to about 00V. That is, the drum potential as seen from the internal potential drops from a potential of about -700 V to near the ground potential. At this time, the negative polarity toner adhering to the surface of the developing roller 4 is removed by the developing bias (2).
00 V) from the drum potential (700 V), and toner adheres from the surface of the developing roller 4 to the exposed portion of the surface of the photosensitive drum 1. I do. Further, for a carrier charged to the opposite polarity, the electric repulsive force from the photosensitive drum 1 becomes stronger, so that the carrier does not transfer to the photosensitive drum 1. That is, in the copying machine according to the present embodiment, even when a certain amount of difference between the developing bias and the drum potential is provided to obtain a sufficient image quality, the developing bias is set to be small because the drum potential of the exposed portion is large. It is also possible to prevent the developer from being unnecessarily scattered on members around the ground potential. In this way, the toner adhered to the exposed portion is transferred to the sheet by the transfer unit 5. In the transfer unit 5, even when the negative polarity toner is used as described above, the bias applied at the time of the transfer can be set to the positive polarity. For this reason, it becomes easy to suppress the generation of ozone and the like. As described above, according to the copying machine of the present embodiment, the internal potential of the photoconductor drum is set to be positive with respect to the ground potential by the internal potential applying means, so that the surface potential of the photoconductor is changed from the internal potential. In fact, since the negative polarity is formed and the reversal development is performed, the potential of the exposed portion can be set to be higher than the potential of the unexposed portion. That is, like regular development,
The potential of the portion where the developer is attached can be set higher than the potential of the portion where the developer is not attached. For this reason, even when shifting from the non-image forming area to the image forming area, the change in the surface potential of the photoconductor is reduced, and unnecessary adhesion of the developer can be prevented without complicated control. Further, since the drum potential of the exposed portion of the photoconductor becomes a large value, the developing bias can be set to a small value correspondingly, and the developer is unnecessarily scattered to peripheral members. Can be prevented. In addition, since the developer adheres to the portion where the potential is large, it is not necessary to apply a reverse bias to prevent unnecessary developer adhesion even in a non-image forming area where no voltage is applied. When shifting from the formation region to the image formation region, there is no need to change the potential across the two polarities. Therefore, an electrical floating state does not occur temporarily at the time of the potential switching, and unnecessary adhesion of the developer in the floating state can be prevented even in the contact developing method in which the photosensitive member and the developing unit are in contact with each other. Further, since the same potential setting as in the normal development can be performed, the system can be shared between the normal development and the reversal development.

[0006]

In the above embodiment, the present invention is applied to a copying machine. However, the present invention is not limited to this, and may be applied to other image forming apparatuses such as a laser printer and a facsimile. In the above embodiment, the case where reversal development is performed using negative polarity toner is described. However, the present invention is not limited to this case, and the case where reversal development is performed on a positively charged photoconductor using positive toner is described. It is also possible to apply the present invention. FIG. 4 shows, in chronological order, the timing of voltage application to each part in the copying machine when a positive polarity toner is used for a positively charged photoconductor. 4A to 4H are the same as those in FIG. As shown in FIG. 4, the timing chart when the positive toner is used for the positively charged photoreceptor is different from the case where the negative toner is used for the negatively charged photoreceptor. Symmetrically. Such an image forming apparatus is also an example of the image forming apparatus of the present invention. In the above-described embodiment, the drum potential of the photosensitive drum 1 is kept close to the ground potential by bringing the grounded conductor 62 into contact with the surface of the photosensitive layer 11, but the present invention is not limited to this. Applying a DC voltage near the ground potential to the conductor 62,
An AC voltage may be applied. Further, these may be combined. Further, the drum potential is not limited to the vicinity of the ground potential. Such an image forming apparatus is also an example of the image forming apparatus of the present invention.

[0007]

As described above, according to the image forming apparatus of any one of claims 1 to 8, the internal potential of the photosensitive drum is negative or positive when the internal potential of the photosensitive drum is viewed from the ground potential by the internal potential applying means. By setting the surface potential of the photosensitive member to a positive or negative polarity in view of the internal potential by performing the reversal development, the potential of the exposed portion is reduced to the potential of the unexposed portion. It can be set higher than the potential. That is, similarly to the normal development, the potential of the portion where the developer is attached can be set higher than the potential of the portion where the developer is not attached. For this reason, even when shifting from the non-image forming area to the image forming area, the change in the surface potential of the photoconductor is reduced, and unnecessary adhesion of the developer can be prevented without complicated control. Further, since the drum potential of the exposed portion of the photoconductor becomes a large value, the developing bias can be set to a small value correspondingly, and the developer is unnecessarily scattered to peripheral members. Can be prevented. In addition, since the developer adheres to the portion where the potential is large, it is not necessary to apply a reverse bias to prevent unnecessary adhesion of the developer even in a non-image forming area where no voltage is applied. When shifting from the formation area to the image formation area, there is no need to change the potential across the two polarities. Therefore, an electric floating state does not occur temporarily at the time of potential switching, and unnecessary adhesion of the developer in the floating state can be prevented even in the contact developing method in which the photosensitive member and the developing unit are in contact with each other. Further, since the same potential setting as in the normal development can be performed, the system can be shared between the normal development and the reversal development.

[Brief description of the drawings]

FIG. 1 is a diagram showing a schematic configuration of a copying machine according to an embodiment of the present invention.

FIG. 2 is a diagram for explaining the copying machine according to the embodiment in detail;

FIG. 3 is a diagram showing, in chronological order, timings of voltage application of negatively charged negative toner to each section of the copying machine.

FIG. 4 is a diagram showing, in chronological order, the timing of voltage application of positively charged positive toner to each section of the copying machine.

FIG. 5 is a diagram illustrating an example of a schematic configuration of a conventional copier that performs reversal development.

FIG. 6 is a diagram showing, in chronological order, the timing of voltage application of positively charged positive toner to each section of the conventional copying machine.

FIG. 7 is a diagram showing, in chronological order, timings of voltage application of the negatively charged negative toner to each section of the conventional copying machine.

FIG. 8 is a diagram showing, in chronological order, the timing of voltage application in each section of a copying machine that performs normal development of positively charged positive toner.

FIG. 9 is a diagram showing, in chronological order, the timing of voltage application in each section of the copying machine that performs normal development of negatively charged negative toner.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 ... Photoconductor 3 ... Laser light source (exposure means) 4 ... Developing means 6 ... Internal potential application means 61 ... High voltage power supply 62 ... Conductor

 ────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Keisuke Oba 1-2-2, Tamazo, Chuo-ku, Osaka-shi, Osaka Mita Industries Co., Ltd. F-term (reference) 2H003 AA12 BB11 CC04 2H073 AA05 AA10 BA02 BA03 BA13 CA03

Claims (8)

[Claims] A charging means for uniformly charging an image forming area on the surface of the photoconductor to a positive or negative surface potential when viewed from an internal potential of the photoconductor; An image forming apparatus comprising: an exposing means for exposing and exposing; and a developing means for developing by attaching a positive or negative toner to a portion exposed by the exposing means. An image forming apparatus, comprising: an internal potential applying means for applying a neutral internal potential to the photosensitive member. 2. The image forming apparatus according to claim 1, wherein said internal potential applying means applies said internal potential higher than said surface potential to said photoconductor. 3. The image forming apparatus according to claim 1, wherein said charging means applies a surface potential near a ground potential to said photosensitive member. 4. The image forming apparatus according to claim 3, wherein said charging means comprises a conductor which comes into contact with said photoreceptor surface. 5. The image forming apparatus according to claim 4, wherein a DC bias near a ground potential is applied to said conductor. 6. The image forming apparatus according to claim 4, wherein an AC bias is applied to the conductor. 7. The image forming apparatus according to claim 1, wherein the toner is contained in a two-component developer mixed with a carrier charged in the opposite polarity. 8. The image forming apparatus according to claim 1, wherein the photoconductor and the developing unit are in contact with each other.