IMAGE FORMING APPARATUS

An additive coating is executed, where an additive is transferred, from a developing roller to a photosensitive drum by setting a surface potential of the drum to be lower than a developing voltage, and from the drum to a charging roller by setting a charging voltage to be lower than the surface potential. By setting a potential difference Vback between the surface potential and a developing voltage in a developing portion to be larger in the external additive coating than that in the image forming, a development amount of the additive is increased. Further, by setting a transfer voltage to be higher than the surface potential, transfer of the additive is suppressed. By setting the potential difference between the surface potential and the transfer voltage to be smaller than a discharge threshold between the transfer roller and the drum, the additive is suppressed from having a strong positive polarity.

BACKGROUND OF THE INVENTION

Field of the Invention

Description of the Related Art

Conventionally, an imaging apparatus, such as a copier and a printer, that forms images using an electrophotographic process is known. Such an image forming apparatus electrostatically transfers a toner image, which is formed on a surface of a photosensitive drum, i.e., image bearing member, onto a recording material or the like in a transfer step, by applying voltage from a power supply to a transfer member which is disposed facing the photosensitive drum. Then, an image is formed by fixing the toner image onto the recording material using a fixing unit.

As means for collecting untransferred toner remaining on the photosensitive drum, Japanese Patent Application Publication No. 2001-183905 proposes a cleanerless (simultaneous developing-cleaning) type image forming apparatus, which collects the toner on the photosensitive drum using a developing apparatus so as to reuse the toner.

In a case of an image forming apparatus that is a cleanerless type and is also a contact charging type which performs charging in a state of contacting the drum, a foreign substance may be transferred to the photosensitive drum through a recording material during an image forming operation, and then be transferred to a charging roller. If the foreign substance remains on the surface of the charging roller thereafter, the foreign substance adhering to the charging roller may scratch the surface of the photosensitive drum. In this case, the desired potential may not be reached at the scratched portion, which may generate an image defect.

SUMMARY OF THE INVENTION

It is an object of the present invention to suppress image defects in a cleanerless type and contact charging type image forming apparatus.

According to an aspect of the present disclosure, an image forming apparatus of the present disclosure includes:a photosensitive drum which is rotatable;a charging unit that rotates in contact with the photosensitive drum, and charges a surface of the photosensitive drum;an exposing unit that exposes the surface of the photosensitive drum charged by the charging unit;a developing unit that supplies toner containing an external additive which has a charging polarity that is an opposite polarity of a charging polarity of the toner to the surface of the photosensitive drum exposed by the exposing unit;a transfer unit that transfers the toner, which has been supplied to the photosensitive drum by the developing unit, to a recording material; anda controller that controls the charging unit, the developing unit and the transfer unit,
wherein, after the toner supplied to the surface of the photosensitive drum is transferred to the recording material, the developing unit is configured to collect toner remaining on the surface of the photosensitive drum,wherein the controller is configured to perform: an image forming operation to form an image on the recording material; and an external additive coating operation to transfer the external additive or the toner containing the external additive from the developing unit to the photosensitive drum, and transfer the external additive from the photosensitive drum to the charging unit, andwherein the controller is configured not to perform exposure by the exposing unit in the external additive coating operation, and configured to control voltages so as to satisfy:

Vrg>Vdg⁢2Vdg⁢1>VcgVback_g>Vback_pVtg>Vdg⁢3Vtg-Vdg⁢3<Vtth,where V1>V2represents a potential relationship in which an electrostatic force, directed from a position of a potential V1to a position of a potential V2, acts on particles having a same charging polarity as a charging polarity of the external additive,Vcg represents a charging voltage in the external additive coating operation,Vdg1represents a surface potential of the photosensitive drum,Vdg2represents a surface potential of the photosensitive drum in a developing portion facing the developing unit,Vdg3represents a surface potential of the photosensitive drum in a transfer portion facing the transfer unit,Vrg represents a developing voltage to be applied to the developing unit,Vtg represents a transfer voltage to be applied to the transfer unit,Vtth represents a threshold of a potential difference with which discharge is generated between the transfer unit and the photosensitive drum,Vback_g represents a potential difference between the developing voltage and the surface potential of the photosensitive drum in the developing portion, andVback_p represents a potential difference between the developing voltage and the surface potential of the photosensitive drum in the developing portion in the image forming operation.

According to another aspect of the present disclosure, an image forming apparatus of the present disclosure includes:a photosensitive drum which is rotatable;a charging unit that rotates in contact with the photosensitive drum, and charges a surface of the photosensitive drum;an exposing unit that exposes the surface of the photosensitive drum charged by the charging unit;a developing unit that supplies toner containing an external additive which has a charging polarity that is an opposite polarity of a charging polarity of the toner to the surface of the photosensitive drum exposed by the exposing unit;a transfer unit that transfers the toner, which has been supplied to the photosensitive drum by the developing unit, to a recording material; anda controller that controls the charging unit, the developing unit and the transfer unit,
wherein, after the toner supplied to the surface of the photosensitive drum is transferred to the recording material, the developing unit is configured to collect toner remaining on the surface of the photosensitive drum,wherein the controller is configured to perform: an image forming operation to form an image on the recording material; and an external additive coating operation to transfer the external additive or the toner containing the external additive from the developing unit to the photosensitive drum, and transfer the external additive from the photosensitive drum to the charging unit, andwherein the controller is configured to perform exposure by the exposing unit on an entire region in a direction intersecting with a circumferential direction of the photosensitive drum in the external additive coating operation, and configured to control voltages so as to satisfy:

Vlg⁢2>VrgVlg⁢1>VcgVtg=Vlg⁢3where V1>V2represents a potential relationship in which an electrostatic force, directed from a position of a potential V1to a position of a potential V2, acts on particles having a same charging polarity as a charging polarity of the external additive,Vcg represents a charging voltage in the external additive coating operation,Vlg1represents a surface potential of the photosensitive drum,Vlg2represents a surface potential of the photosensitive drum in a developing portion facing the developing unit,Vlg3represents a surface potential of the photosensitive drum in a transfer portion facing the transfer unit,Vrg represents a developing voltage to be applied to the developing unit, andVtg represents a transfer voltage to be applied to the transfer unit.

DESCRIPTION OF THE EMBODIMENTS

Preferred embodiments of the present invention will now be described in detail with reference to the drawings. However, dimensions, materials and shapes of composing elements described in the following embodiments, relative positions thereof, and the like may be changed appropriately depending on the configuration and various conditions of an apparatus to which the present invention is applied. Hence, description of the embodiments is not intended to limit the scope of the invention, unless otherwise specified.

FIG.1is a schematic diagram depicting an image forming apparatus of Embodiment 1. The image forming apparatus100of Embodiment 1 is a monochrome laser beam printer which uses the cleanerless system and the contact charging system.

In the image forming apparatus10of Embodiment 1, a rotatable cylindrical electrophotographic photosensitive member (hereafter photosensitive drum)1is disposed as an image bearing member. Around the photosensitive drum1, a charging roller2(charging unit), an exposing apparatus3(exposing unit), a developing apparatus4(developing unit), a transfer roller5(transfer unit), and a pre-charging exposing apparatus6(means for eliminating charging potential) are disposed. In the following description, it is assumed that a lateral direction is a direction vertical to a rotation shaft direction of the photosensitive drum1, and a longitudinal direction is a direction parallel with the rotation shaft direction of the photosensitive drum1.

The photosensitive drum1is an image bearing member that is rotary-driven in the arrow direction and bears a toner image. A control unit99(seeFIG.2), such as a controller, receives image signals from an external apparatus130(seeFIG.2). Thereby the image forming operation is started and the photosensitive drum1is rotary-driven. In the rotating process, the photosensitive drum1is uniformly charged by the charging roller2, to have a predetermined polarity (negative polarity in Embodiment 1) at a predetermined potential. The photosensitive drum1is exposed by the exposing apparatus3in accordance with the image signals. Thereby an electrostatic latent image is formed. Then this electrostatic latent image is developed by the developing apparatus4at a developing position, and is visualized as a toner image.

The charging roller2is rotary-driven in a state of contacting with the photosensitive drum1. The charging roller2is charging unit for charging the surface of the photosensitive drum1at a charging portion which is formed with the photosensitive drum1. The charging roller2may be rotary-driven by the rotation of the photosensitive drum1. The charging roller2has an elastic layer formed of a conductive elastic material of which surface roughness is Ra 1.5 to 2.5 μm. The charging roller2contacts with the surface of the photosensitive drum1at a predetermined contact pressure, and forms a charging portion thereby. In the charging portion, the rotating directions of the charging roller2and the photosensitive drum1are the same. The peripheral speed of the charging roller2is faster than the peripheral speed of the photosensitive drum1. The charging roller2is rotary-driven by a charging roller driver98(seeFIG.2) at a 105% peripheral speed with respect to the photosensitive drum1. To the rotation shaft of the charging roller2, a predetermined DC voltage is applied from a charging voltage power supply92(SeeFIG.2) in accordance with the image forming operation. Here in accordance with the image forming operation, a −1350V DC voltage is applied to the rotation shaft of the charging roller2as the charging voltage Vcp, and the surface of the photosensitive drum1is charged to −780V (predetermined potential). The surface potential of the photosensitive drum1was measured by a surface potential meter, Model344made by Trech Inc. The surface potential−780V of the photosensitive drum1here is a surface potential of the photosensitive drum1in a non-image forming region (dark area potential Vdp2), where a toner image is not developed.

The exposing apparatus3exposes the surface (charging surface) of the photosensitive drum1which is charged by the charging roller2. The exposing apparatus3is exposing unit for exposing the surface of the photosensitive drum1in an exposing portion at the downstream side of a charging portion in the rotating direction of the photosensitive drum1. The exposing apparatus3is a laser scanner apparatus. The exposing apparatus3emits a laser beam based on image information which is inputted from an external apparatus130(e.g. host computer). The exposing apparatus3is exposing unit for forming an electrostatic latent image on the charging surface of the photosensitive drum1, which has been uniformly charged. In Embodiment 1, an exposure amount is adjusted such that the image forming potential of the photosensitive drum1in the electrostatic latent image portion (bright area potential Vlp2) after being exposed by the exposing apparatus3becomes −100V.

The developing apparatus4supplies toner, containing external additive, to the surface (exposure surface) of the photosensitive drum1, which is exposed by the exposing apparatus3. The developing apparatus4is developing unit for supplying toner, containing the external additive, to the surface of the photosensitive drum1in a developing portion at the downstream side of the exposing portion in the rotating direction of the photosensitive drum1. The photosensitive drum1faces the developing apparatus4in the developing portion. The developing apparatus4is constituted of a developing roller41(developer bearing member), a toner supply roller42(developer supply means), a toner storage chamber43to store toner, and a developing blade44. The toner is supplied from the toner storage chamber43to the developing roller41by the toner supply roller42. The toner supplied to the developing roller41passes through a contact portion with the developing blade44, whereby the toner is charged to a predetermined polarity.

The toner is non-magnetic toner of which normal polarity is negative, manufactured by the suspension polymerization method. The volume-average particle diameter of this toner is 6.0 μm, and the toner carried on the developing roller41is charged to negative polarity. An external additive is added to the surface of the toner. The charging polarity of the external additive is the opposite of the charging polarity of the toner. In Embodiment 1, the toner has negative polarity, and the external additive is 200 nm hydrotalcite, of which particles have positive polarity. By externally adding particles of which polarity is opposite that of the toner, polarity of the toner becomes stable. The external additive may also be a different positive type external additive, such as particles using metal (e.g. titanium) or metal oxide.

The developing roller41contacts with the surface of the photosensitive drum1at a predetermined contact pressure, and forms a developing portion thereby. The developing roller41is rotary-driven by a developing roller driver90(seeFIG.2) at 140% peripheral speed with respect to that of the photosensitive drum1. The developing roller41and the photosensitive drum1may be configured to be driven by a common driving source. To a rotation shaft of the developing roller41, a −380V DC voltage (developing voltage Vrp) is applied from a developing voltage power supply93(seeFIG.2). When an image is formed, the toner image carried on the developing roller41is developed in an image forming potential Vlp2portion of the photosensitive drum1by an electrostatic force that is generated by the potential difference between this developing voltage −380V and the image forming potential Vlp2=−100V of the photosensitive drum1.

The transfer roller5transfers the toner, supplied to the photosensitive drum1by the developing apparatus4, to a recording material P, which is a transfer target object. The transfer roller5is transfer unit for transferring the toner on the surface of the photosensitive drum1to the recording material P, which is a transfer target object, in a transfer portion at the downstream side of the developing portion in the rotating direction of the photosensitive drum1. The photosensitive drum1faces the transfer roller5in the transfer portion. The transfer roller5contacts with the surface of the photosensitive drum1at a predetermined contact pressure, and forms the transfer portion thereby. To the rotation shaft of the transfer roller5, a predetermined voltage is applied from a transfer voltage power supply94(seeFIG.2) at a predetermined timing.

The pre-charging exposing apparatus6is pre-exposing unit for exposing the surface of the photosensitive drum1in a pre-charging exposing portion at the downstream side of the transfer portion and at the upstream side of the charging portion, in the rotating direction of the photosensitive drum1. By performing exposure using the pre-charging exposing apparatus6, lack of uniformity of the surface potential of the photosensitive drum1after the recording material P passing through the transfer portion can be eliminated.

The recording material P stored in a cassette10is fed by the paper feeding unit7at a timing when the toner image formed on the photosensitive drum1reaches the transfer portion. The recording material P is conveyed to the transfer portion via a resist roller pair8. The toner image formed on the photosensitive drum1is transferred onto the recording material P by the transfer roller5to which a predetermined transfer voltage is being applied by the transfer voltage power supply94.

Then the recording material P carrying the toner image is conveyed to a fixing unit9. The recording material P is heated and pressed in the fixing unit9. Thereby the toner is melted and fixed to the recording material P. Then the recording material P is discharged from the image forming apparatus100.

By the above operation, a monochrome print image is formed. The image forming apparatus100is a cleanerless type image forming apparatus in which toner, which was not transferred to the recording material P (transfer target object) in the transfer portion, is collected by the developing apparatus4.

In some cases a foreign substance existing in the image forming apparatus100or entering from outside of the image forming apparatus100may be conveyed with the recording material P to the transfer portion, then transferred to the photosensitive drum1in the transfer portion. In another case, the foreign substance may be transferred to the charging roller2in the charging portion, and may adhere to and remain on the charging roller2. In this case, the foreign substance may scratch the surface of the photosensitive drum1. If the scratch is deep, a charge holding capability of the surface of the photosensitive drum1drops. Then the toner is developed in the scratched portion on the photosensitive drum1in the developing portion, and a black spot image may be generated. In particular, under a high temperature high humidity environment, charges around the scratched portion of the photosensitive drum1tend to flow into the scratched portion where resistance is low. Therefore, the black spot image tends to stand out even more. The foreign substance here is, for example, a piece of metal, a piece of resin, a mineral (e.g. quartz) or the like. Such a relatively hard foreign substance easily scratches the photosensitive drum1, causing the black spot image. Adhesion of the foreign substance to the charging roller2is more easily generated if tackiness of the surface of the charging roller2is high. This means that in an early stage of operation, where there is no adhesion of the toner to the surface of the charging roller2and tackiness is high, adhesion of a foreign substance to the charging roller2is likely to occur.

To solve this problem, in Embodiment 1, an external additive coating operation, to adhere a predetermined amount of an external additive to the charging roller2, is performed in an early stage of operation of the charging roller2. In the external additive coating operation, an external additive or a toner containing an external additive is transferred from the developing roller41to the photosensitive drum1. Further, the external additive is transferred from the photosensitive drum1to the charging roller2. In the external additive coating operation, the charging voltage, the developing voltage, the transfer voltage and the exposure amount are controlled so that this transfer of the external additive or the toner containing the external additive is implemented. The “early stage of operation” here is, for example, a case where the charging roller2is brand new, a case where a number of times of rotation of the charging roller2is less than a threshold, or a case where the image forming operation is not yet executed or a number of times of executing the image forming operation is less than a threshold. In the following description, a case of performing the external additive coating operation when the charging roller2is brand new will be described as an example. The effect to decrease the tackiness can be obtained even if toner, instead of the external additive, is adhered to the charging roller2. However, the effect of decreasing the tackiness is higher if the external additive is adhered.

The reason why the effect of decreasing the tackiness is higher in coating with the external additive than with coating toner will be described.FIG.3indicates the surface potential that is formed on the photosensitive drum1in the image forming operation, and the applied voltage to each member. It is assumed that Vdp1is the surface potential immediately before the charging portion of the photosensitive drum1in the image forming operation, Vdp2is the surface potential of a region which is not exposed by the exposing apparatus3immediately before the developing portion, Vlp2is the surface potential of the exposed region, and Vdp3is the surface potential immediately before the transfer portion. It is also assumed that Vcp is the charging voltage, Vrp is the developing voltage, and Vtp is the transfer voltage in the image forming operation. In Embodiment 1, the pre-charging exposure is performed by the pre-charging exposing apparatus6(pre-charging exposure ON), and Vdp1becomes 0V. It is assumed that the charging voltage Vcp is −1350V, the dark area potential Vdp2in the developing portion of the photosensitive drum1is −780V, the bright area potential Vlp2therein is −100V, the dark area potential Vdp3in the transfer portion is −780V, the developing voltage Vrp is −380V, and the transfer voltage Vtp is +1500V.

In Embodiment 1, the charging roller2and the photosensitive drum1are rotated with a peripheral speed difference. Therefore, the toner on the charging roller2is likely to be charged to negative polarity by rubbing. In the image forming operation inFIG.3, the charging voltage Vcp is higher in the negative polarity side than the potential Vdp1of the photosensitive drum1immediately before the charging portion. In the case of this potential relationship, the toner having negative polarity adhering to the charging roller2is discharged to the photosensitive drum1. The external additive, on the other hand, has a charging polarity (positive polarity) which is the opposite of the polarity of the toner. Therefore, the polarity of the external additive does not easily become negative polarity even if it is rubbed on the charging roller2, and the external additive is not easily discharged from the charging roller2to the photosensitive drum1. Furthermore, the particle size of the external additive of Embodiment 1 is 1/10 or less than that of toner. Therefore, regardless the irregularity of the surface of the charging roller2, the external additive can be coated more uniformly than toner. Hence the particles to be adhered to the charging roller2are preferably the external additive, which is less likely to be discharged from the charging roller2, and more easily coated uniformly compared with toner.

The external additive coating operation will be described. The external additive coating operation is an operation sequence to develop the external additive on the photosensitive drum1in the developing portion, and then transfer the external additive to the charging roller2efficiently without generating a discharge in the transfer portion and the charging portion.

The external additive coating operation is executed by the control unit99, which is controller disposed in the image forming apparatus100.FIG.2is a block diagram indicating the control unit99and various composing elements of the image forming apparatus100controlled by the control unit99. The control unit99controls the drum driver91, and rotary-drives the photosensitive drum1. The control unit99also controls the charging voltage power supply92and applies DC voltage (charging voltage) to the charging roller2, and controls the exposing apparatus3and exposes the photosensitive drum1. The control unit99also controls the charging roller driver98, and rotary drives the charging roller2. Further, the control unit99controls the developing voltage power supply93, and applies DC voltage (developing voltage) to the developing apparatus4. The control unit99also controls the pre-charging exposing apparatus6, and exposes the photosensitive drum1at a position on the upstream side of the charging portion. The control unit99also controls the transfer voltage power supply94, and applies the DC voltage (transfer voltage) to the transfer roller5. Furthermore, the control unit99determines whether or not the image forming apparatus100is brand new. For example, information that an image forming apparatus100is brand new is stored in a memory97before shipment, and when the external additive coating operation is correctly executed, this information indicating the brand new state is updated to the information indicating the image forming apparatus100is in the state of operation. The control unit99also acquires information on the operation environment of the image forming apparatus100using an environment sensor96. The environment sensor96is an acquiring unit for acquiring information on the operating environment of the image forming apparatus100. Examples of the environment sensor96are a temperature sensor and a humidity sensor. In the case of the later mentioned configuration equipped with a brush member11, the control unit99controls a brush voltage power supply95, and applies voltage to the brush member11. In the case of the configuration not equipped with the brush member11, the brush voltage power supply95is not included. The power supply to apply voltage to the brush member11may be shared with another power supply (e.g. developing voltage power supply93), so that the power supply to apply voltage to the brush member11is not separately provided.

FIG.4indicates the surface potential that is formed on the photosensitive drum1in the external additive coating operation, and the applied voltage to each member. It is assumed that Vdg1is the surface potential immediately before the charging portion of the photosensitive drum1in the external additive coating operation, Vdg2is the surface potential immediately before the developing portion, and Vdg3is the surface potential immediately before the transfer portion. It is also assumed that Vcg is the charging voltage, Vrg is the developing voltage, and Vtg is the transfer voltage in the external additive coating operation. In the following description, the potential relationship when an electrostatic force acts on particles having the charging polarity of the external apparatus, in the direction from the position at the potential V1to the position at the potential V2, is expressed by V1>V2. In Embodiment 1, the charging polarity of the external additive is positive polarity, hence if V1=+150V and V2=−340V, for example, the potential relationship V1>V2described above is satisfied. In the case where the charging polarity of the external additive is negative polarity, if V1=−380V and V2=−100V, for example, the potential relationship V1>V2described above is satisfied. In the external additive coating operation, exposure by the exposing apparatus3is not performed, and voltages are controlled so as to satisfy Vrg>Vdg2and Vdg1>Vcg. Thereby the external additive is transferred from the developing apparatus4to the photosensitive drum1, and the external additive is transferred from the photosensitive drum1to the charging roller2. In Embodiment 1, exposure by the pre-charging exposing apparatus6is not performed, and the charging voltage Vcg=−900V, Vrg=+150V, and Vdg2=Vdg1=−340V. The charging voltage Vcg in the external additive coating operation is −900V, and the charging voltage Vcp in the image forming operation is −1350V, hence the potential relationship is Vcg>Vcp. The dark area potential Vdp2of the developing portion in the image forming operation is −780V, and the dark area potential Vdg2of the developing portion in the external additive coating operation is −340V.

Here it is assumed that Vback is the potential difference between the developing voltage in the developing portion and the surface potential of the charging surface of the photosensitive drum1, Vback_p is the potential difference of the developing portion in the image forming operation, and Vback_g is the potential difference of the developing portion in the external additive coating operation. In Embodiment 1, the voltages are controlled so that Vback_g>Vback_p is satisfied in the external additive coating operation. Here Vback_g=Vrg−Vdg2and Vback_p=Vrp−Vdp2. Thereby the external additive having the positive polarity can be developed more so than during the image forming operation, and can be transferred from the developing apparatus4to the photosensitive drum1. In Embodiment 1, the potential difference Vback_g of the developing portion in the external additive coating operation is 490V, and the potential difference Vback_p of the developing portion in the image forming operation is 400V.

In the external additive coating operation, the voltages are controlled so that the potential relationship in the transfer portion becomes Vtg>Vdg3. Thereby transfer of the external additive from the photosensitive drum1to the transfer roller5can be prevented. In Embodiment 1, the transfer voltage Vtg in the external additive coating operation is +100V, and the surface potential Vdg3of the photosensitive drum1in the transfer portion is −340V. Therefore, it can be prevented that the external additive having the positive polarity adheres to the transfer roller5in the transfer portion.

If Vtth is the threshold of the potential difference with which discharge is generated between the transfer roller5and the photosensitive drum1, the voltages are controlled so that Vtg−Vdg3<Vtth is established in the external additive coating operation. Then the discharge is not generated between the photosensitive drum1and the transfer roller5in the transfer portion. Hence an increase of the charges of the external additive can be prevented. In Embodiment 1, the transfer voltage Vtg is +100V, the surface potential Vdg3of the photosensitive drum1in the transfer portion is −340V, and the potential difference between the transfer roller5and the photosensitive drum1is 440V The discharge threshold Vtth between the transfer roller5and the photosensitive drum1is 600V Since discharge is not generated in the transfer portion, it can prevent the external additive from having strong positive polarity due to discharge, and prevent the external additive from strongly adhering to the photosensitive drum1.

If Vcth is the threshold of the potential difference with which discharge is generated between the charging roller2and the photosensitive drum1, the voltages are controlled so that Vdg1−Vcg<Vcth is established on the external additive coating operation. Then the discharge is not generated between the photosensitive drum1and the charging roller2in the charging portion. In Embodiment 1, Vdg1−Vcg<Vcth is implemented by not performing exposure by the pre-charging exposing apparatus6(pre-charging exposure OFF) in the external additive coating operation, or by reducing the exposure amount by the pre-charging exposing apparatus6than the amount in the image forming operation. Thereby it can be prevented that the charging polarity of the external additive becomes the same polarity as the charging polarity of the toner in the charging portion. In Embodiment 1, Vcth is 600V, and the pre-charging exposure is not performed (Vdg1=Vdg3=−340V, Vcg=−900V), hence Vdg1−Vcg<Vcth is established. Thereby it can prevent the external additive having positive polarity on the photosensitive drum1from becoming negative polarity, and the external additive having positive polarity is electrically transferred from the surface of the photosensitive drum1to the charging roller2efficiently.

The external additive coating operation is executed for a predetermined time, so that more external additive is adhered to the charging roller2than in the image forming operation, and a sufficient amount of external additive can be adhered to the charging roller2. In Embodiment 1, the execution time of the external additive coating operation is 30 seconds. This execution time is equivalent to the time that the charging roller2rotates about 150 times.

The execution time of the external additive coating operation may be set in accordance with the environment information, such as temperature and humidity, detected by the environment sensor96installed in the image forming apparatus100. For example, temperature may be detected by the environment sensor96, and a longer execution time of the external additive coating operation may be set as the temperature becomes higher. Humidity may be detected instead by the environment sensor96, and a longer execution time of the external additive coating operation may be set as the humidity becomes higher. In Embodiment 1, in a high temperature high humidity environment (temperature: 32.5° C., humidity: 80%) where black spots standout even more, the execution time of the external additive coating operation is set to about 60 seconds (equivalent to the time that the charging roller2rotates about 300 times). The predetermined correspondence of the environment conditions and the execution time of the external additive coating operation is stored in the memory97in advance. Based on the detection result by the environment sensor96and the correspondence stored in the memory97, the control unit99can acquire the execution time of the external additive coating operation. The correspondence is, for example, the higher the temperature the longer the execution time of the external additive coating operation, or the higher the humidity the longer the execution time of the external additive coating operation. The means for controlling the external additive coating operation in accordance with the environment conditions, such as temperature and humidity, is not limited to changing the execution time, but may be changing the rotation speed, voltages, or the like of the photosensitive drum1and the charging roller2. The control of the external additive coating operation in accordance with the environment conditions may be changed depending on the type of the external additive.

The effect of Embodiment 1 will be described next. In Embodiment 1, the external additive coating operation is performed before executing the image forming operation in a brand-new image forming apparatus100. Thereby the external additive is adhered to the charging roller2, and the adhesive force of the surface of the charging roller2is decreased, so as to prevent the adhesion of a foreign substance. In order to confirm the effect of Embodiment 1, a number of black spot lines that were generated was verified in Embodiment 1 and in Comparative Example 1.

This verification will be described in detail.

To verify a number of black spot lines that were generated, one sheet of paper, on which about 500 quartz particles (particle diameter: 100 to 300 μm) are scattered, is passed, then 1000 sheets of paper free of foreign substance are passed, and the image of the 1000th sheet of paper was evaluated. The image forming apparatus that is used here is brand new, and this verification was performed under a high temperature (32.5° C.) high humidity (80%) environment. In Embodiment 1, the above evaluation was performed after the external additive coating operation was performed for 60 seconds on the brand-new image forming apparatus. In the comparative example, the above evaluation was performed on the brand-new image forming apparatus, without performing the external additive coating operation. The “number of black spot lines that were generated” here means a number of lines of black spots that were generated at a 2 mm pitch in the direction vertical to the conveying direction in the plane of the recording material P. For example, the black spot image illustrated inFIG.5is counted as one line. The evaluation was performed three times for Embodiment 1 and in the comparative example respectively.

Table 1 indicates a number of black spot lines that were generated in Embodiment 1 and in Comparative Example 1.

As illustrated in Table 1, in Embodiment 1, a number of black spot lines that were generated is 0, while in Comparative Example 1, a number of black spot lines that were generated is 4 to 6, that is, performing the external additive coating operation has an effect of preventing the generation of black spots.

As described above, by performing the external additive coating operation on a brand-new image forming apparatus, adhesion of a foreign substance to the charging roller2can be prevented, whereby the generation of black spot images can be prevented.

Cleaning Operation

In the case where a large amount of toner has positive polarity, not only external additive but also considerable amount of toner is transferred to the charging roller2in the above-mentioned external additive coating operation. Therefore, after the external additive coating operation, the cleaning operation to control the charging voltage, the developing voltage and the transfer voltage may be performed so that toner adhering to the charging roller2is transferred to the photosensitive drum1.

FIG.6indicates the surface potential that is formed on the photosensitive drum1in the cleaning operation, and the voltage of each member. It is assumed that Vdc1is the surface potential of the photosensitive drum1in the cleaning operation immediately before the charging portion, Vdc2is the surface potential immediately before the developing portion, Vdc3is the surface potential immediately before the transfer portion, Vcc is the charging voltage, Vrc is the developing voltage, and Vtc is the transfer voltage. In the cleaning operation, exposure by the pre-charging exposing apparatus6is performed and exposure by the exposing apparatus3is not performed, and the voltages are controlled so as to satisfy Vdc1>Vcc and Vrc>Vdc2. Thereby the toner can be transferred from the charging roller2to the photosensitive drum1, and the toner can be transferred from the photosensitive drum1to the developing apparatus4. In Embodiment 1, Vdc1is 0V, Vcc is −500V, Vrc is +150V, and Vdc2is 0V.

In the cleaning operation, the charging roller2and the photosensitive drum1are rotary-driven in the state of applying the charging voltage having negative polarity. By rubbing with the peripheral speed difference between the charging roller2and the photosensitive drum1, the toner adhering to the charging roller2is charged to negative polarity. Then static electricity of the photosensitive drum1is eliminated by the pre-charging exposing apparatus6, and history of the charging potential of the photosensitive drum1before executing the clean operation is erased, so that the potential of the photosensitive drum1becomes approximately 0V. The toner can be transferred from the charging roller2onto the photosensitive drum1using the electrostatic repulsive force between the charging roller2and the toner charged to negative polarity.

If Vcth is the threshold of the potential different with which discharge is generated between the charging roller2and the photosensitive drum1, the voltages may be controlled so that Vdc1−Vcc<Vcth is satisfied in the cleaning operation. Thereby it can be prevented that the toner adhering to the charging roller2in the charging portion is charged to an opposite polarity (positive polarity). In Embodiment 1, Vcc is −500V, Vdc1is 0V, and Vcth is 600V Thereby it can prevent the toner adhering to the charging roller2from becoming positive polarity by discharge.

Since the developing voltage Vrc is +150V and the surface potential Vdc2of the photosensitive drum1in the developing portion is 0V, the toner having negative polarity adhering to the photosensitive drum1is transferred to the developing apparatus4.

In the cleaning operation, the voltage may be controlled so that Vtc>Vdc3is satisfied. Thereby it can be prevented that toner charged to the opposite polarity (positive polarity) is transferred from the photosensitive drum1to the transfer roller5. In the cleaning operation, the potential relationship is Vrc>Vdc2, hence the toner having positive polarity is developed in the developing portion, but by setting Vtc>Vdc3, the developed toner having positive polarity is not transferred from the transfer portion. In Embodiment 1, in the cleaning operation, Vtc is +100V, and Vdc3is 0V Thereby a potential relationship that prevents the toner having positive polarity from adhering to the transfer roller5is established.

If Vtth is the threshold of the potential difference with which discharge is generated between the transfer roller5and the photosensitive drum1, the voltages may be controlled so that Vtc−Vdc3<Vtth is satisfied in the cleaning operation. Thereby it can be prevented that charges of the toner charged to the opposite polarity (positive polarity) increase in the transfer portion. In Embodiment 1, Vtc is +100V, Vdc3is 0V, and Vtth is 600V. Thereby it can prevent the toner from becoming strong positive polarity by discharge in the transfer portion.

By performing the above-mentioned cleaning operation for at least one cycle of the charging roller2, the toner on the charging roller2can be cleaned. The time of the cleaning operation can be changed in accordance with the operation environment of the image forming apparatus100, the type of the external additive, and the like.

Brush Member

An example of applying the present invention to an image forming apparatus equipped with a contact member, which contacts with the surface of the photosensitive drum1on the downstream side of the transfer portion and on the upstream side of the charging portion in the rotating direction of the photosensitive drum1, will be described. An example of the contact member is a brush member11, as illustrated inFIG.7. The brush member11is a collection member that selectively collects paper dust and allows toner to pass. The brush member11is, for example, a member constituted of a base fabric11a, which is made of synthetic fibers containing carbon, woven with conductive yarns11bmade of conductive nylon 6. The conductive yarns11bof which brush member11are, for example, 6.5 mm long and two denier thick, and are disposed at a 240 kF/inch2density, with a 1 mm entry amount. The thickness of the conductive yarn11bis preferably 1 to 10 denier, and even more preferably 1 to 6 denier. The density of the conductive yarns11bis preferably 150 kF/inch2or more.

A predetermined brush voltage, i.e., DC voltage, is applied to the brush member11from the brush voltage power supply95(FIG.2).FIG.8Aindicates the surface potential formed on the photosensitive drum1in the image forming operation, and the applied voltage to each member, andFIG.8Bindicates the potential of the photosensitive drum1in the external additive coating operation, and the applied voltage to each member.

As indicated inFIG.8A, if Vbp is the brush voltage in the image forming operation, the voltages are controlled so that Vbp>Vdp3is satisfied in the image forming operation. Because of this, toner having positive polarity passes through the brush member11, and toner on the photosensitive drum1is not collected by the brush member11. In Embodiment 1, Vbp is −380V, and Vdp3is −780V.

As indicated inFIG.8B, if Vbg is the brush voltage in the external additive coating operation, the voltages are controlled so that Vbg>Vdg3is satisfied. As a result, this prevents the toner and external additive having a positive polarity moving from the photosensitive drum1to the brush member11. Since the toner and the external additive having a positive polarity pass through the brush member11, the toner and the external additive on the photosensitive drum1are not collected by the brush member11. In Embodiment 1, Vbg is +150V, and Vdg3is −340V.

In the cleaning operation of the charging roller2as well, if Vbc is the brush voltage in the cleaning operation, the voltages are controlled so that Vbc>Vdc3is satisfied, just like the case ofFIG.8B. Thereby it can be prevented that the toner charged to the opposite polarity (positive polarity) is transferred from the photosensitive drum1to the brush member11. Potential relationships other than with the brush member11are as described in Embodiment 1.

Embodiment 2 will be described. In Embodiment 2, a member the same as Embodiment 1 is denoted with a same reference sign, and description thereof will be omitted. In Embodiment 2, the photosensitive drum1is exposed by the exposing apparatus3in the external additive coating operation, whereby the absolute value of the potential of the photosensitive drum1is decreased from before the exposing portion, and the toner is developed in the developing portion. In the developing portion, essentially toner having negative polarity is developed, but the external additive is also developed along with the toner. The amount of external additive to be developed is in proportion to the amount of toner to be developed. Therefore, by adjusting the exposure amount and changing the absolute value of the potential of the photosensitive drum1, the amount of the external additive to be developed can be controlled. For example, in the case of using the image forming apparatus in a high temperature and high humidity environment, the coating amount of the external additive can be increased by adjusting the exposure amount, without increasing the execution time of the external additive coating operation.

The external additive coating operation of Embodiment 2 is an operation sequence to develop the toner on the photosensitive drum1in the developing portion, and then efficiently transfer the external additive contained in the toner to the charging roller2without generating a discharge in the transfer portion.

FIG.9indicates the surface potential that is formed on the photosensitive drum1in the external additive coating operation of Embodiment 2, and the applied voltage to each member. It is assumed that Vlg1is the surface potential immediately before the charging portion of the photosensitive drum1in the external additive coating operation, Vdg2is the surface potential (dark area potential) immediately before the exposing portion, Vlg2is the surface potential (bright area potential) immediately before the developing portion, and Vlg3is the surface potential immediately before the transfer portion. It is also assumed that Vcg is the charging voltage in the external additive coating operation, Vrg is the developing voltage, and Vtg is the transfer voltage. In the external additive coating operation, the exposing apparatus3performs exposure on the entire range of the photosensitive drum1in a direction intersecting with the circumferential direction of the photosensitive drum1(rotation shaft direction), and the voltages are controlled so as to satisfy Vlg2>Vrg, Vlg1>Vcg, and Vtg=Vlg3. Thereby the toner containing the external additive is transferred from the developing apparatus4to the photosensitive drum1, the external additive is transferred from the photosensitive drum1to the charging roller2, and transfer of the external additive and toner from the photosensitive drum1to the transfer roller5is prevented. In Embodiment 2, the charging voltage Vcg=−1350V is applied in the external additive coating operation, so that the potential Vdg2of the photosensitive drum1on the downstream side of the charging portion is set to −780V, and the potential Vlg2of the photosensitive drum1in the exposing portion is set to −100V by exposure of the exposing apparatus3. Then the developing voltage Vrg is set to −380V, and toner is developed in the developing portion.

Here the region to be exposed by the exposing apparatus3, that is, the region where the toner is developed, is set such that the latitudinal width (one rotation of charging roller2) is 25 mm, and the longitudinal width is the entire range of the imaging region. At the timing when the toner reaches the transfer portion, the transfer voltage Vtg=−100V is applied so as to be the same as the potential Vlg3=−100V of the photosensitive drum1in the transfer portion. By this potential relationship, the external additive having positive polarity and toner having a negative polarity are not transferred to the transfer roller5.

At the timing when the toner reaches the pre-charging exposing portion, the pre-charging exposure by the pre-charging exposing apparatus6is stopped (OFF). Thereby the potential Vlg3=−100V of the photosensitive drum1in the transfer portion remains unchanged, and the potential Vlg1of the photosensitive drum1in the charging portion becomes −100V, and in the charging portion, the external additive having positive polarity is transferred from the photosensitive drum1to the charging roller2by electrostatic force. Here an example of turning the pre-charging exposure OFF by the pre-charging exposing apparatus6was described, but the pre-charging exposure may be performed with an exposure amount that is less than the exposure amount by the pre-charging exposing apparatus6in the image forming operation.

In order to confirm the effect of Embodiment 2, the same verification as Embodiment 1 was performed in Embodiment 2 and in Comparative Example 2. In Embodiment 2, the external additive coating operation was performed on a brand-new image forming apparatus, and then the same evaluation as Embodiment 1 was performed. In Comparative Example 2, the same evaluation as Embodiment 1 was performed without performing the external additive coating operation on the brand-new image forming apparatus. Table 2 indicates a number of black spot lines that were generated in Embodiment 2 and in Comparative Example 2.

As described in Table 2, in Embodiment 2, a number of black spot lines that were generated is 0, while in the Comparative Example 2, a number of black spot lines that were generated is 5 to 6, that is, performing the external additive coating operation has an effect of preventing the generation of black spots.

As described above, by performing the external additive coating operation on a brand-new image forming apparatus, adhesion of a foreign substance to the charging roller2can be prevented, whereby the generation of black spot images can be prevented.

In the external additive coating operation, the voltages may be controlled to satisfy Vlg1−Vcg<Vcth by increasing the charging voltage Vcg at the timing when the toner and the external additive reach the charging portion. In the present description, increasing the voltage V1to voltage V2means changing the voltage V1to voltage V2that satisfies the potential relationship of V2>V1, as described above. Vcth is a threshold of the potential difference at which discharge is generated between the charging roller2and the photosensitive drum1. Thereby it can be prevented that the charging polarity of the external additive becomes the same polarity as the charging polarity of the toner in the charging portion. In Embodiment 2, it is assumed that the charging voltage Vcg is −1350V until the toner reaches the charging portion, and is changed to −600V at the timing when the toner reaches the charging portion. Thereby Vlg1−Vcg=500V is established, which is lower than the discharging threshold of Vcth=600V. As a result, this prevents the external additive having positive polarity, contained in the toner, from becoming negative polarity in the charging portion.

The exposure amount by the exposing apparatus3in the external additive coating operation may be set in accordance with the environment information, such as the temperature and humidity, detected by the environment sensor96. For example, temperature may be detected by the environment sensor96, and the exposure amount in the external additive coating operation may be increased as the temperature becomes higher. Further, humidity may be detected by the environment sensor96, and the exposure amount in the external additive coating operation may be increased as the humidity becomes higher. In Embodiment 2, the exposure amount may be increased in a high temperature high humidity environment (temperature: 32.5° C.; humidity: 80%) in which black spots standout even more, and the exposure amount may be lowered in a low temperature low humidity environment in which black spots standout less.

In some cases, in Embodiment 2, toner may adhere to the charging roller2when the toner passes through the charging portion. Hence, just like Embodiment 1, the cleaning operation of the charging roller2may be performed after the external additive coating operation.

In the configuration of Embodiment 2 as well, a brush member11, as illustrated inFIG.7, may be included, just like Embodiment 1. In the case of the configuration including the brush member11, the brush voltage Vbg in the external additive coating operation may be the same as the potential Vlg3of the photosensitive drum1in the contact portion of the brush member11and the photosensitive drum1. Thereby it can be prevented that toner and the external additive are transferred from the photosensitive drum1to the brush member11, so that the brush member11does not collect the toner and the external additive.

In the configuration described in the above embodiments, the toner image formed on the photosensitive drum1is transferred to the recording material P (transfer target object) in the transfer portion, but the configuration of the image forming apparatus to which the present invention is applicable is not limited to this configuration. For example, the present invention is also applicable to an image forming apparatus configured such that a toner image is primarily transferred to an intermediate transfer belt (transfer target object) in the transfer portion, and the toner image is secondarily transferred from the intermediate transfer belt to the recording material P. Further, in the configuration described in the above embodiments, the image forming apparatus100includes the photosensitive drum1, the charging roller2and the developing apparatus4, but the present invention is also applicable to a process cartridge type image forming apparatus. In this case, when the charging roller2, included in the process cartridge having the photosensitive drum1, is in a brand-new state, the external additive coating operation can be performed on the charging roller2before starting the image forming operation with this process cartridge.

According to the present invention, image defects can be prevented in a cleanerless type and contact charging type image forming apparatus.

Other Embodiments

This application claims the benefit of Japanese Patent Application No. 2023-122745, filed on Jul. 27, 2023, which is hereby incorporated by reference herein in its entirety.