High resolution image forming apparatus

An image forming apparatus includes a photosensitive body (11) having a charge transport layer (11d) as a surface layer. The charge transport layer (11d) as the surface layer is charged and exposed so that a latent image is formed thereon. In order to form a basic dot whose diameter is from 10 μm to 25 μm, a thickness t (μm) of the charge transport layer (11d) and a diameter D (μm) of the basic dot satisfy the relationship of t≦D/2.

BACKGROUND OF THE INVENTION

This invention relates to an image forming apparatus such as a printer, a copier or the like that uses electrophotographic technology.

Conventionally, there is proposed an image forming apparatus that forms a latent image by irradiating a surface layer of a photosensitive body whose thickness is less than or equals to 15 μm with the light beam whose diameter is less than or equals to 50 μm. The amount of the toner that forms a dot image is 1.2 times the amount of the toner that forms an area image, so as to enhance the image quality. Such a technology is disclosed in, for example, Japanese Laid-open Patent Publication No. 2000-108409 (particularly, in Page 3 and FIG. 1).

Recently, the image forming apparatus generally has the resolution of 600 DPI, and the development work to further enhance the resolution of the image forming apparatus has already started. Although an exposure (i.e., scanning) of high resolution image data has been accomplished, it is difficult to reproduce a single dot on a recording medium corresponding to such a resolution, and therefore it is difficult to obtain high quality print output corresponding to the high resolution image data. Particularly, the conventional image forming apparatus is not able to reproduce the minute dot whose diameter is approximately less than or equals to 25 μm, corresponding to the higher resolution of, for example, 1200 DPI and 2400 DPI.

SUMMARY OF THE INVENTION

An object of the present invention is to provide an image forming apparatus capable of printing an image with the resolution of approximately 2400 DPI, and stably keeping the printing performance during the lifetime of the image forming apparatus.

The invention provides an image forming apparatus including a photosensitive body having a charge transfer layer as a surface layer charged and exposed so that a latent image is formed thereon. In order to form a basic dot whose diameter is from 10 μm to 25 μm, a thickness t (μm) of the charge transport layer and a diameter D (μm) of the basic dot satisfy the relationship: t≦D/2.

The invention also provides another image forming apparatus including a photosensitive body having a charge transport layer as a surface layer charged and exposed so that a latent image is formed thereon, and a developer bearing body that contacts said photosensitive body and develops the latent image. In order to form a basic dot whose diameter is from 10 μm to 25 μm, the developer bearing body is urged against the photosensitive body with an urging force from 10 g/cm to 50 g/cm per unit length of a contact portion of the photosensitive body and the developer bearing body.

The invention also provides further image forming apparatus including a photosensitive body having a charge generation/transport layer as a surface layer charged and exposed so that a latent image is formed thereon, and a developer bearing body that contacts said photosensitive body and develops the latent image. In order to form a basic dot whose diameter is from 10 μm to 25 μm, the developer bearing body is urged against the photosensitive body with an urging force from 10 g/cm to 50 g/cm per unit length of a contact portion of the photosensitive body and the developer bearing body.

With such an arrangement, it becomes possible to reproduce the dot corresponding to the resolution of 1200 DPI and 2400 DPI, and to stably keep the printing performance during the lifetime of the image forming apparatus.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Embodiments of the present invention will be described with reference to the attached drawings.

FIG. 1is a side view showing a configuration of a main part of an image forming apparatus1according to Embodiment 1 of the present invention.

As shown inFIG. 1, the image forming apparatus includes a laminate-type photosensitive drum11. The photosensitive drum11has laminated charge generation layer and charge transport layer (described later) at the surface thereof, and the charge transport layer constitutes a surface layer of the photosensitive drum11. The charging roller12applies a negative DC voltage to the photosensitive drum11and applies an electric charge to the surface layer of the photosensitive drum11, so that the surface layer is charged to an electric potential from −300V to −600V. The charging roller12rotates together with the photosensitive drum11in the direction shown by an arrow inFIG. 1, so as to restrict the decrease of the thickness of the surface layer of the photosensitive drum11. The exposing device13includes an optical writing device having laser or LED (Light Emitting Diode) as light sources. The exposing device13selectively exposes the surface layer of the photosensitive drum11to form a latent image on the surface layer of the photosensitive drum11.

A developing device14includes a toner container44, a developing roller41, a toner supply roller42and a developing blade43. The toner container44stores a toner45therein. The toner supply roller42supplies the toner45to the developing roller41. The developing blade43forms a thin layer of the toner45on the developing roller41. The developing device14causes the developing roller41(that bears the toner45) to be urged against the photosensitive drum11so that the latent image on the surface of the photosensitive drum11is developed (i.e., visualized) with the toner45as described later. The developing roller41and the toner supply roller42has rotation axes parallel with each other, and are urged against each other with uniform pressure. The developing roller41and the toner supply roller42rotate in the same directions as shown by arrows inFIG. 1. The developing blade43and the developing roller41are disposed in parallel to each other so that a bent portion of the developing blade43is urged against the developing roller41with a uniform pressure.

The toner image formed on the photosensitive drum11is transferred to the recording medium22by a transfer roller21to which a predetermined high voltage is applied. The transfer roller21is disposed in opposition to the photosensitive drum11so that the recording medium22is nipped between the transfer roller21and the photosensitive drum11. The transfer roller21and the photosensitive drum11rotate together with each other so as to feed the nipped recording medium22. The toner layer that has been transferred to the recording medium22is fixed to the recording medium22when the recording medium22passes a fixing device23. After the transferring of the toner image, the toner45that remains on the photosensitive drum11is scraped by a cleaning blade15. The cleaning blade15is disposed in parallel to the photosensitive drum11so that the cleaning blade15is urged against the surface of the photosensitive drum11with a uniform force. The residual toner scraped by the cleaning blade15is recovered by a toner recovering mechanism65, and is carried by a not-shown toner carrying mechanism to a waste toner accommodating portion.

The photosensitive drum11, the charging roller12, the developing device14, the cleaning blade15and the toner recovery mechanism65(indicated by a dashed line inFIG. 1) are integrally constructed as a drum cartridge2. It is possible that the drum cartridge2is detachably attached to a main body of the image forming apparatus1.

InFIG. 1, X-axis is defined in the feeding direction of the recording medium22, and Y-axis is defined in the direction of the rotation axis of the photosensitive drum11. Z-axis is defined to be perpendicular to both of X-axis and Y-axis. In other figures, the X-axis, Y-axis and Z-axis indicate the same directions as those shown inFIG. 1.

FIG. 2is an enlarged sectional view of a part in the vicinity of the surface of the photosensitive drum11, cut by XZ plane.

The photosensitive drum11includes a conductive base11amade of metal in the shape of cylinder, an undercoat layer11bformed on the conductive base11a, a charge generation layer (CGL)11cformed on the undercoat layer11b, a charge transport layer (GTL)11dformed on the charge generation layer11c. The undercoat layer11bis made of an inorganic material with a predetermined electric resistance, and has a function to prevent the entry of the electric charge moved from the conductive base11aand to cancel the imperfections of the conductive base11a. The charge generation layer11cis made of an organic optical semiconductive material that absorbs the light energy to generate positive and negative electric charge. The thickness of the charge generation layer11cis less than or equals to 1 μm. The charge transport layer11dis made of an organic optical semiconductive material and a resin material, and has a function to transport the electric charge generated (in this case, the positive electric charge) by the charge generation layer11cto the surface of the photosensitive drum11, so as to neutralize the charged voltage (in this case, the negatively charged voltage) of the surface of the photosensitive drum11.

FIG. 3is a top view of an urging mechanism for urging the developing roller41against the photosensitive drum11with a constant pressure. InFIG. 3, the urging mechanism at an end portion of the photosensitive drum11and the developing roller41is illustrated in the direction seen from above along Z-axis.

As shown inFIG. 3, the photosensitive drum11has a rotation axis11grotatably supported by bearings provided on a stationary frame51disposed on a main body of the drum cartridge2(FIG. 1). The developing roller41has a rotation axis41arotatably supported by bearings provided on a movable frame52. The same urging mechanism is provided on the other end portion of the photosensitive drum11and the developing roller41in, for example, a symmetrical manner.

The movable frame52is guided by the main body of the drum cartridge2(FIG. 1), and is movable so that the rotation axis41aof the developing roller41is kept parallel to the rotation axis11gof the photosensitive drum11(i.e., the rotation axes41aand11gare aligned on the same plane). A coil spring53extends between the stationary frame51and the movable frame52, and pulls the frames51and52toward each other. The pressure with which the photosensitive drum11and the developing roller41(bearing the toner45) are urged against each other can be adjusted by varying the strength of the coil spring53.

The reproducing test of a single basic dot using the image forming apparatus1ofFIG. 1will be described. Hereinafter, “the diameter of the basic dot” is used to mean the diameter of the dot as the minimum unit of the printed image.

In this test, an LED head including an array of LED light sources is used as the exposing device13. By varying the areas of the LED light sources, the diameter of the beam spot for exposure is adjusted, with the result that the diameter of the basic dot of the latent image on the surface of the photosensitive drum11is adjusted. A lens array is used as an optical system of the LED head. The diameter of the beam spot for exposure is set according to “1010” pattern. The MFG (Modulation Transfer Function) is 70%. The MFG is calculated based on the strength amplitude reproduced by the optical system or the like as follows:
(SMax−Smin)/(SMax+SMin)×100
where SMaxand SMinare respectively the maximum value and the minimum value of the strength amplitude. Further, the above described “1010” pattern is that dots (i.e., ON dots) and blank dots (i.e., OFF dots) are alternatively arranged in lateral direction and in vertical direction. With the “1010” pattern, the reproducibility can be determined by comparing (and evaluating) the adjacent dots with each other.

The toner used in the test is made by polymerization. The degree of circularity of the toner is 0.95, and the particle diameter (mean volume diameter) of the toner is 5.0 μm. In this regard, the toner can be made of pulverized toner processed into spherical shape. Further, the degree of circularity of the toner can be ranged from 0.90 to 0.99. The particle diameter of the toner can be ranged from 3.0 μm to 9.0 μm. The developing roller41used in the test has the surface roughness from 1 to 10 μm. The surface of the developing roller41is made of urethane rubber, urethane resin, silicone rubber, silicone resin, Epichlorohydrin rubber or the like.

Other test condition will be described below.

The surface roughness of the photosensitive drum11is from 0.1 μm to 2.0 μm, although the photosensitive drum11of Embodiment 1 is not limited to have the surface roughness of this range.

The undercoat layer11bis made of alumite, titanium oxide or the like. The resistance of the undercoat layer11bis from 1 KΩ/cm2to 1 MΩ/cm2. The thickness of the undercoat layer11bis from 1.0 μm to 60 μm.

The charge generation layer11cis made of phthalocyanine pigment whose core is made of aluminum or the like.

The charge transport layer11dis made of a resin of polycarbonate or the like in which a chemical compound (for example, hydrazone) having a charge transport property is included.

According to the above described condition, the thickness of the charge transport layer11dof the photosensitive drum11(FIG. 2) is varied as a parameter, and the basic dots of several diameters are printed on the recording medium22. The reproducibility of the basic dot is evaluated by observing the toner image of the basic dot formed on the recording medium22. The result of the evaluation is shown in Table 1.

In the above Table 1, the mark “o” indicates that the toner is collected to substantially form a circle, and the reproducibility of the basic dot is recognized. The mark “x” indicates that the toner is dispersed, and the reproducibility of the basic dot is not recognized. In the test using the above described “1010” pattern, the evaluation is “o” when the dots (ON dots) are not overlapped with each other, and the evaluation is “x” when the dots are partially overlapped with each other.

Based on the evaluation shown in Table 1, the reproducibility of the basic dot is determined by the thickness of the charge transport layer11dof the photosensitive drum11within the range in which the test is carried out. When the thickness of the charge transport layer11dof the photosensitive drum11is less than or equals to 10 μm, it becomes possible to reproduce the basic dot with the diameter of 22 μm. When the thickness of the charge transport layer11dof the photosensitive drum11is less than or equals to 5 μm, it becomes possible to reproduce the basic dot with the diameter of 11 μm. Based on Table 1, when the evaluation “o” is obtained, the diameter D of the basic dot and the thickness t of the charge transport layer11dsatisfy the relationship: D/2≧t.

It is understood that the reason why the reproducibility of the basic dot is determined by the thickness of the charge transport layer11dof the photosensitive drum11is as follows. The electric charge generated by the charge generation layer11c(that absorbs the light energy of exposure) is diffused in the charge transport layer11dbefore the electric charge approaches the surface of the photosensitive drum11. Thus, as the charge transport layer11dbecomes thicker, the reproduced basic dot is broadened, so that the basic dot is not sharply reproduced.

Next, the test to determine the decreasing amount of the thickness of the surface layer of the photosensitive drum11will be described.

FIG. 4is a graph of the experimentally determined relationship between the urging force with which the developing roller41is urged against the photosensitive drum11and the decreasing amount of the thickness of the surface layer of the photosensitive drum11. InFIG. 4, the lateral axis indicates the urging force generated by the coil spring53(FIG. 3) divided by the length with which the developing roller41contacts the photosensitive drum11. In other words, the lateral axis indicates the urging force per unit length. The vertical axis indicates the decreasing amount of the thickness of the surface layer of the photosensitive drum11, when 10000 pages have been printed.

The urging force is varied by adjusting the strengths of the pair of the coil springs53of the urging mechanisms (one of which is shown inFIG. 3) provided on both end portions of the photosensitive drum11and the developing roller41. The circumferential speed of the developing roller41is set to be 1.3 times the circumferential speed of the photosensitive drum11.

As shown inFIG. 4, the decreasing amount of the thickness of the surface layer of the photosensitive drum11is in proportion to the urging force when the urging force is larger than a predetermined force. When the urging force is set to approximately 50 g/cm (50×9.8 mN/cm), the decreasing amount of thickness of the surface layer of the photosensitive drum11is approximately 1 μm when 10000 pages have been printed. However, when the urging force is gradually reduced, the decreasing amount of the thickness of the surface layer of the photosensitive drum11does not change. It is understood that, as the urging force becomes smaller, the primary factor of the decreasing the thickness of the surface layer shifts from the developing roller41to other components such as the charging roller12, the transfer roller21or the cleaning blade15.

The matters to be considered for determining the urging force of the developing roller41are as follows:(1) In consideration of the variations of the surface condition and the diameter in the axial direction of the photosensitive drum11and the developing roller41, it is necessary to apply a sufficient urging force throughout the axial length of the developing roller41, and the lower limit of the urging force is preferably 10 g/cm.(2) The lifetime of the photosensitive drum11is preferably the same as a replacement period (i.e., lifetime) of the drum cartridge2shown inFIG. 1. For example, the lifetime of the photosensitive drum11is preferably approximately 20000 pages of printing.(3) The photosensitive drum11needs to have a uniform charging property, and therefore the charge transport layer11dwith the thickness of at least 2 μm needs to remain during the lifetime of the photosensitive drum11. This is because if the thickness of the surface layer is further reduced, the electric leakage between the charging roller12and the conductive base11aof the photosensitive drum11may occur, so that the charge transport layer11dcan not hold the electric charge and may cause the failure in development.(4) In order to reproduce the basic dot with the diameter of approximately 11 μm (corresponding to the resolution of 2400 DPI), the thickness of the charge transport layer11dis set to be less than or equal to 5 μm according to the above described test result.

In consideration of the above described matters, if the initial thickness of the charge transport layer11dis 5 μm (when the photosensitive drum11is started to be used), the allowable decreasing amount of the thickness of the charge transport layer11dis 3 μm at the maximum when 20000 pages have been printed. If the decreasing amount of the thickness when 20000 pages have been printed is assumed to be double as the decreasing amount of the thickness shown inFIG. 4(when 10000 pages have been printed), the allowable urging force is 60 g/cm (60×9.8 mN/cm) corresponding to the decreasing amount of the thickness of 1.5 μm. Therefore, the urging force of the developing roller41is set by the coil spring53in the range from 10 g/cm (10×9.8 mN/cm) to 50 g/cm (50×9.8 mN/cm) in consideration of the variation. When the urging force of the developing roller41is set from 10 g/cm to 50 g/cm, the initial thickness of the charge transport layer10dmust be greater than or equals to 4 μm, because the thickness of the charge transport layer11dneeded to remain during the lifetime of the photosensitive drum11is 2 μm, and the decreasing amount of the thickness of the charge transport layer11dis 2 μm.

As described above, according to Embodiment 1 of the present invention, by setting the thickness of the charge transport layer11dof the photosensitive drum11to be less than or equals to the predetermined thickness, it becomes possible to reproduce the basic dot corresponding to 1200 DPI and further to reproduce the basic dot corresponding to 2400 DPI. Although the initial thickness of the charge transport layer11dneeds to be thin (for example, from 5 to 11 μm), the decreasing amount of the charge transport layer11dcan be restricted by setting the urging force of the developing roller41urged against the photosensitive drum11within a predetermined range. Therefore, the lifetime of the photosensitive drum11can be longer than or equals to the lifetime of the drum cartridge2(for example, approximately 20000 prints).

FIG. 5is an enlarged sectional view of a part in the vicinity of the surface of a photosensitive drum71used in an image forming apparatus according to Embodiment 2, cut by XZ plane.

The image forming apparatus according to Embodiment 2 is different from that of Embodiment 1 (FIG. 1) in the structure of the laminated portion of the photosensitive drum71and the polarities of the voltages applied to the photosensitive drum71or the like. The components of the image forming apparatus of Embodiment 2 that are the same as (or correspond to) those of Embodiment 1 are assigned the same reference numerals, and the duplicate explanation and drawing are omitted. The description is emphasized on the difference of the image forming apparatus of Embodiment 2 from that of Embodiment 1.

As shown inFIG. 5, the photosensitive drum71includes a conductive base71amade of metal in the shape of cylinder, an undercoat layer71bformed on the conductive base71a, a charge transport layer71dformed on the undercoat layer71b, and a charge generation layer71c(i.e., a surface layer) formed on the charge transport layer71d. The undercoat layer71bis made of an inorganic material with a predetermined resistance, and has a function to prevent the entry of the electric charge moved from the conductive base71aand to cancel the imperfections of the conductive base71a. The charge generation layer71cis made of an organic optical semiconductive material that absorbs the light energy to generate positive and negative electric charges. The negative electric charge neutralizes the positively charged voltage of the surface of the photosensitive drum71, and the positive electric charge is transmitted to the charge transport layer71d. The thickness of the charge generation layer71cis approximately 3 μm. The charge transport layer71dis made of an organic optical semiconductive material and a resin material, and has a function to transport the electric charge (the positive electric charge) generated by the charge generation layer71cto the undercoat layer71b.

In Embodiment 2, the charging roller12(FIG. 1) applies positive DC voltage to the photosensitive drum71so as to charge the surface layer of the photosensitive drum71from +300 V to +600 V. The charging roller12rotates together with the photosensitive drum71so as to restrict the decrease of the thickness of the surface layer of the photosensitive drum71.

The reproducing test of a single basic dot using the image forming apparatus of Embodiment 2 will be described. The method and condition of the test is the same as those described in Embodiment 1, and the duplicate explanation is omitted.

The thicknesses of the charge transport layer71dand the charge generation layer71cof the photosensitive drum71are varied as parameters, and the basic dots of several diameters are printed on the recording medium22. The reproducibility of the basic dot is evaluated by observing the toner image of the basic dot formed on the recording medium22. As a result of the test, it is recognized that the basic dot with the diameter of 22 μm and the basic dot with the diameter of 11 μm can be reproduced, and the reproducibility does not depend on the thicknesses of the charge transport layer71dand the charge generation layer71c. It is understood that the reason is as follows: In the photosensitive drum71, the charge generation layer71cabsorbs the light energy of exposure, and generates positive and negative electric charge in the vicinity of the surface of the charge generation layer71c. The negative charge generated in the vicinity of the surface of the charge generation layer71cneutralizes the positively charged voltage on the surface of the charge generation layer71c. Accordingly, the diffusion of the electric charge (negative electric charge) is restricted, irrespective of the thicknesses of the charge transport layer71dand the charge generation layer71c, and therefore the reproduced basic dot tends not to be broadened.

Next, the test to determine the decreasing amount of the thickness of the surface layer of the photosensitive drum71will be described. The method and condition of the test is the same as those described in Embodiment 1, and the duplicate explanation is omitted.

FIG. 6is a graph showing the experimental result. As shown inFIG. 6, the decreasing amount of the surface layer of the photosensitive drum71is in proportion to the urging force when the urging force is larger than the predetermined force. When the urging force is set to approximately 50 g/cm, the decreasing amount of the thickness of the surface layer of the photosensitive drum71is approximately 1 μm when 10000 pages have been printed. However, when the urging force is gradually reduced, the decreasing amount of the thickness of the surface layer of the photosensitive drum71does not change. It is understood that, as the urging force becomes smaller, the primary factor of the decreasing the thickness of the surface layer shifts from the developing roller41to other components such as the charging roller12, the transfer roller21or the cleaning blade15.

In the case where the urging force is set from 10 g/cm to 50 g/cm, the decreasing amount when 10000 pages have been printed is approximately 1 μm. Thus, the decreasing amount when 2000 pages have been printed is assumed to be 2 μm. Therefore, if the thickness of the charge generation layer71cis set to be greater than or equal to 3 μm, the charge generation layer71cwith the thickness of approximately at least 1 μm can remain while 2000 pages have been printed. Therefore, the lifetime of the photosensitive drum71can be longer than or equal to the replacement period of the drum cartridge2(for example, approximately 20000 pages of printing).

As described above, according to Embodiment 2 of the present invention, by using the photosensitive drum71having the charge generation layer71cas the surface layer, it becomes possible to reproduce the basic dot corresponding to 1200 DPI, and to further reproduce the basic dot corresponding to 2400 DPI. Further, the decreasing amount of the thickness of the charge transport layer71dcan be restricted by setting the urging force of the developing roller41urged against the photosensitive drum71in the range from 10 g/cm to 50 g/cm, with the result that the lifetime of the photosensitive drum71can be longer than or equals to the lifetime of the drum cartridge2(for example, approximately 20000 prints).

FIG. 7is an enlarged sectional view of a part in the vicinity of the surface of the photosensitive drum81used in an image forming apparatus according to Embodiment 3, cut by XZ plane.

The image forming apparatus according to Embodiment 3 is different from that of Embodiment 1 (FIG. 1) in the structure of the laminated portion of the photosensitive drum81and the polarities of the voltages applied to the photosensitive drum81or the like. The components of the image forming apparatus of Embodiment 3 that are the same as (or correspond to) those of Embodiment 1 are assigned the same reference numerals, and the duplicate explanation and drawing are omitted. The description is emphasized on the difference of the image forming apparatus of Embodiment 3 from that of Embodiment 1.

As shown inFIG. 7, the photosensitive drum81includes a conductive base81amade of metal in the shape of cylinder, an undercoat layer81bformed on the conductive base81a, and a charge generation/transport layer81eformed on the undercoat layer81b. The undercoat layer81bis made of an inorganic material with a predetermined resistance, and has a function to prevent the entry of the electric charge moved from the conductive base81aand to cancel the imperfections of the conductive base81a. The charge generation/transport layer81eis made of an organic optical semiconductive material and a resin material. The charge generation/transport layer81eabsorbs the light energy to generate positive and negative electric charge. The negative electric charge neutralizes the positively charged voltage of the surface of the photosensitive drum11, and positive electric charge is transmitted to the undercoat layer81a. The thickness of the charge generation/transport layer81eis approximately 20 μm.

In Embodiment 3, the charging roller12(FIG. 1) applies positive DC voltage to the photosensitive drum81so as to charge the surface layer of the photosensitive drum81from +300 V to +600 V. The charging roller12rotates together with the photosensitive drum81so as to restrict the decrease of the thickness of the surface layer of the photosensitive drum81.

The reproducing test of a single basic dot using the image forming apparatus of Embodiment 3 will be described. The method and condition of the test are the same as those described in Embodiment 1, and the duplicate explanation is omitted.

The thickness of the charge generation/transport layer81eof the photosensitive drum81is varied as a parameter, and the basic dots of several diameters are printed on the recording medium22. The reproducibility of the basic dot is evaluated by observing the toner image of the basic dot formed on the recording medium22. As a result of the test, it is recognized that the basic dot with the diameter of 22 μm and the basic dot with the diameter of 11 μm can be reproduced, and the reproducibility does not depend on the thicknesses of the charge generation/transport layer81e. It is understood that the reason is as follows. In the photosensitive drum81, the charge generation/transport layer81eabsorbs the light energy of exposure, and generates positive and negative electric charge in the vicinity of the surface of the charge generation/transport layer81e. The negative charge generated in the vicinity of the surface of the charge generation/transport layer81eneutralizes the positively charged voltage of the surface of the charge generation/transport layer81e. Accordingly, the diffusion of the negative electric charge is restricted, irrespective of the thickness of the charge generation/transport layer81e, and therefore the reproduced basic dot tends not to be broadened.

Next, the test to determine the decreasing amount of the thickness of the surface layer of the photosensitive drum81will be described. The method and condition of the test are the same as those described in Embodiment 1, and the duplicate explanation is omitted.

FIG. 8is a graph showing the experimental result. As shown inFIG. 8, the decreasing amount of the thickness of the surface layer of the photosensitive drum81is in proportion to the urging force when the urging force is larger than the predetermined force. When the urging force is set to approximately 50 g/cm (50×9.8 mN/cm), the decreasing amount of the thickness of the surface layer of the photosensitive drum81is approximately 1 μm when 10000 pages have been printed. However, when the urging force is gradually reduced, the decreasing amount of the thickness of the surface layer of the photosensitive drum81does not change. It is understood that, as the urging force becomes smaller, the primary factor of the decreasing the thickness of the surface layer shifts from the developing roller41to other components such as the charging roller12, the transfer roller21or the cleaning blade15.

In Embodiment 3, the thickness of the charge generation/transport layer81eis set to 20 μm. However, Embodiment 3 is not limited to the generation/transport layer81ewith the thickness of 20 μm. By setting the thickness of the generation/transport layer81eto be greater than or equal to the thickness needed for correct operation to which the decreasing amount based onFIG. 8is added, it becomes possible to obtain the lifetime required for he photosensitive drum81. In a particular example, the thickness of the generation/transport layer81ecan be approximately from 20 μm to 50 μm.

As described above, according to Embodiment 3 of the present invention, by using the photosensitive drum81having the charge generation/transport layer81eas the surface layer, it becomes possible to reproduce the basic dot corresponding to 1200 DPI, and further to reproduce the basic dot corresponding to 2400 DPI. Further, the decreasing amount of the charge generation/transport layer81ecan be restricted by setting the urging force of the developing roller41urged against the photosensitive drum81in the range from 10 g/cm to 50 g/cm, with the result that the lifetime of the photosensitive drum81can be longer than or equals to the lifetime of the drum cartridge2(for example, approximately 20000 prints).

While the preferred embodiments of the present invention have been illustrated in detail, it should be apparent that modifications and improvements may be made to the invention without departing from the spirit and scope of the invention as described in the following claims.