Developing devices for use in electrophotographic apparatus

Abstract: A developing device, using a one-component developer composed of colored fine synthetic resin toner particles, comprises a vessel (12) for holding the developer (D) and a developing roller (26) rotatably provided within the vessel (12), a portion thereof being exposed therefrom and resiliently pressed against a surface of a photosensitive drum (24). The toner particles are held by the surface of the developing roller (26) to form a developer layer therearound, and are carried to the surface of the image formation drum (24) for development of an electrostatic latent image formed thereon. The developing device further comprises a blade member (28) provided within the vessel (12) and resiliently engaged with the developing roller (26) for regulating a thickness of the developer layer formed therearound. Means (42,44,46,48,50,52,54,56) are provided whereby the pressures exerted on the developing roller (26) by the photosensitive drum (24) and the blade member (28), respectively, can be at least partially released.

Description: The present invention relates to developing devices for use in electrophotographic apparatus, wherein an electrostatic latent image is visually developed by using a one-component developer, particularly a non-magnetic type one-component developer. As is well known, an electrophotographic printer carries out the processes of: producing a uniform distribution of electrical charges on a surface of an electrostatic latent image carrying body such as an electrophotographic photoreceptor; forming an electrostatic latent image on the electrically charged surface of the electrophotographic photoreceptor by optically writing an image thereon, using a laser beam scanner, an LED (light emitting diode) array, an LCS (liquid crystal shutter) array or the like; visually developing the electrostatic latent image with a developer, i.e. , toner, which is electrically charged to be electrostatically adhered to the electostatic latent image zone; electrostatically transferring the developed visible image to a paper; and fixing the transferred image on the paper. Typically, the electrophotographic photoreceptor is formed as a photosensitive drum having a cylindrical conductive substrate and a photoconductive insulating film bonded to a cylindrical surface thereof. In the developing process, a two-component developer composed of a toner component (colored fine synthetic resin particles) and a magnetic component (magnetic fine carriers) is widely used, as it enables a stable development of the latent image. Note, typically the toner particles have an average diameter of about 10 µm, and the magnetic fine carriers have a diameter ten times larger than the average diameter of the toner particles. Usually a developing device using the two-component developer includes a vessel for holding the two-component developer, wherein the developer is agitated by an agitator provided therein. This agitation causes the toner particles and the magnetic carriers to be subjected to triboelectrification, whereby the toner particles are electrostatically adhered to each of the magnetic carriers. The developing device also includes a magnetic roller, provided in the vessel as a developing roller, in such a manner that a portion of the magnetic roller is exposed therefrom and faces the surface of the photosensitive drum. The magnetic carriers with the toner particles are magnetically adhered to the surface of the magnetic roller to form a magnetic brush therearound, and by rotating the magnetic roller carrying the magnetic brush, the toner particles are brought to the surface of the photosensitive drum for the development of the electrostatic latent image formed theron. In this developing device, a ratio between the toner and magnetic components of the developer body held in the vessel must fall within a predetermined range, to continuously maintain a stable development process. Accordingly, the developing device is provided with a toner supplier from which a toner component is supplied to the two-component developer held in the vessel, to supplement the toner component as it is consumed during the development process, whereby the component ratio of the two-component developer held by the vessel is kept within the predetermined range. This use of a two-component developer is advantageous in that a stable development process is obtained thereby, but the developing device per se has the disadvantages of a cumbersome control of a suitable component ratio of the two-component developer, and an inability to reduce the size of the developing device due to the need to incorporate the toner supplier therein. A one-component developer is also known in this field, and a developing device using same does not suffer from the above-mentioned disadvantages of the developing device using the two-component developer, because the one-component developer is composed of only a toner component (colored fine synthetic resin particles). Two types of the one-component developer are known; a magnetic type and a non-magnetic type. A developing device using the magnetic type one-component developer can be constructed in substantially the same manner as that using the two-component developer. Namely, the magnetic type one-component developer also can be brought to the surface of the photosensitive drum by a rotating magnetic roller as in the developing device using the two-component developer. The magnetic type one-component developer is suitable for achromatic color (black) printing, but is not suitable for chromatic color printing. This is because each of the toner particles composing the magnetic type one-component developer includes fine magnetic powders having a dark color. In particular, the chromatic color printing obtained from the magnetic type one-component developer appears dark and dull, due to the fine magnetic powders included therein. Conversely, the non-magnetic type one-component developer is particularly suitable for chromatic color printing because it does not include a substance having a dark color, but the non-magnetic type one-component developer cannot be brought to the surface of the photosensitive drum by the magnetic roller as mentioned above. A developing device using the non-magnetic type one-component developer is also known, as disclosed in U.S. Patents No. 3,152,012 and No. 3,754,963, Japanese Examined Patent Publication (Kokoku) No. 60-12627, and Japanese Unexamined Patent Publications (Kokai) No. 62-976 (equivalent to GB-A-2176718), No. 62-118372, No. 63-100482, and No. 63-189876, and comprises: a vessel for holding the developer; a developing roller provided within and supported rotatably by the said vessel such that a portion of the roller is exposed from the vessel and pressed, when the device is in use, against the surface of an electrostatic latent image carrying body, the developing roller being formed of a conductive elastic material that serves, when the device is in use, to entrain the toner particles to form a developer layer around the roller and to carry the entrained toner particles to the surface of the said electrostatic latent image carrying body for use in developing an electrostatic latent image formed thereon; and a blade member provided within and supported by the said vessel, which member is maintained, when the device is in use, in a working position in which it is pressed against the developing roller for regulating a thickness of the said developer layer formed around the developing roller. In developing devices of this type the conductive elastic developing roller may be formed of a conductive silicone rubber material or a conductive polyurethane rubber material or the like. When the conductive rubber roller is rotated within the body of the non-magnetic type one-component developer held by the vessel, the toner particles composing the non-magnetic type one-component developer are frictionally entrained by the surface of the conductive rubber developing roller to form a developer layer therearound, whereby the toner particles can be brought to the surface of the photosensitive drum for the development of the electrostatic latent image formed thereon. The blade member, which is resiliently pressed against the surface of the developing roller, serves to uniformly regulate a thickness of the developer layer formed therearound so that an even development of the latent image can be carried out. The blade member also serves to electrically charge the toner particles by a triboelectrification therebetween. In this type of developing device, the development process is carried out in such a manner that, at the area of contact between the photosensitive drum and the conductive rubber developing roller carrying the developer layer, the charged toner particles are electrostatically attracted and adhered to the latent image due to a bias voltage applied to the conductive solid rubber developing roller. To achieve a proper development of the latent image by the developing rubber roller, an elasticity or hardness of the developing roller is an important parameter, because the development quality and the development toner density are greatly affected by a contact or nip width between the photosensitive drum and the solid rubber developing roller pressed thereagainst. Namely, the developing roller must be pressed against the photosensitive drum so that a given nip width by which a proper development is obtained is established therebetween. When the developing roller is formed as a solid rubber roller, it may have a relative high hardness. For example, when measured by an Asker C-type hardness meter, the solid rubber developing roller showed an Asker C-hardness of about 58°. Accordingly, the solid rubber developing roller must be pressed against the photosensitive drum with a relatively high pressure to obtain the required nip width therebetween, but the higher the pressure exerted upon the photosensitive drum by the developing roller, the greater the premature wear of the drum. It has been suggested by the present inventors that the developing roller be formed of a conductive open-cell elastic material, to give it a high softness. Such a conductive open-cell elastic developing roller is constituted to prevent a penetration of the toner particles into an open-cell foam structure thereof, whereby the high softness of the developing roller can be maintained over a long period. In the developing device disclosed in GB-A-2176718, mentioned above, the vessel containing the developing roller is movable with respect to the photosensitive drum so that when the device is in a non-operative condition the developing roller is released from the pressure exerted thereon by the photosensitive drum. However, in this device, if the electrophotosensitive printer is not operated for a long time and/or if deterioration of the developing roller occurs, the developing roller is still subjected to plastic deformation because the developing roller is not released from the pressure exerted thereon by the blade member. In particular, a developing roller using a conductive open-cell elastic material as proposed by the present inventors is susceptible to such a plastic deformation due to the high softness thereof. As is obvious, when the developing roller is deformed, a proper development of the latent image cannot be carried out. Also, when the pressure exerted on the developing roller by the blade member is not released, the toner particles between the developing roller and the blade member may be adhered to each other, due to such pressure, and thus a poor development of the latent image may occur. According to the present invention, there is provided a developing device using a one-component developer composed of toner particles, which device comprises: a vessel for holding the developer; a developing roller provided within and supported rotatably by the said vessel such that a portion of the roller is exposed from the vessel and pressed, when the device is in use, against the surface of an electrostatic latent image carrying body, the developing roller being formed of a conductive elastic material that serves, when the device is in use, to entrain the toner particles to form a developer layer around the roller and to carry the entrained toner particles to the surface of the said electrostatic latent image carrying body for use in developing an electrostatic latent image formed thereon; and a blade member provided within and supported by the said vessel, which member is maintained, when the device is in use, in a working position in which it is pressed against the developing roller for regulating a thickness of the said developer layer formed around the developing roller; characterised in that the said blade member is supported movably with respect to the developing roller by the said vessel, the device having pressure release means operable when the device is in a non-operating state to cause the said blade member to be moved from its said working position to a release position in which the pressure exerted thereby on the said developing roller is reduced as compared to the pressure exerted thereby on that roller when the blade member is in its said working position. In such a developing device the pressure exerted on the developing roller by the blade member can be at least partially released during the non-operative condition of the developing device, whereby a plastic deformation of the developing roller can be avoided. Incidentally, US 4674441 discloses a developing device that has an elastic blade member which is pressed against a metal developing roller for regulating the thickness of the developer formed around the roller when the device is in use. When the device is not in use the pressure exerted by the blade member on the roller is reduced as compared to the working pressure so as to prevent the developer becoming attached to the surface of the roller or the blade member, and to prevent cohesion of the toner particles, which would otherwise lead to non-uniformity in the thickness of the developer layer. However, in this device the developing roller is made of metal and hence is not susceptible to plastic deformation. Preferably, in a developing device embodying the invention the said vessel is supported movably with respect to the electrostatic latent image carrying body and is maintained, when the device is in use, in a working position in which the said exposed portion of the developing roller is pressed against the said surface of the electrostatic latent image carrying body, and the said pressure release means are operable when the device is in the said non-operating state to cause the said vessel to be moved from its said working position to a release position in which the pressure exerted by the electrostatic latent image carrying body on the said developing roller is reduced as to compared to the pressure exerted by the body on that roller when the vessel is in its said working position. In this way the pressure exerted in the developing roller by the photosensitive drum can also be at least partially released during the non-operative condition of the developing device, further reducing the danger of plastic deformation of the developing roller. Conventionally, when the developing device is movable away from and toward the photosensitive drum, to resiliently press the developing roller against the photosensitive drum, a drive motor for the developing roller is supported by a frame structure of the electrophotographic printer, and a gear train for transmitting a rotational drive force from the drive motor to the developing roller is provided on one of the side wall portions of the developing device. The gear train is not engaged with an output gear of the drive motor until the developing device is positioned at a developing position in which the developing roller is resiliently pressed against the photosensitive drum. Nevertheless, even though the developing device is positioned at the developing position, it is still slightly movable toward and away from the photosensitive drum, and accordingly the developing device may be subjected to a twist motion due to the drive force of the motor, and thus it is difficult to resiliently press the developing roller against the photosensitive drum with a uniform linear pressure, resulting in an uneven development of the latent image. In a preferred embodiment of the present invention a drive source, for rotating the developing roller, is supported by the said vessel. When the vessel is mounted in a movable frame, the drive source may be attached to the movable frame. In such an embodiment, a transmission of a rotational drive force from the drive motor to the developing roller can be stably carried out, whereby an even development of the latent image can be ensured. A known developing device, provided with a detector for detecting a lack of the developer held in the vessel and raising an alarm to inform the operator that the consumption of the developer has reached a predetermined level, is disclosed in Japanese Examined Patent Publication No. 62-502, but the conventional detector is very complex and costly. In a preferred embodiment of the present invention the developing device further comprises: an agitator having an agitating element rotatable within the said vessel for agitating the developer; and means for detecting a lack of the developer in the said vessel, which detecting means include a first element which is arranged so that it can be entrained and rotated by the said agitating element when the device is in use, a second element which rotates together with the said first element, a third element which rotates together with the said agitating element, and a detector for detecting when the said second element passes a predetermined position during its rotation. When the said first element is entrained and rotated by the said agitating element, the said second and third elements are aligned with one another so that a detection of such passage of the said second element by the said detector is prevented by the said third element, whereas when an amount of the developer is less than a predetermined level the said first element rotates faster, due to the force of gravity, than the said agitating element so that detection of such passage of the said second element by the said detector becomes possible. In such an embodiment the detector for detecting a lack of the developer, can be simply and economically constructed. Furthermore, a developing device provided with a seal arrangement for preventing a leakage of the developer at locations near to the ends of the developer roller is known. This leakage of the developer is apt to occur at locations near to the ends of the developer roller because, when a thickness of the developer layer is regulated by the blade member, the excess developer has a tendency to be pushed toward the ends of the developer roller. The conventional seal arrangement, however, does not provide an effective and complete prevention of the developer leakage at the ends of the developing roller. In a preferred embodiment of the present invention the respective axial lengths of the said developing roller and the said blade member are both greater than the width of the developer stream directed onto the developing roller in the said vessel, and seal members are applied to end zones of the said developing roller and end zones of the said blade member. In such an embodiment, the seal members constitute a seal arrangement by which a prevention of the developer leakage at the ends of the developing roller can be effectively and completely carried out. Reference will now be made, by way of example, to the accompanying drawings, in which: Figure 1 shows a schematic view of a first developing device embodying the present invention; Figure 2 shows an exploded view of the developing device of Fig. 2; Figure 3 shows a perspective view of a frame casing of the developing device of Fig. 1; Figure 4 shows a view, similar to Fig. 1, of the Figure 1 device when a solenoid actuator thereof is energized; Figure 5 shows a view, similar to Fig. 4, of the Figure 1 device when the solenoid actuator is de-energized; Figure 6 shows a schematic view of a second developing device embodying the present invention, in which a rotary magnet actuator of the device is de-energized; Figure 7 shows a view, similar to Fig. 6, of the Figure 6 device when the rotary magnet actuator is energized; Figure 8 shows a view of a modification of the device of Figs. 6 and 7, in which two solenoid actuators of the modified device are de-energized; Figure 9 shows a view, similar to Fig. 8, of the Figure 8 device with the two solenoid actuators energized; Figure 10 shows a schematic view of a third developing device embodying the present invention, in which a rotary magnet actuator of the device is de-energized; Figure 11 shows a perspective view of a movable frame for a photosensitive drum of the developing device of Fig. 10; Figure 12 shows a perspective view of a vessel of the developing device of Fig. 10; Figure 13 shows a perspective view of a frame lever of the developing device of Fig. 10; Figure 14 shows a view, similar to Fig. 10, of the Figure 10 device when the rotary magnet actuator thereof is energized; Figure 15 shows a schematic view of a fourth developing device embodying the present invention, in which two solenoid actuators of the device are de-energized; Figure 16 shows a view, similar to Fig. 15, of the Figure 15 device when the two solenoid actuators are energized; Figure 17 shows a perspective view of a fifth developing device embodying the present invention; Figure 18 shows a plan view of the device of Fig. 17; Figure 19 shows a perspective view of a modification of the device of Fig. 17; Figure 20 shows a perspective view of a modification of the device of Fig. 19; Figure 21 shows a side view of the device modification of Fig. 20; Figure 22 shows a schematic view of a sixth developing device embodying the present invention; Figure 23 shows a perspective view of parts of the developing device of Fig. 22; Figure 24 shows a view taken along a line X-X of Fig. 22; Figure 25 shows a view, similar to Fig. 22, illustrating a positional relationship between a tongue element, a small plate element, and photo-sensor; Figure 26 shows a view, similar to Fig. 22, of the Fig. 22 developing device at a different developer consumption level; Figure 27 shows a view, similar to Fig. 22, of the Fig. 22 developing device at a further different developer consumption level; Figure 28 shows an exploded view of a seventh developing device embodying the present invention; Figure 29 shows an enlarged perspective view of an end of both a developing roller and a blade member together with a seal member applied thereto; Figure 30 shows a perspective broken view of a seal arrangement of the developing device of Fig. 29; and Figure 31 shows a plan view illustrating the dimensional and positional relationship between the developing roller, the blade member and other elements. Figure 1 schematically shows a developing device 10 using a non-magnetic type one-component developer incorporated into an electrophotgraphic printer (not shown). The developing device 10 comprises a vessel 12 for holding a non-magnetic type one-component developer D composed of colored fine toner particles of a suitable synthetic resin such as polyester or styrene acrylic resin, and having an average diameter of about 10µm. As shown in Figure 2, the vessel 12 has a generally rectangular parallelepiped shape, and is received in and supported by a frame casing 14 in the form of a shelf-like structure having a rectangular bottom plate 14a, side walls 14b extended upward from the shorter sides thereof, and a back wall 14c extended upward from one of the longer sides of the bottom plate 14a; the vessel 12 has a box-like configuration matching that of the frame casing 14. As best shown in Fig. 2, the rectangular bottom plate 14a of the frame casing 14 is provided with a pair of projections 16, 16 and a pair of resilient tongue elements 18, which are disposed along the long side edges of the bottom plate 14a, respectively. When the vessel 12 is received in the frame casing 14, the projections 16, 16 are engaged with the face of an acute shoulder 16a forming a part of a back wall surface of the vessel 12, near the bottom thereof, and the resilient tongue elements 18 are firmly engaged with a semi-circular groove 18a formed in a front bottom edge of the vessel 12, as shown in Fig. 1, whereby the vessel 12 is securely and fixedly supported on the bottom plate 14a of the frame casing 14. Each of the side walls 14b of the frame casing 14 is provided with a hole 20 formed at an upper front corner thereof, and the frame casing 14 is swingably suspended from a shaft 22 extended through the holes 20 of the side walls 14b and supported by a frame structure of the electrophotographic printer (not shown), whereby the frame casing 14, and therefore the vessel 12, can be moved toward and away from a photosensitive drum 24 forming a part of the electrophotgraphic printer. The photosensitive drum 24 comprises a sleeve substrate 24a made of a conductive material such as aluminum, and a photoconductive material film 24b formed therearound. The photoconductive material film 24b of the photosensitive drum 24 may be composed of an organic photoconductor (OPC), a selenium photoconductor or the like. A uniform distribution of electrical charges is produced on a surface of the photoconductive material film 24b of the photosensitive drum 24 by a suitable discharger (not shown), such as a corona discharger, and an electrostatic latent image is then optically written on the charged surface of the photoconductive material film 24b by an optical writing means (not shown) such as a laser beam scanner, an LED (light emitting diode) array, an LCS (liquid crystal shutter) array or the like. In particular, when the charged area of the photoconductive material film 24b is illuminated by the optical writing means, charges are released from the illuminated zone through the grounded sleeve substrate 24a, so that a potential difference between the illuminated zone and the remaining zone forms the electrostatic latent image. The developing device 10 also comprises a developing rubber roller 26 rotatably supported between the side walls of the vessel 12, and having a portion thereof exposed at a front of the vessel 12. The frame casing 14 is resiliently biased toward the photosensitive drum 24 by a pair of coil springs 27 fitted between the printer frame structure and the frame casing 14, and thus the exposed portion of the developing roller 26 is resiliently pressed against the surface of the photosensitive drum 24. Note, in Fig. 1, a portion of the printer frame structure on which the coil springs 27 act is symbolically represented by reference numeral 30, and the coil springs 27 also act on L-shaped shelf elements 30a fixed on the outer wall surfaces of the side walls 14b, respectively, as shown in Figs. 2 and 3. During the operation of the developing device 10, the photosensitive drum 24 and the developing roller 26 are rotated in the directions indicated by arrows A₁ and A₂, respectively, whereby the developing roller 26 entrains the toner particles to form a developer layer therearound, and thus the toner particles are brought to the surface of the photosensive drum 24 for the development of the latent image formed thereon. For example, the photosensitive drum 24 may have a diameter of 60 mm and a peripheral speed of 70 mm/s, and the developing roller 26 may have a diameter of 20 mm and a peripheral speed of from 1 to 4 times that of the photosensitive drum 24. The developing roller 26 comprises a shaft 26a rotatably supported by the side walls of the vessel 12, and a roller element 26b mounted thereon. The roller element 26b is preferably formed of a conductive open-cell foam rubber material based upon polyurethane, silicone, acrylonitrile-butadiene or the like. In this case, the roller element 26b is preferably constituted in such a manner that pore openings appear in an outer surface of the roller element 26b, and the diameter of these pore openings is at most twice the average diameter of the toner particles, so that a penetration of the toner particles to the inside of the open-cell foam structure of the roller element 26b can be effectively prevented, and thus the high softness of the roller element 26b can be maintained over a long period. The roller element 26b formed of the conductive open-cell foam rubber material preferably has an Asker-C hardness of from about 10 to 50°, more preferably 10°, and thus it is possible to press the developing roller 26 against the photosensitive drum 24 at a linear pressure of from about 22 to 50 g/cm, most preferably 43 g/cm, so that a contact or nip width of from about 1 to 3.5 mm can be obtained between the developing roller 26 and the photosensitive drum 24. The contact or nip width of from about 1 to 3.5 mm is necessary to a proper development of the latent image. Also, the roller element 26b preferably has a volume resistivity of from about 10⁴ to 10¹⁰ Ω · m, most preferably 10⁶ Ω · m. Note, the roller element 26b may serve to electrically charge the toner particles by a triboelectrification therebetween. The developing device 10 further comprises a blade member 28 engaged with the surface of the developing roller 26, to render uniform the thickness of the developer layer formed therearound, and thereby ensure an even development of the latent image. The blade member 28 is pivotably mounted between the side walls of the vessel 12 by pivot pins 28a, as shown in Fig. 2. Further, an elongated block member 30 is provided between the side walls of the vessel 12 near the blade member 28 and above the developing roller 26, and has through holes 30a formed therein, each of which receives a coil spring 32 and a stopper element 32a to resiliently bias the blade member 28 in a direction indicated by an arrow A₃. With this arrangement, the blade member 28 may be resiliently pressed against the developing roller 18 at a linear pressure of about 26 g/mm, to regulate the thickness of the developer layer formed therearound. The vessel 12 is provided with a partition 34 disposed therein adjacent to the blade member 28, as shown in Fig. 1, so that a space 34a free from the developer D remains therebetween. The blade member 28 may be formed of a suitable non-conductive or conductive rubber material, but preferably is coated with Teflon, and may be further formed of a suitable metal material such as aluminum, stainless steel, brass or the like. Note, the blade member 28 may also serve to electrically charge the toner particles by a triboelectrification therebetween. The developing device 10 further comprises a toner-removing roller 36 rotatably provided within the vessel 12 and in contact with the developing roller 26 in such a manner that a contact or nip width of about 1 mm is obtained therebetween, and by which remaining toner particles not used for the development of the latent image are removed from the developing roller 26. The toner-removing roller 36 may be formed of a conductive open-cell foam rubber material, preferably a conductive open-cell foam polyurethane rubber material having a volume resistivity of about 10⁶ Ω · m, and an Asker-C hardness of from about 10 to 70°, most preferably 30°. The toner-removing roller 36 is rotated in the same direction as the developing roller 26, whereby the remaining toner particles are mechanically removed from the developing roller 26. For example, the toner-removing roller 36 may have a diameter of 11 mm and a peripheral speed of from 0.5 to 2 times that of the developing roller 26. In the embodiment shown in Fig. 1, the toner-removing roller 36 is partially received in a recess formed in a bottom of the vessel 12, whereby a leakage of the toner particles from a space between the developing roller 26 and the vessel bottom can be prevented. Further, the developing device 10 comprises a paddle roller 38 for moving the toner particles toward the developing roller 26, and an agitator 40 for agitating the developer D to remove a dead stock from the vessel 12. The paddle roller 18 and the agitator 28 are rotated in the directions indicated by arrows A₄ and A₅, respectively. In operation, for example, when the photosensitive film 24b of the photosensitive drum 24 is formed of an organic photoconductor (OPC), a distribution of a negative charge is produced thereon, a charged area of which may have a potential of from about -600 to -650 volts. In this case, the latent image zone formed on the drum 24 by the optical writing means may have a reduced potential of about -50 volts. Note, in this case, the toner particles are given a negative charge. When the developing roller 26 is rotated within the developer D, the toner particles are frictionally entrained by the surface of the roller element 26b, and thus the toner particles are carried to the surface of the photosensitive drum 24. A developing bias voltage of from about -200 to -500 volts is applied to the developing roller 26, so that the toner particles carried to the surface of the drum 24 are electrostatically attracted only to the latent image zone having the potential of about -50 volts, as if the latent image zone were charged with the negative toner particles, and thus the toner development of the latent image is carried out. As mentioned above, the remaining toner particles not used for the development are mechanically removed from the developing roller 26 by the toner-removing roller 36, but the remaining toner particles also can be electrostatically removed from the developing roller 26 by applying a bias voltage of from -150 to -400 volts to the toner-removing roller 36. Since the developer layer formed of the remaining toner particles is subjected to physical and electrical affects during the developing process, it should be removed from the developing roller 26 and a fresh developer layer be formed thereon. On the other hand, when the blade member 28 is formed of the conductive material, a bias voltage of from about -200 to -500 volts is applied to the conductive blade member 28 so that the charged toner particles are not electrostatically adhered to the blade member 28. This is because, when the blade member has an opposite polarity with respect to a potential of the developing bias voltage applied to the developing roller 26, the toner particles are electrostatically adhered to the blade member 28, to thereby hinder an even formation of the developer layer around the developing roller 26. The application of the bias voltage to the blade member 28 may also contribute to the charging of the toner particles by a charge-injection effect. Note, when the photocondutive material film 24b of the photosensitive drum 24 is composed, for example, of a selenium photoconductor, on which a distribution of a positive charge is produced, the toner particles are positively charged and a positive bias voltage is applied to the developing roller 26 and the blade member 36. When the developing operation is stopped, i.e. , when the rotation of the developing roller 26 is stopped, the pressures exerted thereon by the phososensitive drum 24 and the blade member 28, respectively, must be released, because otherwise the roller element 26b will be plastically deformed while the developing roller 26 is at a stop. To this end, the developing device 10 is provided with a pressure release mechanism, to prevent this plastic deformation of the developing roller 26. In this embodiment, the pressure release mechanism includes a pair of generally T-shaped lever members 42, each of which has a stem 42a, and arms 42b and 42c perpendicularly extending from a top end thereof. The T-shaped lever members 42 are disposed between the side walls 14b and the side walls of the vessel 12, respectively, and each of the T-shaped lever members 42 is pivotally attached at a lower end of the stem 42a thereof to the corresponding side wall 14b by a pivot pin 44, as shown in Figs. 2 and 3. The T-shaped lever members 42 are connected to each other by a connecting rod 46, the ends of which are coupled to free ends of the arms 42b, respectively. The pressure release mechanism also includes a link element 48 having one end pivotally connected to the connecting rod 46 at a middle point thereof, and a solenoid actuator 50 having a working rod 50a pivotally connected to the other end of the link 48 by a pivot pin 52. Each of the T-shaped lever members 42 has a protrusion 42d extended upward from a transition region between the stem 42a and the arm 42b. The pressure release mechanism further includes a coil sping 54 acting between a back of the protrusion 42d of each T-shaped lever member 42 and the back wall 14c of the frame casing 14, as shown in Fig. 1, and a generally U-shaped leaf spring 56 secured to the protrusion 42d of each T-shaped lever member 42 at a front thereof. The coil springs 54 serve to resiliently bias the T-shaped lever members 42 in the clockwise direction in Fig. 1, and the U-shaped leaf springs 56 face portions 28c extended from the blade member 28, respectively, as shown in Fig. 1. During the developing operation, the solenoid actuator 50 is electrically energized so that the working rod 50a thereof is retracted, as shown in Figs. 1 and 4. In this case, the developing roller 26 is pressed against the photosensitive drum 24 at the linear pressure of from about 22 to 50 g/cm by the coil springs 27, and the blade member is pressed against the developing roller 26 at the linear pressure of about 26 g/mm by the coil springs 32. On the other hand, when the developing operation is stopped, the solenoid actuator 50 is electrically de-energized and the working rod 50a is moved from the retracted position to an extended position by the coil springs 54, whereby the T-shaped lever members 42 are moved in the clockwise direction in Figs. 1 and 4. This clockwise movement of the T-shaped lever members 42 results in an abutment of the arm portions 42c thereof against a portion 58 of the printer frame structure, so that the frame casing 14 is moved against a spring force of the coil springs 27 in the clockwise direction, as shown in Fig. 5, whereby the developing roller 26 is separated from the photosensitive drum 24. At the same time, the clockwise movement of the T-shaped lever members 42 also results in an abutment of the U-shaped leaf springs 56 against the extended portons 28c of the blade member 28, so that the blade member 28 is moved against a spring force of the coil springs 32 in the clockwise direction, whereby the blade member 28 is separated from the developing roller 26. Therefore, since pressures exerted on the developing roller 26 by the photosensitve drum 24 and the blade member 28, respectively, are released, the developing roller 26 is not subjected to plastic deformation. The developing device 10 shown in Figs. 1 to 5 is further characterized in that an electric motor 60 for driving the developing roller 26 is mounted on one of the side walls 14b of the frame casing 14, whereby the developing roller 26 can be stably and uniformly pressed against the photosensitive drum 24 at a given linear pressure. If the motor 60 is supported by the printer frame structure as in the conventional manner, the developing device 10 will be subjected to a twist motion by the drive force of the motor. Note, the drive motor 60 is operatively connected to the shaft 26a of the developing roller 26 through a gear train (not shown). Figures 6 and 7 show another embodiment of the developing device according to the present invention. The developing device per se of Figs. 6 and 7 is substantially identical to that of Figs. 1 to 5. In Figs. 6 and 7, a stopper member 34b made of a foam rubber material or sponge material is disposed between the partition 34 and the blade member 28 so that the developer is prevented from entering the space 34a therebetween. In the embodiment of Figs. 6 and 7, the frame casing 14 is guided to be moved toward and away from a photosensitive drum 24. As shown in Fig. 6, a coil spring 62 is disposed between the frame casing 14 and a portion 64 of the printer frame structure so that the frame casing 14 is resiliently biased to a position shown in Fig. 6 in which the developing roller 26 is separated from the photosensitive drum 24. Also, a spring 66 is disposed between the blade member 28 and the partition 34 so that the blade member 28 is resiliently biased to a position shown in Fig. 6 in which the blade member 28 is separated from the developing roller 26. The developing device of Figs. 6 and 7 is provided with a cam/link mechanism including a cam element 68 securely mounted on a cam shaft 68a supported by the printer frame structure, and a two-arm element 70 is pivoted on a shaft 70a, which is also supported by the printer frame structure. One end of the two-arm element 70 is engaged with the cam element 68, and the other end thereof is engaged with the blade member 28, so that the two-arm element 70 is resiliently biased in the counterclockwise direction. The cam/link mechanism further includes an arm element 72 having one end securely attached to the cam shaft 68a, and a link element 74 having one end pivotally connected to the other end of the arm element 72. The other end of the link element 74 is formed as an L-shaped portion and is engaged with a back upper edge of the frame casing 14, as shown in Figs. 6 and 7. The cam/link mechanism is driven by a rotary magnet actuator 76 coupled to the cam shaft 68a. When the rotary magnet actuator 76 is electrically de-energized, the cam/link mechanism is in the condition shown in Fig. 6, due to the spring forces of the coil springs 62 and 66, and thus the developing roller 26 is separated from the photosensitive drum 24 and the blade member 28 is separated from the developing roller 26. When the developing operation is started, the rotary magnet actuator 76 is electrically energized so that the arm element 72 is moved from a position shown in Fig. 6 to the position shown in Fig. 7, whereby the frame casing 14 is moved toward the photosensitive drum 24, against the spring force of the coil spring 62, and thus the developing roller 28 is pressed against the photosensitive drum 24 at a given linear pressure. Also, during the energization of the rotary magnet actuator 76, the cam element 68 is moved from a position shown in Fig. 6 to the position shown in Fig. 7, and accordingly, the two-arm element 70 is moved against the spring force of the coil spring 66 in the clockwise direction, and thus the blade member 28 is pressed against the developing roller 26 at a given linear pressure. Namely, when the developing operation is stopped, by electrically de-energizing the rotary magnet actuator 76, the pressures exerted on the developing roller 26 by the photosensitve drum 24 and the blade member 28 , respectively, can be released. Figures 8 and 9 show a modification of the embodiment of Figs. 6 and 7, in which two solenoid actuators 78 and 80 are used in place of the cam/link mechanism shown in Figs. 6 and 7. The solenoid actuator 78 is supported by the printer frame structure, and a working rod 78a thereof is connected to the back wall of the frame casing 14. When the solenoid actuator 78 is electrically de-energized, the working rod 78a is retracted by the spring force of the coil spring 62, and thus the frame casing 12 is resiliently biased to the position shown in Fig. 8 by the coil spring 62 whereby the developing roller 26 is separated from the photosensitive drum 24. The solenoid actuator 80 is supported by the frame vessel 12, and a working rod 80a thereof is pivotally connected to the blade member 28. When the solenoid actuator 80 is electrically de-energized, the working rod 80a is retracted by the spring force of the coil spring 66, and thus the blade member 28 is resiliently biased to the position shown in Fig. 8 to be thereby separated from the developing roller 26. When the developing operation is started, the solenoid actuators 78 and 80 are electrically energized so that the working rods 78a and 80a are extended from the positions shown in Fig. 8 to the positions shown in Fig. 9, respectively, whereby the developing roller 26 is pressed against the photosensitive drum 24 at a given linear pressure and the blade member 28 is pressed against the developing roller 26 at a given linear pressure. With the arrangement mentioned above, when the solenoid actuators 78 and 80 are electrically de-energized, the pressures exerted on the developing roller 26 by the photosensitive drum 24 and the blade member 28, respectively, can be released. Figures 10 to 14 show yet another embodiment of the developing device according to the present invention, which is assembled together with the photosensitive drum 24 as one unit. In this embodiment, the vessel 12 per se is detachably mounted in place in the printer frame structure, and is provided with partitions 82 and 84 by which an interior of the vessel 12 is divided into a developer storage chamber 86 and a drum chamber 88. Namely, in this embodiment, only the portion 86 of the vessel 12 holds the developer. The developing roller 26, the blade member 28, the toner-removing roller 36, and the paddle roller 38′ are provided within the developer storage chamber 86, and the photosensitive drum 24 is provided within the drum chamber 88. A portion of the developing roller 26 is exposed from a space between the partitions 82 and 84 and faces the photosensitive drum 24 In Figs. 10 and 14, 90 indicates a corona discharger provided within the drum chamber 88 and by which a uniform distribution of electrical charges is produced on the surface of the photoconductive drum 24. Note, although not shown in Figs. 10 and 14, the optical writing means such as the LED array for writing an electrostatic latent image on the charged surface of the photoconductive drum 24 is also provided within the drum chamber 88. In the embodiment of Figs. 10 to 14, the photosensitive drum 24 is supported by a movable frame 92. As shown in Fig. 11, the movable frame 92 includes a generally rectangular top plate 92a, a pair of side plates 92b extended downward from short sides of the top plate 92a, an upright plate 92c extended upward from one of longer sides of the top plate 92a, and a flap-like plate element 92d integrally formed along the other longer side of the top plate 92a. Each of the side plates 92b has a through hole 92e formed therein near the lower end thereof, and the photosensitive drum 24 has stub shafts 24c projected from the end faces thereof and having a diameter somewhat smaller than that of the through hole 92e. Accordingly, the photosensitive drum 24 is rotatably supported between the side plates 92b by inserting the stub shafts 24c into the through holes 92e, respectively. The vessel 12 has the appearance as shown in Fig. 12, and is provided with a pair of elongated openings or slots 94 formed in side walls thereof. Note, only one of the elongated openings or slots 94 is shown in Fig. 12. The vessel 12 is also provided with two elongated rectangular openings 96 and 98 formed in a top walls thereof and disposed in parallel to each other. The movable frame 92 with the photosensitive drum 24 supported thereby is arranged within the drum chamber 88 of the vessel 12 in such a manner that the stub shafts 24c of the photosensitive drum 24 are inserted into the elongated slots 94 while the upright plate 92c is partially projected out of the elongated rectangular opening 96. The blade member 28 is also arranged within the developer storage chamber in such a manner that a top portion of the blade member 28 is projected out of the elongated rectangular opening 98. Note, a scraper element 100 is attached to the flap-like plate element 92d of the movable frame 92, and is engaged with the photosensitive drum 24 to remove residual toner particles (not transferred to a sheet or paper during the toner image-transferring process) from the surface thereof. As shown in Fig. 10, a coil spring 102 is disposed between the partition 82 and the upright plate 92c of the movable frame 92, so that the movable frame 92 is resiliently biased to a position shown in Fig. 10, whereby the photosensitive drum 24 is separated from the developing roller 26. Also, a coil spring 104 is disposed between the blade member 89 and a support element 106 fixed on an inner surface of the top wall of the vessel 12, so that the blade member 28 is resiliently biased in the clockwise direction to be separated from the developing roller 26. In this embodiment, the developing device is provided with a cam/lever mechanism including a frame levber 108, and a cam element 110 operatively connected thereto. As shown in Fig. 13, the frame lever 108 has a rectangular frame appearance, and includes a frame 108a, a first front abutment plate 108b extended downward from a front edge of the frame 108a, a second front abutment plate 108c extended downward from the frame 108a at the rear side of the first front abutment plate 108b, and a rod element 108d extended along the rear edge of the frame 108a, the ends of the rod element 108d being securely joined to piece elements 108e extended downward from the rear side corners of the frame 108a. As shown in Fig. 10, the frame lever 108 is provided on the top wall of the vessel 12 in such a manner that the front faces of the first and second front abutment plates 108b and 108c face the top of the blade member 28 and the upright plate 92c of the movable frame 92. The cam element 110 is securely mounted on a shaft 110a supported by the printer frame structure and coupled to a rotary magnet actuator 110b, and has a cam slot 110 formed therein, in which the rod element 108d of the frame lever 108 is received. When the rotary magnet actuator 110b is electrically de-energized, the frame lever 108 is resiliently biased to a position shown in Fig. 10 by the spring forces of the coil springs 102 and 104, so that the photosensitive drum 24 and the blade member 28 are separated from the developing roller 26, as shown in Fig. 10. When the developing operation is started, the rotary magnet actuator 110b is electrically energized so that the cam element 110 is driven in the counterclockwise direction, whereby the frame lever 108 is moved from the position shown in Fig. 10 to a position shown in Fig. 14, so that the movable frame 98 and the blade member 28 are moved against the spring forces of the coil springs 102 and 104, respectively, and accordingly, the photosensitive drum 24 and the blade member 28 are pressed against the developing roller 26 at given linear pressures, respectively. When the developing operation is stopped, by electrically de-energizing the solenoid actuators 78 and 80, the pressures exerted on the developing roller 26 by the photosensitve drum 24 and the blade member 28, respectively, are released. Figures 15 and 16 show yet another embodiment of the developing device according to the present invention, which is also assembled together with the photosensitive drum 24 as a unit. In this embodiment, the vessel 12 is fixed to the frame casing 14, which is detachably mounted in place in the printer frame structure, and has an expanded portion 14d in which the photosensitive drum 24 is received, as shown in Figs. 15 and 16. Although not illustrated, the corona discharger for producing a uniform distribution of electrical charges on the surface of the photoconductive drum 24, the optical writing means such as the LED array for writing an electrostatic latent image on the charged surface of the photoconductive drum 24, and other elements are also provided within the expanded portion 14d of the frame casing 14. In the embodiment of Figs. 15 and 16, the photosensitive drum 24 is supported by inserting the stub shafts 24c into elongated slots 14e formed in side walls of the expanded portion 14d, so that the photosensitive drum 24 is movable toward and away from the developing roller 26. An end of each stub shaft 24c is projected out of the corresponding elongated slot 14e, and is rotatably connected to one end of an L-shaped arm member 112. A solenoid actuator 114 is supported by the printer frame structure or the frame casing 14, and a working rod 114a thereof is coupled to the other end of the L-shaped arm member 112. When the solenoid actuator 114 is electrically de-energized, the working rod 114a thereof is extended so that the photosensitive drum 24 is separated from the developing roller 26, as shown in Fig. 15. On the other hand, the stopper member 34b disposed between the blade member 28 and the partition 34 is made of a rubber material having a high elasticity, so that the blade member 28 is resiliently biased to a position shown in Fig. 15. A solenoid actuator 116 is supported by the frame casing 14 or the printer frame structure, and a working rod 116a thereof is pivotally coupled to the blade member 28. When the solenoid actuator 116 is electrically de-energized, the working rod 116a thereof is retracted so that the blade member 28 is maintained at the position shown in Fig. 15 to be separated from the developing roller 26. When the developing operation is started, the solenoid actuators 114 and 116 are electrically energized so that the working rod 114a is retracted while the working rod 116a is extended, whereby the photosensitive drum 24 and the blade member 28 are pressed against the developing roller 26. When the developing operation is stopped, by electrically energizing the solenoid actuators 114 and 116, the pressures exerted on the developing roller 26 by the photosensitve drum 24 and the blade member 28, respectively, can be released. Note, in the embodiments mentioned above, the pressures exerted on the developing roller may be partially released or reduced to a level at which the prevention of the plastic deformation of the developing roller can ensured, with the developing roller being in contact with the photosensitive drum and the blade. Figures 17 and 18 show another embodiment of the developing device according to the present invention. In this embodiment, the developing device comprises a vessel 118 for holding the non-magnetic type one-component developer, in which a developing roller, a blade member, a toner-removing roller, and other elements are arranged in substantially the same manner as mentioned above. The developing device also comprises a movable frame 120 for receiving the vessel 118. The frame 120 is comprised of a rectangular plate 120a, and a pair of side walls 120b extended upward from shorter sides of the rectangular plate portion 120a. Each of the side walls 120b is provided with a pawl element 122 resiliently biased inward by a suitable spring (not shown) such as a torsion spring. When the vessel 118 is received in the frame 120, the pawls 122 are engaged with the vessel 118, as shown in Fig. 18, and thus it is immovably held in the movable frame 120. The frame 120 bridges a pair of guide rails 124, each of which has a guide groove 124a formed therein, and is provided with guide rollers or slider elements (not shown) engaged in the guide grooves 124a, whereby the frame 120 is movable toward and away from a photosensitive drum 126 installed at a fixed position. Note, the photosensitive drum 126 is also constructed in substantially the same manner as mentioned above. The vessel 118 or the frame 120 is resiliently baised by a suitable spring (not shown) in a direction indicated by an arrow A₆, whereby the developing roller (a portion thereof is indicated by reference numeral 128 in Fig. 18) is pressed against the photosensitive drum 126. In the developing device of Figs. 17 and 18 an electric motor 130 for driving the developing roller 128 is mounted on one of the side walls 120b of the frame 120. In particular, the motor 130 is attached to the outer wall surface of the side wall 120b concerned, with an output shaft of the motor 130 passing therethrough. An output gear 130a is fixed to the end of the motor shaft, and is engaged with a gear 132 attached to the inner wall surface of the side wall 120b concerned. The gear 132 is then engaged with a gear 134 attached to the corresponding side wall of the vessel 118, and thus the gear 134 is engaged with a gear 136 attached to the corresponding end of a shaft of the developing roller shaft 138. With this arrangement, the developing roller can be stably and reliably driven because the movable developing device per se is provided with the motor mounted thereon. Figure 19 shows a modification of the embodiment shown in Figs. 17 and 18, in which the motor 130 is mounted on one of the side walls of the vessel 118. An output shaft of the motor 130 passes through the side wall concerned of the vessel 118, and an output gear (not visible) is fixed to the end of the motor shaft. The output gear is engaged with a gear (also not visible) attached to the inner wall surface of the side wall concerned of the vessel 118 and having a shaft passing therethrough. This shaft also has a gear 138 fixed on the outer end thereof, as indicated by reference numeral 140 in Fig. 19. The gear 138 is engaged with a gear 142 attached to the outer wall surface of the side wall concerned of the vessel 118, and the gear then engaged with a gear 144 attached to the corresponding end of a shaft of the developing roller shaft. Note, the gears provided within the vessel 118 are preferably covered, to be protected from an ingress of the toner particles. The vessel 118 with the motor 130 is received in the movable frame 120 and is immovably held therein by a releasable fixture. In this modified embodiment, since the movable developing device per se is also provided with the motor mounted thereon, the developing roller can be stably and reliably driven. Note, in Fig. 19, the slider element engaged in each guide groove 124a is indicated by reference numeral 120c. Figures 20 and 21 also show a modification of the embodiment of Fig. 19, in which the motor 130 and the gears associated therewith are arranged in the same manner as in Fig. 19. In this modified embodiment, a fixed frame 146 is used in place of the movable frame 140, and includes a pair of bracket elements 146a disposed near the end face of the photosensitive drum 126, and a bar element 146b extended between the bracket elements 146a. The vessel 118 has a pair of hook elements 148 extended upward from the top wall of the vessel 118, and is swingably suspended from the bar elements 146b by engaging the hook elements 146 therewith, as shown in Fig. 21. The vessel 118 is resiliently baised by a coil spring 150 (Fig. 12) in the direction of the arrow A₆, whereby the developing roller 128 is pressed against the photosensitive drum 126. Figures 22 to 27 show yet another embodiment of the developing device according to the present invention. This developing device is provided with a means for detecting a lack of the developer, and is simply and economically constructed according to the invention. In the embodiment of Figs. 22 to 27, the developing device comprises a vessel 152 for holding the non-magnetic type one-component developer, in which a developing roller 154, a blade member 156, and a toner-removing roller 158 are arranged in substantially the same manner as mentioned above, and further, a phososensitive drum 159 is constructed in substantially the same manner as mentioned above. Note, in Figs. 22, 25, 26, and 27, the toner particles are symbolically shown by small open circles. The developing device of Figs. 22 to 27 further comprises an agitator 160 provided within the vessel 152 for agitating the toner developer as mentioned above. The agitator 160 includes a shaft 160a, an end of which passes through a side wall 152a of the vessel 152, and has a gear 160b fixed thereon, as shown in Fig. 24. The developing roller 154 includes a shaft 154a, an end of which also passes through the side wall 152a of the vessel 152 and has a gear 154b fixed thereon. The gear 154b is operatively connected to the gear 160b through a gear train (not shown in Fig. 24) whereby, during the rotation of the develping roller 154, the shaft 160a of the agitator 160 is rotated in a direction indicated by arrow A₇ in Fig. 22. The agitator 160 also includes an rod-like agitating element 160c, radially bent and joined to the shaft 160a, as shown in Fig. 23. By rotating the shaft 160a with the rod-like agitating element 160c, the developer can be effectively agitated in the vessel 152. The agitating element 160c may be provided with a coil wire 160d, illustrated by a chain line in Fig. 22, whereby the agitation of the developer can be facilitated. As mentioned above, the developing device of Figs. 22 to 27 is provided with lack of developer detecting means incorporated into the agitator 160, and generally indicated by reference numeral 162. In particular, the lack of developer detecting means 162 includes a pair of sleeve elements 162a loosely fitted over the shaft 160a of the agitator 160, and a rod-like element 162b having radially bent ends and joined to the sleeve elements 162a. Note, in Fig. 23, only one of the sleeve elements 162a is illustrated. The lack of developer detecting means 162 also includes a gutter-like element 162c extended from an end edge of the sleeve element 162a ( i.e. , that illustrated) along the agitator shaft 160a, as shown in Fig. 23. The gutter-like element 162c also passes thorugh the side wall 152a of the vessel 152, and has a tongue element 162d suspended from the end thereof which projects from the side wall 152a of the vessel 152. The lack of developer detecting means 162 further includes a small plate element 162e attached to the end of the agitator shaft 160a, as best shown in Fig. 23. When the agitator shaft 160a is rotated as mentioned above, the rod-like agitating element 160c (or the coil wire 160d) is abutted against the rod-like element 162b, so that the rod-like element 162b is also rotated together with the agitator shaft 160a, resulting in a rotation of the tongue element 162d about the longitudinal axis of the agitator shaft 160a. The attachment of the small plate element 162e to the end of the agitator shaft 160a is performed in such a manner that the small plate element 162e is aligned with the tongue element 162d along the longitudinal axis of the agitator shaft 160a while the rod-like element 162b is rotated together with the agitator shaft 160a, as shown in Figs. 23 and 25 Furthermore, the lack of developer detecting means 162 includes a well known type of photo-sensor 162f provided at the outside of the vessel side wall 152a, as shown in Figs. 24 and 25, and supported by the vessel 152 or the printer frame structure. The photo-sensor 162f is positioned at a slightly higher level than that of the inner bottom surface of the vessel 152 and at a location on a circle drawn by the end of the tongue element 162d during the rotation thereof. The photo-sensor 162f emits a light, and when the emitted light is reflected back, the photo-sensor 162f detects the reflected light. Note, the small plate element 162d has a non-reflective surface, whereas the tongue element 162f has a reflective surface. In operation, when a sufficient amount of the developer is held in the vessel 152, as shown by a broken line in Fig. 22, the rod-like element 162b is rotated together with the agitator shaft 160a so that the alignment of the tongue element 162d with the small plate element 162e is maintained during the rotation thereof. Accordingly, the photo-sensor 162f cannot detect the reflected light because the small plate element intervenes between the tongue element 162d and the photo-sensor 162f. Also, even though some developer is consumed due to the development of the latent image, as shown in Fig. 26, the rod-like element 162b can be rotated together with the agitator shaft 160a, and thus the photo-sensor 162f cannot detect the reflected light. Nevertheless, when a large amount of the developer is consumed as shown in Fig. 27, the rod-like element 162b cannot be partially rotated together with the agitator shaft 160a. In particular, during the rotation of the rod-like element 162b from the bottom dead point to the top dead point, the rod-like element 162b is lifted up by the agitating element 160c, but just after the rod-like element 162b passes the top dead point, the rod-like element 162b falls down toward the bottom dead point, due to the force of gravity, at a speed higher than the rotational speed of the agitating element 160c. In this case, since the small plate element 162e cannot intervene between the tongue element 162d and the photo-sensor 162f, the photo-sensor 162f can detect the light reflected from the tongue element 162d. For example, the printer may be constructed so that an alarm is raised when the reflected light is detected, and thus the operator is informed that there is a lack of the developer. Figures 28 to 31 show still another embodiment of the developing device according to the present invention. This developing device is provided with a seal arrangement for the developer. This developing device comprises a vessel 164 for holding the non-magnetic type one-component developer, which is assembled from a vessel body 164a having a recess 164b, and a pair of rectangular side wall members 164c securely attached to side faces of the vessel body 164a. As shown in Fig. 28, the recess or developer storage chamber 164b is formed in such a manner that a large opening is formed at a front side of the vessel 164 while a portion of the vessel body 164a forms a rear wall of the vessel 164. The developing device also comprises a developing roller 166 having a shaft 166a rotatably supported by the side wall members 164c at the front side of the vessel 164, and a roller element 166b mounted thereon and formed of a conductive elastic material, preferably a conductive open-cell foam elastic material, for the reason stated hereinbefore. Note, in Fig. 28, through holes 166c are formed in the side wall member to receive the ends of the developing roller shaft 166a. The developing device further comprises a blade member 168 swingably supported between the side wall member 164c at a rear of the developing roller 166. Namely, the blade member 168 has a through hole 168a formed along the longitudinal axis thereof and registered with holes 168b formed in the side wall members 164c, and a pivot pin (not shown) is extended from one of the holes 168b to the other holes 168b through the through hole 168a, whereby the blade member 168 is swingable about the longitudinal axis thereof. Furthermore, the developing device comprises a partition 170 securely supported between the side wall members 164c at a rear of the blade member 168, and a toner-removing roller 172 rotatably supported between the side wall members 164c below the developing roller 166 and engaged therewith. Note, the developing roller 166, the blade member 168, the partition 170, and the toner-removing roller 172 are equivalent to the corresponding elements of Fig. 1. As shown in Fig. 28, the developing roller 166 and the blade member 168 have axial lengths larger than a width of the recess or developer storage chamber corresponding to an axial length of the partition 170. Accordingly, each of the side wall members 164c is formed with a recess 174 at the inner wall surface thereof for receiving the corresponding ends of the developing roller 166 and blade member 168. Each of the recesses 174 is defined by a bottom surface portion 174a and first, second and third side surface portions 174b, 174c, and 174d perpendicularly raised from the bottom surface portion 174a. As apparent from Figs. 29 and 30, a profile defined by the first, second, and third side surface portions 174b, 174c, and 174d substantially comforms with the back profile (defined by a broken line in Fig. 29) of the ends of the developing roller 166 and blade member 168, and the former profile surface is spaced from the latter profile surface to receive a seal member 176 therebetween. The seal member 176 is preferably shaped so as to be received and compressed between the profile surfaces. With this developer seal arrangement, leakage of the developer at locations near the ends of the developer roller 166 can be completely prevented. In summary, as shown in Fig. 31, the developer seal arrangement is based upon the fact that axial lengths ℓ₁ and ℓ₂ of the developing roller 166 and blade member 168 are longer than a width ℓ₃ of a developer stream directed from the developer storage chamber 164b toward the developing roller 166 during the development, and that the seal members 176 are applied to the end zones of the developing roller 166 and the end zones of the blade member 168 having a width ℓ₄ and separated from the developer stream directed to the developing roller 166. The width ℓ₃ of the developer stream is usually represented by a width of the developer storage chamber 164b, the partition 170 or the toner-removing roller 172. As is easily understood, the width ℓ₄ is defined by the following formula: ℓ₄ = (ℓ₁ or ℓ₂ - ℓ₃)/2 In Fig. 31, although the length ℓ₁ of the developing roller 166 is nearly equal to the width ℓ₂ of the blade member 168, they may be different from each other, if necessary. Note, in Fig. 31, ℓ₅ indicates an effective developing range which is narrower by 2 α than the width ℓ₁ of the developing roller 166, because sufficient developer cannot be fed to the end zone having the width α. Although the embodiments of the present invention are explained in relation to a photosensitive drum, they can be also applied to a dielectric drum on which the electrostatic latent image can be formed. Further, although the developing device according to the present invention has been described above for use with the non-magnetic type one-component developer, the magnetic type one-component developer may be also used, if necessary. Finally, it will be understood by those skilled in the art that the foregoing description is of preferred embodiments of the present invention, and that various changes and modifications can be made thereto without departing from the scope thereof as defined in the appended claims.

Claims: A developing device using a one-component developer composed of toner particles, which device (10) comprises: a vessel (12) for holding the developer; a developing roller (26) provided within and supported rotatably by the said vessel (12) such that a portion of the roller is exposed from the vessel and pressed, when the device is in use, against the surface of an electrostatic latent image carrying body (24), the developing roller (26) being formed of a conductive elastic material that serves, when the device is in use, to entrain the toner particles to form a developer layer around the roller and to carry the entrained toner particles to the surface of the said electrostatic latent image carrying body for use in developing an electrostatic latent image formed thereon; and a blade member (28) provided within and supported by the said vessel (12), which member is maintained, when the device is in use, in a working position in which it is pressed against the developing roller (26) for regulating a thickness of the said developer layer formed around the developing roller; characterised in that the said blade member (28) is supported movably with respect to the developing roller (26) by the said vessel (12), the device having pressure release means (42, 44, 46, 48, 50, 56; 68, 70, 76; 78, 80; 108, 110, 110b; 114, 116) operable when the device is in a non-operating state to cause the said blade member to be moved from its said working position to a release position in which the pressure exerted thereby on the said developing roller (26) is reduced as compared to the pressure exerted thereby on that roller when the blade member is in its said working position. A developing device as claimed in claim 1, wherein the said pressure release means (42, 44, 46, 48, 50, 56; 68, 70, 76; 78, 80; 108, 110, 110b; 114 116) are such that, when in its said release position, the blade member (28) is separated from the developing roller (26), whereby the pressure exerted on that roller by the blade member is completely released when the blade member is in its said release position. A developing device as claimed in claim 1 or 2, wherein the said vessel (12) is supported movably with respect to the electrostatic latent image carrying body (24) and is maintained, when the device is in use, in a working position in which the said exposed portion of the developing roller (26) is pressed against the said surface of the electrostatic latent image carrying body (24), and the said pressure release means (42, 44, 46, 48, 50, 56; 68, 70, 76; 78, 80) are operable when the device is in the said non-operating state to cause the said vessel (12) to be moved from its said working position to a release position in which the pressure exerted by the electrostatic latent image carrying body (24) on the said developing roller (26) is reduced as to compared to the pressure exerted by the body on that roller when the vessel is in its said working position. A developing device as claimed in claim 3, wherein the said pressure release means (42, 44, 46, 48, 50, 56; 68, 70, 76; 78, 80) are such that, when the said vessel (12) is in its said release position, the developing roller (26) is separated from the electrostatic latent image carrying body (24), whereby the pressure exerted on that roller by the body is completely released when the vessel is in its said release position. A developing device as claimed in claim 3 or 4, wherein the said pressure release means comprise an actuator (50), and a link/lever mechanism (42, 44, 46, 48) providing an operative connection between, on the one hand, the actuator (50) and, on the other hand, both the said vessel (12) and the said blade member (28), the said link/lever mechanism (42, 44, 46, 48) being arranged so that the said vessel (12) and the said blade member (28) are both positioned in their respective working positions when the said actuator (50) is energised, and that the said vessel (12) and the said blade member (28) are positioned at their respective release positions when the actuator (50) is de-energised. A developing device as claimed in claim 5, wherein the said actuator (50) is a solenoid actuator. A developing device as claimed in claim 3 or 4, wherein the said pressure release means comprise an actuator (76), and a cam/link mechanism (68, 70, 72, 74) providing an operative connection between, on the one hand, the said actuator (76) and, on the other hand, both the said vessel (12) and the said blade member (28), the said cam/link mechanism (68, 70, 72, 74) being arranged so that the said vessel (12) and the said blade member (28) are positioned at their respective working positions when the actuator (76) is energised, and that the said vessel (12) and the said blade member (28) are positioned at their respective release positions when the said actuator (76) is de-energised. A developing device as claimed in claim 7 wherein the said actuator (76) is a rotary magnetic actuator. A developing device as claimed in claim 3 or 4, wherein the said pressure release means have separate mechanisms (78, 80) for moving the blade member (28) and the vessel (12) respectively from their respective working positions to their release positions. A developing device as claimed in claim 9, wherein the vessel-moving mechanism comprises an actuator (78) operatively coupled to the said vessel (12), which actuator is arranged in such a manner that the said vessel (12) is positioned at its said working position when the actuator (78) is energised, and that the said vessel (12) is positioned at its said release position when the actuator (78) is de-energised, and wherein the blade-member-moving mechanism comprises an actuator (80) operatively coupled to the said blade member (28), which actuator (80) is arranged in such a manner that the said blade member (28) is positioned at its said working position when the actuator (80) is energised, and that the said blade member (28) is positioned at its said release position when the actuator (80) is de-energised. A developing device as claimed in claim 10, wherein each of the said actuators (78, 80) is a solenoid actuator. A developing device as claimed in any one of claims 3 to 11, wherein a drive source (130), for rotating the developing roller (128), is supported by the said vessel (118). A developing device as claimed in any one of claims 3 to 11, wherein the said vessel (12;118) is supported by a movable frame (14;120), and a drive source (60;130), for rotating the developing roller, is attached to the movable frame (14;120). A developing device as claimed in claim 1 or 2, wherein the said electrostatic latent image carrying body (24) is supported movably with respect to the developing roller (26) and is maintained, when the device is in use, in a working position in which the said surface thereof is pressed against the said exposed portion of the said developing roller, and the said pressure release means (92, 108, 110, 110b; 114, 116) are operable when the device is in the said non-operating state to cause the electrostatic latent image carrying body (24) to be moved from its said working position to a release position in which the pressure exerted thereby on the said developing roller (26) is reduced as compared to the pressure exerted by the body on that roller when the body is in its said working position. A developing device as claimed in claim 14, wherein the said pressure release means (92, 108, 110, 110b; 114, 116) are such that, when in its said release position, the electrostatic latent image carrying body (24) is separated from the said developing roller (26), whereby the pressure exerted by the body on that roller is completely released when the body is in its said release position. A developing device as claimed in claim 14 or 15, wherein the said pressure release means (92, 108, 110, 110b) comprise an actuator (110b), and a cam/lever mechanism (92, 108, 110) providing an operative connection between, on the one hand, the said actuator (110b) and, on the other hand, both the said electrostatic latent image carrying body (24) and the said blade member (28), the said cam/lever mechanism (92, 108, 110) being arranged so that the said electrostatic latent image carrying body (24) and the said blade member (28) are positioned at their respective working positions when the said actuator (110b) is energised, and that the said electrostatic latent image carrying body (24) and the said blade member (28) are positioned at their respective release positions when the said actuator (110b) is de-energised. A developing device as claimed in claim 16, wherein the said actuator (110b) is a rotary magnetic actuator. A developing device as claimed in claim 14 or 15, wherein the said pressure release means (114, 116) have separate mechanisms (114, 116) for moving the blade member (28) and the electrostatic latent image carrying body (24) respectively from their respective working positions to their release positions. A developing device as claimed in claim 18, wherein the body-moving mechanism comprises an actuator (114) operatively coupled to the said vessel (12), which actuator (114) is arranged in such a manner that the electrostatic latent image carrying body (24) is positioned at its said working position when the said actuator (114) is energised, and that the body (24) is positioned at its said release position when the actuator (114) is de-energised, and wherein the blade-member-moving mechanism comprises an actuator (116) operatively coupled to the said blade member (28), which actuator (116) is arranged in such a manner that the said blade member (28) is positioned at its said working position when the actuator (116) is energised, and that the said blade member (28) is positioned at its said release position when the actuator (116) is de-energised. A developing device as claimed in claim 19, when each of the said actuators (114, 116) is a solenoid actuator. A developing device as claimed in any preceding claim, further comprising: an agitator (160) having an agitating element (160c) rotatable within the said vessel (152) for agitating the developer; and means (162) for detecting a lack of the developer in the said vessel, which detecting means include a first element (162b) which is arranged so that it can be entrained and rotated by the said agitating element (160c) when the device is in use, a second element (162d) which rotates together with the said first element (162b), a third element (162e) which rotates together with the said agitating element (160c), and a detector (162f) for detecting when the said second element (162d) passes a predetermined position during its rotation, wherein, when the said first element (162b) is entrained and rotated by the said agitating element (160c), the said second and third elements (162d, 162e) are aligned with one another so that a detection of such passage of the said second element (162d) by the said detector (162f) is prevented by the said third element (162e), and wherein when an amount of the developer is less than a predetermined level the said first element (162) rotates faster, due to the force of gravity, than the said agitating element (160c) so that detection of such passage of the said second element (162d) by the said detector (162f) becomes possible. A developing device as claimed in claim 21, wherein the said detector (162f) is a photo-sensor which is operable to emit light and to detect a reflection of the emitted light, the said second and third elements (162d, 162e) having reflective and non-reflective surfaces respectively, and the said photosensor (162f) being oriented at the said predetermined position so that the light emitted therefrom is directed perpendicularly to the said reflective surface of the said second element (162d). A developing device as claimed in any preceding claim, wherein the respective axial lengths (ℓ₁, ℓ₂) of the said developing roller (166) and the said blade member (168) are both greater than the width (ℓ₃) of the developer stream directed onto the developing roller (26) in the said vessel (164), and wherein seal members (176) are applied to end zones of the said developing roller (166) and end zones of the said blade member (168).

Inventors: HIROSE KAZUNORI, NISHINO YUKIO, HIROSE, KAZUNORI, NISHINO, YUKIO
Applicants: FUJITSU LTD, FUJITSU LIMITED
Application year: 1990
Main classification: G03G15
IPC codes: G03G15