Developing device

Abstract: A developing device comprises a developer carrying means (30) for carrying developer from a developer supplying means (35,37) to an electrophoto­ graphic light-sensitive member (22) having electro­ static latent image and a developer returning means (41,43) for returning superfluous developer to the developer supplying means, thereby over-electri­ fication of the developer is prevented.

Description: FIELD OF THE INVENTION AND RELATED ART STATEMENT 1. FIELD OF THE INVENTION The present invention relates generally to electrophotography, and more particularly to a developing device thereof. 2. DESCRIPTION OF THE RELATED ART Developing methods using a dry type developer are classified into a method using two-components developer and a method using one-component developer. In the method using the two-components developer, since a mixed developer consisting of toner and carrier is used, a toner density controlling device is required to maintain a mixture ratio of the toner and carrier in a predetermined constant value, and there is a defect that the carrier must be exchanged by a new one every predetermined time period because of its deterioration. Recently, in order to improve the above-mentioned defect, a developing method using one-component developer which does not use the carrier is proposed. The developing method is disclosed in the Japanese patents JP-A- 54-43038 and JP-A- 56-110963, and United States Patent US-A-4,083,326, for example. These constitutions are shown in FIG.1, FIG.2 and FIG.3, respectively. Reference numerals of these figures are given by the inventor of the present application. Referring to FIG.1 (Sho 54-43038), a developing roller 1 is composed of a cylindrical sleeve containing magnets 1A. Magnetic toner 2 is stored in a hopper 3 which is installed under the developing roller 1. A blade 4 contacts the surface of the developing roller 1. A photoreceptor 5 is disposed over the developing roller 1. The developing roller 1 has a rough surface made of a metal material, and the toner 2 is supplied on the surface of the developing roller 1 from the hopper 3. When the developing roller 1 rotates in the direction as shown by an arrow A, the toner 2 is charged with a predetermined polarity by the blade 4 touching and sliding on the surface of the developing roller 1 and is coated on the surface of the developing roller 1. The toner 2 charged with the predetermined polarity flies to the photoreceptor 5 when the toner is faced to an electrostatic latent image on the surface of the photoreceptor 5, and the electrostatic latent image is developed. In the prior art, the toner being on the surface layer of the developing roller 1 contacts the blade 4 and is charged by friction. The toner which is not used to develop on the developing roller 1 is further charged by touching with the blade 4. As a result, the toner is overcharged, and there is a difficulty in reproduction of a high quality image. FIG.2 is a second conventional example in the prior art (Sho 56-110963). Referring to FIG.2, a developing roller 7 made of electroconductive urethane is in contact with a photoreceptor 6. An electroconductive fur brush 8a is in contact with the developing roller 7. A power source 9 applies an electric potential to the fur brush 8a and developing roller 7. An electric potential distributor 10 controls an electric potential of the power source 9. Toner 12 is stored in a hopper 11. Toner 12 supplied to an electroconductive fur brush 8b from the hopper 11 is supplied to the fur brush 8a. The toner supplied on the fur brush 8a via the fur brush 8b is charged by friction of the fur brushes 8a and 8b. Then the toner is coated on the surface of the developing roller 7 from the fur brush 8a by electric potential of the power source 9. The toner on the developing roller 7 adheres on electrostatic latent image of the photoreceptor 6. In the above-mentioned process, a developing density is adjusted by the electric potential which is controlled by the electric potential distributor 10. In the above-mentioned prior art, the toner charged on the fur brush 8a is coated on the developing roller 7 by electric field between the fur brush 8a and the developing roller 7. Therefore, the toner on the developing roller 7 and in the fur brush 8a which is not used to develop is further charged by friction between the developing roller 7 and fur brush 8a. Moreover, since the toner is supplied from the hopper 11 disposed over the fur brush 8a and 8b, surplus toner is likely to be filled in the fur brushes 8a and 8b. As a result, the toner is overcharged, and there is also a difficulty to reproduce a high quality image. FIG.3 shows a prior art in the US patent US-A-4,083,326. Referring to FIG.3, two electroconductive fur brushes 18 and 19 arranged in a hopper 15 are in contact with the developing roller 13. A sheet-shaped photoreceptor 17 is disposed over the developing roller 13. A blade 16 is also in contact with the developing roller 13. A first power source 20 gives an electric potential across the hopper 15 and the fur brush 18. A second power source 21 gives an electric potential across the fur brush 19 and the hopper 15. The voltage of the second power source 21 is higher than that of the first power source 20, and is lower than an electric potential of the electrostatic latent image of the photoreceptor 17. The toner 14 which is charged by friction of the fur brush 18 is supplied to the developing roller 13 via the fur brush 18 by a potential difference between the power sources 20 and 21. The toner supplied on the developing roller 13 is flattened by the blade 16. Then, the toner 14 adheres to the electrostatic latent image on the photoreceptor 17. The toner 14 remained on the developing roller 13 after developing is scraped by the fur brush 19 and a ghost on the developing roller is removed. In the above-mentioned prior art, the charged toner on the fur brush are coated on the developing roller by one of the two fur brushes, and the toner charged and remained on the developing roller after a developing process is removed by the other fur brush. In the example, over-charging of the toner on the developing roller is prevented. However, the toner in the fur brush which is not used in developing is repeatedly rubbed with the developing roller and further charged by friction, thereby over-charging is liable to occur, and there is a difficulty to obtain a high quality image. In JP-A-61-45 257 a developing device is disclosed in which developer is stirred in a hopper 2 and an agitator 4 is mounted in the lower part of the hopper 2. A layer thickness control member consisting of a doctor blade 6 is vertically located in the hopper 2, and a gap is formed between the inner bottom wall of the hopper 2 and the lower end of the doctor blade 6. The developer in the hopper 2 is stirred by the agitator 4 and is sent off toward the developer supply roller 5 through the gap. The lower end of the doctor blade 6 is pressed on the surface of the developer supply roller 5, and thereby a developer layer uniform in thickness is formed on the developer supply roller 5. OBJECT AND SUMMARY OF THE INVENTION An object of the present invention is to provide a developing device capable of reproducing a high quality image by preventing over-charging of toner in the developing device. This object is achieved in a developing device having the features of the preamble of claim 1 by its characterizing features. BRIEF DESCRIPTION OF THE DRAWINGS FIG.1, FIG.2 and FIG.3 are cross-sectional views showing main portion of the developing devices using the one-component developer in the prior art; FIG.4 is a cross-sectional view showing a main portion of a developing device of a first embodiment in accordance with the present invention; FIG.5 is a perspective view showing a main portion of the developing device in the first embodiment; FIG.6 is a perspective view of a fur brush; FIG.7 is a cross-sectional view showing a detailed constitution of the main portion in the first embodiment; FIG.8 is a cross-sectional view showing the main portion of the developing device of the first embodiment; FIG.9 is a cross-sectional view showing a main portion of the developing device in a second embodiment; FIG.10 is a cross-sectional view showing a main portion of the developing device in a third embodiment; FIG.11 is a cross-sectional view on an enlarged scale of the developing device in the third embodiment; FIG.12 is a cross-sectional view showing a main portion of the developing device in a fourth embodiment; FIG.13 is a cross-sectional view showing a main portion of the developing device in a fifth embodiment; FIG.14 is a cross-sectional view showing a main portion of the developing device in a sixth embodiment; FIG.15 is a cross-sectional view showing a main portion of the developing device in a seventh embodiment; FIG.16 is a cross-sectional view showing a main portion of the developing device in an eighth embodiment; FIG.17 and FIG.18 are cross-sectional views showing operation of a thickness restriction means; FIG.19 is a cross-sectional view on an enlarged scale showing operation of the developing device in the eighth embodiment; FIG.20 is a cross-sectional view showing a main portion of the developing device in a ninth embodiment; DESCRIPTION OF THE PREFERRED EMBODIMENT FIG.4 is a cross-sectional view showing a developing device of an electrophotographic copier in a first embodiment in accordance with the present invention. Referring to FIG.4, a photoreceptor drum 22 as an electrostatic charging member is made of photoconductive material 24 such as oxide zinc, selenium or organic photoconductive material which is formed on an aluminum drum 23. The photoconductive material 24 on the whole surface of photoreceptor drum 22 is charged by a charging electrode 25. Polarity of the charging is negative in the oxide zinc, and is positive in selenium. An electrostatic latent image is formed on the photoreceptor drum 22 by projection of a light pattern 27 by an optical means (not shown). In the embodiment, a regular one-component nonconductive toner is used as a developer 28, and a magnetic toner or nonmagnetic toner is usable as the developer 28. A developer carrying means comprises a developing roller 30 and a cylindrical elastic member 31. The developing roller 30 is a metal roller made of stainless steel or aluminum, or is a roller which is coated by a plastics on the surface of the metal roller. FIG.5 is a perspective view of the developing roller 30. Referring to FIG.5, the developing roller 30 has smooth surfaces on both end portions 30a and 30b, and a rough surface having fine protrusions on the surface of the central portion 30c. The developing roller 30 as shown in FIG.4 is placed with a predetermined interval to the photoreceptor drum 22, and is rotated counterclockwise, for example. The cylindrical elastic member 33 is generally composed of a fur brush 31 and a sponge 31A, and a layer of elastic member 33 is formed on the outer surface of a metal shaft 32. In the embodiment, only the fur brush 31 is used an the cylindrical elastic member 33, which is made of electroconductive fur using rayon fiber containing carbon. The fur brush 31 is enclosed in a housing 34. The surface of the fur brush 31 is in contact with the surface of the developing roller 30, and is rotated clockwise with a rotating speed which is, in circumference, faster than that of the developing roller 30, thereby the amount of developer 28 which is supplied to the developing roller 30 increases, and a follow-up characteristic on coating is improved. Moreover, spill of the developer 28 can be reduced, and ghost of the developing roller 30 can be erased by scraping the developer 28 adhered on the surface of the developing roller 30 after the developing process. Fibers of the fur brush 31, as shown in FIG.6, are arranged in spiral shape so that the developer 28 is moved to the central portion from both end portions of the fur brush 31 by rotation thereof. A developer storing container 35 has a first supplying means 37 for supplying the developer 28 to the fur brush 31 therein. The developer storing container 35 is composed as a portion of the housing 34 shown in FIG.8, and an opening 38 (FIG.4) for supplying the developer 28 is disposed on the upper portion thereof. The first supplying means 37 comprises a sheet-shaped elastic member 40 which is fixed on a shaft 39 at an end portion. The elastic member 40 is made of a polyethyleneterephthalate sheet of 30·10⁻⁶ meters (microns) thick. The developer 28 is supplied to the fur brush 31 by rotation or shake of the shaft 39. In the embodiment, the shaft 39 is rotated clockwise. A partition plate 41 (FIG.4) is disposed between the developer storing container 35 and the fur brush 31, and is provided with an opening 42 wherethrough the developer 28 flows in or flows out between the developer storing container 35 and the fur brush 31. A scraper 43 is formed as a part of the housing 34. The scraper 43 makes the amount of the developer 28 contained in the fur brush 31 uniform by rubbing the fur brush 31, and scrapes the overcharged developer 28 off the fur brush 31. The opening 38 for supplying the developer 28 is usually closed by a lid 44. As shown in FIG.5, the developing roller 30 is held by bearings 45, and the fur brush 31 is held by bearings 46. A thickness restriction means 47 serves to restrict the thickness of the developer 28 on the developing roller 30. In the embodiment, a rubber blade made of an elastic material such as urethane rubber is used therefore. The thickness restriction means 47 can be made of other rubber material, elastic plastics such as polyethyleneterephthalate or a metal spring made of phosphorbronze or spring steel. Moreover a spring metal or plastic member coated with fluorine plastic is usable. Since the straightness of the edge of the thickness restriction means 47 sharply influences the forming of a uniform thin layer of the developer 28, the edge of the thickness restriction means 47 is precisely straight within 0.15 mm. The thickness restriction means 47, as shown in FIG.4, is arranged to contact the surface of the developing roller 30 at its edge on a position prior to facing the surface of the photoreceptor drum 22 in rotation thereof. A width of the thickness restriction means 47, as shown in FIG.5, is made to contact both the end portion 30a and 30b of the developing roller 30. Seal members 48 and 49 for preventing leak of the developer 28 made of a fabric, nonwoven fabric (felt), sponge or elastic member such as rubber are mounted on the respective end portions 30a and 30b of the developing roller 30 and the respective side faces of the thickness restriction means 47 are made in contact with the seal members 48 and 49. Scatter preventing means 50 of the developer 28, as shown in FIG. 4, is disposed under to developing roller 30 and, with respect to the direction of rotation of the developing roller, before a position where the fur brush 31 and the developing roller are in contact. The scatter preventing means 50 is composed of a U-shaped scatter preventing member 51 and a scatter preventing sheet 52 touching on the surface of the developing roller 30 at one edge and mounted on the scatter preventing member 51 at another end. The scatter preventing sheet 52 can be fixed on the housing 34 at its one end. As shown in FIG. 7, the width of the scatter preventing sheet 52 is selected to reach both the surfaces 30a and 30b of the developing roller 30. Both ends of the scatter preventing sheet 52 are pressed to the developing roller 30 by the seal members 48 and 49 set on both ends of the developing roller 30. The edge portion of the scatter preventing sheet 52 contacts the surface of the developing roller 30 at a left portion from a line connecting the respective centers of the developing roller 30 and fur brush 31. The scatter preventing sheet 52 is made of an elastic material such as polyethyleneterephthalate or urethane rubber. In the embodiment, urethane rubber, which is capable of closely contacting the developing roller 30, is used. Its preferable thickness is 50--200·10⁻⁶ meter (micron). The scatter preventing member 51 is made of metal such as stainless steel or aluminum. The scatter preventing member 51 receives the developer 28 scattered from a gap of the developing roller 30 and the scatter preventing sheet 52. In usual use, scatter of the developer 28 is perfectly prevented by the scatter preventing sheet 52, and therefore the scatter preventing member 51 having a small capacity is usable. In case that conductive material is used for the fur brush 31, over-charging of the developer 28 by friction on the developing roller 30 is prevented, and its potential is uniformed. Therefore, it is preferable that the fur brush 31 is made of a conductive material having a specific resistance of under 10¹⁰ ohm cm, preferably 10³ ohm cm--10⁷ ohm cm. The fur brush 31 can be made of other conductive fiber in place of the conductive rayon fiber in the embodiment. Using a fur brush made by "electrostatic fur setting method" is effective to uniform coating of the developer 28. Even if an electric conductive sponge, electric conductive cloth or soft wire-brush is used for the elastic member 33 of the fur brush 31, friction charging and coating of the developer are effectively accomplished. In case that the developer 28 is one-component magnetic toner, a magnetic roller for the shaft 32 is used, and the magnetic brush is formed on the outer surface thereof as the elastic member 33. Operation of the developing device in the above-mentioned embodiment is elucidated hereinafter. In the experiment of the embodiment, the photoreceptor 24 on the photoreceptor drum 22 is oxide zinc. The circular elastic member 33 is a fur brush wherein rayon fibers including carbon having about 10⁵ ohm cm in specific resistance are mounted on the shaft 32 made of aluminum in a fiber density of about 3600/cm² as an elastic member 33. Surface roughness of the developing roller 30 is 5 R maximum. A line pressure of the thickness restriction means 47 pressing to the developing roller 30 is 25 g/cm. A gap between the photoreceptor drum 22 and the developing roller 30 is 0.15 mm. Non-magnetic one-component toner of positive charging is used as the developer 28. As shown in FIG.4, the surface of the photoreceptor drum 22 is charged to -600 V by a negative corona by impressing -6 kV on the charging electrode 25 from the first high voltage DC power source 26. Subsequently, a reflected image of an original document (not shown), for example illuminated by a halogen lamp, is projected on the photoreceptor drum 22 through an optical system (not shown), and a charging potential on the surface of the photoreceptor drum 22 corresponding to the white portion of the original document is reduced to almost zero volt by the reflected light, thereby a positive electrostatic latent image is formed. On the other hand, the developer 28 in the storing container 35 is supplied in the fur brush 31 via the opening 42 by the first supplying means 37. Then, superfluous developer 28 in the fur brush 31 is scraped off by the scraper 43, and a suitable amount of the developer 28 is supplied. The developer 28 in the fur brush 31 is charged to a positive potential by friction of the scraper 43 and the housing 34. An electric field is generated between the developing roller 30 and the fur brush 31 by the charged developer 28 in the fur brush 31, and the charged developer 28 is coated on the surface of the developing roller 30. In the step, the developer 28 supplied in the fur brush 31 is further uniformly charged to positive potential by friction of the surface of the developing roller 30 which is caused by the rotation of the fur brush 31. Since the electric potential is not applied across the developing roller 30 and the fur brush 31 and the fur brush 31 is insulated from the housing 34, the developer 28 uniformly charged is coated on the developing roller 30. A thickness of the developer 28 on the surface of the developing roller 30 coated by the above-mentioned step is larger than a desired value, and there is a slight unevenness. Subsequently, the outer surface portion of the developer 28 on the developing roller 30 is scraped by the thickness restriction means 47 by rotation of the developing roller 30, and the thickness is restricted to about 40 µm (10 µm to 70 µm is desirable). Since the straightness of the edge of the thickness restriction means 47 is under 0.15 mm, coating of the developer 28 on the developing roller 30 is realized without an unevenness of the thickness or grooves. In the above-mentioned step, though the developer 28 scraped by the thickness restriction means 47 tends to fall on both end portions of the fur brush 31, the developer 28 is transferred to the central portion from both end portions of the fur brush 31 by rotation of the fur brush, which has fur alligned spirally as shown in FIG.6. As a result, the developer 28 does not gather on neither the end portion of the developing roller 30 nor the fur brush 31, and the developer 28, which is returned in the developer storing container 35, is maintained to an even level. Since the seal members 48 and 49 touching both side faces of the thickness restriction means 47 are in contact with the smooth surfaces 30a and 30b of both end portions of the developing roller 30, wear of the seal members 48 and 49 due to rotation of the developing roller 30 is very little and scattering or leak of the developer 28 are prevented. Since the surfaces of the seal members 48 and 49 which contact the thickness restriction member 47 and developing roller 30 are made smoothly, there are no gaps at those contacting portions. Therefore, the developer 28 flowing along the axis of the developing roller 30 does not leak to the outside, and thereby a high quality image which is free from photographic fog is obtainable. When the electrostatic latent image charged with negative potential on the photoreceptor drum 22 faces to the developer 28 charged with positive potential on the developing roller 30, the developer 28 flows to the photoreceptor drum 22 by static force of the electrostatic latent image on the photoreceptor drum 22, and the latent image is developed. The rest of the developer 28 on the developing roller 30 which is not used to develop is carried with the developing roller 30 downward, and pass the scatter preventing sheet 52. Though the scatter preventing sheet 52 contacts the developing roller 30, since the developer 28 adheres on the developing roller 30 by electrostatic force it does not fall in the scatter preventing member 51. The developer 28 which has passed the scatter preventing sheet 52 does not leak out again of the housing 34. Finally, the developer 28 adhering on the surface of the developing roller 30 is scraped by the fur brush 31, and the ghost is erased. As a result, high quality image is obtainable. The developer 28 scraped by the fur brush 31 is carried by the fur brush 31, and is further scraped by the scraper 43 and is returned to the developer storing container 35 through the opening portion 42. Therefore, the developer 28 on the developing roller 30 is not overcharged. When the developer 28 in the developer storing container 35 is consumed and decreases, the sheet-shaped elastic member 40 is rotated clockwise and reaches the position shown in FIG.8, and thereby the developer 28 is pushed up to the opening portion 42. The whole developer 28 in the developer storing container 35 can be supplied to the fur brush 31, even if the bottom of the developer storing container 35 is flat. Furthermore, since the developer 28 is agitated by the sheet-shaped elastic member 40 in the developer storing container 35, the level of the developer 28 in the developer storing container 35 is maintained evenly. Since the circumference speeds of the developing roller 30 is identical with that of the photoreceptor drum 22, an edge defect in developing can be prevented. In the developing device of the embodiment, since the remains of the developer which is not used in developing is returned in the developer storing container 35 through the opening 42, over-charging of the developer is prevented. Moreover, since supplying of the developer to the developing roller 30 and scraping of the developer 28 after developing step are accomplished by one fur brush, the developing device is simple in constitution and small in size. Furthermore, since scattering and leaking of the developer are prevented, the developing device is not restricted in arrangement. In the embodiment, the present invention is applied to positive-positive dye developing in the electrophotographic copier, but can be applied to negative-positive dye developing in the laser printer, for example. FIG.9 is a cross-sectional view of a developing device of a second embodiment in accordance with the present invention, wherein elements similar to the elements of FIG.4 are identified by like numerals. The first supplying means 37 comprises at least two plates 40a and 40b on the outer surface of the shaft 39, and the shaft 39 is rotated in the developer storing container 35. The fore plate 40a with respect to the rotating direction of the shaft 39 as shown by an arrow C is stronger than the rear plate 40b in an elastic force. The end portion of the plate 40b lightly contacts to an inner wall of the developer storing container 35. On the other hand, the end portion of the plate 40a is apart from the inner wall of the developer storing container 35 with a predetermined short interval. The plate 40a is provided with an opening 40c. In the embodiment, the plate 40b is made of sheet-shaped elastic material such as polyethyleneterephthalate of 30 µm (micron) in thickness, and rubber material such as urethane or neoprene, elastic plastic film such as polystyrene or Teflon (Trade mark), elastic metal such as phosphor bronze or spring steel are usable as elastic material. Though the plate 40a is made of an aluminum solid member, a metal plate such as stainless steel, metal plate coated with fluorine plastics, hard plastic or ceramics are usable as the solid member. The shaft 39 is rotated clockwise. In the embodiment, the thickness restriction means 47 for restricting thickness of the layer of the developer is pressed to the developing roller 30, and thereby, the thickness of the developer 28 is restricted to a predetermined value without precise adjustment. The thickness restriction means 47 comprises at least two plates, and a plate 47a contacting the developing roller 30 is joined on one end of the other plate 47b. The plate 47a has a smaller elastic coefficient than the plate 47b. In the embodiment, though the plate 47a made of urethane rubber and the plate 47b made of phosphor bronze plate for spring are joined in one body, other rubber material or elastic plastic is usable for the plate 47a, and elastic material such as spring steel is usable for the plate 47b. Since the fur brush 31 is slower than the developing roller 30 in the circumference speed, the developer 28 adhered on the surface of the developing roller 30 is scraped after the developing step. A second DC high voltage power source 53 applies a voltage across the developing roller 30 and fur brush 31, thereby to adjust thickness of the developer 28 charged on the surface of the developing roller 30. A sensing means 54 detects a current of the second DC high voltage power source 53, and detects existence of the developer 28. A third voltage applying means 55 applies a voltage across the fur brush 31 and scatter preventing member 51, in order to prevent leak of the developer 28 by attraction of the fur brush 31 to the developer 28. Operation of the second embodiment is elucidated hereinafter. The developer 28 is supplied in the fur brush 31 by the first supplying means 37 through the opening 42. In the supplying step, when a lot of the developer 28 is stored in the developer storing container 35, the developer 28 is mainly transferred by the plate 40a having a high elastic force, and when the developer 28 is consumed and a little developer 28 remains, the developer 28 on the bottom and inner wall of the developer storing container 35 is transferred by the plate 40b having a lower elastic force. Thereby, the developer 28 is not damaged, and can be supplied in the fur brush 31 without remains. Moreover, since the plate 40a has the opening 40c, the developer 28 is not held between the plate 40a and 40b, a constant amount of the developer 28 is transferred. Though the edge of the plate 40b is lightly contacted on the inner wall of the developer storing container 35 and the edge of the plate 40a is arranged with a short interval to the inner wall of the developer storing container 35 in the embodiment, it is available that the plate 40b is longer than the plate 40a in the direction of the inner wall and is arranged so as to close the inner wall with a short interval. In the above-mentioned state, a bending angle of the plate 40b is varied by amount of the developer 28 in the storing portion 36. When a large amount of the developer 28 is stored in the developer storing container 35, the plate 40b is bent by a load of the developer 28, and the greater part of the developer 28 is transferred by the plate 40a. When the developer 28 decreases in the developer storing container 35, the plate 40b returns to an original shape by elastic force, and the developer 28 which is on the inner wall of the developer storing container 35 is transferred by the plate 40b. Consequently, all the developer 28 is supplied in the fur brush 31 without damage of the developer 28. A superfluous developer 28 in the fur brush 31 is scraped by the scraper 43, and a suitable amount of the developer 28 is supplied. The developer 28 is charged with positive potential by friction between the developer and the scraper 43 and housing 34. Then, the developer 28 is transferred to the position contacting to the developing roller 30 by rotation of the fur brush 31, and the charged developer 28 is coated on the developing roller 30. Nextly, a DC voltage from the second high voltage DC power source 53 is applied across the developing roller 30 and the fur brush 31, thereby to adjust thickness of the developer 28 on the surface of the developing roller 30. In the above-mentioned step, the thickness of the developer 28 is adjustable by change of the voltage from ±30 V to ±250 V. For instance, in case that a positive voltage of the second high voltage DC power source 53 is applied to the fur brush 31 and a negative voltage thereof is applied to the developing roller 30, in transferring the developer 28 from the fur brush 31 to the developing roller 30, such developer which is not charged or is charged in reverse polarity is hardly transferred. Consequently, only the developer 28 charged in normal polarity is transferred and contributes to developing, and thereby a fine image is obtainable. In the above-mentioned developing process, since a suitable amount of the developer 28 exists in the fur brush 31, a contact resistance between the fur brush 31 and the developing roller 30 is sufficiently high, and hence a current of the detecting means 54 is a small. When the developer 28 in the developer storing container 35 and the fur brush 31 is consumed by repeated operation, the contact resistance decreases, whereby the current of the detecting means 54 increases. As a result the existence of the developer 28 in the developer storing container 35 can be detected by the current value of the detecting means 54. Since the developer 28 is agitated by the plates 40a and 40b in the developer storing container 35, the developer 28 in the developer storing container 35 is evened. The thickness of the developer 28 on the surface of the developing roller 30 which is adjusted by a voltage applied from the second high voltage DC power source 53 is thicker than a prefered value, and hence the thickness is slightly uneven. Since the developer 28 in the fur brush 31 is flown to the developing roller 30 by electric potential of the second high voltage DC power source 53, a rise time for forming a predetermined thickness of the developer 28 is within one second. The developer 28 on the surface of the developing roller 30 passes the thickness restriction means 47 by rotation of the developing roller 30, and hence it is further charged to positive potential by the thickness restriction means 47. A part of the developer 28 on the developing roller 30 is scraped by the thickness restriction means 47 and is restricted within a predetermined thickness. Namely, since the plate 47a of the thickness restriction means 47 touching to the developing roller 30 is made of urethane rubber layer having a small elastic coefficient, the plate 47a evenly contacts the developer 28 on the developing roller 30. An angle ϑ between the surface of the developing roller 30 and the plate 47a is preferably as large as 30°, and thereby accumulation of the developer 28 between the developing roller 30 and the thickness restriction means 47 is reduced and the thickness of the developer 28 supplied on the developing roller 30 can be restricted to a predetermined thickness. As a result, high quality image having no unevenness in density is obtainable. Subsequently, the electrostatic latent image on the photoreceptor 22 charged with negative potential is faced to the developer 28 on the developing roller 30 which is charged with positive potential, and hence the developer 28 flies to the photoreceptor 22 by the electrostatic force of the electrostatic latent image on the photoreceptor 22, and the latent image is developed. After developing, the developer 28 adhering on the surface of the developing roller 30 is scraped by the fur brush 31, and hence the ghost of the developing roller 30 is erased, and thereby a high quality image having no ghost is obtainable. The developer 28 scraped by the fur brush 31 is transferred by the fur brush 31, then scraped by the scraper 43 and is returned to the developer storing container 35 through the opening 42. Therefore, the developer 28 on the developing roller 30 is not overcharged. FIG.10 is a cross-sectional view showing a third embodiment in accordance with the present invention. Referring to FIG.10, elements similar to the elements of FIG.9 are identified by like numerals, and the description for FIG.9 is applied similarly. The plate 40a of the first supplying means 37 has an opening 40c. The shaft 39 is rotated counterclockwise. The thickness restriction means 47 for restricting thickness of the developer layer, as shown in FIG.11, is composed of the plates 47a and 47b. Then the plate 47a is adjusted so as to part from the developing roller 30 with a small distance L (L is preferably from 0.05 to 0.5 mm) at a corner 62. The plate 47a has a predetermined thickness so that the plate 47b parts from the surface of the developing roller 30. The circumference speed of fur brush 31 is selected equal to the developing roller 30. An AC voltage superimposed on a positive DC voltage is applied across the developing roller 30 and the fur brush 31 by the first voltage applying means 53, thereby to adjust thickness of the developer 28 charged on the surface of the developing roller 30. Operation of the third embodiment is elucidated hereinafter. In the third embodiment, a thick layer of the developer 28 of a predetermined thickness can be coated on the developing roller 30 by pulsating the current of the first voltage applying means 53 wherein AC voltage is superimposed on the DC voltage. The developer 28 adhering on the surface of the developing roller 30 after the developing step is vibratingly moved between the fur brush 31 and the developing roller 30 by effect of the AC voltage, and thereby the ghost on the developing roller 30 is erased, and condensation of the developer 28 is prevented, moreover high adhering force of the developer 28 on the developing roller 30 can be reduced. As a result, the high quality image having no ghost is obtainable. As shown in FIG.11, the superfluous developer 28 on the developing roller 30 is scraped by a corner 62 of the plate 47a, the thickness of the developer 28 is evened to a predetermined thickness corresponding to a distance L between the surface of the developing roller 30 and the corner 62. Subsequently, the developer 28 on the developing roller 30 is scraped by the contact surface of the plate 47a of the thickness restriction means 47, and the thickness of the developer 28 is restricted to the predetermined value. The developer 28 scraped by the corner 62 of the plate 47a is carried upward along a vertical wall of the corner 62 as shown by an arrow F and falls on the fur brush 31. In the embodiment as shown in FIG.10, the shaft 39 of the first supplying means 37 is rotated counterclockwise, thereby the developer 28 in the developer storing container 35 is gathered in the right portion of the developer storing container 35 by the plate 40a and 40b. As a result, the developer 28 does not exist in the vicinity of the opening 42. The developer 28 fallen on the fur brush 31 is scraped by the scraper 43 and is returned to the developer storing container 35 through the opening 42. The developer 28 returned in the developer storing container 35 is transferred rightward of FIG.10 by the plate 40b through the opening 40c of the plate 40a. Consequently, overcharging of the developer 28 is prevented and scraping effect by the scraper 43 is improved. FIG.12 is a cross-sectional view showing a fourth embodiment, where elements similar to the elements of FIG.9 are identified by like numerals. Furthermore, the first voltage applying means 53 and the first supplying means 37 are identical with that of the third embodiment. A voltage which is higher than a predetermined voltage is applied across the developing roller 30 and the fur brush 31 for an initial predetermined time period by the first voltage applying means 53. Then after the initial predetermined time period, the voltage is reduced to the predetermined value. The thickness restriction means 47 is made of a solid member of stainless steel, stainless steel coated by fluorine plastic, solid plastic, ceramics or the like, and is pressed to the developing roller 30 by an elastic force of a spring 56. Operation of the fourth embodiment is elucidated hereinafter. In the embodiment, the amount of the developer 28 which is supplied from the fur brush 31 is increased by increasing the voltage which is applied for the initial predetermined time period, and thereby reduction of the image intensity in the initial stage of developing and thereafter is prevented. Furthermore, the thickness restriction means 47 is made of a solid member of metal, and thereby a smoother surface than that of rubber is obtainable. When the thickness restriction means 47 is contacted on the surface of the developing roller 30, an aven thin layer of the developer 28 which has no stripe can be formed. In the embodiment, an AC voltage is superimposed on the DC voltage in the first voltage applying means 53, and thereby the developer 28 adhered on the surface of the developing roller 30 is vibrated between the fur brush 31 and the developing roller 30 by the AC voltage after the developing step. As a result, ghost of the developing roller 30 is erased. Moreover, solidification of the developer 28 is prevented, and a high adhering force of the developer 28 to the developing roller 30 is prevented, thereby a high quality image having no ghost is obtainable. FIG.13 is a cross-sectional view showing a fifth embodiment, where elements similar to the elements of FIG.4 are identified by like numerals. In the embodiment, a voltage of the first voltage supplying means 53 is progressively increased on an initial stage of operation so that the charged developer 28 on the fur brush 31 is constantly supplied to the developing roller 30. The thickness restriction means 47 consists of a sealed pouch 47d confining a fluid 47c such as air therein. The pouch 47d is made of urethane sheet and is fixed on a lower surface of the end portion of the plate 47e made of urethane rubber. A gas such as nitrogen, a liquid such as water, low molecule organic liquid such as ethylene glycol, polymer diverging liquid or high polymer liquid are usable as replacements for the air confined therein. The pouch 47d can be made of a plastic sheet such as polyethyleneterephthalate. Teflon, vinyl chloride resin or other rubber sheet is usable. The plate 47e can be made of an elastic material such as spring steel, plastic or rubber, or a solid material such as aluminum, steel or plastic. Operation of the fifth embodiment is elucidated hereinafter. Referring to FIG.13, a voltage, which is applied across the developing roller 30 and fur brush 31 by the first voltage applying means 53, is progressively increased from a voltage which is lower than a predetermined voltage to the predetermined voltage. As a result, when the voltage is applied and thereafter, the charged developer 28 on the fur brush 31 can be constantly supplied on the developing roller 30, and is coated evenly on the surface of the developing roller 30. The contacting part of the thickness restriction means 47 contacting the developing roller 29 is the pouch 47d closing a fluid therein, the pouch 47d evenly contacting the developer 28 on the developing roller 30. Furthermore, a contact pressure between the surface of the developer 28 and the pouch 47d can be held in a constant value. Consequently, high quality and even image is obtainable. FIG.14 is a cross-sectional view showing a sixth embodiment. The first supplying means 37 is identical with that of the second embodiment. Other elements similar to the elements of FIG.5 are identified by corresponding numerals in the embodiment. A cylindrical elastic member 31 is made of electroconductive sponge, which is mounted around a shaft 32 of aluminium. In the above-mentioned constitution, the elastic member 31 has a function of charging by friction and supplying the developer on the developing roller as shown in the first embodiment. The elastic member 31 is disposed in parallel with the developing roller 30 with a predetermined interval (0.1--0.5 mm is recommended). According to the experiment by the inventors, the developer 28 charged by the voltage of the first voltage applying means 53 can be transferred to the developing roller 30 in spite of the predetermined interval. A bias voltage applying means 57 applies a DC bias voltage across the photoreceptor drum 22 and the developing roller 30. Thickness restriction means 47 comprises the pouch 47d made of urethane sheet closing air therein which is fixed on one end of a solid plate 47e made of stainless steel. The plate 47e is pressed to the developing roller 30 by a sheet-shaped spring 58 made of an elastic material such as spring steel, plastic or rubber. The pouch 47d contacts the surface of the developing roller 30 between both contact portions of the developing roller 30 against the photoreceptor 22 and the elastic member 31 and at a position prior to the contact portion against the photoreceptor 22 in rotation of the developing roller 30. The plate 47e is mounted at a position of the housing 34 wherein the pouch 47d tends to part from the surface of the developing roller 30 by friction against the surface of the developing roller 30 in rotation of the developing roller 30. Moreover, the pouch 47d has a predetermined height so that the plate 47e maintains a predetermined interval to the surface of the developing roller 30. Operation of the sixth embodiment is elucidated hereinafter. The pouch 47d contacts the surface of the developing roller 30 in parallel with the axis of the developing roller 30, since the plate 47e of the thickness restriction means 47 is made of a metal solid member. Moreover, since an interval is held between the surface of the developing roller 30 and the plate 47e, the developer 28 scraped by the pouch 47d is moved upward along the vertical wall of the pouch 47d as shown by an arrow F in FIG.11 and falls on the cylindrical elastic member 30, and thereby the developer 28 is evenly supplied on the developing roller 30. When the developer 28 on the developing roller 30 faces to the electrostatic latent image of the photoreceptor drum 22, the developer 28 flies to the electrostatic latent image through a combined effect of the electrostatic force of the electrostatic latent image and an electric field of the DC bias voltage which is applied across the photoreceptor drum 22 and the developing roller 30 by the bias voltage applying means 57. An AC voltage or a DC voltage superimposed on an AC voltage is usable in the embodiment. FIG.15 is a cross-sectional view showing a seventh embodiment. Elements similar to the elements of FIG.6 are identified by corresponding numerals. The thickness restriction means 47 comprises an elastic plate 47f. Materials of the plate 47f are identical with that of the first embodiment. The plate 47f contacts the surface of the developing roller 30 at a position between both the contact portions of the developing roller 30 against the photoreceptor 22 and the elastic member 31 and prior to a facing to the photoreceptor 22 in rotation of the developing roller 30. The thickness restriction means 47 is mounted on the housing 34 at a position receiving a force so as to press it to the surface of the developing roller 30 by friction to the developing roller 30 in rotation of the developing roller 30. Operation of the seventh embodiment is elucidated hereinafter. Referring to FIG.15, since the plate 47f tends to approach the surface of the developing roller 30, accumulation of the developer 28 inbetween the developing roller 30 and the plate 47f is prevented. Accordingly, solidification of the developer 28 is effectively prevented. Since the thickness restriction means 47 is apart from the surface of the developing roller 30, the developer 28 charged on the developing roller 30 does not fly to the thickness restriction means 47, and the developer 28 is evenly coated on the developing roller 30. FIG.16, FIG.17, FIG.18 and FIG.19 show an eighth embodiment. Referring to FIG.16, elements similar to the elements of FIG.9 are identified by corresponding numerals. In the embodiment, as shown in FIG.5, both end portions 30a and 30b of the developing roller 30 have smooth surfaces, and the surface of the central portion 30c has a rough surface. FIG.17 and FIG.18 are cross-sectional views of the developing roller 30 on an enlarged scale. Referring to FIG.17 and FIG.18, when an average diameter of the developer 28 is designated by D, an average thickness of the developer 28 is designated by h and an average depth of hollows of the surface is designated by H, and the average depth H is selected as shown by the following relation: 0 ≦ h - D ≦ H . The depth H is the distance from bottom to surface of a hollow on the rough surface of the developing roller 30. Referring to FIG.16, a plate 59 for sputtering the developer 28 is composed on the housing 34 in one body. The plate 59 is disposed at a position touching to the fur brush 31 on a preceding position of the contact portion of the developing roller 30 and the fur brush 31 in rotation of the fur brush 31, thereby the top portion of the fur brush 31 is bent by the plate 59 as shown in FIG.19. Operation of the eighth embodiment is elucidated hereinafter. Referring to FIG.16, when the rotating fur brush 31 contacts the plate 59, the fur brush 31 is bent. When the fur brush 31 passes the plate 59, the fur brush 31 bent by the plate 59 returns to an original shape, and scatters the developer 28 to the developing roller 30 as shown in FIG.19. As a result, solidification of the developer 28 is prevented, and hence a rise-up time for forming a predetermined thickness of the developer 28 is reduced. Moreover, leak of the developer 28 is prevented by the plate 59. A thickness of the developer 28 on the developing roller 30 is restricted to a sum of the depth of the hollow and the diameter of the developer 28 adhering directly on the surface of the developing roller 30. Therefore, an image having fine lines is reproduced by the developer 28 adhered directly on the developing roller 30. Moreover, an image having even density is obtainable by holding a constant amount of the developer 28 in the hollows. FIG.20 is a cross-sectional view showing a ninth embodiment. Referring to FIG.20, where elements similar to the elements of FIG.8 are identified by corresponding numerals, a DC voltage is applied across the developing roller 30 and the plate 59 in order to scatter the developer 28 to the developing roller 30 by a second voltage applying means 60. In the embodiment, though the second voltage applying means is a DC power supply, a power supply wherein AC voltage is superimposed on the DC voltage is usable. Moreover, the plate 59 is formed as a part of the U-shaped leak preventing member 51. A common contact 61 of a magnet relay is switched to a contact S1 when the developing device is in operation, and is switched to a contact S2 when the developing device is not in operation. When the magnet relay 61 switched to the contact S2, a voltage having a reverse polarity is applied across the developing roller 30 and the fur brush 31. In the embodiment, since the voltage is applied across the developing roller 30 and the plate 59 by the second voltage applying means 60, the amount of the developer 28 which is scattered by the fur brush 31 is controlled. Moreover, when the developer 28 is transferred from the fur brush 31 to the developing roller 30 by an electric field between them, the developer 28 which is not charged or is charged in reverse polarity is hardly transferred, only the developer 28 charged normally is transferred and contributes to the development. As a result, a high quality image is obtainable. When the developing device is out of operation, a voltage having reverse polarity is applied across the developing roller 30 and the fur brush 31 by switching the contact 61, and thereby the surplus developer 28 on the developing roller 30 is attracted to the fur brush 31 by the electric field, and the ghost on the developing roller is erased. Furthermore, since the developer 28 does not remain on the developing roller 30, the developer does not undergo influence of humidity conditions or the like. The developing device in accordance with the present invention is not restricted within only the foregoing nine embodiments as mentioned above, and other constitutions by combination of the respective means such as charging of the developer, coating on the developing roller and restriction of the thickness of the developer are available. The present invention is suitable to a non-magnetic one component developer, and is advantageous in developing of a color image which is piled up with a plurality of the developers on the charge holding member, since the developing roller does not contact the charge holding member. As mentioned above, the present invention has a simple constitution wherein a circulating means for operating a flow-in or flow-out of the developer between the developer holding means and the first developer supplying means, thereby over-charging of the developer is prevented and a high quality image is obtainable.

Claims: A developing device for developing an electrostatic latent image on a light-sensitive member (22) with one-component developer (28), including a rotating endless-type developer carrying means (30) for carrying developer to said light-sensitive member (22); a developer storing means (35) for storing developer (28) therein; a first developer supplying means (37) installed in said developer storing means (35) for supplying developer to a second supplying means (31) which includes a cylindrical elastic member (33) for supplying developer from said developer storing means (35) to said developer carrying means (30); and developer returning means (41, 43) for returning developer from said developer carrying means (30) to said developer storing means (35), characterized in that the developer returning means includes a partition (41) having an opening (42) being disposed between said cylindrical elastic member (33) and said first developer supplying means (37); and a scraper (43) disposed adjacent said opening (42) for scraping off developer from said cylindrical elastic member (33). A developing device in accordance with claim 1, wherein said first supplying means (37) comprises at least one plate (40) which rotates in the developer storing means (35), in a manner that an end portion of said plate being in contact with an inner wall of said developer storing means (35) and moves along said partition (41) to supply developer into said opening (42) of said partition (41). A developing device in accordance with claim 1 or 2, wherein said developer carrying means (30) has a rough surface defined by a plurality of hollows, the average depth H of the hollows of the rough surface determined by 0≦h-D<H, where h is the average thickness of the developer layer and where D is the average diameter of the developer particles. A developing device in accordance with one of the claims 1 to 3, further comprising: first voltage applying means (53) for applying a DC voltage, or a DC voltage superimposed to an AC voltage, across said developer carrying means (30) and said second supplying means (31) which controls the supply of the developer. A developing device in accordance with one of the claims 1 to 4, further comprising: spattering means (59) for supplying developer of said cylindrical elastic member (33) to said developer carrying means (30), said spattering means disposed to contact a surface portion of said moving cylindrical elastic member to effect in use spattering prior to that surface portion contacting said developer carrying means. A developing device in accordance with claim 5, wherein a voltage is applied across said spattering means (59) and developer carrying means (30). A developing device in accordance with claim 5 or 6, wherein said developer carrying means (30) contacts said cylindrical elastic member (33) and the contacting part of said developer carrying means (30) moves, in use, in the same direction as said contacting part of said cylindrical elastic member and at a slower speed than that of said cylindrical elastic member. A developing device in accordance with one of the claims 1 to 7, further comprising: thickness restriction means (47) having a straight edge contacting, in use, the developer on the surface of said developer carrying means (30) at a position prior to facing the surface of said light-sensitive member (22), said thickness restriction means adjusting, in use, the thickness of the developer on said developer carrying means. A developing device in accordance with claim 8, wherein said thickness restriction means (47) comprises a contact member (47a) contacting said developer carrying means and a holding member (47b) holding said contact member in contact with said developer carrying means (30). A developing device in accordance with claim 9, wherein said contact member (47a) is made of an elastic member. A developing device in accordance with claim 9 or 10, wherein a leading edge of said contact member (47a) with respect to the rotation of said developer carrying means (30) is spaced from said developer carrying means to define a predetermined gap relative to said developer carrying means. A developing device in accordance with one of the claims 1 to 11, further comprising: scatter preventing means (50) for preventing the scattering of developer, said scatter preventing means installed under said developer carrying means (30) at a position in advance with respect to the rotation of said developer carrying means (30) of the nip defined between said developer carrying means and said cylindrical elastic member (33). A developing device in accordance with claim 12, wherein said scatter preventing means (50) comprises a U-shaped scatter preventing member (51) and a scatter preventing sheet (52) which is fixed on said scatter preventing member at one end thereof and which touches the surface of said developer carrying means (30) at another end to receive the developer from the surface of said developer carrying means (30). A developing device in accordance with claim 13, wherein said scatter preventing sheet (52) is a rubber sheet having a thickness from 50 microns to 200 microns. A developing device in accordance with claim 13 or 14, further comprising: sealing members (48, 49) for preventing the scattering of developer disposed along both end surfaces of said developer carrying means (30) and pressing against said developer carrying means through said scatter preventing sheet (52). A developing device in accordance with claim 15, wherein said sealing members (48, 49) are fabricated forms of an elastic material selected from rubber, sponge, cloth, and nonwoven fabric.

Inventors: KATAYAMA YOSHIHIRO, ONO TAIZO, SAITOH TOSHI, KATAYAMA, YOSHIHIRO, ONO, TAIZO, SAITOH, TOSHI
Applicants: MATSUSHITA ELECTRIC IND CO LTD, MATSUSHITA ELECTRIC INDUSTRIAL CO., LTD.
Application year: 1987
Main classification: G03G15
IPC codes: G03G15