DEVELOPING UNIT FOR USE IN ELECTROPHOTOGRAPHIC PRINTING APPARATUS

Abstract: A developing unit is provided for developing an electrostatic latent image formed on an image forming member, by electrostat­ ically adsorbing powder developer onto the latent image. The developing unit comprises a screw (9) provided with paddles or grooves each having a helix angle with respect to an axis of the screw, the screw transferring and supplying the powder developer stored in a developer reservoir to the developing roller; a blade (6) for regulating a layer thickness of the powder developer to be transferred to the image forming member; and a flow regulating plate (7″) for biasing a flow of the powder developer, which has been removed by the blade (6) and returning to the developer reservoir due to the gravity action, into direction opposite to a direction in which the powder developer is transferred by the screw (9). The developing unit further comprises a positioning structure for positioning the developing roller.

Description: The present invention relates to a developing unit suitable for use in recording apparatus such as copying machines and electrophotographic printers. An electrophotographic recording apparatus generally comprises, as its principal components, a photosensitive drum having an image support or forming member made by, for example, a photosensitive layer of amorphous selenium formed on a metallic cylindrical surface; an electrification or charging unit for uniformly charging the surface of the photosensitive drum by corona discharge, etc., to photosensitize the photosensitive drum; an exposing unit to form an electrostatic latent image on the surface of the photosensitive drum thus photosensitized by irradiating a laser beam modulated by information to be recorded; a developing unit for developing the electrostatic latent image, which has been formed by the exposing unit, with toner; a transferring unit for transferring the toner image formed by the developing unit to paper; and a fixing unit for fixing the toner image, which has been transferred to the paper by the transferring unit, through heat melting, etc. In the developing unit having the above arrangement, the toner, which has an electric charge of opposite polarity to that of the electrostatic latent image formed on the photosensitive drum, is attracted by electrostatic attraction to the latent image. Generally a powdered toner is used, and the powdered toner is mixed with an iron powder, etc., called the "carrier", to provide the powdered toner with the electric charge. This mixture is known as a "two-component developer" or simply a "developer". When the toner and carrier are mixed in the developing unit, frictional charging occurs which provides the electric charge for the developer. Generally, a mixing ratio of the toner (toner concentration) is expressed by weight percentage, and is, for example, 4%. Since the toner concentration is gradually decreased during use, the toner concentration must be monitored and toner supplied when the toner concentration reaches a predetermined limit. In an electrophotographic recording apparatus which allows the use of several kinds of papers having different widths, the toner concentration in the developing unit becomes nonuniform in a paper width direction if papers having a width narrower than the maximum recordable width are used for a long time, and subsequently, if a paper having a width wider than the width of those papers is used, a difference in the recording density will occur in the direction of the paper width of the wider paper. Therefore, particularly in a recording apparatus which uses papers of different widths, it would be desirable to solve the problem of nonuniformity of toner concentration in a paper width direction. A laser printer is a particular example of a recording apparatus where the problem of nonuniformity of toner is especially acute. A laser printer comprises, as its principal components, an image forming member (photosensitive drum) having a photosensitive layer of amorphous selenium formed on a metallic cylindrical surface; a charging unit for uniformly charging the surface of the image forming member by corona discharge, etc., to provide photosensitivity thereto; an exposing unit to form an electrostatic latent image on the surface of the image forming member given the photosensitivity by irradiating a laser beam modulated by information to be recorded; a developing unit for developing the electrostatic latent image formed by toner; a transferring unit for transferring the toner image formed by the developing unit to a recording paper; and a fixing unit for fixing the toner image which has been transferred to the recording paper by the transferring unit. In the developing unit, the toner given an electric charge of an opposite polarity to that of the electrostatic latent image formed on the image forming member is attracted electrostatically to the latent image to obtain the toner image. The toner is used in the form of a mixture (called the "two-component developer") in which the toner is mixed with a fine magnetic substance called the "carrier", or it is used by itself (called the "one-component developer") with magnetism to be provided. To supply the developer to the image forming member, a developing roller is provided for the developing unit. In order to solve the nonuniformity of toner concentration, the applicant of the present invention hitherto considered the following units: Figure 3 of the accompanying drawings shows a partly sectioned perspective view of a first unit considered by the present applicant. The numeral 1 represents a casing; 2 a developer reservoir provided at the bottom of the casing 1; 3 a paddle roller (stirring roller) provided with fins (paddles) extending parallel in an axial direction and rotating in a direction indicated by an arrow mark A to scoop developer contained in the developer reservoir 2 in a circumferential direction; 4 a developing roller rotating in a direction indicated by an arrow mark B to transfer the developer scooped by the paddle roller 3 to a photosensitive drum 5 which is an image forming member; 6 a blade for regulating a layer thickness of the developer to be transferred to the photosensitive drum 5 by the developing roller 4; and 7 a guide plate for separating the developer removed by the regulation of the blade 6 in a direction indicated by an arrow mark C and for returning the developer to the developer reservoir 2. The guide plate 7 is provided with a plurality of fins 7a each being inclined with respect to an axis of the developing roller 4. When the developer removed by the regulation of the blade 6 is returned to the developer reservoir 2 by the guide plate 7, the returning developer is diffused in an axial direction of the developing roller 4 so that the concentration of the developer may be uniformalized. Figure 4 of the accompanying drawings shows an explanatory view of an essential portion of a second unit considered by the present applicant. In the figure, a guide plate 7' is similar to the guide plate of the first example (Fig 3) in their shapes, but the inclination of each fin 7a (only its setting position is indicated by an alternate long and short dash line) is larger than that of the fin of the first example. Accordingly, the developer returning to the developer reservoir 2 is biased further in a direction indicated by an arrow mark D. In addition, a drop hole 7b is provided at an end of the guide plate 7'. The numeral 8 represents a screw conveyer for transferring the developer falling from the drop hole 7b in a direction indicated by an arrow mark E, the screw conveyer comprising a screw 8a and a sleeve 8b. In the second example, the developer which has been removed by the regulation of the blade 6 (Fig. 3) and returning to the developer reservoir 2 is biased in an axial direction of the developing roller 4 by the fins 7a and also is dropped from the drop hole 7b to a biasing end and fed back to the other end, thereby preventing the developer from accumulating at the biasing end, and at the same time, uniformalizing the concentration of developer. The first unit may be effective in eliminating the local nonuniformity of developer in an axial direction of the developing roller 4, but it is not sufficient to eliminate the nonuniformity of developer concentration caused by the use of papers having different widths. The second unit has problems in that the deterioration of developer is hastened, thereby shortening its service life because the screw conveyor 8 provides a stress to the developer, and that the manufacturing cost is unavoidably increased because there are many parts. Therefore, it is desirable to provide a low-cost developing unit which can eliminate greatly the nonuniformity of the concentration in an axial direction of the developing roller without hastening the deterioration of developer. EP-A-0125497 discloses an electrostatic copying apparatus. This apparatus comprises a powder developer regulating unit, for use in apparatus for developing an electrostatic latent image formed on an image support member by employing a developing roller to transfer developer powder onto the latent image, the regulating unit comprising: a stirring roller provided with a plurality of paddles each extending substantially the full length of the roller along a helical path around its axis, which roller is rotatable about its axis to transfer developer powder stored in a developer reservoir of the unit, when it is in use, towards and axially along the surface of the said developing roller; a powder blocking element arranged so as to regulate the thickness of a layer of said developer powder that is brought into contact with said image support member by said developing roller when the unit is in use; and a powder guidance device arranged to impose on a stream of the developer powder, which has been removed from the developing roller by the action of said blocking element and which is returning to said developer reservoir under the influence of gravity, a component of motion in a direction opposite to that in which the developer powder is transferred axially along the surface of the developing roller by said stirring roller. The provision of a stirring roller and powder guidance member serve to some extent to redistribute developer powder to provide a more uniform application to the developer roller. However, this regulating unit has been found to be capable of improvement. It is an object of the invention to improve the capability of such a regulating unit to provide a uniform distribution of developer powder, regardless of the printing paper used. A first aspect of the present invention is characterised in that the paddles of the said plurality are eight in number, one axial end of each paddle being displaced about the axis of the stirring roller by an angle in the range from 120° to 240° in relation to the other axial end of that paddle, that angle being such that the rate of axial transference of developer powder by the said stirring roller from the said developer reservoir, when the unit is in use, is substantially equal to the rate of axial transference of developer powder to the said developer reservoir. According to a second aspect of the present invention there is provided a developing unit comprising a regulating unit embodying the first aspect of the present invention and a developing roller rotatably mounted adjacent the stirring roller. Furthermore, according to a third aspect of the present invention there is provided a printing apparatus of common design and comprising a developing unit embodying the second aspect of the present invention. Reference will now be made, by way of example, to the accompanying drawings, in which: Fig. 1 is a view showing the principle of the present invention; Fig. 2 is a perspective view partly cut away showing an embodiment of the present invention; Fig. 3 is perspective view partly cut away showing a first example of a unit previously considered; Fig. 4 is an explanatory view showing a second example of a unit previously considered; Fig. 5 is a perspective view showing a stirring roller embodying the present invention; Fig. 6 is a graph for determining an optimum number of paddles of the stirring roller, Fig. 7 is a graph for determining an optimum angle of each fin of a powder guidance member; Fig. 8 shows a view for use in explaining the principle of a positioning structure for use in an embodiment of the present invention; Figs. 9(a) and 9(b) are views showing a positioning structure in an embodiment of the present invention; Fig. 10 is a view showing the positioning structure of Figs. 9(a) and 9(b) in more detail; Fig. 11 is a view taken along the arrow marks I-I shown in Fig. 10; Fig. 12 is a view taken along the arrow marks II-II shown in Fig. 10; Fig. 13 is a side view showing the constitution of a portion of an embodiment of the present invention; Fig. 14 is a perspective view showing another portion of the embodiment of Fig. 13; Fig. 15 is a view showing the constitution of yet another portion of the embodiment of Fig. 13; Fig. 16 shows a view for use in explaining the principle behind the portion of Fig. 15; Fig. 17 is a further explanatory view; Fig. 18 is a side view of the portion shown in Fig. 13; Fig. 19 is a view taken along the line G-G in Fig. 18; Fig. 20 shows a view of printing apparatus embodying the present invention; Fig. 21 is a view showing a portion of an embodiment of the present invention; and Fig. 22 is a graph showing the effect of an embodiment of the present invention. Figure 1 is a view showing the principle of the present invention. In the figure, the numeral 9 represents a stirring roller or screw provided with paddles or grooves each having a helix angle with respect to an axis of the screw, the screw transferring developer powder stored in a developer reservoir 2 towards (arrow mark A) and axially along (arrow mark E) the surface of the developing roller 4 to supply the powder developer for the developing roller 4; 7'' a powder guidance member or flow regulating plate for biasing a stream (arrow mark C) of the powder developer, which has been removed by the regulation of a blade 6 and which is moving due to the action of gravity, into a direction (arrow mark D) opposite to the direction (arrow mark E) in which the powder developer is transferred by the screw 9. The flow regulating plate 7'' is provided with a plurality of biasing fins each of which is inclined with respect to an axis of the screw 9 as will be described later. Namely, the developer stored in the developer reservoir 2 is transferred in the direction of arrow mark A to be supplied to the developing roller 4, and the developer biased in the direction of arrow mark D by the flow regulating plate 7'' is fed back in the direction of arrow mark E. Figure 2 is a perspective view partly broken showing an embodiment according to the present invention. A screw 9 is provided with eight paddles each having a helix angle in the range of 120° to 240° with respect to an axis of the screw 9. The screw 9 rotates in a direction indicated by arrow mark A to transfer powder developer stored in a developer reservoir 2 in a direction indicated by arrow mark E and in a direction indicated by arrow mark A toward the circumference of a developing roller 4 to supply the powder developer to the developing roller 4. A flow regulating plate 7'' is provided with a plurality of fins 7a each being inclined with respect to an axis of the developing roller 4 similar to the guide plate 7 shown in the first considered example. As a result, the developer which has been removed by the regulation of a blade 6 and returning to the developer reservoir 2 is biased in a direction indicated by arrow mark D. As described in the above, the screw according to the described embodiment has both the functions of the paddle roller and screw conveyer of the earlier considered example, and reduces the stress applied to the developer by circulating the developer in the developer reservoir. As a result, any nonuniformity of the concentration of developer in an axial direction of the developing roller will be greatly reduced without hastening the deterioration of the developer and without increasing the number of parts. Figure 5 is a perspective view showing a screw embodying the present invention. The screw 9 is provided with eight paddles 9a each of which is twisted by 180° for the total length (about 28 cm) of the screw 9. Figure 6 is a graph showing the relationship between helix angles and transferring forces circumferentially and axially of the screw with respect to the change of the number of paddles of the screw. An ordinate axis indicates the amount transferred in a circumferential direction (the higher up the ordinate axis in Fig. 6, the larger the amount) of the toner which is scooped by the screw and supplied to the developing roller, as well as the amount (the lower down the ordinate axis, the larger the amount) of the toner which is returned in an axial direction of the screw after the toner is fed back from the developing roller. An abscissa axis indicates helix angles. In this case, each helix angle value indicates the degree of twist of each paddle with respect to the whole length of the screw (which is about 28 cm similar to the whole length of the developing roller). An area between straight lines "a" and "b" shown in the graph is the one in which the helix angle causes the amounts transferred in the circumferential and axial directions to balance. This area will be defined as follows: If a single paper having a width which is the same as the total axial length of the developing roller is printed after printing 1000 papers each having a width of a half of the total axial length of the developing roller, a difference in printed density between a portion of the single paper which has been printed by one half of the developing roller used for printing the 1000 papers, and the other portion of the single paper which has been printed by the other half of the developing roller not used for printing the 1000 papers is within 0.2 of the OD value. In the area between the lines "a" and "b", the toner is stirred uniformly for the whole length of the developing roller, and the difference in printing density is so small that it can be ignored in practical use. As apparent from the graph, an optimum constitution will be realized with eight paddles each having a helix angle of 180°. Figure 7 is a graph showing a balance between an amount of toner returned from the developing roller guided by the fins and the amount of toner transferred by the screw in an opposite direction to the biased direction, with respect to the inclination angle of each fin. An area between straight lines "c" and "d" shown in the graph is the one in which the balance between the amounts transferred by the fins and the screw become optimum, and the toner is uniformly distributed over all the developing roller. The graph has been prepared subject to the screw of eight paddles each having a helix angle of 180°. As apparent from the graph, an optimum inclination angle of each fin is 40° to 50°. A positioning structure in a developing unit embodying the second aspect of the present invention, for arranging a developing roller precisely in parallel with an image forming member (photosensitive drum), will be described hereafter. The positioning structure is to be the subject of a European patent application (not yet filed) divided out of the present application. The principle of the positioning structure will be explained with reference to Figure 8, in which the numeral 101 represents an image forming member; 102 a developing roller; 103 a first supporting member disposed at one end of a shaft of the developing roller 102; 104 a second supporting member disposed at another end of the shaft of the developing roller 102; 109 and 110 a pair of holding members (side frames) for holding the image forming member 101; 107 a first fitting member which is fixed to one (109) of the holding members to fit the first supporting member 103 thereto; and 108 a second fitting member which is fixed to the other holding member 110 to fit the second supporting member 104 thereto. In this way, the accumulation of dimensional errors is reduced by providing only two members, i.e., the holding member 109 (or 110) and the fitting member 107 (or 108) which are interposed between the image forming member 101 and the developing roller 102. The periphery of the image forming member 101 to which the developer tends to adhere is not used for positioning, thereby improving the accuracy of a gap between the image forming member 101 and the developing roller 102. Figure 9 is a view showing the construction of another embodiment, in which Fig. 9(A) is a front view and Fig. 9(B) a view taken along the arrow marks A-A. In this embodiment, the positioning structure is applied for a rotary sleeve type magnetic developing roller. In the figure, the reference numerals shown in Fig. 8 represent like parts, and, further, the numerals 111 and 112 represent roller bearings fixed to holding members 109 and 110 to support a rotary shaft of an image forming member 101; 113 a driving gear provided at one end of a shaft of the image forming member 101; 114 a guide pin provided on a stationary magnet side, the guide pin 114 being a first supporting member on one side of a shaft of a developing roller 102; 115 a roller bearing provided on a rotary sleeve side, the roller bearing 115 being a second supporting member on the other side of the shaft of the developing roller 102; 116 a first stopper fixed to the holding member 109, the first stopper 116 being a fitting member to receive the guide pin 114; 117 a second stopper fixed to the holding member 110, the second stopper 117 being a fitting member to receive the roller bearing 115 there; 118 a developing unit casing; and 119 a driving gear for driving a sleeve of the developing roller 102. In the above arrangement, the image forming member 101 is supported by a pair of the holding members 109 and 110 through the high-precision roller bearings 111 and 112. Therefore, simply by fixing the first and second stoppers 116 and 117 to a pair of the holding members 109 and 110 by precision positioning pins, etc., the developing roller 102 can be positioned with high accuracy with respect to the image forming member 101. Since the guide pin 114 and the roller bearing 115 are disposed outside the developing unit casing 118, they will not be contaminated by the developer. As described in the above, the positioning structure can maintain a desired gap between the image forming member and the developing roller with high accuracy so that the recording quality will be stable for a long time. Figure 10 is a top view showing in detail the construction of the positioning structure shown in Figs. 9(A) and 9(B). The developing unit casing 118 accommodates the developing roller 102, a stirring roller 126 comprising the above-mentioned screw, and a toner supplying roller 127 for supplying the toner from a hopper (not shown) located above the casing 118 to the casing 118. The shaft 114 of the developing roller 102 is fixed to a magnetic roller (to be described later) disposed inside the developing roller 102 and not rotatable with respect to the developing unit casing 118. Another shaft 130 of the developing roller 102 is connected to an outer sleeve of the developing roller 102 and is rotatable with respect to the casing 118 through a bearing 128. Around the shaft 130 within the casing 118, there is provided a seal 125 for preventing the toner from entering the bearing 128. The shaft 130 is received in a guide groove 117a (Fig. 11) of the stopper 117 through the bearing 115. The stopper 117 is fixed to the side frame 110 to position precisely an end portion 117b (against which the bearing 115 abuts) of the guide groove 117a at a location spaced from a shaft 129 of the photosensitive drum 101 by a predetermined distance. A driving shaft 131 passes through the stopper 117, and a gear 124 fitted to the driving shaft 131 engages with the gear 119 fixed to the shaft 130 of the developing roller to drive it in rotation. A pulley 123 is fixed to the shaft 131 outside the side frame 110 and connected to a motor 121 through a proper means such as a belt and a gear. The numeral 122 represents a gear box. The motor 121 also drives a pulley 113 fixed to the shaft 129 of the photosensitive drum 101 through a timing belt 120, thereby driving the photosensitive drum 101. Similar to the stopper 117, the other stopper 116 is provided with a guide groove 116a having an end portion 116b against which the shaft 114 of the developing roller 102 abuts (Fig. 12). To maintain a distance "L" between the guide groove end portion 116b of the stopper 116 and the shaft 129 of the photosensitive drum 101 at a predetermined value, the stopper 116 is fixed with a high accuracy to the side frame 109. According to the above arrangement, the developing unit is inserted into a printing apparatus in a direction indicated by an arrow mark P, and the shafts 114 and 130 of the developing roller are received in the guide grooves 116a and 117a of the stoppers 116 and 117 respectively to reach the end portions 116b and 117b respectively. In this state, the developing unit is held by a proper means to maintain the condition that a gap G (Fig. 12) between the surface of photosensitive drum 101 and the surface of developing roller 102 will be maintained precisely as it is. A developing unit structure in which developer stored in a developer casing can be easily replaced will be described hereunder. The developing unit structure is to be the subject of a further European patent application (not yet filed) divided out of the present application. Figure 13 is a vertical cross-sectional view showing a developing unit 216 embodying the second aspect of the present invention. A toner cartridge 202 containing fresh toner is disposed above a developer unit casing 201. Under the toner cartridge 202, there is provided a toner supplying roller 203 for supplying the toner to the developer casing 210. A stirring roller 204 (the screw which has been described before) and a developing roller 205 are disposed in the developer unit casing 201. The stirring roller 204 rotates in a direction indicated by an arrow mark C to supply the toner (not shown) to the peripheral surface of developing roller 205. The peripheral surface of developing roller 205 comprises a rotary sleeve (to be described later) which rotates in a direction indicated by an arrow mark B to supply the toner onto a photosensitive drum (not shown) through an opening 208. The numeral 206 represents a blade for regulating the thickness of toner on the developing roller 205; and 207 a fin for biasing the toner, which has been removed by the blade 206, in a predetermined direction and returning it toward an axis of the stirring roller 204. The developing unit 216 is removably fitted to a printer and provided with a fitting piece 215 which is fixed to a frame of the printer. A magnetic roller (to be described later) disposed inside the developing roller 205 is fixed immovably with respect to the developer casing 201. A minimum magnetic force portion of the magnetic roller is arranged to face substantially vertically downward as indicated by an arrow mark A. A cover 209 is disposed under the developing roller 205 to extend substantially for the whole length of the developing roller 205. The periphery of cover 209 is formed in a step 210 (Fig. 14) to constitute a labyrinth or mating structure which is fitted to the periphery of an opening 201a formed at the bottom of casing 201 to completely prevent the toner from leaking. The cover 209 is fixed to the casing 201 by screws 244 through fitting pieces 245. A magnetic sensor 260 for detecting the toner concentration is fitted to the cover 209. The magnetic sensor 260 is connected to a driving portion of the toner supplying roller 203. It is preferable to locate the magnetic sensor 260 at a position between the developing roller 205 and the stirring roller 204 as will be described later. Figure 15 is a view showing the construction of the developing roller 205. A magnetic roller 211 is magnetized at a plurality of predetermined positions and fixed at both its ends to fixed shafts 212a and 212b. The sleeve 213 is rotatably fitted to the fixed shafts 212a and 212b through bearings 261 and 262. The sleeve 213 surrounds the peripheral surface of magnetic roller 211. The numeral 263 represents a seal. A driving shaft 264 is fixed to the sleeve 213, and a driving gear 214 is fitted to the driving shaft 264. The shafts 212a and 264 and a gear 214 correspond to the shafts 114 and 130 and the gear 119 shown in Fig. 10 respectively. Figure 16 is a view showing an example of a magnetized state of the magnetic roller 211, and Fig. 17 a waveform diagram showing magnetic force. As shown in Fig. 16, a plurality of N and S poles (N₁ , N₂ , and S₁ to S₃) are magnetized on the magnetic roller 211 with predetermined intervals α₁ to α₅. In this case, a minimum magnetic force portion appears between the poles S₂ and S₃ as apparent from Fig. 17. The minimum magnetic force portion is caused to face substantially vertically downward as indicated by the arrow mark A (Fig. 13). As shown in Fig. 13, the cover 209 is arranged to face the minimum magnetic force portion. When the toner is to be replaced, the developing unit 216 is removed from the printer, and the cover 209 is opened to discharge the toner at the bottom of the casing. At this time, if the sleeve 213 is rotated by operating manually the driving gear 214 (Fig. 15) of the developing roller 205, the toner remaining on the sleeve will leave the sleeve at a lower position where the magnetic force is weak and be discharged from the casing. Further, if the developing roller driving gear 214 is connected to the stirring roller 204 through a proper gear means, the stirring roller 204 is rotated together with the developing roller by operating the developing roller driving gear 214. Accordingly, the toner remaining at the bottom of the casing is scooped by the stirring roller 201 to be supplied to the circumference of developing roller 205, and as mentioned above, the toner leaves the roller at the lower position where the magnetic force is weak, to be discharged from the casing. Figure 18 is a view showing the developing unit 216 fitted to a printer. The developing unit 216 is mounted on a frame 217 on the printer side. The reference mark "F" indicates an inserting direction of the developing unit, and "E" the removing direction thereof. As described above, the developing unit 216 is positioned at a predetermined location by abutting the shaft 205a of the developing roller against the stopper 220 on the printer side. A plate 218 having a hole 219 is disposed at an end portion of the frame 217. The fitting piece 215 of developing unit 216 is inserted into the hole 219 to fix the developing unit 216 to the printer. The developing unit 216 can move up and down in a direction indicated by an arrow mark D with the developing roller shaft 205a being the center of the movement. Figure 19 is a view taken along the arrow marks G-G showing the frame on the printer side shown in Fig. 18. The reference marks 217a and 217b represent side frames. The numeral 222 represents a developing unit driving gear which is connected to a motor (not shown) through a belt 221. Figure 20 is a view showing the construction of a laser printer having the developing unit 216 described above, which embodies the third aspect of the present invention. At a paper supplying portion 237, printing papers 236 are taken out one by one by a pick roller 235 and supplied to the printer. Around a photosensitive drum 231, there are successively disposed a uniform charging unit 232; a latent image forming unit 234 for forming a latent image on the photosensitive drum 231 with a laser beam 233; the developing unit 216 for developing the latent image by supplying toner thereto; a transfer charge unit 238 for transferring the toner image onto the printing paper; a separation charge unit 242; a cleaner 239; and a discharging unit 243. The numeral 240 represents a fixing unit, and 241 a stacker for stacking printed papers. Figures 21 and 22 are views showing fitting positions of the toner concentration sensor of the developing unit in an embodiment of the present invention. To obtain a constant printing quality, the toner concentration should be maintained at a constant value. To achieve this, the toner concentration sensor detects the toner concentration status, and the toner is supplied or replaced if the concentration is lowered. Figure 21 is a schematic view showing the developing unit. The arrow marks R and S indicate the flow of toner 330. The numeral 301 represents a photosensitive drum; 321 a stirring roller; 322 a developing roller; 323 a blade; and 324 a toner biasing plate. A toner concentration sensor 331 is disposed at the bottom of a casing 340 and between the developing roller 322 and the stirring roller 321. At this position, the toner from the developing roller and the toner from the stirring roller are mixed and always flow uniformly. Therefore, a highly reliable toner concentration detection will be performed at this position, because erroneous detection due to the turbulence of the flow of toner or due to the nonuniformity of concentration caused by a difference of the toner consumption at various locations of the developing roller is prevented. Figure 22 is a graph showing the relationship between the toner concentration C r and an analog output of the sensor with respect to various installation positions of the toner concentration sensor. In the graph, curves A, B, C, and D correspond to installation positions which are shown in the right upper side of the graph to have the same reference marks respectively. From the graph, the following is apparent: In the case of curve B according to the present invention, the inclination of C r to analog output is steep so that a change in the concentration can be accurately detected. In the case of curve A, the total quantity of toner at the sensor installation position is small so that the detection may be difficult to perform. In the cases of curves C and D, an influence of the paddles of stirring roller is large to vary largely the analog output so that the adjustment may be difficult to perform. Further, the inclination of C r to analog output is decreased so that a highly accurate detection will not be realized. At a position E, the flow of toner is not stable due to a blade 323 causing turbulence in the flow, so that a reliable detection will not be realized. Therefore, the most reliable concentration detection will be performed when the sensor is arranged at the position B.

Claims: A powder developer regulating unit, for use in apparatus for developing an electrostatic latent image formed on an image support member (5) by employing a developing roller (4) to transfer developer powder onto the latent image, the regulating unit comprising: a stirring roller (9) provided with a plurality of paddles (9a) each extending substantially the full length of the roller along a helical path around its axis, which roller is rotatable about its axis to transfer developer powder stored in a developer reservoir (2) of the unit, when it is in use, towards and axially along the surface of the said developing roller (4); a powder blocking element (6) arranged so as to regulate the thickness of a layer of said developer powder that is brought into contact with the said image support member by said developing roller (4) when the unit is in use; and a powder guidance device (7) arranged to impose on a stream of the developer powder, which has been removed from the developing roller by the action of said blocking element (6) and which is returning to said developer reservoir under the influence of gravity, a component of motion in a direction opposite to that in which the developer powder is transferred axially along the surface of the developing roller by said stirring roller; characterised in that the paddles (9a) of the said plurality are eight in number, one axial end of each paddle being displaced about the axis of the stirring roller (9) by an angle in the range from 120° to 240° in relation to the other axial end of that paddle, that angle being such that the rate of axial transference of developer powder by the said stirring roller (9) from the said developer reservoir (2), when the unit is in use, is substantially equal to the rate of axial transference of developer powder to the said developer reservoir (2). A regulating unit as claimed in claim 1, in which the angle of displacement is substantially 180°. A regulating unit as claimed in claim 1 or 2, wherein said powder guidance device is provided with a plurality of fins (7a) each being inclined by an angle in the range from 40 o to 50 o with respect to the axis of said stirring roller. A developing unit comprising a regulating unit as claimed in claim 1, 2 or 3, and a developing roller (102) rotatably mounted adjacent the stirring roller. A developing unit as claimed in claim 4, comprising a positioning structure for positioning the developing roller (102) relative to an image support member (101), which comprises a circumferentially displaceable surface, said positioning structure comprising: first and second supporting members (103, 104; 114,130) disposed at respective ends of the shaft of said developing roller; first and second holding members (109, 110) adapted to hold such an image support member; and first and second locating members (107, 108; 116,117) secured respectively to said holding members to locate the first and second supporting members respectively relative thereto. A developing unit as claimed in claim 4 or 5, which includes a developer casing (118) adapted to contain developer powder and within which the developing roller (102) is rotatably mounted. A developing unit as claimed in claim 6, wherein said developing roller and the developer casing are fitted removably to said holding members (109, 110), said first and second locating members being provided with grooves (116a, 117a) for guiding said first and second supporting members (114, 130) respectively such that said first and second supporting members abut against end portions of said grooves so as to be located thereby. A developing unit as claimed in claim 5, 6 or 7, wherein end portions of said first and second supporting members are located outside said developer casing. A developing unit as claimed in any one of claims 4 to 8, in which the developing roller comprises a magnet (211) immovably fixed relative to the developer casing, and a rotatable sleeve (213) surrounding the magnet, the magnet having been magnetised with variable magnetisation about its periphery, and having a portion of minimum magnetic force, said minimum magnetic force portion being arranged, when the developing unit is located in an apparatus for developing an electrostatic latent image, to face substantially downwardly, and wherein the developer casing comprises a cover (209) arranged to face said minimum magnetic force portion. A developing unit as claimed in claim 9, wherein a peripheral portion of said cover is stepped (210) so as to mate with a correspondingly stepped portion of said developer casing. A developing unit as claimed in claim 6, or any one of claims 7 to 10 when appended to claim 6, wherein a toner concentration sensor for controlling the toner concentration of the developer powder is arranged at a lower region of the developer casing and between the developing roller and the stirring roller. A developing unit as claimed in claim 6, or any one of claims 7 to 11 when appended thereto, which comprises locking means for locking the unit onto the frame of a printing apparatus, the locking means comprising a fitting piece (215) secured to a front side face of the casing of the unit and adapted to fit into a corresponding aperture (219) provided in the frame in such a way as to be slidably movable in the aperture so that the unit can move up and down so as to pivot about the shaft (205a) of the developing roller disposed at the rear side of the unit. A printing apparatus comprising at least an image support member for supporting a latent image to be developed by developer powder transferred thereto; and a developing unit as claimed in any of claims 4 to 12, which unit is removably installed in the printing apparatus. A printing apparatus as claimed in claim 13, in which the developing unit is such that used developer is discarded therefrom when the unit is removed from the printing apparatus. A printing apparatus as claimed in claim 13 or 14, which further comprises a latent image forming unit (234) for forming a latent image on the image forming member; a transfer electrification unit (238) for transferring the latent image onto printing paper; a separation electrification unit (242); a cleaner (239) and a discharging unit (243). A printing apparatus as claimed in claim 15, in which the latent image forming unit comprises means for modulating a laser beam with image information. A printing apparatus comprising a photosensitive drum (231) around which are successively disposed a uniform electrification unit (232); a latent image forming unit (234) for forming a latent image on the photosensitive drum surface; a developing unit (216) for developing the latent image by absorbing toner developer transferred by a developing roller onto the latent image; a transfer electrification unit (238) for transferring the latent image onto a printing paper; a separation electrification unit (242); a cleaner (239); and a discharging unit (243), wherein the said developing unit (216) comprises a developing unit as claimed in any one of claims 4 to 12.

Inventors: HINO, TOSHIO, HOUKI, YOJI, IINO, YOSHIO, IKUTA, HIROTOSHI, NAKANISHI, AKIO, SUETANI, KAZUHISA
Applicants: FUJITSU LIMITED
Application year: 1986
Main classification: G03G15
IPC codes: G03G15