Developing unit for use in electronic printing apparatus

Abstract: A bracket (25) having a high degree of rigidity is provided. A magnet roller (26) is rotatably mounted on the bracket and a doctor blade (27) is fixed to the bracket. A bracket assembly (41) comprising the bracket, the magnet roller and the doctor blade is fixed to a frame of a developing unit. A gap g between the magnet roller (26) and the doctor blade (27) is maintained at a predetermined value even if the frame is deformed due to an external force or vibration, and thus printing at a constant density is ensured.

Description: The present invention relates to a developing unit for use in an electronic printing apparatus. More particularly the present invention relates to a developing unit which is capable of easily adjusting a gap between a doctor blade and a magnet roller and stably maintaining this gap. In an electronic photographic printer or electronic copying machine, an electro-static latent image corresponding to an image to be printed or copied is optically formed on a photo-sensitive drum. Thereafter, the electro-static latent image is developed using powdered toner, and the image produced by the toner is transferred to recording paper and fixed. Thus, a desired image is printed or copied. Fig. 1 is a schematic view illustrating the construction of such an electronic photographic printer. The electronic photographic printer comprises a photo-sensitive drum 1 having a cylindrical surface on which electro-static latent images are formed by projecting optical images; a cleaner 2 for cleaning the cylindrical surface of the photo-sensitive drum 1; a charging unit 3 for uniformly charging the cylindrical surface of the photo-sensitive drum 1 by ultraviolet-ray irradiation or the like; and a scanning unit 4 for scanning the cylindrical surface of the photo-sensitive drum 1 with a light beam RB from a laser diode (not shown) or the like. The scanning unit 4 comprises a polygon mirror 4a for causing the light beam rb to deflect in the axial direction of the photo-sensitive drum 1; a fq lens 4b for compensating for a focal distance; a mirror 4c for converting an optical path; and a cylindrical lens 4d for making the light beam rb converge on the cylindrical surface. When the light beam rb is turned on/off on the basis of image data while the photo-sensitive drum 1 is being rotated and the cylindrical surface of the photo-sensitive drum 1 is being scanned with the light beam rb, an electro-static latent image is formed on the cylindrical surface of the photo-sensitive drum 1. In Fig. 1, reference numeral 5 denotes a developing unit for developing the electro-static latent image by toner; reference numeral 6 denotes a transfer unit for transferring the image formed by toner, transferred onto the cylindrical surface of the photo-sensitive drum 1, to recording paper; reference numeral 7 denotes a discharging unit for removing charges remaining on the recording paper; reference numeral 8 denotes a fixing unit for fixing toner to recording paper; and reference numeral 9 denotes a hopper for recording paper, for example, two hoppers 9a and 9b for supplying sheets of paper of different sizes are provided; reference numeral 10 denotes a guide for feeding recording paper; reference numeral 11 denotes a stacker for recording paper; and reference numeral 12 denotes a top lid. As shown in Fig. 2, the developing unit 5 comprises a container 51 which is filled with powdered toner tn and a frame 52 for holding the container. The container 51 is generally divided into a stirring section 5a for causing frictional electrification of the particles of toner TN by stirring the toner, and a toner-separation section 5b. A sensor 5c for detecting the amount of the toner tn remaining inside the container 51 is disposed in the vicinity of the bottom thereof. The toner tn is stirred as a result of a stirring member 5a-1 being rotated. The granular toner tn is electrified by the friction between the toner tn and the stirring member 5a-1, and sent to the toner-separation section 5b. The electrified toner tn adheres by a magnetic force to a magnet roller 5b-1 in the toner-separation section 5b. in this way, the toner particles adhere to the cylindrical surface of the magnet roller 5b-1 in a layer. After the thickness of the toner particles TN, i.e., the height from the cylindrical surface, is restricted by a doctor blade 5b-2 placed in proximity to the cylindrical surface of the magnet roller 5b-1, the toner particles contact the cylindrical surface of the photo-sensitive drum 1. As a result, the toner tn moves from the magnet roller 5b-1 to the photo-sensitive drum 1. However, the amount of the toner tn moved varies in proportion to the magnitude of the difference between a bias voltage applied to the magnet roller 5b-1 and the electrical potential resulting from the electro-static latent image on the cylindrical surface of the photo-sensitive drum 1. Only those electro-static latent images the difference of which between the bias voltage and the electrical potential is larger than a threshold value are developed by the toner tn. In the conventional developing unit 5, both the magnet roller 5b-1 and the doctor blade 5b-2 are directly fixed to the frame 52. As a lighter weight of an electronic photographic printer or electronic copying machine is demanded, the frame 51 is usually formed from plastics. As a consequence, the stiffness thereof is small, and therefore distortion is likely to occur when an external force is applied thereto. When, for example, a printer is placed on a base which is not flat, the entire chassis and casing of the printer is deformed due to its own weight of the printer, say, tens of kg, causing the frame 52 of the developing unit 5 to be distorted. The gap between the magnet roller 5b-1 and the doctor blade 5b-2 is as narrow as 0.26 to 0.28 mm, and easily affected by distortion of the developing unit 5 as described above. Therefore, the depth of the layer of toner tn on the cylindrical surface of the magnet roller 5b-1 (magnetic roller) is no longer held constant. Thus, the amount of toner tn transferred to the cylindrical surface of the photo-sensitive drum 1 changes not only with the potential difference between the roller 5b-1 and the drum 1 but also with the change of the gap, and the density of the image on the recording paper is not uniform. Also, variations in the gap are sometimes caused by vibrations from outside. If this problem occurs, complex adjustments and maintenance of the printer or copying machine must be performed. According to the present invention, there is provided a developing unit for use in an electronic printing apparatus, comprising:- a frame including a container for toner particles; a magnet roller, onto which the electrified toner particles are deposited, for transporting the toner particles to a photosensitive drum; a doctor blade for restricting the depth of the toner particles deposited on the magnet roller; and a bracket for holding the magnet roller and the doctor blade, the magnet roller and the doctor blade being connected to the frame of the developing unit by means of the bracket. An embodiment of the present invention may provide a construction which is capable of maintaining the gap between a magnet roller and a doctor blade at a constant distance even when the frame of a developing unit is distorted due to an external force. An embodiment of the present invention may also provide a developing unit which is capable of printing at a uniform density without substantially increasing the weight of an electronic printing apparatus (e.g. printer or copying machine). An embodiment of the present invention may also provide a developing unit in which printing density does not change due to outside vibrations, and which is capable of easily adjusting and maintaining the printing density. Reference is made, by way of example, to the accompanying drawings, in which:- Fig. 1 is a schematic view which illustrates the construction of an electronic photographic printer embodying the present invention; Fig. 2 is a view which illustrates the construction of a conventional developing unit of an electronic photographic printer; Fig. 3 is a perspective view of an embodiment of the present invention; Fig. 4 is a cross-sectional view of a developing unit embodying the invention; Fig. 5 is a is a cross-sectional view of a bracket assembly of the developing unit of Fig. 4; Fig. 6(a) is an exploded perspective view of the bracket assembly; Fig. 6(b) is a perspective view illustrating the way in which the bracket assembly is assembled; and Fig. 7(a) and (b) are perspective views showing how the bracket assembly is mounted to the frame of the developing unit. Fig. 3 is a perspective view which illustrates the fundamental construction of a bracket assembly 41 embodying the invention. A bracket 25, produced for example by punching and bending sheet metal, is provided, and a magnet roller 26 (magnetic roller) and a doctor blade 27 are held by the bracket 25. The magnet roller 26 is rotatably supported by the bracket 25, and the doctor blade 27 is fixed to the bracket 25. In this way, the magnet roller 26 and the doctor blade 27 are mounted onto the frame (not shown) of the developing unit by means of the bracket 25. The bracket 25 has a large stiffness (i.e. high degree of rigidity) because it is made of a metallic plate, and the roller 26 and the doctor blade 27 are not directly fixed to the frame of the developing unit. Consequently, even if the developing unit is distorted because the printer casing is deformed, there is no variation in the relative position between the magnet roller 26 and the doctor blade 27. As a result, the gap g between the magnet roller 26 and the doctor blade 27 can be maintained at a constant, and the printing density is uniform and stable. Fig. 4 is a cross-sectional view which illustrates the construction of a developing unit 20 of the present invention. It comprises a container 22 filled with powdered toner tn and a frame 21 for supporting the container in the same manner as in the conventional developing unit which was explained referring to Fig. 2. A stirring member 23 for stirring the toner tn to cause frictional electrification is disposed inside the container 22. A toner sensor 24 for detecting the amount of toner tn remaining inside the container 22 is disposed in the vicinity of the bottom of the container 22. The electrified toner tn is deposited on a magnet roller 26 and is transported to a photosensitive drum (not shown). In the developing unit 20 of the present invention, the magnet roller 26 and the doctor blade 27 for restricting the height of the toner tn deposited on the cylindrical surface of the magnet roller 26 are held by the bracket 25 which is fixed to the frame 21 in the manner as explained later. Reference numeral 28 denotes screws for fixing the doctor blade 27 to the bracket 25. The bracket 25 is produced by punching and then bending a sheet plate, as described above. As a consequence, the bracket 25 has a large stiffness and therefore is not deformed even if a relatively large force is applied to it. Fig. 5 is a cross-sectional view of a bracket assembly 41 with the magnet roller 26 and the doctor blade 27 mounted on the bracket 25. The rotational shaft 26-1 of the magnet roller 26 is supported by a bearing (not shown) provided on the bracket 25. After the position of the doctor blade 27 is adjusted so that the gap g between the doctor blade 27 and the magnet roller 26 becomes a predetermined value, the doctor blade 27 is fixed to the bracket 25 by the screws 28. Fig. 6(a) is an exploded perspective view which illustrates the construction of the bracket assembly 41 in detail. Fig. 6(b) is a perspective view illustrating an assembled bracket assembly. Referring to Fig. 6(a), the bracket 25 is produced by punching a sheet plate into a predetermined form and then bending it, as described above. The bracket 25 has side plate sections 25a and 25b, a top plate section 25c and a doctor blade mounting section 25d which extends obliquely from the top plate section 25c. A circular cutout 25e for rotatably holding the magnet roller 26, a semicircular cutout 25f, fitted to a pin disposed on a development frame (both not shown), for holding the assembly 41, and a projecting portion 25j for positioning the bracket assembly 41 with respect to the frame 21 are disposed on both of the side plate sections 25a and 25b. Tapped holes 25g, 25h and 25i for fixing the doctor blade 27 are provided on the doctor blade mounting section 25d. The magnet roller 26 has a cylindrical main body portion 26a, and a shaft 26b which is fixed to the main body portion 26a and extends from both ends thereof. The doctor blade 27 has a rectangular shape, in which slits 27b, 27c and 27d which go through the plate are provided. The slits 27b, 27c and 27d are positioned in such a manner as to correspond to the tapped holes 25g, 25h and 25i. When assembling, as shown in Fig. 6(b), the shaft 26b of the magnet roller 26 is fitted to the circular cutout 25e of the bracket 25 via the bearings 29 and 30. After the magnet roller 26 is rotatably mounted on the bracket 25 in this manner, the doctor blade 27 is disposed on the doctor blade mounting section 25d, and is tentatively fixed by screws 28a, 28b and 28c. The position of the doctor blade 27 is adjusted so that the gap g between it and the magnet roller 26 becomes a predetermined value, then tightened and fixed by the screws 28a, 28b and 28c. Thus, the bracket assembly 41 is completed. Figs. 7(a) and 7(b) are an exploded perspective view and a perspective view, respectively, which illustrate an example of one method of linking the frame 21 of the developing unit 20 to the bracket assembly 41. The semicircular cutouts 25f provided in both the side plate sections 25a and 25b of the bracket 25 are fitted to a pair of pins 21a and 21b provided on the frame 21. The bracket assembly 41 is connected to the frame 21 by a screw 31 in such a way that the projecting portions 25j provided on the side plate sections 25a and 25b are brought into abutment with the frame 21. in this way, the frame 21 and the bracket assembly 41 are fixed to each other. Alternative methods may be used for linking the frame 21 to the bracket assembly 41, such as a hooking structure employing claws. According to the present invention, as has been explained above, a bracket having a large stiffness for holding a magnet roller and a doctor blade is provided, and the magnet roller and the doctor blade are fixed to a frame of a developing unit by means of the bracket. Therefore, even if a large external force is applied to the frames of an electronic photographic printer or a developing unit thereof, no change occurs in the gap between the magnet roller and the doctor blade. As a consequence, the printer can maintain a constant printing density without being affected by the installation process or environmental vibrations. Furthermore, since the gap can be adjusted at the bracket assembly stage, density adjustment and maintenance operations can be performed easily and efficiently. In addition, since the bracket assembly is manufactured and maintained as a block, improved quality consistency and a reduced cost of the printer are achieved. An increase in the weight due to the addition of a bracket is 0.5 kg or smaller, which is negligible considering the weight of the entire printer. The present invention can be applied to any type of apparatus which prints an image onto a medium using toner particles, and the term "electronic printing apparatus" is to be construed accordingly. Also, the term "magnet roller" embraces any kind of means capable of transporting electrified toner particles.

Claims: A developing unit for use in an electronic printing apparatus, comprising:- a frame (21) including a container (22) for toner particles; a magnet roller (26), onto which the electrified toner particles are deposited, for transporting the toner particles to a photosensitive drum; a doctor blade (27) for restricting the depth of the toner particles deposited on the magnet roller; and a bracket (25) for holding the magnet roller and the doctor blade, the magnet roller and the doctor blade being connected to the frame (21) of the developing unit by means of the bracket (25). A developing unit according to claim 1, wherein the bracket (25) comprises a pair of side plate portions (25a, 25b) for holding a rotational shaft (26b) of the magnet roller (26); a top plate portion (25c) for linking the pair of side plate portions; and a doctor blade mounting section (25d) to which the doctor blade (27) is fixed. A developing unit according to claim 2, wherein the bracket (25) has cutout portions (25f) provided at the lower ends of the side plate portions (25a, 25b) for detachably mounting the bracket to a pair of pins (21a, 21b) disposed on the frame (21).

Inventors: FUJIMOTO TETSUYA C O FUJITSU L, FUJIMOTO, TETSUYA, C/O FUJITSU LIMITED
Applicants: FUJITSU LTD, FUJITSU LIMITED
Application year: 1992
Main classification: G03G15
IPC codes: G03G15, G03D15