Recording apparatus

Abstract: A recording apparatus comprising an image carrying body (1), a device for forming an electrostatic latent image on the image carrying body (1), a developing and cleaning device (4) for supplying a developing agent to the electrostatic latent image to reverse-develop the latent image and for removing developing agent remaining on the image carrying body (1), and a contact-type transfer device (5) for pressing a sheetlike material against the developing agent image on the image carrying body (1), to transfer the developing agent image to the sheetlike material. The developing agent on the elastic developing member (10) is a one-component developing agent charged to the same polarity as the electrostatic latent image.

Description: The present invention relates to an image forming apparatus as defined in the preamble of claim 1 for developing an electrostatic latent image formed on an image carrying body, such as a photo-receptor, and recording the developed image on a transfer material such as paper. Conventional recording apparatuses of this type include electrophotographic devices, electrostatic printers, etc. In the case of conventional apparatuses, an electrostatic latent image is formed on a photo-receptor, and a developing agent is then made to adhere electrostatically to the latent image, as a result of which a developing agent image is formed. Subsequently, the developing agent image is recorded by being transferred to paper. After image transfer, the electrostatic latent image and untransferred particles of the developing agent remain on the photoreceptor, the residual developing agent being removed by means of a cleaning device, and the latent image then removed by means of a de-electrifying device. In recent times, there has been increasing demand for such recording apparatuses in more compact form. In this connection, a method is disclosed in Published Unexamined Japanese Patent Publication No. 47-11538, for example, whereby a recording apparatus is reduced in size through making use of a device which serves as both a developing device and a cleaning device. According to this method, an electrostatic latent image is developed as a photoreceptor drum makes a first passage through the developing device, and a residual image remaining after transfer is cleaned off as the drum makes a second passage therethrough. However, because the cleaning step is effected by means of the photoreceptor drum making a second passage through the developing device, the recording speed is halved, and the recording area cannot be greater than the area of the whole peripheral surface of the drum. To obtain a greater recording area, therefore, the photoreceptor drum must inevitably be made relatively large in size, so that the apparatus cannot be satisfactorily reduced in size. Disclosed in U.S. Pat. No. 3,649,262, on the other hand, is a method in which reduction of the recording speed is prevented by using a developing device which can remove the residual developing agent as it develops an electrostatic latent image. According to this method, however, charging of the photoreceptor drum, formation of the electrostatic latent image, and developing are performed with the residual image left on the drum after the transfer process. In the charging process, therefore, the latent image and developing agent image remaining on the photoreceptor drum are unexpectedly charged, and next image exposure is effected. Accordingly, uniform charging and satisfactory formation of the electrostatic latent image cannot be ensured, and the residual image in the preceding process develops superposed on a so-called ghost image. Thus, the resulting image is not clear. Such a phenomenon is liable to present itself particularly when the solid area of the image (in which the developing agent image spreads over a wide area) overlaps the residual image in the preceding process. Moreover, it sometimes is the case that a residual developing agent image, as well as the residual electrostatic latent image itself, remains as a residual image on account of insufficient cleaning, and sometimes may be transferred to the paper. Thus, the conventional recording apparatuses cannot produce distinct images, and never permit reduction in size. From DE-A-3 706 873 is known an image forming apparatus comprising a cleanerless developing system as defined in the preamble of claim 1. This known device comprises a developing unit with a developing roller which simultaneously performs the developing function as well as the toner removal and which is positioned so that a gap distance is present between the developing roller and an image carrying body. This known structure is designed so as to avoid disadvantages of previous cleaning methods in which a cleaning device is held in contact with the image carrying body thus causing abrasion of drum surface and deterioration of picture quality and lifetime. EP-A-0 313 363 describes the structure of a transfer roller as a prior development made by the applicant of this invention but not a combination of this transfer roller with a cleanerless developing system. An object of the present invention is to provide an image forming apparatus of reduced size and capable of producing clear and distinct images. According to the invention there is provided an image forming apparatus comprising: means for charging a rotatable image carrying body, means for forming a latent image on the image carrying body charged by the charging means, developing and cleaning means for developing the latent image with a one component developing agent and for removing the developing agent remaining on the image carrying body herefrom while the latent image is developed, means for transferring the developed image on the image carrying body to a recording medium, and means provided between the transfer means and the charging means for disordering the one-component developing agent remaining on the image carrying body after transfer of the developed image by the transfer means so as to render the developing agent unreadable or non-patterned, characterized in that said developing and cleaning means includes an integrated developing and cleaning roller having an elastic layer and a conductive layer, formed of a mixture of polyurethane resin and 10 to 30% by weight of conductive carbon, for carrying the one-component developing agent to an exposed portion of the image carrying body and for simultaneously removing residual developing agent from the same exposed portion of the image carrying body, and means for rotating the developing and cleaning roller so that the developing and cleaning roller is in slidable contact with the image carrying body with a predetermined contact width, and means for applying a linear load in a range of 20 to 150 g/cm to said developing and cleaning roller so as to produce a frictional force between the one-component developing agent and the residual developing agent thereby to enhance cleaning of the residual developing agent from the image carrying body, and said transfer means is transferring the developed image on the image carrying body to the recording medium by bringing the recording medium into pressure-contact with the image carrying body and includes an elastic and conductive transfer member formed of a mixture of polyurethane resin and 10 to 30% by weight of conductive carbon. This invention can be more fully understood from the following detailed description when taken in conjunction with the accompanying drawings, in which: Fig. 1 is a sectional view of a recording apparatus according to an embodiment of the present invention; Fig. 2 is a sectional view of a developing roller or transfer roller included in the recording apparatus shown in Fig. 1; Fig. 3 is a diagram showing the range wherein satisfactory transfer efficiency is obtained by use of a transfer roller; Fig. 4 is a diagram showing the range wherein satisfactory transfer efficiency is obtained by use of a conventional transfer charger operating on the principle of corona discharge; Fig. 5 is a graph showing the relationship between the voltage applied to the transfer roller and the efficiency of transfer of a toner image formed on a photo-receptor drum. A preferred embodiment of the present invention will now be described in detail with reference to the accompanying drawings. Fig. 1 is a sectional view of a recording apparatus according to the preferred embodiment of the invention. As can be seen from this figure, a photoreceptor drum 1 is disposed substantially in the center of a housing H of the recording apparatus and rotatable in the direction indicated by arrow A. The photoreceptor drum 1 is formed of a photoconductive material, such as an organic photoconductor (OPC), and is surrounded by a charger 2, a laser device 3, a developing and cleaning device 4, a transfer roller 5, a discharge lamp 6, and a disordering device 7. The charger 2, which is situated above the photo-receptor drum 1, charges the surface of the drum 1 substantially uniformly to -500 to 800 V. The laser device 3 applies a laser beam 8 to the surface of the photoreceptor drum 1, in accordance with the image to be recorded, and thus forms the desired electrostatic image. The developing and cleaning device 4 is provided with a hopper 9 containing a so-called one-component developing agent T capable of being friction charged. A developing roller 10 is disposed in the hopper 9, and transports the developing agent T to the position where it faces the photoreceptor drum 1, and after image transfer, returns developing agent T remaining on the surface of the drum 1 to the hopper 9. As shown in Fig. 2, the developing roller 10 is composed of a metal shaft 10a, an elastic layer 10b surrounding the shaft 10a, and a conductive surface layer 10c formed on the surface of the layer 10b. The developing roller has elasticity as a whole. The elastic layer 10b is formed of polyurethane foam, for example, while the material constituting the conductive surface layer 10c is selected from among materials (listed later) suitable for friction charging the developing agent T and having the required elasticity and friction characteristics. The conductive surface layer 10c may be formed by applying, for example, a mixture of polyurethane resin and 10 to 30% by weight of conductive carbon to the elastic layer 10b. The electric resistance of the layer 10c can range from 10⁵ to 10¹⁰ Ω · cm. The developing roller 10 is in pressure-contact with an elastic blade 13 which serves to form the developing agent T as a thin layer on the surface of the roller 10. The blade 13 may be formed of phosphor bronze, polyurethane resin, or silicone resin. The developing agent T passing through the blade 13 is charged negatively or to the same polarity as the photoreceptor drum 1, thus forming one or two developing agent layers. The developing roller 10 is connected with a bias power source 14, and is connected electrically with the surface layer 10c. By virtue of this arrangement, a predetermined developing bias can be applied to the roller 10 at the time of development and cleaning. A sponge-like developing agent transportation roller 15, which is disposed in the hopper 9, serves to prevent cohesion of the developing agent T in the hopper and to transport the developing agent. The transfer roller 5, which is situated substantially directly beneath the photoreceptor drum 1, faces the peripheral surface of the drum 1 across a paper transportation path 16. The roller 5 has the same construction as the developing roller 10 as shown in Fig. 2, and the electric resistance of its conductive surface layer 10c can range from 10⁵ to 10¹⁰ Ω · cm. A conducting part, made of a mixture of silicone resin and 30 to 40% by weight of conductive carbon, is formed at each end portion of the transfer roller 5. A transfer voltage is applied to the conductive surface layer 10c through the conducting part. The transfer roller 5 applies a voltage of 800 to 1,800 V to the back surface of a transfer paper delivered thereto, causing a toner to be electrostatically attracted to the front surface of the paper, and a toner image to be transferred from the photoreceptor drum 1 to the paper. This contact-type transfer means ensures reliable image transfer even in conditions of high humidity, so that the residual developing agent can be decreased to reduce the cleaning load. Also, paper dust from the transfer paper can be removed and prevented from getting mixed with the developing agent. The disordering device 7 is provided with a conductive brush 18 whose tip end or the vicinity thereof is in sliding contact with the photoreceptor drum 1 as the drum rotates, and applies a voltage of 0 to 400 V thereto. When the voltage is applied thus, developing agent remaining on the drum 1 after image transfer is disordered so as to be rendered unreadable or nonpatterned, the residual electrostatic latent image remaining on the drum 1 also being de-electrified and erased by means of the brush 18. Since the negative charge of the photoreceptor drum 1 is previously erased by means of the discharge lamp 6, the discharge by means of the brush 18 has a primary role to erase the positive charge. Since the disordering device 7 is located above the photoreceptor drum 1, the developing agent T adhering to the brush 18 can be prevented from dropping and being scattered within the apparatus. Thus, even if the developing agent T drops onto the photoreceptor drum 1, it can be transported on the drum 1 to be recovered directly by means of the developing device 4. The photoreceptor drum 1 is underlain by a paper supplying unit 19 containing paper sheets 17 which are fed onto the paper transportation path 16 by means of a paper supplying roller 20 disposed above the paper supplying unit 19. The transportation path 16 is provided with a fixing device 21 for fixing the transferred toner image to each paper sheet 17. The following is a description of the operation of the recording apparatus described above. The photoreceptor drum 1 is rotated in the direction of arrow A, and the peripheral surface of the drum 1 is charged to about -500 to -800 V by means of the charger 2. Subsequently, the laser beam 8 from the laser device 3 is applied to the charged region, thereby forming an electrostatic latent image on the surface of the photoreceptor drum 1. Then, the latent image is transported to a cleaning position where it faces the developing and cleaning device 4. The developing agent (toner) T is delivered from the developing roller 10 in the developing and cleaning device 4, and is caused to adhere to the electrostatic latent image on the surface of the photoreceptor drum 1. At this time, the developing roller 10 is pressed against the drum 1, so that the developing roller 10 undergoes an elastic deformation. As a result, the roller 10 comes into contact with the drum 1 with a predetermined nip width, whereby the toner T is caused to adhere to the latent image, thereby forming a toner image. In this case, the toner T adheres to that portion of the photoreceptor drum 1 exposed to the laser beam 8, thus subjecting the latent image to the so-called reverse development. The average particle size of the toner T used may range from 8 to 15 µm. The toner T is charged to about -5 to -30 µc/g by friction between the blade 13 and the surface layer 10c of the developing roller 10, and a voltage of about -200 to -450 V is applied to the roller 10. The developed toner image is then transported to a transfer region where it faces the transfer roller 5. Meanwhile, as the paper supplying roller 20 rotates, the paper sheet 17 is fed from the paper supplying unit 19 in synchronism with the rotation of the photoreceptor drum 1. When the paper sheet 17 comes into contact with the transfer roller 5, its back surface is positively charged. Accordingly, the toner image on the surface of the photoreceptor drum 1 is electrostatically attracted and transferred to the sheet 17. In this case, a voltage of 1,000 to 2,000 V from a DC power source 23 is applied to the transfer roller 5 via its rotating shaft. This voltage is applied to the conductive surface layer 10c having a resistance of 10⁵ to 10⁹ Ω · cm through the conducting part. To facilitate the cleaning or the removal of the adhering toner, paper dust, or other foreign matter, the surface of the transfer roller 5 should preferably be formed of a material which enjoys smoothness and low friction characteristic. In this embodiment, conductive fluoropolymer or conductive polyester is used as the material of the conductive surface layer 10c, whose surface can be easily cleaned by means of a cleaning blade 22. The rubber hardness of the whole transfer roller 5 preferably ranges from 25 to 50, as measured according a method provided by the Japanese Industrial Standards. With use of such a soft material, the allowance for the force of pressure of the transfer roller 5 on the photoreceptor drum 1 is good enough for a satisfactory result. In Fig. 3, the hatched region represents the range of satisfactory transfer efficiency (operating environment) obtained with use of the transfer roller 5. Likewise, the hatched region of Fig. 4 represents the range of satisfactory transfer efficiency (operating environment) obtained with use of a conventional transfer charger based on corona discharge. As seen from Figs. 3 and 4, a transfer efficiency of 85% or more can be obtained at the relative humidity of 30 to 85% when the transfer roller 5 (Fig. 3) is used, while it can be obtained at the relative humidity of 30 to 50% according to the method using the transfer charger. Thus, according to the conventional method, even if allowable level of a transfer efficiency is lowered to 60%, the relative humidity only spreads to 30 to 70%, indicating that the maximum tolerance of the relative humidity can be increased to 70% at the most. For the recording apparatus having no cleaning device, the above circumstances indicate that the post-transfer residual toner increases at high humidity, thus constituting the most significant cause of defective cleaning of the conventional cleanerless recording apparatus. Using the contact-type transfer system employing the elastic conductive transfer roller 5, the recording apparatus according to the aforementioned embodiment can efficiently render non-patterned the residual toner after the transfer throughout a wide range of environment. Since the transfer roller 5 is directly in contact with the paper sheet 17 during the transfer, moreover, paper dust sticking to the sheet 17 can be efficiently removed by attraction by means of the roller 5. Accordingly, very little extraneous matter remains on the photoreceptor drum 1 after the transfer. Since the paper sheet 17 is pressed by the transfer roller 5, transfer errors (partial omission) can be prevented, and a distinct transfer image can be obtained without being affected by the size or quality of the sheet 17. Fig. 5 is a graph showing test results on the relationship between the voltage applied to conducting part of the transfer roller 5 and the transfer efficiency of the toner image on the photoreceptor drum 1. The transfer efficiency is obtained by thoroughly removing the toner image from the drum 1 before and after the transfer by means of adhesive tape and weighing the removed image. A series of tests indicated that slight memory images, produced by the residual toner, start to develop when the transfer efficiency is not higher than 70%, and that defective images involving conspicuous memory images are produced when the transfer efficiency is not higher than 60%. According to the present invention, in contrast with this, a high transfer efficiency (about 85%) can be maintained in a wide range, as shown in Fig. 3, if a voltage of, for example, 1,800 V is applied to the transfer roller 5. Thus, there may be provided a high-reliability cleanerless system which produces no memory images. After the transfer, the paper sheet 17 is delivered to the fixing device 21, whereupon the toner is fused and fixed to the sheet 17. Then, the sheet 17 is discharged. After the transfer process, a residual toner image or a positive or negative residual electrostatic latent image barely remains on the surface of the photoreceptor drum 1. Although not essentially required, the negative latent image is first erased of its negative latent image by means of the discharge lamp 6. Next, the residual toner reaches the location of the disordering device 7 as the drum 1 rotates, whereupon it is rendered nonpatterned by the device 7. In the disordering device 7, the brush 18 is brought into contact with the photoreceptor drum 1, and the residual electrostatic latent image and toner image are disordered by means of mechanical and electrostatic forces, thereby creating an unreadable state. In this case, the friction charging polarity of the material of the brush 18 is made identical with that of the toner, so that a repulsive force is produced in the toner. Thus, the residual toner on the photo-receptor drum 1 cannot be attracted to the brush 18, that is, the toner is prevented from accumulating on the brush. If a conductive material is used for the brush 18, a repulsive force is produced between the brush 18 and the toner by applying a potential of the same polarity as the toner or ground potential to the brush. Thus, the toner is prevented from accumulating on the brush 18. The residual toner on the photoreceptor drum 1 is disordered in this manner. The disordered residual toner scatters on the surface of the drum 1 and lightly adheres to the surface without being seized by the brush 18. The disordering device 7 does not serve as a cleaning device, but has an only auxiliary function for cleaning. The toner particles scattered on the surface of the photoreceptor drum 1 are distributed in too small dots to form characters or an image. The disordered region of the photoreceptor drum 1 is then transported to a charging position where it faces the charger 2, and is charged by corona discharge. After the charging, the drum 1 is exposed by means of the laser device 3 to form an electrostatic latent image thereon, which then reaches again the developing and cleaning position where it faces the developing and cleaning device 4. In the electrostatic latent image, the residual toner is spread uniformly and thin enough both in an exposed portion, to which the toner is expected to adhere, and in a non-exposed portion, so that there is no possibility of irregular exposure. Thus, the residual potential after exposure is uniform, so that a uniform toner image can be obtained even in a second cycle of development. As described above, the developing roller 10 has a hardness of 30 to 70 (based on the JIS rubber hardness measurement method) and a low resistance of 10² to 10⁸ Ω · cm. If a linear load of 20 to 150 g/cm is applied to the developing roller 10, and if the roller 10 is brought into sliding contact with the photo-receptor drum 1 at a peripheral speed 1.5 to 4 times as high as the peripheral speed of the drum 1, a contact width (nip width) of 1 to 4 mm is formed. When the residual toner and the toner T on the developing roller 10 are in sliding contact at the nip portion, a great frictional force is produced between them, whereby the cleaning capacity can be increased. If the developing agent is formed of the toner T only, reduction of image quality, such as streaks, cannot be caused. In the non-exposed portion, moreover, the force of attraction by the developing bias is greater than that of the photoreceptor drum 1, so that the toner T adhering to the drum 1 is attracted to the developing and cleaning device 4 and recovered. Thus, new toner particles from the developing roller 10 are caused to adhere to the exposed portion by supplying the roller 10 with the developing bias of a proper value intermediate between the residual potential of the exposed portion and the potential of the non-exposed portion. At the same time, the residual toner on the non-exposed portion is attracted to the developing roller 10 and recovered. In this case, the residual toner is in a small quantity, and is scattered in small dots by the disordering device 7, so that it can be efficiently recovered by means of the disordering and charging device 2. Thus, one toner image can be obtained by repeatedly rotating the photo-receptor drum 1. After the developing and cleaning, the toner image is transferred to the paper sheet 17 at the position where it faces the transfer roller 5. Thereafter, the same processes of operation are repeated. According to the recording apparatus of the embodiment described above, the ghost image, which has conventionally been produced, can be eliminated, and defective cleaning can be prevented, despite the use of the photoreceptor drum 1 with a short diameter. When 20,000 copies were taken using an image area of about 7% and size-A4 paper sheets, they all were able to enjoy satisfactory images without entailing defective cleaning. If a bias voltage is applied to the disordering device 7, moreover, it can be adjusted so that the toner is effectively disordered, and the recovery of the toner is positively prevented. Thus, the disordering device 7 can be prevented from being contaminated. In this case, the toner adhering to the disordering device 7 can be forced out onto the surface of the photoreceptor drum 1 by applying a voltage of, for example, about 100 to 300 V to the brush 18 during non-printing operation, that is, while the non-image region is passing the device 7. The discharged toner is transported to the developing and cleaning device 4 to be recovered thereby. In this case, moreover, a voltage of 0 to 100 V is applied to the developing roller 10 without operating the charger 2 in order to prevent the photoreceptor drum 1 from being charged. The toner can be prevented from accumulating in the disordering device 7 by an alternative method. According to this method, a charging region for attracting the toner adhering to the device 7 is formed in the non-image region on the photoreceptor drum 1. In this case, the drum 1 is charged to the polarity opposite to that of the electrostatic latent image by means of the transfer roller 5. This can be easily done in a reverse development system. Although the transfer roller 5 is used as the contact-type transfer means in the device according to the embodiment shown in Fig. 1, it may be replaced by a transfer belt. In the above embodiment, moreover, the conductive elastic roller is used as the transfer roller 5. In the device according to the embodiment described above, furthermore, the nonmagnetic one-component developing system is used as an example which best facilitates the reduction in size. However, the present invention is not limited to this embodiment, and the magnetic one-component brush method may be also employed. Further, a roller-shaped rotating brush of the same material as the brush 18 of the disordering device 7 may alternatively be used as the transfer roller. In this case, the rotating brush is shaped like the transfer roller 5, and the same applied voltage and force of pressure are used. The toner adhering to the rotating brush can be removed by means of a rotating electrode to which is applied a voltage of the polarity opposite to that of the voltage applied to the brush. As described above, the recording apparatus according to the present invention employs the contact-type transfer means, so that the transfer efficiency can be improved and stabilized. Further, the reverse development is used, and the image carrying body and the developing agent are charged to the same polarity, so that the adhesion of the developing agent to the image carrying body can be weakened, and the cleaning efficiency for the residual developing agent can be improved. Furthermore, the apparatus is provided with the disordering means which disorders the developing agent remaining on the image carrying body, thereby rendering the developing agent nonpatterned. Thus, irregular exposure and production of the ghost image can be prevented, so that a distinct image can be obtained.

Claims: An image forming apparatus comprising: means (2) for charging a rotatable image carrying body (1); means for forming a latent image on the image carrying body (1) charged by the charging means (2); developing and cleaning means (4) for developing the latent image with a one component developing agent and for removing the developing agent remaining on the image carrying body (1) therefrom while the latent image is developed; means (5) for transferring the developed image on the image carrying body (1) to a recording medium; and means (7) provided between the transfer means (5) and the charging means (2) for disordering the one-component developing agent remaining on the image carrying body (1) after transfer of the developed image by the transfer means (5) so as to render the developing agent unreadable or non-patterned; characterized in that: said developing and cleaning means (4) includes; an integrated developing and cleaning roller (10) having an elastic layer (10b) and a conductive layer (10c), formed of a mixture of polyurethane resin and 10 to 30% by weight of conductive carbon, for carrying the one-component developing agent to an exposed portion of the image carrying body (1) and for simultaneously removing residual developing agent from the same exposed portion of the image carrying body (1), and means for rotating the developing and cleaning roller so that the developing and cleaning roller (10) is in slidable contact with the image carrying body (1) with a predetermined contact width, and means for applying a linear load in a range of 20 to 150 g/cm to said developing and cleaning roller so as to produce a frictional force between the one-component developing agent and the residual developing agent thereby to enhance cleaning of the residual developing agent from the image carrying body (1); and said transfer means (5) is transferring the developed image on the image carrying body (1) to the recording medium by bringing the recording medium into pressure-contact with the image carrying body (1) and includes an elastic and conductive transfer member formed of a mixture of polyurethane resin and 10 to 30% by weight of conductive carbon. The apparatus according to claim 1, characterized in that said elastic developing roller (10) comprises a metal shaft (10a), the elastic layer (10b) surrounding the metal shaft (10a), and the conductive surface layer (10c) formed on the surface of the elastic layer (10b). The apparatus according to claim 1, characterized in that said conductive surface layer (10c) has an electric resistance ranging from 10² to 10⁹ Ω · cm. The apparatus according to claim 1, characterized in that said developing and cleaning means (4) includes an elastic blade (13) in sliding contact with the elastic developing roller (10). The apparatus according to claim 1, characterized in that said development effected by said developing and cleaning means (4) has reverse development. The apparatus according to claim 1, characterized in that said developing agent on said elastic developing roller (10) is charged to the same polarity as the electrostatic latent image. The apparatus according to claim 1, characterized in that said transfer roller (5) includes a conductive surface layer having an electric resistance ranging from 10⁵ to 10¹⁰ Ω · cm. The apparatus according to claim 1, characterized in that said transfer roller (5) is provided, at each end portion thereof, with a conducting part formed of a mixture of silicone resin and 30 to 40% by weight of conductive carbon.

Inventors: KOHYAMA MITSUAKI C O INTELLECT, KOHYAMA, MITSUAKI, C/O INTELLECTUAL PROPERTY DIV.
Applicants: TOSHIBA KK, KABUSHIKI KAISHA TOSHIBA
Application year: 1990
Main classification: G03G15
IPC codes: G03G15, G03G21, G03G13