Image forming apparatus

Abstract: An image forming apparatus includes an image bearing member for bearing an electrostatic image; a developer carrying member for carrying a developer and for forming a developing zone with the image bearing member, wherein the developer is externally added with additive having a charging polarity opposite from that of the developer; voltage applying means for applying a developing voltage to the developer carrying member; voltage control means for controlling the developing voltage so as to change force for directing the additive toward a non-image portion of the image bearing member in accordance with a number of image forming operations.

Description: FIELD OF THE INVENTION AND RELATED ART The present invention relates to an image forming apparatus such as a copying machine or a printer which employs an electrostatic recording system or an electrophotographic recording system. An electrophotographic image forming apparatus such as a laser beam printer or a copying machine which employs an electrophotographic system uses developer (hereinafter, "toner") in the form of powder. Toner is held in a developer container, which is a developer holding container. It is conveyed to a developer bearing member (hereinafter, "developing sleeve") by a toner conveying means, and is borne on the development sleeve. It is given a predetermined electrical charge by a toner layer regulating member (hereinafter, "doctor blade"), and is transferred onto an image bearing member (hereinafter, "photosensitive member"), to develop an electrostatic latent image on the photosensitive member into a visual image. Thereafter, the visible image is transferred onto a piece of transfer medium such as a sheet of paper by a transferring means, and then is fixed to the transfer medium, in a fixing apparatus. The toner which remains on the photosensitive member without being transferred onto the transfer medium is stripped off from the photosensitive member by a cleaning member placed in contact with the photosensitive member, and is sent to a cleaning container, ending a single cycle of the image forming process, and a user can receive a copy with a desired image. As one of various image developing methods, a jumping developing method has been known. According to this method, a latent image on a photosensitive member is developed by positioning the toner bearing member of an image developing apparatus close to the photosensitive member, that is, without allowing contact between the two members. At this time, a conventional image developing apparatus which employs a jumping developing method will be described with reference to a typical conventional image developing apparatus depicted in Figure 12. In the developing apparatus 7 in Figure 12, negatively chargeable toner 32 contained in a developer container 3 is borne on a development sleeve 10. As the development sleeve 10 is rotated in the direction of an arrow mark b, the toner borne on the development sleeve 10 is conveyed toward an image developing station, in which the peripheral surfaces of the development sleeve 10 and the photosensitive member 1 directly face each other. On its way to the development station, the toner is regulated by a doctor blade 9 placed in contact with the development sleeve 10, being coated in a thin layer on the peripheral surface of the development sleeve 10. In the developing station, a gap of 50 - 500 µm is maintained between the peripheral surfaces of the development sleeve 10 and the photosensitive member 1, and as development bias composed of a DC current and an AC current is applied to the development sleeve 10 from a bias power source 33, the toner coated in a thin layer on the development sleeve 10 jumps over to the electrostatic latent image on the photosensitive member 1, and adheres to it, developing in reverse the latent image into a toner image, i.e., a visible image. The aforementioned development bias is applied to the development sleeve 10 not only during the period in which the photosensitive member is being actively used for image formation, but also during other periods in which the photosensitive member 1 is being idly rotated in terms of image formation; for example, the pre-rotation period in which the photosensitive member 1 is rotated prior to an actual image forming operation, the post-rotation period in which the photosensitive member 1 is rotated after the completion of an image forming operation, the period, or interval, between the preceding and following image formation cycles, and the like. In such an image developing apparatus as the one described above, there sometimes occurs the so-called "flowing image effect", i.e., a phenomenon that certain portions of a latent image formed on the photosensitive member 1 drop out due to the ozonic compounds generated on the photosensitive member 1. In order to prevent the occurrence of the "flowing image effect", it is feasible to externally add abrasive additive to developer so that the ozonic compounds are continuously shaved away from the peripheral surface of the photosensitive member 1 during image formation. Presently, however, the addition of external additive to developer has not produced desirable results. SUMMARY OF THE INVENTION The object of the present invention is to provide an image forming apparatus capable of preventing the flowing image effect caused by the adhesion of ozonic compounds to the image bearing member. This object is achieved by an image forming apparatus having the features of claim 1. Advantageous further developments are set out in the dependent claims. According to the present invention the image forming apparatus is capable of polishing clean the peripheral surface of the image bearing member, with the use of external additive externally added to developer. Furthermore the image forming apparatus is capable of controlling the ratio to toner at which external additive is supplied to the image bearing member. The object, features and advantages of the present invention will become more apparent upon a consideration of the following description of the preferred embodiments of the present invention taken in conjunction with the accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is a schematic section of the image forming apparatus in the first embodiment of the present invention, and depicts the general structure thereof. Figure 2 is a graph which shows the change in the ratio to toner at which positively chargeable external additive jumped onto the photosensitive member when the voltage level of the development bias was kept constant, in the first embodiment. Figure 3 is a graph which shows the change, in the first embodiment, in the ratio to toner at which the positively chargeable external additive jumped onto the photosensitive member when the size of the area of the development bias waveform, correspondent to the jumping of the positively charged external additive, was controlled. Figure 4 is an explanatory drawing which graphically depicts the development bias in the first embodiment. Figure 5 is a block diagram of the image forming apparatus in the first embodiment. Figure 6 is a flow chart for controlling the development bias, in terms of the size of the area of the waveform of the development bias, correspondent to the jumping of the positively charged external additive. Figure 7 is a schematic section of the image forming apparatus in the second embodiment of the present invention, and depicts the general structure thereof. Figure 8 is a graph which shows the change, in the second embodiment, in the amount of the positively charged additive which jumped onto the photosensitive member when the development bias was kept constant. Figure 9 is a block diagram of the image forming apparatus in the second embodiment of the present invention. Figure 10 is a flow chart for controlling the development bias, in terms of the size of the area of the waveform, correspondent to the jumping of the positively charged external additive, in the second embodiment. Figure 11 is a graph which shows the change, in the second embodiment, in the ratio to toner at which the positively charged external additive jumped onto the photosensitive member when the development bias was controlled, in terms of the size of the area of the waveform, correspondent to the jumping of the positively charged external additive. Figure 12 is a schematic section of a conventional image forming apparatus, and depicts the general structure thereof. DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, the image forming apparatus in accordance with the present invention will be described in detail with reference to the drawings. Embodiment 1 The first embodiment of the present invention will be described with reference to Figures 1 - 6. Figure 1 depicts the image forming apparatus in accordance with the present invention. An image forming apparatus 100 comprises a process cartridge 43, a transfer roller 13, a fixing apparatus 19, an optical system consisting of a laser scanner 4 and a mirror 6, and the like. The process cartridge integrally comprises several processing apparatuses: a photosensitive member 1, a charge roller 2, a developing apparatus 7, and a cleaning apparatus 14. The photosensitive member 1 is an image bearing member, and is constituted of an electrically conductive base member 1b, which is an aluminum cylinder, and a photoconductor photosensitive layer 1a, which is laid on the peripheral surface of the base member 1b. It is rotatively driven in the direction indicated by an arrow mark a . The peripheral surface of the rotating photosensitive member 1 is uniformly charged to the negative polarity by the charge roller 2, and then is exposed to a laser beam 5, which is projected from a laser scanner 4 and deflected by the mirror 6 disposed in the main assembly of the image forming apparatus 100. The laser beam 5 is modulated with sequential digital electric image signals sent from a video-controller (unillustrated), based on the image data. As a result, an electrostatic latent image is formed on the peripheral surface of the photosensitive member 1. The electrostatic latent image on the photosensitive member 1 is developed in reverse into a toner image, i.e., a visible image, by the toner 8 borne on the development sleeve 10 within the developing apparatus 7. The toner image is transferred onto a piece of transfer sheet P fed from a sheet feeder tray, by the function of a transfer roller 13. After receiving the toner image, the transfer sheet P is separated from the photosensitive member 1, and is introduced into a fixing apparatus 19, in which the toner image is fixed to the transfer sheet P. Thereafter, the transfer sheet P is discharged from the image forming apparatus main assembly, onto a delivery tray 23. Meanwhile, the residual toner, that is, the toner which remains on the photosensitive member 1 after the toner image transfer, is removed by a cleaning apparatus 14, and then, the next cycle of image formation begins. The charge roller 2 is constituted of a metallic core 2a, and an elastic rubber layer 2b in the form of a roller fitted around the peripheral surface of the metallic core 2a. The electrical resistance of the elastic layer is in the medium range. The charge roller 2 is rotatively supported at both longitudinal ends of the metallic core 2a by bearings, being kept always in contact with the photosensitive member 1. The charge roller 2 is rotated by the rotation of the photosensitive member 1. The metallic core 2a of the charge roller 2 is electrically connected to a charge bias application power source 17 capable of applying compound voltage composed of DC voltage and AC voltage. As charge bias is applied to the charge roller 2 through the metallic core 2a, the peripheral surface of the photosensitive member 1 is charged to a predetermined potential level. The developing apparatus 7 employs a noncontact type developing system. It has a development sleeve 10, which bears the toner 8 and conveys it to the photosensitive member 1, and a developer container 3, which stores the toner 8. The development sleeve 10 is produced by coating carbon dispersed paint on the peripheral surface of a tubular base member, and it is nonmagnetic. The tubular base is formed of aluminum, stainless steel, or the like. The peripheral surface of the development sleeve 10 displays a certain degree of roughness due to the properties of the paint coated thereon, and the roughness contributes to the toner conveyance by the development sleeve 10. The development sleeve 10 is rotatively supported by unillustrated bearings, and is rotated in the direction indicated by an arrow mark b by the photosensitive member 1 through a gear (unillustrated). The development sleeve 10 is connected to a development bias power source 12 capable of applying compound bias composed of DC bias and AC bias, to the development sleeve 10. As bias is applied to the development sleeve 10 by the development bias power source 12, the latent image on the photosensitive member 1 is visualized as a toner image. Further, the development sleeve 10 is supported so that the peripheral surface of the development sleeve 10 holds a predetermined development gap from the peripheral surface of the photosensitive member 1. The doctor blade 9 is a toner layer thickness regulating member which regulates the thickness of the layer of the toner 8 on the development sleeve 10. It gives the toner 8 a proper amount of triboelectrical charge, in cooperation with the development sleeve 10; the doctor blade 9 triboelectrically charges the toner 8 to a proper potential level, in cooperation with the development sleeve 10. As for the material for the doctor blade 9, it is possible to use elastic material such as urethane or silicon rubber, elastic metal such as phosphor bronze or stainless steel, or relatively stiff elastic resin such as polyethylene terephthalate. The doctor blade 9 is welded to a metallic plate 22 fixed to the inside of the developing apparatus 7. The toner 8 is nonmagnetic, negatively chargeable, single component toner, and is stored in the developer container 3. To the toner 8, external additive (unillustrated) is added to prevent the flowing image effect. As for the external additive, it is desirable that the external additive is in the form of positively chargeable particles, and is more likely to jump onto the print-less portions (non-image portions) of the peripheral surface of the photosensitive member (normal development) than onto the print portions (image portions), because the flowing image effect is more likely to occur on the print-less portions. Also, the addition of the external additive to the negatively chargeable toner assures that the toner 8 is triboelectrically charged to a satisfactory potential level from the beginning of the service life of the process cartridge 43, and therefore, desirable images are formed throughout the service life of the process cartridge 43. As for the positively chargeable particles, strontium titanate particles or Melamine particles, are available. In this embodiment, strontium titanate particles are employed (hereinafter, "positive external additive"). The positive external additive is added to the toner by a ratio of 1.3 percent in weight (hereinafter, "wt. %"). Within the development sleeve 10, a magnetic roller 11 is fixedly disposed. The magnetic roller 11 has four magnetic poles: S1, S2, N1 and N2. The pole S1 is positioned immediately next to the photosensitive member 1, so that the fog causing toner particles are kept adhered to the development sleeve 10 while the toner 8 is caused to jump onto the photosensitive member 1 to develop a latent image. The pole S2 is positioned across the magnetic roller 11 from the pole S1, and its function is to attach the toner 8 in the developer container 8 toward the development sleeve 10 so that the toner 8 circulates (in the direction indicated by an arrow mark E in the drawing) adjacent to the development sleeve 10, following the rotation of the development sleeve 10. This circulation of the toner 8 contributes to the triboelectrical charging of the toner 8. The poles N1 and N2 contribute to the conveyance and triboelectrical charging of the toner 8 coated on the development sleeve 10. Although a magnetic roller with four magnetic poles is employed in this embodiment, the number of the magnetic poles does not need to be limited to four; the number does not matter as long as magnetic poles capable of providing the aforementioned functions are present. Within the developer container 3 located at a position below the development sleeve 10, a toner blowout prevention sheet 18 for preventing the toner 8 from being blown out is disposed to prevent the toner from leaking from the bottom of the development sleeve 10. The service life of the process cartridge 43 in this embodiment, in terms of the cumulative number of copies, is 5,000 when the average dot ratio per page is 4 %. Below the developing apparatus 7, a data storing means 50, which employs nonvolatile memories, is located. The data storing means 50 is connected to a CPU 104 located in the main assembly of the image forming apparatus 100 through a connecting device 105. In the data storing means 50, the cumulative number of the copies, which is inputted from the CPU 105, is stored, and is increased by one each time a copy is printed. There is no restriction about the data to be stored in the data storing means as long as the cumulative usage of the process cartridge 43 can be detected by the main assembly of the image forming apparatus 100. For example, the cumulative length of time charge bias was applied to the photosensitive member 1 by the charge roller 2, the cumulative length of time the photosensitive member 1 was rotated, and the like, may be stored, which is obvious. While the process cartridge 43 is in the image forming apparatus 100, the data storing means 50 remains in connection with the CPU 104, and the cumulative number of the printed copies is continuously written into, or read from, the data storing means 50 by the CPU 104. Next, the development bias applying method in this embodiment will be described. This embodiment is characterized in that in order to properly adjust the ratio to the toner at which the positive external additive, i.e., the external additive charged to the polarity opposite to that of the developer, jumps onto the photosensitive member 1, throughout the service life of the process cartridge, that is, through the entire length of time the process cartridge 43 remains fit for practical usage, the size of the area of the waveform of the development bias applied to the development sleeve 10, correspondent to the jumping of the positive external additive onto the photosensitive member, on the print-less portions, (hereinafter, simply, "jumping side area size") is varied in response to the cumulative number of the copies printed by the process cartridge 43. The image forming apparatus 100 in this embodiment was subjected to a durability test, in which 5,000 copies were made, applying a development bias composed of AC and DC components. The AC component had a voltage of 1200 V (Vpp = 1200 V) and a frequency of 1800 Hz (Vf = 1800 Hz), and the DC component had a voltage of -400 V (Vdc = -400 V). Further, the development bias was given a rectangular waveform with a fixed duty ratio of 1:1. During this test, the ratio to the toner at which the positive external additive jumped onto the photosensitive member 1 was confirmed. The results of the test show that improvements were made regarding the problem that image density was low at the beginning of the service life, but the effects of this embodiment upon the flowing image effect did not last until the 5000th copy. Further, the results also showed that the streaky images were made at the beginning of the service life, and the images with white spots began to be made past the midpoint of the durability test. Regarding the streaky images, it was discovered that they were made because a portion of the positive external additive escaped through the cleaning point and interfered with the formation of the latent image. As for the direct cause of the images with white spots, it was discovered that they were made because some of the positive additive particles were buried into the peripheral surface of the photosensitive member 1, becoming nuclei to which the toner particles fused (so-called "image with toner fusion spots"). The change in the ratio at which the positive external additive jumped onto the photosensitive member during the aforementioned durability test is as shown in Figure 2. As is evident from Figure 2, the ratio at which the positive external additive jumped onto the photosensitive member was excessive at the beginning of the durability test, but as the test progressed, it gradually decreased, eventually becoming less than the predetermined ratio by which the positive external additive was initially added to the toner. In other words, the excessive jumping of the positive external additive at the beginning caused the failure in cleaning the photosensitive member by the positive external additive, which in turn caused images to be streaky. The excessive jumping of the positive external additive at the beginning also caused the positive external additive to be buried into the peripheral surface of the photosensitive member, which in turn caused the toner to remain adhered to the peripheral surface of photosensitive member (toner fusion). Further, as the test progressed, the ratio at which the positive external additive jumped onto the photosensitive member decreased below the predetermined ratio, becoming no longer effective against the flowing image effect, and as a result, the flowing image effect worsened. In another durability test, a development bias with a rectangular waveform, the duty ratio of which was variable, was used. In other words, the size of the area of the waveform of the development bias, correspondent to the jumping of the positive external additive, in Figure 4, was varied, and the ratio at which the positive external additive jumped onto the photosensitive member (hereinafter, "the jumping ratio of the positive external additive") was checked in relation to the size of the aforementioned waveform area. Next, referring to Figure 4, the development bias used in this test will be described in detail. Figure 4 is an explanatory drawing which depicts a development bias with a frequency of 1800 Hz applied to a development sleeve. A referential code Vdc represents the time-average voltage level of the development bias, that is, an integrated voltage level obtained by integrating the voltage level of the development bias across a single cycle of the development bias (hereinafter, simply, "integrated voltage level"). Referential codes V1 and V2 represent the highest and lowest voltage levels, that is, the peak voltages of the development bias, and referential codes T1 and T2 represent the periods through which the peak voltages V1 and V2 are applied, respectively. It is possible to control image density using this integrated voltage level. A referential code VL represents the surface potential level of the latent image print portions of the photosensitive member, and a referential code VD represents the surface potential level of the latent image print-less portions of the photosensitive member. The development bias used in this embodiment is such a development bias that has the following specifications: when T1 = T2 (duty ratio is 1:1), |V1-V2| = 1200 V, and Vdc = -400 V. The potential levels VL and VD are: VL = -150 V, and VD = -650 V. When image density greatly changes due to the controlling of the jumping side area size, the amount of light is adjusted so that the value of |Vdc-VL| remains at 250 V, and also, the development bias is adjusted to shift the entire waveform in the negative or positive side so that the value of |Vdc-VD| remains at 250 V. On the print portions of the photosensitive member, a latent image with the negative polarity is developed in reverse using the negatively charged toner. More specifically, in the period T1, an electric field works in the direction to induce the toner 8 to move from the development sleeve 10 to the photosensitive member 1 (direction to develop latent image), with a magnitude correspondent to |VL-V1|, and therefore, the toner 8 is affected by a force which works in the same direction with a magnitude proportional to |VL-V1|. On the other hand, in the period T2, an electric field works in the direction to induce the positive external additive to move from the development sleeve 10 to the photosensitive member 1, with a magnitude correspondent to |V2-VL|, and therefore, the positive external additive is affected by a force which works in the same direction with a magnitude proportional to |V2-VL| (in this period T2, force works in the direction to strip the toner away from the photosensitive member and move it to the development sleeve). On the other hand, on the print-less portions of the photosensitive member, in the period T1, an electrical field works on the toner 8 in the direction to induce the toner 8 to move from the development sleeve 10 toward the photosensitive member 1 (direction to develop latent image on photosensitive member), with a magnitude of |VD-V1|, and therefore, a force with a magnitude proportional to |VD-V1| works on the toner 8 to induce it to move in the same direction, whereas in the period T2, an electric field works on the external additive in the direction to induce the external additive to move from the development sleeve 10 toward the photosensitive member 1 (direction to strip away toner having adhered to photosensitive member), with a magnitude of |V2-VD|, and therefore, a force with a magnitude proportional to |V2-VD| works on the external additive in the same direction. Referring to Figure 4, the jumping side area size may be defined as the product of the contrast V between the surface potential level VD of the print-less portions of the photosensitive member and the highest voltage level V2 of the development bias, and the length of the period T2 through which the voltage level of the development bias is highest. Table 1 presented below shows the results of a test conducted to confirm the correlation between the jumping side area size and the ratio at which the positive external additive jumped onto the photosensitive member. Jump side area size (V.sec) Jump amount of additive (% by wt.) ≧ 0.58 ≧ 3.0 0.50 - 0.58 ≧ 2.0 0.43 - 0.50 ≧ 1.0 0.38 - 0.43 ≧ 0.5 < 0.38 < 0.5 According to Table 1, there is a desirable relationship between the jumping side area size and the ratio at which the positive external additive jumped onto the photosensitive member. As the jumping side area size was reduced, the ratio at which the positive external additive jumped onto the photosensitive member reduced, whereas as the jumping side area size was increased, the ratio at which the positive external additive jumped onto the photosensitive member increased. This implies that the ratio at which the positive external additive jumps can be controlled by controlling the jumping side area size. It was also confirmed that neither of the aforementioned two components of the jumping side area size, i.e., the contrast V and the length of the period T2, displayed a greater correlation with the jumping ratio of the positive external additive, than the other. All that was confirmed was that both the contrast V and the length of the period T2 had some correlation with the jumping ratio of the positive external additive. Therefore, the jumping side area size may be controlled by controlling either the magnitude of the contrast V or the length of the period T2, or by controlling both. Also in the test, the relationship between the ratio at which the positive external additive jumped onto the photosensitive member, and the various image defects (insufficient image density at the beginning of usage, insufficient cleaning of the positive external additive, toner fusion, flowing image effect) was confirmed using the aforementioned development bias, the duty ratio of which is variable. The results of the test are shown in Table 2 given below. In the table, a reference character o means that no image defect occurred; a referential character Δ means that defects insignificant in terms of practical usage, occurred; and a referential character x means that significant defects occurred. Jump amount of additive Initial low density Cleaning defect Fusion Flow ≧ 3.0 % by wt. o x x o ≧ 2.0 % by wt. o Δ Δ o ≧ 1.0 % by wt. o o o o ≧ 0.5 % by wt. o o o o < 0.5 % by wt. Δ o o x According to Table 2, there is a clear correlation between the ratio at which the positive external additive jumped and the various image defects. In other words, in order to prevent the occurrence of the insufficient cleaning by the positive external additive and the occurrence of the toner fusion, control should be executed so that the ratio at which the positive external additive jumps onto the photosensitive member is kept below 2.0 wt. %. In order to prevent image density from becoming too low at the beginning of the service life of the process cartridge 43, or in order to prevent the flowing image effect from occurring, the ratio at which the positive external additive jumps onto the photosensitive member should be kept above 0.5 wt. %. In other words, in order to prevent the occurrence of the above described image defects throughout the service life of the process cartridge 43, i.e., the length of time the process cartridge 43 remains fit, all that is necessary is to keep between 0.5 wt. % to 2.0 wt. %, the ratio at which the positive external additive jumps onto the photosensitive member. Therefore, it is evident, from the above table which shows the correlation among the jumping ratio of the positive external additive, the jumping side area size, and the various image defects, that in order to maintain desirable image quality, that is, to prevent the occurrence of the aforementioned various image defects, throughout the entire service life of the process cartridge 43, control should be executed so that the jumping side area size remains between 0.38 V.sec and 0.58 V.sec. In view of the change in the jumping ratio of the positive external additive in the durability test, the results of which are given in Figure 2, and in which the development bias was fixed, it is evident that the jumping ratio of the positive external additive remained above 2.0 wt. % in the period between the first and 500th copies, and image quality was improved in terms of the insufficient image density at the beginning of the service life of the process cartridge 43, but the insufficient cleaning by the positive external additive occurred. In the period from the 2500th copy to the 5000th copy, the ratio at which the positive external additive jumped onto the photosensitive member remained below 0.5 wt. %, and the flowing image effect began to occur, progressively worsening. Thus, it may be assumed that the occurrence of the flowing image effect can be prevented throughout the service life of the process cartridge 43 as long as control is executed so that, during the initial period up to the 500th copy, the ratio at which the positive external additive jumps remains above 0.5 wt. % but below 2.0 wt. % (jumping side area size being between 0.38 V.sec and 0.50 V.sec), preventing the occurrence of the insufficient cleaning and the toner fusion, while improving image quality in terms of the initial insufficient image density, whereas, during the period from the 2500th copy and thereafter, the ratio at which the positive external additive jumps remains above 0.5 wt. % (jumping side area size being above 0.38 V.sec). Therefore, in this embodiment, in order to output copies with desirable image quality throughout the service life of the process cartridge 43, such an operational sequence is employed that, based on the data stored in the data storing means 50 located in the image forming apparatus 100, the jumping side area size of the development bias is kept at 0.43 V.sec while the cumulative number of printed copies is between 0 and 500; 0.47 V.sec, from 501 to 2500; and 48 V.sec from 2501 to 5000. Next, referring to Figures 5 and 6, the method in this embodiment for controlling the jumping side area size of the development bias in response to the cumulative number of the printed copies will be described in detail. Figure 5 shows the block diagram for the control sequence in this embodiment. Referring to Figure 5, the process cartridge 43 comprises the data storing means 50 which stores the number of the printed copies, and the image forming apparatus 100 comprises a reading/writing means 182, a computing means 183, the development bias power source 12, and the CPU 104. The reading/writing means 182 reads out data from the data storing means 50 or write data into the data storing means 50, and the computing means 183 computes the cumulative usage of the process cartridge 43 based on the data read out of the data storing means 50. The computing means 183 sends to the CPU 104, a signal that represents the cumulative usage of the process cartridge 43, based on the cumulative number of the printed copies stored in the process cartridge 43. Receiving the signal from the computing means 183, the CPU 104 controls the jumping side area size of the development bias outputted by the development bias power source 12. After the printing, the number of the copies just printed is added to the cumulative number of the printed copies read out from the data storing means 50 prior to the current printing operation, and the total is inputted into the data storing means 50 through the reading/writing means 182, and is stored there. Next, the control, in this embodiment, of the image forming apparatus 100 will be described in detail with reference to Figure 6. First, receiving image signal inputted from an image signals inputting means such as a computer, the CPU 104 reads out information regarding the cumulative number of the printed copies from the data storing means 50, through the reading/writing means 182 (Step 1). Next, the computing means 183 determines in which of the following ranges the cumulative number of the printed copies is: (a) 0 - 500, (b) 501 - 2500 or (c) 2501 or more (Step 2). If it is determined that the cumulative number of the printed copies is in Range (a), the output of the development bias power source 12 is set so that the jumping side area size of the development bias becomes 0.43 V.sec. If it is determined that the cumulative number of the printed copies is in Range (b), the output of the development bias power source 12 is set so that the jumping side area size of the development bias becomes 0.47 V.sec. If the cumulative number of the printed copies is in Range (c), the output of the development bias power source 12 is set so that the jumping side area size of the development bias becomes 0.48 V.sec (Step 3). Then, a printing operation is carried out using the above settings (Step 4). During the printing operation, the number of the copies printed in the current printing operation is continuously added to the cumulative number of the printed copies read out of the data storing means 50 (Step 5). Next, the cumulative number of the printed copies is written into the data storing means 50 through the reading/writing means 182 (Step 6), and the printing operation is ended (Step 7). The above-described control method was used to print 5000 copies to test the durability of the process cartridge 43 in terms of image quality. During the test, the ratio at which the positive external additive jumped onto the photosensitive member was also confirmed. The results of the test showed that the insufficient cleaning by the positive external additive, the toner fusion, and the flowing image effect did not occur, and image quality was stable even in the initial period of the process cartridge usage; desirable copies could be outputted throughout the test. In view of the graph in Figure 3, which shows the change in the amount of the jumped positive external additive, it is evident that the ratio at which the positive external additive jumped was kept above 0.5 wt. % but below 2.0 wt. % throughout the test. As described above, in this embodiment, in order to control the ratio at which the positive external additive jumps onto the photosensitive member, development bias, the jumping side area size of which is variable, is used. Therefore, the ratio at which the positive external additive jumps onto the photosensitive member is kept at a proper level throughout the service life of the process cartridge 43, stabilizing image quality during the initial period of the service life of the process cartridge 43, maintaining the effects of the positive external additive upon the flowing image effect, preventing the production of streaky images, and preventing the toner fusion, so that high quality images can be formed throughout the service life of the process cartridge 43. Embodiment 2 Next, referring to Figures 7 - 11, the second embodiment of the present invention will be described. Figure 7 depicts the image forming apparatus 101 in this embodiment. The image forming apparatus 101 comprises a process cartridge 44, a transfer roller 13, a fixing apparatus 19, an optical system consisted of a laser scanner 4, a mirror 6, and the like. The process cartridge 44 integrally comprises processing apparatuses: a photosensitive member 1, a charge roller 2, a developing apparatus 30, and a cleaning apparatus 14. The same components or portions as those in Figure 1 are given the same referential characters as those in Figure 1. In the developer container 3, a toner 21 is held. The positive external additive in the toner 21 is the same as the one in the first embodiment. In this embodiment, the positive external additive is initially added by 0.75 wt. %. The service life of the process cartridge 44 is 4000 copies when the average dot ratio per page is 4 %. Next, the development bias applying method in this embodiment, which is the specific aspect of this embodiment that characterizes it, will be described in detail. This embodiment is characterized in that in order to prevent the occurrence of the flowing image effect which tends to become worse toward the end of the service life of the process cartridge 44, such development bias is applied that increases, throughout the latter half of the service life of the process cartridge, the ratio to the toner at which the positive external additive jumps onto the photosensitive member during the transfer sheet intervals in a continuous printing operation, and the pre-rotation period in which the photosensitive member is rotated prior to the formation of a latent image. The image forming apparatus 101 in this embodiment was subjected to a durability test, in which 4000 copies were made, applying a development bias composed of AC and DC components. The AC component had a voltage of 1600 V (Vpp = 1600 V) and a frequency of 2400 Hz (Vf = 2400 Hz), and the DC component had a voltage of -400 V (Vdc = -400 V). Further, the development bias was given a rectangular waveform with a fixed duty ratio of 1:1. During this test, the ratio at which the positive external additive jumped onto the photosensitive member was confirmed. The results are as follows: image quality could be improved in terms of the image density start-up at the initial period of the service life of the process cartridge 44, but the effect of the positive external additive in terms of preventing the flowing image effect was satisfactory only up to the 2000th copy, failing to remain satisfactory up to the 4000th copy, or the end of the service life of the process streaky during the initial period of the service life, and also, white spots appeared in the images toward the end of the service life, but both defects were at the levels that did not cause any problem in terms of practical usage. It should be noted here that the streakiness and the white spots in this embodiment occurred due to the same causes as those in the first embodiment. The change in the ratio at which the positive external additive jumped onto the photosensitive member in the above endurance test was as shown in Figure 8. In Figure 8, the ratio at which the positive external additive jumped onto the photosensitive member was larger during the initial period of the service life of the process cartridge 44, and gradually decreased, eventually decreasing to a level at which the ratio of the positive external additive to the toner on the peripheral surface of the photosensitive member was less than the ratio by which the positive external additive was initially added to the toner. In other words, the higher jumping ratio of the external additive during the initial period of the process cartridge 44 caused the insufficient cleaning by the positive external additive, leading to the creation of the nuclei which was the cause of the toner fusion to the photosensitive member, whereas toward the end of the process cartridge 44, the jumping ratio of the positive external additive became less than the predetermined ratio by which the positive external additive was initially added to the toner, and as a result, the effects of the positive external additive in terms of preventing the flowing image effect gradually diminished, worsening the flowing image effect. Next, the image forming apparatus 101 in this embodiment was subjected to another durability test which was substantially the same as the first test in this embodiment, except for one aspect of the development bias. More specifically, the development bias applied to the development sleeve 10 had an AC component with a voltage level of 1600 V (Vpp = 1600 V) and a frequency of 2400 Hz (Vf = 2400 Hz), and a DC component with a voltage of -400 V (Vdc = -400 V), as had the development bias in the preceding test in this embodiment. The waveform was also rectangular. However, in this embodiment, the duty ratio of the development bias was rendered variable. More specifically, during the actual developing period, a development bias with a fixed duty ratio of 1:1 was applied, whereas, during the sheet interval and the pre-rotation period, a development bias, the duty ratio of which was variable (hereinafter, "sheet interval development bias"), was applied. Then, the ratio at which the positive external additive jumped onto the photosensitive member was measured, while changing the jumping side area size of the waveform of the sheet interval development bias; in the test, the jumping side area size of the sheet interval development bias was varied, and the ratio at which the positive external additive jumped onto the photosensitive member was measured for each of the various jumping side area sizes. Because this embodiment concerns such flowing image effect that occurs after the printing of the 2000th copy, that is, such flowing image effect that creates a problem in practical usage, this test was carried out after 2000 copies were printed with the use of process cartridge 44. The sheet interval bias in this test was basically the same as that in the first embodiment, except that in this embodiment, |V1-V2| = 1600 V, when T1 = T2 in Figure 4. The frequency of the development bias was 2400 Hz, and Vdc = -400 V. Further, while the sheet interval bias was applied, the surface potential level VD of the photosensitive member was fixed at -650 V. The length of the sheet interval, and the length of the pre-rotation period, were set to be equivalent to the circumference of the photosensitive member, or a single rotation of the photosensitive member. Table 3 given below shows the results of this test carried out to confirm the correlation between the jumping side area size and the ratio at which the positive external additive jumped. Jump side area size (V.sec) Jump count of additive (% by wt.) ≧ 0.42 ≧ 3.0 0.37 - 0.42 ≧ 2.0 0.30 - 0.37 ≧ 1.0 0.25 - 0.30 ≧ 0.5 < 0.25 < 0.5 According to Table 3, there was a desirable relationship between the jumping side area size and the ratio at which the positive external additive jumped onto the photosensitive member, which is similar to the relationship in the first embodiment. As the jumping side area size was reduced, the amount of the jumped positive external additive reduced, whereas as the jumping side area size was increased, the amount of the jumped positive external additive increased. The implies that the ratio at which the positive external additive jumps onto the photosensitive member can be controlled by controlling the jumping side area size. It should be noted here that according to Table 3, the ratio of the jumping side area size relative to the amount of the jumped positive external additive in this embodiment is smaller than that in the first embodiment. This is due to the fact that in this embodiment, the ratio of the positive external additive, relative to the toner, which jumped onto the photosensitive member during the actual developing period, was approximately 0.4 wt. %. It was also confirmed by the test that neither of the aforementioned two components of the jumping side area size, i.e., the contrast V and the length of the period T, displayed a greater correlation with the amount of the jumped positive external additive, than the other. All that was confirmed was that both the contrast V and the length of the period T2 had correlation with the amount of the jumped positive external additive. Therefore, the jumping side area size may be controlled by controlling either the magnitude of the contrast V or the length of the period T2, or by controlling both. The image forming apparatus 101 was subjected to another test, in which the relationship between the ratio at which the positive external additive jumped onto the photosensitive member, and the various image defects (insufficient image density at the beginning of usage, insufficient cleaning of the positive external additive, toner fusion, and flowing image effect), was confirmed using the aforementioned development bias, the duty ratio of which was variable. This test was carried out also after 2000 copies were printed using the process cartridge 44. The results of the test are shown in Table 4 given below. In the table, a reference character o means that no image defect occurred; a referential character Δ means that image defects, insignificant in terms of practical usage, occurred; and a referential character x means that significant image defects occurred. Jump amount of additive Cleaning defect Fusion Flow ≧ 3.0 % by wt. x x o ≧ 2.0 % by wt. Δ Δ o ≧ 1.0 % by wt. o o o ≧ 0.5 % by wt. o o o < 0.5 % by wt. o o x According to Table 4, it is clear that there is a definite correlation between the ratio at which the positive external additive jumped onto the photosensitive member and the various image defects. In other words, in order to prevent the occurrence of the insufficient cleaning by the positive external additive and the occurrence of the toner fusion, control should be executed so that the ratio at which the positive external additive jumps onto the photosensitive member should be kept below 2 wt. %. In order to prevent the flowing image effect from occurring, the ratio at which the positive external additive jumps onto the photosensitive member should be kept above 0.5 wt. %. In other words, in order to suppress the flowing image effect, while preventing the occurrence of the insufficient cleaning of the positive external additive and the toner fusion, during the latter half of the service life of the process cartridge 43, all that is necessary is to keep between 0.5 wt. % to 2.0 wt. %, the ratio at which the positive external additive jumps onto the photosensitive member. In other words, it is evident, from the above described correlation among the ratio at which the positive external additive jumped onto the photosensitive member, the jumping side area size, and the various image problems (traceable to insufficient cleaning of positive external additive, toner fusion, and flowing image effect), that in order to prevent the occurrences of the insufficient cleaning of the positive external additive, the toner fusion, and the flowing image effect, the jumping side area size should be kept above 0.25 V.sec but below 0.42 V.sec during the latter half of the service life of the process cartridge 44. In view of the change in the jumping ratio of the positive external additive in the durability test, the results of which are given in Figure 8, and in which the development bias was fixed, it is evident that the jumping ratio of the positive external additive remained above 0.5 wt. % but below 3.0 wt. % in the period between the first and 2000th copy, and image quality was improved in terms of the problems related to the insufficient image density at the beginning of the service life of the process cartridge 43 was improved. Also during this period from the first to the 2000th copy, the insufficient cleaning by the positive external additive occurred, and the toner fusion nuclei were also created, but they were not severe enough to cause problems in practical usage. In the period from the 2000th copy to the 4000th copy, the jumping ratio of the positive external additive remained below 0.5 wt. %, and the flowing image effect began to occur, progressively worsening. This implies that the occurrence of the flowing image effect can be prevented, while improving image quality in terms of the initial insufficiency in image density, throughout the service life of the process cartridge 44, as long as control is executed so that during the period past the 2000th copy, the ratio at which the positive external additive jumps remains above 0.5 wt. % (jumping side area size being above 0.25 V.sec). Therefore, in this embodiment, in order to output copies with desirable image quality throughout the service life of the process cartridge 44, such an operational sequence is employed that, based on the data stored in the data storing means 50 located in the image forming apparatus 101, the sheet interval development bias is not applied during the period in which the cumulative number of the printed copies is 0 - 1999, and then, the sheet interval development bias is applied during the period in which the cumulative number of printed copies is 2000 - 4000, so that the jumping side area size of the development bias is kept at 0.33 V.sec. Next, referring to Figures 9 and 10, the method in this embodiment for controlling the jumping side area size of the development bias in response to the cumulative number of the printed copies will be described. Figure 9 shows the block diagram for the control sequence in this embodiment. The same components as those in Figure 5 are given the same referential codes as those in Figure 5. The operational structure depicted in Figure 9 is the same as that in FIgure 5, and therefore, its description will be omitted. Next, referring to Figure 10, a flow chart, the control sequence for the image forming apparatus 101 in this embodiment will be described. The first and second embodiments are different only in Steps 2 and 3, and therefore, the descriptions of the steps in this embodiment, other than Steps 2 and 3, which are the same as those in the first embodiment, will be omitted. In Step 2 in this embodiment, the computing means 183 determines whether the cumulative number of the copies printed by the process cartridge 44 is in a range of (a) 0 - 2000 or a range of (b) 2001 or more. In Step 3, an arrangement is made so that the sheet interval development bias is not outputted from the development bias power source 12 if it is determined in Step 2 that the cumulative number of the copies printed by the process cartridge 44 is in Range (a), whereas if it is determined that the cumulative number is in Range (b), the sheet interval development bias is outputted from the development bias power source 12, keeping the jumping side area size at 0.33 V.sec. Using the above-described control method, the image forming apparatus 101 in this embodiment was subjected to a durability test in which 4000 copies were printed. During the test, the ratio at which the positive external additive jumped onto the photosensitive member was also confirmed. The results of the test showed that the insufficient cleaning by the positive external additive, the toner fusion, and the flowing image effect did not occur; desirable copies could be outputted throughout the durability test. In view of the graph in Figure 11, which shows the change in the ratio at which the positive external additive jumped, it is evident that the ratio at which the positive external additive jumped was kept above 0.5 wt. % but below 2.0 wt. % during the latter half of the service life of the process cartridge 44. As described above, in this embodiment, a sheet interval development bias, the jumping side area size of which can be varied in response to the cumulative number of the copies printed by the process cartridge 44, is used so that the ratio at which the positive external additive jumps onto the photosensitive member can be controlled. Therefore, throughout the service life of the process cartridge 44, the effects of the positive external additive upon the flowing image effect can be maintained, while stabilizing image quality during the initial period of the service life of the process cartridge 44; high quality images can be stably outputted. Further, in this embodiment, the ratio at which the positive external additive jumps onto the photosensitive member is controlled during the sheet intervals, assuring that the positive external additive jumps onto the photosensitive member at a proper ratio, regardless of the dot ratio during the actual developing period. Also, the jumping side area size is controlled during the period in which image-less portions of the photosensitive member is in the development station, and therefore, it is unnecessary to consider the change in image density caused by the controlling of the jumping side area size. In other words, it is possible to execute drastic control.

Claims: An image forming apparatus comprising: an image bearing member (1) for bearing an electrostatic image; a developer carrying member (10) for carrying a developer (8) and for forming a developing zone with said image bearing member (1), wherein additive having a charging polarity opposite from that of the developer is added to the developer; voltage applying means (12) for applying a developing voltage to said developer carrying member resulting in a force for directing the additive toward said image bearing member (1); voltage control means (104) for controlling the developing voltage so as to change the force for directing the additive toward a non-image portion of said image bearing member in accordance with a number of image forming operations. An apparatus according to claim 1, wherein the developing voltage is in the form of a rectangular wave, and said control means controls a duty ratio of the developing voltage. An apparatus according to claim 1, wherein said control means increases a size of a jump side area of the additive in a waveform of the developing voltage with the increase of the number of the image forming operations. An apparatus according to claim 1, wherein said developer carrying member (10) is contained in a developing cartridge detachably mountable to a main assembly of said image forming apparatus. An apparatus according to claim 4, wherein said developer carrying member (10) is contained in a unit (43) which contains said image bearing member (1) and said developer carrying member (10) integrally. An apparatus according to claim 5, wherein said unit (43) includes storing means (50) for storing the number of image forming operations, wherein said control means controls the developing voltage in accordance with information stored in the storing means.

Inventors: INAMI SATORU, KATO JUNICHI, NAKAZONO YUSUKE, SHINOHARA SEIICHI, YOSHIDA MASAHIRO, INAMI, SATORU, KATO, JUNICHI, NAKAZONO, YUSUKE, SHINOHARA, SEIICHI, YOSHIDA, MASAHIRO
Applicants: CANON KK, CANON KABUSHIKI KAISHA
Application year: 1998
Main classification: G03G15
IPC codes: G03G15