Apparatus for and method of forming image

Abstract: An image forming apparatus comprises a unit (2) for electrostatically charging the surface of an image carrier (1) uniformly, a unit (3) for forming an electrostatic latent image on the surface of the charged image carrier, a unit (4) for developing the electrostatic latent image formed on the surface of the image carrier thereby to form a toner image, and a roller (5) for transferring and fixing the toner image to a transfer member (6) such as a paper sheet. The developing unit (4) includes a developing roller (4a) which is disposed so as to contact the image carrier. The developing roller is connected to a power source (4b) for electrostatically charging the toner particles on the developing roller to the same polarity as the charging polarity of the image carrier, and for applying an electric potential to the developing roller so that toner particles on the developing roller adhere to an image portion of the image carrier and the toner particles remaining on a non-image portion of the image carrier arre attracted by the developing roller. The developing roller rotates in the direction opposite to that of the image carrier and a peripheral velocity of the developing roller exceeds 1.2 times that of the image carrier.

Description: The present invention relates to an apparatus for and a method of forming an electrophotographic image, and more particularly to those adapted for various business machines and instruments such as a printer. An electrophotographic image processing apparatus has been conventionally utilized as an electrophotographic printer. Such an image processing system carries out the steps of electrically charging an image carrier, i.e. a photoconductor drum uniformly, forming a latent image on the photoconductor drum, developing the latent image using toner, transferring the toner on the photoconductor drum to a transfer member, fixing the toner on the transfer member and removing the toner remaining on the photoconductor drum therefrom. The photoconductor drum is discharged before the next charging process starts upon completion of the transferring process in order to prevent an afterimage from being formed on the photoconductor drum. The charging process and the transferring process are generally performed by utilizing corona discharge. Since harmful substances such as ozone are produced by corona discharge, it has to be collected by a filter. However, prolonged use of a filter causes a decrease in collective efficiency and results in the need for frequent filter replacement. An ozone free process where ozone generation is prevented by the employment of a roller type transfer system or a charging roller system has been proposed. (refer to Electronic Communication Institute Thesis '77/4 Vol. J60-C NO.4 pp213-218). The roller type transfer system performs the steps of placing a transfer member on a toner image formed by development on the surface of a photoconductor drum, pressing a transfer roller on the transfer member and applying a voltage polarity which is opposite to that of the toner, to the transfer roller. In this system, an electric field is generated in a gap between the transfer member and the upper layer of the toner image whereby the toner is transferred to the transfer member by an electrostatic force of the electric field. A charging roller system is also known, which operates according to the same principle as the roller type transfer system for electrically charging the photoconductor drum. In this system, a voltage is applied to a charging roller so that an electric charge is directly applied to the photoconductor drum, which leads to no generation of ozone. There has also been proposed an image forming system eliminating a cleaning process (Refer to Japan Hardcopy '89 Thesis pp 143-146) . In this system, the photoconductor drum is exposed to light after the photoconductor drum is uniformly charged by the corona discharge whereby the surface potential of the exposure portion is attenuated. Toner is stuck to the attenuated portion by reversal development while toner in a thin layer which remains on the photoconductor drum is collected therefrom. That is, since the toner which remains on the nonexposed portion of the photoconductor drum after the completion of the transferring process is electrically charged with the same polarity as in the developing process, the toner is attracted to the developing unit due to the electrostatic force caused by the difference between the surface potential of the electrically charged photoconductor drum and the developing bias. The employment of this cleaningless process can miniaturize the image processing apparatus and the remaining toner on the photoconductor drum can be collected therefrom in the developing process. This avoids having to dispose of the remaining toner and improves efficiency as it can be reused. In the ozone free process the remaining toner is collected by a cleaning blade or a brush or the like, and is disposed of. In the cleaningless process, ozone is generated and is collected by a filter requiring maintenance and frequent replacement to maintain its efficiency. Accordingly, even if both processes are combined as the photoconductor drum is charged by the charging roller before the residual toner is removed from the photoconductor drum by the developing unit, a portion of the photoconductor drum on which the toner particles are stuck is not electrically charged with electricity by the charging roller and therefore the electrostatic force of the developing unit has no effect on these toner particles and they are not attracted to the developing unit and are not completely removed from the photoconductor drum. This can cause the generation of a positive afterimage on the transfer member in the subsequent transferring process. Furthermore, a large amount of uncollected residual toner particles remaining on the photoconductor drum means that when the photoconductor drum is exposed to light, it cannot reach the surface and is absorbed by the residual toner particles, resulting in poor exposure and the generation of a negative afterimage on the transfer member in the subsequent transferring process. An image forming apparatus of the type described above is known from EP-A-0400563 and comprises an image carrier, a charging unit for electrostatically charging the surface of the image carrier, a latent image forming unit for forming an electrostatic latent image on the charged surface of the image carrier, a developing unit disposed adjacent to the image carrier for developing the electrostatic latent image formed on the surface of the image carrier to form a toner image, means for transferring and fixing the toner image formed on the surface of the image carrier to a transfer member and a power source connected to the developing unit for electrostatically charging toner particles on the developing unit with the same polarity as the charging polarity of the image carrier, and operative to set the potential of the developing unit to a value capable of allowing the toner particles to adhere to an image portion of the image carrier and of allowing the toner particles remaining on a non-image portion of the image carrier to be attracted by the developing unit away from the image carrier. A method of forming an image is also known and comprises the steps of: a) electrostatically charging the surface of an image carrier; b) forming an electrostatic latent image on the charged surface of the image carrier; c) developing the electrostatic latent image by sticking toner particles thereto to form a toner image; d) transferring the toner image to a transfer member wherein the toner particles remaining on a non-image portion of the image carrier after the transfer process are attracted to the developing unit, the toner particles on an image portion of the image carrier remaining thereon. The present invention seeks to provide an apparatus and method for forming an image which solves or substantially alleviates the aforementioned problems and in which no ozone or other harmful substance is generated, whereby residual toner remaining on the photoconductor drum is collected and reused without deterioration of the resulting image formed on the transfer member. According to an aspect of the present invention, there is provided an image forming apparatus in which a toner holding unit is disposed between the transfer means and the charging unit for attracting the toner particles thereto and for returning them to the image carrier. According to a second aspect of the present invention there is provided a method for forming an image comprising the step of attracting the toner particles to a toner holding unit and returning them to the image carrier. In a preferred embodiment the turning direction of the developing roller is opposite to that of the image carrier and the peripheral velocity of the developing roller can be set to exceed 1.2 times that of the image carrier. Preferably, the charging unit includes a charging roller in contact with the image carrier. The absolute value of the potential on the charging roller can be decreased during no printing operation while the charging roller is engaging with the surface of the image carrier, or at the end of printing operation. Preferably, the turning direction of the charging roller can be opposed to that of the image carrier and the peripheral velocity of the charging roller and that of the image carrier can be differentiated from each other. For example, the peripheral velocity of the charging roller can be less than that of the image carrier, and vice versa. In a preferred embodiment, the toner holding unit includes a toner holding roller. The peripheral velocity of the toner holding roller being equal to or greater than the peripheral velocity of the photoconductor drum. The toner holding unit is preferably formed from a semi conductive rubber or sponge having an electrical resistance of between 10 3 to 10 9 Ohms. According to the preferred method of carrying out the invention, the charging is performed by contacting a charging member connected to a power source to the surface of the image carrier, without employing a corona discharge system. Although remaining on the image carrier upon completion of the transferring step, the toner particles are not removed by a cleaning device but are collected owing to electrostatic force which is, for instance, generated in the developing step before the transferring step starts after the charging step. Preferably, the method includes the step of positively charging the negatively charged toner particles that adhere to the toner holding unit before returning them to the image carrier. A preferred embodiment of the present invention will now be described, by way of example only, with reference to Figures 5-10 and 12 of the accompanying drawings, Figures 1-4 and 11 showing a prior art arrangement, in which: Figure 1 is a schematic view showing a prior art image forming apparatus; Figure 2 is a block diagram of the prior art image forming apparatus of Figure 1; Figure 3 is a flowchart showing an operation of the prior art image forming apparatus to Figure 1; Figure 4 is an enlarged view of a developing unit of the prior art image forming apparatus of Figure 1; Figure 5 is a schematic view showing an image forming apparatus according to a first embodiment of the present invention; Figure 6 is a time chart of an image forming apparatus according to a second embodiment of the present invention; Figure 7 is a time chart of an image forming apparatus according to a third embodiment of the present invention; Figure 8 is a table showing the characteristics of toner particles employed by the image forming apparatus to the. present invention; Figure 9 is a view showing the relation between the characteristic value of toner particles and the amount of toner particles attached to the charging roller; Figure 10 is a view showing the relation between the characteristic value of toner particles and the surface potential of a photoconductor drum; Figure 11 is a schematic view of an electrophotographic apparatus to which a conventional method for forming an image is applied; and Figure 12 is a view showing the relation between the characteristic value and the density of the toner particles. A prior art image forming apparatus will first be described with reference to Figures 1 to 4 of the accompanying drawings. Figure 1 is a schematic view showing a prior art image forming apparatus and Figure 2 is a block diagram of the prior art image forming apparatus of Figure 1. A drum type image carrier, i.e. photoconductor drum 1 rotates in the direction of the arrow A. An organic photoconductor drum (hereinafter referred to as OPC), with a negative polarity is employed as the drum type image carrier. The dielectric layer on the photoconductor drum 1 has a dielectric constant which is expressed as follows: ε p = 3.5 ε o (ε o =8.855 x 10 -12 [C/V m ] : space dielectric constant and the thickness d p of the photoconductor drum is expressed as d p =20[µm]. A charging roller 2 constituting a charging unit is formed from a conductive rubber roller. The charging roller 2 contacts the photoconductor drum 1 at a given pressure and follows in rotation. The charging roller 2 may be rotated by a driving means, not shown, through a gear, etc. instead of the friction with the photoconductor drum 1. A fixed type contact charging unit such as a brush may replace the charging roller 2. The electric resistance of the charging roller 2 is set to be 10 5 [Ω] but may be set to be approximately on the order of 10 o to 10 9 [Ω]. If the electric resistance is too low, due to a pin hole on the surface of the photoconductor drum 1, a large amount of current is liable to flow into the charging roller 2. On the other hand, if the electric resistance is too high, a stable surface potential is hardly obtained. Accordingly, the electric resistance is preferable to range from 104 to 10 9 [Ω]. The electric resistance mentioned here means that between the contacting plane where the charging roller 2 contacts the photoconductor drum 1 (an area as large a nip width x longitudinal length) and a conductive shaft 2a which supports the charging roller 2. A power source 2b applies a voltage to the conductive shaft 2a. A latent image forming unit 3 subject the photoconductor drum 1 to exposure in light in response to a printing signal and draws an electrostatic latent image comprising an exposure portion and nonexposure portion, on the surface of the photoconductor drum 1. The latent image forming unit 3 is an LED, but it may be a laser beam scanning unit, a liquid crystal shutter array, etc. A toner carrier, i.e. a developing roller 4 constituting a developing unit contacts to the photoconductor drum 1 at a given pressure and rotates in the direction of the arrow B. Thee developing roller is formed of a conductive rubber roller. The electric resistance of the developing roller 4 is set to be 10 6 [Ω] but may be set to be approximately 10 o to 10 p [Ω]. If the electric resistance is too low, a large amount of current flows into the developing roller 4 when the surface of the developing roller directly contacts to the photoconductor drum 1 in case the photoconductor drum 1 has a pin hole or a small amount of toner on the surface thereof locally. On the contrary, if the electric resistance is too high, the developing efficiency is lowered whereby low density in the eventual printed image is liable to occur. Accordingly, the electric resistance is preferable to range from 10 4 to 10 8 [Ω]. The electric resistance mentioned here means that between the contacting plane where the surface of the developing roller 4 contacts the photoconductor drum 1 and the conductive shaft 2a. Toner particles are laminated to several tens µm thick on the developing roller 4 and enter a developing area which contacts the photoconductor drum 1 by a means, not shown, as the developing roller 4 rotates whereby the development is performed. The toner particles carry an electric charge polarity which is the same as the charging polarity of the photoconductor drum 1 so as to perform reversal development between the photoconductor drum 1 and the developing roller 4. In this case, the exposure portion to which toner particles are stuck forms an image portion while the nonexposure portion to which toner particles are not stuck forms a non-image portion. A power source 4b applies an electric potential, which is intermediate between that of the image portion and that of the non-image portion of the photoconductor drum 1, to the developing roller 4. A transfer roller 5 constituting a transfer unit transfers a toner image on the photoconductor drum 1 to a transfer member 6 which is conveyed in the direction of arrow C. The transfer roller 5 contacts the photoconductor drum 1 at a given pressure and is driven thereby. The transfer roller 5 may be replaced by another means if the latter substantially performs the same function as the former. The transfer member 6 may be a recording paper. The electric resistance of the transfer roller 5 means that between the contacting plane where the surface of the transfer roller 5 contacts the photoconductor drum 1 and a conductive shaft 5a. The electric resistance is set to be 10 8 [Ω] but may be set to range approximately from 10 o to 10 9 [Ω]. If the electric resistance is too low, a large amount of current flows when the photoconductor drum 1 has pinholes on the surface thereof. If the transfer member 6 has a width less than those of the photoconductor drum 1 and the transfer roller 5, there is not a likelihood of obtaining a sufficient electric field, which causes a poor transfer. On the contrary, if the electric resistance is too high, most of the voltage is applied to the transfer roller 5 so that sufficient voltage is not applied to the toner layer, which causes poor transfer. The transfer member 6 to which the toner image is transferred is separated from the photoconductor drum 1 and is introduced into a fixing unit, not shown. The transfer member 6 is discharged as a printed matter outside the image forming apparatus upon completion of the fixing process. A power source 5b applies a voltage to the conductive shaft 5a. In Fig. 2, a control portion 11 of the image forming apparatus supplies a printing signal to the latent image forming unit 3 so that an LED array head emits light upon reception of the printing signal. The control portion 11 supplies a driving signal to the photoconductor drum 1 so that the photoconductor drum 1 is driven. The control portion 11 further supplies a high voltage signal to the power sources 2b, 4b and 5b so that these power sources set the potentials of the charging roller 2, the developing roller 4 and the transfer roller 5 to the appropriate values. An operation of the prior art image forming apparatus will be described with reference to Figs. 3 and 4. Fig. 3 is a flowchart showing an operation of the prior art image forming apparatus of Fig. 1 and Fig. 4 is an enlarged view of a developing unit of the prior art image forming apparatus of Fig. 1. In Fig. 4, toner particles 12a are stuck to the image portion of the photoconductor drum 1 from the surface of the developing roller 4. Denoted at 4b is a power source. The toner particles 12b remains on the surface of the photoconductor drum 1 upon completion of the transfer of the toner image on the transfer member 6 (Fig. 1). Since the image forming apparatus has no cleaning means such as a blade, a cleaning brush, the toner particles 12b are stuck to the surface of the photoconductor drum 1 to thereby form a residual toner layer and enter a uniformly charged area where the photoconductor drum 1 contacts the charging roller 2. When the density of the residual toner layer in the uniformly charged area is low, the charged potential difference on the surface of the photoconductor drum 1 due to the presence of the residual toner layer is small so that the surface of the photoconductor drum 1 is uniformly charged with electricity. Thereafter, the surface of the photoconductor drum 1 is subjected to light exposure and is optically drawn on the exposure portion to form a latent image thereon. At this time, if the density of the residual toner layer is low, a spot diameter for optical drawing becomes sufficiently greater than the size of the toner particle 12b, which leads to less influence upon formation of the latent image caused by the presence of the residual toner layer. As a result, an excellent latent image can be obtained. Successively, the toner particles 12b contact the developing roller 4. The potential of the developing roller 4 is controlled to an intermediate value between those of the exposure and nonexposure portions of the photoconductor drum 1 by the power source 4b. Accordingly, the toner particles 12a remaining on the nonexposure portion are attracted by the developing roller 4 owing to the electrostatic force as illustrated in Fig. 4 and are collected by the developing unit. Meanwhile, the toner particles 12b remaining on the exposure portion are not collected by the developing unit but remains stuck to the photoconductor drum 1. The toner particles 12a on the developing roller 4 are attracted by the photoconductor drum 1, contrary to the toner particles 12b, whereby the latent image on the photoconductor drum 1 is developed to thereby form the toner image. Successively, the toner image on the photoconductor drum 1 is transferred to the transfer member 6 by the transfer roller 5, whereby one cycle of image forming operation is completed. A toner image transfer efficiency of the transfer roller 5 is much higher than that by the conventional corona discharge, which allows the toner particles 12b to remain less on the photoconductor drum 1. Since the developing roller 4 develops the latent image by contacting the photoconductor drum 1, a large amount of the toner particles 12b can be collected and the toner particle collection efficiency is much improved compared with that of the conventional non-contact magnetic brushing developing system. If the peripheral velocity of the developing roller 4 in the direction of the arrow B is greater than that of the photoconductor drum 1 in the direction of the arrow A, particularly, if the former exceeds 1.2 times the latter, an experiment data showed that the toner particles 12b on the photoconductor drum 1 move toward the developing roller 4, which leads to a high toner particle collection efficiency. It is possible to develop the latent image on the photoconductor drum 1 with sufficient amount of toner particles stuck to the photoconductor drum 1. Accordingly, even if the amount of toner particles is less supplied to the developing roller 4 so as to form a thin toner layer thereon since the amount of the toner particles 12b which corresponds to the difference in the peripheral velocity between the developing roller 4 and the photoconductor drum 1 is collected by the developing unit 4, so that the collected toner particles are supplied additionally to the thin toner layer thereon. A problem with the prior art image forming apparatus described above is that if the residual toner particles 12 are collected in the developing process and the surface of the photoconductor drum 1 is charged with electricity by the charging roller 2 without generating ozone, the photoconductor drum 1 is prevented from charging with electricity in the charging process when the toner particles 12b remaining on the surface of the photoconductor drum 1 in the transfer process pass between the charging roller 2 and the photoconductor drum 1, whereby the portion to which the tone particles 12b are stuck can not be charged with electricity. Consequently, since the electrostatic force does not influence the toner particles 12b in the developing process, the toner 12b can not be sufficiently collected, which causes the generation of a positive afterimage on the transfer member 6 in the next transferring process. Furthermore, a large amount of residual toner particles 12b remaining on the photoconductor drum causes disadvantages in the exposure process. If toner 12b is covered thickly on the surface of the photoconductor drum 1, light cannot reach the photoconductor drum 1 because the toner 12b absorbs it, thus resulting in poor exposure. The poor light exposure to the image portion causes to collect the toner 12b in the following process, without developing and sticking new toner particles. As a result, the portion corresponding to the previous image portion looms up white, or the so-called negative afterimage, in the present image portion. A first embodiment of the present invention will now be described with reference to Figure 5 of the accompanying drawings in which the insufficient charging and exposure owing to the remaining toner particles 12b is prevented, thus also preventing the positive or negative afterimage from being generated. A drum-type image carried, i.e. a photoconductor drum 1 rotates in the direction of arrow A. A negative type OPC is employed in the first embodiment. The charging roller 2 constituting a charging unit is formed of a semi-conductive rubber roller at the surface thereof. The power source 2b supplies a voltage to the conductive shaft 2a. The charging roller 2 rotates in the direction of the arrow D. Its peripheral velocity is greater than that of the photoconductor drum 1 and is set to be at the ratio of 1:1.1 to 1:2 relative to that of the photoconductor drum 1. The voltage of the power source 2b is 1.3 [kV] and the surface potential of the photoconductor drum is -800[V]. The latent image forming unit 3 subjects the photoconductor drum 1 to light exposure in response to the printing signal supplied by the control portion 11 and drafts an electrostatic latent image comprising exposure portion and nonexposure portion on the surface of the photoconductor drum 1. Although the photoconductor drum 1 employs an LED according to the first embodiment, it may be a laser beam scanning unit, a liquid crystal shutter array, etc. A toner carrier, i.e. a developing roller 4 constituting a developing unit contacts the photoconductor drum 1 at a given pressure and rotates in the direction of the arrow B at the peripheral speed with the ratio of 1:1.1 to 1:1.5 relative to the photoconductive drum 1. The surface of the developing roller is formed of a semiconductor rubber. The power source 4b applies a voltage to the conductive shaft 4a. With the application of the voltage to the conductive shaft 4a, there appears the potential on the developing roller 4 which potential is substantially intermediate between that of the image portion, the exposure portion of the photoconductor drum 1 and that of the non-image portion, i.e., the nonexposure portion of the developing roller 4. The potential on the developing roller 4 is set to be-350[V]. The toner particles 12a on the developing roller 4 are thinned to several tens µm thick by a developing blade 18 and enter the developing area where the developing roller 4 contacts the photoconductor drum 1 as the developing roller 4 rotates and developing takes place. The toner particles 12a have the same negative charges as that of the photoconductor drum 1 and a reversal development is performed. As this state, the toner particles 12a stuck to the exposure portion forms the image portion while the toner particles 12a sticks to the nonexposure portion and forms the non-image portion. The transfer roller 5 constituting the transfer unit transfers the toner image formed on the photoconductor drum 1 to the transfer member 6 which is conveyed in the direction of the arrow C by a means, not shown. The transfer roller 5 is structured so as to contact the photoconductor drum 1 at a given pressure and follows rotatably. The roller 5 may be replaced by other means if the same function can be attained. The transfer member 6 to which the toner image has been transferred is separated from the photoconductor drum 1 and is introduced into a fixing unit, not shown. Thereafter the number 6 is discharged as a printed matter outside the image forming apparatus. The power source 5b applies a voltage to the conductive shaft 5a. A toner holding roller 7 is formed of a semiconductive rubber or a semiconductive sponge at the surface thereof. Electric resistance of the semiconductive rubber or sponge ranges from 10 3 to 10 9 [Ω]. The voltage ranging from +100 to +700 [V] is applied to a conductive shaft 7a which supports a toner holding roller. The toner holding roller 7 rotates in the direction of the arrow F. The peripheral velocity thereof is greater than that of the photoconductor drum 1 and is set to be 1 to 2 times that of the photoconductor drum 1. The toner particles 12b negatively charged remain on the photoconductor drum 1. The operation of the image forming apparatus according to the first embodiment will be described hereinafter. The negatively charged toner particles 12b which remain on the photoconductor drum 1 in the transferring process are attracted by the toner holding roller 7 owing to the electrostatic force. Moreover, the toner particles 12b stuck to the toner holding roller 7 are positively charged using the toner holding roller 7, thus sticking again to the photoconductor drum 1. At this time, the positively charged toner particles 12b on the toner holding roller 7 are stuck to both the image and non-image portions of the photoconductor drum 1. Accordingly, the thickness of the toner particle layer on the toner holding roller 7 is increased. As mentioned above, although the toner particles remaining on the photoconductor drum 1 are once stuck to the toner holding roller 7, they are gradually returned to the photoconductor drum 1 with the toner holding roller 7 rotating. This is particularly effective in case there are many toner particles 12b remaining on the photoconductor drum 1. The toner particles 12b once stuck by the toner holding roller 7 are successively returned to the photoconductor drum 1 and reduce the thickness of the toner layer on the photoconductor drum 1. This is more effective if the peripheral velocity of the toner holding roller 7 is set to be 1:1.3 relative to that of the photoconductor drum 1. In the charging process, the toner particles 12b between the charging roller 2 and the photoconductor drum 1 move on the charged photoconductor drum 1 due to the charging roller 2 since the peripheral velocity of the charging roller 2 is greater than that of the photoconductor drum 1. Accordingly, the surface of the photoconductor drum 1 is charged with electricity uniformly at the portion where the toner particles 12b were stuck before the movement of the toner particles and at the portion where the toner particles 12b have been stuck again after the movement of the toner particles 12b. The larger the peripheral velocity ratio of the charging roller to the photoconductor drum 1 is, the more stably the portion where the toner particles 12b are attached can be charged with electricity. However, it was practically effective when the peripheral velocity ratio is 1:1.3. At this time, the toner particles 12b are negatively charged since the negative charge is introduced thereinto by the charging roller 2. In the exposure process, since the light for exposure is shaded at the portion where the toner particles 12b are attached thicker on the photoconductor drum 1, the same portion is not exposed sufficiently. However, according to the first embodiment, since the toner particles 12 are distributed sparsely on the photoconductor drum 1 owing to the provision of the toner holding roller 7, there is no likelihood of occurrence of insufficient exposure and the so-called negative afterimage. In the developing process, the developing roller 4 has a potential which is intermediate between that of the nonexposure portion and that of the exposure portion of the photoconductor drum 1. The negatively charged toner particles 12b remaining on the nonexposure portion attracted by the developing roller 4 owing to the electrostatic force. Meanwhile, the negatively charged toner particles 12b remaining on the nonexposure portion are not attracted by the developing roller 4 since the exposure portion is at the exposure potential. On the contrary, new toner particles 12a are moved from the developing roller 4 and stuck to the exposure portion owing to the electrostatic force. Thereafter, the toner image is transferred to the transfer member 6 in the transferring process by the electrostatic force caused by the transfer roller 5. The toner image on the transfer member 6 is fixed thereto by a fixing device, not shown. Although the peripheral velocity of the charging roller is greater than that of the photoconductor drum 1 according to the first embodiment, the former can be less than the latter. A second and third embodiment of the present invention will now be described with reference to Figs. 6 and 7. Fig. 6 shows a time chart of an image forming apparatus according to the second embodiment and Fig. 7 shows a time chart of an image forming apparatus according to the third embodiment. The latent image forming unit 3 subjects the photoconductor drum 1 to light exposure upon reception of the printing signal from the control portion 11. At this instance, the printing signal is made valid corresponding to the motion of the transfer member 6 but is made invalid at the gap between the transfer members (hereinafter referred to as a paper gap). A power source 5b of the transfer roller 5 is controlled according to the paper gap. That is, the power source 5b controls to permit the potential TR of the transfer roller 5 to be at the polarity for transferring the toner particles 12a to the transfer member 6 when the transfer member 6 is positioned between the transfer roller 5 and the photoconductor drum 1, while it permits the potential TR to be at the polarity inverse to that at the time of transferring process in order to prevent the tone particles 12a from being transferred to the transfer roller 5 as illustrated in Fig. 6. The potential CH of the charging roller 2 is controlled by the power source 2b so as to be temporarily reduced in absolute value from the value necessary for charging to 0 [V] during the time when the charging roller 2 passes the area of the photoconductor drum 1 corresponding to the paper gap. At this time, the surface of the photoconductor drum 1 is negatively charged since there remains the electric charge, which was supplied thereto at the time when the charging roller 2 passed, on the surface of the photoconductor drum 1. Accordingly, the positively charged toner particles 12b which remains on the photoconductor drum 1 and are attracted by the charging roller 2 are attracted by the photoconductor drum 1 owing to electrostatic force. If the potential CH of the charging roller 2 is set to be 0 [V], the potential of the photoconductor drum 1 is lowered so that the toner particles 12a on the developing roller 4 moves to the photoconductor drum 1 and attached thereto. Therefore, an absolute value of a potential DEV of the developing roller 4 is lessened to be 0 [V] at the time when the developing roller 4 reaches the portion corresponding to the paper gap. Since the portion corresponding to the paper gap moves as the photoconductor drum 1 rotates, the time for setting the potential CH of the charging roller 2 to 0 [V], the time for setting the potential DEV of the developing roller 4 to 0 [V] and the timing for setting the polarity of the potential TR of the transfer roller 5 to the inverse polarity are respectively shifted from one another. In the developing process, most of the toner particles 12a which are moved from the developing roller 4 to the photoconductor drum 1 are negatively charged but some of them are positively charged. The positively charged toner particles 12a remains on the photoconductor drum 1 after completion of the transferring process and are liable to be stuck to the charging roller 2. Since the absolute value of the potential CH of the charging roller 2 is lessened every time the charging roller 2 reaches the portion corresponding to the paper gap, the toner particles 12b stuck to the charging roller 2 are removed so that the amount of the toner particles 12b remaining thereon is decreased, whereby the uniform continuous charging can be performed. As illustrated in Fig. 7, if the absolute value of the potential CH of the charging roller 2 is lessened during a given time T before the photoconductor drum 1 stops rotating, the toner particles 12b stuck to the charging roller 2 can be removed. Since continuous printing is rarely performed, the toner particles 12b stuck to the charging roller 2 can be sufficiently removed in such a manner. A fourth embodiment of the present invention will now be described with reference to Figs. 8-10 and Fig. 12. A polymerizing method for manufacturing the toner particles can eliminate a pulverizing method and can achieve a high productivity compared with a pulverizing method and furthermore sizes of the toner particles can be controlled relatively with ease. Accordingly, it is possible to reduce the sizes of the toner particles to thereby contribute to obtaining a high resolution and a high quality image. The toner particles manufactured by the polymerizing method are spherical or substantially spherically shaped owing to the characteristics of its manufacturing method. The spherical toner particles have a strong Van der Waals attaching force to the photoconductor drum compared with indefinite toner particles in view of its shape, and are hardly caught by a blade, a brush, etc., which causes an inferior cleaning. The smaller the particle size is, the more remarkable this tendency is. There is proposed a method of forming desired shaped toner particles by cohering the minute toner particles which have sizes ranging from 1 to 4 [µm] which were obtained by the polymerizing method and successively by melting the minute particles at the contact points thereof (refer to Japanese Patent Laid-Open Publication No. 63-186253). However, this method complicates for manufacturing the toner particles and costs high. In view of the drawbacks of this method, described hereinafter is a method which is capable of using spherical toner particles which are manufactured by the polymerizing method and is cheap in running costs thereof. Data in the table of Fig. 8 show the result of employment of various toner particles by the image forming apparatus in Fig. 1. Toner particles as denoted at A, E and I are manufactured by the pulverizing method, at B to D, F to H and J to L are respectively manufactured by the polymerizing method. Styrene acrylic copolymer is employed as a binding resin. The amount of charging control agent is regulated so that the tine layer of the toner particles on the developing roller 44 has an average thickness of 20 [µm] and a specific charge per toner q/m establishes the expression of q/m = -10±1[µC/g]. If the average thickness of the toner layer is less than 5 [µm], the toner particles become in short supply so that a sufficient image density can not be obtained. If the average thickness of the toner layer exceeds 30 [µm], an electric field for collecting the toner particles by the developing roller 4 is weakened, so that the toner particles can not be sufficiently collected. If the specific charge per toner q/m is less than -5[µC/g], there is a likelihood of occurrence fog on the surface of the nonexposure portion, which leads to the deterioration of the image. If the specific charge per toner exceeds -20[µC/g], it becomes difficult to transfer the image toner, which causes an inferior transfer. S.d is a product of a BET ratio surface area S[m 2 /g] and a volume average particles size d[µm] and is a characteristic value representing the shape of the toner particles. That is, if the characteristic value S.d become greater, it means that the toner particles are more indefinite while if it becomes smaller, it means that the toner particles are more spherical. S/d is sometimes employed as the characteristic value representing merely the shapes of the toner particles. However, if S/d is employed as such, it is impossible to compare the shapes of those which have different average particles sizes with each other. Accordingly, the S.d is employed as the characteristic value in order to institute the comparison between the toner particles which have different average particle sizes. Fig. 9 is a view showing the relation between the characteristic value S.d and the toner particle deposit per unit area of the charging roller 4. The data in Fig. 9 is a result of test showing the deposit per unit area, i.e. the amount of toner particles attached to the surface of the charging roller (Fig. 1) after the completion of the continuous printing of the 500 pieces of sheets (A4 size) at [25%] duty cycle using various toner particles. Assume that the voltage of the power source 2b is -1.4 [kV], the surface potential of the photoconductor drum 1 is -840 [V] at the state where the toner particles are not supplied to the image forming apparatus, i.e. where the toner particles are neither attached to the charging roller 2 nor to the photoconductor drum 1. The voltage of the power source 4b is -300 [V] and the voltage of the power source 5b is +2 [kV]. As illustrated in Fig. 9, when the characteristic value S.d exceeds about 18, it is understood that the residual toner particles are stuck to the surface of the charging roller 2. If the characteristic value S.d exceeds about 20, it is confirmed that the toner particles remained on the surface of the charging roller 2 form a uniform layer having the thickness which ranges from 10 to 20 [µm] or more. If the characteristic value S.d is less than 18, the toner particles do not remain on the charging roller 2 even if the continuous printing of 10,000 pieces of sheets is performed. Any of the toner particles A to L which remain on the surface of the photoconductor drum 1 is collected by the developing roller 4, which leads to no generation of the afterimage caused by the inferior collection of the toner particles. Successively, another similar test was made under the condition that the voltage of the power source 2b is - 1.1[kV] or -1.6[kV]. This test revealed that there is approximately 2% difference between the mass of deposit per unit area, i.e. the amount of various toner particles to be stuck to the charging roller 2 under this test and that under previous test, i.e. the test as illustrated in Fig. 9. That is, the presence or the amount of the remaining toner particles stuck to the charging roller 2 is not much varied although the voltage variation of the power source 2b varies the electric field at the charging process, which shows that it depends largely on the characteristic value S.d. Fig. 10 is a view showing the relation between the characteristic value S.d and the surface potential of the photoconductor drum 1. The surface potential of the photoconductor drum 1 in Fig. 10 is measured before the exposure process starts upon completion of the charging process when the continuous printing is performed under the condition that the voltage of the power source 2b (Fig. 1) is -1.4[kV]. When the characteristic value S.d is less than 18, the amount of toner particles stuck to the charging roller 2 is substantially zero and the surface potential of the photoconductor drum 1 is - 840[V]±10[V]. If the characteristic value S.d exceeds 20, the surface potential of the photoconductor drum is decreased and much varied. This is caused by the fact that the voltage of the power source 2b is distributed to the dielectric layer of the photoconductor drum 1 and the toner layer on the charging roller 2. It seems that the degree of the variation is caused by the variation of the thickness of the toner layer and the density of filling of the toner particles in the longitudinal direction. In view of the grounds set forth above, if the characteristic value S.d exceeds 28, a solid image appears thick at a part of the non-image portion of the photoconductor drum 1. That is, the amount of the toner particles' to be stuck to the charging roller 2 should be substantially zero in order to stabilize the surface potential of the photoconductor drum 1 in the continuous operation. For this reason, it is necessary for the toner particles to be spherical or to have the shapes close to the spherical shapes. The following comparative test has been made in order to more clarify the phenomenon that the spherical toner particles are not liable to be stuck to the charging roller 2. Fig. 11 is a schematic view of an electrophotographic apparatus to which a conventional method for forming an image is applied and Fig. 12 is a view showing the relation between the characteristic value and density of toner particles caused by the inferior cleaning. A blade-type cleaning device 21 is provided at the side opposite to the photoconductor drum 1. The voltage of the power source 2 b is regulated so that the surface potential of the photoconductor drum 1 becomes -840 [V]. The cleaning device has a cleaning blade 21a which is formed of a urethane rubber having a thickness of 1.8 [mm], and has a hardness of JISA 70 o and a blade length of 11 [mm]. The cleaning blade 21a is disposed along a full width of the photoconductor drum 1 under the condition that an angle for positioning thereof relative to the photoconductor drum 1 is 24 o and deflection thereof is 2[mm]. Denoted at I.D. in the vertical axis of the graph in Fig. 12 is a reflection density representing the amount of toner particles which remain on the photoconductor drum 1 and are poorly cleaned before the developing process starts after passing the cleaning blade 21a provided that the continuous printing is performed in the same way as explained in Fig. 9 and 10 under the condition set forth above. The toner particles employed here are those as denoted at I to L as illustrated in Fig. 8. the graph shows that the toner particles which remain on the photoconductor drum 1 are liable to pass the cleaning blade 21a if the characteristic value S.d is less than 18.2 and are poorly cleaned, which increases the reflection density, i.e. I.D. If the characteristic value S.d exceeds 20, the toner particles are better cleaned, which renders the I.D. to be substantially zero. The result of test reveals the following: The spherical toner particles are not liable to be cleaned compared with the non-spherical toner particles. The reason of the increase of the poor cleaning is that the spherical toner particles are strong in the Van der Waals force to the photoconductor drum and the toner particles slip under the cleaning blade 21 because of the spherical shape. The Van der Waals force to the surface of particles generally depends on the random surface roughness of the particles. Accordingly, if the particle size is same, it is well known that the smoother the surface of the particle is, the stronger the sticking force is. The poor cleaning is specified using a threshold value, on the substantially same characteristic value S.d as illustrated in Fig. 9. It is evident that the toner particles remaining on the photoconductor drum are liable to remain on the photoconductor drum when they are stuck to the charging roller or the cleaning blade. The toner particles stuck to the charging roller 2 is not largely varied even of the electrostatic force which influences the toner particles remaining within the charged area, is varied. The Van der Waals force and the shapes of the toner particles affect largely the behaviour of the toner. As set forth above in detail, since the image carrier is charged with electricity by the charging roller while the latter contacts the surface of the former, there is no likelihood of generation of the harmful substance such as ozone which has been caused by the corona discharge and no likelihood of environmental pollution. Although the toner particles remain on the image carrier upon completion of the transferring process, these toner particles can be collected by the developing roller owing to the electrostatic force. Accordingly, it is not necessary to dispose of the collected toner particles, which improves the efficiency of using the toner particles. The residual toner particles remaining on the image carrier stick to the toner holding roller and are returned to the image carrier little by little as the toner holding roller rotates. Accordingly, the toner layer formed on the surface of the image carrier is thinned which can prevent the insufficient exposure of the image carrier and non-uniform charging of the photoconductor drum. If the turning direction of the developing roller is opposite to that of the image carrier and the peripheral velocity of the developing roller exceeds 1.2 times that of the image carrier, the efficiency of collecting the toner particles is enhanced and the efficiency of using the toner particles is improved. In case that the absolute value of the potential of the charging roller is lessened while no printing operation is performed, the toner particles which are stuck to the charging roller are stuck by the image carrier owing to the electrostatic force, whereby the amount of the toner particles which are stuck to the charging roller can be reduced. Accordingly, it is possible to charge the surface of the image carrier with electricity uniformly, whereby the stable image can be obtained. The charging unit may comprise the charging roller and the turning direction of the charging roller may be opposed to that of the image carrier. Furthermore, the peripheral velocity of the charging roller can be differentiated from that of the image carrier. That is, the peripheral velocity of the charging roller can be less than or greater than that of the image carrier. In this case, the toner particles remaining on the image carrier are moved while the image carrier is charged with electricity by the charging roller. Accordingly, it is possible to sufficiently collect the toner particles in the developing process since not only the portion to which the toner particles have been stuck before the movement of the toner particles but also the portion to which the toner particles are stuck after the movement are uniformly charged with electricity. If the peripheral velocity of the charging roller is less than that of the image carrier, there occurs an effect that the toner particles are stuck to the image carrier which has a high peripheral velocity so that the amount of the toner particles which are stuck to the charging roller can be reduced. As a result, the surface of the image carrier can be uniformly charged with electricity, whereby the stable image can be obtained. Since the charging member connected to the power source charges the surface of the image carrier with electricity in the charging process while the former contacts the surface of the latter, there is no likelihood of generation of the harmful substance such as ozone caused by corona discharge. The toner particles remaining on the image carrier at the time of completion of the transferring process can be collected owing to the electrostatic force before the transferring process starts after the completion of the charging process, e.g. in the developing process. Since the shapes of the toner particles are spherical and the characteristic value S.d which is given by the product of the BET ratio surface area S[m 2 /g] and the volume average particle size d [µm] is less than 18, the amount of toner particles stuck to the charging member can be reduced and the voltage which is applied by the power source connected to the charging member is not distributed to the toner particles on the charging member, whereby the surface potential on the image carrier can be stabilized and also the high resolution and high quality image can be obtained. Since the tone particles remaining on the image carrier can be collected owing to the electrostatic force before the transferring process starts after the completion of the charging process, for instance, in the developing process, they can be sufficiently collected regardless of the shapes thereof. Accordingly, it is possible to recycle the collected toner particles.

Claims: An image forming apparatus comprising: an image carrier (1); a charging unit (2a,2b) for electrostatically charging the surface of the image carrier (1); a latent image forming unit (3) for forming an electrostatic latent image on the charged surface of the image carrier (1); a developing unit (4) disposed adjacent to the image carrier (1) for developing the electrostatic latent image formed on the surface of the image carrier (1) to form a toner image; means (5) for transferring and fixing the toner image formed on the surface of the image carrier (1) to a transfer member (6); and a power source (4b) connected to the developing unit for electrostatically charging toner particles on the developing unit (4) with the same polarity as the charging polarity of the image carrier (1), and operative to set the potential of the developing unit (4) to a value capable of allowing the toner particles to adhere to an image portion of the image carrier (1) and of allowing the toner particles remaining on a non-image portion of the image carrier (1) to be attracted by the developing unit (4) away from the image carrier (1) characterised by a toner holding unit (7a,7b) disposed between the transfer means (5) and the charging unit (2a,2b) for attracting the toner particles thereto and for returning them to the image carrier (1). An image forming apparatus according to claim 1 wherein the charging unit (2a,2b) includes a charging roller (2) in contact with the image carrier (1). An image forming apparatus according to claim 1 or claim 2 wherein the developing roller (4a) rotates in an opposite direction to the image carrier (1), the peripheral speed of the developing roller (4a) exceeding 1.2 times that of the image carrier (1). An image forming apparatus according to any preceding claim wherein the charging unit (2a,2b) is equipped with means for reducing an absolute value of the potential of the charging unit (2a,2b) when the image carrier (1) is rotated and no printing operation is being performed. An image forming apparatus according to claim 2 wherein the charging roller rotates in an opposite direction to the image carrier (1) and the peripheral speed of the charging roller (2) is different from that of the image carrier (1). An image forming apparatus according to any preceding claim wherein the toner holding unit (7a,7b) includes a toner holding roller (7). An image forming apparatus according to claim 6 wherein peripheral speed of the toner holding roller (7) is equal to or greater than the peripheral speed of the image carrier (1). An image forming apparatus according to claim 6 or claim 7 wherein the toner holding roller (7) comprises a surface formed from semiconductive rubber or sponge having an electrical resistance of between 10 3 to 10 9 Ohms. An image forming apparatus according to any preceding claim, wherein the toner particles are spherical and have a characteristic value S.d which is a product of BET ratio surface area S (m 2 /g) and a volume average particle size d[µm] and which is less than 18. A method of forming an image comprising the steps of: (a) electrostatically charging the surface of an image carrier (1); (b) forming an electrostatic latent image on the charged surface of the image carrier (1); (c) developing the electrostatic latent image by sticking toner particles thereto to form a toner image; (d) transferring the toner image to a transfer member wherein the toner particles remaining on a non-image portion of the image carrier (1) after the transfer process are attracted to the developing unit (4), the toner particles on an image portion of the image carrier (1) remaining thereon, the method characterised by the step of attracting the toner particles to a toner holding unit (7a,7b) and returning them to the image carrier (1). A method according to claim 10 characterised by the step of positively charging the negatively charged toner particles that adhere to the toner holding unit (7a,7b) before returning them to the image carrier (1).

Inventors: ISHIHARA TORU, ITO KATSUYUKI, MURANO TOSHIRO, SAKAI MASATO, ISHIHARA, TORU, ITO, KATSUYUKI, MURANO, TOSHIRO, SAKAI, MASATO, Ishihara, Toru, c/o Oki Electric Ind. Co., Ltd., Ito, Katsuyuki, c/o Oki Electric Ind. Co., Ltd., Murano, Toshiro, c/o Oki Electric Ind. Co., Ltd., Sakai, Masato, c/o Oki Electric Ind. Co., Ltd.
Applicants: OKI ELECTRIC IND CO LTD, OKI ELECTRIC INDUSTRY COMPANY, LIMITED
Application year: 1992
Main classification: G03G15
IPC codes: G03G21, G03G15