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 are attracted by the developing roller. A charging roller (2) in contact with the image carrier (1) is contacted by a conductive metal blade (15).

Description: The present invention relates to an apparatus for and a method of forming an electrophotographic image, and mare particularly to those adapted for various business machines and instruments, especially for a storage device 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 uniformly charging an image carrier, i.e. a photoconductor drum with electricity, forming a latent image on the photoconductor drum, developing the latent image using a supply of toner particles, transferring the toner on the photoconductor drum to a transfer member, fixing the toner on the transfer member and removing the residual toner from the photoconductor drum. A known technique is applied to remove residual charge after the transferring process and before a subsequent charging process to prevent an afterimage from being formed on the photoconductor drum. The charging process and the transferring process generally utilise corona discharge. When corona discharge is used, ozone or some other harmful substance is produced and has to be collected by a filter. However, prolonged use of a filter causes a reduction in its ability to collect ozone efficiently and so its frequent replacement is required. An alternative process is known in which generation of ozone is prevented by providing a roller type transfer system or a charging roller system. Such a system is described in the Electronic Communication Institute Thesis '77/4 Vol. J60-C NO.4 pp 213-218. A roller type transfer system performs the steps of positioning atransfer medium on atoner image formed on the surface of a photoconductor drum, urging a transfer roller against the photoconductor drum and transfer medium and applying a voltage to the transfer roller having a polarity opposite to that of the toner. An electric field is generated in the gap between the transfer medium and the upper layer of the toner image whereby the toner is transferred to the toner medium due to the electrostatic force generated by the electric field. The charging roller system has the same principle as the roller type system and is used to charge the photoconductor drum with electricity. In this system, a voltage is applied to a charging roller so that an electric charge is applied directly to the photoconductor drum to prevent the generation of ozone. A conventional image forming system is known (see Japan Hardcopy '89 Thesis pp 143-146) in which a cleaning process is eliminated. The photoconductor drum is exposed to light having been uniformly charged with electricity by corona discharge, whereby the surface potential of the exposure portion is attenuated. Toner is stuck to the attenuated portion by reverse development whilst toner in a thin layer which remains on the photoconductor drum is collected. As the toner remaining on the non-exposure portion of the photoconductor drum after the transfer process has been completed is charged with electricity with the same polarity as in the developing process, the toner is attracted to the developing unit due to the electrostatic force resulting from the difference between the surface potential of the photoconductor drum and the developing bias. The above described process can substantially reduce the size of the imaging processing apparatus and residual toner collected in the developing process can be reused. A problem with the conventional image forming apparatus is that as the photoconductor drum and the charging roller are in contact, residual toner remaining on the photoconductor drum after the transfer step is attracted to the charging roller which reduces the ability of the charging roller to uniformly charge the surface of the photoconductor drum leading to deterioration in the print quality. An image forming apparatus is known from EP-A-0323252 comprising an image carrier, a charging unit including a charging roller in contact with the image carrier 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 thereby 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 operable 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, the charging unit including a conductive blade in contact with the charging roller and connected to a power source. A method of forming an image is also known and comprises the steps as claimed in claim 7. An image forming apparatus according to the present invention is characterised in that the conductive blade is operable to change the polarity of toner particles attracted to the charging roller from the image carrier, the toner particles being returned to the image carrier such that the toner particles remaining on the image carrier have the same polarity as the charging polarity of the image carrier. The developing unit preferably includes a developing roller which is disposed so as to contact the image carrier and is connected to a power source which charges toner particles on the developing unit with electricity having the same polarity as the charging polarity of the image carrier. The power source applies an electrical potential to the developing rollerto allow the toner particles to stick to an image portion of the image carrier and to allow toner particles remaining on a non-image portion of the image carrier to be attracted to the developing unit. Advantageously, 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. In a preferred embodiment, the absolute value of the potential of the charging roller can be decreased whilst no printing is being performed, or at the end of a printing operation. Furthermore, 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 addition, when a developing roller is used and the conductive blade is in contact with the charging roller, the power source connected to the developing roller charges the toner particles on the developing roller with electricity having the same polarity as that of the image carrier. The conductive blade and the charging roller are repeatedly connected to the power source which fits the potential of the conductive blade the same as that of the charging roller, with a large absolute value. Preferably, a power source applies an electric potential to the developing roller, allowing the toner particles to be attracted to an image portion of the image carrier and allowing the toner particles remaining on the non-image portion of the image carrier to be attracted to the developing unit. According to an aspect of the invention, there is provided a method of forming an image comprising the steps of negatively charging positively charged toner particles that are attracted to the charging roller from the image carrier, and returning the negatively charged toner particles to the image carrier so that the toner particles on the image carrier have the same polarity as the charging polarity of the image carrier. Embodiments of the invention will now be described, by way of example only, with reference to Figures 5 to 9 and 11 of the accompanying drawings in which: Figure 1 is a schematic view of 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 of 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 of an image forming apparatus according to a first embodiment of the present invention; Figure 6 is an enlarged view of the charging roller of the image forming apparatus of Figure 5; Figure 7 is a table showing the characteristic of toner particles employed by a image forming apparatus according to a second embodiment of the present invention; Figure 8 is a view showing the relationship between the characteristic value of toner particles and the amount of toner particles attached to the charging roller; Figure 9 is a view showing the relationship between the characteristic value of toner particles and the surface potential of the photoconductor drum; Figure 10 is a schematic view of an electrophotographic apparatus to which a conventional method for forming an image is applied; and Figure 11 is a view showing the relationship between the characteristic value and density of the toner particles. A prior art image forming apparatus will now be described with reference to Figures 1 to 4 of the accompanying drawings. A drum type image carrier, i.e. photoconductor drum 1 rotates in the direction of arrow A. According to the present embodiment, an organic photoconductor drum (hereinafter referred to 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 =3.5ε 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 unit having a charging roller 2 is formed of a conductive rubber and is located in contact with the photoconductor drum 1 at a given pressure. The charging roller 2 may be rotatably driven by driving means, not shown, alternatively, it may be driven by the friction with the photoconductor drum. The electric resistance of the charging roller 2 is set to be 10 5 [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 10 4 to 10 9 [Ω]. The electric resistance means that between the contacting plane where the charging roller 2 contacts the photoconductor drum 1 (an area as large as nip width x longitudinal length). A conductive shaft 2a supports the charging roller 2 and is supplied with a voltage from a power source 2b. A latent image forming unit 3 subjects the photoconductor drum 1 to light in response to a printing signal and draws an electrostatic latent image comprising an exposure potion and a non-exposure portion on the surface of the photoconductor drum 1. The latent image forming unit may be an LED, but it may also be a laser scanning unit or a liquid crystal shutter array. A toner carrier, i.e. a developing unit 4 constituting a developing roller 4a contacts the photoconductor drum 1 at a given pressure and rotates in the direction of the arrow B. The developing roller 4a is formed of a conductive rubber and its electric resistance is set to be 10 6 [Ω] but may be set in the range 10 0 to 10 9 [Ω]. If the electric resistance is too low, a large amount of current flows into the developing roller 4a when the surface of the developing roller is in direct contact with the photoconductor drum 1 and the drum 1 has a pin hole or a small amount of toner on its surface. On the contrary, if the electric resistance is too high, the developing efficiency is lowered and a low density image is liable to occur. Accordingly, the electric resistance is preferable to range from 10 4 to 10 8 [Ω]. The electric resistance means that between the contacting plane where the surface of the developing roller 4a contacts the photoconductor drum 1 and the conductive shaft 2a. Toner particles are laminated to several tens µm thick on the developing roller 4a and enter a developing area where the developing roller 4a is in contact with the photoconductor drum whereby 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 4a. In this case, the exposure portion to which toner particles are stuck form an image portion while the nonexposure portion to which toner particles are not stuck forms a non-image portion. A power source 4b applies a voltage to a shaft 4c of the developing roller 4a. The power source 4b applies an electrical 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 4a. A transfer unit including a transfer roller 5 is in contact with the image carrier 1 transfers a toner image on the photoconductor drum 1 to a transfer medium 6 which is conveyed in the direction of arrow C. The transfer medium 6 may be recording paper. The electric resistance of the transfer roller 5 means that between the contacting plan 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 in a range of approximately 10 0 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 medium 6 has a width less than that of the photoconductor drum 1 and the transfer roller 5, sufficient electric field may not be obtained 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 also causes poor transfer to the transfer medium. The transfer medium 6 to which the toner image is transferred is separated from the photoconductor drum and is introduced into a fixing unit, not shown. The transfer medium 6 is discharged from the image forming apparatus upon completion of the fixing process. A power source 5b applies a voltage to the conductive shaft 5a. As illustrated in Figure 2, a control portion 11 of the image forming apparatus supplies a printing signal to the latent image forming unit 3 so than an LED array head emits light upon reception of the printing signal. It also supplies a signal to the photoconductor drum 1 to cause it to rotate, and a high voltage signal to the power sources 2b, 4b and 5b to set the potential of the charging roller 2, the developing roller 4a and the transfer roller 5 to the appropriate values, respectively. An operation of the prior art image forming apparatus will now be described with reference to Figures 3 and 4. In Figure 4, toner particles 12A adhere to the image portion of the photoconductor drum 1 from the surface of the developing roller 4a. The toner particles 12b remain on the surface of the photoconductor drum 1 upon completion of the transfer of the toner image onto the transfer member 6 (Figure 1). Since the image forming apparatus has no cleaning means such as a blade or brush, the toner particles 12b on the surface of the photoconductor drum 1 form a residual 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 exposed to light and a latent image is drawn. 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, which has a potential intermediate between the potential of the exposure and nonexposure portions of the photoconductor drum 1. Accordingly, the toner particles 12a remaining on the nonexposure portion are attracted by the developing roller 4a owing to the electrostatic force as illustrated in Figure 4 and are collected by the developing unit. The toner particles 12b remaining on the exposure portion are not collected by the developing unit but remain stuck to the photoconductor drum 1. The toner particles 12a on the developing roller 4a are attracted by the photoconductor drum 1 to develop the latent image on the photoconductor drum 1 and form a toner image. The toner image on the photoconductor drum 1 is subsequently transferred to the transfer medium 6 by the transfer roller 5, to complete one cycle of the image forming operation. If the peripheral velocity of the developing roller 4a 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, experimental data has shown that the toner particles 12b on the photoconductor drum 1 move toward the developing roller 4a, leading 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 less toner particles are supplied to the developing roller 4a to form a thin toner layer thereon, the residual toner particles 12b are supplied additionally to the thin toner layer. A problem with the prior art image forming apparatus described above is that some of the residual toner particles 12b remaining on the surface of the photoconductor drum 1 after the transfer process adhere to the charging roller 2 resulting in non-uniform charging of the photoconductor drum 1 and insufficient development of a toner image and the generation of a poor quality image on the transfer medium. A first embodiment of the present invention will now be described with reference to Figure 5 of the accompanying drawings in which non-uniform charging of the photoconductor drum 1 caused by a build up of residual toner particles on the charging roller can be prevented or substantially reduced. A negative type OPC photoconductor drum 1 rotates in the direction of arrow A. The charging roller 2 has a layer of semiconductive rubber 2c around the conductive shaft 2a. The semiconductive rubber layer 2c has a volumetric resistance value which ranges from 10 5 to 10 10 [Ω]. The charging roller 2 rotates in the opposite direction to the photoconductor drum 1. The peripheral velocity of the charging roller 2 is less than that of the photoconductor drum 1 and the former is set to be 0.95 to 0.5 times the latter. The power source 2b is connected and supplies a voltage to the conductive shaft 2a. A conductive blade 15 in the form of a flexible metal plate is fixed so as to press against the surface of the charging roller 2 and is connected to a power source 16. The power source 2b has a voltage of approximately - 1000 [V] and the power source 16 has a voltage of approximately -1200[V] in order to charge the photoconductor drum 1 uniformly with a potential of -600[V]. That is, a potential difference of between -50[V] to -300[V] is applied between the charging blade 15 and the charging roller 2. The arrangements of the latent forming unit 3, the developing roller 4a, the transfer roller 6 and the power source are same as previously described. Operation of the first embodiment will now be described. Toner particles 12b having both positive and negative polarities remaining on the photoconductor drum 1 after transfer of a toner image onto the transfer medium 6 enter a uniformly charged area where the photoconductor drum 1 is in contact with the charging roller 2. The charging roller 2 is charged by the power source 16 and carries negative polarity relative to the photoconductor drum 1. Accordingly, the charging roller 2 charges the photoconductor drum 1 with electricity and attracts the positively charged toner particles 12b owing to the electrostatic force. The negatively charged toner particles 12b remain on the photoconductor drum 1 and pass the uniformly-charged area between the charging roller 2 and the photoconductor drum 1. The peripheral velocity of the charging roller 2 is 0.95 to 0.5 times that of the photoconductor drum 1 which causes some of the residual toner particles 12b stuck to the charging roller 2 to move back onto the photoconductor drum 1 rotating at higher speed. If the difference between the velocity of the charging roller 2 and the photoconductor drum 1 is increased, the amount of the toner particles 12b that move to the photoconductor drum 1 from the charging roller 2 also increases, but the mechanical load applied to the photoconductor drum 1 is increased due to friction. When positively charged toner particles on the charging roller 2 pass the conductive blade 15, they become negatively charged as the potential applied to the conductive blade 15 by a power source 16 is such that it carries negative polarity relative to the charging roller 2. As the charging roller 2 continues to rotate, negatively charged toner particles 12b enter the uniformly charged area between the charging roller 2 and the photoconductor drum and move toward the photoconductor drum 1. Thus, the density of the toner particles 12b stuck to the charging roller 2 is kept to a minimum and uniform charging of the photoconductor drum 1 can be maintained. As the toner particles 12b remaining on the photoconductor drum 1 contact the developing roller 4a, the power source 4b applies an intermediate potential between that of the nonexposure portion and that of the exposure portion of the photoconductor drum 1 that is applied to the developing roller 4a. Accordingly, the toner particles 12b remaining on the nonexposure portion adhere to the developing roller 4a owing to the electrostatic force and are collected by the developing unit and the toner particles move to the exposure portion from the developing roller 4a. The toner image on the photoconductor drum 1 is subsequently transferred to the transfer medium 6 by the transfer roller 5, to complete one cycle of the image forming operation. A second embodiment of the present invention will now be described with reference to Figures 7 to 10. A polymerizing method for manufacturing toner particles can eliminate a pulverizing method and can achieve a high productivity. Furthermore the size of the toner particles can be controlled with relative 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 spherical shaped and have strong Van de Waals attaching forces. Compared with toner particles of indefinite shape they are much easier to clean from a surface using a blade, or a brush. A method of forming desired shape 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 is proposed in Japanese Patent Laid-Open Publication No. 630186253). However, this method is complicated and expensive. In view of the drawbacks of the known method, a method will now be described which is capable of using spherical toner particles manufactured by the polymerizing method. The second embodiment will be described hereinafter with reference to Figures 7 to 11 which show characteristics of the toner particles used in the image forming apparatus. Toner particles as denoted at A, E and I (Figure 7) 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 thin layer of the toner particles on the developing roller has an average thickness of 20 [µm) and a specific charge pertoner q/m establishes the expression of q/m = 10 ± 1 [µC/g]. If the average thickness of the toner layer is less than 15 [µm], the toner particles are in short supply and a sufficient image density cannot be obtained. If the average thickness of the toner layer exceeds 30 [µm], the electric field for collecting the toner particles by the developing roller 4a is weakened, so that the toner particles cannot be sufficiently collected. If the specific charge per toner q/m is less than -5 [µC/g], there is a likelihood of the occurrence fog on the surface of the nonexposure portion, which leads to deterioration of the image. If the specific charge per toner exceeds -20 [µC/g], it also becomes difficult to transfer the image onto the recording medium, which causes an inferior transfer. S.d is a product of the BET ratio surface area S [m 2 /g] and a volume average particle size d[µm] and is a characteristic value representing the shape of the toner particles. That is, if the characteristic value S.d becomes 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 shape 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 particle 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. A view of the relationship between the characteristic value S.d and the toner particle deposit per unit area of the charging roller 2 is shown in Figure 8. Figure 8 is a result of a test showing the deposit per unit area, i.e. the amount of toner particles attached to the surface of the charging roller after the completion of the continuous printing of 500 sheets (A4 size) at [25%] duty cycle using various toner particles. Assuming 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 not 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 Figure 8, when the characteristic value S.d exceeds about 18, the residual toner particles are stuck to the surface of the charging roller 2. If the characteristic value S.d exceeds about 20, the toner particles remain on the surface of the charging roller 2 and form a uniform layer having a 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 continuous printing of 10,000 sheets is performed. Any toner particles A to L which remain on the surface of the photoconductor drum 1 are collected by the developing roller 4a, which leads to no generation of an afterimage caused by an inferior collection of the toner particles. Another similar test was conducted in which 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 the deposit per unit area, i.e. the amount of the various toner particles stuck to the charging roller 2 under this test and that under the previous test. That is, the amount of toner particles stuck to the charging roller 2 does not vary greatly despite the voltage variation of the power source 2b, showing that the amount of particles depends largely on the characteristic value S.d. Figure 9 is a view showing the relationship between the characteristic value S.d and the surface potential of the photoconductor drum 1. The surface potential of the photoconductor drum 1 in Figure 9 is measured before the exposure process starts and upon completion of the charging process when the continuous printing is performed under the condition that the voltage of the power source 2b is -1.4 [Kv]. When the characteristic value S.d is less than 18, the amount of toner particles stuck to the charging roller is substantially zero and the surface potential of the photoconductor drum 1 is -840[V]±[V]. If the characteristic value S.d exceeds 20, the surface potential of the photoconductor drum 1 is decreased and varies greatly. 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. 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. 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 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 stick to the charging roller 2. Figure 10 is a schematic view of an electrophotographic apparatus to which a conventional method of forming an image is applied and Figure 11 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 2b 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° and a blade length of 11 [mm]. The cleaning blade 21 is disposed along a full width of the photoconductor drum 1 at an angle relative to the photoconductor drum 1 of 24° and a deflection of 2 [mm]. Denoted at I.D. in the vertical axis of the graph in Figure 11 is a reflection density representing the amount of the 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 Figure 8 to 9 under the condition set forth above. The toner particles employed here are those denoted as I to L as illustrated in Figure 7. 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 the 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 1 and the toner particles slip under the cleaning blade 21 because of the spherical shape. The Van der Waals force to the surfaces of particles generally depends on the random surface roughness of the particles. Accordingly, if the particle size is the 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 Figure 8. 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 does not vary largely even if 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 in contact with the surface of the image carrier 1, the generation of ozone is prevented. Although the toner particles remain on the image carrier upon completion of the transfer process, and some of them are attracted to the charging roller 2, the uniform charging of the photoconductor drum is not hindered as the polarity of the toner particles on the charging roller is charged so that they are returned to the image carrier. As shapes of the toner particles are preferably 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 the 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 so a high resolution and high quality image can be obtained.

Claims: An image forming apparatus comprising: an image carrier (1); a charging unit (2a, 2b) including a charging roller (2) in contact with the image carrier (1) 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 thereby form a toner image; means (5) fortransferring 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 operable 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), the charging unit (2a, 2b) including a conductive blade (15) in contact with the charging roller (2) and connected to a power source (16), characterised in that the conductive blade is operable to change the polarity of toner particles attracted to the charging roller (2) from the image carrier, the toner particles being returned to the image carrier (1) such that the toner particles remaining on the image carrier (1) have the same polarity as the charging polarity of the image carrier (1). An image forming apparatus according to claim 1, wherein the conductive blade (15) is a metal blade. An image forming apparatus according to claim 1 or 2 wherein the developing unit (4) includes a developing roller (4a) that 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 any preceding claim wherein the charging roller (2) 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 potential of the conductive blade (15) is the same as that of the charging roller (2). A method of forming an image comprising the steps of: (a) electrostatically charging the surface of an image carrier using a charging unit (2a,2b) having a charging roller (2) in contact with the image carrier (1) and a conductive element (15) in contact with the charging roller (2); (b) forming an electrostatic image on the charged surface of the image carrier (1); (c) developing an electrostatic latent image by sticking toner particles thereto to form a toner image; (d) transferring the toner image to a transfer member; and 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 on the image carrier (1), the method being characterised by using the conductive element (15) to negatively charge positively charged toner particles that are attracted to the charging troller (2) from the image carrier, and returning the negatively charged toner particles to the image carrier (1) so that the toner particles on the image carrier (1) have the same polarity as the charging polarity of the image carrier (1). A method of forming an image according to claim 7, wherein the toner particles are spherical and have a characteristic 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.

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