Patent Application
20160048096
1. Field of the Invention
The present invention relates to a color image forming apparatus using an electrophotography process or the like.
2. Description of the Related Art
An image forming apparatus configured to use an intermediate transfer member, such as a copier and a laser beam printer, is known. In such an image forming apparatus, in a primary transfer step, a toner image formed on the surface of a photosensitive drum serving as an image bearing member is transferred onto the intermediate transfer member by applying a voltage by a voltage source to a primary transfer member disposed opposite the photosensitive drum. In a full color printer in which a color image constituted by a plurality of colors is formed, a toner image constituted by a plurality of colors is formed on the intermediate transfer member surface by executing this primary transfer step with respect to each color and overlapping the toner images of the respective colors. In a secondary transfer step, a toner image of a plurality of colors which is formed on the intermediate transfer member surface is transferred onto the surface of a recording material such as paper by applying a voltage to the secondary transfer member. A color image is then formed by permanently fixing the transferred toner image to the recording material by a fixing means.
Japanese Patent Application Laid-open No. 2012-98709 discloses a configuration in which primary transfer is performed by using a belt-shaped member (referred to hereinbelow as “intermediate transfer belt”) as the intermediate transfer member and applying a voltage to a current supply member that is in contact with an outer circumferential surface of the intermediate transfer belt at a position which is set apart from the primary transfer region. In such a configuration, the primary transfer of a toner image from the photosensitive drum surface to the intermediate transfer belt is performed in image forming stations by using a secondary transfer member as the current supply member and allowing a current to flow from the current supply member to the intermediate transfer belt in the circumferential direction of the belt. Such a configuration makes it possible to remove a high-voltage power supply dedicated to the primary transfer from the apparatus configuration and reduce the cost and size of the image forming apparatus.
However, in the abovementioned configuration, in a monochromatic mode in which only a black station is used, a primary transfer current still flows in the photosensitive drums of color stations which are not used. As a result, the primary transfer current in the black station can be insufficient. Since the primary transfer is performed by allowing the secondary transfer current to flow in the belt circumferential direction on the intermediate transfer belt with respect to the photosensitive drum, a voltage cannot be switched independently for each image station. Thus, in the monochromatic mode in which a current flows also in the color stations which are not used, it is difficult to allow a suitable primary transfer current to flow in the photosensitive drum of the black station.
It is an objective of the present invention to provide an image forming apparatus that performs primary transfer by allowing a current to flow to an intermediate transfer belt in the circumferential direction of the belt, the apparatus being capable of ensuring good primary transferability.
In order to attain the abovementioned objective, the image forming apparatus in accordance with the present invention, comprising:
Further features of the present invention will become apparent from the following description of exemplary embodiments (with reference to the attached drawings).
Modes for carrying out the invention will be explained hereinbelow in greater details on the basis of examples thereof with reference to the drawings. The dimensions, materials, shapes, mutual arrangements and the like of the constituent components described in the embodiments should be changed according to the configuration of the apparatus to which the invention is applied, and various conditions. Thus, the scope of the invention is not intended to be limited to the below-described embodiments.
[General Configuration of Image Forming Apparatus]
The configuration and operation of the image forming apparatus of the present embodiment will be explained hereinbelow with reference to
The first image forming station (a) is provided with a drum-shaped electrophotographic photosensitive member (referred to hereinbelow as “photosensitive drum”) 1a, a charging roller 2a which is a charging member, a developing device 4a, and a cleaning device 5a. The photosensitive drum la is an image bearing member that is rotationally driven at a predetermined circumferential speed (process speed) in the direction shown by an arrow and bears a toner image. The developing device 4a serves for developing a yellow toner at the photosensitive drum 1a accommodating the yellow toner. The cleaning device 5a serves for recovering the toner that has adhered to the photosensitive drum 1a. In the present embodiment, the cleaning device 5a is provided with a cleaning blade which is a cleaning member that is in contact with the photosensitive drum 1a, and a waste toner box which accommodates the toner recovered by the cleaning blade.
A CPU (control unit) 9 which is a control IC of the image forming apparatus including a controller or the like starts an image forming operation upon receiving an image signal and rotationally drives the photosensitive drum 1a. The photosensitive drum 1a is subjected to uniform charging treatment to a predetermined potential at a predetermined polarity (negative polarity in the present embodiment) by the charging roller 2a in the rotation process and subjected to exposure corresponding to the image signal by an exposure means 3a. As a result, a latent electrostatic image corresponding to the yellow color component image of the target color image is formed. Then, this latent electrostatic image is developed by the developing device (yellow developing device) 4a at the development position and visualized as a yellow toner image. In this case, the normal charging polarity of the toner accommodated in the developing device is a negative polarity. In the present embodiment, the latent electrostatic image is inversely developed by the toner charged to the same polarity as the electrostatic polarity of the photosensitive drum by the charging member, but the present invention can be also used in an electrophotographic device in which a latent electrostatic image is normally developed by a toner charged to a polarity inverted with respect to the charging polarity of the photosensitive drum.
An intermediate transfer belt 10 is tensioned (supported) by a plurality of rollers 11, 12, 13, which serve as tension members (support members), and rotationally driven at a circumferential speed substantially equal to that of the photosensitive drum 1a in the direction matching the movement direction of the photosensitive drum 1a by a contact region which is in contact with the photosensitive drum 1a. The yellow toner image formed on the photosensitive drum 1a is transferred (primary transfer) onto the intermediate transfer belt 10 in the process of passing through the contact region (referred to hereinbelow as “primary transfer region”) of the photosensitive drum 1a and the intermediate transfer belt 10. In the present embodiment at the time of primary transfer, an electric current flows in the circumferential direction of the intermediate transfer belt 10 from a secondary transfer roller 20 serving as a current supply member that is in contact with the intermediate transfer belt 10, and a primary transfer potential is formed in each primary transfer region of the intermediate transfer belt 10. A method for forming the primary transfer potential in the present embodiment is explained hereinbelow. A primarily untransferred toner remaining on the surface of the photosensitive drum 1a is cleaned and removed with the cleaning device 5a and then supplied to an image forming process performed after charging.
Likewise, a magenta toner image of the second color, a cyan toner image of the third color, and a black toner image of the fourth color are formed by the second, third, and fourth image forming stations (b), (c), and (d) and successively transferred in an overlapping manner onto the intermediate transfer belt 10. As a result, a composite color image corresponding to the target color image is formed on the intermediate transfer belt 10. The four-color toner image on the intermediate transfer belt 10 is entirely transferred (secondarily transferred) to the surface of a recording material P such as paper which is supplied by a paper supply means 50 as the four-color toner image passes through a secondary transfer region formed by the intermediate transfer belt 10 and the secondary transfer roller 20 serving as a secondary transfer member. The recording material P bearing the four-color toner image is introduced into a fixing unit 30 where the toners of four colors are melted, mixed, and fixed to the recording material P by heating and pressurization. The toner remaining on the intermediate transfer belt 10 after the secondary transfer is cleaned and removed with a cleaning device 16. The above-described operations result in the formation of a full-color printed image.
[Configuration of Primary Transfer Region]
The intermediate transfer belt 10, the rollers 11, 12, 13 serving as tension members, and the metal roller 14 which are necessary for forming a primary transfer potential in each primary transfer region will be explained hereinbelow with reference to
As depicted in
As depicted in
The metal roller 14 is constituted by a nickel-plated SUS round rod of a straight shape with an outer diameter of 6 mm and rotates following the rotation of the intermediate transfer belt 10. The metal roller 14 comes into contact over a predetermined area in the longitudinal direction which is perpendicular to the movement direction of the intermediate transfer belt 10. The distance between the photosensitive drum 1b of the second image forming station (b) and the photosensitive drum 1c of the third image forming station (c) is denoted by W, the distance between the photosensitive drum 1b and the metal roller 14 is denoted by T, and the lift height of the metal roller 14 with respect to the intermediate transfer belt 10 is denoted by H1. The distance, as referred to herein, is a distance between the adjacent axial centers in the movement direction of the intermediate transfer belt 10. In the present embodiment W=50 mm, T=25 mm, and H1=2 mm.
Further, as depicted in
The intermediate transfer belt 10 which is used in the present embodiment uses an endless polyimide resin mixed with carbon as an electrically conductive agent and has a circumferential length of 700 mm and a thickness of 90 μm. In the present embodiment, the polyimide resin is used as a material for the intermediate transfer belt 10, but any other material may be used, provided it is a thermoplastic resin. For example, materials such as polyesters, polycarbonates, polyacrylates, acrylonitrile-butadiene-styrene copolymers (ABS), polyphenylene sulfides (PPS), and polyvinylidene fluoride (PVdF), and mixed resins thereof may be used. Further, fine electrically conductive metal oxide particles and ionic conducting agents can be used as the electrically conductive agent instead of carbon.
The intermediate transfer belt 10 of the present embodiment has a volume resistivity of 1×109 Ω·cm. The volume resistivity is measured using a ring probe type UR (system MCP-HTP12) in Hiresta-UP (MCP-HT450) manufactured by Mitsubishi Chemicals Co., Ltd. As for the measurement conditions, the indoor temperature is set to 23° C., the indoor temperature is set to 50%, the applied voltage is 100 V, and the measurement time is 10 sec. In the present embodiment, the intermediate transfer belt 10 with a volume resistivity within a range of 1×107 to 1×1010 Ω·cm can be used. The volume resistivity used herein is a measure of electric conductivity of the material of the intermediate transfer belt 10, and the value of resistance in the circumferential direction is important when determining whether or not the desired primary transfer potential can be actually formed by allowing an electric current to flow in the circumferential direction.
The measurement method is explained hereinbelow. In a state in which the intermediate transfer belt 10 is rotated by the driver roller 102 at a speed of 100 mm/sec, a constant current IL is applied to the inner surface roller 101 and voltage VL is monitored with the high-voltage power supply 103 connected to the inner surface roller 101. The measurement system depicted in
In the configuration according to the present embodiment, the intermediate transfer belt 10 is used that has a resistance value of 1×106 Ω in the circumferential direction which has been determined by the above-described measurement method. The measurements with respect to the intermediate transfer belt 10 of the present embodiment were conducted at a constant current of IL=5 μA, and the monitor voltage VL at this time was 3.25 V. The monitor voltage VL is determined within an interval of one rotation of the intermediate transfer belt 10 from the average value of the values measured in this interval. Concerning RL, since RL=2VL/IL, RL=2×3.25/(5×10−6)=1.5×106 Ω, and where this value is recalculated for an equivalent of 100 mm, the resistance value in the circumferential direction becomes 0.5×106 Ω. In the present embodiment, a conductive belt in which the electric current can thus be allowed to flow in the circumferential direction is used as the intermediate transfer belt 10.
[Method for Forming Primary Transfer Potential]
In the configuration according to the present embodiment, the secondary transfer power supply 21 that applies a voltage as a transfer power supply to the secondary transfer member is also used as a power supply for performing the primary transfer. Thus, the secondary transfer power supply 21 is a transfer power supply that supplies an electric current to the primary transfer region of the secondary transfer roller 20 and the intermediate transfer belt 10, this power supply being used commonly for the primary transfer and the secondary transfer. The secondary transfer roller 20 is a current supply member in the present embodiment.
As described hereinabove, the primary transfer is performed by allowing a current to flow from the secondary transfer power supply 21 to the photosensitive drums 1a to 1d through the secondary transfer roller 20 and in the belt circumferential direction of the intermediate transfer belt 10. At his time, a primary transfer potential is formed in the image forming stations (a), (b), (c), (d), and the toner on the photosensitive drums 1a to 1d moves on the intermediate transfer belt 10 under the effect of the difference between the primary transfer potential and the photosensitive drum potential, thereby performing the primary transfer.
The configuration of the secondary transfer region in the present embodiment is explained hereinbelow. The secondary transfer roller 20 serving as a secondary transfer member in the secondary transfer region has an outer diameter of 18 mm and is obtained by coating a nickel-plated steel rod with an outer diameter of 8 mm with a foamed spongy body having a volume resistance of 108 Ω·cm and a thickness of 5 mm and including an NBR and an epichlorohydrin rubber as the main components. Further, the secondary transfer roller 20 is brought into contact with the outer circumferential surface of the intermediate transfer belt 10 by a pressurizing force of 50 N and forms the secondary transfer region. The secondary transfer roll 20 is rotationally driven with respect to the intermediate transfer belt 10 and configured such that a constant electric current is supplied from the transfer power supply 21 when the toner located on the intermediate transfer belt 10 is secondarily transferred onto the recording material P such as paper.
The transfer power supply 21 is configured to be connected to the secondary transfer roller 20 and supply a secondary transfer voltage outputted from a transformer (not shown in the figure) to the secondary transfer roller 20. The CPU 9 which is the control IC of the image forming apparatus controls the secondary transfer voltage supplied by the transfer power supply 21 by performing feedback of a difference between a preset control current and a monitor current which is an actual output value, such that the secondary transfer current becomes substantially constant. The transfer power supply 21 can output a voltage within a range of 100 V to 4000 V.
The CPU 9 can execute a full-color mode in which a toner image is primarily transferred from the photosensitive drums 1a, 1b, 1c, 1d of all of the stations to the intermediate transfer belt 10, and a monochromatic mode in which a toner image is primarily transferred from the photosensitive drum 1d which is a photosensitive drum of a specific station to the intermediate transfer belt 10. In this case, the specific photosensitive drum 1d is taken as a first photosensitive member, and other photosensitive drums 1a, 1b, 1c are taken as second photosensitive members.
The value of the secondary transfer current in the present embodiment is set to 20 μA according to the transfer efficiency of the secondary colors in the full color mode and to 14 μA according to the monochromatic transfer efficiency (in order to form only a monochromatic image) in the monochromatic mode.
Further, in the configuration of the present embodiment, the intermediate transfer belt 10 and the photosensitive drums 1a, 1b, 1c of the color stations are not separated from each other in the monochromatic mode to reduce cost and dimensions. In other words, in the monochromatic mode, the photosensitive drums 1a, 1b, 1c of the color stations are also brought into contact with the intermediate transfer belt 10, together with the photosensitive drum 1d of the black station (d).
[Features of Potential Control Method in the Present Embodiment]
The specific feature of the present embodiment is that in the monochromatic mode (during single transfer), the absolute value of the charging voltage of the photosensitive drums 1a, 1b, 1c of the color image forming stations (a), (b), (c) which are not used is lowered with respect to that of the photosensitive drum 1d of the black station (d) which is used. As a result, the absolute value of the charging potential of the photosensitive drums 1a, 1b, 1c (second photosensitive members) is reduced with respect to that of the charging potential of the photosensitive drum 1d (first photosensitive member). Explained hereinbelow is the case in which the comparison in the relationship of negative polarities is performed by the absolute values.
In the present embodiment, the charging rollers 2a to 2d are obtained by coating a nickel-plated steel rod with an outer diameter of 6 mm with a nitrile butadiene rubber (NBR) with a thickness of 3 mm as an elastic layer and then coating a polyurethane resin with a thickness of 10 μm as a surface layer. The volume resistivity of the elastic layer is adjusted to 10̂4 Ω·cm and the volume resistivity of the surface layer is adjusted to 10̂11 Ω·cm. The charging roller 2a is brought into contact with the photosensitive drum 1a by a pressurizing force of 6 N and is rotationally driven with respect to the photosensitive drum 1a. A DC voltage is applied by the charging high-voltage power supplies 22a, 22b, 22c, 22d to the charging rollers 2a to 2d. The configuration that charges the photosensitive drums corresponds to the charging device in accordance with the present invention.
In the full color mode (multiple transfer), the charging voltage (charging bias) supplied by the charging high-voltage power supplies 22a to 22d to the charging rollers 2a to 2d is controlled such that the charging voltage of all of the charging rollers 2a to 2d becomes −1000 V. As a result, the charging potential (surface potential) formed on the surface of each photosensitive drum 1a to 1d is −500 V.
In the monochromatic mode, a method for controlling the charging potential of the photosensitive drum differs between the black station (d) and the color image forming stations (a), (b), (c), which are image forming stations other than the black station (d). More specifically, in the black station (d), the charging voltage of the charging roller 2d is controlled to −1000 V which is the same as in the full color mode. Meanwhile, in the color image forming stations (a), (b), (c), the charging voltage of the charging rollers 2a, 2b, 2c is controlled to −800 V. As a result, the charging potential of the photosensitive drums 1a, 1b, 1c of the color image forming stations (a), (b), (c) is −300 V, and the charging potential of the photosensitive drum 1d in the black station (d) is −500 V. Thus, the charging potential of the photosensitive drums 1a, 1b, 1c of the color image forming stations (a), (b), (c) decreases (absolute value decreases) with respect to that of the photosensitive drum 1d of the black image forming station (d).
[Operations Performed by Potential Control Method in the Present Embodiment]
In the configuration of the present embodiment, the charging voltage of the color image forming stations (a), (b), (c) is decreased in the monochromatic mode to lower the surface potential of the photosensitive drums 1a, 1b, 1c and prevent the primary transfer current of the black stations (d) from decreasing.
Table 1 shows the current value of the current supply member, the values of the primary transfer current flowing in the image forming stations (a) to (d), and the primary transfer efficiency in each printing mode in the present embodiment and a comparative example, those results being used for comparative evaluation of the present embodiment and the comparative example. By contrast with the configuration of the present embodiment, in the configuration of the comparative example, the charging voltage control in the monochromatic mode is performed such that the charging voltage of the image forming stations (a) to (d) is controlled to −1000 V in the same manner as in the full color mode. As a result, the surface potential of each of the photosensitive drums 1a to 1d becomes −500 V. Other features are the same as in Embodiment 1.
The evaluation results are explained below. In the full color mode, the control is performed at a current of 20 μA from the current supply member in both the present embodiment and the comparative example.
The evaluation results obtained in the monochromatic mode with the configuration of the present embodiment are explained hereinbelow. In the monochromatic mode, the current value of the current supply member is decreased from 20 μA of the full color mode to 14 μA to improve secondary transferability. Further, as mentioned hereinabove, in the configuration of the present embodiment, since the intermediate transfer belt 10 and the photosensitive drums 1a, 1b, 1c are not separated from each other in the monochromatic mode, the total amount of the electric current flowing in the image forming stations is also decreased. However, in the configuration of the present embodiment, the charging potential of the photosensitive drums 1a, 1b, 1c of the color image forming stations (a), (b), (c) is decreased to −300 V, as compared to the charging potential −500 V of the photosensitive drum 1d of the black station (d). As a result, the primary transfer current flowing in the color image forming stations (a), (b), (c) can be reduced to 3 μA. Therefore, the amount of primary transfer current flowing in the black station (d) can be maintained at 5 μA in the same manner as in the full color mode. As a consequence, good primary transferability can be ensured in the black station (d).
In the configuration of the comparative example, the current value of the current supply member in the monochromatic mode is also reduced to 14 μA in the same manner as in the embodiment. In this case, the charging potential of each of the photosensitive drums 1a, 1b, 1c, 1d becomes −500 V, and therefore, the image forming stations (a), (b), (c), (d) have the same primary transfer current value of 3.5 μA. As a result, since the primary transfer current value of the clack station (d) is 3.5 μA, the primary transfer efficiency decreases and transfer defects occur.
As explained hereinabove, with the configuration of the present embodiment, the charging voltage of the color image forming stations (a), (b), (c) is decreased in the monochromatic mode to lower the charging potential of the photosensitive drums 1a, 1b, 1c. As a result, the decrease in the primary transfer current of the black station (d) can be suppressed and, therefore, good primary transferability can be ensured.
In the configuration of the present embodiment, a method for decreasing the voltage of the charging rollers 2a, 2b, 2c is described as a method for lowering the charging potential of the photosensitive drums 1a, 1b, 1c in the monochromatic mode. Thus, a charging means is caused to function as a potential control means. However, this method for controlling the charging potential of photosensitive drums is not limiting. For example, in the monochromatic mode, the (absolute value of the) charging potential of the photosensitive drums 1a, 1b, 1c can be controlled to decrease by exposing the photosensitive drums 1a, 1b, 1c at a constant exposure amount with exposure means 3a, 3b, 3c. Thus, the exposure means are caused to function as a potential control means. The charging potential on the photosensitive drum which is formed by the exposure can be controlled by the intensity of exposure (laser power), and the exposure means 3a, 3b, 3c are controlled such as to perform the exposure with the laser light of an intensity such that the charging potential of the photosensitive drums 1a, 1b, 1c assumes the above-described value. This method makes it possible to obtain the same effect as that of the present embodiment. The charging potential of the photosensitive drums may be also controlled in combination with the above-described charging voltage control and exposure control.
The image forming apparatus according to Embodiment 2 of the present invention will be explained hereinbelow with reference to
The offset amounts of the metal rollers 14a to 14d are set such as to obtain the positions which are as close as possible in order to stabilize the intermediate transfer belt potential while avoiding any damage caused by contact of the metal rollers 14a to 14d with the photosensitive drums 1a to ld through the intermediate transfer belt 10. Where the distance between the photosensitive drum 1a and the photosensitive drum 1b is denoted by W, the offset distance of the metal roller 23a is denoted by K, and the lift height of the metal roller 23a with respect to the intermediate transfer belt 10 is denoted by H4, in the present embodiment, W=60 mm, K=8 mm, and H4=1 mm.
In the same manner as in Embodiment 1, the metal roller 14a is constituted by a nickel-plated SUS rod of a straight shape with an outer diameter of 6 mm, and this roller is rotationally driven by the rotation (movement) of the intermediate transfer belt 10. The metal roller 14b disposed in the second image forming station (b), the metal roller 14c disposed in the third image forming station (c), and the metal roller 14d disposed in the fourth image forming station (d) are configured similarly to the metal roller 14a.
In the present embodiment, the secondary transfer opposing roller 13 forming the primary transfer surface of the intermediate transfer belt 10 is grounded via the voltage maintaining element 15. The voltage maintaining element 15 serves to maintain the connected member (the secondary transfer opposing roller 13) at a predetermined potential by allowing an electric current to flow from the current supply member to the voltage maintaining element 15 through the intermediate transfer belt 10. More specifically, parts of the electric current flowing from the current supply member to contact portion with the intermediate transfer belt 10 is caused by the voltage maintaining element 15 to flow to the ground, whereby the secondary transfer opposing roller 13 is maintained at a predetermined potential. As a result, the potential of the primary transfer region is maintained at a predetermined level. The predetermined potential of the voltage maintaining element 15 is set such as to maintain a primary transfer potential which makes it possible to obtain the desired transfer efficiency in each primary transfer region. In the present embodiment, a Zener diode 15, which is a constant-voltage element, is used as the voltage maintaining element 15. The Zener diode 15 generates a predetermined voltage (referred to herein as Zener voltage) at the cathode side when a current equal to or higher than a predetermined value flows therein. In the present embodiment, the Zener voltage is set to 300 V in order to obtain the desired primary transfer efficiency.
[Method for Forming Primary Transfer Potential]
In the configuration of the present embodiment, in the same manner as in Embodiment 1, since the secondary transfer power supply 21 that applies a voltage as a transfer power supply to the secondary transfer member is used as a power supply for performing the primary transfer, the secondary transfer roller 20 serves as a current supply member in the present embodiment. As mentioned hereinabove, since the Zener diode 15 is connected to the secondary transfer opposing roller 13 on which the intermediate transfer belt 10 is tensioned, the primary transfer is performed by allowing an electric current to flow from the secondary transfer power supply 21 to the secondary transfer opposing roller 13 through the intermediate transfer belt 10. In this case, since an electric current flows in the Zener diode 15, the secondary transfer opposing roller 13 assumes a potential corresponding to the Zener diode 15. With this potential being a starting point, an electric current flows in the metal rollers 14a to 14d and a primary transfer potential is formed in the image forming stations (a) to (d). The toners of the photosensitive drums 1a to 1d are moved onto the intermediate transfer belt 10 by the difference between this primary transfer potential and the photoelectric drum potential, whereby the primary transfer is performed.
However, in the configuration of the comparative example, the current value of the current supply member in the monochromatic mode is reduced to 14 μA with respect to 20 μA in the full color mode to improve secondary transferability. Further, since the potential of each photosensitive drum 1a, 1b, 1c, 1d of the image forming station is −500 V, a common current of 3.5 μA flows in the image forming stations (a), (b), (c), and (d). In this case, the potential of the metal rollers 14a, 14b, 14c, 14d does not rise to the Zener voltage and becomes 240 V because no current flows to the Zener diode 15 side. As a result, since the primary transfer current value in the black station (d) becomes 3.5 μA, the primary transfer efficiency decreases and transfer defects occur.
Accordingly, the specific feature of the present embodiment is that the potential of the photosensitive drums 1a, 1b, 1c is decreased by setting OFF the charging voltage of the color image forming stations (a), (b), (c) in the monochromatic mode. More specifically, in the monochromatic mode, the charging voltage of the charging roller 2d of the black station (d) is controlled to −1000 V, that is, the same voltage as in the full color mode. Meanwhile, the charging voltage of the charging rollers 2a, 2b, 2c of the color image forming stations (a), (b), (c) is set OFF (no charging bias is applied). As a result, the charging potential of the photosensitive drum 1d of the black station (d) becomes −500 V, whereas, the charging potential of the photosensitive drums 1a, 1b, 1c of the color image forming stations becomes substantially 0 V, and the charging potential of the photosensitive drums 1a, 1b, 1c decreases.
[Operations Performed by the Potential Control Method in the Present Embodiment]
In the configuration of the present embodiment, the charging voltage of the color image forming stations (a), (b), (c) is set OFF in the monochromatic mode to lower the charging potential of the photosensitive drums 1a, 1b, 1c and suppress the decrease in the primary transfer current of the black station (d).
Table 2 shows the current value of the current supply member, the values of the primary transfer current flowing in the image forming stations (a) to (d), and the primary transfer efficiency in each printing mode in the present embodiment and a comparative example, those results being used for comparative evaluation of the present embodiment and the comparative example. The primary transfer efficiency is represented in the same manner as in
The evaluation results are explained below with reference to
In the configuration of the comparative example, the potential of the photosensitive drums 1a to 1d of the image forming stations is −500 V even in the monochromatic mode. Therefore, a common current of 3.5 μA flows in the image forming stations (a) to (d). In this case, the potential of the metal rollers 14a to 14d does not rise to the Zener voltage and becomes 240 V because no current flows at the Zener diode 15 side. As a result, the primary transfer current value of the black station (d) becomes 3.5 μA, which results in the decreased primary transfer efficiency and the occurrence of transfer defects.
By contrast, in the configuration of the present embodiment, the potential of the photosensitive drums 1a, 1b, 1c of the color image forming stations (a), (b), (c) in the monochromatic mode is substantially 0 V. Therefore, no current flows in the color image forming stations (a), (b), (c). As a result, a current flows in the Zener diode 15 and the potential of the metal rollers 14a to 14d rises to 300 V, which is the Zener voltage, thereby allowing a current of 5 μA to flow to the black station (d). At this time, a current of 9 μA flows in the Zener diode 15, this current being part of the current of 14 μA supplied from the current supply member that has not flown to the black station (d). It follows from the above, that the amount of the primary transfer current flowing in the black station (d) can be maintained at 5 μA in the same manner as in the full color mode, and good primary transferability of the black station (d) can be ensured.
As explained hereinabove, with the configuration of the present embodiment, the charging voltage of the color image forming stations (a), (b), (c) in the monochromatic mode is set OFF, thereby reducing the charging potential of the photosensitive drums 1a, 1b, 1c substantially to 0 V. As a result, the decrease in the primary transfer current of the black station (d) can be suppressed and, therefore, good primary transferability can be ensured.
In the configuration of the present embodiment, a method for setting OFF the charging voltage of the charging rollers 2a, 2b, 2c is described as a method for lowering the charging potential of the photosensitive drums 1a, 1b, 1c in the monochromatic mode, but this method for controlling the charging potential of photosensitive drums is not limiting. For example, in the monochromatic mode, the electrical connection of the charging rollers 2a, 2b, 2c and the photosensitive drums 1a, 1b, 1c may be released and the application of the charging voltage to the photosensitive drums 1a, 1b, 1c may be set OFF by separating the charging rollers 2a, 2b, 2c. Thus, a configuration can be realized in which the contact positions and separation positions of at least the charging rollers 2a, 2b, 2c with respect to the photosensitive drums 1a, 1b, 1c are mechanically movable. The control can be also performed to produce a charging potential difference by exposing the photosensitive drums 1a, 1b, 1c at a constant exposure by the exposure means 3a, 3b, 3c, thereby charging the charging potential of the photosensitive drums 1a, 1b, 1c with respect to the photosensitive drum 1d. The effect that can be obtained with those methods is the same as in the present embodiment. The charging potential of the photosensitive drums may be also controlled by a combination of the above-described charging voltage control and exposure control.
In the present embodiment, the configuration is explained in which the secondary transfer roller 13 is used as the current supply member, but such a configuration of the current supply member is not limiting. Thus, as depicted in
The features of the above-described embodiments can be combined with each other whenever possible.
While the present invention has been described with reference to exemplary embodiments, it is to be understood that the invention is not limited to the disclosed exemplary embodiments. The scope of the following claims is to be accorded the broadest interpretation so as to encompass all such modifications and equivalent structures and functions.
This application claims the benefit of Japanese Patent Application No. 2014-165260, filed Aug. 14, 2014, which is hereby incorporated by reference herein in its entirety.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2014-165260 | Aug 2014 | JP | national |
