Patent Application
20180181033
The present invention relates to an electrophotographic image forming apparatus such as a copying machine and printer.
As an electrophotographic image forming apparatus, an image forming apparatus including an intermediate transfer member has been known. In a conventional image forming apparatus, a primary transfer member is disposed in such a manner as to face a photosensitive drum with an intermediate transfer member interposed therebetween, and the photosensitive drum contacting the intermediate transfer member to form a primary transfer nipping unit. To the primary transfer member, voltage is applied by a first high-voltage power supply. This application of voltage generates a primary transfer potential in the primary transfer unit. A potential difference is generated between the photosensitive drum and the intermediate transfer member, causing a toner image formed on a surface of the photosensitive drum serving as an image bearing member to be transferred to the intermediate transfer member (hereafter, referred to as a primary transfer step). The primary transfer step is iteratively executed on a toner image of each of multiple colors, whereby toner images of the respective colors are formed on a surface of the intermediate transfer member. Next, voltage is applied from a second high-voltage power supply to a secondary transfer member, whereby the toner images of the multiple colors formed on the surface of the intermediate transfer member are collectively transferred to a surface of a recording material such as paper (hereafter, referred to as a secondary transfer step). The toner images collectively transferred to the surface of the recording material in the secondary transfer step are fused on the recording material by a fixing device (hereafter, referred to as a fusing step).
There is a configuration in which, for example, use is made of an endless belt (hereafter, referred to as an intermediate transfer belt) as the intermediate transfer member, and the intermediate transfer belt is tensioned by a plurality of tensioning members on an inner peripheral surface of the intermediate transfer belt. Japanese Patent Application Laid-Open No. 2013-231942 discloses a configuration in which a contact member contacting the intermediate transfer belt is connected to a voltage maintaining element in a region on the intermediate transfer belt between a tensioning member and a tensioning member where the toner images are transferred from the plurality of image bearing members. According to Japanese Patent Application Laid-Open No. 2013-231942, primary transfer is performed not by using a high-voltage power supply for the primary transfer but by causing current to flow from a high-voltage power supply for secondary transfer via a secondary transfer member and a tensioning member facing the secondary transfer member into the voltage maintaining element connected to the contact member contacting the intermediate transfer belt. In such a configuration, a primary transfer potential in a primary transfer unit is generated by a constant voltage that occurs when the current is caused to flow into the voltage maintaining element.
However, in the configuration in the conventional example where the primary transfer is performed by causing current to flow from a current supply member into the voltage maintaining element connected to the contact member contacting the intermediate transfer belt, the primary transfer potential in the primary transfer unit maintains a constant potential by the voltage maintaining element. For that reason, when an impedance of the primary transfer unit fluctuates greatly, an image fused on a recording material may incur poor transfer such as poor density.
An aspect the present invention is an image forming apparatus including an image bearing member configured to bear a toner image, an intermediate transfer belt onto which a toner image is primarily transferred from the image bearing member, the intermediate transfer belt having a conductivity and being endless, a secondary transfer member configured to secondarily transfer the toner image from the intermediate transfer belt to the transfer member, the secondary transfer member contacting an outer peripheral surface of the intermediate transfer belt, a transfer power supply configured to apply a voltage to the secondary transfer member, an opposing member supporting an inner peripheral surface of the intermediate transfer belt, the opposing member provided to oppose the secondary transfer member through the intermediate transfer belt, a contact member provided to correspond to oppose the image bearing member through the intermediate transfer belt, the contact member contacting the inner peripheral surface of the intermediate transfer belt, and a current restriction circuit electrically connected to the contact member and the opposing member, to restrict an amount of current flowing from the opposing member to the contact member in a case where a voltage is applied from the transfer power supply to the secondary transfer member, to a predetermined amount of current, independently of variation of resistance value of the intermediate transfer belt.
Further features of the present invention will become apparent from the following description of exemplary embodiments with reference to the attached drawings.
Preferred embodiments of the present invention will now be described in detail in accordance with the accompanying drawings.
[Overview of Color Image Forming Apparatus]
The first image formation station a includes a drum-shaped electrophotographic photosensitive member (hereafter, referred to as a photosensitive drum) 1a as an image bearing member, a charge roller 2a, a developing device 4a, and a cleaning device 5a. The photosensitive drum 1a is an image bearing member configured to be rotary driven in a direction illustrated by an arrow, at a predetermined circumferential speed (hereafter, referred to as a process speed) and configured to bear a toner image. The developing device 4a is a device for storing a yellow toner and developing the yellow toner on the photosensitive drum 1a. The cleaning device 5a is a member for collecting the toner adhered to the photosensitive drum 1a. In Embodiment 1, the cleaning device 5a includes a cleaning blade being a cleaning member adapted to contact with the photosensitive drum 1a and a waste toner box adapted to store the toner collected by the cleaning blade.
Upon receiving an image signal, a controller (see
The intermediate transfer belt 10 is tensioned by a plurality of tensioning members 11, 12 and 13, includes an opposing portion adapted to contact with the photosensitive drum 1a, and is movable in a moving direction of the photosensitive drum 1a at substantially the same circumferential speed as the photosensitive drum 1a. The yellow toner image formed on the photosensitive drum 1a is transferred to the intermediate transfer belt 10 (hereafter, referred to as primary transfer) in a course of passing through a contact portion between the photosensitive drum 1a and the intermediate transfer belt 10 (hereafter, referred to as a primary transfer unit).
In Embodiment 1, the primary transfer involves causing current to flow into the intermediate transfer belt 10 from a current supply member contacting the intermediate transfer belt 10, and the current generates a primary transfer potential in a primary transfer unit of the intermediate transfer belt 10 in each of the image formation stations. A method for generating the primary transfer potential in Embodiment 1 will be described later. Residual toner on a surface of the photosensitive drum 1a is removed by the cleaning device 5a, so that the photosensitive drum 1a is cleaned. The cleaned photosensitive drum 1a is to be subjected to an image formation process that includes charging and subsequent steps. Subsequently, in the second, third, and fourth image formation stations b, c, d, a magenta toner image of a second color, a cyan toner image of a third color, and a black toner image of a fourth color are formed similarly, and transferred to the intermediate transfer belt 10 one by one in the respective primary transfer units. The indexes a to d following reference numerals, which are used corresponding to YMCBk, may be omitted unless the indexes are necessary.
Through the above steps, on the intermediate transfer belt 10, a full-color image equivalent to the intended color images is formed. The toner images of the four colors on the intermediate transfer belt 10 are collectively transferred to a surface of a recording material P fed by a sheet feeder 50 in a course of passing through a secondary transfer unit formed by the intermediate transfer belt 10 and a secondary transfer roller 20 (hereafter, referred to as secondary transfer). The secondary transfer roller 20 as a secondary transfer member contacts an outer peripheral surface of the intermediate transfer belt 10 with a pressing force, forming the secondary transfer unit. The secondary transfer roller 20 is rotated in such a manner as to follow the intermediate transfer belt 10. The secondary transfer roller 20 is a member for transferring a toner image on the intermediate transfer belt 10 onto a recording material P.
A secondary transfer power supply 21 (transfer power supply) being first application unit includes a transformer configured to generate a high voltage, and is configured to supply a secondary transfer voltage to the secondary transfer roller 20. Voltage output from the transformer is controlled to be substantially constant by the controller, whereby the secondary transfer voltage supplied to the secondary transfer roller 20 by the secondary transfer power supply 21 is controlled to be constant. The secondary transfer power supply 21 is capable of outputting a voltage ranging from 100 to 4000 [V].
The recording material P to which the toner images of the four colors are transferred in the secondary transfer unit is heated and pressurized by a fuser 30. This heating and pressurization cause the toners of the four colors to be fused and mixed together, fixed to the recording material P. Toner residing on the intermediate transfer belt 10 after the secondary transfer is removed by a cleaning device 16 including a cleaning blade, so that the intermediate transfer belt 10 is cleaned. Through the above operation, a full-color printed image is formed. A current restriction circuit 17 will be described later.
[Overview of Controller]
Description will be made about a configuration of a controller 100 configured to control the entire image forming apparatus, with reference to
The secondary transfer power supply 21 includes a current detection circuit 21h. The current detection circuit 21h being a detection unit is configured to detect current that flows through the secondary transfer roller 20 as voltage is applied to the secondary transfer roller 20 by the secondary transfer power supply 21. The transfer control unit 201 is configured to control a value of voltage output from the secondary transfer power supply 21, based on a load current flowing through the secondary transfer roller 20 and detected by the current detection circuit 21h. Hereafter, the load current flowing through the secondary transfer roller 20 will be referred to as a secondary transfer current i2 (see
[Overview of Intermediate Transfer Belt]
Next, description will be made in detail about the intermediate transfer belt 10, the tensioning members 11, 12 and 13, and the contact member 14. The intermediate transfer belt 10 is tensioned by three shafts: the tensioning members 11, 12 and 13. Hereafter, the tensioning member 11 will be referred to as a drive roller 11, the tensioning member 12 will be referred to as a tension roller 12, and the tensioning member 13 will be referred to as a secondary transfer opposing roller 13 being a secondary transfer opposing member (hereafter, referred to as an opposing roller 13). Contact members 14a to 14d are members electrically connected to the opposing roller 13, which is one of the tensioning members, and being contact the inner peripheral surface of the intermediate transfer belt 10 in vicinities of the photosensitive drums 1a to 1d, respectively. At positions where the contact members 14a to 14d face the photosensitive drums 1a to 1d, respectively, the intermediate transfer belt 10 is disposed as an intermediate transfer member. The intermediate transfer belt 10 is an endless belt having a conductivity given by adding a conductive agent to a resin material.
The opposing roller 13 is one of the multiple tensioning member and a member through which current flows via the intermediate transfer belt 10 when voltage is applied to the secondary transfer roller 20 by the secondary transfer power supply 21. The opposing roller 13 is a roller forming a nipping unit with the secondary transfer roller 20. The intermediate transfer belt 10 is movable in the same direction as a rotation direction of the photosensitive drums 1a to 1d in opposing portions contacting with the photosensitive drums 1a, 1b, 1c and 1d, at substantially the same circumferential speed as the circumferential speed of the photosensitive drums 1a, 1b, 1c and 1d. The intermediate transfer belt 10 is adapted to move at the substantially the same circumferential speed as the circumferential speed of the photosensitive drums 1a to 1d, by the drive roller 11 adapted to rotate by a driving source (not illustrated).
As illustrated in
The secondary transfer power supply 21 is configured to apply voltage to the secondary transfer roller 20, so as to cause current to flow into opposing roller 13 from the secondary transfer roller 20 via the intermediate transfer belt 10. As viewed from the opposing roller 13, the secondary transfer roller 20 functions as the current supply member. The opposing roller 13 is electrically connected to the metal rollers 14a to 14d. The current flowing into the opposing roller 13 therefore flows into the metal rollers 14a, 14b, 14c and 14d and via the metal rollers 14a, 14b, 14c and 14d, flows into the intermediate transfer belt 10 forming the respective primary transfer units. This current generates primary transfer potentials in the primary transfer units. Potential differences between the primary transfer potentials and photosensitive drum potentials in the primary transfer units cause toners on the photosensitive drums 1a, 1b, 1c and 1d (image bearing members) to move from the photosensitive drums 1a, 1b, 1c and 1d onto the intermediate transfer belt 10. The primary transfer is thus performed in the primary transfer units. The secondary transfer power supply 21 functions as a power supply for applying the secondary transfer voltage to the secondary transfer roller 20 and also functions as a current supply source for supplying current to the intermediate transfer belt 10 so as to generate the primary transfer potentials in the primary transfer units.
[Method for Generating Primary Transfer Potential]
Description will be made in detail about a method for generating a primary transfer potential used for execution of the primary transfer, which is a feature of the present invention, in comparison with a conventional example.
Hence, Embodiment 1 is intended for a stable supply of a proper primary transfer current to the primary transfer units regardless of environment. To this end, the current restriction circuit 17 is connected to a current path between the opposing roller 13 and the metal rollers 14a, 14b, 14c and 14d, as illustrated in
This restriction enables the proper primary transfer current to flow to the primary transfer units regardless of fluctuations in impedance occurring in the primary transfer units due to various factors. The current restriction circuit 17 is a circuit connected in the path of current that flows from the opposing roller 13 to the metal rollers 14 when voltage is applied to the secondary transfer roller 20 by the secondary transfer power supply 21 and is a circuit restriction current flowing from the opposing roller 13 to the metal rollers 14, to a predetermined current. A configuration of the current restriction circuit 17 will be described below.
[Current Restriction Circuit]
Next, the current restriction circuit 17 will be described with reference to
In the current restriction circuit 17, when a secondary transfer current i2 flows from the secondary transfer roller 20 being the current supply member, voltage is applied between the base terminal and the emitter terminal of the transistor 17e, and current flows into the base terminal of the transistor 17e. At this point, a current i1 flowing through the resistor 17f is expressed by Formula (1) below using a base-emitter voltage Vbe of the transistor 17 and a resistance value R1 of the resistor 17f.
i1=Vbe/R1 (1)
Here, a base current in the transistor 17e has a current value sufficiently small as compared with the secondary transfer current i2, and the current i1 calculated by Formula (1) can be regarded as a total of values of currents flowing into the metal rollers 14a, 14b, 14c and 14d. The current i1 will hereafter be referred to as a primary transfer current i1. For example, assuming that a predetermined value of the primary transfer current i1 is 20 [μA], the primary transfer current flowing into the metal rollers 14a, 14b, 14c and 14d and necessary in the primary transfer, the resistance value R1 of the resistor 17f is set as follows. Typically, the base-emitter voltage Vbe of the transistor 17 substantially satisfies Vbe=0.7 [V], and thus, from Formula (1), the resistance value R1 of the resistor 17f is about 35 [kΩ].
[Current Path of Secondary Transfer Power Supply]
Next, a current path from the secondary transfer power supply 21 will be described with reference to
The secondary transfer current i2 is a branched off into a primary transfer current i1 and a surplus current is by an action of the current restriction circuit 17 described above. The primary transfer current i1 is the secondary transfer current i2 converted into a predetermined current value by the above-described resistor 17f of the current restriction circuit 17, flowing from the metal rollers 14a to 14d to the photosensitive drums 1a to 1d, and returning to the secondary transfer power supply 21. The surplus current is a difference of current (i2−i1) between the secondary transfer current i2 and the primary transfer current i1 flowing as a collector current of the transistor 17e, and then returning to the secondary transfer power supply 21. As seen from the above, since the secondary transfer current i2 flowing to the secondary transfer roller 20 matches a summed current of the primary transfer current i1 and the surplus current is (i2=i1+is), the secondary transfer current i2 flowing to the secondary transfer roller 20 can be detected by the current detection circuit 21h.
[Current Detection Circuit]
Next, the current detection circuit 21h will be described. In Embodiment 1, the transfer control unit 201 executes auto transfer voltage control (ATVC) on the secondary transfer roller 20. In the ATVC, the transfer control unit 201 causes the current detection circuit 21h to detect current that flows into the secondary transfer roller 20 when a secondary transfer positive voltage applied to the secondary transfer roller 20. Here, the ATVC is to apply a predetermined voltage to the secondary transfer roller 20, to detect current flowing to the secondary transfer roller 20, and to control a voltage to be applied to the secondary transfer roller 20 in image formation based on a result of the detection of the current. A configuration of the current detection circuit 21h is similar to configurations disclosed in, for example, Japanese Patent Application Laid-Open No. 2013-078252 and the like, and will not be elaborated. The transfer control unit 201 can detect a value of the current flowing into the secondary transfer roller 20 based on the detection result from current detection circuit 21h.
[Current Control by Secondary Transfer Power Supply]
Next, the current control by the secondary transfer power supply 21 will be described. Let TB denote the amount of current flowing into the secondary transfer roller 20 and TA denote a total current amount necessary for executing the primary transfer satisfactorily. Here, the total current amount TA is a total amount of currents flowing into the primary transfer units (primary transfer unit of the four colors). The transfer control unit 201 executes the ATVC to apply the secondary transfer positive voltage to the secondary transfer roller 20, the secondary transfer positive voltage making the amount TB of the current flowing into the secondary transfer roller 20 satisfy a condition that the amount TB is larger than the total current amount TA (TB≥TA). The current amount TB satisfying the condition that the current amount TB is larger than the total current amount TA allows the above-described action of the current restriction circuit 17 of branching off the secondary transfer current i2 into the primary transfer current i1 and the surplus current is, enabling the predetermined primary transfer current i1 to flow into the primary transfer units. In this manner, the transfer control unit 201 is configured to control voltage to be applied to the secondary transfer roller 20 by the secondary transfer power supply 21. The transfer control unit 201 controls the secondary transfer power supply 21 so that the amount TB of the of current flowing into the secondary transfer roller 20 becomes larger than the predetermined current amount TA or larger, the predetermined current amount being needed to transfer toner images formed on the multiple photosensitive drums 1a to 1d on the intermediate transfer belt 10.
[Image Forming Operation]
Next, in the image forming operation in Embodiment 1, description will be made about a relation between the secondary transfer voltage, the potential of the primary transfer units, and the current flowing into the primary transfer units, in a course from start of the image forming operation, via the primary transfer, to completion of the secondary transfer, with reference to a timing chart of
In the image forming apparatus, the image forming operation is started by reception of an image signal from the controller 100. Before the primary transfer is started, at a timing S1, the transfer control unit 201 starts application of a voltage V2 from the secondary transfer power supply 21 to the secondary transfer roller 20. When the voltage V2 is applied to the secondary transfer roller 20, the secondary transfer current i2 flows from the secondary transfer roller 20 to the metal rollers 14a to 14d via the intermediate transfer belt 10 and the opposing roller 13, forming the potential V1 in the primary transfer units. To the current path from the opposing roller 13 to the metal rollers 14a to 14d, the current restriction circuit 17 is connected. The current restriction circuit 17 restricting the secondary transfer current i2 enables the primary transfer current i1 to flow into the primary transfer units. The primary transfer current i1 has a current value larger than a current value at which the predetermined transfer efficient can be obtained. In Embodiment 1, the voltage V2 is set at 2000 V to allow the primary transfer current i1 to flow.
Subsequently, at timing S2, the primary transfer is started with the first image formation station a. Toner images are transferred one by one from the photosensitive drums 1a to 1d to the intermediate transfer belt 10. At timing S3, toners on the intermediate transfer belt 10 reach the secondary transfer unit, where the secondary transfer is performed. The transfer control unit 201 applies a voltage V3 to the secondary transfer roller 20 from the secondary transfer power supply 21, the voltage V3 being necessary for the secondary transfer. The transfer control unit 201 changes, at timing S3, the voltage output from the secondary transfer power supply 21 from the voltage V2 to the voltage V3. This change transfers the toner images on the intermediate transfer belt 10 on the recording material P, in the secondary transfer unit. The voltage V3 output from the secondary transfer power supply 21 in the secondary transfer is set at, for example, 2500 V. At the timing S3, the voltage applied from the secondary transfer power supply 21 is changed from the voltage V2 to the voltage V3, and the secondary transfer current i2 increases. Even in such a case, the primary transfer current i1 is kept constant by the action of the current restriction circuit 17.
Next, at timing S4, the primary transfer is terminated, and the secondary transfer is thereafter terminated at timing S5, so that the image forming operation is terminated. At timing S5, the transfer control unit 201 stops applying the voltage to the secondary transfer roller 20 from the secondary transfer power supply 21. This stop of application causes the secondary transfer current i2 and the primary transfer current i1 not to flow, so that the primary transfer potential becomes 0 V.
As seen from the above, the transfer control unit 201 controls the secondary transfer power supply 21 so that the voltage V2, which is a first voltage, is applied to the secondary transfer roller 20 before toner images formed on the respective multiple photosensitive drums 1a to 1d are transferred to the intermediate transfer belt 10. To transfer the toner images on the intermediate transfer belt 10 on the recording material P, the transfer control unit 201 controls the secondary transfer power supply 21 so that the voltage V3, which is a second voltage higher than the voltage V2, which is the first voltage, is applied to the secondary transfer roller 20.
As illustrated in
[Comparison Results]
Next, comparison results will be described. Table 1 shows a relation between the potential of the primary transfer units and current flowing into the primary transfer units in image formation, in Comparative Example 1 illustrated in
Table 1 shows primary transfer potentials [V] and primary transfer currents [μA] in Comparative Example 1 and Embodiment 1. Table 1 also shows the primary transfer potentials and the primary transfer currents with the impedance of the primary transfer units being 10 MΩ, 30 MΩ and 50 MΩ.
In a configuration of Comparative Example 1, the primary transfer potential in the primary transfer units is at a constant voltage generated by the voltage maintaining element 15 irrespective of the impedance of the primary transfer units. Therefore, when the impedance of the primary transfer units fluctuates due to external factors such as environmental variations, the primary transfer current fluctuates. Since the primary transfer potential is constant, the primary transfer current is decreased with an increase in the impedance of the primary transfer units. If a proper primary transfer current cannot be ensured in the primary transfer units, toners in a required amount cannot be transferred to the intermediate transfer belt 10 from the photosensitive drums 1a to 1d. This failure to ensure the proper primary transfer current leads to a poor transfer such as poor density on an image fused on the recording material P.
For example, in the configuration of Comparative Example 1 (
In contrast, in a configuration of Embodiment 1 (
As described above, according to Embodiment 1, the current restriction circuit 17 is connected in the current path between the opposing roller 13 and the metal rollers 14, and a part of the current restriction circuit 17 is grounded. This configuration suppresses the fluctuations in the primary transfer current, enabling a satisfactory primary transfer property to be ensured regardless of the fluctuations in impedance of the primary transfer units. In Embodiment 1, the configuration of the current restriction circuit 17 has a PNP transistor and a resistor. However, use can be made of other kinds of elements (e.g., an element such as MOSFET) as long as configurations of the circuit can provide the same effect, and such configurations will not be eliminated from the scope of the invention. In Embodiment 1, the metal rollers 14a, 14b, 14c and 14d being the contact members are provided on the photosensitive drums 1a, 1b, 1c and 1d, respectively. However, metal rollers are not necessarily provided in all of the photosensitive drums. As seen from the above, according to Embodiment 1, the primary transfer potential can be generated in such a manner that deals with fluctuations in the impedance of the primary transfer units.
Embodiment 1 is described such that the current restriction circuit 17 connected to the current path from the opposing roller 13 to the metal roller 14 suppresses the fluctuations in the primary transfer current, enabling a satisfactory primary transfer property to be ensured regardless of the fluctuations in impedance of the primary transfer units. In contrast, a feature of Embodiment 2 is that a resistive member such as a thermistor having a temperature coefficient of resistance is applied to the current restriction circuit 17. The rest of the configuration is similar to the configuration of the image forming apparatus in Embodiment 1, and description will be made with similar components denoted by like reference characters. In a case where fluctuations in the impedance of the primary transfer units are increased due to external factors such as environmental variations, Embodiment 2 aims at solving a problem in such a case in that a primary transfer current greatly changes due to an environment, a primary transfer property cannot be ensured, and a required toner amount cannot be transferred to an intermediate transfer belt.
[Difference in Current Restriction Circuit]
The current restriction circuit 17 with a thermistor 17g added thereto will be described with reference to
More in detail, the resistor 17f includes one end connected to the opposing roller 13 and another end connected to the metal rollers 14. The thermistor 17g includes one end connected to the opposing roller 13 and one end of the resistor 17f, and another end connected to the metal rollers 14 and the other end of the resistor 17f. In the transistor 17e, the emitter terminal is connected to the opposing roller 13, the one end of the resistor 17f, and the one end of the thermistor 17g, the base terminal is connected to the metal rollers 14, the other end of the resistor 17f, and the other end of the thermistor 17g, and the collector terminal is grounded.
In the current restriction circuit 17, when a secondary transfer current i2 flows from the secondary transfer roller 20 being the current supply member, voltage is applied between the base terminal and the emitter terminal of the transistor 17e, and current flows into the base terminal of the transistor 17e. Here, a current i1 flowing through the resistor 17f and the thermistor 17g is expressed by Formula (2) below using a base-emitter voltage Vbe of the transistor 17, a resistance value R1 of the resistor 17f, and a resistance value Rth of the thermistor 17g.
i1=Vbe/{(R1×Rth)/(R1+Rth)}=Vbe/Rx (2)
Here, a base current in the transistor 17e is a current sufficiently small as compared with the secondary transfer current i2, and the current i1 calculated by Formula (2) can be regarded as a total of values of currents flowing into metal rollers 14a, 14b, 14c and 14d. The current i1 will hereafter be referred to as a primary transfer current i1.
Next, an advantageous effect of Embodiment 2 will be described. Description will be made below about a case of intending to increase the primary transfer current i1 with an increase in atmosphere temperature, by way of example. In this case, it is understood from Formula (2) that the combined resistance value Rx of the resistor 17f and the thermistor 17g may be reduced according to an atmosphere temperature. As the thermistor 17g, use is therefore to be made of a negative temperature coefficient (NTC) thermistor, which has a negative temperature characteristic. The thermistor 17g is a thermistor a resistance value of which decreases with an increase in temperature.
Here, assuming that a resistance value of the NTC thermistor is R0 [kΩ] at a temperature T0 [° C.], a resistance value Rth [kΩ] of the NTC thermistor at a temperature T [° C.] is typically expressed by Formula (3) below.
Rth=R0×exp(B×((1/(T+273))−(1/(T0+273)))) (3)
Assume that the resistance value R1 of the resistor 17f is 1 [MΩ]. With parameters of the thermistor 17g given as follows: B value: 3500 [K], temperature T0=25 [° C.], and resistance value R0=33 [kΩ], Formula (2) and Formula (3) provide temperature characteristics of the primary transfer current i1 illustrated in
As described above, according to Embodiment 2, adding and connecting the thermistor 17g to the current restriction circuit 17 in Embodiment 1 enables automatic adjustment of the primary transfer current i1 according to the atmosphere temperature. In Embodiment 2, use is made of an NTC thermistor as a resistive member having a temperature coefficient of resistance. However, use can be made of other kinds of elements as long as configurations of the circuit can provide the same effect, and such configurations will not be eliminated from the scope of the invention. The current restriction circuit 17 has the configuration in which the resistor 17f is connected to the thermistor 17g in parallel. However, the configuration does not necessarily include the resistor 17f as long as configurations of the circuit can provide the same effect, and such configurations will not be eliminated from the scope of the invention. As seen from the above, according to Embodiment 2, the primary transfer potential can be generated in such a manner that deals with fluctuations in the impedance of the primary transfer units.
Embodiments 1 and 2 are described such that the current restriction circuit 17 connected to the current path from the opposing roller 13 to the metal roller 14 suppresses the fluctuations in the primary transfer current, enabling a satisfactory primary transfer property to be ensured regardless of the fluctuations in impedance of the primary transfer units. In contrast, a feature of Embodiment 3 is that a voltage maintaining element is additionally connected to an opposing roller 13. The rest of the configuration is similar to the configuration of the image forming apparatus in Embodiment 1, and description will be made with similar components denoted by like reference characters. A current restriction circuit 17 may have the configuration of Embodiment 1 or the configuration of Embodiment 2.
[Overview of Secondary Transfer Opposing Roller]
A configuration of Embodiment 3 will be described below with reference to
[Method for Generating Secondary Transfer Opposing Roller Potential]
Next, a method for generating a potential of the opposing roller 13 will be described in detail in comparison with Embodiment 1. In Embodiment 1 (
Hence, Embodiment 3 has a configuration in which a proper primary transfer current for the primary transfer units is supplied with stability regardless of environment, and at the same time, when the impedance Z1 of the primary transfer units temporarily increases, control is executed as follows. That is, to maintain the potential of the opposing roller 13 at a predetermined potential or lower, the opposing roller 13 is grounded via the Zener diode 15z, which is a constant voltage element and a voltage maintaining element, as illustrated in
Table 2 shows a relation among the potential of the opposing roller 13, the potential of the secondary transfer unit, and the secondary transfer positive voltage, in Embodiment 1 and Embodiment 3.
Here, the primary transfer current is assumed to be 20 [μA]. Table 2 shows the potentials and the voltage with the impedance of the primary transfer units being 10 MΩ, 30 MΩ and 50 MΩ. The secondary transfer positive voltage is a total of the potential of opposing roller 13 and the potential of the secondary transfer unit.
In Embodiment 3, to reduce in size of the image forming apparatus, the secondary transfer power supply 21 is assumed to be a high-voltage power supply capable of outputting a voltage range from 100 to 4000 [V]. As shown in Table 2, in Embodiment 1, when satisfaction of an optimal primary transfer current is intended, the potential of the opposing roller 13 increases with an increase in the impedance of the primary transfer units. For example, when the impedance of the primary transfer units is 200 [Me], the potential of the opposing roller 13 is 4000 [V]. To cause a primary transfer current of 20 [μA] to flow when the impedance of the primary transfer units is 200 [MΩ], the secondary transfer power supply 21 has to output a secondary transfer positive voltage of 5000 [V]. Such a voltage cannot be supported by the secondary transfer power supply 21 capable of outputting a voltage within a range from 1000 to 4000 [V], and may arise a risk of increasing the power supply capacity.
In contrast, the configuration of Embodiment 3 includes the Zener diode 15z connected to the current path from the opposing roller 13 to the current restriction circuit 17. This configuration maintains the potential of the opposing roller 13 at a predetermined potential (1000 V) or lower and enables a proper potential to be generated in the secondary transfer unit irrespective of the impedance of the primary transfer units. For example, as shown in Table 2, when the impedance of the primary transfer units is 200 [MΩ], the potential of the opposing roller 13 is 1000 [V]. To cause a primary transfer current of 20 [μA] to flow when the impedance of the primary transfer units is 200 [Me], the secondary transfer power supply 21 may output a secondary transfer positive voltage of 2000 [V]. With this configuration, even the secondary transfer power supply 21 capable of outputting a voltage within a range from 1000 to 4000 [V] can support the voltage, eliminating the risk of increasing the power supply capacity.
As described above, according to Embodiment 3, the voltage maintaining element is connected to the opposing roller 13. This configuration maintains the potential of the opposing roller 13 at a predetermined potential and enables a proper potential to be generated in the secondary transfer unit while avoiding the increase in the power supply capacity of the secondary transfer power supply 21, irrespective of various fluctuations in the impedance occurring in the primary transfer units. As seen from the above, according to Embodiment 3, the primary transfer potential can be generated in such a manner that deals with fluctuations in the impedance of the primary transfer units.
Embodiment 1 to Embodiment 3 are described such that the current restriction circuit 17 is employed, and a potential having a positive polarity is generated in the intermediate transfer belt 10 and the metal rollers 14. In Embodiment 4, a smoothing element is additionally connected to a current restriction circuit, enabling an intermediate transfer belt 10 and metal rollers 14 connected to the smoothing element to have a potential of a negative polarity.
[Difference in Current Restriction Circuit]
Next, with reference to
In Embodiment 4, the secondary transfer power supply 21 is capable of applying a voltage of a positive polarity and a voltage of a negative polarity, to the secondary transfer roller 20. The diode 18 being a first smoothing element includes a cathode terminal connected to the opposing roller 13 and an anode terminal connected to the metal rollers 14 and is connected to the current restriction circuit 27 in parallel. The current restriction circuit 27 includes the resistor 27f being a first resistor element including one end connected to the opposing roller 13 and another end connected to the metal rollers 14. The current restriction circuit 27 includes the transistor 27e. The transistor 27e includes an emitter terminal connected to the opposing roller 13 and the one end of the resistor 27f, a base terminal connected to the metal rollers 14 and the other end of the resistor 27f, and a collector terminal grounded via the diode 27g being a second smoothing element. The current restriction circuit 27 includes the resistor 27h being a second resistor element connected between the emitter terminal and the collector terminal of the transistor. The current restriction circuit 27 further includes the diode 27g, and an anode terminal of the diode 27g is connected to the other end of the resistor 27h and the collector terminal of the transistor 27e, and a cathode terminal of the diode 27g is grounded.
Meanwhile, when the secondary transfer negative voltage is applied to the secondary transfer roller 20, a negative current iN flows from the metal rollers 14 to the current restriction circuit 27.
[Method for Generating Negative Potential in Intermediate Transfer Belt]
Description will be made below about a case of maintaining the potential of the intermediate transfer belt 10 at a negative polarity, with reference to
The application of a voltage of a negative polarity from the secondary transfer power supply 21 to the secondary transfer roller 20 forms the following a route of a negative current. That is, the formed route of a negative current starts from GNDs (not illustrated) of the photosensitive drums 1, passes through the metal rollers 14, the diode 18, the opposing roller 13, the intermediate transfer belt 10, and the secondary transfer roller 20, and returns to the secondary transfer power supply 21. Assume that the voltage of a negative polarity applied from the secondary transfer power supply 21 to the secondary transfer roller 20 is, for example, −1000 [V]. This route enables the intermediate transfer belt 10 contacting the metal rollers 14 to have a negative potential.
As described above, according to Embodiment 4, the diode 18 being a smoothing element is added to the current restriction circuit 27, and the cathode side of the diode 18 is connected to the opposing roller 13, and the anode side of the diode 18 is connected to the metal rollers 14. The current restriction circuit 27 includes the diode 27g and the resistor 27h so as to protect the transistor 27e by preventing a reverse potential from being generated between the emitter terminal and the collector terminal. This configuration forms a bypass route allowing current to flow from the metal rollers 14 to the opposing roller 13 via the diode 18 when a secondary transfer negative voltage is applied to a secondary transfer roller 20. This bypass route enables the intermediate transfer belt 10 contacting the metal rollers 14 to have a negative potential. To the current path between the opposing roller 13 and the current restriction circuit 27 of Embodiment 4, the Zener diode 15z of the Embodiment 3 may be connected. As seen from the above, according to Embodiment 4, the primary transfer potential can be generated in such a manner that deals with fluctuations in the impedance of the primary transfer units.
Embodiment 1 to Embodiment 4 are described such that use is made of the secondary transfer roller 20 as a current supply member, and current is supplied from the secondary transfer roller 20 to the intermediate transfer belt 10. In contrast, a feature of Embodiment 5 is that use is made of the secondary transfer roller 20 as well as another conductive member as a current supply member from which current is supplied to the intermediate transfer belt 10. Specifically, a feature of Embodiment 5 is that, as the conductive member, use is made of a charge member for removing toner residing on the intermediate transfer belt 10 after the secondary transfer. The rest of the configuration is similar to the configuration of the image forming apparatus in Embodiment 1, and description will be made with similar components denoted by like reference characters.
[Cleaning Intermediate Transfer Belt]
Next, a method for cleaning the intermediate transfer belt 10 will be described. In Embodiment 5, toners are charged to have a negative polarity in developing devices 4a, 4b, 4c and 4d, thereafter developed in the photosensitive drums 1a, 1b, 1c and 1d, and transferred to the intermediate transfer belt 10 in the primary transfer units. The secondary transfer roller 20 to which the positive polarity voltage is applied from the secondary transfer power supply 21 thereafter performs the secondary transfer on a recording material P such as paper, so as to form an image. Toner residing on the intermediate transfer belt 10 after the secondary transfer is easily charged to have a positive polarity under an influence of the voltage of a positive polarity applied to the secondary transfer roller 20. As a result, the toner residing after the secondary transfer has positive and negative polarities intermixedly. The toner residing after the secondary transfer may locally accumulate in a form of multiple layers, residing on the intermediate transfer belt 10, under an influence of unevenness on the surface of the recording material P.
The brush member 19 is disposed in such a manner as to be fixed relatively to the intermediate transfer belt 10 rotary moving and disposed in such a manner as to enter the intermediate transfer belt 10 by a predetermined intrusion amount. The brush member 19 is supported in the image forming apparatus and does not rotate while the intermediate transfer belt 10 moves. Therefore, when toner passes through a charge unit formed by the brush member 19 and the intermediate transfer belt 10, the toner accumulating on the intermediate transfer belt 10 in a form of multiple layers is mechanically scattered to be substantially as high as one layer, by a difference in circumferential speed between the brush member 19 and the intermediate transfer belt 10. To the brush member 19, the voltage of a positive polarity is applied from the high-voltage power supply 60, and constant current control is executed. When the toner residing after the secondary transfer passes through the charge unit, the toner is charged to have a positive polarity being a reversed polarity to a polarity of the toner in the development. The toner having a negative polarity not having completely been charged to have a positive polarity is collected by the brush member 19.
The toner having an optimal charge given by the brush member 19 thereafter moves to the photosensitive drum 1a charged to have a negative polarity in the primary transfer unit. The toner having moved from the intermediate transfer belt 10 to the photosensitive drum 1a is collected by a cleaning device 5a disposed on the photosensitive drum 1a. The movement of the toner charged to have a positive polarity from the intermediate transfer belt 10 to the photosensitive drum 1a may be performed at a timing the same as a timing of transferring a toner image from the photosensitive drum 1a to the intermediate transfer belt 10 (simultaneously with the transfer) or may be performed at a time different from the timing of transferring. As seen from the above, a feature of Embodiment 5 is that use is made of the secondary transfer roller 20 as well as the conductive brush member 19 being a charge member, as a current supply member. The reason for using the conductive brush member 19 will be described below.
[Roles of Current Supply Members in Image Formation]
In Embodiment 1 to Embodiment 3, the secondary transfer roller 20 has two roles. One of the roles is to flow a predetermined current amount for the secondary transfer so as to satisfy the secondary transfer property. Another one of the roles is to a predetermined current amount for the primary transfer to the photosensitive drums 1 so as to maintain a potential of the intermediate transfer belt 10 in the respective primary transfer units. Therefore, in Embodiment 1, the predetermined current amount for the secondary transfer and the predetermined current amount for the primary transfer need to be supplied only from the secondary transfer roller 20 as a current supply member.
Here, a relation between the predetermined current amount for the secondary transfer and the predetermined current amount for the primary transfer will be described. The predetermined current amount for the secondary transfer is desirably set at a current value such that optimizes a transfer efficient for a recording material P in the secondary transfer unit. In Embodiment 5, a current amount optimal for the secondary transfer is assumed to be, for example, 15 μA. Meanwhile, the predetermined current amount for the primary transfer is desirably set at a current value such that optimizes a transfer efficient for the intermediate transfer belt 10 in the primary transfer units. In Embodiment 5, a current amount optimal for the primary transfer is assumed to be, for example, 20 μA. From the above, letting a current amount TA denote an amount of current necessary to execute the primary transfer suitably, and a current amount TB denote an amount of current supplied to the intermediate transfer belt 10, a predetermined primary transfer performance can be obtained when a condition that the current amount TB is equal to or higher than the current amount TA is satisfied.
However, when it is intended to supply the current amount TB from only the secondary transfer roller 20, a current amount of 20 μA or larger needs to be supplied, and the current amount is larger than a current amount of 15 μA with which the secondary transfer property takes an optimal value. As in Embodiment 1, when it is intended to supply current from only the secondary transfer roller 20, the predetermined primary transfer performance needs to be obtained by increasing the amount of current to be supplied to the secondary transfer roller 20 within a tolerable range of a secondary transfer performance. Hence, in Embodiment 5, additional use of the brush member 19 as a current supply member enables the amount of current supplied from the secondary transfer roller 20 to be set optimal for the predetermined current amount for the secondary transfer and at the same time enables the primary transfer property to be satisfied.
The transfer control unit 201 is configured to control the voltage applied to the secondary transfer roller 20 by the secondary transfer power supply 21 and the voltage applied to the brush member 19 by the high-voltage power supply 60. A total of the amount of current flowing through the secondary transfer roller 20 and the amount of current flowing through the brush member 19 is controlled to be a predetermined current amount or larger required for transferring toner images formed on the multiple photosensitive drums 1a to 1d on the intermediate transfer belt 10 (TB≥TA).
[Secondary Transfer Power Supply and Current Control]
Next, description will be made about a current control over the secondary transfer power supply 21 being a first application unit and the high-voltage power supply 60 being a charge power supply. Specifically, a controller 100 being a control unit is configured to control the secondary transfer power supply 21 and the high-voltage power supply 60, so as to supply current from the secondary transfer roller 20 and the brush member 19 to the intermediate transfer belt 10. As described above, a current necessary for the primary transfer is 20 μA. Therefore, when a summed current of a current flowing from the brush member 19 and a current flowing from the secondary transfer roller 20 is 20 μA or larger, a potential necessary for the primary transfer is retained. Hence, supplying a current of 5 μA or larger from the brush member 19 makes the summed current 20 μA or larger even when the current supplied from the secondary transfer roller 20 is 15 μA, and the secondary transfer and the primary transfer can be executed satisfactorily.
[Image Forming Operation]
Next, in the image forming operation in Embodiment 5, description will be made about a relation between the secondary transfer voltage, the potential of the primary transfer units, and the current flowing into the primary transfer units, in a course from start of the image forming operation, via the primary transfer, to completion of the secondary transfer, with reference to a timing chart of
The image forming operation is started by the controller 100 outputting an image signal. Before the primary transfer is started, at a timing S11, application of the voltage V2 from the secondary transfer power supply 21 to the secondary transfer roller 20 is started under control of the transfer control unit 201. Assume that, for example, 13 μA is set to a current flowing through the secondary transfer roller 20 as voltage is applied to the secondary transfer roller 20 from the secondary transfer power supply 21. This setting causes a current supply from the secondary transfer roller 20 to the primary transfer units to be started. At timing S11, a current supply to the primary transfer units is started also from the brush member 19. At timing S11, a current of 13 μA is supplied from the secondary transfer roller 20, and a current of 7 μA is supplied from the brush member 19. Therefore, an optimal primary transfer current of, for example, 20 μA is supplied to the primary transfer units.
At timing S12, the primary transfer is started with the first image formation station a. Toner images are transferred one by one from the photosensitive drums 1a to 1d to the intermediate transfer belt 10. At timing S13, toner images on the intermediate transfer belt 10 reach the secondary transfer unit. The transfer control unit 201 changes the secondary transfer voltage to the voltage V3 necessary for the secondary transfer, transferring the toner images on a recording material P. When the voltage V3 is applied from the secondary transfer power supply 21 to the secondary transfer roller 20, the secondary transfer current i2 flowing into the secondary transfer roller 20 is an optimal current I2 of, for example, 15 μA. Here, since a current of 15 μA is supplied from the secondary transfer roller 20, and a current of 7 μA is supplied from the brush member 19, a total current value of these currents is 22 μA, which is larger than an optimal primary transfer current. However, by the action of the current restriction circuit 17, the optimal primary transfer current, for example, 20 μA is supplied to the metal rollers 14.
Next, at timing S14, the primary transfer is terminated. The current supply from the brush member 19 is terminated. With this termination, the primary transfer current decreases at timing S14. At timing S15, the secondary transfer is terminated, and the current supply from the secondary transfer roller 20 is terminated. With this termination, the primary transfer current becomes zero at timing S15. At timing S15, the image forming operation is terminated.
In this manner, the transfer control unit 201 causes the voltage V2, which is a third voltage, to be applied from the secondary transfer power supply 21 to the secondary transfer roller 20 before toner images formed on the respective multiple photosensitive drums 1a to 1d are transferred to the intermediate transfer belt 10. The transfer control unit 201 causes a fourth voltage to be applied from the high-voltage power supply 60 to the brush member 19 before toner images formed on the respective multiple photosensitive drums 1a to 1d are transferred to the intermediate transfer belt 10. To transfer the toner images on the intermediate transfer belt 10 on the recording material P, the transfer control unit 201 causes the voltage V3 to be applied to the secondary transfer roller 20 while maintaining the application of the fourth voltage from the high-voltage power supply 60. The voltage V3 is a fifth voltage higher than the voltage V2, which is a third voltage.
As illustrated in
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. 2016-249533, filed Dec. 22, 2016 which is hereby incorporated by reference herein in its entirety.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2016249533 | Dec 2016 | JP | national |
