This application is based on and claims priority under 35 USC 119 from Japanese Patent Application No. 2017-197572 filed Oct. 11, 2017.
The present invention relates to image forming apparatuses.
According to an aspect of the invention, there is provided an image forming apparatus including a transfer unit, a contact unit, and a constant-current controller. The transfer unit nips a recording medium by using an image retaining unit, which retains an image by using a charged imaging particle, and a transfer member and generates a transfer electric field in a transfer region between the image retaining unit and the transfer member so as to electrostatically transfer the image retained by the image retaining unit onto the recording medium. The contact unit is provided at an upstream side and a downstream side of the transfer region in a transport direction of the recording medium and comes into contact with the recording medium while the recording medium passes through the transfer region, so as to function as an electrode leading to a ground. The constant-current controller performs constant-current control on a transfer current to be fed to the transfer region by using a transfer voltage applied from a transfer power source in a condition in which the recording medium is a low-resistance recording medium having a predetermined resistance value or lower or having an electrically-conductive layer along a medium base surface.
Exemplary embodiments of the present invention will be described in detail based on the following figures, wherein:
In
In
In such a technical solution, this exemplary embodiment is intended to improve the transfer performance with respect to a low-resistance recording medium S. Although the type of recording medium may be selected as appropriate, the exemplary embodiment is effective especially when adding a low-resistance recording medium, such as a metallic sheet, as a transfer target.
In this example, the image retaining unit 1 may be an intermediate transfer member of an intermediate transfer type or a dielectric member of a direct transfer type, so long as the image retaining unit 1 is configured to retain an image G thereon.
The transfer unit 2 has the transfer member 2a that comes into contact with the recording medium S, and the transfer member 2a may be a roller-shaped member or a belt-shaped member so long as the transfer member 2a has a function for nipping and transporting the recording medium S in cooperation with the image retaining unit 1 and a function for causing a transfer electric field to occur in the transfer region TR between the transfer member 2a and the image retaining unit 1.
Furthermore, although the transfer unit 2 of a widely-used type has the opposing member 2b disposed facing the transfer member 2a at the back surface of the image retaining unit 1, the transfer unit 2 is not limited to this type and may include a type in which an image electrode is incorporated in the image retaining unit 1.
Moreover, a low-resistance recording medium S may be a recording medium having a predetermined resistance value or lower or may be a recording medium having an electrically-conductive layer along the medium base surface. The latter may sometimes be included in the former, but is sometimes not included in the former, such as a case where the recording medium has a high-resistance surface layer (the resistance thereof being measured using a measuring technique set in accordance with the Japanese Industrial Standards (JIS)). However, even if the latter is not included in the former, since the recording medium often apparently shows a low-resistance behavior in which a high transfer voltage applied thereto travels in the planar direction, such a recording medium is also treated as a low-resistance recording medium.
The contact unit 3 widely includes a direct grounded type, a resistance grounded type, and a bias grounded type so long as the contact unit 3 is of a type other than a non-grounded (floating) type. Furthermore, the contact unit 3 may include at least one member provided at each of the inlet and outlet sides of the transfer region TR such that at least one of the members of the contact unit 3 comes into contact with the recording medium S, while the recording medium S passes through the transfer region TR, so as to function as an electrode leading to the ground. In this example, the contact unit 3 includes multiple contact members 3a and 3b at the upstream side of the transfer region TR in the transport direction of the recording medium S, and also includes a single contact member 3c at the downstream side of the transfer region TR in the transport direction of the recording medium S. The contact member 3a is a guide member that guides the recording medium S along a transport path, and the contact member 3b is a positioning member that positions the recording medium S. The contact member 3c is, for example, a belt-shaped transport member that transports the recording medium S. In
Furthermore, in this exemplary embodiment, if a low-resistance recording medium S is to be used, the constant-current controller 4a causes a constant transfer current to flow to the transfer region TR.
Normally, in order to transfer charged imaging particles (such as toner) to the recording medium S from the image retaining unit 1, such as an intermediate transfer member, it is necessary to stably generate an optimal electric field for the type of the recording medium S (1), the width of the recording medium S (2), and the resistance of the transfer member 2a (3), which are variable. For achieving this, there are constant-voltage control and constant-current control. In constant-voltage control, the optimal voltage varies due to being affected by the type of the recording medium S (1) and the resistance of the transfer member 2a (3), but is not affected by the width of the recording medium S (2). In constant-current control, the optimal voltage is affected by the width of the recording medium S (2), and the way in which the optimal voltage is affected varies depending on the resistance of the transfer member 2a (3). However, the optimal voltage is basically less likely to be affected by the type of the recording medium S (1) and the resistance of the transfer member 2a (3).
Normally, constant-voltage control is often employed as a control method for the recording medium S. This is because, even in constant-current control, the type of the recording medium S (1) and the resistance of the transfer member 2a (3) are not made completely ineffective, whereas at least the width of the recording medium S (2) is reliably made ineffective in constant-voltage control.
With reference to an example in which a low-resistance recording medium S has an electrically-conductive layer along the medium base surface, assuming that a constant transfer voltage is applied to the recording medium S, the recording medium S would have the same electric potential at any location within the surface thereof, so that the transfer voltage would spread over the entire surface of the recording medium S during the transfer process. This implies that there is a possibility of leakage of the transfer current to all members of the contact unit 3 (e.g., the contact members 3a to 3c) existing in the entire surface range of the recording medium S. The member receiving the leakage and the amount of leakage depend on the position of the recording medium S being transported and the resistance values of the contact members 3a to 3c and sequentially change in accordance with the transporting process of the recording medium S. Accordingly, in a case where a low-resistance recording medium S is used, since the impedance of the current path through which the transfer current flows changes in accordance with the transporting process of the recording medium S, there is a high possibility that the transfer electric field of the transfer region TR may change when the constant-voltage control method of applying a constant transfer voltage is employed.
In this exemplary embodiment, the constant-current controller 4a performs constant-current control when a low-resistance recording medium S is used, so that even if the impedance of the current path through which the transfer current flows changes in accordance with the transporting process of the recording medium S, a constant transfer current flows to the transfer region TR, whereby a stable transfer electric field is constantly generated.
This situation also occurs in a low-resistance recording medium, such as black paper, containing a large amount of an electrically-conductive material, such as carbon black.
Next, a representative example of the image forming apparatus according to this exemplary embodiment will be described.
One representative example of the image forming apparatus when a freely-chosen type of recording medium S is used has a determining unit 5 that is capable of determining the type of recording medium S traveling toward the transfer region TR and determines whether or not the constant-current controller 4a is necessary based on a determination signal of the determining unit 5.
A representative example of the determining unit 5 is a detector that detects whether or not the traveling recording medium S is a low-resistance recording medium. In this example, it is desirable that the determining unit 5 is capable of determining a low-resistance recording medium S while the recording medium S is traveling.
A representative example that determines whether or not the constant-current controller 4a is necessary includes a selecting unit 6 that selects the constant-current controller 4a when the recording medium S is a low-resistance recording medium and that selects a constant-voltage controller (which includes the transfer power source 2c in this example) when the recording medium S is not a low-resistance recording medium.
In this example, if the constant-current controller 4a is selected, a detector 7 may detect the transfer current flowing to the transfer region TR, and the transfer voltage VTR of the transfer power source 2c may be controlled based on the detection signal such that the transfer current becomes a constant current.
Furthermore, as shown in
Moreover, as a desirable layout of the contact unit 3 at the inlet and outlet sides of the transfer region TR, the members of the contact unit 3 provided at the upstream side and the downstream side of the transfer region TR in the transport direction of the recording medium S are separated from each other by a distance d that is shorter than a length ds of the recording medium S in the transport direction. In addition to being effective when causing a transfer current to flow from the contact unit 3 to the ground over the entire length of a low-resistance recording medium S in a state where the recording medium S extends astride the members of the contact unit 3 disposed at the inlet and outlet sides of the transfer region TR, this example suppresses a leakage of electric current to a non-passing section of the recording medium S in a region other than the contact region between the recording medium S and the contact unit 3.
Furthermore, when a low-resistance recording medium S passes through the transfer region TR, the recording medium S causes at least the contact unit 3 (i.e., the contact members 3a and 3b in this example) located at the inlet side of the transfer region TR and the contact unit 3 (i.e., the contact member 3c in this example) located at the outlet side of the transfer region TR to function as an electrode leading to the ground. In addition to being effective when causing a transfer current to flow to the ground from at least one of the members of the contact unit 3 disposed at the inlet and outlet sides of the transfer region TR, this example prevents the transfer current from flowing toward the transfer member 2a and causes a sufficient amount of constant transfer current to flow as a transfer electric field to the entire surface of the low-resistance recording medium S, thereby achieving a stable image.
When a low-resistance recording medium S is used, the transfer unit 2 may switch the transfer member 2a from a grounded state to a non-grounded state. In this example, the transfer member 2a is switched to a non-grounded state when a low-resistance recording medium S is used, so that the current path to the transfer member 2a is blocked off. This is effective for causing the transfer current of the transfer region TR to stably and entirely flow to the ground from the contact unit 3 located at the inlet and outlet sides of the transfer region TR without flowing to the transfer member 2a, thereby achieving a stable image over the entire surface of the recording medium S.
In this exemplary embodiment, a method of generating a transfer electric field in accordance with constant-current control with respect to a low-resistance recording medium S is employed. When employing this method, a second-transfer section desirably includes the following components.
Specifically, as shown in
Exemplary embodiments of the present invention will be described below in further detail with reference to the appended drawings.
Overall Configuration of Image Forming Apparatus
In
Image Forming Sections
In this exemplary embodiment, each of the image forming sections 22 (22a to 22f) has a drum-shaped photoconductor 23, and the photoconductor 23 is surrounded by a charging device 24 such as a corotron or a transfer roller that electrostatically charges the photoconductor 23, an exposure device 25 such as a laser scanning device that writes an electrostatic latent image onto the electrostatically-charged photoconductor 23, a developing device 26 that uses the corresponding color component toner to develop the electrostatic latent image written on the photoconductor 23, a first-transfer device 27 such as a transfer roller that transfers the toner image on the photoconductor 23 onto the intermediate transfer member 30, and a photoconductor cleaning device 28 that removes residual toner from the photoconductor 23.
The intermediate transfer member 30 is wrapped around multiple (three in this exemplary embodiment) tension rollers 31 to 33. For example, the tension roller 31 is used as a drive roller driven by a drive motor (not shown), and the intermediate transfer member 30 is moved in a circulating manner by the drive roller. Moreover, an intermediate-transfer-member cleaning device 35 for removing residual toner from the intermediate transfer member 30 after a second-transfer process is provided between the tension rollers 31 and 33.
Second-Transfer Device (Collective Transfer Device)
Furthermore, as shown in
Furthermore, the opposing roller 56 (also serving as the tension roller 33 in this example) is supplied with a transfer voltage VTR from a transfer power source 60 via an electrically-conductive feed roller 57, so that a predetermined transfer electric field is generated between the elastic transfer roller 55 and the opposing roller 56.
In this example, the transfer power source 60 is capable of selecting between constant-voltage control and constant-current control. Specifically, in the transfer power source 60, the transfer voltage VTR is set in an adjustable manner based on a signal from an output signal generator 62, and the output signal generator 62 is connected to a constant-current control circuit 61. A feedback ammeter 63 is connected in series between the transfer power source 60 and the feed roller 57, a feedback current path is provided between the ammeter 63 and the constant-current control circuit 61, and a selection switch 64 is provided at an intermediate location of this feedback current path. By turning the selection switch 64 on or off, it is selected whether or not feedback-based constant-current control is to be performed. When the selection switch 64 is turned on, an electric current value monitored at the ammeter 63 is fed back to the output signal generator 62 via the constant-current control circuit 61, and the transfer voltage VTR in the transfer power source 60 is set in an adjustable manner such that a transfer current ITR in the second-transfer region TR becomes a constant current.
Although the transfer roller 55 is disposed in pressure contact with the intermediate transfer member 30 in the second-transfer device 50 in this example, a belt transfer module configured by wrapping a transfer belt between tension rollers, one of which is served by the transfer roller 55, may be used as an alternative.
Fixing Device
As shown in
Sheet Transport System
As shown in
Furthermore, the sheet transport system 80 has a branch transport path 87 that branches off downward from a point of the horizontal transport path 84 located downstream of the fixing device 70 in the sheet transport direction and that is capable of inverting the sheet S. The sheet transport system 80 returns the sheet S inverted in the branch transport path 87 to the horizontal transport path 84 from the vertical transport path 83 via a transport path 88, allows another image to be transferred onto the back face of the sheet S in the second-transfer region TR, and outputs the sheet S to the sheet output tray 86 via the fixing device 70.
In addition to a positioning roller 90 that positions the sheet S and feeds the sheet S to the second-transfer region TR, the sheet transport system 80 is also provided with an appropriate number of transport rollers 91 in the transport paths 83, 84, 87, and 88.
Moreover, at the opposite side from the sheet output tray 86, the image forming apparatus housing 21 is provided with a manual sheet feeder 92 used for manually feeding a sheet S toward the horizontal transport path 84.
Furthermore, at the inlet side of the second-transfer region TR, the horizontal transport path 84 is provided with a guide chute 93 that guides the sheet S that has passed through the positioning roller 90 toward the second-transfer region TR. In this example, a single guide chute 93 is provided between the positioning roller 90 and the second-transfer region TR and is constituted of a pair of metallic chute components that are disposed facing each other, so as to regulate the guide path for the sheet S.
As an alternative to this example in which a single guide chute 93 is provided between the positioning roller 90 and the second-transfer region TR, multiple (e.g., two) guide chutes 93 may be provided. In the case where multiple guide chute 93 are provided, the guide chutes 93 may be disposed at different angles and positions from each other, thus increasing the degree of freedom for adjusting the guide path for the sheet S.
Moreover, at the outlet side of the second-transfer region TR, an antistatic needle 96 as a static eliminating member is provided between the second-transfer region TR and the transport belt 85. When the sheet S is disposed close to the antistatic needle 96 after the second-transfer process, the antistatic needle 96 discharges the electric charge from the electrostatically-charged sheet S so as to remove static electricity therefrom.
Sheet Type
A sheet S usable in this example may be, for example, plain paper with a surface resistance of 1010 to 1012 a/sq. or a low-resistance sheet Sm with a surface resistance lower than that of plain paper.
A representative example of a low-resistance sheet Sm is, for example, a so-called metallic sheet formed by stacking a metallic layer composed of aluminum (e.g., an aluminum deposited surface) 101 on a base layer 100 composed of a sheet base material and coating the metallic layer 101 with a surface layer 102 composed of synthetic resin, such as polyethylene terephthalate (PET), as shown in
Although a metallic sheet of this type may have a predetermined surface resistance value (e.g., 106 to 107 Ω/sq.), the actual resistance value measured in accordance with the surface resistance measuring technique complying with JIS does not fall below the threshold value or lower, as in the above-mentioned metallic sheet including the surface layer 102 composed of a high resistance material, and there are types of metallic sheets that substantially act as a low-resistance sheet when the transfer voltage VTR is applied thereto.
It is also possible to form a color image constituted of, for example, YMCK (yellow, magenta, cyan, and black) colors directly on a metallic sheet serving as a low-resistance sheet Sm of this type. For example, as shown in
Examples of the low-resistance sheet Sm include black paper containing an electrically-conductive material, such as carbon black, and black coated paper in which a coating layer containing an electrically-conductive material, such as carbon black, is formed on a normal paperboard.
Configuration Example of Determining Unit
As shown in
In this example, supposing that plain paper (including a high-resistance sheet other than a low-resistance sheet) is used as the sheet S, since the surface resistance of plain paper is high to a certain extent, the determination current from the determination power source 113 flows across the pair of determination rollers 111, as indicated by a dotted line in
In contrast, supposing that a low-resistance sheet, such as a metallic sheet, is used as the sheet S, since the surface resistance of a low-resistance sheet is lower than that of plain paper, when the low-resistance sheet is disposed astride the pairs of determination rollers 111 and 112, a portion of the determination current from the determination power source 113 flows across the pair of determination rollers 111, as indicated by a solid line in
As an alternative to this example in which the determining unit 110 determines the sheet type by measuring the surface resistance of the sheet S being transported, for example, the sheet type may be determined based on a designation signal when the user designates the sheet type, or the sheet type (especially, a metallic sheet type) may be determined by using a light reflective sensor provided in the sheet transport path.
Sheet Contact Members Located at Inlet and Outlet Sides of Second-Transfer Region
In this exemplary embodiment, as shown in
In this example, the positioning roller 90 is constituted of a metallic roller and is connected to ground via a resistor 94. The guide chute 93 is constituted of metallic chute components that are connected to ground via a resistor 95. The resistor 94 selected for the positioning roller 90 and the resistor 95 selected for the guide chute 93 have resistance values lower than that of the transfer roller 55 (volume resistivity in this example).
As an alternative to this example in which the resistors 94 and 95 are selected by comparing the resistance values thereof with the resistance value of the transfer roller 55, if the second-transfer device 50 is, for example, a belt transfer module, the resistors 94 and 95 may be selected by comparing the resistance values thereof with the resistance value from the belt transfer module to the ground. Furthermore, as an alternative to this example in which a resistance grounding method of connecting the positioning roller 90 and the guide chute 93 to ground via the resistors 94 and 95 is employed, the positioning roller 90 and the guide chute 93 may be directly connected to ground.
Furthermore, in this example, the transport belt 85 is constituted of, for example, a belt member 85a composed of electrically-conductive rubber and tensely wrapped between a pair of tension rollers 85b and 85c. At least one of the tension rollers 85b and 85c (e.g., 85c) is composed of metal, electrically-conductive resin, or a combination of these materials, and the cored bar of the tension roller is directly connected to ground.
Although the antistatic needle 96 is not necessarily a contact member that always comes into contact with the sheet S, the antistatic needle 96 is directly connected to ground. Therefore, when the sheet S that has passed through the second-transfer region TR moves close to the antistatic needle 96, a discharge phenomenon occurs between the two, whereby static electricity is removed from the sheet S.
Furthermore, in this exemplary embodiment, the length d of the sheet transport path between the guide chute 93 and the transport belt 85, which are sheet contact members immediately located at the inlet and outlet sides of the second-transfer region TR, is set to be shorter than the length ds, in the transport direction, of a minimum-size sheet usable as a low-resistance sheet Sm. Therefore, at least in the transporting process in which the sheet S passes through the second-transfer region TR, the sheet S is disposed astride the second-transfer region TR and the guide chute 93 or the transport belt 85.
Drive Control System of Image Forming Apparatus
As shown in
Operation of Image Forming Apparatus
Assuming that sheets S with different surface resistance values are used in a mixed fashion in the image forming apparatus shown in
In this case, a sheet S is fed from the sheet feed container 81 or 82 or from the manual sheet feeder 92 and is transported toward the second-transfer region TR via a predetermined transport path. Before the sheet S reaches the second-transfer region TR, the determining unit 110 measures the surface resistance of the sheet S (i.e., performs a sheet-type determination process).
The controller 120 determines whether or not the sheet S is a low-resistance sheet based on the determination result of the determining unit 110. If the sheet S is a low-resistance sheet, a feedback circuit including the constant-current control circuit 61 is selected by using the selection switch 64, so that constant-current control is executable.
In contrast, if the controller 120 determines that the sheet S is not a low-resistance sheet, the feedback circuit is disabled by using the selection switch 64, and constant-voltage control is executed by the transfer power source 60.
Subsequently, when the sheet S reaches the second-transfer region TR, images G formed at the image forming sections 22 (22a to 22f) and first-transferred to the intermediate transfer member 30 are second-transferred onto the sheet S. Then, the sheet S undergoes a fixing process performed by the fixing device 70 and is output onto the sheet output tray 86, whereby the sequential printing operation (image forming operation) ends.
Second-Transfer Process
High-Resistance Sheet
In a case where the sheet S is a high-resistance sheet St (widely including sheets other than a low-resistance sheet Sm and including plain paper), the feedback circuit including the constant-current control circuit 61 is not selected, as shown in
In this state, the high-resistance sheet St reaches the second-transfer region TR via the positioning roller 90 and the guide chute 93, and the images G on the intermediate transfer member 30 are second-transferred onto the sheet S in the second-transfer region TR. In this case, even if the high-resistance sheet St comes into contact with the positioning roller 90, the guide chute 93, or the transport belt 85 while the high-resistance sheet St passes through the second-transfer region TR, the surface resistance of the high-resistance sheet St is high enough so that the transfer operation is stably performed on the high-resistance sheet St in the second-transfer region TR without a portion of the transfer current in the second-transfer region TR leaking via the high-resistance sheet St as a current path leading to the ground for the positioning roller 90, the guide chute 93, or the transport belt 85, thereby preventing the occurrence of trouble, such as reduced image density in a part of the high-resistance sheet St.
Low-Resistance Sheet
The following description relates to a case where the sheet S is a low-resistance sheet (e.g., metallic sheet) Sm.
In this case, as shown in
Therefore, the transfer voltage VTR constant-current-controlled by the transfer power source 60 is applied from the feed roller 57 toward the opposing roller 56 in the second-transfer region TR, so that a transfer electric field is generated from the intermediate transfer member 30 side.
In this state, the low-resistance sheet Sm passes through the second-transfer region TR via the positioning roller 90 and the guide chute 93 and travels while moving into contact with or in close proximity to the antistatic needle 96 and the transport belt 85. As shown in
ZBUR+ITB: Impedance of Opposing Roller 56 and Intermediate Transfer Member 30
ZBTR: Impedance of Transfer Roller 55
Ztoner: Impedance of Toner
ZSheetBaseMaterial: Impedance of Base Layer 100 of Low-Resistance Sheet Sm
ZMetallicLayer: Impedance of Metallic Layer 101 of Low-Resistance Sheet Sm
ZRoller: Impedance of Positioning Roller 90
ZChute: Impedance of Guide Chute 93
ZBTR: Impedance of Transfer Roller 55
ZDTS: Impedance of Antistatic Needle 96
ZBelt: Impedance of Transport Belt 85
In
In the equivalent circuit shown in
In this state, the currents ITR1 to ITR4 flowing distributively to the contact members coming into contact with the low-resistance sheet Sm or the proximity members coming into close proximity to the low-resistance sheet Sm are set depending on the respective impedances ZRoller, ZChute, ZDTS, and ZBelt, but since the transfer current ITR in the second-transfer region TR is the sum of the currents ITR1 to ITR4 flowing distributively to the contact members coming into contact with the low-resistance sheet Sm or the proximity members coming into close proximity to the low-resistance sheet Sm, the transfer current ITR in the second-transfer region TR is no longer dependent on the contact members or proximity members.
In a case where the low-resistance sheet Sm travels at a position upstream, in the transport direction, of the position of the low-resistance sheet Sm shown in
In a case where the low-resistance sheet Sm travels at a position downstream, in the transport direction, of the position of the low-resistance sheet Sm shown in
Accordingly, in this exemplary embodiment, the length d of the sheet transport path between the guide chute 93 and the transport belt 85 located at the inlet and outlet sides of the second-transfer region TR is set to be shorter than the length ds, in the transport direction, of the low-resistance sheet Sm, so that the low-resistance sheet Sm is in contact with at least one contact member located at the inlet or outlet side of the second-transfer region TR while the low-resistance sheet Sm passes through the second-transfer region TR. Thus, by generating a transfer electric field from the intermediate transfer member 30 side, a constant transfer current ITR stably flows to a toner image as an image G located between the intermediate transfer member 30 and the low-resistance sheet Sm in the second-transfer region TR.
Furthermore, even if the impedance of the current path changes during the transporting process of the low-resistance sheet Sm, the transfer current ITR flowing through the second-transfer region TR is controlled to a constant current by the constant-current control circuit 61. Thus, for example, even in a case where a halftone image is formed on the low-resistance sheet Sm, the transfer current ITR does not change rapidly, and there is no concern that uneven image densities may occur as a result of insufficient transfer current ITR.
Improvement with Regard to Effect on Sheet Width by Constant-Current Control
As shown in
Normally, constant-current control is affected by the sheet width when there is too much transfer current leaking to the non-passing section SB located at the outer side of the passing section SA. If the electric current simply leaks in accordance with the area ratio between the passing section SA and the non-passing section SB, there is no effect on transferability. If a uniform current density is achieved in the axial direction of the nip region of the second-transfer region TR, an electric current (i.e., an electric field) necessary for the toner layer is obtained. However, the impedance of the passing section SA is higher than that of the non-passing section SB by an amount equivalent to the impedances Ztoner and ZsheetBaseMaterial of the toner layer and the sheet base material, thus causing the electric current to flow inevitably toward the non-passing section SB. Therefore, in a case where the same transfer current is fed from the intermediate transfer member 30 side, the current density in the passing section SA inevitably becomes insufficient. This implies that the transfer electric field acting on the toner layer is insufficient, thus causing a transfer defect.
However, in this exemplary embodiment, even when constant-current control is executed on the low-resistance sheet Sm, the phenomenon of the transfer current ITR leaking toward the non-passing section SB is minimized in accordance with the following reasons.
In this exemplary embodiment, the grounding conditions (i.e., impedance conditions) for the contact members (such as the positioning roller 90, the guide chute 93, and the transport belt 85) that are disposed at the inlet and outlet sides of the second-transfer region TR and that are to come into contact with the low-resistance sheet Sm are set to be lower than the impedance of the transfer roller 55, as indicated by expression 1 below.
ZRoller,ZChute,ZBelt<ZBTR (1)
As shown in
Although this example has been described with reference to the transport belt 85 as an example, the effect caused by the width of the low-resistance sheet Sm may be minimized in accordance with similar reasons in a state where the low-resistance sheet Sm comes into contact with the positioning roller 90 or the guide chute 93 located at the inlet side of the second-transfer region TR.
In order to evaluate the minimization capability against the effect caused by the width of the low-resistance sheet Sm in and around the second-transfer section of the image forming apparatus according to this exemplary embodiment, the behavior in and around a second-transfer section of an image forming apparatus according to a first comparative example will be described.
The structure of and around the second-transfer section according to this comparative example is substantially similar to that in the first exemplary embodiment but differs from that in the first exemplary embodiment in that the grounding conditions (i.e., impedance conditions) for the contact members (such as the positioning roller 90, the guide chute 93, and the transport belt 85) that are disposed at the inlet and outlet sides of the second-transfer region TR and that are to come into contact with the passing section SA are set to be higher than the impedance of the transfer roller 55, as indicated by expression 2 below.
ZRoller,ZChute,ZBelt>ZBTR (2)
As shown in
In
This exemplary embodiment exhibits effects substantially similar to those of the image forming apparatus according to the first exemplary embodiment but differs from the first exemplary embodiment in that the transfer roller 55 of the second-transfer device 50 is set in a non-grounded state (i.e., floating state) when a low-resistance sheet Sm is used.
Therefore, in this exemplary embodiment, when the low-resistance sheet Sm, such as a metallic sheet, passes through the second-transfer region TR, the transfer voltage VTR from the transfer power source 60 is applied from the feed roller 57 toward the opposing roller 56 via the constant-current control circuit 61, as in the first exemplary embodiment, so that a transfer electric field is generated in the second-transfer region TR from the intermediate transfer member 30 side. Thus, the transfer current ITR flows along the metallic layer 101 of the low-resistance sheet Sm and flows to a path leading to the ground from the members (i.e., the positioning roller 90, the guide chute 93, the antistatic needle 96, and the transport belt 85) coming into contact with or into close proximity to the low-resistance sheet Sm. Since the transfer roller 55 is in a non-grounded state in this example, a portion of the transfer current ITR does not flow toward the transfer roller 55.
Accordingly, in this exemplary embodiment, the current path to the transfer roller 55 is completely blocked off when a low-resistance sheet Sm is used. Therefore, although there is a concern in the first exemplary embodiment that a portion of the transfer current ITR may flow as a leaking current toward the transfer roller 55 via the passing section and the non-passing section, a portion of the transfer current ITR may be prevented from leaking toward the transfer roller 55 in this exemplary embodiment, regardless of the passing section and the non-passing section. As an alternative to this exemplary embodiment in which the switching to the non-grounded state is performed by using the switch 130, it is possible to perform switching to the ground via a resistor with a resistance value sufficiently higher than the impedances of the contact members.
The foregoing description of the exemplary embodiments of the present invention has been provided for the purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise forms disclosed. Obviously, many modifications and variations will be apparent to practitioners skilled in the art. The embodiments were chosen and described in order to best explain the principles of the invention and its practical applications, thereby enabling others skilled in the art to understand the invention for various embodiments and with the various modifications as are suited to the particular use contemplated. It is intended that the scope of the invention be defined by the following claims and their equivalents.
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