Image forming apparatus to control transfer voltage for secondary transfer to recording material

An image forming apparatus includes an intermediate transfer belt, an outer roller, a power source, and a regulation member. The outer roller transfers a toner image onto a recording material. The power source applies a voltage to the outer roller. The regulation member regulates orientation of the recording material. Information is acquired regarding a timing at which orientation of a recording material first portion, passing through the transfer portion that is located between the outer roller and the regulation member, can change to move closer to the intermediate transfer belt. Based on the acquired information, a transfer voltage target value for transfer onto the recording material is changed between a first target value of a period anterior to the timing and a second target value of a period posterior to the timing that is larger in absolute value than the first target value.

BACKGROUND OF THE DISCLOSURE

Field

The present disclosure relates to an image forming apparatus such as a printer, a copier, or a facsimile apparatus that uses an electrophotographic method or an electrostatic recording method.

Description of the Related Art

An example of image forming apparatuses that use the electrophotographic method is a conventional image forming apparatus that uses an intermediate transfer method. In such an image forming apparatus, a toner image formed on a photosensitive member serving as an image bearing member is primarily transferred onto an intermediate transfer member at a primary transfer portion, and then secondarily transferred from the intermediate transfer member onto a recording material such as paper at a secondary transfer portion. In a color image forming apparatus that uses the intermediate transfer method, a plurality of photosensitive members is arranged in a moving direction of an intermediate transfer member. Toner images of respective colors are primarily transferred from the plurality of photosensitive members onto the intermediate transfer member to be superimposed on one another. As the intermediate transfer member, an endless intermediate transfer belt stretched around a plurality of stretching rollers is widely used. The primary transfer is often performed by a primary transfer voltage being applied from a primary transfer power source to a primary transfer member provided on an opposite side of a photosensitive member across the intermediate transfer belt. The secondary transfer is often performed by a secondary transfer voltage being applied from a secondary transfer power source to a secondary transfer member being in contact with one of the plurality of stretching rollers via the intermediate transfer belt.

Japanese Patent Application Laid-Open No. 2012-98709 proposes a configuration in which primary transfer is performed by applying a voltage to a current supply member being in contact with an outer circumferential surface of an intermediate transfer belt. In the configuration, the intermediate transfer belt is a conductive belt that has low electric resistance (hereinafter, will be simply referred to as resistance) and has conductivity that enables a current flow from the current supply member in a circumferential direction. In addition, in the configuration, a current necessary for primary transfer is supplied from the current supply member to a plurality of photosensitive members via the intermediate transfer belt. In the configuration, a secondary transfer member can be used as the current supply member.

Nevertheless, in the case of using the low-resistance intermediate transfer belt having conductivity that enables the current flow in the circumferential direction as discussed in Japanese Patent Application Laid-Open No. 2012-98709, for example, if the orientation of a recording material changes when passing through a secondary transfer portion, a current flowing in the vicinity of the secondary transfer portion might change. This is because the current flowing in the secondary transfer portion is likely to flow into the vicinity of the secondary transfer portion due to low electric resistance of the intermediate transfer belt. If the current flowing in the secondary transfer portion changes in this manner, in a case where a secondary transfer voltage drops due to a change in load on a secondary transfer power source, a current necessary for secondarily transferring a toner image from the intermediate transfer belt onto the recording material becomes insufficient, and a transfer defect might occur.

SUMMARY OF THE DISCLOSURE

The present disclosure is directed to preventing an occurrence of a transfer defect attributed to a change in the orientation of a recording material conveyed in the vicinity of a secondary transfer portion in a configuration in which an intermediate transfer belt having conductivity that enables a current flow in the circumferential direction is used.

According to an aspect of the present disclosure, an image forming apparatus includes an image bearing member configured to bear a toner image, an intermediate transfer belt that is endless and is configured to form a primary transfer portion by being in contact with the image bearing member, wherein the toner image is primarily transferred onto the intermediate transfer belt from the image bearing member at the primary transfer portion, a plurality of stretching rollers configured to stretch the intermediate transfer belt, the plurality of stretching rollers including an inner roller, an outer roller configured to form a secondary transfer portion by being in contact with the intermediate transfer belt at a position facing the inner roller, and configured to secondarily transfer the toner image from the intermediate transfer belt onto a recording material at the secondary transfer portion, a power source configured to apply a voltage to the outer roller, a regulation member arranged upstream of the secondary transfer portion in a conveyance direction of the recording material, wherein the regulation member is configured to regulate orientation of the recording material by being in contact with the outer roller and the regulation member, and a control unit configured to control a secondary transfer voltage to be output by the power source for secondary transfer, wherein the intermediate transfer belt has conductivity enabling a current flow in a circumferential direction, and wherein the control unit controls to acquire information regarding a timing at which orientation of a first portion of the recording material, passing through the secondary transfer portion that is located between the outer roller and the regulation member, can change to move closer to the intermediate transfer belt and, based on the acquired information, controls to change a target value of the secondary transfer voltage for the secondary transfer onto the recording material between a first target value of a period anterior to the timing and a second target value of a period posterior to the timing that is larger in absolute value than the first target value.

DESCRIPTION OF THE EMBODIMENTS

Hereinafter, an image forming apparatus according to an exemplary embodiment of the present disclosure will be described in more detail with reference to the drawings.

1. Overall Configuration and Operation of Image Forming Apparatus

FIG.1is a schematic cross-sectional diagram of an image forming apparatus100according to a first exemplary embodiment. The image forming apparatus100according to the first exemplary embodiment is a full-color laser printer employing an inline method and an intermediate transfer method that can form a full-color image using an electrophotographic method. The image forming apparatus100can form a full-color image on a recording material P (e.g., recording paper, plastic sheet) based on image information. The image information is input to the image forming apparatus100from an image reading device provided in the image forming apparatus100or connected to the image forming apparatus100, or a host device such as a personal computer connected to the image forming apparatus100in such a manner that communication can be performed.

The image forming apparatus100includes, as a plurality of image forming units, first, second, third, and fourth image forming units (stations) Sa, Sb, Sc, and Sd that respectively form yellow (Y), magenta (M), cyan (C), and black (K) toner images. In the first exemplary embodiment, the first, the second, the third, and the fourth image forming units Sa, Sb, Sc, and Sd are arranged in a row in a direction intersecting with a vertical direction. In the first, the second, the third, and the fourth image forming units Sa, Sb, Sc, and Sd, components having the same or corresponding function or configuration may be collectively described without letters a, b, c, and d added to the end of reference numerals for indicating colors of the stations in which the components are provided.

A photosensitive drum1being a rotatable drum-type (cylindrical) photosensitive member (electrophotographic photosensitive member) serving as an image bearing member bearing a toner image is driven to rotate in an arrow R1direction (counterclockwise direction) indicated inFIG.1, by driving force transmitted from a driving source (not illustrated) included in a driving unit. In the first exemplary embodiment, four photosensitive drums1are provided side by side in the direction intersecting with the vertical direction. The surface (outer circumferential surface) of the rotating photosensitive drum1is uniformly charged to a predetermined potential with predetermined polarity (negative polarity in the first exemplary embodiment) by a charging roller2being a roller type charging member serving as a charging unit. The charged surface of the photosensitive drum1is subjected to scanning exposure performed by an exposure device (laser scanner unit)3serving as an exposure unit, based on image information, and an electrostatic latent image (electrostatic image) corresponding to the image information is formed on the photosensitive drum1. The exposure device3forms the electrostatic latent image on the photosensitive drum1by irradiating the surface of the photosensitive drum1with laser light based on an output calculated by a central processing unit (CPU) circuit unit to be described below, based on the image information input from the host device such as a personal computer. The electrostatic latent image formed on the photosensitive drum1is developed (visualized) with toner serving as developer that is supplied by a development device4serving as a development unit, and a toner image is formed on the photosensitive drum1. In the first exemplary embodiment, toner charged to the same polarity (negative polarity in the first exemplary embodiment) as charging polarity of the photosensitive drum1adheres to an exposure portion (image portion) on the photosensitive drum1in which an absolute value of a potential is decreased as a result of the photosensitive drum1being exposed after being uniformly charged (reversal development). In the first exemplary embodiment, normal charging polarity of toner being charging polarity of toner at the time of development is negative polarity.

An intermediate transfer belt10that is an endless belt serving as an intermediate transfer member is arranged to face the four photosensitive drums1. The intermediate transfer belt10is stretched with predetermined tensional force (tension) around a driving roller11, a tension roller12, and a secondary transfer opposing roller13that serve as a plurality of stretching rollers (support rollers). An image transfer surface M is formed between the secondary transfer opposing roller13and the driving roller11. The intermediate transfer belt10rotates (revolves) in an arrow R3direction (clockwise direction) indicated inFIG.1, by the driving roller11being driven to rotate in an arrow R2direction (clockwise direction) indicated inFIG.1, by driving force transmitted from a driving source (not illustrated) included in the driving unit. Stretching rollers other than the driving roller11are driven to rotate along with the rotation of the intermediate transfer belt10. On the inner circumferential surface of the intermediate transfer belt10, primary transfer rollers14being roller type primary transfer members serving as primary transfer units are arranged corresponding to the respective photosensitive drums1. By pressing the inner circumferential surface of the intermediate transfer belt10toward the photosensitive drum1, each of the primary transfer rollers14forms a primary transfer nip (primary transfer portion) N1at which the photosensitive drum1and the intermediate transfer belt10come into contact. At the time of image formation (at the time of primary transfer), at the primary transfer nip N1, a primary transfer current flows from the intermediate transfer belt10to the photosensitive drum1due to a potential difference (primary transfer potential) between the photosensitive drum1and the intermediate transfer belt10. By the function of the primary transfer current, the toner image formed on the photosensitive drum1is primarily transferred onto the rotating intermediate transfer belt10. The supply of the primary transfer current will be further described below. For example, when a full-color image is formed, toner images of yellow, magenta, cyan, and black colors that are formed on the respective photosensitive drums1are primarily transferred sequentially onto the intermediate transfer belt10to be superimposed on one another.

On the outer circumferential surface side of the intermediate transfer belt10, a secondary transfer roller (outer roller)20being a roller type secondary transfer member serving as a secondary transfer unit is arranged at a position facing the secondary transfer opposing roller (inner roller)13. The secondary transfer roller20is pressed against the secondary transfer opposing roller13, comes into contact with the secondary transfer opposing roller13via the intermediate transfer belt10, and forms a secondary transfer nip (secondary transfer portion) N2at which the intermediate transfer belt10and the secondary transfer roller20come into contact. At the time of image formation (at the time of secondary transfer), at the secondary transfer nip N2, a secondary transfer current flows from the secondary transfer roller20to the intermediate transfer belt10due to a potential difference (secondary transfer potential) between the secondary transfer roller20and the intermediate transfer belt10. By the function of the secondary transfer current, the toner image formed on the intermediate transfer belt10is secondarily transferred onto the recording material P that is conveyed while being pinched between the intermediate transfer belt10and the secondary transfer roller20. The supply of the secondary transfer current will be further described below. Recording materials P are stored in a cassette51serving as a recording material storage portion. The recording materials P stored in the cassette51are picked up one by one by a feeding roller50being a feeding member serving as a feeding unit and fed out from the cassette51. Each of the recording materials P fed out from the cassette51is conveyed to a registration roller pair60being a conveyance member serving as a conveyance unit. As described below in detail, the registration roller pair60conveys the recording material P to the secondary transfer nip N2at a timing synchronized with the toner image on the intermediate transfer belt10.

The recording material P on which the toner image has been transferred is conveyed to a fixing device30serving as a fixing unit. The fixing device30fixes (melts, settles) the toner image onto the recording material P by applying heat and pressure to the recording material P bearing the unfixed toner image. For example, when a full-color image is formed, by heat and pressure being applied to the recording material P in the fixing device30, four-color toner on the recording material P is melted and mixed in color, and fixed onto the recording material P. In the first exemplary embodiment, the fixing device30includes a fixing roller31serving as a fixing member, and a pressing roller32serving as a pressing member that comes into pressure contact with the fixing roller31. In the first exemplary embodiment, the fixing roller31is a roller having an outer diameter of 18 mm and being obtained by forming an elastic layer of insulating silicone rubber around a metal element tube and further covering the outer circumference of the elastic layer with an insulating perfluoroalkoxy alkane (PFA) tube. The fixing roller31includes a halogen heater (not illustrated) as a heating unit. The halogen heater is not in contact with the fixing roller31, and generates heat by being supplied with a voltage from a power source (not illustrated). The pressing roller32is a roller having an outer diameter of 18 mm and being obtained by forming an elastic layer of conductive silicone rubber around a metal core and further covering the outer circumference of the elastic layer with a conductive PFA tube. By being pressed with a pressing force of 10 kgf, the fixing roller31and the pressing roller32form a fixing nip. The pressing roller32is driven to rotate by a motor (not illustrated), and the fixing roller31is driven and rotated by the rotation of the pressing roller32. Then, the recording material P is conveyed while being pinched between the fixing roller31and the pressing roller32at the fixing nip. The metal core of the pressing roller32is connected to the ground via a 1000-MΩ resistive element. By letting out charges on the fixing roller31and the pressing roller32to the ground via the pressing roller32and the resistive element, it is possible to prevent the surfaces of the fixing roller31and the pressing roller32from being charged. The recording material P on which the toner image is fixed is discharged (output) to the outside of the image forming apparatus100.

On the other hand, toner (primary transfer remaining toner) remaining on the photosensitive drum1after the primary transfer is removed and collected from the surface of the photosensitive drum1by a drum cleaning device5serving as a photosensitive member cleaning unit. In addition, toner (secondary transfer remaining toner) remaining on the intermediate transfer belt10after the secondary transfer and an adhering substance such as paper dust are removed and collected from the surface of the intermediate transfer belt10by a belt cleaning device7serving as an intermediate transfer member cleaning unit.

The image forming apparatus100is also configured to be capable of forming a single-color or a multicolor image using only one desired image forming unit S, or using some of the plurality of image forming units S.

In the first exemplary embodiment, the image forming apparatus100is a printer that supports a process speed (corresponding to circumferential velocity of the photosensitive drum1or the intermediate transfer belt10) of 148 mm/sec and A4 size paper.

In each of the image forming units S, the photosensitive drum1, and the charging roller2, the development device4, and the drum cleaning device5that serve as a process unit acting on the photosensitive drum1constitute a process cartridge6integrally attachable to and detachable from with respect to an apparatus main body of the image forming apparatus100. The process cartridge6is made attachable and detachable with respect to the image forming apparatus100via a mounting unit such as a mounting guide and a positioning member that are provided on the apparatus main body of the image forming apparatus100.

2. Transfer Configuration

In the first exemplary embodiment, the primary transfer roller14is a cylindrical metal roller having an outer diameter of 6 mm. In the first exemplary embodiment, steel use stainless (SUS) coated with nickel plating is used as a material of the primary transfer roller14. In the first exemplary embodiment, the primary transfer roller14is arranged to be shifted from the position of the photosensitive drum1toward a downstream side in a moving direction of the intermediate transfer belt10. In the first exemplary embodiment, the primary transfer roller14is arranged at a position at which the rotational center of the primary transfer roller14is shifted by 8 mm from the rotational center of the photosensitive drum1toward the downstream side in the moving direction of the intermediate transfer belt10in a cross section substantially orthogonal to a rotational axis direction of the photosensitive drum1. In the first exemplary embodiment, to prevent a contact region of the photosensitive drum1and the intermediate transfer belt10and a contact region of the primary transfer roller14and the intermediate transfer belt10from overlapping in the moving direction of the intermediate transfer belt10, the latter contact region is arranged on the downstream side of the former contact region. Then, the intermediate transfer belt10is pressed upward by the primary transfer roller14toward the photosensitive drum1to wind around the photosensitive drum1. In the first exemplary embodiment, the primary transfer roller14is arranged at a position at which the intermediate transfer belt10is pressed upward by 1 mm toward the photosensitive drum1with respect to a tangent line of the photosensitive drum1in the cross section substantially orthogonal to the rotational axis direction of the photosensitive drum1, and presses the intermediate transfer belt10with a force of about 200 gf. An amount by which the intermediate transfer belt10winds around the photosensitive drum1is thereby ensured. In the first exemplary embodiment, the primary transfer roller14is driven and rotated by the rotation of the intermediate transfer belt10.

In the first exemplary embodiment, the secondary transfer roller20forms the secondary transfer nip N2by being in contact with the intermediate transfer belt10supported by the secondary transfer opposing roller13from behind with a pressure force of 50 N. In the first exemplary embodiment, the secondary transfer roller20is a roller having an outer diameter of 18 mm and being obtained by covering the circumference of a nickel plating steel rod having an outer diameter of 8 mm with an elastic layer of an elastomeric foam (sponge) mainly containing nitrile butadiene rubber (NBR) and epichlorohydrin rubber, and having a volume resistivity of 1×108Ω·cm and a thickness of 5 mm. In the first exemplary embodiment, the secondary transfer roller20is driven and rotated by the rotation of the intermediate transfer belt10.

In the first exemplary embodiment, the intermediate transfer belt10is constituted of an endless belt having a circumferential length of 695 mm and a thickness of 90 μm. In the first exemplary embodiment, the intermediate transfer belt10is formed using polyimid resin mixed with carbon as a conducting agent. The intermediate transfer belt10exhibits electronically conductive characteristics as electric characteristics, and has a feature that is a small variation in resistance value with respect to temperature and humidity of atmosphere. While polyimid resin is used as a material of the intermediate transfer belt10in the first exemplary embodiment, the material is not limited to this. As the material of the intermediate transfer belt10, thermoplastic resin including the following resin, for example, can be desirably used. For example, resin such as polyimide (PI), polyester, polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polycarbonate, polyarylate, acrylonitrile-butadiene-styrene copolymer (ABS), polyphenylene sulfide (PPS), or polyvinylidene fluoride (PVDF), and mixed resin of these can be used. As a conducting agent, aside from carbon, conductive metallic oxide fine particles can be used.

In the first exemplary embodiment, the volume resistivity of the intermediate transfer belt10is 1×109Ω·cm. The measurement of the volume resistivity was conducted using HIRESTA-UP (MCP-HT450) with a ring probe of type UR (model MCP-HTP12) manufactured by Mitsubishi Chemical Corporation. The measurement was conducted under the condition including an indoor temperature of 23° C., an indoor humidity of 50%, an applied voltage of 100 V, and a measurement time of 10 sec. The volume resistivity is a measurement unit of conductivity for the material of the intermediate transfer belt10. The resistance in the circumferential direction of the intermediate transfer belt10was measured as described below.

FIGS.3A and3Bare schematic diagrams illustrating a measurement system of resistance in the circumferential direction of the intermediate transfer belt10(cross-sectional diagrams substantially orthogonal to the rotational axis direction of each roller to be described below). The resistance in the circumferential direction of the intermediate transfer belt10was measured using a measurement device (circumferential direction resistance measurement tool) illustrated inFIG.3A. First, the configuration of the measurement device will be described. The intermediate transfer belt10whose resistance in the circumferential direction is to be measured is stretched around an inner surface roller101(outer diameter of 30 mm) and a driving roller102(outer diameter 30 mm) without slack. The inner surface roller101formed of metal is connected to a high-voltage power source (high-voltage power source manufactured by TREK, INC.: MODEL_610E)103, and the driving roller102is electrically grounded (connected to the ground). The high-voltage power source (high-voltage power source manufactured by TREK, INC.: MODEL_610E)103can monitor an applied voltage while flowing a constant current. The driving roller102includes a metal core and an elastic layer formed by covering the outer circumference of the metal core with an elastic member. The surface of the driving roller102is covered with a conductive rubber (elastic layer) having a sufficiently-low resistance to the intermediate transfer belt10, and the driving roller102rotates so that a moving speed of the intermediate transfer belt10becomes 100 mm/sec.

Next, a measurement method will be described. In a state in which the intermediate transfer belt10is rotated by the driving roller102at the moving speed of 100 mm/sec, a constant current ILis flowed to the inner surface roller101by the high-voltage power source103, and a voltage VLbetween the inner surface roller101and the ground is monitored by the high-voltage power source103. The measurement system illustrated inFIG.3Acan be regarded as an equivalent circuit illustrated inFIG.3B. More specifically, a resistance RLin the circumferential direction of the intermediate transfer belt10in a distance (rotational center-to-center distance) L between the inner surface roller101and the driving roller102can be calculated by RL=2VL/IL. In the first exemplary embodiment, the distance L between the inner surface roller101and the driving roller102is 300 mm. By converting the above-described resistance RLinto a value corresponding to a length of 100 mm in the circumferential direction of the intermediate transfer belt10, a resistance in the circumferential direction of the intermediate transfer belt10is obtained. To flow a current to the photosensitive drum1from a current supply member through the intermediate transfer belt10, the resistance in the circumferential direction of the intermediate transfer belt10is desirably 1×109Ω or less.

In the first exemplary embodiment, a resistance value in the circumferential direction of the intermediate transfer belt10that has been obtained by the above-described measurement method is 1×108Ω. In the first exemplary embodiment, a voltage VLmonitored by the above-described measurement method was 750 V in a case where a constant current IL=5 μA was flowed. The monitoring of the voltage VLwas performed for a section corresponding to one round of the intermediate transfer belt10, and an average value of measurement values for the section was defined as the voltage VL. Since the resistance RLcan be calculated by RL=2VL/IL, RL=2×750/(5×10−6)=3×108Ω is obtained, and by converting the resistance into a value corresponding to the length of 100 mm in the circumferential direction of the intermediate transfer belt10, a resistance value in the circumferential direction of the intermediate transfer belt10becomes 1×108Ω. In a case where a maximum distance from the current supply member to the photosensitive drum1is 100 mm or more, it is sufficient that a resistance is converted into a resistance in the circumferential direction that corresponds to the length. In the first exemplary embodiment, a conductive belt having conductivity that enables a current flow in the circumferential direction in this manner is used as the intermediate transfer belt10. As described below, in the first exemplary embodiment, the intermediate transfer belt10has conductivity that enables a current flow in the circumferential direction of the intermediate transfer belt10at least between the secondary transfer nip N2and the primary transfer nip N1. The resistance value in the circumferential direction of the intermediate transfer belt10is not limited to this, but the resistance value is typically about 1×104Ω or more.

The intermediate transfer belt10may have a single-layer configuration or a multilayer configuration including a plurality of layers. The intermediate transfer belt10having the multilayer configuration may have the following configuration, for example. The intermediate transfer belt10includes, for example, a base layer obtained by dispersing carbon in a PPS resin having a thickness of about 100 μm and adjusting an electric resistance. Resin to be used may be PI, PVDF, nylon, PET, PBT, polycarbonate, polyether ether ketone (PEEK), or polyethylene naphthalate (PEN). In addition, a surface layer being formed of high-resistance acrylic resin and having a thickness of about 0.5 to 3 μm is provided on the outer surface of the base layer. The high-resistance layer being the surface layer is provided, for example, to improve secondary transferability for a small-sized recording material P by reducing a current difference between a region where the recording material P passes and a region where the recording material P does not pass in a longer direction of the secondary transfer nip N2(rotational axis direction of the secondary transfer roller20). As conductive powder contained in the base layer, carbon black can be used. Nevertheless, an additive agent to be mixed to adjust the electric resistance value of the intermediate transfer belt10is not specifically limited. Examples of conductive filler for adjusting the resistance include carbon black and various conductive metal oxides. Examples of non-filler type resistance adjuster include a low-molecular-weight ion conductive material such as various metal salts and glycols, charging prevention resin containing an ether bond or a hydroxyl group in molecules, or an organic high-molecular compound exhibiting electronic conductivity. In addition, the intermediate transfer belt10having the multilayer configuration may have the following configuration, for example. The intermediate transfer belt10includes a base layer and an inner surface layer. As the base layer, endless PVDF in which ion conducting agent such as multivalent metal salt or quaternary ammonium ion serving as the conducting agent is mixed in is used. As the inner surface layer, an acrylic resin in which carbon serving as the conducting agent is mixed in is used. In this case, the base layer is the thickest layer among layers included in the intermediate transfer belt10, in a thickness direction of the intermediate transfer belt10. The inner surface layer is a layer formed on the inner circumferential surface side of the intermediate transfer belt10. In the thickness direction being a direction intersecting with the moving direction of the intermediate transfer belt10, the base layer is formed at a position closer to the photosensitive drums1than the inner surface layer. As a material of the base layer, PVDF can be used, but another material may be used. For example, a material such as polyester or ABS, and a mixed resin of these may be used. As a material of the inner surface layer, an acrylic resin can be used, but another material may be used. For example, a material such as polyester may be used. Electric resistances of the base layer and the inner surface layer may be different, and the electric resistance of the inner surface layer can be set to a lower electric resistance than the electric resistance of the base layer. A surface resistivity measured from the outer circumferential surface side (base layer side) can be regarded as an electric resistance of the base layer, and a surface resistivity measured from the inner circumferential surface side (inner surface layer side) can be regarded as an electric resistance of the inner surface layer.

3. Transfer Potential Formation

In the first exemplary embodiment, on the inner circumferential surface side of the intermediate transfer belt10, the primary transfer rollers14a,14b,14c, and14dare arranged to correspond to the respective photosensitive drums1a,1b,1c, and1d. In the first exemplary embodiment, the primary transfer rollers14a,14b,14c, and14dare connected to respective primary transfer power sources (one or more high-voltage power source circuits)15a,15b,15c, and15d. In the first exemplary embodiment, the driving roller11, the tension roller12, and the secondary transfer opposing roller13are electrically floating. At the time of image formation (at the time of primary transfer, at the time of secondary transfer), a primary transfer voltage (+100 V in the first exemplary embodiment) with opposite polarity to the normal charging polarity of toner is applied to the primary transfer rollers14from the respective primary transfer power sources15by constant voltage control. Primary transfer potential (potential difference between the photosensitive drum1and the intermediate transfer belt10) is thereby formed at each of the primary transfer nips N1. Due to the potential difference, the current (primary transfer current) flows from the intermediate transfer belt10to each of the photosensitive drums1. By the function of the primary transfer current, primary transfer is performed so that toner images move from the surfaces of the respective photosensitive drums1onto the intermediate transfer belt10. In the first exemplary embodiment, the primary transfer roller14forms the current supply member that comes into contact with the intermediate transfer belt10at a position different from that of the photosensitive drum1, flows a current in the circumferential direction of the intermediate transfer belt10by receiving a voltage, and primarily transfers a toner image from the photosensitive drum1onto the intermediate transfer belt10. In other words, in the first exemplary embodiment, at the time of image formation (at the time of primary transfer, at the time of secondary transfer), the potentials of the primary transfer rollers14are maintained at the predetermined potential (+100 V in the first exemplary embodiment), and the primary transfer potentials at the primary transfer nips N1are maintained at the predetermined potential. The predetermined potential of the primary transfer rollers14maintained by the primary transfer power sources15is set so that a primary transfer potential that enables desired transfer efficiency to be obtained can be maintained at the primary transfer nips N1. By using the intermediate transfer belt10having conductivity that enables a current flow in the circumferential direction, an effect of stabilizing the surface potential (primary transfer potential) of the intermediate transfer belt10by flowing a primary transfer current in the circumferential direction of the intermediate transfer belt10can be obtained. With this configuration, primary transferability at the plurality of primary transfer nips N1can be improved.

A secondary transfer power source (high-voltage power source circuit)21is connected to the secondary transfer roller20. At the time of image formation (at the time of primary transfer, at the time of secondary transfer), a predetermined secondary transfer voltage, to be described below in detail, is applied to the secondary transfer roller20from the secondary transfer power source21by constant voltage control. If the voltage is applied from the secondary transfer power source21to the secondary transfer roller20, the secondary transfer potential (potential difference between the intermediate transfer belt10and the secondary transfer roller20) is formed at the secondary transfer nip N2. Due to the potential difference, the current (secondary transfer current) flows from the secondary transfer roller20to the intermediate transfer belt10. By the function of the secondary transfer current, secondary transfer is performed so that a toner image moves from the surface of the intermediate transfer belt10onto the recording material P.

4. Control Configuration

FIG.2is a block diagram schematically illustrating a control configuration of a main part of the image forming apparatus100according to the first exemplary embodiment. The image forming apparatus100includes a controller200serving as a control unit that controls the entire image forming apparatus100. The controller200incorporates a CPU circuit unit150being a calculation control unit, and a read-only memory (ROM)151and a random access memory (RAM)152being storage units. In accordance with control programs stored in the ROM151, the CPU circuit unit150comprehensively controls components of the image forming apparatus100such as a primary transfer control unit201, a secondary transfer control unit202, a development control unit203, an exposure control unit204, and a charging control unit205. A table for determining a target value of the secondary transfer voltage to be described below is stored in the ROM151. The table is read by the CPU circuit unit150and reflected in the control. The RAM152temporarily stores control data and is used as a work area of calculation processing associated with the control.

Under the control of the controller200, the primary transfer control unit201and the secondary transfer control unit202respectively control the primary transfer power source15and the secondary transfer power source21. The primary transfer control unit201and the secondary transfer control unit202respectively include current detection units (one or more current detection circuits) (not illustrated) and can respectively control voltages to be output from the primary transfer power source15and the secondary transfer power source21, based on current values detected by the current detection units. In addition, the primary transfer control unit201and the secondary transfer control unit202respectively include voltage detection units (one or more voltage detection circuits) (not illustrated) and can respectively control voltages to be output from the primary transfer power source15and the secondary transfer power source21, based on voltage values detected by the voltage detection units. In the first exemplary embodiment, a voltage to be output from the primary transfer power source15at the time of image formation (at the time of primary transfer, at the time of secondary transfer) is subjected to constant voltage control. In the first exemplary embodiment, a voltage to be output from the secondary transfer power source21at the time of image formation (at the time of primary transfer, at the time of secondary transfer) is subjected to constant voltage control. The control of the secondary transfer voltage will be described below in more detail.

An operation unit (operation panel)70provided in the image forming apparatus100is connected to the controller200. The operation unit70includes a display unit that displays information under the control of the controller200, and an input unit that inputs information to the controller200by an operation performed by an operator such as a user or a service person in charge. The operation unit70may include a touch panel having functions of the display unit and the input unit. A host device (external device)300such as a personal computer that is connected to the image forming apparatus100is connected to the controller200. An image reading device (not illustrated) provided in the image forming apparatus100or connected to the image forming apparatus100may be connected to the controller200. In addition, various sensors provided in the image forming apparatus100, such as a registration sensor61and an environment sensor40to be described below, are connected to the controller200. Signals related to detection results of the sensors such as the registration sensor61and the environment sensor40are input to the controller200and used in the control performed by the CPU circuit unit150. In addition, a registration control unit63to be described below is connected to the controller200. The registration control unit63controls a registration driving unit64to be described below under the control of the controller200.

If the controller200receives image information and printing information (printing command) from the host device300such as a personal computer, the controller200performs control to execute a printing operation (job) by controlling the above-described control units. The printing information includes a start instruction (start signal) of the printing operation, and information regarding an image forming condition such as information regarding the recording material P. The information regarding the recording material P includes any type of information that can distinguish the recording material P, such as an attribute (i.e., paper type category) that is based on general characteristics such as plain paper, fine quality paper, glossy paper, gloss paper, coated paper, embossed paper, thick paper, thin paper, and paper quality, numerical values or numerical value ranges of grammage, thickness, size, and rigidity, or brand (including a manufacturer, product name, part number, etc.). The type of recording material P can be determined for each recording material P distinguished by the information regarding the recording material P. The information regarding the recording material P may be included in information regarding an image formation mode that designates an operation setting of the image forming apparatus100, such as a “plain paper mode” and a “gloss paper mode”, or may be replaced with the information regarding an image formation mode. The printing information may be input to the controller200from the operation unit70. The image information may be input to the controller200from the operation unit70or the image reading device (not illustrated).

5. Control of Secondary Transfer Voltage

Next, the control of the secondary transfer voltage according to the first exemplary embodiment will be further described.FIGS.4A and4Bare cross-sectional diagrams illustrating a conveyance orientation of the recording material P in the vicinity on an upstream side of the secondary transfer nip N2according to the first exemplary embodiment (cross section substantially orthogonal to the rotational axis direction of the secondary transfer roller20and the secondary transfer opposing roller13).FIG.4Aillustrates the conveyance orientation of the recording material P before the trailing end of the recording material P passes through a registration roller nip R to be described below, andFIG.4Billustrates the conveyance orientation of the recording material P after the trailing end of the recording material P passes through the registration roller nip R to be described below.

In the first exemplary embodiment, the registration roller pair60also serves as a regulation member that regulates the orientation of the recording material P. The regulation member is arranged on the upstream side of the secondary transfer nip N2, and regulates the orientation of the recording material P being in contact with the secondary transfer roller20and the regulation member. If the trailing end of the recording material P passes through a contact portion with the registration roller pair60, i.e., passes through the registration roller nip R formed by two rollers of the registration roller pair60, the orientation of a portion on the trailing end side of the recording material P in the vicinity on the upstream side of the secondary transfer nip N2becomes free. The portion on the trailing end side of the recording material P is a portion of the recording material P, which is passing through the secondary transfer nip (secondary transfer portion) N2, that is located between the secondary transfer roller (outer roller)20and the registration roller pair (regulation member)60.

The first exemplary embodiment is characterized in that a target voltage of a secondary transfer voltage is increased at a timing at which the portion on the trailing end side of the recording material P moves closer to the intermediate transfer belt10to be along the surface of the intermediate transfer belt10by a change in orientation of the portion on the trailing end side of the recording material P. A state in which the portion on the trailing end side of the recording material P is along the surface of the intermediate transfer belt10typically refers to a state in which the portion on the trailing end side of the recording material P becomes substantially parallel to the surface (pre-secondary transfer nip stretching surface Pt to be described below) of the intermediate transfer belt10. In a state in which the portion on the trailing end side of the recording material P is along the surface of the intermediate transfer belt10, typically, at least part of the portion on the trailing end side of the recording material P comes into contact with the surface of the intermediate transfer belt10, but the entire portion on the trailing end side of the recording material P may have no contact with the intermediate transfer belt10. Hereinafter, more detailed description will be given.

In the first exemplary embodiment, the secondary transfer roller20forms the secondary transfer nip N2by being in contact with the outer circumferential surface of the intermediate transfer belt10with a pressure force of 50 N. The secondary transfer roller20is driven and rotated by the rotation of the intermediate transfer belt10. Then, the recording material P such as paper is conveyed while being pinched between the intermediate transfer belt10and the secondary transfer roller20at the secondary transfer nip N2.

The secondary transfer power source21is connected to the secondary transfer roller20and supplies a secondary transfer voltage, which is output from a transformer, to the secondary transfer roller20. As illustrated inFIG.2, the secondary transfer control unit202is controlled by the CPU circuit unit150of the controller200, which is a control integrated circuit (IC) of the image forming apparatus100. The secondary transfer voltage to be output by the secondary transfer power source21is controlled by the secondary transfer control unit202. In the first exemplary embodiment, the secondary transfer control unit202feeds back a difference between a preset target voltage and a detected voltage being an actual output value of the secondary transfer power source21that is detected by the voltage detection unit, to the transformer of the secondary transfer power source21so that the secondary transfer voltage becomes substantially constant. The secondary transfer control unit202thereby performs constant voltage control of the secondary transfer voltage to be supplied to the secondary transfer roller20from the secondary transfer power source21. The control of adjusting an output from a power source so that a value of a voltage output by the power source becomes closer to a target voltage value, irrespective of a value of a flowing current, is referred to as the constant voltage control. In the first exemplary embodiment, the constant voltage control of a primary transfer voltage to be applied to the primary transfer roller14from the primary transfer power source15is performed similarly to the above-described control by the CPU circuit unit150of the controller200and the primary transfer control unit201.

In the first exemplary embodiment, the resistance in the circumferential direction of the intermediate transfer belt10is so low that a current can flow in the circumferential direction of the intermediate transfer belt10. In the first exemplary embodiment, the intermediate transfer belt10has conductivity that enables a current flow in the circumferential direction of the intermediate transfer belt10at least between the secondary transfer nip N2and the primary transfer nip N1. Thus, the potential of the secondary transfer opposing roller13arranged at a position facing the secondary transfer roller20is set to a potential substantially the same as the potential (primary transfer voltage) of the primary transfer roller14. More specifically, the potential is set to +100 V in the first exemplary embodiment as described above. Then, in the first exemplary embodiment, at the secondary transfer nip N2, the secondary transfer is performed by the secondary transfer current being flowed to a toner image due to the potential difference between the secondary transfer voltage determined by the secondary transfer control unit202and the potential of the secondary transfer opposing roller13. In the first exemplary embodiment, the secondary transfer power source21can output a voltage within the range of +100 V to +4000 V.

As described above, the feeding roller50serving as a feeding unit picks up the recording materials P in the cassette51serving as a recording material storage portion, one by one, and feeds the recording materials P to the registration roller pair60serving as a conveyance unit. The registration roller pair60includes two rollers arranged to face each other. The two rollers form the registration roller nip R in the contact portion by at least one of the two rollers being pressed against the other. At least one of the two rollers is driven and rotated by the registration driving unit64(FIG.2) including a driving source and a chain of drives. In the first exemplary embodiment, the two rollers of the registration roller pair60are individually driven and rotated by the registration driving unit64, but in the case of a configuration in which one of the rollers is driven and rotated by the registration driving unit64, the other one roller is driven and rotated by the rotation of the one roller. A start and a stop of rotation of the registration roller pair60that is driven by the registration driving unit64and a rotation speed thereof are controlled by the registration control unit63(FIG.2) under the control of the controller200.

The conveyance of the recording material P from when the leading end of the recording material P enters the secondary transfer nip N2to when the trailing end of the recording material P passes through the registration roller nip R will be described with reference toFIG.4A. The registration roller pair60conveys the recording material P, which has been fed by the feeding roller50, to the secondary transfer nip N2while nipping the recording material P at the registration roller nip R. At this time, as illustrated inFIG.4A, the orientation of the recording material P is regulated by the secondary transfer nip N2and the registration roller nip R, and substantially constant secondary transferability is maintained.

In the first exemplary embodiment, the registration sensor61that detects the presence or absence of the recording material P on a conveyance path of the recording material P is provided at the position of the registration roller pair60. In the first exemplary embodiment, the leading end of the recording material P is detected by the registration sensor61. Then, the controller200can predict the position of the trailing end of the recording material P by acquiring a signal output by the registration sensor61when the registration sensor61detects the leading end of the recording material P. Specifically, in the first exemplary embodiment, the controller200predicts the position of the trailing end of the recording material P based on a detection signal of the registration sensor61, information regarding a size of the recording material P, and information regarding a conveyance speed of the recording material P. The controller200can acquire the information regarding a size of the recording material P (in particular, information regarding a length in the conveyance direction of the recording material P) from the information regarding the recording material P that is included in the printing information input from the host device300, for example. The controller200can also acquire the information regarding a conveyance speed of the recording material P from a control value of the registration driving unit64that is designated by the registration control unit63, for example. The controller200can thereby control the secondary transfer voltage based on the position of the trailing end of the recording material P as described below in more detail.

In the first exemplary embodiment, the conveyance speed of the recording material P conveyed by the registration roller pair60is changed depending on a timing at which the leading end of the recording material P is detected, so that the leading end of the recording material P and a leading end of a page range on the intermediate transfer belt10match each other at a predetermined position anterior to the secondary transfer nip N2. Hereinafter, the above-described predetermined position anterior to the secondary transfer nip N2will be referred to as a merge point MP. Then, before the leading end of the recording material P reaches the merge point MP, the conveyance speed of the recording material P conveyed by the registration roller pair60is returned to the speed before the change, and the recording material P is conveyed to the secondary transfer nip N2. With this configuration, at the secondary transfer nip N2, the positions of the leading end of the recording material P and the leading end of the page range on the intermediate transfer belt10can be matched without stopping the conveyance of the recording material P. The page range on the intermediate transfer belt10is a range on the intermediate transfer belt10that corresponds to the size of the recording material P, and a toner image is formed within the range. Then, at the secondary transfer nip N2, the toner image formed within the page range on the intermediate transfer belt10is transferred onto the recording material P.

Next, the conveyance of the recording material P from when the trailing end of the recording material P passes through the registration roller nip R to when the trailing end of the recording material P passes through the secondary transfer nip N2will be described with reference toFIG.4B. The orientation of the recording material P during the period is different from the orientation of the recording material P illustrated in FIG.4A. Specifically, if the trailing end of the recording material P passes through the registration roller nip R, the orientation of the portion on the trailing end side of the recording material P on the upstream side of the secondary transfer nip N2becomes the orientation as illustrated inFIG.4B. In other words, the orientation of the portion on the trailing end side of the recording material P changes from the orientation when the recording material P is pinched by the secondary transfer nip N2and the registration roller nip R, and becomes a free state.

In a cross section substantially orthogonal to the rotational axis direction of the secondary transfer roller20and the secondary transfer opposing roller13, a straight line connecting the rotational center of the secondary transfer opposing roller13and the rotational center of the secondary transfer roller20is defined as a secondary transfer nip center line Lc. In the cross section, a straight line orthogonal to the secondary transfer nip center line Lc and passing through a contact point of the intermediate transfer belt10on the secondary transfer nip center line Lc and the secondary transfer roller20is defined as a secondary transfer nip tangent line Ln. In addition, a stretching surface (outer circumferential surface) of the intermediate transfer belt10formed by the tension roller12and the secondary transfer opposing roller13is defined as the pre-secondary transfer nip stretching surface Pt. The tension roller12is an example of a stretching roller arranged next to the secondary transfer opposing roller13on the upstream side of the secondary transfer opposing roller13in the moving direction of the intermediate transfer belt10. At this time, in the first exemplary embodiment, the secondary transfer nip tangent line Ln is located on a secondary transfer opposing roller13(inner roller) side of the pre-secondary transfer nip stretching surface Pt. When the recording material P is pinched at the secondary transfer nip N2, the recording material P tends to take the orientation of being along the secondary transfer nip tangent line Ln. Thus, in the case of a configuration in which the pre-secondary transfer nip stretching surface Pt is on the secondary transfer opposing roller13side of the secondary transfer nip tangent line Ln as in the first exemplary embodiment, the recording material P changes as follows. More specifically, the portion on the trailing end side of the recording material P that has passed through the registration roller nip R moves closer to the intermediate transfer belt10due to the rigidity (stiffness) of the recording material P and changes into a shape of being along the pre-secondary transfer nip stretching surface Pt.

In the case of decreasing the resistance of the intermediate transfer belt10to flow the primary transfer current in the circumferential direction of the intermediate transfer belt10as in the first exemplary embodiment, the following issue arises. More specifically, if the recording material P is along the intermediate transfer belt10as described above, the secondary transfer nip N2apparently widens, and a large amount of current flows to the intermediate transfer belt10via the recording material P. Consequently, the load on the secondary transfer power source21increases, and the secondary transfer voltage drops, and thus a secondary transfer defect may occur because the secondary transfer voltage necessary for a secondary transfer process is not secured at the secondary transfer nip N2.

Addressing the issue by the improvement in performance of the secondary transfer power source21may lead to upsizing and a cost increase of the image forming apparatus100. Thus, in the configuration in which the intermediate transfer belt10having conductivity that enables a current flow in the circumferential direction is used, it is desirable to prevent an occurrence of a secondary transfer defect attributed to the change in the orientation of the portion on the trailing end side of the recording material P in the vicinity on the upstream side of the secondary transfer nip N2with a simple configuration without involving the upsizing and the cost increase of the image forming apparatus100.

In view of the foregoing, in the first exemplary embodiment, the target voltage of the secondary transfer voltage is increased at a timing at which the trailing end of the recording material P passes through the registration roller nip R and the portion on the trailing end side of the recording material P is along the pre-secondary transfer nip stretching surface Pt.

FIG.5is a chart illustrating control of the secondary transfer voltage according to the first exemplary embodiment.FIG.5illustrates transitions of the target voltage in the control of the secondary transfer voltage that is performed by the secondary transfer control unit202and the detected voltage being an actual output value of the secondary transfer power source21that is detected by the voltage detection unit, from when the recording material P enters the secondary transfer nip N2to when the secondary transfer process ends.

In the first exemplary embodiment, based on the information regarding a size of the recording material P in the information regarding the recording material P that is included in the printing information input from the host device300, the controller200predicts a timing at which the trailing end of the recording material P passes through the registration roller nip R. In other words, the controller200predicts a timing at which the trailing end of the recording material P becomes free. After that, during a period from when the leading end of the recording material P enters the secondary transfer nip N2to when the trailing end of the recording material P passes through the registration roller nip R, the controller200controls the secondary transfer control unit202to apply the secondary transfer voltage to the secondary transfer roller20from the secondary transfer power source21by setting the target voltage of the secondary transfer voltage to a first target voltage (first target value) V1. Then, the controller200controls the secondary transfer control unit202to increase the target voltage of the secondary transfer voltage to a second target voltage (second target value) V2at a timing at which the trailing end of the recording material P passes through the registration roller nip R. With this configuration, an effect of preventing a drop in secondary transfer voltage that is attributed to an increase in load on the secondary transfer power source21at a timing at which the trailing end of the recording material P passes through the registration roller nip R is obtained.

Next, setting of the first and second target voltages V1and V2will be described. In the first exemplary embodiment, the controller200selects the first and second target voltages V1and V2based on the information regarding the recording material P that is included in the printing information input from the host device300, and information regarding an environment. In other words, depending on the type of recording material P, (1) rigidity (stiffness) of the recording material P and (2) a resistance value of the recording material P vary. The rigidity of the recording material P varies depending on the size, grammage, paper quality, paper type category, and a brand of the recording material P, for example. In the first exemplary embodiment, as information regarding the rigidity of the recording material P, the grammage or the paper type category (or the brand) is used. The resistance of the recording material P varies depending on the grammage, the paper quality, the paper type category, or the brand of the recording material P, for example. In the first exemplary embodiment, as information regarding a resistance value of the recording material P, the grammage or the paper type category (or the brand) is used. When the paper quality, the paper type category, or the brand of the recording material P is the same (or can be regarded as equivalent), because the grammage and the thickness of the recording material P (paper) tend to be in a substantially proportional relation, the grammage may be used as an index of the thickness, or the thickness may be used in place of the grammage. The resistance value of the recording material P also varies depending on the environment. The information regarding an environment may be information regarding at least one of temperature or humidity on at least one of the inside or outside of the image forming apparatus100. In the first exemplary embodiment, as the information regarding an environment, information regarding an absolute moisture amount that is obtained based on a detection result of the temperature and humidity of ambient atmosphere around the image forming apparatus100by the environment sensor40provided in the image forming apparatus100is used. The absolute moisture amount may be obtained by the environment sensor40, or may be obtained by the controller200based on the detection result of the environment sensor40. With respect to the above-described (1) rigidity of the recording material P and (2) the resistance value of the recording material P, the following tendency can be generally observed although not limited thereto.

As for (1) rigidity of the recording material P, as the rigidity becomes higher (the recording material P becomes thicker and stiffness becomes stronger), a spring constant of the recording material P becomes larger. Thus, as the rigidity becomes higher, after the trailing end of the recording material P passes through the registration roller nip R, the orientation of the portion on the trailing end side of the recording material P swiftly changes, the portion on the trailing end side of the recording material P moves to be along the intermediate transfer belt10more quickly, and a contact area tends to be larger. In other words, a drop amount of the secondary transfer voltage at a timing at which the trailing end of the recording material P passes through the registration roller nip R varies depending on the rigidity of the recording material P. Thus, it is desirable to change the second target voltage V2(a change amount of the second target voltage V2with respect to the first target voltage V1) depending on the rigidity of the recording material P. Since the rigidity of the recording material P varies depending on the type of recording material P, it is desirable to change the second target voltage V2(the change amount of the second target voltage V2with respect to the first target voltage V1) depending on the type of recording material P (the grammage or paper type category (or brand) in the first exemplary embodiment).

As for (2) the resistance value of the recording material P, a secondary transfer voltage for flowing the necessary secondary transfer current at the secondary transfer nip N2varies depending on the resistance value of the recording material P. Thus, it is desirable to change the first target voltage V1depending on the resistance value of the recording material P. In addition, even after the trailing end of the recording material P passes through the registration roller nip R and the portion on the trailing end side of the recording material P moves to be along the intermediate transfer belt10, a drop amount of the secondary transfer voltage varies depending on the resistance of the recording material P. Thus, it is desirable to change the second target voltage V2(the change amount of the second target voltage V2with respect to the first target voltage V1) depending on the resistance value of the recording material P, to sufficiently prevent the drop in secondary transfer voltage. The resistance value of the recording material P varies depending on the type of recording material P. Thus, it is desirable to change the first target voltage V1and the second target voltage V2(the change amount of the second target voltage V2with respect to the first target voltage V1) depending on the type of recording material P (the grammage or paper type category (or brand) in the first exemplary embodiment). In addition, an environment (the absolute moisture amount in the first exemplary embodiment) affects the resistance value of the recording material P (as an absolute moisture amount becomes larger, the resistance becomes lower). Thus, it is desirable to change the first target voltage V1and the second target voltage V2(the change amount of the second target voltage V2with respect to the first target voltage V1) depending on an absolute moisture amount for each type of recording material P.

The change amount of the second target voltage V2with respect to the first target voltage V1is affected by moisture absorbability of each type of recording material P. Thus, a dominantly affecting property changes between the rigidity and the resistance value. Thus, it is desirable to appropriately preset the change amount of the second target voltage V2with respect to the first target voltage V1through experiments so that a secondary transfer defect can be sufficiently prevented. Generally, in the case of the recording material P having a surface on which coating processing is not performed, such as plain paper, a property of increased absorbability as the grammage is small tends to dominantly exert influence compared to an increase in rigidity caused by an increase in grammage. Thus, for the recording material P of this type, the difference between the first target voltage V1and the second target voltage V2for the recording material P having a first grammage is made larger than the difference between the first target voltage V1and the second target voltage V2for the recording material P having a second grammage larger than the first grammage (higher rigidity). Then, the difference between the first target voltage V1and the second target voltage V2when the absolute moisture amount is a first absolute moisture amount is made larger than the difference between the first target voltage V1and the second target voltage V2when the absolute moisture amount is a second absolute moisture amount larger than the first absolute moisture amount (resistance of the recording material P is lower) (refer to Table 1 provided below). On the other hand, in the case of the recording material P having a surface on which coating processing is performed, such as gloss paper, the increase in rigidity caused by an increase in grammage tends to dominantly exert influence compared to the property of increased absorbability as the grammage is small. Thus, for the recording material P of this type, the difference between the first target voltage V1and the second target voltage V2for the recording material P having the second grammage larger than the first grammage (higher rigidity) is made larger than the difference between the first target voltage V1and the second target voltage V2for the recording material P having the first grammage. Then, the difference between the first target voltage V1and the second target voltage V2when the absolute moisture amount is the second absolute moisture amount larger than the first absolute moisture amount (resistance of the recording material P is lower) is made larger than the difference between the first target voltage V1and the second target voltage V2when the absolute moisture amount is the first absolute moisture amount (refer to Table 2 provided below).

In particular, the resistance value of the recording material P varies depending on various factors (e.g., the grammage, paper quality, absolute moisture amount). In the first exemplary embodiment, a table indicating a relationship between each of the grammage and the absolute moisture amount, and the first and second target voltages V1and V2is preliminarily obtained for each type (paper type category (or brand)) of the recording material P, and stored in the ROM151of the controller200. Then, based on the printing information and the detection result of the environment sensor40, the CPU circuit unit150of the controller200reads the corresponding first and second target voltages V1and V2from the above-described table, and reflects the first and second target voltages V1and V2in the control of the secondary transfer voltage.

Tables 1 and 2 each illustrate an example of the above-described table of the first and second target voltages V1and V2. Table 1 is a table for plain paper, and Table 2 is a table for gloss paper. In the first exemplary embodiment, the controller200determines the first and second target voltages V1and V2corresponding to a grammage or an absolute moisture amount that does not exist in the table, such as a grammage or an absolute moisture amount between grammages or absolute moisture amounts in the table, by linear interpolation. As indicated in Tables 1 and 2, the above-described control (target value change control) of changing the target voltage of the secondary transfer voltage between the first target voltage V1and the second target voltage V2needs not be performed under all conditions. Under a condition under which a secondary transfer defect does not occur, the first target voltage V1and the second target voltage V2may be set to the same voltage (in other words, the target value change control needs not be performed). In other words, as indicated in Tables 1 and 2, in the case of forming an image on a preset predetermined recording material P (e.g., the grammage, paper type category (or brand)), the controller200can execute the target value change control. As indicated in Tables 1 and 2, when an environmental condition satisfies a preset predetermined condition (e.g., when the absolute moisture amount is larger than or equal to a predetermined threshold value), the controller200can execute the target value change control.

As described above, for various recording materials P, substantially constant secondary transferability can be ensured from the leading end of the recording material P to the trailing end thereof, and a secondary transfer defect can be prevented.

6. Effect Confirmation

To confirm an effect of the first exemplary embodiment, a test of checking whether an image defect has occurred was conducted using various recording materials P under a high-temperature and high-humidity environment (temperature 30° C., relative humidity 80%, absolute moisture amount 24.3 g/m3). The test was conducted for the first exemplary embodiment and a comparative example.

In an image forming apparatus100according to the comparative example, as illustrated inFIG.6, a target voltage of a secondary transfer voltage is kept constant even after the trailing end of the recording material P passes through the registration roller nip R and the orientation of the portion on the trailing end side of the recording material P changes. The configuration and the operation of the image forming apparatus100according to the comparative example are substantially the same as the configuration and the operation of the image forming apparatus100according to the first exemplary embodiment except for the above-described point.

Table 3 indicates a type (brand, grammage) of the recording material P used in the test, the setting of the first and second target voltages V1and V2, and a result indicating whether an image defect has occurred. The image defect was evaluated by visually observing a secondary transfer defect (density reduction in an image or image deficiency) in a predetermined test image. A case where an image defect did not occur was indicated as “Pass”, whereas a case where an image defect occurred was indicated as “Fail”.

In the configuration of the comparative example, the image defect occurred in part of an image obtained subsequent to a timing at which the trailing end of the recording material P passes through the registration roller nip R and the portion on the trailing end side of the recording material P moves to be along the intermediate transfer belt10, in all of the tests that use the respective recording materials P. It is considered that the image defect has occurred because a large amount of current has flowed to the intermediate transfer belt10via the recording material P and the secondary transfer voltage has dropped at the timing as illustrated inFIG.6.

On the other hand, in the configuration of the first exemplary embodiment, by increasing the target voltage of the secondary transfer voltage at the timing at which the trailing end of the recording material P passes through the registration roller nip R and the portion on the trailing end side of the recording material P moves to be along the intermediate transfer belt10, the drop in secondary transfer voltage has been prevented. Thus, an image defect has not occurred in all of the tests that use the respective recording materials P.

As described above, in the first exemplary embodiment, the image forming apparatus100has a configuration in which the controller200acquires information regarding the timing at which the orientation of a portion of the recording material P passing through the secondary transfer nip N2that is located between the secondary transfer roller20and the registration roller pair60can change to get closer to the intermediate transfer belt10, and can execute control of changing, based on the information, a target value of the secondary transfer voltage for secondary transfer onto the recording material P between a first target value for a period anterior to the above-described timing, and a second target value for a period posterior to the above-described timing that is larger in absolute value than the first target value. In particular, in the first exemplary embodiment, the controller200can execute control of changing the target value of the secondary transfer voltage for the secondary transfer onto the recording material P so that an absolute value thereof becomes large at a predetermined timing that is based on a timing at which the trailing end in the conveyance direction of the recording material P passing through the secondary transfer nip N2passes through the contact portion with the registration roller pair60. The controller200can acquire the information regarding the above-described timing based on the information regarding the recording material P. In the first exemplary embodiment, the information regarding the recording material P includes the information regarding a length in the conveyance direction of the recording material P. In the first exemplary embodiment, the information regarding the recording material P includes the information regarding the rigidity of the recording material P. The information regarding the recording material P may include information regarding at least one of grammage, material, category, or brand of the recording material P. The controller200can change a change amount of the second target value with respect to the first target value based on information similar to the above-described information regarding the recording material P. The controller200can change a change amount of the second target value with respect to the first target value based on the information regarding at least one of temperature or humidity on at least one of the inside or outside of the image forming apparatus100(the information regarding an environment). In the first exemplary embodiment, the controller200performs the constant voltage control of the secondary transfer voltage, and the target value of the secondary transfer voltage is a target voltage value in the constant voltage control.

In the first exemplary embodiment, in a configuration in which the intermediate transfer belt10having conductivity that enables a current flow in the circumferential direction is used, it is possible to prevent the occurrence of a secondary transfer defect attributed to a change in orientation of the portion on the trailing end side of the recording material P in the vicinity on the upstream side of the secondary transfer nip N2with a simple configuration without involving the upsizing and the cost increase of the image forming apparatus100. In other words, in the first exemplary embodiment, in the configuration in which the intermediate transfer belt10having conductivity that enables a current flow in the circumferential direction is used, it is possible to prevent the occurrence of a transfer defect attributed to the change in orientation of the recording material P conveyed in the vicinity of the secondary transfer nip N2.

Next, a modification of the first exemplary embodiment will be described.

In the first exemplary embodiment, examples of tables (Tables 1 and 2) for determining the target value of the secondary transfer voltage for plain paper and gloss paper have been described. A similar table can be prepared for another recording material such as rough paper and plastic paper, and control similar to that in the first exemplary embodiment can be performed. In addition, with respect to plain paper, for example, different tables may be prepared for different brands, and control similar to that in the first exemplary embodiment can be performed.

In the first exemplary embodiment, the first and second target voltages V1and V2corresponding to a grammage or an absolute moisture amount between grammages or absolute moisture amounts in the tables such as Tables 1 and 2 are determined by linear interpolation, but the present disclosure is not limited to such a configuration. For example, the second target voltage V2(change amount of the second target voltage V2with respect to the first target voltage V1) may be determined using another interpolation method for a condition under which a drop in secondary transfer voltage is large. For example, the second target voltage V2may be determined by performing nonlinear interpolation on the second target voltage V2so that a difference between the first and second target voltages V1and V2becomes larger for a condition with a larger drop in secondary transfer voltage.

In the first exemplary embodiment, the description has been given of the case where the orientation of the portion on the trailing end side of the recording material P changes at the timing at which the trailing end of the recording material P passes through the registration roller nip R, but the present disclosure is not limited to such a configuration. For example, as illustrated inFIGS.7A and7B, the present disclosure can also be applied to a case where the orientation of the portion on the trailing end side of the recording material P changes at the timing at which the trailing end of the recording material P passes through a contact portion with a conveyance guide member62serving as a regulation member.FIGS.7A and7Bare cross-sectional diagrams illustrating the vicinity of the secondary transfer nip N2according to this example (cross section substantially orthogonal to the rotational axis direction of the secondary transfer roller20and the secondary transfer opposing roller13). InFIGS.7A and7B, a component having the same or corresponding function or configuration as a component of the image forming apparatus100according to the first exemplary embodiment illustrated inFIG.1is assigned the same reference numeral. An image forming apparatus100illustrated inFIGS.7A and7Bincludes the conveyance guide member62as a regulation member that is arranged on the upstream side of the secondary transfer nip N2and regulates the orientation of the recording material P being in contact with the secondary transfer roller20and the regulation member. The conveyance guide member62is configured to regulate movement of the recording material P in a direction getting closer to the intermediate transfer belt10(pre-secondary transfer nip stretching surface Pt).FIG.7Aillustrates the conveyance orientation of the recording material P before the trailing end of the recording material P passes through the contact portion with the conveyance guide member62, andFIG.7Billustrates the conveyance orientation of the recording material P after the trailing end of the recording material P passes through the contact portion with the conveyance guide member62. In this example, if the trailing end of the recording material P passes through the contact portion with the conveyance guide member62(typically, an end portion on the downstream side of the conveyance guide member62in the conveyance direction of the recording material P), the orientation of the portion on the trailing end side of the recording material P in the vicinity of the secondary transfer nip N2on the upstream side becomes free. Then, by the change in orientation of the recording material P, the portion on the trailing end side of the recording material P moves closer to the intermediate transfer belt10to be along the surface of the intermediate transfer belt10. Thus, in the same way as the first exemplary embodiment, the target voltage of the secondary transfer voltage is to be increased at the timing at which the portion on the trailing end side of the recording material P moves to be along the intermediate transfer belt10. In other words, the target voltage of the secondary transfer voltage is to be increased at the timing at which the portion on the trailing end side of the recording material P moves to be along the intermediate transfer belt10depending on a conveyance path of the recording material P in the image forming apparatus100.

In the first exemplary embodiment, the timing at which the trailing end of the recording material P passes through the registration roller nip R is predicted based on the printing information input from the host device300, but the present disclosure is not limited to such a configuration. For example, the timing at which the trailing end of the recording material P passes through the registration roller nip R may be detected by using a detection result of the trailing end of the recording material P that is obtained by the registration sensor61, and control may be performed to increase the target voltage of the secondary transfer voltage at the timing.FIG.8is a chart illustrating the transition of the detection signal of the registration sensor61, the target voltage of the secondary transfer voltage, and the detected voltage. In this example, the detection signal of the registration sensor61is turned on when the leading end of the recording material P enters the registration roller nip R and is turned off when the trailing end of the recording material P passes through the registration roller nip R. Thus, in this example, the controller200can detect the timing at which the trailing end of the recording material P passes through the registration roller nip R from a timing at which the detection signal of the registration sensor61changes from on to off. For example, in a case where an operator designates a wrong size of the recording material P, or in a case where the recording material P other than a standard-sized sheet is used, in this example, a switching timing of the target voltage of the secondary transfer voltage can be determined based on a detection result of the registration sensor61. In this manner, by determining the timing of increasing the target voltage of the secondary transfer voltage using the detection result of the registration sensor61, an effect of preventing a secondary transfer defect irrespective of the size of the recording material P can be obtained more surely.

In the first exemplary embodiment, the target voltage of the secondary transfer voltage is changed at the timing at which the trailing end of the recording material P passes through the registration roller nip R, but the present disclosure is not limited to such a configuration. The target voltage of the secondary transfer voltage is to be changed at the timing at which the portion on the trailing end side of the recording material P moves closer to the intermediate transfer belt10by a change in orientation of the portion on the trailing end side of the recording material P to be along the surface of the intermediate transfer belt10. In other words, the target voltage of the secondary transfer voltage is to be changed at a timing at which a distance between the portion on the trailing end side of the recording material P and the intermediate transfer belt10becomes smaller to fall below a predetermined value by the change in orientation of the portion on the trailing end side of the recording material P. For example, at a timing at which a conveyance speed of the recording material P conveyed by the registration roller pair60is increased, the portion on the trailing end side of the recording material P may move closer to the intermediate transfer belt10by the change in orientation of the portion on the trailing end side of the recording material P to be along the surface of the intermediate transfer belt10. Thus, the target voltage of the secondary transfer voltage can be changed at a timing earlier or later than the timing, and an effect similar to that of the first exemplary embodiment can be obtained. Also in the case of changing the target voltage of the secondary transfer voltage based on the timing at which the trailing end of the recording material P passes through the registration roller nip R, a change timing of the target voltage of the secondary transfer voltage is not limited to substantially the same timing as the passage timing. The target voltage of the secondary transfer voltage can be changed at a timing shifted to an earlier or later timing from the passage timing, so that a drop in secondary transfer voltage can be sufficiently prevented. For example, as with the target voltage of the secondary transfer voltage, an amount of shift of the timing from the passage timing to an earlier or later timing can be preset based on the type of recording material P or an environment. The same applies to a case where the target voltage of the secondary transfer voltage is changed based on the detection result of the registration sensor61as described above, and a change timing of the target voltage of the secondary transfer voltage is not limited to substantially the same timing as the timing at which the registration sensor61detects the trailing end of the recording material P. The target voltage of the secondary transfer voltage can be changed at a timing shifted from the detection timing to an earlier or later timing, so that a drop in secondary transfer voltage can be sufficiently prevented. For example, as with the target voltage of the secondary transfer voltage, an amount of shift of the timing from the detection timing to an earlier or later timing can be preset based on the type of recording material P or an environment.

In the first exemplary embodiment, the target voltage of the secondary transfer voltage is increased from the first target voltage V1to the second target voltage V2larger in absolute value than the first target voltage V1, but the present disclosure is not limited to such a configuration. Depending on a change configuration of the orientation of the portion on the trailing end side of the recording material P in the image forming apparatus100, the target voltage of the secondary transfer voltage may be decreased from the first target voltage V1to a second target voltage V2′ smaller in absolute value than the first target voltage V1. Specifically, the following case can be considered. For example, there is a case where, from a state in which the portion on the trailing end side of the recording material P is along the surface of the intermediate transfer belt10, the conveyance speed of the recording material P conveyed by the registration roller pair60is decreased. In this case, because the recording material P becomes loose, the portion on the trailing end side of the recording material P moves away from the intermediate transfer belt10by a change in orientation of the portion on the trailing end side of the recording material P and is no longer along the surface of the intermediate transfer belt10. In this case, because the load on the secondary transfer power source21is reduced in contrast to the first exemplary embodiment, the secondary transfer voltage might increase more than expected. If the secondary transfer voltage increases more than expected, an image defect might occur due to an occurrence of abnormal electrical discharge. Thus, in this case, control of decreasing the target voltage of the secondary transfer voltage from the first target voltage V1to the second target voltage V2′ smaller in absolute value than the first target voltage V1can be performed. In addition, there is a case where the image forming apparatus100has a configuration as illustrated inFIGS.9A and9B.FIGS.9A and9Bare cross-sectional diagrams illustrating the vicinity of the secondary transfer nip N2according to this example (cross section substantially orthogonal to the rotational axis direction of the secondary transfer roller20and the secondary transfer opposing roller13). InFIGS.9A and9B, a component having the same or corresponding function or configuration as a component of the image forming apparatus100according to the first exemplary embodiment illustrated inFIG.1is assigned the same reference numeral. In the image forming apparatus100illustrated inFIGS.9A and9B, in a cross section substantially orthogonal to the rotational axis direction of the secondary transfer roller20, the secondary transfer nip tangent line Ln is located closer to the secondary transfer roller20(outer roller) than the pre-secondary transfer nip stretching surface Pt. In this case, at the timing at which the trailing end of the recording material P passes through the registration roller nip R, the portion on the trailing end side of the recording material P changes to a separated state from a state of being along the surface of the intermediate transfer belt10. Then, in this case, because the load on the secondary transfer power source21is reduced in contrast to the first exemplary embodiment, the secondary transfer voltage might increase more than expected. Thus, in this case, control of decreasing the target voltage of the secondary transfer voltage from the first target voltage V1to the second target voltage V2′ smaller in absolute value than the first target voltage V1can be performed. In the case of performing the control of decreasing the target voltage of the secondary transfer voltage, the target voltage of the secondary transfer voltage is to be changed at a timing at which the portion on the trailing end side of the recording material P moves away from the intermediate transfer belt10by a change in orientation of the recording material P and is no longer along the surface of the intermediate transfer belt10. In other words, the target voltage of the secondary transfer voltage is to be changed at a timing at which a distance between the portion on the trailing end side of the recording material P and the intermediate transfer belt10becomes larger than a predetermined value by a change in orientation of the portion on the trailing end side of the recording material P. As for the detection and the setting of the change timing of the target voltage of the secondary transfer voltage, the description in the first exemplary embodiment and the description in the modification about the case of increasing the target voltage of the secondary transfer voltage can be similarly applied to the case of decreasing the target voltage of the secondary transfer voltage.

In the first exemplary embodiment, the target value of the voltage to be applied to the secondary transfer roller20from the secondary transfer power source21is changed relative to the primary transfer voltage (potential of the secondary transfer opposing roller13) maintained at a constant voltage of +100 V. The target value of the secondary transfer voltage being a potential difference between the secondary transfer roller20and the secondary transfer opposing roller13is thereby changed. Nevertheless, the present disclosure is not limited to such a configuration. The target value of the secondary transfer voltage being a potential difference between the secondary transfer roller20and the secondary transfer opposing roller13may be changed by changing the primary transfer voltage (potential of the secondary transfer opposing roller13). Alternatively, both of the potentials of the secondary transfer roller20(voltage to be applied to the secondary transfer roller20from the secondary transfer power source21) and the secondary transfer opposing roller13(voltage to be applied to the primary transfer roller14from the primary transfer power source15) may be changed. This example can be employed in a configuration in which secondary transfer is performed at a timing different from that of primary transfer.

In the first exemplary embodiment, voltages are applied from the individual primary transfer power sources15to the respective primary transfer rollers14, but the present disclosure is not limited to such a configuration. The voltages may be applied from a common primary transfer power source15to all or a part of the plurality of primary transfer rollers14.FIG.10is a schematic cross-sectional diagram of the image forming apparatus100according to this example. InFIG.10, a component having the same or corresponding function or configuration as a component of the image forming apparatus100according to the first exemplary embodiment illustrated inFIG.1is assigned the same reference numeral. In the image forming apparatus100illustrated inFIG.10, the primary transfer rollers14a,14b,14c, and14dare connected to the common primary transfer power source (high-voltage power source circuit)15. In this example, the secondary transfer opposing roller13is connected to the above-described common primary transfer power source15. In this example, the driving roller11and the tension roller12are electrically floating. A configuration in which the secondary transfer opposing roller13is electrically floating without being connected to the above-described common primary transfer power source15, or a configuration in which at least one of the driving roller11or the tension roller12is further connected to the above-described common primary transfer power source15is also conceivable. Also with such a configuration, as with the first exemplary embodiment, at the time of image formation (at the time of primary transfer, at the time of secondary transfer), the primary transfer rollers14and the secondary transfer opposing roller13are maintained at a predetermined potential (for example, +100 V), and the primary transfer potentials at the primary transfer nips N1are maintained at a predetermined potential. In this example, by reducing the number of primary transfer power sources15, the configuration of the image forming apparatus100can be simplified, and cost saving can be achieved.

In the first exemplary embodiment, the primary transfer voltage is applied by providing the primary transfer power source15, but the present disclosure is not limited to such a configuration. In the case of using the intermediate transfer belt10having conductivity that enables a current flow in the circumferential direction, the secondary transfer power source21can be used as a power source for supplying the primary transfer current, without providing the primary transfer power source15.FIG.11is a schematic cross-sectional diagram of the image forming apparatus100according to this example. InFIG.11, a component having the same or corresponding function or configuration as a component of the image forming apparatus100according to the first exemplary embodiment illustrated inFIG.1is assigned the same reference numeral. In the image forming apparatus100illustrated inFIG.11, the secondary transfer opposing roller13, the driving roller11, and the tension roller12are electrically grounded (connected to the ground) via a zener diode16being a voltage maintaining element serving as a voltage maintaining unit (voltage stabilization unit). On the inner circumferential surface side of the intermediate transfer belt10, the primary transfer rollers14each serving as a contact member that is in contact with the inner circumferential surface of the intermediate transfer belt10are arranged between the secondary transfer opposing roller13and the driving roller11so as to correspond to the respective photosensitive drums1. The configuration and arrangement of the primary transfer rollers14are similar to those of the first exemplary embodiment. Then, the primary transfer rollers14are electrically grounded (connected to the ground) via the above-described zener diode16. The zener diode16being a constant voltage element is an element that maintains a predetermined voltage (zener voltage) by flowing a current, and when a current larger than or equal to a fixed value flows, a zener voltage is generated on a cathode side. In other words, one end side (anode side) of the zener diode16is connected to the ground, and the other end side (cathode side) thereof is connected to the secondary transfer opposing roller13, the driving roller11, the tension roller12, and the primary transfer rollers14. By a voltage being applied from the secondary transfer power source21to the secondary transfer roller20, the secondary transfer opposing roller13and the primary transfer rollers14(furthermore, the driving roller11and the tension roller12) are maintained at the zener voltage. In this example, a current flows from the primary transfer rollers (metal rollers)14arranged in the vicinity of the respective photosensitive drums1and maintained at the zener voltage to the respective photosensitive drums1via the intermediate transfer belt10. Toner images are thereby primarily transferred onto the intermediate transfer belt10from the photosensitive drums1. In this example, the secondary transfer roller20constitutes a current supply member that comes in contact with the intermediate transfer belt10at a position different from that of the photosensitive drum1, flows a current in the circumferential direction of the intermediate transfer belt10by receiving an applied voltage, and primarily transfers a toner image from the photosensitive drum1onto the intermediate transfer belt10. A configuration in which at least one of the secondary transfer opposing roller13, the driving roller11, or the tension roller12is electrically floating without being connected to the zener diode16is also conceivable. As described above, since an absolute value of the zener voltage of the zener diode16becomes the primary transfer voltage, the primary transfer voltage can be maintained constant without providing the primary transfer power source15as in the first exemplary embodiment. In this example, downsizing and cost saving of the image forming apparatus100can be achieved. In this example, it is sufficient that the target voltage of the secondary transfer voltage to be applied to the secondary transfer roller20is controlled relative to the primary transfer voltage (potential of the secondary transfer opposing roller13) determined as described above in the same way as in the configuration of the first exemplary embodiment or the modification described herein. In this example, the zener diode is used as the voltage maintaining element, but the voltage maintaining element is not limited to this. Any element can be used as long as the element can obtain a similar effect. For example, a resistive element or a varistor being a constant voltage element can also be used. The primary transfer power source15according to the first exemplary embodiment can also be regarded as a voltage maintaining unit (voltage stabilization unit) for maintaining voltages of the primary transfer roller14and the secondary transfer opposing roller13.

In the first exemplary embodiment, the target voltage of the secondary transfer voltage is changed in two steps, i.e., the first target voltage V1and the second target voltage V2, but the present disclosure is not limited to such a configuration. For example, in the case of a configuration in which a distance (contact state) between the portion on the trailing end side of the recording material P and the intermediate transfer belt10changes in steps, the target voltage of the secondary transfer voltage may be changed in multiple steps of three or more steps. Also in this case, at least one of target voltages of the secondary transfer voltage corresponds to a target voltage in a period before the trailing end of the recording material P passes through the contact portion with the regulation member (e.g., the registration roller pair60). In addition, at least one of the target voltages of the secondary transfer voltage corresponds to a target voltage in a period after the trailing end of the recording material P passes through the contact portion with the regulation member.

In the first exemplary embodiment, as the target value of the secondary transfer voltage, the target voltage value of the secondary transfer voltage to be applied in a case where the secondary transfer voltage is subjected to constant voltage control is changed, but the present disclosure is not limited to such a configuration. As the target value of the secondary transfer voltage, a target current value of the secondary transfer current to be applied in a case where the secondary transfer voltage is subjected to constant current control may be changed. In this case, the secondary transfer power source21includes a current detection unit (ammeter) that detects a current flowing at the secondary transfer nip N2by applying a voltage to the secondary transfer roller20. The current detection unit detects the current flowing at the secondary transfer nip N2at a predetermined cycle (current detection cycle) at the time of image formation (at the time of secondary transfer). Then, based on a difference between a preset target current value and a detected current value detected by the current detection unit, the secondary transfer control unit202determines a voltage to be applied to the secondary transfer roller20from the secondary transfer power source21in the next current detection cycle. In other words, the secondary transfer control unit202adjusts the voltage to be applied to the secondary transfer roller20in the next current detection cycle so that the detected current value becomes closer to the target current value. With this configuration, the secondary transfer voltage to be applied to the secondary transfer roller20from the secondary transfer power source21is controlled so that the current flowing at the secondary transfer nip N2becomes substantially constant. The control of adjusting an output of a power source so that a value of a flowing current becomes closer to the target current value in this manner is referred to as the constant current control. In the case of performing the constant current control of the secondary transfer voltage in this manner, by controlling the target current value in place of the target voltage value that is controlled in the first exemplary embodiment or the modification described herein so that substantially constant secondary transferability is obtained, a similar effect can be obtained.

The effect of the first exemplary embodiment becomes more prominent as the resistance in the circumferential direction of the intermediate transfer belt10becomes lower. In the first exemplary embodiment, the resistance value in the circumferential direction of the intermediate transfer belt10is 1×108Ω. In the case of using the intermediate transfer belt10having a further lower resistance value in the circumferential direction, a change in secondary transfer current at the secondary transfer nip N2that is attributed to a change in orientation of the portion on the trailing end side of the recording material P becomes larger. Thus, the effect of the first exemplary embodiment becomes more prominent. In particular, in a case where a conductive layer (e.g., a resistance value in the circumferential direction thereof is about 1×106Ω or less) exists on the innermost surface of the intermediate transfer belt10, the case is equivalent to a case where an electrode exists on the inner circumferential surface of the intermediate transfer belt10. Thus, by a capacitance change between before and after the portion on the trailing end side of the recording material P moves to be along the intermediate transfer belt10, it becomes easier for a current to flow to the intermediate transfer belt10via the recording material P, and a drop in the secondary transfer voltage becomes more likely to occur. Thus, in the case where the conductive layer exists on the innermost surface of the intermediate transfer belt10, the effect of the first exemplary embodiment becomes more prominent. In a case where the intermediate transfer belt10includes a plurality of layers, if an electric resistance of an innermost layer is lower than an electric resistance of other layers, the effect of the first exemplary embodiment becomes more prominent.

Next, another exemplary embodiment of the present disclosure will be described. A basic configuration and an operation of an image forming apparatus according to a second exemplary embodiment are the same as those of the image forming apparatus according to the first exemplary embodiment. Thus, in the image forming apparatus according to the second exemplary embodiment, a component having the same or corresponding function or configuration as a component of the image forming apparatus according to the first exemplary embodiment is assigned the same reference numeral as that in the first exemplary embodiment, and the detailed description thereof will be omitted.

1. Configuration of Second Exemplary Embodiment

In the first exemplary embodiment, at the timing at which the trailing end of the recording material P passes through the registration roller nip R, the target voltage of the secondary transfer voltage is rapidly increased. In other words, in the first exemplary embodiment, the controller200performs control to change the target voltage of the secondary transfer voltage from the first target voltage V1to the second target voltage V2at once. In contrast, in the second exemplary embodiment, the target voltage of the secondary transfer voltage is gradually changed as the portion on the trailing end side of the recording material P gradually moves to be along the surface of the intermediate transfer belt10after the trailing end of the recording material P passes through the registration roller nip R.

FIG.12is a chart illustrating control of the secondary transfer voltage according to the second exemplary embodiment.FIG.12illustrates transitions of the target voltage in the control of the secondary transfer voltage that is performed by the secondary transfer control unit202and the detected voltage being an actual output value of the secondary transfer power source21that is detected by the voltage detection unit, from when the recording material P enters the secondary transfer nip N2to when the secondary transfer process ends.

In the second exemplary embodiment, based on the information regarding a size of the recording material P in the information regarding the recording material P that is included in the printing information input from the host device300, the controller200predicts a timing at which the trailing end of the recording material P passes through the registration roller nip R. In other words, the controller200predicts a timing at which the trailing end of the recording material P becomes free. After that, during a period from when the leading end of the recording material P enters the secondary transfer nip N2to when the trailing end of the recording material P passes through the registration roller nip R, the controller200controls the secondary transfer control unit202to apply the secondary transfer voltage to the secondary transfer roller20from the secondary transfer power source21by setting the target voltage of the secondary transfer voltage to the first target voltage V1. Then, the controller200controls the secondary transfer control unit202to start to increase the target voltage of the secondary transfer voltage at a timing at which the trailing end of the recording material P passes through the registration roller nip R, and gradually increase the target voltage of the secondary transfer voltage so that the target voltage reaches the second target voltage V2before the trailing end of the recording material P passes through the secondary transfer nip N2. At this time, in the second exemplary embodiment, the controller200performs control to linearly change the target voltage of the secondary transfer voltage from the first target voltage V1to the second target voltage V2. With this configuration, it is possible prevent a drop in secondary transfer voltage that gradually changes to suitably deal with a change in state of the portion on the trailing end side of the recording material P that gradually moves to be along the surface of the intermediate transfer belt10after the trailing end of the recording material P passes through the registration roller nip R.

As described above, in the second exemplary embodiment, in the configuration in which the orientation of the portion on the trailing end side of the recording material P gradually changes, by performing control that suitably deals with the change in orientation, an effect similar to the effect described in the first exemplary embodiment can be obtained.

Next, a modification of the second exemplary embodiment will be described.

In the second exemplary embodiment, the target voltage of the secondary transfer voltage is linearly changed from the first target voltage V1to the second target voltage V2, but the present disclosure is not limited to such a configuration. For example, as illustrated inFIG.13, the controller200may perform control to change the target voltage of the secondary transfer voltage from the first target voltage V1to the second target voltage V2in steps. In a case where the recording material P has small rigidity (e.g., thin paper), when the recording material P passes through the registration roller nip R, because springiness of the recording material P is weak and a rapid orientation change amount is small, the recording material P may gradually move to be along the intermediate transfer belt10. In such a case, as described above, it may be desirable to perform control of changing the target voltage of the secondary transfer voltage in steps. Moreover, in a case where the recording material P is label paper, thickness, resistance, rigidity, or paper quality may vary discontinuously within the surface of the recording material P. In such a case, it may be desirable to change the target voltage of the secondary transfer voltage in steps as described above.

In addition, for example, as illustrated inFIG.14, the controller200may perform control to nonlinearly and continuously change the target voltage of the secondary transfer voltage from the first target voltage V1to the second target voltage V2. In a case where the recording material P has high rigidity (e.g., thick paper), the following change is performed. More specifically, due to springiness of the recording material P, the leading end of the portion on the trailing end side of the recording material P swiftly moves to be along the intermediate transfer belt10at the instant of passage through the registration roller nip R, and then, the recording material P gradually moves to be along the intermediate transfer belt10toward the rearmost trailing end. In such a case, it may be more desirable to perform control of nonlinearly and continuously changing the target voltage of the secondary transfer voltage from the first target voltage V1to the second target voltage V2as described above. More specifically, the target voltage of the secondary transfer voltage is initially increased at a relatively large increase rate, and then, the increase rate is gradually decreased. In this case, a change configuration of the slope of a change in target voltage of the secondary transfer voltage can be varied based on the rigidity of the recording material P.

The control of the second exemplary embodiment and the control of the above-described modification may be combined with the configuration of the modification described in the first exemplary embodiment.

Heretofore, the present disclosure has been described based on specific exemplary embodiments, but the present disclosure is not limited to the above-described exemplary embodiments.

In the above-described exemplary embodiments, the primary transfer member (contact member) is a roller-shaped member formed of metal being a conductive material, but the conductive material is not limited to metal, and the primary transfer member is not limited to a roller-shaped member. The primary transfer member (contact member) may be a roller-shaped member including an elastic layer formed of conductive resin or rubber, a sheet-shaped member formed of conductive resin, or a brush-shaped member including conductive brush fibers.

This application claims the benefit of Japanese Patent Application No. 2020-155905, filed Sep. 16, 2020, which is hereby incorporated by reference herein in its entirety.