JP2015194663A - Transfer device and image forming apparatus - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a transfer device which is capable of suppressing transfer failure when using a transfer bias power supply to which a DC power supply and an AC power supply are electrically connected, and to provide an image forming apparatus.SOLUTION: A transfer device 50 comprises: a nip forming member 56 for forming a transfer nip by coming into contact with an image carrier 51; and a transfer bias power supply 200 to which a DC power supply 201 and an AC power supply 202 are electrically connected and which outputs a transfer bias. The transfer device 50 transfers a toner image on an image carrier to a recording medium sandwiched in the transfer nip by the transfer bias outputted by the transfer bias power supply. The transfer bias power supply starts up a DC component of the transfer bias by constant voltage control so that a previously set target voltage is achieved and then switches to constant current control so that a previously set target current is achieved before the toner image on the image carrier is transferred to the recording medium.

Description

  The present invention relates to a transfer device used in an image forming apparatus such as a printer, a facsimile machine, and a copying machine, and an image forming apparatus including the transfer device.

  2. Description of the Related Art Conventionally, as a transfer device of an image forming apparatus, a transfer nip is formed by a nip forming member abutting on an image carrier to sandwich a recording medium, and a transfer bias output from a transfer bias power source is applied to the transfer nip. There is known one that transfers the above toner image to a recording medium. As an example, there is known a secondary transfer device that secondarily transfers a superimposed toner image obtained by sequentially transferring toner images formed on a plurality of photoconductors onto an intermediate transfer belt to a recording material.

  In the secondary transfer device, as a nip forming member, a secondary transfer roller that abuts on the surface of the intermediate transfer belt as an image carrier, and a secondary transfer counter roller that abuts against the secondary transfer roller and abuts on the back surface of the intermediate transfer belt. Is provided. Then, an intermediate transfer belt is sandwiched between the secondary transfer roller and the secondary transfer counter roller to form a transfer nip.

  Either one of the secondary transfer counter roller or the secondary transfer roller is connected to a transfer bias power source, a transfer bias composed of a DC voltage outputted is applied, and the other is connected to the ground. Thereby, a transfer electric field for electrostatically moving the toner image from the secondary transfer counter roller side to the secondary transfer roller side is formed between the secondary transfer counter roller and the secondary transfer roller. Then, the toner image on the intermediate transfer belt is secondarily transferred to the recording medium fed into the transfer nip at a timing synchronized with the toner image on the intermediate transfer belt by the action of the transfer electric field.

  In such an image forming apparatus, it is known that the transfer bias applied to the secondary transfer roller or the secondary transfer counter roller by the transfer bias power source is controlled at a constant current. In this way, when the transfer bias is controlled at a constant current, even if the electrical resistance of the intermediate transfer belt, the two transfer rollers, etc. fluctuates in a temperature and humidity environment, the applied voltage changes accordingly. As a result, stable transferability can be obtained.

  In such a configuration, if a recording medium such as Japanese paper having a surface with a lot of unevenness is used as the recording medium, it becomes easy to generate a light and shade pattern (density unevenness) in the image. This light / dark pattern (density unevenness) is caused by the fact that a sufficient amount of toner is not transferred to the concave portion on the paper surface, and the image density of the concave portion becomes thinner than the convex portion.

  Therefore, in the transfer device provided in the image forming apparatus described in Patent Document 1, a transfer bias in which an AC voltage is superimposed on a DC voltage is applied using a transfer bias power source in which a DC power source and an AC power source are connected. It has become. By using such a transfer bias, the toner reciprocates between the concave portion on the surface of the recording medium and the image carrier, and the toner can come into contact with the concave portion on the surface of the recording medium. It is said that poor transfer of toner to the recess can be suppressed.

  When applying a transfer bias in which an AC voltage is superimposed on a DC voltage using a transfer bias power source in which a DC power source and an AC power source are connected, the DC component of the transfer bias is output via the substrate of the AC power source. Is done.

  As a result of intensive studies by the inventor of the present application, it has been found that the following problems occur when a transfer bias is applied by constant current control by a transfer bias power source in which a DC power source and an AC power source are connected.

  That is, due to the influence of the capacitor circuit in the substrate of the AC power supply, the output responsiveness of the DC component of the transfer bias becomes slower than when the DC voltage is output using only the DC power supply, and the target voltage required for transfer is reduced. The rise time tended to be longer. For this reason, a necessary transfer bias cannot be secured at the leading edge of the image, and the image density at the leading edge of the image is diluted, resulting in a transfer failure.

  The applicant of the present application has started up a DC component of a transfer bias output from a transfer bias power source in which a DC power source and an AC power source are electrically connected by constant voltage control, and then the toner image on the intermediate transfer belt is recorded on the recording medium. We are developing a transfer device that switches to constant current control before it is transferred.

  As in this transfer device, the DC component of the transfer bias output from the transfer bias power supply is raised by constant voltage control, so that the DC component rises to a target voltage at a steep slope compared to the case where the DC component is raised by constant current control. The rise time can be shortened.

  Further, in this transfer apparatus, aiming at the maximum output value of the DC component of the transfer bias output from the transfer bias power source, the DC component is output by constant voltage control for a preset start-up time, thereby achieving the target voltage. Is to be obtained.

  As a result, it is possible to prevent the density at the front end of the image from being diluted due to insufficient transfer bias due to a delay in the rise time to the target voltage.

  Further, when transferring the toner image on the intermediate transfer belt to a recording medium, a transfer bias is applied by constant current control. As a result, the transfer electric field at the transfer nip can be stabilized and stable transfer performance can be obtained even if the electrical resistance of the intermediate transfer belt, the secondary transfer roller or the like fluctuates due to the temperature and humidity environment. .

  However, the rising slope of the DC component changes due to resistance fluctuations of the intermediate transfer belt, the secondary transfer roller, and the like caused by environmental fluctuations such as temperature and humidity. For this reason, there arises a problem that the voltage value when the same rise time has elapsed from the start of the rise varies and the target voltage cannot be obtained.

  The present invention has been made in view of the above problems, and an object of the present invention is to provide a transfer that can suppress the occurrence of transfer failure when a transfer bias power supply in which a DC power supply and an AC power supply are electrically connected is used. And an image forming apparatus including the transfer device.

  In order to achieve the above object, according to the first aspect of the present invention, a nip forming member for forming a transfer nip in contact with an image carrier is electrically connected to a DC power source and an AC power source. A transfer bias power supply for outputting a bias, wherein the transfer bias output by the transfer bias power supply transfers a toner image on the image carrier to a recording medium sandwiched in the transfer nip. The bias power supply is set in advance before the toner image on the image carrier is transferred to the recording medium after the DC component of the transfer bias is raised by constant voltage control so as to be a preset target voltage. It is characterized by switching to constant current control so that it becomes the set target current.

  As described above, according to the present invention, when a transfer bias power source in which a DC power source and an AC power source are electrically connected is used, there is an excellent effect that occurrence of transfer failure can be suppressed.

The schematic diagram which shows the control signal and output waveform of the direct current | flow component of a superimposition bias. 1 is a schematic configuration diagram of a printer that is an image forming apparatus according to an embodiment. FIG. 2 is a schematic configuration diagram of an image forming unit. The figure which shows the power supply structure of the secondary transfer bias power supply which consists of DC power supply and AC power supply. The wave form diagram which shows an example of the waveform of the superimposition bias output from DC power supply and AC power supply. 6 is a graph showing an example of a time for moving toner from the intermediate transfer belt side to the recording paper side and a time for returning the toner from the recording paper side to the intermediate transfer belt side among AC components. Explanatory drawing of an example of the rise time of the high voltage | pressure output by a superposition bias, and the high voltage | pressure output by a DC bias. The figure which shows the power supply structure of the secondary transfer bias power supply consisting of DC power supply. (A) The figure used for explanation of the rise time when the DC component of the superimposed bias is raised by constant current control, (b) The explanation of the rise time when the DC component of the superimposed bias is raised by constant voltage control Figure used for. The figure which shows the voltage detection timing at the time of DC bias application (DC constant current mode). The figure which shows the voltage detection timing at the time of alternating current bias (superimposition bias) application. (A) The figure which showed the starting time when the target voltage is high, (b) The figure which showed the rising time when the target voltage is low. FIG. 4 is a schematic diagram of a contact / separation mechanism that contacts and separates an intermediate transfer belt and a secondary transfer roller. FIG. 4 is a diagram illustrating a state where an intermediate transfer belt and a secondary transfer roller are separated by a gap amount A. FIG. 4 is a diagram illustrating a state where an intermediate transfer belt and a secondary transfer roller are separated by a gap amount C. FIG. 4 is a diagram illustrating a state where an intermediate transfer belt and a secondary transfer roller are in contact with each other. FIG. 4 is a diagram used for explaining a gap amount between an intermediate transfer belt and an intermediate transfer belt when a sheet enters a secondary transfer nip. FIG. 5 is a diagram used for explaining a gap amount between an intermediate transfer belt and a secondary transfer roller when a front end of a sheet passes through a secondary transfer nip and a toner image on the intermediate transfer belt is transferred to a sheet. 6 is a graph showing how a profile of a gap amount between a secondary transfer roller and an intermediate transfer belt changes depending on a difference in the amount of margin formed at the leading edge of a sheet.

[Embodiment 1]
Embodiment 1 of the present invention will be described below with reference to the drawings.
FIG. 2 is a schematic configuration diagram of a printer that is an image forming apparatus according to the present embodiment.

  This printer has an intermediate transfer belt 51 which is an endless belt member as an intermediate transfer member. Further, four image forming units 1Y, 1M for forming yellow (Y), magenta (M), cyan (C), and black (K) toner images along the upper running side of the intermediate transfer belt 51. , 1C, 1K are arranged in parallel to form a tandem image forming unit.

  Since the image forming units 1Y, 1M, 1C, and 1K have the same configuration except for the color of the toner to be handled, only one image forming unit 1 will be described with reference to FIG. At this time, subscripts (Y, M, C, K) indicating each color are omitted as appropriate.

  As shown in FIG. 3, the image forming unit 1 includes a photosensitive drum 11 as an image carrier, a charging device 21, a developing device 31, a cleaning device 41, and the like. In the present embodiment, the image forming units 1Y, 1M, 1C, and 1K are provided to be detachable from the printer main body.

  The charging device 21 is a charging unit that charges the surface of the photosensitive drum 11 with a charging roller. The developing device 31 is a developing unit that develops and visualizes the latent image on the photosensitive drum 11 with a developer containing toner. The cleaning device 41 is a cleaning unit that cleans the surface of the photosensitive drum.

  The photosensitive drum 11 has a drum shape with an outer diameter of about 60 [mm] in which an organic photosensitive layer is formed on the surface of a drum base, and is rotated in the clockwise direction in the drawing by a driving unit (not shown).

  The charging device 21 generates a discharge between the charging roller and the photosensitive drum 11 while bringing a charging roller to which a charging bias is applied into contact with or close to the photosensitive drum 11, so that the surface of the photosensitive drum is uniform. Charge.

  In this embodiment, the surface of the photosensitive drum is uniformly charged by the charging device 21 so as to have the same negative polarity as the normal charging polarity of the toner. As the charging bias, one in which an AC voltage is superimposed on a DC voltage is employed. Note that a method using a charging charger may be adopted instead of the method using a charging roller.

  The developing device 31 includes a developing sleeve 31a serving as a developer carrier and two screws serving as a stirring member that conveys the developer while stirring in a storage container in which a two-component developer including toner and a carrier is stored. Members 31b and 31c are provided. As the developing device 31, a developing device that uses a one-component developer containing toner may be employed.

  The cleaning device 41 includes a cleaning blade 41a and a cleaning brush roller 41b.

  The cleaning blade 41 a is in contact with the surface of the photosensitive drum from the counter direction with respect to the rotation direction of the photosensitive drum 11. Further, the cleaning brush roller 41 b is in contact with the surface of the photosensitive drum while rotating in the direction opposite to the rotation direction of the photosensitive drum 11. Then, the surface of the photosensitive drum is cleaned by the cleaning blade 41a and the cleaning brush roller 41b.

  Above the image forming units 1Y, 1M, 1C, and 1K, an optical writing unit 80 serving as a latent image writing unit for writing a latent image on the surface of the photosensitive drum charged by the charging device 21 is disposed. .

  The optical writing unit 80 optically scans the surfaces of the photosensitive drums 11Y, 11M, 11C, and 11K with laser light emitted from a laser diode based on image information sent from an external device such as a personal computer. .

  By this optical scanning, electrostatic latent images for Y, M, C, and K are formed on the surfaces of the photosensitive drums 11Y, 11M, 11C, and 11K. Specifically, in the entire area of the uniformly charged surface of the photosensitive drum 11, the portion irradiated with the laser light attenuates the potential. Thereby, the electric potential of a laser irradiation location becomes smaller than the electric potential of an other location (background part), As a result, it becomes an electrostatic latent image.

  The optical writing unit 80 is configured to cause the laser light L emitted from the light source to be polarized in the main scanning direction by a polygon mirror that is rotationally driven by a polygon motor (not shown), and the surface of the photosensitive drum through a plurality of optical lenses and mirrors. Is irradiated. Moreover, you may employ | adopt what performs optical writing with the LED light emitted from several LED of the LED array.

  Below the image forming units 1Y, 1M, 1C, and 1K, a transfer unit 50 is disposed as a transfer device that moves the endless intermediate transfer belt 51 endlessly in the counterclockwise direction in the drawing. .

  In addition to the intermediate transfer belt 51, the transfer unit 50 includes a driving roller 52, a secondary transfer counter roller 53, a cleaning backup roller 54, four primary transfer rollers 55, a secondary transfer roller 56, and a belt cleaning device 57. Have.

  The intermediate transfer belt 51 is stretched by a driving roller 52, a secondary transfer counter roller 53, a cleaning backup roller 54, and four primary transfer rollers 55Y, 55M, 55C, and 55K disposed inside the loop. . The intermediate transfer belt 51 is moved endlessly in the counterclockwise direction in the figure by the rotational force of the driving roller 52 that is driven to rotate counterclockwise in the figure by a driving means (not shown).

  As the intermediate transfer belt 51, a belt having the following characteristics is used. That is, the thickness is about 20 [μm] to 200 [μm], preferably about 60 [μm].

The surface resistance is 9.0 to 13.0 [LogΩ / cm ² ], preferably 10.0 to 12.0 [LogΩ / cm ² ]. In addition, as a measuring method, it measures on the conditions of the applied voltage 500 [V] and 10 [sec] value with Mitsubishi Chemical's Hiresta UP MCP HT45, HRS probe.

  The volume resistivity is about 6.0 to 13.0 [Log Ωcm], preferably about 7.5 to 12.5 [Log Ωcm], and more preferably about 9.0 [Log Ωcm]. In addition, as a measuring method, it measures on the conditions of the applied voltage of 100 [V] and 10 [sec] value with Mitsubishi Chemical's Hiresta UP MCP HT45, HRS probe.

  The material may be a single layer or multiple layers of PI (polyimide), PVDF (vinylidene fluoride), ETFE (ethylene-tetrafluoroethylene copolymer), PC (polycarbonate), etc. Is possible.

  Note that a release layer may be coated on the surface of the intermediate transfer belt 51 as necessary. Examples of the material used for the coating include, but are not limited to, the following. For example, ETFE (ethylene-tetrafluoroethylene copolymer), PTFE (polytetrafluoroethylene), PVDF (vinylidene fluoride), PEA (perfluoroalkoxy fluororesin), FEP (tetrafluoroethylene-hexafluoride) Fluorine resins such as propylene copolymer) and PVF (vinyl fluoride) can be used.

  Examples of the belt manufacturing method include a casting method and a centrifugal molding method, and the surface thereof may be polished if necessary.

  Further, an endless belt member having a three-layer structure of a base layer, an elastic layer, and a coat layer may be used.

  In the case of a belt member having a three-layer structure, the base layer is made of a material obtained by combining, for example, a fluorine-based resin having a small elongation or a rubber material having a large elongation with a material that is difficult to stretch, such as canvas. The elastic layer is made of, for example, fluorine rubber or acrylonitrile-butadiene copolymer rubber, and is formed on the base layer. The coat layer is formed by coating the surface of the elastic layer with, for example, a fluorine resin.

  The resistivity of the intermediate transfer belt 51 is adjusted by dispersing a conductive material such as carbon black in the belt member.

  The primary transfer rollers 55Y, 55M, 55C, and 55K sandwich the intermediate transfer belt 51 that is moved endlessly between the photosensitive drums 11Y, 11M, 11C, and 11K. As a result, primary transfer nips for Y, M, C, and K in which the front surface of the intermediate transfer belt 51 and the photosensitive drums 11Y, 11M, 11C, and 11K abut are formed.

  A primary transfer bias is applied to the primary transfer rollers 55Y, 55M, 55C, and 55K by a transfer bias power source (not shown). As a result, a transfer electric field is formed between each color toner image on the photoconductive drums 11Y, 11M, 11C, and 11K and each primary transfer roller 55, and on each photoconductive drum 11 by the action of the transfer electric field and the nip pressure. From the toner image to the intermediate transfer belt 51.

  Then, the Y toner image, the M toner image, the C toner image, and the K toner image are sequentially superimposed and primarily transferred on the intermediate transfer belt 51, whereby a four-color superimposed toner image is formed on the intermediate transfer belt 51. It is formed.

  In the case of forming a monochrome image, a support plate (not shown) supporting the primary transfer rollers 55Y, 55M, and 55C for Y, M, and C in the transfer unit 50 is moved to move the primary transfer rollers 55Y, 55M, and 55C. Is moved away from the photosensitive drums 11Y, 11M, and 11C.

  As a result, the front surface of the intermediate transfer belt 51 is separated from the photosensitive drums 11Y, 11M, and 11C, and the intermediate transfer belt 51 is brought into contact with only the photosensitive drum 11K.

  In this state, among the four image forming units 1Y, 1M, 1C, and 1K, only the image forming unit 1K is driven to form a K toner image on the photosensitive drum 11K.

  The primary transfer roller 55 includes an elastic roller including a metal core and a conductive sponge layer fixed on the surface thereof. In this embodiment, the primary transfer roller 55 has the following characteristics. Yes.

  The roller outer shape is 16 [mm], and the diameter of the cored bar is 10 [mm]. The volume resistance is 6.0 to 8.0 [Log Ωcm], preferably 7.0 to 8.0 [Log Ωcm].

  The volume resistance of the primary transfer roller 55 is measured by rotation measurement. A load of 10 [N] / one side is applied, a bias of 1 [kV] is applied to the primary transfer roller shaft, and the primary transfer is performed during one minute measurement. The resistance value was measured while rotating the roller 55 once, and the average value was defined as the volume resistance.

  The resistance R of the sponge layer calculated based on Ohm's law (R = V / I) is 1E6 [Ω] to 1E9 [Ω], preferably about 3E7 [Ω].

  A primary transfer bias is applied to such a primary transfer roller 55 by constant current control. In place of the primary transfer roller 55, primary transfer means using a transfer charger, a transfer brush, or the like may be employed.

  The secondary transfer roller 56 of the transfer unit 50 is disposed outside the loop of the intermediate transfer belt 51, and the intermediate transfer belt 51 is sandwiched between the secondary transfer opposing roller 53 inside the loop. As a result, a secondary transfer nip where the front surface of the intermediate transfer belt 51 and the secondary transfer roller 56 abut is formed.

  In the transfer unit 50, the portion where the intermediate transfer belt 51 is sandwiched between the secondary transfer roller 56 and the secondary transfer counter roller 53 is a second toner image transferred from the intermediate transfer belt 51 onto the recording paper P. Next transfer part.

  While the secondary transfer roller 56 is electrically grounded, the secondary transfer bias power source 200 is connected to the secondary transfer counter roller 53 and a secondary transfer bias is applied by the secondary transfer bias power source 200. As a result, a secondary transfer electric field for electrostatically moving the toner from the secondary transfer counter roller 53 side to the secondary transfer roller 56 side is formed between the secondary transfer counter roller 53 and the secondary transfer roller 56. The

  The secondary transfer roller 56 may be connected to the transfer bias power source 200 and the secondary transfer counter roller 53 may be electrically grounded.

  Below the transfer unit 50, a paper feed cassette 100 that stores a plurality of recording papers P in a stacked state is disposed. In this paper feed cassette 100, a paper feed roller 101 is brought into contact with the top recording paper P in the paper bundle, and this recording paper P is fed into the paper feed path by being driven to rotate at a predetermined timing. Send it out.

  A registration roller pair 102 is disposed near the end of the paper feed path. The registration roller pair 102 immediately stops the rotation of both rollers when the recording paper P delivered from the paper feed cassette 100 is sandwiched between the rollers.

  Then, at a timing at which the sandwiched recording paper P can be synchronized with the toner image on the intermediate transfer belt 51 in the secondary transfer nip, the rotational driving of both rollers of the registration roller pair 102 is resumed, and the recording paper P is moved to the secondary transfer nip. Send to the transfer nip.

  The toner image on the intermediate transfer belt 51 brought into close contact with the recording paper P at the secondary transfer nip is collectively secondary transferred onto the recording paper P by the action of the secondary transfer electric field or nip pressure.

  The recording paper P having the full-color toner image or the monochrome toner image formed on the surface in this way is separated from the secondary transfer roller 56 and the intermediate transfer belt 51 by the curvature when passing through the secondary transfer nip.

  The secondary transfer counter roller 53 is formed by laminating a resistance layer on a cored bar made of stainless steel, aluminum, or the like.

  The resistance layer is made by dispersing conductive particles such as carbon or metal complex in polycarbonate, fluorine rubber, silicon rubber or the like, or rubber such as NBR or EPDM, rubber of NBR / ECO copolymer, polyurethane Made of conductive rubber or the like. The volume resistance is 6.0 to 12.0 [Log Ωcm], preferably 7.0 to 9.0 [Log Ωcm].

  Further, a foam type having a hardness of 20 degrees to 50 degrees or a rubber type having a rubber hardness of 30 degrees to 60 degrees may be used. However, since the contact is made with the secondary transfer roller 56 via the intermediate transfer belt 51, even a small contact pressure is not contacted. A sponge type that does not produce a part is desirable. This is because, as the contact pressure between the intermediate transfer belt 51 and the secondary transfer counter roller 53 is larger, characters and fine lines are more likely to be lost, which is prevented.

  The secondary transfer roller 56 is formed by laminating a resistance layer and a surface layer made of conductive rubber or the like on a cored bar made of stainless steel or aluminum.

  In this embodiment, the outer diameter of the secondary transfer roller 56 is 20 [mm], and the core metal is stainless steel having a diameter of 16 [mm]. The resistance layer is a rubber having a hardness of 40 to 60 degrees [JIS-A] made of an NBR / ECO copolymer.

  The surface layer is made of a fluorine-containing urethane elastomer, and the thickness is desirably 8 to 24 [μm]. The reason is that the surface layer of the roller is often manufactured by a coating process. Therefore, when the surface layer thickness is 8 [μm] or less, the influence of the resistance unevenness due to the uneven coating is large, and a leak occurs at a location where the resistance is low. There is a possibility that it is not preferable. In addition, wrinkles are generated on the roller surface and the surface layer is liable to crack.

  On the other hand, when the thickness of the surface layer exceeds 24 [μm], the resistance increases, and when the volume resistivity is high, the voltage when a constant current is applied to the core of the secondary transfer counter roller 53 increases. Since it exceeds the voltage variable range of the constant current power supply, the current is below the target current. In addition, when the voltage variable range is a sufficiently high range, there is a leak due to a high voltage path from the constant current power source to the core of the secondary transfer counter roller 53 or a high voltage on the core of the secondary transfer counter roller 53. Is likely to occur.

  Further, when the thickness of the surface layer of the secondary transfer roller 56 is greater than 24 [μm], the hardness is increased, and the adhesion to the recording paper P and the intermediate transfer belt 51 is deteriorated.

The surface resistance of the secondary transfer roller 56 is 6.5 [LogΩ / cm ² ] or more, and the volume resistance of the surface layer of the secondary transfer roller 56 is 10.0 [LogΩcm] or more, more preferably 12.0 [LogΩcm]. That's it.

  The secondary transfer roller 56 may be a foam type roller that does not have a surface layer. In this case, the volume resistance of the secondary transfer roller 56 is 6.0 to 8.0 [Log Ωcm], preferably 7.0 to 8.0 [Log Ωcm].

  At this time, the secondary transfer counter roller 53 may be a foam type, a rubber type, or a metal roller such as stainless steel, and its volume resistance is 6.0 [lower than that of the secondary transfer roller 56]. Log Ωcm] or less is preferable.

  Note that the volume resistance of the secondary transfer roller 56 and the secondary transfer counter roller 53 is measured by the same rotation measurement method as that of the primary transfer roller 55 described above.

  A fixing device 90 is disposed on the right side of the secondary transfer nip in the drawing. The fixing device 90 forms a fixing nip with a fixing roller 91 containing a heat source such as a halogen lamp and a pressure roller 92 that rotates while contacting with the fixing roller 91 with a predetermined pressure.

  The recording paper P fed into the fixing device 90 is sandwiched between the fixing nips in such a posture that the unfixed toner image carrying surface is in close contact with the fixing roller 91. Then, the toner in the toner image is softened by the influence of heating and pressurization, and the full color image is fixed. The recording paper P discharged from the fixing device 90 is discharged out of the apparatus through a post-fixing conveyance path.

  FIG. 4 shows a power supply configuration of a secondary transfer bias power supply 200 as a secondary transfer bias output means provided in the printer of this embodiment.

  As shown in FIG. 4, the secondary transfer bias power source 200 includes a DC power source (first power source) that outputs a DC component, and an AC power source (second power source) that outputs an AC component or a DC component superimposed with an AC component. It consists of and. As the secondary transfer bias, it is possible to output a DC voltage (hereinafter referred to as a DC bias) and a voltage obtained by superimposing an AC voltage on the DC voltage (hereinafter referred to as a superimposed bias).

  In the secondary transfer bias power supply 200 having such a configuration, when the superimposed bias is output, an output signal is sent from the control unit 300 to the DC power supply 201 and the AC power supply 202, and the superimposed bias is applied to the secondary transfer counter roller 53.

  When a DC bias is output, a signal is sent from the control unit 300 only to the DC power supply 201 and the DC bias is applied to the secondary transfer counter roller 53.

  Here, the configuration using the AC power source 202 that outputs only an AC component is shown as the second power source. However, as the second power source, a power source that outputs an AC component superimposed on the DC component is used. It is also possible. With such a configuration, it is possible to realize superimposed bias application with a low-cost and space-saving power supply configuration.

  FIG. 5 is a waveform diagram showing an example of a superimposed bias waveform output from the DC power supply 201 and the AC power supply 202.

  In FIG. 5, the offset voltage Voff is the value of the DC component of the superimposed bias. The Peak-to-Peak voltage Vpp is a Peak-to-Peak voltage that is an alternating current component of the superimposed bias.

  The superposition bias is a superposition of the offset voltage Voff and the peak-to-peak voltage Vpp, and the time average value thereof is the same value as the offset voltage Voff.

  As shown in the figure, the superimposed bias has a sinusoidal shape, and has a positive peak value and a negative peak value.

  Of these two peak values, Vt in the figure indicates the peak value of the one that moves the toner from the intermediate transfer belt side to the recording paper side in the secondary transfer nip (minus side in this embodiment). It is. In addition, Vr in the figure indicates a peak value on the way of returning the toner from the recording paper side to the intermediate transfer belt side (plus side in this embodiment).

  A superimposed bias including a direct current component is applied so that the time average value of the offset voltage Voff has the same polarity as that of the toner (minus in this embodiment). This makes it possible to move the toner from the intermediate transfer belt side to the recording paper side and transfer it onto the recording paper P while moving the toner back and forth.

  Note that a sinusoidal waveform is used as the AC voltage, but a rectangular waveform may also be used.

  Among the AC components, the time for moving the toner from the intermediate transfer belt side to the recording paper side (minus side in this embodiment) and the time for returning the toner from the recording paper side to the intermediate transfer belt side (plus in this embodiment) It is also possible to set different times.

  In the example shown in FIG. 6, the time A on the transfer direction side for moving the toner from the intermediate transfer belt side to the recording paper side in one cycle of the AC component, and the toner from the recording paper side to the intermediate transfer belt side are moved. It is set larger than the time B on the return side.

  However, the waveform shown in FIG. 6 is an example, and the ratio between the time A on the transfer direction side and the time B on the return side can be set as appropriate.

  When recording paper P having a large surface irregularity, such as Japanese paper or embossed paper, is used, a superimposed bias is applied. As a result, as described above, the toner is moved relatively from the intermediate transfer belt side to the recording paper side and transferred onto the recording paper P while reciprocating the toner, thereby improving the transferability to the paper recess. Therefore, it is possible to improve abnormal images such as an increase in transfer rate and voids.

  However, it is known that applying a superimposed bias as the transfer bias makes it easier to generate transfer dust compared to applying a DC voltage alone. Transfer dust is a phenomenon in which toner is scattered around the image area in the transfer process.

  As the frequency of the AC component of the transfer bias increases, the number of reciprocating movements of the toner between the intermediate transfer belt 51 and the recording paper P in the secondary transfer nip increases, and transfer dust tends to occur. Although transfer dust can be suppressed by optimizing the frequency, transfer dust may occur depending on environmental conditions.

  On the other hand, when recording paper P (smooth paper, coated paper, etc.) with small unevenness such as ordinary transfer paper is used, sufficient transferability can be obtained by applying a secondary transfer bias only with a direct current component.

  Therefore, depending on the paper type of the recording paper P, there is a paper type that does not require a superimposing bias. In this case, since there is sufficient transferability without applying a superimposing bias, a direct current bias is not applied without applying an alternating bias. Apply only. Thereby, generation | occurrence | production of transfer dust can be suppressed.

  Therefore, by switching between applying a DC bias when using a recording paper P with small unevenness such as a normal transfer paper and applying a superimposed bias when using a recording paper P with large unevenness, various recording papers P can be used. In contrast, good transferability can be obtained. In addition, since the AC power source 202 is turned on only when necessary, the service life is also extended.

  Next, the rise time of the high voltage output due to the superimposed bias and the high voltage output due to the DC bias will be described.

  FIG. 7 is an explanatory diagram of an example of the rise time of the high voltage output by the superimposed bias and the high voltage output by the DC bias. FIG. 8 is a diagram showing a power supply configuration of the secondary transfer bias power supply 200 including the DC power supply 201.

  Note that the term “rise” refers to a state in which there is a potential difference regardless of plus or minus from a state without potential difference (0 [kV]). For reference, the term “falling” refers to a state where there is no potential difference (0 [kV]) from a state where there is a potential difference regardless of whether it is positive or negative.

  When only the DC bias is output at high voltage by the constant current control using the secondary transfer bias power source 200 as shown in FIG. 8, the rising time of the DC bias is as shown in FIG.

  That is, 50 [ms] from when the direct bias output instruction is given to the secondary transfer bias power supply 200 until the bias value of the secondary transfer bias power supply 200 reaches a target value (e.g., corresponding to −10 [kV]). I need it.

  Note that a DC bias output instruction to the secondary transfer bias power supply 200 is performed by outputting a DC bias output signal to the secondary transfer bias power supply 200.

  On the other hand, when the superimposed bias is output at a high voltage by the constant current control using the secondary transfer bias power source 200 as shown in FIG. 4, the rising time of the superimposed bias is as shown in FIG.

  That is, after the superimposing bias output instruction is given to the secondary transfer bias power supply 200, the bias value (offset voltage Voff) of the secondary transfer bias power supply 200 reaches a target value (e.g., corresponding to −10 [kV]). Therefore, 600 [ms] is required.

  The superimposing bias output instruction to the secondary transfer bias power supply 200 is performed by outputting a DC bias output signal and an AC bias output signal to the secondary transfer bias power supply 200.

  As described above, when a high voltage is output to the secondary transfer bias power supply 200 with a superposed bias, it takes time for the bias value to reach a target value as compared with a case where a high voltage is output only with a DC bias.

  This is because the AC power supply 202 has a capacitor for load adjustment, and this capacitor has a certain amount of capacity to maintain an AC waveform. On the other hand, the DC component of the superimposed bias is controlled at a constant current, and is output at a predetermined low current to prevent inrush, and it takes time to charge the DC component of the superimposed bias by the load adjustment capacitor. As a result, the voltage rise time is delayed.

  The superimposed bias AC component is also charged to the load adjustment capacitor, but the superimposed bias AC component is controlled at a constant voltage, so there is no problem even if a large voltage is superimposed from the beginning. It takes less time to charge the capacitor.

  Here, in the image forming apparatus described in Japanese Patent Application Laid-Open No. 2008-275844, when the image area of the recording paper passes through the secondary transfer unit, the transfer power source that outputs only a DC voltage is controlled at a constant voltage. A transfer bias is applied. Then, the transfer bias applied when the image area of the recording paper passes through the secondary transfer portion is corrected by the number of printed sheets and the paper type paper thickness from the voltage value measured when there is no recording paper in the secondary transfer portion. Yes.

  On the other hand, in this embodiment, the DC component of the transfer bias applied in the non-image area at the start-up is controlled by constant voltage, and the DC component of the transfer bias applied when the image area of the recording paper passes through the secondary transfer portion. The constant current is controlled.

  FIG. 1 is a schematic diagram showing a control signal and an output waveform of the DC component of the superimposed bias. FIG. 1A shows a waveform of a constant voltage control signal sent from the control unit 300 to the secondary transfer bias power source 200. FIG. 1B shows a waveform of a constant current control signal sent from the control unit 300 to the secondary transfer bias power source 200. FIG. 1C shows the output of the bias (current or voltage) output to the secondary transfer counter roller 53.

  FIG. 9 compares the rise times when the DC component of the superimposed bias is raised by constant current control and when it is raised by constant voltage control. FIG. 9A shows the start-up time when the DC component of the superimposed bias is started by constant current control. FIG. 9B shows the rise time when the DC component of the superimposed bias is raised by constant voltage control.

  In the present embodiment, when the secondary transfer bias power source 200 outputs a high voltage with a superimposed bias, the DC component of the superimposed bias is started up by constant voltage control so as to become a preset target voltage (FIG. 1A and FIG. 1). (Refer FIG.1 (c)). This makes it possible to achieve the target bias value without requiring a rise time as shown in FIG. 9B, compared to the case where the DC component of the superimposed bias shown in FIG. Therefore, it is possible to suppress the dilution of the density at the front end of the image due to the delay in the rise time.

  In addition, since the voltage value of the DC component at the time of start-up is set by the target voltage itself, even if the rising gradient of the DC component changes due to environmental fluctuations or the like, the DC component is targeted. Can be voltage.

  In addition, after starting up with constant voltage control so as to have a preset target voltage, before the toner image on the intermediate transfer belt 51 is transferred onto the recording paper P, the preset target current is set. Switch to current control (see FIGS. 1B and 1C).

  As described above, when the toner image on the intermediate transfer belt 51 is transferred to the recording paper P, the electric resistance of the intermediate transfer belt 51 and the like varies depending on the temperature and humidity environment by applying the transfer bias with constant current control. However, it is possible to stabilize the transfer electric field and obtain a stable transfer property.

  In FIG. 1, when the voltage is raised by constant voltage control (before the leading edge of the paper reaches the transfer nip), the secondary transfer roller 56 and the intermediate transfer belt 51 are separated from each other. The transfer nip may be formed by bringing the secondary transfer roller 56 into contact with the intermediate transfer belt 51 before the image (toner image) on the recording paper P reaches the secondary transfer position.

  Further, when the voltage is raised by constant voltage control (before the leading edge of the paper reaches the transfer nip), the secondary transfer roller 56 is brought into contact with the intermediate transfer belt 51 with a weaker pressure than that at the time of image transfer. The pressure may be increased before the image (toner image) on the recording paper P reaches the secondary transfer position.

  In FIG. 1, when the leading edge of the paper reaches the transfer nip, the secondary transfer bias power source 200 is switched from the low voltage control to the constant current control. However, the present invention is not limited to this. That is, the secondary transfer bias power supply 200 may be switched from constant voltage control to constant current control after the paper leading edge reaches the transfer nip and before the image leading edge reaches the transfer nip.

  In addition, there are various types of recording paper P used for electrophotography, and the optimum transfer bias for good transfer varies depending on the material and thickness of the recording paper P, and the optimum transfer at the leading edge of the image. The bias is also different.

  For this reason, the target voltage for the rising constant voltage control is preferably set to an optimum voltage depending on the type of the recording paper P such as thin paper or thick paper.

  For example, when a constant voltage is used regardless of the type of recording paper P, the image area tip bias is too high for thin paper, resulting in overtransfer, and for thick paper, transfer is insufficient, resulting in abnormal images (white spots, low density, etc.). there's a possibility that.

  Therefore, in the present embodiment, the correspondence due to the difference in the recording paper P is changed to an appropriate target voltage depending on the difference in the paper thickness and paper type, that is, the thickness of the recording paper P and the surface property. In addition, although an example was shown in Table 1, it is not specifically limited to this.

  Here, Japanese Patent Laid-Open No. 2012-42827 discloses changing the voltage of the AC component of the transfer bias in which the DC component and the AC component are superimposed depending on the paper type and paper thickness. There is no disclosure of changing the target voltage of the direct current component in.

  In this embodiment, the output voltage (the resistance value of the secondary transfer portion) is detected to control (correct) the target voltage for rising.

  The transfer resistance is also greatly related to the electrical resistance of transfer members such as the secondary transfer roller 56, the secondary transfer counter roller 53, and the intermediate transfer belt 51.

  That is, if the resistance of the transfer member is too low, the effect of the resistance of the toner layer increases, and the applied voltage changes greatly depending on the image area, and the transfer efficiency changes when the image area is small and large.

  In addition, even when the resistance of the transfer member is too high, there is a problem in that the applied voltage becomes too high, causing current leakage and disturbing the image. Furthermore, when the voltage is increased to the upper limit of the power supply performance, there is a problem in that there is a risk that the current will not flow, transfer will not be performed sufficiently, and the power supply may be broken.

  It is generally seen that the resistance of members constituting the transfer device, such as the intermediate transfer belt 51 and the secondary transfer roller 56, gradually changes due to the application of a transfer bias. Therefore, when the resistance of the intermediate transfer belt 51 and the secondary transfer roller 56 changes with time, the above-described problems may occur.

  Therefore, in the present embodiment, the transfer bias value (the DC component of the superimposed bias), the AC component of the superimposed bias, and the rising target voltage) is corrected by the detected resistance value of the secondary transfer unit.

  The DC power supply 201 and the AC power supply 202 shown in FIG. 4 are controlled by sending a PWM signal (pulse width modulation signal) such as a constant voltage control signal and a constant current control signal from the control unit 300. Of the DC power supply 201 and AC power supply 202 provided in the secondary transfer bias power supply 200, only the DC power supply 201 has the voltage detection means 203.

  The feedback voltage detected by the voltage detection unit 203 is input to the control unit 300 and used for resistance detection at the secondary transfer unit.

  Further, since the AC power source 202 is not provided with voltage detection means (circuit configuration for detecting voltage), it is possible to save space and reduce costs.

  In the present embodiment, in a DC transfer mode in which image transfer is performed by applying a DC bias as a secondary transfer bias, the DC power source 201 is used and the resistance at the secondary transfer unit is determined based on the feedback voltage detected by the voltage detection unit 203. The transfer current value is determined and controlled by calculating the value. The resistance value at the secondary transfer portion is a resistance value including the intermediate transfer belt 51 and the recording paper P. In the present embodiment, constant current control is performed.

  In the present embodiment, detection by the voltage detection unit 203 is performed every predetermined number of outputs (transfer).

FIG. 10 shows voltage detection timing when a DC bias is applied in the DC transfer mode.
Although FIG. 10 shows that voltage detection is performed between the first sheet and the second sheet, as described above, voltage detection is performed every predetermined number of outputs (transfer).

  Here, an example is shown in which voltage detection is performed during a job (between sheets in a job), but voltage detection may be performed after the end of a job that outputs (transfers) a predetermined number of sheets.

  FIG. 10 shows that the output of the DC power supply 201 is turned off when performing voltage detection. However, it is not always necessary to turn it off, and the detection is also performed by lowering the output to some extent (changing the monitor voltage). Is possible.

  Basically, the secondary transfer bias power source 200 is turned off in order to prevent the ground toner on the intermediate transfer belt 51 from being transferred to the secondary transfer roller side between sheets. Therefore, in FIG. 10, the voltage (resistance) is measured with the DC power supply 201 turned on only when voltage detection is performed between sheets.

  In addition, by making the secondary transfer bias power source 200 sufficiently small between sheets, it is possible to suppress the transfer of the intermediate transfer belt 51 to the secondary transfer roller side to the secondary transfer roller side to some extent. . For this reason, it is possible to detect the voltage by making the DC bias between the papers smaller than that at the time of transfer and applying a predetermined DC bias when detecting the voltage.

  On the other hand, in the superimposed transfer mode in which a superimposed bias is applied as the secondary transfer bias and image transfer is performed, the AC power source 202 to which the superimposed bias is applied does not include a voltage detection unit. Therefore, the DC power source 201 is used during the superimposed transfer mode. The output voltage is detected and fed back. Thus, the resistance value at the secondary transfer portion is calculated, and the output of the AC power source 202 is controlled (corrected).

FIG. 11 shows the voltage detection timing when an AC bias is applied in the superimposed transfer mode.
In FIG. 11, voltage detection is performed between the first sheet and the second sheet. However, as described above, voltage detection is performed every predetermined number of sheets output (transfer).

  Here, an example is shown in which voltage detection is performed during a job (between sheets in a job), but voltage detection may be performed after the end of a job that outputs (transfers) a predetermined number of sheets.

  As can be seen from the timing chart shown in FIG. 11, the output of the AC power supply 202 is turned off when the output voltage is detected using the DC power supply 201.

  That is, during the superimposing transfer mode, the output voltage (resistance value of the secondary transfer portion) is detected by temporarily switching the power source from the AC power source 202 to the DC power source 201.

  When the output voltage is detected in the superimposing transfer mode, the output of the AC power source 202 is turned off so that the voltage can be detected without being affected by the output of the AC power source 202.

  In this embodiment, when the output voltage (resistance value of the secondary transfer portion) is detected and the output of the power source is controlled (corrected), if the resistance value is high, the resistance value is low in the direction of decreasing the power output. If so, the power output is controlled to increase.

  By detecting the output voltage (resistance value of the secondary transfer portion) and correcting the output of the power source every time a predetermined number of sheets are formed, it is possible to maintain good transferability over time.

  Further, the target voltage for the rising constant voltage control is also corrected according to the detected output voltage (the resistance value of the secondary transfer portion).

  When the current used for voltage detection is, for example, 25 [μA], the detection voltage is as shown in Table 2.

  The detection voltage varies depending on the resistance value of the secondary transfer portion (in this case, the roller resistance), and the higher the resistance, the higher the voltage. Therefore, the resistance value of the transfer member can be determined from the detection voltage.

  In this way, the resistance value of the transfer member can be determined based on whether the detected voltage is lower or higher than the voltage threshold value, and the optimum rise target voltage is obtained by multiplying the resistance of the transfer member by the optimum resistance correction coefficient. Can be set.

  Furthermore, an environmental condition detection unit having a temperature / humidity sensor (not shown) that detects at least one of temperature and relative humidity is provided, and either temperature, relative humidity, and absolute humidity calculated from the temperature and relative humidity, Or two or more are combined to detect changes in environmental conditions. Then, when the amount of change exceeds a specified value (for example, when the temperature changes by 5 [° C.]), the timing for performing voltage detection may be used.

  Controls (corrects) the transfer bias (DC bias, superimposed bias) applied at the secondary transfer unit by adding the conditions detected by the environmental condition detection means to the feedback voltage detection information performed when applying the DC bias and applying the superimposed bias. )

  The environmental conditions include LL (temperature 19 [° C.] humidity 30 [%]), ML (temperature 23 [° C.] humidity 30 [%]), MM (temperature 23 [° C.] humidity 50 [%]), MH. (Temperature 23 [° C.] humidity 80 [%]), HH (temperature 27 [° C.] humidity 80 [%]), and the like. However, the values and combinations of temperature and humidity are not limited to this.

  This makes it possible to obtain good transferability according to the environmental conditions. Equation 1 shows an example of a calculation formula used when obtaining a target voltage for constant voltage control in consideration of roller resistance and environmental conditions. An example of the target voltage correction coefficient corresponding to the correlation between the roller resistance and the environmental conditions is shown in Table 3, but is not limited to this.

  For example, when the roller resistance and the environmental condition are not considered, as shown in Table 1, it is assumed that the target voltage for plain paper with a paper thickness of 3 is −2.3 [kV], and this is the “voltage standard value” of Equation 1. And Then, considering the roller resistance and environmental conditions, as shown in Table 3, when the detected voltage is 1.0 [kV] or less, the roller resistance is 7.0th power, and the environmental condition is MM, the target voltage The correction coefficient (corresponding to “voltage environment correction coefficient × voltage resistance correction coefficient” in Equation 1) is 90%.

  Therefore, the target voltage in consideration of the roller resistance and the environmental conditions can be obtained from Equation 1 as follows: target voltage = −2.3 [kV] × 0.9 = −2.07 [kV].

  In addition, the target voltage correction factor is not limited to the correlation between roller resistance and environmental conditions, but also the correction factor according to temperature, the correction factor according to humidity, and the correction factor according to temperature and humidity. Alternatively, the target voltage correction control may be performed using a correction coefficient corresponding to the roller resistance.

  As described above, even if the AC power supply 202 of the secondary transfer bias power supply 200 does not have a configuration for detecting the feedback voltage, the resistance value in the secondary transfer portion is calculated in the transfer mode with the superimposed bias applied. Thus, the optimum transfer bias can be applied.

  That is, while realizing space saving and low cost of the AC power source 202, good transferability can be obtained by applying an appropriate superimposed bias.

  Note that constant voltage control may be performed so that the transfer bias becomes the target voltage while detecting the voltage when the transfer bias is applied by the secondary transfer bias power source 200.

  The superimposing bias can provide good transferability on the recording paper P having large irregularities. Therefore, by switching between applying a DC bias when using a recording paper P with small unevenness, such as a normal transfer paper, and applying a superimposed bias when using a recording paper P with large unevenness, various papers can be used. In addition, good transferability can be obtained.

  Further, when the recording paper P with small unevenness is used, when the secondary transfer bias power source 200 having the power source configuration shown in FIG. Sometimes it starts up with constant voltage control to reach the target voltage. Then, after starting up with constant voltage control so as to have a preset target voltage, before the toner image on the intermediate transfer belt 51 is transferred to the recording paper P, the preset target current is set. Switch to current control.

  Even when only a DC bias is applied without applying an AC bias, even when the constant current control is used, the rise time is delayed due to the influence of the load adjusting capacitor of the AC power supply 202 as described above. End up.

  Therefore, when only the DC bias is applied without applying the AC bias in the secondary transfer bias power supply 200 having the power supply configuration shown in FIG. 4, it is started by the constant voltage control, and compared with the case of starting by the constant current control. However, the target bias value can be achieved without requiring rise time. Therefore, it is possible to suppress the dilution of the density at the front end of the image due to the delay in the rise time.

  Furthermore, voltage detection can be performed both when applying a DC bias and when applying a superimposed bias, and the resistance value at the secondary transfer unit can be calculated. Bias control with current value is possible.

  As described above, in the present embodiment, the timing for detecting the voltage when the DC bias is applied and when the superimposed bias is applied is set for every predetermined number of outputs (during the job), but is not limited thereto. For example, it may be performed at an appropriate timing such as after a job for printing a predetermined number of times, at the time of starting up the apparatus, or before the image adjustment control for adjusting the image forming conditions.

  Here, in the image forming apparatus described in Japanese Patent Application Laid-Open No. 7-168403, the transfer voltage is detected and measured and stored in a memory in an adjustment mode performed at the time of product shipment from the factory or maintenance and inspection in the market. . Then, at the time of image formation, a transfer power source that outputs only a DC voltage is controlled to a constant voltage, and the transfer voltage stored in the memory is raised. Therefore, even if the paper type, paper thickness, environmental conditions, etc. change, it cannot be applied immediately.

  On the other hand, in the image forming apparatus according to the present embodiment, the image forming apparatus is started up with an optimum target voltage set in advance according to printing conditions such as paper type, paper thickness, and environmental conditions without performing the adjustment mode. It is possible. Therefore, even if the paper type, paper thickness, environmental conditions, etc. change, it can be automatically and immediately adapted.

  In addition to changing the target voltage value depending on the printing conditions, the time for constant voltage control of the DC component of the transfer bias at the start-up may be changed depending on the printing conditions.

  As described above, there are a wide variety of recording papers P used for electrophotography, and the optimum transfer bias for good transfer varies depending on the material and thickness of the recording paper P. The optimum transfer bias is also different.

  For this reason, as described above, it is possible to perform good transfer by setting the target voltage of the rising constant current control to an optimum target voltage according to the printing conditions. The time it takes to reach will also be different.

  FIG. 12A shows the start-up time when the target voltage is high. FIG. 12B shows the rise time when the target voltage is low.

  For example, when the printing condition is a low temperature and low humidity environment, the optimum transfer bias is increased due to an increase in resistance of the transfer member or the recording paper P. Therefore, although the target voltage at the time of start-up is increased, as shown in FIG. 12A, when the target voltage is high, the time for reaching the target voltage becomes long.

  Conversely, when the printing conditions are a high temperature and high humidity environment, the optimum transfer bias is lowered. Therefore, although the target voltage at the time of start-up is lowered, as shown in FIG. 12B, when the target voltage is low, the time to reach the target voltage is shortened.

  Therefore, the time for constant voltage control of the DC component of the transfer bias at the start-up is changed according to the printing conditions so that the time required to reach a different target voltage depending on the printing conditions can be secured at the start-up.

  Specific printing conditions include paper type and thickness, environmental conditions such as temperature and humidity, and transfer members such as the secondary transfer roller 56, the secondary transfer counter roller 53, and the intermediate transfer belt 51. For example, changes in electrical resistance.

  Regarding the change in the electric resistance of the transfer member, the change in the electric resistance due to the environmental conditions and the use over time is detected by the method described above, and the time for performing the constant voltage control at the start-up is changed based on the detection result.

  In addition, under the environmental conditions, in addition to the change in the electrical resistance of the transfer member described above, the rising gradient also changes. This is because the rising slope changes because the electrostatic capacity of the transfer member varies depending on the environmental conditions. For example, in a high-temperature and high-humidity environment, since the capacitance increases, the rising gradient becomes small.

  By changing the time for constant voltage control of the DC component of the transfer bias at the start-up according to these printing conditions, the time required to reach the target voltage can be secured, and the optimum Good transfer can be performed with a transfer bias. Therefore, for example, it is possible to suppress the density unevenness of the uneven paper and the dilution of the image density at the leading edge of the image.

  On the other hand, as shown in FIG. 12, the toner having a polarity opposite to that at the time of transfer is applied so that the toner adhering to the intermediate transfer belt 51 does not transfer to the surface of the secondary transfer roller 56 except for the time required for rising between the sheets. It is desirable to apply a reverse bias as a bias to the secondary transfer roller 56 or to turn off the secondary transfer bias power source 200. Accordingly, it is possible to suppress the toner attached to the surface of the secondary transfer roller 56 from attaching to the back surface of the recording paper P at the secondary transfer portion and causing the back surface of the recording paper P to be stained.

  In particular, by applying a reverse bias to the secondary transfer roller 56 between paper sheets, even if toner adheres to the surface of the secondary transfer roller 56, the surface of the secondary transfer roller 56 is transferred onto the intermediate transfer belt 51. The toner can be transferred. Then, the toner transferred and adhered onto the intermediate transfer belt 51 is removed from the intermediate transfer belt 51 by a belt cleaning device. As a result, it is possible to ensure the cleanability of the secondary transfer roller 56 and the like between sheets.

  Therefore, by performing the above-described control between papers, it is possible to suppress unevenness in uneven paper density and dilute density at the leading edge of the image while ensuring cleanability of the secondary transfer roller 56 and the like between papers. be able to.

[Modification 1]
Next, a modified example (hereinafter, this modified example is referred to as “modified example 1”) for controlling the target voltage of the rising constant voltage control will be described.
As described above, the printer according to this embodiment includes a full-color printing mode for forming a full-color toner image in which each color toner image of yellow (Y), magenta (M), cyan (C), and black (K) is superimposed, There are two image forming operation modes different in the type of toner or the amount of toner used for image formation, namely a monochrome printing mode for forming a monochrome toner image consisting of only a black (K) toner image. In the full-color printing mode, the amount of toner to be transferred at the secondary transfer nip is larger than that in the monochrome printing mode. Therefore, the optimum secondary transfer bias is set higher than that in the monochrome printing mode. As described above, since the optimum transfer bias differs depending on the printing mode, it is preferable to change the target voltage for the rising constant voltage control depending on the printing mode.

  That is, in the first modification, when printing in the monochrome printing mode, the target voltage of the rising constant voltage control is set to the target voltage corresponding to the secondary transfer bias set corresponding to the monochrome printing mode. To do. Specifically, since the size of the secondary transfer bias corresponding to the monochrome printing mode is smaller than that in the full color printing mode, the target voltage for the rising constant voltage control in the monochrome printing mode is set smaller than in the full color printing mode. To do. Conversely, when printing in the full-color printing mode, the target voltage for the rising constant voltage control is set larger than that in the monochrome printing mode.

  In the first modification, there are two image forming operation modes in which the type or amount of toner used for image formation is different, that is, the monochrome printing mode and the full color printing mode, but are not limited thereto.

  For example, an image forming operation mode in which image formation is performed using special toners other than yellow (Y), magenta (M), cyan (C), and black (K) may be included. This special toner is, for example, a transparent toner used for the purpose of forming a high gloss image, a fluorescent toner used for the purpose of forming a fluorescent image, red, green used for the purpose of expanding the color reproduction region, etc. Color toner (so-called spot color toner).

  Further, for example, an image forming operation mode such as a productivity priority mode that prioritizes productivity and an image quality priority mode that prioritizes image quality may be included.

  If the type of toner and the amount of toner used for image formation are different, the optimum size of the secondary transfer bias changes. By changing the target voltage of the rising constant voltage control according to the difference, it is possible to improve the image at the leading edge of the image.

[Modification 2]
Next, another modified example (hereinafter, this modified example is referred to as “modified example 2”) for controlling the target voltage of the rising constant voltage control will be described.
The printer according to the second modification has a double-sided printing function. That is, after the toner image is secondarily transferred to the first surface of the recording medium and fixed by the fixing device 90, the recording medium is transported again to the secondary transfer nip and separated from the second surface of the recording medium. The toner image is secondarily transferred. When image formation is performed on both sides of the recording medium in this way, the recording medium at the time of secondary transfer to the second surface is the moisture in the recording medium when the toner image on the first surface is fixed by the fixing device 90. Is decreasing, the electrical resistance value is high. Therefore, the optimum secondary transfer is performed between the secondary transfer onto the first surface of the recording medium having a low electrical resistance value and the secondary transfer onto the second surface of the recording medium having a high electrical resistance value. The magnitude of the bias is different.

  Therefore, in the second modification, the target voltage of the rising constant voltage control at the time of secondary transfer to the first surface is set according to the secondary transfer bias set at the time of secondary transfer to the first surface, and the first A target voltage for rising constant voltage control at the time of secondary transfer to the second surface is set according to the secondary transfer bias set at the time of secondary transfer to the second surface. Thus, the target voltage for constant voltage control when transferring the toner image to the first surface of the recording medium, and another toner image on the second surface of the recording medium after the toner image is transferred to the first surface. By differentiating the target voltage of the constant voltage control at the time of transfer, it becomes possible to improve the image at the front end of the image.

[Modification 3]
Next, another modification example (hereinafter, this modification example is referred to as "modification example 3") for controlling the target voltage of the rising constant voltage control will be described.
The printer according to the third modification has two modes with different process line speeds in the full color printing mode. Specifically, the full-color printing mode in which the process linear speed is slowed is a mode in which, for example, the speed of the recording medium passing through the fixing device 90 is slowed down in order to obtain good fixability. When the process linear velocity, that is, the recording medium conveyance speed changes, the optimum size of the secondary transfer bias also changes. Therefore, in the third modification, the target voltage for constant voltage control is changed according to the difference in the conveyance speed of the recording medium. This makes it possible to improve the image at the front end of the image.

[Embodiment 2]
Hereinafter, as in the first embodiment, another embodiment in which the present invention is applied to a printer that is an image forming apparatus (hereinafter, this embodiment is referred to as “second embodiment”) will be described. The printer of the present embodiment is the same as that of the first embodiment except for the fact that it has a contact / separation mechanism for contacting / separating the intermediate transfer belt 51 and the secondary transfer roller 56, etc. . Therefore, in the following description, differences from the printer of the first embodiment will be described.

  In the printer of this embodiment, when the paper enters the secondary transfer nip or when the paper leaves, the secondary transfer roller 56 and the intermediate transfer belt 51 are separated in advance so that the paper P enters the secondary transfer nip. Reduces shock jitter due to shock when moving or pulling out.

  The shock jitter is that a torque load is generated on the intermediate transfer belt 51 due to an impact when the paper P enters or exits the secondary transfer nip, and there is a speed difference between the photosensitive drum 11 and the intermediate transfer belt 51. It occurs, causing horizontal stripe-like density unevenness in the printed image and causing printing failure.

  FIG. 13 is a schematic diagram of a contact / separation mechanism 60 that contacts and separates the intermediate transfer belt 51 and the secondary transfer roller 56. The secondary transfer roller 56 and the secondary transfer counter roller 53 are arranged to face each other so that the secondary transfer roller 56 is positioned on the lower side and the secondary transfer counter roller 53 is positioned on the upper side with the intermediate transfer belt 51 interposed therebetween. The secondary transfer roller 56 is applied with a biasing force toward the secondary transfer counter roller 53 by a spring 67 which is a biasing means.

  The intermediate transfer belt 51 and the secondary transfer roller 56 can be freely contacted and separated within a certain range by a contact / separation mechanism 60 constituted by a stepping motor 63, an eccentric cam 61, and the like. Eccentric cams 61 are installed coaxially with the secondary transfer counter roller 53 at both ends in the axial direction of the secondary transfer counter roller 53.

  Ball bearings 62 are attached to both ends of the secondary transfer roller 56 in the axial direction so as not to prevent the rotation of the secondary transfer roller 56, and the eccentric cam 61 is abutted against the ball bearing 62. ing. When the cam shaft 61a to which the eccentric cam 61 is attached is rotated by the rotational driving force from the stepping motor 63, the eccentric cam 61 and the cam shaft 61a are D-cut so that the eccentric cam 61 also rotates at the same timing and at the same angle. It is fitted with a groove or the like.

  As a shape of the eccentric cam 61, a portion where the distance between the rotation center of the eccentric cam 61 and the outer shape portion is the shortest is shorter than the diameter of the secondary transfer counter roller 53. Further, the longest distance connecting the rotation center of the eccentric cam 61 and the outer shape portion is longer than the diameter of the secondary transfer counter roller 53.

  The rotation of the cam shaft 61a can be freely controlled by the stepping motor 63, and the rotational driving force of the stepping motor 63 is transmitted to the cam shaft 61a via the gears 64 and 65 and the timing belt 66. The stepping motor 63 can be controlled to rotate at a step angle of 1.8 [°], and the eccentric cam 61 is moved by the rotational driving force from the stepping motor 63 before the sheet P enters (enters) the secondary transfer nip. Rotate.

  Here, “distance connecting the contact portion of the eccentric cam 61 with the ball bearing 62 from the rotation center of the eccentric cam 61 + the radius of the ball bearing 62” is defined as a distance L1. Further, “the radius of the secondary transfer counter roller 53 + the thickness of the intermediate transfer belt 51 + the radius of the secondary transfer roller 56” is a distance L2.

  The eccentric cam 61 is abutted against the ball bearing 62. When the relationship of distance L1> distance L2 is satisfied by rotating the eccentric cam 61, the secondary transfer roller 56 resists the bias from the spring 67. It is pushed down in a direction away from the intermediate transfer belt 51.

  When the leading edge of the sheet begins to pass between the intermediate transfer belt 51 and the secondary transfer roller 56, the stepping motor 63 starts the rotation of the eccentric cam 61 again. When the relationship of distance L1 <distance L2 is satisfied, the intermediate transfer belt 51 and the secondary transfer roller 56 come into contact with each other, and a predetermined transfer pressure is applied to the paper P.

  With such a configuration, when the paper enters the secondary transfer nip, the intermediate transfer belt 51 and the secondary transfer roller 56 are separated from each other, so that the impact when the paper enters and the process linear speed fluctuation (speed fluctuation) of the intermediate transfer belt 51 are increased. Can be suppressed.

  Similarly, when the sheet P comes out of the secondary transfer nip, the eccentric cam 61 is rotated by the stepping motor 63 so that the intermediate transfer and the secondary transfer roller 56 are performed before the trailing end of the sheet P passes through the secondary transfer nip. The belt 51 is separated. Thereby, it is possible to reduce an impact when the paper P comes out of the secondary transfer nip.

  In the contact / separation mechanism 60, the configuration in which the secondary transfer roller 56 is pushed down via the ball bearing 62 by the eccentric cam 61 as described above functions as a secondary transfer roller push-down mechanism. Of course, each element constituting the secondary transfer roller pressing mechanism is not limited to that shown in FIG. 13, and various known means can be employed.

  The contact / separation operation between the secondary transfer roller 56 and the secondary transfer counter roller 53 will be described with reference to FIGS. 14, 15, and 16. FIG. 14 is a diagram illustrating a state in which the secondary transfer roller 56 and the secondary transfer counter roller 53 are separated by a gap amount A. FIG. FIG. 15 is a diagram illustrating a state where the secondary transfer roller 56 and the secondary transfer counter roller 53 are separated by a gap amount C. FIG. FIG. 16 is a diagram illustrating a state in which the secondary transfer roller 56 and the secondary transfer counter roller 53 are in contact with each other.

  14 and 15 show a state before the paper P enters the secondary transfer nip, and FIG. 16 shows a state when the toner image is transferred from the intermediate transfer belt 51 to the paper P. Show.

  The eccentric cam 61 has three stop positions, that is, a cam position A, a cam position B, and a cam position C, and the position of the eccentric cam 61 stops at the position of the cam position A as shown in FIG. The secondary transfer roller 56 and the secondary transfer counter roller 53 are separated by a gap amount A.

  Further, as shown in FIG. 15, when the position of the eccentric cam 61 is stopped at the cam position C, the secondary transfer roller 56 and the secondary transfer counter roller 53 are separated by a gap amount C. It has become.

  As shown in FIG. 16, when the position of the eccentric cam 61 is stopped at the cam position B, (the eccentric cam radius from the rotation center of the eccentric cam to the outer periphery of the eccentric cam 61 at the point B + the radius of the ball bearing) <(Radius of secondary transfer counter roller 53 + radius of secondary transfer roller 56). Therefore, the secondary transfer roller 56 and the secondary transfer counter roller 53 are in contact with each other. Then, as described above, the transfer pressure necessary for transferring the toner image from the intermediate transfer belt 51 to the paper P is applied by the spring 37.

  The eccentric cam 61 is rotated about the cam shaft 61a by the rotational driving force transmitted by controlling the stepping motor 63 by the control unit 300, so that the cam position C-cam position B-cam position A-cam position C is obtained. The cam position can be changed continuously.

  The gap amount between the intermediate transfer belt 51 and the secondary transfer roller 56 when the paper P enters the secondary transfer nip will be described with reference to FIG.

  As described with reference to FIGS. 14 and 15, when the sheet P enters the secondary transfer nip, the contact / separation mechanism 60 separates the intermediate transfer belt 51 and the secondary transfer roller 56 from each other so that a gap ( A gap is secured. At this time, since the intermediate transfer belt 51 and the secondary transfer roller 56 are separated from each other, no transfer pressure is generated.

  The gap amount between the intermediate transfer belt 51 and the secondary transfer roller 56 when the sheet P enters the secondary transfer nip is Y1, and the size of the gap amount Y1 is the gap amount A and the gap amount described above. 2 levels with C. The gap amount Y1 is ensured in order to reduce the impact when the paper P enters the secondary transfer nip, and it is desirable to secure a size larger than the thickness of the paper P. Accordingly, the gap amount A and the gap amount C are appropriately set.

  Even if the gap amount Y1 is increased beyond the thickness of the paper P, the impact when the paper P enters the secondary transfer nip does not change. However, the larger the gap amount, the larger the intermediate transfer belt 51. Therefore, the operation required for the contact and separation of the secondary transfer roller 56 with respect to is increased. Therefore, it is desirable to ensure that the gap amount Y1 is slightly larger than the thickness of the paper P.

  Next, with reference to FIG. 18, between the intermediate transfer belt 51 and the secondary transfer roller 56 when the leading edge of the paper passes through the secondary transfer nip and the toner image T on the intermediate transfer belt 51 is transferred to the paper P. The gap amount will be described.

  A gap amount between the intermediate transfer belt 51 and the secondary transfer roller 56 when the toner image T on the intermediate transfer belt 51 is transferred to the paper P is Y2. This gap amount Y2 is a gap amount sufficient to ensure a sufficient transfer pressure for transferring the toner image T on the intermediate transfer belt 51 onto the paper P. After the leading edge of the paper P enters the secondary transfer nip, the contact / separation mechanism 60 performs a contact operation to move the secondary transfer roller 56 toward the intermediate transfer belt 51. The gap amount Y2 between the roller 56 and the roller 56 is equal to or less than the gap amount Y1.

  In order to obtain a good printing result, the paper P is in the state of FIG. 17 when the paper enters the secondary transfer nip and the margin formed at the front end of the paper passes through the secondary transfer nip. , Transition to the state of FIG. The “margin” or “margin portion” described later refers to a portion on the printing surface of the paper P where no image is formed or where no image is formed.

  If the transition from the state shown in FIG. 17 to the state shown in FIG. 18 is delayed, the transfer pressure becomes insufficient when the toner image on the intermediate transfer belt 51 is transferred onto the paper P, and transfer failure occurs and color loss occurs. Will occur.

  On the other hand, if the gap Y1 is not secured between the intermediate transfer belt 51 and the secondary transfer roller 56 when the sheet enters the secondary transfer nip, the impact when the sheet enters the secondary transfer nip increases. . Therefore, the shock jitter due to the impact when the paper enters the secondary transfer nip is deteriorated.

  Assuming that the margin amount of the margin formed at the leading end of the sheet is X and the process linear velocity is p, the time t for which the margin portion passes through the secondary transfer nip is “t = X / p”. Further, the gap amount required when the paper P enters the secondary transfer nip is the gap amount Y1, and the blank portion passes through the secondary transfer nip, and the toner image on the intermediate transfer belt 51 is transferred to the paper P. The gap amount sometimes required is the gap amount Y2.

  For this reason, in order to obtain a good transfer result, the contact / separation mechanism 60 is moved in the direction of contact with the intermediate transfer belt 51 (the approaching direction) at a speed of “V = (Y2−Y1) / (X / p)” or more. It is necessary to move the next transfer roller 56. The speed V of the contact operation becomes kinetic energy, which is an impact when the intermediate transfer belt 51 and the secondary transfer roller 56 come into contact with each other and causes shock jitter. Therefore, by reducing the speed V as much as possible, it is possible to reduce shock jitter caused by an impact when the intermediate transfer belt 51 and the secondary transfer roller 56 come into contact with each other.

  FIG. 19 is a graph showing how the profile of the gap amount between the secondary transfer roller 56 and the intermediate transfer belt 51 changes due to the difference in the amount of margin formed at the leading edge of the paper P.

  The vertical axis in FIG. 19 indicates the gap amount between the secondary transfer roller 56 and the intermediate transfer belt 51. In the figure, “Y1” is the gap amount required when the sheet P enters the secondary transfer nip as described above, and “Y2” in the figure indicates the toner image on the intermediate transfer belt 51 as described above. This is the gap amount required when transferring onto the paper P.

The horizontal axis in FIG. 19 indicates the position from the leading edge of the paper. In the figure, “X ₀ ” and “X ₁ ” represent the margin amounts, respectively, and the margin with the margin amount X _{1 is} the margin with the margin amount X ₀ . It shows that the margin is wider than that.

  Further, the slope of the graph of FIG. 19 represents the contact operation speed of the secondary transfer roller 56 with respect to the intermediate transfer belt 51. The steeper slope of the graph indicates that the contact operation speed is faster.

  As can be seen from FIG. 19, the wider the margin (the larger the amount of margin), the gentler the slope of the graph when changing the gap amount from the gap amount Y1 to the gap amount Y2, and the gap amount Y1 is changed to the gap amount Y2. The minimum required contact operation speed is reduced. Therefore, as the margin is wider (the amount of margin is larger), the control unit 300 controls the stepping motor 63 to slow down the contact operation speed, so that the intermediate transfer belt 51 and the secondary transfer roller 55 are in contact with each other. Impact can be reduced.

  Conversely, the narrower the margin (the smaller the amount of margin), the steeper the graph when changing the gap amount from the gap amount Y1 to the gap amount Y2, and the minimum required to change the gap amount Y1 to the gap amount Y2. The contact operation speed is increased. As the contact operation speed increases, the impact at the time of contact between the intermediate transfer belt 51 and the secondary transfer roller 55 increases, so that shock jitter occurs due to the impact.

Note that when the margin amount is X _0, the minimum edge margin as the contact and separation operation speed shock jitter due to the impact at the time of contact with the intermediate transfer belt 51 and the secondary transfer roller 55 is not visualized is obtained . Therefore, when the margin amount is narrower margin than X _0, the contact and separation with the operating speed is too fast, making it possible to reduce the shock jitter due to the impact at the time of contact with the intermediate transfer belt 51 and the secondary transfer roller 55 It becomes difficult.

Therefore, when the margin amount is narrower margin than X _0, the shock jitter due to the impact at the time of contact with the intermediate transfer belt 51 and the secondary transfer roller 55 that does not elicit a greater gap amount than the gap amount Y2 It is possible to do. However, in this case, when the toner image on the intermediate transfer belt 51 is transferred onto the paper P, the transfer pressure becomes insufficient and a transfer failure occurs, resulting in color loss.

Therefore, in this embodiment, when the margin amount is narrower margin than X _0, than if the margin amount is X ₀ or margins, by increasing the rising target voltage of the transfer bias, the transfer electric field during the transfer The transfer performance is compensated so as not to cause transfer failure due to insufficient transfer pressure.

In this embodiment, X ₀ = 2.0 [mm]. When the margin amount is narrower than 2.0 [mm], the rising target voltage is made larger than that when the margin amount is 2.0 [mm] or more so that the tip density unevenness and the transfer pressure are increased. Insufficiency can be suppressed and shock jitter can be reduced.

What has been described above is merely an example, and the present invention has a specific effect for each of the following modes.
(Aspect A)
A nip forming member such as a secondary transfer roller 56 for forming a transfer nip such as a secondary transfer nip in contact with an image carrier such as the intermediate transfer belt 51, a DC power source such as a DC power source 201, an AC power source 202, etc. A transfer bias power source such as a secondary transfer bias power source 200 that outputs a transfer bias, and a toner image on the image carrier by the transfer bias output from the transfer bias power source. In the transfer device such as the transfer unit 50 that transfers the recording bias to the recording medium such as the recording paper P sandwiched in the transfer nip, the transfer bias power supply has a constant voltage so that the DC component of the transfer bias becomes a preset target voltage. After start-up by control, switch to constant current control so that the toner image on the image carrier becomes a preset target current before being transferred to the recording medium.
In (Aspect A), the DC component of the transfer bias is raised by constant voltage control so as to become a preset target voltage. As a result, it is possible to raise the DC component to the target voltage with a steep slope, compared with the case where the DC component is raised by constant current control, and the rise time to the target voltage can be shortened.
Further, since the voltage value of the DC component at the time of start-up is set not by the rise time but by the target voltage itself, even if the rising slope of the DC component changes due to environmental fluctuations or the like, the DC component Can be set to a target voltage.
Therefore, it is possible to prevent the density at the front end of the image from being diluted due to insufficient transfer bias due to a delay in the rise time up to the target voltage.
Also, when transferring the toner image on the image carrier to the recording medium, a transfer bias is applied under constant current control, so even if the electrical resistance of the image carrier or nip forming member fluctuates, the transfer at the transfer nip is performed. A stable transfer property can be obtained by stabilizing the electric field.
Therefore, when a transfer bias power source in which a DC power source and an AC power source are electrically connected is used, it is possible to suppress the occurrence of transfer failure.
(Aspect B)
In (Aspect A), the target voltage for constant voltage control is changed according to the thickness of the recording medium. According to this, as described in the above embodiment, good transferability can be obtained regardless of the thickness of the recording medium.
(Aspect C)
In (Aspect A) or (Aspect B), the target voltage for constant voltage control is changed according to the type of the recording medium. According to this, as described in the above embodiment, good transferability can be obtained regardless of the type of recording medium.
(Aspect D)
In (Aspect A), (Aspect B), or (Aspect C), it has temperature / humidity detection means such as environmental condition detection means for detecting at least one of temperature and humidity, and is detected by the temperature / humidity detection means. The target voltage for constant voltage control is changed according to at least one of the temperature and humidity. According to this, as described in the above embodiment, good transferability can be obtained regardless of temperature and humidity.
(Aspect E)
In (Aspect A), (Aspect B), (Aspect C) or (Aspect D), it has resistance detection means for detecting the electrical resistance of the members constituting the transfer nip, and the electricity detected by the resistance detection means. The target voltage for constant voltage control is changed according to the resistance. According to this, as described in the above embodiment, good transferability can be obtained regardless of the electrical resistance.
(Aspect F)
In (Aspect A), (Aspect B), (Aspect C), (Aspect D) or (Aspect E), recording is performed with the transfer bias from among a plurality of image forming operation modes different in the type of toner used for image formation. The target voltage for constant voltage control is changed according to the image forming operation mode selected for forming the toner image transferred to the medium. According to this, as described in Modification 1, good transferability can be obtained at the leading edge of the image regardless of the difference in the image forming operation mode in which the type of toner used for image formation is different.
(Aspect G)
(Aspect A), (Aspect B), (Aspect C), (Aspect D), (Aspect E) or (Aspect F) Target for Constant Voltage Control when Transferring Toner Image onto First Surface of Recording Medium The voltage and a target voltage for constant voltage control when transferring another toner image to the second surface of the recording medium after transferring the toner image to the first surface are made different. According to this, as described in Modification 2, good transferability can be obtained at the image leading edge portions of both the first surface and the second surface of the recording medium.
(Aspect H)
In (Aspect A), (Aspect B), (Aspect C), (Aspect D), (Aspect E), (Aspect F), or (Aspect G), conveyance of a recording medium onto which a toner image is transferred by the transfer bias. The target voltage for constant voltage control is changed according to the difference in speed. According to this, as described in the third modification, good transferability can be obtained at the leading edge of the image regardless of the difference in the conveyance speed of the recording medium.
(Aspect I)
(Aspect A), (Aspect B), (Aspect C), (Aspect D), (Aspect E), (Aspect F), (Aspect G) or (Aspect H) The constant voltage control time for the DC component of the transfer bias is changed. According to this, as described in the above embodiment, it is possible to suppress the uneven density reduction of the concavo-convex paper and the dilution of the image leading edge density while ensuring the cleaning property between the recording media.
(Aspect J)
In (Aspect I), the printing conditions include at least one of the type and thickness of the recording medium. According to this, as described in the above embodiment, it is possible to set an optimum rise time according to the type and thickness of the recording medium.
(Aspect K)
In (Aspect I) or (Aspect J), the printing condition includes at least one of temperature and humidity detected by the temperature and humidity detection means. According to this, as described in the above embodiment, it is possible to set an optimum rise time according to temperature and humidity.
(Aspect L)
In (Aspect I), (Aspect J), or (Aspect K), the printing conditions include an electrical resistance of a member constituting the transfer nip. According to this, as described in the above embodiment, it is possible to set an optimum rise time according to the electric resistance.
(Aspect M)
(Aspect A), (Aspect B), (Aspect C), (Aspect D), (Aspect E), (Aspect F), (Aspect G), (Aspect H), (Aspect I), (Aspect J), In (Aspect K) or (Aspect L), the bias applying unit applies only the DC bias component without applying the AC bias component of the superimposed bias depending on the type of the recording medium. According to this, as described in the above embodiment, generation of transfer dust can be suppressed.
(Aspect N)
(Aspect A), (Aspect B), (Aspect C), (Aspect D), (Aspect E), (Aspect F), (Aspect G), (Aspect H), (Aspect I), (Aspect J), In (Aspect K), (Aspect L), or (Aspect M), contact / separation means such as a contact / separation mechanism 60 that contacts and separates the image carrier and the transfer roller, and the image carrier by the contact / separation means Speed control means such as a controller 300 for controlling the speed of contact operation with the transfer roller, and the contact control operation by the speed control means as the margin formed at the paper leading edge of the recording medium is narrower. The speed is increased, and the target voltage of the constant voltage control is changed according to the margin amount from the leading edge of the margin paper. According to this, as described in the above-described embodiment, it is possible to suppress tip density unevenness and insufficient transfer pressure, and to reduce shock jitter.
(Aspect O)
In an image forming apparatus such as a printer that forms an image on a recording medium by transferring an image formed on the surface of an image carrier such as an intermediate transfer belt 51 to a recording medium such as recording paper P using a transfer unit. (Aspect A), (Aspect B), (Aspect C), (Aspect D), (Aspect E), (Aspect F), (Aspect G), (Aspect H), (Aspect I), (Aspect J), The transfer device of (Aspect K), (Aspect L), (Aspect M) or (Aspect N) is used. According to this, as described in the above embodiment, when a transfer bias power supply in which a direct current power supply and an alternating current power supply are electrically connected is used, it is possible to suppress the occurrence of transfer failure and to achieve good image formation. It can be carried out.

DESCRIPTION OF SYMBOLS 1 Image forming unit 11 Photoconductor drum 21 Charging device 31 Developing device 31a Developing sleeve 31b Screw member 31c Screw member 41 Cleaning device 41a Cleaning blade 41b Cleaning brush roller 50 Transfer unit 51 Intermediate transfer belt 52 Drive roller 53 Secondary transfer counter roller 54 Cleaning backup roller 55 Primary transfer roller 56 Secondary transfer roller 57 Belt cleaning device 60 Contact / separation mechanism 61 Eccentric cam 61a Cam shaft 62 Ball bearing 63 Stepping motor 64 Gear 65 Gear 66 Timing belt 67 Spring 80 Optical writing unit 90 Fixing device 91 Fixing roller 92 Pressure roller 100 Paper feed cassette 101 Paper feed roller 102 Registration roller pair 200 Secondary transfer bias power supply 201 Direct Power 202 AC power source 203 voltage detection unit 300 control unit

JP 2006-267486 A

Claims (15)

A nip forming member for forming a transfer nip in contact with the image carrier;
A DC power supply and an AC power supply are electrically connected, and includes a transfer bias power supply that outputs a transfer bias.
In the transfer device for transferring the toner image on the image carrier to the recording medium sandwiched in the transfer nip by the transfer bias output from the transfer bias power source,
The transfer bias power source starts up the DC component of the transfer bias by constant voltage control so as to be a preset target voltage, and then before the toner image on the image carrier is transferred to a recording medium. A transfer device that switches to constant current control so as to obtain a preset target current. The transfer device according to claim 1.
A transfer apparatus, wherein a target voltage for constant voltage control is changed according to the thickness of the recording medium. The transfer device according to claim 1 or 2,
A transfer apparatus, wherein a target voltage for constant voltage control is changed according to a type of the recording medium. The transfer device according to claim 1, 2 or 3,
It has temperature and humidity detection means for detecting at least one of temperature and humidity,
A fixing device, wherein a target voltage for constant voltage control is changed in accordance with at least one of temperature and humidity detected by the temperature and humidity detection means. The transfer apparatus according to claim 1, 2, 3 or 4.
It has resistance detection means for detecting the electrical resistance of the members constituting the transfer nip,
A transfer apparatus, wherein a target voltage for constant voltage control is changed in accordance with the electrical resistance detected by the resistance detection means. The transfer apparatus according to claim 1, 2, 3, 4 or 5.
According to an image forming operation mode selected for forming a toner image to be transferred to a recording medium by the transfer bias from among a plurality of image forming operation modes having different types of toner or amounts of toner used for image formation. A transfer apparatus characterized by changing a target voltage for constant voltage control. The transfer device according to claim 1, 2, 3, 4, 5 or 6.
Target voltage for constant voltage control when transferring a toner image to the first surface of the recording medium and when transferring another toner image to the second surface of the recording medium after transferring the toner image to the first surface A transfer apparatus characterized in that a target voltage for constant voltage control is made different. The transfer device according to claim 1, 2, 3, 4, 5, 6 or 7.
A transfer apparatus, wherein a target voltage for constant voltage control is changed in accordance with a difference in conveyance speed of a recording medium onto which a toner image is transferred by the transfer bias. The transfer apparatus according to claim 1, 2, 3, 4, 5, 6, 7 or 8.
A transfer device characterized in that the time for constant voltage control of the DC component of the transfer bias at startup is changed according to the printing conditions. The transfer device according to claim 9.
The transfer apparatus, wherein the printing conditions include at least one of a type and a thickness of a recording medium. The transfer device according to claim 9 or 10,
The transfer apparatus, wherein the printing conditions include at least one of temperature and humidity. The transfer device according to claim 9, 10 or 11,
The transfer apparatus according to claim 1, wherein the printing condition includes an electrical resistance of a member constituting the transfer nip. The transfer apparatus according to claim 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11 or 12.
The transfer apparatus according to claim 1, wherein the bias applying unit does not apply the AC bias component of the superimposed bias depending on the type of the recording medium, and applies only the DC bias component. The transfer apparatus according to claim 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, or 13.
Contact / separation means for contacting / separating the image carrier and the transfer roller;
A speed control means for controlling a contact operation speed between the image carrier and the transfer roller by the contact / separation means;
The narrower the margin formed at the paper front end of the recording medium, the faster the contact operation speed by the speed control means,
The transfer apparatus, wherein the target voltage for the constant voltage control is changed in accordance with a margin amount of the margin from the front end of the sheet. In an image forming apparatus for forming an image on a recording medium by transferring the image formed on the surface of the image carrier to the recording medium using a transfer unit,
An image forming apparatus using the transfer device according to claim 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13 or 14.

Images