JP2017116802A - Image forming apparatus - Google Patents

Abstract

PROBLEM TO BE SOLVED: To suppress insufficiency in image density at a leading end of a recording sheet.SOLUTION: A control part is configured to perform control of switching both a DC component and an AC component of a secondary transfer bias including a superposed voltage that is output for secondarily transferring a toner image from an intermediate transfer belt to a recording sheet, between a value for a leading end for transferring a toner image to a leading end of the recording sheet and a value for a main body for transferring a toner image to a main body that continues from the leading end of the recording sheet, and making the value for a leading end higher than the value for a main body in terms of an offset voltage Voff being the DC component and making a peak-to-peak value Vpp for a leading end lower than a peak-to-peak value Vpp for a main body in terms of the AC component.SELECTED DRAWING: Figure 11

Description

  The present invention relates to an image forming apparatus.

  Conventionally, as a transfer bias for transferring a toner image on the surface of an image carrier to a recording sheet that has entered the transfer nip, an image using a superimposed voltage obtained by superimposing a DC voltage and an AC voltage is used. Forming devices are known.

  For example, the image forming apparatus described in Patent Document 1 includes an intermediate transfer belt on a recording sheet sandwiched between secondary transfer nips by contact between an intermediate transfer belt as an image carrier and a secondary transfer roller as a nip forming member. The toner image is secondarily transferred. At this time, a secondary transfer bias to be applied to the secondary transfer roller is output from the power source as a superimposed voltage.

  In general, in an image forming apparatus, an image density is likely to be insufficient at the leading end of a recording sheet. This is because when the leading edge of the recording sheet enters the transfer nip, the transfer current concentrates in the area where the image carrier and the nip forming member are in direct contact with each other in the transfer nip. It was found that the transfer current flowing through the tip was insufficient.

  In order to solve the above-described problems, the present invention provides an image carrier, toner image forming means for forming a toner image on the surface of the image carrier, and a nip forming member that forms a transfer nip in contact with the surface. A power supply that outputs a superposed voltage by superimposing a DC voltage and an AC voltage as a transfer bias for transferring the toner image on the surface to the recording sheet that has entered the transfer nip, and the power supply And a control means for controlling the transfer bias output from the image forming apparatus, the transfer bias direct current voltage and the alternating current voltage are values for the front end portion for transferring the toner image to the front end portion of the recording sheet. And the value for the main body part for transferring the toner image to the main body part following the front end part of the recording sheet, and for the DC voltage, the value for the front end part is changed to the main body part. On the other hand, with respect to the AC voltage, the control means is configured to perform control to make the value for the tip portion lower than the value for the main body portion. .

  According to the present invention, there is an excellent effect that an image density deficiency at the front end portion of the recording sheet can be suppressed.

1 is a schematic configuration diagram illustrating a printer according to an embodiment. FIG. 3 is an enlarged configuration diagram illustrating an enlarged image forming unit for K in the printer. FIG. 3 is an enlarged cross-sectional view partially showing a cross section of an intermediate transfer belt of the printer. FIG. 3 is an enlarged plan view showing the intermediate transfer belt partially enlarged. FIG. 2 is a block diagram showing a main part of an electric circuit of a secondary transfer power source in the printer, together with a secondary transfer back roller and a secondary transfer nip backing roller. FIG. 3 is an enlarged configuration diagram showing a secondary transfer nip and its periphery in a configuration using a single-layer structure as an intermediate transfer belt, unlike the printer. FIG. 3 is an enlarged cross-sectional view illustrating a secondary transfer nip and surrounding configuration in the printer according to the embodiment. The graph which shows the waveform of the secondary transfer vise output from the secondary transfer power supply. (A) is a graph which shows an example of a time-dependent change in the various voltage values of the secondary transfer bias which consists of a superposition voltage used when using a smooth sheet in the printer. (B) is different from the printer in that the change over time in various voltage values of the secondary transfer bias composed of the superimposed voltage in which the peak-to-peak value for the front end and the peak-to-peak value for the main body are the same. The graph which shows an example. The graph which shows the time-dependent change of the various voltages at the time of switching simultaneously from the value for front-end | tip parts to the value for main-body parts in the transfer bias which consists of a superimposition voltage with a DC voltage and an AC voltage. 3 is a graph showing changes with time of various voltages of a transfer bias composed of superimposed voltages in the printer according to the first embodiment.

Hereinafter, an embodiment of an electrophotographic color printer (hereinafter simply referred to as a printer) will be described as an image forming apparatus to which the present invention is applied.
First, a basic configuration of the printer according to the embodiment will be described. FIG. 1 is a schematic configuration diagram illustrating a printer according to an embodiment. In the figure, the printer according to the embodiment includes four image forming units 1Y, 1M, 1C, and 1K for forming yellow (Y), magenta (M), cyan (C), and black (K) toner images. It has. Also provided are a transfer unit 30, an optical writing unit 80, a fixing device 90, a feeding cassette 100, a registration roller pair 101, and the like.

  The four image forming units 1Y, 1M, 1C, and 1K use Y, M, C, and K toners of different colors as image forming materials. Is done. Taking an image forming unit 1K for forming a K toner image as an example, as shown in FIG. 2, this includes a drum-shaped photosensitive member 2K as a latent image carrier, a drum cleaning device 3K, a charge eliminating device, and a charging device. 6K, developing device 8K and the like. These devices are held by a common holding body and integrally attached to and detached from the printer main body, so that they can be exchanged at the same time.

  The photoreceptor 2K has an organic photosensitive layer formed on the surface of a drum base, and is driven to rotate clockwise in the figure by a driving means. The charging device 6K generates a discharge between the charging roller 7K and the photosensitive member 2K while bringing the charging roller 7K to which a charging bias is applied into contact with or in proximity to the photosensitive member 2K, thereby making the surface of the photosensitive member 2K uniform. Charge like this. In the embodiment, the toner is uniformly charged to the same negative polarity as the normal charging polarity of the toner. As the charging bias, a superposition of a DC voltage and an AC voltage is employed. The charging roller 7K is formed by coating a metal cored bar with a conductive elastic layer made of a conductive elastic material. Instead of a method in which a charging member such as a charging roller is brought into contact with or close to the photoreceptor 2K, a method using a charging charger may be adopted.

  The uniformly charged surface of the photosensitive member 2K is optically scanned by a laser beam emitted from an optical writing unit, which will be described later, and carries an electrostatic latent image for K. The electrostatic latent image for K is developed by the developing device 8K using K toner to become a K toner image. Then, primary transfer is performed on an intermediate transfer belt 31 described later.

  The drum cleaning device 3K removes the transfer residual toner adhering to the surface of the photoreceptor 2K after the primary transfer process (primary transfer nip described later). It includes a cleaning brush roller 4K that is driven to rotate, a cleaning blade 5K that abuts the free end of the cleaning brush roller 4K in a cantilevered state, and the like. The transfer residual toner is scraped off from the surface of the photoreceptor 2K by the rotating cleaning brush roller 4K, and the transfer residual toner is scraped off from the surface of the photoreceptor 2K by the cleaning blade.

  The static eliminator neutralizes the residual charge on the photoreceptor 2K after being cleaned by the drum cleaning device 3K. By this charge removal, the surface of the photoreceptor 2K is initialized and prepared for the next image formation.

  The developing device 8K includes a developing unit 12K that includes a developing member 9K that is a developer carrier, and a developer transport unit 13K that stirs and transports the K developer. The developer transport unit 13K includes a first transport chamber that houses the first screw member 10K and a second transport chamber that houses the second screw member 11K. Each of the screw members includes a rotary shaft member whose both ends in the axial direction are rotatably supported by bearings, and a spiral blade projecting in a spiral manner on the peripheral surface thereof.

  The first transfer chamber that houses the first screw member 10K and the second transfer chamber that houses the second screw member 11K are partitioned by a partition wall. However, at both ends of the partition wall in the screw axis direction, communication ports are formed for communicating the two transfer chambers. The first screw member 10K conveys the K developer held in the spiral blade from the back side to the front side in the direction orthogonal to the paper surface in the drawing while stirring in the rotation direction with the rotational drive. To do. Since the first screw member 10K and a later-described developing member 9K are arranged in parallel so as to face each other, the conveying direction of the K developer at this time is also a direction along the rotation axis direction of the developing member 9K. . Then, the first screw member 10K supplies K developer along the axial direction to the surface of the developing member 9K.

  After the K developer transported to the vicinity of the front end of the first screw member 10K in the drawing passes through the communication opening provided in the vicinity of the front end of the partition wall in the drawing and enters the second transport chamber. The second screw member 11K is held in the spiral blade. And with the rotational drive of the 2nd screw member 11K, it is conveyed toward the back | inner side from the near side in a figure, stirring in a rotation direction.

  In the second transfer chamber, a toner concentration sensor is provided on the lower wall of the casing, and detects the K toner concentration of the K developer in the second transfer chamber. As the K toner density sensor, a sensor comprising a magnetic permeability sensor is used. Since the magnetic permeability of the K developer containing K toner and magnetic carrier has a correlation with the K toner concentration, the magnetic permeability sensor detects the K toner concentration.

  The printer is provided with Y, M, C, and K toner replenishing means for individually replenishing Y, M, C, and K toners in the second storage chamber of the developing device for Y, M, C, and K, respectively. It has been. The control unit of the printer stores Vtref for Y, M, C, and K, which are target values of output voltage values from the Y, M, C, and K toner density detection sensors, in the RAM. When the difference between the output voltage value from the Y, M, C, K toner density detection sensor and the Vtref for Y, M, C, K exceeds a predetermined value, the Y, M, C, K toner is detected for the time corresponding to the difference. M, C, K toner supply means is driven. As a result, Y, M, C, and K toners are replenished into the second transfer chamber of the developing device for Y, M, C, and K.

  The developing member 9K accommodated in the developing unit 12K faces the first screw member 10K and also faces the photoreceptor 2K through an opening provided in the casing. Further, the developing member 9K includes a cylindrical developing sleeve made of a nonmagnetic pipe that is driven to rotate, and a magnet roller fixed inside the developing sleeve so as not to rotate with the sleeve. Then, the K developer supplied from the first screw member 10K is carried on the sleeve surface by the magnetic force generated by the magnet roller, and is conveyed to the developing region facing the photoreceptor 2K as the sleeve rotates.

  A developing bias having the same polarity as the toner and having an absolute value larger than the potential of the electrostatic latent image on the photosensitive member 2K and smaller than the uniform charging potential of the photosensitive member 2K is applied to the developing sleeve. ing. As a result, a developing potential for electrostatically moving the K toner on the developing sleeve toward the electrostatic latent image acts between the developing sleeve and the electrostatic latent image on the photoreceptor 2K. Further, a non-developing potential that moves K toner on the developing sleeve toward the sleeve surface acts between the developing sleeve and the background portion of the photoreceptor 2K. By the action of the developing potential and the non-developing potential, the K toner on the developing sleeve is selectively transferred to the electrostatic latent image on the photoreceptor 2K, and the electrostatic latent image is developed into the K toner image.

  In FIG. 1, Y, M, and C image forming units 1Y, 1M, and 1C also perform Y, M, and C toner images on photoreceptors 2Y, 2M, and 2C in the same manner as K image forming unit 1K. Is formed. Above the image forming units 1Y, 1M, 1C, and 1K, an optical writing unit 80 serving as a latent image writing unit is disposed. The optical writing unit 80 optically scans the photoreceptors 2Y, 2M, 2C, and 2K with laser light emitted from a laser diode based on image information transmitted 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 photoreceptors 2Y, 2M, 2C, and 2K. The optical writing unit 80 irradiates the photosensitive member through a plurality of optical lenses and mirrors while polarizing the laser light L emitted from the light source in the main scanning direction by a polygon mirror rotated by a polygon motor. It is. You may employ | adopt what performs optical writing by the LED light emitted from several LED of the LED array.

  Below the image forming units 1Y, 1M, 1C, and 1K, a transfer unit 30 is disposed as a transfer unit that moves the endless intermediate transfer belt 31 endlessly in the counterclockwise direction in the drawing. . The transfer unit 30 includes a drive roller 32, a secondary transfer back roller 33, a cleaning backup roller 34, four primary transfer rollers 35Y, 35M, 35C, and 35K in addition to the intermediate transfer belt 31 that is an image carrier. Yes. Further, it also has a belt cleaning device 37 and the like.

  The intermediate transfer belt 31 is stretched by a driving roller 32, a secondary transfer back roller 33, a cleaning backup roller 34, and four primary transfer rollers 35Y, 35M, 35C, and 35K disposed inside the loop. Then, it is moved endlessly in the same direction by the rotational force of the driving roller 32 that is driven to rotate counterclockwise in the figure by the driving means.

  The four primary transfer rollers 35Y, 35M, 35C, and 35K sandwich the intermediate transfer belt 31 that is moved endlessly between the photoreceptors 2Y, 2M, 2C, and 2K. As a result, primary transfer nips for Y, M, C, and K in which the front surface of the intermediate transfer belt 31 and the photoreceptors 2Y, 2M, 2C, and 2K abut are formed. A primary transfer bias is applied to the primary transfer rollers 35Y, 35M, 35C, and 35K by a primary transfer power source. As a result, a transfer electric field is formed between the Y, M, C, and K toner images on the photoreceptors 2Y, 2M, 2C, and 2K and the primary transfer rollers 35Y, 35M, 35C, and 35K. The Y toner formed on the surface of the Y photoconductor 2Y enters the Y primary transfer nip as the photoconductor 2Y rotates. Then, the image is primarily transferred from the photoreceptor 2Y to the intermediate transfer belt 31 by the action of the transfer electric field and nip pressure. The intermediate transfer belt 31 on which the Y toner image has been primarily transferred in this way then passes sequentially through the primary transfer nips for M, C, and K. Then, the M, C, and K toner images on the photoreceptors 2M, 2C, and 2K are sequentially superimposed on the Y toner image and primarily transferred. A four-color superimposed toner image is formed on the intermediate transfer belt 31 by this superimposing primary transfer. In place of the primary transfer rollers 35Y, 35M, 35C, and 35K, a transfer charger, a transfer brush, or the like may be employed.

  Below the transfer unit 30, a sheet conveyance unit 38 including a secondary transfer nip backing roller 36, a sheet conveyance belt (generally referred to as a secondary transfer belt or a transfer member) 41, and the like is disposed. ing. The endless sheet conveying belt 41 is stretched by a plurality of rollers such as a secondary transfer nip backing roller 36 disposed inside the loop, and is rotated by the secondary transfer nip backing roller 36 in the drawing. It can be rotated clockwise. Then, the secondary transfer nip backing roller 36 abuts the area around the secondary transfer back roller 33 in the entire circumferential direction of the intermediate transfer belt 31 to form a secondary transfer nip.

  The secondary transfer nip lining roller 36 disposed in the loop of the sheet conveying belt 41 is grounded, whereas the secondary transfer back roller 33 disposed in the loop of the intermediate transfer belt 31 includes A secondary transfer bias output from the next transfer power supply 39 is applied. As a result, the negative polarity toner is electrostatically moved between the secondary transfer back roller 33 and the secondary transfer nip backing roller 36 from the secondary transfer back roller 33 side to the secondary transfer nip backing roller 36 side. A secondary transfer electric field is formed. Note that a secondary transfer roller may be used as the nip forming member instead of the sheet conveying belt 41, and this may be brought into direct contact with the intermediate transfer belt 31.

  Below the transfer unit 30, a feeding cassette 100 that stores a plurality of recording sheets P in a bundle of sheets is disposed. In the feeding cassette 100, a sheet feeding roller 100a is brought into contact with the uppermost recording sheet P of the sheet bundle, and the recording sheet P is fed to the feeding path by being rotated at a predetermined timing. Send it out. A registration roller pair 101 is disposed near the end of the feeding path. The registration roller pair 101 stops the rotation of both rollers when the recording sheet P fed from the feeding cassette 100 is sandwiched between the rollers. Then, rotation driving is restarted at a timing at which the sandwiched recording sheet P can be synchronized with the four-color superimposed toner image on the intermediate transfer belt 31 in the secondary transfer nip, and the recording sheet P is directed to the secondary transfer nip. Send it out.

  The four-color superimposed toner image on the intermediate transfer belt 31 brought into close contact with the recording sheet P at the secondary transfer nip is secondarily transferred onto the recording sheet P by the action of a secondary transfer electric field or nip pressure, and is full color toner. Become a statue. The recording sheet P having a full-color toner image formed on the surface in this way is separated from the intermediate transfer belt 31 by curvature when passing through the secondary transfer nip. Further, the sheet is separated from the sheet conveying belt 41 by the curvature of the separation roller 42 that is wound around the sheet conveying belt 41.

  Note that the following configuration may be employed instead of the configuration in which the sheet transfer belt 41 as a nip forming member is brought into contact with the intermediate transfer belt 31 to form the secondary transfer nip. That is, the secondary transfer nip is formed by bringing a nip forming roller as a nip forming member into contact with the intermediate transfer belt 31.

  Untransferred toner that has not been transferred to the recording sheet P adheres to the intermediate transfer belt 31 after passing through the secondary transfer nip. This is cleaned from the belt surface by a belt cleaning device 37 in contact with the front surface of the intermediate transfer belt 31. A cleaning backup roller 34 disposed inside the loop of the intermediate transfer belt 31 backs up the cleaning of the belt by the belt cleaning device 37 from the inside of the loop.

  A fixing device 90 is disposed downstream of the secondary transfer nip in the sheet conveyance direction. 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 sheet P fed into the fixing device 90 is sandwiched between the fixing nips in a posture in which 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 sheet P discharged from the fixing device 90 passes through a post-fixing conveyance path and is then discharged outside the apparatus.

  When a monochrome image is formed, the printer according to the embodiment changes the posture of the support plate that supports the primary transfer rollers 35Y, 35M, and 35C for the Y, M, and C in the transfer unit 30 by driving a solenoid or the like. Let As a result, the primary transfer rollers 35Y, 35M, and 35C for Y, M, and C are moved away from the photoreceptors 2Y, 2M, and 2C, and the front surface of the intermediate transfer belt 31 is separated from the photoreceptors 2Y, 2M, and 2C. Let In this way, only the image forming unit 1K for K among the four image forming units 1Y, 1M, 1C, and 1K is driven with the intermediate transfer belt 31 in contact with only the black photoconductor 2K. Thus, a K toner image is formed on the K photoconductor 2K. The present invention can be applied not only to an image forming apparatus that forms a color image but also to an image forming apparatus that forms only a monochrome image.

  FIG. 3 is an enlarged sectional view partially showing a transverse section of the intermediate transfer belt 31. The intermediate transfer belt 31 has an endless belt-like base layer 31a made of a material having a certain degree of flexibility and high rigidity, and an elastic surface made of an elastic material excellent in flexibility laminated on the front surface thereof. Layer 31b. In the elastic surface layer 31b, particles 31c are dispersed, and in the state in which the particles c protrude partly from the surface of the elastic surface layer 31b, as shown in FIG. Are closely lined up. A plurality of irregularities are formed on the belt surface by the plurality of particles 31c.

  Examples of the material of the base layer 31a include a resin in which an electrical resistance adjusting material made of a filler or an additive for adjusting the electrical resistance is dispersed. From the viewpoint of flame retardancy, the resin is preferably a fluorine-based resin such as PVDF (polyvinylidene fluoride) or ETFE (ethylene / tetrafluoroethylene copolymer), a polyimide resin, or a polyamide-imide resin. . Further, from the viewpoint of mechanical strength (high elasticity) and heat resistance, a polyimide resin or a polyamideimide resin is particularly preferable.

  Examples of the electrical resistance adjusting material dispersed in the resin include metal oxides, carbon black, ionic conductive agents, and conductive polymer materials. Examples of the metal oxide include zinc oxide, tin oxide, titanium oxide, zirconium oxide, aluminum oxide, and silicon oxide. In order to improve the dispersibility, the metal oxide previously subjected to surface treatment may be used. Examples of carbon black include ketjen black, furnace black, acetylene black, thermal black, and gas black. Examples of the ionic conductive agent include tetraalkyl ammonium salts, trialkyl benzyl ammonium salts, alkyl sulfonates, and alkyl benzene sulfonates. Alkyl sulfate, glycerin fatty acid ester, sorbitan fatty acid ester, polyoxyethylene alkylamine, polyoxyethylene fatty acid alcohol ester, alkyl betaine, lithium perchlorate and the like may be used. A mixture of two or more of these ionic conductive agents may be used. The electrical resistance adjusting material to which the present invention can be applied is not limited to those exemplified so far.

For the coating liquid that is the precursor of the base layer 31a (in which the electrical resistance adjusting material is dispersed in the liquid resin before curing), a dispersion aid, a reinforcing material, a lubricant, and a heat conducting material are used as necessary. An antioxidant or the like may be added. The addition amount of the electrical resistance adjusting material contained in the base layer 31a of the seamless belt suitably equipped as the intermediate transfer belt 31 is preferably 1 × 10 ⁸ to 1 × 10 ¹³ [Ω / □] in terms of surface resistance, volume resistance 1 × 10 ⁶ to 1 × 10 ¹² [Ω · cm]. However, from the viewpoint of mechanical strength, it is necessary to select and add an amount in a range where the molded film is brittle and does not easily break. In other words, electrical properties (surface resistance and volume resistance) and mechanical strength using a coating liquid in which the blending ratio of resin components (polyimide resin precursor, polyamideimide resin precursor, etc.) and an electrical resistance adjusting material is adjusted appropriately. It is preferable to manufacture and use a seamless belt with a good balance. In the case of carbon black, the content of the electrical resistance adjusting material is preferably 10 to 25 [wt%], more preferably 15 to 20 [wt%] of the total solid content in the coating liquid. Further, the content in the case of a metal oxide is preferably 150 [wt%] of the total solid content in the coating liquid, more preferably 10 to 30 [wt%]. If the content is less than the above-mentioned range, a sufficient effect cannot be obtained. If the content is more than the above-mentioned range, the mechanical strength of the intermediate transfer belt 31 (seamless belt) is remarkably lowered. Absent.

  The thickness of the base layer 31a is not particularly limited and may be appropriately selected depending on the situation, but is preferably 30 μm to 150 μm, more preferably 40 μm to 120 μm, and particularly preferably 50 μm to 80 μm. If the thickness of the base layer 31a is less than 30 μm, the belt may be easily torn by cracking, and if it exceeds 150 μm, the belt may be broken by bending. On the other hand, when the thickness of the base layer 31a is within the particularly preferable range described above, it is advantageous in terms of durability.

  In order to improve the belt running stability, it is preferable to reduce the layer thickness unevenness of the base layer 31a as much as possible. The method for adjusting the thickness of the base layer 31a is not particularly limited, and can be appropriately selected depending on the situation. For example, measurement with a contact type or eddy current type film thickness meter or a method of measuring a cross section of the film with a scanning electron microscope (SEM) can be mentioned.

  As described above, the elastic surface layer 31b of the intermediate transfer belt 31 has a concavo-convex shape of a plurality of dispersed particles 31c on the surface. Examples of the elastic material for forming the elastic surface layer 31b include general-purpose resins, elastomers, and rubbers. In particular, an elastic material excellent in flexibility (elasticity) is preferably used, and an elastomer material or a rubber material is preferable. Examples of the elastomer material include polyester, polyamide, polyether, polyurethane, polyolefin, polystyrene, polyacryl, polydiene, and silicone-modified polycarbonate. A thermoplastic elastomer such as a fluorinated copolymer may be used. Examples of the thermosetting resin include polyurethane, silicone-modified epoxy, and silicone-modified acrylic resins. Examples of the rubber material include isoprene rubber, styrene rubber, butadiene rubber, nitrile rubber, ethylene propylene rubber, butyl rubber, silicone rubber, chloroprene rubber, and acrylic rubber. Furthermore, chlorosulfonated polyethylene, fluorine rubber, urethane rubber, hydrin rubber and the like can also be exemplified. From the materials exemplified so far, it is possible to appropriately select a material capable of obtaining desired performance. In particular, it is preferable to select a material that is as soft as possible in order to follow the surface unevenness of a recording sheet having an uneven surface, such as a leather paper. Further, since the particles 31c are dispersed, a thermosetting material is preferable to a thermoplastic material. This is because the thermosetting material has excellent adhesion to the resin particles and can be reliably fixed by the effect of the functional group contributing to the curing reaction. Vulcanized rubber is also a preferred material for the same reason.

  Among the elastic materials constituting the elastic surface layer 31b, acrylic rubber is most preferable from the viewpoints of ozone resistance, flexibility, adhesion to particles, flame retardancy, environmental stability, and the like. The acrylic rubber may be generally commercially available and is not limited to a specific product. However, among various crosslinking systems (epoxy groups, active chlorine groups, carboxyl groups) of acrylic rubber, those having a carboxyl group crosslinking system are superior in terms of rubber physical properties (particularly compression set) and processability. It is preferable to select a crosslinking type. As the crosslinking agent used for the carboxyl group-based acrylic rubber, an amine compound is preferable, and a polyvalent amine compound is most preferable. Specific examples of such amine compounds include aliphatic polyvalent amine crosslinking agents and aromatic polyvalent amine crosslinking agents. Further, examples of the aliphatic polyvalent amine cross-linking agent include hexamethylene diamine, hexamethylene diamine carbamate, N, N′-dicinnamylidene-1,6-hexane diamine and the like. Examples of the aromatic polyvalent amine crosslinking agent include 4,4′-methylenedianiline, m-phenylenediamine, 4,4′-diaminodiphenyl ether, 3,4′-diaminodiphenyl ether, 4,4 ′-(m- And phenylene diisopropylidene) dianiline. 4,4 '-(p-phenylenediisopropylidene) dianiline, 2,2'-bis [4- (4-aminophenoxy) phenyl] propane, 4,4'-diaminobenzanilide and the like may be used. Furthermore, 4,4′-bis (4-aminophenoxy) biphenyl, m-xylylenediamine, p-xylylenediamine, 1,3,5-benzenetriamine, 1,3,5-benzenetriaminomethyl, etc. Good.

  The appropriate range of the amount of the crosslinking agent is preferably 0.05 to 20 parts by weight, more preferably 0.1 to 5 parts by weight with respect to 100 parts by weight of the acrylic rubber. When the blending amount of the crosslinking agent is too small, crosslinking is not sufficiently performed, so that it is difficult to maintain the shape of the crosslinked product. On the other hand, when there is too much content, a crosslinked material will become hard too much and the elasticity etc. as crosslinked rubber will be impaired.

  The acrylic rubber used for the elastic surface layer 31b may be blended with a crosslinking accelerator for the purpose of promoting the crosslinking reaction of the crosslinking agent described above. The type of the crosslinking accelerator is not particularly limited, but it is preferable that the crosslinking accelerator can be used in combination with the polyvalent amine crosslinking agent described above. Examples of such crosslinking accelerators include guanidine compounds, imidazole compounds, quaternary onium salts, tertiary phosphine compounds, alkali metal salts of weak acids, and the like. Examples of the guanidine compound include 1,3-diphenylguanidine, 1,3-diortolylguanidine and the like. Examples of the imidazole compound include 2-methylimidazole and 2-phenylimidazole. Examples of the quaternary onium salt include tetra n-butylammonium bromide and octadecyltri-n-butylammonium bromide. Examples of the polyvalent tertiary amine compound include triethylenediamine and 1,8-diaza-bicyclo [5.4.0] undecene-7 (DBU). Examples of the tertiary phosphine compound include triphenylphosphine and tri-p-tolylphosphine. Examples of the alkali metal salt of a weak acid include inorganic weak acid salts such as sodium or potassium phosphates and carbonates, and organic weak acid salts such as stearates and laurates.

  An appropriate range of the amount of the crosslinking accelerator used is preferably 0.1 to 20 parts by weight, more preferably 0.3 to 10 parts by weight per 100 parts by weight of the acrylic rubber. When there are too many crosslinking accelerators, the crosslinking rate may become too fast at the time of crosslinking, the bloom of the crosslinking accelerator on the surface of the crosslinked product may occur, or the crosslinked product may become too hard. On the other hand, when there are too few crosslinking accelerators, the tensile strength of a crosslinked material may fall remarkably, and the elongation change or tensile strength change after a heat load may be too large.

  In preparing the acrylic rubber, an appropriate mixing method such as roll mixing, Banbury mixing, screw mixing, and solution mixing can be employed. The order of blending is not particularly limited, but after sufficiently mixing components that are not easily reacted or decomposed by heat, as a component that is easily reacted by heat or a component that is easily decomposed, for example, a crosslinking agent or the like at a temperature at which reaction or decomposition does not occur. What is necessary is just to mix in a short time.

  Acrylic rubber can be made into a crosslinked product by heating. A preferable heating temperature is 130 to 220 ° C, more preferably 140 ° C to 200 ° C. Moreover, a preferable crosslinking time is 30 seconds to 5 hours. As a heating method, a method used for crosslinking of rubber such as press heating, steam heating, oven heating, hot air heating and the like may be appropriately selected. Further, after cross-linking once, post-cross-linking may be performed in order to surely cross-link to the inside of the cross-linked product. The post-crosslinking time varies depending on the heating method, crosslinking temperature, shape, etc., but is preferably 1 to 48 hours. About the heating method and heating temperature at the time of post-crosslinking, it is possible to select suitably. For the selected material, appropriate materials such as an electrical resistance adjusting agent for adjusting electrical characteristics, a flame retardant for obtaining flame retardancy, and an antioxidant, a reinforcing agent, a filler, a crosslinking accelerator, etc., as necessary. You may make it contain. Furthermore, the various materials already described can be used as an electric resistance adjusting agent for adjusting electric characteristics. However, since carbon black, metal oxide, and the like impair flexibility, it is preferable to reduce the amount used, and it is also effective to use an ionic conductive agent or a conductive polymer. Moreover, you may use them together.

  It is preferable to add 0.01 to 3 parts of various perchlorates and ionic liquids to 100 parts by weight of rubber. When the addition amount of the ionic conductive agent is 0.01 parts or less, the effect of reducing the resistivity cannot be obtained. Further, if the addition amount is 3 parts or more, there is a high possibility that the conductive agent will bloom or bleed onto the belt surface.

Regarding the addition amount of the electric resistance adjusting material, the resistance value of the elastic surface layer 31b is 1 × 10 ⁸ to 1 × 10 ¹³ [Ω / □] in terms of surface resistance and 1 × 10 ⁶ to 1 × 10 ^{12 in} terms of volume resistance. It is preferable to adjust so that it may become the range of (ohm * cm). Further, in order to obtain a high toner transfer property to an uneven sheet as required for a recent electrophotographic image forming apparatus, the micro rubber hardness value of the elastic surface layer 31b in a 23 ° C. 50% RH environment is set to 35. The flexibility is preferably adjusted as follows. In so-called microhardness measurement such as Martens hardness and Vickers hardness, only the hardness in the shallow region in the bulk direction of the measurement site, that is, in a very limited region near the surface, is measured, so the deformation performance of the entire belt cannot be evaluated. For this reason, for example, when a flexible material is used for the outermost surface of the intermediate transfer belt 31 having a low deformation performance, the microhardness value is lowered. Such an intermediate transfer belt 31 has low deformation performance, that is, poor followability to the concavo-convex sheet. As a result, the transfer performance to the concavo-convex sheet required for the recent image forming apparatus cannot be sufficiently exhibited. End up. Therefore, it is preferable to evaluate the flexibility of the intermediate transfer belt 31 by measuring the micro rubber hardness that can evaluate the deformation performance of the entire intermediate transfer belt 31.

  The layer thickness of the elastic surface layer 31b is preferably 200 μm to 2 mm, and more preferably 400 μm to 1000 μm. When the layer thickness is smaller than 200 μm, the followability to the surface irregularities of the recording sheet and the effect of reducing the transfer pressure are lowered, which is not preferable. On the other hand, if the layer thickness is larger than 2 mm, the elastic surface layer 31b is easily bent by its own weight, and the running performance becomes unstable, or the belt is liable to be cracked by being wound around a roller that stretches the belt. It is not preferable because it becomes. In addition, as a measuring method of layer thickness, the method of measuring by observing a cross section with a scanning microscope (SEM) can be illustrated.

  The particles 31c dispersed in the elastic material of the elastic surface layer 31b have an average particle diameter of 100 μm or less, have a true spherical shape, are insoluble in organic solvents, and have a 3% thermal decomposition temperature of 200 ° C. or higher. The resin particle which is is used. Although there is no restriction | limiting in particular in the resin material of particle | grains 31c, An acrylic resin, a melamine resin, a polyamide resin, a polyester resin, a silicone resin, a fluororesin, rubber | gum etc. can be illustrated. The base surface of the particles made of these resin materials may be surface treated with a different material. A hard resin may be coated on the surface of spherical base particles made of rubber. Moreover, as a base particle, you may use a hollow thing and a porous thing.

  Among the resin materials exemplified so far, silicone resin particles are most preferable from the viewpoint of excellent lubricity, releasability with respect to toner, abrasion resistance, and the like. Particles obtained by finishing a resin material into a spherical shape by a polymerization method or the like are preferable, and particles closer to a true sphere are more preferable. In addition, as the particles 31c, it is desirable to use particles having a volume average particle diameter of 1.0 μm to 5.0 μm and monodispersed particles. The monodisperse particles are not particles having a single particle size but particles having a very sharp particle size distribution. Specifically, it is a particle having a distribution width of ± (average particle size × 0.5 μm) or less. When the particle size of the particles 31c is less than 1.0 μm, the effect of promoting the transfer performance by the particles 31c cannot be sufficiently obtained. On the other hand, if the particle size is larger than 5.0 μm, the gap between the particles becomes large and the surface roughness of the belt increases, so that the toner cannot be transferred satisfactorily or the intermediate transfer belt 31 is cleaned. It becomes easy to generate a defect. Furthermore, since the particles 31c made of a resin material are generally highly insulating, if the particle size is too large, the charges of the particles 31c tend to cause image disturbance due to the accumulation of charges during continuous printing.

  As the particles 31c, specially synthesized particles or commercially available products may be used. By applying the particles 31c directly to the elastic surface layer 31b and smoothing, the particles can be easily and uniformly aligned. By doing in this way, the overlap of the particles 31c in the belt thickness direction can be almost eliminated. The diameter of the cross section of the particles 31c in the surface direction of the elastic surface layer 31b is desirably as uniform as possible, and specifically, it is preferable to have a distribution width of ± (average particle diameter × 0.5 μm) or less. For this reason, it is preferable to use a powder having a small particle size distribution as the powder of the particles 31c. However, a method for selectively applying only the particles 31c having a specific particle size to the surface of the elastic surface layer 31b should be adopted. For example, a powder having a relatively large particle size distribution can be used. The timing at which the particles 31c are applied to the surface of the elastic surface layer 31b is not particularly limited, and may be any before or after crosslinking of the elastic material of the elastic surface layer 31b.

  In the surface direction of the elastic surface layer 31b in which the particles 31c are dispersed, the projected area ratio between the portion where the particles 31 are present and the portion where the surface of the elastic surface layer 31b is exposed is the presence of the particles 31c. It is desirable that the projected area ratio of the portion that has been set to be 60% or more. If it is less than 60%, the chance of direct contact between the toner and the solid surface of the elastic surface layer 31b is increased, so that good toner transferability cannot be obtained, or toner cleaning performance from the belt surface is reduced. Or decrease the filming resistance of the belt surface. It is also possible to use an intermediate transfer belt 31 in which the particles 31c are not dispersed in the elastic surface layer 31b.

  FIG. 5 is a block diagram showing the main part of the electric circuit of the secondary transfer power supply 39 together with the secondary transfer back roller 33, the secondary transfer nip backing roller 36, and the like. The secondary transfer power supply 39 includes a DC power supply 110, an AC power supply 140 configured to be detachable, a power supply control unit 200, and the like. The DC power supply 110 is a power supply for outputting a DC voltage for applying an electrostatic force from the belt side to the recording sheet side in the secondary transfer nip to the toner on the surface of the intermediate transfer belt 31. A DC output control unit 111, a DC drive unit 112, a DC voltage transformer 113, a DC output detection unit 114, an output abnormality detection unit 115, an electrical connection unit 221 and the like are provided.

  The AC power source 140 is a power source that outputs an AC voltage for forming an alternating electric field in the secondary transfer nip. An AC output control unit 141, an AC drive unit 142, an AC voltage transformer 143, an AC output detection unit 144, a removal unit 145, an output abnormality detection unit 146, an electrical connection unit 242 and an electrical connection unit 243 are provided. Yes.

  The power supply control unit 200 controls the DC power supply 110 and the AC power supply 140. It has a CPU (Central Processing Unit), a ROM (Read Only Memory), a RAM (Random Access Memory), and the like. The DC output control unit 111 receives a DC_PWM signal for controlling the output level of the DC voltage from the power supply control unit 200. Further, the output value of the DC voltage transformer 113 detected by the DC output detector 114 is also input. The DC output control unit 111 performs the following control based on the duty ratio of the input DC_PWM signal and the output value of the DC voltage transformer 113. That is, the driving of the DC voltage transformer 113 is controlled via the DC drive unit 112 so that the output value of the DC voltage transformer 113 is set to the output value indicated by the DC_PWM signal.

  The DC drive unit 112 drives the DC voltage transformer 113 according to the control from the DC output control unit 111. The DC voltage transformer 113 is driven by the DC drive unit 112 and outputs a negative DC high voltage. When the AC power supply 140 is not connected, the electrical connection portion 221 and the secondary transfer back roller 33 are electrically connected by the harness 301, and the DC voltage transformer 113 is connected via the harness 301. A DC voltage is output (applied) to the secondary transfer back roller 33. On the other hand, when the AC power supply 140 is connected, since the electrical connection portion 221 and the electrical connection portion 242 are electrically connected by the harness 302, the DC voltage transformer 113 is connected to the AC power supply 140 via the harness 302. Output DC voltage.

  The DC output detection unit 114 detects the output value of the DC high voltage from the DC voltage transformer 113 and outputs it to the DC output control unit 111. Further, the DC output detection unit 114 outputs the detected output value to the power supply control unit 200 as an FB_DC signal (feedback signal). This is because the power supply control unit 200 controls the duty of the DC_PWM signal so that the transferability does not deteriorate due to the environment and load. In this printer, since the AC power supply 140 can be attached to and detached from the main body of the secondary transfer power supply 39, the impedance of the output path of the high voltage output is determined depending on whether the AC power supply 140 is connected or not. Changes. For this reason, when the DC power supply 110 performs constant voltage control and outputs a DC voltage, the voltage dividing ratio changes due to the impedance in the output path changing according to the presence or absence of the AC power supply 140. Furthermore, since the high voltage applied to the secondary transfer back surface roller 33 changes, the transferability changes depending on whether or not the AC power supply 140 is present.

  Therefore, in this printer, the DC power supply 110 performs constant current control to output a DC voltage, and the output voltage is changed according to the presence or absence of the AC power supply 140. As a result, even if the impedance in the output path changes, the high voltage applied to the secondary transfer back roller 33 can be kept constant, and the transferability can be kept constant regardless of the presence or absence of the AC power supply 140. it can. Furthermore, the AC power supply 140 can be attached and detached without changing the value of the DC_PWM signal. As described above, in this printer, the DC power supply 110 is controlled at a constant current, but the following configuration may be adopted. That is, if the high voltage applied to the secondary transfer back roller 33 can be kept constant by changing the value of the DC_PWM signal when the AC power supply 140 is attached or detached, a configuration in which the DC power supply 110 is controlled at a constant voltage is adopted. May be. The AC power supply 140 may be configured to be fixed in the secondary transfer power supply 39 (not detachable). In this case, the DC power source 110 may perform constant current control or constant voltage control.

  The output abnormality detection unit 115 is disposed on the output line of the DC power supply 110, and when an output abnormality occurs due to a ground fault or the like of an electric wire, an SC signal indicating an output abnormality such as a leak is sent to the power supply control unit 200. Output. As a result, it is possible to perform control for stopping the high voltage output from the DC power supply 110 by the power supply control unit 200.

  The AC output control unit 141 receives an AC_PWM signal for controlling the output level of the AC voltage and the output value of the AC voltage transformer 143 detected by the AC output detection unit 144 from the power supply control unit 200. The AC output control unit 141 performs the following control based on the duty ratio of the input AC_PWM signal and the output value of the AC voltage transformer 143. That is, the driving of the AC voltage transformer 143 is controlled via the AC driver 142 so that the output value of the AC voltage transformer 143 becomes the output value indicated by the AC_PWM signal.

  An AC_CLK signal that controls the output frequency of the AC voltage is input to the AC drive unit 142. The AC driving unit 142 drives the AC voltage transformer 143 based on the control from the AC output control unit 141 and the AC_CLK signal. The AC driver 142 drives the AC voltage transformer 143 based on the AC_CLK signal, so that the output waveform generated by the AC voltage transformer 143 can be controlled to an arbitrary frequency indicated by the AC_CLK signal. .

  The AC voltage transformer 143 is driven by the AC drive unit 142 to generate an AC voltage, and generates a superimposed voltage by superimposing the generated AC voltage and a DC high voltage output from the DC voltage transformer 113. When the AC power supply 140 is connected, that is, when the electrical connecting portion 243 and the secondary transfer back roller 33 are electrically connected by the harness 301, the AC voltage transformer 143 generates the superimposed voltage generated by Applied to the secondary transfer back roller 33 via the harness 301. The AC voltage transformer 143 outputs (applies) the DC high voltage output from the DC voltage transformer 113 to the secondary transfer back roller 33 via the harness 301 when no AC voltage is generated. . The voltage (superimposed voltage or DC voltage) output to the secondary transfer back roller 33 is then fed back into the DC power source 110 via the secondary transfer nip backing roller 36.

  The AC output detection unit 144 detects the output value of the AC voltage of the AC voltage transformer 143 and outputs it to the AC output control unit 141. Further, the detected output value is output to the power supply control unit 200 as an FB_AC signal (feedback signal). This is because the power supply control unit 200 controls the duty of the AC_PWM signal so that the transferability does not deteriorate due to the environment or load. The AC power supply 140 performs constant voltage control, but may perform constant current control. Further, the waveform of the AC voltage generated by the AC voltage transformer 143 (AC power supply 140) may be either a sine wave or a rectangular wave, but this printer employs a short pulse rectangular wave. . This is because it is possible to further improve the image quality by making the waveform of the alternating voltage a short-pulse rectangular wave.

Next, a characteristic configuration of the printer according to the embodiment will be described.
The reason for using a superimposed transfer voltage as the secondary transfer bias varies depending on the configuration of the image forming apparatus. In the image forming apparatus described in the cited document 1, when a recording sheet having a large surface unevenness, such as Japanese paper, is used, in order to transfer a sufficient amount of toner into the surface concave portion, a recording voltage composed of two superimposed voltages is used. The next transfer bias is used. On the other hand, in the printer according to the embodiment, when a recording sheet having excellent surface smoothness is used as the recording sheet, the image density is insufficient due to the toner charge amount Q / M being reduced in the secondary transfer nip. In order to suppress the occurrence, a secondary transfer bias composed of a superimposed voltage is used. Such a difference is caused by a difference in the configuration of the intermediate transfer belt. The difference will be described below.

  FIG. 6 is an enlarged configuration diagram showing the secondary transfer nip and its periphery in a configuration in which the intermediate transfer belt 31 is different from that of the present printer and has a single layer structure. When the intermediate transfer belt 31 having a single layer structure is used, a secondary transfer current flows between the secondary transfer back roller 33 and the secondary transfer nip backing roller 36 as follows. In other words, as indicated by the arrows in the figure, the secondary transfer current flows in a straight line concentrating at the nip center position (center position in the belt movement direction). Does not flow so much. Since the secondary transfer current flows in this manner, the time during which the secondary transfer current is applied to the toner in the secondary transfer nip is relatively short. For this reason, the secondary transfer current hardly injects an electric charge having a polarity opposite to the normal polarity to the toner.

  In the printer according to the embodiment, as described above, the intermediate transfer belt 31 has a multilayer structure. In such a configuration, even when a secondary transfer bias consisting of only a DC voltage is used, the elastic surface layer 31b of the intermediate transfer belt 31 is flexibly deformed by following the surface irregularities of the recording sheet in the secondary transfer nip. A sufficient amount of toner can also be secondarily transferred to the surface recesses of the recording sheet. However, when a recording sheet with excellent surface smoothness is used, due to the characteristics of the flow of the secondary transfer current, in the secondary transfer bias consisting only of DC voltage, the toner charge amount Q / M is generated by injecting the reverse charge in the nip. Thereby reducing the image density. In particular, in a halftone image using area gradation, since the amount of toner per unit area of the image area is small, the amount of reverse charge injected into the toner increases and the toner may be reversely charged. . Then, the secondary transfer property is remarkably lowered, and an insufficient image density is generated in a halftone image.

  FIG. 7 is an enlarged cross-sectional view illustrating the secondary transfer nip and the surrounding configuration in the printer according to the embodiment. In the secondary transfer nip, a secondary transfer current flows between the secondary transfer back surface roller 33 and the secondary transfer nip backing roller 36 as follows. That is, the secondary transfer current flows in the belt thickness direction while spreading in the belt circumferential direction at the interface between the base layer 31a and the elastic surface layer 31b. As a result, the secondary transfer current reaches not only the center position of the nip but also the vicinity of the nip inlet and the nip outlet, so that it takes a long time for the secondary transfer current to act on the toner in the secondary transfer nip. become. Then, it becomes easy to excessively inject a charge having a polarity opposite to the normal polarity due to the secondary transfer current to the toner, so that the charge amount of the normal polarity of the toner is greatly reduced or the toner is reversely charged. Or the secondary transferability is hindered. As a result, it has been found that a recording sheet excellent in surface smoothness tends to cause insufficient image density. Note that the secondary transfer current is hindered by the wraparound of the same secondary transfer current not only in the two-layer belt used in this printer but also in a multilayer belt having three or more layers. I also understood that.

  Therefore, in order to acquire the sheet information of the recording sheet to be used, more specifically, the recording sheet set in the sheet feeding cassette 100, the printer accepts information on the brand of the recording sheet by a user input operation. It has become. Specifically, as shown in FIG. 5, an operation display unit 300 is connected to the power supply control unit 200. The operation display unit 300 includes various keys, a touch panel, and the like, and can input various information by a user input operation. The user can input a brand as the sheet information of the recording sheet P set in the feeding cassette 100 by an input operation on the operation display unit 300 as information acquisition means.

  The power supply control unit 200 stores a sheet data table in which the brand of the recording sheet is associated with the basis weight and the sheet type information (whether the sheet is an uneven sheet or a smooth sheet) in the brand. When the brand information is input by the user, the basis weight and the sheet type information corresponding to the brand are specified. If the sheet type information is a concavo-convex sheet, only the DC voltage is used in subsequent print jobs. The secondary transfer device is output from the secondary transfer power source 39. On the other hand, when the sheet type information is a smooth sheet, the secondary transfer power source 39 outputs a secondary transfer bias including a superimposed voltage in subsequent print jobs.

  FIG. 8 is a graph showing the waveform of the secondary transfer vice composed of the superimposed voltage output from the secondary transfer power supply 39 of the printer according to the embodiment. In order to perform secondary transfer of the toner image on the intermediate transfer belt 31 to the recording sheet P at the secondary transfer nip in the configuration in which the secondary transfer bias is applied to the secondary transfer back roller 33 as in this printer. It is necessary to employ a secondary transfer bias having the following characteristics. That is, the bias is such that the time average polarity is the same as the charging polarity of the toner. Specifically, the secondary transfer bias is composed of an alternating voltage that periodically inverts the polarity by superimposing a DC voltage and an AC voltage. However, in terms of time average, the polarity is the same negative polarity as that of the toner. It is biased. As described above, by adopting the secondary transfer bias in which the time average polarity is a negative polarity, the toner is repelled relatively to the secondary transfer back surface roller 33 and electrostatically moves from the belt side to the recording sheet P side. It can be moved. Note that when a configuration in which a secondary transfer bias is applied to the secondary transfer nip backing roller 36 is adopted, a secondary transfer bias having a time average opposite to that of the toner may be employed. This is because the toner can be moved from the belt side to the recording sheet P side by electrostatically attracting the toner toward the secondary transfer nip backing roller 36 relatively by the secondary transfer bias. As the secondary transfer bias, one that periodically inverts the polarity may be used, or one that does not periodically change polarity (the polarity is always constant) as shown in FIG.

  In the figure, T indicates one cycle of the secondary transfer bias whose polarity is periodically reversed. In the figure, Vt is the transfer peak value of the two peak values that causes the toner to electrostatically move more strongly from the belt side to the recording sheet side. Vr is an inverse peak value opposite to the transfer peak value Vt. Voff is an offset voltage as the value of the DC component of the secondary transfer bias. As an example, for example, the transfer peak value Vt is −7.0 [kV], and the reverse peak value Vr is −0.6 [kV]. The offset voltage Voff is −3.8 [kV], and the average potential Vave (time average) is −2.0 [kV]. Further, the peak-to-peak potential Vpp of the AC component is 6.4 [kV]. The duration A of the transfer peak value Vt is 0.10 [ms]. The period of the waveform is 0.66 [ms] and the duty is 85 [%]. The process linear velocity as the linear velocity of various members of the printer is 600 [mm / s].

  The illustrated secondary transfer vise has a waveform in which the duty in the period T exceeds 50 [%]. Specifically, the duty is expressed based on the following definition. In other words, a position obtained by shifting the reverse peak value toward the transfer peak value by 30% of the peak-to-peak value is defined as the waveform baseline. Further, the time when the waveform is on the reverse peak value Vr side from the baseline is defined as the reverse time B. More specifically, the time from when the waveform starts rising or falling toward the reverse peak value Vr from the base line to immediately before falling to the base line or immediately before rising is defined as the reverse time B. The duty is a ratio in the period T of the reverse time B.

  Vave in the figure indicates the average potential of the secondary transfer bias, and is the same value as the solution of “reverse peak value Vr × duty / 100 + transfer peak value Vt × (1−duty) / 100”. In the figure, A is a forward time obtained by subtracting the reverse time B from the period T.

  As shown in the figure, in the secondary transfer bias, the reverse time B is longer than half of the period T, that is, the duty exceeds 50 [%]. When such a secondary transfer bias is employed, the time during which a positive polarity charge opposite to the charge polarity may be injected into the toner within the period T is shortened. It is possible to suppress a decrease in the toner charge amount Q / M due to the charge injection at. Thereby, it is possible to suppress the occurrence of insufficient image density due to a decrease in secondary transferability due to a decrease in the toner charge amount Q / M.

  The decrease in the toner charge amount Q / M occurs remarkably when the intermediate transfer belt 31 having the structure in which the particles 31c are dispersed in the material of the uppermost layer (elastic surface layer 31b) is used as in this printer. . Specifically, in the above configuration, the particles 31c reduce the contact area between the belt surface and the toner in the secondary transfer nip. Thereby, the toner releasability from the belt surface can be improved and the secondary transfer efficiency can be increased. However, it is easy to inject a charge having a reverse polarity to the toner by causing the secondary transfer current to flow intensively between the regularly arranged insulating particles 31c. For this reason, although the particles 31c are dispersed for the purpose of increasing the secondary transfer efficiency, the secondary transfer efficiency may be deteriorated. Therefore, by adopting a secondary transfer bias with a high duty, it is possible to reliably obtain the effect of improving the secondary transfer efficiency by the particles 31c.

  As the particles 31c, particles having a charging performance opposite to the normal charging polarity of the toner can be used. In this printer, the particles are made of a positively charged melamine resin. In such a configuration, it is possible to suppress the occurrence of a phenomenon in which the secondary transfer current is concentrated between the particles due to the charge of the particles 31c, and to further reduce the amount of reverse charges injected into the toner.

  Further, as the particles 31c, particles having a charging performance of the same polarity as the normal charging polarity of the toner may be used. In this printer, it is a negatively chargeable silicone resin particle (trade name: Tospearl).

  As the intermediate transfer belt 31, a belt provided with a surface layer made of urethane, Teflon (registered trademark), or the like as the uppermost layer may be used. Moreover, you may use what laminated | stacked multiple layers which consist of resin, such as a polyimide and a polyamideimide. Regardless of which belt is used, the occurrence of insufficient image density can be suppressed by employing a high-duty secondary transfer bias.

  In the secondary transfer bias shown in the figure, the polarity of the reverse peak value Vr is the same as the polarity of the forward peak value Vt, but the polarity of the reverse peak value Vr is reversed from the polarity of the forward peak value Vt. A secondary transfer bias may be employed.

  The power control unit 200 is connected to a main control unit that controls driving of each device in the printer. The main control unit transmits a registration signal to the power supply control unit 200 at the moment when the driving of the registration roller pair 101 is started and the recording sheet P is started to be sent out toward the secondary transfer nip. When a registration signal is sent from the main control unit, the power supply control unit 200 starts a time measurement process based on that point. Thereafter, the point in time at which the predetermined time has passed is regarded as the point at which the recording sheet P has reached the upstream position by a predetermined distance from the secondary transfer nip (hereinafter referred to as upstream arrival point). Then, when reaching the upstream, the output of the secondary transfer bias from the secondary transfer power supply 39 is started. At this time, if it is a secondary transfer bias composed of a superimposed voltage, a DC voltage having a value for the leading end for secondary transfer of the toner image to the leading end of the recording sheet and an AC voltage having a value for the leading end are obtained. The superimposed image is output from the secondary transfer power supply 39. In the case of a secondary transfer vice consisting only of a DC voltage, a DC voltage having a value for the tip is output from the secondary transfer power supply 39.

  Further, the power supply control unit 200 regards a point in time when a predetermined time has elapsed from the upstream arrival point as a point in time when the leading end of the recording sheet P passes through the secondary transfer nip (hereinafter referred to as a leading end passing point). Then, the secondary transfer bias output from the secondary transfer power supply 39 is switched from that for the tip portion to that for the main body portion in the vicinity of the time when the tip portion passes. The secondary transfer bias for the main body is for secondary transfer of the toner to the main body on the rear side of the front end of the recording sheet P. In the case of the secondary transfer bias composed of the superimposed voltage, the secondary transfer power supply 39 outputs a superimposed value of the direct current voltage for the main body and the alternating current voltage for the main body. If the secondary transfer bias is composed only of a DC voltage, a DC voltage having a value for the main body is output from the secondary transfer power supply 39.

  By such control, when the leading end portion of the recording sheet P enters the secondary transfer nip, the secondary transfer is performed under a condition using the secondary transfer bias for the leading end portion. On the other hand, when the main body portion following the leading end portion of the recording sheet P enters the secondary transfer nip, the secondary transfer is performed under the condition using the secondary transfer bias for the main body portion.

  As described above, when the toner image is secondarily transferred to the recording sheet P made of a concavo-convex sheet, the power supply controller 200 causes the secondary transfer power supply 39 to output a secondary transfer bias consisting only of a DC voltage. At this time, the value for the tip of the secondary transfer bias is set higher than the value for the main body.

  The reason why the value of the DC voltage is made different is as follows. That is, when the leading end portion of the recording sheet P enters the secondary transfer nip, the region where the intermediate transfer belt 31 and the sheet conveying belt 41 are in direct contact with each other and the leading end portion of the recording sheet P is sandwiched therebetween. Are generated in the secondary transfer nip. At this time, since the secondary transfer current is concentrated in the former area where no recording sheet having a high electrical resistance exists, the secondary transfer power source flowing at the leading end of the recording sheet is likely to be insufficient. If the secondary transfer bias consisting only of the DC voltage is set to a value suitable for the secondary transfer of the toner image to the main body of the recording sheet P, the secondary transfer current becomes insufficient at the leading edge of the recording sheet. .

Table 1 below shows the ratio of the value for the leading end portion to the value for the main body portion for each of the AC component (peak-to-peak value) and DC component of the secondary transfer bias composed of the superimposed voltage, and the leading edge of the recording sheet. 4 is a table showing the relationship with the image density of a portion. Table 1 was prepared based on a print test performed in an environment of temperature = 23 ° C. and humidity = 50% using plain paper having a basis weight of 80 g as a recording sheet (smooth sheet). . The peak-to-peak value Vpp for the main body portion in the AC component is 6.4 [kV]. Further, the output target value (constant current control) for the main body in the DC component is −114 [μA].

  In Table 1, o in the image density item indicates that a good image density is obtained at the leading edge of the recording sheet. On the other hand, x indicates that an image shortage has occurred. As shown in Table 1, if both the alternating current component and the direct current component have the same value for the leading end (100%) as the value for the main body, insufficient image density occurs at the leading end of the recording sheet. On the other hand, when the value for the direct current component is made higher than the value for the main body, a good image density can be obtained at the front edge of the recording sheet. If the degree to which the value for the tip of the DC component is made higher than the value for the main body is relatively large (140%), the value for the tip of the AC component may be slightly lower than the value for the main body. (90%) Good image density can be obtained at the leading edge of the recording sheet.

  Therefore, when the toner image is secondarily transferred to the front end portion of the recording sheet P made of a concavo-convex sheet, the value for the front end portion of the secondary transfer bias consisting only of the DC voltage is made higher than the value for the main body portion. As a result, a sufficient amount of secondary transfer current can be supplied to the leading end portion of the recording sheet P made of a concavo-convex sheet to suppress the occurrence of insufficient image density at the leading end portion of the sheet. Further, when the toner image is secondarily transferred to the main body portion of the recording sheet P, the recording sheet is sandwiched in the entire area of the secondary transfer nip and does not cause concentration of the secondary transfer current. Is lower than the value for the tip. As a result, it is possible to suppress wasteful energy consumption when the toner image is secondarily transferred to the main body of the recording sheet P made of a concavo-convex sheet.

However, in a low-temperature and low-humidity environment, there were cases where many white spots were generated in the image at the leading end of the recording sheet P. Table 2 below shows the ratio of the value for the front end portion to the value for the main body portion for the AC component (peak-to-peak value) and DC component of the secondary transfer bias composed of the superimposed voltage, and the front end of the recording sheet. 4 is a table showing the relationship with the image density of a portion. Table 2 was prepared based on a print test conducted in a temperature = 10 ° C. and humidity = 15% environment using plain paper having a basis weight of 80 g as a recording sheet (smooth sheet). .

  In Table 2, when both the alternating current component and the direct current component have the same value for the tip (100%) as that for the main body, the temperature is 23 ° C. and the humidity is 50% (see Table 1). Insufficient image density occurs at the leading edge of the recording sheet. When the AC component ratio is 140% and the DC component ratio is 140%, the AC component ratio is 120% and the DC component ratio is 140%, and the AC component ratio is 100%, and In each case where the ratio of the DC component was 140%, the image quality was abnormal. Specifically, a large number of white spots are generated in the image at the leading end of the recording sheet.

  As a result of earnest research on the cause of this white spot, in those cases, the transfer peak value Vt is set higher than the discharge start voltage in the secondary transfer nip, and discharge is generated in the secondary transfer nip. It was found that white spots were caused as a result of reverse charging of the toner by discharge. Specifically, as described above, for the DC voltage, constant current control is adopted so that a good secondary transfer current can be applied to the sheet regardless of the environment. In such a configuration, the value of the DC component (Voff) is lower in a low temperature and low humidity (10 ° C., 15%) environment where the electrical resistance of the recording sheet is relatively higher than in a normal temperature and normal humidity (23 ° C., 50%) environment. The desired secondary transfer current is allowed to flow with high control. Then, in a low-temperature and low-humidity environment, the transfer peak value Vt output for the tip portion becomes too high and exceeds the discharge start voltage. The reason why the value for the front end portion of the DC component is higher than the value for the main body portion is to suppress the occurrence of insufficient image density, but in this case, the front end of the recording sheet P in a low temperature and low humidity environment. It becomes easy to generate a white spot due to discharge at the portion. White spots greatly reduce image quality compared to insufficient image density.

  Therefore, the power supply control unit 200 controls the secondary transfer bias composed of the superimposed voltage employed when using a smooth sheet as follows. That is, for the DC voltage, the value for the tip is made higher than the value for the main body as in the case of using the uneven sheet. For the AC voltage, the value for the front end (peak to peak value) is made lower than the value for the rear end (peak to peak value). At the time of secondary transfer to the tip portion where discharge is likely to occur, the peak-to-peak value of the AC voltage is lowered to lower the transfer peak value Vt that becomes the maximum in one cycle. Accordingly, it is possible to suppress the occurrence of white spots in the image at the leading edge of the smooth sheet by making it difficult to generate a discharge in the secondary transfer nip.

  Note that if the transfer peak value Vt is lowered at the leading edge of the smooth sheet, the image density may be lowered at the leading edge, but as described above, the generation of white spots is more than the decrease in image density. However, image quality degradation is remarkable. For this reason, in the printer according to the embodiment, the countermeasure is taken by giving priority to the generation of white spots at the front end rather than the decrease in image density at the front end.

  FIG. 9A is a graph illustrating an example of a change with time in various voltage values of the secondary transfer bias including a superimposed voltage used when a smooth sheet is used in the printer according to the embodiment. FIG. 9B is different from the printer in FIG. 9B. Various voltage values of the secondary transfer bias composed of superimposed voltages in which the peak-to-peak value for the front end and the peak-to-peak value for the main body are the same. It is a graph which shows an example (henceforth a comparative example) in a time-dependent change in. In these figures, Vt is a transfer peak value, Vr is an inverse peak value, and Voff is an offset voltage. Since these are potential values, they can be established with either positive polarity or negative polarity. However, in the examples in these figures, the transfer peak value Vt, offset voltage Voff, and reverse peak value Vr are all set to negative polarity. An example is shown. The transfer peak value Vt and the offset voltage Voff must always have the same polarity, but the reverse peak value Vr may have a reverse polarity. In these figures, Vpp is a peak-to-peak value, but since this is not a potential value but a potential difference value, there is no negative polarity.

  In both the embodiment (FIG. 9A) and the comparative example (FIG. 9B), the offset voltage Voff for the tip is made smaller than the offset voltage Voff for the main body, and accordingly, for the tip. The reverse peak value Vr is higher on the minus side than the reverse peak value Vr for the main body. Further, the transfer peak value Vt for the tip portion is higher than the transfer peak value Vt for the main body portion. However, in the embodiment, the transfer peak value Vt can be lowered by the amount that the peak-to-peak value Vpp for the tip portion is lower than that for the main body portion.

  Next, printers according to the respective examples in which a more characteristic configuration is added to the printer according to the embodiment will be described. Unless otherwise described below, the configuration of the printer according to each example is the same as that of the embodiment.

[First embodiment]
In the secondary transfer power supply 39 including the DC power supply 110 and the AC power supply 140, the output response of the DC voltage is generally slow while the output response of the AC voltage is relatively fast. . In the printer according to the embodiment, only about 10 [msec] is required from the start of the change control of the peak-to-peak value of the AC voltage until the actual output reaches the target value. On the other hand, it takes 27 [msec], for example, until the actual output reaches the target value after the DC voltage change control is started.

  If there is a difference in output responsiveness as described above, the average potential Vave (average of one cycle) of the superimposed voltage deviates more than the target when the secondary transfer bias is started, but at this time, the secondary transfer is still not performed. There is no problem because it is not done. However, when the secondary transfer bias is switched from that for the tip portion to that for the main body portion, there is a risk of causing a problem.

  FIG. 10 is a graph showing changes with time of various voltages when switching from the value for the front end portion to the value for the main body portion at the transfer bias composed of the superimposed voltage is performed simultaneously with the DC voltage and the AC voltage. In the figure, the DC component and the AC component of the superimposed voltage are raised so as to have values for the tip when reaching the upstream as described above. Then, the peak-to-peak value Vpp of the AC voltage quickly rises to the target value (value for the tip), but the DC component (Voff) rises to the target value (value for the tip). It takes a relatively long time to raise. For this reason, the transfer peak value Vt of the superimposed voltage also takes a relatively long time to reach the target value, but during that time, the leading edge of the recording sheet (smooth sheet) has not yet entered the secondary transfer nip. Secondary transfer failure due to insufficient transfer peak value Vt does not occur. Thereafter, the leading edge of the recording sheet enters the secondary transfer nip at a timing at which the transfer peak value Vt of the superimposed voltage becomes substantially the target value.

  Thereafter, when the end portion passage time comes, the value for the tip portion is switched to the value for the main body portion for each of the DC component and the AC component. Specifically, the output target value in the constant current control of the DC component and the output target value in the constant voltage control of the AC component are simultaneously switched from the value for the front end portion to the value for the rear end portion. Then, for the AC component, the actual output voltage after switching the output target value in the constant voltage control is quickly raised to the target value. On the other hand, for the DC component, it takes a relatively long time for the actual output voltage after switching the output target value in the constant current control to fall to a value at which a target current value can be obtained. Then, as shown in the figure, in the region near the leading end of the main body of the recording sheet P, the transfer peak value Vt may exceed the discharge start voltage to cause a discharge and generate a white spot.

  Therefore, the power supply control unit 200 starts the switching of the output target value in the constant current control of the DC voltage with respect to the switching from the value for the front end portion of the secondary transfer chair to the value for the main body portion. This is performed at a timing earlier than the start of switching of the output target value in the control.

  FIG. 11 is a graph showing temporal changes in various voltages of the transfer bias composed of superimposed voltages in the printer according to the first embodiment. As shown in the figure, the power supply control unit 200 starts switching from the tip portion to the main body portion with respect to the output target value in the constant current control of the DC voltage at a timing slightly earlier than the time when the tip portion passes. Slowly start the voltage toward the value for the main unit. After that, at the time of passing through the tip, start switching from the tip to the main unit for the output target value in constant voltage control of the AC voltage, and quickly determine the peak-to-peak value of the AC voltage for the main unit. Start up. At this time, since the DC voltage has already fallen to a certain value, as shown in the drawing, the DC voltage and the AC voltage are maintained while maintaining the transfer peak value Vt of the superimposed voltage lower than the discharge start voltage. It is possible to complete the switch to the value for the main body. Thereby, generation | occurrence | production of the white spot in the image in the area | region near the front-end | tip part in the main-body part of a smooth sheet can be suppressed.

[Second Example]
In the configuration in which the recording sheet P is sandwiched in the secondary transfer nip, the value of the secondary transfer current necessary for transferring the toner image to the recording sheet P increases as the basis weight of the recording sheet P increases. On the other hand, in order to increase the value of the secondary transfer current, as the output target value of constant current control at the DC voltage of the secondary transfer current composed of the superimposed voltage is increased, white spots in the image due to discharge are generated. It becomes easy.

  When the brand of the recording sheet P is input by the user, the power control unit 200 specifies the basis weight and type corresponding to the brand from the above-described sheet data table. The type of recording sheet P is divided into two types: a smooth sheet and an uneven sheet. Therefore, the power supply control unit 200 can specify the basis weight and type (whether it is a smooth sheet or an uneven sheet) of the recording sheet P to be used by inputting a brand by the user. it can.

As described above, when the type of the recording sheet P to be used is a smooth sheet, the power supply control unit 200 causes the secondary transfer power supply 39 to output a secondary transfer bias having a superimposed voltage. At this time, as shown in Table 3 below, as the basis weight of the recording sheet P to be used increases, the value for the front end of the DC component is increased while the value for the front end of the AC voltage is decreased. Implement control.

  Thus, the value for the front end portion of the DC component is increased as the basis weight increases. On the other hand, by reducing the value for the front end of the AC voltage, even in the case of a recording sheet having a large basis weight, it is possible to suppress the image density shortage at the front end, and in the image of the front end due to discharge. Generation of white spots can be suppressed.

  As in this printer, in the configuration in which the DC component of the secondary transfer bias composed of the superimposed voltage is output by constant current control, the electrical resistance of the recording sheet P increases as the environment becomes low temperature, low humidity, or low temperature and low humidity. The output voltage value of the DC component is made higher. For this reason, as the environment becomes low temperature, low humidity, or low temperature low humidity, white spots in the image due to discharge are easily generated at the front end portion of the smooth sheet.

  Therefore, in this printer, as shown in FIG. 5, the environmental sensor 301 for detecting the temperature and humidity is connected to the power supply control unit 200. Then, as shown in Table 3 above, as the environment becomes low temperature and low humidity, the value for the front end of the AC component (the output target value of constant voltage control) is lowered. Thereby, regardless of the environment, it is possible to suppress the occurrence of white spots in the image due to the discharge at the leading edge of the smooth sheet.

  For the environment, absolute humidity is calculated from the detection results of temperature and humidity, and both temperature and humidity are grasped based on this value. That is, as the absolute humidity decreases, the value for the tip portion of the AC component is decreased. When a temperature sensor that detects only temperature or a humidity sensor that detects only humidity is provided instead of the environmental sensor 301, the value for the front end of the AC component may be lowered as the detection result by them decreases. .

What was demonstrated above is an example, and there exists an effect peculiar for every following aspect.
[Aspect A]
Aspect A includes an image carrier (for example, the intermediate transfer belt 31) and toner image forming means (for example, the image forming unit 1, the transfer unit 30, and the optical writing unit 80) for forming a toner image on the surface of the image carrier. As a transfer bias for transferring the toner image on the surface to a nip forming member (for example, a sheet conveying belt 41) that contacts the surface and forms a transfer nip, and a recording sheet that has entered the transfer nip. In an image forming apparatus, comprising: a power source that outputs a superimposed voltage obtained by superimposing a DC voltage and an AC voltage; and a control unit (for example, power source control unit 200) that controls a transfer bias output from the power source. Each of the DC voltage and the AC voltage is transferred to the leading end portion for transferring the toner image to the leading end portion of the recording sheet, and to the recording sheet. Switching between the value for the main body part for transferring the toner image to the main body part following the tip part, and for the DC voltage, the value for the tip part is made higher than the value for the main body part, while the AC voltage is Is characterized in that the control means is configured so as to carry out control to make the value for the tip portion lower than the value for the main body portion.

  In such a configuration, the shortage of the transfer current flowing to the front end portion of the recording sheet that has entered the transfer nip is suppressed by making the value for the front end portion of the DC voltage at the transfer bias composed of the superimposed voltage higher than the value for the main body portion. . Thereby, insufficient image density at the leading end of the recording sheet can be suppressed.

  In the aspect A, it is also possible to suppress the occurrence of white spots in the image at the leading edge of the recording sheet for the reason described below. In other words, the present inventors have conducted experiments to output a DC voltage of the transfer bias when transferring the toner image to the leading end of the sheet and an output when transferring the toner image to the sheet main body on the rear side of the leading end of the sheet. It has been found that a white spot is more likely to occur if the value is larger than. This makes it easy to generate a plurality of white spots in the image at the leading end of the recording sheet. It has also been found that white spots in the image are generated by the occurrence of discharge in the transfer nip. In the configuration using the transfer bias composed of the superimposed bias, two peak-to-peak values are generated in the transfer bias. There are two peak values: the peak value for the electrostatic movement of the toner more strongly from the image carrier side to the nip forming member side, and the other peak value. Among them, the former peak value has a larger value (absolute value), so that discharge is likely to occur at the timing of the former peak value. To suppress the occurrence of insufficient image density at the leading edge of the recording sheet, if the value for the leading edge of the DC voltage is made higher than the value for the main body, the former peak value at the leading edge of the recording sheet It becomes easy to cause discharge exceeding it. Therefore, in the aspect A, with respect to the AC component of the transfer bias, the value for the leading end (peak to peak value) is made smaller than the value for the trailing end (peak to peak value). This suppresses the occurrence of white spots in the image at the front end portion of the recording sheet by suppressing the increase in the former peak value by making the value for the front end portion of the DC voltage higher than the value for the main body portion. be able to.

  As described above, in the aspect A, it is possible to suppress both the image density shortage at the front end portion of the recording sheet and the generation of white spots in the image at the front end portion.

[Aspect B]
Aspect B is the aspect A in which the image carrier has a multilayer structure including a plurality of layers including an elastic surface layer made of an elastic material in which at least a plurality of fine particles are dispersed, and the power source is As the transfer bias, a transfer bias with a duty less than 50%, which is a ratio with respect to one period of time during which electrostatic force that moves toner from the nip forming member side to the image carrier side in the transfer nip is dominant, is output. It is characterized by being. In such a configuration, as described in the embodiment, the toner releasability from the surface of the image carrier is improved by reducing the contact area between the surface of the image carrier and the toner in the transfer nip by the fine particles, Transfer efficiency can be increased. Further, by using an image bearing member having an elastic surface layer, a sufficient amount of toner can be transferred also into the surface recesses of the recording sheet rich in surface unevenness. Furthermore, by outputting a transfer bias with a duty less than 50%, it is possible to suppress the occurrence of transfer failure due to a decrease in toner charge amount when a recording sheet having excellent surface smoothness is used. You can also.

[Aspect C]
Aspect C, in aspect A or B, starts switching from the value for the front end of the DC voltage to the value for the main body, and starts switching from the value for the front end of the AC voltage to the value for the main body The control means is configured to perform control performed at an earlier timing. With this configuration, as described in the first embodiment, it is possible to suppress the occurrence of insufficient image density near the boundary between the recording sheet main body and the leading end.

[Aspect D]
Aspect D is characterized in that in any one of Aspects A to C, an information acquisition means for acquiring sheet information, which is information of a recording sheet to be used, is provided.

[Aspect E]
Aspect E implements control in aspect D to increase the value for the front end of the DC voltage while lowering the value for the front end of the AC voltage as the basis weight as the sheet information increases. As described above, the control means is configured. With this configuration, as described in the second embodiment, it is possible to suppress the occurrence of insufficient image density at the leading end of the recording sheet regardless of the basis weight.

[Aspect F]
In the aspect F, in the aspect D or E, when the type information as the sheet information is a surface uneven sheet, the transfer information including the superimposed voltage is output from the power source, while the type information is a smooth sheet. In this case, the control means is configured to perform control to output a transfer bias composed of only a DC voltage from the power source. In such a configuration, when a surface uneven sheet is used, a sufficient amount of toner can be transferred into the surface recess of the surface uneven sheet by using a transfer bias consisting of only a DC voltage. Furthermore, when a smooth sheet is used, a transfer bias including a superimposed bias can be used to suppress the occurrence of transfer failure due to a reduction in the toner charge amount in the transfer nip.

[Aspect G]
Aspect G provides environment detection means for detecting the environment in any one of aspects A to F. As the detection result by the environment detection means becomes low temperature, low humidity, or low temperature and low humidity, The control means is configured to perform control for lowering the value. With this configuration, as described in the third embodiment, it is possible to suppress the occurrence of image density defects at the front end portion of the recording sheet regardless of the environment.

[Aspect H]
Aspect H is any one of aspects A to G, wherein the toner image forming means develops a latent image carrier, a developing means for developing a latent image on the latent image carrier, and obtaining a toner image; After the toner image on the latent image carrier is transferred to the endless intermediate transfer belt as the image carrier, the toner image is secondarily transferred to the recording sheet in the transfer nip by the contact between the intermediate transfer belt and the nip forming member. And a transfer means that performs the transfer, and the nip forming member is an endless nip forming belt.

1Y, C, M, K: toner image forming unit (toner image forming means)
31: Intermediate transfer belt (image carrier)
31a: Base layer (substrate)
31b: Elastic layer (laminated layer)
39: Secondary transfer power source 41: Sheet conveying belt (nip forming member)
P: Recording sheet

Patent No. 5,489,556

Claims (8)

An image carrier, a toner image forming unit that forms a toner image on the surface of the image carrier, a nip forming member that forms a transfer nip in contact with the surface, and a recording sheet that has entered the transfer nip A power supply for outputting a transfer bias for transferring the toner image on the surface as a transfer bias by superimposing a DC voltage and an AC voltage, and a control means for controlling the transfer bias output from the power supply. In the image forming apparatus provided,
Each of the DC voltage and the AC voltage of the transfer bias is used to transfer the toner image to the value for the leading end portion for transferring the toner image to the leading end portion of the recording sheet and the main body portion following the leading end portion of the recording sheet. For the DC voltage, the value for the tip is made higher than the value for the body while the value for the tip is changed to the value for the body An image forming apparatus characterized in that the control means is configured so as to perform control to be lower. The image forming apparatus according to claim 1,
The image carrier has a multilayer structure including a plurality of layers including an elastic surface layer made of an elastic material in which at least a plurality of fine particles are dispersed;
In addition, the power supply has a duty that is a ratio of one period of time during which electrostatic force that moves toner from the nip forming member side to the image carrier side in the transfer nip is dominant as a transfer bias, less than 50%. An image forming apparatus that outputs the processed image. The image forming apparatus according to claim 1 or 2,
The control for starting the switching from the value for the front end of the DC voltage to the value for the main body is performed at an earlier timing than the start of switching from the value for the front end of the AC voltage to the value for the main body. Thus, an image forming apparatus comprising the control means. The image forming apparatus according to any one of claims 1 to 3,
An image forming apparatus comprising an information acquisition unit that acquires sheet information that is information of a recording sheet to be used. The image forming apparatus according to claim 4.
The control means is configured to perform control to increase the value for the front end of the DC voltage while lowering the value for the front end of the AC voltage as the basis weight as the sheet information increases. An image forming apparatus. The image forming apparatus according to claim 4 or 5,
When the type information as the sheet information is a surface uneven sheet, the transfer bias composed of the superimposed voltage is output from the power source, while when the type information is a smooth sheet, only the DC voltage is output from the power source. An image forming apparatus characterized in that the control means is configured to perform control to output a transfer bias comprising: The image forming apparatus according to claim 1,
An environment detection means is provided to detect the environment.
An image characterized in that the control means is configured to perform control to lower the value for the tip of the AC voltage as the detection result by the environment detection means becomes low temperature, low humidity, or low temperature and low humidity. Forming equipment. The image forming apparatus according to claim 1,
The toner image forming means includes a latent image carrier, developing means for developing a latent image on the latent image carrier to obtain a toner image, and an endless toner image on the latent image carrier as the image carrier. And a transfer means for secondary transfer to a recording sheet in the transfer nip by the contact between the intermediate transfer belt and the nip forming member after being transferred to the intermediate transfer belt,
The image forming apparatus is characterized in that the nip forming member is an endless nip forming belt.

Images