Patent Description:
When paper is cooled in a low temperature environment, toner is difficult to fuse onto the paper, and temperature of a fixing member falls. Therefore, fixing under offset (low-temperature fixing) occurs. To deal with this problem, the paper is heated before the toner is fixed to raise temperature of the paper.

For example, <CIT> describes an image formation apparatus including a paper feeder that feeds continuous paper, a paper ejector that winds the fed continuous paper, a transferrer that is disposed between the paper feeder and the paper ejector in a paper conveyance direction and forms a toner image on paper, a fixer that is disposed on a downstream side of the transferrer and an upstream side of the paper ejector and fixes the toner image on the paper, and a paper pre-heating member that is disposed between the transferrer and the fixer in the paper conveyance direction and heats the paper.

In <CIT>, the paper pre-heating member is disposed between the transferrer and the fixer. Therefore, the paper pre-heating member is brought into contact with a back surface of the paper. Even if only the back surface of the paper is heated, temperature of a front surface of the paper such as thick paper or tack paper does not sufficiently rise, resulting in fixing under offset. Tack paper used for label paper is formed of a base material, adhesive, and release paper in that order from a front-surface side, and thus is thick. Therefore, even if the tack paper is heated from a release paper side on the back surface, the heat is difficult to be transmitted to a base material side. If an amount of heating from a back-surface side of the paper is increased to such an extent that fixing under offset does not occur, moisture contained in the paper is vaporized, and an air bubble is generated in a toner layer at a time of fixing, resulting in image unevenness (toner blister). In addition, in a case of coated paper, the paper may be swollen due to separation of surface coating and a paper base material. In a case of tack paper, the paper may be swollen due to separation of a base material and release paper (paper blister).

If, between the transferrer and the fixer, the paper pre-heating member is brought into contact with the paper, disturbance of an unfixed toner image occurs. In particular, if the paper pre-heating member that slides on paper is used, toner is more easily scattered. Therefore, there has been a problem that disturbance of an unfixed toner image may occur. <CIT> discloses an image forming device to secure satisfactory fixing performance. A preheating device is arranged in front of a secondary transfer part to preheat a transfer material to <NUM> deg. A controller controls preheating amount from the leading edge to the trailing edge of the transfer material. <CIT> discloses a sheet clamping and conveying unit and an image forming apparatus. The sheet clamping and conveying unit is installed in the image forming apparatus together with an image forming unit that prints a toner image onto a recording sheet and a fixing unit that heats and fixes the toner image printed by the image forming unit onto the recording sheet. <CIT> discloses a fixing device including, a transfer section for transferring a toner to a recording medium, a heating section provided at a downstream side of the recording medium from the transfer section, for heating the toner transferred in the transfer section until the toner becomes a softened state or a fluidized state, and a pressing and conveying section provided at a downstream side of the recording medium from the heating section and conveying the recording medium while pressurizing, with a pressure roller, a surface of the recording medium on which the toner is attached. <CIT> discloses an image forming apparatus including an image former, a fixing device, an image forming path, a circulation path and a hardware processor. Along the image forming path, paper is conveyed to the image former and the fixing device. The circulation path diverges from the image forming path on a downstream side of the fixing device in a paper conveying direction and meets the image forming path on an upstream side of the image former in the paper conveying direction so as to circulate the paper such that an image forming side of the paper is unchanged. The hardware processor performs paper heating control to make the paper pass through the fixing device without image forming on the paper, thereby heating the paper, and re-convey the paper via the circulation path to the image former. <CIT> discloses an image forming apparatus includes a fixing unit including at least a pair of rollers which thermally fix a toner image formed on a surface of a recording medium by pressing against the recording medium, and a separating and contacting mechanism supporting the pair of rollers so that the rollers can separate from and make contact with the recording medium, where the separating and contacting mechanism controls the rollers when contacting the rollers to the recording medium, by contacting each of the rollers approximately perpendicularly to the surface of the recording medium and so that the recording medium thereafter makes contact with a surface of each of the rollers for a wrap-around angle greater than <NUM> DEG.

An object of the present invention is to provide an image formation apparatus and image formation method capable of reducing occurrence of image formation failure.

To achieve the abovementioned object, the present invention provides an image formation apparatus according to claim <NUM>. According to a second aspect, the present invention provides an image formation method according to claim <NUM>. Further aspects of the present invention are set forth in the dependent claims, the drawings, and the following description. Further, according to an aspect of the present invention, an image formation apparatus reflecting one aspect of the present invention comprises.

The advantages and features provided by one or more embodiments of the invention will become more fully understood from the detailed description given hereinbelow and the appended drawings which are given by way of illustration only, and thus are not intended as a definition of the limits of the present invention:.

Hereinafter, one or more embodiments of the present invention will be described with reference to the drawings. However, the scope of the invention is not limited to the disclosed embodiments. <FIG> is a diagram schematically showing an overall configuration of an image formation apparatus <NUM> according to an embodiment of the present invention. <FIG> is a diagram showing main units of a control system of the image formation apparatus <NUM> according to the present embodiment. The image formation apparatus <NUM> is an intermediate transfer type color image formation apparatus utilizing electrophotographic process technology. That is, the image formation apparatus <NUM> primarily transfers toner images of colors of yellow (Y), magenta (M), cyan (C), and black (K), which are formed on respective photoreceptor drums <NUM>, onto an intermediate transfer belt <NUM>, superimposes the toner images of the four colors on the intermediate transfer belt <NUM>, and then secondarily transfers the superimposed toner images onto paper, thereby forming a toner image.

In addition, the image formation apparatus <NUM> adopts a tandem system in which the photoreceptor drums <NUM> corresponding to the four colors of YMCK are arranged in series in a running direction of the intermediate transfer belt <NUM>, and the toner images of the respective colors are sequentially transferred onto the intermediate transfer belt <NUM> with one procedure.

As shown in <FIG>, the image formation apparatus <NUM> includes an image reader <NUM>, an operation display <NUM>, an image processor <NUM>, an image former <NUM>, a paper conveyor <NUM>, a fixer <NUM>, a paper pre-heater <NUM>, a controller <NUM>, and the like.

The controller <NUM> includes a central processing unit (CPU) <NUM>, a read only memory (ROM) <NUM>, a random access memory (RAM) <NUM>, and the like. The CPU <NUM> reads, from the ROM <NUM>, a program corresponding to processing content, loads the program into the RAM <NUM>, and centrally controls operation of each block of the image formation apparatus <NUM> in cooperation with the loaded program. At this time, various data stored in a storage <NUM> are referred to. The storage <NUM> includes, for example, a non-volatile semiconductor memory (so-called flash memory) or a hard disk drive.

Via a communicator <NUM>, the controller <NUM> transmits and receives various data to and from an external apparatus (for example, a personal computer) connected to a communication network such as a local area network (LAN) or a wide area network (WAN). For example, the controller <NUM> receives image data transmitted from the external apparatus, and forms a toner image on the paper on the basis of the image data (input image data). The communicator <NUM> includes, for example, a communication control card such as a LAN card.

The image reader <NUM> includes an automatic document feeder <NUM>, which is referred to as an auto document feeder (ADF), a document image scanner (scanner) <NUM>, and the like.

With a conveyance mechanism, the automatic document feeder <NUM> conveys a document placed on a document tray, and sends the document to the document image scanner <NUM>. The automatic document feeder <NUM> can continuously read images (including both sides) of a large number of documents placed on the document tray at once.

The document image scanner <NUM> optically scans a document conveyed onto a contact glass from the automatic document feeder <NUM>, or a document placed on the contact glass, forms, on a light receiving surface of a charge coupled device (CCD) sensor (not shown), an image of light reflected from the document, and reads a document image. The image reader <NUM> generates input image data on the basis of a result of reading by the document image scanner <NUM>. The input image data is subjected to predetermined image processing in the image processor <NUM>.

The operation display <NUM> includes, for example, a liquid crystal display (LCD) with a touch panel, and functions as a display <NUM> and an operation unit <NUM>. The display <NUM> displays various operation screens, an image state, an operation status of each function, and the like according to a display control signal input from the controller <NUM>. The operation unit <NUM> includes various operation keys such as a numeric keypad and a start key, receives various input operations by a user, and outputs an operation signal to the controller <NUM>. The input operations includes operation of selecting any one of a normal mode, a low-gloss mode in which a gloss level is lower than a predetermined value (value of the normal mode), and a high-gloss mode in which the gloss level is higher than the predetermined value.

The image processor <NUM> includes a circuit or the like that performs digital image processing on the input image data according to an initial setting or user setting. For example, the image processor <NUM> performs gradation correction on the basis of gradation correction data (gradation correction table) under control of the controller <NUM>. In addition to the gradation correction, on the input image data, the image processor <NUM> performs various kinds of correction processing such as color correction and shading correction, compression processing, and the like. The image former <NUM> is controlled on the basis of the image data subjected to these processings.

The image former <NUM> includes image formation units 41Y, <NUM>, 41C, and <NUM>, an intermediate transfer unit <NUM>, and the like. The image formation units 41Y, <NUM>, 41C, and <NUM> are for forming, on the basis of the input image data, images with colored toners of a Y component, an M component, a C component, and a K component, respectively.

The image formation units 41Y, <NUM>, 41C, and <NUM> for the Y component, M component, C component, and K component, respectively, have a similar configuration. For convenience of illustration and description, common constituents are denoted by the same reference signs, and Y, M, C, or K is added to a reference sign when distinguishing each component. In <FIG>, only a constituent of the image formation unit 41Y for the Y component is denoted by a reference sign, and reference signs for constituents of the other image formation units <NUM>, 41C, and <NUM> are omitted.

An image formation unit <NUM> includes an exposurer (not shown), a developer (not shown), a photoreceptor drum <NUM>, an electric charger (not shown), a drum cleaner (not shown), and the like.

The photoreceptor drum <NUM> is, for example, a negatively charged organic photo-conductor (OPC) in which an under coat layer (UCL), a charge generation layer (CGL), and a charge transport layer (CTL) are sequentially laminated on a peripheral surface of an aluminum conductive cylindrical body (aluminum tube). The charge generation layer includes an organic semiconductor obtained by dispersing a charge generation material (polycarbonate, for example) in a resin binder (for example, polycarbonate), and generates a pair of positive charge and negative charge with exposure by the exposurer. The charge transport layer is formed by a resin binder (polycarbonate resin, for example) in which a hole-transport material (electron-donating nitrogen-containing compound) is dispersed, and transports a positive charge generated in the charge generation layer to a surface of the charge transport layer.

The controller <NUM> rotates the photoreceptor drums <NUM> at a constant peripheral speed by controlling drive current supplied to a drive motor (not shown) that rotates the photoreceptor drums <NUM>.

The electric charger uniformly charges a surface of the photoreceptor drum <NUM> having photoconductivity to negative polarity. The exposurer includes, for example, a semiconductor laser, and irradiates the photoreceptor drum <NUM> with laser light corresponding to an image of each color component. The positive charge is generated in the charge generation layer of the photoreceptor drum <NUM> and is transported to the surface of the charge transport layer, by which a surface charge (negative charge) of the photoreceptor drum <NUM> is neutralized. An electrostatic latent image of each color component is formed on the surface of the photoreceptor drum <NUM> due to a potential difference from surroundings.

The developer is, for example, a two-component developer, and visualizes the electrostatic latent image by attaching the toner of each color component to the surface of the photoreceptor drum <NUM> to form a toner image.

The drum cleaner includes a drum cleaning blade or the like that comes into sliding contact with the surface of the photoreceptor drum <NUM>, and, after the primary transfer, removes transfer residual toner remaining on the surface of the photoreceptor drum <NUM>.

The intermediate transfer unit <NUM> includes the intermediate transfer belt <NUM>, a primary transfer roller (not shown), a plurality of support rollers (not shown), a secondary transfer roller <NUM>, a belt cleaner (not shown), and the like.

The intermediate transfer belt <NUM> is formed by an endless belt, and is stretched in a loop shape around the plurality of support rollers. At least one of the plurality of support rollers includes a drive roller, and the other support rollers include a driven roller. For example, the drive roller is preferably a roller disposed on a downstream side of the primary transfer roller for the K component in a belt running direction. Thus, running speed of the belt in a primary transferrer can be easily kept constant. As the drive roller rotates, the intermediate transfer belt <NUM> runs at a constant speed in a direction of the arrow A.

The primary transfer roller is disposed on an inner peripheral surface side of the intermediate transfer belt <NUM> so as to face the photoreceptor drums <NUM> of the respective color components. Primary transfer nips for transferring the toner images from the photoreceptor drums <NUM> to the intermediate transfer belt <NUM> are formed by the primary transfer rollers being pressed against the respective photoreceptor drums <NUM> with the intermediate transfer belt <NUM> interposed therebetween.

The secondary transfer roller <NUM> is disposed on an outer peripheral surface side of the intermediate transfer belt <NUM> while facing a backup roller 423B disposed on the downstream side of the drive roller in the belt running direction. A secondary transfer nip for transferring the toner images from the intermediate transfer belt <NUM> onto the paper is formed by the secondary transfer roller <NUM> being pressed against a backup roller (not shown) with the intermediate transfer belt <NUM> interposed therebetween.

When the intermediate transfer belt <NUM> passes through the primary transfer nips, the toner images on the photoreceptor drums <NUM> are sequentially superimposed and primarily transferred onto the intermediate transfer belt <NUM>. Specifically, primary transfer bias is applied to the primary transfer rollers, and charge with polarity opposite to polarity of the toner is applied to a back-surface side of the intermediate transfer belt <NUM> (side in contact with the primary transfer roller), by which the toner images are electrostatically transferred onto the intermediate transfer belt <NUM>.

Thereafter, when the paper passes through the secondary transfer nip, the toner images on the intermediate transfer belt <NUM> are secondarily transferred onto the paper. Specifically, secondary transfer bias is applied to the secondary transfer roller <NUM>, and charge with polarity opposite to polarity of the toner is applied to a back-surface side of the paper (side in contact with the secondary transfer roller <NUM>), by which the toner images are electrostatically transferred onto the paper. The paper onto which the toner images have been transferred is conveyed toward the fixer <NUM>.

The belt cleaner (not shown) includes a belt cleaning blade or the like that comes into sliding contact with a surface of the intermediate transfer belt <NUM>, and, after the secondary transfer, removes transfer residual toner remaining on the surface of the intermediate transfer belt <NUM>. Note that, instead of the secondary transfer roller <NUM>, a secondary transfer belt may be adopted by being stretched in a loop shape around the plurality of support rollers including a secondary transfer roller (so-called belt-type secondary transfer unit).

The fixer <NUM> includes an upper fixer 60A having fixing-surface-side members disposed on a fixing surface (front surface on which the toner images are formed) side of the paper, a lower fixer 60B having a back-surface-side support member disposed on a back surface (surface opposite to the fixing surface) side of the paper, a heating source 60C, and the like. When the back-surface-side support member is pressed against a fixing-surface-side member, a fixing nip that nips and conveys the paper is formed.

After the toner images are secondarily transferred onto the paper, the fixer <NUM> fixes the toner images on the conveyed paper by heating and pressurizing the paper with the fixing nip. The fixer <NUM> is disposed as a unit in a fixing device, that is, a housing.

The paper conveyor <NUM> includes a paper feeder <NUM>, a paper ejector <NUM>, a conveyance path unit <NUM>, and the like. The paper feeder <NUM> includes a delivery roller 51a that delivers, to a downstream side, the paper (continuous paper) wound in a roll.

The paper delivered to the downstream side of the paper feeder <NUM> is conveyed to the image former <NUM> by the conveyance path unit <NUM> via the paper pre-heater <NUM> (described later). In the image former <NUM>, the toner images on the intermediate transfer belt <NUM> are secondarily transferred collectively onto one surface of the paper, and a fixing process is performed in the fixer <NUM>. The paper ejector <NUM> includes a winding roller 52a that winds, into a roll, the paper on which an image is formed. The conveyance path unit <NUM> includes conveyance rollers 53a that hold and convey paper. Note that the conveyance path unit <NUM> may include a paper tension application roller (not shown) that applies tension to the paper and a skew adjustment roller (not shown) that adjusts skew of the paper.

Next, a configuration of the fixer <NUM> will be described in more detail with reference to <FIG> is a diagram schematically showing the fixer <NUM>. In the present embodiment, a fixing method is limited to a belt fixing method. However, the fixing method is not limited to the belt fixing method in the present invention.

The upper fixer 60A includes a fixing belt <NUM> having an endless shape, a heating roller <NUM>, and an upper pressure roller <NUM> (belt heating type), which are the fixing-surface-side members. The fixing belt <NUM> is stretched between the heating roller <NUM> and the upper pressure roller <NUM> under a predetermined tension.

A main body of the fixing belt <NUM> includes polyimide (PI), for example. An outer peripheral surface of the main body is covered with heat-resistant silicone rubber and serves as an elastic layer. Further, a surface layer of the fixing belt <NUM> is covered or coated with a tube of perfluoroalkoxy (PFA) that is heat-resistant resin.

The fixing belt <NUM> comes into contact with the paper on which a toner image is formed, and heats and fixes the toner image on the paper in a predetermined temperature range.

The heating roller <NUM> heats the fixing belt <NUM>. The heating roller <NUM> incorporates the heating source 60C that is, for example, a halogen heater that heats the fixing belt <NUM>. The heating roller <NUM> has a configuration in which an outer peripheral surface at a cylindrical core metal formed of aluminum or the like is covered with a resin layer coated with PTFE.

Temperature of the heating source 60C is controlled by the controller <NUM>. The heating roller <NUM> is heated by the heating source 60C, and as a result, the fixing belt <NUM> is heated. Thus, the toner formed on the paper is heated. The controller <NUM> controls toner fixing temperature by controlling the heating source 60C on the basis of, for example, an on/off pattern in a predetermined duty ratio in half-wave units.

The upper pressure roller <NUM> is formed by covering, with an elastic layer, a solid core metal formed of metal such as iron, for example. As a material of the elastic layer, for example, heat-resistant silicone rubber can be used. In addition, as the elastic layer, the heat-resistant silicone rubber can be covered with a resin layer coated with PTFE that is heat-resistant low friction resin.

The lower fixer 60B includes a lower pressure roller <NUM> that constitutes a back-surface-side support member (roller pressure method). The lower pressure roller <NUM> is formed by covering, with an elastic layer, an outer peripheral surface of a base material layer including aluminum (Al). As a material of the elastic layer, for example, heat-resistant silicone rubber can be used. In addition, as the elastic layer, the heat-resistant silicone rubber can be covered with a resin layer of a PFA tube as a surface release layer.

A heating source such as a halogen heater may be incorporated in the lower pressure roller <NUM>. The lower pressure roller <NUM> is heated by the heating source generating heat. Thus, the toner formed on the paper is heated. The controller <NUM> may control toner fixing temperature by controlling the heating source on the basis of, for example, an on/off pattern in a predetermined duty ratio in half-wave units.

The lower pressure roller <NUM> is pressed against the upper pressure roller <NUM> at a predetermined fixing load via the fixing belt <NUM>. In this manner, a fixing nip NP that nips and conveys the paper is formed between the upper pressure roller <NUM> and the fixing belt <NUM>, and the lower pressure roller <NUM>.

The lower pressure roller <NUM> is connected to a motor, a gear, or the like (not shown), and drive force of the motor is transmitted to the lower pressure roller <NUM>. The controller <NUM> outputs a drive signal to the motor that drives the lower pressure roller <NUM> to control peripheral speed of the lower pressure roller <NUM>.

In the fixer <NUM>, the upper fixer 60A, the lower fixer 60B, and the heating source 60C fix an unfixed toner image on the paper by conveying the paper while heating and pressurizing the paper with the fixing nip NP.

Meanwhile, in a case where the paper is thick paper, tack paper, or the like for example, temperature of the front surface of the paper does not sufficiently rise, and fixing under offset may occur, even if, between the intermediate transfer unit <NUM> and the fixer <NUM>, to heat the paper, the paper pre-heating member is brought into contact with the back surface of the paper. If an amount of heating from the back-surface side is increased so that fixing under offset does not occur, image unevenness due to toner blister may occur at a time of the fixing, or paper blister may occur. If, between the intermediate transfer unit <NUM> and the fixer <NUM>, the paper pre-heating member is brought into contact with the paper, disturbance of an unfixed toner image may occur.

In the present embodiment, in order to reduce occurrence of image formation failure such as fixing under offset, image unevenness, disturbance of an unfixed toner image, or the like, the paper pre-heater <NUM> is disposed on an upstream side of the intermediate transfer unit <NUM> (hereinafter, simply referred to as "upstream side") in the paper conveyance direction, and is disposed on a downstream side of the paper feeder <NUM> (hereinafter, simply referred to as "downstream side") in the paper conveyance direction.

<FIG> is an enlarged view showing an example of a configuration of the paper pre-heater <NUM> according to the present embodiment. As shown in <FIG>, the paper pre-heater <NUM> includes a front-surface-side roller <NUM> and a back-surface-side roller <NUM>. The front-surface-side roller <NUM> is disposed on a downstream side of a back-surface-side roller <NUM>. Here, the front surface of the paper corresponds to an "image formation surface" in the present invention.

The front-surface-side roller <NUM> is disposed so as to come into contact with the front surface of the paper. The front-surface-side roller <NUM> incorporates a front-surface-side heating source <NUM> (a halogen heater, for example) that heats the front-surface side of the paper. A diameter of the front-surface-side roller <NUM> is larger than a diameter of the back-surface-side roller <NUM>. A front-surface-side roller temperature sensor <NUM> (described later) that detects temperature of the front-surface-side roller <NUM> is disposed in a vicinity of the front-surface-side roller <NUM>.

The back-surface-side roller <NUM> is disposed so as to come into contact with the back surface of the paper. The back-surface-side roller <NUM> incorporates a back-surface-side heating source <NUM> (a halogen heater, for example) that heats the back-surface side of the paper. The back-surface-side roller <NUM> is disposed at a position not facing the front-surface-side roller <NUM> with the paper interposed therebetween. Thus, a paper nip is not formed and the paper is not nipped. Therefore, deformation of the paper, such as a wrinkle is less likely to occur.

A back-surface-side roller temperature sensor <NUM> (described later) that detects temperature of the back-surface-side roller <NUM> is disposed in a vicinity of the back-surface-side roller <NUM>.

As shown in <FIG>, the paper sent out from the paper feeder <NUM> is wound around the back-surface-side roller <NUM> in a counterclockwise direction. Then, the paper is passed from the back-surface-side roller <NUM> to the front-surface-side roller <NUM>, and further wound around the front-surface-side roller <NUM> in a clockwise direction. A circumferential length L <NUM> in which the front-surface-side roller <NUM> and the front surface of the paper come into contact with each other is a length from an eleven o'clock position to six o'clock position of the front-surface-side roller <NUM> in the clockwise direction. A circumferential length L2 in which the back-surface-side roller <NUM> and the back surface of the paper come into contact with each other is a length from a twelve o'clock position to five o'clock position of the back-surface-side roller <NUM> in the counterclockwise direction. Because the diameter of the front-surface-side roller <NUM> is larger than the diameter of the back-surface-side roller <NUM>, the circumferential length L1 is equal to or greater than the circumferential length L2 (L1 ≥ L2). Thus, the temperature of the back surface of the paper heated by the back-surface-side roller <NUM> can be set lower than the temperature of the front surface of the paper heated by the front-surface-side roller <NUM>.

<FIG> is a diagram showing paper temperature at each paper position immediately after image formation starts. <FIG> is a diagram showing the paper temperature at each paper position five minutes after the image formation starts. <FIG> is a diagram showing a relationship between paper temperature and an elapsed time from the start of the image formation, for each position on a conveyance path. "paper feeding", "paper pre-heater", "conveyor", "transferrer", and "fixing inlet" shown in <FIG> indicate positions (paper positions) of the "paper feeder <NUM>", "front-surface-side roller <NUM>", "conveyance rollers 53a", "intermediate transfer unit <NUM>", and "inlet of the fixer <NUM>", respectively.

In a case where a machine and the paper are left in a low temperature environment, for example at <NUM>, for a long time, the machine and the paper are also cooled to <NUM>. If image formation is started in this low-temperature environment, the temperature of the paper at the inlet of the fixer falls to <NUM> even in a case where the paper temperature is raised to <NUM> by the paper pre-heater <NUM>, because the paper is cooled by the conveyance rollers 53a disposed between the paper pre-heater <NUM> and the intermediate transfer unit <NUM>, and by the intermediate transfer belt <NUM> (refer to <FIG>). When a predetermined time (five minutes, for example) elapses from the start of image formation, the conveyance rollers 53a and the intermediate transfer belt <NUM> are warmed by the paper. Therefore, at each of the paper positions, the paper temperature does not fall so much from the temperature of the paper pre-heater <NUM> (refer to <FIG> and <FIG>).

Therefore, in the present embodiment, after instruction for the image formation, it is determined whether or not the temperature of the fixing belt <NUM> (hereinafter, referred to as "fixing temperature") detected by a fixing-belt temperature sensor <NUM> (refer to <FIG>) is equal to or lower than a predetermined temperature Tfs (<NUM>, for example). Then, in a case where the fixing temperature is equal to or lower than the predetermined temperature Tfs, paper pre-heating processing for heating the paper is performed at a time of warming up the fixer <NUM> (warm-up time).

<FIG> is a diagram showing an example of an arrangement configuration of temperature sensors according to the present embodiment. As shown in <FIG>, a paper pre-heater outlet paper temperature sensor <NUM>, a fixer-inlet paper temperature sensor <NUM>, the fixing-belt temperature sensor <NUM>, an intermediate-transfer-belt temperature sensor <NUM>, the front-surface-side roller temperature sensor <NUM>, and the back-surface-side roller temperature sensor <NUM> are disposed at predetermined positions of the image formation apparatus <NUM>.

The paper pre-heater outlet paper temperature sensor <NUM> is a contactless temperature sensor, and detects the temperature of the front surface of the paper at an outlet of the paper pre-heater <NUM>. The fixer-inlet paper temperature sensor <NUM> is a contactless temperature sensor, and detects the temperature of the front surface of the paper at the inlet of the fixer <NUM>.

The fixing-belt temperature sensor <NUM> detects the temperature of the fixing belt <NUM> (fixing temperature). The intermediate-transfer-belt temperature sensor <NUM> detects temperature of the intermediate transfer belt <NUM>. The front-surface-side roller temperature sensor <NUM> detects the temperature of the front-surface-side roller <NUM>. The back-surface-side roller temperature sensor <NUM> detects the temperature of the back-surface-side roller <NUM>. The controller <NUM> acquires respective detection results from the paper pre-heater outlet paper temperature sensor <NUM>, the fixer-inlet paper temperature sensor <NUM>, the fixing-belt temperature sensor <NUM>, the intermediate-transfer-belt temperature sensor <NUM>, the front-surface-side roller temperature sensor <NUM>, and the back-surface-side roller temperature sensor <NUM>.

In a case where the fixing temperature is equal to or lower than the predetermined temperature Tfs, the controller <NUM> executes paper pre-heating control and warm-up control for the fixer <NUM> in parallel. For example, the paper pre-heating control includes (<NUM>) transfer press control, (<NUM>) control of the paper conveyor <NUM>, and (<NUM>) lighting control for each of the front-surface-side heating source <NUM> and the back-surface-side heating source <NUM>. The warm-up control for the fixer <NUM> includes (<NUM>) drive speed control for the upper pressure roller <NUM> and the lower pressure roller <NUM>, and (<NUM>) lighting control for the heating source 60C.

The controller <NUM> executes transfer press control. Thus, the secondary transfer roller <NUM> is pressed against the backup roller 423B with the intermediate transfer belt <NUM> interposed therebetween. As a result, the paper conveyed from the paper feeder <NUM> via the conveyance rollers 53a can be brought into contact with the intermediate transfer belt <NUM> and the secondary transfer roller <NUM>.

The controller <NUM> controls the paper conveyor <NUM> so that a paper conveyance speed at a time of paper pre-heating is a predetermined speed WVph. Note that the predetermined speed WVph at the time of paper pre-heating is set to a speed lower than a paper conveyance speed VP at a time of the image formation. Thus, the paper is conveyed at a low speed. Therefore, waste paper can be reduced, and the paper can be efficiently heated.

In a case where the temperature of the front surface of the paper at the inlet of the fixer <NUM> is equal to or higher than a predetermined temperature WTpa2, the temperature of the front surface of the paper being detected by the fixer-inlet paper temperature sensor <NUM>, the controller <NUM> controls the paper conveyor <NUM> to stop conveyance of the paper. Thus, the intermediate transfer belt <NUM>, the secondary transfer roller <NUM>, and the conveyance rollers 53a, which come in contact with the paper on the upstream side of the fixer <NUM>, are sufficiently warmed. As a result, it is possible to reduce a chance of a decrease in the paper temperature immediately after the start of the image formation switched from warm-up (paper pre-heating).

The controller <NUM> performs lighting control for the front-surface-side heating source <NUM> so that the temperature of the front-surface-side roller <NUM> is at a predetermined temperature WTph1, and performs lighting control for the back-surface-side heating source <NUM> so that the temperature of the back-surface-side roller <NUM> is at a predetermined temperature WTph2. The predetermined temperature WTph1 is set to a temperature higher than the predetermined temperature WTph2. Thus, in heating the paper, the temperature of the back surface of the paper can be set lower than the temperature of the front surface of the paper.

The paper conveyed from the paper pre-heater <NUM> via the conveyance rollers 53a warms the conveyance rollers 53a and the intermediate transfer belt <NUM>. However, if the temperature of the intermediate transfer belt <NUM> is too high, cleaned toner is welded. The controller <NUM> performs lighting control for the front-surface-side heating source <NUM> and the back-surface-side heating source <NUM> so that temperature of the intermediate transfer belt <NUM> detected by the intermediate-transfer-belt temperature sensor <NUM> does not exceed a predetermined temperature Tbs. Thus, it is possible to reduce a chance of welded toner.

In a case where the temperature of the front surface of the paper at the inlet of the fixer <NUM> is equal to or higher than a predetermined temperature WTpa2, the temperature of the front surface of the paper being detected by the fixer-inlet paper temperature sensor <NUM>, the controller <NUM> turns off lighting of the front-surface-side heating source <NUM> and the back-surface-side heating source <NUM>.

Next, the warm-up control for the fixer <NUM> will be described.

The controller <NUM> performs control so that drive speeds of the upper pressure roller <NUM> and the lower pressure roller <NUM> are at a predetermined speed WVf.

The controller <NUM> performs lighting control for the heating source 60C so that the fixing temperature is at a predetermined temperature WTf. In a case where the fixing temperature is equal to or higher than the predetermined temperature WTf, the controller <NUM> determines whether or not an elapsed time from the start of the warm-up control for the fixer <NUM> (warm-up time) is equal to or longer than a predetermined time TMw. In a case where the warm-up time is equal to or longer than the predetermined time TMw, the controller <NUM> ends the warm-up control for the fixer <NUM>. Thus, the fixing temperature is set to a predetermined temperature PTf.

As described above, in a case where the fixing temperature is equal to or lower than the predetermined temperature Tfs, the controller <NUM> executes the paper pre-heating control and the warm-up control for the fixer <NUM> in parallel. Thereafter, the controller <NUM> executes image formation control.

Next, an example of operation of the image formation apparatus <NUM> according to the present embodiment will be described with reference to <FIG> is a flowchart showing an example of operation of the image formation apparatus <NUM> according to the present embodiment. This flow is started by an image formation instruction input to the image formation apparatus <NUM>. In <FIG>, the temperature of the front-surface-side roller <NUM> is shown as "pre-heating front". The temperature of the back-surface-side roller <NUM> is shown as "pre-heating back".

First, in step S100, the controller <NUM> determines whether or not the fixing temperature is equal to or lower than the predetermined temperature Tfs (<NUM>, for example).

In a case where the fixing temperature is equal to or lower than the predetermined temperature Tfs (step S100: YES), the paper pre-heating control (steps S110 to S160) and the warm-up control for the fixer <NUM> (steps S200 to S240) are performed in parallel.

In step S110, the controller <NUM> executes transfer press control. Thus, the paper can be brought into contact with the intermediate transfer belt <NUM> and the secondary transfer roller <NUM>.

In step S120, the controller <NUM> controls the paper conveyor <NUM> so that the paper conveyance speed is at the predetermined speed WVph.

In step S130, the controller <NUM> controls the front-surface-side heating source <NUM> so that the temperature of the front-surface-side roller <NUM> is at the predetermined temperature WTph1, and controls the back-surface-side heating source <NUM> so that the temperature of the back-surface-side roller <NUM> is at the predetermined temperature WTph2.

Next, in step S140, the controller <NUM> determines whether or not the paper temperature at the inlet of the fixer <NUM> (fixer-inlet paper temperature shown in <FIG>) is equal to or higher than the predetermined temperature WTpa2. In a case where the paper temperature is equal to or higher than the predetermined temperature WTpa2 (step S140: YES), the processing proceeds to step S150. In a case where the paper temperature is lower than the predetermined temperature WTpa2 (step S140: NO), the processing returns to step before step S130.

In step S150, the controller <NUM> turns off the front-surface-side heating source <NUM> and the back-surface-side heating source <NUM>.

Next, in step S160, the controller <NUM> controls the paper conveyor <NUM> so that the conveyance of the paper is stopped. Thereafter, the image formation is started. At the time of the image formation, a conveyance speed in paper pre-heating is controlled to the same conveyance speed as a conveyance speed in the image formation. In addition, the front-surface-side heating source <NUM> and the back-surface-side heating source <NUM> are controlled so that the temperatures of the front-surface-side roller <NUM> and back-surface-side roller <NUM> are set to PTph1 and PTph2, respectively.

The controller <NUM> executes the following control in parallel with the above paper pre-heating control (steps S110 to S160). In step S200, the controller <NUM> performs control so that drive speeds of the upper pressure roller <NUM> and the lower pressure roller <NUM> are at the predetermined speed WVf.

Next, in step S210, the controller <NUM> performs lighting control for the heating source 60C so that the fixing temperature is at a predetermined temperature WTf.

Next, in step S220, the controller <NUM> determines whether or not the fixing temperature is equal to or higher than the predetermined temperature WTf. In a case where the fixing temperature is equal to or higher than the predetermined temperature WTf (step S220: YES), the processing proceeds to step S230. In a case where the fixing temperature is lower than the predetermined temperature WTf (step S220: NO), the processing returns to step before step S210.

In step S230, the controller <NUM> determines whether or not an elapsed time from start of the warm-up (warm-up time) is equal to or longer than the predetermined time TMw. In a case where the warm-up time is equal to or longer than the predetermined time TMw (step S230: YES), the processing proceeds to step S240. In a case where the warm-up time is shorter than the predetermined time TMw (step S230: NO), the processing returns to step before step S210.

In step S240, the fixing temperature is set to the predetermined temperature PTf. Thereafter, if the paper pre-heating is at step S160, the image formation is started. If the paper pre-heating has not yet reached step S100, the image formation is started after step S160 is reached.

The image formation apparatus <NUM> according to the above embodiment includes the intermediate transfer unit <NUM> that forms the toner image on the front surface of the paper, the fixer <NUM> that fixes the toner image onto the paper, the paper conveyor <NUM> that conveys the paper between an upstream side of the intermediate transfer unit <NUM> and a downstream side of the fixer <NUM> in the paper conveyance direction, and the paper pre-heater <NUM> that heats the front-surface side of the paper on the upstream side of the intermediate transfer unit <NUM> in the paper conveyance direction.

With the above configuration, the front-surface-side roller <NUM> comes into contact with the front surface of the paper on the upstream side of the intermediate transfer unit <NUM>, and the temperature of the front surface of the paper sufficiently rises. Therefore, it is possible to reduce occurrence of fixing under offset and image unevenness. In addition, because the front-surface-side roller <NUM> and the back-surface-side roller <NUM> come into contact with the paper on the upstream side of the intermediate transfer unit <NUM>, it is possible to reduce a chance of disturbance of an unfixed toner image.

In the image formation apparatus <NUM> according to the above embodiment, the paper pre-heater <NUM> heats the front-surface side of the paper at time of warm-up of the fixer. By effectively using time for conventionally performed fixing warm-up as a time for paper pre-heating, it is possible to reduce a chance of a decrease in productivity.

In the image formation apparatus <NUM> according to the above embodiment, a paper conveyance speed at the time of warm-up is lower than a paper conveyance speed at the time of the image formation in which the toner image is formed on the front surface of the paper. Thus, it is possible to shorten length of waste paper.

In the image formation apparatus <NUM> according to the above embodiment, the diameter of the front-surface-side roller <NUM> is larger than the diameter of the back-surface-side roller <NUM>. Thus, the circumferential length L1 in which the front-surface-side roller <NUM> is in contact with the front surface of the paper is equal to or longer than the circumferential length L2 in which the back-surface-side roller <NUM> is in contact with the back surface of the paper. Thus, the temperature of the back surface of the paper heated by the back-surface-side roller <NUM> can be set lower than the temperature of the front surface of the paper heated by the front-surface-side roller <NUM>. As a result, the temperature on the back-surface side of the paper can remain low. Therefore, the paper can be efficiently heated while reducing a chance of toner blister or paper blister.

In the image formation apparatus <NUM> according to the above embodiment, the paper pre-heater <NUM> includes the front-surface-side roller <NUM> disposed so as to come into contact with the front surface of the paper, and the back-surface-side roller <NUM> disposed, at a position not facing the front-surface-side roller <NUM> with the paper interposed therebetween so as to come into contact with the back surface of the paper. Thus, the paper is not sandwiched between the front-surface-side roller <NUM> and the back-surface-side roller <NUM>. Therefore, it is possible to reduce a chance of deformation of the paper, such as a wrinkle.

In the image formation apparatus <NUM> according to the above embodiment, an amount the paper pre-heater <NUM> heats the front-surface side of the paper is equal to or larger than an amount the paper pre-heater <NUM> heats the back-surface side of the paper. Thus, the front-surface side of the paper can be efficiently heated. In addition, the temperature on the back-surface side does not rise excessively. Therefore, it is possible to reduce a chance of toner blister or paper blister.

Next, the image formation apparatus <NUM> according to a modification of the present embodiment will be described with reference to <FIG> is a diagram showing a predetermined distance of repeatedly conveyed paper in the image formation apparatus <NUM> according to a first modification, at the time of the paper pre-heating. In the above embodiment, in order to shorten the length of the waste paper, for example, the paper conveyance speed at the time of the warm-up (at the time of the paper pre-heating executed in parallel) is set slower than the paper conveyance speed at the time of the image formation. Meanwhile, in the first modification, in order to shorten the length of the waste paper, the paper is repeatedly conveyed at the time of the paper pre-heating. Specifically, the paper conveyor <NUM> conveys the paper, which is continuous paper, in a reciprocating manner for the predetermined distance or longer at the time of paper pre-heating. In addition to the paper pre-heater, the fixer also heats the paper. With this arrangement, the length of the waste paper can be further shortened. The predetermined distance of the conveyance in the reciprocating manner is determined by a positional relationship among the paper pre-heater, the fixer, an intermediate transferrer, and the conveyor.

The predetermined distance is set as follows. First, a distance D1 between the front-surface-side roller <NUM> and the intermediate transfer unit <NUM> is compared with a distance D2 between the fixer <NUM> and a conveyance roller 53a on an uppermost stream. Here, the "the conveyance roller on the uppermost stream" refers to a conveyance roller positioned on an uppermost stream side, among a plurality of conveyance rollers disposed between the fixer <NUM> and the front-surface-side roller <NUM>.

Next, in a case where the distance D1 is equal to or longer than the distance D2 (D1 ≥ D2), if the predetermined distance is set to the shorter distance D2, the paper heated by the front-surface-side roller <NUM> does not reach the intermediate transfer unit <NUM>. Therefore, the intermediate transfer unit <NUM> is not warmed by the paper heated by the front-surface-side roller <NUM>. Meanwhile, the paper heated by the fixer <NUM> is warmed. If the predetermined distance is set to the longer distance D1 or longer, the paper heated by the front-surface-side roller <NUM> reaches the intermediate transfer unit <NUM>. Therefore, the intermediate transfer unit <NUM> is warmed by the paper heated by the front-surface-side roller <NUM>. The paper is also warmed by the paper heated by the fixer <NUM>. As a result, the intermediate transfer unit <NUM> and the conveyance rollers 53a can be more efficiently warmed, the length of the waste paper can be shortened, and the paper pre-heating time at the time of warm-up can be shortened. Then, even if an image is formed in a state where the machine or the paper is left at a low temperature, the paper is conveyed to the fixer <NUM> without the temperature of the paper falling at the intermediate transfer unit <NUM>. Therefore, it is possible to reduce occurrence of fixing under offset.

Next, in a case where the distance D1 is shorter than the distance D2 (D1 < D2), if the predetermined distance is set to the shorter distance D1, the paper heated by the front-surface-side roller <NUM> reaches the intermediate transfer unit <NUM>. However, the paper heated by the fixer <NUM> does not reach the intermediate transfer unit <NUM> or a conveyance roller 53a. If the predetermined distance is set to the longer distance D2 or longer, the intermediate transfer unit <NUM> and the conveyance rollers 53a are warmed by the paper heated by both the front-surface-side roller <NUM> and the fixer <NUM>.

In the above embodiment, in a case where the paper pre-heating is executed at the time of warm-up, the fixer <NUM> does not heat the front-surface side of the paper. Meanwhile, in the first modification, the front-surface side of the paper is heated. Specifically, the fixer <NUM> presses the paper at the time of the warm-up, and heats and conveys the paper with the fixing nip NP. Thus, the paper is heated by both the paper pre-heater <NUM> and the fixer <NUM>. Therefore, it is possible to warm the intermediate transfer unit <NUM> and the conveyance rollers 53a more efficiently. Note that the fixer <NUM> heats the paper at a reciprocation cycle the same as a reciprocation cycle of the paper pre-heater <NUM>.

Next, operation of the image formation apparatus <NUM> according to the first modification will be described with reference to <FIG> is a flowchart showing an example of operation of the image formation apparatus <NUM> according to the first modification. In <FIG>, the temperature of the front-surface-side roller <NUM> is shown as "pre-heating front". The temperature of the back-surface-side roller <NUM> is shown as "pre-heating back". In an example of the operation of the image formation apparatus <NUM> shown in <FIG>, also, the paper pre-heating control and the warm-up control for the fixer <NUM> are executed in parallel. In the following description, steps that are the same as the steps shown in <FIG> will be omitted, and different steps will be mainly described.

In the paper pre-heating control, in step S120 shown in <FIG>, the controller <NUM> controls the paper conveyor <NUM> so that the paper conveyance speed is at the predetermined speed WVph. Meanwhile, in step S320 shown in <FIG>, the controller <NUM> controls the paper conveyor <NUM> so that the paper conveyance speed is at the predetermined speed WVph and that the paper is conveyed in the reciprocating manner (repeatedly conveyed) at a predetermined reciprocation cycle LR.

In step S140 shown in <FIG>, the controller <NUM> determines whether or not the fixer-inlet paper temperature is equal to or higher than the predetermined temperature WTpa2. Meanwhile, in step S340 shown in <FIG>, the controller <NUM> determines whether or not the temperature of the intermediate transfer belt <NUM> (transfer belt temperature shown in <FIG>) is equal to or higher than a predetermined temperature WTb.

Next, in the warm-up control for the fixer <NUM> in step S370 shown in <FIG>, the controller <NUM> controls the fixer <NUM> so as to convey (fix-press) the paper while heating and pressurizing the paper with the fixing nip NP.

In step S200 shown in <FIG>, the controller <NUM> performs control so that the drive speeds of the upper pressure roller <NUM> and the lower pressure roller <NUM> are at the predetermined speed WVf. Meanwhile, in step S380 shown in <FIG>, control is performed so that the drive speeds of the upper pressure roller <NUM> and the lower pressure roller <NUM> are at the predetermined speed WVf, and that the upper pressure roller <NUM> and the lower pressure roller <NUM> are driven in a reciprocating manner at the predetermined reciprocation cycle LR. In a case where it is determined in step S340 of the paper pre-heating control that the temperature of the intermediate transfer belt <NUM> (the transfer belt temperature shown in <FIG>) is equal to or higher than the predetermined temperature WTb (the paper pre-heating control is ended), the controller <NUM> performs control so as to switch from the drive in the reciprocating manner as described above to normal drive (drive in a conveyance direction), and so as to release the fixing.

Next, the image formation apparatus <NUM> according to a second modification will be described.

In the first modification, the paper repeatedly conveyed at the time of paper pre-heating is continuous paper. However, in the second modification, the paper is not limited thereto and is cut paper. In a case where the cut paper is conveyed, the paper conveyor <NUM> may also function as, for example, a circulatory conveyance path of the image formation apparatus <NUM>. The circulatory conveyance path is capable of forming images on both surfaces of the paper while circulating and conveying the paper. The circulatory conveyance path circulates and conveys the paper by returning the paper, which is conveyed from the paper pre-heater <NUM> to a downstream side of the fixer <NUM>, to an upstream side of the paper pre-heater <NUM>. The intermediate transfer unit <NUM> and the conveyance rollers 53a are disposed between the downstream side of the paper feeder <NUM> and an upstream side of the paper ejector <NUM>.

The paper conveyor <NUM> repeatedly conveys a predetermined number of sheets of the paper, which is the cut paper, at the time of the paper pre-heating. Thus, the intermediate transfer unit <NUM> and the conveyance rollers 53a are warmed by the paper. As a result, after transition from the paper pre-heating to the image formation, the paper is conveyed to the fixer <NUM> without the temperature of the paper falling at the intermediate transfer unit <NUM> or the conveyance rollers 53a. Therefore, it is possible to reduce occurrence of fixing under offset.

Next, the paper pre-heater <NUM> according to a third modification will be described with reference to <FIG> is an enlarged view showing an example of a configuration of the paper pre-heater <NUM> according to the third modification.

In the above embodiment, as shown in <FIG>, the front-surface-side roller <NUM> incorporates the front-surface-side heating source <NUM>. The back-surface-side roller <NUM> incorporates the back-surface-side heating source <NUM>. Because the diameter of the front-surface-side roller <NUM> is larger than the diameter of the back-surface-side roller <NUM>, the circumferential length L1 in which the front-surface-side roller <NUM> and the front surface of the paper come into contact with each other is equal to or greater than the circumferential length L2 in which the back-surface-side roller <NUM> and the back surface of the paper come into contact with each other (L <NUM> ≥ L2).

Meanwhile, in the third modification, as shown in <FIG>, the front-surface-side roller <NUM> incorporates the front-surface-side heating source <NUM>, but the back-surface-side roller <NUM> does not incorporate the back-surface-side heating source <NUM>. In addition, the diameter of the front-surface-side roller <NUM> is the same as the diameter of the back-surface-side roller <NUM>. Thus, the circumferential length L1 is the same as the circumferential length L2 (L1 = L2).

In the image formation apparatus <NUM> according to the third modification, only the front-surface side of the paper is heated, by which the temperature inside the paper does not rise excessively, and it is possible to reduce occurrence of fixing under offset while reducing a chance of toner blister or paper blister.

Next, the paper pre-heater <NUM> according to a fourth modification will be described with reference to <FIG> is an enlarged view showing an example of a configuration of the paper pre-heater <NUM> according to the fourth modification.

In the third modification, as shown in <FIG>, one front-surface-side roller <NUM> heats the front-surface side of the paper.

Meanwhile, in the fourth modification, as shown in <FIG>, a plurality of front-surface-side rollers <NUM> heats the front-surface side of the paper. Each of the plurality of front-surface-side rollers <NUM> incorporates the front-surface-side heating source <NUM>. The plurality of front-surface-side rollers <NUM> can efficiently heat the front-surface side of the paper.

Next, the image formation apparatus <NUM> according to a fifth modification will be described with reference to <FIG> is a diagram schematically showing the image formation apparatus <NUM> according to the fifth modification.

In the above embodiment, the paper conveyor <NUM> includes a plurality of conveyance rollers 53a disposed between the paper pre-heater <NUM> and the intermediate transfer unit <NUM> so as to come into contact with the paper.

Meanwhile, in the fifth modification, the paper conveyor <NUM> is disposed between the paper pre-heater <NUM> and the intermediate transfer unit <NUM> so as not to come into contact with the paper. Specifically, the conveyance rollers 53a to be in contact with the paper are not disposed between the paper pre-heater <NUM> and the intermediate transfer unit <NUM>. Thus, it possible to reduce a decrease in temperature of the paper heated by the paper pre-heater <NUM> and conveyed from the paper pre-heater <NUM> to the intermediate transfer unit <NUM>.

In the above embodiment and the modifications, aspects in which the paper pre-heater <NUM> is brought into contact with the paper has been described. However, the present invention is not limited thereto, and the paper pre-heater <NUM> may heat the paper without being brought into contact with the paper. In this case, for example, an infrared heater or the like is used as the paper pre-heater <NUM>.

The present invention is suitably used for an image formation apparatus that is required to reduce occurrence of image formation failure.

Claim 1:
An image formation apparatus (<NUM>) comprising:
a transferrer (<NUM>) configured to form a toner image on an image formation surface of paper;
a fixer (<NUM>) configured to fix the toner image on the paper;
a paper conveyor (<NUM>) configured to convey the paper between an upstream side of the transferrer (<NUM>) and a downstream side of the fixer (<NUM>) in a paper conveyance direction; and
a paper pre-heater (<NUM>) configured to
heat, on the upstream side of the transferrer (<NUM>) in the paper conveyance direction, the image formation surface of the paper, and
heat the image formation surface of the paper at time of warm-up of the fixer (<NUM>), wherein, at the time of the warm-up, the paper conveyor (<NUM>) is configured to convey the paper at a conveyance speed lower than a conveyance speed at a time of image formation in which the toner image is formed on the image formation surface of the paper.