Patent Publication Number: US-8971774-B2

Title: Fuser and image forming device including the same

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     The present application is related to, claims priority from and incorporates by reference Japanese Patent Application No. 2011-017247, filed on Jan. 28, 2011. 
     TECHNICAL FIELD 
     The present application relates to a fuser used in an electrographic type image forming device and the image forming device that includes the fuser. 
     BACKGROUND 
     A conventional image forming device using an electrographic method forms an electrostatic latent image that corresponds to image information by exposing a surface of a photosensitive drum using an exposure head, such as a light emitting diode (LED) and the like after uniformly charging the surface of the photosensitive drum by a charging roller. Then, a toner image is formed by electrostatically attaching a thin layer of toner on a development roller to the electrostatic image. After transferring the toner image onto a sheet carried by a carrying belt using a transfer roller, an image is formed on the sheet by fixing the toner image using a fuser. 
     This type of image forming device uses a belt heating type fuser. In such a fuser, a fusion belt formed by an endless belt is heated, and a fusion roller is pressed by a pressure application roller facing across the fusion belt, thereby forming a nip part. The carried sheet is pinched by the nip part, and the toner image is fixed onto the sheet by heat and pressure. See Japanese Laid-Open Patent Application No. 2009-151115 (paragraphs 0012-0020, 0028 and FIG. 1). 
     However, in the above-described conventional technology, because the toner image is fixed onto the sheet by pinching the sheet that has been carried, by the nip part formed by pressing the fusion roller with the pressure application roller facing across the fusion belt, there is a problem that excess reverse curling occurs on the sheet after the fusion if a temperature difference between the pressure application roller and the fusion belt and fusion roller is large at the time of fusion. 
     Such a sheet with a large amount of reverse curling causes carrying ability of the sheet after fusion and stackability of the sheet on a stacker to be reduced. The present application is made in consideration of solving the above-described problem and has an object to provide a device that suppresses the reverse curling amount at the time of fusion at the fuser. 
     SUMMARY 
     In order to solve the above subjects, a fuser of the present invention includes a first roller that includes a first elastic layer, a belt member provided on, and rotates around, the first roller, a second roller that includes a second elastic layer and that forms a nip part by pressing, through the belt member, the first roller, and a heating member that heats the belt member. A thickness of the second elastic layer of the second roller is less than a thickness of the first elastic layer of the first roller. In another view, an image forming device of the present invention includes the fuser discussed above. 
     As a result, the present application as an advantage to increase a surface temperature of the pressure application roller to a temperature needed to start printing by increasing a speed to raise the temperature of the second roller (for example, a pressure application roller) and to suppress the reverse curling amount generated on a sheet by reducing the temperature difference between the first roller (for example, a fusion roller) and the pressure application roller at the time of fusion. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is an explanatory diagram illustrating a side surface of a schematic configuration of a printer of a first embodiment. 
         FIG. 2  is an explanatory diagram illustrating a cross-section of a main part of a fuser of the first embodiment. 
         FIG. 3  is an explanatory diagram illustrating a configuration of a pressure application roller of the first embodiment. 
         FIG. 4  is an explanatory diagram illustrating a configuration of a fusion roller of the first embodiment. 
         FIG. 5  is an explanatory diagram illustrating a heater of the first embodiment. 
         FIG. 6  is an explanatory diagram illustrating a configuration of a fusion belt of the first embodiment. 
         FIG. 7  is a block diagram illustrating a control system of the printer of the first embodiment. 
         FIG. 8  is an explanatory diagram illustrating evaluation results of the pressure application roller of the first embodiment. 
         FIG. 9  is a graph illustrating a relationship, in the first embodiment, between an elastic layer thickness of the pressure application roller and a temperature to which the pressure application roller reaches at the time of turning on. 
         FIG. 10  is a graph illustrating a relationship between a heat capacity of the heat application roller and the temperature to which the pressure application roller reaches at the time of turning on in the first embodiment. 
         FIG. 11  is a graph illustrating a relationship between the temperature to which the pressure application roller reaches at the time of turning on and an amount of reverse curling in the first embodiment. 
         FIG. 12  is an explanatory diagram illustrating evaluation results of the pressure application rollers in the first embodiment with equalized flexure strength. 
         FIG. 13  is an explanatory diagram illustrating another form of the heater of the first embodiment. 
         FIG. 14  is an explanatory diagram illustrating a cross-section of a main part of the fuser of a second embodiment. 
         FIG. 15  is an explanatory diagram illustrating a configuration of a pressure member of the second embodiment. 
         FIG. 16  is an explanatory diagram illustrating evaluation results of the pressure application roller of the second embodiment. 
         FIG. 17  is a graph illustrating a relationship, in the second embodiment, between an elastic layer thickness of the pressure application roller and a temperature to which the pressure application roller reaches at the time of turning on. 
         FIG. 18  is a graph illustrating a relationship between the heat capacity of the heat application roller and the temperature to which the pressure application roller reaches at the time of turning on in the second embodiment. 
         FIG. 19  is a graph illustrating a relationship between the temperature to which the pressure application roller reaches at the time of turning on and the amount of reverse curling in the second embodiment. 
     
    
    
     DETAILED DESCRIPTION OF EMBODIMENTS 
     Embodiments of a fuser and an image forming device according to the present specification are explained below with reference to the drawings. 
     First Embodiment 
     In  FIG. 1 , reference numeral  1  is a printer as an image forming device. The printer  1  of the present embodiment is an electrographic color printer that prints color images. In the printer  1 , a sheet cassette  2  that accommodates sheets P as printing media, such as normal paper and the like, is removably installed in a lower part of a device housing of the printer  1 , and a stacker  3  on which the sheets P with images printed thereon are stacked is provided on an upper surface of the exterior part of the printer  1 . The sheet cassette  2  and the stacker  3  are connected by a sheet carrying path  4  formed in an approximately S shape as shown by a broken line in  FIG. 1  (including a top surface part of a parallel part of the later-discussed carrying belt  9 ). 
     A sheet supply mechanism that is formed by sheet supply rollers  5   a  and  5   b  and a separation piece  6  and that separates and feeds each of the sheets P from the sheet cassette  2  is provided at a connection part between the sheet carrying path  4  and the sheet cassette  2 . Carrying rollers  7  that pinch and carry each sheet P that is fed from the sheet supply mechanism and registration rollers  8  that correct diagonal traveling of and carry the sheet P carried by the carrying rollers  7  are provided on the downstream side of the sheet supply roller  5   b  in the carrying direction of the sheet P (“sheet carrying direction”). 
     A carrying belt  9  that carries the sheet P carried by the registration rollers  8  is positioned on the downstream side of the registration rollers  8  in the sheet carrying path. Above a top surface part of the paralleled part of the carrying belt  9 , a plurality of image forming parts  11  are provided along the carrying belt  9 . An exposure head  12  for forming an electrostatic latent image is provided above each image forming part  11 . On the opposite side of the top surface part of the carrying belt  9 , a transfer roller  13  is provided that transfers a toner image as a developer image formed by the image forming part  11  onto the sheet P. A fuser  14  that fixes the toner image transferred on the sheet P is provided on the downstream side of the transfer belt  9  in the sheet carrying direction. Moreover, a plurality of ejection rollers  16   a  and  16   b  that pinch and carry the sheet P ejected from the fuser  14  to the stacker  3  on a top cover  15  are arranged on the downstream side of the fuser  14  in the sheet carrying direction. 
     In the printer  1  of the present embodiment, there are four independent image forming parts  11   k ,  11   y ,  11   m  and  11   c  that accommodate toners T in black (K), yellow (Y), magenta (M) and cyan (C), respectively, as developers and that are provided in the order along the sheet carrying direction to form toner images. Because these four image forming parts  11  have the same configuration, only one image forming part  11  is explained below. 
     The image forming part  11  includes a photosensitive body, such as photosensitive drum  18 , on which an electrostatic latent image is formed by the exposure head  12 , a charging roller  19  that uniformly charges the photosensitive drum  18 , a development roller  20  that develops an image by attaching the toner T to the electrostatic latent image on the photosensitive drum  18 , a supply roller  21  that supplies the toner T to the development roller  20 , a toner cartridge  22  that accommodates the toner T of a set color, a cleaning blade  23  that removes the toner T remained on the photosensitive drum  18  after transfer by scraping off the toner T from the photosensitive drum  18 , and the like. In addition, each image forming part  11  is integrally configured and is removably installed in the printer  1 . Therefore, the top cover  15  of the printer  1  is configured to be able to open and close. 
     The exposure head  12  as an exposure device is supported by the top cover  15  and is provided above, and to face, the photosensitive drum  18 . The exposure head  12  includes a light emitting body such as light emitting diode (LED) light that emits light, laser light and the like, and forms an electrostatic latent image on the surface of the photosensitive drum  18  based on image information. The transfer roller  13  as a transfer device is provided to face the photosensitive drum  18  across the carrying belt  9  and transfers, by a transfer voltage applied thereto, the toner image formed on the photosensitive drum  18  onto the sheet P carried by the carrying belt  9 . 
     The fuser  14  of the present embodiment is a belt heating type device and is configured from a pressure application roller  30  (as the second roller) and a fusion belt unit  31  as shown in  FIG. 2 . The fusion belt unit  31  is configured from a fusion roller  32  (as the first roller), a fusion belt  33 , a heater  34 , a heater holder  35  that also functions as a guide for the fusion belt  33 , and the like. 
     The pressure application roller  30  and the fusion roller  32  of the fusion belt unit  31  are arranged to face, and parallel with, each other across the fusion belt  33 . The pressure application roller  30  presses the fusion belt unit  31  at a predetermined pressure by a pressure mechanism (not shown) provided to the pressure application roller. As a result, a nip part is formed between the fusion belt  33  and the pressure application roller  30  with a predetermined nip width in the sheet carrying direction. 
     A belt temperature sensor  36  as a belt temperature detection device configured from a thermistor or the like that slides on, and detects a temperature of, an inner circumferential surface of the fusion belt  33  is provided between the heater  34  of the fusion device  14  and the nip part and on the upstream side of, and near, the nip part in the rotational direction (clockwise direction in  FIG. 2 ) of the fusion belt  33 . In addition, a pressure application roller temperature sensor  37  as a pressure application roller temperature detection device configured from a thermistor or the like that slides on, and detects a temperature of, an outer circumferential surface of the pressure application roller  30  is provided on the upstream side of, and near, the nip part of the pressure application roller  30  in the rotational direction of the pressure application roller  30 . The fuser  14  may be integrally or removably installed to the printer  1 . 
     As shown in  FIG. 3 , the pressure application roller  30  is configured from a core shaft  30   a , a heat resistant elastic layer  30   b  as a second elastic layer, and a release layer  30   c  formed of fluorine resin or the like and is rotatably supported by a bearing (not shown). The pressure application roller  30  is driven by a drive force transmitted from a fusion motor  38  (see  FIG. 1 ) to a pressure application roller gear (not shown) provided at the core shaft  30   a  in order to rotate in a rotational direction to carry the sheet P in the sheet carrying direction shown by an arrow in  FIG. 2  (the counterclockwise direction in  FIG. 2  is referred to as a carrying rotational direction). 
     The core shaft  30   a  of the pressure application roller  30  of the present embodiment is configured from a pipe made of an aluminum material (A5052) with a thickness t 1  (maybe referred to as “core shaft thickness t 1 ”) and a length of 230 mm. A silicone rubber layer having a thickness t 2  (maybe referred to as “elastic layer thickness t 2 ”) is formed as an elastic layer  30   b  on the outer circumferential surface of the pipe. The surface of the pressure application roller  30  is covered by a perfluoroalkyl vinyl ether copolymer (PFA) resin tube, which is a type of fluorine resin, as the release layer  30   c  having a thickens of 40 μm. The pressure application roller  30  has an outer diameter of 36 mm. The thickness t 1  of the core shaft  30   a  and the thickness t 2  of the elastic layer  30   b  are discussed later. 
     As shown in  FIG. 4 , the fusion roller  32  is configured form a core shaft  32   a  formed by a pipe or shaft made of a metal such as iron, aluminum alloy and the like, and a heat resistant elastic layer  32   b , such as a silicone rubber, fluorine resin or the like, as a first elastic layer. The fusion roller  32  is rotatably supported by a bearing (not shown) and rotates together with the pressure application roller  30  in accordance with the rotation of the fusion belt  33  that is rotated together by a frictional force at the nip part due to the rotation of the pressure application roller  30 . The core shaft  32   a  of the fusion roller  32  of the present embodiment is configured from a pipe made of an aluminum material (A5052) with a diameter of 26 mm, a thickness t 3  of 1.5 mm (t 3 =1.5 mm; maybe referred to as “core shaft thickness t 3 ”) and a length of 230 mm, and a silicone rubber layer having a thickness t 4  of 5 mm (t 4 =5 mm; maybe referred to as “elastic layer thickness t 4 ”) formed as an elastic layer  32   b . The fusion roller  32  has an outer diameter of 36 mm. 
     As shown in  FIG. 5 , the heater  34  as a heating body is a sheet heater in a slender shape configured from electric insulation layer  34   b , such as a glass or the like, provided on a substrate  34   a , such as stainless steel, ceramic or the like, a resistance heating body  34   d  having an electrode  34   c  formed on the electric insulation layer, and a protective layer  34   e  protecting the resistance heating body  34   d . A material, such as nickel-chrome alloy, silver-palladium alloy and the like may be used for the resistance heating body  34   d . Moreover, a glass coating using a pressure resistant glass is applied on the protective layer  34   e . The sheet heater is disposed such that the longitudinal direction is substantially identical to an axis of the belt  33  that is a heating target. 
     The heater holder  35  is positioned distant from the fusion roller  32  on the opposite side of the pressure application roller  30  and to face the fuser roller  32 . The heater holder  35  supports the fusion belt  33  with the fusion roller  32  so that the fusion belt  33  is rotatably tensioned. The heater holder  35  is configured by a resin with high heat resistance, such as polyether ether ketone (PEEK), liquid crystal polymer (LCP) or the like. The heater  34  is fixedly supported along a longitudinal direction of, and on a top center part of, the heater holder  35  with a heat resistant adhesive or the like. 
     As shown in  FIG. 6 , the fusion belt  33  is formed by a heat resistant elastic layer  33   b , such as a silicone rubber, fluorine resin or the like, provided on an outer circumferential surface of a tubular belt base  33   a  made of a material, such as nickel, polyimide, stainless steel or the like, and a release layer  33   c  made of a fluorine resin or the like on an outer circumferential surface of the elastic layer  33   b . The fusion belt  33  rotates together with the pressure application roller  30  by the frictional force at the nip part due to the rotation of the pressure application roller  30  and is heated by the heater  34 . The fusion belt  33  of the present embodiment is an endless belt with a polyimide steel tubular member having a thickness of 50 μm as the belt base  33   a , a silicone rubber layer having a thickness of 100 μm provided as the elastic layer  33   b  and a PFA resin layer having a thickness of 30 μm formed as the release layer  33   c.    
     Moreover, regarding the inner diameter of the fusion belt  33 , the time to raise the temperature of the fusion belt  33  increases if a circumferential length of the fusion belt  33  is long, and a space would be insufficient if the circumferential length is short, causing the outer diameter of the fusion roller  32  needed for securing a nip width to be reduced. Therefore, the outer diameter of the fusion  32  is configured to 36 mm, and the inner diameter of the fusion belt  33  is configured to 45 mm. In addition, an output of the heater  34  is configured to 900 W. The pressure application roller  30  is configured to press the fusion belt  33  at 10 kgf on each side, or the total of 20 kgf, by a pressure mechanism (not shown). Moreover, for the printer  1  of the present embodiment, the print speed is configured to 30 ppm (page/minute) for A4 (portrait), and the warm-up time is configured to 30 seconds. 
     In  FIG. 7 , reference numeral  40  is a controller for the printer  1  that is connected to a host device, such as a personal computer, via a communication network (not shown). The controller  40  has a function to execute print process and the like by controlling each part in the printer  1  and a function to control the data communication with the host device. Reference numeral  41  is a memory part of the printer  1  that stores programs to be executed by the controller  40 , various data used for the programs, processing results by the controller  40  and the like. 
     Reference numeral  42  is a high voltage power source that applies voltage to the charging roller  19 , the development roller  20 , the supply roller  21 , the transfer roller  13  and the like based on a command from the controller  40 . The charging roller  19 , the development roller  20 , the supply roller  21  and the like are electrically connected to the high voltage power source  42  when the image forming part is installed in the printer  1 . 
     Reference numeral  43  is a fusion controller that supplies power for heating to the heater  34  of the fuser  14  from a power supply circuit (not shown) and rotates the pressure application roller  30  in the carrying rational direction by supplying power to the fusion motor  38  based on a command from the controller  40 . 
     In addition, a surface temperature of the fusion belt  33  detected by the belt temperature sensor  36 , a surface temperature of the pressure application roller  30  detected by the pressure application roller temperature sensor  37 , and the like are inputted to the fusion controller  43 . The controller  40  turns on and off the power supplied to the heater  34  by the fusion controller  43  based on the surface temperature of the fusion belt  33  inputted to the fusion controller  43  and controls the surface temperature of the fusion belt  33  to be maintained in a predetermined fusion temperature. 
     Operation of each part during the printing operation of the printer  1  of the present embodiment is explained below. The controller  40  of the printer  1  starts printing in accordance with a print order when the print order is received from a host device. The controller  40  then feeds the sheet P accommodated in the sheet cassette  2  to the sheet carrying path  4  by separating each sheet using the sheet supply rollers  5   a  and  5   b  and a separation piece  6  and carries sheet P to the carrying belt  9  using the carrying rollers  7  and the registration rollers  8 . 
     In parallel with this, the controller  40  applies predetermined voltage that is configured in advance to each of rollers in each image forming part  11  and the transfer roller  13  using the high voltage power source  42  and uniformly charges the surface of each photosensitive drum  18  by charging voltage applied to the charge roller  19  of each image forming part  11 . The controller  40  then causes each exposure head  12  to emit light in accordance with image information based on the print order and forms an electrostatic latent image on the surface of each photosensitive drum  18  by exposure. The controller  40  develops the electrostatic latent image on the photosensitive drum  18  by attaching to toner T supplied from the supply roller  21  onto the surface of the photosensitive drum  18  using the development roller  20  to form a toner image of the corresponding color on the surface of the photosensitive drum  18 . 
     As the sheet P is carried to the image forming part  11  by the carrying belt  9 , toner images in black, yellow, magenta and cyan are sequentially transferred onto the sheet P by transfer voltage applied to the transfer roller  13  while the sheet P passes between the transfer roller  13  and the photosensitive drums  18  in the respective image forming parts  11   k ,  11   y ,  11   m  and  11   c , and thereby a color toner image is formed. 
     As the sheet P with the toner image transferred thereon is carried to the fuser  14 , the toner image is fixed to the sheet P by the fuser  14  and is ejected and stacked to the stacker  3  on the top cover  15  by the ejection rollers  16   b  after being carried by the ejection rollers  16   a  to complete the print operation. 
     Fusion operation by the fuser  14  in this case is explained below. First, the controller rotates the fusion motor  38  using the fusion controller  43  in accordance of the start of printing in the printer  1 . The controller  40  then rotates a pressure application roller gear of the pressure application roller  30  for the fuser  14  via a drive gear array (not shown) provided in the main body of the printer  1  and causes the fusion belt  33  and the fusion roller  32  to follow and to be rotated by the frictional force at the nip part in accordance with the rotation of the pressure application roller  30 . 
     In addition, the controller  40  supplies power to the heat  34  from the power supply circuit  34  using the fusion controller  43  to generate heat and to heat the fusion belt  33  from the inner circumferential surface side. The temperature of the fusion belt  33  heated by the heater  34  is detected by the belt temperature sensor  36  and is inputted to the fusion controller  43 . The fusion controller  43  turns on and off the power that is supplied to the heater  34  from the power supply circuit based on the detected surface temperature of the fusion belt  33  to control the surface temperature of the fusion belt  33  to be maintained at the predetermined fusion temperature. 
     As the sheet P with the toner image transferred thereon is carried in a state where the surface temperature of the fusion belt  33  is maintained at the predetermined fusion temperature, the sheet P is pinched by the nip part formed by the fusion roller  32  and the pressure application roller  30  via the fusion belt  33 . Then the sheet P is heated by the fusion belt  33  at a predetermined fusion temperature and pressed by the pressure application roller  30  at a predetermined pressure. As a result, the toner image is fixed to the sheet P. 
     In addition, it is preferable that the pressure application roller  30  starts rotating without delay from the time when the heater  34  is turned on because the pressure application roller  30  of the present embodiment does not include a heat generating body. Therefore, in the present embodiment, the pressure application  30  is configured to start rotating at the time when the heater  34  is turned on. Moreover, a target temperature of the fusion belt  33  of the present embodiment is configured to 160° C., and the temperature of the fusion belt  33  is controlled to reach the fusion temperature configured from a predetermined temperature range having the target temperature as a median value at the time of executing fusion after the heater  34  is turned on. 
     For the belt heating type fuser  14  with the configuration of the present embodiment, the evaluation test indicated below was conducted by changing the thickness t 2  (see  FIG. 3 ) of the elastic layer  30   b  of the pressure application roller  30  to study a configuration for suppressing the reverse curling amount. 
     As shown in  FIG. 8 , sample pressure application rollers  30  subject for the evaluation had an outer diameter of 36 mm with a core shaft  30   a  (material: A5052) having the same thickness t 1  of 1.5 mm and an elastic layer  30   b  of various thicknesses t 2  of 2 mm, 4 mm, 6 mm and 8 mm (first to fourth samples (samples 1 to 4)). In addition, the same fusion roller  32  was paired with respective sample pressure application rollers  30 . The fusion roller  32  had an outer diameter of 36 mm and included a core shaft  32   a  (material: A5052) having a thickness of 1.5 mm and an elastic layer  32   b  having a thickness 5 mm. Also, the same fusion belt  33  having the above-described configuration was used. 
     In the print operation, the print can be started when the temperature of the fusion belt  33  reaches the fusion target temperature from the room temperature. The surface temperature of the pressure application roller  30  that is detected by the pressure roller temperature sensor  37  at this time is called a starting pressure application roller end-point temperature. 
     For the evaluation test, the fuser  14  with the sample pressure application roller  30  attached therein was installed in the printer  1 , and 50 A4-size sheets P (Oki Data Excellent Paper) were fed in the portrait orientation and were continuously printed at 30 pages-per-minute (ppm) with a printer pattern that achieves 5% toner duty after turning on the power and completing warm-up. A reverse curling amount after ejection of the first sheet P and a stacking condition of the 50 sheets P after ejection were configured as evaluation items. In addition, the evaluation was conducted under a high-temperature-high-humidity environment (HH environment), under which the sheet P after fusion becomes easily reverse-curled. 
     The reverse curling in the present explanation is a state of the sheet P where the sheet P convexly curls with the surface of the sheet P on which the toner T has been fixed facing upward. Evaluation results of each sample pressure application roller  30  according to the above-described evaluation conditions are shown in  FIG. 8 . 
     As shown in  FIG. 8 , it is observed that the starting pressure application roller end-point temperature at the time of start of printing immediately after the warm-up of the pressure application roller  30  is at a temperature at which the reverse curling amount that causes a stacking failure does not occur, when the elastic layer thickness t 2  of the pressure application roller  30  is less than the elastic layer thickness t 4  of the fusion roller  32 . 
     Explaining in more details, as shown in  FIG. 9 , the starting pressure application roller end-point temperature at the time when the fusion belt  33  reaches the fusion target temperature fusion from the from the room temperature at the time of warming up increases from 70° C. to 110° C. as the elastic layer thickness t 2  of the pressure application roller  30  decreases from 8 mm to 2 mm. Moreover, as shown in  FIG. 10 , the starting pressure application roller end-point temperature increases from 70° C. to 110° C. as a heat capacity of the pressure application roller  30  is decreased from 411 J/K to 230 J/K. 
     As shown in  FIG. 11 , the reverse curling amount of the sheet P at this time decreases from 25 mm to 8 mm as the starting pressure application roller end-point temperature increases. The stacking condition after printing 50 sheets shows no or little disarrangement when the reverse curling amount is 10 mm or less. 
     That is, if the relationship of thicknesses between the elastic layer  30   b  of the pressure application roller  30  and the elastic layer  32   b  of the fusion roller  32  is configured to
 
a. Elastic layer thickness t2 of pressure application roller&lt;Elastic layer thickness t4 of fusion roller  (1)
 
the reverse curling amount at the time of start of printing immediately after the warm-up is controlled at 10 mm or less, resulting in an excellent stacking condition.
 
     In addition, if the relationship of heat capacity of the pressure application roller  30  and heat capacity of the fusion roller  32  is configured to
 
a. Heat capacity of pressure application roller&lt;Heat capacity of fusion roller  (2)
 
the reverse curling amount at the time of start of printing immediately after the warm-up is controlled at 10 mm or less, resulting in an excellent stacking condition.
 
     As described above, it was understood that a large reverse curling amount occurs when the heat capacity is large and that the reverse curling amount is small when the heat capacity of the heat roller  30  is reduced, even with the same configuration. When the relationship of the elastic layer thickness t 4  of the fusion roller  32  and fusibility was studied by another test, occurrence of fusion defects was slightly observed with the elastic layer thickness t 4  of 1 mm. Therefore, it is necessary that a more preferred elastic layer thickness t 4  of the fusion roller  32  is 2 mm or more. 
     In addition to the above-described evaluation test, for the fifth to seventh samples (samples 5 to 7) with the same outer diameter and the core shaft thickness t 1  of the pressure application roller  30  that has been configured so that a flexure strength to be equalized in response to the elastic layer thickness t 2 , with the pressure application roller  20  of the second sample as a reference, a similar evaluation test was conducted based on the combination with the above-described fusion roller  32  and the fusion belt  33 . The evaluation results are shown in  FIG. 12 . The core shaft  30   a  of the pressure application roller  30  of the seventh sample is a solid shaft having an outer diameter of 20 mm. 
     As shown in  FIG. 12 , when the heat capacity of the pressure application roller  30  is less than the heat capacity of the fusion roller  32 , the starting pressure application roller end-point temperature at the time of start of printing immediately after warming up the pressure application roller  30  is at a temperature at which the reverse curling amount that causes the stacking defect does not occur. It was observed that the occurrence of the reverse curling amount is further suppressed when the heat capacity of the core shaft  30   a  is smaller compared with the evaluation results of the samples shown in  FIG. 8  (see sample 5). 
     As described above, in the present embodiment, the thickness of the elastic layer  30   b  of the pressure application roller  30  for the fuser  14  is made less than the thickness of the elastic layer  32   b  of the fusion roller  32 . Therefore, the temperature of the pressure application roller  30  increases fast, and the surface temperature of the pressure application roller  30  is increased, during the warm-up, to the temperature needed for start of printing. As a result, the difference in temperatures of the fusion unit  31  and the pressure application roller  30  at the time of fusion is reduced, and the difference in dryness of the front and back sides of the sheet P are decreased. Accordingly, the fuser  14  that allows the reverse curling amount to be reduced can be provided. In addition, because the temperature increase of the fusion roller  30  is increased, the warm-up time at the time of start of printing is shortened. 
     Furthermore, the printer  1  of the present embodiment, with the fuser  14 , provides excellent sheet carrying ability and stackability for the fusion process after being turned on and recovery from a power saving mode. In addition, the preset embodiment is explained with a sheet heater as the heating member. However, the heater may be a halogen heater  45 . 
     The halogen heater  45  is configured from a halogen lamp  45   b  as a heat generating body built in a heater cover  45   a  as shown in  FIG. 13 . Heat is transmitted from the halogen lamp  45   b  to the fusion belt  33  through a sliding surface between heater cover  45   a  and the fusion belt  33 , and thereby the fusion belt  33  is heated from the inner circumferential surface side. In addition, the heater cover  45   a  of the halogen heater  45  is positioned distant from the fusion roller  32  on the opposite side of the pressure application roller  30  and to face the fuser roller  32 . Similar to the heater holder  35 , the heater cover  45   a  has a function to support the fusion belt  33  with the fusion roller  32  so that the fusion belt  33  is rotatably tensioned. 
     As described above, in the present embodiment, the thickness of the elastic layer of the pressure application roller is made less than the thickness of the elastic layer of the fusion roller in the belt heating type fuser. Therefore, the temperature of the pressure application roller increases fast, and the surface temperature of the pressure application roller is increased to the temperature needed for start of printing during the warm-up. As a result, the difference in temperatures of the fusion unit and the pressure application roller at the time of fusion is reduced. Accordingly, the fuser allows the reverse curling amount to be reduced. In addition, the warm-up time at the time of start of printing is shortened. 
     Second Embodiment 
     The fuser of the present embodiment is explained with reference to  FIGS. 14 to 19  below. Explanation of parts that are similar to the first embodiment is omitted by adding the same reference numerals. As shown in  FIG. 14 , in the fuser  14  of the present embodiment, a pad  50  is provided as a pressure member inside the fusion belt and adjacent to the upstream side of the fusion roller  32  in the rotational direction of the fusion belt  33  (clockwise direction in  FIG. 14 ). The pad  50  is urged in a direction to press the pressure application roller  30  through the fusion belt  33  by a sprint member  51 , such as a compressed coil spring or the like, so as to form the nip part between pressure application roller  30  and the pad  50  and the fusion roller  32 . 
     As a result, the nip width is made longer than that in the above-described first embodiment, resulting in improved fusion speed. In addition, the outer diameter of the fusion roller  32  is made small. Therefore, by reducing the heat capacity of the fusion belt unit  31 , the warm-up time is decreased. Therefore, in the present embodiment, the print speed is configured to 40 ppm for carrying A4 size paper in the portrait orientation, and the warm-up time is configured to 20 seconds. The fusion belt  33  of the present embodiment, which is similar to the first embodiment. has an inner diameter of 45 mm. 
     As shown in  FIG. 15 , the pad  50  is configured from a support base  50   a  made of a metal, such as aluminum, an elastic material  50   b  adhered and fixed to the support base  50   a , and a sliding layer  50   c  provided on a surface layer of the elastic material  50   b . The elastic material  50   b  is formed with an arc surface  50   d  that has the same radius of curvature of the pressure application roller  30  via the fusion belt  33 . 
     In the pad  50  of the present embodiment, the support base  50   a  is made of an aluminum material (material: A6063), and the elastic material  50   b  is formed by a silicone rubber. The sliding layer  50   c  is configured by coating the PFA resin having a thickness of 30 μm, and an arc length of the arc surface  50   d  is configured to 5 mm. 
     The configuration of the fusion roller  32  of the present embodiment is similar to the above-described first embodiment. The core shaft  32   a  is configured from a pipe made of an iron material (material: STKM) with a diameter of 23 mm, a thickness t 3  of 1.5 mm (t 3 =1.5 mm) and a length of 230 mm, and an elastic layer  32   b  formed by a silicone rubber layer having a thickness t 4  of 1 mm (t 4 =1 mm). The fusion roller  32  has an outer diameter of 26 mm. 
     The configuration of the pressure application roller  30  of the present embodiment is similar to the first embodiment but different in the following. The core shaft  30   a  is configured from a pipe formed of an iron material (material: STKM) having a diameter of 28 mm, a thickness t 1  of 0.5 mm (t 1 =50 mm) and a length of 230 mm. A silicone rubber layer having a thickness t 2  is formed as an elastic layer  30   b . The surface of the pressure application roller  30  is covered by a PFA resin tube as a separation layer  30   c  having a thickness of 40 μm. The pressure application roller  30  has an outer diameter of 36 mm. The thickness t 2  of the elastic layer  30   b  is discussed later. 
     Print operation of the printer  1  and fusion operation of the fuser  14  in the present embodiment are the same as those in the above-described first embodiment. Therefore, their explanation is omitted. For the belt heating type fuser  14  with the configuration of the present embodiment, the evaluation test similar to the first embodiment was conducted by changing the thickness t 2  of the elastic layer  30   b  of the pressure application roller  30  to study a configuration for suppressing the reverse curling amount. 
     As shown in  FIG. 16 , sample pressure application rollers  30  subject for the evaluation had an outer diameter of 36 mm with a core shaft  30   a  (material: STKM) having the same thickness t 1  of 1.5 mm and an elastic layer  30   b  of various thicknesses t 2  of 0.5 mm, 1 mm, 2 mm and 3 mm (eighth to eleventh samples (samples 8 to 11)). The fusion roller  32  that was paired with each sample pressure application roller  30  had the configuration of the present embodiment as discussed above. The fusion belt  33  had the same configuration as that in the first embodiment. In addition, the print speed in the evaluation test in the present embodiment was 40 ppm for carrying A4 size paper in the portrait orientation. 
     Evaluation results of each sample pressure application roller  30  according to the above-described evaluation conditions are shown in  FIG. 16 . As shown in  FIG. 16 , it is observed that the starting pressure application roller end-point temperature at the time of start of printing immediately after the warm-up of the pressure application roller  30  is at a temperature at which the reverse curling amount that causes a stacking failure does not occur, when the elastic layer thickness t 2  of the pressure application roller  30  is less than the elastic layer thickness t 4  of the fusion roller  32 . 
     Explaining in more details, as shown in  FIG. 17 , the starting pressure application roller end-point temperature at the time when the fusion belt  33  reaches the fusion target temperature fusion from the from the room temperature at the time of warming up increases from 80° C. to 125° C. as the elastic layer thickness t 2  of the pressure application roller  30  decreases from 3 mm to 0.5 mm. Moreover, as shown in  FIG. 18 , the starting pressure application roller end-point temperature increases from 80° C. to 125° C. as a heat capacity of the pressure application roller  30  is decreased from 240 J/K to 84 J/K. 
     As shown in  FIG. 19 , the reverse curling amount of the sheet P at this time decreases from 25 mm to 5 mm as the starting pressure application roller end-point temperature increases. The stacking condition after printing 50 sheets shows a stacking result with no or little disarrangement when the reverse curling amount is 10 mm or less. 
     That is, if the relationship of thicknesses between the elastic layer  30   b  of the pressure application roller  30  and the elastic layer  32   b  of the fusion roller  32  is configured to
 
a. Elastic layer thickness t2 of pressure application roller&lt;Elastic layer thickness t4 of fusion roller  (3)
 
the reverse curling amount at the time of start of printing immediately after the warm-up is controlled at 10 mm or less, resulting in an excellent stacking condition.
 
     In addition, if the relationship of heat capacity of the pressure application roller  30  and heat capacity of the fusion roller  32  is configured to
 
a. Heat capacity of pressure application roller&lt;Heat capacity of fusion roller  (4)
 
the reverse curling amount at the time of start of printing immediately after the warm-up is controlled at 10 mm or less, resulting in an excellent stacking condition.
 
     As described above, it was understood that a large reverse curling amount occurs when the heat capacity is large and that the reverse curling amount is small when the heat capacity of the heat roller  30  is reduced, even with the same configuration. When the relationship of the elastic layer thickness t 4  of the fusion roller  32  and fusibility was studied by another test, occurrence of fusion defects was slightly observed with the elastic layer thickness t 4  of 1 mm. Therefore, it is necessary that a more preferred elastic layer thickness t 4  of the fusion roller  32  is 2 mm or more. In the embodiment, the outer diameter of the fusion roller  32  is 25 mm. Therefore, the preferred elastic layer thickness is approximately 8% ore more with respect to the outer diameter. 
     As described above, similar to the first embodiment, in the present embodiment, the thickness of the elastic layer  30   b  of the pressure application roller  30  for the fuser  14  is made less than the thickness of the elastic layer  32   b  of the fusion roller  32 . Therefore, the temperature of the pressure application roller  30  increases fast, and the surface temperature of the pressure application roller  30  is increased to the temperature needed for start of printing during the warm-up. As a result, the difference in temperatures of the fusion unit  31  and the pressure application roller  30  at the time of fusion is reduced, and the difference in dryness of the front and back sides of the sheet P. Accordingly, the fuser  14  that allows the reverse curling amount to be reduced can be provided. In addition, because the temperature increase of the fusion roller  30  is increased, the warm-up time at the time of start of printing is shortened. 
     Furthermore, the printer  1  of the present embodiment, with the fuser  14 , provides excellent sheet carrying ability and stackability for the fusion process after being turned on and recovery from a power saving mode. 
     In addition, in the present embodiment, because of the pad  50  added to the fuser  14 , the nip width is configured longer than the first embodiment. Therefore, the fusion speed is improved, and the outer diameter of the fusion roller  32  can be reduced. As a result, the heat capacity of the fusion unit  31  is reduced, and the warm-up time at the time of starting printing is shortened. 
     As described above, in the present embodiment, the thickness of the elastic layer of the pressure application roller is made less than the thickness of the elastic layer of the fusion roller in the belt heating type fuser, and a pad that presses the pressure application roller via the fusion belt is provided adjacent to the fusion roller. Therefore, the temperature of the pressure application roller increases quickly, and the surface temperature of the pressure application roller is increased to the temperature needed for the start of printing during the warm-up. As a result, the difference in temperatures of the fusion unit and the pressure application roller at the time of fusion is reduced. Accordingly, the fuser allows the reverse curling amount to be reduced. In addition, the warm-up time at the time of start of printing is further shortened. 
     The present embodiments are not limited to those described above, and various changes and modifications are available without departing from the scope of the invention. In addition, the description of members disclosed in the present application is examples and are not to be limited to the description. Moreover, in each of the above-described embodiments, the print medium is normal paper. However, the medium is not limited to this and may be an overhead projector (OHP) sheet, a card, a post card, a thickness having a weight of about 200 g/m 2  or more, an envelope, and a special paper such as a coated paper having a large heat capacity and the like. 
     Further, in each of the above-described embodiments, the heating member is explained as a sheet heater or a halogen heater. However, the heating member may be a cylindrical heater having a sliding surface against the fusion belt that has approximately the same radius of curvature as that for the fusion belt. Types and shapes of the heating member are not limited. Furthermore, in each of the above-described embodiments, the heater is described as being provided inside the fusion belt. However, the heater may be provided outside the fusion belt. 
     Concerning the temperature increase of the pressure application roller  30  and the fusion roller  32 , there is a high dependability to the thickness of the elastic layer of each roller. Therefore, materials and characteristics of the elastic layers of the pressure application roller  30  and the fusion roller  32  are not limited, although the same material is preferred for stabilizing the heat transfer. 
     In addition, in the present embodiments, the material of the elastic layers of the pressure application roller  30  and the fusion roller  32  is silicone rubber in consideration of heat tolerance, antifriction, heat resistance and the like. The silicone rubber may be formed by liquid silicone rubber or millable-type silicon rubber. Moreover, a foaming condition of the elastic layer may be a solid state (expansion ratio=1) or a foam state (expansion ratio&gt;1). In the present embodiments, the elastic layer of the pressure application roller  30  is formed by the liquid silicone rubber in the solid state. Further, the elastic layer of the fusion roller  32  is formed by the liquid silicone rubber in the solid state. 
     In the present embodiments, the foaming condition of the pressure application roller  30  and the fusion roller  32  is in the solid state. However, similar effects can be obtained with the combination of any foaming condition in the present application, as long as the expansion ratio is between 1.0 and 5. Here, the expansion ratio is a ratio of volume expansion of a foam plastic having the same mass in comparison with the foam plastic in the solid state, or refers to a value of an apparent density of the foam plastic divided by a density of a synthetic resin before foaming. 
     Further, in the present embodiments, the fusion roller  32  is explained to be driven and rotated by the pressure application roller  30 . However, if the fusion roller  32  is rotated as the driving side, and if the pressure roller  30  is driven and rotated, the fusion belt  33  is evenly carried by the fusion roller  32  with the elastic layer of the pressure application roller  30  being in the solid state and with the elastic layer of the fusion roller  32  being in the foam state. As a result, an effect, such as stable fusion quality, is obtained. 
     Furthermore, as explained in the first embodiment, if the core material of the fusion roller  32  and the pressure application roller  30  is aluminum, the thickness of the core is preferably set to 0.5 to 2.0 mm. In addition, more effects are obtained by setting the thickness of the elastic layer of the pressure application roller  30  by 0.4 to 0.8 times of the thickness of the elastic layer of the fusion roller  32 . 
     Moreover, as explained in the second embodiment, if the core material of the fusion roller  32  and the pressure application roller  30  is iron, the thickness of the core is preferably set to 0.3 to 2.0 mm. In addition, more effects are obtained by setting the thickness of the elastic layer of the pressure application roller  30  by 0.25 to 0.8 times of the thickness of the elastic layer of the fusion roller  32 . 
     In addition, with respect to the diameters of the fusion roller and the pressure application roller, the diameter of the fusion roller  32  and the diameter of the pressure application roller  30  are configured approximately the same in the present embodiments. However, the same effects are obtained as long as the diameter of the fusion roller  32  is in ±10% of the diameter of the pressure application roller  30 . 
     In each of the above-described embodiments, the image forming device is explained as a color printer. However, it is not limited to this and may be a monochrome printer, a copy machine, a facsimile device, a multi function peripheral and the like that uses the electrographic method.