Fixing apparatus

BACKGROUND OF THE INVENTION

00011. Field of the Invention

0002The present invention relates to a fixing apparatus used in an image forming apparatus such as a copying machine or a laser beam printer.

00032. Description of the Related Art

0004An image forming apparatus such as an electrophotographic copying machine or a laser beam printer includes a fixing apparatus that fixes a toner image formed on a recording material by performing heating and pressing. The fixing apparatus is heated by employing a heat roller method that uses a cylindrical member which internally includes a halogen heater. Further, there recently is a film heating method which can save power in the fixing apparatus.

0005The fixing apparatus employing the film heating method includes a sliding nip portion (hereinafter referred to as an inner surface nip portion or a heat transfer nip portion) and a pressing nip portion (hereinafter referred to as a fixing nip portion). The sliding nip portion is formed by a cylindrical belt (hereinafter referred to as a fixing sleeve) that is based on a heat-resistant resin or metal, and a heating member that is made of ceramic or the like, and contact-slides to an inner surface of the fixing sleeve. The pressing nip portion is formed by a pressing member pressing via the fixing sleeve. The recording material carrying the toner image is held between and conveyed through the fixing nip portion, so that the toner image is fixed on the recording material. Since the fixing apparatus employing the film heating method intensively heats an area surrounding the fixing nip portion, power can be saved and wait time can be shortened (i.e., quick start) as compared to a fixing apparatus employing the heat roller method.

0006Further, there are various types of recording materials used in the image forming apparatus. Such recording materials to be used in the image forming apparatus include plain paper commonly used in an office, a medium in which a sheet is in folds, e.g., an envelope, and thin paper whose basis weight is 60 g/m²or less.

0007Japanese Patent Application Laid-Open No. 2007-128037 discusses a fixing apparatus in which a pressing force can be changed to correspond to various media such as an envelope or thin paper. However, if the pressing force is changed according to the type of media in the fixing apparatus employing the film heating method and including the fixing sleeve formed of a highly rigid member such as metal, a region width of the fixing nip portion becomes narrow when the pressing force is reduced. As a result, heat-transfer efficiency of the heating member becomes lowered due to narrowing of the region width in the inner surface nip portion. In other words, if the pressing force of the fixing apparatus employing the film heating method is reduced to prevent wrinkles to be generated in the envelope or prevent curling, the heat-transfer efficiency when fixing the image on the medium may be lowered.

SUMMARY OF THE INVENTION

0008The present invention is directed to a fixing apparatus capable of reducing power consumption when lowering a pressing force applied to a fixing nip portion and performing fixing processing.

0009According to an aspect of the present invention, a fixing apparatus includes a heater, a belt member to which the heater is in contact with on an inner circumferential surface, a back-up member configured to form a fixing nip portion with the heater via the belt member for performing fixing processing on a recording material, and a pressure adjustment mechanism configured to be capable of setting a pressing force to be applied to the fixing nip portion to a first pressing force and a second pressing force that is lower than the first pressing force, wherein the fixing apparatus is capable of executing a first fixing mode in which fixing processing is performed at the first pressing force and a second fixing mode in which fixing processing is performed at the second pressing force, and wherein the heater includes a first heating member pattern and a second heating member pattern, and if the first pressing force is set to the pressing force to be applied to the fixing nip portion, all of the first heating member pattern and the second heating member pattern are contained within a contact region between the heater and the belt member, and if the second pressing force is set to the pressing force to be applied to the fixing nip portion, all of the first heating member pattern is contained within the contact region and at least a portion of the second heating pattern is disposed outside the contact region.

0010Further features and aspects of the present invention will become apparent from the following detailed description of exemplary embodiments with reference to the attached drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

0011The accompanying drawings, which are incorporated in and constitute a part of the specification, illustrate exemplary embodiments, features, and aspects of the invention and, together with the description, serve to explain the principles of the invention.

0012<figref idref="DRAWINGS">FIG. 1</figref> is a schematic cross-sectional view illustrating an image forming apparatus according to a first exemplary embodiment.

0013<figref idref="DRAWINGS">FIG. 2</figref> is a schematic cross-sectional view illustrating a fixing apparatus according to the first exemplary embodiment.

0014<figref idref="DRAWINGS">FIGS. 3A and 3B</figref> are a cross-sectional view and a plain view illustrating a heater according to the first exemplary embodiment.

0015<figref idref="DRAWINGS">FIG. 4</figref> is a schematic diagram illustrating a longitudinal heating distribution of the heater according to the first exemplary embodiment.

0016<figref idref="DRAWINGS">FIGS. 5A</figref>, <b>5</b>B, and <b>5</b>C illustrate a pressing mechanism and statuses in each fixing mode.

0017<figref idref="DRAWINGS">FIGS. 6A and 6B</figref> illustrate areas surrounding an inner surface nip portion according to the first exemplary embodiment.

0018<figref idref="DRAWINGS">FIGS. 7A</figref>, <b>7</b>B, and <b>7</b>C illustrate a plain view of a heater and areas surrounding an inner surface nip portion according to a second exemplary embodiment.

0019<figref idref="DRAWINGS">FIGS. 8A</figref>, <b>8</b>B, and <b>8</b>C illustrate a plain view of a heater and areas surrounding an inner surface nip portion according to a third exemplary embodiment.

0020<figref idref="DRAWINGS">FIG. 9</figref> illustrates another example of the heater according to the third exemplary embodiment.

0021<figref idref="DRAWINGS">FIGS. 10A</figref>, <b>10</b>B, and <b>10</b>C illustrate a plain view of a heater and areas surrounding an inner surface nip portion according to a fourth exemplary embodiment.

DESCRIPTION OF THE EMBODIMENTS

0022Various exemplary embodiments, features, and aspects of the invention will be described in detail below with reference to the drawings.

(1) Entire Configuration of a Multi-Color Image Forming Apparatus

0023The entire configuration of an image forming apparatus according to a first exemplary embodiment will be described below with reference to <figref idref="DRAWINGS">FIG. 1</figref>. <figref idref="DRAWINGS">FIG. 1</figref> is a schematic cross-sectional view illustrating the entire configuration of a full-color laser beam printer (hereinafter referred to as a printer <b>10</b>) that is an example of the image forming apparatus according to the present exemplary embodiment. According to the present exemplary embodiment, the full-color laser beam printer including a plurality of photosensitive drums will be described below. However, the present invention is not limited to this configuration. The present invention can be applied to a monochrome copying machine or printer including a single photosensitive drum.

0024Referring to <figref idref="DRAWINGS">FIG. 1</figref>, a cassette <b>11</b> which can be drawn out is stored in a lower portion of the printer <b>10</b>. Recording materials P are stacked and contained in the cassette <b>11</b>. A pick-up roller <b>13</b> feeds the recording materials P from the cassette <b>11</b>. The recording materials P are then separated sheet-by-sheet by a feed-retard roller pair <b>14</b>, and conveyed to a resist roller <b>15</b>.

0025The printer <b>10</b> includes an image forming unit <b>7</b> in which image forming stations <b>7</b>Y, <b>7</b>M, <b>7</b>C, and <b>7</b>K corresponding to each of yellow, magenta, cyan and black colors respectively are arranged side by side.

0026The image forming unit <b>7</b> includes photosensitive drums <b>1</b>Y, <b>1</b>M, <b>1</b>C, and <b>1</b>K (hereinafter collectively referred to a photosensitive drum <b>1</b>), i.e., image bearing members, and charging devices <b>2</b>Y, <b>2</b>M, <b>2</b>C, and <b>2</b>K that uniformly charges the surface of the photosensitive drum <b>1</b>. Further, the image forming unit <b>7</b> includes developing devices <b>4</b>Y, <b>4</b>M, <b>4</b>C, and <b>4</b>K that attach toner to an electrostatic latent image on the photosensitive drum <b>1</b> and develop the image as the toner image. Furthermore, the image forming unit <b>7</b> includes primary transfer units <b>8</b>Y, <b>8</b>M, <b>8</b>C, and <b>8</b>K (hereinafter collectively referred to as a primary transfer unit <b>8</b>) that transfer the toner image on the photosensitive drum <b>1</b> to an electrostatic transfer belt <b>29</b>.

0027Scanner units <b>3</b>YM and <b>3</b>CK disposed below the image forming unit <b>7</b> emit a laser beam according to image information to form the electrostatic latent image on the photosensitive drum <b>1</b>. Developing rollers <b>5</b>Y, <b>5</b>M, <b>5</b>C, and <b>5</b>K supply toner to the photosensitive drum <b>1</b>, and cleaners <b>6</b>Y, <b>6</b>M, <b>6</b>C, and <b>6</b>K clean the photosensitive drum <b>1</b>.

0028A toner image T transferred to the transfer belt <b>29</b> by the primary transfer unit <b>8</b> is transferred to the recording material P at a secondary transfer unit <b>31</b>. The recording material P is then passed through a fixing apparatus <b>12</b>, so that the toner image T is fixed on the recording material P. The recording material P is conveyed to and passes through a discharge roller pair <b>32</b>, and discharged to a recording material stacking unit <b>33</b>.

0029A sheet-passing width of the recording material P in the printer <b>10</b> according to the present exemplary embodiment is 76 mm to 297 mm in a direction perpendicular to a conveying direction (hereinafter referred to as a longitudinal direction). A sheet-passing reference in the printer <b>10</b> is a proximate center with respect to the direction perpendicular to the conveying direction of the recording material P (hereinafter referred to as center alignment).

(2) Fixing Apparatus

0030The fixing apparatus <b>12</b> according to the present exemplary embodiment will be described below with reference to <figref idref="DRAWINGS">FIG. 2</figref>. The fixing apparatus <b>12</b> according to the present exemplary embodiment employs the film-heating method, and a heating apparatus uses a rotatable pressing member driving method (i.e., tensionless type).

0031Referring to <figref idref="DRAWINGS">FIG. 2</figref>, the fixing apparatus <b>12</b> includes a fixing sleeve <b>20</b> (i.e., a cylindrical belt member) to be described in detail below, and a pressing roller <b>22</b> (i.e., a back-up member). A heater holder <b>17</b> is a heat-resistant, rigid heater holding member whose cross-section has an approximate semicircular arc gutter shape. A heater <b>16</b> is disposed along a longitudinal direction of the heater holder <b>17</b> at a bottom side thereof. The fixing sleeve <b>20</b> is loosely and externally fitted on the heater holder <b>17</b>. The heater <b>16</b> is a ceramic heater.

0032The heater holder <b>17</b> formed of highly heat-resistant liquid crystal polymer resin holds the heater <b>16</b> and guides the fixing sleeve <b>20</b>. According to the present exemplary embodiment, Zenite 7755 (product name) which is manufactured by Dupont is used as the liquid crystal polymer. The upper usable temperature limit of Zenite 7755 is approximately 270° C.

0033A pressing roller <b>22</b> is manufactured by forming on a stainless steel core, a silicone rubber layer of approximately 3 mm thickness by injection molding, and covering with a tetrafluoroethylene-perfluoroalkyl vinyl ether (PFA) resin tube of approximately 40 μm thickness. According to the present exemplary embodiment, an outer diameter of the pressing roller <b>22</b> is 25 mm. Both ends of the core of the pressing roller <b>22</b> are mounted between far side and near side of side plates (not illustrated) of an apparatus frame <b>24</b> to freely rotate.

0034A fixing sleeve unit including the heater <b>16</b>, the heater holder <b>17</b>, and the fixing sleeve <b>20</b> is disposed above the pressing roller <b>22</b>, so that the heater <b>16</b> is disposed facing downwards and parallel to the pressing roller <b>22</b>. A pressing mechanism to be described below applies a force to both ends of the heater holder <b>17</b> towards the pressing roller <b>22</b> with a maximum force of 147 N (15 kgf) for one side, i.e., 294 N (30 kgf) total pressing force.

0035A fixing nip portion N of a predetermined width necessary for heat-fixing is thus formed by press-contacting the heater <b>16</b> onto the pressing roller <b>22</b> via the fixing sleeve <b>20</b>. The pressing mechanism to be described below includes an automatic pressure changing mechanism (i.e., a pressure adjustment mechanism), and is capable of changing the pressing force according to the medium to be passed through.

0036An entrance guide <b>23</b> and a fixing-sheet discharge roller <b>26</b> are mounted on the apparatus frame <b>24</b>. The entrance guide <b>23</b> guides the recording materials so that the recording material P that has passed through a secondary transfer nip portion is correctly guided to the fixing nip portion N. According to the present exemplary embodiment, the entrance guide <b>23</b> is formed of polyphenylene sulfide (PPS) resin.

0037The pressing roller <b>22</b> is driven to rotate by a driving unit (not illustrated) at a predetermined peripheral speed in an anticlockwise direction indicated by an arrow illustrated in <figref idref="DRAWINGS">FIG. 2</figref>. When the pressing roller <b>22</b> rotates, the outer surface of the pressing roller <b>22</b> and the fixing sleeve <b>20</b> generates a press-contact frictional force at the fixing nip portion N. As a result, a rotational force is applied to the cylindrical fixing sleeve <b>20</b> by the press-contact frictional force, and the inner surface side of the fixing sleeve <b>20</b> becomes attached to and slides on the downward side of the heater <b>16</b>. The fixing sleeve <b>20</b> is thus rotatably driven in a clockwise direction as indicated by an arrow illustrated in <figref idref="DRAWINGS">FIG. 2</figref> around the heater holder <b>17</b>. Grease is applied on the inner surface of the fixing sleeve <b>20</b> to ensure slidability between the heater holder <b>17</b> and the inner surface of the fixing sleeve <b>20</b>.

0038When the pressing roller <b>22</b> starts to rotate, the cylindrical fixing sleeve <b>20</b> is rotatably driven. The heater <b>16</b> is then turned on, and the temperature thereof rises. While the temperature of the heater <b>16</b> has risen to and is adjusted to be at a predetermined temperature, the recording material P carrying an unfixed toner image is guided along the entrance guide <b>23</b> between the fixing sleeve <b>20</b> and the pressing roller <b>22</b> of the fixing nip portion N. The toner image carrying side of the recording material P becomes attached to the outer surface of the fixing sleeve <b>20</b> in the fixing nip portion N, and the recording material P is held and conveyed between the fixing nip portion N together with the fixing sleeve <b>20</b>.

0039The heat from the heater <b>16</b> is applied to the recording material P via the fixing sleeve <b>20</b> in the holding and conveying process, so that the unfixed toner image on the recording material P is melted and fixed to the recording material P by the heat and pressure applied thereto. The recording material P which has passed through the fixing nip portion N is curvature-separated from the fixing sleeve <b>20</b>, and discharged by the fixing-sheet discharge roller <b>26</b>.

0040<figref idref="DRAWINGS">FIGS. 3A and 3B</figref> illustrate a cross-sectional view and a plain view of the heater <b>16</b> respectively. The heater <b>16</b> includes the following components.

0000(1) A ceramics substrate <b>41</b> which is a horizontally long flat plate, whose longitudinal direction is perpendicular to the sheet passing direction (according to the present exemplary embodiment, the lengths of the ceramic substrate <b>41</b> is 370 mm in the longitudinal direction and 10 mm in a short-length direction, and the thickness is 0.6 mm); <br /> (2) resistive heating member layers (i.e., heating member patterns) <b>42</b> and <b>43</b> linearly or zonally coated along the longitudinal direction on one surface of the ceramic substrate <b>41</b> by screen printing, which generates heat by applying an electric current, the resistive heating member layers <b>42</b> and <b>43</b> are formed by applying electrically-conductive paste containing silver-palladium alloy (Ag/Pd) on the substrate <b>41</b>, and the resistive heating member layers <b>42</b> and <b>43</b> are of approximately 10 μm thickness, 1 mm width, and 303 mm length; <br /> (3) electrodes <b>44</b><i>a</i>, <b>44</b><i>b</i>, and <b>44</b><i>c </i>which are patterns for supplying electric power to the resistive heating member layers <b>42</b> and <b>43</b>, formed by screen-printing silver paste on one surface of the ceramic substrate <b>41</b>; <br /> (4) thin glass coating <b>45</b> of approximately 30 μm thickness, to protect and ensure insulation of the resistive heating member layers <b>42</b> and <b>43</b>; and <br /> (5) a sliding layer <b>46</b> formed of polyimide, disposed at locations which come into contact with the inner surface of the fixing sleeve <b>20</b>, on the other surface of the ceramic substrate <b>41</b>.

0041<figref idref="DRAWINGS">FIG. 4</figref> illustrates a heating distribution of the heater <b>16</b> in the longitudinal direction, generated when supplying predetermined amount of power to the resistive heating member layers <b>42</b> and <b>43</b>. The resistive heating member layer is formed of two heating members <b>42</b> (i.e., a first heating member pattern) and <b>43</b> (i.e., a second heating member pattern). The heating member <b>42</b> is formed so that a heat generation amount continuously decreases from a longitudinal center, i.e., the sheet-passing reference, to the edge portion (i.e., the heat generation amount per unit length gradually decreases). On the other hand, the heating member <b>43</b> is formed so that the heat generation amount continuously increases from the longitudinal center to the edge portion.

0042The electrode <b>44</b> (i.e., the electrodes <b>44</b><i>a</i>, <b>44</b><i>b</i>, and <b>44</b><i>c</i>) in the heater <b>16</b> are attached to a power-supply connector. The power is supplied from a heater driving circuit unit to the electrode <b>44</b> via the power supply connector, so that the resistive heating member layers <b>42</b> and <b>43</b> generate heat and the temperature of the heater <b>16</b> promptly rises.

0043According to the present exemplary embodiment, the electrode <b>44</b><i>c </i>is a common electrode that causes the heating member <b>42</b> to generate heat via the electrode <b>44</b><i>a</i>, and the heating member <b>43</b> to generate heat via the electrode <b>44</b><i>b</i>. Each of the heating members <b>42</b> and <b>43</b> are independently driven by a heater driving circuit (not illustrated). A time ratio to turn on the heating members <b>42</b> and <b>43</b> is controlled according to a paper size. As a result, a temperature rise in the portion of the fixing apparatus in which the sheet has not passed through can be effectively reduced while corresponding to the paper sizes whose width in the longitudinal direction is 76 mm to 297 mm.

0044In normal use, the fixing sleeve <b>20</b> starts to be rotatably driven when the pressing roller <b>22</b> starts to rotate, and the temperature of the inner surface of the fixing sleeve <b>20</b> also rises along with the temperature rise of the heater <b>16</b>. Power supply to the heater <b>16</b> is controlled using proportional-integral-derivative (PID) control. Power input to the heater <b>16</b> is controlled so that the temperature of the inner surface of the fixing sleeve <b>20</b>, i.e., the temperature detected by a sleeve thermistor <b>18</b> (a first temperature detection element), becomes a target value.

0045Referring to <figref idref="DRAWINGS">FIG. 2</figref>, a plate spring <b>25</b> presses the thermistor <b>18</b> against the inner surface of the fixing sleeve <b>20</b>. A second temperature detection element <b>19</b> detects the temperature of the heater <b>16</b>, and monitors the heater <b>16</b> to prevent an excessive rise in the temperature.

0046A control unit <b>21</b> controls supplying of power to the heater <b>16</b> according to the temperature detected by the first temperature detection element <b>18</b>. A signal line of the second temperature detection element <b>19</b> is also connected to the control unit <b>21</b>, and the control unit <b>21</b> stops supplying power to the heater <b>16</b> when the temperature of the heater <b>16</b> rises excessively.

0047According to the present exemplary embodiment, the fixing sleeve <b>20</b> is a cylindrical member (i.e., an endless belt) in which an elastic layer is formed on a belt-shaped member. More specifically, the fixing sleeve <b>20</b> includes a base layer which is a stainless (SUS) metal endless belt (i.e., a belt base member) formed into a cylindrical shape of 24 mm inner diameter and 30 μm thickness. A silicone rubber layer (i.e., the elastic layer) of approximately 300 μm thickness is formed on the base layer. Further, a PFA resin tube layer (i.e., an outermost layer) of 30 μm thickness is formed to cover the silicone rubber layer.

0048<figref idref="DRAWINGS">FIG. 5</figref> illustrates the pressing mechanism according to the present exemplary embodiment. According to the present exemplary embodiment, the pressing mechanism applies a spring force of a pressing spring <b>81</b> to the external surface of the pressing roller <b>22</b> via a pressing plate <b>82</b>, a flange <b>83</b>, the heater holder <b>17</b>, the heater <b>16</b>, and the fixing sleeve <b>20</b>. More specifically, the pressing spring <b>81</b> disposed between the fixing frame <b>24</b> and the pressing plate <b>82</b> presses the flange <b>83</b> that supports the heater folder <b>17</b> from both of the longitudinal sides, towards the pressing roller <b>22</b> via the heater <b>16</b> and the fixing sleeve <b>20</b>.

0049A cam member <b>84</b> is a part of the pressure adjustment mechanism. The cam members <b>84</b> are disposed facing to the pressing spring <b>81</b> across the pressing plate <b>82</b>, in the near side and the far side of the pressing plate <b>82</b> with respect to <figref idref="DRAWINGS">FIGS. 5A</figref>, <b>5</b>B, and <b>5</b>C. The cam members <b>84</b> in the near side and the far side are of the same size and shape, and are fixed to a camshaft <b>85</b> in the same phase. The camshaft <b>85</b> is supported by a bearing to freely rotate, and is rotated and stopped by a motor (not illustrated).

0050Referring to <figref idref="DRAWINGS">FIG. 5A</figref>, the cam member <b>84</b> is not in contact with the pressing plate <b>82</b>, so that the maximum pressing force is applied on the fixing nip portion (i.e., a first pressing state). According to the present exemplary embodiment, the total pressing force in the first pressing state is 294 N (30 kgf).

0051If the camshaft <b>85</b> is then rotated 90° from the state illustrated in <figref idref="DRAWINGS">FIG. 5A</figref>, the cam member <b>84</b> moves and the state becomes as illustrated in <figref idref="DRAWINGS">FIG. 5B</figref>. The pressing plate <b>82</b> is thus pushed up, and the pressing force applied on the fixing nip portion can be set lower than in the first pressing state (i.e., a second pressing state). According to the present exemplary embodiment, the total pressing force in the second pressing state is 147 N (15 kgf).

0052If the cam shaft <b>85</b> is further rotated 90° from the state illustrated in <figref idref="DRAWINGS">FIG. 5B</figref>, the cam member <b>84</b> also moves and the state becomes as illustrated in <figref idref="DRAWINGS">FIG. 5C</figref>. The pressing plate 82 is thus further pushed up, and the pressing force can be set even lower (i.e., a third pressing state).

0053According to the present exemplary embodiment, a case where a regular size paper commonly used in the office is to be printed will be referred to as normal printing. In the normal printing, fixing processing is performed in a first fixing mode (i.e., the pressing force applied to the fixing nip portion is set to a first pressing force). Further, if an envelope whose width in the longitudinal direction is comparatively narrow is to be printed on, fixing processing is performed in a second fixing mode (i.e., the pressing force applied to the fixing nip portion is set to a second pressing force). This is to prevent wrinkles from occurring. Further, if a paper jam is to be treated, or the power of the main body is to be switched off, the state is set to the third pressing state.

0054As described above, the fixing apparatus includes the heater, the belt member of which the inner peripheral surface is in contact with the heater, and the back-up member forming the fixing nip portion that performs fixing processing on the recording material along with the heater via the belt member. The fixing apparatus further includes the pressure adjustment mechanism that is capable of setting the pressing force applied to the fixing nip portion to the first pressing force and to the second pressing force that is lower than the first pressing force. The fixing apparatus is capable of executing the first fixing mode that performs fixing processing at the first pressing force, and the second fixing mode that performs fixing processing at the second pressing force.

0055<figref idref="DRAWINGS">FIGS. 6A and 6B</figref> illustrate areas surrounding the nip portion in the fixing apparatus <b>12</b> according to the present exemplary embodiment. Referring to <figref idref="DRAWINGS">FIGS. 6A and 6B</figref>, the nip portion in the fixing apparatus <b>12</b> includes an inner surface nip portion Na and a fixing nip portion N. The inner surface nip portion Na is the press-contacting portion between the heater <b>16</b> and the fixing sleeve <b>20</b>, and is a heat transfer channel from the heater <b>16</b> to the fixing sleeve <b>20</b> (i.e., a contact region between the heater and the belt member). The fixing nip portion N is the press-contacting portion between the fixing sleeve <b>20</b> and the pressing roller <b>22</b>. Ranges of the regions of the inner nip portion Na and the fixing nip portion N are determined by a positional relationship between the heater <b>16</b> and the heater holder <b>17</b>, the rigidity of the fixing sleeve <b>20</b>, the elasticity of the pressing roller <b>22</b>, and the pressing force generated by the above-described pressing mechanism.

0056If a fixing sleeve whose base member is SUS is to be used, such as the fixing sleeve <b>20</b> according to the present exemplary embodiment, the fixing sleeve forms a curvature-shaped nip portion as illustrated in <figref idref="DRAWINGS">FIG. 6</figref>, due to the rigidity thereof. Thus, generally, the range of the region of the fixed nip portion N is narrower than the range of the region of the inner surface nip portion Na.

0057<figref idref="DRAWINGS">FIG. 6A</figref> illustrates the positional relationship between an inner surface nip portion Na<b>1</b> and surrounding members in the first fixing mode. Referring to <figref idref="DRAWINGS">FIG. 6A</figref>, all of the heating members <b>42</b> and <b>43</b> of the heater <b>16</b> are contained within the range of the region of the inner surface nip portion Na<b>1</b> in the first fixing mode.

0058According to the present exemplary embodiment, the width of the inner surface nip portion Na <b>1</b> is 5 mm, and the width of the fixing nip portion N<b>1</b> is 9 mm. The fixing nip portion N<b>1</b> and the inner surface nip portion Na<b>1</b> are each positioned to be approximately symmetrical with respect to the center of the nip portion. Further, the width of each of the heating members <b>42</b> and <b>43</b> of the heater <b>16</b> is approximately 1 mm, and a distance between the heating members <b>42</b> and <b>43</b> is 0.5 mm. According to the present exemplary embodiment, a shortest distance between the heating member <b>43</b> and a boundary of the inner surface nip portion Na<b>1</b> is 0.5 mm.

0059<figref idref="DRAWINGS">FIG. 6B</figref> illustrates the positional relationship between an inner surface nip portion Na<b>2</b> and the surrounding members in the second fixing mode. Referring to <figref idref="DRAWINGS">FIG. 6B</figref>, all of the heating member <b>42</b> of the heater <b>16</b> is contained within the range of the region of the inner surface nip portion Na<b>2</b> in the second fixing mode. Further, a portion or the entire heating member <b>43</b> is disposed outside the inner surface nip portion Na<b>2</b>.

0060According to the present exemplary embodiment, the width of the inner surface nip portion Na<b>2</b> is 3 mm, and the width of the fixing nip portion N<b>2</b> is 6 mm. The fixing nip portion N<b>2</b> and the inner surface nip portion Na<b>2</b> are each positioned to be approximately symmetrical with reference to the center of the nip portion. Further, the widths of the heating members <b>42</b> and <b>43</b> of the heater <b>16</b> are each approximately 1 mm, and the distance between the heating members <b>42</b> and <b>43</b> is 0.5 mm. The heating member <b>43</b> is disposed 0.8 mm away from the inner surface nip portion Na<b>2</b>.

0061As described above, the heater includes the first heating member pattern and the second heating member pattern. When the pressing force applied on the fixing nip portion is set to the first pressing force, all of the first heating member pattern and the second heating member pattern are contained within the contact region between the heater and the belt member. When the pressing force applied on the fixing nip portion is set to the second pressing force, the entire first heating member pattern is contained within the contact region between the heater and the belt member. However, at least a portion of the second heating member pattern is disposed outside the contact region.

0062Heating control of the heater <b>16</b> performed in each fixing mode will be described below.

(1) First Fixing Mode

0063In the case of the first fixing mode which is set in normal printing, both of the heating members <b>42</b> and <b>43</b> of the heater <b>16</b> are turned on, so that the heating distribution in the longitudinal direction becomes approximately flat. Printing is then performed. All of the heating members <b>42</b> and <b>43</b> are contained within the range of the region of the inner surface nip portion Na<b>1</b>. The heat generated by the heating members <b>42</b> and <b>43</b> is thus efficiently transferred to the fixing sleeve <b>20</b> via the inner surface nip portion Na<b>1</b>, and then to the recording material P, and the toner image T on the recording material P is fixed.

(2) Second Fixing Mode

0064In the case of the second fixing mode which is set when printing on the envelope, the heating member <b>42</b> is turned on and the heating member <b>43</b> is turned off, so that the heating distribution in the longitudinal direction is high at the center. Printing is then performed. The entire heating member <b>42</b> is contained within the range of the region of the inner surface nip portion Na<b>2</b>. The heat generated by the heating member <b>42</b> is thus efficiently transferred to the fixing sleeve <b>20</b> via the inner surface nip portion Na<b>2</b>, and then to the recording material P, i.e., the envelope. On the other hand, a portion or the entire heating member <b>43</b> is disposed outside the inner surface nip portion Na<b>2</b>. If the heating member <b>43</b> generates heat at a turn-on time ratio similar to that of the heating member <b>42</b> in such a state, the heat outside the inner surface nip portion Na<b>2</b> of the heater is not efficiently transferred to the fixing sleeve <b>20</b>. The ratio of heat transferred to the heater <b>16</b> and the heater <b>17</b>, i.e., the components other than the recording material P, thus increases. As a result, the percentage of the heat amount contributing to fixing of the toner becomes small with respect to the heat generation amount of the heater, so that heat efficiency is lowered. According to the present exemplary embodiment, since the heating member <b>43</b> outside the inner surface nip portion Na<b>2</b> is not turned on, the heat efficiency is hardly lowered.

0065Table 1 compares control temperature of the thermistor <b>18</b>, occurrence frequency of wrinkles on the envelope, and average consumed power of the fixing apparatus for fixing the toner image T on the envelope for various combinations of each fixing mode and heating control. The envelope is printed in an environment in which atmosphere temperature is 30° C. and relative humidity is 80%.

0000<tables id="TABLE-US-00001" num="00001"><table frame="none" colsep="0" rowsep="0"><tgroup align="left" colsep="0" rowsep="0" cols="7"><colspec colname="offset" colwidth="42pt" align="left" /><colspec colname="1" colwidth="28pt" align="center" /><colspec colname="2" colwidth="28pt" align="center" /><colspec colname="3" colwidth="28pt" align="center" /><colspec colname="4" colwidth="28pt" align="center" /><colspec colname="5" colwidth="35pt" align="center" /><colspec colname="6" colwidth="28pt" align="center" /><thead><row><entry /><entry namest="offset" nameend="6" rowsep="1">TABLE 1</entry></row><row><entry /><entry namest="offset" nameend="6" align="center" rowsep="1" /></row><row><entry /><entry /><entry>Heat-</entry><entry>Heat-</entry><entry /><entry /><entry /></row><row><entry /><entry>Press-</entry><entry>ing</entry><entry>ing</entry><entry>Fix-</entry><entry>Wrinkle</entry><entry>Con-</entry></row><row><entry /><entry>ing</entry><entry>member</entry><entry>member</entry><entry>ing</entry><entry>on</entry><entry>sumed</entry></row><row><entry /><entry>force</entry><entry>42</entry><entry>43</entry><entry>temp.</entry><entry>envelope</entry><entry>power</entry></row><row><entry /><entry namest="offset" nameend="6" align="center" rowsep="1" /></row></thead><tbody valign="top"><row><entry /></row></tbody></tgroup><tgroup align="left" colsep="0" rowsep="0" cols="7"><colspec colname="1" colwidth="42pt" align="left" /><colspec colname="2" colwidth="28pt" align="center" /><colspec colname="3" colwidth="28pt" align="center" /><colspec colname="4" colwidth="28pt" align="center" /><colspec colname="5" colwidth="28pt" align="center" /><colspec colname="6" colwidth="35pt" align="center" /><colspec colname="7" colwidth="28pt" align="center" /><tbody valign="top"><row><entry>Exemplary</entry><entry>147N</entry><entry>ON</entry><entry>OFF</entry><entry>160° C.</entry><entry>0%</entry><entry>600 W</entry></row><row><entry>embodiment</entry></row><row><entry>1</entry></row><row><entry>Comparison</entry><entry>294N</entry><entry>ON</entry><entry>ON</entry><entry>150° C.</entry><entry>20% </entry><entry>800 W</entry></row><row><entry>example 1</entry></row><row><entry>Comparison</entry><entry>147N</entry><entry>ON</entry><entry>ON</entry><entry>160° C.</entry><entry>0%</entry><entry>800 W</entry></row><row><entry>example 2</entry></row><row><entry namest="1" nameend="7" align="center" rowsep="1" /></row></tbody></tgroup></table></tables>

0066As illustrated in table 1, the wrinkles are not generated in the envelope according to the first exemplary embodiment, and the consumed power is also reduced. In contrast, since the envelope is passed through the fixing apparatus at high pressing force in the comparison example 1, the wrinkles are easily generated in the envelope. In the case of the comparison example 2, the pressing force is reduced, so that the wrinkles are not generated in the envelope. However, since it is difficult for the heat to be transferred from the heating member <b>43</b> to the envelope, the heat efficiency is low, and the consumed power cannot be reduced.

0067According to the present exemplary embodiment, when low pressing force is applied in performing fixing, the heating members are disposed with respect to the inner surface nip portion and the heat control is performed as described above. As a result, the occurrence frequency of the wrinkles in the envelope and fixing heat efficiency are both improved when applying low pressing force. According to the present exemplary embodiment, the heating members <b>42</b> and <b>43</b> of the heater <b>16</b> are disposed the opposite side to the inner surface nip portion Na with respect to the base member <b>41</b> (i.e., back surface heating is performed). However, the present invention is not limited to this configuration, and the heating members <b>42</b> and <b>43</b> may be disposed on the same side as the inner surface nip portion Na with respect to the base member <b>41</b> (i.e., front surface heating may be performed).

0068Further, according to the present exemplary embodiment, heating control is performed in the second fixing mode without turning on the heating member <b>43</b>. However, if the turn-on time of the heating member <b>43</b> is shortened with respect to the turn-on time of the heating member <b>42</b>, a similar effect can be achieved. Further, various modifications within the technological scope may be performed.

0069The effect achieved in the present invention may also be realized according to a second exemplary embodiment to be described below. A fixing apparatus according to the present exemplary embodiment branches a heating member <b>53</b>, i.e., a second heating member pattern of a heater <b>50</b> into two. The second heating member pattern <b>53</b> is then each disposed at an upstream position and a downstream position in a recording material conveying direction with respect to a heating member <b>52</b>. Other configurations are similar to those of the first exemplary embodiment, and detailed description will thus be omitted.

0070<figref idref="DRAWINGS">FIGS. 7A</figref>, <b>7</b>B, and <b>7</b>C are a plain view of the heater <b>50</b> and schematic cross-sectional views of areas surrounding nip portions according to the present exemplary embodiment.

0071Referring to <figref idref="DRAWINGS">FIGS. 7A</figref>, <b>7</b>B, and <b>7</b>C, the heating members <b>52</b> and <b>53</b> (<b>53</b><i>a </i>and <b>53</b><i>b</i>) are disposed on the heater <b>50</b> as described below. The heating members <b>53</b><i>a </i>and <b>53</b><i>b </i>which are formed so that the heat generation amount continuously increases from the longitudinal center to the edge portion are formed as parallel circuits. The heating members <b>53</b><i>a </i>and <b>53</b><i>b </i>are disposed at the upstream position and the downstream position of the heating member <b>52</b> formed so that the heat generation amount continuously decreases from the longitudinal center to the edge portion.

0072The positional relationship between the inner surface nip portion Na and the heating members <b>52</b> and <b>53</b> are illustrated in <figref idref="DRAWINGS">FIGS. 7B and 7C</figref>. In the first fixing mode illustrated in <figref idref="DRAWINGS">FIG. 7B</figref>, all of the heating members <b>52</b>, <b>53</b><i>a</i>, and <b>53</b><i>b </i>of the heater <b>50</b> are contained within the range of the region of the inner surface nip portion Na<b>1</b>. In the second fixing mode illustrated in <figref idref="DRAWINGS">FIG. 7C</figref>, the entire heating member <b>52</b> is contained within the range of the region of the inner surface nip portion Na<b>2</b>, and a portion or all of the heating members <b>53</b><i>a </i>and <b>53</b><i>b </i>is disposed outside the inner surface nip portion Na<b>2</b>.

0073According to the present exemplary embodiment, the width of the inner surface nip portion Na<b>1</b> is 5 mm, the widths of the heating members <b>52</b>, <b>53</b><i>a</i>, and <b>53</b><i>b </i>are each approximately 1 mm, the distance between the heating members is 0.5 mm, and the shortest distance from the heating members <b>52</b>, <b>53</b><i>a</i>, and <b>53</b><i>b </i>to a boundary of the inner surface nip portion Na<b>1</b> is 0.5 mm. According to the second fixing mode, the width of the inner surface nip portion Na<b>2</b> is 3 mm, the widths of the heating members <b>52</b>, <b>53</b><i>a</i>, and <b>53</b><i>b </i>are each approximately 1 mm, the distance between the heating members is 0.5 mm, and the heating members <b>53</b><i>a </i>and <b>53</b><i>b </i>are disposed 0.75 mm away from the inner surface nip portion Na<b>2</b>.

0074Methods for driving the heating members <b>52</b> and <b>53</b> (<b>53</b><i>a </i>and <b>53</b><i>b</i>) in each fixing mode are similar to those according to the first exemplary embodiment. More specifically, the heating members <b>52</b>, <b>53</b><i>a</i>, and <b>53</b><i>b </i>can be independently turned on via electrodes <b>54</b><i>a</i>, <b>54</b><i>b</i>, and <b>54</b><i>c </i>illustrated in <figref idref="DRAWINGS">FIG. 7A</figref>.

0075According to the present exemplary embodiment, the heat from the heating members <b>52</b> and <b>53</b> can be transferred to a wide range within the region of the inner surface nip portion Na<b>1</b> in the first fixing mode by branching the heating members. As a result, the heat efficiency can be improved in the first fixing mode. Further, in the second fixing mode in which the pressing force is reduced, a similar effect as in the first exemplary embodiment can be achieved. According to the present exemplary embodiment, the heating members <b>53</b><i>a </i>and <b>53</b><i>b </i>are formed as the parallel circuits. However, the present invention is not limited to this configuration, and the heating members <b>53</b><i>a </i>and <b>53</b><i>b </i>may also be formed as serial circuits.

0076The effect acquired in the present invention may also be realized according to a third exemplary embodiment to be described below. According to the present exemplary embodiment, a normal printing heating member <b>62</b> and an envelope printing heating member <b>63</b> are formed as the heating members of a heater <b>60</b> in the fixing apparatus. The other configurations are similar to those of the first exemplary embodiment, and detailed description will thus be omitted.

0077<figref idref="DRAWINGS">FIG. 8A</figref> is a plain view illustrating the heater <b>60</b> according to the present exemplary embodiment. Referring to <figref idref="DRAWINGS">FIG. 8A</figref>, the normal printing heating member <b>62</b> (i.e., <b>62</b><i>a </i>and <b>62</b><i>b</i>) is disposed to generate heat so that the heating distribution becomes approximately flat in the longitudinal direction. According to the present exemplary embodiment, the normal printing heating member <b>62</b> is formed so that the length in the longitudinal direction is 303 mm, which is sufficient to fix an image on A3 size paper of width 297 mm. On the other hand, the envelope printing heating member <b>63</b> is formed so that the length in the longitudinal direction is shortened. According to the present exemplary embodiment, the length of the envelope printing heating member <b>63</b> with respect to the longitudinal center, i.e., the sheet-passing reference, is 182 mm, which is sufficient to fix an image on a B5 size envelope of width 176 mm. The methods for driving the heating members <b>62</b> (<b>62</b><i>a </i>and <b>62</b><i>b</i>) and <b>63</b> in each fixing mode are similar to those according to the first exemplary embodiment. More specifically, the heating members <b>62</b><i>a</i>, <b>62</b><i>b</i>, and <b>63</b> can be independently turned on via electrodes <b>64</b><i>a</i>, <b>64</b><i>b</i>, and <b>64</b><i>c </i>illustrated in <figref idref="DRAWINGS">FIG. 8A</figref>.

0078The heating control of the heating members <b>62</b><i>a</i>, <b>62</b><i>b</i>, and <b>63</b> is performed in each fixing mode as follows. In the first fixing mode, i.e., normal printing, the normal printing heating member <b>62</b> is used, and in the second fixing mode, i.e., envelope printing in which the pressing force is reduced, the envelope printing heating member <b>63</b> is used.

0079According to the present exemplary embodiment, the length of the envelope printing heating member <b>63</b> in the longitudinal direction is shortened to correspond to the width of the envelope. As a result, a region which is uselessly heated in a non-sheet passing portion of the envelope can be reduced in the second fixing mode, in which the pressing force is reduced when the envelope is passed through, so that the power consumption in the second fixing mode can be further reduced.

0080According to the present exemplary embodiment, the longitudinal center is set as the sheet-passing reference. However, the present invention is not limited to this configuration, and the present invention may be applied when the longitudinal edge portion is set as the sheet-passing reference by disposing the heating members in the longitudinal direction accordingly. Further, <figref idref="DRAWINGS">FIG. 9</figref> illustrates a heater <b>65</b> according to the present exemplary embodiment including a heating member <b>68</b> exclusively for an envelope in which the heat generation amount decreases from the sheet-passing reference to the longitudinal edge portion.

0081The effect acquired in the present invention may also be realized according to a fourth exemplary embodiment to be described below. According to the present exemplary embodiment, a normal printing heating member <b>72</b> and a curl prevention heating member <b>73</b> are formed as the heating members of a heater <b>70</b> in the fixing apparatus. The other configurations are similar to those according to the first exemplary embodiment, and detailed description will thus be omitted. According to the present exemplary embodiment, the first fixing mode is a normal processing mode for processing the recording material of a normal thickness, and the second fixing mode is a curl reduction mode for processing a thin recording material.

0082<figref idref="DRAWINGS">FIG. 10A</figref> is a plain view illustrating the heater <b>70</b> according to the present exemplary embodiment. Referring to <figref idref="DRAWINGS">FIG. 10A</figref>, both the normal printing heating member <b>72</b> and the curl prevention heating member <b>73</b> of the heater <b>70</b> according to the present exemplary embodiment have approximately flat heating distribution in the longitudinal direction.

0083Further, according to the present exemplary embodiment, the second fixing mode which is of reduced pressing force is employed as a curl prevention mode (i.e., the curl reduction mode) set when using the recording material that easily becomes curled, such as thin paper. The range of the region of the fixing nip portion N<b>2</b> in the second fixing mode is set narrower than the fixing nip portion N<b>1</b> in the first fixing mode, so that excessive heat is reduced from being supplied to the recording material P. The deformation of the recording material P caused by passing through the fixing apparatus, i.e., curling, can thus be prevented. According to the present exemplary embodiment, the width of the fixing nip portion N<b>1</b> is 9 mm, and the width of the fixing nip portion N<b>2</b> is 6 mm.

0084The methods for driving the heating members <b>72</b> (<b>72</b><i>a </i>and <b>72</b><i>b</i>) and <b>73</b> in each fixing mode are similar to those according to the first exemplary embodiment. More specifically, the heating members <b>72</b> and <b>73</b> can be independently turned on via electrodes <b>74</b><i>a</i>, <b>74</b><i>b</i>, and <b>74</b><i>c </i>illustrated in <figref idref="DRAWINGS">FIG. 10A</figref>. In the first fixing mode, i.e., the normal paper printing mode, only the heating member <b>72</b> or both the heating members <b>72</b> and <b>73</b> are turned on in performing heating control. In the second fixing mode of lower pressing force, i.e., the curl prevention mode for thin paper, the heating member <b>73</b> is turned on.

0085According to the present exemplary embodiment, the second fixing mode of lower pressing force is employed as the curl prevention mode for performing fixing on thin paper. Further, the heating member <b>72</b> of which a portion or the entirety is disposed outside the inner surface nip portion Na<b>2</b> is not turned on. The heat efficiency in the curl prevention mode is thus prevented from becoming lowered.

0086According to the present exemplary embodiment, the normal printing heating members <b>72</b><i>a </i>and <b>72</b><i>b </i>are disposed in both the upstream position and the downstream position of the curl prevention heating member <b>73</b>. However, the present invention is not limited to this configuration, and the normal printing heating member <b>72</b> may be disposed on either the upstream position or the downstream position of the curl prevention heating member <b>73</b>. Other various modifications within the technological scope may also be performed. Further, according to the present exemplary embodiment, only the heating member <b>73</b> is turned on in the second fixing mode. However, the present invention is not limited to this configuration, and a similar effect can be achieved even when both the heating members <b>72</b> and <b>73</b> are turned on, and the turn-on time ratio of the heating member <b>72</b> is reduced as compared to the first fixing mode.

0087While the present invention has been described with reference to exemplary embodiments, it is to be understood that the invention is not limited to the disclosed exemplary embodiments. The scope of the following claims is to be accorded the broadest interpretation so as to encompass all modifications, equivalent structures, and functions.

0088This application claims priority from Japanese Patent Application No. 2010-252471 filed Nov. 11, 2010, which is hereby incorporated by reference herein in its entirety.