Patent Publication Number: US-7725049-B2

Title: Image forming apparatus having duct and exhaust outlet

Description:
CROSS REFERENCE TO RELATED APPLICATION 
   This application claims priority from Japanese Patent Application No. 2006-327431, filed on Dec. 4, 2006, the entire subject matter of which is incorporated herein by reference. 
   FIELD 
   Aspects of the invention relate to an electrophotographic image forming apparatus, such as a tandem-type image forming apparatus. 
   BACKGROUND 
   A known electrophotographic image forming apparatus is designed to form an image on a recording sheet by transferring a developer image formed on a developing device, e.g. a photosensitive drum, onto the recording sheet and fixing the transferred image on the recording sheet by heat. 
   In the electrophotographic image forming apparatus, a great amount of heat is generated by an image forming device including the fixing device and the developing device. Thus, to prevent a temperature of the image forming device from excessively rising, the image forming apparatus includes an exhaust fan configured to discharge hot air from the apparatus. 
   The image forming apparatus also includes a permeable filter in an exhaust duct for discharging hot air. The permeable filter is configured to prevent dust suspended in air, e.g. developer particles and paper dust, from being discharged along with the hot air. 
   The permeable filter is configured to remove dust by filtering the air. 
   To sufficiently remove dust from the hot air, which is discharged from the apparatus, by using a permeable filter, the permeable filter needs to be made of a tight woven fiber such that minute air passages formed in the permeable filter can be decreased in size. However, if the capability of removing dust is improved in this manner, ventilation resistance at which the air passes through the permeable filter will increase. 
   When the ventilation resistance increases, the amount of air passing through the permeable filter may decrease, a sufficient amount of hot air may not be discharged, and the temperature in the image forming device may excessively rise. 
   Such problems may be solved with an exhaust fan having a high ventilation capability. However, to increase the ventilation capability, some measures may be taken, e.g., the exhaust fan may be increased in size or a rotational speed of the exhaust fan may be increased. 
   However, if a larger-sized exhaust fan is used, the image forming apparatus tends to become larger in size. If the rotational speed of the exhaust fan is increased, noise produced by the exhaust fan may become high. 
   SUMMARY 
   Aspects of the invention provide an image forming apparatus configured to remove dust from hot air discharged from the apparatus. An image forming apparatus which forms an image on a recording sheet includes a housing having an exhaust outlet and an image forming unit disposed in the housing and configured to form an image on a recording sheet. A duct in the housing may have an inlet at a first end in the housing, and communicate with the exhaust outlet at a second end in the housing. The image forming apparatus includes an exhaust fan disposed between the exhaust outlet and the duct, and being configured to cause air entering the duct from the inlet to go toward the exhaust outlet. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Illustrative aspects of the invention will be described in detail with reference to the following figures in which like elements are labeled with like numbers and in which: 
       FIG. 1  is a perspective view of a laser printer according to an illustrative embodiment of the invention; 
       FIG. 2  is a side sectional view of the laser printer; 
       FIG. 3  shows an exhaust duct viewed from a drawer unit; 
       FIG. 4  is a top view of the exhaust duct of  FIG. 3 ; 
       FIG. 5  is a cross-sectional view of the exhaust duct along the line A-A of  FIG. 3 ; 
       FIG. 6  is a perspective view of the exhaust duct and an exhaust fan; 
       FIG. 7  is a perspective view of the exhaust duct; 
       FIG. 8  is a perspective view of the exhaust duct without a filter unit; 
       FIG. 9  is a perspective view of the filer unit; 
       FIG. 10  shows positional relationships of the exhaust duct and drawer unit in the housing viewed from the top; 
       FIG. 11  is a front view of a louver according to a second illustrative embodiment of the invention; and 
       FIG. 12  is a perspective view of a housing containing the louver. 
   

   DETAILED DESCRIPTION 
   An illustrative embodiment of the invention will be described in detail with reference to the accompanying drawings. An image forming apparatus according to aspects of the invention is applied to a laser printer used in connection with a computer in this illustrative embodiment. It will be appreciated that aspects of the invention apply to other types of image forming apparatuses as well. 
   An appearance of a laser printer  1  will be now described with reference to  FIG. 1 . 
   An upper side of  FIG. 1  is referred to as the top of the laser printer  1 , and the right side of  FIG. 1  is referred to as the front side of the laser printer  1 . In the following description, top, bottom, rear, and front of objects in the laser printer  1  are used with reference to the arrows in  FIG. 1 . 
   A housing  3  is provided for an apparatus body of the laser printer  1 . A sheet discharge tray  5  may be provided on the top of the housing  3 . Printed recording sheets such as plain paper or transparencies may be ejected from the housing  3  and received on the sheet discharge tray  5 . An apparatus frame member (not shown) made of metal or resin may be provided in the housing  3 . A drawer unit  70  and a fixing unit  80  may be coupled to the apparatus frame member disposed in the housing  3  in a detachable manner as shown in  FIG. 2 . 
   An internal structure of the laser printer  1  will be described with reference to  FIG. 2 . 
   The laser printer  1  may include an image forming unit  10 , a feeder portion  20 , and a feed unit  30 . The image forming unit  10  is configured to form an image on a recording sheet. The feeder portion  20  may function as a part of a feeding device configured to supply a recording sheet to the image forming unit  10 . The feed unit  30  may be configured to feed a recording sheet to four developing cartridges  70 K,  70 Y,  70 M,  70 C included in the image forming unit  10 . 
   After an image has been recorded on a recording sheet, an intermediate feed roller  90  and an ejection chute (not shown) may take the recording sheet and feed it upwards towards ejection rollers  91 . The ejection rollers  91  may cause the sheet to be ejected from the ejection portion  7  and onto the ejection tray  5 . 
   The feeder portion  20  may include a sheet supply tray  21 , a sheet supply roller  22 , and a separation pad  23 . The sheet supply tray  21  may be disposed in the lowermost part of the housing  3 , and may be configured to hold a stack of recording sheets. The sheet supply roller  22  may be disposed at an upper front end of the sheet supply tray  21 , and may be configured to supply or feed a recording sheet from the sheet supply tray  21  to the image forming unit  10 . The separation pad  23  may be disposed downstream of the sheet supply roller  22  in the direction of the roller&#39;s rotation, and may be configured to apply a resistance to separate a topmost sheet from the stack of recording sheets in the sheet supply tray  21 . The recording sheet stored in the sheet supply tray  21  makes a u-turn (e.g., is flipped over) at the front side of the housing  3 , and is conveyed to the image forming unit  10 , which may be centrally disposed in the housing  3 . 
   The feed unit  30  may include a drive roller  31 , a driven roller  32 , and a conveyor belt  33 . The drive roller  31  may be configured to rotate along with an operation in the image forming unit  10 . The driven roller  32  may be spaced away from the drive roller  31  and may be configured to rotate. The conveyor belt  33  may be stretched between the drive roller  31  and the driven roller  32 . 
   The drive roller  31  is rotatably supported by a frame (not shown) of the feed unit  30  with a rotating shaft of the drive roller  31  being fixed. The driven roller  32  is rotatably supported by the frame with a rotating shaft of the driven roller  32  being changeable. The driven roller  32  is urged by a deformable member (not shown), e.g. a spring, in a frontward direction to be separated from the drive roller  31  directly or indirectly. This applies a tension to the conveyor belt  33 . 
   The image forming unit  10  may be a direct-tandem type, where color printing is possible. The image forming unit  10  may include a scanner unit  60 , the drawer unit  70 , and the fixing unit  80 . 
   The scanner unit  60  may be disposed in an upper portion of the housing  3 , and may be configured to form electrostatic latent images on corresponding surfaces of photosensitive drums  71  disposed in the four developing cartridges  70 K,  70 Y,  70 M, and  70 C, respectively. The scanner unit  60  may include a laser light source, a polygon mirror, fθ lenses and reflecting mirrors. 
   A laser beam emitted from the laser light source, based on image data, may be deflected by the polygon mirror, pass through the fθ lenses, and be folded by the reflecting mirrors to be directed to a surface of the photosensitive drum  71 , on which an electrical latent image is formed. 
   The drawer unit  70  may include the four developing cartridges  70 K,  70 Y,  70 M, and  70 C, and a slider casing  75  that stores the cartridges  70 K,  70 Y,  70 M, and  70 C therein. The slider casing  75  may be coupled to the housing  3  so as to move in a horizontal direction, i.e., in a front-rear direction of the laser printer  1  in this illustrative embodiment, while being supported by rails (not shown) disposed in the apparatus frame member of the housing  3 . 
   The four developing cartridges  70 K,  70 Y,  70 M, and  70 C may correspond to four color types of developer, such as black, yellow, magenta, and cyan, respectively, and may be arranged in a line along a sheet feeding direction. The developing cartridges  70 K,  70 Y,  70 M, and  70 C are configured to transfer a developer image of a corresponding color directly onto a recording sheet. 
   The four developing cartridges  70 K,  70 Y,  70 M, and  70 C may be identical in structure, but with different colors of developer. Thus, in the following description, the structure of the developing cartridges will be described by using the developing cartridge  70 C as an example. 
   The developing cartridge  70 C may include a photosensitive drum  71 , a charger  72 , and a developer storing portion  74  inside. 
   The photosensitive drum  71  may be configured to carry an image that is to be transferred onto a recording sheet. The photosensitive drum  71  may be cylindrically shaped, and its outermost layer may be a positively charged photosensitive layer made of polycarbonate. 
   The charger  72  may be configured to charge the surface of the photosensitive drum  71 . The charger  72  may be disposed away from the photosensitive drum  71 , so as to face the photosensitive drum  71  diagonally rearward from above. 
   The charger  72  according to this illustrative embodiment may be a scorotron charger that charges the surface of the photosensitive drum  71  uniformly and positively by corona discharge from a charging wire made of tungsten or the like. 
   A transfer roller  73  may be disposed to face the photosensitive drum  71 , and may be configured to rotate along with the rotation of the conveyor belt  33 . 
   Also, the transfer roller  73  may transfer a developer image adhering on the surface of the photosensitive drum  71  to a print surface of the recording sheet by applying an electrical charge, having a polarity (a negative charge in this illustrative embodiment) opposite to an electrical charge of the photosensitive drum  71 , to the recording sheet from the bottom side (opposite the print surface) of the recording sheet as it passes by the photosensitive drum  71 . 
   A developer storing portion  74  may include a developer chamber  74 A, a developer supply roller  74 B, and a developing roller  74 C. Developer may be stored in the developer chamber  74 A. The developer supply roller  74 B and the developing roller  74 C may be configured to supply developer to the photosensitive drum  71 . 
   Developer stored in the developer chamber  74 A may be supplied to the developing roller  74 C along with the rotation of the developer supply roller  74 B. The developer supplied to the developing roller  74 C may be carried on a surface of the developing roller  74 C, regulated to a uniform thickness by a layer thickness regulating blade  74 D, and then supplied to the surface of the photosensitive drum  71  that is exposed to light by the scanner unit  60 . 
   The fixing unit  80  may be disposed rearward from the photosensitive drum  71  with respect to the sheet feeding direction, and may be configured to melt developer transferred onto the recording sheet by heat and fix it to the recording sheet. The fixing unit  80  may be removable from the apparatus frame member. 
   The fixing unit  80  may include a heat roller  81  and a pressure roller  82 . The heat roller  81  may be disposed to face the print surface of a recording sheet, and may be configured to apply a feeding force to a recording sheet while heating the developer on the recording sheet. The pressure roller  82  may be disposed to face the heat roller  81  from below, and may be configured to press against the heat roller  81 . 
   The heat roller  81  may be rotated in synchronization with the developing roller  74 C and the conveyor belt  33 . The pressure roller  82  may receive a rotational force from the heat roller  81  via a recording sheet that is sandwiched between the rollers  81 ,  82 , and rotate by following the rotation of the heat roller  81 . 
   As shown in  FIG. 1 , an exhaust outlet  3 A is disposed in an upper portion on a side of the housing  3 . Inside and outside of the housing  3  communicate with each other via the exhaust outlet  3 A. Air in the housing  3  is discharged from the exhaust outlet  3 A. As shown in  FIG. 2 , the exhaust outlet  3 A is connected to an exhaust duct  100 . The exhaust duct  100  is configured to direct air in the housing  3  to the exhaust outlet  3 A. The exhaust duct  100  includes a first inlet  101 . The first inlet  101  is disposed in a lower portion of the exhaust duct  100 , located between the fixing unit  80  and the drawer unit  70 , and is opened toward the fixing unit  80 . 
   The first inlet  101  is covered with a first filter  101 A. The first filter  101 A includes a flat adsorptive surface  101 B, which may be electrostatically charged, for collecting dust. The first filter  101 A may be disposed such that the adsorptive surface  101 B is substantially perpendicular (i.e., within 15 degrees of perpendicular) to the direction of air flowing into the exhaust duct  100  from the first inlet  101 . The first filter  101 A is a permeable filter through which air flows. The first filter  101 A is configured to remove dust from the air by catching the dust (e.g., by using a self-created electrostatic charge) at the adsorptive surface  101 B when the air is filtered. The first filter  101 A may be a non-woven fabric made from a polypropylene-based material. 
   As shown in  FIGS. 2 and 10 , the exhaust duct  100  extends in a direction parallel to an axial direction of the drive roller  31  (or a width direction of the laser printer  1 ), and is located in a space above a sheet feed path in the housing  3  between the fixing unit  80  and the drawer unit  70 . 
   Thus, as shown in  FIG. 5 , air flowing through the first inlet  101  into the exhaust duct  100  is directed toward a second filter  103  and an inner wall  102  of the exhaust duct  100  disposed toward the drawer unit  70 . The air flows upward along the second filter  103  and the inner wall  102  toward the exhaust outlet  3 A. 
   In this illustrative embodiment, the inner wall  102  is inclined with respect to the vertical direction such that an air passage area of the exhaust duct  100  widens toward the exhaust outlet  3 A or in an upward direction. This structure reduces pressure loss that is generated when the direction of the air flowing through the first inlet  101  is changed. 
   The second filter  103  includes flat contact surfaces  103 A. The second filter  103  is disposed downstream from the first filter  101 A in the direction of airflow in the exhaust duct  100 . In addition, the second filter  103  is disposed so that the contact surfaces  103 A and the surface  101 B of the first filter  101 A are disposed in array and face each other. Thus, air flowing through the first filter  101 A is received by and contacts the second filter  103 . The second filter  103  is a contact-type filter configured to remove dust from air flowing into the exhaust duct  100  by catching the dust at the contact surfaces  103 A when the air contacts the contact surfaces  103 A. 
   The air having passed through the second filter  103  goes upward in the exhaust duct  100 . The second filter  103  can be positioned in the exhaust duct  100  such that the contact surfaces  103 A are parallel to the upward airflow. In other words, the second filter  103  is positioned such that the air, which flows from the left to the right in  FIG. 5 , just after passing through the first filter  101 A, contacts the contact surfaces  103 A. 
   More specifically, the first inlet  101  is open in a direction to take in air substantially horizontally (i.e., within 15 degrees of the front direction in  FIG. 5 ), and the air flowing into the exhaust duct  100  from the first inlet  101  changes its direction approximately 90 degrees upward just after entering the exhaust duct  100 . The second filter  103  is disposed such that the contact surfaces  103 A are substantially parallel to the airflow direction that has been changed and the width direction of the laser printer  1 . 
   “The contact surfaces  103 A are substantially parallel to the airflow” means that macroscopic airflow is parallel to the contact surfaces  103 A. As the air contacts the contact surfaces  103 A, a large quantity of air contacts the contact surfaces  103 A microscopically. As the air contacts the contact surfaces  103 A, dust suspended in the air adheres to the contact surfaces  103 A due to the momentum caused by the contact. Thus, dust can be removed from the air. 
   Similar to the first filter  101 A, the second filter  103  may be constructed from a non-woven fabric made from a polypropylene-based material. The second filter  103  catches dust by adhesion or adsorption and also filtration by which the air passes through the contact surfaces  103 A of the second filter  103 . 
   The second filter  103  may have electrostatic attraction for collecting dust using static electricity generated when the air contacts the second filter  103 , and/or may employ ozone absorption using activated carbon. 
   A third filter  102 A may be disposed in contact with the inner wall  102 . The third filter  102 A may be the same contact-type filter as the second filter  103 . The first filter  101 A and the second filter  103  may be combined via a filter frame  104  as shown in  FIG. 9 . The filter frame  104  is detachably attached to the exhaust duct  100  via a deformable engaging member. In the following description, the first filter  101 A and the second filter  103  integrally formed via the filter frame  104  are collectively referred to as a filter unit  105 . 
   The filter frame  104  is provided with deformable engaging protrusions  104 A,  104 B. The exhaust duct  100  is provided with recessed portions, not shown, that are engaged with the engaging protrusions  104 A,  104 B. The filter frame  104  is detachably attached to the exhaust duct  100  via the deformable engaging protrusions  104 A,  104 B. 
   The filter unit  105  can be mounted in the exhaust duct  100  as the engaging protrusions  104 A,  104 B engage in the recessed portions. When a pressing portion  104 C is pressed, the engaging protrusions  104 A,  104 B are deformed and disengaged from the recessed portions, and the filter unit  105  can be removed from the exhaust duct  100 . 
   The exhaust duct  100  is provided with a second inlet  106  in the upper portion as shown in  FIG. 2 . The second inlet  106  may be formed with a number of openings as shown in  FIG. 3 . As with the first inlet  101 , the second inlet  106  is provided with a fourth filter  106 A, which is a permeable filter. 
   An intake  3 B is provided at the rear of the housing  3  as shown in  FIG. 2 . The intake  3 B is disposed to allow the inside and outside of the housing  3  to communicate with each other and take in air for cooling into the housing  3 . The intake  3 B is provided with slats  3 C. The slats  3 C are inclined downward from the horizontal direction from the inside to outside of the housing  3 . 
   An exhaust fan  110  is disposed between the exhaust outlet  3 A of the housing  3  and the exhaust duct  100 . The exhaust fan  110  is configured to cause air entering the exhaust duct  100  from the first inlet  101  or the second inlet  106  to go toward the exhaust outlet  3 A. The exhaust fan  110  is constructed of an axial fan which causes air to flow in a direction parallel to a rotating shaft. 
   The exhaust fan  110  is provided, at an exhaust side, with a louver  120  including fins  121 , as shown in  FIG. 6 . The louver  120  is configured to guide or change airflow from the exhaust fan  110  in at least two different directions. 
   More specifically, the fins  121  are disposed to guide airflow from the exhaust fan  110  toward a rotational direction (or a tangential direction) of the exhaust fan  110  and to prevent airflow guided in one of the two different directions from colliding with airflow guided in the other direction. 
   In  FIG. 6 , the fins  121  are divided into upper fins  121 A disposed above a center of rotation of the exhaust fan  110  and lower fins  121 B disposed below the center of rotation of the exhaust fan  110 . Airflow guided by the upper fins  121 A and airflow guided by the lower fins  121 B is controlled to avoid collision with each other. 
   In the louver  120 , the upper fins  121 A and the lower fins  121 B are inclined or slanted in opposite directions with respect to a central portion of the louver  120  in the longitudinal direction or vertical direction in  FIG. 6 . That is, the upper fins  121 A are disposed to guide airflow to the front, while the lower fins  121 B are disposed to guide airflow to rear. It will be appreciated that the upper fins  121 A and the lower fins  121 B may not be divided at the central portion of the louver  120 . For example, the upper fins  121 A and the lower fins  121 B may be divided in an area above or below the central portion of the louver  120 . 
   The louver  120  is mounted to the housing  3  so as to rotate on the center of the rotation of the exhaust fan  110 . In  FIG. 6 , the upper fins  121 A guide airflow to the front and the lower fins  121 B guide airflow to the rear. However, the directions in which the fins  121  guide the airflow are not limited to the front and rear, as shown in  FIG. 6 . 
   The first filter  101 A and the second filter  103  can be disposed in an array, in the direction of the airflow, so that their surfaces  101 B and  101 A face each other. After dust suspended in the air is removed by the first filter  101 A, further dust in the air can be removed by the second filter  103 . Thus, dust can be sufficiently removed from hot air discharged from the housing  3 . 
   Dust can be removed from the air by two different types of filters, that is, a permeable filter and a contact-type filter, which are disposed in an array so that their surfaces face each other to receive air serially in the direction of the airflow. With the use of the two different types of filters together, increased amounts of dust can be filtered from the air. Thus, there is no need to further reduce in size minute air passages formed in the first filter  101 A that is the permeable filter, and the ventilation resistance generated in the first filter  101 A can be prevented from increasing. 
   The second filter  103  is configured to remove dust contained in air contacting the contacting surface  103 A and flowing along the contact surface  103 A, while the first filter  101 A is configured to remove dust in the air when the air flows through the first filter  101 A. Thus, the ventilation resistance to be generated in the second filter  103  can be prevented from increasing. 
   According to the illustrative embodiment, the filters can remove dust, while preventing the ventilation resistance from increasing. Thus, the filters can remove dust from hot air discharged from the housing  3  without having to increase the size and noise of the exhaust fan  110 . 
   The second filter  103  is disposed downstream from the first filter  101 A in the direction of the airflow. In addition, the second filter  103  is disposed so that the air just having flowed through the first filter  101 A contacts the contact surfaces  103 A and the direction that the air flows is changed. Thus, as the air is reliably caused to contact the contact surfaces  103 A of the second filter  103 , the second filter  103  can catch dust and remove dust from the heat. 
   As the second filter  103  is disposed so that the air flowing through the first filter  101 A contacts the contact surfaces  103 A and the direction that the air flows is changed, the ventilation resistance will increase, as compared with a case that the contact surface  103 A is disposed parallel to the direction of airflow. 
   However, the ventilation resistance of the illustrative embodiment is sufficiently small compared to removing dust only with a permeable type filter. 
   The second filter  103  is not intended to prevent all the air flowing through the contact surface  103 A. The second filter  103  includes an air passage that allows some of the air to flow through or permeate the contact surface  103 A. Accordingly, the loss of pressure associated with the air contacting the contact surface  103 A can be reduced. Thus, dust can be sufficiently removed from the air while the ventilation resistance can be prevented from increasing. 
   The second filter  103  is made from a non-woven fabric having electrostatic attraction and ozone absorption. Thus, the second filter  103  can be made inexpensively while effectively removing dust from the air. 
   The first filter  101 A and the second filter  103  can be combined and function as the filter unit  105 . Thus, the first filter  101 A and the second filter  103  can be handled as a single component and easily attached to and removed from the exhaust duct  100 . 
   The second filter  103  also allows air to pass through. The second filter  103  can efficiently remove dust because it can catch the dust by use of both air permeability and through electrostatic adhesion with the contact surface  103 A. 
   The louver  120  is configured to guide airflow from the exhaust fan  110  toward the rotation direction or tangential direction of the exhaust fan  110  and to prevent airflow guided in one of the two different directions from colliding with airflow guided in the other direction. Thus, hot air can be smoothly discharged from the housing  3  without lowering the fan efficiency of the exhaust fan  110 . In addition, the louver  120  covers the exhaust fan  110  so as to prevent foreign matter from entering the exhaust fan  110 . 
   In  FIG. 6 , the length of each fin  121  extends vertically in order to align with protruding ridges  3 D extending vertically on the side of the housing  3  as shown in  FIG. 1 . The fins  121  may extend in a direction (e.g., longitudinal) parallel to each other. To discharge hot air smoothly from the housing  3  without lowering the fan efficiency of the exhaust fan  110 , the fins  121  do not need to be arranged so that their length extends vertically. The fins  121  may be arranged so that their length extends horizontally. 
   A second illustrative embodiment will be described. 
   The louver  120  according to the first illustrative embodiment is configured to guide airflow from the exhaust fan  110  in two different directions only. The exhaust fan  110 , which is an axial fan, cannot guide the entire airflow from the exhaust fan  110  toward the rotation direction or tangential direction of the exhaust fan  110 . 
   In the second illustrative embodiment, a louver  130  includes fins  131  that are spirally shaped like blades of a centrifugal multi-blade fan or a centrifugal pump as shown in  FIGS. 11 and 12 . This structure allows the entire airflow from the exhaust fan  110  to be guided toward the rotational direction or tangential direction of the exhaust fan  110 . 
   Thus, hot air can be smoothly discharged from the housing  3  by increasing the fan efficiency of the exhaust fan  110 . In addition, the louver  130  covers the exhaust fan  110  so as to prevent foreign matter from entering the exhaust fan  110 . 
   The first filter  101 A, the second filter  103 , and the third filter  102 A can be constructed from a non-woven fabric made from a polypropylene-based material. However, the filters may be constructed from a non-woven or woven fabric made from another material, e.g., polyester, polyethylene terephthalate, nylon, cellulose, glass fiber, or metal. Each filter may be constructed from a different material. 
   The first filter  101 A, the second filter  103 , and the third filter  102 A may be multilayer filters. A multi-layer filter can be constructed from a combination of a filter of coarse and robust mesh and a filter of fine, soft, non-woven fabric. With this configuration, the filter of coarse and robust mesh can maintain the entire strength, and the filter of fine, soft, non-woven fabric can efficiently catch dust, e.g. toner particles. 
   The exhaust fan  110  can be disposed at one side of the exhaust duct  100  with respect to the width of the exhaust duct  100  or the axial direction of the drive roller  31 . However, the exhaust fan  110  may be disposed at each side of the exhaust duct  100 . 
   The exhaust fan  110  is an axial fan. However, the exhaust fan  110  may be another type of fan, e.g. a centrifugal multi-blade fan such as a turbo fan and a sirocco fan, and a cross-flow fan. 
   The third filter  102 A may be disposed on the inner wall  102 . However, it is sufficient if the permeable filter (the first filter  101 A) and at least one contact-type filter are disposed in an array so that their surfaces face each other in the direction of the airflow. The second filter  103  or the third filter  102 A may be omitted. The second filter  103  or the third filter  102 A may be disposed upstream from the first filter  101 A. Two or more second filters  103  may be disposed in an array so that their surfaces face each other to receive air serially in the direction of airflow. Also, the second filters may be disposed in parallel to receive air in the direction of airflow concurrently. Also, certain second filters  103  may be disposed in parallel to receive air in the direction of airflow concurrently with those certain filters  103  being in series with one or more other second filters  103  in the direction of airflow. 
   If at least one of the first filter  101 A, the second filter  103 , and the third filter  102 A is fluffy, noise can be absorbed when air is blown toward the exhaust duct  100 . In this case, the length of fibers can be about 1-3 mm. 
   The direction of airflow is changed approximately 90 degrees just after the air passes through the first filter  101 A. It will be appreciated that the change in direction of airflow is not limited to 90 degrees. However, instead of changing the direction of airflow just after the air passes through the first filter  101 A, the first inlet  101  may be opened downward and the contact surface  103 A of the second filter  103  may be disposed parallel to the direction of the airflow. 
   The position of the exhaust duct  100  is not limited to the position shown in  FIG. 2 . The exhaust duct  100  may be disposed in other positions. 
   The second filter  103  applies electrostatic attraction using static electricity, and ozone absorption using activated carbon. However, the second filter  103  may be electrically charged to exert electrostatic attraction. Alternatively, the second filter  103  may not apply either electrostatic attraction or ozone absorption. 
   The first filter  101 A and the second filter  103  can be combined via the filter frame  104 . However, these filters may be attached to and detached from the exhaust duct  100  independently. Alternatively, the first filter  101 A, the second filter  103 , and the third filter  103 A may be combined. 
   While the features herein have been described in connection with various example structures and illustrative aspects, it will be understood by those skilled in the art that other variations and modifications of the structures and aspects described above may be made without departing from the scope of the invention. Other structures and aspects will be apparent to those skilled in the art from a consideration of the specification or practice of the features disclosed herein. It is intended that the specification and the described examples are illustrative only with the true scope of the invention being defined by the following claims.