Patent Publication Number: US-8123333-B2

Title: Image forming apparatus and head unit

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     The present application claims priority from Japanese Patent Application No. 2007-172984, filed on Jun. 29, 2007, the disclosure of which is incorporated herein by reference in its entirety. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an image forming apparatus which forms an image by jetting droplets of a liquid such as an ink from a plurality of nozzles toward a recording medium, while moving a head unit having the plurality of nozzles, and relates to the head unit. 
     2. Description of the Related Art 
     As it is shown in  FIG. 6A  and  FIG. 6B , an ink-jet printer, which forms an image on a recording paper  82  by jetting ink droplets from a plurality of nozzles toward the recording paper  82  while moving a jetting head  81  in a scanning direction, has been used practically. In this ink-jet printer, when the jetting head  81  is moved, air enters relatively between the jetting head  81  and the recording paper  82  from one end of a movement direction of the recording head  81 , and this air flows in one direction of the scanning direction. Since both sides orthogonal to the scanning direction between the jetting head  81  and the recording paper  82  (in other words, both sides in a transporting direction of the recording paper) are open, thereafter, the air escapes to the both sides, in the transporting direction, at which a resistance is low ( FIG. 6A  is a plan view explaining this state). As a result, an air current A, which spreads over the both side in the transporting direction while flowing in one direction of the scanning direction, is formed between the jetting head  81  and the recording paper  82 , on both sides in the scanning direction and nearby. 
     Moreover, generally, it has been known that ink droplets which are jetted from the nozzle include a main droplet  83   b  which is accompanied by a satellite droplet  83   a  having a weight smaller than the main droplet. Since the main droplet  83   b  is heavier in weight, the main droplet  83   b  is not so affected by the air current A, and flies almost straight from the nozzle. On the other hand, the satellite droplet  83   a  jetted from the nozzles on both sides in the transporting direction flies to be deflected in a direction in which the air current A spreads. Consequently, as shown in  FIG. 6B , the satellite droplet  83   a  jetted from the nozzle at an intermediate portion in the transporting direction while flying to be deflected lands on the recording paper  82  at a position which is horizontal in the scanning direction of the main droplet  83   b . On the other hand, the satellite droplet  83   a , in the air current A which spreads toward both sides in the transporting direction, lands at a position which is shifted to be inclined with respect to the main droplet  83   b . As a result, relative positions of landing of the satellite droplet  83   a  and the main droplet  83   b  are different between near the center position of the transporting direction and both sides of the transporting direction. Therefore, a dot which is formed by the satellite droplet  83   a  and the main droplet  83   b  is shifted from a desired position of landing, and a diameter and a shape of the dot are not uniform, thereby causing a defective image quality. 
     When the main droplet  83   b  is fine, or a jetting speed is slow, there is a possibility that the main droplet  83   b  is affected by the air current A. In this case, there is a possibility that the defective image quality becomes more substantial. 
     Moreover, when ink droplets are jetted from a nozzle, not only the main droplet  83   b  and the satellite droplet  83   a , but also more fine ink particles called as a mist are known to be formed. It is considered that the mist is generated by an air current between the jetting head and the recording paper, when the ink is ejected from the nozzle and divided into the main droplets  83   b  and the satellite droplets  83   a . Moreover, the mist is floated by the air current which is generated by the movement of the jetting head. 
     For preventing the mist and the satellite droplets of the ink from being floated over a wide range, in a jetting head according to a conventional technology, a discharge port which is capable of discharging air toward the recording paper is formed at a front side in the scanning direction. For example, in a recording head described in US Patent Application Laid-open No. US-2002089563 (corresponds to Japanese Patent Application Laid-open No. 2002-273859), an air curtain is formed at a front side in the scanning direction with respect to nozzles by discharging air from the discharge port. Due to the air curtain, an air current between the jetting head and the recording paper, which is formed when the jetting head is moved, is blocked, and the mist and the satellite droplets of the ink are prevented from being floated over a wide range. 
     However, in the recording head of the conventional technology, the space between the jetting head and the recording paper is open in the scanning direction and the transporting direction. According to such a structure, an air curtain is formed and air is intercepted at the front side of the jetting head. However, air is blown between the jetting head and the recording paper from both sides of the transporting direction, and a vortex flow is generated at a rear side of the air curtain. The vortex flow disturbs the air current between the jetting head and the recording paper, and the liquid droplets land at an undesired position of the recording paper. In this manner, although the jetting head of the conventional technology is capable of preventing the floating of the mist and the satellite droplets of the ink over a wide range, it is difficult to land a dot accurately at a desired position, and to make uniform a diameter and a shape of the dots. 
     SUMMARY OF THE INVENTION 
     An object of the present invention is to provide an image forming apparatus which is capable of forming a high quality image by landing a dot of ink droplets jetted from each nozzle at a desired position, and by making a diameter and a shape of the dots uniform. 
     According to a first aspect of the present invention, there is provided an image forming apparatus which forms an image by jetting liquid droplets of a liquid onto a recording medium, the apparatus including: a head unit which has, on a bottom wall of the head unit, a jetting head having a plurality of nozzles for jetting the liquid droplets, and which jets the liquid droplets from the nozzles onto the recording medium while moving in a scanning direction; a gas inlet port which is open through a side wall, the side wall being orthogonal to the scanning direction of the head unit, and via which a gas is taken into the head unit when the head unit moves in the scanning direction; and a pair of gas discharge ports which is open through the bottom wall of the head unit, at positions on both sides in a direction orthogonal to the scanning direction, to interpose the nozzles therebetween, and via which the gas, taken from the gas inlet port, is discharged toward the recording medium. 
     When the head unit is moved in the scanning direction, a gas is taken in via the gas inlet port, and the gas taken in is discharged through the pair of gas discharge ports, toward the recording medium. The pair of gas discharge ports is formed in the bottom wall of the head unit, at positions on both sides in a direction orthogonal to the scanning direction, to interpose the nozzles therebetween. Therefore, by the gas which is discharged from the pair of gas discharge ports, it is possible to form a layer of gas in other words, an air curtain, which flows toward the recording medium on both sides with respect to the nozzles in the direction orthogonal to the scanning direction. Accordingly, when the head unit is moved, it is possible to suppress an air current which spreads over the both sides orthogonal to the scanning direction, between the jetting head and the recording medium. Further, it is possible to land the liquid droplets jetted from the nozzle on the recording medium without being deflected in the direction orthogonal to the scanning direction, and thereby facilitating to make uniform a diameter and a shape of dots. 
     Moreover, in the present invention, it is possible to reduce an amount of gas which spreads over both sides orthogonal to the scanning direction from a space between the jetting head and the recording medium by intercepting a gas flow on both sides in the direction orthogonal to the scanning direction, between the jetting head and the recording medium. In other words, it is possible to reduce the amount of gas entering between the jetting head and the recording medium. Accordingly, when the liquid droplets are ejected from the nozzle, it is possible to suppress an effect of the air current on the nozzle, and to suppress the generation of mist of the liquid droplets. Accordingly, it is possible to reduce a defect caused due to the mist of liquid droplets. 
     In the image forming apparatus of the present invention, each of the gas discharge ports may be open to extend in the scanning direction. In this case, since the gas taken via the gas inlet port is discharged through the pair of gas discharge ports each of which extends in the scanning direction, it is possible to form the air curtain along the scanning direction, on both sides in the direction orthogonal to the scanning direction, interposing the nozzles therebetween. Accordingly, the air current which spreads over the both sides orthogonal to the scanning direction is suppressed, and it is possible to land the liquid droplets jetted from the nozzle on the recording medium without being deflected in the direction orthogonal to the scanning direction, and to make a diameter and a shape of dots uniform. 
     The image forming apparatus of the present invention may further include a guide member which guides the gas, taken into the head unit via the gas inlet port, to the pair of gas discharge ports. In this case, it is possible to guide the gas taken via the gas inlet port to the pair of gas discharge ports by the guide member. Accordingly, it is possible to increase a flow of the gas discharged from the pair of gas discharge ports, and to improve a wind-shield effect by the air curtain. 
     In the image forming apparatus of the present invention, the side wall may include a first side wall on one side in the scanning direction of the head unit, and a second side wall on the other side in the scanning direction; and the gas inlet port may have a first gas inlet port which is open in the first side wall, and via which the gas is taken into the head unit when the head unit moves in the one side of the scanning direction, and a second gas inlet port which is open in the second side wall and via which the gas is taken into the head unit when the head unit moves in the other side of the scanning direction. In this case, even when the head unit moves in one side or the other side of the scanning direction, it is possible to take the gas into the head unit from the first gas inlet port or the second gas inlet port. 
     In the image forming apparatus of the present invention, each of the pair of gas discharge ports may have a first gas discharge port via which the gas taken into the head unit from the first gas inlet port is discharged, and a second gas discharge port via which the gas taken into the head unit through the second gas inlet port is discharged; and the guide member may have a first guide member which guides the gas taken via the first gas inlet port to the first gas discharge port, and a second guide member which guides the gas taken via the second gas inlet port to the second gas discharge port. In this case, when the head unit moves in one side of the scanning direction, the gas is taken into the head unit via the first gas inlet port, and the gas taken in is guided to the pair of first gas discharge ports by the first guide member, and discharged. Moreover, when the head unit moves in the other side of the scanning direction, the gas is taken into the head unit via the second gas inlet port, and the gas taken in is guided to the pair of second gas discharge ports by the second guide member, and discharged. In this manner, when the head unit is moved in one side or the other side of the scanning direction, it is possible to form the air curtain by the gas discharged through the pair of gas discharge ports, on both sides, with respect to the nozzles, in the direction orthogonal to the scanning direction. Consequently, it is possible to apply the present invention to the head unit which is movable in both the one side and the other side of the scanning direction. Furthermore, since it is possible to form the air curtain only by moving the head unit by a mechanism which moves the head unit, it is not necessary to provide a mechanism which is to be used only for forming the air curtain, and the purpose is served without making a structure of the entire apparatus complicated. 
     According to a second aspect of the present invention, there is provided a head unit which jets liquid droplets of a liquid while moving in a scanning direction, including: a jetting head which has a plurality of nozzles for jetting the liquid droplets, and which is arranged on a bottom wall of the head unit; gas inlet ports which are formed on side walls on one side and on the other side in the scanning direction of the head unit, respectively; and a pair of gas discharge ports which are formed in the bottom wall, and which extend in the scanning direction to interpose the nozzles therebetween. 
     When the head unit is moved in the scanning direction, the gas is taken in via the gas inlet port, and the gas taken in is discharged through the pair of the gas discharge ports, toward the recording medium. The pair of gas discharge ports is formed in the bottom wall of the head unit, to extend on both sides in the direction orthogonal to the scanning direction, interposing the nozzles therebetween. Therefore, it is possible to form a layer of gas, in other words, an air curtain, flowing through the pair of gas discharge ports toward the recording medium, along the scanning direction, interposing the nozzles. Accordingly, when the head unit is moved, it is possible to suppress the air current which spreads over the both sides orthogonal to the scanning direction, between the jetting head and the recording medium. Further, it is possible to land the liquid droplets jetted from the nozzle on the recording medium without being deflected in a direction orthogonal to the scanning direction, and thereby facilitating to make a diameter and a shape of dots uniform. 
     The head unit of the present invention may further include a guide member which guides a gas taken via the gas inlet ports, to the pair of gas discharge ports. In this case, it is possible to guide the gas which is taken in through the gas inlet port, to the pair of gas discharge ports by the guide member. Accordingly, it is possible to increase a flow of the gas discharged from the pair of gas discharge ports, and to improve a wind-shield effect by the air curtain. 
     In the head unit of the present invention, one of the gas inlet ports formed in the side wall on one side in the scanning direction may be formed as a pair of first gas inlet ports corresponding to the pair of the gas discharge ports; and the other of the gas inlet ports formed in the side wall on the other side in the scanning direction may be formed as a pair of second gas inlet ports corresponding to the pair of the gas discharge ports. In this case, since the gas inlet ports are formed corresponding to the pair of the gas discharge ports, it is possible to guide efficiently the air taken in via each of the gas inlet ports, to the gas discharge port. 
     In the head unit of the present invention, a non-return valve may be provided to each of the first gas inlet ports and each of the second gas inlet ports. 
     In the head unit of the present invention, each of the pair of gas discharge ports may have a first gas discharge port via which discharges the gas taken from the first gas inlet port is discharged, and a second gas discharge port via which the gas taken from the second gas inlet port is discharged; and the guide member may have a first guide member which guides the gas taken from the first gas inlet port to the first gas discharge port, and a second guide member which guides the gas taken from the second gas inlet port to the second gas discharge port. 
     The head unit of the present invention may further include: a circuit element which drives the jetting head; and a heat releasing body which releases heat of the circuit element, and the heat releasing body may be arranged in the vicinity of the guide member. In this case, it is possible to improve further a heat releasing effect of the heat releasing body by the air which is taken in via the gas inlet port, and is guided by the guide member. 
     According to a third aspect of the present invention, there is provided an image forming apparatus which forms an image by jetting liquid droplets of a liquid onto a recording medium, including: the head unit as defined in the second aspect of the present invention; a head-unit moving mechanism which moves the head unit; and a transporting mechanism which transports the recording medium. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic perspective view showing main components of an ink-jet printer according to a first embodiment of the present invention; 
         FIG. 2  is a vertical cross-sectional view of a head unit of the ink-jet printer shown in  FIG. 1 ; 
         FIG. 3  is an enlarged perspective view showing the head unit which is slidably supported by a pair of guide rails; 
         FIG. 4  is a bottom view of the head unit; 
         FIG. 5A  is a cross-sectional view taken along a line VA-VA in  FIG. 4 ,  FIG. 5B  is a cross-sectional view taken along a line VB-VB in  FIG. 4 , and  FIG. 5C  is a cross-sectional view taken along a line VC-VC in  FIG. 4 ; 
         FIG. 6A  is a plan view showing a flow of air between a jetting head and a recording paper in a conventional apparatus, and  FIG. 6B  is a plan view showing a landing position of a main liquid droplet and a satellite liquid droplet; 
         FIG. 7A  is a bottom view of a head unit of a first modified embodiment, and  FIG. 7B  is a cross-sectional view taken along a line VIIB-VIIB in  FIG. 7A ; 
         FIG. 8A  is a bottom view of a head unit of a second modified embodiment,  FIG. 8B  is a cross-sectional view taken along a line VIIIB-VIIIB in  FIG. 8A , and  FIG. 8C  is a cross-sectional view taken along a line VIIIC-VIIIC in  FIG. 8A ; and 
         FIG. 9  is a vertical cross-sectional view of the head unit of the second modified embodiment. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     An embodiment according to the present invention will be described below by referring to the drawings. In the following description, a direction of jetting of an ink from an ink-jet head is described as a downward direction or a lower side, and a side opposite to the downward direction is described as an upward direction or an upper side. 
       FIG. 1  is a perspective view showing main components of an ink-jet printer  1  according the embodiment of the present invention. As shown in  FIG. 1 , the ink-jet printer  1  (image forming apparatus) is provided with a pair of guide rails  2  and  3  which are installed substantially in parallel, and a head unit  4  is slidably supported in a main scanning direction, by the guide rails  2  and  3 . A plurality of ink supply tubes  9  which supply inks of four colors (black, cyan, magenta, and yellow) respectively from an ink tank (not shown in the diagram) is connected to the head unit  4 . An ink-jet head  14  (refer to  FIG. 4 ) having nozzles  49  exposed in a downward direction is mounted on the head unit  4 . At a lower side of the ink-jet head  14 , a recording paper P is transported by a transporting mechanism  10  in a direction orthogonal to the scanning direction (hereinafter, called as a ‘transporting direction’), and the ink is jetted from the ink-jet head  14  toward the recording paper P which is transported. The head unit  4  is joined to a timing belt  7  which is put around a pair of pulleys  5  and  6 , and the timing belt  7  is installed substantially parallel to an extending direction of the guide rail  3 . A motor  8  which drives in normal and reverse rotations is provided to one pulley  5 , and the timing belt  7  reciprocates by the pulley  5  being driven by the normal and the reverse rotations. With the reciprocating of the timing belt  7 , the head unit  4  moves in the scanning direction along the guide rails  2  and  3 . In other words, the head unit  4  is moved by a head unit moving mechanism which includes the pair of pulleys  5  and  6 , the timing belt  7 , the motor  8 , and the guide rails  2  and  3 . 
       FIG. 2  is a vertical cross-sectional view of the head unit  4  of the ink-jet printer  1  shown in  FIG. 1 . As shown in  FIG. 2 , the head unit  4  accommodates a buffer tank  11  inside a box-shaped case which forms a carriage  12 , and the ink-jet head  14  is installed on a lower side of a bottom wall  12   a  of the carriage  12 . The buffer tank  11  has a structure such that the four inks supplied from ink tanks (not shown in the diagram) via the ink supply tubes  9  (refer to  FIG. 1 ) are stored temporarily in four ink chambers respectively, and the ink is supplied appropriately to the ink-jet head  14  through an ink outflow port  11   a.    
     The case which forms the carriage  12  is slidably mounted on the guide rails  2  and  3  (refer to  FIG. 1 ). The case includes the bottom wall  12   a  facing the recording paper P, front and rear walls  12   d  which erect from the bottom wall  12   a  and are parallel to a main scanning direction (refer to  FIG. 4 ), and side walls (a first side wall and a second side wall)  12   c  which are orthogonal to the main scanning direction. As shown in FIG.  4 , in a plan view, the bottom wall  12   a  is formed to be rectangular in shape, and is formed to have outer dimensions greater than outer dimensions of the ink jet head  14 . 
     The ink-jet head  14  has the nozzles  49 , in a lowermost surface, which open downward (a direction toward the recording paper P) and jet ink droplets. The nozzles  49 , as shown in  FIG. 4 , form rows in a direction orthogonal to the main scanning direction, and a plurality of rows for various ink colors are arranged in the main scanning direction. As the ink-jet head  14 , it is possible to use ink-jet heads of various jetting types such as a type in which a piezoelectric element is deformed, a type in which a vibration plate is deformed due to static electricity, or a type in which a pressure generated by boiling the ink by heating. The ink-jet head  14  is fixed to the bottom wall  12   a  of the carriage  12  via a frame plate  13  by an adhesive etc. 
     The head unit  4  structured in such manner moves in the scanning direction by driving the motor  8 , and jets the ink from the nozzles  49  by driving an actuator  18  based on image data etc., and forms an image on the recording paper P. 
     A gas, concretely, an air is taken into the head unit  4  when the head unit  4  is moved in the scanning direction, and the air taken in is discharged toward the recording paper P. A structure of the head unit  4  for forming a high quality image will be described below in further detail by referring to  FIG. 3  to  FIG. 5 . 
     As shown in  FIG. 4 , a pair of first gas discharge ports  51  and a pair of second gas discharge ports  52  are formed in the bottom wall  12   a  of the carriage  12  of the head unit  4 . The pair of the first gas discharge ports  51  and the pair of the second gas discharge ports  52  extend in the scanning direction, to be arranged on both outer sides orthogonal to the scanning direction with respect to each row of the nozzles  49 , in other words, interposing the ink-jet head  14  in the transporting direction of the paper. Moreover, each of the first gas discharge ports  51  and the second gas discharge ports  52  is formed to penetrate through a direction of thickness of the bottom wall  12   a , and to have a length which is greater than a width of the plurality of rows of the nozzles  49  (width in a direction orthogonal to a direction of rows) in the main scanning direction. One of the first gas discharge ports  51  and one of the second gas discharge ports  52  are arranged in order of the first gas discharge port  51  and the second gas discharge port  52  from an outer side toward an inner side, at one side of the rows of the nozzles  49 , and are arranged in order of the other side of the second gas discharge ports  52  and the other of the first gas discharge ports  51  from the outer side toward the inner side, at the other side of the rows of the nozzles  49 . The first gas discharge ports  51  and the second gas discharge ports  52  may be arranged symmetrically interposing the rows of the nozzles  49 . 
     Moreover, as shown in  FIG. 3  and  FIG. 4 , in the both side walls  12   c  in the main scanning direction of the carriage  12 , first gas inlet ports  57  which open on one side in the main scanning direction and second gas inlet ports  58  which open on the other side in the main scanning direction are formed corresponding to the first gas discharge ports  51  and the second gas discharge ports  52 , respectively. The first gas inlet ports  57  and the second gas inlet ports  58 , as shown in  FIG. 4 , communicate with the first gas discharge ports  51  and the second gas discharge ports  52  by first guide members  55  and second guide members  56 . Namely, the first guide members  55  guide the gas, taken into the head unit  4  via the first gas inlet ports  57 , to the first gas discharge ports  51 , and the second guide members  56  guide the gas, taken into the head unit  4  via the second gas inlet ports  58 , to the second gas discharge ports  52 . 
     As shown in  FIG. 4 , each of the first guide members  55  has one end connected to one of the first gas inlet ports  57  of one side wall  12   c , and passes through the carriage  12 , to extend along the front and rear walls  12   d  of the carriage  12  from the first gas inlet port  57 . The other end of the first guide member  55  is connected to one of the first gas discharge ports  51 . At an interior of each of the first guide members  55 , a first air discharge channel  61  which communicates the first gas inlet port  57  and the first gas discharge port  51  is formed. The first guide members  55  are arranged to connect the first gas discharge ports  51  and the first gas inlet ports  57  corresponding to the first gas discharge ports  51  respectively, at one side and the other side of the transporting direction. 
     Each of the second guide members  56  similarly, has one end connected to one of the second gas inlet ports  58  of the other side wall  12   c , and passes through the carriage  12 , to extend along the front and rear walls  12   d  of the carriage  12  from the second gas inlet port  58 . The other end of the second guide member  56  is connected to one of the second gas discharge ports  52 . At an interior of each of the second guide members  56 , a second air discharge channel  62  which communicates the second gas inlet port  58  and the second gas discharge port  52  is formed. The second guide members  56  are also arranged to connect the second gas discharge ports  52  and the second gas inlet ports  58  corresponding to the second gas discharge ports  52  respectively, at one side and the other side of the transporting direction. 
     As it will be described later, for making rapid a flow of air discharged from the first gas discharge ports  51  and the second gas discharge ports  52 , it is preferable to increase an area of opening of the first gas inlet ports  57  and the second gas inlet ports  58 , and to increase an amount of air which is taken in. In the embodiment, both of the first guide members  55  and the second guide members  56  have an enlarged shape toward the first gas inlet ports  57  and the second gas inlet ports  58 , in a plan view as shown in  FIG. 4 . Moreover, it is also possible to form one first gas inlet port  57  and one second gas inlet port  58  throughout an area of the both side walls  12   c  respectively, and to connect the first gas inlet port  57  to the first gas discharge ports  51  and to connect the second gas inlet port  58  to the second gas discharge ports  52 , while narrowing the first air discharge channels  61  and the second air discharge channels  62 . 
     Furthermore, in the embodiment, a ceiling  61   b  of the first guide member  55  forming the first air discharge channel  61 , as shown in  FIG. 5A , is inclined to descend toward the bottom wall  12   a , as advancing toward an inner side of the carriage  12  from the corresponding first gas inlet port  57 , and is curved to descend toward the bottom wall  12   a  as it advances further, and communicates with an end portion on the other side (right side in  FIG. 5A ) in the main scanning direction of the first gas discharge port  51 . A ceiling  62   b  of the second guide member  56  forming the second air discharge channel  62 , as shown in  FIG. 5B , is inclined to descend toward the bottom wall  12   a , as advancing toward the inner side of the carriage  12  from the corresponding second gas inlet port  58 , and is curved to descend toward the bottom wall  12   a  as it advances further, and communicates with an end portion on one side (left side in  FIG. 5B ) in the main scanning direction of the second gas discharge port  52 . Moreover, as shown in  FIG. 5C , between the first air discharge channel  61  and the second air discharge channel  62 , a thin wall portion  64  which forms one side wall of the first air discharge channel  61  and one side wall of the second air discharge channel  62 , extends up to the bottom wall  12   a.    
     A louver  63  or a filter which blocks dust and impurities from entering is installed in each of the first gas inlet ports  57  and the second gas inlet ports  58 . 
     When the head unit  4  is moved in one direction A 1  in the main scanning direction (hereinafter, called as ‘one direction A 1 ’), an air taken in via the pair of first gas inlet ports  57  directed toward the one direction A 1  is discharged downwardly toward the recording paper P from the pair of first gas discharge ports  51  via the pair of first air discharge channels  61 . The air discharged from the first gas discharge ports  51  form layers of air, in other words, a pair of air curtains which extend in the scanning direction interposing the rows of the nozzles  49 , on both outer sides in the direction of the rows of the nozzles  49 . Moreover, when the head unit  4  is moved in the other direction B 1  in the main scanning direction (hereinafter called as the ‘other direction B 1 ’) opposite to the one direction A 1 , an air is taken via the second gas inlet ports  58 . The air is discharged downwardly toward the recording paper P from the second gas discharge ports  52  via the pair of second air discharge channels  62 , and forms an air curtain similarly as when the head unit  4  is moved in one direction A 1 . When the head unit  4  is moved in any of the one direction A 1  and the other direction B 1 , an air flow is generated between the head unit  4  and the recording paper P. However, it is possible to suppress the generation of the air flow which spreads over both sides orthogonal to the scanning direction, by the air curtain. 
     Consequently, ink droplets (including the main liquid droplets and the satellite liquid droplets) are not deflected by the air flow which spreads over both sides orthogonal to the scanning direction, and it is possible to land the ink droplets at desired positions on the recording paper P. When the satellite liquid droplets in particular, are jetted from nozzles at both ends in the row of the nozzles  49  in the conventional structure, the satellite droplets land on an outer side than the main liquid droplets, due to the air flow which spreads toward both sides. In the embodiment, the main liquid droplets and the satellite liquid droplets jetted from the nozzle at any position in the row of nozzles  49  land at relatively almost the same position. Accordingly, it is possible to have a uniform shape and diameter of dots formed by ink droplets jetted from the nozzles  49 . 
     Moreover, by forming pair of air curtains to interpose the ink-jet head  14  on both sides in the transporting direction of the ink-jet head  14 , an amount of air which spreads over both sides in a direction orthogonal to the main scanning direction between the head unit  4  and the recording paper P is reduced, in other words, an amount of air entering between the ink-jet head  14  and the recording paper P is reduced. Accordingly, when the liquid droplets are projected from the nozzle  49  and divided into the main liquid droplets and the satellite liquid droplets, an effect of the air flow on the ink is reduced, and it is possible to suppress the generation of ink mist. Since the generation of the ink mist is suppressed, it is possible to reduce contamination of an interior of the image forming apparatus and occurrence of an electrical fault. 
     Next, a first modified embodiment will be described below by referring to  FIG. 7A  and  FIG. 7B .  FIG. 7A  is a bottom view of the head unit  4  in the first modified embodiment, and  FIG. 7B  is a cross-sectional view taken along a line VIIB-VIIB in  FIG. 7A . In the embodiment, the pair of the first gas discharge ports  51  and the pair of the second gas discharge ports  52  which extend in the scanning direction and interpose the ink-jet head  14  have been formed in the bottom wall  12   a  of the head unit  4 . However, there may be only one pair of such gas discharge ports. In the first modified embodiment, as shown in  FIG. 7A , a pair of gas discharge ports  151   a  and  151   b  is formed to extend in the scanning direction, interposing the ink-jet head  14 , and the gas discharge port  151   a  is connected to gas inlet ports  57   a  and  58   a  on one side in the transporting direction, and the gas discharge port  151   b  is connected to gas inlet ports  57   b  and  58   b  on the other side in the transporting direction. The gas inlet ports  57   a  and  58   a  on one side in the transporting direction are connected to the gas discharge port  151   a  via guide members  155   a  and  156   a  respectively, on one side in the transporting direction, and the gas inlet ports  57   b  and  58   b  on the other side in the transporting direction are connected to the gas discharge port  151   b  via guide members  155   b  and  156   b  respectively, on the other side in the transporting direction. A three-dimensional shape of each guide member is almost same as in the embodiment, Out as shown in  FIG. 7B , a thin wall portion  164  between the guide members  155   a  and  156   a  on the one side in the transporting direction does not extend up to the bottom wall  12   a . Further, the first modified embodiment is different from the embodiment in that the guide members  155   a  and  156   a  are merged mutually in the vicinity of the gas discharge port  151   a . Moreover, also a thin wall portion  164  between the guide members  155   b  and  156   b  on the other side in the transporting direction does not extend up to the bottom wall  12   a , and the guide members  155   b  and  156   b  are merged mutually in the vicinity of the gas discharge port  151   b.    
     Even in the first modified embodiment, when the head unit  4  is moved to one side A 1  in the scanning direction, air is taken in via the gas inlet ports  57   a  and  57   b  on the side of A 1 , and the air which is taken from the gas inlet port  57   a  is discharged through the gas discharge port  151   a  via an air discharge channel  161   a  inside the guide member  155   a , and the air which is taken from the gas inlet port  57   b  is discharged through the gas discharge port  151   b  via an air discharge channel  161   b  inside the guide member  155   b . When the head unit  4  is moved to the other side B 1  in the scanning direction, air is taken in from the gas inlet ports  58   a  and  58   b  on the side of B 1 , and the air which is taken from the gas inlet port  58   a  is discharged through the gas discharge port  151   a  via an air discharge channel  162   a  inside the guide member  156   a , and the air which is taken from the gas inlet port  58   b  is discharged through the gas discharge port  151   b  via an air discharge channel  162   b  inside the guide member  156   b . Since the guide member  155   a  is merged with the guide member  156   a  in the vicinity of the gas discharge port  151   a , it is possible to reduce an amount of air flowing through one of the air discharge channels  161   a  and  162   a  from flowing into the other of the air discharge channels. Similarly, since the guide member  155   b  is merged with the guide member  156   b  in the vicinity of the gas discharge port  151   b , it is possible to reduce an amount of air flowing through one of the air discharge channels  161   b  and  162   b  from flowing into the other of the air discharge channels. As it has been described above, even in the first modified embodiment, it is possible to achieve an effect similar as in the embodiment. Moreover, in the first modified embodiment, since only one pair of gas discharge ports is formed, even when the head unit  4  is moved in any sides of the scanning direction, it is possible to form the air curtain at the same position with respect to the ink-jet head  14 . Accordingly, it is possible to achieve a constant wind-shield effect independent of the movement direction of the head unit  4 . 
     Next, a second modified embodiment which is an embodiment upon making further modifications in the first modified embodiment will be described below by referring to  FIG. 8A  to  FIG. 8C .  FIG. 8A  is a bottom view of the head unit  4  in the second modified embodiment.  FIG. 8B  is a cross-sectional view taken along a line VIIIB-VIIIB in  FIG. 8A .  FIG. 8C  is a cross-sectional view taken along a line VIIIC-VIIIC in  FIG. 8A . Even in the second modified embodiment, similarly as in the first modified embodiment, the pair of gas discharge ports  151   a  and  151   b  is formed to extend in the scanning direction, interposing the ink-jet head  14 , and the gas discharge port  151   a  is connected to the gas inlet ports  57   a  and  58   a  on one side in the transporting direction, and the gas discharge port  151   b  is connected to the gas discharge ports  57   b  and  58   b  on the other side in the transporting direction. The second modified embodiment is different from the first modified embodiment in that non-return valves  171   a ,  171   b ,  172   a , and  172   b  are attached to the gas inlet ports  57   a  and  57   b  on one side A 1  in the scanning direction, and to the gas inlet ports  58   a  and  58   b  on the other side B 1  in the scanning direction, respectively so that the non-return valves  171   a ,  171   b ,  172   a , and  172   b  direct toward the inside of the head unit  4 . Moreover, the second modified embodiment is different from the first modified embodiment in that a guide member  250   a  which connects the gas inlet port  57   a  and the gas discharge port  151   a  also serves as a guide member which connects the gas inlet port  58   a  and the gas discharge port  151   a , and a guide member  250   b  which connects the gas inlet port  57   b  and the gas discharge port  151   b  also serves as a guide member which connects the gas inlet port  58   b  and the gas discharge port  151   b . Moreover, as shown in  FIG. 8C , a ceiling  251   a  of the guide member  250   a  is inclined to be descended from the corresponding gas inlet ports  57   a  and  58   b  toward the bottom wall  12   a , as advancing toward an inner side of the carriage  12 , and unlike in the first modified embodiment, does not communicate with end portions on a side in the scanning direction of the gas discharge port  151   a . A three-dimensional structure of the guide member  250   b  is similar to a three-dimensional structure of the guide member  250   a . The non-return valve  171  has a pair of rotating shafts  181   a ,  181   a  which is provided to the side wall  12   c  at the interior of the head unit  4  and each of which extends along an edge on both sides in the transporting direction of the gas inlet port  57   a , and a pair of opening and closing members  191   a ,  191   a  attached to the rotating shafts  181   a  respectively, and bias members which are attached to the rotating shafts  181   a  respectively and apply biases to the opening and closing members  191   a  in a predetermined direction respectively. The pair of opening and closing members  191   a ,  191   a  is installed to be directed toward the interior of the head unit  4 , and a bias is applied thereon in a direction of closing the gas inlet port  57   a  by the bias members. As a bias member, for example, a coil spring coiled around the rotating shaft  181   a  or a plate spring attached to the rotating shaft  181   a  are usable. Therefore, edges, of the pair of opening and closing members  191   a , on a side opposite to the two rotating shafts  181   a , overlap mutually at the interior of the head unit  4 , and it is possible to prevent the air inside the head unit  4  from flowing out through the gas inlet port  57   a . On the other hand, when the head unit  4  is moved and air enters into the head unit  4  through the gas inlet port  57   a , the pair of opening and closing members  191   a  and  191   a  is pushed open by the air, and the air is taken into the head unit  4 . A structure of the other non-return valves  171   b ,  172   a , and  172   b  is similar to the structure of the non-return valve  171   a.    
     When the head unit  4  moves to one side A 1  in the scanning direction, the non-return valves  171   a  and  171   b  attached to the gas inlet ports  57   a  and  57   b  respectively on the side A 1  are opened, and air is taken in. The air taken in from the gas inlet port  57   a , moves through an air discharge channel  260   a  inside the guide member  250   a . Since the non-return valve  172   a  of the gas inlet port  58   a  is closed, the air does not escape through the gas inlet port  58   a , and is discharged through the gas discharge port  151   a . Similarly, the air taken in from the gas inlet port  57   b , moves through an air discharge channel  260   b  inside the guide member  250   b . Since the non-return valve  172   b  of the gas inlet port  58   b  is closed, the air does not escape through the gas inlet port  58   b , and is discharged through the gas discharge port  151   b . When the head unit  4  is moved to the other side B 1  in the scanning direction, the non-return valves  172   a  and  172   b  attached to the gas inlet ports  58   a  and  58   b  on the side B 1  are opened, and air is taken in. The air taken in from the gas inlet port  58   a  moves through the air discharge channel  260   a  inside the guide member  250   a . Since the non-return valve  171   a  of the gas inlet port  57   a  is closed, the air does not escape through the gas inlet port  57   a , and is discharged through the gas discharge port  151   a . Similarly, the air taken in from the gas inlet port  58   b , moves through the air discharge channel  260   b  inside the guide member  250   b . Since the non-return valve  171   b  of the gas inlet port  57   b  is closed, the air does not escape through the gas inlet port  57   b , and is discharged through the gas discharge port  151   b . By an operation described above, in the second modified embodiment, it is possible to have an effect similar as in the first modified embodiment. Moreover, the guide member  250   a  which connects the gas inlet port  57   a  and the gas discharge port  151   a  also serves as a guide member which connects the gas inlet port  58   a  and the gas discharge port  151   a , and the guide member  250   b  which connects the gas inlet port  57   b  and the gas discharge port  151   b  also serves as a guide member which connects the gas inlet port  58   b  and the gas discharge port  151   b . Accordingly, it is possible to simplify a structure of the air discharge channels as compared to the structure in the first modified embodiment. 
     In the embodiment and the modified embodiments described above, since a temperature of a driving circuit  290  provided for driving the actuator  18  rises when the actuator  18  is driven, a heat releasing body  280  for releasing heat from the driving circuit  290  may be provided. In this case, as shown in  FIG. 9  for example, when the heat releasing body  280  is arranged in the vicinity of the guide member  250   a , due to flowing of the air taken in from the gas inlet port through the air discharge channel  260   a  in the guide member  250   a , it is possible to improve a heat releasing effect of the heat releasing body  250   a.    
     In the embodiment and the modified embodiments described above, the air which is discharged from the air discharge ports may be inclined not only in a direction orthogonal to the recording paper but also in one direction with respect to the scanning direction or the direction orthogonal to the scanning direction. 
     Moreover, in the embodiment and the modified embodiments described above, although air is taken in and discharged when the head unit is moved, the structure is not necessarily restricted to such a structure. For example, the pair of gas discharge ports may be replaced by an air discharge fan, and the pair of gas inlet ports may be replaced by an air intake fan, and the air may be taken in and discharged forcibly. 
     Moreover, the embodiment and the modified embodiments described above are embodiments in which, the present invention is applied to an apparatus using, an ink as a liquid. However, the application of the present invention is not restricted to the embodiments and the modified embodiments described above. The present invention is also applicable to an apparatus which jets droplets of a liquid, used in various fields such as medical treatment and analysis, provided that the apparatus is required to land liquid droplets jetted from a nozzle on an object without the liquid droplets being deflected in a direction orthogonal to the scanning direction, by preventing an air current from spreading over both sides in a direction orthogonal to the movement direction of the head, between the jetting head and the object, when the head unit is moved. Moreover, the present invention may be applied to an image forming apparatus in which a liquid other than ink is used, such as an apparatus which applies a colored liquid at the time of manufacturing a color filter of a liquid-crystal display apparatus.