Patent Publication Number: US-10759197-B2

Title: Digital printing device

Description:
TECHNICAL FIELD 
     The present invention relates to the field of printing technology, and more particularly to a digital printing device. 
     BACKGROUND 
     On Traditional printers (sheet printers), there are generally two methods in transporting sheets to be printed, that is, a roller-to-roller paper transporting method and a roller suction transporting. If the sheet is transported by a roll-to-roller paper transporting method, requirements for the sheets will be strict. It is necessary to ensure that a leading end of a sheet has entered the second group of rollers when the rear end of the sheet leaves the first group of rollers, so as to ensure that the sheet goes forward normally. This may cause a minimum limit on the length of the sheet in a feeding direction. Moreover, a larger gap between the two rollers will cause that the pulling force is insufficient to send out the sheet, and a smaller gap between the two rollers will cause a larger pressure acting on the sheet, thereby the sheet may be crushed and the strength of the sheet will be affected. If the sheet is transported by the roller suction transporting method, the accuracy of transporting will be poor. Usually, the sheet will be printed in more than two colors, so the registration color accuracy between the two rollers will be relatively poor. In addition, in the suction and transportation process of the prior art, although an air door manually opened and closed is provided on a negative pressure suction box, the air door can only roughly adapt to the width of the sheet while being opened and closed. There are still leakages of air at some suction holes on both sides of the sheet, which causes a poor effect of negative pressure suction. 
     SUMMARY 
     The object of the present invention is to provide a digital printing device, which solves the problems in the prior art that the transporting accuracy of sheet to be transported by the negative pressure suction is poor, the requirements for the sheet are strict, and the negative pressure suction effect is poor as the openable/closable door on the negative pressure suction box cannot be precisely adjusted to has same width as the sheet. 
     The technical solution adopted by the present invention to solve the technical problems is a digital printing device, comprising a suction transport device, the suction transport device comprises a transport belt for transporting a sheet, at least one first suction box, which is configured to suck the sheet on the transport belt by negative pressure during transport, is provided in the space defined by the transport belt, multiple first suction holes in communication with the first suction box are provided on the transport belt, at least one side of the first suction box is provided with a movable door which moves in a direction perpendicular to a transporting direction to make the first suction box maintain a negative pressure state in cooperation with the sheet. 
     In the digital printing device of the present invention, the movable door is provided with first nuts, which are sleeved on first screw rods and move along with rotation of the first screw rods, and the first screw rods are driven by a first driver. 
     In the digital printing device of the present invention, the first driver drives the first screw rods to rotate through a transmission mechanism which includes a transmission shaft driven by the first driver to rotate, connecting rods connected in one-to-one correspondence with each of the first screw rods, and a plurality of first bevel gears and second bevel gears engaged with each other, each of the second bevel gears is connected to each of the connecting rods ( 102 ) in a one-to-one correspondence, and each of the first bevel gears is connected to the transmission shaft, the transmission shaft and the connecting rods are interacted with each other by the first bevel gears and the second bevel gears. 
     In the digital printing device of the present invention, a feeding device is disposed at a transport starting end of the suction transport device, the feeding device includes a suction panel and a plurality of feeding wheels, the suction panel is provided with second suction holes, the feeding wheels are disposed at positions corresponding to the second suction holes, and above the second suction hole of an area on at least one side of the suction panel, a windshield cover for covering the area, which is movable to adjust a covering thereof, is mounted. 
     In the digital printing device of the present invention, the windshield cover includes a fixed end portion, a sliding end portion and a telescopic portion, the fixed end portion is fixed on one side of the suction panel, the telescopic portion is connected between the fixed end portion and the sliding end portion, the sliding end portion moves in a direction perpendicular to a feeding direction and drives the telescopic portion to extend and contract, and sliding end portion causes the telescopic portion to cover the area on one side of the suction panel during movement. 
     In the digital printing device of the present invention, a dust removing device is disposed above a transport starting end of the suction transport device, the dust removing device includes a first dust removing portion for removing floating dust on a surface of the sheet, a blowing portion for loosening dust adhered on the surface of the sheet, and a second dust removing portion for removing dust adhered on the surface of the sheet and loosened by the blowing portion. 
     In the digital printing device of the present invention, the first dust removing portion, the blowing portion, and the second dust removing portion are sequentially disposed along the transporting direction of the sheet; the first dust removing portion includes a first dust removing brush, and a fixed member for fixing the first dust removing brush; the second dust removing portion includes an air pumping pipeline, and an air pumping tube is connected to a bottom end of the air pumping pipeline, the bottom end of the air pumping tube is provided with an air pumping opening and a second dust removing brush is disposed around the air pumping opening; the blowing portion includes a blowing pipeline, and a blowing tube is connected to a bottom end of the blowing pipeline, the bottom end of the blowing tube is provided with a blowing opening and the blowing opening is blown toward a side where the second dust removing brush is located; the fixed member for the first dust removing portion is mounted on a side of the blowing pipeline, the second dust removing portion is mounted on a side of the blowing portion in opposition to the first dust removing portion. 
     In the digital printing device of the present invention, at least one jet printing unit is provided above the transport belt, and each of the jet printing units is provided with a nozzle maintenance device comprising a moisturizing assembly; the moisturizing assembly includes a movable body that can be moved between a first position and a second position, the movable body is provided with a maintenance chamber for accommodating nozzles that keep the jet printing unit wet; scrapers for scraping ink from the nozzles in one-to-one correspondence with each nozzle of the jet printing unit, are provided in the maintenance chamber; the nozzles of the jet printing unit are provided above the first position. 
     In the digital printing device of the present invention, a cleaning assembly for cleaning the scrapers is provided above the second position, the cleaning assembly includes a fixed body; nozzles for spraying cleaning liquid in one-to-one correspondence with each of the scrapers, are provided on a side of the fixed body facing the second position, a liquid groove for containing the cleaning liquid, which is connected with each nozzle, is provided on a side of the fixed body in opposition to the nozzles; at an edge of the maintenance chamber of the moisturizing assembly, a seal member is provided. 
     In the digital printing device of the present invention, one end of the scraper is formed with a curved blade; the nozzle includes a main body, on which a containing groove for accommodating the blade is provided, and at least one spray hole is provided at a bottom of the containing groove. 
     Embodying the digital printing device of the present invention has the following beneficial effects: by transporting sheets using the transport belt during printing, the digital printing device of the present invention avoids the problem of poor registration accuracy, and improves the printing effect. Moreover, by providing the movable door on the first suction box, the present invention makes the width of the first suction box to accurately fit the width of the sheet, so that sheets of different specifications can be can be adapted to transport in corresponding negative pressure suction areas, and there will be no air leakage and the negative pressure suction effect is good. 
    
    
     
       DRAWINGS 
         FIG. 1  is a schematic structural view of the digital printing device in the present invention; 
         FIG. 2  is a schematic structural view of a part of the structure of the digital printing device in the present invention; 
         FIG. 3  is a schematic structural view of a suction transport device in the present invention; 
         FIG. 4  is a schematic partially-enlarging structural view of the suction transport device in the present invention; 
         FIG. 5  is a schematic top view of the suction transport device in the present invention; 
         FIG. 6  is a schematic structural view of a combined structure of a feeding device and a dust removing device in the present invention; 
         FIG. 7  is a schematic top view of the feeding device in the present invention; 
         FIG. 8  is a schematic side view of the feeding device in the present invention; 
         FIG. 9  is a schematic front view of the dust removing device in the present invention; 
         FIG. 10  is a schematic side view of the dust removing device in the present invention; 
         FIG. 11  is a schematic structural view of a nozzle maintenance device in a first state in the present invention; 
         FIG. 12  is a schematic structural view of the nozzle maintenance device in a second state in the present invention; 
         FIG. 13  is a schematic structural view of the nozzles of the nozzle maintenance device in the present invention. 
     
    
    
     DETAILED DESCRIPTION 
     The structure and functional principle of a digital printing device of the present invention will be further described below in combination with the accompanying drawings and embodiments: 
     In the description of the present invention, it is necessary to understand that orientations or positional relationships indicated by the terms such as “center”, “longitudinal”, “widthwise”, “up”, “down”, “front”, “rear”, “left”, “right”, “vertical”, “horizontal”, “top”, “bottom”, “inside”, “outside”, “clockwise”, and “counterclockwise” are based on orientations or positional relationships shown in the drawings, and are intended only to facilitate to descript the present invention and simplify the description, rather than to indicate or imply that the device or element referred to must have a specific orientation or be constructed and operated in a specific orientation, therefore, they cannot be understood as a restriction on this invention. 
     As shown in  FIG. 1-2 , the digital printing device includes a suction transport device  1 , a feeding device  2 , a dust removing device  3 , a jet printing unit  4 , and a nozzle maintenance device  5  disposed on the jet printing unit  4 . The feeding device  2  is installed at a transport starting end of the suction transport device  1 . The dust removing device  3  is provided above the transport starting end of the suction transport device  1 . At least one jet printing unit is provided above the transport belt. The nozzle maintenance device is configured in one-to-one correspondence with each printing unit. The jet printing unit can be multiple and be arranged along a transporting direction. On the other hand, there may be one jet printing unit. 
     As shown in  FIG. 3 , the suction transport device  1  includes a transport belt  11  for transporting sheets, a main driving roller  12  for driving the transport belt  11  to move, a follow driving roller  13  rotating along with the movement of the transport belt  11 , and a second driver (not shown in the drawings) driving the main driving roller  12  to rotate. The second driver can be a motor or an electric motor, and can be directly connected with the main driving roller  12 , or can be connected with the main drive roller  12  through a transmission. 
     As shown in  FIGS. 3-4 , at least one negative pressure first suction box  14 , which is configured to suck the sheet on the transport belt  11  by negative pressure during transport, is provided in the space defined by the transport belt  11 . Multiple first suction holes (not shown in the drawings) in communication with the negative pressure first suction box  14  are provided on the transport belt  11 . At least one side of the negative pressure first suction box  14  is provided with a movable door  15  which moves in a direction perpendicular to the transporting direction to make the negative pressure first suction box  14  maintain a negative pressure state in cooperation with the sheet. The negative pressure first suction box  14  is installed with a first vacuum pumping device (not shown in the drawings), which forms a negative pressure by suction. The negative pressure first suction box  14  may be directly connected to the first vacuuming pumping device, or may be connected to the first vacuuming pumping device through an air suction pipe  16 . The top end of the negative pressure first suction box  14  is provided with vents  141  which are communicated with the first suction holes. 
     In this embodiment, as shown in  FIGS. 4 and 5 , the number of the negative pressure first suction box  14  is plural and the negative pressure first suction boxes are arranged sequentially along the transporting direction. The sidewalls of each of the negative pressure first suction boxes  14  located on both sides of the transport belt  11  are provided with movable doors  15 , each of which is provided with a first nut  18 . The first nuts  18  are sleeved on first screw rods  19  and moves along with rotation of the first screw rods  19 . The first screw rods  19  are driven by the first driver  20 . The first driver  20  can be an electric motor or a motor. The first nuts  18  on two opposite movable doors  15  of same negative pressure first suction box  14  are sleeved on same first screw rod  19 . 
     Two ends of the first screw rod  19  are respectively formed with a positive thread segment and a reverse thread segment, and two adjacent first screw rods are configured as follows: an A end of the first one of the first screw rods is formed as the positive thread segment, and an A end of the adjacent second one of the first screw rods is formed as the reverse thread segment; a B end of the first one of the first screw rods is formed as the reverse thread segment, and a B end of the adjacent second one of the first screw rods is formed as the positive thread segment. When the first screw rod  19  is rotated, the first nuts  18  at both ends of the first screw rod  19  will move relative to each other or move away from each other at the same time, which drives the two opposite movable doors  15  on the same negative pressure first box  14  to move relative to each other or to move away from each other at the same time. The first screw rods  19  are disposed perpendicular to the transporting direction of the transport belt  11 . The first nuts  18  drive the movable doors  15  to move along a lengthwise direction of the first screw rods  19 . Each of the negative pressure first suction boxes  14  is configured in one-to-one correspondence with each of the first screw rods  19 . 
     The first driver  20  drives each first screw rod  19  to rotate through the transmission mechanism  10 . The transmission mechanism  10  includes a transmission shaft  101  driven by the first driver  20  to rotate, connecting rods  102  connected in one-to-one correspondence with each of the first screw rods  19 , and a plurality of first bevel gears  103  and second bevel gears  104  engaged with each other. Each of the second bevel gears  104  is connected to each of the connecting rods  102  in a one-to-one correspondence, and each of the first bevel gears  103  is connected to the transmission shaft  101 . The transmission shaft  101  and the connecting rods  102  are interacted with each other by the first bevel gears  103  and the second bevel gears  104 . The connecting rod  102  is connected with the first screw rod  19  through a coupling  105 . The transmission shaft  101  is disposed to extend along the transporting direction of the transport belt  11 . The first bevel gear  103  and the second bevel gear  104  are engaged vertically with each other. 
     When the width of the sheet is smaller than the width of the transport belt  11 , the first driver  20  is started to drive the transmission shaft  101  to rotate, and the transmission shaft  101  drives each connecting rod  102  to rotate through the first bevel gears  103  and the second bevel gears  104 , thereby driving the first screw rod  19  to rotate. Then, the first nuts  18  at both ends of the first screw rod  19  drives the movable door  15  to move, so that the distance between the movable doors  15  is substantially the same as the width of the sheet, and the space formed by the top, bottom and two movable doors  15  of the negative pressure first suction box  14  is located directly below the sheet. When vacuuming begins, the space can be in an approximate vacuum state, so that the sheet is maintained in a negative pressure suction state. 
     In another embodiment, the number of the negative pressure first suction box  14  is one, and it is integrally placed in the space defined by the transport belt  11 . The number of the first screw rod  19  is also one, and both ends are formed with a positive thread segment and a reverse thread segment, respectively. 
     In another embodiment, a sidewall of each of the negative pressure first suction boxes  14  located on one side of the transport belt  11  is set as the movable door  15 , and a sidewall on the other side is fixedly connected to the negative pressure first suction box  14 . One end of the first screw rod  19  near the movable door  15  is formed with a threaded segment. When the number of the first screw rods  19  is plural, two adjacent first screw rods  19  are configured as follows: an A end of the first one of the first screw rods is formed as the positive thread segment, and an A end of the adjacent second one of the first screw rods is formed as the reverse thread segment. When the width of the sheet is smaller than the width of the transport belt  11 , the sheet is placed near an edge of one side of the transport belt  11 . The position of the movable door  15  is designed to be located on the other side of the transport belt  11 . When the first driver  20  is started to drive the transmission shaft  101  to rotate, the transmission shaft  101  drives each of the connecting rods  102  to rotate by the first bevel gears  103  and the second bevel gears  104 , thereby driving the first screw rod  19  to rotate, and the first nut  18  on the first screw rod  19  drives the movable door  15  to move, so that the distance between the movable door  15  and an opposite sidewall of the negative pressure first suction box  14  is substantially the same as the width of the sheet, and the space formed by the top, bottom and a movable door  15  of the negative pressure first suction box  14  is directly below the sheet. When vacuuming begins, the space can be in an approximate vacuum state, so that the sheet is maintained in the negative pressure suction state. 
     In another embodiment, the first screw rod  19  can be directly driven by the first driver  20  to rotate, without the transmission mechanism  10 . When the number of the first screw rods  19  is plural, each of the first screw rods  19  is provided with a first driver  20 . 
     By transporting sheets using the transport belt during printing, the problem of poor registration accuracy can be avoided, and the printing effect can be improved. At the same time, the first suction box is provided with a movable door that can be moved to realize a stepless adjustment, so that the width of the first suction box is accurately matched with the width of the sheet, thereby adapting to transport of the sheets of the different specifications in corresponding negative pressure suction areas. There will be no air leakage and the negative pressure suction effect is good. 
     As shown in  FIGS. 6-8 , the feeding device  2  includes a suction panel  21  and a plurality of feeding wheels  22 . The suction panel  21  is provided with second suction holes  23  that suck the sheet on the suction panel  21  by negative pressure. The feeding wheels  22  are disposed at positions corresponding to the second suction holes  23 . Specifically, there is a gap between the feeding wheel  22  and a corresponding hole wall of the second suction hole  23 . A second suction box  24 , which is assembled with the suction panel  21  together, is provided below the suction panel  21 , for negative pressure suction, and the second suction box  24  is connected with a second vacuum pumping device (not shown in the drawings), so that a negative pressure can be formed in the second suction box  24 . Preferably, a plurality of feeding wheels  22  are arranged in a plurality of rows along the feeding direction on the suction panel  21 . More preferably, two adjacent rows of the feeding wheels  22  are staggered. In other embodiments, the feeding wheels  22  can also be distributed freely. Each row of the feeding wheels  22  is driven by roller shafts (not shown in the drawings) to rotate. The feeding wheels  22  are sleeved on the roller shafts which are driven by a third driver  25  to rotate. The third driver  25  can be a motor or a servo motor. 
     Above the second suction holes  23  of an area on at least one side of the suction panel  21 , a windshield cover  26 , which covers the second suction holes  23  in the area on the one side so as to maintain the sheet in a negative pressure suction state, is mounted. The number of the windshield cover  26  may be one. The sheet covers the second suction holes  23  in the area on the one side of the suction panel  21 . The windshield cover  26  is provided on the other side of the suction panel  21  and covers the second suction holes  23  that are not covered in the area of the one side; the number of the windshield cover  26  also may be two, and the two windshield covers  26  are respectively provided on both sides of the suction panel  21 . The sheet is placed in a middle position in the suction panel  21 , and the two windshield covers  26  respectively cover the second suction holes  23  that are not covered by the sheet on both sides of the suction panel  21 . 
     In this embodiment, the windshield cover  26  includes a fixed end portion  261 , a sliding end portion  262  and a telescopic portion  263 . The fixed end portion  261  is fixed on one side of the suction panel  21 . The telescopic portion  263  is connected between the fixed end portion  261  and the sliding end portion  262 . The sliding end portion  262  moves in a direction perpendicular to the feeding direction and drives the telescopic portion  263  to extend and contract. The sliding end portion  262  causes the telescopic portion  263  to cover the second suction holes  23  in the area on one side of the suction panel  21  during movement. An erected baffle  262   a  or a handle or the like is provided on the sliding end portion  262 . The telescopic portion  263  is folded and telescopic. The sliding end portion  262  is manually moved. 
     In another embodiment, the telescopic portion  263  includes a plurality of plates that are connected to each other in a staggered and overlapped manner, and each of two adjacent plates can be moved relative to each other, and the sliding end portion  262  is also moved manually. 
     In another embodiment, the sliding end portion  262  is provided with a second nut which is sleeved on a second screw rod and is moved along with rotation of the second screw rod. The second screw rod is driven by a fourth driver or manually. The second screw rod extends in a direction perpendicular to the transporting direction of the sheet, that is, the lengthwise direction thereof is the same as the direction perpendicular to the transporting direction of the sheet. The fourth driver can be a motor or an electric motor. In addition, when the number of the windshield covers  26  is two and the two windshield covers are respectively provided on both sides of the suction panel  21 , the thread directions of the second nuts on the sliding end portions  262  of the two windshield covers  26  are opposite, and both of the second nuts are sleeved on the same second screw rod, so that the sliding end portions  262  are moved toward each other or away from each other at the same time. In other embodiments, it can be that both ends of the same second screw rod are formed respectively as the positive thread segment and the reverse thread segment, so that the sliding end portions  262  are moved toward each other or away from each other at the same time. 
     In another embodiment, the windshield cover  26  is a door that can be moved in a direction perpendicular to the feed direction, that is, a common sliding door-like door body, and the windshield cover  26  covers the second suction holes in the area on one side of the suction panel  21  during the movement. 
     By providing a stretchable windshield cover on at least one side of the suction panel of the feeding device, it is possible to accurately adapt to the width of the sheet to be printed, and the stretchable windshield cover can completely cover the suction holes that are not covered by the sheet, so that a better negative pressure suction effect can be maintained. 
     As shown in  FIGS. 6, 9, and 10 , the dust removing device  3  includes a first dust removing portion  31  for removing floating dust on a surface of the sheet, a blowing portion  32  for loosening dust adhered on the surface of the sheet, and a second dust removing portion  33  for removing dust adhered on the surface of the sheet and loosened by the blowing portion  32 . In this embodiment, the first dust removing portion  31 , the blowing portion  32 , and the second dust removing portion  33  are sequentially disposed along the transporting direction of the sheet. In other embodiments, the blowing portion  32 , the second dust removing portion  33 , and the first dust removing portion  31  can be sequentially disposed along the transporting direction. 
     The first dust removing portion  31  includes a first dust removing brush  311 , and a fixed member  312  for fixing the first dust removing brush  311 . The second dust removing portion  33  includes an air pumping pipeline  331 . An air pumping tube  332  is connected to a bottom end of the air pumping pipeline  331 . The bottom end of the air pumping tube  332  is provided with an air pumping opening  333  and a second dust removing brush  334  is disposed around the air pumping opening  333 . The blowing portion  32  includes a blowing pipeline  321 . A blowing tube  322  is connected to a bottom end of the blowing pipeline  321 . The bottom end of the blowing tube  322  is provided with a blowing opening  323  and the blowing opening  323  is blown toward a side where the second dust removing brush  334  is located. The fixed member  312  for the first dust removing portion  31  is mounted on a side of the bottom end of the blowing pipeline  321 . The second dust removing portion  33  is mounted on a side of the blowing portion  32  in opposition to the first dust removing portion  31 . 
     A support portion  34  for supporting the sheet is provided under the first dust removing portion  31 , the second dust removing portion  33  and the blowing portion  32 , and is located between the suction panel  21  of the feeding device  2  and the transport belt  11  of the suction transport device  1 . A tip end of the first dust removing brush  311  and a tip end of the second dust removing brush  314  are maintained at a predetermined distance from the support portion  34 . The predetermined distance is equal to or smaller than a thickness of the sheet, which is being transported. 
     The blowing pipeline  321  includes a blowing pipe  321   a  and a blowing beam  321   b  that are connected with each other. The blowing pipe  321   a  and the blowing beam  321   b  are perpendicular to each other. A top end of the blowing pipe  321   a  is fixed by a fixed portion  35 . The blowing beam  321   b  is connected at the lower end of the blowing pipe  321   a . The blowing pipe  321   a  is connected to a gas generating device (not shown in the drawings) and discharges gas having a certain pressure through the gas generating device. The blowing pipe  321   a  is erected on the blowing beam  321   b . The lengthwise direction of the blowing beam  321   b  extends along a direction perpendicular to the feeding direction, and one or more blowing pipes  321   a  can be erected on the blowing beam  321   b . The blowing tube  322  is connected to a lower end on a side of the blowing beam  321   b  near the second dust removing portion  33  and is connected to the blowing beam  321   b . A side of the blowing beam  321   b  in opposition to the second dust removing portion  33  is connected to the first dust removing brush  311  through the fixed member  312 . The first dust removing brush  311  includes two rows of brushes respectively fixed on both sides of the fixed member  312 , and the length of the first dust removing brush  311  is substantially the same as the length of the blowing beam  321   b . In other embodiments, the first dust removing brush  311  may also include only one brush. The fixed member  321  is provided with a plurality of slender slots (not shown in the drawings) and can be fixed by screws passing through the slender slots. Since the slender slots are provided, the fixed member  312  can be moved up and down as needed, so that the distance between the tip end of the first dust removing brush  311  and the support portion  34  can be adjusted to adapt the sheets with different thicknesses. 
     The blowing tube  322  gradually narrows toward the blowing opening  323  from top to bottom, which facilitates the airflow to be ejected at a large speed, so that an intensity of the airflow that being blown is enhanced. The blowing tube  322  includes an upper air blade  322   a  and a lower air blade  322   b . The upper air blade  322   a  is provided with a plurality of slender slots (not shown in the drawings) and is fixed by screws passing through the slender slots. As needed, the upper air blade  322   a  can be moved up and down through the slender slots, so that a size of the blowing opening  323  can be adjusted. 
     The air pumping pipeline  331  of the second dust removing portion  33  includes an air pumping pipe  331   a  and an air pumping beam  331   b  that are connected with each other. A top end of the air pumping beam  331   b  is fixed by the fixed portion  35 . A bottom end of the air pumping beam  331   b  is connected with a top end of the air pumping pipe  331   a . The top end of the air pumping tube  332  is connected with the bottom end of the air pumping pipe  331   a  and is connected with the air pumping pipe  331   a . The air pumping beam  331   b  and the air pumping pipe  331   a  are perpendicular to each other, that is, the air pumping pipe  331   a  is erected under the air pumping beam  331   b , and one or more air pumping pipe  331   a  may be erected under the air pumping beam  331   b . The lengthwise direction of the air pumping beam  331   b  extends along the direction perpendicular to the feeding direction, that is, it is the same as the lengthwise direction of the blowing beam  321   b . The air pumping tube  332  is generally in a funnel shape. The second dust removing brush  334  includes two brushes respectively provided on both sides of the air pumping opening  333  at the bottom end of the air pumping tube  332 . In other embodiments, the second dust removing brush  334  may also include only one brush. The air pumping beam  331   b  is connected to an air pumping device (not shown in the drawings) which may be an air pump or a blower. The length of the second dust removing brush  334  is substantially the same as that of the air pumping tube  332 . 
     In operation, the sheet is transported onto the support portion  34 , and the surface to be printed of the sheet firstly contacts with the tip end of the first dust removing brush  311  of the first dust removing portion  31 . Since the distance between the tip end of the first dust removing brush  311  and the support portion  34  is equal to or smaller than the thickness of the sheet, the tip end of the first dust removing brush  311  can sweep away dust and paper dust on the surface to be printed; when the sheet continues to move and enter the area where the blowing portion  32  is located, the airflow is ejected from the blowing opening  323 , which can loosen the dust adhered on the surface to be printed; when the sheet continues to move into the area where the second dust removing portion  33  is located, the distance between the tip end of the second dust removing brush  334  and the support portion  34  is equal to or smaller than the thickness of the sheet, so that the dust that has been loosened can be swept away by the second dust removing brush  334  and taken away by the air pumping tube  332 , the air pumping pipe  331   a  and the air pumping beam  331   b.    
     During the movement, the sheet is subjected to dust removals by the first dust removing portion  31  and the second dust removing portion  33 , and the surface to be processed of the sheet is subjected to dust removal twice to meet the requirements for high dust removal effect. 
     As shown in  FIGS. 11-12 , the nozzle maintenance device  5  includes a moisturizing assembly  51  comprising a movable body  511  that can be moved between a first position  100  and a second position  200 . Both ends of the movable body  511  are provided with slide rails (not shown in the drawings) and driving members  512 . The movable body  511  has sliders (not shown in the drawings) which move along the slide rails. The driving members  512  drive the sliders to move, so that the movable body  511  is driven to move along the slider between the first position  100  and the second position  200 . The driving member  512  can be an air cylinder or a hydraulic cylinder, etc. In other embodiments, the movable body  511  can be driven to move by a motor in cooperation with a belt transmission or a nut screw driving device. 
     The movable body  511  is provided with a maintenance chamber  513  for accommodating nozzles that keep the jet printing unit  4  wet. The maintenance chamber  513  contains moisturizing liquid. Scrapers  514  for scraping ink from the nozzles in one-to-one correspondence with each nozzle of the jet printing unit  4 , are provided in the maintenance chamber  513 . The nozzles of the jet printing unit  4  are provided above the first position  100 . The scraper  514  has an end for scraping ink from the nozzle, which is formed with a blade (not shown in the drawings) slightly curved obliquely upward. 
     A cleaning assembly  52  for cleaning the scrapers  514  is provided above the second position  200 . The cleaning assembly  52  includes a fixed body  521 . Nozzles  522  for spraying cleaning liquid in one-to-one correspondence with each of the scrapers  514 , are provided on one side of the fixed body  521  facing the second position  200 . When the movable body  511  is moved to the second position  200 , the nozzles  522  are substantially located directly above the scrapers  514 . A liquid groove  523  for containing the cleaning liquid, which is connected with each nozzle  522 , is provided on one side of the fixed body  521  in opposition to the nozzles  522 . As shown in  FIG. 13 , each of the nozzles  522  includes a main body  522   a , on which a containing groove  522   b  for accommodating the blade of the scraper  514  is provided. At least one spray hole  522   c  is provided at the bottom of the containing groove  522   b . That is, there may be one spray hole  522   c  or a plurality of spray holes  522   c  opened at the bottom of the containing groove  522   b . In addition, in other embodiments, the containing groove  522   b  is not provided on the main body  522   a  of the nozzles  522 , and spray hole (s)  522   c  is (are) directly provided on the main body  522   a . The cleaning liquid flows from the liquid groove  523  to each of the nozzles  522 , and is ejected through the spray hole(s)  522   c  to clean the scrapers  514 . 
     At an edge of the maintenance chamber  513  of the moisturizing assembly  51 , a seal member  515  is provided. The seal member  515  can be a rubber strip, or the seal member  515  can be oil, which can realize a seal with an oil seal. This ensures that the maintenance chamber  513  is kept in a sealed state during the moisturizing process of the nozzles of the jet printing unit  4 , to thereby prevent the moisturizing liquid in the maintenance chamber  513  from volatilizing. 
     As shown in  FIG. 11 , when the nozzles of the jet printing unit  4  are not in operation, the moisturizing assembly  51  enters the first position  100 , and the jet printing unit  4  contacts tightly with the seal member  515  to form a sealed state, to thereby prevent the moisturizing liquid in the maintenance chamber  513  from volatilizing. The nozzles make moisturizing maintenance through the moisturizing liquid. A plurality of avoidance slots (not shown in the drawings) for accommodating the scrapers  514  are provided on bottom plates of the nozzles, ensuring that the nozzles does not collide with the scraper  514  when entering into the moisturized state. 
     As shown in  FIG. 12 , when the nozzles of the jet printing unit  4  is needed to print, the driving member  512  drives the movable body  511  to move to the second position  200 , and the moisturizing assembly  51  enters an automatic cleaning position. At this time, the nozzles  522  are located directly above the scrapers  514 , and the cleaning liquid sprayed from the nozzles  522  sprays and cleans the scrapers  514 . This cleaning liquid is flowing in the liquid groove  523  to ensure the cleaning effect. 
     The above nozzle maintenance device can effectively flush the ink on the scrapers off by spraying the cleaning liquid from top to bottom by the cleaning assembly that can move to the top of the scrapers. In addition, the scrapers are fixed in the moisturizing assembly, which is advantageous to simplify the structure, compared with the traditional scrapers and the moisture-retaining assembly, which are independent assemblies. 
     It should be understood that, modifications or changes may be made in accordance with the above description for the person skilled in the art, but such modifications or changes should be within the scope of the claims appended in the present invention.