Abstract:
An ink-jet recording apparatus comprises an ink-jet head having an ink ejection surface on which a plurality of nozzles are arrayed; a medium carrier forming a carrying surface on which a record medium is carried; a carriage mounted with the ink-jet head such that the ink ejection surface confronts the carrying surface; a carriage drive mechanism including a plurality of parallel guide rods supporting the carriage and extending across the direction where the record medium is carried by the medium carrier, the carriage drive mechanism reciprocating the carriage along the guide rods; and a guide shift mechanism for shifting the plurality of guide rods in a direction where the gap between the ink ejection surface and the carrying surface varies.

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to an ink-jet recording apparatus including an ink-jet head for ejecting ink droplets onto a record medium. 
   2. Description of Related Art 
   A plotter is known that performs recording with a pen that is moved by pulling via a belt a carriage mounted with the pen. Supporting the carriage by two guide rails is disclosed in Japanese Patent No. 2797637 as a technique for reciprocating the carriage in one direction in such a plotter. Application of this technique to a serial printing-type ink-jet printer results in a structure in which the carriage mounted with an ink-jet head is supported by two guide rods and simultaneously reciprocates along the guide rails. In this case, the distance from the ink ejection surface of the ink-jet head to the record medium carrying surface is invariable. 
   However, a record medium for use in the ink-jet printer may have various thicknesses. Therefore, the distance from the ink ejection surface to the record medium surface confronting the ink ejection surface varies depending on the thickness of a record medium. For this reason, when record media having different thickness are used, the printing quality changes depending on the thickness of the printed medium, leading to unstable printing quality. 
   SUMMARY OF THE INVENTION 
   It is a primary object of the present invention to provide an ink-jet recording apparatus capable of printing with a high quality irrespective of the thickness of a record medium. 
   According to an aspect of the present invention, the ink-jet recording apparatus comprises an ink-jet head that has an ink ejection surface on which a plurality of nozzles are arrayed; a medium carrier that forms a carrying surface on which a record medium is carried; a carriage mounted with the ink-jet head such that the ink ejection surface confronts the carrying surface; a carriage drive mechanism that includes a plurality of parallel guide rods supporting the carriage and extending across the direction where the record medium is carried by the medium carrier, the carriage drive mechanism reciprocating the carriage along the guide rods; and a guide shift mechanism that shifts the plurality of guide rods in a direction where the gap between the ink ejection surface and the carrying surface varies. 
   This enables the distance from the ink ejection surface to the record medium surface confronting the ink ejection surface to be kept constant. A high-quality printing can thus be effected on various record media having different thickness. Even when a record medium having a relatively large thickness is used, dirt on the record medium or jamming can be restrained from occurring since the record medium is hard to rub against or to get caught by the ink ejection surface of the ink-jet head. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Other and further objects, features and advantages of the invention will appear more fully from the following description taken in connection with the accompanying drawings in which: 
       FIG. 1  is a schematic front elevational view of an ink-jet printer in accordance with an embodiment of the present invention; 
       FIG. 2  is a bottom plan view of the ink-jet printer shown in  FIG. 1 ; 
       FIG. 3  is a side elevational view of a carriage and its vicinity included in the ink-jet printer shown in  FIG. 1 ; 
       FIG. 4  is a partial side view of the ink-jet printer shown in  FIG. 1 ; 
       FIG. 5  is a cross-sectional view of a guide rod in the vicinity of its one end, taken along line V—V of  FIG. 4 ; 
       FIG. 6  is an enlarged, fragmentary side view of a gear and its vicinity, depicted in  FIG. 4 ; and 
       FIG. 7  is a side elevational view of the ink-jet printer shown in  FIG. 1 . 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   Referring first to  FIGS. 1 and 2 , an ink-jet printer  1  in accordance with an embodiment of the present invention is of a serial printing type having a carriage  20  that reciprocates transversely in  FIG. 1 . The carriage  20  provides a support for two ink-jet heads  21  such that their ink ejection surfaces  21   a  are horizontal. On the ink ejection surface  21   a , a multiplicity of nozzles are arrayed in a matrix. The ink-jet head  21  is of a rectangular geometry whose longitudinal direction is orthogonal to the direction in which the carriage  20  moves. This ink-jet head  21  has a length of, e.g., about of 10 cm or longer and is elongated as compared with ones having in-line nozzles that are arrayed in one or two rows. In  FIG. 1 , a medium carrier is not shown such as a carrying belt  25  which will be described later. 
   As shown in  FIG. 3 , the carriage  20  has circular in section bores  20   a  and  20   b  disposed near its opposite ends along the longitudinal direction of the ink-jet heads  21 . The bores  20   a  and  20   b  receive cylindrical guide rods  18  and  19  in such a manner as to slidably support the carriage  20 . The guide rods  18  and  19  are supported at their respective opposite ends by side frames  2  and  3 , respectively, that are vertical plates extending parallel to each other outside the carrying belt  25 . As will be described later, the guide rods  18  and  19  are supported by the side frames  2  and  3  such that the height of the guide rods is adjustable, The side frames  2  and  3  are coupled to each other via a front frame  4 . 
   Outside the side frame  2  is supported a carriage drive shaft  5  level with the guide rods  18  and  19  such that the carriage drive shaft  5  is orthogonal to the guide rods  18  and  19 . Outside the side frame  3  is supported a coupling shaft  6  level with the guide rods  18  and  19  such that the coupling shaft  6  is orthogonal to the guide rods  18  and  19 . That is, the two shafts  5  and  6  are arranged parallel to the direction (hereinafter, referred to simply as “paper carrying direction”) in which paper, a record medium is carried. 
   The front frame  4  is mounted with a carriage drive motor  7  below the guide rods  18  and  19 . A pulley  9  is fixedly fitted around an output shaft  8  of the carriage drive motor  7 . An endless belt  11  is wrapped around the pulley  9  and a pulley  10  fixedly fitted around one end of the carriage drive shaft  5  so that the carriage drive shaft  5  rotates together with rotation of the output shaft  8 . 
   The carriage drive shaft  5  is fitted with two toothed driving pulleys  12  and  13 . The coupling shaft  6  is also fitted with toothed driven pulleys  14  and  15  positioned corresponding to the driving pulleys  12  and  13 , respectively. Carriage driving belts  16  and  17  in the form of endless belts are respectively wrapped around the driving pulley  12  and the driven pulley  14  and around the driving pulley  13  and the driven pulley  15 . The carriage driving belts  16  and  17  extend level with and parallel to the guide rods  18  and  19 , respectively. All of the four pulleys  12 ,  13 ,  14  and  15  are of the same external diameter. Thus, by driving the carriage drive motor  7  forwardly or reversely, it is possible to rotationally drive the carriage driving belts  16  and  17  at the same speed in both directions, clockwise and counterclockwise directions of  FIG. 1 . 
   As seen in  FIGS. 2 and 3 , the carriage  20  is provided with fixed portions  23  and  24  disposed outside the bores  20   a  and  20   b , respectively. The carriage  20  is secured via the fixed portions  23  and  24  to the carriage driving belts  16  and  17 , respectively. Thus, by driving the carriage drive motor  7  forwardly or reversely, it is possible to reciprocate the carriage  20  in the direction (the direction of a thick arrow of  FIG. 2 ) along the guide rods  18  and  19 . In this manner, use of the two guide rods  18  and  19  allows the carriage  20  supporting the elongated ink-jet heads  21  to travel stably. 
   As indicated by long dashed double-short dashed lines in  FIGS. 2 and 3 , at positions confronting ink ejection surfaces of the ink-jet heads  21  is disposed a carrying surface  25   a  of the carrying belt  25  in the form of an endless belt for carrying the record medium in the direction orthogonal to the guide rods  18  and  19 . The carrying belt  25  is wrapped around two rollers  26  and  27 . A driving force from a motor (not shown) is transmitted to one roller  26  so that while keeping a record medium on the carrying surface  25   a  of the carrying belt  25  the record medium can be carried along the carrying surface  25   a  in the direction of outlined arrows depicted in  FIGS. 2 and 3 . 
   A structure for supporting the guide rods  18  and  19  will then be described with further reference to  FIGS. 4 and 5 . As shown in  FIG. 2 , the guide rods  18  and  19  are provided at their respective opposite ends with cylindrical inserting portions  18   a ,  18   b  and  19   a ,  19   b , respectively, that are smaller in external diameter than parts inside the inserting portions. The inserting portions  18   a ,  18   b ,  19   a  and  19   b  are coaxial with the parts inside the inserting portions. 
   As shown in  FIG. 5 , the inserting portion  19   a  is inserted into a bearing hole  33 , having a diameter larger than the external diameter of the inserting portion  19   a  and smaller than the external diameter of the guide rod  19 , formed in the side frame  3 , in such a manner that the inserting portion  19   a  protrudes in its entirety from the side frame  3  to the exterior. The inserting portion  19   a  protruding from the bearing hole  33  is press-fitted into a support hole  34  formed in a gear  30  acting as a rotator. The center of the support hole  34  is offset from the rotational center of the gear  30 . 
   The gear  30  includes a toothed portion  31  having teeth formed around its circumference, and a boss  32  axially protruding from the toothed portion  31 . The boss  32  is positioned between the inner peripheral surface of the bearing hole  33  formed in the side frame  3  and the outer peripheral surface of the inserting portion  19   a . The other inserting portions  18   a , and  18   b ,  19   b  are also supported via the gears  30  by the side frames  3  and  2 , respectively, similar to the case of the inserting portion  19   a  supported by the side frame  3 . 
   Outside the side frame  3  above the gear  30 , as shown in FIGS.  4  and  5 , the side frame  3  provides a support for an L-shaped bar  35  elongated in the paper carrying direction. The L-shaped bar  35  is of an L-shaped section and has at its opposite ends two elongated holes  36  facing toward the paper carrying direction. Each elongated hole  36  receives a boss  37  having a slightly smaller external diameter than the width of the elongated hole  36 . A screw  38  fitted into the boss  37  is threadedly secured to the side frame  3  such that the L-shaped bar  35  can be displaced by a length of the elongated hole  36  in the direction along the paper carrying direction with respect to the side frame  3 . Stoppers  39  and  40  are disposed at both sides in the direction where the L-shaped bar  35  is displaceable, the stopper  39  and  40  serving to regulate the displacement of the L-shaped bar  35  to thereby define the range of displacement of the L-shaped bar  35 . 
   The undersurface near both extremities of the L-shaped bar  35  is formed with a rack  41  extending over a longer range than the elongated hole  36 . The rack  41  engages with the toothed portion  31  formed around the circumference of the gear  30 . As a result, the gear  30  fitted to the inserting portion  18   a  and the gear  30  fitted to inserting portion  19   a  are coupled together by way of the L-shaped bar  35 . In other words, transverse displacement of the L-shaped bar  35  causes corotation of the two gears  30 . It is to be noted that upon the coupling of these two gears  30 , adjustment is made such that the rotational phases of the support holes  34  coincide. 
   The L-shaped bar  35  is provided at or near the center in the longitudinal direction thereof with an upwardly protruding, substantially cylindrical knob  42 . Thus, arrangement is such that the L-shaped bar  35  can simply be slid along its longitudinal direction by holding the knob  42  with one hand and moving it transversely. 
   In this embodiment, the side frame  2  is not provided with any member similar to the L-shaped bar  35 . Accordingly, the gear  30  fitted to the inserting portion  18   b  is not coupled to the gear  30  fitted to the inserting portion  19   b . This means that these two gears  30  may be a mere eccentric cam having no toothed portion around its outer peripheral surface. In a case where the side frame  2  is fitted with a similar member to the L-shaped bar  35 , however, that member may couple together the gear  30  fitted to the inserting portion  18   b  and the gear  30  fitted to the inserting portion  19   b . 
   By virtue of employment of the above structure for supporting the guide rods  18  and  19 , it is possible to vary the distance between the ink ejection surface  21   a  of he ink-jet head  21  and the carrying surface  25   a  through the slide of L-shaped bar  35 . Reference is then made to  FIGS. 6 and 7  to describe the action when operating the knob  42  fitted to the L-shaped bar  35 . 
   As shown in  FIG. 6  which is an enlarged, fragmentary side view of the inserting portion  19   a  and its vicinity of  FIG. 4 , the center A 1  of the inserting portion  19   a  and of the guide rod  19  is offset from the center O of the gear center  30 . Therefore, when the gear  30  rotates as a result of sliding of the L-shaped bar  35 , the center of the inserting portion  19   a  and of the guide rod  19  moves from A 1  to A 2 . This allows the guide rod  19  to move from the position depicted by a dashed line to the position depicted by a long dashed double-short dashed line. The movement changes the position of the guide rod  19  in the amount of displacement in height H. The guide rod  18  coupled to the guide rod  19  by way of the two gears  30  and the L-shaped bar  35  also moves by the same distance as the distance through which the guide rod  19  moves, in the same direction as the direction in which the guide rod  19  moves. For this reason, the height of the carriage  20  supported by the two guide rods  18  and  19  varies by the amount of displacement in height H of the guide rods  18  and  19 . It is thus possible to vary the distance, the gap G shown in  FIG. 7 , between the ink ejection surface  21   a  of the ink-jet head  21  and the carrying surface  25   a.    
   Referring finally to  FIG. 7 , a status is shown when the knob  42  of the L-shaped bar  35  is moved from right to left. In the state of  FIG. 7 , the knob  42  is at its right-hand position. At that time, the gap, designated at G in  FIG. 7 , between the ink ejection surface  21   a  and the carrying surface  25   a  is relatively narrow, When the knob  42  is moved to its left-hand position from this state, the L-shaped bar  35  also moves jointly therewith so that the rack  41  formed on the L-shaped bar  35  causes simultaneous rotations of the two gears  30 . Then, as described above, the rotations of the gears  30  cause simultaneous variances the amount of displacement in height H of the two guide rods  18  and  19 . Since the carriage  20  is supported by the two guide rods  18  and  19 , the gap G becomes larger by the same amount as the amount of displacement in height H of the guide rods  18  and  19 . 
   Description will then be made briefly of printing action on the record medium in the ink-jet printer  1  thus configured. In the initial state, the knob  42  is operated in advance to adjust the distance between the ink ejection surface  21   a  and the carrying surface  25   a  so that the distance from the ink ejection surface  21   a  to the surface of the record medium is kept at a predetermined distance irrespective of the thickness of the record medium to be printed. When the printing action is started, a record medium fed from a paper feed cassette (not shown) lying on the left hand in  FIGS. 2 and 3  to the carrying belt  25  passes, together with rotation of the carrying belt  25 , through the gap G formed between the undersurface, i.e., the ink ejection surface  21   a  of the ink-jet head  21  and the top surface, i.e., the carrying surface  25   a  of the carrying belt  25 . Simultaneous with this, the carriage  20  travels reciprocatingly in the width direction (the direction of the thick arrow of  FIG. 2 ) of the record medium while ejecting ink droplets from nozzles formed on the ink ejection surface  21   a . As a result, a desired image is formed on the top surface of the record medium. 
   As set forth hereinabove, according to this embodiment, the gap between the ink ejection surface  21   a  and the carrying surface  25   a  can be adjusted with the parallelism therebetween maintained. It is thus possible to keep the distance between the ink ejection surface  21   a  and the record medium surface at a certain level value by increasing the gap between the ink ejection surface  21   a  and the carrying surface  25   a  for a record medium having a large thickness but by reducing the gap between the ink ejection surface  21   a  and the carrying surface  25   a  for a record medium having a small thickness, This enables printing with a high printing quality to be effected on any record medium having a different thickness such as a mere one-ply record medium or a two-ply envelope. Even when using a record medium having a considerably large thickness, the record medium can be prevented from rubbing against or getting caught by the ink ejection surface  21   a  of the ink-jet head  21 . It is accordingly possible to reduce the occurrence of dirt on the record medium or jamming to a large extent. 
   The ink-jet recording apparatus of this embodiment is excellent in practical use since the gap adjustment is possible between the ink ejection surface  21   a  and the carrying surface  25   a  through a simple structure where the guide rods  18  and  19  are supported at eccentric positions on the gears  30 . In addition, the ink-jet recording apparatus of this embodiment is excellent in operability since the L-shaped bar  35  only needs to be slid. Formation of the rack  41  on the L-shaped bar  35  allows the gear  30  to rotate through the engagement of the rack  41  with the toothed portion  31  of the gear  30 , contributing to enhancement of reliability. 
   Furthermore, its operation becomes easier due to the L-shaped bar  35  capable of simultaneously rotating the two gears  30 . Provision of the knob  42  on the L-shaped bar  35  facilitates user&#39;s operation. Since the guide rods  16  and  19  are supported at their respective opposite ends by the corresponding gears  30 , the guide rods  18  and  19  can hardly vary in height, contributing to excellent height stability. 
   In addition, since the carriage  20  is movable by the carriage driving belts  16  and  17  running parallel to the guide rods  18  and  19 , it can hardly occur that the gap between the ink ejection surface  21   a  and the carrying surface  25   a  varies depending on the position of the carriage  20 . 
   Although the above embodiment has included the belt  25  as a carrier for carrying a record medium, the carrying means is not limited thereto but may be a mechanism in which the record medium is carried while being nipped by rollers or roller pairs having cylindrical surfaces as their carrying surfaces. The L-shaped bar  35  need not necessarily be provided for each gear  30  to couple the two gears  30  together. 
   The mechanism for moving the guide rods can variously be modified without being limited to the above mechanism. The record medium can be in the form of paper, plastic film, etc. It is only essential that the record medium is a sheet-like medium having flexibility and allowing images to be recorded on its one surface by the ink-jet heads. The coupling of the two gears  30  may be achieved by wrapping an endless belt around the two gears  30 , instead of using the L-shaped bar  35 . Although in the above embodiments the present invention has been described by way of the ink-jet printer in an example, the present invention is applicable equally to other printing equipment such as facsimiles or copiers mounted with ink-jet heads. 
   While this invention has been described in conjunction with the specific embodiments outlined above, it is evident that many alternatives, modifications and variations will be apparent to those skilled in the art. Accordingly, the preferred embodiments of the invention as set forth above are intended to be illustrative, not limiting. Various changes may be made without departing from the spirit and scope of the invention as defined in the following claims.