Patent Publication Number: US-8528717-B2

Title: Printing apparatus

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     The present application claims priority to Japanese Patent Application No. 2007-224789, filed on Aug. 30, 2007, the entire contents of which are herein incorporated by reference. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a printing apparatus for printing intended characters, images, etc. on a print substrate, such as a T-shirt. 
     2. Discussion of the Background 
     Conventionally, an apparatus and method for printing characters, images, etc. on a print substrate such as a T-shirt, screen printing are known in which a plurality of screens corresponding to printing colors are prepared and are used to print each color on the surface of the print substrate so that printed colors are overlapped. Since the screen printing requires a plurality of exclusive screens corresponding to designs and colors, making the screens is troublesome and expensive. Thus, screen printing is not suitable for high-mix, low-volume production type printing. Recently, methods for printing on a print substrate such as a T-shirt by using an inkjet printer to eject ink droplets directly on the print substrate have been carried out. JP-A-2002-154247 and JP-A-2007-031888 describe such printing apparatuses and methods using an inkjet printer. The contents of JP-A-2002-154247 and JP-A-2007-031888 are incorporated herein by reference in their entirety. 
     For printing characters, images, etc. on a print substrate made of cloth such as a T-shirt by using an inkjet printer, it may required to conduct a lot of processing operations, such as preprocessing for the purpose of preventing ink blurring and post-processing for the purpose of protecting images printed on the surface of the print substrate. Especially in an industrial printing apparatus, there are many situations that the printing mode should be changed according to the kind of print substrate to be printed and changes in the production line. Thus, there is a need for a printing apparatus and method that can efficiently conduct the printing procedure requiring such a lot of processing steps relative to the print substrate, thereby improving the operating efficiency. Conventional printing apparatuses do not allow for efficient changes. 
     SUMMARY OF THE INVENTION 
     The present invention advantageously provides an embodiment in which a printing apparatus includes a pallet configured to receive a print substrate, a conveyer system configured to convey the pallet, and a plurality of work mechanisms arranged along a pallet conveying direction of the conveyer system. The work mechanisms are configured to conduct processing operations to the print substrate. The conveyer system includes an upper conveyer mechanism configured to sequentially convey the pallet to respective working positions of the work mechanisms, and a lower conveyer mechanism arranged below and along the upper conveyer mechanism. The lower conveyer mechanism is configured to convey the pallet in a direction opposite to the pallet conveying direction of the upper conveyer mechanism so that the conveyer system is structured to have an upper stage and a lower stage. 
     The present invention also advantageously provides an embodiment in which a printing apparatus includes a pallet configured to receive a print substrate, means for conveying the pallet, and means for conducting processing operations to the print substrate. The means for conducting processing operations being arranged along a pallet conveying direction of the means for conveying the pallet. The means for conveying the pallet includes an upper means for conveying the pallet sequentially to respective working positions of the means for conducting processing operations, and a lower means for conveying the pallet arranged below and along the upper means for conveying the pallet. The lower means for conveying the pallet is configured to convey the pallet in a direction opposite to the pallet conveying direction of the upper means for conveying the pallet. 
     The present invention further advantageously provides an embodiment in which a method of printing on a print substrate includes conveying a pallet having a print substrate received thereon, using an upper conveyer mechanism to convey the pallet to working positions to sequentially conduct processing operations to the print substrate along a pallet conveying direction, and using a lower conveyer mechanism to covey the pallet in a direction opposite to the pallet conveying direction of the upper conveyer mechanism. The lower conveyer mechanism is arranged below and along the upper conveyer mechanism. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       A more complete appreciation of the invention and many of the attendant advantages thereof will become readily apparent with reference to the following detailed description, particularly when considered in conjunction with the accompanying drawings, in which: 
         FIG. 1  is a side view showing schematic structure of a printing apparatus according to an embodiment of the present invention; 
         FIG. 2  is a perspective view showing structures of a loading station unit, a printing station unit, and a collecting station unit as components of the printing apparatus; 
         FIG. 3  is a perspective view showing a structure of a pallet on which print substrates to be printed by the printing apparatus are placed; 
         FIG. 4  is a perspective view showing a structure of a conveyer unit as a component of the printing apparatus; 
         FIG. 5  is a perspective view showing a structure of each of upper and lower conveyer mechanisms as parts of a conveyer unit; 
         FIG. 6  is a plan view of the conveyer unit; 
         FIGS. 7(A) and 7(B)  are a plan view (A) and a side view (B), respectively, showing a structure of the loading station unit or a collecting station unit; 
         FIG. 8  is a side view showing a structure of an anteroposterior moving mechanism as a part of a printer unit of a printing station unit; 
         FIG. 9  is a perspective view showing a driving system of driving sprockets in the anteroposterior moving mechanism; 
         FIG. 10  is a partial sectional view showing a peripheral structure of a fitting member in the left-side anteroposterior moving mechanism; and 
         FIG. 11  is a partial sectional view showing a peripheral structure of a fitting member in the right-side anteroposterior moving mechanism. 
     
    
    
     DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS OF THE INVENTION 
     Embodiments of the present invention will be described hereinafter with reference to the accompanying drawings. In the following description, the constituent elements having substantially the same function and arrangement are denoted by the same reference numerals, and repetitive descriptions will be made only when necessary. 
     Embodiments of the present invention advantageously provide a printing apparatus that can efficiently conduct a printing procedure requiring a lot of processing operations relative to a print substrate, thereby improving the operating efficiency, and allow a printing mode to be easily changed. 
     An embodiment of the present invention includes a printing apparatus (e.g., the printing apparatus  1  in  FIG. 1 ) having a pallet (e.g., the pallet  6 ) on which a print substrate (e.g., the print substrate  5 ) is placed, a conveyer system (e.g., the conveyer units  10  of the work station units  2   b - 2   g ) for conveying the pallet, and a plurality of work mechanisms (e.g., the work units  3   b - 3   g ) arranged along the pallet conveying direction of the conveyer system, where the printing apparatus prints intended images or the like by substantially conducting a plurality of processing operations to the print substrate, and where the conveyer system includes an upper conveyer mechanism (e.g., the upper conveyer mechanisms  11 ) for sequentially conveying the pallet to respective working positions of the work mechanisms, and a lower conveyer mechanism (e.g., the lower conveyer mechanisms  12 ) that are arranged below and along the upper conveyer mechanism for conveying the pallet in a direction opposite to the pallet conveying direction of the upper conveyer system so that the conveyer system is structured to have two upper and lower stages. 
     According to the above structure, the pallet can be easily conveyed to the working position of each work mechanism by the upper conveyer mechanism, thereby effectively conducting the printing procedure requiring a lot of processing steps relative to the print substrate. Further, the pallet after the print substrate is collected can be conveyed to the starting point of the conveyance of the upper conveyer mechanism by the lower conveyer mechanism, thereby improving the operating efficiency. 
     The printing apparatus further includes delivering mechanisms (e.g., the elevation units  13  of the loading station unit  2   a  and the collection station unit  2   h ) that are disposed on both ends of the conveyer system for delivering the pallet between the upper conveyer mechanism and the lower conveyer mechanism. 
     Since the printing apparatus further includes delivering mechanisms which are disposed on both ends of the conveyer system for delivering the pallet between the upper conveyer mechanism and the lower conveyer mechanism, the conveyance of the pallet between the upper conveyer mechanism and the lower conveyer mechanism is automated, thereby easily achieving the circulating conveyance of the pallet and thus further improving the operating efficiency. 
     In the printing apparatus, the conveyer system is composed of a plurality of conveyer units (e.g., the conveyer units  10  of the work station units  2   b - 2   g ), each of which has the upper conveyer mechanism and the lower conveyer mechanism for each of the work mechanisms and which are abutted to allow delivery of the pallet between the conveyer units. 
     Since the conveyer system is composed of a plurality of conveyer units each of which has the upper conveyer mechanism and the lower conveyer mechanism for each of the work mechanisms and which are abutted to allow delivery of the pallet between the conveyer units, it is possible to change the setting of steps for the printing procedure by replacing or rearranging each of the conveyer units and it is thus possible to easily change the printing mode according to the kind of the print substrate or change of the production line, thereby making the printing apparatus to suit printing of a high-mix, low-volume production type. 
     Hereinafter, a preferred embodiment of the present invention will be described with reference to attached drawings.  FIG. 1  shows a schematic structure of a printing apparatus  1  according to an embodiment of the present invention. 
     The printing apparatus  1  includes eight work station units (a loading station unit  2   a , a preprocessing station unit  2   b , a first drying station unit  2   c , an undercoating station unit  2   d , a printing station unit  2   e , an post-processing station unit  2   f , a second drying station unit  2   g , and a collecting station unit  2   h ) that are aligned adjacent to each other. The station units  2   b  through  2   g  are provided corresponding to respective processing operations to be conducted to a print substrate  5  such as T-shirts and each include a work unit (a preprocessing unit  3   b , drying units  3   c  and  3   g , an undercoating unit  3   d , a printing unit  3   e , an post-processing unit  3   f ) for conducting each processing operation. 
     Each of the station units  2   b  through  2   g  has a conveyer unit  10  for conveying a pallet  6  which holds the print substrate  5  thereon. The conveyer unit  10  includes an upper conveyer mechanism  11  for conveying the pallet  6  holding the print substrate  5  thereon at a working position as will be described later for each work unit (the preprocessing unit  3   b , the drying units  3   c  and  3   g , the undercoating unit  3   d , the printing unit  3   e , the post-processing unit  3   f ), and a lower conveyer mechanism  12  for conveying the empty pallet  6  (without holding the print substrate  5 ). The upper conveyer mechanisms  11  and the lower conveyer mechanisms  12  are placed in a uniform way through the respective station units  2   b  through  2   g  such that ends as the pallet receiving portions and the pallet let-off portions of the respective upper conveyer mechanisms  11  are surely abutted to each other and also ends as the pallet receiving portions and the pallet let-off portions of the respective lower conveyer mechanisms  12  are surely abutted to each other so as to allow the delivery of the pallet  6  between the station units. 
     The loading station unit  2   a  and the collecting station unit  2   h  each have an elevation unit  13  for carrying the pallet  6  between the upper conveyer mechanism  11  and the lower conveyer mechanism  12 . The elevation unit  13  moves up and down to abut itself to the ends as the pallet receiving portion and the pallet let-off portion of the upper conveyer mechanism  11  and the lower conveyer mechanism  12  so as to allow the delivery of the pallet  6  between the upper conveyer mechanism  11  and the lower conveyer mechanism  12 . The pallet  6  can be conveyed and circulated through the respective station units  2   a  through  2   h  by the elevation units  13  and the upper and lower conveyer mechanisms  11 ,  12  (the respective conveyer units  10 ). 
     In each of the station units  2   a  through  2   h , each elevation unit  13 , or each work unit (the preprocessing unit  3   b , the drying units  3   c  and  3   g , the undercoating unit  3   d , the printing unit  3   e , the post-processing unit  3   f ) and each conveyer unit  10  are mounted to a frame body (a conveyer frame  130 , an elevation frame  152  in  FIG. 2 ) of each station unit and thus can be replaced or rearranged integrally with each frame body. Each frame body has wheels  14  (wheels  131   a , wheels  151  in  FIG. 2 ) that are fixed to the bottom thereof and have locking mechanisms, respectively, thereby easily allowing the movement of each of the station units  2   a  through  2   h.    
     In the printing apparatus  1 , a print substrate  5  is first put on the pallet  6  by an operator in the loading station unit  2   a . The pallet  6  holding the print substrate  5  thereon is moved by the elevation unit  13  toward the upper conveyer mechanism  11  of the preprocessing station unit  2   b  and is then conveyed to the next station unit, i.e., the preprocessing station unit  2   b.    
     The preprocessing station unit  2   b  includes the preprocessing unit  3   b  having a preprocessing printer or the like for conducting a preprocessing step by applying base coating material (transparent ink) to the surface of the print substrate  5  (in a range where intended characters, images, etc. are printed) to previously coat the surface of the print substrate  5 . The preprocessing step is conducted for the purpose of preventing ejected inks, applied at the undercoating station unit  2   d  and the printing station unit  2   e , from penetrating inside the print substrate  5  and preventing ink blurring. The pallet  6  conveyed from the loading station unit  2   a  to the preprocessing station unit  2   b  is moved to a working position, as will be described later, of the preprocessing unit  3   b  via a standby position, as will be described later, by the upper conveyer mechanism  11 . The print substrate  5  on the pallet  6  is subjected to the aforementioned preprocessing step while the pallet  6  is held at the working position and is then conveyed to the next station unit, i.e., the first drying station unit  2   c.    
     The first drying station unit  2   c  includes the drying unit  3   c  having a heater or the like for drying the base coating material (transparent ink) which was applied at the preprocessing station unit  2   b  (the preprocessing unit  3   b ) to coat the surface of the print substrate  5 . The drying step is conducted for the purpose of preventing adhesion of inks applied at the undercoating station unit  2   d  and the printing station unit  2   e  from being poor and thus preventing the print quality from being poor. The pallet  6  conveyed from the preprocessing station unit  2   b  to the first drying station unit  2   c  is moved to a working position of the drying unit  3   c  via a standby position by the upper conveyer mechanism  11 . The print substrate  5  on the pallet  6  is subjected to the aforementioned drying step while the pallet  6  is held at the working position and is then conveyed to the next station unit, i.e., the undercoating station unit  2   d.    
     The undercoating station unit  2   d  includes the undercoating unit  3   d  having an undercoat printer or the like for conducting an undercoating process by applying undercoating material (white ink) on the surface (the surface of the base coating material) of the print substrate  5  preprocessed at the preprocessing station unit  2   b  (the preprocessing unit  3   b ). The undercoating process allows the intended color characters, images, etc. to be printed without affecting the color of ink ejected at the printing station unit  2   e  by the color of the print substrate  5  itself, thereby improving the chromogenic characteristics. The pallet  6  conveyed from the first drying station unit  2   c  to the undercoating station unit  2   d  is moved to a working position of the undercoating unit  3   d  via a standby position by the upper conveyer mechanism  11 . The print substrate  5  on the pallet  6  is subjected to the aforementioned undercoating process while the pallet  6  is held at the working position and is then conveyed to the next station unit, i.e., the printing station unit  2   e.    
     The printing station unit  2   e  includes a printing unit  3   e  having an inkjet printer or the like for conducting a printing step for printing intended characters, images, etc. by ejecting ink droplets from inkjet nozzle onto the surface of the print substrate  5  undercoated at the undercoating station unit  2   d  (the undercoating unit  3   d ). The pallet  6  conveyed from the undercoating station unit  2   d  to the printing station unit  2   e  is moved to a working position of the printing unit  3   e  via a standby position by the upper conveyer mechanism  11 . The print substrate  5  on the pallet  6  is subjected to the aforementioned printing step while the pallet  6  is held at the working position and is then conveyed to the next station unit, i.e., the post-processing station unit  2   f.    
     The post-processing station unit  2   f  includes the post-processing unit  3   f  having a post-processing printer or the like for conducting a post-processing step by applying protective coating material (transparent ink) to the surface of the characters, images, etc. of the print substrate  5  printed at the printing station unit  2   e  (the printing unit  3   e ) to protect and coat the surface. The post-processing step is conducted for the purpose of preventing the ejected ink of the intended characters, images, etc. printed at the printing station unit  2   e  (the printing unit  3   e ) from peeling off the surface of the print substrate  5 . The pallet  6  conveyed from the printing station unit  2   e  to the post-processing station unit  2   f  is moved to a working position of the post-processing unit  3   f  via a standby position by the upper conveyer mechanism  11 . The print substrate  5  on the pallet  6  is subjected to the aforementioned post-processing step while the pallet  6  is held at the working position and is then conveyed to the next station unit, i.e., the second drying station unit  2   g.    
     The second drying station unit  2   g  includes the drying unit  3   g  having a heater or the like for drying the protective coating material (transparent ink) protecting and coating the surface of the printed characters, images, etc. of the print substrate  5  applied at the post-processing station unit  2   f  (the post-processing unit  3   f ). By this drying step, all of the processing operations for the print substrate  5  are completed, thus finishing the print substrate  5  with printed intended characters, images, etc. thereon. The pallet  6  conveyed from the post-processing station unit  2   f  to the second drying station unit  2   g  is moved to a working position of the drying unit  3   g  via a standby position by the upper conveyer mechanism  11 . The print substrate  5  on the pallet  6  is subjected to the aforementioned drying step while the pallet  6  is held at the working position and is then conveyed to the next station unit, i.e., the collecting station unit  2   h.    
     In the collecting station unit  2   h , the finished print substrate  5  on the pallet  6  conveyed from the second drying station unit  2   g  after the respective processing operations is collected by the operator. Empty pallet  6  after the print substrate  5  is collected is moved by the elevation unit  13  toward the lower conveyer mechanism  12  of the second drying station unit  2   g  and is conveyed to the loading station unit  2   a  through the post-processing station unit  2   f , the printing station unit  2   e , the undercoating station unit  2   d , the first drying station unit  2   c , and the preprocessing station unit  2   b  in this order by the lower conveyer mechanisms  12 . Then, at the loading station unit  2   a , a new print substrate  5  is put on the empty pallet  6  and is subjected to the aforementioned steps. 
     Then, the structure of the conveyer unit  10  (the upper conveyer mechanism  11  and the lower conveyer mechanism  12 ) of each of the station unit  2   b  through  2   g  and the structure of the elevation unit  13  of each of the loading station unit  2   a  and the collecting station unit  2   h  will be described in detail with regard to  FIG. 2  through  FIG. 7 .  FIG. 2  shows a printing apparatus  1 ′ including a loading station unit  2   a , a printing station unit  2   e , and a collecting station unit  2   h  that are aligned adjacent to each other. For the sake of convenience, in  FIG. 2  through  FIG. 6 , directions indicated by arrows “UP”, “FRONT”, and “LEFT” will be upward, forward, and leftward directions in the following description. 
     The printing station unit  2   e  includes a conveyer unit  10  that is composed of an upper conveyer mechanism  11  for conveying a pallet  6  holding a print substrate  5  thereon, a lower conveyer mechanism  12 , and a printer unit  20  (a printing unit  3   e ) as will be described later. As show in  FIG. 3 , the pallet  6  is formed into a rectangular plate-shape and has a pair of long table supporting members  7  fixed to the upper surface thereof. Disposed on the table supporting members  7  are a plurality of (four in this embodiment) loading tables  8  that are aligned in the lateral direction. Each loading table  8  includes a top panel  8   a  having a rectangular plate shape for holding each print substrate  5  thereon and four leg members  8   b  extending downwardly from the lower surface of the top panel  8   a . The pallet  6  is provided with four cutouts  6   a  two of which are formed in each of the front and rear side surfaces thereof and which can come in contact with to locating pins  141   a  of first sensors  141  and locating pins  146   a  of second sensors  146 . 
     As shown in  FIG. 4 , the conveyer unit  10  has a conveyer frame  130  including a rectangular frame body  131  that has a plurality of wheels  131   a  having locking mechanisms and is located at a predetermined height from a floor, four frame columns  132  standing on side surfaces near the four corners of the frame body  131 , a pair of upper frame members  133  that are fixed to extend between upper portions of the left-side frame columns  132  and between upper portions of right-side frame columns  132 , respectively, and a pair of lower frame members  134  that are fixed to extend between lower portions of the left-side frame columns  132  and between lower portions of right-side frame columns  132 , respectively. An upper conveyer mechanism  11  is supported along the upper frame members  133  and a lower conveyer mechanism  12  is supported along the lower frame members  134 , thereby making a two-stage structure. The upper frame members  133  and the lower frame members  134  have guide walls  133   a ,  134   a  which are disposed on the outer surfaces thereof to extend upward along the outer surfaces. The guide walls  133   a ,  134   a  prevent the pallets  6  on the upper conveyer mechanism  11  and the lower conveyer mechanism  12  from deviating to the left or the right. 
     The upper conveyer mechanism  11  has a pair of upper belt conveyers  110   a ,  110   b  that are supported between the upper frame members  133 ,  133  such that the upper belt conveyers  110   a ,  110   b  are spaced apart from each other by a predetermined distance to extend in parallel to each other and horizontally by using a plurality of long supporting frame members  135 . As shown in  FIG. 5 , the upper belt conveyers  110   a ,  110   b  include long body members  111   a ,  111   b , driving pulleys  113   a ,  113   b  and driven pulleys  114   a ,  114   b  rotatably mounted at the ends of the long body members  111   a ,  111   b  via mounting members  112 , and endless conveyer belts  115   a ,  115   b  which are wound around the driving pulleys  113   a ,  113   b  and the driven pulleys  114   a ,  114   b  to extend therebetween. The left and right driving pulleys  113   a ,  113   b  are connected to each other through a driving shaft  116 . Attached to the driving shaft  116  is a first sprocket  117   c . A driving chain  118  is wound around the first sprocket  117   c  and a second sprocket  117   d  to extend therebetween. The second sprocket  117   d  is rotated by a driving motor  119  that is fixed to the lower surface of the supporting frame member  135 , whereby the first sprocket  117   c  is rotated through the driving chain  118  and the driving pulleys  113   a ,  113   b  are rotated in synchronism with each other via the driving shaft  116 . That is, the left and right conveyer belts  115   a ,  115   b  are rotated and moved in synchronism with each other by the driving motor  119  so as to move the pallet  6  on the conveyer belts  115   a ,  115   b  rearward. 
     At predetermined positions in the feeding direction (the anteroposterior direction) of the upper conveyer mechanism  11 , as shown in  FIG. 6 , a first holding mechanism  140  and a second holding mechanism  145  for locating and holding the pallet  6  conveyed by the upper conveyer mechanism  11  are disposed on the upper conveyer mechanism  11 . The first holding mechanism  140  includes first sensors  141  for sending a signal when the pallet  6  is located at a preset first holding position (a working position where the printer unit  20  as will be described later prints on the print substrate  5 ), first air cylinders  142  for lifting up the pallet  6  to space the pallet  6  apart from the upper conveyer mechanism  11  (the conveyer belts  115   a ,  115   b ) when the pallet  6  is located at the first holding position, and a first controller  143  which receives the signal, indicating that the pallet  6  is located at the first holding position, from the first sensors  141  and thus sends a control signal to the first air cylinders  142 . 
     The first sensors  141  each have a locating pin  141   a  which is expandable in the vertical direction and are disposed on the inner sides of the upper belt conveyers  110   a ,  110   b , respectively (the total number of the first sensors  141  is two in this embodiment). The locating pins  141   a  in the expanded state come in contact with the cutouts  6   a  (see  FIG. 2  or  FIG. 3 ), formed on the rear side of the pallet  6 , when the pallet  6  conveyed on the upper belt conveyers  110   a ,  110   b  is located at the first holding position. It should be noted that the locating pins  141   a  of the first sensors  141  in the contracted state do not come in contact with the pallet  6  (the cutouts  6   a ). The first sensors  141  detect pressing force produced when the cutouts  6   a  of the pallet  6  collide with the locating pins  141   a  and thus sends a signal, indicating that the pallet  6  is located at the first holding position, to the first controller  143 . 
     The first controller  143  is connected to the first sensors  141  and the first air cylinders  142  via cables (not shown). Through the cables, the first controller  143  receives the signal, indicating that the pallet  6  is located at the first holding position, from the first sensors  141  and sends a control signal to the first air cylinders  142 . The first controller  143  is mounted to the supporting frame member  135  between the upper belt conveyers  110   a ,  110   b.    
     The first air cylinders  142  each have a holding pin  142   a  which is expandable in the vertical direction and are disposed on the upper belt conveyers  110   a ,  110   b  two by two at positions corresponding to the position of the pallet  6  being in the first holding position (the total number of the first air cylinders  142  is four in this embodiment). The holding pins  142   a  are expandable according to the control signal of the first controller  143 . The holding pins  142   a  when being expanded come in contact with the lower surface of the pallet  6  and lift up the pallet  6 , thereby holding the pallet  6  spaced apart from the conveyer belts  115   a ,  115   b  of the upper belt conveyers  110   a ,  110   b . As the holding pins  142   a  of the first air cylinders  142  are contracted according to the control signal of the first controller  143  from the state holding the pallet  6  spaced apart from the conveyer belts  115   a ,  115   b , the pallet  6  is returned to be put on the conveyer belts  115   a ,  115   b.    
     The second holding mechanism  145  has a similar structure as the aforementioned first holding mechanism  140  and includes second sensors  146 ,  146  for sending a signal when the pallet  6  is located at a preset second holding position (a position before (in front of) the first holding position in the conveying direction, that is, a standby position before the first holding position not to lap over the working position of the printer unit  20  as will be described later), second air cylinders  147  for lifting up the pallet  6  to space the pallet  6  apart from the upper conveyer mechanism  11  when the pallet  6  is located at the second holding position, and a second controller  148  which receives the signal, indicating that the pallet  6  is located at the second holding position, from the second sensors  146  and thus sends a control signal to the second air cylinders  147 . 
     The second sensors  146  each have a locating pin  146   a  that is expandable in the vertical direction and are disposed on the inner sides of the upper belt conveyers  110   a ,  110   b , respectively (the total number of the second sensors  146  is two in this embodiment). The locating pins  146   a  in the expanded state come in contact with the cutouts  6   a  (see  FIG. 2  or  FIG. 3 ), formed on the rear side of the pallet  6 , when the pallet  6  conveyed on the upper belt conveyers  110   a ,  110   b  is located at the second holding position. It should be noted that the locating pins  146   a  of the second sensors  146  in the contracted state do not come in contact with the pallet  6  (the cutouts  6   a ). The second sensors  146  detect pressing force produced when the cutouts  6   a  of the pallet  6  collide with the locating pins  146   a  and thus sends a signal, indicating that the pallet  6  is located at the second holding position, to the second controller  148 . 
     The second controller  148  is connected to the second sensors  146  and the second air cylinders  147  via cables (not shown). Through the cables, the second controller  148  receives the signal, indicating that the pallet  6  is located at the second holding position, from the second sensors  146  and sends a control signal to the second air cylinders  147 . The second controller  148  is mounted to the supporting frame member  135  between the upper belt conveyers  110   a ,  110   b.    
     The second air cylinders  147  each have a holding pin  147   a  which is expandable in the vertical direction and are disposed on the upper belt conveyers  110   a ,  110   b  two by two at positions corresponding to the position of the pallet  6  being in the second holding position (the total number of the second air cylinders  147  is four in this embodiment). The holding pins  147   a  are expandable according to the control signal of the second controller  148 . The holding pins  147   a  when being expanded come in contact with the lower surface of the pallet  6  and lift up the pallet  6 , thereby holding the pallet  6  spaced apart from the conveyer belts  115   a ,  115   b  of the upper belt conveyers  110   a ,  110   b . As the holding pins  147   a  of the second air cylinders  147  are contracted according to the control signal of the second controller  148  from the state holding the pallet  6  spaced apart from the conveyer belts  115   a ,  115   b , the pallet  6  is returned to be put on the conveyer belts  115   a ,  115   b.    
     As shown in  FIG. 4 , the lower conveyer mechanism  12  has a pair of lower belt conveyers  120   a ,  120   b  which are supported between the lower frame members  134 ,  134  such that the lower belt conveyers  120   a ,  120   b  are spaced apart from each other by a predetermined distance to extend in parallel to each other and horizontally by using a plurality of long supporting frame members  136 . The lower belt conveyers  120   a ,  120   b  have similar structure as the upper belt conveyers  110   a ,  110   b  of the aforementioned upper conveyer mechanism  11 . As shown in  FIG. 5 , the lower belt conveyers  120   a ,  120   b  comprise long body members  121   a ,  121   b , driving pulleys  123   a ,  123   b  and driven pulleys  124   a ,  124   b  rotatably mounted at the ends of the long body members  121   a ,  121   b  via mounting members  122 , and endless conveyer belts  125   a ,  125   b  which are wound around the driving pulleys  123   a ,  123   b  and the driven pulleys  124   a ,  124   b  to extend therebetween. The left and right driving pulleys  123   a ,  123   b  are connected to each other through a driving shaft  126 . Attached to the driving shaft  126  is a first sprocket  127   c . A driving chain  128  is wound around the first sprocket  127   c  and a second sprocket  127   d  to extend therebetween. The second sprocket  127   d  is rotated by a driving motor  129  which is fixed to the lower surface of the supporting frame member  136 , whereby the first sprocket  127   c  is rotated through the driving chain  128  and the driving pulleys  123   a ,  123   b  are rotated in synchronism with each other via the driving shaft  126 . That is, the left and right conveyer belts  125   a ,  125   b  are rotated and moved in synchronism with each other by the driving motor  129  so as to move the pallet  6  on the conveyer belts  125   a ,  125   b  forward. 
     The loading station unit  2   a  and the collecting station unit  2   h  have the same structures. As shown in  FIG. 7 , each of the loading station unit  2   a  and the collecting station unit  2   h  has an elevation frame  152 , including a rectangular plate-shape supporting base  152   a  which has a plurality of wheels  151  having locking mechanisms and is located at a predetermined height from the floor, and a wall-like frame  152   b  which stands on the supporting base  152   a  and includes a frame section on the opposite side from the conveyer unit  10  and left and right side frame sections (having a U-like shape in a plan view), and an elevation unit  13  which is installed on the supporting base  152   a.    
     The elevation unit  13  includes an elevation cylinder  153 , a plurality of (four in this embodiment) guide tubes  154 , an elevation table  155 , and a pair of left and right elevation belt conveyers  160   a ,  160   b . The elevation cylinder  153  and the guide tubes  154  are standing on the supporting base  152   a . The elevation table  155  having a plate-like shape is attached to an upper end of an expandable output rod  153   a  of the elevation cylinder  153  and upper ends of guide rods  154   a , which are slidably fitted to the guide tubes  154 . The elevation table  155  can move up and down in the vertical direction because of the expansion and contraction of the elevation cylinder  153  (the output rod  153   a ). 
     On the left and right sides of the elevation table  155 , the pair of left and right belt conveyers  160   a ,  160   b  are disposed to extend in parallel to each other and horizontally by using a plurality of supporting frame members  156 . The elevation belt conveyers  160   a ,  160   b  have the same structures as the upper belt conveyers  110   a ,  110   b  of the aforementioned upper conveyer mechanism  11  and thus include driving pulleys (not shown), driven pulleys (not shown), and endless conveyer belts  165   a ,  165   b  that are wound around the driving pulleys and the driven pulleys to extend therebetween. The left and right driving pulleys are connected to each other via a driving shaft  166  so that the conveyer belts  165   a ,  165   b  are rotated and moved in synchronism with each other via the driving chain  168  by the rotation of the driving motor (not shown) so as to move the pallet  6  on the conveyer belts  165   a ,  165   b  forward or rearward. 
     The elevation belt conveyers  160   a ,  160   b  move up and down together with the elevation table  155  by the expansion and contraction of the elevation cylinder  153  (the output rod  153   a ) and can abut the upper belt conveyers  110   a ,  110   b  of the upper conveyer mechanism  11  or the lower belt conveyers  120   a ,  120   a  of the lower conveyer mechanism  12  composing the printing station unit  2   e . As a result, the delivery of the pallet  6  is allowed between the elevation belt conveyers  160   a ,  160   b  and the upper belt conveyers  110   a ,  110   b  or the lower belt conveyers  120   a ,  120   b . On the left and right sides of the elevation belt conveyers  160   a ,  160   b , long guide frame members  158 ,  158  extending along the elevation belt conveyers  160   a ,  160   b  using a plurality of supporting frame members  157  are arranged. On the outer surfaces of the guide frame members  158 ,  158 , guide walls  158   a ,  158   a  are formed to extend upward along the outer surfaces of the guide frame members  158 ,  158 , respectively, thereby preventing the pallet  6  on the belt conveyers  160   a ,  160   b  from deviating to the left or the right. 
     Hereinafter, the detailed structure of the printer unit  20  of the printing station unit  2   e  will be described with regard to  FIG. 2 , and  FIG. 8  through  FIG. 11 . As shown in  FIG. 2 , the printer unit  20  (the printing unit  3   e ) includes anteroposterior moving mechanisms  220 ,  230  which are arranged on left and right sides (at the positions corresponding to the first holding mechanism  140  (the first holding position) of the aforementioned upper conveyer mechanism  11 ) of the of the conveyer frame  130  of the conveyer unit  10 , and a printer mechanism  210  which is detachably mounted to the anteroposterior moving mechanism  220 ,  230 . 
     As shown  FIG. 8 , the anteroposterior moving mechanisms  220 ,  230  each include a driving sprocket  221 ,  231 , a first driven sprocket  222 ,  232 , a toothed belt  223 ,  233  that is wound around the driving sprocket  221 ,  231  and the first driven sprocket  222 ,  232  to extend therebetween, a second driven sprocket  224 ,  234 , and a guide roller  225 ,  235  so that the toothed belt  223 ,  233  is arranged in an L-like shape of which top is the second sprocket  224 ,  234 . It should be noted that each driving sprocket  221 ,  231  and each second driven sprocket  224 ,  234  (each guide roller  225 ,  235 ) are rotatably supported to be spaced apart from each other in the vertical direction on the left or right side of the conveyer frame  130 , and each first driven sprocket  222 ,  232  and each second driven sprocket  224 ,  234  (each guide roller  225 ,  235 ) are rotatably supported to be spaced apart from each other in the anteroposterior direction on the left or right side of each upper frame member  133 ,  133 . The toothed belts  223 ,  233  are connected via fitting members  240 ,  245  at their ends, respectively so that each toothed belt  223 ,  233  is formed into a ring shape. The fitting members  240 ,  245  are located between the first driven sprockets  222 ,  232  and the second driven sprocket  224 ,  234 , respectively. 
     As shown in  FIG. 9 , the left and right driving sprockets  221 ,  231  are connected to each other via a driving shaft  229 . Attached to a middle portion of the driving shaft  229  is a driving gear  226 . A train of driving gears  228   a ,  228   b  is disposed to be meshed with the driving gear  226 . The train of driving gears  228   a ,  228   b  is driven to be rotated by the driving motor  227  so as to rotate the driving gear  226 , thereby rotating the left and right driving sprockets  221 ,  231  in synchronism with each other via the driving shaft  229 . That is, the left and right toothed belts  223 ,  233  are rotated or moved in synchronism with each other by the driving motor  227  so that the left and right fitting members  240 ,  245  are moved in synchronism with each other in the anteroposterior direction along the upper frame members  133 ,  133  (the upper conveyer mechanism  11 ). The driving shaft  229 , the driving motor  227 , and the like are disposed below the lower frame members  134 ,  134  (the lower conveyer mechanism  12 ) so as not to interrupt the passage of the pallet  6 . 
     The fitting member  240  for connecting the toothed belt  223  in the left-side anteroposterior moving mechanism  220  is shown in  FIG. 10 . The fitting member  240  includes a body member  241  having a U-like section and two pairs of guide rollers  242   a ,  242   b  attached to the side surface of the body member  241 , in which the guide rollers  242   a  and  242   b  in each pair are aligned to be spaced apart from each other by a predetermined distance in the vertical direction and the pairs are mounted to be spaced apart from each other in the anteroposterior direction as shown in  FIG. 8 . On the other hand, a guide rail  137  extending horizontally in the anteroposterior direction is attached to the outer surface of the left-side upper frame  133  of the conveyer unit  10 . The two pairs of guide rollers  242   a ,  242   b  are fitted into the guide grooves  137   a ,  137   b  which are formed in the upper and lower surfaces of the guide rail  137  so that the fitting member  240  is guided by the guide rail  137  to move in the anteroposterior direction. The fitting member  240  is provided with an engaging hole  241   a  formed in and penetrating the upper surface of the body member  241 . 
     The fitting member  245  for connecting the toothed belt  233  in the right-side anteroposterior moving mechanism  230  is shown in  FIG. 11 . The fitting member  245  includes a body member  246  having a U-like section with a longer lower side and two pairs of guide rollers  247   a ,  247   b  attached to the side surface of the body member  246 , in which the guide rollers  247   a  and  247   b  in each pair are aligned to be spaced apart from each other by a predetermined distance in the vertical direction and the pairs are mounted to be spaced apart from each other in the anteroposterior direction. It should be understood that the guide rollers  247   a ,  247   b  are arranged in a bilaterally-symmetrical pattern with the guide rollers  242   a ,  242   b  of the fitting member  240  of the left-side anteroposterior moving mechanism  220 . On the other hand, a guide rail  138  extending horizontally in the anteroposterior direction is attached to the outer surface of the right-side upper frame  133  of the conveyer unit  10 . The two pairs of guide rollers  247   a ,  247   b  are fitted into the guide grooves  138   a ,  138   b  that are formed in the upper and lower surfaces of the guide rail  138  so that the fitting member  245  is guided by the guide rail  138  to move in the anteroposterior direction. The fitting member  245  is provided with an engaging hole  246   a  formed in and penetrating the upper surface of the body member  246 . 
     Two pairs of guide rollers  248   a ,  248   b  attached to the lower surface of the outwardly extending lower side of the body member  246 , in which the guide rollers  248   a  and  248   b  in each pair are aligned to be spaced apart from each other by a predetermined distance in the lateral direction and the pairs are mounted to be spaced apart from each other in the anteroposterior direction. On the right side of the upper frame  133  of the conveyer unit  10 , a guide surface  133   b  facing upward and extending in the anteroposterior direction is formed and a guide rail  139  extending in the anteroposterior direction is attached to the guide surface  133   b . The two pairs of guide rollers  248   a ,  248   b  are fitted into the guide grooves  139   a ,  139   b  that are formed in the left and right surfaces of the guide rail  139  so that the fitting member  245  is guided by the guide rail  139  to move in the anteroposterior direction. That is, the fitting member  245  can be guided by the guide rail  138  and the guide rail  139  both in the vertical direction and the lateral direction to move in the anteroposterior direction. 
     As is apparent from the description of the structure, the left and right toothed belt  223 ,  233  are rotated and moved by driving the driving motor  227  so that the left and right fitting members  240 ,  245  can be moved in synchronism with each other in the anteroposterior direction. During this, the left side fitting member  240  is guided in the vertical direction accurately by the guide rail  137  and the right side fitting member  245  is guided in the vertical direction and the lateral direction accurately by the guide rails  138 ,  139  to move in the anteroposterior direction. The printer mechanism  210  is detachably attached to the left and right fitting members  240 ,  245  that are moved in synchronism with each other in the anteroposterior direction so that the printer mechanism  210  is entirely moved in the anteroposterior direction. 
     As shown in  FIG. 2 , the printer mechanism  210  includes a long guide bar member  211  that extends in the lateral direction above the upper conveyer mechanism  11  (the upper belt conveyer  110   a ,  110   b ) and is mounted on the anteroposterior moving mechanisms  220 ,  230 , a printer head  212  which is disposed movably along the guide bar member  211 , and an ink supply device  213  and a maintenance station  214  which are mounted on the right and left ends of the guide bar member  211 . 
     On the lower surface of the guide bar member  211 , a pair of engaging projections  211   a ,  211   b  (see  FIGS. 10 ,  11 ) are formed which are spaced apart from each other in the lateral direction (corresponding to a distance between the left and right fitting members  240 ,  245  of the anteroposterior moving mechanisms  220 ,  230 ) and project downward. The engaging projections  211   a ,  211   b  are fitted into the aforementioned engaging holes  241   a ,  246   a  formed in the upper surfaces of the left and right fitting members  240 ,  245 , whereby the guide bar member  211  is detachably mounted. As a result, as the left and right fitting members  240 ,  245  are moved in synchronism with each other in the anteroposterior direction, the guide bar member  211  is also moved in the anteroposterior direction together. 
     The guide bar member  211  is provided with a guide rail (not shown) extending in the lateral direction. A printer head  212  is disposed on the guide bar member  211  such that the printer head  212  can be guided to move in the lateral direction. The guide bar member  211  is also provided with a driving belt (not shown). By controlling the operation of the driving belt, the printer head  212  is controlled to move in the lateral direction along the guide bar member  211 . The printer head  212  has a large number of inkjet nozzles (not shown) formed in its lower surface to face downward. By ejection of ink droplets from the inkjet nozzles, printing of intended characters and/or images on print substrates  5  below the inkjet nozzles is conducted. 
     The ink supply device  213  is mounted on the right end of the guide bar member  211 . The maintenance station  214  is mounted on the left end of the guide bar member  211 . The maintenance station  214  has a device which retracts the printer head  212  into the maintenance station  214 , and aspirates and cleans the remaining ink in the inkjet nozzles. In addition to the ink supply device  213 , disposed on the right end portion of the guide bar member  211  is a controller (not shown) for controlling the movement of the printer head  212  and controlling the ejection of ink droplets from the inkjet nozzles formed in the lower surface of the printer head  212 . 
     A cable guide  215  connecting the guide bar member  211  and the printer head  212  is provided. Wires for sending electric power and signals and flexible tubes for supplying ink are arranged within the cable guide  215 . According to this structure, electric power, control signals, and ink can be supplied from the side of the guide bar member  211  (from the ink supply device  213  and the like) to the printer head  212 . 
     Below the conveyer unit  10 , a power source and a control equipment (not shown) are disposed and flexible cable guide (not shown) is disposed. In the flexible cable guide, cables for sending electric power and control signals from the power source and the control equipment to control the upper and lower conveyer mechanisms  11 ,  12  (the driving motors  119 ,  129 ) and the first and second holding mechanism  140 ,  145  (the first and second controllers  143 ,  148 ) of the conveyer unit  10 , the elevation cylinder  153  of the elevation unit  13 , the anteroposterior moving mechanisms  220 ,  230  (the driving motor  227 ), and the printer mechanism  210  (the ink supply device  213 , the controller, and the maintenance station  214 ) of the printer unit  20  are arranged. 
     A series of actions of the printing apparatus  1 ′ having the aforementioned structure will be described. First, by an operator, print substrates  5  are put on the respective loading tables  8  of the first pallet  6  on the elevation belt conveyers  160   a ,  160   b  of the elevation unit  13  in the loading station unit  2   a . The first pallet  6  holding the print substrates  5  thereon is moved to the upper belt conveyers  110   a ,  110   b  of the upper conveyer mechanism  11  that abut the elevation belt conveyers  160   a ,  160   b , that is, the first pallet  6  is conveyed to the printing station unit  2   e.    
     In the printing station unit  2   e , first the locating pins  141   a  of the first sensors  141  composing the first holding mechanism  140  disposed on the upper conveyer mechanism  11  are in the expanded state and the locating pins  146   a  of the second sensors  146  composing the second holding mechanism  145  disposed on the upper conveyer mechanism  11  are in the contracted state. The first pallet  6  conveyed to the printing station unit  2   e  is moved backward on the upper belt conveyers  110   a ,  110   b  so that the cutouts  6   a  formed in the rear side of the first pallet  6  come in contact with the locating pins  141   a  of the first sensors  141  (the first pallet  6  is located at its working position). As the cutouts  6   a  of the first pallet  6  come in contact with the locating pins  141   a , the holding pins  142   a  of the first air cylinders  142  are expanded according to a control signal from the first controller  143  so as to lift up the first pallet  6 , thereby holding the pallet  6  spaced apart from the conveyer belts  115   a ,  115   b  of the upper belt conveyers  110   a ,  110   b . As the first pallet  6  is held in the state spaced apart from the conveyer belts  115   a ,  115   b , the locating pins  141   a  of the first air cylinders  141  are contracted and the locating pins  146   a  of the second sensors  146  are expanded. 
     As the first pallet  6  is held at the first holding position (the working position) by the first holding mechanism  140 , the printed surfaces of the print substrates  5  on the pallet  6  and the printer head  212  (the inkjet nozzles) of the printer mechanism  210  are held in a state facing each other with a certain distance (about 2 mm) therebetween. In this state, ink droplets are ejected from the inkjet nozzles while the printer mechanism  210  is moved in the anteroposterior direction above the print substrates  5  on the first pallet  6  by the anteroposterior moving mechanisms  220 ,  230 , and the printer head  212  is moved in the lateral direction along the guide bar member  211 , thereby conducting printing of intended characters, images, and the like on the respective print substrates  5 . In a stable state relative to the print substrates  5  without being affected by vibration of the upper belt conveyers  110   a ,  110   b , higher precise printing can be achieved by the printer mechanism  210 . By the drive control of the anteroposterior moving mechanisms  220 ,  230 , the movement control of the printer head  212 , and the ejection control of ink from the inkjet nozzles, different characters, images and the like can be printed on the respective print substrates  5  aligned in the lateral direction and held on the pallet  6 , respectively. 
     On the other hand, when printing step is conducted by the printer unit  20  in a state that the first pallet  6  is held at the first holding position, new print substrates  5  are put on the respective loading tables  8  of the second pallet  6  on the elevation belt conveyers  160   a ,  160   b  of the elevation unit  13  by the operator in the loading station unit  2   a . The second pallet  6  holding the print substrates  5  thereon is moved to the upper belt conveyers  110   a ,  110   b  of the upper conveyer mechanism  11  which about the elevation belt conveyers  160   a ,  160   b , that is, the second pallet  6  is conveyed to the printing station unit  2   e.    
     The second pallet  6  conveyed to the printing station unit  2   e  is moved backward on the upper belt conveyers  110   a ,  110   b  so that the cutouts  6   a  formed in the rear side of the second pallet  6  come in contact with the locating pins  146   a  of the second sensors  146  (the second pallet  6  is located at its standby position). As the cutouts  6   a  of the second pallet  6  come in contact with the locating pins  146   a , the holding pins  147   a  of the second air cylinders  147  are expanded according to a control signal from the second controller  148  so as to lift up the second pallet  6 , thereby holding the second pallet  6  spaced apart from the conveyer belts  115   a ,  115   b  of the upper belt conveyers  110   a ,  110   b . As the second pallet  6  is held in the state spaced apart from the conveyer belts  115   a ,  115   b , the locating pins  146   a  of the second sensors  146  are contracted (the locating pins  141   a  of the first sensors  141  are kept in the contracted state). 
     As the second pallet  6  is held at the second holding position (the standby position) by the second holding mechanism  145  and then the printing step by the printer unit  20  relative to the first pallet  6  held at the first holding position is completed, the holding pins  142   a  of the first air cylinders  142  are contracted according to the control signal of the first controller  143  so that the first pallet  6  is returned onto the conveyer belts  115   a ,  115   b . The first pallet  6  returned onto the conveyer belts  115   a ,  115   b  is moved to be on the elevation belt conveyers  160   a ,  160   b  of the collecting station unit  2   h  which abut to the upper belt conveyers  110   a ,  110   b  so that the first pallet  6  is conveyed to the collecting station unit  2   h . As the first pallet  6  is conveyed to the collecting station unit  2   h , the locating pins  141   a  of the first sensors  141  are expanded (the locating pins  146   a  of the second sensors  146  are kept in the contracted state). 
     As the first pallet  6  is conveyed to the collecting station unit  2   h , the respective print substrates  5  after completing the printing step on the first pallet  6  are collected by the operator. The first pallet  6  after the print substrates  5  are collected is moved to the lower belt conveyers  120   a ,  120   b  of the lower conveyer mechanism  12  which abut the elevation belt conveyers  160   a ,  160   b  by the operation of the elevation unit  13  (the elevation cylinder  153 ) so that the first pallet  6  is conveyed to the printing station unit  2   e  again. The first pallet  6  conveyed to the printing station unit  2   e  is conveyed on the lower belt conveyers  120   a ,  120   b  toward the loading station unit  2   a . In the loading station unit  2   a , new print substrates  5  are put on the first pallet  6  and are then subjected to the same step as mentioned above. 
     On the other hand, as the first pallet  6  is conveyed to the collecting station unit  2   h , the holding pins  147   a  of the second air cylinders  147  are contracted according to a control signal of the second controller  148  so that the second pallet  6  is returned onto the conveyer belts  115   a ,  115   b . The second pallet  6  returned onto the conveyer belts  115   a ,  115   b  are moved backward on the upper belt conveyer  110   a ,  110   b  so that the cutouts  6   a  formed in the rear side of the second pallet  6  come in contact with the locating pins  141   a  of the first sensors  141  (the second pallet  6  is located at its working position). As the cutouts  6   a  of the second pallet  6  come in contact with the locating pins  141   a , the holding pins  142   a  of the first air cylinders  142  are expanded according to a control signal from the first controller  143  so as to lift up the second pallet  6 , thereby holding the second pallet  6  spaced apart from the conveyer belts  115   a ,  115   b  of the upper belt conveyers  110   a ,  110   b . After that, similarly to the aforementioned first pallet  6 , the second pallet  6  is subjected to the printing step by the printer unit  20 , the conveyance to the collecting station unit  2   h , the collection of the print substrates, and the conveyance to the loading station unit  2   a  in this order. Even when processing operations are conducted concurrently by a plurality of working devices like the aforementioned case, it is not necessary to keep the next pallet waiting until all of the processing operations are terminated because it is possible to make the second pallet  6  wait by the second holding mechanism  145 , thereby preventing reduction in the operation rates of the working devices. 
     As mentioned above, though the detailed structure and the actions of the conveyer unit  10  and the elevation unit  13  have been described with regard to the printing apparatus  1 ′ in which the loading station unit  2   a , the printing station unit  2   e , and the collecting station unit  2   h  are aligned to abut each other as shown in  FIG. 2 , work station units such as the preprocessing station unit  2   b , the first drying station unit  2   c , are further aligned to abut in addition to the above three work station units in the printing apparatus of this embodiment as shown in  FIG. 1 . Therefore, in the printing apparatus  1 , besides the actions mentioned above, an action for delivering the pallet from a conveyer unit  10  to a conveyer unit  10  of another work station unit is generated. 
     Hereinafter, for example, the action for delivering the pallet  6  from the conveyer unit  10  (the upper conveyer mechanism  11 ) of the printing station unit  2   e  to the conveyer unit  10  (the upper conveyer mechanism  11 ) of the post-processing station unit  2   f  will be described. In the printing station unit  2   e , as the printing step relative to the first pallet  6  held at the first holding position is completed by the printer unit  20  as mentioned above, the first pallet  6  is returned onto the conveyer belts  115   a ,  115   b  by the first holding mechanism  140 . The first pallet  6  returned onto the conveyer belts  115   a ,  115   b  is moved to the upper belt conveyers  110   a ,  110   b  of the post-processing station unit  2   f  which abut the upper belt conveyers  110   a ,  110   b  of the printing station unit  2   e  so that the first pallet  6  is conveyed to the post-processing station unit  2   f . As the first pallet  6  is conveyed to the post-processing station unit  2   f , the locating pins  141   a  of the first sensors  141  of the printing station unit  2   e  are in the expanded state and the locating pins  146   a  of the second sensors  146  are in the contracted state. 
     In the post-processing station unit  2   f , first the locating pins  141   a  of the first sensors  141  composing the first holding mechanisms  140  disposed on the upper conveyer mechanism  11  are in the expanded state and the locating pins  146   a  of the second sensors  146  composing the second holding mechanism  145  disposed on the upper conveyer mechanism  11  are in the contracted state. The first pallet  6  conveyed to the post-processing station unit  2   f  is moved backward on the upper belt conveyers  110   a ,  110   b  and is held at the first holding position (the working position of the post-processing unit  3   f ) spaced apart from the conveyer belts  115   a ,  115   b  by the first holding mechanism  140 . The first pallet  6  is held at the first holding position by the first holding mechanism  140 , the post-processing unit  3   f  conducts the post-processing step by applying a protective coating material to the surfaces of the print substrates  5 , on which characters, images, and the like were printed by the printer unit  20 , to protect and coat the surfaces. As the first pallet  6  is held at a position spaced apart from the conveyer belts  115   a ,  115   b , the locating pins  141   a  of the first sensors  141  of the post-processing station unit  2   f  are in the contracted state and the locating pins  146   a  of the second sensors  146  of the post-processing station unit  2   f  are in the expanded state. 
     On the other hand, as the first pallet  6  is conveyed to the post-processing station unit  2   f , the second holding mechanism  145  returns the second pallet  6  onto the conveyer belts  115   a ,  115   b  in the printing station unit  2   e . The second pallet  6  returned onto the conveyer belts  115   a ,  115   b  is moved backward on the upper belt conveyers  110   a ,  110   b  and is held at the first holding position (the working position of the printer unit  20 ) spaced apart from the conveyer belts  115   a ,  115   b  by the first holding mechanism  140 . As the second pallet  6  is held at the first holding position by the first holding mechanism  140 , the printer unit  20  prints intended characters, images, and the like on the respective print subjects  5  held on the second pallet  6 . As the second pallet  6  is held at a position spaced apart from the conveyer belts  115   a ,  115   b , the locating pins  141   a  of the first sensors  141  of the printing station unit  2   e  are in the contracted state and the locating pins  146   a  of the second sensors  146  of the printing station unit  2   e  are in the expanded state. 
     As the printing step by the printer unit  20  relative to the second pallet  6  held at the first holding position is terminated, the first holding mechanism  140  returns the second pallet  6  onto the conveyer belts  115   a ,  115   b . The second pallet  6  returned onto the conveyer belts  115   a ,  115   b  is moved to the upper belt conveyers  110   a ,  110   b  of the post-processing station unit  2   f , which abut the upper belt conveyers  110   a ,  110   b  of the printing station unit  2   e . The second pallet  6  conveyed to the post-processing station unit  2   f  is moved backward on the upper belt conveyers  110   a ,  110   b  and is held at a second holding position (the standby position of the post-processing station unit  2   f ) spaced apart from the conveyer belts  115   a ,  115   b  of the upper belt conveyer  110   a ,  110   b  by the second holding mechanism  145 . In case that the first pallet  6  after the post-processing is already conveyed to the following second drying station unit  2   g  when the second pallet  6  is conveyed to the post-processing station unit  2   f , the second pallet  6  is held directly at the first holding position (the working position of the post-processing unit  3   f ) by the first holding mechanism. 
     After the post-processing by the post-processing unit  3   f  relative to the first pallet  6  held at the first holding position is terminated in the post-processing station unit  2   f , the first pallet  6  is returned onto the conveyer belts  115   a ,  115   b  by the first holding mechanism  140 . The first pallet  6  returned onto the conveyer belts  115   a ,  115   b  is moved to the upper belt conveyers  110   a ,  110   b  of the second drying station unit  2   g , which abut the upper belt conveyers  110   a ,  110   b  of the post-processing station unit  2   f  so that the first pallet  6  is conveyed to the second drying station unit  2   g . After that, similarly to the above, the first pallet  6  is subjected to the drying step by the drying unit  3   g , the conveyance to the collecting station unit  2   h , the collection of the print substrates  5 , and the conveyance to the loading station unit  2   a  in this order. 
     In the post-processing station unit  2   f , the second pallet  6  is returned to the conveyer belts  115   a ,  115   b  by the second holding mechanism, is moved backward on the upper belt conveyers  110   a ,  110   b , is held at the first holding position (the working position of the post-processing unit  3   f ) spaced apart from the conveyer belts  115   a ,  115   b  by the first holding mechanism  140 , and is then subjected to the post-processing by applying a protective coating material to the surfaces of the print substrates  5 , on which characters, images, and the like were printed by the printer unit  20 , to protect and coat the surfaces. After that, similarly to the aforementioned first pallet  6 , the second pallet  6  is subjected to the conveyance to the drying station unit  2   g , the drying step by the drying unit  3   g , the conveyance to the collecting station unit  2   h , the collection of the print substrates  5 , and the conveyance to the loading station unit  2   a  in this order. 
     Though the above description has been made with regard to the first pallet  6  and the second pallet  6 , a larger number of pallets  6  such as the third pallet  6 , the forth pallet  6 , etc. can be conveyed sequentially and circulated by the respective station units  2   a  through  2   g  and subjected to respective processing operations by the work units  3   b  through  3   f  in the printing apparatus  1  according to this embodiment. 
     As mentioned in the above, the printing apparatus  1  includes the respective station units  2   a  through  2   h , corresponding to the respective processing operations to be conducted to print substrates  5 , which are aligned adjacent to each other. The station units  2   b  through  2   g  have the conveyer units  10  for conveying the pallet  6  holding the print substrates  5  thereon, respectively. The upper conveyer mechanisms  11  and the lower conveyer mechanisms  12  of the respective conveyer units  10  are placed in a uniform way such that ends as the pallet receiving portions and the pallet let-off portions of the respective upper conveyer mechanisms  11  are surely abutted to each other and also ends as the pallet receiving portions and the pallet let-off portions of the respective lower conveyer mechanisms  12  are surely abutted to each other. Accordingly, the delivery of the pallet  6  between the station units is allowed, whereby intended characters, images, etc. can be printed by the respective work units  3   b  through  3   g  sequentially conducting the processing steps relative to the print substrates  5 . As mentioned above, the printing procedure requiring a lot of processing operations relative to the print substrates  5  can be efficiently conducted, thereby improving the operating efficiency. 
     In the printing apparatus  1 , the loading station unit  2   a  and the collecting station unit  2   h  are arranged on both ends. The loading station unit  2   a  and the collecting station unit  2   h  each include an elevation unit  13  for conveying the pallet  6  between the upper conveyer mechanism  11  and the lower conveyer mechanism  12 . The elevation unit  13  moves up and down to abut itself to the ends as the pallet receiving portion and the pallet let-off portion of the upper conveyer mechanism  11  and the lower conveyer mechanism  12  so as to allow the delivery of the pallet  6  between the upper conveyer mechanism  11  and the lower conveyer mechanism  12 . Therefore, the pallet  6  can be conveyed and circulated through the respective station units  2   a  through  2   h  by the elevation units  13  and the respective conveyer units  10  (the upper conveyer mechanism  11  and the lower conveyer mechanism  12 ), thereby further improving the efficiency of print work. 
     In the printing apparatus  1 , each of the station units  2   a  through  2   h  includes each elevation unit  13  or each work unit (the preprocessing unit  3   b , the drying units  3   c  and  3   g , the undercoating unit  3   d , the printing unit  3   e , the post-processing unit  3   f ) and each conveyer unit  10  (the upper conveyer mechanism  11  and the lower conveyer mechanism  12 ) that are mounted to the frame body (the conveyer frame  130 ) of each station unit and thus can be replaced or rearranged integrally with each frame body, thereby allowing the processing steps for printing operation to be changed and set. Therefore, it is possible to easily change the printing mode according to the kind of the print substrates  5  or change of the production line, thereby making the printing apparatus suitable for high-mix low-volume production type printing. The printing apparatus can be designed and manufactured separately for each step, thereby allowing structure simplification and weight reduction, and thus reducing the installation area. 
     Though the present invention has been described with regard to the preferred embodiments, the printing apparatus of the present invention is not limited to the structures of the aforementioned embodiments and printing apparatuses with various modifications and changes from the aforementioned embodiment are also included in the present invention. It is therefore to be understood that, within the scope of the appended claims, the invention may be practiced otherwise than as specifically described herein.