Patent Publication Number: US-7712989-B2

Title: Ink jet recording apparatus capable of performing a duplex print operation

Description:
CROSS-REFERENCES TO RELATED APPLICATIONS 
   This application claims priority from, and is a continuation of U.S. patent application Ser. No. 10/218,472, filed Aug. 14, 2002, which was a continuation of application Ser. No. 09/330,669, filed Jun. 11, 1999, each of which is incorporated by reference herein. 

   BACKGROUND OF THE INVENTION 
   This invention relates to an ink jet recording apparatus, and more particularly to an ink jet recording apparatus which is capable of performing a duplex print operation such that printing is performed on both sides of a sheet of paper. 
   An ink jet recording apparatus has been used widely as an image forming mechanism in printers, copying machines, and so on. Some ink jet recording apparatuses are capable of operating in a duplex print mode in which a print operation can be performed on both sides of the recording sheet. One example of such a printer is described in Japanese Laid-Open Patent Publication No. JPAP08-337011 (1996). The ink jet recording apparatus according to this example is adapted to perform a duplex print operation by using two recording mechanisms which are positioned in the ink jet recording apparatus so as to be spaced apart from each other. Accordingly, the structure of the ink jet recording apparatus becomes complex and the apparatus itself becomes relatively large and expensive because of the use of two recording mechanisms, as well as, a plurality of paper transportation mechanisms required therein. As a result, the cost of manufacturing such a duplex mode printer is greatly increased. 
   In addition, such an apparatus may have a disadvantage with respect to the quality of a print image. This is because in the above-mentioned ink jet recording apparatus, the sheet is transported to the second recording mechanism while the printed surface of the sheet contacts or is rubbed against a surface of a guide plate connecting the first recording mechanism to the second recording mechanism. Yet, with ink jet recording, time is required to dry the printed surface and, therefore, the printed surface should be protected from contacting any material or any object that might smudge or affect the quality of the printed image on the sheet. 
   SUMMARY OF THE INVENTION 
   In order to overcome the problems described above, preferred embodiments of the present invention provide an ink jet recording apparatus which has a very small size and is constructed to print high quality printed sheets in a duplex print mode such that both sides of a sheet are printed. 
   Further, preferred embodiments of the present invention provide a method and apparatus for ink jet printing in which two sides of a sheet can be printed by ink jet printing but using only a single ink jet print head, thereby eliminating the need for two ink jet print head or printing units as is required in conventional devices. That is, preferred embodiments of the present invention provide a method and apparatus for ink jet printing in which two sides of a sheet can be printed by the same ink jet print head. 
   In addition, preferred embodiments of the present invention provide a method of ink jet printing on both sides of a sheet such that a sheet that has been printed on one side is fed to a location outside of the printer and then is fed back into the printer to have the second side of the sheet printed. 
   In addition, preferred embodiments of the present invention provide a method and apparatus for ink jet printing in which a pair of exit rollers are capable of being driven in a reverse direction so as to feed a sheet that has been printed on one side thereof back into the printer so that the second side of the sheet can be printed. 
   Additional preferred embodiments provide an ink jet printing apparatus in a sheet diverting mechanism diverts the feed of a sheet that has been printed on one side thereof so that the sheet is printed on the second side thereof. 
   According to one preferred embodiment of the present invention, an ink jet recording apparatus performs a print operation by controlling an ink jet recording head so as to eject ink drops there from onto front and back surfaces of a recording sheet. The ink jet recording apparatus transports a portion of the recording sheet to a location outside of the apparatus after a completion of the print operation on the front surface of the recording sheet and before the print operation on the rear surface of the recording sheet. 
   The portion of the recording sheet transported to the location outside of the apparatus may include a surface that has the ink drops disposed thereon. 
   The recording sheet which has been printed on the front surface thereof may be reversely transported to the print position again using a switchback mechanism. 
   The location outside of the apparatus may be positioned below the print position or may be an upper surface of an input sheet cassette that contains a plurality of recording sheets. 
   The recording sheet may be transported to the print position by a transport mechanism that includes a transport belt. 
   The recording sheet may be moved at a speed which is substantially the same as a moving speed of the transport belt when the recording sheet is reversely transported to the print position again. 
   The apparatus may have a single print mechanism that includes the ink jet recording head. 
   Drying elements such as a fan, heater or timing controlled sheet feeding mechanism may be provided in the printer for ensuring that the one-side-printed sheet is dried before the sheet is fed back into the printer for printing of the second side. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     A more complete appreciation of the present invention and many of the attendant advantages thereof will be readily obtained as the same becomes better understood by reference to the following detailed description when considered in connection with the accompanying drawings, wherein. 
       FIG. 1  is an illustration of a perspective front view of an ink jet printer according to a preferred embodiment of the present invention; 
       FIG. 2  is an illustration of a perspective rear view of the ink jet printer of  FIG. 1 ; 
       FIG. 3  is an illustration for explaining a mechanical structure of the ink jet printer of  FIG. 1 ; 
       FIG. 4  is an illustration for explaining a moving component such as a manual input tray, for example; 
       FIG. 5  is a block diagram of a control unit of the ink jet printer of  FIG. 1 ; 
       FIG. 6  is a flowchart for explaining an exemplary procedure of a duplex print operation of the ink jet printer of  FIG. 1 ; 
       FIGS. 7-9  are illustrations for explaining different methods of switchback transportation of a recording sheet to achieve the duplex print operation; 
       FIG. 10  is another preferred embodiment of the present invention showing an alternative arrangement for feeding the one-side-printed sheet to a location outside of the apparatus; 
       FIG. 11  is another preferred embodiment of the present invention showing an alternative arrangement for feeding the one-side-printed sheet to a location outside of the apparatus including a sensor for sensing the sheet and a separation pawl A in a first position; 
       FIG. 12  is another preferred embodiment of the present invention showing an alternative arrangement for feeding the one-side-printed sheet to a location outside of the apparatus including a sensor for sensing the sheet and a separation pawl A in a second position; 
       FIG. 13  is a further preferred embodiment of the present invention including a fan for expediting drying of the ink image on the one-side-printed sheet; 
       FIG. 14  is a further preferred embodiment of the present invention including an alternative arrangement of a fan for expediting drying of the ink image on the one-side-printed sheet; and 
       FIG. 15  is an additional preferred embodiment of the present invention including a heater disposed in a separate heating unit outside of the apparatus for expediting drying of the ink image on the one-side-printed sheet. 
   

   DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
   Referring now to the drawings, wherein like reference numerals designate identical or corresponding parts throughout the several views, and more particularly to  FIG. 1  thereof, front and rear perspective views of an ink jet printer  1  according to a preferred embodiment of the present invention are illustrated, respectively. The ink jet printer  1  shown in  FIG. 1  includes an ink jet print unit  2 . As will be described in more detail below, only a single ink jet print unit is required to achieve two-sided printing as opposed to the conventional device in which two such ink jet print units are required to achieve two-sided printing. Thus, the printer  1  preferably only has a single ink jet print unit  2 . 
   The printer  1  also includes an input sheet cassette  4 , a manual input tray  5 , an output sheet tray  6  (see  FIG. 2 ), and an upper cover  7 . The print unit  2  is installed under the upper cover  7  inside of the ink jet printer  1  such that an operator can access the print unit  2  when the upper cover  7  is opened (see chain lines A in  FIG. 4 ). The print unit  2  includes various elements ( FIG. 3 ), such as a carriage  13  which is movable in the main scanning direction of the print unit  2 , an ink jet recording head  14  which is mounted on the carriage, an ink cartridge  15  arranged to supply ink to the ink jet recording head. 
   The input sheet cassette  4  shown in  FIG. 1 , which can alternatively be a tray type, can hold a plurality of sheets  3 . The input sheet cassette  4  is installed inside the lower part of the ink jet printer  1  from the front side of the ink jet printer  1  when the input sheet cassette  4  has the sheets  3  therein, and is movable outwardly when no sheet is contained therein so that new sheets can be inserted therein. The manual input tray  5  is rotatably mounted in the front of the ink jet printer  1 , above the input sheet cassette  4 . The manual input tray  5  allows an occasional use of different sheets when it is set in an open position. The output sheet tray  6  ( FIG. 2 ) is mounted on the rear side of the ink jet printer  1 , and receives the sheets  3  which are ejected after a printing operation performed by the printer unit  2  is finished. 
   In this ink jet printer  1 , the sheet  3  which is selected either from the input sheet cassette  4  or the manual input tray  5  is transported to the print unit  2  which then records an image on one side of the sheet  3 . Then, the one-side-printed sheet is ejected to the output sheet tray  6 . This is similar to a normal printing operation in a conventional printer that prints on only side of a sheet. 
   As illustrated in  FIG. 3 , the carriage  13  is slidably supported by a main guide rod  11  in the front and a sub-guide rod  12  at the rear thereof, wherein both the main guide rod  11  and the sub-guide rod  12  bridge left and right side plates (not shown) of the ink jet printer  1 . The carriage  13  can thus slide in the main scanning direction (vertical direction relative to the drawing of  FIG. 3 ). The ink jet recording head  14  may be a single color print head (black) or may preferably have nozzles (not shown) for ejecting yellow (Y), cyan (C), magenta (M), and black (B) ink drops. The head  14  is mounted on the bottom surface of the carriage  13  such that the nozzles can eject ink drops in a downward direction. The carriage  13  is provided on the top surface thereof with disposable ink tanks  15  (i.e., ink cartridges) for supplying the color ink to the respective nozzles. The ink tanks  15  are secured to the carriage  13  by a support lever  16 . To release the ink tanks  15 , the support lever  16  is unhooked and rotated (see chain lines B of  FIG. 4 ). 
   Alternatively, the ink jet recording head  14  may be replaced by a plurality of recording heads, aligned in the main scanning direction, each for ejecting each color ink or a recording head having a single nozzle for ejecting a plurality of different color inks. Of course, a single color (black) ink jet head may also be provided alone or in combination with other print heads. 
   Under the print unit  2 , a main transport roller  21  and a sub-transport roller  22  are provided such that a transport belt  23  movably stretched there between can transport the sheet  3  to a print position located immediately under the nozzles of the ink jet recording head  14 . Electrostatic force is used for the transportation of the sheet  3  by the transport belt  23 . A platen plate  24  is provided at a position opposed to the ink jet recording head  14  and positioned relative to the transport belt  23 . Preferably, the main transport roller  21  has a sufficiently large diameter (i.e., about 30 mm or more) to generate enough electrostatic force so as to prevent the sheet  3  from separating from the transport belt  23  during the turning period around the main transport roller  21  of the transportation movement. The transport belt  23  preferably is made of medium resistance substance having a volume resistance range of 10 9  Ωcm to 10 12  Ωcm. In addition, a transportation direction regulating roller  25  is mounted at a location before the print position on the transport belt  23 , pressing the main transport roller  21  via the transport belt  23  and regulating the transportation direction of the sheet  3  so that the sheet  3  is transported in the direction that the transport belt  23  moves. 
   The sheet  3  in the input sheet cassette  4  is picked up and fed into a transportation path in the ink jet printer  1  via a pick-up roller  26  and a friction pad  27 . The sheet  3  is then transported along a guide plate  29  to a midway roller  28 , located before the transportation direction regulating roller  25  on the periphery of the main transport roller  21 , for pressing the sheet  3  onto the surface of the transport belt  23 . Thereby, the sheet  3  from the input sheet cassette  4  is transported to the transport belt  23  which will further transport the sheet  3  to the print position. The input sheet cassette  4  includes a cassette main body  31 , a bottom plate  32 , and an extension bottom plate  33 . The sheets  3  are placed on a planar surface defined by the bottom plate  32  and the extension bottom plate  33 . Such a planar surface for receiving and holding the sheets  3  can be extended by changing the position of the extension bottom plate  33  (see chain lines C in  FIG. 4 ), thereby making it possible to use a sheet having a length longer than the cassette main body  31 . In addition, an end fence  34  is mounted on the upper surface of the extension bottom plate  33 . The end fence  34  can be slid in a stepless manner and can be set at any position within the width of the cassette main body  31  in the vertical direction of the drawing of  FIG. 3 . 
   Also, the sheet  3  which is inserted from the manual input tray  5  when the manual input tray  5  is in an open position (see chain lines D in  FIG. 4 ) is picked up and fed into another transportation path in the ink jet printer  1  via a manual input pick-up roller  35  and a pair of manual input transportation rollers  36  and  37 . The sheet  3  is then transported along a manual input guide plate  38  to the midway roller  28 . Thereby, the sheet  3  from the manual input tray  5  is transported to the transport belt  23  which will further transport the sheet  3  to the print position. 
   After having passed the print position, the sheet  3  is transported along an ejection guide plate  41  to an ejection main roller  42  and an ejection sub-roller  43 . The sheet  3  is then ejected to the output sheet tray  6  via the ejection main roller  42  and the ejection sub-roller  43 . The rollers  42  and  43  are at an ejection position. The output sheet tray  6  can be extended as indicated by broken lines E in  FIG. 4 . 
   In addition to the above sheet ejection path, the present preferred embodiment has another sheet ejection path which is directed to a location between the output sheet tray  6  and the input sheet cassette  4  and is used for the sheet ejection after a front side print operation in a duplex print mode. It should be noted that the location for feeding the one-side-printed sheet outside of the printer before the one-side-printed sheet is fed back into the printer for printing of the second side, can be positioned at a variety of different locations and is not limited to the location shown in  FIGS. 3 and 4 . As will be described below, the output sheet tray  6  may be used for supporting the one-side-printed sheet before the sheet is fed back into the printing of the second side. Also, other locations and supports for the one-side-printed sheet may be used and may be located at different positions. 
   A sheet diverting mechanism is provided for achieving the two-sided printing. More specifically, a first path-selection pawl  46  is mounted to switch between these two sheet ejection paths. Accordingly, the first path-selection pawl  46  is rotated in the simplex or one-side-only print mode so that the tip thereof is set in a lower position, thereby selecting the sheet ejection path for ejecting printed sheets to the output sheet tray  6 . In the duplex or two-sided print mode, the first path-selection pawl  46  is rotated so that the tip thereof is set at an upper position, thereby selecting the sheet ejection path for the duplex print operation. 
   In the duplex mode, after the sheet has passed the print position for the front side print operation, the sheet  3  is transported along a duplex ejection guide plate  45  to a duplex switchback main roller  47  and a duplex switchback sub-roller  48 . The sheet  3  is further transported towards a duplex transit tray provided on the upper surface of the input sheet cassette  4  via the duplex switchback main roller  47  and the duplex switchback sub-roller  43 . The duplex switchback main roller  47  is stopped at a predetermined timing so as to keep holding the sheet  3  at the trailing edge thereof, and is reversely driven to start transportation of the sheet  3  for the print operation on the back side of the sheet  3 . 
   A second path-selection pawl  49  is provided upstream of the duplex switchback main roller  47  and the duplex switchback sub-roller  43  in the sheet ejection path along the duplex ejection guide plate  45 . The second path-selection pawl  49  switches between the sheet ejection path and a duplex print path formed underneath the sheet ejection path. Accordingly, when the sheet  3  is transported to the duplex transit tray, the second path-selection pawl  49  is rotated so that the tip thereof is set at a lower position. Thereby, the sheet ejection path to the duplex transit tray is selected. When the sheet  3  is transported from the duplex transit tray towards the duplex print path, the second path-selection pawl  49  is rotated so that the tip thereof is set in an upper position. Thereby, the duplex print path for the duplex print operation is selected. 
   After having started the reverse rotation, the duplex switchback main roller  47  and the duplex switchback sub-roller  48  transport the sheet  3  along a duplex guide plate  51  to a duplex main roller  52  and a duplex sub-roller  53 . The sheet  3  then contacts the transport belt  23 , and is further transported to a transportation sub-roller  54  and to the midway roller  28  by the duplex main roller  52  and the duplex sub-roller  53 . Thus, the sheet  3  can be transported to the print position so as to be printed on the back side thereof. 
   Next, an exemplary hardware configuration of a control unit of the ink jet printer  1  is explained with reference to  FIG. 5 . As shown in  FIG. 5 , the control unit of the ink jet printer  1  includes a CPU (central processing unit)  80  for controlling all of the operations of the ink jet printer  1 , and a ROM (read only memory)  81  for storing various kinds of information (including programs) related to the operations of the ink jet printer  1 . The control unit further includes a RAM (random access memory)  82  used as a working memory and the like, and an image memory  83  for storing data of processed image information. The control unit further includes a PIO (parallel input and output) port  84 , an input buffer  85 , another PIO port  86 , a head drive circuit  88 , and drivers  89  and  90 . 
   The PIO  84  receives image information sent from a host system as well as information for indicating whether printing is to be done in a simplex or one-side only print mode or a duplex or two-sided print mode, a size of sheet used, commands sent from a console panel (not shown), signals sent from various kinds of sensors such as a home position sensor for detecting a home position of the carriage  13 , and so forth. In addition, the PIO  84  sends information to the host system and console panel. 
   The head drive circuit  88  drives the recording head  14  which includes energy generating devices corresponding to the Y, M, C, and B color nozzles. Each energy generating device preferably includes an electronic-to-mechanical transducer such as a piezoelectric transducer or an electronic-to-heat transducer such as a heating resistor. When driving the recording head  14 , the head drive circuit  88  selects at least one energy generating device from among all the energy generating devices of the recording head  14  in accordance with the information sent from the CPU  80  via the PIO  86  and applies a drive waveform to each selected energy generating device. In this way, the corresponding nozzles are driven. The drive waveform may be a square wave, a deltaic wave, a sine wave, etc. 
   The driver  89  drives a motor  91  for moving the carriage  13  in the main scanning direction, a motor  92  for rotating the main transport roller  21  in the sub-scanning direction, a motor  93  for rotating the duplex switchback main roller  47 , in accordance with the information sent from the CPU  80  via the PIO  86 . The driver  90  drives solenoids  94  and  95  for moving the first and second path-selection pawl is  46  and  49 , respectively, in accordance with the information sent from the CPU  80  via the PIO  86 . 
   Next, an exemplary procedure of the duplex print operation of the ink jet printer  1  is explained with reference to  FIGS. 6-9 . The exemplary procedure of the duplex print operation of the ink jet printer  1  is shown in a flowchart of  FIG. 6 . As shown in the flowchart of  FIG. 6 , when the duplex print mode is instructed, the first path-selection pawl  46  is switched to the upper position in Step S 11 , the second path-selection pawl  49  is switched to the lower position in Step S 12 , and the duplex switchback main roller  47  is driven in a forward rotational direction so as to rotate in the direction to transport the sheet  3  to the duplex transit tray in Step S 13  (see  FIG. 7 ). 
   Then, in Step S 14 , the pick-up roller  26  is driven to send the sheet  3  from the input sheet cassette  4 , so that the sheet  3  is transported to the transport belt  23  which will then transport the sheet  3  in the sub-scanning direction via the electrostatic force. While the sheet  3  is passing through the print position underneath the ink jet recording head  14 , the ink jet recording head  14  performs the print operation on the front surface of the sheet  3  in Step S 15 . In the print operation, the carriage  13  is moved in the main scanning direction, and the energy generating devices of the ink jet recording head  14  are driven in accordance with the recording image. Thereby, the image is printed on the front surface of the sheet  3 . 
   Then, the process of Step S 16  checks the ink jet recording head  14  has finished the print operation on the front side of the sheet  3 . The leading edge of the sheet  3  is led to the duplex ejection path and the sheet  3  itself is transported to the duplex transit tray. Thereby, the sheet  3  is ejected to a location outside of the ink jet printer  1 , as illustrated in  FIG. 8 . After a completion of the print operation, the duplex switchback main roller  47  is stopped at a time when the trailing edge of the sheet  3  is pinched between the duplex switchback main roller  47  and the duplex switchback sub-roller  48  in Step S 17 , as illustrated in  FIG. 8 . This position at which the sheet is pinched by rollers  47  and  48  which are stopped is referred to as a drying position and the rollers  47  and  48  are a drying holding device. In  FIGS. 3 and 4 , it is seen that the rollers  47 ,  48  are below rollers  42 ,  43 . 
   After the duplex switchback main roller  47  is stopped, the process of Step S 18  determines if a predetermined time has elapsed from the that the print operation on the front side of the sheet  3  is completed. This predetermined time includes a time period necessary for drying the ink drops applied onto the surface of the sheet  3 . Accordingly, when a drying process is not needed, the process of waiting for the elapse of the predetermined time is not needed, while in a case that the drying process is needed, some other operation can be performed during the waiting period. Also, the predetermined time can be adjusted in accordance with an amount of the ink drops used in the print operation, which can be measured by calculating the numbers of black dots including the color dots included in the image data. 
   When the predetermined time has elapsed from the time that the print operation on the front side of the sheet  3  is completed, the first path-selection pawl  46  is switched to the lower position in Step S 19 , the second path-selection pawl  49  is switched to the upper position in Step S 20 , and the duplex switchback main roller  47  is reversely driven so as to rotate in the direction to transport the sheet  3  to the duplex print path in Step S 21  (see  FIG. 9 ). Thereby, the sheet  3  is transported to the duplex print path. The sheet  3  is further transported to the transport belt  23  via the rotation of the duplex main roller  52 . In this operation, the sheet  3  is transported towards the transport belt  23  at the same speed as that of the transport belt  23 . Thus, the transport belt  23  can make close contact with the sheet  3  and transport the sheet  3  without making scratches on the surface thereof. 
   Then, while the sheet  3  is passing through the print position underneath the ink jet recording head  14 , the ink jet recording head  14  performs the print operation on the back side surface of the sheet  3  in Step S 22 . After that, the process of Step S 22  checks if the ink jet recording head  14  has finished the print operation on the back side of the sheet  3 . The leading edge of the sheet  3  is led to the ejection path and the sheet  3  is transported to the output sheet tray  6 . Thereby, the sheet  3  is ejected to the output sheet tray  6 , as illustrated in  FIG. 9 . 
   In the manner described above, the ink jet printer  1  has a sheet transportation passage in which a portion of the sheet is transported to a location outside of the apparatus after the completion of the print operation on the front side of the sheet and is returned to the next print operation on the back side of the sheet in a switchback mode. 
   As a result, the ink jet printer  1  has a very simple structure and eliminates the need to have more than one ink jet recording head. In addition, the printed front surface of the sheet by the print operation can be dried during the time that the sheet is transmitted to a location outside on the duplex transit tray. Thus, the print quality is greatly improved. 
   In addition, the predetermined time that is advantageously used for drying the print surface of the sheet in the duplex print mode may used for performing of other operations. 
   Further, the duplex transit tray is preferably mounted below the print position and the sheet ejection path has a downwardly inclined slope leading to the duplex transit tray. With this configuration, the present preferred embodiment can reliably transport the sheet which has been printed on the front side thereof and is therefore, heavier because of the weight of the deposited ink drops to the duplex transit tray. In the present preferred embodiment, the duplex transit tray is not a separate mechanical component but is simply the surface of the input sheet cassette  4 . Thus, the structure is simple. 
   Although the duplex transit tray is defined by the upper surface of the input sheet cassette  4  in the present preferred embodiment, it may alternatively be combined with the output sheet tray  6 . 
   The switchback mechanism transfers the one-side printed sheet onto the surface of the ordinary eject tray and holds the sheet thereon. Thus, the sheet is exposed to air outside of the apparatus so as to expedite drying and to avoid the moist, humid atmosphere inside of the printer which prevents rapid drying of the ink drops on the one-side-printed sheet. 
   Such a switchback mechanism operates in the following manner as seen in  FIGS. 10-12 . 
   In the duplex print mode, the sheet  3  which has been transferred to the print position is printed on the front side of the sheet  3  and is further transferred to the ordinary sheet output tray  6 , as illustrated in  FIG. 10 . Thus, in this preferred embodiment, a separate sheet path leading to a location on top of the sheet cassette is not necessary and the sheet output tray can be used to support the one-side-printed sheet thereon before the sheet is fed back into the printer for printing on the second side thereto. During this operation, a separation pawl A is set at a lower position. 
   A sensor B which is mounted in the paper path between the print head  14  and the separation pawl A, detects a trailing edge of the advancing sheet  3 . Upon such a detection, the sheet  3  is advanced further for a predetermined time and is then stopped so that the trailing edge thereof passes the separation pawl A. Then, the eject rollers  42 ,  43  are activated to hold the sheet  3  at the trailing edge thereof, as illustrated in  FIG. 11 . 
   Then, the separation pawl is set to an upper position and the eject rollers  42  and  43  are driven reversely so as to transfer the one-side-printed sheet  3  to the roller  21 , as illustrated in  FIG. 12 . 
   Then, the sheet  3  is transferred again to the print position and is printed on its back surface. After that, the sheet  3  is transferred to the eject tray  6  and is ejected thereto. 
   In addition, the time period that the eject rollers  42  and  43  grip the sheet  3  at the trailing edge thereof can be changed according to an operator instruction which may be made in accordance with various factors such as characteristics of ink, sheet, and so on. For example, ink on an ordinary plain paper can be dried more quickly than that on a calendared paper and the operator can normally select a type of sheets when selecting the print operation. If such a time period is not sufficiently provided, the ink of the one-side printed sheet  3  will rub against the separation pawl A and/or guide plate C. As a result, the ink image on the sheet  3  will be negatively affected. 
     FIGS. 13-15  show preferred embodiments of the present invention including ink drying mechanisms. When the one-side-printed sheet  3  is to be dried on the eject tray, a fan  100  can be mounted in a position as show in  FIG. 13 . When the one-side-printed sheet  3  is dried on the paper cassette, a fan  100  can be mounted at a position as shown in  FIG. 14 . 
   In conventional devices, for safety reasons, it is difficult to find a safe and reliable configuration to provide a heater to expedite drying the one-side-printed sheet. 
   However, in a preferred embodiment of the present invention shown in  FIG. 15 , when the one-sided printed sheet  3  is dried on the paper cassette, a heater  110  can be mounted at a position as shown in  FIG. 15 . The heater should be located inside of a portion of the printer housing  120  provided with the apparatus for safety reasons. 
   In describing preferred embodiments of the present invention illustrated in the drawings, specific terminology is used for the sake of clarity. However, the present invention is not intended to be limited to the specific terminology so selected and it is to be understood that each specific element includes all technical equivalents which operate in a similar manner. 
   Numerous additional modifications and variations of the present invention are possible in light of the above teachings. It is therefore to be understood that within the scope of the appended claims, the present invention may be practiced otherwise than as specifically described herein. 
   This document claims the priority rights of and is based on the subject matter described in Japanese Patent Application No. 10-165331 filed on Jun. 12, 1998, in the Japanese Patent Office, the entire contents of which are hereby incorporated by reference.