Patent Publication Number: US-7222955-B2

Title: Both-side recording apparatus

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to a both-side recording apparatus in which both-side recording can be performed to a recording medium inverted by a sheet inversion unit, particularly to the both-side recording apparatus with a sheet transport mechanism having a pair of sheet transport rollers including a sheet transport roller and a pinch roller and a pair of sheet discharge roller arranged on the downstream side of the pair of sheet transport rollers in a transport direction. 
   2. Related Background Art 
   In an inkjet recording apparatus which can perform the both-side recording, some methods have been realized or proposed. In these methods, after the recording of the surface (front surface) of the recording paper is terminated, the transport direction of the recording paper is inverted, the recording paper is fed into an inversion device, the recording paper is transported again by the same sheet transport unit after termination of inversion operation, and the recording is performed to a reverse surface of the recording paper by the same recording unit. 
   In the invention disclosed in U.S. Pat. No. 6,332,068, the recording paper is supported by the sheet transport rollers while sandwiched by the sheet transport rollers when recording operation of the front surface of the recording paper is terminated, and a rotating direction of the sheet transport roller is directly reversed to transport the recording paper to the inversion unit. In the invention disclosed in Japanese Patent Application Laid-Open No. 2002-067407, after the recording operation of the front surface of the recording paper is performed while the recording paper is transported to the downstream side where the recording paper has already disengaged from the sheet transport roller, the transport direction of the recording paper is reversed, and the sheet transport rollers support the recording paper while sandwiching the recording paper again by utilizing a guide member. In both the methods, a region where the recording of the front surface is terminated and ink is fixed is supported by the sheet transport roller while sandwiched by the sheet transport roller again, and the recording paper is transported. 
   However, there are some restrictions in the above conventional examples. In the invention disclosed in U.S. Pat. No. 6,332,068, because a paper supporting and sandwiching space for the sheet transport rollers is required, it is impossible that an array of discharge ports of a recording head is arranged to proximity of a nip position. Therefore, a blank space where the recording operation cannot be performed is not prevented from remaining in a rear end portion of the recording paper. In the invention disclosed in Japanese Patent Application Laid-Open No. 2002-067407, the blank space in a rear end portion of the recording paper can be eliminated. However, in the case where the recording operation is performed by the inkjet recording, since deformation such as surface waviness influenced by swelling of the recording paper occurs, there is a possibility that paper jam is generated when the sheet transport rollers support the recording paper while sandwiching the recording paper. Further, since the sheet transport rollers support the region where the recording the ink is fixed while sandwiching the region again to transport, it is necessary to wait until the ink is completely dried to be fixed. In consideration of various kinds of conditions, it is necessary to secure sufficient dry standby time in order not to transfer the ink to the roller side in any case. 
   SUMMARY OF THE INVENTION 
   In view of the foregoing, it is an object of the invention to provide the both-side recording apparatus which can perform the recording without the blank space in an overall range of a recording medium, transport the recording medium to a gap between the sheet transport roller and a pinch roller before the recording medium absorbs the ink and the like to be deformed in transporting the recording paper, securely support the recording medium while sandwiching the recording medium again with the sheet transport roller and the pinch roller, and securely prevent the paper jam and the like. 
   In order to achieve the above object, the invention is a both-side recording apparatus with a sheet transport mechanism having a pair of sheet transport rollers including a sheet transport roller and a pinch roller pressed against the sheet transport roller, at least one pair of sheet discharge rollers arranged on the downstream side of the sheet transport roller in a transport direction, and a pair of sheet discharge rollers including a rotating body pressed against the roller, characterized in that a recording medium can be transported to a position where a rear end of the recording medium is released from the pair of sheet transport rollers when a first surface is recorded at first, and then the recording paper is transported to a paper inversion unit in such a manner that the sheet transport roller is pressed into contact with the pinch roller again to further continue the transport in the reverse direction after the sheet transport roller and the pinch roller are released to transport the recording medium toward a reverse direction of the first surface recording by the pair of sheet discharge rollers. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a schematic perspective view showing an overall configuration of a both-side recording apparatus according to an embodiment of the invention; 
       FIG. 2  is a schematic sectional side view showing the overall configuration of the both-side recording apparatus according to the embodiment of the invention; 
       FIG. 3  is a schematic perspective view showing a pinch roller pressing and separating mechanism of the both-side recording apparatus according to the embodiment of the invention; 
       FIGS. 4A ,  4 B, and  4 C are a schematic sectional side view showing the pinch roller pressing and separating mechanism of the both-side recording apparatus according to the embodiment of the invention; 
       FIGS. 5A and 5B  are a schematic sectional side view showing a PE sensor of the both-side recording apparatus according to the embodiment of the invention; 
       FIGS. 6A and 6B  are a schematic sectional side view showing a sheet passing guide up-and-down movement mechanism of the both-side recording apparatus according to the embodiment of the invention; 
       FIG. 7  is a schematic perspective view showing a guide shaft up-and-down movement mechanism of the both-side recording apparatus according to the embodiment of the invention; 
       FIGS. 8A ,  8 B, and  8 C are a schematic sectional side view showing the guide shaft up-and-down movement mechanism of the both-side recording apparatus according to the embodiment of the invention; 
       FIG. 9  is a schematic perspective view showing a driving mechanism of a cam lift shaft of the both-side recording apparatus according to the embodiment of the invention; 
       FIGS. 10A ,  10 B,  10 C, and  10 D are a schematic sectional side view showing a state at each position of a lift mechanism of the both-side recording apparatus according to the embodiment of the invention; 
       FIG. 11  is a timing chart showing an operational state of the lift mechanism of the both-side recording apparatus according to the embodiment of the invention; 
       FIGS. 12A ,  12 B, and  12 C are a schematic sectional side view showing an operational state of the lift mechanism of the both-side recording apparatus according to the embodiment of the invention; 
       FIG. 13  is a schematic sectional side view showing a configuration of an automatic both-side unit (automatic inversion unit, paper inversion unit) of the both-side recording apparatus according to the embodiment of the invention; 
       FIGS. 14A and 14B  are a schematic sectional side view showing operation of a flap in the automatic both-side unit of the both-side recording apparatus according to the embodiment of the invention; 
       FIG. 15  is a schematic sectional side view showing an automatic both-side unit driving mechanism of the both-side recording apparatus according to the embodiment of the invention; 
       FIGS. 16A ,  16 B,  16 C,  16 D,  16 E and  16 F are a schematic sectional side view sequentially showing the operational state of the automatic both-side unit driving mechanism of the both-side recording apparatus according to the embodiment of the invention; 
       FIGS. 17A ,  17 B,  17 C,  17 D and  17 E are a schematic sectional side view sequentially showing another operational state of the automatic both-side unit driving mechanism of the both-side recording apparatus according to the embodiment of the invention; 
       FIGS. 18A ,  18 B, and  18 C are a schematic sectional side view showing reverse-surface front-end registration operation in the case where a thin recording sheet is used in the both-side recording apparatus according to the embodiment of the invention; 
       FIGS. 19A ,  19 B, and  19 C are a schematic sectional side view showing the reverse-surface front-end registration operation in the case where a thick recording sheet is used in the both-side recording apparatus according to the embodiment of the invention; 
       FIG. 20  which is composed of  FIG. 20A and 20B  are flowcharts showing a sequence of automatic both-side recording operation of the both-side recording apparatus according to the embodiment of the invention; 
       FIG. 21  is a block diagram showing a configuration of a control circuit of the both-side recording apparatus according to the embodiment of the invention; and 
       FIG. 22  is a schematic sectional side view showing another configuration of automatic both-side unit of the both-side recording apparatus according to the embodiment of the invention. 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   A preferred embodiment of the invention will be specifically described below referring to the accompanying drawings. In each drawing, the same constituents and corresponding parts are indicated by the same reference numerals and signs.  FIG. 1  is a schematic perspective view showing an overall configuration of an embodiment of the recording apparatus to which the invention is applied and  FIG. 2  is a schematic sectional side view showing the overall configuration of the recording apparatus according to the embodiment, as viewed from a direction of an arrow A of  FIG. 1 . The recording apparatus shown in  FIGS. 1 and 2  is the inkjet recording apparatus which discharges ink to perform the recording onto the recording medium. In the following description, since the recording paper is a representative example of the recording medium, sometimes the wording in which the recording medium in broad sense of the term should be used is expressed by the recording paper or the paper. However, scope of the recording medium is not limited to the paper (recording paper). 
   In  FIGS. 1 and 2 , the numeral  1  designates a main body of a recording unit, the numeral  2  designates an automatic both-side unit (paper inversion unit, automatic inversion unit), the numeral  10  designates a chassis which supports a structure of the recording unit  1 , the numeral  11  designates a recording head which discharges the ink to perform the recording, the numeral  12  designates an ink tank storing the ink supplied to the recording head  11 , the numeral  13  designates a carriage which perform scanning (main scanning) while holding the recording head  11  and the ink tank  12 , the numeral  14  designates a guide shaft which guides and supports the carriage  13 , the numeral  15  designates a guide rail which guides and supports the carriage  13  in parallel to the guide shaft  14 , the numeral  16  designates a carriage belt (timing belt) which drives the carriage  13 , the numeral  17  designates a carriage motor which drives carriage belt  16  through a pulley, the numeral  18  designates a code strip which detects a position of the carriage  13 , and the numeral  20  designates an idler pulley which is opposed to the puller of the carriage motor  17  to tension the carriage belt  16 . 
   The numeral  21  designates a sheet transport roller which transports the recording medium (recording paper), the numeral  22  designates a pinch roller which is driven by pressing the sheet transport roller  21  against the pinch roller, the numeral  23  designates a pinch roller holder which rotatably holds the pinch roller  22 , the numeral  24  designates a pinch roller spring which presses the pinch roller  22  into contact with the sheet transport roller  21 , the numeral  25  designates a sheet transport roller pulley which is fixed to the sheet transport roller  21 , the numeral  26  designates an LF motor which drives the sheet transport roller  21 , the numeral  27  designates a code wheel which detects a rotational angle of the sheet transport roller  21 , and the numeral  29  designates a platen which is opposed to the recording head  11  to support the recording paper. 
   The numeral  30  designates a first sheet discharge roller which cooperated with the sheet transport roller  21  to transport the recording medium, the numeral  31  designates a second sheet discharge roller which is provided on the downstream side of the first sheet discharge roller  30 , the numeral  32  designates a first star gear train which is of a rotating body being opposed to the first sheet discharge roller  30  to hold the recording medium, the numeral  33  designates a second star gear train which is of the rotating body being opposed to the second sheet discharge roller  31  to hold the recording medium, the numeral  34  designates a star gear base which rotatably holds the first start gear train  32  and the second star gear train  33 , the numeral  36  designates a maintenance unit which prevents the recording head  11  from clogging.(clogging of a discharge port or nozzle) to maintain and recover ink discharge performance and is actuated when the ink is caused to run through ink channels of the recording head  11  in exchanging the ink tanks  12 , and the numeral  37  designates a main ASF (Automatic Sheet Feeder) which is of an automatic sheet supply unit stacking the sheet of recording paper to supply the recording paper one by one to a recording unit during the recording operation. 
   In  FIGS. 1 and 2 , the numeral  38  designates an AFS base which becomes a foundation of the main ASF  37 , the numeral  39  designates a sheet supply roller which supplies the stacked sheets of recording paper while abutting on the stacked sheets of recording paper, the numeral  40  designates a separation roller which separates the plurality of recording mediums one by one when the plurality of recording mediums are simultaneously supplied, the numeral  41  designates a pressing board which stores the recording medium to bias the recording medium toward the sheet supply roller  39 , the numeral  42  designates a side guide which is provided on the pressing plate  41  and can be fixed at an arbitrary position in a width direction of the recording medium, and the numeral  44  designates an ASF flap which controls the paper passing direction of the recording medium into one direction from the main ASF  37 . 
   The numeral  50  designates a lift input gear which engages an ASF planet gear  49 , the numeral  51  designates a lift reduction gear train which conveys power from the lift input gear  50  while reducing speed, the numeral  52  designates a lift cam gear which is directly connected to a lift cam shaft, the numeral  55  designates a guide shaft spring which provides biasing force to move the guide shaft  14  to one side, the numeral  56  designates a guide oblique surface on which a cam of the guide shaft gear  53  slides, the numeral  58  designates a lift cam shaft which lifts the pinch roller holder  23  and the like, the numeral  70  designates a sheet passing guide which guides the front end of the recording medium to the nip portion between the sheet transport roller  21  and the pinch roller  22 , the numeral  72  designates a base which supports the whole of the main body of the recording unit  1 , and the numeral  301  designates a control board in which control units are combined. 
     FIG. 21  is a block diagram showing driving means for driving the whole of the recording apparatus to which the invention is applied. In  FIG. 21 , the numeral  19  designates a CR (carriage) encoder sensor which is mounted on the carriage  13  and reads the code strip  18 , the numeral  28  designates an LF encoder sensor which reads the code wheel  27  attached to the chassis  1 , the numeral  46  designates an ASF motor which drives the main ASF  37 , the numeral  67  designates a PE sensor which detects the operation of a sensor lever  66 , the numeral  69  designates a lift cam sensor which detects the operation of the lift cam shaft  58 , and the numeral  130  designates a both-side unit sensor which detects attachment and detachment of the automatic both-side unit  2 . 
   In  FIG. 21 , the numeral  302  designates a PG motor which drives the maintenance unit  36 , the numeral  303  designates a PG sensor which detects the operation of the maintenance unit  36 , the numeral  305  designates an ASF sensor which detects the operation of the main ASF  37 , the numeral  307  designates a head driver which drives the recording head  11 , the numeral  308  designates a host apparatus which transmits recording data to the recording apparatus, the numeral  309  designates an interface (I/F) which electrically interfaces between the host apparatus  308  and the recording apparatus, the numeral  310  designates CPU which control the recording apparatus and gives a control direction, the numeral  311  designates ROM in which control data and the like are written, and the numeral  312  designates RAM which becomes an area where recording data and the like are loaded. 
   Referring to  FIG. 1 ,  FIG. 2 , and  FIG. 21 , the recording apparatus according to the invention will be briefly described, and then the operation of each unit will be described. At first, a configuration of the commonly used serial scan type of recording apparatus will be described. The recording apparatus according to the embodiment mainly includes a sheet supply unit, a recording medium transport unit (sheet transport unit), a recording unit, a recording means (recording head) maintenance unit, and an automatic inversion unit (automatic both-side unit). When the recording data is transmitted from the host apparatus  308  and stored in RAM  312  through I/F  309 , CPU  310  gives a recording operation start direction to start the recording operation. 
   When the recording is started, the sheet supply operation is performed at first. The sheet supply unit is the main ASF. The sheet supply unit includes the automatic sheet supply unit which draws the recording medium one by one from the plurality of recording mediums (the plurality of sheets of recording paper) stacked on the pressing plate  41  in each recording operation and supplies the recording medium to the recording medium transport unit (sheet transport unit). When the sheet supply operation is started, the ASF motor  46  is rotated in a forward direction, and, the power of the ASF motor  46  rotates the cam holding the pressing plate  41  through the gear train. When the cam is disengaged by the rotation of the ASF motor  46 , the pressing plate  41  is biased toward the sheet supply roller  39  by action of the pressing plate spring (not shown). At the same time, the sheet supply roller  39  is rotated in the direction in which the recording medium (paper) is transported, so that the uppermost recording medium of the plurality of recording mediums stacked on the pressing plate  41  is started to transport. Sometimes the plurality of sheets of paper is simultaneously supplied by conditions of friction force between the sheet supply roller  39  and the recording paper and the friction force between the sheets of paper. 
   In that case, the separation roller  40 , which is pressed against the sheet supply roller  39  and has predetermined return running torque in the direction opposite to the recording sheet transport direction, acts so as to push back the sheets of recording paper except the recording paper located on the most sheet supply roller  39  side onto the original pressing plate. When the ASF sheet supply operation is terminated, the separation roller  40  is released from the state in which the separation roller  40  is pressed into contact with the sheet supply roller  39  by the operation of the cam, and the separation roller  40  is separated from the sheet supply roller  39  with a predetermined distance. At this point, in order to securely push back the recording paper to the predetermined position on the pressing plate, the return pawl  43  is rotated to push back the recording paper. Only one sheet of recording paper is supplied to the sheet transport unit in the above-described manner. 
   When one sheet of recording paper is supplied from the main ASF  37 , the front end of the recording paper abuts on the ASF flap  44  biased in the direction in which the sheet passing path is blocked by the ASF flap  44 . However, the recording paper pushes away the ASF flap  44  to pass through. When the recording operation of the recording paper is terminated and a rear end of the recording paper passes through the ASF flap  44 , since the ASF flap  44  returns to the biased state to close the sheet passing path, even if the recording paper is supplied in the reverse direction, the recording paper never return to the main ASF  37  side. 
   The recording paper supplied from the sheet supply unit is transported toward the nip portion of the sheet transport roller (transport roller)  21  and the pinch roller  22  which are of the sheet transport unit. Because the center of the pinch roller  22  is attached so as to be slightly offset from the center of the sheet transport roller  21  toward the direction which comes close to the first sheet discharge roller  30 , the angle of a tangential direction in which the recording paper is inserted is slightly declined from a horizontal direction. Therefore, in order that the front end of the recording paper is accurately guided into the nip portion, the recording paper is transported while the sheet passing path formed by the pinch roller holder  23  and the guide member (sheet passing guide)  70  is declined. 
   The paper (recording paper) supplied by ASF  37  abuts on the nip portion of the sheet transport roller  21  in the stop state. At this point, a loop of the paper is formed between the sheet supply roller  39  and the sheet transport roller  21  in such a manner that the main ASF  37  supplies the paper by an interval somewhat longer than the predetermined length of the sheet passing path. The force generated by returning the loop to the straight state presses the front end of the paper against the nip portion of the sheet transport roller  21 , which allows the front end of the paper to be parallel along the line of the sheet transport roller  21 , and the so-called registration operation is completed. After the registration operation is completed, the LF motor (transport motor)  26  is started to rotate in the direction in which the recording paper is moved toward the forward direction, i.e. in the direction in which the recording paper proceeds to the first sheet discharge roller  30 . Then, the power of the sheet supply roller  39  is cut, and the sheet supply roller  39  is rotated with the paper. At this point, the recording paper is transported only by the sheet transport roller  21  and the pinch roller  22 . The paper advances to the forward direction in every predetermined amount of linefeed and proceeds along a rib provided in the platen  29 . 
   The front end of the paper engages the nip portion of the first sheet discharge roller  30  and the first star gear train  32  and the nip portion of the second sheet discharge roller  31  and the second star gear train  33 . Circumferential speeds of the first sheet discharge roller  30  and the second sheet discharge roller  31  is set substantially equal to the circumferential speed of the sheet transport roller  21 , and the sheet transport roller  21  is connected to the first sheet discharge roller  30  and the second sheet discharge roller  31  by the gear train, so that the first sheet discharge roller  30  and the second sheet discharge roller  31  are rotated while synchronizing with the sheet transport roller  21 . Therefore, the paper is transported without loosening or tensioning the paper. The sheet transport roller  21  and the pinch roller  22  constitute a pair of sheet transport rollers. The first sheet discharge roller  30  and the first star gear train  32  constitute a first pair of sheet discharge rollers, the second sheet discharge roller  31  and the second star gear train  33  constitute a second pair of sheet discharge rollers, and the first pair of sheet discharge rollers and the second pair of sheet discharge rollers constitute a set of the pairs sheet discharge rollers. 
   The recording unit mainly includes the recording head  11  which is of the recording means performing the recording to the recording paper on the basis of the recording data and the carriage  13  which mounts the recording head  11  to perform the scanning in the direction usually orthogonal to the recording sheet transport direction. The carriage  13  is guided and supported by the guide shaft  14  fixed to the chassis  10  and the guide rail  15  which is a part of the chassis  10 . The carriage  13  performs reciprocating motion (scanning) by transferring the driving force of the carriage motor  17  through the carriage belt  16  tensioned between the carriage motor  17  and the idler pulley  20 . 
   The plurality of ink channels communicated with the ink tank  12  is formed in the recording head  11 , and the ink channel is communicated to the discharge port provided in a surface (discharge port surface) opposite to the platen  29 . An actuator for discharging the ink is arranged in the inside of the plurality of discharge ports forming an array of discharge ports. For example, the actuator utilizing film boiling pressure of liquid by electrothermal conversion material (heating element), electromechanical transducer (electro-pressure conversion element) such as piezoelectric element, or the like is used as the actuator for discharging the ink. 
   In the inkjet recording apparatus which is of the recording apparatus using the recording head  11 , an ink droplet is discharged according to the recording data by transmitting the signal of the head driver  307  to the recording head through a flexible flat cable  73 . The ink droplet can be discharged toward the recording paper at proper timing by reading the code strip  18  tensioned in the chassis  10  with the CR encoder  19  mounted on the carriage  13 . When the recording of one line is terminated, the sheet transport unit (recording medium transport unit) transports the recording paper by the required amount. The recording operation can be performed over the surface of the recording medium by repeating the above operation. 
   The recording head maintenance unit maintains and recovers the recording operation of the recording head  11  in the normal state by preventing the clogging of the discharge port of the recording head  11  or by removing dirt caused by paper powders or the like on the discharge port surface of the recording head  11 . A capping mechanism which covers the discharge ports, a suction recovery mechanism which sucks the ink from the discharge port in the capping state, a wiping mechanism which wipes off a periphery of the discharge port, and the like are used as the recovery mechanism for the recording head maintenance unit. 
   The maintenance unit  36  which is arranged at the standby position of the carriage while opposed to the recording head  11  includes the capping mechanism having a cap which abuts on the discharge port surface of the recording head  11  to protect the discharge ports, the wiping mechanism having a wiper which cleans the discharge port surface, the suction recovery mechanism having a suction pump which is connected to the cap to generate negative pressure in the cap, and the like. When the ink is sucked out in order to refresh the ink in the discharge port of the recording head  11 , the ink is sucked and evacuated in such a manner that the cap is pressed against the discharge port surface and the suction pump is driven to generate the negative pressure in the cap. In the case where the ink adheres to the discharge port surface after the ink suction, or in the case where a foreign material such as the paper powder adheres to the discharge port surface, the ink adhesion and the foreign material are removed in such a manner that the discharge port surface is wiped by the wiper abuts on the discharge port surface to move the wiper in parallel to the discharge port surface. 
   The summary of the recording apparatus was described above. Then, the specific configuration of the embodiment including the configuration of the automatic both-side unit  2  for the purpose of the paper inversion unit or the automatic inversion unit will be described in detail. The recording apparatus according to the invention is characterized in that the recording is automatically performed to the front surface and reverse surface of the recording paper which is of the sheet-like cut paper without troubling an operator, i.e. the automatic both-side recording can be performed. 
   The sheet passing path of the recording paper will be described referring to  FIG. 2 . In  FIG. 2 , the numeral  104  designates a changeover flap including a movable flap which is rotatably supported and determines the direction of the sheet passing path of the recording paper, the numeral  106  designates an outlet flap which is rotatably supported and opens and closes when the recording paper is moved out from the both-side unit  2 , the numeral  108  designates a both-side roller A which is of an inversion roller transporting the recording paper in the both-side unit  2 , the numeral  109  designates a both-side roller B which is of the inversion roller transporting the recording paper in the both-side unit  2 , the numeral  112  designates a both-side pinch roller A which is driven by pressing the both-side roller A  108  against the both-side pinch roller A, and the numeral  113  designates a both-side pinch roller B which is driven by pressing the both-side roller B  109  against the both-side pinch roller B. 
   When the recording operation is started, the recording paper the recording paper is supplied one by one from the plurality of sheets of the recording paper stacked in the main ASF  37  by the action of the sheet supply roller  39 , and the recording paper is transported to the sheet transport roller  21 . The recording paper sandwiched between the sheet transport roller  21  and the pinch roller  22  is transported toward the direction indicated by an arrow a in  FIG. 2 . In the case where the both-side recording is performed, after the recording of the front surface of the recording paper is terminated, the recording paper is transported toward the direction indicated by an arrow b in  FIG. 2  through a horizontal path provided below the main ASF  37 . Since the automatic both-side unit  2  which is of the automatic inversion unit is arranged in the rear direction of the main ASF  37 , the recording paper is introduced from the horizontal path into the automatic both-side unit  2  and transported toward the direction indicated by an arrow c in  FIG. 2 . 
   In the both-side unit  2 , the recording paper changes the proceeding direction while sandwiched between the both-side roller B  109  and the both-side pinch roller B  113 , and then the recording paper is transported toward the direction indicated by an arrow d in  FIG. 2  while sandwiched between the both-side roller A  108  and the both-side pinch roller A  112 . Finally, the proceeding direction of the recording paper is changed to 180 degrees (inversion) to return to the horizontal direction. The recording paper transported toward the direction indicated by the arrow a in  FIG. 2  through the horizontal path is sandwiched by the sheet transport roller  21  and the pinch roller  22  again, and the reverse surface of the recording paper is performed. As described above, after the recording of the front surface is terminated, the recording paper is inverted by the horizontal path provided below the main ASF  37  and the automatic both-side unit  2  provided in the rear direction of the main ASF  37 , and the recording of the reverse surface is performed to automatically perform the recording to both the front surface and the reverse surface. 
   A recording range in recording the surface (first surface, front surface) will be described. The recording head  11  has a discharge port area (recording area, ink discharge area) N between the sheet transport roller  21  and the pinch roller  22 . Usually it is very difficult that the discharge port area N is arranged close to the nip portion of the sheet transport roller  21 , because of ink channel arrangement to the discharge port, an electric lead to the actuator (discharge energy generation means) which discharges the ink, or the like. Therefore, in the range where the recording paper is sandwiched by the sheet transport roller  21  and the pinch roller  22 , the recording can be performed only up to the range separated from the nip portion of the sheet transport roller  21  by a length L 1  as shown in  FIG. 2 . 
   In the recording apparatus according to the embodiment, in order to reduce a blank space of an lower end of the surface, the recording is continued up to the region where the recording paper is separated from the nip portion of the sheet transport roller  21  and transported while sandwiched only by the first sheet discharge roller  30  and the second sheet discharge roller  31 . This allows the recording operation to be performed until the blank space of the lower end of the surface is eliminated. However, when the recording paper is transported from the state in which the blank space of the lower end of the surface is eliminated toward the direction indicated by the arrow b in  FIG. 2 , the recording paper can not be guided to the nip portion of the sheet transport roller  21  and the pinch roller  22 , or it is difficult to guide the recording paper to the nip portion, and there is a possibility that the so-called paper jam is generated. In the embodiment, in order to avoid the paper jam, the pinch roller  22  is released from the sheet transport roller  21  by the means described below to make a predetermined gap, the end portion of the recording paper is brought into the gap, and then the pinch roller  22  is pressed against the sheet transport roller  21  again. This allows the recording paper to be transported toward the direction indicated by the arrow b in  FIG. 2 . 
   Then, a release mechanism of the pinch roller  22 , the release mechanism of the paper sensor lever (PE sensor lever)  66 , a pressure adjustment mechanism of the pinch roller spring  24 , an up-and-down movement mechanism of the guide member  70 , and the up-and down mechanism of the carriage  13  which are the characteristic configuration of the embodiment will be described. As described above, the pinch roller  22  is operated so as to be released from the sheet transport roller  21 . Further, the recording apparatus of the embodiment also includes other mechanisms in order to inverse the recording paper after the recording paper is brought into again. 
   One of the mechanisms is the release mechanism of the PE sensor lever  66  as the paper sensor lever. The PE sensor lever  66  is rockably attached at a predetermined angle relative to the surface of the recording paper so that the position of the front end or the rear end of the recording paper is accurately detected when the recording paper proceeds in the forward direction. This results in the technical problem that the end portion of the recording paper is hooked when the recording paper proceeds in the reverse direction or the front end of the PE sensor lever  66  breaks into the recording paper in transportation. Therefore, in the embodiment, the PE sensor lever  66  is released from the paper pass surface part of the way of the recording paper inversion process so as not to abut on the recording paper. 
   It is not always necessary to form the release mechanism of the PE sensor lever  66 , and it is also possible to substitute the release mechanism of the PE sensor lever  66  to another means or configuration. That is to say, it is also possible to form the means for solving the above technical problem, in which a roller or the like is provided at the front end of the PE sensor lever  66  and the above technical problem is solved by rotating the roller even if the recording paper proceeds toward the reverse direction. Further, it is also possible to form the means, in which the rocking angle of the PE sensor lever  66  is increased and the PE sensor lever  66  is rocked to the angle in the reverse direction when the recording paper is transported toward the reverse direction. 
   Another mechanism is the pressure adjustment mechanism of the pinch roller spring  24 , namely the pressure adjustment mechanism for changing the pressure (spring force) which presses pinch roller  22  against the sheet transport roller  21 . The embodiment is configured so as to rotate the whole of the pinch roller holder  23  to release the pinch roller  22 . In the state in which the pinch roller  22  is pressed against the sheet transport roller  21 , because the pinch roller spring  24  is pressed against the pinch roller holder  23 , when the pinch roller holder  23  is rotated in the release direction, the pressure of the pinch roller spring  24  is increased, which results in an adverse effect such as the increase in load for releasing the pinch roller holder  23  and the increase in stress applied to the pinch roller holder  23  itself. The mechanism (pressure adjustment mechanism) which reduces the pressure of the pinch roller spring  24  in releasing the pinch roller holder  23  is provided in order to prevent the adverse effect. 
   Another mechanism is the up-and-down mechanism of the sheet passing guide. The guide member including the sheet passing guide  70  constitutes a first sheet passing path which guides the recording paper transported from the automatic sheet supply unit  37  and a part of a common portion of the first sheet passing path and a second sheet passing path which transports the recording paper to the automatic inversion unit including the both-side unit  2  or guides the recording paper supplied from the automatic inversion unit. As shown in  FIG. 2 , in order to guide the recording paper supplied from the main ASF  37  to the sheet transport roller  21 , the sheet passing guide  70  is usually located at the position where the angle slightly rises from the horizontal path so that the recording paper is smoothly introduced to the nip portion of the LF roller  21 . As described above, the LF roller  21  has slightly the angle relative to the horizon. However, in this case, when the recording paper is transported toward the direction indicated by the arrow b in  FIG. 2 , the recording paper is guided to toward the main ASF  37  again. Therefore, the configuration in which the angle of the sheet passing guide  70  is changed so that the sheet passing guide  70  becomes horizontal is more prefer in order to prevent this movement to smoothly guide the recording paper to the horizontal path. For this purpose, the up-and down mechanism which raises and lowers the sheet passing guide  70  as the guide member is provided. 
   Final one of the mechanisms is the up-and-down mechanism of the carriage  13 . When the pinch roller holder  23  becomes the release state in which the pinch roller holder  23  is released from the sheet transport roller  21 , the front end of the pinch roller holder  23  comes close to the carriage  13 , and the front end of the pinch roller holder  23  abuts on the carriage  13  not to be moved in the main scanning direction. The up-and-down mechanism of the carriage  13  prevents the abutment of the front end of the pinch roller holder  23  on the carriage  13 . Therefore, the up-and-down mechanism for raising the carriage  13  in synchronization with the release operation of the pinch roller holder  23  is provided. The up-and-down mechanism of the carriage  13  can be also applied to other uses. For example, the up-and-down mechanism of the carriage  13  can be also utilized when the recording head  11  is moved so as to be retracted, in order that the recording head  11  does not come into contact with the recording paper when the recording is performed to the thick recording paper. 
   The above five mechanisms (the release mechanism of the pinch roller  22 , the release mechanism of the PE sensor lever  66 , the pressure adjustment mechanism of the pinch roller spring  24 , the up-and-down mechanism of the sheet passing guide  70 , and the up-and-down mechanism of the carriage  13 ) will be described in detail below.  FIG. 3  is a schematic perspective view showing the outline configuration of the pinch roller release mechanism, the PE sensor lever release mechanism, the pinch roller spring pressure adjustment mechanism, and the sheet passing guide up-and-down mechanism. 
   In  FIG. 3 , the numeral  59  designates a pinch roller holder pressing cam which abuts on the pinch roller holder  23 , the numeral  60  designates a pinch roller spring pressing cam which becomes a point of action of the pinch roller spring  24 , the numeral  61  designates a PE sensor lever pressing cam which abuts on the PE sensor lever  66 , the numeral  62  designates a lift cam shaft shielding plate which shows the angle of the lift cam shaft  58 , the numeral  65  designates a sheet passing guide pressing cam which abuts on the sheet passing guide  70 , the numeral  66  designates a PE sensor lever which comes into contact with the recording paper to detect the front end and rear end of the recording paper, the numeral  67  designates a PE sensor which light is permeated or transmitted and shielded by the PE sensor lever  66 , the numeral  68  designates a PE sensor lever spring which biases the PE sensor lever  66  toward the predetermined direction, the numeral  69  designates a lift cam sensor in which the light is transmitted and shielded by the lift cam shaft shielding plate  62 , and the numeral  71  designates a sheet passing guide spring which biases the sheet passing guide  70  toward the predetermined direction. 
   The pinch roller release mechanism, the PE sensor lever release mechanism, the pinch roller spring pressure adjustment mechanism, and the sheet passing guide up-and-down mechanism are operated by the rotation of the lift cam shaft  58 . In the mechanisms of the embodiment, the pinch roller holder pressing cam  59 , the pinch roller spring pressing cam  60 , the PE sensor lever pressing cam  61 , and the sheet passing guide pressing cam  65  are fixed to the lift cam shaft  58 , so that each cam is formed to be operated in synchronization with one revolution of the lift cam shaft  58 . At this point, the initial angle and the one revolution of the lift cam shaft  58  are recognized in such a manner that the lift cam shielding plate  62  transmits and shields the light of the lift cam sensor  69 . The effect of the invention is not limited to the above configuration, and it is also possible to use the mechanism which independently drives each cam if necessary. 
   The operation of each mechanism will be described.  FIGS. 4A to 4C  are a partially side view showing schematically the operations of the pinch roller release mechanism and the pinch roller spring pressure adjustment mechanism.  FIG. 4A  shows the case in which the pinch roller holder pressing cam  59  is located in the initial position, the pinch roller  22  is pressed against the sheet transport roller  21 , and the pressure of the pinch roller spring  24  is in a standard state. The pinch roller holder  23  is journaled in a bearing portion of the chassis  10  by a pinch roller holder shaft  23   a , and the pinch roller holder  23  is formed to be rocked over the range of the predetermined angle. The pinch roller  22  is journaled in one end of the pinch roller holder  23 , an area abutting on the pinch roller holder pressing cam  59  is provided in the other end of the pinch roller holder  23 . 
   In  FIG. 4A , the pinch roller spring  24  is a helical torsion coil spring in which one end of the pinch roller spring  24  is caused to abut on the pinch roller  22  side as a power point, the other end is supported by the pinch roller spring pressing cam  60 , an intermediate part is supported by a support portion of the chassis  10 . The pinch roller  22  is pressed against the sheet transport roller  21  with the predetermined pressure in the above-described manner. At this point, when the rotation driving mechanism of the sheet transport roller  21  is operated, the recording paper which is supported while sandwiched by the nip portion of the sheet transport roller  21  and the pinch roller  22  can be transported. 
     FIG. 4B  shows the case in which the pinch roller  22  is released and the pinch roller spring  24  releases the force. Namely, when the lift cam shaft  58  is rotated in the direction of an arrow a in  FIGS. 4A to 4C , the pinch roller holder pressing cam  59  abuts on the pinch roller holder  23  to gradually rotate the pinch roller holder  23  in the direction of an arrow b in  FIGS. 4A to 4C , which allows the pinch roller  22  to be released from the sheet transport roller  21 . In the state shown in  FIG. 4B , the abutment surface of the pinch roller spring pressing cam  60  on the pinch roller spring  24  becomes a smaller diameter portion, and a twist angle θ 2  of the pinch roller spring  24  is larger than a twist angle θ 1  in  FIG. 4A , so that the spring weight is decreased and the weight is not substantially applied to the pinch roller holder  23 . Therefore, the stress is not substantially applied to the pinch roller holder  23 . In this state, a predetermined gap H is formed between the sheet transport roller  21  and the pinch roller  22 , even if the recording paper is roughly guided, the front end of the recording paper can be easily inserted into the nip portion. 
     FIG. 4C  shows the light contact state in which, similarly to  FIG. 4A , the pinch roller  22  is pressed against the sheet transport roller  21  but the pressing force is weak. In  FIG. 4C , When the lift cam shaft  58  is further rotated in the direction of the arrow a in  FIG. 4 , the abutment between the pinch roller holder pressing cam  59  and the pinch roller holder  23  is gradually released, and the pinch roller holder  23  is rotated in the direction of an arrow c in  FIG. 4  to return to the original state. The abutment surface of the pinch roller spring pressing cam  60  on the pinch roller spring  24  has an intermediate diameter between the case shown in  FIG. 4A  and the case shown in  FIG. 4B . 
   Accordingly, a twist angle θ 3  of the pinch roller spring  24  is slightly smaller than the twist angle θ 1  in  FIG. 4A , so that the force pressing the pinch roller  22  against the sheet transport roller  21  is slightly decreased. According to the above configuration, in the case where the thick recording paper is supported while sandwiched between the sheet transport roller  21  and the pinch roller  22 , the twist angle of the pinch roller spring  24  becomes larger than normal, which allows the generation weight to the pinch roller holder  23  to be prevented from increasing. Therefore, in the case of both the recording paper having the normal thickness and the thick recording paper, rotational load caused by shaft loss of the sheet transport roller  21  can be leveled. When the one revolution of the lift cam shaft  58  is performed through the above-described states, the pinch roller release mechanism and the pinch roller spring pressure adjustment mechanism return to the state shown in  FIG. 4A  to become the standard state. 
     FIGS. 5A and 5B  show the partially side view showing schematically the operation of the PE sensor lever up-and-down mechanism.  FIG. 5A  shows the case in which the PE sensor lever pressing cam  61  is located in the initial position and the PE sensor lever (paper detecting lever)  66  is free. The PE sensor lever  66  is journaled by supporting a PE sensor lever shaft  66   a  with the bearing portion of the chassis  10 . In the state shown in  FIG. 5A , the PE sensor lever  66  is biased to the position shown in  FIG. 5A  by the action of the PE sensor lever spring  68 , and the shielding plate portion of the PE sensor lever  66  shields the PE sensor  67 . When the recording paper passes through the region of the PE sensor lever  66 , the PE sensor lever  66  is rotated in the clockwise direction in  FIGS. 5A and 5B  to become the state in which the PE sensor lever  66  transmits the light of the PE sensor  67 , which enables the PE sensor  67  to detect the presence of the recording paper. The front end and the rear end of the recording paper can be sensed by the light shielding state or the light transmission state. 
     FIG. 5B  is the partially side view showing schematically the state in which the PE sensor lever  66  as the paper detecting lever is locked. In  FIG. 5B , When the PE sensor lever pressing cam  61  is rotated in the direction of an arrow a, a can follower portion of the PE sensor lever  66  is raised, and the PE sensor lever  66  is rotated in the direction of an arrow b. In this state, the paper detecting portion of the PE sensor lever  66  is hidden in the inside of the pinch roller holder  23 . Even if the recording paper passes through, the recording paper never abuts on the PE sensor lever  66 . Therefore, in this state, even if the recording paper is transported in the direction of the arrow b, the recording paper never collides with the PE sensor lever  66  to become the jam state. 
     FIGS. 6A and 6B  are the partially side view showing schematically the operation of the sheet passing guide up-and-down mechanism.  FIG. 6A  shows the case in which the sheet passing guide  70  as the guide member is in an up-state. In  FIG. 6A , the sheet passing guide  70  is usually biased toward the direction in which the sheet passing guide  70  is raised by the sheet passing guide spring  71 , the sheet passing guide  70  abuts on a stopper (not shown) to determined the position (raised position, up-position). In the case where the recording paper supplied from the main ASF passes through, the sheet passing guide  70  is held at the up-state by the action of the sheet passing guide spring  71  as the elastic member. However, in the case where the force larger than normal acts on the sheet passing guide  70 , the sheet passing guide  70  is configured so as to be able to be lowered (become a down-state) against the biasing force of the sheet passing guide spring  71 . 
     FIG. 6B  shows the case in which the sheet passing guide  70  is in the down-state. In  FIG. 6B , when the sheet passing guide pressing cam  65  fixed to the lift cam shaft  58  is rotated in the direction of an arrow a in  FIGS. 6A and 6B , the sheet passing guide pressing cam  65  abuts on the sheet passing guide follower portion  70   a  which is a part of the sheet passing guide  70  to gradually press the sheet passing guide follower portion  70   a . This allows the sheet passing guide  70  to be rotated in the direction of an arrow b in  FIGS. 6A and 6B  to be lowered against the biasing force of the sheet passing guide spring  71 . In this state, a portion which faces the sheet passing path of the sheet passing guide  70  substantially becomes even, and the sheet passing path substantially completely becomes straight. Therefore, when the recording paper is transported toward the direction of the arrow b in  FIG. 2  by the sheet transport roller  21 , the recording paper is horizontally transported, and a part where the recording has been already performed on the surface of the recording paper is never pressed in the upward direction of the sheet passing path. 
     FIG. 7  is the schematic perspective view showing the carriage up-and-down mechanism. In  FIG. 7 , the numeral  14   a  designates a right guide shaft cam attached to the guide shaft  14 , the numeral  14   b  designates a left guide shaft cam attached to the guide shaft  14 , and the numeral  53  designates a cam idler gear which integrally connects the lift cam gear  52  and the right guide shaft cam  14   a . As shown in  FIG. 1 , the guide shaft  14  is supported by the both side surfaces of the chassis  10 , and the guide shaft  14  is fitted in a guide long hole in vertical direction (not shown), so that the guide shaft  14  can be freely moved in the direction of an arrow Z in FIG.  7  but the movement in the directions of an arrow X and an arrow Y in  FIG. 7  is controlled. 
   In the configuration of the mechanism shown in  FIG. 7 , the guide shaft  14  is usually biased in the downward direction (direction opposite to the arrow Z) by the guide shaft spring  74 . When the cam idler gear  53  is rotated, the right guide shaft cam  14   a  and the left guide shaft cam  14   b  abut on the guide oblique surface  56 , which allows the guide shaft  14  to move in the vertical direction while the guide shaft is rotated. 
     FIGS. 8A to 8C  show the partially side view showing schematically the operation of the carriage up-and-down mechanism.  FIG. 8A  shows the case in which the carriage  13  is located in a first carriage position which is of the standard position. In this state, the guide shaft  14  is positioned by abutting the lowermost of the guide long hole  57  in the chassis  10 , and the guide shaft cam  14   a  does not cone into contact with the guide oblique surface  56 . 
     FIG. 8B  shows the state in which the carriage  13  is moved to a second carriage position slightly higher than the first carriage position. When the lift cam gear  52  fixed to the lift cam shaft  58  is rotated by the rotation of the lift cam shaft  58 , a guide shaft cam  14   c  is rotated through the cam idler gear  53  engaging the lift cam gear  52 . The carriage  13  which is guided and supported by the guide shaft  14  is moved from the first carriage position to the second carriage position. At this point, when the number of teeth of the lift cam gear  52  is set to the same number of teeth of the guide shaft cam gear  14   c , the lift cam shaft  58  and the guide shaft  14  are rotated by the substantially same angle in the same direction. The reason why the lift cam shaft  58  and the guide shaft  14  are not rotated by just the same angle is that, while the rotational shafts of the lift cam gear  52  and the cam idler gear  53  are fixed, the distance between the gears in the guide shaft cam gear  14   c  is fluctuated by the vertical movement of the guide shaft  14  itself which is of the rotational shaft. 
   When the lift cam shaft  58  is rotated in the direction of an arrow a in  FIGS. 8A to 8C , the guide shaft  14  is rotated in the direction of an arrow b in  FIGS. 8A to 8C . The guide shaft cams  14   a  and  14   b  abut on the fixed guide oblique surface  56  by the rotation of the guide shaft  14 . As described above, since the moving direction of the guide shaft  14  is controlled only in the vertical direction of the guide long hole  57  in the chassis  10 , the guide shaft  14  is moved to the second carriage position. In the case where the deformation of the recording paper is large and the recording paper collides with the recording head  11  in the first carriage position, it is preferable to set the carriage  13  to the second carriage position. 
     FIG. 8C  shows the case in which the carriage  13  is located in the highest third carriage position. When the lift cam shaft  58  is further rotated from the second carriage position, radiuses of cam surfaces of the guide shaft cams  14   a  and  14   b  are increased, and the guide shaft  14  is moved to the further higher position. The third carriage position is preferable to the case in which the thick recording paper is used. The details of the five mechanisms, namely the pinch roller release mechanism, the PE sensor lever release mechanism, the pinch roller spring pressure adjustment mechanism, the sheet passing guide up-and-down mechanism, and the carriage up-and-down mechanism were described above. 
   Then, the driving mechanism of the lift cam shaft  58  will be described below.  FIG. 9  shows the schematic perspective view showing the driving mechanism of the lift cam shaft. In the embodiment, the driving source of the lift cam shaft  58  is the ASF motor  46  which also drives the main ASF  37 . The proper control of the rotating direction and the amount of rotation of the ASF motor  46  can operate the main ASF  37  or the lift cam shaft  58 . In  FIG. 9 , the numeral  46  designates the ASF motor which is of the driving source (upper half of the ASF motor is cut in order to show the gears), the numeral  47  designates an ASF pendulum arm which is located in the next stage of the gear attached to the ASF motor  46 , the numeral  48  designates an ASF sun gear which is attached to the center of the ASF pendulum arm  47 , the numeral  49  designates an ASF planet gear which is attached to the end portion of the ASF pendulum arm  47  to engage the ASF sun gear  48 , the numeral  63  designates a pendulum lock cam fixed to the lift cam shaft  58 , and the numeral  64  designates a pendulum lock lever which acts to the pendulum lock cam  63  to be rocked. 
   As described above, the driving force transfer direction is determined by the rotating direction of the ASF motor  46 . In order to operate the lift cam shaft  58 , the ASF motor  46  is rotated in the direction of an arrow a in  FIG. 9 . Then, the gear attached to the ASF motor  46  rotates the ASF sun gear  48 . Because the ASF sun gear  48  and the ASF pendulum arm  47  rotatably engaged with each other with predetermined frictional force, the ASF pendulum arm  47  swings to the rotating direction (direction of an arrow b in  FIG. 9 ) of the ASF sun gear  48 . Then, the ASG planet gear  49  engages the lift input gear  50  of the next stage. This allows the driving force of the ASF motor  46  to be transferred to the lift cam gear  52  through the lift reduction gear train  51 . At this point, the driving force to the gear train driving the main ASF  37  as the automatic sheet supply unit is cut, because the ASF pendulum arm  47  swings to the direction of the arrow b. 
   On the contrary, in the case where the ASF motor  46  drives the main ASF  37  of the automatic sheet supply unit, the ASF pendulum arm  47  swings to the direction opposite to the arrow b in  FIG. 9  by rotating the ASF motor  46  in the direction opposite to the arrow a in  FIG. 9 . Therefore, the engagement between the ASF planet gear  49  and the lift input gear  50  is released, and the other ASF planet gear  49  provided in the ASF pendulum arm  47  engages the gear train on the main ASF  37  side to drive the main ASF  37 . In the embodiment, a stepping motor is used as the ASF motor  46  and the stepping motor is controlled by open loop. Needless to say, it is also possible that an encoder is used in the DC motor to drive the ASF motor  46  by closed loop control. 
   In the case where the planet gear mechanism is used for the driving force transfer, when the load of the driven side becomes negative, the pendulum lock lever  64  is moved to remove the engagement of the gears and the phase of the driven side goes ahead of the driving source, and there is a possibility that the so-called ahead rotation occurs. In order to prevent the ahead rotation, the pendulum lock cam  63  and the pendulum lock lever  64  are provided in the embodiment. In the case where the lift cam shaft  58  is in the predetermined angle range, the pendulum lock lever  64  is rocked in the direction of an arrow c in  FIG. 9  by the cam surface shape of the pendulum lock cam  63 , and the pendulum lock lever  64  engages the ASF pendulum arm  47  to fix the main ASF  37  so that the main ASF  37  does not return to the driving side. Therefore, the ASF planet gear  49  always engages the lift input gear  50 , so that the ASG motor  46  and the lift cam shaft  58  are always synchronously rotated. 
   When the pendulum lock cam  63  return to the predetermined angle range, the pendulum lock lever  64  returns to the direction opposite to the arrow c in  FIG. 9 , and the lock of the ASF pendulum arm  47  is released to return to the state in which the drive can be transferred to the main ASF side when the ASF motor  46  is reversely rotated. The release of the pinch roller  22 , the lock of the PE sensor lever  66 , pressure adjustment of the pinch roller spring  24 , the vertical operation of the sheet passing guide  70 , and the vertical operation of the carriage  13  can be performed by the driving mechanism of the lift cam shaft  58  described above. The above five movable mechanisms are collectively called a lift mechanism. 
   How these five movable mechanisms (lift mechanism) cooperate with one another to operate will be described below.  FIGS. 10A to 10D  are the schematic partially side view showing the operations of the carriage  13 , pinch roller  22 , the PE sensor lever  66 , and the sheet passing guide  70 .  FIG. 10A  shows the case in which the lift mechanism is located in a first position. In this state, the pinch roller  22  is pressed against the sheet transport roller  21 , the PE sensor lever  66  is free, the pinch roller spring  24  ( FIG. 4 ) is pressed with the normal pressure, the sheet passing guide  70  is in the up-state, and the carriage  13  is located in the first carriage position. 
   The state shown in  FIG. 10A  is the position utilized for the recording operation in which the normal recording paper is used or for the registration after the inversion of the recording paper in the automatic both-side unit  2 . The carriage  13  is movably guided and supported along the guide shaft  14 , and the carriage  13  is configured to be vertically moved by moving vertically the guide shaft  14  along the guide long hole  57  made in the chassis  10 . 
     FIG. 10B  shows the case in which the lift mechanism is located in a second position. In this state, the pinch roller  22  is pressed against the sheet transport roller  21 , the PE sensor lever  66  is free, the pinch roller spring  24  ( FIG. 4 ) is pressed with the normal pressure, the sheet passing guide  70  is in the up-state, and the carriage  13  is located in the second carriage position. The second position of the lift mechanism differs from the first position of the lift mechanism only in the height position of the carriage  13 . The state shown in  FIG. 10B  is the position utilized in order to eliminate scraping between the recording paper and the recording head  11  due to the large deformation of the recording paper or to use the slightly thick recording paper. 
     FIG. 10C  shows the case in which the lift mechanism is located in a third position. In this state, the pinch roller  22  is released from the sheet transport roller  21  to make the predetermined gap, the PE sensor lever  66  is retracted upward to be locked, the pressing force of the pinch roller spring  24  ( FIG. 4 ) is weakened, the sheet passing guide  70  is lowered, and the carriage  13  is located in the highest third carriage position. Compared with the second position of the lift mechanism, all the states are changed, and the sheet passing path is opened straight, the recording paper can be retracted. The state shown in  FIG. 10C  is the position utilized in order to transport the recording paper toward the direction of the arrow b in  FIG. 2  after the front surface recording of the recording paper is terminated or to insert the thick recording paper. 
     FIG. 10D  shows the case in which the lift mechanism is located in a fourth position. In this state, the pinch roller  22  is pressed against the sheet transport roller  21 , the PE sensor lever  66  is retracted upward to be locked, the pinch roller spring  24  ( FIG. 4 ) is pressed with slightly weak pressure, the sheet passing guide  70  is lowered, and the carriage  13  is located in the highest third carriage position. Compared with the third position of the lift mechanism, the pinch roller  22  is returned to the state in which the pinch roller  22  is pressed against the sheet transport roller  21  and the pinch roller spring  24  is changed so as to be pressed with slightly weak pressure. The state shown in  FIG. 10D  is the position utilized for the case in which the recording paper is transported toward the automatic both-side unit  2  after re-retracting the recording paper during the automatic both-side recording or the case in which the recording is performed to the thick recording paper. 
   In the embodiment, in consideration of the operation of the recording apparatus, the mechanism is simplified by restricting the four positions of the lift mechanism shown in  FIGS. 10A to 10D . Namely, the position is changed by circulating through the first position, the second position, the third position, the fourth position, and the first position. The invention is not limited to the above mechanism, and it is also possible that each mechanism element is configured to be properly independently moved. The pressure adjustment mechanism of the pinch roller spring  24  is not always necessary. In the case where the pinch roller holder  23  has sufficiently high rigidity, or in the case where the fluctuation in load of the LF motor  26  does not become problematic, the pressure adjustment mechanism of the pinch roller spring  24  can be omitted. Even if the sheet passing guide  70  is even by the arrangement of the main ASF  37  and the like, it is possible that the up-and-down mechanism of the sheet passing guide  70  can be removed, as long as the mechanism can well guide the front end of the recording paper into the nip portion of the sheet transport roller  21 . 
     FIG. 11  is a timing chart showing the operational state of the lift mechanism. In order to further easily recognize the contents described in  FIGS. 4A to 10D , the description is performed again by the timing chart shown in  FIG. 11 . A horizontal axis of  FIG. 11  shows the angle of the lift cam shaft  58  in the range of 360 degrees, a longitudinal axis shows the position of each mechanism element. In  FIG. 11 , the angle of the lift cam shaft  58  is detected with the lift cam sensor  69  ( FIG. 3 ) by operating synchronously the lift cam shaft  58  and the guide shaft  14  and the rotational angle of the ASF motor  46  ( FIG. 21 ) is controlled, which allows the plurality of mechanisms to be simultaneously operated. Up to this point the operation of the lift mechanism was described. 
     FIGS. 12A to 12C  are the schematic side view for explaining the process in which the recording paper is brought into the nip portion of the sheet transport roller  21  again after the recording is performed to the front surface of the recording paper. The automatic both-side recording of the recording paper will be described below referring to  FIGS. 12A to 12C .  FIG. 12A  shows the state in which the first sheet discharge roller  30 , the first star gear train  32 , the second sheet discharge roller  31 , and the second star gear train  33  support recording paper  4  while sandwiching the recording paper  4  after the recording is performed to the front surface of recording paper  4 . The first star gear train  32  and the second star gear train  33  are formed by the rotating body which the corresponding sheet discharge roller is pressed against to be rotated. At this point, the lift mechanism is in state of the first position or the second position. When the recording paper  4  proceeds to this state to perform the recording, since the discharge port string (nozzle string) of the recording head  11  can face the recording paper  4  up to the rear end portion of the recording paper  4 , the recording can be performed without making the blank space of the lower end in the recording paper  4 . 
   Then, the lift mechanism is moved to the third position shown in  FIG. 12B  to make the predetermined amount of large gap between the pinch roller  22  and the sheet transport roller  21 . Therefore, the rear end of the recording paper  4  is easily brought into the gap, even if the rear end of the recording paper  4  undulates or curls. At this point, the pinch roller holder  23  and the carriage  13  do not interfere with each other, so that the carriage  13  is located at any position in the main scanning direction. 
     FIG. 12B  shows the state in which the recording paper  4  is transported toward the direction of the arrow b in  FIG. 2  (hereinafter the transport of the recording paper  4  in the direction of the arrow b in  FIG. 2  is referred to as back feed) by rotating the first sheet discharge roller  30  from the state shown in  FIG. 12A  in the direction of the arrow in  FIG. 12B  and the recording paper  4  is moved to the position below the pinch roller  22  to be stopped at the position. The reason why the recording paper  4  is stopped at this state is that the recording apparatus of the embodiment adopts the wet type of inkjet recording method. Namely, immediately after the recording operation, the surface on which the recording has been performed (the upper surface in  FIGS. 12A to 12C ) of the recording paper  4  is in the state in which the surface is wet. When the recording paper  4  is immediately pressed by the pinch roller  22  and the sheet transport roller  21 , the ink in transferred to the pinch roller  22 , and the ink on the pinch roller  22  is transferred to the recording paper  4  again. As a result, there is a possibility that an ink stain is generated in the recording paper  4 . 
   Whether the ink is transferred to the pinch roller  22  or not, i.e. whether the ink hit in the recording paper  4  is dried or not depends on various conditions. The conditions includes the kind of the recording paper, the kind of the using ink, a method of overstriking the using ink, an amount of using ink hit in the recording paper per unit area (for example, density per unit area of the recorded data), the ambient temperature in which the recording operation is performed, the ambient humidity in which the recording operation is performed, flow velocity of ambient gas in which the recording operation is performed, and the like. When the recording paper having an ink accepting layer to quickly introduce the ink inside the recording paper is used, the ink tends to be quickly dried. When the ink easy to penetrate the recording paper is used, the ink tends to be quickly dried. When the ink system in which the ink generating chemical reaction is used to be solidified by overstriking the ink on the surface of the recording paper is used, the ink tends to be quickly dried. 
   When the amount of ink hit in unit area is decreased, the ink tends to be quickly dried. When the ambient temperature in which the recording operation is performed is increased, the ink tends to be quickly dried. When the ambient humidity in which the recording operation is performed is decreased, the ink tends to be quickly dried. When the flow velocity of ambient gas in which the recording operation is performed is fastened, the ink tends to be quickly dried. Thus, since the required drying time is determined by the conditions, in the embodiment, the drying time required in performing the recording on the normal service condition (normal recording paper and normal recording operation environment) by using a predetermined ink system is specified to standard time, and the drying time is changed by the predictable condition. 
   The predictable condition is the amount of ink hit in unit area. Further, when ambient temperature detecting means, ambient humidity detecting means, ambient flow velocity detecting means, and the like are used with the amount of ink hit in unit area, dry standby time can be further predicted in detail. For example, the invention can adopt the method of determining the dry standby time in such a manner that the data received from the host apparatus  308  ( FIG. 21 ) is stored in RAM  312  ( FIG. 21 ), the amount of ink hit in unit area is calculated, and the maximum value of the amount of ink hit in unit area is compared to a predetermined threshold described in ROM  311  ( FIG. 21 ). Namely, in the case where the amount of ink hit in unit area is large, the dry standby time is lengthened. In the case where the amount of ink hit in unit area is small, the dry standby time is shortened. Therefore, the dry standby time depending on the recording pattern can be optimized. 
   The dry standby time also depends on whether the kind of the ink used for the recording is dye ink or pigment ink. In the case of the dye ink, the dry standby time is shortened because the dye ink is easy to dry. In the case of the pigment ink, the dry standby time is lengthened because the pigment ink is not easy to dry. When the ambient temperature is high, since the ink is easy to dry, the dry standby time is shortened. When the ambient temperature is low, since the ink is not easy to dry, the dry standby time is lengthened. When the ambient humidity is high, since the ink is not easy to dry, the dry standby time is lengthened. When the ambient humidity is low, since the ink is easy to dry, the dry standby time is shortened. In the case of the recording paper having the ink accepting layer on the surface of the recording paper to immediately bring the hit ink inside the recording paper, since the ink is easy to dry on the surface of the recording paper, the dry standby time is shortened. In the case of the recording paper having high water repellency, since the ink is not easy to dry, the dry standby time is lengthened. 
   It is possible that the recording paper  4  is held at the position shown in  FIG. 12A  for the dry standby. Further, it is more preferable that the back feed of recording paper  4  is performed to the position shown in  FIG. 12B  to be held for standby. This is because the recording paper  4  is deformed. When the recording is performed to the recording paper through the wet type inkjet process, sometimes fiber of the recording paper is swollen because the recording paper absorbs moisture and the recording paper stretches. Sometimes a part where the paper stretches and a part where the paper does not stretch are formed in the recording paper. In this case, surface irregularity of the recording paper is remarkably formed. The amount of irregularity depends on elapsed time when the recording paper starts to absorb the moisture, and the amount of irregularity is gradually increased to reach the predetermined amount of deformation. 
   When the amount of deformation of the end portion of the recording paper is increased as the time elapses, even if the pinch roller  22  is sufficiently released from the sheet transport roller  21 , there is a possibility that the end portion of the recording paper interferes with the pinch roller  22  to generate the jam. In order to prevent the jam, after the recording is terminated, the back feed is performed before the irregularity of the recording paper is increased, and the recording paper is moved to the position below the pinch roller  22 . For the above-described reason, the back feed of the rear end of the recording paper  4  is performed to the position shown in  FIG. 12B  to wait until a part of the recording paper where the recording has been performed is dried. The gap between the sheet transport roller  21  and the pinch roller  22  during the release is set to the amount of the deformation of the recording paper after the recording of the front surface. 
     FIG. 12C  shows the state in which the recording paper  4  is being transported toward the automatic both-side unit  2 . After a part of the recording paper  4  where the recording has been performed is dried and the ink is not transferred to the pinch roller  22  even if the pinch roller  22  is pressed against the recording paper  4 , the lift mechanism is moved to the fourth position as shown in  FIG. 10D , and the pinch roller  22  and the sheet transport roller  21  support the recording paper  4  while sandwiching the recording paper  4 . In this state, the sheet transport roller  21  is driven to perform the back feed of the recording paper  4 . At this point, since the PE sensor lever  66  is rotated upward to be locked, the front end of the PE sensor lever  66  does not break into the recording paper  4  nor scrape against the part where the recording has been performed to peel off. 
   Since the sheet passing guide  70  is in the down-state, the sheet passing surface substantially becomes even, and the recording paper  4  can be transported in straight line toward the automatic both-side unit  2 . In the embodiment, normally the sheet passing guide  70  is in the up-state. The invention is not limited to this state, and it is also possible that the normal state of the sheet passing guide  70  is set to the down-state. Namely, it is also possible that the sheet passing guide  70  is configured to be moved to the first position during the sheet supply operation from the main ASF  37 . The recording paper having high rigidity can be smoothly inserted by the above configuration when the recording paper having the high rigidity is inserted from the sheet discharge roller side. The termination of the front surface recording of the recording paper  4  to the transport process of the recording paper  4  to the automatic both-side unit  2  was described above. 
     FIG. 13  is the schematic sectional side view showing an installation state of the sheet passing path and the sheet transport roller in the automatic both-side unit  2  which is of the paper inversion unit or the automatic inversion unit. A recording paper transport mode inside the automatic both-side unit  2  will be described referring to  FIG. 13 . In  FIG. 13 , the numeral  101  designates a both-side unit frame constituting a part of a structure of the automatic both-side unit  2  and a sheet transport path, the numeral  102  designates an inner guide which is fixed to the inside of the both-side unit frame  101  and constitutes a part of the sheet transport path, the numeral  103  designates a rear cover which is openably and closably arranged at the back of the both-side unit frame  101  and constitutes a part of the sheet transport path, the numeral  105  designates a changeover flap spring biasing the changeover flap  104  toward the predetermined direction, the numeral  107  designates an outlet flap spring biasing the outlet flap  106  toward the predetermined direction, the numeral  110  designates a both-side roller rubber A which is a rubber portion of the both-side roller A  108 , and the numeral  111  designates a both-side roller rubber B which is a rubber portion of the both-side roller B  109 . 
   When the recording paper  4  is transported to the automatic both-side unit  2  from the state shown in  FIG. 12C , the introduction path can be clearly determined in only one, because the outlet flap  106  is biased to the position shown in  FIG. 13  by the action of the outlet flap spring  107 . Therefore, the recording paper  4  proceeds toward the direction of an arrow a. In the case where the recording paper  4  abuts on the changeover flap  104  which is of the movable flap and the normal both-side recording can be performed to the recording paper  4 , because the load of the changeover flap spring  105  is set so that the changeover flap  104  is not rotated, the recording paper  4  proceeds along the sheet passing path between the changeover flap  104  and the both-side unit frame  101 . Then, the recorded surface (front surface) of the recording paper  4  abuts on the both-side roller rubber B  111  of the both-side roller B  109 , and the recording paper  4  is supported by the both-side roller B  109  and the both-side pinch roller B  113  while sandwiched in the direction in which the unrecorded surface (reverse surface) abuts on the both-side pinch roller B  113  made of polymer resin having high lubricity. 
   At this point, the circumferential speeds of the both-side roller A  108 , the both-side roller B  109 , and the sheet transport roller  21  are set by the driving mechanism described later so that the both-side roller A  108 , the both-side roller B  109 , and the sheet transport roller  21  are substantially rotated at the same speed, so that the recording paper  4  is transported without generating a slide between the recording paper  4  and the both-side roller B  109 . Since circumferential speeds of these rollers are substantially the same, the recording paper  4  is not loosened or the recording paper  4  is not tensioned. When the proceeding direction of the recording paper  4  is changed by the both-side roller B  109 , the recording paper  4  proceeds along the rear cover  103 , and similarly the recording paper  4  is supported by the both-side roller rubber A  110  of the both-side roller A  108  and the both-side pinch roller A  112  while sandwiched. 
   The proceeding direction of the recording paper  4  is changed by the both-side roller A  108  again, and the recording paper  4  is transported toward the direction of an arrow b in  FIG. 13 . The both-side roller A  108  the both-side roller B  109  constitutes the reverse roller for inverting the surfaces of the recording paper  4  or reversing the transport direction of the recording paper  4 . When the recording paper  4  directly proceeds, the front end of the recording paper  4  abuts on the outlet flap  106 . Because the outlet flap  106  is biased by the outlet flap spring  107  having the very weak load, the recording paper  4  pushes away the outlet flap  106  to go out from the automatic both-side unit  2 . Because the sheet passing path length in the automatic both-side unit  2  is set so that the rear end of the recording paper  4  passes through below the outlet flap  106  when the front end of the recording paper  4  goes out from the outlet flap  106 , the front end and the rear end of the recording paper  4  never scrape with each other. 
   Although the detail flow chart is described later, the length of the recording paper  4  can be measure by the PE sensor lever  66  when the recording is performed to the surface of the recording paper  4 . Therefore, in the case where the recording paper which is shorter than the distance between the sheet transport roller  21  and the both-side roller B  109  or the distance between the both-side roller A  108  and the sheet transport roller  21  or the recording paper which is longer than the distance around the sheet passing path from the outlet flap  106  to the outlet flap  106  is inserted, a warning is given at a stage in which the recording of the surface is terminated, and the recording paper  4  is discharged without transporting the recording paper  4  to the automatic both-side unit  2 . 
   The reason why the recorded surface of the recording paper  4  is transported while the recorded surface faces the both-side roller rubber A  110  or the both-side roller rubber B  111  will be described. The both-side roller rubber A  110  and the both-side roller rubber B  111  are the driving side, and the both-side pinch roller A  112  and the both-side pinch roller B  113  are the coupled driving side, so that the recording paper  4  is transported while following the driving roller side, and the coupled driving side is rotated by the friction force between the recording paper  4  and the roller. At this point, it is necessary that the shaft loss of the rotating shaft which journal the both-side pinch roller A  112  and the both-side pinch roller B  113  is sufficient small. However, when the shaft loss is increased by some kind of a cause, there is possibility that the slide is generated between the recording paper  4  and the both-side pinch roller A  112  or the both-side pinch roller B  113 . The region where the recording is performed in the recording paper  4  is dried to a degree that the ink is not transferred by abutting on the roller. However, when the recording paper  4  is scraped, there is also a possibility that the ink is removed from the surface of the recording paper  4 . 
   When the slide is generated between the recorded surface of the recording paper  4  and the both-side pinch roller A  112  or the both-side pinch roller B  113  while the recorded surface is in contact with the rollers, there is a possibility that the ink is removed from the recorded surface of the recording paper  4 . In order to prevent the ink removal, in the embodiment, the driving side member is arranged so as to abut on the recorded surface (front surface) side, and the coupled driving member is arranged so as to abut on the unrecorded surface (reverse surface) side. The following reason can be also cited as another reason on the arrangement. Namely, since the both-side roller A  108  or the both-side roller B  109  on the driving side has the restriction of a rending radius of the recording paper  4 , the diameter can not be formed lower than a certain degree. However, since the diameters of the both-side pinch roller A  112  and the both-side pinch roller B  113  can be formed in the dimension smaller than the both-side roller A  108  or the both-side roller B  109 , in order to design the compact automatic both-side unit  2 , the diameter of the both-side pinch roller A  112  or the both-side pinch roller B  113  is often designed in the small dimension. 
   Basically the ink is not transferred from the recorded surface of the recording paper  4  to the roller side. However, sometimes the very small amount of ink is transferred and the roller abutting on the recorded surface becomes dirty with the ink. In the case of the roller having the smaller diameter, contact frequency of an outer periphery of the roller with the recording paper is increased, and the soiling speed is fastened compared with the roller having the larger diameter, so that the roller having the smaller diameter is disadvantageous to the soil. In the embodiment, from the viewpoints of miniaturization of the apparatus and the soil of the roller, the both-side roller A  108  and the both-side roller B  109  which have the larger diameter are arranged on the side where the roller abuts on the recorded surface (front surface) of the recording paper. 
   Further, the following reason can be also cited as another reason on the arrangement. Namely, in the case where the pair of rollers in which one roller is driven supports the recording paper while sandwiching the recording paper and transports the recording paper, in order to ensure the accurate amount of transport, in many cases, the roller on the driving side is made of the material having a high friction coefficient, the roller on the coupled driving side is made of the material having the low friction coefficient, and either the roller on the driving side or the roller on the coupled driving side is made of the elastic material to obtain the large area of the nip portion (nip area). Usually the roller on the driving side is made of the rubber materials (rubber-like elastic body). The rubber materials obtain the high friction coefficient at relatively low cost and have the high elasticity. Means in which polishing is performed onto the surface made of rubber including elastomers to intentionally make the minute irregularity of polishing marks is often used in order to increase transport force. In this case, the coupled driving side is usually made of the polymer resin having the relatively small friction coefficient. 
   In the case where the surface of the rubber materials having the minute irregularity and the surface of the polymer resin having the flat and smooth surface are compared, the soil of the ink adheres to both the surfaces of the rollers when the surfaces abut on the recorded surface of the recording paper. In the surface of the rubber materials having the minute irregularity, the small mount of soil is transferred to the recording paper because the minute irregularity holds the soil. On the other hand, in the surface of the polymer resin having the flat and smooth surface, the soil is removed from the surface of the polymer resin to transfer the recording paper again. Therefore, it is advantageous that the rubber materials abut on the recorded surface of the recording paper. In the embodiment, the roller made of the rubber materials is arranged on the side which abuts on the recorded surface (front surface) of the recording paper and the roller made of the polymer resin materials is arranged on the side which abuts on the unrecorded surface (reverse surface) of the recording paper. Up to this point, the reverse operation for performing the both-side recording of the normal recording paper has described. 
   For the case in which the automatic both-side recording is not performed and the recording is performed to the recording medium having the high rigidity, the operation of the automatic both-side unit  2  will be described below. It is assumed that, for example, the recording medium having the high rigidity is the thick paper whose thickness ranges from 2 mm to 3 mm or the circular or irregular-shaped recording medium which is mounted on a predetermined tray. Since the recording medium has high rigidity, the recording paper can not be bent to a degree that the recording medium follows the diameter of the both-side roller of the automatic both-side unit  2  and the both-side recording can not be performed. However, there is a possibility that the recording is performed to the recording medium having the high rigidity while the automatic both-side unit  2  is attached to the recording apparatus. Since the sheet supply can not be also performed by utilizing the main ASF in the case of the recording medium having the high rigidity, the straight sheet passing path is used. This allows the recording medium to be supplied from the sheet discharge roller side toward the sheet transport roller  21  side. For this case, the operation of the automatic both-side unit  2  will be described below. 
     FIGS. 14A and 14B  are the schematic sectional side view for explaining the operation of the changeover flap  104 .  FIG. 14A  shows the case in which the automatic both-side recording is performed with the normal recording paper. In this case, the changeover flap spring  105  continues to bias the changeover flap  104  to the stopper against the pressing force of the recording paper  4 , so that the recording paper  4  is guided to the sheet passing path which reverses the recording paper  4 . 
     FIG. 14B  shows the state in which the recording medium having the high rigidity (including the recording paper) is used. When the recording medium  4  having the high rigidity is transported to the automatic both-side unit  2 , the recording medium passes through below the outlet flap  106  to abut on the changeover flap  104 . In the changeover flap spring  105 , the spring load is set to a degree that, when the recording medium having the high rigidity is inserted to press the changeover flap  104 , the changeover flap  104  is retracted by the pressing force of the recording medium, so that the changeover flap spring  105  is rotated in the counterclockwise direction (arrow direction) in  FIG. 14B  to be retracted as the recording medium having the high rigidity proceeds. Therefore, the recording medium having the high rigidity is guided to a retracting path  131  which is of a second sheet passing path provided between the both-side roller A  108  and the both-side roller B  109 . Because a hole (through hole, opening) is made in the region corresponding to the retracting path  131  of the rear cover  103 , even if the long recording medium having the high rigidity is used, the recording medium does not interfere the automatic both-side unit  2  to restrict the transport. 
   The invention is not limited to the configuration described above referring to  FIG. 14B . Namely, in realizing the invention, it is not always necessary to provide the retracting path  131  between the two upper and lower both-side rollers  108  and  109 , and the following configuration can be performed. FIG.  22  is the schematic sectional side view showing the automatic both-side unit in which the both-side roller having the large diameter is arranged above the substantially horizontal path. In  FIG. 22 , the changeover flap  104  is biased to the position shown in  FIG. 22  by the changeover flap spring (not shown), and the spring force (pressing force) of the changeover flap spring is set to the load so that the changeover flap  104  can be rotated when the recording medium having the high rigidity abuts on the changeover flap  104 . In  FIG. 22 , the components corresponding to those shown in  FIG. 13  and  FIGS. 14A and 14B  are indicated by the same reference numeral, and the detail description of the same component is omitted. 
   In the case of the recording paper having the low rigidity, the recording paper proceeds toward the direction of an arrow a by rotating the both-side roller A  108  in the direction of an arrow c in  FIG. 22 . In the case of the recording medium having the high rigidity, the recording medium pushes away the changeover flap  104  to proceed to the retracting path  131  in the direction of an arrow b in  FIG. 22 . Accordingly, even if the long recording medium having the high rigidity is used, the recording medium does not interfere with the automatic both-side unit to restrict the transport. In the automatic both-side unit of the invention, one-side recording can be performed to the recording medium (including the recording paper) which can not be bent due to the high rigidity without removing the automatic both-side unit. Up to this point, the automatic both-side unit  2  having the two sheet passing paths has described. 
   Then, the driving mechanism of the rollers in the automatic both-side unit  2  will be described.  FIG. 15  is the schematic sectional side view showing the configuration of the driving mechanism of the rollers in the automatic both-side unit  2  while the embodiment ( FIG. 1 ) of the recording apparatus to which the invention is applied is viewed from the side opposite to  FIG. 2 . In  FIG. 15 , the numeral  115  designates a both-side transfer gear train which transfers the power from the LF motor  26  to a both-side sun gear  116 , the both-side sun gear  116  is located in the center of a both-side pendulum arm  117 , the both-side pendulum arm  117  can rock about the both-side sun gear  116 , the numeral  118  designates a both-side planet gear A which is rotatably attached to the both-side pendulum arm  117  and engages the both-side sun gear  116 , and the numeral  119  designates a both-side planet gear B. 
   In  FIG. 15 , the numeral  120  designates a spiral grove gear engaging the both-side sun gear  116  through the idler, the numeral  121  designates a reverse rotation delay gear A engaging the both-side planet gear, the numeral  122  designates a reverse rotation delay gear B which is coaxial with to the reverse rotation delay gear A  121 , the numeral  123  designates a reverse rotation delay gear spring which gives the biasing force between the reverse rotation delay gear A  121  and the reverse rotation delay gear B  122 , the numeral  124  designates a both-side roller idler gear connecting the two both-side roller gears, the numeral  125  designates a both-side roller gear A fixed to the both-side roller A  108 , the numeral  126  designates a both-side roller gear B fixed to the both-side roller B  109 , the numeral  127  designates a stop arm which engages a groove of the spiral groove gear  120  to be rocked, the numeral  128  designates a stop arm spring which centers the stop arm  127 , and the numeral  132  designates a both-side pendulum arm spring attached to the both-side pendulum arm  117 . 
   As described above, in the embodiment, the driving force of the automatic both-side unit  2  is obtained from the LF motor  26  which drives the sheet transport roller  21 . According to the above configuration, when the sheet transport roller  21  and the both-side roller A  108  or the both-side roller B  109  cooperate to transport the recording paper, the start or stop timing and sheet transport speed are substantially completely synchronized with each other. The driving force from the LF motor  26  is transferred to the both-side sun gear  116  through the both-side transfer gear train  115 . The rockable both-side pendulum arm  117  is attached to the both-side sun gear  116 , and the both-side planet gear A  118  and the both-side planet gear B  119  are attached to the both-side pendulum arm  117 . 
   Since the appropriate amount of frictional force acts between the both-side sun gear  116  and the both-side pendulum arm  117 , the both-side pendulum arm  117  is rocked in accordance with the rotating direction of the both-side sun gear  116 . At this point, assuming that the direction in which the LF motor  26  is rotated in the direction in which the sheet transport roller  21  transports the recording paper toward the sheet discharge direction is set to the forward direction and the direction in which the sheet transport roller  21  transports the recording paper to the automatic both-side unit  2  side is set to the reverse direction, when the LF motor  26  is rotated in the forward direction, the both-side sun gear  116  is rotated in the direction of the arrow a in  FIG. 15 . The both-side pendulum arm  117  is also basically rocked in the direction of the arrow a in  FIG. 15  with the rotation of the both-side sun gear  116 . 
   Then, the both-side planet gear A  118  engages the both-side roller idler gear  124  to rotate the both-side roller idler gear  124 . As the both-side roller idler gear  124  is rotated, the both-side roller gear A  125  is rotated in the direction of the arrow c in  FIG. 15  and the both-side roller gear B  126  rotated in the direction of the arrow d in  FIG. 15 . The direction of the arrow c and the direction of the arrow d in  FIG. 15  are one in which the both-side roller A  108  and the both-side roller B  109  transport the recording paper in the automatic both-side unit  2  respectively. 
   When the LF motor is rotated in the reverse direction, the both-side sun gear  116  is rotated in the direction of the arrow b in  FIG. 15 . The both-side pendulum arm  117  is also basically rocked in the direction of the arrow b in  FIG. 15  with the rotation of the both-side sun gear  116 . Then, the both-side planet gear B  119  engages the reverse rotation delay gear A  121 . In the reverse rotation delay gear A  121  and the reverse rotation delay gear B  122 , a projection is protruded from a thrust surface opposite to each other. Assuming that the reverse rotation delay gear B  122  is fixed, when the reverse rotation delay gear A  121  is rotated one turn, the projections act as a clutch engaging each other. 
   Before the both-side planet gear B  119  engages the reverse rotation delay gear A  121 , the reverse rotation delay gear A  121  and the reverse rotation delay gear B  122  are biased toward the direction in which the projection are separated from each other by the reverse rotation delay gear spring  123 , so that the reverse rotation delay gear A B  122  starts the rotation after the reverse rotation delay gear A  121  is substantially rotated one turn from the start of the rotation. Thus, an interval from the start of the rotation in the reverse direction of the LF motor  26  to the start of the rotation of the reverse rotation delay gear B  122  becomes a delay interval when the both-side roller A  108  and the both-side roller B  109  are stopped. 
   When the reverse rotation delay gear B  122 , the both-side roller gear A  125  is rotated in the direction of the arrow c in  FIG. 15  through the both-side roller idler gear  124 , and the both-side roller gear B  126  is rotated in the direction of the arrow d in  FIG. 15  through the both-side roller idler gear  124 . These rotating directions are the same as the rotating direction when the LF motor  26  is rotated in the forward direction. The above described mechanism can always rotate the both-side roller A  108  and the both-side roller B  109  in the sheet transport direction independently of the rotating direction of the LF motor  26 . 
   The action of the spiral groove gear  120  will be described below. In the spiral groove gear  120 , a gear surface is formed on the outer periphery, and the cam is formed on one end face. The spiral groove having caterpillers in the innermost periphery and the outermost periphery is cut in the cam. In the embodiment, because the spiral groove gear  120  is directly connected to the both-side sun gear  116  through the idler gear, the spiral groove gear  120  is rotated in the same direction as the both-side sun gear  116  in synchronization with the both-side sun gear  116 . Because the groove of the spiral groove gear  120  engages a follower pin  127   a  which is of a part of the stop arm  127 , the stop arm  127  is rocked with the rotation of the spiral groove gear  120 . For example, when the spiral groove gear  120  is rotated in the direction of an arrow e in  FIG. 15 , the follower pin  127   a  is brought in the inner periphery, so that the stop arm  127  is rocked in the direction of an arrow g in  FIG. 15 . Even if the spiral groove gear  120  continues to rotate in the direction of the arrow e in  FIG. 15 , since the follower pin  127   a  enters the caterpiller of the innermost periphery, the stop arm  127  stops at the predetermined position. 
   On the contrary, when the spiral groove gear  120  is rotated in the direction of an arrow f in  FIG. 15 , the follower pin  127   a  is moved toward the outer periphery, so that the stop arm  127  is rocked in the direction of an arrow h in  FIG. 15 . When the spiral groove gear  120  continues to rotate in the direction of the arrow f in  FIG. 15 , the follower pin  127   a  enters the caterpiller of the outermost periphery, so that the stop arm  127  stops at the predetermined position. The stop arm spring  128  is attached to the stop arm  127  so that the follower pin  127   a  is smoothly moved from the caterpillers of the innermost periphery and the outermost periphery to the spiral groove, when the rotating direction of the spiral groove gear  120  is changed. The stop arm spring  128  centers the stop arm  127  with respect to the proximity in the center of the moving range of the stop arm  127 . 
   The stop arm  127  performing the above operation acts the both-side pendulum arm spring  132  attached to the both-side pendulum arm  117 . The both-side pendulum arm spring  132  attached to the both-side pendulum arm  117  is the elastic member expanded in the direction of the stop arm  127 . The front end of the both-side pendulum arm spring  132  is always located in the radial direction of the spiral groove gear  120  rather than the stop arm  127 . 
   When the LF motor is rotated in the forward direction, the following action is given by the above positional relationship. Namely, when the LF motor  26  is rotated in the reverse direction to transport the recording paper  4  to the automatic both-side unit  2  and inverts the recording paper  4  to return the recording paper  4  to the sheet transport roller  21 , the stop arm  127  is rotated in the caterpiller of the outermost periphery of the spiral groove gear  120 . Then, the stop arm  127  is moved toward the inner periphery of the spiral groove gear  120  while the recording of the reverse surface is performed by rotating the LF motor  26  in the forward direction. When the LF motor  26  is rotated in the forward direction, since the power transfer is performed by rocking the both-side pendulum arm  117  in the direction of the arrow a in  FIG. 15 , the stop arm  127  abuts on the both-side pendulum arm spring  132  on the way of the stop arm  127  toward the inner periphery. 
   When the LF motor  26  is further rotated in the forward direction, since the stop arm  127  is further moved to the inner periphery to elastically deform the both-side pendulum arm spring  132 , an attitude of the both-side pendulum arm  117  is determined by the balance among the force acting in the direction of an pressure angle in the case where the gears of the both-side planet gear A  118  and the both-side roller idler gear  124  engages with each other, the force rocking the both-side pendulum arm  117  in the direction of the arrow a in  FIG. 15 , and the force of repulsion of the both-side pendulum arm spring  132 . In the embodiment, since the force of repulsion of the both-side pendulum arm spring  132  is set small, even if the stop arm  127  is located in the caterpiller of the innermost periphery, the power transfer between the both-side planet gear A  118  and the both-side roller idler gear  124  can be continued only by elastically deforming the both-side pendulum arm spring  132 . 
   Even if the LF motor  26  becomes the stopped state while the rotation and stop are repeated by the intermittently driving, the tooth surfaces of the both-side planet gear A  118  and the both-side roller idler gear  124  still engage with each other, so that the engagement between the both-side planet gear A  118  and the both-side roller idler gear  124  is never disengaged. However, when the recording of the reverse surface of the recording paper  4  is terminated and the power transfer to the automatic both-side unit  2  becomes unnecessary, it is preferable to cut the power in order to reduce the load of the LF motor  26 . Therefore, the following means is performed in the case where the power transfer is cut. 
   The LF motor  26  is slightly rotated in the reverse direction, while the stop arm  127  enters the caterpiller of the innermost periphery and the both-side pendulum arm spring  132  is elastically deformed. Although the moment of the both-side pendulum arm  117  in the direction of the arrow b in  FIG. 15  by the repulsion of the both-side pendulum arm spring  132  is stopped by the engagement between the both-side planet gear A  118  and the both-side roller idler gear  124 , since the slight rotation of the LF motor  26  in the reverse direction gives the rotation which disengages the gears, the both-side pendulum arm  117  is rotated in the direction of the arrow b in  FIG. 15 . 
   Once the both-side pendulum arm  117  is rotated in the direction of the arrow b in  FIG. 15 , the both-side pendulum arm spring  132  elastically deformed returns to the original shape. Further, even if the LF motor  26  is rotated in the forward direction, the both-side pendulum arm spring  132  interferes with the stop arm  127 . Therefore, the both-side pendulum arm  117  can not be rocked until the both-side planet gear A  118  engages the both-side roller idler gear  124 . As a result, as long as the predetermined amount of rotation of the LF motor  26  in the reverse direction is not performed from this state, the driving force is not transferred to the both-side pendulum arm  117  and after stages in the automatic both-side unit  2 . In the driving up to the both-side pendulum arm  117 , since the gear train is merely rotated, the load applied to the LF motor is small, and the load is substantially equal to the load in the case where the automatic both-side unit  2  is not attached. 
   In the case where the LF motor  26  is rotated in the reverse direction from the state in which the stop arm  127  is located in the caterpiller of the innermost periphery, no action is exerted on the both-side pendulum arm spring  132  and the stop arm  127 , so that the power can be transferred to the reverse rotation delay gear A  121  as described above. Up to this point, the outline of the driving mechanism of the rollers in the automatic both-side unit  2  has been described. 
     FIGS. 16A to 16F  are the schematic sectional side view for explaining the operation of the driving mechanism of the rollers in the automatic both-side unit  2 , and  FIGS. 20A and 20B  are the flowcharts showing the operational sequence of the automatic both-side recording. The operation of the driving mechanism of the rollers in the automatic both-side unit  2  and the operation of the automatic both-side recording will be described in detail below referring to  FIGS. 16A to 16F  and the flow chart shown in  FIGS. 20A and 20B . In  FIGS. 16A to 16F ,  20 A and  20 B, when the automatic both-side recording is started, sheet supply of the recording paper  4  is performed in Step S 1 . For example, the recording paper  4  is supplied from the main ASF  37  toward the sheet transport roller  21 . In Step S 2 , the recording of the front surface is performed. This operation is similar to the case of the one-side recording. At this point, the state of the driving mechanism of the rollers is shown in  FIG. 16A . 
     FIG. 16A  shows the state in which the LF motor  26  is being rotated in the forward direction after the driving mechanism of the automatic both-side unit  2  is initialized. Namely,  FIG. 16A  shows the state in which the recording operation of the front surface is being performed during the automatic both-side recording, the state in which the normal recording operation is being performed while the automatic both-side recording is not used, or the like. Since the follower pin  127   a  of the stop arm  127  is located in the caterpiller of the innermost periphery of the spiral groove gear  120 , when the both-side pendulum arm  117  is rocked in the direction of the arrow a in  FIG. 15 , the both-side pendulum arm  117  abuts on the stop arm  127 , and the both-side pendulum arm  117  can not be rotated any more. Further, since the both-side planet gear A  118  can not engage the both-side roller idler gear  124 , the driving force from the LF motor  26  is not transferred to the both-side roller gear A  125  and the both-side roller gear B  126 . In this state, the both-side roller A  108  and the both-side roller B  109 , in which the shaft loss is generated by receiving the pressure of the both-side pinch roller A  112  or the both-side pinch roller B  113 , are not rotated, so that the load applied to the LF motor  26  is small. 
   In Step S 3 , it is confirmed whether the rear end of the recording paper can be detected by the PE sensor  67  or not at the point that the recording of the front surface is terminated. At this point, when the PE sensor detects the presence of the recording paper  4 , since the rear end of the front surface of the recording paper  4  can not be detected yet, in Step S 4 , the LF motor  26  is still rotated in the forward direction. The recording paper  4  is moved until the rear end of the front surface of the recording paper  4  passes through the PE sensor  66  to reach a position p 2  slightly in front of the PE sensor lever  67 . In Step S 5 , the length of the recording paper  4  is calculated from the amount of transport of the recording paper  4  since the PE sensor  66  detects the front end of the front surface of the recording paper  4  until the PE sensor  67  detects the rear end of the front surface of the recording paper  4 . 
   As described above, in the case where the length of the recording paper  4  is shorter than a predetermined length L 1 , because the recording paper  4  can not reach the roller between the sheet transport roller  21  and the both-side roller B  109  or between the both-side roller A  108  and the sheet transport roller  21 , it is necessary that the recording paper  4  having the length shorter than the predetermined length L 1  is removed from the automatic both-side recording operation. In the case where the length of the recording paper  4  is longer than a predetermined length L 2 , because the recorded surfaces of the recording paper  4  interfere with each other in the sheet passing path from the sheet transport roller  21  to the automatic both-side unit  2 , it is necessary that the recording paper  4  having the length longer than the predetermined length L 2  is removed from the automatic both-side recording operation. In the case where the judgment that the recording paper  4  is removed from the automatic both-side unit  2  is made by the condition of Step S 5 , the operational sequence proceeds to Step S 6 , and the LF motor  26  is rotated in the forward direction to directly remove the recording paper  4 . In the case where the length of the recording paper  4  meets the condition of Step S 5 , the operational sequence proceeds to Step S 7 , and the lift mechanism is set to the third position to release the pinch roller  22 . The release of the pinch roller  22  from the sheet transport roller  21  is performed after the first predetermined time or the third predetermined time elapsed since the surface recording is terminated. 
   In Step S 8 , it is confirmed whether the rear end of the front surface of the recording paper  4  has been already transported to the downstream side of a position p 1  near the pinch roller  22 . In the case where the rear end of the recording paper  4  has been already transported to the downstream side, in Step S 9 , the LF motor  26  is rotated in the reverse direction to perform the back feed of the recording paper  4  until the rear end of the front surface is moved to the position p 1  so that the recording paper  4  is supported by the sheet transport roller  21  and the pinch roller  22  while sandwiched when the pinch roller  22  is returned to the pressing state. At this point, the driving mechanism of the rollers is in the state shown in  FIG. 16B . From Step S 2  to Step S 8 , it is desirable that the operation is not stopped as much as possible and Step S 9  is performed before the recording paper  4  is deformed as described before. In the case where the rear end of the front surface is located on the upstream side of the position p  1 , when the pinch roller  22  is directly pressed, since the pinch roller  22  and the sheet transport roller  21  can securely support the recording paper while sandwiching the recording paper, the operational sequence proceeds to Step S 10 . 
     FIG. 16B  shows the state immediately after the rotation in the reverse direction of the LF motor  26  is started. Namely,  FIG. 16B  shows the case immediately after the back feed is started after the front surface recording of the automatic both-side recording is terminated, the case in which the LF motor  26  is rotated in the reverse direction in order to adjust the start position after the sheet supply from the main ASF  37 , or the like. There is no obstacle preventing the both-side pendulum arm  117  from rocking in the direction of the arrow b in  FIG. 15 , so that the both-side planet gear B  119  engages the reverse rotation delay gear A  121 . Although the reverse rotation delay gear A  121  starts the rotation with the engagement between the both-side planet gear B  119  and the reverse rotation delay gear A  121 , because the driving force is not transferred to reverse rotation delay gear B  122  until the reverse rotation delay gear A  121  is substantially rotated one turn, the both-side roller idler gear  124  is not rotated, and the both-side roller A  108  and the both-side roller B  109  are not operated. 
   Therefore, the load which the LF motor  26  is applied is still small in this state. The reason why this state is set is that the rotation of the both-side roller B  109  is not required until the front end of the recording paper  4  reaches the both-side roller B  109 , because there is a distance between the sheet transport roller  21  and the both-side roller B  109  when the back feed of the recording paper  4  is performed during the automatic both-side recording. Further, another reason is that the both-side roller A  108  or the both-side roller B  109  is not caused to be unnecessarily rotated during adjusting the start positioning the normal recording. 
   In Step S 10 , the time when the operational sequence stands by until the recorded ink on the front surface of the recording paper  4  is dried is provided. As described above, since the required drying time is varied dependent on the some factors, dry standby time t 1  (second predetermined time) can be set to a variable parameter. Specifically, the dry standby time t 1  is determined in consideration of the kind of the recording paper, the kind of the using ink, a method of overstriking the using ink, the amount of using ink hit in the recording paper per unit area, the ambient temperature, the ambient humidity, the flow velocity of ambient gas, and the like. The second predetermined time is longer than the first predetermined time. In Step S 11 , the lift mechanism is set to the fourth position. This allows the sheet transport roller  21  and the pinch roller  22  to support the recording paper  4  while sandwiching the recording paper  4  again. 
   In Step S 12 , dry standby time t 2  (fourth predetermined time) is provided. In the case where the dry standby time t 1  is performed in Step S 10 , it is also possible that the dry standby time t 2  is not used. It is also possible that the dry standby time t 2  is set to t 2 =0 and the operational sequence proceeds to the next step. For example, the dry standby time t 2  is used in the case where the recording operation is not performed in the rear end portion of the recording paper  4  and the blank space is present. In this case, there is no problem, even if the dry standby time t 1  is set to t 1 =0 in Step S 10  and the control is performed so that the pinch roller  22  is pressed against the blank space. However, when the back feed is immediately performed to transport the recording paper  4 , there is a possibility that the ink before drying is transferred to the pinch roller  22 , so that the dry standby time t 2  is used at this point. The fourth predetermined time is longer than the third predetermined time. 
   In Step S 13 , the LF motor  26  is rotated in the reverse direction to perform the back feed of the recording paper  4  by a predetermined amount x 1 . In Step S 13 , the recording paper  4  is transported to the automatic both-side unit  2  to invert the recording paper  4 . After Step S 13 , the front end of the reverse surface returns to the position slightly in front of the sheet transport roller  21 . The driving mechanism of the rollers up to Step S 13  is shown in  FIG. 16C . 
     FIG. 16C  shows the state in which the LF motor  26  further continues to rotate in the reverse direction. Namely,  FIG. 16C  shows the state in which the back feed of the recording paper  4  is performed to invert the recording paper  4  by the automatic both-side unit  2 . After the state shown in  FIG. 16B , when the reverse rotation delay gear A  121  is substantially rotated one turn, the projection protruded toward the thrust direction of the reverse rotation delay gear A  121  engages the projection of reverse rotation delay gear B  122  provided in opposite direction to the projection of reverse rotation delay gear A  121 , which allows the reverse rotation delay gear A  121  and the reverse rotation delay gear B  122  to be rotated in an integrated manner. When the reverse rotation delay gear B  122  starts to rotate, since the reverse rotation delay gear B  122  always engages the both-side roller idler gear  124 , the both-side roller idler gear  124 , the both-side roller gear A  125 , and the both-side roller gear B  126  are rotated. Therefore, the both-side roller A  108  is rotated in the direction of the arrow c in  FIG. 15 , and the both-side roller B  109  is rotated in the direction of the arrow d in  FIG. 15 . 
   Then, the so-called registration operation in the case where the nip portion of the sheet transport roller  21  and the pinch roller  22  supports the front end of the reverse surface while sandwiching the front end of the reverse surface will be described. In Step S 14 , the control is changed on the basis of whether the recording paper  4  is the paper having the low rigidity or the paper having the high rigidity. The judgment of the rigidity of the recording paper  4  can be performed by the kind of the recording paper which a user set by printer driver, or the judgment of the rigidity can be performed by the detecting means for measuring the thickness of the recording paper. The control is divided into the two kinds of the control. This is because behavior depends on the rigidity of the recording paper when the recording paper  4  is bent to form the loop. 
   At first, the case in which the thin recording paper has the relatively low rigidity will be described.  FIGS. 18A to 18C  are the schematic sectional side view showing the registration operation of the front end of the reverse surface in the case the thin recording paper is used. In  FIGS. 20A and 20B  and  FIGS. 18A to 18C , in Step S 13 , the sheet paper inversion transport of  FIGS. 18A to 18C  is performed by rotating the LF motor  26  in the reverse direction. After Step S 13 , the front end of the reverse surface of the recording paper returns to the proximity of the sheet passing guide  70 . In the case of the thin recording paper, the operational sequence proceeds to Step S 15 . In Step S 15 , the lift mechanism is operated to be moved to the first position. This allows the sheet passing guide  70  to be raised. 
     FIG. 18B  shows the state in which Step S 15  is terminated. AS described above, since the sheet transport roller  21  and the pinch roller  22  are arranged on the side of the first sheet discharge roller  30  while the center of the pinch roller  22  is offset relative to the center of the sheet transport roller  21 , the nip portion of the sheet transport roller  21  and the pinch roller  22  slightly has the angle relative to the substantially horizontal line in which the recording paper  4  is transported. The front end of the reverse surface of the recording paper  4  can be smoothly guided to the declined nip portion by returning the sheet transport guide  70  to the raised position before the registration operation. In Step S 16 , the LF motor  26  is rotated in the reverse direction to further transport the recording paper  4  toward the sheet transport roller  21 . In Step S 17 , the PE sensor  67  detects the front end of the reverse surface of the recording paper  4 . When the PE sensor  67  can detect the front end of the reverse surface, the operational sequence proceeds to Step S 18 . 
   In Step S 18 , the recording paper  4  is transported by a distance x 2  slightly longer than the distance between the detection position of the front end of the reverse surface by the PE sensor  67  and the sheet transport roller  21 . As a result, the front end of the reverse surface of the recording paper  4  reaches the nip portion of the sheet transport roller  21  and the pinch roller  22 , and a part of the recording paper  4  which is excessively transported is bent to form the loop.  FIG. 18C  shows the state in which Step S 18  is terminated. Although the gap of the sheet passing path in the height direction is decreased by setting the sheet passing guide  70 , since the recording paper  4  has the relatively low rigidity, the loop is easily formed to push the recording paper  4 . Therefore, the front end of the reverse surface of the recording paper  4  follows the nip portion of the sheet transport roller  21  and the pinch roller  22  to become parallel to the sheet transport roller  21 , and the registration operation is completed. In Step S 19 , the rotating direction of the LF motor  26  is changed to the rotation in the forward direction, the front end of the reverse surface of the recording paper  4  is supported by the nip portion while sandwiched, and the front end of the reverse surface is transported by a predetermined distance x 3  to complete the start of preparation of reverse surface recording. 
   Then, the case in which the thick recording paper (recording medium) has the relatively high rigidity will be described.  FIGS. 19A to 19C  are the schematic sectional side view showing the registration operation of the front end of the reverse surface in the case the thick recording paper is used.  FIG. 19A  shows the state on the way of Step S 13 , and  FIG. 19B  shows the state in which Step S 13  is terminated. In Step S 20 , the LF motor  26  is rotated in the reverse direction while the sheet passing guide  70  is left at the lowered position, and the recording paper  4  is transported by a distance x 4  slightly longer than the distance between the front end of the reverse surface of the recording paper  4  stopped at the position in Step S 13  and the nip of the sheet transport roller  21 . Therefore, similarly to the thin recording paper, the front end of the reverse surface of the recording paper  4  reaches the nip portion of the sheet transport roller  21  which is reversely rotated, and a part of the recording paper  4  which is compressed forms the loop, so that the front end of the reverse surface of the recording paper  4  becomes parallel to the sheet transport roller  21  and the registration operation is completed.  FIG. 19C  shows the state in which Step S 20  is terminated. 
   In Step S 21 , the rotating direction of the LF motor  26  is changed to the forward direction, the front end of the reverse surface of the recording paper  4  is supported by the nip portion while sandwiched, the front end of the reverse surface is transported by the predetermined distance x 3 , and the start of the reverse surface recording is prepared. In Step S 19  or Step S 21 , the LF motor  26  which has been rotated in the reverse direction changes the rotating direction to the forward direction. At this point, the both-side pendulum arm  117  is rocked in the direction of the arrow a in  FIG. 15 . Therefore, the engagement between the both-side planet gear B  119  and the reverse rotation delay gear A  121  is disengaged. The reverse rotation delay gear A  121  and the reverse rotation delay gear B  122  engage with each other by the projections during the rotation of the LF motor  26  in the reverse direction. At the same time, the reverse rotation delay gear spring  124  which is of the helical torsion coil spring sandwiched by the reverse rotation delay gear A  121  and the reverse rotation delay gear B  122  is compressed. The reverse rotation delay gear spring  124  is expand in such a manner that the reverse rotation delay gear A  121  becomes free, so that the reverse rotation delay gear A  121  is substantially rotated one turn to return to the initial state. 
   In Step S 22 , the lift mechanism is set to the first position, and the preparation of the start of the reverse surface recording is completed. In the case where the thick recording paper is used, the reason why the sheet passing guide  70  is set to the lowered position during the registration operation will be described below. In the case where the loop is formed in a manner shown in  FIG. 18C  similar to the thin recording paper, since the recording paper has the high rigidity, the recording paper  4  is transported along the pinch roller holder  23  before reaching the nip portion. Therefore, after the recording paper reaches the nip portion, even if the recording paper is further transported to try to generate the loop, since the space where the loop is generated has been already eliminated, the loop is not generated. Consequently, there is a possibility that the good registration operation is not performed. That is why the sheet passing guide  70  is set to the lowered position. 
   When the loop is not generated, sag is not formed in the recording paper  4  which is simultaneously nipped between the position at the both-side roller A  108  and the position at the sheet transport roller  21 . In the case where the mechanism such as the both-side pendulum arm  117  is used for the driving mechanism of the both-side rollers like the embodiment, the time when the both-side pendulum arm  117  is rocked becomes necessary during the interval from the reverse rotation of the LF motor  26  in Step S 21  to the forward rotation of the LF motor  26  in Step S 21 , and the both-side roller A  108  and the both-side roller B  109  are topped during the interval. 
   Since the sheet transport roller  21  is directly connected to the LF motor  26 , there is no stop interval. Consequently, contradiction is generated in the sheet transport speed. When the sag of the recording paper is generated, the contradiction of the sheet transport speed can be absorbed by taking the margin of the sag during Step S 21 . On the other hand, in the case of no sag, the contradiction of the sheet transport speed can not be absorbed, and the sheet transport roller  21  side forcedly tries to transport the recording paper. However, since rear portion of the recording paper  4  is supported by the both-side roller A  108  and the both-side pinch roller  112  while sandwiched, sometimes actually the recording paper  4  is not transported. This causes the amount of transport of the front end of the reverse surface of the recording paper  4  to go out of adjustment, and sometimes the blank space in the upper end portion of the reverse surface becomes shorter than the expected length. In order to solve the above problem, the gap in the height direction between the pinch roller holder  23  and the sheet transport roller  21  is sufficiently formed by lowering the sheet passing guide  70 , and the loop generation space is secured. Therefore, even in the thick recording paper having the relatively high rigidity is used, the good registration operation can be performed. 
   In Step S 23 , the reverse surface recording of the recording paper  4  is performed. In the most cases, the rear end of the reverse surface of the recording paper  4  is still supported by the both-side roller A  108  and the both-side pinch roller  112  while sandwiched. At this point, when the rotation of the both-side roller A  108  is stopped, since the load pulling the recording paper backward is generated, there is a possibility that the accuracy of the sheet transport is worsened. Therefore, in the embodiment, the driving of the both-side roller A  108  is configured so as to continue in at least the interval when the rear end of the reverse surface the recording paper  4  is supported by the both-side roller A  108  and the both-side pinch roller  112  while sandwiched. The driving mechanism of the both-side rollers is in the state shown in  FIG. 16D . 
     FIG. 16D  is the schematic sectional side view showing the operational state of the driving mechanism of the rollers in the automatic both-side unit  2  while the LF motor  26  is being rotated in the forward direction after the inversion operation of the recording paper. When the LF motor  26  changes the rotation from the state shown in  FIG. 16C  to the forward direction, the both-side pendulum arm is rocked in the direction of the arrow a in  FIG. 15 . At this point, the stop arm  127  is rocked in the direction of the arrow h in  FIG. 15 . Even if the both-side pendulum arm  117  is rocked in the direction of the arrow a in  FIG. 15 , the both-side pendulum arm spring  132  never abuts on the stop arm  127 , so that the both-side planet gear A  118  engages the both-side roller idler gear  124  to transfer the driving force. 
   When the rotation of the LF motor  26  in the forward direction is continued, the follower pin  127   a  is guided by the spiral groove gear  120  to be moved to the inner periphery, and the stop arm  127  is rocked in the direction of the arrow g in  FIG. 15 . On the way of rocking, the stop arm  127  abuts on the both-side pendulum arm spring  132  to deform the both-side pendulum arm spring  132 . The force rocking the both-side pendulum arm  117  in the direction of the arrow b in  FIG. 15  acts on the both-side pendulum arm  117  by reaction force generated by the deformation of the both-side pendulum arm spring  132 . However, because the engaging force between the tooth surfaces of the gears is stronger than the rocking force while the driving force is transferred between the both-side planet gear A  118  and the both-side roller idler gear  124 , the engagement between the both-side planet gear A  118  and the both-side roller idler gear  124  is not disengaged, and the driving is continued.  FIG. 16C  shows this state. 
   AS described above, even if the LF motor  26  is intermittently driven while the rotation and the stop are repeated, the tooth surfaces of the gears of the both-side planet gear A  118  and the both-side roller idler gear  124  engages with each other, so that the engagement between the both-side planet gear A  118  and the both-side roller idler gear  124  is never disengaged. Further, when the recording operation of the reverse surface of the recording paper  4  is continued and the LF motor  26  is rotated in the forward direction, the follower pin  127   a  reaches the innermost periphery of the spiral groove gear  120 . In this case, the driving mechanism of the both-side rollers is in the state shown in  FIG. 16E . At this point, the both-side pendulum arm spring  132  becomes the state of maximum displacement. However, since the load of the both-side pendulum arm spring  132  is set so that the engaging force between the tooth surfaces of the gears is larger than the force rocking the both-side pendulum arm  117 , the engagement between the gears which continues to rotate the LF motor  26  in the forward direction is never disengaged. When the recording operation of the reverse surface of the recording paper  4  is terminated, the operational sequence proceeds to Step S 24 . 
   In Step S 24 , the sheet discharge operation is performed. The sheet discharge operation discharges the recording paper  4  onto a sheet discharge tray (not shown). The sheet discharge operation is performed in such a manner that the rotation of the LF motor  26  in the forward direction is continued and the recording paper  4  is transported outside the recording unit main body  1  by the second sheet discharge roller  31 . In Step S 25 , an absolute position of the front end of the reverse surface is checked. This is because sometimes the follower pin  127   a  does not reach the innermost periphery of the spiral groove gear  120  in the case where the short recording paper is used. Even in such cases, when the reverse surface recording operation of the recording paper  4  is terminated, the follower pin  127   a  is configured so as to always come to the innermost periphery of the spiral groove gear  120  by rotating the LF motor  26  by the predetermined length. 
   In Step S 26 , the initialization of the driving mechanism of the both-side rollers is performed. As described above, since the force charged by the both-side pendulum arm spring  132  is held by the engagement between the both-side planet gear A  118  and the both-side roller idler gear  124 , the engagement is disengaged by rotating the LF motor  26  by the very small amount in the reverse direction. Namely, when the LF motor  26  is rotated in the reverse direction, since the both-side pendulum arm  117  tries to be rocked in the direction of the arrow b in  FIG. 15 , the engagement between the both-side planet gear A  118  and the both-side roller idler gear  124  is disengaged, and the both-side pendulum arm  117  is rocked in a stroke in the direction of the arrow b in  FIG. 15  by the force in which the both-side pendulum arm spring  132  returns to the original state. The driving mechanism of the both-side rollers is in the state shown in  FIG. 16F . 
   In this state, since the both-side pendulum arm spring  132  returns to the original state, in the case where the LF motor  26  is rotated in the forward direction, the both-side pendulum arm  117  tries to be rocked in the direction of the arrow a in  FIG. 15 . However, since the follower pin  127   a  enters the innermost periphery of the spiral groove gear  120 , the both-side pendulum arm spring  132  abuts on the stop arm  127 , and the both-side planet gear A  118  can not engage the both-side roller idler gear  124 . Even if the LF motor is further rotated in the forward direction, since the follower pin  127   a  continues to rotate in the innermost periphery of the spiral groove gear  120 , the both-side roller A  108  and the both-side roller B  109  are never driven. As described above, since the reverse rotation delay gear A  121  is initialized in Step S 19  or Step S 21 , each initialization of the driving mechanism of the both-side rollers is terminated up to Step S 26 . Up to this point, the automatic both-side recording operation is terminated. In the case where the automatic both-side recording operation is continuously performed, the same operational sequence can be repeated. 
   In the embodiment, although the elastic abutment relationship between the both-side pendulum arm  117  and the stop arm  127  is realized by the action of the both-side pendulum arm spring  132 , the invention is not limited to the embodiment, and the following configuration can be also used.  FIGS. 17A to 17E  are the schematic sectional side view showing the operational state of the driving mechanism of the rollers in the automatic both-side unit  2 . The both-side pendulum arm  117  shown in  FIGS. 17A to 17E  includes an arm having the low elasticity, and the arm can abut on the stop arm  127 . The operation of this configuration will be briefly described below. 
   Since the operations from  FIG. 17A  to  FIG. 17C  are similar to the operations from  FIG. 16A  to  FIG. 16C , the description is omitted here.  FIG. 17D  shows the state in which the stop arm  127  is moved toward the inner peripheral direction of the spiral groove gear  120  to abut on the arm of the both-side pendulum arm  117 . Since the arm of the both-side pendulum arm  117  has the low elasticity, when the stop arm is pressed by the stop arm  127 , the force rotating the both-side pendulum arm  117  in the direction of the arrow b in  FIG. 15  acts. The force acts in the direction in which the engagement between the both-side planet gear A  118  and the both-side roller idler gear  124  is disengaged. 
   The force disengaging the engagement balances with the pressure acting between the tooth surfaces of the both-side planet gear A  118  and the both-side roller idler gear  124  and the elastic and sliding forces of the gears. However, the force disengaging the engagement is increased as the follower pin  127   a  is moved to the inner periphery, the force disengaging the engagement overcomes the force between the tooth surfaces and forcedly disengages the engagement between the both-side planet gear A  118  and the both-side roller idler gear  124 . At the same time when the engagement is disengaged, the both-side roller A  108  and the both-side roller B  109  stop to rotate.  FIG. 17E  shows this state. The timing when the rotation of the roller is stopped is performed at a proper time after the rear end of the reverse surface of the recording paper  4  passes through the both-side roller A  108  in Step S 23 . 
   After the disengagement between gears, even if the LF motor  26  is rotated in the forward direction, the stop arm  127  prevents the both-side pendulum arm  117  from rocking in the direction of the arrow a in  FIG. 15 , so that the automatic both-side unit  2  is not driven until the LF motor  26  is rotated by the predetermined amount in the reverse direction. Similarly to the first embodiment, since the initialization of the reverse rotation delay gear A  121  is performed in Step S 19  or Step S 21 , At this point, the initialization of the driving mechanism of the rollers in the automatic both-side unit  2  is completed. Therefore, the load rotating the both-side roller A  108  and the both-side roller B  109  during the recording operation of the reverse surface can be eliminated, and the rotational load can be decreased. Up to this point, another embodiment the driving mechanism of the rollers in the automatic both-side unit  2  was described. 
   The invention is not limited to another embodiment, and it is also possible to configure the control in which the positions of the lift mechanism are changed. Namely, in another embodiment, although the sheet passing guide  70  is in up-state during the normal standby state, it is possible that the sheet passing guide  70  is in down-state during the normal standby state. Specifically, the control in which the normal lift mechanism is set to the third position and the lift mechanism is moved from the third position to the first position before Step S 1  is added to the configuration. Further, it is also possible that the control in which the lift mechanism is moved from the first position to the third position after Step S 26  is added to the configuration. This configuration is preferable to the case in which the thick paper is supplied from the sheet discharge roller side, because the pinch roller  22  is released in the standby state. Up to this point, the automatic both-side recording operation along the flow chart showing the operational sequence was described. 
   Specific forms of the invention cited below are described in the above-described embodiments. Specific Form 1: A both-side recording apparatus with a sheet transport mechanism having a pair of sheet transport rollers including a sheet transport roller and a pinch roller pressed against the sheet transport roller, at least one pair of sheet discharge rollers arranged on the downstream side of the sheet transport roller in a transport direction, and a pair of sheet discharge rollers including a rotating body pressed against the roller, characterized in that a recording medium can be transported to a position where a rear end of the recording medium is released from the pair of sheet transport rollers when a first surface is recorded at first, and then the recording paper is transported to a paper inversion unit in such a manner that the sheet transport roller is pressed into contact with the pinch roller again to further continue the transport in the reverse direction after the sheet transport roller and the pinch roller are released to transport the recording medium toward a reverse direction of the first surface recording by the pair of sheet discharge rollers. 
   The configuration of Specific Form 1 provides the both-side recording apparatus in which the recording can be performed without the blank space in the overall range of the recording medium because the recording can be performed while the recording medium is transported until the recording medium is released once from the sheet transport, and the sheet transport roller and the pinch roller can securely support the recording medium while sandwiching the recording medium and the paper jam can be securely prevented because the recording medium is transported toward the reverse direction while the pinch roller is released to make the gap between the pinch roller and the sheet transport roller. 
   Specific Form 2: A both-side recording apparatus according to Specific Form 1, wherein a gap between the sheet transport roller and the pinch roller is set larger than an amount of deformation of the recording medium after the first surface of the recording medium is recorded. The configuration of Specific Form 2 obtains the effect that the recording medium can be transported to the gap between the sheet transport roller and the pinch roller, the sheet transport roller and the pinch roller can securely support the recording medium while sandwiching the recording medium again, and the paper jam can be securely prevented. 
   Specific Form 3: A both-side recording apparatus according to Specific Form 1 or Specific Form 2, wherein the transportation of the recording medium is started toward the reverse direction after a predetermined time elapsed from termination of the recording of the first surface in the recording medium. The configuration of Specific Form 3 obtains the effect that the ink soil of the sheet transport roller or the pinch roller or the soil of the recording paper itself can be eliminated by sufficiently drying the ink on the first surface. 
   Specific Form 4: A both-side recording apparatus as in any one of Specific Forms 1 to 3, wherein a second predetermined time is longer than a first predetermined time, when the recording medium is transported to a paper inversion unit in such a manner that the sheet transport roller and the pinch roller are released after the first predetermined time elapsed from the termination of the recording of the first surface in the recording medium, a rear end of the first surface is transported toward the reverse direction beyond a nip portion of the sheet transport roller, the transport of the recording medium is stopped for the second predetermined time, and then the sheet transport roller is pressed into contact with the pinch roller again to further continue the transport in the reverse direction. The configuration of Specific Form 4 provides the both-side recording apparatus in which the recording medium can be transported to the gap between the sheet transport roller and the pinch roller before the recording medium is deformed by absorbing the ink and the like and the paper jam can be securely prevented because the rear end of the first sheet is transported toward the reverse direction beyond the nip portion of the sheet transport roller after the shorter first predetermined time elapsed. 
   Specific Form 5: A both-side recording apparatus as in any one of Specific Forms 1 to 3, wherein a fourth predetermined time is longer than a third predetermined time, when the recording medium is transported to the paper inversion unit in such a manner that the sheet transport roller and the pinch roller are released after the third predetermined time elapsed from the termination of the recording of the first surface in the recording medium, the rear end of the first surface is transported toward the reverse direction beyond the nip portion of the sheet transport roller, said sheet transport roller is pressed into contact with the pinch roller again, the transport of the recording medium is stopped for the fourth predetermined time, and then the transport of the recording medium in the reverse direction is resumed. The configuration of Specific Form 5 provides the both-side recording apparatus in which, in the case where there is the blank space in the rear end of the first surface, the pinch roller is pressed against the blank space and the pinch roller waits for the fourth predetermined time, therefore, the amount of deformation of the recording paper caused by absorbing the ink and the like can be decreased and the paper jam can be securely prevented. Specific Form 6: A both-side recording apparatus as in any one of Specific Forms 1 to 5, wherein the recording is performed to the recording paper by inkjet recording means for discharging ink from a discharge port. 
   Specific Form 7: A both-side recording apparatus as in any one of Specific Forms 1 to 6, wherein the total time from the termination of the recording of the first surface to the termination of the second predetermined time or the fourth predetermined time can be changed by density per unit area of data recorded in the first surface. Specific Form 8: A both-side recording apparatus as in any one of Specific Forms 1 to 6, wherein the total time from the termination of the recording of the first surface to the termination of the second predetermined time or the fourth predetermined time can be changed by a kind of the ink used in the recording. Specific Form 9: A both-side recording apparatus as in any one of Specific Forms 1 to 6, wherein the total time from the termination of the recording of the first surface to the termination of the second predetermined time or the fourth predetermined time can be changed by conditions of atmosphere such as ambient temperature and ambient humidity. 
   Specific Form 10: A both-side recording apparatus as in any one of claims  1  to  6 , wherein the total time from the termination of the recording of the first surface to the termination of the second predetermined time or the fourth predetermined time can be changed by the kind of the recording medium. The configurations of Specific Forms 6 to 10 obtain the effect that the recording medium in which the recording is performed to the first surface can be efficiently dried by changing the dry standby time according to atmosphere of the recording apparatus such as the amount of using ink hit in the recording paper per unit area, the kind of the ink, the ambient temperature, and the ambient humidity or the kind of the recording paper, and the dry standby time can be reduced to shorten the recording operation time by cutting out the unnecessary drying processing. 
   In the above embodiments, the serial type recording apparatus in which the recording is performed while the recording head as the recording means is moved in the main scanning direction was described as an example. However, the invention can be also applied to the line type recording apparatus which uses the line type recording means having the length covering a total width or a part of the recording medium and perform the recording only by the sub-scanning (paper transport), and the same effect can be achieved. The invention can be freely carried out independently of the number of recording means. The invention can be also applied to the recording apparatus using one recording means, the recording apparatus for color recording which utilizes the plurality of recording means using different color inks, the recording apparatus for gradation recording which utilizes the plurality of recording means using the inks having the same color and different densities, and the recording apparatus in which the above recording apparatuses are combined, and the same effect can be achieved. 
   In the case where the recording apparatus is the inkjet recording apparatus, the invention can be applied to any arrangement of the recording head and the ink tank such as the configuration using the changeable head cartridge in which the recording head and the ink tank are integrated and the configuration in which the recording head and the ink tank are individually formed and connected with a tube for ink supply, and the same effect can be achieved. In the case where the recording apparatus is the inkjet recording apparatus, in addition to the recording apparatus using the inkjet recording head in which the ink is discharged by utilizing the thermal energy, the invention can be also applied to the recording apparatus utilizing the other ink discharge methods, e.g. the recording apparatus using the inkjet recording head in which the ink is discharged by using the electromechanical transducer body such as a piezoelectric element, and the same operation and effect can be achieved. 
   As can be seen from the above description, according to the invention, the both-side recording apparatus in which the recording can be performed without the blank space in the overall range of the recording medium because the recording can be performed while the recording medium is transported until the recording medium is released once from the sheet transport, and the sheet transport roller and the pinch roller can securely support the recording medium while sandwiching the recording medium and the paper jam can be securely prevented because the recording medium is transported toward the reverse direction while the pinch roller is released to make the gap between the pinch roller and the sheet transport roller can be provided.