Abstract:
A printing apparatus is provided. The printing apparatus includes a printing unit, a sheet feeder, a discharge unit, a sheet path including a feeding path, a discharge path, and a reversing path, a manual sheet inlet, through which a recording sheet is manually inserted, a manual sheet path merging into the sheet path at a merging point and guiding the manually-inserted recording sheet to the merging point, a detecting unit to detect the manually-inserted sheet fed through the manual sheet inlet while a double-face printing operation is conducted with a preceding recording sheet, a judging unit to judge whether the detecting unit detected the manually-inserted sheet, and a conveyer controller unit to reduce a speed to convey the preceding recording sheet in the sheet path when the judging unit judges that the detecting unit detected the manually-inserted sheet.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     This application claims priority from Japanese Patent Application No. 2010-265735, filed on Nov. 29, 2010, the entire subject matter of which is incorporated herein by reference. 
     BACKGROUND 
     1. Technical Field 
     An aspect of the present invention relates to a printing apparatus capable of printing images on either side of a recording sheet. More specifically, the present invention relates to a double-face printable printing apparatus having a sheet path, along which a recording sheet fed manually by a user is guided to a printing unit. 
     2. Related Art 
     A printing apparatus capable of “double-face printing,” i.e., printing an image on an either or both sides of a recording sheet, is known. The printing apparatus may have a sheet-reversing path, in which the recording sheet with an image printed on one side is turned over, so that another image can be printed on the other side of the recording sheet. Meanwhile, a printing apparatus having a manual sheet path to convey a manually-fed recording sheet is known. Further, a printing apparatus, which has both of the double-face printing function and the manual sheet path, is known. 
     SUMMARY 
     In the printing apparatus with the double-face printing function and the manual sheet path, a recording sheet may be manually inserted in the manual sheet path by a user whilst the printing apparatus is conducting a double-face printing operation with a recording sheet having been loaded in the printing apparatus earlier. However, behaviors of the printing apparatus in such a case, in which the preceding recording sheet conveyed in the sheet-reversing path and the manually-inserted recording sheet may collide, are yet to be considered. For example, when the collision does occur, the collided recording sheets may be jammed inside the printing apparatus, and the user may be required to remove the collided recording sheets. 
     In view of such consideration, the present invention is advantageous in providing a printing apparatus, which reduces burden for the user when the recording sheet is manually inserted whilst the printing apparatus is in a double-face printing operation. 
     According to an aspect of the present invention, a printing apparatus, which is capable of double-face printing to print images on both of two sides of a recording sheet, is provided. The printing apparatus includes a printing unit, which is configured to print an image on one of the two sides of the recording sheet, a sheet feeder, which is configured to feed the recording sheet to the printing unit, a discharge unit, in which the recording sheet with the image printed thereon is settled, a sheet path, which includes a feeding path to guide the recording sheet fed from the sheet feeder to the printing unit, a discharge path to guide the recording sheet passing through the printing unit to the discharge unit, and a reversing path diverging from the discharge path and merging into the feeding path at an upstream position with respect to the printing unit along a direction of conveying the recording sheet, the sheet path guiding the recording sheet therealong and reversing the recording sheet by use of the reversing path during a double-face printing operation, a manual sheet inlet, through which a recording sheet is manually inserted in the printing apparatus by a user, a manual sheet path, which is configured to merge into the sheet path at a merging point being in an upstream position with respect to the printing unit along the direction of conveying the recording sheet and guide the manually-inserted recording sheet to the merging point, a detecting unit, which is configured to detect the manually-inserted sheet fed through the manual sheet inlet whilst the double-face printing operation is conducted with a preceding recording sheet, a judging unit, which is configured to judge as to whether the detecting unit detected the manually-inserted sheet, and a conveyer controller unit, which is configured to reduce a speed to convey the preceding recording sheet in the sheet path when the judging unit judges that the detecting unit detected the manually-inserted sheet. 
     According to another aspect of the present invention, a printing apparatus, which is configured to print an image on a recording sheet, is provided. The printing apparatus includes a sheet storage, which is configured to store the recording sheet, a printing unit, which is configured to print an image on one of two sides of the recording sheet, a pick-up unit, which is configured to pick up the recording sheet from the sheet storage, a discharge unit, which is configured to discharge the recording sheet, a conveyer, which includes a first conveyer configured to convey the recording sheet picked up from the sheet storage by the pick-up unit to the printing unit; a second conveyer configured to convey the recording sheet passing through the printing unit to the discharge unit; and a reversing conveyer configured to diverge from the second conveyer and merge into the first conveyer at a merging point, and conveys the recording sheet and reversing the recording sheet by use of the reversing conveyer during a double-face printing operation, a sheet inlet, which is configured to receive a different recording sheet inserted in the printing apparatus, a third conveyer, which is configured to merge into the first conveyer at the merging point and to convey the different recording sheet from the sheet inlet to the merging point, a detector, which is configured to detect the different recording sheet being fed through the sheet inlet, and a conveyer controller, which is configured to reduce a speed to convey the recording sheet in the conveyer if the detector detects the different recording sheet being inserted into the sheet inlet while the conveyer is conveying the recording sheet. 
    
    
     
       BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS 
         FIG. 1  is a perspective view of a printer according to embodiments of the present invention. 
         FIG. 2  is a schematic diagram to illustrate an internal configuration of the printer according to the embodiments of the present invention. 
         FIG. 3  is a block diagram to illustrate an electrical configuration of the printer according to the embodiments of the present invention. 
         FIG. 4  is a first part of a flow of a double-face printing operation to be conducted in the printer according to the embodiments of the present invention. 
         FIG. 5  is a second part of the flow of the double-face printing operation to be conducted in the printer according to the embodiments of the present invention. 
         FIG. 6  is a third part of the flow of the double-face printing operation to be conducted in the printer according to the embodiments of the present invention. 
         FIG. 7  is a fourth part of the flow of the double-face printing operation to be conducted in the printer according to a first embodiment of the present invention. 
         FIG. 8  is a part of a flow of the double-face printing operation to be conducted in the printer according to a second embodiment of the present invention. 
         FIG. 9  is a schematic diagram to illustrate an internal configuration of another example of the printer according to the embodiments of the present invention. 
     
    
    
     DETAILED DESCRIPTION 
     Hereinafter, embodiments of the present invention will be described with reference to the accompanying drawings. A printer  100  is a printing apparatus, which has a double-face printing function and a manual sheet path for conveying a manually fed recording sheet. 
     Overall Configuration of Printer 
     The printer  100  (see  FIG. 1 ) according to the present embodiment includes a main unit  10 , which forms an image on a surface of a recording sheet, a display unit  41  including a liquid crystal display, and an operation panel  40 , through which information concerning operations of the printer  100  is displayed and user&#39;s input is entered. The operation panel  40  includes buttons  42  such as a start key, a stop key, and numerical keys (not shown). 
     The main unit  10  is arranged in a lower section of the printer  100  and includes sheet cassettes  91 ,  92 , a discharge tray  96 , and a manual sheet inlet  93 . The sheet cassettes  91 ,  92  are installable in and removable from the printer  100  and stores unused recording sheets therein. The discharge tray  96  is arranged in a top section of the printer  100 . The recording sheets with images formed thereon are discharged out of the main unit  10  and settled in the discharge tray  96 . The manual sheet inlet  93  is an opening, through which a user manually inserts the recording sheet in the main unit  10 . 
     Internal Configuration of the Printer 
     The printer  100  includes a processing unit  50  (see  FIG. 2 ), which forms a toner image in a known electro-photographic laser-printing method and transfers the toner image onto a surface of the recording sheet. The processing unit  50  may or may not be a laser-printing unit but may be, for example, an inkjet-printing unit. Further, the printer  100  may or may not necessarily be a color printer but may be, for example, a monochrome printer. 
     The printer  100  includes feed rollers  71 ,  72 , a register roller  73 , and a discharge roller  76 . The feed roller  71  picks up the recording sheets stored in the sheet cassette  91  one-by-one, and the feed roller  72  picks up the recording sheets stored in the sheet cassette  92  one-by-one. The register roller  73  conveys the recording sheet to the processing unit  50 . The discharge roller  76  conveys the recording sheet to the discharge tray  96 . Further, the printer  100  includes a plurality of conveyer rollers, including conveyer rollers  77 ,  78 , along sheet paths. The discharge roller  76  is rotatable in two different (normal and reverse) directions. The register roller  73 , the conveyer rollers  77 ,  78  are rotatable in a single (normal) direction. 
     In the printer  100 , a feeding path  11 , in which the recording sheet picked up from the sheet cassette  91 / 92  by the feed roller  71 / 72  is guided through the register roller  73  to the processing unit  50 , are arranged. Further, in the printer  100 , a discharge path  12 , in which the recording sheet is guided from the processing unit  50  through the discharge roller  76  to the discharge tray  96 , is arranged. The feeding path  11 , from the sheet cassette  91 / 92  to the processing unit  50 , and the discharge path  12 , from the processing unit  50  to the discharge roller  76 , is arranged to have a cross-sectional shape similar to a “U,” and a printing path  14  including the feeding path  11  and the discharge path  12  has a cross-sectional shape similar to an “S.” The printing path  14  is indicated in dash-and-dot lines in  FIG. 2 . 
     In the printer  100  according to the present embodiment, the recording sheets stored in the sheet cassette  91 / 92  are picked up one-by-one and fed in the feeding path  11 . The recording sheets are conveyed to the processing unit  50 , in which the toner image is transferred onto the surface of the recording sheet. The recording sheet with the transferred toner image is forwarded to a fixing device (not shown) in the processing unit  50 , and the toner image is thermally fixed on the surface of the recording sheet. The recording sheet with the fixed image is carried in the discharge path  12  to the discharge roller  76 , which ejects the recording sheet out of the main unit  10 . The ejected recording sheet is settled in the discharge tray  96 . 
     The printer  100  has a structure to turn over the recording sheet in order to print an image even on a reversed side of the recording sheet. More specifically, the printer  100  is capable of printing a first image on one side (a first side) of the recording sheet, turning over the recording sheet after the first image is printed on the first side, and printing a second image on the other side (a second side) of the same recording sheet. The recording sheet with the first image printed on the first side is turned over in a reversing path  13  and returned to the processing unit  50  with the second side facing the processing unit  50 . The reversing path  13  is indicated in a double-dotted line in  FIG. 2 . 
     The reversing path  13  is arranged to diverge from the discharge path  12  at a branch point  15 , which is in a lower-stream position with respect to the processing unit  50  and in an upper-steam position with respect to the discharge roller  76  along a direction of a flow of the recording sheet being conveyed. The reversing path  13  diverged from the branch point  15  extends in a position between the processing unit  50  and the sheet cassette  91  and merges into the printing path  14  at a merging point  16 , which is in a lower-stream position with respect to the feed rollers  71 ,  72  in the feeding paths  11  and in an upper-stream position with respect to the register roller  73 . Thus, paths for the recording sheet in the printer  100  include the printing path  14  and the reversing path  13 . 
     A flow of the recording sheet being conveyed in the printer  100  during a double-face printing operation will be described below. Firstly, the recording sheet being picked up from the sheet tray  91 / 92  is carried in the feeding path  11  to the processing unit  50 . The first image is formed on the first side of the recording sheet in the processing unit  50 . Secondly, the recording sheet with the first image printed on the first side is carried in the discharge path  12  to the discharge roller  76 . Thirdly, when the recording sheet reaches the discharge roller  76  and a rear end of the recording sheet passes through the branch point  15 , rotation of the discharge roller  76  in a normal direction is stopped with the recording sheet being nipped between the discharge roller  76  and a paired roller (unsigned). Fourthly, a rotating direction of the discharge roller  76  is switched, and the discharge roller  76  rotates in a reverse direction. Fifthly, according to the reverse rotation of the discharge roller  76 , the recording sheet is conveyed in the reverse direction in the reversing path  13  via the branch point  15 . Sixthly, the recording sheet is returned to the feeding path  11  via the merging point  16 , which is in the upper-stream position with respect to the processing unit  50 . Thus, the recording sheet is turned over to have the second side facing the processing unit  50  when the recording sheet is carried to the processing unit  50 . Seventhly, the second image is printed on the second side of the recording sheet in the processing unit  50 . Finally, the recording sheet with the second image printed on the second side is discharged by the discharge roller  76  and settled in the discharge tray  96 . 
     The printer  100  is further provided with a structure for manual sheet-feeding, which conveys a recording sheet manually fed by a user in a manual sheet path  17  in the printer  100 . The manual sheet path  17 , indicated in a broken line in  FIG. 2 , guides the manually-inserted recording sheet through the manual sheet inlet  93  to the feeding path  11  via the merging point  18 . That is, the manual sheet path  17  ranges from the manual sheet inlet  93 , which is a most upstream point, to the merging point  18 , which is a most downstream point. The merging point  18  is in an upper-stream position with respect to the register roller  73  and a lower-stream position with respect to the feed rollers  71 ,  72  in the feeding path  11 . 
     A flow of the manually-inserted recording sheet being conveyed in the printer  100  during a printing operation will be described below. Firstly, the recording sheet is inserted through the manual sheet inlet  93  by the user. The manually-inserted recording sheet is forwarded by the user along the manual sheet path  17  to the feeding path  11 . When the manually-inserted recording sheet reaches the register roller  73 , the register roller  73  nips the manually-inserted recording sheet in cooperation with a paired roller (unsigned) and stands by. Secondly, when the processing unit  50  is prepared for printing an image, the register roller  73  is rotated, and the manually-inserted recording sheet is carried in the feeding path  11  to be automatically drawn in the printer  100 . Thirdly, when the manually-inserted recording sheet reaches the processing unit  50 , the image is printed on a surface of the manually inserted recording sheet in the processing unit  50 . Thus, until the register roller  73  nips the manually-inserted recording sheet, the user holds and forwards the manually-inserted recording sheet in the manual sheet path  17 . Once the register roller  73  nips the manually-inserted recording sheet, the manually-inserted recording sheet is automatically carried in the feeding path  11 . It is to be noted that images may be formed on the first and second sides of the manually-inserted recording sheet, similarly to the recording sheet fed from the sheet cassette  91 ,  92 , once the manually-inserted recording sheet is drawn automatically in the printer  100 . 
     The printer  100  includes sheet sensors  61 - 69 , which are arranged along the sheet paths  13 ,  14 ,  17 . The sheet sensors  61 - 64  are arranged along the feeding path  11 . More specifically, the sheet sensor  61  and the sheet sensor  62  are arranged in immediate downstream positions with respect to the feed roller  71  and the feed roller  72  respectively. The sheet sensor  63  is arranged in an immediate upstream position with respect to the register roller  73 , and the sheet sensor  64  is arranged in an immediate downstream position with respect to the register roller  73 . The sheet sensors  65 - 67  are arranged along the discharge path  12 . More specifically, the sheet sensor  65  is arranged in an immediate upstream position with respect to the branch point  15 , the sheet sensor  66  is arranged in an immediate upstream position with respect to the discharge roller  76  with reference to the flow of the recording sheet being conveyed in the normal direction. The sheet sensor  67  is arranged in an immediate downstream position with respect to the discharge roller  76  with reference to the flow of the recording sheet being carried in the normal direction. Further, the sheet sensors  68 ,  69  are arranged along the reversing path  13 . More specifically, the sheet sensor  68  is arranged in an immediate upstream position with respect to the conveyer roller  77 , which is in a position closest to the branch point  15  amongst the plurality of conveyer rollers  77 ,  78 . The sheet sensor  69  is arranged in a lower-stream position with respect to the sheet sensor  68  with reference to the flow of the recording sheet being conveyed in the reversing path  13 . 
     Each of the sheet sensors  61 - 69  detects presence of the recording sheet entering a detectable range thereof and changes output signals when the recording sheet entering the detectable range is detected. Therefore, when the signals from the sheet sensors  61 - 69  change, that is, when signals indicating absence of the recording sheet are switched to signals indicating presence of the recording sheet, the front end of the recording sheet reaching the detectable range is detected. When the signals indicating presence of the recording sheet are switched to the signals indicating absence of the recording sheet, the rear end of the recording sheet passing through and exiting the detectable range is detected. 
     Additionally to detection of the front end of the recording sheet, signals output from the sheet sensors  61 ,  62  can be referred to in order to detect a length of the recording sheet being carried in the feeding path  11 . The length of the recording sheet may be obtained, for example, based on a length of a time period between detection of the front end and detection of the rear end of the recording sheet at the sheet sensor  61  and a speed to carry the recording sheet, which is obtained based on a rotation speed of the feed roller  71 . 
     The sheet sensor  63  can detect the manually-inserted recording sheet having been inserted in the manual sheet path  17  at the earliest amongst the sheet sensors  61 - 69 . Therefore, signals output from the sheet sensor  63  can be referred to in order to detect the manually-inserted recording sheet having been inserted. The sheet sensor  63  may be referred to as a “pre-register sensor  63 ” hereinbelow. Meanwhile, signals output from the sheet sensor  64  can be used to determine timings to start feeding the recording sheet to the processing unit  50  and to start forming an image in the processing unit  50 . The sheet sensor  64  may be referred to as a “post-register sensor  64 ” hereinbelow. 
     It is to be noted that a quantity and arrangement of the sheet sensor are not limited to those described above but may be modified arbitrarily. For example, a greater quantity of the sheet sensors may enable to detect an accurate position of a sheet jam, when occurs, in the sheet paths. For example, on the other hand, a smaller quantity of the sheet sensors may enable manufacturing cost for the printers to be reduced. 
     Electrical Configuration of the Printer 
     The electrical configuration of the printer  100  will be described (see  FIG. 3 ). The printer  100  is provided with a controller unit  30 , which includes a CPU  31 , a ROM  32 , a RAM  33 , a non-volatile RAM (NVRAM)  34 , an ASIC  35 , and a network interface (I/F)  36 . The controller unit  30  is electrically connected with the processing unit  50 , the operation panels  40 , the sheet sensors  61 - 69 , and a motor to drive the conveyer rollers  71 - 78 . The rollers  71 - 78  may be driven commonly by a motor. Alternatively, a plurality of motors may be provided, and the rollers  71 - 78  may be driven individually. 
     The CPU  31  is an arithmetic processor, which processes information to be used to achieve functionalities of the printer  100  including image forming. The ROM  32  stores programs to control the printer  100  and information concerning operation settings and initial settings of the printer  100 . The RAM  33  serves as a work area, in which the controlling programs are loaded, and a memory area, in which image data is temporarily stored. The NVRAM  34  is a data storage, in which information concerning operation settings and image data can be stored. 
     The CPU  31  controls behaviors of the printer  100  through the ASIC  35 . In particular, the CPU  31  processes information from the controlling programs and signals obtained from various sensors and stores the information in the RAM  33  and the NVRAM  34  to drive components in the printer  100 . The CPU  31  controls, for example, timing for emitting light from an exposure device, and activation of the motor to drive the rollers  71 - 78 . 
     The network I/F  36  connects the printer  100  with networks, such as a local area network (LAN), to establish communication with other external devices (e.g., personal computer) through the network. The printer  100  can be supplied with print jobs through the network I/F  36 . 
     Double-Face Printing Operation (First Example) 
     Behaviors of the printer  100  in a double-face printing operation according to a first embodiment of the present invention will be described with reference to flowcharts shown in  FIGS. 4 ,  5 ,  6 , and  7 . The flows of double-face printing operation may be controlled and executed by the controller unit  30  upon, for example, receipt of a print job for double-face printing, which is transmitted from an external device. In the print job described below, an image is formed on a recording sheet supplied from the sheet cassette  91 ; however, a recording sheet supplied from the sheet cassette  92  or other additional sheet cassette (not shown) may be similarly handled in the printer  100 . 
     As shown in  FIG. 4 , when the double-face printing operation starts, in S 100 , a recording sheet is picked up from the sheet cassette  91 . In S 101 , it is determined as to whether a recording sheet reached the detectable range for the pre-register sensor  63  based on the signals output from the pre-register sensor  63 . In other words, it is determined as to whether the pre-register sensor  63  detected a front end of a recording sheet. If no recording sheet is detected (S 101 : NO), the flow repeats S 101  until the pre-register sensor  63  detects the recording sheet. 
     If a recording sheet is detected by the pre-register sensor  63  (S 101 : YES), in S 102 , it is judged as to whether the recording sheet is a manually-inserted recording sheet. If a front end of a recording sheet is detected by the pre-register sensor  63  prior to an estimated timing, at which the recording sheet picked up from the sheet cassette  91  is expected to reach the detectable range of the pre-register sensor  63  based on the speed to convey the recording sheet, it is determined that the recording sheet detected by the pre-register sensor  63  is a manually-inserted recording sheet, which is carried via a sheet path other than the feeding path  11 , i.e., the manual sheet path  17 . For example, if the pre-register sensor  63  detects a front end of a recording sheet after the sheet sensor  61  detected a front end of a recording sheet and before a predetermined period elapses, it is determined that the recording sheet detected by the pre-register sensor  63  is a manually-inserted recording sheet. 
     If the detected recording sheet is a manually-inserted recording sheet (S 102 : YES), the manually-inserted recording sheet may collide with the preceding recording sheet, which was picked up from the sheet cassette  91  in S 100 . Therefore, in S 110 , conveyance of the recording sheets is terminated (“error-termination”), and the double-face printing operation is aborted. When the double-face printing operation is error-terminated, the preceding recording sheet and the manually-inserted recording sheets are required to be removed by a user. If the recording sheet detected by the pre-register sensor  63  is not a manually-inserted recording sheet, that is, the detected recording sheet is the recording sheet picked up in S 100  (S 102 : NO), in S 103 , printing an image on a first side of the recording sheet starts. 
     Following S 103 , in S 104 , it is judged as to whether the recording sheet with the image printed on the first side thereof passed through the register roller  73 . In other words, it is judged as to whether a predetermined time period elapsed after a rear end of the recording sheet was detected by the pre-register sensor  63 . If the recording sheet has not passed through the register roller  73  (S 104 : NO), the flow repeats S 104  until the recording sheet passes through the register roller  73 . 
     If the recording sheet has passed through the register roller  73  (S 104 : YES), in S 105 , it is judged as to whether the recording sheet has passed through a transfer area in the processing unit  50 , in which the toner image is transferred onto a surface of the recording sheet. In other words, it is judged as to whether a predetermined time period elapsed after the post-register sensor  64  detected the rear end of the recording sheet. If the recording sheet has not passed through the transfer area (S 105 : NO), the flow repeats S 105  until the recording sheet passes through the transfer area. If the recording sheet has passed through the transfer area (S 105 : YES), in S 106 , printing the image on the first side of the recording sheet is finished. 
     In S 107 , it is judged as to whether the recording sheet reached a reversible position, in which the recording sheet is nipped by the discharge roller  76 , and the rear end of the recording sheet has passed through the branch point  15 . In the reversible position, therefore, the direction of conveying the recording sheet can be switched, and the recording sheet starts being turned over to a reversed orientation. For example, it may be determined that the rear end of the recording sheet has passed through the branch point  15  if a predetermined time period elapsed after the rear end of the recording sheet was detected by the sheet sensor  65 . If the recording sheet has not passed through the branch point  15  (S 107 : NO), the flow repeats S 107  until the recording sheet reaches the reversible position. 
     If the recording sheet reached the reversible position (S 107 : YES), in S 108 , conveyance of the recording sheet is stopped, and it is judged as to whether a manually-inserted recording sheet is detected. For example, when the pre-register sensor  63  detects a recording sheet after the recording sheet passed through the register roller  73  and before the recording sheet is reversed, it is determined that the recording sheet detected by the pre-register sensor  63  is a manually-inserted recording sheet. 
     If no manually-inserted recording sheet is detected (S 108 : NO), in S 109 , the discharge roller  76  is rotated in the reverse direction. Accordingly, the recording sheet in the reversible position is carried in the reverse direction and directed to the reversing path  13 . The flow proceeds to S 120  ( FIG. 5 ). The flow following S 120  will be described later in detail. 
     Meanwhile, in S 108 , if a manually-inserted recording sheet is detected (S 108 : YES), and if the preceding recording sheet in the reversible position is carried in the reverse direction in the reversing path  13 , the preceding recording sheet may collide with the manually-inserted recording sheet being nipped by the register roller  73  when the preceding recording sheet returns to the feeding path  11  via the reversing path  13 . In order to avoid the collision, the flow proceeds to S 150  ( FIG. 6 ), and conveyance of the preceding recording sheet is stopped. On the other hand, the manually-inserted recording sheet is maintained nipped by the register roller  73 . In S 151 , a message to notify the user of presence of the redundant manually-inserted recording sheet and to instruct the user to remove the redundant manually-inserted recording sheet is displayed to the user via the display unit  41 . The user may remove the manually-inserted recording sheet in accordance with the instruction. 
     In S 152 , it is determined as to whether the manually-inserted recording sheet has been removed. Removal (or presence) of the manually-inserted recording sheet can be determined based on the signals from the pre-register sensor  63 . If the manually-inserted recording sheet remains (S 152 : NO), in S 160 , it is judged as to whether a timeout period elapsed. If the timeout period has not elapsed (S 160 : NO), the flow returns to and repeats S 152  until the manually-inserted recording sheet is removed or the timeout period elapses. Once the timeout period elapses (S 160 : YES), in S 161 , conveyance of the recording sheets is error-terminated, and the double-face printing operation is aborted. 
     In S 152 , if the manually-inserted recording sheet has been removed (S 152 : YES), collision of the preceding recording sheet with the manually-inserted recording sheet is avoided, and the double-face printing operation is resumed. Therefore, in S 153 , the discharge roller  76  is rotated in the reverse direction. The flow proceeds to S 120  ( FIG. 5 ). 
     In S 120 , which follows S 109  ( FIG. 4 ) or S 153  ( FIG. 6 ), it is judged as to whether the preceding recording sheet is released from the discharge roller  76 . Release of the recording sheet from the discharge roller  76  may be determined, for example, when a predetermined time period elapsed after the sheet sensor  67  detected the rear end of the recording sheet. If the recording sheet is not released from the discharge roller  76  (S 120 : NO), the flow repeats S 120  until the recording sheet is released from the discharge roller  76 . If the recording sheet is released from the discharge roller  76  (S 120 : YES), in S 121 , rotation of the discharge roller  76  is switched in order for the discharge roller  76  to start rotating in the normal direction. The recording sheet is conveyed in the reversing path  13  and returns to the feeding path  11 . 
     In S 122 , it is judged as to whether the recording sheet returning in the reversing path  13  reaches the merging point  18 . It may be determined that the recording sheet in the reversing path  13  reaches the merging point  18  when, for example, a predetermined time period elapsed after the sheet sensor  69  had detected the front end of the recording sheet. If the recording sheet has not reached the merging point  18  (S 122 : NO), in S 130 , it is judged as to whether a manually-inserted recording sheet is detected. If no manually-inserted recording sheet is detected (S 130 : NO), the flow returns to S 122 . 
     In S 130 , if a manually-inserted recording sheet is detected (S 130 : YES), the preceding recording sheet may collide with the manually-inserted recording sheet being nipped by the register roller  73  when the preceding recording sheet returns to the feeding path  11  via the reversing path  13 . In order to avoid the collision, the flow proceeds to S 170  ( FIG. 7 ), and conveyance of the preceding recording sheet is stopped. In S 171 , a message to notify the user of presence of the redundant manually-inserted recording sheet and to instruct the user to remove the redundant manually-inserted recording sheet is displayed to the user via the display unit  41 . The user may remove the manually-inserted recording sheet in accordance with the instruction. 
     In S 172 , it is determined as to whether the manually-inserted recording sheet has been removed. Removal (or presence) of the manually-inserted recording sheet can be determined based on the signals from the pre-register sensor  63 . If the manually-inserted recording sheet has been removed (S 172 : YES), collision of the preceding recording sheet with the manually-inserted recording sheet is avoided, and the double-face printing operation is resumed. Therefore, in S 173 , conveyance of the preceding recording sheet is resumed. The flow proceeds to S 122  ( FIG. 5 ). 
     Meanwhile, if the manually-inserted recording sheet remains (S 172 : NO), in S 180 , it is judged as to whether a timeout period elapsed. If the timeout period has not elapsed (S 180 : NO), the flow returns to and repeats S 172  until the manually-inserted recording sheet is removed or the timeout period elapses. If the timeout period elapses (S 180 : YES), in S 181 , the manually-inserted recording sheet nipped by the register roller  73  is drawn in and through the main unit  10  and directed along the printing path  14  to be forcibly ejected out of the printer  100 . Therefore, the manually-inserted recording sheet is ejected ahead of the preceding recording sheet. The ejected manually-inserted recording sheet is settled in the discharge tray  96 . 
     In S 182 , it is judged as to whether the manually-inserted recording sheet has been successfully ejected. Forcible ejection of the recording sheet may be determined based on a predetermined period, within which the recording sheet is assumed to pass through the detectable area of the sheet sensor  67 . Additionally, for example, when the sheet sensor  67  detects the manually-inserted recording sheet passing through the detectable range of the sheet sensor  67  within the predetermined period, it is determined that the manually-inserted recording sheet is ejected successfully. On the other hand, the sheet sensor  67  does not detect the manually-inserted recording sheet passing through the detectable range of the sheet sensor  67  within the predetermined period, it is determined that forcible ejection of the manually-inserted recording sheet failed. Once the rear end of the manually-inserted recording sheet passes through the merging point  18 , beyond which collision of the manually-inserted recording sheet with the preceding recording sheet is avoidable, concern for the preceding recording sheet about the collision with the manually-inserted recording sheet is cleared. Therefore, in S 182 , it may be determined that the manually-inserted recording sheet was successfully ejected when the rear end of the manually-inserted recording sheet passing through the merging point  18  is detected. More specifically, when the rear end of the manually-inserted recording sheet passing through the detectable range of the pre-register sensor  63 , which is in the downstream position with respect to the merging point  18  and in a position closest to the merging point  18 , is detected, it may be determined that the manually-inserted recording sheet was successfully ejected. Based on the judgment, conveyance of the preceding recording sheet may be resumed earlier. In any way, once the manually-inserted recording sheets exits a range, in which the manually-inserted recording sheet may collide with the preceding recording sheet, conveyance of the preceding recording sheet can be resumed. 
     In S 182 , if forcible ejection of the manually-inserted recording sheet failed (S 182 : NO), in S 183 , conveyance of the preceding recording sheet is error-terminated, and the double-printing operation is aborted. If the manually-inserted recording sheet has been successfully ejected (S 182 : YES), collision of the preceding recording sheet with the manually-inserted recording sheet is avoided, and the double-face printing operation is resumed. Therefore, in S 173 , conveyance of the preceding recording sheet is resumed. The flow proceeds to S 122  ( FIG. 5 ). 
     In S 122 , if the preceding recording sheet returning in the reversing path  13  reaches the merging point  18  (S 122 : YES), in S 123 , it is judged as to whether the recording sheet reaches the detectable range of the pre-register sensor  63 , that is, whether the pre-register sensor  63  detects the front end of the recording sheet. If the recording sheet does not reach the detectable range of the pre-register sensor  63  (S 123 : NO), the flow repeats S 123  until the recording sheet is detected by the pre-register sensor  63 . 
     If the recording sheet reaches the detectable range of the pre-register sensor  63  (S 123 : YES), in S 124 , printing an image on a second side of the recording sheet returned in the feeding path  11  starts. In S 125 , it is judged as to whether the recording sheet has passed through the transfer area in the processing unit  50 . In other words, it is judged as to whether the predetermined time period elapsed after the post-register sensor  64  detected the rear end of the recording sheet. If the recording sheet has not passed through the transfer area (S 125 : NO), the flow repeats S 125  until the recording sheet passes through the transfer area. 
     If the recording sheet has passed through the transfer area (S 125 : YES), in S 126 , printing the image on the second side of the recording sheet is finished. Thus, the double-face printing operation is completed. 
     Double-Face Printing Operation (Second Example) 
     Behaviors of the printer  100  in a double-face printing operation according to a second embodiment of the present invention will be described with reference to a flowchart shown in  FIG. 8 . In the double-printing operation in the second example, when a manually-inserted recording sheet is detected, the printer  100  enters a low-speed mode, in which a speed to convey the preceding recording sheet is lowered. In the flowchart shown in  FIG. 8 , a flow following detection of a manually-inserted recording sheet ( FIG. 5 , S 130 : YES), whilst a preceding recording sheet is in the reversing path  13 , is different from the flows described in the first embodiment. In other words, the flows of the behaviors of the printer  100  in the double-face printing operation shown in  FIG. 7  are replaced with steps shown in  FIG. 8  in the second embodiment. Therefore, in the following description, the flows of steps to replace those in  FIG. 7  will be described in detail, and description of the common steps shown in  FIGS. 4-6  will be omitted. 
     In S 130  ( FIG. 5 ), if a manually-inserted recording sheet is detected (S 130 : YES), the flow proceeds to S 270 . In S 270 , the printer  100  enters a low-speed mode, in which a speed to convey the preceding recording sheet is lowered than an initial normal conveying speed. In the low-speed mode, by conveying the recording sheet in the reduced speed, extra time for the manually-inserted recording sheet to be removed out of the printing path  14  is created. Thus, collision of the preceding recording sheet with the manually-inserted recording sheet is avoided. The reduced speed in the low-speed mode may be a fixed speed or may be varied according to a distance between the merging point  18  and the preceding recording sheet at the timing, at which the manually-inserted recording sheet is detected. Therefore, when the distance between the merging point  18  and the preceding recording sheet is smaller, the conveying speed may be lower. Following S 270 , in S 171 , a message to notify the user of presence of the redundant manually-inserted recording sheet and to instruct the user to remove the redundant manually-inserted recording sheet is displayed to the user via the display unit  41 . The user may remove the manually-inserted recording sheet according to the instruction. 
     In S 172 , it is judged as to whether the manually-inserted recording sheet has been removed. If the manually-inserted recording sheet has been removed (S 172 : YES), in S 273 , the printer  100  exits the low-speed mode, and the speed to convey the recording sheet is increased, for example, to the initial conveying speed. The flow proceeds to S 122  ( FIG. 5 ). 
     Meanwhile, if the manually-inserted recording sheet remains (S 172 : NO), in S 280 , it is judged as to whether a timeout period elapsed. A length of the timeout period may be fixed or varied according to the distance between the merging point  18  and the preceding recording sheet at the timing, at which the manually-inserted recording sheet is detected. Therefore, when the distance between the merging point  18  and the preceding recording sheet is smaller, the timeout period may be shorter. If the timeout period has not elapsed (S 280 : NO), the flow returns to and repeats S 172  until the manually-inserted recording sheet is removed or the timeout period elapses. If the timeout period elapses (S 280 : YES), in S 181 , the manually-inserted recording sheet nipped by the register roller  73  is drawn in and through the main unit  10  and directed along the printing path  14  to be forcibly ejected out of the printer  100 . Therefore, the manually-inserted recording sheet is ejected ahead of the preceding recording sheet. The ejected manually-inserted recording sheet is settled in the discharge tray  96 . 
     In S 182 , it is judged as to whether the manually-inserted recording sheet has been successfully ejected. If forcible ejection of the manually-inserted recording sheet failed (S 182 : NO), in S 183 , conveyance of the preceding recording sheet is error-terminated, and the double-printing operation is aborted. If the manually-inserted recording sheet has been successfully ejected (S 182 : YES), that is, once the manually-inserted recording sheets exits a range, in which the preceding recording sheet may collide with the manually-inserted recording sheet, collision of the preceding recording sheet with the manually-inserted recording sheet is avoided, and the double-face printing operation is resumed. Therefore, in S 273 , the printer  100  exits the low-speed mode, and the speed to convey the recording sheet is increased to the initial conveying speed. The flow proceeds to S 122  ( FIG. 5 ). 
     According to the second embodiment, when the manually-inserted recording sheet is inserted during the double-face printing operation, extra time for the manually-inserted recording sheet to be removed out of the printing path  14 , either by being removed by the user or by forcibly ejected, is created. Therefore, errors due to collision of the preceding recording sheet with the manually-inserted recording sheet can be avoided whilst the double-face printing with the preceding recording sheet is continued. Further, whilst conveyance of the recording sheet is not stopped but maintained, throughput per unit of time of the printer  100  according to the second embodiment can be greater than throughput of the printer  100  in the first embodiment, in which conveyance of the recording sheet is stopped. In other words, productivity of the printer  100  can be improved. Meanwhile, although the throughput per unit of time may be lower, it is to be noted that the printer  100  in the first embodiment does not require the complicated control of the conveying speed, as required in the printer  100  according to the second embodiment. Therefore, the printer  100  can be controlled in less complicated steps. 
     In the second embodiment, the once-reduced conveying speed in the low-speed mode is increased to the initial speed. However, the conveying speed may not necessarily be increased to the initial speed as long as the conveying speed after exiting the low-speed mode is faster than the conveying speed in the low-speed mode, and the productivity, which is once reduced by the lowered speed, is regained. 
     As has been described above, the printer  100  according to the embodiments of the present invention creates extra time, within which collision between the preceding recording sheet and the manually-inserted recording sheet can be avoided, by reducing and/or stopping the conveying speed when the manually-inserted recording sheet is detected during the double-face printing operation. Accordingly, errors due to collision of the preceding recording sheet with the manually-inserted recording sheet can be avoided, and the user&#39;s manual works such as removing the jammed sheets can be reduced. In this regard, reducing the conveying speed includes stopping conveyance of the recording sheet. 
     Although examples of carrying out the invention have been described, those skilled in the art will appreciate that there are numerous variations and permutations of the printer that fall within the spirit and scope of the invention as set forth in the appended claims. It is to be understood that the subject matter defined in the appended claims is not necessarily limited to the specific features or act described above. Rather, the specific features and acts described above are disclosed as example forms of implementing the claims. 
     For example, the present invention can be similarly effectively applied to other image forming apparatuses having a double-face printing system and a manual sheet-feeding mechanism such as a copier, a multifunction peripheral device, and a facsimile machine. Further, the image forming unit in the image forming apparatus may not necessarily form images electro-photographically, but may form in, for example, inkjets. Furthermore, the image forming apparatus may be either a multi-color image forming apparatus or a monochrome image forming apparatus. 
     For another example, the printer  100  may be equipped with a conveying system, such as conveyer rollers, to assist inserting the recording sheet in the manual sheet path  17  in order for the manually-inserted recording sheet to be forwarded in the manual sheet path  17  smoothly. 
     For another example, the merging point  18 , at which the manual sheet path  17  merges into the feeding path  11 , may not necessarily be arranged in the downstream position with respect to the merging point  16 , at which the reversing path  13  merges into the feeding path  11  but may be arranged in an upstream position with respect to the merging point  16 . 
     Further, the pre-register sensor  63  may not necessarily serve to detect the manually-inserted recording sheet directed in the manual sheet path  17 , but a sheet sensor to specifically detect the manually-inserted recording sheet in the manual sheet path  17  may be provided. With the specifically dedicated sheet sensor, the manually-inserted recording sheet may be detected earlier than the pre-register sensor  63 . Furthermore, with the specific sheet sensor, the manually-inserted recording sheet may be detected whilst the printer  100  waits for the preceding recording sheet to pass by the register roller  73  in S 104  ( FIG. 4 ). When the manually-inserted recording sheet is detected by the specific sheet sensor, conveyance of the recording sheets may be error-terminated. 
     For another example, during the printing operation to print an image on the first side of the recording sheet, judgment to determine presence of the manually-inserted sheet is performed when the recording sheet reaches the reversible position ( FIG. 4 , S 107 ). However, the judgment may not necessarily be triggered by the recording sheet reaching the reversible position but may be made whenever the pre-register sensor  63  detects the manually-inserted recording sheet. In this case, the conveying speed to convey the preceding recording sheet may be reduced immediately after detection of the manually-inserted sheet. On the other hand, if the judgment is to be made when the recording sheet reaches the reversible position, when the user removes the manually-inserted recording sheet immediately after the insertion, necessity to lower the conveying speed is cleared, and the preceding recording sheet may be conveyed in the initial speed. Therefore, productivity of the printer  100  can be maintained. 
     In the embodiments described above, the conveying speed is lowered when the manually-inserted recording sheet is detected whilst the reversed preceding recording sheet is in the reversing path  13 . However, the conveying speed may be reduced when the manually-inserted recording sheet is detected whilst the preceding recording sheet is in the printing path  14 . In this case, it is likely that the preceding recording sheet is in the discharge path  12 ; therefore, it is preferable that forcible ejection of the manually-inserted recording sheet after the timeout period is omitted. Further, changing the conveying speeds whilst the preceding recording sheet is in the processing unit  50  may undesirably affect an outcome and quality of the printed image. Therefore, it is preferable that the conveying speed is not changed and reduction of the speed to convey the preceding recording sheet is canceled whilst the preceding recording sheet is in the processing unit  50 . 
     For another example, the printer  100  may not necessarily convey a single recording sheet at a time to print the first image on the first side and the second image on the second side sequentially. A plurality of (e.g., two) recording sheets may be in the printer  100  at a time, and a first image may be printed on a first side of a succeeding recording sheet in the printing path  14  whilst a preceding recording sheet with a first image printed on a first side thereof is in the reversing path  13 . In other words, first images may be printed sequentially on first sides of a plurality of recording sheets. In the printer  100  configured as above, the succeeding recording sheet may not necessarily wait for the preceding recording sheet to be ejected but may be efficiently fed in the printing path  14  before the preceding recording sheet is ejected. In other words, the recording sheets may be fed in shorter intervals, and productivity of the printer  100  can be improved. 
     Further, the discharge roller  76  may not necessarily serve to reverse the recording sheet, but a specific reversing roller  79  (see  FIG. 9 ) may be provided. For example, as shown in  FIG. 9 , the reversing path  19  may have a switchback path  191 , in which the recording sheet is turned over, and a returning path  192 , along which the recording sheet turned over in the switchback path  191  is guided to the feeding path  11 . In a double-face printing operation, the recording sheet with the first image printed on the first side is carried in the switchback path  191  via the branch point  15 . In this regard, the reversing roller  79  rotates in a normal direction. When the recording sheet is carried in the switchback path  191  with its rear end portion nipped by the reversing roller  79 , rotation of the reversing roller  79  is switched to a reverse direction. According to the reverse rotation, the recording sheet is guided in the returning path  192  and returns to the feeding path  11  via the merging point  16 . In this regard, the second side of the recording sheet comes to face the processing unit  50  to have the second image printed thereon. With this reversing configuration, the discharge roller  76  may not necessarily be rotatable in the normal and reverse directions but may be rotatable solely in the normal direction. According to the above-described configuration, the recording sheet starts to be reversed at the branch point  15 , at which the discharge path  12  and the reversing path  19  diverge.