Abstract:
Sheet inverters and methods for inverting sheets are provided that guide longer length receivers through a 180 degree turn while the receivers are being transported along a transport path while reducing the possibility of jams, the extent of sheet stress experienced during inversion and reducing the risk of creating print artifacts.

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention relates to a sheet inverter and a method for inverting sheets in a printing machine. 
       BACKGROUND OF THE INVENTION 
       [0002]    In the printing industry, different types of sheet inverter are known, which are typically employed in a duplex path of a printing machine. One such type of sheet inverter, which is for example described in U.S. Pat. No. 6,626,103 B1, allows a leading edge of a sheet to remain the same before and after inversion. This may be beneficial to registered printing on the front and back side of the sheet. 
         [0003]    This known sheet inverter has a first pair of spaced apart rollers having a first endless belt entrained thereabout in a figure eight configuration such that the first belt has a 180° twist in each section extending between said first pair of rollers and a second pair of spaced apart rollers having a second endless belt entrained thereabout in a figure eight configuration such that the second belt has a 180° twist in each section extending between said second pair of rollers. The first and second pairs of rollers are arranged adjacent to each other such that one section of said first belt and one section of said second belt, each belt having said 180° twist, contact each other to form a transport path therebetween. This is done by arranging one roller of the first pair of rollers and one roller of the second pair of rollers adjacent to each other to form an entrance group of rollers at an entrance of the transport part and arranging one roller of the first pair of rollers and one roller of the second pair of rollers adjacent to each other to form an exit group of rollers at an exit of the transport path. The transport path thereby also has a 180° twist. The twist forces a sheet sandwiched and transported between the belts to be twisted along the line of contact and to turn by 180° while it is transported along the transport path through the inverter. The inverter also has a guide wire to guide an edge portion of the sheet over and across the transport path during turning of the sheet. The known guide wire is arranged to support the edge portion of the sheet in substance in accordance with the turning force impaired by the twist in the transport path, i.e. it is arranged to provide substantially the same turning rate as the one provided by the twist in the transport path. 
         [0004]    This arrangement is suitable for a wide range of sheets, which may differ with respect to stiffness and dimensions. For long sheets, however, in particular sheets having a length (in the direction of transportation) longer than half the length of the transport path through the inverter, this arrangement may cause problems as described below. In this case, when the leading edge reaches the midpoint of the transport path, both the twist in the transport path and the guide wire will urge the sheet into an upright orientation. While the transport path urges the sheet in an upright orientation only in a middle section thereof, the guide wire urges the edge portions thereof in the upright orientation. When the sheet is longer than half the length of the transport path, the trailing edge will still be held in a horizontal position between the entrance group of rollers at the entrance end of the transport path. This may cause jams, undue stress in the sheet and may especially lead to artifacts in a printed image on a surface of the sheet due to excessive bending thereof. This problem is obviously more pronounced the longer and the stiffer the sheet. 
         [0005]    It is therefore an object of the invention to provide a sheet inverter and a method for inverting a sheet in a printing machine, which may overcome or at least lessen one of the above problems. 
       SUMMARY OF THE INVENTION 
       [0006]    In accordance with one aspect of the invention, a sheet inverter for inverting sheets in a printing machine is provided. The sheet inverter has a first pair of spaced apart rollers having a first endless belt entrained thereabout in a figure eight configuration such that the first belt has a 180° twist in each section extending between said first pair of rollers and a second pair of spaced apart rollers having a second endless belt entrained thereabout in a figure eight configuration such that the second belt has a 180° twist in each section extending between said second pair of rollers. The first and second pairs of rollers are arranged adjacent to each other such that one section of said first belt and one section of said second belt, each having said 180° twist; contact each other to form a transport path therebetween. The transport path thus has a 180° twist, which causes a central portion of a sheet, which is sandwiched between the belts, to turn over by 180° by following the twist in the transport path while the sheet is transported there along through the inverter. The inverter further has at least one guide unit, arranged to provide guidance to a first lateral portion of a sheet while it is transported along the transport path through the inverter. The at least one guide unit has a turning section, which crosses over the transport path from one side to an opposite side thereof and which is arranged to guide the lateral portion of said sheet to turn and move over the transport path by imparting a turning motion to the lateral portion of the sheet, the imparted turning motion having a substantially higher angular rate than a turning motion imparted by the twist in the respective section of the transport path. The term “substantially higher angular rate” is supposed to include the at least one guide to impart an angular turning rate to the edge region of the sheet which is at least 1.5 times higher than the angular twist rate. This allows the imparted turning of the first lateral portion of a sheet, caused by the guide unit, to be moved downstream along the transport path compared to the known guide wire. This may reduce stress in the sheet and in particular in long sheets having a length grater than half the length of the transport path. 
         [0007]    In accordance with another aspect of the invention, a method is provided for inverting a sheet in a printing machine as it is transported through a sheet inverter having a first pair of spaced apart rollers having a first endless belt entrained thereabout in a figure eight configuration such that the first belt has a 180° twist in each section extending between said first pair of rollers and a second pair of spaced apart rollers having a second endless belt entrained thereabout in a figure eight configuration such that the second belt has a 180° twist in each section extending between said second pair of rollers, said first and second pairs of rollers being arranged adjacent to each other such that one section of said first belt and one section of said second belt, each having said 180° twist, contact each other to form a transport path therebetween, said transport path having a 180° twist. In the method, the first and second belts are caused to circulate about the first and second rollers such that the belt sections forming the transport path move n the same direction and at the same speed. A central portion of sheet is transported between the belt sections forming the transport path such that it is sandwiched by the belt section and moved therewith and a turning motion is imparted to the central portion of the sheet by causing the central portion to follow the twist in the transport path while the sheet is transported along the transport path. Also, a turning movement is imparted to a first lateral portion of the sheet by at least one guide to cause the first lateral portion of said sheet to turn and thereby move over the transport path, the imparted turning motion having a substantially higher angular rate than a turning motion imparted to the central portion of the sheet in the respective section along the transport path. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0008]    The above and other objects, features, and advantages of the present invention will become more apparent when taken in conjunction with the following description and drawings wherein identical reference numerals have been used, where possible, to designate identical features that are common to the figures, and wherein: 
           [0009]      FIG. 1  is a schematic side view of a printing machine, in which an inverter according to the invention may be employed; 
           [0010]      FIG. 2  is an enlarged schematic top view of the inverter shown in  FIG. 1 . 
           [0011]      FIG. 3  is an enlarged schematic side view of the inverter shown in  FIG. 1 . 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0012]    Terminology regarding locations and directions such as above, below, right and left, used throughout the description relates to the representations in the drawings and are thus not to be viewed in a limiting sense. However, they may also relate to a preferred final arrangement of components. 
         [0013]      FIG. 1  shows a schematic side view of a multi-color printing machine  1 , having a feeder  3 , an alignment unit  4 , a plurality of printing units  5 , a transport unit  7 , a fusing unit  9 , a duplex path  12  with an inverter  13 , as well as an output tray  15 . Many different types of single or multi-color printing machines are known, and  FIG. 1  depicts only a highly simplified example of such a printing machine  1 . 
         [0014]    The feeder  3  is disposed to hold a stack of sheets and to feed individual sheets to the alignment unit  4 , and is shown at a first end of the printing machine  1 . However, said feeder may also be arranged at any other location and need not feed sheets directly to the alignment unit  4 . The alignment unit  4  is of a suitable type that aligns the sheets supplied thereto in a suitable manner and transfers them to the transport unit  7 . The transport unit  7  is of a known type that is suitable to transport sheets past the printing units  5 . In the depicted embodiment, the transport unit  7  comprises an endless transport belt  17  that is guided around corresponding transport and guide rollers  19 . 
         [0015]    The printing units  5  are arranged adjacent the transport belt  17  and are suitable for printing respective color separation images onto sheets that are transported by the transport belt  17  past the printing units  5 . The multi-color printing machine  1  as shown has five printing units  5  but may have any number of printing units. The printing units  5  are shown as electrophotographic printing units, but they may also be of a different type, such as for example of the ink jet type, capable of transferring a print medium such as toner or ink to a sheet to form an image. 
         [0016]    The fusing unit  9  is arranged downstream of the transport unit  7 , to receive the sheets after printing thereon by the printing units  5 . The fuser  9  may be of any suitable type for fusing or fixing the print medium previously printed onto the sheet. This may be accomplished, for example, by heated pressure rollers or also by other suitable devices such as, for example, a contactless heating device that operates with light or other electromagnetic radiation such as, for example, microwaves. 
         [0017]    The fusing unit  9  is followed by the duplex path  12  that provides a sheet transport path back to the alignment unit  4 . In the duplex path  12 , the inverter  13 , which will be described in more detail hereinbelow, is provided to invert a sheet that is being transported along the duplex path  12 . If a sheet is not to be directed to the duplex path  12  after it has gone through the fusing unit  9 , it is also possible to guide the sheet via an appropriate diverter to the output tray  15 . 
         [0018]    With reference to  FIGS. 2 and 3 , the inverter  13  will be described in more detail. 
         [0019]    The inverter  13  has a first pair of spaced apart rollers  22 ,  23  having a first endless belt  25  entrained thereabout in a figure eight configuration. The first belt  25  has sections  25   a ,  25   b , which in substance extend freely between the rollers  22 ,  23 . The sections  25   a ,  25   b  each provide a 180° twist in the belt  25 . The rollers  22 ,  23  are spaced in a horizontal direction and are offset by approximately the heights of one roller in a vertical direction, such that the belt section  25   a  extends in a substantially horizontal manner. 
         [0020]    The inverter  13  has a second pair of spaced apart rollers  32 ,  33  having a second endless belt  35  entrained thereabout in a figure eight configuration. The second belt  35  has sections  35   a ,  35   b , which in substance extend freely between the rollers  32 ,  33 . The sections  35   a ,  35   b  each provide a 180° twist in the belt  35 . The rollers  32 ,  33  are spaced in a horizontal direction and are offset by approximately the heights of one roller in a vertical direction, such that the belt section  35   a  extends in a substantially horizontal manner. 
         [0021]    The first  22 ,  23  and second  32 ,  33  pairs of rollers are arranged adjacent to each other such section  25   a  of the first belt  25  and section  35   a  of the second belt, contact each other over a substantial portion of the respective sections  25   a ,  35   a  to form a transport path  36  therebetween. At least one of the rollers  22 ,  23 ,  32 ,  33  may be connected to a driving mechanism (not shown) to rotate the same. In the embodiment as shown the rotation is such that the belt sections  25 ;  35   a  of the first  25  and second  35  belt would move from left to right. The transport path  36  has a 180° twist, corresponding to the twist in the sections  25   a , and  35   a  of the first  25  and second  35  belts, respectively. The twist would force a sheet, which is transported into the transport path, such that it is sandwiched between the belts  25 ,  35 , to be twisted and turned over by 180° while the sheet is transported along the transport path  36  through the inverter  13 . The rollers  22  and  32  form an entrance group of rollers for the transport path  36  as can be best seen in  FIG. 3 . Roller  22  is arranged above and slightly offset (to the left in  FIG. 3 ) in the horizontal direction with respect to roller  32 . The horizontal offset is beneficial in forming a flexible entrance nip  36   a  between the belt sections  25   a ,  35   a.  Similarly, rollers  23  and  33  respectively form an exit group of rollers for the transport path  36 . Here, roller  33  is arranged above and slightly offset (to the right in  FIG. 3 ) in the horizontal direction with respect to roller  23 . The horizontal offset is beneficial in forming a flexible exit nip  36   a  between the belt sections  25   a ,  35   a . With respect to  FIG. 2 , it should be noted that sections  25   b  and  35   b  of the belts  25  and  35  were omitted for ease of representation. 
         [0022]    The rollers  22 ,  23  may be of the type as described in U.S. 2003/0034234 A1, which is incorporated herein by reference, having a circumferential recess for at least partially receiving the belt  25  and providing lateral guide surfaces for the belt. Additional guide rollers, contacting and guiding at least some of the belt sections as described in U.S. Pat. No. 6,626,103 B1, which is incorporated herein by reference, may be present. 
         [0023]    The arrangement of the rollers  22 ,  32  of the entrance group and the rollers  23 ,  33  of the exit group in combination with the optional lateral guide surfaces and the optional additional guide rollers influence the twist in the respective sections  25   a  and  35   a , which form the transport path  36 . In particular, there is in substance no twist at the entrance and exit nips and directly adjacent thereto. Only some distance into the transport path, the twist starts and the main portion of the twist, i.e. more than 50% and preferably more than 70% or even more than 80% occurs in a middle section of the transport path. The transport path may be partitioned along its length l extending between the entrance nip  36   a  and the exit nip  36   b  into an entrance section, extending over the first quarter of the length, a midsection extending over the second and third quarter of the length and an exit section extending over the fourth quarter of the length. As indicated above, the main twist occurs in the middle section, which is thus also termed the operating section of the transport path. In the operating section, the angular twist rate provided by the belt sections  25   a  and  35   a  is in substance constant. 
         [0024]    With respect to  FIG. 3 , it is noted that the twist in the belt sections  25   a  and  35   a  is arranged such that an area of a sheet being transported through the transport path  36 , which at the entrance section lies behind the belts  25 ,  35  will be moved from a substantially horizontal orientation upwards and over the belt sections  25   a ,  35   a.  Similarly, an area of the sheet, which at the entrance section lies in front of the belts  25 ,  35  will be moved from a substantially horizontal orientation downwards and under the belt sections  25   a ,  35   a.    
         [0025]    The inverter  13  further has several guide units  40 ,  42 ,  44 ,  46 , and  48  for guiding lateral regions of a sheet being transported through the inverter  13 , i.e. regions which are not sandwiched by the belt sections  25   a  and  35   a.  These lateral regions may also be called edge regions. With respect to  FIG. 3 , it is to be noted that the guide unit  48  was omitted for ease of representation. 
         [0026]    Guide unit  40 , which is best seen in  FIG. 2 , is formed as a guide plate  50  having a leading edge  51 , facing upstream of the transport path  36 , a trailing edge  52 , facing downstream of the transport path  36 , and two lateral edges  53  extending parallel to the transport path  36 . The plate  50  is arranged in substance horizontally and slightly below the transport path  36  in the vicinity of the entrance nip. It is arranged on that side of the transport path  36 , where the edge region of a sheet will move downwards. The leading edge  51  extends at a right angle to the transport path  36 . The trailing edge extends at such an angle to the transport path that the plate tapers outwardly, i.e. away from the transport path  36 . As can be seen by the skilled person, due to the taper, the plate  50  will provide lesser support to outer edge regions of a sheet compared to more central regions thereof. This allows an outer edge of the sheet to move downward earlier than a central portion thereof. The upper surface of the plate  50  and the surface of the trailing edge may be of a material having a low coefficient of friction, such as PTFE. 
         [0027]    Guide unit  42 , which is also best seen in  FIG. 2 , is formed as a guide plate  58  having an elongated rectangular shape. The plate  58  is arranged in substance horizontally and slightly below the transport path  36 . The plate  58  is arranged on that side of the transport path  36 , where the edge region of a sheet will move upwards, and the plate extends lengthwise over approximately the entrance section of the transport path  36 . The upper surface of the plate  58  may be of a material having a low coefficient of friction, such as PTFE. 
         [0028]    Guide unit  44 , is formed as a guide element  60  having a bottom wall  62  and a side wall  63 , the side wall  63  extending in substance perpendicular to the bottom wall  62 . The bottom wall  62  has a first section  65  and an adjacent second section  66 . The first section  65  is arranged upstream of the second section  66  with respect to the direction of the transport path  36 . The first section  65  has a top surface, which is arranged in substance horizontally and slightly below the transport path  36 , at approximately the same height as plate  58 . The second section  66  has planar a top surface, which is angled upwards with respect to the first section  65 , such that it extends from a position below the transport path  36  to a position above the transport path  36 , as can be best seen in  FIG. 3 . In this respect it should be noted that the side wall  63  extends upwards from the bottom wall  62  both in the first  65  and second  66  sections thereof and provides a lateral abutment for a sheet transported through the inverter  13 , in case the sheet is skewed or moves sideways in this direction. The guide element  60  is arranged on that side of the transport path  36 , where the edge region of a sheet will move upwards, i.e. the same side as plate  58 . Guide element  60  is arranged further away from the transport path  36  than plate  58 , i.e. plate  58  is arranged between guide element  60  and transport path  36 . The guide element  60  extends lengthwise over the entrance section of the transport path  36  and approximately half the midsection thereof. The second section  66  extends lengthwise along the middle section of the transport path  36 . The upper surface of bottom wall  62  and the inner surface of side wall  63 , i.e. the surface facing towards the transport path  36 , may be of a material having a low coefficient of friction, such as PTFE. 
         [0029]    Guide unit  46  is formed as a wire  70  having several different sections along its extension. As shown, the wire  70  has a first section  71 , a second section  72 , a third section  73  and a fourth section  74 . Furthermore a separate bridge section  76  is provided, as will be explained hereinbelow. The sections are arranged in the above order along the wire  70  and in the direction of the transport path  36 . 
         [0030]    The first section  71  is in substance a straight wire section, which is arranged in substance horizontally and slightly below the transport path  36 . The first section  71  extends in substance parallel to the transport path (in a lateral sense), as can be seen in the top view of  FIG. 2 . The section  71  basically may be seen as a (partial) extension of the guide surface provided by plate  58 . The plate  58  and the section  71  of the wire  70  thus provide guidance to a lateral portion of a sheet being transported along the transport path in substance parallel to the transport path over a substantial length thereof. Substantially parallel as used herein is supposed to encompass angular deviations of less than 10°, preferably less than 5°. The term over a substantial length is supposed to encompass at least ¼ of a length l of the transport path, and preferably at least ⅓ of said length. 
         [0031]    The second section  72  is in substance also a straight wire section. The second section  72  is angled upwards with respect to the horizontal orientation of the first section  71 . The second section angles upwards at approximately the same angle as the second section  66  of bottom wall  62  of guide element  60 . The second section  72  is also angled with respect to the (lateral) parallel extension of the first section  71 . The second section  72  is angled towards the transport path  36 . A downstream end (in the direction of the transport path) of the section  72  is thus arranged vertically above the transport path  36  (best seen in  FIG. 3 ) and in a lateral sense directly adjacent thereto (best seen in  FIG. 2 ). In the direction of the transport path, the second section  72  may be said to extend in the midsection thereof and in particular extends from the second quarter to the third quarter along the length l of the transport path  36 . 
         [0032]    The third section  73  is a curved wire section. The curve initially defines a slope steeper than a slope of the second section  72  defined by its upward angle. The curve is arranged to change the overall orientation of the wire from an upwards incline to a downwards incline as best seen in  FIG. 3 . The third section  73  is arranged to cross the transport path  36  from a first lateral side thereof to the opposite lateral side, as best seen in the top view of  FIG. 2 . In the direction of the transport path, the third section  73  extends mainly in the third quarter along the length l of the transport path  36  and the third section crosses the transport path in the last third of the midsection and preferably in the second half of the third quarter along the length l of the transport path  36  (corresponding to the last quarter of the midsection, which forms the operating section). 
         [0033]    The fourth section  74  is in substance again a straight wire section. The fourth section  74  is angled downwards (i.e. in heights) towards the transport path  36  (best seen in  FIG. 3 ). In a lateral sense, the fourth section  74  is angled away from the transport path  36  (best seen in  FIG. 2 ). The free end of the fourth section ends in the vicinity of the exit nip  36   b  of the transport path, at a laterally spaced position (best seen in  FIG. 2 ) and just above the transport path (best seen in  FIG. 3 ). 
         [0034]    The bridge section  76  is in substance a straight wire, which extends between the opposite ends of the third section  73  of the wire guide  70  and is connected thereto in any appropriate manner such as soldering, brazing, welding, gluing, etc. 
         [0035]    Guide unit  48 , which is only shown in  FIG. 2 , is formed as a wire  80  having several different sections along its extension. As shown, the wire  80  has a first section  81 , a second section  82 , and a third section  83 . The sections are arranged in the above order along the wire  80  and in the direction of the transport path  36 . 
         [0036]    The first section  81  is in substance a straight wire section, which extends in substance perpendicular from a free end towards the transport path  36 . The first section  81  is inclined upwards with respect to the horizontal orientation of the transport path  36  from said free end. The free end may be positioned just about at the level of the transport path and the opposite end of the section  81  is positioned above the transport path  36 . 
         [0037]    The second section  82  is in substance also a straight wire section. The second section  82  is angled downwards with respect to the horizontal orientation of the transport path  36 . The second section  82  is angled (laterally) with respect to the perpendicular extension of the first section  81  such that it extends at an angle into the direction of the transport path. The second section  82  thus extends laterally towards the transport path. The second section  82  ends before crosses the transport path  36 . The downward angle is chosen such that the end of the second section which is distanced from the first section is positioned below the transport path. In the direction of the transport path, the second section  82  may be said to extend in the midsection thereof and in particular extends from the second quarter to the third quarter along the length l of the transport path  36 . 
         [0038]    The third section  83  is another straight wire section. The third section  83  in angled downward at an angle which is less steep than the downward angle of the second section. The third section is also angled in a lateral sense towards the transport path  36  such that it crosses the same in the lateral direction. As will be understood, the third section crosses under the transport path  36 . The third section is arranged to cross under the transport path  36  at a position along the length l of the transport path which lies in the last third of the midsection and preferably in the second half of the third quarter along the length l of the transport path  36  (corresponding to the last quarter of the midsection, which forms the operating section). The crossing may in the direction of the transport path  36  be at the same position as the crossing of the third section  73  of wire  70 . The third section  83  ends shortly after crossing under the transport path. Another guide unit, not shown, providing an upwardly angled guide such as the one described in DE 10 2007 022 176, which is incorporated herein by reference, may be provided downstream of the third section  83  of wire  80 . Alternatively, the wire  80  may also have a straight or curved fourth section providing an upwardly and outwardly extending guide surface for a sheet. 
         [0039]    Operation of the printing machine  1  and in particular of inverter  13  will be described hereinbelow with reference  FIGS. 1 to 3 , using the example of duplex printing on a sheet, such as a paper sheet, via electrophotographic printing units. 
         [0040]    First, feeder  3  is used to place a sheet against the alignment unit  4 , and said sheet is suitably aligned. Subsequently, the sheet is transferred to the transport belt  17  of the transport unit  7  and, for example, held thereon in an electrostatic manner. The transport belt  17  is transported in a circulating manner in order to guide the sheet along the printing units  5 , which provide a toner image on an upwardly facing first side of the sheet. Due to the plurality of printing units, different color separation images of a multi-color printed picture are suitably transferred to the sheet. Now the sheet with the toner layers applied thereto is guided through the fusing unit  9  in which the toner image is fused. Subsequently, the sheet is guided to the duplex path  12 , where the sheet is turned by the inverter  13  and is then applied to the alignment unit  4  in an inverted manner, i.e. the first side on which was previously printed is now facing downwards. The sheet is again suitably aligned and then transferred to the transport belt  17  to be transported along the printing units  5  for transferring a toner image to the second side of the sheet. This toner image is then fused to the sheet in fusing unit  9  and the sheet is subsequently transported to output tray  15 . 
         [0041]    The inversion process is now described in more detail with reference to  FIGS. 2 and 3 . The sheet is transported into inverter  13 , such that a central portion thereof (i.e. central with respect to a direction transverse to the direction of transport) is received between belt sections  25   a  and  35   a  of belts  25  and  35 . A leading edge of the sheet will first enter the entrance nip  36   a  and will then be transported along the transport path  36 . During the movement of the sheet along the transport path  36 , the central portion thereof will follow in substance the twist in the transport path as described above. 
         [0042]    When the leading edge of the sheet enters nip  36   a , lateral portions of the sheet will be supported by plate  50  of guide unit  40 , plate  58  of guide unit  42  and bottom wall  62  of guide unit  44 . On the side of plate  50 , the support will decrease from the outside edge of the sheet towards the central portion thereof, while the sheet is moved along the transport path  36 , due to the taper of plate  50 . This allows the sheet portion arranged on this side of the transport path to first move downwards at an outer section thereof compared to a more central section. Once the trailing edge of the sheet moves over the plate  50 , the edge  52  avoids a spontaneous rotation of this portion of the sheet, as the support is sequentially decreased towards the central portion. 
         [0043]    On the other side, of the transport path  36  the lateral portion of the sheet will be supported by plate  58  of guide unit  42  and if it has a sufficient width (which is assumed in the following) also by bottom wall  62  of guide element  60  of guide unit  44 . With respect to bottom plate  62 , the sheet will initially supported by the first section  65 , extending horizontally. Upon movement of the sheet along the transport path  36 , the lateral portion will loose support by the plate  58  when the leading edge of the sheet moves over the end of plate  58 . The lateral portion of the sheet will then be supported by the first section  71  of wire  70  of guide unit  46  and also bottom wall  62  of guide element  60 . 
         [0044]    Once the leading edge of the sheet reaches the second section  72  of the wire  70 , the respective lateral portion will be supported mainly by the second section  72  of the wire  70  and an outer edge thereof may still be supported by the bottom wall  62  of guide element  60 , albeit the second, upwardly inclined section  66  thereof. Due to the upward slope of the second section  72  and the second section  72  also being angled (in a lateral sense) towards the transport path, upon further movement of the sheet, the lateral portion will be moved upwards and into a turning motion. The turning motion imparted to the lateral portion of the sheet by the second section  72  of wire  70  starts later than the turning motion imparted to a central portion of the sheet by the twist in the transport path. The turning motion imparted by the second section  72  of wire  70 , however, has a substantially higher angular rate than the one imparted by the twist in the respective section of the transport path. 
         [0045]    The angular rate of the turning motion imparted to the lateral portion of the sheet by the second section  72  of wire  70  while the sheet is transported along the transport path  36 —is at least 1.5 times (preferably at least 2 times) higher than the angular turning rate imparted to a central portion of the sheet imparted by the twist in the transport path. Thus, even though the turning motion imparted to the lateral portion of the sheet starts later than the turning motion imparted to a central portion thereof, the turning rate at the lateral portion is higher, such that it will gain upon the turning of the central portion. Assuming a sheet having a length (in the direction of the transport path) which is longer than half the length of the transport path, this delayed turning motion of the lateral portion may reduce stress in the sheet compared to a turning motion imparted to the lateral portion which in substance follows the turning motion imparted to the central portion of the sheet. As will be realized, the lateral portion may remain in a flatter configuration for a longer time. The higher turning rate imparted by the second section  72  of wire  70  ensures that the lateral portion gains upon the turning of the central portion and thus facilitates in cooperation with the third  73  and fourth  74  sections of wire  70  that the turning motion will be complete before the leading edge of the sheet reaches the exit nip  36   b  of the transport path  36 . 
         [0046]    Once the leading edge of the sheet reaches the third section  73  of wire  70 , the lateral portion will be supported and further turned by the curved section of wire  70  and also the bridge section  76 . The angular turning rate imparted to the lateral portion of the sheet in the third section  73  is again substantially higher than the one imparted to respective central portion of the sheet by the transport path. At least at the point where the third section crosses over the transport path, the lateral portion of the sheet will take an upright orientation or will actually already begin to fall over to the other side. While the bridge section  76  will provide guidance to an intermediate lateral portion of the sheet, the curved portion of wire  70  will draw out any possible curl formed at a lateral edge portion of the sheet to ensure the lateral edge portion to flip over after sheet passes the upright orientation. As previously explained, the third section  72  crosses over the transport path in the last third, preferably the last quarter of the midsection, i.e. the operative section. Thus, the lateral portion will reach the upright orientation downstream of the central portion reaching the upright orientation, which is approximately at the midpoint along the transport path  36 . One the leading edge reaches the crossing over position of the third section (which may correspond to a midpoint of the third section), it is preferred that the trailing edge of the sheet has passed the entrance nip  36   a , i.e. the position of the cross over point may preferably be chosen in accordance with the longest sheet to be guided through the inverter  13 . In so doing, at the point in time when the front edge would start to flip over on its own, the central portion of the sheet at the trailing edge is free to move, i.e. it is no longer tightly clamped between the rollers  22 ,  32  at the entrance nip. However, even if the trailing edge is still in the entrance nip, when the leading edge of the sheet reaches the cross over point of the third section  73 , the delayed upright orientation of the lateral portion of the sheet and the high angular turning rate imparted to the lateral portion are beneficial in reducing stress in the sheet. 
         [0047]    When the leading edge of the sheet moves along the fourth section  74  of wire  70 , the wire ensures the lateral portion at the leading edge to completely turn over before the leading edge of the sheet reaches the exit nip  36   b  of the transport path  36 . 
         [0048]    From the above it becomes clear that the wire  70  guides a lateral portion of the sheet to turn while it passes over the transport path  36 . In a similar manner, wire  80  of guide unit  48  is arranged to guide another lateral portion of the sheet (located on the other side of the transport path to the lateral portion guided by wire  70 ) to turn while it passes under the transport path  36 . 
         [0049]    As described above, the lateral portion on the side of plate  50  is only supported in the vicinity of the entrance nip  36   a  of the transport path. Past the plate  50 , the lateral portion of the sheet may move downwards due to gravity and due to the turning motion imparted by the transport path  36 . This downward movement may be inhibited by the inherent stiffness of the sheet, and in some instances even a slight upward movement of a lateral edge of the sheet may occur due to the sheet curling. Once the leading edge of the sheet reaches the first section  81  of the wire,  80 , the upwards extension of this sections  81  ensures that the respective lateral portion of the sheet will be caught under the wire  80 . When the leading edge of the sheet moves along the second section  82  of wire  80 , the lateral portion of the sheet will be guided downwards and into a turning motion. The angular rate of the turning motion imparted by the second section  82  is again substantially higher than the turning motion imparted to the respective central portion of the sheet. The imparted angular turning rate may be similar to the one imparted by the second section  72  of the wire  70 . When the leading edge of the sheet moves along the third section  83  of the wire  80 , the third section  83  forces the lateral portion of the sheet into a substantially downright orientation and to pass under the transport path  36 . The lateral portion of the sheet will be forced in the downright orientation at approximately the same point along the transport path at which the other lateral portion of the sheet will be forced in the upright orientation. Here, the third section may end or may extend a little bit further and another guide element (not shown may be provided to support an upwards movement and further turning of the lateral section of the sheet at the other side of the transport path  36 . 
         [0050]    The invention has been described in detail with particular reference to certain preferred embodiments thereof, but it will be understood that variations, combinations, and modifications can be effected by a person of ordinary skill in the art within the spirit and scope of the invention.