Patent Publication Number: US-5293205-A

Title: Side to side sheet inverter

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to devices for transporting sheets in or between printers, and more particularly to sheet handlers, feeders, and sheet transport mechanisms which are built into or added onto systems composed of one or more printers. It particularly relates to two-sided printing systems. 
     Among prior art printing or copying systems, it is common to provide, at the output of one print &#34;engine&#34; a transport mechanism to carry sheets that have been printed on one side, and to function as a collating, inverting or other specialized sheet handling system. U.S. Pat. No. 4,879,571 issued to one of the present inventors, Paul Plasschaert, shows one such specialized sheet handling system wherein an entering sheet from the output of one print engine is inverted side-to-side as it undergoes a right angle change in transport direction. 
     The inverting system described in that patent operates by driving a front corner of the moving sheet against a J-bar that extends into the paper path to initiate a curling-over of the corner. As the sheet continues to advance along the transport path, the curled-over edge pushes against an obstruction oriented at 45° across the transport path, which causes the leading end to continue curling over as it makes a right angle turn into the output path. That transport allows the injection of half-printed (e.g., single-side printed) sheets from a side station, into a main printing line with the sheets flipped over for receiving a second print image, thus offering greater flexibility in configuring multi-printer systems. 
     It would also be desirable to flip a printed sheet without changing its transport direction, and preferably to do so in a short transport path. 
     SUMMARY OF THE INVENTION 
     A sheet inverter for use in a printing system in accordance with the present invention includes first and second sheet drivers located along a substantially straight transport path, and a curl bar for curling over a leading edge of a sheet as it is driven by the first sheet driver. The second sheet driver is positioned to capture the curled-over sheet and draw it along at a slightly higher level over a guide bar that angles out from the transport axis. As the sheet is drawn along, the opposite side trailing corner is pulled over the guide bar, completely inverting the sheet in a side-to-side sense. A system according to the invention for producing two-sided prints may use two substantially identical print engines with the inverter connected between the output of the first engine and the input of the second engine. A preferred system, however, employs the inverter in a sheet return path extending from the output to input of a single print engine, to provide already printed sheets back into the printing path. 
     For that embodiment, the inverter preferably further includes a non-inverting path into which the once-printed sheets may be directed when a second printing operation, such as a highlight color operation, is to occur on the same side of the sheet. The non-inverting path feeds the sheet directly in at the second sheet driver, rather than at the input path to the first sheet driver and curl bar. Thus, both an inverting and a non-inverting transport path, can be selected to transport a sheet from the print engine output to its input. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 shows a preferred printing system and sheet inverter in accordance with the present invention; 
     FIG. 2 shows a detail of the sheet inverter with two inlets of FIG. 1; 
     FIG. 3 shows a perspective view of the inverting portion of the mechanism of FIG. 2; 
     FIG. 4 is a top plan view and partially schematic view showing operation of the mechanism of FIG. 3; 
     FIG. 5 is a cross section of a sheet in transit over the roll bar element illustrated in FIGS. 3 and 4; and 
     FIG. 6 shows the profile of a curl guide depicted in FIGS. 3 and 4. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     As shown in FIG. 1, a printing system 10 incorporating a sheet transport according to the invention includes a print engine 1 having an input path 2, an imaging assembly 4, and an output path 6 such that sheets 18 delivered to the input path 2 are imprinted with an image by the imaging assembly and transported along the output path 6. Paths 2, 6 may include feed rollers, moving belts or other known transport mechanisms. A sheet transport unit 20 extends between the output path 6 and the input path 2, and serves to transport already-printed sheets back to the input for further printing. In accordance with a basic embodiment of the invention, the transport unit 20 transports the sheet along a straight line path such that the leading edge of a sheet leaving output path 6 remains the leading edge as the sheet is transported to the input path 2, while unit 20 flips the sheet in a side-to-side sense. 
     This is accomplished by providing a plurality of guide plates that define a sheet transport channel 21a for a sheet input, and a sheet transport channel 22a located at a vertically offset level from path 21a, as shown in FIG. 2. As a sheet 18 is transported along path 21a by transport roller sets 23a, 23b, 23c a side of its leading edge is curled over into the offset path 22a. Thereafter, a second set of transport roller sets 24a, 24b, 24c engage the curled over edge and pull the sheet in alignment along path 22a, completing the flip-over of that sheet. The use of multiple roller pairs in each path serves to maintain a straight orientation of the sheet as it travels along its flipping path between input A and output B. 
     In the illustrated embodiment, a parallel input path 21b with transport roller pairs 23d, 23e provides a second input that also feeds into the output path 22a. This path feeds into the output path 22a at a shallow angle to essentially feed sheets along a straight-through path from the printer output to the input for a second pass without flipping, as described further below. 
     The operation of the lateral flip-over operation occurring as the sheet passes from path 21a to path 22a is best seen in the perspective view of FIG. 3. The upper guide plates and rollers defining input feed path 21b are removed for clarity of illustration. 
     In the illustrated portion of the mechanism, a lower guide plate 26 serves as a platen along which a sheet is driven by the previously described transport rollers in path 21a. As shown, each set of transport rollers consists of one or more upper rollers, each of which is opposed to a corresponding lower roller to grip the sheet therebetween and drive it along the platen. 
     The first set of rollers 23a consists of one drive pair on an upper axle 25a, and one drive pair on a lower axle 25b. The two axles may both be driven, synchronously in opposite senses, so that the sheet is gripped between each set of upper and lower rollers and driven straight along the path. Preferably, however, one axle is driven, while the other carries idler rollers. The second and third sets of drive rollers have a rocking support mechanism, best seen in the upper part of transport drives 24a, 24b, that holds an idler roller above each of the lower driven rollers. All of the lower rollers are driven synchronously, so that the sheet is carried along at a uniform speed by successive engagement with each set of rollers, while the double rollers 23a, 24c at the input and output assure that the sheet enters and leaves the transport in a straight path without fishtailing. 
     Operation of the mechanism proceeds as follows. After the leading edge of sheet 18 passes transport rollers 23a, the right leading corner 18a of the sheet contacts a deflector pin 30 which hooks upward in its path, and curls the sheet over so that it progressively bends as it advances. The curled over corner 18a is caught under a guide cover 27 formed of sheet metal and having a Z-shape in vertical section. The cover forces the corner to lie in a plane parallel to and above its original path. The side wall of the guide cover is disposed obliquely inward toward a stationary roll guide 32 so that the bend in the sheet forms a shoulder 19 which, as it lengthens, fits against the concave face of the roll guide 32, located upstream from the deflector pin and closer to the center line of the transport path. The pressure of the roll guide at this position causes the sheet to curl over further, so that the curled-over right front corner 18a is brought entirely over into a position aligned with the left edge of sheet 18. During this stage of advance, pressure from the transport rollers 23a, 23b cooperates with other elements, as described below with regard to FIG. 4, to bring the sheet into the proper orientation and maintain the sheet in alignment. 
     As the leading corner 18a curls over, it comes to rest on upper guide plate 36 and is gripped by the first of the output transport path drive roller pairs 24a. Shoulder 19, meanwhile, curves around and over a roll bar 34, an elongated curved rod that has a proximal end 34a located to catch and orient the shoulder or bend in the sheet as it advances. Roll bar 34 has an extended central portion 34b oriented parallel to the paper path that helps to maintain the sheet orientation as it is transported by the side rollers 24a, 24b, by guiding the shoulder without exerting any lateral pull that might skew the sheet. 
     At this stage, the turned over leading end of the sheet is engaged by upper transport path rollers 24a, 24b, while the trailing rear edge of the sheet is still engaged by lower transport roller 23c. As the sheet leaves roller 23c, its front end shoulder moves along the third and final portion 34c of the roll bar. This portion angles outwardly to the right, so that as the sheet advances the trailing left underside of the sheet is drawn over the roll bar, completing the side-to-side turnover of the sheet. The edge 18a is held straight all the while by rollers 24a, 24b, so that the completely inverted sheet is then traveling with the correct alignment when it is engaged on both sides by double drive rollers 24c at the output. As the sheet continues to travel, the trailing left undercorner is brought entirely around so that the full sheet lies flat and inverted on the upper guide plate 36. In this manner, the sheet is inverted as it travels along a substantially straight line direction, without switchbacks or branches. The total path length is approximately two to three sheet lengths, about the same as the path length through a typical print engine, so the sheet inverter may conveniently reside above, adjacent to, or below the engine, connected via a switched sheet deflector or simple relay transport. 
     FIG. 4 shows a top plan view, partly schematic, of the transport mechanism and sheet progression as illustrated in FIG. 3. As seen in this top view, the roll bar 34 hooks outwardly at its proximal end 34a to urge the shoulder of the curling sheet into the channel 38 of roll guide 32, which, in turn, is positioned essentially at the centerline of the sheet. Thus, just before the sheet 18 leaves drive roller 23b its leading edge arrives at the straight portion 34b of the roll bar and its middle portion is urged into the channel 38 of the roll guide (FIG. 6), so that the free trailing edge follows and the curling shoulder extends essentially straight parallel to the transport path. This positions the sheet straight, so it travels thus bent into a U-shape as shown in FIG. 5 over the initial output drive rollers 24a, 24b until its leading edge commences to uncurl over the outwardly-angled uncurling bar 34c at the end of roll bar 34. At that time it remains engaged by at least two output drive rollers, so it does not get pulled out of alignment as the sheet is drawn over the oblique portion 34c of the roll bar. 
     The sheet turnover mechanism as just described can extend in a straight path between the output of one printer and the sheet input of a second printer, which is to print a second image on the turned-over side. In a preferred embodiment, however, the mechanism is positioned between output and input of a single print engine, preferably an engine of a type which uses electrographic control signals to form an image. The image signals are changed for each pass of the sheet so that after the sheet has been fed through once, the turnover mechanism inverts it to receive a second image on its other side. A preferred electrographic printer for implementing a system of this type is a so-called &#34;ionographic&#34; printer, of the type described, for example, in U.S. Pat. Nos. 4,160,257, 4,992,807, or the like. Magnetic or laser printers, pin array printers, or copier machines having photoconductive imaging members may also be used. 
     A further preferred embodiment of a printing system utilizing the sheet turnover mechanism of the present invention performs multipass printing on one or both sides of the sheet, utilizing the mechanism of FIG. 3 with a parallel non-inverting input transport 21b as shown in FIGS. 1 and 2. A logic unit controls a path switching element at the output of the printer to feed a sheet to path 21b if further printing is to be performed on the same side, and to path 21a if the next printing operation is to be performed on the other side of the sheet. The further printing operation on the same side may be, for example, a highlight color printing, a magnetic toner character printing as for a check code printing operation, or printing of an additional color of a multipass color printing operation. In another embodiment, a second pass printing with the same color is used to extend the contrast or density range of a one-color printing process. 
     It will be appreciated that the sheet turning mechanism as described above has a straight-through short path geometry adaptable to many printer configurations, and that the invention contemplates diverse systems containing one or more printers, interconnected by one or more such transport mechanisms, to effect plural different printing operations on a single sheet. Further, the basic mechanical arrangement of elements in the transport assembly may be varied by substitution of equivalent elements. For example, the input sheet transport may be positioned above the output sheet path, rather than below it, with the elements arranged to curl a sheet downwardly into the output path. Drive belts may replace the rollers for contacting the sheet at one or more positions, and the guide plates may be modified in shape or even replaced with other types of guiding or channeling elements. Systems may be configured with multiple print engines and different path-changing options. 
     This completes a description of the transport according to the present invention, and an illustrative printing system utilizing that transport. The invention in these embodiments being thus shown and described, various modifications and improvements will occur to those skilled in the art, and all such modifications and improvements are considered to be within the scope of the invention, as defined by the claims appended hereto.