Abstract:
A method and an apparatus for performing a double sided imposition verification on sheets ( 7 ) of paper or another material of impositions ( 9 ) prepared by means of digital data ( 1, 2 ) and comprising several print pages consisting of text and/or images, and comprising a printing unit ( 3 ) in the form of a plotter/printer having a paper magazine ( 4 ) and means ( 5 ) for feeding one sheet ( 7 ) of paper at the time into the printing unit ( 3 ), means ( 17 ) for guiding said sheet ( 7 ) of paper into the printing unit ( 3 ) and for turning said sheet ( 7 ) of paper for making it possible to print said sheet of paper on both sides thereof, means ( 24 ) for observing the existence of sheets ( 7 ) of paper in the feeder unit, and means ( 25 ) for observing the front/rear edge of a sheet of paper, feeder and stop means ( 22, 23, 26 &lt;-&gt; 14 ) for the sheet of paper, and means for turning the direction of rotation of the feeding-in and feeding-out means ( 22, 23 ) for sheets ( 7 ) of paper into and out from, respectively, said printing roll ( 14 ).

Description:
BACKGROUND OF THE INVENTION 
   The present invention relates to a method and an apparatus intended to be used, in the first place, within the reproduction process area in the graphic industry, and more particularly for enhancing the possibility of preparing printing plates, for instance for offset printing, having correct impositions. 
   In planning the layout of magazines and books etc the pages must be placed in such an imposition that said pages, after folding of the printed page appear in a chronological order. As an assisting means for making the layout correct and the pages to follow each other in correct order a verification of the impositions is made. 
   Preparing “impositions” means that, for instance 8 pages (or 16, 32, 64 pages, etc) are placed in such order on the paper to be printed, which paper is of a predetermined size, that the pages, after the print paper has been folded, follow in a chronological order of page numbers, so that, in an 8 page print sheet, pages 1, 4, 5, 8 appear on the front side of the print sheet, and that pages 2, 3, 6 and 7 appear on the rear side of said print sheet. 
   PRIOR ART 
   In presently used methods for verifying the correctness of impositions it has been possible to check only one side of the print sheet. This is unsatisfactory since the rear side of the print sheet (the supplement side) is missing in the verification print out, and that there is consequently no possibility of checking that the impositions both for the front side and for rear side coincide with each other. 
   In conventional printing processes according to modern methods there are used computers for creating the print pages comprising text and images. A print page is what is seen in a book, a magazine, a catalogue, a newspaper etc. Digital data for said print pages are passed through several steps, which result in a so-called imposition. An imposition shows the position of each page of a printing sheet, generally comprising 8, 16, 32 or 64 print pages. The position of each side consisting of digital data is determined both considering the printing machine, for instance an offset printing press, an intaglio printing machine, or in some cases a letterpress printing machine by means of which the sheet is to be printed, and also by the method according to which the printed paper is folded after the printing step. Since the printing is made on both sides of the paper there is a need for two impositions. A first imposition contains data for printing of the front side of the print sheet and the second imposition contains data for printing of the rear side of the print sheet. 
   Previously the impositions used to be made on transparent films which were mounted such as to form a first imposition—for the front side—and a second imposition for the rear side. Said two impositions thereafter were exposed onto a first offset plate for the front side and a second offset plate for the rear side. 
   It is important that it is possible to verify that the pages of the impositions for the front side and the rear side, respectively, fall within the same area. For verifying that this is true it is possible to place the transparent films over each other and to visually check that said films have a mutual perfect register. Often there are used register marks, like hair crosses, which, at perfect register, have to be placed straight on top of each other, what is interpreted as a proof of perfect register between the impositions of the front side and the rear side, respectively, of the print sheet. 
   The work using films for the impositions is time consuming and expensive and requires several work moment which generally have to be performed in a dark room. Modern technics have made it possible to eliminate the need for film handling, and—instead thereof—to expose digital data directly from a computer onto the printing plate, generally by a laser heat print technics according to a method which is generally referred to a CTP technics (Computer To Plate), whereby the costs and the work involved in using transparent film handling processes are completely eliminated. By eliminating the need for use of films there have, however, appeared difficulties to verify or prove that the digital data for the impositions of the front side and the rear side, respectively, have obtained a perfect mutual register. 
   THE INVENTION 
   The invention is intended to solve the above mentioned problem by providing a quick and simple method of preparing a print sheet containing both a front side and a rear side. This is possible by using a type of printer or plotter which is formed to be able to print on both sides of a paper, generally using a rotary printing roll, over which the paper is moved in two steps, a first step whereby the printing is made on one side (front side) and a second step at which the paper is turned and the printing is made of the rear side of the paper. 
   Simple printing apparatus, for instance matrix plotters, are known in the art, which apparatus operate on rolls of paper, generally formed with guide bores along the edges of the path of paper. Said known apparatus generally have been formed so that it is possible to print only on one side of the paper. 
   Since also the printing is generally made on plane sheets of paper there has been a wish that the verification of the impositions for the front and rear sides can be made directly on plane sheets of paper, preferably the same type of paper, cut like the sheet of paper which is to be used in the printing machine. To this end the invention relates to a verification or proof print machine having a cassette in which the print papers are placed, and from which one paper at the time is lifted out and is introduced in the verification print machine, and more particularly in which the printing is made both of the front side and on the rear side of the print paper. 
   The impositions having a predetermined number of pages, each consisting of text and images, are made up as known in the art in a computer, and digital information from the computer is transferred to the printing unit in which the impositions are printed with an accurate positioning, controlled by information from the computer. When the printing of the first page, for instance the front side of the print sheet is ended a feeding in and feeding out unit of the printer/plotter takes over the print sheet and turns same and feeds it back into the printing unit. A printing is thereafter made on the rear side of the sheet of paper corresponding to the imposition for the rear side. 
   When the second (rear) side of the sheet of paper has been printed a double sided print sheet has been simulated. It is now possible to check the positioning of pages on both sides of the print sheet and the positioning of each separate page on the double page. 
   In this connection it should be mentioned that it is important that the print sheet from the paper cassette is introduced in the printing unit with the front edge of the paper exactly parallel to the axis of the printing unit and with the side edges of the print sheet exactly perpendicular to said axis. It may, however, happen that the print sheet is introduced slightly obliquely into the printing unit. If no correction of the position of the print sheet is made this may have as an effect that the impositions are printed at some angle to the front edge of the paper, and that the impositions thereby do not appear at exact mutual registers. It is therefore important that the sheet of paper is oriented so that the front edge and the rear edge of the paper are always arranged straight against a register edge of the printing apparatus. 
   For solving said problem the invention also is directed to a method and an apparatus for positioning the sheet of paper with the front edge and the rear edge thereof, respectively, exactly straight against a register edge of the verification printing machine. 
   A complete method for verifying the correctness of impositions for a front page and a rear page, respectively of a printing sheet is made as follows:
     a) digital data comprising text and images are created in a computer,   b) whereby a verification machine for printing on both the front side and the rear side of a print sheet is used,   c) cut plane sheets to be printed are placed in a cassette, which is connected to the verification print machine,   d) digital data simulating the imposition for the front side is sent from the computer to the verification print machine,   e) a sheet to be printed is lifted out of the paper cassette and is fed, with the “front edge” thereof into the verification print machine,   f) the sheet to be printed is fed to an exact position, or is, upon need, rearranged to take an exact position and is introduced into the verification print machine via a feeding and turning unit,   g) digital data corresponding to the imposition of the front side are printed on the sheet of paper,   h) after said printing of the front side the print sheet is turned,   i) the print sheet is, with precision, fed, with the “rear edge” thereof to the printing machine, eventually after some adjustment of the position like in connection to the printing of the front side,   j) digital data corresponding the imposition of the rear side are transferred from the computer to the verification print machine,   k) the rear side of the sheet of paper is printed using digital data.   

   By now studying the impositions on the front side and the rear side of the printed sheet of paper, eventually after said sheet has been folded, it is possible to accurately verify that the impositions of the front side and rear side show a perfect mutual register. 
   For foreseeing that the sheet of paper comes to engagement with the front edge and the rear edge thereof, respectively, exactly against an engagement edge of the verification print machine, even if said sheets of paper should happen be introduced obliquely, it is made as follows:
         the sheet is fed (at small steps) in the direction towards the engagement edge of the verification print machine ( FIG. 4A ),   the feeding is made by means of step feeding rolls from a position spaced from the front edge,   so that the paper is buckled, which buckling may be greater or less at the left hand edge or at the right hand edge of the paper depending on the oblique positioning of the paper ( FIG. 4B ),   the printing unit locks the front edge thus correctly positioned, for instance in that the print sheet is introduced a very slight distance into the printing unit ( FIG. 4C ),   the step feeding rolls are released from contact with the sheet of paper, whereby said paper is given a possibility of being turned into a correct position, corresponding to the buckling of the paper,   whereby the paper is pulled into the verification print machine and the paper is printed ( FIG. 4D ).       

   
     THE DRAWINGS 
     Now the invention is to be described more in detail with reference to the accompanying drawings, in which  FIG. 1  is a perspective view of a verification or imposition proof machine according to the invention.  FIG. 2  diagrammatically discloses the system for performing the method according to the invention.  FIG. 3  diagrammatically, and in a vertical cross section view, shows an apparatus according to the invention, and illustrates the actual method.  FIG. 4  shows in four different projections, A–D, the method and the apparatus for straightening up an obliquely introduced print paper. 
   

   DETAILED DESCRIPTION 
   As best evident from  FIG. 2  a complete system for preparing a printing plate, using the invention, comprises a computer  1  having a viewing screen and a storing unit  2 , a verification printer  3  with a paper sheet magazine  4  and a feeding and turning unit  5  for the sheets in the printer  3 , and a unit  6  for finally making films and/or printing plates. The verification print system  3 ,  4 ,  5  provides a unit which is directly connected to the storing unit  2  of the computer  1 , and also the plate making unit  6  is connected to the storing unit  2  of the computer  1 . 
   The computer is formed for providing impositions, as known in the art, having a front side and a rear side thereby providing printing sheets, which impositions, according to the invention, can be verified by being printed onto a test print sheet  7  in the printer  3 . Each imposition comprises several pages  8 , for instance 4, 8, 16, 32 or 64 pages, each page comprising text and/or images, which pages  8  are positioned in an accurately determined pattern and in accurately determined locations in relation to each other so that the pages  8 , after the printing sheet  7  has been folded will be located at exactly desired positions of the printing sheet. It is thereby important that the imposition  9  of the pages for the front side of the printing sheet is located in exact register with the imposition of the pages for the rear side of the printing sheet. It is for the verification thereof that the invention has been developed, even if the invention is useful also for other technical fields, especially within the graphic field. 
   As best evident from the diagrammatic illustration in  FIG. 3  the printing part of the printer  3 , which is known per se, comprises a control unit  10  which is connected at the rear side of the printer  3 . At the front side the printer is formed with a carrier  11  for ink cartridges  12 , for instance of ink jet type, preferably the four normal colours yellow, magenta, cyan and black for making 4-colour images. The ink cartridge carrier  11  is mounted on a horizontal lineary guide bar  13  on which the entire cartridge unit  12  is displaceable in two opposite directions while printing a sheet  7  of paper which is rotated stepwise past the ink cartridges  12  with the print paper supported on a printing roll  14 . The printing of the colour or coulors from the cartridges  12  is controlled by the control unit  10  of the printer. 
   As soon as the guide unit  10  of the printer observes that data is coming from the computer  1 ,  2  a central guide unit  15  is activated, which guide unit, in turn, checks that there is paper  7  in the paper magazine or the cassette  4 . In the illustrated case the cassette  4  is mounted at an angle of about 45°. The checking means for paper may be a sensor  16 , which is mounted underneath the cassette  4 . The cassette is mounted at the entrance of a guiding, feeding and turning unit  17  for a paper, which has been connected to the printer. For providing an feeding-in of the uppermost sheet  7  of paper in the cassette a solenoid  18  pushes the front edge of the cassette upwards so that said uppermost sheet  7  can be seized by suction cups  19  and can be lifted up to the upper side of down holding means  20 , whereby a single sheet  7  is separated from the cassette. The solenoid  18  is thereafter de-activated, whereby the remaining sheets  7  of paper in the cassette fall back to the bottom of the cassette. 
   For feeding the separated sheet  7  into the printing unit feeder rolls  21  are actuated to introduce the sheet of paper into the feeder unit  17 , in which the sheet is seized and moved forward by secondary drive rolls  22  and counter rolls  23  in the feeder unit co-operating therewith, so as to feed said sheet forward until a feeder/turning sensor  24  is activated. The feeder rolls  21  of the paper magazine/cassette are stopped, and the solenoid  18  is de-activated, whereby the bottom plate of the cassette falls back to the bottom of the cassette. The feeding and turning sensor  24  observes the presence of the sheet of paper, and the central unit  15  thereby brings the counter roll  23  to become lowered towards the secondary drive roll  22  which starts rotating in the clockwise direction, whereby the sheet  7  of paper is moved on along the lower guide plate  17   b  until said sheet  7  of paper reaches a second sensor, the media sensor  25 , which is arranged to observe the front/rear edge of a sheet of paper and which is mounted at an introduction edge at the printing roll  14 , which edge is formed by counter press rolls  26  engaging the printing roll  14 ,  FIG. 4A . The front edge of the paper is thereby stopped whereas the feeder rolls  22  are stepped forwards, whereby the paper becomes buckled as indicated in  FIG. 3B . The counter press rolls  26  which have so far been still standing in contact with the printing roll  14  now slowly begins to rotate together with the printing roll, so that the straightened up front edge of the sheet of paper is seized, is pulled in a little distance between the printing roll  14  and the counter press rolls  26  and is blocked by the printing unit,  FIG. 4C . The feeder rolls  22 ,  23  are opened and the buckled paper is straightened up rearwardly from the front edge of the paper which is parallel to the axis of the printing roll  14 ,  FIG. 4D . The buckling of the paper eliminates lack of lineary positioning of the front edge of the sheet of paper and foresees that said front edge extends parallelly to the axis of the printing roll  14 . 
   The sheet of paper now is positioned with the front edge thereof exactly parallelly to the axis of the printing roll  14 , and the printing is commenced in that the printing roll  14  starts rotating in the counter clockwise direction, as indicated with the arrow in  FIG. 3 , whereby the paper in pulled round the printing roll  14 . The printing roll  14  now feeds the paper in the counter clockwise direction round said printing roll  14  guided by the guide plates  17  until the media sensor  25  is de-activated in that the rear edge of the sheet of paper reaches as far as to said media sensor  25 . 
   The central control unit  15  counts the number of pulses, and the length of the sheet of paper is calculated by the control unit  10 . 
   The control unit  10  now has full control over the positioning of the paper in the print position. In this position the front edge of the sheet of paper is about 25 mm displaced to the left of the ink cartridges  12 . The ink cartridge carrier  11  is now moved transversally to the sheet of paper parallelly to the axis of the printing roll  14 . A paper sensor  27 , which is mounted on the ink cartridge carrier  11 ,  12  observes the parallellity of the side edges of the sheet of paper and the width of the paper. Said width of the sheet of paper is calculated by the control unit  10  of the printer  3 . 
   Digital data from the computer  1 ,  2  corresponding to the imposition  9  of the front side is now printed on one side of the sheet  7 . As soon as the media sensor  25  has been de-activated, what happens when the rear edge of the sheet passes the sensor  25  and digital data corresponding to the actual side are ready printed the central unit  15  instructs the counter rolls  23  to lower into contact with the secondary feeder rolls  22  and said feeder rolls  22  now start rotating in the counter clockwise direction, whereby the paper is pulled down in the part  17   a  of the turning unit until the rear edge of the paper has reached the turning sensor  24  for turning the sheet of paper. In this position the feeder rolls  22 ,  34  have held the paper secured thereby preventing same from becoming moved to an oblique position. Thereby the first side of the sheet of paper has been printed. 
   The printing of the second side (rear side) is started in that the central unit  15  now brings the secondary feeder rolls  22  to rotate in the clockwise direction in co-operation with the counter rolls  23 , so that the sheet of paper is introduced against the upper guide plate  17   b  of the turning unit  17 . The introduction is prosecuted so that the sheet of paper comes into contact with the feeding edge ( 26 &lt;-&gt; 14 ) and becomes buckled in the same way as described above as concerns the front edge. The rear edge which is now facing upwards passes and activates the media sensor  25  and comes into engagement with the counter rolls  26 . Said counter rolls  26  now once again start rotating slowly together with the printing roll  14 , and the rear edge of the sheet of paper is introduced and is locked between the printing roll  14  and the counter press rolls  26 . The secondary feeder rolls  22 ,  23  are opened and the paper is straightened up—if actual—so that the rear edge (now facing upwards) of the paper is parallel to the axis of the printing roll  14  and the paper  7  is straightened out. The printing roll  14  with the paper  7  is once again rotated in the counter clockwise direction and the paper is guided in the direction towards the lower guide plate  17   a  of the turning unit  17  until the media sensor  25  thereof is de-activated in that the end edge (formerly the front edge) of the sheet of paper passes the sensor  25 . The central control unit  15  counts the number of pulses and the length of the paper is calculated by the control unit  10  of the printer. 
   The secondary feeder rolls  22  and counter rolls  23  are released from each other, and the printing roll  14  now is again rotated in the counter clockwise direction, and the paper is moved round said printing roll  14 . Now the paper is, with the forwards facing edge located parallelly to the axis of the printing roll  14  and moved to a position underneath the ink cartridges  12 . 
   The cartridge carrier  11  with the ink cartridges  12  mounted thereon move reciprocally on the lineary guide bar  13 , parallelly to the axis of the printing roll  14 . The paper sensor  27  on the cartridge carrier  11  observes the side edge parallellity of the sheet of paper and the width of the sheet. The size of said width of the paper is calculated by the control unit  10 . The sheet now has been turned upside down, and therefore the rear side is located underneath and facing the printing unit  12 . Digital data representing the imposition of said rear side now are fed from the computer  1 ,  2  to the printer  3 . As soon as the media sensor  25  is de-activated, what happens when the rear edge of the sheet has passed the sensor  25  and after digital data corresponding to the actual side, the rear side, have been ready printed, the central unit  15  instructs the counter rolls  23  to be lowered into contact with the secondary feeder rolls  22 , whereby the sheet of paper is seized. The feeder rolls now are rotated in the counter clockwise direction whereby the sheet of paper is fed downwards into the lower part  17   a  of the feeding unit  17 . The counter rolls  23  are raised from the paper, and the paper slides out of said feeding unit  17  and drops down in a receiver. The sheet of paper now has been printed on both sides, and it is easy to verify that the impositions are correct both at the front side and at the rear side, and that the impositions for the front and rear sides are in correct register with each other. If this should not be the case a correction is made in the computer, and a renewed imposition verification is made as described above. 
   Even in the invention is described above mainly as concerns verification of impositions on a printing sheet in the graphic industry it is obvious that the invention is useful in other technical fields, especially for introducing flexible sheets of material in exact positions in a processing machine, whereby there is used a buckling of said flexible sheet for automatically direct the sheet into a correct position in the machine. 
   REFERENCE NUMERALS 
   
       
         1  computer 
         2  storing unit 
         3  verification printer 
         4  sheet magazine 
         5  feeding/turning mechanism 
         6  plate/film processor 
         7  printing sheet 
         8  side 
         9  imposition 
         10  control unit 
         11  carrier 
         12  ink cartridge 
         13  bar 
         14  printing roll 
         15  central control unit 
         16  paper sheet sensor 
         17  feeding/turning unit 
         18  solenoid 
         19  suction cup 
         20  down holding means 
         21  feeder roll for magazine 
         22  secondary feeder roll 
         23  counter roll 
         24  sensor for turning media 
         25  media sensor front/rear edge 
         26  counter press roll 
         27  paper sensor (on carrier  11 )