Abstract:
A machine for manufacturing flat bottom bags includes a drum for carrying along at least one blank having a flattened tubular shape in an advancing direction, a first scoring apparatus upstream of the drum for marking on the blank a first folding line which is transverse with respect to the advancing direction, a front gripper on the drum that grips a front edge of the blank on the drum, in an opening step, and and carries along the blank, a first lateral gripping means on the drum that grip the lateral edges of the blank in the region of the first folding line.

Description:
BACKGROUND 
       [0001]    The invention relates to a machine for making flat bottom bags, in particular paper bags, of the type including a drum onto which a blank bag is driven and follows various folding and gluing operations in order to form the bag. 
         [0002]    Flat bottom bags have been known for many years now, used for example to carry provisions or shopping in stores. Such a bag comprises a tubular envelope closed at one end by a flat bottom. The flat bottom is obtained by the pressing, the folding down of the flaps from the envelope and the gluing of these flaps together. 
         [0003]    The manufacture of these bags has been more or less automated and according to various techniques. One of them is detailed, for example, in document U.S. Pat. No. 3,554,099. The machine described in this document shows that a blank is first of all formed and presents two skins connected together along two opposite edges by two bellows. The blank is placed flat on a table, one of the skins, called the lower skin, being against the table, the other skin being above and the bellows being pressed down between the two skins. The blank is driven in translation on the table in a direction parallel to the bellows towards a peripheral drum from which various stations are distributed. 
         [0004]    First scoring means make a first fold line at the bellows, and a second fold line at a predetermined distance from the first line, in a direction perpendicular to the bellows. The scoring means comprise a rotary blade holder and additional slots carried by a counter-rotating drum. As it progresses, the front of the blank is gripped between the drum and a roller. The drum comprises front pinching means to drive the blank by pinching the front edge of the lower skin. To facilitate the separation of the two skins, the opening roller includes suction means which maintain the upper skin against the roller after it has passed between the roller and the drum. Set back in relation to the front pinching means, first lateral pinching means pinch the lateral edge of the blank at the junction between the bellows and the lower skin. Symmetrically, two pinching means pinch the lateral edge of the blank at the junction between the bellows and the upper skin. When the blank continues to pass between the roller and the drum, the lateral pinching means move away from each other, opening the blank and forming a fold on the bellows in a radial plane of the drum and the roller. The two pinching means release the blank whilst in the continuation of the movement of the drum which drives the blank, spatulas press down the open part towards the centre. The upper skin is folded towards the rear at the second fold line. 
         [0005]    In the continuation of the operations, second scoring means mark two transverse lines, on either side of the second fold line. One of these lines coincides with the first fold line. Then, adhesive is applied to the two flaps delimited by these fold lines and they are pressed down one onto the other to close the flat bottom and thus form the bag. 
         [0006]    Such a machine allows a high production rate to be attained as all the manufacturing operations are automated. However, it is designed for a single bag format. When another format is to be produced, many parts must be changed and laborious adjustments must be made. Also, certain characteristics of the bag produced depend on the production rate in such a way that it is difficult to anticipate these characteristics by an adjustment when the machine is stopped. The characteristics must be checked when the machine is in operation, then new adjustments must be done on the stopped machine before a new validation in operation. 
         [0007]    The aim of the invention is therefore to provide a machine for making flat bottom bags allowing the bag format to be changed very rapidly and easy adjustment of the characteristics of the bags produced. 
       SUMMARY 
       [0008]    With these targets in mind, the object of the invention is a machine for making flat bottom bags, comprising a drum to drive at least a blank of flattened tubular form in a direction of advance, first scoring means upstream of the drum to mark on the blank a first transverse fold line in relation to the direction of advance, front pinching means on the drum to pinch a front edge of the blank on the drum, in an opening stage, and to drive the blank, from the first lateral pinching means on the drum to pinch the lateral edges of the blank at the first fold line, the machine being characterised in that it comprises first adjustment means to adjust in operation both the distance between the front edge and the first fold line and the relative angular position of the front pinching means and of the first lateral pinching means. 
         [0009]    The position of the first fold line in relation to the front edge of the blank is related to the position of the first lateral pinching means which must pinch the blank in the alignment of this first fold. By changing these two characteristics in a synchronous manner, this characteristic can be adjusted in operation. By allowing one of the characteristics of the flat bottom bag to be adjusted in operation, means are thus provided which do not require the stoppage of the machine to observe the effect of the adjustment directly on the manufactured product. The adjustment time is thus reduced and the quantity of bags produced during the adjustment phase is reduced. 
         [0010]    According to a constructive arrangement, the drum comprises a first element carrying the front pinching means, a second element carrying the first lateral pinching means pivotally mounted in relation to the first element around the rotational axis of the drum, the first adjustment means determining the relative angular position between the first and the second element. As the first element and the second element can be offset angularly, the relative position of the front pinching means and of the lateral pinching means can be adjusted. The offset can be obtained by a relative actuation between the two elements. It can also be obtained by each being driven by the same motor, the first adjustment means comprising a reduction gear allowing an angular offset to be introduced between the drive of each of the two elements. Such a reduction gear comprises, for example, two planetary gear wheels of same axis, each wheel meshing with a satellite, the satellites being attached to the same rotation shaft. The reduction ratios between one and the other of the wheel/satellite pairs are different. The satellite axle is rotatively mounted on a satellite holder, which is pivotally mounted around the same rotation axis as the planetary gears. One of the planetary gears receives the movement from the motor, whereas the second planetary gear transmits the movement to one of the drum elements. The difference in the reduction ratio introduced by the two planetary/satellite gear pairs is compensated for by an adaptor so that the two elements are finally driven at the same speed. By modifying the position of the satellite holder, an offset is introduced into the drive movement between the two elements and these are angularly offset. 
         [0011]    According to an improvement, the first scoring means are provided to mark in addition a second fold line on the bag at a distance from the first fold line, the machine comprising second adjustment means to adjust in operation the distance between the first and second fold line. The second fold line corresponds to a fold of the upper skin which will be pressed down around this line towards the rear as in the prior art described previously. The distance with the first fold line corresponds to half of the dimension of the flat bottom in the direction of advance. The second adjustment means allow another characteristic of the bag to be modified, that is the position of the press down at the rear of the upper skin. 
         [0012]    In a complementary manner, the machine comprises second scoring means to mark, after the opening stage, a third fold line on the blank, the second adjustment means being provided to adjust, in addition, the distance between the first and third fold line so that the second fold line will be at mid-distance between the first and the third fold line. The third fold line marks the limit of the flat bottom and around which a flap will be pressed down forwards. The adjustment of the position of this third line allows the adjustment of the dimension of the flat bottom to be completed in the direction of advance in a synchronised manner with the position of the second fold line. 
         [0013]    According to a complementary arrangement, the two scoring means are provided to mark a fourth fold line at the same location on the blank as the first fold line. The part included between the front edge of the blank and this fourth fold line being pressed down rearwards in a subsequent operation in such a way that the flat bottom will be delimited by the third and fourth fold line. 
         [0014]    According to a constructive arrangement, the two scoring means comprise a first die on a third drum element to mark the third fold line, the third element being pivotally mounted in relation to the second element around the drum rotational axis, the two adjustment means determining the relative angular position between the second and the third element. The first die is capable of receiving a blade which pushes back the blank into the die to make the scoring, in a manner known itself. The relative position of the second and the third element can be adjusted in the same way as the relative position between the first and the second element, that is with similar components. 
         [0015]    In a complementary manner, the second scoring means comprise a second die on the second element to mark the fourth fold line. The first fold line is maintained by the first pinching means on the second element. The second die is therefore placed on the second element to make the fourth fold line at the same location as the first fold line. 
         [0016]    According to a constructive arrangement, the first scoring means comprise a first marker roller and a counter-roller which are counter-rotative with rotational axes parallel to that of the drum and placed on either side of a trajectory for the blanks, the first marker roller comprising a first blade and a second blade extending parallel to the rotational axis to respectively mark the first and the second fold line by pinching between the blade and the counter-roller, the angular difference between the blades being adjustable by the second adjustment means. The adjustment of the angular difference between the first and the second blade on the first marker roller allows the distance between the first and the second fold line to be adjusted. The counter-roller allows the blank to be accompanied during the blade scoring operation. 
         [0017]    In a complementary manner, the counter-roller comprises a first and a second peripheral indent, the indents extending parallel to the rotational axis of the counter-roller, the relative angular position of the indent being adjusted by the second adjustment means so that the blades do the scoring of the fold lines by pushing the blank against the respective indent. 
         [0018]    According to another constructive characteristic, the machine comprises an opening roller to open the blank, the roller being placed opposite the drum downstream of the first scoring means, the opening roller being rotative around an axis parallel to that of the drum in an opposite direction, the opening roller comprising at least one sucker to suck and move away an upper skin of the blank outer side at the drum near to the front edge. Means are thus provided to separate the upper skin from the lower skin and place the front pinching means to pinch the front edge of the lower skin of the blank. 
         [0019]    According to an improvement, the opening roller in addition comprises two lateral pinching means substantially on the same generatrix as the sucker to grip the lateral edges of the blank and to tension the blank between the first and second pinching means. The first and second lateral pinching means cooperate to deploy the bellows and tension them along a line which has been marked previously with the first fold line. The second pinching means are controlled to release the blank when the bellows are tensioned. These are then maintained in place by the rigidity conferred by the bellows fold line. 
         [0020]    Other advantages may again appear to a person skilled in the art on reading the examples below, illustrated by the appended figures, given for illustration purposes. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0021]      FIG. 1  is a schematic view of an installation for making flat bottom bags according to the invention; 
           [0022]      FIG. 2  is a schematic view of a machine for making flat bottom bags in compliance with the invention; 
           [0023]      FIG. 3  is a kinematic diagram of the elements of the machine of  FIG. 2 ; 
           [0024]      FIG. 4  is a cross-sectional view of the first scoring means; 
           [0025]      FIG. 5  is a schematic view of a reduction gear used for the adjustment means; 
           [0026]      FIGS. 6 to 11  are perspective views of the bag blank at different manufacturing stages. 
       
    
    
     DETAILED DESCRIPTION 
       [0027]    An installation for making flat bottom bags comprises in a classical manner a feed roller  1  of a reel B of film such as paper, a take-up device  2  to store a strip of unwound film, an envelope forming machine  3 , a cutting device  4  and a machine  5  for forming flat bottom bags  6 . A stacking device, not shown, can also be provided to receive the bags produced and form stacks. The envelope forming machine  3  folds the strip to join and glue the two edges together. At outlet, the envelope has two skins  61 ,  62  connected together along two opposite edges by two bellows  63 . The cutting machine  4  makes a transverse cut of the envelope to form blanks  6   a  with a constant transverse section, therefore of tubular form, as shown on  FIG. 6 . The blank  6   a  is presented with the bellows  63  folded between the two skins  61 ,  62  thus with a flattened form. In a conventional manner, it is considered that one of the skins  61  is the lower skin and the other the upper skin  62 . The blank is identified below by reference  6  completed by a letter to distinguish between the various bag  6  manufacturing stages. 
         [0028]    The machine  5  forming bags  6  comprises a drum  50  rotationally mounted around a drum transverse axis A and on the periphery of which the blank  6   a  is gripped and submitted to the transformations detailed below. The blank follows a path in the machine which in part follows the periphery of the drum  50  in a so-called direction of advance. 
         [0029]    The machine  5  first comprises in the direction of the path first scoring means  51  upstream of the drum  50 . The first scoring means  51  comprise a first marker roller  510  and a counter-roller  511  which are counter-rotative with rotational axes C, D parallel to the axis of the drum  50  and placed on either side of the path of the blanks  6   a.  The first marker roller  510  comprising a first blade  5101  and a second blade  5102  extending parallel to the rotational axis C to respectively mark a first and a second fold line on the blank  6   b  by pinching between the blade  5101 ,  5102  and the counter-roller  511 . As shown on the detail of  FIG. 4 , the first marker roller  510  comprises a centre shaft  5103  inserted into a partially open tube  5104 . The centre shaft  5103  carries the first blade  5101  whereas the tube carries the second blade  5102 . The first blade  5101  protrudes from the periphery of the tube passing via the open part of the tube  5104 . The tube  5104  is pivotally mounted on the centre shaft  5103  in such a way that the angular difference between the blades  5101 ,  5102  is adjustable, in a manner detailed below. The counter-roller  511  also comprises a centre shaft and a partially open tube which respectively carry a first indent  5111  and a second indent  5112  extending parallel to the rotational axis of the counter-roller  511 . The first indent  5111  is carried by a quadrant which is flush with the periphery of the tube and which protrudes into the open part of the tube. The first blade  5101  in cooperation with the first indent  5111  allow the first fold line  64  transverse in relation to the direction of advance to be marked on the blank at a predetermined distance from the front edge  68  of the blank  6   b,  whereas the second blade  5102  in cooperation with the second indent  5112  allow a second fold line  65 , parallel to the first fold line  64 , to be marked on the blank  6   b  at a distance from the front edge  68  higher than the first fold line  64 , as shown on  FIG. 7 . 
         [0030]    The drum  50  is formed by three elements  501 ,  502 ,  503 , each pivoting in relation to the other. Each element is driven by a shaft with drum axis  50  as axis. Each element also comprises four regularly spaced branches in order to form four stations for the transformation of the blank as shown on  FIG. 2 . Thus, on reach rotation of the drum  50 , four bags  6  are formed, one per station. 
         [0031]    Each station comprises front pinching means  52  carried by the first element  501  of the drum  50  to pinch a front edge  68  of the blank on the drum  50 , in an opening stage, and drive the blank. The second element  502  of the drum  50  carries first lateral pinching means  53  to pinch the lateral edges of the blank  6   b  at the first fold line  64 . 
         [0032]    The machine  5  comprises an opening roller  54  to open the blank  6   b.  It is placed opposite the drum  50  downstream of the first scoring means  51 , tangent to the path followed by the blank  6   b.  The opening roller  54  rotates around an axis E parallel to that of the drum  50  in an opposite direction. The opening roller  54  comprises at least one sucker  540  to suck and move away the upper skin  62  of the blank  6   b  on outer side of the drum  50  near to the front edge  68 . The opening roller  54  is driven by a specific motor  542  with a variable speed during a given rotation. The opening roller  54  also comprises second lateral pinching means  541  substantially on the same generatrix as the sucker  540  to grip the lateral edges of the blank and to tension the blank between the first and second lateral pinching means  53 ,  541 . The pinching means  52 ,  53 ,  541  are controlled to grip and release the blank  6   c  at positions detailed below. 
         [0033]    The machine  5  also comprises spatulas  55  which are placed in a fixed manner, on the periphery of the drum  50 , tangent to the drum  50 , oriented in a pinching direction in the movement of the drum  50 . The second element  502  has a table  5020  of semi-cylindrical form adjusted to the periphery of the drum  50 , the table  5020  being drilled with suction holes connected to a suction line, not shown. A distributor controls the communication between the holes and the line to cut the suction when the bag must leave the drum  50 . 
         [0034]    The machine  5  also comprises second scoring means  56  to mark, after the opening stage, a third and a fourth fold line  66 ,  67  on the blank  6 . The third fold line  66  is placed symmetrical to the first fold line  64  in relation to the second fold line  65 . The fourth fold line  67  passes via the same location as the first fold line  64  in order to mark the parts of the blank which have been pressed down over the first fold line  64 . The second scoring means  56  comprise a second marker roller  560  of same composition as the first marker roller  510  and comprising a third and fourth blade  5603 ,  5604  carried respectively by a shaft  5601  and a tube  5602 . The second scoring means  56  also comprise a first die  561  on the third element  503  of the drum  50  to mark the third fold line  66  in cooperation with the third blade  5603  and a second die  562  on the second element  502  of the drum  50  to mark the fourth fold line  67  in cooperation with the fourth blade  5604 . The second die  562  comprises tightening means, not shown, to pinch the blank along the fourth fold line  67 . 
         [0035]    The machine  5  also comprises downstream of the second marker roller  560  an adhesive application station  57  and two press down stations  58  which will not be detailed here. 
         [0036]    In order to do an adjustment in operation of certain bag characteristics, the machine  5  comprises first adjustment means  7  and second adjustment means  8 . The first adjustment means  7  are provided to adjust in operation both the distance between the front edge  68  and the first fold line  64  and the relative angular position on the drum  50  of front pinching means  52  and lateral pinching means  53 . The second adjustment means  8  are provided to adjust also in operation the distance between the first and the third fold line  65 ,  66  so that the second fold line  65  will be at mid-distance between the first and the third fold line  64 ,  66 . 
         [0037]    In detail, the first adjustment means  7  determine the relative angular position between the first and the second element  501 ,  502 . For this, a drum motor  505  drives a main shaft  506 , which is connected to the second element  502  by a first set  91  of pulleys and a notched belt. The first element  501  is driven by the main shaft  506  by means of a first reduction gear  71  belonging to the first adjustment means  71 . 
         [0038]    By referring to  FIG. 5 , which shows in a schematic and general manner the composition of the first reduction gear  71 , it can be seen that the reduction gear comprises two planetary gear wheels  711 ,  712  with same rotational axis, each wheel  711 ,  712  meshing with a satellite  713 ,  714 , the satellites being attached to the same rotation shaft  715 . The reduction ratios between one and the other of the wheel/satellite pairs are different. The shaft  715  of the satellites is rotationally mounted on a satellite holder  716 , which is pivotally mounted around the same rotational axis as the planetary gears  711 ,  712 . If: 
         [0039]    Z 1  is the number of teeth of the first planetary gear  711 ; 
         [0040]    Z 2  is the number of teeth of the secondary planetary gear  712 ; 
         [0041]    Za is the number of teeth of the satellite or satellites  713  of the first planetary gear  711 ; 
         [0042]    Zb is the number of teeth of the satellite or satellites  714  of the second planetary gear  712 ; 
         [0043]    and 
         [0000]    
       
         
           
             
               ρ 
               = 
               
                 
                   
                     Z 
                     1 
                   
                   · 
                   
                     Z 
                     b 
                   
                 
                 
                   
                     Z 
                     2 
                   
                   · 
                   
                     Z 
                     a 
                   
                 
               
             
             , 
           
         
       
     
         [0044]    then, when the satellite holder  716  is stationary, ω 2 =ρ.ω 1 , where ω 2  and ω 2  are the rotational speeds of the first and second planetary gear  711 ,  712  respectively. When the first planetary gear  711  is stopped, the rotation of the satellite holder  716  drives the rotation of the second planetary gear  712  according to the following relation: 
         [0000]      ω 2 =(1−ρ).ω ps  
 
         [0045]    where ω ps  is the rotational speed of the satellite holder  716 . In this relation, it can be seen that an angular offset between the elements driven respectively by the first and second planetary gear  711 ,  712  can be obtained by turning the satellite holder  716 . This angular offset is produced, even if the drum is rotating. 
         [0046]    In the setup of the first reduction gear  71 , the first planetary gear  711  is directly driven by the main shaft  506 , the second planetary gear  712  is connected to the first element  501  by means of a second set  92  of two pulleys and a notched belt. The satellite holder  716  is connected to a control knob  72  which can be operated manually to determine the relative angular position between the first and the second element  502 . The various transmission ratios between the main shaft  506  and the second element  502 , and that of the first reduction gear  71  and of the second set  92  of pulleys and belt are chosen so that the first element  501  and the second element  502  rotate at the same speed. 
         [0047]    So that the second adjustment means  8  can adjust in operation the distance between the first and the second fold line  64 ,  65 , they in particular determine the relative angular position between the second and the third element  502 ,  503 . For this, a third set  93  of pulleys connects the main shaft  506  to the first planetary gear of a second reduction gear  82  similar to the first reduction gear  71 . The second planetary gear is connected by a fourth set  94  of pulleys and belt to a shaft  5030  of the third element  503 . The various transmission ratios are also chosen so that the third element  503  is driven at the same speed as the first and the second element  502 . The satellite holder of the second reduction gear  82  is normally stationary, but it can be pivoted to introduce an angular offset between the main shaft  506  and the shaft  5030  of the third element  503 . It is connected to a second control knob  85  which can be operated manually. 
         [0048]    The second adjustment means  8  in addition allow the angular difference to be adjusted between the blades  5101 ,  5102  of the first marker roller  510  of the first scoring means  51  and thus the angular difference between the first and the second indent  5111 ,  5112  of the counter roller  511 . For this, the centre shaft  5103  of the first marker roller  510  is connected to the centre shaft  5113  of the counter roller  511  by a first pair of gears in such a way that the roller and the counter roller  511  are counter rotative and always synchronised in such a way that the first blade  5101  and the second indent  5111  coincide during the rotation. Also, the tube  5104  of the first marker roller  510  is connected to the tube  5114  of the counter roller  511  by a second pair of gears  513  in such a way that the second blade  5102  and the second indent  5112  coincide during the rotation. In addition, a third reduction gear  83 , similar to the first reduction gear  71 , connects the centre shaft  5103  and the tube  5104  of the first marker roller  510 . The centre shaft  5103  is connected by a fifth set  95  of pulleys and belt to the first planetary gear of the third reduction gear  83 , whereas the tube  5104  is connected by a sixth set  96  of pulleys and belt to the second planetary gear of the third reduction gear  83 . The satellite holder of the third reduction gear  830  is controlled by the second control knob. The transmission ratios of the third reduction gear  83 , of the fifth and of the sixth set  95 ,  96  of pulleys and belt are chosen so that when the satellite holder is stationary, the centre shaft  5103  and the tube  5104  of the first marker roller  510  rotate at the same speed. The pivoting of the satellite holder of the third reduction gear  83  induces an angular offset between the centre shafts  5103 ,  5113  and the tubes  5104 ,  5114  of the first marker roller  510  and of the counter roller  511 . The centre shaft  5103  is driven by a first scoring motor  515 . 
         [0049]    The second adjustment means  8  also allow the angular difference between the blades  5603 ,  5604  of the second scoring means  56  to be adjusted. For this, a fourth reduction gear  84 , similar to the first reduction gear  71 , connects the centre shaft  5601  and the tube  5602  of the second marker roller  560 . The centre shaft  5601  is connected by a seventh set  97  of pulleys and belt to the first planetary gear of the fourth reduction gear  84 , whereas the tube  5602  is connected by an eighth set  98  of pulleys and belt to the second planetary gear of the fourth reduction gear  84 . The satellite holder of the fourth reduction gear  84  is controlled by the second control knob. The transmission ratios of the fourth reduction gear  84 , of the seventh and eighth set  97 ,  98  of pulleys and belt are chosen so that when the satellite holder is stationary, the shaft  5601  and the tube  5602  of the second marker roller  560  rotate at the same speed. The pivoting of the satellite holder of the fourth reduction gear  84  induces an angular offset between the shaft and the tube of the second marker roller  560 . The centre shaft  5601  is driven by a second scoring motor  563 . 
       Operation 
       [0050]    During the operation of the machine  5  for making flat bottom bags, the main motor  505  drives the drum  50  at a constant speed, except during transitory phases. The first and the second scoring means  51 ,  56  and the opening roller  54  are rotationally driven by their own respective motors  515 ,  563 , but in a synchronised speed and position manner in relation to the drum  50 . The synchronisation is done for example electronically. The marker rollers  510 ,  560  and the opening roller  54  rotate in the opposite direction to the drum  50 , whereas the counter roller  511  rotates in the same direction. 
         [0051]    The blank  6   a,  from the cutting device arrives between the first marker roller  510  and the counter roller  511 . The first scoring means  51  make a first fold line  64  and a second fold line  65  at a predetermined distance from the first fold line  64 , in a direction perpendicular to the bellows  63 , as shown by the blank  6   b  represented on  FIG. 7 . By progressing, the front of the blank  6   b  passes between the drum  50  and the opening roller  54 . The lower skin  61  of the blank is sucked against the table  5020  of the second element  502  whilst the upper skin  62  is sucked by the sucker  540  of the opening roller  54 . When the upper skin  62  is separated from the lower skin  61 , the front pinching means  52  pinch the front edge  68  of the lower skin  61  against the table  5020  to maintain the blank  6   b  on the drum  50  and drive it. When the opening of the bellows  63  is sufficient, the first lateral pinching means  53  pinch the lateral edge of the blank at the junction between the bellows  63  and the lower skin  61  in the alignment of the first fold line  64 . Symmetrically, the second lateral pinching means  541  pinch the lateral edge of the blank at the junction between the bellows and the upper skin  62  in the alignment of the first fold line  64 . When the blank  6   a  continues its passage between the roller and the drum  50 , the lateral pinching means  541 ,  53  move away from each other, open the blank  6   c  and form a fold on the bellows  63 , as shown on  FIGS. 8 and 9 . The second lateral pinching means  541  then release the blank  6   c  whereas, in the remainder of the movement of the drum  50  which drives the blank  6   c,  spatulas  55  press down the part thus opened towards the centre and the rear. The upper skin  62  is folded towards the rear at the second fold line  65 , as shown by the blank  6   d  on  FIG. 10 . 
         [0052]    Concerning the operations, the second scoring means  56  mark on the blank  6   d  a third and a fourth transverse fold line  66 ,  67  on either side of the second fold line  65 . The fourth fold line  67  coincides with the first fold line  64 . Then, adhesive is applied to the two flaps  60 ,  69  delimited by these fold lines  66 ,  67  at the adhesive application station  57 . A rear flap  69 , beyond the third fold line  66  is pressed down towards the front. The front pinching means  52  release the front flap  60  so that it is pressed down around the fourth fold line  67  onto the rear flap  69  at the press down stations  58  to close the flat bottom and thus form the bag  6 , as shown on  FIG. 11 . 
         [0053]    When the format of the bags to be produced is to be changed, the first adjustment means  7  can be acted upon to modify the position of the front pinching means  52  in relation to the second element  502 , by rotating the first knob, and therefore the satellite holder of the first reduction gear  71 . At the same time, in an automatic manner or manual manner, the relative angular position of the first scoring means  51  can be acted upon to adjust the distance between the front edge  68  of the blank  6   a  and the first fold line  64  which determines the length of the front flap  60 . 
         [0054]    The second adjustment means  8  can also be acted upon to adjust the distance between the first fold line  64  and the third fold line  66 , which determines the width of the flat bottom, by acting on the second scoring means  56 . Acting on the second knob, causes the rotation of the satellite holder of the second, of the third and of the fourth reduction gear  82 ,  83 ,  84 , and therefore the relative angular offset between the second and the third element  502 ,  503 , between the first and the second blade  5101 ,  5102  of the first marker roller  510  and between the first and the second indent  5111 ,  5112  of the counter roller  511 , and between the third and the fourth blade  5603 ,  5604  of the second marker roller  560 . The position of the second fold line  65  which must be found at mid-distance from the first and the third fold line  64 ,  66  is also moved by acting on the first scoring means  51 . 
         [0055]    The invention is not limited to the embodiment described above as an example. The offset between the rotationally driven elements can be introduced by other means, such as the axial offset of a helical-toothed sprocket.