Abstract:
The invention relates to a print unit for a packaging machine, in which a cavity, equipped with an armature made of ferromagnetic material and supporting, in operation, a blank to be printed, is mobile in front of a fixed electromagnet, which can be activated so as to attract the armature against a fixed print head, thus clamping the blank between the armature and the print head.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to a print unit. In particular, the invention relates to a print unit suitable for use in cigarette packaging machines, to which the following text refers, although without limiting the scope of the invention, for printing a code on wrapping material blanks used in the packaging machine. 
     Known cigarette packaging machines normally comprise a print unit for printing a code on the blanks, allowing identification, for example, of the date and/or place of production of the relative packet. 
     In the above-mentioned packaging machines, the print unit in turn comprises a seat designed to hold a blank and mounted on a rotary wheel which moves the blank forward along a path which passes through a print station. The print station houses an actuator which moves a print head away from and towards the seat, so as to print the above-mentioned code on a blank held in the seat. 
     The above-described print unit is normally equipped with a deformable elastic element, which is inserted between the above-mentioned actuator and the print head, or between the seat and the wheel, and can be adjusted to calibrate the force with which the print head acts upon the blank. 
     In operation, the above-described print unit subjects the wheel shaft supports to relatively high levels of stress, particularly in modern packaging machines, which have very high operating speeds (over 500 packets per minute). Since they must stand such stress, the shaft supports and corresponding seats must be very strong and are, therefore, expensive. 
     Moreover, in the above-mentioned print unit, it is very difficult to precisely calibrate the force with which the print head acts upon the blank, since said force depends on the elastic characteristics of the deformable element, on the reciprocal position of the print head and the seat, and on the actuator travel. 
     The aim of the present invention is to provide a print unit which has none of the above-mentioned disadvantages and, at the same time, is simple and economical to produce. 
     SUMMARY OF THE INVENTION 
     Accordingly, the invention provides a print head comprising a print head housed in a print station for printing a graphic symbol on a sheet; an electromagnet housed in the print station; an armature made of a ferromagnetic material which moves away from and towards the print head due to the action of a magnetic field produced by the electromagnet; feed means for feeding the sheet to the print station in a position between the armature and the print head; and control means designed to activate the electromagnet, so as to attract the armature against the print head. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention will now be described with reference to the accompanying drawings, which illustrate an embodiment of the invention, without limiting the scope of its application; and in which: 
     FIG. 1 is a side elevation view, with some parts cut away to better illustrate others, of a preferred embodiment of the unit according to the present invention; 
     FIG. 2 is a scaled-up detail from FIG. 1, in cross-section according to line II—II in FIG. 1; and 
     FIG. 3 is a scaled-up detail from FIG.  2 . 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     With reference to the accompanying drawings, in FIG. 1 the numeral  1  indicates as a whole a continuous packaging machine comprising a motor-driven wrapping wheel  2 , designed to turn with continuous motion (counterclockwise in FIG. 1) about its axis (not illustrated) perpendicular to the plane in FIG.  1 . The wheel  2  has a plurality of evenly distributed peripheral seats  3  (only one of which is illustrated), designed to receive a succession of ready-folded blanks  4  made of cardboard or a similar material. The machine  1  also comprises a magazine  5  for a stack  6  of blanks  4 , and a suction pick-up unit  7 , located between a pick-up station S 1  which is positioned at an open base  8  of the magazine  5 , and a feed station S 2  which is positioned at the wrapping wheel  2 , to take the blanks  4  from the open base  8  and feed them in succession to the seats  3 . 
     The pick-up unit  7  comprises a motor-driven wheel  9  which turns continuously, and clockwise in FIG. 1, about its fixed central axis  10 , parallel with the axis (not illustrated) of the wheel  2 , and perpendicular to a longitudinal axis  11  of the magazine  5 . The wheel  9  has an inner chamber  12  (only partially illustrated), pneumatically connected by a pipe  13  to a suction pump  14  designed to create a vacuum in the inner chamber  12 , so that the pressure in the inner chamber is lower than the outside atmospheric pressure. 
     The chamber  12  is connected (in a known way, not illustrated) to a plurality of pneumatic pick-up heads  15  supported by the wheel  9  and evenly distributed about the axis  10 . The pick-up heads  15  are connected to the wheel  9  by arms  16 , each hinged to the wheel  9  by a pin  17  (illustrated in FIG. 2) in such a way that it oscillates, relative to the wheel  9 , about an axis  18  parallel with the axis  10 . Each pick-up head  15  is, in turn, hinged to the relative arm  16  in such a way that it oscillates, relative to the arm  16 , about an axis  19  parallel with the axis  10 . 
     The angle of each arm  16  about the axis  18  is controlled by a cam control device (of the known type and not illustrated) housed in the wheel  9 . 
     The angle of each pick-up head  15  about the axis  19  is controlled by another cam control device (of the known type and not illustrated) housed in the wheel  9  and mechanically connected to the pick-up head  15  by a lever mechanism  20  comprising a lever  21  hinged to the wheel  9  in such a way that it oscillates about the axis  18  by a hollow pin  22  (illustrated in FIG. 2) which houses the relative pin  17 , and a connecting rod  23 , one end of which is hinged to the lever  21 , the other end being hinged to the pick-up head  15  in such a way that it oscillates, relative to the pick-up head  15 , about an axis  24  parallel with the axis  10 . 
     As is better illustrated in FIG. 2, each head  15  substantially has the shape of a plate, one side of which is connected to the relative arm  16 , and the other side of which is limited by a pick-up surface  25 , in which a cavity  26  is defined, delimited by a ring-shaped edge  27  and connected to the chamber  12  by a pipe  28  made along the arm  16 . 
     In the embodiment illustrated, the pick-up surface  25  is substantially defined by the upper surface of the edge  27 . 
     Inside each pipe  28 , at the joint  29  connecting the pipe  28  to the relative head  15 , there is a valve  30 , designed to open and close the pipe  28 , so that it is communication with or cut off from the cavity  26 . Each head  15  also has a through-hole  31 , which provides permanent communication between the cavity  26  and the outside. 
     Each valve  30  is of the known type with mechanical control, and is controlled by a rod  32  designed so that it can be moved axially in a direction that is radial to the axis  10  by a cam driver device (of the known type and not illustrated) housed in the wheel  9 , between an open position (illustrated in FIG. 2) in which the cavity  26  is pneumatically connected to the pipe  28  and, therefore, to the chamber  12 , and a closed position (not illustrated) in which the cavity  26  is pneumatically isolated from the pipe  28  and, therefore, the chamber  12 . 
     In order to hold a blank  4  in contact with the pick-up surface  25  of a head  15 , the surface  25  is substantially brought into contact with the blank  4 , which seals the cavity  26  and, together with the cavity  26 , forms a vacuum chamber, in which a vacuum is created (relative to the atmospheric pressure), by opening the corresponding valve  30  and, therefore, putting the vacuum chamber in communication with the chamber  12 , in which the pump  14  constantly maintains a vacuum. 
     The head loss caused by the presence of the through-hole  31  permanently connected to the outside does not create particular problems, since the pump  14  is able to compensate even high levels of head loss, such as those induced by the through-hole  31 . 
     In order to release the blank  4 , the atmospheric pressure in the vacuum chamber is automatically restored, thanks to the presence of the hole  31 , by simply closing the valve  30 . 
     As illustrated in FIG. 1, each seat  3  is made on an outer surface  33  of a head  34  opposite an inner portion of the head  34  connected to the wheel  2  in such a way that it oscillates, relative to the wheel  2 , about an axis  35  parallel with the axis  10 , controlled by a cam-rocker switch control device (of the known type and not illustrated). Each seat  3  has two lateral surfaces  36 , which are angled towards one another and towards a lower surface  37 , in which there are holes  38  for communication with a known type of suction device, not illustrated. 
     The continuous rotation of the wheel  9  about the axis  10  causes each head  15  to move forwards along a closed path P, substantially circular and extending about the axis  10 , and through the pick-up station S 1 , in which the head  15  picks up a blank  4  from the open base  8  of the magazine  5 , through a print station S 3 , in which a print unit  39  prints a code (not illustrated) on the blank  4 , and through the feed station S 2 , in which the head  15  feeds the blank  4  to a corresponding seat  3  of the wrapping wheel  2 . 
     The above-mentioned code (not illustrated) is normally alphanumeric, or a bar code, and usually indicates the date and/or place of production of the packet. 
     As illustrated in FIG. 2, the print unit  39  comprises a support pin  40 , mounted on the machine  1  in such a way that it oscillates about an axis  41  parallel with the axis  10 , under the thrust of a cam actuator device (of the known type and not illustrated), and rigidly supporting an electromagnet  42 , which has a core  43  made of ferromagnetic material with a pair of opposite pole shoes  44 , between which there is a chamber  45  through which the pin  40  passes. 
     In the chamber  45  and, therefore, between the pole shoes  44 , is a print device  46  which is rigidly supported by the pin  40  and has a print head  47  designed to print the code on a blank  4 . 
     The electromagnet  42  also comprises a coil  48  designed to produce a magnetic field along the core  43  when an electric current with adjustable intensity is passed through it, said current generated by a control device  49  of the known type electrically connected to the coil  48 . 
     As illustrated in FIG. 3, each pick-up head  15  comprises an armature  50  made of ferromagnetic material, which has a flat outer surface  51  designed to support a blank  4  and is mounted in such a way that it is mobile in the cavity  26 , moving along a direction  52  perpendicular to the axis  19  against the action of a pair of springs  53 , each of which is wrapped around a fixed pin  54 , along which the armature  50  can slide. 
     Each spring  53  is compressed between the armature  50  and an upper head  55  of the relative pin  54 , so that the armature  50  is normally held in a home position (illustrated in FIG. 3 a ), in which the surface  51  is at a given distance other than zero from a relative blank  4 . An upper surface  56  of each head  55  constitutes a base upon which a blank  4  rests, the blank held by the corresponding pick-up head  15 . 
     The operation of the machine  1  is described below with reference to a single pick-up head  15  and starting from a moment in which the pick-up head  15  has released a blank  4  into a seat  3  at the feed station S 2  and moves, under the thrust of the wheel  9  and along the path P, towards the pick-up station S 1 , to pick up another blank  4 . 
     During the transit between the feed station S 2  and the pick-up station S 1 , the valve  30  of the head  15  is kept in the closed position and the cam control devices (of the known type and not illustrated) impart to the arm  16  an early oscillation (clockwise in FIG. 1) about the axis  18 , which continues until the head  15 , moving along the path P, reaches a position close to the feed station S 1 . 
     When the head  15  moves close to the station S 1 , the cam control devices (of the known type and not illustrated) impart to the arm  16  a delay oscillation (counterclockwise in FIG. 1) about the axis  18  and a simultaneous oscillation (clockwise in FIG. 1) of the head  15  about the axis  19 , so that the head  15  is substantially stopped in a position opposite the base  8  of the magazine  5  and, at the same time, the head  15  is pushed outwards in a radial direction until it adheres to the blank  4  which is in contact with the base  8 , and the head  15  is, therefore, pushed in the direction of the axis  11  towards the inside of the magazine  5 , exerting a given pressure on the blank  4 . 
     As the head  15  moves forwards inside the magazine  5  in the direction of the axis  11 , suction is activated through the cavity  26 , bringing the valve  30  into the open position, to hold the blank  4  in contact with the surface  25 . 
     When the blank  4  adheres to the surface  25 , an oscillation (clockwise in FIG. 1) about the axis  19  is imparted to the head  15 , allowing a first edge of the blank  4 , the lower edge in FIG. 1, to be deformed and clear a tooth  5 a delimiting the base  8 , then removal of a second edge (the upper edge in FIG. 1) of the blank  4  from under a tooth  5   b  delimiting the base  8 , thus completing extraction of the blank  4  from the base  8 . 
     At this point, the cam control devices (of the known type and not illustrated) interrupt the delay oscillation of the arm  16  about the axis  18  and, due to the rotation of the wheel  9 , the head  15  leaves the pick-up station S 1 , in which it picked up the blank  4 , and goes to the print station S 3 . 
     During the transit between the pick-up station S 1  and the print station S 3 , the cam control devices (of the known type and not illustrated) impart to the arm  16  an early oscillation (clockwise in FIG. 1) about the axis  18 , which continues until the head  15 , moving along the path P, reaches a position close to the print station S 3 . 
     When the head  15  moves close to the print station S 3 , the cam control devices (of the known type and not illustrated) impart to the arm  16  a delay oscillation (counterclockwise in FIG. 1) about the axis  18  and a simultaneous oscillation (clockwise in FIG. 1) of the head  15  about the axis  19 , so that the head  15  is substantially stopped, for a given interval, in a position opposite the print unit  39  (as illustrated in FIG. 2) which, in turn, and for the same purpose, completes a corresponding oscillation about the axis  41 . 
     In accordance with another embodiment, not illustrated, the pin  40  is a fixed pin and the unit  39  does not oscillate about the axis  41 . 
     In the print station S 3  and during the above-mentioned interval, the blank  4  is in a position (illustrated in FIG. 2) between the armature  50  and the print head  47 . The control device  49  then supplies the coil  48  with a current with preset intensity, to generate at the core  43  a magnetic field which attracts the armature  50  towards the pole shoes  44  against the action of the springs  53 ; the consequent movement of the armature  50  towards the pole shoes  44  brings the surface  51  of the armature  50  into contact with a corresponding inner surface  57  of the blank  4 , bringing an outer surface  58  of the blank  4  into contact with the print head  47  with a preset force, depending on the elastic force generated by the springs  53  and the intensity of the magnetic field, which, in turn, depends on the intensity of the electrical current circulating in the coil  48 . 
     Then, when the magnetic field is interrupted by interrupting the current passing through the coil  48 , the armature  50  returns, due to the action of the springs  53 , to the home position (illustrated in FIG. 3 a ) and the cam control devices (of the known type and not illustrated) interrupt the delay oscillation of the arm  16  about the axis  18  and, due to the rotation of the wheel  9 , the head  15  leaves the print station S 3  and goes to the feed station S 2 . 
     During the transit between the print station S 3  and the feed station S 2 , the cam control devices (of the known type and not illustrated) impart to the arm  16  an early oscillation (clockwise in FIG. 1) about the axis  18 , which continues until the head  15 , moving along the path P, reaches a position close to the feed station S 2 . 
     When the head  15  moves close to the station S 2 , the cam control devices (of the known type and not illustrated) impart to the arm  16  a delay oscillation (counterclockwise in FIG. 1) about the axis  18  and a simultaneous oscillation (clockwise in FIG. 1) of the head  15  about the axis  19 , so that the head  15  is substantially stopped in a position opposite the corresponding seat  3  of the wrapping wheel  2  and, at the same time, the head  15  is pushed outwards until the pick-up surface  25  substantially adheres to the lower surface  37  of the seat  3 . 
     When the pick-up head  15  is in the feed station S 2 , the head  34  in which the seat  3  is made also oscillates about the axis  35 , to keep the pick-up surface  25  and the lower surface  37  opposite one another and substantially parallel. 
     During the substantially radial movement of the surface  25  towards the lower surface  37 , the suction through the cavity  26  is interrupted, bringing the valve  30  into the closed position and, at the same time, suction through the holes  38  is activated, so that the blank  4  is left in the seat  3  and is pneumatically held in the seat  3 . 
     At this point, the cam control devices (of the known type and not illustrated) interrupt the delay oscillation of the arm  16  about the axis  18  and, due to the rotation of the wheel  9 , the head  15  leaves the feed station S 2  and goes to the pick-up station S 1 . The above-mentioned operations are repeated cyclically. 
     Obviously, the above-mentioned oscillations (clockwise in FIG. 1) of the head  15  about the axis  19  during the steps in which the head  15  moves towards the stations S 1 , S 2 , S 3 , are followed by return oscillations (counterclockwise in FIG. 1) about the axis  19  during the subsequent steps in which the head moves away from the stations S 1 , S 2 , S 3 . 
     As illustrated in FIG. 1, insertion of the head  15  in the seat  3  allows the blank  4  to be folded into a U-shape along its pre-folded lines (of the known type and not illustrated) as the blank  4  is fed to the seat  3 . 
     The above description clearly indicates that, during the printing operations, the shaft (not illustrated) of the wheel  9 , the pins  17  and hinges at  19  are not subjected to any impact generated by the contact between the armature  50  and the print head  47 , since such impacts are absorbed by the pin  40 . Moreover, the force with which each blank  4  makes contact with the print head  47  can be regulated, even during operation, in a simple, rapid, precise fashion, by simply adjusting the intensity of the current supplied to the coil  48 .