Abstract:
The invention concerns a direct printing machine by ink jet on a medium, including one or several printing heads with a spray nozzle, each head being dedicated to one shade. The invention is characterized in that the heads are borne by a pen carriage on the medium along three orthogonal axes X, Y and Z. Each of the heads is provided with a surge tank borne by the pen carriage, the surge tanks being supplied from one or several fixed tanks, the heads and the surge tanks are maintained in low pressure and the surge tanks are supplied with ink from the fixed tanks.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention presented here involves a machine for direct printing by projection of ink drops onto a medium 
     2. Description of the Related Art 
     The ink jet technique is known from the prior-art. 
     It makes use of printing heads made of a piezo-electric ceramic having a chamber that, by deformation, projects the ink out of nozzles onto a medium. The projection of the ink can also be obtained, for certain types of ink, by heating that creates the formation of a gaseous bubble whose expulsion causes the ink jet. 
     These techniques are associated with microprocessor-controlled systems which make it possible to create, by ink projection, the desired image that can be, for example, a sign or an alphabetic letter or any other representation. These techniques are mainly used in office automation equipment. 
     In this equipment, the relative displacements of the printing head by the ink jets and the medium to be printed are low in amplitude, which does not harm the quality of the print. The necessary partial vacuum at the outlet of the projection nozzles is not affected by the low displacements. These technologies, however, are only suitable for certain types of specific inks and are not suitable for the projection of inks having a high corrosive capacity, the use of which is sought after for the quality of the printing obtained and for its unalterable adhesion on the medium or for inks to be developed by ultraviolet rays. 
     The invention intends to create a machine that allows equally the direct printing, by corrosive inks, i.e. having a solvent or ultraviolet base, on small formats or very large formats and on any type of medium, notably rigid, planar or formed The creation of a machine of this type that meets this definition is done by mastering a certain number of problems. 
     Since corrosive inks cannot be loaded into the known distributing cartridges of the HEWLETT PACKARD™ type or LEXMARK™. type, it is necessary to design a specific system for supplying ink using devices that make it possible to maintain a partial vacuum of 4 millibars at the level of the projection nozzles, without a variation in the pressure that would be detrimental to good printing, and this must be done independently of the given technique, by which the relative movements between the heads and the projection nozzles must be very fast. As an example, the heads and nozzles can be subjected to movement speeds of 800 millimeters per second and to accelerations of 5000 millimeters per second squared, which generates a pressure variation at the level of the nozzles. 
     Moreover, the nozzles functioning at a slight partial vacuum, with the variations mentioned above, are likely, in the course of the movement as a dynamic function, to pump out air bubbles. 
     This generates damage to their function that is causing stop of the projection, deflected jet, etc. 
     The invention presented here intends to provide a solution to this problem in creating a printing machine. 
     BRIEF SUMMARY OF THE INVENTION 
     For this purpose, the machine according to the invention for the direct printing functioning by projection of ink drops onto a medium, by one or more heads with projection nozzles, the machine being of a type having relative movements between the medium to be printed and the projection heads, each head being preferably dedicated to one color or to one ink, is characterized essentially in that: 
     the head(s) is or are carried by a mobile carriage above the medium to be printed along the three axes X, Y, and Z orthogonally in space, 
     each of the heads is equipped with a buffering reservoir carried by the mobile carriage, and 
     the head(s) and their buffering reservoir are kept at a partial vacuum and the buffering reservoirs are supplied under pressure from one or more stationary reservoirs. 
     According to another characteristic of the invention, the mobile carriage guides the heads and the nozzles cyclically on a system for cleaning and maintenance of the heads which cleans the nozzles and primes them by suction, evacuates the excess drops by doctoring, and makes it possible to keep the nozzles in a good functioning condition. 
     According to yet another characteristic of the invention, the internal volume of each loaded buffering reservoir is divided in order to reduce the amplitude of waves produced by the movements of the reservoirs. 
    
    
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
     Other advantages and characteristics of the invention appear in reading the following description of an embodiment form of the invention given as a non-restrictive example and illustrated by the attached drawings. 
     FIG. 1 is a schematic planar view of a machine according to the invention. 
     FIG. 2 is a schematic view representation of the hydraulic system of the machine according to the invention. 
     FIG. 3 is a schematic view of the buffering reservoirs with the projection head. 
     FIGS. 4 and 5 are planar and sectional views respectively of the insert divider introduced into each of the reservoirs. 
     FIG. 6 shows in a planar view the system for cleaning and maintenance of the heads and projection nozzles which is linked to the hydraulic circuit. 
     FIG. 7 is a sectional view of the device shown in FIG. 6, 
     FIG. 8 is a schematic view of the maintenance cycle. 
     FIG. 9 shows a sectional view of a buffering reservoir and an additional reservoir. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     As shown schematically in the planar view in FIG. 1, the machine according to the invention consists of a rectangular table I consisting of a planar, horizontal plate in a rectangular shape on which the medium is arranged prior to receiving the printing. A transverse frame  2  is mobile in the direction X on the table. 
     On the frame  2 , a loaded carriage  3  is mounted that is mobile by any known mechanism along the Y axis and the Z axis, where the X, Y, and Z axes are orthogonal in space. The mobile carriage can thus be moved in all directions on the table  1  and in height along Z relative to it. 
     These movements are controlled by a command and control unit consisting of a microprocessor with connected equipment. The position of the mobile carriage, notably in height, relative to the table is regulated by any detection system of a known type. 
     From the side of the table, one or more reservoirs  4  is/are arranged to be stationary so as to receive the inks used and supply, by conduits  7 , the reservoir(s)  6  of the mobile carriage  3 , these reservoirs  6  being located lower than the reservoir(s)  4 . 
     Arranged stationary on the table  1  is a system  5  for cleaning the heads and plates of the projection nozzles. 
     The loaded carriage  3  is shown in greater detail in FIG. 2 with the hydraulic system. 
     The high-velocity mobile loaded carriage consists of one Or more buffering reservoirs  6 . 
     Shown in the example of FIG. 2 are four buffering reservoirs  6 , each of them being dedicated to one color of ink or to one shade of ink functioning to compose a color, each of them being connected by a conduit line  7  to the stationary reservoir  4 , which consists of as many chambers and lines as there are buffering reservoirs. 
     Downstream from each buffering reservoir are correspondingly one or two ink projection heads based on a piezoelectric or other system, each head supplying its projection nozzles that are carried by a nozzle plate. According to the preferred embodiment form, each buffering reservoir  6  supplies only one projection head. In addition, the buffering reservoirs  6  are independent from each other and are mounted in a removable manner on the mobile carriage. This arrangement, while allowing the individual replacement of each reservoir, makes the maintenance of the assembly easier, In the same spirit, a similar device is provided for the projection heads and the associated nozzles. Another advantage in the removability of the reservoirs  6  and the projection heads resides in the improvement of the flexibility of the use of the printing machine. 
     Thus, each reservoir  6  and associated projection head can now be affected by only one, color and only one category of ink. Thus, you avoid the obligation of a complete cleaning of the reservoirs and the heads prior to the replacement of the ink, justified by problems of chemical and/or physical incompatibility that can exist between the inks of different categories and/or different colors. 
     The reference item  8  indicates the connections of each of the heads that are supplied by a controlled electrodistributor  9  downstream from each of the buffering reservoirs  6 . The distributors  9  are controlled individually by the command and control unit. By the command of the electrodistributors, the communication between the buffering reservoirs and the associated projection heads is interrupted or established, and thus the supply in ink from the pulverization nozzles is interrupted or established. These electrodistributors are controlled independently from each other. 
     Upstream from each of the buffering reservoirs  6 , on each of the supply lines  7  from the stationary reservoir  4 , a controlled electrodistributor  10  controlled by the command and control unit is mounted. By controlling each one of these electrodistributors, the communication between the associated reservoirs  4  and  6  is established or interrupted and thus the supply or not of the reservoir  6  with ink. 
     In a manner so as to detect the level of ink in the reservoir and to order the supply of ink when the level is at a minimum and to interrupt this supply when the level is at maximum, a level detector is combined with the reservoir  6  and is designed to transmit two distinct electronic signals, one of which is representative of a minimum level and the other is representative of a maximum level. This level detector is connected electronically to the command and control unit in order to transmit to this unit the maximum and minimum level signals. 
     As can be seen in FIG. 2, the buffering reservoirs  6  can be kept in a partial vacuum by a line  11  shared with the mechanism for implementing a partial vacuum  12 , where this line is connected to a compressor  13 . This mechanism for implementing a partial vacuum is advantageously made up of a device using the Venturi effect. Such a device, itself known from the prior-art, consists of a first internal channel provided along one of its ends with a compressed air intake opening and along its other end with a compressed air outlet opening The end of a second channel, which is connected by its other end to line  11 , opens radially into this internal channel. By the passage of an air flow into the first channel, an aspiration effect, generator of the partial vacuum, is created in the second channel. 
     The line  11  is equipped, between the compressor  13  and the device using the Venturi effect  12 , with mechanisms for supplying and controlling, such as a filter, regulator, and manometer of the known types that are not described here. 
     A bypass  14  connected on the line  11  is joined, by the intermediary of a controlled stopcock  15 , to a device using the Venturi effect  16  which sets a partial vacuum in the reservoir  17  with a drain cock  18 . 
     The reservoir at partial vacuum  17  is connected by an electrodistributor  19  to the system  5  for the cleaning and maintenance of the ink pr jection nozzles. 
     The device using the Venturi effect  12  on the line  11  keeps the buffering reservoir at partial vacuum. 
     According to another embodiment form, each buffering reservoir  6  is equipped with its own mechanism for creating a partial vacuum. This mechanism for creating a partial vacuum is made up advantageously of a device using the Venturi effect  12  such as the one previously described. The aspiration opening of this device using the Venturi effect is connected directly by its aspiration opening to a hole passing through the upper wall of the buffering reservoir  6  Thus, a partial vacuum is created in the upper part of the reservoir  6 , above the level of the ink. 
     According to a first embodiment form, the buffering reservoirs are supplied with ink by the lines  7 , the ink supply pressure being on the order of 0.1 to 0.2 bar. 
     According to a second embodiment form, each buffering reservoir  6  is supplied with ink by an additional reservoir  34  that belongs to it, where the additional reservoir  34  is connected to the corresponding conduit  7  by the corresponding electrodistributor  10  This additional reservoir, for example, in the form of a cylinder, consists of a bottom wall, a surrounding wall that is vertically erected on the bottom wall, and a blocking cover that comes to rest on the upper horizontal groove of the surrounding wall in order to block the opening that defines the wall at this level. The additional reservoir  34  accommodates the level detector device and is connected to the buffering reservoir  6  by a lower conduit  34   a  located below the minimum ink level and by an upper conduit  34   b  located above the level of ink that allows respectively the supply in ink of the buffering reservoir  6  and the equilibrium of air pressures in the two reservoirs. Preferentially, the conduits  34   a  and  34   b  ensure a rigid mechanical connection between the buffering reservoir  6  and the additional reservoir  34 . Thus, these two reservoirs  6  and  34  both together comprise a removable assembly that is all in one piece. 
     As can be seen in FIG. 9, the lower part of the surrounding wall of the additional reservoir is equipped with a hole passing through to which the outlet opening of the electrodistributor  10  is connected. This additional reservoir is thus located so that it is supplied from the bottom. This thus avoids, when the ink is supplied, the formation of waves and shock waves, generated from variations in ink pressure in the additional reservoir  34  and in the buffering reservoir  6 . 
     The level detector is advantageously made up of a float  35  and a sensor  36  of the float position. This sensor is connected electrically to the command and control unit, and it is fitted to transmit to the unit a minimum level signal and a maximum level signal. This detector is carried by the cover of the additional reservoir  34 . 
     The projection heads function at a slight partial vacuum. It is necessary for the regularity of the printing that this partial vacuum remains constant at around minus  4  millibars or is subject to lower possible variations, a variation from minus four to minus eight millibars being acceptable. 
     It is necessary that there be, in each of the buffering reservoirs  6 , independently of the variations in the ink level and the movement of it in each reservoir, due to the fact of the movement of the loaded carriage carrying them, a minimum of change in the partial vacuum in order to avoid variations in the partial vacuum at the outlet of the nozzles which would generate printing disorders. 
     For this purpose, each buffering reservoir  6  is divided in order to prevent the formation of waves. 
     FIG. 3 shows a section view of an buffering reservoir among the four loaded buffering reservoirs  6 . 
     Each buffering reservoir  6  preferably has a cylindrical volume whose highest point is blocked and whose bottom  20  is restricted around the electrodistributor and the head. 
     For this purpose, the bottom wall  21  of each buffering reservoir has a cylindrical volume at a low height and a smaller diameter than that of the reservoir with an outlet nozzle  22 . 
     In the cylindrical volume of each of the buffering-reservoirs  6 , a dividing device  23  is introduced which breaks it down into several small reservoirs and performs a function of a wave breaking damping device. 
     As shown in FIGS. 5 and 6, the divider device is a block having a section corresponding to that of the internal volume of the buffering reservoir which is equipped from top to bottom with parallel channels  24  that define as many small reservoirs. 
     Advantageously, the divider has at its bottom, a recess  25  that makes the circulation of ink easier. 
     It goes without saying that any other system creating a division and a partitioning of the reservoir can be adapted. 
     The buffering-reservoirs  6  and additional reservoirs, the dividers and the covers are made of a suitable material, resistant to the ink solvents used. 
     The device for cleaning the nozzle plates and maintaining the nozzles connected to the compressor by the bypass  14  (FIG. 2) and shown schematically in position in FIG. 1 is represented in detail in FIGS. 7 and 8. On this plate, reserved aware arranged in the shape of a rectangle  27 . Two channels  28  and  29 , connected by conduits to the reservoir  17  at partial vacuum, open into each of the reserved areas. 
     The reserved areas  27  are aligned and offset at a slant relative to their axis of alignment. 
     The reserved areas  27  each accommodate a plate  28   a  made of a flexible material, for example, silicone. 
     The plate  28   a  has two parallel oblong openings at a slant. 
     A first short opening  30  corresponds to a conduit nozzle  28 . The opening  30  is equipped with two lips  31  having a low height. 
     A second oblong opening  32  corresponds to the conduit  29 . This opening  32  has a lip for doctoring  33 . 
     The maintenance cycle of the heads and nozzles is shown in FIG. 8 in a continuous line, the different stages of die cycle and the displacement of the head or the heads being shown by the letters A, B, C, D, E, F and G. 
     At A, the projection head is guided at a speed V 1  above the lips  31  of the opening  30 . 
     At B, the head is lowered at a speed V 2  onto the lips  31 . 
     At C, the partial vacuum at the head is interrupted. The suction is implemented by the action of the device using the Venturi effect  16  over a time T 1 . 
     At D, the head is brought back up by a value of  2  millimeters above the lips  31 , the suction is stopped, the partial vacuum is started again. 
     At E, the head is moved at a speed V 2  above the doctoring lip  33 . 
     At F, the head is brought back up again at a speed V 3 . 
     At G, the head is placed above the maintenance and opening zone  32 , and the spitting out of the rasters, i.e. of a quantity of ink over a time T, is started. 
     The maintenance cycle allows the cleaning of the nozzles and the nozzle plates and priming the heads and nozzles while preventing them from becoming sealed, for example, by the evaporation of solvents. 
     Finally, in order to avoid the migration of inks between the different printed zones on the medium, a prepolymerization is performed. For this purpose, behind the projection heads, the machine is equipped with a mechanism fitted to create a polymerization of the ink affixed on the medium, where this mechanism can be made up of a source for radiating ultraviolet light oriented towards the medium. Any other suitable mechanism can be used.