Patent Publication Number: US-2021187961-A1

Title: Printing apparatus and corresponding method

Description:
FIELD OF THE INVENTION 
     The present invention relates to a printing apparatus and a method for printing on supports like paper, cardboard, textile materials, metal materials, wood materials, plastic materials, minerals or the like. 
     BACKGROUND OF THE INVENTION 
     Printing apparatuses are known that operate according to a so-called ink recirculation system. In this kind of printer, the ink is recirculated between two ink tanks passing through the print head in a channel tangent to the nozzles. The flow of ink along the nozzles generates by a sort of Venturi effect a depression which acts on an ink meniscus which bounds up in the nozzles due to the surface tension of the ink. In its basic configuration, this printer comprises at least a tank for a print material, like a dye, paint, or enhancing materials. In particular, printing apparatuses are known having a plurality of tanks, each dedicated to a distinct color, paint or enhancing material. 
     The print material, taken from the tank, is channeled to a recirculating unit, which circulates the print material through a print head. In the art, the fact that print material recirculation is necessary is known, primarily for balancing the ink meniscus in order to be maintained inside the nozzle opening and also due to the tendency of sundry print materials to settle and/or sediment, becoming unusable for print, and requiring a downtime for cleaning the printing machine and providing it with new print material. 
     Application EP2875956A1 by the same applicant describes a printing apparatus comprising a print head for printing a print material on a print support, and a recirculating unit of the print material. The recirculating unit comprises a first container, a second container, a first recirculating device to generate a first stream of said print material from said first container toward said print head and said second container, and a second recirculating device to transfer the print material from the second container to the first container. In particular, the first container of the print material is maintained under atmospheric pressure, while the second container is maintained under negative pressure, i.e. a pressure lower than atmospheric pressure. 
     Print heads have different requirements in terms of print material flow control; in use, it was shown that not all print heads can be used with the recirculating system described in EP2875956A1. Indeed, particularly for special ink formulations such as ceramic inks or inks comprising heavy particulate components the flow of recirculating ink which passes through the print head needs to be maintained slow in relation to traditional ink recirculation systems. Furthermore, the channel for the ink in the print head has to be maintained negative otherwise the ink will flow out of the nozzles of the print head. As disclosed in EP2875956A1 the recirculation of the ink inside the print head is caused by providing two ink tanks respectively connected to an entrance and to the output of the ink channel in the print head e pressure being applied to the ink in each tank in such a way to generate a pressure difference which drives the ink flow from one tank to the other. In printers in which the print head is displaced at least along one direction in relation to the area to be coated, the speed of the print head is about 1 msec. and the ink in the tank is subjected to rapid accelerations and decelerations which causes fluctuations of the pressure in each tank, and thus in the flow rate. This condition further renders the setting of the pressure difference between the two tanks of the recirculation system a particularly critical aspect of the ink recirculation system. 
     Considering slow recirculation flow ink systems, the need of not applying a positive over pressure in relation to the atmospheric pressure in one of the tanks limits the possibility of setting pressures in the two tanks which allows generating slow flow rates or speeds of ink through the print head. One tank needs indeed not to overcome the atmospheric pressure, so that for setting a pressure difference for obtaining a slow recirculation flow the negative pressure in the other tank will be very close to the atmospheric pressure, rendering the setting more critical. Furthermore, a reduced pressure difference between the two tanks renders the ink pressure and thus the ink flow less stable relatively to the pressure fluctuations introduced by the print head displacement with rapid accelerations and decelerations. 
     Moreover, it is worthwhile noting that print materials are often emulsions, and a too energetic recirculation can lead to a separation of emulsion phases, in a way conceptually not different from what occurs in the production of butter through cream churning. After using the apparatus described in EP2875956A1 with a plurality of print materials, the applicant noted that in some cases said recirculating unit can lead to an undesired separation of the phases of some print materials. 
     Documents U.S. Pat. No. 8,408,685, EP1831025 discloses printing apparatus operating according the ink recirculation system. In these documents two tanks or a tank divided into two separate parts is provided which tanks or tank parts are respectively connected to the input and to the output port of an ink flow through channel of the print head. Pressure or depression generating means are provided for applying a depression to one of the tank and a positive pressure to the other tank in order to generate the pressure difference between the two tanks driving the ink to flow from one tank to the other through the print head. 
     SUMMARY OF THE INVENTION 
     Aim of the present invention is providing a recirculating unit capable of working with any type of print materials and print head, and of economic construction. 
     This object is achieved by an apparatus and a method having the features of the independent claims. Advantageous embodiments and refinements are specified in claims dependent thereon. 
     A recirculating unit according to the present invention comprises: 
     at least a first container for print material, 
     at least a second container for print material, both maintained under two distinct negative pressures, i.e. a pressure lower than atmospheric pressure. 
     The fact that both containers are maintained under negative pressure, which can be controlled as desired, allows varying the speed of print material recirculation, working on the quantity of negative pressure applied in order to recirculate print material. This allows building up any kind of pressure difference by controlling actively a pressure in each tank and being completely free from variations of the atmospheric pressure due to environmental causes or to the displacements of the print head. Furthermore, the fact that two negative pressures are applied to the ink avoids that a positive pressure is exercised to the ink and thus breaks the forces maintaining stable the ink meniscus in the nozzles. 
     The method according to the present invention comprises the following steps: 
     Supplying a print material through a supply device to a first container making part of the recirculating unit, 
     Generating a first flow of said print material through a first recirculating sub-unit from said first container to said print head and to a second container of the recirculating unit, and 
     Channeling through a second recirculating sub-unit said print material from said second container to said first container, said print material first container and second container being kept under two distinct negative pressures. 
     According to a further feature which can be provided in combination with the above-mentioned features, the second recirculating sub-unit to transfer the print material from the second container to the first container comprises at least the delivery pipe for feeding the ink into the first container which delivery pipe has an output in the first container which is positioned at a depth inside the said first container below a predetermined minimum level of the ink in the said first container. 
     According to a variant, the recirculation pipe of the second recirculating sub-unit picking the ink from the second container in order to feed the ink to the first container is provided with an inlet which is placed inside the said second container at a depth below a predetermined minimum level of the ink in the said second container. 
     The above embodiments allow compensating possible pressure fluctuations introduced by the recirculation pump needed for transferring the ink from the second container back to the first one. This recirculation takes place in a continuous way. The recirculation pump could in some cases introduce a positive pressure which using two negative pressures might at least partially compensate one of the two negative pressures so that the negative pressure acting on the meniscus in the nozzles is changed and potentially reduced in such a way that ink is not anymore retained in the nozzles against dropping out 
     According to a further feature of the present invention which can be provided in any combination with the above embodiments and features an ink feeding device is provided comprising an accumulation tank, a pumping device and an ink refilling pipe for feeding refilling ink to the first print material container, which ink refilling pipe connects the output of pumping device with the first container, a pumping device control unit which drives the pumping device in a pulsed manner, for feeding in an alternate manner predetermined amounts of ink from the accumulation tank to the first container. 
     According to an embodiment the pumping device driving pulses have a predetermined frequency of the on off cycles of the pumping device with a predetermined repetition frequency and with predetermined time durations of the on and of the off phase of each cycle which are determined according to empirical, particularly experimental ways in relation to the constructive parameters of the print head, of the recirculation unit of the feed device for ink refilling and of the ink kind. 
     According to an embodiment, the pumping device operate with an on/off cycle of about 100 ms to 1000 ms. In a variant embodiment, the pumping device operates with an on/off cycle in the range from 300 to 700 ms. In a preferred embodiment, the on/off cycle duration of the pumping device is about 500 ms. 
     According to still a further embodiment the pressure in the circuit is about 8 to 15 kpa. 
     In an embodiment, the range of the pressure in the second recirculating subunit goes from 0 to 4 Kpa. 
     According to a further feature which can be provided alternatively or in combination with the preceding features, the ink refilling pipe for feeding refilling ink to the first print material container and connecting the output of pumping device with the first container has an outlet opening which is placed a predetermined depth inside the first container, which depth is lower than a predetermined minimum level of the ink inside the said first container and/or inside the said second container. 
     The above further improvements relating to the refilling unit allows to ensure that the refilling unit does not introduce perturbations of the negative pressure, particularly in the first container which are detrimental to the pressure conditions for maintaining the meniscus in the correct position inside the nozzle and compromising heavily the functions of the print head. Indeed, in the devices according to the state of the art, operating with an atmospheric pressure exercised on the ink in the first tank, a further pressure difference is introduced by the pumping devices feeding the refilling ink in the first container which would cause a small fluctuation of the pressure in the first tank between a very small negative and a very small positive pressure without sensibility affecting the pressure balance on the meniscus. This condition changes when considering the teaching according to the present invention in which a negative pressure is exercised also on the ink in the first container since the fluctuations could at least in some cases introduce a positive pressure on the ink which can compensate the negative pressure and thus disturb the pressure balance allowing the maintenance of the meniscus in the nozzles of the print head. 
     A first advantage of the present invention lies in the possibility of using any print head, including print heads that tended to leak print material in undesired way with the previous recirculating system, so leading to a better print quality. 
     A second advantage of the present invention is a better accuracy of print and the possibility of using any print material, particularly print materials that could not be used in the apparatus according to EP2875956A1. 
     A third advantage of the present invention is a more accurate control of the print material in the containers supplying print head, improving their control. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Further advantages and properties of the present invention are disclosed in the following description, in which exemplary embodiments of the present invention are explained in detail based on the drawing: 
         FIG. 1  is a schematic representation of an embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION 
       FIG. 1  shows an embodiment of a printing apparatus  10  according to the present invention, to print a print material on a (not shown) support. 
     The print material can comprise an ink, a paint or an enhancing material suitable to confer, for example, a shiny or opaque effect or other graphic effects, such as glitter, sequins or the like, conferred for example by respective paints. 
     The print support can be made for example of a material chosen from paper, cardboard, textile materials, metal materials, wood materials, plastic materials, mineral materials or ceramic materials, although other types of material can also be used, and are not relevant for the purposes of the present invention. 
     According to an embodiment, the printing apparatus  10  comprises a feed device  11  to feed the print material toward a recirculating unit  15 , which in its turn generates, in a print head  13 , a continuous recirculation of the print material. For the sake of convenience, in the description the recirculating unit  15  is subdivided into two recirculating subunits, a first recirculating subunit  16  and a second recirculating subunit  17 . 
     The print head  13  can be selectively controlled, in a substantially known way, to deposit the print material on a (not shown) print support according to a prefixed pattern. 
     According to a preferred embodiment, the feed device  11  comprises an accumulation tank  12  into which the print material is deposited before printing, and a first pumping device  22  to transfer the print material from the accumulation tank  12  toward the recirculating unit  15 . 
     Said accumulation tank  12  is connected to a filtering device  21  through a pipe  41 . The pumping device  22  takes the print material from the accumulation tank  12  through a first suction pipe  41 , and channels it to the recirculating unit  15  through a second pipe  42 . 
     In an embodiment, the feed device  11  further comprises a mixing device  30  to periodically or continuously mix the print material contained in the accumulation tank  12 . 
     On the first pipe  41  and/or on the second pipe  42  a filtering device  21  can be interposed, e.g. in the form of a filter or the like. 
     According to an embodiment a control unit  33  may be provided which drives the pumping device  22  in a pulsed way, such as to feed the ink for refilling the first tank  14  of the recirculation unit  15  with small ink amounts in an alternate manner. 
     According to a first variant embodiment, the control unit  33  is configured in order to set predetermined repetition frequencies of the on/off deriving cycles of the pumping device  22 . Variant embodiments may provide a control unit configured to set the time duration of the on and off phases of the pumping device  22 . 
     According to an embodiment, the control unit  33  is provided with a user interface allowing to input data relating to the repetition frequency and to the duration of the on and off phases of the pumping device  22 . 
     According to still a further embodiment one or more of the above parameters may be set in an automatic way by measuring the level of the ink inside at least the first container  14  by an ink level sensor  56  and by setting at least one of the repetition frequency and to the duration of the on and off phases of the pumping device  22  needed for refilling the ink in the tank in order the level of the ink falls below a minimum level L1. 
     The algorithm of calculating and setting the above parameters, either manually or automatically is preferably determined in an empirical, especially experimental manner, as a function of the constructive parameters of the system, particularly of the feeding device  11  of the refilling system and of the kind of ink either in house by the system producer or at the system user. 
     The said algorithm can be in the form of a program in which the instructions are coded for carrying out the algorithm by the control unit  33 . 
     Said recirculating unit comprises a first container  14  for print material connected to the feed device  11 . The first container  14  is connected to the accumulation tank  12  through pipe  42 . The first container  14  is upstream a respective print head  13  to which it is connected through a first pipe  43   a . In a preferred embodiment, the first pipe  43   a  is connected in correspondence to a lower portion, or bottom, of the first container  14 . The connection of the first pipe  43   a  in the lower part of the first container  14  allows preventing the forming of air bubbles in the print material contained in the first container  14 . 
     According to still another feature of the present invention, the second pipe  42  connecting the output of the pumping device with the first container  14  of the recirculating unit  15 , has an outlet  142  which is placed at a depth inside the said first container  14  which depth is lower than a predetermined minimum ink level L1 inside the said first container  14 . 
     The recirculating unit further comprises a second container  18  for print material, coming from the print head  13  through pipe  43   b . In a preferred embodiment, the second pipe  43   b  is connected in correspondence to a lower portion, or bottom, of the second container  18 . The connection of the second pipe  43   b  in the lower part of the second container  18  allows preventing the forming of air bubbles in the print material contained in the second container  18 . 
     Print material first container  14  and second container  18  lie on the same plane, and can contain two distinct levels of print material. 
     The recirculating unit is of pneumatic type and comprises a pair of members  19 ,  20  (in the form of a series of solenoid valves) for generating negative pressure connected to said print material first container  14  and second container  18 . In particular, said members  19 ,  20  are configured for generating a negative pressure, or depression, inside first container  14  and second container  18 . The difference of negative pressure between the first container  14  and the second container  18  leads the print material to shift from the first container  14  to the second container  18 , passing through the print head  13 . 
     The members  19 ,  20  for generating negative pressure comprise a series of solenoid valves assembled on a mechanical block configured to exploit Venturi effect. In particular, the member  19  generates negative pressure in the first container  14 , and the member  20  generates negative pressure in the second container  18 ; the two negative pressures are different between the said print material containers  14  and  18 . 
     Typically, the value of the negative pressure in the print material first container  14  is −4 Kilopascal (kPa), while the value in the print material second container  18  is −10 kPa. This difference in the negative pressures activates the shifting of print material from the first container  14  to the second container  18  passing through the print head  13 . 
     Controlling the two members  19  and  20  for generating negative pressure, the negative pressure in said two print material containers  14  and  18  can be varied. Indicatively, the values of negative pressure which can be obtained range 0 to −50 Kilopascal. 
     Different print heads and different print materials require the fine-tuning of the negative pressures in print material first container  14  and second container  18 . 
     The detection of negative pressure values is performed by a pressure sensor  50  detecting the negative pressure values in print material containers  14  and  18 . The detected values are then transmitted to a control unit  33  controlling the members  19  and  20  so that the desired negative pressure values are obtained. 
     Moreover, the members  19  and  20  generating negative pressure have a further valve enabling the cleaning of the print head  13  under positive pressure, modifying the pressure inside print material containers  14  and  18 . Obviously, cleaning cycles, which implies feeding the print head  13  under positive pressure, are performed when the printing apparatus is not printing. 
     Moreover, there are provided two overflow tanks  25   a ,  25   b , which are interposed between print material first container  14  and second container  18 , e.g. in an intermediate position between a first connecting pipe  46  and a second connecting pipe  47 . 
     In particular, in the tank  25   a , connected to the first container  14 , air is kept in a condition of negative pressure, and in the tank  25   b , connected to the second container  18 , a condition of negative pressure is also generated. 
     Said overflow tanks  25   a  and  25   b  can be provided with (not shown) discharge valves, for discharging possible condensation, or print material that could be present inside them. Moreover, said tanks  25   a ,  25   b  can be provided with sensors detecting print material. If such sensors are activated, they control or provide a signal for deactivating said members  19 ,  20  for generating negative pressure. In this way damages to the members  19 ,  20  generating negative pressure can be prevented. 
     In an embodiment of the present invention, the recirculating unit  15  also comprises a second recirculating sub-unit  17 , to transfer the print material from the second container  18  to the first container  14 . 
     The second recirculating subunit  17  comprises a pumping device  23  connected, through a pipe  28 , to the first container  14  and second container  18 , to shift the print material from the second container  18  to the first container  14 . In a preferred embodiment, the suction pipe  28  is connected with one of its end to the lower portion, or bottom, of the second container  18 . This allows preventing that air is suctioned through suction pipe  28 . 
     The pipe  28  connects the pumping device  23  to print material second container  18 , while a pipe  45  connects the pumping device  23  to the first container  14  for print material. In a preferred embodiment, the pipe  45  is connected with one of its end to the lower portion, or bottom, of the second container  18 . This allows preventing that air is suctioned through suction pipe  28 . 
     In an embodiment, at least the delivery pipe  45  is provided with a filtering device  59  for filtering the print material stream that from the second container  18  enters in the first container  14 . Moreover, should the pumping device  23  membrane break, the presence of the filtering device  59  prevents the entrance of undesired particles into the print material first container  14 , and from there to the print head  13 , with ensuing possible damages to the print head. 
     According to an embodiment of the present invention, at least the delivery pipe  45  connecting the output of the pumping device  23  with the first container  14  of the recirculating unit  17 , has an outlet which is placed at a depth inside the said first container  14  which depth is lower than a predetermined minimum ink level L1 inside the said first container  14 . 
     In a further embodiment, also the pipe  28  connecting the second container  18  with the input of the pumping device  23  of the recirculating unit  17 , has an inlet which is placed at a depth inside the said first container  14  which depth is lower than a predetermined minimum ink level L1 inside the said first container  14 . 
     In an embodiment, there are provided detection devices, which comprise a first level sensor  56   a  to detect the level of print material in the first container  14 , and a second level sensor  56   b  to detect the level of print material in the second container  18 . 
     The first level sensor and the second level sensor can be chosen from a group comprising optical sensors, magnetic sensors, inductive sensors, capacitive sensors, floating members, or possible combinations thereof. 
     In the present embodiment, there are provided new dynamic level sensors, which allow, with respect to the previous on/off sensors, a more accurate control of print material level in the feed tank of the print heads, improving their management. 
     In an embodiment, the recirculating unit  15  comprises conditioning devices connected to at least one of either the first container  14  or the second container  18  and configured to modify in a desired way the physical characteristics (e.g. temperature, viscosity) of the print material that circulates through the print head  13 . 
     The conditioning devices can comprise a system to regulate the temperature, for example obtained with a heat-carrying circuit, able to modulate the temperature of the print material contained in at least one of either the first container  14  or the second container  18 , for example by heating and/or cooling action. 
     The conditioning devices comprise e.g. a heating member  52  mounted outside at least one of either the first container  14  or the second container  18  to heat the print material contained in the latter according to pre-set modes. 
     The heating member comprises e.g. an electric resistance  52  as shown in  FIG. 1 , a heat conditioning circuit or other device suitable to the purpose. 
     In a further embodiment, the conditioning devices comprise a cooling member  61  to induce in at least one of either the first container  14  or the second container  18  a cooling of the print material contained therein, and to adjust its temperature to the temperature of use by the print head  13 . In an embodiment, the cooling member  61  comprises a fluid-dynamic conditioning circuit that exploits the principles of a cooling cycle. 
     The printing apparatus comprises a control unit  33 , in this case a PLC, configured to monitor and manage at least the functioning mode of the first recirculating subunit  15  and the second recirculating subunit  17 . In particular, depending on the signals detected by the first level sensor and the second level sensor, the control unit  33  can suitably manage the activation of the first pumping device  22  and of the pumping device  23 , in order to maintain in the first container  14  and the second container  18  a predetermined level of print material, so as to ensure the correct supply to the print head  13 . 
     In an embodiment, the control unit  33  is connected to the components of the apparatus  10  to be controlled and managed through electric connections (not shown), such as electric cables or electric tracks, in the case where the control device comprises integrated boards or PCB. 
     In a further embodiment, the control unit  33  can be served by user interface devices  34  to allow a human user to selectively set some print parameters and monitor alarms and anomalies. 
     In an embodiment, the interface devices  34  are external to the printing apparatus  10 , for example made by means of an electronic calculator such as a PC, or similar device. 
     Even if the present invention was described with reference to a printing apparatus  10  comprising one print head  13  only, an application of the invention to a printing apparatus with more than one print head  13  is possible. 
     In an embodiment, for each print head  13  it is necessary to provide its own feed device  11  and its own recirculating unit  15 , like in the description above. 
     In an alternative embodiment, the recirculating unit  15  according to the present invention can feed a plurality of print heads, typically up to four. 
     According to a variant embodiment of the apparatus of  FIG. 1 , the first and the second container  14  and  18  for the print material are provided with a dedicated print material level sensor  56   a  and  56   b . Each of the two sensors monitors the level of the print material in the corresponding container  14  and  18  separately. The sensors react to a maximum print material L2 level in the respective container  14  ad  18  and the signal generated by each sensor, separately, is transmitted to the control unit  33  which controls the members for generating negative pressure  19  and  20 . When the level of the print material in at least one of the two containers  14 ,  18 , reaches the maximum level L2, the signal of the sensor triggers the stop of the members generating negative pressure allowing the atmospheric pressure to build up into both the containers  14  and  18  and causing the print material to evacuate the containers. Evacuation takes place through the nozzles of the print head  13 , since in absence of the flow through the print head no retain force is applied to the meniscus of print material in each of the nozzles and the print material flows down into a recovery tank  62 . 
     From this tank, the print material can be again pumped into the system. This can be carried out automatically by providing sensors of the print material in the tank  62 , such as level sensors or other kind of sensors which controls a pump for transferring the print material from tank  62  to the containers  14  and  18 . Obviously the transfer of the print material from the retrieval tank  62  into the containers  14  and/or  18  can also be carried out by a manual control of the pump. 
     When the ink level in the containers reaches a minimum level indicated by L1 the level sensors  56   a  and/or  56   b  signals this condition and triggers the activation of the members  19 ,  20  generating the negative pressure in the containers  14  and  18  and the system starts again its normal operation. 
     The above system for controlling the level of the print material in the containers  14  and  18  is particularly relevant when considering that a negative pressure is applied to each of the containers  14  and  18  and since these negative pressures apply a suction action on the print material if the system fails in maintaining the correct balances of the pressures it easily may happen that the level of the print material in one of the two containers or in both rises in an uncontrolled manner leaving the container and filling up the apparatus. Monitoring the level of the print material allows directing the print material exceeding a certain level along a controlled path to a retrieve tank avoiding the flow of rising print material flooding all-over the apparatus causing damages and requesting long lasting recovery interventions. 
     While the invention has been described in connection with the above described embodiments, it is not intended to limit the scope of the invention to the particular forms set forth, but on the contrary, it is intended to cover such alternatives, modifications, and equivalents as may be included within the scope of the invention. Further, the scope of the present invention fully encompasses other embodiments that may become obvious to those skilled in the art and the scope of the present invention is limited only by the appended claims. 
     LIST OF REFERENCE NUMBERS 
     
         
           10  printing apparatus 
           11  feed device 
           12  accumulation tank 
           13  print head 
           14  first print material container 
           15  recirculating unit 
           16  first recirculating sub-unit 
           17  second recirculating sub-unit 
           18  second print material container 
           19  member for generating negative pressure 
           20  member for generating negative pressure 
           21  filtering device 
           22  pumping device 
           23  pumping device 
           25   a  first overflow tank 
           25   b  second overflow tank 
           28  pipe 
           30  mixing device 
           33  control unit 
           34  interface device 
           41  pipe 
           42  pipe 
           142  pipe outlet 
           43   a  pipe 
           43   b  pipe 
           45  delivery pipe 
           46  pipe 
           47  pipe 
           50  pressure sensor 
           52  heating member 
           56  detection device 
           59  filtering device 
           61  cooling member 
           62  print material retrieve tank 
         L1 print material minimum level 
         L2 print material maximum level