Patent Publication Number: US-8974041-B2

Title: Droplet selection mechanism

Description:
This application is the U.S. National Phase of International Application No. PCT/NL2008/050707, filed Nov. 7, 2008, designating the U.S. and published in English as WO 2009/061195 on May 14, 2009 which claims the benefit of European Patent Application No. 07120334.3 filed Nov. 9, 2007. 
     FIELD OF THE INVENTION 
     The invention relates to a droplet selection device for a continuous printing system. In this connection, by a continuous jet printing technique is meant the continuous generation of drops which can be utilized selectively for the purpose of a predetermined printing process. The supply of drops takes place continuously, in contrast to the so-called drop-on-demand technique whereby drops are generated according to the predetermined printing process. 
     BACKGROUND OF THE INVENTION 
     A known apparatus is described, for instance, in U.S. Pat. No. 3,709,432. This document discloses a so-called continuous jet printer for printing materials using a first droplet ejection system arranged to generate a continuous stream of first droplets from a fluid jetted out of an outlet channel. During the exit of the fluid through an outlet channel, a pressure regulating mechanism provides, with a predetermined regularity, variations in the pressure of the viscous fluid adjacent the outflow opening. This leads to the occurrence of a disturbance in the fluid jet flowing out of the outflow opening. This disturbance leads to a constriction of the jet which in turn leads to a breaking up of the jet into drops. This yields a continuous flow of egressive drops with a uniform distribution of properties such as dimensions of the drops. 
     The publication shows a gas jet mechanism to selectively deflect the drops. The fluid jet length is controlled of droplets generated by the regulating mechanism. The deflection properties of the droplets differ from that of the jet, so that droplets can be selectively deflected. 
     SUMMARY OF THE INVENTION 
     In one aspect, the invention aims to provide an alternative to the continuous droplet ejection system that is used to deflect the continuous stream of the first droplets. 
    
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
         FIG. 1  shows schematically a first embodiment of a printing system for use in the present invention; 
         FIG. 2  shows a first embodiment of a deflecting jet system; 
         FIG. 3  shows a second embodiment of deflecting jet system; 
         FIG. 4  shows a third embodiment of deflecting jet system; and 
         FIG. 5  shows an alternative embodiment of deflecting jet system. 
     
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
     According to an aspect of the invention, a droplet selection device for a continuous printer is provided, comprising: a droplet ejection system arranged to generate a continuous stream of droplets from a first fluid jetted out of an outlet channel; and a jet system arranged to generate a second jet for colliding the jet into the stream of droplets wherein the jet system comprises a deflector to selectively deflect the second jet into the continuous stream of droplets 
     According to another aspect of the invention, a method of selecting droplets from a fluid jet ejected from a continuous printer is provided, comprising generating a continuous stream of droplets from a first fluid jet jetted out of an outlet channel, generating a second jet for colliding into the droplets so as to selectively deflect the droplets from a predefined printing trajectory wherein the second jet is selectively deflected and collided with a predefined first droplet. 
     It is noted that in this connection, the term jet is used to identify a continuous longitudinal shaped volume of material moving through space, to denote the contrast with (a series of) droplets, each formed of generally spherical isolated volumes. 
     Without limitation, droplet frequencies may be in the order of 2-80 kHz, with droplets smaller than 80 micron. 
     In addition, by virtue of high pressure, fluids may be printed having a particularly high viscosity such as, for instance, viscous fluids having a viscosity of more than 300·10 −3  Pa·s when being processed. In particular, the predetermined pressure may be a pressure up to 600 bars. 
     Other features and advantages will be apparent from the description, in conjunction with the annexed drawings, wherein: 
       FIG. 1  shows schematically a first embodiment of a printing system for use in the present invention; 
       FIG. 2  shows a first embodiment of a deflecting jet system; 
       FIG. 3  shows a second embodiment of deflecting jet system; 
       FIG. 4  shows a third embodiment of deflecting jet system; and 
       FIG. 5  shows an alternative embodiment of deflecting jet system. 
       FIG. 1  shows a first schematic embodiment of a continuous printer head  1  according to the invention. The print head  1  comprises a first droplet ejection system  10  arranged to generate a continuous stream of first droplets  6  from a fluid jetted out of an outlet channel  5 . The droplet ejection system  10  comprises a chamber  2 , defined by walls  4 . Chamber  2  is suited for containing a pressurized liquid  3 , for instance pressurized via a pump or via a pressurized supply (not shown). The chamber  2  comprises an outlet channel  5  through which a pressurized fluid jet  60  is jetted out of the channel and breaks up in the form of droplets  6 . Schematically shown, actuator  7  is formed near the outlet channel  5  and may be vibrating piezo-electric or magnetostrictive member. By actuation of the actuator  7 , a pressure pulse is formed, breaking up the fluid jet and accordingly forming smaller monodisperse droplets  6 . 
     The outflow opening  5  is included in a relatively thin nozzle plate  4  which can be a plate manufactured from metal foil, of a thickness of 0.3 mm for example 0.1-3 mm. The outflow opening  5  in the plate  4  has a diameter of 50 μm in this example. A transverse dimension of the outflow opening  5  can be in the interval of 2-500 μm. As an indication of the size of the pressure regulating range, it may serve as an example that at an average pressure up to 600 bars [≡600×10 5  Pa]. The print head  10  may be further provided with a supporting plate  40  which supports the nozzle plate  4 , so that it does not collapse under the high pressure in the chamber. Examples of vibrating actuators may be found for example in WO2006/101386 and may comprise a vibrating plunger pin arranged near the outlet channel  5 . 
     The distance interval of the vibrating plunger pin may depend on the viscosity of the fluid. When printing fluids having a high viscosity, the distance from the end to the outflow opening is preferably relatively small. For systems that work with pressures up to 5 Bars [≡5·10 5  Pa], this distance is, for instance, in the order of 1.5 mm. For higher pressures, this distance is preferably considerably smaller. For particular applications where a viscous fluid having a particularly high viscosity of, for instance, 300-900·10 −3  Pa·s, is printed, an interval distance of 15-30 μm can be used. The vibrating pin preferably has a relatively small focusing surface area, for instance 1-5 mm2. In general, suitable ranges of the viscosity may be between 20-900·10 −3  Pa·s. 
     In  FIG. 1  jet system  70  is arranged to generate a second jet  61 . The second jet  61  is directed towards the stream of droplets  6  and is able to collide into a targeted droplet to selectively deflect the droplets from a predefined printing trajectory  3  towards a substrate  8 . The jet is comprised of fluid, typically a gas-fase material. Jet system  70  is provided with deflection system  71 , that deflects the second jet  61  from or into the continuous stream of droplets  6 . The jet  61  accordingly moves in transverse direction relative to the predefined printing trajectory towards substrate  8 . In  FIG. 1 , it is shown that the fluid jet  61  ejected from jet system  70  collides with a specific droplet  62 . Accordingly droplet  62  of a stream of droplets  6  is not received on substrate  8  but for instance in a collection gutter  9 . In a preferred embodiment printing material in collection gutter  9 , comprised of a mixture of jet material  61  and droplets material  62 , is demixed to recirculate printing liquid  3  through the printerhead  10  and/or to provide printing liquid to deflection system  70 . Generally, the printhead  10  can be identified as a continuous print head. Control of the jet system  70 , in particular deflector  71 , is provided by a control circuit  11 . The control circuit  11  comprises a signal output  12  to control actuation of the deflector  71  and signal input  13  indicative of a droplet generating frequency of the first droplet injection system  10 . In addition, control circuit  11  comprises synchronizing circuitry  14  to synchronize a deflection movement of the deflector  71  to deflect jet  61  to an ejection frequency of first droplets  6  of the printhead  10 . By control circuit  11 , droplet  62  can be selectively deflected out of droplet stream  6  of the printhead  10  on individual basis. In one aspect of the invention a droplet frequency of the printhead  10  is higher than 20 kHz. In particular, with such frequencies, a droplet diameter can be below 100 micron, in particular below 50 micron. In addition to a jet speed of 8 m/s or higher, a deflection speed of the deflector  71  is well suited to select a predefined droplet  62  of continuous stream  6  to have it collided with a fluid jet  61  to selectively deflect the droplet  62  from a predefined printing trajectory. In view of selected viscosities of jet material  60 , which may be ranging from 300-900−10 −3  Pa·s, and the fact that they may be formed from an isolated printing material, that is printing material that is non-polar, generated droplets  6  are difficult to deflect by electromagnetic fields. The current inventive principle can provide a suitable alternative, which may be very specific to individual droplets  62 . Accordingly a high dynamic range can be obtained by the deflection method according to the inventive embodiment depicted in  FIG. 1 . In one aspect the first droplets  6  are of a higher viscosity and/of isolating printing material. In that respect, the nature of the fluid jet  61  is typically a gas or a fluid having a very low viscosity. With the arrangement disclosed in  FIG. 1  a method can be provided for selecting droplets  6  from a fluid jet  60  ejected from a continuous printer head  10 . The droplets can be used for many purposes including image printing, rapid manufacturing, medical appliances and polymer electronics. In particular, the method is suited for printing fluids that fail to respond to electrostatic or electrodynamic deflection methods. Accordingly, for a continuous stream of first droplet  6  from a fluid jet  60 , a deflection method is provided by generating a continuous stream  6  of droplets from a first fluid jet  60  jetted out of an outlet channel  5 . A second jet  61  is generated for colliding into the droplets  6  so as to selectively deflect the droplet  6  from a predefined printing trajectory. The second jet  61  is selectively deflected and collided with a predefined first droplet  62 . It is noted that the timescale of the trajectory change is very small so that it can be used for high frequency printing methods, in particular, more than 20 kHz. In addition the deflection method illustrated hereabove, in contrast to prior art methods is relatively insensitive for droplet size variations or droplet charge variations which do not significantly affect the deflection behavior. 
       FIG. 2  shows a specific embodiment of the deflector  71 , depicted in  FIG. 1 . In particular, an air nozzle  73  is provided on a rotating disk  72 . By rotating the air nozzle  73 , the jet  61  can be deflected by synchronizing the rotation with the droplet frequency of stream  6 , droplets  62  can be selectively deflected from the predefined printing trajectory towards substrate  8 . Accordingly nozzle  73  is arranged to rotate the jet into and out of the predefined trajectory of droplets  6 . 
       FIG. 3  shows an alternative embodiment of the deflector  71 . Here the fluid jet  61  is translated sideways by a movement of a nozzle  73 , for instance by a vibrating piezo-element attached to nozzle  73 . Accordingly, a vibrating element  74  is coupled to a nozzle  73  to sideways translate the nozzle respective to the predefined trajectory, to produce a jet  61  that is sideways translated into and out of a droplet stream  6   
       FIG. 4  shows a further alternative embodiment of the deflector  71 . Here a jet  61  produced by jet generator  70 , is deflected by a curved surface  75 , that is arranged to the brought in contact with jet  61 . By “touching” the jet  61 , Coanda&#39;s principle will provide a jet deflection, which can provide lateral displacement of the jet relative to the trajectory of droplets  6 . Accordingly, the deflector  71  is provided by a curved surface  75  to be brought in contact with the fluid jet. 
       FIG. 5  shows an alternative embodiment of the deflector  71 . In particular, an air nozzle  73  is provided that can rotate laterally with respect to an ejection direction of jet  61 . By rotating the air nozzle  73 , the jet  61  can be deflected by synchronizing the rotation with the droplet frequency of stream  6 , droplets  62  can be selectively deflected from the predefined printing trajectory towards substrate  8 . Accordingly nozzle  73  is arranged to rotate the jet into and out of the predefined trajectory of droplets  6 . It is noted that minute rotations or tilts of the nozzle  73  may be sufficient to translate the beam over a relevant distance, depending on the distance of the droplets  62  relative to the nozzle  73 . Accordingly, individual droplet selections may be possible of frequencies higher than 20 kHz 
     In one aspect, deflection by impulse transfer can be used to selectively deflect the first droplets from a predefined printing trajectory towards a print substrate  8 . 
     Alternatively, the jet deflection method can be used to chemically activate first droplets  62 , for example, to selectively change the properties of the droplet  62  by fluid jet  61  in order to obtain a predetermined printing behavior. For example, this could be e.g. changing temperature, or changing the chemical properties by mixing. 
     In addition, by colliding droplets with fluid jet  61 , special forms of encapsulated droplets can be provided. In this way, special droplet compositions can be provided, for example, a droplet having a hydrophile and a hydrophobe side, or a droplet having multiple colored sides, for example, a black and a white side or a droplet having red, green and blue sides. 
     The invention has been described on the basis of an exemplary embodiment, but is not in any way limited to this embodiment. Diverse variations also falling within the scope of the invention are possible. To be considered, for instance, are the provision of regulable heating element for heating the viscous printing liquid in the channel, for instance, in a temperature range of 15-1300° C. By regulating the temperature of the fluid, the fluid can acquire a particular viscosity for the purpose of processing (printing). This makes it possible to print viscous fluids such as different kinds of plastic and also metals (such as solder).