Abstract:
A printer that produces objects from liquid conductive material is disclosed. In one embodiment, the printhead has a chamber for containing liquid conductive material surrounded by an electromagnetic coil. A DC pulse is applied to the electromagnetic coil, resulting in a radially-inward force on the liquid conductive material. The force on the liquid conductive material in the chamber results in a drop being expelled from an orifice. In response to a series of pulses, a series of drops fall onto a platform in a programmed pattern, resulting in the formation of an object.

Description:
BACKGROUND OF THE INVENTION 
     Field of the Invention 
       [0001]    Liquid metal jet printing is, in one embodiment, a type of drop-on-demand printing. It is similar to ink-jet printing in that a drop of liquid to be printed is dispensed from a nozzle at specific intervals to create a figure or object. Typically, a platform beneath the nozzle moves in a pre-programmed pattern to form an object. To produce a pattern on the printing surface, drops are successively ejected from the nozzle after each movement of a printhead. The timing of the movement of the nozzle is often dependent upon the time required to produce a drop of liquid. 
         [0002]    In three-dimensional printing, patterns are generally repeated on a printing surface, where successive drops on top of another eventually produce a three-dimensional object. Three dimensional printing has been most successful, to this point, in creating plastic objects. Three-dimensional printing of metal objects has been limited in its usefulness due to the technical difficulties in working with liquid metal. 
         [0003]    Various methods of producing a liquid metal drop for printing have been developed. A number of devices known in the art utilize mechanical force to propel liquid metal out of a nozzle. Mechanical force for producing a drop can be generated by various means. Some devices related to the present disclosure utilize piezoelectric actuators to generate mechanical force to generate a drop, such as U.S. Pat. No. 7,077,334. The &#39;334 patent is directed to a drop-on-demand printer. The method described in the &#39;334 patent exemplifies the use of a piezoelectric actuator to create pressure in the fluid-containing chamber of a drop-on-demand printing device. Another example of the use of a piezoelectric actuator in drop-on-demand printing is described in U.S. Pat. No. 4,828,886. Means of producing a drop other than piezoelectric have been described in the related art. Ultrasonic means of generating a drop. Examples of this method include U.S. Pat. Nos. 3,222,776 and 4,754,900, which induce vibrations at the nozzle through the use of ultrasound to produce a drop. 
         [0004]    The related art discloses various methods by which devices have utilized electromagnetic coils to produce a force on liquid metal to eject liquid metal out of a nozzle. For example, U.S. Pat. No. 6,202,734 relates to a device for producing liquid metal drops utilizing magentohydrodynamics. The &#39;734 patent also describes the use of electromagnetic force to produce drop-on-demand liquid metal. The patentable improvement over the related art described by the &#39;734 patent generally relates to the use of alternating current and magnetohydrodynamics in liquid metal printing. 
         [0005]    A number of related art devices utilize a magnetic coil adjacent to the liquid metal to induce a field to impose a force on the liquid. In these types of devices, the liquid carries a current flowing in a direction perpendicular to the surrounding magnetic field, thereby generating a force. This type of device is generally known as an electromagnetic (EM) pump. EM pump devices generally rely on alternating current (AC) in the magnetic coil to produce a force on liquid metal. Examples of AC EM pump devices include U.S. Pat. No. 4,842,170; which describes an electromagnetic pump applying an alternating current to an electromagnetic coil adjacent a nozzle. U.S. Pat. No. 3,807,903 describes an electromagnetic pump that relies on varying electrical current to control the liquid flow from a nozzle. 
         [0006]    U.S. Pat. Nos. 8,267,669, 4,818,185, 4,398,589; 4,566,859, 3,515,898 and 4,324,266, 4,216,800 also relate to devices for electromagnetically pumping liquid metal. Generally, these devices utilize alternating current or travelling magnetic fields by physically moving permanent magnets to impart force on a liquid metal. These devices were patentable because they improved upon the prior art by eliminating the need for solid electrodes to produce a current in the metal flow. The patentable improvements over the prior art for the &#39;669 and &#39;185 patents generally relate to the ability of the devices to create a force in the liquid metal stream without electrodes that could corrode, or seals that could fail. 
         [0007]    U.S. Pat. No. 5,377,961 relates to an improvement on an electromagnetic pump type device for producing drops of liquid metal. The &#39;961 patent relates to a soldering device for depositing small amounts of solder on a printed circuit board. The &#39;961 device pinches off drops by a mechanism that propels a drop forward and reverses force on the stream to separate the stream from the drop using an AC current applied to the liquid metal. The improvement of the &#39;961 device relates to the reversal of force to produce a drop in a relatively short period of time. The method utilized by the &#39;961 device reverses the direction of the electric current applied to the system, causing the force exerted on the solder stream to be substantially instantaneously reversed without the necessity of transferring electrical energy to vibratory, ultrasonic or the like. 
         [0008]    The related art described above has several disadvantages. The &#39;734 patent does not utilize direct current (DC) applied to a magnetic coil to produce a force in an annular direction leading to the liquid metal being forced radially toward the nozzle, thereby producing a liquid metal drop. The use of a DC pulse to produce a force simplifies the construction of a drop-on-demand printer. With regard to the relevant art described previously, where mechanical force is used to generate a drop, seals and moving parts are prone to wear and failure. For example, a piezoelectric actuator must be kept below its curie temperature to continue functioning. This requires it to be placed remotely behind insulation and act through rods or linkages. This complexity adds friction, risk of leakage, low performance and more expensive maintenance requirements. 
         [0009]    Similarly, mechanical means of displacing a drop generally involve more moving parts, which can lead to greater wear on the device and greater expense. Ultrasonic methods of mechanically displacing a drop rely on the back and forth motion induced by ultrasonic radiation. Such methods have not been effective enough to produce an economically viable liquid metal jet printer in the marketplace. 
         [0010]    With regard to electromagnetic force devices, the related art described herein generally utilizes alternating current to generate an outward flow from the nozzle and a reverse, inward flow to displace the drop from the liquid stream. With the use of alternating current applied to a magnetic coil, the current must be applied in one direction and then the magnetic field must be reversed, a stepwise process that requires significant time, in terms of drop-on-demand printing and more complex and expensive power electronics. While related devices have addressed this issue, none have been successful in limiting exposure of critical parts to corrosive liquid metal which subjects such devices to significant and expensive wear. 
       SUMMARY OF THE INVENTION 
       [0011]    The present disclosure overcomes the disadvantages of the related art. The present disclosure describes the application of a single pulse of direct current to an electromagnetic coil to create a radial force on a liquid conductive material. This radial force results in a drop of liquid conductive material being expelled from a nozzle onto a platform. As the platform moves relative to the nozzle, a series of drops solidify on the platform to form a 3D object. The present disclosure describes a device that will not corrode or arc like related devices. Further, the device of the present disclosure requires fewer moving parts and is less expensive to build than currently existing related devices. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0012]    The present invention and the manner in which it may be practiced is further illustrated with reference to the accompanying drawings wherein: 
           [0013]      FIG. 1  shows a perspective view of the 3D printer. 
           [0014]      FIG. 2  shows an exploded view of the internal components of the printhead. 
           [0015]      FIG. 3  shows a side view of the internal components of the printhead. 
           [0016]      FIG. 4   a  shows a cross-sectional view taken along line  4   a  from  FIG. 3  illustrating the internal components of the printhead. 
           [0017]      FIG. 4   b  shows cross-sectional view of the lower housing taken along line  4   b  of  FIG. 2  including the pump chamber and damping chamber. 
           [0018]      FIG. 5  shows a side elevational view of the internal components of the printhead including the electromagnetic coil. 
           [0019]      FIG. 6  shows a broken away cross sectional view of the printhead. 
           [0020]      FIG. 7  shows a broken away cross sectional perspective view the nozzle, without liquid conductive material in the chamber, illustrating the flow of inert gas. 
           [0021]      FIG. 8  shows a broken away cross sectional perspective view the nozzle pump containing liquid conductive material. 
           [0022]      FIG. 9  shows a schematic cross sectional view of liquid conductive material in the pump chamber, including the flow of liquid material out of the pump chamber and the electromagnetic coil. 
           [0023]      FIG. 10  shows a schematic cross sectional view of the nozzle pump including magnetic field lines. 
           [0024]      FIG. 11  shows a perspective view of the nozzle pump producing drops forming a 3D object. 
       
    
    
     DETAILED DESCRIPTION OF INVENTION 
       [0025]    The following detailed description is merely exemplary in nature and is not intended to limit the described embodiments or the application and uses of the described embodiments. As used herein, the word “exemplary” or “illustrative” means “serving as an example, instance, or illustration.” Any implementation described herein as “exemplary” or “illustrative” is not necessarily to be construed as preferred or advantageous over other implementations. All of the implementations described below are exemplary implementations provided to enable persons skilled in the art to make or use the embodiments of the disclosure and are not intended to limit the scope of the disclosure, which is defined by the claims. For purposes of description herein, the terms “upper,” “lower,” “left,” “rear,” “right,” “front,” “vertical,” “horizontal,” and derivatives thereof shall relate to the invention as oriented in  FIG. 1 . Furthermore, there is no intention to be bound by any expressed or implied theory presented in the preceding technical field, background, brief summary or the following detailed description. It is also to be understood that the specific systems and processes illustrated in the attached drawings, and described in the following specification, are simply exemplary embodiments of the inventive concepts defined in the appended claims. Hence, specific dimensions and other physical characteristics relating to the embodiments disclosed herein are not to be considered as limiting, unless the claims expressly state otherwise. 
         [0026]    At the outset, it should be clearly understood that like reference numerals are intended to identify the same structural elements, portions, or surfaces consistently throughout the several drawing figures, as may be further described or explained by the entire written specification of which this detailed description is an integral part. The drawings are intended to be read together with the specification and are to be construed as a portion of the entire “written description” of this invention as required by 35 U.S.C. §112. 
         [0027]    Since many modifications, variations, and changes in detail can be made to the described preferred embodiments of the invention, it is intended that all matters in the foregoing description and shown in the accompanying drawings be interpreted as illustrative and not in a limiting sense. Thus, the scope of the invention should be determined by the appended claims and their legal equivalence. 
         [0028]      FIG. 1  illustrates an overview of the liquid metal 3D printer  100  of the present invention. In the preferred embodiment, drops of liquid metal that form a three dimensional metal object are produced by a printhead  102  supported by a tower  104 . The printhead  102  is affixed to vertical z-axis tracks  106   a  and  106   b  and can be vertically adjusted, represented as movement along a z-axis, on tower  104 . Tower  104  is supported by a frame  108  manufactured from steel tubing. 
         [0029]    Proximate to frame  108  is a base  110 , formed of granite. Base  110  supports a platform  112  upon which a 3D object is formed. Platform  112  is supported by x-axis tracks  114   a  and  114   b , which enable platform  112  to move along an x-axis. X-axis tracks  114   a  and  114   b  are affixed to a stage  116 . Stage  116  is supported by y-axis tracks  118   a  and  118   b , which enable stage  116  to move along a y-axis. 
         [0030]    As a drop of molten aluminum  120  falls onto platform  112 , the programmed horizontal movement of platform  112  along the x and y axes results in the formation of a three dimensional object. The programmed movement of stage  116  and platform  112  along x-axis tracks  114   a  and  114   b , and y-axis tracks  118   a  and  118   b  is performed by means of an actuator  122   a  and  122   b , as would be known to a person of ordinary skill in the art. Liquid metal 3D printer  100  was designed to be operated in a vertical orientation but other orientations could also be employed. 
         [0031]    Liquid metal 3D printer  100  requires input from external sources to control its moving parts. Control and coordination of the liquid metal 3D printer  100  comes from a controller which in the preferred embodiment is a computer, as would be known to one of ordinary skill in the art. The computer is used to translate electronic information into signals to control the ejection of droplets, the positioning of stage  116  and platform  112 , as well as the height of printhead  102 . Printhead  102  may remain stationary in the preferred embodiment of the present invention; the movement of stage  116  and platform  112  provides sufficient range of motion. An inert gas supply  140  provides a pressure regulated source of inert gas  142 , such as argon, to the printhead  102  through a gas supply tube  144  to prevent the formation of aluminum oxide.  FIG. 1  also shows a source of aluminum  132  and aluminum wire  130 . 
         [0032]      FIG. 2  shows an exploded view of the internal components of printhead  102 . Alternative embodiments may utilize aluminum in bar, rod, granular or additional forms. In alternative embodiments, any sufficiently conductive liquid or colloidal mixture could be used in place of aluminum with the proper adjustments to the system, as would be known by one of ordinary skill in the art. An upper pump housing  210 , pump partition  204 , and lower pump housing  214  together form a first chamber, herein referred to as a pump chamber  220 . The internal components shown in  FIG. 2  are manufactured from a non-conductive material, which in the preferred embodiment is boron nitride. 
         [0033]      FIG. 3  illustrates internal components of printhead  102  assembled. In the preferred embodiment, the internal components of printhead  102  shown in  FIGS. 2 and 3  are designed to be fitted together by clamping. In alternative embodiments additional means of connecting individual parts of the present invention may be contemplated, and could include adhesives, mechanical connections including screws, bolts, or other means as would be known to a person of ordinary skill in the art. Upper pump housing  210 , pump partition  204  (shown in  FIG. 2 ), lower pump housing  214  assembled together form nozzle pump  300 . 
         [0034]      FIG. 4   a  is a cross-sectional view taken along line  4   a  from  FIG. 3  of the assembled internal components of printhead  102 .  FIG. 4   a  shows a channel  404  extending from a first end where aluminum wire  130  enters printhead  102  and a second end where liquid aluminum leaves channel  404  and enters pump chamber  220 . Adjacent pump chamber  220  is nozzle  410 . Surrounding channel  404  is a tundish  402 . 
         [0035]      FIG. 4   b  shows a cross-sectional view taken along line  4   b  from  FIG. 2 , illustrating lower pump housing  214  and pump chamber  220 . Lower pump housing  214  has ledges  420  to prevent pump partition  204  from falling into pump chamber  220 . Adjacent to pump chamber  220  is nozzle  410 . Contained within nozzle  410  and downstream of pump chamber  220  is a second chamber, herein referred to as a damping chamber  430 . Downstream of damping chamber  430  within the nozzle is a concentric orifice  440  through which liquid conductive material is expelled. 
         [0036]    In the preferred embodiment, located between orifice  440  and damping chamber  430  is a surface extending radially outward and upstream of orifice  440  to the wall of damping chamber  430 . An alternative embodiment may exclude the damping chamber  430 , in which case liquid aluminum would flow directly from pump chamber  220  to orifice  440 . 
         [0037]      FIG. 5  illustrates nozzle pump  300  enclosed by electromagnetic coil  510  which is manufactured from copper, or alternatively tungsten, plasma or other materials known to be suitable by those of skill in the art. Electromagnetic coil  510  has positive electrical connection  504  and a negative electrical connection  506 . 
         [0038]      FIG. 6  illustrates a cross-sectional view of printhead  102 , which shows cooled wire inlet  608 , an outer sleeve  606 , and the nozzle pump  300  enclosed by electromagnetic coil  510 . In the preferred embodiment, aluminum wire  130  is fed into cooled wire inlet  608  and a wire guide and gas seal  610  made of copper. The aluminum wire  130  then passes through an insulating coupler  604 , made of Macor ceramic, where inert gas  142  is supplied through the melt shield gas inlet port  602 , made of Macor ceramic, to apply a protective inert gas  142  shield before the aluminum is melted. 
         [0039]    Melted aluminum, or other electrically conductive liquid, flows downward under gravity and positive pressure exerted by inert gas  142  along a longitudinal z-axis to nozzle pump  300 . Electrical heating elements  620   a  and  620   b , made of nichrome, heat the interior of a furnace  618 , made of firebrick, to above the 660° C. melting point of aluminum. A thermally conductive boron nitride tundish  402  transmits heat to aluminum wire  130 , as supplied from a source of aluminum  132 , causing it to melt as it enters nozzle pump  300 . 
         [0040]    Inert gas  142  is conveyed via melt shield gas inlet port  602  and nozzle shield gas port  630  allowing inert gas  142  to form a shield around the liquid aluminum to prevent the formation of aluminum oxide while in flight. A high purity inert gas  142  atmosphere reduces the potential for clogging as molten aluminum passes into pump chamber  220 . 
         [0041]      FIG. 7  illustrates pump chamber  220 , which serves as a reservoir of molten aluminum, in the downstream portion of nozzle pump  300 . Inert gas  142 , as indicated by arrows, flows inside and outside of nozzle pump  300 . 
         [0042]      FIG. 8  shows molten aluminum flowing downward through upper pump housing  210  around pump partition  204  to form a charge of molten aluminum  710 . Charge of molten aluminum  710  is contained primarily within the pump chamber  220 , with a small amount of the molten aluminum contained in upper pump housing  210  to keep pump chamber  220  fully primed. An excess of molten aluminum in the upper section of pump chamber  220  would increase the inertia of the charge of molten aluminum  710  and cause an undesirable decrease in the firing rate of nozzle pump  300 . In alternative embodiments the number of dividers in the pump partition  204  may be varied. 
         [0043]    Electromagnetic coil  510  is shaped to surround nozzle pump  300 . The pressure on the inert gas  142  inside nozzle pump  300  is adjusted to overcome much of the surface tension at the nozzle  410  in order to form a convex meniscus  810 . The pre-pressure within pump chamber  220  prior to a pulse is set by inert gas  142  to create convex meniscus  810  with a spherical cap that is less than the radius of nozzle orifice  440 . This pressure is determined by Young&#39;s law as P=2×surface tension/orifice  440  radius. 
         [0044]      FIG. 9  is a simplified 3D section through nozzle pump  300  showing only the electromagnetic coil  510  and the charge of molten aluminum  710 . Charge of molten aluminum  710  is shown at an appropriate level in pump chamber  220  for operation. The shape of the upstream portion of charge of molten aluminum  710  conforms to pump partition  204  and partition dividers  206 . 
         [0045]      FIG. 9  further shows electromagnetic coil  510  shaped around nozzle pump  300  in such a way as to focus magnetic field lines  940  vertically through charge of molten aluminum  710 . Nozzle pump  300  is transparent to the magnetic field. The electromagnetic coil  510  applies forces to charge of molten aluminum  710  to pump liquid metal based on the principles of magnetohydrodynamics. A step function direct current (DC) voltage profile applied to the electromagnetic coil  510  causing a rapidly increasing applied current  900  to electromagnetic coil  510 , thereby creating an increasing magnetic field that follows the magnetic field lines  940 . The optimal range of voltage for the pulse and current strength, as well as the range of time durations for the pulse, for effective operation vary depending on the electrical resistivity of the fluid, viscosity and surface tension. The possible effective range is wide, where alternative embodiments could be optimally range from 10 to 1000 volts (V) and 10 to 1000 amperes (A). 
         [0046]    According to Faraday&#39;s law of induction, the increasing magnetic field causes an electromotive force within the pump chamber  220  which in turn causes an induced current in molten aluminum  930  to flow along circular paths through the charge of molten aluminum  710 . The charge of molten aluminum  710  has a length (L) and height (h) dictated by pump chamber  220  height with an electrical resistance (R). The induced current in molten aluminum  930  is also inversely proportional to resistance in the charge of molten aluminum  710 . A magnitude of magnetic field  910  (B) within a given time is also proportional to the DC voltage applied. The induced current in molten aluminum  930  (i) is proportional to the rate of change of magnitude of magnetic field  910  (d/dtB) which is itself proportional to the DC voltage applied. 
         [0047]    The induced current in molten aluminum  930  and the magnetic field produce a resulting radially inward force on molten aluminum  920  (F), known as a Lorenz force, in a ring shaped element through the charge of molten aluminum  710  equal to the vector multiplication iL×B. The radially inward force on molten aluminum  920  is proportional to the square of the DC voltage applied. The incremental pressure contribution by the ring shaped element is F/(L×h). An integration of the pressure contribution of all of those elements through pump chamber  220  results in peak pressure (P) occurring at the inlet to the nozzle  410 . 
         [0048]    Peak pressure (P) is also proportional to the square of the DC voltage applied. This pressure overcomes surface tension and inertia in the molten aluminum to expel the drop of molten aluminum. At the same time, the computer causes stage  116  to move to deposit the drop of molten aluminum in the desired location on platform  112 . After a pulse is sent and the drop of molten aluminum is discharged from the nozzle, damping chamber  430  reduces the resulting negative pressure pulse, thereby allowing nozzle orifice  440  to stay filled with liquid aluminum while awaiting the next pulse. 
         [0049]    In alternative embodiments of the present invention, the shape of the nozzle may be varied to achieve a smooth inlet bell. In one embodiment, an efficient intrinsic electromagnetic heating mode is possible by pulsing the electromagnetic coil at approximately 20 us, 300 amps and 1500 Hz. This creates sufficient heat to maintain the housing and aluminum at 750 C thereby melting the aluminum. The heat is created through resistive losses in the electromagnetic coil and inductive heating within the aluminum. Use of this heating mode eliminates the need for any external heating system. 
         [0050]      FIG. 10  shows patterns of magnetic field lines  940  within the charge of molten aluminum  710  at time equals 6 uS after the beginning of the DC pulse. The arc of the field lines is seen to be deflected due to the current flowing within the charge of molten aluminum. 
         [0051]      FIG. 11  illustrates nozzle pump  300  producing a drop of molten aluminum  120  during formation of a 3D printed object  1100  on platform  112 . The 3D printed object  1100  is the location to which molten metal droplets are directed from nozzle  410 . As each drop of molten aluminum  120  is deposited, it solidifies, thereby increasing the volume of 3D printed object  1100 . The proper orientation of 3D printed object  1100  is maintained by computer programs that control and coordinate the movement of platform  112 . 
         [0052]    In certain embodiments orientation of the components may be altered through additional means, including, but not limited to altering the orientation of 3D printed object  1100  relative to printhead  102  and nozzle  410 . Specific adjustments to 3D printed object  1100  may be made as might occur during 5-axis or 4-axis printing. In certain embodiments, addition of materials to 3D printed object  1100  during formation may also facilitate proper positioning. 
         [0053]    In certain embodiments, platform  112  may be constructed of a material that facilitates heating or cooling to optimize solidification of drop of molten aluminum  120  upon contact, as would be known to one of reasonable skill in the art. Properties of platform  112  or the surrounding environment that facilitate cooling may be adjusted for the particular properties of drop of molten aluminum  120 , or any alternative liquid metal or conductive liquid that may be used to form a drop. 
         [0054]    The preferred embodiment of the present invention describes a single nozzle pump  300  of printhead  102 . In alternative embodiments of liquid metal 3D printer  100 , the printhead  102  may have an array consisting of more than one nozzle pump  300  or more than one printhead  102 . Such an array can be assembled and controlled as would be known to one of ordinary skill in the art. 
         [0055]    Having described the presently preferred embodiments of the invention, it is to be understood that the invention may otherwise be embodied within the scope of the appended claims.