Patent Publication Number: US-2023150026-A1

Title: Metal drop ejecting three-dimensional (3d) object printer and improved method for operating the printer

Description:
TECHNICAL FIELD 
     This disclosure is directed to melted metal ejectors used in three-dimensional (3D) object printers and, more particularly, to the ejection of the melted metal drops to form objects in those systems. 
     BACKGROUND 
     Three-dimensional printing, also known as additive manufacturing, is a process of making a three-dimensional solid object from a digital model of virtually any shape. Many three-dimensional printing technologies use an additive process in which an additive manufacturing device forms successive layers of the part on top of previously deposited layers. Some of these technologies use ejectors that eject UV-curable materials, such as photopolymers or elastomers. The printer typically operates one or more extruders to form successive layers of the plastic material that form a three-dimensional (3D) printed object with a variety of shapes and structures. After each layer of the three-dimensional printed object is formed, the plastic material is UV cured and hardens to bond the layer to an underlying layer of the three-dimensional printed object. This additive manufacturing method is distinguishable from traditional object-forming techniques, which mostly rely on the removal of material from a work piece by a subtractive process, such as cutting or drilling. 
     Recently, some 3D object printers have been developed that eject drops of melted metal through one or more nozzles to form 3D objects. These printers have a source of solid metal, such as a roll of wire or pellets, that is fed into a chamber of an ejector head where an external heater is operated to melt the solid metal. The ejector head is positioned within the opening of an electrical coil. An electrical current is passed through the coil to produce an electromagnetic field that causes the meniscus of the melted metal at a nozzle of the chamber to separate from the melted metal within the chamber and be propelled from the one or more nozzles. This type of metal drop ejecting printer is called a magnetohydrodynamic (MHD) printer by some in the art. 
     A platform is positioned opposite the nozzle(s) of the ejector and the ejector head is moved in a X-Y plane parallel to the plane of the platform by a controller operating actuators so the melted metal drops ejected from the nozzle form metal layers of an object on the platform. Another actuator is operated by the controller to alter the position of the ejector head or platform in the vertical or Z direction to position the ejector head and an uppermost layer of the metal object being formed by a distance appropriate for continuation of the object formation. 
     One type of MHD printer builds parts with drops exiting the nozzle at ˜400 Hz. The bulk metals melted for ejection from the nozzle of this printer include Al 6061, 356, 7075 and 4043. The size of the ejected drops is ˜0.5 mm in diameter and these drops spread to a size of ˜0.7 mm in diameter upon contact with the part surface. The melting temperature of these aluminum alloys is approximately 600° C. Empirical studies have shown that the optimal receiving surface temperature needs to be from −400° C. to −550° C. for good adherence to the previously formed surface. At these object surface temperatures the melted metal drops combine with the build part to form a consistent build structure. 
     To commence printing operations with an MHD printer, the operator of the printer must perform a start-up procedure. This procedure includes bringing the printer to its operational temperature, which is typically about 825° C., filling the supply reservoir within the printer with melted aluminum, and setting the drop mass that is used during the manufacture process to form the layers of the object. After the start-up routine is completed and the operational parameters, such as the melted metal drop mass, are determined, the printer is operated to form the metal object. During the manufacturing process, the operational parameters determined during the start-up procedure are not changed. 
     One issue that arises during the performance of the start-up procedure is the balancing of the operational parameter values required for a part. Parameter values that yield a good surface quality are not the best values for providing the best mechanical properties of the part and vice versa. These conflicting goals lead to a reduced latitude for material properties of the part as well as the surface finish of the part. Thus, the parameter values determined during start-up are a compromise so the range that can be used for the parameter values has to be small. Being able to use a wider range of parameter values to achieve competing goals for object properties would be beneficial. 
     SUMMARY 
     A new 3D metal object printer changes operational parameter values during manufacture of a part. The 3D metal object printer includes an ejector head configured to eject melted metal drops, an electrical power supply configured to supply selectively electrical power to the ejector head at a first voltage level and a second voltage level, the first voltage level being greater than the second voltage level, and a controller operatively connected to the ejector head and the electrical power supply. The controller is configured to operate the electrical power supply to supply electrical power at the first voltage level when the melted metal drops ejected by the ejector head are forming a first portion of a layer of a metal object and to operate the electrical power supply to supply electrical power at the second voltage level when the melted metal drops ejected by the ejector head are forming a second portion of the layer of the metal object, the first portion of the layer having a first measurable value for a physical property that is measurably greater than a second measurable value for the physical property that the second portion of the layer has. 
     A method of operating a 3D metal object printer changes operational parameter values during manufacture of a part. The method includes operating an electrical power supply to supply electrical power at a first voltage level to an ejector head when melted metal drops ejected by the ejector head are forming a first portion of a layer of a metal object, and operating the electrical power supply to supply electrical power at a second voltage level when the melted metal drops ejected by the ejector head are forming a second portion of the layer of the metal object, the first voltage level being greater than the second voltage level. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The foregoing aspects and other features of a 3D metal object printer and its method of operation that change operational parameter values during manufacture of a part are explained in the following description, taken in connection with the accompanying drawings. 
         FIG.  1    is a graph showing the effects of ejection pulse width, ejection pulse voltage, and drop mass velocity on one another. 
         FIG.  2    is a graph showing the effect of ejection pulse voltage on elongation of an ejected melted metal drop and tensile strength of an object. 
         FIG.  3    is schematic diagram of a 3D metal object printer that changes operational parameter values during manufacture of a metal object. 
         FIG.  4    is a flow diagram of a process for operating the 3D metal object printer of  FIG.  3   . 
         FIG.  5    depicts a previously known 3D metal object printer that does not change operational parameter values during manufacture of a metal object. 
     
    
    
     DETAILED DESCRIPTION 
     For a general understanding of the 3D metal object printer and its operation that changes operational parameter values during manufacture of a metal object, reference is made to the drawings. In the drawings, like reference numerals designate like elements. 
       FIG.  5    illustrates an embodiment of a prior art 3D metal object printer  100  that does not change the operational parameter values after their determination during the start-up procedure. The platform  112  is a solid metal plate. The heating elements of the heater  114  are operatively connected to the controller  148  for operation of the heating elements. In the printer of  FIG.  5   , drops of melted bulk metal are ejected from a receptacle of a removable vessel  104  having a single nozzle  108  and drops from the nozzle form swaths for layers of an object on a platform  112 . As used in this document, the term “removable vessel” means a hollow container having a receptacle configured to hold a liquid or solid substance and the container as a whole is configured for installation and removal in a 3D metal object printer. As used in this document, the term “bulk metal” means conductive metal available in aggregate form, such as wire of a commonly available gauge or pellets of macro-sized proportions. A source of bulk metal  116 , such as metal wire  120 , is fed into a wire guide  124  that extends through the upper housing  122  in the ejector head  140  and melted in the receptacle of the removable vessel  104  to provide melted metal for ejection from the nozzle  108  through an orifice  110  in a baseplate  114  of the ejector head  140 . As used in this document, the term “nozzle” means an orifice in a removable vessel configured for the expulsion of melted metal drops from the receptacle within the removable vessel. As used in this document, the term “ejector head” means the housing and components of a 3D metal object printer that melt, eject, and regulate the ejection of melted metal drops for the production of metal objects. 
     With continued reference to  FIG.  5   , a melted metal level sensor  184  includes a light source and a reflective sensor. In one embodiment, the light source is a laser and, in some embodiments, a blue laser. The reflection of the laser off the melted metal level is detected by the reflective sensor, which generates a signal indicative of the distance to the melted metal level. The controller receives this signal and determines the level of the volume of melted metal in the removable vessel  104  so it can be maintained at the upper level  118  in the receptacle of the removable vessel. The removable vessel  104  slides into the heater  160  so the inside diameter of the heater contacts the removable vessel and can heat solid metal within the receptacle of the removable vessel to a temperature sufficient to melt the solid metal. As used in this document, the term “solid metal” means a metal as defined by the periodic chart of elements or alloys formed with these metals in solid rather than liquid or gaseous form. The heater is separated from the removable vessel to form a volume between the heater and the removable vessel  104 . 
     Continuing with the discussion of the printer shown in  FIG.  5   , an inert gas supply  128  provides a pressure regulated source of an inert gas, such as argon, to the ejector head through a gas supply tube  132 . The gas flows through the volume between the heater and the removable vessel and exits the ejector head around the nozzle  108  and the orifice  110  in the baseplate  114 . This flow of inert gas proximate to the nozzle insulates the ejected drops of melted metal from the ambient air at the baseplate  114  to prevent the formation of metal oxide during the flight of the ejected drops. 
     The ejector head  140  of  FIG.  5    is movably mounted within Z-axis tracks for vertical movement of the ejector head with respect to the platform  112 . One or more actuators  144  are operatively connected to the ejector head  140  to move the ejector head along a Z-axis while other actuators  144  are operatively connected to the platform  112  to move the platform in an X-Y plane beneath the ejector head  140 . The actuators  144  are operated by a controller  148  to maintain an appropriate distance between the orifice  110  in the baseplate  114  of the ejector head  140  and an uppermost surface of an object on the platform  112 . 
     Moving the platform  112  of  FIG.  5    in the X-Y plane as drops of molten metal are ejected toward the platform  112  forms a swath of melted metal drops on the object being formed. Controller  148  also operates actuators  144  to adjust the vertical distance between the ejector head  140  and the most recently formed layer on the substrate to facilitate formation of other structures on the object. While the molten metal 3D object printer  100  is depicted in  FIG.  5    as being operated in a vertical orientation, other alternative orientations can be employed. Also, while the embodiment shown in  FIG.  5    has a platform that moves in an X-Y plane and the ejector head moves along the Z axis, other arrangements are possible. For example, the actuators  144  can be configured to move the ejector head  140  in the X-Y plane and along the Z axis or they can be configured to move the platform  112  in both the X-Y plane and Z-axis. 
     The controller  148  operates the switches  152  selectively. One switch  152  can be selectively operated by the controller to provide electrical power from source  156  to the heater  160 , while another switch  152  can be selectively operated by the controller to provide electrical power from another electrical source  156  to the coil  164  for generation of the electrical field that ejects a drop from the nozzle  108 . Because the heater  160  generates a great deal of heat at high temperatures, the coil  164  is positioned within a chamber  168  formed by one (circular) or more walls (rectilinear shapes) of the ejector head  140 . As used in this document, the term “chamber” means a volume contained within one or more walls in which a heater, a coil, and a removable vessel of a 3D metal object printer are located. The removable vessel  104  and the heater  160  are located within this chamber. The chamber is fluidically connected to a fluid source  172  through a pump  176  and also fluidically connected to a heat exchanger  180 . As used in this document, the term “fluid source” refers to a container of a liquid having properties useful for absorbing heat. The heat exchanger  180  is connected through a return to the fluid source  172 . Fluid from the source  172  flows through the chamber to absorb heat from the coil  164  and the fluid carries the absorbed heat through the exchanger  180 , where the heat is removed by known methods. The cooled fluid is returned to the fluid source  172  for further use in maintaining the temperature of the coil in an appropriate operational range. 
     The controller  148  of the 3D metal object printer  100  requires data from external sources to control the printer for metal object manufacture. In general, a three-dimensional model or other digital data model of the object to be formed is stored in a memory operatively connected to the controller  148 , the controller can access through a server or the like a remote database in which the digital data model is stored, or a computer-readable medium in which the digital data model is stored can be selectively coupled to the controller  148  for access. This three-dimensional model or other digital data model is processed by a slicer implemented with the controller to generate machine-ready instructions for execution by the controller  148  in a known manner to operate the components of the printer  100  and form the metal object corresponding to the model. The generation of the machine-ready instructions can include the production of intermediate models, such as when a CAD model of the device is converted into an STL data model, or other polygonal mesh or other intermediate representation, which can in turn be processed to generate machine instructions, such as g-code, for fabrication of the device by the printer. As used in this document, the term “machine-ready instructions” means computer language commands that are executed by a computer, microprocessor, or controller to operate components of a 3D metal object additive manufacturing system to form metal objects on the platform  112 . The controller  148  executes the machine-ready instructions to control the ejection of the melted metal drops from the nozzle  108 , the positioning of the platform  112 , as well as maintaining the distance between the orifice  110  and the uppermost layer of the object on the platform  112 . 
     The controller  148  can be implemented with one or more general or specialized programmable processors that execute programmed instructions. The instructions and data required to perform the programmed functions can be stored in memory associated with the processors or controllers. The processors, their memories, and interface circuitry configure the controllers to perform the operations previously described as well as those described below. These components can be provided on a printed circuit card or provided as a circuit in an application specific integrated circuit (ASIC). Each of the circuits can be implemented with a separate processor or multiple circuits can be implemented on the same processor. Alternatively, the circuits can be implemented with discrete components or circuits provided in very large scale integrated (VLSI) circuits. Also, the circuits described herein can be implemented with a combination of processors, ASICs, discrete components, or VLSI circuits. During metal object formation, image data for a structure to be produced are sent to the processor or processors for controller  148  from either a scanning system or an online or work station connection for processing and generation of the signals that operate the components of the printer  100  to form an object on the platform  112 . 
     Using a high speed camera to observe ejected melted metal drops against a linear measurement scale, the velocity of ejected melted metal drops at a plurality of ejection pulses was determined. A model of the results showing the drop mass, ejection pulse width, and ejection pulse voltage is presented in  FIG.  1   . The black line  104  in the model is a typical target drop mass used for producing metal objects, which is 1.5 grams. As can be seen from the model, an increase in the ejection pulse voltage increases the ejected drop velocity while maintaining the target drop mass. As shown in  FIG.  1   , an ejection pulse voltage of approximately 160 Volts (V) produces a drop velocity of about 2.4 m/s, while a pulse voltage of approximately 200 V produces a drop velocity of about 3.2 m/s. Thus, a voltage increase of about 25% results in a 33% increase in drop velocity. 
     Next, metal objects were formed with melted metal drops having a mass of approximately 1.5 grams that were ejected with 160 V ejection pulses and then metal objects were formed with 1.5 gram melted metal drops ejected with 200 V ejection pulses. Physical properties of the two metal objects were compared to determine the effects of ejecting the melted metal drops at two different velocities. As used in this document, the term “physical property” means a characteristic of matter that can be observed and measured without a chemical change to the matter. Specifically, the objects formed with the drops ejected with the 160 V pulses had smoother surfaces than the objects formed with the drops ejected with the 200 V pulses. The objects were then subjected to tensile strength tests and the results of that testing are shown in  FIG.  2   . The objects formed with the drops ejected by the 200 V pulses exhibit greater tensile strength than the objects formed with the 160 V pulses. Thus, the tensile strength of the object formed with the melted drops ejected at 3.4 m/s is measurably greater. As used in this document, the term measurably greater means the greater measurement value of a physical property is about 10% greater than the lesser measurement value of the same physical property. 
     These results were used to reconfigure the programmed instructions that are stored in a non-transitory computer readable media, such as non-volatile memory  102 , that is operatively connected to the controller  148 ′ of the printer  100 ′ shown in  FIG.  3   . When these instructions are executed by the controller, the power supply  156  is operated at a higher voltage when melted metal drops that form interior portions of the object are being ejected and the power supply is operated at a lower voltage when melted metal drops that form surface features of the object are being ejected. In this manner, the interior portions of the object exhibit greater tensile strength while the exposed areas of the object present better surface characteristics. 
     The controller  148 ′ of the 3D metal object printer  100 ′ requires data from external sources to control the printer for object formation. In general, a three-dimensional model or other digital data model of the object to be formed is received by the controller from a memory, server, remote database, or a computer-readable non-transitory medium operatively connected to the controller. The three-dimensional model or other digital data model is used by the controller to generate machine-ready instructions for execution by the controller  136  to operate the components of the printer  100  and fabricate the object corresponding to the model. The generation of the machine-ready instructions can include the production of intermediate models, such as when a CAD model of the object is converted into an STL data model, or other polygonal mesh or other intermediate representation, which can in turn be processed to generate machine-ready instructions, such as g-code for fabrication of the object by the printer. As used in this document, the term “machine-ready instructions” means computer language commands that are executed by a computer, microprocessor, or controller to operate components of a 3D metal object additive manufacturing system to form a metal object. The controller  148 ′ executes the machine-ready instructions to control the ejection of the metal drops from the printhead  104 , the positioning of stage  148  and the platform  112 , the distance between the printhead  102  and the uppermost layer of the object, and the voltage of the power supply as described previously. 
     A process for operating the printer of  FIG.  3    is shown in  FIG.  4   . In the description of the process, statements that the process is performing some task or function refers to a controller or general purpose processor executing programmed instructions stored in non-transitory computer readable storage media operatively connected to the controller or processor to manipulate data or to operate one or more components in the printer to perform the task or function. The controller  148 ′ noted above can be such a controller or processor. Alternatively, the controller can be implemented with more than one processor and associated circuitry and components, each of which is configured to form one or more tasks or functions described herein. Additionally, the steps of the method may be performed in any feasible chronological order, regardless of the order shown in the figures or the order in which the processing is described. 
       FIG.  4    is a flow diagram  400  of a process that operates the printer  100 ′. The process begins at printer start-up with generation of the machine-ready instructions (block  404 ). The process executes the machine-ready instructions to operate the printer and form the metal object (block  408 ). When the process detects one or more instructions for forming a perimeter of a layer (block  412 ), then the power supply voltage level is set to a lower level, such as 160 V (block  416 ) and the printer is operated to form the perimeter with the power supply set at the lower level (block  408 ). Otherwise, an interior portion of a layer is being formed so the power supply voltage level is set to a higher level, such as 200 V (block  420 ) and the printer is operated to form the perimeter with the power supply set at the higher level (block  408 ). The process continues until the build process is finished. 
     It will be appreciated that variants of the above-disclosed and other features and functions, or alternatives thereof, may be desirably combined into many other different systems, applications or methods. Various presently unforeseen or unanticipated alternatives, modifications, variations or improvements may be subsequently made by those skilled in the art that are also intended to be encompassed by the following claims.