Patent Publication Number: US-2022212257-A1

Title: Metal drop ejecting three-dimensional (3d) object printer with a thermally insulated build platform translational mechanism

Description:
TECHNICAL FIELD 
     This disclosure is directed to melted metal ejectors used in three-dimensional (3D) object printers and, more particularly, to the thermal insulation of build translations mechanisms for build platforms used 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 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. A platform opposite the nozzle(s) of the ejector is moved in a X-Y plane parallel to the plane of the platform by a controller operating actuators so the ejected metal drops 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. This type of metal drop ejecting printer is also known as a magnetohydrodynamic printer. 
     One such magnetohydrodynamic 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 and these drops spread to a size of ˜0.7 mm upon contact with the part surface. The melting temperature of these aluminum types 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 temperatures the melted metal drops combine with the build part in a uniform way that produces bonds that result in a strong and consistent build structure. When the build surface temperatures fall below 400° C., the drops do not combine as smoothly or with the necessary bonding strength required. This lackluster bonding increases porosity in the part, forms uneven build surfaces, produces unwelded drops, and yields shape inconsistencies. All of these unwanted results lead to degraded physical properties, such as low fatigue strength and tensile strength, as well as poor appearance issues in the final part. 
     As noted above, however, empirical studies have shown that if the temperature of the part is maintained at 400° C. or greater, the build quality is improved over the quality of the parts in which the temperature of the part was maintained at less than 400° C. Providing temperatures in the optimal range is possible using known heating methods such as IR heating, injecting a heated noble gas, ceramic heaters, convective heating, and the like. 
     Providing an enclosed environment that enables the part temperature to remain at the optimal level, however, is not a straightforward proposition. The X-Y translation mechanism used to move the build plate during the build process must be protected from the high temperatures required for building the parts. This thermal protection needs to move fluidly with the build platform moved by the X-Y translation mechanism within a confined enclosure to ensure adequate thermal insulation regardless of the position of the build platform. Additionally, the high temperatures optimal for melted metal drop bonding with previously formed layers can degrade the life of the X-Y translation mechanism. Being able to configure an environment for production of a metal part using melted metal drops that ensures optimal temperatures for metal drop bonding without adversely impacting the life of the build platform X-Y translation mechanism would be beneficial. 
     SUMMARY 
     A new 3D metal object printer provides an environment for production of a metal part using melted metal drops that ensures optimal temperatures for metal drop bonding without adversely impacting the life of the build platform X-Y translation mechanism. The 3D metal object printer includes an ejector head, a platform positioned opposite the ejector head, a heater configured to direct heat toward the platform, a translation mechanism configured to move the ejector head, a housing that encloses an internal volume in which the translation mechanism and platform are located, a first actuator operatively connected to the platform, the actuator being configured to operate the translation mechanism to move the platform within the housing, and a thermally insulative fluid that covers the translation mechanism. 
     A method of operating the new 3D metal object printer provides an environment for production of a metal part using melted metal drops that ensures optimal temperatures for metal drop bonding without adversely impacting the life of the platform X-Y translation mechanism. The method includes operating a heater to direct heat toward a platform; and operating a translational mechanism to move the platform through a volume of a thermally insulative fluid within a housing, the movement of the platform being in an X-Y plane opposite an ejector head configured to eject drops of melted metal toward the platform. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The foregoing aspects and other features of a 3D metal object printer that provides an environment for production of a metal part using melted metal drops that ensures optimal temperatures for metal drop bonding without adversely impacting the life of the build platform X-Y translation mechanism are explained in the following description, taken in connection with the accompanying drawings. 
         FIG. 1A  is a front view of a 3D metal object printer that includes a thermally insulative fluid that protects the X-Y translation mechanism for the build platform while enabling the part being formed to maintain a temperature in an optimal range for metal drop bonding to previously formed part layers. 
         FIG. 1B  is a rear view of the printer of  FIG. 1A  that provides a better view of the heat exchanger for the thermally insulative fluid. 
         FIG. 2  is a flow diagram of a process for operating the printer of  FIGS. 1A and 1B . 
         FIG. 3  depicts a previously known 3D metal object printer that cannot maintain the temperature of a part being built in an optimal range for metal drop bonding to previously formed part layers. 
     
    
    
     DETAILED DESCRIPTION 
     For a general understanding of the environment for the 3D metal object printer and its operation as disclosed herein as well as the details for the printer and its operation, reference is made to the drawings. In the drawings, like reference numerals designate like elements. 
       FIG. 3  illustrates an embodiment of a prior art melted metal 3D object printer  100  that can be modified to produce the 3D metal object printer of  FIG. 1A  and  FIG. 1B . In this embodiment, drops of melted bulk metal are ejected from a ejector head  104  having a single nozzle, although the ejector head can be configured with a plurality of nozzles, and the ejected drops form swaths for layers of an object  108  on a platform  112 . 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  160 , such as metal wire  130 , is fed into the ejector head and melted to provide melted metal for a chamber within the ejector head. An inert gas supply  164  provides a pressure regulated source of an inert gas  168 , such as argon or nitrogen, to the chamber of melted metal in the ejector head  104  through a gas supply tube  144  to prevent the formation of metal oxide in the ejector head. 
     The ejector head  104  is movably mounted within Z-axis tracks  116 A and  116 B in a pair of vertically oriented members  120 A and  120 B, respectively. Members  120 A and  120 B are connected at one end to one side of a frame  124  and at another end to one another by a horizontal member  128 . An actuator  132  is mounted to the horizontal member  128  and operatively connected to the ejector head  104  to move the ejector head along the Z-axis tracks  116 A and  166 B. The actuator  132  is operated by a controller  136  to maintain a distance between the nozzle (not shown in  FIG. 3 ) of the ejector head  104  and an uppermost surface of the object  108  on the platform  112 . 
     Mounted to the frame  124  is a planar member  140 , which can be formed of granite or other sturdy material to provide reliably solid support for movement of the platform  112 . Platform  112  is affixed to X-axis tracks  144 A and  144 B so the platform  112  can move bidirectionally along an X-axis as shown in the figure. The X-axis tracks  144 A and  144 B are affixed to a stage  148  and stage  148  is affixed to Y-axis tracks  152 A and  152 B so the stage  148  can move bidirectionally along a Y-axis as shown in the figure. Actuator  122 A is operatively connected to the platform  112  and actuator  122 B is operatively connected to the stage  148 . Controller  136  operates the actuators  122 A and  122 B to move the platform along the X-axis and to move the stage  148  along the Y-axis to move the platform in an X-Y plane that is opposite the ejector head  104 . Performing this X-Y planar movement of platform  112  as drops of molten metal  156  are ejected toward the platform  112  forms a swath of melted metal drops on the object  108 . Controller  136  also operates actuator  132  to adjust the vertical distance between the ejector head  104  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. 3  as being operated in a vertical orientation, other alternative orientations can be employed. Also, while the embodiment shown in  FIG. 3  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 ejector head  104  can be configured for movement in the X-Y plane and along the Z-axis. Additionally, for an embodiment of the ejector head  104  having a plurality of nozzles, the ejector head can configured with an array of valves (not shown) associated with the nozzles in a one-to-one correspondence to provide independent and selective control of the ejections from each of the nozzles. 
     The controller  136  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 a 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  136  from either a scanning system or an online or work station connection for processing and generation of the ejector head control signals output to the ejector head  104 . 
     The controller  136  of the melted metal 3D 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  136 , 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  136  for access. This three-dimensional model or other digital data model can be used by the controller to generate machine-ready instructions for execution by the controller  136  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  136  executes the machine-ready instructions to control the ejection of the melted metal drops from the ejector head  104 , the positioning of stage  148  and the platform  112 , as well as the distance between the ejector head  102  and the uppermost layer of the object  108  on the platform  112 . 
       FIG. 1A  and  FIG. 1B  illustrate an embodiment of a melted metal 3D object printer  100  that provides an environment for production of a metal part using melted metal drops that ensures optimal temperatures for metal drop bonding without adversely impacting the life of the build platform X-Y translation mechanism. In the description of this printer, like reference numbers for components discussed above with reference to  FIG. 3  are used for like components in the printer of  FIG. 1A  and  FIG. 1B . The printer  200  includes an ejector head  104  that is mounted on a support plate  204 . The ejector head  104  and the support plate  204  are configured to move vertically bidirectionally along the Z axis by operation of the actuator  132 . The support plate moves within an internal volume of a housing  208  formed by four standing walls to form a rectangularly shaped housing. The housing  208  in  FIG. 1A  and  FIG. 1B  is made of a transparent material to facilitate viewing of the internal volume of the housing, although the housing can be made of translucent or opaque materials and can have shapes other than the rectangular shape shown in the figure. The wall or walls forming the housing enclose the internal volume except for the upper opening in which the support plate  204  fits. The clearance between the edges of the support plate  204  and the walls of the housing  208  are relatively tight to help hold heat within the housing. The wall or walls of the housing  208  are made of a heat resistant material, such as quartz glass. One or more heating elements  220  are mounted to the side of support plate  204  that faces the internal volume of the housing  208 . These heating elements can be infrared heaters, outlets for noble gases heated outside of the housing, ceramic heaters, convective heaters, and the like. In one embodiment, eight millimeter heating tubes made by Heraeus Noblelight of Gaithersburg, Md. form the heating elements mounted to the support plate  204 . Also, a temperature sensor  230  is operatively connected to the controller  136  to provide the controller with a signal indicative of the temperature within the volume of the housing  208 . The controller  136  is configured to compare the signal from the sensor  230  to an upper temperature limit and lower temperature limit for the internal volume of the housing that maintains the object surface temperature in the range of about 400° to about 550° C. The housing helps maintain the temperature of the object  108  within the optimal range of about 400° C. to about 550° C. because it encloses the space around the object and helps prevent the loss of heat from the internal volume of the housing  208 . The dimensions of the internal volume of the housing  208  can be optimized to help balance the parameters affecting temperatures within the internal volume of the housing. 
     With continued reference to  FIG. 1A  and  FIG. 3 , platform  112  on which the object  108  is formed is supported by the planar member  140  and the X-Y translation mechanism as described above with reference to  FIG. 3 . As noted above with respect to  FIG. 3 , controller  136  operates the actuators  122 A and  122 B to move the platform along the X-axis and to move the stage  148  along the Y-axis to move the platform in an X-Y plane that is opposite the ejector head  104 . Performing this X-Y planar movement of platform  112  as drops of molten metal  156  are ejected toward the platform  112  forms a swath of melted metal drops on the object  108 . This X-Y translation mechanism, although visible, is covered in a volume of thermally insulative fluid  250 . As used in this document, the term “thermally insulative fluid” means a material in the liquid phase that is non-corrosive and non-toxic. This thermally insulative fluid provides a thermal insulation layer through which the components of the X-Y translation mechanism can move without impeding the movement of platform  112 . As the platform slides along the members of the X-Y mechanism, the fluid is displaced by the mechanism components. 
     The thermally insulative fluid  250  in one embodiment is a high temperature rated molten salt fluid that is non-toxic and non-corrosive. As used in this document, the term “molten salt” means a fluoride, chloride, or nitrate salt at a temperature that is greater than the melting temperature of the salt, The fluid allows the platform to move easily while providing full non-corrosive and temperature-controlled coverage for the X-Y translation mechanism. In one embodiment, the gas atmosphere surrounding the part  108  on the platform  112  is an inert gas environment, such as nitrogen or argon. The inert gas supplied to the atmosphere surrounding the part  108  is likely the same gas as being supplied to the ejector head  104 . The molten salt fluid used in one embodiment is Dynalene MS-1, which is available from Dynalene of Whitehall, Pa. This molten salt solution has a maximum operating temperature of 565° C., although this molten salt should not be kept at the maximum temperature for a long period of time as precipitates form. The molten salt becomes a liquid above 225° C. so it needs to be heated to that temperature and maintained at that temperature or higher so the material remains molten in the housing  208 . The melting operation is performed in a heated reservoir that is remote from the system  200  so the molten salt can be cooled during maintenance or other system  200  down times. When the molten salt is permitted to solidify, it expands so the reservoir that is heated to return the salt to its molten state must have a capacity that is greater than the volume of molten salt needed to cover the translation mechanism. It can be used with carbon steel components up to a temperature of about 400° C. Above 400° C., the components within the housing  208  are made of stainless steel, Inconel, or other corrosion-resistant alloys. Since the maximum operating temperature for this molten salt is a little short of 565° C., it is well-suited for maintaining the metal part  108  in the temperature range of about 400° C. to about 550° C. provided the components of the translation mechanism are made of the appropriate corrosion-resistant materials. 
       FIG. 1B  shows the printer  200  in a rear view. In this view, a heat exchanger  248  is fluidly connected to the volume of thermally insulative fluid  250  by a pipe  258 . A pump  244  is fluidly connected to the heat exchanger  248  and the pipe  258  to pull fluid  250  from the housing  208  and recirculate it through the heat exchanger to remove heat from the fluid before returning the fluid to the housing  208  through another pipe  258 . Ambient air in the heat exchanger removes the heat from the fluid passing through the exchanger. Additionally, a fan  240  can be configured to blow air through the heat exchanger  248  to aid in the cooling of the fluid  250 . Both the fan  240  and the pump  244  are connected to the controller  136  so the controller can operate the components to move the fluid  250  through the heat exchanger or blow air through the exchanger. A temperature sensor  254  is also operatively connected to the controller  136  to provide a signal generated by the sensor that is indicative of the temperature of the fluid in the heat exchanger. The controller  136  is configured with programmed instructions, which when executed, compare the signal from the sensor  254  to a maximum temperature, which in one embodiment is 500° C., and when the temperature of the fluid  250  in the exchanger  248  exceeds that maximum temperature, the controller  136  operates the fan  244  to aid in the cooling of the fluid by the heat exchanger  248 . 
     A process for operating the printer shown in  FIG. 1A  and  FIG. 1B  is shown in  FIG. 2 . 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  136  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. 2  is a flow diagram  300  of a process that operates the printer  200 . The process begins with the printer start-up, which includes melting the thermally insulative material in a remote reservoir and supplying the molten salt to the interior volume of housing  208  (block  304 ). Printer operations begin (block  306 ). When the temperature in the housing exceeds a predetermined maximum temperature (block  308 ), the pump is turned on to move fluid through the heat exchanger (block  312 ). The predetermined maximum temperature is less than the temperature than the ejected melted drops but greater than the temperature to which the platform  112  is heated. In one embodiment, the ejected metal drops are ejected at a temperature of about 600° C. to about 650° C., while the platform  112  is maintained at a temperature of about 400° C. In this embodiment, the predetermined maximum temperature for the atmosphere surrounding the part is about 550° C. The temperature of the fluid being returned to the housing from the heat exchanger is monitored until it exceeds a maximum return temperature for the fluid (block  316 ). In the embodiment discussed above that heats the platform  112  to about 400° C. and that uses Dynalene MS-1, this maximum return temperature is about 500° C. Once that temperature is reached, the fan is activated (block  320 ). If the temperature of the returning fluid falls below predetermined lower temperature for the fluid, the fan is deactivated (block  324 ). In the embodiment being discussed in which the optimal temperature range for material bonding within the housing is about 400° C. to about 550° C., this predetermined lower temperature is about 450° C. This process of thermally insulative fluid temperature regulation continues until the printer operations are halted (block  328 ). The fluid is transferred from the housing to the remote reservoir and the heaters for the housing are deactivated (block  332 ). 
     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.