Patent Publication Number: US-2022226888-A1

Title: Method and system for operating a metal drop ejecting three-dimensional (3d) object printer to shorten object formation time

Description:
TECHNICAL FIELD 
     This disclosure is directed to melted metal ejectors used in three-dimensional (3D) object printers and, more particularly, to operation of the ejectors to form three-dimensional (3D) metal objects. 
     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 from one or more ejectors to form 3D objects. These printers have a source of solid metal, such as a roll of wire or pellets, that are fed into a heating chamber where they are melted and the melted metal flows into a chamber of the ejector. The chamber is made of non-conductive material around which an uninsulated electrical wire is wrapped. An electrical current is passed through the conductor to produce an electromagnetic field to cause 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 nozzle. A platform opposite the nozzle 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 and another actuator is operated by the controller to alter the position of the ejector or platform in the vertical or Z direction to maintain a constant distance between the ejector and an uppermost layer of the metal object being formed. This type of metal drop ejecting printer is also known as a magnetohydrodynamic printer. 
     Most metal drop ejecting printers have a single ejector that operates at an ejection frequency in a range of about 50 Hz to about 1 KHz and that eject drops having a diameter of about 50 μm. This firing frequency range and drop size extends the time required to form metal objects over the times needed to form objects made with plastic or other known materials. Although some metal drop ejecting printers have one or more printheads or more than one nozzle fluidly coupled to a common manifold, they still are limited to these ejection frequencies and drop sizes. Three-dimensional object printers having multiple nozzles that form plastic objects and the like are known to use a single nozzle for formation of fine features or the perimeters of layers and then increase the number of nozzles used to infill the layer. By increasing the number of nozzles used, a greater amount of the thermoplastic material can be dispensed into the interior regions of a layer in a short amount of time to improve the production time for the objects manufactured by such printers. Maintaining an adequate supply of melted metal to multiple printheads or nozzles is difficult, especially if the number of nozzles being used is selectively varied during the object formation. Being able to operate a metal drop ejecting printer to provide higher effective melted metal dispensing rates and form larger swaths or ribbons of melted metal to decrease the time for object formation would be beneficial. 
     SUMMARY 
     A new method of operating a metal drop ejecting apparatus to provide higher effective melted metal dispensing rates and form larger swaths or ribbons of melted metal to decrease the time for object formation. The method includes identifying a portion of a layer in an object to be formed on a platform as exterior or interior using a layer model of the object, operating an ejector in an ejection mode when the portion of the object to be formed is identified as being exterior, and operating the ejector in an extrusion mode when the portion of the object to be formed is identified as being interior. 
     A new metal drop ejecting apparatus provides higher effective melted metal dispensing rates and forms larger swaths or ribbons of melted metal to decrease the time for object formation forms. The apparatus includes a melter configured to receive and melt a solid metal, an ejector operatively connected to the melter to receive melted metal from the melter, a platform configured to support a substrate, the platform being positioned opposite the ejector, a user interface configured to receive a digital data model of an object to be formed on the platform, and a controller operatively connected to the melter, the ejector, and the user interface. The controller is configured to generate a layer model of the object to be formed on the platform using the digital data model, identify a portion of the object to be formed on the platform as exterior or interior using the layer model of the object, operating the ejector in an ejection mode when the portion of the object to be formed is identified as being exterior, and operating the ejector in an extrusion mode when the portion of the object to be formed is identified as being interior. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The foregoing aspects and other features of a metal ejecting 3D object printer and its operation that provides higher effective melted metal dispensing rates and forms larger swaths or ribbons of melted metal to decrease the time for object formation are explained in the following description, taken in connection with the accompanying drawings. 
         FIG. 1  depicts an additive manufacturing system that operates a liquid metal drop ejector to provide higher effective melted metal dispensing rates and form larger swaths or ribbons of melted metal to decrease the time for object formation. 
         FIG. 2A  and  FIG. 2B  depict formation of a layer of a metal object using the system of  FIG. 1 . 
         FIG. 3  illustrates how an ejector in the system of  FIG. 1  is supplemented with additional melted metal that is adequate to support the formation of larger swaths or ribbons. 
         FIG. 4  illustrates the parameters for the equation used to regulate the amount of melted metal in the ejector of  FIG. 3 . 
         FIG. 5  is a flow diagram of a process that operates the printing system of  FIG. 1  to infill interior regions of layers in metal objects more quickly. 
     
    
    
     DETAILED DESCRIPTION 
     For a general understanding of the environment for the system and its operation as disclosed herein as well as the details for the device and its operation, reference is made to the drawings. In the drawings, like reference numerals designate like elements. 
       FIG. 1  illustrates an embodiment of a melted metal 3D object printer  100  that has a printhead  104  that operates in two modes, an ejection mode for formation of exterior surfaces and features and an extrusion mode for the infill of interiors. As used in this document, “ejection mode” means operation of a printhead to eject discrete drops of melted metal from a nozzle of the printhead and “extrusion mode” means operation of the printhead to exude a continuous stream of melted metal from the same nozzle of the printhead. A source of bulk metal  160 , such as metal wire  130 , is fed into the printhead and melted to provide melted metal for a chamber within the printhead. 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. An inert gas supply  164  provides a pressure regulated source of an inert gas  168 , such as argon, to the melted metal in the printhead  104  through a gas supply tube  144  to prevent the formation of metal oxide in the printhead. 
     The printhead  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 they are connected to one another by a horizontal member  128 . An actuator  132  is mounted to the horizontal member  128  and operatively connected to the printhead  104  to move the printhead along the z-axis tracks  116 A and  166 B. The actuator  132  is operated by a controller  136  to maintain a predetermined distance between one or more nozzles (not shown in  FIG. 1 ) of the printhead  104  and an uppermost surface of the substrate  108  on the platform  112  and the traces being formed on the substrate  108 . 
     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 printhead  104 . Performing this X-Y planar movement of platform  112  as molten metal  156  is either ejected or extruded toward the platform  112  forms a line of melted metal drops on the substrate  108 . Controller  136  also operates actuator  132  to adjust the vertical distance between the printhead  104  and the most recently formed layer on the substrate to facilitate formation of other structures on the substrate. While the molten metal 3D object printer  100  is depicted in  FIG. 1  as being operated in a vertical orientation, other alternative orientations can be employed. Also, while the embodiment shown in  FIG. 1  has a platform that moves in an X-Y plane and the printhead moves along the Z axis, other arrangements are possible. For example, the printhead  104  can be configured for movement in the X-Y plane and along the Z axis. Additionally, while the depicted printhead  104  has only one nozzle, it is configured in other embodiments with multiple nozzles and a corresponding array of electromagnetic actuators associated with the nozzles in a one-to-one correspondence to provide independent and selective control of the ejections from each of the nozzles and the nozzles can be supplied from different sources of bulk metal and the bulk metals of these metals can be different metals. 
     The system  100  is also provided with a reservoir of melted bulk metal  174  that is connected to the melted metal chamber within the printhead  104  by a conduit  178  having a valve  182 . The controller  136  is operatively connected to the electromagnetic actuator within the printhead  104  and to the valve  182 . When the controller  136  operates the printhead  104  in ejection mode, it generates control signals to operate the electromagnetic actuator to eject drops of melted metal and to keep the valve  182  closed. When the controller  136  operates the printhead  104  in extrusion mode, the controller generates control signals to open the valve  182  while monitoring the signal generated by a pressure sensor  312  ( FIG. 3 ) within the printhead  104  to keep the printhead supplied with an amount of melted metal adequate to extrude melted metal through the nozzle continuously to support the extrusion operation of the printhead. 
     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 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 electronic device 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 control signals used to operate the printhead  104 . 
     The controller  136  of the melted metal 3D object printer  100  requires data from external sources to control the printer for 3D metal object manufacture. In general, a three-dimensional model or other digital data model of the device 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. A known program, sometimes called a slicer, forms from the digital data model a layer model of the object to be manufactured. The layer model identifies the exterior portions of the layers of the object and the interior regions of the layers. The layer model is 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 layer 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 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. The controller  136  executes the machine-ready instructions to control the operations of the printhead  104 , the positioning of stage  148 , and the platform  112 , as well as the distance between the printhead  102  and the uppermost layer of the object on the platform  112 . 
     The formation of a layer  204  is shown in  FIG. 2A  and  FIG. 2B . If the layer  204  is identified as an exterior surface of the object to be manufactured, such as the bottom layer of the object, then the controller  136  operates the printhead  104  in ejection mode to form the entire bottom surface layer. For a subsequent layer  204  that is not an exterior layer, the perimeter  208  of the layer, the feature  212 , and the perimeter  208  of the opening  216  are formed while operating the printhead  104  in ejection mode since the perimeter  208  is part of the exterior of the object, the feature  212  is a solid member, and the perimeter is also on an exposed surface of the object. The controller  136  then operates the printhead  104  in extrusion mode to fill in the interior between the perimeter  208  of the layer and the perimeter  216  of the opening as shown in  FIG. 2B . The operation of the printhead in extrusion mode is now described more fully. As used in this document, the term “exterior” means a surface that contacts ambient air when manufacture of the object is finished and the term “interior” means a portion of the object that does not contact ambient air when the manufacture of the object is finished. 
     The nozzle  304  and feed chamber  308  of the ejector in the printhead  104  are shown in  FIG. 3 . The electrical wire that is wrapped about the ejector to form the electromagnetic field that ejects a drop of melted ink is not shown to facilitate the discussion of the extrusion mode of the printhead. The conduit  178  to the reservoir  174  noted above directs melted metal from the reservoir  174  into the feed chamber  308  when the valve  182  is open. A pressure sensor  312  is positioned within the feed chamber  308  and it generates a signal that is transmitted to the controller  136  that indicates the pressure above the upper surface of the melted metal  316  in the feed chamber. This pressure can be regulated by operating the inert gas source  164  to increase or decrease the flow of inert gas from the gas source into the feed chamber  308 . When the pressure is increased to a predetermined minimum value, the melted metal is extruded continuously from the nozzle  304 . Because the melted metal is being extruded continuously, rather than in discrete drops, the supply of melted metal is diminished more rapidly. To compensate for this loss of melted metal, the controller  136  opens the valve  182  and melted metal from the reservoir  174  is urged by gravity through the conduit  178  into the feed chamber  308 . Thus, continuous ribbons or swaths of melted metal are extruded from the nozzle  304  while operating the actuators that produce relative movement between the printhead  104  and the platform  112  to fill an interior area of a layer. This operation fills the layer more quickly than is possible by operating the printhead in ejection mode. Once the interior area of the layer is filled, the controller  136  closes the valve  182  and operates the inert gas source  164  to decrease the amount of gas supplied to the feed chamber  308 . The controller continues this operation of the inert gas source  164  while monitoring the signal from the pressure sensor  312  until the pressure within the feed chamber  308  returns to a lower pressure that does not force the melted metal from the feed chamber  308  and through the nozzle  304 . Melted metal now remains in the feed chamber  308  until an electromagnetic pulse is generated for ejecting a drop through the nozzle  304 . 
       FIG. 4  is a depiction of the melted metal in the feed chamber  308  and its egress through the nozzle  304 . To regulate the amount of melted metal in the feed chamber, the net flow out of the feed chamber is a function of the height H of the melted metal in the chamber and the volumetric flow of melted metal into the chamber. The volumetric flow out of the nozzle  304  is V=C d  A (2 gH) 1/2 , where the flow volume is measured in m 3 /sec, A is the area of the aperture in m 2  and C d  is the discharge coefficient defined by C c C v  where C c  is the contraction coefficient, which is 0.62 for a sharp edge aperture and 0.97 for a well-rounded aperture, and C v  is a velocity coefficient, which is 0.97 in some embodiments. As used in this document, the term “sharp edge aperture” means an opening in the nozzle of the ejector that is formed with straight lines and “well-rounded aperture” means an opening in the nozzle that is formed with one or more curved lines. Using a level sensor  402  that follows the upper surface of the melted metal in the chamber  308  and generates a signal indicative of the change in the level of the melted metal along with the equations noted above, the controller is configured to determine the volumetric flow out of the feed chamber  308  and operate the valve  182  to replace the displaced volume and maintain the height H of the melted metal in the feed chamber at a constant height during the extrusion mode of printhead operation. 
     A process for operating the printer shown in  FIG. 1  is shown in  FIG. 5 . 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. 5  is a flow diagram  500  of a process that operates the printing system  100  to infill interior regions of layers in metal objects more quickly. The process begins by identifying whether a path for formation of a portion of a layer in the object is on an exterior surface of the object or within an interior portion (block  504 ). For exterior surface formation, the printhead is operated in an ejection mode in a known manner to form the layer portion (block  508 ). If the portion to be formed is an interior portion, then pressure within the feed chamber is monitored while the inert gas supply is operated to increase the pressure to a level that extrudes melted metal from the nozzle (block  512 ). The valve that enables additional melted metal to flow into the feed chamber is opened (block  516 ) and the height of the melted metal in the feed chamber is monitored (block  520 ). If the height changes (block  524 ), then the valve is operated to open and the resulting flow of melted metal into the chamber returns the melted metal height to the constant level (block  528 ). This operation continues until the interior region is filled (block  532 ). 
     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.