Metal drop ejecting three-dimensional (3D) object printer and method of operation for forming metal support structures

Abstract

To provide a molten metal drop ejecting apparatus to form support structures not adhering tightly to object features supported by the structures without adversely impacting the environment of a 3D metal object printer, and an operation method for forming metal support structures.SOLUTION: A three-dimensional (3D) metal object manufacturing apparatus is equipped with two solid metal moving mechanisms that are independently operated to move two different metals into a receptacle of a vessel 104 in a molten metal drop ejecting apparatus. An ejector is operated to form object features with molten metal drops of one of the two different metals and to form support features with molten metal drops of the other of the two different metals. The thermal expansion coefficients of the two metals are sufficiently different to easily separate the support features from the object features after the object and the support features are cooled.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present disclosure is directed to three-dimensional (3D) object printers that eject molten metal droplets to form objects, and more particularly to the formation of metal support structures used to form objects by such printers. [Background technology]

[0002] Three-dimensional printing, also known as additive manufacturing, is a process for creating three-dimensional solid objects from digital models of virtually any shape. Many three-dimensional printing techniques use a layer-by-layer process in which an additive manufacturing device forms successive layers of a part on top of previously deposited layers. Some of these techniques use dispensers that dispense ultraviolet-curable materials, such as photopolymers or elastomers. The printer typically operates one or more extruders to form successive layers of plastic material that build up three-dimensional printed objects with various shapes and structures. After each layer of the three-dimensional printed object is formed, the plastic material is ultraviolet-cured, solidifying and bonding the layer to underlying layers of the three-dimensional printed object. This additive manufacturing method is distinguishable from traditional object-forming techniques, most of which rely on removing material from a workpiece through subtractive processes, such as cutting or drilling.

[0003] Recently, several 3D object printers have been developed that eject droplets of molten metal from one or more dispensers to form 3D objects. These printers have a solid metal source, such as a roll or pellets of wire, which is fed into a heated reservoir within the printer where the solid metal melts and fills the reservoir. The reservoir is made of a non-conductive material around which an electric wire is wound to form a coil. Passing an electric current through the coil generates an electromagnetic field that causes a meniscus of molten metal at the reservoir's nozzle to separate from the molten metal in the reservoir and propel it out of the nozzle. A platform opposite the dispenser's nozzle is moved in an XY plane parallel to the plane of the platform by a controller that operates an actuator, causing the ejected metal droplets to form the object's metal layer on the platform. Another actuator, operated by a controller, changes the position of the dispenser or platform in the vertical or Z direction to maintain a constant distance between the dispenser and the top layer of the metal object being formed. This type of metal droplet ejection printer is also known as a magnetohydrodynamic (MHD) printer.

[0004] In 3D object printing systems using elastomeric materials, temporary support structures are formed by using additional dispensers to dispense droplets of a different material to form supports for overhangs and object features extending away from the object during its formation. Because these support structures are made from a different material than the material forming the object, they do not adequately stick or adhere to the object. As a result, the support structures are easily separated from the object features they support during object fabrication and can be removed from the object after its formation is complete. This is not the case with metal droplet ejection systems. When support structures are formed using molten metal within the printer, the metal ejected to form the object strongly adheres to the object features that require support during solidification. As a result, a significant amount of machining and polishing is required to remove the supports from the object. Damage to the object can occur during this post-fabrication process. It is difficult to coordinate different metal droplet ejection printers using different metals because the thermal conditions of different metals can affect the build environment of the object-forming system. For example, a supporting structure metal with a higher melting temperature may weaken or soften the metal forming the object, or a supporting metal structure with a lower melting temperature may weaken when the object feature contacts the structure. It would be beneficial for a metal drop ejection printer to be able to form supporting structures that allow for the formation of metal object overhangs and other extended features. Summary of the Invention

[0005] A new method for operating a 3D metal object printer forms support structures that are not tightly adhered to object features supported by the structures without adversely affecting the environment of the 3D metal object printer. The method includes operating a first solid metal supply mechanism to move a first solid metal into a receptacle in a dispenser for melting, and operating a second solid metal supply mechanism to move a second solid metal, different from the first solid metal, into the receptacle in the dispenser for melting.

[0006] The new 3D metal object printer forms a support structure that is not tightly bonded to an object feature supported by the structure without adversely affecting the environment of the 3D metal object printer. The new 3D metal object printer includes a dispenser having a container with a receiving portion therein, a heater configured to heat the container to a temperature sufficient to melt a solid metal within the receiving portion of the container, a first solid metal supply mechanism configured to move the first solid metal into the receiving portion of the container in the dispenser for melting, and a second solid metal supply mechanism configured to move a second solid metal, different from the first solid metal, into the receiving portion of the container in the dispenser for melting. [Brief explanation of the drawings]

[0007] The above-mentioned aspects and other features of a method for forming a support structure that is not tightly adhered to object features supported by the structure without adversely affecting the environment of the 3D metal object printer, and a 3D metal object printer that implements the method, are described in the following description, taken in conjunction with the accompanying drawings. [Figure 1] A new 3D metal object printer is presented that forms support structures that are not tightly bonded to the object features supported by the structures without adversely affecting the environment of the 3D metal object printer. [Figure 2] 2 is a schematic diagram of a wire feed mechanism used to provide wire to the wire guide 124 of FIG. 1 and to measure the wire retracted from the receptacle of the printer of FIG. 1. FIG. [Figure 3A] 1A-1C are diagrams illustrating the formation of metal support structures used to form metal overhang structures in an object. [Figure 3B] 10 illustrates the contraction of the metal support structure from the metal overhang structure during cooling of the object and support structure. [Figure 3C] 10 illustrates the formation of a support structure with segmented boundaries to facilitate removal of the support structure after the object fabrication process is complete. [Figure 4A] FIG. 1 is a flow diagram of a process for forming support structures that are not tightly bonded to the object features supported by the structures without adversely affecting the environment of a 3D metal object printer. [Figure 4B] FIG. 10 is a flow diagram of a process for forming support structures with segmented boundaries such that they are not tightly adhered to object features supported by the support structures. [Figure 5] FIG. 1 is a schematic diagram of a prior art 3D metal printer that does not include components for forming support structures made of different metal alloys. DETAILED DESCRIPTION OF THE INVENTION

[0008] For a general understanding of the 3D metal object printer disclosed herein and the environment for its operation, as well as details of the printer and its operation, reference is made to the drawings, in which like reference numbers represent like elements.

[0009] FIG. 5 illustrates an embodiment of a known 3D metal object printer 100 that ejects single droplets of molten metal to form an object without using a support structure. In the printer of FIG. 5 , droplets of molten bulk metal are ejected from a removable reservoir 104 containing a single nozzle 108, and the droplets from the nozzle form a swath for a layer of the object on a platform 112. As used herein, the term “removable reservoir” refers to a hollow container having a reservoir configured to hold a liquid or solid substance, where the container as a whole is configured for installation and removal from a 3D metal object printer. As used herein, the term “reservoir” refers to a hollow container having a reservoir configured to hold a liquid or solid substance that can be configured for installation and removal from a 3D metal object printer. As used herein, the term “bulk metal” refers to a conductive metal available in a mass form, such as commonly available standard wire or pellets of macro-sized proportions.

[0010] With further reference to FIG. 5 , a bulk metal source 116, such as a metal wire 120, is fed to a wire guide 124 extending through an upper housing 122 in the dispenser head 140 and melted within a container in a removable vessel 104 to provide molten metal that is ejected from a nozzle 108 through an orifice 110 in a base plate 114 of the dispenser head 140. As used herein, the term “nozzle” refers to an orifice fluidly connected to a volume within a container containing molten metal, configured to eject molten metal droplets from the container. As used herein, the term “dispenser head” refers to the housing and components of a 3D metal object printer that melt, dispense, and regulate the ejection of molten metal droplets for the production of metal objects. The molten metal level sensor 184 includes a laser and a reflective sensor. Reflection of the laser from the molten metal level is detected by the reflective sensor, generating a signal indicative of the distance to the molten metal level. The controller receives this signal and determines the level of the molten metal volume within the removable vessel 104 so that it can be maintained at an upper level 118 within the removable vessel's housing. The removable vessel 104 slides into the heater 160, and the heater's inner diameter contacts the removable vessel, allowing the solid metal within the removable vessel's housing to be heated to a temperature sufficient to melt the solid metal. As used in this document, the term "solid metal" means a metal defined in the periodic table of elements, or an alloy formed by these metals in solid form rather than liquid or gas. The heater is separated from the removable vessel, forming a volume between the heater and the removable vessel 104. An inert gas supply 128 provides a pressure-regulated source of inert gas, such as argon, to the dispenser head through a gas supply tube 132. The gas flows through the volume between the heater and the removable vessel and exits through an orifice 110 in the dispenser head and base plate 114 around the nozzle 108. This flow of inert gas adjacent to the nozzle insulates the ejected droplets of molten metal from the ambient air at the base plate 114, preventing metal oxides from forming during the flight of the ejected droplets.The gap between the nozzle and the surface on which the ejected metal droplets land is intentionally kept small enough so that the inert gas exiting the nozzle does not dissipate before the droplets in the inert gas stream land.

[0011] The dispenser head 140 is movably mounted in a Z-axis track to accommodate vertical movement of the dispenser head relative to the platform 112. One or more actuators 144 are operatively connected to the dispenser head 140 to move the dispenser head along the Z-axis, and to the platform 112 to move the platform in the X-Y plane below the dispenser head 140. The actuators 144 are operated by a controller 148 to maintain the appropriate distance between the orifice 110 in the base plate 114 of the dispenser head 140 and the top surface of the object on the platform 112.

[0012] As droplets of molten metal are dispensed toward the platform 112, swaths of molten metal droplets are formed on the object being formed by moving the platform 112 in the XY plane. The controller 148 also operates the actuator 144 to adjust the vertical distance between the dispenser head 140 and the most recently formed layer on the substrate to facilitate the formation of other structures on the object. Although the molten metal 3D object printer 100 is depicted in FIG. 5 as operating in a vertical orientation, other alternative orientations can be employed. Also, while the embodiment depicted in FIG. 5 has the platform moving in the XY plane and the dispenser head moving along the Z axis, other arrangements are possible. For example, the actuator 144 can be configured to move the dispenser head 140 along the Z axis in the XY plane, or to move the platform 112 in both the XY plane and the Z axis.

[0013] The controller 148 operates switches 152. One switch 152 can be selectively operated by the controller to provide power from a power supply 156 to a heater 160, and another switch 152 can be selectively operated by the controller to provide power from another power supply 156 to a coil 164 to generate an electric field that ejects droplets from the nozzle 108. Because the heater 160 generates a large amount of heat at a high temperature, the coil 164 is positioned within a chamber 168 formed by one (circular) or two or more (rectilinear) walls of the ejector head 140. As used herein, the term "chamber" refers to a volume contained within one or more walls in a metal drop ejection printer in which the heater, coil, and removable reservoir of the 3D metal object printer are located. The removable reservoir 104 and heater 160 are located within such a chamber. The chamber is fluidly connected to a fluid source 172 via a pump 176 and 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. Heat exchanger 180 is connected via a return to fluid source 172. Fluid from source 172 flows through a chamber to absorb heat from coil 164, and the fluid transports the absorbed heat through exchanger 180, where it is removed by known methods. The cooled fluid is returned to fluid source 172 for further use in maintaining the temperature of the coil within a suitable operating range.

[0014] The controller 148 of the 3D metal object printer 100 requires data from an external source to control the printer for metal object production. Typically, 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 selectively access the digital data model, such as through a server, a remote database on which the digital data model is stored, or a computer-readable medium on which the digital data model is stored. This three-dimensional model or other digital data model is processed by a slicer implemented with the controller to generate machine-compatible instructions that the controller 148 executes in a known manner to operate components of the printer 100 to form a metal object corresponding to the model. The generation of machine-compatible instructions can include the generation of an intermediate model, such as when a CAD model of a device is converted to an STL data model, a polygonal mesh, or other intermediate representation, which can then be processed to generate machine instructions, such as G-code, for manufacturing the object by the printer. As used in this document, the term "machine-enabled instructions" means computer language commands executed by a computer, microprocessor, or controller to operate components of a 3D metal object additive manufacturing system to form a metal object on platform 112. Controller 148 executes the machine-enabled instructions to control the ejection of molten metal droplets from nozzle 108, the positioning of platform 112, and maintaining the distance between orifice 110 and the top layer of the object on platform 112.

[0015] A novel 3D metal object printer 100' is shown in Figure 1, using like reference numerals for like components. A controller 148' is configured with programmed instructions stored in a non-transitory medium operably connected to the controller that, when executed by the controller, causes the controller to detect layers of a metal support structure in the model data and generate machine readiness instructions that operate one or more of the actuators 144 to dispense a support metal alloy from supply 116B into the removable container 104 and change the composition of the molten metal in the container so that subsequent metal drop ejections from the nozzles 108 form the metal support structure. After all or a portion of the metal support structure is formed, the controller 148' then detects machine readiness instructions to form an object layer and operates one or more of the actuators 144 to dispense a build metal alloy from supply 116A into the removable container 104 and change the composition of the molten metal in the container so that subsequent metal drop ejections from the nozzles 108 form the object structure. This transition between forming the object structure and forming the support structure continues until the object is complete, after which the metal support structure, formed of a different metal alloy, can be separated from the object without machining.

[0016] More specifically, printer 100' includes two metal supplies 116A and 116B, which are wire spools as shown, although the supplies can also be volumes of metal powder or metal pellets. Wire from the two supplies 116A and 116B is independently provided to two wire feed mechanisms 200A and 200B, respectively, which are described in more detail below. These mechanisms are connected to one or more actuators to provide motive force for rollers within the mechanisms to push the wire through wire guides 118A and 118B, respectively. Both of these wires directly guide the wire to a common wire guide 122, which feeds the wire into vessel 104 for melting.

[0017] Mechanism 200A for feeding wire from wire supply 116A through wire guide 118A to container 104 is shown in FIG. 2. Mechanism 200A is one embodiment used with mechanisms 200A and 200B to dispense wire into containers. Controller 148′ is operably connected to an actuator, such as a stepper motor 240, to control the rate at which wire is delivered from supply 116A to container 104. Actuator 240 drives roller 224 and is operably connected to controller 148′ so that the controller can adjust the speed at which the actuator drives roller 224. Another roller opposite roller 224 is freewheeling and therefore follows the rotational speed at which roller 224 is driven.

[0018] A slip clutch 244 is operably connected to the drive shaft of the actuator 240 that feeds wire into the vessel. As used herein, the term "slip clutch" refers to a device that applies a frictional force to an object to move it to a predetermined setpoint. When the predetermined setpoint for the frictional force is exceeded, the device slips and therefore no longer applies frictional force to the object. The slip clutch allows the force exerted by the roller 224 on the wire 120 to remain within the constraints of the wire's strength, regardless of how often, how fast, or how long the actuator 240 is driven. This constant force can be maintained by driving the actuator 240 at a speed faster than the fastest expected rotational speed of the drive roller 224 or by placing an encoder wheel 248 on the roller 224 and sensing the rotational speed with a sensor 252. A signal generated by the sensor 252 indicates the angular rotation of the roller 224, and the controller 148' uses this signal and the radius of the roller 224 to determine the length of wire being fed from the wire supply 116A into the vessel 104. That is, mechanisms 200A and 200B act as a type of wire displacement sensor. Alternatively, the freewheeling roller opposite driven roller 224 can have an encoder 248 attached to it that generates a signal indicative of its angular position, so that the length of metal wire dispensed by the mechanism can be determined. In a further alternative design, slip clutch 244 is omitted, and the number of steps of motor movement that stepper motor 240 makes to dispense a segment of wire into a receptacle is recorded and used to determine the length of linear wire movement.

[0019] The length of wire fed into vessel 104 is used to determine the volume of molten metal added to vessel 104. If a metal support structure layer is to be formed, controller 148' operates actuator 144 in mechanism 200B to feed a sufficient length of wire into vessel 104 to convert the volume of molten metal in the vessel into primarily an alloy useful for forming a metal support structure. When controller 148' detects that a portion of the object layer is to be formed, controller 148' operates actuator 144 in mechanism 200A to feed a sufficient length of wire into vessel 104 to convert the volume of molten metal in the vessel into primarily an alloy useful for forming a metal support structure.

[0020] The support structure metal of supply 116B is specifically selected to have a mismatched coefficient of thermal expansion from the construction metal of supply 116A. The mismatch in thermal expansion coefficients allows the support structure to contract at a different rate than the object structure when the object and support structure cool after object fabrication is complete. Depending on the integrity of the support structure, the support structure will collapse or separate from the object structure so that it can be easily removed from the object to produce the final part.

[0021] In one embodiment, the coefficient of thermal expansion of common aluminum alloys used in object construction, such as Al6061, Al356, Al7075, and Al4043, is approximately 13×10 -6 / °C. The metal / alloy for the metal support structure has a thermal expansion coefficient that is mismatched from the thermal expansion coefficient of the metal used in the object structure within the temperature range from the highest melting point of the two metals to room temperature. The support structure metal is also compatible with the object structure metal by dissolving into the molten object structure metal in a container. These criteria identify the following metals (other aluminum alloys, magnesium, zinc, and alloys of magnesium and zinc) as suitable support structure materials for use with the aluminum object structure materials mentioned above. α = approximately 12.9X10 -6 For a metal object structure made of Al-7075, which has a thermal expansion coefficient of α = 1×10 / °C, a useful metal support structure metal is -5 / °C, or α = approx. 14x10-6 / ℃.

[0022] In one embodiment, when a transition is made from one metal to another, the controller operates the actuator to move the dispenser away from the object and then operates the dispenser to expel an amount of material currently in use that is approximately one and a half of the volume of the nozzle to ensure that the next dispensed metal droplet is primarily the metal of the metal object or metal support. To ensure that the amount of molten metal in the container corresponds to the volume of the nozzle, the level of the molten metal in the container is kept at a low level that approximates the volume of the nozzle using the laser level sensor described above.

[0023] An example of the interleaved use of two materials is shown in FIG. 3A . In the leftmost illustration, a U-shaped portion 304 of an object is formed using metal from object build material spool 116A. After the molten metal used to form portion 304 is discharged from the vessel, an amount of support structure material corresponding to support structure 308 is provided into the vessel and melted, such that a dispenser can be operated to form the support structure within the U-shaped recess of portion 304. After the remaining support material is discharged from the dispenser nozzle, an object overhang 312 is formed on support structure 308. As shown in the rightmost illustration of FIG. 3A , the support structure is removed to have an opening where the support structure was formed.

[0024] Figure 3B shows object portion 304 with support structure 308 and overhang 312. When object fabrication is complete, the composite object is cooled. The thermal expansion coefficient of the material used to form support structure 308 allows it to contract more than the object portion and overhang. Therefore, the support structure can be separated from the object portion and overhang and removed to form an opening, as shown on the far right of Figure 3B.

[0025] FIG. 3C illustrates an additional technique for facilitating support structure removal. In addition to forming the support structure from materials with different thermal expansion coefficients, the support structure 308′ is also formed with protrusions 316 to form a segmented boundary 320 between the body of the support structure 308′ and the object portion 304 and overhang 312. This segmented boundary 320 between the support structure 308′ and the object portion 304 and overhang 312 is not as firmly attached to the object and overhang as the solid boundary shown in FIGS. 3A and 3B. Therefore, after the composite object cools, the support structure 308′ can be removed from the object more easily than the support structure shown in FIGS. 3A and 3B. In another embodiment, the support structure 308′ is formed from the same material used to form the object portion 304 and overhang 312. Note that the segmented boundary 320 facilitates separation of the support structure 308′ from the object portion 304 and overhang 312, even though they are made from the same material.

[0026] The controller 148' may be implemented using one or more general-purpose or special-purpose programmable processors that execute programmed instructions. Instructions and data needed to perform programmed functions may be stored in memory associated with the processor or controller. The processors, their memory, and interface circuits configure the controller to perform the operations described above and below. These components may be provided on a printed circuit card or as circuits within an application specific integrated circuit (ASIC). Each of the circuits may be implemented in a separate processor, or multiple circuits may be implemented on the same processor. Alternatively, the circuits may be implemented with individual components or circuits provided within a very large scale integrated (VLSI) circuit. The circuits described herein may also be implemented with a combination of processors, ASICs, individual components, or VLSI circuits. During the formation of the metal object, image data of the structure to be manufactured is sent from either a scanning system or an online or workstation connection to a processor(s) in the controller 148' for processing and generating signals that operate components of the printer 100' to form the object on the platform 112.

[0027] A process for operating a 3D metal object printer 100′ to form a metal support structure using a molten metal different from the molten metal used to form the object structure is shown in FIG. 4A . In a process description, a description of the process performing certain tasks or functions refers to a controller or general-purpose processor executing program instructions stored on a non-transitory computer-readable storage medium operably connected to the controller or processor to manipulate data or operate one or more components in the printer to perform the task or function. The controller 148′ described above can be such a controller or processor. Alternatively, a controller can be implemented with two or more processors and associated circuits and components, each configured to perform one or more tasks or functions described herein. Additionally, the steps of the method can be performed in any feasible chronological order, regardless of the order shown in the figures or the order in which the processes are described.

[0028] FIG. 4A is a flow diagram of a process 400 for operating the printer 100′ to form a support structure using a molten metal different from the molten metal used to form a portion of the metal object. The process begins with the formation of an object layer using a first molten metal (block 404). After the completion of each object layer, the process determines whether the next layer to be formed is a support layer (block 408). If the next layer is not a support layer, the process forms an object layer with the first metal (block 404). If the next layer is a support layer, the process moves the dispenser to a position where the molten metal in the container can be discharged and the dispenser operates to discharge the molten first metal from the dispenser (block 412). A second metal appropriate for forming a structural layer is dispensed into the container and melted, and the dispenser is returned to the object formation area for the production of a layer of the support structure using the molten second metal (block 416). A support structure layer is formed (block 420), and the process determines whether the next layer to be formed is an object layer (block 424). If the next layer is a support layer, the process forms a support layer with a second metal (block 420). If the next layer is an object layer, the process moves the dispenser to a position where the molten second metal in the container can be discharged and the dispenser operates to discharge the molten second metal from the dispenser (block 428). An appropriate amount of the first metal is dispensed into the container, melted, and the dispenser is returned to the object formation area (block 432). The object layer is formed (block 404), and the process continues until all object and support layers have been formed.

[0029] FIG. 4B is a flow diagram of a process 400′ for operating the printer 100′ to form a support structure with a segmented boundary to form a portion of a metal object. Using like reference numbers to describe like operations, the process begins with forming an object layer using a first molten metal (block 404). After completion of each object layer, the process determines whether the next layer to be formed is a support layer (block 408). If the next layer is not a support layer, the process forms an object layer with the first metal (block 404). If the next layer is a support layer, the process forms a support structure with a segmented boundary (block 420′) until the process determines that the next layer to be formed is an object layer (block 424). If the next layer is a support layer, the process continues forming a support structure with a segmented boundary (block 420′). If the next layer is an object layer, the process continues forming object layers (block 404) until another support layer is formed (block 408). This process continues until all object layers and support layers have been formed.

[0030] It will be appreciated that variations 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 subsequently occur to those skilled in the art, which are also intended to be encompassed by the following claims.

Claims

Claim 1 A metal droplet discharging device comprising: a discharger having a container with an accommodation portion therein; a heater configured to heat the container to a temperature sufficient to melt solid metal in the accommodation portion of the container; a first solid metal supply mechanism configured to move a first solid metal into the accommodation portion of the container in the discharger for melting; and a second solid metal supply mechanism configured to move a second solid metal different from the first solid metal into the accommodation portion of the container in the discharger for melting.

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