Improved vessel for melting metal in metal drop ejecting three-dimensional (3D) object printer

Abstract

To provide a metal drop ejecting apparatus in a three-dimensional (3D) metal object manufacturing apparatus which inhibits adverse impact of dross on the ability of a laser level-sensor that measures a molten metal level in a vessel in a printer where a solid metal is melted.SOLUTION: A vessel 104' having a receptacle that holds a molten metal has a divider 192 that prevents metal dross formed at a solid metal inlet of the receptacle from migrating to a portion of the receptacle to which a molten metal level sensor 184 directs light.SELECTED DRAWING: Figure 1

Description

Technical Field

[0001] The present disclosure relates to a three-dimensional (3D) object printer that ejects molten metal droplets to form an object, and more specifically, to a container for melting metal in such a printer.

Background Art

[0002] Three-dimensional printing, also known as additive manufacturing, is a process of creating a three-dimensional solid object from a digital model of virtually any shape. Many three-dimensional printing techniques use a layering process in which a layer-forming device forms successive layers of a part on top of previously deposited layers. Some of these techniques use an ejector that ejects an ultraviolet-curable material such as a photopolymer or an elastomer. The printer typically operates one or more extruders to form successive layers of a plastic material that builds a three-dimensional printed object having various shapes and structures. After each layer of the three-dimensional printed object is formed, the plastic material is ultraviolet-cured, solidified, and adhered to the underlying layer of the three-dimensional printed object. This additive manufacturing method is distinguishable from conventional object-forming techniques that mostly rely on the removal of material from a workpiece by a subtractive process such as cutting or drilling operation.

[0003] Recently, several 3D object printers have been developed that form 3D objects by ejecting droplets of molten metal from one or more extruders. These printers have a solid metal source, such as a roll of wire or pellets, which is supplied to a heated containment chamber in a container within the printer where the solid metal is melted, filling the containment chamber. As used herein, the term “containment chamber” means a cavity in a structure configured to hold the molten metal. The containment chamber is made of a non-conductive material around which electric wire is wound to form a coil. As an electric current passes through the coil, it generates an electromagnetic field, which causes a meniscus of molten metal at the nozzle of the containment chamber to separate from the molten metal within the containment chamber and be propelled out of the nozzle. A platform opposite the nozzle of the extruder is moved in an XY plane parallel to the plane of the platform by a controller that operates actuators, so that the ejected metal droplets form a metal layer of the object on the platform, and another actuator, operated by the controller, changes the position of the extruder or platform vertically or in the Z direction to maintain a constant distance between the extruder and the top layer of the forming metal object. This type of metal droplet printer is also known as a magnetohydrodynamic (MHD) printer.

[0004] The molten metal in the container housing within the printer must be maintained at a level sufficient to support the metal droplet ejection operation without depleting the molten metal supply within the printer. In some metal droplet ejection printers, a blue laser is directed to the surface level of the molten metal in the housing, and a reflection sensor monitors the laser reflection by the surface level to determine the current height of the molten metal in the housing. If the sensor output indicates that the surface level has dropped to a threshold position within the housing, a wire feed actuator is activated to supply more solid metal to the housing.

[0005] During the printing process performed by an MHD printer, metals, typically alloys such as aluminum and magnesium, form oxides as the metal melts at the inlet of the container. These oxides are commonly referred to as dross. As used herein, the term “dross” refers to combinations of materials in the container of an MHD printer that are unsuitable for the formation of objects. These materials include aluminum oxide, magnesium oxide, aluminum trapped by these oxides, and bubbles formed during the melting of solid metals. This dross accumulates in the container during the printing process, and the amount of dross produced corresponds to the amount of metal melted in the container. The dross accumulates on top of the molten metal in the container and causes problems during printing.

[0006] One problem arising from dross formation is its negative impact on the ability of the laser level sensor to measure the molten metal level in the container. Dross is dark and has a rough surface that affects the reflection of the laser and its reception by the reflection sensor. If the level is not accurately monitored, the container may become empty during the printing process, ruining the metal object. All dross-related level sensing failures lead to premature printer shutdown, dross removal, container nozzle replacement, and printer restart. The printer's operating time is limited because it must be shut down to remove the dross. This limited operating time also means that the amount of metal ejected is limited because the number and size of the objects produced are suboptimal. Additionally, the temperature of the molten metal cannot reach the optimal temperature for metal droplet ejection, as higher molten metal temperatures produce more dross. It would be beneficial to find a way to manage the dross without affecting the sensing of the molten metal level and without extending the time for printer production. [Overview of the project]

[0007] A new container for a 3D metal object printer prevents dross generated within the container from interfering with the detection of the molten metal level by a laser level sensor. The new container includes a housing within the container, the housing comprising a wall defining the housing having a first end and a second end, and a partition within the housing for separating the first portion of the housing from the second portion, the partition extending from the first end of the housing at a distance shorter than the distance from the first end to the second end.

[0008] The new 3D metal object printer includes a new container that prevents dross generated within the container from interfering with the detection of the molten metal level by a laser level sensor. The new 3D metal object printer includes a housing configured to hold molten metal, the housing comprising an extruder head defining the housing having a first end and a second end, and a partition within the housing for separating the first portion of the housing from the second portion, the partition extending from the first end of the housing at a distance shorter than the distance from the first end to the second end.

[0009] A new metal insert for filling a new container houses a divider within the new container. The new metal insert includes an elongated portion configured to be received within a first housing of the removable container, a slot formed within the elongated portion of the metal insert configured to receive a divider within the first housing of the removable container, and a bulbous portion configured to be received within a second housing of the removable container. [Brief explanation of the drawing]

[0010] The aforementioned embodiments and other features of a container for a 3D metal object printer that prevent dross generated within the container from interfering with the detection of the molten metal level by a laser level sensor are described below in conjunction with the attached drawings. [Figure 1] This invention demonstrates a novel 3D metal object printer that features a container in which the dross generated within the container does not interfere with the detection of the molten metal level by a laser level sensor. [Figure 2A] This is a top view of the upper housing of a container that has a partition to prevent the dross generated inside the container from interfering with the detection of the molten metal level by the laser level sensor in the printer shown in Figure 1. [Figure 2B] This is a side view of the upper housing of a container that has a partition to prevent dross generated inside the container from interfering with the detection of the molten metal level by a laser level sensor. [Figure 3A] This is a top view of a newly installed container with partitions. [Figure 3B] Figure 3A is a top view of the container during the printing process performed using the printer shown in Figure 1, demonstrating that dross accumulation in the container occurs only on the partition side where solid metal is introduced for melting. [Figure 4A] This is an exploded side view of a two-part container equipped with a metal insert used to fill the container when the printer is started. [Figure 4B] This shows the installation of a metal insert inside the upper housing of the container. [Figure 4C] This shows an assembled container containing a metal insert. [Figure 5] This is a schematic diagram of a 3D metal printer that uses a laser system to determine the position of the surface level of molten metal within the printer's housing, which may be affected by dross inside the printer's container. [Modes for carrying out the invention]

[0011] For a general understanding of the 3D metal object printer and the environment for its operation disclosed herein, as well as the details of the printer and its operation, refer to the drawings. In the drawings, similar reference numbers represent similar elements.

[0012] Figure 5 illustrates an embodiment of a conventionally known 3D metal object printer 100, which uses a light beam and reflection sensors to determine the surface level of molten metal in the housing of a container within the printer. In the printer of Figure 5, droplets of molten bulk metal are ejected from the housing of a removable container 104 having a single nozzle 108, and the droplets from the nozzle form a swath for a layer of object on the platform 112. As used herein, the term “removable container” means a hollow container having a housing configured to hold a liquid or solid substance, and the container as a whole is configured to be installed and removed in a 3D metal object printer. As used herein, the term “bulk metal” means conductive metal available in aggregate form, such as wires of commonly available standard sizes or pellets of macro-size ratios. A bulk metal source 116, such as a metal wire 120, is supplied to a wire guide 124 extending through an upper housing 122 within the dispensing head 140, melted in a housing of a removable container 104, and provided with molten metal to be extruded from a nozzle 108 through an orifice 110 in the base plate 114 of the dispensing head 140. As used herein, the term “nozzle” means an orifice in a removable container configured for extruding molten metal droplets from a housing in the removable container. As used herein, the term “dispensing head” means the housing and components of a 3D metal object printer that melts, extrudes, and adjusts the extrusion of molten metal droplets for the manufacture of metal objects. The molten metal level sensor 184 includes a light source and a reflection sensor. In one embodiment, the light source is a laser, and in some embodiments, a blue laser. The reflection of the laser from the molten metal level is detected by the reflection sensor and generates a signal indicating the distance to the molten metal level. The controller receives this signal and determines the volume level of the molten metal in the removable container 104 so that it can be maintained at the upper level 118 within the housing of the removable container. The removable container 104 slides into the heater 160, the inner diameter of the heater comes into contact with the removable container, and the solid metal in the housing of the removable container is heated to a temperature sufficient to melt the solid metal.As used in this document, the term “solid metal” means a metal as defined in the periodic table of elements, or an alloy formed by these metals in a solid form rather than a liquid or gaseous state. The heater is separated from the removable container, forming a volume between the heater and the removable container 104. The inert gas supply unit 128 provides a pressure-regulated source of an inert gas, such as argon, to the discharger head through the gas supply pipe 132. The gas flows through the volume between the heater and the removable container and exits through the discharger head around the nozzle 108 and the orifice 110 in the base plate 114. This flow of inert gas close to the nozzle insulates the discharged droplets of molten metal from the ambient air around the base plate 114, preventing the formation of metal oxides during the flight of the discharged droplets.

[0013] The discharger head 140 is mounted to move within a Z-axis trajectory to accommodate the vertical movement of the discharger head relative to the platform 112. One or more actuators 144 are operably connected to the discharger head 140 to move the discharger head along the Z-axis, and operably connected to the platform 112 to move the platform in the XY plane below the discharger head 140. The actuators 144 are operated by a controller 148 to maintain an appropriate distance between the orifice 110 in the base plate 114 of the discharger head 140 and the uppermost surface of an object on the platform 112.

[0014] As droplets of molten metal are ejected toward the platform 112, moving the platform 112 in the XY plane creates swaths of molten metal droplets on the object being formed. The controller 148 also operates the actuator 144 to adjust the vertical distance between the ejector head 140 and the layer most recently formed on the substrate, facilitating the formation of other structures on the object. The molten metal 3D object printer 100 is shown in Figure 5 as operating in a vertical orientation, but other alternative orientations can be employed. Also, the embodiment shown in Figure 5 has a platform that moves in the XY plane and the ejector head that moves along the Z axis, but other arrangements are possible. For example, the actuator 144 may be configured to move the ejector head 140 in the XY plane along the Z axis, or it may be configured to move the platform 112 in both the XY plane and along the Z axis.

[0015] The controller 148 operates switches 152. One switch 152 may be selectively operated by the controller to provide power to the heater 160 from a power source 156, and another switch 152 may be selectively operated by the controller to provide power to the coil 164 from another power source 156 to generate an electric field for ejecting droplets from the nozzle 108. Because the heater 160 generates a large amount of heat at a high temperature, the coil 164 is located within a chamber 168 formed by one (circular) or more (linear) walls of the ejector head 140. As used in this document, the term “chamber” means a volume contained within one or more walls in which the heater, coil, and removable container of the 3D metal object printer are located. The removable container 104 and the heater 160 are located within this chamber. The chamber is fluidically connected to a fluid source 172 via a pump 176 and is also fluidly connected to a heat exchanger 180. As used in this document, the term “fluid source” refers to a container of liquid having properties useful for absorbing heat. The heat exchanger 180 is connected via a return to the fluid source 172. Fluid from the source 172 flows through the chamber, absorbing heat from the coil 164, and the fluid carries the absorbed heat through the exchanger 180, where the heat is removed by known means. The cooled fluid is returned to the fluid source 172 for further use in maintaining the coil temperature within a suitable operating range.

[0016] The controller 148 of the 3D metal object printer 100 requires data from an external source to control the printer for the manufacture of metal objects. Generally, a three-dimensional model or other digital data model of the object to be formed is stored in memory operably connected to the controller 148, and the controller may be able to access a remote database where the digital data model is stored via a server or the like, or a computer-readable medium where the digital data model is stored may be selectively connected to the controller 148 and become accessible. This three-dimensional model or other digital data model is processed by a slicer implemented with the controller to generate machine-ready instructions that the controller 148 executes in a known manner, thereby operating the components of the printer 100 and forming a metal object corresponding to that model. The generation of machine-ready instructions may include the creation of an intermediate model, such as when a CAD model of a device is converted to an STL data model, or to another polygonal mesh or other intermediate representation, and this intermediate model may then be processed to generate machine instructions, such as g-code, for the printer to manufacture the device. As used in this document, the term “machine-responsive instruction” means a computer language command executed by a computer, microprocessor, or controller to operate the components of a 3D metal object additive manufacturing system to form a metal object on the platform 112. The controller 148 executes the machine-responsive instruction to control the ejection of molten metal droplets from the nozzle 108, the positioning of the platform 112, and the maintenance of the distance between the orifice 110 and the uppermost layer of the object on the platform 112.

[0017] The controller 148 may be implemented using one or more general-purpose or dedicated programmable processors to execute programmed instructions. Instructions and data required to perform programmed functions may be stored in memory associated with the processor or controller. The processors, their memories, and interface circuits configure the controller to perform the operations described above, as well as those described below. These components may be provided on a printed circuit card or as circuits in an application-specific integrated circuit (ASIC). Each circuit may be implemented on a separate processor, or multiple circuits may be implemented on the same processor. Alternatively, the circuits may be implemented as individual components or circuits provided in a very large-scale integrated (VLSI). Furthermore, the circuits described herein may be implemented in combination of processors, ASICs, individual components, or VLSI circuits. During the formation of a metal object, image data of the structure to be manufactured is transmitted to the processor(s) of the controller 148 from either the scanning system or an online or workstation connection to process and generate signals that operate the components of the printer 100 to form the object on the platform 112.

[0018] Using similar reference numbers for similar components, a new 3D metal object printer 100' is shown in Figure 1. The 3D metal object printer 100' includes a novel removable container 104' containing a partition 192 within the container 104'. This partition 192 separates the portion of the container dwelling into which solid metal is supplied for melting from the portion of the container containing the molten metal surface, to which a laser level sensor 184 directs its laser. Since the partition 192 does not extend the length of the container dwelling, the molten metal surface is the same on both sides of the partition. However, the partition 192 is long enough to prevent dross generated at the solid metal inlet on the feeding side of the partition from reaching the molten metal surface on the side of the partition as measured by the laser level sensor 184. Thus, the accuracy of the measurement of the molten metal surface level is not affected by the generation of dross, and printer 100' continues to operate longer than printer 100 in Figure 5, and the throughput of printer 100' is greater than that of printer 100 in terms of the amount of molten metal extruded. Therefore, printer 100' can produce objects larger than printer 100 and more objects. As used herein, the term “partition” means any structure that separates the upper portion of the container housing into two sides, thereby preventing dross from moving from one side of the partition to the other.

[0019] As will be described below with reference to FIGS. 4A, 4B, and 4C, one embodiment of the removable container 104' has two separate housings that can be joined together to form a removable container for installation within the printer 100'. As shown in the top view of FIG. 2A, the partition 192 extends across the receiving portion 198 of the upper housing 204 from the first side of the inner circumference of the upper housing in a diametrically opposite direction of the inner circumference, and is positioned to separate the portion 194 of the receiving portion 198 from the solid metal input portion 196 of the receiving portion 198 when the laser level sensor 184 faces its laser. As shown in the side view of FIG. 2B, the partition 192 has a length shorter than the length of the upper housing 204. In one embodiment, the partition 192 has a length that is about 20% of the length of the upper housing 204, but the partition can have a greater or shorter length. However, the length is sufficient to prevent dross migration from the solid metal inlet to the sensing side 194 of the receiving portion and not so great as to prevent molten metal from flowing around the lower end of the partition 192.

[0020] FIGS. 3A and 3B show the effect of the partition 192 on the dross generated at the solid metal inlet. In FIG. 3A, the removable container 104' is installed and solid metal is melted within the container to fill the receiving portion. At this point during the operation of the printer, no dross has been generated. As the printer 100' continues to operate by ejecting molten metal droplets and replenishing the molten metal held in the container 104', dross 300 is generated within the solid metal inlet region 196. However, the partition 192 keeps the dross 300 at or near the solid metal inlet and does not transfer it to the level sensing side 194. Thus, the laser level sensor 184 can continue to accurately sense the molten metal level so that an appropriate amount of solid metal is supplied to the container 104' and the operating state of the printer 100' is preserved.

[0021] Figure 4A is a side view of the removable container 104' of the printer 100'. This embodiment of the removable container 104' is a two-part structure including an upper housing 204 and a lower housing 208. As used herein, the term “housing” means a structure having a portion of a housing inside it and configured to be fixed to another structure to form a removable container. The lower housing 208 contains the nozzle 108 (shown in Figure 1). The upper housing 204 includes a collar 228 that is longer than the lower housing 208 and has a circumference equal to the outer circumference of the lower housing 208. The opening of the lower housing 208 opposite the nozzle in the lower housing 208 has a flange that extends from the opening and has a circumference smaller than the inner circumference of the collar 228. The collar 228 has an inner step that is recessed from the end of the upper housing 204 fixed to the lower housing 208 by a distance corresponding to the distance the flange of the lower housing extends from the lower housing. Therefore, the flange of the lower housing slides within the collar 228 until it contacts the inner step of the upper housing 204 so as to fit within the inner circumference of the collar 228. When the upper housing 204 and the lower housing 208 are assembled, they form a housing having a shape corresponding to the metal insert 212. The metal insert 212 is a solid metal piece (such as aluminum or copper) having an elongated, rounded stem 216 and a bulbous portion 220 terminating at a pointed end that fits into the nozzle 108. As used in this text, the term “elongated” means a structure longer than its width, and the term “rounded” means a structure having at least a partially cylindrical shape. As used in this text, the term “bulbous” means a structure having a conical shape along at least a portion of its longitudinal axis. The rounded stem 216 has a slot 214 formed therein for housing the partition 192 when the metal insert 212 is installed in the upper housing 204. In one embodiment, the slot 214 in the metal insert 212 is less than half the length of the metal insert, and in some embodiments, the length of the slot is less than 20 percent of the length of the metal insert.The upper housing 204 is also formed with an induction flange 224. This flange fits into a groove within the dispenser head 140 to correctly orient the removable container 104' within the printer 100' and hold the container in its correct orientation after the container is installed within the dispenser head 140.

[0022] Together with the partition 192, the upper housing 204 is formed of boron nitride, and the lower housing 208 is formed of graphite. In some embodiments, the partition 192 and the upper housing are integrally formed, while in other embodiments, the partition is disposed within the receiving portion and attached to the wall(s) forming the receiving portion. Both of these materials are high-temperature ceramics. In one embodiment, the upper and lower housings are heated to a temperature in the range of about 800 °C to about 850 °C over a period of 8 hours or more. The receiving portion within the removable container 104' can be coated with a suitable antioxidant flame retardant material that helps attenuate the formation of oxides on the metal insert. As used herein, the term "antioxidant flame retardant" means any material that reduces the formation of metal oxides in the type of metal disposed in the receiving portion of the removable container. Since the boron nitride forming the upper housing is not conductive, it does not prevent the generation of an electric field used to eject molten metal droplets from the receiving portion through the nozzle 108 and the orifice 110. The overall dimensions of the assembled removable container are 55 mm, the length of the upper housing is 40 mm, and the length of the lower housing is 15 mm. The outer perimeter of the upper housing at the color 228 is about 50 mm with a diameter of about 16 mm, and the perimeter at the widest part of the lower housing is about 50 mm with a diameter of about 16 mm.

[0023] Before being installed in the ejector head 140 of the printer 100, the metal insert 212 is loaded into the removable container 104. This is done by pressing the stem 216 of the insert 212 into the housing portion of the upper housing 204 (Figure 2B) or by pressing the tip of the bulbous portion 220 into the lower housing. Sometimes several spots of cyanoacrylate adhesive, more commonly known as "super glue," are applied to either the inner step of the lower housing 208 or the inner circumference of the collar 228, and the inner step of the lower housing 208 is then slid within the inner circumference of the collar 228 to fix the lower and upper housings together, as shown in Figure 2C. This adhesive is removed by the heat applied from the heater 160 during printer operation, so that the two housings can be separated for printer maintenance. An inert gas source 128 is connected to the upper housing and supplies insert gas to the housing in the removable container to reduce oxidation of the molten metal in the removable container. A thermocouple (not shown) is placed in the opening 232 (Figure 2A) to provide a signal indicating the heat inside the removable container, so that the controller can adjust the operation of the heater 160.

[0024] It will be understood that variations or substitutions of the features and functions disclosed above and other features and functions may, preferably, be combined into many other different systems, applications, or methods. Various substitutions, modifications, variations, or improvements not currently foreseen or anticipated may be made later by those skilled in the art, and these are also intended to be covered by the following claims.

Claims

Claim 1 A metal droplet discharging device, a housing configured to hold molten metal, the housing defining a discharging head having a first end and a second end, a partition within the housing for separating a first portion of the housing from a second portion, the partition extending at a distance from the first end of the housing that is shorter than the distance from the first end to the second end of the housing, the metal droplet discharging device comprising the partition.

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