Patent Application
20230158575
The present teachings relate generally to liquid ejectors in drop-on-demand (DOD) printing and, more particularly, to a buoyancy-based level sensing system and methods for use within a liquid metal ejector of a DOD printer.
A drop-on-demand (DOD) or three-dimensional (3D) printer builds (e.g., prints) a 3D object from a computer-aided design (CAD) model, usually by successively depositing material layer upon layer. A drop-on-demand (DOD) printer, particularly one that prints a metal or metal alloy, ejects a small drop of liquid aluminum alloy when a firing pulse is applied. Using this technology, a 3D part can be created from aluminum or another alloy by ejecting a series of drops which bond together to form a continuous part. For example, a first layer may be deposited upon a substrate, and then a second layer may be deposited upon the first layer. One particular type of 3D printer is a magnetohydrodynamic (MHD) printer, which is suitable for jetting liquid metal layer upon layer which bond together to form a 3D metallic object. Magnetohydrodynamic refers to the study of the magnetic properties and the behavior of electrically conducting fluids.
In MHD printing, a liquid metal is jetted out through an exit orifice of a nozzle of the 3D printer onto a substrate or onto a previously deposited layer of metal. A printhead used in such a printer is a single-nozzle head and includes several internal components within the head which may need periodic replacement. In some instances, a typical period for nozzle replacement may be an 8-hour interval. During the liquid metal printing process, the aluminum and alloys, and in particular, magnesium containing alloys, can form oxides and silicates during the melting process in the interior of the pump. These oxides and silicates are commonly referred to as dross. The buildup of dross is a function of pump throughput and builds continuously during the print process. In addition to being composed of a combination of aluminum and magnesium oxides and silicates, the dross may also include gas bubbles. Consequently, the density of the dross may be lower than that of the liquid metal printing material and the dross may build at the top of the melt pool, eventually causing issues during printing. Certain DOD printers use, for example, a non-contact red-semiconductor laser sensor, operating at an approximate wavelength of 660 nm, for measuring the melt pool height during printing. This is commonly referred to as a level-sensor. Dross accumulation while printing impacts the ability of the level-sensor to accurately measure the molten metal level of the pump and may lead to prematurely ending the print job. This may also cause the pump to erroneously empty during printing, thereby ruining the part. Dross plugs may also grow within the pump causing issues with the pump dynamics resulting in poor jet quality and additional print defects, such as the formation of satellite drops during printing. The dross could potentially break apart and a chunk of this oxide falls into the nozzle resulting in a clogged nozzle orifice. When the level sense signal “drops out,” this can cause a catastrophic failure condition, leading to printer shut down, requiring clearing or removal of the dross plug, replacing the print nozzle, and beginning start-up procedures again.
Thus, a method of and apparatus for level sense control in a metal jet printing drop-on-demand or 3D printer is needed to provide longer printing times and higher throughput without interruption from defects or disadvantages associated with dross build-up.
The following presents a simplified summary in order to provide a basic understanding of some aspects of one or more embodiments of the present teachings. This summary is not an extensive overview, nor is it intended to identify key or critical elements of the present teachings, nor to delineate the scope of the disclosure. Rather, its primary purpose is merely to present one or more concepts in simplified form as a prelude to the detailed description presented later.
A metal ejecting apparatus is disclosed. The metal ejecting apparatus includes a nozzle orifice in connection with the inner cavity and configured to eject one or more droplets of the liquid metal printing material, a float in contact with a surface of the liquid metal printing material, where the float is buoyant within the liquid printing material, and a filament attached to the float on a first end and attached to a level sensing system on a second end.
Implementations of the metal ejecting apparatus include a hollow float, where a density of the float is less than a density of the liquid metal printing material, or where the float may include boron nitride. The float may be non-wetting when in contact with the liquid metal printing material. The float may be resistant to high temperature. The filament may be under tension. The filament may include a material that may be resistant to high temperature and non-wetting when in contact with the liquid metal printing material. The filament may include an indicating feature in one or more locations along a length of the filament. The indicating feature is configured to alert the level sensing system of a low level of liquid metal printing material. The indicating feature is configured to alert the level sensing system of a sufficient level of liquid metal printing material. The level sensing system may include an ultrasonic sensor, a visual sensor, a mechanical force sensor, a laser sensor, or a combination thereof.
Another exemplary metal ejecting apparatus is disclosed. The metal ejecting apparatus may include a nozzle orifice in connection with the inner cavity and configured to eject one or more droplets of the liquid metal printing material, a float in contact with a surface of the liquid metal printing material where the float is buoyant within the liquid printing material, a filament which may include an indicating feature, attached to the float on a first end, and attached to a level sensing system. The metal ejecting apparatus may include where the level sensing system is configured to receive a signal from the indicating feature.
Implementations of the metal ejecting apparatus may include where the indicating feature is configured to alert the level sensing system of a low level of liquid metal printing material, and the indicating feature is configured to alert the level sensing system of a sufficient level of liquid metal printing material. The level sensing system may include an ultrasonic sensor, a visual sensor, a mechanical force sensor, a laser sensor, or a combination thereof.
A method of sensing and controlling a level of liquid printing material in a metal jetting apparatus is disclosed. The method includes placing a buoyant float onto a surface of a liquid printing material held within an inner cavity of the metal jetting apparatus, receiving a first signal indicative of a vertical position of the buoyant float within the inner cavity of the metal jetting apparatus, and sending a second signal to a printing material feed system.
Exemplary implementations of the method of sensing and controlling a level of liquid printing material in a metal jetting apparatus may include where receiving the first signal is completed with a level sensing system may include an ultrasonic sensor, a visual sensor, a mechanical force sensor, a laser sensor, or a combination thereof. The method of sensing and controlling a level of liquid printing material in a metal jetting apparatus may include adding printing material from the printing material feed system when the first signal indicates that the vertical position of the buoyant float denotes an insufficient level of liquid printing material held within the inner cavity of the metal jetting apparatus. The method of sensing and controlling a level of liquid printing material in a metal jetting apparatus may include stopping an addition of printing material from the printing material feed system when the first signal indicates that the vertical position of the buoyant float denotes a sufficient level of liquid printing material held within the inner cavity of the metal jetting apparatus. The method of sensing and controlling a level of liquid printing material in a metal jetting apparatus may include heating a solid printing material held within the inner cavity of the metal ejecting apparatus, thereby causing the solid printing material held to change to a liquid within the metal ejecting apparatus.
The accompanying drawings, which are incorporated in and constitute a part of this specification, illustrate embodiments of the present teachings and together with the description, serve to explain the principles of the disclosure. In the figures:
It should be noted that some details of the figures have been simplified and are drawn to facilitate understanding of the present teachings rather than to maintain strict structural accuracy, detail, and scale.
Reference will now be made in detail to exemplary embodiments of the present teachings, examples of which are illustrated in the accompanying drawings. Wherever possible, the same reference numbers will be used throughout the drawings to refer to the same, similar, or like parts.
In metal jet printers employing a drop-on-demand printing methodology and technology, a small drop, also referred to as a droplet, of liquid aluminum alloy or other metal is ejected when a firing pulse is applied. Using this technology, a three-dimensional (3D) part can be created from aluminum alloy or another metal by ejecting a series of drops which bond together to form a continuous part. The print-head, or metal ejector jet, used in exemplary printers is a single-nozzle head and certain internal components within the head need periodic replacement. A typical period for nozzle replacement may be on the order of an 8-hour interval. During the metal jet printing process the aluminum and components of the alloys, in particular magnesium, can form oxides during the melting process on the inlet and in the inner cavity of the pump, which is commonly referred to as “dross.” This dross builds up in the inner cavity and other areas of the ejector pump during printing and is a function of printing material throughput through the pump. The dross, in the case of aluminum printing material, is a combination of aluminum oxide, magnesium oxide, aluminum and gas bubbles. The dross builds in the top of the melt pool that resides in the metal ejector pump and causes issues during printing. Exemplary printing systems employ a red-semiconductor laser (660 nm wavelength) non-contact sensor for measuring the melt pool height during printing, and thus is an effective level sensor for the metal jet printing system. Dross accumulation while printing impacts the ability of the level sensor to accurately measure the molten metal level of the pump and leads to prematurely ending the print job. When the level sense signal “drops out” it leads to shutting down the machine, clearing or removing the dross plug, replacing the print nozzle, and beginning the start-up procedure again. This may result in an incomplete part or premature shutdown of a printing operation.
Exemplary embodiments of the present disclosure include a level sensing system for a liquid ejector using a buoyant ceramic float suspended into the upper pump portion of the liquid metal ejector for continuous monitoring of the height of the melt pool. The buoyant construction of the float would constrain it to be located at the top of the melt pool irrespective of the composition of the melt pool. For example, if there were dross or other contamination in the melt pool, the float mechanism would remain operational. The vertical translation of the float is captured externally and converted to a pool height via a continuous line. A connected tensioner apparatus may be used to maintain a constant tension in the lines leading up to the float, and therefore, the height of the top level of the melt pool is maintained within the nominal range via a closed loop system connecting the aluminum wire feed and the level sensing systems.
The 3D printer 100 may also include a power source, not shown herein, and one or more metallic coils 106 enclosed in a pump heater that are wrapped at least partially around the ejector 104. The power source may be coupled to the coils 106 and configured to provide an electrical current to the coils 106. An increasing magnetic field caused by the coils 106 may cause an electromotive force within the ejector 104, that in turn causes an induced electrical current in the printing material 126. The magnetic field and the induced electrical current in the printing material 126 may create a radially inward force on the printing material 126, known as a Lorenz force. The Lorenz force creates a pressure at an inlet of a nozzle 110 of the ejector 104. The pressure causes the printing material 126 to be jetted through the nozzle 110 in the form of one or more liquid drops 128.
The 3D printer 100 may also include a substrate, not shown herein, that is positioned proximate to (e.g., below) the nozzle 110. The ejected drops 128 may land on the substrate and solidify to produce a 3D object. The 3D printer 100 may also include a substrate control motor that is configured to move the substrate while the drops 128 are being jetted through the nozzle 110, or during pauses between when the drops 128 are being jetted through the nozzle 110, to cause the 3D object to have the desired shape and size. The substrate control motor may be configured to move the substrate in one dimension (e.g., along an X axis), in two dimensions (e.g., along the X axis and a Y axis), or in three dimensions (e.g., along the X axis, the Y axis, and a Z axis). In another embodiment, the ejector 104 and/or the nozzle 110 may be also or instead be configured to move in one, two, or three dimensions. In other words, the substrate may be moved under a stationary nozzle 110, or the nozzle 110 may be moved above a stationary substrate. In yet another embodiment, there may be relative rotation between the nozzle 110 and the substrate around one or two additional axes, such that there is four or five axis position control. In certain embodiments, both the nozzle 110 and the substrate may move. For example, the substrate may move in X and Y directions, while the nozzle 110 moves up and/or down in a Y direction.
The 3D printer 100 may also include one or more gas-controlling devices, which may be or include a gas source 138. The gas source 138 may be configured to introduce a gas. The gas may be or include an inert gas, such as helium, neon, argon, krypton, and/or xenon. In another embodiment, the gas may be or include nitrogen. The gas may include less than about 10% oxygen, less than about 5% oxygen, or less than about 1% oxygen. In at least one embodiment, the gas may be introduced via a gas line 142 which includes a gas regulator 140 configured to regulate the flow or flow rate of one or more gases introduced into the three-dimensional 3D printer 100 from the gas source 138. For example, the gas may be introduced at a location that is above the nozzle 110 and/or the heating element 112. This may allow the gas (e.g., argon) to form a shroud/sheath around the nozzle 110, the drops 128, the 3D object, and/or the substrate to reduce/prevent the formation of oxide (e.g., aluminum oxide) in the form of an air shield 114. Controlling the temperature of the gas may also or instead help to control (e.g., minimize) the rate that the oxide formation occurs.
The liquid ejector jet system 100 may also include an enclosure 102 that defines an inner volume (also referred to as an atmosphere). In one embodiment, the enclosure 102 may be hermetically sealed. In another embodiment, the enclosure 102 may not be hermetically sealed. In one embodiment, the ejector 104, the heating elements 112, the power source, the coils, the substrate, additional system elements, or a combination thereof may be positioned at least partially within the enclosure 102. In another embodiment, the ejector 104, the heating elements 112, the power source, the coils, the substrate, additional system elements, or a combination thereof may be positioned at least partially outside of the enclosure 102. While the liquid ejector jet system 100 shown in
The use of a buoyant ceramic float suspended into the upper pump for continuous monitoring of the height of the melt pool, constrained to be located at the top of the melt pool irrespective of the composition of either the float or the contamination or melt pool. Several conventional ceramics, due to their heat and corrosion resistant properties make ideal float materials. The vertical translation of the float is captured externally and converted to a pool height via a continuous line or filament. External level sensing can be accomplished in several ways, depending on the embodiment. A filament with increasing thickness with extremities corresponding to the extremities in the melt pool height can be used in conjunction with an ultrasonic sensor. It should be noted that alternate embodiments may include ultrasonic sensing, force measurement, or other means to detect height. Aside from thickness in the filament, the filament may be patterned, have physical flags, or other indicators such as thickness or color gradients, wherein the accompanying sensor may be visual or colorimetric. The measured thickness, whether based on a physical indicator such as filament diameter that is ramped, stepped, or transitions over a gradient would directly correspond to the pool height in the pump, as shown in
In certain embodiments of the method of sensing and controlling a level of liquid printing material in a metal jetting apparatus 500, the printing material is in the form of a wire feed, while alternate embodiments may utilize printing material made from powder or liquid. The printing material may be composed of metal or metal alloys or combinations thereof as described herein, or alternatively may be composed of plastic or plastic composite materials, one or more polymers, or combinations thereof. Exemplary metal or metal alloys may include aluminum, aluminum alloys, or a combination thereof. In embodiments utilizing wire feed or externally introduced printing materials, the method may include removing the printing material feed from the reservoir or inner cavity of the ejector jet at certain times. Any and all of the steps of controlling a level of liquid printing material in a metal jetting apparatus 500 may be repeated multiple times, up to even ten times as long as the inner cavity or reservoir will accommodate additional volume and not be adversely affected by accumulated dross inside the cavity of the liquid ejector. Alternate embodiments of methods for controlling a level of liquid printing material in a metal jetting apparatus 500 may include filling an inner cavity in communication with a liquid ejector with a printing material, reading a level signal from a surface of a melt pool in the reservoir using a float sensor system as described herein, connecting the level signal generated by the float and detector in the leveling system, to the filling of the inner cavity with the printing material, pausing a jetting operation of the liquid ejector, increasing a quantity of printing material in the inner cavity until a low level signal is no longer received from the float sensor level system, and resuming the jetting operation of the liquid ejector.
The method described herein provides an advantageous, “within print-job” adjustment of a target fluid setpoint level in metal jetting printing systems. A print job may be “paused” any number of times to adjust target fluid level and may be easily implemented into standard control software for a variety of metal jet printers or printers using liquid ejectors. Employing various embodiments of this method may facilitate print run time increases without shutting down due to level-sense failures, which enables and allows for larger size part builds and longer time between pump replacements. This method should also allow for maintaining jetting performance while changing pump level and improving printing system ability to measure and control the level of the melt pool height. The method may further enable running at higher pump temperatures as well, which can lead to improved jet quality, as higher temperature setpoints may lead to faster dross accumulation.
While the present teachings have been illustrated with respect to one or more implementations, alterations and/or modifications may be made to the illustrated examples without departing from the spirit and scope of the appended claims. For example, it may be appreciated that while the process is described as a series of acts or events, the present teachings are not limited by the ordering of such acts or events. Some acts may occur in different orders and/or concurrently with other acts or events apart from those described herein. Also, not all process stages may be required to implement a methodology in accordance with one or more aspects or embodiments of the present teachings. It may be appreciated that structural objects and/or processing stages may be added, or existing structural objects and/or processing stages may be removed or modified. Further, one or more of the acts depicted herein may be carried out in one or more separate acts and/or phases. Furthermore, to the extent that the terms “including,” “includes,” “having,” “has,” “with,” or variants thereof are used in either the detailed description and the claims, such terms are intended to be inclusive in a manner similar to the term “comprising.” The term “at least one of” is used to mean one or more of the listed items may be selected. Further, in the discussion and claims herein, the term “on” used with respect to two materials, one “on” the other, means at least some contact between the materials, while “over” means the materials are in proximity, but possibly with one or more additional intervening materials such that contact is possible but not required. Neither “on” nor “over” implies any directionality as used herein. The term “conformal” describes a coating material in which angles of the underlying material are preserved by the conformal material. The term “about” indicates that the value listed may be somewhat altered, as long as the alteration does not result in nonconformance of the process or structure to the illustrated embodiment. The terms “couple,” “coupled,” “connect,” “connection,” “connected,” “in connection with,” and “connecting” refer to “in direct connection with” or “in connection with via one or more intermediate elements or members.” Finally, the terms “exemplary” or “illustrative” indicate the description is used as an example, rather than implying that it is an ideal. Other embodiments of the present teachings may be apparent to those skilled in the art from consideration of the specification and practice of the disclosure herein. It is intended that the specification and examples be considered as exemplary only, with a true scope and spirit of the present teachings being indicated by the following claims.
