Patent Application
20240001444
The present teachings relate generally to liquid ejectors in drop-on-demand (DOD) printing and, more particularly, to a system for level sensing 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 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. 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 printing system is disclosed, including a metal ejecting apparatus, which may include a structure defining an inner cavity to receive a metal printing material, and a nozzle orifice in connection with the inner cavity and configured to eject one or more droplets of a liquid metal may include the metal printing material. The printing system also includes a first print material feed system configured to supply a first print material into the inner cavity. The printing system also includes a second print material feed system may include a second print material configured to measure a level of metal printing material in the inner cavity, where the second print material is a wire.
Implementations of the printing system may include a pass-through laser sensor, where the second print material is detected by the pass-through laser sensor. The first print material may include a wire. The first print material and the second print material may include the same composition. The first print material may include aluminum.
A method of sensing and controlling level in a metal jetting apparatus is disclosed. The method of sensing and controlling level in a metal jetting apparatus includes advancing a predetermined feed length of a secondary print material may include a wire into an inner cavity of a metal ejecting apparatus where the predetermined feed length corresponds to a target fill level of the inner cavity of the metal ejecting apparatus. The method also includes detecting a first measurement of an end of the secondary print material. The method of sensing and controlling level in a metal jetting apparatus also includes retracting the secondary print material from the inner cavity of the metal ejecting apparatus. The method also includes detecting a second measurement of an end of the secondary print material. The method also includes comparing the first measurement of an end of the secondary print material to a second measurement of an end of the secondary print material to determine a retracted length.
Implementations of the method of sensing and controlling level in a metal jetting apparatus may include repeating the steps of advancing the predetermined feed length of the secondary print material into the inner cavity of the metal ejecting apparatus, detecting the first measurement of the end of the secondary print material, retracting the secondary print material from the inner cavity of the metal ejecting apparatus, detecting the second measurement of the end of the secondary print material, and comparing the first measurement of the end of the secondary print material to the second measurement of the end of the secondary print material to determine a retracted length. The method of sensing and controlling level in a metal jetting apparatus may include pausing after advancing the predetermined feed length of the secondary print material. The method of sensing and controlling level in a metal jetting apparatus may include waiting a predetermined time prior to advancing the predetermined feed length of the secondary print material. The method of sensing and controlling level in a metal jetting apparatus may include increasing the predetermined feed length of the secondary print material if the retracted length is less than or equal to the predetermined feed length of the secondary print material. The method of sensing and controlling level in a metal jetting apparatus may include decreasing the predetermined feed length of the secondary print material is greater than the retracted length of the secondary print material. The method of sensing and controlling level in a metal jetting apparatus may include adjusting a target fill level of a primary print material in the inner cavity of the metal ejecting apparatus. The target fill level of the primary print material can be increased when the predetermined feed length of the secondary print material is less than the retracted length of the secondary print material. The target fill level of the primary print material can be decreased the predetermined feed length of the secondary print material is greater than the retracted length of the secondary print material. The primary print material and the secondary print material may include the same composition.
Another method of sensing and controlling level in a metal jetting apparatus is disclosed, including advancing a predetermined feed length of a secondary print material may include a wire into an inner cavity of a metal ejecting apparatus at a pixel-based feed rate of a primary print material. The method also includes detecting a first measurement of an end of the secondary print material. The method also includes retracting the secondary print material from the inner cavity of the metal ejecting apparatus. The method also includes detecting a second measurement of an end of the secondary print material. The method also includes comparing the first measurement of an end of the secondary print material to a second measurement of an end of the secondary print material to determine a retracted length.
Implementations of the method of sensing and controlling level in a metal jetting apparatus may include adjusting a fill level of the inner cavity of the metal jetting apparatus. The pixel-based feed rate of the primary print material is increased when the retracted length is greater than the predetermined feed length. The pixel-based feed rate of the primary print material is decreased when the retracted length is less than the predetermined feed length. The primary print material and the secondary print material may include the same composition.
The features, functions, and advantages that have been discussed can be achieved independently in various implementations or can be combined in yet other implementations further details of which can be seen with reference to the following description.
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 implementations 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.
Certain examples of drop-on-demand printers eject a small drop or droplet of liquid aluminum alloy when a firing pulse is applied. Using this technology, a 3D part can be created from aluminum alloy by ejecting a series of drops which bond together to form a continuous part. The present disclosure includes a print-head used in the printer that is a single-nozzle head which includes some internal components within the head needing periodic replacement. A typical period for nozzle replacement is in 8-hour intervals. During a standard printing process the aluminum and alloys, and in particular magnesium, can form oxides during the melting process on the inlet of the pump, which is commonly referred to as “dross.” The dross builds up in the pump during printing and is a function of metal throughput. The dross is a combination of materials such as aluminum oxide, magnesium oxide, aluminum, gas bubbles, or a combination thereof. Dross builds in the top of the melt pool that resides in the ejector pump and causes issues during printing. Dross accumulation can impact the ability of a laser level sensor that measures the molten metal level of the pump. Erroneous readings from this laser level sensor can cause the pump to empty during printing ruining the part. Most dross related level-sense failures lead to shutting down the machine, requiring a clearing or removal of the dross plug, replacing the print nozzle, and/or beginning start-up again. Under such conditions, printers cannot sustain printing at, for example, max jetting frequency of 400 Hz that provides a total aluminum throughput of two pounds or greater for an 8-hour print interval due to dross build-up.
The present disclosure provides a method and system that uses a secondary wire feed input to the upper pump of an ejector, located adjacent to the primary wire feed input. A secondary wire feed input, using the same alloy as the primary wire feed input, can be used as a method of making a positive assessment of the height of the liquid print material in the pump. This secondary wire feed input measurement can be used periodically to verify the fluid height, for example, on the order of every couple minutes. During the intervals in-between these measurements, the machine can rely on a feed-forward control to regulate input wire to the upper pump based on a known drop usage over that time and average drop mass being output by the ejector. The second wire feed wire feed input can utilize a pass-through laser sensor which can assess or measure the initial “end-point” of the wire before it is fed into the molten pool at the top of the pump, and then also re-assessing the “end-point” of the wire once it has been removed from the upper pump of the ejector. Feeding the wire into the upper pump by a known length and then retracting the wire can enable determination of a known maximum height of the fluid in the pump. It should be noted that for the purposes of the present disclosure the term “wire” can refer to a wire, a rod, a ribbon, or similar extensible structure that is structurally appropriate to provide a physical probe of a level of molten print material in an ejector jet as described herein.
Operating in this manner allows a portion of the secondary wire feed input that enters the molten pool of aluminum to be melted, and the length of the wire will be shortened accordingly. The wire will be melted in the pool regardless of if the wire encounters liquid, dross, or a combination of these two. Upon retracting the wire and recording a measurement of the new length can result in one of two conclusions. In a first case (Case 1), if it is measured that the length of the wire has not changed, it can be determined the fluid is at or below a known maximum level. In a second case (Case 2), if it is measured that the length of the wire retracted in shorter as compared to the original length, the exact height of the fluid is known relative to a datum. By beginning at a nominal length of wire corresponding to a target pump fill level to feed in, and then continuing repeat the process and lengthen the distance until the second case (Case 2) is encountered, the exact height of the molten aluminum in the pump can be determined. Such a level sensing system and methods as described herein provides a system and method that remains insensitive to dross build up in an ejector pump, along with any associated defects.
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
Next, the wire is fed into the pump of the ejector by the requested “feed length” 906, the wire is retracted until the end of the wire reaches the laser sensor 908, the beam exits the laser sensor, or the beam in the laser sensor goes from broken to unbroken, next it is determined what is the “retracted length” of the wire as compared to the “feed length” 910. The “feed length” was determined in step 904. Based on the information determined in the preceding steps, a first decision point is reached. If the retracted length is equal to feed length 912, the system will add an incremental length amount to the “feed length” 914, and steps 906, 908, and 910 are repeated as needed. Following step 910, a second decision point may be reached. If it is found that the retracted length is less than the current “feed length” 916, then it is determined what is the “retracted length” as compared to “length C” 918. If the retracted length is greater than “length C” 920A, then the actual fluid level is below the target level 922A and the feed-forward algorithm of the primary print material feed is increased on a pixel-based feed rate 924A. If it is found that the retracted length is less than “length C” 920B, then the actual fluid level is above the target level 922B; and therefore, the feed-forward algorithm of the primary print material feed is reduced or decreased on a pixel-based rate 924B. Finally, the printer is instructed to wait a set time 926. This predetermined time interval can be from 1 minute to about 10 minutes, or from about 1 minute to about 5 minutes, or from about 1 minute to about 2 minutes. These operation steps of a level sensing and controlling system in a metal jetting apparatus can include advancing a predetermined feed length of a secondary print material comprising a wire into an inner cavity of a metal ejecting apparatus wherein the predetermined feed length corresponds to a target fill level of the inner cavity of the metal ejecting apparatus, detecting a first measurement of an end of the secondary print material, retracting the secondary print material from the inner cavity of the metal ejecting apparatus, detecting a second measurement of an end of the secondary print material, and comparing the first measurement of an end of the secondary print material to a second measurement of an end of the secondary print material to determine a retracted length.
Methods and apparatus for a level sense for a metal ejector jet and printing system in accordance with the present disclosure provide a secondary wire feed input, using the same aluminum alloy or printing material as the primary wire feed input, which can be used as a means of making a positive assessment of the height of the liquid aluminum in the pump. This secondary wire feed input measurement can be utilized periodically to verify the fluid height in the melt pool in an inner cavity of the ejector jet, at a predetermined time, on the order of every couple minutes, or other interval as needed. During the interval between these measurements the machine can rely on a feed-forward control to regulate input wire to the upper pump based on a known drop usage over that time and average drop mass. The second wire feed wire feed input can utilize a pass-through laser sensor which can assess the initial “end-point” of the wire before it is fed into the molten aluminum pool at the top of the pump, and then also re-assess the “end-point” of the wire once it has been removed or retracted from the upper pump. By beginning at a nominal length, corresponding to a target pump fill level, of wire to feed in, and then continuing repeat the process & lengthen that distance until case 2, where the length of the wire is now shortened, is encountered the exact height of the molten aluminum in the pump can be determined. Such a method and system as described herein provides advantageous print run times that can be increased before an un-planned shut-down, which provides a benefit to a printing system by allowing for larger size part builds and increased overall productivity. The method and system further serves to improve the life of the upper pump as dross build up from longer runs can be detrimental to the operation of upper pumps. It is also known that seasoned or used upper pumps have improved sustained jetting performance. The system and methods of level sense can also improve the ability of the printing system to measure and control the level of the melt pool height in the inner cavity of the ejector, and in certain cases, can enable operation at higher pump temperatures for improved jet quality.
Such a printing system and method and apparatus for level sensing can be used instead of or in addition to the existing level detection system. It should be noted that the second feed is not intended to be a material supply, just used as a probing level sense, which can be employed during a printing operation or in between printing operations, i.e., during a pause in printing operations. It is not necessary to stop printing while measuring or performing methods as described herein. As such, while it may be of interest to use a similar wire material in the secondary feed system as the printing material, it does not necessarily need to be the same. Methods as described herein do not rely on laser reflecting off a surface of a melt pool, which can be inhibited by contamination, such as dross. Even if the second feed wire contacts dross at a surface of the melt pool, it will still melt, thereby still enabling an effective level sense and determination.
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.
