DE102018000172A1 - Method and apparatus for ejecting drops of molten metal by pulsed gas shocks - Google Patents

Abstract

TECHNICAL PROBLEM One of the main problems with all equipment used for ejecting drops of molten metal is the effective heat insulation of the high temperature area in which the molten metal is located. In the previous method, this is usually implemented only inadequate. The process presented here provides a significant improvement in the thermal insulation of the high temperature region. Solution to the Problem The metal is melted in the form of a wire in a special printhead. This is composed of a crucible and two metal sheets. The crucible has a melting chamber from which the melt flows via a channel into a nozzle chamber. The nozzle chamber has a gas inlet in its upper region and an outlet nozzle in the lower region. By inertially directing inert gas onto the melt surface at high speed, drops of molten metal are ejected from the discharge nozzle. The metal sheets serve both as heating elements and as a mechanical bearing of the crucible. For this purpose, the sheets have thin webs at their ends, which are fastened to special connecting bolts. The sheets are heated by electric current. Because the printhead is attached to the rest of the apparatus only by small cross-sectional webs, there is good thermal insulation of the printhead. Field of application is ejecting drops of molten metal.

Description

Technical area

The object of the invention presented here is a method which is used for the targeted ejection of drops of molten metal from a nozzle. The invention further comprises an apparatus implementing this method. A potential field of application of such a method is the additive production of metallic solids. For this purpose, first a computer model of the volume to be produced is subdivided into layers. Based on these data, an apparatus for ejecting the metal drops is moved relative to a suitable base. This means either the apparatus is moved, or the pad or both together. During this movement, metal drops are ejected, which then fall on the surface and solidify on impact. As a result, the solid is printed layer by layer on the substrate. Another possible application is the printing of printed conductors on electronic circuit boards or the application of molten solder on solder joints. Methods for the targeted ejection of liquid metal are generally summarized under the generic term "Liquid Metal Jetting", abbreviated LMJ. A distinction is made between processes in which the discharge occurs continuously and where the liquid metal is ejected drop by drop. In the following, however, only the dropwise methods are to be considered.

State of the art

1. 1. Einführen des Rohmaterials, üblicherweise in Drahtform, in eine Schmelzkammer, die beheizt wird.
2. 2. Aufheizen des Rohmaterials in der Kammer auf eine Temperatur über seinen Schmelzpunkt, so dass es schmilzt.
3. 3. Einleitung der Schmelze in eine Düse. Alternativ kann die Düse auch direkt in die Schmelzkammer eingearbeitet sein.
4. 4. Ausstoß der Schmelze aus der Düse.

The above applications make LMJ a promising technology in a variety of manufacturing processes. Therefore, considerable efforts have been made in industry and research to develop LMJ processes and corresponding apparatuses for their realization. The basic principle is always the same.

1. 1. Introduction of the raw material, usually in wire form, into a melting chamber which is heated.
2. 2. Heating the raw material in the chamber to a temperature above its melting point so that it melts.
3. 3. Introduction of the melt into a nozzle. Alternatively, the nozzle can also be incorporated directly into the melting chamber.
4. 4. ejection of the melt from the nozzle.

In general, it is necessary to run the entire process in an oxygen-poor atmosphere, otherwise oxidation would occur. The aim of all processes is to be able to control the dropwise outflow of the melt in time and space as precisely as possible. Typically, the ejected droplets have a diameter of 40-500 [μm]. The individual methods differ mainly in the way in which the melt is forced out of the nozzle. For this it is necessary to exert a force on the melt. This force can be generated mechanically, pneumatically, electromagnetically or acoustically. In the mechanical process, an actuator is in direct or indirect contact with the molten metal. By pulsing the actuator, pressure pulses are applied to the melt whereby individual drops are expelled from the nozzle. In the processes in which there is direct contact between melt and actuator, piezo actuators are mostly used. For example, in the U.S. Patent No. 4,828,886 , filed by Hieber, describes an apparatus in which drops of molten solder are ejected by being displaced from a glass tube by means of an annular piezoactuator. However, these methods are only applicable to metals having a melting point below 400 C °, since piezoactuators do not work when exceeding their Curie temperature, which is typically between 300 - 400 [C °]. If LMJ is also to be used in refractory metals such as aluminum or copper, it is necessary to heat-isolate the actuator from the molten metal. This can be accomplished by switching a transmission member between the actuator and the melt, the transmission member having to be heat resistant and a poor conductor of heat. Such a method is in US Patent No. 5,598,200 , filed by Gore. Here, one end of a ceramic rod is immersed in a chamber filled with molten metal. The other end is connected to an acoustic pulse generator. The ceramic rod conducts the generated pulses from the generator into the melt, thereby dropping the melt dropwise from a nozzle at the bottom of the chamber. It is also possible to use acoustic waves directly, as in the US Patent 5,722,479 , filed by Oeftering. A special feature of this invention is that the molten metal is not ejected from a nozzle. Instead, the melting chamber has an opening which is coated with a film consisting of the melt and through its Surface tension is maintained. By introducing focused acoustic waves, this film is made to vibrate so that individual drops dissolve. These drops are then electrically charged and their trajectory can be controlled by means of deflection electrodes. in the US Patent 6,446,878 , filed by Chandra et al., a device is presented in which pneumatic pressure surges are used to force the molten metal through nozzles. For this purpose, the melting chamber has an inlet, which is connected via a controllable valve to a container filled with compressed inert gas. By quickly opening and closing this valve, pressure surges are applied to the melt, so that it is driven by nozzles in the lower part of the chamber. Furthermore, it is possible to eject the melt by means of magnetohydrodynamic forces, as in the U.S. Patent No. 4,919,335 , filed by Hobson et al., is demonstrated. In this technique, an electric current is passed through the melt. At the same time a magnetic field is generated, which passes through the melt perpendicular to the direction of flow. Due to the induced Lorentz forces, the melt is pressed out of a nozzle.

Description of the invention

Especially if refractory metals, such as aluminum or copper are to be processed by LMJ, a coherent concept for the thermal insulation of the molten metal is indispensable. Only heat-resistant materials should come into contact with the melt and it must be ensured that no corrosion of components of the apparatus or contamination of the melt occurs. Poor thermal insulation also results in a high power requirement for heating the melt and cooling the heat sensitive components of the apparatus. Although almost all authors are aware of the importance of good thermal insulation, this is only marginally dealt with in the procedures presented so far. One of the main objectives of the procedure presented here is to close this gap.

The main components for implementing the method are a wire feed cooling holder, a print head, and a gas impact generator. The printhead consists of three parts, namely two metal sheets and a crucible. The two metal sheets serve both as bearings of the crucible and as heating elements. All 3 Parts must be resistant to high temperatures. In the crucible, a melting chamber and a nozzle chamber is incorporated, which are connected by a channel. The crucible must be only a few [cm ³ ] in size. By means of the wire feed, a metal wire is introduced into the melting chamber. The metal sheets have at their ends thin webs, which are connected to electrical connections. The electrical connections in turn are bolts that are electrically insulated and heat-conducting attached to the cooling bracket. The cross-sectional area of the webs should be much smaller than that of the connecting bolt. An electrical voltage is applied to the terminals so that a current flows through the metal sheets. The resulting Joule heat is used to heat the crucible to a temperature beyond the melting point of the metal to be processed. The resulting melt flows through the connecting channel into the nozzle chamber. This has in its upper part a gas inlet and in the lower area is the actual nozzle from which the melt can escape. In the two metal sheets are corresponding recesses. Above the gas inlet of the nozzle chamber is the mouth of the gas impact generator. Both are separated by a thin gap. The gas impact generator is a gas-filled container from the mouth of which gas is ejected in a pulse-like manner at high speed and directed into the gas inlet of the nozzle chamber. When the gas impact hits the surface of the melt, an overpressure is generated for a short time, which is large enough to overcome the surface tension of the melt, so that a drop from the nozzle dissolves. The gas is forced sideways and flows out of the gap into the environment. The gas used is an inert gas to prevent chemical reactions with the melt. The pressure amplitude and frequency can be controlled. The gas shocks can be generated both mechanically and pneumatically. But it is important only that the gas impact generator and printhead do not touch, to prevent direct heat conduction between the two components. It is a good thermal insulation of the print head achieved because it is connected only to the webs of the metal sheets to the rest of the apparatus. The thinner the cross-sectional area of the webs, the better the thermal insulation. Since the metal wire to be melted also has a very small cross-sectional area, its heat output, which it transmits to the environment, is low. It is thus possible to heat the print head to very high temperatures, while the rest of the apparatus hardly heats up. This is still supported by active cooling.

embodiment

berechnet werden, wobei R_th den thermischen Widerstand, l die Länge des Steges, A seine Querschnittsfläche und λ des Wärmeleitkoeffizienten des verwendeten Metalls bezeichnet. Der infinitesimale Temperaturabfall dT über einem Abschnitt des Steges mit der infinitesimalen Länge dl berechnet sich zu $$\text{d}T=\frac{Q}{\lambda A}\text{d}l,$$

wobei Q die Wärmeleistung ist, die durch den Abschnitt geleitet wird. Die Wärmeleistung wiederum ist abhängig, von der Stromstärke und dem spezifischen Widerstand p des Steges. Es gilt $$Q=\rho I^2\frac{l}{A}$$

wobei I die Stromstärke im Steg darstellt. Im folgenden wird die Abhängigkeit von p und λ von der Temperatur vernachlässigt. Einsetzen von Formel (3) in (2) und anschließendes Integrieren ergibt dann $$\begin{array}{c}\Delta T={\displaystyle {\int }_0^l{I}^2\frac{\rho \tau }{\lambda A^2}d}\tau ,\\ \Delta T=\frac{1}{2}{I}^2\frac{\rho l^2}{\lambda A^2}.\end{array}$$

In dieser Beziehung stellt ΔT die Temperaturdifferenz zwischen Anschlussbolzen und Schmelztiegel dar. Es seien die Werte aus Tabelle 1 gegeben.In drawing 1 and 2 an apparatus is shown which realizes the invention. It is in a protective gas atmosphere, preferably argon. The printhead 1 is electrically conductive to two connection bolts 6 and 7 connected. The connection bolts are good conductors for electric power and heat and electric attached to the cooling bracket. It must be ensured a sufficiently good heat conduction between the connecting bolt and the cooling bracket. Also attached to the cooling bracket is a heat sink 4 , He is actively cooled, being 41 the input for the cooling medium and 42 is the output. The gas shock generator 2 is stored so that its muzzle 21 just above the nozzle chamber of the print head, wherein the mouth tube is inserted through a recess in the cooling bracket. In drawing 3 is a cross-sectional view of the printhead and the cooling bracket shown. To heat the printhead, an electrical voltage is applied to the terminal studs so that a current flows through the metal sheets of the printhead. About the wire feeder 5 the metal wire gets into the melting chamber 11 directed. For this, the wire is in the opening 53 introduced. The feed wheels 51 and 52 are driven by a stepper motor (not shown) and push the wire forward by means of frictional forces. In addition, the feed wheels carry the excess heat, which is passed from the wire from the melting chamber off. For this purpose, it is advantageous to manufacture the feed wheels of a good heat conductor such as aluminum and store them so that the heat conduction to the cooling bracket is affected as little as possible. After the wire has melted, the melt flows over the connecting channel 12 in the nozzle chamber 13 , By the gas blows coming out of the muzzle 21 hit the surface of the melt, this is dropwise from the nozzle 14 pushed out. In drawing 4 the printhead is shown in detail. The melting pot 15 consists of a high temperature resistant ceramic. The two metal sheets 16 and 17 ideally consist of tungsten. The upper sheet has recesses for the melt and nozzle chamber entry. The lower plate has one for the outlet nozzle. The annular recesses 19 serve both the attachment and the electrical connection to the connecting bolts. The bridges 18 fulfill two functions. First, because of their small cross-section, most of the Joule heat is generated in them. On the other hand, this ensures that only little heat is conducted into the cooling holder, because the thermal resistance of the webs is correspondingly large. He can with the formula $$R_{tH}=\frac{l}{\lambda A}$$

where R _{th is} the thermal resistance, l the length of the ridge, A its cross-sectional area and λ the coefficient of thermal conductivity of the metal used. The infinitesimal temperature drop dT over a section of the ridge with the infinitesimal length dl is calculated to be $$\text{d}T=\frac{Q}{\lambda A}\text{d}l.$$

where Q is the heat output passed through the section. The heat output in turn depends on the current and the resistivity p of the web. It applies $$Q=\rho I^2\frac{l}{A}$$

where I represents the current in the bridge. In the following, the dependence of p and λ on the temperature is neglected. Substituting formula (3) into (2) and then integrating then yields $$\begin{array}{c}\Delta T={\displaystyle {\int }_0^l{I}^2\frac{\rho \tau }{\lambda A^2}d}\tau .\\ \Delta T=\frac{1}{2}{I}^2\frac{\rho l^2}{\lambda A^2},\end{array}$$

In this respect, ΔT represents the temperature difference between the terminal stud and the crucible. The values given in Table 1 are given.

wobei P_S die emittierte Strahlungsleistung, ε der Emissionsgrad, σ die Stefan-Boltzmann-Konstante und A die Oberfläche des Druckkopfes bezeichnet. Für den Druckkopf werden die Werte aus Tabelle 2 vorausgesetzt. Einsetzen der Werte in Formel (5) ergibt eine Strahlungsleistung von 88 [W]. Die Summe aus Strahlungsleistung und Wärmeleitungsleistung ergibt 145 [W]. Diese Angabe ist aber lediglich als grober Schätzwert zu verstehen, da insbesondere der Emissionsgrad ε stark mit der Oberflächenbeschaffenheit und der Temperatur variiert, Zudem sind der spezifische Widerstand und der Wärmeleitkoeffizient ebenfalls temperaturabhängig. Um die Strahlungsverluste gering zu halten, ist ein möglichst kleine Druckkopfoberfläche wünschenswert. Konvektionsverluste durch das umgebende Schutzgas werden vernachlässigt. Obwohl der Querschnitt der Stege gering ist, garantiert die hohe Festigkeit von Wolfram eine gute mechanische Stabilität und das selbst bei hohen Temperaturen von bis zu 2000 [C°].Inserting these values in formula (4) gives a temperature difference of just under 1000 [K]. The converted heat output is according to formula 3 14.25 [W] per bar. With four bars this gives a total of 57 [W] which must be derived from the cooling bracket. Of course, the connecting bolts must have a sufficiently large cross section, so that they are prevented from heating up as well. In reality, one will Table 1: Properties of the tungsten sheets designation Variable name value Specific resistance of tungsten (at 1000 C °) ρ 2.85 · 10 ^-7 [Ω / m] Thermal conductivity of tungsten λ 167 [W / (m · K)] Length of the bridge l 0.02 [m] Cross-sectional area of the bridge A 10 ^-6 [m ² ] amperage I 50 [A] Table 2: Printhead Properties. designation Variable name value emissivity ε 0.8 temperature T 1273 [K] surface A 7.5 · 10 ^-4 [m ² ] greater power needed to maintain the said temperature difference ΔT than the one just calculated, because part of the heat output is emitted by the printhead as radiation. In order to determine the radiation power of the print head as a function of its temperature, the Stefan Boltzmann law is used. This is $$P_{\text{S}}=\in \cdot \sigma \cdot A\cdot {\mathrm{<figure-callout\; id="4"\; label="T"\; filenames="DE102018000172A1\_0006.png,DE102018000172A1\_0007.png"\; state="\{\{state\}\}">T</figure-callout>}}^4$$

where P _S denotes the emitted radiation power, ε the emissivity, σ the Stefan-Boltzmann constant and A the surface of the print head. For the printhead, the values in Table 2 are assumed. Substituting the values in formula (5) gives a radiant power of 88 [W]. The sum of radiant power and heat conduction power gives 145 [W]. However, this information is only to be understood as a rough estimate, since in particular the emissivity ε varies greatly with the surface condition and the temperature. In addition, the specific resistance and the heat conduction coefficient are also temperature-dependent. In order to keep the radiation losses low, the smallest possible print head surface is desirable. Convection losses due to the surrounding inert gas are neglected. Although the cross-section of the webs is low, the high strength of tungsten guarantees good mechanical stability, even at high temperatures of up to 2000 [C °].

• • Die Größe der Kraft, die auf die Membran aufgebracht wird und die Amplitude ihrer Auslenkung.
• • Das Verhältnis der Größen der Oberfläche der Membran und der Austrittsfläche des Mündungsrohrs.
• • Das Volumen in der unteren Behälterhälfte und ihre Form.

In drawing 5 a cross-sectional view of a possible embodiment of the gas shock generator is shown. It comprises a cylindrical container consisting of the two halves 22 and 23 which are bolted together. The lower half 23 tapers conically and ends in the mouth tube 21 , which in turn leads to the gas inlet of the nozzle chamber. In the lower half there is a steel membrane 25 that by a stacked piezoelectric actuator 24 is deflected periodically. These actuators are manufactured by bonding several layers of a piezoelectric ceramic together. Due to the inverse piezoelectric effect, the application of an electrical voltage creates a mechanical stress within the layers, causing them to expand. Stacked piezo actuators can have a travel of several hundred [μm] and generate forces of several [kN]. By electrical high voltage pulses are passed to the actuator, gas shocks are generated because the deflection of the membrane a certain volume of gas is suddenly displaced. In order to increase the gas velocity, the cross section of the container tapers in the direction of the mouth. The larger the ratio of the areas of the membrane and the outlet, the higher the rate of increase of the gas. How high the generated overpressure in the nozzle chamber depends on several factors, some of which are listed below.

• • The magnitude of the force applied to the membrane and the amplitude of its deflection.
• • The ratio of the sizes of the surface of the membrane and the outlet surface of the muzzle tube.
• • The volume in the lower half of the container and its shape.

Another possible embodiment of the gas impact generator is in drawing 6 shown. A small volume is between the rubber membrane 211 and the bottom of the right container included. The Actuator in the container 26 exerts a force on the piston 27 out, rotatable with the lever 29 connected is. The other end of the lever is rotatable with the piston 210 connected, which has contact with the rubber membrane. Due to the different length lever lengths takes place a power and Stellwegwandlung. The pivot bearing 27 the lever is on the tank 25 attached. When the rubber membrane is deflected downwardly, the trapped volume decreases and the displaced gas is expelled from the mouth tube as a gas shock. The gas shocks can be generated not only mechanically, but also pneumatically. This is by drawing 7 clarified. The gas shock generator in this embodiment consists only of the outlet pipe 21 and a connection 212 for compressed gas. The connection is connected to a valve and this in turn to a pressure accumulator. The gas shocks are generated by repeated pulse-like opening and closing of the valve.

QUOTES INCLUDE IN THE DESCRIPTION

This list of the documents listed by the applicant has been generated automatically and is included solely for the better information of the reader. The list is not part of the German patent or utility model application. The DPMA assumes no liability for any errors or omissions.

Cited patent literature

• US 4828886 [0003]
• US 5598200 [0003]
• US 5722479 [0003]
• US 6446878 [0003]
• US 4919335 [0003]

Claims (5)

A method and apparatus for ejecting drops of molten metal by pulsed gas shock using a special print head to melt the supplied metal in wire form. The printhead is composed of a crucible and two metal sheets. The crucible is made of a heat-resistant ceramic and contains a melting chamber and a nozzle chamber, which are connected via a channel. The nozzle chamber has a gas inlet in its upper area and an outlet nozzle in the lower area. The metal sheets serve both as a mechanical bearing and as heating elements of the crucible. Method and apparatus for ejecting drops of molten metal with a printhead according to Claim 1 , wherein the metal sheets are attached via narrow webs to connecting bolts. Narrow means that the lands have a much smaller cross-sectional area than the areas of the metal sheets that are in direct mechanical contact with the crucible. The connecting bolts also have a much larger cross-sectional area than the webs. The connection between the bars and connecting pins is electrically conductive. Method and apparatus for ejecting molten metal with a printhead according to Claim 1 and connecting bolt according to Claim 2 , wherein the connecting pins are electrically isolated but heat-conductively attached to a cooling bracket. On the cooling bracket a heat sink is attached, which can be actively cooled. Method and apparatus according to Claim 1 . 2 and 3 wherein a gas-blast generator is used to generate pulsed gas shocks impinging on the printhead Claim 1 are directed. The gas blows drive the melt out of the discharge nozzle of the print head. The gas shock generator can be designed both mechanically and pneumatically. Method and apparatus according to Claim 1 . 2 . 3 and 4 in which a gas impact generator according to Claim 4 located above the printhead. Its exit orifice is just above the gas inlet of the nozzle chamber, but there is no mechanical contact between the gas shock generator and printhead. Rather, both are separated by a narrow gap.

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