WO2017089176A1 - 3d printer with a printing head without movable parts - Google Patents

Abstract

The invention relates to a 3D printer (1) with a printing head (2) for generating drops (3c) of a liquid phase (3b) of a metal (3). The printing head (2) comprises a reservoir (4) for the metal (3, 3a, 3b), and the reservoir (4) has an outlet opening (5) for the drops (3c). The printing head (2) additionally comprises at least one induction coil (6) for applying a magnetic field Bi to the metal (3, 3a, 3b). At least one capacitor (7) is provided which can be charged selectively from a direct-current source (10) via a switching device (8, 8a, 8b, 8c) and can be short-circuited via the induction coil (6).

Description

 description

Title: 3D printer with printhead without moving parts

The invention relates to a 3D printer for metals with a print head for

Ejection of drops of the metal no moving parts needed.

PRIOR ART 3D printers for metals which deposit drops of a liquid phase of the metal and assemble them into the object to be produced require a metering device for producing the drops. conventional

Metering devices have moving components that are used to either displace or cause the molten metal to vibrate. The moving parts, for example, by the in the

Patent applications DE 10 2010 062 388 AI and DE 10 2011 078 068 Al described eddy current actuators are controlled.

A metering device without moving parts is disclosed in WO 2007/038987 Al. This metering device is based on the fact that an electric current of

Magnitude of 1,000 A passed through the molten metal and a Lorentz force is applied to the metal by a standing perpendicular to this magnetic field magnetic field. This Lorentz force drives the liquid metal out of the discharge opening of the metering device.

Disclosure of the invention

In the context of the invention, a 3D printer with a print head has been developed for producing droplets of a liquid phase of a metal. The printhead includes a reservoir for the metal, this reservoir a Has outlet opening for the drops. Furthermore, the print head comprises at least one induction coil for loading the metal with a magnetic field Bi.

According to the invention, at least one capacitor is provided, which can be charged via a switching device selectively from a DC voltage source and short-circuited via the induction coil.

He was recognized that when shorting the capacitor over the

Induction coil of current flowing through the induction coil current I very quickly increases to a very high value. As a result, a magnetic field Bi is generated, which changes with a very large time gradient. As a result, an eddy current is induced in the metal in the reservoir, which in turn generates a magnetic field BM. This magnetic field BM is the magnetic field of the Bi

Induction coil opposite to Lenz's rule. So there is a repulsive force F between the two magnetic fields Bi and BM. This repulsive force F is large enough to drive drops of a liquid phase of the metal from the outlet of the reservoir.

Compared with the dosage by means of the Lorentz force, this type of metering has the significant advantage that the metal in the reservoir does not have to be contacted electrically. Accordingly, the printhead is much more compact to build. Furthermore, the strength of the repulsive force F depends essentially on the temporal gradient of the current I through the induction coil. It has been recognized that this relationship is particularly well suited to the requirements for the production of the smallest possible droplets: In order to even promote metal from the outlet of the reservoir, a certain minimum force is required. Once this minimum force has been reached, the quantity conveyed through the outlet opening, and thus also the size of the drop, depends on the duration of action of the force. Thus, if a very large force acts for a very short time, it is optimal for producing very small drops with low energy consumption.

Typical meaningful action for the force, the metal from the

Exit hole is floating, ranging in size between 50 Microseconds and one millisecond. The duration of action is according to the invention by the charge and the capacitance of the capacitor, as well as by the

Inductance and the ohmic resistance of the induction coil, given. It is quite easy to adapt these variables so that a predetermined value for the duration of action is realized. There is no electronics required to control the duration of action.

The induction coil may be dedicated to the printhead for propelling metal from the outlet of the reservoir. However, the inventors have recognized that, for example, an induction coil already present for heating the metal on many printheads for generating the force F, which drives the metal out of the outlet opening of the reservoir,

Since the metal is essentially incompressible both in the solid and in the liquid state of matter, it does not matter at which point in the reservoir the force F is introduced. The force F can thus also far away from the outlet opening of the reservoir in the part of the reservoir, in which the metal is melted, are introduced.

Therefore, in a particularly advantageous embodiment of the invention, the induction coil via the switching device optionally connectable to the capacitor and an AC voltage source. If the induction coil is connected to the AC voltage source, a periodic eddy current is generated in the metal, which does not drive the metal net. Due to the ohmic losses due to the finite resistance of the metal, the metal is heated. On the other hand, if the induction coil is connected to the capacitor and shorts the capacitor short, a very high directional eddy current is generated for a very short time, which drives a drop of the metal out of the outlet opening of the reservoir.

Thus, for example, a printhead with induction heating according to the prior art alone by the addition of the capacitor, the

Switching device and the DC voltage source are converted so that it is able to eject drops of the liquid metal from the outlet opening of the reservoir without moving parts. Induction coils for heating the metal are designed for such high continuous currents that they can be easily supplied with the short current pulse from the capacitor.

In a further advantageous embodiment of the invention, the reservoir can have a first region with a heater for melting the metal and a second region with the outlet opening. The magnetic field Bi generated by the induction coil in the second region of the reservoir is then stronger than in the first region of the reservoir. The effect of the heating is then completely independent of the generation of the drops. For example, a first region of the reservoir can simultaneously supply a plurality of second regions of the reservoir with respectively associated induction coils with liquid metal. It can then be added at several points at the same time drops of the liquid metal to the object to be produced. This allows the

Increase production speed.

In a particularly advantageous embodiment of the invention, the capacitor for short-circuiting via at least one thyristor, and / or via at least one bipolar transistor with an insulated gate electrode, IGBT, as a switch in the switching device with the induction coil connected. A thyristor has the advantage here that it only has to be switched on by a control current pulse and switches itself off again automatically after the capacitor has been discharged. By contrast, a bipolar transistor has the advantage that the current flow through the induction coil can be interrupted even before the capacitor has been completely discharged.

When the capacitor is shorted via the induction coil, a maximum current I of at least 100 A, preferably of at least 500 A and very particularly preferably of at least 1000 A, is advantageously drivable by the induction coil. It is advantageous in case of short circuit of the capacitor over the

Induction coil a maximum power P of at least 100 kW, preferably of at least 1 MW, drivable by the induction coil. The high maximum current, or the high maximum power, causes even with a very small size of the print head, a sufficiently large force for ejecting a drop of the liquid metal can be generated. This smaller size can turn into a faster production speed be reversed by having multiple reservoirs on one printhead,

or more outlet openings are provided. Analogous to an inkjet printer with a plurality of nozzles, droplets of the liquid metal can then be added to the object to be produced at several points at the same time.

Advantageously, when the capacitor is shorted via the induction coil, the current I through the induction coil reaches its maximum value within a maximum of 100 microseconds, preferably within a maximum of 50

Microseconds, after the time to the short circuit. Then the temporal

Gradient of the current I through the induction coil sufficiently large to produce a sufficiently large force F at a given maximum current I.

In the case of a short circuit of the capacitor via the induction coil, it is advantageous to act between the magnetic field Bi of the induction coil and the latter

caused magnetic field BM in the metal a repulsive force F of at least 1 N, preferably of at least 10 N. Such a force is reliably sufficient to overcome the surface tension of the liquid metal and to drive a drop from the outlet opening of the reservoir.

Advantageously, the magnetic field Bi of the induction coil on the

Outlet directed component. Then, the repulsive force F immediately drives metal in the direction of the outlet opening of the reservoir. Otherwise, the force F acts in the reservoir as a short-term pressure increase, which propagates through the incompressible liquid metal in the entire reservoir. The

Pressure increase is so even in the area of the outlet opening of the

Reservoirs indirectly effective when the magnetic field Bi is not on the

Outlet opening is directed.

In a particularly advantageous embodiment of the invention, the

Outlet opening of the reservoir has a diameter d in the range of 0.1 mm to 0.5 mm, preferably in the range of 0.2 mm to 0.3 mm, on. In this area for the diameter d, the outlet opening is still reliable through the

Closed surface tension of the liquid metal, so that on a active closing element, such as a nozzle needle can be dispensed with. At the same time, a diameter d is in the claimed range

manufacturing technology comparatively easy to produce. In the prior art, diameters d in the claimed range were too large to produce small drops with diameters of 100 microns or less. It had to be worked with smaller diameters d at the price that producing an outlet with decreasing

Diameter d disproportionately expensive. By created according to the invention possibility to exert on the liquid metal for a very short time a very large force in the direction of the outlet opening can be made with a given outlet significantly smaller drops than in the prior art. With a diameter d in the claimed area, it is possible to produce drops with a diameter of less than 100 micrometers. The magnetohydrodynamic metering device according to the prior art, which makes use of the Lorentz force, can only produce droplet sizes above 800 micrometers with the same outlet opening.

Advantageously, the reservoir and the outlet opening are dimensioned such that the surface tension σ of the liquid metal prevents the liquid metal against its gravitational pressure p from passing through the outlet opening, while the capacitor is not discharged via the induction coil. Then only the balance of the charge on the condenser is decisive for the duration for which liquid metal is ejected from the outlet of the reservoir. An active closing element for the outlet opening is not required.

The induction coil may be embedded, for example, in a ceramic for protection against the heat of the liquid metal. The induction coil can be wound, for example, from a water-cooled waveguide. For this purpose, for example metals with high melting points can be used, such as copper or aluminum. The reservoir and his

Outlet opening preferably have a wall of a

Temperature resistant ceramic to keep the temperature of the liquid metal inside the reservoir can. The reservoir may optionally include an additional induction coil which maintains the liquid metal at a predetermined temperature by inductive heating. The outlet opening of the reservoir can be made very different. It can be conical, centric, stepped, etc. executed and adapted to the respective requirements or operating conditions. Further variations are possible in the form of the

Reservoirs. They can be optimized in terms of repeatability of drop size, drop frequency and efficiency.

For example, the capacitor may have a capacitance between 5 microfarads and 10 microfarads. When charging with a DC voltage of 800 volts can be stored in it then an amount of energy, for example, 2 to 3 joules. Advantageously, the voltage of the DC voltage source is less than 2,000 volts. Beyond this limit are the required components

disproportionately more expensive.

Further measures improving the invention will be described in more detail below together with the description of the preferred embodiments of the invention with reference to figures.

embodiments

It shows:

Figure 1 embodiment of the 3D printer 1 according to the invention with an induction coil 6, which also serves the heating of the reservoir 4.

Figure 2 embodiment of a 3D printer 1 according to the invention with separate from the induction coil 6 heater 9 for the first part 4a of the reservoir 4th

Figure 3 extension of the embodiment shown in Figure 2 on

a plurality of outlet openings 5, which are independently controllable. Figure 4 Time course of the voltage U across the induction coil 6, the current I through the induction coil 6 and the force exerted on the liquid metal 3b force F after short circuit of the capacitor 7 at time to.

FIG. 1 shows an exemplary embodiment of the 3D printer 1 according to the invention. The printhead 2 is stationary in this embodiment. Instead, the work surface 12 on which the object 13 to be produced is displaceable against the print head 2 via a positioning mechanism not shown in detail in FIG. 1 along the three spatial directions x, y and z. The printhead

2 of the 3D printer 1 comprises a reservoir 4, on the input side of which the metal 3 in its solid phase 3a is supplied. In that shown in FIG

Embodiment, this solid phase 3a is a wire. The induction coil 6 is connected via the switch 8a with the AC voltage source

11 connectable. The alternating voltage from the alternating voltage source 11 generates an alternating magnetic field inside the reservoir 4, which excites a periodic eddy current in the metal 3. This periodic eddy current undergoes 3 resistive losses by the resistance of the metal, whereby the metal 3 is heated. Therefore, a liquid phase is formed in the reservoir 4

3b of the metal 3.

The capacitor 7 can be charged by the DC voltage source 10 via the switch 8c. If a drop 3c to be ejected from the outlet opening 5 of the reservoir 4, the capacitor 7 via the switch 8b and the

Induction coil 6 are short-circuited. Here, the leading to the AC voltage source 11 switch 8a and the are advantageous for

DC voltage source leading switch 8c turned off. Due to the low internal resistance of the capacitor 7, a large current I, which generates a correspondingly strong magnetic field Bi in the interior of the metal 3, builds up very quickly in the induction coil 6 during short-circuiting of the capacitor 7. This magnetic field Bi induces a directed eddy current in the metal 3. This eddy current in turn generates a magnetic field BM, which is directed against the original field Bi of the induction coil 6 due to the Lenz rule. Both magnetic fields Bi and BM exert the repulsive force F on each other, the acts on the liquid metal 3b and drives a portion thereof as drops 3c from the outlet opening 5 of the reservoir 4 out.

The discharged during the discharge of the capacitor 7 from the outlet opening 5 with the diameter d liquid metal 3d forms due to its

Surface tension σ with a sufficiently long time of flight to the object to be produced 13, or to the working surface 12, to a single drop 3c. The surface tension σ has, moreover, the further effect of preventing the liquid metal 3b against its gravitational pressure p from passing through the outlet opening 5 of the reservoir 4 as long as the capacitor 7 is not discharged via the induction coil 6. Therefore, the outlet opening 5 is normally closed permanently, without this being a special

Closing element would be necessary. The common ground of the capacitor 7, the induction coil 6 and the

Voltage sources 10 and 11 is denoted by 14.

FIG. 2 shows another embodiment of the print head 2 for the SD printer 1 according to the present invention. In contrast to FIG. 1, the induction coil 6 does not simultaneously serve to heat the reservoir 4.

 Instead, the reservoir 4 is divided into a first part 4a, in which the metal 3, 3a is melted by a heater 9, and in a second part 4b, which contains the outlet opening 5. The second part 4b of the reservoir 4 is shaped such that when unloading the not shown in Figure 2

Condenser 7 acting on the liquid metal 3b force F on the

 Outlet opening 5 is focused out. From the outlet opening 5, a drop 3c of the liquid metal 3b is driven out with each discharge of the capacitor 7. FIG. 3 shows an extension of the print head 2 illustrated in FIG

In contrast to FIG. 2, a single first part 4a of the reservoir 4, which is heated by a single heater 9, supplies a plurality of second parts 4b of the reservoir

Reservoirs 4, each with its own outlet opening 5 and its own induction coil 6. It can then independently of each outlet openings 5 drops 3c are driven out. This can be done in several places to be produced object 13 at the same time each a drop 3c are attached. The object 13 is accordingly faster to produce.

FIG. 4 illustrates the time profile of a discharge of the capacitor 7. Over time t, the voltage U across the induction coil 6, the current I through the induction coil 6, and the force F acting on the liquid metal 3b are plotted. At time to the discharge of the capacitor begins 7. About the induction coil 6, a high voltage U is provided immediately. Due to the inductance of the induction coil 6, the current I builds up in the induction coil 6 with a time delay. The current I reaches a maximum after a short time. The generation of the opposing field BM in the metal 3, and thus the formation of the repulsive force F, takes a short time. Therefore, there is a small delay between the time maximum in the current I and the time maximum in the force F.

Shortly before the current I reaches its maximum, the voltage U across the induction coil 6 already begins to break, because most of the charge has already flowed away from the capacitor 7. With a short time delay to

Incoming voltage U also breaks the current I through the induction coil 6 a. With another little delay, the force F also breaks down on the liquid metal 3d. The exit of liquid metal 3b from the outlet opening 5 is terminated.

The metal 3a does not necessarily have to be added as a wire. It can also be supplied, for example, as a powder or granules. With pulse widths of advantageously between 100 microseconds and one

For example, repetition rates above 500 Hz can be achieved in milliseconds in order to achieve a rapid build-up rate of the object 13 to be produced. The size of the drops 3c can be over the tension with which the

Capacitor 7 is charged from the DC voltage source 10 can be varied. Thus, the 3D printer according to the invention in relation to the

Drop size considerably more versatile than 3D printers according to the prior art.

Claims

claims A 3D printer (1) having a printhead (2) for generating droplets (3c) of a liquid phase (3b) of a metal (3), the printhead (2) including Reservoir (4) for the metal (3, 3a, 3b), said reservoir (4) having an outlet opening (5) for the drops (3c) and wherein the printhead (2) further comprises at least one induction coil (6) for acting of the metal (3, 3a, 3b) with a magnetic field Bi, characterized in that at least one capacitor (7) is provided, which can be charged via a switching device (8, 8a, 8b, 8c) optionally by a DC voltage source (10) and via the induction coil (6) is short-circuited. 2. 3D printer (1) according to claim 1, characterized in that the Induction coil (6) via the switching device (8, 8a, 8b, 8c) optionally with the capacitor (7) and with an AC voltage source (11) is connectable. 3. 3D printer (1) according to one of claims 1 to 2, characterized characterized in that the reservoir (4) has a first area (4a) with a heater (9) for melting the metal (3, 3a) and a second area (4b) with the outlet opening (5), that of the induction coil ( 6) generated magnetic field Bi in the second region (4b) of the reservoir (4) is stronger than in the first region (4a) of the reservoir (4). 4. 3D printer (1) according to one of claims 1 to 3, characterized in that the capacitor (7) can be connected to the induction coil (6) for short-circuiting via at least one thyristor, and / or via at least one insulated gate bipolar transistor, IGBT, as a switch (8b) in the switching device (8). 5. 3D printer (1) according to one of claims 1 to 4, characterized characterized in that in case of short circuit of the capacitor (7) via the Induction coil (6) a maximum current I of at least 100 A, preferably of at least 500 A and most preferably of at least 1000 A, through the induction coil (6) is drivable. 6. 3D printer (1) according to one of claims 1 to 5, characterized characterized in that in case of short circuit of the capacitor (7) via the Induction coil (6) a maximum power P of at least 100 kW, preferably of at least 1 MW, through the induction coil (6) is drivable. 7. 3D printer (1) according to one of claims 1 to 6, characterized characterized in that in case of short circuit of the capacitor (7) via the Induction coil (6) of the current I through the induction coil (6) reaches its maximum value within a maximum of 100 με, preferably within at most 50 με, after the time to the short circuit. 8. 3D printer (1) according to one of claims 1 to 7, characterized characterized in that in case of short circuit of the capacitor (7) via the Induction coil (6) between the magnetic field Bi of the induction coil (6) and the resulting magnetic field BM in the metal (3, 3a, 3b) a repulsive force F of at least 1 N, preferably of at least 10 N, acts. 9. 3D printer (1) according to one of claims 1 to 8, characterized in that the magnetic field Bi of the induction coil (6) has a component directed toward the outlet opening (5) of the reservoir (4). 10. 3D printer (1) according to one of claims 1 to 9, characterized in that the outlet opening (5) of the reservoir (4) has a Diameter d in the range of 0.1 mm to 0.5 mm, preferably in the range of 0.2 mm to 0.3 mm. 11. 3D printer (1) according to one of claims 1 to 10, characterized by such a dimensioning of the reservoir (4) and the Outlet opening (5) that the surface tension σ of the liquid metal (3b) the liquid metal (3b) against its gravitational pressure p at the passage through the outlet opening (5) prevents, while the capacitor (7) is not discharged via the induction coil (6).