DE102018200227A1 - Piston-punch assembly with flow gap for a 3D print head for metals - Google Patents

Abstract

A printhead (10) for a 3-D printer for metals with a liquid metal melt crucible (12) is proposed, wherein a piston (20) is arranged to be movable longitudinally in the crucible (12). A punch (32) protrudes into the molten metal (14). The spring force of a spring (34) pushes the piston (20) away from an exit opening (18) at the lower end of the print head (10). The piston (20) has a flow opening (36) which can be sealed by a lower end of the punch (32) as it moves forward towards the exit port (18). As a result of the forward movement, a droplet (30) is ejected from the outlet opening (18) by compression of the liquid metal (14). The spring (34) is designed so that during a backward movement of the punch (32) and the piston (20) of the punch (32) retracts faster than the piston (20) and thereby a flow gap (42) in the flow opening (36 ) is formed, can flow through the liquid molten metal (14) from the melting chamber (28) in a piston chamber in front of the outlet opening (18).

Description

The invention relates to 3D printers, in particular a printhead of a 3D printer for thermoplastic materials, in particular metals.

State of the art

A 3D printer for a thermoplastic material obtains a solid phase of a material as a raw material, generates a liquid phase therefrom, and selectively deposits this liquid phase at the locations associated with the object to be formed. Such a 3D printer includes a printhead in which the source material is melted. Furthermore, means for generating a relative movement between the print head and a work surface on which the object is to be formed are provided. In this case, either only the print head, only the work surface or both the print head and the work surface can be moved relative to each other.

The printhead has a first mode of operation in which liquid material exits therefrom and a second mode of operation in which no liquid material exits therefrom. The second operating state is taken, for example, in the period when a different position on the work surface or a substrate is approached and on the way there is no material to be applied. For example, it is possible to switch between the two operating states of the print head by switching on or off the propulsion of the solid starting material.

Compared to thermoplastics metals have a much higher melting point and at the same time in the liquid state a much lower viscosity. One research approach to solving the problem of adding liquid metal to the object only in defined places is pneumatic Drop-on-Demand (DoD). This technique is used, for example, in the publication of Han-song Zuo, He-jun Li, Le-juaqi, Jun Luo, Song-yi Zhong, Hai-peng Li, "Effect on wetting behavior on the basis of pneumatic drop-on-demand technique", Journal of Materials Processing Technology 214, 2566-2575 (2014) explained.

Solutions are known where a crucible for a molten metal and an associated actuator are provided. This can for example be a piezo actuator with a stamp, but solenoid valves are used. The stamp protrudes from the actuator in the molten metal in a crucible and triggers the exit of a metal drop from an outlet opening by a displacement movement of a piston.

Disclosure of the invention

Embodiments of the invention may improve a quality and reliability of a printhead as well as a quality of the generated drops. The invention described below is based on the considerations set out below. In the solutions known in the prior art, the movement of a piston arranged in the molten metal leads to a displacement of molten metal in the longitudinal direction of the crucible both in the direction of the outlet opening, but also a frequently far greater proportion in the opposite direction, ie in the direction of the actuator , The known solutions have an annular gap between the piston and a wall of a crucible, over which - after the ejection of a drop - liquid molten metal is to flow into a piston chamber between the piston and the outlet opening compensating.

Since the proportion of molten metal that flows back in the direction of the actuator is to be regarded as undesirable or as a loss, so far this annular gap is often designed so that the proportion of displaced upwards through the annular gap molten metal compared to the total by the movement of the Piston or stamp displaced volume is as small as possible.

However, it has been shown that a reduction of the annular gap leads to the fact that liquid metal melt can not flow into the piston chamber sufficiently quickly within a designated period of a few milliseconds during a return movement of the punch. This can lead to the short-term resulting negative pressure in the piston chamber air or. Gas bubbles arise in the region of the outlet opening, which can adversely reduce the compression properties of the molten metal. Furthermore, cavitation effects can occur, which are noticeable for example on an underside of the piston by gas bubbles.

To solve this problem, a printhead for a 3-D printer for metals is proposed, which has a crucible for receiving a liquid molten metal, wherein an outlet opening is located at a lower end of the crucible. A crucible may be understood to mean a suitable container or vessel that is configured to receive a liquid molten metal. For this purpose, for example, a heating element can be arranged on the crucible, which keeps the molten metal in a liquid state. The outlet opening serves to eject a drop in the direction of a working surface or in the direction of a substrate. The molten metal may for example be aluminum or have similar suitable metals. In principle, other suitable non-metallic thermoplastic materials such as plastics and the like can be used instead of the metals. By a lower end is meant the end of the printhead that faces the substrate or work surface. For the sake of better illustration, it is assumed below that the direction to the outlet opening or in the direction of the substrate is meant "downwards" and a direction to the actuator is correspondingly meant by "above".

A piston is disposed in the molten crucible in the molten metal and is further slidable in a longitudinal direction of the crucible. A width of the ring gap known in the state of the art can here be reduced to a technically necessary minimum, for example in order to keep the volume flow over the annular gap as small as possible or to compensate for thermal expansion effects. In one example, however, it is possible that an annular gap is additionally formed between the inner wall of the crucible and the piston, which is also possible in combination with the solution presented here. On top of the piston, the crucible forms a melting space. This melting space can be seen as a reservoir for liquid molten metal, which can also be subjected to an increased pressure according to one example.

The piston is preferably guided completely within the molten metal or completely surrounded by molten metal. In the area between the piston and the outlet opening, a piston chamber is formed. The piston chamber can be understood as the region of the crucible which is compressed by a stroke of the piston in the direction of the outlet opening and from which the ejected drop is discharged via the outlet opening.

A ram protrudes from an upper side of the crucible into the molten metal and is movable in the longitudinal direction of the crucible. For example, an upper portion of the punch may be located outside the crucible and in mechanical contact with the actuator. According to one example, it is also possible that the plunger is completely outside the liquid molten metal in a second operating state without ejection of a drop and immersed in the liquid molten metal for compression and initiation of the drop ejection.

A spring is arranged in the crucible so that its spring force moves the piston away from the exit opening. For example, a necessary force for further displacement of the piston in the direction of the outlet opening increases due to the compression of the spring. The piston has a continuous flow opening extending in the direction of movement. This flow opening is arranged, for example, as a circular or tubular bore in a center of a piston surface. The flow opening is preferably designed so that liquid molten metal can flow from the melting space into the piston space without great resistance. According to one example, the piston can be understood as a piston sleeve which, together with the piston, represents a function of a piston for compression.

The punch is designed in its cross-sectional shape and diameter so that it moves in a forward movement of the punch in the melting chamber in the direction of the outlet opening the piston in a forward movement in the direction of the outlet opening, thereby sealingly closes the flow opening. In other words, this can be understood as a valve which is closed by the mechanical contact of the punch with the piston. As a result, no liquid molten metal can flow back into the melting chamber, so that an improved compression effect in the piston chamber can be achieved. By this compression, a drop of the liquid metal is expelled from the exit port.

The spring is designed so that when a backward movement of the punch away from the outlet opening a speed of the backward movement of the punch is greater than a speed of the backward movement of the piston. In other words, in the case of a rapid reversal of the direction of movement of the actuator in the opposite direction, the punch retracts much faster than the piston, which follows the backward movement in a time-delayed or slower manner. Due to the relative movement of the plunger away from the piston, a flow gap is formed in the flow opening of the piston between the plunger and the piston, through which molten metal melt from the melting chamber can flow into the piston chamber. As an advantage, it can be seen here that due to the greater degrees of freedom of the dimensioning of the flow gap and the flow opening, an effectively larger cross-section and volume for the inflow of liquid molten metal from the melting space into the piston space is available than in the hitherto known solutions, in particular in the case of previous use an annular gap. This advantageously leads to the fact that no disadvantageous negative pressure conditions or other conditions arise which cause or at least favor a suction of air or the formation of gas bubbles.

According to one embodiment of the invention, the punch is moved by means of a piezo actuator. Such piezobased actuators can be particularly advantageous for the necessary fast Reversing movement of a forward movement to a backward movement of the punch are suitable. According to another example, the punch is moved by means of a magnetic actuator. According to one embodiment of the invention, the spring is configured such that upon advancement of the plunger a pressure of the plunger is created in the reverse direction for sealing the flow opening. Described differently, the plunger and the piston are moved together against the force of the spring downwards in the direction of the outlet opening. Furthermore, the arrangement described can for example be understood that the piston is pressed by the spring against a lower end of the punch and thus an increased sealing effect is achieved. In one example, such a contact pressure 10 to 20 Newton.

According to one embodiment of the invention, a mechanical stop for limiting the rearward movement of the piston is provided on the melting chamber. In other words, it should be prevented that, for example, by the action of the spring, the piston could be lifted too far out of the molten metal. In one example, this can further prevent the volume in the piston chamber from becoming too large. Also advantageously, this can result in an increased degree of freedom in the dimensioning of the spring, since, for example, must not be paid to a maximum deflection.

According to one embodiment of the invention, the spring is arranged in the piston chamber between the piston and a wall of the piston chamber. Thus, for example, the piston can be supported by the spring from a lower end of the piston chamber. According to one embodiment of the invention, the spring is designed as a plate spring or coil spring. As advantages, a comparatively simple structure and good spring action can be seen in proven simple spring designs here. According to one embodiment of the invention, the spring is designed as a tension spring and arranged between the piston and an inner wall of the crucible above the piston. In other words, here the spring is not loaded during compression but to pressure, but on train.

By the arrangement above the liquid molten metal according to another embodiment of the invention, the spring itself need not necessarily be disposed in the hot molten metal, which can increase durability and reliability. In one example, a connection between a top of the piston and a bottom of the spring may be made by using a mechanical extension, such as a pull rod. Here it would be only the pull rod, which would be arranged in the hot liquid molten metal and the spring itself, together with its extension outside the molten metal would be arranged.

In one embodiment of the invention, a pressure of the liquid molten metal in the melting chamber is increased during the backward movement of the punch relative to a pressure in the piston chamber. This may have the advantage that liquid molten metal is forced into the piston chamber at a higher speed and thus higher volume and faster through the flow gap. According to one embodiment of the invention, the pressure increase is generated at least in part by the energy of the backward movement of the punch. For this purpose, various solutions are known in the prior art, which can use the energy of the backward movement of the punch and thus to achieve, for example, a compression of the melting space. This may also have the advantage that the time period in which the punch is in a backward movement corresponds approximately to the time span in which an increase in pressure in the melting chamber would be advantageous.

In one embodiment of the invention, a backward movement of the punch is at least twice as high as a speed of the rearward movement of the piston. In other words, a more pronounced difference in speed between the backward movement of the punch and the backward movement of the piston can have the advantage that the flow gap opens comparatively early in the beginning of the backward movement and so more time can be available for flowing sufficient liquid molten metal into the piston chamber.

In a further aspect of the invention, a method for producing a metallic drop from a liquid molten metal in a printhead is proposed. In a first step, this is done by moving a stamp into a crucible with liquid molten metal. In this case, the plunger sealingly closes a flow opening of a piston in the crucible. The punch displaces the piston in the direction of an outlet opening and thus triggers the ejection of a drop of liquid metal. In a further step, the stamp is moved out of the crucible with a backward movement. At the same time a spring moves the piston with a backward movement away from the outlet opening. Here, the punch is moved at a greater speed of the backward movement than a speed of the backward movement of the piston. This creates a flow gap in the flow opening of the piston, flows through the liquid molten metal from the melting chamber into the piston chamber.

list of figures

• 1 zeigt einen Druckkopf gemäß der Erfindung in Schnittdarstellung während einer Vorwärtsbewegung eines Kolbens.
• 2 zeigt einen Druckkopf gemäß der Erfindung in Schnittdarstellung während einer Rückwärtsbewegung eines Kolbens.
• 3 zeigt einen Druckkopf gemäß der Erfindung mit einer oberhalb einer Metallschmelze angeordneten Zugfeder.
• 4 zeigt ein erfindungsgemäßes Verfahren zum Erzeugen eines metallischen Tropfens aus einer flüssigen Metallschmelze.

Other features and advantages of the present invention will become apparent to those skilled in the art from the following description of exemplary embodiments, which should not be construed as limiting the invention, and with reference to the accompanying drawings.

• 1 shows a printhead according to the invention in a sectional view during forward movement of a piston.
• 2 shows a printhead according to the invention in a sectional view during a backward movement of a piston.
• 3 shows a printhead according to the invention with a arranged above a molten metal spring.
• 4 shows a method according to the invention for producing a metallic drop from a liquid molten metal.

All figures are only schematic and not to scale. In the figures, similar or like elements are numbered with like reference numerals.

Detailed description of the figures

In 1 is a printhead according to the invention 10 with a melting pot 12 shown. The melting pot 12 serves as a container for a liquid molten metal 14 , To obtain a liquid molten metal 14 For example, from a solid starting material, in addition, a heating element 16 at the crucible 12 or in a wall of the crucible 12 to be appropriate. At a lower end of the crucible 12 is an exit opening 18 arranged. This may have a diameter of the order of 0.2-0.4 mm, for example, for an aluminum melt. A piston 20 is inside the liquid molten metal 14 arranged and in the representation shown here vertically along an inner wall 22 of the crucible 12 slidably or movably arranged. The melting pot 12 forms on a top of the piston 20 a melting space and continues to form in the area between the piston 20 and the exit opening 18 a piston chamber 26 out.

The annular gap 24 is here designed such that only one compared to the volume of an ejected drop 30 or Ligaments a lowest possible volume flow as backflow between the piston chamber 26 and the melting room 28 flows. A stamp 32 protrudes in the longitudinal direction of the crucible 12 from a top of the crucible 12 into the liquid molten metal 14 into it. An actor 40 is here outside the crucible 12 arranged and executed the stamp 32 to move. In one example, the actuator is 40 a piezo actuator. Typically, the generated cyclic movements of the stamp are 32 through the actor 40 in the range of about 100 - 2000 Hz and a stroke of the punch is for example in the range of about 3-10 microns.

In the piston chamber 26 are two springs 34 arranged, which are designed in the example shown here as coil springs. In the piston 20 here is a flow opening in the middle 36 arranged without stamp 32 for the molten metal 14 a connection between the melting chamber 28 and the piston chamber 26 forms. Through the continuous flow opening 36 can the piston 20 be sleeve-like or cylindrical. Their diameter and shape are designed such that preferably a sufficiently large cross-sectional area for a rapid flow of liquid molten metal 14 is available. The Stamp 32 is at its lower end 38 in its shape and diameter designed so that it moves forward in the direction of the outlet opening 18 the flow opening 36 mechanically contacted and sealed at the same time sealing. This will be at a forward movement of the punch 38 at the same time the piston 20 in a forward direction toward the exit port 18 postponed. The Stamp 32 closes the flow opening 36 by its shape and by a contact force resulting from the forward movement of the punch 32 and a spring force of the springs 34 , This leads to a compression of the liquid molten metal 14 in the piston chamber 26 , This will be a drop 30 the molten metal melt 14 from the outlet 18 pushed out. The stamp moves 32 together with the piston 20 against one of the springs 34 caused spring force and compresses the springs 34 ,

In 2 is a printhead 10 according to the invention in a sectional view during a backward movement of the punch 32 and the piston 20 shown. The elements shown correspond in principle to those in 1 with the difference that here is a movement of the stamp 32 and the piston 34 towards a top of the crucible 12 or in the reverse direction is shown. This moves the stamp 32 at a higher or higher speed of its backward movement from the exit port 18 away as a speed of backward movement of the piston 20 , In other words, the springs are pressing 34 the piston 20 back up in a reverse direction. The backward movement of the stamp 32 is from the actor 40 generated. This means that the actor 40 preferably a quick reversal of the direction of movement of the punch 32 should run to a higher speed of movement in the reverse direction than the movement of the piston 20 to reach.

By moving the stamp faster 32 creates a flow gap 42 (shown here as a dashed line area) between the lower end 38 of the stamp 32 and the flow opening 36 , through the liquid molten metal 14 from the melting room 28 in the piston chamber 26 can flow into it. An advantage can be seen here that with a corresponding configuration of the flow gap 42 a comparatively large spatial area is created by the liquid molten metal 14 effectively and as quickly as possible can get into the piston chamber and in this way a faster compensation of lost by the ejection of the drop liquid molten metal 14 can be reached. In one example, an ejected drop may be about a diameter double the diameter of the exit orifice 18 respectively.

An attack 44 standing on the inner wall 22 of the crucible 12 is arranged, limits the backward movement of the piston 42 in the direction of the actuator 40 , This can be advantageously prevented that the piston 20 at still acting spring force of the springs 34 is moved too far in the reverse direction and optionally from the molten metal 14 is moved out. The feathers 34 are designed here as coil springs. In one example, the springs are 34 designed as disc springs, with both a single plate spring and a plurality of disc springs are possible. Due to the resulting flow gap 42 can also gas bubbles, for example, by the suction of air through the outlet opening 18 or caused by cavitation, escape upwards.

In 3 is an embodiment of a printhead according to the invention 10 shown, in which the elements shown in principle in their function and arrangement the elements in the 1 and 2 correspond. In particular, here is a modified arrangement of the springs 34 shown between the piston 20 and the inner wall 22 of the crucible 14 above the piston 20 is arranged. There is a relevant for the contraction and expansion area of the spring 34 outside the liquid molten metal 14 , This has the advantage that the spring 34 at least in essential functional areas no longer in the hot molten metal 14 and thereby a reliability and life of the spring 34 can be increased. To provide a mechanical connection to the molten metal in the molten metal 14 located piston 20 can reach, for example, an extension element 46 be used.

In 4 is a method of producing a metallic drop 30 from a liquid molten metal 14 in a printhead according to the invention 10 shown. A first step involves moving in 110 a stamp 32 in a crucible 12 with liquid molten metal 14 , where the stamp 32 a flow opening 36 a piston 20 in the crucible 12 sealingly closes and the piston 20 in the direction of an outlet opening 18 shifts. This will cause the ejection of a drop 30 liquid metal 14 triggered. In a following step, a move out 120 of the stamp 32 from the crucible 12 with a backward motion, being a spring 32 at the same time the piston 20 with a backward movement from the exit opening 18 moved away. The stamp moves 32 with a greater speed of backward motion than a speed of backward movement of the piston 20 , This results in a flow gap 42 in the flow opening 36 of the piston 20 , through the liquid molten metal 14 from a melting room 28 above the piston 20 in a piston chamber 26 flows.

Finally, it should be noted that the terms "comprising", "having", etc., do not exclude the presence of other elements. The term "a" does not exclude the presence of a plurality of objects. The reference numerals in the claims are merely for ease of reading and are not intended to limit the scope of the claims in any way.

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 non-patent literature

• Han-song Zuo, He-jun Li, Le-juaqi, Jun Luo, Song-yi Zhong, Hai-peng Li, "Effect on wetting behavior on the basis of pneumatic drop-on-demand technique", Journal of Materials Processing Technology 214, 2566-2575 (2014) [0004]

Claims (12)

A printing head (10) for a 3D printer for metals, comprising - a crucible (12) for receiving a liquid molten metal (14) having an exit port (18) located at a lower end of the crucible (12); - A piston (20) which is arranged in the crucible in the molten metal (14) and further in a longitudinal direction of the crucible (12) is designed to be displaceable; wherein the crucible (12) forms a melting space (28) on an upper side of the piston (20) and forms a piston space (26) in the area between the piston (20) and the outlet opening (18); - A punch (32) which projects in the longitudinal direction of the crucible (12) movable from an upper side of the crucible (12) into the molten metal (14); - A spring (34) which is arranged in the crucible (12) and whose spring force moves the piston (20) from the outlet opening (18); characterized in that the piston (20) has a continuous flow opening (36) extending in the direction of movement; wherein the punch (32) is configured in its cross-sectional shape and diameter so that upon a forward movement of the punch (32) into the melting space (28) in the direction of the outlet opening (18) the piston (20) in a forward movement in the direction of the outlet opening (18) displaces while sealingly closes the flow opening (36), so that a drop (30) of the liquid metal (14) is ejected from the outlet opening (18) by a consequent compression in the piston chamber (26); wherein the spring (34) is configured such that upon a backward movement of the punch (32) away from the exit port (18), a speed of the backward movement of the punch (32) is greater than a speed of the rearward movement of the piston (20), such that the through-flow opening (36) of the piston (20) forms a flow gap (42) between the ram (32) and the piston (20) through which liquid molten metal (14) flows from the melting chamber (28) into the piston chamber (26). Printhead (10) according to Claim 1 , wherein the punch (32) by means of a piezo-actuator (40) is moved. A printhead (10) as claimed in any one of the preceding claims, wherein the spring (34) is configured so that upon advancement of the plunger (32) a pressure of the plunger (20) is created in the reverse direction for sealing the flow opening (36). Printhead (10) according to one of the preceding claims, wherein a mechanical stop (44) for limiting the backward movement of the piston (20) is provided on the melting chamber (28). Printhead (10) according to one of the preceding claims, wherein the spring (34) in the piston chamber (26) between the piston (20) and a wall of the piston chamber (26) is arranged. Printhead (10) according to one of the preceding claims, wherein the spring (34) is designed as a plate spring or coil spring. Printhead (10) according to one of the preceding claims, wherein the spring (34) is designed as a tension spring and between the piston (20) and an inner wall (22) of the crucible (12) above the piston (20) is arranged. Printhead (10) according to Claim 6 , Wherein at least a portion of the spring (34) in the outside of the molten metal melt (14) is arranged. A printhead (10) according to any one of the preceding claims, wherein a pressure of the liquid molten metal (14) in the melt space (28) is increased during the backward movement against a pressure in the piston space (26). Printhead (10) according to Claim 9 wherein the pressure increase is generated at least in part by the energy of the backward movement of the punch (32). A printhead (10) according to any one of the preceding claims, wherein a backward movement of the plunger (32) is at least twice as high as a speed of the backward movement of the plunger (20). Method (100) for producing a metallic drop (30) from a liquid molten metal (14) in a printhead, comprising the steps of: - moving (110) a punch (32) into a melting pot (12) with liquid molten metal (14) ; wherein the plunger (32) sealingly closes a flow opening (36) of a piston (20) in the crucible (12) and displaces the piston (20) in the direction of an outlet opening (18) and thereby ejecting a drop (30) of liquid metal (14 ) triggered; - Moving (120) of the punch (32) from the crucible (12) with a backward movement, wherein a spring (34) simultaneously moves the piston (20) with a backward movement of the outlet opening (18); wherein the punch (32) is moved at a greater speed of the backward movement than the speed of the backward movement of the Piston (20), so that a flow gap (42) in the flow opening (36) of the piston (20) is formed, flows through the liquid molten metal (14) from the melting chamber (28) in the piston chamber (26).

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