Open Source Prior art for low temperature metal extrusion
Use case - induction heated cast metal casting
Concept: use a slow controlled trickle of molten, low temperature metal such as Zinc Aluminum Alloys (ZA)to pour into plaster of paris casting forms made by melted out 3D printed forms. Allows for automated casting of part using a 3D printing extrusion type of approach
3D Printer prints the plastic casting patterns for making gypsum molds
3D printer has a high temperature heater block, possibly induction based, which melts the metal.
The trick here is to use pressure assist.
In plastic 3D printing, the plastic pushes molten plastic through the extruder.
In metal-melt 3D printing (non-welding-based), metal melts in a heated chamber, and because it is so liquid, it will just flow out without needing to be pushed.
A possibe way to deposit a 3D print would be to dribble droplets or a small stream.
A small precise stream could be used to fill small cavities in casting applications using 3D printer-generated molds.
Droplets could be used for additive manufacturing, but the substrate would probably have to be kept at high temperature. This is where precise induction heating could be useful, or where small lasers (4W) could be used to assist in fusion using hot substrates. This would enable low energy laser-assisted fusion
The question is: what is the behavior of metal droplets in 3D printing?

Pressure Assist Design

Air tight metal chamber with rubber seal for feedstock entry.
Pressurized: pressure hose inlet and valve.
Aperture is the nozzle exit
Elongated rubber seal fits tight against feed wire and makes an air seal.
Extruder drive is sufficiently strong to push ZA or other metal wire through seal.
Induction heater melts metal inside nozzle. Metal drips out.
Pressure regulation assists in rate of metal drip.
Drip control occurs one way by extruder feed motor. Wire feed stops - and metal stops flowing. This assumes that the induction heat is rapid for melting the filament instantly, almost flash melting it.
May require pressure gas feed to be shielding gas if oxidation is an issue
Another way to control deposition rate may be turning off the induction heat to stop the melt.
All together, induction heat control, pressure control, and feed wire rate combine for overall exgtrusion rate control.

Links

Method of magnetohydrodynamic droplet ejection at khz-scale droplet frequency is already known - [1]
Patent on MHD metal deposition - [2]
Recent patent on induction-heated metal deposition - [3]
Induction heated plastic - on RepRap - [4]
Induction for metal concept on Google Groups - from [5]
Inductance-based temperature control - [6]
Nophead 3D printing with solder to make conductive traces - [7]
UNSW paper - induction melting of solder - W. S. Tan, University of New South Wales at the Australian Defence Force Academy. The author wishes to thank his supervisor, Dr Sean O’Byrne and Dr Murat Tahtali for their ongoing research assistance, enthusiasm and supply of equipments. Acknowledgements are also made to Darryl, Ban, Joe Kurtz, Douglas, Tom Thomson and David Sharp for their technical assistance of different hardware.

- [8]