Metal deposition print head
This page is the nucleus on this wiki for development of a metal depositing print head to go along with an XYZ Cartesian robot or robot arm.
Machines are the main physical tools that allow material wealth creation. Good machines require precisely shaped metal parts, made to dimensional tolerances in the 10-30 micron range (roughly).
The MetalicaRap is moving forwards but there are many roads to glory :). Another particularly interesting and promising method is to form the metal object roughly and slightly oversized (near net shape using a solid freeform fabrication method, and then machine the surfaces of the part that must be precisely shaped using a traditional subtractive method like EDM or milling, either after or during freeform fabrication.
While there do not seem to be any precision cnc milling or EDM machines, the know how to make them is widely available, so a serviceably precise Open EDM or milling machine can certainly be made. Development of a suitable open freeform fabrication method suitable will enable the afor mentioned type of printer.
Contents
Existing freeform fabrication processes
Proposed approaches
Wire and induction heating
Wire as a feedstock is a handy way to go, easy to transport and, for the prototyping process easier to buy and handle. Under atmospheric argon.
Primer for uninitiated: Remember current is induced in such a way that it produces a field that counteracts the changes in the primary magnetic field. In the diagram of magnetic field lines, with the higher density of magnetic field lines indicates the areas that are heated more. The higher the frequency, the less the heating tends to penetrate the surface of the inductively heated metal.
Induction heating could either use a plain, rod shaped air core solenoid or non conductive ferrite core magnet, with one pole of the magnet close to the metal surface, and modulated at relatively high frequency (10s of khz) to achieve an annular shaped heating area on the surface of a metal object (the melt pool).
Another way which could give a more uniform, circular heated area could be to have the magnetic field enter the surface over a small area and exit over a large area. Heating occurs at a useful intensity at the small pole.
Resistive heating
Arc
Try to basically scale down and make open one of the freeform fabrication ones methods that use wire under argon, or another arc welding method. Remember precision is not critical since there will be a machining step later.
=electron beam
Electron beam welding, basically. See documents at the main page on existing processes; nasa simply adapted a purchased electron beam welding system to use as a freeform fabricator.
The main problem is that their system used a vacuum of 5x 10-5 torr. While making a chamber for that level is doable, everything inside has to be compatible with that vacuum level too. Sensors, electronics, motors, lubricants, all become harder to integrate with the system inside the chamber when desirable, as buying off the shelf units specially made for high vacuum is probably very expensive. Could try to make our own but it's ultimately a real hurdle however you work it.
However strictly speaking only the electron gun itself needs to be under a high vacuum. There are many electron beam welding rigs that use low vacuum chambers and under normal atmospheric pressure (would still have to be argon though). Either a plasma window or even just a suitable arrangement of powerful pumps and narrow constrictions filled with gas is enough to keep the inside of the gun a vacuum while getting the electron beam out. A thin material though which the beam passes would probably not work due to overheating of it, or scattering or excess absorption of the beam? What if water cooled or it were moved (or the beam redirected) rapidly so the beam only passes through a given section very briefly? The thin material window can move relative to the electron gun easily; there are plastics and other materials which are sufficiently flexible and gas impermeable that they could be used to accommodate the motion while maintaining a seal. Or vacuum oil could be used with a metal to metal joint for a good seal.
Since we don't need a particularly well focused electron beam more research is needed on why NASA chose to use high vacuum in the entire chamber - money was no object maybe and they just didn't want to bother with plasma windows?
We could use the same gun as the MetalicaRap or a smaller one.
Ultimately the main benefit of electron beam for our purposes is probably only precision control of the energy into the melt pool? What else? The SMD and PTA FFF methods supposedly get full strength so it might not have much advantage but be more complex and expensive to prototype.
Summary of comparison between different approaches
I think we should steer clear of the high vacuum approaches anyway for this particular project, because of the cost and difficulty of prototyping and working with high vacuum. GreenAtol 00:00, 14 August 2011 (UTC)
Argon supply
As mentioned in the reprap forum, Argon can be extracted from the air at a small scale economically and with relatively non expensive or complex machinery. So the cost or difficulty of obtaining Argon as a consumable is probably not a major issue ultimately, just the electricity cost. The documentation on the commercial FFF processes indicates high purity argon is desirable, (99.99%) and that 10 volumes of gas exchange in the chamber is typically needed to achieve good metallurgy. But even high purity Argon does not look too hard to make at a small scale. Seems unlikely that even if they start with 99.99% argon that the purity actually in the chamber after 10 exchanges will be near that, maybe though. Successive division operations can add (multiply) up pretty fast. Laminar flow rather than dilution flow in the chamber must be employed most likely.
Approaches include, may need to be combined or repeated for high purity:
- Pressure swing adsorption.
- Membrane separation (maybe with reverse osmosis membranes?)
- Fractionation of air.
