Support structures for metal deposition
Support structures may be useful for a metal deposition print head.
For the uninitiated: When you print from the bottom up, layer by layer starting from the bottom, suppose you are printing a coat hanger, vertically, with the hook a the top. That is, viewing it in profile. The tip of the hook needs to be printed before it is connected in any way to the rest of the coat hanger. So it would need, in any case to be supported against gravity in the meantime. This bit may be called a "leader" before it is connected to the main object. Or you may be printing a large number of small unconnected objects... See fixturing and grounding sections too.
Patents reveal a lot of interesting information about possible materials and methods for support structures. Water soluble ceramics, structures that can be disintegrated with ultrasonic vibrations, high strength wax, and there is probably a lot more gold to be found.
If needed, more than one support structure could be used, each with their own strengths. So if a material was really good for attaching the metal to the base of the print chamber for instance for fixturing, but really slow to deposit.
Support structures perform the following functions/should have the following properties for the printers we are talking about here:
- brace long thin vertical shafts and walls and parts that are not otherwise braced against the base of the print chamber (like the tip of the coat hanger hook before connected to the rest of the hanger, a leader) against the milling forces. This sort of entails being able to stick to the metal reasonably well and also be reasonably strong.
- damp vibrations from the mill in long thin shafts and walls and floating points, again needs strength and stick
- if it's a stout object with no overhangs and is not a leader it doesn't really need any bracing or support material since it can stand up by itself and would be affixed to the base of the print chamber. Those objects may still be useful so as a step of the dev process, printing these might make a good intermediate goal.
- Act as a sort of in situ cast:
- The subtractive machine tool will probably have a hard time in most cases machining the bottom surface of any perfectly horizontal overhangs of any significant length (a mill bit of the right shape or multi axis milling machine could help in many situations but there will still be a lot of geometries where it wouldn't be able to reach under there). In that case the support can be deposited, milled to the desired shape, and then the molten metal deposited on top. The precision would never be as good as milling the metal itself but it's better than nothing. Obviously this requires that it be heat resistant enough.
- When printing a leader, the bottom of the leader would similarly be hard to machine, and could benefit from this.
- Ultimately you still need to be able to machine under ledges at least a bit or you could only machine the top facing surface of any object which is an unacceptably severe limit on printable geometry. Suitable milling bits will probably suffice, or a "bit" with a small 90 degree gear built in, even, or the milling drive motor could be a water turbine or other quite small power plant (so it can reach in places reasonably), and the mill be a 5 axis mill.
A good support structure strategy, in this case, would also entail:
- structures are easy to remove, preferably completely and not damage the surface. Metals are usually soluble in other metals. So if the support material was another metal like a low melting point alloy, it might damage the precisely machined surface anywhere the molten material touches, either when it is being deposited or removed, which is probably not acceptable. Maybe some alloys to choose from that do not have this problem with certain metals though.
- In many cases it will have to disintegrate to be removed.
- structures are manageable cost. Support Material may be recycled in many cases probably.
- main problem is desirable material combined with practical deposition and removal.
Add examples of the water soluble ceramics, low melting alloys, high strength wax, also some sort of fast set gypsum (crystal structure absorbs moisture into it, dissolve to remove? Even if it takes some time to set that may be okay slows things down a bit but they are already pretty slow), materials like concrete which are sort of like epoxy (hardening occurs due to chemical reaction), the roughened stainless steel foil material pressed together one, salt or another material deposited in a solvent and the solvent evaporated, ultrasonic powder consolidation could work (powder plus pressure plus ultrasound), high speed droplets or bits of metal or deformable material fired at the surface (that droplet spray method sometimes used for making liners on engine cylinders?)(coudl be released with ultrasonic shaking maybe), maybe could be electrolytically deposited but probably pretty slow, vaporized and then deposits on there. Anything that liquiefies with cold rather than heat?
Thixotropic materials. Materials which sublimate at low atmospheric pressures? (could pressurize the build chamber too).
Fundamentally it is of course printing all over again, but the precision can be very low and it only has to work with one material with the right properties (although more might be useful of course especially if no perfect material which can also be easily printed with and removed can be found though here's hoping).
Grounding and supports
Unless the support material is a metal, or perhaps a metal trace from the main object is made out to any leaders may be problems grounding the leaders. The deposition method could maybe be designed so that the connection to ground is made right close to the melt pool, so if a GTAW like process were used there might be 2 arcs rather than 1, hitting the workpiece half millimeter apart and with current going down one and up the other. Or maybe better yet the wire could be the ground (the molten metal might be hard to maintain in continuity with the wire though). Induction could avoid this problem maybe.
Also obviously something needs to be figured out even for the main, non leader part, but just a wire sticking up from the bottom of the plate, to which the first bit of metal is welded should be fine.
Approaches that do not require grounding or have the current collector very close to the heated area:
- Non-transfer plasma arc welding - basically super hot gas directed at the workpiece. Gas is heated by the arc, arc does not extend to the workpiece so it does not need grounding.
- Other methods using suitable gas flow patterns could be used to get heating only on small spots where desired e.g. have the gas exit from a small tube which is concentric with another tube which vacuums the gas and plenty of nearby atmospheric gas, back up, or even direct gas in a sort of cone shape towards the same area that the hot gas is being directed, keeping nearby areas exposed only to colder gas? The colder gas acting as a sort of wedge to get the hot gas away from the surface as soon as it has served it's purpose. Relative gas velocities probably have to be appropriate, but the surface of the molten pool would be concave so that might help direct the gas away from the surface again. One issue might be splashing or vaporization of metal from the surface of the melt pool. Might also be possible to heat the gas stream with a tungsten filament?
Conceivably the entire process could be be done at a fraction of atmospheric pressure, so that instead of arcs you can get stable gas discharges. That may allow reduced electrode wear in the case of non-transfer plasma welding, or it could allow the two discharges to be more easily directed to the workpiece without difficult to manage instabilities?
- Chemically heated gas like oxyhydrogen welding - not as hot and probably not as precise as plasma, plus complications of hydrogen or oxygen or other stuff contaminating workpiece.
- Heat the wire not the melt pool directly - need more info on welding, most likely problem is that not enough energy can be transferred (heat capacity of metal is too low)
- Solid state welding like ultrasonic, EM pulse and explosive
- electron beam welding uses only very low currents so it would be relatively easy to ground the object with a small metal probe that touches the metal, which probably doesn't work really with arc welding because the currents are so high. So high pressure ebeam then.