Gearhead_Shem_Tov
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RepRap, Lawrence Welk, and Bubbles, oh my!
Here's a blue sky problem I'm working on: a purely additive fabrication process in the spirit of the Open Source RepRap 3D printer movement, one that
might come closer to fabricating all the components it needs to make a replica of itself with fewer "vitamin" inputs.
In the RepRap world "vitamins" refer to any components other than the feedstock material used to make 3D prints. For current RepRap machines these
include precise steel rods, threaded rods, nuts & bolts, stepper motors, sensors, and all control and power electronics. The ultimate goal of a
RepRap system is zero vitamin input, but all the current working systems are a long, long way away from this goal.
My idea involves using a 6-degree-of-freedom (6-DOF) "printer" head that would produce precisely placed and formed bubbles of some material that can
be made to selectively adhere and harden in contact with previously deposited bubbles. If the bubbles are small enough and deposited in the right
patterns it should be possible to print surfaces which can approach any arbitrary contour, perhaps even smooth enough to dispense with machined rods
and screws.
The first problem, though, is to come up with an inexpensive material with which to make the bubbles. No doubt any number of exotic organic polymers
can be found, along with numerous methods for their selectively adhering and hardening. The sorts of resins used for high-end stereolithography
probably could be made to work with laser or UV curing, but those resins are quite expensive.
The goal is a cheap, versatile material, and ideally, for reasons I'll get to shortly, one that can withstand higher temperatures than are typical for
organic polymers. My first candidate is a sodium silicate solution where the hardening agents are heat and carbon dioxide. The CO2 would be
used to blow the bubble to start the hardening from the inside surface, with a timed application of heated CO2 "puffs" applied externally
to finish the job. It might be possible to do a limited amount of "sculpting" of each bubble with the CO2 puffs such that a smoother
surface results.
So, now for the ideas I'm much less sure of. To make this sodium silicate foam useful, it would be nice to be able next to selectively plate
(electroless or electrolytic) various metals on them. I'm thinking copper or copper alloys for making conductors or bearing surfaces, and nickel or
iron or Ni/Fe alloys to make high-strength surfaces that can also have useful magnetic properties. If this can be accomplished then, in principle, you
should be able to print any electromechanical component desired.
For instance, if among the feedstocks were nickel and chromium salts -- don't ask me which -- then you could deposit a nichrome heating element. The
sodium silicate could withstand the heat (I'm hoping). It could also act as an electrical insulator, so use a different feedstock solution and you lay
down the conductive tracks of a motor coil.
Here is where my ignorance will likely show the most, but here goes: I'm supposing a first electroless pass would be needed, followed by electrolytic
passes to build up thickness. I'm further assuming that the electrolytic plating cell would be a small, continuously-fed "plating bead" held in place
by surface tension between the plating head and the region to be plated -- no container, no large plating bath. The bead would be dragged along as
plating progressed, perhaps encouraged along by puffs of air or CO2.
Those are my basic ideas. Now the more knowledgable folks out there can point out all the flaws, gotchas, and implausibilities implicit in all this. I
welcome the education, painful though it might be. 
Also, to make sure I don't take the project -- or myself -- too deadly seriously, I'm calling it the "Lawrence Welk" RepRap. Or maybe just the
"BubbleRap"...
-Bobby
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12AX7
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A spray or sinter technique might be good to construct things like motor windings. There ought to be a ceramic or glass that sinters at about the
same temperature as copper, and has the same thermal expansion rate. Windings would then be laid down in layers of powders, glued together
temporarily with a suitable organic binder. Sintering would require an inert atmosphere (which for copper, is quite a range; H2O (steam), CO2, N2, or
anything reducing, i.e., containing CO or H2, or any combination of these), but it shouldn't require pressing or molding.
Selective plating can't really be done, or at least, isn't practical as far as I know. Just about everything relies on either masking off areas that
aren't to be plated, or plating the whole thing and masking and etching away the unwanted material. Either way, the resulting pattern is subtractive,
not only additive. It's probably easier to deposit powders, or weld strips on.
Sodium silicate as a binder, I'd be concerned about reaction rate. Does it set up fast enough for printing? Certainly not in bulk (that can take
forever), but you're talking bubbles, only the surface matters -- how fast does it form a skin? Microns per second might be useful enough to blow
semirigid bubbles. And stacking those could certainly make some interesting structures, long as you can stick them together.
Are you familiar with perlite? Suppose you had a nozzle that delivers sodium silicate solution, and it has a little CO2 bubble generator sideways in it. Every time a
bubble is blown, it becomes semirigid inside (hopefully!). The outside remains relatively stickly. This is stuck down on a substrate, building a
lattice of bubbles of controlled size and position. Maybe an elevated CO2 concentration around the construction would be used to encourage the
network to congeal (but without impairing the ability to stick bubbles together). In this manner, you could basically synthesize your own perlite,
with mili-scopic structure, open or closed cell, isotropic or otherwise. That'd be pretty cool.
Does anyone know if sodium can be leached out of hardened water glass? I would guess so, but if the remaining structure is silica gel or something
like that, it might not be too happy to, say, throw in the bathtub for a rinse. If a solid gel lattice can be produced, it would be very easy to
finally sinter the structure into fused silica -- talk about dimensional stability and lightness!
Tim
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Gearhead_Shem_Tov
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Most of what I "know" about sodium silicate is from university, a manufacturing processes class I took in engineering. We used the stuff as a binder
for making hollow cores for sand casting metal. You'd mix up a slurry of sand and sodium silicate, dump it in a properly shaped metal cavity mould,
heat the mould, gas the surface with CO2, then dump out the loose sand from the middle.
It would leave behind a skin of bonded sand that was strong enough to withstand the rigours of handling, placement in a green sand mould, and the
pressure and temperature shock of the molten metal. As the metal solidified and shrank the hollow cores would then collapse, preventing shrinkage
tears in the metal. Small holes allowed you to shake out the remnants of the core. It impressed me how effective a binder sodium silicate could be.
As far as selective plating, I'm hoping that a liquid bead could be kept at the right temperature, pH, and metal salt concentration with continuous
feeding of the electrolyte and plating electrode. I visualise it would be something like a tiny MIG welder-type wire feed, but instead of an electric
arc the current flows through an electrolyte. The electrolyte is a small bead/droplet and thus can only carry the metal cations where electrolyte wets
the surface of the work. That's where the selectivity comes from. How small the bead could be (and consequently how fine a feature you could plate)
would be dictated by current density and temperature rise of the electrolyte and evaporation rates.
Maybe a better way to think of it would be as similar to the process of brush electroplating.
-Bobby
[Edited on 25-8-2013 by Gearhead_Shem_Tov]
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Gearhead_Shem_Tov
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I came across an interesting paper from the '60s that talks about applications of sodium silicate foams as a structural material. One fascinating
wrinkle it mentions is using electrolytic means to produce the foam.
http://contrails.iit.edu/DigitalCollection/1965/AFAPLTR65-10...
-Bobby
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Gearhead_Shem_Tov
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Here's a handy summary resource of all things sodium silicate, the OxyChem Sodium Silicates Handbook:
https://final-test.oxy.com/OurBusinesses/Chemicals/Products/...
Good reading. It's got lots of charts and tables including information on viscosity variation with temperature, which would be important to consider
in making sodium silicate bubble structures. It also talks a bit about application to catalysts using its gelling properties.
-Bobby
[Edited on 28-8-2013 by Gearhead_Shem_Tov]
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Texium
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Thread Moved
21-11-2023 at 13:37 |
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