Inexpensive Ceramic Shell: Aluminum Casting with Drywall Joint Compound

We’ve been aluminum casting at the Milwaukee Makerspace since November, and I have cast several things since then.  For simplicity, we started by using a lost foam casting method, wherein the form to be cast is fabricated in Owens Corning Foamular 150 (Styrofoam), and is then tightly packed in a reusable, oil bonded sand called petrobond.  The molten aluminum is poured directly on the styrofoam, vaporizing it.  Because the mold is made of sand, the surface texture on the cast aluminum part has the “resolution” of the grain size of the sand.

Ceramic shell is another technique often used in art casting.  The positive of the form to be cast in metal is first created in wax, which is then dipped repeatedly in a silica slurry, that slowly builds up to the desired ½” thickness.  The surface detail reproducible is much smaller/better, as the silica has a much finer “grain size.”  The piece is then put in a kiln to burn out the wax and harden the silica, thereby forming an empty mold.  Typically the mold is cooled, inspected for leaks, patched, and then is buried in regular sand.  Note that to avoid fracturing the mold, it must be heated before pouring.  With all these steps, this process is relatively time consuming and is also somewhat expensive.

Recently, I read a blog post about a quick and low cost ceramic shell alternative that substitutes one or two coats of watered down “Hamiltons White Line Drywall Texture mix” for the tedious ceramic shell process outlined above.  While I couldn’t find that exact product, 4.5 gallon buckets of Sheetrock brand lightweight drywall joint compound (DJC) are omnipresent.  Note that some bags of quick setting drywall joint compound are actually just plaster, and cannot be substituted. I first assembled all the parts needed to make a quick test of the process.  I decided to make some aluminum packing peanuts:

Aluminum_Peanuts_parts

I hot glued the pyramid shaped sprues to the round cup and to the peanuts themselves:

Aluminum_Peanuts_together

I removed half of the 43 Lbs of DJC from the bucket, and poured in 10 lbs of water, taking care to mix it thoroughly with a spiral paint mixer connected to a drill.  Then, I just dipped the whole styrofoam assembly into the bucket, let it dry overnight, and dipped it in a second time.  Immediately after the first dip, I took care to brush the surface of any especially undercut areas, to prevent air bubbles from sticking to the surface.  In the future, I may consider pulling a vacuum on the bucket of DJC to de-gas it.  This may help prevent the formation of air bubbles on the surface of the styrofoam parts.  In addition, I could have first dipped the assembly in surfactant. After two dips, the 1/8” thick shell on the assembly looked like this:

Aluminum_Peanuts_coated

It was a week before the next aluminum pour at the Makerspace, during which time I poured a half ounce of acetone into the mold to dissolve the polystyrene packing peanuts and styrofoam, producing an empty mold.  This step is only necessary when casting packing peanuts, as their polystyrene tends to rapidly expand out of the mold and catch fire, while the pink styrofoam (also polystyrene) is made for homes, and so is much better behaved.  I buried the now-empty DJC mold in ordinary sand, and Matt W fired up the Bret’s furnace, melted a #16 crucible of aluminum, and poured it (Thanks guys!).  After fifteen minutes, I pulled the mold out of the sand, and found the DJC was a little darker.  The act of pulling the mold out of the sand an leaving it to cool over night left it somewhat cracked:

Aluminum_Peanuts_cast

The DJC crumbled off so easily that I didn’t even need a brush.  Also, I noticed that there is more yellowish surface tarnish on pieces left in the DJC to fully cool.  I recommend removing the DJC immediately after the aluminum solidifies.

Aluminum_Peanuts_cleaned

After making a few more, I’m almost ready to safely pack valuables, such as my “Marquis, by Waterford” crystal stemware:

Aluminum_Peanuts_in_use

Finally, check out the phenomenal surface detail that this process can reproduce.  For scale, this peanut is 1.5” long.  The surface texture on the front face is about ~0.002”!

Aluminum_Peanut_closeup

Thanks to Jason G for this last photo.  Also, a big thanks to Dave from buildyouridea.com for letting me know that one or two dips will do it!

FEAR

 

I’ve updated Robert Indiana’s iconic sculpture “LOVE” for our times!  While “Love” may have been an appropriate sentiment from 1964 to 1970 when the 2D and 3D versions were made, I think that the revised text is more appropriate for the 2000’s and 2010’s. Fear is 8” tall and 4” deep, and while not a monumental outdoor sculpture, FEAR appears fairly sizable on a table top.

Fear, which is solid aluminum and weighs over 7 lbs, was cast last Thursday with quite a few other pieces.  The great thing about having an aluminum foundry at the Makerspace is that the whole thing cost about $7!  – $4 for propane, $1 for Styrofoam, and $3 for some Rotozip bits.  If FEAR were cast in bronze, it would weigh over 20 lbs, which would cost $200 for the metal alone.  As it is, we melted down old heat sinks, stock cutoffs and hard drive frames, so the metal is essentially free.

In the spirit of Indiana who made his own font, I drew FEAR up in Inkscape using Georgia Bold, but I increased the height of the Serifs a bit.  Shane helped me with the file manipulation and G-code generation (Thanks!), so I could use the CNC router to cut FEAR out of styrofoam.  I exported FEAR’s hairline thickness outline as .dxf so it I could bring it into CamBam to generate the G-code. The outer contour of FEAR was selected, and the following settings were chosen:

  • General -> Enabled -> True
  • General -> Name -> Outside
  • Cutting Depth -> Clearance Plane -> 0.125 (inches)
  • Cutting Depth -> Depth Increment -> 1.05 (inches)
  • Cutting Depth -> Target Depth -> -1.05 (inches)
  • Feedrates -> Cut Feedrate -> 300 (inches per second)
  • Options -> Roughing/Finishing -> Finishing
  • Tool -> Tool Diameter -> 0.125 (inches)
  • Tool -> Tool Profile -> End Mill

Identical settings were chosen for the inner contours of FEAR, with the exception of General -> Name -> Inside.   Then, I just selected “Generate G-code.”  Check out the real-time video of Makerspace CNC router running the G-code and cutting out the 1” thick Styrofoam (Owens Corning Foamular 150).

After cutting four 1” thick pieces, they were stacked and glued together.  I buried the foam FEAR in petrobond, and then attached Styrofoam sprues and vents.  For a more complete explanation of the quick lost-styrofoam casting process, check out this post.   Stay tuned for details of our next Aluminum pour, which will be in January in the New Milwaukee Makerspace!

 

Makerspace Aluminum Casting Foundry

I arrived at the Makerspace on Thursday without an idea of what I would cast in metal, and in less than two hours I was removing my piece from the steaming petrobond! Check out the fruit of two hours of labor cast in metal!

That’s right! The Milwaukee Makerspace had its first (and second) aluminum pour on Thursday! Thanks to the hard work of several members, the Makerspace now has a fully functional aluminum casting foundry.  The custom built propane and diesel powered furnace melted an entire #16 crucible of aluminum in less than 20 minutes.  Check out Brant’s video to see our fearless foundry foreman leading the two pours!

To get the foundry running quickly, we’ve started out by using a lost-styrofoam casting method.  That is, styrofoam is carved into the desired shape and then a sprue and vents are attached with hot glue(!).  This assembly is placed in a wooden form, and is surrounded by tightly packed petrobond, an oil bonded, reusable sand.   Then, the molten aluminum is poured directly onto the styrofoam sprue.  The styrofoam is instantly vaporized by the 1250 degree Fahrenheit aluminum, which fills the void in the petrobond formerly occupied by the styrofoam. The air and perhaps even some of the styrofoam residue escapes from the mold through the vents.  We’ll be phasing in bonded sand and lost wax casting soon, so stay tuned for those details.

Eventually we’ll be having aluminum casting classes; however, we’re definitely going to be having aluminum pours on alternate Thursday evenings for the next few months.  Join our mailing list / google group to get more details.  Metal pours are spectacular to watch, so feel free to stop by to see the action around 7 or 8 pm, or join the Makerspace and participate!

Casting Furnace Update

Despite summer vacation and other obligations, work continues to progress on the Casting Furnace.  In the past few weeks Bret has pinched the end of a metal brake line tube used to feed the furnace diesel fuel and installed a needle valve to better control the fuel flow rate.

Brant has been milling and machining parts for a mechanism that will both lift the lid and turn it out of the way when someone steps on a foot pedal.  The next steps will be to finish the foot pedal, weld it to the rig, and secure the lid to the top of of the lifting post.  Bret also plans to improve the casting tongs so they are more easy to use.

For more information, see the project wiki page: http://wiki.milwaukeemakerspace.org/projects/casting_furnace

Rubens Tube Update!

The Rubens tube I made a while back puts on a fairly impressive show when its speaker is driven with music or a noise box, as was done during the Milwaukee Makerspace Grand Opening.  The story is somewhat different when it is used to display the acoustic standing wave pattern inside the tube.  When a single tone (sine wave) at a resonance frequency of the system is played though the speaker, the heights of the flames map out the sinusoidal shape along the length of the tube. There are two very important variables whose values determine how well this will work.  These are the acoustic pressure in the tube, which is set by the speaker and its input voltage, and the propane gas pressure, which is set by a regulator or the valve on the propane bottle.  Even a professionally made Rubens tube has a relatively small range of these two pressure settings that create a nice sine wave distribution of flame heights.

After running my Rubens tube for a short while, I realized I’d made a few design choices that make the already small range of good operating parameters even smaller.  First, I didn’t actually use a gas regulator, I only used the valve on the propane nozzle.  Note that the propane flow rate is highly affected by the temperature of the nozzle, so the propane tank must be kept in a water-filled bucket to prevent the outlet valve from freezing up.  Just like a gas pressure regulator can be used, so could an acoustic pressure regulator (i.e. a compressor).  This could help prevent the flames from extinguishing during particularly dynamic musical passages.  Alternately, some type of pilot light system could be devised so that the flames automatically relight – perhaps glowing red nichrome wire could be added in a moderately safe way.  I also spaced the fifty 0.043” diameter holes apart by only 0.9 inches.  Having this many holes reduces the amplitude of the resonances, making them more difficult to ‘find’ by simply listening to the amplitude when adjusting the frequency input to the speaker.   Better performance would be achieved by having fewer holes spaced further apart.  Lastly, I’ve used a pipe whose inner diameter is only 2.5 inches.  A larger diameter pipe would further increase the amplitude of the resonances.

I put an electret microphone inside the Rubens tube at the end opposite the speaker, and measured the pressure inside while electrically driving the speaker with pink noise.  I did this with both air and propane inside the tube.  The following graph shows the low amplitude of the tube’s resonances with propane inside — between 4 and 7 dB.

The other interesting bit of data one can find from this graph is the speed of sound in propane.  Knowing the sound speed, one can calculate either the length of pipe needed to have a particular fundamental resonance frequency (n=1) or if a particular speaker has a resonance frequency low enough to excite the fundamental resonance of a particular length Rubens tube.  The resonance frequencies of a tube having uniform cross sectional area and two rigidly closed ends are given by: Fn = nC/(2*L), where n is the nth mode, C is the sound speed, and L is the length of tube.  The Rubens tube doesn’t have two closed ends, it has a paper cone speaker at one end.  It also doesn’t have uniform area – it has a small open volume in front of the speaker.  Both of these will change the “effective” length of the tube.  Don’t worry though, we can use the measured resonance frequencies with air in the tube to calculate the effective length: Leff = nC/(2*Fn).  Knowing C=343 m/s in air, we can use the measured resonances of the first three modes  (144 Hz, 262 Hz and 380 Hz) to find that the (averaged) effective length is 1.28m.  Using this effective length and the lowest three resonance frequencies (108 Hz, 205 Hz, and 300 Hz) with propane in the tube, Fn = nC/(2*Leff) predicts the sound speed to be ~265 m/s.

Makerspace members or any other folks near Milwaukee should feel free to stop by (on Tuesdays or Thursdays evenings) and fire up the Rubens tube.  Just use a small amplifier so you don’t put more than 30 Watts into the 4” speaker!  For some scholarly information about Rubens tubes, check  out the series of articles in the journal “The Physics Teacher:”  M. Iona (14), p325 from 1976; T. Rossing (15), P260 from 1977; R Bauman (15) p448 from 1977; and G. Flicken (17) p306 from 1979.

Jacob’s Ladder

In about 30 minutes after the Makerspace meeting on Tuesday, Ross, Jason and I made a Jacob’s Ladder from an old neon light transformer that I brought in, a couple pieces of wood, and some TIG welding aluminum filler rod.

With 110 Volts at its input, the transformer put out 9000 Volts! The arc formed across the gap, but it wasn’t actually a high enough voltage to make the arc ascend the ladder. When I hooked the transformer through a Variac and turned the knob to 130 Volts, the arc began to ascend the ladder – but only a few inches. The obvious solution was to add more voltage, so I hooked another Variac in series with the first, turning this one to 140 Vac. With the output of the transformer a bit under 12000 Volts, the arc ascended over 2 feet up the ladder. Check out the video below showing the arc’s interaction with wood.

Rubens Flame-Tube

Last Saturday morning I spent 4 hours making a Rubens Tube at the Makerspace while the crew from Pumping Station: One was shooting footage for their documentary entitled The Rebirth of the Maker Movement. Here is the first lighting of the tube:
Milwaukee Makerspace: We play with fire
A Rubens Tube is a pretty flashy piece of physics demo gear that uses fire to show the acoustic standing wave pattern inside an organ-pipe like tube fitted with a loudspeaker at one end. The 48” long, 2.5” diameter tube is filled with propane, which escapes through a series of 50 0.043” diameter holes spaced by 0.9 inches all along the length of the tube. Once the tube is full of propane (with absolutely no air), the 50 propane jets are lit with a striker or match, and all 50 flames have an identical height of 2 or 3 inches. When sound is played though the speaker at one end of the tube, the flame height is modulated by the acoustic pressure from the speaker. When a single tone (sine wave) at a resonance frequency of the system is played though the speaker, the heights of the flames map out the sinusoidal shape along the length of the tube. Playing music with dynamics or signing through the speaker is especially dramatic. I need to spend a few more hours fitting the Rubens Tube with a speaker at the other end and adding an additional propane inlet.