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Mantis

Kiln design with a 3D printer

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Sorry if this is the wrong forum. I'm building a small kiln from a propane bottle. I'm planning on using it exclusively to burn out 3D printed parts from plaster molds, so I can pour aluminum into them. It will need to get up to 600c and maintain a very slow airflow (convection is probably sufficient, or I could install a tiny computer fan) so the PLA vapors can escape. Since I've almost managed this temperature with a large un-insulated gas barbecue, I imagine a small propane camp stove burner will be fine for the well insulated 8 liter volume I'm planning.

The ports on the side and top will be for adjusting airflow, with the side one also giving me a way to light the burner. The bottom one feeds the burner and hopefully stops it overheating. Perhaps I'll make it height adjustable. The lower stick will support a grill to hold my plaster cast and drip pan. My refractory is "Asahi Caster" and is rated to 1500c. I'll also be using rockwool for extra insulation. The top comes off at the seam on the tank. (I forgot to put a seam in the refractory part of the 3D model but it'll be at the same place.)

I already have a 3D printer, so I'm going to use it to print the cavity form, then remove it with a torch and pliers once the refractory is ready to fire. As such the sky is the limit for chamber, vent, etc design. Can someone with more fluid dynamics knowledge than me offer any advice? How does it look as it is now? What would you change?

propane kiln.png

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I have a Johnson 900ss which is a larger volume than what you're looking at, I can do a 16lb crucible no problem. It runs at 240,000 Btu's - You've got a 20lb propane bottle, you'll need at least 2" of refractory, preferably 2.5"-3" for efficiency. Even at 2", that's going to leave you just 8" x about 6" (been a while since I measured a tank) for an inside chamber, it needs air flow around the crucible plus enough wiggle room to grab the crucible with tongs.. Yeah, a camp stove isn't going to cook this job, you'd be lucky to bake a potato in it, maybe melt lead. I'd want a minimum 40,000 Btu's for the volume (1000btu/lb of aluminum minimum and most casting furnaces run magnitudes more), it can always be dialed back but you can't get more if it's not available. There's a lot more to heat in a crucible furnace than just the air inside it, an easy-bake oven it is not. Oh, and you need 743c (1370f) for casting aluminum, not 600.  Your burner should come in off the side, offset to swirl around the crucible not aimed directly at it, no reason for an opening at the bottom. The lid should be removable, I'd do a rotational hinge with just a steel rod and a piece of pipe. Just a small opening, 3" or so, is all you need in the top for off-gassing.

Lastly, no reason to 3d print your interior form, just go to a home improvement store and pick up a cardboard concrete tube form for the appropriate size for your internals (they're used for casting concrete footers for decks etc). Cast the bottom, once set, I would cut your burner opening in the tank, probably about the same size as a toilet paper cardboard roll insert. I'd take the tp roll insert and attach it to the form at the correct orientation for the burner with masking tape, once it's perfect, cover it all with vaseline as a release. Put it in the tank and cast around it. It'll cost pennies and take minutes as opposed to printing it. I did this entire procedure about 15 years back with a 40lb propane tank that I still have as a backup 3 burner forced air forge.

Not to shoot down the idea, it can definitely be done, but the numbers and methods need to be adjusted in my opinion. 

-J

I just realized you're in Japan, so a Home Depot is out, but a home center store like Viva Home should carry the concrete forms. Also, those cardboard tubes we American's are used to in the center of toilet paper rolls might not be that popular in Japan, but I'm sure you can find or make a 40mm cardboard tube for the burner opening, or even just 3d print it - definitely quicker than doing the entire interior!

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I'm not melting aluminum with this kiln. It's for drying 1-2 liter plaster flasks and burning out the PLA plastic investment inside them (less than 100 grams), over the course of 6-12 hours. My btus/hr are about 7000, which pottery websites say is about three times what I need for my volume. My stove-top has no trouble melting 2kg+ of zinc, so there should be no issue getting a couple liters of slowly moving air to the same temperature.

For Aluminum melting I already have an effective, if janky, solution in the form of two earthenware charcoal stoves stacked on top of each other. The charcoal and blower tube goes in the bottom one, the crucible in the top one. It just fits a fire extinguisher crucible. I'm planning on building a better, waste oil powered one soon.

We definitely have toilet paper rolls, but I don't actually recall seeing concrete tube formers, though I'm probably just not looking. I can print the size I need for less than $3 of plastic, which is cheaper than the gasoline to get to a home center. The form gets printed in "vase mode" which only prints the outside lines so it only takes a couple hours. It felt ridiculous to me too at first, but honestly it's cheaper and more convenient to do it this way.

 

A bunch of further questions:

I'd like to find porous investment plaster so I can vacuum my castings after they are poured. I'm having trouble finding it for sale (my Japanese is decent but I'm not a native). There's a plaster mix called Hi-stone C-2 (ハイストーンC-2) that the manufacturer recommends as it's designed for high heat, but they don't say anything about porosity. Could I mix the fine sand I use for sand-casting into that plaster to make it more porous?

Is leaving a freshly poured casting in a vacuum chamber until it cools a good or a bad idea? I'm thinking doing it this way would make my aluminum cool very slowly, with good crystal structure and without oxidation or air absorption. Would this make the aluminum undesirably soft though?

Since my kiln won't be getting anywhere near the limit of my refractory, could I blend a bit of dish soap into my refractory cement to foam it for improved insulation?

Does anyone know of a pottery supply store in Ibaraki? Or at least what word to plug into Google maps?

 

charcoal furnace.jpg

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"We definitely have toilet paper rolls, but I don't actually recall seeing concrete tube formers, though I'm probably just not looking."

I definitely didn't mean that in a negative way, I actually meant that your toilet paper rolls might be a little less wasteful than ours here in the states, like those fancy ones without the hard tube of cardboard in the middle.. God knows we Americans love anything we can throw in a landfill! lol And I didn't understand what the purpose was so I'll apologize for that as well. As far as improving the insulation of your refractory, or making it more porous, see if you can find vermiculite in a gardening shop, it's a natural insulation and can be mixed with refractory cement easily. 

-J

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Have you been over to alloyavenue.com, formerly backyard metal casting?  It's a series of forums dedicated to casting which might be a bit more on target than a series of forums dedicated to blacksmithing.

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