Cast-Iron Furnace Build Using Partially 3D-Printed Molds

Looks great. No need to apologize about that little bit of porosity. Makes no difference. Practically everyone has that amount or more.

Denis

 

The pictures in my last post were actually from yesterday, didn't have time to post them. I left it like that overnight and just now took out the inner mold. It looks like it turned out good. The walls look even thinner in person... not sure about the durability of this design in terms of cracking. It is extremely light though, I could easily lift the entire thing with one hand. Once the water is driven off during final curing it should get even lighter.

There was some surface discoloration where the aluminum backed tape was applied (mostly on the bottom and around plinth). Don't think it will affect anything but interesting nonetheless. The tape itself was also heavily oxidized. I think that the refractory must turn into a strong base when mixed with water and therefore reacted with the aluminum.

Next step is to do the slow heating routine and then apply some ITC-100 (and hopefully fill some of those bubbles to make it smoother inside). Also need to settle on a lid design, I will make another post about that.

Starting to remove the inner mold:

Drilled a hole in the plywood mold bottom to "drain" the sand:

I found that the slightly damp cardboard would peel away easily along the seam and could be rolled up on itself to make it easy to remove.

Inside looks much the same as the outside. That sharp corner around the bottom might prove to be a point of failure:

Very thin:

Corroded aluminum tape:

Need a slightly taller plinth because the crucible is partly in line with the tuyere.

 

Nice job!! I was considering doing something like this to drop into a steel can lined with wool but was hesitant about fragility due to the wall thickness but you did it and it looks great. What is your hot face thickness?

 

1.125", I think if I were to redesign it I would instead do a little thicker, maybe 1.5", just for the extra support.

 

Here is the promised lid post. Since I'm still not sure of the best design, I decided to go the old fashioned way and just wing it (so no CAD this time). I just used some offcuts of the same sonotube as before and found some other shapes around the house that got me to where I wanted it. It has a lip that protrudes into the furnace to prevent hot gases leaking. The vent hole is ~4.5" diameter, just because that was the size of the can I found. I'm thinking it is better to go big and tune the flow by partially closing it with firebrick if necessary. I also cut out the bottom of a paper bowl and glued it upside down to make a tapered section on the bottom of the lid. Not for any particular reason other than maybe helping to improve the exhaust flow a little and looking more professional. I then gave it a coat of spray paint to hopefully seal it from moisture (we'll see how that turned out, not sure if it worked that well).

Provisions for lifting the lid come in the form of two t-shaped handles which I'm thinking will allow my bulky foundry gloves to still be able to grab it. I also added shafts on either side for eventually making a lever type lifting system. A rebar ring imbedded in the refractory provides a location for the handles to attach to that will distribute the weight and hopefully won't come loose. I'm a little worried about the refractory just cracking to pieces around this circle of steel, so I made the lid extra thick (~3.5" of refractory). This means that the lid weighs nearly half of the entire rest of the weight of the refractory.

Ugly welds, but my excuse is that I was out of welding gas on my MIG (plus nobody will ever see these again, hopefully):

T-shaped design allows for easy lifting even with gloves that make it hard to close your fist (these are just my welding gloves btw, I have aluminized sleeve length gloves for the foundry):

Mold ready:

Cast a larger plinth too:

 

I'll be interested in seeing how it holds up long term in regards to cracking as the iron heats and cools inside the lid.

Looks nicely made tho. How tight is the fit with the furnace body? Did that groove you made fit? How'd you make the counter groove on the lid?

 

I should have taken more pictures of the build process, would have saved me trying to put this into words lol. I used the same two sizes of sonotube that I used on the furnace body and just cut out a ring of cardboard the fit between them to make the top of the step (the wall of the stepped part is just a 1" tall piece of the smaller diameter sonotube). I previously molded on the outside of the sonotube to form the inner cavity of the furnace body and I am using the inside of the sonotube for the lid step, so there will be a gap of precisely the wall thickness of the sonotube between the lid groove/step and inner cavity (about 1/8" all around).

 

I think you will get a couple good heats out of that lid..... But, ultimately I think it will crack when the rebar expands quicker than the refractory. My furnace has a bunch of loose bricks held together with large pipe clamps. The side bricks get a little bit loose during the run. My top bricks seem to be holding strong. I am waiting for the total collapse of my top bricks. In hindsight, I wish I designed my furnace with a little bit of pyramid lid. Hindsight is always 2020.

Edit; in post # 43, picture 8, what size crucible do you have inside your new furnace? It looks really tight to lift out with proper tools and it looks a little bit tight to have anything but propane as a fuel.... Also, in my opinion it looks like you will need to have a plinth of 4 to 5 inches in height to raise the crucible. If you are thinking of getting up to cast iron temperatures, you might want to consider using diesel fuel. But you might need more clearances using a smaller crucible and higher plinth. My furnace is about 12 inches wide and 14 inches tall. Using diesel fuel I might be lucky with a A12 with a 3" tall plinth.

 

Are you planning on wrapping this refractory furnace with ceramic wool to keep some of the temperatures restrained? I used 2 inches of ceramic wool and I've left a provisions to add 2.5" of Rock-wool. But, right now I'm not sure if it's needed.

 

I was also thinking the space was pretty tight for a crucible and tongs. Tho I did something similar with less than an inch to spare on either side on my A16. So its possible if you cleverly design the tongs. But it isn't comfortable. I wish I had another inch of space.

 

The crucible is a Procast #10. There is about 1.125" around the crucible. I am going to be using diesel fuel. That picture was taken on a ~3" plinth that was too small. I recently cast a taller plinth to get it above the burner. Also, the step on the lid will protrude into the furnace 1", so that is the reason for the extra height. I'm planning to make the crucible lifting tongs out of 1/4" steel sheet, so it shouldn't be a problem to fit around the crucible with that much gap right?

I have two rolls of 1" ceramic wool 24"x 6' this should let me do about 3" of insulation. I have a bunch of rockwool I can add on top of this too.

Would you mind posting a picture of your tongs? I'd love to see how you designed them.

 

I welded a triangle strut for the jaw of the arms. Stiff enough to hold the crucible and thin enough to get past the crucible and wall barely.

 

Very interesting, this is very similar to the design I am imagining for my own pair. I figure I might be able to get away with even a little less thickness since my crucible will hold half the weight of an A16. I need to pick up some more welding gas, but then this will be the first item on my to-do list.

 

I am very interested in your initial trials with using diesel. It is a tricky fuel to get lit and dialled in, but once you get it dialled in it is somewhat easy.

Closing the lid prematurely what is problematic for me. The only advice I can give here is get it hot, then reduce the fuel and get ready to reduce air, before closing the lid. Then tweak them up till you have a blow torch coming out of the lid hole.

I personally wish I went a little bigger with my lid hole. I understand it would be less efficient but easier to dial-in.

 

Lighting a siphon nozzle with diesel is easy and should rarely blow out. Just turn the compressed atomizing air to 10 pounds. Open the combustion air "Just a little" and drop into the furnace a paper towel partially soaked in diesel after setting it on fire with a match. Crack open the fuel and then gradually open the combustion air. In a short time the fire will be roaring nicely. Now open the fuel to about .15 to .18L/M. Now close the lid and tune the combustion air so that about 6 inches of flame is visible coming out of the vent in daylight. Turning the combustion air down a tick should just start to show some black smoke traces. Open it so that the smoke just goes away. You should be in the ball park for optimal combustion. Experience will help you tune it closer to more optimal quicker.

If you don't have a flow meter, some experimentation shooting fuel into a measuring container using a stopwatch should give you an idea of flow rate vs needle valve setting. However, needle valves seem to be touchy things and a flow meter really helps hit a flow rate accurately and repeatably.

Denis

 

Started the first part of the heating cycle on the main body of the furnace last night. I used a PID temperature controller with a coil of kanthal resistance wire (I forget exactly what numbers I used, but I think it draws less than 500W). I let it sit at 150C for several hours, but it struggled to get the entire thing up past 250C (no insulation yet), so the rest will have to be done using the burner.

It actually worked pretty well overall, which makes me think it might be worthwhile to make a more permanent solution so that the furnace could be used for precise temperature control for heat treating. Something like a removable heating element that could be swapped in after/during a heat to maintain the temp at a certain value more precisely.


I used thicker gauge kanthal to attach the thin wire of the coil to the relay:

Very simple PID controller, just turns on when its plugged in and triggers a relay until it hits the right temperature (it was about $20 with the solid state relay).


I recently got a thermal camera–which is a really cool piece of tech–so I thought I might as well take some pictures and share them. All the pictures were taken when the PID controller was set to 150C (later I tried to get up to 250C).

Large hotspot on side where I think the coil was closer to the wall:

Insulation around the kanthal makes for a really high temperature locally:

The tuyere was significantly colder, probably should have pulled that plug of insulation I had around the thermocouple a little further out to allow the heat to reach it.

My extension cable was feeling it too:

This is very interesting: the bubbles (divots) in the refractory seem to be causing hot and cold spots. My theory for the hot spots is simply that there is less material insulating in these spots, allowing heat to escape. The cold spots have me stumped; I'm not sure what would be causing them (like I said, the cold spots are also divots in the surface). The difference between the hot and cold spots on the wall is only about 10*C.

The solid state relay that came with my PID controller. Note how hot the high current heating element side gets as compared to the low voltage controller leads (which are just at ambient):

 

Did the first firing on propane with nothing inside today. I haven't built an actual propane burner, so I just used one of those cheap weed burners and pointed my blower fan at the tuyere, which seemed to work good enough. I started with barely any flame and slowly increased the flow to increase the temperature. The inside reached much more than 500C by the time I was done (~3 hours), probably close to 1000C. I stuck the thermocouple in there after it was done and it read 500C after it had cooled down a bit. I also demolded the lid and put it on during the firing to cure that as well.

Something that was very concerning was the amount and size of the cracks that formed in the body, particularly at the hottest part of the furnace which was where the flame was directly contacting in front of the tuyere. I was originally very concerned that these cracks would cause the furnace to just crumble once I lifted it, but once it cooled down the cracks largely went away and it seems to be still be quite strong, although I am going to be very careful with it as I wrap it with insulation and set it up. Likely won't survive very long, just too thin.

Overall the lid turned out good and the heat was pretty successful. I now need to build a base plate and wrap it with insulation, and also make some tongs before I am ready to do the first melt.


Same curious corrosion left over from aluminum tape:

Temporary burner set up:

During heat, just turned up the fuel after having it on low heat for about an hour or so:

After heat, cooling down, used that half of a failed plinth cast to cover the hole and trap in some more heat:

Severe cracking that really has me worried (still hot):

Same spot (directly in the path of the flame, where it got the hottest) after cooling to ~100C: