I've been using firebricks as a makeshift melter for aluminum (primarily lost-PLA castings)... but I've gotten tired of a few things:
- Time to build / takedown (firebricks aren't waterproof at these temps; water in them would cause them to explode)
- Long melt time due to lots of thermal mass and not-very-insulating firebricks
- It took a while to find a good stacking configuration for my flask...
- Opening up for the pour is hot and dangerous
So I made a melter (underglorified kiln -- the heat's definitely not even on this one) from a 20 gal trashcan.
Step 1: Materials & Tools
- 1 20 Gallon Galvanized Trashcan
- 2 Bags Perlite
- 2 Tubs of Refractory Cement
- Stainless steel wire (lockwire)
- Cooking spray (if using a mailing tube)
- Mailing tube used as form for inlet
- Propane tank used as form for chamber (ick. see step 7)
- Wine bottle used as form for vent (No!)
- I used a Harbor Freight flamethrower for the source of heat...
- Gloves, buckets, eye protection, and a dust mask
- Sawzall (or a cut off wheel or a Dremel)
- Tin snips (or scissors you don't care about)
- Drill (or a punch that can make small holes in the trashcan)
Step 2: Measure... and Cut
I placed a set of firebricks in the bottom to space the propane tank off the bottom and give me a defined floor. Turns out, that's a bad thing (you can't remove the firebricks afterwards).
I then put my flask on top of the bricks and figured out where I should cut the trashcan (2" from the top of my flask).
I then put the propane tank in place and measured where I should cut the inlet so it would be tangent to the tank just after the tank became a cylinder (Lesson: Also make the inlet tangent to the floor).
Using a sawzall (but probably easier with a cutting wheel), I cut the opening for the mailing tube, and I cut the top off the trashcan (luckily for me, the cut line was just before the propane tank started curving on top).
I coated the mailing tube heavily with cooking spray so it wouldn't absorb the water (the refractory cement said it was important not to absorb the water) and inserted it in place.
You probably should wrap the propane tank. The weld made a bump that cracked the refactory when it was removed...
Step 3: Mix, Wet, and Pack the Bottom
Refractory cement is unlike normal concrete, so I hope I didn't do anything wrong here....
Note that perlite is not useful for higher temperatures (it melts). But I'm using this for Aluminum and Brass, so hopefully it's ok...
To make "insulating cast refractory", I mixed 3:1 by volume perlite and the refractory cement. To keep things even, I scooped 3 scoops of perlite, followed by 1 scoop of refractory cement, mix, 3 more scoops perlite, 1 more scoop refractory, and mix again.
The goal is to have a nice even mixture/coating of the perlite with the refractory cement. Then add water (I ended up using 3 pts for each bucket, but I started with 2 pt) -- unlike concrete, if you add too much water, the refractory cement will not be strong. Stir. To keep the water right, I then scooped out of the bucket into a plastic container to stir again and add more water if needed or more drier mix from the bucket if too wet.
Then scoop into the waiting trashcan and pound it in [ramming] (most people use a block of wood. I used my hands). The goal is to have the coated perlite in great contact with each other for strength. Since the perlite is mostly air, the insulation is its job (if you have voids in the wall, it will be brittle and tend to explode when heated).
At the top, I used a brick to tamp it down and make it smoother.
All in all, this used most of the 2 buckets of cement and ~1.4 bags of perlite.
Step 4: Prepare the Top...
Cut 1-2" up the sides of the lid on every pleat using tin snips (or scissors -it's thin metal). Fold those strips to the inside and hammer out to make a rim on the inside. This gives a ledge to hold the refractory in place
Drill a bunch (I drilled every 3 pleats) of holes about 3/4" above the bottom, and string the wire through in a square box pattern. Twist the wire together on the inside to have some tension. Pay more attention than I did and your lid will stay more round! This gives reinforcing wires to hold the refractory in place.
Place a wine bottle in the middle so you can make a vent... and since I was forming this on concrete, I put everything on a 10 gal plastic kitchen bag so my driveway wouldn't absorb the water.
Actually, you should use the mailing tube (or something 4.25 - 5 in diameter). On initial firing a 750ml wine bottle was too small... and you can always block the vent a bit with a firebrick.
Step 5: Ram the Top!
Mix more refractory like before and ram it into place (so you get a good surface at the bottom and hold the bottle in place so it doesn't shift). I was shooting for 2" insulating cast all the way around, but I ran out again. I hope it's good enough!
Step 6: Prepare to Wait...
But we're not going to wait yet...
- Let's free up the propane cylinder: whack the top of the tank (NOT the valve assembly), tapping it in all directions so a small gap appears between the refractory and the tank.
- Let's remove the wine bottle -- tap it with your fingers like the tank, then slowly pull it out.
- Let's cover our work -- the cement says it's important to keep water in play for 24 hrs, so I draped the top of the bottom with a plastic bag and put the lid on the top.
Then I waited 24 hrs (more like 48 -- Monday had day job issues) and removed the propane cylinder and all the plastic and set it to air-dry for the next 3 days.
Step 7: Hot Waiting... Initial Heat Treating
The instructions on the refractory cement say to "be gentle on the initial firing so steam escaping doesn't cause the casting to explode". I only have a Harbor Freight flamethrower (no gentle setting), so I put a hot plate inside (running the cord out the inlet), and put the melter together.
Then I waited a week... the outside gets warm....
Step 8: Initial Firing
I connected and started the flamethrower... set it to about 2/3rds power and found that the sound was very different outside vs inside, and it stunk badly, burning my eyes; I didn't make the vent hole large enough!
For the initial test, I put a piece of clunker to separate the top from the bottom and continued on... 30 minutes later, I had liquid on top...
After shutting the gas off and lifting the top off (put on something fireproof -- it's hot!) -- use welder gloves, as the handles are hot (but not as hot as a firebrick), I found my crucible wasn't very hot at the bottom...
So I made changes:
- Carved the bottom part of the inlet -- now I can aim the flamethrower down to hopefully heat more evenly
- Carved the vent hole out to 4.25" dia... would've gone bigger (still chokes the flamethrower at full power), but ran into the wires....
Wait for it to cool completely or you'll have a fire when vacuuming the refractorybits out.