I always slit the pvc on one side lengthwise on the tablesaw and put a 3/8 or 1/2" strip of plywood between it. Tape it off and pull the strip out when the castable has set. Pvc pops right out.
I did pretty much the the same thing.
Using 3.5" and 6" PVC pipe tubes (makes a 4" chamber and 1" thick liner walls), temporarily screw each piece of PVC to a wooden board, place the board against the fence, and split the PVC pipes in half on the bandsaw.
The inner pieces get spaced with a 1/4" thick slat of plywood ripped to be the same width as the tube diameter. Place the slat between the cut pieces, making it a circle again (sand width as needed to get a pretty good fit), and run a wrap of masking tape on the top and bottom to make it stay together. Cover one end with duct tape. Don't go up the side more than 1/4".
The larger piece of PVC makes the outer casting shell. Hinge the pieces with duct tape and strap the outer shell shut with wraps of duct take around the top and bottom.Tape off the bottom with duct tape.
Set the inner tube in place using three pieces of dowel or wood strips as spacers. Push down to stick to the duct taped bottoms together. Remove spacers.
Pour Cast-o-lite 30 ( or whatever other castable refractory you have) and fill the space between the tubes. This prevents the refractory running out the bottom. Use your fingers to keep the top properly spaced.
When fully dry, peel off the duct tape on the bottoms, tap/pull out the spacer slat and pop the two halves out of the inside. Then take the outside pieces apart and let it dry for a week or more.
To cure the refractory, there are several ways. One is to set it on a turkey fryer and turn the flame on low. Let it run for half an hour, then turn up end for end for a second half hour. Turn up to high and repeat the half hour bakes.
Once cured, test fit to the forge shell using spacers the same way you kept the tubes spaced in pouring. Once aligned, mark the holes for burners and TC sheaths. Remove and drill the holes. The dry refractory drilles easily with a masonry bit. Clean up the holes with a round/half round file and chamfer the edges smooth.
Wrap tightly with the insulation wool using paracord or similar small cord to bind the wool tight, and slide into the shell. Once in place, pull the cord out slowly.
The rest is pretty much the same as any forge build.
TIP:
One really good trick is to get a ceramic burner tube off ebay. It is merely a ceramic tube, usually sold as
alumina ceramic tubing (same thing the TC sheath is made of)
. One suitable for a forge burner port runs between $10 and $30, depending on your source. There was a fellow at Ashokan a few years back with a big box of 6"X2" tubes for something like $10 each. Make the burner mount pipe on the shell just big enough to slide the ceramic tube through. Install the burner port as normal, and slide the ceramic tube through the wool and refractory liner. The TC sheath should be mounted through a steel tube similarly. Use satanite on the inside to make a smooth seal. Any extra space from the outside can be packed with bits of refractory wool. Using this setup, you can run the burner farther back in the tube. Play with position and find the sweet spot. Stainless pipe burners are also a good idea. (I have titanium tubes that I haven't experimented with yet, but am curious how they will do.)
