My studio assistant, John McGeen, and I started making molds from the Duchamp Readymake chess set on Thursday. You can see that we started with a cast iron flask with cope and drag halves. This particular mold has an opening formed into it for a pour hole and "button". It uses rolls pins for the indexing of the cope and drag.
For our "sand" we are using delft clay for the molding process.
I usually figure out where my parting line will be and then I create a board (with a cutout in the sale of the piece at the halfway mark) that will establish that parting line in the object and I can ram the sand on top of that board. In this case I wanted to do this quickly, so I ram one half of the mold and then strike off the back side of that half, flip the mold over and then physically press the object to be molded into the smoothly packed sand.
I press it deep enough to be at the halfway mark of the chess piece and am careful not to let the piece "roll" toward one side or the other. You want a direct downward pressure if you are going to mold something in this "ghetto" fashion. I have also established that I am going to pour the piece from the bottom up since the opening in my flask will feed the flow of metal from bottom to top in the piece. I also am careful to situate the piece fairly close to the opening so I can keep my sprue or gate channels short. If these gates are too long the metal might "freeze" before it fills the mold. In the case of these small pieces and the fact that I am doing this quickly, I do not put any vent passages into the mold as I would with a larger more critical piece.
Once the piece has been pressed into one half of the mold, I dust the surface of the mold with corn starch (baby powder or talc would be a fine alternative to the corn starch). The powder will keep the sand from sticking to the part and the sand when you ram the other half of the mold.
I make sure and chop the delft clay very fine so that I am able to place fine powder directly on top of the part. The first layers of sand are delicately rammed and then I am able to continue to place more sand into the mold.
After that side has been fully rammed, then I strike the back side of the mold off.
Next, it's time to open the mold by placing a wedge on each side of the mold where the roll pins are and carefully work both sides until the mold is split.
I carefully lift one side of the mold off and then proceed with cutting gates into the sand that will feed metal to the piece to be cast. I am able to carefully extract the chess piece that is being molded and I am careful to watch for any small pieces of sand that can fall into the mold. I make sure that the passage way to the gates is clear and tapered so that the piece will have a smooth flow of metal into the mold.
Once everything has been checked for small pieces of sand or particles, I'm ready to close the mold up. The two halves are brought together and then I use two pieces of wood to clamp the mold half together with a c-clamp. The force of the molten metal can sometimes cause a blow-out or a bulge to form, so the wood will keep constant pressure against the exposed sand in the mold. It will also make sure that the cope and drag are held tight against each other to reduce the potential of flashing (or a thin sliver of metal formed usually at the parting line) to occur. Now we're ready to pour some molten metal...