I won't be able to use the MDF with Bronze, too hot. May try plaster or make a wood positive and cast it in sand.
All of my aluminum casting attempts have been done in sand. Although, the very first time I tried pouring some molten- I had a couple of pieces of plywood that were left over scraps from cutting some wooden gears out on the CNC Router table. I will never forget that experience! I knew that the molten would probably burn the plywood, but I THOUGHT that it would at least solidify before too much burning occurred. WRONG! That plywood did not even slow the molten's descent down, on it's way to the ground! (It literally VAPORIZED the plywood on contact!).
Since learning from that experience, I have been making sand molds. And being retired, (and unable to afford Petrobond casting sand), I learned how to make my own foundry sand. After trying many different recipes, the one that finally worked with good results was: 80% sand, mixed with 15% Bentonite clay (CLUMPING type Kitty Litter), and 5% Charcoal ashes. I had to construct a special machine to use for grinding the kitty litter into powder, which consisted of a steel turn-table, with two grind stones (like those on a bench grinder) which ROLL against the surface of the steel turn-table. A variable-speed DC motor was used to drive the turn-table, at the slowest possible speed, while friction drives the grind stones.
The slow speed is needed to prevent centrifugal force from simply slinging the kitty litter to the outer rim of the turn-table, (and thereby OUT of the path of the grind stones). Only small amounts can be ground at a time, as it is after all, Bentonite clay, and will begin to clump onto the grind stones. A knife-edge was later added to the stones, to keep the clay scraped off during the grinding process. It is also helpful, to first DRY the kitty litter, by placing it in a pot, and into the foundry furnace, stirring constantly until it is completely dry. Once the kitty litter clay has been ground into the finest possible powder consistency, then pour it into the sand, add the charcoal ashes, and begin mixing by hand, until all three elements are evenly mixed.
I use one of those plastic tubs, with a snap-on lid to store the foundry sand in, to prevent (at least as much as possible) the sand mix from becoming too dry, or too wet. When using the sand to make the sand molds, I use a spray bottle of water, and lightly moisten the sand, stirring constantly by hand, until it becomes wet enough to form cakes when squeezed in my hand. At that point, it is ready to pour into the flasks to make the sand mold. The sand mix is re-usable, however (because of the kitty litter) has to be re-ground to break up the clumps back into powder form. I have learned, that this step is only necessary for a small amount. (The sand that actually contacts against the pattern being molded). The rest of the sand acts as just a "filler" of the flask. And when rammed up (packed into the flask) properly, it will ALL become very similar to concrete. Especially after a pour, and must again be broken up, and ground. A five gallon plastic bucket, with a few rocks thrown in, and turned tumbler-style (again, S-L-O-W-L-Y) will speed up the process.
Sorry to ramble, but I know that others are reading this with interest, and I just wanted to warn folks that metal casting is VERY DANGEROUS. I am actually amazed at the results you were able to achieve with the MDF, that it did not Vaporize on contact with the molten metal! I would hate to hear that someone tried this, and lost a foot or something, as the molten metal poured right through the MDF or other non-fire proof mold material. Metal Casting is very dangerous, and must be done with respect, paying attention at all times to what you are doing. Serious injury can occur!