Foam can be run in a variety of molds, including Ultracal to fiberglass. Ultracal is porous, so it needs to be properly sealed and released to prevent the foam latex from adhering to the mold. Every bit as important as sealing and releasing the mold is making certain that there is no moisture left in the mold before baking foam latex in it. This is important for two reasons. Residual moisture in the mold will prevent moisture from the latex being absorbed by the mold, thereby causing the foam to shrink more after curing. Moisture in the mold can also cause steam pockets to form within the mold, which can ruin the foam. Water heated under pressure (as in a clamped, sealed mold) can boil at a lower temperature than normal (212°F—100°C), such as those needed for baking foam latex (under 200°F). To prevent that from happening, your stone molds should be heated for several hours at nearly 200°F (93°C) to remove any residual moisture. This is particularly true of new molds.
The same is true of fiberglass molds—not to remove residual moisture (because there is none) but to vent off styrene remaining in the mold, which can react badly with the sulfur given off during the foam latex-curing process and transfer to the foam.
Okay, back to the process:
1. Turn the speed down to 4 for 1 minute. This stage will begin to refine the foam, breaking up the biggest bubbles.
2. Turn the speed down to 1 for the last 4 minutes to further refine the foam. When there are 2 minutes left, begin adding the gelling agent and continue mixing for 12 minutes. It is critical that the gelling agent be mixed well, and depending on what mixer you use, the methods of assuring that the gelling agent is sufficiently mixed can vary.
3. At 12 minutes, turn off the mixer and remove the bowl and you are ready to carefully fill your molds.
Once the foam has gelled (you can tell by gently pressing on the foam; it should give a little and bounce back), you can place the molds in the oven and heat them until the foam is fully cured.