Once the mold is closed and the foam has gelled, it’s time to pop it into the oven. But first, you need to understand the following: (1) Higher temperatures make the foam gel faster, and (2) higher humidity makes the foam gel faster. To correct for these conditions, follow these tips: In hot and/or muggy conditions use less gelling agent into the mix and pour it sooner. Another way to extend your working time is to add extra foaming agent, which will prevent the latex from gelling too fast. GM Foam offers a product called Foam Stabilizer that is designed for use in high-humidity, high-temperature environments.
The following points are also very important to understand before embarking on a foam run:
■ The cure "window" is larger at lower temperatures. At 185°F (85° C), foam can take 3 hours to cure, but at 4 hours it could be over-cured. This over-cured foam loses tear strength and in extreme cases becomes crumbly. The same foam, cured at 165°F (about 78°C), could take 5 hours to cure, but even if cured for
7 hours it would still be fine. In other words, a low-temperature cure could have a 3-hour window where the cured foam would be usable. A 200°F (93° C) cure may only have a 20-minute window where the foam is usable.
■ Steam lakes are areas of foam that have been pushed away from the mold surface by pockets of steam, then cured into that incorrect shape. These areas have all the detail of the sculpture, but they are depressed and too dense. This is a hazard in nonporous molds, such as epoxical and fiberglass, or molds that have not been properly predried. However, it is a problem that can be remedied:
1. The first step is mold preparation. Nonporous surfaces are to be coated with a thin solution of paste wax (such as Johnson’s wax for floors) that has been cut with 99 percent alcohol. This "alcowax" should be thin and runny. Brush it into the inside of the mold, do not allow to pool, and when it’s dry, brush it out with a dry brush. The mold surface will become polished and shiny. More important, the mold surface will be sealed from outgassing, which we feel causes sites for steam laking to begin.
2. Cure at a lower temperature (for a longer time). GM Foam recommends curing at 165°F (78°C) for 5 to 7 hours for this kind of mold.