Venting your foam pattern
Here's what I do: drill a hole straight down through the sprue into the pattern. Find areas that would collect an air bubble, like a bump sticking up off a flat horizontal surface, and drill holes vertically into them. Now, connect the holes with a horizontal hole, so you can blow into the sprue hole and air will come out of the other holes. Put soda straws into the holes to keep sand out of the pattern and provide a path for the gas. Holes should be slightly undersized so the straw will stay in the hole without needing glue. The straw should stick up out of the sand.
Patterns with a wax coating or areas with lots of wax will need more venting. Most thick (1/2" or greater) foam patterns with a good sized sprue won't need any vents at all, the gas goes out the sprue. It catches on fire and spews black smoke out of right where you're trying to see, though, so you may want to vent it anyway.
