I don't understand; 1. why the loose sand holds it's shape long enough for the metal to solidify, once the foam has evaporated. 2. What are ingredients of the slurry that they coat the foam with, that it's both hard and porous to gases
The exact composition is trade secret (read: NOT pattened as Aldo claims, as that would require disclosure of the blend, similar to why fragrances aren't pattened). What I can share is the primary 'special' components, excluding its water base are graphene and iron. A combination of the refractory coating and the density of the molten metal is sufficient to maintain the mould shape. the sand is compacted via vibration of the flask prior to casting.
@2:22 you see the foam parts dipped into a slurry. this coats the foam in a refractory that acts as a barrier between the loose sand and the foam mold.