Hi Marcos, I use a heat cured silicone rubber (vulcanized) It is
better to solder the sprue onto the model, But if you have no idea
what the material is, then, with silicone heat cured powder seperated
molds, it is easier to place the sprue in the mold about 1/16" away
from the model and cut , grind or burn the connection to the model
after the mold is cured.