First step in creating a bonded sand mold is to create a pattern to be cast. In this case I decided to experiment with party balloons. I chose this material because the patter needs to be made from something that will be completely evaporated by the metal being poured into the mold. In most cases, polystyrene is used because molten metal will burn the pattern away and the cavity will be replaced with metal. This is real nasty for the environment and your health. So I encourage creative use of less harmful materials for your pattern. You do not need to think about undercuts but holes can be tricky to pack.