Polyurethane Foam: Available in higher densities, stronger and more durable than EPS foam, poly foams are high-end materials for specialty projects or project pieces. Due to their higher densities these foams often can be machined smooth and their chemical makeup is such that they can be used with a number of solvents and coatings that would literally eat through EPS foam (although it should be noted that numerous coatings have been developed that work perfectly for EPS products). This expanded durability of poly foams makes these foams the best choice for projects that require substantial chemical finishing, which most often occurs if they are intended for an extreme location (underwater, intense heat, etc.) or another specialty purpose. The higher density of poly foams also often requires specialty cutting equipment such as serrated wire machines, which most shops do not have, and which are very expensive. A final consideration is the fact that most poly foams only come in sheets, unlike the large blocks of EPS foam. In order to fabricate a 3D object, it therefore must be assembled in layers, the layers laminated together and the whole project mudded and sanded smooth. Thus, the benefits of poly foams need to be weighed against the higher costs of both materials and production before you choose poly over EPS.