A method of reducing the number or size of voids in a polyurethane comprises reacting the formulation components in a container having an inner surface, or having a liner that has an inner surface, that has been modified to reduce the shear forces that normally accompany introduction of such components into a container. This is done by modifying the inner container's or container liner's surface by profiling and/or engraving it, or by including as a liner a mesh. The profiling and/or engraving or mesh serves to alter the formulation's flow dynamics such that the polyurethane has fewer and/or smaller voids, i.e., has a more uniform density, than polyurethanes formed without the modification.