Casting polyurethanes that have a thick cross-section (300 mm plus) can present some problems. Polyurethane has a low thermal conductivity and the curing reaction is exothermic (gives off heat). This can lead to problems when casting thick cross-sections. The normal aim is to have the polyurethane hot enough to reduce the viscosity to allow the easy filling of the mold and for all the entrapped air to escape. This has to occur before the polyurethane starts to increase in viscosity and gel.