Described is a method of preparing a cured, non-elastomeric polymeric sheet derived from a polyisocyanate. The method comprises the following steps: combining a first component and second, separate component to form a reaction mixture; introducing the reaction mixture into a preheated sheet mold at a certain minimum fill rate, allowing the reaction mixture to gel; heating the reaction mixture to a temperature and for a time sufficient to yield a cured sheet having a thickness of at least 6.35 mm (0.25 in); and removing the cured sheet from the mold to yield a non-elastomeric polymeric sheet. When the active hydrogen functional groups in the second component include hydroxyl groups, the first and second components are initially heated to a temperature of at least 50° C., Polyurethane sheets formed by such processes demonstrate minimal optical defects and the process allows for the production of superior sheets of higher thicknesses than previously possible.