Abstract: Urea-modified bubble-free moldings based on polyisocyanate polyaddition products are made by reacting (a) an organic polyisocyanate with (b) a polyhydroxyl compound and/or polyepoxide in the presence of (c) a water chain extender in the form of an aqueous alkali aluminosilicate or an aqueous alkali-alkaline earth aluminosilicate of the zeolite type in a mold using cast resin technology. The reaction components are used in quantities such that the equivalent ratio of isocyanate to hydroxyl and/or epoxide groups is from 1.1:1 to 6.1. The chain extender is used in a quantity such that the amount of water is at least equivalent to the excess of isocyanate. These moldings may be made by a one-shot or a two-shot process. These moldings are characterized by better mechanical properties such as tensile strength and elongation than prior art moldings.