Heat-curable plasticised mixtures for the manufacture of moulding materials based on polyurethanes, containing (a) a polyisocyanate, (b) a slightly branched polyester possessing terminal hydroxyl groups and (c) a defined aromatic polycarboxylic acid hydroxyester containing at least one secondary hydroxyl group (for example a hydroxyester manufactured by reacting 1 mol of trimellitic anhydride and 1 mol of 1,2propanediol in a first stage, and further reaction with 2 mols of cresylglycid), with the mixture containing, per 1 equivalent of isocyanate group, 0.05 to 0.3 hydroxyl equivalents of the polyester (b) and 0.3 to 1.0 hydroxyl equivalents of the aromatic polycarboxylic acid hydroxyester (c). The mouldings obtained by hot curing of the moulding compositions are flexible, tough and impact-resistant, and these mechanical properties are largely independent of temperature; a transition to the soft rubberyelastic state only occurs above 150*C.