The effect of thermal stresses in moulded components has been demonstrated by measuring the warpage of centre-gated discs which were injected in a mould, the two sides of which were regulated at different temperatures. Polypropylene discs exhibited larger warpage than polystyrene ones. The measured curvature increased with the difference in mould temperature, and decreased when the packing pressure increased. Thermal stresses and strains have been computed assuming an orthotropic thermo-elastic behaviour for the polymer. A three-dimensional finite element method was used. The different steps of the moulding process, and associated changes of boundary conditions, have been taken into account. The results are qualitatively in agreement with the measurements. Finally the calculation was applied to a box mould, leading to a characteristic box deformation in "diamond ace" form which is consistent with the experiments.