This paper contains a systematic study on warpage of injection moulded plates and corner products as a function of mould temperature difference and holding pressure. The study is restricted to unfilled amorphous materials. As expected, experiments showed that warpage increased linearly with the applied temperature difference between mould halves. Nor expected was that at low holding pressures rite plates curved towards the hot side, whereas at high holding pressures they warped towards the cold side. A simple thermoelastic theory is proposed which could explain all observations qualitatively and quantitatively. Standard simulation software, on the other hand was seen to predict warpage behaviour at the higher holding pressures which was completely opposite to the experimental observations.