A detailed 3-dimensional analysis of cyclic heat transfer in a complex injection moulding mould tool has been carried out, using a general-purpose commercial heat transfer code (FIDAP). A novel, generally applicable technique for modelling of heat loss to ambient from the tool parting surfaces is introduced, to facilitate inclusion of the mould-open period in the modelling. Simulations are continued through 30 cycles of moulding, to attainment of steady periodic conditions. During this period temperatures at the polymer-steel interface rose by between 10 °C and 40°C above the coolant temperature. At the start of cooling, values of cavity surface heat transfer coefficients referred to the coolant temperature ranged from approximately 4000 W/m 2 K to 400 W/m 2 K, with the highest values occurring directly below the cooling channels and the lowest near a heated sprue bush. At the end of the cooling period, values at each location had dropped by approximately an order of magnitude. These results illustrate the limitations of plastics cooling analyses that use constant values of temperature or heat transfer coefficients as boundary conditions on the cavity surface. Comparison with results obtained omitting the modelling of the mould-open period shows how, in the latter case, higher tool temperatures are attained in the steady periodic conditions, and a larger number of cycles are required to reach steady periodic conditions.