The thermal conductivity of thermoplastics is measured under moulding conditions (high pressure and high temperature). A specific apparatus has been designed and is described. A parameter estimation method is used for the experimental data processing. It is based on the resolution of a I-D non-linear inverse conduction problem. The optimisation algorithm developed to solve the problem, is efficient and stable. confidence intervals of the estimated parameters take into account errors both in the temperature measurements and in the parameters of the model. Heat capacity is estimated from calorimetric measurements, error analysis on the estimated conductivity is discussed, especially in the phase change interval. Numerical validation and experimental examples are presented.