The flow of molten polymers in corotating twin screw extruders has been largely studied, but little attention has been paid until now to the melting process. In the present work, we develop an experimental study of melting in twin screw extruders, including pressure and temperature measurements, dead-stop experiments and sampling, observation of cross sections and quantification of solid fraction. The influences of process parameters (screw speed and feed rate), screw profile, pellets size and extruder size are also investigated. An original model of the melting process is proposed, deduced from these observations. This model allows one to calculate the evolution of mean shear rate, pressure gradient, dissipated energy, pellet radius and both solid and liquid temperatures along the screws, for a sequence of right- and left-handed screw elements and blocks of kneading discs. The results of the model are in good agreement with the experiments.