Unlike the melting in single-screw extruders (SSEs), which has been extensively studied and modeled, the so-called "dissipative melting" occurring in intermeshing co-rotating twin-screw extruders (Co-TSEs) is still not well understood and modeled. In this article, the heating/melting phenomena and mechanisms in Co-TSEs are briefly examined from the energy point of view. Experiments are carried out on the heating/melting behavior of a typical semicrystalline polymer (polypropylene) with two different particulate forms (pellets, powder) and an amorphous polymer (polystyrene), both in different screw configurations. The results of this work show clearly the importance of plastic energy dissipation (PED) of individual pellets in the partially filled kneading section and polymer particulate assemblies, when densified, to the heating/melting of polymer particulates in Co-TSEs.