Plastic energy dissipation (PED) of individual polymer pellets as well as of densified polymer particulate assemblies may be the dominant heating/melting mechanisms in intermeshing corotating twin-screw extruders (Co-TSEs). In the present paper, the amount of PED is evaluated by conducting the uniaxial unconfined compression experiments on both molded cylindrical polymer samples and polymer particulate assemblies. The compression experiments were extended to a much higher deformation (strain) range. The results of this work clearly demonstrate the significant contribution to melting from even a single deformation of individual pellets. One severe deformation of PP pellets can provide almost one sixth of the energy required for their melting. For PS pellets, one severe deformation alone can provide sufficient energy to heat up the pellets to above their glass-transition temperature. On the other hand, the mechanical energy input during the compressive deformation of particulate assemblies is much lower than that of solid polymer samples, depending, to a great extent, on the degree of the particulate densification/void ratio of the particulate system. There are two types of PED in the melting section of Co-TSEs, localized PED of individual pellets in the partially filled kneading section and global PED of consolidated particulate assemblies in the fully filled section. The mandatory deformation of solid particulates, caused by the interaction of kneading paddle pairs, and the associated PED for both individual pellets and consolidated particulate assemblies, provides very efficient heating/melting of polymer feedstock in Co-TSEs.