The planetary roller extruder is well known for its superior mixing performance and its exact temperature control capabilities. Therefore it is widely used in the polymer industry for the compounding of shear- and temperature sensitive materials like PVC or Wood Plastic Composites. In contrast to the considerable amount of scientific work that deals with investigation, modeling or simulation of the process behavior of single and twin screw extruders there are only few publications about the planetary roller extruder. Due to increased quality requirements and the trend to cost reduction by process optimization, this is increasingly becoming a problem for plant construction and processing companies. In our former work first analysis of the plastics melting process in planetary roller extruders were conducted and models describing these processes were published. However the influences of pellet diameter and inorganic filler were not considered. As a consequence the process models are only valid for a partial amount of the materials processed on planetary roller extruder. This paper presents experimental melting behavior investigations of powder, micro pellets and standard pellets. Furthermore the influence of filler amount is examined with standard pellets. At these experiments the melting initiation and the melting propagation is determined. Based on the experimental results, a theoretical consideration for the melting in planetary roller extruders is proposed.