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Due to increased quality requirements and the trend to cost reduction by process optimization, the modeling of plastic processing by means of simulation software becomes more and more important to predict process behavior. Most tools are based on a physical analysis of the process conditions and a reflection of those in a mathematical model, either based on FE methods or an approach to approximation or complete analytical models. First models were published for planetary roller extruders. However, these models deal primarily with the melt conveying behavior and have not yet been developed for the melting process which in many cases is critical to address homogenization features of such machines in the melt phase. This paper presents an approach to calculate the melting degree along the barrel of a planetary roller extruder. Therefore, models that are used to describe the melting process of single and twin screw extruders are adjusted to the conditions in the planetary roller extruder. At first the relevant process was divided in the three steps solid conveying, melting initiation and melting propagation. The solid conveying is described by the Archimedes solid conveying model. In order to estimate the melting initiation the solid particles temperature increase was used for partial filled sections. Further, it was assumed that the melting cannot start later than at the point where the extruder flow channels are fully filled for the first time. The melting propagation was described by a modified disperse melting model. The developed models were implemented into a simulation tool. The models were verified by experimental investigations. A comparison between simulated results and experimental data shows a good agreement.