The co-kneader 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 high filled compounds. 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 co-kneader. Due to increased quality requirements and the trend for cost reduction by process optimization, this is increasingly becoming a problem for plant construction and processing companies. To address this problem, experimental investigations of the melting behavior of polymer materials in the co-kneader had been conducted. In order to determine the melting degree along the extruder length a special barrel was used which can be opened in axial direction. Based on the experimental results, a theoretical consideration for co-kneaders that are operated as plastification extruders is proposed. At first the relevant process is divided into three parts consisting of 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 is used for partially filled sections. Furthermore, it is 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 is described by a modified disperse melting model. The developed models are implemented into a simulation tool. A comparison between simulated results and experimental data shows a descent agreement.