Wood-plastic composites (WPC) are enjoying growing popularity due to their advantageous properties. They consist of a thermoplastic polymer matrix and wood fibers or particles. For example, due to the plastic component, WPC has lower water absorption and greater three-dimensional design freedom than pure wood. The wood component, on the other hand, improves the mechanical properties and sustainability of the finished products compared to pure plastic. Particularly in injection molding, the processing of highly filled WPC poses a major challenge. At wood contents above 40 wt.-%, flow anomalies already become apparent. Along the flow path, the plastic and wood components segregate and lead to a tearing melt front. At the same time, partial wall sliding leads to breakouts in the form of a free jet, the appearance of which intensifies with increasing wood content. The free jets lead to weld lines whose positions, on the one hand, cannot be accurately predicted and, on the other hand, represent a mechanical weak point in the final component. A more promising approach is offered by the sandwich injection molding process. By pre-sending a skin material with good filling behavior, flow anomalies or free jets and thus weld lines in the component can be prevented with the right process settings. In this work, the influence of different highly filled core and skin materials including their prefill levels on the mold filling behavior is investigated. For this purpose, a specially developed injection mold with glass insert is used, which allows video recording of the injection process. The one-component WPC specimens with free-jet formation and corresponding weld lines are compared with the sandwich components in terms of optical and mechanical properties.