In this study, the special injection molding process, inline injection molding compounding (IIMC), and the conventional twin-screw process using powdered flame retardants are compared for material preparation. The demonstration of the homogenization of flame retardants for the same material formulations within the different processes is aimed. Compared to conventional material preparation in the twin screw, the mixing effect of direct injection molding compounding with a single screw plasticizing unit is low, which is why processing aids are indispensable. Parameter studies are carried out within inline injection molding compounding in order to evaluate the dispersion and distribution of the fillers in relation to the process parameters. Furthermore, the inline injection molding compounding with flame retardant masterbatch is investigated with regard to the mixing effect. For the evaluation of the individual studies, a transmitted light method is used to detect agglomerations of fillers in the test specimens. The material formulations are also compared using reflection spectroscopy within the individual preparation processes. Finally, mechanical properties are determined in order to verify the degree of homogenization in relation to the mechanical parameters.