Filaments and short fibers as fillers are one of the most common ingredients to thermoplastics modification in order to impact on mechanical properties of injection molded parts and maintain cost efficiency at minimum part weight. In this study, glass and basalt fibers are processed into a thermoplastic matrix to compare achievable mechanical property improvements as a result of fiber type, content, length and orientation in various plastic matrix materials. Fibers are processed into the thermoplastics matrix by a twin screw extruder, regranulated and then injection molded into components. Mechanical properties of these components are measured. Results of experimental studies are compared and correlated in order to derive quantitative relationships to be able to predict properties for different material combinations of glass and basalt fibers and a variety of thermoplastics. Major parameters of the model are fiber content, length and orientation as a result of processing conditions. Comparable to the CHAMIS laws, a relationship is established to predict achievable Young´s modulus and tensile strength for both types of fibers used. Based on the material model, a determination of required fiber content, length and orientation is feasible. The desired configuration of the compounded fiber filled material and its alteration in the injection molding process needs to be taken into account to assert the desired property enhancement in the injection molded part. The steps of the process are analyzed and methods to predict fiber length reduction during processing and resulting orientation are comprehended. The study will introduce ways to predict properties of fiber filled thermoplastics more accurately and thus allow comparing glass and basalt fiber effectiveness.