Although molding of thermoplastics is very productive method of machinery manufacturing, pure plastics are almost not used in so much quantity. Particle reinforced composites are more popular, because presence of solid inclusions reduces volume of organic matrix and usually improve strength and stiffness. Final properties are strongly influenced by manufacturing process which affects inner material structure. In composites where fibers are continuous in one direction or placed in layers, i.e. the fibers do not end inside the composite and it`s length is close to the dimension of machinery parts, elastic   and thermal properties  can be predicted quite easily by Halpin-Tsai equations or derived simplified methods  with high accuracy. This paper is focused on prediction of density and material stiffness of composites, which are reinforced with very short glass fibers in thermoplastic matrix. In the first part we define the field of problem. Then we present simplified analytical calculus in compare to finite element method. We focus on ABAQUS 2018 and its features, which can be used for solving those problems. Estimation approaches such as representative volume element method and mean field homogenization are also studied. After this presented methods are confronted with selected material datasheet of Ultramid® A3WG6  and Zytel® 70G35HSLRA4  composite material. The effect of fiber randomization on material stiffness is introduced . At the end of thesis we use previously calculated material data for modal analysis of real parts which are made from molded thermoplastic with short glass as well.