During injection molding of thermoplastics the filling depends on the material's rheological properties, on the characteristics of the mold and also on the process parameters. Thanks to the elaboration of short shots, it is possible to reconstruct the main successive steps of the filling and to know the advancing melt flow. The choice of specific cavities and particular injection gates allow for filling to be studied in many different cases. The disposition of barriers inside the cavities gives opportunity to orient the flow.Experimental studies consider the injection of polyester and polyamide unfilled and reinforced with short glass fibres. Original results in terms of filling are shown on images. We compare experimental fillings with theoretical fillings obtained with a rheological simulation software. Good agreement is proved for unfilled materials. For reinforced materials results diverge because jetting observed on the short shots can not be taken into account during simulation.The regime of melt flow entering the cavity has an important influence on the structure, properties and appearance of injection molded parts. That is why correlation between filling and mechanical properties are examined thanks to ultrasonic measurements of modulus. A study in terms of anisotropic properties is investigated.