Engineering plastics, such as polybutylene terephthalate (PBT) with high levels of short glass fibers exhibit unusual flow behavior during injection molding which may affect the final properties of moulded pieces. Through short-shots experiments it is obvious that jetting during cavity filling was systematically associated with highly filled materials, while neat polymers fill cavities through the simple laminar flow. Jetting phenomenon was related with very low, if any, post-extrusion swelling of highly filled materials. In order to study the high shear rheological properties of engineering plastics, in conditions similar to industrial processing, an injection capillary rheometer was developed and fixed on an injection machine. In that case, pressure, shear rate, material plastification and injection are representative of the processing. For comparison, standard laboratory rheometer was performed. A commercial PBT, either neat or filled with 30% glass fiber, was extensively tested for flow properties with this system, using several entrance cones and 2 series of various L/D dies. The experimental swelling study was based on the observation of the flow at the end of the die, thanks to an infra-red camera positioned on the top of the injection machine. Images showing the thickness and the behavior of flow at the die exit prove that swelling depends particularly on glass fiber percent, injection shear rate and die length. All these characteristics are important to take into account during the injection to optimize the relation between processing parameters and final part quality.