Reactive extrusion modelling is essential to help analyze phenomena taking place in the extruder, optimization and scale up. Gel concentration in the polymer dependent on the type of polymer, concentration and type of crosslinking agent and processing conditions. Fluid flow in extruders has been described kinematically using residence time distributions (RTDs). Residence Distributions (RxD), as expressed by Residence Volume Distributions and Residence Revolution Distributions, have shown to collapse the RTD curves onto single master curves based on specific throughput. The values of mean residence time (tn) as a function of the number of 45 degrees kneading elements in the screw profile, screw speed, and feeding rate. When the same extruder is used for processing different polymers, differences in RxDs have been attributed mainly to differences in material viscosity and melt flow index. An axial dispersion model can be used to predict the flow characteristics from control parameters (screw speed and flow rate) and geometrical parameters (screw profile and die design). Viscoelastic nature of the polymer needs to be taken into account in analyzing extrusion flows. Viscoelastic models Maxwell Model, FENE Model, and Giesekus model are used to determine the effect of elasticity on RTD. Weissenberg number and Deborah numbers are calculated to gage differences in RxD. The characteristic relaxation time λ of the polymer has proved to be the key quantity. When is small and viscous effects dominate, the RVD model approach is valid. When λ is large, the elastic effects of the polymer may dominate under typical flow conditions. Using cross-linked polyester as an example, materials were reacted with peroxide in a twin-screw extruder at various peroxide levels, screw speed and material through put. Compared to viscosity, elasticity dominated Resident Time Distributions.