Rheological properties play an important role during plastic processing. In the context of thermoforming a product for the automotive industry, the rheological properties of an industrial grade elastomer are optimised. The elastomer in this case is EPDM (Ethylene-Propylene Diene Monomer), a thermoplastic-polyolefin (TPO) offering significant advantages such as lightweight, low-cost and ease of processing. To obtain specific shear-rheological properties, we reinforce EPDM with a nano-scale fractal filler. It is well documented in literature that the extensional behaviour of filled molten polymer systems exhibiting an yield stress is primarily strain softening. Such a behaviour poses challenges for products which need to be thermoformed where an extensional strain-hardening behaviour is preferred. The extensional behaviour of EPDM is identified to be linear i.e. neither strain-hardening nor strain-softening. Addition of nano-scale fractal filler at varying concentrations leads to a strain-softening behaviour. In order to improve the processability for thermoforming, the EPDM-nanocomposites are cross-linked via an electron beam. This results in a complex extensional behaviour where strain softening is observed at low strains followed by a hardening behaviour due to an elastic response at large strain. We present our work on the effect of cross-linking on the rheological behaviour of EPDM-nanocomposites. Additionally, we discuss the validity of changing hydrodynamic reinforcement during deformation in contrast to the classical Leonov model for molten filled polymer systems involving free and trapped chains.