Rotomolding, also known as rotational molding, is a versatile manufacturing process used to produce hollow plastic products with complex shapes. This work aims in studying the process as a whole, to have a better understanding of the performance of polymer powder (PA11), modeled using a macroscopic approach, thus modeling the powder state as a continuum medium, through the use of numerical simulations. Also attempted to control the thickness of the final product through a parametric study. The primary motivation is to gain a comprehensive understanding of this phenomenon in rotomolding, which is difficult to study experimentally. Numerical simulation using the Eulerian approach with the finite element method with anisotropic mesh refinement and the immersed boundary method has proven to be a powerful tool for analyzing and optimizing the rotomolding process. Particularly, the movement of molten polymers within the mold is crucial during the heating and cooling phases of the polymer. The finite element method allows the combination of several physical, which helps identifying critical process variables and optimize design choices. A macroscopic approach is used in \cite{Riber} to study powder flow, taking into account factors such as powder sintering, polymer degradation, crystallization, bubble collapse ... The Herschel-Bulkley law will be used to model the viscosity during the transition from powdered to molten polymers. Simulation allows for the anticipation of polymer behavior during the molding process, including cooling and flow characteristics. Overall, numerical simulation holds immense potential for advancing rotomolding technology and improving the quality and performance of rotomolded products. The proposed model and numerical simulation approach will be presented and discussed through several application examples. To verify our methodology, we begin by examining a 2D segment of the mold. Initially, we simulate a single-phase scenario, followed by the incorporation of a two-phase problem involving the polymer/air interface. Once validated, we extend our simulation to the rotomolding process of a small 3D cylinder, validating the results against experimental data obtained from Forvia, before proceeding to simulate a complete 3D scenario involving a full-scale hydrogen tank once we having a robust code that takes into account all the physical phenomena.