The mixing of wood-polymer composites inside an extruder was modelled using Computational Fluid Dynamics (CFD) and the software Ansys CFX. The motions of the mixing blocks inside the extruder were modelled using the immersed boundary method, which avoids re-meshing in each time step. Two different models were used; one with a stopping block and one without. The purpose of the stopping block is to ensure that the flow stays inside the mixing region for a longer time. The flow field was initialized in two different ways; one starting from a steady state-solution, and one starting directly from a specified flow field and specific volume fraction. The results indicate that the stopping block works as intended. The general conclusion is that immersed solids is a promising method, but more work is required.