This paper demonstrates the importance of the careful selection of the input material data for the calculation of the flow behavior of the wood polymer composites (WPCs) with different numerical schemes. To this goal, the rheological data of pre-dried and un-dried WPC samples were measured, utilizing a commercial high pressure capillary rheometer equipped with slit dies of different gap heights. The data were incorporated in finite-difference and finite-element methods in order to predict the pressure drop along a profile die and compared to measurements on an extruder at different flow rates. While the un-dried WPC sample is expected to undergo wall slippage during flow, the simulation results on the slit and profile dies indicate that neither a fully slipping plug flow nor a fully adhering shear flow is capable of providing reasonable results. By utilizing ANSYS Polyflow software, a combination of both flow types was incorporated by a 3-dimensional FEM analysis to simulate the pressure drops. The results show that by using the shear viscosity data from capillary measurements the calculated pressure drops are lower than the experimental data and close to the fully shear flow case. It was noted that the traditional experimental method of shear viscosity measurement could not be applied in the presence of wall slippage phenomenon due to the fact that the entire velocity profile in the slit die changes and the resulting flow curves of the material is gap-dependent. Therefore, an optimization procedure was used to evaluate the shear viscosity which was found to be capable of capturing the flow behavior of the WPC in the presence of wall slippage. This research clearly points out the significance of using a combination of shear and plug flows in such simulations through applying shear viscosity and wall slippage coefficients, respectively. Moreover, it emphasizes that the data from the capillary rheometry should be treated carefully in the presence of the wall slip.