This article presents a simulation study of the injection molding of a macroscopic part with microfeatures: an optical grating with amplitude 0.6 mu m and period 3 mu m. The aim of the simulations is to develop a procedure capable of predicting the replication of optical gratings based on the injection molding processing parameters. Three coupled simulations are performed. 1.) A macroscopic simulation of the flow in the part without microfecuures. 2.) A microscale flow simulation in a domain around a single microfeature using the pressure from the macroscopic simulation as input. 3.) A thermal simulation of the mold wall coupled to the microscale simulation. In the simulations the polymer melt is treated as a generalized Newtonian find. Shear viscosity and thermal properties were characterized as part of the study. Other physical effects such as adhesion and elasticity are discussed, but not included in the present model. The simulation results agree well with the experimental replication data. and correctly describe how small changes in the mold temperature can lead to large changes in the replication.