Sammendrag
We present simulations and injection moulding experiments for a diffractive grating. The grating consists of an irregular wave pattern with wave height 0.6 μm and wavelength 3 μm on a 1.2 mm thick part. This microfeatured part was replicated in a DVD grade polycarbonate. The degree of replication, as measured with white light interferometry and atomic force microscopy, increases with increasing mould temperature, injection velocity and holding pressure, and is high at optimal settings. 2D simulations of the filling were performed in CFX, a general Computational Fluid Dynamics code. The focus is on the coupling between the filling of the main part and the filling of the microfeatures. These filling simulations must be performed separately because of the different size scales. The rheology of the polymer is represented by a Cross – WLF model. A VOF method is used to capture the free surface and the wall has a constant heat transfer coefficient. We use as boundary condition in a local simulation around the microfeatures, the velocity field from an analytical solution of a fluid injected between two parallel plates. The reported simulations indicate that the common approach to use the pressure field at the wall as a boundary condition in the microscale simulation is neglecting important effects.