Techniques for the three dimensional simulation of gas assisted injection molding (GAIM) are presented, including analysis of primary and secondary gas penetration and the use of time and position dependent heat transfer boundary conditions, computed in a segregated 3D analysis of conduction in the tool, through many cycles of moulding. Predictions are compared with results of experimental trials using a mould tool with a glass window that allows video recording of the process. Accurate agreement is obtained for gas penetration distances in primary and secondary penetration and for the forms of the gas channel cross sections, in a part that incorporates typical features of a GAIM application. The success of the simulations relies on careful detailing of the computational model and numerical procedures, including the use of dynamically updated flow and heat transfer boundary conditions, the use of sufficiently high resolution meshes, and efficient, high accuracy iterative solution techniques.