The problem of production flow in steel casting foundry is analysed in this paper. Because of increased demand and market competition, a reorganisation of the foundry process is required, including the elimination of manual labour and the implementation of automation and robotisation of certain processes. The problem is how to determine the real difference in work efficiency between human workers and robots. We show an analysis of the production efficiency of steel casting foundry operated by either human operators or industrial robots. This is a problem from the field of Operations Research for which the Discrete Event Simulation (DES) method is used. Three models are developed, including the foundry before and after automation when taking into consideration parameters of the availability of machines, operators and robots. We apply the OEE (Overall Equipment Effectiveness) indicator to present how the availability, performance and quality parameters influence the foundry’s productivity. In addition, stability of the simulation model was analysed. This approach allows for a better representation of real production processes and the obtained results can be used for further economic analysis.