This paper considers the process of extrusion of aluminium alloy 6061 bars aimed at finding a way to minimize the in-process metal loss due to the use of an additional work piece (in the form of back plug) made of cheaper material. Numerical modelling of the extrusion process was carried out for three different heating modes applied to both the tooling and the work pieces. The authors analyzed the mutual flow of the ingot and plug material at the final stage of extrusion in order to determine the moment when the back end defect starts to form and how it forms. The stress-strain state for three process options was investigated. Thus, the authors looked at the colour gradient pictures showing the distribution of strain and strain rate longitudinally along the extrusion centerline. It was found that when a pre-heated additional work piece is used, the butt-end should be made larger because of the early formed back end defect. When a cold plug and a hot dummy block are used, the back end defect forms early as well, as the back plug heats up at the final process stage reaching almost the same temperatures as the main work piece. It was established that the optimum option would be not heating the back work piece or the dummy block. The modelling helped reveal that the size of the buttend in this case is smaller. The obtained modelling data can be used for conducting pilot studies aimed at reducing the size of the butt-end when producing extruded aluminium bars. This will help improve the metal utilization factor.