Weld lines are formed whenever two or more polymer melt flow fronts are merged inside of a flow channel. In injection moulding this occurs when the flow is separated and merged in order to contour an obstacle, when several gates are used or when there are filling paths with different flow restrictions that promote the development of several flow fronts advancing at different velocities. In extrusion, these lines are formed in hollow profiles, which use torpedoes to shape their internal surface, or when flow separators are used with a view to control the flow distribution in dies that are highly non-balanced. Weld lines zones are weaker than the remaining ones, their presence may also affect the aesthetics of the product, but their formation is unavoidable in many situations. This subject has been relatively well studied for the injection moulding case, but information concerning extrusion is scarce and the results gathered in injection moulding cannot be extended to the extrusion process, due to the huge differences on the relative thermo-mechanical conditions. To overcome this limitation, a prototype extrusion die system, consisting of an extrusion die having a torpedo that can be displaced during extrusion, was conceived aiming to produce tapes with a weld line. The system enables to produce tapes without weld line or with a weld line formed in different and controlled conditions, namely axial location of the torpedo, flow rate and temperature. The subsequent mechanical characterization of these tapes enables to determine the effect of the conditions in which weld lines are formed on their quality. In this work the prototype system is described and results obtained with polystyrene will also be presented. Further future exploration of this system is expected to contribute for the establishment of useful guidelines for the design of extrusion dies for hollow profiles or comprising flow separators.