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This paper deals with an innovative technique concerning CO2 laser machining in order to create 3D cavities to be used as moulds for the casting of polymer resins, vaporizing PMMA layer by layer. The removal of a single layer is achieved using multiple overlapping straight grooves. The groove profile (depth and width) has been predicted by theoretical models which investigate the influence of cutting sequence, number of passes, laser beam scanning speed and radiant flux. The removal depth varies proportionally with the number of layers, while the surface roughness, influenced by the grooves spacing and the orientation of the scanning direction between successive layers, cannot be decreased under a minimum value. It has been shown that infrared laser systems could represent an effective tool for 3D machining of cavities in PMMA to be used as moulds for casting of polymeric resins, not only for the high speed of the fabrication process itself, but also for the high flexibility of the method. The lack of cutting forces allows a quick positioning of the workpiece which does not need to be clamped. Results from this study showed that complex shapes can be machined even with sharp corners, due to the small radius of the focused spot (about 350 micron). Further investigations should be performed to obtain cavities characterized by back drafts or machined in technopolymers and thermosets which have different behaviours to infrared radiations.