A process for continuous welding of a sheet of plastic material for manufacturing flexible packages, comprising a step of positioning the sheet relative to the welding device, putting ends of the sheet into contact, forming a weld by heating, pressing and cooling the ends of the sheet that are in contact, acquiring a primary temporal signal with a sensor, the primary temporal signal proportional to a thickness of the weld, transforming the primary temporal signal into a primary frequency signal and defining a low-frequency spectrum, a medium-frequency spectrum, and a high-frequency spectrum from the primary frequency signal, reconstructing a secondary low-frequency signal from the low-frequency spectrum, and determining the thickness of the weld based on the secondary low-frequency signal.