The invention relates to a welding string (28) for forming transverse welding seams in a sheet of film (F) that comprises at least one film layer, said sheet of film (S) being moved continuously through a welding unit, and wherein pairs of welding strings (28) are moved in a curve at each their side of the sheet of film (F) by means of movement means arranged at each their rim of the sheet of film (F) whereby the welding strings (28) are - along a part of the curve - caused to extend parallel with the sheet of film (F) and at the same rate, and wherein at least the one welding string (28) comprises a heater element, and means configured for ensuring the mutual abutment of the welding strings against the sheet of film (F) at each their side thereof with a certain force. According to the invention the welding string (28) comprises a welding body (35) in the form of a thin-walled, hollow metal profile wherein the heater element is configured interiorly of the profile, and the welding body (35) is supported by a rigid carrier beam (30, 31) configured for transmitting the movement in the curve to the welding body (35), and the support of the welding body (35) comprises spring elements (10, 39) that are distributed in the longitudinal direction of the welding string between the welding body (35) and the carrier beam (30), and adjustable attachment elements (42, 50) that are distributed in the longitudinal direction of the welding string (28) and configured for securing the welding body (35) relative to the carrier beam against the spring force of the spring elements (42, 50).