An unwinding unit (400) for unwinding a strip (S) of heat sealing material for quadrangular machines (1) comprises a casing (401) adapted to be mounted on a quadrangular machine (1) and a puller (402) operatively associated with the casing (401) and configured to pull the strip (S) relative to the casing (401) along a feed path (P) and in a feed direction (V). A diverter (407) is configured to divert the strip (S) between an initial, preferably horizontal, stretch (P1) of the feed path and a final, substantially vertical stretch (P2) of the feed path, defining a curved, connecting stretch (P3) between the initial stretch and the final stretch. A deformer (413) operatively associated with the casing (401) are configured to make on the strip (5) an extended crease line (414) running along the strip itself starting from the curved stretch (P3) or downstream of the curved stretch (P3), in the feed direction (V) of the strip (S).