A food-safe container (2) for liquids is composed of a tubular body (3) referable to a longitudinal axis (2a), and a neck (7) presenting a quadrangular base (16) fastened coaxially to an open top end (5) of the tubular body (3) afforded by a frustopyramidal connecting portion (6) of which the four tapered corner edges are pinched initially and left with four relative fins (22); the container (2) is formed by a machine (1) equipped with a folding and sealing or welding unit (24) including twin pairs of arms (26) by which the single fins (22) are folded flat against corresponding side walls (6a, 6b, 6c, 6d) of the connecting portion (6), and a pair of heat-sealers or ultrasonic welders (29) operating in conjunction with the arms (26), by which the fins (22) flattened against the respective side walls (6a, 6b, 6c, 6d) are secured in place.