a running web of thermoplastic material is uniformly relaxed across its width by monitoring the tension in its marginal sections and by adjusting the corresponding heaters adjacent the path of the marginal sections . the median section of the web is relaxed in response to generation of a signal which denotes the tension profile across the entire web and is obtained as a result of the processing of signals which denote the monitored tension in marginal sections of the web . those sections of the web wherein the tension is higher are heated less intensively than the sections wherein the tension is lower .