Zonewise film thickness control in biaxially oriented film prodn. - using hot air shafts to produce zonewise heating prior to transverse stretching

摘要

A large number of elongated, pref. rectangular, air shafts are mounted in the longitudinal direction of the thermoplastic film above and/or below the film strip, and are pref. mutually parallel. The air shafts each have inlet and outlet for the hot air supplied, the outlet opening being directed towards the film strip. Individually movable louvre-type sheets are provided in the air shafts perpendicular to the direction of flow of the hot air and located upstream of the outlet opening, for the control of the quantity of hot air supplied. System enables the transverse variations in film thickness which normally results after transverse stretching to be very considerably reduced using a much simpler and more easily operated appts. than those currently used.