Method for stretching web of plastic sheet into arcuate form

Abstract

The invention relates to stretching of a continuous web of a thermoplastic sheet, such as polyvinyl butyral sheet, having straight longitudinal edges into an arcuate form to use it as an interlayer of curved and laminated windshields of vehicles. Usually the web has a colored band to serve as a glare shield band along its one longitudinal edge. First the web travels vertically downward and is heated from its both sides by a vertical heater to produce a gradient of temperature across the width of the web such that the temperature at the web edge remote from the colored band becomes higher than the temperature at the opposite edge. Under longitudinal tension the heated web is passed around a first heated tapered roll and then around a second heated tapered roll having a higher temperature. Over these tapered rolls the web edge remote from the colored band is located relatively near the large diameter end of each tapered roll. The web stretched by the two heated rolls is successively passed over a plurality of cooled tapered rolls. By this method the web is smoothly stretched into a desired arcuate form without appearance of defects such as wrinkles or streaks on the stretched web.