Method for the manufacture of plastic sheets of good optical quality

摘要

Plastic sheets of good optical quality suitable for use in the glazings of automotive vehicles, eye glasses etc., are made by depositing a hardenable liquid mass upon a taut moving ribbon of plastic which has been pretreated by tensioning and heating to remove wrinkles and creases which otherwise would render optically unsatisfactory the sheet formed on the ribbon upon hardening of the liquid mass. The temperature range within which the pretreatment should be conducted is 60 DEG -100 DEG C., with 70 DEG -90 DEG C. being preferred; the tension applied in the pretreatment should be a dekanewton or more per square millimeter, preferably 1.5-7.5 dekanewtons per mm2; and the time of pretreatment is at least a minute and preferably 5 or more minutes. The lateral tension is preferably applied by means of pairs of opposed idler rollers positioned along the opposite side edges of the moving ribbon and biased toward each other to pinch the ribbon between them, with their axes angled outwardly by a few degrees from a direction normal to the direction of travel of the ribbon. The pretreatment heating is preferably applied in two successive ovens providing successive temperatures of about 70 DEG and 90 DEG C., respectively.