GB808182A - Improvements in or relating to a process for forming stretched thermoplastic sheets into curved contours - Google Patents

Abstract

808,182. Bending stretched thermoplastic sheets. SWEDLOW PLASTICS CO. March 1, 1956 [Oct. 28, 1955], No. 6442/56. Class 87(2). A process of forming to a curved contour a stretched thermoplastic sheet comprises mechanically bending the sheet to the contour, holding the sheet to the contour whilst heating it to a temperature and for a sufficient period to remove most of the stresses imparted by the bending but below the temperature at which large shrinkage occurs, and holding the sheet to the contour during cooling. The heating conditions may be such that a flat specimen of the same sheet, heated under the same conditions as the bent sheet, would not shrink more than three per cent. The sheet may, prior to bending, be pre-heated or have its surfaces preheated and the bending may be effected in stages. A mould has a bottom section 10, covered with rubber impregnated suede cloth 12 and having a heating or cooling chamber 16, and a top section 20 with a similar covering 21. The sheet 30 is inserted between the sections 10, 20 and deformed to the curvature of the moulding surface of the bottom section 10 by the closing of clamps 25. The mould may be preheated prior to use and during bending and when bending of the sheet is complete heating fluid is passed through the chamber 16 and, when the bending stresses in the sheet are relieved after four to twenty hours cooling fluid is passed through the chamber 16. Suitable thermoplastic sheet materials are polymethyl methacrylate, polymethyl alpha-chloracrylate and polystyrene. Examples of heating times and temperatures for different materials are given.