Patent GB-818847-A (GB)
Title: Prepressing laminated glass assemblies
IPC: B29C33/10
Filed: 1956-07-05
Abstract: 818,847. Laminating glass. PITTSBURGH PLATE GLASS CO. July 5, 1956 [Aug. 19, 1955], No. 20851/56. Class 140. A method of prepressing at least the marginal area of an assembly comprising two matched sheets of glass and a thermoplastic interlayer comprises supporting the assembly on a flexible sheet within an area of the sheet defined by a continuous rib on the top surface of the sheet and spacing the periphery of the assembly from the rib, placing a flexible cover sheet over the assembly and the rib to cover the entire area of the flexible sheet defined by the rib and to maintain a conduit around the periphery of the assembly, evacuating the conduit through the rib while maintaining air passages from the periphery of the assembly to the point of evacuation at the rib, and maintaining the vacuum while heating at least the marginal area of the glass sheets and interlayer, whereafter the vacuum may be released, the cover sheet removed and the prepressed assembly of glass sheets and interlayer removed. As shown, a support table is provided with a flexible top surface by stretching a flexible sheet 16 across support members 12 and 13. A flexible sheet 20 is draped across this surface, and is provided with a perimeter tubing 22 which has a flat on its bottom surface for sealing to the top surface of sheet 20. Outlet tubes (not shown) are connected to the perimeter tubing at two points on opposite sides of the assembly and to a vacuum pump. A flexible corrugated sheet 25 rests on flexible sheet 20 and extends as far as the tubing 22, which is provided with holes 27 spaced along it and passing from its interior downwards through its inner wall. A thermally-insulating flexible sheet 29 rests on sheet 25, and like it extends as far as tubing 22. Sheet 29 is sufficiently porous to provide passage for air from the valleys in corrugated sheet 25. A flexible sheet of heat-resistant, preferably thermally-insulating, material 34 rests on sheet 29 and has approximately the outline of the assembly 40 to be laminated. A flexible electric strip heater 45 is affixed to sheet 34 and occupies an area coextensive and in alignment with the marginal area of assembly 40. A flexible sheet 48 having an area slightly greater than assembly 40 lies above sheet 34, a flexible sheet 50, considerably more flexible than sheet 48, lies over sheet 48 and extends sufficiently beyond tubing 22 to form a seal therearound when vacuum is applied. The heater 45 is connected through wires 54 and 55, which pass between tubing 22 and sheet 20, with an electrical source (not shown). Sheets 16, 25, 48 and 50 may be of rubber, sheet 48 being thicker than sheet 50. The tubing 22 may be of circular (as shown) or triangular cross-section and may be stuck to sheet 20 with rubber cement. Sheets 29 and 34 may be of asbestos cloth, and the strip heater 45 may be a strip of fibre-glass cloth having an insulated copper wire sewn to it along a zigzag path. In modifications, (a) a cottonbacked rubber sheet is used with its cotton side upwards in place of corrugated sheet 25, (b) a single flexible sheet is used in place of sheets 48 and 50, it may be of greater thickness in the central area, and may be fastened at one end to a roller rotatably mounted to a fixed support while its other end is fastened to a clamp which is raised and lowered by a hydraulic cylinder, (c) two strip heaters are used, connected in parallel, each extending half-way round the periphery of assembly 40 instead of the one extending the whole way round, (d) flexible sheet 34 is reversed and placed above the assembly 40 which rests directly on sheet 29, and (e) sheet 16 is the roof of a chamber into which air can be pumped to exert pressure on assembly 40. In operation, the apparatus is first assembled as shown, except that sheets 48 and 50 are omitted. The assembly 40 is then placed in position with its marginal area resting on heater 45. Sheets 48 and 50 are then placed in position. The vacuum pump is started to evacuate air from the assembly, and when the pressure is low enough, electricity is passed through the heater. After heating for the desired time, the heat is turned off and vacuum released. Sheets 50 and 48 are removed and assembly 40 removed from the apparatus. The assembly may then be edge-rolled as necessary and autoclaved by known methods. When pressing curved assemblies, e.g. motor-car windscreens, the supports 12 and 13 for flexible sheet 16 are so shaped as to approximate to the projected shape of the assemblies, which may have corvex sides up or down. The apparatus may also be used to prepress a plurality of small laminated glass assemblies, e.g. safety goggles.