Patent GB-1208073-A (GB)
Title: Method of casting a film
IPC: B28B1/30
Filed: 1968-03-21
Abstract: 1,208,073. Casting flat films. CORNING GLASS WORKS. 21 March, 1968 [28 March, 1967], No. 13614/68. Heading B5B. [Also in Division C1] A film of uniform thickness is oast by moving a flexible belt upwardly at an angle to a horizontal plane through a pool of film casting material so that the belt forms at least one side of the pool, moving the belt horizontally after it passes out of the pool with the film cast thereon, drying the film and stripping the film from the belt. The thickness of the film cast is controlled by controlling the angle the belt forms with reference to the horizontal plane and/or the speed of the belt. As shown 11 in side elevation in Fig. 1, and in a top plan view in Fig. 2, an apparatus for carrying out the method of the invention includes a coating table 10, having a flat horizontal top 14. A pool P of the material to be cast is carried at one end of the table (Figs. 2, 4 and 5) and a belt B is unwound from a roll 15, passes upward through the pool forming one side wall of the pool at a variable angle to the vertical, and then horizontally over the table top 14 and is taken up at a roll 17. The speed of the belt is controlled by drive rollers 19, while the roll 15 is braked. The angle the belt B forms with a vertical plane is controlled by pivoting a belt support plate 20 about a pivot shaft 22 and holding it in an adjusted position by means of side plates 24 having adjustment holes 26 therein. The pool P may be provided with a cover 36 in three jointed sections. This cover covers the pool and the first part of the belt during the coating to act as a dust cover and enclose the pool P. As the belt moves upwards over plate 20 the material in pool P adheres to the surface of the belt not in contact with the plate and forms a coating of constant thickness in this surface. As the belt moves horizontally over the table top 14 it is dried either by a heater 40 under the table top 14 or by heating means such as infra-red lamps 42. At the outlet end of the table, the belt B is bent over a sharp edge 46 to be taken up at a constant rate by take-up rolls 19. The cast film F separates from the belt and may either be passed to another support table or taken up itself with interleaving paper material I from roll 48 on roll 50. A thickness measuring gauge 60 is used to monitor the thickness of the film cast on the belt B before the coated belt passes over the table top 14. This gauge may be an X-ray gauge mounted over the belt and reciprocated in a horizontal plane in a path transverse to the belt. A source of X - ray emitting material is positioned below the belt. An output from the gauge may be utilized to control and monitor the thickness of the cast film by feedback from the thickness measured to control the speed of the belt. Since the thickness of the cast film is proportional to the viscosity of the coating material, and the viscosity varies with temperature, the pool P may be provided with a thermostat. In the example the liquid to be cast is a latex base suspension of 94% alumina. The belt is a polyethylene terephthalate belt and the angle of the belt to the vertical is 60 degrees. The belt is driven at 10 feet per minute and the viscosity of the material being cast is 2000 cp. The material is dried while the belt is stationary on the horizontal top of the drying table for two hours at 65‹ C. and a green ceramic sheet material 0À030 inch thick results. The green ceramic material may be later cut, punched, fired and glazed to provide high dielectric strength ceramic plates of uniform density for use as substrates for microcircuits.