GB855393A - Improvements in or relating to the manufacture of glass tubing - Google Patents

Abstract

855,393. Glass manufacture. GENERAL ELECTRIC CO. Ltd. June 8, 1959 [June 12. 1958], No. 18788/58. Class 56. In the coating of a first glass tubing internally with a layer of a second glass, particularly for making sodium vapour discharge lamps, the tubing is drawn downwardly from a tank containing the first glass in a molten condition through an annular gap between the sides of an orifice in the base of the tank and the external surface of a co-operating hollow mandrel extending vertically downwards into the orifice through the molten glass and the molten second glass is fed downwards through the mandrel and out through an annular aperture in the vicinity of the orifice into contact with the first glass to form a layer on and covering the inner surface of the drawn tubing. Molten first glass 10 in the foreheater 6 of a tank 1 is drawn by any suitable means such as endless belts through a circular orifice 2 in the base, formed by a ring 5, over the lower end of a tubular mandrel 3 adjustably mounted to vary the annular space and therefore the thickness of the tubing 14. Molten second glass 13 is stored in an enlarged upper part 20 of the mandrel 3 and kept at a controlled level from a crucible 54, the glass 13 being fed downwardly between the mandrel 3 and a hollow cylindrical member 11, through which air is blown, held spaced from the mandrel by lugs 18 and flaring outwardly at its lower end to provide a control valve 19, the member 11 being adjustable relative to the mandrel 3 to vary the size of the annular aperture and therefore the thickness of the layer 16 on the inner surface of tube 14. The mandrel 3 is surrounded by a sleeve 9 which is continuously rotated to stir the molten glass 10. The glass 13 is a soft glass melting at a lower temperature than the glass 10 and it is maintained molten by the heat from the glass 10, the sleeve 9 being provided with apertures 55 which can be partially or wholly closed as required by a sleeve 57. Additionally the reservoir part 20 of the mandrel 3 may be heated electrically. The mandrel 3, ring 5, member 11 and sleeve 9 are preferably of "Nichrome" (Registered Trade Mark) coated with platinum or of other refractory material such as fused quartz. In an alternative embodiment, the reservoir for glass 13 surrounds the mandrel and is fed through a number of small holes in the mandrel to the space between it and the member 11, the reservoir being fed from a hopper with finely powdered material. The glass 10 is preferably, in percentages by weight, SiOz 72.0; Al 2 O 3 1À5; CaO 5À0; MgO 3À5; NazO 17À2; K 2 O 0À2; B 2 O 3 0À7; The inner layer is of sodium resistant glass either (1) SiOz 8À0; AI 2 O 3 24À0; B 2 O 3 48À0; CaO 6.0; NazO 14À0; Fe 2 O 3 0.02 and 0À03; or (2) B 2 O 3 25À0; K 2 O 5À5; CaO 9À0; BaO 36À0; Al 2 O 3 24À5.