GB893651A - Method of forming lens blanks - Google Patents

Abstract

893,651. Glass manufacture. PITTSBURGH PLATE GLASS CO. Aug. 10, 1960 [Aug. 17, 1959], No. 27682/60. Addition to 891,822. Class 56. A method of manufacturing a multifocal lens blank comprises placing a blank 10 of glass having a ground and polished countersink 12 formed therein below a previously sheared stream of glass issuing from an orifice O, causing the rate of flow across the cross-section of the orifice to vary in an asymmetrical pattern, directing the stream of glass on the supported blank 10 so that the initial shear mark 56 impinges upon the blank adjacent and outside an edge of the countersink 12, flowing the deposited glass across at least a portion of the countersink 12 to fuse thereto, providing relative radial movement between the blank 10 and the axis of the stream, while continuing the flow thereon, to assist the flow of glass across the countersink 12, stopping the relative movement and shearing the stream at 56<SP>1</SP>, so that the resulting gob of glass flows over and fills any remaining unfused surface of the countersink whereby the shear marks 56, 56<SP>1</SP> on the gob are positioned outside the countersink 12. The blanks 10 are mounted on support blocks 20 so that the countersink 12 is directed downwardly to one side of the block 20 and centrally of a groove 26 by which the block 20 is mounted on complementary lugs 24 on a conveyer 18 for passage through a tunnel lehr L to preheat the blanks, the blocks 20 being aligned against a stop 28. The heated blank 10 emerges from the lehr L under the orifice O of a melting tank MT, the orifice O being provided with a spreader tube 50 as disclosed in the parent Specification. The glass stream, which has been cut by shears 32 to leave a shear mark 56, descends to contact the countersink 12 as in Fig. 6 with the mark 56 outside the countersink. After a short period while the glass spreads across the blank 10, the block 20 is moved radially of the orifice O by a pusher device 36 from the dotted to the full line position of Fig. 8 and after a further short period of time the glass is sheared by the shears 32 to leave a mark 56<SP>1</SP> which is on the outer surface of the glass, both marks 56, 56<SP>1</SP> being removed in subsequent operations to form the lens. The pusher 36 then operates again to move the block 20 on to a transfer belt 34 from which it is pushed by a pusher device 38 on to the conveyer belt 40 of an annealing lehr A, the timing of the operations being controlled by a timer T. The blank 10 may be of glass with the following composition in percentages by weight: SiO 2 70.4; Na 2 O 8.4; K 2 O 7.7; CaO 11.9; Na 2 SO 4 0.4; Al 2 O 3 0.1; Sb 2 O 5 1.1 and the minor segment of the following composition: SiO 2 45.0; PbO 46.0; Na 2 O 2.1; K 2 O 6.4; Sb 2 O 5 0.5. The glass for the minor segment is rolled into flat sheets which are annealed before breaking to form cullet. The cullet is melted in a platinum-lined furnace and passes to a refining chamber at about 2550‹F. and then to a stirring chamber at about 2250‹F. Specification 749,169 also is referred to.