Abstract:
A method and apparatus for manufacturing spheres from molten material, such as glass, by allowing drops of the molten material to pass from one end to another of a column containing a liquid, such as a mixture of salts, the temperature of the column being so graduated that the drops harden during passage through the column. The column can be bifurcated at the lower end into legs provided with cooling cuffs for alternate use.

Description:
FIELD OF THE INVENTION 
     The present invention concerns a method of manufacturing spheres from a molten material, and apparatus for carrying out the method. The invention is particularly, though not exclusively, concerned with making glass spheres. 
     BACKGROUND OF THE INVENTION 
     It is known that such spheres may be manufactured by a number of casting or molding methods using molds specifically adapted in each case to the material used. Such known methods and the apparatus utilizing them have a number of disadvantages, chief among them being the fact that it is extremely difficult to eliminate the parting line on the final sphere, corresponding to the line on which the two halves of the mold close, and the protuberence left by the so-called sprue or entry port of the molten material. The accuracy of the resulting sphere is determined by the condition and the shape of the mold, which in turn are adversely effected by frequent use, quite apart from the difficulty in initially arriving at a perfect shape. Another method which is used particularly with glass spheres and which may also supplement the previously described method is to grind a large number of rough spheres in a rotating drum together with an abrasive powder. This method produces spheres with matt surfaces which are also relatively inaccurate and exhibit considerable differences between their major and minor diameters. 
     SUMMARY OF THE INVENTION 
     Accordingly, in one aspect the present invention consists of a method of manufacturing spheres from molten material in which a drop of the molten material is allowed to sink or to rise through a column filled with a liquid with which said molten material either does not react chemically at all, or does so only to an insignificant extent, the temperature of the column being so graduated that when the drop has passed along the column, it will have hardened sufficiently to be handled. 
     In another aspect the present invention consists of apparatus for carrying out the above method comprising means for melting the material and arranged to deliver molten drops of the material to one end of a substantially upright tube carrying a liquid material through which the drops can move either by falling under gravity or by floating, means being provided for maintaining a temperature gradient in the liquid material such that the liquid at the end of the tube where the drops are introduced is hotter than the liquid at the other end, the temperature of the liquid at the said other end being below the point at which the molten material hardens. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     An embodiment of the present invention will now be described, by way of example, and with reference to the accompanying drawings, in which: 
     FIG. 1 is a diagrammatic side view of apparatus for manufacturing glass spheres constructed in accordance with the present invention, and 
     FIG. 2 is a diagrammatic front view of the apparatus of FIG. 1. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The apparatus shown in the accompanying drawings is for use in a process for making spheres from a fusible material such as glass. A typical glass would be of the kind known as &#34;Sovirel MO.&#34; Thus, in the drawings, 1 represents the hearth floor of a conventional furnace for melting glass and capable, for example, of heating the molten glass to 1100° C. The molten glass is allowed to drip through a nozzle 2 of suitable heat and chemical resistant material into a sphere-forming column generally indicated at 3 and comprising a tubular body 4 containing a liquid material 5. A constant head of liquid glass is provided in the furnace, and the temperature of the glass and the nozzle diameter are so chosen that the drops of molten glass falling from the nozzle 2 are of the required size. 
     The liquid material 5 is a salt mixture selected to give a uniform melt at all temperatures down to at least a few degrees below the annealing point of the glass whilst having at this temperature a density slightly less than that of the glass at the same temperature. The precise value of the density difference is not critical and depends to a small extent on the size of the spheres to be produced. Nevertheless the density difference should be of the order of 0.05 to 0.2. The faster the rate of fall of the drops through the liquid salt the greater is the distortion of shape. The rate of fall is governed by the relative densities, but the high viscosity of glass avoids practical difficulties in this matter. 
     The liquid material 5 is preferably a mixture of salts and one such mixture consists of NaCl, KCl and CsCl. For Sovirel MO glass a mixture of ZnCl 2  and NaCl may also be advantageous. The temperature of the salt mixture 5 is adjusted so that the drops of molten glass cool slowly as they fall through the mixture, allowing the drops to assume nearly perfect sphericity. In the present example the temperature of the salt mixture at the top of the column 3 is maintained at 750° C, and it gradually drops to 380° C at the bottom of the column. To counteract the continuous input of heat with the molten glass, cooling fins 12 are provided at the top of the column 3. The annealing temperature of the molten glass is approximately 560° C. The level of melt in the column 3 is maintained roughly constant by being replenished with melt from a continuously heated reservoir 6. Close to its lower end the column 3 is bifurcated so that a substantially continuous throughput of glass spheres can be obtained in a manner to be described. 
     When the installation is put into operation, the salt mixture 5 in the column 3 is brought up to temperature by suitable external heating means, such as insulated electrical resistance wire wound on the outside 13, while a flow of cooling water is maintained through cooling cuffs indicated at 7, 8, 9 and 10. The effect of these cooling cuffs is to cool the salt mixture adjacent thereto so that plugs of solid salt are obtained. When stable temperatures have been achieved, the cooling water in cuff 9 is turned off, the result of which is that the salt plug adjacent to the cuff 9 is melted. The nozzle 2 is opened and the molten glass drips into the salt mixture 5. The drops descent at a decreasing rate down the column 3 and are deflected by the solid, still cooled plug of salt mixture at the left-hand leg so that the drops pass into the right-hand leg. As this leg fills up with spheres the level of the salt mixture rises in the column 3, the initial level being selected so that the final level is still a few centimeters below the nozzle 2. 
     The cooling water in cuff 7 is now turned off and that in cuff 9 turned on so that the plug of solid mixture in the upper end of the left-hand leg of the column 3 melts while a solid plug is formed in the upper end of the right-hand leg, thus isolating the hardened spheres which have fallen into the right-hand leg. Furthermore fresh drops entering the column are now deflected into the left-hand leg. The cooling water in the plug 10 is now turned off and a removable sealing plate 11 at the bottom of this leg heated electrically so that the lower plug in this leg melts and the spheres and salt now isolated in this leg can be discharged through a metal mesh into a heated container which is not shown. The spheres are then passed through a lehr and washed free of salt while the discharged salt is returned to the reservoir. This process can be repeated indefinitely so that a substantially continual throughput of spheres is achieved. 
     Although the column described is vertical, it may alternatively be slightly inclined in whole or in part so as to allow the disposition of a number of columns around closely disposed groups of nozzles in the hearth floor of a single furnace. 
     All the metal parts of the apparatus described must be of a material which is highly resistant to corrosion. The alloy known as Inconel 600 has been found suitable for this purpose.