Page:EB1922 - Volume 31.djvu/324

290 ing treatment. The tank is furnished with an extension at the refining end into which the glass flows and cools sufficiently to be drawn. When in a proper condition, an iron bait in the form of a narrow iron plate is lowered into the molten glass, which welds to it. By means of a hand-actuated device the bait is raised ; the sheet of glass following it is drawn through a pair of narrow water-coojed rollers arranged at each side of the sheet, which assist in maintaining its width, and then over a hard and highly polished roller situated about 30 in. above the drawing pot. Here the glass assumes a hori- zontal position. In the neighbourhood of the bending roller, addi- tional heat is applied to the sheet to prevent any possibility of crack- ing ; the sheet of glass then passes over a flattening plate and enters the annealing lehr. At this point a caterpillar drive pulls the sheet along and furnishes the power for the automatic drawing of the sheet. It will be seen, therefore, that the process is continuous so long as a supply of glass is available. By this process sheet-glass can be produced which may be of any predetermined thickness within wide limits, the governing factors being the speed of drawing and the temperature of the glass in the draw pot. The width of the sheet is about 6 ft., and the speed of drawing for the thin variety is about 2 ft. 6 in. per minute. At first there was a tendency for the glass manufactured by this process to be somewhat cordy, probably due to the surface cooling in the drawing chamber, but this has been overcome and the product is now of very good quality.

In the Belgian method of drawing sheet-glass the space above the glass in the tank is divided into two parts by means of a brick curtain which depends from the roof to a short distance below the surface of the glass; by this means the flame and hot gases are re- stricted to the melting end. Subsidiary ports are provided in the refining end to regulate temperature. In the refining end of the furnace two further similar brick curtains are arranged parallel and comparatively close together; between them floats a debiteuse, a hollow vessel made of fireclay or similar refractory material, rect- angular in plan and with rectangular ends, but having a section to within a short distance of each end somewhat like an inverted M with the apex of the central angle cut off, thus leaving a long narrow slit giving access from the outside to the inside of the receptacle. This device has a specific gravity slightly less than that of glass; it floats, therefore, in such a position that the narrow central slit is just below the surface of the glass. Above the refining or drawing end of the tank, an erection in the form of a square tower about 13 or 14 ft. high, made of sheet iron lined with refractory material, is pro- vided. On opposite sides of this tower are sets of double resilient rollers disposed vertically, and so arranged that when the sheet of glass is being drawn the edges of the sheet will pass between, and be gripped by, the rollers. The rollers on one side are driven by suitable gearing from an electric motor.

In drawing a sheet of glass a bait consisting of a narrow flat woven iron sheet of a length equal to the length of the slit in the debiteuse is lowered within the lips forming the slit. When the glass has welded to the bait the latter is raised, lifting with it a sheet of glass. By means of a water-circulating system the glass is chilled sufficiently to retain its form and then passes up between the rollers. When once gripped by the rollers the upward draw is continuous so long as the motive power is applied to the rollers. The bait is removed when the sheet reaches the top of the tower. The tower is provided with a series of inclined iron diaphragms, the upper part of each of which is flush with the rollers. These diaphragms serve the double purpose of preventing broken glass from falling into the tank, and of preventing the heat from the tank ascending the tower. By this means a rough annealing is performed, since the ascending sheet of glass is subject to a gradually falling temperature. When the sheet reaches the top of the tower it is cut to size and packed.

Plate Glass. No special innovations have been introduced in recent years in the methods of manufacturing plate glass, with the exception of the means for annealing the plates. In the older method the plates are placed on the floor of a kiln when the latter is at a dull red heat; the opening is then built up and luted with fireclay. The heat is shut off, and the kiln allowed to cool gradually over a long period. Recently, however, a plant has been installed in the United States for annealing the plates in a continuous lehr, and it is claimed that the glass is equally well annealed as in the old process. The time saved is considerable, being five hours as against three days by the kiln method.

After the glass has been melted in a pot, the latter is taken bodily from the furnace, and the glass poured on to the rolling table, about 28 teet x 16 feet. This consists of a large cast-iron bed, usually made up in segments, carefully bolted together so as to give an even smooth surface and cooled by a water circulating system. A large roller extending the full width of the table, and weighing from 5 to 6 tons, is mechanically driven forward and spreads the glass out into a sheet. Guides are provided at each side of the table upon which the roller bears; the height of the guides governs the thickness of the sheet formed. The plate having been rolled is moved forward into the first section of the lehr, which is maintained at a temperature of about 600 C., and then progresses by an intermittent motion through the other sections of the lehr. The floor of the first sections of the lehr is made up of fireclay slabs, and, in the cooler sections, the glass moves forward on wooden slats or battens, the total length of the lehr being about 400 feet. As a fresh plate is rolled about every

ten minutes, this fixes the period during which a plate remains in any one section of the lehr. After leaving the lehr the plates are carried by a travelling crane to the grinding and polishing shop.

Tube Drawing Machine. There are two types of machine, the semi-automatic and the fully automatic. In the semi-automatic machine the mass of glass on the blowing iron is prepared as in the case of drawing by hand. The drawing machine is installed in a tower about 170 ft. high, in the basement of which is a motor-driven winding drum. A steel wire rope connected to the drum runs straight to a fixed pulley at the top of the tower and down again to the blow- pipe carriage. The carriage is therefore raised or lowered when the drum is operated. The carriage is provided with means for securing the blowpipe, and also with four rollers which permit it to move freely between vertical guides. The glass having been prepared on the blowing iron, a punty is secured to a socket between the vertical guides; the glass, still on the blowing iron, is lowered on to the upper face of the punty and adheres to it ; the blowing iron is then locked in its carriage and the motor started. The speed of the draw governs the size of the tube, which may be regulated by means of a variable speed on the motor. The tube having been drawn, it is parted from the punty, and by means of a band brake gradually lowered and cut up into lengths. Practically any type of tubing can be drawn on this machine, inasmuch as the finished product depends upon the form imposed upon the glass by marvering and blowing prior to being put into the machine.

In the case of the fully automatic machine only tubing having a circular section can be drawn. Glass is ladled from the melting furnace into a specially constructed pot, heated by a system of burners and provided with a baffle extending from the top of the pot down into the glass, and also with an orifice from which the glass flows regularly into a rectangular clav trough. From a small opening in the trough, the size of which can be controlled, the glass flows, in the form of a ribbon, on to a revolving cone. The cone is hollow and made of fireclay, and varies in size according to the tube to be drawn. Longitudinally through the centre of the cone is a steel tube with a nicrome steel cap. This tube is for supplying air to the interior of the glass tube being drawn, and also serves as a means for rotating the cone. The speed of revolution can be governed by the motor. The axis of the cone is inclined so that the apex is depressed. The ribbon of glass from the pot flowing on to the larger diameter of the cone tends to flow by gravity towards the apex, and soon after starting the whole cone is covered with molten glass; the flow con- tinues beyond the end of the cone and maintains its form of a hollow cylinder owing to the air under pressure which is admitted to the central tube. At this stage the glass tube is much larger in diameter than the finished tube, but by the time it has reached a series of pulleys in line the diameter has been reduced to the desired size, and it has cooled sufficiently to retain its form. It continues to pass over the series of pulleys until at about 150 ft. from the pot the tube passes between, and is gripped by, two endless chain belts faced with asbestos sheet pads. As soon as the tube is gripped by the belts a steady pull is maintained. The speed at which the belts travel, combined with the temperature of the glass at the cone, determines the size of the tube. After passing the belts the tube is cut into lengths automatically; they fall into a tray of a rotary conveyor, where they are automatically sorted into separate racks.

Automatic Flowing and Feeding Devices. Various forms of feeds have been devised for delivering a pre-determined quantity of molten glass from a tank furnace to the glass-forming machine. In all of these it is essential that the supply of glass should be maintained at a constant level, and it follows, therefore, that these devices have been applied either to tank furnaces or subsidiary containers which are constantly replenished.

The most primitive form of feed consisted of a simple overflow from a spout or lip with a stream of glass cut at periodical intervals by means of a pair of blades actuated automatically and water- cooled, and an improvement on this form of feed provides for the substitution of one of the blades by a series of cup-like devices fixed at the ends of radial arms rotating from a common centre, in such a way as to bring the cups in succession under the stream of glass. Each of these cups rotates about its own horizontal axis.

At the moment when a mould has been sufficiently charged the edge of a cup meets the moving blade, scissor fashion, in the line of the stream of glass. The glass now falls into the cup which is gradually rotating about its axis. During this part of the operation a new mould has taken up its correct position, the cup continues to rotate and pours the accumulated glass into the mould, into which the now unimpeded stream of glass also falls. The next following cup again intercepts the stream and so completes the cycle.

In another type of feed a spout is provided at the working end of the furnace. On the under-side of the spout there is a hole capable of being varied in size. A pair of shears automaticallv cuts off the glass as it flows from the hole, after which a timing device retards the flow of glass. This governs the quantity of glass delivered.

In another type of feed the glass is delivered by a reciprocating paddle working in a specially devised extension to the tank. The mass of glass which is forced over the lip of the receptacle by the paddle is severed by suitably actuated blades. The mass of glass falls on to an inclined chute, water lubricated, and is delivered into the glass-forming machine.