BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is an overall view of an apparatus to produce glass shaped bodies;
FIG. 2 is a cross-section view of a conventional nozzle;
FIG. 3 is a cross-section view of a nozzle of the present invention;
FIG. 4 is a cross-section view of a nozzle of the present invention;
FIG. 5 is a cross-section view of a nozzle of the present invention; and
FIG. 6 is a cross-section view of a nozzle of the present invention.
DETAILED DESCRIPTION OF THE INVENTION
Embodiments of the present invention are explained with reference to FIGS. 1 to 6.
FIG. 1 is an overall view of a glass production apparatus that utilize a nozzle of the present invention. The glass production apparatus includes a melting device, a flow-out device (nozzle) and a molding device. Raw materials of glass are typically poured into a crucible within a melting device, then heated and molten at a predetermined temperature to prepare molten glass. The flow-out device is typically a nozzle made of heat-resistant metal; the molten glass flows out from another end into a molding tool within the shaping device through a flow-out device after optional treatments of clarification, defoaming, stirring, etc. The molding tool may be of various configurations depending on the preforms to be produced. For example, when plate glass is produced, the molten glass flows down as a continuous flow onto an substantially quadrangle molding tool, and when being float-shaped, in general, the molten glass is dropped on a porous molding tool having circular depressions.
FIG. 2 represents a cross section of a conventional nozzle. 1 represents molten glass, 2 represents a nozzle, 3 represents a dropping glass gob, and 4 represents a molding tool to receive the dropped glass gob. A plurality of arrows within the molten glass expresses the temperatures of molten glass in relation to the lengths of the arrows.
As shown in FIG. 2, the temperature of molten glass is higher toward the central portion of the nozzles, and the temperature decreases and thus the viscosity increases toward the inner wall of nozzles; therefore, the dropped glass gob is mainly of the higher temperature portion that drops from around the central portion. The reason to take such a distribution is that higher temperature portions and lower temperature portions are not significantly mixed and far from being able to exchange heat in the configuration of conventional nozzles. It is also difficult with conventional nozzles to correctly measure and control the temperature of glass stream center. The resulting glass forming glass gobs tends to cause striae, because of no control into an appropriate temperature distribution.
FIG. 3 represents a cross section of an embodiment of the nozzle of the present invention. In the nozzle shown in FIG. 3, a baffle plate 5 is provided at a site inside the nozzle that restricts a portion of flow path to rapidly change the flow direction. Therefore, the flow path is discontinuously narrowed at the site where the baffle plate 5 is disposed, and the center of the flow path at the site is shifted from the center of the flow path at the upstream.
When such a configuration is employed, the flow path of the higher-temperature glass stream, which has flowed through around the upstream center of the nozzle, is forcibly shifted to around the inner wall of the nozzle; in this case, the difference of flow velocity from the lower-temperature glass stream, which had flowed originally through around the inner wall of the nozzle, is reduced. Consequently, the temperature of the glass is more uniformalized in the nozzle. When passing through the baffle plate 5, the glass stream is uniformalized in terms of the temperature distribution, and also the temperature can be measured and controlled more accurately; therefore, when the glass gobs are flowing down, a higher-temperature glass stream or inadequate temperatures, which may induce striae, is unlikely to occur. The glass stream passes through the narrow flow path formed by the baffle plate 5 in FIG. 3; accordingly, the flow path formed by one baffle plate 5 is not defined as one site, as long as the effect to shift the flow path is adequately derived.
FIGS. 4 and 5 represent cross sections of other embodiments of the nozzles of the present invention. In the embodiment shown in FIG. 3, the center of gravity of the cross section aligns relative to the upstream and the downstream of the site where changing the flow path of the glass stream, whereas, in the embodiments shown in FIGS. 4 and 5, the flow path does not align once being changed and the glass stream flows in another flow pass; either embodiments of the present invention may be employed. In these cases, the narrow flow path formed by shifting the flow path is not defined as one site.
FIG. 6 shows an embodiment in which a plurality of baffle plates 5 is disposed within a nozzle, and the normalizing of the temperature distribution is promoted by way of increasing the stirring effect on the glass stream using two baffle plates. The baffle plates may be disposed in the same or different directions to each other.
EXAMPLES
Specific examples of the present invention are explained in the following.
Example 1
In this Example, an optical glass was molten within a crucible, the molten glass was flowed through a nozzle connected to the crucible and flowed out from the terminal flow outlet, and was float-shaped on a molding tool, which was ejecting gas and made of porous stainless steel, thereby obtaining glass gobs to use as a preform for precision press molding.
The nozzle in use was a platinum nozzle having the same structure as shown in FIG. 3 described above. The inner diameter of the nozzle without the baffle plate 5 was 3 mm (cross-section area: 7.07 mm2), and the flow outlet was expanded to 6 mm. The total length of the nozzle, i.e. the length from the outlet of the nozzle to the flow outlet of the nozzle end, was 2 m.
The baffle plate was attached within the nozzle at the site of 47 mm from the flow outlet, and the thickness of the baffle plate was 1 mm. The area of the glass flow path was 0.79 mm2 at the portion where the baffle plate was attached. That is, the cross-sectional area of the flow path where the baffle plate is attached was about 11% based on that of the other area.
The receiving mold was made of porous stainless steel; glass gobs were obtained by way of receiving the molten glass under a condition of ejecting air from the receiving surface, thereby receiving the molten glass in a condition of floating on the receiving mold.
The glass in use was prepared by melting an optical glass mainly containing boron oxide and lanthanum oxide. The crucible was maintained at approximately 1200° C., and the flow-out pipe was maintained at approximately 1100° C. The molten glass was made under the condition of separating into droplets from the flow outlet. The flow rate of the molten glass was 80 grams per minute at this time.
The glass gobs were visually observed with respect to optical defects such as devitrification and striae; as a result, such defects could not be found, and the glass gobs were of high quality available as preforms for forming optical elements.
Comparative Example
A Comparative Example is shown in comparison to Example 1. Glass gobs were obtained in the same manner as Example 1, except that no baffle plate was disposed within the nozzle.
The glass gobs were visually observed with respect to optical defects such as devitrification and striae; as a result, the existence of striae could be visually confirmed, and the quality of the glass gobs was inadequate for a raw material to form optical elements.
Example 2
Glass gobs were obtained in the same manner as Example 1, except that baffle plates were disposed at two sites of 30 mm and 90 mm from the flow outlet within the nozzle. The thickness of each of the baffle plates was 1 mm. The resulting glass gobs were high-quality glass gobs similar to Example 1, and no optical defects such as devitrification and striae could be visually observed.