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Soda—lime glass accounts for over 90% of the glass produced worldwide due to its low melting temperature which facilitates continuous high volume production. Nearly all the flat soda-lime glass is now produced on the float process, eliminating all mechanical surface grinding and polishing operations. This has kept the price of the glass nearly constant over the last twenty-five years. The float process was developed in the late 1950's and was in full utilization by the end of the 1970's. A brief overview of the float process is included, along with key statistics regarding the current capacity, efficiencies, etc. Advances in float technology now result in very parallel high quality surfaces free from major distortions and lenses. However, it is known that this glass is not normally suitable for critical optical applications and we have been exploring +e flatness issue culminating in the introduction of LOF Ultrafloat Glass in 1991. Our efforts to produce a flatter float glass have centered on two critical production areas, as well as the instrumentation/QC procedures needed to insure compliance. The two critical production areas are: 1) hot end lehr roll quality and 2) bath exit/lehr zone temperatures. We have been continually refining these on production runs of the Ultrafloat product, hampered only by the high fixed cost of the float process and the need to service our conventional flat glass customer base with minimal downtime. Details are discussed in this paper. Additional effort was invested in training the plant QC personnel in operation of the interferometer and the interpretation/statistical treatment of the massive amount of interferometric data generated on a continuous glass ribbon 3.3 meters across. Our experience has shown that we can now produce high quality glass with flatness better than 11 fringes across 4" (at the HeNe wavelength), with some areas considerably flatter. These figures apply generally to a 5mm thick glass; thinner glass is appreciably worse as the equilibrium thickness of molten glass on molten tin is about 6.25mm and thin glass undergoes considerable stretching to form the thin ribbon. Future work will include exploring the absolute flatness limit on the float process, the repeatability and process control parameters, as well as the ability to produce larger plates within a given flatness tolerance. Related work will include the effect on parallelism and minimization of internal glass defects.
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