Glass manufacturing – Processes – Reshaping or surface deformation of glass preform
Reexamination Certificate
2000-04-28
2002-06-25
Colaianni, Michael (Department: 1731)
Glass manufacturing
Processes
Reshaping or surface deformation of glass preform
C065S104000, C065S114000, C065S115000, C065S117000, C065S268000, C065S269000, C065S271000, C065S033200, C065S355000, C264S432000
Reexamination Certificate
active
06408649
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to the thermal treatment of any type of glass and glass-like materials, preferably of a glass sheet for shaping, bending, tempering, annealing, coating and float processing by rapidly and uniformly heating the glass sheet with microwave radiation so that the glass sheet can be processed without cracking. Glass and glass-like materials which may be thermally treated by the inventive method include flat glass sheets, glass fibers, organic mixtures incorporating glass and glass-like materials and the like. Glass sheets treated by this method can be used in the production of windshields, side windows and rear windows in vehicles such as automobiles and the like as well as for the production of architectural window glass and the like.
BACKGROUND OF THE INVENTION
Thermal treatment of glass and glass-like materials is widely used for the production of vehicle windows, architectural glass, fiberglass ceilings and composites is and the like. A huge number of glass and glass-like materials and especially glass sheets are thermally treated each year worldwide.
One of the main problems in the thermal treatment of glass (e.g., the shaping or bending of glass sheets), is that the increasing heating rate of these glass sheets must be kept comparatively low when the glass is heated from room temperature to a softened temperature to prevent the glass sheet from cracking. Otherwise, cracking of the sheet can occur if different areas of the glass sheet are heated to different temperatures. This temperature differential between different areas or layers of the sheet raises the internal stresses in the glass to the point where these stresses become higher than the sheet modulus of rupture of the heated glass causing the glass to crack or shatter.
Generally, glass sheets are thermally treated by; conveying the sheets via an appropriate support mechanism through a horizontal tunnel-type furnace; heating them while in the furnace using infrared, hot air, gas or a combination of these methods to a heat softened temperature; and then shaping or bending the glass sheets. After shaping, the glass sheet is transferred to a cooling station where the sheet is controllably cooled. The described process assures that the thermal treatment is conducted at appropriate production rates. Numerous patents (see, e.g., U.S. Pat. Nos. 5,882,370, 5,858,047, 5,743,931, 5,352,263, 5,022,908, 5,079,931, 5,078,774, 5,066,320, 5,059,233, 5,057,138, 5,057,137, 5,032,162, 5,005,318, 4,986,842, 4,983,201, 4,976,762, 4,881,962, 4,816,055, 4,767,439 and 3,951,634) describe different methods of thermally treating glass sheets using tunnel-type furnaces. In all of these methods, the total heating time for each sheet while resident in the furnace is hundreds of seconds.
Productivity of tunnel-type furnaces can be increased, but only in limited ways. The simplest way to increase productivity is to make the furnace longer. A longer furnace allows the conveyer's speed to be increased because the total heating time for each sheet is correspondingly increased, allowing the necessary low temperature differential between the external surfaces and inside layers of the glass sheet to be maintained. However, even without this modification, existing furnaces are too long, massive, expensive and inefficient and have material handling problems.
For example, there is high heat conduction from rollers to glass in tunnel-type furnaces, which together with the convection and radiation heat below the glass results in the amount of heat transferred from the lower surface of the glass exceeding that transferred to the upper surface of the glass through convection and radiation alone. In addition, there is non-uniformity of the glass temperature in the conveying direction that leads to deterioration of the surface quality and optical quality properties of the glass sheets.
Even if the glass sheet is evenly heated within this type of furnace, when the glass sheet is delivered out of the furnace, the leading and trailing ends of the glass sheet are cooled for different periods of time before the glass sheet reaches the pressing/bending position in the shaping/bending device. This can result in cracks in the glass sheet when it is pressed and bended.
Additionally, it is often difficult to reliably accomplish local heating for combined shaping and bending of glass sheets especially in auto and structural glass production when a massive furnace is used. These localized heating operations require expensive furnace modification (see, e.g., U.S. Pat. Nos. 5,735,922, 5,591,245 and 5,755,845) and still cannot guarantee the effective overheating of the limited locations and temperatures needed for bending.
Rather than eliminate the use of tunnel-type furnaces in the thermal processing of glass sheets, most improvements in the art have focused on reducing the size of the furnaces (which reduces the total heating time) without reducing production rate. One of the ways to achieve this goal is to increase the power of the heat applied to the glass at the moment the glass temperature exceeds around 450° C. by creating a separate chamber at the end of the furnace heated by powerful electric or gas heaters having a temperature range of 800° C. to 900° C. (U.S. Pat. No. 5,232,482) or 800° C. to 1000° C. (U.S. Pat. No. 5,306,324). Microwave energy has also been used to reduce the total heating time by treatment with microwave radiation only at the end of the glass heating process starting at a temperature around 420-450° C. (U.S. Pat. Nos. 4,838,915, 4,471,192, 5,656,053 and 5,822,879).
Another way to reduce furnace length and heating time is to increase heat uniformity. Many patents focus on the solution of problems related to the non-uniform heating of glass including designing heating elements that are separately and independently controllable and which create the necessary distribution of heat by joining infrared and hot air heating (U.S. Pat. Nos. 5,908,000, 5,368,624 and 5,296,270), designing special heating devices (U.S. Pat. Nos. 6,005,230 and 4,888,038), creating a special temperature gradient over a glass surface (U.S. Pat. No. 5,149,352) and using furnace roller heat (U.S. Pat. No. 4,591,374).
However, as noted above, the common feature of all of the abovementioned patents is that they describe methods to reduce the total heating time of a glass sheet in tunnel furnaces and are not directed to (or capable of) eliminating this type furnace in glass processing. Thus, there is a clear need in the art for a method for the rapid heating of glass sheets which eliminates tunnel type furnaces or which shortens their length considerably to make them more effective and less expensive. A need also exists for a treatment method that can ensure high-speed heating of narrow, localized places on a glass sheet.
The main problem with increasing the rate of heating is the inevitability of creating temperature differences between the interior and the external surfaces of the glass sheets. As discussed above, different patents describe the equalization of infrared or convection heat on glass surfaces. Microwave heaters can be employed for this purpose as well (see, e.g., U.S. Pat. No. 5,828,042, U.S. patent application Ser. No. 09/439,533 filed Nov. 12, 1999). Hypothetically, the methods described therein at least provide a way to keep this temperature differential to a minimum on the external glass surface or surfaces. However it is extremely difficult to reduce the temperature differential throughout the thickness of the glass sheet without changing its properties.
The unsuitability of conventional radiation sources for the rapid heating of a glass sheet is illustrated by the following example and FIG.
1
. Glass highly absorbs infrared radiation and is opaque to hot air and gas as well. Thus, when infrared or convection heat
1
transmitted by hot air, a gas, or the like interacts with a glass sheet
2
the main portion of the power is absorbed by the thin layers of the glass located at the external surfaces of the glass sheet
Shevelev Mykhaylo
Sklyarevich Vladislav E.
Colaianni Michael
Gyrotron Technology, Inc.
Morgan & Finnegan , LLP
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