Production of translucent sheet glass by grinding

Details Production of translucent sheet glass by grinding Production of translucent sheet glass by grinding

2000-03-17

2003-01-28

Vincent, Sean (Department: 1731)

Glass manufacturing

Processes

With wearing away of surface material

C065S059700, C065S099200, C065S094000, C451S041000, C451S259000

Reexamination Certificate

active

06510708

ABSTRACT:

FIELD OF THE INVENTION
This invention relates to translucent smooth glass sheets, to novel processes for their production and to novel methods of operating a glass grinding and polishing production facility so as to produce translucent smooth glass sheets.
DESCRIPTION OF THE PRIOR ART
Translucent glass sheets find use in a variety of applications in which their is a need for light transmission coupled with a desire for a degree of privacy. Products of this type can be made by a variety of processes. Shot blasting processes are used to produce a translucent sheet product having a relatively rough surface. The roughness of the surface means that these products are not acceptable for all applications. Acid etching processes are used and produce a product having a relatively smooth surface. However acid etching processes present obvious safety and environmental hazards and are thereby expensive to operate. Rolling processes using an appropriate roller may also be used but product a produce with a relatively rough surface.
Clear smooth glass sheets are currently produced almost exclusively by the float glass process. Prior to the advent of the float glass process in the late 1950's clear smooth glass sheets were produced by grinding and polishing relatively rough glass plates. Typical grinding and polishing processes are described at page 705 of “The Handbook of Glass Manufacture” Volume II, edited by Fay V Tooley and published by Books for Industry, 777 Third Avenue, New York, N.Y. 10017. The use of glass grinding and polishing facilities to produce sheet glass is now largely confined to the production of glass products which cannot be produced using the float process. The most prominent example is the production of polished wired safety glass.
SUMMARY OF THE INVENTION
We have now discovered a novel method for the production of a translucent glass sheet which comprises grinding at least one surface of a clear glass sheet with a suitable abrasive. These methods may be and preferably are carried out using an existing facility for the grinding and polishing of sheet glass. The clear glass sheet may be subjected to all or part of the grinding process but is not polished. The product of these grinding processes preferably have a smoothness close to that of acid etched glass. The ground surface has characteristics which distinguish those products from the acid etched glass sheets. Certain of those ground glass sheets are believed to be novel and those sheets comprise a second aspect of the invention.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT OF THE INVENTION
From one aspect this invention provides a process for the production of a translucent smooth glass sheet which comprises grinding at least one surface of a clear flat glass sheet under controlled conditions so as to produce a translucent sheet having a smoothness better than 5.0 &mgr;M Ra.
Ra is a measure of roughness of the glass surface which is the arithmetic mean of the absolute departures of the roughness profile from the main line. The translucent glass sheet will generally be less smooth than the clear sheet from which it is produced. In this specification all measurements of the surface of the translucent glass sheet were obtained using a KSA Tencor P10 profiler (both to produce a three-dimensional image and a two-dimensional one line scan profile) unless specifically stated otherwise. A stylus having a radius of 0.1 &mgr;M was employed using a loading of 0.5 mg. The short range filter was maintained at the instrument's default setting of 8.3 &mgr;M corresponding to a scan rate of 50 &mgr;M per second and the data point sampling rate of 50 Hz. No long-range filter was employed. Three-dimensional images were recorded over an evaluation area of 250 &mgr;M×250 &mgr;M. Fifty scans were collected per image giving a scan line spacing of 5 &mgr;M. The two dimensional profiles were recorded over an elongation length of 4 mm.
The ground glass sheets produced by the processes of this invention preferably have Ra values falling within the range 0.5 &mgr;M to 5.0 &mgr;M. Preferably they will have an Ra value falling within the range 0.5 &mgr;M to 3.0 &mgr;M or most preferably 0.1 &mgr;M to 2.0 &mgr;M.
The preferred type of clear flat glass sheet for use in the processes of this invention is a sheet of float glass. Float glass has a very smooth surface, better than 0.1 &mgr;M Ra and is widely available in a variety of thicknesses. In the preferred embodiments the clear float glass sheet will have a thickness of from 4 mm to 25 mm. Thinner sheets may be employed but are more liable to break during the grinding process. Thicker sheets may be ground if desired. In a second preferred embodiment the clear flat glass sheet is a rolled wired glass sheet. Safety glazing with a wire grill insert is widely employed. Rolled wired flat glass is conventionally ground and polished so as to remove the pattern applied by the roller to produce a clear polished wired glass. In the processes of this invention the clear wired glass is ground under controlled conditions so as to produce a translucent wired glass. The clear wired flat glass generally has a thickness of from 5.0 to 8.0 mm.
The clear glass sheets will preferably be ground on one surface only. Both sides can be ground if necessary. The desirability of grinding on one or both sides will normally depend upon the intended use of the ground glass sheet.
The grinding process reduces the thickness of the glass sheet. In general grinding one face of a glass sheet will reduce the thickness of the sheet by approximately 0.5 mm.
The grinding processes of this invention may be carried out by mounting the clear flat glass sheets upon a suitable flat and level table and passing them under a grinding head which is fed with a suitable abrasive medium. The grinding head may conveniently be made from cast iron. A wide variety of materials have been proposed for use as abrasives in glass grinding processes. Any of these abrasives are potentially useful in the processes of this invention. The most commonly used abrasive and the preferred abrasive in the processes of this invention is sand. The sand is fed to the grinding head in the form of an aqueous suspension of sand.
Normally the grinding operation will be carried out using a progressively finer abrasive during the process. The coarser more abrasive particles are fed to the initial stage of the grinding process and are replaced by finer particles as the grinding proceeds. The process may be carried out using two main variants of the production process. In the first variant the clear glass sheet is passed under a series of grinding heads each of which is fed with a specific grade of abrasive. The abrasive particles from each stage are recovered, regraded and passed to a subsequent stage until they are too abraded to be of further use. In the second variant the glass sheet is placed under a single grinding head and a progressively finer grade of abrasive is fed to that head as the grinding progresses.
The nature of the abrasive, the pressure on the grinding head and the duration of the grinding process all have a significant effect upon the properties of the ground glass sheet which is produced. It is within the routine skill of the art to adjust these parameters in such a way as to produce the desired product. In particular the roughness of the ground side of the glass may be controlled to fall within the desired region.
The degree of translucency is influenced by the parameters of the grinding process and the process is controlled so as to produce a desired translucent product. For the intended purpose of privacy glazing the panels have a milky appearance which prevents an observer seeing an object on the far side of the glazing unless the object is immediately adjacent to the glazing.
The grinding processes of the present invention may impart novel characteristics to the surface of the glass and lead to the production of novel glass sheets having new and useful properties.
Ground translucent float glass itself is believed to be novel a

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