Electric heating – Metal heating – By arc
Reexamination Certificate
2002-08-02
2003-10-21
Heinrich, Samuel M. (Department: 1725)
Electric heating
Metal heating
By arc
Reexamination Certificate
active
06635846
ABSTRACT:
FIELD OF INVENTION
This invention relates to the field of marking materials with lasers and more particularly to a method of vitrifying heat sensitive coatings to materials.
BACKGROUND OF THE INVENTION
Bricks can be formed from a variety of clay-based materials and in a variety of colors and sizes. Primary materials include surface clays, fire clays, shales or combinations of these. For proper forming, such materials must have plasticity when sufficiently wetted, rigid when dried. Bricks can be then formed by extrusion, molding or dry-pressing and are fired in a kiln at temperatures between 1800° F. and 2100° F. (980° C. and 1150° C.) These variables in manufacturing produce units with a wide range of colors, textures, sizes and physical properties.
Naturally occurring clays are divided into specific types having particular properties. For example, clay is defined as a natural, mineral aggregate consisting essentially of hydrous aluminum silicate. It is a product of decomposition and alteration of feldspathic rocks and contains a mixture of particles of different sizes and widely differing physical, chemical and mineralogical properties. The non-plastic portion consists of altered and unaltered rock particles of which the most common and abundant substances are quartz, micas, feldspars, iron oxides, and calcium and magnesium carbonates. Organic matter usually is present in greater or lesser amounts, and frequently plays an important role in determining clay properties. The essential constituents of clays are hydrated silicates of aluminum, of which there are several, but the most important and widespread are the kaolinite group. The typical clay minerals—kaolinite, montmorillonite, etc., have microscopic plate-like structures which are chiefly responsible for their plasticity (formability) when wetted with water. The fineness of a clay's grain influences not only its plasticity but also such properties as drying performance, shrinkage, warping, strength and quality of marking achievable by laser energy. Clay's with high aggregate contents, sands and organic matter are prone to poor glassy vitrification by laser energy. Clays occur in three principal forms, all of which have similar chemical compositions but different physical characteristics. Surface clays may be the upthrusts of older deposits or of more recent, sedimentary formation. As the name implies, they are found near the surface of the earth.
Shales are clays that have been subjected to high pressures until they have hardened almost to the form of slate. Fire clays are usually mined at deeper levels than other clays and have refractory qualities. As a rule, they contain fewer impurities than shales or surface clays and have more uniform chemical and physical properties. Surface clays and fire clays differ from shales more in physical structure than in chemical composition. Chemically, all three are compounds of silica and alumina with varying amounts of metallic oxides and other impurities. Although technically metallic oxides are impurities, they act as fluxes, promoting fusion at lower temperatures. Metallic oxides. (particularly those of iron, magnesium and calcium) influence the color of the finished fired product. The brick manufacturer minimizes variations in chemical composition and physical properties by mixing clays from different locations in the pit and from different sources. However, because clay products have a relatively low selling price, it is not economically feasible to refine clays to produce uniform raw materials. Since variations in properties of raw materials must be compensated for by varying manufacturing processes, properties of finished products from different manufacturers will also vary somewhat. The widespread usage of bricks as a building material on highly visible areas such as walkways and building fronts has led artisans to attempt to etch and decorate such materials with letters and/or graphical patterns.
It is known that glass can be formed by melting or fusing materials under extremely high temperatures by a process called vitrification. It is also known that lasers produce intensely focused beams of light at specific wavelengths which results in localized heating of an object which falls in the path of the active beam. Laser light can be produced and amplified by a variety of sources including, for example, Nd:YAG lasers produce laser light at a principal wavelength of 1064 nanometers (nm). Nd:YAG lasers can be operated as a continuous wave (CW) laser or with pulse or frequency modification. In the latter instance, a Q-switch is used to reflect the laser beam back into the lasing chamber to build up more power before the beam is released. The result is a pulsed laser with each pulse being more intense than a continuous wave beam from the same laser unit. Other lasers, such as carbon dioxide (CO
2
), can be constructed and configured for different wavelengths and power outputs.
Laser marking of bricks, pavers, terra-cotta tiles, and other high clay content materials is known in the art. U.S. Pat. No. 6,064,034 previously issued to the Applicant, the contents of which are incorporated herein by reference, discloses a process for the vitrification of bricks and other vitrescent objects by use of a laser. In particular, a continuous wave beam such as that provided from a ND:Yag or CO
2
laser operating in a power range of 50-250 watts with a collimator and lens provides a laser beam intensity in the range of about 1.6×10
5
-1.4×10
6
watts/cm
2
for use in marking of a vitrescent object. The laser beam is steered by a computer to produce lettering and graphical patterns on the vitrescent object. For process efficiency and enhancement of the appearance, the surface of the vitrescent object is heated to a temperature of about 100° F. prior to vitrification, or dried so as to achieve a predetermined moisture content.
U.S. Pat. No. 5,538,764 discloses a method of removing a surface layer from a hydraulically bonded material such as concrete. The disclosure is directed to the use of a laser having a power density between 100 W/cm
2
and 800 W/cm
2
causing surface layer removal between a depth of 1 mm to about 4 mm.
U.S. Pat. No. 5,554,335 is directed to the marking of ceramics by three distinct laser power conditions. A first laser power condition is carried out in a first finite time period, a second laser power condition is carried out in a second time period, and a third laser power condition is carried out in a third finite time period. This process requires an exceedingly long residence time with a concomitant loss of economic efficiency. Additionally, the quality of marking which results from the practice of this invention does not result in a smooth glassy appearance.
U.S. Pat. Nos. 4,769,310 and 5,030,551 discloses a method for marking ceramic materials, glazes, glass ceramics and other glasses. Ceramic and glass materials have been treated at high temperatures in their formation, and thereby have a glassy surface, or a glaze over their entire surface. Such glassified surfaces are difficult to mark, even with a laser. As a result, the '310 and '551 patents disclose a method of applying a transparent layer of material (e.g. 100 to 10,000 angstroms thick) such as titanium dioxide to the outer surface of the ceramic or glass object, and then irradiating the oxide layer with a pulsed laser beam. The irradiation causes discoloration of the applied oxide layer at the irradiated areas.
Glazed ceramic materials, such as whiteware, often develop regions of cracking due to wear, impacts, thermal stresses and the like. U.S. Pat. No. 5,427,825 discloses a method of repairing such glaze defects by preheating a glaze defect area with radiant energy. The glaze defect area is then treated by applying higher power radiant energy (such as infrared) from a laser to provide localized heating of the glaze material. The surface is further treated with radiant energy at a lower power density so as to limit the rate of cooling of the fusion zone and the immediately surrou
Heinrich Samuel M.
McHale & Slavin P.A.
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