Patent Application
20100127420
The invention relates generally to methods and apparatus for forming shaped articles. More specifically, the invention relates to a method and an apparatus for reforming a thin sheet of material into a shaped article.
Molding is a common technique used to make shaped objects. Precision molding is suitable for forming shaped glass articles, particularly when the final glass article is required to have a high dimensional accuracy and a high-quality surface finish. In precision molding, a glass preform having an overall geometry similar to that of the final glass article is pressed between a pair of mold surfaces to form the final glass article. The process requires high accuracy in delivery of the glass preform to the molds as well as precision ground and polished mold surfaces and is therefore expensive.
Press molding based on pressing a gob of molten glass into a desired shape with a plunger can be used to produce shaped glass articles at a relatively low cost, but generally not to the high tolerance and optical quality achievable with precision molding. Where the molten glass has to be spread thinly to make a thin-walled glass article having complex curvatures, the molten glass may become cold, or form a cold skin, before reaching the final desired shape. Shaped glass articles formed from press molding a gob of molten glass may exhibit one or more of shear marking, warping, optical distortion due to low surface quality, and overall low dimensional precision.
In one aspect, the invention relates to an apparatus for forming a shaped article having a first surface with a first surface profile and a second surface with a second surface profile. The apparatus comprises a first end mold having a cavity formed therein. The cavity is defined by a surface having at least a portion of the first surface profile. The apparatus further includes an intermediate mold having a hole formed therein. The intermediate mold is distinct from the first end mold and is configured for stacking against the first end mold such that the hole is aligned with the cavity. The apparatus includes a second end mold having a protuberance formed on a surface thereof. The protuberance is defined by a surface having at least a portion of the second surface profile and is sized for insertion into the hole and cavity.
In another aspect, the invention relates to an apparatus for forming a plurality of shaped articles, wherein each shaped article has a first surface with a surface profile and a second surface with a second surface profile. The apparatus comprises a first end mold having a plurality of cavities formed therein. Each of the cavities is defined by a surface having at least a portion of the first surface profile. The apparatus includes an intermediate mold having a plurality of holes formed therein. The intermediate mold is distinct from the first end mold and is configured for stacking against the first end mold such that each of the holes is aligned with one of the cavities. The apparatus includes a second end mold having a plurality of protuberances formed on a surface thereof. Each of the protuberances is defined by a surface having at least a portion of the second surface profile and is sized for insertion into one of the holes and one of the cavities.
In yet another aspect, the invention relates to a method of making a shaped article having a first surface with a first surface profile and a second surface with a second surface profile. The method comprises aligning a cavity in a first end mold with a protuberance in a second end mold. The cavity is defined by a surface having at least a portion of the first surface profile. The protuberance is defined by a surface having at least a portion of the second surface profile. The method includes placing a sheet of glass-based material at a bottom of the cavity. The method further includes compressing the sheet between the surface having at least a portion of the first surface profile and the surface having at least a portion of the second surface profile to impress at least a portion of the first surface profile and at least a portion of the second surface profile on a first surface and a second surface, respectively, of the sheet, thereby forming a shaped article.
Other features and advantages of the invention will be apparent from the following description and the appended claims.
The accompanying drawings, described below, illustrate typical embodiments of the invention and are not to be considered limiting of the scope of the invention, for the invention may admit to other equally effective embodiments. The figures are not necessarily to scale, and certain features and certain views of the figures may be shown exaggerated in scale or in schematic in the interest of clarity and conciseness.
The invention will now be described in detail with reference to a few embodiments, as illustrated in the accompanying drawings. In describing the embodiments, numerous specific details are set forth in order to provide a thorough understanding of the invention. However, it will be apparent to one skilled in the art that the invention may be practiced without some or all of these specific details. In other instances, well-known features and/or process steps have not been described in detail so as not to unnecessarily obscure the invention. In addition, like or identical reference numerals are used to identify common or similar elements.
The apparatus 100 further includes an intermediate mold 108. In one example, the intermediate mold 108 may be in a plate form. In one example, the intermediate mold 108 is distinct from the bottom mold 102 and is selectively (or temporarily) stacked on the bottom mold 102 as shown in
The hole 110 in the intermediate mold 108 and the cavity 104 in the bottom mold 102, when stacked and aligned as illustrated in
The apparatus 100 further includes a top mold 124 having a base 122, which may be in the form of a plate, and a protuberance 126 formed on a surface 125 of the base 122. In the illustrated example, the protuberance 126 has a top protuberance portion 128 and a bottom protuberance portion 130. The bottom protuberance portion 130 is defined by a surface 132 having a surface profile matching the top surface profile of the shaped article. The protuberance 126 is sized for insertion into the hole 110 and cavity 104 in the intermediate mold 108 and bottom mold 102, respectively. The top protuberance portion 128 is sized to plug the hole 110 by insertion in the hole 110. In general, the top protuberance portion 128 is larger in size, or diameter, than the bottom protuberance 130 to allow the bottom protuberance portion 130 to pass through the hole 110 in the intermediate mold 108.
For an apparatus 100 for making a plurality of shaped articles, as illustrated in
The bottom, intermediate, and top molds 112, 108, 124 may be made of a suitable heat resistant material, i.e., one that would not interact with the material to be used in forming the shaped article(s). Typically, the mold material is selected such that there isn't a large mismatch in coefficient of thermal expansion (CTE) between the mold material and the material of the shaped article(s). In one non-limiting example, the mold material is selected such that the absolute CTE mismatch between the mold material and the material of the shaped article(s) is less than about 1×10−6/°C. In one non-limiting example, the shaped article is made of a glass-based material, such as a glass or glass-ceramic. For glass-based materials, examples of suitable material for the molds include, but are not limited to, stainless steel and graphite. The surface of the molds including the shaping profiles may be coated with a non-stick material, such as, but not limited to, boron nitride, calcium hydroxide, and carbon soot to facilitate separation of the shaped article from the molds.
The amount of force applied to the sheet 134 through the protuberance 126 in
The pressed sheet 134 in
In one example, the sheet 134 used in making the shaped article is made of a glass-based material that can be chemically strengthened by ion-exchange. Typically, the presence of small alkali metal ions such as Li+ and Na+ in the glass structure that can be exchanged for larger alkali metal ions such as K+ render the glass composition suitable for chemical strengthening by ion-exchange. The base glass composition can be variable. For example, U.S. patent application Ser. No. 11/888,213, assigned to the instant assignee, discloses alkali-aluminosilicate glasses that are capable of being strengthened by ion-exchange and down-drawn into sheets. The glasses have a melting temperature of less than about 1650° C. and a liquidus viscosity of at least about 1.3×105 Poise and, in one embodiment, greater than about 2.5×105 Poise. The glasses can be ion-exchanged at relatively low temperatures and to a depth of at least 30 μm. Compositionally the glass comprises: 64 mol %≦SiO2≦68 mol %; 12 mol %≦Na2O≦16 mol %; 8 mol %≦Al2O3≦12 mol %; 0 mol %≦B2O3≦3 mol %; 2 mol %≦K2O≦5 mol %; 4 mol %≦MgO≦6 mol %; and 0 mol %≦CaO≦5 mol %, wherein: 66 mol %≦SiO2+B2O3+CaO≦69 mol %; Na2O+K2O+B2O3+MgO+CaO+SrO>10 mol %; 5 mol %≦MgO+CaO+SrO≦8 mol %; (Na2O+B2O3)—Al2O3≦2 mol %; 2 mol %≦Na2O—Al2O3≦6 mol %; and 4 mol %≦(Na2O+K2O)—Al2O3≦10 mol %.
The ion-exchange process typically occurs at an elevated temperature range that does not exceed the transition temperature of the glass. The glass is dipped into a molten bath comprising a salt of an alkali metal, the alkali metal having an ionic radius that is larger than that of the alkali metal ions contained in the glass. The smaller alkali metal ions in the glass are exchanged for the larger alkali metal ions. For example, a glass sheet containing sodium ions may be immersed in a bath of molten potassium nitrate (KNO3). The larger potassium ions present in the molten bath will replace smaller sodium ions in the glass. The presence of the large potassium ions at sites formerly occupied by sodium ions creates a compressive stress at or near the surface of the glass. The glass is then cooled following ion exchange. The depth of the ion-exchange in the glass is controlled by the glass composition. For potassium/sodium ion-exchange process, for example, the elevated temperature at which the ion-exchange occurs can be in a range from about 390° C. to about 430° C., and the time period for which the sodium-based glass is dipped in a molten bath comprising a salt of potassium can range from about 7 up to about 12 hours (with less time being required at higher temperatures, and more time being required at lower temperatures). In general, the deeper the ion-exchange, the higher the surface compression and the stronger the glass can be.
While the invention has been described with respect to a limited number of embodiments, those skilled in the art, having benefit of this disclosure, will appreciate that other embodiments can be devised which do not depart from the scope of the invention as disclosed herein. Accordingly, the scope of the invention should be limited only by the attached claims.
