Method and system for making a glass article with uniform mold temperature

A glass sheet is formed on a mold into a glass article having a three-dimensional shape. The mold, with the glass article thereon, is arranged within an interior space of a radiation shield such that the mold is between a leading end barrier and a trailing end barrier of the radiation shield. The mold, glass article, and radiation shield are translated through a sequence of cooling stations while maintaining the mold between the leading and trailing end barriers, wherein the leading and trailing end barriers inhibit radiation heat transfer at leading and trailing ends of the mold.

FIELD

The present disclosure relates to a method and system for thermally reforming glass.

BACKGROUND

Glass articles, such as cover glasses, for handheld electronic devices may be made by thermal reforming, which involves heating a glass sheet to a temperature at which the glass can be deformed without damage and then forming the heated glass sheet into a glass article having a three-dimensional (“3D”) shape.

SUMMARY

The present disclosure describes a method of making glass articles. In one aspect, the method includes forming a glass sheet on a mold into a glass article having a three-dimensional shape. The method further includes arranging the mold, with the glass article on the mold, within an interior space of a radiation shield such that the mold is between a leading end barrier and a trailing end barrier of the radiation shield. The mold, glass article, and radiation shield are translated through a sequence of cooling stations while maintaining the mold between the leading and trailing end barriers, wherein the leading and trailing end barriers inhibit radiation heat transfer at leading and trailing ends of the mold.

In at least one embodiment of the method, the arranging of the mold is such that the radiation shield extends by a height greater than zero above a top end of the mold.

In at least one embodiment of the method, the arranging of the mold is such that the radiation shield extends by a height greater than zero below a bottom end of the mold.

In at least one embodiment of the method, the arranging of the mold is such that there is an air gap between an outer circumferential edge of the mold and an inner surface of the radiation shield facing the interior space.

In at least one embodiment of the method, the forming of the glass article includes using vacuum to pull the glass sheet against a surface of the mold having a three-dimensional surface profile that defines the three-dimensional shape of the glass article.

In at least one embodiment, the method further includes maintaining temperatures in the cooling stations during the translation such that by the time the glass article reaches an end of the last cooling station in the sequence of cooling stations, the temperature of the glass article would have dropped to a temperature at which the viscosity of the glass article is greater than 1013poise.

In at least one embodiment of the method, the leading and trailing end barriers inhibit heat transfer such that a maximum temperature differential across a surface of the mold adjacent to the glass article is less than 5° C.

In at least one embodiment of the method, the leading and trailing end barriers inhibit heat transfer such that a maximum temperature differential across a surface of the mold adjacent to the glass article is less than 2° C.

The present disclosure further describes an apparatus for making glass articles. In one aspect, the apparatus includes a mold having a mold surface for forming a glass sheet into a glass article having a three-dimensional shape. The apparatus further includes a radiation shield comprising a leading end barrier and a trailing end barrier arranged in opposing, spaced-apart relation to define an interior space in which the mold is received.

In at least one embodiment, the apparatus further includes a sequence of cooling stations arranged in order of decreasing temperature.

In at least one embodiment, the apparatus further includes a conveyor system for translating the radiation shield and mold along the sequence of cooling stations such that the mold remains within the interior space during the translation.

In at least one embodiment of the apparatus, at least a portion of the radiation shield forming the leading and trailing end barriers is coated with a material having an emissivity in a range from 0.1 to 0.4.

In at least one embodiment of the apparatus, the radiation shield is made of a material resistant to oxidation in a temperature range of 500° C. to 900° C.

In at least one embodiment of the apparatus, a reflective material is applied on at least a portion of the radiation shield forming the leading and trailing end barriers.

In at least one embodiment of the apparatus, the radiation shield extends by a height greater than zero above a top surface of the mold.

In at least one embodiment of the apparatus, the radiation shield extends by a height greater than zero below a bottom surface of the mold.

In at least one embodiment of the apparatus, the interior space is sized such that there is an air gap between the outer circumferential edge of the mold and an inner surface of the radiation shield facing the interior space.

In at least one embodiment of the apparatus, at least a portion of the radiation shield is configured as a cooling plate.

In at least one embodiment of the apparatus, at least a portion of the radiation shield includes an isothermal heat transfer device.

In at least one embodiment of the apparatus, the radiation shield has a structure selected from a pair of parallel walls, a box, a tube, and a dome.

It is to be understood that both the foregoing summary and the following detailed description are exemplary of the invention of the present disclosure and are intended to provide an overview or framework for understanding the nature and character of the invention as claimed. The accompanying drawings are included to provide a further understanding of the invention and are incorporated in and constitute a part of this disclosure. The drawings illustrate various embodiments of the invention and together with the description serve to explain the principles and operation of the invention.

DETAILED DESCRIPTION

In the following detailed description, numerous specific details may be set forth in order to provide a thorough understanding of embodiments of the invention. However, it will be clear to one skilled in the art when embodiments of the invention may be practiced without some or all of these specific details. In other instances, well-known features or processes may not be described in detail so as not to unnecessarily obscure the invention. In addition, like or identical reference numerals may be used to identify common or similar elements.

To meet design specifications, glass articles for handheld devices have to meet very tight shape accuracy on the order of ±50 μm. To achieve such shape accuracy, the glass article, after forming, is cooled and/or annealed while on the mold until the glass reaches a temperature below the glass transition temperature at which the glass article can be safely removed from the mold. The cooling phase typically involves transporting the mold, with the glass article thereon, along a succession of cooling stations. To prevent warping of the glass article and to achieve the required shape accuracy, the temperature differentials across the surface of the glass article should be very small, e.g., not greater than 5° C., and the temperature differentials across the thickness of the glass article should be very small, e.g., not greater than 2° C., during the cooling phase.

Temperature decreases progressively along the succession of cooling stations, which means that any downstream cooling station will have a lower temperature than the adjacent upstream cooling station. If there are no physical barriers between the cooling stations or the cooling stations are not physically separated, the leading end of the mold would be exposed to a lower temperature environment while the trailing end of the mold would be exposed to a higher temperature environment. This would induce a temperature differential between the leading and trailing ends of the mold. The induced temperature differential will influence the surface and thickness temperature differentials of the glass article on the mold, possibly resulting in unacceptable surface and thickness temperature differentials.

In the invention of the present disclosure, radiation barriers are formed at the leading and trailing ends of a mold while cooling a glass article on the mold. Modeling results show that temperature differential across the glass article of less than 2° C. can be achieved using this method and without a need for actively managing the temperature differential across the mold. This method eliminates the need to physically separate the cooling stations with insulation material that could potentially give rise to glass contamination. This method also minimizes warping of the glass article, making it possible to produce the glass article with shape accuracy that meets required specifications.

FIG. 1shows stations100-108in which a glass article having a 3D shape can be made. The first station100is a forming station where a glass sheet109will be formed into the glass article having a 3D shape using a mold112. The forming station100is equipped with heating means111, e.g., infrared heaters, resistive heaters, or induction heaters, for heating the glass sheet to a temperature at which it can be formed into the glass article. The remaining stations102,104,106,108are cooling stations where the glass article will be cooled down to a temperature at which it can be safely separated from the mold112. The cooling stations102,104,106,108are each controlled to a different temperature, with the temperature of the cooling stations decreasing progressively from the first cooling station102closest to the forming station100to the last cooling station108. That is, T(102)>T(104)>T(106)>T(108), where T is temperature. The cooling stations102,104,106,108may incorporate heating elements and/or insulation (not identified separately) to provide a desired thermal environment in the cooling station. In some embodiments, there are no physical or insulating partitions between the cooling stations102,104,106,108. However, if there are physical partitions between the cooling stations102,104,106,108in other embodiments, these partitions may be in the form of doors that can be selectively opened to allow passage of the mold112and glass on the mold from one station to another.

Extending through the stations100-108is a running table (or conveyor)110, which may be a linear table (or conveyor) or a rotary table (or conveyor), which means that the stations100-108are not restricted to the linear arrangement ofFIG. 1. As shown inFIG. 1, the mold112is in the second cooling station104, having been translated from the forming station100into the first cooling station102and from the first cooling station102into the second cooling station104. The mold112is supported on a mold support114, which is supported on or otherwise coupled to the running table110. The mold support114may incorporate rotating means so that the mold112is rotatable while in or traveling through the cooling stations102-108. The mold112is carrying a glass article115, which was formed from the glass sheet109in the forming station100. The mold112and glass article115may be referred to as glass/mold assembly113.

Surrounding at least the leading and trailing ends124a,124bof the mold112is a radiation shield200. With the radiation shield200surrounding the mold112, the radiation shield200and glass/mold assembly113are translated through the sequence of cooling stations102-108together. The radiation shield200may be supported on or otherwise coupled to the running table110so as to travel with the glass/mold assembly113through the cooling stations102-108. Alternatively, the radiation shield200may be provided with a separate running table (or conveyor), with the operation of the running table of the radiation shield200coordinated with that of the mold112so that the relationship between the radiation shield200and mold112is preserved through translating the radiation shield200and glass/mold assembly113through the sequence of cooling stations102-108. The radiation shield200may be disposed around the mold112after the mold112has been translated from the forming station100into the first cooling station102. Alternatively, the radiation shield200may be disposed around the mold112before the mold112and glass sheet109are loaded into the forming station100, in which case the radiation shield200will also surround the mold112while the mold112is in the forming station110.

The radiation shield200comprises a front surface202and a back surface204, which are in opposing relation, and an interior space222, which is large enough to accommodate the mold112, defined between the surfaces202,204. When the mold112is arranged in the interior space222, the front surface202is adjacent to the leading end124aof the mold112and forms a barrier between the leading end124aand the radiation view factor to the cooling station downstream of the leading end124a. Hence, the front surface202may be referred to as a leading end barrier. Also, when the mold112is arranged in the interior space222, the back surface204is adjacent to the trailing end124bof the mold112and forms a barrier between the trailing end124band the radiation view factor to the cooling station upstream of the trailing end124b. Hence, the back surface204may be referred to as a trailing end barrier. It should be noted that the parts of the mold112corresponding to the leading and trailing ends124a,124bmay not be fixed, e.g., if the mold112is being rotated while arranged within the interior space222. Therefore, what is regarded as the leading end124aof the mold112at any instance will be whatever end of the mold112is facing the direction in which the mold112is traveling. Similarly, what is regarded as the trailing end124bof the mold112at any instance will be whatever end of the mold112is facing a direction opposite to the direction in which the mold112is traveling.

FIG. 2ashows radiation shield200.1, an embodiment of the radiation shield200, in form of two parallel walls. The radiation shield200.1has opposing front and back walls206.1,208.1. The surfaces202.1,204.1of the front and back walls206.1,208.1provide the barrier-forming front and back surfaces mentioned above. The front and back walls206.1,208.1are spaced apart to define an interior space222.1within which the mold112is received. The dimensions of the front and back walls206.1,208.1may be selected such that in use the radiation shield200.1extends by a height ht>0 above the top end126aof the mold112and/or by a height hb>0 below the bottom end126bof the mold112. The spacing between the front and back walls206.1,208.1may be selected such that air gaps128a,128bexist between the front and back walls206.1,208.1(or inner surfaces120a,120bfacing the interior space222.1) and the leading and trailing ends124a,124bof the mold112. The air gaps128a,128bmay or may not have the same width. The front and back walls206.1,208.1are shown as flat and vertical inFIG. 2a, but in alternate embodiments, they could be curved and/or slanted.

FIG. 2bshows radiation shield200.2, which is one embodiment of the radiation shield200, in form of a box. The radiation shield200.2has opposing front and back walls206.2,208.2and opposing side walls210.2,212.2. The surfaces202.2,204.2of the front and back walls206.2,208.2provide the barrier-forming front and back surfaces mentioned above. The front and back walls206.2,208.2are spaced apart to define an interior space222.2within which the mold112is received. The side walls210.2,212.2extend between and adjoin the ends of the front and back walls206.2,208.2, thereby enclosing the sides of the interior space222.2. As in the case of the radiation shield200.1, the dimensions of the walls206.2,208.2,210.2,212.2can be selected such that in use the radiation shield200.2extends by a height ht>0 above the top end of the mold and/or by a height hb>0 below the top end of the mold. Also, as shown inFIG. 2c, the spacing between the front and back walls206.2,208.2and between the side walls210.2,212.2may be selected such that an air gap128exists between the circumferential edge127of the mold112and the inner surface120of the radiation shield200.2facing the interior space222.2. The air gap128may or may not have a uniform width around the mold112.

FIG. 2dshows radiation shield200.3in box form, with front and back walls206.3,208.3and side walls210.3,212.3. A top cover wall214is secured to or integrally formed with the top of the walls206.3,208.3,210.3,212.3. In alternate embodiments, a bottom cover wall (not shown) may be secured to or integrally formed with the bottom of the walls206.3,208.3,210.3,212.3. Thus, the radiation shield200.3may have a top and/or bottom cover wall. Where the radiation shield200.3has both top and bottom covers, a door should be provided in at least one of the walls206.3,208.3,210.3,212.3and cover walls to allow the mold112to be arranged in the interior space of the radiation shield. Except for the top cover wall214, the radiation shield200.3has a structure similar to that of radiation shield200.2(inFIG. 2b).

FIG. 2eshows another radiation shield200.4that has a structure that is similar to that of radiation shield200.2(inFIG. 2b). The main difference between the radiation shields200.4,200.2is that the front and back walls206.4,208.4and side walls210.4,212.4of the radiation shield200.4are slanted. In alternate embodiments, the radiation shield200.4may also be provided with a top and/or bottom cover wall as described with respect to radiation shield200.3(inFIG. 2d).

FIG. 2fshows a radiation shield200.5having a curved wall216in tubular form. The mold will be arranged in the interior space222.5formed by the curved wall216. The cross-sectional shape of the tube formed by the curved wall216may be circular, oval, or even an irregular closed shape. In use, the front and back wall sections206.5,208.5of the curved wall216will provide the leading and trailing end barriers. If desired, the radiation shield200.5may be provided with top and/or bottom cover walls, as described with respect to radiation shield200.3(inFIG. 2d).

FIG. 2gshows radiation shield200.6having a curved wall218in dome or paraboloid form. The mold will be arranged in the interior space222.6of the dome. In use, the front and back wall sections206.6,208.6of the curved wall or dome218will provide the leading and trailing end barriers of the radiation shield. If desired, the radiation shield200.6may be provided with a bottom cover wall, as described with respect to radiation shield200.3(inFIG. 2d).

In some embodiments, any of the radiation shields described above may incorporate one or more isothermal heat transfer devices, such as heat pipes, in their walls. For illustration purposes,FIG. 2dshows heat pipes121in the front wall206.3. This does not necessarily mean that isothermal heat transfer device(s) have to be located in front walls. The location of the isothermal heat transfer devices would be selected based on the desired temperature distribution within the interior space of the radiation shield. Alternatively, any of the walls of the radiation shields described above may be cooling plates.

In some embodiments, any of the walls of the radiation shields described above may have holes or openings or doors, which may be strategically positioned for various uses such as venting the interior space of the radiation shield, measuring conditions within the interior space of the radiation shield, performing operations within the interior space of the radiation shield, and supplying fluid, such as a cooling fluid, to the radiation shield, e.g., where the radiation shield incorporates isothermal heat transfer devices or cooling plates.

Returning toFIG. 1, the radiation shield200achieves mold/glass article temperature uniformity by blocking thermal radiation view factors to upstream and downstream cooling stations at the leading and trailing ends124a,124bof the mold112. Further improvements to the mold/glass article temperature uniformity can be realized when the walls or wall sections of the radiation shield200are coated with a material of low emissivity. The coating may be applied to the entire radiation shield200or limited to the walls or wall sections providing the front and back surfaces202,204that serve as the leading and trailing end barriers. In some embodiments, the low-emissivity coating material has an emissivity in a range from 0.1 (e.g., platinum) to 0.4 (e.g., gold).

In some embodiments, the wall(s) of the radiation shield200is made of a material that is resistant to oxidation at the high temperatures that would be encountered in the cooling stations, such as temperatures in a range from 500° C. to 900° C. For example, the wall(s) of the radiation shield200may be made of a superalloy, such as INCONEL® 600 alloy (which is a nickel-chromium alloy). Other materials that do not oxidize at temperatures as high as 900° C. may alternately be used for the wall(s) of the radiation shield200.

The ability of the radiation shield200to achieve mold/glass article temperature uniformity can be enhanced by applying a reflective material to the inner surface (e.g.,120inFIG. 2b) and/or outer surface (e.g.,129inFIG. 2b) of the radiation shield. In one example, the wall(s) of the radiation shield200is made of INCONEL® 600 alloy, and gold is deposited on the wall(s) by first spraying the wall(s) with a suspension of silicon carbide powder in epoxy resin, followed by vacuum deposition of gold. In another example, the wall(s) of the radiation shield200is made of INCONEL® 600 alloy, and Jade mirrors from colloidal solution or thin, e.g., 0.5 μm thick, foil are applied to the wall. Other combinations of materials besides those specifically mentioned above may alternately be used for the wall(s) of the radiation shield200.

Depending on the mold/glass article dimensions, cooling station dimensions, and mold placement in the cooling stations, the values of ht, hb, and d, and ε can be selected to achieve a particular level of temperature uniformity across the top surface of the mold112. The parameter htis the height by which the radiation shield200extends above the top end of the mold112(see, e.g., htinFIG. 2a). The parameter hbis the height by which the radiation shield200extends below the bottom end of the mold112(see, e.g., hbinFIG. 2a). The parameter d is the smallest width of the air gap between the inner surface of radiation shield200facing the interior space of the radiation shield200and the outer circumferential edge of the mold112(see, e.g., air gaps128a,128binFIG. 2aor air gap128inFIG. 2c). The parameter c is the emissivity of the radiation shield200, especially the emissivity of the portion of the radiation shield200providing the leading and trailing end barriers. In one or more embodiments, each of ht, hb, and d is greater than zero. In general, the optimal values of ht, hb, and d will depend on the dimensions of the cooling stations (102-108inFIG. 1), dimensions of mold112, and placement of mold112in the cooling stations.

InFIG. 3, the mold112has a mold surface130, whose 3D surface profile will determine the shape of the glass article115(inFIG. 1). To allow forming by vacuum, holes131may be formed in the mold112. Through the holes131, vacuum can be applied to the cavity133above the mold surface130in order to pull the glass sheet109into the cavity133and against the mold surface130. The mold112is made of a material that can withstand high temperatures such as would be encountered in the stations100-108. The mold material may be one that will not react with (or stick to) the glass109under forming conditions, or the mold surface130may be coated with a coating material that will not react with (or stick to) the glass109under the forming conditions. In one example, the mold is made of a non-reactive carbon material, such as graphite, and the mold surface130is highly polished to avoid introducing defects into the glass when the mold surface is in contact with the glass. In another example, the mold is made of a dense ceramic material, such as silicon carbide, tungsten carbide, or silicon nitride, and the mold surface130is coated with a non-reactive carbon material, such as graphite. In another example, the mold130is made of a superalloy, such as INCONEL® 718 alloy, a nickel-chromium alloy, and the molding surface130is coated with a hard ceramic material, such as titanium aluminum nitride. The atmosphere in the forming station100should be inert if the mold112is made of a carbon material or if the mold surface130is coated with a carbon material.

The composition of the glass sheet109, which will be formed into the glass article115(inFIG. 1) is selected based on the desired properties of the glass article. For applications requiring high strength and scratch-resistance, the glass sheet may be made of an ion-exchangeable glass, i.e., a glass containing relatively small alkali metal or alkaline-earth metal ions that can be exchanged for relatively large alkali or alkaline-earth metal ions. Examples of ion-exchangeable glasses can be found in the patent literature, e.g., U.S. Pat. No. 7,666,511 (Ellison et al; 20 Nov. 2008), U.S. Pat. No. 4,483,700 (Forker, Jr. et al.; 20 Nov. 1984), and U.S. Pat. No. 5,674,790 (Araujo; 7 Oct. 1997), all incorporated by reference in their entireties, and are also available from Corning Incorporated under the trade name GORILLA® glass. Typically, these ion-exchangeable glasses are alkali-aluminosilicate glasses or alkali-aluminoborosilicate glasses. The ion-exchangeable glass will allow chemical tempering of the glass article by ion-exchange, which would improve both the strength and scratch resistance of the glass article.

To form the glass article115, the glass sheet109is placed on the mold112, as shown inFIG. 3. Then, the glass sheet109and mold112are loaded into the forming station100, as shown inFIG. 1. Prior to loading the glass sheet109and mold112into the forming station100, the glass sheet112is preheated to some initial temperature, i.e., in order to minimize the residence time of the glass/mold assembly in the forming station100. This initial temperature will be below a forming temperature at which the glass can be deformed. At the forming station100, the glass sheet109is heated to the forming temperature at which the glass can be formed into the glass article. Typically, the forming temperature will be between a temperature corresponding to a glass viscosity of 1013poise and a temperature corresponding to a glass viscosity of 107poise. Preferably, the forming temperature will be between a temperature corresponding to a glass viscosity of 1011poise and a temperature corresponding to glass viscosity of 107poise. More preferably, the forming temperature will be between a temperature corresponding to a glass viscosity of 109.1poise and a temperature corresponding to a glass viscosity of 107.7poise. The glass article115is formed by allowing the glass sheet109to slump against the mold surface130(inFIG. 3) and/or by using vacuum to pull the glass sheet109against the mold surface130. Depending on the forming technique, the glass article115can be formed in 5 to 45 seconds. Depending on the cooling method (active versus passive), cooling of the glass article115on the mold112can take 2 to 10 minutes.

The combination of the glass article115and mold112may be referred to as glass/mold assembly113. After the glass article115is formed, the glass/mold assembly113is transported from the forming station100to the first cooling station102. The radiation shield200is arranged to surround the mold112either before the glass/mold assembly113enters the first cooling station102or as soon as the glass/mold assembly113enters the first cooling station102. Within the first cooling station102, the temperature of the glass article115is allowed to drop by some predetermined amount. The glass/mold assembly113may include a cooling plate119below the mold112. A coolant, such as cooled air, can be circulated through the cooling plate119to reduce the temperature of the mold112. The air temperature in the cooling station will attempt to reach equilibrium with the mold temperature, and the temperature of the glass article will be between the air temperature and mold temperature.

Then, the glass/mold assembly113is moved into the second cooling station104, where the temperature of the glass article is again allowed to drop by some predetermined amount. Again, cooling of the glass article115may be assisted by operation of the cooling plate119. This moving of the glass/mold assembly113from one station to the next continues until the glass/mold assembly113is in the last cooling station108. At the end of the last cooling station108, the glass article115will have a freezing temperature, which is a temperature at which the glass article115can be separated from the mold112. In one embodiment, the freezing temperature is below a temperature corresponding to a glass viscosity of 1012poise. Preferably, the freezing temperature is below a temperature corresponding to a glass viscosity of 1013poise.

While the glass/mold assembly113is moving through the sequence of cooling stations102-108in the direction indicated by the arrow125, the radiation shield200travels with the glass/mold assembly113and blocks radiation view factors from the leading and trailing ends124a,124bof the mold112. That is, the leading end124aof the mold112will be shielded from direct exposure to the thermal environment in the downstream cooling station, and the trailing end124bof the mold112will be shielded from direct exposure to the thermal environment in the upstream cooling station or forming station. By blocking the radiation view factors, the radiation shield200inhibits radiation heat transfer at the leading and trailing ends124a,124bof the mold112that would have otherwise occurred due to exposure to the upstream and downstream stations. This means that the leading and trailing ends124a,124bof the mold112will see substantially the same temperature conditions as the remainder of the mold112, which ultimately leads to a more uniform temperature distribution across the mold112, which leads to reduced temperature differential across the glass article115carried by the mold112. Additionally, the mold112may be rotated within the interior space of the radiation shield200to further improve temperature uniformity in the mold112and glass article115.

Maximum temperature differential across the glass article of less than 5° C. can be achieved by cooling the glass article115with the radiation shield200blocking radiation view factors from the leading and trailing ends of the mold112. When the radiation shield walls are coated with reflective material and their dimensions are optimized to the dimensions of the cooling stations and placement of the mold in the cooling stations, maximum temperature differential across the glass article of less than 2° C. can be achieved.

After the glass/mold assembly113is unloaded from the last cooling station108, the glass article115is separated from the mold112. The glass article115may then be subjected to various post-forming processes. For example, the edges of the glass article115could be trimmed to size, e.g., if the glass sheet used in forming the glass article did not have a net size prior to the forming of the glass article. The trimmed edges of the glass article115could be ground and polished. After this, the glass article115could be strengthened by chemical tempering, e.g., ion-exchange, or thermal tempering. Coatings such as anti-smudge coating or optical coatings may be applied on the surface of the glass article after strengthening. The glass article may also be annealed. However, it has been found that post-annealing may be bypassed due to the high mold temperature uniformity achievable using the radiation shield.

EXAMPLES

A study was carried out to investigate the effectiveness of the radiation shield on mold temperature uniformity. For the study, a thermal model of the cooling process was developed and numerical simulations were performed using FLUENT® software by Ansys, Inc.FIG. 4shows a schematic of the model used in the study, with the mold112and radiation shield200translated through a sequence of cooling stations138-142in the direction indicated by the arrow144. For the study, the temperatures of the cooling stations138,140,142were controlled to 700° C., 660° C., and 620° C., respectively. The dimension of each of the cooling stations was 20 inches by 20 inches by 16 inches, with 16 inches being the height of the cooling station. The dimension of the mold was 8.5 inches by 8.5 inches by 0.5 inches, with 0.5 inches being the height of the mold.

FIG. 5ashows a contour plot of the mold temperature in the second cooling station140if the mold112is cooled for a time period of 1 minute without the radiation shield200surrounding the mold112.FIG. 5bshows a contour plot of the mold temperature in the second cooling station140if the mold112is cooled for the same time period with the radiation shield200surrounding the mold112. The plots show that the temperature of the mold when cooled with a radiation shield is much more uniform than when not cooled with a radiation shield. The maximum temperature differential across the top surface of the mold in the plot ofFIG. 4is less than 2° C.

Another study was carried out to determine the effect of the radiation shield parameters ht, hb, and d on mold temperature uniformity, where htis the height of the radiation shield wall above the top end of the mold, hbis the height of the radiation shield wall below the bottom end of the mold, and d is the width of the smallest air gap between the radiation shield and the outer circumferential edge of the mold. For the study, the model ofFIG. 4and the temperature conditions mentioned above in connection withFIG. 4were used. The modeling was performed using FLUENT® software.

FIG. 6shows a plot of the temperature differential on the top surface of the mold as a function of radiation shield height ht. The temperature differential (Δt) indicated inFIG. 6is the difference between the maximum and minimum temperatures observed on the top surface of the mold. Lines150-156are shown in the plot, with line150corresponding to hb=0, d=0.5 inches, and ε=0.8, line152corresponding to hb=1 inch, d=0.5 inches, and ε=0.8, line154corresponding to hb=1 inch, d=0.25 inches, and ε=0.8, and line156corresponding to hb=1 inch, d=0.25 inches, and ε=0.4 (where c is the radiation shield wall emissivity).

From the plot ofFIG. 6, it can be concluded that significant improvement in mold temperature uniformity is achieved by increasing the radiation shield height until the radiation shield wall extends 1.5 inches above the top surface of the mold, i.e., ht=1.5 inches. Extending the radiation shield beyond 1.5 inches above the top end of the mold does not seem to provide any additional benefit to the mold temperature uniformity. From the plot ofFIG. 6, it can also be concluded that further improvement to the mold temperature is realized by extending the radiation shield box below the mold (i.e., increasing hb) and by positioning the radiation shield box as close as possible to the mold (i.e., reducing d). A further discovery was made that the mold temperature differential can be reduced below 2° C. by making the radiation shield wall reflective or with low emissivity of 0.4 or less.