Patent Publication Number: US-2010126222-A1

Title: Method and apparatus for forming and cutting a shaped article from a sheet of material

Description:
FIELD 
     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. 
     BACKGROUND 
     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. 
     SUMMARY 
     In one aspect, the invention relates to an apparatus for forming shaped articles from a sheet of material which comprises a first mold having a mold surface and a network of gutters formed in the mold surface. The network of gutters has a network gutter profile and defines an array of islands on which an array of bumps is formed. Each of the bumps has a surface with a shaped profile. The apparatus further includes a second mold adapted for positioning on the mold surface. The second mold has a network of protuberances defining a plurality of cavities. Each of the cavities is sized to overlap one of the bumps of the first mold. The network of protuberances has a network protuberance profile complementary to the network gutter profile. 
     In another aspect, the invention relates to an apparatus for forming a shaped article from a sheet of material which comprises a first mold having a mold surface and a network of gutters formed in the mold surface. The network of gutters has a network gutter profile and defines an island on which a bump is formed. The bump has a surface with a shaped profile substantially matching a surface profile of the shaped article. The apparatus further includes a second mold adapted for positioning on the mold surface. The second mold has a network of protuberances defining a cavity sized to overlap the bump. The network of protuberances has a network protuberance profile complementary to the network gutter profile. 
     In another aspect, the invention relates to a method of making a shaped article which comprises positioning a sheet of material on a mold surface of a first mold such that a first portion of the sheet overlies a network of gutters in the mold surface and a second portion of the sheet of material overlies a bump on the mold surface. The network of gutters defines an island on which the bump is formed, and the bump has a surface with a shaped profile. The method further includes positioning a second mold having a network of protuberances defining a cavity on the sheet of material such that the network of protuberances contacts the first portion of the sheet of material and the cavity overlaps the bump. The method further includes pressing the network of protuberances against the sheet of material. The pressing results in thinning out of the sheet of material between the first portion and the second portion of the sheet of material, squeezing of excess sheet of material from the thinning out into the network of gutters, and molding of the second portion of the sheet of material to the bump, thereby forming the shaped article. 
     Other features and advantages of the invention will be apparent from the following description and the appended claims. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
       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. 
         FIG. 1  is a cross-sectional view of an apparatus for forming a shaped article. 
         FIG. 2  is a top view of the bottom mold of an apparatus for forming a shaped article. 
         FIG. 3  is a bottom view of the top mold of an apparatus for forming a shaped article. 
         FIG. 4  is a top view of a bottom mold of an apparatus for forming a plurality of shaped articles. 
         FIG. 5  is a bottom view of a top mold of an apparatus for forming a plurality of shaped articles. 
         FIG. 6  shows a sheet of material positioned on a bottom mold of an apparatus for forming a shaped article. 
         FIG. 7  shows a top mold suspended over the sheet of material of  FIG. 6 . 
         FIG. 8  shows the top mold of  FIG. 7  in contact with the sheet of material of  FIG. 6 . 
         FIG. 9  shows a shaped article formed between a top mold and a bottom mold of an apparatus for forming a shaped article. 
     
    
    
     DETAILED DESCRIPTION 
     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. 
       FIG. 1  is a cross-sectional view of an apparatus  100  for making a shaped article. In general, a shaped article will be considered herein as having a top surface with a top surface profile and a bottom surface with a bottom surface profile. The terms “top surface” and “bottom surface” are arbitrary. Each of the top surface and bottom surface may be the inner or outer surface (front or back surface) of the shaped article. 
     Apparatus  100  includes a bottom mold  102  having a base portion  104 , which may be generally planar. Base portion  104  has a mold surface  106  in which a network of gutters  108  is formed. The gutters in the network of gutters  108  extend from the mold surface  106  into the base portion  104 . The network of gutters  108  defines an island  110  on the mold surface  106 . A bump  112  is formed on the island  110 . The bump  112  has an outer surface  114  characterized by a shaped profile matching the top surface profile or bottom surface profile of the shaped article to be formed. The network of gutters  108  has a gutter profile, which is more clearly shown in  FIG. 2 . It should be noted that the shaped profile of the bump  112  and the gutter profile of the gutter  108  are not limited to the examples depicted in  FIGS. 1 and 2 . For example, the shaped profile of the bump  112  may be convex and smooth, as shown in  FIG. 1 , or may be more complex, e.g., including concave and/or textured portions. In general, the shaped profile of the bump  112  and the gutter profile of the network of gutters  108  will depend on the shape of the shaped article to be formed. 
     Referring to  FIGS. 1 and 2 , the network of gutters  108  also defines an island  115  on the mold surface  106 . The island  115  encircles the island  110  and is separated from the island  110  by the network of gutters  108 . Side bumps  116  are formed on the island  115 . The side bumps  116  form an arrangement of bumps encircling the bump  112 . Like the bump  112 , the side bumps  116  also have shaped profiles. However, the shaped profiles of the side bumps  116  do not have to match the top surface profile or bottom surface profile of a shaped article to be formed using apparatus  100  because shapes formed by the side bumps  116  would generally be discarded. 
     Referring to  FIG. 1 , apparatus  100  further includes a top mold  118 . In one non-limiting example, the top mold  118  includes a base portion  120  and a network of protuberances  122  formed on the base portion  120 . The network of protuberances  122  defines a first cavity  124  sized to overlap (fit over) the bump  112 . The network of protuberances  122  may further define a plurality of cavities  126 , each of which may be sized to overlap (fit over) one of the side bumps  116 . In general, the arrangement of the cavities  124 ,  126  defined by the network of protuberances  122  is complementary with the arrangement of the bumps  112 ,  116  in the bottom mold  102 . This means that when the top mold  118  is aligned with the bottom mold  102  (as indicated by the broken arrows in  FIG. 1 ), the cavities  124 ,  126  are aligned with and in a position to overlap the bumps  112 ,  116 , respectively. This is better seen by comparing  FIG. 2  with  FIG. 3 , where  FIG. 2  shows a top view of the bottom mold  102  and  FIG. 3  shows a bottom view of the top mold  118 . Also, the network of protuberances  122  has a protuberance profile ( FIG. 3 ) that is complementary to the gutter profile ( FIG. 2 ) of the network of gutters  108 . This means that when the top mold  118  is aligned with the bottom mold  102  (as indicated by the broken arrows in  FIG. 1 ), the protuberances  122  are also aligned with the gutters  108 , and the gutters  108  are in a position to accept the protuberances  122 . 
     Still referring to  FIG. 1 , the top mold  118  is brought into contact with the bottom mold  102 , with the network of protuberances  122  resting on the mold surface  106 , in order to form a shaped article. When forming the shaped articles, the cavities  124 ,  126  fit over the bumps  112 ,  116 , respectively. Alignment features may be included in apparatus  100  to facilitate alignment of the cavities  124 ,  126  with the bumps  112 ,  116 , respectively. In one non-limiting example, the alignment features may include a pin  128  formed on the top mold  118  and a hole  130  formed on the bottom mold  102  to receive the pin. Alternatively, the pin  128  may be formed on the bottom mold  102  and the hole  130  formed in the top mold  118  to fit over the pin  128 . A plurality of alignment features  128 ,  130  may be provided in the top mold  118  and bottom mold  102  as desired. The protuberances  132  on the inside of the network of protuberances  122  may be sized to slide into the network of gutters  108  when the top mold  118  is mounted on the mold surface  106  of the bottom mold  102 . Alternatively, the protuberances  132  on the inside of the network of protuberances  122  may simply rest on the islands  110 ,  115 . 
     As illustrated in  FIG. 4 , bottom mold  102  may include an array of islands  110  on which bumps  112  are formed. Each bump  112  shown in  FIG. 4  has a shaped profile as described above. The shaped profile of the bumps  112  may be the same or may be different. Similarly, as illustrated in  FIG. 5 , top mold  118  may include a plurality of cavities  122  sized to overlap the bumps ( 112  in  FIG. 4 ), where the plurality of cavities  122  are defined by the network of protuberances  122 . The bottom mold  102  shown in  FIG. 4  and top mold  118  shown in  FIG. 5  allow multiple shaped articles to be formed in a single operation or step. 
     The bottom mold  102  and top mold  118  in  FIGS. 1-5  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&#39;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 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. 
       FIGS. 6 and 7  illustrate a method of making a shaped article. In  FIG. 6 , a sheet of material  140  is placed on the bottom mold  102 . In one non-limiting example, the sheet of material  140  is a sheet of glass-based material, e.g., a sheet of glass or a sheet of glass-ceramic. At this point, sheet  140  is a flat piece of material (as opposed to a preform having a shape that approximates the shape of the shaped article to be formed). Sheet  140  is placed on the bottom mold  102  such that a first portion  140   a  of the sheet  140  overlies the network of gutters  108 , a second portion  140   b  of the sheet  140  overlies the bump  112 , and a third portion  140   c  of the sheet  140  overlies the side bumps  116 . In this position, the sheet  140  is heated to a temperature above the softening temperature of the glass-based material. Typically, heating of the sheet  140  also includes heating of the bottom mold  102 . In one non-limiting example, sheet  140  is heated to a temperature of about 10° C. higher than the softening point of the glass-based material. Sheet  140  may also be heated prior to being placed on the bottom mold  102 , but not necessarily to a temperature above the softening temperature of the glass-based material. Additional heating of the preheated sheet  140  may be used to achieve the desired temperature at which the sheet  140  will be molded to form shaped articles. 
       FIG. 7  shows the top mold  118  suspended over the sheet  140 . It is noted that the top mold  118  may be suspended over the sheet  140  prior to or after heating the sheet  140 . In the former case, the top mold  118  may be heated along with the sheet  140 . In  FIG. 8 , the top mold  118  is brought into contact with the sheet  140 , with the cavities  124 ,  126  aligned with the bumps  112 ,  116 , respectively. The pins  128  in the top mold  118  may be aligned with the holes  130  to achieve the proper alignment between the top mold  118  and the bottom mold  102 . When properly aligned, the cavity  124  protects the upper surface  141  of the second sheet portion  140   b  overlying the bump  112  from being touched by the network of protuberances  122 . The cavity  124  is deep enough that there is clearance between its wall and the upper surface  141  both before and after the second sheet portion  140   b  is molded to the bump  112 , as will be described below. In other words, the height of the cavity  124  is greater than the sum of the height of the bump  112  and thickness of the sheet  140 . This allows the upper surface  141  of the second sheet portion  140   b,  which will become a surface of a shaped article, to remain in a pristine condition. 
     In  FIG. 9 , the network of protuberances  122  is pressed against the sheet  140 . If top mold  118  is not sufficiently heavy, such pressing may include applying an external load to the top mold  118 . The network of protuberances  122  is pressed against the sheet  140  until the network of protuberances  122  encounters the mold surface  106  on the bottom mold  102 . Several events occur during this pressing. One event is molding of the sheet  140  to the bumps  112 ,  116  as the sheet  140  is being pushed downwardly and around the periphery of the bumps  112 ,  116  by the network of protuberances  122 . Another event is thinning of the sheet  140  in the region where it is in contact with the network of protuberances  122 . This region is between the first portion  140   a  of the sheet  140  which overlies the network of gutters  108  and the second portion  140   b  of the sheet  140  which overlies the bump  112 . This region is also between the first portion  140   a  of the sheet  140  which overlies the network of gutters  108  and the third portion  140   c  of the sheet  140  which overlies the side bumps  116 . The thinning path will generally trace the edges of the network of gutters  108  (refer to  FIG. 2  for the network gutter profile). In some examples, the localized thinning out of the sheet  140  effectively results in cutting or shearing of the sheet  140  along the thinning path. Such cutting or shearing may be achieved by applying enough force to the network of protuberances  122  to pinch the sheet  140  between the network of protuberances  122  and the mold surface  106 . In some examples, the protuberances in the network  122  may slide into the gutters in the network  108 , creating a scissor-like action that shears the sheet  140 . Another event that occurs during pressing of the sheet  140  is that the excess sheet material produced by the thinning out of the sheet  140  is squeezed into the network of gutters  108 . 
     The portion of the sheet  140  molded onto the bump  112  becomes the shaped article  142 . After pressing, the shaped article  142  is allowed to cool down between the top mold  118  and the bottom mold  102 . The shaped article  142  may be allowed to cool down to a temperature below the strain point of the glass-based material from which the shaped article is made. For example, the shaped article may be cooled to a temperature of about 50° C. below the glass strain point. Then, the top mold  118  is separated from the bottom mold  102 . Next, the shaped article  142  is popped from the surrounding sheet of material. Additional processing of the shaped article  142  may include annealing the shaped article  142  and chemically strengthening the shaped article  142 . The shaped article may also be finished, e.g., by fire polishing, to improve its surface quality. The method described can be used to form a plurality of discrete shaped articles  142  in a single operation or step using the top mold  118  in  FIG. 5  and the bottom mold  102  in  FIG. 4 . Further, a stack of apparatus  100  as explained above can be used to make several discrete shaped articles  142  in a single operation or step. 
     In one non-limiting example, the sheet  140  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 1.3×10 5  Poise and, in one embodiment, greater than about 2.5×10 5  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 %≦SiO 2 ≦68 mol %; 12 mol %≦Na 2 O≦16 mol %; 8 mol %≦Al 2 O 3 ≦12 mol %; 0 mol %≦B 2 O 3 ≦3 mol %; 2 mol %≦K 2 O≦5 mol %; 4 mol %≦MgO≦6 mol %; and 0 mol %≦CaO≦5 mol %, wherein: 66 mol %≦SiO 2 +B 2 O 3 +CaO≦69 mol %; Na 2 O+K 2 O+B 2 O 3 +MgO+CaO+SrO&gt;10 mol %; 5 mol %≦MgO+CaO+SrO≦8 mol %; (Na 2 O+B 2 O 3 )−Al 2 O 3 ≦2 mol %; 2 mol %≦Na 2 O−Al 2 O 3 ≦6 mol %; and 4 mol %≦(Na 2 O+K 2 O)−Al 2 O 3 ≦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 (KNO 3 ). 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 high 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.