Patent Publication Number: US-2011067450-A1

Title: Method and apparatus for forming shaped articles from sheet material

Description:
PRIORITY 
     The application claims the priority and benefit of PCT Application No. PCT/IB2008/003701 titled “Method and Apparatus for Forming Shaped Materials from Sheet Material” filed on Nov. 26, 2008 in the name of inventors Allan Mark Fredholm, Christophe Pierron, Patrick Jean Pierre Herve and Thierry Luc Alain Dannoux. 
    
    
     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 forming a shaped glass-based article which may have a thin wall. 
     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. Shaped glass articles have also been formed by pressing glass plates into molds. 
     SUMMARY 
     In one aspect, the invention relates to a method of making shaped articles which comprises providing a container containing an array of spaced-apart positive molds, each of the positive molds having an exterior surface including a profile defining an interior of a shaped article. The method further includes positioning a sheet of glass-based material on the container such that a closed volume is defined between the sheet and the container, and the closed volume encloses the array of spaced-apart positive molds. The method includes applying vacuum to the closed volume and sagging the sheet by vacuum onto the exterior surfaces of the positive molds and into spaces between the positive molds to form an array of shaped articles interconnected by sagging webs in a portion of the sheet, where the sagging webs extend below a base of the array of shaped articles. The method includes separating the array of shaped articles from the positive molds and trimming off the sagging webs to separate the array of shaped articles into individual shaped articles. 
     In another aspect, the invention relates to an apparatus for making shaped articles which comprises a container having at least one vacuum port and a surface for receiving a sheet of glass-based material. The apparatus includes at least one positive mold supported in the container, where the at least one positive mold has an exterior surface including a profile defining an interior of a shaped article. The apparatus includes an open volume defined between the container and the at least one positive mold. The open volume is in communication with the vacuum port. 
     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 top view of an apparatus for making shaped articles. 
         FIG. 2  is a cross-section of  FIG. 1  taken along line  2 - 2 . 
         FIG. 3  is a perspective view of a positive mold. 
         FIG. 4  shows a sheet of material suspended over an apparatus for making shaped articles. 
         FIG. 5  shows the sheet of  FIG. 4  positioned on the container of the apparatus. 
         FIG. 6  shows the sheet of  FIG. 5  sagged onto positive molds. 
         FIG. 7  shows a force applied to a web in the sheet of  FIG. 6 . 
         FIG. 8  is a partial array of interconnected shaped articles. 
         FIG. 9  shows individual shaped articles. 
     
    
    
     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 top view of an apparatus  100  for making shaped articles. The shaped articles may be made from a glass-based material, such as glass or glass-ceramic. Apparatus  100  includes a container  102  having a side wall  104  and base wall  106 . Container  102  may be made of a heat-resistant, sturdy material.  FIG. 2  is a vertical cross-section of apparatus  100 . As shown in  FIG. 2 , the container  102  includes vacuum ports  108 . In general, the container  102  may have one or more vacuum ports  108 . The vacuum ports  108  may be located in the base wall  106 , as shown, and/or may be located in the side wall  104 . 
     Referring to  FIGS. 1 and 2 , a plurality of positive molds  116  is supported in the container  102 . In general, one or more positive molds  116  may be supported in the container  102 . The positive molds  116  may be arranged such that there is a gap G between each positive mold  116  and its neighboring positive molds. The width of gap G may be the same or different across the apparatus. The shape of each positive mold  116  will depend on the desired shaped article to be formed by the positive mold. For illustration purposes,  FIG. 3  shows an example of a positive mold  116  for forming a shaped article. The positive mold  116  has an exterior surface  118 , which includes a profile of the interior of the shaped article to be formed by the positive mold  116 . The mold  116  is described as “positive” because the exterior surface  118  which bears the profile of the shaped article is generally concave. The exterior surface  118  may be smooth or textured. The positive mold  116  also has a base surface  120 , which may be arranged on a support as will be explained below. 
     Returning to  FIGS. 1 and 2 , the positive molds  116  may be made of a heat-resistant material, preferably one that would not react with the glass-based material that will be used in making the shaped articles under the conditions at which the shaped articles would be made. Such conditions will become apparent during subsequent descriptions of how the shaped articles are made using the apparatus. As an example, the positive molds  116  may be made of high-temperature steel, cast iron, or ceramic. To extend the life of the mold, the exterior surfaces  118  of the positive molds  116  may be coated with a hard heat-resistant material that would not react with the glass-based material that will be used in making the shaped articles. An example of such a material is diamond chromium coating. 
     Referring to  FIG. 2 , the positive molds  116  are supported on a plurality of pillars  110 . The pillars  110  are supported on the base wall  106  of the container  102 . The base wall  106  may include recesses  112  for receiving an end of the pillars  110 . The positive molds  116  may similarly include recesses  121  for receiving an end of the pillars  110 . The pillars  110  may have a circular cross-section or other type of cross-section, e.g., elliptical or annular. The size, e.g., diameter, of the pillars  110  may be the same or may be different across the apparatus. The pillars  110  may be arranged such that there is a gap g between each pillar  110  and its neighboring pillars. The width of gap g may be the same or vary across the apparatus. The pillars  110  may be made of a heat-resistant sturdy material. 
     Referring to  FIGS. 1 and 2 , in one example, a spacer ring  124  is disposed in an annular gap  123  between the side wall  104  of the container  102  and the positive molds  116 . The spacer ring  124  is spaced from the positive molds  116  such that an annular gap  125  is formed between the spacer ring  124  and the positive molds  116 . As more clearly shown in  FIG. 2 , the spacer ring  124  may include a stop  128 , which is a surface that can oppose a force between the spacer ring  124  and the positive molds  116 , as will be explained below. 
     Referring to  FIG. 1 , the container  102  provides a surface  119  on which a sheet of glass-based material can be positioned. In one example, ejectors  127  are located on the surface  119 . The ejectors  127  may be operated to assist in unloading a sheet of glass-based material from the container  102 . 
     Referring to  FIG. 2 , an open volume, generally identified at  115 , is defined between the container  102  and the positive molds  116 . The volume  115  is “open” because of the gaps G between the positive molds  116  and the gaps g between the pillars  110 . The open volume  115  is in communication with the vacuum ports  108 . The annular gap  125  may also contribute to the openness of the open volume  115  where the annular gap  125  is interconnected with the gaps G and g. If the annular gap  125  is not interconnected with the gaps G and g, a separate vacuum circuit may be connected to the annular gap  125  for providing vacuum in the annular gap  125 . 
       FIGS. 4-9  illustrate a method of making shaped articles. In  FIG. 4 , a sheet  130  made of a glass-based material is suspended over the container  102  of apparatus  100 . The sheet  130  may be suspended over the container  102  using any suitable method, such as by suction cups. The suction cups or other gripping device may be applied from above, below, or at the edges of the sheet  130 . Where the top surface  132  of the sheet  130  is pristine, the suction cups or other gripping device may contact the top surface  132  near the edges of the sheet  130  that will not be formed into shaped articles. The sheet  130  may be transported to the container  102  from a sheet forming station using any suitable translation device, such as a set of rollers. The sheet  130  may be made by any suitable process, such as fusion draw process or float glass process. The sheet  130  may be transported to the container  102  as a discrete sheet or as a continuous sheet. The sheet  130  may have one pristine surface or two pristine surfaces. A sheet  130  having pristine surface(s) can be made, for example, by a fusion draw process. 
     The material of sheet  130  may be any glass-based composition suitable for the application in which the shaped articles are to be used. The glass-based material may be glass or glass-ceramic. In one example, the glass-based material is a glass composition that is capable of being chemically strengthened by ion-exchange. Typically, the presence of small alkali ions such as Li +  and Na +  in the glass structure that can be exchanged for larger alkali 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. 11/888213, 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 1.3×10 5  Poise and, in one embodiment, greater than 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 ≦3mol %; 2 mol %≦K 2 O≦5 mol %; 4 mol %≦MgO≦6 mol %; and 0 mol %≦CaO≦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 &lt;2 mol %; 2 mol %≦Na 2 O−Al 2 O 3 ≦6 mol %; and 4 mol %≦(Na 2 O+K 2 )−Al 2 O 3 ≦10 mol %. 
     In order to form the glass sheet  130  into shaped articles, the sheet  130  has to be at an elevated temperature at which it can be molded. Arrows  134  show that the sheet  130  may be heated to an elevated temperature while being suspended over the container  102 . Sheet  130  may also be heated to an elevated temperature prior to being suspended over container  102 . In one example, sheet  130  is heated to a temperature at which the viscosity of the glass-based material is approximately 10 9  Poise or lower. In general, this temperature will depend on the composition of the glass-based material. 
     In  FIG. 5 , sheet  130  is brought into contact with the container  102 , thereby defining a closed volume, generally identified by  135 , between the container  102  and the sheet  130 . The temperature of the positive molds  116  may be lower than the temperature of the sheet  130 . In the position depicted in  FIG. 5 , the sheet  130  overlies the positive molds  116 , the gaps G between the positive molds  116 , and the annular gap  125  between the positive molds  116  and the spacer ring  124 . 
     The method includes applying vacuum to the closed volume  135  through the vacuum ports  108 . This can be achieved, for example, by connecting a vacuum pump to the vacuum ports  108  and using the vacuum pump to remove air and other gases from the closed volume  135 . As shown in  FIG. 6 , application of the vacuum results in sagging of the sheet  130  onto the exterior surfaces  118  of the positive molds  116  and into the gaps G and  125 . The portion of the sheet  130  sagged onto the exterior surfaces  118  forms shaped articles  144 . The portion of the sheet  130  sagged into the gaps G results in sagging webs  146  (concave webs), which interconnect the shaped articles  144 . In one example, the sagging webs  146  extend below a base of the array of shaped articles  144  so that they can be trimmed off, as will be further explained below, to separate the shaped articles  144  into individual pieces. 
     The portion of the sheet  130  sagged into the annular gap  125  results in a sagging web  148  between the shaped articles  144  (i.e., the ones adjacent to the spacer ring  124 ) and the remainder  130   a  of the sheet  130 .  FIG. 7  shows that a force F may be applied to the sagging web  148  either to press (thin out) the sagging web  148  or cut through the sagging web  148 . In the latter case, the interconnected shaped articles  144  would be separated from the remainder  130   a  of the sheet  130 . Force F may be applied with a tool having a blunt or sharp edge. 
     The method includes keeping the interconnected shaped articles  144  on the positive molds  116  until the glass-based material cools down, typically to a temperature at which the glass-based material has a viscosity of approximately 10 13  Poise or greater. Vacuum may be maintained in the closed volume ( 135  in  FIG. 5 ) while the glass-based material cools down to the desired temperature. Next, the cooled interconnected shaped articles  144  are unloaded from the positive molds  116 . Unloading may include pressurizing the closed volume ( 135  in  FIG. 5 ) and/or activating the ejectors ( 127  in  FIG. 1 ). 
       FIG. 8  shows a portion of the interconnected shaped articles  144  formed as described above, after unloading from the positive molds ( 116  in  FIG. 7 ). The interconnected shaped articles  144  may be annealed. After annealing, the sagging webs  146  are trimmed off to separate the shaped articles  144  into individual pieces. When the sagging webs  146  extend below the base of the shaped articles  144  as shown in  FIG. 8 , trimming off can be accomplished, e.g., by grinding. This avoids the use of complex machinery to dice the interconnected shaped articles  144  into individual pieces. The sagging web  148  is also trimmed off. The molding process may also be such that the sagging web  148  extends below the base of the shaped articles  144 , thereby simplifying the trimming off process. 
       FIG. 9  shows the individual shaped articles  144 . The method may include finishing the trimmed edges of the individual shaped articles  144 . The method may further include chemically strengthening the shaped articles  144 , as will be explained below. After chemical strengthening, techniques such as fire-polishing may be used to finish the shaped articles. 
     In one example, chemical strengthening is by ion-exchange. 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 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 390° C. to 430° C., and the time period for which the sodium-based glass is dipped in a molten bath comprising a salt of potassium can be 7 to 12 hours (less time at high temperature, more time at lower temperature). In general, the deeper the ion-exchange, the higher the surface compression and the stronger the glass can be. 
     The method and apparatus described above can allow forming of thin-walled shaped glass-based articles (e.g., having wall thickness &lt;2 mm) at high precision and low cost. The exterior of the shaped articles does not come into contact with the positive molds and therefore can be pristine if the original sheet from which they are made has at least one pristine surface. The method is reproducible and consistent and flexible. Flexibility may be realized in the ability to form shaped articles with different shapes in a single process. 
     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.