Patent Publication Number: US-9896367-B2

Title: Methods and apparatuses for producing laminated glass sheets

Description:
This application is a divisional of U.S. application Ser. No. 14/413,625 filed on Jan. 8, 2015, which claims the benefit of priority under 35 U.S.C. § 371 of International Application Number PCT/IB2012/001715 filed on Jul. 13, 2012, the content of each of which is relied upon and incorporated herein by reference in its entirety. 
    
    
     BACKGROUND 
     Field 
     The present specification generally relates to laminated glass sheets and, more specifically, to methods and apparatuses for producing laminated glass sheets by float processes. 
     Technical Background 
     Glass articles, such as cover glasses, glass backplanes and the like, are employed in both consumer and commercial electronic devices such as LCD and LED displays, computer monitors, automated teller machines (ATMs) and the like. Some of these glass articles may include “touch” functionality which necessitates that the glass article be contacted by various objects including a user&#39;s fingers and/or stylus devices and, as such, the glass must be sufficiently robust to endure regular contact without damage. The glass articles incorporated in these devices may be susceptible to damage during transport and/or use of the associated device. Accordingly, glass articles used in such devices may require enhanced strength to be able to withstand not only routine “touch” contact from actual use, but also incidental contact and impacts. 
     Various processes may be used to strengthen glass articles, including chemical tempering and thermal tempering. Chemical and thermal tempering processes may be used to strengthen a glass article after the article is formed, thereby requiring additional processing steps and handling of the glass article, both of which may result in damage to the glass article which increases production costs and decreases productivity, particularly for larger glass articles. 
     Accordingly, a need exists for alternative methods and apparatuses for forming strengthened glass sheets. 
     SUMMARY 
     According to one set of embodiments, a method for forming a laminated glass sheet may include forming a multi-layer glass melt from a molten core glass and at least one molten cladding glass. The multi-layer glass melt may have a width W m , a melt thickness T m  and a core to cladding thickness ratio T c :T cl . The multi-layer glass melt may be directed onto the surface of a molten metal bath contained in a float tank having a width W f . The width W m  of the multi-layer glass melt is less than the width W f  of the float tank prior to the multi-layer glass melt entering the float tank. The multi-layer glass melt may flow over the surface of the molten metal bath such that the width W m  of the multi-layer glass melt increases, the melt thickness T m  decreases, and the core to cladding thickness ratio T c :T cl  remains constant as the multi-layer glass melt solidifies into a laminated glass sheet. 
     In another set of embodiments, a method for forming a laminated glass sheet may include forming a molten core glass from a core glass composition and forming a molten cladding glass from a cladding glass composition. A slot draw apparatus comprising a core glass slot and at least one cladding glass slot may be provided. The core glass slot and the at least one cladding glass slot may be oriented in parallel with one another. The slot draw apparatus may be positioned over a float tank containing a molten metal bath and oriented at a slot angle greater than or equal to 0° and less than 90° with respect to a surface of the molten metal bath. A width W s  of the slot draw apparatus may be less than a width W f  of the float tank. The molten core glass and the molten cladding glass may be delivered to the slot draw apparatus such that the molten core glass passes through the core glass slot and the molten cladding glass passes through the at least one cladding glass slot. The molten cladding glass and the molten core glass may form a multi-layer glass melt with a width W m , a melt thickness T m , and a core to cladding thickness ratio T c :T cl  upon exiting the slot draw apparatus. The width W m  of the multi-layer glass melt is less than the width W f  of the float tank. The multi-layer glass melt may be directed onto the surface of the molten metal bath. As the multi-layer glass melt flows over the surface of the molten metal bath, the width W m  of the multi-layer glass melt increases, the melt thickness T m  decreases, and the core to cladding thickness ratio T c :T cl  remains constant as the multi-layer glass melt solidifies into a laminated glass sheet. 
     In yet another set of embodiments, an apparatus for forming a laminated glass sheet may include a core glass melting vessel, a cladding glass melting vessel and a slot draw apparatus comprising a core glass slot and at least one cladding glass slot. The core glass slot and the at least one cladding glass slot may be oriented in parallel with one another. The core glass slot may be fluidly coupled to the core glass melting vessel such that molten core glass can be delivered from the core glass melting vessel to the core glass slot. The at least one cladding glass slot may be fluidly coupled to the cladding glass melting vessel such that molten cladding glass can be delivered from the cladding glass melting vessel to the at least one cladding glass slot. The apparatus may further include a float tank containing a molten metal bath. The float tank may have a width W f  which is greater than a width W s  of the slot draw apparatus. The slot draw apparatus may be positioned over the float tank and oriented at a slot angle greater than or equal to 0° and less than 90° with respect to a surface of the molten metal bath. 
     Additional features and advantages of the methods and apparatuses for forming laminated glass sheets will be set forth in the detailed description which follows, and in part will be readily apparent to those skilled in the art from that description or recognized by practicing the embodiments described herein, including the detailed description which follows, the claims, as well as the appended drawings. 
     It is to be understood that both the foregoing general description and the following detailed description describe various embodiments and are intended to provide an overview or framework for understanding the nature and character of the claimed subject matter. The accompanying drawings are included to provide a further understanding of the various embodiments, and are incorporated into and constitute a part of this specification. The drawings illustrate the various embodiments described herein, and together with the description serve to explain the principles and operations of the claimed subject matter. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1A  schematically depicts an exemplary glass manufacturing apparatus for forming laminated glass sheets, according to one or more embodiments shown and described herein; 
         FIG. 1B  schematically depicts a portion of the glass manufacturing apparatus of  FIG. 1A ; 
         FIG. 2  schematically depicts a top view of the glass manufacturing apparatus of  FIG. 1A ; 
         FIG. 3  schematically depicts a front view of a slot draw apparatus for forming a multi-layer glass melt; 
         FIG. 4  schematically depicts a cross section of the slot draw apparatus of  FIG. 3 ; and 
         FIG. 5  schematically depicts a cross section of a multi-layer glass melt according to one or more embodiments shown and described herein. 
     
    
    
     DETAILED DESCRIPTION 
     Reference will now be made in detail to methods and apparatuses for forming laminated glass sheets, embodiments of which are schematically depicted in the accompanying drawings. Whenever possible, the same reference numerals are used throughout the drawings to refer to the same or like parts. One embodiment of a method for forming a laminated glass sheet is schematically depicted in  FIG. 1A . The method generally includes forming a multi-layer glass melt from a molten core glass and at least one molten cladding glass. The multi-layer glass melt may have a width W m , a melt thickness T m  and a core to cladding thickness ratio T c :T cl . The multi-layer glass melt may be directed onto the surface of a molten metal bath contained in a float tank having a width W f . The width W m  of the multi-layer glass melt is less than the width W f  of the float tank prior to the multi-layer glass melt entering the float tank. The multi-layer glass melt may flow over the surface of the molten metal bath such that the width W m  of the multi-layer glass melt increases, the melt thickness T m  decreases, and the core to cladding thickness ratio T c :T cl  remains constant as the multi-layer glass melt solidifies into a laminated glass sheet. Various embodiments of methods for forming laminated glass sheets and apparatuses for performing the method will be described in more detail herein with specific reference to the appended drawings. 
     The term “liquidus viscosity,” as used herein, refers to the shear viscosity of the glass composition at its liquidus temperature. 
     The term “liquidus temperature,” as used herein, refers to the highest temperature at which devitrification occurs in the glass composition. 
     The term “CTE,” as used herein, refers to the coefficient of thermal expansion of the glass composition averaged over a temperature range from about 20° C. to about 300° C. 
     Strengthened laminated glass articles may be formed by fusing one or more glass cladding layers having a relatively low average coefficient of thermal expansion to a glass core layer which has a relatively high average coefficient of thermal expansion. As the laminated structure cools, the differences in the coefficients of thermal expansion of the glass core layer and the glass cladding layer create compressive stresses in the glass cladding layers. 
     Laminated glass sheets have been formed by a fusion lamination process, such as the fusion lamination process disclosed in U.S. Pat. No. 4,214,886 and similar fusion lamination processes. Glass compositions used in conjunction with fusion lamination processes generally have a high liquidus viscosity of greater than 100 kpoise such that the glass is able to be drawn vertically downward at elevated temperatures. In comparison, glasses with lower viscosities tend to “run” at the temperature of the fusion lamination process making it difficult to draw such glass compositions at elevated temperatures. Further, it has been found that reducing the temperature of the fusion lamination process to accommodate glasses with lower viscosities (i.e., increasing the viscosity of the glass by lowering the processing temperatures) may increase the number of defects in the glass as the lower temperatures encourage the nucleation and growth of crystals on the ceramic forming equipment of the fusion apparatus which can become dislodged and embedded in the glass. In addition, the shear mass of the fusion forming equipment, such as the isopipe, used in fusion forming processes makes it difficult to scale the processes to form large-width glass sheets. The methods and apparatuses described herein enable the formation of laminated glass sheets from glass compositions with low liquidus viscosities and also enable the formation of large-width laminated glass sheets. 
     Referring now to  FIG. 1A , an exemplary glass manufacturing apparatus  100  for forming laminated glass sheets from molten glass is schematically depicted. The glass manufacturing apparatus generally comprises a core glass delivery system  110 , a cladding glass delivery system  120 , a slot draw apparatus  140 , and a float tank  160 . The float tank contains a molten metal bath  162 , such as molten tin or the like. 
     The core glass delivery system  110  generally includes a core melting vessel  101 , a core fining vessel  103 , a core mixing vessel  104 , a core delivery vessel  108 , and a core feed pipe  109  coupled to a core slot of the slot draw apparatus  140 . The cladding glass delivery system  120  generally includes a cladding melting vessel  121 , a cladding fining vessel  123 , a cladding mixing vessel  124 , a cladding delivery vessel  128 , and a cladding feed pipe  129  coupled to at least one cladding slot of the slot draw apparatus  140 . 
     The float tank  160  is generally positioned below the core glass delivery system  110  and the cladding glass delivery system  120  such that molten core glass  106  and molten cladding glass  126  can be delivered to the float tank by gravity. In the embodiment of the float tank  160  depicted in  FIG. 1A , the float tank  160  includes a receiving plane  161  suspended over the surface of the molten metal bath  162 . The receiving plane  161  includes a receiving surface  163  for receiving a multi-layer glass melt  300  discharged from the slot draw apparatus  140 . The receiving surface  163  is positioned at an angle with respect to the surface of the molten metal bath  162  such that the multi-layer glass melt discharged from the slot draw apparatus  140  flows over the receiving surface  163  and onto the surface of the molten metal bath  162  in a controlled manner. 
     While the float tank  160  of  FIG. 1A  is depicted with a receiving plane  161  having a receiving surface  163 , it should be understood that, in other embodiments (not shown), the float tank  160  may be constructed without a receiving plane  161 . In these embodiments, the multi-layer glass melt discharged from the slot draw apparatus  140  may be deposited directly on the surface of the molten metal bath  162 . 
     Referring to  FIGS. 1A and 2 , in some embodiments, the float tank  160  may also include one or more top rolls  170  (one depicted in  FIG. 1A ) for contacting the multi-layer glass melt  180  as it flows over the surface of the molten metal bath  162 . The top rolls  170  are each coupled to a rotating shaft  171  such that, as the top rolls rotate, the multi-layer glass melt is drawn in a direction of the width W f  of the float tank  160  and/or a length L f  of the float tank  160  to encourage the glass melt to spread over the surface of the molten metal bath  162 . 
     Referring to  FIGS. 1A-1B and 3-4 , the slot draw apparatus  140  is disposed over the float tank  160  and oriented such that a slot angle θ between the slot draw apparatus  140  and the surface of the molten metal bath  162  is greater than or equal to 0° and less than 90°. The slot draw apparatus is formed from a precious metal, such as platinum, platinum alloys or other precious metals suitable for use at the elevated temperatures of a glass forming process. The slot draw apparatus  140  generally comprises a core slot  142  and at least one cladding slot which is substantially parallel with the core slot  142 . The width W s  of the slot draw apparatus  144  (i.e., the width of the core slot  142  and the width of the cladding slot(s)  144 ) is less than the width of the float tank  160  (see, e.g.,  FIG. 2 ). 
     In the embodiment of the slot draw apparatus  140  shown in  FIGS. 1A-1B and 3-4 , the slot draw apparatus is constructed with a first cladding slot  144   a  positioned over the core slot  142  and a second cladding slot  144   b  positioned beneath the core slot  142 . In this embodiment, the slot draw apparatus  140  may be used to produce a multi-layer glass melt with a central core disposed between two cladding layers. However, it should be understood that the slot draw apparatus  140  may be constructed with a single cladding slot positioned either over the core slot  142  or below the core slot  142 , such as when the slot draw apparatus is  140  is used to form a multi-layer glass melt with a single core layer and a single cladding layer. Further, it should also be understood that the slot draw apparatus may be formed with greater than three slots, such as when the slot draw apparatus is used to form a multi-layer glass melt with more than three layers. 
     The core slot  142  of the slot draw apparatus  140  has a core height H c  and the at least one cladding slot has a height H cl . In embodiments where the slot draw apparatus contains a first cladding slot  144   a  and a second cladding slot  144   b , as depicted in  FIGS. 3 and 4 , the first cladding slot may have a height H cla  and the second cladding slot may have a height H clb . In some embodiments, the height H c  of the core slot may be equal to the height of each of the cladding slots. In other embodiments, the height H c  of the core slot may be different than the height of the cladding slots. In still other embodiments, the height H cla  of the first cladding slot  144   a  may be different than the height H clb  of the second cladding slot. 
     In the embodiment of the slot draw apparatus  140  shown in  FIGS. 3-4 , the core feed pipe  109  is coupled to the core slot  142  of the slot draw apparatus  140 , as depicted in  FIG. 4 , such that molten core glass delivered to the slot draw apparatus  140  with the core feed pipe  109  flows through the core slot  142 . The cladding feed pipe  129  is coupled to the first cladding slot  144   a  and the second cladding slot  144   b  with feed plenums  145   a ,  145   b , respectively. Accordingly, the molten cladding glass delivered to the slot draw apparatus  140  through the cladding feed pipe  129  flows through the plenums  145   a ,  145   b  and through the first cladding slot  144   a  and the second cladding slot  144   b , respectively. 
     In general, the pressure drop of the molten glass flowing through the slots is greater than the pressure drop of the molten glass in the respective feed pipes. For example, in some embodiments, the pressure drop of the molten glass through the slot draw apparatus is at least 10× greater than the pressure drop of the molten glass in the corresponding feed pipe. This pressure drop in the slot draw apparatus  140  encourages the molten glass to fill each of the slots across the entire width W s  of the slot draw apparatus  140  thereby promoting uniformity in the multi-layer glass melt formed by the slot draw apparatus. 
     Despite being formed from metals and/or alloys suitable for use at high temperatures, the dimensions of the slot draw apparatus  140  may change over time due to elevated temperature exposure. In order to minimize distortions in the resultant multi-layer glass melt, the core slot  142  and the cladding slots  144   a ,  144   b  of the slot draw apparatus may include a plurality of reinforcing webs  147  positioned in each of the slots, as depicted in  FIGS. 3-4 . The reinforcing webs improve the mechanical rigidity of the slot draw apparatus and also minimize dimensional changes in the slot draw apparatus due to prolonged elevated temperature exposure. 
     In some embodiments of the slot draw apparatus  140 , the reinforcing webs  147  are recessed from the slot openings, as depicted in  FIG. 4 . This configuration allows the molten glass to flow around the webs and re-knit into a continuous mass prior to exiting each of the slots. The process of re-knitting the glass web generally occurs between the end of the reinforcing webs  147  and the exit of the slot draw apparatus  140  and is assisted by the pressure drop in this portion of the slot draw apparatus as well as the surface tension of the molten glass. The re-knitting process may be further assisted by the specific geometry of the webs as well as gravity as the molten glass exits the slot draw apparatus and is deposited in the molten metal bath. However, in other embodiments (not shown), the reinforcing webs  147  may extend to the slot opening. 
     While  FIGS. 3-4  schematically depicts a slot draw apparatus  140  which includes reinforcing webs  147  in each of the slots, it should be understood that the reinforcing webs are optional and that, in some embodiments, the slot draw apparatus  140  may be formed without reinforcing webs. 
     Referring again to  FIGS. 1A and 2 , in operation, core glass batch materials are introduced into the core melting vessel  101  as indicated by arrow  102 . The core glass batch materials are melted in the core melting vessel  101  to form molten core glass  106 . The molten core glass  106  flows into the core fining vessel  103  which has a high temperature processing area that receives the molten core glass  106  from the core melting vessel  101 . The core fining vessel  103  removes bubbles from the molten core glass  106 . The core fining vessel  103  is fluidly coupled to the core mixing vessel  104  by a connecting tube  105 . That is, molten glass flowing from the core fining vessel  103  to the core mixing vessel  104  flows through the core connecting tube  105 . The core mixing vessel  104  is, in turn, fluidly coupled to the core delivery vessel  108  by a connecting tube  107  such that molten glass flowing from the core mixing vessel  104  to the core delivery vessel  108  flows through the connecting tube  107 . The core delivery vessel  108  supplies the molten core glass  106  to the core slot of the slot draw apparatus  140 . 
     Simultaneously, cladding glass batch materials are introduced into the cladding melting vessel  121  as indicated by arrow  122 . The cladding glass batch materials are melted in the cladding melting vessel  121  to form molten cladding glass  126 . The cladding fining vessel  123  has a high temperature processing area that receives the molten cladding glass  126  from the cladding melting vessel  121 . The cladding fining vessel  123  removes bubbles from the molten cladding glass  126 . The cladding fining vessel  123  is fluidly coupled to the cladding mixing vessel  124  by a connecting tube  125 . That is, molten cladding glass flowing from the cladding fining vessel  123  to the cladding mixing vessel  124  flows through the cladding connecting tube  125 . The cladding mixing vessel  124  is, in turn, fluidly coupled to the cladding delivery vessel  128  by a connecting tube  127  such that molten glass flowing from the cladding mixing vessel  124  to the cladding delivery vessel  128  flows through the connecting tube  127 . The cladding delivery vessel  128  supplies the molten cladding glass  126  to at least one cladding slot of the slot draw apparatus  140 . 
     The molten core glass and the molten cladding glass flow through the slot draw apparatus  140  in the respective core and cladding slots. The relative orientation of the slots in the slot draw apparatus  140  causes the molten core glass and the molten cladding glass to be layered together upon exiting the slot draw apparatus  140 , thereby forming a multi-layer glass melt  300 , such as the multi-layer glass melt  300  depicted in cross section in  FIG. 5 . The multi-layer glass melt  300  discharged from the slot draw apparatus has a melt thickness T m  and includes a core layer  302  disposed between a first cladding layer  304   a  and a second cladding layer  304   b . The core layer  302  has a thickness T c , the first cladding layer  304   a  has a thickness T cla  and the second cladding layer  304   b  has a thickness T clb . The thickness of each of these layers is generally proportional to the cube of the height of the corresponding slot (i.e., the core layer  302  has a thickness T c ≈H c   3 , the first cladding layer  304   a  has a thickness T cla ≈H cla   3 , and the second cladding layer  304   b  has a thickness T clb ≈H clb   3 ). Further, the multi-layer glass melt  300  has a core to cladding thickness ratio T c :T cl  where T c  is the thickness of the core layer  302  and T cl  is the sum of the thicknesses of the cladding layers  304   a ,  304   b . Accordingly, in embodiments in which two cladding slots are disposed on either side of a core slot, the core to cladding thickness ratio Tc:Tcl of the multi-layer glass melt  300  can be approximated by the equation H c   3 /(H cla   3 +H clb   3 ). The width W m  of the multi-layer glass melt  300  is generally the same as the width W s  of the slot draw apparatus  140  as the core layer  302  and the cladding layers  304   a ,  304   b  are discharged from the slot draw apparatus. Accordingly, it should be understood that the width W m  of the multi-layer glass melt  300  is generally less than the width of the float tank W f  prior to the multi-layer glass melt  300  entering the float tank  160 . 
     In the embodiment of the glass manufacturing apparatus  100  depicted in  FIG. 1A , the slot draw apparatus  140  is oriented at a slot angle greater than or equal to about 0 degrees and less than 90 degrees relative to the surface of the molten metal bath to facilitate depositing the multi-layer glass melt  300  onto the surface of the molten metal bath  162  while still maintaining the orientation and integrity of the layered structure imparted to the multi-layer glass melt  300  by the slot draw apparatus  140 . Further, the non-perpendicular orientation of the slot draw apparatus  140  with respect to the surface of the molten metal bath encourages the multi-layer glass melt  300  to flow over the surface of the molten metal bath  162  in a direction away from the slot draw apparatus  140 . 
     As noted above, the embodiment of the float tank  160  depicted in  FIG. 1A  includes a receiving plane  161  suspended over the surface of the molten metal bath  162 . The receiving plane  161  includes a receiving surface  163  which is angled downward, into the molten metal bath  162 . In this embodiment, the multi-layer glass melt  300  discharged from the slot is deposited on to the receiving surface  163  of the receiving plane  161  in order to introduce the multi-layer glass melt  300  into the molten metal bath  162  in a controlled manner. Specifically, the multi-layer glass melt  300  is discharged from the slot draw apparatus  140  onto the receiving surface  163  such that the multi-layer glass melt  300  flows over the receiving surface  163  and onto the surface of the molten metal bath  162 , thereby maintaining the orientation and integrity of the individual layers of the multi-layer glass melt  300 . 
     While  FIG. 1A  depicts the multi-layer glass melt  300  as being deposited on the receiving surface  163  of the receiving plane  161  before entering the molten metal bath  162 , it should be understood that this step is optional. For example, in some embodiments (not shown) the multi-layer glass melt  300  may be deposited directly into the molten metal bath  162  without first being deposited onto the receiving surface  163  of a receiving plane  161  suspended over the molten metal bath  162 . 
     Stiller referring to  FIGS. 1A and 2 , upon being deposited on the molten metal bath  162 , the multi-layer glass melt  300  flows over the surface of the molten metal bath  162 . As the multi-layer glass melt flows over the surface of the molten metal bath  162 , the multi-layer glass melt  300  spreads over the surface of the molten metal bath  162  in the direction of both the length L f  and width W f  of the float tank  160  such that the width W m  of the multi-layer glass melt  300  increases and the thickness T m  of the multi-layer glass melt  300  decreases as the multi-layer glass melt reaches both an equilibrium width and an equilibrium thickness on the surface of the molten metal bath  162 . However, while the melt thickness T m  of the multi-layer glass melt  300  decreases and the width W m  of the multi-layer glass melt increases, the core to cladding thickness ratio T c :T cl  remains constant as the multi-layer glass melt solidifies into a laminated glass sheet. 
     As noted hereinabove, the glass manufacturing apparatus  100  may include one or more top rolls  170  which may be used to contact the multi-layer glass melt  300  and draw the multi-layer glass melt  300  over the surface of the molten metal bath  162 , thinning the multi-layer glass melt  300  and, optionally, increasing the width W m  of the multi-layer glass melt  300 . Accordingly, in some embodiments, the top rolls  170  may be used to draw the multi-layer glass melt  300  in a direction of the width W f  of the float tank as the multi-layer glass melt flows over the surface of the molten metal bath. In some other embodiments, the top rolls  170  may be used to draw the multi-layer glass melt  300  in a direction of the width W f  of the float tank and in a direction of the length L f  of the float tank as the multi-layer glass melt  300  flows over the surface of the molten metal bath  162 . 
     As the multi-layer glass melt flows and/or is drawn over the surface of the molten metal bath  162 , the multi-layer glass melt  300  gradually cools and solidifies, forming a laminated glass sheet. In some embodiments described herein, the cladding glass has a first glass composition which has an average cladding coefficient of thermal expansion CTE clad  and the core glass is formed from a second, different glass composition which has an average coefficient of thermal expansion CTE core . In these embodiments, the CTE core  may be greater than the CTE clad  such that, when the multi-layer glass melt  300  solidifies, the difference in the coefficients of thermal expansion results in the cladding glass being compressively stressed thereby increasing the mechanical strength of the laminated glass sheet without the glass sheet being ion exchanged or thermally tempered. 
     The methods and apparatuses described herein may be used to produce laminated glass sheets of varying thicknesses and widths. In particular, the methods and apparatuses described herein may be scaled to produce laminated glass sheets having widths on the order of several meters. For example, in some embodiments, the width of the resultant glass sheet may be greater than 1 meter or even greater than 3 meters. In some embodiments, the width of the resultant glass sheet may be greater than 4 meters or even greater than 5 meters. 
     Further, the thicknesses of the resultant laminated glass sheets may be less than 1 cm. For example, in some embodiments, the thickness of the resultant laminated glass sheet may be less than or equal to 7 mm or even less than or equal to 5 mm. In some embodiments, the thickness of the resultant laminated glass sheet may be less than or equal to 2.5 mm. In still other embodiments the thickness of the resultant laminated glass sheet may be less than or equal to 1 mm. 
     Further, the methods and apparatuses described herein may also be used to form laminated glass sheets with different structures. For example, any number of cladding slots may be added on either side of the core slot in order to produce a laminated glass sheet having the desired structure. The methods and apparatuses described herein may be used to produce laminated glass sheets with symmetrical claddings (i.e., the same number of cladding layers on either side of the glass core) or asymmetrical claddings (i.e., a different number of cladding layers on either side of the glass core). Further, the methods and apparatuses described herein may also be used to produce laminated glass sheets wherein the thicknesses of the cladding layers are symmetrical or asymmetrical about the glass core. 
     While the methods and apparatuses described herein are compatible with glass compositions of varying liquidus viscosities, the methods and apparatuses described herein are particularly well suited for use with core glass compositions and cladding glass compositions which have lower liquidus viscosities which are not generally suitable for use with fusion forming processes such as fusion lamination processes. For example, in the embodiments described herein, the core glass compositions and the cladding glass compositions may have liquidus viscosities less than 100 kpoise. In some embodiments, the core glass compositions and the cladding glass compositions may have liquidus viscosities less than or equal to 100 kpoise, or even less than or equal to 50 kpoise. In some embodiments, the core glass compositions and the cladding glass compositions may have liquidus viscosities less than or equal to 30 kpoise or even less than or equal to 20 kpoise. 
     EXAMPLES 
     The methods and apparatuses will be further clarified by the following hypothetical example. 
     Example 1 
     While not wishing to be bound by theory, it is believed that the methods and apparatuses described herein may be used to form laminated glass sheets as exemplified by the following hypothetical example. The hypothetical slot draw apparatus has a core slot disposed between a first cladding slot and a second cladding slot. The core slot had a height H c  of 0.0125 m. The cladding slots each had a height H cla =H clb =0.006 m. The hypothetical slot draw apparatus had a width W s  of 0.4 m. Based on the foregoing, the core to cladding ratio T c :T cl  of the glass melt discharged from the slot draw apparatus is 4.5. In this hypothetical, the slot draw apparatus may be coupled to a core glass delivery system and a cladding glass delivery system which, combined, are capable of delivering 37 metric tons of glass per day to the slot draw apparatus. The core glass and the cladding glass compositions hypothetically have identical viscosities and the temperature of the glass manufacturing system is maintained at a temperature such that both the core glass and the cladding glass have viscosities of 2000 poise. It is believed that the glass manufacturing system of this hypothetical example is suitable for forming a glass sheet having a width of 4 m or greater. 
     It should now be understood that the methods and apparatuses described herein may be utilized to produce laminate glass sheets from core and cladding glass compositions having a broad range of liquidus viscosities, including liquidus viscosities of less than 100 kpoise or even less than 20 kpoise. Further, the methods and apparatuses described herein may be scaled to produce glass sheets having widths on the order of several meters, including, without limitation, glass sheets with widths greater than about 4 meters. 
     Further, the methods and apparatuses described herein may be utilized during formation of the glass sheet to produce a strengthened laminated glass sheet and, as such, the need for secondary processing steps may be eliminated. Accordingly, the risk of damaging the glass sheet as the glass sheet is transferred to different processing areas may also be eliminated thereby decreasing production losses and production costs. 
     It will be apparent to those skilled in the art that various modifications and variations can be made to the embodiments described herein without departing from the spirit and scope of the claimed subject matter. Thus it is intended that the specification cover the modifications and variations of the various embodiments described herein provided such modification and variations come within the scope of the appended claims and their equivalents.