Patent Publication Number: US-2019185363-A1

Title: Apparatus and method of managing mechanically induced stress on a crack tip when separating a flexible glass ribbon

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit of priority under 35 U.S.C. § 119 of U.S. Provisional Application Ser. No. 62/345,146 filed on Jun. 3, 2016, the content of which is relied upon and incorporated herein by reference in its entirety. 
    
    
     TECHNICAL FIELD 
     The present specification generally relates to apparatuses and methods for managing stress on a crack tip when separating a flexible glass ribbon. 
     BACKGROUND 
     Glass manufacturing apparatus are commonly used to form various glass products for example LCD sheet glass. Glass substrates in flexible electronic applications are becoming thinner and lighter. Glass substrates having thicknesses lower than 0.5 mm, for example less than 0.3 mm, for example 0.1 mm or even thinner can be desirable for certain display applications, especially portable electronic devices for example laptop computers, handheld devices and the like. To achieve these smaller thicknesses, it is known to manufacture glass ribbon by downwardly flowing molten glass over a forming wedge and using edge rollers to engage beads formed at opposite edge portions of a glass ribbon. After attaining a final desired ribbon thickness in the quality portion thereof, the beads are removed by slitting the ribbon in a longitudinal direction. The manner of slitting the ribbon affects the strength of the edges on the quality portion of the ribbon which, in turn, affects the ability to use the ribbon for a desired purpose. There is thus desired improved manners of slitting a ribbon so as to attain a strong edge on the quality portion so that the ribbon may be used in desired applications for example in portable electronic devices, laptop computers, handheld devices and the like. 
     SUMMARY 
     The present concept involves managing mechanically induced stress on a crack tip when separating a flexible glass ribbon during an edge trim removal process. The edge trim may be removed from a central portion of the flexible glass ribbon in a continuous fashion by an edge trimming apparatus. The continuous strip of edge trim is then collected while the continuous strip of edge trim remains connected to the central portion of the flexible glass ribbon and can eliminate any need for handling relatively small pieces of thin flexible glass edge trim. 
     During edge trim removal, particularly during a laser cutting process, it can be desirable to control mechanically generated stress on the crack tip at the cutting location, such that the mechanically induced stresses do not overly influence crack tip formation, and thereby to improve edge quality. In other words, the thermal stress (for example as imparted by a laser beam and coolant jet) should be the primary driving stress which maintains crack tip propagation and stability, as opposed to mechanically induced stresses imparted by physical edge separation. However, edge separation of the edge trim from the central quality portion of the flexible glass ribbon is desirable to inhibit edge quality reduction by, for example, crack formation and propagation resulting from contact between the edges. Additionally, edge separation is desirable to avoid particle generation from the edges rubbing together, as such particles undesirably wind up on the major surfaces of the glass ribbon and may thereby inhibit the production of desired structures on the glass ribbon. 
     According to a first aspect, a method of managing mechanically induced stress on a crack tip comprises: 
     directing a flexible glass ribbon to an edge trimming apparatus including a cutting device, the flexible glass ribbon including a first broad surface and a second broad surface that extend laterally between a first edge and a second edge; 
     separating the first edge of the flexible glass ribbon as the flexible glass ribbon moves by the cutting device so as to form a continuous strip of edge trim connected to a central portion of the flexible glass ribbon at a crack tip; 
     detecting a width of a gap between the first edge and the central portion; and 
     controlling the width of the gap. 
     According to a second aspect, there is provided the method of aspect 1, wherein the separating further comprises directing a laser beam onto at least one of the first broad surface and the second broad surface. 
     According to a third aspect, there is provided the method of aspect 2, further comprising directing a cooling jet onto the at least one of the first broad surface and the second broad surface thereby cooling the flexible glass ribbon at a location proximate the laser beam. 
     According to a fourth aspect, there is provided the method of any one of aspects 1-3, wherein the step of detecting the width of the gap includes detecting the width of the gap at a predetermined distance downstream of the crack tip. 
     According to a fifth aspect, there is provided the method of aspect 4, wherein the predetermined distance is about 30 cm downstream from the crack tip. 
     According to a sixth aspect, there is provided the method of aspect 5, wherein the width of the gap at the predetermined distance is maintained below about 0.11 mm. 
     According to a seventh aspect, there is provided the method of any one of aspects 1-6, wherein detecting the width of the gap includes using a laser displacement sensor. 
     According to an eighth aspect, there is provided the method of any one of aspects 1-7, wherein controlling the width of the gap comprises doing so such that mechanically induced stress at the crack tip is maintained below 30 MPa. 
     According to an ninth aspect, an apparatus that manages mechanically induced stress on crack tip comprises: 
     a conveying assembly that directs a flexible glass ribbon in a conveying direction; 
     an edge trimming apparatus including a cutting device that receives the flexible glass ribbon and that separates a first edge of the flexible glass ribbon to form a continuous strip of edge trim connected to a central portion of the flexible glass ribbon at a crack tip; 
     a gap measurement device that provides a signal indicative of a width of a gap between the first edge and the central portion of the flexible glass ribbon; and 
     a gap adjustment device that adjusts the width of the gap between the strip of edge trim and the central portion based on the signal. 
     According to a tenth aspect, there is provided the apparatus of aspect 9, wherein the cutting device comprises a laser. 
     According to an eleventh aspect, there is provided the apparatus of aspect 10, further comprising a cooling fluid jet proximate the laser. 
     According to a twelfth aspect, there is provided the apparatus of any one of aspects 9-11, wherein the gap adjustment device detects the width of the gap at a predetermined distance downstream of the crack tip. 
     According to a thirteenth aspect, there is provided the apparatus of any one of aspects 9-12, wherein the predetermined distance is about 30 cm downstream from the crack tip. 
     According to a fourteenth aspect, there is provided the apparatus of aspect 13, wherein the width of the gap at the predetermined distance is maintained below about 0.11 mm. 
     According to a fifteenth aspect, there is provided the apparatus of any one of aspects 9-14, wherein the gap measurement device comprises a laser displacement sensor. 
     According to a sixteenth aspect, there is provided the apparatus of any one of aspects 9-15, wherein mechanical stress at the crack tip is maintained below 30 MPa. 
     According to a seventeenth aspect, a flexible glass structure has a thickness of at most about 0.3 mm and a cut edge formed by a laser cutting device in a continuous laser cutting process, the flexible glass structure having an as cut particle density of no more than about 0.0015 particles per cm 2  determined in accordance with Particle Density TEST #1. 
     According to an eighteenth aspect, there is provided the apparatus of aspect 17, having an as cut particle density of no more than about 0.001 particles per cm 2  determined in accordance with Particle Density TEST #1. 
     According to a nineteenth aspect, there is provided the apparatus of aspect 17, having an as cut particle density of no more than about 0.0005 particles per cm 2  determined in accordance with Particle Density TEST #1. 
     Additional features and advantages will be set forth in the detailed description which follows, and in part will be readily apparent to those skilled in the art from the description or recognized by practicing the embodiments as exemplified in the written description and the appended drawings and as defined in the appended claims. It is to be understood that both the foregoing general description and the following detailed description are merely exemplary, and are intended to provide an overview or framework to understanding the nature and character of the claims. 
     The accompanying drawings are included to provide a further understanding of principles of the present disclosure, and are incorporated in and constitute a part of this specification. The drawings illustrate one or more embodiment(s), and together with the description serve to explain, by way of example, principles and operation of the embodiments described herein. It is to be understood that various features disclosed in this specification and in the drawings can be used in any and all combinations. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a partial view of an apparatus for processing a flexible glass ribbon, according to some embodiments; 
         FIG. 2  is a section view along line  2 - 2  of  FIG. 1  illustrating a cutting support member with an upwardly extending convex support surface, according to some embodiments; 
         FIG. 3  illustrates a schematic view of an apparatus for processing a flexible glass ribbon according to some embodiments; 
         FIG. 4  illustrates a gap measurement device for use in some embodiments, including the apparatus of  FIG. 1 ; 
         FIG. 5  illustrates a schematic view of a gap adjustment device for use in some embodiments, including the apparatus of  FIG. 1 ; and 
         FIG. 6  illustrates graph showing an exemplary correlation between gap width at a predetermined location downstream of a crack tip and corresponding mechanically induced stress due to toe out at the crack tip, as well as an exemplary correlation between gap width at two different distances downstream from the crack tip. 
     
    
    
     DETAILED DESCRIPTION 
     Embodiments described herein generally relate to apparatuses and methods for managing mechanically induced stress on a crack tip when separating a flexible glass ribbon by controlling lateral motion of edge strips of the flexible glass removed from a central quality portion of the flexible glass ribbon in a laser cutting process. As described herein, numerical and analytical models may be used to define a relationship between a gap or “toe out” between an edge of the edge strip and an edge of the quality portion of the flexible glass ribbon, and how that gap relates to a mechanically induced stress at the crack tip. Using this modeling data, a measurement system is provided that is capable of measuring the gap continuously in a steady-state run condition. This measurement can, in turn, be used to close steering control loops, which define the lateral position of the edge strip relative to the central quality portion (i.e., the amount of toe-out or gap) at the crack tip and generally downstream of laser separation. 
     Referring to  FIG. 1 , a flexible glass ribbon  10  is illustrated being conveyed through a glass processing apparatus  12 , only a portion of which is illustrated by  FIG. 1 . The flexible glass ribbon  10  may be conveyed in a continuous fashion from a glass ribbon source  14  (FIG.  3 ) through the glass processing apparatus  12 . The glass ribbon source  14  may be either a forming process, or may be a roll of glass. The flexible glass ribbon  10  includes a pair of opposed first and second edges  16  and  18  that extend along a length of the flexible glass ribbon  10  and a central portion  20  that spans between the first and second edges  16  and  18 . In some embodiments, the first and second edges  16  and  18  may be covered in an adhesive tape  25  that is used to protect and shield the first and second edges  16  and  18  from contact. The tape  25  may be applied to one or both of the first and second edges  16  and  18  as the flexible glass ribbon  10  moves through the apparatus  12 . A first broad surface  22  and an opposite, second broad surface  24  also span between the first and second edges  16  and  18 , forming part of the central portion  20 . Typically, the tape  25  would be applied to edges of a glass ribbon  10  that has been cut already, i.e., after beads  26 ,  28  have been removed. However, the tape  25  may also be applied to a glass ribbon  10  having beads  26 ,  28  as shown. 
     In embodiments where the flexible glass ribbon  10  is formed using a down draw fusion process, which is shown in part in  FIG. 3 , the first and second edges  16  and  18  may include beads  26  and  28  with a thickness T 1  that is greater than a thickness T 2  within the central portion  20 . The central portion  20  may be “ultra-thin” having a thickness T 2  of about 0.3 mm or less, or may be up to 0.5 mm, including but not limited to thicknesses of, for example, from about 0.01 to about 0.5, from about 0.01 to about 0.4, from about 0.01 to about 0.3 mm, from about 0.01 to about 0.275 mm, from about 0.01 to about 0.25 mm, from about 0.01 to about 0.225 mm, from about 0.01 to about 0.2 mm, from about 0.01 to about 0.175 mm, from about 0.01 to about 0.15 mm, from about 0.01 to about 0.125 mm, from about 0.01 to about 0.1 mm, from about 0.01 to about 0.075 mm, from about 0.01 to about 0.05 mm, from about 0.01 to about 0.025 mm, from about 0.025 to about 0.5 mm, from about 0.05 to about 0.5 mm, from about 0.075 to about 0.5 mm, from about 0.1 to about 0.5 mm, from about 0.125 to about 0.5 mm, from about 0.15 to about 0.5 mm, from about 0.175 to about 0.5 mm, from about 0.2 to about 0.5 mm, from about 0.225 to about 0.5 mm, from about 0.25 to about 0.5 mm, from about 0.275 to about 0.5 mm, from about 0.025 to about 0.275 mm, from about 0.5 to about 0.25 mm, from about 0.75 to about 0.225 mm, from about 0.1 to about 0.2 mm, from about 0.125 to about 0.175 mm, although flexible glass ribbons  10  with other thicknesses may be formed in other examples. 
     The flexible glass ribbon  10  is conveyed through the apparatus  12  using a conveyor system  30  ( FIG. 1 ). Lateral guides  32  and  34  may be provided to orient the flexible glass ribbon  10  in the correct lateral position relative to the machine or travel direction  36  of the flexible glass ribbon  10 . For example, as schematically shown, the lateral guides  32  and  34  may include rollers  38  that engage the first and second edges  16  and  18 . Opposed forces  40  and  42  may be applied to the first and second edges  16  and  18  using the lateral guides  32  and  34  that help to shift and align the flexible glass ribbon  10  in the desired lateral orientation in the travel direction  36 . 
     As further illustrated, the lateral guides  32  and  34  can engage the first and second edges  16  and  18  on the tape  25  without engaging the central portion  20  of the flexible glass ribbon  10 . As such, the pristine or quality surfaces of the opposed first and second broad surfaces  22  and  24  of the central portion  20  of the flexible glass ribbon  10  can be maintained while avoiding undesired scratching or other surface contamination that might otherwise occur if the lateral guides  32  and  34  were to engage either of the first and second broad surfaces  22  and  24  of the central portion  20 . Moreover, the lateral guides  32  and  34  may engage the flexible glass ribbon  10  as it is being bent about an axis  46  transverse to the travel direction  36  of the flexible glass ribbon  10 . Bending the flexible glass ribbon  10 , can increase the rigidity of the glass ribbon  10  throughout the bend. As such, the lateral guides  32  and  34  can engage the glass ribbon  10  in a bent condition. The forces  40  and  42  applied by the lateral guides  32  and  34  are therefore less likely to buckle or otherwise disturb the stability of the glass ribbon profile when laterally aligning as the flexible glass ribbon  10 . 
     The apparatus  12  can further include a cutting zone  50  downstream from the axis  46 . In one example, the apparatus  12  may include a cutting support member  52  configured to bend the flexible glass ribbon  10  in the cutting zone  50  to provide a bent target segment  54  with a bent orientation. Bending the target segment  54  within the cutting zone  50  can help stabilize the flexible glass ribbon  10  during the cutting procedure. Such stabilization can help prevent buckling or disturbing the flexible glass ribbon profile during the procedure of separating at least one of the first and second edges  16  and  18  from the central portion  20  of the flexible glass ribbon  10 . 
     The cutting support member  52  can comprise a non-contact support member designed to support the glass ribbon  10  without touching the first and second broad surfaces  22  and  24  of the flexible glass ribbon  10 . For example, referring to  FIG. 2 , the non-contact cutting support member  52  can comprise one or more curved air bars configured to provide a cushion of air to space between the flexible glass ribbon  10  and the cutting support member  52  to prevent the central portion  20  of the flexible glass ribbon  10  from contacting the cutting support member  52 . 
     Referring briefly to  FIG. 2 , the cutting support member  52  can be provided with a plurality of passages  58  configured to provide positive pressure ports  60  such that an air stream  62  can be forced through the positive pressure ports  60  toward the bent target segment  54  to create an air cushion  66  for non-contact support of the bent target segment  54 . Optionally, the plurality of passages  58  can include negative pressure ports  68  such that an air stream  71  can be drawn away from the bent target segment  54  to create a suction to partially counteract the force from the air cushion  66  created by the positive pressure ports  64 . A combination of positive and negative pressure ports can help stabilize the bent target segment  54  throughout the cutting procedure. Indeed, the positive pressure ports  64  can help maintain the desired air cushion  66  height between the central portion  20  of the flexible glass ribbon  10  and the cutting support member  52 . At the same time, the negative pressure ports  68  can help pull the flexible glass ribbon  10  toward the cutting support member  52  to prevent the flexible glass ribbon  10  from undulating or having portions of the bent target segment  54  from floating away from other portions of the target segment  54  when traversing over the cutting support member  52  in the travel direction  36 . 
     Providing the bent target segment  54  in the cutting zone  50  can also increase the rigidity of the flexible glass ribbon  10  throughout the cutting zone  50 . As such, as shown in  FIG. 1 , optional lateral guides  70 ,  72  can engage the flexible glass ribbon  10  in a bent condition as the flexible glass ribbon  10  passes over the cutting support member  52  within the cutting zone  50 . Forces  74  and  76  applied by the lateral guides  70  and  72  are therefore less likely to buckle or otherwise disturb the stability of the glass ribbon profile when laterally aligning as the flexible glass ribbon  10  passes over the cutting support member  52 . The optional lateral guides  70  and  72  can therefore be provided to fine tune the bent target segment  54  at the proper lateral orientation along a direction of the axis  46  transverse to the travel direction  36  of the flexible glass ribbon  10 . 
     As set forth above, providing the bent target segment  54  in a bent orientation within the cutting zone  50  can help stabilize the flexible glass ribbon  10  during the cutting procedure. Such stabilization can help prevent buckling or disturbing the glass ribbon profile during the procedure of separating at least one of the first and second edges  16  and  18 . Moreover, the bent orientation of the bent target segment  54  can increase the rigidity of the bent target segment  54  to allow optional fine tune adjustment of the lateral orientation of the bent target segment  54 . As such, the flexible glass ribbon  10  can be effectively stabilized and properly laterally oriented without contacting the first and second broad surfaces  22  and  24  of the central portion  20  during the procedure of separating at least one of the first and second edges  16  and  18 . 
     Increased stabilization and rigidity of the bent target segment  54  of the flexible glass ribbon  10  can be achieved by bending the target segment  54  to include an upwardly convex surface and/or an upwardly concave surface along a direction of the axis  46 . For example, as shown in  FIG. 2 , the bent target segment  54  includes a bent orientation with an upwardly facing convex surface  80 . Examples of the disclosure can involve supporting the bent target segment  54  with an upwardly facing convex support surface  82  of the cutting support member  52 , for example the illustrated air bar. Providing the cutting support member  52  with an upwardly facing convex support surface  82  can likewise bend the flexible glass ribbon  10  in the cutting zone  50  to achieve the illustrated bent orientation. 
     The apparatus  12  can further include a wide range of edge trimming apparatus configured to separate the first and second edges  16  and  18  from the central portion  20  of the glass ribbon  10 . In one example, as shown in  FIG. 3 , edge trimming apparatus  100  can include an optical delivery apparatus  102  for irradiating and therefore heating a portion of the upwardly facing surface of the bent target segment  54 . In one example, optical delivery apparatus  102  can comprise a cutting device for example the illustrated laser  104  although other radiation sources may be provided in further examples. The optical delivery apparatus  102  can further include a circular polarizer  106 , a beam expander  108 , and a beam shaping apparatus  110 . 
     The optical delivery apparatus  102  may further comprise optical elements for redirecting a beam of radiation (e.g., laser beam  112 ) from the radiation source (e.g., laser  104 ), for example mirrors  114 ,  116  and  118 . The radiation source can comprise the illustrated laser  104  configured to emit a laser beam having a wavelength and a power suitable for heating the flexible glass ribbon  10  at a location where the beam is incident on the flexible glass ribbon  10 . In one embodiment, laser  104  can comprise a CO 2  laser although other laser types may be used in further examples. 
     The laser  104  may initially emit the laser beam  112  with a substantially circular cross section (i.e. the cross section of the laser beam at right angles to the longitudinal axis of the laser beam). The optical delivery apparatus  102  transforms laser beam  112  such that the beam  112  has a significantly elongated shape when incident on glass ribbon  10 . As shown in  FIG. 1 , the elongated shape can produce an elongated radiation zone  120  that may include the illustrated elliptical footprint although other configurations may be provided in further examples. The elliptical foot print can be positioned on the upwardly facing surface (either convex or concave) of the bent target segment. 
     The boundary of the elliptical footprint can be determined as the point at which the beam intensity has been reduced to 1/e 2  of its peak value. The laser beam  112  passes through circular polarizer  106  and is then expanded by passing through beam expander  108 . The expanded laser beam  112  then passes through beam shaping apparatus  110  to form a beam producing the elliptical footprint on a surface of the bent target segment  54 . The beam shaping apparatus  110  may, for example, comprise one or more cylindrical lenses. However, it should be understood that any optical elements capable of shaping the beam emitted by laser  104  to produce an elliptical footprint on the bent target segment  54  may be used. 
     The elliptical footprint can include a major axis that is substantially longer than a minor axis. In some embodiments, for example, major axis is at least about ten times longer than minor axis. However, the length and width of the elongated radiation zone are dependent upon the desired separating speed, desired initial crack size, thickness of the glass ribbon, laser power, etc., and the length and width of the radiation zone may be varied as needed. 
     As further shown in  FIG. 3 , the example edge trimming apparatus  100  can also include a coolant fluid delivery apparatus  122  for cooling the heated portion of the upwardly facing surface of the bent target segment  54 . The coolant fluid delivery apparatus  122  can comprise a coolant nozzle  124 , a coolant source  126  and an associated conduit  128  that may convey coolant to the coolant nozzle  124 . 
     With reference to  FIGS. 1-3 , the coolant nozzle  124  can be configured to deliver a coolant jet  130  of coolant fluid to the upwardly facing surface of the bent target segment  54 . The coolant nozzle  124  can have various internal diameters to form a cooling zone  132  of a desired size. As with elongated radiation zone  120 , the diameter of coolant nozzle  124 , and the subsequent diameter of coolant jet  130 , may be varied as needed for the particular process conditions. In some embodiments, the area of the glass ribbon immediately impinged upon by the coolant (cooling zone  132 ) can have a diameter shorter than the minor axis of the radiation zone  120 . However, in certain other embodiments, the diameter of the cooling zone  132  may be larger than the minor axis of elongated radiation zone  120  based on process conditions for example glass ribbon traveling speed, glass thickness, laser power, etc. Indeed, the (cross sectional) shape of the coolant jet may be other than circular, and may, for example, have a fan shape such that the cooling zone forms a line rather than a circular spot on the surface of the glass ribbon. A line-shaped cooling zone may be oriented, for example, perpendicular to the major axis of elongated radiation zone  120 . Other shapes may be beneficial in certain circumstances. 
     In one example, the coolant jet  130  comprises water, but may be any suitable cooling fluid (e.g., liquid jet, gas jet or a combination thereof) that does not stain or damage the upwardly facing surface of the bent target segment  54  of the flexible glass ribbon  10 . The coolant jet  130  can be delivered to a surface of the flexible glass ribbon  10  to form the cooling zone  132 . As shown, the cooling zone  132  can trail behind the elongated radiation zone  120  to propagate an initial crack or crack tip  180  formed by aspects of the disclosure described more fully below. 
     The combination of heating and cooling with the optical delivery apparatus  102  and the coolant fluid delivery apparatus  122  can effectively induce thermal stress to separate the first and second edges  16  and  18  from the central portion  20  while minimizing or eliminating undesired residual stress, microcracks or other irregularities in the opposed edges  140 ,  142  of the central portion  20  that may be formed by other separating techniques. Moreover, due to the bent orientation of the bent target segment  54  within the cutting zone  50 , the glass ribbon  10  can be properly positioned and stabilized to facilitate precise separating of the first and second edges  16  and  18  respectively from the first and second edges  140 ,  142  during the separating process. Still further, due to the convex surface topography of the upwardly facing convex support surface, the continuous strips of edge trim  176  and  178  can travel away from the central portion  20  (in an out of plane direction and/or laterally in an in-plane direction), thereby reducing the probability that the first and second edges  16  and  18  will subsequently engage (and therefore damage) the first and second broad surfaces  22  and  24  and/or the high quality opposed edges  140 ,  142  of the central portion  20 . 
     Referring to  FIGS. 1 and 4 , the glass processing apparatus  12  further includes a gap measurement and adjustment apparatus  160  that includes a gap measurement device  162  and a gap adjustment device  164 . The gap measurement device  162  may be supported by a support assembly  166  that includes a support arm  168  that positions the gap measurement device  162  over a gap  170  between cut edges  172  and  140  of the edge trim  176  and the central portion  20 , respectively. In other embodiments, the gap measurement device  162  may be positioned below the gap  170  by the support arm  168 . Whether above or below the gap  170 , the gap measurement device is configured to intermittently or continuously measure the width of the gap  170  between the edges of the edge trim  176  and the central portion  20  in the cross-machine direction, i.e., in the direction of axis  46  (i.e., in a lateral, in-plane direction). A suitable gap measurement device  162  may include an LJ series laser displacement sensor commercially available from Keyence Corporation. It should be noted that while the gap measurement device  162  and gap adjustment device may be described primarily in connection with cut edges  172  and  140  and edge trim  176 , their description and operation may apply equally with respect to cut edges  182  and  142  and edge trim  178 . 
     The width of the gap  170  is measured at a location downstream of the cutting zone  50  where crack tip formation occurs, which is shown more clearly by  FIG. 4 . Measuring at a predetermined distance downstream of the crack tip  180  (e.g., about 12 cm (5 inches) or more, for example about 20 cm (8 inches) or more, for example about 30.4 cm (12 inches) or more, for example from about 12 to about 38 cm (about 5 inches to about 15 inches)) can provide a more measurable gap  170  between cut edges  172  and  140  of the edge trim  176  and the central portion  20  since the gap  170  may widen at locations downstream from the crack tip  180 . In some embodiments, it may be desirable to measure the width of the gap  170  at a position closer to the crack tip  180 . 
     Referring again to  FIG. 1 , the gap measurement and adjustment apparatus  160  further includes a controller  184  (e.g., a proportional-integral-derivative (PID) controller) that receives a signal or input from the gap measurement device  162  that is indicative of width of the gap  170  at the predetermined location. The controller  184  then adjusts a position of the gap adjustment device  164  to thereby adjust a position of the edge trim  176  relative to the central portion  20  in the cross-machine direction. This adjustment of the edge trim  176  relative to the central portion  20  results in a change in the width of the gap  170  measured by the gap measurement device  162 , which, in turn, adjusts the amount of toe out at the crack tip  180 . This process may be repeated continuously or intermittently to adjust the width of the gap  170  at the predetermined distance from the crack tip, wherein the adjustment moves the amount of the gap toward a predetermined gap width that is saved in memory of the controller  184 . The amount of the predetermined gap may vary based on the glass thickness, the width of the central portion, and/or other processing parameters. Alternatively, or in addition, the memory could store a look-up table having predetermined gaps that correspond to a particular desired mechanically induced stress levels as explained further below. 
     Referring briefly to  FIG. 5 , the gap adjustment device  164  may include a motor  190  that controls angular position of a roller  192  (e.g., a top roller of a pinch roller set that also includes a bottom roller  194 ) about a pivot axis A. The roller  192  may be rubberized or otherwise formed with a compliant material that can grip the bead  26  and change the lateral orientation of the edge trim  176  through turning of the roller  192  about the pivot axis A in response to a command from the controller  184 . Changing the lateral position of the edge trim  176  relative to the central portion  20  of the flexible glass ribbon  18  also affects the width of the gap  170  at the gap measurement device  162  and the degree of toe out at the crack tip  180 . In some embodiments the top roller  192  is driven, whereas the bottom roller  194  is an idle roller that rotates from contact with the glass ribbon  18 . Although  FIG. 5  shows rollers  192 ,  194  as engaging the edge trim  176 , similar rollers may be used in other embodiments to engage the edge of central portion  20  to more accurately control the width of the gap  170 . Additionally, although  FIG. 5  shows the rollers  192 ,  194  for one edge portion (including  16 ,  26 ,  140 ,  176 ) of the glass ribbon, a similar arrangement of rollers may be used on the other edge portion (including  18 ,  28 ,  142178 ) of the glass ribbon. 
     Controlling the gap  170  can produce high quality edges on the central portion  20  of the glass ribbon. In embodiments, controlling the gap  170  can reduce the effect that mechanically induced stresses have at the crack tip, thereby allowing the thermally induced stresses to propagate the crack tip, which results in higher quality edges. Alternatively, or in addition, controlling the gap  170  can reduce the undesirable effects produced from the edge trim  176 ,  178  rubbing against the edges  140 ,  142  of the central portion  20 , namely, physical damage to the edges  140 ,  142  from the contact, and/or the generation of particles that undesirably adhere to the broad surfaces  22 ,  24  of the central portion. 
     First, controlling the gap  170  can reduce the effect that mechanically induced stresses have at the crack tip. Without wishing to be bound by theory, the width of the gap or toe out at the crack tip  180  can provide an indication of mechanically induced stress due to ribbon separation. The ribbon separation at the crack tip  180  can be almost exclusively an in-plane separation as the edge trim  176  may remain in the same plane of the central portion  20  until farther downstream of the crack tip  180 . In some embodiments, gap measurement device  162  is located about 12 in (about 30.4 cm) downstream from the crack tip  180 , and measures the in-plane separation. In other embodiments the edge trim  176  is moved out of the plane of the central portion  20  almost immediately downstream from the location of the crack tip  180 , for example about 25 mm downstream of the crack tip  180 ; in such cases, controlling the toe out or gap is still beneficial in managing mechanically induced stresses at the crack tip, particularly so that the mechanically induced stress does not adversely affect edge quality and/or thermally induced stresses that beneficially separate the glass ribbon and that result in high edge quality. 
     Referring to  FIG. 6 , a graph is illustrated showing an exemplary correlation between gap width (in mm, along the X-axis) at a predetermined location downstream of the crack tip  180  (e.g., 12 in) and mechanically induced stress (in MPa, along the left-hand Y-axis) due to toe out at the crack tip  180 . 
     In this example, two plots are illustrated for mechanically induced stresses on opposite sides (top or first broad surface  22 , i.e., the surface on which the laser beam  112  and coolant jet  130  impinge, and bottom or second broad surface  24 , i.e., the surface on which air streams  62  impinge) of the ribbon because the mechanically induced stresses can be different at these locations depending, at least in part, on the processes used. Plot line  200  illustrates mechanically induced stress (in MPa, on the left-hand Y-axis) at the crack tip at the bottom side of the flexible glass ribbon  10  when the web separation (gap  170 , in mm as shown on the X-axis) is measured at a position of 12 inches (about 30.4 cm) downstream of the crack tip. Plot line  202  illustrates mechanically induced stress (in MPa, on the left-hand Y-axis) at the crack tip at the opposite, top side, of the flexible glass ribbon  10  when the web separation (gap  170 , in mm as shown on the X-axis) is measured at a position 12 inches (about 30.4 cm) downstream of the crack tip. In this example, mechanically induced stress is generally greater along plot line  200  than along plot line  202 , illustrating that the mechanically induced crack tip stress is greater at the bottom of the flexible glass ribbon  10  than it is at the top of the flexible glass ribbon  10 , at the same location downstream of the crack tip. Thus, a target gap width at the predetermined location can be identified for a particular mechanically induced stress. For example, to maintain a target stress of about 30 MPa, it can be seen that a gap of about 100 μm (about 0.100 mm, as measured at a point 12 inches (30.4 cm) downstream from the crack tip) should be maintained using the gap measurement and adjustment apparatus  160 . Any suitable target stress can be selected, for example less than about 100 MPa, for example less than about 70 MPa, for example less than about 50 MPa, for example less than about 30 MPa, for example less than about 25 MPa. For 100 micron thick glass, it was found that maintaining the mechanically induced stresses below about 30 MPa produced high quality edges from the laser separation. That is, a mechanically induced stress of about 30 MPa or less allowed the thermally induced stress (from the laser and coolant jet) to open the crack tip  180 , thereby producing high quality edges  140 ,  142  on the central portion  20  (quality web, or desired web). When mechanically induced stresses exceeded 30 MPa, the quality of edges  140 ,  142  is degraded, because the mechanically induced stresses start to overwhelm the thermally induced stresses at the crack tip. 
     Further,  FIG. 6  illustrates the correlation between various distances downstream from the crack tip. That is, although the gap  170  width was measured at a point 12 inches (about 30.4 cm) downstream from the crack tip, such was done for convenience based on the physical arrangement of the equipment used (both that used to measure gap width and that used to sever the glass ribbon), and need not be the case. Instead, the gap width measured at a particular point downstream may be correlated (for example based on Pythagorean theorem or finite element analysis(FEA)). The relationship between gap width at various distances downstream of the crack tip and the mechanically induced stress at the crack tip can be predicted for example, using FEA, as shown in  FIG. 6 . Plot line  204  illustrates web separation (gap  170  width) at a position that is 15 mm downstream of the crack tip (right-hand Y-axis) versus web separation 12 inches downstream of the crack tip (X-axis). That is, the mechanically induced stress (about 20 MPa) from a gap width of 0.064 mm as measured at a point 12 inches (30.4 cm) downstream from the crack tip is equivalent to the mechanically induced stress from a gap width of 0.003 mm as measured at a point 15 mm downstream from the crack tip. As can be seen from  FIG. 6 , the gap distance as measured near the crack tip (15 mm) is significantly smaller than that measured at a distance (12 inches) farther from the crack tip, but both mechanically induce the same stress at the crack tip. By way of further example, a gap of about 100 microns (0.1 mm) as measured at a distance of 12 inches (about 30.4 cm) downstream of the crack tip produces a stress of about 30 MPa according to line  200 , but the gap as measured at a distance of 15 mm downstream of the crack tip is only about 0.004 mm (see the points where the vertical dashed line crosses each the plot lines  200  and  204 ). Because maintaining adequate separation near the crack tip (so as to reduce particle generation, as discussed below) is balanced against increasing stress at the crack tip, the primary interest in the amount of gap separation is near the crack tip. Thus, the gap width at a point 15 mm downstream of the crack tip is of interest but, again, it may not be practical to measure the gap width at that point due to physical machine constraints. Accordingly, one may measure the gap width at a different distance downstream of the crack tip and correlate that to a particular gap width as measured nearer to the crack tip (for example, at 15 mm downstream of the crack tip). 
     Second, controlling the gap  170  can reduce the undesirable effects produced from the edge trim  176 ,  178  rubbing against the edges  140 ,  142  of the central portion  20 . As indicated above, separating the edge trim  176  from the central portion  20  laterally has advantages of reducing defects (in the edges of the central or quality portion, for example) and reducing glass particles (that disadvantageously stick to the broad surfaces of the central or quality portion, for example) due to contact between the cut edges after laser separation. 
     Particles having a size on the order of 20 microns are of interest to the electronic display industry. That is, display pixels are of such a size that particles on the order of 20 microns noticeably affect the pixel performance in an undesired manner. Accordingly, the density of particles on the order of 20 microns is typically measured for glass substrates. Particle density per unit area can be determined using any suitable flat display glass inspection system, for example as commercially available from Dr. Schenk GmbH. Particle densities are greatest adjacent the cut edges of the central portion  20 , which portion is not likely to wind up in an end use device anyway. Thus, measuring particle density directly at the edges is not necessarily advantageous. Instead, with reference to  FIG. 1 , and according to Particle Density TEST #1 as described here, it is more useful to measure particle density in a zone  198  that extends over the length of the glass ribbon  18  in the direction of arrow  36  and that is bounded by dashed lines  195  and  197 . Line  195  is parallel to and offset from the second edge  142  of the central portion  20  by a distance of 20 mm in the direction of arrow  199  toward the center of the glass ribbon. Similarly, line  197  is parallel to and offset from the second edge  142  by a distance of 70 mm in the direction of arrow  199 . Accordingly zone  198  is a 50 mm wide stripe extending along the length of the ribbon, and it is in this zone that particles are counted. Particles having a size on the order of 20 microns are counted, and the value is normalized according to area sampled so as to give a particle density per unit area. According to Particle Density Test #1, the number of particles per square centimeter is measured by counting the number of particles (having a size on the order of 20 microns) in zone  198  over a window length of several centimeters along the length of the ribbon. The number of particles (having a size on the order of 20 microns) is then divided by the area sampled (in cm) to provide a number of particles per square centimeter. For example, if 1000 particles (having a size on the order of 20 microns) were counted in an area bounded by lines  197 ,  198  (spaced by 5 cm, or 50 mm, as described) and a length of 1000 cm along the length of the web, the particle density would be 0.2 particles per cm 2 . Particle density may also be measured in a similar zone adjacent to first edge  140  of the central portion  20 . Particle densities herein were measured with a GlassInspect system having particle counting capability from Dr. Schenk GmbH. The camera resolution was a 20 micron square pixel oriented so that the sides of the square pixel were oriented generally in the cross-web and length of the web directions. 
     By controlling the gap  170  as described above, when severing a glass ribbon into a central portion and strips of edge trim, the inventors obtained (immediately after severing the edge trim, i.e., before washing) a particle density in zone  198 , according to Particle Density TEST #1, of less than about 0.0015 particles per cm 2 , in some embodiments less than about 0.001 particles per cm 2 , and in other embodiments less than about 0.0005 particles per cm 2 , wherein particles having a size of 20 microns or greater were measured. The thickness of the glass ribbon was 100 microns, and the width of the central portion (i.e., distance in direction of arrow  199  from second edge  142  to first edge  140 ) was 2600 mm. 
     In light of the above, controlling the amount of the gap  170  is a delicate balance. That is, a sufficient amount of gap is desirable to prevent the edge trim from rubbing against the edges of the central portion, and generating unwanted particles and/or edge damage from physical contact. However, if the amount of the gap  170  is too large, then mechanically induced stress at the crack tip can disadvantageously disturb the thermally induced stresses resulting in poor edge quality on the central portion. In some embodiments, it is desirable to operate the severing operation at a minimum amount of gap, i.e., just enough to minimize the generation of unwanted particles, thereby minimizing the amount of mechanically induced stress at the crack tip. 
     In the above detailed description, for purposes of explanation and not limitation, example embodiments disclosing specific details are set forth to provide a thorough understanding of various principles of the present description. However, it will be apparent to one having ordinary skill in the art, having had the benefit of the present disclosure, that embodiments described herein may be practiced in other variations that depart from the specific details disclosed herein. Moreover, descriptions of well-known devices, methods and materials may be omitted so as not to obscure the description of various principles. Finally, wherever applicable, like reference numerals refer to like elements. 
     Ranges can be expressed herein as from “about” one particular value, and/or to “about” another particular value. When such a range is expressed, another embodiment includes from the one particular value and/or to the other particular value. Similarly, when values are expressed as approximations, by use of the antecedent “about,” it will be understood that the particular value forms another embodiment. It will be further understood that the endpoints of each of the ranges are significant both in relation to the other endpoint, and independently of the other endpoint. 
     Directional terms as used herein—for example up, down, right, left, front, back, top, bottom—are made only with reference to the figures as drawn and are not intended to imply absolute orientation. 
     Unless otherwise expressly stated, it is in no way intended that any method set forth herein be construed as requiring that its steps be performed in a specific order. Accordingly, where a method claim does not actually recite an order to be followed by its steps or it is not otherwise specifically stated in the claims or descriptions that the steps are to be limited to a specific order, it is no way intended that an order be inferred, in any respect. This holds for any possible non-express basis for interpretation, including: matters of logic with respect to arrangement of steps or operational flow; plain meaning derived from grammatical organization or punctuation; the number or type of embodiments described in the specification. 
     As used herein, the singular forms “a,” “an” and “the” include plural referents unless the context clearly dictates otherwise. Thus, for example, reference to a “component” includes aspects having two or more such components, unless the context clearly indicates otherwise. 
     It should be emphasized that the above-described embodiments of the present description, particularly any “preferred” embodiments, are merely possible examples of implementations, merely set forth for a clear understanding of various principles of the description. Many variations and modifications may be made to the above-described embodiments without departing substantially from the spirit and various principles of the description. All such modifications and variations are intended to be included herein within the scope of this disclosure and the following claims. 
     For example, although the methods of, and apparatus for, managing mechanically induced stresses at a crack tip were described in the context of separating an edge (that may include a bead) portion from a central portion, such methods and apparatus may be used to separate different portions of the quality area itself as in a slitting operation. For example, a slitting operation may separate one or more quality portions of smaller width from a central portion of a ribbon having a wider quality portion. That is, the “edge” trimmed from the central area of the ribbon may or may not include a bead and, thus, the strip of edge trim may be a desired portion of the ribbon itself to be used in manufacturing other devices, for example, consumer electronic devices, photovoltaic devices, or electrochemical energy storage devices.