Patent Publication Number: US-2018036937-A1

Title: Extrusion-to-sheet production line and method

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of U.S. patent application Ser. No. 15/047,673, filed Feb. 19, 2016, which is a continuation of U.S. patent application Ser. No. 13/772,613, filed Feb. 21, 2013, which claims the benefit of U.S. Provisional Application Ser. No. 61/608,686, filed Mar. 9, 2012, the entire disclosures of which are incorporated herein by reference. 
    
    
     BACKGROUND 
     It is known to continuously emboss patterns of micro-prismatic elements on one or more surfaces of sheets or films using one or more embossing bands or belts. However, there is a need to be able to produce thicker polymer sheets of a single material containing a pattern of optical elements at a relatively high rate while maintaining high tolerances on the geometry of the optical elements. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic side view of an exemplary extrusion-to-sheet production line embodiment. 
         FIGS. 2A and 2B  are enlarged exploded fragmentary cross-sections through the embossing belt, plastic sheet, and carrier film in the flat cooling area of the extruder-to-sheet production line of  FIG. 1 , taken on the plane of the line  2 - 2  of  FIG. 1 . 
         FIGS. 3A and 3B  are enlarged fragmentary cross-sections through the plastic sheet and carrier film of  FIG. 1  downstream of the flat cooling area, taken on the plane of the line  3 - 3  of  FIG. 1 . 
         FIG. 4  is a flow chart of an example of a method to produce embossed plastic sheet from a continuously-extruded sheet of molten plastic using a continuous embossing belt. 
     
    
    
     DETAILED DESCRIPTION 
     The embodiments will now be described with reference to the drawings, wherein like reference numerals are used to refer to like elements throughout. The figures are not to scale. Features that are described and/or illustrated with respect to an exemplary embodiment may be used in the same way or in a similar way in one or more other embodiments and/or in combinations with or instead of the features of other embodiments. 
     As described in detail below, the extrusion-to-sheet production line and method comprise a first roll and a second roll set to a predetermined gap through which a continuously-extruded sheet of molten plastic material passes to calender the sheet to a predetermined thickness. The sheet is caused to pass through a nip formed between the second roll and a continuous belt looped around a third roll and a fourth roll spaced apart from one another. The nip is where the belt is at its closest point between the second roll and the third roll. The belt comprises an embossing pattern of optical element shapes that is an inverse pattern of a pattern of optical element shapes to be embossed at a first major surface of the sheet. The sheet remains in contact with the second roll until the sheet passes through the nip, where the pattern of optical element shapes on the belt is embossed into the first major surface of the sheet. Downstream of the third roll is a flat cooling area through which the belt passes while the first major surface of the sheet is still in contact with the embossing pattern on the belt for cooling the sheet and completing the set of the pattern of optical element shapes into the sheet while the sheet is in a flat configuration. Downstream of the flat cooling area is a separation area where the belt separates from the sheet after completing the set of the pattern of optical element shapes into the sheet. 
     The extrusion-to-sheet production line has the advantage that relatively thick polymer sheets of a specified thickness containing a precise pattern of optical element shapes can be continuously produced at a relatively high rate. Typical production rates range from about 1.5 millimeters per second for sheets near the maximum of the thickness range described below to about 500 millimeters per second for sheets near the minimum of the thickness range. 
       FIG. 1  shows an example of an extrusion-to-sheet production line or apparatus  10  comprising a first roll  12  and a second roll  14  set to a predetermined gap  16  through which a continuously-extruded sheet  18  of molten plastic material passes to calender the sheet to a predetermined thickness. The gap  16  is settable to calender the sheet  18  to a defined thickness. In an example, the calendered sheet  18  has a thickness of between about 0.3 millimeters and about 15 millimeters. 
     In the example shown in  FIG. 1 , the sheet  18  of molten plastic material is continuously extruded through a gap die  20  into the gap  16  between the first and second rolls  12  and  14 . The plastic material is comprised of a single optical material (for example, acrylic, polycarbonate or other appropriate material) which may be rigid or flexible depending on thickness. 
     After passing through the gap  16 , the sheet  18  remains in contact with the second roll  14  and rotation of the second roll  14  causes the sheet  18  to pass through a nip  22  formed between the second roll  14  and a continuous belt  24  looped around a heated third roll  26  and a cooled fourth roll  28  in spaced relation from the third roll  26 . The nip  22  is where the belt  24  is at the closest point between the second roll  14  and the third roll  26 . The first, second and third rolls  12 ,  14  and  26  are located in order adjacent one another, and the third and fourth rolls  26  and  28  are offset from one another and configured to receive the continuous embossing belt  24 . In the example shown in  FIG. 1 , the first, second and third rolls are stacked vertically one above the other. However, other arrangements of the rolls are possible and may be used. Moreover, the second roll may include a first sub-roll and a second sub-roll adjacent one another. In this case, the gap is between the first roll and the first sub-roll of the second roll, and the nip is between the second sub-roll of the second roll and the third roll. 
     The belt  24  comprises an embossing pattern  30  of optical element shapes  32  that is an inverse of a pattern  34  of optical element shapes  36  to be embossed at a first major surface  38  of the sheet  18  (see  FIG. 2 ). The rolls described herein as being heated may be electrically heated, heated by circulating hot oil, or heated in another suitable way. The rolls described herein as being cooled may be cooled by circulating coolant, such as water. 
     Rolls  12 ,  14 ,  26  and  28  are rotatably driven in the direction of the arrows shown in 
       FIG. 1  using any suitable drive (including but not limited to a chain drive or synchronous hydraulic or electric motors, not shown) to advance the belt  24  and cause the continuously-extruded sheet  18  to pass through the gap  16  between the first and second rolls  12  and  14  and remain in contact with the second roll  14  until the sheet passes through the nip  22 . As the sheet  18  passes through the nip  22 , the nip transfers the pattern of optical element shapes  32  from the belt  24  to the extruded sheet  18  while the sheet is near or above the glass transition temperature of the plastic material. 
     In an example, the first roll  12  is cooled and the second roll  14  is heated to a temperature to maintain the extruded sheet  18  near, at or above its glass transition temperature upstream of the nip. In another example, both the first roll  12  and the second roll  14  are cooled to cool the extruded sheet  18  to a temperature at which the plastic material has sufficient structural integrity to form the sheet but is still malleable enough to emboss. 
     In the example shown in  FIG. 1 , a belt adjuster  39  is connected to the fourth roll  28  to steer the belt  24  around the third and fourth rolls  26  and  28 , and to adjust the spacing between the third and fourth rolls  26  and  28  to accommodate belts of different lengths and to allow the belt  24  to be installed and removed. 
     Downstream of the third roll  26  is a flat cooling area  40  through which the belt  24  passes while the first major surface  38  of the sheet  18  is still in contact with the embossing pattern on the belt. In the example shown in  FIG. 1 , at least one pressure roller  59  is provided in the region of the flat cooling area  40  for pressing the sheet against the belt to assist in maintaining the sheet in contact with the embossing pattern on the belt during passage through the flat cooling area. 
     The flat cooling area  40  is located between the third and fourth rolls  26  and  28  such that the third roll rotates towards the flat cooling area, which is for cooling the sheet and completing the set of the pattern of optical element shapes  36  into the sheet while the sheet is in a flat configuration in order to maintain high geometrical tolerances on the individual optical element shapes of the pattern. In the example shown in  FIG. 1 , the flat cooling area  40  includes a cooling element  41  through which a coolant is circulated to cool the sheet. The cooling element  41  is located close to the second major surface  48  of the sheet  18 . 
     In another example also shown in  FIG. 1 , at least one pressure roller is provided downstream of the nip  22  and upstream of the flat cooling area  40  for pressing the sheet  18  against the embossing pattern on the belt  24  while the sheet is still in a hot state to set the pattern of optical element shapes  36  into the sheet.  FIG. 1  shows three circumferentially-spaced pressure rollers  42 ,  44  and  46  that are movable into and out of engagement with a second major surface  48  of the sheet for sequentially pressing the sheet against the belt downstream of the nip and upstream of the flat cooling area while the sheet is still in a hot state to set the pattern of optical element shapes into the sheet. A greater or lesser number of pressure rollers may be provided as desired. The pressure rollers  42 ,  44  and  46  as well as the pressure roller  59  are each typically faced with rubber or another compliant material. 
     In another example also shown in  FIG. 1 , a carrier film  50  is superimposed into direct contact with the second major surface  48  of the sheet  18  downstream of the nip  22  and upstream of the flat cooling area  40 .  FIG. 1  shows the carrier film  50  being fed from a supply reel  52  around a guide roller  54  for superimposing the carrier film  50  into direct contact with the second major surface  48  of the sheet  18  upstream of the first pressure roller  42 . In another example, the supply reel and guide roller are located to superimpose the carrier film into direct contact with the second major surface of the sheet upstream of the second roll  14 . 
     The carrier film  50  is made of a suitable protective material such as biaxially oriented polyethylene terephthalate that has a glass transition temperature higher than the temperature of the sheet at the nip so the carrier film will not melt or fuse to the sheet. The carrier film  50  has a surface  55  with a finish that is transferred onto the second major surface  48  of the sheet by pressure asserted by one or more of the pressure rollers.  FIGS. 2A and 2B  schematically show examples of surface finishes on the carrier film  50 , a smooth finish  56  shown in  FIG. 2A , and a matte finish  58  shown in  FIG. 2B . In another example, the surface  55  of the carrier film  50  that is superimposed into direct contact with the second major surface  48  of the sheet  18  comprises an additional embossing pattern of optical element shapes that is an inverse pattern of an additional pattern of optical element shapes to be embossed at the second major surface  48  of the sheet. 
     Exemplary optical element shapes  36  that are set into the first major surface  38  of the sheet  18  (and if desired also into the second major surface  48  of the sheet) include light-scattering elements, which are typically features of indistinct shape or surface texture, such as printed features, ink-jet printed features, selectively-deposited features, chemically etched features, laser etched features, and so forth. Such optical element shapes are typically formed in a master (not shown) by the above-mentioned processes and are transferred from the master to the belt  24  by a suitable process such as electro-forming. Other exemplary optical element shapes include features of well-defined shape such lenticular or prismatic grooves and features of well-defined shape that are small relative to the linear dimensions of the major surfaces of the sheet, which are sometimes referred to as micro-optical element shapes. The smaller of the length and width of micro-optical element shapes is less than one-tenth of the width of the sheet and the larger of the length and width of the micro-optical element shapes is less than one-half of the width of the sheet. The length and width of the micro-optical elements are measured in a plane parallel to the major surfaces of the sheet. Micro-optical elements are shaped to predictably reflect or refract light. However, one or more of the surfaces of the micro-optical elements may be modified, such as roughened, to produce a secondary effect on the light reflected or refracted by the micro-optical elements. 
     At least one of the size, shape, depth, density and orientation of the optical element shapes  36  set into the sheet  18  may vary across the width and/or the length of the sheet. In the examples shown in  FIGS. 2A and 2B , the size and density of the optical element shapes  36  set into the first major surface  38  of the sheet vary across the width of the sheet. The optical element shapes  36  set into the sheet  18  can be protrusions from the sheet as shown in  FIG. 2A , or indentations into the sheet as shown in  FIG. 2B . The maximum size of a light guide that can be made using the production line  10  is nominally equal to the width and length of the belt  24 . The production line  10  can be used to make light guides smaller than the maximum-size light guide by locating multiple discrete patterns of optical element shapes along the length and/or width of the belt. The patterns of optical element shapes need not be the same. For example, the patterns of optical element shapes for the light guides of several tablet devices can be located on the belt alongside a pattern of optical element shapes for the light guide of a large-screen television. 
     Downstream of the flat cooling area  40  is a separation area  60  where the belt  24  separates from the sheet  18  and the superimposed carrier film  50  after completing the set of the pattern of optical element shapes into the sheet.  FIGS. 1, 3A and 3B  show the sheet  18  and the carrier film  50  passing through a sheet output area  62  aligned tangentially with the third and fourth rolls  26  and  28  after the belt  24  has separated from the sheet  18  in the separation area  60 . The carrier film  50  is a disposable protective layer that can be subsequently removed from the sheet  18  whenever desired. 
       FIG. 4  is a flow chart  68  of an example of a method to produce embossed plastic sheet from a continuously extruded-sheet of molten plastic using a continuous embossing belt. 
     In block  70  the continuously-extruded sheet  18  of molten plastic material passes between the first and second rolls  12  and  14  set to a predetermined gap to calender the sheet to a predetermined thickness (see  FIG. 1 ). 
     In block  72  the sheet passes through the nip  22  formed between the second roll  14  and the continuous embossing belt  24  looped around the heated third roll  26  and cooled fourth roll  28  spaced apart from one another. The nip is where the belt is at the closest point between the second and third rolls. The belt comprises an embossing pattern of optical element shapes to be embossed at the first major surface of the sheet. 
     In block  74  the sheet is kept in contact with the second roll  14  until the sheet passes through the nip, where the pattern of optical element shapes on the belt is embossed into the first major surface of the sheet. 
     In block  76  the belt passes through the flat cooling area  40  downstream of the third roll  26  while the first major surface of the sheet is still in contact with the embossing pattern of optical element shapes on the belt to cool the sheet and complete the set of the pattern of optical element shapes into the sheet while the sheet is in a flat configuration. 
     In block  78  the belt separates from the sheet after completing the set of the pattern of optical element shapes into the sheet. 
     The orientation of the extrusion-to-sheet production line is merely exemplary and different orientations can be used. For example, the line can be inverted so that the embossed sheet exits the line at an area above the area through which the continuously-extruded sheet of molten plastic material enters the line or the line can be rotated through a suitable angle. 
     In this disclosure, the phrase “one of” followed by a list is intended to mean the elements of the list in the alternative. For example, “one of A, B and C” means A or B or C. The phrase “at least one of” followed by a list is intended to mean one or more of the elements of the list in the alternative. For example, “at least one of A, B and C” means A or B or C or (A and B) or (A and C) or (B and C) or (A and B and C). 
     Although this disclosure has described certain embodiments, equivalent alterations and modifications will become apparent upon the reading and understanding of the specification. In particular, with regard to the various functions performed by the above-described components, the terms used to describe such components are intended to correspond, unless otherwise indicated, to any component which performs the specified function of the described component (e.g., that is functionally equivalent), even though not structurally equivalent to the exemplary embodiments. In addition, while a particular feature may have been disclosed with respect to only one embodiment, such feature may be combined with one or more other features as may be desired and advantageous for any given or particular application.