Abstract:
An improved method and apparatus is provided for continuously embossing a precision pattern of micro-prismatic elements on a surface of a resinous sheeting material with the aid of an endless metal embossing belt. The method includes the steps of moving the belt along a closed path through a heating station and a cooling station, conveying superimposed resinous film and sheeting material into proximity with the belt, passing the film and sheeting between the belt and a series of sonic welding heads to thereby begin to impress a pattern of micro-prismatic formations of the belt into one surface of the sheeting, pressing the film and sheeting against the heated belt until the one surface of the sheeting fully conforms to the embossing pattern, and stripping the film and embossed sheeting from the belt.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    The present invention relates generally to an improved method and apparatus for producing sheeting having precision patterns of micro-prismatic elements formed therein and, more particularly, to an improved method and apparatus for continuously embossing the surface of a film or film laminate with a pattern of precisely formed cube-corner retroreflective elements.  
           [0003]    2. Description of the Related Art  
           [0004]    Cube-corner type reflectors have long been in use in such applications as pavement markers, automobile reflectors and retroreflectors for use in highway signage construction, for example. The phrases “cube-corner,” or “trihedral” or “tetrahedron” are well recognized terms in the art for structure consisting of three mutually perpendicular faces, without regard to the size or shape of each face, or the optical axis of the element so provided. One early example of a cube-corner type reflector is disclosed in the patent to Stimson, U.S. Pat. No. 1,906,655, issued May 2, 1933.  
           [0005]    In more recent times, cube-corner retroreflective elements have been used advantageously not only in pavement markers and automobile reflectors, but also in flexible retroreflective sheeting suitable for use in highway signage construction, for example. Retroreflective sheeting requires, among other things, a drastic reduction in the size of the cube-corner elements by comparison to the elements used typically in pavement markers and automobile reflectors. Cube-corner type reflectors, to retain their functionality of reflecting light back generally to its source, require that the three reflective faces be maintained flat and within several minutes of 90° relative to each other. Spreads beyond this, or unevenness in the faces, results in significant light spread and a drop in intensity at the location desired. A more detailed description of the optics of cube and microcube structures are found in commonly assigned copending application U.S. Ser. No. 08/655,545 (as published in PCT case US97/08806), the disclosure of which is incorporated herein by reference.  
           [0006]    For many years, it was suggested that cube-corner retroreflective sheeting could not be manufactured successfully using embossing techniques (e.g., Rowland, U.S. Pat. No. 3,684,348, column 5, lines 30-42). However, embossing techniques were perfected such that embossed microcube retroreflective sheeting is now readily available. An example of a successful method for embossing sheeting is disclosed in U.S. Pat. No. 4,601,861, issued to Pricone et al. and assigned to the common assignee herein, the disclosure of which is incorporated specifically herein by reference.  
           [0007]    While the method and apparatus disclosed in the aforementioned Pricone et al. patent performs effectively in continuously producing high quality microcube retroreflective sheeting, a disadvantage of such a system is the time involved in forming the prismatic elements. Generally, such a system is only capable of producing the embossed film at a rate of no more than thirty lineal inches per minute. The principal time factor in this system is that required to heat the film to its glass temperature, to enable formation of the microprismatic elements. This requires multiple embossing machines if high volume production is desired. Consequent cost in terms of machine maintenance and floor space, for example, also is therefore required. Accordingly, it is desirable to provide a method and apparatus capable of increasing production capacity of microcube retroreflective sheeting over prior known technology. Further, it is desirable to provide such a method and apparatus which produces a high quality finished product. It is further desirable to provide such a method and apparatus which is practical and relatively inexpensive to use. Still further, it is desirable to provide for the high production of embossed sheeting or laminates formed with precise patterns of micro-prismatic cells which can serve functions other than in retroreflective sheeting.  
         SUMMARY OF THE INVENTION  
         [0008]    The present invention improves over the prior art by providing an improved method for continuously embossing a precision pattern of micro-prismatic elements on a surface of a resinous sheeting material with the aid of an endless metal embossing belt. The method includes the steps of moving the belt along a closed path through a heating station and a cooling station, conveying superimposed resinous film and sheeting material into proximity with the belt, passing the film and sheeting between the belt and a series of sonic welding heads to thereby begin to impress a pattern of micro-prismatic formations of the belt into one surface of the sheeting, pressing the film and sheeting against the heated belt until the one surface of the sheeting fully conforms to the embossing pattern, and stripping the film and embossed sheeting from the belt.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0009]    The foregoing and other novel features and advantages of the invention will be better understood upon a reading of the following detailed description taken in conjunction with the accompanying drawings in which:  
         [0010]    [0010]FIG. 1 is a fragmentary plan view, greatly enlarged, of the embossed surface of one form of microcube retroreflective sheeting produced by the present invention;  
         [0011]    [0011]FIG. 2 is a fragmentary side schematic view, greatly enlarged, showing the embossing pattern of one form of an embossing tool for embossing the retroreflective pattern of the sheeting of FIG. 1, as though taken along the line  2 - 2  of FIG. 1, except that the tool is of female microcubes and the finished film is of male microcubes;  
         [0012]    [0012]FIG. 3 is a schematic perspective view of one form of retroreflective sheeting produced by the present invention, after further processing has rendered the sheeting ready for installation;  
         [0013]    [0013]FIG. 4 is a schematic view of one form of apparatus constructed in accordance with the principles of the invention for producing the retroreflective sheeting of FIGS. 1 and 3;  
         [0014]    [0014]FIG. 5 is a schematic view of a second form of apparatus constructed in accordance with the principles of the invention for producing the retroreflective sheeting of FIGS. 1 and 3;  
         [0015]    [0015]FIG. 6 is a top schematic view of an embossing roller in accordance with the invention showing one form of orientation of multiple ultrasonic vibration heads;  
         [0016]    [0016]FIG. 7 is a side schematic view of a micro-prismatic laminated product other than retroreflective sheeting; and  
         [0017]    [0017]FIG. 8 is a side schematic view of another micro-prismatic laminated product other than retroreflective sheeting.  
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0018]    The present invention will first be described in connection with the production of high quality retroreflective sheeting, although other sheeting applications will be discussed hereinafter.  
         [0019]    Referring now to the drawings, and initially to FIG. 1, a portion of retroreflective sheeting is designated generally by the reference numeral  12 . The sheeting  12  is preferably of thermoplastic material having embossed on one surface thereof a repeating pattern of retroreflective microcube-corner type reflector elements  14 . The thermoplastic material may advantageously be acrylic. Sheeting  12  initially had smooth front and back surfaces and was on the order of 0.006 inch (0.15 mm) thick. Alternatively, the sheeting  12  may consist of a laminate of different thermoplastic materials having different characteristics, as hereinafter described.  
         [0020]    The retroreflective pattern of elements  14  was formed with the aid of embossing tool  16  of a thin flexible belt or cylinder of the type produced in accordance with that invention entitled Embossing Tool and Method of Producing Same, U.S. Pat. No. 4,478,769, and assigned to applicant&#39;s assignee. Other shapes and arrays of microcube elements may be formed on the tool. Such shapes may be hexagons, triangles, rectangles or the like as disclosed in aforesaid U.S. Ser. No. 08/655,545.  
         [0021]    As shown in FIG. 2, the embossing tool  16  has on one surface an embossing pattern  18 , the depth of which is indicated by dimension A. One example for dimension A may be 0.00338 inch (0.085 mm). Dimension B of FIG. 1 represents the distance between parallel grooves which, for the “A” dimension provided, would be on the order of 0.0072 inch (0.18 mm).  
         [0022]    [0022]FIG. 3 shows one form of sheeting  12  produced by the present invention, after further processing and ready for use. More specifically, the retroreflective pattern of cube corner elements  14  may be covered with a metalized layer  19 , which in turn may be covered by a suitable backing material  20 , in turn covered by a suitable adhesive  22  for mounting, in turn covered by release paper  24 . The thickness of the metalizing layer  19  is essentially immeasurable. Backing material  20  may have a thickness, dimension C, of about 0.001 inch (0.025 mm) and the thickness of the adhesive layer  22  may be about 0.0015 inch (0.038 mm). The total thickness of the complete structure  25  is about 0.010 inch (0.25 mm) and the structure  25  is flexible enough so it can be rolled and readily stored on a supply reel  26 . Another version may consist of air cells formed by sonic welding of a rear film layer to the embossed layer, as disclosed in applicants&#39; copending application Ser. No. 08/566,006, commonly assigned.  
         [0023]    In accordance with the invention, one form of machine  30  for producing the cube corner sheeting  12  is shown schematically in elevation in FIG. 4. A supply reel  32  of unprocessed acrylic web  34  is mounted above the machine as is a supply reel  36  of transparent plastic film  38 , such as Mylar. In the illustrated embodiment, the web  34  may be 0.006 inch (0.15 mm) thick and the film  38  may be 0.002 inch (0.05 mm) thick. The flat web  34  and the film  38  are fed from the reels  32  and  36 , respectively, to a guide roller  40  positioned in close proximity to the embossing means  16 .  
         [0024]    The embossing means  16  includes an embossing tool in the form of an endless metal belt  44  which may be about 0.020 (0.5 mm) inch in thickness and 54 inches in circumference and 22 inches wide. The width and circumference of the belt  44  will depend in part on the width of the material to be embossed, as well as on the desired embossing speed and the thickness of the belt  44 . The belt  44  is mounted on and supported for rotation by a heating roller  46  and a post-cooling roller  48  having parallel axes. Rollers  46  and  48  may be driven by chains (not shown) to advance the belt  44  in the direction of the arrow. Belt  44  is provided on its outer surface with a continuous female embossing microprismatic pattern such as the cubes  18  (FIG. 2).  
         [0025]    Evenly spaced around the belt for about 180° around the heating roller  46  are a plurality, at least three, and as shown five, pressure rollers  60  of a resilient material, preferably silicone rubber, with a durometer hardness ranging from Shore A 20 to 90, and preferably from Shore A 60 to 90. While the rollers  46  and  48  could be the same size, the diameter of heating roller  46  is about 10½ inches (26.6 cm) and the diameter of the post-cooling roller is about 8 inches (20.3 cm). The diameter of each pressure roller  60  is about 6 inches (15.2 cm). The heating roller  46  or the post-cooling roller  48  may have axial inlet and outlet passages joined by an internal spiral tube for circulation therethrough of hot oil (in the case of the heating roller) or other liquid (as in the case of the cooling roller) supplied through appropriate lines.  
         [0026]    The web  34  and film  38  are fed over guide roller  40  where they are superimposed to form a laminate  62  which then is conveyed over the belt  44 . In preferred form, the machine  30  is provided with a series of infrared heaters  64  which serve to preheat the laminate  62  after it has passed around the guide roller  40 . In accordance with the invention the laminate  62  then passes between heating roller  46  and a series of sonic welders  70 . The sonic welders  70  may be of a type operated by a 120 volt 60 Hertz power supply designed to vibrate at 20,000 cycles per second with horns  72  that move through 0.010 inch. Although only one sonic welder  70  is shown, in practice, the machine  30  will comprise several welders  70  positioned in staggered relation to cover the full width of the laminate  62 . The welders  70  serve to essentially drive the heated web  34  into the embossing tool  44  to initiate formation of the microcube corner retroreflective elements  14 .  
         [0027]    The laminate  62  then passes under pressure rollers  60  and is moved with the belt  44  around the heating roller  46  and then along the belt  44  through a generally planar cooling station  76 . The film  38 , which has a higher glass transition temperature than the web  34 , performs several functions during this operation. First, it serves to maintain the web  34  under pressure against the belt  44  while traveling around the heating roller  46 , thus assuring conformity of the web  34  with the precision pattern  16  of the tool during the change in temperature gradient as the web  34  drops below the glass transition temperature of the material. Second, the film  38  maintains what will be the outer surface of the sheeting in a flat and highly finished surface for optical transmission. Finally, the film  38  acts as a carrier for the web  34  in its weak “molten” state and prevents the web  34  from otherwise adhering to the pressure rollers as the web  34  is heated above the glass transition temperature. The cooling station  76  is preferably of a type disclosed in the aforementioned U.S. Pat. No. 4,601,861 which operates with chilled fluid.  
         [0028]    The machine  30  includes a stripper roller  80  around which the laminate  62  passes to remove the laminate  62  from the belt  44  shortly before the belt  44  itself contacts the post-cooling roller  48 . The laminate  62  then is fed from stripping roller  80  over further guide rollers  82  to an annealing means  84 . The laminate  62  then emerges from the annealing means  84  guided by additional guide rollers  86  with the film  38  facing outwardly, past a monitoring device  88  which continuously monitors the optical performance of the sheeting. From there, the finished laminate  62  having the embossed sheeting  13  may be transferred to a wind-up roller (not shown) for removal and further processing.  
         [0029]    A second form of embossing machine constructed in accordance with the principles of the invention is illustrated in FIG. 5 and designated generally by the reference numeral  100 . The machine  100  includes as a principal component a heated roller  102  which is much larger than the roller  46  and is preferably on the order of 34 inches (86.4 cm) in diameter. As in the machine  30 , an endless metal belt  104  provided with an embossing pattern passes around the roller  102  and is heated thereby. The machine  100  also includes a cooling shoe  106  over which the belt  104  passes, as will be described in detail hereinafter.  
         [0030]    A supply reel  108  of unprocessed acrylic web  110  is mounted over the machine as is a supply reel  112  of transparent Mylar  114 . In this embodiment of the invention, an intermediate supply reel  116  of UV stabilized face film  118  is also provided. The resulting composite  120  passes around a guide roller  122  and beneath a series of essentially aligned sonic welders  124 , only one of which can be seen, which essentially begins to drive the web  110  into the embossing belt  104 . The laminate  120  then passes around the heater roller  102  beneath a series of pressure rollers  126  where the web  110  is fully impressed into the belt  104 .  
         [0031]    The cooling shoe  106  is an arcuate, hollow member through which chilled fluid flows. The shoe  106  serves to lower the temperature of the laminate to preferably on the order of 100° F. aided by a cold air plenum  128  which blows on the laminate  120 . The laminate  120  then passes around a stripper roller  130  and is drawn to a wind-up roller  132 . As in the machine  30 , a series of infrared heaters  134  may be provided to preheat the web  110 .  
         [0032]    [0032]FIG. 6 shows a top schematic view of a heated roller  102  illustrating one form of orientation of multiple ultrasonic welders  124  spaced along the width of the roller  102 . Preferably, the welders  124  are positioned in staggered overlapping relation so that the welders  124  act on the laminate  120  continuously across its entire width.  
         [0033]    Referring now to FIG. 7, another form of laminate, shown greatly enlarged, is designated by the reference numeral  140 . This form of laminate  140  has a layer of thermoplastic material  142  embossed with a pattern of microprismatic type channels  144  defining upstanding support portions  146 . A cover layer  148  is later thermally welded to the support portions  146 . The channels  144  may in this form of laminate contain a deposit of a suitable chemical composition  150  which changes color in the presence of a bodily fluid, which is drawn into the channels  144  by capillary action. An application for such a device may, for example, be a home pregnancy test kit. The layer  142  is readily embossed using the ultrasonic technique as hereinabove described.  
         [0034]    [0034]FIG. 8 illustrates yet another laminate  160  comprising two sheets of spaced thermoplastic material  162  embossed with a pattern of microprismatic type projections  164 . This structure  160  is suitable for use as a fuel cell in accordance with well-known electrochemical technology and the sheets  162  are also readily embossed using the ultrasonic technique hereinabove described.  
         [0035]    It can now be appreciated that embossing machines  30  and  100  constructed in accordance with the invention provide considerable improvement over prior art devices in terms of production output capacity. A typical embossing machine of the type disclosed, for example, in aforementioned U.S. Pat. No. 4,601,861 has a sheeting production rate of three feet (0.91 m) per minute. In contrast, with the present machines, production rates as high as 30 feet (9.1 m) per minute are believed readily attainable. This production rate increase is directly attributable to the preheating of the film together with the initial forming of the cube corner retroreflective elements by the sonic welding heads prior to conveying the laminate under the pressure rollers and around the heated roller.  
         [0036]    In preferred form, the machines  30  and  100  may use five welding heads  124  having a nominal width each of 11.5 inches (29.2 cm) as are presently commercially available. An embossing belt  104  may thereby be used having a width on the order of 52 inches (1.32 m) to form finished film  120  having a width on the order of 48½ inches (1.123 m).  
         [0037]    It can further be appreciated that the machines  30  and  100  are also capable of producing sheeting of high optical intensity at considerably greater speed than heretofore known. One advantage of the machine  100  is that the large diameter roller  102  and shoe  106  arrangement greatly increases the life of the generally cylindrical metal embossing belt  104  by reducing bending stresses on the belt  104  as are present in the machine  30 . The large roller  102  also increases the working area of the belt  104  to help speed production.  
         [0038]    While the present invention has been described in connection with preferred embodiments thereof, it will be apparent to those skilled in the art that many changes and modifications may be made without departing from the true spirit and scope of the present invention. Accordingly, it is intended by the appended claims to cover all such changes and modifications as come within the true spirit and scope of the invention.