CA1241514A - Methods and apparatus for embossing a precision optical pattern in a resinous sheet or laminate - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE
Disclosed herein are an improved method and apparatus for continuously embossing a repeating pattern of precise detail, in particular, cube-corner type reflector elements, on one surface of a single sheet or on one surface of a laminate of transparent thermoplastic materials, to form retroreflective sheeting. A con-tinuous embossing tool in the form of a flexible thin metal belt or cylinder has on its outer surface an embossing pattern which is the reverse of the precision optical pattern to be formed. The embossing tool is continuously moved at a predetermined speed along a closet course through a heating station where the temperature of a portion of the embossing tool is raised to be above the glass transition temperature of the sheeting or laminate and a cooling station where the heated portion of the embossing tool is cooled while in a relatively planar condition to be below that glass transi-tion temperature. The sheeting is continuously moved at the predeter-mined speed from a supply thereof into engagement with the embossing pattern on the tool and is pressed there-against continuously at a plurality of pressure points sequentially spaced along the heating station, with the one surface of the sheeting confronting and engaging the embossing pattern until the sheeting is raised above its glass transition temperature and conforms to the embossing pattern on the one face. The sheeting is maintained in engagement with the tool until the tool passes through the cooling station and the sheet-ing is lowered below its glass transition temperature and the pattern solidifies. The sheeting thereafter is stripped from the tool and 2.
next passed through a reheating station where the embossed sheeting is heated to an annealing temperature where the stresses formed in the thermoplastic material during embossing and cooling are relieved, while preserving and enhancing the precision optical pattern previously formed.

Description

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CROSS-REFERENCE TO RELATED APPLICATION

This application is related to the U.S. Patent of Sidney A. Heenan and Robert M. Pricone, No. 4,486,363, issued December 4, 1984, and assigned to the same assignee as the present application.

BACRGROUND OF INVENTION
This invention relates to improved methods and apparatus for producing sheeting having precision patterns where flatness and angular accuracy are important, such as for optical purposes, such as Fresnel lenses incorporating catadioptrics, precise flats, angles and uniform detail, and, more particularly, to improved methods and apparatus for continuously embossing a repeating retrore1ecting pattern of fine or precise detail on one surface of sheeting of transparent thermoplastic material or a laminate of such materials to form the sheeting into the desired pattern. Specifically, the techniques are applicable to produce cube-corner type retroreflective sheeting.
Cube-corner type reflectors have been known for many years and many millions have been sold. The phrase "cube-corner" or "tri-hedral", or "tetrahedron" are art recognized terms for structure consisting of three mutually perpenticular faces, without regard to the size or shape of each face, or the optical axis of the element so provided. Each of the faces can assume a different size and shape relative to the other two, depending upon the angular reflective characteristlcs deaired, and the moldlng techniques employed One example of a cube-corner type reflector i9 provided by Stimson U.S.

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Patent No. 1,906,655, issued May 2, 1933, wherein there is disclosed a reflex light reflector including an obverse face and a reverse light reflecting face consisting of a plurality of cube-corner reflec-tor elements, each having three mutually perpendicular surfaces adapted for total internal reflection of light impinging thereon from the obverse face. Reflectors, as taught by the Stimson patent, are individually molded and are relatively quite thick and rigid.
For many years now, the preferred material for cube-corner type reflectors has been methyl methacrylate. Another example of a cube--corner type reflector is the rectangular parallelepiped disclosedin Heasley U.S. Patent No. 4,073,568.
It long has been desired to obtain the benefits of cube-corner reflectors as used in pavement marKers or for automotive purposes, but with the reflector in the form of flexible sheeting. This involves, among other things, a drastic reduction in the size of the cube-corner element.
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.
Prior attempts have been made to produce reflective sheeting wherein the reflective elements are of the cube-corner type. For many years, it was suggested that cube-corner sheeting could not be manufactured using embossing techniques (e.g. Rowland U.S. Patent No. 3,684,348, Col. 5 ii. 30-42).
A more recent attempt at embossing cube-corner sheeting is that of Rowland U.S. Patent No. 4,244,683, issued January 13, 1981.

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However, the method and apparatus of Rowland U.S. Patent No.
4,244,683 are relatively quite complex and only seml-continuous or sequential in nature. Consequently, the Rowland teaching is quite costly to implement, maintain and operate. The operation is slow and the resultant reflective sheeting it quite costly.
Moreover, to produce sheetLng 48 inches wide, to be economically feasible, would be prohibitively expensive and complicated using the sequential mold technique of Rowland 4,244,683.
Also known are other prior techniques for embossing repeating patterns on thermoplastic sheeting, among which other prior techniques are those taught by the following:
Swallow U.S. Patent No. 2,442,443, issued June 1, 1948;
Hochberg U.S. Patent No. 3,157,723, issued November 17, 1964;
Kloender U.S. Patent No. 3,246,365, issued April 19, 1966;
Bergh U.S. Patent No. 4,097,634, issued June 27, 1978; and Nyfeler et al. U.S. Patent No. 4,223,050, issued September 16, 1980.
These other prior techniques do not involve the production of retro-reflective sheeting or the precision patterns required for optical purposes. As noted, in order for cube-corner reflective sheeting to be successful, the embossed cube-corner elements must be extremely accurately formed, much more so thaa is required of the embossed elements of these "other prior techniques", which, therefore, although they may be satisfactory for producing the intended products, may not be adaptable to the production of cube-corner reflective sheeting.
In Patent No. 4,486,363 there is disclosed a novel method and apparatus for continuously embossing a precision optical pattern on one surface of a continuous resinous sheeting material. The present invention discloses and claims improved methods and apparatus ~2~15~

capable of producing embossed cubc-corner type sheeting having significantly higher degrees of reflective efficiency.
Accordingly, the present invention seeks to provide improved methods and apparatus for embossing a repeating retroreflective pattern of cube-corner reflecting elements on one face of sheeting of transparent thermoplastic material, or a laminate of such materials, which methods and apparatus operate continuously and are greatly simplified with respect to the prior art.
Further, the invention seeks to provide such improved methods and apparatus which are relatively inexpensive, in terms of implementation and operation, yet when used in conjunction with the inventions disclosed and claimed in applicant's U.S. Patent 4,486,363, operate to provide significant increases in reflectivity in the final embossed products.
Still further, the invention seeks to provide such improved methods and apparatus enabling continuous production of cube-corner reflective sheeting of reduced costs.
The foregoing and other aspects and advantages will appear from the following description of examples of the invention.
SUMMARY OF THE INVENTION
The invention in one aspect pertains to a method for continuously embossing a precision optical pattern requiring sharp angles and flatness of faces in certain detail on one surface of a continuous resinous sheeting material, the method being performed with the aid of a generally cylindrical endless metal embossing element having an inner surface and an outer surface, the outer surface having a precision optical embossing pattern which is the reverse of the precision optical pattern to be formed on one surface of said sheeting, and wherein the method includes the steps of continuously 6a moving the endless embossing element along a closed course through a heating station, where the embossing element is heated through its inner surface to a predetermined temperature and then to a cooling station where the embossing element is cooled below the predetermined temperature, continuously feeding onto the embossing element as it passes througha part of the heating station superimposed resinous film and sheeting materials, the resinous materials of the film and the sheeting each having different glass transition temperatures, the sheeting being in direct contact with the outer precision patterned surface of the embossing tool, continuously heating the embossing element to the predetermined temperature at the heating station 9 the temperature being greater than the glass transition temperature of the sheeting and less than the glass transition temperature of the resinous film, pressing the superimposed film and sheeting against the embossing element at a plurality of pressure points sequentially spaced along the heating station with one surface of the sheeting confronting and engaging the precision optical pattern on the embossing element until the one surface of the sheeting conforms to the precision optical embossing pattern, continuously passing the embossing element and the superimposed film and sheeting through the cooling station where the temperature of the embossing element and the sheeting is lowered below the sheeting glass transition temperature with the film serving to substantially continuously maintain the sheeting in engagement with the embossing element through the heating station and through the cooling station, and continuously stripping the superimposed layer of film and embossed sheeting from the embossing element, the film being later strippable from the other face of the sheeting without destroying the optical pattern formed on the one face of the sheeting.

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6b The improved method in one aspect includes the cooling step being substantially effected while the superimposed film and sheeting and embossing tool are disposed in a generally planar condition, thereby to schieve an increase in the optical efficiency of the embossed sheeting.
The method improvement in another aspect includes reheating the embossed sheeting and film to a temperature in the range of about 82 to 93 C. thereby to relieve any strain in the film caused by cooling thereof at the cooling station.
Another aspect of the invention pertains to apparatus for continuously embossing a precision optical pattern on one surface of transparent resinous material or materials, the apparatus includes embossing means including a continuous seamless embossing tool in the form of a thin metal element having an inner surface and an outer surface, the outer surface having a precision optical embossing pattern thereon which is the reverse of the precision optical pattern to be formed in the resinous material. There are means for continuously moving the embossing element along a closed course, and means for introducing superimposed film and sheeting of resinous materials onto the embossing element with one face of the sheeting in direct contact with the optical pattern on the embossing element. A
heating means is provided for raising the temperature of the embossing pattern to be above the glass transition temperature of the sheeting and below the glass transition temperature of the film while the embossing element is in a first portion of its course. Cooling means is provided for lowering the temperature of the sheeting to be below the glass transition temperature while the element and the sheeting are in a generally planar condition in their course, thereby to .

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rigidify the precision pattern while in an undistorted condition. A
plurality of pressure means are sequentially spaced along the first portion of the course for pressing the superimposed film and sheeting against the embossing element with the one surface of the sheeting confronting and engaging the embossing pattern until the one surface conforms to the embossing pattern, with the film serving to substantially continuously maintain the sheeting in engagement with the embossing element until the latter passes the second portion of the course. Means thereafter strip the superimposed film and sheeting from the embossing element.
The apparatus may also include means for reheating the superimposed sheeting and film to a temperature in the range of 82 to 93 C. after stripping thereof from the embossing element.
More particularly, the present invention relates ta improved methods and apparatus for continuously embossing a repeating retroreflecting pattern of cube-corner reflector elements on one surface of sheeting of thermoplastic material to form the sheeting into retroreflective sheeting. A continuous embossing tool in the form of a thin metal element has on its outer surface an embossing pattern which is the reverse of the retroreflecting pattern. The tool is continuously moved at a predetermined speed along a closed course through a heating station where the temperature of a portion of the embossing tool and pattern is raised to be above the glass transition temperature , - . .
'; , if 7 ~241514 of the sheeting and a cooling station where the temperature of that portion of the embossing tool i9 lowered to be below that glass transition temperature. The sheeting is continuously moved at the predetermined speed from a supply thereof into engagemene with the embossing element and is pressed agaLnst the element either continu-ously, or at a plurality of pressure points sequentially spaced along said heating station, with one surface of the sheeting confront-ing and engaging the embossing pattern until the sheeting softens and the one surface conforms to the embossing pattern. The sheeting 0 i8 maintained in engagement with the tool untLl the tool passes through a cooling station where the embossed material is abruptly and continuously cooled to a temperature significantly below the glass transition temperature of the sheeting, while in a relatively planar condition, and the sheeting solidifies. The sheeting is thereafter stripped from the tool and, in a preferred embodiment, is reheated to an annealing temperature where internal stresses caused by embossing and cooling are relieved, while preserving and enhancing the reflective efficiency of the precision optical pattern so formed.
A preferred material for the sheeting is acrylic. The emboss-; ing tool preferably is a continuous belt, having the embossing pattern on its outer surface. The heatLng station is provided by a roller, and the cooling station may comprise a manifold that directs a cooled fluid (liquid or gag) against the embossed sheeting material while the belt and the formed material are in a generally planar condition . The post-cooling annealing step is best accomplished after the embossed thermopIastic material is stripped from the embossing tool and while the embossed material is still under moderate tension As disclosed in Patent No. 4,486,363, it i3 preferable that 241~jl4 8.
the sheetlng, prlor to engaging the embossing tool, be engaged on it surface remote from the one surface, with a film of thermoplastic materlal, such a polyester (Mylar), having a glass tran~itlon temper-atur~ whlch l hlgher than that of the sheeting and hi8her than thy temperature of the embo~ing pattern at the heatlng statlon, 80 that the pressure points exert pressure on the sheetlng through the film to cause the one surface of the sheeting to conform to the embossing pattern. The film act as a carrier for the sheeting ln lts weak, ~olton contitlon And turlng and after cooling and anneal-ing and keep the sheetlng from tearin8. The fit alto acts asan lnterleaf between the sheeting and the pressuse points, which preferably are pressure rollers of silicone rubber with a durometer hartnes- from Shore 60 to 90, which would otherwise tend to stick to the sheeting.

The inventlon will be descrlbed with reference to the accom-panying trawings in which FIG. l. is a plan view, greatly enlarget and somewhat fragmen-tary, of tho embossed surface of one form of reflectlve sheeting protuced by the present invention;
FIG. 2. 18 a slde elevation, somewhat fragmentary and somewhat schematic and very enlarged view, showing the embossln3 pattern of one form of an embosslng tool for embossing the retroreflecting pattern of the sheeting of FIG. l, as though taken in the tirection of the arrows 2-2 in FIG.l, except that the tool ig of female cubes and the sheetin8 of male cubes;
FIG. 3. is a perspective, somewhat schematic vlew of one form of reflective sheetin8 produced by the present invention, after further processing ha rendered the sheeting reaty for installation;
* trade mark .3 9 ~2~1514 and FIG. 4. is a schematic representatlon of preferred apparatus constructed in accordance wlth the lnvention for producing ehe reflec-tive sheetin8 of FIGS. 1. and 3, the machine including embossing means comprising en embossLng tool ln the form of a contlQuou~ flex-ible cylinder, or belt, cooling mcsns, and annealing means.
DESCRIPTION OF THE ILLUSTRATED EMBODIMENTS
FIG. 1. shows in plan view the rear surface of a portion of flexible reflective sheeting 12 of transparent thermoplastlc material having embossed on one surface thereof a repeating retro-reflecting pattern of cube-corner type reflector elements 14. The thermoplastic material may advantageously be acrylic. Sheeting 12 initially had parsllel front and back surfaces ant was initially on the order of 0.1524 mm tblck. Alternatively, the sheeting 12 may consist of a laminate of different transparent thermoplastic materials having different characteristics, as hereinafter discussed.
The retroreflective pattern of elements 14 was formed with the aid of an embossing tool 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 applicants' assignee. As shown in FIG. 2, the emboss-ing tool has on one surface an embossing pattern 16, the depth of which is indicated by dimension A. one example for dimension A
may be 0.0859 mm. Dimension B on FIG 1. represents the distance between parallel grooves which, for the "A" dimension provided, would be about 0.183 mm.
In order for sheeting 12 to have adequate optical properties, ehe embossing pattern 16 must be extremely accurately formed and ~LX4151~
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ehe retroreflectlve p~teern of the cube-corner elementa 14 muqc be an extremely accurate reverse reproduction of the embossing pattern 16. Thus, the embossed surface of the sheetlng 12 must conform to the embossing pattern 16 with an extremely high degree of accuracy.
FIG. 3. shows one form of sheeting 12 produced by the present invention, after further processing ant ready for use. More specifi-cally, the retroreflectlve pattern of cube-corner elements 14 may be covered with a metallized layer 18, which in turn may be covered by a 3uitable backing material 20, in turn covered by a suitable adhesive 22 (for mounting), in turn covered by release paper 24.
The thickness of the metallizing layer 18 is i measurable. Backing material 20 may have a thickness, dimension C, of about 0.0254 mm and the thickness of adhesive 22 may be about 0.0381 I. The total thickness of the complete structure 25 is about 0.254 em, and it is flexible enough 80 it can be rolled and readily stored on a supply reel 26. The sheeting 12 may be any desired color, to impart that color to retroreflected light. The details of applying a back coat and adhesive are well known in the art and aimilar to that used in the manufacture of "glass bead" type sheeting. In lieu of metal-lizing, oeher materials and/or back coatings may be applied to thecube-corner elements, such post forming steps not forming part of the present invention.
A preferred machine for producing the cube-corner sheeting 12 is shown schematically in elevation in JIG. 4. It will be under-stood that the specific const uctional details of the basic embossing machine are substantially as dlsclosed in U.S. Patent No.
4,486,363, and for purposes of convenience, a like numbering sy4tem for malor components of such system is adopted herein.
A supply reel 36 of unprocessed acrylic web 13 18 mounted ~Z~5~

on the righthand end of the machine, a0 is a supply reel 40 of trans-parent plastic fllm such a Mylar film 42. In the illustrated embodi-ment, the web 13 may be 0.1524 mm thick and the fllm 42 may be 0.0508 mm thick. The flat web 13 and the film 42 are fed from reels 36 and 40, respectively, to the embosslng means 34, over guide rollers (not shown), in the direction of the arrows.
The embossing means 34 inclutes an embossing tool or element in the form of an endless metal belt 48 which may be about 0.508 mm in thickness and 54 inches in "circumference" and 22 inches wide.
The width and circumference of the belt 48 will tepent, in part, upon the width or material to be embossed and the desired embossing speet snd the thickne3s of the belt 48. Belt 48 i8 mounted on and carried by a heating roller 50 and a po~t-cooling roller 52 having parallel axes. Rollers 50 and 52 may be triven by chains (not shown), to advance belt 48 at a predetermined linear speed in the direction of the arrow. Belt 48 i8 provided on it outer surface with a contin-UOU8 female embossing pattern 16 (FIG. 2.).

Evenly spaced sequentially around the belt, for about 180 around the heating roller 50, are a plurality, at least three, and as shown five, pressure rollers 58 of a resilient material, preferably silicone rubber, with a durometer hardness ranging from Shore A 20 to 90, but preferably, from Shore A 60 to 90.
While rollers 50 and 52 could be the same size, in machine 28 as constructed, the diameter of heating roller 50 is about 26.67 cm and the diameter of post cooling roller 52 is about 20.32 cm.
The diameter of each roller 58 is about 15.24 cm. For purposes of illustration, the spacing between rollers 50 and 52 is shown as grestly exaggerated, given the dimension of the rollers 50, 52 and the belt 48. It will be understood that the gap or free area * trade mark , between the rollers will dlffer depending upon the selected d~mensLons of Che belt 48 and roller 50 and 52.
It shoult be understood that elther the heatlng roller 50 or the post-cooling roller 52, may have axial inlet and outlet pa-ssages joined by an internal spirsl tube for circulation theretbrough of hot oil (ln the case of heating roller 50) supplied through a supply line or other material (in the csse of cooling roller 52) also supplied through appropriate lines.
The web 13 and the film 42, a ~tatet, are fed to embossing means 34, where they are superimposed to form a laminate 69 which 18 introduced between the belt 48 and the leading pressure roller 58a, with the web 13 between the film 42 and the belt 48. One face of web 13 directly confronts ant engages embossing pattern 16 and one face of the film 42 directly confronts and engages pressure rollers 58. The laminate 69 i9 moved with the belt 48 to pass under the remaining pressure rollers 58 and around the heating roller 50 and from thence along belt 40 through a general planar cooling station 80 located between heating roller 50 and post-cooling roller 52.
The film 42 performs several functions during this operation.
First, it serves to maintain the web 13 under pressure against the belt 48 while travelling around the heating and post-cooling rollers 50 and 52 and while traversing the distance between them, thus assur-ing conformity of the web 13 with the precision pattern 16 of the tool during the change in temperature gradient as the web (now embossed sheet) drops below the glass transition temperature of the material. Second, the film maintain what will be the outer surface of the sheetin8 in a flat and highly finished surface for optical transmission. Finally, the film 42 acts as a carrier for ~L24~514 13.
the web in its weak "molten" state and prevents the web from otherwise adhering to the pressure rollers 58 as the web i9 heated above the glass transition temperature.
The embossing means 34 includes a stripper roller 70, around which laminate 69 is passed to remove the same from the belt 48, shortly before the belt 48 itself ContactY post-cooling roller 52 on its return path to the heating roller 50.
The laminate 69 is then fed from stripper roller 70 over further guiding rollers 44, to an annealing means 90. The laminate 69 then emerges from the annealing means 90, guided by additional guiding rollers 44, with the film 42 facing outwardly, past a monitor-ing device 74 for continuously monitoring the optical performance of the embossed reflective sheeting. From there, the finished lami-nate 69 having the embossed sheeting 13, may be transferred to a ; wind-up roller (not shown) for removal and further processing.
; The heating roller 50 is internally heated (as aforesaid) so that as belt 48 passes thereover through the heating station, the temperature of the embossing pattern 16 at the portion of the tool is raised sufficiently so that web 13 is heated to a temperature above its glass transition temperature, but not sufficiently high as to exceed the glass transition temperature of film 42. For the acrylic web (or sheeting) 13 and polyester film 42, a suitable temper-ature for heating roller 50 in the heating station is in the range from Z18C to 246C, and preferably about 218C.
The post-cooling roller 52 also may be internally heated (as aforesaid) 80 that as belt 48 passes thereover through the cooling station, the temperature of the portion of the tool embossing pattern 16 i9 maintained at about the same temperature to which the belt 48 is lowered at the cooling station 80.

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As prevlougly noted, the prevent Lnvention provLded signl~icant snd unexpected lmprovements ln the reflectlve efElclency of the sheetlng producet thereby.
The first such lmprovement 18 schleved by causlng the embo0sed laminate 69 ant belt 48 to be abrupely ant signiflcantly cooled whlle the belt 48 and lamlnste 69 are on 8 generally planar positLon.
Applicsnts have discovered that by promptly effecting such cooling in the "flat", a three-folt increase in ~peciflc lntensity of the ~heetLng can be achieved, as compared to coollng around the roller 52.
In order to effect such promFt and full coollng, the cooling statlon designated generally 80 is provided on the emboAslng apparatus.
The cooling station may consist of a simple shroud or manifold 81 closely spaced to the outer face of the carrler fllm 42 at the area located between the rollers 50 and 52, where the belt 48 is under tension and planar, ant wieh the web 42 holdlng the formet sheetlng 13 thereagalnst. A sultable source for chllled fluid 83 and appropriate inlet and outlet ductwork 84 and 85 and a pump 86 for circulation of the chilLet fluLd also are provided. The chilled fluid may be water, air, or, for example, other gases or Çluids such as liquid nitrogen. Satisfactory result have been achieved when the chilled fluld is on the order of about 10C so as to cause the laminate 69 and the belt 48 to quickly drop below 82C ln cempera-ture, and preferably cooled to a range of approximately 38C to 49C. This rapld cooling below the gla~g transition temperature of the sheetLng 13, while the formed cubeg 14 and film carrier 42 are in a generally flat and undlstorted cond$tion, apparently effec-tlvely rlgidlfies or "freezes" the precisLon formed cube-corner elements 14 of the sheetLn8 13. Because the belt 48 19 extremely 15. ~415~4 thin, It i8 de~lrable to maincain iC9 temperature at about 49C
as lt passes over the post-coollng roller 52 and back toward the heating roller 50.
As prevlously noted, the space between rollers 50 and 52 ~8 greatly exaggerated for illustrative purposes only. For roller3 50 and 52 of the tiameters previously indicated, and the belt having the clrcumference previously lndicated, the actual distance between the rollers, at the closest point, may be less than 25.4 , and the planar area may be on the order of about 25.4 mm. For larger diameter rollers and a larger belt, this space and consequèntly the "flat" area available for cooling, will vary. The infusLon of heat through the plastic web effectively controls the cooling in the planar area and therefore a larger plansr area would be neces-sary to effectively cool at higher running speeds. The important aspect to achieve this unexpected improvement is that the cooling be effected while the tool and film are in a generally planar area and in an undistortet condition. The belt speed, di3tances and cooling temperature will then be correlated to achieve the maximum increase in reflective efficiency.
A second important and unexpected improvement in reflective efficiency i9 provided by subsequently reheating the formed film to a relatively high temperature, in the range of 82C to 93C, after the sheeting 13 and film 42 is cooled and stripped from the embossing belt 48. It has been found that this reheating, in the range indicated, generally provides an additional 25~ or more increase in the reflective efficiency in sheeting which is cooled in the "flat", and an even greater percentage increase for sheeting which ;~ i9 simply cooled by passing it over the post-cooling roller 52.
While the particular phenomenon 19 not understood, it is believed 16 ~2~15~'L
ehat lt Ls glmilar to an annealing process, wherein any stresses whlch are "frozen" into the fllm during the cooling stage are re-lieved, 90 that the cube-corner elements can relax to a condition very highly approximatlng the precl~ion sngles formed during the embossing sts8e. For purposes of this application therefore, thls reheating step also may be referred to as annealing. The annealing step can be accomplishet by running the material directly through the annealing or rehestihg oven 90, posLtioned directly near the embossing machine , Jo that anneal$ng can be done in a continuous fashion. For example, the material may run through at a rate of 1.2 meters per minute, and the sheetin8 material would be subject to the annealing temperatures for at least ten minutes. Where contin-UOU8 annealing i8 performed, it is desirable that there be some tension on the laminate 69, but it should be a very low tension which would be approximately less than 0.0893 kgms of tension per cm of width of laminate.
It also has been found that it is desirable to run the material through the annealing oven while the Mylar* carrier film 42 still is associated with the formed sheeting, and that a lower reflective efficiency improvement is accomplished if performed without the film 42.
Alternatively, the finished roll of film with the Mylar*thereon can be subjected to a static heating technique, where the entire roll is placed in an oven and allowed to be heated for an extended time period, until the entire roll reaches tbe designated cemperature range. No set time can be provided since it will depend upon the size of the roll.
It has been found that if rehesting occurs below 82C, there apparently is lnsufflclent "rellef" of the cube-corner elements, * trade mark ~24~51~

and thae if reheated above 93C, there i8 a rather rapid drop in the reflective capability of the cube-corner elements, presumably because the material then loses it critical shape. It has been found that 91C is the optimum temperature for providing significant stability in the annealed cube-corner elements, while preserving the greatest degree of improved reflectivity. The concept of, and result of, the annealing step i8 unexpectet. It has heretofore been believed that any reheating of any acrylic material used in forming cube-corner reflectors, whether injection molded or embossed, above 82C, generally would cause those cube-corner elements to be distorted, either by sink marks or the like in the individual cube faces, or by changes in the dihedral angles between reflective faces, and therefore that this generally would result in a significant reduction in reflective efficiency. Thus, applicants present improved process, and the apparatus provided, enhance the reflective effi-ciency, provided the same is accomplished within the specified temper-ature range.
While the annealing step may prove beneficial in itself, without cooling of the sheeting in the "flat", it is believed that the combination of both cooling the film, while in its planar condi-tion, and subsequently annealing same on a continuous basis, together provide unexpected and improved results in the reflective efficiency of the embossed sheeting 13.
It also should be understood that it is possible that for certain enviromental conditions, a second layer of thermoplastic material, having either specific W inhibitors or otherwise somewhat dissimilar from the web 13, will simultaneously be run through the embossing equipment with the film 13 and the web 42. Under these circumstances, an addltional feed roller may be utilized or, alterna-18. ~2~ 4-tively, the additional layer of thermoplastic material may be prelAmi-nated to the web ~3 before it is provided as roll 36.
The Lmprovet results obtalned by the improved methods and apparatus clslmed herein also are achieved when a laminate of such thermoplastic materials is used. As an example of the laminate that mlght be used, the film or web 13 could be rubber modified polymethylmethacrylate, sold by the Roh~ L Haas Company, under its designation Plexiglas*DR, and it wlll be about 0.1525 ; thick.
An sdditional layer of thenmoplagtic material about 2 mils. thick may be applied directly from a separate feet roller, or prevlously laminated to sheet 13, and may consist of an acrylic materisl such as Korat* D, sold by Polymer Extruded Products, Inc., of Newark, Jew Jersey. This material then serves as the outer surface of the finished sheeting snd has significant W inhibitors therein whereby the sheeting may be uset to meet specific adverse environmental characteri8tics.
While relatively high pressures should be used informing the precision cube-corner elements, pursuant to the existing embossing techniques and apparatus, a minimum of 3.49 kgm/cm2 gauge pressure should be applied through the pressure rollers 50 to the web 13, film 42 and the tool 58, as they pass through the embossing equipment, in order to achieve a reasonable initial minimum level of reflective intensity for the film. It has been found that the laminate 69 can be processed through the embossing means 23 at the rate of about 0.91 to 1.2 meter3 per minute, with saeisfactory results in terms sf the optical performance and other pertinent properties of the finished refleccive sheeting. Prior to shipping the reflective sheeting 12, the film 42 may be stripped therefrom.
It should be noted that reference numeral 13 may refer indis-Jo * trade mark ~241~-14 criminately hereln to the embos3ed sheeting or web In Lt~ initial Norm, to les In-process Eonm or to its fLnal reflectlve fonm, a3 appropriate.
The term "glass transition temperature" i8 a welL known ten of art snd i8 applied to Chermoplaseic materials a3 well as glass.
For purposes hereln, it iB the temperature at which the material is vf8cou8 and begins to flow when heated. For varlous extendable types of acry~lc, ehe gla 8 transitlon eemparatures begin at about 93C. For polyeater (~ylar~, ie beglns at about 249C. to 254C.
A preferred materlal for the embosslng tool disclosed hereln 1J nlckel. The very thin tool (about 0.254 mm to about 0.762 mm) permlts the rapid heatlng ant coollng of ehe tool, and the sheet, through the requlred temperature gradlents vhile pressure iB applied by the pressure rollers and the carrler fllm. The result 19 the contlnuous productlon of a preclslon pattern where flatness and angular accuracy are important while permittlng formatlon of sharp corn2rs wlth mlnlmal distortlon of optical surfaces, whereby the flnished sheet provldes hlgh optlcal efficiency.
The lnventlon, in lts varlous aspects and disclosed forms, is well adapted to the atCainment of the stated ob3ects and advantages and others. The dlsclosed detail are not to be taken as limitations on the invention, except as those details may be includet in the appended claims.

* trade mark ,

Claims (22)

19a.

What is claimed is:

1. An improved method for continuously embossing a precision optical pattern requiring sharp angles and flatness of faces in certain detail on one surface of a continuous resinous sheeting material, the method being performed with the aid of a generally cylindrical endless metal embossing element having an inner surface 20.
and an outer surface, the outer surface having a precision optical embossing pattern which is the reverse of the precision optical pattern to be formed on one surface of said sheeting, and wherein the method includes the steps of:
(a) continuously moving the endless embossing element along a closed course through a heating station, where said embossing element is heated through its inner surface to a predetermined temper-ature and then to a cooling station where said embossing element is cooled below said predetermined temperature;
(b) continuously feeding onto said embossing element as it passes through a part of said heating station superimposed resinous film and sheeting materials, said resinous materials of said film and said sheeting each having different glass transition temperatures, said sheeting being in direct contact with the outer precision pat-terned surface of said embossing tool;
(c) continuously heating said embossing element to said predetermined temperature at said heating station, said temperature being greater than the glass transition temperature of said sheeting and less than the glass transition temperature of said resinous film;
(d) pressing said superimposed film and sheeting against said embossing element at a plurality of pressure points sequentially spaced along said heating station with one surface of said sheeting confronting and engaging said precision optical pattern on said embossing element until said one surface of said sheeting conforms to said precision optical embossing pattern;
(e) continuously passing said embossing element and said superimposed film and sheeting through said cooling station where the temperature of said embossing element and said sheeting is lowered 21.
below said sheeting glass transition temperature, with said film serving to substantially continuously maintain said sheeting in engagement with said embossing element through the heating station and through said cooling station; and (f) continuously stripping said superimposed layer of film and embossed sheeting from said embossing element, said film being later strippable from the other face of said sheeting without destroy-ing said optical pattern formed on said one face of said sheeting, the improvement comprising:
(g) said cooling step being substantially effected while said superimposed film and sheeting and embossing tool are disposed in a generally planar condition, thereby to achieve an increase in the optical efficiency of the embossed sheeting.

2. The method of Claim l, wherein said course is cylindrical through the heating station and said pressure points are provided by at least three spaced pressure rollers, and said course is general-ly planar through said cooling station.

3. The method of Claim 1, wherein said cooling is achieved by directing a chilled fluid against said sheeting and said film, thereby to effect an abrupt and substantial drop in temperature thereof and thereby to quickly rigidify the precision optical elements while the embossing element and formed sheeting are in a generally undistor-ted condition.

4. The method of Claim 1, wherein said embossing tool is a thin flexible seamless metal belt, the heating station is a heated roller, and said cooling station is in juxtaposition with said film as said element leaves said heating roller and passes through said planar area, and includes means for directing a chilled fluid against said film as it passes thereover.

22.

5. The method of Claim 1, wherein the temperature of said sheeting is lowered to about 49°C at said cooling station.

6. The method of Claim 1, wherein said sheeting is acrylic, said film is polyester, the temperature of said embossing pattern at said heating station is sufficiently high to raise the temperature of said sheeting to a range between 218°C and 246°C, and the temper-ature at said cooling station is sufficiently low to lower the temper-ature of said sheeting to 82°C or below as said sheeting passes through said cooling station.

7. The method of Claim 1, wherein said precision optical pattern is in the form of an array of female cube-corner type elements whereby the sheeting formed thereby has male cube-corner elements on the one face thereof in contrast with said tool, and the finished sheeting thereby is provided with an array of retroreflective cube-corner elements thereon.

8. The method set forth in Claim 1, and further including the step of reheating said embossed sheeting and film to a temperature in the range of about 82°C to 93°C thereby to improve the optical efficiency of said embossed sheeting.

9. The method set forth in Claim 8, wherein the temperature achieved in said sheeting at reheating is about 91°C.

10. The method set forth in Claim 8, wherein said reheating is accomplished on a continuous basis by causing said superimposed layer of embossed sheeting and film to be continuously directed through a reheating station after said film and sheeting are stripped from said embossing element.

11. The method set forth in Claim 10, wherein said stripped film and sheeting are reheated long enough to assure that all of the film and embossed sheeting reaches a temperature in the range between 23.
82°C and 93°C.

12. An improved method for continuously embossing a precision optical pattern requiring sharp angles and flatness of faces in certain detail on one surface of a continuous resinous sheeting material, the method being performed with the aid of a generally cylindrical endless metal embossing element having an inner surface and an outer surface, the outer surface having a precision optical embossing pattern which is the reverse of the precision optical pattern to be formed on one surface of said sheeting, and wherein the method includes the steps of:
(a) continuously moving the endless embossing element along a closed course through a heating station, where said embossing element is heated through its inner surface to a predetermined temper-ature and then to a cooling station where said embossing element is cooled below said predetermined temperature;
(b) continuously feeding onto said embossing element as it passes through a part of said heating station superimposed resinous film and sheeting materials, said resinous materials of said film and said sheeting each having different glass transition temperatures, said sheeting being in direct contact with the outer precision pat-terned surface of said embossing tool;
(c) continuously heating said embossing element to said predetermined temperature at said heating station, said temperature being greater than the glass transition temperature of said sheeting and less than the glass transition temperature of said resinous film;
(d) pressing said superimposed film and sheeting against said embossing element at a plurality of pressure points sequentially spaced along said heating station with one surface of said sheeting 24.
confronting the engaging said precision optical pattern on said embossing element until said one surface of said sheeting conforms to said precision optical embossing pattern;
(e) continuously passing said embossing element and said superimposed film and sheeting through said cooling station where the temperature of said embossing element and said sheeting is lowered below said sheeting glass transition temperature, with said film serving to substantially continuously maintain said sheeting in engagement with said embossing element through the heating station and through said cooling station; and (f) continuously stripping said superimposed layer of film and embossed sheeting from said embossing element, said film being later strippable from the other face of said sheeting without destroy-ing said optical pattern formed on said one face of said sheeting, the improvement comprising the step of;
(g) reheating said embossed sheeting and film to a temperature in the range of about 82°C to 93°C, thereby to relieve any strain in said film caused by cooling thereof at said cooling station.

13. The method set forth in Claim 12, wherein the temperature achieved in said reheating is about 91°C.

14. The method set forth in Claim 12, wherein said reheating is accomplished on a continuous basis by causing said superimposed layer of embossed sheeting and film to be continuously directed through a reheating station after said film and sheeting are stripped from said embossing element.

15. The method set forth in Claim 13, wherein said stripped film and sheeting are heated long enough to assure that all of the film and embossed sheeting reaches a temperature in the range between 82°C and 93°C.

25.

16. Apparatus for continuously embossing a precision optical pattern on one surface of transparent resinous material or materials, said apparatus comprising:
(a) embossing means including a continuous seamless embossing tool in the form of a thin metal element having an inner surface and an outer surface, said outer surface having a precision optical embossing pattern thereon which is the reverse of the precision optical pattern to be formed in the resinous material;
(b) means for continuously moving said embossing element along a closed course;
(c) means for introducing superimposed film and sheeting of resinous materials onto said embossing element with one face of said sheeting in direct contact with said optical pattern on said embossing element;
(d) heating means for raising the temperature of said emboss-ing pattern to be above the glass transition temperature of said sheeting and below the glass transition temperature of said film while said embossing element is in a first portion of its course;
(e) cooling means for lowering the temperature of said sheet-ing to be below said glass transition temperature while said element and said sheeting are in a generally planar condition in their course, thereby to rigidify said precision pattern while in an undistorted condition;
(f) a plurality of pressure means sequentially spaced along said first portion of said course for pressing said superimposed film and sheeting against said embossing element with said one surface of said sheeting confronting and engaging said embossing pattern until said one surface conforms to said embossing pattern, with said film serving to substantially continuously maintain said sheeting 26.
in engagement with said embossing element until the latter passes said second portion of said course; and (g) means for thereafter stripping said superimposed film and sheeting from said embossing element.

17. The aparatus of Claim 16, wherein said embossing element is a thin seamless flexible metal belt.

18. The apparatus of Claim 16, wherein said heating means is provided by an internally heated heating roller and said cooling means includes a manifold adopted to direct a chilled fluid against said film and said sheeting and said tool as said film passes there-over.

19. The apparatus of Claim 16, wherein said precision optical pattern comprises an array of cube-corner type reflective elements.

20. The apparatus set forth in Claim 16, and further including means for reheating said superimposed sheeting and film to a tempera-ture in the range of 82°C to 93°C after stripping thereof from said embossing element.

21. The apparatus set forth in Claim 16, wherein said reheating means is arranged to continuously reheat said superimposed film and sheeting as the same are continuously stripped from said embossing element.

22. Apparatus for continuously embossing a precision optical pattern on one surface of transparent resinous material or materials, said apparatus comprising;
(a) embossing means including a continuous seamless embossing tool in the form of a thin metal element having an inner surface and an outer surface, said outer surface having a precision optical embossing pattern thereon which is the reverse of the precision optical pattern to be formed in the resinous material;

27.
(b) means for continuously moving said embossing element along a closed course;
(c) means for introducing superimposed film and sheeting of resinous materials onto said embossing element with one face of said sheeting in direct contact with said optical pattern on said embossing element;
(d) heating means for raising the temperature of said emboss-ing pattern to be above the glass transition temperature of said sheeting and below the glass transition temperature of said film while said embossing element is in a first portion of its course;
(e) cooling means for lowering the temperature of said sheet-ing to be below said glass transition temperature while said element and said sheeting are in a generally planar condition in their course, thereby to rigidify said precision pattern while in an undistorted condition;
(f) a plurality of pressure means sequentially spaced along said first portion of said course for pressing said superimposed film and sheeting against said embossing element with said one surface of said sheeting confronting and engaging said embossing pattern until said one surface conforms to said embossing pattern, with said film serving to substantially continuously maintain said sheeting in engagement with said embossing element until the latter passes said second portion of said course;
(g) means for thereafter stripping said superimposed film and sheeting from said embossing element; and (h) means for reheating said superimposed sheeting and film to a temperature in the range of 82°C to 93°C after stripping thereof from said embossing element.