Patent Application
20150140309
1. Technical Field
The present invention relates to a process and apparatus for embossing material with precise detail, and more particularly, to a process and apparatus for making relatively rigid panel products of thermoplastic material having surfaces with precision microstructures, as defined below.
2. Background Art
Processes and apparatus for embossing precision optical patterns such as microcubes, in a thin film resinous sheet or laminate, are well known, as referenced in U.S. Pat. Nos. 4,486,363; 4,478,769; 4,601,861; 5,213,872; 6,015,214, and more recently U.S. Pat. No. 6,908,295, which patents are all incorporated herein by reference. In the production of such synthetic resin optical sheeting film, highly precise embossing (generally exceeding the capabilities of the current micromolding processing techniques for synthetic resins, is required because the geometric accuracy of the optical elements determines its optical performance. The above referenced patents disclose in particular methods and apparatus for continuously embossing a repeating retro-reflective cube-corner pattern of fine or precise detail on one surface of a transparent and thin thermoplastic film to form the surface of the film into the desired microstructure pattern.
However, besides precision optical retro-reflective sheeting, various other applications have been envisioned which would be highly enhanced by the formation of highly precise shapes and structures in resinous relatively rigid panels. Currently, for example, in the manufacture of road signs the processed cube-corner thin film is normally adhered to an underlying metal or other rigid substrate, so that the laminated panel has enough structural integrity to be mounted as a road sign. One proposed alternative to traditional reflective sheeting can be accomplished by embossing similar cube-corner structures directly on a polymeric rigid sheet at least 2.5 mm or thicker. Other applications include solar panels in which an array of Fresnel type lenses are continuously formed in a thin film and then laminated to a rigid transparent polymer substrate. Such applications require the embossing of thin thermoplastic material film to provide the precisely formed and spaced functional geometric elements, or arrays of such functional geometric elements on the film surface. In the case of solar panels, not only must the lens element be optically accurate to focus light on the target area for energy conversion, but the spacing of the lens elements relative to each other also is of critical importance to achieve the necessary efficiency of light directed to the receiving energy converting junction.
These geometric elements, or precision microstructures, are defined by any or all of the following characteristics: precise embossing depths; flat surfaces with precise angular orientation; fine surface smoothness; sharp angular features with a very small radius of curvature; and precise dimensions of the elements and/or precise separation of the elements, within the plane of the film. The precise nature of the formed surface is critical to the functional attributes of the formed products, whether used for microcubes or other optical features such as the radial Fresnel lenses in solar panels; or as light directing or diffusing panels for lighting fixtures; or as channels for microfluidics, or in fuel cells; or for accurate dimensions, flatness and spacing when providing a surface for holding nanoblocks in Fluidic Self Assembly (FSA) techniques; or imparting a microtextured surface that is not optically smooth within an array that includes, or excludes additional microarchitecture.
U.S. patents describing some uses of precise microstructures include: U.S. Pat. Nos. 4,486,363; 6,015,214 (microcubes); U.S. Pat. Nos. 5,783,856; 6,238,538 (microfluidics); and U.S. Pat. No. 6,274,508 (FSA).
As described in some of the above mentioned patents, such as U.S. Pat. Nos. 4,486,363, 4,601,861, and 4,478,769, embossed microstructure film may be made on a machine that includes two supply reels, one containing an unprocessed film of thermoplastic material, such as acrylic or polycarbonate, or even vinyl, and the other containing a transparent and optically smooth plastic carrier film such as PET (trade name Mylar), which should not melt or degrade during the embossing process. These films are fed to and pressed against a heated embossing tool in the form of a thin endless flexible metal belt. The belt creates the desired embossed pattern on one surface of the thermoplastic film, and the carrier film makes the other surface of the thermoplastic film optically smooth.
The belt moves around two rollers, which advance the belt at a predetermined linear controlled speed or rate. One of the rollers is heated and the other roller is cooled. An additional cooling station, e.g. one that blows cool air, may be provided between the two rollers. Pressure rollers are arranged about a portion of the circumference of the heated roller. Embossing occurs on the web as it and the continuous tool pass around the heated roller while pressure is applied by one or more pressure rollers, causing the film to be melted and pressed onto the tool. The embossed film (which may have been laminated to other films during the embossing process), is cooled, monitored for quality and then moved to a storage winder. At some point in the process, the PET carrier film may be stripped away from the embossed film.
The prior apparatus and process work well to produce rolls of film that are effectively 48″ (122 cm) wide (52″/132 cm at selvage), but such equipment and processes have several inherent disadvantages. First, the process speed (and thus the volume of material) is limited by the time needed to heat, mold and “freeze” the film. Also, the pressure surface area and thus the time available to provide adequate pressure by the pressure rollers impose certain special constraints; and then cooling the material is required before separation from the tool. Finally the formation of some embossed surfaces while the tool is in a curved condition requires complex modification of the geometry of the tool surface, because the thermoplastic elements are formed while on a curved surface but generally used later while on a flat surface.
One earlier prior device for forming microcubes while in a planar condition is illustrated in U.S. Pat. No. 4,332,847, and involves indexing of small (9″×9″ or 22.86 cm×22.86 cm) individual molds at a relatively slow speed (See Col. 11, lines 31-68). That process is not commercially practical because of its perceived inability to accurately reproduce microstructures because of indexing mold movement and the relatively small volume (caused by mold size) and speed.
It became apparent that there was a need for equipment and processes that permit a larger volume of precision microstructured material to be produced in a given time, and using tools that may heat, emboss and cool the film while in a planar condition. In this regard, U.S. Pat. No. 6,908,295 developed the technology for embossing thin film in a double band continuous press.
Continuous press machines have been used in certain industries. These machines include double band presses which have continuous flat beds with two endless bands or belts, usually steel, running above and below the product and around pairs of upper and lower drums or rollers. An advantage of such presses is the mainly uniform pressure that can be provided over a large area. These machines form a pressure or reaction zone between the two belts and have the advantage that pressure is applied to a product when it is flat rather than when it is curved. The double band press also allows pressure to be adjusted over a wide range and the same is true of temperature variability. Dwell time or time under pressure also is controllable for a given press by varying the production speed or rate, as is capacity, which may be changed by varying speed and/or length and/or width of the press. Another advantage of the double band press is that the raw material may be heated first and then cooled, while the product is maintained under pressure. Heating and cooling elements may be separately located one after the other in line behind the belts. The steel press belts are first heated and then cooled, thereby efficiently heating and then cooling the material in the reaction zone and all while under pressure.
Continuous press machines, fitting the general description provided herein, are made by Hymmen GmbH of Bielefeld, Germany (U.S. office: Hymmen International, Inc. of Duluth, Ga.) as models ISR and HPL. These are double belt presses and also appear under such trademarks as ISOPRESS and ISOROLL. Typically they have been used to produce relatively thick laminates, primarily for the furniture industry, but have also been used to form polymer materials for use in the luggage industry, for example. Prior to the '295 invention they had not been considered for use in making microstructured products.
The bands with embossing patterns formed therein as disclosed and claimed in the aforesaid '295 patent had microstructured surfaces for forming the desired structure in the product passing through the press. These surfaces were either proposed to be the direct bands placed on the machine, or in one (or more) overlay band that was to be a continuous band placed over the regular smooth band(s) of the press.
In all the prior art versions of microstructured embossing noted above the metal embossing tools must be replaced at intervals, therefore a more efficient and effective method and apparatus for forming rigid panels with a microstructured surface has been invented.
While the apparatus disclosed in the '295 patent will work, it is neither cost effective nor an efficient way to produce rigid panels having a microstructured surface on one face, such as used in solar panels or roadway signs. In the first instance the thin film, after formation from the press, must be relocated to a different assembly station, where an adhesive is applied and a relatively rigid substrate panel adhered and then cured. This is both time consuming and labor intensive.
Secondly, and perhaps most importantly, the cost in time and labor to make the large belts required under the '295 patent also renders the film production inefficient and more costly. Over time in the prior art machines it was observed that the thin belts both lose some element of accuracy and also suffer some “creep” which, in the case of solar panels requiring light to be focused on a designated area, can render the panels less efficient. The longer the belt, such as disclosed in the '295 patent, the more exacerbated this problem becomes.
As described in the '295 patent, to provide the necessary formed belts or overlay tools that would fit on the Hymmen press, a belt having a perimeter of 419″ (1247 cm) was produced. In that case the belt also was 29.53″ (75 cm) wide. Not only is such a large belt unwieldy, the number of steps and complexity of formation of each such large belt is very time consuming and expensive as can be appreciated by the description of assembly in the '295 patent. Further, to preserve accuracy of the finished film, tracking elements for the large belts are required on the press, and the large belt makes tracking more complex. Finally, in embossing a thicker film, such as used in solar panels or for traffic signs, it is more difficult to separate the finished film continuously from a moving belt. The peel angle is difficult when having to remove a film from facets that have a low draft angle, as frequently found in solar panel designs. Moreover, as these large belts must be replaced with some regularity, the '295 apparatus using the continuous band tool, is not commercially practical.
Thus a primary object of the present invention is to provide a process and apparatus for efficiently, effectively, and inexpensively embossing thermoplastic materials with precise microstructure detail into a relatively rigid panel and at relatively high speeds.
For purposes hereof, a relatively rigid panel is a panel that, while it may have some degree of flexibility, it is sufficiently self supporting to be considered as a structural unit without any additional material laminated or adhered to it to render it functional for mounting. This does not preclude additional layers being adhered to the formed panel as part of a mounted structure, or to form a more complex multilayer object, it being the intent that the current manufacturing steps of adhering a thin film to a thicker substrate by lamination or otherwise to provide structural integrity will have been eliminated.
Also in considering the phrase relatively rigid or rigid herein the panel stiffness may depend both on the thickness and elasticity modulus of the material to be embossed and wherein the thickness or rigidity is so stiff it would not permit continuous embossing off a roll of supply material.
The present invention not only obviates the problems caused by large belts with microstructured surfaces, as suggested by the '295 patent, because it eliminates overlay bands or providing the microstructured surface on the continuous bands of the press, it also speeds up production of finished rigid solar or highway sign panels by eliminating a number of other manufacturing steps.
These advantages are accomplished by providing individual tool elements that match the panel to be formed and to serially feed the tool/panel combination into the double band press, minimizing the cost/time to prepare large continuous bands and the extended down time in changing the bands as they creep or lose accuracy and by directly embossing the rigid panel the subsequent current thin film production and then laminating steps are obviated.
It is a primary object of the invention to provide a process for forming thermoplastic products having precision microstructured surfaces, comprising the steps of: providing a continuous press having opposed parallel continuous bands having upper and lower press surfaces defining a relatively flat reaction zone therebetween; serially feeding individual tools, each having a defined microstructured array thereon and a rigid thermoplastic material as a panel having one face juxtaposed with the tool surface, between the bands and through the reaction zone; and causing at least one surface of the panel material to be heated to its embossing temperature Te while applying pressure to at least one press surface to form the precise microstructure surface in the panel as it moves through the reaction zone; and moving the tool and embossed panel to an adjacent area of the reaction zone and cooling the tool and embossed panel while concurrently maintaining pressure on the panel.
Another object of invention is to provide an apparatus for continuously forming thermoplastic products having precision microstructured surfaces thereon, comprising a continuous double band press having spaced upper and lower primary bands, means are provided for serially feeding individual tool elements having the desired microstructured surface formed thereon and juxtaposed with a relatively rigid panel of a thermoplastic material through the press and between the bands. Heating means are provided for heating the tool and thus at least one surface of the panel to its embossing temperature Te; as are pressure means for applying sufficient pressure to the belts to cause the precise engagement of the heated thermoplastic panel with the belts and the tool surface to emboss the material with the precise microstructured pattern. The apparatus has cooling means for cooling the tool and the embossed panel while maintaining pressure on the panel while it is cooled, and while it is moving through the press.
As noted, the present invention offers numerous advantages and relates to a process and apparatus for making thermoplastic products having precision microstructured patterns in a relatively rigid material, comprising the steps of providing a continuous press with an upper set of rollers, a lower set of rollers, an upper belt disposed about the upper set of rollers, a lower belt disposed about the lower set of rollers, the upper and lower belts defining a relatively flat reaction zone therebetween, the reaction zone including a heating station, a cooling station and pressure producing means; feeding a metallic tool having the inverse of the desired microstructured form juxtaposed with a relatively rigid polymeric panel between the bands and through the reaction zone; heating the tool and at least the juxtaposed surface of the panel to an embossing temperature Te above the glass transition temperature Tg of the thermoplastic material, (e.g. around 100° to 150° F./38° C. to 66° C. above Tg); applying an elevated embossing pressure to the panel, (e.g. about 250 psi/1.7 MPa); cooling the panel (e.g. well below Tg); while maintaining the elevated pressure on the panel.
The present invention adapts a known type of continuous machine press, known as an isobaric double band continuous press, to the embossing of precision microstructured thermoplastic panels. As noted, one well-known type of precision microstructured sheeting is optical sheeting. Flatness and angular accuracy are important in precision optical sheeting including, for example, cube corner retroreflective films for road reflectors or signage, and Fresnel lenses incorporating catadioptrics for solar panels. For purposes of this application, the term “panel” is used to describe any relatively rigid polymeric material having a predetermined size and shape and thickness into which the microstructured surface is to be formed on at least one side of the panel. The term is not to be limited by size or shape or intended use of the panel, or the particular polymer of which it is formed. Moreover, while the preferred tools used are metallic, a tool formed of a much higher melt point than the panel material may be used. In a preferred embodiment of the invention the feeding, heating and removal of the tools and panels is automated to a great degree to further enhance machine capacity.
Besides precision optical sheeting for use in solar panels, various other applications have been developed requiring the formation of highly precise shapes and structures in resinous optical film. In particular the invention permits the embossing of thermoplastic material to provide precision microstructures comprising microscopic embossed elements of elements, or arrays of microscopic recessed and/or raised embossed element having applications to optical, micro-fluidic, micro-electrical, micro-acoustic, and/or micro-mechanical fields. It would be particularly useful in forming large microprismatic panels for use as traffic signs.
As used in the present application, “precision microstructured” material generally refers to a resinous polymer material having an embossed precise geometric pattern of very small elements or shapes, and in which the precision of the formation is essential to functionality of the product. In this instance the precision of the embossed panel is a function of both the precise geometry required of the product, and the capability of the embossing tool, process and apparatus to conserve the geometric integrity from tool to article formed in the panel (on one or both sides thereof):
In certain embodiments of precision microstructured panel, discrete elements and/or arrays of elements may be defined as embossed recessed regions, or embossed raised regions, or combinations of embossed recessed and raised regions, relative to the unembossed regions of the panel. In other embodiments, all or portions of the precision microstructured panel may be continuously embossed with patterns of varying depths comprising elements with the characteristics described above. Typically, the discrete elements or arrays of elements are arranged in a repetitive pattern; but the invention also encompasses non-repetitive arrays of precision microstructured shapes.
Exemplary types of precision microstructured panels, and their requirements of precision, include:
Retroreflective materials for road reflectors or signage; and Fresnel lenses for optical solar array applications. In each instance precise flatness, angles and uniform detail are important. Cube-corner type reflectors, to retain their functionality of reflecting light back generally to its source, require that the three reflective faces of the cube 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. Also, surface smoothness is required so light is not diffused.
Feature to feature accuracy for LCD display systems and for solar panels in which adjacent embossed recesses not only have to be precisely shaped, the spatial relations of the array of recesses also must be closely adhered to.
The ability to manufacture microstructures with an edge radius of less than 0.001 inches (25 microns) and with very sharp points and sharp ridges (less than 0.00028 inches (7 microns).
Volumetric accuracy for microfluidic and microwell applications with 90% or greater accuracy of the cross sectional area being conserved through the length of channel; and from channel to channel, and/or well to well, in which dimensions range from 0.00020 to 0.008 inches (5-200 microns) depth; 0.00020 inches to 10 inches (5 microns to 25.4 cm). The channels may have convoluted shapes and microtextured shapes.
Surface roughness for microfluidic applications that allow for low friction and minimal surface drag, all resulting in smooth continuous non-diffusive flow, allowing the fluid flowing laminar.
The avoidance of residual stresses by providing essentially stress free microstructures. This is critical for some optical, FSA, and for microfluidic applications were the detection mechanisms uses fluorescent polarization technology. Materials with stress generally have strand orientation, which acts like a polarizing lens. Materials that contain residual stresses may relax that stress during subsequent processing or during the life cycle of the product, resulting in dimensional instability.
For Fresnel lenses, either radial or lenticular.
The precision microstructured pattern typically is a predetermined geometric pattern that is replicated from the tool. It is for this reason that the tools of the preferred embodiment are produced from electroformed masters that permit the creation of precisely designed structures.
A more complete understanding of the present invention and other objects, aspects, aims and advantages thereof will be gained from a consideration of the following description of the preferred embodiments read in conjunction with the accompanying drawings provided herein.
While the present invention is open to various modifications and alternative constructions, the preferred embodiments shown in the drawings will be described herein in detail. It is understood, however, that there is no intention to limit the invention to the particular form disclosed. On the contrary, the intention is to cover all modifications, equivalent structures and methods, and alternative constructions falling within the spirit and scope of the invention as expressed in the appended claims.
Referring now to
The tool in this case may be 39.37″ (1 M) in both length L and width W, and may have six or more rows of lenses 110 in each direction depending on the lens and panel sizes required by the customer. In some instances small flat areas may be provided between rows of lenses to facilitate later mounting of a completed panel to a frame for mounting in a much larger rotatable structure. As best seen in
The tool 100 may be formed by first diamond cutting a master lens for the pattern generally depicted in
Referring now to
In
Because of the advantages of the present process, it is possible to form a rigid polymeric microprismatic traffic sign panel in a manner that will eliminate the need for the typical aluminum backing member which is normally used to provide rigidity to the thin films heretofore used. This may result in a cost savings to the customer of as much as 35-40% based on elimination of an expensive material (aluminum) and the extra labor and handling involved in applying the film to the aluminum panel.
Referring now to
An upper plain surfaced belt 210 is mounted around the upper rollers 202, 204 and a lower plain surfaced belt 215 is mounted around the lower rollers 206, 208. The direction of rotation of the drums or rollers, and thus the belts or bands 210 and 215, is shown by the curved arrows in
It is to be understood that each of the pressure sections may be heated or cooled; i.e., the temperature of each press section can be independently controlled, which is particularly advantageous as will be later explained. Thus, for example, the first two upstream pressure sections, upper sections 230, 232 and the first two lower sections 240, 242 may be heated whereas the downstream sections 234 and 244 may be cooled or maintained as a relatively constant but lower temperature than the upstream heated sections. Further, each of the upper and lower sections of a pair also may be controlled to a different temperature. It will be observed from
The upper surface 214 of lower belt 215 may be smooth to facilitate movement of the carrier material for the tool/panel/overlay stack (hereinafter described as the “sandwich”), as it moves through the press 200.
When embossing thicker panels, it is desirable to have the lower belt section operate at a much higher temperature than the upper section (unless embossing is to take place on both sides of the panel), as it is undesirable to heat the entire panel to above its melting point. Thus having a temperature gradient from high temperature at the embossing surface to much lower temperature at the opposite panel surface during embossing accelerates cooling of the embossed panel thereby accelerating removal time of the finished panel from the tool.
The process for embossing the relatively rigid polymer panel 150 to precise microstructure formation consists of feeding a sandwich 300 (
In one set of experimental runs, panels of PMMA having a thickness of 3 mm and length and width of 1.5 m by 0.990 m, coupled with a nickel tool 100 and having thirty Fresnel lenses in the array, were passed through a press 200 of the type described. Pressure in the ranges of 30 to 40 bar were found satisfactory with the best result at 40 bar. The temperature changes between upper and lower stations may have varied between 210° C. for the upper section 230 and 210° C. for the lower section 240, and temperatures of 70° C. for the second station and 20° C. at the third station where cooling was taking place. Various combinations of temperature, speed and pressure may be utilized depending upon the desired finish and the thickness of the panel to be embossed.
With the dimensions and reaction zones stated above, the process rate by serially feeding sandwiches into the press may move at about 7.5 to 10 feet (2.5 to 3.5 meters) per minute. Even though the experiment was limited by having to manually feed sandwiches 300 into the press the rate of speed was much greater than the rate of existing prior art machines, which for a 0.5 mm thick film used for example in solar panels runs at 0.66 meters per minute and are then laminated, to 3 (mm) sheet, at an average rate of 1.3 meters/minute the average speed of the two operations is 0.98 meters per minute.
After passing through the press 200 and being sufficiently cooled, the tool 100 and formed panel (now designated 170 in
The overlay also assures that the upper surface of the panel 150 as it moves through the press will have a smooth unblemished surface that will not pick up any of the graininess in the steel bands, no matter how well polished. Where the panel 170 is comprised primarily of PMMA, than the overlay material may be of Mylar. Other combinations of course are likely.
For a given size embossing tool and panel, and press machine, the embossing goal is to maximize production. Other things being equal, the design that uses more of the press belt's width and length is better. Length might be used for forming or for cooling. At the maximum running speed, these two minimum times (forming and cooling) occupy all the available length. The minimum time (length) required for forming may be less than, equal to, or greater than the minimum time (length) required for cooling. The present equipment permits some variation of these distances by virtue of the pressure plate arrangements. Additional pre-heating of the tool and panel before entry to the reaction zone, or post-reaction zone cooling also may be provided, depending on the materials and thicknesses used. It may also be desirable to temporarily connect adjacent tools as they run through the press to both minimize damage to the belts (by avoiding discontinuity gaps as pressure is applied) and disconnecting the adjacent tools as they exit the press.
In isobaric double band presses such as that of Hymmen GmbH, the bands serve to seal in the pressurized fluid (oil or air), which can be under an elevated pressure as great as 1000 psi/68 bar/6.8 MPa). This requires that the belt have adequate mechanical strength (tensile strength and yield strength) to withstand the high pressures.
The reaction zone 220 is formed between the lower run of the upper press band 210 and the upper run of the lower press band 215 in which the material panel is fed, which in this case was a synthetic thermoplastic resin.
The reaction zone pressure can be applied hydraulically to the inner surfaces of the endless press belts 210 and 215 by the opposing pressure plates 230, 232, 234, and 240, 242, 244 and is transferred from the belts to the sandwich 300 fed therebetween (see
The pressing force is provided on the sandwich 300 in the reaction zone 220, 221 by a fluid pressure medium introduced into the space between the upper and lower pressure plates and the adjacent inside surfaces of the press belts located between the rollers, which portions of the belts form the reaction zone. The space forming the so-called pressure chamber (exemplified for the lower belt as 260) is defined laterally by sliding seals. In order to avoid contamination of the panel 150, desirably compressed air or other gases (as opposed to liquids) are used as the pressure medium in the pressure chamber of the reaction zone.
As the continuous press includes polished and plain surfaced bands, a very smooth surface finish is required that may be provided for example using a polished chrome surface of a stainless steel band. In the case of the Hymmen isobaric press, a surface finish of 0.00008-0.00016 inches (2-4 micron) Rz is desired, which is equivalent to 80-160 microinch rms in English units. Cf. American National Standards Institute, “Surface Finish”, ANSI B46.1. Surface treatment techniques such as polishing, electropolishing, superfinishing and liquid honing, can be used to provide the highly smooth surface finishes of belts 210, 215.
Considering now the resinous panel material in greater detail; for purposes of the present invention, two temperature reference points are used: Tg and Te.
Tg is defined as the glass transition temperature, at which plastic material will change from the glassy state to the rubbery state. It may comprise a range before the material may actually flow.
Te is defined as the embossing or flow temperature where the material flows enough to be permanently deformed by the continuous press of the present invention, and will, upon cooling, retain form and shape that matches or has a controlled variation (e.g. with shrinkage) of the embossed shape. Because Te will vary from material to material and also will depend on the thickness of the panel material and the nature of the dynamics of the continuous press, the exact Te temperature is related to conditions including the embossing pressure(s); the temperature input of the continuous press and the press speed, as well as the extent of both the heating and cooling sections in the reaction zone, both at the upper and lower levels of the press elements.
The embossing temperature must be high enough to exceed the glass transition temperature Tg, so that adequate flow of the material can be achieved to provide highly accurate embossing of the film by the continuous press.
Numerous thermoplastic materials may be considered as polymeric materials to provide precision microstructure panels. Applicant has experience with a variety of thermoplastic materials to be used in embossing under pressure at elevated temperatures. These materials include thermoplastics of a relatively low glass transition temperature (up to 302° F./150° C.), as well as materials of a higher glass transition temperature (above 302° F./150° C.).
Typical lower glass transition temperature (i.e. with glass transition temperatures up to 302° F./150° C.) include materials used for example to emboss cube corner sheeting or Fresnel lenses for solar panels, such as vinyl, polymethyl methyacrylate (PMMA or Acrylic), low Tg polycarbonate, polyurethane, and acrylonitrile butadiene styrene (ABS). The glass transition Tg temperatures for such materials are 158° F., 212° F., 302° F., and 140° to 212° F. (272° C., 100° C., 150° C., and 60° to 100° C.).
Higher glass transition temperature thermoplastic materials (i.e. with glass transition temperatures above 302° F./150° C.) which applicant's assignee has found suitable for embossing precision microstructures, may include polysulfone, polyarylate, cyclo-olefinic copolymer, high Tg polycarbonate, and polyether imide.
A table of exemplary thermoplastic materials, and their glass transition temperatures, appears below as Table I:
The thermoplastic panel also may comprise a filled polymeric material, or composite, such as a microfiber filled polymer, and may comprise a multilayer material, such as a coextrudate of PMMA and BPA-PC.
A variety of thermoplastic materials such as those listed above in Table I may be used in the press 200 (or the other embodiments described). Relatively low Tg thermoplastic materials such as polymethyl methacrylate, ABS, polyurethane and low Tg polycarbonate may be used in the press 200. Additionally, relatively high Tg thermoplastic materials such as polysulfone, polyarylate, high Tg polycarbonate, polyetherimide, and copolymers also may be used in the press 200. Applicants have observed as a rule of thumb that for good fluidity of the molten thermoplastic material in the reaction (embossing) zone, the embossing temperature Te should be at least 50° F. (10° C.), and advantageously between 100° F. to 150° F. (38° C. to 66° C.), above the glass transition temperature of the thermoplastic sheeting.
With such thermoplastic material the pressure range is approximately 150 to 700 psi (10.3 to 48 bar/1.03 to 4.82 MPa), and potentially higher, depending on factors such as the operational range of the continuous press; the mechanical strength of the embossing tool (high pressure capacity); and the thermoplastic material and thickness of the thermoplastic panel.
It may be desirable that the panel be cooled under low or no pressure, after being exposed to heat and pressure during the forming process, to minimize potential residual stress in the final product. Cooling under low or no pressure may differ from product to product. Thus, under some circumstances the cooling station will be maintained in the range of 35° F. to 41° F. (2° C. to 5° C.) and the pressure range approximately 150 to 200 psi (10.3 to 13.7 bar/1.03 to 1.37 MPa). The pressure in the reaction zone will be similar for heating and cooling.
The planar surface of products such as Fresnel lenses must have a surface roughness of 10-15 nm Ra or lower in order to have an acceptable range of light transmission through the lens. It has been determined that in order to adequately form the planar side of the product in a double belt press the belt on the planar side must also have a surface roughness in the range of Ra 10 to 15 nm or lower. Using other methods of replicating optical lenses such as hot polymer embossing the planar surface of the product is determined by the surface roughness of the carrier film such as optical grade PET. As an example the polymer in a hot polymer embossing process is heated to a temperature above the Tg while in contact with the PET carrier and is then cooled below the Tg so the new surface roughness of the planar surface is determined by the surface roughness of the PET. It is therefore required the surface roughness of the upper belt in the double belt press be 10-15 nm or less surface roughness.
Thermoplastic materials of thicknesses of up to 0.250 inches (6.35 mm) may be embossed with precise formations in the range of 0.0004 to 0.010 inches (0.1 to 250 microns) deep.
The apparatus of the present invention allows for the thermoplastic panel to be relatively thick and yet still have precision microstructures in one or both major surfaces. This allows products as diverse as solar panels, office light diffusers, reflective signage, compact disks, flat panel displays, high-efficiency lighting systems for internally illuminated signs and medical diagnostic products to be efficiently, effectively and inexpensively manufactured. Another exemplary application is retroreflective lenses for road markers, which are more than 0.04 inches (1 mm) thick. The embossing is on the order of 0.006 inches (0.15 mm) deep.
In embossing relatively thick thermoplastic panels, the apparatus of the invention can emboss both sides of the sheeting without heating the center. This can be accomplished using a sandwich of a panel juxtaposed between two tools, and with no overlay. Besides double sided embossing of a monolayer material, the embossing process of the invention permits the embossing of two polymer layers separated by a separator sheet, which are later stripped apart; an example is a sandwich of PMMA, PET, and PMMA films.
The use of the phrase polymer in the appended claims is intended to cover all of the foregoing possibilities—single layer panels; laminates; use of a strippable carrier and registered and unregistered embossing.
A typical Fresnel lens pattern for a solar panel 170 with lens groove elements 128 formed with the aid of the embossing tool 100 such as that depicted in
There is depicted in
The specification describes in detail several embodiments of the present invention. Other modifications and variations will, under the doctrine of equivalents, come within the scope of the appended claims. For example, presses having somewhat different geometries and/or different dimensions are considered equivalent structures. Different thermoplastic material may affect pressure and temperature as well as process speed. Further, different material densities and thicknesses may also affect the apparatus and process. There is no desire or intention here to limit in any way the application of the doctrine of equivalents.
This application claims the benefit of provisional application Ser. No. 61/646,027 filed May 11, 2012. This application relates to significant improvements to the method and apparatus of prior patent, U.S. Pat. No. 6,908,295, issued Jun. 21, 2005, of which the current inventor is a named co-inventor thereof.
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/US13/31918 | 3/15/2013 | WO | 00 |
| Number | Date | Country | |
|---|---|---|---|
| 61646027 | May 2012 | US |
