Machining techniques for retroreflective cube corner article and method of manufacture

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FIELD OF THE INVENTION

This invention relates to retroreflective cube corner element articles having prismatic retroreflective elements.

BACKGROUND OF THE INVENTION

Many types of retroreflective elements are known, including prismatic designs incorporating one or more geometric structures commonly known as cube corners. Retroreflective sheeting which employs cube corner type reflecting elements is well-known. Cube corner reflecting elements are trihedral structures which have three approximately mutually perpendicular lateral faces meeting in a single corner. Light rays are typically reflected at the cube faces due to either total internal reflection or reflective coatings. The manufacture of directly machined arrays comprising retroreflective: cube corner elements has many inefficiencies and limitations. Percent active aperture, flexibility, and manufacturing ease are adversely affected by these limitations, and overall production costs versus performance are often higher relative to the new class of articles and methods of manufacture taught below.

SUMMARY OF THE INVENTION

The invention comprises a method of manufacturing a cube corner article comprising the steps of providing a machinable substrate of material suitable for forming reflective surfaces, and creating a plurality of geometric structures including cube corner elements in the substrate by directly machining at least two sets of parallel grooves in the substrate. The direct machining forms at least one geometric structure side surface having both an optical portion and a non-optical portion.

The invention also comprises a method of machining a cube corner article comprising the steps of providing a directly machinable substrate in which a plurality of initial groove sets are machined to produce a plurality of geometric structures including cube corner elements, and adjusting the height of at least one of the geometric structures by directly machining at least one additional groove in at least one groove set.

The invention also comprises a method of machining a cube corner article comprising the steps of providing a directly machinable substrate in which a plurality of groove sets are machined to produce a plurality of geometric structures including cube corner elements, and machining at least one of the grooves in each of at least two of the groove sets along partially overlapping paths in the substrate but at different depths of groove to form a final groove.

The invention also comprises a retroreflective cube corner article which is a replica of a directly machined substrate in which a plurality of geometric structures including cube corner elements are machined in the substrate. At least one of the geometric structures is height adjusted by directly machining at least one additional groove in at least one groove set.

The invention also comprises a retroreflective cube corner article which is a replica of a directly machined substrate in which a plurality of geometric structures including cube corner elements are machined in the substrate. Each geometric structure is bounded by at least one groove from each of at least two sets of parallel final grooves in the substrate, and at least one geometric structure comprises a side surface having both an optical portion and a non-optical portion.

The invention also comprises a retroreflective cube corner element composite sheeting comprising a plurality of zones of geometric structures including retroreflective cube corner elements. Each zone comprises a replica of a directly machined substrate in which a plurality of initial groove sets are machined to produce a plurality of geometric structures including cube corner elements. The composite sheeting comprises at least one zone with height adjusted geometric structures including cube corner elements formed by directly machining at least one additional groove in at least one groove set.

The invention also comprises a retroreflective cube corner element composite sheeting comprising a plurality of zones of geometric structures including retroreflective cube corner elements. Each zone comprises a replica of a directly machined substrate in which a plurality of cube corner elements are bounded in the substrate by a plurality of grooves from a plurality of groove sets. The composite sheeting comprises at least one zone with at least one geometric structure side surface having both an optical portion and a non-optical portion.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a plan view of a directly machined three groove set retroreflective cube corner element array.

FIG. 2 is a section elevation view taken along line 2--2 of FIG. 1.

FIG. 3 is a plan view of some of the active apertures of the array shown in FIGS. 1 and 2.

FIG. 4 is a plan view of a directly machined multiple groove set array having a 3.degree. relief angle.

FIG. 5 is a section elevation view taken along line 5--5 in FIG. 4.

FIG. 6 is a plan view of some of the active apertures of the array shown in FIG. 4.

FIG. 7 is a plan view of a directly machined retroreflective cube corner element array.

FIG. 8 is a section elevational view taken along line 8--8 in FIG. 7.

FIG. 9 is a plan view of some of the active apertures of the array shown in FIG. 7 and FIG. 8.

FIG. 10 is a plan view of a directly machined canted retroreflective cube corner element array.

FIG. 11 is a plan view of some of the active apertures of the array shown in FIG. 10 at zero entrance angle.

FIG. 12 is a graph depicting percent active aperture versus entrance angle for the arrays shown in FIGS. 1, 4, and 7.

FIG. 13 is a graph depicting percent active aperture versus entrance angle for arrays shown in FIGS. 7 and 10.

FIG. 14 is a section elevational view depicting use of a sealing medium.

FIG. 15 is a section elevational view depicting a retroreflective cube corner element array having a separation surface.

FIG. 16 is a schematic view of a machine tool for grooving directly machined arrays.

FIG. 17 is a plan view of a composite array comprising several zones of arrays.

FIG. 18 is a plan view of a directly machined array with variable groove spacing.

DETAILED DESCRIPTION OF THE INVENTION

The manufacture of retroreflective cube corner element micro-cube arrays is accomplished using molds made by different techniques, including those known as pin bundling and direct machining. Molds manufactured using pin bundling are made by assembling together individual pins which each have an end portion shaped with features of a cube corner retroreflective element. Examples of pin bundling include U.S. Pat. No. 3,926,402 to Heenan et al., and United Kingdom Patents 423,464 and 441,319 to Leray.

The direct machining technique, also known generally as ruling, comprises cutting portions of a substrate to create a pattern of grooves which intersect to form cube corner elements. The grooved substrate is referred to as a master from which a series of impressions, i.e., replicas, may be formed. In some instances, the master is useful as a retroreflective article, however, replicas, including multigenerational replicas, are more commonly used as a retroreflective article. Direct machining is an excellent method for manufacturing master molds with small micro-cube arrays. Micro-cube arrays are particularly beneficial for producing thin replica arrays with improved flexibility. Micro-cube arrays are also conducive to continuous process manufacturing. The process of manufacturing large arrays is also relatively easier using direct machining methods rather than other techniques. Examples of direct machining are shown in U.S. Pat. No. 4,588,258, issued to Hoopman, and U.S. Pat. No. 3,712,706 issued to Stamm, which disclose single or multiple passes of a machine tool having two opposing cutting surfaces for cutting grooves to form cube corner optical faces in a substrate. An example of direct machining involving only two sets of grooves is shown in U.S. Pat. No. 4,895,428 to Nelson et al.

FIG. 1 discloses one embodiment of a retroreflective cube corner element array 12 manufactured from a directly machinable substrate 13 by use of at least three groove sets each comprising a plurality of parallel non-overlapping grooves. Preferably, secondary groove sets consisting of evenly spaced secondary grooves 14, 16, are arranged in non-parallel relation, and a primary groove set consists of a plurality of parallel evenly spaced primary grooves 20 centered between secondary groove intersections 22. An alternate embodiment groove spacing comprises varied rather than evenly spaced grooves. In the embodiment disclosed in FIG. 1, a plurality of raised discontinuous geometric structures including retroreflective cube corner elements are formed. In this Figure the intersections of the grooves within two groove sets are not coincident with at least one groove in a third groove set. Also, the separation between the intersections of the grooves within two groove sets with at least one groove in a third groove set is preferably greater than about 1.times.10.sup.-2 millimeters. All of these geometric structures are similar to cube corner elements 24, 26, and 30. FIG. 1 illustrates a multiple structure array in which the cube corner elements are shown formed from primary and secondary grooves with a uniform depth of cut. The grooves intersect with included angles of 60.degree..

FIG. 2 is a cross section elevation view taken along lines 2--2 of FIG. 1. FIG. 2 illustrates the difference in heights of cube peaks 34, 36, and 38 corresponding to cube corner elements 24, 26, and 30. Cube peak 38 illustrates a very high point of the directly machined substrate relative to all other surfaces. In addition, formation of the structure depicted in FIG. 1 and FIG. 2 results in vertical surfaces 41 which create difficulties during processing of arrays of this type. Vertical surfaces contribute to interlocking of mating faces during replication of these arrays, which in turn results in labor inefficiencies, material waste, and slow down of manufacturing.

For these arrays, optical performance is conveniently defined by the percent of the surface area that is actually retroreflective, i.e. which comprises an effective area or active aperture. The percent active aperture varies as a function of the amount of canting, refractive index, and the entrance angle. The structure of array 12 shown in FIG. 1 and FIG. 2 demonstrates an exceptional approximately 91% active aperture, as schematically shown in the percent active aperture depiction of FIG. 3. FIG. 3 also depicts multiple active aperture sizes which result when using the geometric structures and method of manufacturing described above. In particular, differently sized apertures 47, 49, and 53, are intermixed and arranged in close proximity, and correspond to the different types of retroreflective cube corner elements 24, 26, and 30 shown in FIG. 1. Array 12 is quite useful in applications requiring high brightness at zero or low entrance angles such as photoelectric sensors, traffic control materials, directional reflectors, and retroreflective markings.

FIG. 4 discloses retroreflective cube corner element array 56 formed using multiple groove sets in similar manner to that shown and described above in relation to FIG. 1 to create retroreflective cube corner elements 24, 76, and 77. However, array 56 is formed by machining each of the grooves 94, 95, 96 with a 3.degree. relief angle. As shown in FIG. 5, this relief angle results in a less vertical orientation of surface 62 as compared with surface 41, shown in FIG. 2. This less vertical orientation of surface 62 enhances ease of manufacturing and permits considerable improvements during the replication process of array 56.

Use of a relief angle also results in a reduction in percent active aperture corresponding to such arrays. As shown in FIG. 6, array 56 comprises multiple differently sized and shaped apertures 47, 79, and 83. As shown in FIG. 3, the apertures depicted in FIG. 6 are also intermixed and arranged in close proximity to provide relatively high brightness at low entrance angles. However, the maximum percent active aperture of array 56 is reduced to only about 84% due to the use of relief angles eliminating some optical surface area. Increased relief may be utilized to further enhance ease of manufacturing and replication, but it also results in additional reduction in maximum percent active aperture. Sufficiently large relief angles may lower some of the higher structures within arrays. However, the resulting trihedral structures will no longer be cube corner retroreflective elements.

FIG. 7 discloses yet another embodiment of a retroreflective cube corner element array 88 manufactured in similar manner to array 12 and array 56 with a plurality of secondary and primary grooves. Single or multiple passes of a machine tool may be used to produce the shape of the grooves which form geometric structure side surfaces which may include cube corner element optical surfaces. Final grooves form all the geometric structure side surfaces and may be comprised of one or more grooves. Directly machine