CA2173229A1

CA2173229A1 - Novel machining techniques for retroreflective cube corner article and method of manufacture

Info

Publication number: CA2173229A1
Application number: CA002173229A
Authority: CA
Inventors: Gerald M. Benson; Kenneth L. Smith
Original assignee: Gerald M. Benson; Kenneth L. Smith; Minnesota Mining And Manufacturing Company
Current assignee: 3M Co
Priority date: 1993-10-20
Filing date: 1994-10-20
Publication date: 1995-04-27
Also published as: JP3590064B2; JPH09504384A; CN1133637A; US6080340A; US6168275B1; DE69415991D1; EP0724736A1; EP0724736B1; DE69415991T2; US5600484A; CN1040695C; WO1995011470A2

Abstract

A method of machining a substrate to produce a cube corner element optical array (12). The method includes steps of directly machining a plurality of groove sets (14, 16, 20) into a directly machinable substrate to form an array having a plurality of geometric structures including cube corner elements (24, 26, 30), and machining at least two of the groove sets along the same path in the substrate but at different depths of groove to produce a geometric structure side surface having both an optical portion and a non-optical portion.

Description

2~73~9 NOVEL MACHINING TECHN~OUES FOR RETROREFLECT~VE
CUBE CORNER ARTICLE AND METHOD OF MANUFACTURE

5 Field of the Invention This invention relates to r~lrûle[lective cube corner elpmpnt articles having pricm~tic ~ ol~ nective ~olem~pnt Back~round of the Invention Many types of retroreflective elPmentc are known, inclurling pricm~tiC designs h~ccjl~uld~ g one or more geometric structures commonly known as cube corners. Retroreflective chP~ting which employs cube corner type reflPcting cle...~tc is well-known. Cube corner reflecting el~m~ntc are trihedral structures which have three approximately mutually perpen~lic~ r lS lateral faces m~ting in a single corner. Light rays are typically rPfl~PCtP~ at the cube faces due to either total internal refle~tion or reflective co~tingC~ The m~nuf~ctllre of directly m~hinPcl arrays comprising rtLIwt;flective cube corner mt~ntc has many inefficien~ies and limit~tions Percent active ap~,lu~, fleYibility, and manufacturing ease are adversely affected by these limit~tions,20 and overall production costs versus ~lrùrl,.ance are often higher relative to the new class of articles and metho~s of m~nllf~ct--re taught below.

Summary of the Invention The present invention provides cube corner articles made up of 25 an array of cube corner elompntc defined by at least two intcla~;ling sets ofdirectly rn~hine~ parallel grooves. The height of at least one of the structuresin the array is adju~led or different from other structures in the array as (1ei,c.;he~ below. The invention also provides metho-lc for making such cube corner articles. In brief summ~ry, the method typically comprises m~hining a 30 series of intersecting sets of parallel grooves as described herein into a substrate and forming one or more generations replicas of the substrate.
Retroreflective articles of the invention ov~co,.,e many structural and optical limit~tionc of convention~l cube corner l_t~ùrenective el-o-m~nt . ~173~2,~ PCT/US94/12016 des~ c. The new class of multiple structure cube corner arrays provided by the invention permit diverse cube corner ~haping and permit m~mlf~ture of cube corner arrays having highly tailorable optical ~lÇ~",nance.

5 Brief Desc,i~lion of the Drawin~s The invention will be further eYpl~inPd with l~f~ ,ence to the drawing, wherein:
Figure 1 is a plan view of an illllst~tive directly m~rhinP~ three groove set rcLlo~nective cube corner elemPnt array of the invention.
Figure 2 is a section elevation view taken along line 2-2 of Figure 1.
Figure 3 is a plan view of some of the active apertures of the array shown in Figures I and 2.
Figure 4 is a plan view of an illustrative directly m~ in multiple groove set array of the invention having a 3 relief angle.
Figure S is a section elevation view taken along line 5-5 in Figure 4.
Figure 6 is a plan view of some of the active ap~.~ures of the array shown in Figure 4.
20 Figure 7 is a plan view of an illustrative directly m~chin c;t,o,enective cube corner elemPnt array of the invention.
Figure 8 is a section elevational view taken along line 8-8 in Figure 7.
Figure 9 is a plan view of some of the active apertures of the array shown in Figure 7 and Figure 8.
Figure 10 is a plan view of an illustrative directly m~hin canted ~e~urenective cube corner elPmPnt array of the invention.
Figure 11 is a plan view of some of the active apertures of the array shown in Figure 10 at zero entr~nre angle.
Figure 12 is a graph depicting percent active al~e~LIn~; versus entr~nce angle for the arrays shown in Figures 1, 4, and 7.

WO95/11470 2 ~ 7 3 2 2 9 PCT/US9~/12016 Figure 13 is a graph depicting percent active ~."Lurc versus ~ntr~nce angle for arrays shown in Figures 7 and 10.
Figure 14 is a section elevational view depicting use of a sealing mPditlm.
S Figure 15 is a section elevational view ~epicting a .cllo.eilective cube corner elemPnt array having a separation s-~
Figure 16 is a scllP .~l;c view of a m~hine tool for grooving directly m~rhin~d arrays.
~;igure 17 is a plan view of an illustrative colllposiLe array of the invention comprising several zones of arrays.
Figure 18 is a plan view of an illustrative directly m~hinPd array of the invention with variable groove spacing.
These figures, which except for Figures 12 and 13 are i~e~li and not to scale, are intPndçd to be merely illustrative and non-limiting.
Detailed Descliplion of Illustrative Embodiments of the Invention The invention provides a method of m~nl~f~tllring a cube corner article comprising the steps of providing a m~chin~hle s~bstr~tç of material suitable for forming reflective surfaces, and creating a plurality of g~~ ,lliC
- 20 structures including cube corner elçmçnt~ in the s.lbslldte by directly m~hining at least two sets of parallel grooves in the ~.lbs~ e. The direct m~chining forms at least one ge~lllellic structure side surface having both an optical portion and a non-optical portion.
The invention also provides a method of m~rhinin~ a cube comer article comprising the steps of providing a directly m~chin~hle substrate in which a plurality of initial groove sets are m~rhined to produce a plurality of geometric structures including cube corner elçm~nts, and adjusting the height ofat least one of the g~mellic structures by directly m~rhining at least one ~rlr~itjon~l groove in at least one groove set.
The invention also provides a method of m~t hining a cube corner article comprising the steps of providing a directly m~hin~hle substrate in which a plurality of groove sets are m~hin~A to produce a plurality of ., .

WO9S/11470 21~ 3 2 ~ ~ PCT/US94/12016 g~lnel~ic SLll~Ctul~,S inrluding cube corner plPmPnt~, and m~rhinin~ at least one of the grooves in each of at least two of the groove sets along partially ov~llal)ping paths in the substrate but at different depths of groove to form a final groove.
S The invention also provides a rellolcllective cube corner article which is a replica of a directly m~rhine~d substrate in which a plurality of geometric structures inrlut~ing cube corner elPmentc are marhinP~I in the substrate. At least one of the geometric structures is height adjusted by directly m~rhinin~ at least one additional groove in at least one groove set.
The invention also provides a lc llor~flective cube corner article which is a replica of a directly m~rhine~d substrate in which a plurality of geometric structures including cube corner elemPnt~ are rn~rhinP~ in the ~.lbs~t~P. Each geollleL~ic structure is bounded by at least one groove from each of at least two sets of parallel final grooves in the substr~te, and at least one geometric structure comrri~Çs a side surface having both an optical portion and a non-optical portion.
The invention also provides a l~lluf~nective cube corner e1Pment composite ~heeting compAsing a plurality of zones of geollletAc structures inrlu~1ing retroreflective cube corner el~Pmpnt~. Each zone comprises a replica of a directly m~rhine~ b~Lldte in which a plurality of initial groove sets are m~chined to produce a plurality of geometric structures inr~ ing cube corner elements. The composite ~hP~tine compAses at least one zone with height adjusted geometAc structures inclll-ling cube corner elelnPnt~ formed by directly m~rhining at least one ~ tion~l groove in at least one groove set.
The invention also provides a ~l-o-eQective cube corner elemPnt composite shPeting compAsing a plurality of zones of geometric ~Lll~(;tule,S
including le~ror,llective cube corner elemPnt~. Each zone comprises a replica of a directly m~rhine~ ~I,shdte in which a plurality of cube corner PlPmpnts are bounded in the substrate by a plurality of grooves from a plurality of groove sets. The composite sheetine comprises at least one zone with at least one geometric structure side surface having both an optical portion and a non-optical portion.

WO 95/11470 ; . PCT/US94/12016 ~7322~
The m~mlf~tllre;of l~hu,cnective cube corner el~mçnt micro-cube arrays is accompli~hçd using molds made by different techniques, including those known as pin bundling and direct m~chining. Molds m~nuf~ctllred using pin bundling are made by assembling togPth~sr individual S pins which each have an end portion shaped with fcdtules of a cube cornero~eflective ~l~m~ont Fy~mples of pin bundling include U.S. Patent No.

3,926,402 (Heenan et al.) and United Kingdom Patent Nos. 423,464 and 441,319 (Leray).
The direct m~chining technique, also known generally as ruling, comprises cutting portions of a substrate to create a pattern of grooves which intersect to form cube corner elem~ont~. The grooved substrate is lefer,~ to as a master from which a series of impressions, i.e., replicas, may be formed. In some inct~nces, the master is useful as a retroreflective article, however, replicas, including multigenerational replicas, are more commonly used as a retroreflective article. Direct m~chininE is an çl~ççllçnt method for mam~fartur1ng 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 m~nuf~rtllring. The process of m~ f~tllring large arrays is also relatively - 20 easier using direct m~chining metho-l~ rather than other techniques. Examples of direct rn~rhinin~ are shown in U.S. Patent No. 4,588,258 (Hoopman) and U.S. Patent No. 3,712,706 (Stamm), which disclose single or multiple passes of a m~chine tool having two opposing cutting surfaces for cutting grooves to form cube corner optical faces in a substrate. An example of direct m~.^hinjn~
involving only two sets of grooves is shown in U.S. Patent 4,895,428 (Nelson et al.).
Figure 1 di~closes one embo-~imçnt of a retrùl~nective cube corner el~ment array 12 manufactured from a directly m~hin~hle substrate 13 by use of at least three groove sets each comprising a plurality of parallel non-overlapping grooves. Preferably, sçoond~ry groove sets collsisting of evenly spaced secondary grooves 14, 16, are arranged in non-parallel relation, and a pli~Jal,~ groove set cQ~ t~ of a plurality of parallel evenly spaced primary WO95/11470 ~73~ PCTIUS94/12016 ~

grooves 20 centered between sP~conrl~ry groove intG~ ions 22. An alternate embodiment groove spacing comprises varied rather than evenly spaced grooves. In the embodiment ~ close~ in Figure 1, a plurality of raised ~i~ontinuous geometric structures inClu~ling rel.orenective cube corner S el~mPnt~ are formed. In this Figure the intersectio~ of the grooves within twogroove sets are not coincident with at least one groove in a third groove set.
Also, the separation between the intPrse~tions of the grooves within two groove sets with at least one groove in a third groove set is preferably greater than about 0.01 millimPters. All of these geometric structures are similar to cube corner elementc 24, 26, and 30. Figure 1 illustrates a multiple structure array in which the cube corner el~mPnt~ are shown formed from primary and se~o~d~ry grooves with a uniform depth of cut. The grooves intersect with included angles of 60-.
Figure 2 is a cross section elevation view taken along lines 2-2 of Figure 1. Figure 2 illustrates the dirr~ rence in heights of cube peaks 34, 36, and 38 col-~sponding to cube corner elPmPnt~ 24, 26, and 30. Cube peak 38 illustrates a very high point of the directly m~hined substrate relative to all other surfaces. In addition, formation of the structure ~iepictPd in Figure 1 and Figure 2 results in vertical surfaces 41 which create difficulties during 20 procescing of arrays of this type. Vertical surfaces contribute to interloc~ing Of mating faces during replication of these arrays, which in turn results in labor inefficiçncies, material waste, and slow down of manufacturing.
For these arrays, optical p~lr~,-,l,al1ce is conveniently defined by the percent of the surface area that is actually rctlurenective~ i.e., which 25 comprises an effective area or active apelLu~e. The percent active ape"ure varies as a function of the amount of canting, ~f,~cli~,re index, and the e~ n angle. The structure of array 12 shown in Figure 1 and Figure 2 ~lemo~ctr~tes an exceptional approxim~t~ly 91% active aperture, as schPrn~tic~lly shown in the percent active aperture depiction of Figure 3. Figure 3 also depicts multiple 30 active aperture sizes which result when using the geometric structures and method of mannf~t~t~lnng described above. In particular, differently sized a~.~ures 47, 49, and 53, are intermixed and arranged in close proximity, and wo 9~/11470 2 ~ 7 ~ ~ 2 9 PCT/US94/12016 co"~ond to the different types of retroreflective cube corner Plempnt~ 24, 26, and 30 shown in Figure 1. Array 12 is quite useful in applications re~uiring high bri~htnç~ at zero or low entrance angles such as photoelectric sensors, traffic control materials, dir~l;ol-~l re~Pctors, and leLlu~flective markings.
S Pigure 4 discloses r~t.ur~llective cube corner elemPnt array 56 formed using multiple groove sets in similar manner to that shown and described above in relation to Figure 1 to create retroreflective cube comer ehPmP~nt~ 24, 76, and 77. However, array 56 is formed by m~chining each of the grooves 94, 95, 96 with a 3- relief angle. As shown in Figure 5, this reliefangle results in a less vertical orientation of surface 62 as co.l.pal~d with surface 41, shown in Figure 2. This less vertical oriPnt~tion of surface 62 enhances ease of m~mlf~tllring and permits considerable improvements during the replication process of array 56.
Use of a relief angle also results in a reduction in percent active a~ Lure coll~ 5po~ ing to such arrays. As shown in Figure 6, array 56 comprises multiple differently sized and shaped apertures 47, 79, and 83. As shown in Figure 3, the ap~lures depicted in Figure 6 are also intermixed and arranged in close proximity to provide relatively high brightnP~ at low entr~neeangles. However, the m~timum percent active a~e-lule of array 56 is reduced to only about 84% due to the use of relief angles elimin~ting some optical surface area. Increased relief may be utilized to further enh~nce ease of m~n~ tnring and replication, but it also results in additional re~uction in m~imum 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 elPmPn~c.
Figure 7 ~i~closes yet another embo limPnt of a retroreflective cube corner elem~Pnt array 88 manufactured in similar manner to array 1~ and array 56 with a plurality of secont~ry and primary grooves. Single or multiple passes of a m~hine tool may be used to prûduce the shape of the grooves 30 which form geometric structure side surfaces which may include cube corner el~PmPnt optical surfaces. Final grooves form all the geometric structure side surfaces and may be comprised of one or more grooves. Directly m~rhinP~

wo 95/11470 ~ 2 ~ 9 PCT/US94/12016 array 88 is subst~ti~lly identic~lly formed as array 56, with the exception of further adjusting the height of at least one of the structures formed in the array.
This is accomplished in one of several different possible manners. One embo~iimpnt comprises m~hining a plurality of groove sets to produce a S plurality of geometric structures in~ in~ cube corner çlPmPntc, and m~chinin at least one of the grooves in each of at least two of the groove sets along o~.l~ping or partially ovell~ing paths in the substr~te but at different depths of groove. Another embo~liment compri~P~ creating a plurality of geometric structures incl~l~ing cube corner el~m~ntC by directly m~hining at least two - 10 sets of parallel grooves in the substrate so that groove m~chining forms a final groove with at least one geG.,l~l.ic structure side surface having both an optical portion and a non-optical portion. In this context, "optical portion" refers to a surface which is actually retroreflective at some entrance angle. Preferably, these portions intersect along an axis that is parallel to the axis of the groove(s) 15 which form the geometric structure side surface. This may be accomplished using a novel m~ ine tool to form the final groove using only two groove sets, or by simply using more than two groove sets to form the final groove, as described below.
For eY~mple, primary groove 94 shown originally in Figure 5 20 and also partially in Figure 8, is m~ inP~ into substrate 13. Then, in a subsequent processing step, an a~prop.iate machine tool forming a subsequent groove 96 is passed through the substrate in an overlapping or partially - ovellapying manner to the primary groove path or subst~nti~lly parallel to primary groove 94 at a depth sufficiPnt to reduce the height of cube corner 25 el~mPnt 76 (Figure 5) but not to a depth which would cut optical surfaces of other previously formed cube corner elPment~ such as el~-..e.-t- 24 and 77. It is .ecogni,P~ that in this subsequent ~,oce~;.\g step, which compri~es a subsequent groove set, a groove 96 is formed in partial overlap of groove 94.
Groove 96 is likely to be formed only by cutting substrate sl-rf~es on one side 30 of previous groove 94. The included angle of groove 96 may be of any value although it should preferably not cut surfaces of adjacent cube corner PlPmPnt~.This results in final groove 97, shown in Figure 8 in side view, which is the wo95/11470 ~ 3 2 2 9 PCT/US94/12016 product of the m~hining operations which form the final surfaces of geometric structures along the groove. Similar ~ ition~l m~hininE operations may be pelro,-l-ed on the ~ races along the se~o~ ry groove sets. As shown in Figure 8, the height Hl of the cube corner elemPnt cle.picted as cube comer elem~nt 76 5 in Figure 5 and now formed in a new shape as cube corner element 99 with a peak 101 is less than height H2 of cube corner element 76. Final groove 97 forms at least one ge~mellic structure side surface 98 which has both an opticalportion and a non-optical portion, i.e., a surface that is not one of the three s.,bsl;~..t;~lly orthogonal surfaces which form a cube corner.
Adjuctment of the height of at least one of the cube corner el~m~ontc by use of direct m~rhining techniques provides subst~nti~ C!ce~>;ng advantages, and improves me~h~nic-~l and optical l,e.Ço,lnallce. A lower height eases the separation of replir~t~S from master arrays during the replication process. Indeed, the replication quality is also greatly improved with a thinner, 15 height reduced array. Reduction of the height also generally results in an overall thinner construction array 88 than those described above in relation to Figures 1-6. This enh~ncPs the ease of m~nl-f~r-t--ring, p,oce~;ng, and h~n~ling. In ~irlitio~, a thinner array, yet one which comprises substantial optical advantages over known arrays, is advantageous in order to reduce the 20 effect of vignPtting~ which otherwise reduces the amount of light against theoptical retroreflective surfaces of the array due to the ch~nnel effect of a very long (e.g., high) structure through which the light must travel.
Another advantage of adjusting the height using the novel cutting metho~c described above is the increased percent active aperture of re~u1tinE~
25 arrays, particularly for arrays using non-zero relief angles. These arrays may exhibit up to about a maximum of 91% active apc.Lil,c, although this value is reduced when a relief angle is used, for example, as shown in Figure 8. With the relief angle, array 88 has a maximum percent ape.Lur~ of about 88%.
Increased relief angles could decrease the percent active apelLu,G at zero 30 entrance angle, although it is possible to m~in~in at least about 70% or greater active aperture using these novel cutting methods. Figure 9 shows the active al)e,Lu,~_s, viewed at æro entrance angle, of array 88. The percent active wo 95/11470 ~ ~ 7 3 2 2 9 PCT/US94/12016 ~

ape.lul~ of array 88 is l~fcse.lled by multiple differently sized and shaped apcllules 47, 79, and 106. These apertures COll spond to cube corner Plem~Pntc 24, 77, and 99. Array 88 may be used in a variety of applications, and is particularly useful for those applications l~uiling high brightnPcc and improvedS ,.~P~h~ l flexibility.
Ml-ltigPnPrational repli~tion of cube corner elçment master arrays is greatly enh~n~ed by use of arrays without vertical s~lrf~es and without deep grooves or high geo,llellic structures. Shorter structures simplifythe task of separating a replicate from a master without ~l~m~ging the optical 10 surfaces. Shorter structures also result in less -.e~h~niç~l interlocking between a replicate and a master. Shorter structures are also less likely to have entrapped bubbles belween a replicate and a master, may be processed at lower te..-p~At~es than higher structures of the same material, and are compatible with higher speed processing. It is recognized that the highest structures referred to may comprise either cube corner elPmPntC or other geometric structures, and that substantial advantages during l)rocPc~;ng occur when the height of the highest structure is reduced to at least about the height of the next highest structure(s). This, of course, recognizes that the plurality of geometric structures may comprise one or more different geometric structures.
Array 12, array 56, and array 88 are examples of cube corner ~lPmPnt retroreflective arrays which comprise non-canted cubes having individual symmetry axes that are perpendicular to a base plane 110. The symmetry axis is a central or optical axis which is a trisector of the internal or dihe~r~l angles defined by the faces of the elPment However, in some practical applications it is advantageous to cant or tilt the symmetry axes of the cube corner reLr~,rellective Plement~ to an orientation which is not perpen~ic~ r to the base plane. The reS~ltin~ canted cube corner el~Prn~Pnt~ combine to produce an array which retroreflects over a different range of entrance angles.
Figure 10 discloses a canted retlul~nective cube corner elPmP,nt array 116 which comprises a plurality of cube corner elPments each formed from primary and secQ~ ry grooves intersecting with includ~P~ angles 58.5 -58.5 -63 .
Each of the primary grooves 118 and each of the secondary grooves 117, 119, W O 9S/11470 ~ 1 7 3 2 ~ 9 PCTrUS94/12016 are evenly spaced and have a 3 relief angle. Array 116 has all of the advantages of array 88, but it also exhibits peak brightne~s at a non-zero entrance angle. This is particularly useful in applications such as highway signage in which a non-zero entrance angle is most likely to occur. The primary S grooves 118 are centered between seco~ary groove intersection~ 120.
- Figure 11 ~ clos~Ps the percent active aperture of array 116 at zero er,ll~nce angle. Array 116 comprises multiple differently shaped and sized active apertures 122, 125, and 129 co,lcsponding to r~trorenective cube corner ~lernent~ 131, 133, and 137. Figure 11 illustrates the reduction at 0 entrance 10 angle of percent active ~ellule, as shown by the size of the non-active zones 141, caused by the canting of array 116.
Figure 12 discloses percent active apellur~; versus entrance angle for arrays with a refractive index of 1.59 and entrance angles of 0 to + 20-for non-canted arrays. Curve lSl discloses the percent active aperture for a conventional 60--60--60- array, such as described in U. S. Patent No.
3,712,706 to Stamm. Curve 153 ~i~çlosPs the percent active ape,lu,~ for the 60--60--60- asymmetric array 12 of Figure 1. Curve 155 ~ closes the percent active a,ue,lure for 60--60--60- height-adjusted 3- relief array 88 of Figure 7,and curve lS9 ~ closes the percent active aperture for 60--60--60- non-height 20 adjusted 3- relief array 56 of Figure 4.
Figure 12 i~ t~ s a high bri~htnP~ array having a m~imum percent active aperture of about 91%, shown by curve 153, which is achieved in an array formed with grooves having no relief angle. An array with a relief angle improves processability but it also results in a relative reductiorl of 25 percent active ap~lL-I~ as depicted by curve 159. This reduction in percent active ap~l~ule is caused by using grooves with a relief angle without also incorpor~ting any height adjustm~nt to the highest sLluctule(s) in the array.
However, as shown in curve 155, improved brightnP~c and ~n,ces~ability is possible by providing groove relief angles and by reduçing the height of the 30 highest structures within the array. These nl~nufaçhlring techniques yield ~ignific~nt increase in percent active aperture in a range of between about -15-to about 20- entrance angle for the arrays di~closed above. ~riition~l wo 95/11470 ~ ~ 7 3 ~ 2 9 PCT/US94/12016 ~,loces~ to reduce the height of the highest structure within an array may be used with arrays having a wide range of relief angles, including zero.
Figure 13 also dicsloses percent active ape,lure versus entrance angle for arrays with a refractive index of 1.59. Curve 155 riicsloses the percent active apGllure for the 60--60--60- asymmPtric~ 3- relief angle array 88shown in Figure 7, which insl~ldes a height adjusted groove set, or a controlleddepth of cut groove set, either of which pr~duces the novel geometric structure side surface having at least one optical portion and at least one non-optical portion. Curve 163 diccl(-ses the percent active apellul~ for a canted array having letrolenective cube corner elemPntc formed by grooves having insll~de~
angles of 58.5 -58.5 -63 , cGll~ ~onding to array 116 shown in Figure 10. As shown in Figure 13, curve 163 has subst~nsi~lly i~lPntis~l features to curve 155except that it achieves peak brightnPss at a non-zero entrance angle. Both curves 155 and 163 exhibit asymmetric entrance angularity when rotated about an axis within the plane of the substrate. Other amounts of canting may be used advantageously to control the entrance angularity ~Ccoci~tp.d with the peak percent active aptllule.
Total light return for retroreflective shP~ting is derived from the product of percent active al)ellu~ and rellu~ ted light ray intensity. For some combinations of cube geometries, entrance angles, and refractive index, cignific~nt reductions in ray intensity may result in relatively poor total light return even though percent active aperture is relatively high. One example is retroreflective cube corner elemPnt arrays which rely on total internal reflection of the lellulc;nected light rays. Ray intensity is subst~nti~lly reduced if the critical angle for total internal reflection is eYce~de~ at one of the cube faces.
~fPt~lli7e~ or other reflective co~tingc on a portion of an array may be utili_ed advantageously in such sit--~tionc. For example, a particular portion of an array which has cube surfaces cont~sting a sealing mPAhlm will often be more reflective when the surfaces have a reflective coating. Alternately, a portion may comprise an entire array.
As shown above, l~lloreflective directly m~hinecl cube corner articles are often deci~nP~ to receive a sealing film which is applied to the WO 9S111470 21~7 3 2 2 9 - PCT/US94/12016 retn,.~llective article in order to m~int~in a low .erl~c~ e index material, such as air, next to the cllu~ ective el~mentc for improved pelro~l,lance. In convention~l arrays this ~ J;~ is often placed in direct contact with the cube corner element~ in ways which degrade total light return. However, as shown in Figure 14, a sealing medium 175 may be placed on the highest s~ll~ces 181 of an array without collt~cting and clegrading the optical ~rvl)clLies of lower r~rùlcllective cube corner elemPntc~ such as el~mPntc 24 and 99. The highest surfaces may comprise cube corner elementC, non-lcllurenective pyramids, frustums, posts, or other structures. Although slight height v~ri~tiQnc may result from slight non-uniformity of groove positions or included angle of cube corner elemPntc due to m~- hining tolerances or intentional inducement of non-orthogonality, these v~ri~tions are not analogous to the v~ri~tiolls ~licclose~ and taught in this invention. When using a sealing me~ m, the highest surface may be shaped, for example as shown by surface 191 in Figure 15, to both hold the sealing my1illm and to increase the light tr~ncmicsivity of the ~heeting. Light tr~nsmiC~ivity of the s~le~ting may be increased through use of a transparent orpartially transparent sealing rnedillm.
It is also recognized that reduction of height of the highest structures has a dramatic effect on reducing flexural rigidity particularly for cube chP~ting utili7-ing a sealing mPAillm. Even a moderate reduction in thicl~ne~ of a she~ting has a significant effect on rigidity since flexural rigidity is plu~ lional to the cube of the thicknecs for a sheet in bending. For example,- - a 20% reduction in overall thi-kne~c will result in roughly a 50% decrease in fl~oYIl~l rigidity.
Figure lS is a schem~tic side view of another embodiment of the invention. This view shows part of an array 200 similar to a portion of array 88shown in Figure 14 but inclu(lin~ the use of a separation surface 206. The side surfaces 210, 213 of geometric structures 218, 219 form the boundary edges 221, 223 for the separation surface. The side surfaces may include cube corner elem~nt optical surfaces as well as non-optical surfaces on cube corner and other geometric structures. Separation surface 206 may have flat or curved portions when viewed in cross section. Sep~tion s,~l~ces may be WO 95/11470 ~ ' PCTJUS94/12016 advantageously utilized to in~;,case light tr~nsmiccion or tr~nS~ n~;y in ShPeting, inr,lllrling flexible shPeting, utili7ing the array structures tlicrlosed above. Separation surface 206 may be formed using a m~hin~ tool with a flat or curved tip, or by further removal of m~t~ri~l from a replica of the array 5 master. This construction is particularly useful in appliç~tionc such as int~ lly illl-...;n~t~ signs and raised pavement ll-~k~
Suitable m~teri~lc for lelrorenective articles or shçeting of this invention are preferably transparent materials which are riim~ncion~lly stable, durable, weatherable, and easily repli~t~d into the desired configuration.
10 FY~mrlçs of suitable materials include glass; acrylics, which have an index of refraction of about 1.5, such as PLEXIGLAS Brand resin m~mlf~ctured by Rohm and Haas Company; polycarbonates, which have an index of refraction of about 1.59; reactive materials such as taught in United States Patents Nos.

4,576,850, 4,582,885, and 4,668,558; polyethylene based ionomers, such as 15 those Illarkeled under the brand name of SURLYN by E. I. Dupont de Nemours and Co., Inc.; polyesters, polyu,e~ Ps; and cellulose acetate butyrates. Polyc~l~ollates are particularly suitable because of their toughn~sc and relatively higher refractive index, which generally contributes to improved lelro~nective pe,ro,mance over a wider range of entrance angles. These 20 m~t~ni~lc may also include dyes, colorants, pigm~ntc, UV stabilizers, or other additives. Transparency of the materials ensures that the separation or other shaped surfaces will t~ncmit light through those portions of the article or ch~eting.
The inco~o~dlion of either truncated and/or separation surfaces 25 does not elimin~te the ~elroreflectivity of the article, but rather it renders the entire article partially transparent. In some applications requiling partially transparent materials, low indices of refraction of the article will improve therange of light tr~ncmitted through the article. In these applications, the increased tr~ncmiCcit~n range of acrylics (refractive index of about 1.5) is 30 desirable. In fully r~tlurellective articles, m~t~ri~ls having high indices of refraction are p,efe,led. In these appli~tionc, materials such as polycarbonates, with refractive indices of about 1.59, are used to increase the difference wo 95111470 ~17 3 2 ~ 9 PCT/US94/12016 between the indices of the material and air, thus increasing let,.,lcnection.
Polyc~l,onates are also generally ~ fe.led for their ~e~l~pel~lu~ stability and impact resict~nce.
Directly m~chined arrays according to the invention are formed

5 by adjusting the height of at least one of the structures in the array. As - described above, one technique for m~nUf~rh~ring such arrays comprises creating a plurality of geometric structures inClu~ing cube corner el~mPntc by directly m~hining at least two sets of parallel grooves in the substrate so thatgroove ."acllining forms at least one geometric structure side surface having both an optical portion and a non-optical portion. This maçhininE may be accomplished using a novel m~t hin~ tool having groove cutting means for simultaneous cutting of a plurality of different geo"~el,ic structure surfaces forming multiple side surfaces on at least one side of a final groove. One eY~mp1e of this type of tool is shown in Figure 16 in which tool 230 comprises groove cutting means having a first cutting surface 235, a second cutting surface 237, and a third cutting surface 239. In this embo liment~ first cuttingsurface 235 and second cutting surface 237 are configured to form at least one geometric structure side surface having both an optical portion and a non-optical portion.
Other emboflimentc of this invention include creation of an article, or replicas of the article, which further modify the shape of the retrorefle~t~ light pattern. These embo limPnt~ comprise at least one groove side angle in at least one set of grooves which differs from the angle n~$~.y to produce an ollhogonal inle.~lion with other faces of elemPntc defined by the groove sides. Similarly, at least one set of grooves may comprise a l.pe~ g pattern of at least two groove side angles that differ from one another.Shapes of grooving tools, or other techniques, may create cube corner elementc in which at least a significant portion of at least one cube corner el~ment optical face on at least some of the cubes are arcuate. The arcuate face may be concave or convex. The arcuate face, which was initially formed by one of the grooves in one of the groove sets, is flat in a direction subst~nti~lly parallel to said groove. The arcuate face may be cylin~lril~l, with the axis of the cylinder WO 95/11470 ~ ~ PCT/US94/lZ016 ~
2 2 ~
parallel to said groove, or may have a varying radius of curvature in a direction perpen~icul~r to said groove.
Co",pG~ite tiling is the technique for combining zones of cube corner elçm~ntc having different oriçnt~tionC. This is used, for example, with S convention~l arrays to provide che~ting with a uniform a~pe~.i.nce at high angles of incidence regardless of oriçnt~tion. Referring to Figure 17, co",posile array 244 comprises several zones of arrays 88. Compocite arrays may comprise adjacent zones of direct m~ hin~ cube corner ,~h("cnecting el~oment arrays inclu~ling either converltion~l or height adjusted arrays having different configurations, or arrays with at least one geon.~ll;c structure side surface having both an optical and a non-optical portion. The size of the zones should be selected according to the requirements of particular appliç~tiQnc. For eY~mple, traffic control applications may require zones which are snfficiently small that they are not visually resolvable to the un~ide~ human eye at the minimum eYpected viewing llict~nce. This provides a co,n~site array with a uniform appearance. Alternatively, c~nnçl m~rking or direction~l reflector applit~tionc may require zones which are sufficiently large that they can be easily resolved by the unaided human eye at maximum required viewing ~ict~nce Figure 18 discloses array 254 which is similar to array 88 in Figure 7 but with variable groove spacing. Grooves 257, 258, and 259 are all in the same groove set. However, as shown in this portion of the array, the spacing of grooves within at least one of the groove sets in the array is variedso that the spacing between a first groove 257 and an ~ e-nt second groove 258 (L,) differs from the spacing between the second groove 258 and an çnt third groove 259 (L2).
The process of adjusting the height of geometric structures within a ,~;~o~nective cube corner Pl~m~ont optical array by either adjusting depth of cut or by conducting an additional height adjustment grooving step results in substantial advantages. These advantages include higher percentage active apcl~ur~ at various entrance angles, thinner construction of arrays, improved proceccing, replication, and h~n-lling of arrays, improved optical ~lro"nance 2~32~
of arrays, improved levels of t~n~ ,nc;f s of arrays, and improved flPYih~ y of arrays. It is ~ ized that the above processes may be accompliched using m~chin~ tools of various shapes.
Various modifi~tions and ~ innC of this invention will S become app~,nt to those skilled in the art without dep~ling from the scope and spirit of this invention.

Claims

NEW CLAIMS

1. A cube corner article comprising a substrate (13) having a base surface disposed in a base plane (110) and a structured surface opposite said base surface, said structured surface including an array of cube corner elements (24, 77, 99) formed by at least two intersecting sets of grooves in substrate (13), wherein:
at least one side surface (98) of at least one groove (97) has a first portion which is not a surface of a cube corner element and extends from the groove vertex at a first angle relative to base plane (110), and a second portion which is a surface of a cube corner element and extends from said first portion and forms a second angle with base plane (110) which is substantially less than the first angle formed by said first portion.

2. The cube corner article of claim 1, wherein:
said second portion forms a reflective surface of a cube corner element.

3. The cube corner article of Claim 1 or 2, wherein:
the intersections of the grooves within two groove sets are displaced from one groove in a third groove set by at least 0.01 millimeters.

4. The cube corner article of any of Claims 1 to 3, wherein:
the groove vertex of a first groove is disposed at a first distance above a reference plane (110) and the groove vertex of a second groove is disposed at a second distance, different from said first distance, above said reference plane (110).

5. The cube corner article of any of Claims 1 to 4, wherein:
at least one of the groove sets includes, in a repeating pattern, at least two groove side angles that differ from one another.

6. The cube corner article of any of Claims 1 to 5, wherein:
said substrate (13) comprises a substantially optically transparent material suitable for use in retroreflective sheeting.

7. The cube corner article of Claim 6, wherein:
a portion of said substrate (13) is coated with a specularly reflective material.

8. The cube corner article of Claims 6 or 7, wherein:
said article exhibits greater than 80 % active aperture in response to light incident on said base surface approximately perpendicular to said base plane (110).

9. The cube corner article of Claims 6 or 7, wherein:
said article exhibits greater than 90 % active aperture in response to light incident on said base surface approximately perpendicular to said base plane (110).

10. The cube corner article of Claim 6 or 7, wherein:
said article exhibits the maximum brightness in response to light incident on said base surface at an oblique angle with respect to base plane (110).

11. The cube corner article of any of Claims l to 10, wherein:
said article exhibits asymmetric entrance angularity when rotated about an axis within the plane of said substrate (13).

12. The cube corner article of any of Claims 1 to 11, wherein:
at least one surface of at least some of the cube corner elements is arcuate over a significant portion of said surface.

13. The cube corner article of Claim 12, wherein:
the shape of said arcuate surface is substantially cylindrical so that the axis of the cylinder is approximately parallel to the groove which bounds said arcuate surface.

14. The cube corner article of any of Claims 1 to 13, wherein:
at least two geometric structures of said array of cube corner elements (24, 77, 99) have different heights above said base plane (110).

15. The cube corner element of claim 14, wherein:
a plurality of said geometric structures provide support for a sealing medium (175).

16. A composite cube corner article comprising a substrate (13) having a base surface disposed in a base plane (110) and a structured surface opposite said base surface, said structured surface including:
a first array of cube corner elements formed by at least two intersecting sets of grooves in substrate (13); and a second array of cube corner elements formed by at least two intersecting sets of grooves in substrate (13), said second array being disposed at a different orientation on substrate (13) than said first array, wherein:
at least one side surface (98) of at least one groove (97) in said first array has a first portion which is not a surface of a cube corner element and extends from the groove vertex at a first angle relative to base plane (110), and a second portion which is a surface of a cube corner element and extends from said first portion and forms a second angle with base plane (110) which is substantially less than the first angle formed by said first portion.

A method of manufacturing a cube corner article comprising the steps of:
providing a machinable substrate (13);
machining at least two intersecting groove sets in said substrate (13) to form an array of cube corner elements (24, 77, 99) on one surface of substrate (13); wherein:
said step of machining said groove sets comprises the step of machining, in at least one groove in at least one groove set an additional groove (96) to form a final groove (97) having on one side thereof a compound cube corner element side surface (98) having a first portion which is not a surface of a cube corner element and extends from the groove vertex at a first angle relative to base plane (110), and a second portion which is a surface of a cube corner element and extends from said first portion and forms a second angle with base plane (110) which is substantially less than the first angle formed by said first portion.

18. The method of Claim 17, wherein:
said final groove (97) is formed using a machining tool (230) having a compound cutting surface including a first cutting surface (235) and a second cutting surface (237).

19. The method of Claim 17 or 18, wherein:
said first portion and said second portion intersect along an axis that is parallel to the axis of the groove forming said compound cube corner element side surface (98).

20. A cube corner article manufactured by the method of any of Claims 17 to 19.

21. A cube corner article which is a replica of the article of Claim 20.