U.S. patent application number 09/764477 was filed with the patent office on 2001-07-26 for retroreflective cube corner sheeting mold and method for making the same.
This patent application is currently assigned to 3M Innovative Properties Company. Invention is credited to Benson, Gerald M..
Application Number | 20010009475 09/764477 |
Document ID | / |
Family ID | 25390408 |
Filed Date | 2001-07-26 |
United States Patent
Application |
20010009475 |
Kind Code |
A1 |
Benson, Gerald M. |
July 26, 2001 |
Retroreflective cube corner sheeting mold and method for making the
same
Abstract
A method is disclosed for manufacturing a plurality of laminae
for use in a mold suitable for use in forming retroreflective cube
corner articles. Each lamina has opposing first and second major
surfaces defining therebetween a first reference plane. Each lamina
further includes a working surface connecting the first and second
major surfaces. The working surface defines a second reference
plane substantially parallel to the working surface and
perpendicular to the first reference plane and a third reference
plane perpendicular to the first reference plane and the second
reference plane. The method includes orienting a plurality of
laminae to have their respective first reference planes parallel to
each other and disposed at a first angle relative to a fixed
reference axis. At least two groove sets are formed in the working
surface. Each groove set includes at least two parallel adjacent
V-shaped grooves in the working surface of the laminae. The at
least two groove sets form first, second and third groove surfaces
that intersect substantially orthogonally to form a plurality of
cube corner elements. Each of the plurality of cube corner elements
is preferably located on essentially one of the plurality of
lamina. The plurality of laminae can be oriented at a second angle
relative to the fixed reference axis prior to forming at least one
of the groove sets. A mold according to the present invention and a
retroreflective article made therefrom are also disclosed.
Inventors: |
Benson, Gerald M.;
(Woodbury, MN) |
Correspondence
Address: |
Office of Intellectual Property Counsel
3M Innovative Properties Company
PO Box 33427
St. Paul
MN
55133-3427
US
|
Assignee: |
3M Innovative Properties
Company
|
Family ID: |
25390408 |
Appl. No.: |
09/764477 |
Filed: |
January 17, 2001 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
09764477 |
Jan 17, 2001 |
|
|
|
08887074 |
Jul 2, 1997 |
|
|
|
Current U.S.
Class: |
359/530 ;
359/529 |
Current CPC
Class: |
Y10T 29/49995 20150115;
Y10T 29/49984 20150115; B29D 11/00625 20130101; Y10T 409/303752
20150115 |
Class at
Publication: |
359/530 ;
359/529 |
International
Class: |
G02B 005/122 |
Claims
What is claimed is:
1. An article comprising an array of nonrulable cube corner
elements, the article comprising a plurality of laminae, each such
lamina having opposed parallel major surfaces and a working surface
connecting the major surfaces, the array having rows of rectangular
cube corner elements formed along the working surfaces of the
laminae.
2. An article comprising a plurality of laminae, each lamina having
opposed parallel major surfaces and a working surface connecting
the major surfaces, the working surface of each lamina having an
inclined surface extending along the working surface and a set of
parallel grooves defining groove surfaces orthogonal to each other
and to the inclined surface so as to form a row of rectangular cube
corner elements.
3. A lamina having opposed parallel major surfaces and a working
surface therebetween, the working surface having an inclined
surface extending therealong and a set of parallel grooves defining
groove surfaces orthogonal to each other and to the inclined
surface so as to form a row of rectangular cube corner
elements.
4. A plurality of laminae as set forth in claim 3, the plurality of
laminae defining in the working surfaces thereof a nonrulable array
of cube corner elements.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This is a continuation of pending U.S. patent application
Ser. No. 08/887,074, filed Jul. 2, 1997.
FIELD OF THE INVENTION
[0002] The present invention relates generally to molds suitable
for use in forming cube corner retroreflective sheeting, to methods
for making the same, and to retroreflective sheeting formed from
such molds. In particular, the invention relates to molds formed
from a plurality of thin laminae and to methods for making the
same.
BACKGROUND OF THE INVENTION
[0003] Retroreflective materials are characterized by the ability
to redirect light incident on the material back toward the
originating light source. This property has led to the wide-spread
use of retroreflective sheeting in a variety of conspicuity
applications. Retroreflective sheeting is frequently applied to
flat, rigid articles such as, for example, road signs and
barricades; however, it is also used on irregular or flexible
surfaces. For example, retroreflective sheeting can be adhered to
the side of a truck trailer, which requires the sheeting to pass
over corrugations and protruding rivets, or the sheeting can be
adhered to a flexible body portion such as a road worker's safety
vest or other such safety garment. In situations where the
underlying surface is irregular or flexible, the retroreflective
sheeting desirably possesses the ability to conform to the
underlying surface without sacrificing retroreflective performance.
Additionally, retroreflective sheeting is frequently packaged and
shipped in roll form, thus requiring the sheeting to be
sufficiently flexible to be rolled around a core.
[0004] Two known types of retroreflective sheeting are
microsphere-based sheeting and cube corner sheeting.
Microsphere-based sheeting, sometimes referred to as "beaded"
sheeting, employs a multitude of microspheres typically at least
partially embedded in a binder layer and having associated specular
or diffuse reflecting materials (e.g., pigment particles, metal
flakes or vapor coats, etc.) to retroreflect incident light.
Illustrative examples are disclosed in U.S. Pat. No. 3,190,178
(McKenzie), U.S. Pat. No. 4,025,159 (McGrath), and U.S. Pat. No.
5,066,098 (Kult). Advantageously, microsphere-based sheeting can
generally be adhered to corrugated or flexible surfaces. Also, due
to the symmetry of beaded retroreflectors, microsphere based
sheeting exhibits a relatively orientationally uniform total light
return when rotated about an axis normal to the surface of the
sheeting. Thus, such microsphere-based sheeting has a relatively
low sensitivity to the orientation at which the sheeting is placed
on a surface. In general, however, such sheeting has a lower
retroreflective efficiency than cube corner sheeting.
[0005] Cube corner retroreflective sheeting comprises a body
portion typically having a substantially planar base surface and a
structured surface comprising a plurality of cube corner elements
opposite the base surface. Each cube-corner element comprises three
mutually substantially perpendicular optical faces that intersect
at a single reference point, or apex. The base of the cube corner
element acts as an aperture through which light is transmitted into
the cube corner element. In use, light incident on the base surface
of the sheeting is refracted at the base surface of the sheeting,
transmitted through the bases of the cube corner elements disposed
on the sheeting, reflected from each of the three perpendicular
cube-corner optical faces, and redirected toward the light source.
The symmetry axis, also called the optical axis, of a cube corner
element is the axis that extends through the cube corner apex and
forms an equal angle with the three optical faces of the cube
corner element. Cube corner elements typically exhibit the highest
optical efficiency in response to light incident on the base of the
element roughly along the optical axis. The amount of light
retroreflected by a cube corner retroreflector drops as the
incidence angle deviates from the optical axis.
[0006] The maximum retroreflective efficiency of cube corner
retroreflective sheeting is a function of the geometry of the cube
corner elements on the structured surface of the sheeting. The
terms `active area` and `effective aperture` are used in the cube
corner arts to characterize the portion of a cube corner element
that retroreflects light incident on the base of the element. A
detailed teaching regarding the determination of the active
aperture for a cube corner element design is beyond the scope of
the present disclosure. One procedure for determining the effective
aperture of a cube corner geometry is presented in Eckhardt,
Applied Optics, v. 10, n. Jul. 7, 1971 pp. 1559-1566. U.S. Pat. No.
835,648 to Straubel also discusses the concept of effective
aperture. At a given incidence angle, the active area can be
determined by the topological intersection of the projection of the
three cube corner faces onto a plane normal to the refracted
incident light with the projection of the image surfaces for the
third reflections onto the same plane. The term `percent active
area` is then defined as the active area divided by the total area
of the projection of the cube corner faces. The retroreflective
efficiency of retroreflective sheeting correlates directly to the
percentage active area of the cube corner elements on the
sheeting.
[0007] Predicted total light return (TLR) for a cube corner matched
pair array can be calculated from a knowledge of percent active
area and ray intensity. Ray intensity may be reduced by front
surface losses and by reflection from each of the three cube corner
surfaces for a retroreflected ray. Total light return is defined as
the product of percent active area and ray intensity, or a
percentage of the total incident light which is retroreflected. A
discussion of total light return for directly machined cube corner
arrays is presented in U.S. Pat. No. 3,712,706 (Stamm).
[0008] Additionally, the optical characteristics of the
retroreflection pattern of retroreflective sheeting are, in part, a
function of the geometry of the cube corner elements. Thus,
distortions in the geometry of the cube corner elements can cause
corresponding distortions in the optical characteristics of the
sheeting. To inhibit undesirable physical deformation, cube corner
elements of retroreflective sheeting are typically made from a
material having a relatively high elastic modulus sufficient to
inhibit the physical distortion of the cube corner elements during
flexing or elastomeric stretching of the sheeting. As discussed
above, it is frequently desirable that retroreflective sheeting be
sufficiently flexible to allow the sheeting to be adhered to a
substrate that is corrugated or that is itself flexible, or to
allow the retroreflective sheeting to be wound into a roll for
storage and shipping.
[0009] Cube corner retroreflective sheeting is manufactured by
first manufacturing a master mold that includes an image, either
negative or positive, of a desired cube corner element geometry.
The mold can be replicated using nickel electroplating, chemical
vapor deposition or physical vapor deposition to produce tooling
for forming cube corner retroreflective sheeting. U.S. Pat. No.
5,156,863 to Pricone, et al. provides an illustrative overview of a
process for forming tooling used in the manufacture of cube corner
retroreflective sheeting. Known methods for manufacturing the
master mold include pin-bundling techniques, direct machining
techniques, and laminate techniques. Each of these techniques has
benefits and limitations.
[0010] In pin bundling techniques, a plurality of pins, each having
a geometric shape on one end, are assembled together to form a
cube-corner retroreflective surface. U.S. Pat. No. 1,591,572
(Stimson), U.S. Pat. No. 3,926,402 (Heenan), U.S. Pat. No.
3,541,606 (Heenan et al.), and U.S. Pat. No. 3,632,695 to Howell
provide illustrative examples. Pin bundling techniques offer the
ability to manufacture a wide variety of cube corner geometries in
a single mold. However, pin bundling techniques are economically
and technically impractical for making small cube corner elements
(e.g. less than about 1.0 millimeters).
[0011] In direct machining techniques, a series of grooves are
formed in a unitary substrate to form a cube-corner retroreflective
surface. U.S. Pat. No. 3,712,706 to Stamm and U.S. Pat. No.
4,588,258 to Hoopman provide illustrative examples. Direct
machining techniques offer the ability to accurately machine very
small cube corner elements which are compatible with flexible
retroreflective sheeting. However, it is not presently possible to
produce certain cube corner geometries that have very high
effective apertures at low entrance angles using direct machining
techniques. By way of example, the maximum theoretical total light
return of the cube corner element geometry depicted in U.S. Pat.
No. 3,712,706 is approximately 67%.
[0012] In laminate techniques, a plurality of laminae, each lamina
having geometric shapes on one end, are assembled to form a
cube-corner retroreflective surface. German Provisional Publication
(OS) 19 17 292, International Publication Nos. WO 94/18581 (Bohn,
et al.), WO 97/04939 (Mimura et al.), and WO 97/04940 (Mimura et
al.), each disclose a molded reflector wherein a grooved surface is
formed on a plurality of plates. The plates are then tilted by a
certain angle and each second plate is shifted crosswise. This
process results in a plurality of cube corner elements, each
element formed by two machined surfaces on a first plate and one
side surface on a second plate. German Patent DE 42 36 799 to
Gubela discloses a method for producing a molding tool with a
cubical surface for the production of cube comers. An oblique
surface is ground or cut in a first direction over the entire
length of one edge of a band. A plurality of notches are then
formed in a second direction to form cube corner reflectors on the
band. Finally, a plurality of notches are formed vertically in the
sides of the band. German Provisional Patent 44 10 994 C2 to Gubela
is a related patent. The reflectors disclosed in Patent 44 10 994
C2 are characterized by the reflecting surfaces having concave
curvature.
BRIEF SUMMARY OF THE INVENTION
[0013] The present invention relates to a master mold suitable for
use in forming retroreflective sheeting from a plurality of laminae
and methods of making the same. Advantageously, master molds
manufactured according to methods disclosed herein enable the
manufacture of retroreflective cube corner sheeting that exhibits
retroreflective efficiency levels approaching 100%. To facilitate
the manufacture of flexible retroreflective sheeting, the disclosed
methods enable the manufacture of cube corner retroreflective
elements having a width as small as 0.010 millimeters.
Additionally, the present application enables the manufacture of a
cube corner retroreflective sheeting that exhibits symmetrical
retroreflective performance in at least two different orientations.
Efficient, cost-effective methods of making molds formed from a
plurality of laminae are also disclosed.
[0014] A plurality of laminae are machined simultaneously to form a
plurality of cube corner elements. The three mutually perpendicular
optical faces of each cube corner element are preferably formed on
one of the plurality of laminae. That is, individual or discrete
cube corner elements preferably do not extend across more than one
lamina. All three optical faces are preferably formed by the
machining process to ensure optical quality surfaces. A planar
interface is preferably maintained between adjacent laminae during
the machining phase and subsequent thereto so as to minimize
alignment problems and damage due to handling of the laminae.
[0015] A plurality of laminae are manufactured for use in a mold
suitable for use in forming retroreflective cube corner articles.
Each lamina has opposing first and second major surfaces defining
therebetween a first reference plane. Each lamina further includes
a working surface connecting the first and second major surfaces.
The working surface defines a second reference plane substantially
parallel to the working surface and perpendicular to the first
reference plane and a third reference plane perpendicular to the
first reference plane and the second reference plane. The method
includes orienting a plurality of laminae to have their respective
first reference planes parallel to each other and disposed at a
first angle relative to a fixed reference axis. At least two groove
sets are formed in the working surface. Each groove set includes at
least two parallel adjacent V-shaped grooves in the working surface
of the laminae. The at least two groove sets form first, second and
third groove surfaces that intersect substantially orthogonally to
form a plurality of cube corner elements. Each cube corner element
is preferably located on essentially one of the plurality of
lamina. The plurality of laminae can be oriented at a second angle
relative to the fixed reference axis prior to forming at least one
of the groove sets.
[0016] In one embodiment, the step of forming at least two groove
sets includes forming a first groove set including at least two
parallel adjacent V-shaped grooves in the working surface of each
of the laminae. Each of the adjacent grooves defines a first groove
surface and a second groove surface that intersect substantially
orthogonally to form a first reference edge on each of the
respective laminae. A second groove set is formed including at
least one groove in the working surfaces of the plurality of
laminae. Each groove in the second groove set defines a third
groove surface that intersects substantially orthogonally with the
first and second groove surfaces to form at least one first cube
corner element located on essentially a single lamina.
[0017] The first cube corner element preferably comprises a
plurality of cube corner elements. Each of the plurality of cube
corner elements are located on essentially one lamina. An interface
between adjacent first and second major surfaces is preferably
planar. Each lamina measures between about 0.025 millimeters and
about 1.0 millimeters in thickness, and more preferably from about
0.1 to about 0.6 millimeters.
[0018] The method includes the step of orienting the plurality of
laminae including assembling the laminae in a fixture defining a
base plane. The first angle measures between about 5.degree. and
about 85.degree. from a fixed reference axis normal to the base
plane, and more preferably between about 10.degree. and about
65.degree. and most preferably about 25.degree. to about
45.degree..
[0019] The step of forming the groove sets comprises forming at
least one of the groove sets parallel to the base plane defined by
the fixture. Alternatively, the groove sets can be formed at an
acute angle relative to the base plane defined by the fixture. The
groove sets can also be formed to vary the distance between
adjacent grooves at different depths in the working surface of the
laminae.
[0020] The process of forming the groove sets can comprise removing
portions of each of the plurality of lamina proximate the working
surface of the plurality of laminae using a material removal
technique. The first, second and third groove surfaces are formed
essentially from the material removal technique. The groove sets
can be formed by inducing relative motion between the plurality of
laminae and a cutting tool. The step of forming the groove sets
comprises a machining operation selected from the group of
machining operations consisting of ruling, fly-cutting, grinding,
and milling. The grooves preferably have an included angle that
measures between about 10.degree. and about 170.degree..
[0021] In one embodiment, the plurality of lamina can be oriented
to have their respective first reference planes parallel to each
other and disposed at a second angle relative to the fixed
reference axis prior to forming the second groove set. The step of
orienting the plurality of laminae to have their respective first
reference planes parallel to each other and disposed at a second
angle relative to the fixed reference axis comprises re-assembling
the plurality of lamina in a suitable fixture. In one embodiment,
the step of orienting the plurality of laminae to have their
respective first reference planes parallel to each other and
disposed at a second angle relative to the fixed reference axis
comprises rotating a plurality of the laminae 180.degree. about an
axis perpendicular to the second reference plane.
[0022] The cube corner elements are typically arranged in opposing
pairs. In an alternate embodiment, optical axes of the cube corner
elements can be generally parallel to provide an asymmetrical total
light return about a 360.degree. range of orientation angles.
[0023] Also disclosed is a method of replicating the working
surface of the mold to form a negative copy of the plurality of
cube corner elements suitable for use as a mold for forming
retroreflective articles, and a mold formed therefrom. A
retroreflective article can be formed from the mold forming the
negative copy.
BRIEF DESCRIPTION OF THE DRAWINGS
[0024] FIG. 1 is a perspective view of a single lamina suitable for
use in the disclosed methods.
[0025] FIG. 2 is a perspective view of a plurality of such
laminae.
[0026] FIG. 3 is an end view of the plurality of laminae oriented
in a first orientation.
[0027] FIG. 4 is an end view of the plurality of laminae following
a first machining operation.
[0028] FIG. 5 is a side view of the plurality of laminae following
a first machining operation.
[0029] FIG. 6 is an end view of the plurality of laminae depicted
in FIG. 5 oriented in a second orientation.
[0030] FIG. 7 is an end view of the plurality of laminae oriented
in a second orientation, wherein alternating lamina have been
rotated 180.degree..
[0031] FIG. 8 is an end view of the plurality of laminae following
a second machining operation.
[0032] FIG. 9 is a top view of the plurality of laminae following a
second machining operation.
[0033] FIG. 10 is an end view of the plurality of laminae oriented
in a first orientation.
[0034] FIG. 11 is an end view of the plurality of laminae following
a first machining operation.
[0035] FIG. 12 is a side view of the plurality of laminae following
a first machining operation.
[0036] FIG. 13 is an end view of the plurality of laminae oriented
in a second orientation.
[0037] FIG. 14 is an end view of the plurality of laminae following
a second machining operation.
[0038] FIG. 15 is a side view of the plurality of laminae following
a second machining operation.
[0039] FIG. 16 is an end view of the plurality of laminae following
a third machining operation.
[0040] FIG. 17 is a top view of the plurality of laminae following
a third machining operation.
[0041] FIG. 18 is a perspective view of a single lamina according
to the method of FIGS. 10-17.
[0042] FIG. 19 is an end view of the plurality of laminae oriented
in a first orientation.
[0043] FIG. 20 is an end view of the plurality of laminae following
a first machining operation.
[0044] FIG. 21 is a side view of the plurality of laminae following
a first machining operation.
[0045] FIG. 22 is an end view of the plurality of laminae oriented
in a second orientation.
[0046] FIG. 23 is an end view of the plurality of laminae following
a second machining operation.
[0047] FIG. 24 is a side view of the plurality of laminae following
a second machining operation.
[0048] FIG. 25 is a side view of the plurality of laminae following
a third machining operation.
[0049] FIG. 26 is a top view of the plurality of laminae following
a third machining operation.
[0050] FIG. 27 is a perspective view of a single lamina according
to the method of FIGS. 19-26.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0051] A plurality of laminae are machined simultaneously to form a
plurality of full cube corner elements. The three mutually
perpendicular optical faces of each cube corner element are
preferably formed on a single lamina. All three optical faces are
preferably formed by the machining process to ensure optical
quality surfaces. A planar interface is preferably maintained
between adjacent laminae during the machining phase and subsequent
thereto so as to minimize alignment problems and damage due to
handling of the laminae.
[0052] In describing the various embodiments, specific terminology
will be used for the sake of clarity. Such terminology is not,
however, intended to be limiting and it is to be understood that
each term so selected includes all technical equivalents that
function similarly. The disclosed methods can be used to form
retroreflective elements of a variety of sizes and shapes, such as
fall cube corner elements and truncated cube corner elements. The
base edges of adjacent truncated cube corner elements in an array
are typically coplanar. The base edges of adjacent full cube corner
elements in an array are not in the same plane. Related
applications filed on the same date herewith include: Cube Corner
Sheeting Mold and Method Making the Same (Atty. Docket No.
51946USA9A); Retroreflective Cube Corner Sheeting Mold and Sheeting
Formed Therefrom (Atty. Docket No. 53305USA5A); Retroreflective
Cube Corner Sheeting, Molds Therefore, and Methods of Making the
Same (Atty. Docket No. 53318USA8A); Tiled Retroreflective Sheeting
Composed of Highly Canted Cube Corner Elements (Atty. Docket No.
53285USA9A); Dual Orientation Retroreflective Sheeting (Atty.
Docket No. 52303USA8B).
[0053] For purposes of description, a Cartesian coordinate system
can be superimposed onto lamina 10. A first reference plane 24 is
centered between first major surface 12 and second major surface
14. First reference plane 24, referred to as the x-z plane, has the
y-axis as its normal vector. A second reference plane 26, referred
to as the x-y plane, extends substantially co-planar with working
surface 16 of lamina 10 and has the z-axis as its normal vector. A
third reference plane 28, referred to as the y-z plane, is centered
between first end surface 20 and second end surface 22 and has the
x-axis as its normal vector. Although various geometric attributes
will be described herein with reference to such Cartesian reference
planes, it will be appreciated that they can be described using
other coordinate systems or with reference to the structure of the
lamina.
[0054] One embodiment of a lamina, as well as a method of making
the same, will now be described with reference to FIGS. 1-9. In
FIG. 1, a representative lamina 10 useful in the manufacture of a
mold suitable for forming retroreflective sheeting includes a first
major surface 12 and an opposing second major surface 14. Lamina 10
further includes a working surface 16 and an opposing bottom
surface 18 extending between first major surface 12 and second
major surface 14. Lamina 10 further includes a first end surface 20
and an opposing second end surface 22. In a one embodiment, lamina
10 can be a right rectangular polyhedron wherein opposing surfaces
are substantially parallel. However, it will be appreciated that
opposing surfaces of lamina 10 need not be parallel.
[0055] FIGS. 2-9 illustrate one embodiment of the formation of a
structured surface comprising a plurality of optically opposing
cube corner elements in the working surface 16 of lamina 10. In
brief, the plurality of laminae 10 are oriented such that their
respective first reference planes 24 are disposed at a first angle
.theta..sub.1, relative to a fixed reference axis (FIG. 3). A first
groove set comprising a plurality of parallel, adjacent grooves
30a, 30b, 30c, etc. (collectively referred to by the reference
numeral 30) is formed in the working surface 16 of the plurality of
laminae 10 (FIGS. 3-5). The grooves of the first groove set 30
define respective first groove surfaces 32a, 32b, 32c, etc. and
respective second groove surfaces 34b, 34c, 34d, etc. Importantly,
the respective first groove surfaces 32a, 32b, 32c, etc. intersect
the respective second groove surfaces 34b, 34c, etc. substantially
orthogonally to define respective first reference edges 36a, 36b,
36c, etc. As used herein, the terms `substantially orthogonally` or
`approximately orthogonally` shall mean that the dihedral angle
between the respective surfaces measures approximately 90.degree.;
slight variations in orthogonality as disclosed and claimed in U.S.
Pat. No. 4,775,219 to Appeldorn are contemplated by the present
invention. A second groove set comprising a plurality of parallel
adjacent grooves 46a, 46b, 46c, etc. is also formed in the working
surface 16 of lamina 10 (FIGS. 6-8). The grooves 46 divides and/or
bisects the first and second groove surfaces 32, 34. For the sake
of clarity, groove surfaces on one side of the groove 46 are
referred to as the first and second groove surfaces 32, 34 and the
groove surfaces on the other side of the groove 46 are referred to
as the third and forth groove surfaces 40, 42.
[0056] The grooves of the second groove set define respective fifth
groove surfaces 48a, 48b, 48c, etc. (collectively referred to by
the reference numeral 48) and sixth groove surfaces 50a, 50b, 50c,
etc. (collectively referred to by the reference numeral 50). The
fifth groove surfaces 48a, 48b, 48c, etc. intersect the respective
first groove surfaces 32a, 32b, 32c, etc. and second groove
surfaces 34b, 34c, etc. substantially orthogonally to form a
plurality of cube corner elements 60a, 60b, 60c on the working
surfaces 16 of the respective laminae. Similarly, the sixth groove
surfaces 50a, 50b, 50c, etc. intersect the respective first groove
surfaces 40a, 40b, 40c, etc. and second groove surfaces 42b, 42c,
etc. substantially orthogonally to form a plurality of cube corner
elements 70a, 70b, etc. on the working surfaces 16 of the
respective laminae. As used herein, the term `groove set` refers to
all parallel grooves formed in working surface 16 of the laminae
10.
[0057] The embodiment will now be explained in greater detail.
Turning back to FIG. 2, a plurality of thin laminae 10 are
assembled together such that the first major surface 12 of one
lamina 10 is adjacent the second major surface 14 of an adjacent
lamina 10. Preferably, the plurality of laminae 10 are assembled in
a fixture of conventional design capable of securing the plurality
of laminae adjacent one another. The fixture preferably defines a
base plane 80 (FIG. 3) which is preferably substantially parallel
to the bottom surfaces 18 of the laminae 10 when the laminae 10 are
positioned as shown in FIG. 2. The plurality of laminae 10 can be
characterized by a Cartesian coordinate system as described above.
Preferably, working surfaces 16 of the plurality of laminae 10 are
substantially coplanar when the laminae are positioned with their
first reference planes 24 perpendicular to base plane 80.
[0058] In FIG. 3, the plurality of laminae 10 are oriented to have
their first reference planes 24 disposed at a first angle
.theta..sub.1 from a fixed reference axis 82 normal to base plane
80. In one embodiment, the first angle .theta..sub.1 is
approximately 27.80.degree.. However, in practice .theta..sub.1 can
be between about 1.degree. and about 85.degree., and more
preferably between about 10.degree. and about 60.degree., and most
preferably between about 25.degree. and about 45.degree..
[0059] Referring to FIGS. 4-5, a first groove set comprising a
plurality of parallel adjacent V-shaped grooves 30a, 30b, 30c, etc.
(collectively referred to by reference numeral 30) is formed in the
working surfaces 16 of the plurality of laminae 10 with the lamina
disposed at angle .theta..sub.1. At least two such grooves 30 are
formed in working surface 16 of the plurality of laminae 10. The
grooves 30 define first groove surfaces 32a, 32b, 32c, etc.
(collectively referred to by reference numeral 32) and second
groove surfaces 34b, 34c, 34d, etc. (collectively referred to by
reference numeral 34) that intersect as shown at groove vertices
33b, 33c, 33d, etc. (collectively referred to by the reference
numeral 33). At the edge of the laminae, the groove forming
operation may form a single groove surface 32a. Groove surfaces 32a
and 34b of adjacent grooves 30a, 30b intersect approximately
orthogonally along a reference edge 36a. Similarly, adjacent groove
surfaces 32b and 34c intersect approximately orthogonally along
reference edge 36b. This can be accomplished by forming grooves 30
using a cutting tool having a 90.degree. included angle. Preferably
this pattern is repeated across the entire working surfaces 16 of
the plurality of laminae 10. Groove vertices 33 are preferably
spaced apart by between about 0.01 millimeters and about 1.0
millimeters, however these values are not intended to be
limiting.
[0060] Grooves 30 are formed by removing portions of working
surface 16 of the plurality of laminae using suitable material
removal techniques including precision machining techniques such as
milling, ruling, grooving and fly-cutting. Chemical etching or
laser ablation techniques can also be used. In one embodiment,
grooves 30 are formed in a high-precision machining operation in
which a diamond cutting tool having a 90.degree. included angle is
repeatedly moved transversely across the working surfaces 16 of the
plurality of laminae 10 along an axis that is substantially
parallel to base plane 80. The diamond cutting tool could, however,
be moved along an axis that is non-parallel to base plane 80 such
that the tool cuts at a varying depth across the plurality of
laminae 10. Further, the machining tool can be held stationary
while the plurality of laminae are placed in motion; any relative
motion between laminae 10 and the machining tool is
contemplated.
[0061] In the embodiment of FIGS. 2-5 , the grooves 30 of the first
groove set are formed at a depth such that the respective first
reference edges 36 intersect the first major surface 12 and the
second major surface 14 of each lamina. Thus, in the end view
depicted in FIG. 4, the reference edges 36 and groove vertices 33
form substantially continuous lines that extend along an axis
parallel to base plane 80. Further, grooves 30 are formed such that
the respective reference edges 36 are disposed in a plane that
intersects the respective first reference planes 24 and the second
reference plane 26 at orthogonal angles. Thus, in a top plan view
the respective first reference edges 36 would appear perpendicular
to the respective first reference planes 24 of the plurality of
laminae 10. However, grooves 30 can also have lesser depths. For
example, if the depth of the tool is decreased, the groove vertices
33 will be formed closer to the working surface 16 and flat,
transmissive regions will be formed.
[0062] To complete the formation of cube corner elements on the
working surfaces 16 of the laminae 10, a second groove set is
formed by machining a single groove in each lamina 10 along an axis
substantially parallel with first reference plane 24. In the
embodiment illustrated in FIGS. 6-8, the plurality of lamina 10 are
removed from the assembly and alternating laminae (10b, 10d, etc.)
are rotated 180.degree. about an axis perpendicular to second
reference plane 26. The plurality of laminae are then reassembled
with their respective first reference planes 24 preferably disposed
substantially perpendicular to base plane 80 as depicted in FIG.
7.
[0063] Referring to FIGS. 8 and 9, a second groove set that
preferably includes at least one groove 46 in each lamina 10 is
formed in the working surface 16 of the plurality of laminae 10. In
the disclosed embodiment the second grooves 46a, 46b, 46c, etc.
(collectively referred to as 46) define respective fifth groove
surfaces 48a, 48b, 48c, etc. (collectively referred to as 48) and
sixth groove surfaces 50a, 50b, 50c, etc. (collectively referred to
as 50) that intersect at respective groove vertices 52a, 52b, 52c,
etc. (collectively referred to as 52) along axes that are
perpendicular to the third reference plane 28.
[0064] The second grooves 46 are formed such that fifth groove
surfaces 48 are substantially orthogonal to the respective first
groove surfaces (e.g. 32a, 32b, etc.) and second groove surfaces
(e.g. 34a, 34b, etc.). Formation of the fifth groove surfaces 48 as
described yields a plurality of cube corner elements 60a, 60b, etc.
(collectively referred to as 60) in working surface 16 of
alternating laminae 10. Each cube corner element 60 is defined by a
first groove surface (32a, 32b, etc.), a second groove surface
(34a, 34b, etc.) and a portion of a fifth groove surface 48 that
mutually intersect at a point to define a cube corner peak, or apex
62. Similarly, the sixth groove surfaces 50 are substantially
orthogonal to the respective third groove surfaces (e.g. 40a, 40b,
etc.) and fourth groove surfaces (e.g. 42a, 42b, etc.). As noted
above, third and fourth groove surfaces 40, 42 were formed by the
first groove set 30. Formation of the sixth groove surface 50 also
yields a plurality of cube corner elements 70a, 70b, etc.
(collectively referred to as 70) in working surface 16 of
alternating laminae 10. Each cube corner element 70 is defined by a
third groove surface (40a, 40b, etc.), a fourth groove surface
(42a, 42b, etc.) and a portion of sixth groove surface 50 that
mutually intersect at a point to define a cube corner peak, or apex
72. Preferably, both groove surfaces 48 and 50 form a plurality of
cube corner elements on the working surface 16 of lamina 10.
However, it will be appreciated that the second groove 46 can be
formed such that only groove surface 48 or groove surface 50 forms
cube corner elements.
[0065] The cube corner elements 60, 70 are opposing pairs that
generate opposing, although not necessarily identical,
retroreflection patterns. The cube corner elements 60, 70
preferably generate symmetrical or mirror image retroreflection
patterns, such as elements that are substantially identical but are
rotated 180.degree. relative to each other. In an alternate
embodiment, the second groove set 46 can be cut in the stack of
laminae shown in FIG. 6 so that the resulting cube corner elements
60, 70 are all aligned in the same direction. That is, the symmetry
axes or optical axes of the cube corner elements 60, 70 are
generally parallel. Similarly, the laminae 10b, 10d, etc. can be
rotated 180.degree. after the second groove set 46 is cut (see FIG.
8). The total light return for cube corner elements 60, 70 aligned
in the same direction is asymmetrical about a 360.degree. range of
orientation angles. An asymmetrical retroreflection pattern can be
desirable for some applications, such as pavement markers or other
items that are viewed from a narrow range of orientation
angles.
[0066] A method of the present disclosure involves simultaneously
machining a plurality of laminae, each lamina comprising one or
more discrete cube corner elements. The cube corner elements
preferably do not extend across more than one lamina. For example,
the three mutually perpendicular optical faces 32, 34, 48 of cube
corner elements 60 are machined on a single lamina. Similarly, the
three optical faces 40, 42, 50 of the cube corner elements 70 are
machined on a single lamina. The cube corner elements 60, 70 can be
located on the same or different laminae. The cube corner elements
60, 70 are advantageously formed with only two groove sets 30, 46
by the machining process to ensure an optical quality surface. A
planar interface between major surfaces 12, 14 is maintained
between adjacent laminae during the machining phase and in the
subsequent mold formed therefrom so as to minimize alignment
problems and damage due to handling of the laminae, to minimize
gaps between adjacent laminae that would degrade the quality of
negative copies, and to minimize flash from migrating into the gaps
between the laminae.
[0067] FIGS. 10-18 illustrate an alternate method of forming the
mold of FIGS. 1-9 on a plurality of laminae as illustrated in FIG.
2, using three groove sets 130, 138, 146. Preferably, the
respective working surfaces 116 of the plurality of laminae 110 are
substantially coplanar when the lamina are positioned with their
respective first reference planes 124 perpendicular to base plane
180. The reference planes 124, 126, 128 correspond to the reference
planes 24, 26, 28, respectively, discussed above.
[0068] Referring to FIG. 10, the plurality of laminae 110 are
oriented to have their first reference planes 124 disposed at a
first angle .beta..sub.1, from a fixed reference axis 182 normal to
base plane 180. In one embodiment, .beta..sub.1 is approximately
27.8.degree.. However, .beta..sub.1 can alternately be between
about 1.degree. and about 85.degree., and more preferably between
about 10.degree. and about 60.degree..
[0069] Referring to FIGS. 11-12, a first groove set comprising a
plurality of parallel adjacent V-shaped grooves 130a, 130b, 130c,
etc. (collectively referred to as 130) is formed in the working
surfaces 116 of the plurality of laminae 110 with the lamina
disposed at angle .beta..sub.1. At least two such grooves 130 are
formed in working surface 116 of the plurality of laminae 110. The
grooves 130 define first groove surfaces 132a, 132b, 132c, etc.
(collectively referred to as 132) and second groove surfaces 134b,
134c, 134d, etc. (collectively referred to as 134) that intersect
as shown at groove vertices 133b, 133c, 133d, etc. (collectively
referred to as 133). At the edge of the lamina, the groove forming
operation can form a single groove surface 132a. Groove surfaces
132a and 134b of adjacent grooves intersect approximately
orthogonally along a reference edge 136a. Similarly, adjacent
groove surfaces 132b and 134c intersect approximately orthogonally
along reference edge 136b. Preferably this pattern is repeated
across the entire working surfaces 116 of the plurality of laminae
110.
[0070] Grooves 130 are formed by removing portions of working
surface 116 of the plurality of laminae using suitable material
removal techniques including precision machining techniques such as
milling, ruling, grooving and fly-cutting. Chemical etching or
laser ablation techniques can also be used. In one embodiment, the
grooves 130 are formed in a high-precision machining operation in
which a diamond cutting tool having a 90.degree. included angle is
repeatedly moved transversely across the working surfaces 116 of
the plurality of laminae 110 along an axis that is substantially
parallel to base plane 180. The diamond cutting tool could,
however, alternately be moved along an axis that is non-parallel to
base plane 180 such that the tool cuts at a varying depth across
the plurality of laminae 110. Further, the machining tool could be
held stationary while the plurality of laminae are placed in
motion; any relative motion between the laminae 110 and the
machining tool is contemplated.
[0071] In the embodiment of FIGS. 11-12, the grooves 130 are formed
at a depth such that the respective first reference edges 136
intersect the first major surface 112 and the second major surface
114 of each lamina. Thus, in the end view of FIG. 11, the reference
edges 136 and groove vertices 133 form substantially continuous
lines that extend along an axis parallel to base plane 180.
Further, grooves 130 are formed such that the respective reference
edges 136 are disposed in a plane that intersects the respective
first reference planes 124 and the second reference plane 126 at
orthogonal angles. Thus, the respective first reference edges 136
would appear perpendicular to the respective first reference planes
124 of the plurality of laminae 110. However, grooves 130 can also
have lesser depths so as to form flat transmissive regions.
[0072] Referring to FIG. 13, the plurality of laminae 110 are then
oriented to have their respective first reference planes 124
disposed at a second angle .beta..sub.2, from fixed reference axis
182 normal to base plane 180. In one embodiment, .beta..sub.2 is
approximately 27.8.degree.. However, in practice .beta..sub.2 can
be between about 1.degree. and about 85.degree., but preferably
between about 10.degree. and about 60.degree.. The angle
.beta..sub.2 is independent of angle .beta..sub.1 and need not
equal .beta..sub.1. To orient the plurality of laminae 110 at angle
.beta..sub.2, the laminae 110 are preferably removed from the
fixture and reassembled with their respective first reference
planes disposed at angle .beta..sub.2.
[0073] Referring to FIGS. 14-15, a second groove set comprising a
plurality of parallel adjacent V-shaped grooves 138b, 138c, etc.
(collectively referred to as 138) is formed in the working surfaces
116 of the plurality of laminae 110 with the lamina disposed at
angle .beta..sub.2. At least two adjacent grooves 138 are formed in
working surface 116 of the plurality of laminae 110. The grooves
138 define third groove surfaces 140a, 140b, 140c, etc.
(collectively referred to as 140) and fourth groove surfaces 142b,
142c, 142d, etc. (collectively referred to as 142) that intersect
as shown at groove vertices 141b, 141c, 141d, etc. (collectively
referred to as 141). At the edge of the lamina, the groove forming
operation can form a single groove surface 140a. Groove surfaces
140a and 142b of adjacent grooves intersect approximately
orthogonally along a reference edge 144a. Similarly, adjacent
groove surfaces 140b and 142c intersect approximately orthogonally
along reference edge 144b. Preferably this pattern is repeated
across the entire working surfaces 116 of the plurality of laminae
110.
[0074] Grooves 138 of the second groove set are also preferably
formed by a high-precision machining operation in which a diamond
cutting tool having a 90.degree. included angle is repeatedly moved
transversely across the working surfaces 116 of the plurality of
laminae 110 along a cutting axis that is substantially parallel to
base plane 180. Again, it will be noted that it is important that
the surfaces of adjacent grooves 138 intersect along the reference
edges 144 to form orthogonal dihedral angles. The included angle of
each groove can measure other than 90.degree.. Grooves 138 are
preferably formed at approximately the same depth in working
surface 116 of the plurality of laminae 110 as grooves 130 in first
groove set. Additionally, the grooves 138 in the second groove set
are preferably formed such that the respective groove vertices
(e.g. 141a, 141b, etc.) and the respective reference edges (e.g.
144a, 144b, etc.) are substantially coplanar with respective groove
vertices (e.g. 133a, 133b, etc.) and the respective reference edges
(e.g. 136a, 136b, etc.) of the grooves 130 in the first groove
set.
[0075] Referring to FIGS. 16-17, a third groove set that preferably
includes at least one groove 146 in each lamina 110 is formed in
the working surface 116 of the plurality of laminae 110. In the
disclosed embodiment the third grooves 146a, 146b, 146c, etc.
(collectively referred to as 146) define respective fifth groove
surfaces 148a, 148b, 148c, etc. (collectively referred to as 148)
and respective sixth groove surfaces 150a, 150b, 150c, etc.
(collectively referred to as 150) that intersect at respective
groove vertices 152a, 152b, 152c, etc. (collectively referred to as
152) along axes that are parallel to the respective first reference
planes 124. The third grooves 146 are formed such that respective
fifth groove surfaces 148 are disposed in a plane that is
substantially orthogonal to the respective first groove surfaces
(e.g. 132a, 132b, etc.) and the respective second groove surfaces
(e.g. 134a, 134b, etc.). Formation of the fifth groove surfaces 148
in this way yields a plurality of cube corner elements 160a, 160b,
etc. (collectively referred to as 160) in working surface 116 of
the respective lamina 110.
[0076] Each cube corner element 160 is defined by a first groove
surface (132a, 132b, etc.), a second groove surface (134b, 134c,
etc.) and a portion of a fifth groove surface 148 that mutually
intersect at a point to define a cube corner peak, or apex 162.
Similarly, sixth groove surface 150 is disposed in a plane that is
substantially orthogonal to the respective third groove surfaces
(e.g. 140a, 140b, etc.) and the respective fourth groove surfaces
(e.g. 142b, 142c, etc.). Formation of the sixth groove surface 150
also yields a plurality of cube corner elements 170a, 170b, etc.
(collectively referred to as 170) in working surface 116 of lamina
110. Each cube corner element 170 is defined by a third groove
surface (140a, 140b, etc.), a fourth groove surface (142a, 142b,
etc.) and a portion of sixth groove surface 150 that mutually
intersect at a point to define a cube corner peak, or apex 172.
Preferably, both fifth groove surface 148 and sixth groove surface
150 form a plurality of cube corner elements on the working surface
116 of lamina 110. However, it will be appreciated that third
groove 146 can be formed such that only fifth groove surface 148 or
sixth groove surface 150 forms cube corner elements.
[0077] In a preferred method the plurality of laminae 110 are
re-oriented to have their respective first reference planes 124
disposed approximately parallel to reference axis 182 before
forming the plurality of grooves 146. However, the grooves 146 can
be formed with the lamina oriented such that their respective first
reference planes are disposed at an angle relative to reference
axis 182. In particular, in some embodiments it may be advantageous
to form the respective third grooves 146 with the respective lamina
110 disposed at angle .beta..sub.2 to avoid an additional
orientation step in the manufacturing process. Preferably, grooves
146 are also formed by a high precision machining operation. In the
disclosed embodiment a diamond cutting tool having an included
angle of about 55.6.degree. is moved across the working surface 116
of each lamina 110 along an axis that is substantially contained by
the first reference plane 124 of the lamina 110 and that is
parallel to base plane 180. Grooves 146 are preferably formed such
that the respective groove vertices 152 are slightly deeper than
the vertices of the grooves in the first and second groove sets.
Formation of grooves 146 result in a plurality of laminae 110
having a structured surface substantially as depicted in FIG.
18.
[0078] As discussed in connection with FIGS. 1-9, the method of
FIGS. 10-18 results in simultaneously machining a plurality of
laminae, each having cube corner elements 160 with three mutually
perpendicular optical faces 132, 134, 148 on a single lamina.
Similarly, the three optical faces 140, 142, 150 of the cube corner
elements 170 are machined on a single lamina. A planar interface
between major surfaces 112, 114 is maintained between adjacent
laminae during the machining phase and in the subsequent mold
formed therefrom so as to minimize alignment problems and damage
due to handling of the laminae.
[0079] FIGS. 19-27 illustrate an alternate embodiment of
simultaneously forming a plurality of cube corner elements on a
plurality of laminae, such as illustrated in FIG. 2. Preferably,
the respective working surfaces 216 of the laminae 210 are
substantially coplanar when the lamina are positioned with their
respective first reference planes 224 perpendicular to base plane
280. The reference planes 224, 226, 228 correspond to the reference
planes 24, 26, 28, respectively, discussed above.
[0080] Referring to FIG. 19, the plurality of laminae 210 are
oriented to have their first reference planes 224 disposed at a
first angle .theta..sub.1, from a fixed reference axis 282 normal
to base plane 280. In one embodiment, .theta..sub.1 is
approximately 54.74.degree.. In theory, .theta..sub.1 can be any
angle between about 45.degree. and about 90.degree., however, in
practice it is typically between approximately about 45.degree. and
about 60.degree.. Referring to FIGS. 20-21, a first groove set
comprising a plurality of parallel adjacent V-shaped grooves 230a,
230b, 230c, etc. (collectively referred to as 230) is formed in the
working surfaces 216 of the plurality of laminae 210 with the
lamina disposed at angle .theta..sub.1. The grooves 230 define
first groove surfaces 232a, 232b, 232c, etc. (collectively referred
to as 232) and second groove surfaces 234b, 234c, 234d, etc.
(collectively referred to as 234) that intersect at groove vertices
233b, 233c, 233d, etc. (collectively referred to by the reference
numeral 233) as shown. At the edge of the lamina, the groove
forming operation can form a single groove surface, e.g. 232a,
234d. Preferably this pattern is repeated across the entire working
surfaces 216 of the plurality of laminae 210.
[0081] Grooves 230 are formed by removing portions of working
surface 216, as discussed above. In one embodiment, the grooves 230
are formed in a high-precision machining operation in which a
diamond cutting tool having a 120.degree. included angle repeatedly
moves transversely across the working surfaces 216 of the plurality
of laminae 210 along an axis that is substantially parallel to base
plane 280. It will be appreciated, however that the diamond cutting
tool can move along an axis that is non-parallel to base plane 280
such that the tool cuts at a varying depth across the plurality of
laminae 210.
[0082] In the embodiment of FIGS. 20-21, the grooves 230 are formed
at a depth such that the respective groove vertices 233 intersect
the first major surface 212 and the second major surface 214 of
each lamina. Thus, in the end view depicted in FIG. 20, groove
vertices 233 form substantially continuous lines that extend along
an axis parallel to base plane 280. Further, grooves 230 are formed
such that the groove vertices 233 and the edges 236 are disposed in
planes that intersect the first reference planes 224 and the second
reference planes 226 at orthogonal angles. The respective groove
vertices appear perpendicular to the respective first reference
planes 224 of the plurality of laminae 210. However, grooves 230
can be formed at lesser depths or along different axes.
[0083] Referring to FIGS. 22-23, the plurality of laminae 210 are
then oriented to have their respective first reference planes 224
disposed at a second angle .theta..sub.2, from fixed reference axis
282 normal to base plane 280 and a second groove set comprising a
plurality of parallel adjacent V-shaped grooves 238a, 238b, 238c,
etc. (collectively referred to as 238) is formed in the working
surfaces 216 of the plurality of laminae 210. In the disclosed
embodiment, .theta..sub.2 is approximately 54.74.degree.. As
discussed above, in theory, .theta..sub.2 can be any angle between
about 45.degree. and about 90.degree., however, in practice it is
preferably between about 45.degree. and about 60.degree.. To orient
the plurality of laminae 210 at angle .theta..sub.2, the laminae
210 are preferably removed from the fixture and reassembled with
their respective first reference planes disposed at angle
.theta..sub.2. The grooves 238 define third groove surfaces 240a,
240b, 240c, etc. (collectively referred to as 240) and fourth
groove surfaces 242b, 242c, 242d, etc. (collectively referred to as
242) that intersect at groove vertices 241b, 241c, 241d, etc.
(collectively referred to as 241) and along edges 247a, 247b, 247c,
etc. as shown. At the edge of the lamina, the groove forming
operation can form a single groove surface. Preferably this pattern
is repeated across the entire working surfaces 216 of the laminae
210.
[0084] Grooves 238 of the second groove set are also preferably
formed by a high-precision machining operation in which a diamond
cutting tool having an included angle of about 120.degree.
repeatedly moves transversely across the working surfaces 216 of
the laminae 210 along a cutting axis substantially parallel to base
plane 280. Grooves 238 are preferably formed at approximately the
same depth as grooves 230. Additionally, grooves 238 are preferably
formed such that the groove vertices (e.g. 241a, 241b, etc. ) are
substantially coplanar with respective groove vertices (e.g. 233a,
233b, etc.) of the grooves 230. After forming the grooves 238 in
the second groove set, each lamina 210 preferably appears as shown
in FIG. 27.
[0085] Referring to FIG. 25-26, a third groove set comprising a
plurality of parallel adjacent V-shaped grooves 246a, 246b, 246c
etc. (collectively referred to as 246) is formed in the working
surfaces 216 of the plurality of laminae 210. The third grooves 246
define fifth groove surfaces 248a, 248b, 248c, etc. (collectively
referred to as 248) and respective sixth groove surfaces 250a,
250b, 250c, etc. (collectively referred to as 250) that intersect
at groove vertices 252a, 252b, 252c, etc. (collectively referred to
as 252). The third grooves 246 are formed such that the fifth
groove surfaces 248 are disposed substantially orthogonal to the
respective first groove surfaces 232 and the respective third
groove surfaces 240.
[0086] Formation of the fifth groove surfaces 248 as described
yields a plurality of cube corner elements (e.g. 260a, 260b, 260c,
etc.), collectively referred to by reference numeral 260, in
working surface 216 of the respective lamina 210. Each cube corner
element 260 is defined by a first groove surface 232 a third groove
surface 240 and a fifth groove surface 248 that mutually intersect
at a point to define a cube corner peak, or apex 262. Similarly,
the sixth groove surfaces 250 are disposed substantially orthogonal
to the respective second groove surfaces 234 and the respective
fourth groove surfaces 242. Formation of the sixth groove surfaces
250 also yields a plurality of cube corner elements 270a, 270b,
etc. (collectively referred to by reference numeral 270) in working
surface 216 of lamina 210. Each cube corner element 270 is defined
by a second groove surface 234, a fourth groove surface 242 and a
sixth groove surface 250 that mutually intersect at a point to
define a cube corner peak, or apex 272. Preferably, both fifth
groove surface 248 and sixth groove surface 250 form a plurality of
optically opposing cube corner elements on the working surface 216
of lamina 210. However, it will be appreciated that third groove
246 could be formed such that only fifth groove surfaces 248 or
sixth groove surfaces 250 forms cube corner elements.
[0087] In a preferred method the plurality of laminae 210 are
re-oriented to have their respective major planes 224 disposed
approximately parallel to reference axis 282 before forming the
plurality of grooves 246. In a preferred embodiment a diamond
cutting tool having an included angle of 90.degree. moves across
the working surfaces 216 of the plurality of laminae 210 along an
axis that is substantially parallel to base plane 280. However, the
grooves 246 can be formed with the lamina oriented such that their
respective major planes are disposed at an angle relative to
reference axis 282. Grooves 246 are preferably formed such that the
respective groove vertices 252 are slightly deeper than the
vertices of the grooves in the first and second groove sets.
Formation of grooves 246 result in a plurality of laminae 210
having a structured surface substantially as depicted in FIG.
27.
[0088] Working surface 216 exhibits several desirable
characteristics as a retroreflective article. The cube corner
element geometry formed in working surface 216 of lamina 210 may be
characterized as a `full` or `high efficiency` cube corner element
geometry because the geometry exhibits a maximum effective aperture
that approaches 100%. Thus, a retroreflective article formed as a
replica of working surface 216 will exhibit high optical efficiency
in response to light incident on the retroreflective article
approximately along the symmetry axes of the cube corner elements.
Additionally, cube corner elements 260 and 270 can be disposed in
opposing orientations and are symmetrical with respect to first
reference plane 24 and will exhibit symmetric retroreflective
performance in response to light incident on the retroreflective
article at high entrance angles.
[0089] The laminae are preferably formed from a dimensionally
stable material capable of holding precision tolerances, e.g.
machinable plastics (for example, polyethylene teraphthalate,
polymethyl methacrylate, and polycarbonate) or metals (for example,
brass, nickel, copper, or aluminum). The physical dimensions of the
laminae are constrained primarily by machining limitations. Each
lamina preferably measures between about 0.025 millimeters and
about 1.0 millimeters in thickness, and more preferably about 0.1
to about 0.6 millimeters, between about 5 and about 100 millimeters
in height, and between about 10 and about 500 millimeters in width.
These measurements are provided for illustrative purposes only and
are not intended to be limiting.
[0090] In the manufacture of retroreflective articles such as
retroreflective sheeting, the structured surface of the plurality
of laminae is used as a master mold which can be replicated using
electroforming techniques or other conventional replicating
technology. The plurality of laminae can include substantially
identical cube corner elements or may include cube corner elements
of varying sizes, geometries, or orientations. The structured
surface of the replica, referred to in the art as a `stamper`,
contains a negative image of the cube corner elements. This replica
can be used as a mold for forming a retroreflective article. More
commonly, however, a large number of positive or negative replicas
are assembled to form a mold large enough to be useful in forming
retroreflective sheeting. Retroreflective sheeting can then be
manufactured as an integral material, e.g. by embossing a preformed
sheet with an array of cube corner elements as described above or
by casting a fluid material into a mold. See, JP 8-309851 and U.S.
Pat. No. 4,601,861 (Pricone). Alternatively, the retroreflective
sheeting can be manufactured as a layered product by casting the
cube corner elements against a preformed film as taught in PCT
application No. WO 95/11464 and U.S. Pat. No. 3,648,348 or by
laminating a preformed film to preformed cube corner elements. By
way of example, such sheeting can be made using a nickel mold
formed by electrolytic deposition of nickel onto a master mold. The
electroformed mold can be used as a stamper to emboss the pattern
of the mold onto a polycarbonate film approximately 500 .mu.m thick
having an index of refraction of about 1.59. The mold can be used
in a press with the pressing performed at a temperature of
approximately 175.degree. to about 200.degree. C.
[0091] Useful materials for making such reflective sheeting are
preferably materials that are dimensionally stable, durable,
weatherable and readily formable into the desired configuration.
Examples of suitable materials include acrylics, which generally
have an index of refraction of about 1.5, such as Plexiglas resin
from Rohm and Haas; thermoset acrylates and epoxy acrylates,
preferably radiation cured, polycarbonates, which have an index of
refraction of about 1.6; polyethylene-based ionomers (marketed
under the name `SURLYN`); polyesters; and cellulose acetate
butyrates. Generally any optically transmissive material that is
formable, typically under heat and pressure, can be used. Other
suitable materials for forming retroreflective sheeting are
disclosed in U.S. Pat. No. 5,450,235 to Smith et al. The sheeting
can also include colorants, dyes, UV absorbers, or other additives
as needed.
[0092] It is desirable in some circumstances to provide
retroreflective sheeting with a backing layer. A backing layer is
particularly useful for retroreflective sheeting that reflects
light according to the principles of total internal reflection. A
suitable backing layer can be made of any transparent or opaque
material, including colored materials, that can be effectively
engaged with the disclosed retroreflective sheeting. Suitable
backing materials include aluminum sheeting, galvanized steel,
polymeric materials such as polymethyl methacrylates, polyesters,
polyamids, polyvinyl fluorides, polycarbonates, polyvinyl
chlorides, polyurethanes, and a wide variety of laminates made from
these and other materials.
[0093] The backing layer or sheet can be sealed in a grid pattern
or any other configuration suitable to the reflecting elements.
Sealing can be affected by use of a number of methods including
ultrasonic welding, adhesives, or by heat sealing at discrete
locations on the arrays of reflecting elements (see, e.g. U.S. Pat.
No. 3,924,928). Sealing is desirable to inhibit the entry of
contaminants such as soil and/or moisture and to preserve air
spaces adjacent the reflecting surfaces of the cube corner
elements.
[0094] If added strength or toughness is required in the composite,
backing sheets of polycarbonate, polybutryate or fiber-reinforced
plastic can be used. Depending upon the degree of flexibility of
the resulting retroreflective material, the material can be rolled
or cut into strips or other suitable designs. The retroreflective
material can also be backed with an adhesive and a release sheet to
render it useful for application to any substrate without the added
step of applying an adhesive or using other fastening means.
[0095] The cube corner elements disclosed herein can be
individually tailored so as to distribute light retroreflected by
the articles into a desired pattern or divergence profile, as
taught by U.S. Pat. No. 4,775,219. Typically the groove half-angle
error introduced will be less than .+-.20 arc minutes and often
less than .+-.5 arc minutes.
[0096] For convenience, the working surfaces of a plurality of
laminae when considered collectively can be referred to as a
collective working surface. Thus, for example, grooves 30 of FIG.
5, having groove bottoms 33 and reference edges 36 (see FIGS. 4 and
5), are formed in the collective working surface of the plurality
of laminae 10 (see FIGS. 1-4) and also are formed in the working
surface 16 of each such lamina 10. As another example, grooves 46
of FIG. 8, having groove bottoms 52, are also formed in the
collective working surface of the plurality of laminae 10, but are
not fully formed in the working surface 16 of each such lamina
10.
[0097] All patents and patent applications referred to, including
those disclosed in the background of the invention, are hereby
incorporated by reference. The present invention has now been
described with reference to several embodiments thereof. It will be
apparent to those skilled in the art that many changes can be made
in the embodiments described without departing from the scope of
the invention. Thus, the scope of the present invention should not
be limited to the preferred structures and methods described
herein, but rather by the broad scope of the claims which
follow.
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