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BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention is directed to the field of continuous embossing of sheeting
or webs and more particularly to methods and apparatus of producing large
scale, flexible, and generally cylindrical embossing tools to emboss
continuous plastic webs or the like with a highly accurate pattern of
cube-corners useful in the manufacture of retroreflective sheeting.
2. Description of the Prior Art
Some presently employed techniques for the production of retroreflective
sheeting include the casting of cube corners on cylindrical drums,
followed by an application of secondary material, whereby the cube corner
elements are adhered to a different back-up material. (e.g., U.S. Pat. No.
3,935,359).
Sequential embossing of cube corner type sheeting material has been
suggested by using a series of tooled plates and molds. The web of
material is embossed on one stroke of the press and then indexed to the
next station for a further pressing operation (U.S. Pat. No. 4,244,683).
This process, while operating on a continuous strip of material, is only
sequential in nature and has all of the economic and manufacturing
deficiencies inherent in such a process. Moreover, to be economically
feasible, the width of film or sheeting to be produced, such as 48",
requires extensive mold handling capability not contemplated by the
Rowland '683 structure and process.
Small, rigid cylindrical rolls also are available for the general
continuous embossing of webs of sheet material but, because of the high
degree of optical accuracy required in reproducing cube corner elements,
this technique has not been used to produce continuous sheeting.
Continuous belt type embossing tools also have been disclosed for embossing
non-optically critical patterns in thermoplastic or other materials, such
as shown in Bussey et. al., U.S. Pat. No. 3,966,383. It also is well known
in the cube-corner reflector art to use electroformed tools for producing
mold elements. However, the relatively small area encompassed by the
typical reflective area permits the easy separation of the electroformed
part from its "master" or from pins. To produce a tool required to emboss
large areas of sheet, it would be possible to assemble larger and longer
groups of masters, but minute seams would be found at the junction lines.
Those seams in a final tool could produce stress risers, flash or fins,
leaving the assembled tool with possible fatigue areas. In accordance with
the present invention, the pieces are reproduced by eliminating the "fin"
or seam and then by producing a cylindrical mother and electroforming
internally of the tool mother. A problem then encountered is the removal
of the cylindrical electroformed tool from the tool mother because of the
very accurate but tightly interfitting male and female faces. The present
invention discloses techniques and apparatus for producing a cylindrical
embossing a tool by electroforms; and a method of separating the finished
tool from the cylindrical tool mother.
SUMMARY OF THE INVENTION
The present invention overcomes the difficulties noted with respect to
prior art embossing tools by providing methods and apparatus for making
large scale, flexible, generally cylindrical embossing tools for embossing
highly accurate cube corner or other types of patterns requiring extremely
accurate precision formations continuously upon a moving web of plastic or
other suitable material.
One or more highly accurate optical quality master elements is cut into
suitable substrates. Each master consists of a precision pattern which, in
the specific disclosed embodiment, may take the form of tetrahedrons or
the like formed when three series of parallel grooves are scribed into the
substrate along each of three axes, each axis being spaced from the other
two by 120.degree.. Each master element has a series of marginal edges of
a geometric figure, such as a triangle, rectangle, square, hexagon, etc.
so that the masters can be placed in an abutting contiguous relationship
without any gaps therebetween. The masters (or copies of the master) are
combined in a cluster to provide a desired pattern in a fixture, and an
electrofore strip is made of the cluster. The electroformed strip is thin
and flexible and with a proper support could itself be used directly as an
embossing or compression molding tool but in a non-continuous manner, such
as the sequential type embossing disclosed in U.S. Pat. No. 4,244,683.
Alternately, a number of electroform copies can be made from a single
master and these copies combined as a desired cluster in a fixture and an
electroform strip made of such copies. This electroform strip also can be
used as an embossing tool or in compression molding. In order economically
to provide a continuous sheet of material, it is desirable to continuously
emboss the thermoplastic substrate without indexing a plurality of molds.
Method and apparatus for accomplishing this is disclosed in copending
application Ser. No. 06/430,866, since issued as U.S. Pat. No. 4,478,769
on Oct. 23, 1984. That apparatus may utilize a tool of the type produced
by the present invention, in which the tool pattern may be at least 48"
wide and have a total circumference of 115".
When producing cube-corner sheeting, the high optical quality of the master
required, permits only a relatively small master to be produced, such as
5" on a side. Accordingly, to produce an embossing tool of sufficient size
to permit embossing of wide webs from the electroform produced from the
electroform copy of the masters or the electroform copy of the copies of
the ruled master, it is necessary to duplicate and enlarge the copies
until a tool of the desired size is achieved.
It therefore is an object of the present invention to produce an improved
embossing tool including novel techniques for assuring accuracy of the
tool master.
It is another object of the invention to produce an embossing tool large
enough to continuously emboss a wide web of material.
It is another object of this invention to produce a large scale, flexible,
generally cylindrical embossing tool.
It is yet a further object of this invention to provide a novel method to
produce an improved embossing tool for embossing cube-corner sheeting.
It is another object of the invention to provide a novel method of
producing a large scale, flexible, seamless cylindrical embossing tool
employing a plurality of individually formed masters, replicating such
masters and through successive combination and replication of such masters
and the resultant copies, produce such an embossing tool.
A further object of the invention is to provide a novel method of producing
a large scale, flexible, seamless cylindrical embossing tool employing a
single master, replicating and combining such and resultant copies to
produce such embossing tool, and a method of separating such large tool
from a cylindrical tool mother.
Other objects and features of the invention will be pointed out in the
following description and claims and illustrated in the accompanying
drawings which disclose, by way of example, the principles of the
invention and the best modes which have been contemplated for carrying
them out.
BRIEF DESCRIPTION OF THE DRAWINGS
In the drawings in which similar elements are given similar reference
characters:
FIG. 1 is a diagrammatic flow chart illustrating the various steps in
producing a cylindrical embossing tool in accordance with the present
invention.
FIG. 1A is a partial top plan view of a completed master for producing an
embossing tool for embossing cube-corner sheeting, in which the master is
prepared according to the method of the invention.
FIG. 2 is an elevational view of the master of FIG. 1A, partly in section,
taken along the line 2--2 in FIG. 1A,
FIG. 3 is a fragmentary top plan view of a partially completed master.
FIG. 4 is a top plan view of a series of blank elements having different
geometric shapes suitable for use as masters in accordance with the method
of the invention.
FIG. 5 is a top plan view of the ruled masters formed of the blanks of FIG.
4, indicating how each of the respective types of ruled masters can be
organized into a cluster with like masters to provide contiguous and
continuous surfaces without gaps therebetween.
FIG. 6 is a perspective representation of the manner in which triangular
masters of the type shown in FIG. 5 can be organized to permit the
production of electroform copies of a ruled master.
FIG. 7 is a schematic representation of the technique employed to create
semi-cylindrical segments according to the present invention.
FIGS. 8 to 11 show the progressive positions and size of a shield used
during the electroforming creation of the semi-cylindrical segment copies.
FIG. 12 is a representational plan view of a tank and shield demonstrating
expansion of the shield's position during electroforming of the
cylindrical tool master;
FIG. 13 is a schematic representation of the arrangement of the
semi-cylindrical segment copies into a complete cylinder.
FIG. 14 is a schematic representation of the cylinder produced by the
semi-cylindrical segment copies.
FIG. 15 is a front perspective view of a suction tool used to remove a
completed cylindrical embossing tool from its mother.
FIG. 16 is a side view of the tool of FIG. 15 with the cylindrical
embossing tool removed to better display the construction of such tool;
and
FIG. 17 is a diagrammatic representation of a collapsed cylindrical tool
prior to removal from its mother.
DESCRIPTION OF THE REFERRED EMBODIMENTS
Turning now to FIGS. 1 to 5, there are shown various aspects of a blank
element which is the basic building element for producing large scale,
flexible, seamless cylindrical embossing tools according to the process of
the present invention. The overall length and width of the element which
becomes the ruled master usually is determined by the type of ruling
device used to cut the master, and the element may be on the order of one
to seven inches on a side. The outline shape of the element, as is shown
in FIG. 4, may be triangular, as at 20, square as at 23, or hexagonal as
at 25. The three shapes of the elements as shown in FIG. 4, as well as
others, for example, the rectangle or the octagon, or other shapes and
combinations also may be employed, but preferably the shape chosen should
be a regular geometric figure which can be combined with other similar
figures without permitting a gap to exist between adjacent sides of such
figures. FIG. 5 illustrates how a number of triangular ruled masters 21
can be arranged into a cluster 22. Similarly a number of square ruled
masters 27 and hexagonal masters 29 can be positioned to form clusters 24
and 26 respectively.
Each element such as 20 is chosen of a thickness such that it remains rigid
during the removal of metal while undergoing generation of the ruled
master and during subsequent electro-forming processes. The element
preferably is of aluminum or electro-deposited copper.
Ruling machines used in forming scribing grooves to provide a ruled master
to make a tool for cube-corner sheeting are well known in the art. Such
machines are capable of positioning the workpiece and a cutter within the
optical tolerances necessary to scribe the grooves to optical requirements
including proper depth, angle, spacing and a mirror finish. Typical groove
spacings to form cube-corner type reflector elements range from 0.003 to
0.0065 inches.
As used herein, the phrases "cube-corner", or "trihedral" or "tetrahedron"
are art recognized terms for retroreflective elements comprising three
mutually perpendicular faces, without reference to the size or shape of
the faces or the portion of the optical axis of the cube-corner element so
formed. Stamm U.S. Pat. No. 3,712,706 discloses various scientific
principles and techniques for ruling a master.
The ruling device must be such that a groove uniform in angle and depth is
created along its entire length, and that each successive groove also is
properly spaced and uniform. The ruling device can be of the type where
the cutter is moved while the workpiece remains stationary or, conversely,
the workpiece is moved with respect to a stationary, usually a diamond
tipped cutter. Further, the ruling device must be capable of accurately
indexing to the second and third or more cutting positions different from
the initial set of grooves.
The element 20 for the ruled master 21 of FIG. 3 is positioned upon a
workpiece support of an appropriate ruling device (schematically at "A" in
FIG. 1) and the cutter thereof set to cut a first series of parallel
grooves 31, arbitrarily selected, along the axis in the direction of the
arrow 32. The cutter has a V-shaped cutting edge of desired pitch and
depth.
In accordance with one aspect of the invention, before cutting of the
second set of grooves 34, the first set of grooves 31 is filled along the
axis 32 with a material of appropriate hardness and machinability to allow
a second set of grooves 34 to be cut without interruption, as if the first
set of grooves 31 did not exist. This allows the material being removed
during cutting to be pushed directly ahead of the cutter instead of into
the first set of grooves at each groove intersection, and thereby possibly
distorting the intersections. The fill also serves to support the faces of
the tetrahedral elements being created and prevents their distortion.
Epoxy or curable polyesters can be used as the fill materials. As noted, a
second set of grooves 34 is then cut along the axis in the direction of
the arrow 35. The remaining fill material (i.e., that portion not at the
intersections of the second grooves 34 with the first grooves 31) then is
removed. Fill material then is applied to both sets of grooves and 34
prior to the cutting of a third set of grooves. The element 20 (or tool)
is then indexed to proper position to cut a third set of grooves. When the
cutting of the third set of grooves is complete, all of the fill material
is removed and the ruled master 21 is ready for the next step. One
suitable material is a casting polyester known as Decra-Coat manufactured
by Resco. A suitable epoxy is Hardman No. 8173.
FIGS. 1A and 2 show a complete ruled master 27 having a square
configuration. A first set of grooves 31 was cut along the axes 32,
followed by a second set 34 along the axes 35 and a third set of grooves
37 along the axes 38. The intersections of the three grooves creates a
base 41 for each of the tetrahedrons or cube-corner elements 40, while the
pitch of the cutting tool determines the slope of the three mutually
perpendicular faces 42, 43 and 44 of the cube corner elements 40. The
intersection of the planes of the faces 42, 43 and 44 is the apex 45 of
the tetrahedron 40.
The ruling devices presently available to cut masters to the optical
accuracy required for cube-corner retroreflectors are not capable of
cutting a single master large enough to be used directly to emboss a web
of the desired width and of a length large enough to permit efficient
operation. Accordingly, the master such as 21 or 27 must be used to
produce copies which can be grouped together to form larger areas until a
tool of the desired dimensions is created. Two options are possible at
this stage.
In the first approach, a number of ruled masters 21 (which may or may not
be identical) are produced and then are arranged in a cluster such as 22,
24 or 26, and assembled in a fixture as at 49 (See FIG. 6), and a thick
nickel electroform solid copy is made by techniques known to those in the
electroforming arts. By the selective shielding of the solid copies, the
deposited nickel can be controlled to produce a solid copy without
interfaces and of uniform thickness throughout. This solid copy then can
be used to generate additional copies needed for the next step, and the
clusters 22 can be disassembled and used for other configurations. The
first solid copy then will be a female having been formed from a number of
the male masters 21.
A second approach employs a single master 21 which is used to generate a
mother copy 19 (FIG. 1) which then is replicated to generate a number of
electro-deposited nickel copies 28 (shown at C on FIG. 1) and the copies
28 of the master 21 then are arranged in a cluster 22 and assembled into a
fixture 49. A solid copy then is made from the clustered copies of the
ruled master 21 (steps D, E, F in FIG. 1). Two successive electroform
steps are performed so that strip 50 of male cube corner elements
corresponding exactly to the ruled master 21 is produced. As noted, the
solid copies 28 are used to generate the thin electroform copy or strip
50. The thin strip 50 is then employed to form a plurality of strips 51 of
female cube-corner elements as shown at H, I and J of FIG. 1. The strips
51 are then ground on their rear surfaces to a specific thickness to
provide the desired flexibility whereby the strips 51 can be formed about
an appropriate mandrel 53 (FIG. 1, step K) for succeeding steps. For
example, four strips 51, each approximately 5 inches in width and 18
inches long, may be produced from the solid copies 28 and arranged about a
cylindrical mandrel 53 so as to provide a cylindrical segment copy 55
(FIG. 1 step M) which is 20 inches wide and 18 inches long. Three
cylindrical segment copies 55 then are employed to produce a final
embossing tool which is 20 inches wide and approximately 54 inches in
circumference. Larger strips and more numerous strips 51 will be used to
produce larger tools.
FIGS. 7 to 11 (and steps J-M of FIG. 1) show the method by which the
segment copies 55 are generated. Each cylindrical segment copy 55 is about
1/3 of the circumference of the final mother tool for generating the
embossing tool. However, different sized segments could be made for
specific applications, such as 1/4 segments or the like. The segment copy
55 could be made thin and bent into its desired shape by an outer support
or it could be produced as a relatively thick member formed in its desired
shape in order to retain the optical accuracy and provide strength for
later operations. In the latter approach, the female strips 51 are placed
about mandrel 53 and both mandrel 53 and strips 51 are placed in the
electrodeposition tank 57 (see step L of FIG. 1) adjacent the nickel
anodes 61. In such position, the central portion of strips 51 are closer
to the anodes 61 than are the ends of strips 51. As a result such ends
will be plated to a lesser degree, giving the cylindrical segment copy 55
little strength at its ends. To obviate this problem, a shield 60 (FIG. 7)
of nonconductive material (e.g. plastic), is placed between the nickel
anodes 61, and the assembled strips 51 on the mandrel 53. The position and
width of the shield 60 is controlled so that at the final stages of the
electro-deposition most of the nickel ions are directed to the strip ends
to increase the thickness of the deposited nickel thereat. FIGS. 8 to 11
show the successive positions of the shield 60 during plating. In FIGS.
8-11, the mandrel 53 is rotated on a vertical axis for representational
purposes. The strips 51 are positioned on the mandrel 51 initially and no
shield is employed as shown in FIG. 8. The anodes 61 have been omitted
from FIGS. 8-11 for the sale of clarity. The anodes 61 normally would be
aligned with the strips 51 and exist above the plane of the paper. With
such an arrangement, the greatest nickel build up would be about the
central portion of the strips 51. As the electro-deposition progresses, it
is desirable to direct more and more of the nickel ions toward the strip
ends. Accordingly, the shield 60 is placed over the central portion of the
strips 51, as is shown in FIG. 9. The shield 60 is supported by two
support rods 62 and 63, which also define the extent of the shield 60.
Since the nickel ions do not pass through the shield 60, they travel
towards the ends of the strips 51 which are furthest away from the
electrodes, to build up the thickness of the electro-deposition in such
areas. FIGS. 10 and 11 show the further extent of the shield 60. A
diagrammatic representation of the shield 60 on successive time periods 2,
3 and 4 is illustrated in plan view in FIG. 12. During period 1, the strip
51 is fully exposed (no shield).
When completed, the segment copies 55 (FIG. 1) with their precision
patterns on the inside, and each comprising 1/3 of the circumference of a
final cylinder, are placed within fixtures (not shown) for support to
define a segment cylinder 65 (FIG. 14). Using the assembled segment
cylinder 65 as the negative electrode with an accurately positioned nickel
anode in the center of the cylinder 65, the segment cylinder 65 will be
plated on its inside diameter to generate a thin flexible but solid
seamless master cylinder 70 having flash or fins which can be ground off
so no stress risers are transferred to the next part. This cylinder then
could be used as a model to produce similar cylinders without repeating
all of the previous steps (steps A-N in FIG. 1). The segment disassembled
into the segment copies 55, leaving the tool master cylinder 70.
The master cylinder 70 now consists of female cubes which are situated on
the outside diameter. This cylinder 70 is identical in configuration to
that which is required as an embossing tool, however, tools produced by
this method (using several segments 55 joined together as a mandrel) have
a number of disadvantages. They require an intricate assembly and
disassembly of the segment fixture which requires precision alignment.
Also of concern is the interface between the thin segments 55 which
contain extremely small fissures. Although this discontinuity is almost
non-detectable, it causes a difference in the crystalline structure of the
metal deposited over it. This change results in stress-risers which become
lineal imperfections causing early fatigue failures in parts that will be
flexed during embossing.
These problems of assembly and metallurgy are avoided by the present
invention. The tool master cylinder 70 will have surface imperfections
such as flash, due to the fissures in the mother fixture, removed by
grinding. Once this flash is removed, subsequent copies made from such
part will not contain stress risers or alterations in the metallurgical
structure, although this cylinder 70, when used as a mandrel does have
these imperfections.
With proper fixturing (not shown), the tool master cylinder 70 then is
rotated during subsequent electroforming, with nickel anodes adjacent its
outer surface to form a thick electroform mother cylinder 75, on the order
of 0.050" to 0.100", as compared to the tool master cylinder 70 which is
of a thickness of only 0.010" to 0.030".
The thick mother cylinder 75 then becomes the negative of a cylindrical
embossing tool 80. Both mother cylinder 75 and the cylindrical embossing
tool 80 formed on the inner surface thereof are continuous and seamless.
The present invention utilizes a novel method to separate the seamless
embossing tool 80 from the mother cylinder 75, without damage to either.
Normal "sweating" techniques (expanding one cylinder and contracting the
other by temperature differential), cannot be employed because of the
depth of the male cubes plated into the female counterparts. To provide
separation, the inner cylinder is fixtured with a vacuum apparatus 90
(FIGS. 15-17). The vacuum apparatus 90 consists of a tube 91 to which is
affixed several hollow suction cups 92. Independently controlled hoses 93
and 94 are affixed one to each cup 92 (see FIG. 16) to create a vacuum.
Each cup 92 is secured to the tube 91 by threaded rods 95 and nuts 96.
Mechanically raising cup 92 by rotating one nut 96 at one end of one cup,
causes the cup to lift the underlying cylindrical tool 80 from the mother
75. This then lifts a portion of the thin and flexible inner tool cylinder
80 away from the rigid outer cylinder 75. Normally, the negative effect of
a vacuum in this direction would be cancelled by the intimate contact of
the two parts and not allow separation. In this case, the breaking force
is initially applied mechanically at the very edges of the cylinders which
allows air to enter between the two cylinders. Once the edges are
separated, the additional cup or cups 92 that run along the line of
separation are mechanically adjusted to continue to apply the vertical
vacuum force along a wider path, stripping the inner tool part 80 along
this line (FIG. 16). Once the length of the cylinder 80 has been separated
along this line, the ends of the inner tool cylinder 80 can be held up
mechanically or manually and the vacuum apparatus 90 removed. The
thickness of the tool 80 (about 0.010" to 0.030") permits it to flex
without damage to the cube corner elements.
The inner cylinder 80 then is totally collapsed, (FIG. 17), either manually
or by mechanical means. At this point, a thin protective film such as
Mylar is positioned between the two cylinders 75 and 80 to insure removal
without digging either surface.
The inner tool cylinder 80 then is pulled clear from the outer cylinder 75
and recovers its shape.
Once removed, the heavy mother cylinder 75 can continue to be used to
produce similar embossing tools 80 at a rate of 12 to 48 hours per copy,
depending on the plating rate used.
The process disclosed herein can be varied along the various steps if a
smaller embossing tool is required or extended if a larger tool is
desired. Although the tool 80 is described as a cylinder during its
production, because of its ability to flex, it may be employed in other
forms. For example, it may be used as a belt having two long sides with
short curved ends where it passes over drive rollers (step T of FIG. 1).
It will be understood that while the present invention describes the making
of a cylindrical tool for embossing cube corner sheeting, the principles
of the invention are applicable to any type of tool in which accuracy of
the embossed surfaces are desired for a specific reason, and the noted
technique for separation of the cylindrical tool from its mother is
applicable to any cylindrical parts formed with an interfering pattern
that prevents "sweating" or other simple separation of two seamless
cylinders.
While there have been shown and described and pointed out the fundamental
novel features of the invention as applied to the preferred embodiments,
it will be understood that various omissions and substitutions and changes
of the form and details of the devices illustrated and in their operation
may be made by those skilled in the art, without departing from the spirit
of the invention.
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