U.S. patent application number 09/795699 was filed with the patent office on 2001-09-27 for cutting and edge sealing cellular retroreflective sheeting.
This patent application is currently assigned to 3M Innovative Properties Company. Invention is credited to Janovec, Jeffrey D..
Application Number | 20010024324 09/795699 |
Document ID | / |
Family ID | 23115328 |
Filed Date | 2001-09-27 |
United States Patent
Application |
20010024324 |
Kind Code |
A1 |
Janovec, Jeffrey D. |
September 27, 2001 |
Cutting and edge sealing cellular retroreflective sheeting
Abstract
A method for cutting and edge sealing cells of cellular
retroreflective sheeting is disclosed using a specially designed
apparatus and tool. Sheeting having a thickness T is heated to a
thermoforming temperature to form a heated sheeting. Pressing the
unheated tool, the heated sheeting, and a substrate together
results in cutting and edge sealing of the heated sheeting. The
tool has a radiused ridge for cutting and edge sealing with the
shape of the ridge similar to the shape desired for the edge seal.
A height H of the ridge is defined relative to the thickness of the
sheeting. The fraction of the ridge height to the thickness of the
sheeting is less than one and more than 0.2. The tools may be
mounted on a perimeter of a wheel for continuously slitting and
edge sealing strips of cellular sheeting. The retroreflectivity of
the sheeting is maximized by (1) minimizing the width of the sealed
edges while (2) maximizing the number of closed cells along the
sealed edges that can withstand a water holdout test.
Inventors: |
Janovec, Jeffrey D.; (River
Falls, WI) |
Correspondence
Address: |
Attention: F. Andrew Ubel
Office of Intellectual Property Counsel
3M Innovative Properties Company
P.O. Box 33427
St. Paul
MN
55133-3427
US
|
Assignee: |
3M Innovative Properties
Company
|
Family ID: |
23115328 |
Appl. No.: |
09/795699 |
Filed: |
February 28, 2001 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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09795699 |
Feb 28, 2001 |
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09290289 |
Apr 13, 1999 |
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6224792 |
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Current U.S.
Class: |
359/529 ;
156/271; 264/1.9; 264/160; 264/163; 359/530; 359/534; 359/535 |
Current CPC
Class: |
B29C 65/7894 20130101;
B29C 66/71 20130101; B29C 66/45 20130101; B29L 2011/0091 20130101;
B29C 66/723 20130101; B29C 66/137 20130101; Y10T 156/1054 20150115;
Y10T 156/1087 20150115; B29C 66/1122 20130101; B29C 66/83413
20130101; Y10T 156/1313 20150115; B29D 11/00605 20130101; B29C
65/7435 20130101; B29C 66/83415 20130101; B29C 66/0242 20130101;
B29C 66/71 20130101; B29K 2001/12 20130101; B29C 66/71 20130101;
B29K 2001/14 20130101; B29C 66/71 20130101; B29K 2023/06 20130101;
B29C 66/71 20130101; B29K 2023/0633 20130101; B29C 66/71 20130101;
B29K 2027/00 20130101; B29C 66/71 20130101; B29K 2033/12 20130101;
B29C 66/71 20130101; B29K 2067/003 20130101; B29C 66/71 20130101;
B29K 2075/00 20130101; B29C 66/71 20130101; B29K 2096/005
20130101 |
Class at
Publication: |
359/529 ;
264/1.9; 264/160; 264/163; 156/271; 359/530; 359/534; 359/535 |
International
Class: |
G02B 005/122; B29D
011/00; G02B 005/124; G02B 005/126 |
Claims
What is claimed is:
1. A method of cutting and edge sealing cellular retroreflective
sheeting, comprising the steps of: positioning a cellular
retroreflective sheeting on a substrate, wherein the sheeting has
an initial thickness T; heating the sheeting to a thermoforming
temperature to yield a heated sheeting; providing at least one tool
having a radiused ridge thereon, the ridge having a height H,
wherein the fraction of H/T is less than one and at least 0.2;
maintaining the tool at a temperature below the thermoforming
temperature; and pressing the tool, the heated sheeting, and the
substrate together until the tool cuts and edge seals the cells to
form sealed edges.
2. The method of claim 1, wherein the sheeting comprises: a
substantially transparent face member; a base member having a front
side and a back side; a network of interconnecting seal leg members
bonding the front side of the base member to the face member to
form sealed cells; and retroreflective elements disposed within the
cells, wherein the elements are arranged in substantially a layer
with an air interface, and wherein at least one of the face member,
base member, and seal leg member is thermoformable at the
thermoforming temperature.
3. The method of claim 1, wherein the sealed edges have an average
width less than 1.5 mm.
4. The method of claim 1, wherein at least 95% of the cells along
the sealed edges are unfilled with water after being subjected to a
water holdout test.
5. The method of claim 1, wherein the temperature of the tool is
less than 30.degree. C. and the thermoforming temperature of the
heated sheeting is more than 70.degree. C.
6. The method of claim 1, wherein the fraction of H/T is within the
range of 0.3 to 0.8.
7. The method of claim 1, further comprising the steps of: rotating
the substrate, the substrate comprising an internally heated can;
unwinding the sheeting from a roll with the width of the sheeting
rotating on the substrate to yield a heated sheeting; rotating a
wheel having a perimeter with at least one tool mounted thereon,
each tool spaced across the width of the sheeting; and pressing the
tool, the heated sheeting, and the substrate together until the
tool cuts and edge seals the cells to form sealed edges.
8. The method of claim 7, comprising the additional step of:
increasing a diameter of at least one of the wheels for maintaining
the temperature of the tool lower than the thermoforming
temperature of the heated sheeting.
9. The method of claim 7, comprising the additional step of:
blowing gas onto the tool for maintaining the temperature of the
tool below the thermoforming temperature of the heated sheet.
10. The method of claim 6, comprising the additional step of:
adding heat to the heated sheeting using a heat source external to
the substrate.
11. Cellular retroreflective sheeting made by a process of cutting
and edge sealing, comprising the steps of: positioning a cellular
retroreflective sheeting on a substrate, wherein the sheeting has
an initial thickness T; heating the sheeting to a thermoforming
temperature to yield a heated sheeting; providing at least one tool
having a radiused ridge thereon, the ridge having a height H,
wherein the fraction of H/T is less than one and at least 0.2;
maintaining the tool below the thermoforming temperature; and
pressing the tool, the heated sheeting, and the substrate together
until the tool cuts and edge seals the cells to form sealed
edges.
12. Cellular retroreflective sheeting, having: sealed edges,
wherein the average width of the sealed edges is less than 1.5 mm;
and at least 80% of the cells along the sealed edges are unfilled
with water after being subjected to a water holdout test.
13. Cellular retroreflective sheeting, having: a substantially
transparent face member; a base member having a front side and a
back side; a network of interconnecting seal leg members bonding
the front side of the base member to the face member to form sealed
cells; retroreflective elements disposed within the cells, wherein
the elements are arranged in substantially a layer with an air
interface; and heat sealed edges, wherein the average width of the
sealed edges is less than 1.5 mm; and at least 80% of the cells
along the sealed edges are unfilled with water after being
subjected to a water holdout test.
Description
TECHNICAL FIELD
[0001] This invention relates to cellular retroreflective sheeting
and to a method of thermally sealing cells formed along an edge
while cutting cellular retroreflective sheeting.
BACKGROUND
[0002] Cellular retroreflective sheeting comprises a base member, a
layer of retroreflective elements, and a transparent face member in
spaced relation away from the base member by a network of narrow
intersecting seal leg members that form hermetically sealed cells
within which the retroreflective elements are isolated from
retroreflective elements of different cells. The layer of
retroreflective elements comprises either glass microspheres or
cube corner elements. In plan view, the network of seal leg members
can form patterns, such as, for example, square, rectangular,
circular, hexagonal, or chain link. Examples of cellular
retroreflective sheeting are described in U.S. Pat. Nos. 4,025,159
(McGrath) and 5,706,132 (Nestegard). This type of sheeting may also
be called encapsulated lens sheeting.
[0003] Cellular retroreflective sheeting must sometimes be cut to
fit, for example, a sign. A process called slitting may be used to
cut sheeting into strips having predetermined widths. Depending on
customer requirements, the sheeting may need to be cut to widths as
little as approximately one centimeter. Cutting of retroreflective
sheeting is described in Information Folder 1.1 "Cutting, Matching,
Premasking, and Prespacing of SCOTCHLITE.TM. Reflective Sheetings
and Films" (April, 1998) available from Minnesota Mining and
Manufacturing Company (3M) of Saint Paul, Minn. Sheeting may be cut
with a knife having a blade with a sharp edge or point. Single
sheets can be hand cut, die cut, or cut electronically using a
computer controlled machine. Volume cutting can be accomplished by
methods such as band sawing, roll cutting, or guillotining.
[0004] When cellular sheeting is cut, a cut edge is formed and the
cells along the edge are no longer sealed. These open cells allow
water and dirt to enter the edge of the sheeting and destroy the
effectiveness of the retroreflective elements. For one example, the
open cells may be exposed to adverse weather conditions. For
another example, the sheeting may be subjected to adverse handling
conditions, such as being cleaned by high pressure washing with
water. This cleaning procedure is typically done to the sheeting
after it is adhered to a substrate, for example, the canvas used
for truck covers. Numerous unsatisfactory attempts have been made
to minimize the width of the sealed edge while hermetically sealing
the open cells along the cut edge of the sheeting. Some examples of
prior methods for cutting and sealing of various materials are as
follows:
[0005] (1) A sharp blade is used to cut the material and a liquid
sealer is brushed onto the cut edge. This method is time consuming,
depends on the installer's skill, and the sealer may contain
solvents harmful to the environment.
[0006] (2) A two step process is used in which the material is
first sealed with heat and pressure followed by cutting through the
sealed area. This method requires a wide seal and accurate
registration between the seal and the cut.
[0007] (3) The material may be cut and then the cut edge sealed via
an ultrasonic technique. However the ultrasonic technique has a
very small process window that changes over time, thereby resulting
in a edge sealing process that is difficult to control.
[0008] (4) The material may be thermal pinch cut by bringing two
pieces of the material together between two heated anvils. It is
difficult to control and keep this type of method operating
continuously without fouling of the anvils with plastic debris.
[0009] (5) Fabric is heated by a hot anvil with the cutting blade
always in contact with the anvil. This method results in a rough
cut edge, with debris along the edge. Further, the hot blade can
retain melted sheeting after a time and thus loses its
effectiveness in cutting.
[0010] (6) A heated blade may be used to cut the material and seal
simultaneously as disclosed in publication WO9526870 (Luhman).
While having significant advantages over other prior methods, this
method can result in having some of the same problems cited in
method 5 above.
[0011] The numerous disadvantages associated with these prior
methods indicate the need for a new, effective, and efficient
cutting and edge sealing method.
[0012] Thus there remains a need to cut and edge seal cellular
sheeting so as to retain maximum retroreflectivity regardless of
subsequent exposure to adverse handling and/or weather
conditions.
SUMMARY OF THE INVENTION
[0013] Cellular retroreflective sheeting is made having sealed
edges with a width preferably less than 1.5 mm, and preferably with
at least 95% of the cells along the sealed edges unfilled with
water after being subjected to a water holdout test. A method of
cutting and edge sealing cellular retroreflective sheeting includes
the steps of positioning the sheeting on a substrate, wherein the
sheeting has an initial thickness T; heating the sheeting to a
thermoforming temperature to yield a heated sheeting; providing at
least one unheated tool having a radiused ridge thereon, with the
ridge having a height H, wherein the fraction of H/T is less than
one and at least 0.2; maintaining the tool at a temperature below
the thermoforming temperature; and pressing the tool, the heated
sheeting, and the substrate together until the tool cuts and edge
seals the cells to form sealed edges.
[0014] Preferably, the tool shape approximates the desired shape of
the sealed edge. The tool is preferably maintained at a temperature
below the thermoforming temperature. A preferred embodiment is to
cool the tool to a temperature less than 30.degree. C. with the
thermoforming temperature of the heated sheeting at more than
70.degree. C. Suitable methods for cooling the tool include, for
example, (a) increasing the diameter of the wheels, and/or (b)
blowing air onto the tool. In a preferred embodiment, the fraction
of H/T is maintained within the range of about 0.3 to 0.8.
[0015] One embodiment of the method comprises the steps of rotating
a substrate (such as a can having an internal means of heating,
e.g., the hot can contains a hot fluid); unwinding the sheeting
from a roll with the width of the sheeting rotating on the
substrate to yield a heated sheeting; rotating at least one wheel
having a perimeter with a tool mounted thereon, each wheel spaced
across the width of the sheeting, and pressing the tool, the heated
sheeting, and the substrate together until the tool cuts and edge
seals the cells to form sealed edges. If desired, heat may be added
to the heated sheeting using a heating means external to the
substrate. This embodiment permits the continuous sealing and
slitting of multiple strips of sheeting.
[0016] In preferred embodiments, the method provides cellular
sheeting having a sealed edge with a width preferably less than 1.5
mm, more preferably less than 0.6 mm, and most preferably less than
0.2 mm. After being subjected to a water holdout test, preferably
at least 80%, more preferably at least 90%, and most preferably at
least 95% of the cells along the sealed edge are found to be
unfilled with water.
BRIEF DESCRIPTION OF DRAWINGS
[0017] The present invention will be described in greater detail in
regard to the attached drawings, in which:
[0018] FIG. 1 is a sectional view of cellular sheeting having glass
beads as retroreflective elements;
[0019] FIG. 2 is a sectional view of cellular sheeting having cube
corners as retroreflective elements;
[0020] FIG. 3 is a schematic side view of apparatus used for
continuously heating, slitting, and edge sealing cellular
sheeting;
[0021] FIG. 4 is a schematic side view of a preferred variation of
the apparatus illustrating external means for heating a
substrate;
[0022] FIG. 5 is a cross sectional view of a tool;
[0023] FIG. 6 is a cross sectional view of a tool and cellular
sheeting showing the cutting and edge sealing step;
[0024] FIG. 7 is a cross sectional view of a non-symmetrical shaped
tool;
[0025] FIG. 8 is a cross sectional view of the tool showing
opposing flat portions;
[0026] FIG. 9 is a cross sectional view of an elongated shaped
tool;
[0027] FIG. 10 is a side view of a wheel having at least one tool
mounted on its perimeter such that rotation of the wheel presses
the tool, the cellular sheeting, and the substrate together until
each tool cuts and edge seals the cellular sheeting;
[0028] FIG. 11 is the same side view as that shown in FIG. 10,
except that the wheel has a larger diameter; and
[0029] FIG. 12 is a plan view of cut and edge sealed cellular
sheeting showing a representative pattern of seal leg members, a
knife cut edge, and a sealed edge.
[0030] These figures, which are idealized and not to scale, are
intended to be merely illustrative and non-limiting.
DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS
[0031] The present invention provides a method for cutting and
thermally edge sealing cellular retroreflective sheeting.
Retroreflectivity of the sheeting is maximized both by (1)
minimizing the width of the sealed edge, and by (2) eliminating
open cells along the edge of the sheeting that collect dirt and
water during subsequent adverse handling, cleaning, and/or outdoor
exposure to weather. Moreover, this invention solves problems in
cutting and edge sealing cellular sheeting as follows:
[0032] (1) A face member and a base member of different chemical
compositions can be used.
[0033] (2) Brittle members that tend to fracture away from the edge
seal during cutting can be used.
[0034] (3) An adhesive layer on a protective liner may be disposed
beneath the base member during the cutting and edge sealing
step.
[0035] (4) The temperature required for edge sealing preferably did
not destroy the effectiveness of the retroreflective elements,
and
[0036] (5) A reliable procedure, a water holdout test, is disclosed
to ascertain whether edge sealing is satisfactory without waiting
for long periods of exposure to outdoor weather.
I Types of Cellular Sheeting
[0037] A. Glass Microspheres as Retroreflective Elements
[0038] FIG. 1 shows a sectional view of a suitable cellular
retroreflective sheeting 10 having a substantially transparent face
member 11, a base member 12, and supporting seal leg members 13
holding the face member in spaced relation to the base member. The
face member, base member, and seal leg members form air cells 15.
The members forming cells are called encapsulating members. At
least one of the encapsulating members is thermoformable, that is,
it deforms under thermal conditions with or without the addition of
pressure. Thermoplastic polymers are especially suitable for
thermal edge sealing. The term "thermoplastic" is used in its
conventional sense to mean a material that softens when exposed to
heat and returns essentially to its original condition when cooled.
The temperature required to cut and seal the edges on the cellular
sheeting is called the thermoforming temperature. The thermoforming
temperature will vary depending on the materials used for the
encapsulating members. The thermoforming temperature is
sufficiently high to result in the encapsulating members forming
sealed edges after the edge has been cut, but yet not so high as to
damage the retroreflectivity of the sheeting. Thermoforming
temperatures may range from about 60 degrees C. to about 150
degrees C. More preferably the thermoforming temperature is in a
range from about 80 degrees C. to 120 degrees C. An example of
cellular retroreflective sheeting of this type is available from 3M
under the designation SCOTCHLITE.TM. High Intensity Grade Sheeting
Series 3870 white. Description of retroreflection and
retroreflective sheeting is found in "Standard Specification for
Retroreflective Sheeting for Traffic Control" ASTM D 4956-94
(November, 1994). Cellular sheeting, classified as Type III, is
illustrative of one type of material suitable for cutting and edge
sealing according to the present invention.
[0039] Illustrative examples of cellular sheeting using
microspheres, also called glass beads, as the retroreflective
elements are disclosed in U.S. Pat. Nos. 3,190,178 (McKenzie);
4,025,159 (McGrath); 4,897,136 (Bailey); 5,064,272 (Bailey);
5,066,098 (Kult); 5,069,964 (Tolliver); 5,714,223 (Araki); and
5,378,520 (Nagaoka); the contents of each of which is incorporated
by reference.
[0040] As shown in FIG. 1, the left cell 15A along the left edge
and the right cell 15C along the right edge are open because seal
leg members are not present on all sides of the cell, whereas the
central cell 15B is closed or sealed because the encapsulating
members are present on all sides of cell 15B. The base member has a
first side and a second side. Optional members on the second side
of the base member are an adhesive layer 17 and a protective liner
18. Usually, though not necessarily, both the adhesive layer and
the protective liner are present during the cutting and edge
sealing of the sheeting.
[0041] The face member may also be called the cover film, top film,
or top coat. The face member serves as a substantially transparent
layer that protects the optical elements from a variety of possible
destructive effects, such as dirt, water, and exposure to weather
and outdoor conditions. The face member may be a single material
but may also comprise layers of different materials. Materials
selected for the face member are preferably dimensionally stable,
durable, weatherable, and readily formable into a desired
configuration. A representative material that is especially useful
includes polymethylmethacrylate. Illustrative examples of other
face member materials are fluorinated polymers, ionomeric ethylene
copolymers, low density polyethylenes, plasticized vinyl halide
polymers, polyethylene copolymers, polyethylene terephthalate,
cellulose acetate, cellulose acetate butyrate, ethylene/acrylate
acid copolymer, and aliphatic and aromatic polyurethanes. The face
member is selected to be sufficiently thick to provide the above
desirable properties. The thickness of the face member may
preferably be between about 0.025 millimeters to 0.25 millimeters
thick, and more preferably will be between 0.05 millimeters to 0.1
millimeters. In addition to thermoplastic face member materials,
other materials that undergo reaction both internally and with
material used for the seal leg members may be used. The face member
may be multilayer as disclosed in U.S. Pat. No. 5,066,098 (Kult).
For example, suitable face members may have a Vicat softening point
between 140.degree. C. to 170.degree. C. when tested according to
ASTM D1525.
[0042] The base member may also be called the binder layer or
cushion coat. The base member typically has a first side containing
a layer of retroreflective elements and a second side typically for
disposing an adhesive layer and a protective liner thereon. The
base member preferably comprises a durable polymeric material that
provides adhesion to the glass beads. Some illustrative examples
include thermoplastic, heat-activated, ultraviolet cured, and
electron beam cured polymer systems. Preferred base member
materials soften sufficiently to flow under pressure at between
about 75.degree. C. to 95.degree. C. but remain substantially firm
to retain the glass beads in a layer at temperatures below about
65.degree. C. The base member may be subsequently cured or
crosslinked as taught in U.S. Pat. No. 4,025,159 (McGrath). Typical
useful materials are acrylic-based monomers, such as polyethylene
glycol diacrylates and hydroxymethyl diactone acrylamide and
acrylic-based polymeric materials, such as acrylate or
methylacrylate polymers or copolymers. The base member may further
comprise adjuvents, for example, a whitening pigment, such as
titanium dioxide, or other suitable colorants. The base member is
sufficiently thick to retain the glass beads and yet not so thick
that material is wasted or the sheeting becomes so thick that edge
sealing becomes more difficult. Thus, the thickness of the base
member is at least 0.03 mm, more preferably at least 0.06 mm, but
generally the thickness does not exceed about 0.3 millimeters.
[0043] The seal leg members may also be called supporting walls,
bonds, septa, or simply seal legs. The seal leg members are
typically formed by application of heat and pressure to the face
member and the base member as disclosed in U.S. Pat. No. 3,190,178
(McKenzie). For example, the face member and the base member may be
laminated together and pressed between two platens heated to
150.degree. C. When one platen is smooth surfaced and the other has
a pattern of 0.75 millimeter high by 0.25 millimeter wide
protrusions, a corresponding pattern of seal leg members is formed
which results in the formation of hermetically sealed air cells
between the face member and the base member. An alternate method to
form the seal leg members is to coat a pattern of narrow lines of
the base member material onto the base member and then thermally
laminate the face member to the base member. These methods and
other variations for forming seal leg members are known to those
skilled in the art. Seal leg members have a height sufficient to
provide an air interface for the glass microspheres. The width of
the seal leg members is preferably less than 1.5 millimeters and
more preferably between about 0.75 millimeters to 1 millimeters,
but width may be selected as desired.
[0044] A layer of retroreflective elements as glass microspheres 19
is contained within each cell so that the glass beads have an air
interface. Preferably, the glass microspheres are partially
embedded (e.g., to approximately half of their diameter) into the
first side of the base member. Glass microspheres have diameters
preferably between about 0.05 to 0.15 millimeters, with diameters
more preferably between about 0.06 to 0.08 millimeters. A
reflective layer is on the embedded portion of the glass
microspheres. The other portion of the glass microspheres is
exposed to air.
[0045] The total thickness T of the cellular sheeting containing
glass microspheres as retroreflective elements is typically between
about 0.25 to 0.75 millimeters, but may be more or less depending
on the members used.
[0046] B. Cube Corners as Retroreflective Elements
[0047] FIG. 2 shows a sectional view of a suitable cellular
retroreflective sheeting 20 having a body member 21 comprising a
substantially transparent face member 22 and an opposing layer of
retroreflective elements as cube corner elements 23, a base member
24, and supporting seal leg members 25 holding the face member in
spaced relation to the base member. Preferably, the face member,
base member, and seal leg members form air cells. The members
forming such cells are called encapsulating members. The
description regarding thermoformability and thermoforming
temperature of the encapsulating members can be found in Section A
above.
[0048] Suitably, the base member has a first side and a second
side. Preferably, the first side of the base member is one of the
encapsulating members for an air interface for the layer of
retroreflective elements contained within each cell 26. FIG. 2
shows an open cell 26A on the left edge and only one closed cell
26B. If desired, the second side of the base member may have an
adhesive layer 27 and a protective liner 28 disposed thereon.
Usually, though not necessarily, both the adhesive layer and the
protective liner are present during the cutting and edge sealing of
the sheeting. An example of cellular cube corner retroreflective
sheeting is available from 3M under the designation SCOTCHLITE.TM.
DIAMOND GRADE.TM. Conspicuity Grade Sheeting Series 960 white.
Description of retroreflection and retroreflective sheeting is
found in "Standard Specification for Retroreflective Sheeting for
Traffic Control" ASTM D 4956-94 (November, 1994). Cellular
sheeting, classified as Type IV, Type V, and Type VI, are examples
of sheeting useful for cutting and edge sealing according to the
present invention.
[0049] Illustrative examples of cube corner-based retroreflective
sheeting are disclosed in U.S. Pat. Nos. 5,138,488 (Szczech );
5,450,235 (Smith); 5,614,286 (Bacon ); 5,706,132 (Nestegard);
5,714,223 (Araki); and 5,754,338 (Wilson); the contents of each of
which is incorporated by reference.
[0050] As shown in FIG. 2, the body member comprises a
substantially transparent face member, and an opposing cube layer
which is a layer of retroreflective elements as cube corner
elements. The face member may also be called an overlay film. The
face member may be the same as previously discussed in the Section
A above. The body member may have a land layer between the cube
layer and the face member. The base triangles of the cube corner
elements form part of the land layer. The land layer has a
thickness preferably less than 0.25 millimeters and more preferably
is desired to be at a minimum. The polymeric materials selected for
the cube layer tend to be hard rigid materials with a high Vicat
softening temperature relative to other polymers. Some of these
materials may be brittle or easily fractured when at room
temperature or lower temperatures. Illustrative examples of
suitable materials for the cube layer include acrylic polymers,
acrylic epoxy, polycarbonates, polyimides, and mixtures
thereof.
[0051] The base member may also be called a backing sheet or
sealing film. The base member has a first side in contact with
either the air cells or the seal leg members. The base member
preferably has a second side typically having an adhesive layer
with a protective liner disposed thereon. The base member may be
disposed beneath the cube layer for the purpose of hermetically
sealing the bottom of the air cells. Preferably the base member
comprises a thermoplastic material. Preferred polymers for use as
the base member are within the styrenic family of multiphase
copolymer resins as described in U.S. Pat. No. 5,754,338 (Wilson).
Typically the Vicat softening temperature of the base member is
about 30.degree. C. less than that of the cube layer. The thickness
of the base member is sufficient to provide an air interface for
the optical elements and to protect the optical elements from
exposure to factors that lower their optical efficiency, such as
dirt and water. The thickness of the base member may be in the same
ranges as described for the face member.
[0052] The seal leg members are typically formed by heat and
pressure applied through the base member using a patterned
embossing roll heated to a temperature above the temperature at
which the base member thermoforms. In a typical thermal/mechanical
method of forming the seal leg members, the temperature of the
embossing roll is at least 10.degree. C. higher, preferably
30.degree. C. higher, and more preferably 50.degree. C. higher than
the Vicat softening temperature of the base member. The base member
is forced into depressions of the cube corner elements to form the
seal leg members. The tips of the cubes in the seal leg member may
deform to form a flange as described in U.S. Pat. No. 5,754,338
(Wilson). If the cube corner elements are parted in places, the
base member may also seal to the face member in these places. In
addition to thermoforming techniques, other techniques, such as
ultrasonic welding, radio frequency welding, thermal fusion, and
reactive welding, may be used with various degrees of success. The
width and height of the seal leg members was discussed in Section A
above.
[0053] The cube corner elements may also be called prisms,
microprisms, or triple mirrors. The basic cube corner
retroreflective element is generally a tetrahedral structure having
a base triangle and three mutually substantially perpendicular
optical faces that cooperate to retroreflect incident light. The
optical faces preferably intersect at an apex, with the base
triangle lying opposite the apex. Each cube corner element also has
an optical axis, which is the axis that extends through the cube
corner apex and trisects the internal space of the cube corner
element. Light incident on the base triangle is transmitted into
the internal space, is reflected from each of the three optical
faces, and is redirected back in the same general direction as the
incoming incident light. As noted before, the faces of the cubes
are usually exposed in the air cells to enable the sheeting to
exhibit total internal reflection or "TIR". The height of the cube
corner elements, defined as the length of the optical axis, is
preferably as small as manufacturable for ease of sealing but may
be as large as necessary while recognizing the desirability of
avoiding waste of material and of increasing the thickness of the
sheeting. The minimum height is preferably about 0.01 mm and the
maximum height is preferably less than 1 mm. The height of the cube
elements is more preferably between 0.02 to 0.5 millimeters. These
elements are disposed in a cube layer. This microstructured layer
is molded into the body member to yield a cube layer using any of a
variety of techniques known to those skilled in the art.
[0054] The total thickness T of the cellular sheeting containing
cube corner elements as the retroreflective elements is typically
between about 0.25 to 0.75 millimeters, but may be more or less
depending on the components used. As the thickness of the cellular
sheeting decreases, the difficulty in achieving a narrow
hermetically sealed edge may also decrease.
[0055] Cellular retroreflective sheeting of the type described in
Section I may be cut and edge sealed by the apparatus, tools, and
methods described in the next sections II and III, and according to
specific illustrative examples given in section V.
II. Continuous Process of Heating, Cutting, and Edge Sealing
Cellular Retroreflective Sheeting
[0056] FIG. 3 is a schematic drawing of an apparatus 30 used for
unwinding, heating, slitting, and edge sealing cellular
retroreflective sheeting 31. As initially manufactured, the
sheeting may have, for example, a width of 60 to 120 centimeter and
a length of 50 meters and be wound on a roll 32. If desired, the
sheeting may be pre-heated in an oven or heated by some source as
it proceeds to the cutting and edge sealing step. As shown in FIG.
3, a preferred method of heating the sheeting is by wrapping a
portion of the sheeting on the surface of a rotating hot can 33
with the sheeting in contact with the surface of the can for a
sufficient time to form a heated sheeting. The can or drum then
serves as both the heat source for the sheeting and as the
substrate for the cutting and edge sealing step. The sheeting may
be unwound from the roll on to the hot can. A wheel 34 has a
perimeter with at least one tool mounted thereon. Suitable
exemplary tool designs are described in detail in Section III. At
least one tool is spaced across the width of the sheeting,
according to the desired width of each strip of sheeting. As the
wheel rotates, each tool initially contacts the sheeting, thereby
pressing each tool, the sheeting, and the substrate together. Each
tool preferably is placed under sufficient pressure to have its
tool press against and through a majority of the thickness of the
sheeting. Generally a pressure of 2.8 to 3.5 kilograms per square
centimeters is sufficient to push the tool through a majority of
the thickness of typical sheeting materials, but any suitable
combination of pressure and sheeting may be used. Wheel diameter
may be chosen to enhance the temperature regulation of the tool.
For example, a larger wheel diameter turning at a given revolution
per minute may result in cooling of the tool. Preferably each wheel
has a diameter between about 7 to 15 centimeters. The spacing
between the tools is usually between about 5 to 10 centimeters,
depending on the width of the sheeting desired and/or its
particular specifications. The heated sheeting exits this cutting
and edge sealing operation with substantially complete sealing of
the edges.
[0057] A specific application for the slitting and edge sealing
methods described above is disclosed in the "Code of Federal
Regulations" Transportation Section, Part 49, pages 226, 227, and
273 (October, 1996). For example, for Type V sheeting,
specifications for the final product Grade DOT-C2 sheeting state
that its width is not less than 50 millimeters. The length is 300
millimeters.+-.150 millimeters. Moreover, at an entrance angle of
45 degrees and an observation angle of 0.5 degrees, the final
product Grade DOT-C2 sheeting has a minimum photometric performance
or coefficient of retroreflectivity in candela/lux/square meter of
15 for white and 4 for red. Since the preferred sealed edge
typically has a width of 1.5 millimeters or less and there is a
sealed edge on each side of the strip of sheeting, the area of the
sealed edges compared to the area of the strips of sheeting is
small so that retroreflectivity is retained.
[0058] FIG. 4 is a preferred variation of the apparatus shown in
FIG. 3. The sheeting 41 is passed around a tensioning roll 42 and
then wrapped onto a rotating hot can 43 as before. A wheel 44
having at least one tool thereon is mounted as before to provide
means for cutting and slitting the heated sheeting. Additional
external heating means 46, such as a quartz heater, is mounted as
shown to quickly raise the temperature of the hot can when the
sheeting is moving around the hot can. When the sheeting is not
moving but remains in contact with the hot can, the external
heating means may be turned off so that the heated sheeting does
not degrade while resting for extended periods on the hot can.
III. Tool Designs
[0059] FIG. 5 is a cross section of a suitable tool 50 having a
first end 51 and a second end 52, the first end for cutting and
edge sealing having a radiused ridge 53 thereon, and the second end
for mounting onto a device (not shown). The device is usually
movable, such as by a press or a wheel. Preferably the tool has a
symmetrical cross section. The ridge has a cutting and edge sealing
portion, which has a shape approximately corresponding to that
desired for the edge seal. The ridge is smooth and rounded, with
the highest point of the ridge labeled A. Preferably the ridge is
semi-circular in cross-section with a radius R. The radius R forms
a half circular sector, with a quarter of the circle defined by the
points A and B. The radius of the ridge is preferably between about
0.05 to 0.3 millimeters. It is surprising that an unheated tool
having a radiused ridge can cut the sheeting. There are opposing
shoulders 54 on either side of point A. The shoulders are defined
by the radii R1 and R2. Typical values for R1 and R2 are preferably
between about 0.05 to 0.3 millimeters and more preferably between
about 0.25 to 0.5 millimeters, respectively. Preferably the center
for the quarter circle sector formed by R1 is spaced approximately
0.5 millimeters apart and 0.1 millimeter below the center for the
radius R of the ridge. The radius R1 forms a quarter circle sector
shown by points C and D. Preferably the center for the quarter
circle sector formed by R2 is spaced approximately 1 millimeter
apart and 0.6 millimeters below the center for the radius R of the
ridge. The radius R2 forms a quarter circle sector shown as points
E and F. A height H of the ridge was defined as the vertical
distance between the point A of the ridge and the point E of the
shoulder. Using preferred dimensions above, the height of the ridge
is 0.36 millimeters. A preferred height is defined relative to the
thickness T of the sheeting. Thus the fraction of H/T is less than
one but more than 0.2. More preferably, the fraction of H/T is
between about 0.3 to 0.8. Thus, if the height of the tool is 0.36
millimeters and the thickness of the sheeting is 0.5 millimeters,
H/T is calculated to be 0.7. There may be curvilinear portions,
such as a curved portion between points B and C or a curved portion
between points D and E. These curvilinear portions may be slanted
to either the horizontal or vertical directions. For example,
curved portion BC is preferably slanted at an angle of 5 degrees
from the vertical direction. Curved portion DE is preferably
slanted at an angle of 5 degrees to the horizontal direction. The
width W of the tool is as wide or wider than the desired width of
the sealed edge. The width of the tool is typically about 1.8
millimeters. Since the preferred width of the sealed edge is less
than 1.5 millimeters, this width of the tool is sufficient to
ensure a satisfactory edge seal. This tool is symmetrical because
the shoulders are centered on the centerline 56.
[0060] The inventive shape of the tool described above results in
the sealed edges of the heated sheeting having no sharp corners or
flats. Rounding of the sealed edge is also believed to be assisted
by the surface tension effect depending on the chemical
compositions of the encapsulating members. It will be appreciated
that although one preferred shape of the tool has been specifically
defined, any shape of tool that results in smooth curved surfaces,
such as drawn by a French curve, is preferred for cutting and edge
sealing of the heated sheeting. Representative materials for the
tool are carbide steel and high carbon steel, although any suitable
durable material can be used. These materials are selected because
the components of the sheeting are extremely abrasive and
destructive to the tool. For example, glass microspheres are
abrasive and the various members may contain fillers, colorants, or
other additives dispersed therein that are abrasive.
[0061] FIG. 6 is a cross section of the tool 60, the sheeting 61,
and the substrate 62 being pressed together until the ridge 63
contacts the substrate. Ideally contact between the tool and the
substrate is minimized, which minimizes wear on either the tool or
the substrate. A radiused ridge on the tool also minimizes wear.
FIG. 6 is a schematic depiction of what is believed to happen to
the sheeting during the cutting and edge sealing step. The
temperature of the sheeting increases when in contact with the hot
substrate to form a heated sheeting. Alternately the sheeting may
be pre-heated to form a heated sheeting. Preferably the tool, the
heated sheeting and the substrate are pressed together. The tool
initially presses against the face member 64. Pressure on the tool
forces the tool through a majority of the thickness of the heated
sheeting and squeezes the various encapsulating members of the
heated sheeting together until the various members seal to each
other and seal the edges. The edges are formed by the tool cutting
the sheeting. During and after the sheeting is cut, a sealed edge
results on each side of the heated sheeting. The temperature of the
tool preferably remains at a lower temperature than the
thermoforming temperature of the heated sheeting and/or temperature
of the substrate partly because only the cutting and sealing
portion of the tool is in contact with the heated sheeting and/or
substrate. While not intending to be bound by theory, it is
believed to be important to maintain the temperature of the tool
lower than the temperature of the heated sheeting. The step of
maintaining the temperature of the tool near room temperature is
preferred because this step prevents debris from the sheeting from
building on the tool and on the sealed edges. Preferably the tool
is at a temperature of 30.degree. C. or less and the heated
sheeting is at a temperature of 70.degree. C. or more. However the
heated sheeting preferably should not be at a temperature so high
that retroreflectivity is permanently damaged. It is surprising
that in the Examples provided herein the thermoforming temperature
did not significantly destroy the retroreflectivity of the
sheeting.
[0062] FIG. 7 is a cross sectional view of a non-symmetrical tool
70 having a left shoulder 72 that is narrower than the right
shoulder 74 because the radiused ridge 76 is not centered on the
tool. The centerline 78 of the radiused ridge divides the shoulder
into uneven portions. A non-symmetrical tool cuts the sheeting and
gives different widths of sealed edges because, as shown in this
case, the left shoulder is narrower than the right shoulder. The
sealed edge on the sheeting resulting from the left shoulder of the
tool would tend to be narrower than the sealed edge from the right
shoulder of the tool. The sealed edge on the right side of the
sheeting results from the left side of the tool, with the left side
of the tool comprising the left shoulder and the left half of the
radiused ridge.
[0063] FIG. 8 is a cross sectional view of a tool 80 having a
radiused ridge 81 with opposing shoulders wherein each shoulder has
an flat portion 83 after the curved portion 82, with the curved
portion of each shoulder jointing a side of the radius ridge.
Although the radiused ridge and the curved portion of the shoulder
function to seal the edge of the sheeting as before, the flat
portion of the shoulder may assist in providing more surface area
of the tool in contact with the sheeting for holding the sheeting
in place during the sealing process.
[0064] FIG. 9 is a cross sectional view of a tool 90 having a
radiused ridge 91 with opposing shoulders 92 such that each side of
the ridge and each corresponding shoulder are formed as smooth
curves, such as those drawn by a French curve. The tool may or may
not be symmetric depending on the symmetry of the opposing
shoulders around the center line 93. The advantage of this tool
design is that a minimum area is in contact with the heated
sheeting during cutting and edge sealing, thus maintaining the tool
at a lower temperature than that of the heated sheeting without the
need for exterior cooling of the tool.
[0065] FIG. 10 is a side view of a device 100 in the form of a
rotating wheel 101 having at least one annular or
semi-cylindrically shaped tool 102 mounted on the perimeter of the
wheel. Alternately each tool may be mounted on the perimeter of a
wheel and the wheels spaced across the width of the sheeting.
Preferably the cross section of the tool is as shown in FIG. 5. The
sheeting 103 is shown on the substrate 104. Preferably the
substrate is heated and thus the temperature of the sheeting
increases when in contact with the hot substrate to form a heated
sheeting. Alternately the sheeting may be pre-heated to form a
heated sheeting. As shown, the tool gradual squeezes the heated
sheeting toward the substrate. As the wheel presses against the
heated sheeting, the face member 105 is contacted and compressed.
Further rotation of the wheel squeezes all of the members of the
sheeting together to initiate sealing. As the tool either nearly
contacts or completely contacts the substrate, cutting results to
form two edges, one on either side of the tool. The two edges
formed by the cutting are completely sealed by the tool.
[0066] FIG. 11 is a side view of the same device 110 as shown in
FIG. 10, except the rotating wheel 111 has a larger diameter. The
cutting and edge sealing step is believed to remain the same as
previously described, except the squeezing of the heated sheeting
112 is much more gradual because the tool 113 rotates slower into
the heated sheeting. This assumes the small diameter wheel of FIG.
10 and the large diameter wheel of FIG. 11 have the same rotational
speed or revolutions per minute. Given that the rotational speed of
the wheel is fixed, the advantage of a large diameter wheel is that
the cutting and edge sealing process is slower and the tool remains
at a lower temperature than with a small diameter wheel.
Alternatively, the speed of the heated sheeting past the tool could
be increased for the larger diameter wheel and still maintain the
same temperature of the tool.
IV Results of Cutting and Edge Sealing Cellular Retroreflective
Sheeting
[0067] FIG. 12 is a plan view of the sheeting 120 after the cutting
and edge sealing step. The shape of the interior encapsulated cells
121 formed by a pattern of seal leg members 122 is shown as square.
However any closed continuous pattern of the seal leg members is
suitable, including for example, rectangular, hexagonal, chain
link, and circular. The square pattern shown is typically
approximately 4 by 4 millimeters. If the pattern were an elongated
hexagonal pattern, then each air cell is typically 5 by 10
millimeters. The width of the seal leg members is typically 0.75 to
1 millimeters. The sealed edge 123 is parallel to the length of the
sheeting. The width of the edge seal is preferably less than 1.5
millimeters. More preferably, the width of the edge seal is 0.75 to
1 millimeters, similar to the width of the seal leg members.
However edge seals as narrow as only 0.25 millimeters are
attainable with such narrow edge seals passing the water holdout
test, which is described in Section V. The width of the sealed edge
can be measured with a ruler. The sealed edge tends to have a
different color than the remainder of the sheeting. Since the
sealed area has little or no retroreflectivity, measuring the
contrast between the brightness in retroreflection is another way
to ascertain the width of the sealed edge or the percentage of
sealed edge surface area to retroreflective sheeting surface
area.
[0068] Normal slitting or just cutting 124, shown on the left side
of the sheeting, leaves open cells along the edge, which is
undesirable for the reasons previously described.
EXAMPLES
[0069] The present invention will now be described with reference
to certain Examples, which are illustrative only. The following
tests were used to evaluate cut and edge sealed samples of cellular
retroreflective sheeting of the present invention.
[0070] A. Water Holdout Test for Evaluating Cell Integrity of
Cellular Retroreflective Sheeting
[0071] The purpose of the water holdout test is to evaluate the
resistance of edge cells to withstand penetration by water. This
test method is an evacuated and then water immersion method to
evaluate the extent to which the process conditions yielded a
hermetically sealed edge. The equipment consists of one 200-250
millimeter inside diameter PYREX glass desiccator, vacuum pump
(0-1.0.times.10.sup.5 Pascals) vacuum gauge, valve, and associated
vacuum tubing, liquid dish soap (mild surfactant), and
approximately 50-gram weight.
[0072] The desiccator is filled half full with tap water and 4-6
drops of the liquid dish soap is then added to the water. The test
is run at room temperature (typically about 20-25.degree. C.). The
sample is immersed in the soap solution and held submerged by the
50-gram weight. The desiccator is closed. A vacuum pump is attached
to the nozzle of the desiccator lid via a vacuum hose containing a
gauge and a bleeder valve. The pressure in the desiccator is
reduced to 85.times.10.sup.3 Pascals below ambient pressure and
maintained for one minute. The vacuum pump is then turned off and
the bleeder valve is opened to allow air into the desiccator until
the pressure is equalized to that of atmospheric pressure. The
desiccator is opened and the sample removed. The amount of water
intrusion into the edge cells is observed. Then the percent of
unfilled edge cells is calculated by measuring the number of
unfilled edge cells times 100 divided by the total number of edge
cells along the length of the sample. Unfilled edge cells are those
edge cells not containing water. The sheeting is considered
satisfactory when at least 80 percent, preferably at least 90
percent and most preferably at least 95 percent of the edge cells
are found to be unfilled with water. Desirably all of the edge
cells are found to be hermetically sealed or unfilled with
water.
[0073] B. Vicat Softening Point Test
[0074] The Vicat Softening Point of the indicated materials is
determined according to ASTM D1525.
[0075] C. Retroreflective Brightness Test
[0076] The coefficient of retroreflection R.sub.A is measured in
accordance with standardized test ASTM D4956-94. R.sub.A values are
expressed in candelas per lux per square meter (cd/lux/m2). The
entrance angle is the angle between an illumination axis from a
light source and a retroreflector axis normal to the surface of the
retroreflective article. The entrance angle was selected to be -4
degrees. The observation angle is the angle between the
illumination axis from the light source and the observation axis.
The observation angle was chosen to be 0.2 degrees.
Example 1
[0077] A 120 centimeter wide roll of cellular cube corner
retroreflective sheeting, available from 3M under the designation
SCOTCHLITE.TM. DIAMOND GRADE.TM. Conspicuity Sheeting Series 981-32
red/white, was provided. This sheeting is designed for truck and
trailer markings for enhanced visibility. The face member was a
polymethylmethacrylate plastic that was thermally deformable at a
temperature selected for edge sealing. The cube layer was
polycarbonate, a relatively stable plastic at the temperature
selected for edge sealing. This sheeting also had a sealing member,
an adhesive layer, and a protective liner. The sealing member was
polyethylene terephthalate, a plastic that also is thermally
deformable at the temperature selected for edge sealing. The
thickness T of the sheeting was approximately 0.45 millimeters. The
sheeting had a length of approximately 45 meters and was wound into
a roll.
[0078] The apparatus used for this experiment is shown in FIG. 3.
The sheeting was unwound from the roll at approximately 13 meters
per minute and then wrapped around a portion of the diameter of a
hot can containing hot oil. The diameter of the can was
approximately 60 centimeters. The surface temperature of the can
was approximately 150.degree. C. The temperature of the sheeting in
contact with the hot can increased to approximately 110.degree. C.,
thus forming a heated sheeting. Thus the thermoforming temperature
was 110.degree. C. The moving device was a wheel as shown in FIG.
3. The diameter of each wheel was approximately 7.5 centimeters.
The wheel had a tool mounted thereon with the tools spaced at 50
millimeter intervals across the width of the heated sheeting. The
preferred dimensions for the tool were used, as illustrated in FIG.
5. Thus the ridge of the tool had a radius R of approximately 0.13
millimeters. The shoulders were defined by R1 equal to 0.13
millimeters, and R2 equal to 0.38 millimeters. The center for the
quarter circle sector formed by R1 was spaced approximately 0.5
millimeters apart and 0.1 millimeter below the center for the
radius R of the ridge. The center for the quarter circle sector
formed by R2 was spaced approximately 1 millimeter apart and 0.6
millimeters below the center for the radius R of the ridge The
height H was equal to 0.26 millimeters. The fraction of H/T was
0.58. The width W of the tool was 1.8 millimeters. The temperature
of each tool was approximately 30.degree. C. The slit and sealed
heated sheeting was then wound up as rolls 50 millimeters wide.
When viewed from the side, each roll appeared to be smoothly slit
and had a sharp, clean color because of the absence of sheeting
debris. The width of the sealed edge was measured with a ruler to
be 0.5 millimeters. When a sample of the sheeting, 6.25 centimeters
wide by 15 centimeters long, was subjected to the water holdout
test procedure, more than 95 per cent of the edge cells were found
to be unfilled with water.
Example 2
[0079] This example used a different type of sheeting for cutting
and edge sealing than used in Example 1. A 120 centimeter wide roll
of cellular cube corner retroreflective sheeting, available from 3M
under the designation SCOTCHLITE.TM. DIAMOND GRADE.TM. Conspicuity
Sheeting Series 960 white, was provided. The face member comprised
a polyurethane that was thermally deformable at the temperature
selected for edge sealing. This sheeting also had an acrylic cube
layer that was relatively stable at the temperature selected for
sealing. A sealing member comprised a multiphase styrenic
thermoplastic copolymer which was thermally deformable at the
temperature selected for edge sealing. An adhesive layer and a
protective liner were disposed beneath the sealing member. The
thickness T of the sheeting was approximately 0.45 millimeters. The
length of the sheeting was approximately 45 meters and was wound
into a roll.
[0080] Using a method similar to that described in Example 1, the
sheeting was unwound from the roll at approximately 13 meters per
minute onto a hot can containing hot oil. The surface temperature
of the can was approximately 100.degree. C. The thermoforming
temperature of the heated sheeting was approximately 75.degree. C.
The moving device was a wheel having a diameter of approximately
7.5 millimeter with tools mounted thereon and each tool spaced at
50 millimeters as described in Example 1. The tool design was also
the same as described in Example 1. The temperature of each tool
was approximately 25.degree. C. The slit and sealed heated sheeting
was then wound up as rolls 50 millimeters wide. When viewed from
the side, each roll appeared to be smoothly slit and had a sharp,
clean color because of the absence of sheeting debris. The width of
the sealed edge was measured with a ruler to be 0.2 millimeters.
When a sample of the sheeting, 6.25 centimeters wide by 15
centimeters long, was subjected to the water holdout test
procedure, more than 95 per cent of the edge cells were found to be
unfilled with water.
Example 3
[0081] This example is to show that the process need not be
continuous. A press was used to press the tool into the sheeting to
form a desired shape. A piece of the sheeting, as described in
Example 2, which was 21 centimeters wide and 28 centimeters long,
was placed onto a substrate that was rigid and flat. The
temperature of both the substrate and the sheeting was adjusted to
be approximately 75.degree. C. The thermoforming temperature of the
heated sheeting was 75.degree. C. The tool had the same design as
previously described in Examples 1 and 2. The second end of the
tool was mounted on a stamper attached to a press, with the stamper
shaped as a semi-circular segment of a cone having a width of 15
centimeters wide and arcuate length of 45 centimeters. The design
of the tool, shown in FIG. 5, remained the same as in Example 1.
The stamper was lowered, thereby pressing the tool through the
heated sheeting. Lowering of the tool was stopped when the top of
the ridge contacted the substrate. The tool was at a pressure of 3
kilograms per square centimeter. The tool was in contact with the
heated sheeting for only a few seconds and thus remained near room
temperature. On withdrawal of the tool from the heated sheeting, a
segment of sheeting was stamped out of the original piece. Such
segments of sheeting are useful to wrap around traffic cone
devices. The sealed edge of this segment was found to have (1) a
width of only 0.2 millimeters, and (2) more than 95 per cent of the
edge cells unfilled with water when subjected to the water holdout
test procedure.
[0082] The various modifications and alterations of this invention
will be apparent to those skilled in the art without departing from
the scope and spirit of this invention and this invention should
not be restricted to that set forth herein for illustrative
purposes only.
* * * * *