Plastic and nonmetallic article shaping or treating: processes – Optical article shaping or treating – Continuous or indefinite length
Reexamination Certificate
1999-04-13
2001-05-01
Vargot, Mathieu D. (Department: 1732)
Plastic and nonmetallic article shaping or treating: processes
Optical article shaping or treating
Continuous or indefinite length
C156S251000, C156S515000, C264S001900, C264S002700, C264S163000
Reexamination Certificate
active
06224792
ABSTRACT:
TECHNICAL FIELD
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
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. No. 4,025,159 (McGrath) and U.S. Pat. No. 5,706,132 (Nestegard). This type of sheeting may also be called encapsulated lens sheeting.
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™ 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. 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:
(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.
(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.
(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.
(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.
(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.
(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.
The numerous disadvantages associated with these prior methods indicate the need for a new, effective, and efficient cutting and edge sealing method.
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
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.
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° C. with the thermoforming temperature of the heated sheeting at more than 70° 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.
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.
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.
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3M Innovative Properties Company
Gwin Doreen S. L.
Vargot Mathieu D.
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