US 3907193 A
Flexible fold lines in semi-rigid and rigid plastic sheet material (e.g., polyvinyl chloride) are provided with stress-relief indentations so that the tensile and compressive strains normally produced at the folds are substantially relieved, thereby permitting efficient production of a variety of sheet plastic articles such as folding containers and the like. The stress-relief indentations are simultaneously formed on opposite surfaces of the sheet material at the fold line by an apparatus having a die containing a heated scoring blade and a platen which can be preheated by contact with the heated scoring blade prior to insertion of the plastic sheet material between the scoring blade and platen. The temperature of the scoring blade and platen and the pressure with which they are brought to bear on the plastic sheet material disposed between them during the scoring operation are such that the thickness of the sheet material is substantially reduced at the fold line by compression and removal of material without any substantial lateral displacement flow thereof away from the fold line. In this way, parallel indentations are formed on both surfaces of the sheet material at the fold line which serve to relieve the tensile strains on the outer fold surface and compression strains on the inner fold surface normally produced when the material is bent about the fold line.
Claims available in
Description (OCR text may contain errors)
United States Patent Heller 1 Sept. 23, 1975 PLASTIC FOLDING CONTAINERS AND  ABSTRACT PROCESS AND APPARATUS FOR MAKING Flexible fold lines in semi-rigid and rigid plastic sheet SAME material (e.g polyvinyl chloride) are provided with [75 Inventor; J l Heller, Greenlawn, stress-relief indentations so that the tensile and compressive strains normally produced at the folds are  Assgnem Au toplex Corporauon Roslyn substantially'relieved, thereby permitting efficient pro- Helghts duction of a variety of sheet plastic articles such as [22 Filed; Apt 8, 1974 folding containers and the like. The stress-relief indentations are simultaneously formed on opposite sur- [211 Appl' 458838 faces of the sheet material at the fold line by an appav ratus having-a die containing a heated scoring blade 152 US. Cl. 229/16; 229/010. 4; 220/31 s; and platen which can be preheated y Contact with 150/5; 229/33 the heated scoring blade prior to insertion of the plas-  Int. Cl. B65D 5/00; B65D 43/16 tic Sheet material between the Scoring blade and 58 Field of Search 220/31 s, 62, 16 A, 16 B; p The temperature of the Scoring blade and 229/30 38 25 IG 4 platen and the pressure with which they are brought to bear on the plastic sheet material disposed between  References Cit d them during the scoring operation are such that the UNITED STATES PATENTS thickness of the sheet material is substantially reduced 2,144,318 1/1939 Kryder 150/.5 thefold l ccfmprfasslon and l of matc' 3221437 12/1965 Schilling zzg/Dla 4 r1al w1thout any substannal lateral displacement flow 3,344,913 10/1967 Best 220/31 s x thereof away from fold P"lrallel 3,446.415 5/1969 Bromley 229/30 indentationsv'are formed both surfaces of the Sheet 3 727 325 4 973 Tmth l l y v 229/1)[(; 4 material at the fold line which serve to relieve the ten- 3.786.982 1/1974 Rakes ct al. 229/25 sile strains on the Outer fold surface and compression Primary ExaminerDavis T. Moorhead Attorney, Agent, or Firm-Pennie & Edmonds QCltiims, 8 Drawing Figures US Patent Sept. 23,1975 Sheet 1 of2 FIG.
US Patent Sept. 23,1975 Sheet 2 on 3,907,193
PLASTIC FOLDING CONTAINERS AND- PROCESS This invention relates to the art of forming fold lines in semi-rigid and rigid plastic sheet material, and more particularly to a process and apparatus for forming fold lines in plastic sheet material to facilitate the formation of novel products such as plastic boxesor containers.
With the development ofa variety of synthetic plastic materials available in sheet form, attempts have-b'een made to utilizethem in the fabrication offolding car-' tons or boxes. Thus, for example, as reported in Mdern Packaging, April, 1964, page 161 and October, l966,-pages 127-129 and Paper, Film and Foil Converter, October,'- 1966, pages 68-71, attempts were made to make a see through folding box of transparent synthetic resin sheet material such as poly(vinyl chloride) by cutting out a blank, folding it along scored fold lines, cementing together two opposite free edges of the blank, and then flattening the assembly sothat the item can be transported and stored. When ready for use, the flattened box is erected, filled with material, and then closed by inserting foldingflaps formed on the bottom and top ends of the box blank. 3
Previous efforts at forming fold lines in a sheet plastic carton blank to provide the necessary flexibility. at the fold line to permit setting up the box from the flat blank have proved to be unsatisfactory due to the fact that damage or undesired discoloration (e.g., blushing or whiteness) of the sheet material occurs at or near the fold line during folding. Also, when the box blank is flattened for storage or shipment or when the flattened box is'erected preparatory'to filling, the panels and flaps of the box do not remain in the-desired condition but rather, tend to revert to a state intermediate of the flattened and opened conditions clue to the springiness orelastic recovery tendency of the material being used to form the box. This elastic behavior tends to interfere with the automatic folding, filling, and closing operations currently being practiced in commercial packaging facilities using paperboard boxes and cartons.
Scoring of sheet material to form a fold line has heretofore been performed by bringing a scoring blade against the sheet material, which is backed up by a platen so that the sheet material is deformed out of the plane of the sheet to form the desired score line. In forming conventional paperboard boxes or cartons, the blades are unheated,-and the fold line is formed by what is referred to as cold scoring, as described for example in U.S. Pat. Nos. 3,137,217 and 3,529,516. Similar cold scoring techniques have been attempted in connection with plastic sheeting as indicated, for exam' along the score line, when the sheet material is folded or set-up into the carton. This problem is recognized in U.S. Pat. No. 3,589,022, which describes attempts to obviate this difficulty by employing" heated scoring blades to form the desired score line indentation on one side of the sheet. It has, however, been found that even where the, scoring blade is heated and brought down on one side of the sheet material sandwiched between the scoring blade and an unheated platen, with the scoring blade penetrating the sheet material on one side of the sheet, there is still produced a weakness and crazing discoloration of the plastic material, though perhaps not-as great as in connection with cold scoring.
Other methods of hot-scoring plastic sheet material are described in'U.S. Pat. No. 3,379,814. The use of plastic; molding techniques for imparting fold lines is described in U.S. Pat. Nos. 3,132,649 and 3,201,145.
Therefore, a need has existed in the packaging field foramethod and apparatus for imparting fold lines to plastic sheet material to permit the production of folding containers and-boxes which are free of the aforesaid drawbacks heretofore experienced in the prior art.
Accordingly, it is an object of the present invention to provide an improved fold line for use in plastic articles, with the material being subjected to minimum distortion at the fold line when the material .is folded, thereby improving its strength and appearance.
Another object of the present invention to provide an improved process for forming fold lines in plastic sheet material which impart sufficient flexibility to permit setting up a carton or box from a flattened or knockeddown condition prior to filling without causing damage, tensile or compressive stresses, strain hardening, crazing, or discoloration of the sheet material at or near the fold line while atthe same time permitting the box to remain flattened until opened. and thereafter remain in the intended open condition with minimum elastic recovery to facilitate machine loading. 1
Another object is to provide an improved plastic folding container from sheet material whose walls are joined at foldlines which impart sufficient flexibility to permit setting up the'container from a flattened or knocked-down condition prior to filling without causing damage, tensile or compressive stresses, strain hardening, crazing or discoloration of the sheet material at or near the fold lines while at the same time permitting the box to remain flattened until opened and thereafter remain in the intended open condition with minimum elastic recovery to facilitate machine loading.
Yet another object of. the invention is to provide an apparatus for cutting plastic sheet container blanks and imparting fold lines thereto having sufficient flexibility to permit setting up the container from a flattened or knocked-down condition prior to filling without causing damage, tensile or compressive stresses, strain hardening, crazing, ordiscoloration of the sheet material at or near the fold line while at the same time permitting the box to remain flattened until opened and thereafter remain in the intendedopen condition with mimimum elastic recovery to facilitate machine loading; 1
These and other objects of the invention as well as a fuller understanding of the advantages thereof can be had by reference to the following detailed description, drawings and claims.
BRIEF DESCRIPTION OF THE DRAWINGS The specific details of the method, product and apparatus of the invention willbe particularly described in conjunction with the accompanying drawings wherein:
FIG. 1 is a perspective view of a section of a sheet of plastic material with strain relief indentations formed in accordance with the teachings of this invention;
FIG. 2 is a cross-sectional end-view of the sheet of material shown in FIG. 1 bent through 90, illustrating the nature of the strain relief provided by the invention;
FIG. 3 is a cross-sectional end-view of the sheet of material in FIG. 1 bent through 180, illustrating the nature of the strain relief provided by the invention;
FIG. 4 depicts a photomicrograph taken through a cross-section of plastic material having strain relief indentations formed in accordance with the teachings of this invention;
FIG. 5 is a top plan view showing a typical box blank formed in accordance with the teachings of the invention;
FIG. 6 is a top plan view of a platen provided with heat-retaining strips along the desired fold lines, with the platen shown arranged for simultaneously forming two box blanks of the type shown in FIG. 5;
FIG. 7 is a plan view of the die used in connection with the platen of FIG. 6 with the single line thicknesses in the drawing representing the edges of the cutting blades of the die, and the double line thicknesses representing the edges of the scoring blades of the die; and
FIG. 8 is a cross-sectional view through the die and platen of FIGS. 6 and 7.
SUMMARY OF THE INVENTION The foregoing objects are achieved according to the present invention wherein a fold line in plastic sheet material is provided with strain relief indentations at opposite sides of the sheet material at the fold line to relieve both tensile and compressive stresses normally produced at opposed surfaces of a fold line during bending, thereby minimizing crazing, material weakening, and stress discoloration or whitening during folding. The sheet material thus treated can be formed into a box or carton blank which can be shipped in a flat or knocked-down condition for set-up or erection when desired for use without exhibiting the undesirable elastic tendency to assume a position intermediate of the fully opened and closed conditions.
The fold lines are provided with stress relief indentations by heat scoring applied at opposite faces along the intended fold line. Heat scoring of the plastic sheet material is accomplished according to the present invention by the use of a heated scoring blade and a platen member. The platen surface supporting the plastic material, against which the scoring blade is brought to bear in a transverse direction, is also heated in the vicinity of the score line. The scoring blade is brought against the previously unheated material to be scored at a pressure and temperature which substantially reduces the material thickness between the scoring blade and the platen, desirably to a value of between about 25 and 75 percent of the original thickness, and preferably to a value of between about 40 and 60 percent of said original thickness. In this way, fold lines in plastic sheet material can be readily formed without requiring the use of expensive equipment, and are of a strength and cost which permit the fabrication of plastic boxes and cartons of significantly improved appearance and which are competitive with previously employed paperboard containers.
The temperature of the scoring blade and platen and the pressure with which they are brought to bear (preferably with minimum dwell time) against the plastic sheet material must be adjusted, depending upon the composition of the plastic material, so that the stress relief indentations are created through compaction and removal of material from the site of scoring without any substantial lateral displacement flow thereof away from the fold line. Although suitable temperature and pressure combinations can vary over a wide range, it is desirable to maintain the temperature of the scoring blade within i 10F. of the deflection range of the sheet material and to adjust the pressure accordingly. The meaning of the term deflection range used herein is that employed by the American Society for Testing Materials in its Standard D648-72. Deflection ranges for a variety of plastic sheet materials suitable for use according to the present invention can be found in the Modern Plastics Encyclopedia, pages 549-569 (1971-2 edition). Although the precise chemical or molecular behavior of the plastic sheet material at the scoring temperatures found satisfactory for practicing the invention is not clearly understood, it has been empirically determined that at temperatures more than about 10F. above the maximum deflection range temperature for a given material set forth in the ASTM deflection ranges, or more than 10F. below the minimum ASTM deflection range temperature, satisfactory scoring is more difficult to attain. Thus, when the temperature exceeds by more than 10F. the upper limit of the ASTM deflection range temperature, a tackiness results tending to cause adhesion between the scoring blades and the material. At temperatures more than 10F. below the lower limit of the ASTM deflection range, crazing of the material tends to occur with apparent strain hardening, producing brittleness.
The present invention can be practiced in scoring a wide variety of plastic sheet materials suitable for use in making boxes and cartons, including but not limited to thermoplastic sheet materials such as those containing polystyrene, polyethylene, polypropylene, nylon, formaldehyde polymers, poly(vinyl chloride), poly(- vinylidene chloride) and related vinyl polymers, nitro cellulose, ethyl cellulose, cellulose acetate, cellulose acetatebutyrate, poly(methyl methacrylate), polyesters, vinyl acetate, and aerylonitrile-butadiene-styrene resins. Such materials can be composed of homopolymers, copolymers or various blends thereof; they may also contain various additives known in the art, including colorants, plasticizers, heat stabilizers, extenders, fillers, and inhibitors against degradation due to oxidation, ultraviolet light, and the like.
Among the commercially available PVC sheet materials, Stauffers X-l5 rigid PVC film, Tennecos Mirrex PVC clear film and American I-Ioechst Corporation Genotherm l00l/l002 PVC films have been found to be eminently suitable for use in the present invention. Plastic sheeting consisting of a copolymer of vinyl acetate and vinyl chloride together with vinylidine chloride is also suitable for use in the present invention.
Satisfactory results can be obtained utilizing a variety of plastic materials, of which the above are given by way of example, ranging in thickness from less than about 0.005 and greater than about 0.015 inch. The apparatus of the present invention is designed so that the scored material is substantially reduced in thickness between the scoring blade and platen. Desirably, the material is reduced in thickness to between about 25 percent and about percent of the original thickness of the material, so that the depth of each indentation on opposite faces of the material along the score line is between about l2.5 and 37.5 percent of the total thickness of the material away from the score line. Preferably, the scored material is reduced to a thickness of between about 40 and 60 percent of the original thickness of the material, so that the depth of each indentation on opposite faces of the material along the score line will be between 20 and 30 percent of the thickness of the material away from the score line. The width of the stress-relief indentations generally depends upon the thickness of the plastic sheet material being scored; the thicker the material the wider the scoring blades. In particular, a scoring blade width of between about 0.025 and about 0.030 inch produces satisfactory stress-relief indentations on sheet material between about 0.005 and about 0.010 inch in thickness; a scoring blade width of between about 0.040 and about 0.045 inch works well on sheet material between about 0.010 and about 0.015 inch in thickness. As noted, earlier, the temperature of the scoring blades is desirably (but not necessarily) maintained within about i F. of the deflection range of the plastic sheet material being scored. Ideally, the temperature of the platen member along the intended score line is the same as that of the corresponding scoring blade, although the platen in actual practice can be as much as about 50F. cooler than the blade. In order to reduce the cost of operation, it is preferred to maintain the scoring blades at a temperature near the lower end of the deflection range. In order to obtain desired scoring blade temperatures, it is advantageous to maintain the heaters on the presses at a temperature approximately 100F. higher than the desired scoring blade temperature, as indicated later in the examples.
It is a feature of the present invention that the heat which must be applied at the platen side of the material can be obtained, if desired, by securing heat-retaining metal tapes to the platen along the desired fold lines, aligned with the scoring blades, using a suitable adhesive. The thickness of these tapes is not critical and tapes having a thickness of about 0.015 inch are found to be suitable. By bringing a heated scoring blade against the tape before positioning each sheet of material to be scored on the platen, the tape can be sufficiently heated so that continuous production is obtained by relying on the heat previously stored in the heat-retaining tapes.
Photomicrographic examination of the score lines under polarized light indicates that there is an increase in density at the score line. Without wishing to be bound by theory, it is believed that when the material at both sides of the score line is heated and compressed at a temperature and pressure according to the present invention, the crazing conventionally encountered in cold scoring and in heretofore known hot-scoring is eliminated, and a laminar continuity of molecular arrangements is attained within the plastic material. Examination under polarized light further indicates that, when the material is bent about the score lines formed in accordance with the invention, since the radius of curvature at the outer surface of the bend is substantially reduced by the indentation at this outer surface, the tensile strains at the outer surface of the material are substantially reduced. Similarly, the indentation on the inner surface at the bend reduces compressive strains thereat due to the fact that there is no material to be compressed at the inner material surface. Furthermore, it is observed that the strain relief indentations formed by heat scoring under pressure at the fold line according to the present invention are brought about by a combination of compaction of plastic material between the indentations (resulting in increased density thereat) and physical removal of material from the scoring site. Surprisingly, these phenomena are unaccompanied by lateral displacement flow of material to the sides of the score line, which would result in the formation of a pair ridges at the fold line as is observed, for example, in the process taught by U.S. Pat. No. 3,379,814. Such ridges are undesirable since they detract from the appearance of the material at the outer surface of the fold and tend to interfere with the operation of the stress-relief indentation at the inner surface of the fold.
As mentioned earlier, it has been discovered that provision of stress relief indentations on opposite surfaces of a rigid or semi-rigid plastic sheet material along the intended line of fold serves to facilitate the production in the sheet material of a fold line which, when the material is folded, minimizes undesired cracking or crazing and discoloration of the material. This apparently results from the fact that the radius of the curvature of the outer surface of the material is substantially reduced at the indentation, thus mimimizing the tensile stress at this outer surface, and reducing the strain in the material. Similarly, at the interior corner of the fold, the absence of material at the indentation eliminates any material which would otherwise offer resistance to compressive forces of the surface material adjacent this inner fold, thus minimizing compressive stresses and strains in the material.
DESCRIPTION OF THE DRAWINGS Referring now to the drawings, like numerals will be employed to" designate like parts.
As shown in FIG. 1, a sheet of thermoplastic material 10 is provided with strain relief indentations l2 and 14 on opposite surfaces of the sheet material along a desired fold lin'e.
As seen in FIG. 2, the strain relief indentations l2 and 14 serve to eliminate material at the points of maximum stress when the sheet material 10 is folded about the score line. Thus, on the upper surface viewed in FIG. 2, the radiusof curvature of the fold is reduced, and any tensile" strain which would normally tend to be greatest at the outer corner of the fold is accommodated by the surface along indentation 12. Similarly, the compressive strain normally produced at the inner corner of the'fold is reduced, since material is removed at indentation 14, thus eliminating material which would otherwise be subject to compression and preventing the transmission of any compressive forces between the innersurfaces l1 and 13 on the sheet material as viewed i'rif FIG. 2.
Referring to FIG. 3, the strain relief indentations l2 and 14 continue to eliminate material which would otherwise be present at the points of maximum stress when the sheet material 10 is folded about the score line. The tensile strain which would normally build up at the outer corner of the fold is accommodated by the surface along indentation 12. In a like manner, the compressive strain normally created at the inner corner where sides 11 and 13 join together is accommodated by stress relief indentation 14.
Referring now to FIG. 4, there is depicted a photomicrograph, in transverse cross-section, of a thermoplastic sheet material. such as poly(vinyl chloride), which has been heat-scored using the process and apparatus of the present invention. As can be seen from FIG. 4, stress reliefi'ndentation 12 appears circular in crosssection to reflect the corresponding curvature (preferably semi-circular) of the working end of the scoring blade which forms it. Stress relief indentation 14 formed by the heated platen member is. for reasons which are not entirely clear, likewise circular in crosssection, although the radius of curvature of indentation 14 is generally slightly less than the radius of curvature of indentation 12. Stress relief indentations 12 and 14 are further characterized by the removal of thermoplastic material at the score line without any corresponding buildup of material along the edges l6, l7, l8 and 19 of the indentations. This evidences the observed 'fact that the heat-scoring process of the present inven tion is not accompanied by undesirable lateral flow dis placement of plastic material away from the site of indentation;
Referring to FIG. 5, the invention is illustrated in the formation of a box blank provided with side panels 21, 22, 23 and 24, and with a securing flap 25. End flaps 26 and 27 are formed on opposite ends of panel 22; end flaps 28 and 29 are formed on opposite ends of panel 24. Closure flap 30 with sealing wing 31 is provided on panel 23, and a similar closure flap 34 with sealing wing 35 is provided on panel 21.
A suitable die and platen arrangement for forming the box blank 15 is illustrated in FIGS. 6, 7 and 8.
Referring to FIGS. 6 and 7, the platen is formed with heated ridges 42 aligned with the scoring blades 48 of the die. Together the heated ridges 42 and heated scoring blades 48 form the stress relief indentations on opposite faces of the material along the score line. As illustrated in FIGS. 6 and 8, it has been found that satisfactory results can be obtained by forming the ridges 42 using a plurality of heat-retaining steel tapes on the surface of the platen 40 aligned with the scoring blades 48, and underlying the lower surface of the material where scores 14 are to be formed. Thus, by arranging a plurality of steel tapes 42 with a thickness of about 0.015 inch and from about /3 to A inch in width, die costs can be reduced, since the heat required to produce the lower indentations 14 can be obtained, prior to feeding the material into the press, by bringing the heated scor ing blades 48 against the steel tapes 42, which act as heat sinks, retaining sufficient heat so that the sheet plastic between the die and the platen is deformed on both the upper and lower surfaces thereof to form and anneal the indentations l2 and 14 as illustrated in FIG.
Referring to FIG. 7, the die 45 is provided with cutting blades 47 denoted by single lines and shown in elevation in FIG. 8, and scoring blades 48 denoted by dou ble lines shown in elevation in FIG. 8. As will be understood by those skilled in the art, the cutting blades '47 are formed with a sharp edge, as opposed to the relatively blunted, preferably semi-circular edge of the scoring blades 48. The cutting blades 47'are arranged to extend from the die 45 a distance greater than the extension of scoring blades 48 from the die. With a 10 mil-thick (i.e., 0.010 inch) material, for example, it is found desirable to set the cutting blades 47 to extend 5 mils beyond the scoring blades 48, so that the sheet material will be pierced by the cutting blades, with the scoring blades compressing the material between the scoring blade 48 andthe tapes 42 to a thickness of 5 mils. Although ideally, the width of the scoring blades 48 and the width of the tapes 42 should be the same, it has been found that satisfactory results can be obtained and alignment problems mimimized by utilizing a relatively thin scoring blade bearing against a tape of considerably greater width.
It is preferred that the cutting blades 47 be unheated as shown, since cold cutting is found to provide a smoother edge.
Referring to FIG. 8, the elements of which are not necessarily drawn to scale, the die 45 is formed with a cover plate 46 and a phenolic asbestos insulation plate 49 arranged over nichrome wired electrical heating element 51. Heating element 51 is supplied with electric current in a conventional manner (not shown) and is insulated from cutting blades 47 and in heat-exchange relationship with scoring blades 48 which are supported by two sandwiched layers 52 and 53 of phenolic asbestos. Compressible rubber cushion members 54, when placed on the cutting blades 47 and on at least some of the scoring blades 48, have been found to improve operations by increasing heat retention in the scoring blades, by facilitating separation of the plastic sheet material from the scoring and cutting blades, and by preventing distortion of the plastic sheet material during scoring. Desirably, the compressible rubber cushion members are of 30 to 40 durometer hardness and extend about one-sixteenth inch beyond the cutting blades 47 and scoring blades 48. As will be understood by those skilled in the art, the aforementioned difference in extension of the cutting blades 47 and scoring blades 48 is modified where the rubber cushioning-insulating members 54 are employed. Preferably, the blades are set to ensure formation of indentations 12 and 14 the combined depth of which is between about 25 and percent of the thickness of the plastic sheet material. The sandwiched elements of die assembly 45 are held in place by chase 55 during actual operation.
DESCRIPTION OF PREFERRED EMBODIMENTS EXAMPLE I Material Poly( vinyl chloride) Thickness l0 mils Heater temperature 250F. Score blade temperature 150F.
Blade pressure lbs. per linear inch EXAMPLE Il Material Polycarbonate Thickness l0 mils Heater temperature 375F. Score blade temperature 275F.
Blade pressure 90 lbs. per linear inch EXAMPLE Ill Material Polyester Thickness l0 mils Heater temperature 300F. Score blade temperature 200F.
Blade pressure 90 lbs. per linear inch The foregoing disclosure and examples have been given for purposes of illustration and elucidation, and
not by way of limitation. lt is understood that changes and variations can be made without departing from the scope of the invention as defined in the following claims.
1. A container comprising walls of plastic sheet material joined to each other by fold lines comprising heatscored linear indentations on opposite faces of the sheet material, the thickness of the sheet material between the indentations being substantially less than the thickness of the sheet material away from the fold lines, the density of the sheet material between the indentations being substantially greater than the density of the sheet material away from the fold lines, and the quantity of plastic material along each of said fold lines being less than the quantity of material through an equal cross-sectional area of sheet material located away from the fold lines, said fold lines being further characterized by the absence of ridges of laterally displaced plastic material along the heat-scored linear indentations.
2. A container according to claim 1 wherein: the plastic sheet material is thermoplastic sheet material; and the thickness of the sheet material between the heatscored linear indentations is between about 25 and about percent of the thickness of the sheet material away from the fold lines. 3. A container according to claim 2 wherein: the thermoplastic sheet material is selected from the group consisting of poly(vinyl chloride), polycarbonate and polyester; and the thickness of the sheet material between the indentations is between about 40 and about 60 percent of the thickness of the sheet material away from the fold lines.