Relatively rigid sheet plastic folding method, apparatus and article

A method, apparatus for forming, and article formed of folded, relatively rigid sheet plastic material (e.g. polyolefin is disclosed). A hinging membrane forming a flexible fold line provides stress-relief indentations in the sheet material so that the tensile and compressive strains normally produced when bending the sheet material at the folds are substantially relieved, thereby permitting efficient production of a variety of sheet plastic articles, such as folding containers and the like. Two embodiments of the invention are disclosed. In one embodiment, the stress-relief indentations are simultaneously formed on opposed surfaces of the sheet material at the desired fold line by an apparatus having an unheated die containing a scoring blade mounted for movement against the material to be scored, which is supported on a hard and polished platen, with the plastic sheet material arranged between the scoring blade and platen. The scoring blade and platen are brought to bear on the plastic sheet material disposed between them during the scoring operation to substantially reduce the material thickness along the fold line without substantial lateral displacement flow of the material away from the fold line. In this first embodiment, indentations are formed on opposed surfaces of the sheet material at the fold line which serves 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. According to a second embodiment, the desired hinging membrane is formed along the desired fold line by compressing the relatively rigid sheet material by means of a wedge shaped creasing blade dimensioned to compress the sheet material along the desired line of folding to between 10% and 75% of its nominal thickness and flaring from the compressed surface to the material surface.

BACKGROUND OF THE INVENTION 
This invention relates to the art of forming fold lines in relatively rigid 
plastic sheet material such as polyolefins, and more particularly to a 
process and apparatus for forming fold lines in such material and to an 
improved container formed thereby. 
With the recent development of extruding synthetic plastic materials, such 
as those in the polyolefin family, in which uniformity of sheet thickness 
and flatness are now attainable, it has now become a suitable material for 
the fabrication of folding cartons. 
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 cracking of the sheet material occurs at or near the 
fold lines 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 the flattened 
and opened conditions, due to the springness or elastic 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 and 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. No. 3,137,217 and U.S. Pat. No. 3,529,516. Similar 
cold scoring techniques have been attempted in connection with plastic 
sheeting as indicated, i.e., in U.S. Pat. No. 3,334,802; U.S. Pat. No. 
3,292,513; U.S. Pat. No. 3,594,464 and U.S. Pat. No. 3,350,492. 
Unfortunately, cold scoring has heretofore proven unsatisfactory when used 
in conjunction with existing thermoplastic sheet materials due to the fact 
that the cold deformation of these sheet materials, along the intended 
fold line, appears to produce a strain hardening of the material, 
resulting in fracturing and crazing 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 for a plastic material 
suitable for use in manufacturing folding cartons and boxes that would 
work in the identical way paperboard boxes do, and in existing folding 
carton making machinery, without the drawbacks of the prior art, which 
would be durable, easily handled and decorated in the same manner as 
folding paperboard containers. 
BRIEF DESCRIPTION OF THE INVENTION 
Accordingly, it is an object of the invention to provide an improved 
hinging membrane forming a fold line for use in plastic articles formed of 
relatively rigid plastic materials, such as polyolefins, 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 invention is to provide an improved process for 
forming fold lines in a relatively rigid plastic sheet material, such as 
an olefin, which imparts sufficient flexibility to permit formation of a 
carton or box from the material. 
Another object is to provide an improved plastic folding container from 
relatively rigid sheet material such as an olefin whose walls are joined 
at fold lines with 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 cracking of the sheet material at or near the fold lines while 
at the same time permitting the container 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 forming 
fold lines in relatively rigid plastic sheet materials. 
These and other objects which will become hereafter apparent are achieved 
according to the present invention by forming a hinging membrane along a 
desired fold line in relatively rigid plastic sheet material. According to 
one embodiment of the invention, the hinging membrane is arranged to 
provide strain relief indentations at opposite sides of the sheet to 
relieve both tensile and compressive stresses normally produced at opposed 
surfaces of a fold line during bending, thereby minimizing crazing, 
material weakening and cracking 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. 
Scoring of the plastic sheet material to form the hinging membrane is 
accomplished according to one embodiment of the invention by the use of a 
blunt scoring blade mounted to press against the sheet material supported 
by a platen, preferably polished in the vicinity of the score line. The 
unheated scoring blade is brought against the unheated material to be 
scored at a pressure which substantially reduces the material thickness 
between the scoring blade and the platen, to a value of between 10% and 
75% and preferably between 15% and 30% of the original nominal sheet 
thickness, with a value of between about 20%-25% being generally 
preferred. 
According to the second embodiment, the scoring blade is formed with a tip 
formed in the shape of an inverted truncated equilateral triangle, which 
compresses the sheet material into a hinging membrane, along a line having 
a width between 10% and 75% of the nominal thickness of the sheet and 
forming an opening flaring out from the hinging membrane to the material 
surface.

DESCRIPTION OF PREFERRED EMBODIMENTS OF THE INVENTION 
Referring now more particularly to the drawings, like numerals will be 
employed to designate like parts. 
The invention is directed to creasing relatively rigid sheet plastic 
materials, such for example as the olefins, and more particularly the 
polyolefins, including copolymers thereof. 
High density polyethylene, extruded without orientation and polypropylene 
having a specific gravity of 0.90 to 0.96 lends itself well to this 
application. Obviously, the combination of both materials mixed by 
polymerization and extruded into a copolymer gives us the best results. 
Polyethylene, because of its nature, flexibility and strength and other 
properties, and polypropylene, because of its stiffness, hinge flex and 
stability together, extruded into a copolymer sheet with a percentage of 
50-50, gives us the ideal combination and qualities required for folding 
carton application. 
Although the precise chemical or molecular behavior of the plastic sheet at 
the score line is not clearly understood, it has been determined that 
optimum hingeability and flex-life of the score is obtained by practicing 
the invention on polyolefins with a specific gravity of between 0.85 and 
0.98. 
Additionally, 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, polyester, 
nylon, formaldehyde polymers, poly (vinyl chloride), poly (vinylidene 
chloride) and related vinyl polymers, nitro cellulose, cellulose acetate, 
cellulose acetate butyrate, poly (methyl methacrylate), vinyl acetate, and 
acrylonitrile-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, impact modifiers, fillers, and inhibitors 
against degradation due to oxidation, ultraviolet light and the like. 
Satisfactory results can be obtained utilizing a variety of plastic 
material, of which the above are given by way of example, ranging in 
thickness from less than about 0.005 and greater than about 0.055 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 sheet material 10 as seen in FIGS. 1-3 is 
reduced in thickness to between about 10% and about 75% of the original 
nominal thickness of the sheet material. Preferably, the scored line 
forming the hinging membrane 15 is reduced to a thickness of between about 
20% and 25% of the original thickness of the material. The width of the 
hinging membrane 15 is preferably of a width between 15% and 85% of the 
nominal thickness of the plastic sheet material being scored; the thicker 
the material the wider the scoring blades 20. In particular, a scoring 
blade width of 0.042 with a flat area on the creasing surface of 0.021 
works well on sheet material between about 0.025 and 0.055. 
As seen in FIGS. 103, the scoring blade 20 is formed with a relatively 
blunt bottom, preferably rounded at its edges 21 to facilitate penetration 
and release of the sheet material 10. This scoring blade 20 is mounted to 
move against sheet 10 supported at least along the desired fold line on 
platen 25, which is preferably formed of steel polished along the desired 
sheet fold line. 
In the embodiment of the invention illustrated in FIGS. 5 and 6, the 
relatively hard plastic sheet 510 of olefin or the like is shown supported 
on relatively hard platen 515 of steel or the like, polished in the area 
of material compression, with creasing blade 520 and cutting blade 525 
mounted to move against the sheet 510, to form the hinging membrane 530. 
In FIG. 6, the sheet material 510 is shown bent about the hinging membrane, 
illustrating how the hinging membrane facilitates desired folding. 
FIG. 4 illustrates a typical container 40 which may be suitably formed of 
the scored and cut sheet material, whether the score lines are formed by a 
scoring blade as in FIGS. 1 and 2 or as in FIG. 5. 
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 compressed with the blunt creasing rule 
and pressure according to the present invention, the cracking and crazing 
conventionally encountered in cold 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 of 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 scoring under 
pressure at the fold line according to the 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 of 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. 
The above disclosure has been given by way of illustration and elucidation, 
and not by way of limitation, and it is desired to protect all embodiments 
of the herein disclosed inventive concept within the scope of the appended 
claims.