Note: Descriptions are shown in the official language in which they were submitted.
P-2663
~35~6
1 This invention rela-tes to a novel process and
apparatus for the stretching of a tubularly--formed sheet
of thermoplastic material and more particularly to a novel
process and apparatus for the bi-axial stretching of a
tubularly-formed sheet of orientable, polymeric thermoplastic
material to form bags of improved strip tensile breaking strength.
Thermoplastic bags for diverse use, such as sandwich
bags, garbage bags, leaf bags and the like, are produced by
extruding a tube of thermoplastic ma~erial, such as high
density polyethylene, with the resulting tubularly-formed
material being cooled, heat sealed and either scored or cut
to the desired length. The thus formed bag exhibits a strip
tensile breaking length representative of the processed
thermoplastic material. The end use of the thermoplastic
bag normally dictates the selection of the thermoplastic
!~ material, e.g. a~ a sandwich bag, a low porosity and normal
strength thermoplastic material is selected whereas a garbage
bag would require the selection of a thermoplastic material
exhibiting high strength characteristics.
It is an object oE the present invention to provide
a novel process and apparatus for stretching a heat-sealable
and tubularly-formed sheet of a s~nthetic material.
Another object of the present invention is to provide
a novel process and apparatus for bi-axially strstching a
collapsed heat-sealable and tubularly-formed sheet or web
of thermoplastic material to form a bag having an improved
strip tensile breaking strength of at least twice that of the
tubularly formed sheet of thermoplastic material being trea-ted;
in effect, providing a significant and unexpected result in
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1 that both the heat seal area and the fold area increase
in streng-th, in addition to the film area.
Various other objects and advantages of the present
inven-tion will become apparent from the following detailed
5 description of an exemplary embodiment thereof with the novel
features thereof being particularly pointed out in the appended
claims.
In accordance with the present invention, there is
provided a process and apparatus for the selective stretching
10 of a tubularly-formed sheet or web of thermoplastic material
in a station provided with a set of groovecl, generally
sinosoidally~shaped rolls, whereby the sheet or web of thermo-
plastic material is stretched in a manner to effect uniform
stretching thereby producing a sheet or web of larger dimension
15 in the direction of stretch.
`. In accordance with a preferred embodiment of the
present invention, there is provided a process and apparatus
for bi axially stretching a tubularly-formed and heat-
sealed sheet o thermoplastic material in a first station
20 and a plurality of second stations wherein the first and
second stations are provided with sets of rolls having
grooves perpendicular and parallel, respectively, to the
axis of each set of rolls. The groove pattern of each set
of rolls is such that the distance between grooves is less
than l.0 millimeters times the sheet or web basis weight
in grams per square meter. The sheet or web of tubularly-
formed thermoplastic material is stretched in a manner to
effect uniform stretching to produce a thermoplastic bag
of substantially improved strip tensile breaking strength.
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1 The inven-tion will be more clearly understood by
reference to the following de-tailed description of an exemplary
embodiment thereof in conjunction with the accompanying
drawi.ngs wherein:
Figure l is a schematic s.ide elevational view of
the apparatus and.process of the present invention; and
Figure 2 is a schematic top elevational view of
the apparatus and process of the present invention.
Drive and support assemblies, timing and safety
1~ circuits and the like, known and used by those skilled in
the art, have been omitted in the interest of clarity.
Referring to Figure l, there is illustrated a
preferred embodiment o the process and apparatus of the
present invention including a circular blown film die assembly
15 and a stretching clssembly, generally indicated as lO and 12,
respectively. The circular blown film die assembly lO forming
the blown film 12 may be any one of the types of assemblies
sold by the Stexlin~ Extrucler Corporation of South Plainfield,
New Jersey~ The blown film 12 is passed about the roller 14
20 to form a flat two-layered sheet 16 prior to introduction
into the heat sealing assembly~ generally indicated as 18,
as known and used by those skilled in the art, wherein the
two layered sheet is heat sealed at selected i.ntervals on
a line perpendicular to the movement of the sheet 16.
The thus heat-sealed, two-layered sheet 20 i5 coursed
in a first station, generally indicated as "I" between a nip 22
of a palr of rollers 24 having a plurality of grooves 2~
perpendicularly formed to the axis of the rollers 24, as seen
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in Figure 2. The sheet 20 is maintained against the lower
grooved roller 2~ by a pair of press rollers 28 to hold
the sheet 20 against the lower roller 28 to thereby prevent
the sheet 20 from narrowing prior to introduction. Once in
the nip 22, the sheet 20 assumes the shape of the groove
pattern and becomes stretched by a factor of the draw ratio
as hereinafter more clearly described.
In the first station, in effect, lateral stretching,
the sheet 20 is wound up at about the same velocity as the
feed velocity. The crimp pattern is flattened out by stretching
the sheet 20 laterally, such as by means of tenter clamps or
curved Mount Hope rolls, generally indicated as 32, such as
known and used by one skilled in the art.
The grooves 26 of the rollers 24 are intermeshed
like gears, as known -to those skilled in the art. As the
sheet 20 enters the nip 22, the sheet 20 assumes the shape
of a groove 26 and is stretched by a factor determined by
the length of the sinus wave "~" of the groove divided by
the original length of the web "~ " between contact points
of each respective groove tip as disclosed in Canadian Patent
No. 1,075,870 issued April 22, 1980 to E.C.A. Schwarz.
The ~raw ratio,~/~ is calculated by the following
~ equation where
; a = ~ d/2 ~ , and the sinus wave of the groove is
~ .
~/~ 1 1 + a cos x dx
D ~
Thus ~or d/~ ratios of 1.0, 0.75 and 0.5 the draw ratios
are 2.35, 2.0 and 1.6, respectively.
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1 ~ laterally s~retched sheet 34 is passed from the
rollers 32 and is coursed between a nip 40 of a first pair of
rollers 42 of a second station "II" with said rollers 42 having
a plurality of grooves 44 parallel to the axis of the rollers 42.
5 The sheet 32 is maintained against the lower grooved roller 42
by a pair of press rollers 46 to ensure that the velocity V
of the sheet 32 is substantially identical to the surface
velocity Vl of the grooved rollers 42. The grooves 44 of
the rollers 42 are intermeshed like gears, as known to those
10 skilled in the ar-t. As the sheet 34 enters the nip 40, the
sheet 34 assumes the shape of a groove 44 and is stretched by
a factor determined by the length of the sinus wave " ~ " of
the groove divided by the original length o~ the web " ~ "
between contact points of each respective groove tip, as
~5 hereinabove discussed with reference to the passage sheet 20
through station I rollers 24.
, .
The sheet 34, after passage through the nip 40 of
the rollers 42, is pulled away by a pair of tension rollers 48
having a surface velocity V2 greater than the surface velocity
20 of the rollers 42, but not greater than a factor of the draw
ratio affected in the nip 40 of the rollers 42.
Accordingly, the length of the sheet 34 is therefore
increased by such factor. It is noted that the sheet 34 does
: not undergo narrowing while being longitudinally stretched or
25 extended, as is the case with conventional roller systems.
In a preferred embodiment of the present invention, the
sheet 3~ is passed through two further pairs of grooved
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,, .......... , . .... ...... ,. ..... ..... . ....... .. ~ ........... ~
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1 rollers 42 to further stretch lengthwise the sheet 34
which is eventually collected on a roller 50.
The maximum permissible draw ratio can easily be
determined by measuring the residual elongation of the thermo-
plastic ma-terial. For best resul-ts, the grooves 44 of the
rollers 42 should be as fine as possible, with groove distance
being increased if heavy basis weight factors are to be orientedO
From experience, good results are obtained~ if the distance
between grooves ( in mm) is less than 1.0 times the fabric
10 basis weight (in gram/m2). With the process and apparatus of
the present invention, a bag is produced having a much higher
strip tensile breaking length (STBL - expressed as meters)
than a normal produced blown ~ilm bag.
; Operation of the process and apparatus is described
15 in the following examples which are in~ended to be merely
: illustrative, and the invention is not to be regarded as
limited thereto.
Exam~le l
A 7" x 7" double layer sheet is for~ed by
extruding a mixture of 90% polypropylene ~melt flow rate
of 0.5 gjmm~ and l0~ clay and had the following properties:
; thickness 0.050" : 127 micron
film basis weight: 125 g~m2
STBI. . 3570 m
break elongation ; 1600%
initial modulus : 1600 m
STBL over seal : ~560 m
STBL over fold : 3050 m
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1 The bag was lleat-sealed and stretched in the
apparatus 1.5 times on a lateral direction and 3.3 times on
a longitudinal direction (3 passes) to a final dimension of
10.5" wide and 23" long. It was noted that the heat-sealed
5 area also stretched. The stretched bag had the following
properties:
STBI. : 23550 m
break elongation : 40%
initial modulus : 185500 m
1~ STBL over seal : 15500 m
STBL over fold : 16600 m
Exampl e I I
The process of Example I was repeated on a similar
15 7l~ x 7" sheet with heat sealing being effected after st.retching
vice before wi.th the STBI, over seal being 2200 m ; i.e~ less
than the STBL over seal of a bag heat sealed hefore stretching.
Example III
~ A 7" x 7" double layer sheet is formed by
extruding a 100~ polypropylene (melt flow rate of 6.0 g/10 mm)
and had the following properties:
thickness 0.050" : 150 micron
film basis weight: 142g/m
2~ STB~ : 3200 m
break elongation : 1400%
initial modulus : 14500 m
STBI. over seal : 14500 m
STBL over fold : 3040 m
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1 The bag was heat-sealed and stretched in the
apparatus 1.5 times on a lateral direction and 4.5 times
on a longitudinal direction (3 passes) to a final dimension
of 10.5" wide and 31.5" long. It was noted that the heat-
5 sealed area also stretched. The stretched bag had the
following properties:
thickness : 25 microns
basis weight : 21 g/m2
STBL : 35700m(length~ -
lCI break elongation : 25%
initial modulus : 225000 m
STBI. over seal : 18400 m
STBI, over ~ol~ : 22050 m
Exam~)le IV
A 7" x 7" double layer sheet is formed by
ex~ending a mixture of 95~ high density polypropylene
(melt flow rate of 2.0 g/lOmm) and 5~ titanium dioxide and
had the following properties:
thickness : 200 micron
film basis weight : 170 g/m2
STBL : 3800 m
break elongation : 1800~
initial modulus : 12000 m
:STBL over seal : 2600 m
STBL over fola : 2650 m
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1 , The bag ~as heat-sealed and stretched in the
apparatus 2.0 times (2 passes) on a lateral direction and
5.0 times on a longitudinal direction (3 passes) to a final
dimension of 14.0" wide and 25" long. It was noted that
5 the heat-sealed area also stretched. The stretched bag had
the following properties:
thickness : 22 microns
basis weight : 18 g/m
STBL : 38500 m
break elongation : 40~
initial modulus : 17500 m
STBL over seal : 28500 m
STBL over fold : 30400 m
Thus, it is readily apparent to one skilled in
15 the art that a novel bag is produced of a light weight per
dimension exhibiting substantially improved strip tensile
,......... . .
breaking length. It will be readily apparent to one skilled
in the art, that depending on end use, that a bag of improved
strip tensile breaking length may be produced by passiny a
20 tubularly-formed thermoplastic sheet, preferably heat-sealed,
either through laterally or longitudinally grooved rollers or
sequentiall,y through such grooved roller~ or as described
herein with reference to the preferred embodiment.
While the invention has been described in connection
25 with an exemplar.y embodiment thereof, it will be understobd that
many modifications will be apparent to those of ordinary skill
in the art and that this application is intended to cover any
adaptations or variations thereof.
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