Note: Descriptions are shown in the official language in which they were submitted.
ih~
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METHOD AND APPARATUS FOR
PRODUCING MESH FILM
The present invention relates to a method and
apparatus for producing a tube of a heat-plastified,
synthetic resinous material, mesh film and, more
particularly, to such a method and apparatus in which
various tube dlameters may be produced.
A number of different techniqueq have been used
~or forming extrudable, synthetic resinous,
thermoplastic compositions into film. Such film finds
utility in numerou~ applications such as, for example,
wrapping material, garbage bags, and the like. It is
known that the tear resistance o~ such film may be
increased significantly by incorporating ridges of
increased film thickness in the film, producing a
product sometimes referred to as mesh film. The ridges
may be configured in various patterns, including
parallel stripes, criss-cross patterns, and
combinations thereof.
U.S. Patent No. 4,265,853, issued May 5, 1981,
to Havens, discloses an apparatus for producing a ridge
pattern in extruded film by differential cooling of the
film during a stretching process. The film is
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extruded through a tubular die and stretched as it
leaves the die. A plurality of nozzles, spaced around
the tubular film, rotate around the film as it is
stretched, cooling the film to produce a plurality of
narrow strips of increased thickness and achieving a
sharply defined ridge effect. A criss-cross pattern of
ridges may be obtained by using two sets of nozzles
which are rotated around the film in opposite
directions. In Havens' apparatus, the tubular film
tO passe~ around the outside surface of a cylindrical
cooling mandrel after it is enlarged to its desired
diameter. Therea~ter, the tubular film is split into
two strips of film which are collected for f urther
processing. While providing a reinforced film product
of high quality, Havens' apparatus is limited in that
it is capable of producing tubular film of only a
specific diameter. It is seen, therefore, that a need
exists for a method and apparatus for making mesh film
of variou~ selected diameters without changing the
apparatus.
This need i~ met by an apparatus according to
the present invention for producing a mesh film which
includes an extrusion die for extruding a tube o~ a
heat-plastified, synthetic resinous material, film, and
first cooling means centrally located within the
extruded tube, for cooling the internal surface of the
tube. The first cooling means has an outer diameter
3 substantially equal to the inner diameter of the
extruded tube. A plurality of cooling nozzles,
disposed radially outward from the first cooling meansS
direct a plurality of streams of cooling gas toward the
exterior surface of the extruded tube. An annular gas
bearing is spaoed axially from the first cooling means
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such that the extruded tube passe~ therethrough after
leaving the first cooling means. This permits the
diameter of the extruded tube to be increased
subsequently by means of a trapped air bubble
therewithin.
The first cooling means may comprise a
generally cylindrical cooled mandrel and a pair of
generally cylindrical gas bearings, positioned on
either side of the mandrel and axially aligned
therewith. Alternatively, the first cooling means may
comprise ~ generally cylindrical gas bearing. At least
some of the plurality of cooling nozzles may be
moveable circumferentially around the extruded tube.
,- 15
The apparatus may further comprise second
cooling means for cooling the tube after it pa~ses
through the annular ga~ bearing a~ the diameter of the
tube is enlarged. The second cooling means may
compri~e mean~ for directing a cooling gas against the
exterior surface of the extruded tube.
The apparatus may further comprise third
cooling means for cooling the tube as it emerges ~rom
the die. The third cooling means may comprise means
for directing a cooling ga~ against the exterior
surface of the extruded tube immediately after it
emerges from the die.
3 The apparatus may ~urther comprise'fourth
cooling means, centrally located within the extruded
tube, for cooling the internal surface of the tube
after the tube emerges from the annular gas bearing.
The exterior diameter of the fourth cooling means may
be substantially greater than the interior diameter of
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the annular gas bearing. The fourth cooling means may
comprise a cooled mandrel, and a gas bearing associated
therewith.
The extrusion die is capable of extruding a
relatively large diameter tube of a heat-plastified,
synthetic resinous material, film . A second annular
gas bearing i9 po~itioned adjacent the die and is
coaxially aligned therewith for receiving the tube
therethrough from the extrusion die. The second
annular ga~ bearing has an inner diameter which is less
than the diameter of the tube as it leaves the die and
i~ ~ub~tantially equal to the outer diameter of the
first cooling means.
- 15
The presen~ in~enti~n particularly re~ides in a
method for producing a mesh film comprising the step~
o~:
a) extruding a heat-pla~tified, synthetic
reqinou~ material, film to form a film tube;
b~ passing the film tube over a centrally
located generally cylindrical cooling means to cool the
interior surface of the tube while directing a
plurality of streams of cooling gas toward the exterior
~urface of the tube;
c) pa~qing the tube through an annular gas
bearing having an inner diameter approximately equal to
the outer diameter of the cooling means; and
d~ thereafter, increasing the diameter of the
tube by means of a trapped air bubble therewithin.
The cooling means may comprise a generally
cylindrical cooled mandrel and a pair of generally
cylindrical gas bearing~ positioned on either side of
the mandrel and axially aligned therewith.
34,394-F -4-
Alternatively, the cooling means may comprise a
generally cylindrical gas bearing. At least some of
the streams of cooling gas may be moved
circumferentially around the extruded tube.
The method may further include the step of
cooling the tube, a~ its diameter is increased, by
directing a cooling ga~ again~t its exterior sur~ace.
The method may further include the ~tep of cooling the
~0 tube, immediately after it is extruded, by directing a
cooling gas against its exterior surface. Furthermore,
the method may further comprise the step of cooling the
tube, after its diameter is increased, by means of a
- cooled mandrel, and a gas bearing associated therewith
positioned within the tube.
The step of extruding the film tu~e may include
the s~ep o~ extruding a rela~i~e~y large diameter ~ube
and then reducing the diame~er of the relatively large
diameter tube by strstching the tube axially.
Accordingly, it is an object of the present
invention to provide a method and apparatus for
producing meqh ~ilm as extruded tubes of various
diameters; to provide such a method and apparatus in
which the tube passes around a cooling means having an
outer diameter substantially equal to the inner
diameter of the tube with a plurality of cooling
nozzles disposed radially outward from the cooling
means, and in which the tube thereafter passes through
an annular gas bearing to facilitate subsequent
enlargement of the tube by means of a trapped air
bubble; to provide such a method and apparatus in which
the cooling means includes a generally cylindrical
cooled mandrel and a pair of generally cylindrical gas
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bearings positioned on either side of the mandrel and
axially aligned therewith; to provide such a method and
apparatus in which a further cooling means is centrally
located within the extruded tube for cooling the
interior surface of the tube after the tube emerges
~rom the annular gas bearing; to provide such a method
and apparatus in which the extrusion die extrudes a
relatively large diameter tube and in which a second
annular gas bearing receives the tube from the
extrusion die to reduce the diameter of the tube to
approximately that of the cooling means; and to provide
such a method and apparatus in which additional cooling
may be provided at various points along the path of the
~- 15 extruded tube.
Other objects and advantages of the invention
will be apparent from the following description, the
accompanying drawings, and the appended claims.
Fig. 1 is a side view of a first embodiment of
the present invention, with portions broken away and in
section;
Fig. 2 i~ a side view of a second embodiment of
the present invention, with portions broken away and in
section; and
Fig. 3 is a side view of a third embodiment of
the present invention, with portions broken away and in
section.
3o
Fig. 1 of the drawings illustrates a first
embodiment of the apparatus of the present invention
for producing a mesh film in which an extrusion die 10
extrudes a tube 12 of a heat plastified, synthetic,
resinous, material, such as for example polyethylene.
The extruded tube 12 moves generally upward, as seen in
.
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Fig. 1, past a first cooling means 14 which is located
centrally within the extruded film tube 12 to cool the
internal surface of the tube 12. The first cooling
means 14 has an outer diameter substantially equal to
the inner diameter of the extruded tube 12.
A plurality of cooling nozzles 16 and 18 are
disposed radially outwardly and in an opposing
relationship to the first cooling means 14, for
directing a plurality of streams of cooling gas toward
the exterior surface of the extruded tube. The cooling
nozzles are arranged in a circle around the tube 12 and
extend radially inwardly from annular elements 20 and
24 de~ining air passage3 22, 26. A support ring 28
~urro~nd~ the annular elements 20 and 24 while
permitting them to rotate freely. Support ring 28
receives air under pressure via conduits 30 and 32 and
supp1ie~ the air through internal passages (not shown)
to air pas~a~es 22 and 26 via openings (not shown) in
the annular elements 20 and 24. Annular elements 20
and 24 may be freely rotated circumferentially around
the tube 12 in the same, or in oppo~ite directions.
The first cooling means 14 includes a generally
cylindrical water cooled mandrel 34 and a pair of
generally cylindrical gas bearings 36 and 38, which are
positioned on either side of the mandrel 34 and which
are axially aligned with the mandrel. The gas bearings
36 and 38, of conventional design, provide a thin
cushion of air over which the tube 12 passes without
appreciable drag.
It will be appreciated that the surface of the
extruded tube 12 is selectively contacted by the air
streams from nozzles 16 and 18 so as to form a
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plurality of narrow strips where the film is cooled.
As may be appreciated, selective cooling raises the
melt tension of the material in the cooled areas so
that the film does not stretch as much during a
subsequent film stretching operation as the adjacent
warmer film areas. As a result of this higher melt
ten~ion, 3harp ridges are formed in the Pilm. rf
nozzles 16 and 18 are caused to rotate in oppo~ite
directions around the tube 12, a criss-cros3 ridge
1a pattern is produced, providing substantially increased
mechanical ~ilm toughnes~. An appropriate motor drive
mechanism (not shown) of conventional design may be
provided for the purpose of rotating the annular
f 15 elements 20 and 24, if desired.
Enlargement or stretching of the tube 12 occurs
a~ter passage of the tube 12 through an annular ga~
bearing ~0 which is spaced axially from the first
2~ cooling means 14. This permits the diameter of the
extruded tube 12 to be increased subse~uently by means
of a trapped air bubble, indicated generally at 42,
within ~he enlarged section 44 of the extruded tube,
while keeping the tube 12 in proper position between
nozzles 16 and 18 and cooling means 14. As
illustrated, enlargement of the tube in section 44 may
be effected to various diameters, as desired. This
enlargement process is accomplished in a conventional
manner.
A second cooling means 46 may be provided for
cooling the tube 12 after it passes through the annular
gas bearing 40. This cooling means may include means
for directing a cooling gas against the exterior
surface of the extruded tube as it is enlarged. In
similar fashion, a third cooling means 48, may be
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provided for directing a cooling gas against the
exterior surface of the extruded tube after it emerges
from the die. Preferably, the second and third cooling
means 46 and 48 direct a generally even flow of air
toward the sur~ace of the tube 12 or section 44 in
order to ensure that differential cooling of the film
is not ef~ected.
The external gas bearing 40 makes it possible
to inflate ~he extruded tube 12 in the section 44
beyond the bearing 40 to ~ario~s desired diameters, as
shown by the dashed line and solid line representations
of section~ ~4. Thus, with a die 10 which provides an
extruded film tube 12 having a specific diameter, the
r 15 tube 12 may be expanded in the section 44, for example,
by factors ranging from 1.5:1 up to 3:1, and above.
The expansion ratio selected will, Gf courqe, depend on
a number of factor~, including the specific film
material being utilized and the application for the
~ilm produced.
A~ may be noted from Fig. 17 the water cooled
mandrel 34 is slightly smaller in diameter than either
of the gas bearings 36 and 38, although this difference
is exaggerated slightly in the drawing for purposes of
clarity. Preferably, the mandrel 34 has a low friction
exterior surface. The air from nozzles 16 and 18
brings the tube 12 into close proximity to the surface
of the mandrel 34 so as to enhance cooling of those
portion~ o~ the film tube against which the air is
directed, producing sharper ridges and permitting
higher extrusion rates.
~eference is now made to Fig. 2 which
illustrates a second embodiment of the present
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invention. The apparatus shown in Fig. 2 is similar in
many respects to that of Fig. 1 and, accordingly, like
structures have been given corresponding reference
numerals, and will not be described again. The
embodiment of Fig. 2 differs from that of Fig. 1,
however, in that it includes a fourth coo~ing means 50,
centrally located within the extruded tube 12 and,
specifically, within the expanded or enlarged section
44 of the tube, for cooling the internal surface of the
tube after the tube emerges from the annular gas
bearing 40. As may be noted, the outer diameter of the
fourth cooling means 50 is substantially greater than
the inner diameter of the annular gas bearing 40.
The fourth cooling means 50 comprises a mandrel
52, which may be water-cooled, and a gas bearing 54
associated therewith. The mandrel 52 is used when
producing the smallest size tube section 44, indicated
in solid llnes, with the larger sizes, as indicated in
da~hed lines, not being cooled by the mandrel 52. This
allows higher production rates when the smallest size
expanded mesh film tube is produced, while retaining
the ability to produce larger sizes of mesh film tube
without the necessity of equipment changes. If
desired, an airbox (not shown) may be provided
surrounding the mandrel 52 when the smallest size tube
is produced to urge the tube against the mandrel 52.
Such an airbox is constructed in two semi-circular
3~ sections to facilitate removal when larger sizes of
mesh film tube are to be produced. Alternatively, the
fourth cooling means 5~ may comprise a gas bearing 54
alone without a mandrel 52 associated therewith.
Fig. 3 of the drawings illustrates a third
embodiment of the present invention. Many of the
34, 394-F - 1 0-
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components are similar to those shown in Figs. 1 and 2
and, therefore, have been given correqponding reference
numerals. The e~bodiment of Fig. 3 differs from the
embodiments of Figs. 1 and 2, however, in that the
extrusion die 55 produces a relatively large diameter
tube 56 of a heat-plastified, synthetic resinous
material, film. The extrusion die ~urther includes a
second annular gas bearing 58, adjacent the die 55 and
coaxially aligned therewith. The second annular gas
bearing 58 receives the tube 56 therethrough from the
extrusion die 55. The bearing 58 has an inner diameter
which i~ less than the diameter of the tube 56 as it
leaves the die 55, and is ~ubstantially equal to the
outer diameter of the first cooling means 14. The tube
56 is reduced in diameter by stretching the tube
between the die 55 and the gas bearing 58.
As a result of the substantially larger die 55,
lower pre~sure~ and lower melt temperatures are
experienced ~y the film ma~erial for any given
extru~ion rate than is the case with the previously
deqcribed embodiments. As a con~equence, the
configuration of Fig. 3 has a higher production
capability. If desired, a further cooling means,
qimilar to means 48 in Fig. ~, may be provided adjacent
the die 55. By varying the amount of cooling provided,
the amount of retained orientation of the film material
in the direction of extrusion can be varied to achieve
3 optimum propertie~ for a given application. An
external airbox may also be positioned surrounding the
tube 56 to minimize pressure differences between the
in9ide and outside of the tube 56.
5
ln some instances, it may be desired to
simplify the apparatus of Figq. 1-3 by replacing the
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mandrel 34 and associated gas bearings 36 and 38 with a
single larger gas bearing of approximately the same
length. It will be appreciated, however, that thi~
will result in reduced cooling capability and a
corresponding reduction in attainable extrusion rates.
rt will further appreciated that a number of other
variations may be made in the apparatus of Figs. 1-3
within the scope of the present invention. For
example, the fourth cooling means 50 of Fig. 2 may, if
desired, be ;ncorporated into the apparatus of Figs. 1
and 3. The apparatus and method of the present
invention may be utilized with any synthetic rasinous
material which is capable of being heat plastified and
extruded a~ a filml
Having de cribed the invention in detail and by
reference to preferred embodiments thereof, it will be
apparent that modifications and variations are possible
withaut departing from the scope of the invention
defined in the appended claims.
34, 394-F -12-
