Note: Descriptions are shown in the official language in which they were submitted.
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This invention relates to a method of and apparatus
for blow molding a plastic article, and more particularly,
to such a method and apparatus in which -fluorine is employed
to impart barrier properties to the article.
One of the chief disadvantages of many thermoplastic
synthetic resinous materials is that they are permeable by
nonpolar solvents at substantial rates of transmission.
This is particularly true of a group of resins which, because
of their many adv~ntageous properties, are commonly employed
in the manufacture of containers and receptacles.
Referring for example to U.S. Patent 2,811,4689
issued October 29, 1957 to S. P. Joffre, it has been known
for some time that barrier properties can be imparted to
articles formed of certain thermoplastics, including articles
which have been blow molded, by exposing a surface of such
an article to fluorine. The fluorine reacts with the material
of the article to form a relatively impermeable surface
barrier.
U.S. Patent 3,862,28~, issued January 21, 1975 to
D. D. Dixon et al., discloses a number of methods of blow
molding thermoplastic articles in which a fluorine mixture
is employed as the blowing medium. More particularly, in
one of these methods, a tube or parison is expanded to
conform to an interior surface of a mold within which it is
enclosed by introducing under pressure to the interior of
the parison a mixture of from about 0.1% to about 10% by
volume fluorine and the remainder an inert gas.
It has been found, however, that when a fluorine
mixture is employed as the blowing medium in high-speed blow
molding systems presently in commercial use, the barrier
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properties imparted to the blown article may be far from
uniform over the extent of its wall, with the result that
various portions of the wall are permeated by nonpolar
solvents at widely differing rates of transmission. In many
instances these permeation rates approach or equal those
exhibited by wholly untreated articles, whereby the usefulness
of the article for containing nonpolar solvents is gravely
impaired or destroyed.
It has also been found that the presence of air,
or more particularly, oxygen, during exposure of the interior
of the tube or parison to fluorine has a similarly deleterious
effect on the barrier properties imparted.
In accordance with the present invention, and
~ntrary to the teaching of the aforementioned patent to
Dixon et al., an inert gas is employed as the blowing medium,
a fluorine mixture being introduced to the interior of the
tube or parison before it is enclosed within the mold.
Like the aforementioned patent to Dixon et al.,
the method according to the present invention begins with an
expandable tube or parison of synthetic resinous material
which is enclosed within a mold and expanded to conform to
an internal surface of the mold. However, in accordance
with the invention, the parison is closed to the ambient 9 a
mixture of from about 0.1% to about 10~ by volume ~luorine
and the remainder an inert gas is introduced to the interior
of the parison before it is enclosed within the mold, and
the step of expanding the parison within the mold is carried
out by introducing an inert gas under pressure to the interior
of the parison.
~ The method according to the invention has been
found to overcome the deficiencies of the prior art described
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hereinabove. SpeciEically, an article produced by the
method of the invention exhibits uniform barrier properties
over the extent of its wall to a degree suitable for containment
of nonpolar solvents.
Preferably the parison is partially expanded
before it is enclosed within the mold by introducing the
fluorine mixture under pressure.
The thermoplastic synthetic resinous material is
preferably selected from the group consisting of acrylonitrile
polymers, styrene polymers, vinyl chloride polymers, and
particularly polymers of olefin monomers having 2 to 8
carbon atoms per molecule.
The invention also provides apparatus for successively
blow molding articles, comprising extrusion means for continuously
extruding an expandable tube of synthetic resinous material,
and a plurality of molds arranged in a series thereof. Each
mold has an internal surface complementary to the outer
surface of an article to be blow molded, the molds heing
adapted to successively enclose therewithin successive
portions of the extruded tube whereby to maintain the interior
of each tube portion closed from the ambient before said
tube portion is enclosed within one of the molds and to
isolate the interiors of the tube portions from one another.
The apparatus also includes first and second fluid delivery
systems adapted to be closed from the ambient, the first
fluid delivery system compri,ing first fluid passage means
associated with the extrusion means for continuously introduc-
ing a first fluid medium to ~he interior of the tube as it
is extruded~ and first conduit means communicating at one
end thereof with the first fluid passage means and adapted to
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communicate at the other end thereof with a source of the
first fluid medium. The second fluid delivery system comprises
second fluid passage means associated with each of the molds
and adapted to sequentially introduce a second fluid medium
under pressure to the interiors of the respective tube
portions enclosed within the molds whereby to expand each
tube portion in sequence to conform to the internal surface
of the respective mold, and second conduit means communicating
at one end thereof with the second fluid passage means and
adapted to communicate at the other end thereof with a
source of the second fluid medium.
Apparatus in accordance with the invention is
admirably suitable for continuously carrying out the method
of the invention to produce a succession of aTticles which
exhibit the advantageous barrier properties described herein-
above.
The drawing is a partial schematic representation
of a blow molding system constructed and adapted to operate
in accordance with a preferred embodiment of the invention.
As disclosed herein, the invention is applied to a
blow molding system of a general type which is otherwise
largely conventional. Examples of systems of this type are
disclosed with particu]arity in U.S. Patents 2,515,093 and
2,579,390, issued ~uly 11, 1950 and December 18, 1951,
respectively, both to E. E. Mills, and in U.S~ Patent 2,784,452,
issued March 12, 1957 to H. S. Ruekberg et al. Because blow
molding machines of this type of system are now well known
and in widespread commercial use, their principal conventional
elements can be described bricfly, as follows.
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An endless series of partible blow molds arranged
in a continuous circular array is rotatable in vertical
planes about a horizontal central axis (not shown) normal to
the plane of the drawing. One such mold in the closed
position is indicated generally at 10 in the drawing. A
second mold 12 is shown in vertical section in a partially
open position. A third mold 14 is represented in fragmentary
form in the fully open position. The direction of rotation
of the molds is clockwise as viewed in the drawing.
Each mo]d comprises a pair of mold halves 16, 18.
The radially inner mold half 16 of each mold is radially
immovable relative to the axis of rotation, whereas each
outer mold half 18 is radially reciprocable between the
closed and fully open positions shown in the drawing. Re-
ciprocation of the outer mold half is normally effected by
means of a simple cam-and-follower arrangemen~ or a fluid-
actuated piston-cylinder device, neither of which is shown.
Ihe mold halves 16, 18 of each mold are provided
with contoured surfaces 20, 22, respectively, which cooperate
when the mold is closed to form a continuous internal surface
thereof complementary to the outer surface of an article to
be blow molded. The internal surface so formed defines the
mold cavity. Each mold half is also provided with uppcr and
lower pinch-off surfaces 24, 26, respectively, the function
; of which will be pointed out hereinafter~
Each of the inner mold halves 16 carries therewithin
a fluid passage in the form of a hollow hlow needle 28
mounted for limited reciprocating movement along its longi-
tudinal axis which is normal to the parting plane 30 of the
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mold. The blow needle is connected to a source of blowing
medium, as will be described with particularity hereinafter.
Also carried by each mold half 16 outwardly thereof is a
needle actuating device 32 which acts intermittently in
accordance with a predetermined program to move the associ-
ated blow needle 28 between the retracted position indicated
in the drawing and an advanced position in which the distal
end of the needle obtrudes into the mold cavity. Details of
the construction and operation of similar blow needles and
needle actuating devices are disclosed in U.S. Patent
3,513,502, issued May 26, 1970 to W. A. Chambers, and U.S.
Patent 3,571,848, issued March 23, 1971 to J. L. Szajna.
~ epresented schematically at 34 in the drawing is
a stationary extruder die head which forms the terminus of
a conventional plasticizer-extruder, the remaining Eeatures
of which are not shown. Die head 34 comprises essentially a
hollow outer die 36, normally of cylindrical form, and an
inner die or core 38 disposed concentrically within and
spaced from the outer die to define therewith an orifice 40
for the extrusion of plastic melt 42 delivered by the
plasticizer. Orifice 40 is normally annular or of similar
configuration. Core 38 is affixed by any suitable means to
a core stem 44, the core and core stem being mounted for
limited reciprocating axial movement in a core support 46
fixedly carried by die head 34. This reciprocating movement
may be effected by any one of a number of well-known core
actuating devices as represented schematically at ~8. Such
a device operates in accordance with a predetermined program
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in synchronism with operation of the molds to vary the
dimensions o-f orifice 40 and thus the wall thickness of the
extrudate 50, thereby affecting the blowing characteristics
of the extrudate and the wall thickness of the resulting
blown article at various locations. Because the variations
so provided are small relative to the average or nominal
wall thickness, they are not represented in the drawing. It
will be apparent that extrudate 50 is of generally tubular
form owing to the configuration of orifice 40.
Formed in core 38 and core stem 44 is a fluid
passage 52 having an inlet 54 and an outlet 56, the latter
being located radially inwardly of and concentric with
orifice 40 whereby fluid passage 52 is disposed to communicate
with the interior of extrudate 50.
The apparatus described thus far is well known and
widely employed in the blow molding of hollow plastic articles,
particularly containers. Conventionally blow needle 28 and
fluid passage 52 are connected to a source or sources of
ordinary air under pressure. (The expression "under pressure"
is employed throughout this specification to mean at a
pressure greater than the opposing ambient pressure.) In
accordance with the present invention, however, blow needle
28 and fluid passage 52 are closed to the ambient and to
other sources of air, or more particularly, oxygen; instead
they are incorporated in novel fluid delivery systems in
which other gases are employed and from which air and oxygen
are excluded.
Specifically, a pair of fluid delivery systems,
indicated generally at 58 and 60, respectively, are closed
56
to the ambient and comprise well-known elements which in
themselves require little description. Delivery systcm 58
begins with a source 62 of inert gas, preferably nitrogen in
the liquid state. (It will be apparent from the foregoing
statement that the expression "inert gas" as used throughout
this specification is intended to include not only the rare
gases but also gases such as nitrogen which under expected
conditions of operation are virtually inactive chemically.)
Source 62 is connected with each blow needle 28 by means of
a fluid conduit 64 which includes in series a nitrogen
vaporizer 66, a solenoid shutoff valve 68 selectively energized
to the open position by a manually controllable switch 70, a
rotary union 72 which provides communication between stationary
and rotatable portions of fluid conduit 64, and a shu~off
valve 74 controlled by a cam-and-follower device 75 in a
manner such that flow of gas through each blow needle 28 is
synchTonized with operation of the mold with which the blow
needle is associated, as will be described with greater
particularity hereinafter. Fluid conduit 64 may also be
provided with shutoff or isolation valves 76 where appropriate
or desirable.
Delivery system 60 begins with a source 78 of a
mixture of fluorine and an inert gas. Commercially available
in the United States and suitable for the purposes of the
invention is a gaseous mixture of 10~ by volume fluorine and
the remainder nitrogen, which is shipped and stored in
relatively small cylinders at a pressure of 2400 psi. Under
this system of containerization source 78 will normally
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comprise a manifold of such cylinders or a plurality of
manifolds connected in parallel. In any case, source 78 as
well as source 62 is readily replaceable when depleted.
Because most applications of the invention do not
require the full ten-percent concentration of fluorine,
inert gas source 62 is included in delivery system 60 as
well as in delivery system 58, though a separate source of
inert gas may be employed in system 60 if desired.
Sources 62 and 78 are connected with fluid passage
52 by means of a fluid conduit 80 which comprises a number
of segments 80a-80h, some of which are connected in parallel
to provide alternative routes, as will become clear from the
following description. Like fluid conduit 64, fluid conduit
80 is provided with shutoff or isolation valves 76 where
appropriate or desirable.
Under normal conditions of continuous operation
conduit segmen~ 80a provides communication, by way of a
series of pressure-reducing valves 82 and 84, between fluorine
source 78 and a blender 86. A suitable blender for this
purpose, or instructions for the construction of SUC}l a
blender, can be obtained from Air Products and Chemicals,
Inc. of Allentown, Pennsylvania. Similarly, conduit segment
80b provides communication between inert gas source 62 and
the blender. Conduit segments 80c, 80e, 80f and 80g, which
are connected in series, provide communication between
blender 86 and fluid passage 52.
Downstream of blender 86, conduit segment 80c
includes a source 88 of a dilute mixture of fluorine in
inert gas, source 88 being supplied directly by blender 86
g
and acting as a surge chamber. Conduit segment 80e includes
in series a check valve 90 and a solenoid shutoff valve 92.
Conduit segment 80f includes a metering valve 94. Conduit
segment 80g is in communication with inlet 54 of fluid
passage 52 and is preferably formed of flexible material to
accommodate the reciprocating movement of core stem 54.
It will be apparent that when the gas is to follow
the route just described, valves 76a, 76b and 76c must be
open and valves 76d and 76h must be closed.
It should be noted that blender 86 is adjustable
to provide any desired concentration of fluorine below 10%
by volume. Concentrations of from about 2% to about 8% by
volume are preferred. A gas analyzer 96 of any suitable
type may be provided in communication with conduit segment
80e to monitor or sample the fluorine concentration.
If the full concentration of ten-percent fluorine
is required, valves 76a, 76b and 76c may be closed and valve
76d opened, whereby conduit segment 80d will interconnect
conduit segments 80a and 80e and thereby act as a bypass
around blender 86 and dilute gas source 88.
Similarly, if only an inert gas is to be introduced
to fluid passage 52, as might be done for reasons of safety
and economy upon commencement of operation and continued
until a stable operating condition is achieved, valves 76a,
76b, 76c and 76d are maintained closed and valve 76h opened,
whereby conduit segment 8~h will interconnect nitrogen
vaporizer 66 and conduit segment 8Qf, thus bypassing conduit
segments 80c, 80d and 80e and the elements included therein.
Like conduit segment 80e, conduit segment 80h includes in
30 series a check valve 98 and a solenoid shutoff v~lve 100.
Pressure gauges 101 may be provided in fluid conduit 80
where desired.
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Solenoid shuto-ff valves 92 and 100 are selectively
energized to the open position by means of a manual selector
switch 102, which is so arranged that flow cannot occur
simultaneously in conduit segments 80e and 80h. The selector
switch also includes an OFF position to prevent flow alto-
gether in fluid passage 52 if desired, even when the molds
are being rotated. Selector switch 102 is in turn energized
by a machine automatic mode relay 104 which also energizes
mold and plasticizer operation during the normal continuous
operating condition. Thus when relay 104 is not energized,
flow in fluid passage 52 is precluded.
In the ensuing description it is assumed that the
blow molding system of the invention is in continuous,
stable, automatic operation; that is to say, the normal
operating condition. In this condition, relay 104 and thus
selector switch 102 are energized. Switch 70 is also
energized.
Plastic melt 42 is delivered at a constant pre-
determined rate to extruder die head 34 where it is continuously
formed into tubular extrudate 50 by way of orifice 40. The
temperature of the melt is maintained at a level high enough
that the extrudate is readily expandable but not so high as
to imperil the integrity of the wall of the extrudate. The
plastic is preferably a polymer of ethylene, particularly
such a polymer having a density of at least 0.94.
The molds are rotated at a speed synchronous with
the extrusion rate. In succession the molds approach the
extruder die head in the fully open position represented by
mold 14. From the open position the outer halves 18 of the
112~ 6
molds are successively directed towards the closed position
represented by mold 10 to close upon successive portions of
the extrudate at a location some distance below the extruder
die head. As each mold is closed its pinch-off sllr-faces 24
and 26 act to close to the ambient the interior o-E the
enc].osed portion of the extrudate and to isolate it from the
interiors of other portions of the extrudate. Thus the
interior of that porti.on of the extrudate between the extruder
die head and the most recently closed mold is maintained
closed to the ambient and provides a tubular parison to be
enclosed by the next succeeding mold.
Because extrudatc SO is hollow as it emerges from
orifice ~0, it will be obvious that it will collapse upon
itself if its internal pressure does not at least equal
ambient pressure. In conventional systems, therefore,
provision is made to i.ntroduce air continuously to the
interior of the extrudate, as by a fluid passage similar to
passage 52. Further, the air is customarily introduced at a
constant rate under pressure to partially expand each
successive portion of extrudate 50 before it is enclosed
within one of the molds. This partial pre-expansion imparts
improved blowing characteristics to the extrudate and permits
closer control of the wall thickness of the blown article at
various critical locations.
In accordance with the present invention, however,
in place of the air a mixture of fluorine and an inert gas
is continuously introduced at a constant rate, and preferably
under pressure, to the interior of the extrudate, where the
fluorine reacts with the inner surface of the extrudate to
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provide the barrier properties mentioned previously. The
fluorine mixture is delivered by way of delivery system 60
as follows.
The ten-percent concentration by volume o-f fluorine
in inert gas is conducted at 2400 psi from source 78 thereof
to pressure reducing valve 82 where its pressure is brought
down to, say, 200 psi. From there it is led to pressure
reducing valve 84 where its pressure is again reduced, in
this instance to, say~ 50 psi, and whence it is introduced
to blender 86. In the blender the mixture is diluted by
inert gas introduced from source 62 thereof, or more precisely,
from vaporizer 66, to a predetermined concentration of
fluorine preferably between about 2% and about 8% by volume.
Blender 86 delivers the dilute gas mixture ~o
dilute gas source 88, from which it is conducted to the
interior of extrudate 50 by way of check valve 90, solenoid
shutoff valve 92 and metering valve 94. Selector switch 102
is in the condition represented in the drawing whereby to
maintain solenoid valve 92 in the open posi~ion and solenoid
valve 100 in the closed position. The rate of flow in -fluid
passage 56 may be established, and adjusted as necessary, by
means of metering valve 94.
When each mold is closed in succession upon the
corresponding portion of extrudate 50, the blow needle 28
carried by that mold is moved to its advanced position by
the corresponding needle actuating device 32, whereby the
needle pierces the wall of the enclosed parison; that is,
the portion of the extrudate enclosed within the mold; the
distal end of the needle thus obtruding into the interior of
the parison. Simultaneously with movement o-f the needle to
the aclvanced position, the associated valve 74 is moved to
the open position by its cam-and-follower device 75 to admit
blowing gas under pressure to the interior of the parison by
way of the blow needle, thereby expanding the parison to
conform to the internal surface of the mold formed by surfaces
20 and 22 thereof; thus is formed a hollow article having an
outer surface complementary to the internal mold surface.
If desired, cam-and-follower device 75 may be timed to hold
valve 74 in the open position for a predetermined period
after the parison has been fully expanded, thereby maintaining
a positive pressure within the hollow article long enough to
alleviate shrinkage as the article begins to cool within the
mold. In any event, blow needle 28 is retracted at some
point after the article has been blownS and the pressure
within the article is relieved, for example in the manner
disclosed in the previously mentioned U.S. Patent 3,571,848.
The excess gas thus released is conducted to a remote location
in a manner similar to ~hat described hereinbelow. The mold
is opened when the article has cooled to a degree such that
it is self-supporting, and the article is removed from the
mold, whereupon it may be subjected to any number of well-
known finishing steps.
As indicated hereinbefore, the blowing gas conven~ion-
ally comprises ordinary air. In the aforementioned patent
to Dixon et al. it comprises a mixture of -fluorine and inert
gas. In accordance with the present invention, on the other
hand, the blowing gas comprises an inert gas alone, preferably
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nitrogen. Specifically, liquid nitrogen is directed from
source 62 thereof to vaporizer 66, where it is converted to
the gaseous state. From the vaporizer the nitrogen gas is
conducted to rotary union 72 by way of solenoid valve 68
which is maintained in the open position by the energized
switch 70~ The gas is distributed from the rotary union to
the various blow needles by way of valves 74, each of which
is operated intermittently in the manner described hereinbefore.
I'he mixture of fluorine and inert gas contained
in each parison when it is enclosed within one of the molds
is substantially diluted upon the introduction of inert gas
by way of the blow needle; that is to say, the fluorine
concentration is substantially reduced. Nevertheless, for
the protection of personnel and property the blown article
should be purged of rluorine. This is most conveniently
done after the article has been remo~ed from the mold and
trimmed to provide an opening (the neck opening in the case
of a container) for reception of a purging wand. It may be
necessary to conduct large articles to two or three purging
stations in succession in order to ensure thorough purging.
The purging is carried out using ordinary air in a shrouded
and vented environment. The gas so removed is conducted to
a remote location, if necessary by way of suitable treating
stations, and exhausted to the atmosphere in innocuous form
or in harmless concentrations.
Operation of the system at the full ten-percent
concentration of fluorine is carried out simply by closing
valves 76a, 76b and 76c and opening valve 76d.
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Regardless of the flworine concentration to be
employed, upon commencement of operation valves 76a, 76b,
76c and 76d are preferably maintained closed, valve 76h is
opened, and selector switch 102 is positioned to open solenoid
valve 100 and maintain solenoid valve 92 closed. The parisons
and blown articles are thereby subjected only to nitrogen
until stable operation is achieved and acceptable articles
are being produced. At this point valves 76a, 76b, and 76c
are opened, valve 76h is closed, and selector switch 102 is
positioned to close valve 100 and open valve 92 to commence
the fluorine tr~atment. The few untreated articles produced
by this procedure may be treated as scrap and reground or
otherwise disposed of.
~ ile the invention has been particularly described
in connection with a certain specific embodiment thereof, it
is to be understood that this is by way of illustration and
not of limitation~ and that the scope of the appended claims
should be construed as broadly as the prior art will permit.
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