Note: Descriptions are shown in the official language in which they were submitted.
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BLOW MOLD FOR FORMING A
REFILLABLE POLYESTER CONTAINER
This is a division of Canadian Patent
Application Serial No. 2,031,033 filed November 28,
1990.
This invention relates to new and useful
improvements in plastic containers, and more
particularly to a blow molded polyester container
having an improved base construction which is
resistant to strain cracks, and the preform from
which such container is blow molded.
An economically and commercially viable,
refillable plastic bottle reduces the existing land
fill and recycle problems associated with disposable
plastic containers, and more particularly with
plastic containers formed of PET. In addition, a
refillable container will permit the entry of the
safer, lighter weight plastic containers into those
markets currently dominated by glass, where
legislation prohibits use of non-returnable
packages.
The desirability of a refillable container
formed of a polyester is specifically discussed, for
example, in U.S. Patent No. 4,334,627 granted June
15, 1982.
At the present, it is to be understood
that several polyester candidates provide the
desired clarity and physical properties deemed
necessary to produce refillable plastic containers.
These polymers include polyethylene terephthalate
(PET) acrylonitrile, polyarylate, polycarbonate,
etc. Of the polymers commercially available, PET,
at the present, offers the best balance of
properties and cost/performance ratios.
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The container specifically disclosed in
U.S. Patent No. 4, 334,627 has been improved and is
specifically disclosed in U.S. Patent No. 4, 725,464,
granted February 16, 1988. This patent makes
specific reference to U.S. Patent No. 4,334,627.
This invention relates to an improvement
in the container specifically disclosed in U.S.
Patent No. 4,725,464 and the preform from which such
container is blow molded.
A refillable container must go through a
loop each time it is reused. The loop is comprised
of (1) an empty caustic wash followed by ( 2)
contaminant inspection and product filling/capping,
(3) warehouse storage, (4) distribution to wholesale
and retail locations, and (5) purchase, use and
empty storage by the consumer followed by eventual
return to the bottler. It is the hot caustic wash
which is most detrimental to the reuse of a blow
molded polyester container. It has been found that
failure (via crack initiation and propagation) of
biaxially oriented blow molded polyester containers
exposed to the caustic wash baths occurs primarily
in the base area and most particularly in the
central part of the base area which has little or no
orientation. It is to the improvement of the
resistance of biaxially oriented blow molded
polyester containers, particularly PET bottles and
the like, that this invention relates.
In accordance with this invention, the
specific configuration of the base forming portion
of the preform has been modified so as to provide
for an improved container base construction.
The invention relates to a blow mold for
molding a heated preform to form a container having
an internally domed base, the blow mold defining a
cavity including a base forming portion, which has a
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central substantially hemispherical dome portion
and, at the center of the dome portion, a gradually
sloping inverted portion with a central
substantially axial recess. The latter matches an
intended shape of an original depending axial
portion on an intended preform.
With the above and other objects in view that
will hereinafter appear, the nature of the invention
will be more clearly understood by reference to the
following detailed description, the appended claims
and the several views illustrated in the
accompanying drawings.
FIG. 1 is a schematic view showing the
typical cycle of loop to which a refillable
container must pass.
FIG. 2 is a vertical sectional view taken
through a container of a configuration to which this
invention specifically relates.
FIG. 3 is an enlarged fragmentary vertical
sectional view taken through the base construction
of the container of FIG. 2.
FIG. 4 is a vertical sectional view taken
through a preform for forming the container of FIG.
2.
FIG. 5 is a horizontal sectional view
taken generally along the line 5-5 of FIG. 4 and
shows the specific cross section of the body portion
of the preform.
FIG. 6 is a transverse horizontal
sectional view taken generally along the line 6-6 of
FIG. 4 and shows specifically the increase in
thickness of the container base forming portion of
the preform.
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FIG. 7 is an enlarged fragmentary vertical
sectional view taken through the base portion of the
preform and shows the specific configuration of the
improved preform construction.
FIG. 8 is a fragmentary vertical sectional
view showing generally the base construction of FIG.
7 with the bottom central part of the base being
engaged by a stretch blow centering rod.
FIG. 9 is a fragmentary vertical sectional
view taken through a base portion of a blow mold in
which the preform of FIG. 4 iS blow molded to form
the container of FIG. 2.
First of all, each container is subjected
to a typical commercial caustic wash solution which
is prepared 3.5 sodium hydroxide by weight with tap
water. The wash solution is maintained at 140 ~F and
the containers are submerged uncapped in the wash
for 15 minutes in accordance with a commercial
bottle wash system.
After removal from the wash solution, the
containers are rinsed in tap water and then filled
with a desired liquid product which may be in the
form of a carbonated drink which is packaged at 4.0
+ 0.2 atmospheres and capped. Such a filled
2 5 container must withstand a temperature of 100~F at
50% RH for 24 hours.
The general loop through which a container
passes in a single use of the container is shown in
FIG. 1.
Generally, the degree of molecular
orientation in the upper shoulder section and in the
base section of a stretch blow molded container is
lower than that in the body or panel section of such
container. In a refillable container, all features
such as flutes, grooves and steps, which are
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potential stress risers, are avoided in the upper
shoulder region and in the base section. Any
decorative embossing, flutes, grooves, etc., should
be restricted to sections having higher degrees of
orientation.
To achieve a good balance between creep-
volume growth and thermal shrinkage in the hot
caustic wash, the preform segment, which forms the
panel section of the container, is configurated to
give a total thickness reduction ratio of 7.5 to
10.5, with a stretch ratio of 3.2 to 3.5 in the hoop
direction and 2.3 to 2.9 in the axial direction.
The lower body and base forming portion of
the preform (see FIG. 7) is configurated to have
sections PQ, QR, RS and ST. These form the segments
pq, qr, rs and st, respectively, of the container
shown in FIG. 3.
Referring now to the drawings in detail,
reference is first made to FIG. 4 wherein there is
illustrated the vertical cross section of a preform
formed in accordance with this invention, the
preform being generally identified by the numeral
10. The upper part of the preform 10 is of the same
configuration as the preform of U.S. Patent No.
4,725,464 and includes a threaded neck finish 12
which terminates at its lower end in a capping
flange 14. Below the capping flange 14, there is a
generally cylindrical section 16 which terminates in
a section 18 of gradually increasing external
diameter so as to provide for an increasing wall
thickness. Below the section 18, there is an
elongated body section 20, the extreme lower part of
which forms an upper part of a container base
forming portion generally identified by the numeral
22. The base forming portion 22 includes at least
the section QR (FIG.7) at the top thereof. As is
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best shown in FIG. 7, in the section QR, the
internal diameter of the preform 10 gradually
decreases as at 24 wherein the section RS is of
increased wall thickness.
It will also be seen from FIG. 7 that the
diameter of the section ST gradually reduces so as
to reduce the wall thickness of the section ST.
Additionally, the lower part of the section ST is
generally of a hemispherical configuration as at 26.
Furthermore, the inner surface of the section ST as
at 28 iS also of a generally hemispherical
configuration, but with he surfaces 26 and 28 having
different centers so that there is a continued
gradual decrease in wall thickness.
At the center T of the section ST, there
is a smoothly rounded upstanding projection 30 which
is to be received in the lower end of a stretch blow
centering rod, as shown in FIG. 8 and to be
described in more detail hereinafter. Further, at
the bottom of the center T, there is a downwardly
directed axial projection 32 which is in the form of
an elongated gate. The outer surface 26 of the
preform 10 smoothly turns to form the outer surface
of the projection 32.
Reference is now made to FIG. 9 wherein
there is illustrated the configuration of the base
portion of a blow mold cavity 34 defined by a
customary type of blow mold 36. The blow mold
cavity 34 in the base of the blow mold 36 has a
lower outer generally hemispherical surface 38 which
terminates in a base contact radius C which is
centered a distance B which is 50 to 75% of the
general overall radius A of the blow molded
container. The radius C is 5 to 15% of the radius
A.
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It will be seen that the blow mold 36 is
of a configuration so as to have a central dome 40
which is of a height D having a dimension which is
20 to 40% of the radius A.
It will also be seen that the dome 40 is
provided with a central recess-42 of a size and
shape to receive the projection 32. Further, the
domed surface of the cavil 34 also includes a
surface portion 44 which gradually slopes into the
recess 42.
At this time it is also pointed out that
the wall thickness of the preform at the cross
section U in FIG. 7 is about 50 to 70% of the cross
section of the preform in the segment RS.
The preform 10 is reheated to the required
glass transition temperature, placed within the blow
mold 36 in the conventional manner, engaged by a
stretch blow centering rod 50, as is shown in FIG.
8, inflated by internal pressurization so as to be
stretched in both the axial and hoop direction to
match the configuration of the cavity 34 and form a
container such as the container of FIG. 2 which is
identified by the reference numeral 52. At this
time, it is to be noted that the rod 50 has a recess
54 in the bottom thereof so as to clear the
projection 30. At the same time, the projection 32
is received in the recess 42. As a result, when the
container 52 is blow molded within the blow mold 36,
both the inside surface and the outside surface of
the base, generally identified by the numeral 56 of
the container 52, will be free of stress
concentrations.
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The container 52 will include the threaded
neck 12 and the capping or support flange 14 in
their original configurations. Immediately below
the flange 14, the container 52 Will have a
generally unoriented thick wall portion 58 which
terminates in a thin wall shoulder 60 which is
formed from the preform section 18. Beneath the
shoulder 60, this is the container body 62. In the
illustrated form of container, the container body 62
is provided with an elongated panel section 64 which
is of a reduced cross section and is intended to
receive a suitable label (not shown). The body 62
continues below the panel section 64 as at 66 and
terminates in he base 56, which, as previously
described, is of the domed type.
The wall thicknesses of the various
portions of the container 52 above the base
construction 56 are clearly shown in FIG. 2. At
this time it is pointed out that the container 52 iS
a 1. 5 liter bottle and the preform 10 is
configurated to form the container 52.
Reference is now made to the enlarged
showing of the base construction 56 found in FIG. 3.
It is to be noted that the various lines P,Q,R,S,
and T find their equivalent lines in the container
52 as lines p,q,r,s and t, respectively, The base
construction 56 starts generally at the line p and
includes an axially extending and radially inwardly
directed outer surface 68 which is generally
hemispherical. It will be seen that there is a very
slight increase in wall thickness between lines p
and q while the section q,r has a gradual increase
in wall thickness with the base construction
terminating in a chime c which forms the support
radius. In the section r,s, the wall thickness
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g_
continues to increase with the base construction 56
being domed as at 72 and the wall thickness
continuing to gradually increase until it reaches
the point 70 which corresponds to the outer edge of
the sloping surface 44. Thereafter, the base
construction 56 continues to smoothly increase but
at a slightly greater rate until the outer surface
terminates in the projection 32 and the inner
surface terminates in the projection 30.
To achieve the required wall thickness at
chime c of the base construction 56, FIG. 3, the
wall thickness of segment RS is about 1.10 to 1.30 x
that of the segment PQ. The transition from thin to
thick wall section is gradual and smooth as
described above. This eliminates any abrupt wall
thickness change in the segment qr.
A higher degree of orientation in the dome
segment st of the base 56 is achieved by optimizing
the base configuration and by the gradual decrease
of the preform wall thickness in the segment ST.
The projections 30, 32 at t insures that a
crystalline region, if any, in this area does not
weaken the section. As pointed out above, the
elongated gate 32 is not crushed during blow
molding.
Inasmuch as the base construction
gradually increases in wall thickness as the
orientation decreases, it will be seen that the
desired strength requirement is obtained for the
base construction 56 by way of smoothly curved
surfaces which greatly reduce the possibility of
stress cracking.
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It is believed that 28 to 30%
crystallinity is an optimum level for a refillable
PET container which is preferably in the form of a
bottle having an injection molded threaded neck
finish. Further, it is to be understood that the
preform, and thus the container, may be formed of a
multilayer construction including internal barrier
layers so as to extend shelf life. The application
of such multilayer preforms may be utilized to
reduce contaminate absorption, if filled with non-
food products, and subsequent product contamination
after washing and filling.
Although only a preferred embodiment of
the refillable plastic bottle and the preform from
which it is formed have been specifically set forth
herein, it is to be understood that minor variations
may be made in either the container, the preform or
the blow mold in which the preform is blow molded to
form the container without departing from the spirit
and scope of the invention as defined by the
appended claims.
