Note: Descriptions are shown in the official language in which they were submitted.
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POLYESTER OONTAINER FCR HOT ~ILL LIQUIDS
BACKGROUND AND SUMMARY OF THE INVENTION
is invention relates to a polyester container and particularly
to such a container having an improved base configuration.
~ Polyester containers have been replacing metal and glass
containers with increasing frequency. The popularity of these products
stems in part to improvements in resin composition, manufacturing
processes, and container designs. TypiGal polyester containers SuGh as
those made from polyethylene terephthalate (PET) material are formed in a
process in which an elongated tubular preform made by injection molding or
other processes is heated and placed into a blow molding cavity. A
pressure differential is applied which causes it to expand to conform to
the inside surface of the mold cavity, thus providing a semi-rigid
thin-walled container. Since the container is exposed to various pressures
and forces during processing and use as will better be explained below, it
must be designed to respond to such physical influences while maintaining a
designed configuration. Random or asymmetrical buckling or deformation of
the container would produce an esthetically and con~ercially unacceptable
product.
Cbntainers must be designed to be stable when set on a horizontai
surface. In the past, n~ny polyester containers were designed to have a
rounded bottom which required a separate base component which was glued to
the container to provide a flat support plane. More recent polyester
container designs, however, are integral structures having a bottom which
forms an outer support ring with a central outwardly concave depress~d
center, often referred to as a "champagne bottom". In addition to the
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requirements of maintaining a desired configuration, there is a further
need to design the container to minimize the quantity of material needed to
; form it. In the past, polyester containers were designed with a reinforced
base having ribs or webs of increased thickness of polyester material which
tended to increase the mass of raw material needed to form the product.
Dur mg the production cycle of a blow molded polyester container,
~- the preform is typically axially stretched and inflated to impart radial
~; elongation to the material. In the art, such forming is known as biaxial
elongation. Such elongation imposes retractive stresses in the material
which, if not relaxed or physically restrained, tend to cause the article
to shrink and deform in certain conditions in the directions of elongation.
e influence of such unrelaxed retractive stresses is particularly
significant during certain-phases of the production cycle of the container.
Immediately after demolding of the containex, the elevated temperature of
the material causes it to be less rigid than the final product.
Accordingly, such unrelaxed retractive stresses tend to have more influence
during this phase of the production cycle.
In the past, most polyester containers were used to contain
quid~ that are initially dispensed into the container at room temperature
or chilled. Presently, however, there is more interest in using polyester
containers for so-called "hot-fill" applications where the beverage or
product is dispensed in the container initially at an elevated temperature
and is then immediately sealed. Hot-fill applications impose additional
mechanical stress inputs to the container structure. Immediately after the
hot liquid is dispensed into the container, its temperature decreases the
rigidity of the p~lyester n~terial, thus making it more subject to the
unrelaxed retractive stresses mentioned preYiously. me container n~st
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sustain internal pressure changes while maintaining its
configuration. For example, as the hot-filled liquid
cools, it shrinks in volume which has the effect of
producing a negative pressure in the container. In use,
~ 5 the container must also be resistant to deformation when
being handled or dropped which causes sudden increases in
internal pressure.
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~ Generally speaking, the present invention provides a
`! PET container formed by blow molding and adapted to be
:~ 10 filled with liquid at an elevated temperature above room
. temperature, the container comprising an upper portion
. defining a sealable closure, a sidewall portion, and
a base portion closing the bottom of the
container and formed integral with the sidewall portion,
the base portion having a generally flat outer support
ring at the lower end of the sidewall portion that is
substantially concentric with the sidewall portion, a
dome formed integral with the outer ring and extending
upwardly into the container and terminating in a central
disc portion that is also substantially concentric with
the sidewall portion, the dome also including an annular
wall extending between the disc portion and the outer
ring, a portion of the annular wall being subject to
deformation by virtue of the presence therein of
unrelaxed retractive stresses resulting from blow molding
and the heating effect of the filling liquid at the
elevated temperature, the annular wall being shaped to
resist deformation by the stresses by reducing the area
of the dome in which the stresses may be formed by
providing a series of alternately arranged radially
upwardly sloping and radially downwardly sloping portions
in the annular wall which provide the annular wall with a
serpentine appearance extending radially from the disc
portion along the dome down to the outer ring when viewed
in radial cross section, the upwardly and downwardly
sloping portions thereafter forming at least one inwardly
concave reinforming ring and at least one inwardly convex
. reinforcing ring being substantially concentrically
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positioned around the central disc portion to thereby
reinforce the ability of the annular wall to resist
deformation during filling of the container with liquid
at the elevated temperature.
Additional benefits and advantages of the present
invention will become apparent to those skilled in the
art to which this invention relates from the subsequent
description of the preferred embodiments and the appended
claims, taken in conjunction with the accompanying
drawings.
BRIEF DE8CRIP~ION OF THE DRAWING5
Figure 1 is a side elevational view of a container
having a base in accordance with a first embodiment of
the present invention with the bottom cut-away and
sectioned.
Figure 2 is a bottom view of the base of the
container shown in Figure 1.
Figure 3 is a cross-sectional view of a preform of
polyester material used in a blow molding process to form
containers according to this invention.
Figure 4 is a cross-sectional view through a blow
molding cavity showing the container of Figure 1 in its
final configuration and showing, in phantom lines, axial
stretching of the preform.
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Figure 5 is a bottom view of a container base in accordance with
a second embodiment of this in~ention.
Figure 6 is a cross-sectional view taken along line 6-6 of Figure
!~ 5.
Figure 7 is a bottom view of a container base in accordance with
a third embodiment of this invention.
Figure 8 i5 a cross-sectional view taken along line 8-8 of Figure
7.
Figure 9 is a bottom view of a container base in accordance with
a fourth embodiment of this invention.
Figure 10 is a cross-sectional view taken along line 10-10 of
Figure 9.
DETAILED DESCFIPTICN OF TXE INVENTIoN
Figures 1 and 2 illustrates an example of a polyester bottle made
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from PET material which is generally designated by reference number 10.
Container 10 generaily includes sidewall portion 12, an upper closure nlouth
14, and a base portion 16. Sidewall 12 can be formed to a multitude of
different configurations to provide the desired structural characteristics,
and product identification and aesthetic intent. ~outh 14 is adapted to
receive a threaded closure cap (not shown) and is a rigid ring which
restrains the mechanical loads imposed by such closures. Base portion 16
generally forms an outer ring 18 which defines support plane 20 and a
central outwardly concave dome region 22. me configuration of base
portion 16 which m~o~porates the features of the present invention will be
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described in greater detail below.
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Figures 3 and 4 illustrate a fabrication process for
forming container lo. Figure 3 shows preform 26 having a shape
similar to a laboratory test tube except that closure mouth 14
is fully formed. In Figure 4, preform 26 is loaded into blow
molding mold halves 28 and 30. Preform 26 is heated and
plunger 32, as shown in Figure 4, is used to axially elongate
the preform as it is expanded through differential pressure to
conform to the inside surface of mold halves 28 and 30. During
such expansion, container 10 undergoes a combination of radial
and axial elongation. As mentioned previously, such elongation
gives rise to retractive stresses in the final product. The
retractive stresses become particularly significant in the
radially outer portions of center dome 22 since that material
undergoes increased elongation as compared with the center area
and is therefore subject to significant shrinkage. The
transition region 24 sho~n in Figure 4 between the center of
bottom portion 16 where the material is substantially
unoriented and the outer area at ring 18 where the material is
highly oriented is particularly susceptible to random and
unsymmetrical buckling.
Mold halves 28 and 30 are shown with coolant passages
38 which are provided to control the temperature of the molds
and may be used to provide differential temperatures within the
mold to provide various material characteristics in designated
areas of the container, such as described in U.S. Patents
4,497,855 and 4,318,882. Those patents describe a container
which is molded in a first configuration and then remolded to
a larger volume configuration, such that when the hot-fill
liquid contracts during cooling, the container returns to its
original configuration in response to the
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plastio's structural "memory" of the first configuration. Bottle 10 i.n
accordance with this invention may be forn~d using this technology.
~ ase portion 16 according to a first embodiment of this invention
is best described with reference to Figures 1 and 2. The radially outer
portion of base portion 16 is rounded inwardly to define ring 18. Dome 22
has a corrugated appearance defined by a plurality of concentric
reinforcing rinys. Tangent points designated by letters A through J in
`~ Figure 1 are used to describe the configuration of dome 22 and desiqnates
intersections of tangent lines identified by the same letters as shown in
Figure 2. me tangent lines define a point of inflection or change in
radius of the container shape. I,ine A represents the inner boundary of
ring 18. Concave ring 40 extends between lines A and B. A large radius
` convex ring 42 ex*ends between lines B and C. Outwardly concave ring 44
extends between lines C and D and merges into convex ring 46. Wall 48
between lines E and F is generally vertical with respect to container 10,
and transitions to rings 50, 52 and 54 between lines F through J which are
outwardly concave, convex and concave, respectively. The center of dome 22
is defined by a flat center disk 56. Tangent lines A through I are all
~ concentric about disk center point 58 and provide an accordion-like or
5~ serpentine cross-sectional configuration for the container base.
~ me ~onfiguration of base portion 16 provides a number of
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structural benefits. D~e to the rigidity provided by the co~cave and
convex rings, base portion 16 is reinforced against dimensionai changes
, caused by the presence of unrelaxed retractive stresses within the
container material when its ~emperature is elevated, particularly during
demolding and hot-filling operations as mentioned above. ~rhis
~ reinforcement effect is provided irlt the critical transition area of ~ase 16
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1327323
where it is particularly needed. Furthermore, the reinforcing rings act as
a plurality of concentric pressure responsive pistons or diaphragm areas
which are able to undergo limited excursion to acco~v~ate changes in
container internal pressure ~aused by volume shrinkage, carbonation of
filled liquid, external force inputs, etc. Although such limited excursion
of areas of dome 22 is permitted in response to such pressure changes, it
maintains a regular and ordered appearance without random buckling,
bulging, pinching, etc. me curved portions of bottom 16 also form stiff
rings which resist forces imposed by unrelaxed contractive forces which, as
mentioned previously, form a gradient in the radial direction from center
point 58. Significantly, the mechanical characteristics of base portion 16
are provided with a thin-walled configuration without the requirement for
increased thickness ribs or other reinforcing features.
Figure 5 illustrates base portion 110 in accordance with a second
emkodiment of this invention which, like the previously described base
portion 16, can be used with containers 10 of various configurations. Base
portion 110 varies principally from that previously described in that the
reinforcing ring features are interrupted at regularly spaced intervals as
`- shown in Figure 5.
In Figure 6, letters are also used to identify the position of
tangent or break lines as previously defined. me section lines of Figure
6 are taken such that the left-hand portion of the section is taken through
outer reinforcing domes 112, whereas the right-hand portion of the sect:ion
line shows the configuration of inner ring of domes 114~ As shown in
Figure 6, the outermost concave ring 16 is generally similar to ring 40
according to the first embodiment which merges into a large radius convex
ring 118 between tangent lines L and M which is between adjacent domes 112.
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132732~
Tangent lines M through P define dome 114 and rings 122 and 124. On the
left-hand side of the section of Figure 6, the area corresponding to ring
118 has tangent linss Q and R defining dome 112/ whereas a flat portion 126
is present in the place of dome 114. As shown in Figure 5, outer domes 112
are interrupted by generally smooth areas 118, whereas domes 114 are
interrupted by areas 126. mis configuration also provides excellent
stability in response to thermal and mechanical loadings on the base
portion 116. As shown in Figure 5, this embcdiment is also characterized
by concentric tangent lines centered at the center of base 110.
A container base portion in accordance with a third embodiment of
this invention is shown in Figure 7 and is generally designated by
reference number 210. This embodiment is also designated by tangent lines
as the earlier embodiments. ~ase portion 210 is similar to base 110 in
that the concentric reinforcing features formed in the base are interrupted
at regular intervals. For bottom 110, however, the interruptions are
for~ed by generally smooth conical surfaces which interrupt the reinforcing
domes. For base portion 210, however, the reinforGing rings are
interrupted with generally spherical outwardly convex protrusions which are
form~d in the molding die using a ball milling tool. Iike the first
embodiment, base 210 initially forms a ring 212 between tangent lines P~ and
S ~ollo-wed by a slightly outwardly convex ring 214 between tangent lines S
and T. An uninterrupted outwardly concave ring 216 is provided between
tangent lines T and U. A second concave ring 218 is positioned bet~leen
tangent lines V and W, and is interrupted at spherical pockets 220 which
are equally angularly spaced about the periphery of hase 210. The
innermost concave ring 222 is similarly interrupted at regularly angularly
spaced spherical pockets 224 between tangent lines W and X. Like the
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second en~odiment, the interruptions in the reinforcing rings are radially
offset as indicated by the positioning of the section lines for forming
Figure 8. Pockets 220 and 224 of base portion 210 can be forn~d from a
variety of tools but are spheri~al in configuration as sho~n in the
figures. The rings 218 and 222 between spherical pockets 220 and 224,
respe~tively, are formed to blend smoothly into the pockets to prevent the
generation of stress concentrations caused by sharp corners.
A container base configuration in accordance with a third
en~odiment of this invention is shown ln Figures 9 and 10 and is generally
designated by reference number 310. Like the previously described
embodiments, tangent lines are used to designate changes in the curvature
of the reinforcing features of the base. Base portion 310 varies from the
prior en~odiments in that it includes a fewer number of reinforcing ring
features. For this en~xdiment, two rather than three rings 312 and 314 are
provided with an outwardly concave configuration. Ring 312 is forn~d
between tangent lines A' and B', whereas ring 314 is fornled between tangent
lines D' and E' with outwardly eonvex ring 316 formed therebetween. mis
embodiment also varies somewhat from the prior embodiments in that a
generally flat circular band 318 is formed between tangent points D' and
E', rather than providing a circular cross-section ring in that area. In
other respects, however, base 310 performs like the previously descri~d
embodiments for providing rigidity and reinforcement for the base portion
in the area where unrelaxed retractive stresses are predominant.
While the above description constitutes the preferred embodmlents
of the pxesent invention, it will be appreciated that the invention is
susceptible to n~dification, variation and change without departing from
the proper scope and fair meaning of the accompanying claims.
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