Plastic container for pressurized fluids

A blow-molded plastic container having a body comprising a neck portion, a generally cylindrical sidewall portion and a bottom structure. The bottom structure comprises a central portion, a plurality of ribs extending downwardly from the bottle sidewall to the central portion, and a plurality of feet extending below the central portion from the sidewall portion. The ribs are defined by an upper curvilinear surface and, in cross-section are of an substantially U-shape having a relatively tight radius, the upper rib surfaces lying on a generally hemispherical curvature in the interior of said container. Each foot is positioned between two ribs and has a pair of rib-defining endwalls connected to and continuous with the ribs on each side of a curvilinear outer wall connected to and continuous with the sidewall portion, a generally horizontal base surface joined to said outer wall, a generally vertical first inner surface forming a lip extending upwardly from the base surface, and a second inner surface extending from the lip to the central portion.

BACKGROUND OF THE INVENTION 
This invention relates to plastic containers, especially plastic containers 
for pressurized fluids, and more particularly, to an improved bottom 
structure for plastic bottles of the type suitable for containing 
effervescent or carbonated beverages. 
Blow-molded plastic bottles for containing liquids at elevated pressures 
are known and have found increasing acceptance. Such containers are 
accepted particularly in the beverage industry for use as one-way 
disposable containers for use with effervescent or carbonated beverages, 
especially carbonated soft drinks. Plastic bottles of this type are 
subject to a number of structural and functional criteria which have 
presented many problems not previously solved. Solutions to the problems 
offered by the prior art have yielded bottles which are not entirely 
satisfactory. 
Because many of the pieces of the equipment used in the handling and 
filling of such bottles are costly and were manufactured to work with 
glass bottles, attempts were made to conform the plastic bottles to the 
size and shape of prior art glass bottles employed for the same purpose. 
However, it has been found that a mere replication of the prior art glass 
bottles in plastic is not entirely satisfactory. The replication of the 
glass structure in plastic is not possible due to the resilient nature of 
the plastic materials and the distortion and creep which the plastic 
materials can exhibit at elevated pressure especially when such bottles 
are subjected to elevated temperatures. Further, the plastic bottle is 
limited to certain modification by the very nature of the blowing process 
and the available materials for use in forming such a bottle. 
The overwhelming use for the bottles of this type are where the contained 
liquid will be carbonated. When used with carbonated beverages, the 
bottles may be subjected to internal pressures normally between 40 and 100 
pounds per square inch and occasionally as high as 200 psi under severe 
conditions of elevated temperature, especially during transportation. In 
such a condition, the bottle is presented with an elevated pressure within 
the bottle when filled. This pressure, however, will be absent both prior 
to sealing and subsequent to the opening of the bottle. The potential for 
failure in the plastic bottle when pressurized is greatest at the bottom 
of the container. Various designs have been employed to effectively deal 
with this condition. 
One of the initial plastic bottle designs had a bottom design consisting 
generally of a hemispherical bottom to which was added as a separate 
member a base cup which supports the bottle in an upright position. This 
design is shown for example, in U.S. Pat. No. 3,722,725. This design has 
been widely used and adopted in the industry. It provides a strong bottle 
because the hemispherical bottom is the geometric shape which most 
uniformly adapts to pressure. However, this basic design has several 
significant disadvantages. 
Initially, the design requires the separate manufacture of the bottle and 
the base cup. It also requires the additional mechanical step of attaching 
the base cup to the bottle. In addition, the amount of material used in 
the bottle and in the base cup is beginning to cause concern among the 
ever more environmentally-conscious public. Compounding the environmental 
problem, in commercial embodiments, the bottle and base cup are generally 
made from dissimilar plastic materials. In such a case, the reclamation or 
recycling of the plastic used in the bottles is difficult if not 
impossible. 
Due to the manufacturing and disposal problems inherent in the two-piece 
construction, the art turned to the manufacture of one-piece bottles. Such 
bottle designs have generally taken the form of bottles where the bottom 
design is a plurality of feet integrally formed in the base of the bottle 
upon which the bottle rests, for example U.S. Pat. No. 3,759,410. Other 
designs for one-piece bottles include a continuous peripheral seating ring 
upon which the bottle rests surrounding a generally concave central 
portion, e.g., U.S. Pat. No. 4,247,012. 
In existing one-piece bottle bottom constructions three general problems 
have been identified in the art. Initially, such plastic bottles have not 
had enough bottom strength to withstand the impact of falling from a 
moderate height onto a hard surface when filled with a carbonated 
beverage. Further, because the bottles are often subjected to extreme 
temperatures, it has been found in some designs that the bottom of the 
bottle everts or otherwise distorts producing a bottle known in the 
industry as a "rocker" where the bottle wobbles in transportation or 
display. Finally, another problem is the stress cracking of such bottles, 
especially under extremes of temperature or pressure or when exposed to 
any stress cracking agent during filling, handling or transportation. 
Moreover, as is known in the art, it is highly desirable that any bottle 
design be of a type which is aesthetically pleasing as the bottle's design 
is used as one feature in the marketing and sale of the contained liquid. 
One known bottom structure which is generally considered aesthetically 
pleasing is the so-called "champagne" bottom. Based upon the traditional 
design of glass champagne bottles, the champagne bottom has a central 
upwardly convex portion which extends up into the bottle interior from the 
continuous base which is a continuation from the bottle sidewall. 
Polyethylene terephthalate (PET) is the preferred plastic used in the 
formation of bottles for carbonated beverages. PET is a desirable material 
to use in such bottles because, when properly processed it has the 
requisite clarity, strength, and resistance to pressure leakage necessary 
for such bottles. Specifically, when blow-molded, PET is essentially 
completely transparent. The PET material has sufficient gas barrier 
properties so that carbonated beverages can be stored for extended periods 
of time without losing any significant amount of the CO.sub.2 pressure 
given by carbonation. Commonly, bottles are blow molded from injection 
molded "preforms" of PET. 
Blow molded bottles formed from injection molded preforms tend to have a 
particularly acute stress cracking problem in the area of the bottle 
bottom portion which includes and lies adjacent to the nib remaining on 
the preform from the sprue or "gate" through which the molten polymer is 
injected into the preform mold. This gate area is manifest in the 
blow-molded bottle by a clouded circlet at or very near the center of the 
bottle bottom. In the prior art bottles, this gate area contains far less 
biaxial orientation than is present in the bottle sidewall or in the 
remainder of the bottom. As a result of this deficiency, the gate area of 
a bottle blow molded from an injection molded preform is more likely to 
fail under stress, particularly under the extreme conditions experienced 
in the transportation and storage especially in geographical areas where 
the ambient temperature exceeds 100.degree. F., than other areas of the 
bottle sidewall and bottom. The beverage industry suffers substantial 
losses due to this stress-cracking problem. 
Thus, the present invention provides a design for a blow-molded one-piece 
plastic beverage container having a bottom design overcoming the problems 
of the prior art. Specifically, the container of the present invention is 
strong enough to withstand a blow from a fall, will not evert under 
pressure, is resistant to stress cracking, and is aesthetically pleasing. 
SUMMARY OF THE INVENTION 
The present invention provides for a plastic bottle which has a neck 
portion, a generally cylindrical sidewall portion and a bottom structure. 
The neck and sidewall portions are conventional while the bottom is 
unique. The bottom structure comprises a plurality of ribs extending from 
the sidewall to a central portion of the bottom structure where the ribs 
intersect. The upper curvilinear surface of the ribs lie on an essentially 
hemispherical curve in the interior of the bottle. The bottom further 
comprises, alternating between the ribs, a plurality of uniquely designed 
feet which extend along a curved path from the sidewall, have endwalls 
connected to adjacent ribs and include a generally horizontal base 
surface. 
Upon pressurization of the bottle, the radial position of the base surface 
from the central portion is displaced slightly outwardly and the base 
surface of each foot assumes a saddle-like contour with two contact points 
at each end of the saddle. These contact points on all the feet lie in a 
common horizontal plane perpendicular to the central vertical axis of 
bottle. 
The bottom presents a pseudo-champagne appearance wherein the feet contain 
a substantially vertical inner surface or lip positioned radially inwardly 
from the base surface and connected to a second inner surface which 
extends from the substantially vertical lip to the central portion of the 
bottom structure. Thus, the inner surfaces of the feet define a 
pseudo-champagne dome below the central portion and below the 
hemispherical bottom contour defined by the upper rib surfaces. 
It has been found that this structure prevents the bottom from everting and 
induces sufficient biaxial orientation in the bottle to improve stress 
crack resistance. The bottle of the present invention has sufficient 
strength to be able to withstand the stress of a pressurized fluid. In 
particular, the bottle is found to have sufficient biaxial orientation in 
the gate area so that the bottom is strengthened in that area.

DETAILED DESCRIPTION 
The processing of the bottles of the present invention involves the 
injection molding of PET into what is commonly referred to as a "preform" 
and then blow-molding such preform into the bottle. 
PET is a polymer with a combination of properties that are desirable for 
the packaging of carbonated beverages including toughness, clarity, creep 
resistance, strength, and a high gas barrier. Furthermore, because PET is 
a thermoplastic it can be recycled by the application of heat. Solid PET 
exists in three basic forms: amorphous, crystalline, and biaxially 
oriented. 
PET in the amorphous state is formed when molten PET is rapidly cooled to 
below approximately 80.degree. C. It appears clear and colorless and is 
only moderately strong and tough. This is the state that preforms are in 
upon being injection molded. 
Crystalline PET is formed when molten PET is cooled slowly to below 
80.degree. C. In the crystalline state, PET appears opaque, milky-white 
and is brittle. Crystalline PET is stronger than amorphous PET and thus it 
is desirable to minimize or eliminate the presence of any crystalline 
material in a preform. Because crystalline PET is stronger than amorphous 
PET, badly formed bottles will result from the blow molding process if a 
significant amount of crystalline PET is present in the preform. 
Oriented PET is formed by mechanically stretching amorphous PET at above 
80.degree. C. and then cooling the material. Biaxially oriented PET is 
usually very strong, clear, tough, and has good gas barrier properties. It 
is generally desirable in order to obtain sufficient biaxial orientation 
that the amount of stretch being applied to the amorphous PET be on the 
order of at least three times. 
While biaxially oriented PET is exceptionally clear and resistant to stress 
cracking, non-biaxially oriented crystalline PET is neither clear nor 
resistant to stress cracking. Further, amorphous PET, although clear, is 
not resistant to stress cracking. One easy test used in the industry to 
determine the stress crack resistance of a PET bottle is to apply an 
acetone-containing solution to a pressurized bottle. Material which is 
amorphous or crystalline in nature will show cracking in a relatively 
short amount of time, on the order of minutes, as compared to the 
resistance of biaxially oriented PET. 
Thus, in the design of plastic containers made of PET it is desirable to 
obtain as much biaxial orientation as is possible. 
Various types of PET material can be used in the manufacture of the bottles 
of the present invention. One important measure of the PET material which 
is used by those skilled in the art is the intrinsic viscosity. Typical 
values of intrinsic viscosity for PET bottle manufacture are in the range 
of 0.65 to 0.85. It has been found preferable in the bottle of the present 
invention to use a PET material with an intrinsic viscosity of not less 
than 0.8. 
In the present invention, a conventionally made injection-molded preform 
can be used. As one skilled in the art knows, various configurations of 
preforms for a desired bottle can be used to make various bottle designs. 
The use of a particular preform with a particular bottle design is a 
matter of design and the selection criteria are known to those of skill in 
the art. It may be advantageous to alter the design of the preform to 
optimize the final bottle. For example, it may be advantageous to taper 
the bottom of the preform to allow better orientation and distribution of 
material. 
In the injection-molding of the preform the molten polymer is injected into 
the mold through a sprue or gate. As a result of this, a nib of polymer 
remains on the preform. The "gate" area of the preform, includes and lies 
adjacent to this nib, and tends not to be biaxially oriented to the same 
degree as the rest of the bottle and, therefore, tends to be a point of 
potential stress cracking. 
Sometimes the gate area of the preform contains a small amount of 
crystalline material as it is difficult in the injection molding process 
to cool that portion of the material rapidly enough to allow it to become 
amorphous. More importantly, in the prior art, the gate area was not 
stretched when the bottle was blow-molded and, therefore, the 
crystallinity was deemed acceptable for the formation of an appropriate 
bottle. The non-oriented area must, therefore, be restricted to a very 
small area around the gate and even if it is so restricted, the area of 
crystallinity introduces potential stress cracking problems in the bottle. 
The bottom structure of the present invention is such that the PET material 
in and around the gate area of the preform is sufficiently biaxially 
oriented in the blow-molding process to improve stress crack resistance 
over the prior art. Thus, the PET material in the entire bottle, including 
that material in the gate area is sufficiently stretched in molding to 
form a bottle which is substantially resistant to stress cracking. 
The bottles of this invention can be formed by a conventional stretch 
blow-molding process. In such a process, biaxial orientation is introduced 
into the PET by producing stretch along both the length of the bottle and 
the circumference of the bottle. In stretch blow-molding, a stretch rod is 
utilized to elongate the preform and air or other gas pressure is used to 
radially stretch the preform, both of which happen essentially 
simultaneously. Prior to blow-molding, the preforms are preheated to the 
correct temperature, generally about 100.degree. C., but this varies 
depending upon the particular PET material being used. 
It is known in the art that the temperature and temperature profile of 
heating of the preform is important to achieve the intended distribution 
of the material over the bottle wall during forming. It also is well known 
in the art how to alter such a temperature profile to produce an 
acceptable bottle once the design of the mold is known. The temperature 
profile is used to control material distribution. 
Once the PET preform is at the desired temperature it is secured by its 
neck in a mold which has a cavity of the desired bottle shape. A stretch 
rod is introduced into the mouth of the bottle to distribute the material 
the length of the bottle and orient the molecules of PET longitudinally. 
Simultaneously, air is blown into the bottle from around the stretch rod 
to distribute the material radially to give the radial or hoop 
orientation. 
Air pressure pushes the bottle walls against the mold, generally 
water-cooled, causing the biaxially oriented PET to cool. Ideally, as is 
known in the art, the bottle wall should touch the mold at all points of 
the bottle at approximately the same time. After sufficient cooling has 
taken place, to avoid bottle shrinkage, the mold is opened and the bottle 
discharged. 
Referring to FIG. 1, a container in the form of a bottle 10 is constructed 
having a body which comprises generally cylindrical sidewall portion 12 
and a neck portion 14. The upper neck portion 14 can have any desired neck 
finish, such as the threaded finish which is shown, and is generally 
closable to form a pressurizable bottle. A bottom portion 16 is provided 
at the lower end of the sidewall portion 12. The bottom portion 16 
comprises a plurality of feet 18. Alternating between said plurality of 
feet 18 are ribs 20 which extend from sidewall 12. The ribs 20 of the 
present invention are defined by an upper curvilinear surface. As can best 
be seen in FIG. 2, in cross section, ribs 20 have an inverted U-shaped 
cross-section with a relatively tight radius. Referring to FIGS. 1-3 it 
can be seen that ribs 20 are continuous and merge into endwalls 22 of feet 
18. 
The bottom section 16 can be comprised of four feet 18 as shown in FIGS. 
1-5 or as shown in FIGS. 6-10 the bottom section 116 can be comprised of 
six feet 118. It is to be understood that the embodiments herein described 
and shown in the drawings are preferred embodiments only and the number of 
feet is primarily a function of the desired aesthetics. However, it is 
preferred to use a larger number of feet in a larger bottle to provide 
more ribs which provide increased stability and rigidity in the bottom 
section. Moreover, the number of feet used must be sufficient so that the 
structure of the feet as hereinafter described is able to cause the PET 
material within the gate area to be sufficiently stretched so as to cause 
biaxial orientation. 
Referring to FIG. 3, the bottom section 16 is seen in a bottom view in an 
embodiment where there are four feet 18 with four corresponding ribs 20. 
As can be seen by referring to FIG. 4, the upper curvilinear surfaces 24 
of ribs 20 form a generally hemispherical curve in the interior of the 
container 10. The ribs 20 are of a substantially inverted U-shape in cross 
section, and define a somewhat tight curve in order to induce biaxial 
orientation of the PET and provide rigid structural support in the bottom. 
The ribs 20 merge smoothly from the sidewall portion 12 of the bottle 10 
and extend to a central portion 26 which can be seen by reference to FIGS. 
3-5. The central portion 26 is generally circular in shape and includes 
the gate area of the preform. 
The upper curvilinear surface 24 of a rib 20 follows a generally 
semicircular path connected to and continuous with sidewall 12 and has a 
radius substantially equal to the radius of the cylindrical sidewall 
portion 12. Alternatively, the path defined by the surface 24 of the ribs 
20 can have two or more arcuate sections of differing radii or can include 
straight sections tangent with curved sections. For example, in FIG. 4 
there is a first arcuate section 28 of radius, r1, equal to that of the 
cylindrical sidewall portion 12. Connected to and continuous with the 
first arcuate section 28 is a second arcuate section 30 of relatively 
smaller radius, r2. This smaller radius second arcuate section 30 is 
connected to and continuous with first arcuate section 28 on one end and 
on its other end is connected to and continuous with central portion 26. 
The size of the radius of arcuate portion 30 relative to arcuate portion 
28 can vary, for example, in the range of from 7 to 15% of the radius of 
the first arcuate section 28. Also central portion 26 has an upper surface 
inside the bottle which is a continuation of the rib curvature, or it can 
be slightly flattened as produced by the contour of the stretch rod. 
Having a central portion 26 which is slightly domed is also within the 
scope of the invention. 
As can be seen by referring to FIGS. 4-5, the feet 18 extend below central 
portion 26 and are defined on their outer surface by a curvilinear outer 
wall 32. This outer foot wall 32 can follow any smooth curvature from the 
bottle sidewall to the foot base surface 40. 
In a preferred embodiment, as shown, the curvilinear outer foot wall 32 is 
comprised of three arcuate sections, the first arcuate section 34 of a 
relatively small radius, r3, the second arcuate section 36 of a relatively 
large radius, r4, and the third arcuate section 38 of a relatively small 
radius, r5. As used in connection with wall 32, relatively large radius is 
meant to indicate a radius of curvature well in excess of the radius of 
the cylindrical section 12 of the bottle and can be larger even than the 
diameter of the cylindrical sidewall portion 12 of the bottle. The first 
arcuate section 34 is connected to and continuous with the sidewall 12. 
Connected to and continuous with the first arcuate section 34 is the 
second arcuate section 36 and connected thereto is third section 38. The 
first arcuate section 34 is connected to and continuous with i.e., 
tangential to, sidewall 12. The third section 38 is connected to and 
continuous with, i.e., tangential to, the horizontal base surface 40 which 
is provided as the bottom of foot 18. In a preferred embodiment, the radii 
of the first and third arcuate portions 34 and 38 can be in the range of 
between 10 and 25% of the radius of second arcuate section 36. 
The bottom of foot 18 is defined by horizontal base surface 40. The 
diameter d shown in FIG. 5 is the effective diameter of the contact 
surface of bottle 10 when the bottle is non-pressurized. As will be 
discussed more fully later, when pressurized, the diameter d increases to 
provide increased stability. The psuedo-champagne dome effect is provided 
by the radially inward surface of the feet 18. A generally vertical first 
inner surface 42 is connected to and extends upwardly from the base 
surface 40 forming a lip. In the embodiment shown, the first inner surface 
42 is 3.degree. off of vertical. A second inner surface 44 extends from 
the substantially vertical lip 42 to the central portion 26. 
In a preferred embodiment, there is an arcuate transition section 46 
joining the second inner surface 44 to the lip 42. A second arcuate 
transition section 48 is located at the opposite end of the second inner 
surface 44 and joins the second inner surface 44 to central portion 26. In 
a preferred embodiment, the angle between the plane extending horizontally 
through the center most point of central portion 26 and the plane defined 
by secondary surface 44 is between about 10.degree. and about 35.degree., 
this angle generally being higher in smaller diameter bottles and lower in 
larger diameter bottles. 
It has been found that the bottom structure 16 depicted in the figures 
provides severe enough curving and provides a mold wherein even the 
central portion 26 is substantially transformed into biaxially oriented 
material in the blow-molding process. Thus, the central portion 26, unlike 
in prior art embodiments, has all of the mechanical property advantages of 
biaxially oriented PET, especially superior stress crack resistance. 
FIGS. 6-10 relate to another embodiment of the container 110 according to 
the present invention. The features which are the same as those described 
in FIGS. 1-5 have the addition of 100 to the respective reference 
numerals. In the embodiment shown in FIGS. 6 through 10, six feet 118, 
along with six ribs 120 are used. As noted above, the specific number of 
feet 118 used in any given embodiment is a matter of choice. However, it 
has been found that for a container of volume of about 16 ounces or 500 
milliliters, a four-footed design is desirable. Correspondingly, for a 
larger container, such as a two-liter bottle, it has been found that a 
six-footed embodiment is preferred. While the choice of the number of feet 
is a design variable adjustable by those skilled in the art, it is noted 
that generally it is desirable to have a smaller number of feet on smaller 
containers so as not to require overly intricate molds which could result 
in a large number of malformed bottles. Correspondingly, in larger 
containers it is desirable to have a larger number of feet to allow the 
number of ribs to be sufficient to define the hemispherical curve which 
gives the bottle of the present invention its strength and also to create 
enough convolution in the bottom design to induce sufficient biaxial 
orientation throughout the bottom of the container, including in the gate 
area. 
Turning to FIG. 6, it can be seen that in the six-footed embodiment of 
bottle 110, there is again a substantially cylindrical sidewall portion 
112 a neck portion 114 of conventional construction and a bottom portion 
116. The bottom portion is comprised of feet 118 and ribs 120. Referring 
back to FIG. 2, it has been found the angle .alpha. between the two rib 
defining endwalls 22 of adjacent feet 18 is approximately 30.degree. for a 
four-footed design in a 16 ounce or 500 milliliter bottle. 
Correspondingly, referring to FIG. 6, it has been found that the angle 
.alpha. between two adjacent rib-defining endwall portions 122 of feet 118 
is about 24.degree., an appropriate amount for a six-footed design in a 
two-liter bottle. 
As shown in FIGS. 8-10, the construction of a bottle with an embodiment of 
six feet is substantially similar to the construction of the four-footed 
bottle. As seen in FIG. 8, the bottom portion 116 of the bottle 110 
contains feet 118 with ribs 120. Central portion 126 can be seen in FIG. 
8. As seen in FIGS. 9-10, the construction of the ribs 120 as well as the 
construction of the feet 118 are similar in both the four-footed and 
six-footed embodiments of the bottle of this invention. 
The bottom construction of the bottle of the present invention not only 
induces sufficient biaxial orientation to increase the stress-crack 
resistance of the bottle, especially the gate area of the bottle, above 
the prior art, but also produces a pseudo-champagne bottom which is 
prevented from everting even under the highest pressures generally 
experienced by such bottles. When the bottle of FIG. 1 was filled with 
carbonated fluid and pressurized, the bottom did not evert. 
Under pressure, the structure of the bottom does alter slightly as shown in 
FIGS. 11-12. As seen in FIG. 11, when pressurized, the curvature of the 
curvilinear outer wall 232 of the foot 218 changes so that the horizontal 
base surfaces 240 are moved radially outwardly toward the sidewall 
portions. This results in the effective diameter d' of the base of the 
bottle increasing from the diameter d as shown in FIG. 5. Generally, 
diameter d' is approximately 8-10% greater than diameter d. Moreover, as 
seen in FIG. 13, even when central portion 226 is slightly flattened in an 
unpressurized bottle, the pressure exerted on central portion 226 in a 
pressurized bottle results in the depression of central portion 226 to 
form a more nearly perfect hemispherical curve as defined by the upper 
surfaces 224 of ribs 220 in the pressurized bottle. In so doing the second 
inner surface 244 of the foot 218 substantially decreases in angle as 
compared to the plane defined horizontally through the center point of 
central portion 226 as best seen in FIG. 11. It is to be noted that the 
curvilinear outer foot wall 232 does not extend radially outside the 
sidewall 212 of the bottle. Any bulge in wall 232 extending past the 
diameter of the sidewall portion 212 would be undesirable from both an 
aesthetic and transportation point of view. 
As seen in FIGS. 11 and 12, when the bottle is pressurized foot 218 takes 
on a saddle-like configuration with the base surface 234 turning into an 
curved surface 246 with two contact points 248 at each end of foot 218. 
This saddle-like contour of foot 218 results in further stability in the 
bottle 210 and further aesthetically pleasing characteristics. 
Furthermore, when the bottle is pressurized, the angle .alpha. between 
adjacent rib wall-defining endwall portions 222 of feet 218 increases over 
the .alpha. of the non-pressurized bottle resulting from the fact that 
these end walls spread somewhat. Thus, the bottom configuration of the 
present invention results in a stable, strong, stress-crack resistant, 
aesthetically pleasing bottle. 
As shown in FIG. 14 and as previously described, the positioning of the 
material within the final blow-molded container product can be controlled 
by the temperature control on the preform used in the blow-molding 
process. As shown in FIG. 13, in a typical cross-section of the bottle 
310, the thickness of the curvilinear outer wall 332 of the foot 318 
varies from the thickness of the sidewall 312 of the container 310 and 
also varies as the foot progresses to its base 340 and to its lip 342, 
second inner wall 344 and central portion 326. Other combinations of 
bottom wall thickness gradation are possible. One of the significant 
advantages of the present invention is that less PET is required in the 
manufacture of the bottles than in prior art bottles. Thus, the 
aforementioned property advantages are augmented by significant cost 
savings.