Structural ribs for hot fillable containers

Abstract

A plastic container includes an upper portion having a mouth defining an opening into the container. A shoulder region extends from the upper portion. A sidewall portion extends from the shoulder region to a base portion. The base portion closes off an end of the container. The sidewall portion is defined in part by at least one arcuately formed rib having a body portion and opposite ends. The body portion curves from a central portion toward the base portion at each of the opposite ends.

Description

TECHNICAL FIELD

This disclosure generally relates to plastic containers for retaining a commodity, and in particular a liquid or semi solid commodity. More specifically, this disclosure relates to a plastic container having a sidewall portion defining arcuate ribs that promote improved material distribution during container formation.

BACKGROUND

The statements in this section merely provide background information related to the present disclosure and may not constitute prior art.

As a result of environmental and other concerns, plastic containers, more specifically polyester and even more specifically polyethylene terephthalate (PET) containers are now being used more than ever to package numerous commodities previously supplied in glass containers. Manufacturers and fillers, as well as consumers, have recognized that PET containers are lightweight, inexpensive, recyclable and manufacturable in large quantities.

Blow-molded plastic containers have become commonplace in packaging numerous commodities. PET is a crystallizable polymer, meaning that it is available in an amorphous form or a semi-crystalline form. The ability of a PET container to maintain its material integrity relates to the percentage of the PET container in crystalline form, also known as the “crystallinity” of the PET container. The following equation defines the percentage of crystallinity as a volume fraction:

$\%\mathrm{Crystallinity}=\left(\frac{\rho -{\rho }_a}{{\rho }_c-{\rho }_a}\right)x100$

where ρ is the density of the PET material; ρ_a is the density of pure amorphous PET material (1.333 g/cc); and ρ_c is the density of pure crystalline material (1.455 g/cc).

Container manufacturers use mechanical processing and thermal processing to increase the PET polymer crystallinity of a container. Mechanical processing involves orienting the amorphous material to achieve strain hardening. This processing commonly involves stretching an injection molded PET preform along a longitudinal axis and expanding the PET preform along a transverse or radial axis to form a PET container. The combination promotes what manufacturers define as biaxial orientation of the molecular structure in the container. Manufacturers of PET containers currently use mechanical processing to produce PET containers having approximately 20% crystallinity in the container's sidewall.

Thermal processing involves heating the material (either amorphous or semi-crystalline) to promote crystal growth. On amorphous material, thermal processing of PET material results in a spherulitic morphology that interferes with the transmission of light. In other words, the resulting crystalline material is opaque, and thus, generally undesirable. Used after mechanical processing, however, thermal processing results in higher crystallinity and excellent clarity for those portions of the container having biaxial molecular orientation. The thermal processing of an oriented PET container, which is known as heat setting, typically includes blow molding a PET preform against a mold heated to a temperature of approximately 250° F.-350° F. (approximately 121° C.-177° C.), and holding the blown container against the heated mold for approximately two (2) to five (5) seconds. Manufacturers of PET juice bottles, which must be hot-filled at approximately 185° F. (85° C.), currently use heat setting to produce PET bottles having an overall crystallinity in the range of approximately 25%-35%.

Food and juice suppliers often fill these products into the containers while the product is at an elevated temperature, typically between 155° F.-205° F. (68° C.-96° C.) and usually at approximately 185° F. (85° C.). When packaged in this manner, the hot temperature of the commodity sterilizes the container at the time of filling. The bottling industry refers to this process as hot filling, and the containers designed to withstand the process as hot-fill or heat-set containers.

One challenge associated with thermal processing of an oriented PET container is the ability to direct material throughout the critical areas of the container during formation. One approach includes preferential heating of the preform to control material distribution wherein different portions of the preform are heated to different temperatures to encourage material distribution in critical areas of the container. However, such as with formation of some rectangular or oval containers, this approach is costly and does not always yield satisfactory material distribution throughout the container. Additionally, in some instances it may be desirable to incorporate ribs at various locations on the container to improve structural integrity of the container as a whole. While such ribs may provide improved durability to the container, the ribs may make it more difficult to successfully distribute the material throughout the container during formation. Explained further, containers incorporating horizontal ribs (i.e., in a direction generally parallel to a container base), may result in material being trapped at the ribs thereby inhibiting material distribution in the heel or base of the container during formation. Similarly, containers incorporating vertical ribs (i.e. in a direction generally transverse to a container base), may result in material being trapped at the ribs thereby causing an uneven distribution of material throughout the sides of the container during formation.

After being hot-filled, the heat-set containers may be capped and allowed to reside at generally the filling temperature for approximately five (5) minutes at which point the container, along with the product, is then actively cooled prior to transferring to labeling, packaging, and shipping operations. The cooling reduces the volume of the commodity in the container. This product shrinkage phenomenon results in the creation of a vacuum within the container. Generally, vacuum pressures within the container range from 1-380 mm Hg less than atmospheric pressure (i.e., 759 mm Hg-380 mm Hg). If not controlled or otherwise accommodated, these vacuum pressures result in deformation of the container, which leads to either an aesthetically unacceptable container or one that is unstable. Hot-fillable plastic containers must provide sufficient flexure to compensate for the changes of pressure and temperature, while maintaining structural integrity and aesthetic appearance. Typically, the industry accommodates vacuum related pressures with sidewall structures or vacuum panels formed within the sidewall of the container. Such vacuum panels generally distort inwardly under vacuum pressures in a controlled manner to eliminate undesirable deformation.

While such vacuum panels allow containers to withstand the rigors of a hot-fill procedure, the panels have limitations and drawbacks. First, such panels formed within the sidewall of a container do not create a generally smooth glass-like appearance. Second, packagers often apply a label to the container over these panels. The appearance of these labels over the vacuum panels is such that the label often becomes wrinkled and not smooth. Additionally, one grasping the container generally feels the vacuum panels beneath the label and often pushes the label into various panel crevasses and recesses.

Thus, there is a need for an improved plastic container, which allows for improved material distribution during formation and accommodates the vacuum pressures which result from hot filling.

SUMMARY

Accordingly, the present disclosure provides a plastic container including an upper portion having a mouth defining an opening into the container. A shoulder region extends from the upper portion. A sidewall portion extends from the shoulder region to a base portion. The base portion closes off an end of the container. The sidewall portion is defined in part by at least one arcuately formed rib having a body portion and opposite ends. The body portion curves from a central portion toward the base portion of the container at each of the opposite ends.

According to additional features, a series of arcuately formed ribs are defined on the sidewall portion. Each of the ribs define an inboard depression along the sidewall portion. The sidewall portion may be defined in part by at least two vacuum panels formed therein. The vacuum panels are movable to accommodate vacuum forces generated within the container resulting from heating and cooling of its contents. The sidewall portion may define a label panel area on an area across the vacuum panels. The label panel area may be adapted to accept a label over the series of ribs.

A method of making a blow-molded PET plastic container includes disposing a preform into a mold cavity. The mold cavity has a surface defining a shoulder forming region, a sidewall forming region and a base forming region. The sidewall forming region defines at least one arcuate extension rib having a body portion and opposite ends. The opposite ends of each extension rib curve from a central portion toward the base forming region at each of the opposite ends. The preform is blown against the mold surface to form a resultant container having a shoulder, a sidewall and a base. The sidewall defines at least one arcuate depression rib corresponding to the at least one arcuate extension rib of the mold cavity.

The configuration of the arcuate extension ribs in the mold cavity are consistent with the directional flow of material during container formation thus facilitating consistent material flow during formation of the container. Moreover, the arcuate extension ribs are generally curved from a central portion at an upstream area to opposite ends at a downstream area. In this way, material may flow smoothly around the arcuate surfaces without becoming substantially impeded or trapped at the ribs.

Additional benefits and advantages of the present disclosure will become apparent to those skilled in the art to which the present disclosure relates from the subsequent description and the appended claims, taken in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view of a plastic container constructed in accordance with the teachings of the present disclosure;

FIG. 2 is a front view of the container of FIG. 1;

FIG. 3 is a side view of the container of FIG. 1;

FIG. 4 is a cross-sectional view of the plastic container, taken generally along line 4-4 of FIG. 2; and

FIG. 5 is a sectional view of an exemplary mold cavity used during formation of the container of FIG. 1 and shown with a preform positioned therein.

DETAILED DESCRIPTION

The following description is merely exemplary in nature, and is in no way intended to limit the disclosure or its application or uses.

FIGS. 1-4 show one example of the present container. In the figures, reference number 10 designates a plastic, e.g. polyethylene terephthalate (PET), hot-fillable container. As shown in FIG. 2, the container 10 has an overall height A of about 8.15 inch (207 mm), and a sidewall and base portion height B of about 4.50 inch (114 mm). The height A may be selected so that the container 10 fits on the shelves of a supermarket or store. As shown in the figures, the container 10 is substantially oval in cross sectional shape including opposing longer sides 14 each having a width C of about 3.00 inch (76.4 mm), and opposing shorter, parting line sides 15 (FIG. 3) each having a width D of about 1.65 inch (42.0 mm). The widths C and D are selected so that the container 10 can fit within the door shelf of a refrigerator. As with typical prior art bottles, opposing longer sides 14 of the container 10 of the present disclosure are oriented at approximately 90 degree angles to the shorter, parting line sides 15 of the container 10 so as to form a generally oval cross section as best shown in FIG. 4. As such, the container 10 further includes a diagonal width G of about 3.04 inch (77.3 mm). In this particular example, the container 10 has a volume capacity of about 14 fl. oz. (414 cc). Those of ordinary skill in the art would appreciate that the following teachings of the present disclosure are applicable to other containers, such as round, square or rectangular shaped containers, which may have different dimensions and volume capacities. It is also contemplated that other modifications can be made depending on the specific application and environmental requirements.

As shown in FIGS. 1-3, the plastic container 10 of the invention includes a finish 12, a shoulder region 16, a sidewall portion 18 and a base 20. Those skilled in the art know and understand that a neck may also be included having an extremely short height, that is, becoming a short extension from the finish 12, or an elongated height, extending between the finish 12 and the shoulder region 16. The plastic container 10 has been designed to retain a commodity during a thermal process, typically a hot-fill process. For hot-fill bottling applications, bottlers generally fill the container 10 with a liquid or product at an elevated temperature between approximately 155° F. to 205° F. (approximately 68° C. to 96° C.) and seal the container 10 with a closure (not illustrated) before cooling. As the sealed container 10 cools, a slight vacuum, or negative pressure, forms inside causing the container 10, in particular, the sidewall portion 18, as will be described, to change shape. In addition, the plastic container 10 may be suitable for other high-temperature pasteurization or retort filling processes, or other thermal processes as well.

The finish 12 of the plastic container 10 includes a portion defining an aperture or mouth 22, a threaded region 24 having threads 25, and a support ring 26. The aperture 22 allows the plastic container 10 to receive a commodity while the threaded region 24 provides a means for attachment of a similarly threaded closure or cap (not illustrated). Alternatives may include other suitable devices that engage the finish 12 of the plastic container 10. Accordingly, the closure or cap (not illustrated) engages the finish 12 to preferably provide a hermetical seal of the plastic container 10. The closure or cap (not illustrated) is preferably of a plastic or metal material conventional to the closure industry and suitable for subsequent thermal processing, including high temperature pasteurization and retort. The support ring 26 may be used to carry or orient a preform 28 (the precursor to the plastic container 10, shown in FIG. 5) through and at various stages of manufacture. For example, the preform 28 may be carried by the support ring 26, the support ring 26 may be used to aid in positioning the preform 28 in the mold, or an end consumer may use the support ring 26 to carry the plastic container 10 once manufactured.

Integrally formed with the finish 12 and extending downward therefrom is the shoulder region 16. The shoulder region 16 merges into and provides a transition between the finish 12 and the sidewall portion 18. The sidewall portion 18 extends downward from the shoulder region 16 to the base 20. The specific construction of the sidewall portion 18 of the heat-set container 10 allows the shoulder region 16 and the base 20 to not necessarily require additional vacuum panels and therefore, the shoulder region 16 and the base 20 are capable of providing increased rigidity and structural support to the container 10. The base 20 functions to close off the bottom portion of the plastic container 10 and, together with the finish 12, the shoulder region 16, and the sidewall portion 18, to retain the commodity.

The plastic container 10 is preferably heat-set according to the above-mentioned process or other conventional heat-set processes. To accommodate vacuum forces, the sidewall portion 18 may include vacuum panels 30 formed therein. As illustrated in the figures, vacuum panels 30 may be generally rectangular in shape and are formed in the opposing longer sides 14 of the container 10. It is appreciated that the vacuum panels 30 may define other geometrical configurations. Accordingly, the container 10 illustrated in the figures has two (2) vacuum panels 30. The inventors however equally contemplate that more than two (2) vacuum panels 30, such as four (4), can be provided. That is, that vacuum panels 30 can also be formed in opposing shorter, parting line sides 15 of the container 10 as well. Vacuum panels 30 may also include an underlying surface 34. Surrounding vacuum panels 30 is land 32. Land 32 provides structural support and rigidity to the sidewall portion 18 of the container 10.

The plastic container 10 according to the present teachings provides a series of arcuately formed ribs 40. The ribs 40 generally define a body 42 having opposite ends 44 curved in a direction away from the finish 12. As best illustrated in FIG. 3, the ribs 40 define inboard depressions on the sidewall portion 18. As will become more appreciated from the following description, the ribs 40 facilitate an even distribution of material during formation of the container 10. In the example shown, three ribs 40 are formed on the sidewall portion 18 on each of the vacuum panels 30. As shown, the ribs 40 generally define a consistent radius. It is appreciated however, that the ribs 40 may alternatively define an increasing radius or a decreasing radius from a central portion. Preferably, the ribs 40 define a length L (FIG. 2) that is about 50% to 90% of the width of opposing longer sides 14, and more preferably 60% to 80%. The present disclosure is especially effective for producing containers that are substantially rectangular, oblong or oval in shape such as containers wherein the width of opposing longer sides 14 is 1.5 to 2.5 times greater than the width of opposing parting line sides 15. It is appreciated that fewer or more ribs 40 may be incorporated on the container 10. Furthermore, it is appreciated that the ribs 40 may define alternate configurations and/or be located elsewhere on the container 10.

The specific height and resulting radius of curvature of the ribs 40 is dependent on container design aspects that affect the amount of stretching the material undergoes during blow molding of the container 10 from the preform 28. The preferred range of a height H (FIG. 2), for a given length L of the ribs 40, is defined by the following equations:

H_MIN=[(L/C)×(SPL−FPL)]×(C/D);

and

H_MAX=[(L/C)×(CPL−FPL)]×(G/D)

where CPL (FIG. 1) represents the corner profile length which is the length of a vertical profile of the container 10 measured from the shoulder region 16 to the base 20 at a corner or an intersection of an opposing parting line side 15 and an opposing longer side 14, SPL (FIG. 2) represents the side profile length which is the length of a vertical profile of the container 10 measured from the shoulder region 16 to the base 20 at a midpoint of the opposing parting line sides 15, and FPL (FIG. 3) represents the front profile length which is the length of a vertical profile of the container 10 measured from the shoulder region 16 to the base 20 at a midpoint of the opposing longer sides 14.

Accordingly, by way of example, the container 10, may have a length L of the ribs 40 measuring approximately 2.18 inch (55.3 mm), representing about 72.4% of the width C, measuring approximately 3.00 inch (76.4 mm). Similarly, the container 10 may also include a width D measuring approximately 1.65 inch (42.0 mm), a diagonal width G measuring approximately 3.04 inch (77.3 mm), a corner profile length CPL measuring approximately 9.52 inch (241.73 mm), a side profile length SPL measuring approximately 9.37 inch (237.90 mm), and a front profile length FPL measuring approximately 9.00 inch (228.74 mm). Thus, by way of example, using the above-described equations and dimensions, the preferred range of the height H for the ribs 40 generally may be approximately 0.49 inch (12.06 mm) (H_MIN) to 0.70 inch (17.31 mm) (H_MAX). The above and previously mentioned dimensions were taken from a typical 14 fl. oz. (414 cc) container. It is contemplated that comparable dimensions are attainable for containers of varying shapes and sizes.

A label panel area 50 is defined at the sidewall portion 18. The label panel area 50 may generally overlay the vacuum panels 30. As is commonly known and understood by container manufacturers skilled in the art, a label may be applied to the sidewall portion 18 at the label panel area 50 using methods that are well known to those skilled in the art, including shrink-wrap labeling and adhesive methods. As applied, the label may extend around the entire body or be limited to a single side of the sidewall portion 18.

Upon filling, capping, sealing and cooling, as illustrated in FIG. 4 in phantom, the underlying surface 34 of vacuum panels 30 is pulled radially inward, toward a central longitudinal axis 46 of the container 10, displacing volume, as a result of vacuum forces. In this position, the underlying surface 34 of vacuum panels 30, in cross section, illustrated in FIG. 4 in phantom, forms an underlying surface 34′. The greater the inward radial movement between underlying surfaces 34 and 34′, the greater the achievable displacement of volume. The configuration of the sidewall portion 18, vacuum panels 30 and ribs 40 allow the vacuum reaction to be absorbed in a controlled manner. Furthermore, the container 10 maintains its outwardly curved cross-sectional shape during vacuum absorption providing a desirable surface for applying a label.

The amount of volume which vacuum panels 30 of the sidewall portion 18 displaces is also dependant on the projected surface area of vacuum panels 30 of the sidewall portion 18 as compared to the projected total surface area of the sidewall portion 18. The generally rectangular configuration of the container 10 creates a large surface area on opposing longer sides 14 of the sidewall portion 18, thereby promoting the use of large vacuum panels. This large surface area promotes the placing of large vacuum panels 30 in this area. Accordingly, as illustrated in FIG. 2, this results in vacuum panels 30 having a width E and a height F. In one example, for the container 10 having a nominal capacity of approximately 14 fl. oz. (414 cc), the width E is about 2.8 inch (71.12 mm) while the height F is about 3.74 inch (95.00 mm).

Turning now to FIG. 5, the preform 28 used to mold the exemplary container 10 in a mold cavity 60 is shown. The plastic container 10 of the present invention is a blow molded, biaxially oriented container with a unitary construction from a single or multi-layer material. A well-known stretch-molding, heat-setting process for making the hot-fillable plastic container 10 generally involves the manufacture of the preform 28 of a polyester material, such as polyethylene terephthalate (PET), having a shape well known to those skilled in the art similar to a test-tube with a generally cylindrical cross section and a length typically approximately fifty percent (50%) that of the resultant container height. A machine (not illustrated) places the preform 28 heated to a temperature between approximately 190° F. to 250° F. (approximately 88° C. to 121° C.) into the mold cavity 60 having a shape similar to the plastic container 10.

The mold cavity 60 generally defines a shoulder forming region 62, a sidewall forming region 64 and a base forming region 66. The sidewall forming region 64 includes arcuate extension ribs 70 thereon corresponding to the ribs 40 formed on the resultant container 10. The arcuate extension ribs 70 slope generally from a central portion 80 downward and away to ends (not specifically shown in FIG. 5) corresponding to the ends 44 of the ribs 40. The arcuate nature of the extension ribs 70 facilitate material flow from an area generally upstream (central portion 80) to an area generally downstream toward and beyond the ends of the ribs 70. In this way, material is discouraged from being impeded or trapped at the ribs 70 during formation of the container 10. As a result, an even material distribution is realized throughout the container 10.

During formation, the mold cavity 60 may be heated to a temperature between approximately 250° F. to 350° F. (approximately 121° C. to 177° C.). A stretch rod apparatus (not illustrated) stretches or extends the heated preform 28 within the mold cavity 60 to a length approximately that of the container 10 thereby molecularly orienting the polyester material in an axial direction generally corresponding with the central longitudinal axis 46 (FIGS. 2 and 3) of the container 10.

While the stretch rod extends the preform 28, air having a pressure between 300 PSI to 600 PSI (2.07 MPa to 4.14 MPa) assists in extending the preform 28 in the axial direction and in expanding the preform 28 in a circumferential or hoop direction thereby substantially conforming the polyester material to the shape of the mold cavity 60 and further molecularly orienting the polyester material in a direction generally perpendicular to the axial direction, thus establishing the biaxial molecular orientation of the polyester material in most of the container 10. Typically, material within the finish 12 and a sub-portion of the base 20 are not substantially molecularly oriented. The pressurized air holds the mostly biaxial molecularly oriented polyester material against the mold cavity 60 for a period of approximately two (2) to five (5) seconds before removal of the container 10 from the mold cavity 60. This process is known as heat setting and results in a heat-resistant container suitable for filling with a product at high temperatures.

Alternatively, other manufacturing methods, such as for example, extrusion blow molding, one step injection stretch blow molding and injection blow molding, using other conventional materials including, for example, high density polyethylene, polypropylene, polyethylene naphthalate (PEN), a PET/PEN blend or copolymer, and various multilayer structures may be suitable for the manufacture of plastic container 10. Those having ordinary skill in the art will readily know and understand plastic container manufacturing method alternatives.

While the above description constitutes the present disclosure, it will be appreciated that the disclosure is susceptible to modification, variation and change without departing from the proper scope and fair meaning of the accompanying claims.

Claims

1. A plastic container comprising:

an upper portion having a mouth defining an opening into said container;

a shoulder region extending from said upper portion;

a sidewall portion extending from said shoulder region to a base portion, said base portion closing off an end of said container, said sidewall portion defined in part by at least one arcuately formed rib having a body portion and opposite ends; and

wherein said body portion of said at least one arcuately formed rib curves from a central portion toward the base portion at each of said opposite ends.

2. The plastic container of claim 1 wherein said at least one arcuately formed rib defines an inboard depression along said sidewall portion.

3. The plastic container of claim 2 wherein the container defines a first pair of opposing sidewalls and a second pair of opposing sidewalls, wherein said first pair of opposing sidewalls are longer in length than said second pair of opposing sidewalls.

4. The plastic container of claim 3 wherein said at least one arcuately formed rib defines a series of ribs.

5. The plastic container of claim 4 wherein said series of ribs are defined on said first pair of opposing sidewalls.

6. The plastic container of claim 5 wherein said series of ribs each define a length that is about 50% to about 90% of a width of said first pair of opposing sidewalls.

7. The plastic container of claim 6 wherein said sidewall portion is defined in part by at least two vacuum panels formed therein, said vacuum panels being movable to accommodate vacuum forces generated within the container resulting from heating and cooling of its contents.

8. The plastic container of claim 7 wherein said series of ribs are defined on said vacuum panels.

9. The plastic container of claim 8 wherein said sidewall portion defines a label panel area on an area across said vacuum panels, said label panel area adapted to accept a label covering said series of ribs.

10. A plastic container comprising:

an upper portion having a mouth defining an opening into said container;

a shoulder region extending from said upper portion;

a sidewall portion extending from said shoulder region to a base portion, said base portion closing off an end of said container, said sidewall portion defined in part by at least one arcuately formed rib having a body portion and opposite ends, wherein said at least one arcuately formed rib curves away from said mouth toward each of said opposite ends; and

wherein said sidewall portion is further defined in part by at least two vacuum panels formed therein, said vacuum panels being movable to accommodate vacuum forces generated within the container resulting from heating and cooling of its contents.

11. The plastic container of claim 10 wherein said at least one arcuately formed rib defines an inboard depression along said sidewall portion.

12. The plastic container of claim 11 wherein the container defines a first pair of opposing sidewalls and a second pair of opposing sidewalls, wherein said first pair of opposing sidewalls are longer in length than said second pair of opposing sidewalls.

13. The plastic container of claim 12 wherein said at least one arcuately formed rib defines a series of ribs.

14. The plastic container of claim 13 wherein said series of ribs are defined on said first pair of opposing sidewalls.

15. The plastic container of claim 14 wherein said sidewall portion defines a label panel area on an area across said vacuum panels, said label panel area adapted to accept a label covering said series of ribs.

16. A method of making a blow-molded PET plastic container, said method comprising the steps of:

disposing a preform into a mold cavity having a surface defining a shoulder forming region, a sidewall forming region and a base forming region, the sidewall forming region defining at least one arcuately formed extension rib having a body portion and opposite ends, the opposite ends of said at least one arcuately formed extension rib curves from a central portion toward the base forming region at each of said opposite ends;

blowing the preform against the mold surface to form a resultant container having a shoulder, a sidewall and a base, the sidewall defining at least one arcuate depression rib corresponding to the at least one arcuately formed extension rib of the mold cavity; and

removing the resultant container from the mold cavity.

17. The method of claim 16 wherein the sidewall forming region defines a series of arcuately formed extension ribs each having a body portion and opposite ends.

18. The method of claim 17 wherein the step of blowing the preform further includes blowing material comprising the preform around the series of arcuately formed extension ribs, the material generally flowing from the body portion of each extension rib at an upstream area toward the opposite ends of each extension rib at a downstream area.

19. The method of claim 18 wherein the step of blowing further includes forming at least two vacuum panels at the sidewall.

20. The method of claim 19 wherein forming at least two vacuum panels includes forming the at least two vacuum panels along a surface encompassing the at least one arcuate depression rib.