WO1988005747A2

WO1988005747A2 - Base structure for containers

Info

Publication number: WO1988005747A2
Application number: PCT/US1988/000302
Authority: WO
Inventors: Malek Basbous
Original assignee: Weggeman, Peter, J.; Standard Packing Corp. S.A.L.
Priority date: 1987-02-04
Filing date: 1988-02-02
Publication date: 1988-08-11
Also published as: AU1341888A; WO1988005747A3; CA1238872A

Abstract

A container such as a bottle or a can for liquid or the like, preferably cylindrical in which at least the lower portion is defined by a substantially vertical closed surface hollow body and which is unitary with a bottom structure. The latter consists of a preferable convex bottom (7) surrounded by a double wall skirt (9), the outer edge (13) of the bottom wall being sealed to the inner wall (15) of the container. A blow molding apparatus to produce such container as well as a process of producing same is also disclosed.

Description

BASE STRUCTURE FOR CONTAINERS

BACKGROUND OF INVENTION

(a) Field of the Invention

This invention relates to containers such as bottles, cans or the like which are pressure resistant or non pressure resistant, to a process of producing same by blow molding and to a blow molding apparatus therefor. More particularly, the present invention relates to an improved bottom structure for pressure resistant or non pressure resistant containers for liquid or the like, made for example of (PET) polyethylene terephthalate or any other similar thermoplastic material such as polyesters, polyamides, polyvinyl chloride (PVC), and in general any flowable polymeric or non polymeric material. The invention is also directed to a new base for bottles or cans which are especially suitable for containing liquids which may or may not be under pressure, such as water, carbonated beverages, and the like more precisely when these containers are produced by operations involving material flowing, such as blow molding, drawing, extruding, injection molding, and the like.

(b) Description of Prior Art

Bottled liquids especially plastic containers holding carbonated beverages present different problems in relation to the base of the bottles. It has indeed been realised that under a pressure generated by the carbonation of the liquid, and containers undergoing changes in atmospheric pressure, elevated temperature, the base has a tendency to creep or to become distorted. Furthermore, under these conditions the container often becomes distorted and unstable when standing on a horizontal surface. Recently, over the years, there have been many developments in the formation of the base for such containers. These developments were particularly concerned with the solving of the distortion produced by the pressure inside the bottom of the containers.

The first such development was the making of a hemispheric or parabolic bottom with an externally applied base cup made of polyethylene or polyethylene terephthalate which can be glued or welded to the base of the bottle in order to provide a flat base surface and a container with a good standing stability. Such a development has been dis¬ closed in DE 29 50 267 published June 11, 1986.

Many other developments relate to the production of a one-piece container with a special reinforced bottom structure commonly known as "petaloid" base or "champagne bottom".

The following Canadian Patents are directed to such types of improvements: 837,924 1970 Chmielowiec

898,718 1972 Adoumaitis et al 910,015 1972 Turnek et al 993,814 1976 Adomaitis 1,056,318 1979 Das 1,075,626 1980 Krishmakumar et al

However, none of these references are directed at containers which overcome some or all of the above-mentioned disadvantages of the prior art.

A blown plastic container has been disclosed in U.S. Patent No. 3,843,005, issued October 22, 1974, inventor Albert R. Uhlig. This container has an arcuate bottom but most importantly is provided with an outwardly flaring multiple thickness skirt, wherein the plies are integrally joined together. This has obvious desadvantages. For example, if the body of the container has arcuate •upstanding walls, as described in the patent, the volume of the liquid that it can contain is much less than in a corresponding container with straight vertical walls and which is made with the same amount of material. In addition, along a production line, the containers according to Uhlig would be unstable with respect to one another and therefore would be unfit for use on a large production assembly. On the other hand, if such containers would be provided with straight vertical walls and an outwardly flaring skirt, there would still be a problem of space because the skirt would prevent the containers from resting against one another. Also, an outwardly, flaring skirt offers a limited resistance to weight so that such containers could not be adapted to contain large volumes of liquid. Finally, since the skirt is made by integrally joining two or more thicknesses of material, it has limited resistance as compared to a thicker skirt which offers more stability when standing on a surface and against deformation.

U.S. Patent No. 3,059,810 issued October 23, 1962, inventor B. Edward discloses a container with a double wall base but with outwardly flaring, vertical walls. In addition the skirt is inwardly offset with respect to the vertical walls. In addition, the invention of Edward is concerned with a cup as opposed to a bottle or a can according to the present invention. It is an object of the present invention to provide a container incorporating a structure in the form of a thick skirt surrounding the base, which is preferably made of separated double wall, thereby giving better standing ability to the container. It is another object of the present invention to provide a container with high stability and having a base which is wider than the petaloid or champagne base. It is another object of the present invention to provide a container which is provided with a bottom surrounding skirt that lies substantially on the same vertical plane as the walls of the container. It is another object of the present invention to provide a unitary one-piece container which contains no extraneous materials such as glues or adhesions thereby favoring better recycling and offering higher resale value. It is another object of the present invention to . provide a container for carbonated beverages and other liquids, which is resistant to inner pressure and is indeformable, that is cheaper to produce in that it avoids the production and/or application of base cups, and uses relatively less material than the present petaloid base containers.

It is another object of the present invention to provide a container with a solid and resistant base skirt, which is reinforced, stronger than the rest of the container body in that it consists of spaced apart rigid double walls, such skirt giving more strength and better stability to the container.

It is another object of the present invention to provide a totally transparent container including the base thereof, offering better guarantees to the consumer should he decide to inspect or verify the contents of the containers especially at the bottom thereof. It is another object of the present invention to provide a container which is pressure resistant and is more stable while on conveyor belts or sitting on shelves. It is another object of the present invention to provide a simple manufacturing process for producing the container according to the invention.

It is another object of the present invention to provide a blow molding apparatus for preparing the container according to the invention. SUMMARY OF INVENTION-

In accordance with the present invention, there is provided a container for liquid or the like, such as bottle or can,- which is pressure resistant or non pressure resistant having at least a^' lower portion defined by a substantially vertical closed surface hollow body, and a bottom structure integrally associated with the lower portion, - the bottom structure to close the bottom end of the container. The bottom structure comprises a bottom wall having an outer edge, a peripheral groove formed in the hollow body short of the lower end thereof, and opposite the outer edge of the bottom wall, the groove forming a peripheral inward projection inside the hollow body. The groove defines an upper and a lower extension of the hollow body, both extensions lying substantially in the same vertical plane. The bottom structure also comprises an inner closed wall essentially inwardly spaced from the lower extention, first means to cause the lower end of the inner closed wall to merge with the lower end of lower extension to form a lower peripheral rib thereby defining a lower peripheral support edge for the container, second means to cause the upper end of the inner closed wall to merge with the outer edge of the bottom wall to form an upper peripheral rib, thereby resulting in a two ply substantially vertical closed skirt at the lower end of the container, the bottom wall being at a higher level than the lower peripheral support edge, and means defining a seal joint between the outer edge of the bottom wall and the inner projection formed in the hollow body.

In accordance with a preferred embodiment of the invention, the hollow body and the skirt are both circular and the bottom wall is convex, for example, it may be hemispheric, parabolic, and the like. However, it is within the scope of the present invention to provide oval or substantially square containers with rounded corners, such as bottles or cans, in which case a hemispheric or parabolic or the like, bottom could still be provided especially for non pressure resistant containers.

In accordance with another preferred embodiment of the invention, the groove is V-shaped, although any other suitable shape falls within the scope of the present invention.

Preferably, the wall merges with the lower end of the cylindrical lower extension by folding to define a rib shaped circular support edge, and it also merges into the outer edge of the bottom wall.

The container according to the invention is preferably made of plastic, such as polyethylene terephthalate, although any other suitable plastic or non plastic material may also be used. In accordance with a preferred embodiment of the invention, the groove measured from the vertical plane to the bottom thereof has a depth which is substantially equal to the distance separating the lower extension and the inner closed wall of the closed skirt. The seal joint is preferably a seal formed by ultrasonic welding between the upper peripheral rib and inner tip of said V-shaped groove. Of course other types of welding can be used as long the resulting seal is sufficiently perfect to prevent any infiltration of liquid or gas into the skirt which could deform the latter. Other types of welding that can be used in accordance with the present invention include radiation processing, electromagnetic bonding, friction joining, magnetic heat-sealing and the like which are well known to those skilled in the art.

In accordance with another preferred embodiment of the invention, the lowermost portion of the convex bottom wall lies at a higher level than the rib shaped circular support edge.

In accordance with another broad aspect of the invention, there is provided a pressure resistant or non pressure resistant container for liquid or the like, such as bottles or cans, having at least a lower portion defined by a substantially vertical closed surface hollow body, and a bottom structure associated with the lower portion, the bottom structure to close the bottom end of the container, the bottom structure comprising a bottom wall having an outer edge, the outer edge being at a level higher than the bottom end of the lower portion, an inner closed wall essentially inwardly spaced from the lower extension, first means to cause the lower end of the inner closed wall to merge with the lower end of the hollow body to form a lower peripheral rib thereby defining a lower peripheral support edge for the container, second means to cause the upper end of the inner closed wall to merge with the outer edge of the bottom wall to form an upper peripheral rib substantially fitting within the hollow body thereby resulting in a two ply substantially vertical closed skirt at the lower end of the container, and means defining a seal joint between the upper peripheral rib and the inner face of the hollow body. In accordance with another embodiment of the invention, there is also provided a blow molding apparatus for producing a pressure resistant or non pressure resistant container for liquid or the like, such as bottles or cans, the container including a bottom wall, which apparatus comprises complementary first and second blow mold parts, means for associating the blow mold parts to form a blow molding unit, the blow molding unit including means to receive a preform and to blow it into a blown preform intended to constitute the pressure resistant or non pressure resistant container, the associated blow mold parts having a continuous peripheral projection formed along the inner walls thereof at the lower portion thereof but short of the lower end thereof, a movable part inserted at the lower end of the blow molding unit, the movable part having a head shaped to form the bottom wall of the container, means to move the movable part from a lowermost position at the lower end of the blow molding unit upwardly to an uppermost position where the outer edge of the head is adjacent the continuous peripheral projection, means enabling a lowermost portion of the blown preform to fold inwardly and upwardly as the movable part is moved upwardly until reaching the level of the peripheral projection, and means to seal together the portion of the preform overlying the continuous horizontal projection and the portion constituting the outer edge of the head.

Preferably, the blow molding unit is cylindrical, the continuous projection is circular and has a V-shaped cross-section. In accordance with a preferred embodiment of the invention, the head of the movable part is circular in cross-section, although any other suitable shape can be used and has a concave face to produce a corresponding convex bottom wall.

Preferably, the lower end of the blow molding unit has a smaller diameter that in the portion above the circular projection.

In accordance with a preferred embodiment of the invention, the intermediate portion of the blow molding unit between the circular projection and the lower end has the same diameter as the portion above the circular projection.

In accordance with another preferred embodiment of the invention, the intermediate portion of the blow molding unit between the circular projection and the lower end has a first section immediately below the circular projection whose diameter is the same - as the portion above the circular projection, followed by a second section between the first section and the lower end whose diameter is intermediate between that of the first section and the diameter at the lower end of the blow molding unit, which should normally be the same as the diameter of the inner wall.

The sealing means preferably consist of an ultrasonic device disposed all around the blow molding unit and means in said sealing means such as an ultrasonic sealing device to cause sealing of the two portions of the blown preform. If desired, the sealing means could be extraneous to the blow molding unit, particularly if sealing is to be carried out after having produced the blown preform and having folded and formed the bottom structure. The blow molding apparatus according to the invention preferably comprises means for heating the intermediate portion of the blow molding unit when folding the lowermost portion of the blown preform. It is also within the scope of the present invention to provide means associated with the intermediate portion of the blow molding unit to keep the blown preform hot when folding the lowermost portion of the blown preform. According to another embodiment of the invention, there is provided a process for producing a pressure resistant container for liquid or the like which comprises providing a tubular preform made of plastic material, introducing the preform in a blow molding apparatus as defined above with the movable part at its lowermost position, blow molding the preform to the shape of the blow molding unit thereby obtaining a blown preform, providing folding conditions in the intermediate portion of the blown preform between the peripheral projection and the lower end of the blown preform, moving the movable part to the uppermost position so as to inwardly and upwardly fold the lowermost portion of the blown preform, and sealing together the portion of the blown preform overlying to continuous horizontal projection and the portion lying on the outer edge of the head.

In other words, the last operation of the process according to the invention consists in welding and sealing the upper part of the skirt in order to produce a folded and sealed container at the base thereof.

Although the sealing operation could be performed in the blow molding unit, it is within the scope of the present invention to provide an operation which could be carried out immediately after blow molding and formation of the bottom structure without the seal has been completed. This may be achieved in accordance with a preferred embodiment according to the invention by providing means such as adjustable rings capable of applying pressure all around the upper portion of the skirt, and applying an ultrasonic treatment for performing welding and complete sealing at this point. BRIEF DESCRIPTION OF DRAWINGS The invention will now be illustrated by means of the annexed drawings which are only given for the purpose of illustration without limiting the scope of the invention. In the drawings,

FIGURE 1 is a perspective view of the bottom part of a container according to the invention;

FIGURE 2 is a cross-section view through a blow molding apparatus according to the invention illustrating the formation of the bottom part of a container according to the invention;

FIGURE 3 is a cross-section view through a device adapted for sealing the joint at the upper end of the skirt outside the blow molding unit; and

FIGURE 4 is a schematic illustration in cross-section of another embodiment of blow molding unit according to the invention;

FIGURE 5 is a schematic representation of a modified embodiment; and

FIGURE 6 is a schematic representation of another modified embodiment.

DESCRIPTION OF PREFERRED EMBODIMENTS

The container will first be described with particular reference to Figure 1 but also to Figure 2 which shows a cross-section view of the bottom part of the container in the process of being formed in the blow molding unit according to the invention. The pressure resistant container according to the invention is shown only in part it being understood that it will normally comprise a tubular body of the same cross-section. However, the invention is not limited to that particular shape and this is the reason why only the bottom portion of the container has been illustrated. The container according to the invention has a lower portion 1 which is defined by a cylindrical member 3. The container also comprises a bottom structure 5 which will be described in detail later, and which is intended to close the bottom end of the container.

The bottom structure is generally made of a convex bottom wall 7 and a two ply skirt 9. Also included in the bottom structure of the container, is a V-shaped peripheral groove 11.

More specifically, it will be realized that the peripheral groove 11 is formed in the hollow cylindrical body 3 short of the lower end thereof, but that it is located exactly opposite the outer edge 13 of the bottom wall 7. Inside the container, the groove 11 forms a circular inward projection 15.

To better describe the container, we shall assume that the groove 11 defines an upper extension 17 and a lower extension 19 of the cylindrical body. Also, it will be realized that the upper and lower extensions 17,19 lie substantially in the same vertical plane.

The bottom structure, at the level of the skirt 9, also comprises a cylindrical wall 21. As shown in the drawings, this wall 21 is inwardly spaced a distance P from the lower extension 19. It will be understood that this distance P can vary to some extent, but that generally, it should be about the same as the depth K of the circular groove 11. The circular wall 21 merges at 23 with the lower end of the lower extension 9 to form a lower circular rib 25 which constitutes the lower circular support edge for the container. On the other hand, the upper end of the circular wall 21 merges with the outer edge 13 of the bottom wall 7 to form an upper peripheral rib 27. It will therefore be seen, that as constituted, the lower extension 9 and the circular wall 21 will result a two ply vertical circular skirt at the lower end of the container 1. In addition, the lowermost portion 29 of the convex bottom wall 7 lies at a higher level than the lower peripheral rib 25.

Although the convex bottom can be of any arcuate shape, it has been found preferable for economical and other reasons to make it into a hemisphere, as shown in the drawings.

Finally, it is essential that the circular rib 27 be joined with the tip of the inward projection 15 so as to prevent any escape of liquid or gas in the space inside the skirt 9 between the lower extension 19 and the inner wall 21. For this purpose, there is provided an ultrasonically formed seal 31 between the upper circular rib 13 and the inner tip 15 of the V-shaped groove 11.

Although the material used to manufacture the container can vary at great length, it has been found that better results can be obtained by utilizing polyethylene terephthalate, and the model illustrated is made of such material.

We shall now describe the blow molding apparatus to producing the pressure resistant container according to the invention with particular reference to Figure 2 of the drawings.

The apparatus comprises two complementary blow mold parts 33 and 35. In operation, these blow mold parts are joined together or associated in any manner known to those skilled in the art to form a blow molding unit. We shall thereafter describe this portion of the apparatus as a blow molding unit. The latter, also in a manner known to those skilled in the art, is constructed and shaped to receive a preform 36 illustrated in cross- section. It will be understood that this preform 36 will be used to finally produce the pressure resistant container according the invention.

Each blow mold part 33,35 is a geometrical image of the other and it will be sufficient to describe only one. We shall therefore restrict the description of the mold parts with reference to item 35 and it will be understood that the same reference numerals should thereafter apply to both parts 33,35. Each part 33,35 has a continuous circular

V-shaped projection 37 forms along the inner walls of the parts 33,35 in the exact location shown in the drawings so as to divide the eventually formed container into an upper portion 17 and a lower portion 19.

At the lower end of the blow molding unit, there is a movable part 39 which is circular in cross-section and has a concave head 41 to produce a corresponding convex bottom wall 7 in the container 1 according to the invention. Means not shown, well known to those skilled in the art, are provided to move the movable part 39 from a lowermost position. a at the lower end of the blow molding unit upwardly to an- uppermost position b shown in dotted lines in the drawings where the outer edge 43 of the head 41 is adjacent the continuous V-shaped circular projection 37.

Referring again to the blow molding unit, more particularly with reference to its lower end, it will be shown that the latter has a smaller diameter than that in the portion above the projection 37. With respect to the intermediate portion 45 of the blow molding unit between the circular V-shaped projection 37 and its lower end, it will be noted that there is a first section 47 immediately below the circular projection 37 whose diameter is the same as the portion above the circular projection. This section 47 is followed by a section 49 which is located exactly between the section 47 and the lower end of the blow molding unit, whose diameter is intermediate between that of section 47 and the diameter c at the lower end of the blow molding unit. This arrangement is intended to facilitate the folding of the preform into skirt 9 as particularly shown in the drawings, to give a blown preform. Once the container has been shaped, the rib

13 and the tip 15 must be sealed together. For this purpose, there is provided an ultrasonic device 51 of standard construction and which is well known to those skilled in t ιe art disposed all around the blow molding unit at the level of the projection 37 and which, in operation, will cause joining together and sealing of the rib 13 and the tip 15. This type of sealing ensures that no liquid or gas will be permitted to flow into the space between the lower portion 19 and the inner wall 21 of the skirt 9.

Finally, once the blown preform has been molded in the shape shown when the movable part is in its lowermost position, the portion of the walls of the container covered by the section 49 must be heated or remain heated in order to enable it to be folded as shown in the drawings. For this purpose, there is provided a conventional heating means 53 well known to those skilled in the art. This will enable an easy folding of the preform in its final position. Alternatively, the conventional means 53 may be constituted of a device known to those skilled in the art which will merely keep the section 49 heated while folding takes place.

The process of producing the container is as outlined before and is merely carried out in the blow molding apparatus which has just been described. Minor modifications are however possible without departing from the scope and spirit of the invention. For example, it may be desirable and even preferable to cause the joining and sealing to take place outside the blow molding unit. Also, instead of having step portion 49 following portion 47, the wall in that region could be uninterrupted and be of the same diameter as the portion of the blow molding apparatus above the projection. In this case, if it is still desired to decrease the diameter of the blown preform at the level thereof corresponding to portion 49, it might be just sufficient to heat the blown preform in that area which will produce a smaller diameter especially when polyethylene terephthalate is used, and will facilitate the folding operation.

Also, according to an alternate embodiment of the invention, a container according to the invention could be produced without a peripheral groove 11, by sealing the outer edge 13 directly against the inner wall of the container.

Referring to Figure 3, it will be seen the once the blown preform 53 has been obtained in a blow molding unit containing no sealing device, the blown preform is disposed on a shaped member 55 whose head 57 fits exactly underneath the skirt 9 and against the bottom wall 7 all as shown in the drawings. The upper edge 59 then faces the outer edge 13 of the blown preform. A pair of adjustable rings 61,63 then presses the groove 11 against rib 13. An ultrasonic sealing device 65 well known to those skilled in the art, is provided to cause welding and sealing to take place.

Referring now to Figure 4, this embodiment teaches to produce a container according to the invention without a groove. This modification is made possible by providing a mold unit with peripheral projection and a movable part 67 whose head 69 has an extendable outer edge 71. In this manner, the rib 13 merely applies against the inner wall of the blow preform and thereafter sealing and welding is carried out as indicated above.

It has also been realised that the container according to the present invention can be provided without the peripheral groove shown in Figures 1, 2 and 3. According to the embodiments illustrated in Figures 5 and • 6 the lower portion 1 defined by cylindrical member 3 is merely extended by means of the two ply skirt 9. The inner closed wall 21 is provided inwardly with respect to the lower portion 1 up to the outer edge 13 of the bottom wall 7.

In the embodiment illustrated in Figure 5 the inner closed wall 21 is as close as possible to the inner face of the lower portion 1, without providing a seal therebetween.

In the embodiment illustrated in Figure 6 there is provided a weld 101 of a certain length between the inner closed wall 21 and the inner face of the lower portion 1.

Claims

I CLAIM:

1. A container for liquid or the like, having at least a lower portion defined by a substantially vertical closed surface hollow body, and a bottom structure associated with said lower portion, said bottom structure to close bottom end of said container, said bottom structure comprising a bottom wall having an outer edge, a peripheral groove formed in said hollow body short of lower end thereof, and opposite said outer edge of said bottom wall, said groove forming a peripheral inward projection inside said hollow body, said groove defining an upper extension and a lower extension of said hollow body, said upper and lower extension lying substantially in same vertical plane, an inner closed wall essentially inwardly spaced from said lower extension, first means to cause lower end of said inner closed wall to merge with lower end of said lower extension to form a lower peripheral rib thereby defining a lower peripheral support edge for said container, second means to cause upper end of said inner closed wall to merge with said outer edge of said bottom wall to form an upper peripheral rib, thereby resulting in a two ply substantially vertical closed skirt at lower end of said container, said bottom wall being at a higher level than said lower peripheral support edge, and means defining a seal joint between said outer edge of said bottom wall and the inner projection formed in said hollow body.

2. A container according to claim 1, wherein said closed surface hollow body consists of a substantially cylindrical member, and said closed skirt and said bottom wall are substantially circular.

3. A container according to claim 2, wherein said bottom wall is convex.

4. A container according to claim 3, wherein said convex bottom wall is shaped to define a hemispheric or parabolic base.

5. A container according to claim 1, wherein said groove is V-shaped.

6. A container according to claim 2, wherein said lower extension is cylindrical, said inner closed wall is also cylindrical, said inner cylindrical wall merging with the lower end of the cylindrical lower extension by folding to define a rib shaped circular support edge.

7. A container according to claim 6, wherein said inner cylindrical wall folds into said outer edge of said bottom wall.

8. A container according to claim 1, which is made of plastic.

9. A container according to claim 8, which is made of polyethylene terephthalate.

10. A container according to claim 1, wherein said groove measured from said vertical plane to bottom thereof has a depth k which is substantially equal to distance p separating the lower extension and the closed wall of said closed skirt.

11. A container according to claim 5, wherein said seal joint consists of an ultrasonically formed seal between said upper peripheral rib and inner tip of said V-shaped groove.

12. A container according to claim 3, wherein the lowermost portion of said convex bottom wall lies at a higher level than said lower peripheral support edge.

13. A container for liquid or the like, having at least a lower portion defined by a substantially vertical closed surface hollow body, and a bottom structure associated with said lower portion, said bottom structure to close bottom end of said contianer, said bottom structure comprising a bottom wall having an outer edge said outer edge being at a level higher than bottom end of said lower portion, an inner closed wall essentially inwardly spaced from said lower extension, first means to cause lower end of said inner closed wall to .merge with ldwer end of said hollow body to form a lower peripheral rib thereby defining a lower peripheral support edge for said container, second means to cause upper end of said inner closed wall to merge with said outer edge of said bottom wall to form an upper peripheral rib substantially fitting within said hollow body thereby resulting in a two ply substantially vertical closed skirt at lower end of the container, and means defining a seal joint between said upper peripheral rib and the inner face of said hollow body.

14. A blow molding apparatus for producing a container for liquid or the like, said container including a bottom wall which comprises complementary first and second blow mold parts, means or associating said blow mold parts to form a blow molding unit, said blow molding unit including means to receive a preform intended to be shaped as a blown preform to constitute said container, said associated blow mold parts having a continuous peripheral projection formed along inner walls thereof at the lower portion thereof but short of the lower end thereof, amovable part inserted at the lower end of said blow molding unit, said movable part having a head shaped to form the bottom wall of said container, means to move said movable part from a lowermost position at the lower end of said blow molding unit upwardly to an uppermost position where outer edge of said head is adjacent said continuous peripheral projection, means enabling a lowermost portion of said preform to fold inwardly and upwardly as said movable part is moved upwardly until reaching the level of said peripheral projection, and means to seal together the portion of the preform overlying said continuous horizontal proijection and the portion on the outer edge of said head.

15. A blow molding apparatus according to claim 14, wherein said blow molding unit is substantially cylindrical, said continuous projection is circular and has a V-shaped cross-section.

16. A blow molding apparatus according to claim 15, wherein the head of said movable part is circular in cross-section and has a concave face to produce a corresponding convex bottom wall.

17. A blow molding apparatus according to claim 16, wherein the lower end of said blow molding unit has a smaller diameter that in the portion above said circular projection.

18. A blow molding apparatus according to claim 17, wherein the intermediate portion of said blow molding unit between said circular projection and said lower end has the same diameter as the portion above said circular projection.

19. A blow molding apparatus according to claim 17, wherein the intermediate portion of said blow molding unit between said circular projection and said lower end has a first section immediately below said circular projection whose diameter is the same as the portion above said circular projection, followed by a second section between said first section and said lower end whose diameter is intermediate between that of said first section and the diameter at the lower end of the blow molding unit.

20. A blow molding apparatus according to claim 15, wherein the sealing means consist of an ultrasonic device disposed all around said blow molding unit and means in said sealing means to cause sealing of the two portions of the preform.

21. A blow molding apparatus according to claim 15, wherein said sealing means are separate from said blow molding unit and consist of a shaped member to receive said blown preform at the bottom wall thereof, a pair of adjustable harms adapted to press fit into said groove and an ultrasonic device to seal the outer edge of the bottom wall against the inner cylindrical body.

22. A blow molding apparatus according to claim 15, which comprises means for heating said intermediate portion of said blow molding unit when folding the lowermost portion of said preform.

23. A process for producing a pressure resistant container for liquid or the like which comprises providing a tubular preform made of plastic material introducing the preform in a blow molding apparatus as defined in claim 14, with the movable part at its lowermost position, blow molding said preform to the shape of said blow molding unit • to form a blown preform, providing folding conditions in the intermediate portion of the blown preform between the peripheral projection and the lower end of the blown preform, moving said movable part to said uppermost position so as to inwardly and upwardly fold the lowermost portion of said blown preform, and sealing together the portion of . the blown preform overlying the continuous horizontal projection and the portion lying on the outer edge of the head, to produce a folded and sealed base.

24. A container for liquid or the like, having at least a lower portion defined by a substantially vertical closed surfaces hollow body, and a bottom structure associated with said lower portion, said bottom structure to close bottom end of said container, said bottom structure comprising a bottom wall having an outer edge, an inner closed wall provided inwardly with respect to said lower portion, first means to cause lower end of said inner closed wall to merge with lower end of said lower portion to form a lower peripheral rib theeby defining a lower peripheral support edge for said container, second means to cause upper end of said inner closed wall to merge with said outer edge of said bottom wall to form an upper peripheral rib, thereby resulting in a two ply substantially vertical closed skirt at lower end of said container, said bottom wall being at a higher level than said lower peripheral support edge.

25. A container according to claim 24, which comprises means defining a seal between at least said outer edge of said bottom wall and the inner face of said lower portion opposite said outer edge.

26. A container according to claim 25, wherein said seal is formed by welding.

27. A container according to claim 26, wherein weld portion extends downwardly from said outer edge.