Abstract:
A thermoplastic container, in particular a bottle, made by blow-molding or stretch blow-molding of a heated preform and including a bell-shaped convex base with its concavity facing outward and provided with reinforcing impressions projecting inward and radiating around a central recess, the impressions having a trapezoid shape with its smaller dimension located toward the center of the base; the impressions include respective arches which form convex trapezoid segments with concavity facing inward and extending from the seat to the central recess by being connected thereto immediately proximate the upper end of the central recess.

Description:
FIELD OF THE INVENTION 
     The present invention relates to the general field of containers, especially bottles, made of a thermoplastic such as PET, manufactured by the blow molding or stretch blow molding of a heated perform, and it relates more specifically to improvements made to those of such containers which possess a bottom which is domed, so as to have the general shape of a spherical cap with its concavity facing the outside of the container, and which defines an annular seat, said bottom having reinforcing impressions protecting toward the inside of the container and radiating around a central indentation or central recess, said impressions having an approximately trapezoidal general shape with the short side located toward the center of the bottom of the container, the long side located toward the periphery of the bottom of the container and a bottom that constitutes the arch of the impression. 
     BACKGROUND OF THE INVENTION 
     Containers intended for containing a still liquid (for example bottles designed to contain table water) are, in most cases, provided with a domed bottom having the general shape of a spherical cap with its concavity facing the outside and of relatively small height. Such bottoms are often provided with radiating ribs spaced apart around a central indentation, it being possible for said ribs to have various conformations and optionally to extend over the base of the wall of the body so as to reinforce the seat (the peripheral region via which the bottom rests on a support). Typically, the height of the bottoms of this kind, central indentation included, is typically around 10 mm, and may be up to 15 mm. 
     Such bottoms are shaped so as to support, without deforming, the column of flat liquid sitting on top of them. However, they do not have sufficient strength to withstand an additional stress, for example due to internal overpressure, even slight overpressure. 
     In the case of carbonated liquids (for example those under a pressure of around 3 to 4×10 5  Pa, or even up to 10×10 5  Pa), it is known to design containers with a bottom having a much more pronounced curvature (called “champagne” bottom or the like) capable of withstanding high pressures without deforming. However, such bottoms require a larger amount of thermoplastic, both because of their greater height and because of the greater wall thickness, at least locally. Containers provided with such bottoms therefore prove to be more expensive and it is more difficult to shape them correctly during molding. 
     It is known, when packaging certain easily oxidizable still liquids, to pour a small amount of liquid (for example one drop) of an inert and rapidly vaporizing substance (for example liquid nitrogen) on the surface of the still liquid at the end of the container-filling phase so as to remove the air (and therefore the oxygen contained in the air) from the free volume sitting on top of the liquid surface immediately before stoppering the container (an operation called “inerting”) or to improve the pressurization of the container in the case of lightly carbonated liquids. This small amount of inert substance ends up by being vaporized once the stoppering operation has been completed, so that there remains, in the closed container, some inerting gas under a low residual pressure of less than 2×10 5  Pa, typically around 1×10 5  Pa or even around 0.5×10 5  Pa. 
     The slightly domed bottoms conventionally provided for containers intended for still liquids are not capable of certainly withstanding, without deforming, a pressure as low as that mentioned above. As regards the use of more resistant bottoms, such as champagne bottoms, their high strength and the additional cost associated with them appear excessive for the envisaged application. 
     SUMMARY OF THE INVENTION 
     It is in this context that the invention aims to provide improved shaping of the domed bottom for containers suitable for being filled with flat liquids and closed off in the presence of a relatively low pressure, not exceeding 2×10 5  Pa, and more generally around 1×10 5  Pa, requiring only a minimum of thermoplastic, easy to shape correctly under the usual conditions for the blow molding or stretch blow molding of containers intended for still liquids, and having a height of the same order of magnitude as that of the bottoms of conventional containers for still liquids. 
     For these purposes, a container designed as mentioned in the preamble, being designed in accordance with the invention, is characterized in that the impressions comprise respective arches that form domed trapezoidal segments with their concavity facing toward the inside of the concavity of said spherical cap formed by said bottom of the container (in other words facing toward the outside of the container) and extending from the seat to the central indentation by being connected to the latter in the immediate vicinity of the upper end of the central indentation. 
     Admittedly, it is known to provide certain types of container, such as those designed to be filled with a hot liquid, with bottoms having inwardly projecting reinforcing impressions radiating around a central indentation, said impressions having an approximately trapezoidal general shape with the short side located toward the center of the bottom of the container. 
     However, in these known containers, such impressions have an arch of substantially flat general shape. Container bottoms thus designed with approximately trapezoidal impressions with a substantially flat arch are admittedly quite satisfactory for being filled with a hot liquid, but they are not however suitable within the envisioned context of a residual overpressure of relatively small value. 
     A bottom designed according to the invention has on the contrary a double structure of domed panels, both as regards the panels defined between the projecting impressions in the bottom of the container itself and as regards the arches of said impressions having a double concavity with the same orientation facing toward the outside of the container. This constitutes a double buttressing system on two levels which, without being capable of withstanding high pressures, is however capable of withstanding, without deforming appreciably, a relatively low residual pressure not exceeding about 2×10 5  Pa, such as that which remains after an inerting operation. 
     In a preferred embodiment, the arches of the impressions have a variable curvature, being greater toward the seat than toward the central indentation. 
     To increase the deformation resistance of the container bottom, it is advantageous for said panels defined between the projection impressions in the bottom of the container to be provided with at least one respective radiating rib. In one specific exemplary embodiment, a single radiating rib is provided on each bottom panel. 
     In practice, it is simple to ensure that the ribs are substantially straight, that they advantageously have a concavity facing toward the outside and that, preferably, their inner end is located at some distance from the central indentation, in other words that the ribs extend radially only over part of the gap between the seat and the central indentation (for example over about one half of this gap) starting from the seat. 
     However, if so needed, it is possible to provide, instead of ribs, reinforcing reliefs having any other desirable shape (for example triangular shape). 
     Depending on the desired degree of stiffening, provision may be made for the ribs to extend only along the bottom as far as close to the seat of the latter or else, alternatively, to extend beyond the seat, rising up over the base of the body of the container if it is desirable to strengthen the seat. 
     In a current field of application, especially as regards bottles, the bottom is of round general shape and the impressions are angularly spaced apart equidistantly. Typically, for current bottles with a diameter of around 50 to 70 mm, the number of impressions is between 3 and 8. 
    
    
     
       BRIEF DESCRIPTION OF THE INVENTION 
       The invention will be more clearly understood upon reading the following detailed description of certain preferred embodiments given solely by way of purely illustrative examples. In this description, reference is made to the appended drawing in which: 
         FIG. 1  is a top view of a preferred embodiment of a container bottom designed in accordance with the invention; 
         FIG. 2  is a side view of the base of a container equipped with the bottom of  FIG. 1  (the side wall of the container only being suggested); 
         FIG. 3  is a three-quarter perspective view from above of the bottom shown in  FIG. 1 ; and 
         FIG. 4  is a side view of an alternative embodiment of the base of a container designed according to the invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Although the provisions according to the invention may be applicable in bottoms of various shapes equipping containers of various shapes and dimensions, the figures illustrate, merely by way of example, a bottom with a round general outline for equipping a container of axisymmetric cylindrical general shape, such as a bottle intended to contain a still liquid before undergoing an inerting treatment at the bottling stage. 
     Referring now to  FIGS. 1 to 3 , the reference  1  denotes a container in its entirety, such as a bottle, made of a thermoplastic such as PET and manufactured by blow molding or stretch blow molding a heated perform. The container  1  has a bottom  2  which is domed, so as to have the general shape of a spherical cap with its concavity facing the outside of the container (it is therefore the convex face of this bottom which is seen in  FIG. 1 ), and which defines an annular seat  10 , that is to say an annular part of the bottom via which the latter rests on a support. The bottom  2  has reinforcing impressions  3 , projecting toward the inside of the container and radiating around a central indentation  4  or central recess. The impressions  3  have an approximately trapezoidal general shape with the short side located toward the center of the bottom and the long side located toward the seat  10  of the bottom. As may be better seen in  FIGS. 1 and 3 , the short side of each approximately trapezoidal impression  3  has, in the example illustrated, a very small dimension so that the impression has an almost triangular general shape with the apex located toward the center. 
     Under these conditions, the impressions  3  define, between them, bottom panels  5  which are themselves also of approximately trapezoidal general shape with the short side located toward the center of the bottom and the long side located toward the seat. The panels  5 , being portions of the bottom  2 , are domed, as can be clearly seen in  FIGS. 2 and 3 . 
     According to the invention, the arches  6  of the impressions  3  form domed trapezoidal segments with their concavity facing toward the inside of the concavity of said spherical cap formed by the bottom  2  of the container (in other words with its concavity facing toward the outside of the container) and extend from the seat  10  to the central indentation by being joined to the latter in the immediate vicinity of the upper end  7  of the central indentation  4 . 
     As may be particularly clearly seen in  FIG. 3 , the panels  5  of the bottom and the arches  6  of the impressions  3 , which alternate with one another, form a domed double structure lying on two levels and centered on the central indentation  4  with a double concavity of the same orientation facing toward the outside of the container. Such an arrangement provides greater mechanical strength, which allows the bottom to be able to withstand, without appreciable deformation, when the container is filled with a still liquid, a slight overpressure not exceeding about 2×10 5  Pa and in practice around 0.5 to 1×10 5  Pa, as is the case for example following an inerting operation or pressurizing operation in order to stiffen the container. 
     Furthermore, such a container bottom may be produced without substantial localized overthicknesses—in other words it may be formed under substantially the same molding conditions as for standard bottoms. 
     Finally, the height h of a container bottom  2  designed according to the invention may be of the same order of magnitude as that of a standard container bottom for a flat liquid, that is to say around 10 to 15 mm for a container such as a bottle with a diameter of 50 to 70 mm. 
     In order for the arches of the impressions to have greater strength, it is advantageous for them to have a variable curvature, being greater toward the seat than toward the central indentation  4 . 
     Preferably, to obtain greater stiffness of the panels  5  of the container bottom  2  between the impressions  3 , the impressions  3  of the bottom  2  are separated by panels  5  that may be provided with at least one respective radiating rib  8 . In practice, to make it easier to form the bottom  2  correctly, only a single radiating rib  8  is provided in each panel  5 . 
     Although many configurations of ribs may be employed, it is however simpler, when etching the mold, for the ribs  8  to be substantially straight and advantageously for them to have a concavity facing toward the outside, as shown in  FIGS. 1 to 3 . Should it be necessary, the panels  5  may be reinforced by means of reliefs having other shapes (for example reliefs of triangular shape). It is not essential for the ribs  8  to extend over the entire gap separating the seat  10  from the central indentation  4 . In the exemplary embodiment illustrated in the appended figures, the ribs  8  extend only over part of the aforementioned gap (about one half of this gap in the configuration shown) and the end of the ribs  8  lying radially toward the inside is at a distance from the central indentation  4 . 
     Moreover, on the side where the ribs  8  have their ends facing radially outward, it is conceivable for them to extend to a point close to the seat  10  or, as an alternative, to extend beyond the seat  10 , rising up over the base of the body  9  of the container  1  as illustrated in  FIG. 4 , the purpose of the latter arrangement being to strengthen the seat  10 . 
     One common, although not exclusive, application of the provisions of the invention is for containers of approximately axisymmetric cylindrical general shape such as bottles. The bottom  2  then generally has a round shape, as shown in  FIGS. 1 to 4 , and in this case the impressions  3  are angularly spaced apart equidistantly. In practice, to reach an acceptable compromise between sufficient strengthening of the bottom  2  (which would require a relatively large number of impressions  3  and interspersed panels  5 ) and correct shaping of the bottom  2  during molding (which would require a relatively small number of reliefs), the number of impressions provided is between 3 and 8, the specific examples shown in  FIGS. 1 to 4  having five impressions  3  alternating with five bottom panels  5 .