PREFORM FOR PLASTIC CONTAINERS

Description

[0001] The field of the invention is that of the design and manufacture of plastic containers from preforms.

[0002] More specifically, the invention relates to a preform having a base with properties enabling the production of high-quality containers.

[0003] Preforms are obtained by injecting their constituent material into an injection mold and each comprises, in a known manner: a body having a cylindrical side wall of revolution about an axis of revolution of the preform; at a first end of the body, a neck separated from the body by a collar, the neck being intended to form a mouth of the final container and to receive a closing element of the container (for example a stopper), the injected neck not undergoing any modification during the container forming process; at a second end of the body, opposite to the first, a bottom which closes the body, the bottom being intended to form the bottom of the container on which the latter rests.

[0004] The body and the bottom of the preform meet at a junction plane which is substantially perpendicular to the axis of revolution of the preform.

[0005] Typically, a plastic container is obtained by blowing (combined in most cases with a stretching step) a preform into a mold with an internal cavity in the shape of the final container to be obtained.

[0006] More specifically, the preforms are first heated in an oven to soften them. For this purpose, the heating is such that the temperature of the preform material exceeds the glass transition temperature (Tg) of its constituent material by several tens of degrees (the preform is, for example, heated to approximately 120°C for PET). PolyEthylene Terephthalate ) whose Tg is 70°-77°C). Next, the preforms are placed in a mold for a forming step during which the preforms are blown until the final container is formed.

[0007] During blow molding, pressurized air is injected into the preform so that its constituent material flows into the mold until it comes into contact with the internal cavity of the mold against which it is held temporarily.

[0008] When, as in most cases, the containers are manufactured by a combination of stretching and blow molding, a rod called an elongation rod or stretching rod is inserted into the preform to stretch it towards the bottom of the mold, in addition to the injection of pressurized air into the preform.

[0009] During the forming process (by blow molding or stretch blow molding), the material may not flow correctly into the mold. The resulting container may then exhibit structural defects that weaken it and / or render it unsuitable for the desired mechanical and aesthetic qualities. These forming defects can, in particular, cause cracking in certain areas of the container's base.

[0010] These cracks, initially non-open, can, in the short or long term and depending on the type of product packaged in the container, develop to the point of generating leaks of product outside the container or even, in some extreme cases, causing the bottom of the container to burst.

[0011] This is particularly important when the container is intended for highly carbonated beverages such as soda. Furthermore, the use of chemical lubricants on post-forming conveyor lines, which come into contact with the formed container, can increase the risk of the container bottom bursting.

[0012] Indeed, a container holding a liquid or carbonated beverage typically has a petaloid base, meaning a base with a series of bumps and valleys around its periphery. These bumps form feet on which the container rests once filled.

[0013] When the base has cracks, these are usually located on the inner edges of the feet, and more particularly at the junction of the feet with a central portion of the base of the container.

[0014] It is also possible that cracks may appear at the center of the bottom, which corresponds to the point of injection of the material into the injection mold during the manufacturing, by molding, of the preform.

[0015] This is the area where a high concentration of stresses exists. In the case of carbonated liquids, the concentration of stresses is primarily due to the presence of carbon in the beverage, which creates gaseous pressure.

[0016] However, a poor distribution of material in the bottom of a formed container (for example, too much material in some areas or, on the contrary, too little material in other areas) does not allow the bottom of the container to withstand this concentration of stresses.

[0017] Thus, when the container is filled and then sealed, the gas contained in the beverage presents a risk to the mechanical strength of the container, especially if the latter is subjected to shocks, even slight ones, or is subjected to difficult transport and storage conditions (high temperature or humidity for example) in its distribution chain.

[0018] In other words, the gas from the drink and the pressure exerted by the drink on the bottom of the container may create too much stress on the cracks, potentially leading to a tear (or in extreme cases a burst) of the bottom of the container at the cracks.

[0019] Uneven distribution of material at the bottom of the container is not sufficient on its own to cause cracking. Other phenomena or parameters, such as those listed below, can also play an influential role: the constituent material of the preform; the preform molding process; the storage conditions of the preforms; the storage conditions of the filled containers; the conditions of the distribution chain; the use of lubricants and their nature on the container production lines; the shape of the bottom of the containers. Furthermore, the following two parameters are particularly highlighted to explain the presence of cracks in the bottom of the formed containers: the quality of heating of the preform prior to its transformation into a container; the shape of the bottom of the preform.

[0020] Indeed, it has been shown that the combination between the shape of the bottom of the preforms and the heating profile, prior to their blowing, had a significant impact on the appearance of cracks in the bottom of the formed containers.

[0021] The heating profile is directly impacted by the design, and therefore the shape, of the bottom of the preform, since an unequal amount of material on the bottom of the preform will require a different heating profile for each part of the bottom of the preform.

[0022] To address heating quality issues, specific heating profiles have been developed that prevent the appearance of so-called cold zones in preforms, areas in which the material has difficulty flowing during container forming.

[0023] However, these specific heating profiles put in place can lead to overconsumption of energy in the container forming installation.

[0024] Furthermore, a heating profile suitable for manufacturing one particular type of container is not necessarily suitable for other types of containers, making it necessary to adapt the heating profile for each type of container to be manufactured. In addition, modifying or redesigning the preform heating unit can incur a significant cost that is difficult to recoup.

[0025] Regarding the base shape of the preforms, the patent document published under number FR 3 045 434 describes a plastic container preform, comprising: a cylindrical body extending along an axis of revolution of the preform; a base closing the cylindrical body, the bottom having an external profile presenting a central portion and a curved portion joining the central portion to the cylindrical body, the central portion presenting, in longitudinal section, a concave section.

[0026] Such a shape of the bottom of the preforms makes it possible in particular to limit the cracking phenomena visible on the formed containers, by optimizing the distribution (in other words the creep) of the plastic material in the mold.

[0027] However, other forms of preform bottoms are being studied in particular to allow the use of preforms for the manufacture of different containers.

[0028] Furthermore, such a preform base presents a disadvantage in terms of manufacturing.

[0029] Indeed, such a base shape is difficult to achieve; moreover, the presence of a concave, rounded shape complicates the injection molding of the preforms, resulting in a significant risk of defects in the injected preforms. In particular, such shapes exhibit residual stresses that can impact the mechanical strength of the container bases after forming.

[0030] To limit or avoid defects, it might be necessary to significantly reduce preform production rates. However, such a reduction would run counter to current manufacturer demands, which seek ever-increasing production rates and therefore shorter cycle times for preforms and / or containers, without having to increase the size of their manufacturing machinery.

[0031] Document EP2316626A1 presents a preform that significantly improves cracking problems. The preform has a bottom with an external profile having a central portion and a curved portion joining the central portion to the cylindrical body, the central portion having, in longitudinal section, a first concave section, the curved portion having successively, in longitudinal section, from the first concave section towards the cylindrical body, a first convex or frustoconical section, a second concave section and a second convex or frustoconical section.

[0032] However, the second convex section, which is in fact closest to the body of the preform, has a simple curvature which does not ensure a gradual transition between the body and the second concave section, so that the appearance of cracking is still not excluded, as creep may not be optimal.

[0033] The invention aims in particular to overcome the drawbacks of the prior art.

[0034] More specifically, the invention aims to provide a preform whose base has a geometry that prevents the formation of cracks when blowing a container.

[0035] The invention also aims to provide such a preform which facilitates a distribution of the material in the mold for manufacturing the container, so as to avoid any risk of deformation of the bottom of the container as it exits the mold.

[0036] The invention also aims to provide such a preform which does not or very little impact manufacturing cycle times, particularly by injection molding.

[0037] These objectives, as well as others that will emerge subsequently, are achieved through the invention, which relates to a preform of a plastic container, comprising: a cylindrical body extending along an axis of revolution of the preform; a base closing the cylindrical body, the base having an external profile presenting a central portion and a curved portion joining the central portion to the cylindrical body, the central portion presenting, in longitudinal section, a first concave section, the curved portion presenting successively, in longitudinal section, from the first concave section towards the cylindrical body, a first convex or frustoconical section, a second concave section and a second convex or frustoconical section, characterized in that the second section, convex or frustoconical, presenting successively, in longitudinal section, from the cylindrical body towards the second concave section, a first radius of curvature, a second radius of curvature and a third radius of curvature and in that it presents a plane of intersection defined by the junction between the cylindrical body and the base, the plane of intersection being perpendicular to the axis of revolution of the preform,and in that the value of the first radius of curvature is between 60% and 100% of the value of a radius of the cylindrical body measured on the plane of intersection, and in that, in longitudinal section, the second concave section exhibits a setback, relative to a hemispherical trace connecting the central portion to the cylindrical body, of between 1% and 35% of a thickness of the cylindrical body, measured on the plane of intersection.

[0038] Thanks to this base, the preform according to the invention allows better material creep during the blowing of the container.

[0039] Moreover, for petaloid-type bottom containers, the presence of the second concave section has a shape memory effect, meaning that it makes it easier to create the feet and prevents the bottom of the container from sagging in its central part.

[0040] Indeed, since the junction between the central portion of the base and the feet is generally concave, the second concave section is interposed in the mold between the central portion and the feet of the container, thus ensuring continuity between the central portion and the feet.

[0041] In addition, such a preform helps to prevent the central area of ​​the bottom of the container from sagging as it exits the mold, the central area being the thickest and therefore most malleable area of ​​the bottom (due to its thermal inertia) as it exits the mold.

[0042] In other words, the presence of the second concave section makes it possible to avoid an angular junction between the feet and the central portion, in which the material would have difficulty flowing, thus causing cracks in the bottom of the blown container.

[0043] Furthermore, this second concave section allows the use of a "memory function" of PET in order to limit the sagging of the bottom of the container when exiting the mold, and therefore limit, or even eliminate, the appearance of cracks.

[0044] The succession of radii of curvature facilitates the creep of the material against the walls of the bottom of the container forming mold and allows in particular a good formation of the feet of the containers, the feet having an essential role in holding the container on a placement surface.

[0045] To obtain good blowing conditions and in particular good material creep, the following dimensional values ​​should preferably be chosen: the value of the first radius of curvature is preferably equal to 77% of the value of a radius of the cylindrical body measured on the plane of intersection; the value of the second radius of curvature is less than or equal to the value of a diameter of the cylindrical body measured on the plane of intersection; the second concave section has a maximum penetration point in the preform, located at a distance from the axis of revolution of the preform, between 20% and 40%, and preferably equal to 25%, of a diameter of the cylindrical body measured on the plane of intersection;In longitudinal section, the second concave section shows a setback, relative to a hemispherical trace connecting the central portion to the cylindrical body, of between 1% and 35%, and preferably 20%, of a thickness of the cylindrical body, measured on the plane of intersection: the first convex or truncated conical section has a fourth radius of curvature equal to half the first radius of curvature of the second convex or truncated conical section.

[0046] Advantageously, the bottom has an internal profile which, together with the second concave section, reduces the thickness of the bottom.

[0047] Reducing the thickness also promotes material creep. Indeed, the thinner the preform, the more the material can be stretched axially and radially (relative to its axis of revolution) thanks to compressed air and / or stretching by the drawing rod.

[0048] Other features and advantages of the invention will become more apparent upon reading the following description of a preferred embodiment of the invention, given by way of illustrative and non-limiting example, and the accompanying drawings, among which: there figure 1 is a schematic cross-sectional view of a preform according to the invention; the figure 2 is a detailed cross-sectional view, at an enlarged scale, of the bottom of the preform according to the invention; the figure 3 is a detailed, enlarged view of a portion of the external profile of the bottom of the preform according to the invention; the figure 4 is a schematic cross-sectional view of a petaloid base of a container obtained from a preform according to the invention.

[0049] On the figure 1 A preform 1 (also called a blank in some patent documents or in specialized literature) is shown for producing a container by blow molding or stretch blow molding in a forming unit. The preform 1 is made of a thermoplastic material such as polyethylene terephthalate (PET). This material, after being preheated, can be blown using a pressurized fluid, typically air, to stretch and conform to the walls of a mold (not shown) in the shape of the final container.

[0050] The preform 1 extends along an axis of revolution A and comprises, from an upper end to a lower end: a neck 2; a body 3, cylindrical; a base 4.

[0051] Neck 2 has an upper end defining a rim 21 of the final container. Generally, neck 2 is the part of the preform that maintains its geometry and dimensions throughout the container forming cycle. In other words, neck 2 is not intended to be deformed once preform 1 is obtained, usually by injection molding.

[0052] Below the rim 21, the neck 2 has a threaded opening 22 for receiving means for sealing the container once it has been formed and filled with contents. Instead of the threaded opening 22, the neck 2 may be provided with a raised edge to receive a sealing cap.

[0053] The neck 2 is connected to the cylindrical body 3 by means of a flange 23, which notably allows the preform 1 to be transported from a loading hopper to a mold in a blow molding unit, passing through an oven to soften the material of the preform 1 and facilitate the forming of the container. The flange 23 also allows the container to be transported in subsequent stages after its formation.

[0054] The cylindrical body 3 is formed of a cylindrical wall 31 of revolution around the axis of revolution A of the preform 1. At an upper end 32, the cylindrical body 3 has a shoulder 33 flaring out towards the rim 21. The shoulder 33 ensures in particular the junction between the cylindrical body 3 and the neck 2 of the preform.

[0055] At a lower end 34, the cylindrical body 3 is extended by the bottom 4 which closes the preform 1. The junction between the cylindrical body 3 and the bottom 4 forms a plane of intersection P, substantially perpendicular to the axis of revolution A of the preform ( figure 2 ).

[0056] The bottom 4 has an external profile 41 of the preform 1 and an internal profile 42 of the preform 1.

[0057] With reference to figures 2 And 3 , the external profile 41 of the preform 1 comprises a central portion 5 and a curved portion 6 joining the central portion 5 to the cylindrical body 31.

[0058] According to a longitudinal cross-sectional view, as shown on the figure 2 , the central portion 5 presents, from the axis of revolution A of the preform 1 towards the cylindrical body 3: a pellet 51 (caused by the injection of material into the preform forming mold 1); a first concave section 52, between the pellet 51 and the curved portion 6.

[0059] The pellet 51 (also called the injection point) is due in particular to the presence of a plug through which the softened plastic material is injected into the mold for manufacturing the preforms 1. This plug forms a cavity in the injection mold, in retreat from the hemispherical bottom plane of the mold, which explains the presence of the pellet, which forms an overthickness from the bottom 4 towards the outside of the preform.

[0060] The curved portion 6, for its part, presents successively, according to the principle of the invention, from the first concave section 52 towards the cylindrical body 3: a first convex or truncated conical section 61; a second concave section 62; a second convex or truncated conical section 63.

[0061] It is specified that a concave section is understood to be a section that forms, with respect to a tangent at the junction between the immediately preceding section and the concave shape, and to a tangent at the junction between the immediately following section and the concave shape, a reduction in material. Thus, a frustoconical portion positioned between two convex sections can be considered a concave section.

[0062] With reference to the figure 3 , the second convex or truncated conical section 63 presents successively, from the cylindrical body 3 towards the second concave section 62, a first radius of curvature 631, a second radius of curvature 632 and a third radius of curvature 633.

[0063] Advantageously, the first radius of curvature 631 is between 60% and 100% of a radius of the cylindrical body 3 measured on the plane of intersection P.

[0064] As an example, for a preform 1 comprising a cylindrical body radius 3 of 13 mm, measured on the plane of intersection P, the first radius of curvature 631 is at least 7.8 mm and at most 13 mm.

[0065] Preferably the first radius of curvature 631 is between 75% and 80% of the radius of the cylindrical body 3 measured on the plane of intersection P.

[0066] Even more preferably, the target value of the first radius of curvature 631 is 77% of the radius of the cylindrical body 3 measured on the plane of intersection P, i.e. 7.7 mm for a radius of the cylindrical body 3 of 10 mm.

[0067] According to a particularly advantageous embodiment, the second radius of curvature 632 is less than or equal to a diameter of the cylindrical body 3 measured on the plane of intersection.

[0068] Advantageously still, the second concave section 62 has a maximum penetration point 621 in the preform 1, located at a distance D from the axis of revolution A, between 20% and 40% of the diameter of the cylindrical body 3 measured on the plane of intersection P.

[0069] Preferably, the maximum penetration point 621 in the preform 1 is located at a distance D from the axis of revolution A, equal to 25% of the diameter of the cylindrical body 3.

[0070] The second concave section 62 has a recess, relative to a hemispherical trace 7 connecting the central portion 5 to the cylindrical body 3, between 1% and 35% of the thickness of the cylindrical body 3, measured on the plane of intersection P.

[0071] As illustrated on the figure 2 , the hemispherical tracing 7 includes a first line 71 defining the shrinkage equal to 1% of the thickness of the cylindrical body 3, and a second line 72 defining the shrinkage according to the preferred embodiment illustrated, i.e. a shrinkage equal to 20% of the thickness of the cylindrical body 3.

[0072] As illustrated on the figure 2 , the second concave section 62 has a shrinkage equal to 20% of the thickness of the cylindrical body 3, the maximum shrinkage being reached in particular at the maximum penetration point 621.

[0073] Advantageously, the first convex or truncated conical section 61 has a fourth radius of curvature equal to half of the first radius of curvature 631 of the second convex or truncated conical section 63.

[0074] As illustrated on the figure 2 in particular, the internal profile 42 defines, with the second concave section 62, a reduction in the thickness of the bottom 4.

[0075] According to the embodiment illustrated in figure 2 , the internal profile 42 is of the hemispherical type and has in its center a bump 421 forming a reserve of material useful in particular when the preform is stretched by a rod during forming, in order to avoid a reduction in material thickness which is detrimental to the mechanical strength of the container formed.

[0076] In an alternative not shown, the inner profile 42 could include, opposite the second concave section 62 of the outer profile 41, a concave section (having a radius greater than that of the second concave section 62), a frustoconical section or a convex section, or any combination of radii of convex and / or concave curvatures and frustoconical portions.

[0077] There figure 4 illustrates a base of container 8 which has a central portion 81 of convex shape, and feet 82 (only one being shown) which extend over a periphery radially external to the central portion 81.

[0078] The feet 82 are connected to the central portion 81 via a concave portion 83 formed by the second concave section 62 of the bottom 4 of the preform 1 (boxes IV on the figures 2 And 4 ).

[0079] During the blowing of preform 1 into a mold to form the final container, the base 4 of preform 1 deforms to conform to the mold walls. The second concave section 62, thanks to the shape memory of PET, retains its concavity so as to perfectly conform to the base of the mold and define the concave portion 83 of the container base 8.

[0080] Thanks to the presence of the second concave section 62 of the preform 1, the base of the container 8 retains its shape as it exits the mold. In other words, the second concave section 62 prevents the central portion 81 of the base of the container 8 from sagging (also called unwinding) towards the feet 82 after the container has been manufactured. Such sagging would be detrimental to the container in terms of mechanical strength, stability, and aesthetics.

Claims

1. Container preform (1) made of plastics material, comprising: - a cylindrical body (3) extending along an axis of revolution (A) of the preform; - a bottom (4) closing the cylindrical body (3), the bottom (4) having an outer profile (41) that has a central portion (5) and a curved portion (6) joining the central portion (5) to the cylindrical body (3), the central portion (5) having, in longitudinal section, a first concave section (52), the curved portion (6) having in succession, in longitudinal section, from the first concave section (52) towards the cylindrical body (3), a first convex or frustoconical section (61), a second concave section (62) and a second convex or frustoconical section (63), characterized in that the second convex or frustoconical section (63) has in succession, in longitudinal section, from the cylindrical body (3) towards the second concave section (62), a first radius of curvature (631), a second radius of curvature (632) and a third radius of curvature (633) and in that it has a plane of intersection (P) defined by the junction between the cylindrical body (3) and the bottom (4), the plane of intersection (P) being perpendicular to the axis of revolution (A) of the preform (1), in that the value of the first radius of curvature (631) is between 60% and 100% of the value of a radius of the cylindrical body (3) measured on the plane of intersection (P), and in that, in longitudinal section, the second concave section (62) has a setback, with respect to a hemispherical outline (7) connecting the central portion (5) to the cylindrical body (3), of between 1% and 35% of a thickness of the cylindrical body (3), measured on the plane of intersection (P).

2. Preform (1) according to Claim 1, characterized in that the value of the first radius of curvature (631) is equal to 77% of the value of the radius of the cylindrical body (3) measured on the plane of intersection (P).

3. Preform (1) according to Claim 1, characterized in that it has a plane of intersection (P) between the cylindrical body (3) and the bottom (4), the plane of intersection (P) being perpendicular to the axis of revolution (A) of the preform (1), and in that the value of the second radius of curvature (632) is less than or equal to the value of a diameter of the cylindrical body (3) measured on the plane of intersection (P).

4. Preform (1) according to any one of the preceding claims, characterized in that the second concave section (62) has a point (621) of maximum penetration into the preform (1), situated at a distance from the axis of revolution (A) of the preform (1) of between 20% and 40% of a diameter of the cylindrical body (3) measured on the plane of intersection (P).

5. Preform (1) according to Claim 4, characterized in that the point (621) of maximum penetration into the preform (1) is situated at a distance from the axis of revolution (A) of the preform (1) that is equal to 25% of the diameter of the cylindrical body (3) measured on the plane of intersection (P).

6. Preform (1) according to any one of the preceding claims, characterized in that the setback of the second concave section (62), with respect to the hemispherical outline (7) connecting the central portion (5) to the cylindrical body (3), is 20% of the thickness of the cylindrical body (3), measured on the plane of intersection (P).

7. Preform (1) according to Claim 1 or Claim 2, characterized in that the first convex or frustoconical section (61) has a fourth radius of curvature equal to half the first radius of curvature (631) of the second convex or frustoconical section (63).

8. Preform (1) according to any one of the preceding claims, characterized in that the bottom (4) has an inner profile (42) that defines, with the second concave section (62), a reduction in the thickness of the bottom (4).