Abstract
The invention relates to a new PET preform design that enables a more precise distribution of material in the bottle bottom and avoids the waste of material around the injection point. The thickness BWT.sub.min of the wall at the center of the gate or tip (3) (injection point) is reduced to a minimum in order to avoid the waste of material around the injection point when the preform is blown. This is particular important for carbonated soft drink application since the reduced amount of amorphous material at the center of the gate (3) helps to reduce the risk of stress cracking on the bottle base. Furthermore a step (4) having thickness WT.sub.max in the body wall thickness, in the region of the periphery of the base with WT.sub.max>WT, allows enough material to be available for the proper blowing of the bottle bottom making the bottle more stable.
Claims
1. A preform made of PET and having a weight lower than 20 g, adapted to make through a blow molding process a blown bottle for carbonated soft drinks, said preform comprising: a neck portion, a bottom portion with a preform tip and with a point of a gate at the preform tip, and a body portion extending between the bottom portion and the neck portion, and having a side wall thickness WT, wherein the bottom portion has a bottom wall thickness BWT with a thinner point at the point of the gate, said thinner point defining a minimum bottom wall thickness BWT.sub.min, wherein, starting at the point of the gate, the bottom wall thickness BWT of the preform increases continuously to the side wall thickness WT, and wherein the ratio BWT.sub.min/WT is between 0.20 and 0.30.
2. The preform according to claim 1, wherein, at an end of the bottom portion, in a region where the side wall thickness WT ends, there is provided a wall step having a thickness WT.sub.max higher than said side wall thickness WT.
3. The preform according to claim 2, where the thickness WT.sub.max of the wall step is between 4% and 20% higher than said side wall thickness WT.
4. The preform according to claim 1, adapted to make a bottle having a capacity of less than 0.75 L.
5. The preform of claim 1, wherein the preform tip is shaped convexly in a direction of a longitudinal axis of the preform.
Description
BRIEF DESCRIPTION OF FIGURES
(1) Further characteristics and advantages of the invention will become apparent in light of the detailed description of a preferred embodiment, but not exclusive, of a preform in plastic material, illustrated by way of example and not limitation, with the aid of the accompanying drawings in which:
(2) FIG. 1 represents the BWT/WT ratio for typical CSD application,
(3) FIG. 2 represents the BWT/WT ratio for typical water application,
(4) FIG. 3 represents the shape and the BWT.sub.min/WT ratio of the preform according to the invention for CSD application,
(5) FIG. 4 represents the shape and the BWT.sub.min/WT ratio of the preform according to the invention for water application,
(6) FIG. 5 represents the formation of the bottom of the bottle by the operation of stretch blow molding of a preform of the standard technology,
(7) FIG. 6 represents the formation of the bottom of the bottle by the operation of stretch blow molding of a preform according to the invention.
(8) The same reference numbers in the figures identify the same elements or components.
DESCRIPTION IN DETAIL OF A PREFERRED EMBODIMENT OF THE INVENTION
(9) The injection compression process (ICP) technology developed by the authors of the present invention offers new possibilities for the design of preforms since the traditional limitations on the preform wall thicknesses can be overcome. Since the position/distance of the core towards the injection point on the gate insert side is not fixed during the entire injection-compression process, which means that the room/space between the core tip and the gate insert is bigger at the start of the injection-compression process and only gets reduced at the last stage of the process when the compression phase occurs, the following BWT.sub.min/WT ratio is now applicable:
0.20≤BWT.sub.min/WT≤0.55, preferably 0.25≤BWT.sub.min/WT≤0.55.
where BWT.sub.min is the minimum bottom wall thickness, that is the bottom wall thickness at the preform tip. This means that the preform wall thickness in the preform tip area can be optimized and wasted material around the injection point can be substantially reduced. If we compare two preforms producing the same bottle, one produced with the traditional injection technology, FIGS. 1 and 2, and the other one produced with ICP, FIGS. 3 and 4, the one produced with ICP will have a lighter base since there is no need to guarantee a minimum ratio of 0.7 in this case, so the wall thicknesses in the preform tip area will be smaller. FIG. 1 represents a state of the art design of the final part 100 of a preform for carbonated soft drinks (CSD), where reference 1 indicates a body portion and reference 2 indicates the bottom portion terminating in the point of support of the petals, where the body 1 begins. In this example the bottom wall thickness BWT is constant and the ratio BWT/WT is of the order of 0.8 and the weight of the final bottle is 21 grams. FIG. 2 shows a preform for water bottle of the state of the art. In this example the bottom wall thickness BWT is constant and the ratio BWT/WT is of the order of 0.83 and the weight of the final bottle is 7.2 grams. FIGS. 3 and 4 are two examples of preform according to the invention, where the bottom wall thickness BWT is not constant, to be compared with the examples given in FIGS. 1 and 2. The CSD preform, FIG. 3, according to the new design presents a thinner zone around the gate 3, with the thinner point at the tip of the gate 3. For the preform of FIG. 3 the ratio BWT.sub.min/WT, where BWT.sub.min represents the Bottom Wall Thickness at the point on the gate 3, where the thickness has the minimum value, is of the order of 0.25 and the weight of the preform is only 19.6 grams. FIG. 4 represents the same concept for a preform of a water bottle: in this case the ratio BWT.sub.min/WT is of the order of 0.55 and the weight is of the order of 6.0 grams. It is worth repeating that the numbers given are only an example that not limit in any way the extent of the invention.
(10) Preferably a preform according the invention, suitable for a carbonated soft drink bottle, has a ratio BWT.sub.min/WT between 0.20 and 0.30 (limit values included). Preferably a preform according the invention, suitable for a non-carbonated soft drink bottle, has a ratio BWT.sub.min/WT between 0.50 and 0.55 (limit values included). From the center of the gate 3, where the bottom wall thickness has the minimum value BWT.sub.min, the thickness of the wall increases gradually to reach the side wall thickness WT of the preform body at the point of support of the petals. The preform thickness at the center of the gate 3, that is at the preform tip, is reduced to a minimum in order to reduce the amount of amorphous material around the injection point of the preform thus leading to less wasted material in this area and enhanced bottle performances in terms of thermal stability and stress cracking, that is particularly important for CSD applications since the risk of stress cracking on the bottle base is one of the main risks for products in PET containing CSD. A challenge in the design of the preform is to achieve a distribution of the PET material in the wall zones where it is needed, which means exactly in the peripheral part of the base and not around the injection point (excess material around the injection point is useless for the bottle mechanical performance). To improve this aspect the body 1 of the preform, according to the invention, presents, in the region where the preform portion having side wall thickness WT ends, a wall step 4 having a thickness WT.sub.max higher than the side wall thickness WT. Said thickness WT.sub.max is between 4% and 20% higher than said side wall thickness WT.
(11) The step 4 is therefore placed between the body portion 1 and the bottom portion 2 of the preform, that is between the end of the body portion 1 and the end of the bottom portion 2, corresponding to the point of support of the petals of the blown container in the case of a petaloid bottom.
(12) This “step core” design with two different thicknesses (WT and WT.sub.max), the wall step being indicated with 4 in the FIGS. 3, 4 and 6, makes it possible to have in the region of the “Step Core” enough material available for the proper blowing of the bottle base (petals), thus improving the stability of the bottle that is very dependent on the thickness of the bottom. With traditional injection technology, it is usually very complicated to have a base strong enough without having wasted material as well around the injection point. Thanks to the ICP preform design, it is now possible to distribute the material more precisely in the bottle base and to avoid a waste of material around the injection point. This aspect is also particularly important for applications suitable for CSD, because the thermal stability of the bottle base is very dependent on the rigidity of the bottle base, so enough material must be distributed into the base to feature a good thermal stability. However, with traditional injection technology, as described before, having base strong enough usually means that some wasted material is placed around the injection point that corresponds to the center of the gate. This is generally accepted, but this wasted material is very sensitive to stress cracking since it is mainly in an amorphous state. When making a preform suitable for ICP, thanks to the invention a better solution is now achieved in terms of thermal stability and less sensitivity to stress cracking. FIGS. 5 and 6 show a scheme that represents how the preform is transformed in the final bottle during the process of stretch-blow molding. In FIG. 5, where reference 5 indicates the petals, it is apparent that the old technique leaves too much material at the center C of the bottom of the bottle, where not only it is not needed but also it increases the possibility of stress cracking. In FIG. 6, where reference 4 indicates the wall step in the region of the “Step Core” it is possible to see that a preform with a gradual transition from the minimum bottom wall thickness BWT.sub.min to the side wall thickness WT leaves the center C of the bottom of the bottle with much less material. The new design, named “Thin Wall Step Core” or TWSC, can be used with advantages, for application in products such as “CSD”, in which the beverage is additive-containing CO.sub.2 in the measurement expressed in g/l or volumes of gas, for all levels of carbonation, from the lowest e.g. 2 g/l up to 9 g/l. The new design TWSC allows to make lightweight containers, increases blower performance and broadens the process window of the distribution of the plastic on the base of the container in order to exceed the specifications of caustic stress cracking (CSC) and thermal stability (TS). Stress cracking is a test designed to assess the quality of a container subjected to internal pressure in its most rigid part, namely the central gate present in the base plate. Since the lines of transport of containers, in general, use basic-type slip agents, the carbonated containers must pass a test simulation called caustic stress cracking. This test requires that the container is filled with the drink/water to the designated level of carbonation and immersed for the height of the petal in an aqueous solution of NaOH. For the time of the test the gate must not break, causing the explosion of the bottle. Since the amorphous material is microporous and permeable, if the transition from the gate to the petals is not smooth and sufficiently cooled during the process of stretch blow molding, the transition zone is particularly attackable in the “critical point” by etching of NaOH (base-catalyzed hydrolysis). The alkaline etching leads to rupture of the ester bond with cleavage of the polymer chains and, if the region between petals and injection point presents sparsely oriented zones, the probability that the base of the bottle fails the test of stress cracking is very high. The mechanism takes place with a first step of surface degradation with formation of local craze that can quickly and easily propagate along the profile of the material leading to brittle fracture. In the case of smooth transition as in the invention, however, the greatest stretching confers preferential orientation to the polymeric chains and lowers susceptibility to deterioration due to basic etching. Thermal stability is another test which allows to evaluate the mechanical strength of the bottle, containing carbonated drink, that is placed for a given time in a thermostat at a temperature higher than the ambient temperature of 20° C. The test allows to simulate the storage of the bottle in hot environments. Since carbon dioxide is a gas soluble in water or water-based beverages, there is established an equilibrium between CO.sub.2 in the liquid state and CO.sub.2 in the gaseous state in function of the temperature, because the solubility decreases with increasing temperature of the gas. This causes an increase of the gaseous phase in the confined volume and an increase of the internal pressure. A consequence is a partial dilatation of the container, with volumetric increase, which in turn significantly lowers the internal pressure, decreases the stiffness due to the interaction PET bottle and heat with possible irreversible deformation of the base called rocker bottom. The stress to which is subjected the bottle bottom interests particularly the structure of the petals and the first part of the transition, and sufficient thickness is necessary to prevent the enlargement of the petals of the container base and the subsequent eversion of the gate that would make the bottle unstable. It is clear that the container must exceed the above-mentioned test of CSC and TS simultaneously, then it becomes necessary to obtain a base for the bottle that satisfies both characteristics, namely that the transition of the gate is tightly stretched and cooled for CSC and that the petals are sufficiently thick and strong to TS. Since the thickness of the gate in a standard preform has a magnitude far superior to the TWSC design, it is difficult to stretch the area of the gate and find a good compromise between the trend of the transition and the thickness on the petals. Precisely because of the binding geometry standard, in state of the art containers very thick transitions are made, in which there is excessive presence of amorphous material, which entails a substantial cooling, to the detriment of the machine cycle time. The advantage of a preform according to the invention is that it optimizes the use of PET, thus obtaining lighter and more performing blown final containers using also the maximum performance of the blower. The “Thin Wall Step Core” preform is designed so that the profile of “Thin Wall” on the gate allows to obtain, on the base of a blown container, a thin and gradual transition that starts at the gate up to the point of support of the petals on the surface, while the profile “Step Core” intakes greater amount of material, compared to state of the art preforms, over the whole profile of the petals. The result translates into a base globally light, in which the amount of material accumulated in the gate, due to traditional preform geometry, through the “TWSC” design is reduced and partially redistributed along the “Step Core” where it will be positioned on the petals profile of the blown bottle, making it robust and easy to be cooled. The advantages of the invention, however, are obtained not only for containers with a petaloid base, but also for containers that may not have a petaloid base, as those used for the mineral water.
(13) According to a preferred embodiment, the preform is provided with a rigid neck portion having a neck ring, wherein the neck is threaded, ad is adapted to make a bottle having a capacity of less than 0.75 liters (L).
