MXPA06010292A - Method for producing heat-set base of a plastic container and the produced container - Google Patents

Abstract

A method for making a blow molded plastic container that can withstand the rigors of pasteurization of retort processing. The method includes introducing an injection molded plastic preform into a blow mold;circulating heated oil through the sidewall mold halves at a temperature of at least about 225°F (107°C) and circulating heated oil through the base push up at a temperature of at least about 275°F (135°C). The container is blow molded from the preform and heat set within the mold. The container has enhanced crystallinity in the base as compared to when heated water is circulated through the mold.

Description

METHOD TO PRODUCE THE THERMICLY HARDENED BASE OF A PLASTIC CONTAINER, AND THE RECIPIENT PRODUCED BACKGROUND OF THE INVENTION FIELD OF THE INVENTION The present invention relates to a method for producing a thermally hardened base in a blow molded plastic container during the molding process. More particularly, the present invention is directed to a method for producing a thermally hardened base by heating the upsetting mold using oil.

THE RELATED TECHNIQUE The use of plastic containers to pack liquid products, such as juices, has become commonplace in recent years. Many of these products are packaged using a hot fill process. In a hot fill process, hot liquid is added to a molded container at approximately 83 ° C (182 ° F). The container is then covered and allowed to cool. During the cooling process the liquid and gaseous contents of the container contract, resulting in a decrease in the internal pressure and the volume of the container. This decrease in pressure and volume can cause the container to deform. For example, the side walls of the container and the dome may be squashed inwardly, causing a cylindrical container to become oval. In extreme cases, the top, where a lid would be fixed, can be brought down to the container. In order to accommodate the forces associated with this pressure and volume reduction, and to prevent deformation of the container, various design aspects may be incorporated into a container. For example, ribs and other structural aspects are incorporated into the sidewall and dome to prevent distortion. These aspects work by adding rigidity. As an alternative to increasing stiffness to prevent deformation, some aspects of the container can be designed to move in response to changes in pressure and volume. For example, there may be thin-walled regions and they may be designed with specific geometries, which are capable of moving in and out, in response to changes in pressure and volume. Additionally, particularly in the sidewalls, there may be vacuum relief panels that flex inwardly in a controlled manner to accommodate changes in volume and pressure. Although the deformation by ovalization is the main problem that can occur in the side wall and the dome of a container, the deformation of the base manifests itself in different ways. For example, when the base is moved in response to the reduction in pressure and volume, the surface at which the container sits can be distorted, resulting in a container settling at an angle. Additionally, when the base responds to volume changes, the center of the base may bulge outwards, so that the base no longer sits uniformly on a flat surface, but swings over rounded portions of bulge. This phenomenon is known as "wobble". Additional aspects can be incorporated into the base of the container to alleviate those problems. The most common structural change in a base is the use of a base crimp with ribs. Base stresses are usually an indented central portion of the base, and usually contain several ribs. Base stressing resists deformation in several ways. First, as they do in a container side wall, the ribs create rigidity and prevent deformation. Additionally, in particular with the containers of polyethylene terephthalate (PET), the formation of ribs by stretching and forming during the blow molding process, index of biaxial orientation of the normally amorphous plastic. This increase in orientation manifests as an increase in the crystallinity of the plastic at the base. As the biaxial orientation and crystallinity increase, the plastic becomes more rigid and, therefore, more resistant to deformation. Such design aspects are well known in the art, although container manufacturers seek to make further improvements to maintain the structural integrity of the containers and accommodate the volumetric and pressure changes. Unlike liquid products, such as juices, other products, particularly solid or semi-solid products, such as pickles and sour cabbage, are processed using different methods. Typically these products are processed by pasteurization and retort processes. Using these procedures a product can be packaged in the container together with a liquid, at a temperature below 82 ° C (180 ° F), or the product can be placed in the container, which is then filled with liquid that had been pre-heated Pasteurization and retort treatment differ from the hot fill process by heating the contents of the filled and capped container to a specific temperature after it is filled and the container is capped. Typically, the filled container is heated to a temperature of more than 93 ° C (200 ° F). In pasteurization processes, temperatures can reach the boiling point of water and, in retorting processes, where excess pressure is applied, the temperature can exceed the boiling point of water. Heating is continued until the contents reach a specified temperature, which is called the central can temperature or core temperature, for a predetermined time. Typical core temperatures that must be reached may be, for example, around 70 to 80 ° C (155-175 ° F), and the time that should remain at the core temperature may be around 20 to 60 minutes. The use of a plastic container for packaged food products using pasteurization or retorting presents several additional challenges for the manufacturer and the designers. For example, the temperatures found in the container during pasteurization or retort processing may be greater than the temperatures encountered during the hot fill process. These temperatures, which can be close to 100 ° C (212 ° F), can be approximated by the transition temperature (Tg) of the plastic material used to make the container, or can even exceed it. For example, the Tg of amorphous polyethylene terephthalate (PET) is 67.2 ° C (153 ° F). Depending on the level of crystallinity and the orientation achieved during processing in a thermal hardening mold, this value can reach up to about 121 ° C (250 ° F) in the side walls of the container. However, due to the lack of stretching and the lower temperatures of the base upset mold, the base region will have a glass transition temperature somewhere between those two values. The increased temperatures, coupled with the positive internal pressures, unique to pasteurization, can cause the base to wobble if the Tg of the material in the base is not high enough. Increasing the temperature of thermal hardening in the base upset elevates the Tg of the unstretched material near the center of the base upset, which makes it more resistant to wobbling at high temperatures and pressures, such as those found during pasteurization . Additionally, pressure changes induced during pasteurization or retort treatment, differ from those of the hot-fill process because, instead of simply occurring a volumetric reduction, there is an increase in the internal pressure developed in the capped container and heated. Because the container is heated after filling and plugging, the container must resist not only vacuum or sub-barrel pressures and volume reductions within the container, but must also withstand higher or subarachal pressures and increases in volume. Despite these differences in demands, plastic containers designed for use in pasteurization and retort processing typically use vacuum absorption panels, similar to those for hot fill containers, to accommodate changes in pressure and volume when the sealed container is heated and / or when the contents are cooled inside the sealed container. The design of the container can be modified to accommodate these additional changes in pressure and volume, for example, by using a thicker plastic. These containers must have a greater rigidity, with respect to a container designed to be processed in hot filling. The stiffness of a container can be increased in other ways. For example, during a typical blow molding process, a piece of hot plastic is extruded into a container mold or a preformed plastic tube (the preform) is inserted into a mold. The preform may be made, for example, by injection molding or extrusion. Within the blow mold the pre-heated plastic can be further heated to soften it, and hot air can be blown into the container to stretch the plastic softening against the side walls of the mold, conforming to those side walls and, in such a way, forming the container. The act of stretching a piece of plastic and conforming it to a mold, mechanically induces the biaxial orientation and crystallinity in the amorphous plastic. As described above, the oriented and crystallized plastic is denser and more rigid than amorphous plastic. The orientation and the crystallinity of the plastic material can be increased in other ways than mechanical processing. The most common way to achieve this is through thermal hardening of the material. The thermal hardening is obtained by heating the plastic material after molding. In one example of a thermal hardening process, the mold is maintained at a slightly elevated temperature, for example, at about 79.4 ° C (175 ° F). This can be obtained by flowing hot water through channels formed inside the mold. As a result of the mold walls becoming hot when the plastic meets the mold wall, it is subjected to a slightly higher temperature, which induces a certain crystallinity and some orientation of the crystals in the side walls of the container. Other processes for heat hardening are known. An example is described in the method of U.S. Patent No. 6,485,669 to Boyd and co-inventors. According to the method described in that patent, hot air is blown into the container, after it is formed. A disadvantage of the thermal hardening process may be the induction of opacity in the container. Non-oriented plastic, such as PET, is typically a clear substance. In the use of plastic containers it is generally convenient to maintain the clarity of the container so that the consumer can see the contents of the container. The PET without orienting is quite clear. However, as the crystallinity and the biaxial orientation increase, by means of a thermal hardening process, for example, the opacity of the container also increases. As long as the crystallinity remains below about 30 percent, the container remains relatively clear. However, as the crystallinity increases, for example up to 30 percent and more, the opacity of the container increases. At about 30 percent crystallinity, this opacity is remarkable as a turbidity in the vessel. As the crystallinity approaches values closer to 100 percent, the container can become substantially opaque or even take on a white appearance. There remains a need in the art for methods to induce crystallinity in a controllable manner. Suitable methods would produce crystallinity that would allow the reinforcement of structural aspects of the container, without unduly increasing the opacity. Additionally, methods in which crystallinity can be increased in regions of the container not visible to the consumer, for example, by limiting the crystallinity to the base of the container, can keep the clarity of the container in the side walls or in other parts of the container. which is more convenient to have a clear container.

BRIEF DESCRIPTION OF THE INVENTION In summary, a method for thermally hardening a portion of a blow molded container includes the use of a blow mold which is constituted by at least a pair of sidewall mold halves and a base upset. Warm oil is circulated through the sidewall mold halves and the base upset, where the oil flowing through the pair of sidewall mold halves is maintained at a temperature of at least 107 ° C ( 225 ° F), and oil is circulated through the base crimp, which is maintained at a temperature of at least about 1 35 ° C (275 ° F). A plastic injection-molded preform is introduced into the blow mold; and a container is blow molded. During the process the container is thermally hardened and then the container of the blow mold is ejected. The oil circulating through the pair of sidewall mold halves is maintained at a temperature of about 135 ° C (275 ° F). The oil circulated through the base crimp is maintained at a temperature of at least about 140.5 ° C (285 ° F), and can be 162.7 ° C (325 ° F) or more. In a particular embodiment of the invention, the oil circulating through the pair of sidewall mold halves is maintained at a temperature of about 135 ° C (275 ° F), and the oil circulating through the upsetting of base is maintained at a temperature of about 162.7 ° C (325 ° F). A container manufactured according to the invention can be filled with a food product and can be processed using a pasteurization or retort treatment method, without undesirable deformation of the container. In another aspect, the invention is a container made according to the method described above. The container according to this aspect includes a manufactured container, a filled container and a filled container that has been processed by pasteurization or by a retort method. In another aspect, a method for increasing the crystallinity in at least a portion of a container includes circulating a hot non-aqueous fluid through a component of a blow mold, using the fluid to heat the component to a temperature of 200. ° C or greater; blow molding a container from a plastic; and thermally hardening at least a portion of the container in contact with the hot component. The hot component can be, for example, a base upset. The base upset can be heated to a temperature higher than the temperature at which the sidewall mold half is heated. For example, the base upset can be heated to a temperature of 121 ° C (250 ° F) or more, 140 ° C (285 ° F) or more, or 162 ° C (325 ° F) or more. By using said method, the crystallinity of the heated component can be increased by at least about 25 percent. The degree of crystallinity can be controlled so that it increases to less than about 30 percent. In a typical container, prepared in a manner described herein, the plastic in the side wall of the container can have an approximate crystallinity of 22 to 25 percent, and the plastic in the base of the container can have an approximate crystallinity of 24 to 35 percent. hundred. In particular, the crystallinity in the base can be about 24 percent.

BRIEF DESCRIPTION OF THE DRAWINGS The above aspects and advantages, and others, of the invention will be apparent from the following more detailed description of a preferred embodiment of the invention, which is illustrated in the accompanying drawings, in which the same reference numbers indicate in general identical, functionally similar and / or structurally similar elements. Figure 1 illustrates a side view of an exemplary embodiment of a container manufactured in accordance with the present invention; and Figure 2 illustrates a bottom perspective view of an exemplary embodiment of a container manufactured in accordance with the present invention.

DETAILED DESCRIPTION OF THE INVENTION The present invention is directed to a method for producing a plastic container with increased crystallinity. The increased crystallinity induced according to the method can be limited to the base of the container. In most embodiments of the present invention, the crystallinity is increased, but maintained at a level that does not induce undesirable opacity in the container structure. In typical blow molding processes, hot water is circulated through the mold during the mold during blow molding. For example, channels can be cut in the side wall of the mold and the base upsetting, for the circulation of hot fluid. In most processes, the fluid used to heat the molds is water. The temperature reached in the thermal hardening is limited by the degree to which the water can be heated; that is, it is limited to the boiling point of water. In such a way, the fluid circulating inside the mold is usually at a temperature of 93 ° C (200 ° F) or less and, because it is circulating through the mold, it is typically on the 65 ° C scale at 82 ° C (150 to 180 ° F). This is less than the temperature at which the mold halves are maintained (approximately 1 35 ° C (275 ° F), and limits the amount of heat induced crystallinity that can be achieved.) Water can be circulated through the mold in For example, in the side walls of the mold, the water circulates through a series of channels in the mold, which extend from the base to the top of the mold, form a channel in the shape of "U" Through the upper part of the mold, and circulate again downwards, to the lower part of the mold, this upward and downward circulation of the water can occur several times before the hot water eventually leaves the mold to be reheated and recirculated. In this embodiment, the fluid is circulated in a spiral pattern, injected near the center of the base and exiting, after forming the spiral, towards the eyebrow of the base upset. Prior art techniques, and handling the walls and base of the container at approximately 82 ° C (1 80 ° F), the crystallinity of both the side walls of the container and the base region of the container may be approximately 19 hundred. In accordance with the present invention, the water that generally circulates to heat the mold is replaced using circulating hot oil. When using oil to heat the molds, higher base mold temperatures are obtained, which leads to higher values of crystallinity in the base. The use of oil has several advantages. First, heat transfer can be better controlled. This is because the oil temperature can be maintained more easily than the water temperature. In particular, oil circulating through the mold is heated to a temperature of more than 93 ° C (200 ° F), can be controlled at 121 ° C (250 ° F) or more, including 140 ° C (285 °) F) or more, 162 ° C (325 ° F) or, if desired, more than 162 ° C (325 ° F). In a particular embodiment of the present invention, the mold halves forming the side walls of the container are heated with water, and the base upset mold is heated with oil. According to this embodiment, the sidewalls of the container are cured with heat during the blow molding process, at a temperature of about 93 ° C (200 ° F), while the base is heated to an elevated temperature of 93 ° C. C (200 ° F) or more. In other embodiments of the invention, both the side walls of the container and the base of the container are heated with circulating oil to increase orientation and crystallinity. For example, the side walls of the container and the base region can each be heated to more than 93 ° C (200 ° F) or more than 121 ° C (250 ° F). For example, in one example embodiment, the side walls of the vessel are heated to approximately 135 ° C (275 ° F). By using the present invention, the temperature of the base and the temperature of the side walls may differ in order to control the crystallinity of the container in different regions. For example, the molds forming the side walls of the container can be heated to approximately 135 ° C (275 ° F) and the base can be heated to a higher temperature, for example, at 162 ° C (325 ° F). In a particular method according to the present invention, a blow molded container is made from polyethylene terephthalate, from an injection molded preform, generally using standard blow molding techniques. The water circulating in the mold halves and in the upset portion of the base is replaced by hot oil. The oil circulating through the sidewalls is maintained at a temperature of approximately 135 ° C (275 ° F), and the oil circulating through the base crimp is maintained at approximately 162 ° C (325 ° F). A container prepared according to this method can have a sidewall crystallinity of about 23 to 25 percent, and an increased base crystallinity of about 24 percent. After blow molding, the container can be filled with a product, for example, pickles or sour cabbage, and can be processed under pasteurization or retort conditions. A container thus manufactured and processed is capable of resisting distortion and base wobble.

The degree of crystallinity required to prevent the base wobble and / or oil temperature required to obtain a desired degree of crystallinity, may vary depending on the design of the base and the manufacturing conditions. For example, if the container is held in the mold for a longer period of time, a lower temperature may be necessary to obtain a particular degree of crystallinity. Additionally, different base designs may require a variable degree of crystallinity, in order to maintain structural integrity during processing. It is within the skill of the art to vary the processing conditions while using the present invention, to obtain the desired operation. Because the present invention allows the crystallinity to be controlled by temperature in particular regions of the container, the crystallinity can be induced in areas of a container where deformation is problematic. Other methods have previously been used to obtain higher thermal hardening temperatures. For example, hot air may be blown into the container after it has been formed, as described, for example, in U.S. Patent No. 6,485,669, identified above, and in U.S. Patent No. 6,585, 124, to Boyd. and co-inventors. However, there are several disadvantages in such methods. For example, the method requires an additional step, with additional time for the container to remain in the mold. This slows down the manufacturing lines, which results in lower production and a decrease in efficiency. Additionally, while air can be blown in a manner that focuses on a limited area of the mold, it is difficult to limit the area where increased thermal hardening occurs. Thus, although design aspects can be incorporated into the container to reduce crystallinity and concomitantly increase opacity in areas where thermal hardening and opacity are not necessary or desirable, for example, in side walls, there is always something of crystallinity and induced biaxial orientation in these other areas. The present invention solves these disadvantages. For example, because the walls of the container, including the base, are subjected to higher temperatures than those at which the blow molding occurs, a sufficient thermal hardening is obtained, during the normal time of the molding process cycle. blown. There is no need to stop production for the incorporation of an additional step. Also in accordance with the present invention, the thermal hardening temperature of different regions of the container can be controlled separately. Therefore, the increased thermal hardening and opacity can be limited to the area where it is convenient, ie, the base; and the rest of the container, where the increased thermal curing and clarity are desirable, ie, the vertical walls, may remain unchanged, as compared to the existing containers. Containers prepared according to the methods of the prior art can experience varying degrees of base wobble when subjected to pasteurization after filling. Improvements in the basic design can minimize the wobble, but nevertheless, it can still be a problem. Additionally, these alternative designs can result in a thicker plastic and prevent or hinder efforts to make the containers lightweight. The ability to produce structurally sound and lightweight containers is of paramount importance to the container manufacturing industry, as well as to manufacturers and processors of food products packaged in plastic containers. By using the present invention, lightweight containers capable of withstanding the rigors of pasteurization and retort processing can be suitably manufactured.

EXAMPLE 1 Figures 1 and 2 illustrate an example container 100, prepared according to a method of the present invention. The container was blown from an injection molded preform, made of Heatwave® resin (obtainable as CF746 from Voridian Corporation, a division of Eastman Chemical Company, Kingsport, TN), having an amorphous density of 1.3291 g / cc. and a crystalline density of 1.4550 g / cc. The blow molding process was carried out using typical conditions and typical process parameters. The sidewall mold halves were maintained at a constant temperature of 135 ° C (275 ° F), and the base temperature was varied as shown in Table 1. After blow molding, the container was cut into several sections, representing different regions of the side wall and the base: the upper panel 1 02, the intermediate panel 1 04, the lower panel 106 and the base 108. The base 08 included the underside of the container and the entire region 108a of upsetting the base. The crystallinity of the container in those regions was measured. The percentage of crystallinity is defined as follows: P - Pa Crystallinity = Pe "Pa where p is the measured density of the PET material, pa is the density of the pure amorphous PET material, and pc is the density of the pure crystalline material Table 1 presents experimental sample data, obtained using various thermal hardening temperatures.

TABLE 1 - Summary of crystallinity TABLE 1 (continued) In table 1 the first data series is for a typical blow molding process, with thermal hardening. The temperatures of the side wall and the base are maintained at approximately 82 ° C (180 ° F). In the remaining data, the side wall is maintained at these typical temperatures, and the base temperature is increased as indicated. As can be seen from these data, the increase in the temperature of the base has little effect on the crystallinity of the side wall, even in the lower portion of the side wall; the crystallinity induced in the side wall of the container remains approximately 23 percent, ranging from approximately 22.9 percent to approximately 25 percent. As the temperature of the base upset increases during blow molding, the crystallinity in the base region is substantially increased, when the present process is used. When the typical thermal hardening process is used, where base temperatures are maintained at 82 ° C (180 ° F), the crystallinity of the sidewall varies from 23 percent to around 25 percent; while the crystallinity in the base remains relatively low, at around 19 percent. As the temperature at which the base upset portion of the mold is maintained increases the crystallinity of the base. For example, when the base setting temperature is maintained at about 121 ° C (250 ° F), the crystallinity of the base increases from 18.9 percent to about 21.9 percent. If the temperature is further increased to 140 ° C (285 ° F), an even higher crystallinity of about 23.4 percent is obtained as a result. In the base setting, temperatures of approximately 162 ° C (325 ° F) can be obtained, with an approximate crystallinity of 24.2 percent in the base region.

EX EMPLO 2 A container as shown in Figures 1 and 2 was blow molded according to the present invention; where the oil circulating inside the side walls was maintained at around 135 ° C (275 ° F), and where the oil circulating within the base upset was maintained at approximately 162 ° C (325 ° F). After molding, the container was filled with pickles and brine, and pasteurized using typical processing parameters. After processing the container showed no visible signs of deformation, wobble or instability. The modalities illustrated and discussed in this description are intended only to teach those who have experience in the subject, the best way that inventors know how to make and use the invention. Nothing in this description should be considered as limiting the scope of the present invention. All the examples presented are representative and not limiting. The embodiments of the invention described above can be modified or varied without departing from the invention, as will be appreciated by those having experience in the art, in light of the foregoing teachings. Therefore, it should be understood that, within the scope of the claims and their equivalents, the invention may be put into practice in a manner other than as specifically described.

Claims (20)

1. CLAIMS 1 .- A method, characterized in that it comprises: providing a blow mold comprising: a pair of side wall mold halves; and a base stress; inserting an injection-molded plastic preform into the blow mold; circulate hot oil through the pair of sidewall mold halves and through the base upset; where the oil circulated through the pair of mold halves of the side wall is maintained at a temperature of at least 1 07 ° C (225 ° F); and the oil circulated through the upsetting of the base is maintained at a temperature of at least about 135 ° C (275 ° F); blow molding a container from the preform; harden the container by heat; and ejecting the blow mold container.
2. 2. The method according to claim 1, further characterized in that the oil circulating through the pair of sidewall mold halves is maintained at a temperature of about 135 ° C (275 ° F).
3. 3. The method according to claim 1, further characterized in that the oil circulating through the base upset is maintained at a temperature of at least 140 ° C (285 ° F).
4. 4. The method according to claim 1, further characterized in that the oil circulating through the base upset is maintained at a temperature of about 162 ° C (325 ° F).
5. 5. The method according to claim 1, further characterized in that the oil circulated through the pair of sidewall mold halves is maintained at a temperature of about 135 ° C (275 ° F), and the oil circulated at Through the base upset is maintained at a temperature of approximately 162 ° C (325 ° F).
6. 6. A container, characterized in that it is manufactured according to the method of claim 1.
7. 7. The method according to claim 1, further characterized in that it further comprises: filling the container with a food product and processing the filled container using a pasteurization or retort method.
8. 8. A full container, characterized in that it is manufactured according to the method of claim 7.
9. 9. A method, characterized in that it comprises: circulating a hot non-aqueous fluid through a component of a blow mold; heating the component with the fluid at a temperature of 200 ° C or more; blow molding a container from a plastic found in the blow mold; heat hardening at least a portion of the container that is in contact with the component; and thereby increase the crystallinity in the at least one portion.
10. 10. The method according to claim 9, further characterized in that the component comprises a base upset.
11. The method according to claim 10, further characterized in that the base upset is heated to a temperature higher than the temperature at which a sidewall mold half is heated.
12. 12. The method according to claim 9, further characterized in that the component is heated to a temperature of 121 ° C (250 ° F) or more.
13. 13. The method according to claim 1 1, further characterized in that the base upset is heated to a temperature of 140 ° C (285 ° F) or more.
14. 14. The method according to claim 1, further characterized in that the base upset is heated to a temperature of 162 ° C (325 ° F) or more.
15. 15. The method according to claim 1, further characterized in that the crystallinity is increased to at least about 25 percent.
16. 16. The method according to claim 1, further characterized in that the crystallinity is increased to less than about 30 percent.
17. 17. - The method according to claim 1, further characterized in that the plastic in a side wall of the container has a crystallinity of about 22 to 25 percent, and the plastic in the base of the container has a crystallinity of about 24 to 35 percent.
18. 18. A container, characterized in that it comprises: a side wall; and a base; where the container is hardened with heat during blow molding; and the side wall has an approximate crystallinity of 22 percent to 25 percent; and the base has a crystallinity of at least about 20 percent. 9.
19. The container according to claim 18, further characterized in that the crystallinity of the base is approximately 24 percent.
20. 20. The container according to claim 19, further characterized in that the crystallinity of the base is less than about 30 percent.