MXPA99001762A - Method for preventing uncontrolled polymer flow in preform neck finish during packing and cooling stage - Google Patents

Abstract

A method of reducing uncontrolled flow of a molten polymer during the packing and cooling stage of an injection molding cycle. A plurality of polymers are injected between a mold cavity and core for making a multilayer plastic article such as a preform, having an exterior layer of a first polymer and an interior layer of a second polymer. In a tapered region of the mold, a minimum width of the tapered region is selected to prevent the second polymer from developing an enhanced leading/trailing edge effect which causes the second polymer to reverse flow during the packing and cooling stage. The method is particularly useful for making preforms for blow molded containers, such as a ketchup container, having a relatively long and thin neck profile. In addition, the method maintains the interior (barrier) layer a minimum distance from the top sealing surface of the preform, in order to prevent defective heat-bonded seals.

Description

METHOD FOR PREVENTING THE UNCONTROLLED FLOW OF THE POLYMER AT THE FINISHING OF THE NECK OF A PREFORM DURING THE STAGE OF PACKING AND COOLING FIELD OF THE INVENTION The present invention is concerned with a method for preventing structural defects in injection molded articles, such as preforins, caused by uncontrolled counterflow and / or erosion on the upper sealing surface that occurs during the packing and cooling stage of the injection cycle.

BACKGROUND OF THE INVENTION Continental PET Technologies, Inc. (CPT) developed and marketed a sequential injection process for manufacturing multilayer plastic containers (see U.S. Patent Nos. 4,550,043, 4,609,516 and 4,781,954). These containers are currently in use as hot fill and ketchup juice containers. The CPT process allows the use of thin layers of expensive barrier materials (for oxygen-sensitive products), outer layers of thermally resistant materials (for high-temperature filling and / or caustic-fill applications) and / or inner layers of recycled materials (for example, central layers not in contact with the food product).

REF. 29585 U.S. Patent No. 4,781,954 discloses a sequential injection process of CPT to manufacture a five-layer container having internal and external layers of polyethylene terephthalate (PET), a core core layer of PET, and first and second intermediate layers of a barrier polymer. The intermediate layers can be made very thin, for example, 0.01-0.15 mm based on the relative melting points of the different polymers and the solidification / tunnel flow characteristic of the sequential process layer - where the last melted polymers The injected ones push the molten polymers injected before between the outer layers that have solidified on the cold mold cavity and the core walls. More specifically, a first metered PET load is injected to the end layer (via the gate or injection channel), from the mold of the preform and flows approximately halfway to the side wall where it is braked or stopped momentarily, before a second injection is made. The internal and external solidified PET layers are formed along the mold cavity and cold core walls, while the inner PET remains hot and fluid. Then, a second metered charge of a barrier polymer is made through the composite or orifice, which forms a melt deposit at the bottom of the preform. The resistance to flow The first charge (PET) in the sequential injection process has a self-leveling aspect on the second load, to cause the second load (barrier) to form a melt deposit that is substantially evenly distributed at all points (360). ° C) around the circumference at the end cap of the cavity. Finally, a third metered PET load is injected, which drives the barrier melt tank to the side wall to form two thin intermediate layers adjacent to the solidified internal and external PET layers, with the melted PET core layer (third load) between them. The barrier material (eg, EVOH) typically has a lower melting temperature than the first injected material (PET), and consequently, the cooling effect of the first solidified layers on the barrier material is not as great as the cooling effect of the mold surfaces on the first material (PET). Thus, there will be some solidification of the barrier material as it is brought into contact with the internal and external solidified PET layers - the third indicated material (PET) will remelter some of the solidified barrier material and make it advance together with the remaining barrier molten material to the center of the preform (tunnel flow), to also reduce the thickness of the intermediate barrier layers. The result is a relatively simple and highly reproducible process with a diversity of important benefits. For example, PET / EVOH five-layer ketchup bottles, made by this process, have been extensively replaced by commercial polypropylene / EVOH / adhesive bottles of the prior art, for at least three reasons. First, the five layer PET / EVOH container is transparent. PET provides a sparingly clear container that is aesthetically superior to the translucent polypropylene container of the prior art. Secondly, the EVOH layers in the PET container constitute only 1.5 percent of the weight of the bottle and do not require adhesive layers to adhere the EVOH to the PET. Instead, the CPT process maintains the ratio of the PET / EVOH layers during manufacturing and use, but allows the layers to separate, when the bottle is re-milled for recycling; then, the two polymers are separated by conventional gravimetry and other means and the PET reprocessed as part of the recycle stream of the bottle with PET soda. In contrast, the polypropylene bottle of the prior art uses approximately 6 to 10 percent EVOH barrier and adhesive layers olefinic, which, not only are more expensive, but also prevent the post-use segregation of the constituent polymers. As a result, most of these bottles end up in municipal waste deposits. A third important commercial benefit is that the PET / EVOH container (as opposed to the polypropylene version) is substantially leak-tight or splinter-proof when dropped on a hard surface. For at least the above reasons, the PET container has a significant commercial success and is recognized by the industry with several design awards. A problem that has been presented to manufacturers of multilayer plastic containers, by using sequential and simultaneous injection processes, is a phenomenon of uncontrolled natural flow known as "backflow" which occurs at the terminal end of an injection molded article during the packing and cooling stage of the injection cycle. This is described in U.S. Patent No. 4,627,952, issued to Ophir, column 1, lines 17-31, as an interruption of the laminar flow of the polymer in the mold cavity when it strikes the terminal end of the cavity and reverses its direction. of flow in the package. As described by Ophir, in a conventional injection molding process, the melt of the polymer is first indicated to a closed mold and subsequently, the additional melt is packaged into the cavity to compensate for the densification (shrinkage) of the melt during the cooling step. In the terminal or "dead" zone of the mold cavity, where the molten layers begin to pack, the flow of the polymer collides with the terminal wall and reverses its flow direction to produce "rebound wave configurations" on a scale molecular in the melt; These wave configurations are points of structural weakness because the multiple layers (waves) are prone to separate. The proposed solution of Ophir is to open the dead zone prior to the mold by providing an outlet, which allows the molten polymer to exit continuously through the terminal end of the mold. Ophir's solution of opening the terminal end of the mold may be acceptable in certain applications, but obviously increases the complexity of the mold apparatus and introduces new variables in the process, which include the need to trim and remove material from the end. terminal "in excess". Stronger controls on temperature, pressure, viscosity, etc., may be another possible way to eliminate backflow; however, such controls reduce the "processing space" available to the container manufacturer and thus inherently increase the manufacturing cost and / or defective container numbers. This is particularly true with today's large-scale, multi-cavity injection molding systems at high performance. Thus, there is a need for a better understanding of the phenomenon of undesirable and substantially uncontrollable counterflow, which occurs during packaging and a method for avoiding the same. Another serious problem experienced by many manufacturers of multi-layer containers is the inability to provide an effective sheet sheet seal on the upper sealing surface of the container. For example, thermally bonded metal sheet seals are used in commercial ketchup containers to seal against oxygen. Any deficiency in the seal between the upper end of the container and the thin film coating leads to the exposure of the product to oxygen, with the resulting degradation and / or leakage. Again, the causes and ability to control the defective seal are not well understood yet.

BRIEF DESCRIPTION OF THE INVENTION In accordance with the present invention, injection molding methods, articles and molds are provided which reduce or eliminate the backflow problems and sealing defects made during the packaging stage and cooling of the injection molding cycle. The source of these problems and methods to prevent them have not been discovered. It has been determined that tapered (ie, restricted) reactions in the injection mold have caused smaller leading edge effects in the polymer flow fronts to increase unacceptability. A smaller front edge effect can be caused, for example, by a slight misalignment between the core and the outer mold cavity, or by slight temperature differences around the circumference of the mold. When these minor leading edge effects are performed by travel through a restricted region, it can lead to an inevitable backflow during the subsequent packing step of the injection molding process. Accordingly, an injection molding method of a multilayer article is provided, wherein a first flow front of a first material precedes a second flow front of a second material in the mold. If the first flow front develops a leading edge effect, a tapered region in the mold can cause a circumferential flow of the material in the first flow front; this will reduce the leading edge effect on the first flow front, but unfortunately, it leads to development / enhancement of a more significant front edge effect on the second (or subsequent) flow front. It has been found that this circumferential flow of the first material can be reduced by increasing the minimum width of any tapered region or construction in the mold. Note that, as used herein, "first" and "second" are relative to the sequence of injections into the mold and not means to exclude previous, subsequent or intermediate injections of other materials. For example, a multilayer preform has (from top to bottom) a neck finish, a tapered neck region, a side wall and a base. The wall thickness of the tapered neck region is selected such that an inner barrier layer (second material) extends substantially towards the neck finish at all points around the circumference and the second material does not exhibit an inversion of the neck. flow during the packing and cooling stage. This is particularly useful in the manufacture of preforms for stretch blow molded containers having a relatively long and thin (narrow) neck profile, such as ketchup containers. The tapered neck region of the preform can be adapted to be radially stretched on the order of 1 to 2 times and axially stretched on the order of 2 to 3 times in order to provide the orientation biaxial desired for resistance; note that the radial stretch is low due to the profile of the long and thin neck of the container. In a preferred embodiment, a minimum wall thickness of the tapered neck region of the preform is at least of the order of 2 mm or greater. In addition, there are certain proportions by weight for which the present invention is particularly advantageous. More specifically, the region of the tapered neck, side wall and base together comprise a body portion having a body weight; The remaining neck finish has a finished weight. For a ratio of finished to body weight of 1: 4, there is generally no problem with backflow (the reason is the relatively large finish). A ratio of 1: 6 is a transition region for which backflow may or may not be a problem; at a higher proportion than this, there is likely to be a backflow problem. A ratio of 1: 8 can produce backflow most of the time. Still further, for a neck finish of a given finished height and a body of a given body height, the present invention is useful where a proportion of the finishing height at the height of the body is not greater than the order of 0.2. :1; that is, a height of the finished Small is more likely to have a counterflow problem. It has also been found that excessive backflow during the packing step can cause an erosion of the first (external) material layer in the upper sealing surface of the finish, such that the second (inner) material layer is too close or it passes through the upper sealing surface, either one or the other of which can lead to a defective seal. To avoid this problem, the counterflow must be minimized and preferably, the counterflow of the inner layer should not be allowed to proceed in such a way that the cap ring (flange), at the lower end of the neck finish, exhibits counterflow around of the entire circumference of the ring. These and other features of the present invention will be understood more in particular from the following detailed description and drawings.

DETAILED DESCRIPTION OF THE DRAWINGS Figure 1A is a schematic illustration of the melt flow in a preform neck finish during an injection molding cycle, and Figure IB is a graph of the pressure versus time for the injection cycle; Figures 2A-7A and 2B-7B are similar to Figures 1A and IB respectively, but each taken at a time successively later in the injection cycle and show the counterflow development during the cooling and packing step; Figures 8A and 8B are similar to Figures 1A and IB respectively, but for a different injection cycle without a prolonged retention step and wherein a molded article with sinking marks is shown after the cycle is complete. Figure 9 is a front elevation view of a ketchup; Figure 10A is a schematic cross-sectional view taken through an injection mold showing a slight misalignment or lack of concentricity between the core and the walls of the cavity, and Figure 10B is a cross-sectional view taken through of a lower section of the mold; Figure 11 is a schematic cross-sectional view taken through an injection and preform nozzle during filling, showing first and second loads in a sequential injection process; Figure 12 is a schematic illustration of a multilayer preform during injection, showing in vertical cross section a first melt front of a first material and a second front of the melt of a second material in a region of the tapered or tapered neck of the preform, with an edge effect developed in the second flow front; Figure 13 is an amplified diagram of a front of the melt having a front edge effect and showing the components of the axial and circumferential plane of the front speed of the melt; Figures 14A and 14B are schematic side and top plan views respectively of a neck and flange termination (support ring) and Figure 14C is a vertical sectional view through the neck finish of Figure 14A that shows the counterflow amount of EVOH and distance between the EVOH and the upper sealing surface of the finish; Figs. 15A-17, 15B-15B and 15C-17C are similar to Figs. 14A, 14B and 14C respectively, but show additional amounts of counterflow at the neck and rim termination; Fig. 18 is a fragmentary, enlarged cross-sectional view of a neck finish with a metal foil seal attached to the upper sealing surface and a lid; Y Figure 19 is an enlarged fragmentary cross-sectional view of an alternative neck finish, seal and cap.

DETAILED DESCRIPTION In an attempt to better understand the backflow problem, a series of short loads were carried out to illustrate what happens during the end of the filling stage and during the packing and cooling stage. These short loads are illustrated in Figures 1-7 and include a series of graphs of the cross sections of the neck finish of the preform and the pressure related to time. Each of Figures 1B-7B shows the same pressure / time cycle, with a movable arrow that indicates the time in the cycle, in which the cross section of the preform was evaluated (shown above in Figures 1A-7A ). The pressure / time curve of Figures 1-7 is typical for the CPT injection sequential molding processes, to manufacture a five layer PET / EVOH container wherein; a first load of virgin PET forms the outer and inner outer layers of the preform; a second EVOH load forms first and second intermediate barrier layers adjacent to the inner and outer layers; and a third load of virgin or recycled PET forms a layer of the central core between the intermediate layers. The fill portion of the cycle mold takes approximately 5.5 seconds and includes a first load of virgin PET starting at t = 0 seconds; a second charge of EVOH starting at t = 2.1 seconds; and a third load of virgin or recycled PET starting at t = 2.4 seconds. There are two light pressure drops shown in Figures IB, indicating the completion of the first load and the start of the second and third loads respectively. The pressure in the cavity increases rapidly during the initial portion of the first load and is then leveled at approximately 281.2 Kg / cm2 (4000 pounds / inch2) for the remainder of the 5.5 seconds fill time. Then, during the packing and cooling stage, the pressure is rapidly increased to approximately 843.6 Kg / cm2 (12,000 pounds / inch2) and maintained for approximately 2 seconds; this ensures a complete filling of the mold; then, at t = 7.5 seconds, the pressure is rapidly lowered to approximately 492.1 Kg / cm2 (7000 pounds / inch2) and held at this value for approximately 7 seconds. The holding pressure is maintained to prevent the formation of depressions or sink marks in the final molded article. Finally, at approximately t = 15 seconds, the pressure is released and falls to zero.

Figures 1A-1B illustrate a point near the end of the filling stage (at = 4.5 seconds), where a first flow front 9 of the first charge (virgin PET) approaches a lower surface 11 of a ring support (flange) 12 of a neck finish 10. The second and third loads are substantially below this point and not visible in Figure 1A. Next, Figures 2A-2B, which are taken even closer to the end of the filling stage (at = 5 seconds), show that a first flow front 15 (virgin PET) has expanded to fill most of the flange 12. and continues to rise to the lower portion 14 of the neck finish (above the flange 12). A second flow front 16 (EVOH) reaches the lower end 11 of the flange, followed by a third flow front 17 (of virgin or recycled PET). In Figures 3A-3B, taken at the end of the filling stage (t = 5.5 seconds), the first load (PET) has formed the inner and outer layers (20, 21) substantially throughout the neck finish and filled the flange 12. One second, flow front 22 (EVOH) is located near an upper sealing surface 13 of the preform, but has not broken the first load. A third flow front 23 (PET) is close behind the second load. There are some corners 25-28 of the neck finish which have not been filled. It should be understood that Figures 2-8, that the lines 18, 19 represent the layers of the second charge of the intermediate internal and intermediate intermediate material (EVOH) in the preform, respectively, surrounding a layer 30 of the core of the material of the third load (PET). Figures 4A-4B show the neck finish after the packing stage (reinforcement), at t = 8 seconds. Here, the first load has completely filled all the contact areas with the core and walls of the cavity and the second and third loads (layers 18, 19, 30) extend substantially vertically throughout the neck finish if penetrate to the top sealing surface 13. If the cycle is now modified (see Figure 8B) to eliminate much of the subsequent prolonged retention stage (after t = 8 seconds) then as shown in Figure 8A, it would form in the depressions of the neck finish or marks of Sinking 35-38, where the outer layer has shrunk from the wall of the cavity. This is particularly noticeable at the flange, top and bottom surfaces at 37 and 38, because the flange 12 is a portion of relatively thick walls of the neck finish (as compared to the top portion 14). These sinking marks are undesirable because they reduce the tightness of the fit between the rocks on the finished neck and the cap; also the subsidence in the wall of the body of the preform can adversely influence the characteristics of reheat and stretch blow of the preform. Thus, a retention stage is necessary. Returning to the previous sequence, Figures 5A-5B show a point near the retention stage (at t = 9 seconds) where the undesirable backflow begins to develop. It is proposed that the holding pressure keep the preform in contact with the cold mold cavity and the core walls and consequently the inner and outer preform layers (20, 21) are colder than the second and third layers of the preform. the interior cargo (18, 19) and (30) If the second and third loads are still fused, they look for a minimum resistance flow path which in this case is back towards the flange 12 (where the greatest amount of contraction occurs in the outer layer). As a result, spill or overflow or portion of handle 40 is formed at the terminal end that includes the second and third load layers; the hidden hand portion, which is the start of the counterflow, is directed radially outward and back towards the flange. As shown in Figures 6A-6B (at t = 10 seconds), the second and third load layers have now backward travel (in counter flow) a substantial portion of the neck finish towards the shoulder 12 to form counterflow layers 18a, 30a and 19a (barrier, PET, barrier, respectively). These excess layers in the neck flange, caused by the backflow, reduce the mechanical strength of the neck finish and are thus undesirable. As shown in Figures 7A-7B, taken near the middle of the retention stage at t = 11 seconds, the counterflow layers (18a, 30a, 19a) have now traveled to the flange 12, but still remain within the layer 21 of the first external charge. Although the diagrams above establish a flow invention that occurs during the packing and cooling stage, a solution to the problem was not obvious and took six years to discover. It has been found that backflow is a particular problem with preforms that have a finished short neck (reduced height) (compared to body height) and / or preforms designed to make containers that have a long, thin neck. The preforms for containers with long and thin neck tend to have a wall thickness in the region that forms the neck that is relatively thin, because this region that forms the neck must suffer relatively large axial stretch to compensate for the relatively low amount of radial stretch. It was not clear why these particular preform designs have a problem with backflow. Still further, it was found that the backflow problem records the erosion of the upper sealing surface by the internal barrier layer, such that an effective seal could not be provided with 100% assurance. It has been found that in a preform for applying a container having a relatively tall and slender neck portion (eg, a ketchup container), the tapered neck region in the mold of the preform has a surprising effect as far as it is concerned. considered a relatively minor leading edge effect caused for example by a misalignment between the core and the cavity. When multiple layers are injected into a mold to make such a preform, a relatively small leading edge effect can be developed on the first flow front (caused by misalignment of the core) before reaching the tapered region. Then, as the first flow front advances through the tapered region, there is an increase in flow velocity that causes an increased circumferential flow of material from the leading edge to the trailing edge on the first flow front. This effectively reduces the effect of the leading edge on the first front of the flow. However, it will aggravate (increase) a front edge effect at any second or later flow front, such that once the second flow front reaches the upper end of the tapered region, there is now a significant leading edge effect . In turn, this is likely to induce the counterflow of the second front flow of the leading edge during the packing and cooling stage. Figure 9 shows an exemplary ketchup 50 having an upper neck finish 52, a long thin neck 52, a sidewall 53 and a base 54. The neck finish includes threads 55 and a lower support ring or flange 56. There is a transition region below the ridge 56, marked by the reference number 57, which substantially defines the area below which the radial and axial stretching of the preform begins. Due to the profile of the long and thin neck of the container, a preform, such as that shown in Figures 10-12, having a tapered neck region 82 is used. Because the long and thin container neck 52 undergoes relatively little radial stretching, it is necessary to improve the axial stretching in order to obtain the necessary biaxial orientation (and resultant mechanical strength) in the neck 52. For this reason, a region of tapered neck is provided. in the preform that decreases in the wall thickness going towards the end of the neck 81 (see Figure 12); the decrease in wall thickness is obtained by reducing the diameter of the external wall 85 as it approaches the neck finish. The reduced thickness portion of the neck region is caused to undergo a greater axial stretch during the initial axial lengthening of the preform (because it is thinner than the remaining side wall and the outer base portions and thus easier to stretch. ). Within the neck region 82 itself, an upper portion 95 undergoes relatively greater axial stretching than a lower portion 96, due to the thickness of the tapered wall. Figure 10 shows a potential cause of a leading edge effect, i.e. a misalignment of the core and the cavity. Core displacement is a common phenomenon that occurs to some degree in all injection tools - single-cavity or multi-cavity mono or multilayer. Figure 10A is a cross-sectional view along the length of the mold 60 of the preform. A core 58 is positioned in an external cavity to define a space for molding a preform between the wall 61 of the core and the wall 62 of the cavity. A misalignment of the core and the cavity is illustrated in a circumference 59 near the lower end of the preform, which is illustrated in cross section in FIG.

Figure 10B. The core is displaced slightly to the left in such a way that the distance between the wall of the core and the wall of the cavity on the right side (ta) is greater than the distance between the wall 61 of the core and the wall 62 of the cavity on the left side (tb). The manufacturers of the container impose the limits on the maximum wall variation in the preform, which is defined by: ya2 + h2, the minimum and maximum radial wall thicknesses. Other potential causes of a leading edge effect include non-isometric cooling, that is, temperature differences at various points along the core and walls of the cavity, and non-uniformity of the melt, that is, temperature differences of polymer due to variations in the hot operating system. However, misalignment of the core / cavity has now been identified as a primary cause of the leading edge effect in sequential co-injection. Figure 11 schematically illustrates the mold cavity of the preform defined by the core wall 61 and the wall 62 of the injection cavity 65 for sequentially injecting different polymeric materials into an orifice 66 in the bottom of the mold of the preform. In the process of mold by sequential injection of CPT, because the first charge is of only one layer, there is no greater difficulty to ensure a substantial flow Balancing the first load to the wall in each cavity (of a multi-cavity apparatus). This is illustrated in Figure 11, wherein the first loading material 70 is injected from a central channel 67 in the nozzle, and flows to approximately half the wall of the preform. It has been found that even if there is some misalignment of the core / cavity, any difference of the leading edge in the first load is very small, this is about 1 mm, when it reaches the upper end of the side wall, in the lower border 63 of the region of tapered neck. In addition, when examining short loads it has been found that this small difference does not increase as the first load advances through the tapered region to the upper border 64. In the CPT process, the second load (barrier) 72 Nor does it require complex equipment or process controls. In sequential molding, it is possible to introduce the barrier melt through a single unbalanced and concentric perforated channel 68, as shown in Figure 11. Despite the introduction of the barrier melt displaced into the cavity, the The flow provided by the virgin PET (first charge) already in the cavity drives the barrier material to form a circumferentially balanced disk-like melt accumulation within the frozen PET surface layers. This "Self-leveling" characteristic of the sequential injection means that there is substantially no effect of the leading edge due to the introduction of the materials into the mold cavity. Finally, a third load of virgin or recycled PET can be injected via the displaced channel 69 and likewise, forms an accumulation of self-leveling melt, which subsequently drives the first and second loads forward of it to fill the mold. Figure 12 shows the first, second and third layers of the charge as they move through the tapered region. In Figure 12, the profile 80 of the preform is defined by the wall 61 of the core and the wall 62 of the cavity, and includes the neck finish 81, the tapered region 82, the side wall 83 and the base 84. In this step in the mold filling cycle, the first material of the charge has a first flow front 100 with a difference in height of the advancing front (as measured from the bottom of the preform) between the top point 101 ( on the left) and the bottom point 102 (on the right), at different points on the circumference. Meanwhile, a second material charge with a second flow front 104 has developed an enhanced difference between the upper point 105 (left) and the lower point 106 (right). As shown in figure 12, the second front 104 develops a larger front edge effect (greater difference between the front and rear points) around the circumference than the first 100 front. It was thought that perhaps the third of virgin or recycled PET caused the unbalanced flow of the second material load, because the third load effectively drives the second (barrier) load upwardly of the preform sidewall. For example, it was thought that if the temperature of the third load was varied around the circumference, due to thermal imbalances imparted in the hot operating system, then, the mold cavity would have a hot side and the second and third loads they will move faster to the hot side. Many efforts were made over an extended period of time to try to understand why the effect of the leading edge developed in the second load and how it could be eliminated. For example, changes were made to the hot operating system that include moving the heater bands, adding melt mixers, changing the barrier flows around the nozzle, modifying the materials used in the manifolds, and removing the rods from The valve. However, none of the above solutions were found to effectively influence or correct the problem.

After much experimentation, it is believed that the source of the leading edge effect in the second load has been determined and a solution was also found. Figure 13 is a longitudinal cross-sectional view showing a front 90 of the melt through a width of a preform having a forward point 91 at the center of a vertical axis 97 and rear points 92, 93 at each of the outer side edges of the melting front. The direction of the fade front is represented by a number of parallel vertical arrows 94 pointing upwards. Due to the difference of the front point / rear point, the velocity of the front of the melt is not parallel to the direction of the flow of the melt, but rather to an angle with respect to it. As shown, the melting rate (Vfundido) has an axial component (Vaxiai) and a circumferential component (Vc? R). The circumferential component causes the material to flow circumferentially in the cavity, transverse to the filling direction. As described previously, the first charge of Virgin PET can develop a front edge effect as it moves up into the injection cavity, due to the non-symmetrical conditions that may exist in the system, such as the lack of concentricity between the core and the walls of the cavity , conditions of non-symmetrical temperature, etc. The circumferential component of the melting speed causes the virgin PET in the vicinity of the forward point to flow towards the rear point. Such a circumferential flow would increase the size of the melt accumulation in the front (front) of the second and third fronts of the in-line fade (circumferentially) with the back point of the first face and at the same time, decrease in the forward direction of the seconds and third melting fronts in line (circumferentially) with the front point of the first front. The circumferential flow of the first material would thus reduce the difference between the front and rear points in the first flow front and thus reduce the edge effect in the first flow front. However, it has the opposite effect on the second and subsequent flow fronts. Due to the transfer of the material circumferentially, there would be unbalanced circumferential flow in front of the second (and subsequent) flow front which may cause or effect a leading edge effect on the second (and subsequent) flow front. Surprisingly, it has been found that the width of the tapered region has a substantial effect on the development of a leading edge effect on the second flow front. It has been found that increasing the The minimum width in this region of the tapered neck, within the limits allowed by the axial stretch ratios of the desired preform / container, has a substantial effect in reducing the development of a leading edge effect in the second or subsequent flow fronts. For example, it has been found that in a case where a second flow front of the load has a leading / trailing edge difference of about 1 mm at the start (lower boundary 63) of the tapered region for the time between the second load reaches the upper border 64 of the tapered region, the edge difference (of the second flow front) has increased to 5-10 mm and still as high as 20 mm, depending on the amount of the wall thickness reduction experienced through the critical tapered neck region. The adjustment of the minimum wall thickness of the tapered neck region can substantially reduce or eliminate the problem. However, for certain preform designs, it may not be possible to completely eliminate the counterflow because the thickness of the tapered neck region would be greater to provide the necessary axial stretch in a corresponding portion of the blow molded container. In this case, the counterflow can be reduced by increasing the wall thickness of the tapered neck region to the extent possible.

It has also been found that backflow often causes a concurrent problem with defective welded seals, since the erosion of the upper sealing surface by the barrier material that accompanies the counterflow of the barrier material at the neck finish. This problem is illustrated in Figures 14-17. In general, the close proximity of the barrier material to the top sealing surface (TSS) is undesirable for those container applications, where a thermally bonded seal is employed. An excessive induction heating time and / or temperature may melt the outer PET layer in the TSS, thereby exposing the barrier material to the sheet adhesive which may in turn weaken or destroy the integrity of the seal. As such, it is desirable to maintain the flow front of EVOH at a distance of at least about 0.1 mm below the TSS. Figures 14A-14C schematically show in side plan view and in cross section a neck finish 103 in which a minimum counterflow amount has been presented (see backflow 109a), and there is a substantial distance di between the upper part 107a of the internal EVOH barrier layer and the TSS 108. Here, dx = 1.0 mm and there has been no backflow to the ridge 110.

Figures 15A-17A, 15B-17B and 15C-17C are similar to Figures 14A, 14B and 14C respectively, but with their increased counterflow amounts of the barrier layer (109b-109d) which not only structurally weaken the finish of the neck, but cause the top or top (107b-107d) of the EVOH barrier layer to approach the TSS. In Figure 15, the EVOH has flowed counterflow (109b) to the flange at an angle greater than 180 °, but less than 270 ° around the flange and the distance di from the upper sealing surface is reduced to approximately 0.4. mm. This still allows the formation of an effective thermal seal. In Figure 16, the EVOH has flowed countercurrently (109c) in addition to the flange and the distance d3 now extends to approximately 320 ° around the flange and has been approximated 0.1 mm away from the upper sealing surface. This distance is the minimum for an acceptable seal. Finally, in Figure 17, the EVOH barrier has flowed countercurrently (109d) in such a way that. it extends completely around the entire flange or flange of 360 ° and the distance d is less than 0.1 mm; This is close to the TSS and will lead to inevitable seal failures. Figure 18 is an enlarged cross-sectional view of a neck finish with thermally bonded seal and cap, according to one embodiment. Plus specifically, a neck finish 120 has a TSS 122, with a thermally bonded laminate sheet seal or liner 124. The liner 124 lies within an interior surface of a cap 126. The liner 124 is designed to ensure a tight seal to prevent leaks, in case there is a deformation in the neck finish. Figure 19 shows a termination 130 of the neck of alternative low height. In order to minimize the weight of the finish, not only is the height of the finish reduced, but there are no threads in the portion 131 of the upper neck finish and no upper flange. Instead of the flange, a radially indented slot 133 is provided, which is engaged by a projection 135 on a snap-fit cover 136. Again, a metal sheet liner 134 is thermally bonded to an upper sealing surface 132 of the neck finish. The present invention has many potential applications for preventing the significant leading edge effects that cause backflow or sealing effects in a multi-layer article, in which preforms and blow-molded containers are included. For example, it is common to use recycled PET as one or more layers in order to reduce the overall cost of the container. However, because recycled PET has a higher color component than virgin PET, it has been found that recycled PET heats up more quickly (ie, during the reheating process, before blow molding). If there is a circumferential difference in the final flow front of the recycled PET layer, the circumferential imbalance can lead to a circumferential imbalance in the heating of the layer and a resultant imbalance in the amount of stretch during blow molding. Another problem that may result from an unequal circumferential distribution of a polymeric layer in a preform or container is a visual "stratification" effect that binds the unacceptable preform / container. For example, a difference in a barrier flow front (e.g., EVOH) can produce an oriented bottle wall with a distortion or visual defect in the oriented neck region. These and other problems can be reduced and / or avoided by the use of the methods of the present invention. Although particular embodiments of the present invention have been described, various modifications will be apparent to those skilled in the art, and are included herein.

In several alternative embodiments, one or more layers of the container preform or portions thereof, may be made of various polyethers and various other polymers, such as polyolefins (eg, polypropylene and polyethylene), polyvinyl chloride, polyacrylate, etc. . Suitable polyesters include homopolymers, copolymers or blends of polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polypropylene terephthalate (PPT), polyethylene naphthalate (PEN), and a copolymer of cyclohexane dimethanol / PET, known as PETG (available from Eastman Chemical Company, Kingsport, Tennessee). The polyesters based on terephthalic or isophthalic acid are commercially available and are convenient. The hydroxy compounds are usually ethylene glycol and 1,4-di- (hydroxymethyl) cyclohexane). In general, the phthalate polyester can include polymeric linkages, side chains and end groups, unrelated to the formal precursors of a previously specified simple phthalate polyester. Conveniently, at least 90 mole percent will consist of terephthalic acid and at least 90 mole percent of a glycol or aliphatic glycols, especially ethylene glycol. Recycled or post-consumer PET (PC-PET) is prepared from plastic PET containers and other recyclable products that are returned by consumers for a recycling operation, and has now been approved by the FDA for use in certain food containers. It is known that PCT-PET has a certain level of I.V. (intrinsic viscosity), moisture content and contaminants. For example, the typical PC-PET, which has a flake size of approximately 1.27 cm (one-half inch) maximum), has an I.V. average of approximately 0.66 dl / g, a moisture content of less than 0.25% and the following levels of contaminants PV: < 100 ppm aluminum: < 50 ppm olefin polymers (HDPE, LDPE, PP): < 500 ppm paper and labels: < 250 ppm colored PET: < 2000 ppm other contaminants: < 500 ppm PC-PET can be used alone or in one or more layers to reduce cost or for other benefits. Although it is useful as a high oxygen barrier layer it is a bottle-grade polyester packaging material, with physical properties similar to PET, ie polyethylene naphthalate (PEN). The PEN provides an improvement of 3 to 5X (three to five times) in the barrier properties and improved thermal resistance, some additional expense. Polyethylene naphthalate (PEN) is a polyester produced when the dicarboxylate of dimethyl-2,6-naphthalene (NDC) is reacted with ethylene glycol. The PEN polymer comprises repeating units of 2,6-ethylene naphthalate. PEN resin is available and has an inherent viscosity of 0.67 dl / g and a molecular weight of about 20,000 from Amoco Chemical Company, Chicago, Illinois. The PEN has a vitreous transition temperature Tg of about 123 ° C, and a melting temperature Tm of about 267 ° C. PET and PEN can be combined or copolymerized in various amounts. In the ranges of about 0-20% PEN and 80-100% PEN, the material can be crystallized, while of about 20-80% PEN, the material can not be crystallized and remains substantially amorphous. The PET and PEN structures are shown below: pe / Suitable polyamides (PA) include PA6, PA6.6, PA6.4, PA6.10, PA11, PA12, etc. Other options include acrylic / amide, amorphous nylon, polyacrylonitrile (PAN), polystyrene, crystallizable nylon (MXD-6), polyethylene (PE); polypropylene (PP), and polyvinyl chloride (PVC). The multilayer preform / container may also include one or more layers of an oxygen barrier material such as ethylene / vinyl alcohol (EVOH), PEN, polyvinyl alcohol (PVOH), polyvinylidene chloride (PVDC), nylon 6, crystallizable nylon (MXD-6), LCP (liquid crystal polymers), amorphous nylon, polyacrylonitrile (PAN) and styrene acrylonitrile (SAN). The intrinsic viscosity (I.V.) effects the processing capacity of the resins. Polyethylene terephthalate having an intrinsic viscosity of about 0.8 is widely used in the light carbonated beverage (CSD) industry. Polyester resins for various applications can range from about 0.55 to about 1.04 and more particularly about 0.65 to 0.85 dl / g. Measurements of the intrinsic viscosity of polyester resins are made according to the procedure of ASTM D-2857, by employing 0.0050 ± 0.0002 g / ml of the polymer in a solvent comprising o-chlorophenol (melting point 0 ° C), respectively at 30 ° C. The Intrinsic viscosity (I.V.) is given by the following formula: I.V. = (In (Vsoln./VSol)) / C where: VSoln is the viscosity of the solution in any unit; Vso ?. it is the viscosity of the solvent in the same units; and C is the concentration in grams of the polymer per 100 ml of solution. The container body blown in one embodiment is substantially transparent. A measure of the transparency in the percent of the haze for the light transmitted through the wall (Ht) that is given by the following formula: where Yd is the diffuse light transmitted by the sample and Ys is the specular light transmitted by the sample. The diffuse and specular light transmission values are measured according to the method of ASTM D 1003, by using any standard color difference meter such as the D25D3P model manufactured by Hunterlab, Inc. The container body in this mode , must have a percent of haze (through the panel wall) of less than about 10% and more preferably less than about 5%. The portion that forms the body of the preform in this embodiment must also be substantially amorphous and transparent, having a percent of fog through the wall of no more than 10% and more preferably no more than about 5%. The container will have variable levels of crystallinity at various positions along the bottle height from the end of the neck to the base. The percent crystallinity can be determined in accordance with ASTM 1505 as follows:% crystallinity = [(ds - da) / (dc - da)] X 100 where ds = sample density in g / cm3, gives = density of an amorphous film of zero percent crystallinity and dc = crystal density calculated from unit cell parameters. For the 5 liter PET / EVOH ketchup container previously described, the panel or portion 53 of the side wall of the container is stretched as much as possible and preferably has an average center of crystallinity of at least about 10% and more preferably at least about 15%. The percent crystallinity in the region 52 of the neck is preferably 5-10%.

Additional increases in crystallinity can be obtained by thermofixing to provide a combination of stress induced crystallization and thermally induced crystallization. The thermally induced crystallinity is obtained at low temperatures to preserve the transparency, that is to keep the container in contact with a blow mold at low temperature. In some applications, a high level of crystallinity on the surface of the side wall alone is sufficient. As a further alternative, the preform may include one or more layers of an oxygen sequestering material. Suitable oxygen sequestering materials are described in U.S. Patent Application Serial No. 08 / 355,703, filed December 14, 1994 by Collette etc. al., entitled "Oxigen Scavenging Composition For Multilayer Preform And Container", which is incorporated herein by reference in its entirety. As described therein, the oxygen scavenger may be an oxidizable organic polymer catalysable by metal, such as a polyamide. The oxygen scavenger can be mixed with PC-PET to accelerate the activation of the sequestrant. The oxygen scavenger can be advantageously combined with other thermoplastic polymers to provide the desired injection molding and blow molding charaistics of stretch to manufacture substantially amorphous injection molded preforms and substantially transparent biaxially oriented polyester containers. The oxygen scavenger can be provided as an inner layer to retard the migration of the oxygen scavenger or its byproducts and to prevent premature activation of the sequestrant. Although certain preferred embodiments of the invention have been specifically illustrated and described herein, it will be understood that variations may be made without departing from the invention as defined by the appended claims. It is noted that, in relation to this date, the best method known to the applicant to carry out the aforementioned invention, is that which is clear from the present description of the invention.

Claims (7)

Claims The invention having been described as above, property is claimed as contained in the following 1. A method for reducing the uncontrolled flow of a molten polymer during the packing and cooling step of an injection molding cycle, wherein a plurality of polymer are injected between a cavity of the mold and core to manufacture a multilayer plastic article having upper and lower regions and a tapered region therebetween which decreases wall thickness as it approaches the upper region, where during the molding cycle, at least one first polymer is injected to form an outer layer and at least one second polymer is injected to form a lower layer and wherein the second polymer forms a flow front having a leading / trailing edge the method is characterized in that it comprises the step of: selecting the width of the tapered region, as defined in between the mold cavity and the core, such that the inner layer extends substantially to the upper region at all points around the circumference and the second polymer does not reverse its flow during the packing and cooling stage. 2. The method according to claim 1, characterized in that the article is an adapted preform for stretch blow molding a container and the tapered region is a tapered neck region adapted to make a relatively long and thin container neck. 3. The method of compliance with the claim 2, characterized in that the region of the tapered neck is adapted to be radially stretched on the order of 1 to 2 times and axially stretched on the order of 2 to 3 times. 4. The method according to claim 1, characterized in that the minimum thickness of the region of the tapered neck is at least 2 mm or greater. The method according to claim 1, characterized in that the tapered and lower regions comprise a body having a body weight, the upper region is a neck finish having a neck weight and wherein a weight proportion of the finished at neck weight is greater in the order of 1: 6. The method according to claim 1, characterized in that the tapered and lower regions comprise a body having a body height, the upper region is a neck finish having a neck height and wherein a proportion of the height of the finished at the height of the body is not greater than the order of 0.2: 1.

1 . A method for reducing the uncontrolled flow of a molten polymer during the step of packing and cooling an injection molding cycle, wherein a plurality of polymers are injected between a mold cavity and a core to make a multilayer plastic preform having a neck finish, a region of the tapered neck that decreases the wall thickness as it approaches the neck finish, a side wall and a base, wherein, during the molding cycle at least one first polymer is injected to form an outer layer and at least one second polymer is injected to form an inner layer and wherein the second polymer forms a flow front having a leading / trailing edge, the method is characterized in that it comprises the step of: selecting the width of the tapered neck region, as defined between the mold cavity and the core, such that the inner layer extends substance Until the end of the neck at all points around the circumference without exceeding a minimum distance from an upper surface of the neck finish. 8. The method according to claim 7, characterized in that the minimum distance allows the effective application of a thermal bond seal to the top surface of the neck finish. 9. An improved method for molding a multi-layer article in an injection mold, wherein a first flow front of a first material precedes a second flow front of a second material and the first flow front has a leading / trailing edge the improvement is characterized by comprising: retarding the development of a leading edge / back in the second flow front by reducing a circumferential flow of the first material from the leading edge to the rear of the first flow front. The method according to claim 9, characterized in that the circumferential flow in the first flow front is reduced by adjusting the shape of the mold. The method according to claim 9, characterized in that the circumferential flow in the first flow front is reduced by increasing the minimum width of any tapered region in the mold. 1

2. An improved method for molding a multilayer article in an injection mold, wherein a first flow front of a first material precedes a second flow front of a second material, the improvement is characterized in that it comprises: increase the minimum width of any tapered region in the mold to retard the development of a leading / trailing edge on the second flow front. 1

3. An improved method for molding a multilayer article in an injection mold, the article has an inner layer and an upper sealing surface, the improvement is characterized in that it comprises: keeping the inner layer at a minimum distance from the surface of the Superior sealing by reducing the amount of counterflow of the inner layer during a packing and cooling stage of the injection molding cycle. The method according to claim 13, characterized in that the article has a circumferential ridge below the upper sealing surface and the counterflow of the inner layer does not extend completely around the circumferential rim. 15. A multi-layer injection molded plastic preform for stretch blow molding of a container, the preform has a neck finish, a tapered neck region decreasing in width as it approaches the neck finish, a lateral wall and a base, the region of the tapered neck is adapted to form a relatively long and thin neck of the container by radial stretching of the order of 1 to 2 times and axial stretching of the order of 2 at 3 times, the neck finish has an outer layer of at least a first polymer and an inner layer of at least one second polymer, such that the inner layer extends substantially to the neck finish at all points around the circumference and without exceeding a minimum distance from a top surface of the neck finish. The preform according to claim 15, characterized in that there is no reversal of flow of the inner layer at the neck finish during injection molding, such that the neck finish has only one inner layer. A container made by stretch blow molding the preform according to claim 15, characterized in that it has a biaxially oriented neck and a side wall and a metallic thin leaf seal attached to an upper sealing surface of the neck finish .