MXPA94008315A

MXPA94008315A - Method to form molecularly oriented plastic receptacles.

Info

Publication number: MXPA94008315A
Application number: MXPA94008315A
Authority: MX
Inventors: W Steele Scott
Original assignee: Bekum Maschf Gmbh
Priority date: 1993-10-27
Filing date: 1994-10-27
Publication date: 2002-07-15
Also published as: KR960705669A; JPH09504240A; DE69404474D1; ATE155727T1; DE69404474T2

Abstract

The present invention discloses a method an apparatus to form molecularly oriented plastic receptacles. A hollow preform, generally cylindrical, with a closed end and an open end, is molded by injection, starting from a partially crystalline molecularly oriented plastic material. The preform is subsequently passed along a series of heating and cooling zones and is reheated at a temperature at which the inter-crystalline structures in the preform melt, without melting the crystal cores of the same. This preform is quickly stretch blow molded and quickly cooled against a polished mold surface, in order to produce a molecularly oriented receptacle having clarity, impact-resistance, and barrier properties to gas and water vapor, as well as improved rigidity.

Description

METHOD FOR FORMING MOLECULARLY ORIENTED CONTAINERS HEADLINE: BEKUM MASCHINENFABRIKEN GMBH GERMAN NATIONALITY WITH ADDRESS: LANKWITZER STRASSE, 14 • 15 D-12107 BERLIN, GERMANY INVENTOR: SCOTT W. STEELE NATIONALITY OF THE UNITED STATES WITH ADDRESS: 3232 RIVER ROAD TOLEDO, OHIO 43614 E.U.A. ÜWE-VOLKER ROOS GERMAN NATIONALITY WITH DOMICILIO: HASENHEIDE # 2 J¿ D-29389 BODENTEICH GERMANY FRANZ GITTNER GERMAN NATIONALITY WITH ADDRESS: lM GRUNDFELD 33 D-29594 SOLTENDIECK GERMANY EXTRACT OF THE INVENTION An improved method and apparatus for forming plastic, molecularly oriented containers is disclosed. A hollow preform, generally cylindrical, that has a * Closed end and an open end, is molded by injection, from a material of partially crystalline plastic, molecularly orientable. The preform is then passed through a series of heating and cooling zones and heated again to a temperature at which the intercrystalline structures in the preform melt, without melting the crystal nuclei thereof. This preform is then rapidly molded by stretching and blowing and rapidly cooled against a polished mold surface, to produce a molecularly oriented vessel, which has clarity, impact resistance, and gas and vapor barrier properties, as well as an improved rigidity. BACKGROUND OF THE INVENTION 1. Field of the invention The present invention relates to a method and apparatus for forming molecularly oriented plastic containers and, more particularly, to a method and apparatus for forming oriented containers, from a polypropylene containing polymer. , by reheating and a process of stretch and blow molding. 2, Summary of the Prior Art Stretch and blow molding has rapidly gained acceptance as a method to produce oriented plastic containers in a wide range of configurations and sizes. In stretch and blow molding, a pattern or preform is first formed by an extrusion or injection molding operation, then stretched, mechanically, or otherwise, and blown in both the axial and radial directions. In doing so, the molecules are aligned along two planes, an arrangement that advantageously improves clarity, impact resistance, gas and vapor barrier properties, and stiffness, in the finished container. The resulting containers are of a lighter weight, resulting in substantial savings in the amounts of the resin used, and the amounts of the process additives, otherwise required, can also be reduced. Resins, normally considered for stretch and blow molding, include: polyethylene terephthalate (PET), polyvinyl chloride (PVC), polypropylene (PP), high density polyethylene (HDPE), and acrylonitrile copolymers (PAN) . Currently, PET is the most widely used, especially in food packaging applications. However, PET lacks the hot filling capacity and is a relatively high cost material. The biaxially oriented PP is a potentially lower cost alternative, which exhibits a good hot filling capacity. However, oriented PP containers have never really had a high degree of commercial success. This is due, in part, to the fact that conventional PP stretch and blow molding methods have produced containers that have only regular optical clarity. More importantly, conventional PP stretch and blow molding processes have not been economically attractive, since none of these processes has achieved the extremely precise temperature control required to obtain the orientation of the PP, except for the use of very long heating times. There are basically two types of processes for the stretch and blow mold: 1) the one-stage process, in which the preforms are first manufactured and then allowed to cool to the correct temperature of the blow molding, before being blown into the molds. containers in the same machine; and 2) the two-stage process, in which the preforms are first manufactured in a machine, they are allowed to cool to ambient temperature outside that machine, and then they are reheated and blown in another machine. The two-stage extrusion type machines have generally been used to obtain preforms and oriented PP containers. In this process, preforms that are extruded, are generally cooled, cut to length, overheated and stretched, while compression molding the neck finish, then blow, cut and ejected. The reheating of the PP preform is critical, because an accurate temperature of the preform must be achieved, to which the intercrystalline material begins to melt, but to which the crystallites do not melt by themselves. Thus, the PP preform must achieve a uniform temperature just below its crystalline melting peak point. Any increase in temperature above this point will cause the crystallites to melt and thus eliminate any possibility of obtaining optimum orientation during the subsequent operation of stretch and blow molding. This effect is particularly important for resin preforms containing PP, and is not yet well understood in the industry. In any case, the result is that conventional reheating methods have been used with furnaces adjusted to the exact final temperature desired for the preform. As the experts in the F material will appreciate, with such heater settings, as the actual average temperature of the preform approaches the desired final temperature, the impulse force to continue any temperature increase in the preform will decrease. The result is that a very long time is required to asymptotically approach the desired final temperature and, in practice, this desired final temperature is never completely obtained. Various methods of reheating have been described in relation to the blow molding of oriented PP containers. Examples include the patents of E. U. A., of Wiley, Nos. 3,496,258 and Re. 26,956 and the patent of E. U. A., of Seeluth, No. 3,950,459. In the aforementioned patents, Wiley describes a method of conditioning an extruded tube of thermoplastic material, prior to its orientation, by blowing molding. The reheating of the extruded tube is controlled, so that the outer portion of the tube is in a crystalline condition and oriented, while the interior of the tube remains relatively tacky and in a sealable condition. According to Wiley, after the preform has been extruded, the outer surface cools until it reaches a crystalline state. Next, the outer surface is reheated to bring the temperature of the outer surface to within a few degrees of the melting point of the crystals. The heat flow is reduced to leave the inside of the preform in a • Sealable, sticky condition. The reheated preform is then molded by stretching and blowing in the conventional manner. Since the Wiley preform is not at the proper orientation temperature over the entire wall thickness, the orientation is not achieved through the wall thickness and the clarity of the product is correspondingly deteriorated. Seeluth, in the patent of E. U. A., No. 3,950,459, • describes a method for producing a preform and blow molding the preform, in which a tubular preform is extruded and cooled below its orientation temperature. The preforms are then passed through a heating zone maintained at an elevated temperature, while preventing the preforms from having an angular rotation, to heat the preforms to an average temperature within the orientation range. In a second heating zone, Seeluth teaches directing radiant energy on at least three peripheral radial portions of each preform, so that each portion is heated to a different orientation temperature. The preforms are then blow molded and the thickness of the wall is measured in each of the portions. The amount of radiant energy applied to the various portions of the preforms is adjusted in response to these measurements. As mentioned before, the two-stage processes can also use preforms that are injection molded into multi-cavity molds, then cooled to room temperature before introduction into a reheat and blow molding machine. The injection molding of the preforms provides certain advantages over the extrusion of these preforms. In contrast to the extruded tubular preforms, the molds for the injection molded preforms can be formed in the desired final configuration. Also, injection molding eliminates the need to close the end of each preform, as required with the extruded preforms. Oas et al., For example, in U.S. Patent No. 4,357,288 discloses a method for forming PP preforms by an injection molding operation. The melting temperatures of the plastic material are controlled to be just above the melting point index of the PP formulations. These temperatures are said to be sufficiently above the crystalline melting temperature to avoid melt fractures during injection, and sufficiently below the temperature at which the formulations will degrade. The preform is separated from the mold, it is heated to a temperature just below its amorphous flow temperature, and then placed in a stretching and blowing molding apparatus. Despite the efforts of those skilled in the art, an efficient method for forming molecularly oriented polypropylene containers having satisfactory clarity is not yet available. SUMMARY OF THE INVENTION The present invention relates to an improved method and apparatus for forming molecularly oriented plastic containers. A hollow preform, generally cylindrical, having a closed end and an open end, is preferably formed by injection molding from a partially crystalline, molecularly orientable plastic material for subsequent molding by stretching and blowing. The molten material is injected into a preform mold cavity at a predetermined filling rate, which is controlled so as to minimize the orientation of the molecular flux induced in the plastic material by its injection into the mold. The injection-fill rate is also controlled in such a way as to minimize the residual stress that could otherwise develop in the plastic material during the injection. The preform is then reheated before the stretch and blow molding operation, at a temperature below the crystalline melting point of the material. The preform is then preferably passed through a series of heating zones, each with a radiant heat source, which heats the preform to a temperature at which the intercrystalline structures in the preform melt, without melting the crystal cores , which are connected by the intercrystalline regions. To avoid the so-called edge effects, which result from the geometry of the heating zone, it has been found to be advantageous to form the injection molding cavity so that the resulting preform decreases in thickness at both of its ends. The preform is then rapidly transferred to the molding zone and secured within the mold of the stretch and blow molding apparatus. The preform is axially stretched with an internal stretching rod, which is attached to a closed end of the preform, and a pressure differential is created, which is sufficient for the expansion of the preform in compliance with the mold walls. The preform is rapidly cooled against the surface of the polished mold, to produce a molecularly oriented vessel, which has improved clarity, impact resistance and gas and water vapor barrier properties, as well as improved stiffness. Therefore, an object of the present invention is to provide a molecularly oriented plastic container that has improved clarity. A further object is to provide a method for producing a plastic container, molecularly oriented, in an efficient manner of time and cost. Another object of the invention is to provide an improved method for forming the preforms for the subsequent stretch and blow molding of these molecularly oriented plastic containers. Yet another object is to provide an improved method for reheating the preforms immediately prior to the stretch and blow molding of this molecularly oriented plastic container. Still another object of the invention is to provide an improved method of stretch and blow molding of reheated preforms, to produce these molecularly oriented plastic containers. Other objects and advantages will become apparent during the course of the following description, when taken in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS The foregoing objects, as well as other advantages of the present invention, will become readily apparent to those skilled in the art from the following detailed description of a preferred embodiment, when considered in conjunction with the accompanying drawings, in which: Figure 1 is a longitudinal sectional view of a preform, according to the present invention; and Figure 2 is a schematic top view of a portion of the reheat system of the invention. DESCRIPTION OF THE PREFERRED MODALITY According to the present invention, a preform is first obtained by injection molding a plastic material, partially crystalline, molecularly orientable, then it is reheated, then it is stretched and blown in the axial direction as well as radial, and it cools quickly. In doing so, the molecules are preferably aligned along two planes, an arrangement that advantageously improves clarity, impact resistance, gas and vapor barrier properties, and stiffness in the finished container. The methods of the present invention can be used to form molecularly oriented containers of any partially crystalline, molecularly orientable plastic material. Exemplary polymers are polymers of at least one mono-1-olefin, having 2 to 8 carbon atoms per molecule. Polypropylene-containing materials are preferred, examples of which include PP homopolymers, copolymers containing more than 50% by weight of propylene, or a mixture of polypropylene homopolymers or copolymers with at least one material such as polyethylene, polybutene , poly-4-methylpentene-1 and the ethylene / propylene elastomeric copolymer. Suitable resins containing PP, according to the invention, may include lubricants, pigments, dyes, inorganic or organic fillers, polymer additives or other additives that are conventionally used in the resins to be blow molded. A particularly preferred material for use according to the present invention is a propylene / ethylene copolymer, which has more than 50% by weight of propylene. The melt index of the propylene / ethylene copolymer is from about 2 to 50, and preferably from about 10 to 30. It has been determined that in the model of the preform and the crystal morphology, the method for reheating and heating the geometry of the area, and the methods for stretch and blow molding, all have influence on the obtained vessel. Improvements in each stage of the general process have been discovered, and the present invention thus provides a method for producing oriented PP containers having good clarity, without unduly prolonged cycle times. 1. PRODUCTION OF THE PREFORM It is known that an injection molded preform must have a correct crystal morphology to be properly oriented during the stretching and blowing molding operation. That is, the crystal structures must be evenly distributed through the preform and the crystallites must be relatively small. It has been determined that the crystal morphology of the preform is dependent on injection molding conditions. According to the present invention, a closed-end preform is injection molded from a partially crystalline, molecularly orientable plastic material. The injection molding of the preform is carried out with process conditions that produce a minimum flow orientation, due to the injection filling process, which results in crystal structures evenly distributed through the preform and of relatively small dimensions. It has also been determined that it is important that the preform be substantially free of stress. Residual stresses have at least two disadvantages. FirstWhen a tensioned preform becomes partially fused, as it is reheated to a temperature below the crystalline melting point, the shape of the preform can change substantially as the internal stresses are relieved. When this happens, the preform will become shorter when measured and the walls will be thicker, and this preform will be difficult to reheat uniformly and optimally by conventional methods of radiant energy. In addition, the residual voltages can induce¬ * Cry to an increase in the recrystallization regime in reheating. This effect can also interfere with the achievement of optimal clarity. Better results have been achieved at filling rates of approximately 3 to 5 grams per second for a single injection cavity, followed by rapid cooling at cooling rates of 4 to 82C / second. The injection rates above about 5 grams per second F result in residual stresses that detrimentally affect the clarity of the container formed subsequently. Injection rates below about 3 grams per second result in a significant number of short injections, and can also cause detrimentally high residual stresses in the preform. The melt is preferably injected into the mold cavity having cooled mold surfaces, which are maintained at a temperature range between about 5 to 202C. These conditions have been found to produce a uniformly distributed crystal structure with relatively small crystallite dimensions. The melt is introduced into the cavity of the preform at the closed end of this preform and force to fill the remaining cavity by the corresponding injection pressure. Injection molds that operate cold or operate hot, and thermal or mechanical closure devices, can be used. Furthermore, the exact geometry of the flow channels of the melt does not seem to be critical, as long as a good balance of flow and cooling is achieved, between the preforms from one cavity to another. Because the geometry of the typical reheat system has an influence on the heating pattern along the length of the preform, it has been determined that the wall thickness of the preform must vary throughout the length, in order to compensate the variations in the energy which makes an impact on the preform and which is imposed by the design of the heating system itself. As illustrated in Figure 1, a preform 10 is obtained, generally cylindrical, with a closed end 12, a main body portion 14, a neck 16 and a threaded open end 18. The walls of the preform, # both in the Closed end 12 as in neck 16, are thinner than the wall of the preform of main body portion 14, to compensate for the fact that the energy intensity is lower at the edges of a heater panel. This variation in heat absorption and the resulting change in the axial temperature profile, in accordance with the change in axial thickness, has been found to be very important in creating a uniform resistance for blowing over the entire length of the preform . This results in a • reheated preform which is more uniform in resistance for the length of it. The variation in the wall thickness of the preform is obtained more easily by forming the cavity of the injection mold, so that the resulting preform decreases in thickness at both ends thereof. The preform is obtained with a decrease of the thickness in both of its ends between approximately 40 to 70 percent. To produce a • Typical container, molecularly oriented, the preform can have a decrease in thickness of approximately 5 mm in half to 3 mm in both of its ends. 2. REHEATING THE PREFORMS Once the preform has been produced with the geometry and morphology of the desired crystals, this preform is allowed to cool to a substantially uniform temperature, which is at least below the temperature at which the melting begins. of the crystals in the material. The preforms are allowed to cool generally at room temperature. The preforms must then be reheated before the stretch and blow molding operation. The reheating of a plastic material, partially crystalline, molecularly orientable, in particular the PP, of the preform, before stretch and blow molding, is critical. This is because an accurate temperature of the preform must be achieved, which is uniform across the wall of the preform and to which the intercrystalline material melts, but to which the crystallites or crystal nuclei by itself do not they melt Thus, the PP preform must be brought to a temperature just below its peak crystalline melting point. Also, the temperature across the entire wall thickness of the preform must be brought within this relatively narrow temperature range, in order to achieve F the optimal orientation and clarity of the container. Any increase in temperature above this point will cause the crystallites to melt, thus eliminating any possibility of obtaining optimal orientation during the subsequent operation of stretch and blow molding. The preform is preferably exposed to a controlled array of infrared heaters, to reheat the preform to a temperature just below the melting point of the crystals of the material. It has been found that PP is very transparent to the energy of I-rays at wavelengths of 1.2 to 1.6 microns. Therefore, heating a relatively thick preform with IR energy, which is not easily absorbed by the preform, results in rapid and uniform heating through the wall thickness, although this process by itself is fundamentally ineffective since the point of view of total energy absorption. The heating system is designed to partially melt the polypropylene crystals, leaving a fine structure of crystal cores for subsequent crystallization, during the stretching / orientation process. It has also been discovered that the latent heat of the crystallization of the PP can be used to "dampen" the overheating condition, in combination with the cooling air, which is directed over the surface «Of the preform, as long as this preform is inside the heating zone. That is, the preforms are introduced to an IR-ray heater arrangement, while simultaneously submitting to the surface cooling air to prevent overheating / melting of the outer wall of the preforms. The amount of heat absorbed on the external surface is greater than the heat absorbed by the underlying layers, resulting in the outer surface being heated * at a higher temperature, more rapidly than the underlying layers. As illustrated schematically in Figure 2, the preforms 10 are loaded onto a series of rotary mandrels, mounted to a conveyor chain 20, which transports the preforms 10 through the following heating stages. The preforms are held in proximity to a first IR heating panel 22, until the temperature of the preform, near its outer surface, exceeds the temperature at which the intercrystalline material begins to melt, and begins to approach the temperature at which all the crystalline material, near the outer surface, melts. As mentioned above, however, this overheating condition is partially controlled by the fact that the melting of crystals is an endothermic process, and for a short time the additional heat absorbed on the surface of the preforms does not cause a rise in temperature. # temperature. The heating panels all preferably comprise high intensity, IR heating quartz tubular panels. A reflective protector 24, preferably formed of a highly polished aluminum sheet, extends substantially along the length of the conveyor chain and is parallel to each of the heating panels with the conveyor chain that goes approximately to the end of the conveyor belt. Tad between them. The reflective protector is provided with slits which communicate with a plurality of cooling air supply ducts, 26a-26f, and serve to direct this cooling air to the outer surface of the preforms. There is preferably a separate conduit for each heating and cooling zone, so that the amount of the cooling air can be controlled separately for each zone. The preforms are removed from the first panel of • heating 22 and are introduced to a cooling zone 23, where a second source of cooling gas, preferably air at room temperature, is supplied through conduit 26b, in order to allow the temperature gradient developed through of the wall thickness of the preforms is substantially equalized by thermal conduction. The amount of cooling air, used during this stage, is greater than that used «While the preforms are adjacent to the heating panel, which is supplied by the duct 26a, but can not be so excessive as to re-cool the surface of the preform, since doing so could impair the final clarity of the bottles blown These reheating and cooling steps are repeated at least once more and, more preferably, twice as long, the preforms advancing by additional panels, 25 and 30, of IR heating, the latter having a relatively lower heating intensity than the initial panel of heating, and followed by the associated cooling zones, 29 and 31. During the cooling stages, the temperature of the preform close to its outer surface is decreased, while the inner layers of the preform wall continue to raise the temperature . Thus, the average temperature of the preform rises relatively quickly, while the temperature gradient % oil per wall of the preform decreases, as this preform advances through the reheat system. The preforms are then preferably introduced into an air convection tempering furnace, while adjusting the desired temperature for the orientation in the stretching and blowing molding, which is a temperature above the temperature at which the structures are structured.

Inter-crystalline ras in the preform begin to melt, but below the temperature at which all the crystal structures in the preform melt. The preforms remain inside the oven for a sufficient time to allow any remaining temperature gradient, which developed through the thickness of the preform, to be substantially equalized.

* The preforms, which have been reheated uniformly at an appropriate orientation temperature, are rapidly transferred to a blow-stretch molding apparatus. 3. STRETCH MOLDING AND BLOWING OF PREFORMS Once reheated to the desired orientation temperature, the preform is immediately placed, in a known manner, into the mold cavity, of the stretching and blowing molding apparatus. The preform is stretched * axially by the use of an internal stretching rod, which joins the closed end of the preform. This preform is radially stretched by the introduction of an internal blow pressure at the open end of the preform, until it is in compliance with the walls of the mold cavity. The axial stretching and radial blowing of the preform can be performed either in sequence or simultaneously.

* According to a preferred embodiment of the present invention, the axial stretching is provided by an internal stretching rod, which is operated at rates of about 110 to 330 cm / sec, and preferably at about 170 to 270 cm / sec. Some volume of blowing air is introduced simultaneously into the open end of the preform to radially expand the preform and substantially conform it to the mold. After the stretching rod is fully extended, a higher blowing pressure is introduced in order to completely shape the preform to the mold. Finally, the stretched material is rapidly cooled against a polished mold surface, which is preferably at least in an S finish. P. I. of grade A.3. Such a finish can be produced, for example, with a diamond polish of grade # 15. EXAMPLE * The following example is illustrative of the present invention and does not constitute any limitation with respect to the subject matter of this invention. A 50 ton horizontal Boy injection press, commercially available from Boy Machines Inc., was used to mold a preform of a propylene / ethylene copolymer resin, grade 5746, from Hoechst AG, which contains 2.5 to 3.0 % ethylene and 97.0 to 97.5% of % propylene, and has a melt flow index of about 15. The injection press was equipped with a cylinder with a diameter of 38 mm and a screw for general purposes. The temperature of the cylinder was maintained at approximately 2102C. The injection press was operated at a general injection cycle of 40 seconds, as follows: the injection fill time was 7 seconds; the injection relay regime was 4.7 grams / second; the injection retention time was 7 seconds, and the cooling time was 23 seconds. The temperature of the mold remained at about 5.72c. The mold cavity was formed so as to produce a hollow preform, generally cylindrical, having a closed end and an open end, and weighing approximately 33 g. The preform has a length, if finished, of 9.12 cm, an internal diameter of the main body portion of the preform of 1.98 cm and an external diameter of the main body portion of the preform d 3.06 cm. The thickness of the wall of the preform was 0.54 cm in the main body portion, 0.20 cm in the neck and 0.30 in the closed end or end cap. The preform was allowed to cool to room temperature and transferred to a reheat station. This preform was placed with the open end downward * on a mandrel, which extends vertically from a conveyor. The conveyor speed is 4.39 c every 4.58 seconds, taking the preform between a series of IR, high intensity, tubular, quartz heating panels (available from Innovative Industries, Inc.) and its parallel reflective protector, formed from a sheet of highly polished aluminum. Each of the heating panels was arranged, generally vertically, with its bottom * approximately at the level of the open end of the preform. The reflector shield was provided with slits, which communicate with a series of cooling air supply ducts. The heating panels and the parallel reflective protector are spaced approximately 10 cm apart, with the conveyor line placed approximately halfway between them. The preforms were previously rotated for about 18 seconds, as they approach the first heating panel, the mandrel rotates at a speed of approximately 50 to 60 rpm. The preform was then heated for approximately 41 seconds, as it passes through the first heating panel in 9 inadvertent stages, while continuously rotating. The first heating panel was supplied with 1,000 Watt heaters at a central spacing of 16 mm, with the following power settings: Level 1 54.0% (adjacent to the neck) * Level 2 40.0% Level 3 28.0% Level 4 28.5% Level 5 26.0% Level 6 30.0% Level 7 10.0% While passing through the first heating panel, the cooling air, at room temperature, • was directed to the preform through the slits in the reflector shield at a speed established approximately 40% of the blower capacity of the systems. The preform was then passed beyond the first heating panel to a section of the line devoid of any heating panel, and was thus cooled for about 18 seconds. The cooling air at room temperature was directed to the preform through the slits in the reflector shield at a set rate which is about 60% of the blower capacity of the systems. The preform was then heated for about 41 seconds, as it passes through a second heating panel in 9 indexed stages and while continuously rotated. The second heating panel was supplied with 1,000 Watt heaters at a center spacing of 16 mm, with the following adjustments: Level 1 54.0% (adjacent to the neck) Level 2 40.0% Level 3 28.0% Level 4 28.5% Level 5 26.0 % Level 6 30.0% Level 7 10.0% While passing through the second heating panel, the cooling air, at room temperature, was directed to the preform through the slits in the reflector shield at a set speed, which It is approximately 40% of the blower capacity of the systems. The preform was cooled again for about 18 seconds by its passage beyond the second heating panel to a section of the line devoid of any • heating panel. The cooling air, at room temperature, was directed to the preform through the slits in the reflector shield at a set speed, which is about 60% of the blower capacity of the systems. The preform was then heated for about 41 seconds in 9 indexed stages, going through a third heating panel, while continuously rotating. The third panel of # heating was supplied with 1,000 Watt heaters at a center spacing of 16 mm, with the following adjustments: Level 1 47.0% (adjacent to the neck) Level 2 24.0% Level 3 22.0% Level 4 22.0% Level 5 25.0% Level 6 25.0% Level 7 0%. While passing through the third heating panel, the cooling air, at room temperature, was directed to the preform through the slits in the reflector shield at a set speed of approximately 40% of the blower capacity. of the sis topics. The preform was cooled again for about 18 seconds by passing it beyond the third heating panel a section of the line devoid of any heating panel. The cooling air, at room temperature, it was directed to the preform through the slits in the reflector shield at a set speed which is approximately 60% of the blower capacity of the systems. The mandrel, and thus the preform, was continuously rotated at a speed of 50 to 60 rpm, while advancing through the heating and cooling zones, described above. The preform was then transferred to a convection tempering oven, set at 1352C, for approximately 27.48 seconds. The preform was removed from the furnace and quickly transferred to the stretch blow molding apparatus from the RBU 225 system, available from Bakum% Maschinenfabriken GmbH. The preform was molded by stretching and blowing under the following conditions: the speed of the stretching rod was 0.11 seconds for a full travel of 245 mm; the low pressure was adjusted to 30% of the maximum of about 40 bars; and the low pressure time was 0.11 seconds. Once the stretch rod was fully extended and the preform had expanded to conform substantially to the mold, the pressure greater than about 70% of the maximum of about 40 bars was applied for about 2.5 seconds, to cause the preform to be completely conform to the mold. The temperature of the mold was about 5.72c. A container of exceptional clarity and good distribution of the material was obtained. In accordance with the provisions of the patent statutes, the present invention has been described in what is considered to represent its preferred embodiment. However, it should be noted that the invention can be practiced otherwise than specifically illustrated and described, without departing from its spirit or scope.

NOVELTY OF THE INVENTION Having described the present invention, it is considered as a novelty and, therefore, the content of the following is claimed as property:

Claims

CLAIMS 1. In a method for the injection molding of a closed end preform, from a plastic material, partially crystalline, molecularly orientable, and the subsequent molding by stretching and blowing, in which a molten mass of material is injected within a mold cavity of the preform at a predetermined filling rate, the improvement comprising controlling the filling rate to minimize the induced orientation of the molecular flow and the stresses developed in the plastic material by its injection into the mold , this filling rate is approximately 3 to 5 grams / second. A method, according to claim 1, further comprising the step of rapid cooling, after its injection into the mold of the preform, at a cooling rate of about 4 to 8 se / second. * 3. A method, according to claim 1, in which the partially crystalline, molecularly orientable plastic material is a polymer containing polypropylene. 4. A method, according to claim 3, in which the plastic, partially crystalline, molecularly orientable material is a propylene / ethylene copolymer, which has more than 50% by weight of propylene. A method, according to claim 4, wherein the copolymer has a melt flow index of about 2 to 50. 6. A method, according to claim 4, wherein the copolymer has a flow index of melting from about 10 to 30. 5. In a method for injection molding a hollow preform, generally cylindrical, having a closed end and an open end, from a plastic material, partially crystalline, molecularly orientable, for the subsequent molding by stretching and blowing, in which a molten mass of material is injected into a mold cavity of the preform, the improvement comprising forming the mold cavity so that the resulting preform decreases in thickness at the closed end and at the open end of this preform. A method, according to claim 7, in which the preform is obtained with a decrease in thickness in both its open end and its closed end, in an amount of about 40 to 70%. 9. A method, according to claim 8, in which the preform is obtained with a decrease in the thickness of both of its ends in an amount of 2 to 4 mm. # 10. A method, according to claim 9, wherein the preform is obtained with a decrease in thickness in both of its ends of about 3 mm. 11. A closed-end preform, of a partially crystalline, molecularly orientable plastic material, for subsequent molding by stretching and blowing, this preform comprises an open end and a closed end, with a hollow body portion, generally F Cylindrical, among them, the thickness of the body portion decrease the open end and at the closed end. 12. A preform, according to the claim 11, in which the partially crystalline, molecularly orientable plastic material is a polymer containing polypropylene. 13. A preform, according to the claim 12, in which the partially crystalline, molecularly orientable plastic material is a propylene / ethylene copolymer, which has more than 50% by weight of propylene. . 14. A preform, according to claim 11, in which this preform is obtained with a decrease in thickness in both its closed end and its open end, in an amount between approximately 40 and 70%. 15. A method for reheating a hollow preform, generally cylindrical, having a closed end and an open end and made of a partially crystalline, molecularly orientable plastic material, prior to stretch casting and blow molding the preform, to produce a molded, molecularly oriented container, which comprises, in sequence, the steps of: I) exposing the preform to a radiant heat source, for a time sufficient to raise the temperature, near the outer surface of the preform , at a temperature at which the intercrystalline structures begin to melt and which approaches, but does not exceed, the temperature at which all crystal structures near the outer surface of the preform melt; II) directing a cooling gas against the outside of the prm; and III) repeating steps 1 and 2, at least one more time. 16. A method, according to claim 15, wherein following the step III, the prm is transferred to a convection oven, maintained at the desired final temperature, for molding, by stretching and blowing, this prm. 17. A method, according to claim 15, wherein the cooling gas is directed against the prm during step I). 18. One method, according to the claim 17, in which the speed of the cooling gas directed against the preform, during stage I), is less than the speed of the cooling gas directed against the preform during stage II). 19. One method, according to the claim 18, in which the cooling gas directed against the preform during both stages I) and II), is comprised of air at ambient temperature. 20. A method, according to claim 15, wherein the radiant heat source is an arrangement of high intensity quartz infrared quartz heaters. 21. A method, according to claim 20, wherein the preform is exposed to infrared energy at wavelengths of about 1.2 to 1.6 microns. 22. A method, according to claim 15, wherein the preform is heated to a temperature, which minimizes the melting of the crystal cores through the wall of this preform. 23. A method, according to claim 15, wherein the preform comprises an open end and its closed end, with a hollow body portion, generally cylindrical, between them, the thickness of this body portion decreases at the open end and at the closed end. 24. A method, according to claim 15, in which the preform is mounted on a conveyor and passed through a series of infrared heating panels, spaced apart by the length of the conveyor. 25. A method, according to claim 15, wherein the preform is mounted on a conveyor and passed through a series of three infrared heating panels, for the length of the conveyor, this heating panel to which it is first exposed is adjusts to an energy greater than the energy at which at least one of the other heating panels fits. 26. A method, according to claim 15, wherein the angular rotation is imparted to the preform while this preform is exposed to a source of radiant energy. 27. A method, according to claim 26, wherein the preform is provided with an angular rotation before being exposed to the source of radiant energy. 28. A method for forming a hollow preform, generally cylindrical, having a closed end and an open end and obtained from a plastic material, partially crystalline, molecularly orientable, reheating this preform and molding it by stretching and blowing, in compliance with a mold, to produce a molded, molecularly oriented container, this method comprises the steps of: I) injection molding the preform; II) cooling the preform to a uniform temperature, below its crystalline melting point; III) and simultaneously: i) passing the preform through a heating zone, containing a source of radiant energy: ii) imparting an angular rotation to the preform, both of which is within the heating zone; and iii) directing a gas, which is at a temperature below that which would cause the fusion of the entire crystal structure in the preform to the outer surface of this preform; and IV) rapidly transferring the preform to a mold zone and securing it within the mold: V) stretching the preform axially, with an internal stretching rod, which is attached to the closed end of this preform; and VI) create a pressure differential, sufficient to expand the preform in accordance with the mold. 29. A method, according to claim 28, in which the preform is provided with an angular rotation, before being transferred to the heating zone. 30. A method according to claim 28, further comprising the step of removing the preform from the radiant heat zone and directing a cooling gas to a temperature below which would cause the melting of the entire crystal structure in the preform, against the outer surface of this preform, before stage IV). 31. One method, according to the claim 30, which further comprises repeating step III) and then removing the preform from the radiant heat zone and directing a cooling gas to a temperature below which would cause the melting of the entire crystal structure in the preform against the outer surface of this preform, before stage IV). 32. One method, according to the claim 31, which further comprises the step of introducing the preform into a convection tempering furnace, which is adjusted to a temperature at which the intercrystalline structures of the preform melt, but which is below the temperature at which all the crystal structures in the preform melt, allowing any temperature gradient, developed through the thickness of the preform, to be matched substantially by the thermal conduction, and then transferring the preform to, the mold zone in the stage IV). 33. A method for forming a molded, molecularly oriented vessel, this method comprises the steps of: I) injection molding a hollow, generally cylindrical preform, having a closed end and an open end, from a partial plastic material crystalline, molecularly orientable mind, in which the mold cavity is formed such that the resulting preform decreases in thickness at the closed end and the open end of this preform; II) cooling the preform to a uniform temperature, below the temperature at which the intercristall material not in the preform begins to melt; III) and simultaneously: i) passing the preform through a heating zone containing a radiant heat source; ii) imparting an angular rotation to the preform, t while this preform is within the heating zone; and iii) directing a cooling gas to the outer surface of this preform; and IV) removing the preform from the radiant heat zone and directing a cooling gas against the outer surface of the preform; V) quickly transferring the preform to a mold zone and securing this preform within a mold; VI) and simultaneously: i) stretching the preform axially with an internal stretching rod, which is attached to the closed end of the preform; and ii) introducing a certain volume of blowing air into the open end of the preform to radially expand this preform; and VII) create a pressure differential, enough torque to expand the preform in full compliance with the mold. 34. A method, as defined in claim 33, wherein steps III) and IV) are sequentially imparted at least once before transferring the preform to the molding zone. In testimony of which we sign the present in Mexico, D.F. to October 27, 1994.