Heat set blow molding process

Details Heat set blow molding process Heat set blow molding process

2001-07-06

2002-06-18

McDowell, Suzanne E. (Department: 1732)

Plastic and nonmetallic article shaping or treating: processes

Direct application of fluid pressure differential to...

Differential temperature conditioning

C264S904000

Reexamination Certificate

active

06406661

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates to a heat set blow molding process. More particularly, the invention is directed to pretreatment of a preform just prior to heat set blow molding same, including increasing the temperature of the exterior surface layer of the preform before it is placed within a blow mold.
BACKGROUND OF THE INVENTION
Biaxially oriented bottles may be manufactured from plastic materials such as polyethylene terephthalate (PET) using both single stage and two-stage machinery. For example, when using the two-stage process, bottles can be manufactured using either of two distinctly different blowing methods. One method of blowing bottles is accomplished by heating preforms from ambient conditions to the lowest possible temperature (but above the glass transition temperature) which will allow for the proper stretching of the material followed by blowing the heated preform into a cold blow mold as rapidly as possible. This process can produce a bottle that has excellent properties for use in many packaging applications, especially for use as a carbonated soft drink bottle. An additional step of conditioning the preform to provide a homogeneous temperature or a temperature distribution across the wall of the preform may be combined with the basic process. The molecular orientation of the material improves the mechanical and optical properties of the ultimately produced container.
This biaxial orientation, however, also increases internal stresses within the container, thereby resulting in dimensional instability under hot conditions. The oriented material has a tendency to shrink, for example during hot filling of a so-produced biaxially oriented container, which relieves the internal stresses but which causes distortion and deformation of the container. This phenomenon is particularly evident when using amorphous polymer preforms which undergo strain induced crystallization during the drawing process, such as for example those made from polyesters, particularly PET.
Biaxially oriented containers which are manufactured for use as bottles for pressurized liquid are conventionally made using a process wherein the preform is blown into conformance with a chilled mold. But in order to increase the dimensional stability of biaxially oriented containers to make them useful for “hot fill” operations, it is necessary to thermo-fix or “heat set” the biaxially oriented container in such a way that substantially relieves the remaining internal stresses.
Heat set blow molding also enhances the crystallinity of the structure of a biaxially oriented container, as well as relieves some of the internal residual stresses so that the container will retain its shape when exposed to hot fill temperatures. Generally, the blowing process is modified to heat the preform to a temperature higher than that normally required to stretch the preform. This hotter preform is then blown into as hot a blow mold as possible. Typically during such a heat set blow molding process, great quantities of air or another gas are blown across the inner surface of the container being formed. The gas acts to cool and to set the inner surface of the container to produce a rigid structure. The introduction of the gas is necessary to prevent the container from shrinking since the outer surface of the container is still hot (above Tg) at the time the blowing gas pressure is relieved and the mold is opened. Moreover, the blowing speed is frequently reduced in a heat set blow molding process, to allow the blown container to maintain contact with the hot blow mold for a long period of time.
However, even utilizing the heat set blow molding process, there are limits to how hot the so-produced containers may be filled. Even heat set blow molded containers will exhibit some amount of shrinkage during the hot filling process, and the containers must, therefore, be designed to account for such shrinkage.
The overall blow mold quality of a well-designed heat set blow molded container may be determined by a combination of factors direct to the economics of the process (impacted by the speed of blowing) and the surface temperature of the hot blow mold (limited by the amount of cooling gas which can be blown through the container). For example, it is known that it is possible to manufacture superior heat set blow molded containers by utilizing hot blow molds at higher than normal temperatures, but this requires the use of great quantities of super-cold blowing gasses. These processes are more complicated and may not be cost effective.
U.S. Pat. No. 4,385,089 to Bonnebat et al. discloses a heat set blow molding process wherein the temperature of the hot blow mold is maintained at a level that is lower than that desirable for manufacturing high quality blow molded containers.
U.S. Pat. No. 4,883,631 to Ajmera discloses a heat set blow molding process wherein severe cooling fluids, such as for example liquid carbon dioxide, are utilized to off-set the tendency for shrinkage of the container in contact with the hot blow mold.
It must be noted that the prior art referred to hereinabove has been collected and examined only in light of the present invention as a guide. It is not to be inferred that such diverse art would otherwise be assembled absent the motivation provided by the present invention, nor that the cited prior art when considered in combination suggests the present invention absent the teachings herein.
It would be desirable to develop a heat set blow molding process for making high quality containers suitable for hot fill operations, which process employs high heat set temperatures without excessive use of cold blowing gasses.
SUMMARY OF THE INVENTION
Accordant with the present invention, an improved process for heat set blow molding a plastic container has surprisingly been discovered. The process comprises the following steps:
providing a plastic preform;
thermally conditioning the preform, to achieve a temperature profile across the wall of the preform wherein the inner surface of the preform is at a higher temperature than the outer surface of the preform, the temperature profile being within the molecular orientation temperature range of the plastic;
heating the outer surface of the preform, to increase the temperature of the outer surface of the preform by a minimum of about 10° C.; higher than the temperature that would normally result from a heat-set preform temperature profile;
enclosing the preform in a hot mold;
expanding the preform to form a container within the hot mold, by introducing a blowing gas into the preform to induce biaxial orientation in the preform and force the preform into conforming contact with the hot mold; and
maintaining the expanded preform in conforming contact with the hot mold for a time sufficient to induce at least partial crystallization of the plastic material.
The present invention is particularly well-suited for preparing plastic containers for hot fill applications, such as for example for filling with hot juices or sauces.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
The present invention is directed to an improved heat set blow molding process, comprising providing a plastic preform, thermally conditioning the preform to achieve a temperature profile across the wall of the preform wherein the inner surface of the preform is at a higher temperature than the outer surface of the perform, said temperature profile being within the molecular orientation temperature range of the plastic, then heating the outer surface of the preform to increase the temperature of the outer surface of the preform by a minimum of about 10° C., enclosing the preform in a hot mold, expanding the preform to form a container within the hot mold by introducing a blowing gas into the preform to induce biaxial orientation in the preform and forcing the preform into conforming contact with the hot mold, and maintaining the expanded preform in conforming contact with the hot mold for a time sufficient to induce at least partial crystallization of the plastic material.
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