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Reference is made to copending application Ser. No. 10/329,666 filed Dec. 24, 2002.
The present application is directed to plastic containers and preforms for blow molding plastic containers, and more particularly to a method of forming internal threads on the preform or container finish.
In the manufacture of plastic containers, such as monolayer or multilayer polyethylene terephthalate (PET) containers, it is conventional to mold a container preform having a body and a finish with one or more external threads. The finish typically is molded to its final geometry in the preform, while the body of the preform is subsequently blow molded to the desired geometry of the container body. However, this manufacturing technique is not well suited for fabrication of preforms or containers having internal threads because, among other reasons, it is difficult to strip a preform having molded internal threads from its forming mold. It is therefore a general object of the present invention to provide a method of forming internal threads in the finish of a molded plastic preform or container, and to provide a preform or container finish manufactured in accordance with such method.
A method of making a plastic container having a body and an internally threaded finish, in accordance with one presently preferred aspect of the invention, includes providing a preform of molded plastic construction having a body and an integral neck, and expanding the body of the preform within a mold cavity to form the body of the container. Either prior to or subsequent to blow molding the container body, at least one internal thread is formed in the neck by engaging an inner surface of the neck with a first rotary tool having a peripheral channel, engaging an outer surface of the neck with a second rotary tool having a peripheral bead, and causing rotation of the neck and tools to swage at least one internal thread in the neck.
The invention, together with additional objects, features, advantages and aspects thereof, will be best understood from the following description, the appended claims and the accompanying drawings, in which:
FIG. 1 is a perspective view of a container preform molded without threads in accordance with one aspect of the present invention;
FIG. 2 is a schematic diagram that illustrates swaging internal threads in the preform of FIG. 1;
FIG. 3 is a schematic diagram that illustrates swaging internal threads in a container molded from the preform of FIG. 1;
FIG. 4 is a sectional view of the portion of FIG. 3 within the area 4;
FIG. 5 is an exploded fragmentary perspective view of a preform or container finish assembly having both internal and external threads in accordance with another aspect of the present invention;
FIG. 6 is a schematic diagram that illustrates swaging internal threads in a preform having molded external threads; and
FIG. 7 is a sectional view of a preform having molded external threads and swaged internal threads.
FIG. 1 illustrates an exemplary container preform 10 having a body 12 and an integral neck 14. Neck 14 preferably is cylindrical in contour, and has no internal or external threads in this embodiment as molded. The specific preform 10 illustrated in FIG. 1 is molded with an external stop bead 16 for cooperating with tamper-indicating structure in a closure secured to the final container, and a support flange 18 that facilitates handling of the preform and the container. Bead 16 and flange 18 are optional. Cylindrical neck 14 is of generally uniform radial wall thickness. Preform 10 can be injection molded or compression molded of monolayer plastic construction such as PET, or of multilayer plastic construction having matrix or structural layers of PET, for example, and one or more intermediate layers of barrier material such as ethylene vinyl alcohol (EVOH) or nylon. The barrier layers may or may not extend into preform neck 14.
In accordance with one aspect of the present invention, internal threads are swage-formed in neck 14 either prior to or subsequent to blow molding preform body 12. As illustrated in FIG. 2, one or more internal threads 20 are swaged in preform neck 14 prior to blow molding preform body 12. These threads 20 are swaged into the preform neck by means of a first rotary tool 22 that engages the inner surface of neck 14, and a second rotary tool 24 that engages the outer surface of neck 14. Inner tool 22 includes a disk 26 with a peripheral channel 28. Outer tool 24 includes a disk 30 having a peripheral bead or rib 32. With tool 22 in engagement with the internal surface of neck 14 and tool 24 engaged with the external surface of neck 14, the tools (and preform) are rotated so that bead 32 deforms or swages the plastic material of neck 14 into channel 28 on tool 22. This deformation can be assisted and promoted, if desired, by heating one or both of the tools 22, 24. After the internal threads 20 are formed in neck 14, preform 10 is placed in a suitable blow mold, and preform body 12 is blow molded to form the body 12a (FIG. 3) of a container.
FIG. 3 illustrates an alternative technique for swaging internal threads 20 in preform neck 14 after blow molding container body 12a. Reference numerals in FIG. 3 that are identical to those in FIGS. 1-2 indicate identical parts. Whether the internal threads are swaged in the preform neck prior to or subsequent to blow molding the preform body, the neck 14 remains of substantially uniform radial thickness as illustrated in FIG. 4.
FIG. 5 illustrates a method of forming a preform or container having both internal and external threads in the preform or container finish. After swaging one or more internal threads 20 into the neck 14 of the preform or container (FIG. 2 or FIG. 3), a separately made finish ring 34 is secured over the external surface of neck 14. Finish ring 34 has one or more external threads 36 preformed therein. As a modification to FIG. 5, stop bead 16 and/or support flange 18 may be formed in ring 34 rather than in the preform. Thus, the final container in accordance with the embodiment of FIG. 5 includes both internal and external threads, with the internal threads being swaged into the container neck and the external threads being formed on a separate finish ring.
FIGS. 6 and 7 illustrate another method of making a preform having both internal and external threads. In this preform 40, the preform is molded with external threads 42 on the preform neck 44. After molding, and either prior to or subsequent to blow molding the preform body, swage tools 22, 24 are used to form internal threads 46 on neck 44.
It may be desirable to heat neck 14 during the thread forming operation. This heating preferably is carried out by conduction from tools 22, 24. The neck preferably is heated to a level above room temperature, such as in the range of 50° C. to 100° C. for PET finishes. The temperature of the finish neck preferably should not exceed the glass transition temperature Tg of the neck material by more than about 40 to 50° C. For example, finish neck heating to a level of 120° C. or above can cause distortion and warping of the neck. Five seconds of contact time is ample for thread formation when neck 14 has a thickness of 0.5 to 1 mm (0.019 to 0.039 inch). A lesser contact time, such as on the order of one second, may not be adequate unless the finish neck is very thin, which would not be desirable in the embodiments of FIGS. 1-4 but could be accommodated in the embodiment of FIG. 5.
Thus, one or more internal threads are swage formed on the E wall of preform and container neck 14. All or a portion of the preform or container neck can be crystallized before or during formation of internal threads 20 to prevent shrinkage or distortion. This would be particularly desirable in hot-fill applications, where the temperature of the material placed in the container otherwise might cause shrinkage or distortion of the container finish. Crystallization of the container finish can extend throughout the radial thickness and the axial length of the neck, including stop bead 16 and/or support flange 18 where provided, if desired.
There have thus been disclosed a preform, a container and a method of manufacture that fully satisfy all of the objects and aims previously set forth. The preform, container and method of manufacture have been disclosed in conjunction with several presently preferred embodiments thereof, and a number of modifications and variations have been discussed. Other modifications and variations will readily suggest themselves to persons of ordinary skill in the art. For example, tooling can be provided on rotary tools 22, 24 to swage-form the top sealing surface at the axially upper edge of neck 14. The invention is intended to embrace all such modifications and variations as fall within the spirit and broad scope of the appended claims.