Patent Application
20050158495
The present invention relates to blow molded plastic containers, to preforms for blow molding such containers, and to methods of making such preforms and containers.
In the manufacture of plastic containers, it is conventional to injection mold or compression mold a container preform having a body and a finish with one or more external threads. The preform finish typically is molded to its final geometry, while the body of the preform subsequently is blow molded to the desired geometry of the container body. The preform may be of monolayer construction, or may be of multilayer construction in which one or more intermediate layers in the preform body may or may not extend into the finish area of the preform. U.S. Pat. Nos. 4,609,516, 4,710,118 and 4,954,376 illustrate injection molding of multilayer container preforms.
Molding the finish portion of the container as part of the preform presents a number of problems. For example, when the preforms are formed by injection molding, the plastic material typically is injected into a mold cavity at the closed end of the preform body, so that the material must flow along the sides of the preform cavity into the area in which the finish is molded. The finish typically requires more accurate and stable dimensioning than the body of the preform, which may limit the cycle time of the molding process. Furthermore, the difficulty in forming the finish limits how thin the wall of the preform body can be. By the time a thick preform finish fills, the sidewall of the preform body can begin to cool if it is too thin. If the finish is made thin to limit the amount of time needed to fill the finish, the finish may not fill fully, and may not have the desired strength for securement of a container closure. It is a general object of the present invention to provide a method of making container preforms and containers, a preform and a container that address one or more of the noted problems in the art.
A hollow plastic container in accordance with a presently preferred aspect of the invention includes an open finish, a closed bottom and a sidewall connecting the finish to the bottom. The container sidewall is ultra thin, preferably having a radial wall thickness of not more than 0.008 inch, and more preferably in the range of about 0.003 to 0.004 inch thick. (All references to radial sidewall thickness refer to average radial wall thickness, and not to isolated thin or thick spots in the wall.)
A method of making a container in accordance with another aspect of the present invention includes molding a preform having a neck with an open end, a closed bottom and a sidewall with a thickness of not more than 0.08 inch, more preferably in the range of about 0.04 to 0.05 inch. The sidewall and bottom of the preform are blow molded to form a container having a wall thickness of not more than 0.008 inch, and more preferably in the range of about 0.003 to 0.004 inch. In the preferred embodiments in accordance with this aspect of the invention, a separately formed finish ring is attached to the neck of the preform either prior to or subsequent to blow molding the preform body.
In accordance with another aspect of the present invention, a thin-wall container is provided that has particular utility for hot-fill applications—i.e., applications in which the container is filled with hot fluid product at a temperature of about 185° F. (Different packagers use different hot-fill temperatures, which typically are about 185° F.) The container sidewall has a thickness of not more than 0.008 inch, and a stretch ratio of about eight to ten as compared with the preform prior to blow molding.
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:
Finish ring 14 may be circumferentially continuous or circumferentially split to facilitate attachment to preform 12. Finish ring 14 includes an annular cylindrical wall 22 having one or more external threads or thread segments 24. A support flange 26 extends radially outwardly from the lower end of wall 22, giving finish ring 14 a generally L-shaped cross section in the illustrated embodiment of the invention. Finish ring 14 may also include a bead for cooperating with tamper-indicating structure on the package closure. Finish ring 14 is externally secured to the surface of preform neck 18 by any suitable means, such as insert molding, press-fit, adhesive, ultrasonic welding, etc. Finish ring 14 may be secured to neck 18 either prior to or subsequent to blow molding container body 28 (
Preform 12 may be of monolayer construction as illustrated in
In accordance with one aspect of the present invention, preform 12 has a wall thickness that is greatly reduced as compared with the prior art—i.e., an ultra thin wall thickness. Wall 16 has a radial thickness of not more than 0.08 inch, most preferably in the range of about 0.04 to 0.05 inch. Such ultra thin wall thicknesses can be employed in accordance with this aspect of the invention because finish ring 14 is separately formed and secured to the neck of the preform body, rather than being molded integrally with the preform body and thereby limiting the thinness of the preform body as previously discussed. When subsequently blow molded to form a container 28 (
The preform and container geometries illustrated in the drawings are intended to be generic, with the principles of the present invention not being limited to any specific preform or container geometry.
Provision of very thin-walled preforms in accordance with one aspect of the present invention has a number of advantages over the prior art. In the prior art, very small preforms of normal wall thickness are used to blow mold thin-walled containers. However, the blow molding operation results in high levels of molecular orientation in the container wall, with the preform-to-container stretch ratio typically being on the order of about ten to twelve. These high stretch ratios result in severe shrinkage and distortion during hot fill applications. However, a larger preform having thin walls is used in accordance with the present invention, reducing the stretch ratio to a range of about eight to ten, which will not shrink and distort during hot fill. Furthermore, the use of a thin-wall preform yields more uniform stretch orientation levels across the thickness of the container wall.
Thin-wall preforms and containers have a further advantage in hot fill applications in that vacuum panels molded into the container wall operate very efficiently because of the reduced wall thickness. That is, the reduced thickness of the panels permits the panels readily to flex inwardly as the hot-filled package cools, reducing distortion to the remainder of the container. Whereas containers for hot fill applications normally have a wall thickness of about 0.012 to 0.015 inch and are blown from a preform having a wall thickness of about 0.15 to 0.18 inch, the present invention provides a container for hot fill applications having a wall thickness of not more than 0.008 inch blown from a preform having a wall thickness of about 0.08 inch.
The thin-wall preforms of the present invention possess the further advantage that they are less sensitive to moisture than are preforms having thicker walls. For example, it has been found that preforms having a moisture content of 2000 to 3000 ppm can be blow molded satisfactorily. Thin-wall preforms also possess the advantage that they can be blow molded around tighter radii on the mold face, providing additional flexibility in container design.
It has been found that the thin-wall preforms of the present invention possess an unexpected additional advantage. During reheat of the preforms prior to blow molding, thermal crystallization occurs in the preform wall, followed by strain crystalization during blow molding. During tests, up to 28% to 35% total sidewall crystallization was obtained, which improved hot-fill performance of the resulting container without affecting the clarity of the container wall. Total sidewall crystalinity can be measured employing any suitable technique, such as the ASTM 1505 density gradient column technique.
There have thus been disclosed a container, a preform assembly, and methods of making the container and preform assembly that fully satisfy all of the objects and aims previously set forth. The invention has been disclosed in conjunction with a presently preferred embodiment thereof, and modifications and variations have been discussed. Other modifications and variations will readily suggest themselves to persons of ordinary skill in the art. The invention is intended to embrace all such modifications and variations as fall within the spirit and broad scope of the appended claims.
