Plastic and nonmetallic article shaping or treating: processes – Direct application of fluid pressure differential to... – Including application of internal fluid pressure to hollow...
Reexamination Certificate
2001-04-13
2003-05-27
McDowell, Suzanne E. (Department: 1732)
Plastic and nonmetallic article shaping or treating: processes
Direct application of fluid pressure differential to...
Including application of internal fluid pressure to hollow...
C264S532000, C264S535000, C264S537000
Reexamination Certificate
active
06569376
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to a process for improving material thickness distribution within a molded bottle and the bottle made with the process. The bottle has a closed base or bottom, an open-end, a longitudinal axis, and a generally square, generally rectangular, or generally oval or other noncircular cross-sectional shape typically perpendicular to the longitudinal axis. In particular, the process is for improving material thickness distribution within the bottle base portion or bottom. The bottle base portion contains a chime, contact surface, and push-up region.
The process relies on a mold-cavity base or bottom region of a blow-molding tool modified with at least one generally straight standing rib. The rib (or two or more parallel ribs) alters mold-cavity bottom geometry that redirects material flow during bottle manufacture allowing better placement and distribution of material that in turn minimize unwanted shape distortions in the molded bottle that could otherwise occur.
The invention is suited for bottles made of polyester materials, such as polyethylene terephthalate (PET), or other polymeric materials. The invention is further suited for bottles generally made with an injection based manufacturing process or its equivalent.
2. Description of the Prior Art
Processors generally make bottles from a relatively hot pliable parison or preform using one of several well-known process technologies for making, heating or re-heating the preform, and forming the preform into the bottle. Air pressure inflates this hot pliable preform against a relatively cool cavity surface within the blow-molding tool to form the bottle configuration having approximately the same surface area and shape. Usually, the blow-molding tool is metal and typically aluminum. A molding technician sizes the preform so that, once inflated, the bottle has an appropriate wall thickness and reasonably uniform material thickness distribution throughout its surface.
Achieving good material thickness distribution using the injection based manufacturing process featuring an injection-molded preform is relatively easy for a bottle with a substantially circular cross-sectional configuration. An injection-molded preform is generally a tube with a longitudinal axis, a circular cross-sectional shape perpendicular to the longitudinal axis, and having a sidewall with a substantially uniform material thickness distribution, one open end, and one closed-end. The longitudinal axis of the preform before inflation typically will coincide with the longitudinal axis of the molded bottle made from that preform.
In general, industry uses two injection based blow-molding process technologies. In the first process, preform length is approximately the same as bottle height. Before inflation, the preform closed-end is adjacent or in close proximity to the mold-cavity section forming the bottle push-up. In the second process, preform length is substantially less than bottle height. Before inflation, a stretch-rod stretches the hot pliable preform in an axial direction corresponding with the preform longitudinal axis, generally pinning the preform closed-end against the mold-cavity section forming the bottle push-up before air pressure completely expands the preform in other directions. The industry generally refers to this second process as “stretch” blow molding or “biaxial molecular orientation” blow molding. The stretch blow molding process is particularly suited for manufacturing bottles of PET polymer materials.
While the technician strives for the uniform material thickness distribution in the molded bottle, compromises are still often necessary. Regions within the push-up, for example, will tend to have a relatively thicker wall section than most other regions of the molded bottle.
Bottles with generally rectangular or oval cross-sectional shapes, shapes with its inherent major or primary axis and minor or secondary axis generally perpendicular to the bottle longitudinal axis, are often difficult to blow-mold when made with the injection-molded preform. Regions of the inflating preform that must move and stretch a greater distance in a direction generally corresponding with the major or primary axis of the bottle cross-sectional shape will tend to thin more than regions that move in a direction generally corresponding with the minor or secondary axis. Consequently, material thickness distribution is not uniform. The bottle wall thickness adjacent to the ends of the primary axis will tend to be thinner than the bottle wall thickness adjacent to the ends of the secondary axis.
Molding technicians have a number of techniques to improve the material thickness distribution of rectangular or oval bottles, that is, techniques to establish a reasonably uniform material thickness distribution. One approach involves changing how quickly selected regions within the preform stretch by changing the material temperature in that region. A slightly cooler preform region in the preform sidewall will tend to resist and stretch less than adjacent warmer regions. Aligning the cooler preform regions with corresponding areas of the bottle that tend to have an otherwise relatively thinner wall thickness will consequently stretch less thus improving material thickness distribution uniformity.
While this approach, sometimes known as “heat profiling,” is effective for improving material thickness distribution uniformity within the sidewall, it is generally not effective for improving material thickness distribution uniformity within the push-up and the molded bottle base or bottom portion. This ineffectiveness is primarily for two reasons.
First, the preform closed-end, the region that forms the bottle base and push-up and having a generally hemispherical shape, is relatively small. While it is feasible to heat profile the entire closed-end to a specific temperature, it is not practical, because of its small size, to heat profile sub regions within the closed-end. Consequently, the heat profiling of the entire preform closed-end region is a compromise generally favoring a need for greater movement in the direction corresponding with the major axis of the bottle cross-sectional shape.
Second, the wall thickness in an area of the push-up surrounding the longitudinal axis of the molded bottle remains relatively thick because the preform closed-end region of the inflating preform has little opportunity to stretch or thin before contacting the relatively cool cavity surface of the blow-mold tool forming the bottle configuration. The wall thickness of preform areas initially contacting portions of the bottle cavity surface will not significantly thin further as the remainder of the inflating preform continues to stretch and come in contact with remaining portions of the bottle cavity surface.
Consequently, the lack of effective heat profiling and the lack of sufficient stretch or thinning of the preform with its circular cross-sectional shape causes the material distribution surrounding the longitudinal axis of the bottle push-up and bottom to have a predominantly thick circular character. The molding process for bottles having a generally square, rectangular, or oval cross-section places this thick circular material distribution within the push-up and base having a corresponding square, rectangular, or oval character.
Overall, the relatively thick areas of the molded bottle tend to cool during manufacture at a slower rate. Consequently, the material within these thick areas is prone to warp and distort. Furthermore, molding technicians, attempting to increase production output, will often remove the bottle from the blow-mold tool before thick areas have sufficiently cooled risking additional distortion of those areas.
When the bottle stands in a typical upright fashion, a region of the base or bottom contacts a supporting surface. The distortions from differences in wall section thickness are generally not a problem with bottles having the circular cross-sectional configuration because these distortion
Gaydosh Kevin D.
LaBombarbe Christopher C.
Wurster Michael P.
Harness & Dickey & Pierce P.L.C.
McDowell Suzanne E.
Schmalbach-Lubeca AG
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