This invention relates to preforms for blow-molded articles, and more particularly, to antinesting preforms for blow-molded synthetic resinous containers.
Food and beverage containers formed of PET (polyethylene terephthalate), and particularly PET bottles for carbonated or effervescent beverages, have become ubiquitous throughout the principal industrial countries of the world.
PET bottles are commonly formed by the reheat-stretch-and--blow procedure. Though this procedure is well known, it may be described briefly as follows.
Tubular preforms, closed at one end and open at the other, are injection-molded of a suitable grade or formulation of PET. The neck finish, that is, the finish at the open end, is formed in the injection mold in a configuration to receive a suitable closure when the resulting container is filled.
The preforms are removed from the injection mold and allowed to cool to ambient temperature, and are later introduced to a blow-molding system where they are first heated to a temperature within the glass-transition range of the material, then mechanically stretched in the longitudinal direction, and finally blow-molded to final container configuration and dimensions. The mechanical stretching and blow-molding impart biaxial molecular orientation to the material of the container body, thereby enhancing the tensile strength of the body wall while reducing its thickness.
A number of machines and systems for performing this procedure are well known and commercially available.
The finished containers take a great variety of sizes and configurations. The problem of designing a PET container for a particular purpose presents a bewildering variety of considerations, among them not only capacity and aesthetic appeal, but economy of material, and importantly, whether the container is to be filled with contents under pressure, as in the case of carbonated beverages, or under vacuum, as in the case of hot-filled products.
When container configuration has been determined, an appropriate preform must be designed. Because of the need to conserve material and yet ensure that the material will be so distributed in the finished container that it will exhibit adequate resistance to deformation and rupture under all conditions of expected use, preform design has become an exact science, and an exacting one.
In the case of certain PET bottles intended to package highly carbonated beverages subject to internal pressures of three to four atmospheres, a preform configuration has evolved in which the outer diameter of the closed end is smaller than the internal diameter of the open end, whereby one preform may, by way of its open end, receive the closed end of an adjacent preform. Simply stated, such preforms are subject to nesting with one another.
Nesting is an impediment to safe and efficient handling. Heretofore, it was necessary to detect and separate nested preforms before damage could occur to them or to the equipment in which they were being processed. In many cases, detection and separation depended exclusively upon the watchfulness and dexterity of a human operator.