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163 PACKAGING OF FOOD IN GLASS CONTAINERS can detect foreign materials such as ferrous and stainless metals, glass particles, stone, bone and plastic materials. This equipment runs at 100 m minâ1. 6.4 Closure selection Closures for glass packaging containers are usually metal or plastic, though cork is still widely used for wines and spirits. Effecting a seal is achieved either by a tight fitting plug, a screw threaded cap applied with torque in one of several ways or a metal cap applied with pressure and edge crimping. Hermetic or airtight sealing can be achieved by heat sealing a flexible barrier material to the glass usually with an overcap for protection and subsequent reclosing during use. The aluminium foil cap applied to a milk bottle is one of the simplest forms of closure. All these closures are applied to what is known as the finish of the container. This may seem an odd name for the part of the container which is formed first but in fact this name goes back to the time of blowing and forming glass containers by hand when the rim was the last part to be formed and therefore called the finish. Four key dimensions determine the finish as shown in Figure. 6.8. Industrywide standards for these dimensions have been agreed upon. The contour of glass threads are round, and closures, both metal and plastic, with symmetrical threads will fit the appropriate containers. T E l T E H H Lug closure Continous thread (C.T) screw closure I Diameter at smallest opening inside finish T Thread diameter measured across the threads E Thread root diameter H Top of finish to top of bead or to intersection with bottle shoulder on beadless designs Figure 6.8 Standard finish nomenclature (courtesy of The Institute of Packaging).