black Art of In 1952, when Al Glessner and his research group at Crown Cork & Seal Co. developed their 12oz “Spratainer” can with a novel “one inch” curled opening—to be used with valves seated in “one inch” mounting cups—they probably had no idea of the complexities that would later evolve. Most mounting cups were made with a “Flowed In D&A GK-45” neoprene rubber gasket with a wet weight of about 0.55 g of a typically 62% solids dispersion in an aromatic solvent mixture. A crimping process was required to hermetically seal the valve cup to the can curl. This was done by lowering a sixsegment collet to a precise depth in the mounting cup well, then expanding the segments to a precise diameter. The company provided guidelines of 0.185" for the depth and 1.065" for the diameter, noting that these were not specifications. In the 62 years since then, the “one inch” can opening has become the standard for all metal aerosol cans over 35mm in diameter. While the can bead (curl) dimensions are still patterned after the original 12oz Spratainer, there are small and subtle differences. We now use C/CO steel, tinplate and aluminum cans of various thicknesses. Cans are made plain, with various linings or polyethylene terephthalate (PET) laminates. The Flowed In gaskets for valve cups vanished by about 1975, and were replaced by polyethylene (PE) laminates, polypropylene (PP) laminates and PE Sleeves—of which only the 0.006" to 0.008" thick PP (bottom side) laminate survives today. The venerable lathe cut gasket, used for such products as pure propellants since 1953, still enjoys limited uses today. The usual thickness is 0.039" (1mm), but some are 0.045" and a Chinese import is 0.059" with a rounded top. They profoundly influenced the optimum crimp depth dimensions. There have been changes in crimping collets as well. Original collets were prepared from 1.065" diameter steel bar stock, with one end tapered and sawed into six tines. By about 1970 these collets were still the standard, but the preferred crimping diameter had changed from 1.065" to 1.070", so bar stock of this new diameter was used. It was also found that very thin abrasive wheels could be used to produce not only the six-segment collet, but an eight-segment and even a ten-segment tool. The eight-segment collet, with its smaller kerf or gap when expanded, became popular during the last few decades. To be complete, at least one major firm uses a customized collet, made from about 1.060" diameter steel bar stock, designed to produce crimp diameters of 1.055" ± 0.01". For quite a long time, collets have no longer been made as a single piece of tempered steel, but rather in the form of a header section, to which the individual segments are firmly attached by the use of a lock-ring. These are easier to produce and have less breakage. Another advantage of this design is that when a segment breaks (usually after millions of crimps), it can be quickly replaced at a low cost. In doing this, the replacement segment should be one that has seen about the same degree of wear as the one that was fractured. If a new segment had been used, the toe would extend to a slightly greater distance than the others, producing a bilobate, less reliable crimp. In the U.S., there are two major collet designs. The 1.75" long (Terco), and the 3" long (Aerofill-R.A Jones) are about equally popular. Both use a toe radius of 0.047" (3/64"). In other countries, collets may have a toe radius of 0.039". In collet manufacture, the oil annealing process is critical to success. Collets that are too soft will wear the toe (foot) too quickly, while very hard steel collets will become brittle and suffer from premature The 28 Spray March 2014
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