black Art of Aerosol Crimping The a range of 0.006". A common practice is for the quality control person to check one can for each rotary crimper or U-t-C gasser station, recording the results on paper or by computer. Additionally, if a gas house operator is utilized, that individual should take readings as time permits. They need not be recorded, but any anomalous results should be reported immediately. Some defects, such as crimps from a collet with a broken segment, will be detected right away as gross leakers in the hot water bath. Because of the multiplicity of can constructions, valve mounting cup designs, metal thickness, gaskets and so forth—each with a dimensional tolerance—the aerosol industry is still actively working toward the development of crimp recommendations that may or may not include depth and diameter suggestions for various combinations of cans and valve mounting cups. This is a delicate subject for legal reasons. A crimp diameter of about 1.07" is often used, but ones centering on 1.05", 1.065" and 1.075" exist as well. The shallower crimps normally have a slightly reduced depth so that they will strongly contact the metal of the can throat. At an excessive diameter, there may be more of a chance that the cup metal will be fractured, or that the gap between collet segments will be humped up sufficiently to cause problems. The crimp depth is often established as about 0.183" for ETP cans and ETP mounting cups having PP laminates, sometimes noted as “ETP/ETP-PP”. The dimension has come about empirically, and will be somewhat different for various fillers and marketers, due to the larger number of elements involved and their dimensional tolerances. For example, the total thickness of both the can top and PP laminated mounting cup can vary by up to about ± 0.002", and this is before all three are deformed to various degrees. The crimping toe should first touch the pressed down valve cup metal at the PHC, actually an imaginary horizontal line, at a depth about even with the seven o’clock position of the outside of the can curl. When the toe pushes out to its full diameter, the curl will then be only negligibly deformed. If the toe contacts the mounting cup higher up, say in the eight- or nine-o’clock curl levels, the can curl will be seriously deformed and the downward force vector diminished so that propellant leakage may occur, either immediately or latently. The misshapen curls are sometimes called “pear curls” or even “banana curls.” The higher back pressure on the toe, from deforming not only the cup but the can curl, will also cause excessive collet segment breakage in many designs. On the other hand, if the crimping collet depth is set too deeply, the expanding toes will catch more air than metal, as the can throat widens. The pull-down force vector is then diminished and the PP laminate is squeezed to a much lesser extent at the crimp. Both of these factors invite gas leakage. When crimping ETP cans with ETP mounting cups that use a lathe cut sealing gasket, shown as ETP/ETP-LC, the gasket is much thicker than the PP laminate, causing the cup to ride higher in the can opening or plug. The crimp depth must be increased to account for this difference. A dimension of about 0.203" is often used. This was determined empirically by tedious trial and error studies and is not a specification, but merely a guideline. Aluminum cans now make up 21.2% of U.S. aerosol volume. Since the metal is not as strong as steel it, is made thicker. The curl thickness will vary with can diameter and designed pressure resistance. While the “one inch” hole specification of 1" ± 0/004" 32 Spray March 2014
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