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makes it unwise to try and crimp them with valve cups that are PP laminated for sealing purposes. Unless the relatively stiff polypropylene can be compressed down to the bottom of every groove, completely filling them, the product will leak propellant gas. If the leakage rate is less than about 10 to 15 grams per year, the product will almost certainly pass through the hot water bath with the defective situation undetected, since there will typically be only one gas bubble for about every 40" (1.0 m) of travel. This much leakage can be disastrous for many products and even faster leakages may not be detected if the operator is bored, distracted or absent. The relative hardness of PP laminates can sometimes be illustrated, even with aerosol cans having smooth and compatible curl surfaces. During hot water bathing, some of these cans may be seen emitting gas bubbles from around the cut edge (skirt) of the mounting cup, but after the first five or ten feet of travel the typical 136°F (58°C) hot water will soften the plastic sufficiently so that it will react to the crimp compression and move to seal off any tiny passageways. Any bubbling will then cease and long term weight loss will be negligible. Aluminum cans up to 38mm (1.50") in diameter may be safely clinched with valves having PP laminate mounting cups. The 45mm (1.77") cans are probably also acceptable, assuming good crimp dimensions and hot water bath surveillance. There are mixed opinions regarding the 50mm (1.97") diameter can, but those in larger diameters should definitely be crimped with valves where the cups are snugly fitted with lathe-cut rubber ring gaskets. One option, applied to larger diameter aluminum cans, is to lathe cut a diagonal chamfer into the outside area of the top of the textured curl. An angle of 21° may be used, and the metal thickness, while variable, is reduced by about 15%. The download resistance of the curl remains essentially unchanged. The cut is typically made with a special tungsten carbide or Stellite-tipped tool bit that is honed to produce an extremely smooth and sharp edge. The resulting aluminum/gasket interface will then hold back any propellant that has managed to seep through the micro-grooves. Considering that the compressed PP laminate thickness is about 0.006" at the midpoint of the top of the mounting cup, while the thickness of the lathe cut gasket is about 0.03" at the same location, it follows that the mounting cup will be about 0.024" higher with the lathe-cut gasket. The crimp depth should then be made about 0.024" deeper to meet the same PHC. This differential compares fairly well with the empirical figure of about 0.021". The lathe cut gasket is usually produced from fairly soft buna rubbers, although neoprene and butyl may be needed on rare occasions. The primary seal is made at the very top of the can curl, where (very roughly) over 250lb of compression acts to squeeze the gasket very hard against the apposing metal surfaces. The crimp is made metal-to-metal and is not a sealing factor. The crimp depth helps to provide gasket compression, while the crimp diameter assists and also prevents "pop-out" of the mounting cup under very high pressure conditions. Aerosol cans with good crimps have been destructively pressurized to over 300 psi-g (20.7 bars) without dislodging the valve mounting cup. On the other hand, mounting cups that utilize a PP laminate gasket depend mainly on the crimp for hermetic sealing. It is essential that the valve cup, when expanded by the crimp, distorts the PP laminate gasket sufficiently so that any potential channels caused by dimples and (particularly) the gaps between expanded collet segments are The Industry Recommendations Dimension B: ETP with Lathe Cut Gasket 0.988" to 0.994" EP with PP Laminate Gasklet 0.988" to 0.994" Aluminum with Lathe Cut Gasket 0.988" to 0.994" Dimension J: ETP with Lathe Cut Gasket 0.948" to 0.970" EP with PP Laminate Gasklet 0.933" to 0.972" Aluminum with Lathe Cut Gasket 0.938" to 0.962" Metal Thickness: ETP (Plain or Lined) 0.0090" to 0.0120" Aluminum (Lined) 0.0135" to 0.0175" Gasket Thickness PP Laminate 0.006" to 0.008 Lathe Cut Gasket 0.032" to 0.044" 30 Spray March 2014 black Art of Aerosol Crimping


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