Aerosol Crimping By Montfort A. Johnsen, Ph.D., Technical Ed itor March 2014 Spray 29 breakage. Some 3" collets can be produced in heavy-duty designs to limit breakage. A process for chrome-plating a worn toe has been considered, since chromium is much harder than steel. It is not yet commercial. The Crimping Process The goal of crimping (also called clinching by some, and sometimes swaging in the UK) is to create a hermetic seal between valve cup and can bead that is highly pressure resistant and will prevent leakage during the service life of the dispenser. High speed aerosol production crimping is performed by rotary crimpers, rotary vacuum crimpers or Underthe Cup (U-t-C) gassers, which include vacuum and crimping stations. Individual crimping heads are “boilerplated” to produce 20 cans per minute (cpm), and some machines can be tweaked by fillers to produce more than that. Rotaries with six, nine, 12 and 18 heads are readily available. A difference between crimpers and U-t-C machines is that regular crimpers perform their crimping action on cans that are nonpressurized— perhaps under a partial vacuum—while the U-t-C device must crimp cans that are already pressurized to sometimes as high as 120 psi-g (8.3 bars). Springs are generally used to overcome the lifting pressure of the propellant and to also provide very desirable download pressure to aid in gasket compression. Springs developing 175 lb are often used in U-t-C machines. When the collet toes expand the valve mounting cup hard against the slanted can wall at what is termed the “The Point of Hard Contact” (PHC), two force vectors are produced. The primary one is the horizontal vector. However, there is also a downward vertical vector, pulling the valve cup down on the can curl with considerable force and compressing the gasket in the process. For non-pressurized cans, successful crimps can usually be produced on this basis alone. For added gasket compression, however, it is best to provide ancillary air or spring pressure, limited by the ability of the dispenser to withstand the download without permanent deformation of the curl or throat areas. This is especially important for smaller aluminum cans, but much less of a concern for strong DOT Specification 2Q steel or tinplate cans, where the metal is at least 0.008" (0.20 mm) thick. Some one-piece and two-piece cans (but especially larger diameter aluminum cans) have an idiomorphic curl texture that results from the metal being progressively squeezed into itself by dome and curl forming operations. The final result is the formation of hundreds of tiny radial gouges, ranging from 0.0001" to 0.0008" deep as a rule, separated by flat-topped low ridges. Trivial terms for this effect are “eyelashing” (from Bill Gregg— and the most popular), “hula skirting” (Jim Greenebaum) and “micro-grooving” (M. Johnsen). In contrast, the curls and chins of three-piece tinplate cans are very smooth and shiny, being formed with dies on both sides of the can dome, and with only minor metal compression. The grooved curl topography of large diameter aluminum cans
SprayMarch14
To see the actual publication please follow the link above