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completely blocked. By looking into the crimped-valve mounting cup, one can easily see the six or eight depressions caused by the collet segments as they expanded, typically into the 1.04" to 1.08" diameter at the mid-point of each depression. The partially-expanded areas, generally called gaps or voids, can cause propellant leakage if they are not filled up. By example, if an eight-segment collet with a closed circumscribing diameter of 0.958" (so it can fit into the well of the valve cup) is expanded to a diameter of 1.07", the gaps between segments will average 0.0429". If expanded to 1.075", the gaps will average 0.0488". These mathematically-derived figures do not tell the whole story, since some gaps in a set will be wider than the average, and the segments will have blunted or beveled edges to prevent them from slicing holes in the mounting cup during the expansion process. The key factor in any gap survey is metal humping—or the degree that the mounting cup metal is not expanded to the designated crimp diameter. The difference, using a 1.07" crimp diameter, is about 0.002". This means that the PP laminate must be compressed to about this same degree for hermetic sealing at the crimp itself. Otherwise, sealing must occur above the crimp locus, where the gasket is compressed between can curl and the mounting cup. Gap mechanics is an important factor in comparing the performance of six- and eightsegment collets, when used for valves with PP laminate gaskets. While the six-segment tool has two fewer gaps, each one is somewhat larger, leading to a more pronounced hump at each gap. In collet design, the completely closed format will have a circumscribing circle of toes that typically measures not more than about 0.954". This will permit it to slip into the well of the mounting cup without scraping or binding, even for cups made slightly smaller because they are PP laminated, yet it still must fit into the “one inch” hole of the can. That hole has a minimum diameter of 0.996". A collet with a maximum closed circumscribing diameter of 0.972" would work for tinplate valve mounting cups fitted with lathe cut gaskets, but it would not work for any valve with an aluminum mounting cup, or a tinplate valve with a PP laminate gasket. Ordinary epon-phenolics and other can and valve cup linings do not present problems with collet fit. They are almost never more than about 0.0005" (12.57 microns) thick. Crimp Measurements In the early days of the aerosol industry, the Federal Instrument firm was the only supplier of gauges for crimp diameter and crimp depth. They were delivered with a gauge block for recalibration purposes. After about ten years, the German firm H.C. Kroplin, GmbH introduced excellent, lightweight gauges, although they were quite fragile and broke if dropped. Other instrument makers soon followed. Total gauge sets can now be purchased through several equipment companies. A sophisticated compound gauge with electronic components may be purchased from Japan. Gauges using the Federal designs are available from the P.R. of China. Pamasol gauge sets, made in Switzerland, are in use throughout the world. Many quality control and production people use gauges rather haphazardly, taking one or two readings per can. Ideally, for an eight segment collet imprint, the valve cup or can dome should be marked at the centerline of each crimp depression, with each mark then numbered from one to eight. Gauges are then calibrated, if necessary. Those calibrating should hold the crimp diameter gauge parallel to the can axis and take readings from locations 1-5, 2-5, 3-7 and 4-8. Then, a reading should be taken with the crimp depth gauge at every indentation. Each set of readings should be within a range of 0.006". A common March 2014 Spray 31


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