are of dimensions such that the width height and length are two to three feet or more and the walls have a thickness capable of withstanding significant pressures given the need to sand cast the housings desirably the units are constructed of 356 aluminum alloy which comprises 6 5 to 7 5 silicon 0 6 iron 0 25 copper 0 35 manganese 0 2 to 0 25 magnesium 0 35 zinc 0 25 titanium 0 05 other elements and the remainder aluminum this aluminum alloy has been found to have the best properties for production of relatively large thick walled sand castings which are used for housing of electrical components where heat is dissipated and there is a need to provide a relatively complex inner and outer surface configuration to accomodate structural shapes such as cooling fins reinforcing ribs mounting bosses component receptacles board slots and similar designs furthermore this aluminum alloy is available at a reasonable cost has the necessary overall physical properties and lends itself to required heat treating for strength and non detrimental brittleness a preferred casting material is ga1si7mg wa din 17 25 2 german specification or 356 al per astm b26 u s specification however because there is a need to provide for relief of pressure from the interior of electrical component housings sand cast from.