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The last factor in selecting the optimum number of cavities must be an economic decision. For extremely low volumes, anything more than a single-cavity mold is not economically justifiable. At the other extreme, very high volumes of product call for large numbers of cavities. Molds are normally built in multiples that result in a balanced cavity layout, with 1,2, 4, 8, 16, 32, 64, and 128 cavities normally constructed. A general rule is that each time the number of cavities is doubled, the additional cavity cost is approximately 65% of the preceding cavity cost. Typically, the additional costs for a greater number of cavities must be justified against manufacturing costs over a given length of time, with three years being the average.