The paper describes a series of blow molding trials on two different polymers under a variety of processing conditions. The tests were performed to investigate factors that affect the variation of wall thickness in a blow molded part. In addition, a numerical analysis was performed to predict the wall thicknesses. A major outcome of the tests was that the relative thickness distributions in the parts were independent of processing conditions and of the material used to form the part. The primary influence of the processing conditions was to put more or less material into the part, but they did not significantly affect the relative distribution of the polymer. The analysis gave a good overall prediction of wall thickness distributions.