The effects of random noise shrinkage on rapid tooling accuracy

This paper describes the effects of shrinkage variation on the accuracy of rapid tooling inserts. For processes involving a phase change, such as sand casting, investment casting, die casting, plastic injection molding, blow molding, and stereolithography, available data indicate that a component of ‘random noise shrinkage’ is superimposed on the mean process shrinkage. Analysis of data indicated that the standard deviation σs, of the random noise shrinkage, is directly proportional to the mean process shrinkage, S̄. Assuming the noise is Gaussian, the maximum linear dimension, Lmax satisfying a given tolerance, is shown to be inversely proportional to S̄. A plot of Lmax (S̄) is shown for various tolerance levels. A conclusion of this work is that tooling concepts based on sintering metal powders must achieve S̄<0.1% to satisfy production tooling tolerances for dimensions up to 20 inches.