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Improving dimensional accuracy in the production of a CastForm™ polystyrene pattern for investment casting

Abstract The focus of this paper was to analyse all stages of the CastForm™(CF) polystyrene pattern fabrication process, identify the reasons leading to poor dimensional accuracy, and outline techniques for its improvement. Rapid manufacturing of patterns for shell or flask investment casting using the laser sintering (LS) technique with CF material involves LS fabrication of a part, cleaning, and wax infiltration. Wax infiltration consists of preheating the part, the application of the liquefied wax in a convection oven, and the cooling of the part to room temperature. To study the dimensional accuracy of the process, a benchmark part was designed in such a way that its thickness could be modified from 2 mm to 20 mm and fabricated with the optimum processing parameters. The experiments conducted show that different wall thicknesses of the parts shrink differently after wax infiltration, particularly in the Z (vertical) direction, and this was found to be the major factor contributing towards poor dimensional accuracy. Using the Pro/Mechanica software package, transient heat transfer analysis has been conducted as an aid to understanding the effect of time and temperature on different wall thicknesses of a part during wax infiltration. In doing so, the thermal properties of CF material were investigated. The paper reports shrinkage factors for different wall thicknesses; the data can be applied in adjusting the appropriate shrinkage factor for a part with uniform thickness. For a part having non-uniform thickness, it was found that hollowing the part to leave a uniform thickness is an option to improve the final dimensional accuracy of the part.