Theoretical model of the two-chamber pressure casting process
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INTRODUCTION
TITANIUM has found increasing use in dental prosthetics, principally as an alternative to allergy-producing casting alloys.[1] Although titanium is difficult to cast using the lost-wax method,[2] technical improvements have led to various methods for preparing acceptable clinical dental prostheses.[2] Dental appliance casting units can be categorized as one of two types, depending on how liquid metal is forced into the cast mold: pressure difference casting units and centrifugal casting machines. This study will consider the former, focusing in particular on the two-chamber pressure casting process. A typical two-chamber casting unit is shown schematically in Figure 1. Here, the upper and lower chambers within the casting unit are separated by an impermeable wall, which supports the cast mold. Both chambers are filled with an inert gas, typically argon, with the lower chamber pressure maintained at approximately 20 kPa (Pl) and the upper chamber set at approximately 200 kPa (Pu). Due to the pressure difference between the mold’s interior and exterior, gas flows through the mold’s porous bottom wall into the lower chamber. Casting is initiated when a titanium (or, more generally, a metal) ingot is arc melted within a copper crucible in the upper chamber; a molten drop eventually forms and falls into R.G. KEANINI, Associate Professor, is with the Department of Mechanical Engineering & Engineering Science, University of North Carolina at Charlotte, Charlotte, NC 28223. K. WATANABE, Associate Professor, Division of Dental Biomaterials Science, Graduate School of Medical and Dental Sciences, Niigata University, Japan, 5274 Gakkoucho-dori 2 Niigata, Japan 951-8514. T. OKABE, Regents Professor and Chairman, Department of Biomaterials Science, Baylor College of Dentistry, Texas A&M University System Health Science Center, Dallas, TX 75246. Manuscript submitted March 29, 2004. METALLURGICAL AND MATERIALS TRANSACTIONS B
the conical crucible positioned immediately above the mold cavity. Although the pressure within the upper chamber and that within the mold cavity are initially equal, once the drop falls into the crucible, the gas trapped within the mold cavity continues to pass through the porous mold into the lower chamber. Due to a combination of gas leakage from the mold cavity and compression due to the drop’s downward motion, a time-dependent pressure difference, Pu P(t), is created across the molten drop. Considering the dynamics of the drop, it is apparent that at least two forces drive the molten metal into the mold cavity: the weight of the metal itself and the time-varying pressure difference that develops across the melt. Other forces that potentially play a role in cast filling dynamics include surface tension, impulse due to metal evaporation, and friction. Each of these will be discussed subsequently. With regard to the pressure force, once a molten drop falls into the conical crucible above the mold cavity, the drop’s upper free surface is subject to the relatively fixed pressure, Pu, within
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