Metalcast S.A. de C.V. is an investment caster with 200 employees in Puebla, Mexico, that is casting increasingly complex parts in more challenging alloys. Stefan Plotz, an engineering consultant, worked with Metalcast engineers to reduce the proportion of rejects in the production of a valve body. "Computer simulation of the casting process helps understand the root cause of defects and evaluate the performance of modified designs without having to build tooling," Plotz emphasized. "The result is a substantial reduction in rejects, from 25% to 3%, which has helped improve Metalcast's competitive position."
Metalcast's speciality are parts up to 1 kg in size, especially for the medical device industry (about 25% of its output), but also the oil equipment and the valve industries, using investment casting to produce geometrically more complex castings. Because investment casting results in parts with very close tolerances, it makes it possible to reduce or eliminate secondary machining.
In the past, Metalcast engineers designed molds according to experience, and produced wax models via manual machining. Plotz worked with them to implement CNC machining that improved the accuracy of the models, leading to closer-tolerance finished parts. He also helped them develop the ability to simulate casting.
He identified ESI Group's ProCAST (www.esi-group.com) as a tool that addresses the specific needs of investment casting foundries. For example, it lets users automatically generate a mesh representing the shell mold, which can include non-uniform thickness and multiple shell layers. The software calculates radiation with view factors, including shadowing effects that are critical for high-temperature alloys. ProCAST uses a physics-based approach to model shrinkage and gas porosity. A dedicated module models gas porosity by computing interdendritic shrinkage while taking into account gas content. The software also models gas segregation, along with the proper treatment of solubility limit and pore nucleation.
Simulating valve bodies — Plotz and two Metalcast engineers completed a one-week, ProCAST training session, and began running the software right away to evaluate several parts with high scrap rates. One of these was a valve body that Metalcast had been producing for some time but had difficulty maintaining yields due to excessive porosity.
Plotz, with engineers Omar and Miguel Martinez, began by importing the part geometry into ProCAST from their CAD program. They used ProCAST's automatic shell generator to create the investment casting shell. They ran the simulation based on the existing production conditions, and ProCAST then predicted the metal flow inside the mold based on the full Navier-Stokes equations. The thermal solver computed heat flow by taking into account conduction, convection, and radiation. The heat release associated with phase changes, such as solidification and solid phase transformations, was described by an enthalpy formulation.
The simulation results showed the filling sequence for the metal arriving into each of the mold cavities. The software provided a time-sequence animation of the filling process with the solidification lines and the solid and liquid fractions plotted. Engineers looked at the solidification line to identify borders where no more filling can occur. The software calculated porosity percentages for areas that were separated from the main filling line.