There’s no place for trial-and-error in the modern investment casting foundry. Industry leaders like Impro use simulation to optimize every aspect of the process, avoiding defects, reducing development lead time, and raising efficiency. Here’s a closer look.

The Complexity of Investment Casting

How metal flows into a mold, then cools and solidifies, determines the efficiency of the process and whether defects form in the parts being cast. Accordingly, foundry engineers devote time and effort to optimizing gates and runners and determining ideal cooling rates.

In the past this was an experience-based, trial-and-error process. A wax pattern was produced and used to form the ceramic shell. Then the wax was melted out and metal poured in and allowed to solidify. After breaking apart the shell and cutting away the metal feeding channels, the cast parts would be inspected thoroughly. This might just be a visual examination, but for safety-critical parts, it could include x-ray inspection, sectioning, and analysis under a microscope.

Depending on the results of the inspection and the time available for process development, pattern molding and shell coating processes might be tweaked, and the cycle repeated. More often though, time pressures meant little could be done to optimize the process of investment casting this particular part.

Simulation Capabilities

Modeling casting processes, especially investment casting, involves addressing both fluid and heat flows. This begins by breaking the shape into a series of elements, each connected at nodes. Physical laws (fluid dynamics, thermal conductivity and radiation), and material properties are applied to each element and the results aggregated to predict how liquid wax or molten metal will fill and solidify in the molds.

Wax Pattern Molding

Errors and defects in the pattern will appear as errors and defects in the cast metal part. Wax patterns are injection molded, and the process is modeled to ensure the mold fills completely and that shrinkage doesn’t affect the geometry.

Shell Molding

Shell thickness determines both mold strength and cooling and solidification rates. By modeling the coating process, it’s possible to optimize thickness to meet objectives for these parameters. However, results from casting simulation are required to determine if changes are needed.

Casting Simulation

This involves looking at fluid flows as the mold fills, and the rate at which the metal cools. The goal is to have solidification occur only after the cavity is full, and to fill slowly enough to minimize turbulence. At the same time, in the interests of efficiency, the process needs to happen as quickly as possible.

Solidification modeling considers heat flows through the ceramic shell. Shell thickness is one variable to include, (which is arrived at by modeling the shell-making process), but another is radiation from the shell. (At high temperatures more heat is lost through radiation than conduction into the atmosphere.)

Complicating this is the tree-like appearance of investment casting molds that results from attaching multiple wax patterns to a central sprue. With this, heat radiates from one ceramic surface onto adjacent surfaces, changing and slowing the cooling rate. Similarly, airflow over the shell is affected by multiple hot surfaces, which makes conduction effects hard to model.

The bottom line is, investment casting, especially solidification, is extremely difficult to model with any degree of accuracy. However, foundries strive to do it because the benefits that result.

Benefits of Simulation in Investment Casting

Simulating each process step results in a form of virtual prototyping. This permits experimentation to evaluate the impact of changing various process aspects. Some examples would be:

• Altering wax solidification rates to reduce the incidence of shrinkage defects in the pattern
• Varying the placement of patterns on the sprue to increase the number of parts cast at one time
• Adjusting runner and gate cross-sections to speed up or slow down cavity filling
• Evaluating how changes in pattern dipping can alter local shell thicknesses and hence solidification rates

Overall, simulation lets foundry engineers explore more options for process optimization, without the time and waste of the old trial-and-error methods. This means shorter development times, higher quality castings, less waste and improved efficiency throughout the process.

Quality Results For Every Project

Investment casting can produce finely-detailed, high quality metal parts, but it’s a complicated process to get right. Impro uses advanced simulation tools to optimize every aspect, minimizing defects while maximizing efficiency. Contact us to learn more or to discuss your investment casting needs.