Most products follow the same lifecycle: production begins with small quantities, ramping-up as demand grows, and finally settling into a gradual decline. Each stage needs careful management if quality, cost and delivery targets are to be met consistently.

For parts produced by investment casting, scaling-up poses a number of challenges. These relate mostly to maintaining the same standard of quality, but some are economic as larger quantities provide opportunities for automation and unit cost reduction. This blog identifies what those challenges are and shares how Impro ensures they are handled smoothly.

Implications of Scaling Production

The core of the investment casting process is forming a ceramic shell around a wax pattern. Once the ceramic, deposited over multiple dipping cycles, has dried, the wax is melted out. This leaves a cavity into which molten metal is poured. The metal is left to solidify, after which the shell is broken apart to release the part.

These aspects of the process remain unchanged, regardless of the quantities being cast. What may change are:

• Pattern production process: For quantity production patterns are injection molded. This yields excellent repeatability. Prior to this patterns may be machined from solid, 3D printed, or even hand carved.
• “Tree” configuration and assembly: Wax patterns are attached to a central sprue that will form the metal delivery passage. Once assembled, this is referred to as the “tree”. For large quantity production, economics dictate maximizing the number of parts cast in a single pour, which means mounting as many patterns as possible on the sprue.
• The dipping process: As the tree becomes larger and increasingly complex, more attention is needed to the dipping process. In particular, shell thickness is the dominant factor in cooling rate, so scaling involves ensuring this doesn’t change from small quantity production. An additional consideration is making sure the shell is strong enough to handle a larger mass of metal.
• Metal pouring: The quantity of metal poured has implications for its temperature when it reaches the mold cavity. A larger mass will cool more slowly, but will take longer to move from the pouring cup at the top of the sprue into the cavity.
• Solidification rate: This affects the grain structure, and hence mechanical properties, of the parts being cast. The temperature of the metal as it flows into the cavity is one factor in the solidification rate, but so too is the complexity or packing density of the tree. As the number of patterns on the sprue increases, airflow is reduced and heat can radiate from one region of the shell onto others. Together these can reduce the cooling rate.

Solutions to Scaling Challenges

Given a goal of having no discernible difference between parts cast in small or large quantities, the primary focus areas are shell dipping, metal pouring, and solidification. For each of these, rather than taking a trial-and-error approach to process development, a better solution is to use advanced modeling tools.

Starting with a CAD model of the tree, it’s possible to simulate dipping to assess impacts on shell thickness. Further modeling work will show how metal flows and temperatures vary with a higher density tree, and permit study of solidification rates.

A benefit of scaling-up is that it creates opportunities for automation, which lowers unit costs and improves consistency. This is seen mainly in injection molding of patterns, where higher quantities help amortize tooling costs, and robotic shell dipping. As technology develops, automated tree assembly and de-gating (cutting cast parts from the sprue), will almost certainly follow.

Your Source for Small and Large Quantity Production

Demand for investment cast parts typically starts with small quantities and rises as sales increase. This requires the investment casting foundry to manage the scaling-up process in a way that maintains or improves quality and costs.

As foundry industry leaders, the experts at Impro have a lot of experience with these issues and understand what’s needed. If you need parts producing by the investment casting process we can support all your quantity needs. Contact us for details.