Sand casting is an economical process for making metal parts, but not everyone understands the complexity it’s capable of. At Impro, our skilled patternmakers and experienced foundry personnel work together to cast intricate shapes ranging from brackets and housings to complex engine blocks and cylinder heads. This blog showcases the talents they bring to the task and the challenges they deal with.

Achieving Precision in Sand Casting

Sand casting is an old process in which molds are formed by packing sand around a pattern. Dry sand needs a binder to hold it together. For a very long time, that binder was bentonite clay, and sand made with it is called “green” sand because of its moisture content.

A step up from green sand is resin sand. This is sand in which the grains are coated with an adhesive. The sand is packed around the pattern, and the adhesive is activated, usually by heat, to hold it in place. Resin sand molds are more accurate than those made from green sand and can form more complex parts.

Complexity in Sand Casting

When people talk about complex castings they’re usually referring to features like fins, ribs, webs and galleries. These are areas with narrow cross-sections where some alloys are reluctant to flow.

Complexity can also mean large changes in cross-section and features that need a lot of cores to produce. Changes in section pose challenges because the metal can tear as it contracts. Cores, like those used to form water passages, need precise positioning to create regions with narrow sections.

The filling problem varies by the type of metal being cast. Foundry experts characterize metals in terms of their “fluidity”. This describes how easily a metal flows into narrow spaces and galleries.

Fluidity is inversely proportional to viscosity and is influenced by factors like metal temperature and solidification behavior. Metal composition affects fluidity, and even changes in ambient conditions can alter how a metal flows. In general, nonferrous alloys have better fluidity than steels, although cast iron also flows well.

In the foundry workers will perform a spiral flow test to check the fluidity of an alloy before pouring.

Optimizing the Sand Casting Process

Designing mold tooling and developing a process for casting a specific part requires a number of tradeoffs. To prevent defects like hot tears it’s sometimes necessary to modify the part design from what the engineering team wanted. Likewise, depending on the alloy chosen, the foundry team may advise widening galleries and ribs to ensure a good fill.

Foundries use sophisticated software to evaluate how a mold cavity will fill and what problems or defects might occur. (This is the science part.) Then they use their experience to decide on the optimal process parameters, such as pouring temperature and fill rate. They might also recommend changes to the patterns and cores to modify thermal gradients and lower the risk of casting defects. (This is where the art comes in.)

Only after the design and process have been thoroughly considered will the foundry team move to actually casting a part. This is inspected carefully and often sectioned to check for completeness and presence of defects. If problems are found, the process, and possibly also the tooling, will be adjusted, and another test run conducted.

Using all the Tools Available

Sand casting is complicated, and the challenges grow bigger as the parts being cast get more intricate. Powerful computer programs analyze part designs and predict how metal will flow into mold cavities. Analytical laboratory tools characterize metal composition and properties.

All this technology is essential, yet it only takes the foundry part way towards an optimal design and process. Ultimately, achieving high quality results needs the input of experienced foundry workers who understand the complex, sometimes capricious, behavior of liquid metal.

Successful foundries, like those that Impro runs, understand the importance of combining foundry science with metal casting art. Technology is very important, but so too is experience. We use both to the very best of our abilities. Contact us to discuss your next sand casting project.