That ensures that the bottom cools first, Smith said, so the grain or grains begin growing from the bottom and work their way upward. Exothermics are high flash point elements, he added. The intention is for any shrinkage to go up into the gating and for the gating to remain hottest longest. The casting is able to draw from the gating to get the metal it needs to solidify without shrinkage or high porosity. Ceramic cores form blade passages.
Air circulating through the blade cools the metal in a turbine's hottest stages.
Once the mold has cooled, any accessible sprue and gating is cut away. Then, through a combination of hammering, blasting, and etching, the remaining shell and ceramic core come off the finished part. Boutot said that any hammering is kept light to avoid part damage. But high-pressure water jet blasting and chemical etching gnaw away at any tenacious bits of shell and ceramic core to reveal a smooth casting. Finally, it's on to a series of nondestructive tests, such as ultrasonic wall thickness checks and fluorescent dye penetrant exams.