Seeing Behind the Scenes
I was recently watching a television program on how a particular casting was made, and the narrator stated, “A worker pours molten metal into a mold and shakes out a casting when the metal has cooled.” While this is accurate, it has been over-simplified and is at such a high level all the detail is lost. Having grown up in the foundry industry, I see something completely different than what the narrator described; I see all the work that has gone on in the background. As a casting buyer, you need to be aware of everything that happens behind the scenes in the production of your part.
In the video, you can see the molding machine and the pattern used to make the molds. The molding machine has gone through a series of adjustments and test runs to verify that each mold is made correctly, the mold halves line up and no variation is being introduced into the process. Preventive maintenance programs have been put in place to make sure the machine runs at peak performance levels.
The pattern used to create the sand mold is a highly engineered tool. Skilled engineers have designed the pattern so the parting line, draft additions, core supports and shrink rule all come together to create a casting that will be dimensionally correct. The gating system has undergone several computer simulation iterations to ensure the metal flows into the mold cavity in a precise manner that will eliminate turbulence and the creation of potential defects. The risers have been simulated to ensure they supply the correct amount of feed metal to the casting as it solidifies to eliminate any internal shrinkage and provide a sound casting.
The next scene is a line of green sand molds moving away from the molding machine along a shuttle conveyor. The molds themselves have been produced from sand, water and bentonite. The percentages of these materials have been carefully measured, and the resulting sand mixture goes through scheduled checks to verify the process is under control. The hardness of the mold is measured to ensure strength and stability. The sand itself has undergone a number of checks including grain size distribution, grain shape and pH balance. The temperature of the sand is monitored and adjusted to keep it within a specified range. Sand cores placed in the mold to create internal features have gone through the same verification steps as the molds. The line of molds all have jackets around them and heavy weights have been placed on top of them to keep metallostatic pressure from separating the halves of the mold.
A skilled worker with a ladle full of molten metal now approaches the molds. The chemistry of the metal has been checked and verified to have the correct elemental makeup that will meet the casting material specification. The metal temperature has been taken and is within the engineered pouring range for the specific casting being produced. Any slag that has formed on the surface of the molten metal has been removed in preparation for pouring the molds.
The metal pourer lines up the ladle and begins to pour the metal into the mold, carefully keeping the pouring basin full so no air is entrained into the sprue. With a close eye on the height of the ladle to avoid excessive head pressure, the pourer watches as the mold vents start to fill, which indicates the mold is almost full. Once the mold is full, the metalcaster continues down the line, repeating the process and pouring the balance of the molds.
The poured molds move down the line into the cooling zone. The time the castings remain in the mold has been engineered to allow the metal to solidify and cool to a subcritical temperature before being moved into the shakeout. Dump the mold too early and the castings can be too hard; too late, and they could self-anneal in the mold.
You can quickly see the foundry has performed a significant amount of engineering in order to produce a casting that conforms to their customer’s specification. The manufacturing process has been locked down and nothing has been left to chance. All the upstream processes and checks are coming together to produce a conforming casting. The next time you watch a mold being poured, be sure to pay attention to what goes on behind the scenes. CS