Eliminating Alloy Defects Through Mold Design

Typically, my main concern is the quality of our own end product. But every now and then our attention shifts to improving the quality of other companies’ end product. Occasionally, our company has been used as the laboratory where potential breakthrough innovations are put to the test.

Recently, an engineer from a multinational corporation came to me and asked me to help in their consideration of a major change in their operation.  Previously, they had been using sand molds to cast their product, but were considering a move to graphite molds.

Our customer’s engineers hypothesized that a switch from sand molds to graphite molds would improve the surface quality of their product thereby reducing defective product. Going from sand to graphite required a change in mold geometry, but consideration also had to be given to the different heat transfer rate of metal through graphite versus sand.

Also, through their study, they planned to determine the best way to reduce centerline shrinkage thus cutting down on scrap. There is always heat shrinkage whenever molten metal solidifies, but metal shrinks differently depending on how heat is transferred from the mold. When metal goes from liquid to solid, there is a corresponding volume change and that volume needs to be continually supplied with a source of liquid metal otherwise a shrinkage void will result.  Controlling the amount of shrinkage and where it occurs depends on the shape of the mold as well as the way heat is transferred from the metal as it fills the mold.

Computer models can predict the effects of these design changes.  Our customer developed a computer simulation that models the heat transfer that takes place when their materials are melted in various graphite mold designs as well as the way various alloy compositions solidify in the graphite molds.  While our customer has vast experience with mold design, they simply didn’t have the facilities to carry out the experimentation needed to verify their designs.

After we conducted the first trial, we shared the results with them and they provided us with additional parameters to test.  We conducted trials with various shapes and compositions.  Our research will help them finalize the design rules for their new graphite molds.  Additionally, they will be able to consistently predict the amount of heat shrinkage that will occur, and, ultimately, control the shrinkage to eliminate the centerline shrinkage that results in defective material. Once these design rules are determined, then our customer can move forward in their goal of casting in graphite on a larger scale.

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