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The success of the thixocore was incorrectly seen as an indicator of success for the whole process.
Because of the inherent difficulties involved with manufacturing large parts in this way, few thought it could be done successfully. Bill Stewart, a manufacturer’s representative for Husky, the company who built the thixomolding machine, provided some perspective. “There were questions and doubts about a piece this size, but when we all sat down with our applications engineers, they thought with Sheryar’s expertise in magnesium that the design was there. It needed a few tweaks, but it was there.” Durrani’s longtime associations in the magnesium world created a fruitful basis for partnering in the development of this technology. In fact, the companies heavily invested in this technology were eagerly waiting for a pioneering champion to mainstream this process for large production parts.
The design of the mold had to take into account the demands of the product (strength and shock absorption), and the intricacies of the thixomolding process. This requires the flow of the metal to take place in a manner that precludes the convergence of two cooling flows of metal within the wheel. The transition states or directional solidification that would result creates unacceptable weaknesses in the metal.
Durrani described the difficulty of designing the mold as being comparable to “the level of love and passion that would be put into a sonnet. There’s so much in this to make it work properly… these (pointing out curves, thicknesses, fillets, and radii in the wheel) are here for very, very critical reasons. There are 23 things I have identified that, if not done right, would produce a crack, every time.”
The speed with which this aspect of the wheel was accomplished was also unprecedented, according to Husky’s Stewart. “The mold for this wheel was done in five weeks… No one has ever done that for a magnesium thixomold. Typically you do a prototype tool first, and that takes months. Sheryar didn’t go that traditional route: he went right to the production tool, saving a lot of time and money due to his experience.”
Durrani expounded:“Getting that right normally takes six months for small parts, trying and retrying the placement and sizing of the overflows. The (mold design for) larger stuff could take people up to one and a half years. We were very, very detailed on the mathematics of it. This is epinephrine inducing, headache level, PhD math!”
Where do you get that skill? “After 20 years of doing nothing but magnesium and making it your one mission in life…then, you can do an application where your first shot is pretty damned close to where it needs to be.”
The artist's rendering
“We went from an artist’s rendering dated January 15, 2007 to computer aided design, and then Finite Element Analysis. We put the loads on it to see where the stresses were, and discovered that there was some twisting going on when cornering if it hit a bump, so we added a cross brace. It completely eliminated the problem and doubled the strength of the wheel.” After several speedy revisions, Husky received tooling on February 19th, and on February 28th, Durrani had his parts in hand.
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