implications on process cost and time. If a customer specifi es fi ve parts out of each heat must be x-rayed, the fewer castings you are able to pour in a heat, the more testing that will be required down the road. Oehrlein said the best way to achieve the most economi-
cal gating would be to start with a small gating system and work your way up to determine how little gating you can get away with to cast a successful part. This practice, how- ever, involves iterations and tooling costs, which adds time and money up front. Process development engineers often encounter this “spend it now or spend it forever” paradox, Oehrlein said. Also affecting the economics of a gating system is the
ease or diffi culty of removing the system after the part has solidifi ed. “It costs money to remove gating,” Oehrlein said. “Obvious
things to avoid are where you know a saw won’t be able to make a cut or that you will have to come up with specialty tools or fi xtures to remove. But another consideration is that saw cuts and chips are lost metal.” In some instances, extensive gating is unavoidable. In
these cases, accounting for the metal used in both the part and the gating system is important when costing the part. “For one casting, we poured 22 lbs. of metal into the mold
to get a 2-lb. casting,” Reamer said. “The reason was the wall section on the casting was 0.08-0.11 in., and it was just a lattice of metal. It had seven sand cores. It was important we poured that much metal just to keep the mold hot. It was that or we didn’t make the part. The gating and the remelt dictates what the casting is going to cost you.”
Goal #5: A Simple System in Which a Design Can Be Quickly Generated, Eliminating Trial and Error
A system that can be constructed quickly and easily will
come in handy when it is time to replace the system following mold erosion that may require re-cutting the gating in the mold. It also allows easier experimentation with alternative gating designs. “Starting simpler means more iterations,” Oehrlein said. “Instances of short deadlines support the use of computer modeling. We fi nd it invaluable to help us sneak up to a good gating system on the computer rather than by iterations of cutting the tool.”
MC For More Information
“Gating the Turbulence in Your Runner System,” R. Pischel, MOD- ERN CASTING July 2006, p. 24.
Carley Foundry utlized a riser to increase the feed and heat mass to improve the castability on this oil tank for Harley-Davidson.
Grenville Castings Ltd. gated a four-cavity mold with eight pulls to economically produce this snowmobile chassis suspension support.
MODERN CASTING / June 2010 37
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