“As pouring is started, maybe


that metal will remelt, maybe it won’t,” Oehrlein said. “Or maybe it will remelt with oxides in it. You don’t want to start filling the mold before the tilting starts.”


Goal #2: Repeatable Flow and Filling of the Mold Cavity


The time it takes to fill a mold


should be consistent. Temperature balances in permanent molds change slowly over time, so mini- mizing the variation of each element (mold temperature, metal tempera- ture, mold dwell time, mold open time, etc.) is important. “Having a good gating system


is part of keeping things moving speedily,” Oehrlein said. “The operator should not be correcting what was inherently wrong with the tool, otherwise you end up with jobs where only one specific person


A one-point gating system that attaches to the machined flange surface was designed for this part in order to reduce cleaning room costs.


can run it successfully. That’s the sign of a weak process.” Cycle time, metal temperature, mold temperature, and


spray coatings can affect the ultimate quality of the casting. “Once you get a permanent mold hot, the only thing


that keeps it hot is a regular cycle,” Oehrlein said. “Molten metal transfers its heat to the mold, and the mold gives off heat through radiation and convection off the surface area.” If a mold is open for too long between cycles, it can cool


down too much, making it difficult to maintain casting qual- ity in subsequent cycles. While an optimum gating system should be able to deliver non-turbulent metal without filters, they are a common, effec- tive way of fixing or improving a system. The use of filters, however, can affect other process variables. “You want the filter to be located in such a way that it is easy to place within seconds, doesn’t leak and is easy to remove from the part as it is ejected from the mold and when you are removing the gating,” Oehrlein said. “Typically you want to put the filter close to the part, as long as it is not tough to remove from that location during gate removal.” Process parameters should


be outlined and strictly ad- hered to in order to ensure a repeatable, successful part. When part of the process is not followed correctly, scrap can result. Reamer recalled a particularly difficult turbo- charger housing cast in low pressure permanent mold, which required a sand core. “We made sure we had a


checklist for the setup,” he said. “The machine had to be inspected to make sure all the elements were working properly before we started.” After the machine was


36


checked over, the metalcaster made 103 castings, of which only one was scrapped due to an error when the gating was cut off. “But the next time they put it on


the machine, [a different operator] didn’t follow the process, and the casting was garbage again,” Reamer said. “Everything has to be absolutely right. The gating system is important, but it isn’t the total panacea—it’s the process [that is critical].”


Goal #3: Progressive and Directionally Controlled Solidification


A well designed gating and


risering system should fill the cast- ing bottom to top and promote solidification with the use of risers. Risers are incorporated into gat-


ing systems to act as a reservoir of hot metal to feed a hard-to-reach section. Rather than a blind riser,


which does not reach the exterior of the mold, Reamer sug- gests using a runner from the gating system to feed the riser. “Risers have to be fed,” he said. “If they are cold, they are not going to give you the same feeding as a hot metal riser.” For other portions of the casting, additional risers can compensate for solidification shrinkage. In a well-designed system, the risers will solidify last. Other techniques can aid in controlling directional solidi- fication, including: Mold Coating—the reduction or elimination of mold coat- ing in places can accelerate chilling. Cooling Lines—lines located below the surface of the tool


can circulate air or water to cool the mold. Insulation—More mold coating or heating elements can be used on areas that need to be kept warmer longer.


Goal #4: An Economical System With High Yield and Low Finishing Costs


To minimize cost, use as little metal as possible by avoiding


heavy gating and risers, and configure the contact between casting and gating and risers for quick and easy removal. “It’s a sin that in the metal-


casting industry, you may put more gating on than you need because you are far more like- ly to be held responsible for the part quality than casting yield,” Oehrlein said. “Don’t forget, anytime you are using more metal than you have to, you are slowing the process down. And the more metal you melt, the more furnaces you need to use.” For parts with strict qual-


Automatic pouring aids in the repeatable flow and filling of the perma- nent mold cavity.


ity requirements, heavy gating has even further


MODERN CASTING / June 2010


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