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R 34


oger Reamer, retired vice president of Progress Casting Group, Plymouth, Minn., spent a large portion of his metalcasting career solving gat- ing puzzles for permanent mold castings. But ask him for a step-by-step method for gating a


casting, and he will be hard-pressed to answer. “I always hesitate to get into a conversation


about this because it’s temperature dependent, coating dependent, cooling and cycle time dependent,” Reamer said. “All those things enter into making a good casting.” Nevertheless, good permanent mold gating principles can help metal- casters produce a cost-effective, quality part. How the principles are applied to each casting may differ, but every successful gating system should achieve three basic goals, according to the AFS Aluminum Permanent Mold Handbook: 1. The tranquil fi lling of the mold cavity. (Quiescent fi lling will help minimize the formation of oxide skins during fi lling.) 2. Repeatable fl ow and fi lling of the mold cavity that produces consistent cycle times.


3. Progressive, directionally-controlled so- lidifi cation by including adequate risers and feeding.


To achieve an economical gating system, start with a small system and work your way up to avoid using too much metal.


All three basic goals must be satisfi ed for a gating and risering system to be effective. Beyond the basic goals, secondary, desirable goals include:


• an economical system with high yield and low fi nishing costs;


• a simple system in which a design can be quickly gener- ated, eliminating trial and error.


Goal #1: Tranquil Filling of the Mold Cavity


Tranquil fi lling occurs when the mold cavity is completely fi lled at a rate slow enough to avoid turbulence that can lead to the formation of oxide fi lm inclusions, typically less than 20 in. (508 mm) per second, according to the AFS Aluminum Permanent Mold Handbook.


For static pour permanent mold, the ideal gating system has a short drop from the top of the sprue to the bot- tom of the mold. For castings with little height in the mold, Reamer found that the Alcoa gating system almost always resulted in an oxide-free casting. The gating system calls for a 1 in. wide, 1.8 in. thick runner that feeds the riser and the connection into the casting.


MODERN CASTING / June 2010


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