HERRON CASTING SERVICES

HERRON CASTING SERVICES
P.O. Box 3403
Barrington, Illinois 60011

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TIPS ON RISERING GREY AND DUCTILE IRON

by David J. Herron, B.S. MTE.

Gating goes hand-in-hand with risering especially when dealing with graphitic irons. You cannot properly deal with one without also dealing with the other. For this reason we do not refer to "gating and risering" but rather more simply to "GATING". The term for us involves the successful filling and solidification of the casting mold cavity.

Therefore, the first tip in risering ductile iron is to consider the effect of the sprue, runners, and ingates. There will always be a certain amount of feeding from the sprue, as the sprue is the highest column of liquid metal available to the entire complex. As soon as any point along the way freezes, the sprue stops feeding and some other point starts (usually the riser). If this does not happen soon enough, a layer of solid metal will form over the top of the risers that may yield to the vacuum of internal metal contraction but may not actually break and form the desired "pipe".

Secondly, consider the effect of varying section thicknesses usually found in today's casting designs. (You do realize few castings are designed by foundrymen don't you?) Here, we are not just speaking of the normal concern for directional solidification (again, heavily influenced by the "gating" system) but rather for the more or less simultaneous contraction and expansion going on inside the casting/riser complex. The net effect, or balance of pressure, varies with cooling time and is never the same from one casting design to another. This is why a set percentage feed metal requirement cannot be meaningfully applied to a graphitic iron casting. We rarely produce simple blocks, spheres or plates.

Thirdly, begin to consider the effect of convective heat flow in the casting/riser complex. Castings do not cool evenly in all directions. This fact greatly complicates our attempts to computer simulate cooling and solidification. Proper understanding however can lead us to logically alter riser shapes and make relative decisions as to which way we should go with positioning castings in the mold, with sizing of riser connections (contacts), and with locating risers on the casting. We introduced these riser shape concepts in major Wisconsin foundries in the late 1980's and the practice is widespread today.

Certainly much more could be said in a full seminar, but for now remember lastly...

The gating system should address:

(1) the feed requirement,

(2) the heat requirement,

(3) and the liquid metal (ferrostatic) pressure requirement

that all casting designs pose for proper solidification.

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