By far the most significant aspect of designing feeding and gating systems for graphitic cast irons is the requirement to maintain positive pressure on the liquid throughout the solidification process. Initially, atmospheric pressure must be allowed to act on the liquid in the feeder, for this to happen, the feeders must pipe. Failure of the feeders to pipe is perhaps one of the commonest causes of porosity defects in graphitic cast irons. This is particularly so with ductile irons, which are predominantly of hypereutectic composition, the "mushy" freezing mode of these alloys can cause feeders to "skull" readily, thus preventing atmospheric pressure from acting on the liquid.
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