![]() ![]() If metal cannot flow from one area of the casting to another, then each of these areas forms a separate feed zone and will require its own feeder (but no more than one.)Īnalysis of a casting begins with the Casting Modulus (M c). To make this determination, we use the Transfer Modulus.įeed zones are defined by knowing where it is possible for liquid metal to flow from one point to another in response to expansion pressures. To correctly design a feeder system for iron castings, it is necessary to analyze the cast shape and determine the location and size of feed zones in the casting. The tendency of many foundry engineers to add more feeders to try to resolve the porosity issue is, in fact, exactly the wrong approach and will worsen the situation. Designs where two or more feeders are feeding the same zone a casting results in porosity, often at the contact point of one of the feeders. The requirement for a single feeder within a single zone of the casting is the rule that is violated most often in iron foundries. Often, porosity will be seen at the contact point of non-piping feeders. This means that only one feeder should be used on each “feeding zone” in an iron casting if multiple feeders are used, one feeder will begin piping while the others will not. Once one riser punctures, the pressure is equalized so there is no longer a higher external pressure to cause other feeders to pipe. This renders atmospheric cores ineffective with these alloys.įor blind feeders to pipe effectively, atmospheric pressure must be able to collapse the weak plastic skin after the internal pressure drops below atmospheric. Cast irons (particularly ductile iron) do not readily form a solid skin during solidification the freezing mechanism is often described as “mushy” or “pasty”. With shrinking alloys, feed metal must be supplied during the entire solidification time.Īnother difference has to do with the “piping” mechanism in the feeder. Once expansion begins, a feeding system should control the expansion pressure to ensure that the casting is self-feeding from that point forward. The objects of feeding systems for iron castings are 1) to provide feed metal only for contraction of the liquid alloy and 2) the contraction of the solidifying iron prior to the start of expansion. ![]() This is significant because in most situations the casting becomes “self-feeding” after the onset of expansion, and no further feeding is required. ![]() The biggest difference between iron and other alloys is graphite expansion during solidification. Spending a short amount of time up front to prevent ongoing problems in foundry production over months or years results in an extremely high return on investment. Most of these problems could have been prevented had the foundry engineers applied correct design methodology from the beginning. The design methods are generally quite easy to implement and require only a minimal investment of time. Often the suggested remedies for these defects worsen the situation, due to the same lack of understanding. Many iron foundries do not take into account the solidification characteristics of iron when designing feeding systems feeders for iron castings are designed essentially as feeders for steel castings, resulting in defects in production castings. ![]()
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