TECHNICAL TOOLBOX ISSUE 37 Feed zones within the casting are defined by knowing where in the casting it is possible for liquid metal to flow from one point to another in response to expansion pressures. If there is no possibility of metal flowing from one area of the casting to another as expansion begins, then each of these areas forms a separate feed zone and each may require its own correctly-designed feeder (but no more than one). The analysis of a casting begins with consideration of the Casting Modulus. This is defined as the volume:surface area ratio of various areas of the casting, and has been used for many years to estimate the order of solidification of different parts of the casting. The Casting Modulus (Mc) allows us to estimate which part of the casting will solidify first and which will solidify last. In steel castings, this information is immediately useful to indicate where risers should be placed and what size they should be (the Modulus of the riser should be greater than the Modulus of the casting). In iron castings, the Casting Modulus is used to estimate when expansion will begin, expressed as a percentage of complete solidification. Prior to development of computers, calculation of Mc was tedious and time-consuming; it required the foundry engineer to estimate volumes and surface areas by approximating various parts of the casting with relatively simple shapes. With modern casting simulation DAVID C. SCHMIDT Vice President Finite Solutions Inc. ARTICLE TAKEAWAYS: • Risers are designed to feed initial metal shrinkage • Gates and Contacts should freeze off as graphite expansion begins • One riser per feeding zone – Too many risers CAUSE shrinkage in cast irons Continued on next page RISER DESIGN BASICS FOR CAST IRONS DESIGN PRICIPLES FOR CAST IRON The fundamental difference between gray and ductile cast iron and other alloys is the expansion that occurs as graphite precipitates during solidification. In most situations, the casting can become “self-feeding” after the onset of expansion and no further feeding is required. The object of designing a risering system for iron castings is to provide feed metal for the contraction of the liquid alloy as well as the contraction of the solidifying iron prior to the start of expansion; once the expansion begins, a well-designed risering system should control the expansion pressure to ensure that the casting is self-feeding during the remainder of solidification. This is in contrast to other alloys such as steel, where feed metal must be supplied to the casting during most or all of solidification and there is no expansion involved. Another major difference between graphitic cast irons and other alloys has to do with the mechanism involved in “piping”, or the onset of feeding behavior in the riser. In practice, only one riser should be used on each “feeding zone” in an iron casting; if multiple risers are placed on the same zone of a casting, then typically one riser will begin piping while the other will not. Often, porosity will be seen at the contact point of non-piping risers. The requirement for a single riser per feeding zone is probably the design rule most often violated in iron foundries. We see designs where two or more risers are feeding the same zone within a casting, and the resulting casting exhibits porosity, often at the contact point of one of the risers. The tendency of many foundry engineers is to add more risers to try and resolve the porosity issue; in fact, this is exactly the wrong approach and will worsen the situation. To correctly design a risering system, we must answer the question: Is this casting composed of a single feed zone, or are there multiple zones and, if so, what is the location and size of each zone? To make this determination, we introduce the concept of the Transfer Modulus.
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