Contact: DAVID C. SCHMIDT dave@finitesolutions.com TECHNICAL TOOLBOX ISSUE 39 Figure 3. Appearance of porosity on machined surface. Close-up inspection of the areas of porosity showed what appeared to be primary shrinkage as shown in Figure 4. A very strong clue to the cause of this porosity is that these defective areas were found at the location of the risers on top of the casting (which were removed after casting shakeout). This suggests the phenomenon which has been discussed earlier in this paper, that multiple risers are being used on a single common feed zone and only one riser is showing piping behavior with shrinkage formation under the non-piping risers. Figure 4. Porosity on machined face, under riser location. An analysis of this casting was performed, involving a solidification simulation and calculation of the MTR. The value of MTR was determined to be 0.96 cm. A plot of MTR in the casting is seen in Figure 5. When this design was adopted in the foundry and the riser and contact were sized correctly, the result was a shrinkage-free casting. It is worth noting that the cores which were originally used by the foundry to create the riser-casting contacts were intended for production of steel castings, where the contact diameter was 50% of the riser diameter. Consideration that the Modulus of the contact should be equal to MTR resulted in a much smaller contact diameter. In this case the foundry produced cores which were specialized for this particular casting to ensure the correct contact size. The analysis of this casting to produce the correct riser design required 15 minutes of time. The foundry could have saved considerable costs over a long period of time had they performed this quick and simple analysis before finalizing the production design for the casting. SUMMARY Understanding the solidification mechanisms of graphitic iron alloys in terms of expansion/contraction behavior, feeding mechanisms and control of expansion pressure is critical to correct design of risering systems. Quick and simple analysis is available which will help the foundry engineer to design the production process correctly at the beginning of production, thereby avoiding major costs involved in producing defective castings. Figure 5. Plot of MTR at a value of 0.96cm. This image shows very clearly that the entire casting consists of a single feed zone, and that only a single riser should be used on this casting. The final revised design for this casting is shown in Figure 6. Figure 6. Single riser design.
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