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Metal Casting Technologies : March 2009
Figure 2a. Model of casting with chill on isolated hot-spot Figure 3. Blow-holes at chill contact Figure 2b. Solidification pattern through centre of casting Figure 4. Cracking at edge of chills positioned on the upper surface of a casting are largely ineffective due to the rapid formation of an air-gap and as such should not be used. 3. Blow-holes at chill contact, such as shown in figure 3 are generally caused by moisture. This can be the result of using damp or rusty chills, condensation on chills positioned in warm moulds or undried coating on the surface of the chill. Chills should be thoroughly ground or shot-blasted before use and stored in a warm dry atmosphere. Chills should be discarded if the surface deteriorates through continual usage. Coatings used on chills are sometimes difficult to dry, it is good practice to precoat chills prior to use and dry by torching or placing in a warm oven. Figure 2c. Plot of shrinkage through centre of casting showing shrinkage eliminated from isolated hot-spot 2. Chills ineffective or not working at all, is again usually the result of undersized chills. If an air-gap exists between the chill and casting surface, such as would be the case if a chill was positioned on the side wall of a casting, then the above recommended sizes should be increased. Chills 4. Cracking around chills such as shown in figure 4, is usually the result of too severe chilling. Chill thickness should not exceed the section of the casting being chilled. Incorporating a buffer of chromite or zircon sand around the chill can also help to eliminate this problem. When multiple chills are used on a section, the distance between adjacent chills should be equal to the chill length and the space between chills filled with a buffer material. METAL Casting Technologies March 2009 51