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Metal Casting Technologies : June 2007
METAL Casting Technologies June 2007 55 Reference: Briggs, Charles Willers, The Metallurgy of Steel Castings, McGraw-Hill Book Company, New York. NAME & DESCRIPTION OF DEFECT CAUSES Sags • Poor wall section. Changes in the appearance of the desired casting. • Poorly positioned core prints • Shifted chaplets • General casting design. Scabs An irregularly shaped mass of metal that usually lies above the casting surface. • High hot shrinkage due to low The defect is due to a combination of spalling and hot shrinkage of the sand. refractoriness of the sand. • Molding sand is too fine. • Moisture in the cores. • Too slow pouring of mold. • Minimize reflected heat on the cope by shading it with internal chills. Seams • Spalling of the cope surface of the mold. Seams are similar to rat tails, except that they are broader and more confined in appearance. Shrinkage • Inadequate size of risers. A cavity caused by inadequately feeding the casting during its • Wrong location of risers. contraction in solidification. • Large differences in section thicknesses of adjoining sections. • Uncontrolled temperature gradient in the mold. - Wrong positioning of gates and risers. - Improper speed of pouring. - Wrong pouring temperature. - Too thin sections being poured. - Improper directional solidification (use chills if possible). - Molding materials of different thermal characteristics. Slag Spots See and refer to "Dirt Spots." Swells • Uneven ramming of the mold. Metal lumps or bulges on the casting surface or on the walls of inner passage • Metal was able to compress that faces. They are formed when the mold settles over a small localized area, or portion of the mold. when the metal compresses the mold in a localized spot. Thick-thin Sections • Shifting of the core either before or during One section of the casting is thinner than that of the pattern; while a the pouring operation. corresponding section is thicker than designed. • Avoid loosely set cores before mold closing. Veining • Too rapid collapsibility of the molding sand. Small, raised lines of metal similar to veins standing out on parts of the • High expansion characteristics of the human body. These are formed when metal enters cracks in the molding sand. molding sand. • Low green and dry strength of the sand. • Not enough fines in the molding sand.