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Metal Casting Technologies : March 2007
50 Back to the www.metals.rala.com.au NAME & DESCRIPTION OF DEFECT CAUSES Misruns and Cold shuts A misrun casting lacks completeness due to failure of the metal to fill the • Mold or core shift. mold cavity. There may be a round-edge hole through the casting wall. • Interrupted metal flow during pouring. A cold-shut casting shows a definite discontinuity due to imperfect fusion • Improper gating. where two streams of metal have converged. The defect may have the • Uneven ramming of mold. appearance of a crack or seam with smooth, rounded edges. • Too low pouring temperature. • Casting section too thin. • Insufficient venting. • Too low head pressure. Metal penetration Metal penetration is a condition in which the metal or metallic oxides have • Openness of the mold surface. filled the voids between the sand grains without displacing them or by • Oxidized metal. chemically changing the silica or binder. • Uneven ramming of mold. • Overheating of the molding sand. • Too high pouring temperature. • Too high metal pressure in mold. Off dimension (core defect) An off-dimension defect is an incorrect dimension occurring as a result of the • Distorted cores. wrong core used, correct cores improperly set, cores omitted, cores incorrectly • Wrong core prints. assembled, or thermal instability of cores. • Improperly set cores. • Excessive metal pressure in mold. • Improper use of rods in core. • Under-cured cores. Open-grain structure A condition where a casting, when machined or fractured, is too coarse-grained. • Too much difference in metal sections. It may be throughout the casting or it may be localized. This is primarily a • Improper coring. metallurgical-type of defect. It is often difficult to differentiate between • Under-baked oil-sand cores. open-grain, porosity, and micro-shrinkage. • Excessive graphitizers in the metal. • Hot spots in the mold. • Too slow cooling in the mold. • Non-use of chills where needed. Poured short A casting which lacks completeness due to the mold not being • Insufficient metal. completely filled with molten metal. • Inadequate mold permeability. • Lack of pop-offs or bleeders in mold. Ram-offs or Ram-aways A ram-off or ram-away is an incorrect casting dimension resulting from the • Loose pattern plate. sand moving away from the pattern during molding. • Inadequate support under the pattern. • Rough mold handling. • Filling the flask too full. • Improper tucking of sand in mold. • Patterns too close together or to edge of flask. Rough surface A casting that lacks the required degree of surface smoothness for • Fillets are too small. a specific application. • Too high metal velocity and temperature. • Coarse sand. • Soft ramming of mold. • Improper mold or core coating. • Dirty patterns or core-boxes.