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Metal Casting Technologies : March 2007
48 Back to the T www.metals.rala.com.au A foundryman s Guide for Analysing Casting Defects in Iron Castings (Compiled and edited by Prof. J. Hermes D. Bautista, Technical Consultant, PMAI) NAME & DESCRIPTION OF DEFECT CAUSES Broken castings • Rough handling. Castings which have been broken or cracked by mechanical action, • Too hard ramming. rough handling, or thermal shock. • Shakeout too soon. • Lack of fillets. • Poor sand collapsibility. • Too tight banding. • Excessive carbide stabilizers in metal. Carbon flotation (Kish) • Too high pouring temperature. Segregation is the metallurgical or mechanical separation of • Heat retention by mold. one or more elements during solidification. Normally this is a • Under-baked cores. function of metal composition and cooling rates. • Carbon content of metal too high. Kish is free graphite separated from the molten iron. • Cooling in mold too slow. • Hot spots. Crushes, Push-ups, Clamp-offs • Mold weights too heavy. These are indentations in the casting surface. • Worn-out patterns. These are caused by disruption of the mold surface • Misalignment of equipment. due to external or internal force or weight. • Careless mold handling. The major cause is careless-ness particularly related • Dropping of weights. to flask equipment, rigging, and molding practice. • Low green strength of molding sand. Cuts or Washes • Too soft ramming of mold. These are rough spots and areas of excess metal caused by • Excessive metal velocity in pouring erosion of the mold or core surface by metal flow. • Incorrect gating. This definition differentiates between a cut and an erosion scab, which are • Too hot molding sand. similar in appearance, but whose cures may be diametrically opposite. • Soft cores. • Excessive use of parting compounds. • Too high pouring temperature. Drops • Rough mold handling. In appearance, this defect resembles a sticker. • Poor pattern draft. It is caused by the loss of a portion of sand • Weak cores. from the cope or other overhanging section. • Poorly prepared molding sand. • Too soft ramming of molds. • Mold explosion when gas ignites. Erosion scabs • Gating through thin sections. This is a defect usually occurring in the drag in which the loosened • Interrupted metal flow during pouring. sand has been eroded away by the motion of the metal and has left • Excessive moisture in the mold or core. a solid junction between the casting and the defect. • Non-uniform ramming of mold. The defect can usually be removed leaving a solid surface. The scab may • Excessive pouring temperatures. result in sand holes or sand inclusions in some other part of the casting. he very first step in analyzing defective castings in cast irons is to correctly identify the nature of the defect. Only correct identification will lead to the proper solution of the problem. A good foundry should be equipped with the proper laboratory and inspection equipment to effectively control casting defects. Guesswork will prove to be very expensive in terms of high rejection rates and unreliable castings. A good cast iron foundry should have a rejection rate of no more than 5% at the worst; average rate would be 21/2%.