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CAUSE DEFECTS


AND T


Detect the causes of and determine remedies for common surface flaws in sand castings. BY DENISE KAPEL, SENIOR EDITOR


here is an explanation for every cast- ing defect. In sand casting, flaws on the surface might indicate faulty properties of the mold, a deeper metallurgical problem or an error in how the part was poured.


Te choice of sand and binders, and how they are


mixed and compacted, can affect the surface quality of a casting greatly. For example, sand with too much low fusion material content—too little hot strength and low green strength—can produce material inclusions in the casting surface. Gates and risers can pose a challenge in preventing defects related to temperature variation. Loose sand in the mold is another typical cause of a variety of surface defects. Molding, melting and pouring techniques all affect


the quality of the finished metal product, whatever the alloy. Planning ahead can help prevent surface defects, but improper melting and pouring methods ranging from low temperature to inoculation errors or sloppy practices will defeat the best design. Te “Analysis of Casting Defects” guide published by the American Foundry Society offers these examples: “In gray iron, a high sulfur may lead to manganese-sulfide slags” and gas defects; and “Leaded


54 | MODERN CASTING March 2014


bronzes may show separation under conditions of imper- fect balance, e.g., yellow brass.” Some alloys produce dross or slag (impurities caused by oxidation) more readily than others, presenting an obstacle to clean casting production. Te following is a look at common sand casting surface defects, along with potential remedies.


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