CAUSE DEFECTS
AND
Plan ahead to prevent surface flaws in sand cast components. BY DENISE KAPEL, SENIOR EDITOR
T
here is an explanation for every casting defect. In sand casting, flaws on the surface might indicate faulty prop- erties of the mold, a deeper metallurgical problem or an error in how the part was poured. Discussion with your casting supplier during the planning and design stages can help eliminate costly quality issues.
Common causes 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. Once the correct alloy is determined for a part based on its performance
requirements, the next step is to consider the process by which it will be cast. Molding, melting and pouring techniques all have an effect on the quality of the finished metal product. 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
bronzes may show separation under conditions of imperfect balance, e.g., yellow brass.” Some alloys produce dross or slag (impurities caused by oxidation) more readily than others, presenting an obstacle to clean cast- ing production. Planning ahead can prevent surface
defects. Improper melting and pouring methods ranging from low tempera- ture to inoculation errors and sloppy work will defeat the best casting de- sign. Talk with your supplier up front about quality expectations and the metalcasting process. Te following are common sand
casting surface defects, courtesy of S&B Industrial Minerals N.A. Inc., Cincinnati, “Manual of Casting De- fects,” 3rd
edition. Jan/Feb 2014 | METAL CASTING DESIGN & PURCHASING | 35
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