Plan ahead to prevent surface flaws and defects in sand cast components.

Denise Kapel, Senior Editor

(Click here to see the story as it appears in January/February 2014's Metal Casting Design & Purchasing.)

There is an explanation for every casting 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. Discussion with your casting supplier during the planning and design stages can help eliminate costly quality issues. 

Common causes

The 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. The “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 casting production.

Planning ahead can prevent surface defects. Improper melting and pouring methods ranging from low temperature to inoculation errors and sloppy work will defeat the best casting design. Talk with your supplier up front about quality expectations and the metalcasting process. Click here to see common sand casting surface defects, courtesy of S&B Industrial Minerals N.A. Inc., Cincinnati, “Manual of Casting Defects,” 3rd edition.