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Excerpt from the June 2001 Modern Casting article by Ian Kay, Mark Nagel and Alfred T. Spada
Casting defect recognition is one of the most difficult tasks facing a metal caster. With the multitude of processes (core making, molding, melting, etc.) used to manufacture a casting, determining which is responsible for a defect requires analysis, testing and, most importantly, experience.
Of all the departments within a foundry, more defects can be attributed to molding and the sand system than any other. This is due in part to the high number of components that make up a greensand mold. From the sand, clay and water to the carbon, cereal and other additives, each component has properties that serve to reduce or control specific defects in castings. However, when the amount of any one component is out of balance with regard to the casting being poured, the potential for defects arises.
This article examines the causes of common green sand casting defects related to expansion, metal penetration, gas and weak sand while offering possible remedies. Although every casting operation is different, common themes tie many of these defects together, allowing a foundry to follow a simple step-by-step remedy progression to determine which area(s) of mold development is responsible for the defect.
Expansion defects are a family of defects that include rat tails, buckles and scabs. These defects originate, in part, from the expansion of the sand gains when heated by the metal entering the mold. Silica sand expands the greatest amount when in contact with the molten metal, as compared to olivine, chromite and zircon sands, which expand less.
Beyond sand expansion, these defects also are moisture related. As molten metal runs over the surface of a green sand mold, moisture in the sand is converted to steam that permeates between the sand grains. When the steam reaches a point in the mold where the sand temperature is less than 212F (100C), it re-condenses, creating a wet layer. This wet layer is weaker than the normal greensand or the hot, dry sand layer directly beneath the metal. As the hot sand expands, the wet layer shears to allow the expansion. The small ridge of sand that extends into the mold cavity as a result of the expansion can create a line on the surface of the casting called a rat tail (Fig. 1). This defect usually is formed on the drag portion of the casting.
In further filling of the mold cavity, the molten metal radiates heat toward the cope casting surface. The moisture on this surface vaporizes and permeates into the sand where it condenses to form the wet layer. In the same manner as in the drag portion of the mold, as the molten metal nears the cope surface of the mold, the intensity of the radiant heat increases and the sand in the dry sand layer expands. The wet layer splits or shears to accommodate this expansion.
As the metal completes the filling of the mold cavity, the sand buckles, creating a deep groove on the casting surface called a buckle (Fig. 2). Sometimes the buckle will open up, allowing the metal to run through the crack in the sand and fill the void behind the buckle to create a scab (Fig. 3). Although the rat tail is synonymous with the drag and the buckle and scab with the cope, the three expansion defects may be found on either casting surface. When foundries are faced with these defects, the following remedy progression should be applied to the sand system:
- make an addition of cellulose or cereal to the sand to provide a place for expansion to occur;
- lower the moisture content of the molding sand, which increases the overall mold strength;
- lower the pouring temperature of the metal (eliminate excess super-heat), which reduces the amount of sand expansion;
- lower the temperature of the molding sand from the return sand system to increase the strength properties of the sand;
increase the clay content of the sand, especially sodium(western) bentonite, for better hot strength properties;
improve the sand distribution to at least three screens to stagger the expansion and create a linear expansion curve;
decrease the amount of fines in the sand. Fines tend to soak up water, increasing overall mold moisture without increasing mold strength;
avoid over-ramming or over-squeezing the mold. This pushes the moisture closer to the mold surface, increasing the probability for defects;
improve the sand mulling practice to create a more homogenous sand mixture with better developed bond;
fill the mold faster by increasing the flow rate of the gating system to leave less time for the heat to act on the sand without pressure from the metal.
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