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REMEDIES FOR SAND ADHERENCE DEFECTS


Sand Adherence Adhering sand defects are com-


mon in all alloys cast in green sand molds and are characterized by a rough casting surface or by sand sticking to the casting surface. Tese defects can cause surface and sub- surface defects. Two of the most com- mon ways these defects are produced are mechanical penetration and chemical reaction. In mechanical penetration, the metal penetrates into the green sand mold because of the metallostatic pressure of the molten metal. It usu- ally is seen when the sprue height is too large. The greater the height of the metal in the mold from the top of the pouring cup to the bot- tom of the casting, the greater the pressure exerted on the liquid. High metal pressures force molten metal between the sand grains where the metal solidifies, holding sand on the casting surface. During a sand adhering chemical


reaction, a reaction between the liq- uid metal (iron) and the sand (sili- con) molding material occurs. These reactions may produce products that act as glue, adhering the molding sand to the casting. These reactions occur only in ferrous alloys. There are two types of chemical reactions: burn-on and burn-in. Te difference between the two is the cooling rate. Burn-on cools slowly and forms crystals. Burn-in cools fast and forms a glass-like and possibly pock-marked surface. Shot blasting will take off a burn-


• Reduce the moisture content of the sand because the moisture fills the space between the sand grains. When the moisture evaporates after being hit by molten metal, it then leaves open space for the metal to penetrate.


• Improve mold compaction to increase density and leave less room for the metal to penetrate.


• Reduce metal pouring speed because higher velocities create more pressure, allowing the metal to penetrate more easily.


• Improve the casting design and avoid metal reentrant angles. Sharp internal cor- ners create hot spots, which are areas where penetration is more likely occur.


• Reduce the metallostatic head pressure because the higher the pressure, the easier metal can penetrate the mold wall. Shorten the sprue.


• Use a mold coating as a preventative barrier against the metal at the mold interface. • Check metal chemistry and temperature, especially in cast iron, to ensure proper fluidity. • Improve the mold filling with better gating to reduce velocities and avoid hot spots. • Increase carbon additives (seacoal) in the mold to create the reducing atmosphere in the mold that produces better surface finish.


• Add a finer sand to the mix if the GFN is too coarse for better mold compaction.


on defect. A burn-in defect usually requires a grinding operation to remove it, if it can be removed at all.


Weak Sand Strengths Two types of sand strength defects


occur in green sand molds: lack of green strength and too little hot strength. Green strength represents the


ability of the green sand to hold the constructed mold. A lack of green strength can be caused by poor sand composition, not enough clay, improper clay proportions, too much or too little water, improper sand mixture, and lack of mulling time. If sand does not have sufficient


green strength, the mold may tear up when the pattern is stripped. Loose sand also may result, leading to inclu- sions in the casting.


REMEDIES FOR GAS POROSITY DEFECTS


• Reduce the combustible level of the sand because combustibles create gas during pouring. • Reduce the moisture content of the sand because moisture means more steam. • Increase the sand permeability to allow the gases to escape through the mold. • With entrapped gases, increase the metal pouring temperature to increase metal fluid life, which provides entrapped gases more time to escape the mold and/or metal.


• With soluble gases, reduce the pouring temperature to reduce the chance for gases to be dissolved in the metal.


• Vent molds and cores to provide the gases a highway to escape through. • Reduce the binder level of cores to reduce the gas producing materials in the mold. • Fill the mold quickly, but quietly to avoid turbulence and entrained oxides. Also, the quicker a mold fills, the less time allowed for gases to form.


• Use a mold or core coating to prevent gases from escaping into the molten metal. Tear-ups/drops, stickers, and


inclusions are defects caused by low green compressive strength. Hot strength is the ability of the


green sand to hold the constructed mold under high temperatures. Low hot strength is a common


cause of defects for cast iron and steel metalcasting facilities where higher metal temperatures are required. Te defects that occur due to this problem are erosion and run outs. Erosion is the wearing away of the mold as the liquid metal fills the mold cavity. Without sufficient hot strength, high metal velocity washes the mold material away, leaving an erosion scab. A run-out defect is a defect


caused by incomplete filling of the mold cavity due to draining or leak- ing. While a majority of run outs occur due to a lack of mold weight being placed in the mold prior to pouring, a run out also can occur with low hot strength. Table 1 provides a helpful chart


to connect the type of defect with the area of the green sand molding process that might have been a factor in caus- ing the defect.


Tis article is based off the AFS Institue e-Learning module, “Introduction to Green Sand Defects.” Alan Jacobson, Grede St. Cloud, Ed Marczyszak, Burndy Foundry, and Jerry Tiel, University of Northern Iowa, contributed to the development of the module. Go to www.afsinc. org/e-learning for more information about this and other e-Learning modules.


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