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Metal Casting Technologies : December 2007
METAL Casting Technologies December 2007 47 mould production greensand contains 2.5 - 3.5% water and between 5 to 10% of active (live) montmorillonite type clay known as bentonite. For example, high pressure box-less moulding uses 2.5 -- 3% water and 7-10% clay. There are two main types of natural bentonites: sodium (Na) bentonite and calcium (Ca) bentonite. Na bentonite provides moulds with good green strength and high hot and dry strength. It is tolerant to water variations and is durable producing low dead burnt fines. It gives greensand moulds resistance to the effects of drying out and to problems such as mould erosion and resistance to expansion defects. It is ideally suited for large or heavy sectioned iron and steel castings. For production of smaller castings, especially at high volume, its high dry strength becomes a disadvantage since more energy is required at shakeout when the castings are removed from the mould after solidification and cooling. In contrast, Ca bentonite produces moulds with high green strength but low dry strength. Ca bentonite can be converted to synthetic Na bentonite using additions of soda ash; this synthetic material, called Na-activated bentonite, has properties near to those of natural Na bentonite. Foundries mass producing Iron castings always use blended mixtures of Na bentonite with Ca bentonite or with Na- activated bentonite. Blending is aimed at producing optimum moulding and mould properties at an economic cost. The target properties are rapid bond development, high green strength, good tolerance to moisture variations, maximum flowability plus the optimum levels of hot and dry strengths to give high thermal stability, high resistance to erosion and distortion combined with good breakdown and easier shakeout. The suppliers of bentonites work together with foundries to tailor particular blends to suit the particular properties required in a specific moulding plant and casting line. When producing iron castings, carbon containing materials called "lustrous carbon formers" are included in the