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Metal Casting Technologies : December 2007
METAL Casting Technologies December 2007 46 1. INTRODUCTION reensand, the traditional moulding process, remains the most popular method for the production of high volume iron castings, for iron and steel castings in many jobbing foundries, and, in recent years as an alternative to diecasting for high volumes of certain aluminium base castings. It is estimated that around 65% of world castings production is still produced using greensand moulds. Greensand is foundry sand that uses clay-water bonding thus enabling moulds to be produced at high production rates by simple squeeze and compaction methods. These days natural clay bonded sands, that is, quarried sands with suitable natural clay contents are seldom used. Modern foundry greensands are made up of special mixtures based on washed and graded silica sand with measured additions of specially blended bentonite clays, water, and other materials such as lustrous carbon formers and cereal. The main advantage of clay bonded sands is that they can be continually re-used by cooling and then remixing with additions of water, new clay and fresh coal dust, etc. to replace the relatively small amounts lost or destroyed during casting, together with additions of some new sand. The continual conditioning and re-use of the sand is carried out within a sand system, which in a typical automotive iron foundry may contain up to around 300 tonnes of sand: hence greensand is often called "system" sand. The raw materials used are relatively inexpensive and are only completely broken down in the very high thermally loaded parts of the mould that come into contact with molten metal. Chemically bonded sands in which bonding is based on chemical reactions such as the hardening of sodium silicate or the polymerization of resin type materials cannot be readily re-used. When using chemical binders the sand has to be reclaimed by removing the bond material by thermal or mechanical means. Unlike the clay in greensand the bond material is lost during reclamation of the sand and cannot be used again. The raw materials for chemically bonded sand also tend to cost more than those for greensand. Hence chemically bonded sands are only used where their higher strengths and superior thermal properties are needed, e.g. as cores to make the internal shapes of castings or for moulds in which improved control of dimensional tolerances is required, or where extra resistance to high temperatures is needed. The metal casting industry has to pay ever-increasing attention to the environmental aspects of their processes and products particularly in terms of emissions and "waste" by-product. In Europe and the US foundries have had to clean up their processes to meet environmental standards so that they can stay in business. Environmental aspects of the preparation of moulds and cores, the pouring, cooling and knock out of moulds [1-3] and the disposal/re-use of spent moulding materials [4-6] have become key areas for R&D in castings production. In general greensand presents fewer health & safety and environmental problems than chemically bonded sands but foundries using greensand have also had to make improvements in environmental compatibility. Castings buyers have become more demanding of casting quality especially with regard to dimensional accuracy, surface finish and consistency -- all which depend essentially on the performance of the mould. Although greensand is traditional, technological improvements to the process therefore continue to be made to improve both its casting and its environmental performance. 2. MATERIALS USED IN GREENSAND PREPARATION Most sand moulds, including greensand, and cores use suitable silica sand since it is readily available and is cheaper than zircon, chromite and olivine sands. To ensure sufficient refractoriness the silica sand must contain a minimum of 96% SiO2 and its size distribution must be controlled. The size, shape and size distribution of sand affect the amount of binder that is required and the packing efficiency, and also have a significant influence on mould permeability and resistance to expansion defects. For greensand moulding the sand normally has an average grain size of 220-250μm with 95% having a grain size spread across 4 screens between 150-350μm. The grains should be rounded with medium to high sphericity. Depending on the method of compaction to be used for A review of greensand moulding basics with reference to some environmental aspects By Dr. John Pearce