Metals Handbook VOL 15

• Molding Methods and Materials, American Foundrymen's Society, 1962 • C.F. Walton and T.J. Opar, Ed., Iron Casting Handbook, Iron Castings Society, 1981 Aggregate Molding Materials Thomas S. Piwonka, University of Alabama Introduction THE CASTING PROCESS involves the pouring of molten metal into a mold; therefore, the mold material and molding method must be selected with care. Most castings are made in sand molds because metallic molds wear out too quickly to be economical for ferrous metals production. For low and medium production runs, the lower tooling costs of sand molding give it an overwhelming cost advantage over permanent molds or die casting. Selection of the molding material and its bonding system depends on the type of metal being poured, the type of casting being made, the availability of molding aggregates, the mold and core making equipment owned by the foundry, and the quality requirements of the customer. A thorough understanding of all of these factors is necessary to optimize the molding system used in the foundry. This article will discuss the various materials used to produce molds and cores for sand casting. These materials include sands, clays, additions to sand mixes, and plastics. The principles that explain how these materials are bonded together are discussed in the articles "Bonds Formed in Molding Aggregates" and "Resin Binder Processes" in this Section. Additional information on the preparation, mulling, handling, and reclamation of sands is available in the article "Sand Processing" in this Volume. Sands The refractory molds used in casting consist of a particulate refractory material (sand) that is bonded together to hold its shape during pouring. Although various sands can be used, the following basic requirements apply to each (Ref 1): • Dimensional and thermal stability at elevated temperatures • Suitable particle size and shape • Chemically unreactive with molten metals • Not readily wetted by molten metals • Freedom from volatiles that produce gas upon heating • Economical availability • Consistent cleanliness, composition, and pH • Compatibility with binder systems Many minerals possess some of these features, but few have them all. Silica Sands Most green sand molds consist of silica sands bonded with a bentonite-water mixture. (The term green means that the mold, which is tempered with water, is not dried or baked.) The composition, size, size distribution, purity, and shape of the sand are important to the success of the moldmaking operation. Sands are sometimes referred to as natural or synthetic. Natural sands contain enough naturally occurring clays that they can be mixed with water and used for sand molding. Synthetic sands have been washed to remove clay and other impurities, carefully screened and classified to give a desired size distribution, and then reblended with clays and other materials to produce an optimized sand for the casting being produced. Because of the demands of modern high-pressure molding machines and the necessity to exercise close control over every aspect of casting production, most foundries use only synthetic sands. Composition. Foundry sands are composed almost entirely of silica (SiO2) in the form of quartz. Some impurities may be present, such as ilmenite (FeO-TiO2), magnetite (Fe3O4), or olivine, which is composed of magnesium and ferrous orthosilicate [(Mg,Fe) SiO4]. Silica sand is used primarily because it is readily available and inexpensive. However, its various shortcomings as a foundry sand necessitate the addition of other materials to the sand mix to produce satisfactory castings, as described later in this article. Quartz undergoes a series of crystallographic transitions as it is heated. The first, at 573 °C (1064 °F), is accompanied by expansion, which can cause mold spalling. Above 870 °C (1598 °F), quartz transforms to tridymite, and the sand may actually contract upon heating. At still higher temperatures (> 1470 °C, or 2678 °F), tridymite transforms to cristobalite. In addition, silica reacts with molten iron to form a slag-type compound, which can cause burn-in, or the formation of a rough layer of sand and metal that adheres to the casting surface. However, because these problems with silica can be alleviated by proper additions to the sand mix, silica remains the most widely used molding aggregate. Shape and Distribution of Sand Grains. The size, size distribution, and shape of the sand grains are important in controlling the quality of the mold. Most mold aggregates are mixtures of new sand and reclaimed sand, which contain not only reclaimed molding sand but also core sands. In determining the size, shape, and distribution of the sand grains, it is important to realize that the grain shape contributes to the amount of sand surface area and that the grain size distribution controls the permeability of the mold. As the sand surface area increases, the amount of bonding material (normally clay and water) must increase if the sand is to be properly bonded. Thus, a change in surface area, perhaps due to a change in sand shape or the percentage of core sand being reclaimed, will result in a corresponding change in the amount of bond required. Rounded grains have a low surface-area-to-volume ratio and are therefore preferred for making cores because they require the least amount of binder. However, when they are recycled into the molding sand system, their shape can be a disadvantage if the molding system normally uses a high percentage of clay and water to facilitate rapid, automatic molding. This is because rounded grains require less binder than the rest of the system sand. Angular sands have the greatest surface area (except for sands that fracture easily and produce a large percentage of small grains and fines) and therefore require more mulling, bond, and moisture. The angularity of a sand increases with use because the sand is broken down by thermal and mechanical shock. The subangular-to-round classification is most commonly used, and it affords a compromise if shape becomes a factor in the sand system. However, control of grain size distribution is more important than control of grain shape. The grain size distribution, which includes the base sand size distribution plus the distribution of broken grains and fines from both molding sand and core sands, controls both the surface area and the packing density or porosity of the mold. The porosity of the mold controls its permeability, which is the ability of the mold to allow gases generated during pouring to escape through the mold. The highest porosity will result from grains that are all approximately the same size. As the size distribution broadens, there are more grains that are small enough to fill the spaces between large grains. As grains break down through repeated recycling, there are more and more of the smaller grains, and the porosity of the mold decreases. However, if the porosity of the mold is too great, metal may penetrate the sand grains and cause a burn-in defect. Therefore, it is necessary to balance the base sand distribution and continue to screen the sand and use dust collectors during recycling to remove fines and to determine the proper bond addition. Most foundries in the United States use the American Foundrymens' Society (AFS) grain fineness number as a general indication of sand fineness. The AFS grain fineness number of sand is approximately the number of openings per inch of a given sieve that would just pass the sample if its grains were of uniform size, that is, the weighted average of the sizes of grains in the sample. It is approximately proportional to the surface area per unit weight of sand exclusive of clay. The AFS grain fineness number is determined by taking the percentage of sand retained on each of a series of standard screens, multiplying each by a multiplier, adding the total, and then dividing by the total percentage of sand retained on the sieves (Ref 2). Table 1 lists the series of sieves used to run the standard AFS standard sieve analysis. A typical calculation of the AFS fineness number, which includes the multiplier factor, is given in Table 2. Table 1 Screen scale sieves equivalent USA series No. Tyler screen Sieve Sieve Sieve Permissible variation Wire scale sieves, opening, opening, opening, in average opening, diameter, openings per mm μm ±mm mm in., ratio 2 , lineal inch or 1.414 6 6 3.35 3350 . 0.11 1.23 8(a) 8(a) 2.36 2360 0.0937 0.08 1.00 12 10 1.70 1700 0.0661 0.06 0.810 16(a) 14(a) 1.18 1180 0.0469 0.045 0.650 20 20 0.850 850 0.0331 0.035 0.510 30 28 0.600 600 0.0234 0.025 0.390 40 35 0.425 425 0.0165 0.019 0.290 50 48 0.300 300 0.0117 0.014 0.215 70 65 0.212 212 0.0083 0.010 0.152 100 100 0.150 150 0.0059 0.008 0.110 140 150 0.106 106 0.0041 0.006 0.076 200 200 0.075 75 0.0029 0.005 0.053 270 270 0.053 53 0.0021 0.004 0.037 Note: A fixed ratio exists between the different sizes of the screen scale. This fixed ratio between the different sizes of the screen scale has been taken as 1.414, or the square root of 2 ( 2 ). For example, using the USA series equivalent No. 200 as the starting sieve, the width of each successive opening is exactly 1.414 times the opening in the previous sieve. The area or surface of each successive opening in the scale is double that of the next finer sieve or one-half that of the next coarser sieve.

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