Sand casting, though an ancient technique, has seen remarkable advancements and remains highly relevant in the 21st century. This process, with roots tracing back nearly 5,000 years to ancient Egypt, has evolved considerably through the centuries while preserving its core principles.<\/p>\n
In this article, we\u2019ll dive into the modern sand casting process, examining how it works and why it remains one of the most widely used metal casting techniques<\/a> even thousands of years after its inception.<\/p>\n Sand casting, or sand moulded casting, is a metal fabrication<\/a> process that uses dies made from sand<\/strong> to create metal products. Sand is easily mouldable with common objects. After shaping it as desired, hot molten metal is poured into the mould. As the metal cools, it takes on the shape of the mould as it solidifies. The final metal product is then extracted from the sand mould and undergoes finishing<\/a> before being put to use.<\/p>\n Using the sand casting technique, products of all sizes and complexity levels can be manufactured. It can be used to cast items as small as pins or as large as railway locomotives<\/strong>. Additionally, it is compatible with a wide range of metals<\/strong>, including cast iron, malleable iron, steel, nickel alloys, copper alloys, aluminium alloys<\/a>, and magnesium alloys.<\/p>\n Its versatility makes sand casting the most widely-used casting process in the world, with over 60% of all cast metal products being made using this method.<\/p>\n While there are many different types of sands to choose from, not all of them are suitable for the casting process. Each of them exhibits certain traits that may or may not align with the project requirements.<\/p>\n Depending on the casting process and metal, the right combination of properties in the moulding sand is needed. To enhance cohesion, special additives and binders can be added<\/strong> to the basic sand, clay, and water mixture.<\/p>\n These binders, which can be organic or inorganic, include cereals, ground pitch, sea coal, gilsonite, fuel oil, wood flour, silica flour, iron oxide, and pearlite among others. Some of these binders have contradicting effects but trade-offs are often necessary<\/strong> to balance the properties for specific casting applications.<\/p>\n The four most common types of sand<\/strong> used for casting are green sand, dry sand, sodium silicate and resin sand:<\/p>\n Green sand<\/strong> is called so because it is essentially raw sand mixed with water. It exhibits a clay-like consistency. Green sand offers a low-cost option for casting sand with good reusability and is compatible with high production rates. However, its low malleability can cause the mould to collapse under certain situations. It also has a propensity to create surface defects.<\/p>\n<\/li>\n Dry sand<\/strong>, on the other hand, is a moulding sand that uses clay and contains very little water acting as the binding agent. It holds its shape quite well once rammed into place. A benefit of dry sand is that it creates very little steam during the filling stage, which in the case of more moist sands, can lead to mould fracturing if left unchecked.<\/p>\n<\/li>\n Sodium silicate<\/strong>, aka water glass, is a special sand that uses a sodium silicate binder to integrate the sand. Sodium silicate can create more intricate or complex shapes than green and dry sand.<\/p>\n<\/li>\n Resin sand<\/strong>, aka Furan sand<\/em>, is another popular sand used for sand casting It uses a furfuryl alcohol resin to make a coherent mass out of the sand. The robust and highly stable compound has very little tendency to break down and provides excellent dimensional accuracy and surface finish. However, it is the most expensive of all sands. It also has a higher lead time.<\/p>\n<\/li>\n<\/ol>\n Casting engineers choose the type of sand based on the metal, product geometry and tolerance limits<\/a>. But no matter the specifications, they always screen a sand type for the following properties:<\/p>\n Refractoriness is a material’s property that allows it to withstand thermal shocks and burning when subjected to high temperatures. The sand serves as a refractory material for molten metal and thus needs to have a high melting point. The selection of sand for casting is largely influenced by the process temperature. If the refractoriness is insufficient, the sand will melt and fuse to the casting.<\/p>\n For many metals, such as aluminium, the sand only needs to withstand a temperature of 700 degrees Celsius. However, sands for steel castings need to withstand at least 1500 degrees Celsius as the melting point is higher<\/a>.<\/p>\n The sand must vent steam and other gases generated during the casting process. Each unit of water in the mould produces 1600 units of steam.<\/p>\n If the gases and steam get trapped, it can cause defects such as blowholes and gas holes in the final product. Their removal is crucial to obtain a good quality finish<\/a>. Thus, permeability is a crucial property in sand for achieving a high-quality finished product.<\/p>\n The properties of moulding sand are influenced by the grain size and distribution: small grains increase mould strength but reduce permeability, while larger grains enhance permeability. Fine grains yield a smooth surface finish on castings but have low permeability, whereas coarse grains provide higher permeability but a rougher finish.<\/p>\n Thermal conductivity is the property of a material that allows heat to flow through it. Efficient heat flow leads to quicker cooling of the molten metal. Sands with high thermal conductivity are required to transfer the heat from the molten metal to the atmosphere.<\/p>\n Thermal expansion is the increase or decrease in the volume of a material when heat is added or removed, respectively. For the sand casting process, sand with a very low thermal expansion is needed. This type of sand will maintain its shape during various stages of the casting process, including the pouring of liquid metal and subsequent cooling.<\/p>\n The ability of sand to flow into smaller areas, such as cavities, depressions, and deep channels of the pattern, determines whether we can use it for casting complex shapes. Sand with higher flowability will fill all the features of a pattern without requiring external pressure.<\/p>\n A highly flowable sand is not needed for simple shapes, such as a manhole cover, but it may be required for something more complex like a sculpture. The level of flowability needed may therefore be determined on a case-by-case basis.<\/p>\n The cost can also vary significantly from sand to sand. Silica sand, for instance, has high availability but also issues like dimensional instability and the tendency to burn or even explode to form sub-micron-sized particles under thermal shock. Silicosis<\/a> is also an extremely concerning issue with workers being exposed to silica for extensive periods. Olivine and chromite sand, on the other hand, are more expensive and less available but do not have the above-mentioned issues.<\/p>\n\n
What Is Sand Casting?<\/h2>\n
Properties of Moulding Sand<\/h2>\n
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Refractoriness<\/h3>\n
Permeability<\/h3>\n
High Thermal Conductivity<\/h3>\n
Low Thermal Expansion<\/h3>\n
Flowability<\/h3>\n
Cost<\/h3>\n
Sand Casting Process<\/h2>\n