Green Sand: An Introduction to Its Use in Foundries
Introduction to Green Sand and Castings
Green sand is a compound mixture used for casting of metals in foundry applications as a mould. It is ‘green’ not by colour, but takes the moniker due to the fact that it is not ‘set’ when the metal is poured into it, rather it is still in the ‘green’ or uncured state. The name derives from green wood, that is, wood still containing a large quantity of water. Green sand is mostly made up of sand, clay, sludge, anthracite and water. The identity of the sand component is crucial. Green sand casting is a simple, reliable and widely used method of casting metals. Green sand should not be confused with greensand, a type of sandstone with a greenish colour, which is not used in foundry applications.
What is Green Sand?
Green sand is a mixture typically containing a sand (75 to 85% by mass), bentonite or kaolinite clay (5 to 11%, used as a binder), water (2 to 4%), sludge (3 to 5%, acting mostly as filler) and anthracite (<1%, as a carbonaceous additive). The sand itself is one of three main types, depending on the casting material; silica-based, chromite-based or zircon-based. Again, depending on the nature of the casting material, other inorganic compounds may be added(1). As casting deals with high-temperature molten metals/alloys, a high capacity for heat is critical. Green sand is advantageous due to its porous nature, allowing gases produced in the mould to escape.
How is it Used?
Casting of metals is a six-step process.
- A pattern is placed into sand,
- The pattern shape is incorporated into the sand as a mould using a flask,
- The pattern is removed,
- The mould is filled with molten metal,
- Allowed to cool/solidify,
- Finally the mould is removed leaving the finished casting.
During the process of casting, the latent heat from the molten metal cures the mould in situ. The molten metal/alloy is poured into the mould directly after it has been compacted into shape. After cooling of the casting, the mould can be removed(2). Spent moulds are typically ground down and re-used, especially if based on one of the more expensive sand types, though it is noteworthy that the US and UK send on average 500kg and 250kg respectively of sand to landfill for every tonne of cast metal(1). Studies have shown that spent foundry sand can be re-used in concrete production(3). Green sand casting is used for a wide variety of casting applications, from small and detailed, to large moulds up to 500 kg in size.
Advantages Over Other Materials and Processes
Green sand casting is a simple, scalable and resilient method for the casting of metals. Casting itself is a millenniums-old technique(4). Depending on the choice of sand, the process can be incredibly inexpensive, particularly if silica is the major sand component due to its abundance. Aspects of the casting such as the surface finish of the product can be easily modulated by the grind size of the sand component, which can also be impacted by the addition of ‘cereal binders’ like dextrin and molasses. Furthermore, the addition of these additives can enhance removability properties. Iron oxide can be added to the green sand in small quantities to prevent metal penetration due to cracking of the mould(5). One notable disadvantage of using silica-type sands as the major green sand component is the potential for the escape of silica particles during the pouring of the metal, which could result in silicosis for foundry workers close by. Green sand moulds, on the small scale, have been produced via 3D printing methods(6) with high degrees of precision and success.
Specialist Components of Green Sand Moulding
As mentioned, various components of the green sand can be changed, or specific compounds used to deliver a specific outcome, or to provide a green sand that is best suited to a particular metal/alloy, or for better environmental performance in the overall green sand moulding process. Two of the most common changes in green sand moulding are using a chromite-type sand, and by using coal dust (anthracite) in the mix.
Coal dust, or anthracite, is fast becoming a common additive in the green sand moulding process. It is a carbonaceous additive that under foundry conditions combusts and oxidises. Historically, the leading additive in green sand casting, as a carbonaceous material, was highly volatile bituminous coal, often referred to as sea coal. Bituminous coal combusts in the mould upon heating and releases hazardous pollutants such as benzene, xylene and toluene. It is imperative that bituminous coal is replaced with an equally well performing carbonaceous material, but with a less environmentally damaging profile. An experimental study showed that under laboratory conditions, anthracite and lower-grade lignite-type coals(7) emitted significantly less hazardous pollutants than did bituminous coal(8), therefore it can be stated that the incorporation of coal dust into a green sand mould is advantageous from an environmental perspective. It is noted that the casting industry wishes to move away from bituminous coal because of its poor environmental performance (9) and fewer hazardous pollutants mean fewer resources need to be dedicated to scrubbing of the foundry’s exhaust overall.
The use of ground anthracite decreases burned-on defects, improves the finish on the surface of the product, and decreases metal penetration. Perhaps its major use, however, is to prevent wetting. Wetting is the process by which the liquid metal/alloy sticks to the sand particles in the green sand mould, leaving them present on the surface of the casted product. Whilst in many cases, mechanical polishing or further machining of the casted product can be performed to remove errant sands, prevention is better than cure. Coal dust provides an effective and inexpensive method of achieving this.
When the mould is heated by the presence of the melted metal/alloy, the coal present decomposes (burns) and gives off carbon dioxide and a variety of volatile organic compounds. The production of such reducing gases inhibits the production of iron oxide (in iron and steel casting applications) which have been hypothesised as an intermediate in the production of ‘burn on’ - impurity deposits on the surface of the casting(10). In addition, due to the reducing environment, a layer of highly lustrous carbon is deposited on the mould surface, affording a refractory barrier between molten metal and sand, thus improving the surface finish(11) and making the casting easier to remove from the mould.
Chromite (FeCr2O4, iron chromium oxide) is an inorganic material most commonly used as an ore in the production of stainless steel; it is the only ore of chromium. As a refractory material in powdered/granulated form, it is particularly attractive due to its stability at high heat, boasting a melting point of 2150 °C. Due to its relatively low abundance (relative to silica), it is a higher-priced sand. As such, chromite is only used in applications requiring the highest quality alloy steels, or for the manufacture of cores. Chromite has low thermal expansion and high thermal conductivity (12).
Valued for its use as a green sand for the production of heavy, sectioned, ferrous-type castings, chromite is not easily wetted and, unlike silica, is basic in nature (13). It is particularly useful in the production of high-manganese content steel (Hadfield steel), as silica does not offer the same levels of chemical resistance(14). It is also commonly used as a mould for aluminium casting.
Chromite moulds can be hard to recover and reuse should there be moderate-to-high amounts of silica present, as this reduces its refractoriness should the chromite become contaminated (15). Despite having more advantageous properties compared to silica, chromite is typically used as a ‘facing sand’ for silica moulds. That is, where silica sand is used for the bulk of the mould and chromite is used at the interface of green sand and the molten metal (16).
High purity chromite occurs naturally in South Africa (17). It is noted that lower quality chromite - such as that reclaimed from other industrial processes - does not have the same levels of thermomechanical robustness and hence sintering during the casting process can be observed (18).
- Green sand is a mixture of materials used to produce moulds for the casting of metals
- It is typically composed of sand, clay, water and other additives
- The process of using green sand moulds is resilient, scalable and reliable
- Silica is the most commonly used sand
- Chromite is an alternative sand that can be used, particularly to take advantage of its superior thermal properties and where chemical resistance is required
- Coal dust (anthracite) is an additive in the green sand-casting process that has a better environmental performance profile than what it replaces, and overall enhances the casting process by improving the finish, reducing burned-on defects and by eliminating wetting
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