Excerpt from Casting Practice: The Ten Rules of Casting by Stephanie Dalquist
2. System Boundaries: Process Materials and Energy Use
The manufacturing process (Figure 2) begins with the formation of a single-use mold from sand and binders, which hold the sand mold together. A lot of the sand comes from the sand reclamation process, where sand from previous molds gets reclaimed for use in new molds. Cores are also made at this point for parts that have internal cavities.
In the green sand casting process, molds are made from a mixture of sand, clay, water, and carbonaceous additives (e.g. bituminous seacoal, anthracite, or ground coke). About 85% of the mold (EPA, 1998), by mass, is sand. The clay (4 to 10%) and water (2 to 5%) act as the binder system from which the mixture derives its strength. Although the carbonaceous additives are a very small component by mass, they are needed to prevent the metal from oxidizing as it solidifies. They burn off on contact with the molten metal, creating an assortment of hazardous air pollutants (HAPs).
Green sand casting is often accompanied by the use of chemical binding systems. Many parts require cores, internal cavities that must be strong enough to hold together as the metal falls in around it. Therefore, binders other than clay are used, including synthetic resins. Many of these binders have to be cured at high temperatures, though new techniques are being adopted to allow curing at room temperature. The resulting core is harder and stronger than the green sand mold. A few foundries use chemical binder systems in molds, too, but this is uncommon because the green sand process is inexpensive, relatively clean, and flexible.
At the same time, the metal is being prepared for casting. The metal is melted in a furnace and is sometimes shuffled to holding furnaces to keep it ready for production. Furnaces vary significantly in size, fuel, and efficiency. Sand casting can be used for almost any metal, but iron, aluminum, and steel are the most common. A high percentage of the metal feed is recycled material.
The molten metal is then poured into the mold. Many foundries use non-contact cooling water systems to hasten the process, bringing the cast product to shakeout that much faster. In shakeout, the sand mold is broken with vibration or high-pressure water to remove the cast product. The sand is sent to reclamation, where it will be cleaned for use in another mold. Even after shakeout, the product is not yet ready for the customer. Sprues, runners, flashing, and other excess metal are removed by cutting or grinding. Additional improvements can be made to the surface before cleaning fluids or resin coatings are applied to protect the finished product.
3. Energy use in Sand Casting
Sand casting is an energy-intensive manufacturing process because it requires the melting and shaping of significant quantities of metal. According to EIA results (EIA, 2001) from self-reported questionnaires sent to foundries nationwide, US foundries a used 216 trillion BTUs in 1998, or 14.6 million BTU per ton of saleable casting. 1997 data (154 trillion BTU, 14.1-ton cast metal) gives only 11 million BTU per ton. The discrepancy, over such a short time frame and from the same data sources, suggests that an aggregate national value may not tell the whole story over the years. Though a good approximation, it does not make distinctions on the effects of, for example, the market’s shift towards lighter metals or the changing efficiency of production.
Government survey data can be compared with published data from industry and researchers. The EIA results are comparable to typical industry estimates within the range of 13 to 15 million Btu per ton (Stevenson, 1995).
Mold and core preparation are becoming increasingly benign in energy use with new developments. Green sand molds can be used only once, which may be considered inefficient compared to permanent molds, but they are also inexpensive and easy to make and change. Each mold made requires little energy, but repeated mold making time and costs can add up. In addition, high curing temperatures are often required to cure binders in green sand cores. Increased use of no-bake binders, which cure at room temperature, has reduced energy requirements of this step.
Green sand can be reused many times without significant treatment. It is filtered to remove binder remnants and fine-grained particles too fine for remolding. After each run, 10% of the green sand is removed from the foundry cycle (DOE, 1999). Green sand filtration produces lots of dust, which needs to be controlled and disposed of properly. For chemically bound sand, reclamation includes the removal of binder residue from sand before reuse within the industry, recycling to other industries, or landfilling. Thermal sand reclamation is the most common method used. Heat or infrared radiation can be applied to combust binders and contaminants (Heine, 1983). The application of heat is similar to what happens near the sand/metal interface during casting, and leads to the release of many of the same organic air pollutants. Heat reclamation changes the properties of the sand over time, eventually preventing its reuse in casting. These processes take, on the outside, 1 MBTU per ton of sand processed (EPA, 1995b), or approximately 5.5 MBTU per ton of metal cast (ETBP, 1998). Thus the 10% sand loss is not insignificant, as each ton of cast metal requires about 5.5 tons of sand, of which approximately half a ton is disposed of.
The mold preparation stage, in total, requires about 1.0-3.0 MBTU/ton saleable casting (DOE, 1999) for green sand foundries and additional energy in foundries using chemically bound sand. This is in line with industry group analysis (CMC, 2002), which concludes that mold making and core making contribute about 20% of total energy use (Figure 3).
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