Bentonite Clay's Role in Green Sand Metalcasting Foundry Systems
(This post is from American Foundry Society's Technical Department's Casting 101)
More than 60% of castings are made using the green sand metalcasting foundry molding process, but what is that? Green sand is a mixture of sand or aggregate, clay and water. In a green sand system, the clay and water combine to form a clay bridge that binds sand grains to one another. This bonding mechanism is a hydrated system that requires water, energy, time and technical expertise.
Two types of bentonite clay are typically used: calcium bentonite and sodium bentonite.
Sodium Bentonite (also known as Western Bentonite): Western bentonite is known for its ability to absorb and hold large amounts of water and for the resulting qualities of its high swell (bigger increase in volume) and high viscosity. It is good for green sand mixtures requiring high dry strength and hot strength, such as for iron and steel casting. It is more difficult to mull and help the clay to attach to the sand grains, so more mull energy is required.
This sodium-based clay is capable of swelling approximately 15 times its dry volume. In molding sand, it helps molding maintain its dimensional accuracy and provides a greater dry/hot strength.
Calcium Bentonite (aka Southern Bentonite): The calcium ion concentration in this variety creates a lower swell green sand system with low viscosity, lower water absorption and the resulting quicker water loss. It is generally considered to be easier to mull and provides higher green strengths but lower dry and hot strengths.
Calcium bentonite swells only twice its unwetted volume, though it provides greater compression strength and permeability. Primarily used in nonferrous castings, its lower hot retaining strength helps in shakeout.
These bentonite clays often are used in mixtures of the two at defined ratios, to maximize the benefits of each.
Other materials can be added to improve certain properties.
Preblends: Preblends are clays that are blended with other sand additives so they become an easy additive to the existing sand system during mixing. Typical preblends can be custom blended to include cellulose, cereals, sea coals and other additives in addition to the sodium and/or calcium bentonite.
New clay technologies have also created more options in the use of clay bonding, such as activated and modified clays that behave with different characteristics than their natural tendencies. Clays are treated and perform differently, so sodium bentonite can achieve some of the beneficial calcium bentonite properties and vice versa.
Reclaimed and Rejuvenated Clays: New technologies allow metalcasting facilities to recycle the clays often lost through ventilation as fine participates are pulled out in sand mixing and transport ventilation systems. The ability to recover "wasted" clays reduces costs and improves sustainability efforts in sand metalcasting operations. Typically, the baghouse dust (with bentonite fines) is mixed into a slurry (either off-site or at the metalcasting facility) and added back into the sand system at the muller.
Carbon: The use of special carbon treated clays can reduce the overall quantity of carbon additives (such as sea coal) into the green sand system and help lower overall volatile carbon, reducing volatile organic compound (VOC) emissions.
Chemically Treated Clay: Bentonite can be treated with various chemicals to provide better flowability and enhanced mulling.