casting 101 C
Bentonite Clay’s Role in Green Sand Systems AMERICAN FOUNDRY SOCIETY TECHNICAL DEPARTMENT
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. Tis bonding mechanism is a hydrated sys- tem that requires water, energy, time and technical expertise. Two types of bentonite clay are
G
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, i.e., more mull energy is required. Tis 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.
Calcuim Bentonite (aka Southern Bentonite): Te calcium ion concen- tration in this variety creates a lower swell green sand system with low vis- cosity, lower water absorption and the resulting quicker water loss. It is gen- erally 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 pro- vides greater compression strength and permeability. Primarily used in nonfer- rous castings, its lower hot retaining strength helps in shakeout. Tese bentonite clays often are used
in mixtures of the two at defined ra- tions, to maximize the benefits of each.
Bentonite clay’s differing properties can have a major impact on a final casting. Preblends: Preblends are clays that
are blended with other sand additives so that they become an easy additive to the existing sand system during mix- ing. 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 modi- fied clays that behave with different characteristics than their natural ten- dencies. 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. Te abil- ity 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: Te 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: Ben-
tonite can be treated with various chemicals to provide better flowability and enhanced mulling. ■
Mar/Apr 2015 | METAL CASTING DESIGN & PURCHASING | 47
reen sand, which is used for molds for the most widely used casting process, is a mixture of sand
Page 1 |
Page 2 |
Page 3 |
Page 4 |
Page 5 |
Page 6 |
Page 7 |
Page 8 |
Page 9 |
Page 10 |
Page 11 |
Page 12 |
Page 13 |
Page 14 |
Page 15 |
Page 16 |
Page 17 |
Page 18 |
Page 19 |
Page 20 |
Page 21 |
Page 22 |
Page 23 |
Page 24 |
Page 25 |
Page 26 |
Page 27 |
Page 28 |
Page 29 |
Page 30 |
Page 31 |
Page 32 |
Page 33 |
Page 34 |
Page 35 |
Page 36 |
Page 37 |
Page 38 |
Page 39 |
Page 40 |
Page 41 |
Page 42 |
Page 43 |
Page 44 |
Page 45 |
Page 46 |
Page 47 |
Page 48 |
Page 49 |
Page 50 |
Page 51 |
Page 52 |
Page 53 |
Page 54 |
Page 55 |
Page 56 |
Page 57 |
Page 58 |
Page 59 |
Page 60