Sand preparation affects quality.
Much of the credit for the improved quality, dimensional accuracy and surface finish is owed to better sand technology. Driven by rising sand acquisition and disposal costs, sand suppliers and foundries have cooperated to extend the useful life of molding sand.
Before sand can be used in molds, however, it has to be prepared to accept the speed and temperatures of current foundry practices. The economics of metalcasting are unforgiving: when the metal is ready, so must be the mold. Amid rising zero defect expectations and resources conservation, sand preparation is vital.
Sand and clay are abundant, inexpensive and nearly chemically inert. Combined with water in the modern mulling process, each sand grain is smeared with a mixture of water and bentonite clay. The muller has replaced physical mixing of naturally bonded sands. Foundries have moved to the stronger, more sensitive bentonite molding sand, and the development of mullers has grown rapidly.
The combination of high-purity bentonite clay with high fusion point washed silica sand allows casting of iron and steel into green or water-bearing molding sands. In naturally bonded sands, with their high clay and moisture contents, the molds had to be dried before use to ensure high-quality iron and steel castings.
Mixing and Mulling
Mixing and mulling are different processes. Mixing is the intermingling of unlike particles to attain an average composition. In molding sand, this is a physical act, and rarely are chemical reactions involved. It is done by various motions that separate and displace one particle in relation to another. Low-viscosity liquids can be dispersed easily throughout a sand mass to establish coatings on the grain surfaces.
Simple blades and paddles distribute additives such as water, core oil or liquid resins. Grain-against-grain impingement readily transfers excess coating from one grain to another, giving uniform coatings regardless of differences in grain size, shape or surface condition. As the dispersion of clay, additives and water progresses, the mix becomes more resistant to compression and shearing, and the mixing device must compensate for that change.
Mulling is the application of work forces to cause kneading, smearing, compression and shear. The strength of the additive films controls the amount of mulling required. For example, mixing flour and water requires varying work effort, depending on whether there is more flour or water in the mix. As in a stiff bread dough with very low water content, simple mixers don't work. A much larger work force, like kneading, is necessary.
Kneading and mulling are mechanically similar. Both processes continuously compress a different portion of their mixes between two solid surfaces and then shear and recompress them.
Three basic processes must be performed to prepare clay bonded sands: 1) binders, additives and water must be blended uniformly; 2) pressure must be applied to overcome water surface tension and the tendency of binders to agglomerate; and 3) abrasion or smearing of the plasticized clay over sand grain surfaces must take place.
Mullers use plows to blend and compress the sand mix continuously. Sand, water, clay and other additives are placed in the muller in a grossly segregated manner. The mulling process causes complete disorder so that when the final product is sampled, the mulled product is uniform.
The changes in green sand as it is being mulled involve:
* agglomeration * attrition * temperature changes * aeration * cumulative mixing
Agglomeration is the gathering of individual grains into clusters that must be considered in green sand bonding. They can be 1/8 to 1/4 in. in diameter or larger and are made up of sand grains, small clay clusters, silt fines and water. Two major causes of agglomeration are high critical moisture content and excessive fines.
Attrition (grinding or fragmentation of sand grains and clay particles) also occurs during agglomeration. Small batch size, angular sand, narrow sand distribution and heavy mulling pressure increase attrition. The sand grain surface area is increased and appears to dry. The sand will have an increased green strength.
The temperature of returning sand will vary unless sand cooling is provided in the system. If it is hot, the moisture added evaporates rapidly. Effective mulling requires sand cooled below 150F. Excessive mulling may increase sand temperature. Hot sand will produce condensation on the pattern, causing sticking, and it will not have consistent properties. Excessive heat drying of the sand during handling gives crumbly edges, sand rain from core surfaces, low dry strength, brittleness in pattern drawing and other problems.
Aeration consists of screening, beating or agitating the sand during or after mixing to eliminate gross lumps. It fluffs the sand, minimizes super-voids and produces a uniform pattern face density.
Cumulative mixing occurs in system sands that are reused continuously. Because of the cumulative mixing, a high percentage of the total clay in the system is developed, and properties are considerably different from new sand mixes.
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|Title Annotation:||preparing high quality molding sand|
|Date:||Aug 1, 1992|
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