Abstract from the Western Michigan University doctoral dissertation by Ananda Mani Paudel.
Aeration sand filling is a new molding technique in foundry. Using this technique, sand with smooth flow fills in any orientation and shape using low-pressure air. This is not possible by conventional gravity and high-pressure blow filling techniques. Aeration was introduced as an energy-efficient and environmentally-friendly sand molding technique. In addition, aeration has its niche on quality of molds it could produce. Friability, one of the crucial green properties for the quality mold was significantly low in aeration in comparison to the gravity and high-pressure blow filling. The fluidization action in aeration acted upon the sand clay interfaces and created the interactions with them, and induced better surface abrasive property. In other words, aeration lowered the friability in the green sand allowing a lower compactibility levels in green sand molding, which was not possible with the conventional molding techniques. The range of 30-35% was suggested as the optimal working range of compactibility for aeration molding technique for selected sand and clay composition.
Advanced cone jolt and thermal erosion tester were developed and used to examine the green sand properties of the foundry sand. Advance cone jolt was sensitive to the clay composition and contamination in green sand, whereas thermal erosion tester demonstrated its relevance in evaluating mold surface behavior at an elevated temperature. Thermal erosion test displayed less sand erosion in the molds built in the aeration.
Green sand in aeration was benefited by the favorable clay orientation. Homogeneous and isotropic distribution of clay platelets occurred during fluidization, which produced a better clay coating on the sand grains and increased the grain to grain bonding. Scanning electron microscope displayed a uniform clay coating and universal micro-tribometer showed greater bonding strength in the surface of the molds produced in aeration. Casting trial along with the relevant standard AFS tests for green sand properties were carried out, and analyzed using design of experiments and statistical tools.
Sand casting, the oldest manufacturing technique, is still popular among metal casters due to its low cost, high productivity and flexibility afforded by the molding process (Schleg, 2003). Among the wide variety of molding techniques in use today, green sand is by far the most diversified and widely used. Green sand in particular is of interest because it is available naturally and is environmentally friendly. The term "green" denotes the presence of moisture in the molding sand and indicates that the mold is not baked or dried. Molding techniques and sand control are the major contributors in quality and productivity of foundries. Development of newer molding techniques to get better castings from the green sand casting is continuing.
Various methods have been employed to shape green sand into molds such as gravity filling and high pressure blowing. Gravity filling and ramming is the traditional method, which is followed by gravity filling and squeezing. The majority of today's modern foundries use a high pressure blowing and squeezing technique to produce either horizontally or vertically parted green sand molds (Ramrattan, 2008). Lately, flaskless molding techniques are introduced. To improve the flowability and bonding of the green sands, various additives are introduced in the sand system (Kuz'min, 1987), (Lafay, 2009). However, none of the existing sand molding technologies has the capability to evenly fill or produce uniformly dense sand molds. Insufficient filling of sand to a complicated shape or deep pockets of small diameter will ultimately lead to casting 1 defects (Dietert, 1974). Aeration technique is proposed as a new sand molding method, which can fill complex shapes and produce uniformly dense molds. Aeration fluidizes the green sand before filling into the mold.
In aeration sand filling, green sand is mixed with air in a fluidizing chamber. The fluidized sand flows smoothly into the mold cavity with low pressure. This is a promising way to fill sand in any shape and orientation. Making molds with complex shapes and deep pockets is now possible (Hirata & Sugita, 2005). Further, the aeration technique is environmentally friendly because it produces molds with better green sand properties and eliminates the requirement of chemicals. Krysiak, et al, (2002) have shown that properly controlled green sand also can produce certain smaller near-net-shape castings comparable to chemically bonded sand. Casting quality is directly related to mold quality and mold quality largely depends on sand control.
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