Today's foundries cannot afford to sacrifice the efficiency and profits that are the consequences of poorly controlled hot molding sand.
Excerpt from the June 2004 Foundry Management & Technology article by FM&T Staff
Metalcasting researchers say hot sand is the leading sand-related problem for today’s foundries. Most metalcasters recognize a relationship between reduced casting quality, increased process variation, and other molding inefficiencies that result from using hot molding sands. Studies show that hot sand affects virtually every operation in the production line.
System sand returned to the muller for reconditioning with a temperature of 160°F (approximately 70°C) or greater is considered to be excessively hot. Return sand ranging from 120° to 160°F (approx. 49°-70°C) is hot enough to demonstrate inconsistent properties during mixing and to be very difficult to control.
Where does it come from?
In a regenerative green-sand system the composition of the sand mixture is formulated to meet the requirements of the molding method being used and the type of casting being produced. The typical system sand consists of varying percentages of sand, clay, organic additives, and water. The system muller and molding machine process this mixture into a finished mold. To form a casting, molten metal is introduced to the mold, and in the process extensive damage occurs to the mold material (the green sand mixture). The damage (heat is the primary concern here) will vary according to the location of the material in the mold.
As molten metal is poured into the mold the sand grains expand - some by more than 10%. During this expansion some sand grains will experience thermal shock and crack apart. The effect of this shock is the most severe at the mold/metal interface anddiminishes further from the mold face. As the sand grains at the mold/metal interface are maintained at a high temperature for a period of time, they may go through a physical modification. The clay additives in the green-sand mold lose their combined water and structure. At this point the clay has lost its plasticity and it cannot be regained by subsequent new water addition.
Calcium bentonite is destroyed at 850°F (455°C) and sodium bentonite at 1,150°F (620°C). The water in the mold evaporates as the temperature in the sand mixture exceeds 212°F (100°C), until all the free water has evaporated.
Some of the water that has evaporated is carried through the mold with hot air and condenses in a layer behind the mold face. As a result of this overall process, the green-sand mold now consists of many layers of sand with properties ranging from the very hot, hard, and brittle layer at the mold face, to the relatively cool, wet, and plastic layer at the mold back.
Once the casting cools the sand is separated from the casting at shakeout and re-enters the sand system. Typically, the hot dry sand is the first part of the mold to re-enter the system followed by the colder, wetter backing sand. The return sand is segregated in surges of sand with vastly different properties, in terms of temperature, moisture content, and clay level.
A sand system may also gain heat from rapid sand system turnover, low sand-to-metal ratios, or high ambient temperatures. Combinations of all of these factors can be found in many metal casting plants.
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