Todays foundries cannot afford to sacrifice the efficiency and profits that are the consequences of poorly controlled hot molding sand.
Excerpt from the June 2003 issue of Foundry Management & Technology
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 and diminishes 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.
There are several types of equipment to cool shakeout sand:
Rotating Cooling Drums. The cooling drum is popular because it’s simple, but its inefficiencies outweigh its apparent benefits. In the cooling drum, sand is fed into one end of a large, inclined rotating drum. Often the downhill side of this drum incorporates a rotary screen. Air is pulled through the drum by an exhaust fan. Because sand moves slowly through the drum, a major requirement for effective sand cooling is ignored: the constant exposure of fresh surface area to the air stream in order to achieve the highest degree of cooling. Plus, due to the slow movement, the sand actually moves through the drum as a ‘slug’ and variations of sand are not mixed.
Mixing sand from various zones of the mold, or from unpoured molds, into hot sand originating from the sand/metal interface is considerably less than satisfactory. Sand leaving the cooling drum exhibits inconsistent properties (temperature, moisture content, etc.)
Stationary Cooling Drums with Rotating Paddles
Vibrating Fluid-Bed Coolers
The Effects of Hot Sand
Hot sand will effect every aspect of a green-sand molding operation. Depending on the foundry’s ability to control the sand temperature, the effects may range from higher scrap rates and casting costs to a complete loss of system control. Most important, though, are casting defects, such as:
Pin Holes — Cavities in the casting surface, either spherical, flattened, or elongated.
Dirt Inclusions — Surface or sub-surface particles of sand, slag, dross, or oxides which are embedded in the metal.
Washes — Rough spots and areas of excess metal caused by erosion of the mold.
Rough surface — A casting surface that lacks the required degree of smoothness.
Sand stickiness — Excess metal on the surface of the casting caused by a portion of the mold face remaining on the pattern.
Broken molds — Molds that are not strong enough to withstand pouring stresses.
Crushes — Indentations in the castings.
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