In one of our last blog posts, we discussed how to make a high-quality mold out of fire clay, silica sand, and a special molding box. Yet you can make the most precise mold in the world and ruin it with a mistake in temperature or a bad pour. Several times in our casting history, we’ve made incomplete buckets, where the aluminum doesn’t quite reach every part of the mold (e.g., bucket number 10 in “Buckets Through the Ages”)—or a misguided pour has wasted the aluminum, forcing the hydro team to make two pours for a single bucket (e.g., bucket number 9 in “Buckets Through the Ages). Thus, great care must be taken both in monitoring the aluminum’s temperature in the furnace and pouring it from a crucible into the mold.
Melting temperature of aluminum: 660º C, 1220°F
Melting temperature of steel crucible: 1370º C, 2500ºF
The Steel Bucket Furnace
When we first decided to try sand casting, we settled on a simple steel bucket design that would efficiently melt the aluminum and give us the chance to try out our molds. With a hole at the bottom for a steel pipe and three inches of fire cement insulation, the steel bucket provided fast and even—but more heat than we expected. In one pour, the hydro team managed to heat the crucible to over 2000ºF; and by the time we attempted to pour, we didn’t have enough aluminum for the bucket. Closer inspection revealed that we had melted the bottom of our crucible, and a layer of molten aluminum covered the bucket’s bottom and leaked into the pipe, through which a hairdryer provided airflow.
Originally, the airflow was provided by a hair dryer duct-taped to a pipe. The only problem is, hairdryers need electricity—that useful commodity we’re trying to provide. This led to research into alternative methods for supplying air. We considered several designs, and ultimately decided that box bellows, which only require plywood and a dowel, would be the easiest to construct and maintain.
So for a more sustainable design, we focused on making a brick furnace to replace the steel bucket. Bricks are more readily available in Rwanda, and could thus eliminate our dependence on fire cement and a specific bucket—so in about five minutes after assembling our materials, we stacked forty bricks into a square furnace with a 6”x6”x20” cavity for the fuel and crucible.
Managing the fire proved more involved than the team expected. A layer of hot coals must surround the crucible as much as possible for a speedy melting process. For our casting, we use a steel crucible, a 2-inch diameter threaded steel pipe with an end cap attached. It has taken anywhere from fifteen to thirty minutes to melt a full crucible of aluminum; throughout heating, one member of the team keeps the fire roaring by pumping air through the furnace with wooden bellows.
The bellows that supply the furnace with air are derived from a design used by Japanese swordsmiths in their forges. These bellows consist of two linked chambers, which allow continuous air flow into the furnace through a steel pipe. The larger chamber contains a moving plate attached to a dowel which can be pushed and pulled by the operator. As the plate moves, it pushes air from one section of the main chamber (the section that is decreasing in volume) through a one-way flap into the secondary chamber. Simultaneously, it draws more air into the other section of the main chamber through another one-way flap. When the plate changes direction, the sections reverse roles, thus providing continuous airflow into the secondary chamber, which then routes the air out of a single pipe into the furnace itself.
The aluminum is ready to pour once it is molten enough to slosh around a little bit inside the crucible. You do not want to allow the aluminum to get much hotter once it reaches this point because it will cause the texture of the poured bucket to be very rough. When pouring the aluminum, it is important to move quickly enough that the surface does not re-solidify, but not so quickly that you spill molten aluminum everywhere. If the molten aluminum touches the wooden edges of the box, it must be quickly scraped away to prevent the wood from catching fire.
Once your aluminum is ready to pour, aim carefully for the pouring hole of your mold—and after a few minutes of cooling followed by quenching in water, your sand-casted object will be ready for use.
Will Hickman ’16, Spencer Chu ’16, Cecilia Robinson ’16