They can be found here: http://www.flickr.com/photos/thayerschool/sets/72157631991926519/

Here’s a taste of some of what can be found on the page.

Local friends we made while waiting for an interview.

View of the site from downstream.

An interview.

Natalie Burkhard ’12 and Adam Khamis (Imperial College London, e.quinox, civil engineer) grout the rebar frame into the bedrock.

Mayor aka Pierre Niyomwungeri—our foreman and translator as MC.

Kevin Francfort `15 and Natalie Burkhard `12 work on the rebar framework for the settling tank.

http://www.flickr.com/photos/thayerschool/8178945821/in/set-72157631991926519/

Kurt Kostyu ’12 chatting with a villager wearing Kostyu’s sweatshirt from his home state.

The battery they already have and use to charge cell phones and power lights. They had to go several hours away to Ruramba to get it charged, before our system became operational.

The team discusses the project.

The site and the river valley.

Ideas for design improvement and further testing

• Design a simple method to warm secondary air and to protect secondary air holes from wind
• Experiment using different types of fuel, such as hay, in the kiln
• Consider new options for secondary-air hole location
• Measure the temperature in the chimney and metal drum during a burn; use chimney temperatures to identify if combustion is possibly occurring in the chimney

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.

The Brick Furnace with Bellows

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.

Bellows design on SolidWorks.

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

A small group meet this afternoon to work on the pyrolyzer. Tasks completed:

• primary door was made and intended location was marked on barrel
• holes and round stock was prepared for the grate support to suspend the biomass above the primary air door
• holes for secondary air were marked with future hole modifications in mind

Goals for next work session (probably Thursday morning):

• weld primary air door to hinge and hinge to barrel
• use plasma torch to cut primary air opening at the base of the barrel
• weld supports for grate to barrel
• plasma torch the hole in the barrel lid for the chimney
• drill holes for secondary air into the chimney

And then the first version of DHE pyrolysis kiln will be ready for a test run!

• The object that you want to cast
  Must have a simple shape. Any cavities must exist on the same plane; for instance, a standard coffee mug cannot be casted because the cup’s main cavity runs perpendicular to that of the handle’s.
• Casting flask
  Make a box with walls and a bottom, and another with only four walls. The top and bottom boxes are called the “cope” and the “drag,” respectively.
  The size of the box will differ according to the size of the object being cast—allow at least two inches on all sides of the object to allow for the heat of the aluminum pour.
  The cope and the drag should be the same length and width, although height can vary. Must be of a sturdy material. The reasons for this will become clear in the step-by-step instructions.
• Something that can be used to tightly pack sand into the mold, a “rammer”
  The sand must be packed very tightly into each part for the mold to stay in place. While we used our hands to press the sand in early one-part molds, we found that the mold maintained its shape much better when we had a solid object to physically pack the sand into the box with. For the two-part mold, working without a rammer is virtually impossible.
• Fine silica sand
  Playground sand, available at low prices, works well.  Silica sand is chosen because of its relatively low porosity; sand made from other rocks can absorb moisture and explode when it comes into contact with molten aluminum.
• Fire clay
  This may be slightly more difficult to obtain. It must be able to withstand the high melting point of aluminum.
• Water

Recommended:

• A mesh sheet to sift sand through
  Anything with holes small enough to allow through grains of sand but not small pebbles should work. We have found that a kitchen sifter with fine, double-layer mesh serves the purpose well.

Steps:
Making a Sand-Clay Mixture

1. Make a 1:9 ratio mixture of clay and sand. Mixing in small quantities is recommended to ensure that the materials are mixed together well.  When you think you’re done, mix again; inconsistent mixtures are more likely to crumble apart in a two-part mold. Each time that you reuse a sand-clay mixture, make sure you sift the material well to make sure that you are not working with any clumps.
2. Mix water into the mixture. Make sure that the added water is thoroughly mixed into your sand before adding more. For the amount of sand-clay we generally deal with, we add our water in roughly tablespoon increments. Use the test below to determine when your sand is ready.
3. Squeeze (hard!) a handful of sand into a hotdog shape. You should be able to pick up the shape and break it in half without crumbling. However, when loose, the mixture should flow through your fingers similar to dry sand. Adding too much water during this step will lead to bubbling during the cast.

Mold-Making
Here, the mold-making method will differ slightly according to the object being cast.

4. Once the mixture is ready, fill the drag to the brim with the mixture. Pack the mixture very tightly into the drag using the rammer. The mixture should not move in the mold when the box is jostled. The sand will compress; continue to fill and ram until the entire drag is filled.
5. Level the sand, using a rod to scrape off excess sand.
6. Pack sand tightly into any of the object’s cavities and scrape off excess sand, so that any cavity or valley in the object is filled and leveled.
7. Place the object cavity-side down on the flat sand surface of the drag.At this point, it is wise to lightly tap the object and make sure that you can raise it without taking the sand with it—just as if you were building a sandcastle.
8. Sprinkle a bit of dry sand on the drag’s surface. This will help keep the two parts of the mold from adhering to one another.
9. Place the cope on top of the drag and situate them so that they are exactly on top of one another. Aligning them correctly at this stage is crucial, as you will have to realign them later. Slippage will result in a skewed cast.
10. Pack sand into the cope. When sprinkling the firstlayer of the sand-clay ixture into the cope,becareful not to disturb the dry sand layer.
11. When adding sand to the ope, include at least half an inch of extra sand-clay mixture above the top of the object. This will depend on the size of the object being cast. Using the rammer, pack the mixture tightly into the cope.
12. Once the surface of he mixture in the cope has been rammed smooth, identify where the highest point on one end of
    yourobject is in the sand and poke a hole here, using a pointed tool such as a pen.Make it pinky-sized—this is the hole that you will pour your molten aluminum into.
13. Find the object’s highest point on the opposite side and also open a hole here. This will be used to allow steam to escape from the sand as molten aluminum flows into your mold.
14. Gently gently, making sure not to shift it horizontally, lift the cope straight up off of the drag.
15. Gently gently put the cope down on a smooth surface, being careful to not to jostle it. Unnecessary movement here may cause your entire mold to fall through.
16. That process should have left your drag and object looking just as it was before you added the top half of the mode.
17. Lift the object straight off of the drag, being careful not to let any of the mold mixture move out of place.
18. Replace the top mold to exactly where it was before removal of the object.