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.
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
We gave a primitive overview of the casting process in our first blog post, but to further clarify the technique to readers, we decided to prove an in-depth guide. Online resources for sand casting are hard to come by, and those that exist are mainly semi-professional—far beyond our skill level. Hopefully, this sand casting guide can help students like us learn from our triumphs – and (many, many) mistakes – and try sand casting for themselves. Materials needed:
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.
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.
This may be slightly more difficult to obtain. It must be able to withstand the high melting point of aluminum.
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.
Making a Sand-Clay Mixture
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.
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.
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.
Here, the mold-making method will differ slightly according to the object being cast.
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.
Level the sand, using a rod to scrape off excess sand.
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.
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.
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.
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.
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.
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.
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.
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.
Gently gently, making sure not to shift it horizontally, lift the cope straight up off of the drag.
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.
That process should have left your drag and object looking just as it was before you added the top half of the mode.
Lift the object straight off of the drag, being careful not to let any of the mold mixture move out of place.
Replace the top mold to exactly where it was before removal of the object.
After this step, all you have left to do is pour molten aluminum into the pour hole. Details about building a furnace to melt aluminum are soon to follow!
The Bioenergy Project recently received its delivery for four 55-gallon steel drums for the purposes of making compost tumblers and pyrolysis kilns. The are currently safe and sound in Thayer’s large frame lab but will soon be met with plasma torches and drills next week after our new members have gone through machine safety training.
The Bioenergy Project is off to a great start this winter term! The fall served as a time of reflection and strategic planning after our back-to-back spring and summer trips. Now that we are on campus for 2013, we are charging ahead with renewed focus on improving our biomass briquetting program and developing biochar production for fuel and fertilizer uses. We feel that by focusing on the fuel source, rather than solely improved cookstoves, we may have a lasting economic, environmental, and health impacts on communities which we serve in a time frame which is achievable and without changing traditional cooking methods.
Tuesday, January 15th was our first Bioenergy Project meeting of the term. We had a terrific turnout with over 30 interested new members. James, Julie Ann, Ayushi, and Amelia gave an overview of the goals of the Bioenergy Project. Below are some of the work team divisions for hands-on projects throughout the term. We can’t wait for all the new members to jump in and get their hands dirty!
Also known pyrolysis kilns, pyrolyzers are chemical reactors that thermally drive decomposition of a feedstock in the absence of oxygen. Pyrolyzers can be optimized for all types of products, but ours will be designed to produce charcoal (stabilized carbon), CO2, and water vapor. Members will build a pyrolyzer over the course of the term in the machine shop.
Also known pyrolysis kilns, pyrolyzers are chemical reactors that thermally drive decomposition of a feedstock in the absence of oxygen. Pyrolyzers can be optimized for all types of products, but ours will be designed to produce charcoal (stabilized carbon), CO2, and water vapor. Members will build a compost tumbler then start decomposing biomass found in the the New England area.
Briquetting Molds and Techniques
Briquettes are traditionally a flat, donut shape. But would alternative shapes and sizes be easier to make or burn? This work team will experiment with different types of briquette molds. We will also explore alternative briquette presses, including a metal ratchet press.
Education and Outreach
In order to effectively transfer briquetting technology to local communities, DHE must develop an appropriate curriculum. This term we will be exploring how to convey such information through alternative mediums such as short radio messages and videos for farmers.
Biochar and Fertilizer Research
This research-based team will help us identify techniques for making, using, and selling biochar and fertilizer in East Africa and other regions of the world. We plan to talk to experts in the field on campus and abroad via Skype.
Join us on Tuesday evenings to find out how YOU can be involved in the Bioenergy Project!
We’ve come a really long way since our first cast, and we wanted to share some of the Hydro buckets that we churned out as we figured out how to sand cast.
As explained in the last blog post, the sand-clay mixture that we used in our casting attempts back in September had water content that was too high. When we poured aluminum into the sand mold, the water evaporated upon contact and bubbled through the aluminum. The aluminum solidified as the bubbles were pushing through it, and we ended up with this.
In our second attempt, we still weren’t quite sure what was going on. We now know in retrospect that too much clay traps air and that too much water causes rapid evaporation. The combination caused bubbling in the poured aluminum. Regardless, it appeared slightly more solid than our first messy attempt, so we agreed to try less water next time and hope for the best.
Since this made our first cast, we were getting more comfortable with the one-part mold–dangerously so. Enough that we thought we could re-shape the stem of the bucket after the mold slightly collapsed. This resulted in a scrawny stem. We did, however, manage to significantly cut down on bubbling by lowering moisture content.
Bucket 4, 5, and 6:
Since our previous attempts prompted more reading, the DHE Hydro team looked to text and online resources for advice. We quickly learned that a 1:9 part clay to sand mixture was the ideal, with barely enough water for the sand to clump together. This revelation vastly improved our casting and led to consistent buckets with a one-part mold.
This half-bucket represents the hydro team’s first attempt at the intricate, tricky, patience-strangling two-part mold. Its sleeker sides meant that it required half the molten aluminum to cast, and thus led to a turbine of half the weight. Unfortunately, we believe that the aluminum solidified before reaching the rest of the bucket, resulting in our stubby bucket. But it put the two-part mold within our reach.
Following our first limited success with the two-part mold, we found it more finicky than ever. Our attempts resulted in collapsed messes, to the point that–due to time constraints–we agreed to make just another one-part mold. In that, this cast wasn’t especially special; trying to quench it too soon, however, we accidently broke off its handle before it solidified.
Bucket 9: After some serious two-part mold practice during the Holiday break, DHE members returned to campus to redeem our last attempt. The molding went well enough, but during our first pour the aluminum solidified before filling the entire negative space. Unwilling to leave any bucket unfinished, we made a second pour into the air holes on the bucket’s opposite side, resulting in a patchwork bucket but a successful one overall.
Similarly, the aluminum in our next pour hadn’t reached a high enough temperature to seep into every corner of the bucket. This resulted in one of the best surface textures we’ve achieved, but a bucket that would be useless in the field.
In a cast done not an hour later, DHE members managed to form our first full bucket, casted in one pour, made in a two-part mold. We made sure that the aluminum had been heated well enough to correct our last two mistakes, which resulted in a grainier texture but a decent bucket. Since we now knew we could make a successful two-part cast, we turned our focus to the surface texture—aiming for a smooth surface that would lend well to a turbine.
In our past two-part molds, we had found ourselves having difficulties with sand sticking to the template bucket’s stem when we lifted the top mold. This ruined our clean edges and often meant starting the whole mold over—a process that could take as much as twenty minutes. To rectify this concern, we placed a small slip of paper on top of the bucket’s handle, which the wet sand would cling to instead of our template bucket. The quick-fix resulted in our cleanest, most beautiful bucket yet.
Sand Casting: the process of pouring molten metal into a sand mold to make a metal casting.
This project’s value lies not only in its end product, a working hydropower station for a rural village in Rwanda, but also in its ability to be replicated on-site. Sand casting requires only basic materials: bricks for a furnace, fuel to burn, aluminum to melt, sand to mold, and wood for frames. By using such readily accessible materials, DHE hopes to instruct the local residents to be able to fix their own turbines and start new hydro projects on their own.
Basic Sand Casting
Minimally, sand casting requires two parts: a furnace and a mold. Stock aluminum is collected in a steel crucible, which is then placed inside the furnace and heated to roughly eight-hundred degrees. During this melting process – which can take fifteen minutes or much longer, depending on the fuel and furnace – a mold of the object is made out of sand. At the beginning of our sand casting practices, we used a simple one-part mold which worked by impression, similar to footsteps on a beach. The melted aluminum is then poured into this impression, and after a few minutes of cooling and quenching, the object is finished.
This type of casting – i.e., implementing a one-part mold – resulted in turbine buckets with vertical sides: well-defined buckets, but too heavy for real use. As casting progressed, we moved to the two-part mold, a trickier but more accurate version that replicated full 3-D objects. Just like the name suggests, a two-part mold involves two parts which result in a cavity between two layers of sand: three air holes leading to the cavity allow room for aluminum to be poured and air to escape. Our resulting buckets have been lighter, smoother, and much more suitable for turbine use. This more complicated procedure, however, has presented new challenges that the DHE hydro team continues to meet and resolve.
When the Hydro Team started out this term, we had minimal experience with sand casting. Previously, DHE had made hydropower turbines in Engineering Sciences 89-90 classes or at a professional foundry, and we only had books and internet tutorials on which to rely. We spent one of the first meetings exploring sand casting in front of computers, googling “How to sandcast aluminum;” and for the freshman, our first “homework” was to choose a specific component of sand casting and research it before the weekend.
When we started, we knew two things: that we would be using a sand clay mixture to make a mold in the box, and that we needed to melt aluminum. This turned out to be more difficult than we’d anticipated.
We were proud of our first attempt, but looking back, the bucket that we ended up with looks almost hazardous. In making our sand clay mixture, we thought that in order for the mold to retain its shape, it would need a high water and clay content. We were wrong. As we poured the molten aluminum into the one-part mold that we’d crafted, the excess water started evaporating upon contact, making bubbles in the aluminum. But molten aluminum solidifies rapidly at room temperature—and even faster in snowy Hanover. The water vapor didn’t bubble through the aluminum so much as push it to the side, where the aluminum solidified. As you can see, there are holes in the bottom of our first bucket, and when flipped over, the aluminum bits are sharp and barely stuck together.
It took us a few more attempts before we realized youtube’s potential as a resource. In one trial, the mold-making took so long that the group working at the furnace to melt the aluminum (melting point ~660°C) accidentally melted the steel crucible (melting point ~1370°C) containing the aluminum. For this furnace, we’d used a steel bucket insulated with concrete, leaving a cylindrical hole in which to place fuel and the crucible. Toward the bottom of the bucket, a pipe circulated air – supplied by a hairdryer on the other end – into the furnace, giving us greater heating efficiency. Of course, aluminum came seeping out from the melting crucible and clogged the hairdryer-pipe combination that we used to feed air into the furnace. Lopsided casting boxes also did not help our progress.
A few instructional videos later, we learned that the sand clay mixture should have about a 1:9 ratio of clay to sand as opposed to the 1:1 ratio that we’d previously been practicing. As for water, we only needed enough for the sand to be able to maintain a “hot dog” shape after squeezing it with our hands. When broken in half, this ideal hot dog would break cleanly without crumbling. Our previous mixtures had the consistency of snowballs; but this new mixture would still flow smoothly through your fingers if you scooped some up.
Our sand casting improved greatly after this revelation, and our casting became fairly consistent.
We then decided to move on to the two-part mold.
This first two-part mold was surprisingly successful. The aluminum was poured in through the main pouring hole and flowed into the mold up until the air holes—but no farther. The highest point on the bucket is on its back, and we put the air holes at the farthest end of this back; however, the lip of the bucket continues on. The aluminum was flowing through the mold slowly and solidifying quickly, so that once the aluminum reached the air holes, it couldn’t move into the lips since there was no place for the air there to escape.
Several of our members worked over break to replace our hair dryer as our furnace’s air circulator, instead using wooden billows to manually pump air through the system. Manual billows relinquish our dependence upon electricity for a hair dryer and can also be made on-site in Rwanda.
Additionally, two winter projects tested the viability of a brick furnace. Both successfully melted aluminum in a brick furnace, so the practice has carried over into our work for this term.
One DHE member with ceramic training constructed several clay crucibles, as our steel crucible sometimes proved difficult to pour, and we thought we would give it a try. On returning for winter, we tested one larger clay crucible, but removed it from the furnace due to increased melting time.
Lastly, as our two-part molds had little success before break, one project focused solely upon the two-part mold, pouring wax instead of aluminum for ease of process. This has also translated into the greater success of our two-part molds this term.
13W; Winter term
Our original steel-bucket furnace has now been replaced by a brick furnace, which forms a square around the crucible and will hopefully increase efficiency and reduce melting time.
The one-part mold has been abandoned: future casting will be exclusively two-part, as the two-part mold shows promise for excellent turbine buckets following more practice. On Jan. 9th, three days after returning to campus for the winter term, we headed to the woodshop to make new boxes to replace the parellelogram-esque boxes that we’d used in the fall term. Two days later, we did our first cast. Unfortunately, the aluminum had not melted enough when we did our pour and filled only half of the mold. We filled the rest of the mold from its air holes a few minutes later.
This morning, we conducted two more trials, and on the second, successfully made our first full bucket. On the first bucket, we encountered the same problem we had before: the aluminum did not reach the extremities of the mold. The surface of the bucket was rough and felt similar to some of the buckets that we’d made earlier in the casting process. We believe that this is because there was higher water content in the sand clay mixture that we used, and this should be an easy enough issue to avoid for the next round.
For this cast, we melted almost double the amount of aluminum necessary to do a single cast. The aluminum was able to maintain a higher temperature throughout the duration of the pour, and the aluminum flowed smoothly into the mold. We believe that this is why we were successful in this attempt.
More tests will be done using ceramic crucibles, including three different sizes.
A separating agent may be employed within the two-part mold, which would result in cleaner separation of the bucket from the sand and give a finer finish to our end bucket.
For future tests, more documentation will be made concerning several variables which might be affecting our molds: e.g., pouring temperature, aluminum amount, and exact water-sand ratio.
In the summer of 2008, DHE installed two hydropower sites in a village called Banda. We returned in the summer of 2011 to do general repair-work and to install a locally-fabricated turbine. In 2012, DHE collaborated with e.quinox, a student group from Imperial College London, to install a third hydropower site in the village of Rugote.
Local villagers take advantage of a business model developed by e.quinox and DHE. It uses of electricity from a turbine to charge car batteries, which in turn can be sold to villagers to power lights and charge cellular telephones. Small businesses and local entrepreneurship, such as barbershops and cellphone charging stations, have opened up because of the newfound access to electricity. Many other aspects of village life have been impacted by the lighting that the batteries provide, from schools and churches to homes and town administrative centers.