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.
Cecilia Robinson ’16, Shinri Kamei ’16