This is the completed biosand filter after the second revision. This is a proof of concept demonstrating that a 3D-printed mold can be used for concrete.
There were numerous improvements over the past filter. The 3D printed parts were redesigned such that they were easily removable after use, instead of being glued into one piece that adhered to the concrete too well. There was also much less use of mechanical ties such as wood staples; instead painters tape more than sufficed to hold the mold together. Instead of plastic sheeting covering the concrete-side of the interior and exterior mold, a form release agent made up of a combination of mineral spirits and petroleum jelly was spread onto the concrete, making it easier to remove as well as much faster to apply.
Additionally, the concrete mix itself contains far more water (often a quart or more for each bag). This allows the concrete to run into spots and harden into a solid block, instead of acting as many pieces of gravel. The amount of water that is used in the concrete mix can be seen in the below image.
The water pools at the top of the concrete as the concrete is saturated with water. This is accurate with other concrete pours from other people. Since I left the filter to dry for three weeks, the amount of water remaining is trivial.
One problem that I did experience is vibrating the concrete before it dried. Since the base of the filter is very small compared to the exterior mold, I was unable to screw in secure supports. When I stuck the vibrator in the concrete, the mold broke and the concrete poured out the broken sides. This happened twice before I decided to screw in as many steel fasteners to the sides together as I could and not vibrate the concrete. This worked and while the concrete did shrink some and contain bubbles, it did dry together successfully.
This project is complete and is awaiting pickup from the nonprofit. Possible next steps include a fully-3D-printed biosand mold on a large-scale 3D printer, which would cost around $5 and would be reusable. This would enable modularity as well as prototyping of unique aspects (vibrator attachment on the side of the exterior mold, handles, etc.). Water tanks/raised granary stands can also be made as well.
I’m quite happy with how this project has turned out and look forward to seeing how 3D printing and international development will work together at Duke in the near future.