For my MechE Capstone, I worked in a team with 7 other students to design & fabricate a plastic recycling center for Northwestern adapted from Precious Plastic. This project encompassed designing the system-level process, designing the components of the system, and fabricating those components.
Our client, Sarah Levesque of Compass Group, came to us with the exciting idea for repurposing plastic waste at Northwestern. She was intrigued by the work that Precious Plastic was doing, and wanted to bring those ideas to life on our campus. However, this would not be as simple as gathering the plans Precious Plastic provides and building machines. The machines and processes of Precious Plastic had to be adapted to fit the needs of Northwestern.
In tackling a project of such large scope, we were faced with many challenges. Here were some of our most defining challenges:
We identified our process' critical system to be 2 parts - the shredding & cleaning of the plastic. Without clean, processed plastic, there would be no end product. In the end, we decided that our forming machine recommendation would be Precious Plastic's extruder because of its versatility in creating stock materials. So, we needed to build 3 machines - a shredder, extruder, and cleaner - and integrate them into 1 system.
The shredder and extruder were to be built primarily following the instructional materials provided by Precious Plastic. However, since Precious Plastic is modeled after sourcing already clean plastic and does not offer a developed cleaning system, our cleaner needed to be designed and built from scratch.
Throughout the first quarter of the project, I spent most of my time leading our testing efforts surrounding cleaning plastic. This involved washing shredded plastic covered in mayo and oil in a variety of ways - dishwasher, laundry machine, hand-washing, and current market solutions for portable washing machines. But no matter what we tried, it seemed that we just couldn't get our plastic clean. The shredded bits always came out of the wash still slicked with oil.
Traditional methods of cleaning seemed to be no match for shredded, oily plastic. A laundry machine was the closest we could come to clean plastic, but the sub-optimal water and energy consumption that would be needed to clean large amounts of plastic pointed us in a different direction.
Continuing onto the second quarter of our capstone, we ventured into the even murkier waters of designing and building a cleaner from scratch. We needed a machine that would provide sufficient agitation to each shredded granule of plastic and not only remove contaminants, but effectively rinse them away.
We came up with 2 concepts to test - a horizontal and vertical cleaner:
Portable washer
The Horizontal Cleaner featured a perforated chamber (containing the dirty plastic is) inside of a larger, outer container (containing water and cleaning agent) that was water-sealed. The inner chamber would be rotated via a crank arm that could rotate freely in both directions. The main objective of this orientation was to provide agitation via tumbling motion - similar to compost tumblers. To drain, the perforated chamber would be removed and the clean plastic could be accessed via a resealable opening.
Our testing mockup consisted of 2 nested buckets and a PVC crank arm. The perforated bucket had drilled holes with a layer of mesh to keep the small pieces of plastic from escaping.
The Vertical Cleaner featured 1 large container where water, cleaning agent, and plastic were contained altogether. An agitator post was rotated from the top and additional agitation of the container was provided via a rotating plate (meant to simulate the dual action of top loading washing machines). A spigot at the bottom of the container allowed for draining contaminated water.
After testing both our horizontal and vertical concepts, we reached the conclusion that the horizontal cleaner performed better than the vertical concept. We used a qualitative touch test, mass comparison, and oil blotting sheet test (shown to the right) to evaluate performance.
There could have been a variety of reasons why the horizontal configuration performed better than its vertical counterpart, but here were a few of our hypotheses:
Horizontal (left), Vertical (middle), Control/Dirty Plastic (right)
Using the conclusions from our comparison testing of the vertical and horizontal cleaners, we refined our horizontal cleaner design for improved robustness and functionality. Unfortunately, due to the spread of COVID-19, our final quarter of capstone was cut short and we were unable to carry out the fabrication of this final prototype.