By Matt Bell
The first machine in our recycling process is the “shredder”. The purpose of this machine is to break down the collected plastic into small pellets. This serves three main functions. First, it makes the plastic of unknown number easier to separate. As each type of plastic has different properties (including density, melting temperature, etc.), a number of tests can be used to differentiate between them once the plastic is broken down. For example, if placed in a liquid of density 1.05 g/cm3, polypropylene (PP) will float, but ABS plastic will sink, allowing you to distinguish between the two. Secondly, the shredder makes the plastic easier to wash and store. Instead of having hundreds of plastic bottles lying around, we can have neat containers of plastic pellets, separated by type (or also by other things like color). Finally, the shredder allows the plastic we collect to be used in the injection molder, our second machine. These small pellets will be fed into a tube, which is then heated up. Once the plastic has melted, it is then pressed by a piston into a mold, where it solidifies and becomes the final product. The small pellets are easy to feed into the machine and have more surface area, allowing the plastic to melt more quickly in the melting chamber.
The design of our shredder closely follows that of Precious Plastics, a large inspiration for ARC as a whole. While eventually we want to completely design our own machines, we felt it more important to get fully-function machine right now, so we can get the ball rolling on the rest of the process. In simplest terms, the shredder has to break larger plastic parts into smaller ones. In order to accomplish that goal, this solution has a hopper to feed plastic parts into the box, where metal blades turn and press the plastic against a piece of perforated steel (sheet metal with small, uniform holes). Plastic is squeezed through the holes, creating equally-sized pellets, that fall out the bottom of the shredder into a collection bin. Most of the parts for this machine could be made with sheet metal, which was then manufactured using a waterjet cutter in the Clark Hall machine shop on Cornell’s campus. A sample of the resulting parts are seen below:
The box itself was then welded together:
The only additional parts were the custom driveshaft which holds the blades, perforated sheet metal (purchased), and a hand-crank (purchased) to turn the driveshaft. The blades were attached to the driveshaft and the driveshaft was placed in the box, through the bearings. Then, the hand crank was attached to the shredder.
The next immediate steps are assembling the blade guards, attaching the perforated sheet metal, adding a simple hopper, and making sure we can provide enough torque with the hand-crank to shred the plastic. In the long-term, we plan to revisit the shredder, making it more affordable and easy to make, without specialized equipment (i.e. remove welding steps), so that people without those resources can make a shredder, too. In the end, the shredder cost only about $200 and we think we can make that even lower!
We plan to have the shredder completed by the end of October and hope to make even more improvements by the end of the semester! We’re looking forward to having our first machine built and starting our personal recycling process.