This is a Mechanical Engineering Senior Design Project done in Spring 2015 semester. This Low Volume Plastic Shredder is a machine designed to take every-day recyclables such as soda bottles and plastic bags, and break them down into small pellets.  These pellets can then be used in the Polymer Engineering Center to research the uses of extruding new plastics directly from recyclables.  The machine is designed to stand over an existing recycling bin, to be used by anyone passing by.  The project was proposed by Professor Natalie Rudolph, and is intended to first see use in the Mechanical Engineering Building at the University of Wisconsin-Madison.

The machine was to be designed within numerous constraints, some of the key ones being: the machine should shred plastic to pellets below 7mm in diameter, it must fit over an existing bin in the Mechanical Engineering Building at the University of Wisconsin-Madison, it should process numerous kinds of plastic, both bottles and bags, and it should handle sizes up to 2 liter bottles. In addition, the machine is to operate quickly, quietly, and in a compact enclosure while also ensuring the safety of the user.

The process of design began with analysis of different plastic cutting methods.  After some research into existing machines, a counter-rotating blade design was developed as our core mechanism. From our initial brainstorms we also produced the method by which the material would safely be moved from the user to the machine, and the method for supporting the machine over a bin.  After concept development, CAD for a prototype was created and a fixture to test the blades was designed.  

 The shaft, spacers, and blades were all machined from steel, with the blades being done via CNC milling.  The prototype frame was constructed from aluminum extrude and fasteners, with supporting parts and walls being laser cut from MDF and plastic.  The drive mechanism is a gear, chain, and sprocket system driven by a hand crank (a motor in the final design).  After making our prototype test fixture, we were able to test how well our initial design shredded.  The machine worked, but it did not shred as well as we hoped.  Some problems arose with pieces of thin plastic getting stuck between blades, and larger chunks getting stuck in the bottom grate.  These issues were then addressed in the final design and future recommendations.

While manufacturing our test prototype, the final design was developed.  Much of the CAD was redone and many of the prototype-friendly parts were replaced to be more manufacturable on a larger scale.  The supporting structure is now welded aluminum channel, while the plates and walls are now thin polycarbonate.  The chute and back cover are a thermoformed plastic, and the hand crank was replaced with a 115v motor.  Controls for user operation were added including an on-button and an emergency-stop.  

The final design for the Low Volume Plastic Shredder met most of our objectives set at the start, but still has room for improvements.  With more time for development and testing, the shredding mechanism could be fine tuned with a combination of blade geometry optimization, and testing different spacing tolerances.  

Fortunately, the project was continued by another senior design group that manufactured and validated our design. Below are the pictures of the prototype and the videos showing the cutting mechanism.