While 3D printing continues to decrease in cost, it is still a large investment. Even after the initial purchase of a printer, purchasing plastic filament still remains a costly affair. One of the best goals now in the printing community is to allow the user to lower the cost by producing their own filament. Many companies and individuals are trying to fine-tune designs on filament extruders, including some excellent designs seen on this site. Eventually, the settings and methods for printing will allow for almost any type of plastic to be used, making any recyclable plastic a source of printing material!
One item that seems to be lacking is an efficient means of granulating plastic in order to feed into an extruder. Most designs use pre-formed plastic pellets, which are also expensive. If a person could shred waste plastic or failed prints to feed into an extruder, the cost of printing material would be greatly reduced. Searching the internet reveals some impressive and well-build plastic shredders, but the complexity and cost are significant. This Instructable will show a proof-of-concept of a simple, low-cost, hand-operated plastic shredder for starting the process of reusing waste plastic for printing.
Two coaxial hollow cylinders (steel pipe in this case) are used as a cutting mechanism. The outer cylinder remains fixed in place, while the inner cylinder is free to rotate within. A slot is cut into both cylinders, such that when aligned, plastic from a hopper above is allowed to fall into the opening. The edges of the slot on the inner cylinder are sharpened to create a cutting edge. As the inner cylinder is turned, the plastic will be caught between the stationary outer cylinder and the rotating inner cylinder, and will be cut much like using a pair of scissors. The cut plastic will be contained within the cylinders, and by mounting the cylinders on an incline, the pieces will be able to fall through the cylinders and out to a collection container. By varying the size of the opening, larger pieces of plastic can be restricted from falling into the cylinders, giving control over the size of the plastic pieces produced.