Plastic Shredder out of Paper shredder
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This is a tested and proven method of shredding old prints and discarded plastics to a size where they can be used with filament extruders. Unlike most other methods of plastic shredding, this one is easy, reliable, and most importantly, cheap. There are options out there that may do a better job like wood chippers and professional grade grinders, but those methods cost large amounts of money and are unwieldy compared to this method.
The method described involves a paper shredder. Paper shredders are very similar to professional plastic shredders and are essentially a scaled down version for home and office use. Outfitting one of these shredders for use with plastics makes sense. (Waning: removing the safeties on these machines makes them quite dangerous to use, although much more fun).
Choosing a shredder
Before any modification can be done, a shredder has to be chosen. This project has been previously done on an 8 and a 10 sheet cross-cut shredder. With this example, a generic 10 piece cross cut shredder is used. In general, the higher the sheet count, the better it will be able to handle the load of plastics. Strip-cut shredders have not yet been tried, but because they only “strip” the material, they, in theory would not be able to create as fine of a shred. Micro-cut shredders would be the most effective as they would dice the plastic into very small pieces, but they would struggle with large or high infill parts.
Modifying the shredder
Once the proper shredder has been selected, it is necessary to remove the safety locks and plastic covers on the shredder to allow prints to fit into the blades. With most shredders there are several safeties to be removed for this to work:
- The shredder casing will have molded plastic that feeds into a narrow slit to keep fingers out.
- There is a paper detection switch that only turns the shredder on if there is paper in the slot.
- The gearbox itself will be constricted to a narrow slit.
The molding is the easiest to remove. Just cut around the perimeter of the slope. The example shredder had a convenient “notch” that separated where the paper funnel began and the cover stopped. That is where it was cut. Removing the switch isn’t that challenging either. Several screws and some wire cutting is all that is needed to be rid of it (if you wish to, you could short the wires previously connected by the switch to make the “auto” mode be on 24/7). The gearbox casing is the hardest to prepare and cut. It helps to take plenty of pictures along the way so it is easier to reassemble. With the example, there are 2 plastic inserts within the blade gearbox that come together to a narrow slit. Both of these need to be cut so the blades can be accessible (They cannot be removed, because, at least with the example shredder, they provide structure support for the blades of the gearbox).
Once all this is done, the shredder should be open enough for most models and discarded plastics to fit into the blades, but there is a large, open gap between the top of the casing and the gearbox where bits can get stuck and may even damage electronics. A wall can easily be made around the gearbox with whatever material is on hand, be it plastic, 3D printed, or other. With the example, an abundance of popsicle sticks were at disposal, so those were used for the wall.
At this point, the shredder should be prepared enough to be used for shredding prints. Before shredding, it is good to separate the plastics by color if the end product is intended to be anything other than black or brown. It is absolutely essential that the plastics be sorted by type beforehand to prevent them mixing as granules. Now that the shredder is minus its safety protection, be extra careful around the blades while it is plugged in. Try, if possible, to keep fingers out of the holding pit whenever it is running, for your safety. It is also possible to add a hinging safety lid/screen to put over the shredder while it is in operation (this cover will also help keep shreds of plastic from bouncing out onto the floor or into operator’s face).
Run the discarded bits through the shredder one at a time. For solid or high infill plastics, it is necessary to pulsate the blades on and off to allow time for the plastic to yield and break. Initial product will look quite large, and that is OK. For smaller granules, simply run the shreds back through the shredder several more times. For fine shreddings, 4 or 5 rounds will do the trick.
If one has the money to do so, they can stack several shredder heads on top of one another to get finder pellets with less passes. They could even change the head type to further this effect (I.E strip-cut head, cross-cut head, micro-cut head).