A Different Kind Of Plastic Shredder For 3D Filament Making

Haven’t you heard? You can make your own 3D filament nowadays from plastic granules (10X cheaper than filament), or even by recycling old plastic! Except if you’re recycling plastic you will have to shred it first…

[David Watkins] came up with a different way of shredding plastic. Typically we’ve seen shrunken versions of giant metal shredders used to dice up plastic into granules that can be melted down and then extruded back into filament. These work with a series of sharp toothed gears that kind of look like a stack of circular saw blades put together inside of a housing.

But that can be rather pricey. [David’s] method is super cheap, and you can do it at home with minimal tools, and maybe $10 or less worth of parts?

It’s pretty simple actually. All you need are two steel pipes, one that will fit inside of the other. Cut a slot into each pipe, sharpen the edges of the inner pipe’s slot, and spin the inner pipe with a bit of mechanical leverage — or you can get fancy and automate it with a high-torque geared motor.

plastic shredder diagram

The shredded plastic does seem a bit big, but you could have various sizes of pipe in order to get it down to a smaller granule size. Regardless, it’s gotta be the least expensive way to shred plastic we’ve ever seen. Short of doing it by hand with a pair of tin snips anyway.

What do you think? [David] would love to hear some ideas from you on ways to improve the system — and if you like it, don’t forget to vote for him in the current Instructables contest — he could win a 3D printer!

46 thoughts on “A Different Kind Of Plastic Shredder For 3D Filament Making

      1. It is not stronger than regular galvanized, it is used for hydraulics because hydraulics+galvanizing=bad news. If they need to handle higher pressures they they go with thicker wall, but generally the steel is all the same between galv and black.

        If you want this thing to last longer you might try some chrome-moly pipe and harden it after machining or make it out of something like 4140PreHard.

    1. It’s 10x cheaper if:
      * You buy a ton of pellets (yes, 1000 kg. That when you go to the 4 euro a kg price)
      * Your time costs nothing (running costs as well as setup/calibration costs)
      * Your extruder costs nothing (home extruders currently cost a few 100. Pro machines go into the 10k or more as far as I know)
      * You add no color or other additives. Most filaments have some additives, as 100% natural PLA does not print that well. (Don’t know the exact mixes, and if I knew I wouldn’t be allowed to tell)

      If you want small quantities:
      Should save you about 50%, if you have the equipment and the time.

      (Once again, trying to pitch in with information, straight from what I know from working at Ultimaker for 3 years)

  1. I had the same question. I looked at building a Fila strider but the cost of pellets was almost the same as filament. Now maybe wholesale bulk is cheaper but that shouldn’t count for a casual 10x cheaper statement like had and many other outlets use.

    Can we get a post with a comparison of pellets dealers and prices had?? That’s called useful to your readers, a popular thing to do some would say.

    1. If you intend to buy in couples (2 – 10) of kg at once, you can expect PLA and ABS pellets to be roughly twice as cheap as regular filament (maybe ~3 times cheaper than premium filament).

      Extruding your own filament is interesting if you either print a lot, want to tinker with custom colors or materials, or like to tinker with new hardware in general :)

  2. Only reason I would think about making or getting a shredder and extruder is to re-use the failed prints.
    I have an old paper shredder that was strong enough to shred CD’s, so I will be testing it to see if it will shred prints. When and if I finally get my printer.

    1. it’ll work if you mostly have thin and flat prints to break apart. might be able to flake your bad prints into layers for that purpose. it should be plenty powerful enough, if it could deal with CDs – they’re polycarbonate which is a VERY tough material.

  3. Thanks, HackaDay! It’s an honor to be on my favorite website! And thanks, everyone, for taking a look!

    This project was just to see if the concept works… and it does, although not great. But it’s a start, and hopefully I can improve it, or maybe it will trigger an idea for someone else…. I want dirt cheap plastic!

    Just to clarify, I had 4 main goals for the device:

    1. Cheap
    2. Low or no complexity
    3. Hardware store parts
    4. Only basic tools needed

    If anyone has any ideas for me, I would greatly appreciate it!

  4. I like shredder designs that take the principal of some sort of grate that refeeds teh plastic into the chipper untill it fits thru the grate. But latley I been thinking, its not really that outlandish to have a fixed-volume melt chamber you just drop some chips of palstic in, the printer melts the whole thing, and pumps it out with a displacement piston. Most of my prints are less than 10g of plastic, which isn’t a verry large melt chamber…

    1. They could, if the parts are very flat. If they aren’t flat (as is the case with most 3D prints) then it won’t work, as it won’t be able to cut/shred. It’d be like trying to use an aluminum can crusher on a steel can. Might be possible, but just as likely to damage the crusher.

      1. Maybe you could just heat up the plastic your using, and smash it flat? if you happen to have a large vice or hyrdraulic press you could probably smash it flat with little to no heat even.Then you’d have plastic pancakes to feed the shredder. Not sure if they make a crosscut shredder that can handle cd’s though, but even if you had a single cut type shredder you could use a paper cutting machine to make pretty quick work of cutting the strips into smaller pieces I would think

  5. might be interesting to do something like this with a disk shaped mechanism so one could use a circular saw motor. a major hurdle would be keeping the mechanism below the melting point of the plastic so it doesn’t get stuck after stopping. Maybe a cheese grater disk would be enough for recycling prints in a pinch.

    1. Wow, that is a great idea! You’ve already got my mind spinning for a possible version 2…. I’m already thinking that a DIY disk shouldn’t be too difficult, thinking about the flare-out and burs made from drilling or punching through metal, which could easily be amplified with a hammer and larger center punch…. maybe using heat to do that on a thicker, more rigid disk…. Thanks you for the idea!

      [Dave Watkins]

  6. What about a compost shredder? I just sold my old one unfortunatly. But they are strong, durable and easy to modify. The One i had could adjust the grinding wheels/blades to cut very small sticks and branches while still being able to chew larger ones too.

    1. That is a great idea! It would certainly be effective for first-pass shredding. A brand-new mulcher would be nce and clean, too, as well as readily available and cheap.

  7. I’ve tried a Royal paper shredder that can do CDs. It’s even one rated to shred a high number of pages at once. It gets bogged down pretty quickly. You want to avoid blenders as they would tend to melt plastic due to the speed. There’s a couple designs out there that look like scaled doesn’t versions of shredders that shred metal for remelting. I’d say investigate that kind of design.

    1. Yeah, that was the first concept I had, and probably the best method out there. Browsing on Google and Youtube for “DIY shredder”, there are some awesome designs out there. But without good machining tools, and the cost of having someone else make the parts put that way out of my budget. I’m still entertaining the idea of just taking the angle grinder to a bunch of large fender washers to create the toothed cutting disks, but i’m not sure about finding appropriate gears cheaply to mach the sizes of the disks…

      [Dave Watkins]

  8. Looks like starting point for a real power house plastic shredding system for recycling 3d printer rejects and or house hold scrap into pelletized raw material for filament extruder use. As to getting pipe to hold an edge, a person could attempt metal hardening via heat treating, or a trick I learned from a welding course.. run a bead of a Hard or high tensile strength type bead on the wear surfaces and then grind an edge for cutting surface. Specialty rod types for a extra tough wear surface called Hard facing are available but probably not at a hardware store like home depot, these would be found at the local welding supply dealer like Airgas or Lincoln welding dealers. More slots in the pipes could improve operation as could a third inner pipe with smaller slots or even a pattern of holes or holes mixed with slots, rotating the third pipe opposite the second pipe may be something to look at to improve pellet uniformity and reduce problems like jammed or clogged slots.

  9. Great idea! You can also find a practical example of a guy using his wife’s Vitamix to chop up plastic here. The ‘dry cup’ would work better, as it is intended to turn dry goods like grains into powder (i.e. flour). Be sure to read the comments as well, as there is lots of good advice there. (…and in case you are wondering how tough a Vitamix really is, one of the demos that they do at Costco is to turn concrete chunks into powder. I doubt that the blade is still sharp after, but the point is that not only does the motor have the power to perform, but the plastic cup is capable of withstanding the pummeling of pieces of concrete hitting it after being propelled by a blade whose tip is traveling at 435kph!)

  10. You could heat the inner or outer pipe by running some current through it (at low voltage of course). Then the pipe could cut through plastic like butter. The pipe would need to be vertical so the pieces immediately fall out without melting more, and everything else would need to be thermally insulated from the pipes so you don’t burn yourself. It might get clogged up with molten plastic though…

  11. Ref the left photo… what is that 10mm threaded bar sticking out there for?
    Great idea. I’m needing to shred HDPE prior to re-melting into block ready for machining of various types.
    I got mostly milk containers and their screw-on caps, this type of cutter/shredder would be mint!

    1. That was one pic from the Instructable just showing how to unclog the shredder if it jams… just push a rod through to push the plastic out the back.

      [Dave Watkins]

    2. Also, just keep in mind the cautions I posted… Way too much gap between the cylinders using the sized pipes I used… It cut, but not well…. That’s my start though!

    1. Not sure if I would buy one, but I would love to know if it works for plastic… Maybe I’ll carry some scrap plastic will me. If I see a grain mill at a yard sale… “Hey, you mind if I try this?”

      [Dave Watkins]

  12. Great idea. I would consider cutting longitudinal slots on the inner and outer tubes which are connected to a smaller diameter set of the original design. I think this would provide a grating filter effect. The finished cut up pieces would exit thru these slots i.e. not the end. Additional stages may be practical an effective.

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