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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.

Step 1: Concept

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.

<p>Hi guys, i am an industrial design student in my final year, i need something like this for my design so bottle caps can be ground down, but i would much prefer it to be motorized. Can this be motorized safely? and if so with what kind of motor?<br><br>Any help would be appreciated, Thanks!</p>
It would be very easy to add a mottos to this, even if it was a drill or a motor with a controller board.
<p>I am currently in a debate about if we should get a 3D printer for our school and I just have to say, this helped SO much prove my point</p>
<p>Perhaps 2 opposing large drill bits setting a hairs breadth away from each other would be good for grinding as well. Most of the extruders on here use a single bit as part of their design.</p>
Thanks! Stay tuned for version 2, in progress ;)
<p>I think my wood chipper works much better and is safer and cheaper. I can shred hundreds of pounds of plastic scrap in an hour. If i shovel it back in for another pass it makes plastic snow.</p>
<p> I like your off the shelf esthetic.</p><p>My suggestion for closing the gap between two loose fitting cylinders is to coat the inside of the larger pipe with a release agent and clamp the slotted side of the inside pipe against the inner surface of the larger pipe.Then fill the space between pipes with concrete.</p><p>The result would be a inner pipe that was closely fitted against the outer pipe.</p>
Thanks for the suggestion!
<p>I recently had another project that required concentric pipes of close tolerance, and found good cheap ones at autozone and my local muffler shop. They have a surprizing number of sizes and I got 2 pipes that were made for each other...</p>
Oh wow, thanks! That would be a little thinner-walled, but this whole project is just to see if the concept is worthwhile... I think I'll try that.... Thanks!
<p>Wouldn't a cheese grater do the job better and easier? </p><p><a href="http://www.walmart.com/ip/22080765?wmlspartner=wlpa&adid=22222222227015943585&wl0=&wl1=g&wl2=c&wl3=42973918832&wl4=&wl5=pla&wl6=69920711540&veh=sem" rel="nofollow">http://www.walmart.com/ip/22080765?wmlspartner=wlpa&amp;adid=22222222227015943585&amp;wl0=&amp;wl1=g&amp;wl2=c&amp;wl3=42973918832&amp;wl4=&amp;wl5=pla&amp;wl6=69920711540&amp;veh=sem</a></p>
Hi! Thanks for reading and commenting! I've had a lot of feedback suggesting the use of various blenders, food processors, mills, etc. I think the problem would be long term durability. Even the infomercial food processing devices that can withstand a minute of grinding concrete are most likely not going to have a good long life afterward. And if they are repeatedly used on hard objects, I would not expect the blades and motor to last long at all.
<p>I think a Straight Tooth Side Milling Cutter (used for cheap) would be your best bet. Maybe a few in a row on a common axel. Check this one out from ebay. </p><p>http://www.ebay.com/itm/UNION-4-x-5-8-x-1-1-4-STRAIGHT-TOOTH-Side-Milling-Cutter-/291489172151?pt=LH_DefaultDomain_0&amp;hash=item43de1bcab7</p>
<p>Or use quite a few and simplify this design. </p><p>https://www.youtube.com/watch?v=Lze-UDPPyYk</p>
<p>Ok yeah I saw that video before I started and had actually wanted to replicate it, but I couldn't source gears within my budget that would work with the parts I wanted to use for the rest. So it was either spend a lot to get gears that worked with the other materials I had, or go cheap on gears locally and have to buy additional materials for the rest. </p><p>What I was going to use for the cutters was just a bunch of large fender washers, and take the angle grinder to them, making 3, 4, or 5 notches into the edge at even spacing around the washer. That would pretty much make the same idea as the cutters on industrial shredders. I may still do that in the future when funds allow the purchase of appropriate gears, and to have someone weld them and the cutters onto shafts. </p><p> Thanks for checking out my project!</p>
<p>Think about recycled bicycle sprockets and chain instead of costly <br>gears. The sprockets don't need to be exactly the same size as the two <br>axles of the grinder don't need to be rotating at the same speed. They <br>just need to be rotating in opposite directions.</p>
And cheap is good so I'm curious!
Thanks for the feedback! It's gonna be a little while until I can check out your link, but I will!
<p>Nice idea. Is this an original idea or is it based on another design? How well does it work? And are there any changes you would make?</p>
Thanks! This is my original design. I had wanted to make a cheap copy of the multi-bladed counter-rotating design, but without a lathe, mill, drill press, and welder, it seemed rather daunting. The real kicker was the gears.... Without the resources to make my own gears to a specified diameter and strength, I was limited to purchasing. Nothing found locally, and without paying a lot, all the other dimensions would have to be based off of gear sizes I could afford, which further limited the availability of locally-sourced parts for the rest.<br><br>So, that began a series of changes in design concept. One cutting shaft instead of two meant no gears. No second meshing cutters meant gaps between disks (good idea for that below), so the series of cutters became a continuous shaft with a slot cut in. The slot became increasingly wider and deeper, until the sketches suddenly seemed more like a pipe. Using a pipe caused issues with a central axle, so the body became the exterior retention system. That meant issues with the load on the walls, so things started getting reinforced to the point it just seemed to need to be surrounded. That's where the second pipe came in.<br><br><br>As far as how well it works? At this point not well. The sloppy tolerances lead to the plastic bending and tearing in chunks, and getting caught between the two pipes. But its a start, and enough to feel good about.<br><br>And changes? I listed some in my last step, and we've been having quite a good discussion below!
<p>You could fit some kind of shim in the bottom of the outer tube, so the inner tube is pushed up to the slot in the outer one. Not perfect but it would help with your &quot;loose scissors&quot; problem. Bias it to one side or the other so that one set of edges closes properly, depending on which way you want to wind it. With a bit of thought you could probably come up with an auto-biasing version...</p>
I did think about a shim, but I was picturing more evenly spaced around the circumference. Putting it just on the bottom to force the top together is a good idea.... I'll probably try that. Thanks!
<p>The mechanical cutter concept is excellent. Next step is to make an extruder for heating this shredded plastic and forming it into filament for use in 3D printers.<br>If you want inexpensive 3D printer filament it might be interesting to try weed-eater string. </p>
<p>Ok good, I don't have to lose sleep.... here you go <a href="https://www.instructables.com/member/arvevans/" rel="nofollow" style="">arvevans</a>:</p><p><iframe allowfullscreen="" frameborder="0" height="281" src="//www.youtube.com/embed/SzUGHrEqEUk" width="500"></iframe></p><p>http://forums.reprap.org/read.php?1,141946,141946</p>
<p>Some weed-beater string is high density PVC, but I am not sure about all of it.</p><p>Seems that an extruder could be something as simple as a 3D printer with heated hopper to melt chopped plastic and a ram for forcing it out a nozzle to &quot;print&quot; the filament into a water bath (bucket) for cooling. Print in a circle to automatically coil the filament in the bottom of the bucket.</p>
Thanks! I'm hoping to eventually make an extruder, and will probably follow one of the Instructables covering that... There are some awesome designs out there! <br><br>Weed whacker string.... wow, I wonder if anyone has tried that... I think you just cost me some more sleep lol
<p>How about using old skill circle saw, one that you can put multiple blades in it 3-5 or more (use longer bolt )have a metal plates on both sides of blades, with just part of the blades sticking out. then a wood frame around that for hopper . use long bolts with wing nuts to hold together ...I'm going to play around with some ideas and will post results .....I'm thinking a shaft with bearings with a stack of circle saw blades 10-20 with jam hopper.</p>
Great! Please keep me posted!
<p>Just to picture what others have said, to make cutting easier:<br>Inward angle on inner cutter edge, <br>Angled inner (or outer) cutter slot.</p>
<p>Thanks!</p>
<p>As far as the pipes not <br>fitting together well, the angled cutter may cut better and fit the pipes <br>together better, too. If you take your grinder and cut a flap on the <br>inner pipe that you bend up and sharpen the edge of, it can take up the <br>gap between the pipes. I, too, thought <br>about the eccentric pattern but that would definitely take some precision not always <br>easily come by in the garage shop.</p><p>I am fortunate to have one <br>of the super-duper 5hp Vitamix blend anything blenders and I have tried it on <br>some plastics. I saw another user use his on straight-up plastic, and I have to <br>say that while it is more work (because of straining and drying the plastic), I <br>put water in mine along with the plastic to keep the plastic from melting in <br>the blades and to help it maintain motion and hopefully hit the blades more <br>times. It still didn&rsquo;t cut it up as much <br>as I wanted, so more work is to be done there on blend time/ speed/ water level/ <br>etc. </p><p>A dedicated filament grinder <br>would be nice if it can get the shred small enough for a filament extruder. Industrial <br>shredders use counter-rotating meshed blades, so maybe using a simpler version <br>with a set of fixed blades with a rotating second set of blades would work or <br>simply a fixed, slotted plate with the rotating blades that go through the <br>slots. That wouldn&rsquo;t have to require <br>welding- just space saw blades on an axle with washers/nuts but I'd make it <br>drill/ motor powered for sure.</p>
<p>Apparently I didn't put my reply in the right box, so you didn't get a notification of my reply, <a href="https://www.instructables.com/member/wilwrk4tls/" rel="nofollow" style="">wilwrk4tls</a>:</p><p>Here it is:</p><p>You have some good ideas there, and some good CAD work as well :) Yeah, I'd love to make a multiple cutting blade device, and I like your idea of using the single shaft to eliminate gearing (lowering cost). I may look into that as well! I never expected so much feedback, but I'm loving the ideas!!!</p>
<p>for the money, just go to a thrift shop and buy a blender or a food chopper. Faster, louder. and quicker.</p>
<p>I think you would be replacing it pretty quickly... unless you were referring to my cat litter comment below</p>
<p>In the lab we use heavy duty electric coffee grinder but we freeze the plastic in liquid nitrogen first. We also feed liquid nitrogen thriough the machine while we grind. Once I tried to use a domestic food processor but it couldn't handle it. I imagine some of the really heavy duty food blenders would work. But these solutions all require big money and are to hard to do at home.</p><p>I really like dmwatkins idea, I wonder how it would go powered by a hand held power drill.</p><p>Has anyone ever fed plastic into a garden shredder?</p>
Thank you! I appreciate that! Someone on HackaDay also suggested a garden (compost) shredder, and others liked that too! If somebody tries it, I'd love to know how it works out!
<p>If the method was to extrude plastic pellets, why not use the same sort of method as shredding meat? You would have to heat the extruding/shredding apparatus to ensure that the plastic would stay melted, but they have kitchen aid meat shredder apparatuses to experiment with for relatively cheap. The trick is to find one made entirely of metal. Good luck!</p>
Thanks for the read and feedback! I did think about the way a meat grinder works as well, and whenever I go to the store, I'm always looking at various items like that for ideas (for this project and 50 others as well lol)... I got inspired by various blenders for a way to grind cat litter for green sand for casting.... probably would've been cheaper to just buy a coffee grinder.... but where's the fun in that? ;)
<p>This is a great new idea. I have been searching for several years for an inexpensive way to grind plastic. I posted this thread in 2009<br><br><a href="https://www.instructables.com/community/Inexpensive..." rel="nofollow">https://www.instructables.com/community/Inexpensive...</a></p><p>The only home shredder available is the FilaMaker mini-shredder that costs about $400. <a href="http://filamaker.eu/" rel="nofollow"> http://filamaker.eu/ </a> A heavy duty &quot;will it blend blender&quot; might work but it is also expensive, a few hundred dollars. There has to be a better way.</p><p>An inexpensive home shredder is something there is a huge demand for that no one had come up with a design for. If someone could invent one that costs under $100 it would be revolutionary. No more buying filament. Just recycle plastic bottles, etc. 3D printing would essentially be free once you had a printer. I wish Instructables would have a design contest for it specifically.</p>
<p>Thanks for the read! Yeah, the amount of plastic we already buy in the way of packaging and cheap items that later break.... why buy filament? I'm excited that you're out there thinking about it also! Thanks!</p><p>I tried to open your link, but I got the 400....</p>
<p>https://www.instructables.com/community/Inexpensive-way-to-grind-plastic-bottles/</p>
OK thanks! I read through the post... funny thing is that I even searched Instructables quite a while ago, but I never used the search word &quot;grind.&quot;. I'll use that on Google when I get a chance...<br><br>I also started thinking about this during a previous project about injection molding. I had saved up a bunch of HDPE I planned on using, but after cutting one container into 1/4&quot; squares with tin snips, I knew I never wanted to do that again.
<p>This is a very clever concept you've created here! Now you've got me thinking and I don't have a 3D printer! I think your idea of scissor action could work instead of two tubes, have one tube and one blade, the tube can have a spiral slit instead of a straight edge and the blade would go on the side of the hopper. I hope this makes sense?</p>
Yes, that makes sense. Thanks for the feedback! I love the ideas that people are giving me.... it's putting my mind in overdrive, which is something I love!<br><br>If you are gonna spin your gears on it, a HackaDay reader suggested a flat disk like a circular saw blade, but with cheese-grater-like cutters on the disk face.... Then I started thinking about Tunnel Boring Machines....
<p>I think the hardest part about a cheese grater is sharpening the blades unless you make it interchangeable so you can throw out the old one and put in the new one easily. You are very good at drawing out ideas from people.......hmmmmm!</p>
What an amazing idea! You have put a huge amount of work in here.
Thanks for the encouragement! It really didn't take all that long... more time staring at it is blankly lol.
Nice and simple design! Good luck in your contest! You got my vote :-)
Thank you so much!
<p>Gee, thanks, now I have another project for my inbasket... :-)</p><p>A really nice idea! I am thinking of making a larger one to handle the milk jugs and other plastic we recycle---they take up lots of room in the bin. And it just occurred to me...if it's possible to make the cutting edge slanted instead of parallel to the pipe axis, it would reduce the effort needed. Hmm....</p>

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