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Revisiting the recycling of Reprap plasticsSaturday, July 12. 2008
The question of recycling waste products from a Reprap machine is revisited...
When the Reprap project was less than a year old the question of how one was to get the filament that the Adrian Bowyer's Mk II extruder, variations of which are almost universally used by Reprap and Repstrap machines today, required was entirely open. We were using polycapralactone plastic at that time. Polycapralactone would melt in a hot cup of coffee and Adrian, Simon, Vik and Ed were simply melting it and rolling it out in 3 mm rods on a sheet of glass. I did the maths on that approach and soon realised that we were going to have to have continuous filament if we were going to print anything significant. That entailed us having some way to extrude filament. Adrian designed one for 3 mm granules and I designed another, smaller one, for powder resin shortly thereafter. We would have probably gone a lot further with this work except that the next month I discovered that the makers of plastic welding rods could provide us with all the plastic filament we wanted at very reasonable prices. That allowed the Reprap project to focus on actually making the printer. Last Fall, Adrian revisited the idea but this time decided to go directly from granules to printing on the Reprap machine instead of making filament for a Mk II. He published the results of his work on the Reprap website. The availibility of a granule or powder resin supply begs another question, viz, are people printing with a Reprap machine like Darwin or Tommelise going to be buying such resin directly or are they also going to be attempting to recycle appropriate plastics, either scrap from their own printing efforts or scrap from elsewhere? If they are going to be recycling plastic they have to have a capability of reducing the scrap into a form that either their direct granule or filament extruder can cope with. That implies that you have some sort of shredder or grinder than can reduce the scrap. A few days ago, in priority setting for R&D for the core Reprap project Dr. Bowyer included the development of a plastics scredder and further work on the granule extruder that he had published earlier. His rationale is very compelling... But shredding RepRap's own products would be immediate, of course. I think this is worth doing intrinsically, and - less importantly - it is politically useful: "RepRap instantly and locally recycles its own products." Heretofore, my thinking had been aimed at converting a entry level garden shredder for the task. You can get into the game with those for about US$200. Dr. Bowyer's sense was that the cost should be somewhere around US$40. He expanded on that... I also think it should work very slowly; almost imperceptibly so, like a clock minute hand. Two advantages to this: it'll be a lot quieter, and it can be run at much lower power. And I don't see why one would be in a rush. Indeed. I'd never thought of it in that light. Adrian's feedback got me to rethinking the whole plastics grinding issue. Recently, I bought a rotary rasp and a rotary file while thinking of new approaches to pumping filament into heated extruder barrels. The rotary rasp was designed for my Dremel and was rather pricey at US$17 while the file was much cheaper at about US$3.50 but had a 1/4" shank that meant that I could only run it in an electric drill. Keeping in mind Adrian's notion that such a shredder, grinder in this case, should operate very slowly I did some informal experiments with that in mind. I first tried some scraps that I had of HDPE left over from a cutting board that I'd bought and cut up into several parts that I needed. HDPE is a tough plastic. I'd tried grinding both HDPE and polycapralactone (CAPA) before with a hand cranked, old-fashioned meat and grain grinder some two years ago with indifferent results. Since then I'd tried using several different kinds of grinders in a desultory fashion and noted that some of the HDPE grit that was produced tended to melt during the grinding and clog the grinder surface. I began with the rotary rasp in my Dremel and ran it at the slowest setting possible with that very high speed machine. 
Imagine my surprise when the rasp produced a relatively fine powder of consistent quality without clogging the rasp. Thus encouraged I tried the rotary file in my electric drill with much the same result. 
The file, operating at much lower rpm ratings didn't clog either. While the grit that it produced was of a coarser quality than that produced by the rasp it would have worked perfectly in my old filament extruder. This morning I decided to try to get a handle on how much power these rotary grinders were demanding. Repeating the experiment with the Dremel-mounted rasp while monitoring energy usage with my clamp meter I noted that depending on the speed I ran it at it drew somewhere between 0.21 and 0.5 amps (23-55 watts). The rotary file drew anywhere between 0.4 and 3 amps (44-330 watts). Interestingly, the rotary rasp tended to clog as the speed pushed up the load towards 0.5 amps. It ran quite happily without grit melt at the lowest speed setting (0.21 amps). The file, on the other hand, was able to grind without melting the grit at upwards of 2 amps. The file, with its few cutting edges and larger mass seems to be subject to less mechanical heating and is, in any case, less prone to clogging by virtue of how it is designed. At 2 amps it was producing an impressive amount of grit. While I haven't solved the problem of producing 3 mm granules from scrap plastic suitable for Adrian's granule extruder, I have produced a much finer resin grit which I suspect that his extruder would use just as happily. Finally, assuming a very small, cheap grinder presumes that it is going to only grind small chunks of scrap plastic successfully due to its low capacity. That limitation assumes that one has a means of reducing larger chunks of plastics into something that the shredder/grinder can eat. Heretofore, I'd been achieving this end by using a hacksaw and heavy duty side cutters to achieve this end. Needless to say, this approach was both time-consuming and tedious. In that my brain was fizzing with ideas last night I wondered whether a simple bolt cutting tool could be applied to this problem. Indeed, it could. 
My US$15 bolt cutters quickly reduced 3/8" HDPE to manageable chunks. I also used it to reduce an ABS inkjet printer housing. While the bolt cutter handled this job it seemed a bit overpowered for the task of cutting this much thinner plastic housing. I expect a heavy set of gardener's shears would do the job as well or better much more handily. I did some tentative grinding experiments using the Dremel-mounted rotary rasp with the resulting ABS scrap with results very similar to what I got with HDPE. Trackbacks
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If you have ever milled plastic these results are not surprising. Using a milling cutter you also get a consistent fine grit.
That brings to mind that perhaps we could grind up large things with a small moving head rather than having to cut them up with bolt cutters.
Imagine a low sided wide box with a pusher at one end that slowly pushed things towards a grinding head the runs up and down the opposite end.
It could grind anything that fits in the box, the only limitation is the height has to be less that the grinding bit unless it also moves up and down.
It would be cool if we had a rotary file as wide as the box. I wonder if they make such a thing?
If the grinding head moved up and down you could kind of build an "un-printer"...object goes in the box, grinder does the work.
Actually, if you angled the box and made it so that the object fell towards the head (with some sort of shield protecting the mechanism from grit), then it would be kind of a gravity feed...?
Kickback would be a problem. You'd want a weighted pusher plate pressing the feedstock into the grinder, with a linear ratchet so it couldn't push back. This adds complexity, and thus cost, however. Hmm.
Crushing corn for chicken feed is commonly achieved using a couple of narrow 3-4 inch rollers at the bottom of a hopper. You could cover a couple of rollers with fine grade-x (expanded steel) and have them just pass by each other. Power from a 12 volt motor like a car starter or heater motor geared way down would give you industrial scale production from the roughest of scrap for about the 40$ Adrian is looking for (assuming the re-use of scavenged parts)
I've made similar devices before with low duty rpm and I can say for certain you don't even need bearings, well greased oak will last just about forever.
It seems worth note that files are not terribly complex devices to make by hand, especially if they are going to be used on something soft like plastic (for harder materials you start needing to fiddle with high-carbon steel and hardening/tempering, to make the file soft enough that your chisel works while you make it, and then hard enough to file the chisel when you use it). The basic process is to use a chisel at an angle to cut each groove in the file, working such that the chisel is slanted away from the grooves already made (so the side of the chisel doesn't mash and ruin them). I'm pretty certain you can get good results with a plain cold chisel - but in general some fiddling is required to get parameters like the angle of the chisel right.
Another suggestion: paper shredders that are also capable of shredding CDs are increasingly common - and they're pretty cheap. http://www.office-discount.de/webapp/wcs/stores/servlet/ProductDisplay?storeId=45&langId=-3&catalogId=&productId=93405&topCategoryStyleId=ff is EUR30 and can shred CDs. I've just tried half of a HDPE shampoo bottle in one here and got (for the most part) strips of 30mm x 5mm out of it. It's not dust, but I figure it should be small enough to enable easy further working?
paper shredder / cd shredder sounds perfect, especially if it's rejigged with a higher power geared down motor it could steadily chew it's way through stuff. In terms of getting it smaller, surely popping it back through the shredder again and again will get it reasonably small - I would have thought 5mm strips would eventually end up as 3mm 'bits' after 2 or 3 passes.
I wonder what the impact on the rasp/file is like?
In following a CNC milling forum, I read that dark colored plastic can be quite hard on milling cutters, as the pigments can be quite abrasive. Black can apparently sometimes have large quantities of carbon black in them.
So, reducing to a powder may put a lot of wear on cutting edges. Might need to strike a balance between size and cutter wear, and perhaps shredding and melting small chunks may be more effective than grinding to powder.
D.
Powder is probably a poor term for what the rasp and file produced. The consistency was more like a fine lint than a powder. Mind, the pigment issue is one I'd heard about before. Thanks for the heads-up! :-)
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