Friday, 29 April 2011

On Small-Scale Plastic Recycling

While struggling through my last exams ever this week, I've been coming down with a fever, so I hope this blag makes sense, as I can't concentrate on much else right now.

Continuing along my previous train of thought (A Mission Statement), I've been considering how one might best go about recycling such sometimes 'non-recyclable' recyclable plastic bits as small bottle caps, yoghurt pots, or some of the chunkier waste material produced by failed prints.

People on the RepRap Wiki have mainly been suggesting two ways of approaching this so far.
One way of printing with recycled plastic is to grind it down into very small bits (granules) and then pour it into an extruder that feeds material in from a hopper, rather than a coil of filament.
Another way is to melt waste plastic down and extrude it into the 3mm thick filament feedstock that can be used by the most common reprap extruders.
The students that I mentioned in my first post went a sort of third combined way, where they ground plastic down and then extruded that into a thick welding rod. I think this overcomplicated things a bit, but I'll get onto that in a minute.

A good way to start looking at this, as with any decision, would be to describe one's requirements or specification.
So, the way I approach this project, I think that the end result should be a machine that:
  • Can be built using materials easily available to most reprappers (ideally scrap).
  • Does not pose a risk to the environment or the user's health.
  • Is simple and hence robust in its construction.
  • Uses a process as material and energy efficient as possible.
A major problem with the first method of approaching this is that it creates exactly some of the type of waste that I am trying to prevent. That is, it creates tiny particles of plastic that can become circulated in the ecosystem.
Nice part, but nasty mess. I doubt that stuff on the cuff and bearing is some epic dandruff.
Whether people add methods for containing such a system or not, such as having a vacuum suck dust from a grinder into a filter bag, they still run the risk of losing some and releasing it into the environment, or inhaling dust themselves. That would also be adding complexity, in conflict with the third requirement, and using up a lot of energy to produce any significant suction power.

Another problem with the first option, is that the most efficient ways to grind up the plastic would involve either designing a highly complex system of sharpenable blades, causing trouble similar to that with the custom-made drive screw on the TUDelft recycler (difficult to acquire, bad design tolerances caused metal abrasion), or continually buying something that was not designed for the task and will need to keep being replaced at great expense, like the various types of grinding attachments suggested on the wiki that are meant for shaping wood.

So, I'll leave grinding things aside until some genius can come up with a layman's way to easily cut waste plastic into homogeneous granules, without spreading fine dust (I don't think anyone wants to go at it with a pair of scissors all day).

As for purely melting plastic down to get it out of its previous shape, this concept seems promising to me because all reprappers will have access to and experience with the gear necessary to melt plastic down in a controlled fashion, by virtue of the type of extruders that they use for printing.
It should be entirely feasible to apply roughly the same design used in the various brass hot-end designs, such as wrapping nichrome wire around a heating chamber, or coupling it with a resistor inside a heating block, and applying a DC current controlled by pulse-width-modulation with the reprap electronics.

In fact, if someone has their extruder connected to the motherboard or controlling board by a detachable connector, like the sort of mini-molex used on gen6 electronics, running the recycler could be as easy as unplugging the extruder cables and then controlling the recycler from software like RepSnapper.

Another possible benefit would be the ability to chuck in really difficult to recycle things, like the PET films from ready meal trays in the example back at the start of this post, or any polyethylene bag of frozen food, for that matter.

On melting plastics like HDPE down at small scale, I have some experience in this area since I experimented with it a couple of years ago. Removing the old shape of bottle caps was literally as easy as cake, for all it took was for me to fold a piece of aluminium foil into a small tray, and heat the caps to slightly above their melting temperature in an electric oven. There were no nasty fumes, the product didn't have any significant voids or bubbles despite the messy pile I put the caps in, and had I set aside a clean baking tray for the purpose, I could have produced an even flatter workpiece. Meanwhile Forrest did something similar here, albeit with powder.

I then tried my clumsy hands at milling that sheet of HDPE, into the shape of a bespoke part I wanted that was in low supply and hence horribly overpriced, using a dremel rotary tool. I don't have that part with me, but when I can dig it up I'll probably post a photo here.
Suffice to say, the material was nice to work with provided you use low speeds and the finished product was fairly high-strength for its purpose, however it produced the same kind of gritty plastic mess shown above. Since then I had been pretty much searching for a method to use plastic recycled like that in a way that didn't create a difficult-to-control mess of dust, and was delighted when I heard of RepRap about a year ago.

The only issue I can think of with this concept is with recycling some plastics like ABS that give off nasty fumes when you heat them up. It's interesting to note a possible end-result of repeatedly recycling some ABS in this way, like in this incident, where cooking out one part of the A-B-S mix created a different plastic with significantly different properties.
Of course, this is a constant issue that reprappers and other fabbers are having with ABS anyway, and is typically patched over by using some form of air-filtering contraption to cut out the heady fumes generated. I also noticed in my experiments with melting down black-dyed PolyPropylene in an oven that that produces a bit of a smell, but nothing that would make you keel over.

You could suggest that we just give up on using ABS, and stick to things like PE and PLA that can be produced without oil, and while that's probably a wise idea in the long run, it should be noted that there is a great wealth of ABS waiting to be used in landfills around the globe, since it has been in widespread use on the cases of various electronics like phones and consoles that were designed to become obsolete and get thrown away in favour of newer models.
Pic from Treehugger.

If we can find better ways to handle the resulting fumes than constantly using up filter materials like activated carbon, then we have plenty of good material to build more repraps from. However, for now I would be cautious with melting that stuff down, and hope that someone can find a clean way to cut it into granules.


  1. I Wonder if a wood chipper would chop up plastic?

  2. Tree chipper. Precisely what I was thinking. There are (relatively) small units that will turn hardwoods into sawdust... though the sharpening problem will still exist, it might be a good way to go if enough 'feedstock' of waste is available.

  3. I should probably write another update, the most effective machine I've seen for this so far was this chopper made from water-jet-cut steel, which is supposed to be OSH, though the designer hasn't made many files available yet: