Lessons In Printer Poop Recycling

The fundamental problem with multi-color 3D printing using a single hotend is that they poop an awful lot. Every time they change filaments, they’ve got to purge the single nozzle, which results in a huge number of technicolor “purge poops” which on some machines are even ejected out a chute at the back of the printer. The jokes practically write themselves.

What’s not a joke, though, is the sheer mass of plastic waste this can produce. [Stefan] from CNC Kitchen managed to generate over a kilo of printer poop for a 500-gram multi-color print. So he set about looking for ways to turn printer poops back into filament, with interesting results. The tests are based around a commercial lab-scale filament extruder, a 3Devo Composer, but should apply to almost any filament extruder, even the homebrew ones. A few process tips quickly became evident. First, purge poops are too big and stringy (ick) to feed directly into a filament extruder, so shredding was necessary.

Second, everything needs to be very clean — no cross-contamination with plastics other than PLA, no metal bits in the chopped-up plastic bits, and most importantly, no water contamination. [Stefan]’s first batch of recycled filament came from purge poops that had been sitting around a while, and sucked a lot of water vapor from the air. A treatment in a heated vacuum chamber seems to help, but what worked best was using purge poops hot and fresh from a print run. Again, ick.

[Stefan] eventually got a process down that produced decent, usable filament that would jam the printer or result in poor print quality. It even had a pretty nice color, which of course is totally dependent on the mix of colors you start with. Granted, not everyone has access to a fancy filament extruder like his, so this may not be practical for everyone, but it at least shows that there’s a path to reducing the waste stream from any printer, especially multi-material ones.

32 thoughts on “Lessons In Printer Poop Recycling

    1. Looks like this is extremely energy intensive. Seems like more damage to recycle the plastic than it would cause on its own. One could melt the poop into one large blob the it won’t be microplastic any longer.

  1. Recycling is definitely necesary if your doing lots of swaps.
    Some colors swaps are faster than others but still waste the same poop. It would be nice if theyd add a camera based color swap detector.

  2. I kind of wish they’d do a multi nozzle setup, 2 or 3 nozzles would mean greatly reduced poops. You could even do things where like if the model is mostly yellow with some red and black bits, keep one nozzle yellow and swap red and black on the other nozzle

    1. Problems to solve in a single 3 color hotend design:
      1: 3 heated hotends could be one triangular heatblock with 3 heaters switching active heater to the current nozzle.
      2: 3 thermocouples to sense accurately the active heatblock chamber. Maybe a single thermocouple located in the center of the triangular heatblock could work.
      3: 3 extruders on the printer head because bowdens are a compromise. An extruder with sliding gears to switch to the active filament would only require a added servo to the printer head.
      4: jams will be a PIA, maybe the nozzles are tied to the heat break so they can be dealt with outside the triangular heatblock monstrosity.

        1. If you don’t purge you will always get a blended mix zone where the chamber still has the old filament in it – might not bother you though and there are ways around it. For instance I know I read of somebody tweaking the slicer to put all that blend zone into the infill, might even have had variable infill as a concept so there is always at least Xmm of extrusion as the purge in the infill before the surface is printed in the new colour.

          1. +1

            I’m not a slicer programmer, so I don’t know how they determine infill, but I wouldn’t think it would be too hard to measure the average “poop”, add 10 – 20 % , and then predict where the infill needs to change colors so that it exits and starts the new layer with the new color. Of course this would only work on horizontal color changes. (A more complex program would do multiple strategic infill changes by region – so not impossible?) I know FORTRAN. are any slicers programmed in it? :-)

  3. I believe there is an option in the slicer to use a separate object as a purge tower. Therefore, if you don’t mind random colors, and both objects fit on the plate, you can actually cut the waste drastically and with no additional hardware :)

    1. A, this entirely depends upon the slicer having this option.
      B, what do you do with a multicoloured block tower of random purged filament?
      C, you still end up with waste, lots of waste.
      D, if you are printing with random colours then you can use a primer and car spray paint to get the colours you really want.

      1. I think Nick means that you can be printing an actual second object “AS a purger tower”. So it actually eliminates waste, assuming you have a second useful object on the bed.

  4. Or, crazy thought, learn to design models that have different parts printed in different colors that assemble into the whole. Or print in one color and paint it.

    Unless it’s mission critical the final object be multiple colors printed in place, 1000 grams of waste for a 500 gram finished product sounds foolhardy.

    1. That doesn’t answer to the best use of multi-filament printing in my thinking though – material property selection. Which gives you options like dissolving support material, flexible elements enmeshed with the stiff, potentially areas that are ‘food safe’ within the the bulk of the object printed in a cheaper filament.

      Even for pure ascetics it has use, while you won’t easily beat even a mediocre paint job in detail and finish quality with printing there is still something to be said for multi-colour printing – it is durable in a way paint is not.

  5. There are two obvious solutions when printing in a single material (PLA):

    – purge into a pellet mold (think axial displacement pump or candy roller) to diectly produce moldable pellets
    – purge by printing a continuous filament onto a rotating drum

    The concept of printing infinite objects should be familiar since the 3DPrintMill, SunShine demo’ed printed filament 4 years ago (https://www.youtube.com/watch?v=6kbjZobJtbM) and a flat spot for adhesion during printing can be tolerated (looking at you, hexagonal Prusament).


  6. >just buy $1000+ worth of gadgets to recycle $20 worth of filament
    This is eco-feelgood nonsense for rich people. I 3d print because it’s cheap. At the end of the day I know it’s plastic, and that my trash will probably choke a sea turtle somewhere.

    1. Rather a negative spin, for many maker spaces and school type environment this concept is almost certainly a major winner fiscally if you can get it working reliably enough and educational enough its a worthwhile demo to have on hand even if you can’t – will really bring home how important properly separating by plastic type is etc. At which point it doesn’t even have to work that well – you can hand the bulk of your waste off to the industrial scale recycler.

      And for folks that do a great deal of their work rapid prototyping with 3d prints even an individual can see a good return on their investment – I’d be shocked if somebody like James Bruton or Ivan Miranda who seem to make almost the entirety of every rather large project they make with 3d printers wouldn’t be winning inside a year, maybe even only a month (though they may choose to keep with fresh plastic being youtubers where pretty colours is important?).

    2. That is a little short sighted. Projects like this are expensive because this entire concept is fairly new. It will how ever inspire other people to do similar projects and from that, the community learns more methods to bring down the costs money, time, and material. Companies will also start to come up with solutions if they see a customer base that is interested in it and economies of scale make things cheaper.

      The entire 3D printer scene would have never happened if people didn’t spend the time and money developing them because just making a mold and pouring melted plastic in it is cheaper. Now you can get 3D printers for less than a couple hundred bucks and they get cheaper daily.

  7. Polylactic Acid (PLA) filament is a recyclable, natural thermoplastic polyester that is derived from renewable resources such as corn starch or sugar cane. The filament is biodegradable under certain conditions

    So the filament is already sustainable, the only advantage of this process is to reduce waste. Which is a great idea, even if not all of us can justify the cost involved

    1. I see someone has pasted from a wiki, so let me explain a bit further. PLA is a bit more than corn squeezes. As you mention “certain conditions” it really means high heat and moisture which don’t occur in nature. Additionally, the “+” in PLA+ is more than adding cost. PLA should really be ID’d as a “bio-polymer” to let people know this isn’t just something you throw into a compost pile, degrading into something earth friendly. And this is just PLA, where other filaments are used by the X1 and P1P. (Let’s be honest; we are talking about these printers as these are the most egregious poopers.)

      In this day and age, we’ve all been tasked with reducing waste and limiting plastics in our food and ground water. 3D printing haven’t drawn attention to the Thunbergs of this world. But that can change easily.

    2. PLA will not break down in your body if it gets through the walls of your GI track in the form of microplastics. That is currently the biggest issue with plastics in general, and will likely be the leaded gasoline for millennials and zoomers as a few more decades pass and we all start to get the related but still unknown health problems. There are already diseases and health issues that basically don’t exist in 3rd world countries, that are occurring in first world countries in droves that could be related. That said, I still love my 3D printer.

  8. As someone who makes things from resin (molds not 3d printed resin) I could sure use some of that nice colorful poop in some of my artwork. That colorful poop would look nice in the right pieces

    1. I’ve seen someone melt the poop into silicone molds. I believe you can buy these molds for making cake decorations and resin molds for not to much money. Seems like an easy process to pour in some plastic and bake at 400° and see what happens. I ordered a mold off Amazon just the other day to try.

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