V4 BeyondPlastic – [PROCESS] Extruding PotatoPeels
Potato peels can turn into a hard material once you heat it up. But what are the ways to actually cook it? And can it be done with some of the Precious Plastic machines?
We made this experiment to find out how potato peels work in the extrusion process and also to learn more about the material properties – how it behaves during the process and after it. The spork mold we used was lent to us by amazing @pauldufour
Since we are nowhere close to being done with this workflow, it still needs further experimentation with its variables like time, temperature, speed and ingredient mixture. Still, we want to already share our process with you since it is different from the pressure molding we did for the other materials.
So far, it looks like this:
Ingredients: ⅔ potato peels mixed with ½ water.
To make two sporks 95 mm x 35 mm we take 60 g of peel powder and mix it with 30 ml of water. This amount is a bit extra than the actual outcome, as some of it stays in the extruder nozzle.
Temperature: the extruder is heated up to 100°C at the nozzle and 30°C – 50°C in the tube.
It is better to keep a mild temperature in the tube, otherwise, the material hardens and gets stuck inside. You probably don’t want to spend next hour or two cleaning up the extruder (we’ve been there).
Optional: preheating the mold up to 180°C-210°C.
This step can be skipped, but we experienced that a preheated mold gives a smoother surface.
Extrusion: with a low speed, it takes 4 min for the material to reach the nozzle and 2 min to fill up the mold.
If the speed is too high, the starch doesn’t have enough time to get cooked so the material stays weak and crumbly.
Taking out: the material is a bit flexy at first but becomes very hard after a day or two.
This is because some leftover moisture evaporates slowly.
Some general learnings that we have:
Shrinking. When the material dries out completely, it becomes about 10% smaller, also because of the water content that leaves after a while.
The spork final size is 90 mm x 32 mm.
Pressure. More pressure gives a more sealed surface.
If we stop the extruder too early, the spork looks rougher and is a bit weaker.
Overheating. Don’t overheat!
Potato peels become very sticky when the temperature is too high, even when you try to be quick. 130°C is already too much.
Mould preheating. It makes the surface more sealed and shiny.
It also finishes cooking. We got a black (burnt) sample from a mold pre-heated up to 250°C. Looked good though.
Coffee grounds… Don’t forget to also grow Oyster mushrooms !
For now I use my gounds I have to fetilise my garden, but I could imagine growing pots made of coffee in my future.
Hi Andy, I think it would be cool to try to make it more low-tech. You don’t have to actually cook it in an oven in the mould – the mixture can be pre-cooked in a pot and then injected and taken out. However, what we learned is that the surface quality and strength appear better when pressure is high, so pressure from a single bolt might happen to be not enough for a complex mould.
If you try it out please share your results, we would be happy to learn:)
Hi Marina, This is a very interesting process! (I have no experience working with bio-plastics) Do you think it would be possible to do this with a more low-tech approach? Perhaps skipping the extruder completely, just inject the potato mixture straight into the mould with a syringe, apply some pressure by screwing a bolt into the entrance hole, and then cook in a normal oven?
Ok, interesting that you can mould the pre-cooked mixture and then take it straight out. I might try the oven method, though it will just be some random mould not designed specially for use with this method. I think if the mould can be completely filled with the mixture, you would be surprised how much pressure you can generate by screwing in a single bolt, it’s probably way higher than you will get with the injection or extrusion machines as you can put a lot of force on a very small area.
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