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I don't understand why it's so hard to build a machine that separates plastic from other materials. I mean in agriculture we have machines that separate tiny se

I don't understand why it's so hard to build a machine that separates plastic from other materials. I mean in agriculture we have machines that separate tiny seeds from anything else, I don't see why it wouldn't be possible to have a plastic sorting machine.

lbriner 8y ago
Because of the massive variation in shapes and sizes, the fact that some plastics might have other trash inside them and that plastic drinks bottles are usually more than one type of plastic. There are machines but they can only perform basic sorting so you still need people to do the "intelligent" part.
schiffern 8y ago
>I don't understand why it's so hard ... we have machines that separate tiny seeds Not hard, just expensive. And understanding how seeds are separated (not only that they are separated) makes it clear why plastics require so many more steps. For seeds, it's a two-step process. First comes threshing[1], which is the act of physically breaking the seeds off the plant. Basically they solved this problem by hitting the stalks against a solid surface. This works because all the plants are the same type and age, so you can dial in a force that will break the seeds _off_ without breaking them _apart_. This won't work for plastics, because they're combined with all sorts of other materials that whacking won't dislodge. One shot of that "Plastic China" documentary[2] shows a worker using a box cutter to remove a plastic label that's shrink-wrapped around (not glued to) a crushed container. The second step is called winnowing[3], and it's basically just blowing air through the falling seeds to separate them aerodynamically from the straw and chaff. This works because seeds have a higher ballistic coefficient (mass/drag). Winnowing is also done with screens or slats, relying on the fact that seeds are of uniform size. But plastics have all different shapes and sizes. Wind winnowing can certainly be used as part of a processing chain (eg to separate films from bulk plastics), and so can screens, but they're nowhere near a complete solution. Because of these complexities, automatically processing plastic at scale requires a lot more than just those two steps (which nowadays are combined together with reaping in the cleverly-named "combine harvester" or combine[4]). That means it's expensive, and that means someone in China with cheap labor and "externalized" pollution costs (read: make your neighbors get sick and die, but don't even have the decency to pay their doctor bills or keep their windowed spouses/orphaned children off the street) can outbid you. [1] https://en.wikipedia.org/wiki/Threshing https://en.wikipedia.org/wiki/Threshing [2] https://news.ycombinator.com/item?id=17678513 https://news.ycombinator.com/item?id=17678513 [3] https://en.wikipedia.org/wiki/Winnowing https://en.wikipedia.org/wiki/Winnowing [4] https://en.wikipedia.org/wiki/Combine_harvester https://en.wikipedia.org/wiki/Combine_harvester
newnewpdro 8y ago
There's obviously complicating factors. If a consumer thinks recycling is some sophisticated magic, they're likely to use the plastic bag they think is recyclable to encapsulate a load of mixed recycling while it accumulates indoors. Assume this bag now contains a combination of paper, plastic, and cans. Or if they have a plastic container full of fresh fruit with a tuna can from lunch, they might put the empty metal can within the now empty plastic container, closing the container to limit the odor. Eventually this winds up in the recycling bin as metal nested within plastic. Automatically identifying and separating these instances intact, I imagine, is non-trivial. What I don't understand is why we need to separate everything intact and can't just shred it all to a uniform size then automate sorting and washing that product.

lbriner 8y ago

Because of the massive variation in shapes and sizes, the fact that some plastics might have other trash inside them and that plastic drinks bottles are usually more than one type of plastic. There are machines but they can only perform basic sorting so you still need people to do the "intelligent" part.

schiffern 8y ago

>I don't understand why it's so hard ... we have machines that separate tiny seeds Not hard, just expensive. And understanding how seeds are separated (not only that they are separated) makes it clear why plastics require so many more steps. For seeds, it's a two-step process. First comes threshing[1], which is the act of physically breaking the seeds off the plant. Basically they solved this problem by hitting the stalks against a solid surface. This works because all the plants are the same type and age, so you can dial in a force that will break the seeds _off_ without breaking them _apart_. This won't work for plastics, because they're combined with all sorts of other materials that whacking won't dislodge. One shot of that "Plastic China" documentary[2] shows a worker using a box cutter to remove a plastic label that's shrink-wrapped around (not glued to) a crushed container. The second step is called winnowing[3], and it's basically just blowing air through the falling seeds to separate them aerodynamically from the straw and chaff. This works because seeds have a higher ballistic coefficient (mass/drag). Winnowing is also done with screens or slats, relying on the fact that seeds are of uniform size. But plastics have all different shapes and sizes. Wind winnowing can certainly be used as part of a processing chain (eg to separate films from bulk plastics), and so can screens, but they're nowhere near a complete solution. Because of these complexities, automatically processing plastic at scale requires a lot more than just those two steps (which nowadays are combined together with reaping in the cleverly-named "combine harvester" or combine[4]). That means it's expensive, and that means someone in China with cheap labor and "externalized" pollution costs (read: make your neighbors get sick and die, but don't even have the decency to pay their doctor bills or keep their windowed spouses/orphaned children off the street) can outbid you. [1] https://en.wikipedia.org/wiki/Threshing https://en.wikipedia.org/wiki/Threshing [2] https://news.ycombinator.com/item?id=17678513 https://news.ycombinator.com/item?id=17678513 [3] https://en.wikipedia.org/wiki/Winnowing https://en.wikipedia.org/wiki/Winnowing [4] https://en.wikipedia.org/wiki/Combine_harvester https://en.wikipedia.org/wiki/Combine_harvester

newnewpdro 8y ago

There's obviously complicating factors. If a consumer thinks recycling is some sophisticated magic, they're likely to use the plastic bag they think is recyclable to encapsulate a load of mixed recycling while it accumulates indoors. Assume this bag now contains a combination of paper, plastic, and cans. Or if they have a plastic container full of fresh fruit with a tuna can from lunch, they might put the empty metal can within the now empty plastic container, closing the container to limit the odor. Eventually this winds up in the recycling bin as metal nested within plastic. Automatically identifying and separating these instances intact, I imagine, is non-trivial. What I don't understand is why we need to separate everything intact and can't just shred it all to a uniform size then automate sorting and washing that product.