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DIY method for sorting shredded plastic flakes by resin type? Answered

I've seen thatthe Coca-Cola company is promoting the keep the cap on PETE bottles, and that they use sink/float methods to separate the bottle from the cap after shredding since PET sinks and HDPE floats, now is there a similar (easy/ homebrew) test for other plastics like PP or LDPE, and by the way can 2 & 4 mix on a Melt? looking into doing some layered DIY proyect that involves the DIY Extrution of PET filament and other plastics for experimenting purposes.


Jack A Lopez

4 years ago

There exist methods for identifying unknown plastics, but as far as I know, none of these methods are quick and easy enough to lend themselves to automation, not without significant a prior knowledge; e.g. like the example you gave where you know in advance your plastic waste contains exactly two kinds of plastic, PET and HDPE, which in shredded form, can be separated by the sinks-or-floats-in-water method.

I think the easiest way for a home brew plastic recycler to get the exact plastic he or she wants, is going to be by sorting it by hand.

As I was saying before, there are methods for identifying a given sample of totally unknown plastic. The poor man's way to do this, is usually a series of simple tests, including: sinks-or-floats-in-water test, melting temperature tests, and also a test that involves burning a small sample, to watch how it burns, and also to smell the smells emitted.

There are a bunch of web pages that explain these tests, but the best summary I ever found was in flowchart form, and was impressed enough by this document that I've decided to just up it to my Instructables picture library, and attach it here.

Also the web site where I found it has gone offline. Although archive.org also saved a copy of that page, here:


Anyway, I think the whole point of that system of numbered resin codes,


was so that we wouldn't have to mess around with burning plastic, just to smell burning plastic fumes to discover what it is.


By the way, there are also high tech ways of doing this, like

fourier transform infrared spectroscopy,

but like I was saying before, even tests like this are not fast enough for automated sorting of plastic.

Also by the way, I remember, a few months ago, noticing that someone in Israel was trying to come up with a cheap, consumer oriented, IR spectroscopy tool, via Kickstarter. I have no idea if that gizmo would actually tell you anything useful about samples of plastic placed in front of it, but now I kinda have to link to it. Because how can I mention something that weird without providing the link?


AskDaDocJack A Lopez

Answer 4 years ago

Great response Jack, I do believe this to be the most helpfull and insigthfull so far, the chart will most defenitely will set me straight on the correct path.



4 years ago

AFAIK it is almost impossible to sort plastics other than by gross characteristics eg density. this is why plastic objects are marked with the plastic type to make recycling easier. BUT then you need to get at it and sort before it gets shredded.


Answer 4 years ago

Given that pre-sorting would be a solution, but sometimes it's not an option, now not all of the pre-shred material is identified as to resin type, so how could one tell what it is by just looking at it? (soda/water bottles are PETE, but is the cap HDPE or LDPE? soap/deterget bottles are HDPE, but what about the caps? some are PP, and some are other stuff.