How To Identify Plastics Before Laser Cutting Them

If you own a pickup truck, you’ll quickly find yourself making friends with people who just happen to need help moving next weekend. Trust me, it’s almost magical. And if you own a laser cutter (or work in a hacker/maker space that has one) you’ll get some odd requests to cut or engrave plastic items of unknown type. Before you do, you should read this (pdf) chemistry lab written by [David A. Katz] to learn how to identify what type of plastic it is.

There are several reasons why you don’t want to cut or engrave some types of materials. A few make a gooey mess that you’ll regret even trying. Others make a horrendous odor. Some hackerspaces will even charge you extra if you stink up the place (aka: malodorous material charge.) Some tend to catch on fire. Yikes.

But that’s not the worst of it. Some types of plastic release potentially deadly hydrogen chloride gas. It’s bad for the optics, it’s wreaks havoc on the electronics and mechanics of the machine, and could do a really good job of messing up your lungs forever. In the video after the break, you can see the flame test for such plastics in action at the NYC Resistor as they test several common items using nothing more than a blow torch and some copper wire. In short, if the flame test produces a green flame, do not put it in the laser.

If you want to see a good list of what is and what isn’t ok to cut, head on over to ATXHackerspace’s wiki. They will give you a nice run down with lots of notes and helpful hints as well.

29 thoughts on “How To Identify Plastics Before Laser Cutting Them

  1. Not all PVC sinks in water. Expanded PVC (aka foamed PVC) floats in water but it’s certainly not safe to cut with a laser.
    The copper wire test would identify ePVC as a problem. Hopefully people won’t assume all floating plastic is safe to cut with a laser.

    1. And not all plastics that soften in hot water are PET(E).

      PCL for example isn’t. Trying to laser cut PCL and derivatives thereof thinking it’s PET is just going to end up with a hot-glue-like blob.

  2. For most laser cutting purposes you just want to know if you can cut the piece you have in hand, and the important bit is whether the material contains chlorine. For that you do the flame test as mentioned above.

    I put together a document explaining the flame test in pictures for our hackerspace:

    We keep a torch, lighter, and pieces of copper wire (stripped Romex work well) on a shelf next to the laser just so people won’t have an excuse not to do the test, and laser training includes the flame test.

    Cutting chlorine-based plastics is one of the three laser-based things that can get you walked out the door.

    1. That ATX hackerspace list is pretty good.

      I just now converted their table into 3 PDF’s which is going right onto the laser housing this afternoon. Thanks!

      (Note: I couldn’t find a CC licensing statement on their website, but I don’t think anyone would mind reproducing this info – everything there is pretty-much common knowledge.)

      1. It’s likely to cut perfectly fine — except it’ll release a bunch of nearly invisible poisonous gas that’ll etch glass and electronics. So the real danger isn’t that it’ll cut bad, or that it’ll burst into flame, but that it’ll irreparably damage your laser cutter, your ventilation system, your lungs. At least one of those can’t be easily replaced.

      2. Shit I just realized this is so asinine when cutting PVC all that is made is HCl vapor. Want to have an equally dangerous situation boil some muriatic acid. lol hackaday usually does pretty well on chemistry stuff but they fail hard on this article.

        1. Makes your laser go rusty really fast.

          It’s not that bad, you just need to do your extraction & filtering properly.

          What isn’t mentioned that’s more important, most of the ‘bad’ plastics are crap to cut by laser. They melt rather than vaporise, so poor quality edges. Mind you the ‘good’ plastics like to catch on fire, so it all evens out.

          There’s never a dull moment with a laser cutter.

        2. From what I’ve read chlorine gas is the main danger but even if HCl were the gas released it would still be a bad idea to breath the fumes. I don’t think you’ll find much debate (except from the exceptionally ignorant) that cutting PVC with a laser, without proper precautions, is a bad idea .
          I hope you boil your muriatic acid (HCl) in a fume hood.

  3. I wonder where polyoxymethylene (POM), trade name Delrin, would fit into this. It is known to burn with an invisible flame; probably wouldn’t want to put it in a laser cutter, either.

        1. No, that’s acrylic you’re thinking of. ABS does cut reasonably well, but it’s expensive and sometimes melts while cutting – it’s more often used for injection moulding.

  4. I like the graph as a general outline on what kinds of plastic you have before you cut it. I know people who have had problems caused by cutting into the wrong types of plastics and having a gooey mess everywhere. I’ll have to hang this up and find all the exemptions that I can for it.

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