Smoothing PLA Printed Parts

We’ve seen a few advances in the finishing processes of 3D prints over the last few months that result in some very attractive parts that look like they were injection molded. Smoothing ABS prints is now a necessary skill for anyone looking to produce professional parts, but those of us using PLA for our RepRaps have been left in the cold. After some experimentation, the guys over at protoparadigm have come up with a way to smooth out those PLA prints, using the same technique and a chemical that’s just as safe as acetone.

Instead of acetone, the guys at protoparadigm are using tetrahydrofuran, or THF, as a solvent for PLA. Other PLA solvents aren’t friendly to living organisms or are somewhat hard to obtain. THF has neither of these qualities; you still need to use it in a well ventilated area with nitrile gloves, but the same precautions when using acetone or MEK still apply. It’s also easy to obtain, as well: you can grab some on Amazon, even.

The process for smoothing PLA prints with THF is the same as smoothing ABS prints with acetone. Just suspend the print in a glass container, pour in a tiny amount of the solvent, and (gently) heat it. The evaporated solvent will smooth all the ridges out of the print, leaving a shiny and smooth surface. You can, of course, hand polish it by dedicating a lint-free cloth and a pair of gloves to the task.

40 thoughts on “Smoothing PLA Printed Parts

  1. I’m no toxicologist, but speaking as a former chemist, THF is about an order of magnitude more toxic than acetone or MEK. Sure, it’s no Benzene, but I’d rather if my metabolic pathways didn’t have that as a potential input mmmmkay?

    1. THF exposure limits 200-250 ppm
      also causes cumulative kidney, liver and lung damage, known mammalian mutagen
      Acetone exposure limits 750-1000ppm
      makes you dizzy, can cause temporary deafness in acute exposure cases.

      THF is nasty stuff and requires hazmat disposal in most states. Anyone thinking about using this process should do themselves and their neighbors the favor of reading the material safety data sheet (MSDS) for THF.
      This article is borderline misinformation.

      1. From what I know, Acetone is not known to be a mutagen, unlike THF. And with mutagens, there’s no safe threshold – they will more or less introduce bugs somewhere in DNA, and each one of these bugs can potentially lead to cancer. (Potentially, even a Darwin Award!)
        To sum up: I totally agree one should stay away from this shit, unless there’s a good chemical lab. It’s just not worth it.

        1. I have never heard of the being a mutagen and this property is not listed in the MSDS for thf. I think the real danger with thf is that it is a peroxide former which will cause explosions when heated.
          Overall thf is not as bad a DCM or benzene but it has other hazards that are more dangerous.

  2. THF is an ether and like all ethers it has a nasty habit of forming peroxides which are explosive. You should be very careful and avoid evaporating THF dry as peroxides are relative stable in solution but when dry they are quite dangerous. It is very simble tu test THF for peroxides, just mix small amount of THF with potassium (or sodium) iodide if solution turns brown it shows indication of peroxides (the browner the more peroxides). This solvent should be kept in the dark (brown glass bottles, storage in dark place, avoid direct sun light – that stuff)

    1. This is sort of a reply to all of the previous comments…THANK YOU! I read this article and the link. I’ll admit that I was excited. I print in PLA and while it’s not critical for me to smooth parts, I imagined that it would be fun to have a relatively safe way to do this. Honestly, I get better information from people with some expertise than I do reading MSDS for this type of product. I understand the dangers better with the explanations. I don’t feel comfortable with this process, but I feel comfortable with the information, so thank you. Valuable information. Thank you.

  3. You should probably change “just as safe as acetone” to “just as dangerous as acetone”. Then you could write an article about how to mix ammonia and bleach and use that to smooth out your life!

  4. Why not glacial acetic acid? As I understand, it won’t break down the PLA like a hydroxide solution would. (Correct me if I’m wrong on this.)

    Sure, it’s a rough chemical to use and you’ll get acid burns if you’re not careful, but it’s not notably toxic. (Hell, if you go with the food grade stuff, you can sprinkle it on your salad after diluting it.) I guess you’d also need a fume hood, since you wouldn’t want to catch even a whiff of those fumes.

    1. Acetic acid, the stuff that gives vinegar its smell and ‘bite’. It’s also used in most single component RTV silicones, which is why they smell like vinegar. The curing process is driven by the evaporation of the acid.

      Like anything containing any sort of acid (like acid soldering flux) you do not use acetic acid RTV silicone on electronics.

      1. It’s a quick way to smooth out your fingerprints, too.

        When I was in high school, I watched a kid ‘exfoliate’ most of the skin from his hands using acetic acid.
        He asked me what acetic acid was. Not knowing he had just stolen a bottle of it from the science lab, I tell him it was the acid in vinegar. He then opens the bottle, gets some on his hands, and learns the hard way that unlike the stuff in the container, vinegar is like 97% water.

  5. Reading wikipedia (yes, I know it’s not considered a “reliable” source), it does seem to imply THF is fairly safe, especially when inhibited. And the safety warnings indicate that only intense or continued exposure would be dangerous.

    Is there a site someone can link to that explains the whens/do’s/don’ts/hows of using THF?

    1. Is a few drops in a jar, done in someone’s garage, really going to take 5 minutes off their life? Everything gives you cancer, in fact there’s a great website that lists everything the Daily Mail (a “news” paper) has claimed either cures or causes cancer. The list is enormous and many things are on both lists.

      I’d be more convinced if someone had figures for how much it increases one’s chance of cancer. Then compare it to the usual things, background radiation, a dental X-ray, two weeks in sunny Chernobyl etc.

  6. I work in an organic chemistry lab, and I absolutely hate working with THF. Its a dehydrant, you have to be careful with its storage because of peroxide formation, and if you don’t wear JUST the right kind of glove, it will swell the material (consequently, it feels like your hand is on fire when this happens). Just stick with Acetone.

  7. This is why 3D Printing is still in the stone age.

    Can you imagine if the first laser printers came out and you had to take each printed page and hand correct each and every letter so they’d be smooth?

    I appreciate all the effort the bleeding edge crowd puts into them, but for now, I’ll stick to either molds or machining.

    1. That’s kind of ironic though considering that machines parts and molded parts have their own obvious qualities to them. You can clearly see tool paths of machined parts and molded parts have seams. Injection molded parts have to be designed in specific ways and you can see the marks from the ejector pins.

      Honestly, I feel like since 3d printing is fundamentally about a relationship between a digital design and a physical part, this influences people into a line of thinking where the nature of the creation process is overly scrutinized.

      I care about the lines on a printed part about as much as I care about the seams on injection molded parts.

    2. I worked on copiers in the early 80’s and if the paper was damp it curled into the open toaster style heaters and then the pages came out flaming. Those machines required a lot of nursing, the companies that sold them did not expect users to tinker but instead employed lots of technicians to keep them running in an acceptable manner. Many of them did not have drums but instead had photo sensitive plastic sheets that could be replaced at every service which could be once a month. A bit like saying you need a new hot end and extruder once a month lol

    3. I’d say FDM 3D printers are more akin to dot matrix printers than laser printers, and smoothing the prints is akin to going over printed graphics by hand with ink to compensate for a low resolution. It’s not something you need to do to have a good piece. It’s just an aesthetic detail.

  8. Please change the summary, THF is terrible stuff. I had to work with it once, and after reading the MSDS, I *really* didn’t want to. This is not a chemical you want to use without a proper fume hood and specific protective equipment.

  9. Try PVC pipe “primer”; this is a witches brew of solvents (acetone, MEK, THF) that might soften PLA as well. It’d still have similar toxicity worries, but as a widely available product you should be able to find “reasonable” consumer-level safety info. And you can get small cans at your local hardware store for a couple bucks…

  10. Maybe it’s been mentioned already in this thread and I just missed it, but isn’t that the main active ingredient in most over the counter PVC solvent cements? I work at a hardware store, and that’s what is in two of the brands of pvc cement that we have. Could someone just pop open the can, use the brush in it to slather the stuff between the PLA parts, and glue it together like your average sprinkler system? The cans are small, inexpensive, and do a pretty good job at keeping fumes/exposure down when not in use.

  11. I took a completely different approach…. steam. Smooths it out nice with a smooth cloth right after it’s had steam contact. I use my clothes steamer with just the 1.5″ hose end… hold the parts with a small pliers or surgical clamp in the steam for a couple of seconds on the piece I want to smooth, and then pull out, smooth with cloth, and repeat till I have done all of areas of the part. Be careful as warping can happen. I wouldn’t use this method on anything that has to be extremely precise as you could warp, but for figurines, models, toys… it’s a safer method.

  12. Try Steam!!!! Seriously… it smooths the PLA out. I took my clothes steamer (Bissel) and took the attachment off so all I had was the 1.5″ hose end. Hold your PLA printed part with a pliers or surgical clamp and put over the steam for just a few seconds. Smooth with a fine cloth, repeat until the part is finished to your satisfaction. It can cause warping, so be very careful. Great for figurines, models, etc…. but be especially careful for parts needing machine tolerances.

  13. My wife just found out that a regular dishwasher can do the trick. Just put the parts as often as needed into the dishwasher (while using at with your dishes to save energy), the more often the smoother they get :)

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