How to dissolve PLA (polylactic acid)?

PLA is biodegradable in the sense that it takes less time to biodegrade than other plastics. It normally takes around 80 years to biodegrade if I remember correctly, compared to a few hundred years for most other plastics. Acetone is able to partially melt PLA, but it won't dissolve; instead it will make the PLA sticky and soft, so I would normally use acetone for smoothing print layers.

I realise this post is over 2 years old but since nobody had mentioned this I'm gonna add it here for any future readers.

DCM (DichloroMethane) is a solvent for PLA and PLA+. I realise not everyone has access to it, but for those who do, it is great for solvent-welding of PLA parts. it dissolves PLA almost instantly and produces a seamless and clean weld once it evaporates away. (works on acrylic and Polycarbonate as well)

Just a safety note, DCM is fairly nasty stuff, you don't want it on your skin, it dissolves and damages many paints and coating as well as epoxies and plastics. the fumes are also fairly toxic. do your homework before getting or using it.

DCM is not really suitable for removing support materials. it's hard to get, it is fairly expensive and it's a toxic, also the waste is expensive to get rid of properly.

As for where to get DCM, it is the main ingredient in some brands of paint stripper. there are instructions online as to how to distill it for use.

Cheers

Ethyl acetate disolves PLA - which is typically the ingredient in non-acetone nail polish removers.

https://en.wikipedia.org/wiki/Ethyl_acetate

According to Wikipedia:

"If the filament is soaked in a small amount of ethyl acetate, it will dissolve and can be used to clean 3D printing extruder heads or remove PLA supports. The boiling point of ethyl acetate is low enough to also smooth PLA in a vapor chamber, similar to ABS and acetone."

When I started 3D printing I did some trials to dissolve PLA. I tried acetone, caustic and some other solvent. The only one that worked very well at room temperature was methylamine in water.

Unfortunately, it is not a product to use at home. Caustic is corrosive, but the vapours of methylamine can irritate the eyes and the respiratory system. It should be used in a fume cupboard.

It is very effective because instead of a saponification, the amine reacts with the ester forming an amide, which is much more efficient.

Simple Solution to Dissolve PLA is THF (Tetrahydrofuran) whose preferred IUPAC name was changed in 2013 to oxolane. It is used in the Pool industry, as well as in 3D Printing, and vinyl printing (like Vehicle wrapping)

See https://en.wikipedia.org/wiki/Tetrahydrofuran

alphonse is right: What you get with NaOH (or KOH) is not dissolved PLA, but a solution of saponified lactic acid. The good news is you can just pour that down the drain as long as we don't speak of many liters per day and your sewage gets treatment and doesn't just flow into the next river.

You can calculate the amount of NaOH needed from the mass of the PLA printed. I'll leave that little stochiometric exercise to you, just use a reasonably small excess of NaOH.

With any organic solvent (if it works), you will soon sit on liters of waste for which you have no legal and affordable way of disposal.

It is true that a mist of aqueous sodium hydroxide can be hazardous to eyes or lungs (or other mucous membranes, even skin when present in large amounts for extended periods of time.) (It is not true that NaOH vapor poses any significant risk at or near room temperature, including at 65°C.) My first suggestion would be to stop the use of the ultrasonics, and try gentle stirring instead. If you must use an ultrasonic bath, then it should be sealed to prevent leakage of the mist into unprotected areas. There are many other chemicals which can provide a high pH.

I doubt your explanation that it is simply the pH which "dissolves" the PLA is correct. Na⁺ will react with PLA to form the salt in a de-esterfication reaction. It is possible that KOH may be more effective, either on a mass or a cost basis - you'd have to try it. The most obvious high pH chemicals are NaOH, KOH, and Ammonia (NH₄OH/NH₃(aq)), but you could also try lye, dry lime, CaO and Ca(OH)₂. You could try mixed salts such as Sodium bicarbonate or sodium biphosphate to increase the metal ion concentration. There are also known catalysts for the desterification reaction, one well known one is iron (but many catalysts may discolor the plastic).

De-esterification also occurs in acidic conditions. You should consider the use of sulfuric acid or acetic acid (perhaps even hydrochloric acid) as well as the bases I've mentioned. In either case (acid or base), any residual contamination (due to poor rinsing or even absorption by the remaining part) can cause various structural and aesthetic problems over time. Be warned.

To answer your question in general (I do not have any specific experience with PLA), NaOH will be quite effective, it is unlikely that other possible bases or even KOH will improve your situation. CaO has obvious advantages in terms of both waste disposal and possibly for effects of aging, but it is also less soluble than K/Na hydroxides, so might be more difficult to rinse away, and it may not result in equally effective decomposition. (And note that your principle waste problem is the PLA waste.)