PLA (polylactic acid) starts to soften at temperatures as low as 50°C to 60°C (122°F to 140°F) and fully melts between 150°C and 180°C (302°F to 356°F). But heat isn’t the only thing that can break down this popular 3D printing material. Certain chemical solvents dissolve PLA effectively, and even friction can generate enough heat to melt and fuse it. What you use depends on whether you’re trying to smooth a print, join parts together, or just understand why your PLA object warped on a hot day.
Heat: PLA’s Low Softening Point
PLA has two critical temperature thresholds. The first is its glass transition temperature, which sits between 50°C and 80°C. This is the point where PLA stops being rigid and begins to soften, losing its structural integrity. A pure PLA made entirely from one stereochemistry (L or D form) has a glass transition around 60°C, which is only slightly hotter than water from a very hot tap. This is why PLA parts left in a car on a summer day or washed in a dishwasher can warp or deform.
The second threshold is the actual melting point, which ranges from 150°C to 180°C. Most commercial PLA filaments melt around 170°C to 175°C. At this temperature, PLA becomes a flowing liquid suitable for 3D printing, injection molding, or recycling. PLA+ (sometimes called PLA Plus or Enhanced PLA) melts at a slightly higher range of 170°C to 190°C due to additives that improve toughness, but the difference is modest.
If you need to intentionally melt PLA for a project, a heat gun, soldering iron, or even a standard kitchen oven can reach these temperatures. For joining two PLA pieces, a soldering iron at low settings works well to fuse surfaces together. Just be aware that overheating PLA produces fumes. The dominant compound released is methyl methacrylate, which accounts for roughly 44% of the volatile organic compounds PLA gives off when heated. Working in a ventilated space is important whenever you’re melting PLA outside of an enclosed 3D printer.
Annealing: Using Heat Without Melting
If your goal is actually the opposite of melting, you can use controlled heat to make PLA more heat-resistant through a process called annealing. This involves baking a finished PLA print at 60°C to 80°C for one to two hours, which allows the plastic’s internal structure to crystallize and become more rigid. After annealing, PLA parts can handle higher temperatures before they start to deform.
The trade-off is shrinkage. Annealed PLA parts typically shrink by 2% to 5%, so you’ll need to scale your prints slightly larger to compensate. An ordinary kitchen oven set to its lowest temperature works, though a small toaster oven gives you more precise control for small parts.
Chemical Solvents That Dissolve PLA
Several chemical solvents will dissolve PLA, but the most effective and widely discussed are dichloromethane, ethyl acetate, and tetrahydrofuran (THF).
Dichloromethane (also called methylene chloride or DCM) is the most aggressive option. It dissolves PLA quickly and completely, making it useful for vapor smoothing or creating a PLA “slurry” to use as glue. However, it is genuinely dangerous. The EPA classifies it as a significant inhalation hazard, with workplace exposure limits set at just 2 parts per million over an eight-hour period. It’s a colorless liquid with a sweet smell, which means you can be exposed without immediately realizing it. If you use DCM at all, it should be outdoors or under a fume hood with proper respiratory protection.
Ethyl acetate is a much safer alternative. It’s the primary solvent in many nail polish removers (not the acetone-based ones) and dissolves PLA slowly enough to give you control over the smoothing process. You can brush it on, dip parts into it, or use it in a vapor bath. The fumes are less toxic than DCM, though ventilation is still a good idea.
THF is another strong PLA solvent used in industrial settings, but it’s volatile and creates peroxides over time if stored improperly, making it a poor choice for casual use.
Why Acetone Doesn’t Work on PLA
This is one of the most common misconceptions in 3D printing. Acetone is the go-to solvent for smoothing ABS prints, and many people assume it works the same way on PLA. It doesn’t. Acetone technically degrades PLA, but rather than dissolving it into a smooth finish, it turns the surface rubbery and sticky. The result usually ruins both the appearance and structural strength of the print. If you’re looking to smooth PLA, ethyl acetate or dichloromethane are the correct solvents. Acetone should be reserved for ABS.
Friction Welding for Joining PLA Parts
Mechanical friction generates enough localized heat to melt PLA at the contact point, which is the principle behind friction welding. For hobbyists, this usually means chucking a small piece of PLA filament into a rotary tool or drill and pressing it against the joint between two parts. The spinning filament melts from friction and acts as a filler, bonding the pieces together.
Industrial friction stir welding of PLA has been studied at rotational speeds between 1,250 and 2,250 RPM with slow feed rates around 20 mm per minute. You don’t need to be that precise at home. A rotary tool spinning a short length of PLA filament at moderate speed, pressed firmly into a seam, will soften and fuse the material in seconds. The bond is surprisingly strong compared to superglue, because you’re actually melting the PLA rather than just adhering surfaces together.
Everyday Situations That Soften PLA
Because PLA’s glass transition temperature is so low, several everyday scenarios can accidentally soften your prints. A closed car in direct sunlight can easily reach 60°C to 80°C inside, which is right in PLA’s softening zone. Dishwashers typically run between 50°C and 75°C. Even hot beverages at 65°C to 70°C can deform a PLA cup or container.
If you’re making functional parts that might encounter heat, consider annealing them first, or switch to a higher-temperature material like PETG (glass transition around 80°C) or ASA. Standard PLA is best suited for decorative items, prototypes, and objects that will stay at room temperature.