PLA is polylactic acid, and it is the reason desktop 3D printing became approachable. It melts low, sticks to a bare plate without help, barely warps, and forgives a badly tuned profile. If you are choosing a first spool, or a spool for a part that lives on a desk, this is the correct answer and there is not much more to say about it.
The interesting parts are the three places it fails, and the one number that quietly breaks cost estimates.
The density number, and why it matters more than it should
The headline figure for PLA is 1.24 g/cm³, and across real spools the range runs roughly 1.17 to 1.26 depending on pigment load. That spread is small enough to ignore for most purposes.
What is not small enough to ignore is that many things sold as PLA are not plain PLA. Matte grades carry a mineral filler and run up to about 1.35; glow-in-the-dark grades carry a ceramic phosphor and can reach 1.40. Your slicer multiplies extruded volume by whatever density sits in the filament profile, so if the profile says 1.24 and the spool is 1.35, the real mass is 8.9% higher than the slicer reports, and every cost estimate goes wrong with it. Setting the real figure takes ten seconds and is the highest-value calibration available.
Temperatures
Nozzle between roughly 195 and 225 °C, bed anywhere from cold to 60 °C. PLA tolerates a wide window, which is a large part of why it is easy — a profile anywhere inside that range still produces a usable part, where the same error in nylon produces a failure.
Run a temperature tower anyway, and choose the lowest temperature that still gives good layer bonding. Lower is better for stringing, for overhangs and for small features, and PLA is one of the few materials where the trade is genuinely available.
Shrinkage
Between 0.2% and 0.5%, and not isotropic — the plate restrains the base while upper layers contract more freely. On a 40 mm part that is a 0.08 mm correction at the bottom of the range and 0.20 mm at the top, small enough that almost nobody compensates for it. On a 200 mm press fit the same range is 0.40 to 1.01 mm, which is enough to matter — and it is one of the reasons a long part fits differently from the short test coupon you checked it against.
What PLA is genuinely bad at
Heat. It softens between 52 and 60 °C. A parked car's interior comfortably exceeds that on a sunny day, and so does a windowsill in summer. A PLA part left in either will sag, and no PLA+ or high-speed reformulation changes this — they share the same base polymer and the same glass transition.
Sunlight. Ultraviolet attacks it. Outdoor PLA goes chalky, discolours and becomes brittle over months. If the part lives outside, PETG or ASA is the answer and PLA is not.
Flexing. PLA is stiff and it is brittle. A clip designed to spring will survive a handful of cycles and then snap. This is a material property, not a printing problem, and the fix is a different material rather than a different profile.
Practical notes
Bed adhesion is straightforward: a clean PEI plate, smooth or textured, at 55 to 60 °C, and no adhesive. If the first layer is not sticking, wash the plate with dish soap and hot water — isopropyl alcohol does not remove skin oils, and a fingerprint is the usual culprit.
Drying is optional rather than required, but a spool left open in a humid room for months will print visibly rougher and snap more easily between the spool and the extruder. Four to six hours at 45 °C restores it, unless the polymer has genuinely aged, in which case nothing will.
Where PLA sits against the alternatives
Against PETG, PLA is stiffer, prints more accurately, bridges better and has cleaner overhangs; PETG survives heat, sunlight and impact where PLA does not. For anything indoors and decorative, PLA. For anything outdoors or functional, PETG. That single decision covers most of what people are choosing between, and the full comparison covers the cases where it flips.
Against ABS, it is not close on printability — PLA needs no enclosure, produces no strong smell, and warps a fraction as much. ABS wins only on heat resistance and on being solvent-smoothable, and both of those are real reasons that come with a real cost in machine requirements.
Against PLA+, the honest answer is that the difference is smaller than the marketing implies. The toughening additive makes parts less likely to shatter when dropped. It does not raise the temperature at which they sag, and any claim that it does is worth checking against a data sheet.
Cost, in practice
PLA is one of the cheapest filaments available, typically between fifteen and twenty-eight dollars a kilogram for mainstream brands. At a density of 1.24 g/cm³, a twenty-dollar kilogram works out to two cents a gram, so a 40-gram part is around eighty cents of plastic.
Against that, a twelve-hour print on a typical machine consumes somewhere in the region of one to one and a half kilowatt-hours, which at most residential rates is a fraction of the filament cost. On short prints in PLA, filament dominates and electricity is close to noise. That balance inverts on long prints with a large heated bed, which is why the cost calculator computes both terms rather than assuming either one is negligible.
Storage
Sealed box, fresh desiccant, and clip the free end into the spool's hole before putting it away. That last habit prevents the tangle that traps a loop under an adjacent wrap and stalls a print at hour six, and it costs nothing.