Does PLA shrink in heat? The temperature where parts start to move

Yes. Printed PLA (polylactic acid) shrinks once it passes its glass transition temperature, 55 to 61 °C on the data sheets checked here. It gets shorter along its print lines and taller in Z, often sagging or warping, and it stays that way after it cools. Below that temperature PLA only expands slightly and returns to size.

At what temperature does PLA start to shrink?

PLA starts to shrink at its glass transition temperature (Tg), which makers list between 55 and 61 °C. Bambu Lab gives 60 °C for PLA Basic, with heat deflection at 54 to 57 °C. Above that point the stretch frozen into each printed line relaxes, so the part shortens along its lines and thickens in Z.

Material	Glass transition (DSC)	Heat deflection, 0.45 / 1.8 MPa	Vicat softening	Source
PLA resin (Ingeo 3052D)	55 to 60 °C	55 °C (one value)	not listed	NatureWorks
PLA Basic	60 °C	57 / 54 °C	57 °C	Bambu Lab
PolyLite PLA	61 °C	60 / 58 °C	63 °C	Polymaker
PETG Basic	69 °C	71 / 68 °C	69 °C	Bambu Lab
PolyLite PETG	81 °C	78 / 75 °C	84 °C	Polymaker
ABS	not listed	87 / 84 °C	94 °C	Bambu Lab
PolyLite ABS	101 °C	100 / 98 °C	104 °C	Polymaker

Heat deflection is the temperature at which a loaded test bar bends by a set amount, so it is the number to use for a bracket or hook that carries weight. Tg, measured by differential scanning calorimetry (DSC), is the number for an unloaded part.

Why printed PLA shrinks instead of expanding

The nozzle stretches each line slightly as it lays it down, and the line cools before the stretch can relax. Zhang et al. (Scientific Reports, 2015) heated printed PLA strips to 90 °C and measured contraction of up to 22.7% along their length. Faster printing stored more strain: their fit gives about 16% at 30 mm/s and 20% at 120 mm/s.

A solid part moves less, but in the same directions. CNC Kitchen held PLA parts at 100 °C for 45 minutes. A 50 mm part came out near 45 mm in X and Y and near 55 mm in Z. Prusa’s annealing tests on PLA, PETG (polyethylene terephthalate glycol), ABS (acrylonitrile butadiene styrene) and ASA (acrylonitrile styrene acrylate) found every sample shrank most along its longest dimension and grew in Z.

Time matters as well as temperature. Bambu Lab suggests annealing PLA Basic at 50 to 60 °C for 6 to 12 hours, and warns that some prints deform and warp in the process. A part left at 50 °C for hours gets a mild version of the same treatment.

Below the glass transition

Under about 50 °C a PLA part expands as it warms and contracts by the same amount as it cools, with no lasting change. The same holds in the cold. The PLA sheets checked here list no expansion coefficient. For scale, INEOS lists 80 to 110 × 10⁻⁶ per °C for its Terluran GP-35 ABS. At that rate a 100 mm part warmed from 20 to 60 °C grows 0.32 to 0.44 mm and shrinks back.

Does PLA shrink in the sun or in a hot car?

On a car dashboard in summer sun, yes. Vanos et al. (Temperature, 2018) parked six cars in Tempe, Arizona, on summer days when the air reached 33 to 40 °C. After an hour in the sun the dashboards averaged 68.9 °C, about 9 °C above Bambu Lab’s PLA Tg.

Surface after about 60 minutes	Car in sun	Car in shade
Dashboard	68.9 °C	47.8 °C
Steering wheel	53.3 °C	41.7 °C
Seat	50.6 °C	41.1 °C
Cabin air (steady state)	47.6 °C	39.5 °C

• Dashboard in sun: above PLA’s Tg. Expect a phone mount or clip left there to shrink and warp.
• Seat or steering wheel in sun: 50 to 53 °C, inside Bambu Lab’s PLA annealing range. A part left for hours can still move.
• Car in shade: 47.8 °C at most, under every PLA figure in the table.

Outside a car, no published measurement of a printed part’s temperature in direct sun turned up. Check it with an infrared or probe thermometer on the part itself during the hottest hour of the day.

Does PETG shrink in the sun?

PETG survives most sun but is marginal on a summer dashboard. Its glass transition is 69 °C for Bambu Lab PETG Basic and 81 °C for Polymaker PolyLite PETG. The Arizona study measured dashboards at 68.9 °C after an hour of sun. That puts PETG Basic at its limit, while PolyLite PETG keeps about 12 °C of margin.

Above its Tg, PETG shrinks the same way PLA does: Prusa’s PETG annealing samples shrank most along their length and grew in Z. Bambu Lab advises against annealing PETG Basic at all and warns that complex parts “may deform obviously” at 60 to 70 °C.

With 12 °C between two brands, one PETG figure does not fit every spool. Read the Tg on the data sheet for the filament you printed.

Does ABS shrink when heated?

Yes, but only well above anything the Arizona cars reached. Polymaker lists PolyLite ABS at a Tg of 101 °C, and Bambu Lab puts its ABS Vicat softening point at 94 °C. Both sit more than 25 °C above the sunny dashboard average.

Above that range ABS behaves like PLA. In Prusa’s annealing tests ABS and ASA warped badly enough that Prusa rated them unsuitable for annealing. Under about 94 °C, an ABS part only grows and shrinks back by the small reversible amount worked out above.

How to check whether a part will hold its size

1. Measure the real temperature at the spot where the part will sit, at the hottest time of day.
2. Compare it with your spool’s data sheet, not a generic figure. Use Tg for a part that carries no load and the heat deflection value at 1.8 MPa for one that does.
3. Test one part. Print a 100 mm bar, measure it, leave it in place through a hot day, and measure again. Enter the before and after lengths in measure mode of the shrinkage compensation calculator to get the change as a percentage.

Scaling the model will not fix heat shrinkage. The change is uneven: X and Y shrink while Z grows, and the part warps. If the test bar moves, print the part in a material whose Tg clears the measured temperature.