2 Simple Methods to Anneal 3D Printed PLA

As-printed PLA is mostly amorphous. The extruder lays down molten polymer, it cools fast, and the molecular chains freeze wherever they happened to land. That frozen disorder is why a printed bracket can start to sag at 55–60 °C, well below the 170–180 °C melting range you see on a filament datasheet. The material is not melting; it is passing its glass transition temperature, Tg, which for standard PLA sits near 60 °C.

Annealing holds the part above Tg for a set time, then cools it slowly. The chains get enough mobility to slide into more ordered regions, so the crystalline fraction rises. A higher crystalline fraction means a higher heat deflection temperature, typically 100–120 °C after a good anneal. It also means the part becomes stiffer and more brittle, and it will shrink 1–3% as the chains pack tighter.

That shrinkage is the trade you are making. Every dimension on the part moves, and the move is not uniform in all directions. XY dimensions usually shrink more than Z, because the layer bonds and the raster orientation give the Z axis a different constraint path. Plan for it in the model, or accept that holes and press fits will change size.

A convection oven with a stable setpoint is enough. Ramp from room temperature to 60–70 °C, hold for 30 minutes so the whole part reaches temperature, then step up to 90–110 °C and hold for 1–4 hours depending on wall thickness. A 3 mm wall needs roughly 1 hour; a 10 mm solid block needs closer to 4. Cool inside the oven with the door closed, at no more than 1 °C per minute down to 40 °C.

The ramp and the cooldown matter more than the peak temperature. Skipping the slow cool re-quenches the part and puts most of the crystallinity back where it started. Pulling a hot part out onto a bench also makes it warp as the outside skins cool faster than the core.

Support the part during the whole cycle. PLA is soft above 60 °C, so a flat panel will sag under its own weight if it is only held at the edges. Fine sand, ceramic powder, or a machined fixture all work as a support. Do not use a metal tray in direct contact with the part if you can avoid it; the tray heats faster than the plastic and leaves marks.

Oven choice is a safety question as much as a quality one. A dedicated lab oven with a PID controller holds ±2 °C, which is good enough. A kitchen oven with a 25 °C swing will overshoot and slump thin sections. Measure the actual air temperature with a thermocouple, not the dial.

Packing the part in a bed of fine salt or sand does two jobs at once. The media supports the walls against gravity, and it slows heat transfer so the part sees a gentler ramp than it would in open air. This is the method to use for tall, thin, or hollow parts where oven annealing alone would collapse them.

Use dry, fine-grain salt or clean kiln sand. Fill the container in layers and tap it down so there are no voids, and bury the part at least 20 mm below the surface on all sides. The part itself goes in at room temperature, not preheated. Bring the whole bed up to 100–110 °C and hold for 1–3 hours, then let the container cool on the bench overnight.

Salt wins on support, but it has a cost. Salt is mildly abrasive and slightly hygroscopic, so rinse the part in warm water and dry it after the cycle. Sand leaves grit in small holes and threads. Neither media gives you a clean surface on fine detail, so mask or plug any feature you care about.

Media packing also limits how fast you can run the cycle. A 5 L container of salt takes 40–60 minutes just to reach setpoint, and longer to cool. That is fine for a batch of small parts packed together. For a single thin part, the oven method with a fixture is usually faster and easier to control.

Skip it when the part is a fit check, a jig, or anything that has to mate with a machined counterface. A 2% shrink on a 50 mm bolt pattern moves each hole 0.5 mm, which is past most clearance allowances. Re-printing is cheaper than reworking a warped bracket.

Skip it when the geometry is mostly thin ribs, unsupported overhangs, or long slender pins. Those features are the first to sag and the last to hold tolerance, and no amount of salt packing fixes a 1 mm pin that bows.

Skip it when the service temperature is below 50 °C. A desk organizer or a display model gains nothing from the extra stiffness and loses surface finish to the media bed.

Use it when the part sees heat in service: an under-hood clip, a fixture near a heated bed, a duct that carries warm air. Those are the cases where the 30–50 °C gain in deflection temperature pays for the shrinkage and the cycle time.

If the part needs both heat resistance and tight tolerance, the honest answer is often a different process. Machined PEEK, POM, or a filled nylon will hold ±0.05 mm and take 150 °C without a shrink allowance. At GreatLight we run both routes, so we can tell you which one is cheaper for your geometry rather than pushing one over the other.