Annealing prints: how to raise the heat resistance of PLA

How annealing works

The term comes from metallurgy, where annealing means heat treatment of a material to reduce internal stress. With plastics the principle is similar. PLA cools so fast in printing that it stays mostly amorphous. When you then heat it above the glass transition, roughly above 60 °C, the chains can move and partly arrange themselves into crystals. This is called crystallinity.

The result is a part that does not soften at 60 °C but holds up to considerably more. It depends on the type of PLA and how well the crystallisation goes.

What annealing does to dimensions and strength

During crystallisation the material shrinks, usually by a few percent, and unevenly. Thin walls and flat surfaces can warp, holes get smaller and precise tolerances are lost. A part after annealing tends to be harder but also more brittle.

That is why annealing is worth it more for simple shapes, where a few tenths of a millimetre do not matter. For assemblies that fit together it is a risk. In that case it is simpler to choose a material with higher heat resistance from the start.

How annealing is done at home

The part is held in an oven or in warm air, usually at 80 to 110 °C for about half an hour to an hour, and then left to cool slowly. To keep it from deforming, some people pack it in salt or sand. The exact temperatures differ by filament maker, so look them up in the datasheet of the particular material.

It is a procedure at your own risk, and an ordinary home oven is not ideal for it, because the temperature is not accurate.

Annealing and your order

Annealing makes sense for simple, thicker parts that will be exposed to heat, such as a holder in a car or a cover near a heat source. For thin-walled, long or very precise parts the risk of warping is greater than the benefit. Infill and the number of walls help too: a thicker part anneals more evenly.

So when ordering, first decide whether the part really needs higher resistance. If it does, ask yourself whether it is simpler to choose PETG, which is not annealed. If you want advice, write to jakub.novak@boostello.com.

We do not offer annealing as a service, like any other finishing. You get the part straight from the printer. If you need a part to survive heat, for example in a car or by a kitchen counter, consider PETG right away, which withstands more than PLA without any treatment. The differences are in the comparison of PLA and PETG.

If you want to anneal the part yourself after delivery, order it a little thicker or try it on one piece first.