Annealing, when referring to metals, is when you heat up a material to make it less brittle and more pliant. In order for this physical change to occur, you have to heat the material to a sweet spot that’s cooler than its melting point but hotter than the temperature it recrystallizes at. You have to maintain this temperature for a while and then let it naturally cool down.
The question is, why would we want to do that? Well, when metals are cast or formed, tiny crystalline structures, called “grains” are formed within the metal. If the metal is cooled quickly after being cast, these grains tend to be small, which makes the metal hard but brittle. These kinds of metals will crack under stress along the junction lines between individual grains.
Metal annealing involves reheating the metal to a temperature below its melting point and allowing it to cool slowly. This causes larger crystals to form from original small ones – larger crystals make the metal softer and more ductile.
Though this definition refers mostly to metals, a similar process can also be used with plastic. The technology behind plastic annealing is not new. It has been used for decades in the production of injection-molded plastic objects. Annealing is used to reduce internal stress in molded parts and make those parts stronger and less prone to warping.
With 3D printing, plastic is melted so it can be extruded and then cooled relatively quickly to form the desired shape. Since plastic is a poor heat conductor, parts of the extruded plastic are cooled unevenly. This leads to internal stress build-up, the worst of which occur between layers. To combat these stress points, you can anneal your 3D printed object.
As with metal, you should heat the plastic to its glass transition temperature, but below its melting point. This will reorganize the internal crystalline structure and cause bigger grains to form, which will result in much stronger objects that are less prone to layer separation.