The Science of Semi-Crystalline Polymers
Standard 3D printed PLA is predominantly amorphous because the rapid cooling from the nozzle freezes polymer chains in disorganized states before they can organize into crystalline structures. This limits standard PLA's heat deflection temperature to around 55 °C.
Annealing (controlled heat treatment above the glass transition temperature) gives polymer chains the thermal energy needed to organize into crystalline lamellae, dramatically raising heat resistance and stiffness.
The Salt Bed Annealing Protocol
If you put an unsupported PLA part into a 100 °C oven, it softens and sags under gravity. To preserve exact geometry:
- Pack the printed part tightly inside an oven-safe container filled with fine baking flour or ultra-fine table salt. Tamp it down so salt fully supports every overhang and internal cavity.
- Place the container in a preheated convection oven at 100 °C to 110 °C for 45 to 60 minutes.
- Turn off the oven and allow the container to cool slowly to room temperature over several hours.
Predicting Dimensional Shrinkage
During recrystallization, parts shrink slightly along the X and Y axes (typically 1.0% to 1.5%) and expand along the Z axis (0.5% to 1.0%). By applying inverse scaling factors in your slicer prior to printing, annealed parts emerge dimensionally accurate and capable of surviving boiling water.
