Introduction to Annealing:
Annealing is most commonly applied in metallurgical operations to increase the physical properties of metals. Similarly, annealing can also be used to increase the properties of many plastics. Polylactic acid (PLA) is a very common 3D printing material which can benefit significantly from high temperature annealing. PLA is strong but brittle to some extent and has a very low glass transition temperature when initially extruded as an amorphous material. It also has the ability to transition to a crystalline material under proper post processing conditions. This makes PLA a perfect candidate for the annealing process, which works by transitioning disorganized amorphous polymer chains to highly organized crystalline microstructures. Annealing PLA 3D prints provides a significant increase to the effective operating temperature and the ultimate strength of the part.
How Annealing PLA Works:
The FDM 3D printing process works by heating the feedstock plastic material to beyond its melting temperature so that extrusion can occur under positive pressure. Once the extrusion is deposited into the desired location, the material is allowed to cool to form the final solid object. Since plastics are generally very bad conductors of heat, this cooling process tends to occur unevenly throughout the part. This uneven cool down causes stresses in the printed object. These internal stresses reduce the theoretical ultimate tensile strength of the bulk material.
To understand why the part strength increases, we need to cover some basic material science. Polymers can exist in two or more distinct microstructure formations. When using FDM extrusion methods, on the microscopic level, the material structure is not properly organized and is found to be in an amorphous form when initially extruded and allowed to cool. When the plastic is again heated to above the glass transition temperature and then it is allowed to cool down slowly and evenly, its internal structure becomes more organized and transitions to a crystalline form. Once PLA is annealed, there is a significant improvement in part strength and stiffness due to this transition from amorphous to crystalline microstructures.
Experimental Methods for Annealing 3D Printed PLA Parts:
As noted above, polylactic acid (PLA) is used throughout this article particularly for its ability to form crystalline microstructures once annealed. An electric convection style oven is most typically used for annealing PLA 3D prints. Since PLA has a very low glass transition temperature in amorphous form, the initial temperature set point used is typically 170°F (76.7°C). This particular experiment was run using 3-time intervals (10, 30 and 60 minutes) at 170°F to better understand the impact of the time based variable on the formation of the crystalline microstructures.
The samples in this experiment were printed with a 0.4mm nozzle at 0.1mm layer heights, 0% infill with three shells. The purpose of this was to measure the influence of rapid annealing on the outer surface of the part. This observation is particularly useful for thin walled objects. For every material, ten samples were created which did not pass through any treatment and those act as the experimental control. This experiment specifically tested the Z-axis strength, or interlayer bonding of both standard and annealed PLA. 
Data that was collected during the experiment is shown in graph against stress strain value.
Graphs showing annealed PLA at 170F for 30 minutes vs PLA control 
Graphs showing softening temperature and dimensional change after annealing PLA 
Results from Annealing PLA:
In numerous controlled experiments, heat treatment of 3D printed PLA parts has been found to improve the ultimate strength as well as elastic modulus of the part. Annealing PLA at temperatures as low as 170°F has been shown to enhance the tensile strength as well as compressive strength when compared to control samples that did not receive heat treatment.  High temperature annealing of 3D printed PLA has also been shown to significantly enhance the heat tolerance of the parts.
In the experiment detailed above, within just 10 minutes of heat treatment at 170°F using a common convection oven, the annealed PLA parts were found to have increased tensile strength by nearly 8% while reducing strain by 12%. This results in a significantly stronger and stiffer 3D printed part when compared to the control samples.  In the second experiment focusing on the softening temperature, we can see a drastic increase in the heat tolerance of the annealed PLA samples. After undergoing high temperature annealing at 150°C, softening temperatures of the annealed PLA parts can be seen to increase to 160°C-180°C. 
Conclusions on Annealed PLA:
When polylactic acid is annealed under properly controlled conditions, it provides a significantly stiffer and stronger part with increased heat deflection temperatures. Strength is found to increase by up to 40% and stiffness can be shown to increase up to 25%. Furthermore, heat deflection temperatures can be shown to increase by up to 300%. Annealed PLA parts have demonstrated heat deflection temperatures of up to 180°C.  Some shrinkage can occur from the annealing process, but this depends significantly upon the geometry of piece as well as the specific material formulation and the annealing methods used.  Collectively, annealing PLA 3D printed parts significantly enhances the structural geometry of the piece. This enhanced strength along with a higher glass transition temperature, allows for a much broader set of practical applications.
Comparing Annealed PLA Parts to Other 3D printing Materials
Empirical test data from 3D printed sample parts in various materials (100% infill) 
All data points were measured using ASTM D638 testing standards. Max stress refers to the maximum stress the specimen undergoes before breaking when slowly pulling on it. The yield stress refers to the maximum stress the specimen undergoes before being deformed irreversibly. The Young Modulus refers to the pressure needed to compress or stretch the specimen. The glass transition temperature is the temperature at which the material starts softening.
Max and Yield Stress
Max stress and yield stress values for high strength 3D printed materials 
Parts made from annealed PLA perform better than polycarbonate (PC) and SLS nylon PA2200 in XY axis strength. The only 3D printing material that boasts a higher max stress is Ultem. However, Annealed PLA provides the highest yield stress in the XY axis. This makes annealed PLA the best 3D printing material for applications that require extremely high strength and rigidity. In terms of Z-axis strength, Annealed PLA out-performs all of the other 3D printing material options available. This graph shows that annealed PLA is much more isometric than the other FDM material options as well. This is particularly true if we again compare annealed PLA to the Ultem test results. Annealed PLA loses roughly 12% of its strength in the Z axis while the strength of Ultem is reduced by 54%, which is due to discontinuities and internal stresses built up in the microstructure between each layer.
In other words, annealed PLA behaves more similarly to SLS or SLA parts in terms of isometry. This isometry provides additional freedom to the engineer or designer. They no longer need to consider build and load orientations while designing.
Graph of Young Modulus for high strength 3D printed materials 
Another area where we see 3D printed annealed PLA outperform is in looking at the Young Modulus. Put simply, this value provides insight into how rigid a material is. Standard 3D printed PLA already boasts a very high Young Modulus even without annealing. The crystallization that occurs during the annealing process increases this value even further. This provides annealed PLA with extreme rigidity and a Young Modulus value that exceeds Ultem, the next most rigid material, by 57%.
Graph of heat resistance (Tg) for high strength 3D printed materials 
Annealed PLA provides some of the best heat resistance characteristics available for 3D printing materials. Only Ultem provides a higher heat resistance by ~10°C. As shown earlier in this article, some of the annealed PLA specimens were actually able to achieve heat resistance temperatures equal to Ultem at 180°C.
High Temperature Annealing for PLA is Now Available as a Material Option at SD3D. Contact us to discuss your next 3D printing project to see if annealed PLA will be the right fit.
 “UROP- Annealed 3D printed parts- Final Write up”, Michael Rosplock.
 “Influence of Annealing and Printing Directions on Mechanical Properties of PLA Shape Memory Polymer Produced by Fused Deposition Modeling,” no. June, 2017.
 “What’s the temperature resistance of annealed PLA, PETG and ABS?”, CNC Kitchen
 “Material Forecast”, 3D Printing Material Testing Database, Optimatter