"Mmm, who loves the smell of freshly baked prints?" *
*(Nobody loves the smell of heated plastic, always print and anneal in well ventilated areas.)
We’ve all been there. You take the time to design an object so that it fulfills a specific function. You carefully select slicer settings that will optimize that function. You choose a print material that compliments the object’s purpose. The printing job itself goes well and the object looks how it was designed to look. However, despite your planning and care, when you put the object to use it fails due to an inherent lack of strength and stiffness.
Depending on the object that you’re creating and it’s end purpose, sufficient strength and stiffness can sometimes be difficult to achieve. Even in applications where the object won’t be subjected to high levels of stress, extra strength can always come in handy. You always wonder, just how strong are 3D printed objects?
So, how to strengthen 3D printed parts?
One of the best ways to increase the strength and stiffness of the your printed objects is by annealing them. In this article were going to look at the process of annealing in general, examining what it is and how it works to increase strength and ductility. Then we’ll look at the specifics involved in annealing some of the most popular printing materials, so that you can begin annealing your 3d prints for strength.
Through a relatively simple process, you can actually turn a standard material like PLA into one of the strongest 3d printer filament.
What is Annealing?
Annealing is an ancient process, originally used in metallurgy to increase the strength of metal objects. Annealing one of several “heat treatments” that are used to change the physical properties of metal without changing the metal’s existing shape. In essence, annealing increases the desirable characteristics of a given metal.
The fundamentals of the annealing process have been adapted by for use with plastics to also increase their strength after an object has been formed. Primarily an industrial plastics technique used as a finishing process, annealing can also be used by anyone with access to a kitchen oven to harden 3d prints.
How Does Annealing Work?
In metallurgy, the process of initially molding and forming a metal object causes crystalline structures, called “grains”, to form within the metal. These grains tend to be large. As a result, the metal is brittle and will crack under stress along the juncture lines between individual grains.
Annealing 3D printed parts involves reheating the metal to a temperature below its melting point and then allowing it to slowly cool. This reheating causes smaller crystals to form within the larger original grains. This tighter crystalline matrix makes the metal stronger and increases ductility. The metal’s shape hasn’t been changed, but it’s desirable characteristics have been increased.
With plastics, the process is essentially the same with some slight differences. FDM printing necessarily involves heating the print material so that it can be extruded. Once extruded, the material then cools to form the printed object. Plastic is a fairly poor conductor of heat. This means that heated plastic tends to cool unevenly. This uneven cooling introduces stress into a printed object. Let’s take a closer look at the nature of this stress.
As you may know, most thermoplastics used in FDM printing are polymers. A polymer consists of two or more substances. Each substance is made up of long molecular chains. These chains are interwoven around each other, creating the polymer.
At a microscopic level, the structure of the plastic is typically unorganized and rather amorphous. Heating the plastic, extruding and cooling it reorganizes this structure into a more organized crystalline form. These crystals tend to be large, broadly similar to those that exist in metal after initial heating and cooling.
The large crystalline-like structure of the plastic makes it prone to failure along the lines between each crystal. Also, uneven cooling due to poor heat conduction results in the polymer shrinking in different ways. This, in turn, causes different tensile forces and compression forces building up in the polymer structure.
Annealing plastic involves gently reheating the substance to a point above its glass transition temperature but below its melting temperature and then slowly allowing it to cool. Like the annealing of metal, this reheating and cooling increases the amount of crystalline structures in the plastic.
Also, when the polymer approaches or reaches its glass transition temperature, the molecular chains have enough energy to enter into an amorphous state. In this state, they are able to rotate, move, stretch, etc. This releases some of the tensile and compression forces that resulted from uneven cooling. Both of these things, in turn, makes the plastic stronger, stiffer and more resistant to the stresses that cause failure.
Now that you know what annealing is and why it works to make the strongest 3d printing material, let’s take a look at the specific techniques that you can use to anneal objects printed from some commonly used thermoplastics.
How to Anneal PLA
PLA tends to be strong, but somewhat brittle. It also has a relatively low melting temperature. Both of these facts make it an excellent candidate for annealing. It’s low glass transition temperature makes it easier to anneal. In addition, annealing reduces the brittle tendencies of PLA by increasing ductility.
In effect, annealing is the best way to make PLA stronger.
The glass transition temperature of PLA is 60C to 65C (140F to 150F). It’s melting temperature is 173C to 178 C (343F to 352F). Therefore, you want to set your oven temperature to about 110C (230F) when annealing regular PLA.
It’s worth noting that our PLA is a lower temperature grade, so annealing would be best done around 70C. Generally if your PLA requires a higher printing temperature, it's of a lower quality. Different ovens have different accuracy in thermostats, so it’s always worth testing with a scrap print first.
If you're looking to make the strongest prints, you're best off starting with the strongest PLA filament. Our standard PLA has been consistently recognized as the strongest PLA available. You can even order a free sample.
This temperature is high enough to allow the PLA to soften and become amorphous, releasing some of the stress caused by printing and increasing its crystalline structure. It is also low enough that the PLA will not melt and flow, losing its shape.
Let the oven come to temperature and then let it sit for about an hour. This waiting period will help insure that the oven temperature is as uniform as possible, preventing hot and cold spots that can negatively affect the annealing process. Use an accurate thermometer to confirm that the oven is at the correct temperature before putting your PLA object in the oven.
It should be noted here that when annealing PLA plastic, or any other filament, ovens with convective heating (fan oven) will produce superior annealing results for the same reason that it produces superior bread. Instead of the heat radiating from one direction, a convective oven circulates heated air around an object, providing a 360° heat source which produces more uniform results.
Note: It’s important you never try annealing prints in a Gas fired oven. The reading may be X degrees, but the flames will be much hotter – potentially melting or setting fire to your prints. Only anneal in an electric oven, and always ensure the heating elements are turned off before placing your prints in. Again, heating elements can get much hotter than the oven reading.
Once the oven is at temperature, place your PLA object or objects on an oven safe surface and put them in the oven until it has cooled each time. Repeat the process a few times, heating up the oven (without your print inside) and turning it off when you place the annealed part back inside. Do this until total annealing time for your print is around an hour. Or longer for larger prints.
This will give the objects enough time to absorb enough heat energy to allow the polymer chains to move, stretch and realign and to recrystallize, forming a sturdier internal matrix. Do not open the oven door during this time, as any loss of heat will result in inferior annealing results.
As the oven gradually loses heat, so will the object or objects. A gradual cooling process will avoid a reoccurrence of the internal stresses that occurred during the more abrupt cooling that happened after printing. It will also reduce the incidence of warping which can still occur at annealing temperatures.
Once the oven is back at room temperature, remove the objects. What you will notice is that the object or objects will have shrunk slightly along the line of its print layers. In addition, you will also notice some expansion perpendicular to the print line. In other words, the dimensions along the X and Y coordinates will have gotten smaller, while the Z dimension will have grown.
These changes are due to the changes in tension, or rather the changes caused by the release of the internal tensile and compressive forces discussed above. On average, you’ll see shrinkage of about 5% and growth of about 2% along the relevant axes. If this shrinkage and growth is going to be a problem, you can compensate for it beforehand during the design and printing processes.
After annealing, you can expect to see some significant improvements in the strength of your PLA object. A 40% increase in strength and durability is not uncommon. Likewise, you can also expect to see good improvement in stiffness. A 25% improvement here is not unexpected. Finally, the stability of the annealed PLA at higher temperatures will also be improved. So, if you need stiff, high tensile PLA parts with good heat resistance, annealing can be the answer.
How to Anneal ABS
ABS tends to be stronger and less brittle than PLA. It also has a higher melting temperature than PLA. Despite these facts, ABS is also an excellent candidate of annealing. While ABS will need to be annealed at a higher temperature than PLA, due to its higher glass transition temperature, the annealing process will provide ABS the same increases in desirable material properties that were seen with PLA.
The glass transition temperature of ABS is 105C (221F). It’s melting temperature is 210C to 240C (410F to 464F). Because of this, you want to set your oven temperature to around 100C (210F). Again, this temperature is high enough to allow the material to soften which will release the stress caused by extrusion while increasing crystallization. If you want to try higher ABS annealing temperatures you can do, but you may find warp and deformation increase. As was the case with PLA, it is also low enough so that the material will not melt, flow or significantly deform.
As always, let the oven come to temperature and then let it sit for about an hour in order to insure temperature uniformity. Use an accurate thermometer to confirm that the oven is at the correct temperature before putting your ABS object in the oven. As with any annealing project a convection oven is always preferable.
When the oven is at temperature, place your ABS object in the oven on an oven safe surface for 30 minutes , plus an addition 15 minutes for every 3mm (1/8 inch) of object cross section. Again, this will give the object enough time to absorb enough heat energy to allow the polymer chains to move, stretch and realign and to recrystallize, forming a sturdier internal matrix. As always, do not open the oven door during the annealing process.
Once the time is up, turn the oven off, leaving your object in the oven. This gradual cooling process will reduce warping and the reoccurrence of internal stress caused by abrupt cooling.
Once the oven is at room temperature, remove your objects. Once again, you may notice shrinkage and growth along the X, Y and Z axes. As was the case with PLA, this shrinkage can be compensated for in the design and printing processes. Also expect to see an improvement in the strength, stiffness and stability of the annealed object, along with improvements in temperature stability.
How to Anneal Nylon 12
Nylon 12 is stronger and less brittle than either PLA or ABS. It’s melting temperature is lower than ABS and is comparable with PLA. Like PLA, it has a low glass transition temperature which makes it easier to anneal. In addition, annealing can significantly increase its heat deflection temperature. When you combine this with its high strength, annealed Nylon 12 makes a great choice for applications where heat and durability are issues.
The glass transition temperature of Nylon 12 is a surprising 41C (105F). However, it’s melting temperature is a respectable 178C – 181C (352F to 358F). Because of this, you want to set your oven temperature to 130C – 140C (266F to 284F). Again, this temperature is high enough to allow the material to soften which will release the stress caused by extrusion while increasing crystallization.
As is always the case when annealing plastics, it is also low enough so that the material will not melt, flow or significantly deform.
Again, once the oven is at temperature let it sit for an hour for the sake of temperature uniformity. Use an accurate thermometer to determine temperature prior to starting the annealing process and use a convection oven if possible.
Place the nylon object on an oven safe surface and put it in the oven for two hours. This time is necessary to allow polymer chain realignment and recrystallization. Don’t open the oven while annealing is underway.
When the two hours are up, turn the oven off, leave the object inside and let the oven cool incrementally to room temperature. This is to reduce warping and to maximize the benefits gained through annealing.
When the oven is at room temperature, remove your objects. Once again, you may notice shrinkage and growth along the X, Y and Z axes. As was the case with PLA, this shrinkage can be compensated for in the design and printing processes.
While you will see a slight improvement in strength, the real improvement will be in temperature stability. Annealing generally increases a nylon object’s heat deflection temperature by over 40%.
If you found this article useful, you may want to download our free 48 page Ultimate Guide To 3D Printing here. Please let us know what your thought, or any questions in the comments below.