How to Apply Annealing to 3D Prints

Hello Machine Bros!
As you well know, not everything ends when you finish printing your piece or model in 3D. There is a wide variety of post-processing that can be applied to 3D prints, either to give them a better aesthetic finish, to assemble a set of parts, to provide special characteristics to printed objects, or even to provide better mechanical and thermal properties to our 3D prints. This is what this article is about, a post-processing which is not so popular in 3D printing, but has been applied to metals and plastic injection for years. This post-processing is known as Annealing, and I will talk about how to apply annealing to 3D prints.

What is Annealing?

Annealing is a thermal treatment whose purpose is the reorganization of the molecular structure of the material (in our case a polymer) and/or the elimination of internal stresses.

The internal stresses in FDM 3D printing are usually generated due to the thermal stress suffered by the 3D prints due to the temperature changes that we expose to the molten filament when leaving the nozzle.

The fact that there is more or less thermal stress and internal stress when 3D printing a part is directly related to the following 3 factors:

• CLTE (Coefficient of Linear Thermal Expansion): It indicates how much a material could vary in size in proportion to changes in temperature.

• Tg (glass transition temperature): It tells us at what temperature a polymer decreases its density, hardness, stiffness and its percentage of elongation.

• Young’s Modulus: It gives us an idea of the level of elasticity that a material has.

The higher the values of any of these 3 characteristics mentioned above, the greater the chances that the piece will suffer from greater thermal stress and therefore greater internal stresses.

We have an article that explains in more detail how heat stress affects 3D printing: Tips to prevent warping and cracking

With reference to the “reorganization of the molecular structure of the material”, there are generally three forms or configurations in which the molecules in a material are organized, these are:

1. Crystal structure: In this configuration, the atoms, molecules, or ions, are in an orderly manner and with repeating patterns that extend in all three dimensions of the material.
   
   
2. Amorphous structure: In this configuration the atoms, molecules, or ions, are in disorder and misaligned even when in their solid-state. They do not have an orderly internal disposition and therefore do not have any specific pattern.
   
   
3. Semi-crystalline structure: In this configuration, the atoms, molecules, or ions are distributed in such a way that two clearly defined regions can be seen in their solid-state. One of these regions is amorphous and the other is crystalline.

The polymers we 3D print with are generally amorphous (ABS for example), some semi-crystalline (PLA for example) and very few crystalline (eg polypropylene, which is not very popular for 3D printing).

When we decide to apply annealing to a material it is in order to reduce internal stresses (in our case generated when printing by FDM).

We also seek to reorganize the molecular structure of semi-crystalline (PLA for example, a very common material in 3D printing) or crystalline polymers.

This is because when the filament is melted at the moment of passing through the extruder, the molecular structure becomes amorphous and as in the most accessible 3D printers we do not have heating chambers, the plastic cools very quickly without giving enough time to that the molecular structure reorganizes to its semicrystalline or crystalline state, thus originating a 3D printed part with an amorphous molecular structure, when in reality if we print with PLA for example, the structure of the part should be semi-crystalline.

Because of what was mentioned in the previous paragraph, we usually see the greatest benefits of applying annealing to a 3D print in PLA and Nylon, since both are semi-crystalline polymers. By applying annealing, we are able to reduce internal stresses (thermal stress) and reorganize the model’s molecular structure.

If we apply annealed to amorphous plastics (PETG, ABS, among others) we will only be able to reduce thermal stress, since the molecular structure of these materials is amorphous.

Next, we you will find 2 tables. One where you can see the predisposition of the materials that we commonly use to suffer from thermal stress, and another where we categorize 3D printing materials based on the way their molecular structure is organized.

How to Anneal a 3D Print?

It consists of gradually and progressively heating the 3D print to a temperature that is between Tg (glass transition temperature) and the melting temperature of the material (this only applies to materials with a crystalline or semi-crystalline structure, materials with an amorphous structure do not have a defined melting temperature, for this type of material a temperature where the deformation would already be excessive is used as a reference).

Once that temperature is reached, we will maintain that temperature for a certain time.

When the time that we have predefined based on the material and the annealing that we wanted to apply has ended, we let the piece cool down very slowly.

All this in order to allow the reorganization of the molecular structure of the material (in case the material is not amorphous), and we also managed to reduce the thermal stress caused by 3D printing by FDM without heating chambers (You can read about these heating chambers and the 3D printers that have them, in the article Tips to prevent warping and cracking).

• Glass transition temperature (Tg): It is a value that tells us at what temperature the polymer decreases its density, hardness, stiffness and its percentage of elongation.
  
  
• Melting temperature: It is the temperature at which the polymer enters a liquid state (this temperature cannot be clearly defined in polymers with an amorphous structure).

What is the Purpose of Annealing?

The objective of applying annealing is “the reorganization of the molecular structure of the material and/or the elimination of internal stresses”.

Applying annealing to a 3D printed part allows it to provide better mechanical and/or thermal properties to said part.

Once we apply annealing to our 3D print, it should be able to withstand greater mechanical stress and higher temperatures.

This does not occur equally in all materials; normally better benefits are obtained by applying annealing to a material with a crystalline or semi-crystalline structure than to a material with an amorphous structure.

It is necessary to take this into account because annealing has its disadvantages, that is why we should analyze the pros and cons of applying this heat treatment to a certain material.

Or even to be more specific to a certain 3D print, carefully study what function will be given to that object that we materialize with our 3D printer, to see if the benefits of annealing really outweigh the disadvantages.

We will talk about the advantages and disadvantages of annealing later in this article.

What Materials Can I Apply Annealing to?

To all, just remember that in amorphous materials you will only manage to reduce heat stress.

In crystalline or semi-crystalline materials, you will be able to reduce thermal stress and also reorganize the molecular structure.

Advantages and Disadvantages of Annealing

To reduce the possibility of suffering deformations during annealing is recommended to sink the piece in sand or salt, to subsequently enter the whole set (piece + sand) or (piece + salt) in the oven to apply the respective heat treatment.

Another important point to take into account is that it is recommended to 3D print the pieces with 100% infill, in order that the annealing is the best possible.

With the intention of anticipating the dimensional changes caused by annealing, the percentage of change of the part when applying annealing is usually analyzed first.

If for example, it changes in the X-axis by 5%, in the Y-axis by 3% and in the Z-axis by 2%, what we do is scale the piece to those dimensions so that after applying the annealing the object will have the desired dimensions.

The obvious disadvantage of this is that we would have to make 2 prints, this only if we are too interested in the piece being the exact dimensions of the design.

What do I Need to Apply Annealing?

You need an electric oven. It is not advisable to do this with gas ovens, because when we involve fire or flames in the annealing process, it is very possible that the heat is not distributed evenly in the 3D print, so they would melt or excessively deform the parts that are closer to the fire.

A thermometer that should be as close to the 3D print as possible, to have a more accurate reading of the temperature at which we are exposing the 3D print.

A stopwatch to measure the annealing time.

A heat-insulating material is usually used to place the 3D printed piece, otherwise, the piece could adhere to the surface where we place it.

It may be useful to submerge the piece in sand or salt to reduce or prevent possible deformations caused by applying this post-processing.

You should also know that they sell electric ovens that fulfill this function, that is, they are designed to program the temperature to be reached, the time they will stay at a certain temperature, and finally when to start cooling down.

We could also manufacture such an oven ourselves. In our case, we made one with a microwave casing, an electrical resistance, an Arduino and among other things.

If you are interested in knowing how to build it, please leave us a message in the comments and we could do another article on how to build this oven for annealing.

How to Anneal PLA?

A good starting point is to anneal the PLA at an approximate temperature between 65°C (149°F) and 70°C (158°F), for a time of between 40 minutes and 80 minutes, then let it cool very slowly.

The parameters “time” and “temperature” can be varied, you can experiment with them, analyze with which you get better results, we simply try to give you an idea of what values to start testing with.

How to Apply Annealing to PETG?

A good starting point is to anneal the PETG at an approximate temperature between 75°C (167°C) and 78°C (172°F), for a time of between 40 minutes and 80 minutes and then let it cool very slowly.

The parameters “time” and “temperature” can be varied, you can experiment with them, analyze with which you get better results, we simply try to give you an idea of what values to start testing with.

How to Apply Annealing to Nylon?

With Nylon it is more difficult to give them a starting point, because in the market there are different types of filaments made of this material that melt at different temperatures.

We are talking about that there are some filaments that can be melted in 3D printers at 220°C (428°F) and others at temperatures as high as 270°C (518°F).

For example, for a Taulman 230 Nylon, a good starting point is to anneal it at an approximate temperature between 70°C (158°F) and 73°C (163°F), for a time of between 50 minutes and 120 minutes.

Remember that you can vary these values, you can experiment with them to see which ones offer you the best results. With Nylon you will have to choose a temperature depending on the type of Nylon with which your filament is made.

How to Apply Annealing to ABS and ASA?

For the annealing of both materials an approximate temperature between 105°C (221°F) and 107°C (224°F) can be a good starting point, for a time of between one hour and two hours, then allow to cool very slowly.

The parameters “time” and “temperature” can be varied, you can experiment with them, analyze with which you get better results, we simply try to give you an idea of what values to start testing with.

How to Apply Annealing to the PC?

A good starting point is to anneal the PC at a temperature between approximately 123°C (253°F) and 125°C (257°F), for a time between one hour and two hours, then allow it to cool very slowly.

The parameters “time” and “temperature” can be varied, you can experiment with them, analyze with which you get better results, we simply try to give you an idea of what values to start testing with.

Example of Annealing Application to a 3D Print in PLA

To determine how feasible it is to apply annealing to the pieces 3D printed by FDM, and also to be able to show you how the process of applying this heat treatment to the prints is, we decided to print on PLA eight test specimens, four of the test specimens they are of a black PLA filament and four of the test specimens are of an orange PLA filament.

Of the eight test specimens, we will apply annealing to four of them, to two black ones and two orange ones, to later make mechanical and thermal resistance tests in order to compare the behavior of the annealed test specimens with the non-annealed test specimens.

The test specimens were printed with a 100% infill, this allows to obtain better results during annealing.

Once the test pieces that we want to anneal have been 3D printed, we place and bury them in a container with sand, then we proceed to apply the annealing, which consists of gradually heating the oven to a temperature of 65°C (149°F) and we maintain that temperature for 80 minutes.

Subsequently, the oven will cool slowly until it reaches room temperature.

Below we leave you a video explaining the process.

Find next a video about applying annealing to a PLA 3D print (activate the subtites in case you can’t see them):

Once we have the 4 annealed test specimens, we start the mechanical and thermal tests.

Thermal and Mechanical Tests to Annealed 3D Prints

1. Mechanical Strength Test on the Black Test Specimens

Find below the video of the mechanical test of an annealed PLA 3D print (Black specimens) and one without annealing:

(Remember to activate the subtitles)

The result of this test was as follows:

• The unannealed test specimen broke at 5.5 kg.
• The annealed test specimen broke at 7,2Kg.
• The difference is 1,7Kg.

2. Mechanical Test on the Orange Test Specimens

Find below the video of the mechanical test of an annealed PLA 3D print (Orange specimens) and one without annealing:

The result of this test was as follows:

• The unannealed test specimen broke at 5,6Kg.
• The annealed test specimen broke at 6,6Kg.
• The difference is 1Kg.

3. Thermal Test on the Black Test Specimens

Find below the video of the thermal test of an annealed PLA 3D print and one without annealing (Black specimens):

The result of this test was as follows:

• The unannealed specimen was deformed first.
• The nut that was on the unannealed specimen fell.
• The annealed test specimen practically underwent no heat deformation compared to the unannealed test specimen.

4. Thermal Test on the Orange Test Specimens

Find below the video of the thermal test (Activate the subtitles by clicking on the CC button):

The result of this test was as follows:

• The unannealed test specimen was deformed first.
• None of the nuts on the test specimens fell off.
• The annealed test specimen practically underwent no heat deformation compared to the unannealed test specimen.

We also want to mention that we did not find measurable dimensional changes with the vernier (caliper) in the annealed specimens, of course, this was comparing them with the specimens that were not annealed.

In the experiment, we were able to determine that improvements were achieved in terms of mechanical and thermal properties in the annealed parts.

Conclusions About Applying Annealing to 3D Prints

We can conclude that annealing can be useful in certain cases. We must take into account the disadvantages of annealing and analyze considering the function that the 3D print will have if it is worthwhile or not to apply this heat treatment.

In our experiment by annealing PLA we observed that the annealed test specimens obtained better mechanical and thermal performances. Therefore, we can affirm that the improvements after annealing are appreciable.

In the same way, the temperature and the annealing time can be varied, it is possible to experiment with different values with the intention of trying to obtain better results, but remember that at higher temperatures, the greater the possibilities that the 3D print will suffer from important dimensional changes and also of greater deformations.

When you are going to apply annealing to a 3D print, we recommend that the piece has 100% Infill.

Also, that you take into consideration that there could be dimensional changes in the piece after finishing the heat treatment. In our case, this did not happen because we use a good annealing temperature for the PLA 65°C (149°F), in which the piece did not suffer almost deformations or dimensional changes.

In addition, we placed the pieces in sand, which we also advise as this also helps prevent these deformations from occurring.

Cheers

See you soon Machine Bros!