How to improve strength through post print annealing
In the 3D printing world the annealing cycle, is a post-production process that allows different plastic materials to refine the mechanical and thermal properties of the filament. The annealing method is used to eliminate or release the internal tensions of the product and to avoid its breakage in post production. Annealing is fundamental and very important when we talk about semi crystalline materials because it is known that they change physical properties when heated to high temperatures. The annealing process increases the crystalline level and brings the artefact back into balance, reducing and even eliminating internal tensions.
How it works
The annealing cycle on amorphous materials, as we have said, has the sole function of stretching and not of crystallizing. It is still important, but it does not affect in a decisive way as it happens in semi crystalline materials.
Each semi-crystalline material has the property of crystallizing under certain conditions and temperatures as a function of the chemical structure and the molecular chain.
The process is directly proportional: the higher the percentage of crystallinity, the greater the increase in mechanical performance.
They increase strength, elasticity and resistance to pressure, also improve dimensional stability. This is very important for those components that work at high temperatures and need to keep the volume constant.
Crystallization is controlled by the cooling cycle.
To reach a good level of crystallization, the annealing process temperature must be kept within a window between the glass transition temperature (Tg) and the melting temperature (Tm). This promotes the correct level of mobility of the molecules to allow crystals to form.
EXAMPLE: PPS AND ANNEALING TIMES
The melting point (Tm) of PPS is 280° C while the Tg is about 130° C. Therefore, the guideline for setting the annealing cycle temperature to ensure proper crystallization is a minimum of 135° C up to a maximum of 150° C.
3D printing offers a natural advantage for increasing the level of crystallinity: printing time. The duration of the print takes a long time and allows the formation of a high number of crystals inside the product. This implies that the material remains inside the printing machine for a long time and if it is equipped with a heated chamber, with suitable temperatures, it favors the annealing cycle.
Otherwise it is possible to print the product and proceed with the annealing cycle through the oven at a later time. In this case the same rule applies: keep the temperature between the values of Tg and Tm, considering the time to reach the temperature, sometimes even 30 minutes; as well as the cooling time, at least 30 minutes from the end of the annealing cycle.
GOOD TO KNOW: POST-TREATMENT DEFORMATIONS
The dimensions after an annealing treatment can vary, it is good to make a sample print and intervene accordingly.
The annealing temperature depends on the application. If our product will be printed at 200 ° C then the annealing must be carried out at at least 205° C, in this way we will avoid deformations. It should be borne in mind that if the annealing takes place at a lower temperature than the printing one, the annealing cycle will lead to a reduction in size.
How to limit deformation
The 3D printed component with FFF technology may deform during the annealing cycle. To avoid unpleasant surprises, we advise you to place the application in a metal container and cover it entirely with sand. The pressure that the sand exerts on the product protects it from any deformation. But be careful because, as is known, sand is a thermal insulator. For correct annealing, double the annealing cycle time.
THE ANNEALING CYCLE
How to improve resistance through post-press annealing
For regular annealing, the heat distribution must be uniform. As we have said, it is necessary to be very careful and keep under control the level of the annealing temperature. For semi crystalline polymers the temperature, if too high, can be even harmful.
An example: Nylon
The midpoint between Tg and Tm for nylon is 160° C. At this temperature, the risk of nylon oxidation is considerable and also rapid. This can lead to changes in the color of the material and cause a permanent loss of mechanical properties such as ductility. It therefore loses processing capacity.
For materials such as nylon it is best to anneal in inert environments, under vacuum or immersed in a fluid. This is because nylon needs a protective barrier that does not alter the natural properties of the material. Nylon works best and prevents oxidation when annealed in hot mineral oil.
Annealing offers indisputable advantages. The most modern FFF 3D printers offer the possibility of correctly processing many semi crystalline polymers, alternatively the process can be carried out thanks to a forced air circulation oven, ventilated.
There is no precise rule to determine the annealing time in the oven, generally 30 minutes are taken per millimeter of thickness, considering the maximum thickness. All semi crystalline materials can use the annealing process, always respecting the correct settings, from PLA to PEEK.