PEEK 3D printing: how to meet challenges and print PEEK
Many industries find numerous advantages in 3D printing PEEK parts: this is due to the impressive properties of PEEK and to the recent advancements of 3D printing technology that is widening the portfolio of 3D printed materials.
But why is PEEK widely known as a difficult material to 3D print? And what are the characteristics that a printer should have to solve these issues? How do you print PEEK?
Let’s deep dive into these aspects, starting with the nature of the technology.
When using FFF technology, the filament to be printed is subjected to two phase changes: it goes from solid to liquid when the filament is brought above the melting temperature and from liquid to solid when it cools down after the extrusion.
Due to the high melting temperature (Tm) of PEEK, its extrusion temperature is very high, usually up to 450°C. For PEEK polymers the thermal gap from the extrusion temperature to room temperature is larger compared to other plastics like PA and ABS.
The greater the difference between the extrusion temperature and the chamber temperature, the higher the thermal shock undergone by PEEK. This shock freezes the molecules in a disordered position and causes the generation of retraction forces and residual stresses in the printed part.
Peek crystallization temperature and its high viscosity
As a result, if the chamber temperature is at room temperature, PEEK parts cool down too quickly and do not have enough time to fully crystallize, showing lower chemical, thermal and mechanical properties.
Moreover, PEEK is one of the most difficult thermoplastics to print, unlike PLA or Nylon (PA), mainly considering its high viscosity. It is also difficult to achieve good dimensional tolerances because of its high shrink rate.
To overcome these problems, multiple features can be implemented to ensure the 3D printing of high-performance materials with the best quality possible.
Currently there are only a few companies capable of printing PEEK with great accuracy and high crystallinity rate, and the reason for this lies in the particularities of this high-performance material.
What is the viscosity of a polymer?
When the filament is melted and extruded, it becomes a fluid, therefore its behaviour will be impacted by its viscosity.
Viscosity is the measure of resistance to flow that arises due to internal friction between fluid laminas as they slip past one another. The higher the viscosity of the polymer, the greater the difficulties that can be encountered while printing.
For instance, with a traditional extrusion system, the material flow coming out of the nozzle is limited by the friction generated on the internal surface of the channel, leading to a continuous clogging of the extruder. Furthermore, during the deposition of fused filament, air tends to remain trapped inside it, generating vacuum areas between following layers. This causes a drastic reduction of the mechanical properties of the printed part. With smaller nozzle diameters and higher printing speeds, the apparent viscosity of the polymer would increase, worsening the issues.
PEEK 3D printing: the Roboze way
Hence the need for a new extrusion system, the HVP extruder, designed by Roboze’s R&D department. Thanks to the internal ceramic channel designed to reduce friction, the extrusion process of high viscosity polymers is facilitated.
This device is designed to impart acceleration to the polymer flow. Narrowing the extrusion section leads to an increase of the shear stress acting on the pseudoplastic polymer that reduces its viscosity; therefore, the shape memory becomes more labile. Compressed air cooling is fundamental for the process: when the temperature increases in a section other than the melting chamber, it can lead to the dilatation of the polymer that clogs the passage channel that would block the pipe and consequently interrupt the printing process.
Furthermore, having a thinner straw simplifies the cooling process, since the specific area exposed to heat exchange is greater.
These features, together with a printing temperature up to 450°C, create an optimized polymer flow and allow the reduction of shrinkage phenomena, ensuring low dimensional tolerances and the preservation of the material chemical-physical properties.
Polymers shrinkage rate and warpage
Shrink rate (also called shrinkage rate), expressed in percent, is a property of polymers that determines the parts’ accuracy and dimension. It is calculated as the volume contraction rate during the processing of a polymer from the melt phase to the cooled solid state.
Due to the high thermal gap from the extrusion temperature to room temperature, the cooling rate of PEEK is high, so it cools down more quickly, affecting the material crystallization. The correlation seems easy: the higher the temperature required to print, the higher the cooling rate and the higher the shrink rate.
Commonly, depending on the material, shrink rate of polymers varies in the 0.2-1% range. Shrinkage with PEEK is up to 2%, making it one of the most difficult materials to print if you do not have the right tools to handle it.
What is warpage?
Let’s define warpage: it is one of the effects of a high shrinkage rate on a printed part. When the 3D printed parts cool after deposition, if the shrink rate is high the component can detach from the build plate. This defect is called warping.
Why does warpage happen at the edges of a printed part? Warpage occurs at the edges because the notch areas are known to be the ones with maximum stress concentration. These stresses, added to the retraction forces generated after cooling, lead to overcome the interfacial adhesion force between the build plate and the polymer, creating the detachment of the printed part from the build plate.
As the polymer grows in z, the number of strands that tend to retract increases, eventually overcoming the interfacial bond strength of the polymer with the build plate.
But warpage is not the only defect that might occur with a high shrink rate: due to the residual stresses that are generated the components might also tend to delaminate, and it eventually results in lower part accuracy.
What is a heated chamber and why do you need it?
With the aim to solve these issues, Roboze has developed a heated chamber in its Argo printers. This insulated heated chamber, capable of reaching 180°C in just two hours, ensures a controlled and repeatable process.
Inside, the air is sucked in by a collector positioned on the bottom, is heated in the interspace, and is fed back through the holes on the side walls. This process allows to reduce the time needed to heat the chamber, increasing the productivity and efficiency of the device. Through the holes on the side walls, the air flow inside the chamber is bi-directional, ensuring even heating and avoiding possible shielding phenomena that occur with a unidirectional flow.
Without a heated chamber, printed PEEK would have a low crystallinity rate and the shrinking rate would be uncontrollable, resulting in low tolerances and an accentuated anisotropy of the part. Furthermore, it would need an additional heat treatment process, called annealing, to optimize the structure of the internal grains and to release the residual stresses accumulated during printing.
On the other hand, printing in a heated chamber allows to manage the shrinkage rate, leading to the full crystallization of the polymers directly during deposition. This ensures enhanced properties of the polymers, together with high accuracy of the printed part, with no need for further heat treatments.
Annealing definition: what is it?
Annealing is a thermal post-process necessary to increase the crystallinity of a material, to remove thermal stresses and to limit dimensional changes at high temperatures.
It is performed by putting the parts in an oven, raising the temperature above the glass transition temperature (Tg), usually around 200°C, and holding them at temperature for 1 hour for each millimetre of wall thickness. The set oven temperature may vary, depending on the final use temperature of the parts to anneal. Common rule is to use a holding temperature at least 20°C greater than the maximum service temperature. The second step is cooling down the oven by 10°C per hour until the oven falls below 140°C, then finally turning off the oven and waiting until the part cools till room temperature. Slow cooling is a mandatory step to reduce internal stresses, hence requiring long post-processing times. To shorten the production times, annealing should be avoided whenever possible.
Annealing 3D printed part: complications
Annealing a 3D printed part is possible but some complications might occur during this process:
- Dimensional changes: depending on the annealing temperature, the dimensions could shrink or expand in any of the three axes (x, y, z), resulting in deformation of the final part due to the rearrangement of the grains at a microscopic level.
- Discrepancy in the crystallinity between the inside and the outside of the component: in case of thick walls, the thinner wall represents the bottleneck for the annealing process, hence leaving the thicker walls unannealed.
- Failure of the process: in achieving the desired properties when the target temperature is excessively high or the holding temperature is wrong, resulting in yellowing or part deformation.
Annealing depends on the minimum wall thickness of the component, which defines the temperatures and holding time that must be respected so as not to have a deformed part. Being the part minimum wall thickness the bottleneck, there might be dissimilarities between the surface and the core of the part, in case of thick walls. Furthermore, depending on the thickness of the part, the annealing process can take a long time, rarely less than 8 hours.
3D printer heated chamber: benefits and advantages
A process that crystallizes the part strand by strand, as the heated chamber, is preferred, because in this way the part is homogeneous as soon as the printing process is done and does not need further process. Argo systems produce PEEK parts that have a very high crystallinity (about 35%), meaning that there is no necessity for an annealing process.
In short, the advantages of the heated chamber are the following:
- High crystallinity reached on printed part, that result in improved characteristics of the material, in terms of chemical, mechanical, and thermal performance.
- Homogeneous crystallinity achieved layer by layer, resulting in the same properties throughout the printed part.
- No need to perform heat treatment processes, like annealing, resulting in lesser step in the supply chain and minimum production time, turning out in higher process control on the final part.
- No limit due to annealing on the minimum thickness to print.
- No deformation due to the heat treatment, resulting in better tolerances.
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