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PEEK 3D Printing

What does a 3D printer need to print PEEK? How to meet challenges and print PEEK?

Numerous industries are discovering the manifold advantages of 3D printing PEEK parts, owing to the remarkable properties of PEEK and the recent strides in 3D printing technology, which have expanded the array of printable materials.

However, why is PEEK often regarded as a challenging material for 3D printing? And what characteristics should a printer possess to mitigate these challenges? Let's delve into these questions, commencing with an exploration of the technology itself.

When employing FFF (Fused Filament Fabrication) technology, the filament undergoes two crucial phase changes: it transitions from solid to liquid when heated above its melting temperature and reverts to a solid state upon cooling post-extrusion. The high melting temperature (Tm) of PEEK necessitates extrusion temperatures reaching up to 450°C. Unlike plastics such as PA and ABS, PEEK polymers experience a significant thermal gap between extrusion and room temperatures.

The greater this temperature differential, the more severe the thermal shock experienced by PEEK. This shock freezes the molecules in a disordered state, resulting in retraction forces and residual stresses within the printed part.

Peek crystallization temperature and its high viscosity

Consequently, if the chamber temperature remains at room temperature, PEEK parts cool rapidly, hindering complete crystallization and leading to inferior chemical, thermal, and mechanical properties.

Moreover, PEEK stands out as one of the most challenging thermoplastics to print due to its high viscosity, unlike PLA or Nylon (PA). Achieving precise dimensional tolerances is difficult owing to its high shrink rate. To address these issues, various features can be incorporated to ensure the high-quality 3D printing of performance materials.

Presently, only a select few companies boast the capability to print PEEK with exceptional accuracy and high crystallinity rates, owing to the unique characteristics of this high-performance material.

Unterstanding Polymer Viscosity

When a polymer filament is melted and extruded, it transforms into a fluid, and its behavior is influenced by viscosity.

Viscosity denotes the measure of resistance to flow arising from internal friction between fluid layers as they slide past each other. Higher polymer viscosity poses greater challenges during printing.

Traditional extrusion systems encounter limitations in material flow due to friction on the internal channel surfaces, leading to extruder clogs. Additionally, during fused filament deposition, air tends to get trapped, creating vacuum areas between successive layers and compromising the mechanical properties of the part. Smaller nozzle diameters and higher printing speeds exacerbate these issues by increasing apparent polymer viscosity.

Polymers shrinkage rate and warpage

Shrinkage rate, expressed as a percentage, characterizes the volume contraction during a polymer's transition from molten to solid state. PEEK's high thermal gap leads to rapid cooling, resulting in increased shrinkage rates and, consequently, challenges in maintaining part accuracy.

Warping, a consequence of high shrinkage rates, manifests as detachment of printed parts from the build plate during cooling. This occurs primarily at edges due to stress concentration, compounded by retraction forces post-cooling.

In addition to warping, high shrinkage rates induce delamination and lower part accuracy due to residual stresses.

The importance of a Heated Chamber

A heated chamber is indispensable for printing PEEK as it facilitates controlled cooling rates, enabling optimal crystallization and minimizing shrinkage-induced deformities. This obviates the need for post-print annealing processes, streamlining production and enhancing part quality.

Annealing: Definition and Complications

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.

Benefits of a 3D Printer with a Heated Chamber

A heated chamber ensures homogeneous crystallization layer by layer, obviating the need for post-print annealing and yielding parts with superior properties and dimensional accuracy. This minimizes production steps, enhances process control, and eliminates constraints associated with annealing, resulting in streamlined production and superior part quality.

In short, the advantages of the heated chamber are the following:

  1. High crystallinity reached on printed part, that result in improved characteristics of the material, in terms of chemical, mechanical, and thermal performance.

  2. Homogeneous crystallinity achieved layer by layer, resulting in the same properties throughout the printed part.

  3. 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.

  4. No limit due to annealing on the minimum thickness to print.

  5. No deformation due to the heat treatment, resulting in better tolerances.

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