Image source: 3ntr, Italy
After getting acquainted with high performance thermoplastics and their advantages, let’s have a look on the printing challenges and the possible solutions.
A part quality in any production technology is measured by its performance (mechanical, chemical, optical, etc.) and dimensional accuracy. Achieving those requirements in additive manufacturing is challenging not only with high-performance polymers like PEI and PAEK family members but also with more common polymers such as ABS and PP.
The main challenges, as we will discuss here are 'warpage' and 'layer adhesion'.
Shrinkage and warpage – the basics
Shrinkage or thermo-mechanical strain of thermoplastic materials is composed of 3 elements: Thermo-elastic (CTE -Coefficient of Thermal Expansion), Crystallization and/or Viscoelastic (“flow” behavior of a solid). The latter two are irreversible and have to do with molecular re-arrangement, which are highly affected by process parameters. However, CTE is an intrinsic property that we have no influence on.
Layers are constrained one to another by molecular diffusion (which forms the layer adhesion), hence, shrinkage of one layer will also affect the other. The thermally dynamic FDM process as shown in Figure 2, causes uneven cooling, that promotes uneven shrinkage. For ABS or PEI (both amorphous) the effects are mainly viscoelastic, whereas, for PEEK and PP the crystallinity is a major factor. Crystalline regions are more packed, and have higher density than the amorphous regions, hence, shrink more.
Shrinkage causes internal stresses
Stress (σ) equals Modulus (E) multiplied by strain(ε) (=shrinkage). -Equation 1
Since shrinkage is higher with increasing temperature difference (∆T=T ambient -
T solidification), it is possible to say that stresses are ∆T dependent. - Equation 2
1) σ=E·ε 2) ε=CTE·∆T
Tsolidification would be considered as Tg for amorphous and Tc for Semi-crystalline (SC) polymers. Above these temperatures, shrinkage does not induce stress since the polymer can flow. That is another reason SC polymers tend to shrink more (Tc>Tg). Nevertheless, CTE is higher above Tg and is more relevant for stresses induced in high Tg SC polymers.
So, if uneven cooling causes uneven shrinkage and shrinkage causes internal stresses, a part with different or asymmetric internal stresses is received. This is where warpage comes into rescue (Really??).
Warpage is the result of a mechanism that balances asymmetric internal stresses by increasing the strain (change in part dimensions) in less stressed regions.
OK, so shrinkage should be controlled to reduce warpage but what about the Modulus? is it possible to control that as well?
Two reminders from previous articles before moving on to solutions:
- Higher degree of crystallinity will promote better mechanical properties (higher Modulus).
- Crystallization occurs above Tg, so in an uneven cooling process like FDM, differences in crystallinity might also be observed. Especially, when dealing with cooling from high processing temperatures of PEEK to room temperature (way below Tg), and then heating and cooling of next layers like the profile in Figure. 2. This re-heating of previous layers may increase crystallinity to create asymmetric layers properties.
To conclude these notes, mechanical performance (E Modulus) is different within the part due to non-uniform thermal process. Crystalline regions induce more stresses due to higher modulus and more shrinkage.
Solutions
Sooner or later, we need the part at room temperature, so what should we do?
Reduce internal stresses and promote uniform morphology by keeping ∆T as low as possible until the part is finished printing, and cool down the full part to room temperature evenly.
How we do it? We choose the right machine and materials for our parts.
Heated bed
Having higher bed temperature in values close to and below Tg (for amorphous) will help to promote a more relaxed molecular state and when the top layers cool down the 1st layer can “flow” (viscoelastic) and contain the shrinkage of the 2nd layer without warping. This will keep the first layer flat on the bed. The bed temperature is relevant only for the first few printed layers due to low thermal conductivity of polymers.
Too high temperature will make the polymer too soft and it will curl so take it into consideration.
Heated chamber
While working with a controlled heated chamber, several advantages are achieved:
1. Improving layer adhesion by keeping molecular mobility and diffusion. Since temperature is high, the healing time or diffusion happens faster so it is possible to print faster.
2. Lowering ∆T and shrinkage.
3. Promoting uniform morphology. For SC polymers, like PEEK, printing at ambient somewhere in the middle between Tg and Tm will promote faster crystallization. As previously stated, the uniform morphology enhances more uniform mechanical properties and more even shrinkage, that will reduce internal stresses differences to prevent warpage.
For amorphous polymers, the temperature should be lower than Tg.
4. Lowering modulus. At higher temperatures, the mechanical properties (E) are lower, hence, internal stresses are lower.
Once the part is finished, it can be cooled down to room temperature more evenly.
Heated Chamber. Image source: Apium, Germany
Materials
SC polymers tend to warp more than amorphous polymers, so material suppliers developed polymers that have the easy processing of amorphous polymers and the final properties of SC ones. The PEKK and other PAEK’s were discussed in previous articles but a quick reminder: these new polymers crystallize very slowly and can be printed while remaining fully amorphous. This enables printing with lower chamber temperature than required for PEEK. The slow crystallization enables longer time for molecular diffusion and better interlayer adhesion.
In order to achieve the benefits of a SC material, the part made with these polymers must be annealed after printing above Tg to promote crystallization. If PEEK is printed in a relatively low ambient temperature, it will crystallize only partially and the post process of annealing will cause more warpage, so be aware.
Although it was not discussed, do not forget part design and other process parameters that have a crucial influence on part quality.
I hope you liked this “High performance polymers for FDM” series. Next article will have a glance on thermoplastics for SLS.
