Image source: miniFactory, Finland
After the first introduction of plastic processing for additive manufacturing, focusing on what amorphous and semi-crystalline polymers are and their transition temperatures, let's move on to discuss the heating and cooling phases and related phenomena by demonstrating the process parameters of two of the rising stars in FFF/FDM, the high-performance polymers, ULTEM™ & PEEK.
Let’s be practical. We’d like to 3D print with ULTEM™, what should we do?
The material must flow in the extruder through the nozzle onto the bed. ULTEM™ (PEI) is an amorphous polymer (Pyramid left side, remember?) and only has a Tg (217°C). To reduce a thermoplastic polymer resistance to flow, heat should be applied. For ULTEM™, exceeding a temperature much higher than 217°C is required, even up to 350°C or more. It is important to distinguish between transition and processing temperatures, the latter is higher for melt processing.
At this temperature, the intermolecular bonds are weak enough to push the material through the extruder and the nozzle, to form a printed layer. The polymer meets the ambient (bed and chamber) temperature, which is lower than the material’s Tg. The polymer cools down and solidifies.
As mentioned previously, thermoplastic polymers must flow to enable processing, so fluid mechanics must be considered. Let’s recap the basics:
Resistance to flow is defined as Viscosity. Thermoplastic polymers behave in a Non-Newtonian shear thinning manner, or pseudoplastic manner which means that higher shear rates reduce viscosity and help for easier processing. It is worth mentioning that shear rates in AM are relatively moderate.
Newtonian fluid- a fluid that its shear stress is linearly correlated to the shear rate. The viscosity is defined as the slope and it is constant for Newtonian fluids.
What happens in the following layer?
Pretty much the same but this time the material meets a solid polymer. The hot material from the nozzle, heats up the interphase between the 2 filaments. Surprisingly, they are bonded. what happened? The “hot” molecules heat the “cold” ones (Fig.1 left side), diffuse to one another, and entangle (Fig.1 right side). Entanglements occur by random rearrangement of the long molecular chains and contribute the material’s strength. This phenomenon is
called molecular interdiffusion. This process is basically a fusion or welding of thermoplastics. Many process parameters affect the molecular interdiffusion including extruder temperature, printing speed, chamber temperature, material viscosity and more. The level of molecular interdiffusion will determine porosity, mechanical properties, and isotropy.
Cooling and crystallization
If a Semi-crystalline polymer is your choice, PEEK for example, a temperature higher than Tm, is required. PEEK Tm is around 343°C, so an extruder temperature around 400°C would be sufficient for printing. As we recall from the previous article, crystals may form when the
material is between its Tm and Tg. Furthermore, the size of the crystals is directly correlated to the cooling rate, demonstrated in Figure 2.
Figure 2 illustrates the correlation between cooling rate and crystallinity level and crystals’ size. The top-left image shows bigger spherulites for slow cooling rate of 1°C/min, and decreased spherulite size for faster rates (Up-right & Down-left). At a cooling rate of 2,000°C/min the polymer remains amorphous (Down-right image).
Each Semi-crystalline polymer has different crystallization kinetics. Some polymers, like PEEK, crystallize fast, while others are slower like PPS or PEKK. This property influences on the processing window of each polymer and the machine setup.
In general, polymers are considered to have poor thermal conductivity which means they cool down slowly. Therefore, the cooling rates (°C/min) in the FFF process are relatively slow, and most semi-crystalline polymers will crystallize at least partially.
Some semi-crystalline polymers, like PEKK, can be printed and remain amorphous. Would it be possible to increase crystallinity after the part is finished? Yes. Here is how. This post-treatment is called Annealing. The finished part is placed in an oven above the Tg, where chains mobility increases, rearrange and may form crystals. This property allows for easier printing process without losing the materials’ advantages. This will be discussed further in next articles.
To conclude, we reviewed the heating and cooling phases of FFF, in a molecular level. Both, sufficient molecular interdiffusion and controlled cooling rates will enable improved mechanical properties and part quality.
Now that the terms in polymers processing are clear (hopefully, and if not please reach out), next article will discuss the structure- properties relationship of high performance polymers.
* ULTEM™ is a brand name of SABIC