A team of researchers from the University of Nantes have recently published a study investigating the heat transfer and adhesion between layers in FFF 3D printing. With the aim of understanding, modeling and actually quantifying the heat exchanges in the printing process, the researchers set out to find the optimal set of print parameters to maximize mechanical properties in 3D printed parts.
FFF vs. Injection molding
With today’s technology, the difference between FFF and injection molding is like night and day. While 3D printing does offer a great deal of design freedom, the porosities in the parts and the poor adhesion between the thermoplastic layers mean FFF printed parts simply can’t take as much force as injected parts.
The adhesion between the layers is predominantly determined by the temperature of the nozzle during extrusion. Too low, and the adhesion is weak. Too high, and the polymer starts to decompose, resulting in low viscosity and a subsequent structural collapse. The Nantes researchers – seeking that ‘sweet spot’ – believed diving deep and investigating the heat transfer at each stage of the process would lead them there.
The predictive numerical model they initially built assumed heat transfer occurred at a number of points during the printing process. This included heat from the nozzle to the polymer, convection currents from the polymer to the air, heat exchanges between polymer layers, heat from the build plate, radiation losses from the polymer to the air, and heat losses from exothermic crystallization.
The influence of heat transfer on adhesion
Throughout the experiment, the team used a Creality CR-10 3D printer with ABS and carbon fiber reinforced PEKK. The 3D model chosen as the test specimen was very basic as it would allow them to apply the heat transfer model very easily. A thermally controlled bench and enclosure were used to help take precise temperature measurements. The temperature measurements themselves were taken using an infrared camera and pyrometer. Once the experimental measurements were taken, the team compared them to the predictive numerical model they had previously developed.
The researchers concluded that their numerical model was sufficiently correct in predicting the heat transfer during the printing process as the results fell in line with the physical measurements. Despite this, their “poor knowledge of the rheological properties” stopped them from being able to accurately predict the adhesion between the layers – at least quantitatively. The team explained that their next steps would be to study the coalescence evolution of the polymers to be able to predict the formation of macro-porosities. This would make the “global degree of adhesion calculable”, giving insight into the process parameters necessary to produce high-performance parts.
Further details of the study can be found in the paper titled ‘Heat Transfer and Adhesion Study for the FFF Additive Manufacturing Process’. It is co-authored by Arthur Lepoivre, Nicolas Boyard, Arthur Levy, and Vincent Sobotka.
While printing temperature is a key player, it is not the only factor that can have a major effect on the strength of a 3D printed part. A recent study by researchers in Greece investigates the change in ABS filament’s mechanical properties in response to recycling. Interestingly, the team found that the stability and overall strength of the ABS increased until cycle five, after which chemical degradation began to take hold. Elsewhere, the U.S. Army experimented with filament additives, developing a high-strength filament with an ABS shell and star-shaped polycarbonate core.
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Featured image shows IR imaging from the experiment. Image via University of Nantes.