French Researchers Examine Heat Transfer & Adhesion in FFF 3D Printing

Researchers from Laboratoire de thermique et énergie de Nantes uncover some of the challenges in 3D printing versus thermoplastic injection, releasing the findings of their recent study in ‘Heat Transfer and Adhesion Study for the FFF Additive Manufacturing Process.’

Mechanical properties are often the topic of study today—from researching helpful additives to studying the influences of color, to issues with porosity, and far more—as users attempt to improve the functionality of parts. Adhesion between layers is a common problem, usually leading to further examination of technique and materials. In this study, the researchers focused on heat exchanges in an attempt to improve 3D printing.

Temperature remains one of the most important settings for users, leading to good quality and performance in printed parts—or in other unfortunate cases, major structural issues.

“To find precisely the limit of this optimal processing area,  the thermal history needs to be predicted accurately,” stated the researchers.

Polymer printability rules for FFF process.

With a better understanding of thermal factors, users may be able to avoid macro-porosities and adhesion problems. During FFF 3D printing, the following heat transfers occur:

  • Heat from the extruder
  • Convection cooling of filament
  • Exchanges between filaments
  • Heat from the support plates
  • Radiative losses
  • Heat from exothermal crystallization for semi-crystalline polymers

At least 6 different heat transfer phenomena are identified in the FFF process.

(a) Comparison of the heat transfer model existing in the literature of FFF process and (b) geometry for the 2D analytical model. Adapted from [7]

While controlling the 3D printing process with high temperatures, the researchers also reinforced PEKK materials with short carbon fibers. In the beginning of the experiment, however, the team used ABS due to ‘greater ease of implementation.’ An experimental bench was 3D printed on a CR-10 3D Printer from Creality3D for measurements of temperature, and then a simulation model was created via COMSOL Multiphysics® v5.4 for predicting temperature and healing.

Experimental bench showing the heated chamber for 3D printing of high temperature polymers and the infrared camera for temperature measurements.

Before printing, the authors customized the 3D printer in their lab, modifying the hardware so it would be able to attain the proper temperatures of up to 400°C.

“The extruder was changed, for a full-metal unit, with a water-cooling closed circuit system. A closed insulated chamber maintains the part in a 200°C atmosphere. It does not block the three translation moving system of the 3D printer  inside  the  chamber. Electronics and mechanical parts are kept   outside the chamber.  This heating chamber is mandatory for printing polymers like PEKK,” said the researchers.

Experimental set-up. A single filament wall was 3D printed. The pyrometer measures the temperature from the side in situ.

Geometry and boundary conditions used in the heat transfer model

The other specimen was a basic structure 3D printed with both ABS and PEKK, in the form of a 60×2.2×50 mm wall. For ABS, the researchers took qualitative measurements with a pyrometer, with quantitative measurements taken for both ABS and PEKK.

IR camera qualitative analysis for ABS.

“Because of the poor knowledge of the rheological properties,  the calculated degree of healing was found to be equal to 1 very quickly for ABS. However, this is the opposite for PEKK material, which reaches only a degree of healing of 0.45 after the cooling-down of the filament,” concluded the researchers.

“The  bench  was  designed to handle high temperature and future work will consist in studying deposition of PEKK more precisely,  and  also  for  carbon  fibers  reinforced  PEKK  with  different process parameters. The  short-term  perspectives  are  to  use  the  model  with  the  thermo-dependent  thermal properties, which were characterized in the  LTeN  laboratory  on  PEKK  polymer.”

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[Source / Images: ‘Heat Transfer and Adhesion Study for the FFF Additive Manufacturing Process’]

 

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