COOLING THEORY FOR THERMOPLASTIC MATERIALS USED IN SCREW EXTRUSION ADDITIVE MANUFACTURING

Large format 3D printing of thermoplastic polymers is a fast growing technology for industrial tools manufacturing and enables the production of meters long workpiece in a fraction of time, material and cost than conventional subtractive solutions. Due to the scale and timing imposed by the industry, Large Format Additive Manufacturing (LFAM) is mostly based on screw extrusion of thermoplastic pellets offering a significantly higher deposition rate and lower material costs compared to the well-known filament extrusion 3D printing (FFF). Carbon fiber reinforced polymers are commonly used in large-scale 3D printing as they minimize distortions and internal stresses during deposition preventing delamination and failure of the printed component. The technology stands out for the exceptional melt deposition rate; the lack of a temperature-controlled build chamber, and the low surface-to-volume proportion of the printed strand, making the temperature management of the deposited material particularly challenging in large-scale 3D printing. Print overcooling may lead to poor adhesion between layers eventually resulting in delamination, excessive heat build-up, on the other hand, is likely to result in sagging and print failure. Print thermal behavior and temperature management are closely related to material, part design and deposition strategy. Even though numerous software solutions for predictive process simulation as well as active feedback print controls for parameters optimization are emerging, common practice still relies on restricted set of strategies deduced by trial and error testing sessions; the best printing configuration is specifically custom-made for each print, an approach that could severely hinder the technology potential. This research is conducted as part of the project of CMS S.p.a., a company specialized in the production of CNC multi-axis machining centers, to develop and market an all-around tool manufacturing solution that would combine milling and Screw Extrusion Additive Manufacturing (SEAM). The study aims to develop a flexible and versatile cooling model that can predict the best process window for large-scale additive manufactured parts and automatically generate the best printing parameters for a generic printing strategy according to part material and shape. Next, the model was incorporated inside a path generation slicing software that operates with the same process parameters, unique solution on the market. Any given material is described by a specific set of variables that can be experimentally derived using a simple standardized procedure. Four industrially relevant materials were investigated for thermal model and software validation. In the framework of large format 3D printed tool manufacturing, 40 wt% carbon fiber reinforced polyamide 6 (PA6) and 20 wt% carbon fiber reinforced acrylonitrile-butadiene-styrene (ABS), polycarbonate (PC), and polyetherimide (PEI) play a strategic role in most applications. In addition, the research offers a physical, mechanical and thermal characterization of the printed workpiece providing a comprehensive guideline for part design, arrangement, and thermal compensation for traditional CFRP manufacturing tools. Finally, for each material, a real tool manufacturing case study and post-processed surface qualification is presented.