Additive manufacturing (AM) or 3D-printing has developed from a research topic to an emerging manufacturing technology in the last decades. Many different technologies and applications have been found, ranging from design prototypes, and small batch products to highly complex lightweight aircraft parts. Especially polymers play an important role in AM in general. In recent years, 3D-printing has undergone a significant market growth due to unique applications. The novel AM technology, ARBURG Plastic Freeforming (APF), was developed by ARBURG GmbH Co & KG (Lossburg, Germany) in 2006. Due to its relatively young age, only a small amount of research has been performed.
The APF technology has the potential to process any thermoplastic polymer material available in granulate shape, hence no development of a filament must be done. This technology uses an injection molding unit and a piezo-electronic shut-off nozzle for the deposition of molten polymer droplets. To guarantee a proper part performance, proper processing parameters must be specified. In this context, a procedure must be researched for this technology that allows a suitable set of parameters to be determined. Therefore, high-influencing parameters must be detected as well as a proper qualification procedure. Within the scope of this Ph.D. thesis, these two questions have been answered by describing a possible parameter qualification procedure and the demonstration of parameter optimization.
Therefore, the Drop Aspect Ratio (DAR) was identified as the most critical processing parameter for geometrical and mechanical properties. The DAR is defined by the ratio of the drop width (W) to the drop height (H) and is used for the definition of the machine path. In general, a low DAR led to high part filling and part density. It was found that the part density can be directly related to the tensile properties of the printed part. Furthermore, it has been demonstrated that the morphology of semi-crystalline polymers can be influenced by the print envelope temperature and part properties can thus be optimized. At evaluated print envelope temperature, weld lines disappeared and a homogeneous morphology in the cross-section of printed parts could be achieved. A real isotropic material behavior was not accomplished, but an improvement was also achieved here.
AM also finds increasing numbers of applications in the medical and healthcare sectors. Medical applications like models, tools, prostheses, and even implants can be printed. These devices must fulfill many requirements like mechanical integrity, functionality, and repeatability. In this work, different polymers have been studied for cranial implants. Therefore, one standard prothesis was printed in different orientations and materials and tested on impact strength. The results were compared with those of a bone cement sample. Although the bone cement sample achieved a higher impact strength, the printed samples also met the requirements and can be used after further parameter and design optimization. The results thus showed that polycarbonate, polycarbonate urethane or polymethyl methacrylate can be established for the manufacture of cranial implants.
A novel concept of a multi-material rib replacement implant was presented, to imitate soft and hard tissue within a single part. This further enhances the possibility of AM for medical devices. Therefore, the bonding between two similar polymers with different stiff- and hardnesses was studied by tensile testing. Distinctive geometrical contact areas were designed to ensure the best load transfer between the materials. Simple plain contact areas led to the best results. Based on these findings a model of the rib implant was designed. The implant was successfully printed and implanted in a human body donor. A reanimation trial was performed, and the movements of the implant were observed. The implant could withstand the upcoming forces and prove its integrity. In conclusion, it was possible to fabricate a multi-material rib replacement implant.
To summarize, this Ph.D. thesis highlights the potential of the APF process for medical applications from a technical point of view. Proper parameter qualification for any given polymer was proposed and the most critical factors influencing the process on the resulting mechanical properties were determined. Parameter optimization was performed on amorphous and semi-crystalline polymers and can serve as a guideline for other parameter optimization using the APF technology. Further, implants were produced and tested mechanically for their impact strength, showing the feasibility of such applications.