Experimental Optimization of Polymer Jetting Additive Manufacturing Process Using Taguchi Design

The overarching goal of this research work is to fabricate patient-specific bone scaffolds and implants for the treatment of osseous fractures and defects. In pursuit of this goal, the objective of the work is to investigate the influence of consequential parameters of polymer jet printing (PJP) process, i.e., print direction, resolution factor, UV light intensity, and deposition head temperature, on the mechanical properties of fabricated femur bone structures. This work tests the following hypothesis: patient-specific bone scaffolds and implants with tunable functional properties are additively fabricated using the PJP process.

PJP is a direct-write, droplet-based additive manufacturing method, emerging as a rapid high-resolution process of choice, particularly in the medical field for the fabrication of a wide spectrum of products, e.g., anatomical models, tissue scaffolds, implants, and prosthetics. PJP allows for non-contact multi-material deposition of functional polymer inks. PJP-deposited structures have a minimum feature size and layer thickness of approximately 150 μm and 15 μm, respectively. The PJP process centers on simultaneous deposition of both build and support materials on a free surface, followed by UV photopolymerization process. Inkjet heads are utilized to deposit liquid photopolymers (also known as photocurable polymers) onto a build platform. The deposited photopolymers are immediately cured in situ using a UV light source, allowing for solid-freeform fabrication. The PJP process is inherently complex, governed by a multitude of process parameters as well as material-machine-process factor interactions, which collectively affect the functional properties of fabricated bone structures. Consequently, physics-based characterization and optimization of the PJP process is inevitable.

In this study, a new test standard was forwarded for the bone structure characterization, designed on the basis of an X-ray μ-CT scan of a femur bone in addition to the ASTM D638-14 standard. Furthermore, the Taguchi L9 orthogonal array design (which consisted of nine experimental runs, each repeated five times) was used to investigate the effects of the influential PJP process parameters on the mechanical properties of fabricated bone structures, including Young’s modulus of elasticity, 0.2% yield stress, tensile strength, and ductility. The selected process parameters (each at two levels) include: (i) print direction (bidirectional and unidirectional); (ii) resolution factor (0.25 and 0.5), (iii) UV light intensity (50 and 250 [mJ/cm2]), and (iv) deposition head temperature (expressed in terms of two normalized values of 0.5 and 1). The mechanical properties of the femur bone structures (having a gauge length of 570 mm and composed of Verowhite Super RGD610 photopolymer) were measured using a tensile testing machine; the samples were subjected to a tensile load, applied with a velocity of 0.08 mm/s. It was observed that the UV light intensity most significantly influences the tensile strength, the modulus of elasticity, as well as the ductility. In addition, the influence of the deposition head temperature on the tensile strength of the fabricated bone structures was almost insignificant. Besides, the unidirectional material deposition resulted in relatively high tensile strength and modulus of elasticity. Overall, the results of this study pave the way for future investigation of the nonlinear effects of PJP process parameters toward optimal fabrication of bone tissue scaffolds and implants.