(187c) Development of Experimentally Validated CFD Model to Study the Drop Dynamics during Inkjet Based 3D Printing of Tablets

Three-dimensional printing in pharmaceuticals offers multiple advantages such as personalization of medicine, fabrication of complex dosage forms and high drug loading in comparison of conventional dosage forms. Binder jet 3D printing involves jetting of binder solution on top of the powder layer in layer-by-layer fashion. The properties of the formulation are highly dependent upon jetting of the ink/binder solution from the printhead nozzle. In this study a finite volume based computational fluid dynamics (CFD) model has been developed to simulate liquid drop formation in a piezoelectric inkjet printhead. Finite-volume based CFD model works by solving continuity and momentum equations to obtain the fluid flow field in the Eulerian coordinate system in conjunction with appropriate physical models in each control volume. In addition to that, drug solutions utilized by our group for 3D printing of pharmaceutical tablets were also evaluated for the drop properties.

Methods:

In this research, Volume of fluid (VOF) and Continuum Surface Force (CSF) model was used to model the drop formation from the printhead nozzle using commercial software ANSYS Fluent. VOF model is used to track interface and CSF model accounts for the surface tension effects. The piezoelectric nozzle simulated here works by creating pressure inside the nozzle when voltage is applied which consequently leads to drop jetting. The system geometry was created in ANSYS Space Claim with a nozzle diameter of 40µm and a stand-off distance of 5mm (Fig. 1). Meshing was performed using ANSYS meshing (Fig 2), and model setup was performed in ANSYS Fluent. The pressure generation inside the nozzle was implemented by corresponding inlet velocity boundary condition. Experimental determined ink solution properties were used as the input for the CFD model.

Results:

Initially the mesh independence study was performed by analyzing different mesh element sizes and different inlet velocity values to find the optimum mesh element size to avoid the effect of different mesh sizes on the simulation results. Various simulations were performed with different inlet velocities and parameters such as drop diameter, drop velocity, and drop shape were compared with the experimental data for model fluid (Fig. 3) and water as the ink solution. It was observed that as the ink surface tension increases the drop volume and drop velocity decreases. Lower surface tension also increases the number of satellite droplets. It can be observed from the Fig. 4 that 5 mg/ml concentration ink solution with the highest surface tension demonstrated drop volume of 24.10 pL and a drop velocity of 6.95 m/s whereas 40 mg/ml drug concentration ink solution with lowest surface tension showed drop volume of 18.80 pL and drop velocity of 6.32 m/s.

Conclusions:

The drop formation is known to be influenced by the fluid properties such as surface tension, viscosity, and density of the ink solution which were studied using this model for multiple ink solutions used during the experiments. This CFD model will help in evaluating the jettability of different ink solutions thereby helping to improve the printing process. The simulation results and experimental data demonstrated a good agreement indicating the applicability of CFD model to study drop properties during inkjet-based 3D printing process.