Study of inkjet printing as additive manufacturing process for gradient polyurethane material

Reactive inkjet printing as additive manufacturing technique is evaluated for generation of gradient material made from polyurethane. Therefore dots are fabricated in situ by drop-into-drop inkjet printing of three separate components (2 polyols, 1 isocyanate) successively from three separate print heads. Changing the mixing ratio of the three components enables tailoring of the mechanical properties for each voxel, which raises the potential for printing of gradient materials. This study does not examine the complete additive manufacturing process to build whole 3D objects but its focus is set on the behavior of the printing heads and the printability of the components as well as on properties of the produced material. Regarding the gradient properties it is necessary to ensure that the stoichiometric ratio is given for all mixtures and that the printable masses of each component comply with the desired resolution, i.e. the smallest size of one voxel. This requires determination of the printing parameters and a definition of the achievable range of mechanical properties (e.g. hardness, elastic modulus) which is done in this study by evaluation of casted specimen from polyurethane. Additionally in this study inkjet printability of the polyols and the polyisocyanate in dependence of different fractions of toluene as a solvent is evaluated by determination of the Ohnesorge number Oh. Easily measurable physical properties as density, surface tension and viscosity are determined for several fractions of toluene to calculate 1/Oh. Finally reactive inkjet printing technology with the optimized liquids is used to create dots to form polyurethane.