Modelling of the capillarity effect in Multi Jet Fusion technology

Multi Jet Fusion is a powder-based Additive Manufacturing technology patented by Hewlett-Packard Inc. It is characterised by the use of lamps instead of lasers to heat and melt polymers and by fusing and detailing agents that are jetted on the polymeric particles to modify and to control their heat absorption and thus selectively melt them. The high production rate and excellent mechanical properties of the manufactured parts, even in com- parison with Laser Sintering, together with the overall product quality make this technology effective for a production of small series of end-parts rather than functional prototypes. In the present paper, the so-called capillarity effect is investigated. Capillarity derives from the interaction between the detailing and the fusing agents during the layer-based part building and it determines a deviation of the upper planar geometries when close to the border edges between the molten material and the powder bed. A model for the estimation of the capillarity effect is here proposed by adopting the free liquid meniscus theory. A benchmark geometry was designed to be affected by the capillarity effect and then manufactured by the MJF process. Values of the contact angle and of the characteristic length of the capillary, which are necessary to implement the analytical model, were obtained by experimental measurements made on the benchmark geometry. As a result the capillarity effect showed a dependence on the border edge orientation. The comparison be- tween calculated shapes of the plane affected by the capillarity effect through the analytical model was in accordance with the experimental measurements thus allowing a reliable prediction to be made.