Strip formation mechanisms and characteristics models in 3D printing of viscous polymer inks

Abstract

The strip formation process on a glass surface in 3D printing of viscous droplets is presented with the relevant strip characteristics modelled. PEDOT:PSS polymer inks at different viscosities are used. It is found that unlike the symmetric circulation flow developed by a single droplet deposition, strip-printing by multiple droplets is achieved by the coalescence of droplets spreading unequally in different directions. Whereas the liquid viscous cohesion retards the flow in the direction aligning with the centre line of the coalesced droplets, the side spreading driven by liquid surface tension fills up the space on the target surface along the bridge between two coalesced droplets. When the deposition distance or space of two consecutive droplets is set at greater than a critical value, this flow inequality results in strips of scallop shape. With a deposition distance below the critical value, the strips formed tend to have parallel edges and the strip width increases with a decrease in the deposition distance. However, if the distance is set below a saturate level at which the liquid surface tension cannot withstand the deposited liquid volume, bulges are detrimentally formed randomly along the printed strips. Because of the viscous retardation to the spreading of drop liquid, the strips printed using a higher viscosity ink is narrower than that of a lower viscosity. Based on the strip formation process, the relevant characteristics of the printed strips are mathematically modelled using dimensional analysis. The developed models are found in good agreement with the experimental data and can be used to guide the selection of the operating parameters for strips with desired characteristics.