The Complete Guide to Direct Ink Writing

Direct ink writing (DIW) is an extrusion-based additive manufacturing technique where ink is extruded through a fine nozzle that follows a digitally defined path to build a three-dimensional structure layer by layer. One of the key characteristics of this technique is its ability to print customizable inks at the meso- and microscale (essentially, not large-scale projects). The technology was first patented in 1997 by Joe Cesarano and Paul Calvert at the Sandia National Laboratory, where they developed it as a technique for printing complex ceramic structures. Since then, DIW has been applied broadly across a range of studies and fabrication processes beyond ceramics. It has mostly been used in research labs for small-scale fabrication and prototyping, but the technique has the potential to create efficient, industrial-grade parts. Here, we’ll take a closer look at the process, materials, and applications behind DIW, highlighting its advantages and limitations.

How Does Direct Ink Writing Work? Processes and Materials:

Typically, the fundamental process for DIW is the same as any 3D printing process. Users need a 3D model created via computer-aided design (CAD) and a movement path file from a slicing software. DIW can process almost any material, provided the ink exhibits the right rheological behavior, namely, the appropriate yield stress under shear and compression, along with suitable viscoelastic properties. As a result, DIW enables a wide range of inks to be printed into complex 3D structures with high-resolution patterning, architectural flexibility, and tailored material characteristics. This sets it apart from other AM technologies like FDM and SLA, which are constrained by material class. Additionally, DIW is versatile because of its ability to use multiple nozzles to create multi-material structures.

Overview of the direct ink writing process. (Image Credits: Sandia National Laboratory)

In DIW, applied pressure forces liquid inks through a nozzle, and this pressure can alter the ink’s viscosity. This is a key distinction of DIW: instead of using heat, DIW prints inks at room temperature, meaning the rheological properties of the ink are critical. After exiting the nozzle and before final deposition, the ink is not completely at rest. Rather, the material bends and stretches depending on the ratio between the extrusion rate and the speed of the printhead movements. Once deposited, the ink solidifies either naturally or through external processes like evaporation, phase changes, heat treatment, or gelation.