Re-entrant solid behavior of 3D-printable epoxy inks

Illinois researchers identify reasons for clogged 3D printers by applying rheological creep tests to analyze ink properties

It’s a common problem in 3D printing that inks get clogged at some point during printing, but scientists haven’t had a good understanding of why this happens, until now.

Researchers at the Illinois Applied Research Institute, in collaboration with Prof. Simon Rogers (Department of Chemical and Biological Engineering), have demonstrated the effectiveness of a rheological test that can be used to better characterize the printability of an ink for Direct Ink Writing (DIW).

In a recently published paper in the journal Rheologica Acta titled “Re-entrant solid behavior of 3D-printable epoxy inks”, the research team showed that rheological creep testing can be used to determine how well a material will print in DIW. “We were investigating the effects of nanostructure in 3D printable epoxy inks, and the results from the traditional rheological tests showed that the inks were effectively the same, yet we were seeing very large differences in the consistency of the prints”, says Dr. Dan Krogstad, lead principal investigator on the study. “With the help of Prof. Rogers’ team, we started to look for another rheological measurement that we could use to describe their properties that better correlates to how well they can be printed.” The result of this work was the identification of transient creep testing.

DIW is a highly versatile 3D printing technique that can be used to print a wide range of materials including polymers, hydrogels, composites, and metal/ceramic slurries. While many 3D printing techniques can only be used on a small range of materials with specific thermal properties or UV-reactivity, DIW can work on any material that can be extruded from a syringe and retain their extruded shape after printing. So, the flow characteristics of the inks during printing, known as the rheological properties, is the most important thing in determining if an ink can be printed by DIW. However, there has not been any universally accepted measurements that can be used to determine whether the materials can be printed in DIW.

This work identified that creep testing results compared well with the printing responses. In creep testing, a constant load is applied to a material and the deformation (strain) is measured with time. This measurement method is very similar to what the inks experience during printing, so the creep results match the printing results better than the most commonly used measurement methods. Transient creep testing is not a new measurement, it is used regularly for the characterization of other materials that require similar flow properties, but had not been used much in the characterization of the DIW inks.

“The use of creep testing allowed us to better understand how much pressure would be needed to extrude the materials through the printer, and how consistent the flow would be over time”, says Dr. Krogstad. The characterization of the time-dependent flow proved very important. “Using creep testing, we can observe this time dependent process, which will allow us to better select printing pressures and printing times that will minimize clogging and will result in better quality prints.”

The team plans to continue to use these tests on a wide range of inks to understand whether this test can be universally used to determine ink printability, or if it is most appropriate for a smaller class of DIW inks.

This work was funded under a 2017 Air Force Office of Scientific Research Young Investigator Program award.