Additive Manufacturing (AM). Cavity Vat Photopolymerization (CVPP)
What does a coconut, an egg, a turtle, a tooth and a man have in common? Correct. They are all tough on the outside and soft on the inside. Several applications from the engineering field require the same material behaviour. CVPP offers a simple but effective AM method of producing such parts.
It was an ordinary day in the lab. I was doing rheology investigations of a very unique dual curing resin material system called RPU 70 from Carbon Inc. In contrast to usual UV-curing resins, dual curing resin formulations are unique because after the shape giving UV-curing step, they require an additional thermal curing step to attain their final properties. Because of my background as a chemical engineer I was always very fascinated by this second curing step and wanted to learn more about the chemistry behind it. Obviously, applying heat to such a system would trigger this reaction and I wanted to verify my hypothesis of an increased viscosity by applying heat to the system and observing the rheologic behavior. At first everything looked very promising and I found a non-linear increasing correlation between temperature and viscosity. However, I forgot one sample in the oven. The next day I found it and I tried to measure it but the rheometer was showing "Error". At first I thought the rheometer was broken but after investigating the sample a little closer I found out that the resin had transformed into an elastomer.
Even though, at that time I did not quite understand the utility of this observation I thought it was pretty cool to transform one starting material by either UV-light or temperature into different solid states.
The next step was to see if I could produce hard shell - soft core parts by creating the solid body first by UV-curing in the 3D printing process and filling that body with liquid resin and thermally curing both.
The results were very promising. After cutting the first test sample in half the two material states were fully developped and there was no liquid resin remaining. Furthermore, it looked like the elastomer core was chemically bonded to the thermoset shell. However, the intermediate step of filling of the body with liquid resin was an subotimal solution as it required additional time and effort.
As I was showing these first results to my PhD colleagues Philip Obst and Lukas Knorr, we were brainstorming on how to improve this method to eliminate the suboptimal intermediate step. The result was a new additive manufacturing method we called cavity vat photopolymerization (CVPP).
If you want to learn more about it you can read our article.