Thomas Hafkamp, a Ph.D. researcher at Eindhoven University of Technology, is developing a novel closed-loop control system for SLA 3D Printing.
Detailed in his thesis titled ‘Towards closed-loop control in photopolymer-based additive manufacturing’, Hafkamp’s work focuses on the resin-based SLA process. The work stems from the fact that most 3D printers today do not utilize sensor data to correct print jobs in real-time, essentially operating on an open-loop whereby the predefined print parameters determine the quality and success of the part.
![Eindhoven Researcher Develops Closed-Loop Control System For SLA 3D Printing 1 The ideal closed-loop control system. Image via the Eindhoven University of Technology.](https://amchronicle.com/wp-content/uploads/2020/10/The-ideal-closed-loop-control-system.-Image-via-Eindhoven-University-of-Technology..png)
The power of the feedback loop
While the SLA systems of 2020 are more sophisticated than they’ve ever been, there is still the ongoing issue of print failures. It takes a great deal of skill and experience (and luck) to fabricate a part perfectly on the first run, calling for what is essentially a trial-and-error approach. In the words of Hafkamp, you cannot “CTRL + P and have a defect-free product roll out of the 3D printer”.
Feedback loops, or closed-loops, could be the answer to this, using sensor data and intelligent algorithms to counteract disturbances that would otherwise result in the deterioration of a part’s quality. In the context of SLA, the feedback control would need to address a system’s process models, sensors, and actuators all at once.
Closed-loop control of photopolymerization
Hafkamp’s work starts with a thorough literature review, whereby the researcher demonstrates theoretically that such a control system is possible. However, he determined that the main limitation would be the measurement aspect of the system, as the relevant machine-implementable sensors are not exactly readily available off the shelf. He took it upon himself to design, manufacture, and test a new optical measurement instrument to remedy the problem.
The novel piece of kit is capable of indicating to what extent a vat of photopolymer resin has been cured into its solid state – in other words, a measurement of its cure state. Its measuring principle is based on a molecular absorption photometer operating in the near-infrared (NIR) range and acts as a faster, more cost-effective alternative to similar general-purpose chemical analysis instruments.
![Eindhoven Researcher Develops Closed-Loop Control System For SLA 3D Printing 2 The photopolymerization process measured by Hafkamp’s instrument. Image via Eindhoven University of Technology.](https://amchronicle.com/wp-content/uploads/2020/10/The-photopolymerization-process-measured-by-Hafkamps-instrument.-Image-via-Eindhoven-University-of-Technology..png)
As a proof-of-concept, Hafkamp set up an experiment at the TNO Materials Solutions lab comprising an infrared spectrometer, a UV LED actuator, and an embedded controller. The apparatus was complete with his own piece of self-developed software enabling real-time data communication. The results showed that the instrument was capable of taking measurements at a rapid rate of 20,000 per second, paving the way for a more comprehensive, 3D printer-ready real-time control system in the future.
![Eindhoven Researcher Develops Closed-Loop Control System For SLA 3D Printing 3 The NIR measurement instrument. Photo via Eindhoven University of Technology.](https://amchronicle.com/wp-content/uploads/2020/10/Screenshot-199-1.png)