The influence of temperature variations on photopolymerization in phase transition stereolithography

The high precision and accuracy of polymer parts manufactured using stereolithography have established this process as the most advanced 3D manufacturing method for hearing aid, dental, casting, and prototyping applications. Manual post-processing steps caused by necessary support structures significantly increase effort and currently prevent fully automated stereolithography based manufacturing. This work presents a novel process—phase-transition stereolithography- that enables a support structure free fabrication method. Due to process-related heat sources and sinks, the temperature distributions in the curing layer are highly inhomogeneous. Therefore, this study aims to provide a deeper understanding of the influence of temperature and exposure strategies on curing behavior, lateral and longitudinal resolution, as well as on mechanical properties and conversion rates. Two resins were developed as base materials according to the application requirements of dental and otoplastic applications. The influence of temperature on the processability of both materials was systematically investigated. Photorheological measurements demonstrated the dependence of the critical energy E_c on temperature at different irradiances levels. Working curves were determined for two irradiation intensities over a material specific temperature range. The curing behavior was examined under various exposure strategies and temperatures. In this context, the effects on lateral resolution, mechanical properties, and changes in the degree of conversion in green and post-cured parts were quantified. It was demonstrated that an increase in process temperature lowers the critical energy E_c, thereby increasing the curing depth under constant irradiation parameters. Lateral blur at constant fluence was influenced mainly by increasing exposure time, and to a lesser extent by increasing temperature. Higher temperatures led to higher conversion in the green parts, with increasing Young’s moduli and decreasing elongation at break as temperature increased. Post-processed parts did not show any significant dependence of mechanical properties on processing temperatures.