Investigation of the temperature influence on the dual curing urethane-methacrylate resin Rigid Polyurethane 70 (RPU 70) in digital light synthesis (DLS)

The two part urethane-methacrylate resin RPU 70, used in DLS, seems to have superior material properties. These properties are achieved by a dual curing process of ultraviolet (UV) and thermal curing. The temperature influence on the processability of the mixed resin and the resulting mechanical properties are still unknown. It was possible to solidify the liquid RPU 70 resin into an elastomer at 60 °C for 15 h without the influence of UV light by solely harnessing the temperature sensitive crosslinking reaction between part A and part B. The ongoing thermal conversion of the liquid resin into an elastomer was analyzed with viscosity measurements, Fourier-transform infrared spectroscopy (FT−IR), differential scanning calorimetry (DSC) and Photo-DSC. The latter method proved to be effective in tracking the thermal conversion and to identify optimal exposure times for preheated resins. Furthermore, thermogravimetric analysis (TGA) showed a linear evaporation of reactive diluents in RPU 70 over time. In order to investigate the processability and to understand the effects of the thermal curing on mechanical properties, the resin was preheated to 30 °C, 40 °C, 50 °C and 60 °C for one hour before the dual curing DLS process. Even though, the viscosity of the resin was increased by preheating and additionally by the released reaction heat of the photopolymerization during DLS, the build job could be produced with the same part quality as the unheated resin. Tensile tests were conducted on dual cured specimen and the results showed that with a preheating of the resin for one hour at 50 °C, the tensile modulus of RPU 70 increased 19.56%, while tensile strength and elongation at break values remained in the standard deviation range of the comparison group. The preheating at 60 °C improved elongation at break and tensile modulus 47.61% and 5.54% respectively, while at the same time tensile strength dropped −5.02% compared to no preheating. Apart from the possibility to slightly modify the mechanical properties by preheating the resin before DLS, there are two temperature-dependent challenges for RPU 70 during the DLS process, namely evaporation and undesirable thermal curing. However, there are also new innovative ways to utilize the result of this study that RPU 70 resin can be thermally solidified into an elastomer: the creation of parts that have a hard shell and a soft core. The hard shell can be produced by using DLS, the soft core by pouring the liquid resin in the shell and thermally curing both.