Comparative study of additively manufactured, state-of-the-art photopolymers and their applicability in prototype mould making

Additive manufacturing is a state-of-the-art technology widely used for mould making. Material jetting (MJ) is one of the best additive technologies because of the dimensional accuracy and the outstanding mechanical properties of the printed parts. MJ is a high-end additive technology, requiring expensive printers and resins. Because of this, MJ is typically limited to special, technical applications. Another limitation of MJ-printed resins is their low glass transition temperature ( T g ), and the decrease in strength, stiffness and high creep above T g . An important development direction of photopolymer resins is increasing the T g and enhancing resistance against heat. This article presents a thermal post-curing method, which aims to improve the thermomechanical performance of photopolymers. This article presents a comparative study of two commercial resins for injection mould making: DigitalABS and Loctite3955. The mechanical and thermomechanical properties of both resins can be modified after printing by thermal post-curing, which results in higher strength, stiffness and resistance against heat, and reduced creep, which are all beneficial for mould making. Thermomechanical material characterisation was performed first, to prove the effect of thermal post-curing on the temperature-dependent stiffness and creep properties. The temperature dependence of stiffness was measured by dynamic mechanical analysis in temperature sweep mode. Both as-printed resins showed a decrease in stiffness with increasing temperature but Loctite3955 post-cured above 160 °C, causing an increase in stiffness. Thermal post-curing drastically increased the storage modulus of Loctite3955 from 2041 to 3934 MPa at room temperature and the difference remained nearly the same in the 20–160 °C temperature range. Following the successful material tests, we performed injection moulding tests to analyse the effect of thermal post-curing on mould insert deformations. Thermal treatment resulted in a considerably (by more than 80%) reduced deformation of the mould insert made from Loctite3955 compared to DigitalABS. Also, there was no residual deformation on the post-cured Loctite3955 insert in contrast to the DigitalABS inserts, which had considerable deformations. The results presented in this article clearly highlight the beneficial effects of thermal post-treatment on the thermomechanical properties of photopolymer parts. Our findings allow the more widespread use of these photopolymers in applications where signifcant thermal and mechanical loads occur.