Photopolymer formulation towards large scale additive manufacturing of autoclave capable tooling

Abstract

Large-scale polymeric additive manufacturing (AM) is emerging as a method for producing prototype tooling in the aerospace industry. However, material options for oven/autoclave-capable tooling are currently limited to costly thermoplastics such as polyetherimide (PEI) and polyphenylene sulfide (PPS). Photopolymers can be formulated for oven/autoclave use and have advantages over thermoplastics such as ambient processing and rapid curing under low-energy ultraviolet light emitting diode (UV LED) lamps. This study presents initial photopolymer formulation work towards large-scale AM with bisphenol A ethoxylate dimethacrylate (Bis-EMA), a vinyl ester suitable for moderate autoclave/oven-curing temperatures (100–135 °C, 212–275°F). Bis-EMA photopolymers were prepared and cured in 3 mm thick specimens by UV irradiation to simulate large-scale AM conditions. Bis-EMA was also blended with pentaerythritol triacrylate (PETIA) to determine the influence of a high-functionality acrylate on cure response and material properties. For the same UV dose, an increase in PETIA content up to 50 wt% increased the Vickers Hardness Number (VHN) on the irradiated surface from 22.6 to 25.0. It also improved the ratio of top to bottom surface hardness from 65% to 91%, indicating possible improvement in through-thickness cure response. The addition of PETIA decreased the tensile modulus from 2.8 GPa to 2.1–2.3 GPa, and PETIA content above 20 wt% reduced the glass transition temperature (T g ) below the Bis-EMA control. A blend of 80 wt% Bis-EMA with 20 wt% PETIA yielded an optimal balance of improved through-thickness hardness over the Bis-EMA control while retaining thermal stability. To investigate suitability for oven/autoclave-capable tooling, this 80:20 photopolymer blend was reinforced with fiberglass and used to produce UV cured demonstration tooling via a pseudo-additive process. The tools were subjected to five oven cure molding cycles, and laser scanning showed an average deviation < 0.05 mm, indicating negligible degradation in the tool geometry. The hardness values of the tool stock material did not change significantly with thermal cycling, and the T g increased from 154 °C to 172 °C. This study is the first report of Bis-EMA/PETIA blends towards use in photopolymers for large-scale AM. The data confirm that Bis-EMA photopolymer formulations can achieve adequate through-thickness cure at the layer thicknesses typical of large-scale AM. An 80:20 Bis-EMA/PETIA blend was thermally stable within the low volume thermal cycles expected of a 3D printed prototype tool.