Adjustment of the thermal expansion coefficient of 3D printed composite tools for composite overwrapped pressure vessels

A thermal mismatch problem between a curable composite and the tool material can negatively impact the cured product's geometrical precision and structural integrity, leading to insufficient quality or, even worse, reduced performance. While keeping the thermal expansion coefficient (CTE) of tooling material as low as possible, for example by using INVAR or ceramic tools (with respective CTE ranges of 0.5-2 and 0-10 µm/mºC), has been one of the viable solutions, both costs of such tools and the fact that through-thickness CTE of the cured composite (typically around 30-42 µm/mºC depending on types of fiber and resin) can still be significantly different from the tools', call for alternative solutions. In this thesis, we hypothesize that by 3D printing a chopped micro carbon fiber filled thermoplastic in combination with continuous glass fiber, low-cost detachable mandrels that match the through-thickness CTE (TT-CTE) of the composites can be produced and used in producing high-performance composite overwrapped pressure vessels. TMA analysis, three-point bending tests, and density calculations were carried out eliminating eight specimens that were designed to vary for their printing orientation, infill pattern, and reinforcement distribution to three. Two detachable mandrels with solid isotropic (S) and triangular (T) infill patterns without any reinforcement and the third one with an isotropic infill pattern reinforced with five concentric rings (S-I-5) were manufactured. With its slightly higher CTE compared to the final composite overwrapped vessel TT-CTE, T gave the highest gains increase in burst performance (50%) and was the lowest-cost alternative. This study laid out the design and materials selection principles for precise, cost-effective, fast, and scalable composite tool production.