Patented water‑soluble expanding mandrels made from an innovative 3D‑printable polymer for manufacturers of hollow, complex composite components. Mandrels permit prepreg lay‑up, simplify mold closing, expand to apply curing pressure, dissolve in tap water to reduce scrap and speed production; tailored design, large‑format additive manufacturing (up to 1000×1000 mm) and long‑life high‑temperature molds offered.