Manufacturing of complex shape composite parts through the combination of pull-braiding and blow moulding

The goal of the funded project PulForm is the development of a reliable and economical processing technology for the production of hollow thermoset CFRP parts of complex shape. This is achieved through the combination of pull-braiding, i.e. an assembly consisting of a braiding and a pultrusion machine, and blow moulding processes. The resin is a hybrid two-step-curing thermosetting resin. In the first reaction step (polyurethane addition reaction), taking place in the pultrusion die at 100 °C, a B-stage of rubber-like consistency is reached. In the second reaction step, the B-staged part is heated to a temperature above 100 °C, triggering a radical polymerization reaction to full curing of the matrix. Selection of an appropriate resin formulation was done by means of rheology measurements during cure and picture frame tests on braided preforms wetted out with resin and brought to a B-stage state. In the first processing step, a tube (outer diameter 58 mm) with a fiber structure consisting of layers of unidirectional carbon fiber rovings and braided layers is continuously produced by pultrusion. The braided layers are produced in-process by a braiding machine assembled upstream of the pultrusion line. The resin is injected into a closed impregnation chamber attached to the heated pultrusion die. In the second processing step, the pultruded B-staged tube is placed inside a heated mould mounted to a press. With the mould closed, pressurized air is blown into an inflatable bladder, pressing it against the tube and the mould inner walls, and thus giving the part its final shape. The moulding process step is evaluated through trials in a mould with a conical expansion geometry. The processing-related material behaviour is evaluated through curing of coupon layers under pressures similar to the applied in blow moulding and subsequent mechanical testing (mode I and mode II interlaminar fracture toughness).