Development of a novel method and apparatus for analysis of die dynamics of an isothermal thermoset pultrusion process

Abstract

Pultrusion is a continuous process to manufacture constant cross-sectional fibre reinforced composite profiles. The profiles take their shape as the continuously moving fibre-matrix combination consolidates inside the cavity of a pultrusion die. The temperature-induced viscosity and volumetric changes of the fibre-matrix during the consolidation as well as the friction between the die wall and the moving fibre-matrix generates shear and normal forces that act on the die wall - phenomena known as “die dynamics”. Quantification and analysis of the die dynamics are crucial to understand and control the pultrusion process. However, state-of-the-art methods available to characterise the pultrusion process have limited capability to record the forces that act on the die wall at each position along the length of the pultrusion die. Further, the on-line measurement techniques demand full-scale pultrusion line which, in general, are resource intensive. In addition, the available methodologies have not considered the impact of process additives on the die dynamics. This research work, therefore, focuses on developing a resource-efficient offline testing method to characterise the die dynamics of a thermoset pultrusion process and to pre-determine the required process parameters for a given fibre-matrix combination. In the newly developed approach, called rotating core method, pre-impregnated rovings wound on a solid core with defined fibre volume fraction rotates about the axis of the core inside a hollow cylindrical heated die. The rotational velocity of the rotating core is set to be identical to the line speed of the pultrusion process. The rotating fibre-matrix undergo temperature-induced polymerisation leading to the transformation of the fibre-matrix into a solid composite within the cylindrical die. This mimic the dominant phenomena that occur inside a pultrusion die in the pultrusion process. An apparatus developed within the scope of this work, the Die Dynamics Simulator (DDS), for the first time allows to continuously measure the torque exerted by the rotating fibre-matrix on the DDS die during the polymerisation process. The measured torque represents the resistive forces that arise within the die during the consolidation of the fibre-matrix combination. Evaluation of the curing kinetics and rheology of the resin formulations facilitates the characterisation of their polymerisation behaviour enabling identification of the components of the resistive forces. Further, this work investigates the influence of the following parameters on die dynamics using the developed apparatus: (1) die temperature, (2) velocity of the fibre-matrix, (3) contact area of the die and the fibre-matrix, (4) part thickness, (5) fibre volume fraction, and (6) process additive - internal mould release (IMR). Subsequently, the developed methodology is validated against the pultrusion process using a lab-scale pultrusion line. The results show the dependency of the resistive forces on the individual and the interactions between multiple parameters. More importantly, the experiments conducted with varying concentrations of IMR permitted to evaluate the implication of the mould release on the evolving resistive forces within the die. The results further provide insight at which phase of the polymerising matrix is the IMR most effective. Comparison of the measured forces on the DDS and from that measured on the pultrusion line show good fit for higher fibre volume fractions of the consolidated composite.