Co-authors​ :

 Lucie LABORDERIE (DENMARK), Robert PIERCE (DENMARK), Philipp Ulrich HASELBACH (DENMARK) 

Vacuum Assisted Resin Infusion (VARI) is a composite manufacturing process commonly used for wind turbine rotor blades, where dry reinforcements are placed on a rigid half-mold sealed by a flexible vacuum bag. Liquid resin flows through and impregnates the reinforcement lay-up, driven by the pressure difference between the inlet – typically at atmospheric pressure – and the outlet – connected to a vacuum pump. The lay-up classically consists of dry layers of fibrous reinforcements, along with any core materials or pre-cast elements. In VARI processes, a resin flow-enhancing distribution mesh is often used to top the lay-up and reduce the filling time. An interlayer (typically peel-ply or perforated release film) is used to ensure easy removal of the mesh from the cured laminate. Peel-ply is a woven and permeable product that increases resin consumption and waste but leaves a rough surface suitable for further adhesion processing. Alternatively, perforated release films have a much lower resin uptake, as they are impermeable membranes with a pattern of tiny discrete perforations.

Preliminary experiments showed that varying the arrangement of release film perforations can lead to very different flow front patterns during infusion (see Fig. 1). A relatively uniform flow front is obtained with one choice of perforated release film (Fig. 1, left), whereas a different perforation pattern leads to several isolated flow fronts, which later converge as the infusion continues (Fig. 1, right). However, previous studies highlighted that such converging flow fronts induced a higher risk of void formation [1]. Therefore, suspicions are raised about the influence of the release film perforation pattern on the infusion quality. Despite different perforation patterns being available, no widely spread recommendations indicate which pattern is adequate for a given application.

This work aims to formulate guidelines for choosing an appropriate perforation pattern for a specific application to avoid isolated and converging flow fronts (Fig. 1, right) and ensure a uniform flow front in the fabric (Fig. 1, left). Based on the lay-up properties (stack height and fabric in-plane and through-thickness permeability), a spacing criterion for selecting an appropriate perforation pattern is derived analytically. Numerical simulations are also performed to validate and verify the criterion. Additionally, when the criterion is met and the flow front is uniform, the perforated release film can also be homogenized for modelling purposes: an essential simplification for computational efficiency and large-scale simulations. A coupled analytical-numerical method is also presented to evaluate the through-thickness permeability of the homogenized release film, and the numerical and analytical methods are both compared against experimental results, in terms of flow front shapes and void generation.