Set-up for experimental measurements of permeability in LCM processes based on unidirectional flow measurements

Abstract

Liquid Composite Molding (LCM), which includes Resin Transfer Molding(RTM), Vacuum As-sisted Resin Transfer Molding(VARTM), Reactive Injection Molding(RIM) and others subsets is considered as one of the most viable and eco-friendly process for manufacturing high quality parts. All the LCM applications involve resin flow into a fibrous network i.e the reinforcement, which can induce defects such as voids and dry spots. Resin flow induced defects are known as the largest source on quality and reproducibility problems. These defects are often caused by unbalanced resin flows, which are directly related to the fiber permeability. Currently many computers simulations tools for predicting LCM processes behavior are available. Using these tools, process engineers can replace the traditional time-consuming trial-and-error pro-cess design procedures by optimized ones. One of the most critical input parameters with respect to the LCM modeling tools is the permeability of the reinforcement.Without a reliable character-ization of the fiber preform neither mathematical models nor computer simulations would be able to provide useful information . To fully characterize a fiber reinforcement it is necessary to measure its principal permeabilities. However, since LCM molded parts are, most of the time, thin shells, the through the thickness permeability is usually neglected and only planar permeabilities need to be evaluated. Permeability measurements have always been a source of disagreement among researchers of LCM process, results obtained from various installations show significant contra-dictions. As permeability is an intrinsic property of a fibrous material, it ought to be evaluated experimentally following a reliable and standardized procedure. The main objective of this thesis is the design of a set-up to measure in-plane (parallel to woven plane) permeabilities of LCM reinforcements by performing unidirectional injections. This method uses a rectangular mold of constant width, the conservation of mass equation and the well know Darcy’s law for the numerical determination of principal permeabilities. As the most critical part of the set-up is the mold, it is verified that the design of the mold meet the requirements to get a good accuracy for permeability measurements. In order to record the injection pressure in real time during the experiments, the set-up is also equipped with a data acquisition system based on a low-cost data acquisition device which include pressure transducers and a flow-meter. A manual scanning method have been implemented to acquire the position of the fluid flow front. Finally the software package Polyper is then used for a post-treatment based on a statistical analysis of the flow velocity during impregnation to evaluate the permeability of the LCM reinforcements. It is also possible to model permeability behavior as a function of the fiber fraction.