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23 Nov 2005  -  United States

VARTM, variability and substantiation

Vacuum-Assisted Resin Transfer Molding (VARTM) has the potential advantage of relatively low cost with sufficiently high volume fractions of reinforcement and the process can be readily applied to large-scale structures. VARTM is an infusion process where a vacuum draws resin into a one-sided mold. A cover, either rigid or flexible, is placed over the top to form a vacuum-tight seal. However, for many aircraft applications, VARTM does not currently provide sufficient repeatability or control of variability. This unpredictable variability is commonly observed when processing with the traditional VARTM process. In order to produce VARTM parts of aircraft quality on a routine basis, the variability must be understood.

Objective:
The long-term objectives of this research are repeatability equivalent to autoclave processing with specific properties (property/weight) that are close to autoclave processed part levels at a lower cost.
There are many factors that influence the variability of the final part. The factors that play a major role in the cause of this variation need to be identified and the causes and effects of changes in these factors understood. Three main VARTM process variations will be considered:

1) The SCRIMP process, patent owned by TPI Composites, is a vacuum infusion process using a high-permeability layer to rapidly distribute the resin on the part surface and then allow through-thickness penetration;
2) The CAPRI process, patented by The Boeing Company, is a SCRIMP variation where vacuum debulking and a reduced pressure difference is used to minimize thickness gradients and resin bleeding;
3) The VAP process, patented by EADS, uses an air-permeable membrane is used on top of the distribution media to allow continuous and aerial venting reducing void content and creating a robust process variant. 

The research team is using and developing advanced simulation tools including Liquid Injection Molding Simulation (LIMS), a comprehensive simulation software for mold filling, to explore a variety of filling scenarios and investigate the influence of various processing conditions in VARTM and the SMARTMolding Intelligent Process Control (IPC) system. This approach enables material, process and part traceability along with semi-automated material lay-up, automated resin mixing and resin infusion and control of dwell times and cure cycles. The automation capabilities enable monitoring of cycle times for all processing steps, sensing of the important process parameters through embedded sensors and QA/QC of the complete process.

The present work provides for an understanding of the sources of variability in VARTM processing. The primary sources are linked to variation in thickness, fiber volume fraction and void content. Work at CCM as part of the FAA Center of Excellence leveraged with the Office of Naval Research Advanced Material Intelligent Processing Center is showing that the VAP and the CAPRI process produce significant reductions in both thickness variation and void content. The program has developed experimental characterization work cells to evaluate the effect of debulking on fiber volume fraction and permeability. Research is also being conducted to characterize and optimize membranes for various aerospace resin systems. Models capable of predicting processing dependent thickness variations have been developed and incorporated into CCM's tool kit for VARTM capabilities. Together, the fundamental science approach can optimize the process setup and predict variability for the various VARTM process variations.

Two industrial partners are supporting the on-going efforts: W.L.Gore & Assoc. GmbH and Cytec Engineered Materials Inc. The capabilities developed in this program are being used to optimize processing approaches for current and future aerospace components.

Source : CECAM: Center of Excellence for Composites and Advanced Materials