Vacuum Assisted Resin Transfer Molding (VARTM)
“Vacuum infusion is a process used for moulding fibre composite mouldings, where uniformly distributed fibres are layered in one of the mould parts, said fibres being rovings, i.e. bundles of fibre bands, bands of rovings or mats, which are either felt mats made of single fibres or woven mats made of fibre rovings. The second mould part, which is often made of a resilient vacuum bag, is subsequently placed on top of the fibre material. By generating a vacuum, typically 80 to 90% of the total vacuum, in the mould cavity between the inner side of the mould part and the vacuum bag, the liquid polymer can be drawn in and fill the mould cavity with the fibre material contained therein. So-called distribution layers and distribution tubes, also called inlet channels, are used between the vacuum bag and the fibre material in order to obtain as sound and efficient a distribution of polymer as possible. In most cases the polymer applied is polyester or epoxy, and the fibre reinforcement is most often based on glass fibres or carbon fibres.”
“During the process of filling the mould, a vacuum is generated by the vacuum channels in the mould cavity, said vacuum in this connection being understood as a negative pressure, whereby liquid polymer is drawn into the mould cavity via the inlet channels in order to fill said mould cavity. From the inlet channels the polymer disperses in the mould cavity as a flow front moves towards the vacuum channels. Thus it is important to position the inlet channels and the vacuum channels optimally in order to obtain a complete filling of the mould cavity. Ensuring a complete distribution of the polymer in the entire mould cavity is, however, often difficult, and accordingly this often results in so-called dry spots, i.e. areas with fibre material not being sufficiently impregnated with resin. Thus dry spots are areas, where the fibre material has not been impregnated, and where there can be air pockets, which are difficult to remove by controlling the vacuum pressure and possibly an overpressure at the inlet side. In connection with vacuum infusion employing a solid mould part and a resilient mould part in the form of a vacuum bag, the dry spots can be repaired after the process of filling the mould by for example perforating the cloth in the respective locations and by sucking out air by means of a syringe needle. Liquid polymer can optionally be injected at the relevant location, which can for example be done by means of a syringe needle as well. This is a time-consuming and tiresome process. In the case of large mouldings, the staff has to stand on the vacuum bag, which is not desirable, especially not when the polymer has not hardened, as it can result in deformations in the inserted fibre material and thus result in local weakenings of the structures.”
Processing
Resin Transfer Molding (RTM)
Recent US Patents
9/28/2010
7,803,302
Vacuum infusion by means of a semi-permeable membrane
Hansen of LM Glasfiber, Denmark has developed a method for producing an oblong reinforced shell structure by vacuum assisted resin transfer molding. Layers of fibers are placed with a distribution layer covered by semipermeable membrane impermeable to resin but permeable to gases. Liquid resin flows into the distribution layer and into the fibers before curing. (RDC 12/8/2010)
Recent Journal Articles
Cure kinetics and chemorheological behavior of a wind epoxy resin system and its viscoelastic properties reinforced by glass fiber matt with process of vacuum assisted resin transfer molding
(477-484) High Performance Polymers 23 #6 (2011)
Abstract
Wen et al ofSouth China University of Technology, China, studied the processability of a wind epoxy resin for the vacuum-assisted resin transfer molding (VARTM) process. No obvious vitrification appeared in the epoxy system and the apparent active energy of the cure reaction calculated by gel time was almost the same as the differential scanning calorimetry result. What is more, the viscoelastic properties of the composite reinforced by glass fiber matt was characterized through dynamic mechanical analysis, and it proved that post-cure was necessary to ensure complete cure and that the stiffness increased several magnitudes after glass fiber reinforcement. (RDC 10/11/2011)
Resin flowing analysis in sandwich laminates under VARTM process
(533-545) Journal of Reinforced Plastics and Composites 30 #6 (2011)
Abstract
Jhan, Lee and Chung National Taiwan University and the United Ship Design & Development Center, Taiwan did sandwich structure infusion experiments under the VARTM process, and defined four resin saturation stages inside the sandwich assembly by observing and explaining the nonlinear experimental flowing fronts. This study also discussed the race-tracking phenomenon of the sandwich structure in detail. This research executed infusion simulations in the 3D sandwich model, including fiber layers and grooves replaced by the circular pipe elements, to compare with the experimental results. Realization of the permeant characteristics in the sandwich structure establishes a base to ensure complete saturation and to analyze the manufacture of large sandwich structures. (RDC 5/25/2011)
High temperature vacuum assisted resin transfer molding of phenylethynyl terminated imide composites
(52–58)Polymer Composites 32 #1 (2011)
Fu et al of Florida State University, Florida and Air Force Research Laboratory, Ohio used Vacuum Assisted Resin Transfer Molding (VARTM) for molding phenylethynyl terminated imides (PETIs). Two processing methods, in-plane and through-thickness resin flow, were proposed and tested. Both methods are capable of fabricating polyimide matrix composites; and the carbon fiber laminates yield good fiber-resin interfacial bonding and comparable mechanical properties to those laminates fabricated using RTM. (RDC 1/28/2011)