A closed-mold/pressure injection system  allows for faster gel and cure times compared to open molded/contact molded FRP parts. The process uses many of the same polyester/vinylester resin matrix materials (continuous strand, cloth, woven roving, long fiber and chopped strand) associated with open molding.



 Vacuum Infusion, also called resin infusion, utilizes a vacuum bag to debulk or compact an FRP laminate sequence of fiberglass reinforcements and/or core materials laid in a female mold.  After debulking, a flowable resin is infused by vacuum to completely wetout the fiberglass reinforcements and eliminate all entrapped air voids in the laminate structure.  High quality composite parts up to 6" thick can be constructed using the vacuum infusion process.  Typically, flowable isophthalic, vinylester and/or polyester resins  are used in the vacuum infusion fabrication process.  Since the vacuum infusion process eliminates the need for secondary bonding, a vacuum infused FRP part is stronger, lighter and less expensive than a comparable open molding FRP part.



Vacuum Bag Molding is a refinement of hand lay-up/open molding using a vacuum to eliminate entrapped air and excess resin.  After the lay-up is fabricated using either a male or female mold from precut plies of fiberglass mat and resin, a nonadhering film of polyvinyl alcohol or nylon is placed over the lay-up and sealed at the mold flange. A vacuum is drawn on the bag formed by the film while the composite is cured at room or elevated temperatures. Compared to hand lay-up/open molding fabrication, the vacuum bag method provides higher fiberglass reinforcement concentrations and better adhesion between layers.  With the vacuum bag molding method, the FRP fabricator has greater process control over resin/glass ratios which results in a stronger, lighter fabricated part.



Resin transfer molding (RTM) is a low pressure closed molding process for moderate volume production quantities, filling the gap between the slow, open molding/contact molding processes and the faster, compression molding process, which requires substantially higher tooling costs. Continuous strand mats and woven reinforcements are laid up dry in the bottom mold half. Preformed glass reinforcements are frequently used in the production of complex mold shapes. The mold is closed and clamped, and a low viscosity, flowable catalyzed resin is pumped in, displacing the air through strategically located vents. Metered mixing equipment is required  to control resin/catalyst ratios which are mixed through a motionless/static mixer and injected into the mold port. Common matrix resins include polyester, vinyl ester, epoxy, and phenolics. Advantages over contact molding methods are a uniform thickness and two finished sides.. For optimum surface finish, a gel coat is applied to the mold surface prior to molding.




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