Open molding

Open contact molding in one-sided molds is a low-cost, common process for making fiberglass composite products. Typically used for boat hulls and decks, RV components, truck cabs and fenders, spas, bathtubs, shower stalls and other relatively large, noncomplex shapes, open molding involves either sprayup or hand layup. In an open-mold sprayup application, the mold is first treated with mold release. If a gel coat is used, it is typically sprayed into the mold after the mold release has been applied. The gel coat then is cured and the mold is ready for fabrication to begin. In the sprayup process, catalyzed resin (viscosity from 500 cps to 1,000 cps) and glass fiber are sprayed into the mold using a chopper gun, which chops continuous fiber into short lengths, then blows the short fibers directly into the sprayed resin stream so that both materials are applied simultaneously. To reduce VOC emissions, piston pump-activated, non-atomizing spray guns and fluid impingement spray heads dispense gel coats and resins in larger droplets at low pressure. Another option is roller impregnators, which pump resin into a roller similar to a paint roller.

In the final steps of the sprayup process, workers compact the laminate by hand with rollers. Wood, foam or other core material may then be added, and a second sprayup layer imbeds the core between the laminate skins. The part is then cured, cooled and removed from the reusable mold.

Hand layup and sprayup methods are often used in tandem to reduce labor. For example, fabric might first be placed in an area exposed to high stress; then, a spray gun might be used to apply chopped glass and resin to build up the rest of the laminate. Balsa or foam cores may be inserted between the laminate layers in either process. Typical glass fiber volume is 15 percent with sprayup and 25 percent with hand layup.