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Figure 3-5 A closed shape like a sphere or a hull with a molded deck requires two molds separated at the widest point. A 6" wide shelf at the edge of the mold allows the laminate to run beyond its trim line and provides a clean area to seal the bag to the mold. When trimmed, the laminate extension provides a flange around the edge of each laminate half that may be used to bond the two halves together. After the top half mold is completed, the plug and mold are turned upside down. The shelf is removed, and the holes from the drywall screws are filled and faired. The casting process is repeated for the bottom half mold, before the plug and top mold are separated. The top mold’s 6" lip takes the place of the temporary shelf for casting the bottom mold’s lip. 3.4 Building a mold Building a mold over a plug is very similar to laminating a part in a mold. After the plug has been completed, the mold shell is built up in layers, or laminated, over the plug. Hull molds are generally built upside down. A framework is bonded to the completed mold shell to help keep it rigid (Figure 3-6) and to provide legs for level support when it is turned right side up (Figure 3-7). The schedule of materials for a mold shell varies depending on the size of the mold. A typi- cal schedule begins with an epoxy gelcoat to provide a high density surface. One layer of light fiberglass cloth followed by multiple layers of heavier cloth will make an adequate skin for small molds. Larger molds may require additional layers of glass, or a core material and additional layers of glass. The following describes one procedure for building a mold over a plug. This procedure may be modified or other procedures may be used as long as the mold provides an airtight surface that holds the object’s shape until the laminate has cured. Apply two coats of thickened epoxy “gelcoat” to the waxed surface of the plug. Thicken the epoxy to a catsup consistency with 420 Aluminum Powder and 404 High-Density Filler to increase toughness and reduce fisheyeing when coating the waxed plug. This gelcoat layer will be the inside surface of the mold. After the gelcoat layer reaches its initial cure, apply the first cloth layers—4 oz cloth followed by several progressively heavier layers of cloth. Take care to eliminate any air voids in the fiberglass/epoxy layers. When the cloth layers have reached an initial cure, apply a 18"–14" thick layer of epoxy/407 (thickened to a peanut butter consistency) over the cloth and allow it to cure. This thick fairing compound layer Vacuum Bagging Molds 13 Added Framework Top Mold Half Plug Temporary shelf is placed around the plug at its widest dimension (points tangent to a plumb line) Bottom Mold Half Added Framework Shelf is removed after plug is inverted Mold shelf provides area to run out laminate beyond the trim line and seal the bag 3PDF Image | Vacuum Bagging Techniques 002150
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