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One of the simplest and most useful molds is a flat, rigid table faced with a smooth plastic laminate (Figure 3-1). This mold is useful for producing flat laminates or panels for bulk- heads, doors, beams, and a wide range of custom structural components. Any portion of the table may be used, and multiple lay-ups of different sizes can be vacuum bagged at one time. 3.2 Curved molds Curved parts can be laminated over male or female molds. A female mold’s surface is gener- ally concave, producing a laminated part with the smooth finish on the convex or outside–a boat hull for example. A male mold generally has a convex mold surface, producing a part with a smooth surface on the concave side–a bathtub or cockpit well. A male mold may also be used to produce a boat hull. An existing hull, for example, can be used as a mold to re- produce a slightly larger version of itself. However, when a part is laminated over a male mold, the rougher bag side of laminate will be the outside of the laminated part (the hull in this case) and will require additional fairing and finishing. A curved mold can be lofted and built in wood or other low density material, with a layer of fiberglass cloth and several coats of epoxy to provide a smooth airtight molding surface. Some parts, because of their shape or size, must be laminated in two separate molds. An open or bowl shaped part, such as a small open boat hull, can be easily pulled from a one piece mold if the opening of the mold is wider than any point on the inside. A closed object, such as an enclosed boat, requires at least two molds. The part is divided at its widest point so that both molds will be wider at the opening than any point inside the mold. A typical small boat is widest at the shear. (The catamaran plug in Figure 3-2 is widest about a foot above the waterline, which is where the deck mold and hull mold are separated). The part will then be laminated in two halves and bonded together after the halves are pulled from the mold and trimmed. Curved molds are often built in a two stage process. In the first stage, a plug or form is built to the exact dimensions and finish of the final object. In some cases an existing object, a hull for example, can be used as the plug. In the second stage, a mold is cast from the plug. In the case of a boat hull, a male plug (essentially a male mold) produces a female mold. To sim- plify construction, the female mold may be built upside down over the top of the plug, then flipped over after it is completed. For all but the simplest of forms, it’s much easier to build, fair and finish a male plug than it is to build, fair and finish a female mold from scratch. 3.3 Building a master plug The plug is an exact, full sized model or pattern of the finished part. A hull plug, for exam- ple, may be lofted and built in much the same way as a one-off hull, with frames, stringers and a skin. It may also be carved free form, using templates or calipers if necessary to trans- fer profiles, establish critical dimensions or keep the plug symmetrical. The strength and durability of the plug should be determined by the number of molds that will be made from it and how long it will have to last. A plug may be used to build many molds for production manufacturing or from time to time replace a damaged or worn out mold. The plug may be altered after molds are made from it to create variations or revisions of a design. Although any number of molds may be cast from a plug, a plug is often used only once. Any material or method of construction is acceptable, as long as the plug is fair, smooth and strong enough to accurately cast the required number of molds from it. Plywood frames and easy to shape materials like cedar or foam will help to reduce the costs and time to build the plug (Figure 3-2). The plug (and mold) should be extended at least 1" past the finished laminate edge to allow for trimming of the laminate. A 6" wide plywood shelf, attached to Vacuum Bagging Molds 11 3PDF Image | Vacuum Bagging Techniques 002150
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