John Marples Made from a quick custom mold, structural angles will serve as step supports on a custom runabout. Flat sheet was cut from 1⁄4″-thick (6mm) G10 plate from Jamestown Distributors.
When boat builders or repairers can’t source a needed composite part or component from a supplier, building them from quick custom molds is a good alternative. Often the obvious choices for custom construction—wood, metal, or sheet plastic—just don’t seem appropriate, especially where visual and material continuity with a FRP or wood/epoxy hull structure is important. Building an efficient composite solution may require creativity to fit the location and function. If it’s hidden under the floorboards, it could be left with a rough finish, but if it’s in plain view, finish and shape need to match the surrounding boat structure. Additionally, when building a quickly molded part in fiberglass, carbon fiber, or any other material, it needs to be made economically. I have found that even when I can source a component from an outside supplier, I can often better their price by building it in my shop.
This article presents a few shop-floor tricks that can yield a cured part on a tight timeline when the boss asks you to “just make it happen.” Let’s start with some essentials of molding small composites parts.
Most molded parts are produced with a finished exterior surface formed against the mold. That means the conventional process starts with a plug—a carved and smoothly finished version of the part—from which a female mold is made. It takes a lot of time to produce a smooth plug and then lay up and cure the mold over it before starting work on the final part. But by using the plug as a male mold, you can save time. This trade-off will yield a part with a rough exterior that must be sanded and prepared for the desired finish, but if the layup is carefully done, the cured exterior surface can be relatively smooth, minimizing finishing time.
I use any inexpensive material for molds that comes easily to hand and provides the shape and finished waxable surface I need to form the part. Wood, foam, and metal are the most common. Because the part’s exterior surface is not formed directly against the male mold, the plug/mold’s surface finish is not critical, so you don’t need multiple coatings of epoxy or paint for that perfect mold finish. If your plug has a simple shape, you can efficiently cover it with aluminum-foil self-adhesive duct tape, which will almost instantly seal and smooth the surface. I have applied this thin aluminum tape over compound curved shapes and burnished it with a wood or plastic tool handle to smooth most wrinkles. Overlaps practically disappear, particularly when wax is added to fill persistent wrinkles.
It’s important to remember that the plug has the same basic requirements whether it is used as a male mold or to make a female mold: It must have draft angles (tapered sides) and no undercuts, so the finished part will easily release. For a laminate layup with fiber cloth, the surface intersections should have generously radiused fillets and rounded exterior corners. Because the layup is easier to apply to convex parts, you should avoid concave hollows if possible. Note that the mold surface can extend beyond the finished part’s dimensions, so you needn’t accurately cut and apply the laminate cloth.
When your mold is ready for use, apply a mold-release wax—standard on any part molded with liquid resins. In conventional composites manufacturing, female molds must be waxed and then sprayed with PVA (polyvinyl alcohol). For our quick molds, you can skip the PVA and just rub the wax in a circular motion onto the mold with a clean cotton cloth. Let it dry for about half an hour at room temperature, longer at colder temps. Buff it lightly with a clean cotton cloth to remove any ridges or wet wax. Repeat the application three more times. Each time the mold is reused, apply a new coat of wax, let it dry, and buff it lightly.
For flat or simple curved surfaces, you can use plastic packing tape as a mold release. It doesn’t require wax and comes in 3″ (76mm) widths.
If the final part is to be adhesively bonded or painted, the wax coating must be removed with a mildly abrasive household cleaner before sanding. Wash it afterward to make sure it is free of water breaks, meaning if water clings to the surface and doesn’t immediately slide off, it is clean enough to proceed with sanding and preparation for painting or bonding.
When assembling layers of fiber laminate, try to saturate them with a minimum amount of resin, without leaving air bubbles or dry areas in the stack. I use a disposable chip brush with about 3⁄8″ (10mm) cut off the working end. The remaining short bristles are stiffer, so you can stipple the layup—tapping with the bristle ends—to encourage bubbles to exit through the surface. If the part is flat, use a squeegee to work out air bubbles and excess resin during layup. Sometimes you can assemble the full laminate layup on a bench surface covered with poly film and then transfer the whole thing to the mold. Be careful to avoid trapping bubbles under the layup.
Determining the optimal laminate thickness for a new part requires a little experience or experimentation, such as making a test part or two. To get started, I often refer to the laminate-thickness chart in Dave Gerr’s book The Elements of Boat Strength (see the sidebar below).
What follows are a few working examples of quick molds, starting with simple ones and progressing to more complex molding methods.
John Marples A length of steel bed-rail angle clamped to square tubing with round corners and resting in custom wood brackets makes a
simple mold.
John Marples A finished part from the plank-on-edge mold.
John Marples A small plank-on-edge mold is vacuum-bagged to compact the epoxy and carbon fiber layup
Flat Fiberglass Sheet
You can buy G10 sheets of fiberglass/epoxy, but you can make your own fiberglass sheet using a piece of window glass or a flat metal surface as a mold. Cover with release wax an area larger than needed. Lay up the laminate to the desired thickness with multiple layers of cloth, coating each with epoxy as you go. This is a simple way to use up cloth scraps, and the new FRP sheet can be cut with a bimetal blade in a jigsaw to fabricate flat components and web material in structural supports for seats, shelves, etc.
Structural Shapes of Fiberglass
The mold in this case can be any commercially available shape of metal angle, channel, or square tubing with rounded corners, or wood. The laminate will be compressed between two shapes to achieve either an angle or a channel shape. Make the layup on polyethylene film; cover it with the same film, and lay it over one mold shape; cover it with a second piece of the same mold shape, and clamp the two lightly to squeeze out excess resin to improve the fiber/resin ratio. When it’s cured, remove the part, pull off the film, and trim the edges. The film will leave a smooth finish on both sides, so sanding is necessary before painting or bonding. You can shorten the curing time by adding some heat or setting the clamped assembly out in the hot sun. For increasing temperature in the shop, set a big cardboard box upside down on the table with an incandescent light bulb in it. For more heat, cover with the box with a blanket and plastic. Use a thermometer: 80°F to 90°F (26.7°C to 32.2°C) seems like plenty to speed up cure times, and won’t burn down the shop.
Boatbuilder Russell Brown is an expert at making small molded parts (see “Carbon from the Coop,” Professional BoatBuilder No. 175, page 92). To make a channel, he laminates cloth over a simple plank edge with eased corners. The plank is smooth coated with epoxy, and waxed. He vacuum-bags the layup and adds heat to speed the cure. The parts shown were used for a carbon fiber masthead assembly for his Gougeon 32 catamaran.
Making Sandwich Beams, Straight or Bowed
There are a couple of ways to laminate a sandwich beam with FRP outer layers to add strength and reduce deflection under load. The sandwich could have a wood or foam core, laminated to the required curve, and the FRP added afterward. That is fairly straightforward, and no prepared mold or fixture is required.
Bow-shaped structures, on the other hand, have more curvature—sometimes a full 180° arch—and tend to be smaller in section. The aircraft windshield bow shown here is a good example. I made it with a carbon unidirectional and cloth layup in epoxy for the skins. The molding surface was a pair of plywood arches laid flat and screwed to the table, leaving a trough between them. I coated the ply edges with epoxy to a smooth finish and waxed them, and then covered the table with aluminum-foil duct tape and waxed it. Each skin—eight layers of 1″-wide (25mm) carbon tow with two layers of carbon cloth—was a separate, long, and narrow layup on polyethylene film. I removed each one from the film and placed it wet against one side of the mold. I disassembled the mold beforehand, so I could lay the laminate on each surface and smooth it before setting the mold back in place on the table to let it cure. The laminates should not slump into the trough. When both skins were applied and cured, I dammed the ends of the trough with sealant putty (Mortite caulking cord). I poured the core into the trough to fill up the remaining volume, using epoxy with WEST System high-density filler mixed to a consistency of pancake batter. This trough was 3⁄4″ (19mm) deep and about 5⁄8″ (16mm) wide, but this same method would also work for a wide range of dimensions.
I removed the mold to free the cured arch. The inner and outer edges on the mold and the side against the table were smooth. I trimmed the rough upper surface to thickness on the tablesaw and sanded off any remaining rough edges. The thin custom bow-shaped support was incredibly stiff and very light compared to other methods of construction.
Functionally, the bow required a bevel on the outer side to match the windshield it would support. That could have been molded in if I had taken more time to build the mold, but the bevel changed from top to bottom and was easy to shape in place with a sander. The exact bevel could also be checked for accuracy with the part in place. I molded in the shape of the attachment surface where it overlapped the fuselage skin. The attachment bolt holes were drilled through the laminate in place for the correct fit.
John Marples The bow-shaped molding channel between carbon fiber outer skins is dammed with putty at the ends, and thickened epoxy core filling is added.
Fiberglass Tubes
You can buy FRP tubes of course, but often the diameter isn’t quite right. Luckily, making your own limber hole liners or stern tubes for power vessels doesn’t take long
You will need a mandrel—a dowel, a plastic pipe, or a tube of any type (even heavy cardboard)—at least 4″ (102mm) longer than the tube you want to build. Next, you’ll need to support the mandrel near the ends, about 1″ above the bench top. I suggest wood V-blocks screwed to the bench. The layup will be a wetted strip of cloth, as wide as the desired tube is long, wrapped tightly around the mandrel. The thickness desired will be a function of the weight of the cloth and the number of turns. Each layer will be about three times the diameter of the mandrel, and a little more as the layers accumulate. (This is a good place to use up long scraps.) The number of wraps is equal to the desired thickness divided by the thickness of each individual layer (from Table 1 in the sidebar).
Here’s how to prep: Cut the cloth’s width and length, and lay it on a piece of film. Next, tape a piece of film to one end, about 10 times the diameter of the mandrel and 2″ (51mm) wider than the cloth. Position the masking tape under the cloth and film with about 1⁄4″ onto the cloth edge along the full width of the cloth. Masking-tape the other end of this film, centered, to the mandrel at the ends only so you can cut off the tape and film after the layup has cured. You now have a mandrel, with film leader and cloth ready to roll up.
The layup procedure is quick. Roll the film leader onto the mandrel and gently start wetting the cloth by painting on a light coating of epoxy as you go. Continue until all the cloth is wrapped on around the mandrel. Pull the cloth lightly to even the end and compress the laminate. Wipe off the extra resin. You could be done here and just let it cure, but if you want a better finish and a smoother tube, rotate the layup while holding a plastic squeegee against it with light pressure. That will compact the cloth and squeeze out some more resin. To truly compact the layup, cut a long strip of film about 1″ wide and spirally wrap the layup from one end to the other, overlapping the edges slightly. Secure the ends with masking tape and pull lightly on the film strip as you wrap it on. It will squeeze out some resin and smooth the exterior of the finished part. To speed the cure, put the part out in the sun or add a little heat as described above.
To remove the part from the mandrel, cut off the taped-on film leader at the mandrel ends. Then twist the mandrel to tighten the leader and slowly pull the mandrel out while rotating. The film leader is free from the mandrel because the masking tape has been cut off, and that should aid sliding the mandrel out.
If you’re using plastic pipe for a mandrel, run the pipe lengthwise through the tablesaw. Cut a strip of wood about the kerf width, place it in the slot, and cover it with masking tape to hold it in place. After the layup is cured, pull out the wood strip, twist the pipe to collapse its diameter, and pull out the pipe.
Brown uses a slightly different method. His mandrel is a piece of aluminum or copper tube—steel does not work well here. The mandrel must be very smooth and without dents or dings. It is waxed in the standard manner before using it as a mold. He stretches out a piece of poly film on the table, held in place with masking tape, then carefully wets out a piece of fiber cloth with resin and roll-wraps it onto the waxed mandrel. The wrap is started by placing the mandrel across the wet cloth near the end and then lifting the cloth edge with a putty knife onto the mandrel. From there, the mandrel is rolled over the cloth to collect it. He cures the wrap-up at about 100°F (38°C) in a hot box. After curing and cooling, the tube slides off because the cooled mandrel shrinks more than the tube. The thermal coefficient of expansion of a metallic mandrel is key to its successful removal, which is why aluminum or copper is best for this application.
John Marples Dry glass laminate is ready to be wet out as it is rolled onto a plastic pipe mandrel protected by a layer of polyethylene film to aid in release
John Marples More poly film is spiral-wound over the layers of wet laminate to yield a smoother finish
John Marples Consistent-quality fiberglass tubing can be molded over metal mandrels, which must be kept smooth, without dents or dings.
John Marples Rolling wet cloth onto a metal mandrel.
Metal Molds
Sheet metal makes a great mold surface because it is nonporous and smooth, requiring only waxing for mold release. You can bend it into any desired developable surface shape and hold it in position for use as a mold. I made 54″-diameter, 31′-long (1.4m, 9.45m) cylinders for a Flettner rotor experiment. The rotors were positioned vertically on a mast and designed to spin at up to 300 rpm, so the shape had to be accurate to avoid vibration. They also needed to be as light as possible to limit weight aloft.
Rather than attempting to build them as a single piece, our team made quarter sections and fastened them together in the same mold used to make the pieces. The mold comprised 3⁄4″-particleboard forms—transverse sections with the radius cut accurately into one edge. They supported a 0.040″-thick (1mm) sheet aluminum mold surface pushed down into the curve and pop-riveted to aluminum square-tube stringers at the top edges. I butt-joined the metal sheets where they were supported by the forms. Then I taped the joints with aluminum-foil duct tape to make them airtight. One side of the mold surface had another strip of metal attached flat to create a “joggle” for attaching its neighbor, leaving the exterior surface lap joint perfectly flush. The mold was quick to build because all the forms were identical, but our team spent considerable time to accurately align and position the forms to create a straight cylinder before gluing each particleboard form to the cement floor with Bondo.
John Marples A cylinder mold section of the foam-cored carbon fiber Flettner rotor part after first layup with the vacuum bag removed.
John Marples The ultralight skin sections were bonded together in the mold, and bulkheads were added to ensure cylindrical accuracy.
The laminate schedule consisted of two layers of carbon fiber cloth on the exterior, 1⁄2″ kerf-cut Divinycell foam core, followed by another layer of carbon cloth on the interior. The layup was a two-step process. First, the exterior layers were saturated and the foam core added. The foam core was prepared with beveled edges and accurately positioned over a coating of “micro” (epoxy mixed with phenolic microballoons into a paintable slurry). To compress the laminate, the layup was vacuum-bagged to pull out the air bubbles and ensure that the core was seated on the skin laminate. The vacuum was maintained during the cure. Second, the final interior laminate of carbon cloth was added in a wet layup without vacuum. The result was a very light (17 lbs/7.7 kg per 31′ quarter section) and stiff partial cylinder. After joining the section panels and adding interior bulkheads to mount bearings, I spun the cylinder horizontally on a test setup and measured the surface run-out (deviation from a perfect cylinder) to be less than 1⁄8″ (3mm). The molding system was accurate considering how little time it took.
John Marples The finished rotors operate on the 34′ (10.36m) trimaran Cloudia for a demonstration in the Discovery Channel series Project Earth.
Contoured Molds
The most common contoured mold shape is a blister, or bump, designed to add clearance between moving parts. For instance, after a repower, you might find a new alternator bracket extends past the confines of the original engine cover. Rather than building a larger cover, you can add a blister to solve the problem. Also, a blister shape can be cut to make an open cowl vent or scupper cover.
John Marples Fiberglass parts formed on an air-intake-vent mold are combined with tubes to create a custom cabin vent system.
There are two ways to build blisters quickly. The first is to make a hole in a flat surface—hardboard, metal, or wood—the size of the blister; the flat surface and edges of the hole mold will need to be waxed in the standard way. Drape the wetted cloth over the hole, letting it sag into a natural blister shape. Lightly push the edges of the cloth toward the hole to increase the depth. When you have the desired shape, let it cure, adding more cloth to build thickness later.
The second method is to create a male mold on a flat or contoured surface. The bump can be shaped from soft wood or foam and glued in place. With wood putty or Bondo, sculpt radius corners where the shape meets the bottom surface. After sanding the contours smooth, cover the form with aluminum duct tape and burnish the surface smooth. Foam molds should be coated with epoxy and then sanded smooth. Apply the standard wax coating, and it’s ready to use as a mold.
Most lightweight fiberglass cloths—6 oz/sq yd–10 oz/sq yd (200 g/m2–340 g/m2)—will conform to contours fairly well. As a rule, the tighter the radius in your mold, the lighter the cloth should be. Experiment to find the best weight to get the desired thickness with the least layers of cloth. Typically, plain-weave cloths work well, but for large layups with large contours (hemispherical shapes), twill weaves arranged on the bias will drape better because their weave structure allows more bias stretch.
John Marples Finished halves of the molded aircraft wheel fairings are fit with temporary brackets while on the plug. In this case, the bracket pairs are fastened with aircraft Clecos, not bolts.
Closed-Shape Parts
A closed shape, like a bottle or a sphere, can be molded over a simple plug in two or more layups. The trick is reassembling all the parts after molding them separately without enclosing the plug. My most recent example of a closed shape mold is a pair of small-plane wheel fairings. In this scenario, the finished part had to be formed from multiple layups, each limited to a portion with enough draft for the plug to be removed when the laminate is cured. I started with an estimate of where the dividing line should be and drew a dark parting line on the plug. Next, I positioned the waxed plug to allow draping the cloth down around the perimeter of the area to receive the layup. After the layup cured, I drew the parting line onto the part before removing it.
If the draft angles on the part are near zero, you may want to use compressed air to help remove it from the mold. To do this, drill a small hole at the center of the cured part, but try to avoid drilling into the plug. Inject air through a hand nozzle into the hole to dislodge the plug. Slipping a thin piece of sheet metal between the part and the plug around the edges of the layup may help too. Once the part is free, trim it to the parting line.
John Marples The wheel-fairing plug is ready for molding the forward half. The parting line drawn on the plug ensures that the draft angles will allow the cured part’s removal from the plug.
Reposition the plug to lay up the second piece (or half) of the part. Repeat the layup as before for all parts. Then trim both parts to the parting lines, and temporarily return them to the plug to see how they fit. If there’s a small gap between the two parts, no problem, but if the parts touch, trim the edges until there is a slight gap. Because these part sections will be glued together by adding reinforcing tape over the joint, the area on either side should be cleaned and sanded. While the parts are in position on the plug, add a few bolt brackets in pairs along the joint area, glued with 5-minute epoxy to the part on either side. Bond them across the joint with a bolt in place.
Now remove the plug and reassemble the parts with bolts in the brackets. If you have access to the interior, apply a wetted reinforcing tape over the joint. If you can’t get to the inside joint, place lengths of reinforcing tape between the brackets on the exterior, and let them cure. Remove the bolts and knock the brackets off with a light tap of a hammer. At this point, the assembly should be in one piece but lacking a full layup over the joint. Add more tape over the joint, staggering the edges, and then add fairing filler before sanding and painting.
John Marples The finished pair of aircraft wheel fairings shows the added blister to cover the disc brake calipers on the inboard side
In-Place Fairings
Faired joints from hulls to appendages are common on boats and aircraft. Putting a cove radius between hull and keel or wing and fuselage is an effective way to reduce drag. Fairings of this type can be quickly made in place by molding potter’s clay without letting it dry.
First, cover the area around the molding with masking tape, and apply a standard wax coating. Before adding clay, cut a layer of fiber cloth to approximate size and prepare to begin the layup. Gather all the materials, and add protective masking to any areas in the drip zone.
John Marples This custom clay fin fairing mold, surrounded by tape to protect the fuselage, is ready for the wet cloth laminate.
Apply small amounts of clay to the fairing area, and smooth it with your fingers, dipping them in water as needed. Any time spent now to make the fairing curves smooth will save a lot of effort later by not having to add fairing compounds or sanding the cured molding.
Once you are satisfied with the shape, mix the resin and wet a single cloth layer on poly film on the bench. Carefully transfer the layup onto the wet clay, and work out the bubbles with a brush. Try not to dent the clay by pushing too hard. Let the laminate cure before adding more layers of cloth to achieve the desired thickness.
John Marples The finished fin fairing is installed.
After the final curing, pry up the edges and remove the molding. The clay will likely stick to the inside and have to be removed with a chisel. Finally, the molding can be washed, trimmed, finished, and fastened in place.
About the Author: John Marples has designed, built, and rigged many sailing vessels. His portfolio includes dozens of wood-epoxy composite sailing and power multihulls to 110‘ (33.5m). He operates Marples Marine, a multihull design and engineering firm in Penobscot, Maine.
Determining Laminate Thickness
[T]he total laminate thickness of liquid resins and woven fiber cloth is determined by the weight of the cloth in ounces per square yard (imperial) or grams per square meter (metric) and the number of layers. In his book The Elements of Boat Strength, Dave Gerr provides an easy chart (Table 1) for this purpose. These estimates might vary slightly for different layup methods. A resin-rich wet layup will be thicker than a vacuum-bagged, resin-starved layup of the same total weight of cloth. Use the chart as a guide to get you in the ballpark.
Russell Brown adds this simple calculation for determining thickness in metric. He says it is amazingly accurate when vacuum-bagging: 100 g/m2 of cloth equals 0.1mm of thickness. Example: 5 layers of 200-g/m2 (6-oz/sq-yd) cloth equal 1mm of thickness.
John Marples 
