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Last Update: 26 December 2005

Mold Construction

This is a brief description of the steps involved in making a simple fiberglass mold, preparing it for use, and maintaining it in good condition. I will stick strictly to the KISS principle. This method is suitable for a backyard builder or small fiberglass shop. It will produce a mold that should last for a hundred or more parts if handled carefully. Professional molds for industrial production are much more complex and many times more expensive.

The focus is on molding a decked canoe or kayak. Some of the details will be different for molding an open canoe, or anything else that would be a one-piece mold, but the basic procedures are identical. In fact, the basic procedures are sound for molding anything from a thimble to a yacht but care must be taken to scale up or scale down the reinforcement materials used to suit the dimensions of the project. The products mentioned below are those that I have used satisfactorily in my own work and which we sell. Other products may be equally satisfactory.

Plug Preparation

The plug is an exact model of the boat to be built. It can be made of almost anything that can be brought to a smooth finish, but most commonly it is fiberglass, with modifications done with automotive body putty.   [Construction of the plug from scratch is beyond the scope of this article as it could be done by any of a dozen or more widely-differing methods.]   It should be puttied and sanded until the shape is correct, all defects are removed, and it is smooth to 220 grit. If the dimensions of the finished product are critical, such as for racing boats, it is necessary to design the plug slightly oversize to allow for the shrinkage of the resin as it cures. This is a small effect,less than 1%, but it occurs twice, once when the mold is built and again when a boat is built in the mold. The degree of perfection of the plug will be directly transferred to the mold and thence to the finished boat. Since body putty is slightly porous, which can create some release problems, the plug should be sprayed with a fast-build automotive primer-surfacer. It should then be wet sanded with progressively finer paper to 600 grit, repainting as necessary to cover exposed putty. Small areas of exposed putty, such as on sharp radii, can be tolerated.

The plug should now be machine buffed with TR-311, heavy-duty cleaner, and allowed to sit overnight for solvent evaporation. This removes the 600 grit scratches, leaving a highly polished surface. Next it should be machine buffed with TR-301 sealer-glaze to seal the surface and provide a base for the wax. This should also be allowed to sit overnight for solvent evaporation. Be sure to use separate buffing pads for the two operations!

Next apply five coats of TR-108 basic mold wax and hand buff. This wax is different from most mold waxes in that it should not be allowed to dry before buffing. Start at one end of the plug and apply a thin, uniform coat, working toward the far end. You should be able to cover a typical canoe or kayak plug in 5 to 10 minutes. After about that amount of time, go back to where you began and start buffing with a soft cotton cloth, such as terry cloth or a piece of a cotton [not polyester] sweatshirt. Since the wax is rather sticky, this requires some elbow grease and it also gums up the cloth. Go over it three or more times, using a fresh cloth each time, until the surface is uniformly shiny. Allow it to stand for several hours for solvent evaporation and hardening before applying the next coat (five coats in all). Allow the plug to sit overnight after the final coat. This is a deep-penetrating wax, which is especially recommended for new surfaces.

If the mold is to be built in two halves, as for typical decked boats, the plug must be fitted with a temporary flange to make the parting plane. There are many ways to do this. A “quick-and-dirty” method is to hot-glue strips of Minicel® along the parting plane, and cover them with PVC tape to make the flange surface. The flange is usually glued to the hull, so the deck is molded first.

Once the flange is in place, spray the deck of the plug with PVA mold release and allow to dry thoroughly. Be certain to wear a respirator for this as well as for all operations using gelcoat or resin. Application of PVA is a black art at best, with innumerable things that can go wrong. Be sure that the plug is clean, well polished, and that the wax has cured for at least six hours. PVA does not go on well in cold conditions, so maintain the materials and work area at 70°F [21°C] or greater. If dilution seems necessary a little water may be added. It is best to use a high-quality paint gun with a small orifice. Use at least 90 psi [620 KPa] air pressure at the gun and close the needle about halfway. Apply a light mist coat first and follow with a heavier coat just sufficient for the droplets to flow together to form a continuous film. Such a film should be 0.005 to 0.010 mm thick. One quart will cover a typical kayak mold. Drying time averages a half hour, but varies greatly with temperature, humidity, and air flow. For more detailed information, Rexco data sheets for Partall® #10 and for the newer Coverall® are available on line. They also have a trouble-shooting guide for use of these products. These documents recommend the use of Rexco Partall® Paste #2 prior to applying the PVA. We have found that TR-108 mold wax works at least as well, perhaps better.

Mold Construction

1.   Apply a layer of tooling gelcoat approximately 0.5 mm thick. It is possible to brush this on, but vastly superior to spray it. A special gelcoat gun must be used, not a paint sprayer. The simplest form is a cup gun, which requires minimal cleanup since the gelcoat is contained in a paper cup which can be discarded after use. We use a clear NPG gelcoat that should be catalyzed with not more than 2% MEKP. Excessive MEKP could cause premature gelation to occur. Before adding catalyst the gelcoat should be warmed [if it is cool] to at least 70-75°F [21-24°C] and stirred thoroughly, preferably with an electric mixer, to bring it to a uniform consistency. If it seems too viscous warm it and stir it some more, but do not dilute it. It must be applied uniformly, as thin areas will wrinkle (“alligator”) when a second gelcoat, or laminating resin, is applied. Also, it is important to allow the gelcoat to cure before proceeding, as inadequately cured gelcoat will alligator. On the other hand, too long an interval will result in a weaker interlaminar bond between the gelcoat and the next layer. Cure cycles can range from one to eight hours, depending upon the shop temperature and other factors. Gelcoating is a technically demanding job. A good gelcoat is essential for the construction of a quality mold.

2.   After the gelcoat is adequately cured, the corner of the flange (where parts will be trimmed) should be reinforced. This is done with milled glass fibers or strips of glass mat worked tightly into the corner where the flange and plug meet. If a second gelcoat is desired, it can be applied immediately, without waiting for the reinforcement to cure. While it is not essential, I recommend a second gelcoat on a production mold.

3.   Next the back-up laminate is applied. For the first layer I use a fine-weave 4 or 6 oz glass cloth (1522 or 3733), carefully saturated to obtain a high glass content, and allowed to cure. Standard polyester or vinylester resin works adequately for this job but special tooling resin will have less shrinkage. A layer of light-weight mat could be used, but it is harder to achieve good saturation without voids. If index marks or documentation in the mold are desired, this is the time to apply them. At minimum, it is very convenient to have the center lines of the boat marked. I recommend use of black ¼" wide polyester twill tape, available from a sewing store, for this purpose.

4.   The remainder of the back-up laminate is now applied. This should consist of at least 3 oz of mat over the surface of the mold, and at least 5 oz on the flange for extra stiffness. The shape of the mold may dictate more stiffness for certain areas. It is best to apply this material in two steps with a cure cycle between to minimize heat buildup during cure. All layers should be trimmed off around the flange as they gel.

5.   It is now time to flip the whole thing over and mold the other half. First, carefully remove the flange without allowing the newly-created mold to release from the plug. Wax the new flange, and the edge of the plug where the temporary flange had been attached. A single coat should be sufficient. Spray PVA and go through steps 1 - 4 for the second half of the mold.

6.   While the mold could now be considered done, it is best to apply a supporting structure to further stiffen the mold and to support it during use. Plywood cross-sections are typically used. Extra strips of mat should be applied across the mold where these are to be attached to stiffen it in order to reduce print-through of the supports to the mold surface. The plywood should also be spaced away from the skin of the mold with strips of Minicel to further reduce print-through. It is worth the extra effort to cut and apply these supports plumb and true so the mold parts will be stable and not wobble or distort while in use. If the mold is to be car-topped at any time, it is nice to laminate in rope endloops for tiedowns. Drill holes in the flange so the mold can be bolted back together for transportation and storage. Now the two parts of the mold can be released from the plug. Either before or after this step the mold should be set aside in a warm place for a week or two for post curing (final hardening). Be sure it is firmly bolted for this step. Getting in a hurry and omitting post cure is one of the main causes of stuck parts on first use.

Finishing & Break-in

After release from the plug, wash the PVA out of the mold and inspect the surface. Sand out all imperfections using the finest wet sandpaper that will be effective, and slightly round the corner of the flange. 220 grit is usually needed for a few spots, as well as the flange. Work through the finer grits until the whole surface has been treated with 600. Now go through the same protocol with TR-311, TR-301, wax, and PVA as described under plug preparation. The mold is now ready for layup of a boat. However, it is better to first layup a “splash layer” — a layer or two of cheap material that will be pulled out and discarded after curing. This serves to help “season” the mold and also to assure that the laminate will not stick in the mold. One will feel much more comfortable doing an expensive layup with Kevlar or graphite if one has first tested the release system with a cheap throwaway laminate.

Between boats the mold should be waxed with one coat of TR-108. Allow at least six hours for the wax to harden, then apply PVA. After five boats have been laid up, and if there have been no release problems, the use of PVA can be omitted. At this point, TR-102 (regular) mold wax may be used between boats. This wax should be allowed to dry before buffing and is considerably easier to buff out than is the TR-108.

Mold Maintenance

In normal use the mold will be simply wiped with a clean, dry cloth and rewaxed between parts. After several parts, the surface may become hazy, particularly in tight-radius areas. When this happens, machine buff the mold with TR-502 wax-buildup remover and rewax with two or three coats of either mold wax. TR-502 does not break the surface seal, so it is not necessary to repeat the finishing process. However, the first part after this process will usually have a harder-than-normal release.

If the mold surface has been allowed to deteriorate to where TR-502 does not restore it, or if it is scratched or otherwise damaged, it is necessary to start over. Sand out the scratches, and go through the cleaning, sealing, and break-in protocol again. Anytime the sealer is removed it is necessary to treat the mold as if it was new, though it should be possible to go through less than five boat layups before omitting the PVA.

John R. Sweet, Mustoe, VA

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