Boat Manufacturing: virtual tour
Open and Closed Molding
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Open Molding
In the open molding process, the surface of the mold is polished
and coated with a wax, which allows easy removal of the finished
product from the mold after curing. Next gel coat, consisting of
unsaturated polyester resin, catalyst, and pigments, is sprayed onto
the waxed mold. This forms the outer, visible surface of the boat.
After the gel coat is fully cured, fiberglass reinforcing material
saturated with catalyzed polyester resin is applied over it. Two
common methods of application are:
- Chopper gun (spray coat) - the resin, catalyst and fiberglass
strand are fed into a gun, and the fiberglass is chopped into
short segments and sprayed with the resin into the mold;
- Hand layup - the resin is catalyzed, then brushed on to layers
of glass mat or roving which have been hand fitted into or over
the mold.
Air bubbles and other imperfections are removed by hand with rollers.
Tools are cleaned periodically of resin, using acetone or a substitute
cleaner before the resin begins to polymerize.
 Styrene
is emitted both during the application stage when gel coat or resin
material is applied to the mold and during the post-application
period when the material cures. Most boat building facilities use
high ventilation rates to ensure that styrene levels are below
the 100-ppm worker exposure limit established by the Occupational
Safety and Health Administration (OSHA). Dilution increases the
volume of contaminated air and, because the cost of an add-on emission
control system is a strong function of the total air flow, these
diluted air streams are more costly to control.
Some facilities designate certain areas for gel coat or resin
spraying to reduce the contamination of plant air. In these cases, a spray booth equipped with a dry filter medium may be used to
reduce particulate emissions, but diluted styrene emissions are
typically vented directly to the atmosphere. Spraying emissions
can be controlled through spray gun pressure calibration, operator
training and overspray containment.
To review, some of the major steps in making open mold-based products
are:
- Mold Preparation
- Mold Waxing
- Resin Preparation
- Gel Coat Application
- Skin Coat
- Fiberglass Application
 Closed Molding
In closed molding, the resin is applied to fabric placed between
the halves of a two-piece mold. Three basic types of closed molding
used in boat manufacturing are resin infusion molding, resin transfer
molding, and compression molding with sheet molding compounds.
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