Note: Descriptions are shown in the official language in which they were submitted.
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METHOD OF MANUFACTURING A BOAT HULL
BACKGROUND OF THE INVENTION
The invention relates to the manufacturing of boat
hulls, and more particularly to the manufacturing of fiber-
reinforced polymer or fiber-reinforced plastic boat hulls.
The spray-up technique, lay-up technique, or a
combination of the two is the most common method of manufacturing
fiber-reinforced polymer (FRP) boat hulls. In both techniques, a
mold release agent (wax) is applied to an open, single-sided,
female mold, and then a layer of gelcoat, which becomes the outer
surface of the hull, is sprayed onto the mold. Typical gelcoat
thicknesses are from 0.4mm to 0.8mm. After the gelcoat has
hardened, a layer ("skincoat") of chopped glass fiber and
polyester resin with catalyst is sprayed onto the gelcoat with a
thickness of approximately 2mm. The skincoat is allowed to cure
before the remaining layers (the bulk laminate) of the hull are
applied. The purpose of the skincoat is to protect the gelcoat
from the high temperatures which may result from the exothermic
reaction of the resin in the bulk layer as the resin cures. The
lay-up technique involves placing glass mat or other reinforcing
materials in the mold and saturating the reinforcement material
with a mixture of liquid polymer resin and catalyst. In the
spray-up technique, chopped glass fibers, liquid resin and the
catalyst are sprayed into the mold. In both methods, air and
excess resin are manually forced out of the reinforcing material
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using hand-held rollers. Layers are added and allowed to cure as
needed to build the hull to the desired thickness. Finally,
wooden stringers are placed in the hull in the mold and
additional layers are added over the stringers to secure the
stringers to the FRP shell.
The resin most commonly used is polyester. Polyester
resins used in boat building may contain styrene monomer in
varying amounts. The styrene monomer reduces the viscosity of
the liquid polyester resin, improving its flowability, and the
styrene also cross-links with the polyester as the resin cures,
resulting in a strong final product. Disadvantageously, styrene
monomer is quite volatile and is an air pollutant. The spray-up
and lay-up techniques result in significant styrene emissions.
It is also known to manufacture a boat hull without
stringers by resin transfer molding. See, for example, U.S.
Patent No. 4,902,215. In resin transfer molding (RTM), dry
reinforcement material is placed in a molding cavity defined by
one or more mold surfaces, and liquid resin is then injected into
the cavity to form the FRP product. See U.S. Patent No.
4,762,740, which is incorporated herein by reference, for further
explanation of conventional resin transfer molding. In one type
of resin transfer molding, one of the mold halves is a flexible
bag or sheet known as a vacuum bag.
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SUMMARY OF THE INVENTION
The invention provides an improved method for
manufacturing an FRP boat hull. This method requires less labor
and less material (particularly resin), is faster, and results in
less pollution than conventional spray-up or lay-up techniques.
This method also provides a stronger hull than is provided by
known RTM techniques for manufacturing boat hulls. The invention
also provides an improved method for making a vacuum bag for
manufacturing a boat hull in an RTM process.
Specifically, the method of the invention is for
manufacturing a boat hull including fiber-reinforced polymer and
a plurality of stringers embedded in the polymer. First, a
female mold is manufactured. The mold has a peripheral flange
and an inner surface having the shape of the outer surface of the
hull to be manufactured. Next, the mold inner surface is coated
with a release agent, a gelcoat and a skincoat if desired. After
the gelcoat and skincoat are cured, dry reinforcement material,
such as various types of strand mat or woven roving, is placed in
the mold. Stringers, with additional dry reinforcement material
(preferably glass mat) thereon, are placed on the dry
reinforcement material and a vacuum bag is placed over the
stringers and the reinforcement material. In order to prevent
resin from pooling beneath the stringers, a material such as
Coremat is placed between the stringers and the dry reinforcement
material.
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The vacuum bag has a peripheral flange which is sealed
to the peripheral flange of the mold so as to provide a manifold
or plenum in which a vacuum can be drawn around the entire
perimeter of the mold cavity or space between the mold and the
vacuum bag. The plenum is preferably formed by providing inner
and outer endless seals at the peripheral flanges. Bleeder cloth
can be placed between the vacuum bag and the mold and extending
from the mold cavity into the plenum. The bleeder cloth prevents
the vacuum bag from sealing against the perimeter of the mold and
assures that the entire mold cavity communicates with the plenum.
A vacuum is then drawn in the mold cavity by evacuating the
plenum, and catalyzed resin is injected into the mold cavity
through ports in the vacuum bag. The ports are closed off at all
times, except when resin is being injected, to maintain a vacuum
in the mold cavity. The vacuum is maintained until the resin has
cured.
The mold used in this process is preferably identical
to a mold that would be used in the conventional spray-up or lay-
up process except that the peripheral flange is significantly
wider in order to permit formation of the plenum.
The vacuum bag is reusable with a minimum of
maintenance. The bag is preferably made of reinforced clear
silicone rubber. The bag is fabricated to conform to the shape
of the inner surface of the boat hull including the stringers and
any other components that may be molded into the hull, such as
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fuel tank mounting plates or a nose block. It is important that
the bag be flexible and elastic. It must also be compatible with
the resin used in the molding process (in both the liquid and
solid states), and the bag must not adhere to the cured resin if
it is to be reusable. The bag is preferably made with silicone
RTV caulking material. This facilitates repairing the bag
because silicone bonds very well to silicone. A stretchable
reinforcing material, such as polyester fishnet fabric, is
preferably embedded in the silicone to increase tear resistance.
To make the bag, a complete hull including stringers
and any other components is built in the mold. The hull can be
made by the conventional spray-up or lay-up processes. The inner
surface of the hull is finished to make it relatively smooth and
free of sharp edges. The bag is made by spreading layers of the
caulking material on the inside of the hull while the hull is
still in the mold. To aid in spreading the silicone, a solvent
such as naphtha, toluene or methylene hydrate is used to thin the
silicone until a "paintable~ consistency is achieved. Several
layers of the silicone mixture are "painted~ onto the inner
surface of the hull, with time allowed between coats to let the
silicone cure. The layer of reinforcing material is then laid
over the silicone, and several more layers of silicone mixture
are applied. The clear silicone and fishnet fabric result in a
relatively transparent bag that allows resin flow to be observed
during the RTM process. Injection ports are either added after
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the foregoing or molded in with the bag. Any suitable method can
be used to form the injection ports.
Other features and advantages of the invention will
become apparent to those skilled in the art upon review of the
following detailed description, claims and drawings.
DESCRIPTION OF THE DRAWINGS
Fig. 1 is an exploded perspective view of apparatus
used in performing the method of the invention.
Fig. 2 is a top plan view of the apparatus shown in
Fig. 1.
Fig. 3 is a view taken along line 3-3 in Fig. 2.
Fig. 4 is an enlarged portion of Fig. 2.
Fig. 5 is a view taken along line 5-5 in Fig. 4.
Fig. 6 is an enlarged portion of Fig. 3.
Fig. 7 is a partial perspective view of the vacuum bag.
Before one embodiment of the invention is explained in
detail, it is to be understood that the invention is not limited
in its application to the details of the construction and the
arrangements of components set forth in the following description
or illustrated in the drawings. The invention is capable of
other embodiments and of being practiced or being carried out in
various ways. Also, it is to be understood that the phraseology
and terminology used herein is for the purpose of description and
should not be regarded as limiting.
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DESCRIPTION OF THE PREFERRED EMBODIMENT
Apparatus 10 for performing the method of the invention
is illustrated in the drawings. The apparatus 10 includes (see
Fig. 1) a female mold 14 having a periphery 18 and peripheral
flange 22. The mold 14 is the same as would be used in a
conventional spray-up or lay-up process except that, whereas the
flange around the perimeter of a conventional mold is typically
50 or 75mm wide, the peripheral flange 22 of the mold 14 is
approximately 125 to 15Omm wide. The mold 14 has an inner
surface 26 having the shape of the outer surface of the hull to
be manufactured.
The apparatus 10 also includes a male mold 30 having a
periphery 32 and a peripheral flange 34. The male mold 30 is
preferably a flexible vacuum bag 38 made of silicone rubber in a
manner described below. The bag 38 is partially illustrated in
Fig. 7. As shown in Fig. 5, the peripheral flange 34 of the
vacuum bag 38 is preferably less than half as wide as the flange
22 of the mold 14. The vacuum bag 38 has an inner surface 42
(the lower surface in Fig. 1) having the shape of the inner
surface of the hull to manufactured.
The bag 38 is preferably made with clear silicone RTV
caulking material. A suitable silicone is available from General
Electric Corporation and is sold as RTV 700 with a beta 2 cure
and 910 diluent. To make the bag 38, a complete hull (not shown)
including stringers and any other components is built in the mold
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14. The conventional spray-up or lay-up processes can be used to
manufacture the hull used in making the bag 38. The inner
surface of this hull should be finished to make it relatively
smooth. The silicone caulking material is thinned to a paintable
consistency with a solvent such as naphtha, toluene or methylene
hydrate. Roughly three parts naphtha to two parts silicone has
been found to be a good mixture. Several coats (approximately
eight) of the silicone mixture are painted onto the inner surface
of the hull, with the silicone being allowed to cure between
coats. Next, a layer of reinforcing material, such as
stretchable polyester fishnet fabric 46 (Fig. 7), is laid over
the silicone, and then several more layers of silicone mixture
are applied. The resultant reinforced silicone bag 38 (partially
shown in Fig. 7) is relatively transparent. Injection ports 50
(see Fig. 3) can be either molded into the bag 38 while the bag
is being formed or added to the bag 38 later. In theory, a
single injection port 50 in the center of the bag 38 is adequate.
In practice, however, it may be desirable to have several
injection ports 50 at various locations. The optimum number and
location of injection ports 50 depends on the size and shape of
the hull being manufactured and can be determined easily through
experimentation.
To manufacture a boat hull in accordance with the
invention, the mold inner surface 26 is coated with (see Fig. 6)
a release agent 54, a gelcoat 58 and a skincoat (if desired).
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The release agent 54 should be applied as needed. Next, dry
reinforcement material 62 is placed in the mold 14. The dry
reinforcement material 62 can be various types of strand mat,
woven roving, or a combination of these. Any other suitable dry
reinforcement material can be employed. Next, structural
members, such as wooden stringers 66 (see Figs. 3 and 6), are
placed on top of the dry reinforcement material 62. The
stringers 66 are surrounded by additional dry reinforcement
material 70, preferably glass mat. To prevent resin from pooling
beneath the stringers 66 (to prevent excessive shrinkage and
resultant print-through), and as shown in Fig. 6, approximately
three layers of a material 74 such as Coremat, which is available
from E.F. Walters Company of Montreal or Coremat U.S.A., are
placed between the stringers 66 and the dry reinforcement
material 62 in the mold 14. The Coremat 74 can be fixed to the
underside of the stringers 66 before the stringers 66 are placed
in the mold 14. The Coremat 74 prevents resin pooling because
the Coremat does not absorb a significant amount of resin.
The vacuum bag 38 is then placed in the mold 14 over
the stringers 66 and the dry reinforcement material 62, with the
peripheral flange 34 of the vacuum bag 38 partially overlapping
the peripheral flange 22 of the mold 14. A mold space or cavity
containing the dry reinforcement material 62 and the stringers 66
is defined between the vacuum bag 38 and the mold 14.
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As shown in Figs. 3 and 5, the peripheries of the
vacuum bag 38 and the mold 14 are sealed by inner and outer
endless seals 78 and 82 and a sealing member or plate 86 having
the shape of the mold flange 22. The outer seal 82 is located
between the upper surface of the mold flange 22 and the underside
of the sealing member 86, and the inner seal 78 is located
between the upper surface of the vacuum bag flange 34 and the
underside of the sealing member 86. The sealing member 86 is
clamped to the mold flange 22 by clamps 90 so that the seals 78
and 82 are compressed to define an airtight plenum or manifold 94
between the seals 78 and 82 and between the sealing member 86 and
the mold flange 22. As shown in Figs. 2, 4 and 6, strips of
bleeder cloth 98 (preferably Coremat) spaced around the periphery
of the vacuum bag 38 overlap the dry reinforcement material 62
and c~mml]nlcate with the plenum 94 to facilitate air flow from
the mold space to the plenum 94. The plenum 94 comml-nlcates with
a vacuum pump 102 which creates a vacuum in the plenum 94 and
thus in the mold space. A conventional three-quarters horsepower
combined electric motor/vacuum pump capable of twenty-six to
twenty-eight inches Hg has been used. The vacuum pump 102 (see
Fig. 1) must be able to establish and maintain a vacuum under the
vacuum bag 38. The purpose of the vacuum is to remove air from
the dry reinforcement material 62, to hold the dry reinforcement
material 62 in place during resin injection and resin cure, and
to enhance movement of the injected resin throughout the mold
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space. The strips of bleeder cloth 98 around the bag periphery
create a uniform vacuum throughout the mold space.
After a vacuum has been drawn in the mold space, resin
(identified by reference numeral 106 in Fig. 6) is injected into
the injection ports 50, through one port at a time. Any suitable
resin, such as an unsaturated polyester resin, can be employed.
Unused ports 50 are clamped off. Because of the reduced pressure
in the mold space, the resin does not have to be injected under
significant pressure. An injection pressure of up to twenty-
eight inches Hg is sufficient. If the injection ports 50 are
properly located, resin will completely fill the mold space,
saturating the dry reinforcement material 62. If necessary,
hand-held rollers can be rolled over the vacuum bag 38 to promote
resin flow throughout the mold space. Finally, the resin is
allowed to cure, and then the bag 38 is removed. The vacuum is
maintained until the resin has cured.
As mentioned above, this method requires less labor and
less material (particularly resin), is faster, and results in
less pollution (lower styrene emissions) than conventional spray-
up or lay-up techniques.
Various features of the invention are set forth in the
following claims.
