Note: Descriptions are shown in the official language in which they were submitted.
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Method of Making
Fiber Reinforced Articles
Technical Field
This invention relates to a method for making
articles and more specifically relates to a me~hod
for making articles having a fiber reinforred plastic
bonded to metal. The invention relates more specific-
ally, though not exclusively, to an irnproved method
for manufacturing an airfoïl.
Background Art
A varie~y of methods exist for forming or fabri-
cating various structures from fiber reinforced plastic
(FRP). Articles of FRP may exhibit cost advantages
over similar articles manufactured entirely of metal
because of a reduced number of pieces re~uired, which
may result in reduced labor. The production of an
article with fewer parts is due to the wide latitude of
complex part configurations permissible with FRP molding.
However, on structures incorporating metal parts, secon-
dary bondin~ operations are necessary for high strengthbonds between the FRP and metal and generally restrict
molding or laminating directly on the metal part.
Although primers have been de~eloped for coati.ng the
metal, the resulting bond with a laminating resin is
generally not adequate for certain types of structural
bonds.
For instance, U.S. Patent 3,321,019 issued May 23,
1967 to G. A. Dimitroff et al for Fiberglass Blade,
owned by the assignee of the present application, dis-
closes the formation of a reinforced fiberglass bladefor use on helicopters, which blade includes a particular
reinforced plastic blade root construction. More specific-
ally, that blade root construction comprises alternate
H-979
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layers of aluminum reinforcing plates and plastic im-
pregnated fibexglass cloth. A suitable primer is pro-
vided between each metal plate and layer of fiberglass
cloth to prevent metal oxidation and to impxove the
adhesion between the metal plates and the plastic im-
pregnated fiberglass cloth. Such primers typically
take the form of coatings having a thickness less than
1 or 2 mils. The alternate metàl and fiberglass sheet
laminations are arranged over a spar member and are
then adhesively bonded to one another by a vacuum in-
jection process in which liquid plastic impregnates the
fiberglass cloth and fills a mold. The resulting struc-
ture provides sufficient strength and integrity for its
intended function, that being the resistance of shear
stresses imposed by bolts which couple the helicopter
blade to a central hub. However, thP bond between the
alternate metal plate and fiberglass laminations may
not provide the desired degree of strength and integrity
or other applications.
For example, fixed wing aircraft have employed FRP
propeller blades for nearly 20 years. These blades have
generally comprised a pre-molded FRP shell securely
bonded to a central metal spar, with certain spaces be-
tween the shell and spar completely filled with a foam-
type material. Adequate structural integrity between
the fiberglass shell and the metal spar were obtained
using certain adhesives, as for instance a thermosetting,
non-volatile, modified epoxy resin such as AFlll manu-
factured by Minnesota Mining & Manufacturing Company
- 30 which was placed as a film on the spar prior to the
bonding of the fiberglass shell therewith. Such adhe-
sives provide ~ bonding ~trength which far exceeds that
possible by khe primers utili2ed in the aforementioned
U. S. Patent 3,321,019 and are thus a~le to provide the
spar-to-shell bond for structures of this particular
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type and utilization. It will be appreciated, however,
that this long standing practice, first required the
formation of a molded FRP shell and then required the
secondary bonding operation through which the shell
S and spar were integrally joined.
Accordingly, it is a principal object of the pre-
sent invention to provide an improved method for manu
facturing various fiber-reinforced airfoils, such as
propeller blades. Included in this object is the reduc-
tion of manufacturing labor while retaining or improvingthe structural characteristics and dimensional uniformity
or repeatability of the product.
In accordance with a particular embodiment of the
invention there is provided a met~od of forming an article
comprising a fiber reinforced plastic bonded to a metal
object. The method includes the step of coating the sur-
face metal object with a curable bonding adhesive, the
adhesive being an epoxy. The adhesive w~ich coats the
object is precured and at least one layer of reinforcing
fibers impregnated with a synthetic polymeric material is
applied to the adhesively coated surface of the object
and constrained within a mold to a desired shape. The
synthetic polymeric material so impregnated into the at
least one fiber layer within the mold is cured to form
the article in accordance with the shape of the mold.
In accordance with a further embodiment of the inven-
tion there is provided a method for manufacturing an air-
foil. The method includes the step of coating the external
surface of the spar with a curable bonding adhesive and
transferably coating the interior surface of a first mold
with a curable bonding adhesive The coated spar is dis-
posed in the mold, the mold interior surfac~`being spac-
ed from at least a portion of the coated spar to define
a void region. A curable lightweight filler material is
introduced into the mold void region and the curable
bonding adhesive and lightweight filler material within
the mold is precured thereb~ to form an airfoil subassembly
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having the ~iller ma-terial adhesively bonded to the spar
and including an adhesive coating on the outer surface
thereo~. Substantially the entire airfoil subassembly
is enveloped within a covering of fibrous reinforcing
material impregnated with a synthetic polymeric material
and constrained within a second mold to a desired shape.
The synthetic polymeric material impregnating the reinforc-
ing material covering within the second mold is cure~ to
thereby produce an airfoil o~ the desired shape.
Disclosure of Invention
In accordance with the present invention, there is
provided an improved method for forming articles comprised
of a fiber reinforced plastic bonded to a metal object.
The surface of the metal object is coated with an adhes-
ive, and particularly a thermosetting, non-volatile,
modified epoxy liquid adhesive with enhanced bonding
properties following precuring. The adhesive thickness
may vary from 5 to ~0 mils as required to compensate
for dimensional variations in the metal and can be
formed to shape in a mold. The adhesive is precured
and one or more layers of reinforcing fibers such as
fiberglass cloth, are applied to the adhesively coated
surface of the metal object. The fibers are previously
or subsequently impregnated with a liquid plastic.
Preferably, the fiber-covered object is placed i~ a mold
and a liquid plastic such as a synthetic polymeric mater-
ial, as for instance a thermosetting resin, is introduc-
ed to the mold to thereby impregnate the fiber layer on
the object. Following impregnation of the fiber layer,
the polymerizable material is cured within the mold to
form the article in accordance with the shape of the
mold. The in-situ molding of the fiberglass with the
adhesive-coated metal object creates a stron~ bond there-
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between and resultS in a substantial reduction in the
labor which otherwise would have been required for a
secondary bonding operation.
In a preferred embodiment of the invention, the
f. 5 method is utlized in the manufacture of airfolis such
as propeller blades. A blade subassembly may first be
made by coating a metal spar with the requisite adhe-
sive, partly to compensate for dimensional variations,
then transferably coating the interior of a subassembly
mold with the adhesive, then placing the spar within the
mold, partially precuring the adhesives, introducing a
- foaming material thereinto and finally precuring the foam
and adhesives such that the foam is bonded to certain
portions of the spar for jointly forming the blade sub-
assembly. The blade subassembly is subsequently envelop-
ed with reinforcing fibers, as for in-stance fiberglass
cloth, and placed in a final mold into which a liquid
plastic such as a thermosetting resin is injected and
cured to thereby complet~ the formation of the blade.
The resin-impregnated fiberglass is bonded, via the ad-
hesive, to the foam at some locations and to the metal
spar at other locations. A protective metal ~heath also
having the precured adhesive on its inner surface may
be placed on the outer surface of the fiberglass prior
to resin impregnation and will be included as an integral
part of the resulting blade.
Brief Description of Drawings
Fig. 1 is a transverse sectional view o~ a propeller
blade made in accordance with a prior art process;
Fig. 2 is a transverse sectional view of a propeller
blade made in accordance with the process of the present
invention;
Fig. 3 is an enlarged view of a portion of the blade
illustrated in Fig. 2;
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Fig. 4 is a perspective view of an adhesive-coated
blade spar;
Fig. 5 is a perspe.ctive view of-one-half of an
adhesive-coated mold for making a blade subassembly;
Fig. 6 is a perspective view of a blade subassem-
bly;
Fig. 7 is a perspective view of a blade subassem-
bly with fiber reinforcement prior to final molding;
Fig. 8 is a diagrammatic sectional view of the
final mold showing the blade assembly of Fig. 7 therein
during the first phase of final molding; and
Fig. g is a view similar to that of Fig. 8 but
illustrating the mold in a secondary phase of the final
molding operation.
Best Mode for Carrying Out the Invention
Referring to Fig. 1, there is illustrated in trans-
verse section~ an airfoil, such as propeller blade 10
manufacturedin accordance with the prior art. Propeller
~ blade 10 includes an outer shell 12 of fiber reinforced
- 20 plastic, which shell is bonded, via a bonding adhesive
14, to an aluminum spar 16 extending substantlally cen-
trallY therewithin and attaining airfoil accuracy no
better. than the cumulative tolexances of the spar, shell
and bond thickness. A lightwaight filler material suc~
as a rigid urethane foam 17 is formed within the voids
remaining between shell 12 and adjacent to spar 1~
following their bonding to one another. A protective
metal sheath 18 is subsequently fitted and bonded to
the leading edge of blade 10 by means of an adhesive
bond 19.
The FRP shell 12 was molded by vacuum bag molding
on a mal.e shell mold or man~rel which fiberglass was
impregnated with a thermosetting resin which was subse-
quently cured. The shell 12 is generally formed as a
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single structure haviny su~ficient slotting along its
forward or leading eage to ~acilitate the intr~duction
of spar 16. The traili~g edge is usually slo~ted the
entire length and is subséquently joined by the adhesive
bond 15~ following insertion thereinto of the spar 16
having the adhesive 14 thereon in the form of a film~
The adhesive 14 is then cured under heat and pressure
to provide the desired bond, A tie-coat mixture of epoxy
resin, polyamide hardener and toluene is used to coat
the surfaces which define the voids between shell 12
and spar 16 to enhance retention of the foam 17 which is
subsequently formed in those voids, as by pouring liquid
foam in and heat curing. Significant time is th~n still
required to prepare the blade and sheath 18 for bonding,
complete that bond, and "finish" the blade.
In contrast with the relatively complex prior art
process for manufacturing blade 10, the blade 30 of
Fig. 2 is made in accordance with the in-situ molding
process of the present in~ention and thereb~ substan;
2~ tially reduces the labor involved. Blade 30 is struc-
turally similar to prior art blade 10 and includes an
outer fiber-reinorced plastic shell 32 bonded to an
aluminum spar 36 via a layer of particular bonding ad-
hesive 34 in mutual bonding contact therewith~ Moreover,
the voids between the F~P shell 32 and spar 36, forward~
ly and rearwardly of the spar, are illed with light-
weight filler material such as rigid urethane foam 37.
A protective nickel sheath 38 is also bonded via a suit-
able adhesive 39 to portion of the leading edge of the
FRP shell 32. However, the process of the invention
considerably simplifies the manufacture of blade 30
relative to prior art blade 10 while also affording
certain improvements in the attainment and repeatability
of dimensional tolerances from blade to blade.
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Fig, 3 illustrates in ~reater detail the bonded
rela~ionship of the fiber-.reinforced plastic 32 with
the aluminum spar 36 via adhesive 34. The selection
of the adhesive 34 used to practice the method of the
invention is relatively important and in the preferxed
embodiment comprises a thermosetting, non-volatile,
modified epoxy liquid adhesive, and particularly that
sold by Minnesota Mining & Manufacturing Company under
the trade name EC-2214 R~ This adhesive and those of
- 10 the same modified epoxy class, when precured, provide
a particularly strong bond between the metal substrate
36 and the resin of the FRP 32~ This adhesive 34 facili-
tates the in-situ molding method in which the adhesive
34 is first applied to the metal su~strate 36 and is
precured and the reinforcing fiber, as for instance
fiberglass cloth, is laid up on the adhesively coated
surface of the substrate and is impregnated with a liquid
plastic, as for instance a thermosetting epoxy resin,
which is subsequently cured while contained within a
mold having the desired shape of the article.
Referring in greater detail to the specific process
for making the airfoil or propeller blade 30, attention
is called to Figs. 4-9 which sequsntially illustrate
various aspects of the blade manufacturing process.
Fig. 4 illustrates a conventional elongated aluminum
spar 36 having a root portion 50 and a tip portion 52.
Spar 36 pro~ides the principal strength member for blade
30~ Spar 36 is coated over substantially its entire
surface, except for root poxtion 50, with the adhesive
34. The adhesive 34 is conveniently thinned with methyl
ethyl ketone and is initially spray-deposited on the
spar to a thickness of about 3-5 mils, following which
it i5 cured at a temperature of about 250F for about
45 minutes, then wiped with methyl-ethyl ketone and
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sandblasted to abrade the surface. An additional
coating of adhesive 34 is sprayed thereover to an
additional thickness of about 10-12 mils. Finally
the adhesive 34 may be applied in a paste form to
the adhesive already on the top and bottom surfaces of
the spar 36 such that it "sizes" the spar to fill any
uneveness and bring the final dimension to one which
will at least continuously contact the upper and lower
inner surfaces of a first mold. The thickness of
adhe5ive 34 on these upper and lower surfaces of spar
36 is generally in the range of about 5 mi~s to 40
mils or more.
The opposing halves of a blade subassembly ~old
are similarly coated with 10-12 mils of adhesive 34~
one-half of such mold being illustrated as element 60
in Fig. 5. A conventional transfer or release agent,
such as Kanstik LM followed by a coat of Arcon 5003, is
first applied to the cavities of mold halves 60.
The adhesive 34, thinned as previously mentioned with
methyl ethyl ketone, i~ spray-deposited upon the
- release agent within the mold over substantially the
total blade area excepting the root portion 50.
The adhesively-coated spar 36 of Fig. 4 is positi-
. oned in the adhesive-coated mold halves 60 which are
then closed about the spar~ ~ partial curing of the
adhesive 34 is then obtained by heating to 210F for
about 30 minutes. The mold cavities of the blade
subassembly mold 60 contact the adhesive 34 on the
upper and lower surfaces of spar 36, but are sub-
stantially wider than the ~par in the fore and a~t
directions of the blade such that vo~ds are created
thereat for the foxmation th~rein o the foam sesments
37. After purging the mold with nitrogen, a two
component, semi-prepolymer rigid urethane foam 37
is mixed in the proper proportion and is then intro-
duced to the closed mold 60, as by injection at its
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bottom, to fill the voids not occupied by spar 36.
The foam 37 and the adhesive 34 on spar 36 and trans-
ferably on the mold 60 are then finally cured, as
by heating to a temperature of 250F for 45-60 minutes.
,~ 5 The mold 60 is then cooled, following which the spar/
foam blade subassembly 70 illustrated in Fig. 6 may
be removed therefrom.
The blade subassembly 70 is only slightly smaller
than the final blade 30 and includes the adhesive 34,
now precured, extending over substantially its entire
outer surface, except for root portion 50, the adhesive
on the foam portion 37 having been transferred thereto
from the interior of mold 60 and the adhesive on the
uppex and lower surfaces (relative to Fig. 2) of spar
36 having been applied directly thereon. This
precured coating of adhesive 34 is typically at least
5 to 10 mils and may be 40 mils or more in thickness
to fill and smooth any irregularities such as scratches
and gouges which may have been in the surface of spar
36, thereby to provide blade subassembly 70 with a
surface of relatively high quality and accuracy. In
this way, the adhesive 34 performs a sizing function
to assure a repeatable size of the subassembly 70.
Moreover, this coating of adhesive 34 protects the spar
- 25 36 and foam 37,to some degree, from damage due to
handling in the subsequent stages of the manufacturing
process.
The precured coating o~ adhesive 34 may then be
cleaned by wiping with methyl~ethyl ketone and sand-
blasted to prepare it for subsequent bonding. The
root portion 50 of subassembly 70 may then be mounted
in a suitable fixture, not shown, for the application
or laying-up of the reinforcing fiber to be used~
Referring to Fig. 7, the blade subassembly is
illustrated as having been covered or enveloped with
one or more layers o~ the reinforcing fiber. In the
present instance, 4-7 layers of woven fiberglass
cloth 71, as for instance Style 1581 available from
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the United Merchants Company, ha~e been very tightly
wound about the blade subassembly at the proper cloth
warp-to-blade axis angle, i.e, 35, and stitched,
as at 72, to retain it in position thereon. The
i~ 5 fiberglass cloth 71 envelopes the entirety of the foam
37 and extends inwardly thereof toward root portion 50
and into engagement with the adhesive 34 thereat on
spar 36 for totally e~closing the foam. It will
be appreciated that other high strength reinforcing
fibers such as aramid, graphite, boron or the like
may be substituted for the fiberglass 71. The
protective nickel sheath 38 may then be pressed onto
the leading edge of the fiberglass-covered subassembly
near the outer tip 52 where it is t~mporarily held in
;15 position by its somewhat resilient engagement with the
fiberglass bulk extending thereinto. The fiber-
glass-engaging surface of sheath 38 will have been
previously dip coated with an adhesive 39 which is
substantially the same as adhesive 34 and which will
have been precured in substantially the same manner
as for adhesive 34. Qptional heater wires 76 may
be placed against the leading edge of the wrapped
fiberglass 71 toward the root end 50 of the blade
subassembly and may be temporarily retained thereat by
attaching with cotton thread.
Referring to Fig. 8, the fibergl-ass-covered
blade subassembly of Fig. 7 is then placed in the
cavity formed by the opposing halves of a final mold 80
to which a suitable mold release agent(s) has been
3~ previously applied~ In the primary phase of this
final mold operation, the opposing halves o~ mold 80
are moved from a fully open position to a partly open
position and retained thereat in a slightly spaced
relationship/ as by two or more spacers 82 which may
be about .020 inch thick. A compressible O-ring 84
positioned between the halves of mold 80 serves to
1 1 .1 ~ 7 3 ~ 13 9
sealingly enclose the mold cavity in a known manner
when the halves of the mold are drawn against the
spacers 82, as by suitable clamps 86. A vacuum source
(not shown) is applied to vacuum port 88 and the desired
~ 5 liquid plastic is then introduced to the mold cavity
via inlet 89. The liquid plastic is pxeferably a syn-
thetic polymerizable material, as ~or instance a thermo-
setting epoxy resin such as APC0 434 marketed by Applied
Plastics Company.
It is normally difficult to obtain the high glass-
to-resin ratio which is required for various airfoils
including propeller blades and the like, particularly
where the number of layers of fiberglass is increased~
However, by providing spacers 82 in accordance with the
present process, complete wet-out of all layers of the
fiberglass 71 can be quickly and easily accomplished
even with higher viscosity resins that are not otherwise
normally used.
~ Referxing to Fig, 9, when the injection of resin
has been completed, the mold spacers 82 are removed and
the mold 80 is fully cloased; The excess resin is
forced out of the mold cavity through the vacuum and
inlet ports 88 and 89 respectively. In this phase of
the final molding operation in which the mold 80 is
fully closed, the mold cavity very precisely and durably
defines the geometry to be given the resulting blade 30.
To polymerize and thereby cure the resin, it is heated
within mold 80 to a temperature of about 250F for approx-
imately 45-60 minutes. This curing of the resin about
the fiberglass results in the fiber reinforced shell 32
forming a particularly strong bond with the precured
adhesive 34 and thus with the spar 36. A similar bond
is obtained between the shell 32 and the foam 37 through
the agency of the adhesive 34 in that region. Addition-
ally, the sheath 38 becomes strongly bonded to the shell
32 through the agency of the adhesive 39 thereat. The
mold 80 is cooled and the blade 30 is then removed in a
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substantially completed form. Generally, only a very
small amount of flashing remains about its midline and
is easily removed.
Although this invention has been shown and
- 5 described with respect to a preferred embodiment thereof,
it should be understood by those skilled in the art
that various changes and omissions in the form and detail
thereof may be made ~herein without departing from ~he
spirit and scope of the invention, For instance; it will
be appreciated that the impregnation of the reinforcing
fibers with liquid plastic may be accomplished by means
other than injection into the mold~ For example, the
glass fiber cloth may be preimpregnated with epoxy resin
and partially cured prior to its placement on the sub-
strate or subassembly, or the resin may be applied by
brush to the glass fiber cloth while on the subassembly
and subsequently cured in the mold~ These techniques
may not, however, realize some of the manufacturing
economies realized by the preferred process.
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