Note: Descriptions are shown in the official language in which they were submitted.
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441-003-6
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TITLE OF THE INVENTION
THERMAL EXPANSION RESIN TRANSFER MOLDING
~ack~round of the Invention
Field of_the Invention
The present ;nvention relates to a method of
preparing shaped objects from a thermally expandable
foam.
Description of the Prior Art
One technique for preparing molded objects of
various shapes is known as the thermal expansion
moldin~ process (TEM). This method has the unqiue
feature of providing pressure necessary for formin~
finished shapes by use of a thermally expandable
material placed within the female cavity of a mold. In
the process, a mandrel (of a thermally expandable
material sucl- as silicone rubber) is wrapped with a
resin preimpregnated Eabric material. The wrapped
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mandrel is then inserted into a closed cavity mold.
Then, the mandrel, mold or both man(1rel an;1 mold are
~ ~ heated with the result that the material of the mandrel
; . expands thereby causing the material to e~pand against
the female cavity mold. Pressure is created witilin the
cavity by the confining surfaces of the mold.
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Significant pressures can be generated by this process
which leads to high quality molded object, even of
complex shapes which are free of voids.
The conventional TEI~ process has several
advantages over vacuum bag and autoclave curing of
objects. An important advantage is that the TEM
process obviates the need for the capital investment
required to support autoclaves. Moreover, significant
labor savings can be realized by the fact that the
expensive bagginy and debagging operations associated
with autoclave processes are not necessary. Still
further, the rate of rejection of product is reduced
because of the above operational advantages, since bag
leaks are a significant problem of autoclave curing
operations. TEM processing is especially well suited
in manufacturing parts of complex shapes because it is
especially difficult to bag parts of complex shapes.
Disadvantages of conventional TE,~ processing which
utllizes silicone rubber mandrels include the fact that
rubber tooling does not have sufficient long term
stahility. That is, during repeated thermal pr(?SSUre
cycling, the rubber mandrels can experit?nce a permanent
compression set, which limits their li~e. Provisions
must also be made in the part being manuEactured to
remove the mandrels aeter curing. This can compromise
~he shape or Eunction of the part. Another
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disadvantage of the conventional TEM process is that a
resulting hollow part of the object may lack suficient
structural strength. Still another disadvantage of
rubber tooling is that i~ the rubber tooling îs
improperly sized, or if the cure cycle temperatures are
exceeded, rubber .tooling is capable of generating
pressures sufficiently high to deform the tooling or
ruin the mandrelO Yet another disadvantaye-of
conventional TEM processing is that since the mandrels
provide an interior mold surface, any imperfections in
ths rubber such as cuts, gouges! and the like will be
transmitted to the part.
Several techniques are known for manufacturing
paddles, which are useful in rowing, canoeing and the
like, that are light in -~eight but yet structurally
strong. Virtually all of these paddles are made either .
of wood, or they have an aluminum or fiberglass shaft
with a blade of compression molded laminate or
injection molded plastic. These paddles typically
weigh between 1 to ~.5 pounds. In another method of
light weiyht paddle construction as described in U.~.
Patent 4,061,106, a core material such as balsa wood is
cut to tle shape of a paddle blade and then the blade
is formed to ~he paddle sha~t. Thereafter, the core îs
coated witll a first layer of resin which wat3rproofs
the wood and provides the required strenyt;l for the
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paddle. After the applied resin layer dries, the
paddle blade is coated with a second layer of resin and
then reinforcing fibers are laid into the surface of
the still wet resin cbating. Upon drying of the resin
coating, a complete paddle is obtained. While this
method of paddle construction provides paddles of
lighter weight construction than conventional paddles,
the process still is relatively complex and involves a
number of operational steps. A need therefore
continues to exist Eor a relatively simpler way of
manufacturing light weight paddles, as well as other
structures, which involve ~ewer manufacturing steps.
SUM~IARY OF THE INVENTION
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Accordingly, one object of the present invention
is to provide a method of manufacturing light weiyht
objects of complex shape by a simple molding process
from synthetic resin materials.
Another object of the present invention is to
provide a method of manufacturing paddles or oars of
lighter weight which are use~ul in aquatic e~ents by a
molding process involving synthetic resin materials.
Briefly, these objects and ot'ner objects o~ the
present invention as h0reina~ter will become more
readily apparent can be attained by a met`nod of formint~
composite molded articles-comprising formillg a tllerno-
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elastic rigid foam core into a desired shape, wrapping
the pre-formed foam core with a fabric, placing said
wrapped pre-formed foam core in a mold whose inner
confining surfaces form the shape of the final article
and which provides the capability of heating selected
areas of said pre formed foam core~ injecting a liquid
thermosetting resin into the mold such that the
thermosetting resin surrounds and wets the fabrlc
wrapped about the pre-formed foam core, selectively
heating desired areas of t!-e mold to a temperature
sufficient to expand the riyid foa-n core to compress
the fabric wrapped surface of the foam core against the
inner confining surfaces of the mold and completing the
heating of the rigid foam core to complete the molding
process, and cooling the mold and removing the article
from the mold. This process is hereinafter referred to
as the thermal expansion resin transEer molding process
or TERTM.
BRIEF DESCRIPTION OF THE DRA~ING~
A more complete appreciation of the invention and
many of the attendant advantages thereof will be
readily obtained as the same becomes better understood
by reference to the following detailed description when
considered in connection with the accompanying
drawings, wherein:
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FIGURE 1 is a cross-sectional view of a mold for
conducting the TERT~ process of the invention; and
FIGURE 2 is a top vie~ of the mold of Figure 1.
DETAILED_DESCRIPTION OF THE_ PREFERRE~ EMBODIMENTS
The fabrication process of the present invention
can be successfully used to prepare consumer products
of widely differing shapes which are usually made from
wood, metal and plastic. In ~act, an advantage of the
present TERTl~ process is that it provides for
flexibility in the manufacture of new products which
are unsuitable or impractical ~or manufacture from
wood, metal or many plastic materials. Moreover, the
TERTM process effectively utilizes lightweight
materials in an economicall~ attractive manner to
prepare sandwich core products of light weight,
strength and stiffness. Thus, the utility of the
process is in the manufacture of articles and objects
of widely varying design and shape which are themselves
useful in many applications such as parts in the
construction of aircraft, motorized, as well as
unmotorized vehicles, recreational Aevices, and the
like~
In the first stage of the TE~TM process oE the
invention, a thermo-elastic rigid foam core is or~ed
into a desired shape by any convenient metllod.
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Usually, the rigid foam is shaped by direct molding or
compression molding with heat. Since the TERTM process
can be utilized to Eorm products oF widely varying
shapes, it is apparent that the shape of the riyid foam
core can be of any convenient shape to expand into the
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shape of the final product within the mold.
In selecting the polymer material for the core,
factors which enter into the determination are t'ne heat
deflection temperature of the finished product, the
weight desired Eor the final product, the cost oE the
polymer material, and the cycling time desired. The
~oam core material must be a m~terial which is heat
expandable at elevated temperatures and yet is stable
at ambient air temperatures. Suitable polymer
~aterials include polyvinyl chloride, selected
polyurethane materials, polyimides, and the like. All
of these materials have thermo-elastic properties which
make them suitable ~or use as core materials.
Polyvinyl chloride and the polyurethanes have the
advantages that they can be cycled much faster at lower
processing temperatures and they cost less than the
polyimides. However, these materials pro-1uce heavier
products because oE the increased .3en~sities re~uire-1 to
compensate for lower compressive and shear
properties. The other Ina jor diEEerences hetwee~ the
foa~able materials are the expansion pressures -hich
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they exert on the internal laminating materials as they
expand within the mold. The higher density polyimide
material having a density up to 7 lb/ft3 is capable of
exerting over 100 psi of pressure, while polyvinyl
chloride and polyurethane pressures are significantly
lower with polyurethane expansion pressures being as
low as several psi.
When either polyvinylchloride or polyimide is used
as the core material, a shaped object is cut fro.n slab
stock of the material and roughly shaped if
necessary. The roughly shaped material is then heated
in an oven for a time sufficient to soften the cell
structure of the object, and the object is tnen quickly
transferred to a room-temperature compression mold for
more precise shaping. In fact, for articles of
relatively complex shape, several compressions Molds my
have to be used to fonn segments which may be joined to
provide a complete core for later use. On the other
hand, lf a polyurethane material is selected as the
core or mandrel material, mecilanical ~re-shaping of the
foam mandrel ~rom slab stock can be eliminated by
molding the mandrel directly Erom the closed
~ cs~iQ~ mold. Polyurethane is availahle in free
rlse densities ran~ing from 2 to 10 lb/ft3. Upon
subsequent exposure to heat in a production mold, the
polyurethane material will expand in a ~as'lion similar
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to pre-compressed polyvinyl chloride and polyimide, but
at significantly lower pressures.
After appropriate core material selection, the
preformed rigid core is ~rapped with fabric
reinforcement. The fabric can be dry or prepregged if
desired or a combination of both dry fabric and preprey
may be used. The dry fabric reinforcing material can
be unidirectional, woven, knitted or braided
material. In fact a suitable structure can be attained
by wrapping the core with filaments. The dry fabric
which is selected can be prepared from a variety of
materials such as graphite, ~evlar, fiberglass and
polyester. Fabric selection also depends on how they
perform when impregnated with the tlermoset resin used
subsequently in the process. Awareness of the various
forms of fabric such as braids, knits, woven r
unidirectional materials and traditional woven
materials will allow the manufacturer to more fully
utilize speclfic properties of the fabric to the best
advantage possible, particularly with respect to raw
material costs and processing.
The utilization o~ hybrid braided f~bric
reinforcements presents particular opportunities in the
fabrication of complex tabular-shaped pro~ucts by
~ERTM. The typt?~ orient~tion and quantity o~
reinforcing fibers all contribute a siynii_al-t role in
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the finished product properties of strength, stiffness
and durability.
One embodiment of wrapping material is a prepreg
which is an epoxy resin soaked fabric. A prepreg is
prepared by immersing a fabri-c into an organic solvent
(acetone) solutio,n of epoxy resin, then removing the
cloth once impregnated and finally dried. The prepreg
which is obtained is tac~;y and semisolid. (Since the
epoxy resin contains a curing agent, the prepreg is
usually stored at low temperatures in order to prevent
premature curing of the resin.) The nature o the
curing process of the prepreg material must be such
that its curing temperature matches closely with the
temperature at which the core or mandrel material
expands during the actual molding process.
The preferred embodiment of core wrapping material
as far as the TERTM process is concerned is dry fabric
because of its lower costs in comparison to prepregs
and also because many more ~orms o~ ~abric are
available in the dry form than are availahle in the
prepreg.
Once the mandrel or core has heen wrapped in the
~; fabric, the wrapped structure is placed withir) a mold
cavity. With regard to the placement of the ~ahric
wrapped mandrel in t~e ~nold cavity, it is important
that the wrapped mandrel be slightly undersized
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relative to the interior size of the mold cavity such
that the structure slightly rattles within the mold
cavity. If the wrapped mandrel is oversi~ed or if the
mold is undersized such that no rattling of the wrapped
mandrel is observed when the mold is shaken, this means
that when the liquid epoxy is injected into the mold,
it will not wet the entire surface of the wrapped
fabricO This in turn means that an article of
incomplete, irregular or imperfect surface Eeatures
will be obtained which makes the product unacceptable.
The clearance between the wrapped mandrel and the
conEining mold surfaces should especially be no lass
than about 0.127 ~n nor ~ore than about 0.76 mm.
Once the wrapped mandrel is positioned within the
mold, a low viscosity thermosetting resin is injected
into the mold such that the fabric and mandral is
surrounded and wetted (impregnated and covered) by the
thermosetting resin. The thermosetting resin which is
used should have a viscosity of about 8000 cp or less
at ambient temperatures and preEerably decrease to less
than 100 cp at the elevat~d processing temperature.
The resin is preferably an epoxy resill, althougll other
thermosetting resins such as polyurethanes or
polyesters can also be employed. The thermosettiny
resin which is used also should be one whose gel time
which is sufficiently lon9 such that ~he Eoam core can
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expand ~ithout interference by a prematurely gelling
thermosetting resin. The rheology of the resin, its
gel time as a function of temperature and its curing
properties all contribute to the manufacturing process
and the resulting product properties, particularly the
stiffness and impact strength.
Following the injection of the epoxy resin, the
mold is heated. Heat is transferred Erom the mold
through the thermosetting resin to the foam core. Once
the témperature of the core reaches the temperature of
expansion of the core material, the foam core expands
such that the wrapped surface of the core is forced
against the confining surfaces of the mold. The major
factor which regulates the temperature oE the molding
step is that the temperature must be at least that
which is the temperature at which the particular core
material being used will expand. The thermosettin~
resin which is injected into the mold must Inaintain its
liquid st~te long enough until the foam has fully
expanded against the interior mold surfaces and
expelled all excess resin from the mold throuyll a vellt
provided in the mold. Once the foam core llaS eXP~I)ded,
the thermosetting resin should be formulated to rapidly
harden and cure. Accordingly, the curiny properties of
the thermosettill-~ rcsin m~lst be consistent ~ith this
stated profile of the moldin~ step. The mold core is
then cooled an~ the object is removed from the mold.
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The thickness of the outer layer oE the cured
product laminate which consists of hardened thermoset
resin impregnated fabric is determined by the thickness
of the wrapped layer. The thickness of the layer may
vary widely, but usually ranges from a minimum
thickness of abou~ 0.25 ~n to just about any greater
thickness desired with a usual maxirnum thickness of up
to about one cm.
The temperature whicll is used in the molding
process should closely match the expansion temperature
of the core or mandrel material as stated. Obviously,
if the temperature is not high enough, the foaln core
will never expand and the process .~ill not ~ork. For
the specific core materials described above, polyimides
generally expand at temperatures ranging from about
149 to about 205C, while polyvinyl chloride resins
generally expand at temperatures ranging rom about
104 to about 163C and polyurethane resins expand at
temperatures ranging from about 65 to about 107C.
An aspect o~ the moldiny process is that the mold
portions provide the means of heating selected ~reas o~
the riyid foam core within the mold such that areas oE
the rigid ~oam core can be uniEormly heated to allo~ a
- controlled rate of expansion of the riyid foaln core
before other areas oE the riyid ~oam core are heated.
~~ In fact, in e~treme cases the selective he;ltiny process
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can be conducted so as to only apply heat to a given
area of the rigid foam core, while other areas of the
rigid foam core exparience no heating whatsoever until
the aforesaid heating and expansion of rigid foam cora
material is accomplished.
The selective heating aspect of the method is
provided by the construction of the mold portions which
are exemplified in the mold half of Figure 1 and the
cross-section of a complete mold sllown in Figure 2.
Vertical baffles 7 are placed in spaced apart
relationship within the hollow interior of each mol'd
portion between the exterior and interior metal
surfaces o~ each mold portion. These baffles provide
the mold portions or halves, as the case may be, with
strength which prevents deformation of the mold
portions or halves during use, while at the same time
defining the flow pattern of the heated fluid which is
passed into each mold portion which provides the heat
required for uniEorm and controlle~ expansion of tha
rigid foam core. As the heate(1 Eluid passes into a
given mold uortion, the bafflcs define the channelcd
flow pattern of the ~eate~ fluid vlitl~ tll* mold
interior so that a selected portion of the interior
; mold surface is heated to supply the heat necessary to
~ expand th~ corresponding .selected arca of tlle rigid
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foam core within the mold.~ If a non-salective heatin~
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regimen is desired, enough heated fluid is supplied to
each mold portion to fill the entire hollow interior of
each mold portion so that the entire mass of rigid foam
core is expanded.
Various techniques can be employed to selectively
heat a rigid foam core within a mold. Referring to
Figure 2, selective heating of a rigid foam core can be
achieved by injecting heated fluid through inlet 13
into the hollow interior of the mold portion in an
amount and with a channeled flow pattern such that the
heated fluid provides the heat necessary for the lower
and middle regions of the rigid foam core. Once these
areas of the foam core have expanded, additional heated
fluid can be passed into the hollow interior of the
mold portion such that the remaining top portion of the
rigid foam core is heated for expansion. To complete
the selective heating process enough heated fluid is
passed into the hollow interior of each mold portion
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until excess ~ escapes from a vent in the top of
the mold.
Another alternative mold construction for
selective heating would be to so construct ~he mold
with Interior baffles in appropriate positions such
that two or more isolated interior regions are
established within the hollow interior of a given mold
portion with each of the interior regions being
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provided ~ith a fluid inlet and a fluid vent. This
technique provides for the selective heating of a given
area of the rigid foam core as each individual interior
region of a mold portion is filled with heated ~luid.
The present invention also embraces any other mold
modification and fluid low pattern which achieves th~
heating of selected areas of the riyid foam core within
a mold.
The mold which is used in the process of the
present invention can be constructed of most any kind
of commonl~ available metals with nickel and aluminum
being the preferred metals. Most preEerred of the
metals is nickel because of its excellent durability
and heat transfer characteristics, and because of
economic considerations.
Fiyures 1 and 2 show typical cross-sectional and
top views respectively of a mold oE preferred
construction which in this case is used in the
production of paddles. The mold provides for the
fabrication of the blade portion of the paddle. The
view of ~iyure 1 shows the internal confininy surEaces
3 of a Inold 1 which ellclosl3 void sL)ace ~ in wllictl cl
wrapped mandrel is expanded. The ~nol,~ Ilas external
surfaces 5. The internal and e~ternal surfaces within
each half of the mold arc separated ~ a hollow
interior cosltaininy spacing baffles 7.
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Figure 2 is a top view of a mold half having
external surface 5 under which is palced a series of
baffles 7 within the hollow interior of the mold
half~ Bolt holes 9 are shown around the peri~hery of
the mold which provides sites by which the mold halves
can be secured in.position against each other around
sealing groove 8. A flexible elastomer or rubber cord
such as a silicone rubber cord is placed within the
groove such that when the two halves of the mold are
bolted together, the cord is co!npressed and seals
against any leakage of the therlnosetting resin after it
has been injected into the cavity. Injection port 11
in communication with the interior of the mold through
channel 10 i5 shown where low viscosity liquid
thermosetting resin is injected into the mandrel fil~ed
void space of the mold. Inlet port 13 provides an
opening into the void space between the outer and inner
surface of the mold which contains the supporting
baffles through which a liquid can be injected in order
to provide heat for the moldin~ process.
The preferred nickel mold of the ~rcsent invention
is prepared by electrodepositing a layer oE metal on
the conforminy surface of a master to form the inner
surface of one half of the mold. Normally, the meta1
; is deposited to a thickness oE ahout one quarter of an
~ inch. ThereaEter, st~el baEEles 7 as shown in Fi~ures
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1 and 2 are placed in position over the
electrodeposited metal layer and are ~elded into
position on the metal surface. The baffles normally
are about one hal inch tall although the height factor
may be varied as required. -The void spaces between the
baffles are then ~illed with a wax and then the
fabrication is immersed into a nickel electroplating
bath where another nickel layer is deposited over the
wax and baffles to form the exterior housing of the
mold half. The fabrication is removed from the
electroplating bath and then is warmed in order to melt
the wax for its removal fro~ the interior of the mold
half. The other half of the mold can ~hen he
fabricated in the same manner as described above. The
baffles within the mold halves serve the important
functions of being elements of construction which add
strength to the mold halves and control the flow path
of the heating 1uid which can be the likes of steam
or oil, throu~h the mold half.
Having now fully described this invention it will
be apparent to one of ordinary skill in the art that
many changes and modi~ications can be Inade thereto
without depa~tiny from the spirit oE the inv ntioll as
set forth herein.
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