U.S. patent application number 13/625598 was filed with the patent office on 2014-03-27 for compliant layer for matched tool molding of uneven composite preforms.
This patent application is currently assigned to THE BOEING COMPANY. The applicant listed for this patent is Erika L. Carter, Panagiotis E. George, Marc R. Matsen. Invention is credited to Erika L. Carter, Panagiotis E. George, Marc R. Matsen.
Application Number | 20140083155 13/625598 |
Document ID | / |
Family ID | 48918473 |
Filed Date | 2014-03-27 |
United States Patent
Application |
20140083155 |
Kind Code |
A1 |
Matsen; Marc R. ; et
al. |
March 27, 2014 |
Compliant Layer for Matched Tool Molding of Uneven Composite
Preforms
Abstract
A method for consolidating a preform made of composite material.
The preform and a compliant metal alloy sheet are placed between
less compliant matched confronting forming/molding surfaces with
the preform being sandwiched between the metal alloy sheet and a
matched confronting surface. The matched confronting surfaces and
compliant metal alloy sheet are heated until the preform reaches at
least its consolidation temperature. During heating, force is
applied so that the matched confronting surfaces exert sufficient
compressive force on the preform and metal alloy sheet to cause the
composite material to consolidate at the consolidation temperature.
The metal alloy sheet has a tensile yield point in a range of
25-300 psi at the consolidation temperature at a strain rate of
about 1% to 10% strain per minute.
Inventors: |
Matsen; Marc R.; (Seattle,
WA) ; Carter; Erika L.; (Seattle, WA) ;
George; Panagiotis E.; (Lake Tapps, WA) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Matsen; Marc R.
Carter; Erika L.
George; Panagiotis E. |
Seattle
Seattle
Lake Tapps |
WA
WA
WA |
US
US
US |
|
|
Assignee: |
THE BOEING COMPANY
Seal Beach
CA
|
Family ID: |
48918473 |
Appl. No.: |
13/625598 |
Filed: |
September 24, 2012 |
Current U.S.
Class: |
72/342.1 ;
72/364 |
Current CPC
Class: |
B29C 70/48 20130101;
B29C 2035/0811 20130101 |
Class at
Publication: |
72/342.1 ;
72/364 |
International
Class: |
B21D 31/00 20060101
B21D031/00 |
Claims
1. A method for consolidating a preform made of composite material,
comprising: placing the preform and a metal alloy sheet between
first and second tool assemblies having matched confronting
surfaces which are less compliant than the metal alloy sheet, with
the preform being sandwiched between the metal alloy sheet and one
of the matched confronting surfaces; heating the matched
confronting surfaces of the first and second tool assemblies and
the metal alloy sheet until the preform reaches at least a
consolidation temperature of the composite material; and applying
force to one or both of the first and second tool assemblies so
that the matched confronting surfaces exert sufficient compressive
force on the preform and metal alloy sheet to cause the composite
material to consolidate at the consolidation temperature, wherein
the metal alloy sheet has a tensile yield point in a range of
25-300 psi at the consolidation temperature at a strain rate of
about 1% to 10% strain per minute.
2. The method as recited in claim 1, wherein the metal alloy sheet
is made of magnesium base alloy.
3. The method as recited in claim 2, wherein a chemical composition
of the magnesium base alloy includes magnesium, aluminum, zinc and
manganese.
4. The method as recited in claim 1, wherein the metal alloy sheet
is made of aluminum base alloy.
5. The method as recited in claim 1, wherein the metal alloy sheet
is very soft at the consolidation temperature.
6. The method as recited in claim 1, wherein the composite material
comprises a matte product.
7. The method as recited in claim 6, wherein the matte product
comprises recycled graphite fibers.
8. The method as recited in claim 7, wherein the matte product
further comprises thermoplastic fibers.
9. The method as recited in claim 6, wherein the composite material
comprises graphite fibers and plastic material.
10. An apparatus for consolidating a preform made of composite
material at a consolidation temperature, comprising: first and
second tool assemblies having matched confronting surfaces; a metal
alloy sheet disposed between said matched confronting surfaces,
wherein said metal alloy sheet has a tensile yield point in a range
of 25-300 psi at the consolidation temperature at a strain rate of
about 1% to 10% strain per minute; means for heating at least said
matched confronting surfaces of the first and second tool
assemblies; and means for applying force to one or both of the
first and second tool assemblies so that said matched confronting
surfaces are capable of exerting compressive force on the preform
and metal alloy sheet.
11. The apparatus as recited in claim 10, wherein said metal alloy
sheet is made of magnesium base alloy.
12. The apparatus as recited in claim 11, wherein a chemical
composition of the magnesium base alloy includes magnesium,
aluminum, zinc and manganese.
13. The apparatus as recited in claim 10, wherein said metal alloy
sheet is made of aluminum base alloy.
14. The apparatus as recited in claim 10, wherein the metal alloy
sheet is very soft at the consolidation temperature.
15. The apparatus as recited in claim 10, wherein each of said
first and second tool assemblies comprises a respective susceptor,
said susceptors forming said matched confronting surfaces.
16. A method for consolidating a composite preform made of recycled
graphite fibers and organic resin fibers, comprising: placing the
composite preform and a metal alloy sheet between matched
confronting surfaces of first and second tool assemblies, the
matched confronting surfaces being less compliant than the metal
alloy sheet; heating the matched confronting surfaces of the first
and second tool assemblies and the metal alloy sheet during a
heating cycle; and applying force to one or both of the first and
second tool assemblies so that the matched confronting surfaces
exert sufficient compressive force on the composite preform and
metal alloy sheet to cause one side of the composite preform to be
thermally coupled to one side of the metal alloy sheet, the other
side of the composite preform to be thermally coupled to one of the
matched confronting surfaces, and the other matched confronting
surface to be thermally coupled to the other side of the metal
alloy sheet, said force being applied during at least a portion of
the heating cycle, wherein the metal alloy sheet has a tensile
yield point in a range of 25-300 psi at a consolidation temperature
at a strain rate of about 1% to 10% strain per minute.
17. The method as recited in claim 16, wherein the metal alloy
sheet is made of magnesium base alloy.
18. The method as recited in claim 17, wherein a chemical
composition of the magnesium base alloy includes magnesium,
aluminum, zinc and manganese.
19. The method as recited in claim 16, wherein the metal alloy
sheet is made of aluminum base alloy.
20. The method as recited in claim 16, wherein the metal alloy
sheet is very soft at the consolidation temperature.
Description
BACKGROUND
[0001] This disclosure generally relates to fabrication of parts
made of composite material. More specifically, this disclosure
relates to apparatus and methods for consolidating and forming a
pre-form made of fiber-reinforced plastic material (also referred
to herein as a composite preform) to reduce voids and/or
porosity.
[0002] Fiber-reinforced organic resin matrix composites have a high
strength-to-weight ratio or a high stiffness-to-weight ratio and
desirable fatigue characteristics that make them increasingly
popular as a replacement for metal in aerospace applications.
Organic resin composites may comprise thermoplastics or
thermosetting plastics.
[0003] Prepregs combine continuous, woven, or chopped reinforcing
fibers with an uncured, matrix resin, and usually comprise fiber
sheets with a thin film of the matrix. Sheets of prepreg generally
are placed (laid-up) by hand or with fiber placement machines
directly upon a tool or die having a forming surface contoured to
the desired shape of the completed part or are laid-up in a flat
sheet which is then draped and formed over the tool or die to the
contour of the tool. Then the resin in the prepreg layup is
consolidated (i.e., pressed to remove any air, gas, or vapor) and
cured (i.e., chemically converted to its final form usually through
chain-extension) in a vacuum bag process in an autoclave (i.e., a
pressure oven) to complete the part.
[0004] In hot press forming, the prepreg is laid-up, bagged (if
necessary) and placed between matched metal tools that include
forming surfaces that define the internal, external, or both mold
lines of the completed part. The tools and composite preform are
placed within a press and then the tools and preform are heated
under pressure to produce a consolidated, net-shaped part.
[0005] It is known to consolidate and form composite preforms using
inductively heated consolidation tools. Induction heating is a
process in which an electrically conducting object (usually a
metal) is heated by electromagnetic induction. During such heating,
eddy currents are generated within the metal and the electrical
resistance of the metal leads to Joule heating thereof. An
induction heater typically comprises an electromagnet through which
a high-frequency alternating current is passed. Most organic matrix
composites require a susceptor in or adjacent to the composite
material preform to achieve the necessary heating for consolidation
or forming. The susceptor is heated inductively and transfers its
heat principally through conduction to the preform sandwiched
between opposing susceptor facesheets. During heating under
pressure, the number of voids and/or the porosity of a composite
preform can be reduced.
[0006] Recycled graphite fibers can be used in the fabrication of
composite aircraft parts, such as lightweight seat back components.
First, a matte product is fabricated using recycled graphite fibers
and virgin thermoplastic fibers; then the matte product is
consolidated and formed in matched tooling comprising opposing
susceptor facesheets. These matte products produced with recycled
graphite fibers can exhibit a rather matted and tangled fiber
architecture. These products can also have undesirable unevenness
and thickness/density variations. Furthermore, since the graphite
fibers are tangled, they do not facilitate the flow of the
thermoplastic material. These characteristics lead to the formation
of voids and/or porosity in the final composite product due to the
fact that thickness and density distribution are not uniform across
the matte product. The formation of voids and/or porosity is
further aided by the fact that the fibers are entangled and flow is
limited and unable to "heal" porosity very effectively. Even
prepreg has some variation in thickness and density distribution,
which could lead to the formation of voids and/or porosity during
matched tool molding of prepreg composites.
[0007] Accordingly, there is a need for a method and an apparatus
that can reduce the number of voids and/or the porosity during the
consolidation and formation of a composite preform having uneven
thickness and/or density distribution.
SUMMARY
[0008] The subject matter disclosed herein is directed to a method
for reducing the number of voids and/or porosity during the
consolidation and formation of a composite preform having uneven
thickness and/or density distribution, such as a matte product
comprising recycled graphite fibers and virgin thermoplastic
fibers. This method is carried out using an apparatus that
comprises matched molding tools. The apparatus further comprises a
compliant layer that is situated between the composite preform and
one of the matched molding tools for the purpose of providing a
more even pressure over the entire area of the preform during the
consolidation process. The compliant layer should have an offset
tensile yield point (0.2% of the strain) in a range of 25-300 psi
at the temperature of consolidation of the preform at strain rates
of about 1% to 10% strain per minute.
[0009] In accordance with one embodiment, a sheet of magnesium base
alloy is used to act as the compliant layer or shim to compensate
for uneven thickness or density over the area of a composite
preform, for example, a matte product comprising recycled graphite
fibers and virgin thermoplastic fibers. Magnesium base alloy makes
an excellent candidate for a compliant layer for high-performance
thermoplastic resins due to the fact that some magnesium alloys
become very soft at temperatures useful for assisting the
consolidation and molding of thermoplastic composites (i.e.,
600-750.degree. F.) and do not melt until above 1000.degree. F. As
the temperature and pressure increase inside the apparatus during
consolidation of the composite preform, the magnesium alloy sheet
softens and forms into the areas of relatively lower pressure. The
magnesium alloy sheet can be reused due to the soft nature of the
material. Other alloys can be used instead of a magnesium base
alloy provided that the alloy has an offset tensile yield point
(0.2% of the strain) in a range of 25-300 psi at the temperature of
consolidation of the preform at strain rates of about 1% to 10%
strain per minute
[0010] In accordance with one aspect, a method is disclosed for
consolidating a preform made of composite material with reduced
number of voids and/or porosity. The preform and a compliant metal
alloy sheet are placed between less compliant matched confronting
forming/molding surfaces with the preform being sandwiched between
the metal alloy sheet and one of the matched confronting surfaces.
The matched confronting surfaces are inductively heated (which
heats the compliant metal alloy sheet by conduction) until the
preform reaches at least a consolidation temperature of the
composite material. During heating, force is applied so that the
matched confronting surfaces exert sufficient compressive force on
the preform and metal alloy sheet to cause the composite material
to consolidate at the consolidation temperature. The compliant
metal alloy sheet has a tensile yield point in a range of 25-300
psi at the consolidation temperature at a strain rate of about 1%
to 10% strain per minute.
[0011] Another aspect of the disclosed subject matter is an
apparatus for consolidating a preform made of composite material at
a consolidation temperature, comprising: first and second tool
assemblies having matched confronting surfaces; a metal alloy sheet
disposed between said matched confronting surfaces, wherein said
metal alloy sheet has a tensile yield point in a range of 25-300
psi at the consolidation temperature at a strain rate of about 10%
strain per minute; means for heating at least the matched
confronting surfaces of the first and second tool assemblies; and
means for applying force to one or both of the first and second
tool assemblies so that the matched confronting surfaces are
capable of exerting compressive force on the preform and metal
alloy sheet.
[0012] A further aspect is a method for consolidating a composite
preform made of recycled graphite fibers and organic resin fibers,
comprising: placing the composite preform and a metal alloy sheet
between matched confronting surfaces of first and second tool
assemblies, the matched confronting surfaces being less compliant
than the metal alloy sheet; heating the matched confronting
surfaces of the first and second tool assemblies and the metal
alloy sheet during a heating cycle; and applying force to one or
both of the first and second tool assemblies so that the matched
confronting surfaces exert sufficient compressive force on the
composite preform and metal alloy sheet to cause thermal coupling
of one matched surface to one side of the composite preform, the
other side of the composite preform to the metal alloy sheet, and
the metal alloy sheet to the other matched confronting surface. The
force is applied during at least a portion of the heating cycle.
The metal alloy sheet has a tensile yield point in a range of
25-300 psi at a consolidation temperature at a strain rate of about
1% to 10% strain per minute.
[0013] Other aspects of the invention are disclosed and claimed
below.
BRIEF DESCRIPTION OF THE DRAWINGS
[0014] Various embodiments will be hereinafter described with
reference to drawings for the purpose of illustrating the foregoing
and other aspects of the invention.
[0015] FIG. 1 is a diagram showing a sectional view of portions of
a known apparatus, the apparatus comprising upper and lower tool
assemblies with matched surfaces designed to consolidate and form a
composite preform. The tool assemblies are shown in their retracted
positions and the preform is shown in an uncompressed state.
[0016] FIG. 2 is a diagram showing a sectional view of the
apparatus depicted in FIG. 1, except that the tool assemblies are
in their extended positions with the preform compressed
therebetween.
[0017] FIG. 3 is a diagram showing an end view of a portion of a
lower tooling die in accordance with one embodiment.
[0018] FIG. 4 is a diagram showing a sectional view of a portion of
the lower tooling partially depicted in FIG. 3, the section being
taken along line 4-4 seen in FIG. 3.
[0019] FIG. 5 is a diagram showing the placement of an
unconsolidated matte product between a pair of opposing susceptors
in their retracted positions, the matte product comprising recycled
graphite fibers and virgin thermoplastic fibers.
[0020] FIG. 6 is a diagram showing a consolidated matte product
between a pair of susceptors in their extended positions.
[0021] FIG. 7 is a diagram showing a compliant layer and a
consolidated matte product between a pair of susceptors in their
extended positions.
[0022] FIG. 8 is a graph showing the effect of temperature on the
tensile properties of a magnesium base alloy having a chemical
composition of 2.5-3.5% aluminum; 0.7-1.3% zinc; 0.20-1.0%
manganese; balance magnesium. The 0.2% proof stress curve has been
extrapolated to the right of the vertical axis labeled "ELONGATION"
to show the anticipated effect of temperatures in excess of
300.degree. C. on the 0.2% proof stress.
[0023] FIG. 9 is a block diagram showing components of a system
comprising upper and lower tool assemblies with matched surfaces
and a compliant metal alloy sheet disposed therebetween for use in
consolidating composite preforms.
[0024] Reference will hereinafter be made to the drawings in which
similar elements in different drawings bear the same reference
numerals.
DETAILED DESCRIPTION
[0025] The following detailed disclosure describes a method and an
apparatus for consolidating and molding/forming a composite preform
having an uneven thickness and/or density distribution. In
accordance with the disclosed method, a compliant layer is placed
between the composite preform and one heated consolidation tool.
The compliant layer is designed to distribute the molding pressure
more evenly over the entire area of the uneven preform during the
consolidation process.
[0026] One known apparatus for matched tool consolidation of
composite preforms is partly depicted in FIGS. 1 and 2. FIG. 1
shows the apparatus in a pre-consolidation stage, while FIG. 2
shows the apparatus while consolidation is under way. The apparatus
comprises a lower die frame 2, a lower tooling die 4 supported by
the lower die frame 2 and having a first contoured die surface 6,
an upper die frame 8, and an upper tooling die 10 supported by the
upper die frame 8 and having a second contoured die surface 12
which is complementary to the first contoured die surface 6. The
contoured die surfaces 6 and 12 may define a complex shape
different than what is depicted in FIGS. 1 and 2. However, the
novel means disclosed herein also have application when the die
surfaces are planar. The die frames 2 and 8 may be coupled to
hydraulic actuators (not shown in FIGS. 1 and 2), which move the
dies toward and away from each other. In addition, one or more
induction coils (not shown in FIGS. 1 and 2) may extend through
each of the tooling dies 4 and 10 to form an induction heater for
raising the temperature of the resin in a composite preform to at
least its consolidation temperature. A thermal control system (not
shown) may be connected to the induction coils.
[0027] Still referring to FIGS. 1 and 2, the apparatus further
comprises a lower susceptor 18 and an upper susceptor 20 made of
electrically and thermally conductive material. The susceptors and
the induction coils are positioned so that the susceptors can be
heated by electromagnetic induction. The lower susceptor 18 may
generally conform to the first contoured die surface 6 and the
upper susceptor 20 may generally conform to the second contoured
die surface 12. In some cases, it is preferred that the temperature
at which a composite preform is consolidated should not exceed a
certain temperature. To this end, susceptors 18 and 20 are
preferably so-called "smart susceptors". A smart susceptor is
constructed of a material, or materials, that generate heat
efficiently until reaching a threshold (i.e., Curie) temperature.
As portions of the smart susceptor reach the Curie temperature, the
magnetic permeability of those portions falls to unity (i.e., the
susceptor becomes paramagnetic) at the Curie temperature. This drop
in magnetic permeability has two effects: it limits the generation
of heat by those portions at the Curie temperature, and it shifts
the magnetic flux to the lower temperature portions, causing those
portions below the Curie temperature to more quickly heat up to the
Curie temperature. Accordingly, thermal uniformity of the heated
preform during the forming process can be achieved irrespective of
the input power fed to the induction coils by judiciously selecting
the material for the susceptor. In accordance with one embodiment,
each susceptor is a layer or sheet of magnetically permeable
material. Preferred magnetically permeable materials for
constructing the susceptors include ferromagnetic materials that
have an approximately 10-fold decrease in magnetic permeability
when heated to a temperature higher than the Curie temperature.
Such a large drop in permeability at the critical temperature
promotes temperature control of the susceptor and, as a result,
temperature control of the part being manufactured. Ferromagnetic
materials include iron, cobalt, nickel, gadolinium and dysprosium,
and alloys thereof.
[0028] The consolidation/molding apparatus shown in FIGS. 1 and 2
further comprises a cooling system 14 comprising respective sets of
cooling conduits 16 distributed in the tooling dies 4 and 10. Each
set of coolant conduits 16 is coupled via respective manifolds to a
source of cooling medium, which may liquid, gas or a gas/liquid
mixture such as mist or aerosol.
[0029] In a typical implementation of a composite consolidation and
molding process, the composite preform 22 is initially positioned
between the upper and lower tooling dies of the stacked tooling
apparatus, as shown in FIG. 1. Then the tooling dies 4 and 10 are
moved toward each other, as shown in FIG. 2, as the induction coils
heat the susceptors 18 and 20. Therefore, as the tooling dies close
toward each other, the susceptors rapidly heat the composite
preform 22. During this process, the composite preform will be
molded by the opposing contoured (or planar) surfaces of the
susceptors 18 and 20.
[0030] After a predetermined interval of time, the cooling system
14 will be operated to apply a cooling medium to the tooling dies 4
and 10, thereby also cooling the susceptors 18 and 20 and the
composite preform 22 therebetween. The composite preform 22 remains
sandwiched between the susceptors for a predetermined period of
time until complete cooling of the composite preform has occurred.
This allows the molded and consolidated composite preform 22 to
retain the structural shape which is defined by the contoured
surfaces of the susceptors 18 and 20. The tooling dies are then
opened and the composite preform can be removed. The formed and
cooled composite preform is removed from the stacked tooling
apparatus without loss of dimensional accuracy when it is cooled at
an appropriate property-enhancing rate.
[0031] FIG. 3 is an end view of a portion of a lower tooling die 4
in accordance with one embodiment. The upper tooling die may have a
similar construction. Each tooling die comprises a multiplicity of
cavities 32, which may be mutually parallel. FIG. 3 shows only two
such cavities 32, the upper portion of each cavity 32 having a
portion of a respective turn of an induction coil 34 which passes
through the uppermost portion of the cavity.
[0032] The sectional view shown in FIG. 4 is taken along line 4-4
seen in FIG. 3 and passes through a cavity 32, but not through the
portion of inductive coil 34 therein. One or more coils can be
used. As the parts requiring fabrication get bigger, it may be
necessary to break the coil into multiple coils connected in
parallel in order to limit the voltage required by each coil.
Without the smart susceptors, control of the current (and resulting
temperature) to each parallel coil could become problematic. For
the sake of simplicity, FIGS. 3 and 4 show a portion of a lower
tooling die for which the corresponding portion of the attached
susceptor is horizontal rather than angled.
[0033] Still referring to FIGS. 3 and 4, the lower tooling die may
comprise a lamination of alternating metal (e.g., stainless steel)
plates 28 and dielectric spacers 30 which are trimmed to
appropriate dimensions to form a plurality of parallel longitudinal
cavities 32 in which the turns of one or more induction coils 34
reside. Each metal plate 28 may have a thickness in the range of
about 0.0625 to about 0.5 inch. Air gaps 36 (see FIG. 4) may be
provided between the upper portions of metal plates 28 to
facilitate cooling of the susceptors. The stacked metal plates 28
may be attached to each other using clamps (not shown), fasteners
(not shown) and/or other suitable means known to persons skilled in
the art. The stacked metal plates 28 may be selected based on their
electrical and thermal properties and may be transparent to the
electromagnetic field produced by the induction coils.
[0034] As best seen in FIG. 4, the smart susceptor 18 is attached
directly to the metal plates 28 of the lower tooling die. (The
smart susceptor 20 seen in FIG. 1 is likewise attached directly to
the metal plates of the upper tooling die.) In accordance with one
embodiment, the metal plates 28 are made of austenitic
(non-magnetic) stainless steel and each plate is 0.185 inch thick,
which is suitable when the induction heater is operated at a
frequency of 1 to 3 kHz and with smart susceptors having
thicknesses of 0.125 inch. These lamination plates can be thicker
than 0.185 inch for lower induction heater frequencies and would
need to be thinner when using higher frequencies. These laminations
can (and should) have a space 36 between them to allow the
quenching fluid (gas or liquid) to have direct impingement against
the surface of the heated susceptor 18. This spacing is dictated by
the thickness and strength of the smart susceptor surface shell and
the consolidation pressures used. In addition, the susceptors do
not require an electrical connection to one another. The metal
plates 28 are interleaved with dielectric spacers 30 except near
the susceptor and in places that are needed to allow the quenching
medium to flow to the susceptor. The same considerations apply to
the upper tooling die and the susceptor attached thereto.
[0035] Preferably each induction coil 34 is fabricated from copper
tubing which is lightly drawn. A lightly drawn condition of the
tubing enables precision bending by numerically controlled bending
machines. Numerically controlled bending of the tubes allows
accurate placement of the tubing relative to the changing contours
of the susceptors, thereby improving the degree to which the each
susceptor is uniformly inductively coupled to the induction heater
across the length and width of the susceptor. However, it should be
understood that the compliant layer disclosed hereinafter can be
employed also in cases wherein the susceptors are planar rather
than concave/convex. Optionally the coils 34 also remove thermal
energy by serving as a conduit for a coolant fluid, such as water.
After being bent and installed, the coils include straight tubing
sections connected by flexible tubing sections. The flexible tubing
sections connect the straight tubing sections and also allow the
dies to be separated. The accurate placement of the tubing of the
induction coils 34 promotes uniformity in the amount of heat
generated by the magnetic flux field and the amount of heat removed
by flow of the coolant fluid.
[0036] As disclosed in U.S. Pat. No. 6,528,771, the induction coils
34 can be connected to a temperature control system that includes a
power supply, a controlling element, a sensor and a fluid coolant
supply preferably containing water (not shown). The power supply
supplies an alternating current to the induction coils 34 which
causes the coils to generate the electromagnetic flux field. The
fluid coolant supply supplies water to the induction coils 34 for
circulation through the coils and the removal of thermal energy
from the dies. The sensor is capable of measuring the power
supplied by the power supply. Alternatively, or in addition to
measuring the power supply, the sensor may include a voltmeter that
can measure the voltage drop across the induction coils 34. The
controlling element receives the sensor output and uses the
measurements in a feedback loop to adjust the power being supplied
by the power supply. The controlling element can include hardware,
software, firmware, or a combination thereof that is capable of
using feedback to adjust the voltage output by the power
supply.
[0037] The system described with reference to FIGS. 1-4 can be
enhanced to facilitate the processing of composite preforms having
uneven thickness and/or density across the extent of the preform by
adding a compliant layer between the composite preform and one
susceptor. For example, it is known to fabricate aircraft parts
from a matte product that comprises recycled graphite fibers and
virgin thermoplastic fibers. FIG. 5 shows the placement of such a
matte product 40 (in an unconsolidated state) between a pair of
opposing susceptors 18 and 20 in their retracted positions. The
matte product 40 can have a matted and tangled fiber architecture
and undesirable unevenness and thickness/density variations.
Furthermore, since the graphite fibers are tangled, they do not
facilitate the flow of the thermoplastic material. As seen in FIG.
6, which shows the matte product 40 at the end of the consolidation
process, these characteristics can lead to the formation of voids
42 and/or porosity in the final composite product due to the fact
that thickness and density distribution were not uniform across the
matte product. The formation of voids and/or porosity is further
aided by the fact that the fibers are entangled and flow is limited
and unable to "heal" porosity very effectively.
[0038] In accordance with various embodiments, the consolidation
apparatus further comprises a compliant layer that is situated
between the composite preform and one of the matched molding tools
for the purpose of providing a more even pressure over the entire
area of the preform during the consolidation process. The compliant
layer should have an offset tensile yield point (0.2% of the
strain) in a range of 25-300 psi at the temperature of
consolidation of the preform at strain rates of about 1% to 10%
strain per minute.
[0039] In accordance with one embodiment shown in FIG. 7, a sheet
44 of magnesium base alloy approximately 0.125 inch thick is used
to act as the compliant layer or shim to compensate for uneven
thickness or density over the area of a matte product 40 comprising
recycled graphite fibers and virgin thermoplastic fibers. The matte
product 40 and the magnesium alloy sheet 44 are shown sandwiched
between a pair of susceptors 18 and 20 in their extended position,
i.e., the molding apparatus is closed. A magnesium base alloy is
selected which becomes very soft at temperatures useful for
assisting the consolidation and molding of thermoplastic composites
(i.e., 600-750.degree. F.) and does not melt until above
1000.degree. F. As the temperature and pressure increase inside the
molding apparatus during consolidation of the resulting composite
product, the magnesium alloy sheet 44 softens and forms into the
areas of relatively lower pressure. The magnesium alloy sheet 44
can be reused due to the soft nature of the material.
[0040] More specifically, a suitable magnesium alloy sheet material
is Elektron AZ31B Sheet, which is commercially available from
Magnesium Elektron UK in Manchester, England. AZ31B is a wrought
magnesium base alloy, is non-magnetic, has high electrical and
thermal conductivity, and has a melting range of 1050-1170.degree.
F. Superplastic forming of AZ31B can occur during the preform
consolidation process. The chemical composition of AZ31B magnesium
base alloy is: 2.5-3.5% aluminum; 0.7-1.3% zinc; 0.20-1.0%
manganese; balance magnesium. FIG. 8 is a graph showing the effect
of temperature on the tensile properties of this particular
magnesium base alloy. The 0.2% proof stress curve has been
extrapolated to the right of the vertical axis labeled "ELONGATION"
to show the anticipated effect of temperatures in excess of
300.degree. C. on the 0.2% proof stress.
[0041] Other magnesium base alloys can be used instead of AZ31B
provided that the alloy has an offset tensile yield point (0.2% of
the strain) in a range of 25-300 psi at the temperature of
consolidation of the preform at strain rates of about 1% to 10%
strain per minute. Alternatively, metal alloys having a base
element different than magnesium, such as aluminum, can be used
provided that they have the aforementioned tensile yield
property.
[0042] A system incorporating a compliant layer 44 the type
described above is shown in FIG. 9. In this embodiment, the
compliant layer 44 is attached to the upper tool die 10 by means of
clamps, fasteners or other known means (not shown in FIG. 9), with
an upper susceptor 20 disposed between the compliant layer 44 and
the upper tool die 10. Alternatively, the compliant layer could be
attached to the lower tool die 4 with the lower susceptor 18
disposed therebetween. During the consolidation process, the upper
and lower tool dies are moved toward each other by hydraulic
actuators 46, which tool closing motion is indicated by arrows in
FIG. 9. Electrical power is supplied to the induction coils (not
shown) by a power supply 48 in the manner previously described.
After consolidation and cooling, the hydraulic actuators 46 move
the tool dies apart to allow removal of the consolidated product
from the mold. The compliant layer can be reused.
[0043] A compliant layer of the type described above also has
application in the consolidation and forming/molding of composite
preforms other than the matte product described herein. For
example, the compliant layer can be used in the consolidation of
composite preforms that comprise reinforcing fibers embedded in a
matrix made of either thermoplastic or thermosetting plastic
material.
[0044] While the invention has been described with reference to
various embodiments, it will be understood by those skilled in the
art that various changes may be made and equivalents may be
substituted for elements thereof without departing from the scope
of the invention. For example, a compliant layer can be used with
heated consolidation tools that lack susceptors. In cases wherein
the tooling dies have heated matched surfaces, the compliant layer
can be placed between one of those heated matched surfaces and the
composite preform. In addition, many modifications may be made to
adapt a particular situation to the teachings herein without
departing from the essential scope thereof. Therefore it is
intended that the claims not be limited to the particular
embodiments disclosed.
[0045] The method claims set forth hereinafter should not be
construed to require that the steps recited therein be performed in
alphabetical order or in the order in which they are recited, and
should not be construed to exclude two or more steps being
performed concurrently.
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