U.S. patent application number 09/222182 was filed with the patent office on 2002-01-10 for three-dimensional integrated composite surface structures.
Invention is credited to HILLIER, LOREN C..
Application Number | 20020004120 09/222182 |
Document ID | / |
Family ID | 22831220 |
Filed Date | 2002-01-10 |
United States Patent
Application |
20020004120 |
Kind Code |
A1 |
HILLIER, LOREN C. |
January 10, 2002 |
THREE-DIMENSIONAL INTEGRATED COMPOSITE SURFACE STRUCTURES
Abstract
A surface structure including a skin (13) a build-up (18) and a
backing (20). A skin is formed and a build-up layer is applied to
the skin. In one embodiment, gas pressure is applied to form the
skin in a single-sided die. A nozzle on a five-axis arm then
applies resin to the skin in a selected pattern. The selected
pattern may form a hexagonal "honeycomb" cell structure, for
example, to support the skin and may transition to a columnar cell
structure for better crush resistance. The hexagonal-to-columnar
structure provides superior crush resistance in automotive body
panels, for example, and improved solar energy concentration in
solar energy applications.
Inventors: |
HILLIER, LOREN C.; (SAN
DIEGO, CA) |
Correspondence
Address: |
SCOTT W HEWETT
400 WEST THIRD STREET
#223
SANTA ROSA
CA
95401
|
Family ID: |
22831220 |
Appl. No.: |
09/222182 |
Filed: |
December 29, 1998 |
Current U.S.
Class: |
428/73 ;
428/116 |
Current CPC
Class: |
B32B 7/00 20130101; Y10T
428/236 20150115; Y10T 428/24157 20150115; Y10T 428/12493 20150115;
Y10T 428/24579 20150115; E04C 2/328 20130101; Y10T 428/24149
20150115; E04C 2/34 20130101; E04C 2/292 20130101; Y10T 428/1234
20150115 |
Class at
Publication: |
428/73 ;
428/116 |
International
Class: |
B32B 003/12 |
Claims
What is claimed is:
1. An article of manufacture comprising: a skin having a surface; a
build-up applied to the surface of the skin, the build-up being
bonded to the skin; and a backing conformed to and bonded to the
build-up wherein the skin, the build-up, and the backing form a
unitary surface structure.
2. The article of claim 1 wherein the skin comprises metal
sheet.
3. The article of claim 2 wherein the surface of the metal sheet
includes a curve.
4. The article of claim 1 wherein the build-up comprises a
polymer.
5. The article of claim 4 wherein the build-up further comprises a
second phase.
6. The article of claim 5 wherein the second phase comprises
microspheres or microballons.
7. The article of claim 5 wherein the second phase comprises
fibers.
8. The article of claim 1 wherein the build-up comprises a first
zone forming a sheet in contact with essentially the entire surface
of the skin.
9. The article of claim 1 wherein the build-up comprises a second
zone having a plurality of walls defining voids, the voids being
sealed by the backing.
10. The article of claim 9 wherein the build-up is applied in a
selected pattern.
11. The article of claim 10 wherein the build-up is applied in a
liquid state through a nozzle.
12. The article of claim 10 wherein the build-up is applied in a
powder state.
13. The article of claim 10 wherein the build-up is a metal, the
metal being applied to the skin in a molten state with a flame
sprayer.
14. The article of claim 9 wherein each of the walls is essentially
perpendicular to the surface of the skin.
15. The article of claim 9 wherein a first section of the walls
defines a hexagonal pattern and a second section of the walls
defines a columnar pattern, the walls transitioning from the
hexagonal section to the columnar section.
16. A surface structure comprising: a metal skin having a surface,
the surface including a curved portion; a polymer build-up applied
to the surface in a selected pattern, the build-up having a first
zone forming a sheet in contact with essentially the entire surface
and a second zone having a plurality of walls, each wall being
essentially perpendicular to the surface, the walls proximal to the
first zone defining a hexagonal pattern and transitioning to a
columnar pattern distal from the first zone; and a backing
conformed to the polymer build-up to seal voids between the walls
and to form a unitary surface structure.
17. An article of manufacture comprising: a skin with a surface,
the surface including a curved section; a build-up applied to the
skin in a selected pattern, the build-up having a plurality of cell
walls, each of the plurality of cell walls being essentially
perpendicular to the surface over the curved section.
18. The article of claim 14 wherein the selected pattern is a
hexagonal pattern.
19. An article of manufacture comprising; a skin with a surface; a
build-up applied to the surface in a selected pattern to form a
plurality of cell walls, the cell walls defining a hexagonal
section and a columnar section, the hexagonal section being
proximate to the surface, and the cell wall transitioning from the
hexagonal section to the columnar section.
Description
CROSS REFERENCE TO RELATED APPLICATIONS
[0001] This application is being filed concurrently with the
following U.S. patent applications: AN ALUMINUM-THERMOPLASTIC PANEL
AND METHOD OF MANUFACTURE by Hillier (Atty. Docket No.
018981-000100); APPARATUS FOR FABRICATING SURFACE STRUCTURES by
Hillier (Atty. Docket No. 018981-000300); and METHOD AND APPARATUS
FOR FORMING SURFACE STRUCTURES FROM A SINGLE-SIDED MOLD by Hillier
(Atty. Docket No. 08981-000400).
STATEMENT AS TO THE RIGHTS TO INVENTION MADE UNDER FEDERALLY
SPONSORED RESEARCH AND DEVELOPMENT
BACKGROUND OF THE INVENTION
[0002] This invention is concerned with surface structures having
improved structural characteristics over conventional shaped
panels, such as may be used in the manufacture of vehicles or in
other applications, such as aerospace, industrial applications,
military applications, and recreational products. The invention is
directed at replacing currently used materials in existing
applications, as well as providing new materials for use in new or
existing applications.
[0003] Panels are used in a wide variety of applications, and come
in a wide variety of materials and shapes. Some panels are flat,
and some are bent or stamped into a shape. For example, a body
panel on an automobile might be stamped from sheet steel or other
alloy. In some cases, such a panel might be too weak to act as a
structural component of the assembly, and is fastened to a frame or
chassis. In other instances, the panel may be sufficiently strong
to provide structural support, or may be welded or otherwise joined
with other sheet steel parts to form an assembly that includes the
panel portion for use in the final product.
[0004] Unfortunately, a stronger panel generally means a thicker
panel. It takes more energy to form a panel into a given shape if
the material is thicker, and thicker material might limit the
shapes that can be formed. Also, a thicker panel is heavier. Thus,
simply making a thicker panel to obtain strength adds to both the
material costs and the fabrication costs, as well as the weight of
the final product, particularly if a panel is made from steel
sheet.
[0005] Thinner panel pieces are often shaped and then spot-welded
into a box-like assembly. The various pieces support each other to
make a structural element of desired strength and rigidity, which
often weighs less than either a thin sheet attached to a frame, or
a thick structural panel. However, thinner panels are not without
problems, such as denting and rust-through. Thin sheet steel dents
relatively easily, even from minor impacts, and might rust entirely
through in a short time if corrosion protection fails.
[0006] Alternatives to making body panels from stamped steel sheet
have been developed to overcome some of the limitations of steel
panels. One approach has been to fabricate panels, or even complete
bodies, out of re-enforced resin (so-called "fiber-glass")
composites. Fiber-glass parts are generally lighter than
comparative steel parts, allow greater choice in the types of
shapes that may be fabricated, and do not rust. Fiber-glass parts
are typically attached to a frame, as they are typically not
structural, as they tend to fail under strain when in sheet form,
although sometimes a shape is built up or adhesively attached to
provide mechanical strength. However, fiber-glass parts tend to
crack or splinter on impact, rather than absorbing much energy from
the impact, and are considered to be relatively fragile, and
scratch easily. Other re-enforced resin systems, such as
"carbon-fiber" systems have been developed to improve some of the
shortcomings of fiber-glass parts, but are typically more difficult
to work with, and more expensive. In some applications, fiber-glass
or other composite parts are molded into a thick, structural part
of complex (multiple curved surfaces) part.
[0007] Molded parts and stamped steel or other alloy parts both are
typically made in a process using a two-part mold or two-part
stamping die, respectively. Making two-part mold tooling is fairly
straight forward, but the resulting sprues and seams must be
trimmed from the part before the part can be considered finished.
Also, the material, which is typically injected into the mold in a
fluid form, must typically be left in the mold long enough to
solidify, either by cooling or by chemical reaction, to solidify
enough to retain the shape of the mold upon removal. The mold dwell
time can slow down the entire fabrication process, thus increasing
costs.
[0008] Stamping steel or other alloys also requires substantial
tooling costs. A stamp and die are both precision tools that must
match, and that typically accept a particular material of a
particular thickness. Changing the design of a stamped part is
expensive and time consuming. Stamping steel sheet or other alloy
has other problems that limit the type of shapes that can be
formed. For example, there is a certain maximum depth, also known
as aspect ratio, that a particular sheet can be drawn to. Trying to
stamp sheet into certain shapes can cause pulling and stretching of
the sheet, particularly puckering, or webbing, in areas adjoining
the seam of a curved area.
[0009] Therefore, a panel that may take a shape with complex curves
that is light, strong, dent resistant, and corrosion resistant is
desirable, and a method for making such panels that is efficient
and adaptable to various materials and shapes is further
desirable.
SUMMARY OF THE INVENTION
[0010] A surface structure including a skin portion bonded to a
build-up section and a backing provides a light, strong, versatile
structural element. In one embodiment, the build-up section
includes cells, the cell walls being essentially perpendicular to a
curved surface of the skin portion. In another embodiment, the
build-up section includes cells that transition from a hexagonal
configuration (cross section) to a columnar (round) configuration.
These embodiments of the invention can be combined for further
benefit.
[0011] In one instance, gas pressure is used to deform a skin
preform to a desired shape and the build-up is applied, as a
settable liquid, semi-liquid, or powder, for example, in a desired
pattern to the formed skin. In another instance, the skin is
applied as a liquid or powder to a single-sided mold and the
build-up, of similar or different material, is then applied in the
desired pattern. In yet another instance, a casting pattern is made
that includes a skin section and at least the build-up section. The
casting pattern is made from wax, for example, which is then used
in a lost-wax process or other process to cast a preliminary
surface structure, to which the backing may be applied.
[0012] These and other aspects of the invention will be better
understood by reference to the following detailed description in
connection with the accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
[0013] FIG. 1A is a simplified cross section of a composite panel
according to the present invention;
[0014] FIG. 1B is a simplified cross section of a portion of a
surface structure with a graded filler build-up section;
[0015] FIG. 1C is a simplified cross section of flat surface
structure according to one embodiment of the present invention;
[0016] FIG. 1D is a simplified cross section of a flat surface
structure with a hexto-round build-up cell, according to another
embodiment of the present invention;
[0017] FIG. 2 is a simplified cross section of a curved surface
structure, according to another embodiment of the present
invention;
[0018] FIG. 3A is a simplified cross section of a skin preform in a
single-sided mold;
[0019] FIG. 3B is a simplified representation of build-up being
applied to a curved skin;
[0020] FIG. 3C is a simplified cross section of a skin and build-up
section in a single-sided mold with a backing preform;
[0021] FIG. 3D is a simplified cross section of a surface structure
in a single-sided mold after application of the backing to the
build-up section;
[0022] FIG. 4A is a simplified flow chart of a process for making a
surface structure according to one embodiment of the present
invention;
[0023] FIG. 4B is a simplified flow chart of a process for making a
surface structure with an explosive-set skin;
[0024] FIG. 5A is a simplified illustration of a three-station
apparatus for fabricating surface structures;
[0025] FIG. 5B is a simplified flow chart of a three-station
fabrication process; and
[0026] FIG. 6 is a simplified illustration of a system for forming
surface structures.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0027] A composite panel including a skin, a center build-up
section, and a backing can be fabricated into a variety of shapes,
including shapes having complex curves, from a variety of
materials. The composite panel results in a surface structure with
desirable and selectable characteristics, such as weight,
stiffness, corrosion resistance, impact resistance, and cost. The
skin can be plastic or metal, such as aluminum, titanium, or
stainless steel, for example, and the center can be thermoplastic
resin, thermosetting resin, or combinations thereof, including
resin-filler composites, as well as other materials, such as metal.
In one embodiment, a nozzle on a multiple-axis carriage discharges
heated thermoplastic resin to build up a polymer center in a
particular pattern on a shaped skin. The polymer center has
sufficient strength to serve as a die for setting, i.e., shaping,
the backing, which may be aluminum sheet, for example. This allows
the center to be directly bonded with the skin and the backing, and
also results in conformal fidelity and integration of the skin,
center, and backing without the use of intermediary adhesives.
[0028] FIG. 1A is a simplified cross section of a composite panel
10 according to one embodiment of the present invention. A skin 12
is joined to a center section 14, also known as a "build-up", which
is joined to an encapsulating skin 16, or backing. The skin can be
metal sheet, such as 6061-O condition aluminum, other aluminum,
such as 1100-O, 3003-O, 5052-O, and 7075-O, titanium, stainless
steel, such as type 4300, plain steel, such as ASTM-A620,
super-plastic alloy, polymer, such as a thermoplastic or
thermosetting plastic, including ABS, polycarbonate, polyethylene,
polyurethane, and type I or II PVC, or other material, such as a
woven material, including materials woven from ceramic or glass. It
is generally desirable that the skin material have good tensile
strength.
[0029] The build-up 14 is a polymer material that is built-up on
the skin, rather than being adhesively attached to the skin with an
intermediary adhesive layer. The build-up is applied to the skin
while the skin is in condition for the build-up to bond to the
skin, either through mechanical bonding, chemical bonding, or both.
In the case of a metal or thermoplastic skin, the skin can be
heated prior to applying the build-up material. In the case of a
thermosetting skin, the skin can be partially un-cured while the
build-up material is applied, or treated with a surface activator,
thus allowing the skin to intimately bond with the build-up.
Examples of thermoplastic materials include polyvinyl chloride,
polycarbonate, acrylic resins, acrylonitrile-butadiene-styrene
("ABS"), polyethlyene, and polypropylene. Examples of thermosetting
materials include epoxies, vinyl esters, polyesters, urethanes,
phenolics, and polyimides.
[0030] The build-up material is applied to the skin as by spraying,
flame-spraying, extruding, or otherwise dispensing, and is
generally applied in a fluid state, such as a liquid, although some
materials, including composite materials, may be applied in a
powder state, and then processed to bond to the skin. Of course,
not all build-up materials will be compatible with all skin
materials, but several examples are provided below. In an
alternative embodiment, a casting pattern is made, such as by
dispensing liquid casting wax on a wax sheet in the mold to produce
a casting pattern for a surface structure.
[0031] The backing 16 is typically aluminum sheet, polyamide film,
or the like, and is intimately bonded to the build-up 14. The
backing preferably has good tensile properties, and is at least
slightly conformable, so that intimate contact with the build-up
can be obtained. The back-up material can be heated or treated with
a surface sensitizer, or only partially cured, when the backing is
applied to obtain good chemical bonding between the build-up and
the backing. Additionally, pressure is used to press the backing
against the build-up to bring the two layers in contact. The
pressure may be mechanically applied, such as by rolling, or may be
hydrostatically applied, such as by gas pressure. The build-up has
sufficient strength to support the applied pressure. During this
process, the skin is externally supported, by a table or die, for
example, so that the composite surface structure does not deform.
The combination of the skin 12, build-up 14, and backing 16 form a
box-like structure in which the skin and backing, separated by the
build-up, provide tensile strength to resist deformation of the
composite surface structure in various directions.
[0032] FIG. 1B is a simplified cross section of a portion of a
surface structure 11 according to another embodiment of the present
invention. The build-up layer 15 is graded to obtain a selected
property. For example, the buildup layer may start as a single
phase 17 (pure build-up material), and then transition to a filled
material 19. The filling material 21 may be gas bubbles, such as
nitrogen that is entrained in the stream of build-up material as it
is applied to the skin, microballons, such as glass microballons,
microshpheres, such as aluminum, glass, or ceramic microspheres, or
gas bubbles may be generated in the build-up material itself. For
example, if polycarbonate is used as the build-up material, the
single phase portion of the build-up layer can be applied at a
temperature that liquefies the polycarbonate, and the temperature
can be raised when applying the filled material to spontaneously
cause bubbles to form and become entrapped in the polycarbonate,
such as supplying the polycarbonate at a temperature of about
485.degree. F. with the nitrogen at about 210.degree. F. at a
pressure of about 4-5 psi.
[0033] Selected superior material properties may be achieved
depending on the desired characteristics of the surface structure
11. For example, entraining or generating gas in the filled
material section can lighten or improve the thermal insulation
properties of the surface structure. A reactive gas, such as
oxygen, can be entrained in a build-up material, such as polyester,
to improve the cure time. In a preferred embodiment, microspheres
are mixed with the polymer or thermosets used in the build-up layer
to provide compressive strength. Other solid forms, such as fibers,
particularly carbon fibers about 0.0003 inches in diameter and less
than about 0.063 inches long, may be used alternatively or in
addition to the microspheres, but microspheres generally provide
better compressive strengthening than fibers as the microspheres do
not exhibit as much of a problem arising from inter-laminate shear.
Generally, fill ratios range between about 20-33 volume %, but
other fill ratios may be appropriate depending on the base material
and filler used The microspheres may be solid, such as spherical
aluminum metal powder, or may be hollow, such as zirconia or glass
bubbles. The microspheres range in diameter from approximately
0.0015-0.008 inches, and a single size of microspheres may be used
in the build-up layer, or microspheres of various sizes may used
for better packing density of the spheres. Additionally, spheres of
various materials may be combined in a single build-up layer.
[0034] FIG. 1C is a simplified cross section of a flat surface
structure having a having a skin 13, such as sheet metal, a
build-up section 18, and a backing 20. The build-up section 18
includes a first zone 31 contacting the skin 13 essentially over
the entire interface between the skin and the build-up section, and
a second zone having walls 33 that form box-like voids 35 in the
build-up section. The backing 20 seals the voids, the combination
of which provides stiffness to the surface structure. The walls of
the voids also provide "give" to the surface structure for improved
dent resistance by transferring loading to global buckling and then
post-catastrophic failure of the hexagonal cells to local buckling.
In some embodiments the voids may be evacuated to provide improved
thermal insulation or mechanical properties, or may be pressurized
with air, nitrogen, water, oil, or other fluid to improve thermal
conductivity, impact resistance, or other mechanical
properties.
[0035] FIG. 1D is a simplified cross section of a portion of a
surface structure showing cells that transition from a hexagonal
cross section 37, or pattern, to a columnar (round) cross section
39. The hexagonal cell pattern contacts the skin 13 in a continuous
fashion, that is, the walls of the hexagonal cells provide support
to the skin without gaps between cells, thus improving the strength
of the surface structure, while the columnar cells provide superior
crush resistance. The hex-to-round cell structure combines these
characteristics to result in a superior support for a skin.
[0036] FIG. 2 is a simplified cross section of a portion of a
surface structure 22 with a curve. The curve is shown only in the
plane of intersection, but may also have components of curvature
that are not co-planar with the plane of intersection, and the
surface of the skin can define a number of curves, such as may be
defined with a non-uniform rational B-spline ("NURBS") surface. The
skin 23 is 6061 aluminum sheet that was originally about 0.060
inches thick., but thinner aluminum alloy sheet could be used, such
as 0.009 inch thick, or steel sheet about 0.010 inches thick, or
thermoplastic sheet 0.0045 inches thick. The 6061 aluminum sheet is
in the O condition, as that condition is less prone to cracking
when formed than aluminum sheet in the T6 condition. The build-up
24 is polycarbonate, for example, and is dispensed in a selected
pattern on the skin from a computer-controlled 5-axis nozzle to a
final thickness of approximately 0.090 inches The nozzle and skin
are heated so that the polycarbonate may be easily dispensed
through the nozzle and form a good chemical bond with the skin. The
surface of the skin has been textured during a forming process that
will be described in further detail below.
[0037] The polycarbonate, which may contain fillers is dispensed on
the skin in a pattern resulting in three zones. Some types of
fillers, such as carbon fibers, may be used to strengthen the
composite resin, other types of fillers, such as microballons, may
reduce the effective viscosity of the resin to aid in dispensing
the resin, as well as strengthen the resin or improve other
properties, such as thermal insulation. The first zone 25 is a
sheet of polycarbonate contacting the skin over its entire surface.
Complete contact with the skin provides good adhesion and shear
strength between the skin and the build-up section 24, which is
about 0.004 to 0.030 inches, preferably at least 0.010 inches,
thick. The second zone 26 is a region of hexagonal cells, or
"honeycomb". The hexagonal cells are formed by dispensing the
polycarbonate in a hexagonal pattern over the first zone sheet 25
in multiple passes. As the polycarbonate cools and solidifies after
extrusion from the nozzle, the hexagonal cells are built up. The
walls 26 of the hexagonal cells are essentially perpendicular to
the surface 27 of the skin around the curve to provide good support
to the skin. Other laminates with hexagonal cores are generally
limited to flat sections, so that the cell walls are perpendicular
to the surface, or are machined to fit curved surfaces, in which
case the cell walls are not always perpendicular to the surface. It
is desirable to have the cell walls perpendicular to the surface to
resist buckling upon impact. If desired, the hexagonal cells may be
tapered to improve impact deceleration, or may remain constant in
cross section to support higher static loads.
[0038] The third zone is a region of columnar cells 28. The
build-up transitions from the hexagonal zone 26 to the columnar
zone 28. The columnar cells are less likely to collapse than
similar hexagonal cells, while the hexagonal cells provide superior
support to the skin than columnar cells, thus a build-up of
superior properties is obtained by transitioning from the hexagonal
structure to the columnar structure so that global loading is
converted to localized loading, and critical deformation occurs on
the backing, rather than the skin. The transition from one zone to
another is achieved by controlling the delivery rate, direction,
and speed of the delivery nozzle.
[0039] Applying the build-up to the skin offers many advantages
over conventional laminates. As discussed above, all of the cell
walls are essentially perpendicular to the skin, even the curved
portions of the skin. Also, the thickness of the cell walls may be
adjusted along the length of the cell by varying the dispensing
rate, e.g. pressure or nozzle speed, to optimize characteristics of
the surface structure for various combinations of materials and
configurations. The cell dimensions, or pitch, can also be varied,
to provide more cells in areas requiring greater support, for
example. The cross-cell dimension for hexagonal cells is generally
between about 0.125-1.00 inches, and the cells typically have an
aspect ratio of about 4.
[0040] A backing 29 of 6061 type O aluminum approximately 0.010
inches thick is formed over the build-up 24. The backing is applied
to the buildup while the build-up is still warm enough to deform
and bond to the backing, but strong enough to provide at least
about 70% of the ultimate (cold) compressive strength of the
build-up. Gas pressure is used to press the backing against and
into the build-up, conforming the backing to the surface of the
buildup and intimately bonding the backing to the build-up to
achieve a unitary surface structure 22 comprising the skin 23,
build-up 24, and backing 29. The above structured build-up, or
other structured build-ups, can be applied to flat panels, as well.
In alternative embodiments, metal is flame-sprayed from a nozzle to
form the build-up section on a formed skin. Higher aspect ratios
may be obtained if the nozzle is used to flame-spray a skin section
on the single-sided mold prior to forming the build-up section.
Similarly, a plastic skin section may be dispensed onto a
single-sided mold prior to forming a build-up section on the skin.
The material used for the skin and the build-up may be the same, or
different, and a single nozzle or multiple nozzles may be used.
When the skin is dispensed on the mold, differential thermal
expansion/contraction between the build-up material and the mold
material can be used to assist in removal of the build-up
material.
[0041] FIGS. 3A to 3D are simplified cross sections illustrating a
process according to the present invention. FIG. 3A shows a skin
preform 30, in this case a sheet of 6061 O-type aluminum 0.010
inches thick, in relation to a single-sided die 32. The
single-sided die 32 is shown with a simple curve 34 for purposes of
illustration only, and may define a NURBS surface with an aspect
ratio appropriate for the selected skin preform, and may be a
"live" die (not shown). A live die has movable portions that can
further shape the skin preform after forming it to the initial
single-sided die. It will be appreciated that a single-sided die is
much less expensive to fabricate than an equivalent stamp-die. In
fact, the single sided die may be fabricated using computer
numerical control ("CNC") machining methods and similar software as
was used to design the surface structure, thus simplifying the
design-tooling process. Similar software may then also be used to
control the build-up material delivery nozzle.
[0042] Cartridge heaters 36 inside the die heat both the die and
the skin preform, in this case to about 450.degree. F., to soften
the skin preform and assist in the formation process. A vacuum port
38 delivers a vacuum between the skin preform and the die, and a
clamp 40 holds the skin preform in place and provides a nominal
metal-to-metal seal 42. An inlet port 44 in the clamp 40 admits
gas, such as heated nitrogen, on the distal side 46 of the skin
preform at a pressure between 20-110 psi, preferably about 45 psi,
and at a controlled rate for a ramp-up period of about 20 minutes
to conform the skin preform to the shape of the die. The length of
the ramp depends on the desired aspect ratio. A 20 minute ramp is
appropriate for an aspect ratio of 0.6 in 0.060 inch thick sheet of
6061-O, for example, while an aspect ratio of 0.1 requires only a 2
minute ramp. The gas flow is controlled by a dome pilot operated
gas valve, available from TESTCOM of Elk River, Minn., for example,
the valve being computer controlled to provide the selected gas
flow rate. For other skin preforms, such as a MYLAR.TM. skin
preform, the skin is formed in as little as two minutes, and a
computer-controlled valve is optional. When the aluminum skin
preform is conformed to the die, the surface of the skin becomes
slightly textured, like an orange peel. This texturing enhances the
subsequent bonding between the skin and the build-up. Furthermore,
the use of a gas to conform the skin preform to the die results in
a clean surface, unlike the surface that would be obtained if
another fluid, such as an oil, were used to hydroform the skin.
Additionally, the gas has relatively little thermal capacity, and
is easily vented, thus the cool-down of the skin in preparation for
the build-up is relatively fast.
[0043] When using other metals, such as titanium or stainless
steel, for the skin preform, an additional step of explosively
setting the skin preform to the die may be desired to overcome
spring-back. An explosive set is a rapid discharge of gas applied
to the distal side of the skin preform after the skin preform has
been formed to the die by the initial gas ramp. For example, a skin
preform of 3045 stainless steel 0.006 thick and approximately six
inches in diameter may be explosively set to the die with a burst
of heated nitrogen at reservoir pressure of 650 psi that releases
1,500 cubic inches in about 0.14 seconds to the distal side of the
skin preform. The gas is transferred from the reservoir to the skin
preform via a burst-diaphragm poppet valve, also known as a
"rupture valve", available from PETER-PAUL ELECTRONICS COMPANY of
New Britain, Conn.
[0044] FIG. 3B shows the skin 48 after forming to the die 32. The
skin is dry, clean, and warm, and is ready for the build-up to be
applied. A nozzle 50 applies polymer to the skin in a selected
pattern, as described above. The nozzle has two inputs 52, 54 for a
two-component (A and B) polymer 56, 58, such as an epoxy system or
a styrene-acrylonitrile/polybu- tadiene system, but could easily be
used to dispense single-component polymers, such as
polyvinylchloride or polycarbonate. Additional nozzle inputs (not
shown) may be present to add accelerators, initiators,
plasticizers, or the like to the polymer before applying the
polymer to the build-up layer. The nozzle has a heater 60 for
heating some polymers to lower their viscosity. The nozzle has an
orifice 62 approximately 0.090 inches in diameter and the nozzle
can travel at speeds up to 20 feet-per-second, although these
specifics are given as examples only and are not meant to limit the
invention. The nozzle has delivered polymer to build up the first
zone 25 next to the skin 23 and has started to build-up a second
zone 26 of honeycomb cells, which are exaggerated in height for
ease of illustration.
[0045] Filler 61, such as microspheres or fibers, is supplied to
the nozzle 50 by a positive-displacement pump 63, where the filler
combines with the resin. In some instances, it may be desirable to
combine the filler with the resin in an optional premixer 64, which
can also be used to mix multi-component resins or a resin and other
substance, such as an accelerator or plasticizer. In other
instances, multi-component resins may be supplied to the nozzle,
where they mix prior to being dispensed on the skin. In this
instance, glass spheres are used as a filler. The spheres are
approximately 0.00015 to 0.008 inches in diameter and are sold
under the name K-20 ZEEOSHPERES.TM. available from 3M/Specialty
Additives Division of Saint Paul Minn., for example. Alternatively,
zirconia microballons or ceramic spheres of similar dimensions may,
be used. The microspheres improve the fluidity, i.e. lower the
viscosity, of the dispensed polymer. Lowering the viscosity of the
applied polymer by the addition of microspheres or microballons is
particularly desirable when dispensing thermoplastic resins, which
generally have a higher viscosity than many of the thermosetting
resins at similar temperatures. Lowering the viscosity of the
applied resin also allows use of a smaller orifice on the nozzle
(for given delivery pressure) for more precise dispensing of the
build-up material, and also results in a higher flow rate, thus
reducing the time required to build-up a given structure. Glass
microballons are preferred to use in conjunction with polycarbonate
polymer because of the high delivery pressures used to extrude the
polycarbonate through the nozzle. These delivery pressures are
obtained by pulse-width modulation before the nozzle.
[0046] Other types of spherical fillers can be used, for example,
solid aluminum microspheres will withstand high delivery pressures
used in conjunction with some resins, or other types of
microballons may be used to fill other polymers, such as epoxy
resins, which can be delivered at a lower nozzle pressure. Fillers
may be combined for some applications, including a variety of
different microspheres, or microspheres in combination with other
fillers, such as fibers to improve tensile strength, or other
solids, such as clay, cellulose, or silica may be used.
[0047] FIG. 3C shows the skin 23 and the build-up 25, which have
not been removed from the die 32 since the skin was formed to the
die, and a backing blank 66 positioned for assembly to the
build-up. The backing blank is a sheet of 6160-O aluminum
approximately 0.010 thick, but could be other material, as
discussed above in relation to the skin. In general, it is
preferred that the material of the backing blank provide tensile
strength to the back of the surface structure. In this instance, in
which the build-up has been patterned into a honeycomb
transitioning to columns, another desirable feature of the backing
blank is that it is deformable.
[0048] As for the skin-forming process, described above, the
backing blank 66 is heated to about 250.degree. F. by hot nitrogen.
The build-up 25 is maintained at a temperature of approximately
210.degree. F. by the cartridge heaters 36 in the die. This
temperature allows the build-up layer to attain approximately 70%
of its compressive strength while remaining slightly softened, so
that the backing will bond to the build-up during the
backing-forming process. Other build-up materials may be have
greater or lesser portions of their ultimate compressive strength
during the backing-forming process, depending on the strength of
the build-up and the type of backing blank used.
[0049] FIG. 3D shows the surface structure 68 in the die 32 after
the backing 70 has been formed and bonded to the build-up 25. The
backing has been formed to the build-up by applying a burst of gas
to the distal side 72 of the backing preform (see FIG. 3C). The gas
was applied at a pressure of approximately 30 psi. Of course, if
the build-up is substantially flat, such as is shown in FIG. 1C,
the backing would not have to deform to the structure of the
build-up, but should still conform to the surface of the build-up.
In the present example, the backing 70 acts as an encapsulating
skin seals gas in the voids 74 of the build-up layer 25. These
gas-filled voids enhance shock absorption of the surface structure
68 and also provide thermal insulation between the skin 23 and the
backing 70. In some embodiments, forming a metal backing to the
build-up work-hardens, or strain-hardens, the backing to provide
additional strength.
[0050] FIG. 4A is a flow chart of a process 400 used for
fabricating a surface structure according to an embodiment of the
present invention. A skin preform is formed to a single-sided die
by applying gas pressure (step 402). Then, without removing the
skin from the die, the build-up layer is formed on the skin (step
404). Next, the backing is conformed to the build-up layer (step
406).
[0051] FIG. 4B is a flow chart of another process 410 used for
fabricating a surface structure according to another embodiment of
the present invention. A skin preform is formed to a single-sided
die with a ramp of gas pressure (step 412) starting at between
20-110 psi increasing at 2-9 psi per minute. The skin preform is
then explosively set to the die (optional) with a burst of gas
(step 414) at 45-650 psi, increasing at 45-650 psi per second. A
first zone of a build-up layer is formed on the skin by applying
polymer to the surface of the skin (step 416). A second, patterned,
zone is formed on the first zone of the build-up layer by applying
polymer through a nozzle that is controllably moved to create the
desired pattern (step 418). Optionally, additional zones may be
applied, either by applying polymer through the nozzle, or by other
means (not shown). A backing is conformed to the build-up layer
(step 420), and the surface structure is removed from the die (step
422).
[0052] FIG. 5A is a simplified diagram of a three-station surface
structure fabrication apparatus 500. The apparatus has three
stations 502, 504, 506 and three die 503, 505, 507. After
processing at each station, the die are rotated 508 to the next
station so that the surface structure is formed without removing
any component of it from the die until the structure is completed.
The first station 502 includes a first die 503 and cap 510 and
applies gas from a gas reservoir 512 to set a skin preform (not
shown) to the first die. The second station 504 includes a second
die 505 that has a skin 514 that was formed to the second die when
the second die was at the first station. A delivery nozzle 516
mounted on a crane arm 518 applies polymer to the skin to form a
build-up layer on the skin.
[0053] The third station 506 includes a third die 507 and a cap
520. The die has a skin and build-up layer that were formed at the
first and second stations. At the third station, a backing preform
(shown) 6061-O aluminum alloy is conformed to the build-up layer by
applying a burst of gas from the gas reservoir 512. A diverter
valve 522 selects whether gas from the reservoir will be supplied
to the first station 502 or the third station 506. Alternatively,
two separate gas reservoirs could be used, one for each
station.
[0054] The build-up step at the second station typically requires
more time than the combined times at the first and third stations,
so a single reservoir with the diverter valve simplifies equipment
requirements.
[0055] FIG. 5B is a simplified flow chart of a process 501 for
fabricating a surface structure using a three-station line. At the
first station, the skin preform is formed to a single-sided die
using gas pressure (step 503). The die is then moved to the second
station (step 505), where a build-up section is applied to the skin
(step 507). The die is then moved to the third station (step 509),
where the backing is applied (step 511). The surface structure is
then removed from the die (step 513), and the die is readied for
another assembly sequence. In a preferred embodiment, the die is
moved from one station to the next by rotating a platen supporting
at least three single-sided dies. The dies can be all of the same
shape, or may define different shapes.
[0056] FIG. 6 is a simplified diagram of an apparatus 600 for
fabricating surface structures. The apparatus may be used in the
three-station apparatus described in conjunction with FIG. 5, or
may be used in other configurations, such as a linear assembly line
or a single-die system. An operator 602 enters commands to a
controller 604 through a user interface 606. The controller could
be a personal computer, a single-board computer ("SBC"), or
programmable logic control ("PLC") module, for example. The
controller includes a processor 601 and a memory 603 that contains
an operating program 605 that has been entered into the memory.
[0057] The controller controls several functions of the apparatus
according to the control program 605 stored in the memory 603 via
control lines 608, only a few of which are shown for purposes of
illustration. The controller can control the operating pressure of
the gas reservoir 512 according to a feedback signal from a
pressure gauge 519, the operating temperatures of the gas heating
element 608, nozzle heater 60, and die heaters 36, according to
feedback signals from temperature sensors, such as thermocouples
(not shown), the delivery rate of polymer from the resin source 515
by a gas-pressure control valve 517, and three-dimensional
positioning and travel of the nozzle 50, among other functions. The
controller can control the addition of filler, such as microspheres
or fibers, from a filler source 632 by controlling the operation of
a positive-displacement pump 634. Gas pressure may be applied to
the die for skin forming or backing operations through a valve 630,
while a different valve 632 may be used for explosively setting
some skin materials, as described above. The gas reservoir 512
receives process gas from a gas source 636, which could be a gas
tank, gas line, or pump, such as a high-pressure pump attached to a
liquid nitrogen source.
[0058] The nozzle 50 is carried on a crane arm 618. The crane arm
is controllably movable with respect to the workpiece 620, and the
position and travel of the nozzle is controlled by motors 622, 624,
626, such as stepper motors, servo motors, hydraulic motors, or
voice coil motors. In a preferred embodiment, servo motors coupled
to the crane 622, are used in conjunction with servo motors mounted
on the crane 624, and coupled to the nozzle to position and move
the nozzle tangentially to the surface of the skin, while a voice
coil motor 626 positions the nozzle perpendicularly from the
surface of the skin. A hydraulic ram 628 is also used to adjust the
height of the crane arm. Thus, the nozzle may traverse a selected
path, such as to define a hexagonal web, while the nozzle maintains
a selected distance from the surface of the skin, even on surfaces
with multiple and complex curves.
[0059] The invention has now been explained with reference to
specific embodiments; however, other various modifications,
alternatives and equivalents may be used. For example, the
composition of material dispensed from the nozzle may be
intentionally varied to optimize selected parameters of the surface
structure. Similarly, fillers may be combined with the resin in a
pre-mixer, rather than at the nozzle. Other embodiments will be
apparent to those skilled in the art. Therefore, this application
should not be limited except by the following claims.
* * * * *