Plastic and nonmetallic article shaping or treating: processes – Mechanical shaping or molding to form or reform shaped article – To produce composite – plural part or multilayered article
Reexamination Certificate
2002-08-30
2004-11-09
Staicovici, Stefan (Department: 1732)
Plastic and nonmetallic article shaping or treating: processes
Mechanical shaping or molding to form or reform shaped article
To produce composite, plural part or multilayered article
C264S258000, C264S292000, C264S324000
Reexamination Certificate
active
06814916
ABSTRACT:
FIELD OF THE INVENTION
This invention relates generally to forming composite materials, and, more specifically, to forming shaped composite parts.
BACKGROUND OF THE INVENTION
Formed composite parts are commonly used in applications, such as aircraft and vehicles, where light weight and high strength are desired. These applications typically utilize complex contoured parts or channels which must be formed and then cured. Historically, complex contoured composite structures have entailed extensive hand labor to form prior to curing. Typically, pre-impregnated composite fiber plies (“pre-pregs”) such as epoxy impregnated carbon fiber laminates are laid by hand over a shaped form or mandrel. Then the part is cured, often by heat curing. Alternately, dry fabric plies (“dry fabric”) may be laid-up, and then a bonding material is added. This results in a contoured part that matches the shape of the mandrel. However, manual lay-up of pre-preg plies or dry fabric is time-consuming.
An alternate forming method known as drape forming uses vacuum bagging. Drape forming has been used successfully to form composite parts where the parts being formed have only a few pre-preg plies. This method involves heating a flat laminate pre-preg composite blank or charge and forcing it around a mandrel with the use of a vacuum bag. However, this method has met with limited success on very thick laminates or more complex shapes. More complex shapes include beams of various shapes such as C, I, or L shapes, with long flange lengths, contours along their length, variable thicknesses, joggles or offsets. Composite parts which are thicker in some areas and thinner in others have “ply-drops” where plies end. This leaves the final cured part thicker in some areas and thinner in others. Long flange lengths add strength to composite members such as those used in aircraft structures. In many applications, the composite parts to be formed need to be contoured or have joggles or direction changes internal to the part.
Vacuum bag drape forming of such parts often results in wrinkling of the plies. Wrinkles occur because some laminate plies are in compression when bent or urged over the mandrel, and buckle when there is no constraint on the bending portion to prevent out-of-plane-buckling. Similarly, on long flange parts, slip resistance between the plies during bending becomes too great, and inner plies buckle. Buckling or wrinkling of the plies also occurs over tools or mandrels that are curved or contoured, or have joggles along their length. Even slight contours of a radius on the order of thousands of inches is enough to initiate wrinkles. As the composite pre-preg charge is bent over the mandrel, if the length of the flange is too long or slip resistance between the plies is too great, out-of-plane-buckling of the laminate will occur.
Current state-of-the-art drape forming techniques using vacuum bags have not been able to control the stress state and shear forces occurring during the composite forming process. As a result, complex contoured shapes are typically manufactured by ply-by-ply hand lay-up techniques. An improvement to vacuum bagging uses an inflated bag under the bending portions of composite charge as it is formed. This inflated bag progressively deflates as the vacuum bag forces the composite charge over the mandrel. This method has been found to slightly decrease out-of-plane buckling. However, hand forming of thick laminates and more complex shapes is still performed to minimize out-of-plane buckling.
Compression molding techniques also have been utilized to form composite pre-preg and dry fabric charges over a tool or a mandrel. However, such methods have encountered the same difficulties in preventing out-of-plane buckling of the laminate during the forming process. In compression molding, a female mold matching the forming mandrel is forced over the composite charge and the mandrel to form the charge.
FIG. 1
is a cross-sectional view of a prior art vacuum bag forming system for forming composite materials. A composite charge
20
is placed over a mandrel
10
. It will be appreciated that the composite charge may be any suitable material for forming composite parts, including, without limitation, dry fabric or pre-preg plies. The mandrel
10
rests upon or is linked to a vacuum base
26
. The vacuum base
26
, mandrel
10
, and composite charge
20
are covered by a vacuum bag
24
. During forming of the composite charge
20
over the mandrel
10
, the charge
20
is heated and air is evacuated from beneath the vacuum bag
24
, This forms the overhanging portions
21
of the composite charge
20
that overhang the top of the mandrel
10
. In this example, vacuum bagging is used to form the flanges of a C-shaped beam or spar. The laminate plies in the overhanging portion
21
of the composite charge
20
shear past one another as composite charge is formed by the vacuum bag
24
over the mandrel
10
.
FIG. 2A
illustrates the prior art method of compression molding a composite charge
20
over a mandrel
10
. A composite charge
20
is placed over a forming tool or mandrel
10
. A compression mold
30
is forced over the composite charge
20
and the mandrel
10
, pressing the composite plies against the mandrel
10
and forming the part.
FIGS. 2B and 2C
show improved methods of compression molding. In
FIG. 2B
, a composite charge
20
is placed over a mandrel
10
. A compression mold
30
with flexible tips
32
bends the composite charge
20
by being forced over the composite charge
20
and the mandrel
10
. The flexible tips
32
at the corners of the mold
30
decrease out-of-plane buckling in the composite charge as it is formed, by smoothing the plies as they are formed over the mandrel
10
.
FIG. 2C
shows a further variation of prior art compression molding of a composite charge over a mandrel. In
FIG. 2C
, the composite charge
20
is placed over the mandrel
10
. A compression mold
30
with forming bladders
34
is forced over the composite charge
20
and the mandrel
10
to form the composite part. The forming bladders
30
press downward and laterally against the bending portions of the composite charge thus decreasing out-of-plane buckling. In
FIGS. 2A
,
2
B, and
2
C, the laminate plies of the composite charge
20
overhanging the mandrel
10
shear past one another over the entire overhang or flange length during the forming process. This creates a tendency for out-of-plane buckling, especially with thick laminates, long flange lengths, contoured parts, joggles or parts with inflections.
FIGS. 3A
,
3
B, and
3
C illustrate the large surface area where laminate plies shear past one another during forming of a composite charge
20
over a mandrel
10
, utilizing the prior art methods of vacuum bagging or simple compression molding illustrated in FIG.
1
and
FIGS. 2A
,
2
B, and
2
C. In
FIG. 3A
, a flat composite charge
20
is placed over the mandrel
10
. In
FIG. 3B
, as bending of the composite charge
20
occurs, a shear zone
22
exists where the laminate plies shear past one another. This inter-ply shear zone encompasses the entire overhang length or flange length of the part being formed. The magnitude of the shearing increases towards the edge of the flange.
In
FIG. 3
, shearing between the laminate plies in the shear zone
22
continues as the composite charge
20
is forced down over the mandrel
10
. Shearing within the shear zone
22
results in out-of-plane buckling of laminate plies. Under prior art methods of vacuum bagging and compression molding, inner plies of the composite charge laid against the mandrel, are in compression from shearing against the outer plies as the composite charge
20
is formed over the mandrel
10
. This is shown in prior art
FIGS. 3B
, and
3
C, where the entire flange area
22
has slipping between the plies.
Therefore, an unmet need exists for a composite forming method and system which forms thick laminate charges and parts with contours, joggles, or long flanges, without out-of-plane buckling of the laminate plies.
Harris Christopher G.
Van West Barry P.
Willden Kurtis S.
Beaufait Mark S.
Black Lowe & Graham PLLC
Staicovici Stefan
The Boeing Company
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