Method of fabricating composite structures

A method of fabricating a composite structure having at least one skin molecularly bonded to a core. A skin is provided of reinforcing fibrous material and a thermoplastic matrix. The skin is thermoformed to the general exterior shape of the composite structure. A core having a thermoplastic matrix complementary to the matrix of the skin is molded into the general shape of the composite structure. The skin and the core are placed in juxtaposition in a final forming mold. A thermoplastic material is injected into the final forming mold under isostatic conditions. The thermoplastic material is complementary to the thermoplastic matrices of the skin and the core.

FIELD OF THE INVENTION 
This invention generally relates to a method of fabricating a composite 
structure and, particularly, to a method of isostatic thermoforming of a 
composite structure having at least one skin bonded to a core. 
BACKGROUND OF THE INVENTION 
Composite structures often are fabricated in a laminated array, sometimes 
including a core, such as of foam, sandwiched between a pair of outer 
structural skins and bonded thereto by some sort of adhesive. The skins 
often are reinforced with fibrous material, and the composite structures 
typically are generally pla ar in geometry due to limits in processing 
technology. Thermoset resin systems predominantly are used with such 
composite structures. 
Due to labor intensity in processing composite structures of the character 
described, such as those fabricated from thermosetting resins, the 
structures usually are relatively expensive. They also are not readily 
repairable. Fully cured structures offer few free polymer chains for 
molecular level bonding to a repair material. 
On the other hand, thermoplastics are reformable, readily repairable and 
inherently much more impact-resistant than thermosets. The use of 
thermoplastics for continuous fiber reinforced structures has, however, 
been limited by an inability to achieve consolidated structures with 
suitable dispersion of resin and fiber reinforcements. There is a need for 
a new and improved method of fabricating advanced composite structures 
using thermoplastic materials or matrices. 
An example of an advanced composite structure with which the invention is 
readily applicable is shown in copending application Ser. No. 191,250 , 
filed May 6, 1988, assigned to the assignee of this invention, and which 
is incorporated herein by reference now abandoned. That application 
involves an irregularly shaped housing, such as for a dynamoelectric 
machine, and includes various components such as conduits, electrical 
devices, etc. embedded within the core of the composite structure. 
This invention is directed to satisfying the above needs and solving the 
problems described. 
SUMMARY OF THE INVENTION 
An object, therefore, of the invention is to provide a new and improved 
method of fabricating a composite structure embodying extensive use of 
thermoplastic materials. 
In the exemplary embodiment of the invention, the composite structure has 
at least one skin molecularly bonded to a core. Usually, the core is 
sandwiched between a pair of skins. 
The method includes the steps of providing a skin composed of reinforcing 
material and a thermoplastic matrix. The skin is thermoformed to the 
general exterior shape of the composite structure. A core is molded into 
the general shape of the composite structure, the core having a 
thermoplastic matrix complementary to the matrix of the skin. The 
composite structure is final-formed by placing the skin and the core in 
juxtaposition in a forming mold, and a thermoplastic material is injected 
into the mold under isostatic conditions. The thermoplastic material is 
complementary to the thermoplastic matrices of the skin and the core. 
Therefore, the complementary thermoplastic matrices molecularly bond the 
entire composite structure into an integral, substantially homogeneous 
construction. 
The reinforcing material of the skin may include such materials as 
continuous or chopped graphite fibers or other fibrous material which may 
be woven or braided and interspersed with the thermoplastic matrix. The 
thermoplastic matrix of the various components may be such as 
polyetheretherketone or liquid crystal polymer. All of these materials are 
capable of being advantageously used to molecularly bond the construction 
in the final forming step wherein the thermoplastic is injected under 
conditions of pressure and heat. 
Other objects, features and advantages of the invention will be apparent 
from the following detailed description taken in connection with the 
accompanying drawings.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
Referring to the drawings in greater detail, the invention is directed to a 
method of fabricating a composite structure having at least one skin 
molecularly bonded to a core. To that end, and referring to FIG. 1, the 
method includes providing a skin 10 composed of reinforcing material and a 
thermoplastic matrix. Preferably, the reinforcing material is a fibrous 
material such as a chopped, woven or braided fabric. The material may be 
of graphite, glass or like fibers. The thermoplastic matrix may be of 
polyetheretherketone, liquid crystal polymer or the like. The skin is just 
rigid enough to be manageable or manipulatable for placing in a tool, such 
as a mold 12. The mold has projections 14 for forming or receiving holes 
16 in the skin. Larger projections 18 may be provided to receive or form 
bores 18 of the skin. The size, shape, configuration, apertures, etc. of 
the skin would depend upon the final composite structure to be formed. For 
instance, in the advanced composite structure context to which the 
invention is applicable, the composite structure may be quite detailed and 
intricate, as shown in the aforementioned application Ser. No. 191,250, 
now abandoned. The mold then is heated to thermoform skin 10 to the 
general exterior shape of the composite structure, as described. Under 
normal circumstances, two skins would be thermoformed to sandwich a core 
(described hereinafter) therebetween. 
The next step is to mold a core 20 (FIG. 2) by an injection molding process 
in a tool 22. The tool may be an appropriate injection mold, the lower 
half only being shown in FIG. 2, with a mold cavity 24 complementary to 
the mold cavity of tool 12 in FIG. 1. The core, of course, is molded with 
complementary bores 26, apertures or other features commensurate to 
fabricating the final composite structure. The foam core has a 
thermoplastic matrix complementary to the matrix of skin 10. For instance, 
a polyetheretherketone thermoplastic matrix might be used with both the 
skin and the core. The core could include a filler material, such as 
hollow glass microspheres. An appropriate core composition is shown in 
copending applicatin Ser. No. 153,332, filed Feb. 8, 1988, assigned to the 
assignee of this invention and which is incorporated herein by reference 
now U.S. Pat. No. 4,837,251. 
The method then includes a final forming procedure which is generally, 
schematically illustrated in FIG. 3 This step generally includes placing 
one or two skins 10 and core 20 in juxtaposition within a molding tool 28 
which has a mold cavity 30 shaped to the final, dimensioned configuration 
of the finished composite structure. A thermoplastic material is injected 
into mold cavity 30, as through a conduit 32, of tool 28 under isostatic 
conditions and heat. Inserts (not shown) may be employed in the mold 
cavity as standoffs to maintain the skin spaced from the walls of the 
cavity. Very viscous materials may be used by providing additional 
positions of injection into the mold. In the most extreme case, very 
little flow of the material is required. In that case, the injected 
thermoplastic material acts primarily as a mechanism for application of 
the required consolidation pressure and temperature. Void content in the 
final structure can be very highly controlled by suitable application of 
pressure and temperature. This injected thermoplastic material may be 
reinforced or filled, as with glass fiber, graphite fibers, microballoons 
and other components which increase the strength or stiffness of the final 
structure. Alloyed or interpenetrating network (IPN) materials may also be 
used for the matrices of the skin 10 and the core 20. 
More particularly, a resin chamber 34 is provided for containing a volume 
of the thermoplastic injecting material 36. The chamber is heated and a 
compressor or other motive force 38 may be used to force the thermoplastic 
resin material through conduit 32 into mold cavity 30. The entire mold 
cavity thereby is under a pressure condition which can be monitored, as at 
gage 40. In other words, an isostatic condition is created in mold cavity 
30 so as to provide equal pressure around and throughout the skin(s) and 
core juxtaposed therewithin. 
Thermoplastic injecting material 36 is complementary to the thermoplastic 
matrices of skin 10 and core 20. The material is effective to form the 
final composite structure 40 to very precise dimensions and would even 
provide a finished or smooth coating surface for the structure. By 
providing complementary thermoplastic matrices throughout the injected 
material, the skin and the core, a molecular bond is created throughout 
and between the components of the final composite structure. No adhesives 
or additional steps in the process are necessary to bond the skins to the 
core and, in fact, the molecular bonding creates a homogeneous structure 
which has no adhesion interfaces between the components of the composite 
structure as with prior art methods where skins, cores or other components 
are prefabricated and subsequently bonded together by adhesives or the 
like. 
It will be understood that the invention may be embodied in other specific 
forms without departing from the spirit or central characteristics 
thereof. The present examples and embodiments, therefore, are to be 
considered in all respects as illustrative and not restrictive, and the 
invention is not to be limited to the details given herein.