Plastic and nonmetallic article shaping or treating: processes – With step of making mold or mold shaping – per se – Utilizing surface to be reproduced as an impression pattern
Reexamination Certificate
1998-11-06
2001-07-24
Silbaugh, Jan H. (Department: 1732)
Plastic and nonmetallic article shaping or treating: processes
With step of making mold or mold shaping, per se
Utilizing surface to be reproduced as an impression pattern
C264S257000, C264S258000, C264S275000, C264S317000, C264SDIG004
Reexamination Certificate
active
06264868
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to methods for manufacturing hollow components made of laminated composite materials, particularly laminated composite materials comprising reinforcing fiber layers embedded in a hot-polymerized resin. More specifically, it relates to manufacturing methods using cores to form the components' cavities.
The invention also relates to methods for manufacturing such hollow components having walls enclosing the cavities which are adjacent to non-enclosing walls, which do not subtend the cavities.
2. Description of the Related Art
Laminated composite components comprising reinforcing fiber layers embedded in a hot-polymerizing resin are used, in particular, in the automobile, space and aeronautic industries because of their excellent strength-to-weight ratio. In general, the reinforcing fibers are carbon fibers or silicon carbide fibers and the resins are epoxides, bismaleimides or poly-imides. In particular, one seeks to manufacture components such as reservoirs, ducts, air manifolds etc. with the demands of material quality, surface condition, mechanical and thermal strength, and high dimensional accuracy. These components comprise thin enclosing walls; that is, walls enclosing and subtending cavities, which may possibly communicate with the outside through minute apertures. Such components are desired to have the enclosing walls adjacent to non-enclosing walls (ones not subtending cavities). Illustratively, they may be a casing comprising an air manifold at its surface.
A well known procedure for making hollow components composed of fiber/resin laminated composites uses a mold matching the outer shape of the component and generally being at least in two parts for the purpose of removing the component. This known procedure in particular includes the following essential operations:
making a core in the shape of the component's cavity;
arranging an inflatable balloon around the core;
cladding the core; that is, placing the resin-preimpregnated reinforcing fibers constituting the composite around the combination of the core and the balloon;
arranging the combination of the core, the balloon and the composite in the mold;
hot-polymerizing the resin with the balloon being pressurized;
hot removal of and cooling of the component;
withdrawing the core; and
withdrawing the balloon through the cavity aperture.
The core's melting point may be lower than the onset temperature of resin-polymerization, for instance paraffin or wax. Following the enclosing process, the core may be withdrawn in molten form. Moreover, resin cores are known which dissolve in an organic solvent; thus, after component removal and cooling, the core may be removed in dissolved form from the cavity. However, this procedure has the drawback of degrading the resin of the component, hence the component's surface quality and its strength. Moreover, water-soluble ceramic cores are known. This latter approach appears to be the most appropriate one for the case under consideration. Pressurizing the balloon allows pushing and compressing the composite against the wall of the mold to reduce the inherent swelling of the preimpregnated fiber layers. The expression “swelling” herein denotes the additional thickness of the preimpregnated fiber layers before compression. In this manner, a homogeneous material is produced free of micro-cavities between the fibers. Typical pressures run between 8 and 15 bars, the thickness of the composite thus being reduced by between 15 and 30%. In other words, a density increase, i.e. compaction, is involved.
Nevertheless, this technique has the following intrinsic drawbacks:
1) the inside surfaces are rough and fairly inaccurate because the pressurized balloon follows the deviations in the cladding; furthermore, the balloon poorly pushes the composite into the angular recesses such that the component consequently remains imprecise at those sites;
2) removal of the balloon after molding may be difficult when the cavity is large and the aperture small, thus constraining the designer to provide adequate aperture dimensions;
3) the balloon must be kept in perfect condition; that is, it must remain sealed and its flexibility must be preserved as long as the resin is above its solid-state polymerizing temperature; however, some resins, such as the polyimides which are kept at high temperatures, have polymerization melting points higher than 300° C. The inventor is unaware of any elastomers with which to make balloons withstanding such temperatures; consequently, the procedure cannot be carried out using such resins.
The components consisting of thin and non-enclosing walls (walls not forming cavities) are conventionally manufactured by a so-called “bag” molding technique using an open mold in the shape of the wall, more specifically in the shape of one of the two surfaces of the wall. This procedure in particular comprises the following stages:
cladding; that is, applying the resin-impregnated fiber layers against the mold to form the composite;
covering the composite with a film which is impermeable relative to the mold, the expert previously having arranged a conduit connected to a partial-vacuum source as well as different fabrics which promote evacuation and removal (such techniques are mentioned merely for elucidation but are unrelated to the implementation of the invention);
hot, autoclave polymerization at the required temperature and pressure, the pressure in the autoclave forcing the composite, by the film against the mold surface, to compact this composite and to shape it as required.
However the bag-molding technique cannot be combined with the balloon-molding technique to manufacture an integral component comprising at least one first hollow part (derived by the balloon molding technique) and at least one thin wall not forming a cavity (derived by the bag molding technique). At least one portion of the enclosing wall subtending the cavity will simultaneously be between the bag and the balloon, that is between two pressurized flexible membranes. That wall portion will assume a significantly random shape even when there is rigorous equilibrium between the pressure exerted on the bag and that on the balloon, for instance, when bag and balloon are made to communicate. Consequently, the geometry of a component manufactured by such a combined technique will always be unreliable.
SUMMARY OF THE INVENTION
The invention proposes a method for manufacturing hollow components made of a hot-polymerizing resin-fiber composite which is free of the drawbacks of the balloon manufacturing method, the component comprising enclosing walls subtending at least one cavity, the method using a mold matching the external shape of the component and comprising, in particular, the following stages:
a) manufacturing a core of a shape which corresponds to that of the cavity to be made;
b) cladding the core with at least one layer of resin-preimpregnated fibers to form a fiber composite;
c) arranging the core clad in the composite in the mold; and
d) hot-polymerizing the resin.
Such a method is characterized in that the core applies an outward compression, exerted by its thermal expansion during resin polymerization, on the composite. The core comprises a body covered by layer of silicone elastomer of a high thermal coefficient of expansion a and of a thickness “e” appropriate for the compression percentage applied to the composite.
Conventionally, the silicone elastomer is used as a seal. The invention makes use of a special property of this material, namely a very high coefficient of thermal expansion up to 1,000×10
−6
/° C. Contrary to the case of a core enclosed by a pressurized balloon, the dual-element core of the invention expands thermally as a solid body, thereby imparting its shape to the composite which it compresses against the mold's surface.
Depending on the core rather than on the cladding, the inside surfaces so made are uniform and accurate. The angular recesses are easily shaped because the cor
Bacon & Thomas
Silbaugh Jan H.
Society Hispano Suiza Aerostructures
Staicovici Stefan
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