Adhesive bonding and miscellaneous chemical manufacture – Methods – Surface bonding and/or assembly therefor
Reexamination Certificate
2001-01-05
2003-01-28
Sells, James (Department: 1734)
Adhesive bonding and miscellaneous chemical manufacture
Methods
Surface bonding and/or assembly therefor
C156S308200, C156S580100, C156S580200
Reexamination Certificate
active
06511563
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to a bi-directional horn for ultrasonically consolidating a fiber reinforced composite structure.
BACKGROUND OF THE INVENTION
Composite materials are becoming more and more attractive for a wide variety of uses, from aircraft and automobiles to sporting goods and toys, because of their high stiffness and strength-to-weight ratio. A composite material is a combination of fibers in a matrix or resin. Typically, a composite structure is made of a number of layers or plies of such composite materials. Typically, composite materials include a combination of fibers or fibrous tows in a matrix of thermoplastic or thermosetting resin. Dry fabric with unidirectional fibers or fibrous tows or woven fibers is often precombined with resin as a “PREPREG”. Examples include carbon, glass or graphite fibers in a resin matrix. The fibers typically comprise more than 35% of the material volume. Dry fabric with unidirectional fibers or fibrous tows or woven fibers can also be precombined with at thermosetting resin. This type of composite, a thermoset composite, generally requires that the fiber/resin plies be laid-up, debulked, and then cured—a process which can take a matter of hours. These composites are contrasted with thermoplastic composites which are generally faster to fabricate because there is no curing involved. The thermoplastic resin infused plies need only be heated to melt the plastic matrix and then pressed together or consolidated to the other plies before cooling. With thermosetting composites, on the other hand, heating to a high enough temperature invokes an exothermic reaction causing the molecules of the resin to cross link. Once this chemical cross linking occurs, the viscosity of the resin cannot be lowered. This is not the case with thermoplastic resins.
As used herein, consolidation means laminating two or more plies together to form a part or structure. Good consolidation implies a low level of voids (typically less than 3%) and a shear strength of the ply interfaces after curing which approaches that of the resin matrix.
Heating the plies, to consolidate them however, is troublesome: a number of different heating techniques have been tried but have met with mixed success. Laser heating in the nip between the previous ply and the ply being applied or laid down has not been wholly successful due to the practical problems of applying the energy at the nip. A laser apparatus with all the necessary controls is also quite expensive. Infrared devices, which depend upon radiant heating, suffer from poor heat moduability and can easily damage the composite structure.
Microwave devices suffer similar shortcomings and are potentially hazardous to surrounding personnel as well. A hot shoe technique, which is commercially available, employs a series of massive, heated, iron-like devices. This method relies on conduction through the ply to heat the interface which makes the process a slow one. Because of size and geometry, this method has only been applied to the production of flat panels, thereby restricting its usefulness.
The most evaluated technique presently in use is hot gas heating. In that process, a stream of hot air or gas is aimed into the nip between the new ply (layer or tape or tow) and the substrate and thereafter the new ply is pressed or ironed onto the substrate using a pinch roller or a shoe. While the consolidation levels achieved using this approach are high, the heating is difficult to modulate with respect to rapid changes in the material feed rate. This complicates the practical integration of convective, hot gas heating with standard computer-numerical-control fabrication equipment. Moreover, despite the high consolidation, some reports on the mechanical properties of the resulting composites have been disappointing. This may be due to damage or degrading of the surface of the material at the nip, especially due to the high heat level applied and the large temperature differential (300° C. or more) between the hot gas stream and the melt temperature of the thermoplastic material.
Filament winding, tape placement and tow placement are also common methods for fabricating parts from fiber reinforced composites.
Filament winding involves winding a filament bundle known as a ‘tow’, to which resin has previously been applied, around a mandrel. Multiple turns around the mandrel are used to build up the required part thickness after which the part is cured in an oven or autoclave.
During winding, thicker parts may require intermediate consolidation or compaction steps known as ‘debulks’ using heat in conjunction with pressure and/or a vacuum. Thick parts cured without any intermediate debulks often develop fiber wrinkling which degrades the mechanical properties of the cured part.
In tape or tow placement, a robotic head is used to place a narrow prepreg tow or tape (typically 0.125-2 inches in width) against a tool which defines the desired part shape. Multiple layers are placed at different orientations to obtain the required ply construction and part thickness. A combination of downward pressure on the tow, applied by the head, and tack (stickiness of the tow) is required to insure the tow remains in location after placement, particularly when placing a tow on concave portions of the tool.
Usually the tow, and the previously deposited ply layers, are heated to increase the tack prior to placement by the robotic head.
Current tow placement machines use separate mechanisms, placed in close proximity, to apply heat and pressure. Commonly, heat is applied by a jet of hot gas directed onto the tow and pressure is applied by one or more rollers or shoes which ride against the surface of the tow. The levels of consolidation achieved in this manner are such that thick tow or tape placed parts also require intermediate debulking to prevent fiber movement or wrinkling during cure.
One obstacle to consistently achieving higher levels of consolidation with these processes is the difficulty inherent in controlling temperature. Because of the heat capacity present in a hot gas system, the temperature of the gas jet, and hence the heat input to the tow cannot be easily modulated to allow for starts, stops or changes in advance rate of the robotic head.
Intermediate debulking typically involved applying a vacuum bag along with associated bag sealants, vacuum lines, connections, etc. to the layup tool or mandrel, and transfer of the tool from the tow placement machine to an oven or autoclave where it is heated to 180-250° F. and held under vacuum pressure for up to four hours. The part is then returned to the tape placement machine to continue the lay-up process. Current thick parts such as the V-22 spindle and the F-22 pivot shaft require numerous intermediate debulks which adds substantial cost.
A method of applying heat and pressure which achieves high levels of consolidation during tape or tow placement, thus eliminating the need for intermediate debulks, is desired and could result in substantial cost savings. The current invention relates to such method which uses a bi-directional device to generate the heat and pressure required for consolidation. Further, the method has the potential, in certain cases, to replace autoclave curing with curing in an oven. Moreover, the unique design of the bi-directional device of this invention allows the ultrasonic horn to be driven over the laminate bi-directionally while remaining in constant contact to the top surface of the composite.
A bi-directional device utilizing ultrasonic energy to heat the plies is appealing for a number of reasons. Unlike convection (hot gas), conduction (hot shoes/irons), or radiation (infrared), ultrasonic consolidation does not depend upon a thermal driver to effect energy transfer to the composite material. Ultrasonic heating is instantaneously modulatable, and it provides deep, penetrating heating in the polymeric matrix beyond mere surface heating.
Ultrasonic welding has long been used to weld or bond neat (unreinforced) plastics with no or little fiber co
Leemon Victor
Player John C.
Roylance Margaret E.
Thomson Douglas T.
Foster-Miller Inc.
Iandiorio & Teska
Sells James
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