Adhesive bonding and miscellaneous chemical manufacture – Methods – Surface bonding and/or assembly therefor
Reexamination Certificate
2000-10-19
2002-08-13
Sells, James (Department: 1734)
Adhesive bonding and miscellaneous chemical manufacture
Methods
Surface bonding and/or assembly therefor
C156S308200, C156S580100
Reexamination Certificate
active
06432236
ABSTRACT:
FIELD OF INVENTION
This invention relates to an improved method of ultrasonically consolidating layers or plies of fiber-reinforced thermosetting resin matrix composite materials and more particularly to such a method which applies the ultrasonic energy generally parallel to the surface of the layer to produce substantial shear in the plies to effect heating of the resin matrix. The invention also relates to the product made by that method.
BACKGROUND OF 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. One type of composite material includes a combination of fibers or fibrous tows in a matrix of thermosetting resin. Typically, such a composite structure is made of a number of layers of plies of “prepreg” tape. As used herein, a composite material means a structure composed of a plurality of plies of fiber-reinforced fabric or tape in a thermosetting resin matrix. Dry fabric with unidirectional fibers or fibrous tows or woven fibers is often precombined with thermosetting resin as a “prepreg”. Examples include carbon, glass or graphite fibers in a staged thermosetting resin matrix. The fibers typically comprise more than 35% of the material volume. Thermoset composites generally requires that the fiber/resin plies be laid-up, debulked, and then cured. This process can take a matter of hours. Such composites are to be contrasted with thermoplastic composites which are generally faster to fabricate because there is no curing involved. The thermoplastic 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 type 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.
Filament winding, tape placement and tow placement are 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 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 on location after placement, particularly when placing 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 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 an ultrasonic horn 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.
Ultrasonic devices used to heat the plies have appeal 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 content. Such welding is done by placing an ultrasonic horn perpendicular to two plastic layers, pressing down on the layers and energizing the horn. Obeda, U.S. Pat. No. 4,713,131, teaches joining large sheets of polypropylene plastic by overlapping the sheets of plastic and welding their edges together using an ultrasonic horn placed between the sheets. Obeda, however, teaches nothing about composite materials.
But, others have attempted to use an ultrasonic horn to fabricate composite parts. See
Joining Methods for Plastic and Plastic Composites: An Overview
, Vijay Stokes,
Polymer Engineering and Science
, Mid-October 1989, vol. 29, no. 19, p. 1310-1324, specifically pp. 1322-1324, items 168-236. These previous attempts to weld even thermoplastic composites during the lamination process, using conventional ultrasonic perpendicularly disposed horn welding techniques, have yielded disappointing results because, it is speculated, the presence of the fibers alters the energy transfer in the material. Moreover, these conventional ultrasonic welding techniques set up a compression wavefront in the material which does not transmit well through the material. In 1987, engineers at Martin Marietta attempted to use an ultrasonic horn to consolidate composite thermoplastic resin-fiber plies. The horn was placed on the top of two moving plies to be consolidated in a direction perpendicular to the plies. A range of different pressures, energy levels, and feed rates were tried. The result, however, was not satisfactory: “C-Scan results have shown that attempts to produce consolidated or near-consolidated laminates have not been successful thus far . . . ” Sonic
Assisted Process Development
”, Interim Technical Report,” contract No. F 33615-86-5041, Martin Marietta Baltimore for Material Laboratory Air Force Wright labs., March 1987.
Therefore, although ultrasonic horns have been used to weld plastic sheets together and, to some extent, have been successfully used to weld thermoplastics containing up to abo
Boyce Joseph S.
Bullock Daniel E.
Leemon Victor
Player John C.
Roylance Margaret E.
Foster-Miller Inc.
Iandiorio & Teska
Sells James
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