Plastic and nonmetallic article shaping or treating: processes – Stereolithographic shaping from liquid precursor
Reexamination Certificate
2000-08-17
2003-10-07
Tentoni, Leo B. (Department: 1732)
Plastic and nonmetallic article shaping or treating: processes
Stereolithographic shaping from liquid precursor
C264S136000, C264S137000, C264S160000, C264S162000, C264S236000, C264S257000, C264S308000, C264S313000, C264S497000, C264S510000, C700S118000, C700S119000, C700S120000
Reexamination Certificate
active
06630093
ABSTRACT:
TECHNICAL FIELD
This invention relates to the manufacture of freeform-fabricated core composite articles, and more specifically, to a method of manufacturing composite articles wherein the core is fabricated in a freeform-fabrication machine, which exactly matches the article's geometry and dimensional specifications. This method eliminates the need to construct rigid molds, sculpted plugs, sculpted cores or preformers.
BACKGROUND OF THE INVENTION
Composite articles are used to produce items that must be lightweight but very strong and very durable. For example, composite fabricated articles have been embraced by the aerospace industry in recent years, where advanced composites have been used to build airframe structures such as wing skins, fuselage sections and many other internal structures and components. In fact, several newly designed general aviation aircraft and most new military aircraft are comprised largely of composite materials. While expensive to develop and perfect, composite-framed aircrafts are lighter, stronger, and more durable than metal-framed aircrafts. Advanced composites are also used to produce a wide-range of other products, such as top-of-the-line sporting goods and racing bikes, but their use in these products are normally limited to applications where the superior properties of the composite (i.e.: high strength to a weight ratio, resistance to corrosion and ease of repairability) is of greater importance than the relatively higher cost of the composite and overall greater cost of manufacturing with composite materials when compared to traditional materials.
Unfortunately, the higher cost of composite product development and manufacturing, compared to manufacturing with traditional materials, is not sufficiently offset by their desirable qualities to be a practical alternative for most mass production durable goods industries.
Much of the higher cost attributed to manufacturing products in advanced composites is due, in large part, to the current method of production that relies upon the use of part specific rigid molds or sculpted plugs. When the product involves complex geometry or when design changes are likely during development and testing, the tooling cost alone can render all but the “highest valued” products uneconomical :using current composite manufacturing methods. Many products could benefit, however, from the added strength, better durability and lighter weight that is realized using composite materials if a method were available that would reduce the cost to manufacture the article using composite materials.
Composite articles typically comprise a core, a plurality of composite skins and an adhesive film. The core is either formed in the basic shape of the desired composite article or is held to a desired shape by a rigid mold. The composite skin comprises one or more thin layers of material used to cover the exterior of the core and is comprised of one or more laminates of impregnated fiber reinforced composite material having a fiber reinforcement such as graphic, aramid or fiberglass fibers disposed in a binding matrix such as epoxy, phenolic or other similar resinous material. Most of the composite articles produced with present methods require a multi-layered composite skin in order to provide the compressive reinforcement required for the composite article. The adhesive film holds the composite skins in place against the exterior surface of the core during the fabrication process and promotes the bond between the composite skins and the core to produce a composite article with the desired physical properties and shape. In certain applications, reinforcing core sheeting material is “sandwiched” in between the composite skins, thus creating a sheet of reinforced core composite material, which can then be used in place of other sheeting materials. The composite skins are positioned directly against the mold or sculpted core by hand to create an assembly referred to as a “lay-up”. In a few industries, the manual lay-up process has been replaced by the use of robotic automated tow placement machines.
The most common method of manufacturing composite articles n involves the use of a vacuum bag assembly wherein an impervious membrane or “vacuum bag” is employed to consolidate the plurality of composite skins with the core to ensure proper adhesion thereof. The lay-up is placed inside a vacuum bag assembly where vacuum pressure consolidates the composite skins with the core. To improve surface finish, the lay-up assembly is sometimes positioned directly against female molds before being placed in a vacuum bag. Then the lay-up assembly, inside the vacuum bag assembly, is “cured” by placing it under constant vacuum pressure within an autoclave, thereby subjecting the lay-up assembly to a higher than ambient temperature and/or higher than ambient atmospheric, pressure for a prescribed period of time. Some binding matrix and composite material combinations are able to cure at ambient atmospheric temperature and pressure and thus may be oven or dry air cured.
Rigid molds are most commonly formed using a “part positive” as a pattern made from plaster, silicone or other similar material. The resultant negative mold is then used to form a more durable mold. When a 3D CAD design of the article is available, negative molds may also be milled or machine sculpted from a block of solid material. Rigid production molds are formed using various materials and methods depending upon the industry and the application. Rigid molds are commonly constructed from steel, cured composite, wood, ceramic or other rigid material.
Aerospace advanced composite parts, where quality and repeatability are vitally important, are commonly fabricated by first creating “outer skins” by shaping composite skins against rigid part specific molds, hollow structures, or tubes made from shaped plugs, or by “build-ups” that are fabricated using multiple layers of the composite skins over a mandrel. In some instances, such as in helicopter or propeller blades, the build-up is fabricated by placing multiple layers of the composite skins over shaped wood cores where the wood cores are sometimes referred to as “pre-formers”. To achieve the desired level of repeatability necessary to meet government or customer requirements, rigid mold produced composite articles represent the largest portion of the current market. For this process, a composite skin is produced first with the focus of capturing the external geometry of the article. Internal structures are mated to the skin using adhesives. The rigid mold itself is often used as a fixture to hold the various composite and non-composite component parts in place during the bonding and curing process. The resulting complex bonded assemblies, which due to the nature of their assembly, may suffer de-bonding or delamination over time. Take the example of an aircraft wing. In a simple design, the upper and lower wing surfaces are formed by “laying up” composite skins over a sculpted foam plug. The foam material adds compressive strength to the wing, but precludes the use of the internal space to house fuel or other components. In more complex aircraft, aircraft wings must carry fuel, landing gear, electronics, electrical wiring, hydraulic lines, fuel pumps and control system components to name a few. Individual composite components mated in a wing assembly may total eighty or more separate components. Currently, composite product designers are confronted with the potential hazard of de-bonding and dissimilar materials corrosion as well as the challenges of accurate placement-of parts within the assembly.
As an alternative to rigid molds, the part positive can be produced as a composite article using a sculpted plug as a core. Sculpted plugs are formed in various ways. Computer controlled routers, CNC mills and lathes are often employed to create an accurately shaped plug. Hand shaping, using spaced sectional patterns as a guide are still common in certain industries. Plugs are also shaped using a “hot wire” device as a cutting tool to sc
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