基本成型工艺
There are numerous methods for fabricating compositecomponents. Some methods have been borrowed (injection molding, for example),but many were developed to meet specific design or manufacturing challenges.Selection of a method for a particular part, therefore, will depend on thematerials, the part design and end-use or application (see “Part designcriteria,” under "Editor's Picks," at right).
Composite fabrication processes involve some form ofmolding, to shape the resin and reinforcement. A mold tool is required to givethe unformed resin /fiber combination its shape prior to and during cure. Foran overview of mold types and materials and methods used to make mold tools.
The most basic fabrication method for thermoset compositesis hand layup, which typically consists of laying dry fabric layers, or“plies,” or prepreg plies, by hand onto a tool to form a laminate stack. Resinis applied to the dry plies after layup is complete (e.g., by means of resininfusion). In a variation known as wet layup, each ply is coated with resin and“debulked” or compacted after it is placed.
Several curing methods are available. The most basic issimply to allow cure to occur at room temperature. Cure can be accelerated,however, by applying heat, typically with an oven, and pressure, by means of avacuum. For the latter, a vacuum bag, with breather assemblies, is placed overthe layup and attached to the tool, then evacuated using a vacuum pump beforecure. The vacuum bagging process consolidates the plies of material andsignificantly reduces voids due to the off-gassing that occurs as the matrixprogresses through its chemical curing stages.
Many high-performance thermoset parts require heat and highconsolidation pressure to cure — conditions that require the use of anautoclave. Autoclaves, generally, are expensive to buy and operate.Manufacturers that are equipped with autoclaves usually cure a number of partssimultaneously. Computer systems monitor and control autoclave temperature,pressure, vacuum and inert atmosphere, which allows unattended and/or remotesupervision of the cure process and maximizes efficient use of the technique.
When heat is required for cure, the part temperature is“ramped up” in small increments, maintained at cure level for a specifiedperiod of time defined by the resin system, then “ramped down” to roomtemperature, to avoid part distortion or warp caused by uneven expansion andcontraction. When this curing cycle is complete and after parts are demolded,some parts go through a secondary freestanding postcure, during which they aresubjected for a specific period of time to a temperature higher than that ofthe initial cure to enhance chemical crosslink density.
Electron-beam (E-beam) curing has been explored as anefficient curing method for thin laminates. In E-beam curing, the compositelayup is exposed to a stream of electrons that provide ionizing radiation,causing polymerization and crosslinking in radiation-sensitive resins. X-rayand microwave curing technologies work in a similar manner. A fourthalternative, ultraviolet (UV) curing, involves the use of UV radiation toactivate a photoinitiator added to a thermoset resin, which, when activated,sets off a crosslinking reaction. UV curing requires light-permeable resin andreinforcements.
An emerging technology is the monitoring of the cure itself.Dielectric cure monitors measure the extent of cure by gauging the conductivityof ions — small, polarized, relatively insignificant impurities that areresident in resins. Ions tend to migrate toward an electrode of oppositepolarity, but the speed of migration is limited by the viscosity of the resin —the higher the viscosity, the slower the speed. As crosslinking proceeds duringcure, resin viscosity increases. Other methods include dipole monitoring withinthe resin, the monitoring of microvoltage produced by the crosslinking,monitoring of the exothermic reaction in the polymer during cure and,potentially, the use of infrared monitoring via fiber-optic technology (see"Monitoring the cure itself,” under "Editor's Picks," at right).
A notable phenomenon gaining momentum in the industry isthat of out-of-autoclave (OOA) curing for high-performance compositecomponents. The high cost and limited size of autoclave systems has promptedmany processors, particularly in aerospace, to call for OOA resins that can becured with heat only in an oven (less capital-intensive and less expensive tooperate than an autoclave, particularly with very large parts), or at roomtemperature., Cytec Industrial Materials (formerly Advanced Composites Group,Heanor, Derbyshire, U.K.) introduced the first OOA resin, an epoxy designed foraerospace applications. OOA tooling epoxies and adhesives also are coming tomarket (see “Autoclave quality outside the autoclave?” under "Editor'sPicks").
Open molding
Open contact molding in one-sided molds is a low-cost,common process for making fiberglass composite products. Typically used forboat hulls and decks, RV components, truck cabs and fenders, spas, bathtubs,shower stalls and other relatively large, noncomplex shapes, open moldinginvolves either hand layup or a semi-automated alternative, sprayup.
In an open-mold sprayup application, the mold is firsttreated with mold release. If a gel coat is used, it is typically sprayed intothe mold after the mold release has been applied. The gel coat then is curedand the mold is ready for fabrication to begin. In the sprayup process,catalyzed resin (viscosity from 500 to 1,000 cps) and glass fiber are sprayedinto the mold using a chopper gun, which chops continuous fiber into shortlengths, then blows the short fibers directly into the sprayed resin stream sothat both materials are applied simultaneously. To reduce VOCs, pistonpump-activated, non-atomizing spray guns and fluid impingement spray headsdispense gel coats and resins in larger droplets at low pressure. Anotheroption is a roller impregnator, which pumps resin into a roller similar to apaint roller.
In the final steps of the sprayup process, workers compactthe laminate by hand with rollers. Wood, foam or other core material may thenbe added, and a second sprayup layer imbeds the core between the laminateskins. The part is then cured, cooled and removed from the reusable mold.
Hand layup and sprayup methods are often used in tandem toreduce labor. For example, fabric might first be placed in an area exposed tohigh stress; then, a spray gun might be used to apply chopped glass and resinto build up the rest of the laminate. Balsa or foam cores may be insertedbetween the laminate layers in either process. Typical glass fiber volume is 15percent with sprayup and 25 percent with hand layup.
Sprayup processing, once a very prevalent manufacturingmethod, has begun to fall out of favor. Federal regulations in the U.S. andsimilar rules in the EU have mandated limits on worker exposure to, andemission into the environment of VOCs and hazardous air pollutants (HAPs).Styrene, the most common monomer used as a diluent in thermoset resins, is onboth lists. Because worker exposure to and emission of styrene is difficult andexpensive to control in the sprayup process, many composites manufacturers havemigrated to closed mold, infusion-based processes, which better contain andmanage styrenes.
Although open molding via hand layup is being replaced byfaster and more technically precise methods (as the following makes clear), itis still widely used in the repair of composite parts. For more informationabout “Composites repair” see the so-named article under "Editor'sPicks."
Resin infusion processes
Ever-increasing demand for faster production rates haspressed the industry to replace hand layup with alternative fabricationprocesses and has encouraged fabricators to automate those processes whereverpossible.
A common alternative is resin transfer molding (RTM),sometimes referred to as liquid molding. RTM is a fairly simple process: Itbegins with a two-part, matched, closed mold that is made of metal or compositematerial. Dry reinforcement (typically a preform) is placed into the mold andthe mold is closed. Resin and catalyst are metered and mixed in dispensingequipment, then pumped into the mold under low to moderate pressure throughinjection ports, following predesigned paths through the preform. Extremelylow-viscosity resin is used in RTM applications for thick parts to permeate preformsquickly and evenly before cure. Both mold and resin can be heated, asnecessary, for particular applications. RTM produces parts without anautoclave. However, when cured and demolded, a part destined for ahigh-temperature application usually undergoes postcure. Most RTM applicationsuse a two-part epoxy formulation. The two parts are mixed just before they areinjected. Bismaleimide and polyimide resins also are available in RTMformulations. Light RTM is a variant of RTM that is growing in popularity. InLight RTM, low injection pressure, coupled with vacuum, allow the use ofless-expensive, lightweight two-part molds or a very lightweight, flexibleupper mold.
The benefits of RTM are impressive. Generally, the drypreforms and resins used in RTM are less expensive than prepreg material andcan be stored at room temperature. The process can produce thick, near-netshape parts, eliminating most post-fabrication work. It also yieldsdimensionally accurate complex parts with good surface detail and delivers asmooth finish on all exposed surfaces. It is possible to place inserts insidethe preform before the mold is closed, allowing the RTM process to accommodatecore materials and integrate “molded in” fittings and other hardware into thepart structure. Moreover, void content on RTM’d parts is low, measuring in the0 to 2 percent range. Finally, RTM significantly cuts cycle times and can beadapted for use as one stage in an automated, repeatable manufacturing processfor even greater efficiency, reducing cycle time from what can be several days,typical of hand layup, to just hours — or even minutes. A recent variant ofRTM, called high-pressure RTM (HP-RTM), is gaining attention for its potentialto quickly produce automotive parts. Typically designed as a completelyautomated system including mold shuttles, the ability to rapidly fill a moldloaded with a preform with a very fast curing resin shows promise for highproduction.
In contrast to RTM, where resin and catalyst are premixedprior to injection under pressure into the mold, reaction injection molding(RIM) injects a rapid-cure resin and a catalyst into the mold in two separatestreams. Mixing and the resulting chemical reaction occur in the mold insteadof in a dispensing head. Automotive industry suppliers combine structural RIM(SRIM) with rapid preforming methods to fabricate structural parts that don’trequire a Class A finish. Programmable robots have become a common means tospray a chopped fiberglass/binder combination onto a vacuum-equipped preformscreen or mold. Robotic sprayup can be directed to control fiber orientation. Arelated technology, dry fiber placement, combines stitched preforms and RTM.Fiber volumes of up to 68 percent are possible, and automated controls ensurelow voids and consistent preform reproduction, without the need for trimming.
Vacuum-assisted resin transfer molding (VARTM) refers to avariety of related processes that represent the fastest-growing new moldingtechnology. The salient difference between VARTM-type processes and RTM is thatin VARTM, resin is drawn into a preform through use of a vacuum only, ratherthan pumped in under pressure. VARTM does not require high heat or pressure.For that reason, VARTM operates with low-cost tooling, making it possible toinexpensively produce large, complex parts in one shot.
In the VARTM process, fiber reinforcements are placed in aone-sided mold, and a cover (typically a plastic bagging film) is placed overthe top to form a vacuum-tight seal. The resin typically enters the structurethrough strategically placed ports and feed lines, termed a “manifold.” It isdrawn by vacuum through the reinforcements by means of a series of designed-inchannels that facilitate wetout of the fibers. Fiber content in the finishedpart can run as high as 70 percent. Current applications include marine, groundtransportation and infrastructure parts. A twist on the VARTM process is theuse of two bags, termed double-bag infusion, which uses one vacuum pumpattached to the inner bag to extract volatiles and entrapped air, and a secondvacuum pump on the outer bag to compact the laminate. This method has beenemployed by The Boeing Co. (Chicago, Ill.) and NASA, as well as smallfabricating firms, to produce aerospace-quality laminates without an autoclave.
Resin film infusion (RFI) is a hybrid process in which a drypreform is placed in a mold on top of a layer, or interleaved with multiplelayers, of high-viscosity resin film. Under applied heat, vacuum and pressure,the resin liquefies and is drawn into the preform, resulting in uniform resindistribution, even with high-viscosity, toughened resins, because of the shortflow distance.
High-volume molding methods
Compression molding is a high-volume thermoset moldingprocess that employs expensive but very durable metal dies. It is anappropriate choice when production quantities exceed 10,000 parts. As many as200,000 parts can be turned out on a set of forged steel dies, using sheetmolding compound (SMC), a composite sheet material made by sandwiching choppedfiberglass between two layers of thick resin paste. To form the sheet, theresin paste transfers from a metering device onto a moving film carrier.Chopped glass fibers drop onto the paste, and a second film carrier places anotherlayer of resin on top of the glass. Rollers compact the sheet to saturate theglass with resin and squeeze out entrapped air. The resin paste initially isthe consistency of molasses (between 20,000 and 40,000 cps); over the nextthree to five days, its viscosity increases and the sheet becomes leather-like(about 25 million cps), ideal for handling.
When the SMC is ready for molding, it is cut into smallersheets and the charge pattern (ply schedule) is assembled on a heated mold(121°C to 262°C or 250°F to 325°F). The mold is closed and clamped, andpressure is applied at 24.5 to 172.4 bar (500 to 2,500 psi). As materialviscosity drops, the SMC flows to fill the mold cavity. After cure, the part isdemolded manually or by integral ejector pins.
A typical low-profile (less than 0.05 percent shrinkage) SMCformulation for a Class A finish consists, by weight, of 25 percent polyesterresin, 25 percent chopped glass, 45 percent fillers and 5 percent additives.Fiberglass thermoset SMC cures in 30 to 150 seconds and overall cycle time canbe as low as 60 seconds. Other grades of SMC include low-density, flexible andpigmented formulations. Low-pressure SMC formulations that are now on themarket offer open molders low-capital-investment entry into closed-moldprocessing with near-zero VOC emissions and the potential for very high-qualitysurface finish.
Automakers are exploring carbon fiber-reinforced SMC, hopingto take advantage of carbon’s high strength- and stiffness-to-weight ratios inexterior body panels and other parts. Newer, toughened SMC formulations helpprevent microcracking, a phenomenon that previously caused paint “pops” duringthe painting process (surface craters caused by outgassing, the release ofgasses trapped in the microcracks during oven cure).
Composites manufacturers in industrial markets areformulating their own resins and compounding SMC in-house to meet needs inspecific applications that require UV, impact and moisture resistance and havesurface-quality demands that drive the need for customized materialdevelopment.
Injection molding is a fast, high-volume, low-pressure,closed process using, most commonly, filled thermoplastics, such as nylon withchopped glass fiber. In the past 20 years, however, automated injection moldingof BMC has taken over some markets previously held by thermoplastic and metalcasting manufacturers. For example, the first-ever BMC-based electronicthrottle control (ETC) valves (previously molded only from die-cast aluminum)debuted on engines in the BMW Mini and the Peugeot 207, taking advantage ofdimensional stability offered by a specially-formulated BMC supplied byTetraDUR GmbH (Hamburg, Germany), a subsidiary of Bulk Molding Compounds Inc.(BMCI, West Chicago, Ill.,).
In the BMC injection molding process, a ram- or screw-typeplunger forces a metered shot of material through a heated barrel and injectsit (at 5,000 to 12,000 psi) into a closed, heated mold. In the mold, theliquefied BMC flows easily along runner channels and into the closed mold.After cure and ejection, parts need only minimal finishing. Injection speedsare typically one to five seconds, and as many as 2,000 small parts can beproduced per hour in some multiple-cavity molds.
Parts with thick cross-sections can be compression molded ortransfer molded with BMC. Transfer molding is a closed-mold process wherein ameasured charge of BMC is placed in a pot with runners that lead to the moldcavities. A plunger forces the material into the cavities, where the productcures under heat and pressure.
Filament winding is a continuous fabrication method that canbe highly automated and repeatable, with relatively low material costs. A long,cylindrical tool called a mandrel is suspended horizontally between endsupports, while the “head” — the fiber application instrument — moves back andforth along the length of a rotating mandrel, placing fiber onto the tool in apredetermined configuration. Computer-controlled filament-winding machines areavailable, equipped with from 2 to 12 axes of motion.
In most thermoset applications, the filament windingapparatus passes the fiber material through a resin “bath” just before thematerial touches the mandrel. This is called wet winding. However, a variationuses towpreg, that is, continuous fiber pre-impregnated with resin. Thiseliminates the need for an onsite resin bath. In a slightly different process,fiber is wound without resin (dry winding). The dry shape is then used as apreform in another molding process, such as RTM.
Following oven or autoclave curing, the mandrel eitherremains in place to become part of the wound component or, typically, it isremoved. One-piece cylindrical or tapered mandrels, usually of simple shape,are pulled out of the part with mandrel extraction equipment. Some mandrels,particularly in more complex parts, are made of soluble material and may bedissolved and washed out of the part. Others are collapsible or built fromseveral parts that allow its disassembly and removal in smaller pieces.Filament-winding manufacturers often “tweak” or slightly modify off-the-shelfresin to meet specific application requirements. Some composite partmanufacturers develop their own resin formulations.
In thermoplastics winding, all material is in prepreg form,so a resin bath is not needed. Material is heated as it is wound onto themandrel — a process known as curing “on the fly” or in-situ consolidation. Theprepreg is heated, layed down, compacted, consolidated and cooled in a single,continuous operation. Thermoplastic prepregs eliminate autoclave curing(cutting costs and size limitations) and reduce raw material costs, and theresulting parts can be reprocessed to correct flaws.
Filament winding yields parts with exceptionalcircumferential or “hoop” strength. The highest-volume single application offilament winding is golf club shafts. Fishing rods, pipe, pressure vessels andother cylindrical parts comprise most of the remaining business.
Pultrusion, like RTM, has been used for decades with glassfiber and polyester resins, but in the last 10 years the process also has foundapplication in advanced composites applications. In this relatively simple,low-cost, continuous process, the reinforcing fiber (usually roving, tow orcontinuous mat) is typically pulled through a heated resin bath and then formedinto specific shapes as it passes through one or more forming guides orbushings. The material then moves through a heated die, where it takes its netshape and cures. Further downstream, after cooling, the resulting profile iscut to desired length. Pultrusion yields smooth finished parts that typicallydo not require postprocessing. A wide range of continuous, consistent, solidand hollow profiles are pultruded, and the process can be custom-tailored tofit specific applications.
Tube rolling is a longstanding composites manufacturingprocess that can produce finite-length tubes and rods. It is particularlyapplicable to small-diameter cylindrical or tapered tubes in lengths as greatas 20 ft/6.2m. Tubing diameters up to 6 inches/152 mm can be rolledefficiently. Typically, a tacky prepreg fabric or unidirectional tape is used,depending on the part. The material is precut in patterns that have beendesigned to achieve the requisite ply schedule and fiber architecture for theapplication. The pattern pieces are laid out on a flat surface and a mandrel isrolled over each one under applied pressure, which compacts and debulks thematerial. When rolling a tapered mandrel — e.g., for a fishing rod or golfshaft — only the first row of longitudinal fibers falls on the true 0° axis. Toimpart bending strength to the tube, therefore, the fibers must be continuouslyreoriented by repositioning the pattern pieces at regular intervals.
Automated fiber placement (AFP). The fiber placement processautomatically places multiple individual prepreg tows onto a mandrel at highspeed, using a numerically controlled, articulating robotic placement head todispense, clamp, cut and restart as many as 32 tows simultaneously. Minimum cutlength (the shortest tow length a machine can lay down) is the essentialply-shape determinant. The fiber placement heads can be attached to a 5-axisgantry, retrofitted to a filament winder or delivered as a turnkey customsystem. Machines are available with dual mandrel stations to increaseproductivity. Advantages of fiber placement include processing speed, reducedmaterial scrap and labor costs, parts consolidation and improved part-to-partuniformity. Often, the process is used to produce large thermoset parts withcomplex shapes.
Automated tape laying (ATL) is an even speedier automatedprocess in which prepreg tape, rather than single tows, is laid downcontinuously to form parts. It is often used for parts with highly complexcontours or angles. Tape layup is versatile, allowing breaks in the process andeasy direction changes, and it can be adapted for both thermoset andthermoplastic materials. The head includes a spool or spools of tape, a winder,winder guides, a compaction shoe, a position sensor and a tape cutter orslitter. In either case, the head may be located on the end of a multiaxisarticulating robot that moves around the tool or mandrel to which material isbeing applied, or the head may be located on a gantry suspended above the tool.Alternatively, the tool or mandrel can be moved or rotated to provide the headaccess to different sections of the tool. Tape or fiber is applied to a tool incourses, which consist of one row of material of any length at any angle.Multiple courses are usually applied together over an area or pattern and aredefined and controlled by machine-control software that is programmed withnumerical input derived from part design and analysis. Capital expenditures forcomputer-driven, automated equipment can be significant.
Although ATL generally is faster than AFP and can place morematerial over longer distances, AFP is better suited to shorter courses and canplace material more effectively over contoured surfaces. These technologiesgrew out of the machine tool industry and have seen extensive use in themanufacture of the fuselage, wingskin panels, wingbox, tail and otherstructures on the forthcoming Boeing 787 Dreamliner and the Airbus A350 XWB.ATL and AFP also are used extensively to produce parts for the F-35 LightningII fighter jet the V-22 Osprey tiltrotor troop transport and a variety of otheraircraft.
Centrifugal casting of pipe from 1 inch/25 mm to 14inches/356 mm in diameter is an alternative to filament winding forhigh-performance, corrosion-resistant service. In cast pipe, 0°/90° wovenfiberglass provides both longitudinal and hoop strength throughout the pipewall and brings greater strength at equal wall thickness compared to multiaxialfiberglass wound pipe. In the casting process, epoxy or vinyl ester resin isinjected into a 150G centrifugally spinning mold, permeating the woven fabricwrapped around the mold’s interior surface. The centrifugal force pushes theresin through the layers of fabric, creating a smooth finish on the outside ofthe pipe, and excess resin pumped into the mold creates a resin-rich,corrosion- and abrasion-resistant interior liner.
Fiber-reinforced thermoplastic components now can beproduced by extrusion, as well. Breakthrough material and process technologyhas been developed with long-fiber glass-reinforced thermoplastic (ABS, PVC orpolypropylene) composites to provide profiles that offer a tough, low-costalternative to wood, metal and injection-molded plastic parts used in officefurniture, appliances, semitrailers and sporting goods. A huge market hasemerged in the past decade for extruded thermoplastic/wood flour (or otheradditives, such as bast fibers or fly ash) composites. These wood plasticcomposites, or WPCs, used to simulate wood decking, siding, window and doorframes, and fencing.
Safety and environmental protection
Fabricators and OEMs must address health, safety andenvironmental concerns when producing and handling composite materials. Theirmethods for maintaining a safe workplace include periodic training, adherenceto detailed handling procedures, maintenance of current toxicity information,use of protective equipment (gloves, aprons, dust-control systems andrespirators) and development of company-wide monitoring policies. Bothsuppliers and OEMs are working to reduce emissions of highly volatile organiccompounds (VOCs) by reformulating resins and prepregs and switching towater-dispersible cleaning agents.
The U.S. Environmental Protection Agency has continued tostrengthen its requirements to meet the mandates of the Clean Air ActAmendments, passed by Congress in 1990. Specifically, the agency’s goal is toreduce the emission of hazardous air pollutants (HAPs), a list of approximately180 volatile chemicals that are considered to pose health risks. Some of thecompounds used in resins and released during cure contain HAPs. In early 2003,the EPA enacted regulations specifically for the composites industry, requiringemission controls using maximum achievable control technology, or MACT. Theregulations took effect in early 2006.
