Composites Manufacturing Process Checklist

 

There are many methods for making composite parts. Therefore, the method selection for a particular part will depend on the materials, part design, and end use or application. This is a selection guide.

 

There are many methods for making composite parts. Some methods are borrowed (e.g., injection molding from the plastics industry), but many methods were developed to meet specific design or manufacturing challenges faced by fiber-reinforced polymers. Therefore, the method selection for a particular part will depend on the materials, part design, and end use or application.

 

The manufacturing process for composites typically includes some form of forming to give the resin and reinforcements shape. A mold is required to give the unformed resin/fiber combination shape before and during curing. The most basic manufacturing method for thermoset composites is hand layup, which typically involves hand placement of layers called dry fabric plies or prepreg plies (fabric pre-impregnated with resin) on a tool to form a laminate layup.

 

After layup is complete (e.g., by resin infusion), resin is applied to the dry plies. In a variation called wet laying, each ply is coated with resin and compacted after forming. Although compaction can be done manually with a roller, most manufacturers today use a vacuum bagging technique, which involves placing a plastic sheet over the layup, sealing at the tool edge, adding one or more ports for air hoses, and then using a vacuum pump to evacuate the air from the space between the sheet and the layup). Deburring not only consolidates the layup, it also removes air from the resin matrix, which would otherwise create undesirable voids (air pockets) in the laminate that would weaken the composite.

 

 

The most basic is to allow curing at room temperature (initiated by a catalyst or hardener additive premixed into the resin). However, curing can be accelerated by applying heat (usually in an oven) and vacuum pressure. For the latter, a vacuum bag with a vent assembly is placed over the stack and attached to the tool (in a manner similar to that used in compaction), and the vacuum is applied before curing begins. The vacuum bagging process here further consolidates the layers of material and significantly reduces voids created by outgassing that occurs as the matrix proceeds through its chemical curing phase.

 

 

In November 2014, Globe Machine Manufacturing Co. (Tacoma, WA, U.S.) piloted its second-generation RapidClave system, an out-of-autoclave hybrid molding process that successfully formed a unidirectional carbon fiber/epoxy prepreg (6-8 ply, 0°/90° layup) part in a 6-minute cycle, a first for thermoset composites and a major step toward mass production expectations for the automotive industry.

 

 

Many high-performance thermoset parts require heat and high consolidation pressures to cure, conditions that necessitate the use of an autoclave. In general, autoclaves are expensive to purchase and operate. Manufacturers equipped with autoclaves typically cure multiple parts simultaneously. Computer systems monitor and control the autoclave’s temperature, pressure, vacuum, and inert atmosphere, which allows for unattended and/or remote supervision of the curing process and maximizes the efficient use of the technology.

 

 

When heat is required for curing, part temperature is “ramped up” in small increments, held at the cure level for a specific time dictated by the resin system, and then “ramped down” to room temperature to avoid part deformation or warping due to differential expansion and contraction. When this cure cycle is complete and the parts are demolded, some parts undergo a secondary, independent post-cure during which they are subjected to a higher temperature than the initial cure for a specific period of time to increase chemical cross-link density.

 

 

Electron beam curing has been explored as an effective method for curing thin laminates. In electron beam curing, the composite layup is exposed to a stream of electrons that provide ionizing radiation, causing polymerization and crosslinking in the radiation-sensitive resin. X-ray and microwave curing techniques work in a similar manner.

 

A fourth alternative, UV (ultraviolet) curing, involves the use of UV radiation to activate photoinitiators added to thermosetting resins that, when activated, initiate a crosslinking reaction. UV curing requires light-transmitting resins and reinforcements.

 

 

An emerging technology is monitoring of the cure process itself. Dielectric cure monitors measure the extent of cure by measuring the conductivity of ions, which are small, polarized, relatively insignificant impurities present in the resin. Ions tend to migrate toward electrodes of opposite polarity, but the rate of migration is limited by the viscosity of the resin—the higher the viscosity, the slower the rate. As crosslinking progresses during the cure process, the resin viscosity increases. Other methods include dipole monitoring within the resin, monitoring of microvoltages generated by crosslinking, monitoring of exothermic reactions in the polymer during the cure process, and using infrared monitoring via fiber optic technology.

 

 

Out-of-autoclave (OOA-) curing is a dramatic phenomenon in the high-performance composite parts industry. The high cost and limited size of autoclave systems has led many processors, particularly in the aerospace sector, to request OOA resins that can only be cured by heat in an oven (less capital intensive and less expensive to operate than an autoclave, especially with very large parts) or at room temperature. Saida Aerospace Materials (now Solvay Composites) has introduced the first OOA resin, an epoxy designed specifically for aerospace applications. OOA tooling epoxies and adhesives are also coming to market.

 

 

Open contact molding in a single-sided mold is a low-cost, common process for manufacturing fiberglass composite products. Open molding is often used for boat hulls and decks, RV components, truck cabs and fenders, spas, tubs, showers, and other relatively large, uncomplicated shapes, including manual layups or semi-automatic alternatives to spraying.

 

In open mold spray applications, the mold is first demolded. If a gel coat is used, it is typically sprayed into the mold after demolding. The gel coat is then cured, and the mold is ready to begin manufacturing. In the spray process, catalyzed resin (viscosity 500-1000cps) and fiberglass are sprayed into the mold using a cut-off gun that cuts continuous fiber into short lengths and then blows the short fibers directly into the sprayed resin stream, applying both materials simultaneously. To reduce VOCs, piston pump-activated non-atomizing spray guns and fluid impingement spray tips dispense gel coat at low pressure and, after the gel coat cures, dispense the resin in larger droplets. Another option is a drum impregnator, which pumps the resin into a drum similar to a paint roller.

 

In the final steps of the spray process, workers compact the laminate by hand with rollers. Wood, foam or other core materials can then be added, and a second spray coat embeds the core between the laminate skins. The part is then cured, cooled and removed from a typically reusable mold. Hand layup and spray methods are often used simultaneously to reduce labor.

 

For example, fabric might be placed first in areas exposed to high stress; then, a spray gun can be used to apply chopped glass and resin to build up the rest of the laminate. Balsa or foam cores can be inserted between laminate layers in either process. Typical fiberglass volumes are 15% when spraying and 25% when hand-laying. Spray processing, once a very popular manufacturing method, has begun to fall out of favor.

 

Federal regulations in the United States and similar regulations in the European Union set limits on worker exposure to volatile organic compounds and hazardous air pollutants (HAPs) and emissions to the environment. Styrene, the most common monomer used as a diluent in thermoset resins, is on both lists.

 

Because worker exposure and emissions of styrene during the spraying process are difficult and costly to control, many composite manufacturers have turned to closed-mold, infusion-based processes to better contain and manage styrene.

 

Although open molding by hand layup is being replaced by faster, more technically precise methods (shown below), it is still widely used to repair damaged parts, including those made from other commonly used materials such as steel and concrete.

 

 

Increasing demand for faster production rates is forcing the industry to replace manual lay-up with alternative manufacturing processes and encouraging manufacturers to automate these processes wherever possible.

 

 

A common alternative is resin transfer molding (RTM), sometimes called liquid molding. RTM is a fairly simple process: It starts with a two-part, matched, closed mold made of metal or composite. The dry reinforcement, usually a preform, is placed into the mold, and then the mold is closed.

 

The resin and catalyst are metered and mixed in a dispensing device and then pumped into the mold through an injection port at low to moderate pressure, following a pre-designed path through the preform. Very low viscosity resins are used in RTM applications, especially for thick parts, to ensure that the resin quickly and thoroughly penetrates the preform before curing begins. Both the mold and the resin can be preheated as needed for a specific application.

 

RTM produces high-quality parts without the need for an autoclave. However, parts for high-temperature applications are often post-cured when curing and demolding. Most RTM applications use a two-part epoxy resin formulation. The two parts are mixed together before injection. Bismaleimide and polyimide resins can also be used in the RTM process.

 

 

Light RTM is a variation of RTM that is growing in popularity. In Light RTM, low injection pressures coupled with vacuum allow for the use of less expensive, lightweight two-part molds or very light, flexible upper molds. The benefits of RTM are impressive. Typically, dry preforms and resins used in RTM are less expensive than prepreg materials and can be stored at room temperature.

 

The process can produce thick, near-net-shape parts, eliminating most post-manufacturing work. It also produces dimensionally accurate, complex parts with good surface detail and, unlike open molding techniques, typically produces contoured but flat parts with A- and B-sides (finished and unfinished, respectively).

 

RTM can provide the desired cosmetic finish on all exposed surfaces of complex, three-dimensional parts. Inserts can also be placed within the preform before the mold is closed, allowing the RTM process to accommodate core materials and integrate “molded-in” accessories and other hardware into the part structure. In addition, RTM molded parts have low void content, measuring ≤2%.

 

Finally, RTM significantly reduces cycle times and can be used as a stage in an automated, repeatable manufacturing process to gain greater efficiency, reducing cycle times from days (typical of hand layup) to hours or even minutes.

 

 

One of the newest variants of RTM, called High-pressure RTM, has attracted attention for its potential to rapidly produce automotive parts. “The standard cycle time for low-pressure RTM (resin injection at 10 to 20 bar) is 30 to 60 minutes,” he said. “It can be as low as 5 minutes, but only for very small parts.” “High pressure,” Mayr said, “means pressures of up to 150 bar in the mixhead and from 30 to 120 bar in the mold, depending on part size and geometry.”

 

HP-RTM is typically designed as a fully automated system including a mold shuttle, which is able to quickly fill a mold containing a preform with very fast-curing resin, promising high production volumes. HP-RTM still includes a fiber preform, a closed mold, a press, and a resin injection system, but the latter is now an impact mixhead, like the ones first developed for polyurethane (PU) foam applications in the 1960s.

 

In fact, meter/mix/inject suppliers of PU and reaction injection molding (RIM, see next item) processes were among the early developers of HP-RTM, including KraussMaffei Technologies GmbH (Munich, Germany), Hennecke Inc. (Sankt Augustin, Germany), Frimo Inc. (Rakuten, Germany), Cannon USA Inc. and Cannon SpA (Cranberry Township, Pennsylvania, U.S., and Borromeo, Italy.

 

 

Molding complex parts in large quantities: A variant of resin transfer molding (RTM), high-pressure RTM (HP-RTM), has been used for the mass production of large-scale integrated carbon fiber reinforced plastic automotive components, such as the side frame of the BMW (Wolfsburg, Germani) i8 sports car.

 

 

Unlike RTM, where the resin and catalyst are premixed before being injected into the mold under pressure, reaction injection molding (RIM) injects fast-curing resin and catalyst into the mold in two separate streams. The mixing and resulting chemical reaction occurs in the mold, not in a dispensing head.

 

Automotive industry suppliers have combined structural RIM (SRIM) with rapid preforming methods to make structural parts that do not require Class A surface finishes. Programmable robots have become a common means of spraying chopped glass fiber/binder combinations onto vacuum-equipped preform screens or molds.

 

Robotic spraying can be used to control fiber orientation. A related technology, dry fiber placement, combines stitched preforms with RTM. Fiber volumes of up to 68% are possible, and automated control ensures low voids and consistent preform replication without trimming.

 

 

Vacuum-assisted resin transfer molding (VARTM- Vacuum-assisted resin transfer molding) refers to various related processes and represents one of the fastest growing molding technologies. The significant difference between a VARTM-type process and RTM is that in VARTM, the resin is simply pumped into a preform by using a vacuum rather than being pumped under pressure. VARTM does not require high heat or pressure. For this reason, VARTM uses low-cost tooling that makes it possible to produce large, complex parts in one go.

 

 

Vacuum infusion has important applications in shipbuilding because it allows manufacturers to infuse entire hulls, deck structures and flat profile parts in a single step. But aerospace structures, another group of typically large components, are also being developed using the vacuum infusion process.

 

In the VARTM process, fiber reinforcements are placed in a single-sided mold and a cover (usually a plastic bag film) is placed on top to create a vacuum seal. Resin is usually introduced into the structure through strategically placed ports and feed lines, called a “manifold.” Resin is vacuum-drawn through the reinforcements through a series of designed internal channels to promote wetting of the fibers. The fiber content in the finished product can be as high as 70%.

 

Current applications include marine, land transportation and infrastructure segments. Resin infusion has important applications in shipbuilding because it allows manufacturers to infuse entire hulls, deck structures and flat profile parts in a single step. But aerospace structures, another group of typically large components, are also being developed using VARTM. A twist on resin infusion is to use two bags, called double-bag infusion, which uses a vacuum pump connected to the inner bag to extract volatiles and trapped air and a second vacuum pump on the outer bag to compact the laminate.

 

Boeing (Chicago, IL, U.S.) and NASA, as well as smaller manufacturing companies, have adopted this approach to produce aerospace-quality laminates without an autoclave. Aerospace quality has also been achieved in the development of an out-of-autoclave (OOA) CFRP wing for the MS-21 single-aisle jetliner by Russian OEM Irkut and manufacturer Aerocomposit, both based in Moscow.

 

A key step was the development of a one-piece CFRP wing box by FACC AG (Ried im Innkreis, Austria) using its proprietary membrane-assisted resin infusion (MARI) process, which uses a semipermeable membrane to achieve a stable, robust process that provides 100% impregnation (no dry spots or voids).

 

OOA infusion has also been demonstrated on large tools and structures for NASA’s Space Launch System (SLS) program using epoxy and bismaleimide (BMI) resins, and similar work with benzoxazine resins is progressing rapidly.

 

 

Resin film infusion (RFI) is a hybrid process where a dry preform is placed on top of a layer of high viscosity resin film or in a mold interlaced with multiple layers. Under applied heat, vacuum and pressure, the resin liquefies and is drawn into the preform, resulting in uniform resin distribution even for high viscosity toughening resins due to the short flow distance

 

 

Compression molding is a high-volume thermoset molding process that uses expensive but very durable metal molds. It is an appropriate choice when production quantities exceed 10,000 parts. Using sheet molding compound (SMC), a composite sheet made of chopped glass fibers sandwiched between two thick layers of resin paste, up to 200,000 parts can be produced on a set of forged steel molds.

 

To form the sheet, the resin paste is transferred from a metering device to a moving film carrier. Chopped glass fibers fall onto the paste, and a second film carrier places another layer of resin on the glass fibers. Rollers compact the glass fibers, saturating them with resin and squeezing out trapped air. The resin paste is initially the consistency of molasses (20,000-40,000 cps); over the next three to five days, its viscosity increases and the sheet becomes leathery (about 25 million cps), perfect for handling.

 

When the SMC is ready for molding, it is cut into smaller sheets and assembled in a filling pattern (ply sheet) on a heated mold (121°C to 262°C). The mold is closed and clamped, and a pressure of 24.5 to 172.4 bar is applied. As the viscosity of the material decreases, the SMC flows to fill the mold cavity.

 

After curing, the part is demolded manually or by an integrated demolding pin. A typical low-profile (less than 0.05% shrinkage) SMC formulation for a Class A finish consists of 25% polyester resin, 25% chopped glass fiber, 45% filler, and 5% additives by weight. Glass-fiber thermoset SMC cures in 30-150 seconds, and total cycle times can be as low as 60 seconds. Other grades of SMC include low-density, flexible, and pigmented formulations.

 

Low-pressure SMC formulations currently on the market offer open molders a low capital investment to enter closed mold processing, with near-zero VOC emissions and the potential for very high-quality surface finishes. Automakers are exploring carbon fiber reinforced SMCs, looking to take advantage of the high strength and stiffness-to-weight ratio of carbon fiber in exterior body panels and other parts.

 

Newer toughened SMC formulations help prevent microcracking, a phenomenon that previously caused paint “pops” during the painting process (surface pits caused by outgassing, which is the release of gases trapped in microcracks during oven curing).

 

Composite manufacturers in the industrial market are formulating their own resins and SMCs in-house to meet the needs of specific applications that require UV, impact and moisture resistance, with surface quality requirements driving the need for custom material development.

 

 

Injection molding is a fast, high-volume, low-pressure, closed process that most commonly uses filled thermoplastics such as nylon with chopped glass fibers. However, over the past 20 years, automated injection molding by BMC has captured some markets previously occupied by thermoplastic and metal casting manufacturers.

 

For example, the first BMC-based electronic throttle control (ETC) valves (previously made only of die-cast aluminum) debuted on the engines of the BMW Mini and Peugeot 207, taking advantage of the dimensional stability provided by specially molded BMCs supplied by TetraDUR GmbH (Hamburg, Germany), a subsidiary of Bulk Molding Compounds Inc. (BMCI, West Chicago, IL, USA).

 

In the BMC injection molding process, a plunger or screw-type piston forces metered material through a heated barrel and injects it (34.47-82.74MPa) into a closed, heated mold. In the mold, the liquefied BMC flows easily along the runners and into the closed mold.

 

After curing and demolding, the parts require only minimal finishing. Injection rates are typically one to five seconds, and in some multi-cavity molds, up to 2,000 small parts can be produced per hour. Parts with thick cross-sections can be compression molded or transfer molded with BMC.

 

Transfer molding is a closed mold process in which a measured charge of BMC is placed in a pot with runners leading to the mold cavity. A plunger forces the material into the cavity, where the product cures under heat and pressure.

 

 

Hybrid injection-molding/thermoforming is an example of the automotive industry seeking short mold cycles (<2 minutes) through hybrid plastic and composite processes. SpriForm is a process developed by HBW Gubesch Thermoforming GmbH (Wilhelmsdorf, Germany) for the CAMISMA automotive seat back project led by Johnson Controls (JCI, Burscheid, Germany).

 

The process preheats a custom blank made of carbon fiber (CF) reinforced polyamide 12 (PA12) organosheet, compression molds it in a matching metal mold and tool, and then injection molds a 30% short glass fiber reinforced PA12 compound that fills the mold cavity to produce a fully overmolded edge as well as ribs and other functional elements.

 

The process is easily automated using two robots and can save 40-50% weight compared to steel seat backs, adding less than $5/kg of incremental cost for the weight saved. While the continuous CF/PA12 tape provides tailored stiffness and strength, the lower-cost injection-molded material accounts for half the mass of the seat back.

 

The one-shot molding process takes about 90 seconds and produces geometrically detailed parts without the need for secondary operations. The base layer of the organosheet preform is a PA12 impregnated mat made from recycled carbon fiber (RCF), which is also a means of reducing part cost and carbon footprint.

 

 

Filament winding is a continuous manufacturing method that can be highly automated and repeatable with relatively low material costs. A long cylindrical tool called a mandrel is suspended horizontally between end supports, while the “head” – the fiber application machine – moves back and forth along the length of the rotating mandrel, placing fibers on the tool in a predetermined configuration.

 

Computer-controlled filament winding machines are available with 2 to 12 axes of motion. In most thermoset applications, filament winding equipment passes the fiber material through a resin “bath” before the material contacts the mandrel. This is called wet winding. However, one variation uses tow prepreg, which is continuous fiber that is pre-impregnated with resin. This eliminates the need for an on-site resin bath.

 

In a slightly different process, the fibers are wound without resin (dry winding). The dried shape is then used as a preform in another molding process, such as RTM. After oven or autoclave curing, the mandrel either remains in place, becoming part of the wound part, or it is usually removed.

 

A one-piece cylindrical or tapered mandrel, usually of simple shape, that is pulled from the part with a mandrel extraction device. Some mandrels, especially in more complex parts, are made of soluble materials that can be dissolved and flushed out of the part. Others are foldable or made of several pieces that can be disassembled and disassembled into smaller parts.

 

Filament winding manufacturers often “tune” or slightly modify off-the-shelf resins to meet specific application requirements. Some composite part manufacturers develop their own resin formulations. In thermoplastic winding, all materials are prepregs, so no resin bath is required. The material is heated as it is wound onto the mandrel, a process known as “dynamic curing” or in-situ consolidation.

 

The prepreg blanks are heated, laid up, compacted, consolidated, and cooled in one continuous operation. Thermoplastic prepregs eliminate autoclave curing (cutting costs and size limitations) and reduce raw material costs, and the resulting parts can be reprocessed to correct defects. Filament winding can produce parts with exceptional circumferential or “hoop” strength.

 

The largest single application for filament winding is golf club shafts. Fishing rods, pipes, pressure vessels and other cylindrical parts make up the bulk of the remaining business.

 

 

Pultrusion, like RTM, has been used for decades with fiberglass and polyester resins, but over the past 10 years the process has also found its way into advanced composite applications. In this relatively simple, low-cost, continuous process, reinforcement fibers (usually rovings, tows, or continuous mats) are typically pulled through a heated resin bath and then formed into a specific shape as it passes through one or more forming guides or bushings.

 

The material then passes through a heated die, where it is formed into a web and solidifies. Further downstream, after cooling, the resulting profile is cut to the desired length. Pultrusion produces a smooth finished part that generally requires no post-processing. A wide range of continuous, consistent, solid and hollow profiles are pultruded, and the process can be tailored to specific applications.

 

 

Tube rolling is a long-standing composites manufacturing process that produces tubes and rods in limited lengths. It is particularly useful for small diameter cylindrical or tapered tubes up to 6.2 m in length. Tubes up to 152 mm in diameter can be rolled efficiently.

 

Typically, either sticky prepreg fabric or unidirectional tape is used, depending on the part. The material is precut into patterns designed to achieve the ply schedule and fiber structure required for the application. The pattern pieces are placed on a flat surface and a mandrel is rolled around each pattern piece under applied pressure, compacting and breaking up the material.

 

When rolling a tapered mandrel, for example, for a fishing rod or golf club, only the first row of longitudinal fibers falls on the true 0° axis. Therefore, to give the tube bending strength, the fibers must be continuously reoriented by repositioning the pattern pieces at regular intervals.

 

Automatic Fiber Placement (AFP). The fiber placement process automatically places multiple individual prepreg tows onto a mandrel at high speed, using a CNC articulated robotic placement head to dispense, clamp, cut and restart up to 32 tows simultaneously. Minimum cut length (the shortest tow length the machine can lay) is an important factor in determining ply shape.

 

The fiber placement head can be attached to a 5-axis gantry, retrofitted to a filament winding machine, or delivered as a turnkey custom system. The machine is equipped with a dual mandrel station for increased productivity. The benefits of fiber placement include processing speed, reduced material scrap and labor costs, part consolidation and improved part-to-part consistency. The process is typically used to produce large thermoset parts with complex shapes.

 

 

Automated tape laying (ATL) is a faster automated process in which prepreg tapes are laid down continuously rather than individual strands to form a part. It is often used for parts with highly complex contours or angles. The tape layup is versatile, allows for in-process breaks, and is easily changed in direction, and it can be adapted for both thermoset and thermoplastic materials.

 

The head includes one or more tape reels, winders, winder guides, compaction shoes, position sensors, and a tape cutter or slitter. In either case, the head can be located at the end of a multi-axis articulated robot that moves around a tool or mandrel to which the material is applied, or the head can be located on a gantry suspended above the tool.

 

Alternatively, the tool or mandrel can be moved or rotated to allow the head to access different parts of the tool. The tape or fiber is applied to the tool in a course, which consists of a row of material of any length at any angle. Multiple courses are often applied together to an area or pattern and are defined and controlled by machine control software that is programmed using digital inputs obtained from part design and analysis.

 

The capital outlay for computer-driven automated equipment can be significant. Although ATL is generally faster than AFP and can place more material over longer distances, AFP is better suited for shorter routes and can more efficiently place material on contoured surfaces.

 

These technologies originated in the machine tool industry and have been widely used in the manufacture of fuselages, wing panels, wing boxes, empennages, and other structures for the upcoming Boeing 787 Dreamliner and Airbus A350 XWB. ATL and AFP are also widely used to produce parts for the F-35 Lightning II fighter, V-22 Osprey tilt-rotor troop transport, and various other aircraft.

 

The latest equipment trends enable both AFP and ATL to switch between the two in minutes by replacing a fixable head. Another area of ​​development is the pursuit of composite aircraft structures that are primarily loaded out-of-autoclave (OOA) with high-performance thermoplastics. Airbus (Toulouse, France) is working with FIDAMC (Madrid, Spain), supported by MTorres (Navarre, Spain), and Technocus EMC2 (Nantes, France), supported by Coriolis Composites SAS (Queven, France), to develop long-strut-stiffened fuselage skin panels that are laser-cured in situ using automated machinery.

 

FIDAMC and MTorres announced at JEC 2014 a CF/polyetheretherketone (PEEK) fuselage panel with 35-40% matrix crystallinity and sufficient degree of consolidation (DOC) to eliminate the need for further heating, vacuum bagging or autoclaving. Real-time temperature control is being integrated into the equipment. Materials are provided by Cytec Aerospace Materials HQ (Woodland Park, NJ, USA) and Toho Tenax Europe GmbH (Wuppertal, Germany).

 

 

Centrifugal casting of 25 mm to 356 mm diameter pipe is an alternative to filament winding in high performance, corrosion resistant service. In cast pipe, 0°/90° braided glass fibers provide longitudinal and hoop strength throughout the pipe wall and greater strength at the same wall thickness compared to multiaxial fiberglass wound pipe.

 

During the casting process, epoxy or vinyl ester resin is injected into a 150G centrifugal rotating mold, penetrating the braid wrapped around the inner surface of the mold. Centrifugal force pushes the resin through the fabric layers to form a smooth surface on the outside of the pipe, and excess resin is pumped into the mold to form a resin-rich, corrosion and wear resistant lining.

 

Fiber reinforced thermoplastic components can now also be produced by extrusion. Breakthrough material and process technologies have been developed using long fiber glass reinforced thermoplastic (ABS, PVC or polypropylene) composites to provide a strong, low-cost profile that replaces wood, metal and injection molded plastic parts used in office furniture, appliances, semi-trailers and sporting goods.

 

Over the past decade, a huge market has emerged for extruded thermoplastic/wood flour (or other additives such as bast fiber or fly ash) composites. These wood-plastic composites, or WPCs, are used to simulate wood panels, siding, door and window frames, and fencing.

 

 

Also known as 3D printing, this newer form of composite part production stems from efforts to reduce costs during the design-to-prototype phase of product development, particularly in the material-, labor-, and time-intensive areas of toolmaking.

 

Additive manufacturing is a step change in the evolution of the concept of rapid prototyping, introduced more than 20 years ago, which is a group of similar but separately developed additive manufacturing technologies, namely the automated process of assembling a three-dimensional (3D) object from a series of layers of specialized material with nominally two-dimensional (2D) cross-sections.

 

All additive manufacturing technologies start with a CAD drawing. Special software is used to convert the solid model CAD data into a file format that represents a three-dimensional surface as a combination of flat triangles.

 

Additional, often proprietary software is then used to “slice” this virtual image into very thin 2D cross-sectional patterns. This layer data is used to guide the additive manufacturing machinery to build a 3D physical model by “stacking” the 2D slices. Currently, five additive manufacturing methods are used:

 

 

Patented in 1986, stereolithography (SLA) was the first fully commercialized rapid prototyping technology and remains the most widely used technology today.

 

In the SLA process, a model of the part is built on a platform that sits just below the surface in a vat of liquid photocurable polymer (usually epoxy or acrylate resin).

 

A low-powered ultraviolet (UV- ultraviolet) laser, programmed with previously created CAD slice data, traces the first layer of the part with its highly focused UV beam, scanning and curing the resin within the boundaries of the slice outline until the entire area within the slice cross-section is cured.

 

An elevator then gradually lowers the platform into the liquid polymer to a depth equal to the slice thickness, and a sweeper re-coats the cured layer with liquid polymer.

 

The laser then scribes a second layer on top of the first. The process is repeated until the part is complete. Depending on the geometry of the part, it may be necessary to build mechanical supports into the part to contain the liquid during the build process.

 

After removal from the vat, the supports are removed from the part, which is then placed in a UV oven for additional curing.

 

 

Fused Deposition Modeling (FDM-Fused Deposition Modeling) is the second most widely used AM process. FDM is made from ABS (acrylonitrile-butadiene-styrene), polycarbonate, and other resins known for their toughness.

 

It is usually chosen when part durability is critical. FDM builds three-dimensional objects one layer at a time. The plastic filament unwraps from the coil, feeding the material to a heated extrusion nozzle, which controls the flow rate.

 

The nozzle is mounted on the machine table and can be moved horizontally and/or vertically. The nozzle moves over a table coated with support material, depositing a thin bead of extruded plastic. For ABS, the thickness of this layer is typically 0.25mm/0.010 inches, which roughly defines the tolerances on FDM parts. Successive extruded layers bond to the previous layer and then harden immediately.

 

The entire system is contained in a chamber at a temperature just below the melting point of the plastic. No post-processing is required after parts are removed from the chamber.

 

 

Laser sintering (LS- Laser Sintering ) was developed in the late 1980s by the American company DTM in Austin, Texas. The technology was acquired by 3D Systems in 2001. In an approach similar to stereolithography, 3D’s Selective Laser Sintering (SLS- Selective Laser Sintering) process uses the heat of a CO2 laser to process a variety of materials in powdered rather than liquid form, including nylon and glass- or carbon-fiber-filled nylon.

 

In an enclosed unit about the size of a print-shop copier, a CO2 laser and a mirror system are mounted on a build table or base that supports the part. Rollers spread a thin layer of powdered material across the surface of the base, and then a mirror system directs the laser beam onto the powder layer. As the beam scans back and forth over the material, the laser turns on and off, selectively sintering the powder (heating the powder particles to melting or fusing temperature) in a pattern identical in size and shape to the cross-sectional slices derived from the converted CAD file.

 

The base is then lowered the distance of the layer thickness, another layer of powder is rolled over the cooled and now solidified first layer, and the sintering process is repeated, bonding the second layer to the first.

 

The process is repeated in layers 0.08 mm to 0.15 mm (0.003 in. to 0.006 in.) thick until the part is complete.

 

 

Digital Light Processing (DLP) was developed by Texas Instruments Inc., Austin, TX, USA, and supports a range of computer-aided modeling devices (CAMOD) developed by EnvisionTEC (Ferndale, MI, USA).

 

Like stereolithography platforms, this technology uses photocurable resins, but reportedly processes them faster (about 25mm/1 in/hour) using a continuous process involving mask projection rather than incremental layering, i.e., an entire image is projected onto a bath of liquid photopolymer rather than scanning or depositing layers of material and applying heat with a point energy source over successively applied layers of powder or liquid resin.

 

In addition, continuous build technology eliminates the visible and tactile stepped part surfaces that are characteristic of layer-based additive manufacturing. EnvisionTEC’s Perfactory Xede machine uses single or multiple DLP-based projectors to produce multiple parts within a relatively small 457×304×508 mm (18×12×20 in) build envelope. Finished parts reportedly have the same properties as engineering plastics, such as ABS, high-density polyethylene or polypropylene.

 

 

3D printing is the newest entrant into the market, having made its debut in late 2007 when Objet Geometrys (Rehovot, Israel) introduced the Connex500 3D system, which builds 3D parts by jetting successive layers of material.

 

Designed to print one or two building materials simultaneously, the system is based on Objet’s PolyJet Matrix printing technology, an advanced version of the inkjet technology with which most people are familiar.

 

Objet Studio for Connex software manages the process, creating print files from converted CAD data. In operation, the system delivers one or two materials to a dedicated fluid system attached to a PolyJet Matrix block, which contains eight printheads, each containing 96 nozzles. Two fully synchronized printheads are assigned to each material, including an easily removable, water-soluble, gel-like support material.

 

These processes were originally intended, and still are, to enable part designers and engineers to bypass the need for prototype tooling, enabling them to produce prototypes in a matter of hours to evaluate form and fit characteristics and, in some cases, as test articles, such as those used in wind tunnel evaluations of part aerodynamics.

 

However, designers have realized that it is also possible to use additive manufacturing systems to create production parts. Fused deposition modeling is a method that has become the most widely used mode of production for fiber-reinforced plastic parts.

 

 

Manufacturers and OEMs must address health, safety and environmental issues when producing and handling composites. Their methods of maintaining workplace safety include regular training, following detailed handling procedures, maintaining current toxicity information, using protective equipment (gloves, aprons, dust protection systems and respirators) and developing company-wide monitoring policies.

 

Suppliers and OEMs are working to reduce emissions of high-volatile organic compounds (VOCs) by reformulating resins and prepregs and switching to water-dispersible cleaners. The U.S. Environmental Protection Agency continues to strengthen its requirements to meet the requirements of the Clean Air Act Amendments passed by Congress in 1990.

 

Specifically, the agency aims to reduce emissions of hazardous air pollutants (HAPs), a list of about 180 volatile chemicals that are considered to pose a risk to health. Some of the compounds used in resins and released during the curing process contain HAPs.

 

In early 2003, the EPA issued regulations specifically for the composites industry requiring the use of maximum achievable control technology (MACT) for emission controls. The regulation came into effect in early 2006.