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THERMOPLASTICS FOR HIGH-TEMPERATURE COMPOSITE PROCESSES & APPLICATIONS

Charlie Costello

Ticona Engineering Polymers

Abstract

The high-temperature, high-mechanical performance end of the composite materials spectrum – so-called advanced composites – has long been dominated by thermoset matrices, primarily epoxy and urethane chemistries with carbon or aramid continuous-strand, unidirectional fiber or fabric weave reinforcements. However, that is starting to change as thermoplastic resin suppliers begin to investigate and more aggressively position their own high-temperature offsets in this segment – not just as lower cost, lower weight, faster processing replacement for thermosets, but as direct metal replacements themselves.

By offering molders and OEMs the option to produce performance parts with higher productivity thermoplastic matrices that do not need to be polymerized in the tool, numerous benefits are gained, including reducing cycle times, volatile-organic compound (VOC) emissions, energy consumption, spoilage, and finishing steps – all of which help drive down costs and make advanced composites more affordable for higher volume applications than metals. This can be particularly attractive for processors without autoclaves or without sufficient autoclave resources to process traditional high-performance thermoset composite matrices, or where producing parts via these or other traditional thermoset processes are difficult and/or costly.

This paper will provide an overview of current market pressures supporting the growth of high-performance thermoplastics, and then will review various processing options for both thermoset and thermoplastic high-performance composites. Next, several short case histories involving conversions to thermoplastic matrices directly from metals will be presented. Last, future trends that could impact this segment will also be considered.

Trends Supporting Thermoplastic Alternatives

While high-performance thermoset matrices like epoxies and urethanes plus some polyimides have long been used for the most demanding spectrum of polymer composites applications (in markets such as aerospace, extreme sports, infrastructure, motorsports, and supercars) there are a number of converging trends that are favoring exploration of non-traditional polymer options such as high-temperature thermoplastics for both thermoset and direct metal conversions.

So-called lightweighting is an important issue in many industries at present, particularly for all forms of personal and mass transportation – from airplanes to truck and bus, to cars. Just as thermoset composites of comparable performance offer significant opportunities to reduce component weight vs. metals, so too do lower specific gravity (SG) thermoplastics provide further chances to take additional weight out of components while offering similar or better performance. The advantage of doing so is a reduction in the amount of increasingly costly energy required to transports goods and people, and to provide services over the life of the vehicle, plus resultant reductions in greenhouse gas emissions as well. Not only does this help protect the environment and enable OEMs to meet tougher emissions and fuel-economy standards, but it also facilitates reduction of operating costs for the ultimate vehicle owner and –

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in the case of commercial trucks and planes, where payload limits are restricted by law – helps ensure that commercial vehicles are able to carry more of what brings profit without exceeding maximum carrying capacity. In other words, taking weight out of vehicle components (and hence vehicles) not only is good for the environment but is good for the bottom line.

Another important trend that also has legal implications in some industries is the desire to reduce volatile-organic compound (VOC) emissions during production of parts and during their subsequent use life. For components used inside buildings and vehicles, where air quality as well as fogging are becoming more heavily regulated, this is an important consideration for OEMs as well as consumers during the life of components. For manufacturers of those parts, whose industries already face or are soon to face increasingly tough regulations on worker safety and environmental protection, there is a high desire to find material alternatives that perform comparably but do not necessitate maintenance of, or installation of new air- and water-handling and worker-safety equipment. Most thermoplastics have a clear advantage over thermosets when it comes to VOC emissions, since polymerization is done by the resin producer under highly controlled conditions in industrial-scale plants rather than in small-scale open- or closed-mold processing operations. Hence, VOC reduction also favors increased use of thermoplastics.

Still another issue with environmental mandates is the end-of-life reclamation requirements in Europe that affect automakers selling into that geography. Again, thermoplastic polymers, that can be remelted and reused multiple times – both during initial part production and after consumer use is done – are favored over thermosets, which, once cross-linked, cannot be remelted and have fewer commercially viable options for reuse. The ability to reclaim polymer from industrial scrap and post-use life promises to help reduce production costs at a time of rising petroleum-input prices and possible reduction in available supplies. For all but the most demanding applications, most thermoplastics still have significant performance to offer even after half-a-dozen heat histories.

And let us not forget about cost reduction or cost containment, a persistent challenge facing virtually all OEMs as raw materials and energy costs fluctuate considerably on the global market, and labor costs can change and production operations can be disrupted or destroyed by regional political unrest, extreme weather, or geologic instability with little or no warning. This is particularly an issue for automakers struggling to return to profitability after the 2008-2009 global crash and finding themselves in a radically different business environment. Because build volumes are much lower now, manufacturing methods that used to be profitable may no longer be so now that long-amortized equipment and facilities have been sold or closed and written off a company’s books. Automakers that were forced to scrap older plants and production equipment during the severe downturn must reinvest with new capital expenditures as the industry rebuilds, particularly as production moves away from traditional manufacturing centers in Detroit, Tokyo, and Wolfsburg to other geographies where labor is more competitive. What they are finding, in many cases, is that the true cost of making parts has changed or is being seen accurately for the first time, since all capital costs now hit the bottom line, and metal stamping or casting may no longer be the lowest cost options. Since tooling for composites is typically much less expensive than that for metals, particularly in the lower production volumes that seem to be the new “normal,” polymeric systems are favored. And since thermoplastics do not need to go through a cross-linking/polymerization cycle in the tool, and hence can be processed so much faster than thermosets, fewer tools and presses are required to produce comparable numbers of parts, or far more parts can be produced on existing equipment. Should build volumes return to their previous highs in the late 1990s and early 2000s, thermoplastics are even better positioned than thermosets owing to their rapid molding cycles.

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A related issue is the desire to reduce or eliminate secondary-finishing operations, which increase handling (and hence time and costs) and also boost the risk of spoilage of parts. Here, too, thermoplastics are favored. Although initial costs for most thermoset resins and processing systems tend to be lower than that for comparably performing thermoplastics, finished part costs can be significantly higher because thermoplastics offer faster processing and fewer post-mold finishing steps (see Figure 1). Additionally, most thermoplastic molding processes can produce parts with better surfaces (owing to the ability to reflow resin and produce a resin- rather than fiber-rich surface even at high fiber volume fractions (FVFs), and the majority of these processes can easily be moderately to highly automated, reducing costs further. This saves significant sanding and filling work needed with most highly reinforced thermosets. Also, the increased ductility of thermoplastics vs. thermosets makes fastening systems like snap-fits realistic, often eliminating the need for adhesive bonding and use of other mechanical fasteners like screws.

Figure 1: Relative cost to produce high-performance carbon fiber-reinforced composites in thermoset (epoxy) and thermoplastic (polyetheretherketone (PEEK), polyetherimide (PEI), and linear polyphenylene sulfide (PPS)) matrices

showing relative cost of prepreg and processing

With all these trends supporting the increased use of thermoplastic materials in the high-performance segment of the composites industry, why are more of these materials not being used there? The biggest problem is a lack of familiarity, which papers like this one seek to help resolve. In the next section we will review molding processes for high-performance thermosets and thermoplastics and compare the relative benefits and challenges of each. After this, we will review several case studies where metals were directly replaced by thermoplastic composites to great benefit for processor, OEM, and customer alike.

Processing Options for High-Performance Composites

Depending on the geometry and performance requirements of a given part, many process options are available for high-performance, continuously reinforced composites – some exclusive to thermoset matrices and some exclusive to thermoplastic matrices, and a few that can be used with either. Each process offers its own set of benefits and challenges.

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Vacuum-Bag / Bladder Molding (Thermosets & Thermoplastics)

Whether cured in an oven or in an autoclave, a significant amount of high-performance thermoset composites and a few thermoplastics are produced via hand layup using layers of prepreg (resin-impregnated fabrics or tow) or bonded assemblies (sandwich-panel constructions). Vacuum bag / bladder molding is favored for large parts used in low-volume applications (e.g. aerospace and defense, racecar and supercar, wind energy, and large boat components) thanks to its ability to produce relatively low-cost, single-sided tooling, which allows moderate to extremely large parts to be molded in a single piece for higher structural integrity and lower finishing costs. The process begins with kit cutting, followed by hand layup of multiple layers of prepreg in a tool. Next, the laminate is sealed and a vacuum is applied to extract air and apply pressure to achieve consolidation. Vacuum bagging uses a number of consumables to seal the laminate prior to applying a vacuum and is favored for lower production volumes and parts that don’t require exacting surface finishes; bladder molding uses a reusable silicone rubber bag, reducing the waste and cost of consumables and enabling higher pressures (hence better consolidation and surface finish) to be achieved. Thermoset parts produced in either process are then generally cured via oven or autoclave at elevated temperature and pressures, although newer resin technology allows for room-temperature cure in the layup buck for parts with lower strength / weight requirements. With thermosets, control of the exotherm during polymerization / cross-linking is critical to preventing carbonization / burning, shrink, cracking and other issues that can affect structural integrity. Thermoplastics can also be processed in ovens or autoclaves to achieve consolidation, to reflow skins for better surfaces, and to stress-relieve critical parts, although this is less common and process cycles are much shorter and process temperatures are generally lower. Once parts are cooled enough to demold, they generally require post-mold trimming and finishing.

Benefits of the hand layup process are that very-large parts can be produced in a single piece; soft / low-cost tooling is generally used, so design changes late in the program mean tooling is less costly to update. Hand layup is also capable of high fiber volume fractions (HVFs). If oven cure is used, equipment investments are modest; and if autoclave curing is used, highly consolidated primary structures can be produced with excellent performance. With the ability to control pressure and heat very closely, autoclave cure provides for excellent flatness and the highest strength / weight ratio in finished parts (due to high consolidation). It also produces parts with fewer voids and dry spots and less molded-in-stress than oven or room-temperature curing processes. Newer out-of-autoclave processes are being advanced that promise quality approaching or matching that of the autoclave but at far faster cycle times.

Process challenges are that hand layup is time intensive and requires skilled labor, and it is not easy to get prepreg to lay down properly across complex surfaces (so parts tend to have fairly simple geometry) and only a single finished surface is produced. With vacuum bagging, there is the mess and cost of consumables; and when autoclave cure is used, capital equipment costs can be significant. In the case of thermosets, open tools can pose health and environmental issues due to VOCs, so off-gasing must be controlled. Cycle times are typically quite long (60-90 minutes for modest-size parts and days for really large structures), hence energy costs are high. Also with thermosets, prepreg materials usually have storage and expiration issues, and the prepreg is costly owing to significant scrappage at the prepreggers when producing B-stage intermediates from the raw materials. With thermoplastics, cycle times are shorter since polymerization has already occurred. Post-mold cutting and finishing also can add additional labor and costs to the production process. Hand layup tends to be best for very-low-to-low volume applications that can tolerate high piece costs to reduce total program costs.

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Resin Infusion (Thermosets & Thermoplastics)

Resin infusion processes also begin with single-sided tooling. Here, a dry preformed reinforcement (often a woven fabric sleeve without resin pre-impregnation) is infused with a very-low viscosity thermoset or thermoplastic resin, which is slowly released into the tool and reinforcement, then allowed to consolidate and cure at elevated temperature and pressure.

This process lends itself well to quite large parts (that may be hollow or thin-walled) and more complex shapes than hand layup owing to the ability of the low viscosity resin to flow into intricate design features. Tooling can be fairly simple, so is low cost, but cycle times typically are quite long, especially for large parts with thicker cross-sections. As with hand layup processes, thermoset processing can pose environmental and health issues owing to single-sided tooling, so VOC outgasing must be controlled. Typical high-performance parts molded via resin infusion include bridge struts and arches, and large wind blades.

Resin Transfer Molding - RTM (Thermosets)

Although it has been around for more than 4 decades, RTM is gaining interest with molders of high-performance composites thanks to its ability to produce net-shape thermoset parts with good surfaces, low void content, and controllable FVF levels rapidly in a process that is similar to thermoplastic injection molding. It is used to produce dimensionally stable and complex parts for airplanes and automobiles, as well as train seating. Here, a dry perform is placed in one half of a two-piece, matched metal die, the mold closes, and low-viscosity resin is injected into the cavity under pressure. Then, vacuum pressure is applied to the tool, removing entrapped air (that otherwise causes voids) and speeding consolidation. Resin cure can be done either based on time (at room temperature) or at elevated temperature (for faster processing). Once the part is fully cured, the tool is open and the part is removed, trimmed, and finished.

Since forming occurs in a closed mold and can be automated, RTM has fewer health and environmental risks than with traditional open-mold thermoset processes like resin impregnation or vacuum bag / bladder molding. Also, 2-piece matched metal dies ensures parts have good surfaces on both sides. In addition, raw materials are less costly than those associated with prepreg used in hand layup processes, although it does offer the opportunity to use dry fiber or braided preforms, which – since they are not pre-impregnated with thermoset resin – no longer have the storage and shelf-life issues of traditional prepregs. Unlike hand layup processes, with RTM it is very easy to achieve quite complex, 3D parts.

Challenges are that the preform can move inside the tool during rein injection, so fiber placement is more challenging to control. Also, to manage good wetout of the preform and obtain repeatable, high-quality parts, control of a number of process parameters is required, although once the right process settings are determined, automation makes it relatively easy to maintain those parameters. While tooling is more costly than that with many other thermoset processes, particularly for larger parts, the speed of the process and lower cost materials help make it cost-effective up to medium production volumes. However, cycle times are still fairly long compared to those achieved with thermoplastics, since resin must be allowed to polymerize and cross-link in the tool.

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Filament Winding (Thermosets & Thermoplastics)

Filament winding is primarily used to produce hollow parts that typically have a round or oval cross-section (e.g. pipes, storage tanks, pressure vessels, gas cylinders, rocket motors, launch tubes, drive shafts, and fishing rods). Continuous fiber tows pass through a resin bath or over a resin-covered drum or through a die prior to moving into a feed eye (that sets bandwidth) before being wound around a heated mandrel in a sequence of changing orientations / winding patterns. This is controlled by the fiber-feed mechanism, rate of rotation of the mandrel, and resin cure time (with thermoplastics allowing much faster winding speeds), until the mandrel is completely covered to a specified thickness. A critical process parameter – affecting fiber-volume fraction and void content, which, in turn, affect part stiffness and strength – is fiber tension, which also causes compaction / consolidation and is controlled by closed-loop, servo-driven tensioners. Winding speed is affected by fiber type, part diameter, cure time for the resin involved, plus part thickness and the winding pattern used. Once would, most parts (whether thermoset or thermoplastic) are placed in an oven and allowed to consolidate and cure. Once the resin is solidified or cured, the mandrel is removed.

Benefits of this process are its ability to achieve very-high FVFs and to control fiber patterns precisely. It also yields parts with very-high mechanical properties. Cycle times can be significantly faster than for other thermoset processes, and raw materials are less costly since they are essentially produced just prior to use.

Limitations to this process are that geometry is limited to fairly round or cylindrical shapes with hollow cores. While mandrels are relatively low cost vs. other types of tooling, the filament winding system can be extremely sophisticated and hence costly.

Tape Layup (Thermosets & Thermoplastics)

Tape layup begins as a film of either a B-stage thermoset or a thermoplastic resin and unidirectional fiber, multidirectional mat, or fabric weaves produced on an extruder. Once the web of film is produced, it is slit into long strips, which are subsequently braided into tapes. Key process variables include braid angle, braid diameter, and spacing. These can be used as a prepreg or preform for hand layup in a tool. For lower cost parts with lower surface finish requirements and flat or slightly contoured geometries, single-sided tooling with a plug or vacuum assist thermoforming can be used. For parts needing a higher quality surface and / or more complex geometry, matched metal dies mounted in a low-pressure thermostamping press are used and additional film or a nonwoven layer may be added to the tool on the show face of the part to produce a resin-rich surface. Cycle time depends on the type of resin used and the times required to heat up and cool down the tool prior to demolding the part. For thermosets, processing typically takes 1 hr or longer owing to the need to polymerize and cross-link the polymer; for thermoplastics, comparable parts can be processed in 1/10th the time and energy, although some portion of time is required to achieve good crystallization with semi-crystalline (rather than amorphous) thermoplastics. Tools or presses equipped with inductive mold heating and cooling (rather than conductive or radiant heating / cooling) benefit by faster processing cycles for either thermoset or thermoplastic tapes. Tubular parts – with and without cores – can be produced in a tool with a split mandrel for easy removal. In such cases, braid diameter can be tailored to meet the radial location in the final part and a silicone sleeve can be used to provide additional expansion of the tool to accommodate a larger inner diameter. Heating / cooling channels in the tool can help speed up process time.

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Benefits of tape layup are that it provides excellent uniformity of fiber / matrix distribution and high fiber wetout to yield parts with fairly high FVFs. Tooling can be as simple as soft-tooling for plug-assist thermoforming or mandrels for tape winding, or as complex as inductively heated / cooled steel dies mounted to a high-speed thermostamping press. And compared with other processes for advanced composites can be considered quite fast, particularly when thermoplastic matrices are used. The process is used to produce reinforced pipe for corrosion-prone environments, composite seat frames, tubing and molded parts for oil and gas exploration/handling, components for aircraft interiors and exteriors, and automotive underhood.

Challenges of this process are that sufficient pressure must be applied to the laminate to achieve resin flow for good consolidation and to eliminate porosity, but too much pressure can result in a dry part by squeezing resin out from between the layers. Good layup technique can help by minimizing air gaps as can molding to stops or using an elastomeric insert. Additionally, thermoset tapes and prepregs have to be stored at low temperatures and have a expiration date. With thermoplastic resins, prepreg freezers (and their accompanying energy costs) are eliminated and production is much faster.

Figure 2 compares typical processes used to produce high-performance thermoset and thermoplastic composite parts in terms of performance properties, relative finished part cost, part design complexity, and production volume.

Figure 2: Performance vs. part cost vs. design complexity vs. production volume for processes used to produce high-performance composite parts

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Successful Metal to High-Temperature Thermoplastic Conversions

While high-performance thermoplastic composites used to be considered only when more cost and weight needed to be taken out of an application, or when an application was thought to be over-engineered in either metals or high-performance thermoset composites, as industry grows more familiar with these materials and the significant performance, processing, and cost benefits they can bring to applications, they are actually becoming the first, rather than second choice in many industries. For example, despite having properties similar to aluminum, it took nearly a decade before linear polyphenylene sulfide (PPS) and glass fabric composites were approved for aviation and defense applications. Once that happened, however, aviation applications proliferated for PPS both glass and carbon reinforcement. There are 252 PPS composites applications totaling 1,000 kg on Airbus S.A.S.’ A340 planes alone.

PPS /Glass Composites Replace Aluminum in Airplane Wing Leading Edge

Fokker Special Products was the first manufacturer to replace aluminum for thermoformed high-performance linear-PPS/continuous glass fiber composites on the inboard leading edge “nose” of wings for the Airbus’s A340-500 and -600 series commercial planes (shown, respectively, in Figures 3&4). This was the first large-scale use of thermoplastic composites in a commercial airplane wing. Starting from pelletized resin, a crystal-clear film of PPS is formed by Lipp-Terler GmbH and the rollstock is shipped to Royal TenCate, who produces a preform of the PPS film and carbon fiber by bonding the layers together in a press under high pressure and high temperature. This produces a high-strength, dimensionally stable and resistant sheet-form composite blanks in the desired layer thickness. The blanks are then heated prior to being placed in a 2-sided tool and thermoformed under high temperature but moderate pressure. The resultant parts reduce weight of the structural part by 20%, increasing flight range while reducing fuel costs. It also improves speed and ease of fabrication, holds tight tolerances, offers excellent resistance to aggressive chemicals like hydraulic fluids, fuel, and deicing agents, and is far more damage tolerant than the aluminum honeycomb structure previously used for the application. High-performance linear PPS thermoplastic is tougher, stronger, and more ductile than similar materials considered for the application and maintains these properties over the broad range of temperature (-40 to 240C) seen by this giant aircraft.

An additional benefit that thermoplastic resin brought to the application was its ability to be welded, allowing ribs and skins to be fused together. Not only is a chemical bond produced in welding much stronger and lower stress than point fasteners, but it is significantly faster to produce. Further, welding saves the time and costs of drilling and installing rivets, plus subsequent maintenance, and it also eliminates the stress concentrations that occur around mechanical fasteners. PPS composites were also used to form flaps / ailerons and keel beams on these planes by the French fabricator, Aerospatiale.

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Figures 3 & 4: The inboard leading-edge nose for wings of the Airbus A340-500 & -600 series commercial planes (left) was converted directly from aluminum to high-performance thermoplastic composites of PPS/glass fabric.

Weight was reduced 20%, and impact strength was increased significantly, all while providing required mechanical, thermal, and chemical performance. Since thermoplastic parts can be welded, the cost, time, and stress

concentrators that are incurred when components are assembled with rivets were eliminated. PPS composites are also being used to produce flaps / ailerons and keel beams on commercial aircraft, replacing aluminum.

PPS / Carbon Composites Replace Aluminum in Airplane Rudder & Elevator

Another interesting aerospace primary structure that was converted from aluminum to thermoplastic composite were the rudders and elevator on the tail sections of Gulfstream Aerospace Corp.’s G650 high-speed business jets (shown in Figure 5&6). The application was developed by Fokker Aerostructures using a carbon fiber / PPS semipreg and plate material developed by TenCate Aerospace Composites, the molded components were subsequently induction welded via a process developed by KVE Composites Group, creating an unbreakable bond while eliminating the cost and time required to drill and install rivets, and the stress concentrators that results from mechanical fasteners. Composites made from PPS / carbon fiber remain hard, impact resistant, stiff and dimensionally stable, even when exposed to wide temperature fluctuations and aggressive fluids.

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Figures 5 & 6: A PPS / carbon composite was used for both the rudders and elevator (left) in the tail section of Gulfstream’s G650 corporate jets (right).

PPS / Carbon Composites Replace Aluminum in Airplane Seat Elements

The airline industry, like automotive, is scrambling to take weight out of components to increase flying range of vehicles by improving fuel efficiency. Inside the cabin, an area of special interest is passenger seating. By replacing seating components currently made of aluminum with composites, not only can weight be reduced, and production costs be lowered, but it is far easier to create more ergonomic shapes that are more comfortable to sit in for extended periods of time. A major aerospace seating supplier switched lumbar and thigh supports, armrest table covers, and shields for in-flight video screens (several of which are shown in Figures 6 & 7) from aluminum to a thermoformed composite comprised of PPS and carbon fiber and reduced weight of these components an average of 40-50%. PPS was selected for the matrix of this composite, produced by Bond-Laminates GmbH and molded by DTC, owing to its high strength, chemical resistance, toughness, and – since this was for an interior cabin application – its excellent flame / smoke / toxicity (FST) properties as well as high limiting oxygen index (LOI) of 40. The seating supplier benefited not only by lighter and less costly parts, but they were also faster and easier to process than comparable aluminum parts yet offered higher mechanical performance and a cost savings ranging from 20-25% depending on the part. Other benefits include weldability (for faster, better joining of parts), rapid molding cycles, toughness, and the ability to be melt-reprocessed (recycled) at the end of part use life.

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Figures 6 & 7: A thermoformed PPS / carbon composite replaced aluminum in numerous airline seating components (3 of which are shown on the left), resulting in 40-50% weight savings, 20-25% cost savings, higher mechanical performance, excellent toughness, as well as inherent, non-halogenated flame retardance and excellent FST

performance. Several of the parts are shown assembled into airline seats (right - without bottom seat cushion).

Future Trends & Opportunities

There are many trends that currently favor increased use of high-performance thermoplastic composites, including the need to reduce weight, lower costs, increase manufacturing productivity (processing speeds), reduce finishing steps, lower VOC emissions (during manufacturing and throughout part use life), and more easily recover / reuse material at end of component life. Fortunately, there are many process options for producing prepreg, semipreg, and preforms from these resins, as well as many forming methods for shaping high-performance thermoplastic composite parts. In every case, cycle times are faster, process steps are fewer, and finished part weight and costs are lower than thermoset composite offsets. As industry successfully uses these materials and acceptance grows, more opportunities are expected to open up and more applications are expected to go straight from metals to thermoplastic composites (without first being converted to thermosets, as was done in the past). Fortunately, there is no need to sacrifice stiffness, strength, damage tolerance, dimensional stability, and resistance to chemicals and temperature fluctuations to save weight and costs when converting from metals to high-performance thermoplastics.

References

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