Using Thermoplastics in Aerospace Applications
In August 2017, Qantas Airlines laid down the challenge to both Boeing and Airbus to offer an aircraft that can cross one of aviation's “last frontiers.” That “last frontier” was an aircraft capable of flying an economical passenger load non-stop for over 20 hours. This would allow Qantas to offer direct service from Sydney to London or New York. Weight reduction through the use of thermoplastics and other technologies would be the key to any chance of success in this endeavor.
Aluminum, steel, and titanium used to reign supreme in the world of aerospace manufacturing, taking up 70% of the average aircraft. Yet as demands for weight reduction and fuel efficiency increase, metals are losing ground to the new kid on the block – thermoplastic polymers and composites. You need only consider the latest generation of modern aircraft to see the impact these materials are having on aerospace manufacturing.
For example, look at the A350 XWB. Over 50% of this fuel-efficient aircraft is built from carbon-reinforced plastic composites, while its competitor, the Boeing 787, is also roughly 50% composite materials. Notably, this trend isn't limited to Airbus and Boeing; other companies such as Bombardier, BAE Systems, Raytheon, GE Aviation, and Lockheed Martin have also leaned into using thermoplastics and composites in their aircraft and defense related systems.
What is the reason behind this drastic shift from aluminum and steel to thermoplastics like PEEK, PPSU, PEI, and other polymer materials? As it turns out, there's more than one explanation.
With fuel costs representing the highest of all operating costs for aircraft, it's not surprising that the demand for lighter aircraft has risen with it. To put this in perspective, it is interesting to note that years ago MIT researchers estimated that for each passenger to carry a cell phone, it cost Southwest Airlines and additional $1.2m annually in fuel costs. If the passengers each carried a laptop then the cost jumped to $21.6m!
Polymer and composite materials meet the challenge of helping reduce aircraft weight by being up to ten times lighter than metal. This sharply lowers lifetime fuel costs, reduces emissions, and extends flight range. By and large, the most efficient airframes contain large amounts of carbon-fiber reinforced polymers and composites. These airframes and components can be responsible for reducing aircraft weight by as much as 20%.
For this reason, the market for machining components from lightweight, high-performance thermoplastics is growing, especially for aerostructure applications. Aerospace-grade polymers such as polyetheretherketone (PEEK), polyphenylsulfone (PPSU), polyetherimide (PEI), and polyetherketoneketone (PEKK) provide a reliable and cost-effective way to reduce weight. More importantly, they add value beyond weight reduction for many applications due to unique properties advantageous to metallic components where their superior corrosion and fatigue resistance, tensile strength, and durability can lead them to outperform metal.
High-performance thermoplastics meet more stringent flame, smoke, and toxicity (FST) standards due to their inherent flame resistance or, in some cases, flame retardancy. A few standouts are PPSU (RADEL), PEI (ULTEM), PPS (RYTON), and PEEK (VICTREX 450G), which have UL94 V-0 flammability ratings without any flame-retardant additives.
It shouldn't be surprising then to know that thermoplastics can survive in extreme temperatures. Two striking examples are polyimide (PI) (VESPEL) and polybenzimidazole (PBI) (CELAZOLE), which can operate uninterrupted from cryogenic temperatures to over 550°F, with intermittent exposure to over 900°F. This, combined with resistance to high wear and friction, gives PI and PBI impressive longevity in hostile environments.
Hostile environments aren't always made so by temperature, though. Resistance is key for aerospace applications which involve exposure to harsh chemicals. The high chemical resistance of thermoplastics like PPS means it can operate even when submerged in a severe chemical environment, where metals are prone to dissolving. PPS's dimensional and density stability has also made it a favorite for aircraft components, whether for interior, mechanical, or exterior.
Another crucial feature for aerospace is corrosion resistance. This is something that thermoplastics excel at, especially when compared to aluminum and steel. One of the primary causes of structural failure for aircraft is galvanic corrosion between dissimilar materials. As aerospace manufacturing leans towards using thermoplastic composite fuselages, metallic structural brackets and other associated causes of galvanic corrosion are being replaced, creating overall safer aircraft.
Both the Boeing 787 and Airbus A350 XWB offer composite fuselages which are able to operate at a higher-pressure differential, which in turn results in a cabin altitude lower than with previous aluminum fuselages. The composite materials allow for higher strength, lower fatigue and no corrosion, allowing for a lower cabin altitude with higher humidity resulting in a less fatigued passenger upon arrival.
Insulation and Radar-Absorption
While metallic components require extensive and costly secondary processing and coating to achieve their insulating properties, polymers and composites are inherently thermally and electrically insulating.
A perfect example of this is PEI, which has one of the highest dielectric strengths of any thermoplastic material. Its low rate of thermal conductivity makes it a frequent choice for aircraft galley equipment, while its UL94 V-O flame rating makes it ideal for aircraft interior components. In fact, ULTEM 2300 – a 30% glass-reinforced grade of ULTEM PEI – is often used as a direct replacement for aluminum due to its similar coefficient of thermal expansion to 6061-T6.
Along with their insulation properties, polymer components have the additional benefit of being radar-absorbent. This makes thermoplastics useful for stealth military aircraft applications, where evading radar detection is mission-critical. Metals, on the other hand, tend to be strong reflectors of electromagnetic waves, making them easy to detect by radar.
Manufacturing and Design Flexibility
One reason aluminum had been so frequently used for aerospace is that it was considered easy to manufacture into aircraft components. A misconception is that thermoplastics don't share that quality. Advances in thermoplastic manufacturing and processing have allowed for great flexibility in both manufacturing and design. One major thermoplastic and composite manufacturing misconception is that plastic parts cannot be easily machined. On the contrary, thermoplastic components have been machined into geometrically complex mission critical components over the past few decades.
Thermoplastics can be machined to extremely tight tolerances up to 0.002mm, which can be critical for aerospace applications. Processes such as rapid thermoforming, autoclave processing, tape and fiber placement techniques and press forming are also all possible with thermoplastic polymers and composites.
Many thermoplastics also have better fatigue properties than metals do, and they tolerate larger deflections without deforming. To prove this fact, just look at the wing flex on a Boeing 787 in flight as shown in the accompanying illustration.
PEEK is one thermoplastic that has fast become a popular replacement for metal in aerospace. It's a natural choice since PEEK's lightweight nature, mechanical strength, creep and fatigue resistance, and ease of processing all give it great versatility. PEEK's diversity of applications includes flight control, fuel systems, aircraft interiors, and engine and aerodynamic-related components.
There are numerous types of thermoplastics gaining ground in aerospace, many more than are named in this article. Yet it's important to remember that each thermoplastic, though sharing broad characteristics, has its own unique strengths that make it better suited for some applications over others. It's not unexpected that an aircraft engine has different needs than a RADEL aircraft galley bezel or a landing gear component.
When determining thermoplastic solutions, it's crucial to practice due diligence and partner with an experienced aerospace plastics manufacturer. They should be able to offer material consultancy as part of their expertise and discuss the pros and cons of each selection, as well as display experience in manufacturing it. Take a careful look at their industry standards and certifications, like the ISO9001:2015 AS9100D, to be sure that they understand the regulatory requirements for aerospace applications. Most importantly, look at the supplier's experience in manufacturing mission critical thermoplastic aerospace components, as many times the actual machining talent and experience is the difference between a failed or successful thermoplastic component.
In a field as mission-critical as aerospace, success often relies on choosing the material best suited for an application. Increasingly, that material is thermoplastic.
This article was written by John Macdonald, President, AIP Precision Machining (Daytona Beach, FL). For more information, visit here .