Plastic Materials Proving to Be Good Fit for Aerospace Parts
As engineers look toward next generation designs, modern aircraft are making greater use of engineering plastics and composites to achieve higher levels of performance.
Materials science has always played a critical role in advancing aerospace technology. In the early 20th century, aircraft were constructed using wood frames with fabric surfaces. These primitive designs were later replaced by aircraft made primarily from metal. Today, modern aircraft and spacecraft are manufactured from a wide range of materials, including many plastics.
The use of engineering plastics has allowed aerospace engineers to achieve greater aircraft performance than would be possible using the older all-metal designs. The engineers who are developing the next generation of commercial aircraft, military aircraft, and advanced spacecraft will almost certainly need to make even more extensive use of innovative polymers to push the envelope of aircraft and spacecraft performance.
This article discusses the advantages of plastics for aerospace designs, provides an overview of some of the ways that plastics are used in the construction of aerospace components, and offers some design considerations when selecting a plastic material for an aerospace application.
Advantages of Plastics in Aerospace Applications
Plastics offer a number of advantages for the aerospace industry when compared with other materials, such as metals and ceramics. One important advantage of plastics is their low specific gravity. A typical plastic will have half the density of aluminum and one-sixth (1/6) the density of steel. When fillers such as glass or carbon fibers are used, plastics can achieve relatively high modulus (stiffness), and the combination of high stiffness and light weight results in a high specific modulus. This is beneficial for aerospace applications because low weight tends to result in fuel efficiency.
Some plastics are extremely tough, which allows them to withstand impact without failure. For example, many military and civilian aircraft canopies are now manufactured from acrylic or polycarbonate plastics, which are significantly lighter than glass and have many times the impact strength of glass. The interiors of aircraft are often subject to impact as travelers manipulate carry-on luggage, and high-toughness plastics, such as KYDEX® Thermoplastics, are robust enough to stand up to these conditions. In both of these examples, the use of plastics has resulted in lightweight, robust parts that are well suited for use in aerospace applications.
Plastics are often chosen for use in electrical applications, such as the housings of aircraft electrical connectors. These parts require plastics with high dielectric strength, such as polyetherimide, PEEK, or DuPont™ Vespel® Polyimide. In some cases, electrical connectors must be able to operate at low or elevated temperatures, and they may be exposed to chemicals during service. Additionally, the plastic materials used in electrical applications generally have to meet stringent flammability requirements.
Plastics are also valuable in aircraft designs because of their ability to interface with mating metal parts without galling or otherwise damaging the metal components. DuPont™ Vespel® Polyimide connecting splines and locking fasteners are two examples of applications where this is important. Additionally, plastics are used for bearings and bushings in aircraft, particularly in operating environments where designers don't want to rely on external lubrication for moving parts. Plastics with friction and wear additives, such as bearing grades of PEEK and friction and wear grades of DuPont™ Vespel®, are well suited for many aerospace bearing applications.
Locking fasteners with DuPont™ Vespel® locking elements are widely used in aerospace applications because of their creep resistance, high temperature capability, and their ability to be repeatedly assembled and disassembled without damaging mating metal surfaces.
One of the most prominent uses of plastics for aerospace applications is for seals that manage the flow of liquids and gasses throughout aircraft systems. Plastic seals are selected for their chemical resistance, high and low temperature capabilities, creep resistance, and their ability to conform to mating metal surfaces and create tight seals. Additionally, specialty seals manufactured from high-performance polymers can operate in extreme environments, including high pressure or vacuum conditions.
Design Considerations for Aerospace Parts
There are a number of important design issues–including operating conditions, part geometry, tolerance issues, and flammability requirements–that must be considered when choosing a plastic material for an application. Aerospace applications may require parts to operate throughout a broad temperature range, from very cold temperatures high in the atmosphere to very hot temperatures when parts must operate near jet engines. Plastic materials will generally become more brittle when they are cold and less stiff when heated. Additionally, plastics that are exposed to high temperatures over a long period of time may creep and/or thermally degrade. Therefore, it is important to look at both low temperature and high temperature properties data when selecting a plastic for an aerospace design.
Also, plastics tend to have much higher coefficients of thermal expansion than metals, which can create tolerance problems when designing assemblies with plastics and metals that must fit together over a broad temperature range. That being said, there are a number of filled plastics that have lower CTEs that more closely match the thermal expansion of metals.
For some applications, the operating environment may include exposure to ultraviolet radiation and, possibly, exposure to chemicals such as cleaners, lubricants, hydraulic fluids, and fuels. Some polymers have excellent resistance to chemicals and radiation, and others will quickly fail in these environments, so it is important to check on the chemical and radiation resistance of a plastic prior to specifying it for an application where service in these conditions will be required.
When designing with plastics, it is important to have "plastic-friendly" part geometries that avoid stress concentrations. For example, fasteners that distribute stress and internal corners with generous radii tend to create better results than flat-head screws and sharp corners, where stress concentrations can cause cracking issues. Cracking problems can become even more severe if an application involves thermal cycling or vibration, or if the parts will be exposed to aggressive chemicals.
Finally, it is important to consider the flammability requirements for the plastics to be used in an aerospace design. Some plastics will resist burning and others will burn very readily under certain conditions. Civilian aircraft, military aircraft, and spacecraft will generally have flammability requirements for the materials of construction, and it is important to consider the flammability performance requirements for a given material prior to specifying it in an aircraft.
In conclusion, plastics provide a number of benefits for aerospace applications, and modern aircraft make extensive use of plastics to achieve high levels of performance. That being said, there is some complexity involved with selecting the correct plastic for an application and designing the parts with the appropriate part geometry and tolerances to ensure performance throughout the service life of the device.
ABOUT THE AUTHOR
Dr. Keith Hechtel is Senior Director of Business Development for Curbell Plastics, Inc., based in Orchard Park, NY. Dr. Hechtel is a recognized speaker on plastic materials and plastic part design. He has conducted numerous presentations for engineers, designers, and fabricators in both industrial and academic settings.
With degrees in geology, industrial technology, and business administration, plus 25-plus years of plastics industry experience, Keith leads the Curbell Plastics Business Development team and offers a unique and practical perspective to material selection and application counseling. He has helped hundreds of businesses achieve quality improvements and cost savings through the use of performance plastics. Contact Keith.