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Imagine the space shuttle lifting off in an explosion of cryogenic fuel from its external tank and solid rocket boosters, climbing across the sky, entering space at hypersonic speed and then returning to earth through the scorching friction of earth's atmosphere.
You would have to think long and hard to imagine a more severe testing ground for engineering materials. And if someday the space shuttle was strong enough and light enough to power itself into orbit under its own store of fuels, the miracle will be made possible through the innovative use of lightweight plastic materials known as composite thermoplastics. This paper explores the expanding role of these materials in today's high performance air, sea, and space ships.
For many people, "plastic" means "cheap and breakable." But when engineers search for new ways to enhance weight savings, corrosion resistance, shock and vibration dampening and stealth they immediately turn to plastic - the only alternative material capable of meeting, and beating, the established performance of aluminum, brass, titanium and steel.
The name "plastic" refers to the ability to form or shape a material, or to the moldability a material adopts under forces such as pressure or heat. Engineers often use the term "polymer" when referring to plastic materials, because it more clearly describes how many (poly) chemical units (mers) form up in complex chains to create modern plastic resins.
Polymers are created by subjecting various chemical and petroleum-based ingredients to heat and pressure in sealed vats or vessels. Specific chemical additives control how the polymer is formed and contribute to its performance in such areas as surface hardness and flame resistance. The process of mixing base materials with chemical additives to create specific types of plastic resins is called "polymerization." The resulting plastic materials can be classified in various ways - by chemical or physical structure, by strength or thermal performance and by optical or electrical properties.
The most significant structural classification for polymers has to do with their shape at the molecular level. Polymers whose long, linear shaped molecules fold tightly together into packed and ordered areas are classified as semicrystalline. Polymers with bulkier molecular shapes not inclined to fold up into spaghetti-like crystals are classified as amorphous.
Semicrystalline polymers are characterized by very good to excellent wear resistance and the ability to withstand high heat and exposure to caustic chemicals. Semicrystalline resins are however relatively more difficult to mold and also tend to exhibit uneven mold shrinkage with elevated stress levels.
Amorphous materials are known for their excellent strength, stiffness and dimensional stability. Amorphous resins are generally easier to mold into tubular shapes and have a good "knit" or weld line strength coefficient. Amorphous resins have good dimensional stability and exhibit even mold shrinkage with lower stress levels.
Strength and thermal resistance are the most sought after characteristics in polymers used in high-performance applications. While consumer products may be produced using "commodity" plastics such as polyethylene and polypropylene, products intended for commercial and military aerospace applications, for example, must be produced using "engineering plastics" or other specialized, high temperature polymers.
Engineering plastics such as Polyetherimide (PEI), polyphthalamide (PPA), and polyphenylene sulfide (PPS) are designed specifically for use in high operating temperature environments. Resins such as polyetheretherketone (PEEK) and various liquid crystal polymers (LCP) are also capable of withstanding extremely high temperatures. These later, high-performance plastics also meet stringent outgassing and flammability requirements.
Historically, "thermoplastic" resins, or those which may be melted and remelted in injection molding machinery are preferred for durable goods manufacturing over "thermoset" resins, such as epoxy, which are hardened via chemical reaction. Thermoplastic polymers generally have shorter processing times than thermosetting polymers and can be reheated and reformed repeatedly if required.
Strength Versus Weight
Let's return for a moment to our dream of a space shuttle strong enough and yet light enough to operate exclusively on its own store of fuels:
To propel itself into space, our "supershuttle" would need to be built from the lightest weight materials available. But to withstand the extreme aerodynamic forces of space flight, it would also need to be incredibly stiff and strong. Materials used throughout the ship would need to be capable of withstanding extremely high operating temperatures. And any material which might be exposed to caustic chemicals and electrolytes would need to be completely corrosion resistant.
If you have been paying attention, you will have already guessed that plastic will play an important role in the construction of this ship. But plastic alone is only part of the story. The class of materials which will make up the bulk of this spacecraft - from the airframe itself down to the engine parts and electrical components - are known as composites.
In scientific terms, composites are materials in which particles or fibers are dispersed in a matrix. This simple definition would encompass such things as concrete (a composite made up of particles of sand and gravel mixed into a matrix of cement and water), and particle board (wood chips and particles suspended in glue).
The purpose of a composite is, of course, to create a substance which combines the constituent parts in some beneficial way. Concrete is harder and more durable than any of the individual ingredients could ever be on their own. Particle board is far stiffer and stronger than wood chips and glue could ever be if applied individually in a construction project.
So it is with composite plastics: Polymer resins can act as a matrix for a wide range of particle and fiber additives. Polymers can be reinforced with glass, minerals, and both conductive and nonconductive graphite fibers to meet a diverse range of mechanical, physical, chemical, thermal and electrical requirements. While certain fiber additives provide additional strength, others address electromagnetic and radio frequency shielding. Additives can also be used to increase flame retardency, to improve lubricity or, in the case of pigments, simply to change the color of the final product.
As was stated before, thermal properties are extremely important when selecting plastic materials for use in high-performance applications. For many such projects, a composite's glass transition temperature (the point at which the heated material softens) will dictate whether or not the material is suitable for use. But other properties, such as its specific gravity, hardness, refractive index, dielectric strength, conductivity, chemical resistance, UV and flame resistance are also critical in deciding which recipe of resins, fibers and additives will be selected for a particular project.
The benefits of modern plastic materials have not yet led to the wholesale elimination of metal from high-performance air, sea and space applications. Aluminum, for example, is still the material of choice for most high-density connectors and accessories. But several factors, including the drive to develop cadmium-free alternatives to plated aluminum parts, have contributed to the wider use of composites. Other important benefits of composites over metal materials include corrosion resistance, vibration dampening, weight reduction and stealth.
Corrosion Resistance: One of the most appealing attributes of composites is their unlimited corrosion resistance as compared to conventional materials. Aluminum interconnect components, for example, are subject to galvanic coupling which causes the metal material to be "sacrificed" to its cadmium/nickel plating. Since high-temperature plastic is not sacrificial to plating, finished products last longer, require less maintenance and directly reduce the overall cost of ownership of the interconnect system.
Vibration Dampening: Another major benefit of composite thermoplastics is vibration dampening. Unlike metals, polymer plastics are less subject to harmonic resonance due to their lighter weight and inherent attenuating properties. Which means threaded components made from these materials are far less likely to vibrate loose when subjected to prolonged periods of vibration and shock. Again, reduced maintenance and reduced cost of ownership are the major benefits realized by systems built from vibration dampening thermoplastics.
Weight Reduction: Next to their anticorrosive capabilities, the characteristic of composites that makes them most attractive is their ability to provide increased strength and stiffness at lighter weights than conventional materials. The typical weight savings for composites over aluminum is approximately 40% (depending on component design). Weight savings versus other materials are even more pronounced: 60% for titanium, 80% for stainless steel, and 80% for brass. Composite materials directly reduce aircraft empty weights and increase fuel fractions. For the aerospace engineer, this leads directly to smaller, lower-cost aircraft that use less fuel to perform a given mission.
Stealth: The reduction of magnetic signatures, corrosion related magnetic signatures and acoustic signatures is critical to the development of stealth applications. Signatures are those characteristics by which systems may be detected, recognized, and engaged. The reduction of these signatures can improve survivability of military systems, leading to improved effectiveness and fewer casualties. Composite thermoplastics are at the heart of a number of advanced stealth application development projects. Forty percent of the structural weight of the new F-22 will be polymer composites, and other systems such as the B-2 and F-117A are expanding their use of stealth technologies beyond basic shaping and material coating techniques to include the use of structural and component composite thermoplastics.
Glenair is the recognized leader in composite thermoplastic research and development for the interconnect accessory industry. In fact, no one else has tooled even a small fraction of the composite thermoplastic accessories available today from Glenair. The product line includes circular and rectangular connectors and accessories, cable junction boxes, conduit, conduit fittings, protective covers, shielding, shielding support rings, and more.
Glenair composite components are produced in injection molded and machined versions and are ideally suited for use in harsh environments where resistance to high temperatures, outgassing, corrosive fluids, fire, shock and vibration is required. Glenair composites are ASTM E595 space rated, and are qualified to the shock, vibration, thread strength and bend moment requirements of MIL-C-38999 and MIL-C 85049. The materials also meet stringent EMI/RFI/HIRF and indirect lightning strike performance specifications.
When selecting composite materials, it is essential to understand - at the molecular level - how performance properties such as elasticity and strength are provided for in each material type. Equally important, it is critical to understand the product design and manufacturing nuances of working with the various types of fibers and polymers. Tom Young, Glenair's senior composite products engineer heads up the company's research and development into composite component design, manufacturing, and plating, and is recognized as the interconnect accessory industry's foremost expert on composite thermoplastics. Glenair has the largest and most experienced staff of composite engineers and manufacturing experts in the interconnect accessory industry. Their combined expertise insures Glenair composite products mate correctly with both metal and composite connectors and meet the customer's most stringent performance requirements. All Glenair designs provide a dimensionally stable and cadmium-free alternative to plated aluminum and brass.
Versus Common Metal Materials
|Aluminum||Zinc Cobalt||350-500 Hrs.|
|Aluminum||Cadmium Nickel||500-1000 Hrs.|
|Aluminum||Zinc Nickel||500 Hrs.|
|Stainless Steel||Nickel||500-1000 Hrs.|
Glenair composite material options include Ultem® (PEI), Amodel® (PPA), Ryton® (PPS), Torlon® (PAI), PEEK and LCP. Base materials can be augmented with conductive and nonconductive additives and reinforcing fibers to meet specific functional specifications. As was mentioned before, each composite material has its own specific structural properties. The following descriptions provide a brief introduction to composite materials selection:
Ultem® (PEI) is an amorphous thermoplastic available both in extruded bars as well as pellets for injection molding. The material combines high performance with good processing characteristics and offers a combination of high heat resistance, high strength modulus and broad chemical resistance. Ultem 2300 is a 30 percent glass filled thermoplastic which displays excellent property retention and resistance to environmental stress. Ultem can be further reinforced with conductive fibers, or plated, for EMI resistance. Ultem performs in operating environments up to 378° F long term and 410° F short term. Ultem meets ASTME595 outgassing requirements, UL94 flammability requirements as well as zero halogen outgassing requirements.
Amodel® (PPA) is a primarily semicrystalline thermoplastic available in pellet form for injection molding. Amodel resins have excellent mechanical properties - strength, stiffness, fatigue and creep resistance - over a broad temperature range. Amodel AFA-6133VO is a 33 percent glass filled thermoplastic which contains a flame retardant additive to meet UL94 tests. The material also meets ASTME595 outgassing requirements. Amodel offers higher operating temperatures than Ultem: up to 392° F long term and 500° F short term. Amodel can be custom colored and can be augmented with conductive fibers, or plating, for EMI applications.
Ryton® (PPS) is a high temperature, injection molded material. It has good mechanical properties and excellent chemical resistance at elevated temperatures. Different grades are available including glass filled and glass/mineral filled versions. Ryton R4-XT is a 40 percent glass filled version engineered for improved knit-weld line characteristics. As a semicrystalline material, Ryton exhibits excellent resistance to prolonged exposure to high temperatures, up to 500° F. Ryton also provides outstanding resistance to a broad spectrum of aggressive chemicals and has very stable dielectric and insulating properties. Ryton is very sensitive to molding conditions and must be processed properly to achieve its maximum potential. Ryton meets ASTME595 outgassing requirements and UL94 flammability tests.
Torlon® (PAI) provides exceptional strength at high temperatures and excellent resistance to chemical solvents. Torlon is also very wear and friction resistant which makes it an ideal material for mechanical retention components such as anti-decoupling springs and contact retention springs. The injection moldable material is nonconductive and operates at temperatures up to 500° F. Torlon is not rated for ASTME595 outgassing requirements but meets UL94 flammability tests.
Polyetheretherketone (PEEK) is a semicrystalline thermoplastic which operates at extremely high temperatures, 500° F long term and 600 degrees F short term. PEEK is an injection moldable material and can be reinforced with glass, mineral, and graphite fibers. PEEK has one of the lightest strength to weight ratios and exhibits outstanding resistance to aggressive chemicals. PEEK 450GL30 is a 30 percent glass filled thermoplastic. The material can be custom colored and metalisized with electroless plating. PEEK meets ASTME595 outgassing requirements and UL94 flammability tests as well as zero halogen requirements.
Liquid Crystal Polymer (LCP) trade name Zydar is available from Glenair in pellet form for injection molded parts. Zydar is a crystalline thermoplastic with extremely good dimensional stability, which makes the material ideally suited for intricate thin wall components. The base resin can be glass or mineral filled. Zydar G-330, for example, is a 30 percent glass filled thermoplastic which operates at temperatures up to 610° F. Zydar meets ASTME595 outgassing requirements and UL94 flammability tests.
Re-Designing for Composites
It's a simple matter to compare thermoplastic materials against steel or aluminum and point out the advantages plastics bring to the table: They're lighter. They don't rust. They don't rattle loose. They can hide from radar. It's much more difficult to actually design composite components which take advantage of these properties while still meeting the requirements of the application for form, fit and function.
Every connector accessory, no matter what it is made of, still needs to thread onto the back of a connector. It must also intermate with other accessories, including both composite and metal versions. Other dimensional standards, such as the number of teeth on an interlock or the location and shape of polarizing keyways on a connector must be observed - again, regardless of what material is used to build the part.
Composite component design is further complicated due to the unique strengths and weakness of the material: Abrupt changes in wall thicknesses, for example, can lead to stress problems in both manufacture and use. Sharp, un-radiused angles can create stress and cause cracking. The length, shape, orientation and distribution of reinforcing fibers is also a critical concern, as is the impact of other additives, such as colorizers and flame retardants, on the behavior of the material during manufacture and use.
The fact that composites are increasingly being specified in interconnect systems, despite the complications of the design and manufacturing process, is a testimony to the true value the materials provide. The "supershuttle" may be just a dream for now. But the weight savings and corrosion resistance composites can bring to today's high-performance interconnect applications are not a dream. They represent real, out-of pocket savings in fuel consumption and system maintenance for a broad range of air, sea and space applications.
Glenair has been the leading manufacturer and supplier of commercial and Mil-Spec connector accessories since 1956. Building on that foundation, the company offers a dozen, full-spectrum product lines designed to meet every interconnect requirement. From hermetic connectors to fiber optics, from conduit systems to Micro-D assemblies, from composite enclosures to assembly tooling. And throughout the years, we've made outstanding customer service our approach to maintaining our position as the industry's best-value interconnect supplier:
- By designing quality into every part we ship.
- By delivering the fastest turnaround on quotes and custom orders.
- By establishing an unsurpassed sales, support and engineering presence in every major market in the world.
- By building the largest capacity, broadest capability factory in the interconnect accessory industry.
- By avoiding cumbersome business practices such as minimum orders and expediting charges, and...
- By maintaining the world's largest "Same-Day Shipment" inventory.