Designing With Plastics for Military Equipment
U.S. soldiers carry loads of equipment and protective gear that can weigh anywhere from 45 to 130 pounds. This is more than enough to quickly tire even the most well-conditioned personnel, contributing to reduced mobility and impaired decision-making that could result in casualties.
Meanwhile, in the skies there has been a proliferation of unmanned aircraft and remotely controlled jet fliers. These aircraft need to travel long distances with limited fuel on board and need materials that make them “invisible” to enemy radar.
To meet the challenges of evolving – often dangerous – circumstances, and to leverage advancing technologies that provide equipment to help the military operate safer, manufacturers need to deliver products that are lighter, stronger and highly functional while simultaneously reducing the total cost of ownership (TCO) associated with their products, including design, production, delivery and maintenance.
Injection-molded plastic parts made from advanced materials are increasingly replacing metal in a variety of military applications, providing a practical solution that accommodates the needs of manufacturers, contractors and users.
Metals vs. Plastics
Metals going back to the Iron Age have been the materials of choice for equipping the military. Metals offer lower thermal expansion and high strength to survive the rugged conditions of combat.
Many metals also offer high electrical conductivity, making them ideal for shielding electronic equipment; others offer high thermal conductivity, so they are well-suited to applications requiring the rapid dissipation of heat, such as engine cooling. Finally, metal parts can be produced to very tight tolerances, though the secondary machining operations required in achieving that precision could be costly.
Today, plastics are replacing metals in military applications due to a growing number of advantages, including the following:
Plastics are much lighter than metals, making them a better choice for a wide variety of uses, from armor and clothing to equipment, protective gear and vehicles. Not only does plastic significantly reduce equipment carry weights – by as much as 20 pounds in some military situations – it increases vertical and horizontal user agility and safety in accord with the U.S. Department of Defense Joint War Fighting Science and Technology Plan.
Freedom of Design and Assembly
Plastics are subject to fewer assembly constraints, enabling manufacturers to consolidate multiple parts into a single, injection-molded plastic part. The ability to design plastic parts with complex geometries also means multiple parts can be assembled using the method best suited to a particular application – such as welding, heat staking or mechanical snap-fit.
Today's sophisticated plastics are extremely durable and outperform comparable metals in resistance to heat, chemicals, moisture and impact. (See “Choosing the Best Plastic” and “Additives for Options” sections, below, for discussion about the characteristics of the major plastics families and how complementary materials can be added to further improve strength and durability.)
Ease of Finish
The color of equipment used in the field or in the air can be an important element of its utility – from a matte finish that decreases glare to a camouflage pattern that enables stealth. With plastics, manufacturers and molders can create virtually any color or finish during the production process and eliminate the need for expensive secondary painting or coating operations otherwise required by metal parts. Moreover, the finish color on plastics will not wear off from rugged wear.
Total Cost of Ownership
Taken together, the lower weight, design, assembly, durability and finishing advantages of injection-molded plastic parts result in dramatically lower total costs. Compared with metal, plastic parts do not require expensive secondary operations, such as machining or painting. In addition, because plastic parts can be designed in much more complex shapes, the manufacturer can reduce the total number of parts produced by as much as 70%.
Similarly, the weight savings created by transitioning to plastic reduces overall fuel and transportation costs. In the case of the military, this is essential since it is the world's single largest consumer of oil at more than 340,000 barrels per day, and the U.S. Department of Defense is requiring its contractors to help improve fuel efficiency.
As explained in a recent blog from Sandvik Coromant, a Sweden-based global tool supplier to the metal cutting industry:
In the modern age, the aerospace industry is also looking ahead to tougher, lighter, and more heat-resistant materials that would lessen emissions, cut fuel costs, and enable higher speeds. So far, in the aviation industry, composites have been the go-to material.
Per Dr. Eleanor Merson, the company's composite research specialist, “Thirty years ago, five to six percent of an aircraft was made up of composites; now, a plane is made up of about 50% composite material.”
Remaining undetected gives fighting forces an upper hand, but traditional metal components leave large electromagnetic radar/sonar echo signatures and infrared heat source footprints. A variety of non-conducting and insulating resins have been introduced to increase the stealth capabilities for military applications, including:
Domes constructed from polymer matrix composites that shield detection equipment and deaden position-revealing vibration on military ships and aircraft.
Military helicopters outfitted with polymer foam blades and Kevlar-car-bon fiber structural materials to enhance multi-spectral stealth capabilities (radar, infrared and acoustic).
Flexible, polymer matrix-based coatings used on a number of military vehicles to thwart “normal” and “thermal” visual detection.
Keys to Conversion
There are several considerations that impact successful metal-to-plastic conversion:
The part design process starts with identifying application requirements in three dimensions: mechanical, thermal and environmental. Will the part or product be dropped? Will it be used in an extremely hot or cold environment? Will it be exposed to harsh chemicals? How long does it have to last? The answers to these and other related questions provide a focal point for the next steps in part development and meeting functional and cost requirements.
Choosing the Best Plastic
Choosing the best plastic for your application requires deep knowledge of the wide range of possible materials. In addition to relying upon an experienced injection-molding partner to guide selection, possessing a basic understanding of the types of plastics is essential. Here's an overview:
Plastics are made up of polymers, meaning long chains of repeated molecule units. The ways in which the chains intertwine determine the plastic's macroscopic properties.
Typically the polymer chain orientations are random, giving the plastic an amorphous, structure. Amorphous plastics have good impact strength and toughness. Examples include acrylonitrile-butadiene-styrene (ABS), styrene-acrylonitrile copolymer (SAN), polyvinyl chloride (PVC), polycarbonate (PC) and polystyrene (PS).
If the polymer chains arrange themselves in an orderly, densely packed way, the plastic is said to be crystalline. Crystalline plastics share many properties with crystals, and generally will have lower elongation and flexibility properties than amorphous plastics, but better chemical resistance. Examples of crystalline plastics include acetal (POM), polyamide (PA; nylon), polyethylene (PE), polypropylene (PP), polyester (PET, PBT) and polyphenylene sulfide (PPS).
Additives for Options
Chemists can change plastics’ characteristics for military application by mixing in different types of polymers or by adding non-plastic materials. For example, particulate fillers increase the formula's modulus and electrical conductivity, improve resistance to heat or ultraviolet light, and reduce cost. Plasticizers go into the mix to decrease modulus and increase flexibility. Other additives can increase resistance to ultraviolet light and heat or prevent oxidation.
Glass fibers, carbon, stainless steel, and various coated fibers or Kevlar all have high reinforcing properties of tensile strength, increased tensile and flexural modulus, good toughness, and stress/strain behavior similar to that of metals. They can improve mechanical properties provided specific part and tool design is used to position the fibers in areas where the application requires added strength.
Designers are taking plastics beyond Kevlar. A colloid blend of silica nanoparticles and polyethylene glycol (PEG) is being tested as a type of “passive and intelligent” body armor, meaning it is semi-viscous when equipment is not in active use, but hardens immediately upon impact.
A short list of popular additives and their properties includes:
Glass fibers improve stiffness and increase heat resistance.
Stainless steel fillers improve conductivity and shielding.
Lubricant fillers reduce wear and friction.
Mineral fillers improve electrical performance and sound dampening, reduce cost, and improve dimensional stability.
Impact modifiers improve toughness.
Flame retardants increase resistance to burning.
Part Design and Analysis
Part design and analysis is a critical aspect of any metal to plastic conversion. Experienced complex injection molders can identify any potential issues early, modify the design to resolve them, then reevaluate and validate through a process of continuous improvement and tight quality control.
The tools available for part analysis include mold-filling simulation, cooling simulation, predictive shrinkage and warping, and finite element analysis. All provide assurance that the part will perform as intended for lower failure risk, and to safeguard large investments in equipment and tooling.
Plastics Respond to Changing Conditions
The “battlefield” has changed significantly since the beginning of this century, and to meet the new demands our military must possess greater levels of flexibility and agility to respond to new challenges. The availability of myriad plastics and trusted relationships with experienced complex injection molders enable designers for military equipment and aircraft to think more about solutions and less about limitations, with the promise of even more developments in plastics in the future.
This article was written by Al Timm, Business Development Engineer, Kaysun Corporation (Manitowoc, WI). For more information, Click Here .